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

Network Working Group                                             J. Ott
Request for Comments: 4585             Helsinki University of Technology
Category: Standards Track                                      S. Wenger
                                                                   Nokia
                                                                 N. Sato
                                                                     Oki
                                                           C. Burmeister
                                                                  J. Rey
                                                              Matsushita
                                                               July 2006

                        Extended RTP Profile for
 Real-time Transport Control Protocol (RTCP)-Based Feedback (RTP/AVPF)

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   Real-time media streams that use RTP are, to some degree, resilient
   against packet losses.  Receivers may use the base mechanisms of the
   Real-time Transport Control Protocol (RTCP) to report packet
   reception statistics and thus allow a sender to adapt its
   transmission behavior in the mid-term.  This is the sole means for
   feedback and feedback-based error repair (besides a few codec-
   specific mechanisms).  This document defines an extension to the
   Audio-visual Profile (AVP) that enables receivers to provide,
   statistically, more immediate feedback to the senders and thus allows
   for short-term adaptation and efficient feedback-based repair
   mechanisms to be implemented.  This early feedback profile (AVPF)
   maintains the AVP bandwidth constraints for RTCP and preserves
   scalability to large groups.

Ott, et al.                 Standards Track                     [Page 1]
RFC 4585                        RTP/AVPF                       July 2006

Table of Contents

   1. Introduction ....................................................3
      1.1. Definitions ................................................3
      1.2. Terminology ................................................5
   2. RTP and RTCP Packet Formats and Protocol Behavior ...............6
      2.1. RTP ........................................................6
      2.2. Underlying Transport Protocols .............................6
   3. Rules for RTCP Feedback .........................................7
      3.1. Compound RTCP Feedback Packets .............................7
      3.2. Algorithm Outline ..........................................8
      3.3. Modes of Operation .........................................9
      3.4. Definitions and Algorithm Overview ........................11
      3.5. AVPF RTCP Scheduling Algorithm ............................14
           3.5.1. Initialization .....................................15
           3.5.2. Early Feedback Transmission ........................15
           3.5.3. Regular RTCP Transmission ..........................18
           3.5.4. Other Considerations ...............................19
      3.6. Considerations on the Group Size ..........................20
           3.6.1. ACK Mode ...........................................20
           3.6.2. NACK Mode ..........................................20
      3.7. Summary of Decision Steps .................................22
           3.7.1. General Hints ......................................22
           3.7.2. Media Session Attributes ...........................22
   4. SDP Definitions ................................................23
      4.1. Profile Identification ....................................23
      4.2. RTCP Feedback Capability Attribute ........................23
      4.3. RTCP Bandwidth Modifiers ..................................27
      4.4. Examples ..................................................27
   5. Interworking and Coexistence of AVP and AVPF Entities ..........29
   6. Format of RTCP Feedback Messages ...............................31
      6.1. Common Packet Format for Feedback Messages ................32
      6.2. Transport Layer Feedback Messages .........................34
           6.2.1. Generic NACK .......................................34
      6.3. Payload-Specific Feedback Messages ........................35
           6.3.1. Picture Loss Indication (PLI) ......................36
           6.3.2. Slice Loss Indication (SLI) ........................37

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