5. Interworking and Coexistence of AVP and AVPF Entities

The AVPF profile defined in this document is an extension of the AVP profile as defined in [2]. Both profiles follow the same basic rules (including the upper bandwidth limit for RTCP and the bandwidth assignments to senders and receivers). Therefore, senders and receivers using either of the two profiles can be mixed in a single session (see Example 3 in Section 4.4).

AVP and AVPF are defined in a way that, from a robustness point of view, the RTP entities do not need to be aware of entities of the respective other profile: they will not disturb each other's functioning. However, the quality of the media presented may suffer.

The following considerations apply to senders and receivers when used in a combined session.

• AVP entities (senders and receivers)

  AVP senders will receive RTCP feedback packets from AVPF receivers and ignore these packets. They will see occasional closer spacing of RTCP messages (e.g., violating the five-second rule) by AVPF entities. As the overall bandwidth constraints are adhered to by both types of entities, they will still get their share of the RTCP bandwidth. However, while AVP entities are bound by the five-second rule, depending on the group size and session bandwidth, AVPF entities may provide more frequent RTCP reports than AVP ones will. Also, the overall reporting may decrease slightly as AVPF entities may send bigger compound RTCP packets (due to the extra RTCP packets).

  If T_rr_interval is used as lower bound between Regular RTCP packets, T_rr_interval is sufficiently large (e.g., T_rr_interval > M*Td as per Section 6.3.5 of [1]), and no Early RTCP packets are sent by AVPF entities, AVP entities may accidentally time out those AVPF group members and hence underestimate the group size. Therefore, if AVP entities may be involved in a media session, T_rr_interval SHOULD NOT be larger than five seconds.

• AVPF entities (senders and receivers)

  If the dynamically calculated T_rr is sufficiently small (e.g., less than one second), AVPF entities may accidentally time out AVP group members and hence underestimate the group size. Therefore, if AVP entities may be involved in a media session, T_rr_interval SHOULD be used and SHOULD be set to five seconds.

  In conclusion, if AVP entities may be involved in a media session and T_rr_interval is to be used, T_rr_interval SHOULD be set to five seconds.

• AVPF senders

  AVPF senders will receive feedback information only from AVPF receivers. If they rely on feedback to provide the target media quality, the quality achieved for AVP receivers may be suboptimal.

• AVPF receivers

  AVPF receivers SHOULD send Early RTCP feedback packets only if all sending entities in the media session support AVPF. AVPF receivers MAY send feedback information as part of regularly scheduled compound RTCP packets following the timing rules of [1] and [2] also in media sessions operating in mixed mode. However, the receiver providing feedback MUST NOT rely on the sender reacting to the feedback at all.