RFC 4585, "Extended RTP Profile for Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)", July 2006

Note: This RFC has been updated by RFC 5506, RFC 8108

Errata ID: 3313
Status: Verified
Type: Technical
Publication Format(s) : TEXT
Reported By: Mo Zanaty
Date Reported: 2012-08-11
Verifier Name: Robert Sparks
Date Verified: 2012-11-26

Section 4.2 says:

      rtcp-fb-ack-param  = SP "rpsi"
                         / SP "app" [SP byte-string]
                         / SP token [SP byte-string]
                         / ; empty

It should say:

      rtcp-fb-ack-param  = SP "rpsi"
                         / SP "app" [SP byte-string]
                         / SP token [SP byte-string]

Notes:

RFC 4585 defines and allows generic NACK (with no further parameters) but not generic ACK (which always requires further parameters). Section 4.2 says:
...
Parameters MUST be provided to further distinguish different types
of positive acknowledgement feedback.
...
The feedback type "nack", without parameters, indicates use of the
Generic NACK feedback format as defined in Section 6.2.1.
...
The ABNF incorrectly allows nothing after "ack", implying that generic ACK is valid. Even the approved errata, which replaces the invalid empty alternate with optional [], still allows nothing after "ack". Note that recent interest in RTP congestion control (e.g. RMCAT BOF) may lead to defining a standard ACK.

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