1.1. Document Structure

This document describes the core QUIC protocol and is structured as follows:¶

• Streams are the basic service abstraction that QUIC provides.¶

  • Section 2 describes core concepts related to streams,¶
  • Section 3 provides a reference model for stream states, and¶
  • Section 4 outlines the operation of flow control.¶

• Connections are the context in which QUIC endpoints communicate.¶

  • Section 5 describes core concepts related to connections,¶
  • Section 6 describes version negotiation,¶
  • Section 7 details the process for establishing connections,¶
  • Section 8 describes address validation and critical denial-of-service mitigations,¶
  • Section 9 describes how endpoints migrate a connection to a new network path,¶
  • Section 10 lists the options for terminating an open connection, and¶
  • Section 11 provides guidance for stream and connection error handling.¶

• Packets and frames are the basic unit used by QUIC to communicate.¶

  • Section 12 describes concepts related to packets and frames,¶
  • Section 13 defines models for the transmission, retransmission, and acknowledgment of data, and¶
  • Section 14 specifies rules for managing the size of datagrams carrying QUIC packets.¶

• Finally, encoding details of QUIC protocol elements are described in:¶

  • Section 15 (versions),¶
  • Section 16 (integer encoding),¶
  • Section 17 (packet headers),¶
  • Section 18 (transport parameters),¶
  • Section 19 (frames), and¶
  • Section 20 (errors).¶

Accompanying documents describe QUIC's loss detection and congestion control [RFCYYY3], and the use of TLS and other cryptographic mechanisms [RFCYYY2].¶

This document defines QUIC version 1, which conforms to the protocol invariants in [RFCYYY1].¶

To refer to QUIC version 1, cite this document. References to the limited set of version-independent properties of QUIC can cite [RFCYYY1].¶

1.2. Terms and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶

Commonly used terms in this document are described below.¶

QUIC:

The transport protocol described by this document. QUIC is a name, not an acronym.¶

Endpoint:

An entity that can participate in a QUIC connection by generating, receiving, and processing QUIC packets. There are only two types of endpoints in QUIC: client and server.¶

Client:

The endpoint that initiates a QUIC connection.¶

Server:

The endpoint that accepts a QUIC connection.¶

QUIC packet:

A complete processable unit of QUIC that can be encapsulated in a UDP datagram. One or more QUIC packets can be encapsulated in a single UDP datagram.¶

Ack-eliciting packet:

A QUIC packet that contains frames other than ACK, PADDING, and CONNECTION_CLOSE. These cause a recipient to send an acknowledgment; see Section 13.2.1.¶

Frame:

A unit of structured protocol information. There are multiple frame types, each of which carries different information. Frames are contained in QUIC packets.¶

Address:

When used without qualification, the tuple of IP version, IP address, and UDP port number that represents one end of a network path.¶

Connection ID:

An identifier that is used to identify a QUIC connection at an endpoint. Each endpoint selects one or more Connection IDs for its peer to include in packets sent towards the endpoint. This value is opaque to the peer.¶

Stream:

A unidirectional or bidirectional channel of ordered bytes within a QUIC connection. A QUIC connection can carry multiple simultaneous streams.¶

Application:

An entity that uses QUIC to send and receive data.¶

This document uses the terms "QUIC packets", "UDP datagrams", and "IP packets" to refer to the units of the respective protocols. That is, one or more QUIC packets can be encapsulated in a UDP datagram, which is in turn encapsulated in an IP packet.¶

1.3. Notational Conventions

Packet and frame diagrams in this document use a custom format. The purpose of this format is to summarize, not define, protocol elements. Prose defines the complete semantics and details of structures.¶

Complex fields are named and then followed by a list of fields surrounded by a pair of matching braces. Each field in this list is separated by commas.¶

Individual fields include length information, plus indications about fixed value, optionality, or repetitions. Individual fields use the following notational conventions, with all lengths in bits:¶

x (A):

Indicates that x is A bits long¶

x (i):

Indicates that x holds an integer value using the variable-length encoding described in Section 16¶

x (A..B):

Indicates that x can be any length from A to B; A can be omitted to indicate a minimum of zero bits, and B can be omitted to indicate no set upper limit; values in this format always end on a byte boundary¶

x (L) = C:

Indicates that x has a fixed value of C with the length described by L, which can use any of the three length forms above¶

x (L) = C..D:

Indicates that x has a value in the range from C to D, inclusive, with the length described by L, as above¶

[x (L)]:

Indicates that x is optional (and has a length of L)¶

x (L) ...:

Indicates that zero or more instances of x are present (and that each instance is length L)¶

This document uses network byte order (that is, big endian) values. Fields are placed starting from the high-order bits of each byte.¶

By convention, individual fields reference a complex field by using the name of the complex field.¶

Figure 1 provides an example:¶

Example Structure {
  One-bit Field (1),
  7-bit Field with Fixed Value (7) = 61,
  Field with Variable-Length Integer (i),
  Arbitrary-Length Field (..),
  Variable-Length Field (8..24),
  Field With Minimum Length (16..),
  Field With Maximum Length (..128),
  [Optional Field (64)],
  Repeated Field (8) ...,
}

When a single-bit field is referenced in prose, the position of that field can be clarified by using the value of the byte that carries the field with the field's value set. For example, the value 0x80 could be used to refer to the single-bit field in the most significant bit of the byte, such as One-bit Field in Figure 1.¶

2.1. Stream Types and Identifiers

Streams can be unidirectional or bidirectional. Unidirectional streams carry data in one direction: from the initiator of the stream to its peer. Bidirectional streams allow for data to be sent in both directions.¶

Streams are identified within a connection by a numeric value, referred to as the stream ID. A stream ID is a 62-bit integer (0 to 2^62-1) that is unique for all streams on a connection. Stream IDs are encoded as variable-length integers; see Section 16. A QUIC endpoint MUST NOT reuse a stream ID within a connection.¶

The least significant bit (0x1) of the stream ID identifies the initiator of the stream. Client-initiated streams have even-numbered stream IDs (with the bit set to 0), and server-initiated streams have odd-numbered stream IDs (with the bit set to 1).¶

The second least significant bit (0x2) of the stream ID distinguishes between bidirectional streams (with the bit set to 0) and unidirectional streams (with the bit set to 1).¶

The two least significant bits from a stream ID therefore identify a stream as one of four types, as summarized in Table 1.¶

The stream space for each type begins at the minimum value (0x0 through 0x3, respectively); successive streams of each type are created with numerically increasing stream IDs. A stream ID that is used out of order results in all streams of that type with lower-numbered stream IDs also being opened.¶

2.2. Sending and Receiving Data

STREAM frames (Section 19.8) encapsulate data sent by an application. An endpoint uses the Stream ID and Offset fields in STREAM frames to place data in order.¶

Endpoints MUST be able to deliver stream data to an application as an ordered byte stream. Delivering an ordered byte stream requires that an endpoint buffer any data that is received out of order, up to the advertised flow control limit.¶

QUIC makes no specific allowances for delivery of stream data out of order. However, implementations MAY choose to offer the ability to deliver data out of order to a receiving application.¶

An endpoint could receive data for a stream at the same stream offset multiple times. Data that has already been received can be discarded. The data at a given offset MUST NOT change if it is sent multiple times; an endpoint MAY treat receipt of different data at the same offset within a stream as a connection error of type PROTOCOL_VIOLATION.¶

Streams are an ordered byte-stream abstraction with no other structure visible to QUIC. STREAM frame boundaries are not expected to be preserved when data is transmitted, retransmitted after packet loss, or delivered to the application at a receiver.¶

An endpoint MUST NOT send data on any stream without ensuring that it is within the flow control limits set by its peer. Flow control is described in detail in Section 4.¶

2.3. Stream Prioritization

Stream multiplexing can have a significant effect on application performance if resources allocated to streams are correctly prioritized.¶

QUIC does not provide a mechanism for exchanging prioritization information. Instead, it relies on receiving priority information from the application.¶

A QUIC implementation SHOULD provide ways in which an application can indicate the relative priority of streams. An implementation uses information provided by the application to determine how to allocate resources to active streams.¶

2.4. Operations on Streams

This document does not define an API for QUIC; it instead defines a set of functions on streams that application protocols can rely upon. An application protocol can assume that a QUIC implementation provides an interface that includes the operations described in this section. An implementation designed for use with a specific application protocol might provide only those operations that are used by that protocol.¶

On the sending part of a stream, an application protocol can:¶

• write data, understanding when stream flow control credit (Section 4.1) has successfully been reserved to send the written data;¶
• end the stream (clean termination), resulting in a STREAM frame (Section 19.8) with the FIN bit set; and¶
• reset the stream (abrupt termination), resulting in a RESET_STREAM frame (Section 19.4) if the stream was not already in a terminal state.¶

On the receiving part of a stream, an application protocol can:¶

• read data; and¶
• abort reading of the stream and request closure, possibly resulting in a STOP_SENDING frame (Section 19.5).¶

An application protocol can also request to be informed of state changes on streams, including when the peer has opened or reset a stream, when a peer aborts reading on a stream, when new data is available, and when data can or cannot be written to the stream due to flow control.¶

3.1. Sending Stream States

Figure 2 shows the states for the part of a stream that sends data to a peer.¶

       o
       | Create Stream (Sending)
       | Peer Creates Bidirectional Stream
       v
   +-------+
   | Ready | Send RESET_STREAM
   |       |-----------------------.
   +-------+                       |
       |                           |
       | Send STREAM /             |
       |      STREAM_DATA_BLOCKED  |
       v                           |
   +-------+                       |
   | Send  | Send RESET_STREAM     |
   |       |---------------------->|
   +-------+                       |
       |                           |
       | Send STREAM + FIN         |
       v                           v
   +-------+                   +-------+
   | Data  | Send RESET_STREAM | Reset |
   | Sent  |------------------>| Sent  |
   +-------+                   +-------+
       |                           |
       | Recv All ACKs             | Recv ACK
       v                           v
   +-------+                   +-------+
   | Data  |                   | Reset |
   | Recvd |                   | Recvd |
   +-------+                   +-------+

The sending part of a stream that the endpoint initiates (types 0 and 2 for clients, 1 and 3 for servers) is opened by the application. The "Ready" state represents a newly created stream that is able to accept data from the application. Stream data might be buffered in this state in preparation for sending.¶

Sending the first STREAM or STREAM_DATA_BLOCKED frame causes a sending part of a stream to enter the "Send" state. An implementation might choose to defer allocating a stream ID to a stream until it sends the first STREAM frame and enters this state, which can allow for better stream prioritization.¶

The sending part of a bidirectional stream initiated by a peer (type 0 for a server, type 1 for a client) starts in the "Ready" state when the receiving part is created.¶

In the "Send" state, an endpoint transmits -- and retransmits as necessary -- stream data in STREAM frames. The endpoint respects the flow control limits set by its peer and continues to accept and process MAX_STREAM_DATA frames. An endpoint in the "Send" state generates STREAM_DATA_BLOCKED frames if it is blocked from sending by stream flow control limits (Section 4.1).¶

After the application indicates that all stream data has been sent and a STREAM frame containing the FIN bit is sent, the sending part of the stream enters the "Data Sent" state. From this state, the endpoint only retransmits stream data as necessary. The endpoint does not need to check flow control limits or send STREAM_DATA_BLOCKED frames for a stream in this state. MAX_STREAM_DATA frames might be received until the peer receives the final stream offset. The endpoint can safely ignore any MAX_STREAM_DATA frames it receives from its peer for a stream in this state.¶

Once all stream data has been successfully acknowledged, the sending part of the stream enters the "Data Recvd" state, which is a terminal state.¶

From any state that is one of "Ready", "Send", or "Data Sent", an application can signal that it wishes to abandon transmission of stream data. Alternatively, an endpoint might receive a STOP_SENDING frame from its peer. In either case, the endpoint sends a RESET_STREAM frame, which causes the stream to enter the "Reset Sent" state.¶

An endpoint MAY send a RESET_STREAM as the first frame that mentions a stream; this causes the sending part of that stream to open and then immediately transition to the "Reset Sent" state.¶

Once a packet containing a RESET_STREAM has been acknowledged, the sending part of the stream enters the "Reset Recvd" state, which is a terminal state.¶

3.2. Receiving Stream States

Figure 3 shows the states for the part of a stream that receives data from a peer. The states for a receiving part of a stream mirror only some of the states of the sending part of the stream at the peer. The receiving part of a stream does not track states on the sending part that cannot be observed, such as the "Ready" state. Instead, the receiving part of a stream tracks the delivery of data to the application, some of which cannot be observed by the sender.¶

       o
       | Recv STREAM / STREAM_DATA_BLOCKED / RESET_STREAM
       | Create Bidirectional Stream (Sending)
       | Recv MAX_STREAM_DATA / STOP_SENDING (Bidirectional)
       | Create Higher-Numbered Stream
       v
   +-------+
   | Recv  | Recv RESET_STREAM
   |       |-----------------------.
   +-------+                       |
       |                           |
       | Recv STREAM + FIN         |
       v                           |
   +-------+                       |
   | Size  | Recv RESET_STREAM     |
   | Known |---------------------->|
   +-------+                       |
       |                           |
       | Recv All Data             |
       v                           v
   +-------+ Recv RESET_STREAM +-------+
   | Data  |--- (optional) --->| Reset |
   | Recvd |  Recv All Data    | Recvd |
   +-------+<-- (optional) ----+-------+
       |                           |
       | App Read All Data         | App Read Reset
       v                           v
   +-------+                   +-------+
   | Data  |                   | Reset |
   | Read  |                   | Read  |
   +-------+                   +-------+

The receiving part of a stream initiated by a peer (types 1 and 3 for a client, or 0 and 2 for a server) is created when the first STREAM, STREAM_DATA_BLOCKED, or RESET_STREAM frame is received for that stream. For bidirectional streams initiated by a peer, receipt of a MAX_STREAM_DATA or STOP_SENDING frame for the sending part of the stream also creates the receiving part. The initial state for the receiving part of a stream is "Recv".¶

For a bidirectional stream, the receiving part enters the "Recv" state when the sending part initiated by the endpoint (type 0 for a client, type 1 for a server) enters the "Ready" state.¶

An endpoint opens a bidirectional stream when a MAX_STREAM_DATA or STOP_SENDING frame is received from the peer for that stream. Receiving a MAX_STREAM_DATA frame for an unopened stream indicates that the remote peer has opened the stream and is providing flow control credit. Receiving a STOP_SENDING frame for an unopened stream indicates that the remote peer no longer wishes to receive data on this stream. Either frame might arrive before a STREAM or STREAM_DATA_BLOCKED frame if packets are lost or reordered.¶

Before a stream is created, all streams of the same type with lower-numbered stream IDs MUST be created. This ensures that the creation order for streams is consistent on both endpoints.¶

In the "Recv" state, the endpoint receives STREAM and STREAM_DATA_BLOCKED frames. Incoming data is buffered and can be reassembled into the correct order for delivery to the application. As data is consumed by the application and buffer space becomes available, the endpoint sends MAX_STREAM_DATA frames to allow the peer to send more data.¶

When a STREAM frame with a FIN bit is received, the final size of the stream is known; see Section 4.5. The receiving part of the stream then enters the "Size Known" state. In this state, the endpoint no longer needs to send MAX_STREAM_DATA frames; it only receives any retransmissions of stream data.¶

Once all data for the stream has been received, the receiving part enters the "Data Recvd" state. This might happen as a result of receiving the same STREAM frame that causes the transition to "Size Known". After all data has been received, any STREAM or STREAM_DATA_BLOCKED frames for the stream can be discarded.¶

The "Data Recvd" state persists until stream data has been delivered to the application. Once stream data has been delivered, the stream enters the "Data Read" state, which is a terminal state.¶

Receiving a RESET_STREAM frame in the "Recv" or "Size Known" state causes the stream to enter the "Reset Recvd" state. This might cause the delivery of stream data to the application to be interrupted.¶

It is possible that all stream data has already been received when a RESET_STREAM is received (that is, in the "Data Recvd" state). Similarly, it is possible for remaining stream data to arrive after receiving a RESET_STREAM frame (the "Reset Recvd" state). An implementation is free to manage this situation as it chooses.¶

Sending a RESET_STREAM means that an endpoint cannot guarantee delivery of stream data; however, there is no requirement that stream data not be delivered if a RESET_STREAM is received. An implementation MAY interrupt delivery of stream data, discard any data that was not consumed, and signal the receipt of the RESET_STREAM. A RESET_STREAM signal might be suppressed or withheld if stream data is completely received and is buffered to be read by the application. If the RESET_STREAM is suppressed, the receiving part of the stream remains in "Data Recvd".¶

Once the application receives the signal indicating that the stream was reset, the receiving part of the stream transitions to the "Reset Read" state, which is a terminal state.¶

3.3. Permitted Frame Types

The sender of a stream sends just three frame types that affect the state of a stream at either the sender or the receiver: STREAM (Section 19.8), STREAM_DATA_BLOCKED (Section 19.13), and RESET_STREAM (Section 19.4).¶

A sender MUST NOT send any of these frames from a terminal state ("Data Recvd" or "Reset Recvd"). A sender MUST NOT send a STREAM or STREAM_DATA_BLOCKED frame for a stream in the "Reset Sent" state or any terminal state -- that is, after sending a RESET_STREAM frame. A receiver could receive any of these three frames in any state, due to the possibility of delayed delivery of packets carrying them.¶

The receiver of a stream sends MAX_STREAM_DATA frames (Section 19.10) and STOP_SENDING frames (Section 19.5).¶

The receiver only sends MAX_STREAM_DATA frames in the "Recv" state. A receiver MAY send a STOP_SENDING frame in any state where it has not received a RESET_STREAM frame -- that is, states other than "Reset Recvd" or "Reset Read". However, there is little value in sending a STOP_SENDING frame in the "Data Recvd" state, since all stream data has been received. A sender could receive either of these two frames in any state as a result of delayed delivery of packets.¶

3.4. Bidirectional Stream States

A bidirectional stream is composed of sending and receiving parts. Implementations can represent states of the bidirectional stream as composites of sending and receiving stream states. The simplest model presents the stream as "open" when either sending or receiving parts are in a non-terminal state and "closed" when both sending and receiving streams are in terminal states.¶

Table 2 shows a more complex mapping of bidirectional stream states that loosely correspond to the stream states discussed in HTTP/2 [HTTP2]. This shows that multiple states on sending or receiving parts of streams are mapped to the same composite state. Note that this is just one possibility for such a mapping; this mapping requires that data be acknowledged before the transition to a "closed" or "half-closed" state.¶

3.5. Solicited State Transitions

If an application is no longer interested in the data it is receiving on a stream, it can abort reading the stream and specify an application error code.¶

If the stream is in the "Recv" or "Size Known" state, the transport SHOULD signal this by sending a STOP_SENDING frame to prompt closure of the stream in the opposite direction. This typically indicates that the receiving application is no longer reading data it receives from the stream, but it is not a guarantee that incoming data will be ignored.¶

STREAM frames received after sending a STOP_SENDING frame are still counted toward connection and stream flow control, even though these frames can be discarded upon receipt.¶

A STOP_SENDING frame requests that the receiving endpoint send a RESET_STREAM frame. An endpoint that receives a STOP_SENDING frame MUST send a RESET_STREAM frame if the stream is in the "Ready" or "Send" state. If the stream is in the "Data Sent" state, the endpoint MAY defer sending the RESET_STREAM frame until the packets containing outstanding data are acknowledged or declared lost. If any outstanding data is declared lost, the endpoint SHOULD send a RESET_STREAM frame instead of retransmitting the data.¶

An endpoint SHOULD copy the error code from the STOP_SENDING frame to the RESET_STREAM frame it sends, but it can use any application error code. An endpoint that sends a STOP_SENDING frame MAY ignore the error code in any RESET_STREAM frames subsequently received for that stream.¶

STOP_SENDING SHOULD only be sent for a stream that has not been reset by the peer. STOP_SENDING is most useful for streams in the "Recv" or "Size Known" state.¶

