| RFC 8841 | SDP Offer/Answer for SCTP over DTLS | May 2020 |
| Holmberg, et al. | Standards Track | [Page] |
The Stream Control Transmission Protocol (SCTP) is a transport protocol used to establish associations between two endpoints. RFC 8261 specifies how SCTP can be used on top of the Datagram Transport Layer Security (DTLS) protocol, which is referred to as SCTP-over-DTLS.¶
This specification defines the following new Session Description Protocol (SDP) protocol identifiers (proto values): "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP". This specification also specifies how to use the new proto values with the SDP offer/answer mechanism for negotiating SCTP-over-DTLS associations.¶
This is an Internet Standards Track document.¶
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841.¶
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8841.¶
Copyright (c) 2020 IETF Trust and the persons identified as the document authors. All rights reserved.¶
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶
The Session Description Protocol (SDP) [RFC4566] provides a general-purpose format for describing multimedia sessions in announcements or invitations. "TCP-Based Media Transport in the Session Description Protocol (SDP)" [RFC4145] specifies a general mechanism for describing and establishing TCP [RFC0793] streams. "Connection-Oriented Media Transport over the Transport Layer Security (TLS) Protocol in the Session Description Protocol (SDP)" [RFC8122] extends [RFC4145] to describe TCP-based media streams that are protected using TLS.¶
The Stream Control Transmission Protocol (SCTP) [RFC4960] is a reliable transport protocol used to transport data between two endpoints using SCTP associations.¶
[RFC8261] specifies how SCTP can be used on top of the Datagram Transport Layer Security (DTLS) protocol, an arrangement referred to as SCTP-over-DTLS.¶
This specification defines the following new SDP [RFC4566] protocol identifiers (proto values): "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP". This document also specifies how to use the new proto values with the SDP offer/answer mechanism [RFC3264] for negotiating SCTP-over-DTLS associations.¶
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.¶
This section defines the following new SDP media description ("m=" line) protocol identifiers (proto values) for describing an SCTP association: "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP". The section also describes how an "m=" line associated with the proto values is created.¶
The following is the format for an "m=" line, as specified in [RFC4566]:¶
m=<media> <port> <proto> <fmt> ...¶
The "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" proto values are similar to both the "UDP" and "TCP" proto values in that they only describe the transport-layer protocol and not the upper-layer protocol.¶
The new proto values are defined as below:¶
[RFC4566] states that specifications defining new proto values must define the rules by which their media format (fmt) namespace is managed.¶
An "m=" line with a proto value of "UDP/DTLS/SCTP" or "TCP/DTLS/SCTP" always describes a single SCTP association.¶
In addition, such an "m=" line MUST further indicate the application-layer protocol using an "fmt" identifier. There MUST be exactly one fmt value per "m=" line associated with the proto values defined in this specification. The "fmt" namespace associated with those proto values describes the generic application usage of the entire SCTP association, including the associated SCTP streams.¶
When the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" proto values are used, the "m=" line fmt value, which identifies the application-layer protocol, MUST be registered by IANA. Section 15.3 defines the IANA registry for the media-format namespace.¶
This section defines the values that can be used within an SDP media description ("m=" line) associated with an SCTP-over-DTLS association.¶
This specification creates an IANA registry for "association-usage" values.¶
| "m=" line parameter | parameter value(s) |
|---|---|
| <media> | "application" |
| <proto> | "UDP/DTLS/SCTP" or "TCP/DTLS/SCTP" |
| <port> | UDP port number (for "UDP/DTLS/SCTP") TCP port number (for "TCP/DTLS/SCTP") |
| <fmt> | A string denoting the association-usage, limited to the syntax of a "token" as defined in RFC 4566 |
m=application 12345 UDP/DTLS/SCTP webrtc-datachannel a=sctp-port:5000 a=max-message-size:100000¶
This section defines a new SDP media-level attribute, "sctp-port". The attribute can be associated with an SDP media description ("m=" line) with a "UDP/DTLS/SCTP" or a "TCP/DTLS/SCTP" proto value. In that case, the "m=" line port value indicates the port of the underlying transport-layer protocol (UDP or TCP), and the "sctp-port" value indicates the SCTP port.¶
