Internet Engineering Task Force (IETF) H. Kaplan, Ed.
Request for Comments: 6849 Acme Packet
Category: Standards Track K. Hedayat
ISSN: 2070-1721 EXFO
N. Venna
Saperix
P. Jones
Cisco Systems, Inc.
N. Stratton
BlinkMind, Inc.
February 2013
Internet Engineering Task Force (IETF) H. Kaplan, Ed.
Request for Comments: 6849 Acme Packet
Category: Standards Track K. Hedayat
ISSN: 2070-1721 EXFO
N. Venna
Saperix
P. Jones
Cisco Systems, Inc.
N. Stratton
BlinkMind, Inc.
February 2013
An Extension to the Session Description Protocol (SDP) and Real-time Transport Protocol (RTP) for Media Loopback
用于媒体环回的会话描述协议（SDP）和实时传输协议（RTP）的扩展
Abstract
摘要
The wide deployment of Voice over IP (VoIP), real-time text, and Video over IP services has introduced new challenges in managing and maintaining real-time voice/text/video quality, reliability, and overall performance. In particular, media delivery is an area that needs attention. One method of meeting these challenges is monitoring the media delivery performance by looping media back to the transmitter. This is typically referred to as "active monitoring" of services. Media loopback is especially popular in ensuring the quality of transport to the edge of a given VoIP, real-time text, or Video over IP service. Today, in networks that deliver real-time media, short of running 'ping' and 'traceroute' to the edge, administrators are left without the necessary tools to actively monitor, manage, and diagnose quality issues with their service. The extension defined herein adds new Session Description Protocol (SDP) media types and attributes that enable establishment of media sessions where the media is looped back to the transmitter. Such media sessions will serve as monitoring and troubleshooting tools by providing the means for measurement of more advanced VoIP, real-time text, and Video over IP performance metrics.
IP语音（VoIP）、实时文本和IP视频服务的广泛部署在管理和维护实时语音/文本/视频质量、可靠性和总体性能方面带来了新的挑战。特别是，媒体交付是一个需要关注的领域。应对这些挑战的一种方法是通过将介质循环回传送器来监控介质传输性能。这通常被称为服务的“主动监视”。媒体环回在确保到给定VoIP、实时文本或IP视频服务边缘的传输质量方面尤其流行。如今，在提供实时媒体的网络中，由于缺少到边缘的“ping”和“traceroute”，管理员没有必要的工具来主动监控、管理和诊断服务质量问题。本文定义的扩展添加了新的会话描述协议（SDP）媒体类型和属性，这些媒体类型和属性能够建立媒体会话，其中媒体被循环回发送器。此类媒体会话将作为监控和故障排除工具，提供更先进的VoIP、实时文本和IP视频性能指标的测量手段。
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/rfc6849.
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/rfc6849.
Copyright Notice
版权公告
Copyright (c) 2013 IETF Trust and the persons identified as the document authors. All rights reserved.
版权所有（c）2013 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。
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.
本文件可能包含2008年11月10日之前发布或公开的IETF文件或IETF贡献中的材料。控制某些材料版权的人员可能未授予IETF信托允许在IETF标准流程之外修改此类材料的权利。在未从控制此类材料版权的人员处获得充分许可的情况下，不得在IETF标准流程之外修改本文件，也不得在IETF标准流程之外创建其衍生作品，除了将其格式化以RFC形式发布或将其翻译成英语以外的其他语言。
Table of Contents
目录
1. Introduction ....................................................3
1.1. Use Cases Supported ........................................4
2. Terminology .....................................................6
3. Overview of Operation ...........................................6
3.1. SDP Offerer Behavior .......................................6
3.2. SDP Answerer Behavior ......................................7
4. New SDP Attributes ..............................................7
4.1. Loopback-Type Attribute ....................................7
4.2. Loopback-Role Attributes: loopback-source and
loopback-mirror ............................................8
5. Rules for Generating the SDP Offer/Answer .......................9
5.1. Generating the SDP Offer for Loopback Session ..............9
5.2. Generating the SDP Answer for Loopback Session ............10
5.3. Offerer Processing of the SDP Answer ......................12
5.4. Modifying the Session .....................................12
5.5. Establishing Sessions between Entities behind NATs ........12
6. RTP Requirements ...............................................13
7. Payload Formats for Packet Loopback ............................13
7.1. Encapsulated Payload Format ...............................14
7.2. Direct Loopback RTP Payload Format ........................16
8. SRTP Behavior ..................................................17
9. RTCP Requirements ..............................................18
10. Congestion Control ............................................18
11. Examples ......................................................18
11.1. Offer for Specific Media Loopback Type ...................19
11.2. Offer for Choice of Media Loopback Type ..................19
11.3. Answerer Rejecting Loopback Media ........................20
12. Security Considerations .......................................21
13. Implementation Considerations .................................22
14. IANA Considerations ...........................................22
14.1. SDP Attributes ...........................................22
14.2. Media Types ..............................................23
15. Acknowledgements ..............................................31
16. References ....................................................31
16.1. Normative References .....................................31
16.2. Informative References ...................................32
1. Introduction ....................................................3
1.1. Use Cases Supported ........................................4
2. Terminology .....................................................6
3. Overview of Operation ...........................................6
3.1. SDP Offerer Behavior .......................................6
3.2. SDP Answerer Behavior ......................................7
4. New SDP Attributes ..............................................7
4.1. Loopback-Type Attribute ....................................7
4.2. Loopback-Role Attributes: loopback-source and
loopback-mirror ............................................8
5. Rules for Generating the SDP Offer/Answer .......................9
5.1. Generating the SDP Offer for Loopback Session ..............9
5.2. Generating the SDP Answer for Loopback Session ............10
5.3. Offerer Processing of the SDP Answer ......................12
5.4. Modifying the Session .....................................12
5.5. Establishing Sessions between Entities behind NATs ........12
6. RTP Requirements ...............................................13
7. Payload Formats for Packet Loopback ............................13
7.1. Encapsulated Payload Format ...............................14
7.2. Direct Loopback RTP Payload Format ........................16
8. SRTP Behavior ..................................................17
9. RTCP Requirements ..............................................18
10. Congestion Control ............................................18
11. Examples ......................................................18
11.1. Offer for Specific Media Loopback Type ...................19
11.2. Offer for Choice of Media Loopback Type ..................19
11.3. Answerer Rejecting Loopback Media ........................20
12. Security Considerations .......................................21
13. Implementation Considerations .................................22
14. IANA Considerations ...........................................22
14.1. SDP Attributes ...........................................22
14.2. Media Types ..............................................23
15. Acknowledgements ..............................................31
16. References ....................................................31
16.1. Normative References .....................................31
16.2. Informative References ...................................32
The overall quality, reliability, and performance of VoIP, real-time text, and Video over IP services rely on the performance and quality of the media path. In order to assure the quality of the delivered media, there is a need to monitor the performance of the media transport. One method of monitoring and managing the overall quality of real-time VoIP, real-time text, and Video over IP services is
VoIP、实时文本和IP视频服务的总体质量、可靠性和性能取决于媒体路径的性能和质量。为了确保交付介质的质量，需要监控介质传输的性能。监控和管理实时VoIP、实时文本和IP视频服务总体质量的一种方法是
through monitoring the quality of the media in an active session. This type of "active monitoring" of services is a method of proactively managing the performance and quality of VoIP-based services.
