Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 6015 Cisco
Category: Standards Track October 2010
ISSN: 2070-1721
Internet Engineering Task Force (IETF) A. Begen
Request for Comments: 6015 Cisco
Category: Standards Track October 2010
ISSN: 2070-1721
RTP Payload Format for 1-D Interleaved Parity Forward Error Correction (FEC)
用于1-D交错奇偶校验前向纠错（FEC）的RTP有效负载格式
Abstract
摘要
This document defines a new RTP payload format for the Forward Error Correction (FEC) that is generated by the 1-D interleaved parity code from a source media encapsulated in RTP. The 1-D interleaved parity code is a systematic code, where a number of repair symbols are generated from a set of source symbols and sent in a repair flow separate from the source flow that carries the source symbols. The 1-D interleaved parity code offers a good protection against bursty packet losses at a cost of reasonable complexity. The new payload format defined in this document should only be used (with some exceptions) as a part of the Digital Video Broadcasting-IPTV (DVB-IPTV) Application-layer FEC specification.
本文档为前向纠错（FEC）定义了一种新的RTP有效负载格式，该格式由封装在RTP中的源媒体的1-D交错奇偶校验码生成。1-D交织奇偶校验码是一种系统代码，其中从一组源符号生成多个修复符号，并在与携带源符号的源流分离的修复流中发送。1-D交错奇偶校验码以合理的复杂度为代价提供了良好的保护，防止突发性数据包丢失。本文件中定义的新有效负载格式应仅作为数字视频广播IPTV（DVB-IPTV）应用层FEC规范的一部分使用（某些例外情况除外）。
Status of This Memo
关于下段备忘
This is an Internet Standards Track document.
这是一份互联网标准跟踪文件。
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). Further information on Internet Standards is available in 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。有关互联网标准的更多信息，请参见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/rfc6015.
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/rfc6015.
Copyright Notice
版权公告
Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2010 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 ....................................................4
1.1. Use Cases ..................................................6
1.2. Overhead Computation .......................................8
1.3. Relation to Existing Specifications ........................8
1.3.1. RFCs 2733 and 3009 ..................................8
1.3.2. SMPTE 2022-1 ........................................8
1.3.3. ETSI TS 102 034 .....................................9
1.4. Scope of the Payload Format ...............................10
2. Requirements Notation ..........................................10
3. Definitions, Notations, and Abbreviations ......................10
3.1. Definitions ...............................................10
3.2. Notations .................................................11
4. Packet Formats .................................................11
4.1. Source Packets ............................................11
4.2. Repair Packets ............................................11
5. Payload Format Parameters ......................................15
5.1. Media Type Registration ...................................15
5.1.1. Registration of audio/1d-interleaved-parityfec .....15
5.1.2. Registration of video/1d-interleaved-parityfec .....16
5.1.3. Registration of text/1d-interleaved-parityfec ......18
5.1.4. Registration of
application/1d-interleaved-parityfec ...............19
5.2. Mapping to SDP Parameters .................................20
5.2.1. Offer-Answer Model Considerations ..................21
5.2.2. Declarative Considerations .........................22
6. Protection and Recovery Procedures .............................22
6.1. Overview ..................................................22
6.2. Repair Packet Construction ................................22
6.3. Source Packet Reconstruction ..............................24
6.3.1. Associating the Source and Repair Packets ..........25
6.3.2. Recovering the RTP Header and Payload ..............25
7. Session Description Protocol (SDP) Signaling ...................27
8. Congestion Control Considerations ..............................27
9. Security Considerations ........................................28
10. IANA Considerations ...........................................29
11. Acknowledgments ...............................................29
12. References ....................................................29
12.1. Normative References .....................................29
12.2. Informative References ...................................30
1. Introduction ....................................................4
1.1. Use Cases ..................................................6
1.2. Overhead Computation .......................................8
1.3. Relation to Existing Specifications ........................8
1.3.1. RFCs 2733 and 3009 ..................................8
1.3.2. SMPTE 2022-1 ........................................8
1.3.3. ETSI TS 102 034 .....................................9
1.4. Scope of the Payload Format ...............................10
2. Requirements Notation ..........................................10
3. Definitions, Notations, and Abbreviations ......................10
3.1. Definitions ...............................................10
3.2. Notations .................................................11
4. Packet Formats .................................................11
4.1. Source Packets ............................................11
4.2. Repair Packets ............................................11
5. Payload Format Parameters ......................................15
5.1. Media Type Registration ...................................15
5.1.1. Registration of audio/1d-interleaved-parityfec .....15
5.1.2. Registration of video/1d-interleaved-parityfec .....16
5.1.3. Registration of text/1d-interleaved-parityfec ......18
5.1.4. Registration of
application/1d-interleaved-parityfec ...............19
5.2. Mapping to SDP Parameters .................................20
5.2.1. Offer-Answer Model Considerations ..................21
5.2.2. Declarative Considerations .........................22
6. Protection and Recovery Procedures .............................22
6.1. Overview ..................................................22
6.2. Repair Packet Construction ................................22
6.3. Source Packet Reconstruction ..............................24
6.3.1. Associating the Source and Repair Packets ..........25
6.3.2. Recovering the RTP Header and Payload ..............25
7. Session Description Protocol (SDP) Signaling ...................27
8. Congestion Control Considerations ..............................27
9. Security Considerations ........................................28
10. IANA Considerations ...........................................29
11. Acknowledgments ...............................................29
12. References ....................................................29
12.1. Normative References .....................................29
12.2. Informative References ...................................30
This document extends the Forward Error Correction (FEC) header defined in [RFC2733] and uses this new FEC header for the FEC that is generated by the 1-D interleaved parity code from a source media encapsulated in RTP [RFC3550]. The resulting new RTP payload format is registered by this document.
本文档扩展了[RFC2733]中定义的前向纠错（FEC）报头，并将此新FEC报头用于FEC，该FEC由RTP[RFC3550]中封装的源媒体的1-D交错奇偶校验码生成。由此产生的新RTP有效负载格式由本文档注册。
The type of the source media protected by the 1-D interleaved parity code can be audio, video, text, or application. The FEC data are generated according to the media type parameters that are communicated through out-of-band means. The associations/ relationships between the source and repair flows are also communicated through out-of-band means.
受1-D交错奇偶校验码保护的源媒体类型可以是音频、视频、文本或应用程序。根据通过带外装置传送的媒体类型参数生成FEC数据。源流和修复流之间的关联/关系也通过带外方式进行沟通。
The 1-D interleaved parity FEC uses the exclusive OR (XOR) operation to generate the repair symbols. In a nutshell, the following steps take place:
1-D交错奇偶校验FEC使用异或（XOR）操作来生成修复符号。简而言之，将执行以下步骤：
1. The sender determines a set of source packets to be protected together based on the media type parameters.
1. 发送方根据媒体类型参数确定要一起保护的一组源数据包。
2. The sender applies the XOR operation on the source symbols to generate the required number of repair symbols.
2. 发送方对源符号应用异或操作，以生成所需数量的修复符号。
3. The sender packetizes the repair symbols and sends the repair packet(s) along with the source packets to the receiver(s) (in different flows). The repair packets may be sent proactively or on demand.
3. 发送方将修复符号打包，并将修复包连同源包一起发送到接收方（以不同的流）。可以主动或按需发送修复包。
Note that the source and repair packets belong to different source and repair flows, and the sender needs to provide a way for the receivers to demultiplex them, even in the case in which they are sent in the same transport flow (i.e., same source/destination address/port with UDP). This is required to offer backward compatibility (see Section 4). At the receiver side, if all of the source packets are successfully received, there is no need for FEC recovery and the repair packets are discarded. However, if there are missing source packets, the repair packets can be used to recover the missing information. Block diagrams for the systematic parity FEC encoder and decoder are sketched in Figures 1 and 2, respectively.
请注意，源数据包和修复数据包属于不同的源数据包和修复数据包流，发送方需要为接收方提供一种将它们解复用的方法，即使它们是在相同的传输流中发送的（即，具有UDP的相同源/目的地地址/端口）。这是提供向后兼容性所必需的（参见第4节）。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。系统奇偶校验FEC编码器和解码器的框图分别如图1和图2所示。
+------------+
+--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Encoder |
| (Sender) | --> +==+ +==+
+------------+ +==+ +==+
+------------+
+--+ +--+ +--+ +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ +--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Encoder |
| (Sender) | --> +==+ +==+
+------------+ +==+ +==+
Source Packet: +--+ Repair Packet: +==+
+--+ +==+
Source Packet: +--+ Repair Packet: +==+
+--+ +==+
Figure 1: Block diagram for systematic parity FEC encoder
图1：系统奇偶校验FEC编码器框图
+------------+
+--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Decoder |
+==+ +==+ --> | (Receiver) |
+==+ +==+ +------------+
+------------+
+--+ X X +--+ --> | Systematic | --> +--+ +--+ +--+ +--+
+--+ +--+ | Parity FEC | +--+ +--+ +--+ +--+
| Decoder |
+==+ +==+ --> | (Receiver) |
+==+ +==+ +------------+
Source Packet: +--+ Repair Packet: +==+ Lost Packet: X
+--+ +==+
Source Packet: +--+ Repair Packet: +==+ Lost Packet: X
+--+ +==+
Figure 2: Block diagram for systematic parity FEC decoder
图2：系统奇偶校验FEC解码器的框图
Suppose that we have a group of D x L source packets that have sequence numbers starting from 1 running to D x L. If we apply the XOR operation to the group of the source packets whose sequence numbers are L apart from each other as sketched in Figure 3, we generate L repair packets. This process is referred to as 1-D interleaved FEC protection, and the resulting L repair packets are referred to as interleaved (or column) FEC packets.
