Internet-Draft S. Bale Intended Status: Informational R. Brebion Expires: January 30, 2023 G. Bichot Broadpeak July 29, 2022 MSYNC draft-bichot-msync-05 Abstract This document describes the Multicast Synchronization (MSYNC) Protocol that aims at transferring video media objects over IP multicast operating preferably RTP. Although generic, MSYNC has been primarily designed for transporting HTTP adaptive streaming (HAS) objects including manifest/playlists and media segments (e.g. MP4, CMAF) according to an HAS protocol such as Apple HLS or MPEG DASH between a multicast server and a multicast gateway. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Copyright and License Notice Copyright (c) 2022 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 Bale et aL. Expires January 30, 2023 [Page 1] Internet-Draft MSYNC July 29, 2022 Provisions Relating to IETF Documents (http://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. Table of Contents 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Definitions . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. MSYNC Protocol . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. MSYNC Packet Format . . . . . . . . . . . . . . . . . . . . 6 3.2. Object Info Packet . . . . . . . . . . . . . . . . . . . . 7 3.3. Object Data Packet . . . . . . . . . . . . . . . . . . . . 9 3.4. Object HTTP Header Packet . . . . . . . . . . . . . . . . . 9 3.5. Object Data-part Packet . . . . . . . . . . . . . . . . . . 10 3.6. Maximum Size Of A MSYNC Packet . . . . . . . . . . . . . . 11 3.7. Sending MSYNC Objects Over IP/Transport Multicast Sessions . . . . . . . . . . . . . . . . . . . . . . . . . 12 3.8. HAS Protocol Dependency . . . . . . . . . . . . . . . . . . 13 3.8.1. Object Info Packet . . . . . . . . . . . . . . . . . . 13 3.8.1.1. Media Sequence . . . . . . . . . . . . . . . . . . 13 3.8.1.2. Object URI . . . . . . . . . . . . . . . . . . . . 14 3.8.2. Sending Rules . . . . . . . . . . . . . . . . . . . . . 15 3.9. RTP As The Transport Multicast Session Protocol . . . . . . 15 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17 5. Security Considerations . . . . . . . . . . . . . . . . . . . 17 5. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 5.1. Normative References . . . . . . . . . . . . . . . . . . . 17 5.2. Informative References . . . . . . . . . . . . . . . . . . 18 6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 18 7. Change Log . . . . . . . . . . . . . . . . . . . . . . . . . . 18 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18 Bale et aL. Expires January 30, 2023 [Page 2] Internet-Draft MSYNC July 29, 2022 1 Introduction MSYNC relies preferably on RTP that makes it particularly suited for transitioning IPTV legacy (MPEG2 TS/RTP) to the HAS ecosystem. MSYNC is simple (no flow control, no forward error correction) although reliable, flexible and extensible; it has been experimented and deployed over IPTV infrastructure (xDSL, cable, fiber) and satellite. 1.1 Terminology The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. 1.2 Definitions manifest: A file gathering the configuration for conducting a streaming session; corresponds to a play list as defined by HLS [RFC8216]. During a HAS streaming session, a manifest or playlist can be modified. media chunk: A piece of a media segment of a fixed duration as specified in [MPEGCMAF]. media segment: A piece of a media sub-stream of a fixed duration (e.g. 2s) as specified in [MPEGCMAF]. init segment: A piece of a media sub-stream used to initialize the decoder as specified in [MPEGCMAF]. media: A digitalized piece of video, audio, subtitle, image, .... media stream: Gathers one or more media sub-streams. media sub-stream: A version of a media encoded in a particular bit- rate, format and resolution; also called representation or variant stream. variant stream : A media sub-stream as defined by HLS [RFC8216]; corresponds to a representation as defined by [MPEGDASH]. representation: A media sub-stream as defined by [MPEGDASH]; Corresponds to a variant stream as defined by HLS [RFC8216]. HTTP Adaptive Streaming (HAS) session: Transport one or more media Bale et aL. Expires January 30, 2023 [Page 3] Internet-Draft MSYNC July 29, 2022 streams (e.g. one video, two audios, One subtitle) according to HTTP. A HAS session is triggered by a player downloading first a manifest file(s), then init and/or media segments (belonging to possibly different sub-streams according to the selected representation) and possibly more manifest files according to the HAS protocol. MSYNC object: As part of a HAS session carried over MSYNC, an MSYSNC object can be an addressable HAS entity