An endpoint is expected to send another STOP_SENDING frame if a packet containing a previous STOP_SENDING is lost. However, once either all stream data or a RESET_STREAM frame has been received for the stream -- that is, the stream is in any state other than "Recv" or "Size Known" -- sending a STOP_SENDING frame is unnecessary.¶

An endpoint that wishes to terminate both directions of a bidirectional stream can terminate one direction by sending a RESET_STREAM frame, and it can encourage prompt termination in the opposite direction by sending a STOP_SENDING frame.¶

4.1. Data Flow Control

QUIC employs a limit-based flow control scheme where a receiver advertises the limit of total bytes it is prepared to receive on a given stream or for the entire connection. This leads to two levels of data flow control in QUIC:¶

• Stream flow control, which prevents a single stream from consuming the entire receive buffer for a connection by limiting the amount of data that can be sent on each stream.¶
• Connection flow control, which prevents senders from exceeding a receiver's buffer capacity for the connection by limiting the total bytes of stream data sent in STREAM frames on all streams.¶

Senders MUST NOT send data in excess of either limit.¶

A receiver sets initial limits for all streams through transport parameters during the handshake (Section 7.4). Subsequently, a receiver sends MAX_STREAM_DATA frames (Section 19.10) or MAX_DATA frames (Section 19.9) to the sender to advertise larger limits.¶

A receiver can advertise a larger limit for a stream by sending a MAX_STREAM_DATA frame with the corresponding stream ID. A MAX_STREAM_DATA frame indicates the maximum absolute byte offset of a stream. A receiver could determine the flow control offset to be advertised based on the current offset of data consumed on that stream.¶

A receiver can advertise a larger limit for a connection by sending a MAX_DATA frame, which indicates the maximum of the sum of the absolute byte offsets of all streams. A receiver maintains a cumulative sum of bytes received on all streams, which is used to check for violations of the advertised connection or stream data limits. A receiver could determine the maximum data limit to be advertised based on the sum of bytes consumed on all streams.¶

Once a receiver advertises a limit for the connection or a stream, it is not an error to advertise a smaller limit, but the smaller limit has no effect.¶

A receiver MUST close the connection with an error of type FLOW_CONTROL_ERROR (Section 11) if the sender violates the advertised connection or stream data limits.¶

A sender MUST ignore any MAX_STREAM_DATA or MAX_DATA frames that do not increase flow control limits.¶

If a sender has sent data up to the limit, it will be unable to send new data and is considered blocked. A sender SHOULD send a STREAM_DATA_BLOCKED or DATA_BLOCKED frame to indicate to the receiver that it has data to write but is blocked by flow control limits. If a sender is blocked for a period longer than the idle timeout (Section 10.1), the receiver might close the connection even when the sender has data that is available for transmission. To keep the connection from closing, a sender that is flow control limited SHOULD periodically send a STREAM_DATA_BLOCKED or DATA_BLOCKED frame when it has no ack-eliciting packets in flight.¶

4.2. Increasing Flow Control Limits

Implementations decide when and how much credit to advertise in MAX_STREAM_DATA and MAX_DATA frames, but this section offers a few considerations.¶

To avoid blocking a sender, a receiver MAY send a MAX_STREAM_DATA or MAX_DATA frame multiple times within a round trip or send it early enough to allow time for loss of the frame and subsequent recovery.¶

Control frames contribute to connection overhead. Therefore, frequently sending MAX_STREAM_DATA and MAX_DATA frames with small changes is undesirable. On the other hand, if updates are less frequent, larger increments to limits are necessary to avoid blocking a sender, requiring larger resource commitments at the receiver. There is a trade-off between resource commitment and overhead when determining how large a limit is advertised.¶

A receiver can use an autotuning mechanism to tune the frequency and amount of advertised additional credit based on a round-trip time estimate and the rate at which the receiving application consumes data, similar to common TCP implementations. As an optimization, an endpoint could send frames related to flow control only when there are other frames to send, ensuring that flow control does not cause extra packets to be sent.¶

A blocked sender is not required to send STREAM_DATA_BLOCKED or DATA_BLOCKED frames. Therefore, a receiver MUST NOT wait for a STREAM_DATA_BLOCKED or DATA_BLOCKED frame before sending a MAX_STREAM_DATA or MAX_DATA frame; doing so could result in the sender being blocked for the rest of the connection. Even if the sender sends these frames, waiting for them will result in the sender being blocked for at least an entire round trip.¶

When a sender receives credit after being blocked, it might be able to send a large amount of data in response, resulting in short-term congestion; see Section 7.7 of [RFCYYY3] for a discussion of how a sender can avoid this congestion.¶

4.3. Flow Control Performance

If an endpoint cannot ensure that its peer always has available flow control credit that is greater than the peer's bandwidth-delay product on this connection, its receive throughput will be limited by flow control.¶

Packet loss can cause gaps in the receive buffer, preventing the application from consuming data and freeing up receive buffer space.¶

Sending timely updates of flow control limits can improve performance. Sending packets only to provide flow control updates can increase network load and adversely affect performance. Sending flow control updates along with other frames, such as ACK frames, reduces the cost of those updates.¶

4.4. Handling Stream Cancellation

Endpoints need to eventually agree on the amount of flow control credit that has been consumed on every stream, to be able to account for all bytes for connection-level flow control.¶

On receipt of a RESET_STREAM frame, an endpoint will tear down state for the matching stream and ignore further data arriving on that stream.¶

RESET_STREAM terminates one direction of a stream abruptly. For a bidirectional stream, RESET_STREAM has no effect on data flow in the opposite direction. Both endpoints MUST maintain flow control state for the stream in the unterminated direction until that direction enters a terminal state.¶

4.5. Stream Final Size

The final size is the amount of flow control credit that is consumed by a stream. Assuming that every contiguous byte on the stream was sent once, the final size is the number of bytes sent. More generally, this is one higher than the offset of the byte with the largest offset sent on the stream, or zero if no bytes were sent.¶

A sender always communicates the final size of a stream to the receiver reliably, no matter how the stream is terminated. The final size is the sum of the Offset and Length fields of a STREAM frame with a FIN flag, noting that these fields might be implicit. Alternatively, the Final Size field of a RESET_STREAM frame carries this value. This guarantees that both endpoints agree on how much flow control credit was consumed by the sender on that stream.¶

An endpoint will know the final size for a stream when the receiving part of the stream enters the "Size Known" or "Reset Recvd" state (Section 3). The receiver MUST use the final size of the stream to account for all bytes sent on the stream in its connection-level flow controller.¶

An endpoint MUST NOT send data on a stream at or beyond the final size.¶

Once a final size for a stream is known, it cannot change. If a RESET_STREAM or STREAM frame is received indicating a change in the final size for the stream, an endpoint SHOULD respond with an error of type FINAL_SIZE_ERROR; see Section 11. A receiver SHOULD treat receipt of data at or beyond the final size as an error of type FINAL_SIZE_ERROR, even after a stream is closed. Generating these errors is not mandatory, because requiring that an endpoint generate these errors also means that the endpoint needs to maintain the final size state for closed streams, which could mean a significant state commitment.¶

4.6. Controlling Concurrency

An endpoint limits the cumulative number of incoming streams a peer can open. Only streams with a stream ID less than (max_stream * 4 + initial_stream_id_for_type) can be opened; see Table 1. Initial limits are set in the transport parameters; see Section 18.2. Subsequent limits are advertised using MAX_STREAMS frames; see Section 19.11. Separate limits apply to unidirectional and bidirectional streams.¶

If a max_streams transport parameter or a MAX_STREAMS frame is received with a value greater than 2^60, this would allow a maximum stream ID that cannot be expressed as a variable-length integer; see Section 16. If either is received, the connection MUST be closed immediately with a connection error of type TRANSPORT_PARAMETER_ERROR if the offending value was received in a transport parameter or of type FRAME_ENCODING_ERROR if it was received in a frame; see Section 10.2.¶

Endpoints MUST NOT exceed the limit set by their peer. An endpoint that receives a frame with a stream ID exceeding the limit it has sent MUST treat this as a connection error of type STREAM_LIMIT_ERROR (Section 11).¶

Once a receiver advertises a stream limit using the MAX_STREAMS frame, advertising a smaller limit has no effect. A receiver MUST ignore any MAX_STREAMS frame that does not increase the stream limit.¶

As with stream and connection flow control, this document leaves implementations to decide when and how many streams should be advertised to a peer via MAX_STREAMS. Implementations might choose to increase limits as streams are closed, to keep the number of streams available to peers roughly consistent.¶

An endpoint that is unable to open a new stream due to the peer's limits SHOULD send a STREAMS_BLOCKED frame (Section 19.14). This signal is considered useful for debugging. An endpoint MUST NOT wait to receive this signal before advertising additional credit, since doing so will mean that the peer will be blocked for at least an entire round trip, and potentially indefinitely if the peer chooses not to send STREAMS_BLOCKED frames.¶

5.1. Connection ID

Each connection possesses a set of connection identifiers, or connection IDs, each of which can identify the connection. Connection IDs are independently selected by endpoints; each endpoint selects the connection IDs that its peer uses.¶

The primary function of a connection ID is to ensure that changes in addressing at lower protocol layers (UDP, IP) do not cause packets for a QUIC connection to be delivered to the wrong endpoint. Each endpoint selects connection IDs using an implementation-specific (and perhaps deployment-specific) method that will allow packets with that connection ID to be routed back to the endpoint and to be identified by the endpoint upon receipt.¶

Multiple connection IDs are used so that endpoints can send packets that cannot be identified by an observer as being for the same connection without cooperation from an endpoint; see Section 9.5.¶

Connection IDs MUST NOT contain any information that can be used by an external observer (that is, one that does not cooperate with the issuer) to correlate them with other connection IDs for the same connection. As a trivial example, this means the same connection ID MUST NOT be issued more than once on the same connection.¶

Packets with long headers include Source Connection ID and Destination Connection ID fields. These fields are used to set the connection IDs for new connections; see Section 7.2 for details.¶

Packets with short headers (Section 17.3) only include the Destination Connection ID and omit the explicit length. The length of the Destination Connection ID field is expected to be known to endpoints. Endpoints using a load balancer that routes based on connection ID could agree with the load balancer on a fixed length for connection IDs or agree on an encoding scheme. A fixed portion could encode an explicit length, which allows the entire connection ID to vary in length and still be used by the load balancer.¶

A Version Negotiation (Section 17.2.1) packet echoes the connection IDs selected by the client, both to ensure correct routing toward the client and to demonstrate that the packet is in response to a packet sent by the client.¶

A zero-length connection ID can be used when a connection ID is not needed to route to the correct endpoint. However, multiplexing connections on the same local IP address and port while using zero-length connection IDs will cause failures in the presence of peer connection migration, NAT rebinding, and client port reuse. An endpoint MUST NOT use the same IP address and port for multiple concurrent connections with zero-length connection IDs, unless it is certain that those protocol features are not in use.¶

When an endpoint uses a non-zero-length connection ID, it needs to ensure that the peer has a supply of connection IDs from which to choose for packets sent to the endpoint. These connection IDs are supplied by the endpoint using the NEW_CONNECTION_ID frame (Section 19.15).¶

5.2. Matching Packets to Connections

Incoming packets are classified on receipt. Packets can either be associated with an existing connection or -- for servers -- potentially create a new connection.¶

Endpoints try to associate a packet with an existing connection. If the packet has a non-zero-length Destination Connection ID corresponding to an existing connection, QUIC processes that packet accordingly. Note that more than one connection ID can be associated with a connection; see Section 5.1.¶

If the Destination Connection ID is zero length and the addressing information in the packet matches the addressing information the endpoint uses to identify a connection with a zero-length connection ID, QUIC processes the packet as part of that connection. An endpoint can use just destination IP and port or both source and destination addresses for identification, though this makes connections fragile as described in Section 5.1.¶

Endpoints can send a Stateless Reset (Section 10.3) for any packets that cannot be attributed to an existing connection. A stateless reset allows a peer to more quickly identify when a connection becomes unusable.¶

Packets that are matched to an existing connection are discarded if the packets are inconsistent with the state of that connection. For example, packets are discarded if they indicate a different protocol version than that of the connection or if the removal of packet protection is unsuccessful once the expected keys are available.¶

Invalid packets that lack strong integrity protection, such as Initial, Retry, or Version Negotiation, MAY be discarded. An endpoint MUST generate a connection error if processing the contents of these packets prior to discovering an error, or fully revert any changes made during that processing.¶

5.3. Operations on Connections

This document does not define an API for QUIC; it instead defines a set of functions for QUIC connections that application protocols can rely upon. An application protocol can assume that an implementation of QUIC provides an interface that includes the operations described in this section. An implementation designed for use with a specific application protocol might provide only those operations that are used by that protocol.¶

When implementing the client role, an application protocol can:¶

• open a connection, which begins the exchange described in Section 7;¶
• enable Early Data when available; and¶
• be informed when Early Data has been accepted or rejected by a server.¶

When implementing the server role, an application protocol can:¶

• listen for incoming connections, which prepares for the exchange described in Section 7;¶
• if Early Data is supported, embed application-controlled data in the TLS resumption ticket sent to the client; and¶
• if Early Data is supported, retrieve application-controlled data from the client's resumption ticket and accept or reject Early Data based on that information.¶

In either role, an application protocol can:¶

• configure minimum values for the initial number of permitted streams of each type, as communicated in the transport parameters (Section 7.4);¶
• control resource allocation for receive buffers by setting flow control limits both for streams and for the connection;¶
• identify whether the handshake has completed successfully or is still ongoing;¶
• keep a connection from silently closing, by either generating PING frames (Section 19.2) or requesting that the transport send additional frames before the idle timeout expires (Section 10.1); and¶
• immediately close (Section 10.2) the connection.¶

6.1. Sending Version Negotiation Packets

If the version selected by the client is not acceptable to the server, the server responds with a Version Negotiation packet; see Section 17.2.1. This includes a list of versions that the server will accept. An endpoint MUST NOT send a Version Negotiation packet in response to receiving a Version Negotiation packet.¶

This system allows a server to process packets with unsupported versions without retaining state. Though either the Initial packet or the Version Negotiation packet that is sent in response could be lost, the client will send new packets until it successfully receives a response or it abandons the connection attempt.¶

A server MAY limit the number of Version Negotiation packets it sends. For instance, a server that is able to recognize packets as 0-RTT might choose not to send Version Negotiation packets in response to 0-RTT packets with the expectation that it will eventually receive an Initial packet.¶

6.2. Handling Version Negotiation Packets

Version Negotiation packets are designed to allow for functionality to be defined in the future that allows QUIC to negotiate the version of QUIC to use for a connection. Future Standards Track specifications might change how implementations that support multiple versions of QUIC react to Version Negotiation packets received in response to an attempt to establish a connection using this version.¶

A client that supports only this version of QUIC MUST abandon the current connection attempt if it receives a Version Negotiation packet, with the following two exceptions. A client MUST discard any Version Negotiation packet if it has received and successfully processed any other packet, including an earlier Version Negotiation packet. A client MUST discard a Version Negotiation packet that lists the QUIC version selected by the client.¶

How to perform version negotiation is left as future work defined by future Standards Track specifications. In particular, that future work will ensure robustness against version downgrade attacks; see Section 21.12.¶

6.3. Using Reserved Versions

For a server to use a new version in the future, clients need to correctly handle unsupported versions. Some version numbers (0x?a?a?a?a, as defined in Section 15) are reserved for inclusion in fields that contain version numbers.¶

Endpoints MAY add reserved versions to any field where unknown or unsupported versions are ignored to test that a peer correctly ignores the value. For instance, an endpoint could include a reserved version in a Version Negotiation packet; see Section 17.2.1. Endpoints MAY send packets with a reserved version to test that a peer correctly discards the packet.¶

7.1. Example Handshake Flows

Details of how TLS is integrated with QUIC are provided in [RFCYYY2], but some examples are provided here. An extension of this exchange to support client address validation is shown in Section 8.1.2.¶

Once any address validation exchanges are complete, the cryptographic handshake is used to agree on cryptographic keys. The cryptographic handshake is carried in Initial (Section 17.2.2) and Handshake (Section 17.2.4) packets.¶

Figure 5 provides an overview of the 1-RTT handshake. Each line shows a QUIC packet with the packet type and packet number shown first, followed by the frames that are typically contained in those packets. So, for instance, the first packet is of type Initial, with packet number 0, and contains a CRYPTO frame carrying the ClientHello.¶

Multiple QUIC packets -- even of different packet types -- can be coalesced into a single UDP datagram; see Section 12.2. As a result, this handshake could consist of as few as four UDP datagrams, or any number more (subject to limits inherent to the protocol, such as congestion control and anti-amplification). For instance, the server's first flight contains Initial packets, Handshake packets, and "0.5-RTT data" in 1-RTT packets.¶

Client                                                  Server

Initial[0]: CRYPTO[CH] ->

                                 Initial[0]: CRYPTO[SH] ACK[0]
                       Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                 <- 1-RTT[0]: STREAM[1, "..."]

Initial[1]: ACK[0]
Handshake[0]: CRYPTO[FIN], ACK[0]
1-RTT[0]: STREAM[0, "..."], ACK[0] ->

                                          Handshake[1]: ACK[0]
         <- 1-RTT[1]: HANDSHAKE_DONE, STREAM[3, "..."], ACK[0]

Figure 6 shows an example of a connection with a 0-RTT handshake and a single packet of 0-RTT data. Note that as described in Section 12.3, the server acknowledges 0-RTT data in 1-RTT packets, and the client sends 1-RTT packets in the same packet number space.¶

Client                                                  Server

Initial[0]: CRYPTO[CH]
0-RTT[0]: STREAM[0, "..."] ->

                                 Initial[0]: CRYPTO[SH] ACK[0]
                                  Handshake[0] CRYPTO[EE, FIN]
                          <- 1-RTT[0]: STREAM[1, "..."] ACK[0]

Initial[1]: ACK[0]
Handshake[0]: CRYPTO[FIN], ACK[0]
1-RTT[1]: STREAM[0, "..."] ACK[0] ->

                                          Handshake[1]: ACK[0]
         <- 1-RTT[1]: HANDSHAKE_DONE, STREAM[3, "..."], ACK[1]

7.2. Negotiating Connection IDs

A connection ID is used to ensure consistent routing of packets, as described in Section 5.1. The long header contains two connection IDs: the Destination Connection ID is chosen by the recipient of the packet and is used to provide consistent routing; the Source Connection ID is used to set the Destination Connection ID used by the peer.¶

During the handshake, packets with the long header (Section 17.2) are used to establish the connection IDs used by both endpoints. Each endpoint uses the Source Connection ID field to specify the connection ID that is used in the Destination Connection ID field of packets being sent to them. After processing the first Initial packet, each endpoint sets the Destination Connection ID field in subsequent packets it sends to the value of the Source Connection ID field that it received.¶

When an Initial packet is sent by a client that has not previously received an Initial or Retry packet from the server, the client populates the Destination Connection ID field with an unpredictable value. This Destination Connection ID MUST be at least 8 bytes in length. Until a packet is received from the server, the client MUST use the same Destination Connection ID value on all packets in this connection.¶

The Destination Connection ID field from the first Initial packet sent by a client is used to determine packet protection keys for Initial packets. These keys change after receiving a Retry packet; see Section 5.2 of [RFCYYY2].¶

The client populates the Source Connection ID field with a value of its choosing and sets the Source Connection ID Length field to indicate the length.¶

0-RTT packets in the first flight use the same Destination Connection ID and Source Connection ID values as the client's first Initial packet.¶

Upon first receiving an Initial or Retry packet from the server, the client uses the Source Connection ID supplied by the server as the Destination Connection ID for subsequent packets, including any 0-RTT packets. This means that a client might have to change the connection ID it sets in the Destination Connection ID field twice during connection establishment: once in response to a Retry packet and once in response to an Initial packet from the server. Once a client has received a valid Initial packet from the server, it MUST discard any subsequent packet it receives on that connection with a different Source Connection ID.¶