No default value is defined for the SDP "sctp-port" attribute. Therefore, if the attribute is not present, the associated "m=" line MUST be considered invalid.¶
The definition of the SDP "sctp-port" attribute is:¶
Christer Holmberg¶
sctp-port-value = 1*5(DIGIT) ; DIGIT defined in RFC 4566¶
The SCTP port range is between 0 and 65535 (both included). Leading zeroes MUST NOT be used.¶
Example:¶
a=sctp-port:5000¶
The mux category [RFC8859] for the SDP "sctp-port" attribute is CAUTION.¶
As the usage of multiple SCTP associations on top of a single DTLS association is outside the scope of this specification, no mux rules are specified for the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" proto values. Future extensions that define how to negotiate multiplexing of multiple SCTP associations of top of a single DTLS association need to also define the mux rules for the attribute.¶
This section defines a new SDP media-level attribute, "max-message-size". The attribute can be associated with an "m=" line to indicate the maximum SCTP user message size (indicated in bytes) that an SCTP endpoint is willing to receive on the SCTP association associated with the "m=" line. Different attribute values can be used in each direction.¶
An SCTP endpoint MUST NOT send a SCTP user message with a message size that is larger than the maximum size indicated by the peer, as it cannot be assumed that the peer would accept such a message.¶
If the SDP "max-message-size" attribute contains a maximum message size value of zero, it indicates that the SCTP endpoint will handle messages of any size, subject to memory capacity, etc.¶
If the SDP "max-message-size" attribute is not present, the default value is 64K.¶
The definition of the SDP "max-message-size" attribute is:¶
Christer Holmberg¶
max-message-size-value = 1*DIGIT ; DIGIT defined in RFC 4566¶
Leading zeroes MUST NOT be used.¶
Example:¶
a=max-message-size:100000¶
The mux category for the SDP "max-message-size" attribute is CAUTION.¶
As the usage of multiple SCTP associations on top of a single DTLS association is outside the scope of this specification, no mux rules are specified for the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" proto values.¶
The UDP/DTLS/SCTP transport is realized as described below:¶
The TCP/DTLS/SCTP transport is realized as described below:¶
This section describes how to manage an SCTP association, DTLS association, and TCP connection using SDP attributes.¶
The SCTP association, the DTLS association, and the TCP connection are managed independently from each other. Each can be established and closed without impacting others.¶
The detailed SDP offer/answer [RFC3264] procedures for the SDP attributes are described in Section 10.¶
This specification does not define semantics for the SDP direction attributes [RFC4566]. Unless the semantics of these attributes for an SCTP association usage have been defined, SDP direction attributes MUST be ignored if present.¶
When an SCTP association is established, both SCTP endpoints MUST initiate the SCTP association (i.e., both SCTP endpoints take the "active" role). In addition, both endpoints MUST use the same SCTP port as client port and server port, in order to prevent two separate SCTP associations from being established.¶
As both SCTP endpoints take the "active" role, the SDP "setup" attribute [RFC4145] does not apply to SCTP association establishment. However, the "setup" attribute does apply to establishment of the underlying DTLS association and TCP connection.¶
The SDP "connection" attribute [RFC4145] does not apply to the SCTP association. In order to trigger the closure of an existing SCTP association and establishment of a new SCTP association, the SDP "sctp-port" attribute (Section 5) is used to indicate a new (different than the ones currently used) SCTP port. The existing SCTP association is closed, and the new SCTP association is established, if one or both endpoints signal a new SCTP port. The "connection" attribute does apply to establishment of underlying TCP connections.¶
Alternatively, an SCTP association can be closed using the SDP "sctp-port" attribute with an attribute value of zero. Later, a new SCTP association can be established using the procedures in this section for establishing an SCTP association.¶
SCTP associations might be closed without SDP signaling -- for example, in case of a failure. The procedures in this section MUST be followed to establish a new SCTP association. This requires a new SDP offer/answer exchange. New (different than the ones currently used) SCTP ports MUST be used by both endpoints.¶