通过监视活动会话中媒体的质量。这种类型的服务“主动监控”是一种主动管理基于VoIP的服务的性能和质量的方法。
The goal of active monitoring is to measure the media quality of a VoIP, real-time text, or Video over IP session. A way to achieve this goal is to request an endpoint to loop media back to the other endpoint and to provide media statistics (e.g., RTP Control Protocol (RTCP) [RFC3550] and RTCP Extended Reports (RTCP-XR) [RFC3611] information). Another method involves deployment of special endpoints that always loop incoming media back for all sessions. Although the latter method has been used and is functional, it does not scale to support large networks and introduces new network management challenges. Further, it does not offer the granularity of testing a specific endpoint that may be exhibiting problems.
主动监控的目标是测量VoIP、实时文本或IP视频会话的媒体质量。实现此目标的一种方法是请求一个端点将媒体循环回另一个端点，并提供媒体统计信息（例如，RTP控制协议（RTCP）[RFC3550]和RTCP扩展报告（RTCP-XR）[RFC3611]信息）。另一种方法涉及部署特殊端点，这些端点总是将所有会话的传入媒体循环回。尽管后一种方法已被使用且功能齐全，但它无法扩展以支持大型网络，并带来了新的网络管理挑战。此外，它没有提供测试可能出现问题的特定端点的粒度。
The extension defined in this document introduces new SDP media types and attributes that enable establishment of media sessions where the media is looped back to the transmitter. The SDP offer/answer model [RFC3264] is used to establish a loopback connection. Furthermore, this extension provides guidelines on handling RTP [RFC3550], as well as usage of RTCP [RFC3550] and RTCP-XR [RFC3611] for reporting media-related measurements.
本文档中定义的扩展引入了新的SDP介质类型和属性，可在介质循环回传送器的情况下建立介质会话。SDP提供/应答模型[RFC3264]用于建立环回连接。此外，此扩展还提供了有关处理RTP[RFC3550]的指南，以及使用RTCP[RFC3550]和RTCP-XR[RFC3611]报告媒体相关测量的指南。
As a matter of terminology in this document, packets flow from one peer acting as a "loopback source", to the other peer acting as a "loopback mirror", which in turn returns packets to the loopback source. In advance of the session, the peers negotiate to determine which one acts in which role, using the SDP offer/answer exchange. The negotiation also includes details such as the type of loopback to be used.
作为本文档中的术语，数据包从一个作为“环回源”的对等方流向另一个作为“环回镜像”的对等方，后者反过来将数据包返回到环回源。在会话之前，对等方使用SDP提供/应答交换进行协商，以确定谁扮演哪个角色。协商还包括要使用的环回类型等详细信息。
This specification supports three use cases: "encapsulated packet loopback", "direct loopback", and "media loopback". These are distinguished by the treatment of incoming RTP packets at the loopback mirror.
本规范支持三种用例：“封装包环回”、“直接环回”和“媒体环回”。通过在环回镜像处处理传入的RTP数据包来区分它们。
In the encapsulated packet loopback case, the entire incoming RTP packet is encapsulated as payload within an outer RTP packet that is specific to this use case and specified in Section 7.1. The encapsulated packet is returned to the loopback source. The loopback source can generate statistics for one-way path performance up to the RTP level for each direction of travel by examining sequence numbers
在封装的数据包环回情况下，整个传入RTP数据包被封装为外部RTP数据包内的有效负载，该外部RTP数据包特定于该用例，并在第7.1节中规定。封装的数据包返回到环回源。通过检查序列号，环回源可以为每个行程方向的单向路径性能生成统计数据，直至RTP级别
and timestamps in the encapsulating outer RTP header and the encapsulated RTP packet payload. The loopback source can also play back the returned media content for evaluation.
以及封装的外部RTP报头和封装的RTP分组有效载荷中的时间戳。环回源还可以播放返回的媒体内容以进行评估。
Because the encapsulating RTP packet header extends the packet size, it could encounter difficulties in an environment where the original RTP packet size is close to the path Maximum Transmission Unit (MTU) size. The encapsulating payload format therefore offers the possibility of RTP-level fragmentation of the returned packets. The use of this facility could affect statistics derived for the return path. In addition, the increased bit rate required in the return direction may affect these statistics more directly in a restricted-bandwidth situation.
由于封装RTP分组报头扩展了分组大小，因此在原始RTP分组大小接近路径最大传输单元（MTU）大小的环境中可能会遇到困难。因此，封装有效负载格式提供了返回数据包的RTP级分段的可能性。使用此工具可能会影响为返回路径派生的统计信息。此外，在带宽受限的情况下，返回方向上所需的增加的比特率可能会更直接地影响这些统计数据。
In the direct loopback case, the loopback mirror copies the payload of the incoming RTP packet into a new RTP packet, using a payload format specific to this use case and specified in Section 7.2. The loopback mirror returns the new packet to the packet source. There is no provision in this case for RTP-level fragmentation.
在直接环回情况下，环回镜像使用第7.2节中规定的特定于此用例的有效负载格式，将传入RTP数据包的有效负载复制到新RTP数据包中。环回镜像将新数据包返回到数据包源。在这种情况下，没有关于RTP级碎片的规定。
This use case has the advantage of keeping the packet size the same in both directions. The packet source can compute only two-way path statistics from the direct loopback packet header but can play back the returned media content.
该用例的优点是在两个方向上保持数据包大小相同。数据包源只能从直接环回数据包报头计算双向路径统计信息，但可以播放返回的媒体内容。
It has been suggested that the loopback source, knowing that the incoming packet will never be passed to a decoder, can store a timestamp and sequence number inside the payload of the packet it sends to the mirror, then extract that information from the returned direct loopback packet and compute one-way path statistics as in the previous case. Obviously, playout of returned content is no longer possible if this is done.
有人建议，环回源在知道传入分组永远不会被传递到解码器的情况下，可以在其发送到镜像的分组的有效载荷内存储时间戳和序列号，然后从返回的直接环回分组中提取该信息，并像前一种情况那样计算单向路径统计。显然，如果这样做的话，将不再可能播放返回的内容。
In the media loopback case, the loopback mirror submits the incoming packet to a decoder appropriate to the incoming payload type. The packet is taken as close as possible to the analog level, then re-encoded according to an outgoing format determined by SDP negotiation. The re-encoded content is returned to the loopback source as an RTP packet with payload type corresponding to the re-encoding format.
