Network Working Group A. Li
Request for Comments: 4756 Hyervision
Category: Standards Track November 2006
Network Working Group A. Li
Request for Comments: 4756 Hyervision
Category: Standards Track November 2006
Forward Error Correction Grouping Semantics in Session Description Protocol
会话描述协议中的前向纠错分组语义
Status of This Memo
关于下段备忘
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.
本文件规定了互联网社区的互联网标准跟踪协议，并要求进行讨论和提出改进建议。有关本协议的标准化状态和状态，请参考当前版本的“互联网官方协议标准”（STD 1）。本备忘录的分发不受限制。
Copyright Notice
版权公告
Copyright (C) The IETF Trust (2006).
版权所有（C）IETF信托基金（2006年）。
Abstract
摘要
This document defines the semantics that allow for grouping of Forward Error Correction (FEC) streams with the protected payload streams in Session Description Protocol (SDP). The semantics defined in this document are to be used with "Grouping of Media Lines in the Session Description Protocol" (RFC 3388) to group together "m" lines in the same session.
本文档定义了允许将前向纠错（FEC）流与会话描述协议（SDP）中受保护的有效负载流分组的语义。本文档中定义的语义将与“会话描述协议中的媒体行分组”（RFC 3388）一起使用，以将同一会话中的“m”行分组在一起。
Table of Contents
目录
1. Introduction ....................................................2
2. Terminology .....................................................2
3. Forward Error Correction (FEC) ..................................2
4. FEC Grouping ....................................................3
4.1. FEC Group ..................................................3
4.2. Offer / Answer Consideration ...............................3
4.3. Example of FEC Grouping ....................................3
5. Security Considerations .........................................4
6. IANA Considerations .............................................4
7. Acknowledgments .................................................5
8. References ......................................................5
8.1. Normative References .......................................5
8.2. Informative References .....................................5
1. Introduction ....................................................2
2. Terminology .....................................................2
3. Forward Error Correction (FEC) ..................................2
4. FEC Grouping ....................................................3
4.1. FEC Group ..................................................3
4.2. Offer / Answer Consideration ...............................3
4.3. Example of FEC Grouping ....................................3
5. Security Considerations .........................................4
6. IANA Considerations .............................................4
7. Acknowledgments .................................................5
8. References ......................................................5
8.1. Normative References .......................................5
8.2. Informative References .....................................5
The media lines in an SDP [3] session may be associated with each other in various ways. SDP itself does not provide methods to convey the relationships between the media lines. Such relationships are indicated by the extension to SDP as defined in "Grouping of Media Lines in the Session Description Protocol" (RFC 3388) [2]. RFC 3388 defines two types of semantics: Lip Synchronization and Flow Identification.
SDP[3]会话中的媒体线可以以各种方式彼此关联。SDP本身不提供传递媒体线路之间关系的方法。如“会话描述协议中的媒体线分组”（RFC 3388）[2]中所定义，SDP的扩展表明了这种关系。RFC3388定义了两种类型的语义：唇同步和流识别。
Forward Error Correction (FEC) is a common technique to achieve robust communication in error-prone environments. In this document, we define the semantics that allows for grouping of FEC streams with the protected payload streams in SDP by further extending RFC 3388.
前向纠错（FEC）是在容易出错的环境中实现健壮通信的常用技术。在本文中，我们通过进一步扩展RFC3388，定义了允许将FEC流与SDP中受保护的有效负载流分组的语义。
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD, "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [1].
本文件中的关键词“必须”、“不得”、“要求”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照RFC 2119[1]中所述进行解释。
Forward Error Correction (FEC) is a common technique to achieve robust communication in error-prone environments. In FEC, communication uses a bandwidth that is more than payload to send redundantly coded payload information. The receivers can readily recover the original payload even when some communication is lost in the transmission. Compared to other error correction techniques (such as retransmission), FEC can achieve much lower transmission delay, and it does not have the problem of implosion from retransmission requests in various multicast scenarios.
前向纠错（FEC）是在容易出错的环境中实现健壮通信的常用技术。在FEC中，通信使用超过有效载荷的带宽来发送冗余编码的有效载荷信息。即使在传输过程中某些通信丢失时，接收机也可以随时恢复原始有效载荷。与其他纠错技术（如重传）相比，FEC可以实现更低的传输延迟，并且在各种多播场景中不存在重传请求内爆的问题。
In general, the FEC data can be sent in two different ways: (1) multiplexed together with the original payload stream or (2) as a separate stream. It is thus necessary to define mechanisms to indicate the association relationship between the FEC data and the payload data they protect.
