Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 6683 Cisco
Category: Informational T. Stockhammer
ISSN: 2070-1721 Nomor Research
August 2012
Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 6683 Cisco
Category: Informational T. Stockhammer
ISSN: 2070-1721 Nomor Research
August 2012
Guidelines for Implementing Digital Video Broadcasting - IPTV (DVB-IPTV) Application-Layer Hybrid Forward Error Correction (FEC) Protection
数字视频广播实施指南.IPTV（DVB-IPTV）应用层混合前向纠错（FEC）保护
Abstract
摘要
Annex E of the Digital Video Broadcasting - IPTV (DVB-IPTV) technical specification defines an optional Application-Layer Forward Error Correction (AL-FEC) protocol to protect the streaming media transported using RTP. The DVB-IPTV AL-FEC protocol uses two layers for FEC protection. The first (base) layer is based on the 1-D interleaved parity code. The second (enhancement) layer is based on the Raptor code. By offering a layered approach, the DVB-IPTV AL-FEC protocol offers good protection against both bursty and random packet losses at a cost of decent complexity. This document describes how one can implement the DVB-IPTV AL-FEC protocol by using the 1-D interleaved parity code and Raptor code that have already been specified in separate documents.
数字视频广播-IPTV（DVB-IPTV）技术规范的附录E定义了可选的应用层前向纠错（AL-FEC）协议，以保护使用RTP传输的流媒体。DVB-IPTV AL-FEC协议使用两层进行FEC保护。第一（基本）层基于1-D交错奇偶校验码。第二层（增强）基于Raptor代码。通过提供分层方法，DVB-IPTV AL-FEC协议以相当复杂的代价提供了针对突发性和随机数据包丢失的良好保护。本文档描述了如何通过使用已经在单独文档中指定的1-D交错奇偶校验码和Raptor码来实现DVB-IPTV AL-FEC协议。
Status of This Memo
关于下段备忘
This document is not an Internet Standards Track specification; it is published for informational purposes.
本文件不是互联网标准跟踪规范；它是为了提供信息而发布的。
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
本文件是互联网工程任务组（IETF）的产品。它代表了IETF社区的共识。它已经接受了公众审查，并已被互联网工程指导小组（IESG）批准出版。并非IESG批准的所有文件都适用于任何级别的互联网标准；见RFC 5741第2节。
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6683.
有关本文件当前状态、任何勘误表以及如何提供反馈的信息，请访问http://www.rfc-editor.org/info/rfc6683.
Copyright Notice
版权公告
Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2012 IETF信托基金和确定为文件作者的人员。版权所有。
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
本文件受BCP 78和IETF信托有关IETF文件的法律规定的约束(http://trustee.ietf.org/license-info)自本文件出版之日起生效。请仔细阅读这些文件，因为它们描述了您对本文件的权利和限制。从本文件中提取的代码组件必须包括信托法律条款第4.e节中所述的简化BSD许可证文本，并提供简化BSD许可证中所述的无担保。
Table of Contents
目录
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. DVB-IPTV AL-FEC Specification . . . . . . . . . . . . . . . . 5
2.1. Base-Layer FEC . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Enhancement-Layer FEC . . . . . . . . . . . . . . . . . . 7
2.3. Hybrid Decoding Procedures . . . . . . . . . . . . . . . . 7
3. Session Description Protocol (SDP) Signaling . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . . 10
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. DVB-IPTV AL-FEC Specification . . . . . . . . . . . . . . . . 5
2.1. Base-Layer FEC . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Enhancement-Layer FEC . . . . . . . . . . . . . . . . . . 7
2.3. Hybrid Decoding Procedures . . . . . . . . . . . . . . . . 7
3. Session Description Protocol (SDP) Signaling . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
6.1. Normative References . . . . . . . . . . . . . . . . . . . 10
6.2. Informative References . . . . . . . . . . . . . . . . . . 10
In 2007, the IP Infrastructure (IPI) Technical Module (TM) of the Digital Video Broadcasting (DVB) consortium published a technical specification [ETSI-TS-102-034v1.3.1] through the European Telecommunications Standards Institute (ETSI). [ETSI-TS-102-034v1.3.1] covers several areas related to the transmission of MPEG2 transport stream-based services over IP networks.
