RFC-4585ExtendedRTPProfileforRTCP-BasedFeedback(RTCP-FB资源-CSDN文库

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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.

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

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

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

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

8. Security Considerations ........................................42

9. IANA Considerations ............................................43

10. Acknowledgements ..............................................47

11. References ....................................................48

11.1. Normative References .....................................48

11.2. Informative References ...................................48

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

(QoS). However, except for a few payload-specific mechanisms [6],

RTP makes no provision for timely feedback that would allow a sender

to repair the media stream immediately: through retransmissions,

retroactive Forward Error Correction (FEC) control, or media-specific

mechanisms for some video codecs, such as reference picture

selection.

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.

This document specifies a modified RTP profile for audio and video

conferences with minimal control based upon [1] and [2] by means of

two modifications/additions: Firstly, to achieve timely feedback, the

concept of Early RTCP messages as well as algorithms allowing for

low-delay feedback in small multicast groups (and preventing feedback

implosion in large ones) are introduced. Special consideration is

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 [1] and the "RTP Profile for Audio and

Video Conferences with Minimal Control" [2] apply. In addition, the

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,

RTCP packets are transmitted according to the timing rules defined

in this document.

Early RTCP packet:

An Early RTCP packet is a packet which is transmitted earlier than

would be allowed if following the scheduling algorithm of [1], the

reason being an "event" observed by a receiver. Early RTCP

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

information about events observed at a receiver -- in addition to

long-term receiver status information that is carried in RTCP

receiver reports (RRs) -- back to the sender of the media stream.

For the sake of clarity, feedback message is referred to as FB

message throughout this document.

Feedback (FB) threshold:

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

on all senders and receivers agreeing on the same value for this

threshold. It is merely intended to provide conceptual guidance

to application designers and is not used in any calculations. For

the sake of clarity, the term feedback threshold is referred to as

FB threshold throughout this document.

Ott, et al. Standards Track [Page 4]

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