rfc4585-rtp协议补充的重要资料

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

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

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

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

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

6.3.3. Reference Picture Selection Indication (RPSI) ......39

6.4. Application Layer Feedback Messages .......................41

7. Early Feedback and Congestion Control ..........................41

Ott, et al. Standards Track [Page 2]

RFC 4585

RTP/AVPF July 2006

1. Introduction

Real-time media streams that use RTP are, to some degree, resilient

against packet losses. RTP [

1] provides all the necessary mechanisms

to restore ordering and timing present at the sender to properly

reproduce a media stream at a recipient. RTP also provides

continuous feedback about the overall reception quality from all

receivers -- thereby allowing the sender(s) in the mid-term (in the

order of several seconds to minutes) to adapt their coding scheme and

transmission behavior to the observed network quality of service

Current mechanisms available with RTP to improve error resilience

include audio redundancy coding [

13], video redundancy coding [14],

RTP-level FEC [

11], and general considerations on more robust media

streams transmission [

12]. These mechanisms may be applied

proactively (thereby increasing the bandwidth of a given media

stream). Alternatively, in sufficiently small groups with small

round-trip times (RTTs), the senders may perform repair on-demand,

using the above mechanisms and/or media-encoding-specific approaches.

Note that "small group" and "sufficiently small RTT" are both highly

application dependent.

given to point-to-point scenarios. Secondly, a small number of

general-purpose feedback messages as well as a format for codec- and

application-specific feedback information are defined for

transmission in the RTCP payloads.

1.1. Definitions

The definitions from RTP/RTCP [

Video Conferences with Minimal Control" [

following definitions are used in this document:

Ott, et al. Standards Track [Page 3]

RFC 4585

RTP/AVPF July 2006

Early RTCP mode:

The mode of operation in that a receiver of a media stream is

often (but not always) capable of reporting events of interest

back to the sender close to their occurrence. In Early RTCP mode,

packets may be sent in Immediate Feedback and in Early RTCP mode.

Sending an Early RTCP packet is also referred to as sending Early

Feedback in this document.

Event:

An observation made by the receiver of a media stream that is

(potentially) of interest to the sender -- such as a packet loss

or packet reception, frame loss, etc. -- and thus useful to be

reported back to the sender by means of a feedback message.

Feedback (FB) message:

An RTCP message as defined in this document is used to convey

The FB threshold indicates the transition between Immediate

Feedback and Early RTCP mode. For a multiparty scenario, the FB

threshold indicates the maximum group size at which, on average,

each receiver is able to report each event back to the sender(s)

immediately, i.e., by means of an Early RTCP packet without having

to wait for its regularly scheduled RTCP interval. This threshold

is highly dependent on the type of feedback to be provided,

network QoS (e.g., packet loss probability and distribution),

codec and packetization scheme in use, the session bandwidth, and

application requirements. Note that the algorithms do not depend

RFC 4585

RTP/AVPF July 2006

Immediate Feedback mode:

A mode of operation in which each receiver of a media stream is,

statistically, capable of reporting each event of interest

immediately back to the media stream sender. In Immediate

role and that the same RTP entity may at the same time act as an

1.2. Terminology