A DTLS association is managed according to the procedures in [RFC8842]. Hence, the SDP "setup" attribute is used to negotiate the (D)TLS roles ("client" and "server") [RFC8122].¶
The TCP connection is managed according to the procedures in [RFC4145]. Hence, the SDP "setup" attribute is used to negotiate the TCP roles ("active" and "passive"), and the SDP "connection" attribute is used to indicate whether to use an existing TCP connection or create a new one. The SDP "setup" attribute "holdconn" value MUST NOT be used.¶
This section defines the SDP Offer/Answer [RFC3264] procedures for negotiating and establishing an SCTP-over-DTLS association. Unless explicitly stated, the procedures apply to both the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" "m=" line proto values.¶
Each endpoint MUST associate one or more certificate fingerprints using the SDP "fingerprint" attribute with the "m=" line, following the procedures in [RFC8122].¶
The authentication certificates are interpreted and validated as defined in [RFC8122]. Self-signed certificates can be used securely, provided that the integrity of the SDP description is assured, as defined in [RFC8122].¶
Each endpoint MUST associate an SDP "tls-id" attribute with the "m=" line, following the procedures in [RFC8842].¶
When the offerer creates an initial offer, the offerer:¶
When the answerer receives an offer that contains an "m=" line describing an SCTP-over-DTLS association, if the answerer accepts the association, the answerer:¶
Once the answerer has sent the answer:¶
If the SDP "sctp-port" attribute in the answer contains an attribute value of zero, the answerer MUST NOT establish an SCTP association. If an SCTP association exists, the offerer MUST close it.¶
If the answerer does not accept the "m=" line in the offer, it MUST assign a zero port value to the corresponding "m=" line in the answer, following the procedures in [RFC3264]. In addition, the answerer MUST NOT initiate the establishment of a TCP connection, a DTLS association, or a DTLS association associated with the "m=" line.¶
Once the offerer has received the answer:¶
If the SDP "sctp-port" attribute in the answer contains an attribute value of zero, the offerer MUST NOT establish an SCTP association. If, in addition, an SCTP association exists, the offerer MUST close it.¶
If the "m=" line in the answer contains a zero port value, the offerer MUST NOT initiate the establishment of a TCP connection, a DTLS association, or an SCTP association associated with the "m=" line. If, in addition, a TCP connection, DTLS association, or SCTP association exists, the offerer MUST close it.¶
When an offerer sends an updated offer, in order to modify a previously established SCTP association, it follows the procedures in Section 10.2, with the following exceptions:¶
The offerer follows the procedures in [RFC8842] regarding the DTLS association impacts when modifying a session.¶
In the case of TCP/DTLS/SCTP, the offerer follows the procedures in [RFC4145] regarding the TCP connection impacts when modifying a session.¶
Multihoming is not supported when sending SCTP on top of DTLS, as DTLS does not expose address management of the underlying transport protocols (UDP or TCP) to its upper layer.¶
When SCTP-over-DTLS is used in a NAT environment, it relies on the NAT traversal procedures for the underlying transport protocol (UDP or TCP).¶
When SCTP-over-DTLS is used with UDP-based ICE candidates [RFC8445], then the procedures for UDP/DTLS/SCTP (Section 7) are used.¶
When SCTP-over-DTLS is used with TCP-based ICE candidates [RFC6544], then the procedures for TCP/DTLS/SCTP (Section 8) are used.¶
In ICE environments, during the nomination process, endpoints go through multiple ICE candidate pairs until the most preferred candidate pair is found. During the nomination process, data can be sent as soon as the first working candidate pair is found, but the nomination process still continues, and selected candidate pairs can still change while data is sent. Furthermore, if endpoints roam between networks -- for instance, when a mobile endpoint switches from mobile connection to WiFi -- endpoints will initiate an ICE restart. This will trigger a new nomination process between the new set of candidates, which will likely result in the new nominated candidate pair.¶