A client MUST change the Destination Connection ID it uses for sending packets in response to only the first received Initial or Retry packet. A server MUST set the Destination Connection ID it uses for sending packets based on the first received Initial packet. Any further changes to the Destination Connection ID are only permitted if the values are taken from NEW_CONNECTION_ID frames; if subsequent Initial packets include a different Source Connection ID, they MUST be discarded. This avoids unpredictable outcomes that might otherwise result from stateless processing of multiple Initial packets with different Source Connection IDs.¶

The Destination Connection ID that an endpoint sends can change over the lifetime of a connection, especially in response to connection migration (Section 9); see Section 5.1.1 for details.¶

7.3. Authenticating Connection IDs

The choice each endpoint makes about connection IDs during the handshake is authenticated by including all values in transport parameters; see Section 7.4. This ensures that all connection IDs used for the handshake are also authenticated by the cryptographic handshake.¶

Each endpoint includes the value of the Source Connection ID field from the first Initial packet it sent in the initial_source_connection_id transport parameter; see Section 18.2. A server includes the Destination Connection ID field from the first Initial packet it received from the client in the original_destination_connection_id transport parameter; if the server sent a Retry packet, this refers to the first Initial packet received before sending the Retry packet. If it sends a Retry packet, a server also includes the Source Connection ID field from the Retry packet in the retry_source_connection_id transport parameter.¶

The values provided by a peer for these transport parameters MUST match the values that an endpoint used in the Destination and Source Connection ID fields of Initial packets that it sent (and received, for servers). Endpoints MUST validate that received transport parameters match received connection ID values. Including connection ID values in transport parameters and verifying them ensures that an attacker cannot influence the choice of connection ID for a successful connection by injecting packets carrying attacker-chosen connection IDs during the handshake.¶

An endpoint MUST treat the absence of the initial_source_connection_id transport parameter from either endpoint or the absence of the original_destination_connection_id transport parameter from the server as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

An endpoint MUST treat the following as a connection error of type TRANSPORT_PARAMETER_ERROR or PROTOCOL_VIOLATION:¶

• absence of the retry_source_connection_id transport parameter from the server after receiving a Retry packet,¶
• presence of the retry_source_connection_id transport parameter when no Retry packet was received, or¶
• a mismatch between values received from a peer in these transport parameters and the value sent in the corresponding Destination or Source Connection ID fields of Initial packets.¶

If a zero-length connection ID is selected, the corresponding transport parameter is included with a zero-length value.¶

Figure 7 shows the connection IDs (with DCID=Destination Connection ID, SCID=Source Connection ID) that are used in a complete handshake. The exchange of Initial packets is shown, plus the later exchange of 1-RTT packets that includes the connection ID established during the handshake.¶

Client                                                  Server

Initial: DCID=S1, SCID=C1 ->
                                  <- Initial: DCID=C1, SCID=S3
                             ...
1-RTT: DCID=S3 ->
                                             <- 1-RTT: DCID=C1

Figure 8 shows a similar handshake that includes a Retry packet.¶

Client                                                  Server

Initial: DCID=S1, SCID=C1 ->
                                    <- Retry: DCID=C1, SCID=S2
Initial: DCID=S2, SCID=C1 ->
                                  <- Initial: DCID=C1, SCID=S3
                             ...
1-RTT: DCID=S3 ->
                                             <- 1-RTT: DCID=C1

In both cases (Figures 7 and 8), the client sets the value of the initial_source_connection_id transport parameter to C1.¶

When the handshake does not include a Retry (Figure 7), the server sets original_destination_connection_id to S1 and initial_source_connection_id to S3. In this case, the server does not include a retry_source_connection_id transport parameter.¶

When the handshake includes a Retry (Figure 8), the server sets original_destination_connection_id to S1, retry_source_connection_id to S2, and initial_source_connection_id to S3.¶

7.4. Transport Parameters

During connection establishment, both endpoints make authenticated declarations of their transport parameters. Endpoints are required to comply with the restrictions that each parameter defines; the description of each parameter includes rules for its handling.¶

Transport parameters are declarations that are made unilaterally by each endpoint. Each endpoint can choose values for transport parameters independent of the values chosen by its peer.¶

The encoding of the transport parameters is detailed in Section 18.¶

QUIC includes the encoded transport parameters in the cryptographic handshake. Once the handshake completes, the transport parameters declared by the peer are available. Each endpoint validates the values provided by its peer.¶

Definitions for each of the defined transport parameters are included in Section 18.2.¶

An endpoint MUST treat receipt of a transport parameter with an invalid value as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

An endpoint MUST NOT send a parameter more than once in a given transport parameters extension. An endpoint SHOULD treat receipt of duplicate transport parameters as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

Endpoints use transport parameters to authenticate the negotiation of connection IDs during the handshake; see Section 7.3.¶

ALPN (see [ALPN]) allows clients to offer multiple application protocols during connection establishment. The transport parameters that a client includes during the handshake apply to all application protocols that the client offers. Application protocols can recommend values for transport parameters, such as the initial flow control limits. However, application protocols that set constraints on values for transport parameters could make it impossible for a client to offer multiple application protocols if these constraints conflict.¶

7.5. Cryptographic Message Buffering

Implementations need to maintain a buffer of CRYPTO data received out of order. Because there is no flow control of CRYPTO frames, an endpoint could potentially force its peer to buffer an unbounded amount of data.¶

Implementations MUST support buffering at least 4096 bytes of data received in out-of-order CRYPTO frames. Endpoints MAY choose to allow more data to be buffered during the handshake. A larger limit during the handshake could allow for larger keys or credentials to be exchanged. An endpoint's buffer size does not need to remain constant during the life of the connection.¶

Being unable to buffer CRYPTO frames during the handshake can lead to a connection failure. If an endpoint's buffer is exceeded during the handshake, it can expand its buffer temporarily to complete the handshake. If an endpoint does not expand its buffer, it MUST close the connection with a CRYPTO_BUFFER_EXCEEDED error code.¶

Once the handshake completes, if an endpoint is unable to buffer all data in a CRYPTO frame, it MAY discard that CRYPTO frame and all CRYPTO frames received in the future, or it MAY close the connection with a CRYPTO_BUFFER_EXCEEDED error code. Packets containing discarded CRYPTO frames MUST be acknowledged because the packet has been received and processed by the transport even though the CRYPTO frame was discarded.¶

8.1. Address Validation during Connection Establishment

Connection establishment implicitly provides address validation for both endpoints. In particular, receipt of a packet protected with Handshake keys confirms that the peer successfully processed an Initial packet. Once an endpoint has successfully processed a Handshake packet from the peer, it can consider the peer address to have been validated.¶

Additionally, an endpoint MAY consider the peer address validated if the peer uses a connection ID chosen by the endpoint and the connection ID contains at least 64 bits of entropy.¶

For the client, the value of the Destination Connection ID field in its first Initial packet allows it to validate the server address as a part of successfully processing any packet. Initial packets from the server are protected with keys that are derived from this value (see Section 5.2 of [RFCYYY2]). Alternatively, the value is echoed by the server in Version Negotiation packets (Section 6) or included in the Integrity Tag in Retry packets (Section 5.8 of [RFCYYY2]).¶

Prior to validating the client address, servers MUST NOT send more than three times as many bytes as the number of bytes they have received. This limits the magnitude of any amplification attack that can be mounted using spoofed source addresses. For the purposes of avoiding amplification prior to address validation, servers MUST count all of the payload bytes received in datagrams that are uniquely attributed to a single connection. This includes datagrams that contain packets that are successfully processed and datagrams that contain packets that are all discarded.¶

Clients MUST ensure that UDP datagrams containing Initial packets have UDP payloads of at least 1200 bytes, adding PADDING frames as necessary. A client that sends padded datagrams allows the server to send more data prior to completing address validation.¶

Loss of an Initial or Handshake packet from the server can cause a deadlock if the client does not send additional Initial or Handshake packets. A deadlock could occur when the server reaches its anti-amplification limit and the client has received acknowledgments for all the data it has sent. In this case, when the client has no reason to send additional packets, the server will be unable to send more data because it has not validated the client's address. To prevent this deadlock, clients MUST send a packet on a Probe Timeout (PTO) (see Section 6.2 of [RFCYYY3]). Specifically, the client MUST send an Initial packet in a UDP datagram that contains at least 1200 bytes if it does not have Handshake keys, and otherwise send a Handshake packet.¶

A server might wish to validate the client address before starting the cryptographic handshake. QUIC uses a token in the Initial packet to provide address validation prior to completing the handshake. This token is delivered to the client during connection establishment with a Retry packet (see Section 8.1.2) or in a previous connection using the NEW_TOKEN frame (see Section 8.1.3).¶

In addition to sending limits imposed prior to address validation, servers are also constrained in what they can send by the limits set by the congestion controller. Clients are only constrained by the congestion controller.¶

Client                                                  Server

Initial[0]: CRYPTO[CH] ->

                                                <- Retry+Token

Initial+Token[1]: CRYPTO[CH] ->

                                 Initial[0]: CRYPTO[SH] ACK[1]
                       Handshake[0]: CRYPTO[EE, CERT, CV, FIN]
                                 <- 1-RTT[0]: STREAM[1, "..."]

8.2. Path Validation

Path validation is used by both peers during connection migration (see Section 9) to verify reachability after a change of address. In path validation, endpoints test reachability between a specific local address and a specific peer address, where an address is the 2-tuple of IP address and port.¶

Path validation tests that packets sent on a path to a peer are received by that peer. Path validation is used to ensure that packets received from a migrating peer do not carry a spoofed source address.¶

Path validation does not validate that a peer can send in the return direction. Acknowledgments cannot be used for return path validation because they contain insufficient entropy and might be spoofed. Endpoints independently determine reachability on each direction of a path, and therefore return reachability can only be established by the peer.¶

Path validation can be used at any time by either endpoint. For instance, an endpoint might check that a peer is still in possession of its address after a period of quiescence.¶

Path validation is not designed as a NAT traversal mechanism. Though the mechanism described here might be effective for the creation of NAT bindings that support NAT traversal, the expectation is that one peer or another peer is able to receive packets without first having sent a packet on that path. Effective NAT traversal needs additional synchronization mechanisms that are not provided here.¶

An endpoint MAY include other frames with the PATH_CHALLENGE and PATH_RESPONSE frames used for path validation. In particular, an endpoint can include PADDING frames with a PATH_CHALLENGE frame for Path Maximum Transmission Unit Discovery (PMTUD) (see Section 14.2.1); it can also include its own PATH_CHALLENGE frame with a PATH_RESPONSE frame.¶

An endpoint uses a new connection ID for probes sent from a new local address; see Section 9.5. When probing a new path, an endpoint can ensure that its peer has an unused connection ID available for responses. Sending NEW_CONNECTION_ID and PATH_CHALLENGE frames in the same packet, if the peer's active_connection_id_limit permits, ensures that an unused connection ID will be available to the peer when sending a response.¶

An endpoint can choose to simultaneously probe multiple paths. The number of simultaneous paths used for probes is limited by the number of extra connection IDs its peer has previously supplied, since each new local address used for a probe requires a previously unused connection ID.¶

9.1. Probing a New Path

An endpoint MAY probe for peer reachability from a new local address using path validation (Section 8.2) prior to migrating the connection to the new local address. Failure of path validation simply means that the new path is not usable for this connection. Failure to validate a path does not cause the connection to end unless there are no valid alternative paths available.¶

PATH_CHALLENGE, PATH_RESPONSE, NEW_CONNECTION_ID, and PADDING frames are "probing frames", and all other frames are "non-probing frames". A packet containing only probing frames is a "probing packet", and a packet containing any other frame is a "non-probing packet".¶

9.2. Initiating Connection Migration

An endpoint can migrate a connection to a new local address by sending packets containing non-probing frames from that address.¶

Each endpoint validates its peer's address during connection establishment. Therefore, a migrating endpoint can send to its peer knowing that the peer is willing to receive at the peer's current address. Thus, an endpoint can migrate to a new local address without first validating the peer's address.¶

To establish reachability on the new path, an endpoint initiates path validation (Section 8.2) on the new path. An endpoint MAY defer path validation until after a peer sends the next non-probing frame to its new address.¶

When migrating, the new path might not support the endpoint's current sending rate. Therefore, the endpoint resets its congestion controller and RTT estimate, as described in Section 9.4.¶

The new path might not have the same ECN capability. Therefore, the endpoint validates ECN capability as described in Section 13.4.¶

9.3. Responding to Connection Migration

Receiving a packet from a new peer address containing a non-probing frame indicates that the peer has migrated to that address.¶

If the recipient permits the migration, it MUST send subsequent packets to the new peer address and MUST initiate path validation (Section 8.2) to verify the peer's ownership of the address if validation is not already underway.¶

An endpoint only changes the address to which it sends packets in response to the highest-numbered non-probing packet. This ensures that an endpoint does not send packets to an old peer address in the case that it receives reordered packets.¶

An endpoint MAY send data to an unvalidated peer address, but it MUST protect against potential attacks as described in Sections 9.3.1 and 9.3.2. An endpoint MAY skip validation of a peer address if that address has been seen recently. In particular, if an endpoint returns to a previously validated path after detecting some form of spurious migration, skipping address validation and restoring loss detection and congestion state can reduce the performance impact of the attack.¶

After changing the address to which it sends non-probing packets, an endpoint can abandon any path validation for other addresses.¶

Receiving a packet from a new peer address could be the result of a NAT rebinding at the peer.¶

After verifying a new client address, the server SHOULD send new address validation tokens (Section 8) to the client.¶

9.4. Loss Detection and Congestion Control

The capacity available on the new path might not be the same as the old path. Packets sent on the old path MUST NOT contribute to congestion control or RTT estimation for the new path.¶

On confirming a peer's ownership of its new address, an endpoint MUST immediately reset the congestion controller and round-trip time estimator for the new path to initial values (see Appendices A.3 and B.3 of [RFCYYY3]) unless the only change in the peer's address is its port number. Because port-only changes are commonly the result of NAT rebinding or other middlebox activity, the endpoint MAY instead retain its congestion control state and round-trip estimate in those cases instead of reverting to initial values. In cases where congestion control state retained from an old path is used on a new path with substantially different characteristics, a sender could transmit too aggressively until the congestion controller and the RTT estimator have adapted. Generally, implementations are advised to be cautious when using previous values on a new path.¶

There could be apparent reordering at the receiver when an endpoint sends data and probes from/to multiple addresses during the migration period, since the two resulting paths could have different round-trip times. A receiver of packets on multiple paths will still send ACK frames covering all received packets.¶

While multiple paths might be used during connection migration, a single congestion control context and a single loss recovery context (as described in [RFCYYY3]) could be adequate. For instance, an endpoint might delay switching to a new congestion control context until it is confirmed that an old path is no longer needed (such as the case described in Section 9.3.3).¶

A sender can make exceptions for probe packets so that their loss detection is independent and does not unduly cause the congestion controller to reduce its sending rate. An endpoint might set a separate timer when a PATH_CHALLENGE is sent, which is canceled if the corresponding PATH_RESPONSE is received. If the timer fires before the PATH_RESPONSE is received, the endpoint might send a new PATH_CHALLENGE and restart the timer for a longer period of time. This timer SHOULD be set as described in Section 6.2.1 of [RFCYYY3] and MUST NOT be more aggressive.¶

9.5. Privacy Implications of Connection Migration

Using a stable connection ID on multiple network paths would allow a passive observer to correlate activity between those paths. An endpoint that moves between networks might not wish to have their activity correlated by any entity other than their peer, so different connection IDs are used when sending from different local addresses, as discussed in Section 5.1. For this to be effective, endpoints need to ensure that connection IDs they provide cannot be linked by any other entity.¶

At any time, endpoints MAY change the Destination Connection ID they transmit with to a value that has not been used on another path.¶

An endpoint MUST NOT reuse a connection ID when sending from more than one local address -- for example, when initiating connection migration as described in Section 9.2 or when probing a new network path as described in Section 9.1.¶

Similarly, an endpoint MUST NOT reuse a connection ID when sending to more than one destination address. Due to network changes outside the control of its peer, an endpoint might receive packets from a new source address with the same Destination Connection ID, in which case it MAY continue to use the current connection ID with the new remote address while still sending from the same local address.¶

These requirements regarding connection ID reuse apply only to the sending of packets, as unintentional changes in path without a change in connection ID are possible. For example, after a period of network inactivity, NAT rebinding might cause packets to be sent on a new path when the client resumes sending. An endpoint responds to such an event as described in Section 9.3.¶

Using different connection IDs for packets sent in both directions on each new network path eliminates the use of the connection ID for linking packets from the same connection across different network paths. Header protection ensures that packet numbers cannot be used to correlate activity. This does not prevent other properties of packets, such as timing and size, from being used to correlate activity.¶

An endpoint SHOULD NOT initiate migration with a peer that has requested a zero-length connection ID, because traffic over the new path might be trivially linkable to traffic over the old one. If the server is able to associate packets with a zero-length connection ID to the right connection, it means that the server is using other information to demultiplex packets. For example, a server might provide a unique address to every client -- for instance, using HTTP alternative services [ALTSVC]. Information that might allow correct routing of packets across multiple network paths will also allow activity on those paths to be linked by entities other than the peer.¶

A client might wish to reduce linkability by switching to a new connection ID, source UDP port, or IP address (see [RFC4941]) when sending traffic after a period of inactivity. Changing the address from which it sends packets at the same time might cause the server to detect a connection migration. This ensures that the mechanisms that support migration are exercised even for clients that do not experience NAT rebindings or genuine migrations. Changing the address can cause a peer to reset its congestion control state (see Section 9.4), so addresses SHOULD only be changed infrequently.¶

An endpoint that exhausts available connection IDs cannot probe new paths or initiate migration, nor can it respond to probes or attempts by its peer to migrate. To ensure that migration is possible and packets sent on different paths cannot be correlated, endpoints SHOULD provide new connection IDs before peers migrate; see Section 5.1.1. If a peer might have exhausted available connection IDs, a migrating endpoint could include a NEW_CONNECTION_ID frame in all packets sent on a new network path.¶

9.6. Server's Preferred Address

QUIC allows servers to accept connections on one IP address and attempt to transfer these connections to a more preferred address shortly after the handshake. This is particularly useful when clients initially connect to an address shared by multiple servers but would prefer to use a unicast address to ensure connection stability. This section describes the protocol for migrating a connection to a preferred server address.¶

Migrating a connection to a new server address mid-connection is not supported by the version of QUIC specified in this document. If a client receives packets from a new server address when the client has not initiated a migration to that address, the client SHOULD discard these packets.¶

9.7. Use of IPv6 Flow Label and Migration

Endpoints that send data using IPv6 SHOULD apply an IPv6 flow label in compliance with [RFC6437], unless the local API does not allow setting IPv6 flow labels.¶

The flow label generation MUST be designed to minimize the chances of linkability with a previously used flow label, as a stable flow label would enable correlating activity on multiple paths; see Section 9.5.¶

[RFC6437] suggests deriving values using a pseudorandom function to generate flow labels. Including the Destination Connection ID field in addition to source and destination addresses when generating flow labels ensures that changes are synchronized with changes in other observable identifiers. A cryptographic hash function that combines these inputs with a local secret is one way this might be implemented.¶

10.1. Idle Timeout

If a max_idle_timeout is specified by either peer in its transport parameters (Section 18.2), the connection is silently closed and its state is discarded when it remains idle for longer than the minimum of both peers' max_idle_timeout values.¶

Each endpoint advertises a max_idle_timeout, but the effective value at an endpoint is computed as the minimum of the two advertised values (or the sole advertised value, if only one endpoint advertises a non-zero value). By announcing a max_idle_timeout, an endpoint commits to initiating an immediate close (Section 10.2) if it abandons the connection prior to the effective value.¶

An endpoint restarts its idle timer when a packet from its peer is received and processed successfully. An endpoint also restarts its idle timer when sending an ack-eliciting packet if no other ack-eliciting packets have been sent since last receiving and processing a packet. Restarting this timer when sending a packet ensures that connections are not closed after new activity is initiated.¶

To avoid excessively small idle timeout periods, endpoints MUST increase the idle timeout period to be at least three times the current Probe Timeout (PTO). This allows for multiple PTOs to expire, and therefore multiple probes to be sent and lost, prior to idle timeout.¶