2007年，数字视频广播（DVB）联盟的IP基础设施（IPI）技术模块（TM）通过欧洲电信标准协会（ETSI）发布了一份技术规范[ETSI-TS-102-034v1.3.1]。[ETSI-TS-102-034v1.3.1]涵盖了与通过IP网络传输基于MPEG2传输流的服务相关的几个领域。
Annex E of [ETSI-TS-102-034v1.3.1] defines an optional protocol for Application-Layer Forward Error Correction (AL-FEC) to protect the streaming media for DVB-IP services transported using RTP [RFC3550]. In 2009, DVB updated the specification in a new revision that is available as [ETSI-TS-102-034v1.4.1]. Among others, some updates and modifications to the AL-FEC protocol have been made. This document describes how one can implement the DVB-IPTV AL-FEC protocol by using the 1-D interleaved parity code [RFC6015] and Raptor code specifications [RFC6681] [RFC6682].
[ETSI-TS-102-034v1.3.1]的附录E定义了应用层前向纠错（AL-FEC）的可选协议，以保护使用RTP[RFC3550]传输的DVB-IP服务的流媒体。2009年，DVB在一个新版本中更新了规范，该版本作为[ETSI-TS-102-034v1.4.1]提供。其中，对AL-FEC协议进行了一些更新和修改。本文档描述了如何使用1-D交错奇偶校验码[RFC6015]和Raptor码规范[RFC6681][RFC6682]实现DVB-IPTV AL-FEC协议。
The DVB-IPTV AL-FEC protocol uses two layers for protection: a base layer that is produced by the 1-D interleaved parity code (also simply referred to as "parity code" in the remainder of this document), and an enhancement layer that is produced by the Raptor code. Whenever a receiver supports the DVB-IPTV AL-FEC protocol, the decoding support for the base-layer FEC is mandatory while the decoding support for the enhancement-layer FEC is optional. Both the interleaved parity code and the Raptor code are systematic FEC codes, meaning that source packets are not modified in any way during the FEC encoding process.
DVB-IPTV AL-FEC协议使用两层进行保护：由1-D交错奇偶校验码产生的基本层（在本文档的其余部分中也称为“奇偶校验码”），以及由Raptor码产生的增强层。每当接收机支持DVB-IPTV AL-FEC协议时，基本层FEC的解码支持是强制性的，而增强层FEC的解码支持是可选的。交织奇偶校验码和Raptor码都是系统的FEC码，这意味着在FEC编码过程中，源分组不会以任何方式被修改。
The DVB-IPTV AL-FEC protocol considers protection of single-sequence source RTP flows only. In the AL-FEC protocol, the source stream can only be an MPEG-2 transport stream. The FEC data at each layer are generated based on some configuration information, which also determines the exact associations and relationships between the source and repair packets. This document shows how this configuration may be communicated out-of-band in the Session Description Protocol (SDP) [RFC4566].
DVB-IPTV AL-FEC协议仅考虑单序列源RTP流的保护。在AL-FEC协议中，源流只能是MPEG-2传输流。每一层的FEC数据是基于一些配置信息生成的，这些配置信息还确定了源数据包和修复数据包之间的确切关联和关系。本文档说明了如何在会话描述协议（SDP）[RFC4566]中进行带外通信。
In DVB-IPTV AL-FEC, the source packets are carried in the source RTP stream and the generated FEC repair packets at each layer are carried in separate streams. At the receiver side, if all of the source packets are successfully received, there is no need for FEC recovery and the repair packets may be discarded. However, if there are missing source packets, the repair packets can be used to recover the missing information.
在DVB-IPTV AL-FEC中，源分组在源RTP流中承载，并且在每一层生成的FEC修复分组在单独的流中承载。在接收机侧，如果成功接收到所有源分组，则不需要FEC恢复，并且可以丢弃修复分组。然而，如果存在丢失的源数据包，则可以使用修复数据包来恢复丢失的信息。
The block diagram of the encoder side for the systematic DVB-IPTV AL-FEC protection is described in Figure 1. Here, the source packets are fed into the parity encoder to produce the parity repair packets. The source packets may also be fed to the Raptor encoder to produce the Raptor repair packets. Source packets as well as the repair packets are then sent to the receiver(s) over an IP network.