like an init segment, a media segment (or fragment, or chunk), a manifest. An MSYNC object can also be a non-addressable transport entity like a part of a segment (an HTTP2 frame or an HTTP 1.1 CTE block). As part of the control channel, an MSYNC object may transport some control plane information (for the receiver as e.g. the multicast gateway configuration). An MSYNC object is typically associated with metadata (aka info), data and possibly an HTTP header. MSYNC packet: The transport unit of MSYNC. Several MSYNC packets mays be used to transport an MSYNC object. transport multicast session: Operating a transport protocol that is (possibly based on) UDP over IP multicast. A session is identified by the transport (UDP) port number, the source IP address and the IP multicast address. RTP multicast session: A transport multicast session based on RTP as defined in [RFC3550]. IP multicast session: A session gathering transport multicast sessions having the same source IP address and destination multicast IP address. MSYNC channel: The set of transport multicast sessions carrying a HAS session as a set of MSYNC objects. MSYNC control channel: the transport multicast session carrying control plane MSYNC objects. 2. Overview MSYNC is a simple protocol typically used between a multicast server (the MSYNC sender) and a multicast gateway (the MSYNC receiver). The multicast server gets ingested with a unicast HAS session conforming to a HAS protocol as e.g. MPEG DASH [MPEGDASH] or HLS [RFC8216] and sends the HAS session elements over a broadcast/multicast link according to MSYNC supporting [possibly RTP/] UDP/IP multicast up to Bale et aL. Expires January 30, 2023 [Page 4] Internet-Draft MSYNC July 29, 2022 the multicast gateway(s) that serve the HAS player(s) in unicast conforming to the same HAS protocol. MSYNC can serve simultaneously multiple gateways conforming to one or several HAS protocols and formats. The Multicast server is configured in order to get the unicast HAS feeds. Considering one among several possible ingest methods (e.g. HTTP GET), for each ingested feed, the multicast server behaves as a sort of player, reading the manifest, discovering the available representations and downloading concurrently media segments of all (or a subset) of the available representations. Finally the multicast server is configured for sending all those HAS session elements over [possibly RTP/]UDP/IP multicast according to a certain UDP flow arrangement (for example all the objects related to each video representation are sent over a separate multicast transport session (multicast IP address + port number) whereas all audio representations are sent over the same transport multicast session. The multicast gateway is configured accordingly in order to be attached to the transport multicast sessions (in particular, it has to subscribe to the corresponding multicast IP group address). Note that the multicast gateway might not be capable of being attached to all the concurrent transport multicast sessions in the same time per bandwidth restriction (e.g. ADSL). In that case, the multicast gateway attaches to the transport multicast session corresponding to the player's request (and detaches from the other previous one). At any time, the multicast gateway can detect corrupted and lost packets and attempt to repair using a repair protocol. This is possible thanks to the RTP protocol if used as the transport layer over UDP . The multicast gateway receives the MSYNC objects and is ready to serve it (e.g. feeds a local cache). Whenever a HAS request is sent by a media player and received by the multicast gateway, the latter reads first its local cache. In case of cache hit, it returns the object. In case of cache miss, the multicast gateway can possibly retrieve the requested object from the associated CDN (or a dedicated server) over an unicast interface (if existing) through operating HTTP conventionally and forwards back to the media player the object once retrieved. With MSYNC deployed over a multicast link/network, the end user media player gets basically the HAS content in full transparency (i.e. the player is absolutely unaware of getting the content through MSYNC or not). Note that nothing precludes the multicast gateway to be co-located Bale et aL. Expires January 30, 2023 [Page 5] Internet-Draft MSYNC July 29, 2022 with the media player and therefore embedded in the end-user terminal. Note that nothing precludes application dependent features in the multicast server and/or the multicast gateway that may adapt/modify the content delivered to the end-user player. 3. MSYNC Protocol 3.1. MSYNC Packet Format The MSYNC packet has the following format. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | version | packet type | object identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | sub-header | | .... | +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ | data | | .... | version: 8 bits version of the MSYNC protocol = 0x3 packet type: 8 bits Defines the MSYNC packet type. The sub-header and the associated data (if any) are dependent on the packet type. The following types are defined. 0x01: object info 0x02: object info redundancy packet 0x03: object data 0x04: Reserved 0x05: object http header 0x06: object data-part as a piece of an object data for transporting e.g. an MPEG CMAF chunk, an HTTP 1.1 chunk or yet an HTTP2 frame. object identifier: 16 bits The field identifies the object being transferred. All MSYNC packets associated with the same object carry the same object identifier in their MSYNC packet header. sub-header: series of N x 32 bits The packet sub-header is linked to the packet type. The details of Bale et aL. Expires January 30, 2023 [Page 6] Internet-Draft MSYNC July 29, 2022 each packet type is given in the next sections. data: series of D x 8 bits This field is optional and is present depending on the packet type. D is bounded by the maximum size of a transport multicast session protocol packet size and the MTU (Maximum Transfer Unit) otherwise as depicted in 3.6. 3.2. Object Info Packet The Object info packet is used to transport the meta-data associated with an object. It permits to characterize the object in term of e.g. size and type. The object information is carried over one object info packet only. The object info packet is typically sent along with the object data it describes. The object identifier corresponds to the object identifier of the object data packets or the object data-part packets the object info packet relates to. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | version | 0x1 or 0x2 | object identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | number of MSYNC packets | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object CRC | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object type | Reserved | mtype | object URI size | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | media sequence | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | | object URI | : : : : | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ object size: 32 bits The number of bytes that compose the object payload transported with a MSYNC object data packet (Section 3.3) or MSYNC object data-part packet (Section 3.5). The size may be 0 indicating that there is no corresponding object's payload transmission foreseen (i.e. no expected MSYNC data or MSYNC data-part packets) . In case Bale et aL. Expires January 30, 2023 [Page 7] Internet-Draft MSYNC July 29, 2022 of a super object transmission (Section 3.5), If the object URI of an object info with an object size set to 0 matches the super object URI then it MUST be interpreted as the end of the super object transmission (Section 3.8.1.2). number of MSYNC packets: 32 bits Number of MSYNC packets that compose the transported object. If the object size is null (set to 0) then the number of MSYNC packets MUST be null (set to 0). object CRC: 32 bits A CRC applied to the object data payload for corruption detection. mtype: 4 bits The manifest (playlist) type, the MSYNC INFO is compatible with. The field can take the following values. 0x00: Not Applicable 0x01: MPEG Dash as specified in [MPEGDASH]. 0x02: HLS as specified in [RFC8216]. 0x03-0xF: Reserved object URI size: 12bits The size in bytes of the object URI field. The value MUST guarantee that the MSYNC info packet size is not greater than the network MTU. object type : 8 bits Defines the type of MSYNC data object associated with this MSYNC info packet 0x00: Reserved 0x01: media manifest (playlist) 0x02: Reserved 0x03: media data or data-part: Transport stream (MPEG2-TS) 0x04: media data or data-part: MPEG4 (CMAF) 0x05: control: control plane information (e.g. multicast gateway configuration) 0x06-0xFF: Reserved media sequence: 32 bits It is a sequence number associated with the MSYNC object data (segment or manifest). It is dependent on the mtype value. It is used to synchronize unicast and multicast receptions in the multicast gateway. The values and rules are detailed in the section 3.8 dedicated to the HAS protocol dependencies. The default value is 0x00. object URI: Quotient(object URI size/32) bits This the path name associated with the object. It MAY corresponds Bale et aL. Expires January 30, 2023 [Page 8] Internet-Draft MSYNC July 29, 2022 to a storage/Cache path. There SHOULD be a direct relationship between this URI and the URL associated with the addressable object (e.g. HAS segment or CMAF chunk and/or a manifest). The rules are detailed in the section 3.8 dedicated to the HAS protocol dependencies. 