10.2. Immediate Close

An endpoint sends a CONNECTION_CLOSE frame (Section 19.19) to terminate the connection immediately. A CONNECTION_CLOSE frame causes all streams to immediately become closed; open streams can be assumed to be implicitly reset.¶

After sending a CONNECTION_CLOSE frame, an endpoint immediately enters the closing state; see Section 10.2.1. After receiving a CONNECTION_CLOSE frame, endpoints enter the draining state; see Section 10.2.2.¶

Violations of the protocol lead to an immediate close.¶

An immediate close can be used after an application protocol has arranged to close a connection. This might be after the application protocol negotiates a graceful shutdown. The application protocol can exchange messages that are needed for both application endpoints to agree that the connection can be closed, after which the application requests that QUIC close the connection. When QUIC consequently closes the connection, a CONNECTION_CLOSE frame with an application-supplied error code will be used to signal closure to the peer.¶

The closing and draining connection states exist to ensure that connections close cleanly and that delayed or reordered packets are properly discarded. These states SHOULD persist for at least three times the current PTO interval as defined in [RFCYYY3].¶

Disposing of connection state prior to exiting the closing or draining state could result in an endpoint generating a stateless reset unnecessarily when it receives a late-arriving packet. Endpoints that have some alternative means to ensure that late-arriving packets do not induce a response, such as those that are able to close the UDP socket, MAY end these states earlier to allow for faster resource recovery. Servers that retain an open socket for accepting new connections SHOULD NOT end the closing or draining state early.¶

Once its closing or draining state ends, an endpoint SHOULD discard all connection state. The endpoint MAY send a stateless reset in response to any further incoming packets belonging to this connection.¶

10.3. Stateless Reset

A stateless reset is provided as an option of last resort for an endpoint that does not have access to the state of a connection. A crash or outage might result in peers continuing to send data to an endpoint that is unable to properly continue the connection. An endpoint MAY send a stateless reset in response to receiving a packet that it cannot associate with an active connection.¶

A stateless reset is not appropriate for indicating errors in active connections. An endpoint that wishes to communicate a fatal connection error MUST use a CONNECTION_CLOSE frame if it is able.¶

To support this process, an endpoint issues a stateless reset token, which is a 16-byte value that is hard to guess. If the peer subsequently receives a stateless reset, which is a UDP datagram that ends in that stateless reset token, the peer will immediately end the connection.¶

A stateless reset token is specific to a connection ID. An endpoint issues a stateless reset token by including the value in the Stateless Reset Token field of a NEW_CONNECTION_ID frame. Servers can also issue a stateless_reset_token transport parameter during the handshake that applies to the connection ID that it selected during the handshake. These exchanges are protected by encryption, so only client and server know their value. Note that clients cannot use the stateless_reset_token transport parameter because their transport parameters do not have confidentiality protection.¶

Tokens are invalidated when their associated connection ID is retired via a RETIRE_CONNECTION_ID frame (Section 19.16).¶

An endpoint that receives packets that it cannot process sends a packet in the following layout (see Section 1.3):¶

Stateless Reset {
  Fixed Bits (2) = 1,
  Unpredictable Bits (38..),
  Stateless Reset Token (128),
}

This design ensures that a stateless reset packet is -- to the extent possible -- indistinguishable from a regular packet with a short header.¶

A stateless reset uses an entire UDP datagram, starting with the first two bits of the packet header. The remainder of the first byte and an arbitrary number of bytes following it are set to values that SHOULD be indistinguishable from random. The last 16 bytes of the datagram contain a Stateless Reset Token.¶

To entities other than its intended recipient, a stateless reset will appear to be a packet with a short header. For the stateless reset to appear as a valid QUIC packet, the Unpredictable Bits field needs to include at least 38 bits of data (or 5 bytes, less the two fixed bits).¶

The resulting minimum size of 21 bytes does not guarantee that a stateless reset is difficult to distinguish from other packets if the recipient requires the use of a connection ID. To achieve that end, the endpoint SHOULD ensure that all packets it sends are at least 22 bytes longer than the minimum connection ID length that it requests the peer to include in its packets, adding PADDING frames as necessary. This ensures that any stateless reset sent by the peer is indistinguishable from a valid packet sent to the endpoint. An endpoint that sends a stateless reset in response to a packet that is 43 bytes or shorter SHOULD send a stateless reset that is one byte shorter than the packet it responds to.¶

These values assume that the Stateless Reset Token is the same length as the minimum expansion of the packet protection Authenticated Encryption with Associated Data (AEAD). Additional unpredictable bytes are necessary if the endpoint could have negotiated a packet protection scheme with a larger minimum expansion.¶

An endpoint MUST NOT send a stateless reset that is three times or more larger than the packet it receives to avoid being used for amplification. Section 10.3.3 describes additional limits on stateless reset size.¶

Endpoints MUST discard packets that are too small to be valid QUIC packets. To give an example, with the set of AEAD functions defined in [RFCYYY2], short header packets that are smaller than 21 bytes are never valid.¶

Endpoints MUST send stateless reset packets formatted as a packet with a short header. However, endpoints MUST treat any packet ending in a valid stateless reset token as a stateless reset, as other QUIC versions might allow the use of a long header.¶

An endpoint MAY send a stateless reset in response to a packet with a long header. Sending a stateless reset is not effective prior to the stateless reset token being available to a peer. In this QUIC version, packets with a long header are only used during connection establishment. Because the stateless reset token is not available until connection establishment is complete or near completion, ignoring an unknown packet with a long header might be as effective as sending a stateless reset.¶

An endpoint cannot determine the Source Connection ID from a packet with a short header; therefore, it cannot set the Destination Connection ID in the stateless reset packet. The Destination Connection ID will therefore differ from the value used in previous packets. A random Destination Connection ID makes the connection ID appear to be the result of moving to a new connection ID that was provided using a NEW_CONNECTION_ID frame (Section 19.15).¶

Using a randomized connection ID results in two problems:¶

• The packet might not reach the peer. If the Destination Connection ID is critical for routing toward the peer, then this packet could be incorrectly routed. This might also trigger another Stateless Reset in response; see Section 10.3.3. A Stateless Reset that is not correctly routed is an ineffective error detection and recovery mechanism. In this case, endpoints will need to rely on other methods -- such as timers -- to detect that the connection has failed.¶
• The randomly generated connection ID can be used by entities other than the peer to identify this as a potential stateless reset. An endpoint that occasionally uses different connection IDs might introduce some uncertainty about this.¶

This stateless reset design is specific to QUIC version 1. An endpoint that supports multiple versions of QUIC needs to generate a stateless reset that will be accepted by peers that support any version that the endpoint might support (or might have supported prior to losing state). Designers of new versions of QUIC need to be aware of this and either (1) reuse this design or (2) use a portion of the packet other than the last 16 bytes for carrying data.¶

11.1. Connection Errors

Errors that result in the connection being unusable, such as an obvious violation of protocol semantics or corruption of state that affects an entire connection, MUST be signaled using a CONNECTION_CLOSE frame (Section 19.19).¶

Application-specific protocol errors are signaled using the CONNECTION_CLOSE frame with a frame type of 0x1d. Errors that are specific to the transport, including all those described in this document, are carried in the CONNECTION_CLOSE frame with a frame type of 0x1c.¶

A CONNECTION_CLOSE frame could be sent in a packet that is lost. An endpoint SHOULD be prepared to retransmit a packet containing a CONNECTION_CLOSE frame if it receives more packets on a terminated connection. Limiting the number of retransmissions and the time over which this final packet is sent limits the effort expended on terminated connections.¶

An endpoint that chooses not to retransmit packets containing a CONNECTION_CLOSE frame risks a peer missing the first such packet. The only mechanism available to an endpoint that continues to receive data for a terminated connection is to attempt the stateless reset process (Section 10.3).¶

As the AEAD for Initial packets does not provide strong authentication, an endpoint MAY discard an invalid Initial packet. Discarding an Initial packet is permitted even where this specification otherwise mandates a connection error. An endpoint can only discard a packet if it does not process the frames in the packet or reverts the effects of any processing. Discarding invalid Initial packets might be used to reduce exposure to denial of service; see Section 21.2.¶

11.2. Stream Errors

If an application-level error affects a single stream but otherwise leaves the connection in a recoverable state, the endpoint can send a RESET_STREAM frame (Section 19.4) with an appropriate error code to terminate just the affected stream.¶

Resetting a stream without the involvement of the application protocol could cause the application protocol to enter an unrecoverable state. RESET_STREAM MUST only be instigated by the application protocol that uses QUIC.¶

The semantics of the application error code carried in RESET_STREAM are defined by the application protocol. Only the application protocol is able to cause a stream to be terminated. A local instance of the application protocol uses a direct API call, and a remote instance uses the STOP_SENDING frame, which triggers an automatic RESET_STREAM.¶

Application protocols SHOULD define rules for handling streams that are prematurely canceled by either endpoint.¶

12.1. Protected Packets

QUIC packets have different levels of cryptographic protection based on the type of packet. Details of packet protection are found in [RFCYYY2]; this section includes an overview of the protections that are provided.¶

Version Negotiation packets have no cryptographic protection; see [RFCYYY1].¶

Retry packets use an AEAD function [AEAD] to protect against accidental modification.¶

Initial packets use an AEAD function, the keys for which are derived using a value that is visible on the wire. Initial packets therefore do not have effective confidentiality protection. Initial protection exists to ensure that the sender of the packet is on the network path. Any entity that receives an Initial packet from a client can recover the keys that will allow them to both read the contents of the packet and generate Initial packets that will be successfully authenticated at either endpoint. The AEAD also protects Initial packets against accidental modification.¶

All other packets are protected with keys derived from the cryptographic handshake. The cryptographic handshake ensures that only the communicating endpoints receive the corresponding keys for Handshake, 0-RTT, and 1-RTT packets. Packets protected with 0-RTT and 1-RTT keys have strong confidentiality and integrity protection.¶

The Packet Number field that appears in some packet types has alternative confidentiality protection that is applied as part of header protection; see Section 5.4 of [RFCYYY2] for details. The underlying packet number increases with each packet sent in a given packet number space; see Section 12.3 for details.¶

12.2. Coalescing Packets

Initial (Section 17.2.2), 0-RTT (Section 17.2.3), and Handshake (Section 17.2.4) packets contain a Length field that determines the end of the packet. The length includes both the Packet Number and Payload fields, both of which are confidentiality protected and initially of unknown length. The length of the Payload field is learned once header protection is removed.¶

Using the Length field, a sender can coalesce multiple QUIC packets into one UDP datagram. This can reduce the number of UDP datagrams needed to complete the cryptographic handshake and start sending data. This can also be used to construct Path Maximum Transmission Unit (PMTU) probes; see Section 14.4.1. Receivers MUST be able to process coalesced packets.¶

Coalescing packets in order of increasing encryption levels (Initial, 0-RTT, Handshake, 1-RTT; see Section 4.1.4 of [RFCYYY2]) makes it more likely that the receiver will be able to process all the packets in a single pass. A packet with a short header does not include a length, so it can only be the last packet included in a UDP datagram. An endpoint SHOULD include multiple frames in a single packet if they are to be sent at the same encryption level, instead of coalescing multiple packets at the same encryption level.¶

Receivers MAY route based on the information in the first packet contained in a UDP datagram. Senders MUST NOT coalesce QUIC packets with different connection IDs into a single UDP datagram. Receivers SHOULD ignore any subsequent packets with a different Destination Connection ID than the first packet in the datagram.¶

Every QUIC packet that is coalesced into a single UDP datagram is separate and complete. The receiver of coalesced QUIC packets MUST individually process each QUIC packet and separately acknowledge them, as if they were received as the payload of different UDP datagrams. For example, if decryption fails (because the keys are not available or for any other reason), the receiver MAY either discard or buffer the packet for later processing and MUST attempt to process the remaining packets.¶

Retry packets (Section 17.2.5), Version Negotiation packets (Section 17.2.1), and packets with a short header (Section 17.3) do not contain a Length field and so cannot be followed by other packets in the same UDP datagram. Note also that there is no situation where a Retry or Version Negotiation packet is coalesced with another packet.¶

12.3. Packet Numbers

The packet number is an integer in the range 0 to 2^62-1. This number is used in determining the cryptographic nonce for packet protection. Each endpoint maintains a separate packet number for sending and receiving.¶

Packet numbers are limited to this range because they need to be representable in whole in the Largest Acknowledged field of an ACK frame (Section 19.3). When present in a long or short header, however, packet numbers are reduced and encoded in 1 to 4 bytes; see Section 17.1.¶

Version Negotiation (Section 17.2.1) and Retry (Section 17.2.5) packets do not include a packet number.¶

Packet numbers are divided into three spaces in QUIC:¶

Initial space:

All Initial packets (Section 17.2.2) are in this space.¶

Handshake space:

All Handshake packets (Section 17.2.4) are in this space.¶

Application data space:

All 0-RTT (Section 17.2.3) and 1-RTT (Section 17.3.1) packets are in this space.¶

As described in [RFCYYY2], each packet type uses different protection keys.¶

Conceptually, a packet number space is the context in which a packet can be processed and acknowledged. Initial packets can only be sent with Initial packet protection keys and acknowledged in packets that are also Initial packets. Similarly, Handshake packets are sent at the Handshake encryption level and can only be acknowledged in Handshake packets.¶

This enforces cryptographic separation between the data sent in the different packet number spaces. Packet numbers in each space start at packet number 0. Subsequent packets sent in the same packet number space MUST increase the packet number by at least one.¶

0-RTT and 1-RTT data exist in the same packet number space to make loss recovery algorithms easier to implement between the two packet types.¶

A QUIC endpoint MUST NOT reuse a packet number within the same packet number space in one connection. If the packet number for sending reaches 2^62 - 1, the sender MUST close the connection without sending a CONNECTION_CLOSE frame or any further packets; an endpoint MAY send a Stateless Reset (Section 10.3) in response to further packets that it receives.¶

A receiver MUST discard a newly unprotected packet unless it is certain that it has not processed another packet with the same packet number from the same packet number space. Duplicate suppression MUST happen after removing packet protection for the reasons described in Section 9.5 of [RFCYYY2].¶

Endpoints that track all individual packets for the purposes of detecting duplicates are at risk of accumulating excessive state. The data required for detecting duplicates can be limited by maintaining a minimum packet number below which all packets are immediately dropped. Any minimum needs to account for large variations in round-trip time, which includes the possibility that a peer might probe network paths with much larger round-trip times; see Section 9.¶

Packet number encoding at a sender and decoding at a receiver are described in Section 17.1.¶

12.4. Frames and Frame Types

The payload of QUIC packets, after removing packet protection, consists of a sequence of complete frames, as shown in Figure 11. Version Negotiation, Stateless Reset, and Retry packets do not contain frames.¶

Packet Payload {
  Frame (8..) ...,
}

The payload of a packet that contains frames MUST contain at least one frame, and it MAY contain multiple frames and multiple frame types. An endpoint MUST treat receipt of a packet containing no frames as a connection error of type PROTOCOL_VIOLATION. Frames always fit within a single QUIC packet and cannot span multiple packets.¶

Each frame begins with a Frame Type, indicating its type, followed by additional type-dependent fields:¶

Frame {
  Frame Type (i),
  Type-Dependent Fields (..),
}

Table 3 lists and summarizes information about each frame type that is defined in this specification. A description of this summary is included after the table.¶

The format and semantics of each frame type are explained in more detail in Section 19. The remainder of this section provides a summary of important and general information.¶

The Frame Type in ACK, STREAM, MAX_STREAMS, STREAMS_BLOCKED, and CONNECTION_CLOSE frames is used to carry other frame-specific flags. For all other frames, the Frame Type field simply identifies the frame.¶

The "Pkts" column in Table 3 lists the types of packets that each frame type could appear in, indicated by the following characters:¶

I:

Initial (Section 17.2.2)¶

H:

Handshake (Section 17.2.4)¶

0:

0-RTT (Section 17.2.3)¶

1:

1-RTT (Section 17.3.1)¶

ih:

Only a CONNECTION_CLOSE frame of type 0x1c can appear in Initial or Handshake packets.¶

For more details about these restrictions, see Section 12.5. Note that all frames can appear in 1-RTT packets. An endpoint MUST treat receipt of a frame in a packet type that is not permitted as a connection error of type PROTOCOL_VIOLATION.¶

The "Spec" column in Table 3 summarizes any special rules governing the processing or generation of the frame type, as indicated by the following characters:¶

N:

Packets containing only frames with this marking are not ack-eliciting; see Section 13.2.¶

C:

Packets containing only frames with this marking do not count toward bytes in flight for congestion control purposes; see [RFCYYY3].¶

P:

Packets containing only frames with this marking can be used to probe new network paths during connection migration; see Section 9.1.¶

F:

The contents of frames with this marking are flow controlled; see Section 4.¶

The "Pkts" and "Spec" columns in Table 3 do not form part of the IANA registry; see Section 22.4.¶

An endpoint MUST treat the receipt of a frame of unknown type as a connection error of type FRAME_ENCODING_ERROR.¶

All frames are idempotent in this version of QUIC. That is, a valid frame does not cause undesirable side effects or errors when received more than once.¶

The Frame Type field uses a variable-length integer encoding (see Section 16), with one exception. To ensure simple and efficient implementations of frame parsing, a frame type MUST use the shortest possible encoding. For frame types defined in this document, this means a single-byte encoding, even though it is possible to encode these values as a two-, four-, or eight-byte variable-length integer. For instance, though 0x4001 is a legitimate two-byte encoding for a variable-length integer with a value of 1, PING frames are always encoded as a single byte with the value 0x01. This rule applies to all current and future QUIC frame types. An endpoint MAY treat the receipt of a frame type that uses a longer encoding than necessary as a connection error of type PROTOCOL_VIOLATION.¶

12.5. Frames and Number Spaces

Some frames are prohibited in different packet number spaces. The rules here generalize those of TLS, in that frames associated with establishing the connection can usually appear in packets in any packet number space, whereas those associated with transferring data can only appear in the application data packet number space:¶

• PADDING, PING, and CRYPTO frames MAY appear in any packet number space.¶
• CONNECTION_CLOSE frames signaling errors at the QUIC layer (type 0x1c) MAY appear in any packet number space. CONNECTION_CLOSE frames signaling application errors (type 0x1d) MUST only appear in the application data packet number space.¶
• ACK frames MAY appear in any packet number space but can only acknowledge packets that appeared in that packet number space. However, as noted below, 0-RTT packets cannot contain ACK frames.¶
• All other frame types MUST only be sent in the application data packet number space.¶

Note that it is not possible to send the following frames in 0-RTT packets for various reasons: ACK, CRYPTO, HANDSHAKE_DONE, NEW_TOKEN, PATH_RESPONSE, and RETIRE_CONNECTION_ID. A server MAY treat receipt of these frames in 0-RTT packets as a connection error of type PROTOCOL_VIOLATION.¶

13.1. Packet Processing

A packet MUST NOT be acknowledged until packet protection has been successfully removed and all frames contained in the packet have been processed. For STREAM frames, this means the data has been enqueued in preparation to be received by the application protocol, but it does not require that data be delivered and consumed.¶

Once the packet has been fully processed, a receiver acknowledges receipt by sending one or more ACK frames containing the packet number of the received packet.¶

An endpoint SHOULD treat receipt of an acknowledgment for a packet it did not send as a connection error of type PROTOCOL_VIOLATION, if it is able to detect the condition. For further discussion of how this might be achieved, see Section 21.4.¶

13.2. Generating Acknowledgments

Endpoints acknowledge all packets they receive and process. However, only ack-eliciting packets cause an ACK frame to be sent within the maximum ack delay. Packets that are not ack-eliciting are only acknowledged when an ACK frame is sent for other reasons.¶

When sending a packet for any reason, an endpoint SHOULD attempt to include an ACK frame if one has not been sent recently. Doing so helps with timely loss detection at the peer.¶

In general, frequent feedback from a receiver improves loss and congestion response, but this has to be balanced against excessive load generated by a receiver that sends an ACK frame in response to every ack-eliciting packet. The guidance offered below seeks to strike this balance.¶