系统DVB-IPTV AL-FEC保护的编码器侧框图如图1所示。这里，源分组被馈送到奇偶校验编码器以产生奇偶校验修复分组。源分组还可以馈送到猛禽编码器以产生猛禽修复分组。然后，通过IP网络将源分组以及修复分组发送到接收机。
+--------------+
+--+ +--+ +--+ +--+ --> | Systematic | -> +--+ +--+ +--+ +--+
+--+ +--+ +--+ +--+ |FEC Protection| +--+ +--+ +--+ +--+
+--------------+
| Parity | -> +==+ +==+ +==+
| Encoder | +==+ +==+ +==+
+--------------+
| Raptor | -> +~~+ +~~+
| Encoder | +~~+ +~~+
+--------------+
+--------------+
+--+ +--+ +--+ +--+ --> | Systematic | -> +--+ +--+ +--+ +--+
+--+ +--+ +--+ +--+ |FEC Protection| +--+ +--+ +--+ +--+
+--------------+
| Parity | -> +==+ +==+ +==+
| Encoder | +==+ +==+ +==+
+--------------+
| Raptor | -> +~~+ +~~+
| Encoder | +~~+ +~~+
+--------------+
Source Packet: +--+
+--+
Source Packet: +--+
+--+
Base-layer Repair Packet: +==+
+==+
Base-layer Repair Packet: +==+
+==+
Enhancement-layer Repair Packet: +~~+
+~~+
Enhancement-layer Repair Packet: +~~+
+~~+
Figure 1: Block Diagram for the DVB-IPTV AL-FEC Encoder
图1:DVB-IPTV AL-FEC编码器的框图
The block diagram of the decoder side for the systematic DVB-IPTV AL-FEC protection is described in Figure 2. This is a minimum performance decoder since the receiver only supports decoding the base-layer repair packets. If there is a loss among the source packets, the parity decoder attempts to recover the missing source packets by using the base-layer repair packets.
图2中描述了系统DVB-IPTV AL-FEC保护的解码器侧的框图。这是性能最低的解码器，因为接收机仅支持解码基本层修复包。如果源分组之间存在丢失，奇偶校验解码器尝试通过使用基本层修复分组来恢复丢失的源分组。
+--------------+
+--+ X X +--+ --> | Systematic | -> +--+ +--+ +--+ +--+
+--+ +--+ |FEC Protection| +--+ +--+ +--+ +--+
+--------------+
+==+ +==+ +==+ --> | Parity |
+==+ +==+ +==+ | Decoder |
+--------------+
+--------------+
+--+ X X +--+ --> | Systematic | -> +--+ +--+ +--+ +--+
+--+ +--+ |FEC Protection| +--+ +--+ +--+ +--+
+--------------+
+==+ +==+ +==+ --> | Parity |
+==+ +==+ +==+ | Decoder |
+--------------+
Lost Packet: X
丢失的数据包：X
Figure 2: Block Diagram for the DVB-IPTV AL-FEC Minimum Performance Decoder
图2:DVB-IPTV AL-FEC最低性能解码器的框图
On the other hand, if the receiver supports decoding both the base-layer and enhancement-layer repair packets, a combined (hybrid) decoding approach is employed to improve the recovery rate of the lost packets. In this case, the decoder is called an enhanced decoder. Section 2.3 outlines the procedures for hybrid decoding.
