RFC 4756

RFC 4756: Forward Error Correction Grouping Semantics in Session Description Protocol 🎀✨

Okay bestie, let's get into it! 💖 This is RFC 4756, a Standards Track document from the IETF Trust era, dated November 2006. It was written by A. Li (Adam H. Li, HyerVision), and it doesn't obsolete or update any other RFCs. Iconic, right? 🦋

Quick vibe check on the title block: it's giving "official standards doc with a cute new feature," and the whole point is to tell SDP (Session Description Protocol) how FEC streams and the payload streams they protect are related. 🧚

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Status of This Memo 📋

This document is an Internet standards track protocol for the Internet community, and it's asking everyone to share discussion and suggestions for improvements. 💅 For the current standardization state and status, check the current edition of the "Internet Official Protocol Standards" (STD 1). Distribution of this memo is unlimited! 🌸

Copyright Notice 🔒

Copyright (C) The IETF Trust (2006). ✨

Abstract 🌈

So this doc defines the semantics that let you group Forward Error Correction (FEC) streams together with the protected payload streams in SDP. 💜 You use these semantics together with "Grouping of Media Lines in the Session Description Protocol" (RFC 3388) so that you can group together "m" lines in the same session. Main character energy! 👑

Table of Contents 📚

1. Introduction ....................................................2
2. Terminology .....................................................2
3. Forward Error Correction (FEC) ..................................2
4. FEC Grouping ....................................................3
   4.1. FEC Group ..................................................3
   4.2. Offer / Answer Consideration ...............................3
   4.3. Example of FEC Grouping ....................................3
5. Security Considerations .........................................4
6. IANA Considerations .............................................4
7. Acknowledgments .................................................5
8. References ......................................................5
   8.1. Normative References .......................................5
   8.2. Informative References .....................................5

1. Introduction 🦋

Okay so SDP [3] media lines can be associated with each other in all kinds of ways, but SDP on its own doesn't give you a way to say how they relate. 🙈 That's what the extension in RFC 3388 [2] is for. RFC 3388 defines two types of semantics: Lip Synchronization and Flow Identification. 🎀

FEC is a super common technique for robust communication in error-prone environments, so this document adds semantics that let you group FEC streams with the protected payload streams in SDP, building on RFC 3388. 🔥

2. Terminology 📖

Here's the key word rule, fr fr: 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 RFC 2119 [1]. 💅

3. Forward Error Correction (FEC) 💖

FEC is a common technique for robust communication in error-prone environments, and it's giving "send a little extra so you can fix the mess." ✨ Communication uses a bandwidth that is more than payload, so it can send redundantly coded payload information. 🧚 Receivers can readily recover the original payload even when some of the communication gets lost in transmission. 🫶

Compared to other error correction techniques (like retransmission), FEC can achieve much lower transmission delay, and it doesn't have the problem of implosion from retransmission requests in various multicast scenarios. 😭 Honestly iconic.

The FEC data can be sent in two different ways: (1) multiplexed together with the original payload stream, or (2) as a separate stream. 🌈 So you need mechanisms to indicate the association relationship between the FEC data and the payload data they protect. 🍓

When FEC data are multiplexed with the original payload stream, the association relationship may, for example, be indicated as specified in "An RTP Payload for Redundant Audio Data" (RFC 2198) [4]. The generic RTP payload format for FEC [5] uses that method. 🦋

When FEC data are sent as a separate stream from the payload data, the association relationship can be indicated in various ways. This document specifies a mechanism for indicating those relationships in the FEC media line grouping. 💜

4. FEC Grouping 🎀

4.1. FEC Group 🌸

Each "a=group" line is used to indicate an association relationship between the FEC streams and the payload streams. 💖 The streams included in one "a=group" line are called a "FEC Group". ✨

Each FEC group MAY have one or more than one FEC stream, and one or more than one payload stream. For example, one payload stream can be protected by more than one FEC stream, or multiple payload streams can share one FEC stream. 🫶