假设我们有一组序列号从1到dxl的dxl源数据包。如果我们对序列号彼此相差L的源数据包组应用异或操作，如图3所示，我们生成L个修复数据包。该过程被称为1-D交错FEC保护，并且产生的L修复分组被称为交错（或列）FEC分组。
+-------------+ +-------------+ +-------------+ +-------+
| S_1 | | S_2 | | S3 | ... | S_L |
| S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL |
| . | | . | | | | |
| . | | . | | | | |
| . | | . | | | | |
| S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL |
+-------------+ +-------------+ +-------------+ +-------+
+ + + +
------------- ------------- ------------- -------
| XOR | | XOR | | XOR | ... | XOR |
------------- ------------- ------------- -------
= = = =
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| ... |C_L|
+===+ +===+ +===+ +===+
+-------------+ +-------------+ +-------------+ +-------+
| S_1 | | S_2 | | S3 | ... | S_L |
| S_L+1 | | S_L+2 | | S_L+3 | ... | S_2xL |
| . | | . | | | | |
| . | | . | | | | |
| . | | . | | | | |
| S_(D-1)xL+1 | | S_(D-1)xL+2 | | S_(D-1)xL+3 | ... | S_DxL |
+-------------+ +-------------+ +-------------+ +-------+
+ + + +
------------- ------------- ------------- -------
| XOR | | XOR | | XOR | ... | XOR |
------------- ------------- ------------- -------
= = = =
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| ... |C_L|
+===+ +===+ +===+ +===+
Figure 3: Generating interleaved (column) FEC packets
图3：生成交织（列）FEC数据包
In Figure 3, S_n and C_m denote the source packet with a sequence number n and the interleaved (column) FEC packet with a sequence number m, respectively.
在图3中，S_n和C_m分别表示序列号为n的源分组和序列号为m的交织（列）FEC分组。
We generate one interleaved FEC packet out of D non-consecutive source packets. This repair packet can provide a full recovery of the missing information if there is only one packet missing among the corresponding source packets. This implies that 1-D interleaved FEC protection performs well under bursty loss conditions provided that a large enough value is chosen for L, i.e., L packet duration should not be shorter than the duration of the burst that is intended to be repaired.
我们从D个非连续源数据包中生成一个交错FEC数据包。如果在相应的源分组中只有一个分组丢失，则该修复分组可以提供丢失信息的完全恢复。这意味着1-D交错FEC保护在突发性丢失条件下表现良好，前提是为L选择足够大的值，即L分组持续时间不应短于预期要修复的突发的持续时间。
For example, consider the scenario depicted in Figure 4 in which the sender generates interleaved FEC packets and a bursty loss hits the source packets. Since the number of columns is larger than the number of packets lost due to the bursty loss, the repair operation succeeds.
例如，考虑图4中描述的场景，其中发送器生成交错的FEC分组，突发性的损失命中源分组。由于列数大于由于突发性丢失而丢失的数据包数，因此修复操作成功。
+---+
| 1 | X X X
+---+
+---+
| 1 | X X X
+---+
+---+ +---+ +---+ +---+
| 5 | | 6 | | 7 | | 8 |
+---+ +---+ +---+ +---+
+---+ +---+ +---+ +---+
| 5 | | 6 | | 7 | | 8 |
+---+ +---+ +---+ +---+
+---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+
+---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 4: Example scenario where 1-D interleaved FEC protection succeeds error recovery
图4：1-D交错FEC保护成功恢复错误的示例场景
The sender may generate interleaved FEC packets to combat the bursty packet losses. However, two or more random packet losses may hit the source and repair packets in the same column. In that case, the repair operation fails. This is illustrated in Figure 5. Note that it is possible that two or more bursty losses may occur in the same source block, in which case interleaved FEC packets may still fail to recover the lost data.
发送方可以生成交织的FEC分组来对抗突发性分组丢失。然而，两个或多个随机数据包丢失可能会击中源并修复同一列中的数据包。在这种情况下，修复操作将失败。这如图5所示。注意，在同一源块中可能发生两个或多个突发性丢失，在这种情况下，交织的FEC分组仍然可能无法恢复丢失的数据。
+---+ +---+ +---+
| 1 | X | 3 | | 4 |
+---+ +---+ +---+
+---+ +---+ +---+
| 1 | X | 3 | | 4 |
+---+ +---+ +---+
+---+ +---+ +---+
| 5 | X | 7 | | 8 |
+---+ +---+ +---+
+---+ +---+ +---+
| 5 | X | 7 | | 8 |
+---+ +---+ +---+
+---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+
+---+ +---+ +---+ +---+
| 9 | | 10| | 11| | 12|
+---+ +---+ +---+ +---+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
+===+ +===+ +===+ +===+
|C_1| |C_2| |C_3| |C_4|
+===+ +===+ +===+ +===+
Figure 5: Example scenario where 1-D interleaved FEC protection fails error recovery
图5：1-D交错FEC保护错误恢复失败的示例场景
The overhead is defined as the ratio of the number of bytes that belong to the repair packets to the number of bytes that belong to the protected source packets.
开销定义为属于修复数据包的字节数与属于受保护源数据包的字节数之比。
Assuming that each repair packet carries an equal number of bytes carried by a source packet and ignoring the size of the FEC header, we can compute the overhead as follows:
假设每个修复数据包承载的字节数与源数据包相同，并且忽略FEC报头的大小，我们可以按如下方式计算开销：
Overhead = 1/D
Overhead = 1/D
where D is the number of rows in the source block.
其中D是源块中的行数。
This section discusses the relation of the current specification to other existing specifications.
本节讨论当前规范与其他现有规范的关系。
The current specification extends the FEC header defined in [RFC2733] and registers a new RTP payload format. This new payload format is not backward compatible with the payload format that was registered by [RFC3009].
当前规范扩展了[RFC2733]中定义的FEC报头，并注册了新的RTP有效负载格式。此新的有效负载格式与[RFC3009]注册的有效负载格式不向后兼容。
In 2007, the Society of Motion Picture and Television Engineers (SMPTE) - Technology Committee N26 on File Management and Networking Technology - decided to revise the Pro-MPEG Code of Practice (CoP) #3 Release 2 specification (initially produced by the Pro-MPEG Forum in 2004), which discussed several aspects of the transmission of MPEG-2 transport streams over IP networks. The new SMPTE specification is referred to as [SMPTE2022-1].
2007年，电影和电视工程师协会（SMPTE）——文件管理和网络技术技术委员会N26——决定修订专业MPEG实践规范（CoP）#3第2版规范（最初由专业MPEG论坛于2004年制定），其中讨论了通过IP网络传输MPEG-2传输流的几个方面。新的SMPTE规范称为[SMPTE2022-1]。
The Pro-MPEG CoP #3 Release 2 document was originally based on [RFC2733]. SMPTE revised the document by extending the FEC header proposed in [RFC2733] (by setting the E bit). This extended header offers some improvements.
Pro MPEG CoP#3第2版文档最初基于[RFC2733]。SMPTE通过扩展[RFC2733]中建议的FEC头（通过设置E位）修改了文档。这个扩展头提供了一些改进。
For example, instead of utilizing the bitmap field used in [RFC2733], [SMPTE2022-1] introduces separate fields to convey the number of rows (D) and columns (L) of the source block as well as the type of the repair packet (i.e., whether the repair packet is an interleaved FEC packet computed over a column or a non-interleaved FEC packet computed over a row). These fields, plus the base sequence number, allow the receiver side to establish associations between the source
例如，[SMPTE2022-1]不使用[RFC2733]中使用的位图字段，而是引入单独的字段来传递源块的行数（D）和列数（L）以及修复包的类型（即，修复数据包是通过列计算的交织FEC数据包还是通过行计算的非交织FEC数据包）。这些字段加上基本序列号，允许接收方在源之间建立关联
and repair packets. Note that although the bitmap field is not utilized, the FEC header of [SMPTE2022-1] inherently carries over the bitmap field from [RFC2733].
和修理包。注意，尽管未使用位图字段，[SMPTE2022-1]的FEC报头固有地从[RFC2733]携带位图字段。
On the other hand, some parts of [SMPTE2022-1] are not in compliance with RTP [RFC3550]. For example, [SMPTE2022-1] sets the Synchronization Source (SSRC) field to zero and does not use the timestamp field in the RTP headers of the repair packets (receivers ignore the timestamps of the repair packets). Furthermore, [SMPTE2022-1] also sets the CSRC Count (CC) field in the RTP header to zero and does not allow any Contributing Source (CSRC) entry in the RTP header.
另一方面，[SMPTE2022-1]的某些部分不符合RTP[RFC3550]。例如，[SMPTE2022-1]将同步源（SSRC）字段设置为零，并且不使用修复数据包的RTP报头中的时间戳字段（接收器忽略修复数据包的时间戳）。此外，[SMPTE2022-1]还将RTP标头中的CSC计数（CC）字段设置为零，并且不允许RTP标头中的任何贡献源（CSC）条目。
The current document adopts the extended FEC header of [SMPTE2022-1] and registers a new RTP payload format. At the same time, this document fixes the parts of [SMPTE2022-1] that are not compliant with RTP [RFC3550], except the one discussed below.
当前文档采用[SMPTE2022-1]的扩展FEC标头，并注册了新的RTP有效负载格式。同时，本文件修复了[SMPTE2022-1]中不符合RTP[RFC3550]的部分，以下讨论的部分除外。
The baseline header format first proposed in [RFC2733] does not have fields to protect the P and X bits and the CC fields of the source packets associated with a repair packet. Rather, the P bit, X bit, and CC field in the RTP header of the repair packet are used to protect those bits and fields. This, however, may sometimes result in failures when doing the RTP header validity checks as specified in [RFC3550]. While this behavior has been fixed in [RFC5109], which obsoleted [RFC2733], the RTP payload format defined in this document still allows this behavior for legacy purposes. Implementations following this specification must be aware of this potential issue when RTP header validity checks are applied.