另一方面，如果接收机支持解码基本层和增强层修复分组，则采用组合（混合）解码方法来提高丢失分组的恢复率。在这种情况下，解码器被称为增强型解码器。第2.3节概述了混合解码的程序。
+--------------+
+--+ X X X --> | Systematic | -> +--+ +--+ +--+ +--+
+--+ |FEC Protection| +--+ +--+ +--+ +--+
+--------------+
+==+ +==+ +==+ --> | Parity |
+==+ +==+ +==+ | Decoder |
+--------------+
+~~+ +~~+ --> | Raptor |
+~~+ +~~+ | Decoder |
+--------------+
+--------------+
+--+ X X X --> | Systematic | -> +--+ +--+ +--+ +--+
+--+ |FEC Protection| +--+ +--+ +--+ +--+
+--------------+
+==+ +==+ +==+ --> | Parity |
+==+ +==+ +==+ | Decoder |
+--------------+
+~~+ +~~+ --> | Raptor |
+~~+ +~~+ | Decoder |
+--------------+
Lost Packet: X
丢失的数据包：X
Figure 3: Block Diagram for the DVB-IPTV AL-FEC Enhanced Decoder
图3:DVB-IPTV AL-FEC增强解码器的框图
The DVB-IPTV AL-FEC protocol comprises two layers of FEC protection: 1-D interleaved parity FEC for the base layer and Raptor FEC for the enhancement layer. The performance of these FEC codes has been examined in detail in [DVB-A115].
DVB-IPTV AL-FEC协议包括两层FEC保护：用于基本层的一维交错奇偶校验FEC和用于增强层的Raptor FEC。[DVB-A115]中详细研究了这些FEC码的性能。
The 1-D interleaved parity FEC uses the exclusive OR (XOR) operation to generate the repair symbols. In a group of D x L source packets, the XOR operation is applied to each group of D source packets whose sequence numbers are L apart from each other to generate a total of L repair packets. Due to interleaving, this FEC is effective against bursty packet losses up to burst sizes of length L.
1-D交错奇偶校验FEC使用异或（XOR）操作来生成修复符号。在一组D×L源分组中，对其序列号彼此相隔L的每组D源分组应用异或操作以生成总共L个修复分组。由于交织，该FEC可有效防止长度为L的突发数据包丢失。
The DVB-IPTV AL-FEC protocol requires that the D x L block of the source packets protected by the 1-D interleaved FEC code be wholly contained within a single source block of the Raptor code, if both FEC layers are used.
DVB-IPTV AL-FEC协议要求，如果使用两个FEC层，则由1-D交错FEC代码保护的源分组的D x L块全部包含在Raptor代码的单个源块中。
Originally, the DVB-IPTV AL-FEC protocol had adopted the 1-D interleaved FEC payload format from [SMPTE2022-1] in [ETSI-TS-102-034v1.3.1]. However, some incompatibilities with RTP [RFC3550] have been discovered in this specification. These issues have all been addressed in [RFC6015] (for details, refer to Section 1 of [RFC6015]). Some of the changes required by [RFC6015] are, however, not backward compatible with the existing implementations that were based on [SMPTE2022-1].
最初，DVB-IPTV AL-FEC协议采用了[ETSI-TS-102-034v1.3.1]中[SMPTE2022-1]中的一维交错FEC有效载荷格式。但是，在本规范中发现了与RTP[RFC3550]的一些不兼容之处。这些问题均已在[RFC6015]中解决（详情请参阅[RFC6015]第1节）。然而，[RFC6015]要求的一些更改与基于[SMPTE2022-1]的现有实现不向后兼容。
In a recent liaison statement from the IETF AVT WG to DVB TM-IPI, it has been recommended that DVB TM-IPI define a new RTP profile for the
在IETF AVT工作组最近与DVB TM-IPI的联络声明中，建议DVB TM-IPI为项目定义一个新的RTP配置文件
AL-FEC protocol since in the new profile, several of the issues could easily be addressed without jeopardizing the compliance to RTP [RFC3550].
AL-FEC协议，因为在新的配置文件中，一些问题可以很容易地解决，而不会影响RTP的合规性[RFC3550]。
At the writing of this document, it was not clear whether or not a new RTP profile would be defined for the AL-FEC protocol. DVB TM-IPI attempted to address some of the issues in the updated specification [ETSI-TS-102-034v1.4.1]; however, there are still outstanding issues.
在编写本文件时，不清楚是否会为AL-FEC协议定义新的RTP配置文件。DVB TM-IPI试图解决更新规范[ETSI-TS-102-034v1.4.1]中的一些问题；然而，仍然存在一些悬而未决的问题。
The following is a list of the exceptions that need to be considered by an implementation adopting [RFC6015] to be compliant with the DVB-IPTV AL-FEC protocol as specified in [ETSI-TS-102-034v1.4.1].