Grouping streams in a FEC group only indicates the association relationship between streams. The detailed FEC protection scheme and parameters are conveyed through the mechanism of the particular FEC algorithm used. 🧚 For example, FEC grouping is used for the generic RTP payload for FEC [5] to indicate the association relationship between the FEC stream and the payload stream. The detailed protection level and length information for the Unequal Loss Protection (ULP) algorithm is communicated in band within the FEC stream. 🔥

4.2. Offer / Answer Consideration 💅

Backward compatibility in offer/answer is generally handled as specified in RFC 3388 [2]. 🌈

Depending on the implementation, a node that doesn't understand FEC grouping (either it doesn't understand line grouping at all, or it just doesn't understand the FEC semantics) SHOULD respond to an offer containing FEC grouping either (1) with an answer that ignores the grouping attribute, or (2) with a refusal to the request (e.g., 488 Not acceptable here or 606 Not acceptable in SIP). 🙈

In the first case, the original sender of the offer MUST establish the connection without FEC. In the second case, if the sender of the offer still wants to establish the session, it SHOULD retry the request with an offer without FEC. 😭

4.3. Example of FEC Grouping 🍓

This example shows a session description of a multicast conference. 🎀 The first media stream (mid:1) contains the audio stream. The second media stream (mid:2) contains the Generic FEC [5] protection for the audio stream. These two streams form an FEC group, and the relationship between them is indicated by the a=group:FEC 1 2 line. 💖

The FEC stream is sent to the same multicast group and has the same Time to Live (TTL) as the audio, but on a port number two higher. ✨ Likewise, the video stream (mid:3) and its Generic FEC protection stream (mid:4) form another FEC group, indicated by the a=group:FEC 3 4 line. 🦋 That FEC stream is sent to a different multicast address, but it has the same port number (30004) as the payload video stream. 🧚

    v=0
    o=adam 289083124 289083124 IN IP4 host.example.com
    s=ULP FEC Seminar
    t=0 0
    c=IN IP4 224.2.17.12/127
    a=group:FEC 1 2
    a=group:FEC 3 4
    m=audio 30000 RTP/AVP 0
    a=mid:1
    m=audio 30002 RTP/AVP 100
    a=rtpmap:100 ulpfec/8000
    a=mid:2
    m=video 30004 RTP/AVP 31
    a=mid:3
    m=video 30004 RTP/AVP 101
    c=IN IP4 224.2.17.13/127
    a=rtpmap:101 ulpfec/8000
    a=mid:4

5. Security Considerations 🔒

Okay, this part is serious, bestie. 😭 There is a weak threat for the receiver: the FEC grouping can be modified to indicate FEC relationships that don't exist. 💀 Such attacks may result in failure of FEC to protect, and/or mishandling of other media payload streams. 🙈 It is recommended that the receiver SHOULD do an integrity check on SDP and follow the security considerations of SDP [3] so it only trusts SDP from trusted sources. 🫶 Protect your SDP, periodt! 🛡️

6. IANA Considerations 📝

This document defines the semantics to be used with grouping of media lines in SDP as defined in RFC 3388. ✨ The semantics defined in this document are to be registered by IANA when they're published in standards track RFCs. 🌸

The following semantics have been registered by IANA in the Semantics for the "group" SDP Attribute under SDP Parameters. 🦋

Semantics                  Token   Reference
------------------------   -----   ----------
Forward Error Correction   FEC     RFC 4756

7. Acknowledgments 💜

The author would like to thank Magnus Westerlund, Colin Perkins, Joerg Ott, and Cullen Jennings for their feedback on this document. 🫶👑

8. References 📚

8.1. Normative References 💖

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.

[2] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne, "Grouping of Media Lines in the Session Description Protocol (SDP)", RFC 3388, December 2002.

[3] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.

8.2. Informative References 🌈

[4] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, September 1997.

[5] Li, A., "An RFC Payload Format for Generic FEC", Work in Progress.

Author's Address 📮

Adam H. Li HyerVision 10194 Wateridge Circle #152 San Diego, CA 92121 U.S.A.

Tel: +1 858 622 9038 EMail: adamli@hyervision.com

Full Copyright Statement ©️

Copyright (C) The IETF Trust (2006). 🌸

This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 💅

This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST, AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 😭

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