[RFC2733]中首次提出的基线报头格式没有字段来保护与修复数据包相关联的源数据包的P和X位以及CC字段。相反，修复包的RTP报头中的P位、X位和CC字段用于保护这些位和字段。然而，这有时可能导致在执行[RFC3550]中规定的RTP标头有效性检查时失败。虽然[RFC5109]中已修复了此行为，该行为已被[RFC2733]淘汰，但本文档中定义的RTP有效负载格式仍允许此行为用于遗留用途。当应用RTP头有效性检查时，遵循此规范的实现必须意识到这一潜在问题。
In 2009, the Digital Video Broadcasting (DVB) consortium published a technical specification [ETSI-TS-102-034] through the European Telecommunications Standards Institute (ETSI). This specification covers several areas related to the transmission of MPEG-2 transport stream-based services over IP networks.
2009年，数字视频广播（DVB）联盟通过欧洲电信标准协会（ETSI）发布了一份技术规范[ETSI-TS-102-034]。本规范涵盖与通过IP网络传输基于MPEG-2传输流的服务相关的几个领域。
Annex E of [ETSI-TS-102-034] defines an optional protocol for Application-layer FEC (AL-FEC) protection of streaming media for DVB-IP services carried over RTP [RFC3550] transport. The DVB-IPTV AL-FEC protocol uses two layers for protection: a base layer that is produced by a packet-based interleaved parity code, and an enhancement layer that is produced by a Raptor code [DVB-AL-FEC]. While the use of the enhancement layer is optional, the use of the base layer is mandatory wherever AL-FEC is used. The DVB-IPTV AL-FEC protocol is also described in [DVB-AL-FEC].
[ETSI-TS-102-034]的附录E定义了通过RTP[RFC3550]传输的DVB-IP服务的流媒体应用层FEC（AL-FEC）保护的可选协议。DVB-IPTV AL-FEC协议使用两层进行保护：由基于分组的交织奇偶校验码产生的基本层和由Raptor码[DVB-AL-FEC]产生的增强层。虽然增强层的使用是可选的，但无论在哪里使用AL-FEC，基层的使用都是强制性的。DVB-IPTV AL-FEC协议也在[DVB-AL-FEC]中描述。
The interleaved parity code that is used in the base layer is a subset of [SMPTE2022-1]. In particular, the AL-FEC base layer uses only the 1-D interleaved FEC protection from [SMPTE2022-1]. The new RTP payload format that is defined and registered in this document (with some exceptions listed in [DVB-AL-FEC]) is used as the AL-FEC base layer.
基本层中使用的交织奇偶校验码是[SMPTE2022-1]的子集。具体而言，AL-FEC基层仅使用来自[SMPTE2022-1]的1-D交错FEC保护。本文件中定义和注册的新RTP有效载荷格式（DVB-AL-FEC中列出的一些例外情况除外）用作AL-FEC基层。
The payload format specified in this document must only be used in legacy applications where the limitations explained in Section 1.3.2 are known not to impact any system components or other RTP elements. Whenever possible, a payload format that is fully compliant with [RFC3550], such as [RFC5109] or other newer payload formats, must be used.
本文件中规定的有效载荷格式必须仅用于已知第1.3.2节中解释的限制不会影响任何系统组件或其他RTP元件的传统应用中。只要可能，必须使用完全符合[RFC3550]的有效负载格式，如[RFC5109]或其他较新的有效负载格式。
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 [RFC2119].
本文件中的关键词“必须”、“不得”、“必需”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照[RFC2119]中所述进行解释。
The definitions and notations commonly used in this document are summarized in this section.
本节概述了本文件中常用的定义和符号。
This document uses the following definitions:
本文件使用以下定义：
Source Flow: The packet flow(s) carrying the source data to which FEC protection is to be applied.
源流：承载要应用FEC保护的源数据的数据包流。
Repair Flow: The packet flow(s) carrying the repair data.
修复流：承载修复数据的数据包流。
Symbol: A unit of data. Its size, in bytes, is referred to as the symbol size.
符号：一种数据单位。其大小（以字节为单位）称为符号大小。
Source Symbol: The smallest unit of data used during the encoding process.
源符号：编码过程中使用的最小数据单位。
Repair Symbol: Repair symbols are generated from the source symbols.
修复符号：修复符号是从源符号生成的。
Source Packet: Data packets that contain only source symbols.
源数据包：仅包含源符号的数据包。
Repair Packet: Data packets that contain only repair symbols.
修复数据包：仅包含修复符号的数据包。
Source Block: A block of source symbols that are considered together in the encoding process.
源块：在编码过程中一起考虑的源符号块。
o L: Number of columns of the source block.
o L：源块的列数。
o D: Number of rows of the source block.
o D：源块的行数。
This section defines the formats of the source and repair packets.
本节定义了源数据包和修复数据包的格式。
The source packets need to contain information that identifies the source block and the position within the source block occupied by the packet. Since the source packets that are carried within an RTP stream already contain unique sequence numbers in their RTP headers [RFC3550], we can identify the source packets in a straightforward manner, and there is no need to append additional field(s). The primary advantage of not modifying the source packets in any way is that it provides backward compatibility for the receivers that do not support FEC at all. In multicast scenarios, this backward compatibility becomes quite useful as it allows the non-FEC-capable and FEC-capable receivers to receive and interpret the same source packets sent in the same multicast session.
源数据包需要包含标识源块和数据包占用的源块内的位置的信息。由于RTP流中携带的源数据包在其RTP头[RFC3550]中已经包含唯一的序列号，因此我们可以以简单的方式识别源数据包，并且不需要附加额外的字段。不以任何方式修改源数据包的主要优点是，它为根本不支持FEC的接收器提供向后兼容性。在多播场景中，这种向后兼容性非常有用，因为它允许不支持FEC和支持FEC的接收器接收和解释在同一多播会话中发送的相同源数据包。
The repair packets MUST contain information that identifies the source block to which they pertain and the relationship between the contained repair symbols and the original source block. For this purpose, we use the RTP header of the repair packets as well as another header within the RTP payload, which we refer to as the FEC header, as shown in Figure 6.
修复包必须包含标识其所属源块的信息，以及包含的修复符号与原始源块之间的关系。为此，我们使用修复包的RTP报头以及RTP有效负载中的另一个报头，我们称之为FEC报头，如图6所示。
+------------------------------+
| IP Header |
+------------------------------+
| Transport Header |
+------------------------------+
| RTP Header | __
+------------------------------+ |
| FEC Header | \
+------------------------------+ > RTP Payload
| Repair Symbols | /
+------------------------------+ __|
+------------------------------+
| IP Header |
+------------------------------+
| Transport Header |
+------------------------------+
| RTP Header | __
+------------------------------+ |
| FEC Header | \
+------------------------------+ > RTP Payload
| Repair Symbols | /
+------------------------------+ __|
Figure 6: Format of repair packets
图6：修复包的格式
The RTP header is formatted according to [RFC3550] with some further clarifications listed below:
RTP标头根据[RFC3550]进行格式化，下面列出了一些进一步的说明：
o Version: The version field is set to 2.
o 版本：版本字段设置为2。
o Padding (P) Bit: This bit is equal to the XOR sum of the corresponding P bits from the RTP headers of the source packets protected by this repair packet. However, padding octets are never present in a repair packet, independent of the value of the P bit.
o 填充（P）位：此位等于受此修复数据包保护的源数据包的RTP头中相应P位的异或和。然而，填充八位字节永远不会出现在修复包中，这与P位的值无关。
o Extension (X) Bit: This bit is equal to the XOR sum of the corresponding X bits from the RTP headers of the source packets protected by this repair packet. However, an RTP header extension is never present in a repair packet, independent of the value of the X bit.
o 扩展（X）位：此位等于受此修复数据包保护的源数据包的RTP头中相应X位的异或和。然而，RTP报头扩展永远不会出现在修复包中，与X位的值无关。
o CSRC Count (CC): This field is equal to the XOR sum of the corresponding CC values from the RTP headers of the source packets protected by this repair packet. However, a CSRC list is never present in a repair packet, independent of the value of the CC field.
o CSC计数（CC）：此字段等于受此修复数据包保护的源数据包的RTP头中相应CC值的异或和。但是，与CC字段的值无关，CSC列表永远不会出现在修复数据包中。
o Marker (M) Bit: This bit is equal to the XOR sum of the corresponding M bits from the RTP headers of the source packets protected by this repair packet.
o 标记（M）位：此位等于受此修复数据包保护的源数据包的RTP报头中相应M位的异或和。
o Payload Type: The (dynamic) payload type for the repair packets is determined through out-of-band means. Note that this document registers a new payload format for the repair packets (refer to Section 5 for details). According to [RFC3550], an RTP receiver that cannot recognize a payload type must discard it. This action provides backward compatibility. The FEC mechanisms can then be used in a multicast group with mixed FEC-capable and non-FEC-
o 有效负载类型：通过带外方式确定修复包的（动态）有效负载类型。请注意，本文件为维修数据包注册了新的有效载荷格式（详情请参阅第5节）。根据[RFC3550]，无法识别有效负载类型的RTP接收器必须丢弃它。此操作提供向后兼容性。然后，可以在具有混合FEC能力和非FEC能力的多播组中使用FEC机制-
capable receivers. If a non-FEC-capable receiver receives a repair packet, it will not recognize the payload type, and hence, discards the repair packet.