以下是采用[RFC6015]以符合[ETSI-TS-102-034v1.4.1]中规定的DVB-IPTV AL-FEC协议的实施需要考虑的例外情况列表。
o SSRC (synchronization source) The DVB-IPTV AL-FEC protocol requires that the SSRC fields of the FEC packets be set to zero.
o SSRC（同步源）DVB-IPTV AL-FEC协议要求FEC分组的SSRC字段设置为零。
This requirement conflicts with RTP [RFC3550]. Unless signaled otherwise, RTP uses random SSRC values with collision detection. An explicit SSRC signaling mechanism is currently defined in [RFC5576] and can be used for this purpose.
此要求与RTP[RFC3550]冲突。除非另有指示，RTP使用随机SSRC值进行碰撞检测。[RFC5576]中目前定义了明确的SSRC信号机制，可用于此目的。
o CSRC (contributing source) The DVB-IPTV AL-FEC protocol does not support the protection of the CSRC entries in the source packets. Thus, in an implementation compliant to DVB-IPTV AL-FEC protocol, the source stream must not have any CSRC entries in its packets, and subsequently the CC fields of the source RTP packets will be zero.
o CSC（贡献源）DVB-IPTV AL-FEC协议不支持保护源数据包中的CSC条目。因此，在符合DVB-IPTV AL-FEC协议的实现中，源流的分组中不得有任何csc条目，并且随后源RTP分组的CC字段将为零。
Note that if there are no RTP mixers used in a system running the DVB-IPTV AL-FEC protocol, the CC field of the source RTP packets will be zero and this is no longer an issue. Thus, if defined, the new RTP profile for the DVB-IPTV AL-FEC protocol should forbid the use of any RTP mixers.
注意，如果在运行DVB-IPTV AL-FEC协议的系统中没有使用RTP混频器，则源RTP分组的CC字段将为零，这不再是问题。因此，如果定义，DVB-IPTV AL-FEC协议的新RTP配置文件应禁止使用任何RTP混频器。
o Timestamp In the DVB-IPTV AL-FEC protocol, the timestamp fields of the FEC packets are ignored by the receivers.
o 时间戳在DVB-IPTV AL-FEC协议中，接收机忽略FEC分组的时间戳字段。
o Payload Type The DVB-IPTV AL-FEC protocol sets the PT fields of the FEC packets to 96.
o 有效负载类型DVB-IPTV AL-FEC协议将FEC分组的PT字段设置为96。
A static payload type assignment for the base-layer FEC packets is outside the scope of [RFC6015]. If defined, the new RTP profile for the DVB-IPTV AL-FEC protocol may assign 96 as the payload type for the base-layer FEC packets.
基本层FEC数据包的静态有效负载类型分配不在[RFC6015]的范围内。如果定义了，则DVB-IPTV AL-FEC协议的新RTP简档可以将96分配为基本层FEC分组的有效负载类型。
In implementations that are based on [RFC6015] and are willing to be compliant with the DVB-IPTV AL-FEC protocol as specified in [ETSI-TS-102-034v1.3.1], all these exceptions must be considered as well; however, in this case, the sender does not have to select a random initial sequence number for the FEC stream as suggested by [RFC3550].
在基于[RFC6015]并愿意遵守[ETSI-TS-102-034v1.3.1]中规定的DVB-IPTV AL-FEC协议的实施中，也必须考虑所有这些例外情况；然而，在这种情况下，发送方不必如[RFC3550]所建议的那样为FEC流选择随机初始序列号。
Note that neither [ETSI-TS-102-034v1.3.1] nor [ETSI-TS-102-034v1.4.1] implements the 1-D interleaved parity code as specified in [RFC6015]. Thus, the payload format registered in [RFC6015] must not be used by the implementations that are compliant with the [ETSI-TS-102-034v1.3.1] or [ETSI-TS-102-034v1.4.1] specification.
请注意，[ETSI-TS-102-034v1.3.1]和[ETSI-TS-102-034v1.4.1]均未实现[RFC6015]中规定的一维交错奇偶校验码。因此，符合[ETSI-TS-102-034v1.3.1]或[ETSI-TS-102-034v1.4.1]规范的实施不得使用[RFC6015]中注册的有效负载格式。
The Raptor code is a fountain code where as many encoding symbols as needed can be generated by the encoder on-the-fly from source data. Due to the fountain property of the Raptor code, multiple enhancement layers may also be specified, if needed.
