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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