有能力的接受者。如果不支持FEC的接收器接收到修复数据包，它将无法识别有效负载类型，因此丢弃该修复数据包。
o Sequence Number (SN): The sequence number has the standard definition. It MUST be one higher than the sequence number in the previously transmitted repair packet. The initial value of the sequence number SHOULD be random (unpredictable) [RFC3550].
o 序列号（SN）：序列号具有标准定义。它必须比先前发送的修复数据包中的序列号高一个。序列号的初始值应为随机（不可预测）[RFC3550]。
o Timestamp (TS): The timestamp SHALL be set to a time corresponding to the repair packet's transmission time. Note that the timestamp value has no use in the actual FEC protection process and is usually useful for jitter calculations.
o 时间戳（TS）：时间戳应设置为与维修包传输时间相对应的时间。请注意，时间戳值在实际FEC保护过程中没有用处，通常用于抖动计算。
o Synchronization Source (SSRC): The SSRC value SHALL be randomly assigned as suggested by [RFC3550]. This allows the sender to multiplex the source and repair flows on the same port or multiplex multiple repair flows on a single port. The repair flows SHOULD use the RTP Control Protocol (RTCP) CNAME field to associate themselves with the source flow.
o 同步源（SSRC）：应按照[RFC3550]的建议随机分配SSRC值。这允许发送方在同一端口上多路复用源和修复流，或在单个端口上多路复用多个修复流。修复流应使用RTP控制协议（RTCP）CNAME字段将其自身与源流关联。
In some networks, the RTP Source (which produces the source packets) and the FEC Source (which generates the repair packets from the source packets) may not be the same host. In such scenarios, using the same CNAME for the source and repair flows means that the RTP Source and the FEC Source MUST share the same CNAME (for this specific source-repair flow association). A common CNAME may be produced based on an algorithm that is known both to the RTP and FEC Source. This usage is compliant with [RFC3550].
在一些网络中，RTP源（生成源分组）和FEC源（从源分组生成修复分组）可能不是同一主机。在这种情况下，对源和修复流使用相同的CNAME意味着RTP源和FEC源必须共享相同的CNAME（对于此特定的源修复流关联）。可以基于RTP和FEC源都知道的算法生成公共CNAME。此用法符合[RFC3550]。
Note that due to the randomness of the SSRC assignments, there is a possibility of SSRC collision. In such cases, the collisions MUST be resolved as described in [RFC3550].
注意，由于SSRC分配的随机性，存在SSRC冲突的可能性。在这种情况下，必须按照[RFC3550]中所述解决碰撞。
Note that the P bit, X bit, CC field, and M bit of the source packets are protected by the corresponding bits/fields in the RTP header of the repair packet. On the other hand, the payload of a repair packet protects the concatenation of (if present) the CSRC list, RTP extension, payload, and padding of the source RTP packets associated with this repair packet.
注意，源分组的P位、X位、CC字段和M位由修复分组的RTP报头中的相应位/字段保护。另一方面，修复分组的有效载荷保护与该修复分组相关联的源RTP分组的csc列表、RTP扩展、有效载荷和填充（如果存在）的级联。
The FEC header is 16 octets. The format of the FEC header is shown in Figure 7.
FEC头是16个八位字节。FEC头的格式如图7所示。
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SN base low | Length recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| PT recovery | Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|N|D|Type |Index| Offset | NA | SN base ext |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SN base low | Length recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|E| PT recovery | Mask |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS recovery |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|N|D|Type |Index| Offset | NA | SN base ext |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: Format of the FEC header
图7:FEC头的格式
The FEC header consists of the following fields:
FEC标头由以下字段组成：
o The SN base low field is used to indicate the lowest sequence number, taking wraparound into account, of those source packets protected by this repair packet.
o SN base low字段用于指示受此修复数据包保护的源数据包的最低序列号（考虑到环绕）。
o The Length recovery field is used to determine the length of any recovered packets.
o 长度恢复字段用于确定任何已恢复数据包的长度。
o The E bit is the extension flag introduced in [RFC2733] and used to extend the [RFC2733] FEC header.
o E位是[RFC2733]中引入的扩展标志，用于扩展[RFC2733]FEC头。
o The PT recovery field is used to determine the payload type of the recovered packets.
o PT recovery字段用于确定已恢复数据包的有效负载类型。
o The Mask field is not used.
o 未使用掩码字段。
o The TS recovery field is used to determine the timestamp of the recovered packets.
o TS恢复字段用于确定恢复的数据包的时间戳。
o The N bit is the extension flag that is reserved for future use.
o N位是保留供将来使用的扩展标志。
o The D bit is not used.
o 不使用D位。
o The Type field indicates the type of the error-correcting code used. This document defines only one error-correcting code.
o 类型字段指示所用纠错码的类型。本文档仅定义一个纠错码。
o The Index field is not used.
o 未使用索引字段。
o The Offset and NA fields are used to indicate the number of columns (L) and rows (D) of the source block, respectively.
o 偏移量和NA字段分别用于指示源块的列（L）和行（D）的数量。
o The SN base ext field is not used.
o 未使用SN base ext字段。
The details on setting the fields in the FEC header are provided in Section 6.2.
第6.2节提供了有关设置FEC标头中字段的详细信息。
It should be noted that a Mask-based approach (similar to the one specified in [RFC2733]) may not be very efficient to indicate which source packets in the current source block are associated with a given repair packet. In particular, for the applications that would like to use large source block sizes, the size of the Mask that is required to describe the source-repair packet associations may be prohibitively large. Instead, a systematized approach is inherently more efficient.
应注意，基于掩码的方法（类似于[RFC2733]中规定的方法）可能无法非常有效地指示当前源块中的哪些源分组与给定修复分组相关联。特别地，对于希望使用大源块大小的应用，描述源修复分组关联所需的掩码的大小可能大得令人望而却步。相反，系统化方法本质上更有效。
This section provides the media subtype registration for the 1-D interleaved parity FEC. The parameters that are required to configure the FEC encoding and decoding operations are also defined in this section.
本节提供了1-D交错奇偶校验FEC的媒体子类型注册。本节还定义了配置FEC编码和解码操作所需的参数。
This registration is done using the template defined in [RFC4288] and following the guidance provided in [RFC4855].
此注册使用[RFC4288]中定义的模板并遵循[RFC4855]中提供的指南完成。
Type name: audio
类型名称：音频
Subtype name: 1d-interleaved-parityfec
子类型名称：1d交错parityfec
Required parameters:
所需参数：
o rate: The RTP timestamp (clock) rate in Hz. The (integer) rate SHALL be larger than 1000 to provide sufficient resolution to RTCP operations. However, it is RECOMMENDED to select the rate that matches the rate of the protected source RTP stream.
o 速率：RTP时间戳（时钟）速率，单位为Hz。（整数）速率应大于1000，以便为RTCP操作提供足够的分辨率。但是，建议选择与受保护源RTP流的速率匹配的速率。
o L: Number of columns of the source block. L is a positive integer that is less than or equal to 255.
o L：源块的列数。L是小于或等于255的正整数。
o D: Number of rows of the source block. D is a positive integer that is less than or equal to 255.
o D：源块的行数。D是小于或等于255的正整数。
o repair-window: The time that spans the FEC block (i.e., source packets and the corresponding repair packets). An FEC encoder processes a block of source packets and generates a number of repair packets, which are then transmitted within a certain duration not larger than the value of the repair window. At the
o 修复窗口：跨越FEC块的时间（即，源数据包和相应的修复数据包）。FEC编码器处理源分组块并生成多个修复分组，然后在不大于修复窗口值的特定持续时间内发送这些修复分组。在
receiver side, the FEC decoder should wait at least for the duration of the repair window after getting the first packet in an FEC block to allow all the repair packets to arrive (the waiting time can be adjusted if there are missing packets at the beginning of the FEC block). The FEC decoder can start decoding the already received packets sooner; however, it SHOULD NOT register an FEC decoding failure until it waits at least for the repair-window duration. The size of the repair window is specified in microseconds.
在接收端，FEC解码器应当在获得FEC块中的第一个分组之后至少等待修复窗口的持续时间，以允许所有修复分组到达（如果在FEC块的开始处存在丢失的分组，则可以调整等待时间）。FEC解码器可以更快地开始解码已经接收到的分组；但是，在至少等待修复窗口持续时间之前，不应注册FEC解码故障。修复窗口的大小以微秒为单位指定。
Optional parameters: None.
可选参数：无。
Encoding considerations: This media type is framed (see Section 4.8 in the template document [RFC4288]) and contains binary data.
编码注意事项：此媒体类型为框架（见模板文档[RFC4288]第4.8节），包含二进制数据。
Security considerations: See Section 9 of [RFC6015].
安全注意事项：见[RFC6015]第9节。
Interoperability considerations: None.
互操作性考虑：无。
Published specification: [RFC6015].
已发布规范：[RFC6015]。
Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media.
使用此媒体类型的应用程序：希望通过在源媒体之外发送冗余数据来提高数据包丢失恢复能力的多媒体应用程序。
Additional information: None.
其他信息：无。
Person & email address to contact for further information: Ali Begen <abegen@cisco.com> and the IETF Audio/Video Transport Working Group.
联系人和电子邮件地址，以获取更多信息：Ali Begen<abegen@cisco.com>以及IETF音频/视频传输工作组。
Intended usage: COMMON.
预期用途：普通。
Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP [RFC3550].
使用限制：此媒体类型取决于RTP帧，因此仅定义用于通过RTP传输[RFC3550]。
Author: Ali Begen <abegen@cisco.com>.
作者：阿里·贝根<abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group delegated from the IESG.