3.3. Object Data Packet This MSYNC packet carries part or all of the the object's data payload. The type of data and the way to process the object's data packets is function of the associated object's info packet. Object payload is transported through a series of object data packets. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | version | 0x3 | object identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data | : : : : object offset: 32 bits The index from which the MSYNC object data packet payload is to be written in order to compose the object data at the receiver side (i.e. the multicast gateway). The first data packet of an object has an offset equal to 0. data: N x 8bits The size N is not declared; it is bounded by the maximum size of the under-laying transport multicast session packet (e.g. RTP) as depicted in Section 3.6. The total size (number of bytes) of the object data is indicated in the associated object info (field object size). 3.4. Object HTTP Header Packet The HTTP header packet carries part or all of an HTTP header related to the object (data) to be sent. There is at most one HTTP header per object that can be repeated. The object identifier is the same than the one present in the object data packets or object data-part packets it relates to. Bale et aL. Expires January 30, 2023 [Page 9] Internet-Draft MSYNC July 29, 2022 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | version | 0x5 | object identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | header size | header offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data | : : : : header size: 16 bits An object HTTP header can be transported over one or several under-laying transport packets. This field indicates the total size of the HTTP header in bytes and it is indicated in each the HTTP header's packet. header offset: 16 bits The index from which this HTTP header MSYNC packet payload data is to be written in order to complement the HTTP header at the receiver side (i.e the multicast gateway). The first packet of the HTTP header has an offset equal to 0. data: N x 8bits The size N is not declared; it is bounded by either the header size field value or by the maximum size of the under-laying transport packet(e.g. RTP)as depicted in Section 3.6. 3.5. Object Data-part Packet This MSYNC packet carries part or all of the media data-part object payload. The type of data and the way to process the object's data-part packets is function of the associated info packet. Object payload is transported through a series of object data-part packets. The data-part is used when the object corresponds to a "part" (a block) of a super object for which the size is unknown (a super object may correspond to a stream or a media segment not yet complete and for which the size is therefore unknown). All data-part packets belonging to the same data part object have the same object identifier that is the same one present in the object info packet and HTTP header (if any) packets the data-part object relates too. All data-part objects composing a super object have a different object identifier. The way to link data-part objects with a super object is thanks to the object info packet (object URI) as Bale et aL. Expires January 30, 2023 [Page 10] Internet-Draft MSYNC July 29, 2022 explained in Section 3.8.1.2. The end of super-object transmission is signaled with an object info packet having both the object size and the number of MSYNC packets set to 0 and having the object URI matching the object URI of the already received parts according to Section 3.8.1.2. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | version | 0x6 | object identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | object offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | super object offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | data | : : : : object offset: 32 bits The index from which the data-part packet payload is to be written in order to compose the object data-part at the receiver side (i.e. the multicast gateway). The first packet of the data-part has an offset equal to 0. super object offset: 32 bits The index from which the object part-data packet payload is to be written in order to compose the super object data at the receiver side (i.e. the multicast gateway). The first data-part object composing a super object has the super object offset equal to 0. The super object offset is the same for all object data-part packets composing the same object data-part. data: N x 8bits The size N is not declared; it is bounded by the maximum size of the under-laying transport packet (e.g. RTP) as depicted in the section 3.6. The total size (number of bytes) of the object data is indicated in the associated object info (field object size). 