在媒体环回情况下，环回镜像将传入分组提交给适合于传入有效负载类型的解码器。数据包尽可能接近模拟级别，然后根据SDP协商确定的传出格式重新编码。重新编码的内容作为具有与重新编码格式对应的有效负载类型的RTP分组返回到环回源。
This usage allows troubleshooting at the codec level. The capability for path statistics is limited to what is available from RTCP reports.
这种用法允许在编解码器级别进行故障排除。路径统计功能仅限于RTCP报告提供的功能。
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
本文件中的关键词“必须”、“不得”、“要求”、“应”、“不应”、“应”、“不应”、“建议”、“可”和“可选”应按照RFC 2119[RFC2119]中所述进行解释。
SDP: Session Description Protocol, as defined in [RFC4566]. This document assumes that the SDP offer/answer model is followed, per [RFC3264], but does not assume any specific signaling protocol for carrying the SDP.
SDP：会话描述协议，定义见[RFC4566]。本文件假设按照[RFC3264]遵循SDP提供/应答模型，但不假设任何用于承载SDP的特定信令协议。
The following terms are borrowed from [RFC3264] definitions: offer, offerer, answer, answerer, and agent.
以下术语取自[RFC3264]定义：要约人、要约人、应答人、应答人和代理人。
This document defines two loopback 'types', two 'roles', and two encoding formats for loopback. For any given SDP offerer or answerer pair, one side is the source of RTP packets, while the other is the mirror looping packets/media back. Those define the two loopback roles. As the mirror, two 'types' of loopback can be performed: packet-level or media-level. When media-level is used, there is no further choice of encoding format -- there is only one format: whatever is indicated for normal media, since the "looping" is performed at the codec level. When packet-level looping is performed, however, the mirror can either send back RTP in an encapsulated format or direct loopback format. The rest of this document describes these loopback types, roles, and encoding formats, and the SDP offer/answer rules for indicating them.
本文档定义了回送的两种“类型”、两种“角色”和两种编码格式。对于任何给定的SDP提供方或应答方对，一侧是RTP数据包的源，而另一侧是镜像循环数据包/媒体。它们定义了两个环回角色。作为镜像，可以执行两种“类型”的环回：数据包级别或媒体级别。当使用媒体级别时，没有更多的编码格式选择——只有一种格式：为普通媒体指定的格式，因为“循环”是在编解码器级别执行的。但是，当执行数据包级循环时，镜像可以以封装格式或直接循环格式发送回RTP。本文档的其余部分描述了这些环回类型、角色和编码格式，以及用于指示它们的SDP提供/应答规则。
An SDP offerer compliant to this specification and attempting to establish a media session with media loopback will include "loopback" media attributes for each individual media description in the offer message that it wishes to have looped back. Note that the offerer may choose to only request loopback for some media descriptions/streams but not others. For example, it might wish to request loopback for a video stream but not audio, or vice versa.
符合本规范并试图建立媒体回送媒体会话的SDP报价人将在其希望回送的报价消息中包含每个单独媒体描述的“回送”媒体属性。请注意，报价人可以选择仅请求某些媒体描述/流的环回，而不请求其他媒体描述/流的环回。例如，它可能希望为视频流而不是音频请求环回，反之亦然。
The offerer will look for the "loopback" media attributes in the media description(s) of the response from the SDP answer for confirmation that the request is accepted.
报价人将在SDP应答的媒体描述中查找“环回”媒体属性，以确认请求已被接受。
In order to accept a loopback offer (that is, an offer containing "loopback" in the media description), an SDP answerer includes the "loopback" media attribute in each media description for which it desires loopback.
为了接受环回报价（即，在媒体描述中包含“环回”的报价），SDP应答器在其希望环回的每个媒体描述中包括“环回”媒体属性。
An answerer can reject an offered stream (either with loopback-source or loopback-mirror) if the loopback-type is not specified, the specified loopback-type is not supported, or the endpoint cannot honor the offer for any other reason. The loopback request is rejected by setting the stream's media port number to zero in the answer as defined in RFC 3264 [RFC3264] or by rejecting the entire offer (i.e., by rejecting the session request entirely).
如果未指定环回类型、不支持指定的环回类型或端点因任何其他原因无法接受提供，应答者可以拒绝提供的流（使用环回源或环回镜像）。通过在RFC 3264[RFC3264]中定义的应答中将流的媒体端口号设置为零或通过拒绝整个提供（即，完全拒绝会话请求）来拒绝环回请求。
Note that an answerer that is not compliant to this specification and that receives an offer with the "loopback" media attributes would ignore the attributes and treat the incoming offer as a normal request. If the offerer does not wish to establish a "normal" RTP session, it would need to terminate the session upon receiving such an answer.
请注意，不符合本规范且收到具有“环回”媒体属性的报价的应答者将忽略这些属性，并将传入报价视为正常请求。如果报价人不希望建立“正常”RTP会话，则需要在收到此类答复后终止会话。
Three new SDP media-level attributes are defined: one indicates the type of loopback, and the other two define the role of the agent.
定义了三个新的SDP媒体级属性：一个表示环回的类型，另两个定义代理的角色。
This specification defines a new "loopback" attribute, which indicates that the agent wishes to perform loopback, and the type of loopback that the agent is able to do. The loopback-type is a value media attribute [RFC4566] with the following syntax:
该规范定义了一个新的“loopback”属性，该属性指示代理希望执行环回，以及代理能够执行的环回类型。环回类型是具有以下语法的值媒体属性[RFC4566]：
a=loopback:<loopback-type>
a=loopback:<loopback-type>
Following is the Augmented BNF [RFC5234] for loopback-type:
以下是环回类型的扩充BNF[RFC5234]：
attribute =/ loopback-attr ; attribute defined in RFC 4566
属性=/loopback attr；RFC 4566中定义的属性
loopback-attr = "loopback:" SP loopback-type
loopback-type = loopback-choice [1*SP loopback-choice]
loopback-choice = loopback-type-pkt / loopback-type-media
loopback-type-pkt = "rtp-pkt-loopback"
loopback-type-media = "rtp-media-loopback"
loopback-attr = "loopback:" SP loopback-type
loopback-type = loopback-choice [1*SP loopback-choice]
loopback-choice = loopback-type-pkt / loopback-type-media
loopback-type-pkt = "rtp-pkt-loopback"
loopback-type-media = "rtp-media-loopback"
The loopback-type is used to indicate the type of loopback. The loopback-type values are rtp-pkt-loopback and rtp-media-loopback.