变更控制员：IESG授权的IETF音频/视频传输工作组。
Type name: video
类型名称：视频
Subtype name: 1d-interleaved-parityfec
子类型名称：1d交错parityfec
Required parameters:
所需参数：
o rate: The RTP timestamp (clock) rate in Hz. The (integer) rate SHALL be larger than 1000 to provide sufficient resolution to RTCP operations. However, it is RECOMMENDED to select the rate that matches the rate of the protected source RTP stream.
o 速率：RTP时间戳（时钟）速率，单位为Hz。（整数）速率应大于1000，以便为RTCP操作提供足够的分辨率。但是，建议选择与受保护源RTP流的速率匹配的速率。
o L: Number of columns of the source block. L is a positive integer that is less than or equal to 255.
o L：源块的列数。L是小于或等于255的正整数。
o D: Number of rows of the source block. D is a positive integer that is less than or equal to 255.
o D：源块的行数。D是小于或等于255的正整数。
o repair-window: The time that spans the FEC block (i.e., source packets and the corresponding repair packets). An FEC encoder processes a block of source packets and generates a number of repair packets, which are then transmitted within a certain duration not larger than the value of the repair window. At the receiver side, the FEC decoder should wait at least for the duration of the repair window after getting the first packet in an FEC block to allow all the repair packets to arrive (the waiting time can be adjusted if there are missing packets at the beginning of the FEC block). The FEC decoder can start decoding the already received packets sooner; however, it SHOULD NOT register an FEC decoding failure until it waits at least for the repair-window duration. The size of the repair window is specified in microseconds.
o 修复窗口：跨越FEC块的时间（即，源数据包和相应的修复数据包）。FEC编码器处理源分组块并生成多个修复分组，然后在不大于修复窗口值的特定持续时间内发送这些修复分组。在接收机侧，FEC解码器应当在获得FEC块中的第一分组之后至少等待修复窗口的持续时间，以允许所有修复分组到达（如果在FEC块的开始处存在丢失的分组，则可以调整等待时间）。FEC解码器可以更快地开始解码已经接收到的分组；但是，在至少等待修复窗口持续时间之前，不应注册FEC解码故障。修复窗口的大小以微秒为单位指定。
Optional parameters: None.
可选参数：无。
Encoding considerations: This media type is framed (see Section 4.8 in the template document [RFC4288]) and contains binary data.
编码注意事项：此媒体类型为框架（见模板文档[RFC4288]第4.8节），包含二进制数据。
Security considerations: See Section 9 of [RFC6015].
安全注意事项：见[RFC6015]第9节。
Interoperability considerations: None.
互操作性考虑：无。
Published specification: [RFC6015].
已发布规范：[RFC6015]。
Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media.
使用此媒体类型的应用程序：希望通过在源媒体之外发送冗余数据来提高数据包丢失恢复能力的多媒体应用程序。
Additional information: None.
其他信息：无。
Person & email address to contact for further information: Ali Begen <abegen@cisco.com> and the IETF Audio/Video Transport Working Group.
联系人和电子邮件地址，以获取更多信息：Ali Begen<abegen@cisco.com>以及IETF音频/视频传输工作组。
Intended usage: COMMON.
预期用途：普通。
Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP [RFC3550].
使用限制：此媒体类型取决于RTP帧，因此仅定义用于通过RTP传输[RFC3550]。
Author: Ali Begen <abegen@cisco.com>.
作者：阿里·贝根<abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group delegated from the IESG.
变更控制员：IESG授权的IETF音频/视频传输工作组。
Type name: text
类型名称：text
Subtype name: 1d-interleaved-parityfec
子类型名称：1d交错parityfec
Required parameters:
所需参数：
o rate: The RTP timestamp (clock) rate in Hz. The (integer) rate SHALL be larger than 1000 to provide sufficient resolution to RTCP operations. However, it is RECOMMENDED to select the rate that matches the rate of the protected source RTP stream.
o 速率：RTP时间戳（时钟）速率，单位为Hz。（整数）速率应大于1000，以便为RTCP操作提供足够的分辨率。但是，建议选择与受保护源RTP流的速率匹配的速率。
o L: Number of columns of the source block. L is a positive integer that is less than or equal to 255.
o L：源块的列数。L是小于或等于255的正整数。
o D: Number of rows of the source block. D is a positive integer that is less than or equal to 255.
o D：源块的行数。D是小于或等于255的正整数。
o repair-window: The time that spans the FEC block (i.e., source packets and the corresponding repair packets). An FEC encoder processes a block of source packets and generates a number of repair packets, which are then transmitted within a certain duration not larger than the value of the repair window. At the receiver side, the FEC decoder should wait at least for the duration of the repair window after getting the first packet in an FEC block to allow all the repair packets to arrive (the waiting time can be adjusted if there are missing packets at the beginning of the FEC block). The FEC decoder can start decoding the already received packets sooner; however, it SHOULD NOT register an FEC decoding failure until it waits at least for the repair-window duration. The size of the repair window is specified in microseconds.
o 修复窗口：跨越FEC块的时间（即，源数据包和相应的修复数据包）。FEC编码器处理源分组块并生成多个修复分组，然后在不大于修复窗口值的特定持续时间内发送这些修复分组。在接收机侧，FEC解码器应当在获得FEC块中的第一分组之后至少等待修复窗口的持续时间，以允许所有修复分组到达（如果在FEC块的开始处存在丢失的分组，则可以调整等待时间）。FEC解码器可以更快地开始解码已经接收到的分组；但是，在至少等待修复窗口持续时间之前，不应注册FEC解码故障。修复窗口的大小以微秒为单位指定。
Optional parameters: None.
可选参数：无。
Encoding considerations: This media type is framed (see Section 4.8 in the template document [RFC4288]) and contains binary data.
编码注意事项：此媒体类型为框架（见模板文档[RFC4288]第4.8节），包含二进制数据。
Security considerations: See Section 9 of [RFC6015].
安全注意事项：见[RFC6015]第9节。
Interoperability considerations: None.
互操作性考虑：无。
Published specification: [RFC6015].
已发布规范：[RFC6015]。
Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media.
使用此媒体类型的应用程序：希望通过在源媒体之外发送冗余数据来提高数据包丢失恢复能力的多媒体应用程序。
Additional information: None.
其他信息：无。
Person & email address to contact for further information: Ali Begen <abegen@cisco.com> and the IETF Audio/Video Transport Working Group.
联系人和电子邮件地址，以获取更多信息：Ali Begen<abegen@cisco.com>以及IETF音频/视频传输工作组。
Intended usage: COMMON.
预期用途：普通。
Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP [RFC3550].
使用限制：此媒体类型取决于RTP帧，因此仅定义用于通过RTP传输[RFC3550]。
Author: Ali Begen <abegen@cisco.com>.
作者：阿里·贝根<abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group delegated from the IESG.
变更控制员：IESG授权的IETF音频/视频传输工作组。
Type name: application
类型名称：应用程序
Subtype name: 1d-interleaved-parityfec
子类型名称：1d交错parityfec
Required parameters:
所需参数：
o rate: The RTP timestamp (clock) rate in Hz. The (integer) rate SHALL be larger than 1000 to provide sufficient resolution to RTCP operations. However, it is RECOMMENDED to select the rate that matches the rate of the protected source RTP stream.
o 速率：RTP时间戳（时钟）速率，单位为Hz。（整数）速率应大于1000，以便为RTCP操作提供足够的分辨率。但是，建议选择与受保护源RTP流的速率匹配的速率。
o L: Number of columns of the source block. L is a positive integer that is less than or equal to 255.
o L：源块的列数。L是小于或等于255的正整数。
o D: Number of rows of the source block. D is a positive integer that is less than or equal to 255.
o D：源块的行数。D是小于或等于255的正整数。
o repair-window: The time that spans the FEC block (i.e., source packets and the corresponding repair packets). An FEC encoder processes a block of source packets and generates a number of repair packets, which are then transmitted within a certain duration not larger than the value of the repair window. At the receiver side, the FEC decoder should wait at least for the
o 修复窗口：跨越FEC块的时间（即，源数据包和相应的修复数据包）。FEC编码器处理源分组块并生成多个修复分组，然后在不大于修复窗口值的特定持续时间内发送这些修复分组。在接收器端，FEC解码器应至少等待
duration of the repair window after getting the first packet in an FEC block to allow all the repair packets to arrive (the waiting time can be adjusted if there are missing packets at the beginning of the FEC block). The FEC decoder can start decoding the already received packets sooner; however, it SHOULD NOT register an FEC decoding failure until it waits at least for the repair-window duration. The size of the repair window is specified in microseconds.
在获得FEC块中的第一个数据包后，修复窗口的持续时间，以允许所有修复数据包到达（如果在FEC块的开头有丢失的数据包，则可以调整等待时间）。FEC解码器可以更快地开始解码已经接收到的分组；但是，在至少等待修复窗口持续时间之前，不应注册FEC解码故障。修复窗口的大小以微秒为单位指定。
Optional parameters: None.
可选参数：无。
Encoding considerations: This media type is framed (see Section 4.8 in the template document [RFC4288]) and contains binary data.
编码注意事项：此媒体类型为框架（见模板文档[RFC4288]第4.8节），包含二进制数据。
Security considerations: See Section 9 of [RFC6015].
安全注意事项：见[RFC6015]第9节。
Interoperability considerations: None.
互操作性考虑：无。
Published specification: [RFC6015].
已发布规范：[RFC6015]。
Applications that use this media type: Multimedia applications that want to improve resiliency against packet loss by sending redundant data in addition to the source media.
使用此媒体类型的应用程序：希望通过在源媒体之外发送冗余数据来提高数据包丢失恢复能力的多媒体应用程序。
Additional information: None.
其他信息：无。
Person & email address to contact for further information: Ali Begen <abegen@cisco.com> and the IETF Audio/Video Transport Working Group.
联系人和电子邮件地址，以获取更多信息：Ali Begen<abegen@cisco.com>以及IETF音频/视频传输工作组。
Intended usage: COMMON.
预期用途：普通。
Restriction on usage: This media type depends on RTP framing, and hence, is only defined for transport via RTP [RFC3550].