3.6. Maximum Size Of A MSYNC Packet An MSYNC packet is composed with a header part and a data part for which the size is bounded by the transport multicast session packet. In case there is no particular restriction as with RTP and/or UDP (which authorize up to 65235 bytes), then the maximum Bale et aL. Expires January 30, 2023 [Page 11] Internet-Draft MSYNC July 29, 2022 size is linked to the path MTU (Maximum Transfer Unit) as the largest transfer unit supported between the source (the multicast server) and the destination (the multicast gateway) without fragmentation. An MSYNC packet MUST fit within a link layer packet. For Ethernet, as an example, the MTU is typically 1500 bytes, assuming a 20 bytes IPv4 header, a 8 bytes UDP header and the 8 bytes MSYNC object data packet header, it gives an MTU of 1464 bytes for the MSYNC object data packet. Operating RTP, the MSYNC object data MTU is decreased by 12 bytes (= 1452 bytes). 3.7. Sending MSYNC Objects Over IP/Transport Multicast Sessions The following considerations are linked to the multicast server configuration. Per MSYNC channel, the way to map MSYNC objects related to a media stream with an IP or transport multicast session is not constrained. The arrangement is chosen function of the network architecture and capacity. For example, in xDSL, the capacity dedicated to multicast is limited which may drive to an arrangement where each sub-stream/representation of a HAS session/MSYNC channel matches with one dedicated IP multicast session. The MSYNC receiver switches to the IP transport session corresponding to the sub-stream/representation it should serve to the user terminal/player. Alternatively, one would like to have one IP multicast session (with possibly multiple transport multicast sessions, each having a different destination port number) for the entire HAS session/MSYNC channel that is an arrangement a la "IPTV", less bandwidth efficient but where only one multicast IP address is allocated per HAS session/MSYNC channel. Considering a satellite network, as all transport multicast sessions are carried simultaneously, all arrangements may make sense. Regarding the mapping with a transport multicast session, the triplet: source IP address, destination multicast IP address and destination transport port number is the discriminator. It is recommended to carry media sub-streams and the control channel in separate transport multicast channels. It facilitates potential error correction procedures. The following granularity is possible: - One IP multicast session per media (audio or video or subtitle) sub-stream (representation); each transport multicast Bale et aL. Expires January 30, 2023 [Page 12] Internet-Draft MSYNC July 29, 2022 session having a different destination multicast IP address. - One transport multicast session for the MSYNC control channel. - It is perfectly possible to send all the MSYNC packets in only one transport multicast session. For each MSYNC object (see object type in 3.2) to be sent, the sender MUST send the following MSYNC packets in the specified order: one object info packet, zero or more object info redundant packets, zero or more HTTP header packets and one or more object data packets or object data-part packets. When the MSYNC object is a media data-part object of size null (used to signal the end of the transmission of a super object) then only one object info packet MUST be sent. 3.8. HAS Protocol Dependency A certain number of MSYNC packet header fields have a dependency on the HAS protocol and therefore on the manifest type. Similarly the sending rules may also depend from the HAS protocol. 