Raptor码是一种喷泉码，编码器可以根据需要从源数据动态生成尽可能多的编码符号。由于Raptor代码的喷泉特性，如果需要，还可以指定多个增强层。
The details of the Raptor code are provided in [RFC6681]. The FEC scheme for the enhancement layer SHALL be the Raptor FEC scheme for a Single Sequenced Flow with FEC encoding ID 5. The RTP payload format for Raptor FEC is specified in [RFC6682].
猛禽代码的详细信息见[RFC6681]。增强层的FEC方案应为单个序列流的Raptor FEC方案，FEC编码ID为5。[RFC6682]中规定了猛禽FEC的RTP有效载荷格式。
It is important to note that the DVB-IPTV AL-FEC protocol in the latest specification [ETSI-TS-102-034v1.4.1] allows both UDP-only and RTP-over-UDP encapsulations for the enhancement-layer FEC stream. The initial specification [ETSI-TS-102-034v1.3.1] exclusively permits UDP-only encapsulation for the enhancement-layer FEC stream.
需要注意的是，最新规范[ETSI-TS-102-034v1.4.1]中的DVB-IPTV AL-FEC协议允许对增强层FEC流进行仅UDP和RTP-over-UDP封装。初始规范[ETSI-TS-102-034v1.3.1]只允许对增强层FEC流进行UDP封装。
When SDP is used for signaling, the transport protocol identifier indicates whether an RTP-over-UDP or UDP-only encapsulation is used. In case of any other signaling framework, the differentiation of the protocol for the enhancement-layer stream is achieved either explicitly through a protocol identifier or implicitly by the version number of the DVB IPTV Handbook. If none of the above signaling is provided, the receiver shall concur from the packet size of the repair packets if RTP-over-UDP or UDP-only encapsulation is used.
当SDP用于信令时，传输协议标识符指示是使用UDP上的RTP还是仅使用UDP的封装。在任何其他信令框架的情况下，用于增强层流的协议的区别要么通过协议标识符显式地实现，要么通过DVB IPTV手册的版本号隐式地实现。如果未提供上述信令，则如果使用UDP上的RTP或仅UDP封装，则接收器应同意修复数据包的数据包大小。
The receivers that support receiving and decoding both the base- and enhancement-layer FEC perform hybrid decoding to improve the repair performance. The following steps may be followed to perform hybrid decoding:
支持同时接收和解码基本层和增强层FEC的接收机执行混合解码以改进修复性能。可以遵循以下步骤来执行混合解码：
1. Base-layer (Parity) Decoding: In this step, the repair packets that are encoded by the parity encoder are processed as usual to repair as many missing source packets as possible.
1. 基本层（奇偶校验）解码：在这一步中，奇偶校验编码器编码的修复数据包按常规处理，以修复尽可能多的丢失源数据包。
2. Enhancement-layer (Raptor) Decoding: If there are still missing source packets after the first step, the repair packets that are Raptor encoded are processed with the source packets already received and the source packets that are recovered in the first step.
2. 增强层（Raptor）解码：如果在第一步之后仍然存在丢失的源数据包，则使用已经接收到的源数据包和在第一步中恢复的源数据包处理Raptor编码的修复数据包。
3. Hybrid Decoding: If there are still missing source packets after the second step, the unprocessed base-layer (parity) repair packets are converted to a form in which they can be added to the Raptor decoding process. With this additional information, Raptor decoding may potentially recover any remaining missing source packet.
3. 混合解码：如果在第二步后仍有丢失的源数据包，则未处理的基本层（奇偶校验）修复数据包将转换为一种形式，可以将其添加到Raptor解码过程中。有了这些附加信息，Raptor解码可能会恢复任何剩余的丢失源数据包。
The procedure that should be followed to benefit from the base-layer repair packets in the Raptor decoding process is explained in detail in Annex E.5.2 of [ETSI-TS-102-034v1.4.1].