13.3. Retransmission of Information

QUIC packets that are determined to be lost are not retransmitted whole. The same applies to the frames that are contained within lost packets. Instead, the information that might be carried in frames is sent again in new frames as needed.¶

New frames and packets are used to carry information that is determined to have been lost. In general, information is sent again when a packet containing that information is determined to be lost, and sending ceases when a packet containing that information is acknowledged.¶

• Data sent in CRYPTO frames is retransmitted according to the rules in [RFCYYY3], until all data has been acknowledged. Data in CRYPTO frames for Initial and Handshake packets is discarded when keys for the corresponding packet number space are discarded.¶
• Application data sent in STREAM frames is retransmitted in new STREAM frames unless the endpoint has sent a RESET_STREAM for that stream. Once an endpoint sends a RESET_STREAM frame, no further STREAM frames are needed.¶
• ACK frames carry the most recent set of acknowledgments and the acknowledgment delay from the largest acknowledged packet, as described in Section 13.2.1. Delaying the transmission of packets containing ACK frames or resending old ACK frames can cause the peer to generate an inflated RTT sample or unnecessarily disable ECN.¶
• Cancellation of stream transmission, as carried in a RESET_STREAM frame, is sent until acknowledged or until all stream data is acknowledged by the peer (that is, either the "Reset Recvd" or "Data Recvd" state is reached on the sending part of the stream). The content of a RESET_STREAM frame MUST NOT change when it is sent again.¶
• Similarly, a request to cancel stream transmission, as encoded in a STOP_SENDING frame, is sent until the receiving part of the stream enters either a "Data Recvd" or "Reset Recvd" state; see Section 3.5.¶
• Connection close signals, including packets that contain CONNECTION_CLOSE frames, are not sent again when packet loss is detected, but as described in Section 10.¶
• The current connection maximum data is sent in MAX_DATA frames. An updated value is sent in a MAX_DATA frame if the packet containing the most recently sent MAX_DATA frame is declared lost or when the endpoint decides to update the limit. Care is necessary to avoid sending this frame too often, as the limit can increase frequently and cause an unnecessarily large number of MAX_DATA frames to be sent; see Section 4.2.¶
• The current maximum stream data offset is sent in MAX_STREAM_DATA frames. Like MAX_DATA, an updated value is sent when the packet containing the most recent MAX_STREAM_DATA frame for a stream is lost or when the limit is updated, with care taken to prevent the frame from being sent too often. An endpoint SHOULD stop sending MAX_STREAM_DATA frames when the receiving part of the stream enters a "Size Known" or "Reset Recvd" state.¶
• The limit on streams of a given type is sent in MAX_STREAMS frames. Like MAX_DATA, an updated value is sent when a packet containing the most recent MAX_STREAMS for a stream type frame is declared lost or when the limit is updated, with care taken to prevent the frame from being sent too often.¶
• Blocked signals are carried in DATA_BLOCKED, STREAM_DATA_BLOCKED, and STREAMS_BLOCKED frames. DATA_BLOCKED frames have connection scope, STREAM_DATA_BLOCKED frames have stream scope, and STREAMS_BLOCKED frames are scoped to a specific stream type. New frames are sent if packets containing the most recent frame for a scope is lost, but only while the endpoint is blocked on the corresponding limit. These frames always include the limit that is causing blocking at the time that they are transmitted.¶
• A liveness or path validation check using PATH_CHALLENGE frames is sent periodically until a matching PATH_RESPONSE frame is received or until there is no remaining need for liveness or path validation checking. PATH_CHALLENGE frames include a different payload each time they are sent.¶
• Responses to path validation using PATH_RESPONSE frames are sent just once. The peer is expected to send more PATH_CHALLENGE frames as necessary to evoke additional PATH_RESPONSE frames.¶
• New connection IDs are sent in NEW_CONNECTION_ID frames and retransmitted if the packet containing them is lost. Retransmissions of this frame carry the same sequence number value. Likewise, retired connection IDs are sent in RETIRE_CONNECTION_ID frames and retransmitted if the packet containing them is lost.¶
• NEW_TOKEN frames are retransmitted if the packet containing them is lost. No special support is made for detecting reordered and duplicated NEW_TOKEN frames other than a direct comparison of the frame contents.¶
• PING and PADDING frames contain no information, so lost PING or PADDING frames do not require repair.¶
• The HANDSHAKE_DONE frame MUST be retransmitted until it is acknowledged.¶

Endpoints SHOULD prioritize retransmission of data over sending new data, unless priorities specified by the application indicate otherwise; see Section 2.3.¶

Even though a sender is encouraged to assemble frames containing up-to-date information every time it sends a packet, it is not forbidden to retransmit copies of frames from lost packets. A sender that retransmits copies of frames needs to handle decreases in available payload size due to changes in packet number length, connection ID length, and path MTU. A receiver MUST accept packets containing an outdated frame, such as a MAX_DATA frame carrying a smaller maximum data value than one found in an older packet.¶

A sender SHOULD avoid retransmitting information from packets once they are acknowledged. This includes packets that are acknowledged after being declared lost, which can happen in the presence of network reordering. Doing so requires senders to retain information about packets after they are declared lost. A sender can discard this information after a period of time elapses that adequately allows for reordering, such as a PTO (Section 6.2 of [RFCYYY3]), or based on other events, such as reaching a memory limit.¶

Upon detecting losses, a sender MUST take appropriate congestion control action. The details of loss detection and congestion control are described in [RFCYYY3].¶

13.4. Explicit Congestion Notification

QUIC endpoints can use ECN [RFC3168] to detect and respond to network congestion. ECN allows an endpoint to set an ECN-Capable Transport (ECT) codepoint in the ECN field of an IP packet. A network node can then indicate congestion by setting the CE codepoint in the ECN field instead of dropping the packet [RFC8087]. Endpoints react to reported congestion by reducing their sending rate in response, as described in [RFCYYY3].¶

To enable ECN, a sending QUIC endpoint first determines whether a path supports ECN marking and whether the peer reports the ECN values in received IP headers; see Section 13.4.2.¶

14.1. Initial Datagram Size

A client MUST expand the payload of all UDP datagrams carrying Initial packets to at least the smallest allowed maximum datagram size of 1200 bytes by adding PADDING frames to the Initial packet or by coalescing the Initial packet; see Section 12.2. Initial packets can even be coalesced with invalid packets, which a receiver will discard. Similarly, a server MUST expand the payload of all UDP datagrams carrying ack-eliciting Initial packets to at least the smallest allowed maximum datagram size of 1200 bytes.¶

Sending UDP datagrams of this size ensures that the network path supports a reasonable Path Maximum Transmission Unit (PMTU), in both directions. Additionally, a client that expands Initial packets helps reduce the amplitude of amplification attacks caused by server responses toward an unverified client address; see Section 8.¶

Datagrams containing Initial packets MAY exceed 1200 bytes if the sender believes that the network path and peer both support the size that it chooses.¶

A server MUST discard an Initial packet that is carried in a UDP datagram with a payload that is smaller than the smallest allowed maximum datagram size of 1200 bytes. A server MAY also immediately close the connection by sending a CONNECTION_CLOSE frame with an error code of PROTOCOL_VIOLATION; see Section 10.2.3.¶

The server MUST also limit the number of bytes it sends before validating the address of the client; see Section 8.¶

14.2. Path Maximum Transmission Unit

The PMTU is the maximum size of the entire IP packet, including the IP header, UDP header, and UDP payload. The UDP payload includes one or more QUIC packet headers and protected payloads. The PMTU can depend on path characteristics and can therefore change over time. The largest UDP payload an endpoint sends at any given time is referred to as the endpoint's maximum datagram size.¶

An endpoint SHOULD use DPLPMTUD (Section 14.3) or PMTUD (Section 14.2.1) to determine whether the path to a destination will support a desired maximum datagram size without fragmentation. In the absence of these mechanisms, QUIC endpoints SHOULD NOT send datagrams larger than the smallest allowed maximum datagram size.¶

Both DPLPMTUD and PMTUD send datagrams that are larger than the current maximum datagram size, referred to as PMTU probes. All QUIC packets that are not sent in a PMTU probe SHOULD be sized to fit within the maximum datagram size to avoid the datagram being fragmented or dropped [RFC8085].¶

If a QUIC endpoint determines that the PMTU between any pair of local and remote IP addresses cannot support the smallest allowed maximum datagram size of 1200 bytes, it MUST immediately cease sending QUIC packets, except for those in PMTU probes or those containing CONNECTION_CLOSE frames, on the affected path. An endpoint MAY terminate the connection if an alternative path cannot be found.¶

Each pair of local and remote addresses could have a different PMTU. QUIC implementations that implement any kind of PMTU discovery therefore SHOULD maintain a maximum datagram size for each combination of local and remote IP addresses.¶

A QUIC implementation MAY be more conservative in computing the maximum datagram size to allow for unknown tunnel overheads or IP header options/extensions.¶

14.3. Datagram Packetization Layer PMTU Discovery

DPLPMTUD [DPLPMTUD] relies on tracking loss or acknowledgment of QUIC packets that are carried in PMTU probes. PMTU probes for DPLPMTUD that use the PADDING frame implement "Probing using padding data", as defined in Section 4.1 of [DPLPMTUD].¶

Endpoints SHOULD set the initial value of BASE_PLPMTU (Section 5.1 of [DPLPMTUD]) to be consistent with QUIC's smallest allowed maximum datagram size. The MIN_PLPMTU is the same as the BASE_PLPMTU.¶

QUIC endpoints implementing DPLPMTUD maintain a DPLPMTUD Maximum Packet Size (MPS) (Section 4.4 of [DPLPMTUD]) for each combination of local and remote IP addresses. This corresponds to the maximum datagram size.¶

14.4. Sending QUIC PMTU Probes

PMTU probes are ack-eliciting packets.¶

Endpoints could limit the content of PMTU probes to PING and PADDING frames, since packets that are larger than the current maximum datagram size are more likely to be dropped by the network. Loss of a QUIC packet that is carried in a PMTU probe is therefore not a reliable indication of congestion and SHOULD NOT trigger a congestion control reaction; see Bullet 7 in Section 3 of [DPLPMTUD]. However, PMTU probes consume congestion window, which could delay subsequent transmission by an application.¶

17.1. Packet Number Encoding and Decoding

Packet numbers are integers in the range 0 to 2^62-1 (Section 12.3). When present in long or short packet headers, they are encoded in 1 to 4 bytes. The number of bits required to represent the packet number is reduced by including only the least significant bits of the packet number.¶

The encoded packet number is protected as described in Section 5.4 of [RFCYYY2].¶

Prior to receiving an acknowledgment for a packet number space, the full packet number MUST be included; it is not to be truncated, as described below.¶

After an acknowledgment is received for a packet number space, the sender MUST use a packet number size able to represent more than twice as large a range as the difference between the largest acknowledged packet and packet number being sent. A peer receiving the packet will then correctly decode the packet number, unless the packet is delayed in transit such that it arrives after many higher-numbered packets have been received. An endpoint SHOULD use a large enough packet number encoding to allow the packet number to be recovered even if the packet arrives after packets that are sent afterwards.¶

As a result, the size of the packet number encoding is at least one bit more than the base-2 logarithm of the number of contiguous unacknowledged packet numbers, including the new packet. Pseudocode and an example for packet number encoding can be found in Appendix A.2.¶

At a receiver, protection of the packet number is removed prior to recovering the full packet number. The full packet number is then reconstructed based on the number of significant bits present, the value of those bits, and the largest packet number received in a successfully authenticated packet. Recovering the full packet number is necessary to successfully remove packet protection.¶

Once header protection is removed, the packet number is decoded by finding the packet number value that is closest to the next expected packet. The next expected packet is the highest received packet number plus one. Pseudocode and an example for packet number decoding can be found in Appendix A.3.¶

17.2. Long Header Packets

Long Header Packet {
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type (2),
  Type-Specific Bits (4),
  Version (32),
  Destination Connection ID Length (8),
  Destination Connection ID (0..160),
  Source Connection ID Length (8),
  Source Connection ID (0..160),
  Type-Specific Payload (..),
}

Long headers are used for packets that are sent prior to the establishment of 1-RTT keys. Once 1-RTT keys are available, a sender switches to sending packets using the short header (Section 17.3). The long form allows for special packets -- such as the Version Negotiation packet -- to be represented in this uniform fixed-length packet format. Packets that use the long header contain the following fields:¶

Header Form:

The most significant bit (0x80) of byte 0 (the first byte) is set to 1 for long headers.¶

Fixed Bit:

The next bit (0x40) of byte 0 is set to 1, unless the packet is a Version Negotiation packet. Packets containing a zero value for this bit are not valid packets in this version and MUST be discarded. A value of 1 for this bit allows QUIC to coexist with other protocols; see [RFC7983].¶

Long Packet Type:

The next two bits (those with a mask of 0x30) of byte 0 contain a packet type. Packet types are listed in Table 5.¶

Type-Specific Bits:

The semantics of the lower four bits (those with a mask of 0x0f) of byte 0 are determined by the packet type.¶

Version:

The QUIC Version is a 32-bit field that follows the first byte. This field indicates the version of QUIC that is in use and determines how the rest of the protocol fields are interpreted.¶

Destination Connection ID Length:

The byte following the version contains the length in bytes of the Destination Connection ID field that follows it. This length is encoded as an 8-bit unsigned integer. In QUIC version 1, this value MUST NOT exceed 20 bytes. Endpoints that receive a version 1 long header with a value larger than 20 MUST drop the packet. In order to properly form a Version Negotiation packet, servers SHOULD be able to read longer connection IDs from other QUIC versions.¶

Destination Connection ID:

The Destination Connection ID field follows the Destination Connection ID Length field, which indicates the length of this field. Section 7.2 describes the use of this field in more detail.¶

Source Connection ID Length:

The byte following the Destination Connection ID contains the length in bytes of the Source Connection ID field that follows it. This length is encoded as an 8-bit unsigned integer. In QUIC version 1, this value MUST NOT exceed 20 bytes. Endpoints that receive a version 1 long header with a value larger than 20 MUST drop the packet. In order to properly form a Version Negotiation packet, servers SHOULD be able to read longer connection IDs from other QUIC versions.¶

Source Connection ID:

The Source Connection ID field follows the Source Connection ID Length field, which indicates the length of this field. Section 7.2 describes the use of this field in more detail.¶

Type-Specific Payload:

The remainder of the packet, if any, is type specific.¶

In this version of QUIC, the following packet types with the long header are defined:¶

The header form bit, Destination and Source Connection ID lengths, Destination and Source Connection ID fields, and Version fields of a long header packet are version independent. The other fields in the first byte are version specific. See [RFCYYY1] for details on how packets from different versions of QUIC are interpreted.¶

The interpretation of the fields and the payload are specific to a version and packet type. While type-specific semantics for this version are described in the following sections, several long header packets in this version of QUIC contain these additional fields:¶

Reserved Bits:

Two bits (those with a mask of 0x0c) of byte 0 are reserved across multiple packet types. These bits are protected using header protection; see Section 5.4 of [RFCYYY2]. The value included prior to protection MUST be set to 0. An endpoint MUST treat receipt of a packet that has a non-zero value for these bits after removing both packet and header protection as a connection error of type PROTOCOL_VIOLATION. Discarding such a packet after only removing header protection can expose the endpoint to attacks; see Section 9.5 of [RFCYYY2].¶

Packet Number Length:

In packet types that contain a Packet Number field, the least significant two bits (those with a mask of 0x03) of byte 0 contain the length of the packet number, encoded as an unsigned two-bit integer that is one less than the length of the packet number field in bytes. That is, the length of the packet number field is the value of this field plus one. These bits are protected using header protection; see Section 5.4 of [RFCYYY2].¶

Length:

This is the length of the remainder of the packet (that is, the Packet Number and Payload fields) in bytes, encoded as a variable-length integer (Section 16).¶

Packet Number:

The packet number field is 1 to 4 bytes long. The packet number is protected using header protection; see Section 5.4 of [RFCYYY2]. The length of the packet number field is encoded in the Packet Number Length bits of byte 0; see above.¶

Version Negotiation Packet {
  Header Form (1) = 1,
  Unused (7),
  Version (32) = 0,
  Destination Connection ID Length (8),
  Destination Connection ID (0..2040),
  Source Connection ID Length (8),
  Source Connection ID (0..2040),
  Supported Version (32) ...,
}

Initial Packet {
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 0,
  Reserved Bits (2),
  Packet Number Length (2),
  Version (32),
  Destination Connection ID Length (8),
  Destination Connection ID (0..160),
  Source Connection ID Length (8),
  Source Connection ID (0..160),
  Token Length (i),
  Token (..),
  Length (i),
  Packet Number (8..32),
  Packet Payload (8..),
}

0-RTT Packet {
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 1,
  Reserved Bits (2),
  Packet Number Length (2),
  Version (32),
  Destination Connection ID Length (8),
  Destination Connection ID (0..160),
  Source Connection ID Length (8),
  Source Connection ID (0..160),
  Length (i),
  Packet Number (8..32),
  Packet Payload (8..),
}

Handshake Packet {
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 2,
  Reserved Bits (2),
  Packet Number Length (2),
  Version (32),
  Destination Connection ID Length (8),
  Destination Connection ID (0..160),
  Source Connection ID Length (8),
  Source Connection ID (0..160),
  Length (i),
  Packet Number (8..32),
  Packet Payload (8..),
}

Retry Packet {
  Header Form (1) = 1,
  Fixed Bit (1) = 1,
  Long Packet Type (2) = 3,
  Unused (4),
  Version (32),
  Destination Connection ID Length (8),
  Destination Connection ID (0..160),
  Source Connection ID Length (8),
  Source Connection ID (0..160),
  Retry Token (..),
  Retry Integrity Tag (128),
}

17.3. Short Header Packets

This version of QUIC defines a single packet type that uses the short packet header.¶

1-RTT Packet {
  Header Form (1) = 0,
  Fixed Bit (1) = 1,
  Spin Bit (1),
  Reserved Bits (2),
  Key Phase (1),
  Packet Number Length (2),
  Destination Connection ID (0..160),
  Packet Number (8..32),
  Packet Payload (8..),
}

17.4. Latency Spin Bit

The latency spin bit, which is defined for 1-RTT packets (Section 17.3.1), enables passive latency monitoring from observation points on the network path throughout the duration of a connection. The server reflects the spin value received, while the client "spins" it after one RTT. On-path observers can measure the time between two spin bit toggle events to estimate the end-to-end RTT of a connection.¶

The spin bit is only present in 1-RTT packets, since it is possible to measure the initial RTT of a connection by observing the handshake. Therefore, the spin bit is available after version negotiation and connection establishment are completed. On-path measurement and use of the latency spin bit are further discussed in [QUIC-MANAGEABILITY].¶

The spin bit is an OPTIONAL feature of this version of QUIC. An endpoint that does not support this feature MUST disable it, as defined below.¶

Each endpoint unilaterally decides if the spin bit is enabled or disabled for a connection. Implementations MUST allow administrators of clients and servers to disable the spin bit either globally or on a per-connection basis. Even when the spin bit is not disabled by the administrator, endpoints MUST disable their use of the spin bit for a random selection of at least one in every 16 network paths, or for one in every 16 connection IDs, in order to ensure that QUIC connections that disable the spin bit are commonly observed on the network. As each endpoint disables the spin bit independently, this ensures that the spin bit signal is disabled on approximately one in eight network paths.¶

When the spin bit is disabled, endpoints MAY set the spin bit to any value and MUST ignore any incoming value. It is RECOMMENDED that endpoints set the spin bit to a random value either chosen independently for each packet or chosen independently for each connection ID.¶

If the spin bit is enabled for the connection, the endpoint maintains a spin value for each network path and sets the spin bit in the packet header to the currently stored value when a 1-RTT packet is sent on that path. The spin value is initialized to 0 in the endpoint for each network path. Each endpoint also remembers the highest packet number seen from its peer on each path.¶

When a server receives a 1-RTT packet that increases the highest packet number seen by the server from the client on a given network path, it sets the spin value for that path to be equal to the spin bit in the received packet.¶