3.8.1. Object Info Packet 3.8.1.1. Media Sequence The media sequence is used by the multicast gateway to synchronize the MSYNC (i.e. multicast) reception with unicast reception. The multicast gateway may operate jointly MSYNC and unicast retrieval of HAS objects. This is useful in some occasions like processing a new streaming session request (i.e. a manifest request after a channel switch) or in the case of segment repair. The multicast gateway may attempt to retrieve a manifest object or segment(s) through a unicast mean (e.g. a CDN server or a repair server) in order to speed up the start of the session or to repair damaged object(s). Consequently, the multicast gateway needs to understand the freshness of the HAS object received through multicast with regard to unicast. If no unicast reception is used jointly with MSYNC in the multicast gateway (e.g. like in one way delivery only), the default value MAY be used: 0x00 HLS master playlist: 0x00 HLS variant playlist; MUST contain the value of EXT-X-MEDIA-SEQUENCE added with the position in the playlist of the last segment Bale et aL. Expires January 30, 2023 [Page 13] Internet-Draft MSYNC July 29, 2022 transmitted. HLS segment: MUST contain the value of EXT-X-MEDIA-SEQUENCE added with the position of the segment in the playlist. DASH manifest: MUST contain $time$/scale or $Number$ corresponding to the last segment transmitted or under transmission (and possibly received partially) and declared by the manifest. DASH segment: MUST contain the $time$/scale or $Number$ value 3.8.1.2. Object URI In the context of HTTP adaptive streaming, The object URI is a URI reference. if the object is a HAS addressable entity (e.g. a segment or a CMAF chunk), the object URI MUST match (be a sub-string) with the URL announced in the corresponding manifest/playlist. Examples: - The object URI: /tvChannel1/Q1/S_2 matches with the segment's URL that is computed from the associated manifest/playlist: ".../tvChannel1/Q1/S_2.mp4" - The object URI /tvChannel11/Q1/S_2_3 matches with the CMAF chunk URL that is computed from the associated manifest/playlist: ".../tvChannel11/Q1/S_2_3.mp4". If the object is a non-addressable HAS entity (e.g. a HTTP 1.1 CTE block), the object URI is composed with a sub-string (that MUST match with the URL announced in the corresponding manifest) and a suffix composed with the hash sign/character (#) and the block number). Example: - The object URI of the 3rd HTTP CTE block of the segment S_2: tvChannel11/Q1/S_2.mp4#2 matches with the segment's request URL that terminates with ".../tvChannel1/Q1/S_2.mp4" The block number of an object URI attached to a media data-part object MUST be incremented for each subsequent transmission. When all the MSYNC data-part packets for all the media data-part objects (e.g. CTE blocks) composing a super object (e.g. a media Bale et aL. Expires January 30, 2023 [Page 14] Internet-Draft MSYNC July 29, 2022 segment) have been sent, the MSYNC sender MUST signal the end of the MSYNC super object transmission through sending an MSYNC object info packet with the object size set to zero (0) . In addition, the object URI MUST contain the URI reference of the next block (never transmitted). see 3.2. Example: - The object URI of the object info packet associated with the 1st HTTP CTE block of the segment S_2: tvChannel11/Q1/S_2.mp4#0 - The object URI of the object info packet associated with the 2nd HTTP CTE block of the segment S_2: tvChannel11/Q1/S_2.mp4#1 - The object URI of the object info packet associated with the 3rd HTTP CTE block of the segment S_2: tvChannel11/Q1/S_2.mp4#2 - The object URI of the object info packet associated with the 4st HTTP CTE block of the segment S_2: tvChannel11/Q1/S_2.mp4#3 - The object URI of the object info packet associated with the 5st HTTP CTE block (of size null) signaling the end of the super object (i.e. segment) transmission: tvChannel11/Q1/S_2.m4s_4 3.8.2. Sending Rules Whenever a manifest (playlist) has to be sent: - The manifest (playlist) object MUST be sent within (duplicated in) all the transport multicast sessions related to the transmission of the video segment objects the manifest/playlist refers to. - It MUST reference addressable objects (segment or CMAF chunk) that have already been sent or for which the transmission has started. 3.9. RTP As The Transport Multicast Session Protocol RTP [RFC3550] MAY be used as part of the transport multicast session protocol according to the following. - The payload type (PT) value is not specified. It may correspond to one of the values specified in [RFC3551]. Its value should be communicated to the MSYNC receiver through a separate unspecified offline mean. - Each RTP multicast session MUST operate a unique different Bale et aL. Expires January 30, 2023 [Page 15] Internet-Draft MSYNC July 29, 2022 SSRC number [RFC3550]. This allows packet retransmission (if used) on the RTP transport multicast session basis. - RTCP usage is not required. Packet retransmission MAY be used in association with the RTP multicast session for packet loss recovery. If this is the case then the RTP Repair client and RTP repair server MUST be compliant with [RFC4585], [RFC4588], [RFC5506] and [RFC5761] according to the following. - The RTP Repair client (i.e. the MSYNC Receiver) submits