环回类型用于指示环回的类型。环回类型值为rtp pkt环回和rtp媒体环回。
rtp-pkt-loopback: In this mode, the RTP packets are looped back to the sender at a point before the encoder/decoder function in the receive direction to a point after the encoder/decoder function in the send direction. This effectively re-encapsulates the RTP payload with the RTP/UDP/IP headers appropriate for sending it in the reverse direction. Any type of encoding-related functions, such as packet loss concealment, MUST NOT be part of this type of loopback path. In this mode, the RTP packets are looped back with a new payload type and format. Section 7 describes the payload formats that are to be used for this type of loopback. This type of loopback applies to the encapsulated and direct loopback use cases described in Section 1.1.
rtp pkt环回：在此模式下，rtp数据包在接收方向编码器/解码器功能之前的一点环回发送方，在发送方向编码器/解码器功能之后的一点环回发送方。这有效地用适合反向发送的RTP/UDP/IP头重新封装RTP有效负载。任何类型的编码相关功能，例如包丢失隐藏，都不能是这种类型的环回路径的一部分。在此模式下，RTP数据包以新的有效负载类型和格式进行环回。第7节描述了用于此类环回的有效负载格式。这种类型的环回适用于第1.1节中描述的封装和直接环回用例。
rtp-media-loopback: This loopback is activated as close as possible to the analog interface and after the decoder so that the RTP packets are subsequently re-encoded prior to transmission back to the sender. This type of loopback applies to the media loopback use case described in Section 1.1.3.
rtp媒体环回：该环回尽可能靠近模拟接口并在解码器之后激活，以便rtp数据包随后在传输回发送方之前重新编码。这种类型的环回适用于第1.1.3节所述的介质环回用例。
The loopback role defines two property media attributes [RFC4566] that are used to indicate the role of the agent generating the SDP offer or answer. The syntax of the two loopback-role media attributes is as follows:
环回角色定义了两个属性媒体属性[RFC4566]，用于指示生成SDP报价或应答的代理的角色。两个环回角色介质属性的语法如下：
a=loopback-source
a=环回源
and
和
a=loopback-mirror
a=环回镜
Following is the Augmented BNF [RFC5234] for loopback-source and loopback-mirror:
以下是环回源和环回镜像的扩充BNF[RFC5234]：
attribute =/ loopback-source / loopback-mirror
; attribute defined in RFC 4566
loopback-source = "loopback-source"
loopback-mirror = "loopback-mirror"
attribute =/ loopback-source / loopback-mirror
; attribute defined in RFC 4566
loopback-source = "loopback-source"
loopback-mirror = "loopback-mirror"
loopback-source: This attribute specifies that the entity that generated the SDP is the media source and expects the receiver of the SDP message to act as a loopback mirror.
环回源：该属性指定生成SDP的实体是媒体源，并期望SDP消息的接收方充当环回镜像。
loopback-mirror: This attribute specifies that the entity that generated the SDP will mirror (echo) all received media back to the sender of the RTP stream. No media is generated locally by the looping-back entity for transmission in the mirrored stream.
环回镜像：该属性指定生成SDP的实体将所有接收到的媒体镜像（回显）回RTP流的发送方。环回实体不会在本地生成介质，以便在镜像流中传输。
The "m=" line in the SDP includes all the payload types that will be used during the loopback session. The complete payload space for the session is specified in the "m=" line, and the rtpmap attribute is used to map from the payload type number to an encoding name denoting the payload format to be used.
SDP中的“m=”行包括环回会话期间将使用的所有有效负载类型。会话的完整有效负载空间在“m=”行中指定，rtpmap属性用于从有效负载类型编号映射到表示要使用的有效负载格式的编码名称。
If an offerer wishes to make a loopback request, it includes both the loopback-type and loopback-role attributes in a valid SDP offer:
如果报价人希望提出环回请求，则在有效的SDP报价中包括环回类型和环回角色属性：
Example: m=audio 41352 RTP/AVP 0 8 100
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:100 G7221/16000/1
Example: m=audio 41352 RTP/AVP 0 8 100
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
a=rtpmap:8 pcma/8000
a=rtpmap:100 G7221/16000/1
Since media loopback requires bidirectional RTP, its normal direction mode is "sendrecv"; the "sendrecv" direction attribute MAY be encoded in SDP or not, as per Section 5.1 of [RFC3264], since it is implied by default. If either the loopback source or mirror wishes to disable loopback use during a session, the direction mode attribute "inactive" MUST be used as per [RFC3264]. The direction mode attributes "recvonly" and "sendonly" are incompatible with the loopback mechanism and MUST NOT be indicated when generating an SDP offer or answer. When receiving an SDP offer or answer, if "recvonly" or "sendonly" is indicated for loopback, the SDP-receiving agent SHOULD treat it as a protocol failure of the loopback negotiation and terminate the session through its normal means (e.g., by sending a SIP BYE if SIP is used) or reject the offending media stream.
由于媒体环回需要双向RTP，其正常方向模式为“sendrecv”；根据[RFC3264]第5.1节的规定，“sendrecv”方向属性可以用SDP编码，也可以不用SDP编码，因为默认情况下它是隐含的。如果环回源或镜像希望在会话期间禁用环回使用，则必须按照[RFC3264]使用方向模式属性“inactive”。方向模式属性“RecvoOnly”和“sendonly”与环回机制不兼容，在生成SDP报价或应答时不得指示。当接收到SDP提供或应答时，如果环回指示为“recvonly”或“sendonly”，则SDP接收代理应将其视为环回协商的协议失败，并通过其正常方式（例如，如果使用SIP，则通过发送SIP BYE）终止会话或拒绝有问题的媒体流。
The offerer may offer more than one loopback-type in the SDP offer. The port number and the address in the offer (m/c= lines) indicate where the offerer would like to receive the media stream(s). The payload type numbers indicate the value of the payload the offerer expects to receive. However, the answer might indicate a subset of payload type numbers from those given in the offer. In that case, the offerer MUST only send the payload types received in the answer, per normal SDP offer/answer rules.
报价人可以在SDP报价中提供多个环回类型。报价中的端口号和地址（m/c=行）表示报价人希望接收媒体流的位置。有效载荷类型编号表示报价人预期收到的有效载荷值。但是，答案可能表示报价中给出的有效负载类型编号的子集。在这种情况下，报价人必须根据正常的SDP报价/应答规则，仅发送应答中接收的有效负载类型。
If the offer indicates rtp-pkt-loopback support, the offer MUST also contain either an encapsulated or direct loopback encoding format encoding name, or both, as defined in Sections 7.1 and 7.2 of this document. If the offer only indicates rtp-media-loopback support, then neither encapsulated nor direct loopback encoding formats apply and they MUST NOT be in the offer.
如果报价表明支持rtp pkt环回，则报价还必须包含本文件第7.1节和第7.2节中定义的封装或直接环回编码格式编码名称，或两者都包含。如果报价仅表示支持rtp媒体环回，则封装或直接环回编码格式均不适用，且不得包含在报价中。
If loopback-type is rtp-pkt-loopback, the loopback mirror MUST send, and the loopback source MUST receive, the looped-back packets encoded in one of the two payload formats (encapsulated RTP or direct loopback) as defined in Section 7.
