RFC Errata
RFC 5576, "Source-Specific Media Attributes in the Session Description Protocol (SDP)", June 2009
Source of RFC: mmusic (rai)
Errata ID: 7544
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Philipp Hancke
Date Reported: 2023-06-15
Section 4.2 says:
Every <ssrc-id> listed in an "ssrc-group" attribute MUST be defined by a corresponding "ssrc:" line in the same media description
It should say:
Every <ssrc-id> listed in an "ssrc-group" attribute MUST be defined by a corresponding "ssrc:" line in the same media description and MUST appear only once in this ssrc-group
Notes:
The goal is to clarify that something like
a=ssrc-group:FID 1234 1234
is not valid. While this is demuxable (in the BUNDLE sense) it would require chaining of ssrc-demuxing and payload type demuxing which is a lot of complexity.
The uniqueness is already implied by the following sentence (emphasis is mine):
The SDP media attribute "ssrc-group" expresses a relationship among *several* sources of an RTP session
earlier in the section.