transport layer feedback (FB) messages in NACK mode (Generic NACK) to the RTP Repair Server according to [RFC5506] and [RFC4585]. - The RTP Repair server receives, processes and responds to the feedback NACK messages (FB) according to [RFC4588]. The RTP Repair server MAY be the MSYNC sender or it MAY be any intermediate entity acting as a multicast RTP receiver (i.e. capable of receiving the multicast RTP packets). In any case, the RTP Repair server and the RTP Repair client MUST operate a unicast interface. - The Session-multiplexing scheme [RFC4588] MUST be applied: the RTP retransmission (repair) stream MUST be sent on a different RTP session than the original (multicast) RTP stream. - The retransmission stream MUST support multiplexing the RTP and RTCP traffic on a single port according to [RFC5761]. Note that instead of relying on "RTP retransmission", an MSYNC receiver (i.e. the multicast gateway) could attempt to recover HAS elements (segments, manifest) through HTTP (aka "HTTP repair"). However the latter method requires a CDN and is less reactive than operating RTCP. Bale et aL. Expires January 30, 2023 [Page 16] Internet-Draft MSYNC July 29, 2022 4. IANA Considerations This document has no actions for IANA. 5. Security Considerations The multicast communication between the MSYNC sender (multicast server) and the MSYNC receiver (the multicast gateway) SHOULD be protected for confidentiality, message corruption and replay attacks. The MSYNC protocol does not specify any security mechanism. MSYNC relies on possibly content protection (Digital Right Management) and on the underlying transport layer and security extensions for providing message integrity/authentication and replay. Secure RTP (SRTP) [RFC3711] and IPSec applied to multicast [RFC5374] are potential candidates for providing such extensions. 5. References 5.1. Normative References [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels", RFC 2119, March 1997, . [RFC3550] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", RFC 3550, July 2003, . [MPEGDASH] "Information technology - Dynamic adaptive streaming over HTTP (DASH) - Part1:Media presentation description and segment formats",ISO/IEC23009-1 [MPEGCMAF] "Information technology - Multimedia application format (MPEG-A) - Part 19:Common media application format (CMAF) for segmented media"ISO/IEC 23000-19 [RFC5506] I. Johansson, M. Westerlund. "Support for Reduced-Size Real-Time Transport Control Protocol(RTCP): Opportunities and Consequences", RFC 5506, April 2009, . [RFC4585] J. Ott, S. Wenger, N. Sato, C. Burmeister, J. Rey. "Extended RTP Profile for Real-time Transport Control Protocol(RTCP)-Based Feedback (RTP/AVPF)", RFC 4585, July 2006, . Bale et aL. Expires January 30, 2023 [Page 17] Internet-Draft MSYNC July 29, 2022 [RFC4588] Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R. Hakenberg, "RTP Retransmission Payload Format", RFC 4588, July 2006, . [RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and Control Packets on a Single Port", RFC 5761, April 2010, . 5.2. Informative References [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", RFC 3551, July 2003, . [RFC3711] M. Baugher, D. McGrew, M. Naslund, E. Carrara, K. Norrman. "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004, . [RFC5374] B. Weis, G. Gross, D. Ignjatic. "Multicast Extensions to the Security Architecture for the Internet Protocol", RFC 5374, November 2008, . [RFC8216] R. Pantos, Ed., W. May, "HTTP Live Streaming", RFC 8216, August 2017, . 6. Acknowledgments The authors will be ever grateful to their late colleague Arnaud Leclerc who has been the initiator of that work. The authors would like to thank the following people for their feedback: Yann Barateau (Eutelsat). 7. Change Log - 05: Updated section 3.9 adding reference (RFC4588) and details for RTP retransmission. Updated/normalized references in section 5.1 and 5.2. - 04: Added detection of super object transmission (Section 3.2 and Section 3.8.1.2); several adjustments regarding RFC style; Section numbering correction.(Sections 3.9 and 3.10 are now section 3.8 and 3.9 respectively). Authors' Addresses Bale et aL. Expires January 30, 2023 [Page 18] Internet-Draft MSYNC July 29, 2022 Sophie Bale Broadpeak 15 rue Claude Chappe Zone des Champs Blancs 35510 Cesson-Sevigne France Email: sophie.bale@broadpeak.tv Remy Brebion Broadpeak 15 rue Claude Chappe Zone des Champs Blancs 35510 Cesson-Sevigne France Email: remy.brebion@broadpeak.tv Guillaume Bichot (Editor) Broadpeak 15 rue Claude Chappe Zone des Champs Blancs 35510 Cesson-Sevigne France Email: guillaume.bichot@broadpeak.tv Bale et aL. Expires January 30, 2023 [Page 19]