如果环回类型为rtp pkt环回，则环回镜像必须发送且环回源必须接收以第7节中定义的两种有效负载格式之一（封装rtp或直接环回）编码的环回数据包。
Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-source
a=rtpmap:112 encaprtp/8000
Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-source
a=rtpmap:112 encaprtp/8000
Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-source
a=rtpmap:112 rtploopback/8000
Example: m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-source
a=rtpmap:112 rtploopback/8000
As with the offer, an SDP answer for loopback follows SDP offer/answer rules for the direction attribute, but directions of "sendonly" or "recvonly" do not apply for loopback operation.
与offer一样，环回的SDP应答遵循direction属性的SDP offer/answer规则，但“sendnonly”或“recvonly”的方向不适用于环回操作。
The port number and the address in the answer (m/c= lines) indicate where the answerer would like to receive the media stream. The payload type numbers indicate the value of the payload types the answerer expects to send and receive.
应答中的端口号和地址（m/c=行）指示应答者希望接收媒体流的位置。有效负载类型编号表示应答者期望发送和接收的有效负载类型的值。
An answerer includes both the loopback-role and loopback-type attributes in the answer to indicate that it will accept the loopback request. When a stream is offered with the loopback-source attribute, the corresponding stream in the response will be loopback-mirror and vice versa, provided the answerer is capable of supporting the requested loopback-type.
应答者在应答中包括环回角色和环回类型属性，以指示其将接受环回请求。当提供具有环回源属性的流时，响应中的对应流将是环回镜像，反之亦然，前提是应答者能够支持请求的环回类型。
For example, if the offer contains the loopback-source attribute:
例如，如果报价包含环回源属性：
m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-source
m=audio 41352 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-source
The answer that is capable of supporting the offer must contain the loopback-mirror attribute:
能够支持报价的答案必须包含环回镜像属性：
m=audio 12345 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-mirror
m=audio 12345 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-mirror
If a stream is offered with multiple loopback-type attributes, the answer MUST include only one of the loopback types that are accepted by the answerer. The answerer SHOULD give preference to the first loopback-type in the SDP offer.
如果提供的流具有多个环回类型属性，则答案必须仅包括应答者接受的一种环回类型。应答者应优先考虑SDP报价中的第一种环回类型。
For example, if the offer contains:
例如，如果报价包含：
m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
a=rtpmap:112 encaprtp/8000
m=audio 41352 RTP/AVP 0 8 112
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
a=rtpmap:112 encaprtp/8000
The answer that is capable of supporting the offer and chooses to loopback the media using the rtp-media-loopback type must contain:
能够支持报价并选择使用rtp介质环回类型环回介质的答案必须包含：
m=audio 12345 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-mirror
m=audio 12345 RTP/AVP 0 8
a=loopback:rtp-media-loopback
a=loopback-mirror
As specified in Section 7, if the loopback-type is rtp-pkt-loopback, either the encapsulated RTP payload format or direct loopback RTP payload format MUST be used for looped-back packets.
如第7节所述，如果环回类型为rtp pkt环回，则环回数据包必须使用封装的rtp有效负载格式或直接环回rtp有效负载格式。
For example, if the offer contains:
例如，如果报价包含：
m=audio 41352 RTP/AVP 0 8 112 113
a=loopback:rtp-pkt-loopback
a=loopback-source
a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
m=audio 41352 RTP/AVP 0 8 112 113
a=loopback:rtp-pkt-loopback
a=loopback-source
a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The answer that is capable of supporting the offer must contain one of the following:
能够支持报价的答案必须包含以下内容之一：
m=audio 12345 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:112 encaprtp/8000
m=audio 12345 RTP/AVP 0 8 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:112 encaprtp/8000
m=audio 12345 RTP/AVP 0 8 113
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:113 rtploopback/8000
m=audio 12345 RTP/AVP 0 8 113
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:113 rtploopback/8000
The previous examples used the 'encaprtp' and 'rtploopback' encoding names, which will be defined in Sections 7.1.3 and 7.2.3.
前面的示例使用了“encaprtp”和“rtploopback”编码名称，这将在第7.1.3节和第7.2.3节中定义。
If the received SDP answer does not contain an a=loopback-mirror or a=loopback-source attribute, it is assumed that the loopback extensions are not supported by the remote agent. This is not a protocol failure and instead merely completes the SDP offer/answer exchange with whatever normal rules apply; the offerer MAY decide to end the established RTP session (if any) through normal means of the upper-layer signaling protocol (e.g., by sending a SIP BYE).
如果收到的SDP应答不包含a=环回镜像或a=环回源属性，则假定远程代理不支持环回扩展。这不是协议故障，而是仅使用任何正常规则完成SDP提供/应答交换；报价人可以通过上层信令协议的正常方式（例如，通过发送SIP BYE）决定结束所建立的RTP会话（如果有的话）。
At any point during the loopback session, either participant MAY issue a new offer to modify the characteristics of the previous session, as defined in Section 8 of RFC 3264 [RFC3264]. This also includes transitioning from a normal media processing mode to loopback mode, and vice versa.
在环回会话期间的任何时候，任一参与者均可发布新的要约，以修改RFC 3264[RFC3264]第8节中定义的前一会话的特征。这还包括从正常媒体处理模式转换到环回模式，反之亦然。
Interactive Connectivity Establishment (ICE) [RFC5245], Traversal Using Relays around NAT (TURN) [RFC5766], and Session Traversal Utilities for NAT (STUN) [RFC5389] provide a general solution to establishing media sessions between entities that are behind Network Address Translators (NATs). Loopback sessions that involve one or more endpoints behind NATs can also use these general solutions wherever possible.
交互式连接建立（ICE）[RFC5245]，使用NAT（TURN）周围的中继进行遍历[RFC5766]，以及NAT的会话遍历实用程序（STUN）[RFC5389]提供了在网络地址转换器（NAT）后面的实体之间建立媒体会话的通用解决方案。涉及NAT后面的一个或多个端点的环回会话也可以尽可能使用这些通用解决方案。
If ICE is not supported, then in the case of loopback, the mirroring entity will not send RTP packets and therefore will not automatically create the NAT pinhole in the way that other SIP sessions do. Therefore, if the mirroring entity is behind a NAT, it MUST send some
如果不支持ICE，则在环回的情况下，镜像实体将不会发送RTP数据包，因此不会像其他SIP会话那样自动创建NAT针孔。因此，如果镜像实体位于NAT后面，它必须发送一些
packets to the identified address/port(s) of the peer, in order to open the NAT pinhole. Using ICE, this would be accomplished with the STUN connectivity check process or through a TURN server connection. If ICE is not supported, either [RFC6263] or Section 10 of ICE [RFC5245] can be followed to open the pinhole and keep the NAT binding alive/refreshed.
