Skip to main content

3.1. Compound RTCP Feedback Packets

Two components constitute RTCP-based feedback as described in this document:

  • Status reports are contained in sender report (SR)/received report (RR) packets and are transmitted at regular intervals as part of compound RTCP packets (which also include source description (SDES) and possibly other messages); these status reports provide an overall indication for the recent reception quality of a media stream.

  • FB messages as defined in this document that indicate loss or reception of particular pieces of a media stream (or provide some other form of rather immediate feedback on the data received). Rules for the transmission of FB messages are newly introduced in this document.

RTCP FB messages are just another RTCP packet type (see Section 4). Therefore, multiple FB messages MAY be combined in a single compound RTCP packet and they MAY also be sent combined with other RTCP packets.

Compound RTCP packets containing FB messages as defined in this document MUST contain RTCP packets in the order defined in [1]:

  • OPTIONAL encryption prefix that MUST be present if the RTCP packet(s) is to be encrypted according to Section 9.1 of [1].
  • MANDATORY SR or RR.
  • MANDATORY SDES, which MUST contain the CNAME item; all other SDES items are OPTIONAL.
  • One or more FB messages.

The FB message(s) MUST be placed in the compound packet after RR and SDES RTCP packets defined in [1]. The ordering with respect to other RTCP extensions is not defined.

Two types of compound RTCP packets carrying feedback packets are used in this document:

a) Minimal compound RTCP feedback packet

A minimal compound RTCP feedback packet MUST contain only the mandatory information as listed above: encryption prefix if necessary, exactly one RR or SR, exactly one SDES with only the CNAME item present, and the FB message(s). This is to minimize the size of the RTCP packet transmitted to convey feedback and thus to maximize the frequency at which feedback can be provided while still adhering to the RTCP bandwidth limitations.

This packet format SHOULD be used whenever an RTCP FB message is sent as part of an Early RTCP packet. This packet type is referred to as minimal compound RTCP packet in this document.

b) (Full) compound RTCP feedback packet

A (full) compound RTCP feedback packet MAY contain any additional number of RTCP packets (additional RRs, further SDES items, etc.). The above ordering rules MUST be adhered to.

This packet format MUST be used whenever an RTCP FB message is sent as part of a Regular RTCP packet or in Regular RTCP mode. It MAY also be used to send RTCP FB messages in Immediate Feedback or Early RTCP mode. This packet type is referred to as full compound RTCP packet in this document.

RTCP packets that do not contain FB messages are referred to as non-FB RTCP packets. Such packets MUST follow the format rules in [1].

Last updated on