When a client receives a 1-RTT packet that increases the highest packet number seen by the client from the server on a given network path, it sets the spin value for that path to the inverse of the spin bit in the received packet.¶

An endpoint resets the spin value for a network path to 0 when changing the connection ID being used on that network path.¶

18.1. Reserved Transport Parameters

Transport parameters with an identifier of the form 31 * N + 27 for integer values of N are reserved to exercise the requirement that unknown transport parameters be ignored. These transport parameters have no semantics and can carry arbitrary values.¶

18.2. Transport Parameter Definitions

This section details the transport parameters defined in this document.¶

Many transport parameters listed here have integer values. Those transport parameters that are identified as integers use a variable-length integer encoding; see Section 16. Transport parameters have a default value of 0 if the transport parameter is absent, unless otherwise stated.¶

The following transport parameters are defined:¶

original_destination_connection_id (0x00):

This parameter is the value of the Destination Connection ID field from the first Initial packet sent by the client; see Section 7.3. This transport parameter is only sent by a server.¶

max_idle_timeout (0x01):

The maximum idle timeout is a value in milliseconds that is encoded as an integer; see (Section 10.1). Idle timeout is disabled when both endpoints omit this transport parameter or specify a value of 0.¶

stateless_reset_token (0x02):

A stateless reset token is used in verifying a stateless reset; see Section 10.3. This parameter is a sequence of 16 bytes. This transport parameter MUST NOT be sent by a client but MAY be sent by a server. A server that does not send this transport parameter cannot use stateless reset (Section 10.3) for the connection ID negotiated during the handshake.¶

max_udp_payload_size (0x03):

The maximum UDP payload size parameter is an integer value that limits the size of UDP payloads that the endpoint is willing to receive. UDP datagrams with payloads larger than this limit are not likely to be processed by the receiver.¶

The default for this parameter is the maximum permitted UDP payload of 65527. Values below 1200 are invalid.¶

This limit does act as an additional constraint on datagram size in the same way as the path MTU, but it is a property of the endpoint and not the path; see Section 14. It is expected that this is the space an endpoint dedicates to holding incoming packets.¶

initial_max_data (0x04):

The initial maximum data parameter is an integer value that contains the initial value for the maximum amount of data that can be sent on the connection. This is equivalent to sending a MAX_DATA (Section 19.9) for the connection immediately after completing the handshake.¶

initial_max_stream_data_bidi_local (0x05):

This parameter is an integer value specifying the initial flow control limit for locally initiated bidirectional streams. This limit applies to newly created bidirectional streams opened by the endpoint that sends the transport parameter. In client transport parameters, this applies to streams with an identifier with the least significant two bits set to 0x0; in server transport parameters, this applies to streams with the least significant two bits set to 0x1.¶

initial_max_stream_data_bidi_remote (0x06):

This parameter is an integer value specifying the initial flow control limit for peer-initiated bidirectional streams. This limit applies to newly created bidirectional streams opened by the endpoint that receives the transport parameter. In client transport parameters, this applies to streams with an identifier with the least significant two bits set to 0x1; in server transport parameters, this applies to streams with the least significant two bits set to 0x0.¶

initial_max_stream_data_uni (0x07):

This parameter is an integer value specifying the initial flow control limit for unidirectional streams. This limit applies to newly created unidirectional streams opened by the endpoint that receives the transport parameter. In client transport parameters, this applies to streams with an identifier with the least significant two bits set to 0x3; in server transport parameters, this applies to streams with the least significant two bits set to 0x2.¶

initial_max_streams_bidi (0x08):

The initial maximum bidirectional streams parameter is an integer value that contains the initial maximum number of bidirectional streams the endpoint that receives this transport parameter is permitted to initiate. If this parameter is absent or zero, the peer cannot open bidirectional streams until a MAX_STREAMS frame is sent. Setting this parameter is equivalent to sending a MAX_STREAMS (Section 19.11) of the corresponding type with the same value.¶

initial_max_streams_uni (0x09):

The initial maximum unidirectional streams parameter is an integer value that contains the initial maximum number of unidirectional streams the endpoint that receives this transport parameter is permitted to initiate. If this parameter is absent or zero, the peer cannot open unidirectional streams until a MAX_STREAMS frame is sent. Setting this parameter is equivalent to sending a MAX_STREAMS (Section 19.11) of the corresponding type with the same value.¶

ack_delay_exponent (0x0a):

The acknowledgment delay exponent is an integer value indicating an exponent used to decode the ACK Delay field in the ACK frame (Section 19.3). If this value is absent, a default value of 3 is assumed (indicating a multiplier of 8). Values above 20 are invalid.¶

max_ack_delay (0x0b):

The maximum acknowledgment delay is an integer value indicating the maximum amount of time in milliseconds by which the endpoint will delay sending acknowledgments. This value SHOULD include the receiver's expected delays in alarms firing. For example, if a receiver sets a timer for 5ms and alarms commonly fire up to 1ms late, then it should send a max_ack_delay of 6ms. If this value is absent, a default of 25 milliseconds is assumed. Values of 2^14 or greater are invalid.¶

disable_active_migration (0x0c):

The disable active migration transport parameter is included if the endpoint does not support active connection migration (Section 9) on the address being used during the handshake. An endpoint that receives this transport parameter MUST NOT use a new local address when sending to the address that the peer used during the handshake. This transport parameter does not prohibit connection migration after a client has acted on a preferred_address transport parameter. This parameter is a zero-length value.¶

preferred_address (0x0d):

The server's preferred address is used to effect a change in server address at the end of the handshake, as described in Section 9.6. This transport parameter is only sent by a server. Servers MAY choose to only send a preferred address of one address family by sending an all-zero address and port (0.0.0.0:0 or [::]:0) for the other family. IP addresses are encoded in network byte order.¶

The preferred_address transport parameter contains an address and port for both IPv4 and IPv6. The four-byte IPv4 Address field is followed by the associated two-byte IPv4 Port field. This is followed by a 16-byte IPv6 Address field and two-byte IPv6 Port field. After address and port pairs, a Connection ID Length field describes the length of the following Connection ID field. Finally, a 16-byte Stateless Reset Token field includes the stateless reset token associated with the connection ID. The format of this transport parameter is shown in Figure 22 below.¶

The Connection ID field and the Stateless Reset Token field contain an alternative connection ID that has a sequence number of 1; see Section 5.1.1. Having these values sent alongside the preferred address ensures that there will be at least one unused active connection ID when the client initiates migration to the preferred address.¶

The Connection ID and Stateless Reset Token fields of a preferred address are identical in syntax and semantics to the corresponding fields of a NEW_CONNECTION_ID frame (Section 19.15). A server that chooses a zero-length connection ID MUST NOT provide a preferred address. Similarly, a server MUST NOT include a zero-length connection ID in this transport parameter. A client MUST treat a violation of these requirements as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

Preferred Address {
  IPv4 Address (32),
  IPv4 Port (16),
  IPv6 Address (128),
  IPv6 Port (16),
  Connection ID Length (8),
  Connection ID (..),
  Stateless Reset Token (128),
}

active_connection_id_limit (0x0e):

The active connection ID limit is an integer value specifying the maximum number of connection IDs from the peer that an endpoint is willing to store. This value includes the connection ID received during the handshake, that received in the preferred_address transport parameter, and those received in NEW_CONNECTION_ID frames. The value of the active_connection_id_limit parameter MUST be at least 2. An endpoint that receives a value less than 2 MUST close the connection with an error of type TRANSPORT_PARAMETER_ERROR. If this transport parameter is absent, a default of 2 is assumed. If an endpoint issues a zero-length connection ID, it will never send a NEW_CONNECTION_ID frame and therefore ignores the active_connection_id_limit value received from its peer.¶

initial_source_connection_id (0x0f):

This is the value that the endpoint included in the Source Connection ID field of the first Initial packet it sends for the connection; see Section 7.3.¶

retry_source_connection_id (0x10):

This is the value that the server included in the Source Connection ID field of a Retry packet; see Section 7.3. This transport parameter is only sent by a server.¶

If present, transport parameters that set initial per-stream flow control limits (initial_max_stream_data_bidi_local, initial_max_stream_data_bidi_remote, and initial_max_stream_data_uni) are equivalent to sending a MAX_STREAM_DATA frame (Section 19.10) on every stream of the corresponding type immediately after opening. If the transport parameter is absent, streams of that type start with a flow control limit of 0.¶

A client MUST NOT include any server-only transport parameter: original_destination_connection_id, preferred_address, retry_source_connection_id, or stateless_reset_token. A server MUST treat receipt of any of these transport parameters as a connection error of type TRANSPORT_PARAMETER_ERROR.¶

19.1. PADDING Frames

A PADDING frame (type=0x00) has no semantic value. PADDING frames can be used to increase the size of a packet. Padding can be used to increase an initial client packet to the minimum required size or to provide protection against traffic analysis for protected packets.¶

PADDING frames are formatted as shown in Figure 23, which shows that PADDING frames have no content. That is, a PADDING frame consists of the single byte that identifies the frame as a PADDING frame.¶

PADDING Frame {
  Type (i) = 0x00,
}

19.2. PING Frames

Endpoints can use PING frames (type=0x01) to verify that their peers are still alive or to check reachability to the peer.¶

PING frames are formatted as shown in Figure 24, which shows that PING frames have no content.¶

PING Frame {
  Type (i) = 0x01,
}

The receiver of a PING frame simply needs to acknowledge the packet containing this frame.¶

The PING frame can be used to keep a connection alive when an application or application protocol wishes to prevent the connection from timing out; see Section 10.1.2.¶

19.3. ACK Frames

Receivers send ACK frames (types 0x02 and 0x03) to inform senders of packets they have received and processed. The ACK frame contains one or more ACK Ranges. ACK Ranges identify acknowledged packets. If the frame type is 0x03, ACK frames also contain the cumulative count of QUIC packets with associated ECN marks received on the connection up until this point. QUIC implementations MUST properly handle both types, and, if they have enabled ECN for packets they send, they SHOULD use the information in the ECN section to manage their congestion state.¶

QUIC acknowledgments are irrevocable. Once acknowledged, a packet remains acknowledged, even if it does not appear in a future ACK frame. This is unlike reneging for TCP Selective Acknowledgments (SACKs) [RFC2018].¶

Packets from different packet number spaces can be identified using the same numeric value. An acknowledgment for a packet needs to indicate both a packet number and a packet number space. This is accomplished by having each ACK frame only acknowledge packet numbers in the same space as the packet in which the ACK frame is contained.¶

Version Negotiation and Retry packets cannot be acknowledged because they do not contain a packet number. Rather than relying on ACK frames, these packets are implicitly acknowledged by the next Initial packet sent by the client.¶

ACK frames are formatted as shown in Figure 25.¶

ACK Frame {
  Type (i) = 0x02..0x03,
  Largest Acknowledged (i),
  ACK Delay (i),
  ACK Range Count (i),
  First ACK Range (i),
  ACK Range (..) ...,
  [ECN Counts (..)],
}

ACK frames contain the following fields:¶

Largest Acknowledged:

A variable-length integer representing the largest packet number the peer is acknowledging; this is usually the largest packet number that the peer has received prior to generating the ACK frame. Unlike the packet number in the QUIC long or short header, the value in an ACK frame is not truncated.¶

ACK Delay:

A variable-length integer encoding the acknowledgment delay in microseconds; see Section 13.2.5. It is decoded by multiplying the value in the field by 2 to the power of the ack_delay_exponent transport parameter sent by the sender of the ACK frame; see Section 18.2. Compared to simply expressing the delay as an integer, this encoding allows for a larger range of values within the same number of bytes, at the cost of lower resolution.¶

ACK Range Count:

A variable-length integer specifying the number of ACK Range fields in the frame.¶

First ACK Range:

A variable-length integer indicating the number of contiguous packets preceding the Largest Acknowledged that are being acknowledged. That is, the smallest packet acknowledged in the range is determined by subtracting the First ACK Range value from the Largest Acknowledged.¶

ACK Ranges:

Contains additional ranges of packets that are alternately not acknowledged (Gap) and acknowledged (ACK Range); see Section 19.3.1.¶

ECN Counts:

The three ECN Counts; see Section 19.3.2.¶

ACK Range {
  Gap (i),
  ACK Range Length (i),
}

   smallest = largest - ack_range

   largest = previous_smallest - gap - 2

ECN Counts {
  ECT0 Count (i),
  ECT1 Count (i),
  ECN-CE Count (i),
}

19.4. RESET_STREAM Frames

An endpoint uses a RESET_STREAM frame (type=0x04) to abruptly terminate the sending part of a stream.¶

After sending a RESET_STREAM, an endpoint ceases transmission and retransmission of STREAM frames on the identified stream. A receiver of RESET_STREAM can discard any data that it already received on that stream.¶

An endpoint that receives a RESET_STREAM frame for a send-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

RESET_STREAM frames are formatted as shown in Figure 28.¶

RESET_STREAM Frame {
  Type (i) = 0x04,
  Stream ID (i),
  Application Protocol Error Code (i),
  Final Size (i),
}

RESET_STREAM frames contain the following fields:¶

Stream ID:

A variable-length integer encoding of the Stream ID of the stream being terminated.¶

Application Protocol Error Code:

A variable-length integer containing the application protocol error code (see Section 20.2) that indicates why the stream is being closed.¶

Final Size:

A variable-length integer indicating the final size of the stream by the RESET_STREAM sender, in units of bytes; see Section 4.5.¶

19.5. STOP_SENDING Frames

An endpoint uses a STOP_SENDING frame (type=0x05) to communicate that incoming data is being discarded on receipt per application request. STOP_SENDING requests that a peer cease transmission on a stream.¶

A STOP_SENDING frame can be sent for streams in the "Recv" or "Size Known" states; see Section 3.1. Receiving a STOP_SENDING frame for a locally initiated stream that has not yet been created MUST be treated as a connection error of type STREAM_STATE_ERROR. An endpoint that receives a STOP_SENDING frame for a receive-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

STOP_SENDING frames are formatted as shown in Figure 29.¶

STOP_SENDING Frame {
  Type (i) = 0x05,
  Stream ID (i),
  Application Protocol Error Code (i),
}

STOP_SENDING frames contain the following fields:¶

Stream ID:

A variable-length integer carrying the Stream ID of the stream being ignored.¶

Application Protocol Error Code:

A variable-length integer containing the application-specified reason the sender is ignoring the stream; see Section 20.2.¶

19.6. CRYPTO Frames

A CRYPTO frame (type=0x06) is used to transmit cryptographic handshake messages. It can be sent in all packet types except 0-RTT. The CRYPTO frame offers the cryptographic protocol an in-order stream of bytes. CRYPTO frames are functionally identical to STREAM frames, except that they do not bear a stream identifier; they are not flow controlled; and they do not carry markers for optional offset, optional length, and the end of the stream.¶

CRYPTO frames are formatted as shown in Figure 30.¶

CRYPTO Frame {
  Type (i) = 0x06,
  Offset (i),
  Length (i),
  Crypto Data (..),
}

CRYPTO frames contain the following fields:¶

Offset:

A variable-length integer specifying the byte offset in the stream for the data in this CRYPTO frame.¶

Length:

A variable-length integer specifying the length of the Crypto Data field in this CRYPTO frame.¶

Crypto Data:

The cryptographic message data.¶

There is a separate flow of cryptographic handshake data in each encryption level, each of which starts at an offset of 0. This implies that each encryption level is treated as a separate CRYPTO stream of data.¶

The largest offset delivered on a stream -- the sum of the offset and data length -- cannot exceed 2^62-1. Receipt of a frame that exceeds this limit MUST be treated as a connection error of type FRAME_ENCODING_ERROR or CRYPTO_BUFFER_EXCEEDED.¶

Unlike STREAM frames, which include a Stream ID indicating to which stream the data belongs, the CRYPTO frame carries data for a single stream per encryption level. The stream does not have an explicit end, so CRYPTO frames do not have a FIN bit.¶

19.7. NEW_TOKEN Frames

A server sends a NEW_TOKEN frame (type=0x07) to provide the client with a token to send in the header of an Initial packet for a future connection.¶

NEW_TOKEN frames are formatted as shown in Figure 31.¶

NEW_TOKEN Frame {
  Type (i) = 0x07,
  Token Length (i),
  Token (..),
}

NEW_TOKEN frames contain the following fields:¶

Token Length:

A variable-length integer specifying the length of the token in bytes.¶

Token:

An opaque blob that the client can use with a future Initial packet. The token MUST NOT be empty. A client MUST treat receipt of a NEW_TOKEN frame with an empty Token field as a connection error of type FRAME_ENCODING_ERROR.¶

A client might receive multiple NEW_TOKEN frames that contain the same token value if packets containing the frame are incorrectly determined to be lost. Clients are responsible for discarding duplicate values, which might be used to link connection attempts; see Section 8.1.3.¶

Clients MUST NOT send NEW_TOKEN frames. A server MUST treat receipt of a NEW_TOKEN frame as a connection error of type PROTOCOL_VIOLATION.¶

19.8. STREAM Frames

STREAM frames implicitly create a stream and carry stream data. The Type field in the STREAM frame takes the form 0b00001XXX (or the set of values from 0x08 to 0x0f). The three low-order bits of the frame type determine the fields that are present in the frame:¶

• The OFF bit (0x04) in the frame type is set to indicate that there is an Offset field present. When set to 1, the Offset field is present. When set to 0, the Offset field is absent and the Stream Data starts at an offset of 0 (that is, the frame contains the first bytes of the stream, or the end of a stream that includes no data).¶
• The LEN bit (0x02) in the frame type is set to indicate that there is a Length field present. If this bit is set to 0, the Length field is absent and the Stream Data field extends to the end of the packet. If this bit is set to 1, the Length field is present.¶
• The FIN bit (0x01) indicates that the frame marks the end of the stream. The final size of the stream is the sum of the offset and the length of this frame.¶

An endpoint MUST terminate the connection with error STREAM_STATE_ERROR if it receives a STREAM frame for a locally initiated stream that has not yet been created, or for a send-only stream.¶

STREAM frames are formatted as shown in Figure 32.¶

STREAM Frame {
  Type (i) = 0x08..0x0f,
  Stream ID (i),
  [Offset (i)],
  [Length (i)],
  Stream Data (..),
}

STREAM frames contain the following fields:¶

Stream ID:

A variable-length integer indicating the stream ID of the stream; see Section 2.1.¶

Offset:

A variable-length integer specifying the byte offset in the stream for the data in this STREAM frame. This field is present when the OFF bit is set to 1. When the Offset field is absent, the offset is 0.¶

Length:

A variable-length integer specifying the length of the Stream Data field in this STREAM frame. This field is present when the LEN bit is set to 1. When the LEN bit is set to 0, the Stream Data field consumes all the remaining bytes in the packet.¶

Stream Data:

The bytes from the designated stream to be delivered.¶

When a Stream Data field has a length of 0, the offset in the STREAM frame is the offset of the next byte that would be sent.¶

The first byte in the stream has an offset of 0. The largest offset delivered on a stream -- the sum of the offset and data length -- cannot exceed 2^62-1, as it is not possible to provide flow control credit for that data. Receipt of a frame that exceeds this limit MUST be treated as a connection error of type FRAME_ENCODING_ERROR or FLOW_CONTROL_ERROR.¶

19.9. MAX_DATA Frames

A MAX_DATA frame (type=0x10) is used in flow control to inform the peer of the maximum amount of data that can be sent on the connection as a whole.¶

MAX_DATA frames are formatted as shown in Figure 33.¶

MAX_DATA Frame {
  Type (i) = 0x10,
  Maximum Data (i),
}

MAX_DATA frames contain the following field:¶

Maximum Data:

A variable-length integer indicating the maximum amount of data that can be sent on the entire connection, in units of bytes.¶

All data sent in STREAM frames counts toward this limit. The sum of the final sizes on all streams -- including streams in terminal states -- MUST NOT exceed the value advertised by a receiver. An endpoint MUST terminate a connection with error FLOW_CONTROL_ERROR if it receives more data than the maximum data value that it has sent. This includes violations of remembered limits in Early Data; see Section 7.4.1.¶