[ETSI-TS-102-034v1.4.1]的附录E.5.2详细说明了在Raptor解码过程中受益于基层修复包应遵循的程序。
This section provides an SDP [RFC4566] example for [ETSI-TS-102-034v1.4.1]. The example uses the FEC grouping semantics [RFC5956].
本节提供了[ETSI-TS-102-034v1.4.1]的SDP[RFC4566]示例。该示例使用FEC分组语义[RFC5956]。
In the example, we have one source video stream (mid:S1), one FEC repair stream (mid:R1) that is produced by the 1-D interleaved parity FEC code, as well as another FEC repair stream (mid:R2) that is produced by the Raptor FEC code. We form one FEC group with the "a=group:FEC-FR S1 R1 R2" line. The source and repair streams are sent to the same port on different multicast groups. The source, base-layer FEC, and enhancement-layer FEC streams are all encapsulated in RTP.
在该示例中，我们有一个源视频流（mid:S1）、一个由1-D交错奇偶校验FEC代码生成的FEC修复流（mid:R1）以及另一个由Raptor FEC代码生成的FEC修复流（mid:R2）。我们用“a=组：FEC-FR S1 R1 R2”行组成一个FEC组。源和修复流被发送到不同多播组上的同一端口。源、基本层FEC和增强层FEC流都封装在RTP中。
Due to the exceptions described in Section 2.1, a [ETSI-TS-102-034v1.4.1]-compliant implementation must not use the RTP payload format defined in [RFC6015]. Instead, it may use the payload format that has been registered by DVB TM-IPI for [ETSI-TS-102-034v1.3.1].
由于第2.1节所述的例外情况，[ETSI-TS-102-034v1.4.1]兼容实施不得使用[RFC6015]中定义的RTP有效负载格式。相反，它可以使用DVB TM-IPI为[ETSI-TS-102-034v1.3.1]注册的有效载荷格式。
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=DVB-IPTV AL-FEC Example
t=0 0
a=group:FEC-FR S1 R1 R2
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=mid:S1
m=application 30000 RTP/AVP 96
c=IN IP4 233.252.0.2/127
a=rtpmap:96 vnd.dvb.iptv.alfec-base/90000
a=mid:R1
m=application 30000 RTP/AVP 111
c=IN IP4 233.252.0.3/127
a=rtpmap:111 vnd.dvb.iptv.alfec-enhancement/90000
a=mid:R2
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=DVB-IPTV AL-FEC Example
t=0 0
a=group:FEC-FR S1 R1 R2
m=video 30000 RTP/AVP 100
c=IN IP4 233.252.0.1/127
a=rtpmap:100 MP2T/90000
a=mid:S1
m=application 30000 RTP/AVP 96
c=IN IP4 233.252.0.2/127
a=rtpmap:96 vnd.dvb.iptv.alfec-base/90000
a=mid:R1
m=application 30000 RTP/AVP 111
c=IN IP4 233.252.0.3/127
a=rtpmap:111 vnd.dvb.iptv.alfec-enhancement/90000
a=mid:R2
Note that in the example above, the payload type has been chosen as 96 for the base-layer FEC stream and there is no "a=fmtp:" line to specify the format parameters. Due to the lack of the format parameters for "vnd.dvb.iptv.alfec-base", it is not possible to learn the FEC parameters from the SDP description.
注意，在上面的示例中，对于基本层FEC流，有效负载类型被选择为96，并且没有“a＝fmtp：”行来指定格式参数。由于缺少“vnd.dvb.iptv.alfec base”的格式参数，无法从SDP描述中学习FEC参数。
This specification adds no new security considerations to the DVB-IPTV AL-FEC protocol.
本规范未向DVB-IPTV AL-FEC协议添加新的安全注意事项。
This document is based on [ETSI-TS-102-034v1.3.1] and [ETSI-TS-102-034v1.4.1]. Thus, the authors would like to thank the editors of [ETSI-TS-102-034v1.3.1] and [ETSI-TS-102-034v1.4.1]. The authors also would like to thank those who reviewed earlier versions of this document.