将数据包发送到对等方的标识地址/端口，以打开NAT针孔。使用ICE，这将通过STUN连接检查过程或通过TURN服务器连接来完成。如果不支持ICE，则可以按照[RFC6263]或ICE的第10节[RFC5245]打开针孔并保持NAT绑定活动/刷新。
Note that for any form of NAT traversal to function, symmetric RTP/RTCP [RFC4961] MUST be used, unless the mirror can control the NAT(s) in its path to create explicit pinholes. In other words, both agents MUST send packets from the source address and port they receive packets on, unless some mechanism is used to avoid that need (e.g., by using the Port Control Protocol).
请注意，任何形式的NAT遍历都必须使用对称RTP/RTCP[RFC4961]，除非镜像可以控制其路径中的NAT以创建显式针孔。换句话说，两个代理都必须从它们接收数据包的源地址和端口发送数据包，除非使用某种机制来避免这种需要（例如，通过使用端口控制协议）。
A loopback source MUST NOT send multiple source streams on the same 5-tuple, since there is no means for the mirror to indicate which is which in its mirrored RTP packets.
环回源不能在同一个5元组上发送多个源流，因为镜像无法指示其镜像RTP数据包中的哪个是哪个。
A loopback mirror that is compliant to this specification and accepts media with the loopback type rtp-pkt-loopback loops back the incoming RTP packets using either the encapsulated RTP payload format or the direct loopback RTP payload format as defined in Section 7 of this specification.
符合本规范并接受环回类型为rtp pkt环回的介质的环回镜像使用本规范第7节中定义的封装rtp有效负载格式或直接环回rtp有效负载格式环回传入rtp数据包。
A device that is compliant to this specification and performing the mirroring using the loopback type rtp-media-loopback MUST transmit all received media back to the sender, unless congestion feedback or other lower-layer constraints prevent it from doing so. The incoming media is treated as if it were to be played; for example, the media stream may receive treatment from Packet Loss Concealment (PLC) algorithms. The mirroring entity re-generates all the RTP header fields as it would when transmitting media. The mirroring entity MAY choose to encode the loopback media according to any of the media descriptions supported by the offering entity. Furthermore, in cases where the same media type is looped back, the mirroring entity can choose to preserve the number of frames/packets and the bit rate of the encoded media according to the received media.
符合本规范并使用环回类型rtp介质环回执行镜像的设备必须将所有接收到的介质传输回发送方，除非拥塞反馈或其他较低层限制阻止其这样做。传入的媒体被视为要播放的媒体；例如，媒体流可以从分组丢失隐藏（PLC）算法接收处理。镜像实体会像传输媒体一样重新生成所有RTP头字段。镜像实体可以选择根据提供实体支持的任何介质描述对环回介质进行编码。此外，在相同媒体类型被环回的情况下，镜像实体可以根据接收到的媒体选择保留帧/分组的数量和编码媒体的比特率。
The payload formats described in this section MUST be used by a loopback mirror when 'rtp-pkt-loopback' is the specified loopback-type. Two different formats are specified here -- an encapsulated RTP payload format and a direct loopback RTP payload format. The encapsulated RTP payload format should be used when the incoming RTP header information needs to be preserved during the
当“rtp pkt loopback”是指定的环回类型时，环回镜像必须使用本节中描述的有效负载格式。这里指定了两种不同的格式——封装RTP有效负载格式和直接环回RTP有效负载格式。当在传输过程中需要保留传入的RTP报头信息时，应使用封装的RTP有效负载格式
loopback operation. This is useful in cases where the loopback source needs to measure performance metrics in both directions. However, this comes at the expense of increased packet size as described in Section 7.1. The direct loopback RTP payload format should be used when bandwidth requirements prevent the use of the encapsulated RTP payload format.
环回操作。这在环回源需要在两个方向上测量性能指标的情况下非常有用。然而，如第7.1节所述，这是以增加数据包大小为代价的。当带宽要求阻止使用封装的RTP有效负载格式时，应使用直接环回RTP有效负载格式。
A received RTP packet is encapsulated in the payload section of the RTP packet generated by a loopback mirror. Each received packet is encapsulated in a separate encapsulating RTP packet; the encapsulated packet would be fragmented only if required (for example, due to MTU limitations).
接收到的RTP分组被封装在由环回镜像生成的RTP分组的有效负载部分中。每个接收到的分组被封装在单独的封装RTP分组中；只有在需要时（例如，由于MTU限制），封装的数据包才会被分割。
Payload Type (PT): The assignment of an RTP payload type for this packet format is outside the scope of this document; it is either specified by the RTP profile under which this payload format is used or more likely signaled dynamically out-of-band (e.g., using SDP; Section 7.1.3 defines the name binding).
有效负载类型（PT）：此数据包格式的RTP有效负载类型的分配不在本文档的范围内；它或者由使用该有效负载格式的RTP配置文件指定，或者更可能在带外动态发出信号（例如，使用SDP；第7.1.3节定义了名称绑定）。
Marker (M) bit: If the received RTP packet is looped back in multiple encapsulating RTP packets, the M bit is set to 1 in every fragment except the last packet; otherwise, it is set to 0.
标记（M）位：如果接收到的RTP数据包在多个封装RTP数据包中循环，则M位在除最后一个数据包外的每个片段中设置为1；否则，它将设置为0。
Extension (X) bit: This bit is defined by the RTP profile used.
扩展（X）位：该位由使用的RTP配置文件定义。
Sequence Number: The RTP sequence number SHOULD be generated by the loopback mirror in the usual manner with a constant random offset as described in RFC 3550 [RFC3550].
序列号：RTP序列号应由环回镜像按照RFC 3550[RFC3550]中所述的常规方式以恒定随机偏移量生成。
Timestamp: The RTP timestamp denotes the sampling instant for when the loopback mirror is transmitting this packet to the loopback source. The RTP timestamp MUST use the same clock rate as that of the encapsulated packet. The initial value of the timestamp SHOULD be random for security reasons (see Section 5.1 of RFC 3550 [RFC3550]).
时间戳：RTP时间戳表示环回镜像将此数据包传输到环回源时的采样瞬间。RTP时间戳必须使用与封装数据包相同的时钟速率。出于安全原因，时间戳的初始值应为随机值（见RFC 3550[RFC3550]第5.1节）。
Synchronization source (SSRC): This field is set as described in RFC 3550 [RFC3550].
同步源（SSRC）：按照RFC 3550[RFC3550]中的说明设置此字段。
The CSRC count (CC) and contributing source (CSRC) fields are used as described in RFC 3550 [RFC3550].