使用限制：此媒体类型取决于RTP帧，因此仅定义用于通过RTP传输[RFC3550]。
Author: Ali Begen <abegen@cisco.com>.
作者：阿里·贝根<abegen@cisco.com>.
Change controller: IETF Audio/Video Transport Working Group delegated from the IESG.
变更控制员：IESG授权的IETF音频/视频传输工作组。
Applications that use RTP transport commonly use Session Description Protocol (SDP) [RFC4566] to describe their RTP sessions. The information that is used to specify the media types in an RTP session has specific mappings to the fields in an SDP description. In this section, we provide these mappings for the media subtype registered by this document ("1d-interleaved-parityfec"). Note that if an application does not use SDP to describe the RTP sessions, an
使用RTP传输的应用程序通常使用会话描述协议（SDP）[RFC4566]来描述其RTP会话。用于指定RTP会话中的媒体类型的信息具有到SDP描述中的字段的特定映射。在本节中，我们将为本文档注册的媒体子类型（“1d交错parityfec”）提供这些映射。请注意，如果应用程序不使用SDP来描述RTP会话，则
appropriate mapping must be defined and used to specify the media types and their parameters for the control/description protocol employed by the application.
必须定义并使用适当的映射来指定应用程序使用的控制/描述协议的媒体类型及其参数。
The mapping of the media type specification for "1d-interleaved-parityfec" and its parameters in SDP is as follows:
SDP中“1d交错parityfec”的媒体类型规范及其参数映射如下：
o The media type (e.g., "application") goes into the "m=" line as the media name.
o 媒体类型（例如，“应用程序”）作为媒体名称进入“m=”行。
o The media subtype ("1d-interleaved-parityfec") goes into the "a=rtpmap" line as the encoding name. The RTP clock rate parameter ("rate") also goes into the "a=rtpmap" line as the clock rate.
o 媒体子类型（“1d交错parityfec”）作为编码名称进入“a=rtpmap”行。RTP时钟速率参数（“速率”）也作为时钟速率进入“a=rtpmap”行。
o The remaining required payload-format-specific parameters go into the "a=fmtp" line by copying them directly from the media type string as a semicolon-separated list of parameter=value pairs.
o 其余所需的特定于有效负载格式的参数直接从媒体类型字符串复制到“a=fmtp”行，作为参数=值对的分号分隔列表。
SDP examples are provided in Section 7.
第7节提供了SDP示例。
When offering 1-D interleaved parity FEC over RTP using SDP in an Offer/Answer model [RFC3264], the following considerations apply:
在提供/应答模型[RFC3264]中使用SDP通过RTP提供1-D交错奇偶校验FEC时，应考虑以下因素：
o Each combination of the L and D parameters produces a different FEC data and is not compatible with any other combination. A sender application may desire to offer multiple offers with different sets of L and D values as long as the parameter values are valid. The receiver SHOULD normally choose the offer that has a sufficient amount of interleaving. If multiple such offers exist, the receiver may choose the offer that has the lowest overhead or the one that requires the smallest amount of buffering. The selection depends on the application requirements.
o L和D参数的每个组合产生不同的FEC数据，并且与任何其他组合不兼容。只要参数值有效，发送方应用程序可能希望提供具有不同L和D值集的多个报价。接收方通常应选择具有足够交织量的报价。如果存在多个这样的提议，则接收机可以选择开销最低的提议或需要最小缓冲量的提议。选择取决于应用要求。
o The value for the repair-window parameter depends on the L and D values and cannot be chosen arbitrarily. More specifically, L and D values determine the lower limit for the repair-window size. The upper limit of the repair-window size does not depend on the L and D values.
o 修理窗口参数的值取决于L和D值，不能任意选择。更具体地说，L和D值确定维修窗口大小的下限。维修窗口大小的上限不取决于L和D值。
o Although combinations with the same L and D values but with different repair-window sizes produce the same FEC data, such combinations are still considered different offers. The size of the repair-window is related to the maximum delay between the
o 尽管具有相同L和D值但具有不同修复窗口大小的组合产生相同的FEC数据，但此类组合仍被视为不同的报价。维修窗口的大小与两次维修之间的最大延迟有关
transmission of a source packet and the associated repair packet. This directly impacts the buffering requirement on the receiver side, and the receiver must consider this when choosing an offer.
传输源分组和相关联的修复分组。这直接影响接收机侧的缓冲需求，并且接收机在选择要约时必须考虑这一点。
o There are no optional format parameters defined for this payload. Any unknown option in the offer MUST be ignored and deleted from the answer. If FEC is not desired by the receiver, it can be deleted from the answer.
o 没有为此负载定义可选的格式参数。报价中的任何未知选项都必须忽略并从答案中删除。如果接收机不需要FEC，可以从应答中删除。
In declarative usage, like SDP in the Real-time Streaming Protocol (RTSP) [RFC2326] or the Session Announcement Protocol (SAP) [RFC2974], the following considerations apply:
在声明性使用中，如实时流协议（RTSP）[RFC2326]或会话公告协议（SAP）[RFC2974]中的SDP，以下注意事项适用：
o The payload format configuration parameters are all declarative and a participant MUST use the configuration that is provided for the session.
o 有效负载格式配置参数都是声明性的，参与者必须使用为会话提供的配置。
o More than one configuration may be provided (if desired) by declaring multiple RTP payload types. In that case, the receivers should choose the repair flow that is best for them.
o 通过声明多个RTP有效负载类型，可以提供多个配置（如果需要）。在这种情况下，接收方应选择最适合自己的维修流程。
This section provides a complete specification of the 1-D interleaved parity code and its RTP payload format.
本节提供了1-D交织奇偶校验码及其RTP有效负载格式的完整规范。
The following sections specify the steps involved in generating the repair packets and reconstructing the missing source packets from the repair packets.
以下部分指定生成修复包和从修复包重建丢失的源包所涉及的步骤。
The RTP header of a repair packet is formed based on the guidelines given in Section 4.2.
修理包的RTP报头是根据第4.2节给出的指南形成的。
The FEC header includes 16 octets. It is constructed by applying the XOR operation on the bit strings that are generated from the individual source packets protected by this particular repair packet. The set of the source packets that are associated with a given repair packet can be computed by the formula given in Section 6.3.1.
FEC报头包括16个八位字节。它是通过对由该特定修复包保护的各个源包生成的位字符串应用异或操作来构造的。可通过第6.3.1节中给出的公式计算与给定修复数据包相关的源数据包集。
The bit string is formed for each source packet by concatenating the following fields together in the order specified:
通过按指定顺序将以下字段串联在一起，为每个源数据包形成位字符串：
o Padding bit (1 bit) (This is the most significant bit of the bit string.)
o 填充位（1位）（这是位字符串的最高有效位。）
o Extension bit (1 bit)
o 扩展位（1位）
o CC field (4 bits)
o CC字段（4位）
o Marker bit (1 bit)
o 标记位（1位）
o PT field (7 bits)
o PT字段（7位）
o Timestamp (32 bits)
o 时间戳（32位）
o Unsigned network-ordered 16-bit representation of the source packet length in bytes minus 12 (for the fixed RTP header), i.e., the sum of the lengths of all the following if present: the CSRC list, header extension, RTP payload, and RTP padding (16 bits).
o 源数据包长度的无符号网络顺序16位表示，字节数减去12（对于固定RTP报头），即以下所有数据包长度之和（如果存在）：CSC列表、报头扩展、RTP有效负载和RTP填充（16位）。
o If CC is nonzero, the CSRC list (variable length)
o 如果CC为非零，则为CSC列表（可变长度）
o If X is 1, the header extension (variable length)
o 如果X为1，则标头扩展名（可变长度）
o Payload (variable length)
o 有效载荷（可变长度）
o Padding, if present (variable length)
o 填充，如果存在（可变长度）
Note that if the lengths of the source packets are not equal, each shorter packet MUST be padded to the length of the longest packet by adding octet(s) of 0 at the end. Due to this possible padding and mandatory FEC header, a repair packet has a larger size than the source packets it protects. This may cause problems if the resulting repair packet size exceeds the Maximum Transmission Unit (MTU) size of the path over which the repair flow is sent.
请注意，如果源数据包的长度不相等，则每个较短数据包必须通过在末尾添加0的八位字节来填充到最长数据包的长度。由于这种可能的填充和强制FEC报头，修复数据包的大小大于它所保护的源数据包的大小。如果产生的修复包大小超过发送修复流的路径的最大传输单元（MTU）大小，则这可能会导致问题。
By applying the parity operation on the bit strings produced from the source packets, we generate the FEC bit string. Some parts of the RTP header and the FEC header of the repair packet are generated from the FEC bit string as follows:
通过对源数据包生成的位字符串应用奇偶校验操作，我们生成FEC位字符串。修复分组的RTP报头和FEC报头的某些部分由FEC比特串生成，如下所示：
o The first (most significant) bit in the FEC bit string is written into the Padding bit in the RTP header of the repair packet.
o FEC位字符串中的第一个（最高有效）位写入修复数据包RTP报头中的填充位。
o The next bit in the FEC bit string is written into the Extension bit in the RTP header of the repair packet.
o FEC位字符串中的下一位写入修复数据包RTP报头中的扩展位。
o The next 4 bits of the FEC bit string are written into the CC field in the RTP header of the repair packet.
o FEC位串的下4位写入修复包的RTP报头中的CC字段。
o The next bit of the FEC bit string is written into the Marker bit in the RTP header of the repair packet.
o FEC位字符串的下一位写入修复数据包RTP报头中的标记位。
o The next 7 bits of the FEC bit string are written into the PT recovery field in the FEC header.
o FEC位串的下7位写入FEC报头中的PT recovery字段。
o The next 32 bits of the FEC bit string are written into the TS recovery field in the FEC header.