本文件基于[ETSI-TS-102-034v1.3.1]和[ETSI-TS-102-034v1.4.1]。因此，作者要感谢[ETSI-TS-102-034v1.3.1]和[ETSI-TS-102-034v1.4.1]的编辑。作者还想感谢审查本文件早期版本的人员。
[ETSI-TS-102-034v1.3.1] ETSI TS 102 034 V1.3.1, "Transport of MPEG 2 TS Based DVB Services over IP Based Networks", October 2007.
[ETSI-TS-102-034v1.3.1]ETSI TS 102 034 V1.3.1，“通过基于IP的网络传输基于MPEG 2 TS的DVB服务”，2007年10月。
[ETSI-TS-102-034v1.4.1] ETSI TS 102 034 V1.4.1, "Transport of MPEG 2 TS Based DVB Services over IP Based Networks", August 2009.
[ETSI-TS-102-034v1.4.1]ETSI TS 102 034 V1.4.1，“通过基于IP的网络传输基于MPEG 2 TS的DVB服务”，2009年8月。
[RFC6015] Begen, A., "RTP Payload Format for 1-D Interleaved Parity Forward Error Correction (FEC)", RFC 6015, October 2010.
[RFC6015]Begen，A.，“用于1-D交错奇偶校验前向纠错（FEC）的RTP有效载荷格式”，RFC6015，2010年10月。
[RFC6681] Watson, M., Stockhammer, T., and M. Luby, "Raptor FEC Schemes for FECFRAME", RFC RFC6681, August 2012.
[RFC6681]Watson，M.，Stockhammer，T.和M.Luby，“FECFRAME的猛禽FEC方案”，RFC RFC6681，2012年8月。
[RFC6682] Watson, M., Stockhammer, T., and M. Luby, "RTP Payload Format for Raptor Forward Error Correction (FEC)", RFC 6682, August 2012.
[RFC6682]Watson，M.，Stockhammer，T.和M.Luby，“猛禽前向纠错（FEC）的RTP有效载荷格式”，RFC 6682，2012年8月。
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson, "RTP: A Transport Protocol for Real-Time Applications", STD 64, RFC 3550, July 2003.
[RFC3550]Schulzrinne，H.，Casner，S.，Frederick，R.，和V.Jacobson，“RTP：实时应用的传输协议”，STD 64，RFC 35502003年7月。
[RFC5576] Lennox, J., Ott, J., and T. Schierl, "Source-Specific Media Attributes in the Session Description Protocol (SDP)", RFC 5576, June 2009.
[RFC5576]Lennox，J.，Ott，J.，和T.Schierl，“会话描述协议（SDP）中的源特定媒体属性”，RFC 55762009年6月。
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session Description Protocol", RFC 4566, July 2006.
[RFC4566]Handley，M.，Jacobson，V.，和C.Perkins，“SDP：会话描述协议”，RFC4566，2006年7月。
[RFC5956] Begen, A., "Forward Error Correction Grouping Semantics in the Session Description Protocol", RFC 5956, September 2010.
[RFC5956]Begen，A.“会话描述协议中的前向纠错分组语义”，RFC 59562010年9月。
[DVB-A115] "DVB Application Layer FEC Evaluations (DVB Document A115)", May 2007, <http://www.dvb.org/technology/ standards/a115.tm3783.AL-FEC_Evaluation.pdf>.
[DVB-A115]“DVB应用层FEC评估（DVB文件A115）”，2007年5月<http://www.dvb.org/technology/ 标准/a115.tm3783.AL-FEC_Evaluation.pdf>。
[SMPTE2022-1] SMPTE 2022-1-2007, "Forward Error Correction for Real-Time Video/Audio Transport over IP Networks", 2007.
[SMPTE2022-1]SMPTE 2022-1-2007，“IP网络实时视频/音频传输的前向纠错”，2007年。
Authors' Addresses
作者地址
Ali Begen Cisco 181 Bay Street Toronto, ON M5J 2T3 Canada
Ali Begen Cisco位于加拿大多伦多湾街181号M5J 2T3
EMail: abegen@cisco.com
EMail: abegen@cisco.com
Thomas Stockhammer Nomor Research Brecherspitzstrasse 8 Munich, 81541 Germany
Thomas Stockhammer Nomor Research Brecherspitzstrasse 8慕尼黑，81541德国
EMail: stockhammer@nomor.de
EMail: stockhammer@nomor.de