通常，FEC数据可以以两种不同的方式发送：（1）与原始有效负载流复用在一起，或者（2）作为单独的流。因此，有必要定义机制来指示FEC数据与其保护的有效载荷数据之间的关联关系。
When FEC data are multiplexed with the original payload stream, the association relationship may, for example, be indicated as specified in "An RTP Payload for Redundant Audio Data" (RFC 2198) [4]. The generic RTP payload format for FEC [5] uses that method.
当FEC数据与原始有效负载流复用时，例如，关联关系可以如“冗余音频数据的RTP有效负载”（RFC 2198）[4]中所指定的那样指示。FEC[5]的通用RTP有效负载格式使用该方法。
When FEC data are sent as a separate stream from the payload data, the association relationship can be indicated in various ways. This document on the FEC media line grouping specifies a mechanism for indicating such relationships.
当FEC数据作为独立于有效载荷数据的流发送时，可以以各种方式指示关联关系。关于FEC媒体行分组的本文档指定了一种指示此类关系的机制。
Each "a=group" line is used to indicate an association relationship between the FEC streams and the payload streams. The streams included in one "a=group" line are called a "FEC Group".
每个“a＝组”行用于指示FEC流和有效负载流之间的关联关系。包括在一个“a＝组”行中的流被称为“FEC组”。
Each FEC group MAY have one or more than one FEC stream, and one or more than one payload stream. For example, it is possible to have one payload stream protected by more than one FEC stream , or multiple payload streams sharing one FEC stream.
每个FEC组可以具有一个或多个FEC流，以及一个或多个有效负载流。例如，可以使一个有效负载流受到多个FEC流的保护，或者多个有效负载流共享一个FEC流。
Grouping streams in a FEC group only indicates the association relationship between streams. The detailed FEC protection scheme/parameters are conveyed through the mechanism of the particular FEC algorithm used. For example, the FEC grouping is used for generic RTP payload for FEC [5] to indicate the association relationship between the FEC stream and the payload stream. The detailed protection level and length information for the Unequal Loss Protection (ULP) algorithm is communicated in band within the FEC stream.
FEC组中的分组流仅表示流之间的关联关系。详细的FEC保护方案/参数通过所使用的特定FEC算法的机制传达。例如，FEC分组用于FEC的通用RTP有效载荷[5]，以指示FEC流和有效载荷流之间的关联关系。不平等丢失保护（ULP）算法的详细保护级别和长度信息在FEC流内的频带内进行通信。
The backward compatibility in offer / answer is generally handled as specified in RFC 3388 [2].
报价/应答中的向后兼容性通常按照RFC 3388[2]中的规定处理。
Depending on the implementation, a node that does not understand FEC grouping (either does not understand line grouping at all, or just does not understand the FEC semantics) SHOULD respond to an offer containing FEC grouping either (1) with an answer that ignores the grouping attribute or (2) with a refusal to the request (e.g., 488 Not acceptable here or 606 Not acceptable in SIP).
根据具体实现，不理解FEC分组（或者根本不理解行分组，或者只是不理解FEC语义）的节点应该响应包含FEC分组的提议（1）回答忽略分组属性，或者（2）拒绝请求（例如，此处不接受488或SIP中不接受606）。
In the first case, the original sender of the offer MUST establish the connection without FEC. In the second case, if the sender of the offer still wishes to establish the session, it SHOULD re-try the request with an offer without FEC.
在第一种情况下，要约的原始发送者必须在没有FEC的情况下建立连接。在第二种情况下，如果要约的发送方仍希望建立会话，则应使用不带FEC的要约重新尝试请求。
The following example shows a session description of a multicast conference. The first media stream (mid:1) contains the audio stream. The second media stream (mid:2) contains the Generic FEC [5] protection for the audio stream. These two streams form an FEC group. The relationship between the two streams is indicated by the "a=group:FEC 1 2" line. The FEC stream is sent to the same multicast
以下示例显示了多播会议的会话描述。第一个媒体流（mid:1）包含音频流。第二媒体流（mid:2）包含音频流的通用FEC[5]保护。这两个流形成一个FEC组。两个流之间的关系由“a=组：FEC 1 2”行表示。FEC流被发送到同一个多播
group and has the same Time to Live (TTL) as the audio, but on a port number two higher. Likewise, the video stream (mid:3) and its Generic FEC protection stream (mid:4) form another FEC group. The relationship between the two streams is indicated by the "a=group:FEC 3 4" line. The FEC stream is sent to a different multicast address, but has the same port number (30004) as the payload video stream.
