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3.7. Summary of Decision Steps

3.7.1. General Hints​

Before even considering whether or not to send RTCP feedback information, an application has to determine whether this mechanism is applicable:

  1. An application has to decide whether -- for the current ratio of packet rate with the associated (application-specific) maximum feedback delay and the currently observed round-trip time (if available) -- feedback mechanisms can be applied at all.

This decision may be based upon (and dynamically revised following) RTCP reception statistics as well as out-of-band mechanisms.

  1. The application has to decide -- for a certain observed error rate, assigned bandwidth, frame/packet rate, and group size -- whether (and which) feedback mechanisms can be applied.

Regular RTCP reception statistics provide valuable input to this step, too.

  1. If the application decides to send feedback, the application has to follow the rules for transmitting Early RTCP packets or Regular RTCP packets containing FB messages.

  2. The type of RTCP feedback sent should not duplicate information available to the sender from a lower layer transport protocol. That is, if the transport protocol provides negative or positive acknowledgements about packet reception (such as DCCP), the receiver should avoid repeating the same information at the RTCP layer (i.e., abstain from sending Generic NACKs).

3.7.2. Media Session Attributes​

Media sessions are typically described using out-of-band mechanisms to convey transport addresses, codec information, etc., between sender(s) and receiver(s). Such a mechanism is two-fold: a format used to describe a media session and another mechanism for transporting this description.

In the IETF, the Session Description Protocol (SDP) is currently used to describe media sessions while protocols such as SIP, Session Announcement Protocol (SAP), Real Time Streaming Protocol (RTSP), and HTTP (among others) are used to convey the descriptions.

A media session description format MAY include parameters to indicate that RTCP feedback mechanisms are supported in this session and which of the feedback mechanisms MAY be applied.

To do so, the profile "AVPF" MUST be indicated instead of "AVP". Further attributes may be defined to show which type(s) of feedback are supported.

Section 4 contains the syntax specification to support RTCP feedback with SDP. Similar specifications for other media session description formats are outside the scope of this document.

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