Implementations MUST treat all ICE candidate pairs associated with an SCTP association on top of a DTLS association as part of the same DTLS association. Thus, there will only be one SCTP handshake and one DTLS handshake even if there are multiple valid candidate pairs; shifting from one candidate pair to another, including switching between UDP and TCP candidate pairs, will not impact the SCTP or DTLS associations. If new candidates are added, they will also be part of the same SCTP and DTLS associations. When transitioning between candidate pairs, different candidate pairs can be currently active in different directions, and implementations MUST be ready to receive data on any of the candidates, even if this means sending and receiving data using UDP/DTLS/SCTP and TCP/DTLS/SCTP at the same time in different directions.¶
In order to maximize the likelihood of interoperability between the endpoints, all ICE-enabled SCTP-over-DTLS endpoints SHOULD implement support for UDP/DTLS/SCTP.¶
When an SDP offer or answer is sent with multiple ICE candidates during initial connection negotiation or after ICE restart, UDP-based candidates SHOULD be included, and the default candidate SHOULD be chosen from one of those UDP candidates. The proto value MUST match the transport protocol associated with the default candidate. If UDP transport is used for the default candidate, then the "UDP/DTLS/SCTP" proto value MUST be used. If TCP transport is used for the default candidate, then the "TCP/DTLS/SCTP" proto value MUST be used. Note that under normal circumstances, the proto value for offers and answers sent during ICE nomination SHOULD be "UDP/DTLS/SCTP".¶
When a subsequent SDP offer or answer is sent after ICE nomination is complete, and it does not initiate ICE restart, it will contain only the nominated ICE candidate pair. In this case, the proto value MUST match the transport protocol associated with the nominated ICE candidate pair. If UDP transport is used for the nominated pair, then the "UDP/DTLS/SCTP" proto value MUST be used. If TCP transport is used for the nominated pair, then the "TCP/DTLS/SCTP" proto value MUST be used. Please note that if an endpoint switches between TCP-based and UDP-based candidates during the nomination process, the endpoint is not required to send an SDP offer for the sole purpose of keeping the proto value of the associated "m=" line in sync.¶
SDP Offer:¶
m=application 54111 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP6 2001:DB8::A8FD
a=tls-id:abc3de65cddef001be82
a=setup:actpass
a=sctp-port:5000
a=max-message-size:100000¶
SDP Answer:¶
m=application 64300 UDP/DTLS/SCTP webrtc-datachannel
c=IN IP6 2001:DB8::001D
a=tls-id:dbc8de77cddef001be90
a=setup:passive
a=sctp-port:6000
a=max-message-size:100000¶
[RFC4566] defines general SDP security considerations, while [RFC3264], [RFC4145], and [RFC8122] define security considerations when using the SDP offer/answer mechanism to negotiate media streams.¶
[RFC4960] defines general SCTP security considerations, and [RFC8261] defines security considerations when using SCTP on top of DTLS.¶
This specification does not introduce new security considerations in addition to those defined in the specifications listed above.¶
This document updates the "Session Description Protocol (SDP) Parameters" registry, following the procedures in [RFC4566], by adding the following values to the table in the SDP "proto" field registry:¶
| Type | SDP Name | Reference |
|---|---|---|
| proto | UDP/DTLS/SCTP | RFC 8841 |
| proto | TCP/DTLS/SCTP | RFC 8841 |
This document defines a new SDP media-level attribute,"sctp-port". The details of the attribute are defined in Section 5.2.¶
This document defines a new SDP media-level attribute,"max-message-size". The details of the attribute are defined in Section 6.2.¶
Per this specification, a new IANA registry has been created, following the procedures in [RFC8126], for the namespace associated with the "UDP/DTLS/SCTP" and "TCP/DTLS/SCTP" protocol identifiers. Each fmt value describes the usage of an entire SCTP association, including all SCTP streams associated with the SCTP association.¶
The fmt value "association-usage" used with these "proto" values is required. It is defined in Section 4.¶
As part of this registry, IANA maintains the following information:¶
association-usage names are to be subject to the "First Come First Served" IANA registration policy [RFC8126].¶
IANA has added the following initial values to the registry.¶
| Name | Reference |
|---|---|
| webrtc-datachannel | RFC 8832, RFC 8841 |
The authors wish to thank Harald Alvestrand, Randell Jesup, Paul Kyzivat, Michael Tüxen, Juergen Stoetzer-Bradler, Flemming Andreasen, and Ari Keränen for their comments and useful feedback. Ben Campbell provided comments as part of his Area Director review. Brian Carpenter performed the Gen-ART review.¶