组和具有与音频相同的生存时间（TTL），但端口号更高。类似地，视频流（mid:3）及其通用FEC保护流（mid:4）形成另一FEC组。两个流之间的关系由“a=组：FEC 3 4”行表示。FEC流被发送到不同的多播地址，但具有与有效负载视频流相同的端口号（30004）。
v=0
o=adam 289083124 289083124 IN IP4 host.example.com
s=ULP FEC Seminar
t=0 0
c=IN IP4 224.2.17.12/127
a=group:FEC 1 2
a=group:FEC 3 4
m=audio 30000 RTP/AVP 0
a=mid:1
m=audio 30002 RTP/AVP 100
a=rtpmap:100 ulpfec/8000
a=mid:2
m=video 30004 RTP/AVP 31
a=mid:3
m=video 30004 RTP/AVP 101
c=IN IP4 224.2.17.13/127
a=rtpmap:101 ulpfec/8000
a=mid:4
v=0
o=adam 289083124 289083124 IN IP4 host.example.com
s=ULP FEC Seminar
t=0 0
c=IN IP4 224.2.17.12/127
a=group:FEC 1 2
a=group:FEC 3 4
m=audio 30000 RTP/AVP 0
a=mid:1
m=audio 30002 RTP/AVP 100
a=rtpmap:100 ulpfec/8000
a=mid:2
m=video 30004 RTP/AVP 31
a=mid:3
m=video 30004 RTP/AVP 101
c=IN IP4 224.2.17.13/127
a=rtpmap:101 ulpfec/8000
a=mid:4
There is a weak threat for the receiver that the FEC grouping can be modified to indicate FEC relationships that do not exist. Such attacks may result in failure of FEC to protect, and/or mishandling of other media payload streams. It is recommended that the receiver SHOULD do integrity check on SDP and follow the security considerations of SDP [3] to only trust SDP from trusted sources.
对于接收方来说，FEC分组可能会被修改以指示不存在的FEC关系的威胁很小。此类攻击可能导致FEC无法保护和/或错误处理其他媒体有效负载流。建议接收方对SDP进行完整性检查，并遵循SDP[3]的安全注意事项，仅信任来自受信任来源的SDP。
This document defines the semantics to be used with grouping of media lines in SDP as defined in RFC 3388. The semantics defined in this document are to be registered by the IANA when they are published in standards track RFCs.
本文档定义了与RFC 3388中定义的SDP中的媒体行分组一起使用的语义。本文档中定义的语义将在标准跟踪RFCs中发布时由IANA注册。
The following semantics have been registered by IANA in Semantics for the "group" SDP Attribute under SDP Parameters.
IANA已在SDP参数下的“group”SDP属性的语义中注册了以下语义。
Semantics Token Reference
------------------------ ----- ----------
Forward Error Correction FEC RFC 4756
Semantics Token Reference
------------------------ ----- ----------
Forward Error Correction FEC RFC 4756
The author would like to thank Magnus Westerlund, Colin Perkins, Joerg Ott, and Cullen Jennings for their feedback on this document.
作者要感谢Magnus Westerlund、Colin Perkins、Joerg Ott和Cullen Jennings对本文件的反馈。
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[1] Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3月。
[2] Camarillo, G., Eriksson, G., Holler, J., and H. Schulzrinne, "Grouping of Media Lines in the Session Description Protocol (SDP)", RFC 3388, December 2002.
[2] Camarillo，G.，Eriksson，G.，Holler，J.，和H.Schulzrinne，“会话描述协议（SDP）中媒体线路的分组”，RFC 3388，2002年12月。
[3] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.
[3] Handley，M.，Jacobson，V.，和C.Perkins，“SDP：会话描述协议”，RFC4566，2006年7月。
[4] Perkins, C., Kouvelas, I., Hodson, O., Hardman, V., Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-Parisis, "RTP Payload for Redundant Audio Data", RFC 2198, September 1997.
[4] 帕金斯，C.，库维拉斯，I.，霍德森，O.，哈德曼，V.，汉德利，M.，博洛特，J.，维加·加西亚，A.，和S.福斯·帕里斯，“冗余音频数据的RTP有效载荷”，RFC 21981997年9月。
[5] Li, A., "An RFC Payload Format for Generic FEC", Work in Progress.
[5] Li，A.，“通用FEC的RFC有效载荷格式”，正在进行中。
Author's Address
作者地址
Adam H. Li HyerVision 10194 Wateridge Circle #152 San Diego, CA 92121 U.S.A.
Adam H.Li HyerVision 10194 Wateridge Circle#152美国加利福尼亚州圣地亚哥92121。
Tel: +1 858 622 9038
EMail: adamli@hyervision.com
Tel: +1 858 622 9038
EMail: adamli@hyervision.com
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