19.10. MAX_STREAM_DATA Frames

A MAX_STREAM_DATA frame (type=0x11) is used in flow control to inform a peer of the maximum amount of data that can be sent on a stream.¶

A MAX_STREAM_DATA frame can be sent for streams in the "Recv" state; see Section 3.1. Receiving a MAX_STREAM_DATA frame for a locally initiated stream that has not yet been created MUST be treated as a connection error of type STREAM_STATE_ERROR. An endpoint that receives a MAX_STREAM_DATA frame for a receive-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

MAX_STREAM_DATA frames are formatted as shown in Figure 34.¶

MAX_STREAM_DATA Frame {
  Type (i) = 0x11,
  Stream ID (i),
  Maximum Stream Data (i),
}

MAX_STREAM_DATA frames contain the following fields:¶

Stream ID:

The stream ID of the affected stream, encoded as a variable-length integer.¶

Maximum Stream Data:

A variable-length integer indicating the maximum amount of data that can be sent on the identified stream, in units of bytes.¶

When counting data toward this limit, an endpoint accounts for the largest received offset of data that is sent or received on the stream. Loss or reordering can mean that the largest received offset on a stream can be greater than the total size of data received on that stream. Receiving STREAM frames might not increase the largest received offset.¶

The data sent on a stream MUST NOT exceed the largest maximum stream data value advertised by the receiver. An endpoint MUST terminate a connection with error FLOW_CONTROL_ERROR if it receives more data than the largest maximum stream data that it has sent for the affected stream. This includes violations of remembered limits in Early Data; see Section 7.4.1.¶

19.11. MAX_STREAMS Frames

A MAX_STREAMS frame (type=0x12 or 0x13) informs the peer of the cumulative number of streams of a given type it is permitted to open. A MAX_STREAMS frame with a type of 0x12 applies to bidirectional streams, and a MAX_STREAMS frame with a type of 0x13 applies to unidirectional streams.¶

MAX_STREAMS frames are formatted as shown in Figure 35.¶

MAX_STREAMS Frame {
  Type (i) = 0x12..0x13,
  Maximum Streams (i),
}

MAX_STREAMS frames contain the following field:¶

Maximum Streams:

A count of the cumulative number of streams of the corresponding type that can be opened over the lifetime of the connection. This value cannot exceed 2^60, as it is not possible to encode stream IDs larger than 2^62-1. Receipt of a frame that permits opening of a stream larger than this limit MUST be treated as a connection error of type FRAME_ENCODING_ERROR.¶

Loss or reordering can cause a MAX_STREAMS frame to be received that state a lower stream limit than an endpoint has previously received. MAX_STREAMS frames that do not increase the stream limit MUST be ignored.¶

An endpoint MUST NOT open more streams than permitted by the current stream limit set by its peer. For instance, a server that receives a unidirectional stream limit of 3 is permitted to open streams 3, 7, and 11, but not stream 15. An endpoint MUST terminate a connection with error STREAM_LIMIT_ERROR if a peer opens more streams than was permitted. This includes violations of remembered limits in Early Data; see Section 7.4.1.¶

Note that these frames (and the corresponding transport parameters) do not describe the number of streams that can be opened concurrently. The limit includes streams that have been closed as well as those that are open.¶

19.12. DATA_BLOCKED Frames

A sender SHOULD send a DATA_BLOCKED frame (type=0x14) when it wishes to send data but is unable to do so due to connection-level flow control; see Section 4. DATA_BLOCKED frames can be used as input to tuning of flow control algorithms; see Section 4.2.¶

DATA_BLOCKED frames are formatted as shown in Figure 36.¶

DATA_BLOCKED Frame {
  Type (i) = 0x14,
  Maximum Data (i),
}

DATA_BLOCKED frames contain the following field:¶

Maximum Data:

A variable-length integer indicating the connection-level limit at which blocking occurred.¶

19.13. STREAM_DATA_BLOCKED Frames

A sender SHOULD send a STREAM_DATA_BLOCKED frame (type=0x15) when it wishes to send data but is unable to do so due to stream-level flow control. This frame is analogous to DATA_BLOCKED (Section 19.12).¶

An endpoint that receives a STREAM_DATA_BLOCKED frame for a send-only stream MUST terminate the connection with error STREAM_STATE_ERROR.¶

STREAM_DATA_BLOCKED frames are formatted as shown in Figure 37.¶

STREAM_DATA_BLOCKED Frame {
  Type (i) = 0x15,
  Stream ID (i),
  Maximum Stream Data (i),
}

STREAM_DATA_BLOCKED frames contain the following fields:¶

Stream ID:

A variable-length integer indicating the stream that is blocked due to flow control.¶

Maximum Stream Data:

A variable-length integer indicating the offset of the stream at which the blocking occurred.¶

19.14. STREAMS_BLOCKED Frames

A sender SHOULD send a STREAMS_BLOCKED frame (type=0x16 or 0x17) when it wishes to open a stream but is unable to do so due to the maximum stream limit set by its peer; see Section 19.11. A STREAMS_BLOCKED frame of type 0x16 is used to indicate reaching the bidirectional stream limit, and a STREAMS_BLOCKED frame of type 0x17 is used to indicate reaching the unidirectional stream limit.¶

A STREAMS_BLOCKED frame does not open the stream, but informs the peer that a new stream was needed and the stream limit prevented the creation of the stream.¶

STREAMS_BLOCKED frames are formatted as shown in Figure 38.¶

STREAMS_BLOCKED Frame {
  Type (i) = 0x16..0x17,
  Maximum Streams (i),
}

STREAMS_BLOCKED frames contain the following field:¶

Maximum Streams:

A variable-length integer indicating the maximum number of streams allowed at the time the frame was sent. This value cannot exceed 2^60, as it is not possible to encode stream IDs larger than 2^62-1. Receipt of a frame that encodes a larger stream ID MUST be treated as a connection error of type STREAM_LIMIT_ERROR or FRAME_ENCODING_ERROR.¶

19.15. NEW_CONNECTION_ID Frames

An endpoint sends a NEW_CONNECTION_ID frame (type=0x18) to provide its peer with alternative connection IDs that can be used to break linkability when migrating connections; see Section 9.5.¶

NEW_CONNECTION_ID frames are formatted as shown in Figure 39.¶

NEW_CONNECTION_ID Frame {
  Type (i) = 0x18,
  Sequence Number (i),
  Retire Prior To (i),
  Length (8),
  Connection ID (8..160),
  Stateless Reset Token (128),
}

NEW_CONNECTION_ID frames contain the following fields:¶

Sequence Number:

The sequence number assigned to the connection ID by the sender, encoded as a variable-length integer; see Section 5.1.1.¶

Retire Prior To:

A variable-length integer indicating which connection IDs should be retired; see Section 5.1.2.¶

Length:

An 8-bit unsigned integer containing the length of the connection ID. Values less than 1 and greater than 20 are invalid and MUST be treated as a connection error of type FRAME_ENCODING_ERROR.¶

Connection ID:

A connection ID of the specified length.¶

Stateless Reset Token:

A 128-bit value that will be used for a stateless reset when the associated connection ID is used; see Section 10.3.¶

An endpoint MUST NOT send this frame if it currently requires that its peer send packets with a zero-length Destination Connection ID. Changing the length of a connection ID to or from zero length makes it difficult to identify when the value of the connection ID changed. An endpoint that is sending packets with a zero-length Destination Connection ID MUST treat receipt of a NEW_CONNECTION_ID frame as a connection error of type PROTOCOL_VIOLATION.¶

Transmission errors, timeouts, and retransmissions might cause the same NEW_CONNECTION_ID frame to be received multiple times. Receipt of the same frame multiple times MUST NOT be treated as a connection error. A receiver can use the sequence number supplied in the NEW_CONNECTION_ID frame to handle receiving the same NEW_CONNECTION_ID frame multiple times.¶

If an endpoint receives a NEW_CONNECTION_ID frame that repeats a previously issued connection ID with a different Stateless Reset Token or a different sequence number, or if a sequence number is used for different connection IDs, the endpoint MAY treat that receipt as a connection error of type PROTOCOL_VIOLATION.¶

The Retire Prior To field applies to connection IDs established during connection setup and the preferred_address transport parameter; see Section 5.1.2. The Retire Prior To field MUST be less than or equal to the Sequence Number field. Receiving a value greater than the value in the Sequence Number field MUST be treated as a connection error of type FRAME_ENCODING_ERROR.¶

Once a sender indicates a Retire Prior To value, smaller values sent in subsequent NEW_CONNECTION_ID frames have no effect. A receiver MUST ignore any Retire Prior To fields that do not increase the largest received Retire Prior To value.¶

An endpoint that receives a NEW_CONNECTION_ID frame with a sequence number smaller than the Retire Prior To field of a previously received NEW_CONNECTION_ID frame MUST send a corresponding RETIRE_CONNECTION_ID frame that retires the newly received connection ID, unless it has already done so for that sequence number.¶

19.16. RETIRE_CONNECTION_ID Frames

An endpoint sends a RETIRE_CONNECTION_ID frame (type=0x19) to indicate that it will no longer use a connection ID that was issued by its peer. This includes the connection ID provided during the handshake. Sending a RETIRE_CONNECTION_ID frame also serves as a request to the peer to send additional connection IDs for future use; see Section 5.1. New connection IDs can be delivered to a peer using the NEW_CONNECTION_ID frame (Section 19.15).¶

Retiring a connection ID invalidates the stateless reset token associated with that connection ID.¶

RETIRE_CONNECTION_ID frames are formatted as shown in Figure 40.¶

RETIRE_CONNECTION_ID Frame {
  Type (i) = 0x19,
  Sequence Number (i),
}

RETIRE_CONNECTION_ID frames contain the following field:¶

Sequence Number:

The sequence number of the connection ID being retired; see Section 5.1.2.¶

Receipt of a RETIRE_CONNECTION_ID frame containing a sequence number greater than any previously sent to the peer MUST be treated as a connection error of type PROTOCOL_VIOLATION.¶

The sequence number specified in a RETIRE_CONNECTION_ID frame MUST NOT refer to the Destination Connection ID field of the packet in which the frame is contained. The peer MAY treat this as a connection error of type PROTOCOL_VIOLATION.¶

An endpoint cannot send this frame if it was provided with a zero-length connection ID by its peer. An endpoint that provides a zero-length connection ID MUST treat receipt of a RETIRE_CONNECTION_ID frame as a connection error of type PROTOCOL_VIOLATION.¶

19.17. PATH_CHALLENGE Frames

Endpoints can use PATH_CHALLENGE frames (type=0x1a) to check reachability to the peer and for path validation during connection migration.¶

PATH_CHALLENGE frames are formatted as shown in Figure 41.¶

PATH_CHALLENGE Frame {
  Type (i) = 0x1a,
  Data (64),
}

PATH_CHALLENGE frames contain the following field:¶

Data:

This 8-byte field contains arbitrary data.¶

Including 64 bits of entropy in a PATH_CHALLENGE frame ensures that it is easier to receive the packet than it is to guess the value correctly.¶

The recipient of this frame MUST generate a PATH_RESPONSE frame (Section 19.18) containing the same Data value.¶

19.18. PATH_RESPONSE Frames

A PATH_RESPONSE frame (type=0x1b) is sent in response to a PATH_CHALLENGE frame.¶

PATH_RESPONSE frames are formatted as shown in Figure 42. The format of a PATH_RESPONSE frame is identical to that of the PATH_CHALLENGE frame (Section 19.17).¶

PATH_RESPONSE Frame {
  Type (i) = 0x1b,
  Data (64),
}

If the content of a PATH_RESPONSE frame does not match the content of a PATH_CHALLENGE frame previously sent by the endpoint, the endpoint MAY generate a connection error of type PROTOCOL_VIOLATION.¶

19.19. CONNECTION_CLOSE Frames

An endpoint sends a CONNECTION_CLOSE frame (type=0x1c or 0x1d) to notify its peer that the connection is being closed. The CONNECTION_CLOSE frame with a type of 0x1c is used to signal errors at only the QUIC layer, or the absence of errors (with the NO_ERROR code). The CONNECTION_CLOSE frame with a type of 0x1d is used to signal an error with the application that uses QUIC.¶

If there are open streams that have not been explicitly closed, they are implicitly closed when the connection is closed.¶

CONNECTION_CLOSE frames are formatted as shown in Figure 43.¶

CONNECTION_CLOSE Frame {
  Type (i) = 0x1c..0x1d,
  Error Code (i),
  [Frame Type (i)],
  Reason Phrase Length (i),
  Reason Phrase (..),
}

CONNECTION_CLOSE frames contain the following fields:¶

Error Code:

A variable-length integer that indicates the reason for closing this connection. A CONNECTION_CLOSE frame of type 0x1c uses codes from the space defined in Section 20.1. A CONNECTION_CLOSE frame of type 0x1d uses codes from the application protocol error code space; see Section 20.2.¶

Frame Type:

A variable-length integer encoding the type of frame that triggered the error. A value of 0 (equivalent to the mention of the PADDING frame) is used when the frame type is unknown. The application-specific variant of CONNECTION_CLOSE (type 0x1d) does not include this field.¶

Reason Phrase Length:

A variable-length integer specifying the length of the reason phrase in bytes. Because a CONNECTION_CLOSE frame cannot be split between packets, any limits on packet size will also limit the space available for a reason phrase.¶

Reason Phrase:

Additional diagnostic information for the closure. This can be zero length if the sender chooses not to give details beyond the Error Code value. This SHOULD be a UTF-8 encoded string [RFC3629], though the frame does not carry information, such as language tags, that would aid comprehension by any entity other than the one that created the text.¶

The application-specific variant of CONNECTION_CLOSE (type 0x1d) can only be sent using 0-RTT or 1-RTT packets; see Section 12.5. When an application wishes to abandon a connection during the handshake, an endpoint can send a CONNECTION_CLOSE frame (type 0x1c) with an error code of APPLICATION_ERROR in an Initial or Handshake packet.¶

19.20. HANDSHAKE_DONE Frames

The server uses a HANDSHAKE_DONE frame (type=0x1e) to signal confirmation of the handshake to the client.¶

HANDSHAKE_DONE frames are formatted as shown in Figure 44, which shows that HANDSHAKE_DONE frames have no content.¶

HANDSHAKE_DONE Frame {
  Type (i) = 0x1e,
}

A HANDSHAKE_DONE frame can only be sent by the server. Servers MUST NOT send a HANDSHAKE_DONE frame before completing the handshake. A server MUST treat receipt of a HANDSHAKE_DONE frame as a connection error of type PROTOCOL_VIOLATION.¶

19.21. Extension Frames

QUIC frames do not use a self-describing encoding. An endpoint therefore needs to understand the syntax of all frames before it can successfully process a packet. This allows for efficient encoding of frames, but it means that an endpoint cannot send a frame of a type that is unknown to its peer.¶

An extension to QUIC that wishes to use a new type of frame MUST first ensure that a peer is able to understand the frame. An endpoint can use a transport parameter to signal its willingness to receive extension frame types. One transport parameter can indicate support for one or more extension frame types.¶

Extensions that modify or replace core protocol functionality (including frame types) will be difficult to combine with other extensions that modify or replace the same functionality unless the behavior of the combination is explicitly defined. Such extensions SHOULD define their interaction with previously defined extensions modifying the same protocol components.¶

Extension frames MUST be congestion controlled and MUST cause an ACK frame to be sent. The exception is extension frames that replace or supplement the ACK frame. Extension frames are not included in flow control unless specified in the extension.¶

An IANA registry is used to manage the assignment of frame types; see Section 22.4.¶

20.1. Transport Error Codes

This section lists the defined QUIC transport error codes that can be used in a CONNECTION_CLOSE frame with a type of 0x1c. These errors apply to the entire connection.¶

NO_ERROR (0x0):

An endpoint uses this with CONNECTION_CLOSE to signal that the connection is being closed abruptly in the absence of any error.¶

INTERNAL_ERROR (0x1):

The endpoint encountered an internal error and cannot continue with the connection.¶

CONNECTION_REFUSED (0x2):

The server refused to accept a new connection.¶

FLOW_CONTROL_ERROR (0x3):

An endpoint received more data than it permitted in its advertised data limits; see Section 4.¶

STREAM_LIMIT_ERROR (0x4):

An endpoint received a frame for a stream identifier that exceeded its advertised stream limit for the corresponding stream type.¶

STREAM_STATE_ERROR (0x5):

An endpoint received a frame for a stream that was not in a state that permitted that frame; see Section 3.¶

FINAL_SIZE_ERROR (0x6):

(1) An endpoint received a STREAM frame containing data that exceeded the previously established final size, (2) an endpoint received a STREAM frame or a RESET_STREAM frame containing a final size that was lower than the size of stream data that was already received, or (3) an endpoint received a STREAM frame or RESET_STREAM frame containing a different final size than the one already established.¶

FRAME_ENCODING_ERROR (0x7):

An endpoint received a frame that was badly formatted -- for instance, a frame of an unknown type or an ACK frame that has more acknowledgment ranges than the remainder of the packet could carry.¶

TRANSPORT_PARAMETER_ERROR (0x8):

An endpoint received transport parameters that were badly formatted, included an invalid value, omitted a mandatory transport parameter, included a forbidden transport parameter, or were otherwise in error.¶

CONNECTION_ID_LIMIT_ERROR (0x9):

The number of connection IDs provided by the peer exceeds the advertised active_connection_id_limit.¶

PROTOCOL_VIOLATION (0xa):

An endpoint detected an error with protocol compliance that was not covered by more specific error codes.¶

INVALID_TOKEN (0xb):

A server received a client Initial that contained an invalid Token field.¶

APPLICATION_ERROR (0xc):

The application or application protocol caused the connection to be closed.¶

CRYPTO_BUFFER_EXCEEDED (0xd):

An endpoint has received more data in CRYPTO frames than it can buffer.¶

KEY_UPDATE_ERROR (0xe):

An endpoint detected errors in performing key updates; see Section 6 of [RFCYYY2].¶

AEAD_LIMIT_REACHED (0xf):

An endpoint has reached the confidentiality or integrity limit for the AEAD algorithm used by the given connection.¶

NO_VIABLE_PATH (0x10):

An endpoint has determined that the network path is incapable of supporting QUIC. An endpoint is unlikely to receive a CONNECTION_CLOSE carrying this code except when the path does not support a large enough MTU.¶

CRYPTO_ERROR (0x1XX):

The cryptographic handshake failed. A range of 256 values is reserved for carrying error codes specific to the cryptographic handshake that is used. Codes for errors occurring when TLS is used for the cryptographic handshake are described in Section 4.8 of [RFCYYY2].¶

See Section 22.5 for details on registering new error codes.¶

In defining these error codes, several principles are applied. Error conditions that might require specific action on the part of a recipient are given unique codes. Errors that represent common conditions are given specific codes. Absent either of these conditions, error codes are used to identify a general function of the stack, like flow control or transport parameter handling. Finally, generic errors are provided for conditions where implementations are unable or unwilling to use more specific codes.¶

20.2. Application Protocol Error Codes

The management of application error codes is left to application protocols. Application protocol error codes are used for the RESET_STREAM frame (Section 19.4), the STOP_SENDING frame (Section 19.5), and the CONNECTION_CLOSE frame with a type of 0x1d (Section 19.19).¶

21.1. Overview of Security Properties

A complete security analysis of QUIC is outside the scope of this document. This section provides an informal description of the desired security properties as an aid to implementers and to help guide protocol analysis.¶

QUIC assumes the threat model described in [SEC-CONS] and provides protections against many of the attacks that arise from that model.¶

For this purpose, attacks are divided into passive and active attacks. Passive attackers have the ability to read packets from the network, while active attackers also have the ability to write packets into the network. However, a passive attack could involve an attacker with the ability to cause a routing change or other modification in the path taken by packets that comprise a connection.¶

Attackers are additionally categorized as either on-path attackers or off-path attackers. An on-path attacker can read, modify, or remove any packet it observes such that the packet no longer reaches its destination, while an off-path attacker observes the packets but cannot prevent the original packet from reaching its intended destination. Both types of attackers can also transmit arbitrary packets. This definition differs from that of Section 3.5 of [SEC-CONS] in that an off-path attacker is able to observe packets.¶

