Specify need for TWCC and CCFB to be consistent
Both TWCC and CCFB need to be consistent across all media sections
in a session description, and in answers, only one of them must be chosen.
Specify this as part of the TWCC spec.
No-try: True
Bug: webrtc:42225697
Change-Id: Ida8f9f96fb2f3b66dd040c01d99cd24344c5afda
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/422320
Auto-Submit: Harald Alvestrand <hta@webrtc.org>
Reviewed-by: Per Kjellander <perkj@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#46188}
diff --git a/docs/native-code/rtp-hdrext/transport-wide-cc-02/README.md b/docs/native-code/rtp-hdrext/transport-wide-cc-02/README.md
index 8dc8261..d55463e 100644
--- a/docs/native-code/rtp-hdrext/transport-wide-cc-02/README.md
+++ b/docs/native-code/rtp-hdrext/transport-wide-cc-02/README.md
@@ -60,3 +60,14 @@
generated, which is equivalent of sending the two-byte extension above.
This is added as an option to allow for a fixed packet header size.
+## Usage and compatibility
+
+Since the transport-wide CC extension is designed to cover all RTP data
+on a connection, it should be specified on all media sections in an SDP
+description. However, omitting it for audio has been supported in the past;
+in this case, congestion control will ignore the audio.
+
+The transport-wide CC extension cannot be used at the same time as the
+CCFB feedback format (RFC 8888). It is legal to make an offer that gives
+both transport-wide CC and CCFB, but in an answer, only one of them can
+be specified, and that choice has to be consistent across all media sections.