o FEC位字符串的下32位写入FEC报头中的TS恢复字段。
o The next 16 bits are written into the Length recovery field in the FEC header. This allows the FEC procedure to be applied even when the lengths of the protected source packets are not identical.
o 接下来的16位写入FEC报头中的长度恢复字段。这允许即使在受保护源分组的长度不相同时也应用FEC过程。
o The remaining bits are set to be the payload of the repair packet.
o 剩余的位被设置为修复包的有效载荷。
The remaining parts of the FEC header are set as follows:
FEC头的其余部分设置如下：
o The SN base low field MUST be set to the lowest sequence number, taking wraparound into account, of those source packets protected by this repair packet.
o SN base low字段必须设置为受此修复数据包保护的源数据包的最低序列号（考虑到环绕）。
o The E bit MUST be set to 1 to extend the [RFC2733] FEC header.
o E位必须设置为1以扩展[RFC2733]FEC头。
o The Mask field SHALL be set to 0 and ignored by the receiver.
o 掩码字段应设置为0，并由接收器忽略。
o The N bit SHALL be set to 0 and ignored by the receiver.
o N位应设置为0，并由接收器忽略。
o The D bit SHALL be set to 0 and ignored by the receiver.
o D位应设置为0，并由接收器忽略。
o The Type field MUST be set to 0 and ignored by the receiver.
o “类型”字段必须设置为0并被接收器忽略。
o The Index field SHALL be set to 0 and ignored by the receiver.
o 索引字段应设置为0，并由接收器忽略。
o The Offset field MUST be set to the number of columns of the source block (L).
o 偏移字段必须设置为源块的列数（L）。
o The NA field MUST be set to the number of rows of the source block (D).
o NA字段必须设置为源块（D）的行数。
o The SN base ext field SHALL be set to 0 and ignored by the receiver.
o SN base ext字段应设置为0，并由接收器忽略。
This section describes the recovery procedures that are required to reconstruct the missing source packets. The recovery process has two steps. In the first step, the FEC decoder determines which source
本节介绍重建丢失的源数据包所需的恢复过程。恢复过程有两个步骤。在第一步中，FEC解码器确定哪个源
and repair packets should be used in order to recover a missing packet. In the second step, the decoder recovers the missing packet, which consists of an RTP header and RTP payload.
为了恢复丢失的数据包，应该使用修复数据包。在第二步中，解码器恢复丢失的数据包，该数据包由RTP报头和RTP有效负载组成。
In the following, we describe the RECOMMENDED algorithms for the first and second steps. Based on the implementation, different algorithms MAY be adopted. However, the end result MUST be identical to the one produced by the algorithms described below.
下面，我们将介绍第一步和第二步的推荐算法。根据实现，可以采用不同的算法。但是，最终结果必须与下面描述的算法产生的结果相同。
The first step is to associate the source and repair packets. The SN base low field in the FEC header shows the lowest sequence number of the source packets that form the particular column. In addition, the information of how many source packets are available in each column and row is available from the media type parameters specified in the SDP description. This set of information uniquely identifies all of the source packets associated with a given repair packet.
第一步是关联源数据包和修复数据包。FEC报头中的SN base low字段显示形成特定列的源分组的最低序列号。此外，可以从SDP描述中指定的媒体类型参数中获得关于每列和每行中有多少个源数据包可用的信息。这组信息唯一地标识与给定修复数据包关联的所有源数据包。
Mathematically, for any received repair packet, p*, we can determine the sequence numbers of the source packets that are protected by this repair packet as follows:
从数学上讲，对于任何接收到的修复数据包p*，我们可以确定受该修复数据包保护的源数据包的序列号，如下所示：
p*_snb + i * L (modulo 65536)
p*_snb + i * L (modulo 65536)
where p*_snb denotes the value in the SN base low field of the FEC header of the p*, L is the number of columns of the source block and
其中p*_snb表示p*的FEC报头的SN base low字段中的值，L是源块的列数，并且
0 <= i < D
0 <= i < D
where D is the number of rows of the source block.
其中D是源块的行数。
We denote the set of the source packets associated with repair packet p* by set T(p*). Note that in a source block whose size is L columns by D rows, set T includes D source packets. Recall that 1-D interleaved FEC protection can fully recover the missing information if there is only one source packet missing in set T. If the repair packet that protects the source packets in set T is missing, or the repair packet is available but two or more source packets are missing, then missing source packets in set T cannot be recovered by 1-D interleaved FEC protection.
我们用集合T（p*）表示与修复分组p*相关联的源分组集合。注意，在大小为L列乘以D行的源块中，集合T包括D个源数据包。回想一下，如果集合T中只有一个源数据包丢失，则1-D交错FEC保护可以完全恢复丢失的信息。如果保护集合T中源数据包的修复数据包丢失，或者修复数据包可用，但两个或更多源数据包丢失，然后，集合T中丢失的源数据包无法通过1-D交错FEC保护恢复。
For a given set T, the procedure for the recovery of the RTP header of the missing packet, whose sequence number is denoted by SEQNUM, is as follows:
对于给定的集合T，恢复丢失分组的RTP报头（其序列号由SEQNUM表示）的过程如下：
1. For each of the source packets that are successfully received in set T, compute the bit string as described in Section 6.2.
1. 对于在集合T中成功接收到的每个源数据包，按照第6.2节所述计算位字符串。
2. For the repair packet associated with set T, compute the bit string in the same fashion except use the PT recovery field instead of the PT field and TS recovery field instead of the Timestamp field, and set the CSRC list, header extension and padding to null regardless of the values of the CC field, X bit, and P bit.
2. 对于与集合T相关联的修复数据包，以相同的方式计算位字符串，除了使用PT恢复字段而不是PT字段和TS恢复字段而不是时间戳字段外，并将CSC列表、报头扩展和填充设置为null，而不考虑CC字段、X位和P位的值。
3. If any of the bit strings generated from the source packets are shorter than the bit string generated from the repair packet, pad them to be the same length as the bit string generated from the repair packet. For padding, the padding of octet 0 MUST be added at the end of the bit string.
3. 如果从源数据包生成的任何位字符串短于从修复数据包生成的位字符串，则将其填充为与从修复数据包生成的位字符串相同的长度。对于填充，必须在位字符串的末尾添加八位字节0的填充。
4. Calculate the recovered bit string as the XOR of the bit strings generated from all source packets in set T and the FEC bit string generated from the repair packet associated with set T.
4. 将恢复的位字符串计算为集合T中所有源数据包生成的位字符串与集合T关联的修复数据包生成的FEC位字符串的异或。
5. Create a new packet with the standard 12-byte RTP header and no payload.
5. 创建一个具有标准12字节RTP报头且无有效负载的新数据包。
6. Set the version of the new packet to 2.
6. 将新数据包的版本设置为2。
7. Set the Padding bit in the new packet to the first bit in the recovered bit string.
7. 将新数据包中的填充位设置为恢复的位字符串中的第一位。
8. Set the Extension bit in the new packet to the next bit in the recovered bit string.
8. 将新数据包中的扩展位设置为恢复位字符串中的下一位。
9. Set the CC field to the next 4 bits in the recovered bit string.
9. 将CC字段设置为恢复的位字符串中的下4位。
10. Set the Marker bit in the new packet to the next bit in the recovered bit string.
10. 将新数据包中的标记位设置为恢复位字符串中的下一位。
11. Set the Payload type in the new packet to the next 7 bits in the recovered bit string.
11. 将新数据包中的有效负载类型设置为恢复的位字符串中的下7位。
12. Set the SN field in the new packet to SEQNUM.
12. 将新数据包中的SN字段设置为SEQNUM。
13. Set the TS field in the new packet to the next 32 bits in the recovered bit string.
13. 将新数据包中的TS字段设置为恢复位字符串中的下一个32位。
14. Take the next 16 bits of the recovered bit string and set the new variable Y to whatever unsigned integer this represents (assuming network order). Convert Y to host order and then take Y bytes from the recovered bit string and append them to the new
14. 取恢复的位字符串的下16位，并将新变量Y设置为它表示的无符号整数（假设网络顺序）。将Y转换为主机顺序，然后从恢复的位字符串中提取Y字节并将其附加到新的
packet. Y represents the length of the new packet in bytes minus 12 (for the fixed RTP header), i.e., the sum of the lengths of all the following if present: the CSRC list, header extension, RTP payload, and RTP padding.
小包裹Y表示新数据包的长度，以字节减去12为单位（对于固定RTP报头），即以下所有数据包的长度之和（如果存在）：CSC列表、报头扩展、RTP有效负载和RTP填充。
15. Set the SSRC of the new packet to the SSRC of the source RTP stream.
15. 将新数据包的SSRC设置为源RTP流的SSRC。
This procedure completely recovers both the header and payload of an RTP packet.
此过程完全恢复RTP数据包的报头和有效负载。
This section provides an SDP [RFC4566] example. The following example uses the FEC grouping semantics [RFC5956].
本节提供了一个SDP[RFC4566]示例。以下示例使用FEC分组语义[RFC5956]。
In this example, we have one source video stream (mid:S1) and one FEC repair stream (mid:R1). We form one FEC group with the "a=group: FEC-FR S1 R1" line. The source and repair streams are sent to the same port on different multicast groups. The repair window is set to 200 ms.