如RFC 3550[RFC3550]所述，使用CSC计数（CC）和贡献来源（CSC）字段。
The outer RTP header of the encapsulating packet is followed by the payload header defined in this section, after any header extension(s). If the received RTP packet has to be looped back in multiple encapsulating packets due to fragmentation, the encapsulating RTP header in each packet is followed by the payload header defined in this section. The header is devised so that the loopback source can decode looped-back packets in the presence of moderate packet loss [RFC3550]. The RTP payload of the encapsulating RTP packet starts with the payload header defined in this section.
封装包的外部RTP报头后面是本节中定义的有效负载报头，在任何报头扩展之后。如果由于碎片，接收到的RTP数据包必须在多个封装数据包中环回，则每个数据包中的封装RTP报头后面紧跟着本节中定义的有效负载报头。报头的设计使得环回源可以在存在中度分组丢失的情况下解码环回分组[RFC3550]。封装RTP数据包的RTP有效负载从本节中定义的有效负载报头开始。
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| receive timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F | R | CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| transmit timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| receive timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F | R | CC |M| PT | sequence number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| transmit timestamp |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| synchronization source (SSRC) identifier |
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
| contributing source (CSRC) identifiers |
| .... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. Encapsulating RTP Packet Payload Header
图1。封装RTP数据包有效负载报头
The 12 octets after the receive timestamp are identical to the encapsulated RTP header of the received packet except for the first 2 bits of the first octet. In effect, the received RTP packet is encapsulated by creating a new outer RTP header followed by 4 new bytes of a receive timestamp, followed by the original received RTP header and payload, except that the first two bits of the received RTP header are overwritten as defined here.
接收时间戳后的12个八位字节与接收数据包的封装RTP报头相同，但第一个八位字节的前2位除外。实际上，通过创建一个新的外部RTP报头，后跟接收时间戳的4个新字节，后跟原始接收RTP报头和有效负载，来封装接收的RTP数据包，但接收RTP报头的前两位被覆盖，如本文所定义。
Receive timestamp: 32 bits
接收时间戳：32位
The receive timestamp denotes the sampling instant for when the last octet of the received media packet that is being encapsulated by the loopback mirror is received from the loopback source. The same clock rate MUST be used by the loopback source. The initial value of the timestamp SHOULD be random for security reasons (see Section 5.1 of RFC 3550 [RFC3550]).
接收时间戳表示从环回源接收到由环回镜像封装的接收媒体分组的最后八位字节时的采样时刻。环回源必须使用相同的时钟速率。出于安全原因，时间戳的初始值应为随机值（见RFC 3550[RFC3550]第5.1节）。
Fragmentation (F): 2 bits
碎片（F）：2位
Possible values are First Fragment (00), Last Fragment (01), No Fragmentation (10), or Intermediate Fragment (11). This field identifies how much of the received packet is encapsulated in this packet by the loopback mirror. If the received packet is not fragmented, this field is set to 10; otherwise, the packet that contains the first fragments sets this field to 00. The packet that contains the last fragment sets this field to 01, and all other packets set this field to 11.
可能的值为第一个片段（00）、最后一个片段（01）、无片段（10）或中间片段（11）。此字段标识环回镜像将多少接收到的数据包封装在此数据包中。如果接收到的数据包没有分段，则该字段设置为10；否则，包含第一个片段的数据包将此字段设置为00。包含最后一个片段的数据包将此字段设置为01，所有其他数据包将此字段设置为11。
The payload type number for the encapsulated stream can be negotiated using SDP. There is no static payload type assignment for the encapsulating stream, so dynamic payload type numbers MUST be used. The binding to the name is indicated by an rtpmap attribute. The name used in this binding is "encaprtp".
封装流的有效负载类型号可以使用SDP协商。封装流没有静态有效负载类型分配，因此必须使用动态有效负载类型编号。与名称的绑定由rtpmap属性指示。此绑定中使用的名称是“encaprtp”。
The following is an example SDP fragment for encapsulated RTP.
以下是封装RTP的SDP片段示例。
m=audio 41352 RTP/AVP 112
a=rtpmap:112 encaprtp/8000
m=audio 41352 RTP/AVP 112
a=rtpmap:112 encaprtp/8000
The direct loopback RTP payload format can be used in scenarios where the 16-byte overhead of the encapsulated payload format is of concern, or simply due to local policy. When using this payload format, the receiver loops back each received RTP packet payload (not header) in a separate RTP packet.
直接环回RTP有效负载格式可用于封装有效负载格式的16字节开销值得关注的场景，或者仅仅是由于本地策略。当使用此有效负载格式时，接收器在单独的RTP数据包中循环回每个接收到的RTP数据包有效负载（而不是报头）。
Because a direct loopback format does not retain the original RTP headers, there will be no indication of the original payload-type sent to the mirror, in looped-back packets. Therefore, the loopback source SHOULD only send one payload type per loopback RTP session if direct mode is used.
由于直接环回格式不保留原始RTP头，因此在环回数据包中不会指示发送到镜像的原始有效负载类型。因此，如果使用直接模式，则每个环回RTP会话中环回源只应发送一种有效负载类型。
Payload Type (PT): The assignment of an RTP payload type for the encapsulating packet format is outside the scope of this document; it is either specified by the RTP profile under which this payload format is used or more likely signaled dynamically out-of-band (e.g., using SDP; Section 7.2.3 defines the name binding).
有效负载类型（PT）：为封装数据包格式分配RTP有效负载类型超出了本文档的范围；它或者由使用该有效负载格式的RTP配置文件指定，或者更可能在带外动态发出信号（例如，使用SDP；第7.2.3节定义了名称绑定）。
Marker (M) bit: This bit is set to the value in the received packet.
标记（M）位：该位设置为接收数据包中的值。
Extension (X) bit: This bit is defined by the RTP profile used.
扩展（X）位：该位由使用的RTP配置文件定义。
Sequence Number: The RTP sequence number SHOULD be generated by the loopback mirror in the usual manner with a constant random offset, as per [RFC3550].
序列号：根据[RFC3550]，环回镜像应以常规方式以恒定随机偏移量生成RTP序列号。
Timestamp: The RTP timestamp denotes the sampling instant for when the loopback mirror is transmitting this packet to the loopback source. The same clock rate MUST be used as that of the received RTP packet. The initial value of the timestamp SHOULD be random for security reasons (see Section 5.1 of RFC 3550 [RFC3550]).
时间戳：RTP时间戳表示环回镜像将此数据包传输到环回源时的采样瞬间。必须使用与接收到的RTP数据包相同的时钟速率。出于安全原因，时间戳的初始值应为随机值（见RFC 3550[RFC3550]第5.1节）。
SSRC: This field is set as described in RFC 3550 [RFC3550].
SSRC：按照RFC 3550[RFC3550]中的说明设置此字段。
The CC and CSRC fields are used as described in RFC 3550 [RFC3550].