Properties of the handshake, protected packets, and connection migration are considered separately.¶

21.2. Handshake Denial of Service

As an encrypted and authenticated transport, QUIC provides a range of protections against denial of service. Once the cryptographic handshake is complete, QUIC endpoints discard most packets that are not authenticated, greatly limiting the ability of an attacker to interfere with existing connections.¶

Once a connection is established, QUIC endpoints might accept some unauthenticated ICMP packets (see Section 14.2.1), but the use of these packets is extremely limited. The only other type of packet that an endpoint might accept is a stateless reset (Section 10.3), which relies on the token being kept secret until it is used.¶

During the creation of a connection, QUIC only provides protection against attacks from off the network path. All QUIC packets contain proof that the recipient saw a preceding packet from its peer.¶

Addresses cannot change during the handshake, so endpoints can discard packets that are received on a different network path.¶

The Source and Destination Connection ID fields are the primary means of protection against an off-path attack during the handshake; see Section 8.1. These are required to match those set by a peer. Except for Initial and stateless reset packets, an endpoint only accepts packets that include a Destination Connection ID field that matches a value the endpoint previously chose. This is the only protection offered for Version Negotiation packets.¶

The Destination Connection ID field in an Initial packet is selected by a client to be unpredictable, which serves an additional purpose. The packets that carry the cryptographic handshake are protected with a key that is derived from this connection ID and a salt specific to the QUIC version. This allows endpoints to use the same process for authenticating packets that they receive as they use after the cryptographic handshake completes. Packets that cannot be authenticated are discarded. Protecting packets in this fashion provides a strong assurance that the sender of the packet saw the Initial packet and understood it.¶

These protections are not intended to be effective against an attacker that is able to receive QUIC packets prior to the connection being established. Such an attacker can potentially send packets that will be accepted by QUIC endpoints. This version of QUIC attempts to detect this sort of attack, but it expects that endpoints will fail to establish a connection rather than recovering. For the most part, the cryptographic handshake protocol [RFCYYY2] is responsible for detecting tampering during the handshake.¶

Endpoints are permitted to use other methods to detect and attempt to recover from interference with the handshake. Invalid packets can be identified and discarded using other methods, but no specific method is mandated in this document.¶

21.3. Amplification Attack

An attacker might be able to receive an address validation token (Section 8) from a server and then release the IP address it used to acquire that token. At a later time, the attacker can initiate a 0-RTT connection with a server by spoofing this same address, which might now address a different (victim) endpoint. The attacker can thus potentially cause the server to send an initial congestion window's worth of data towards the victim.¶

Servers SHOULD provide mitigations for this attack by limiting the usage and lifetime of address validation tokens; see Section 8.1.3.¶

21.4. Optimistic ACK Attack

An endpoint that acknowledges packets it has not received might cause a congestion controller to permit sending at rates beyond what the network supports. An endpoint MAY skip packet numbers when sending packets to detect this behavior. An endpoint can then immediately close the connection with a connection error of type PROTOCOL_VIOLATION; see Section 10.2.¶

21.5. Request Forgery Attacks

A request forgery attack occurs where an endpoint causes its peer to issue a request towards a victim, with the request controlled by the endpoint. Request forgery attacks aim to provide an attacker with access to capabilities of its peer that might otherwise be unavailable to the attacker. For a networking protocol, a request forgery attack is often used to exploit any implicit authorization conferred on the peer by the victim due to the peer's location in the network.¶

For request forgery to be effective, an attacker needs to be able to influence what packets the peer sends and where these packets are sent. If an attacker can target a vulnerable service with a controlled payload, that service might perform actions that are attributed to the attacker's peer but are decided by the attacker.¶

For example, cross-site request forgery [CSRF] exploits on the Web cause a client to issue requests that include authorization cookies [COOKIE], allowing one site access to information and actions that are intended to be restricted to a different site.¶

As QUIC runs over UDP, the primary attack modality of concern is one where an attacker can select the address to which its peer sends UDP datagrams and can control some of the unprotected content of those packets. As much of the data sent by QUIC endpoints is protected, this includes control over ciphertext. An attack is successful if an attacker can cause a peer to send a UDP datagram to a host that will perform some action based on content in the datagram.¶

This section discusses ways in which QUIC might be used for request forgery attacks.¶

This section also describes limited countermeasures that can be implemented by QUIC endpoints. These mitigations can be employed unilaterally by a QUIC implementation or deployment, without potential targets for request forgery attacks taking action. However, these countermeasures could be insufficient if UDP-based services do not properly authorize requests.¶

Because the migration attack described in Section 21.5.4 is quite powerful and does not have adequate countermeasures, QUIC server implementations should assume that attackers can cause them to generate arbitrary UDP payloads to arbitrary destinations. QUIC servers SHOULD NOT be deployed in networks that do not deploy ingress filtering [BCP38] and also have inadequately secured UDP endpoints.¶

Although it is not generally possible to ensure that clients are not co-located with vulnerable endpoints, this version of QUIC does not allow servers to migrate, thus preventing spoofed migration attacks on clients. Any future extension that allows server migration MUST also define countermeasures for forgery attacks.¶

21.6. Slowloris Attacks

The attacks commonly known as Slowloris [SLOWLORIS] try to keep many connections to the target endpoint open and hold them open as long as possible. These attacks can be executed against a QUIC endpoint by generating the minimum amount of activity necessary to avoid being closed for inactivity. This might involve sending small amounts of data, gradually opening flow control windows in order to control the sender rate, or manufacturing ACK frames that simulate a high loss rate.¶

QUIC deployments SHOULD provide mitigations for the Slowloris attacks, such as increasing the maximum number of clients the server will allow, limiting the number of connections a single IP address is allowed to make, imposing restrictions on the minimum transfer speed a connection is allowed to have, and restricting the length of time an endpoint is allowed to stay connected.¶

21.7. Stream Fragmentation and Reassembly Attacks

An adversarial sender might intentionally not send portions of the stream data, causing the receiver to commit resources for the unsent data. This could cause a disproportionate receive buffer memory commitment and/or the creation of a large and inefficient data structure at the receiver.¶

An adversarial receiver might intentionally not acknowledge packets containing stream data in an attempt to force the sender to store the unacknowledged stream data for retransmission.¶

The attack on receivers is mitigated if flow control windows correspond to available memory. However, some receivers will overcommit memory and advertise flow control offsets in the aggregate that exceed actual available memory. The overcommitment strategy can lead to better performance when endpoints are well behaved, but renders endpoints vulnerable to the stream fragmentation attack.¶

QUIC deployments SHOULD provide mitigations for stream fragmentation attacks. Mitigations could consist of avoiding overcommitting memory, limiting the size of tracking data structures, delaying reassembly of STREAM frames, implementing heuristics based on the age and duration of reassembly holes, or some combination of these.¶

21.8. Stream Commitment Attack

An adversarial endpoint can open a large number of streams, exhausting state on an endpoint. The adversarial endpoint could repeat the process on a large number of connections, in a manner similar to SYN flooding attacks in TCP.¶

Normally, clients will open streams sequentially, as explained in Section 2.1. However, when several streams are initiated at short intervals, loss or reordering can cause STREAM frames that open streams to be received out of sequence. On receiving a higher-numbered stream ID, a receiver is required to open all intervening streams of the same type; see Section 3.2. Thus, on a new connection, opening stream 4000000 opens 1 million and 1 client-initiated bidirectional streams.¶

The number of active streams is limited by the initial_max_streams_bidi and initial_max_streams_uni transport parameters as updated by any received MAX_STREAMS frames, as explained in Section 4.6. If chosen judiciously, these limits mitigate the effect of the stream commitment attack. However, setting the limit too low could affect performance when applications expect to open a large number of streams.¶

21.9. Peer Denial of Service

QUIC and TLS both contain frames or messages that have legitimate uses in some contexts, but these frames or messages can be abused to cause a peer to expend processing resources without having any observable impact on the state of the connection.¶

Messages can also be used to change and revert state in small or inconsequential ways, such as by sending small increments to flow control limits.¶

If processing costs are disproportionately large in comparison to bandwidth consumption or effect on state, then this could allow a malicious peer to exhaust processing capacity.¶

While there are legitimate uses for all messages, implementations SHOULD track cost of processing relative to progress and treat excessive quantities of any non-productive packets as indicative of an attack. Endpoints MAY respond to this condition with a connection error or by dropping packets.¶

21.10. Explicit Congestion Notification Attacks

An on-path attacker could manipulate the value of ECN fields in the IP header to influence the sender's rate. [RFC3168] discusses manipulations and their effects in more detail.¶

A limited on-path attacker can duplicate and send packets with modified ECN fields to affect the sender's rate. If duplicate packets are discarded by a receiver, an attacker will need to race the duplicate packet against the original to be successful in this attack. Therefore, QUIC endpoints ignore the ECN field in an IP packet unless at least one QUIC packet in that IP packet is successfully processed; see Section 13.4.¶

21.11. Stateless Reset Oracle

Stateless resets create a possible denial-of-service attack analogous to a TCP reset injection. This attack is possible if an attacker is able to cause a stateless reset token to be generated for a connection with a selected connection ID. An attacker that can cause this token to be generated can reset an active connection with the same connection ID.¶

If a packet can be routed to different instances that share a static key -- for example, by changing an IP address or port -- then an attacker can cause the server to send a stateless reset. To defend against this style of denial of service, endpoints that share a static key for stateless resets (see Section 10.3.2) MUST be arranged so that packets with a given connection ID always arrive at an instance that has connection state, unless that connection is no longer active.¶

More generally, servers MUST NOT generate a stateless reset if a connection with the corresponding connection ID could be active on any endpoint using the same static key.¶

In the case of a cluster that uses dynamic load balancing, it is possible that a change in load-balancer configuration could occur while an active instance retains connection state. Even if an instance retains connection state, the change in routing and resulting stateless reset will result in the connection being terminated. If there is no chance of the packet being routed to the correct instance, it is better to send a stateless reset than wait for the connection to time out. However, this is acceptable only if the routing cannot be influenced by an attacker.¶

21.12. Version Downgrade

This document defines QUIC Version Negotiation packets (Section 6), which can be used to negotiate the QUIC version used between two endpoints. However, this document does not specify how this negotiation will be performed between this version and subsequent future versions. In particular, Version Negotiation packets do not contain any mechanism to prevent version downgrade attacks. Future versions of QUIC that use Version Negotiation packets MUST define a mechanism that is robust against version downgrade attacks.¶

21.13. Targeted Attacks by Routing

Deployments should limit the ability of an attacker to target a new connection to a particular server instance. Ideally, routing decisions are made independently of client-selected values, including addresses. Once an instance is selected, a connection ID can be selected so that later packets are routed to the same instance.¶

21.14. Traffic Analysis

The length of QUIC packets can reveal information about the length of the content of those packets. The PADDING frame is provided so that endpoints have some ability to obscure the length of packet content; see Section 19.1.¶

Note, however, that defeating traffic analysis is challenging and the subject of active research. Length is not the only way that information might leak. Endpoints might also reveal sensitive information through other side channels, such as the timing of packets.¶

22.1. Registration Policies for QUIC Registries

All QUIC registries allow for both provisional and permanent registration of codepoints. This section documents policies that are common to these registries.¶

22.2. QUIC Versions Registry

IANA has added a registry for "QUIC Versions" under a "QUIC" heading.¶

The "QUIC Versions" registry governs a 32-bit space; see Section 15. This registry follows the registration policy from Section 22.1. Permanent registrations in this registry are assigned using the Specification Required policy (Section 4.6 of [RFC8126]).¶

The codepoint of 0x00000001 for the protocol is assigned with permanent status to the protocol defined in this document. The codepoint of 0x00000000 is permanently reserved; the note for this codepoint indicates that this version is reserved for Version Negotiation.¶

All codepoints that follow the pattern 0x?a?a?a?a are reserved, MUST NOT be assigned by IANA, and MUST NOT appear in the listing of assigned values.¶

22.3. QUIC Transport Parameters Registry

IANA has added a registry for "QUIC Transport Parameters" under a "QUIC" heading.¶

The "QUIC Transport Parameters" registry governs a 62-bit space. This registry follows the registration policy from Section 22.1. Permanent registrations in this registry are assigned using the Specification Required policy (Section 4.6 of [RFC8126]).¶

In addition to the fields listed in Section 22.1.1, permanent registrations in this registry MUST include the following field:¶

Parameter Name:

A short mnemonic for the parameter.¶

The initial contents of this registry are shown in Table 6.¶

Each value of the format 31 * N + 27 for integer values of N (that is, 27, 58, 89, ...) are reserved; these values MUST NOT be assigned by IANA and MUST NOT appear in the listing of assigned values.¶

22.4. QUIC Frame Types Registry

IANA has added a registry for "QUIC Frame Types" under a "QUIC" heading.¶

The "QUIC Frame Types" registry governs a 62-bit space. This registry follows the registration policy from Section 22.1. Permanent registrations in this registry are assigned using the Specification Required policy (Section 4.6 of [RFC8126]), except for values between 0x00 and 0x3f (in hexadecimal), inclusive, which are assigned using Standards Action or IESG Approval as defined in Sections 4.9 and 4.10 of [RFC8126].¶

In addition to the fields listed in Section 22.1.1, permanent registrations in this registry MUST include the following field:¶

Frame Name:

A short mnemonic for the frame type.¶

In addition to the advice in Section 22.1, specifications for new permanent registrations SHOULD describe the means by which an endpoint might determine that it can send the identified type of frame. An accompanying transport parameter registration is expected for most registrations; see Section 22.3. Specifications for permanent registrations also need to describe the format and assigned semantics of any fields in the frame.¶

The initial contents of this registry are tabulated in Table 3. Note that the registry does not include the "Pkts" and "Spec" columns from Table 3.¶

22.5. QUIC Transport Error Codes Registry

IANA has added a registry for "QUIC Transport Error Codes" under a "QUIC" heading.¶

The "QUIC Transport Error Codes" registry governs a 62-bit space. This space is split into three regions that are governed by different policies. Permanent registrations in this registry are assigned using the Specification Required policy (Section 4.6 of [RFC8126]), except for values between 0x00 and 0x3f (in hexadecimal), inclusive, which are assigned using Standards Action or IESG Approval as defined in Sections 4.9 and 4.10 of [RFC8126].¶

In addition to the fields listed in Section 22.1.1, permanent registrations in this registry MUST include the following fields:¶

Code:

A short mnemonic for the parameter.¶

Description:

A brief description of the error code semantics, which MAY be a summary if a specification reference is provided.¶

The initial contents of this registry are shown in Table 7.¶

A.1. Sample Variable-Length Integer Decoding

The pseudocode in Figure 45 shows how a variable-length integer can be read from a stream of bytes. The function ReadVarint takes a single argument -- a sequence of bytes, which can be read in network byte order.¶

ReadVarint(data):
  // The length of variable-length integers is encoded in the
  // first two bits of the first byte.
  v = data.next_byte()
  prefix = v >> 6
  length = 1 << prefix

  // Once the length is known, remove these bits and read any
  // remaining bytes.
  v = v & 0x3f
  repeat length-1 times:
    v = (v << 8) + data.next_byte()
  return v

For example, the eight-byte sequence 0xc2197c5eff14e88c decodes to the decimal value 151,288,809,941,952,652; the four-byte sequence 0x9d7f3e7d decodes to 494,878,333; the two-byte sequence 0x7bbd decodes to 15,293; and the single byte 0x25 decodes to 37 (as does the two-byte sequence 0x4025).¶

A.2. Sample Packet Number Encoding Algorithm

The pseudocode in Figure 46 shows how an implementation can select an appropriate size for packet number encodings.¶

The EncodePacketNumber function takes two arguments:¶

• full_pn is the full packet number of the packet being sent.¶
• largest_acked is the largest packet number that has been acknowledged by the peer in the current packet number space, if any.¶

EncodePacketNumber(full_pn, largest_acked):

  // The number of bits must be at least one more
  // than the base-2 logarithm of the number of contiguous
  // unacknowledged packet numbers, including the new packet.
  if largest_acked is None:
    num_unacked = full_pn + 1
  else:
    num_unacked = full_pn - largest_acked

  min_bits = log(num_unacked, 2) + 1
  num_bytes = ceil(min_bits / 8)

  // Encode the integer value and truncate to
  // the num_bytes least-significant bytes.
  return encode(full_pn, num_bytes)

For example, if an endpoint has received an acknowledgment for packet 0xabe8bc and is sending a packet with a number of 0xac5c02, there are 29,519 (0x734f) outstanding packets. In order to represent at least twice this range (59,038 packets, or 0xe69e), 16 bits are required.¶

In the same state, sending a packet with a number of 0xace8fe uses the 24-bit encoding, because at least 18 bits are required to represent twice the range (131,182 packets, or 0x2006e).¶

A.3. Sample Packet Number Decoding Algorithm

The pseudocode in Figure 47 includes an example algorithm for decoding packet numbers after header protection has been removed.¶

The DecodePacketNumber function takes three arguments:¶

• largest_pn is the largest packet number that has been successfully processed in the current packet number space.¶
• truncated_pn is the value of the Packet Number field.¶
• pn_nbits is the number of bits in the Packet Number field (8, 16, 24, or 32).¶

DecodePacketNumber(largest_pn, truncated_pn, pn_nbits):
   expected_pn  = largest_pn + 1
   pn_win       = 1 << pn_nbits
   pn_hwin      = pn_win / 2
   pn_mask      = pn_win - 1
   // The incoming packet number should be greater than
   // expected_pn - pn_hwin and less than or equal to
   // expected_pn + pn_hwin
   //
   // This means we cannot just strip the trailing bits from
   // expected_pn and add the truncated_pn because that might
   // yield a value outside the window.
   //
   // The following code calculates a candidate value and
   // makes sure it's within the packet number window.
   // Note the extra checks to prevent overflow and underflow.
   candidate_pn = (expected_pn & ~pn_mask) | truncated_pn
   if candidate_pn <= expected_pn - pn_hwin and
      candidate_pn < (1 << 62) - pn_win:
      return candidate_pn + pn_win
   if candidate_pn > expected_pn + pn_hwin and
      candidate_pn >= pn_win:
      return candidate_pn - pn_win
   return candidate_pn

For example, if the highest successfully authenticated packet had a packet number of 0xa82f30ea, then a packet containing a 16-bit value of 0x9b32 will be decoded as 0xa82f9b32.¶

A.4. Sample ECN Validation Algorithm

Each time an endpoint commences sending on a new network path, it determines whether the path supports ECN; see Section 13.4. If the path supports ECN, the goal is to use ECN. Endpoints might also periodically reassess a path that was determined to not support ECN.¶

This section describes one method for testing new paths. This algorithm is intended to show how a path might be tested for ECN support. Endpoints can implement different methods.¶

The path is assigned an ECN state that is one of "testing", "unknown", "failed", or "capable". On paths with a "testing" or "capable" state, the endpoint sends packets with an ECT marking -- ECT(0) by default; otherwise, the endpoint sends unmarked packets.¶

To start testing a path, the ECN state is set to "testing", and existing ECN counts are remembered as a baseline.¶

The testing period runs for a number of packets or a limited time, as determined by the endpoint. The goal is not to limit the duration of the testing period but to ensure that enough marked packets are sent for received ECN counts to provide a clear indication of how the path treats marked packets. Section 13.4.2 suggests limiting this to ten packets or three times the PTO.¶

After the testing period ends, the ECN state for the path becomes "unknown". From the "unknown" state, successful validation of the ECN counts an ACK frame (see Section 13.4.2.1) causes the ECN state for the path to become "capable", unless no marked packet has been acknowledged.¶

If validation of ECN counts fails at any time, the ECN state for the affected path becomes "failed". An endpoint can also mark the ECN state for a path as "failed" if marked packets are all declared lost or if they are all CE marked.¶

Following this algorithm ensures that ECN is rarely disabled for paths that properly support ECN. Any path that incorrectly modifies markings will cause ECN to be disabled. For those rare cases where marked packets are discarded by the path, the short duration of the testing period limits the number of losses incurred.¶