在本例中，我们有一个源视频流（mid:S1）和一个FEC修复流（mid:R1）。我们用“a=组：FEC-FR S1 R1”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。维修窗口设置为200毫秒。
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=Interleaved Parity FEC Example
t=0 0
a=group:FEC-FR S1 R1
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 110
c=IN IP4 233.252.0.2/127
a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L=5; D=10; repair-window=200000
a=mid:R1
v=0
o=ali 1122334455 1122334466 IN IP4 fec.example.com
s=Interleaved Parity FEC Example
t=0 0
a=group:FEC-FR S1 R1
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 110
c=IN IP4 233.252.0.2/127
a=rtpmap:110 1d-interleaved-parityfec/90000
a=fmtp:110 L=5; D=10; repair-window=200000
a=mid:R1
FEC is an effective approach to provide applications with resiliency against packet losses. However, in networks where the congestion is a major contributor to the packet loss, the potential impacts of using FEC SHOULD be considered carefully before injecting the repair flows into the network. In particular, in bandwidth-limited networks, FEC repair flows may consume most or all of the available bandwidth and may consequently congest the network. In such cases, the applications MUST NOT arbitrarily increase the amount of FEC
FEC是为应用程序提供抗数据包丢失的弹性的有效方法。然而，在拥塞是分组丢失的主要原因的网络中，在将修复流注入网络之前，应仔细考虑使用FEC的潜在影响。特别地，在带宽有限的网络中，FEC修复流可能消耗大部分或全部可用带宽，并且可能因此导致网络拥塞。在这种情况下，应用程序不得随意增加FEC的量
protection since doing so may lead to a congestion collapse. If desired, stronger FEC protection MAY be applied only after the source rate has been reduced.
保护，因为这样做可能导致拥塞崩溃。如果需要，只有在源速率降低后，才能应用更强的FEC保护。
In a network-friendly implementation, an application SHOULD NOT send/ receive FEC repair flows if it knows that sending/receiving those FEC repair flows would not help at all in recovering the missing packets. Such a practice helps reduce the amount of wasted bandwidth. It is RECOMMENDED that the amount of FEC protection is adjusted dynamically based on the packet loss rate observed by the applications.
在网络友好的实现中，如果应用程序知道发送/接收FEC修复流对恢复丢失的数据包毫无帮助，则不应发送/接收FEC修复流。这种做法有助于减少浪费的带宽。建议根据应用程序观察到的丢包率动态调整FEC保护量。
In multicast scenarios, it may be difficult to optimize the FEC protection per receiver. If there is a large variation among the levels of FEC protection needed by different receivers, it is RECOMMENDED that the sender offers multiple repair flows with different levels of FEC protection and the receivers join the corresponding multicast sessions to receive the repair flow(s) that is best for them.
在多播场景中，可能很难优化每个接收器的FEC保护。如果不同接收器所需的FEC保护级别之间存在较大差异，建议发送方提供具有不同FEC保护级别的多个修复流，并且接收器加入相应的多播会话以接收对其最有利的修复流。
RTP packets using the payload format defined in this specification are subject to the security considerations discussed in the RTP specification [RFC3550] and in any applicable RTP profile.
使用本规范中定义的有效负载格式的RTP数据包应遵守RTP规范[RFC3550]和任何适用RTP配置文件中讨论的安全注意事项。
The main security considerations for the RTP packet carrying the RTP payload format defined within this memo are confidentiality, integrity, and source authenticity. Confidentiality is achieved by encrypting the RTP payload. Altering the FEC packets can have a big impact on the reconstruction operation. An attack that changes some bits in the FEC packets can have a significant effect on the calculation and the recovery of the source packets. For example, changing the length recovery field can result in the recovery of a packet that is too long. Depending on the application, it may be helpful to perform a sanity check on the received source and FEC packets before performing the recovery operation and to determine the validity of the recovered packets before using them.
携带本备忘录中定义的RTP有效载荷格式的RTP数据包的主要安全注意事项是机密性、完整性和源真实性。机密性是通过加密RTP有效负载来实现的。改变FEC数据包会对重建操作产生很大影响。改变FEC数据包中某些位的攻击会对源数据包的计算和恢复产生重大影响。例如，更改长度恢复字段可能导致恢复过长的数据包。根据应用，在执行恢复操作之前对接收到的源和FEC分组执行健全性检查以及在使用它们之前确定恢复的分组的有效性可能是有帮助的。
The integrity of the RTP packets is achieved through a suitable cryptographic integrity protection mechanism. Such a cryptographic system may also allow the authentication of the source of the payload. A suitable security mechanism for this RTP payload format should provide source authentication capable of determining if an RTP packet is from a member of the RTP session.
RTP数据包的完整性是通过适当的密码完整性保护机制实现的。这样的密码系统还可以允许对有效载荷的源进行认证。此RTP有效负载格式的适当安全机制应提供能够确定RTP分组是否来自RTP会话的成员的源认证。
Note that the appropriate mechanism to provide security to RTP and payloads following this memo may vary. It is dependent on the application, transport and signaling protocol employed. Therefore, a
请注意，根据本备忘录为RTP和有效负载提供安全性的适当机制可能会有所不同。它取决于所采用的应用、传输和信令协议。因此,
single mechanism is not sufficient, although if suitable, using the Secure Real-time Transport Protocol (SRTP) [RFC3711] is RECOMMENDED. Other mechanisms that may be used are IPsec [RFC4301] and Transport Layer Security (TLS) [RFC5246]; other alternatives may exist.
单一机制是不够的，但如果合适，建议使用安全实时传输协议（SRTP）[RFC3711]。可使用的其他机制包括IPsec[RFC4301]和传输层安全（TLS）[RFC5246]；可能存在其他替代方案。
If FEC protection is applied on already encrypted source packets, there is no need for additional encryption. However, if the source packets are encrypted after FEC protection is applied, the FEC packets should be cryptographically as secure as the source packets. Failure to provide an equal level of confidentiality, integrity, and authentication to the FEC packets can compromise the source packets' confidentiality, integrity or authentication since the FEC packets are generated by applying XOR operation across the source packets.
如果对已经加密的源数据包应用FEC保护，则不需要额外加密。然而，如果在应用FEC保护之后对源数据包进行加密，则FEC数据包应在加密方面与源数据包一样安全。未能向FEC分组提供同等级别的机密性、完整性和认证可损害源分组的机密性、完整性或认证，因为FEC分组是通过在源分组上应用异或操作生成的。
New media subtypes are subject to IANA registration. For the registration of the payload format and its parameters introduced in this document, refer to Section 5.
新媒体子类型需要IANA注册。有关本文件中介绍的有效载荷格式及其参数的注册，请参阅第5节。
A major part of this document is borrowed from [RFC2733], [RFC5109], and [SMPTE2022-1]. Thus, the author would like to thank the authors and editors of these earlier specifications. The author also thanks Colin Perkins for his constructive suggestions for this document.
本文件的主要部分来自[RFC2733]、[RFC5109]和[SMPTE2022-1]。因此，作者要感谢这些早期规范的作者和编辑。作者还感谢Colin Perkins对本文件提出的建设性建议。
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2119]Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3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。
[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 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。
[RFC4288] Freed, N. and J. Klensin, "Media Type Specifications and Registration Procedures", BCP 13, RFC 4288, December 2005.
[RFC4288]Freed，N.和J.Klensin，“介质类型规范和注册程序”，BCP 13，RFC 4288，2005年12月。
[RFC4855] Casner, S., "Media Type Registration of RTP Payload Formats", RFC 4855, February 2007.
[RFC4855]Casner，S.，“RTP有效负载格式的媒体类型注册”，RFC 48552007年2月。
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002.
[RFC3264]Rosenberg，J.和H.Schulzrinne，“具有会话描述协议（SDP）的提供/应答模型”，RFC 3264，2002年6月。
[DVB-AL-FEC] Begen, A. and T. Stockhammer, "Guidelines for Implementing DVB-IPTV Application-Layer Hybrid FEC Protection", Work in Progress, December 2009.
[DVB-AL-FEC]Begen，A.和T.Stockhammer，“实施DVB-IPTV应用层混合FEC保护的指南”，正在进行的工作，2009年12月。
[RFC2733] Rosenberg, J. and H. Schulzrinne, "An RTP Payload Format for Generic Forward Error Correction", RFC 2733, December 1999.
[RFC2733]Rosenberg，J.和H.Schulzrinne，“通用前向纠错的RTP有效载荷格式”，RFC 2733，1999年12月。
[RFC3009] Rosenberg, J. and H. Schulzrinne, "Registration of parityfec MIME types", RFC 3009, November 2000.
[RFC3009]Rosenberg，J.和H.Schulzrinne，“parityfec MIME类型的注册”，RFC 3009，2000年11月。
[RFC5109] Li, A., "RTP Payload Format for Generic Forward Error Correction", RFC 5109, December 2007.
[RFC5109]Li，A.“通用前向纠错的RTP有效载荷格式”，RFC 5109，2007年12月。
[ETSI-TS-102-034] ETSI TS 102 034 V1.4.1, "Transport of MPEG 2 TS Based DVB Services over IP Based Networks", August 2009.
[ETSI-TS-102-034]ETSI TS 102 034 V1.4.1，“基于IP网络的基于MPEG 2 TS的DVB服务传输”，2009年8月。
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2326]Schulzrinne，H.，Rao，A.，和R.Lanphier，“实时流协议（RTSP）”，RFC2326，1998年4月。
[RFC2974] Handley, M., Perkins, C., and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000.
[RFC2974]Handley，M.，Perkins，C.，和E.Whelan，“会话公告协议”，RFC 2974，2000年10月。
[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。
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. Norrman, "The Secure Real-time Transport Protocol (SRTP)", RFC 3711, March 2004.
[RFC3711]Baugher，M.，McGrew，D.，Naslund，M.，Carrara，E.，和K.Norrman，“安全实时传输协议（SRTP）”，RFC 37112004年3月。
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the Internet Protocol", RFC 4301, December 2005.
[RFC4301]Kent，S.和K.Seo，“互联网协议的安全架构”，RFC 43012005年12月。
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5246]Dierks，T.和E.Rescorla，“传输层安全（TLS）协议版本1.2”，RFC 5246，2008年8月。
Author's Address
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