CC和CSC字段的使用如RFC 3550[RFC3550]所述。
This payload format does not define any payload-specific headers. The loopback mirror simply copies the RTP payload data from the payload portion of the RTP packet received from the loopback source.
此有效负载格式不定义任何特定于有效负载的标头。环回镜像只是从从环回源接收的RTP数据包的有效负载部分复制RTP有效负载数据。
The payload type number for the payload loopback stream can be negotiated using a mechanism like SDP. There is no static payload type assignment for the stream, so dynamic payload type numbers MUST be used. The binding to the name is indicated by an rtpmap attribute. The name used in this binding is "rtploopback".
有效负载环回流的有效负载类型号可以使用类似SDP的机制进行协商。流没有静态有效负载类型分配，因此必须使用动态有效负载类型编号。与名称的绑定由rtpmap属性指示。此绑定中使用的名称是“rtploopback”。
The following is an example SDP fragment for the direct loopback RTP format.
以下是直接环回RTP格式的SDP片段示例。
m=audio 41352 RTP/AVP 112
a=rtpmap:112 rtploopback/8000
m=audio 41352 RTP/AVP 112
a=rtpmap:112 rtploopback/8000
Secure RTP (SRTP) [RFC3711] MAY be used for loopback sessions. SRTP operates at a lower logical layer than RTP, and thus if both sides negotiate to use SRTP, each side uses its own key and performs encryption/decryption, authentication, etc. Therefore, the loopback function on the mirror occurs after the SRTP packet has been decrypted and authenticated, as a normal cleartext RTP packet without a Master Key Identifier (MKI) or authentication tag; once the
安全RTP（SRTP）[RFC3711]可用于环回会话。SRTP在比RTP更低的逻辑层上运行，因此，如果双方协商使用SRTP，则每一方使用其自己的密钥并执行加密/解密、身份验证等。因此，镜像上的环回功能发生在SRTP数据包解密和身份验证之后，作为没有主密钥标识符（MKI）或认证标签的普通明文RTP数据包；一旦
cleartext RTP packet or payload is mirrored -- either at the media-layer, direct packet-layer, or encapsulated packet-layer -- it is encrypted by the mirror using its own key.
明文RTP数据包或有效负载被镜像——在媒体层、直接数据包层或封装数据包层——它由镜像使用自己的密钥进行加密。
In order to provide the same level of protection to both forward and reverse media flows (media to and from the mirror), if SRTP is used it MUST be used in both directions with the same properties.
为了对正向和反向介质流（进出镜像的介质）提供相同级别的保护，如果使用SRTP，则必须在具有相同属性的两个方向上使用SRTP。
The use of the loopback attribute is intended for the monitoring of media quality of the session. Consequently, the media performance information should be exchanged between the offering and the answering entities. An offering or answering agent that is compliant to this specification SHOULD support RTCP per [RFC3550] and RTCP-XR per RFC 3611 [RFC3611]. Furthermore, if the offerer or answerer chooses to support RTCP-XR, they SHOULD support the RTCP-XR Loss Run Length Encoding (RLE) Report Block, Duplicate RLE Report Block, Statistics Summary Report Block, and VoIP Metrics Report Block per Sections 4.1, 4.2, 4.6, and 4.7 of RFC 3611 [RFC3611]. The offerer and the answerer MAY support other RTCP-XR reporting blocks as defined by RFC 3611 [RFC3611].
“环回”属性的使用旨在监视会话的媒体质量。因此，媒体性能信息应在提供和应答实体之间交换。符合本规范的产品或应答代理应根据[RFC3550]支持RTCP，根据RFC 3611[RFC3611]支持RTCP-XR。此外，如果报价人或应答人选择支持RTCP-XR，他们应根据RFC 3611[RFC3611]第4.1、4.2、4.6和4.7节支持RTCP-XR损失游程编码（RLE）报告块、重复RLE报告块、统计摘要报告块和VoIP度量报告块。报价人和应答人可支持RFC 3611[RFC3611]定义的其他RTCP-XR报告块。
All the participants in a media-level loopback session SHOULD implement congestion control mechanisms as defined by the RTP profile under which the loopback mechanism is implemented. For audio/video profiles, implementations SHOULD conform to the mechanism defined in Section 2 of RFC 3551 [RFC3551].
媒体级环回会话中的所有参与者都应实现由RTP配置文件定义的拥塞控制机制，在RTP配置文件下实现环回机制。对于音频/视频配置文件，实施应符合RFC 3551[RFC3551]第2节中定义的机制。
For packet-level loopback types, the loopback source SHOULD implement congestion control. The mirror will simply reflect back the RTP packets it receives (either in encapsulated or direct modes); therefore, the source needs to control the congestion of both forward and reverse paths by reducing its sending rate to the mirror. This keeps the loopback mirror implementation simpler and provides more flexibility for the source performing a loopback test.
对于包级环回类型，环回源应实现拥塞控制。镜像将简单地反射它接收的RTP数据包（以封装或直接模式）；因此，源需要通过降低其到镜像的发送速率来控制前向和反向路径的拥塞。这使得环回镜像实现更简单，并为执行环回测试的源代码提供了更大的灵活性。
This section provides examples for media descriptions using SDP for different scenarios. The examples are given for SIP-based transactions; for convenience, they are abbreviated and do not show the complete signaling.
本节提供了针对不同场景使用SDP进行媒体描述的示例。给出了基于SIP的事务的示例；为方便起见，它们是缩写的，不显示完整的信号。
An agent sends an SDP offer that looks like:
代理发送的SDP报价如下所示：
v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
The agent is offering to source the media and expects the answering agent to mirror the RTP stream per the loopback type rtp-media-loopback.
代理正在提供媒体源，并希望应答代理根据环回类型RTP媒体环回镜像RTP流。
An answering agent sends an SDP answer that looks like:
应答代理发送的SDP应答如下所示：
v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror
a=rtpmap:0 pcmu/8000
v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-mirror
a=rtpmap:0 pcmu/8000
The answerer agrees to mirror the media from the offerer at the media level.
回答者同意在媒体层面上向报价者反映媒体。
An agent sends an SDP offer that looks like:
代理发送的SDP报价如下所示：
v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 112 113
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0 112 113
a=loopback:rtp-media-loopback rtp-pkt-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000
a=rtpmap:113 rtploopback/8000
The offerer is offering to source the media and expects the answerer to mirror the RTP stream at either the media or RTP level.
报价人提供媒体来源，并期望应答人在媒体或RTP级别镜像RTP流。
An answering agent sends an SDP answer that looks like:
应答代理发送的SDP应答如下所示：
v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000
v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 49270 RTP/AVP 0 112
a=loopback:rtp-pkt-loopback
a=loopback-mirror
a=rtpmap:0 pcmu/8000
a=rtpmap:112 encaprtp/8000
The answerer agrees to mirror the media from the offerer at the packet level using the encapsulated RTP payload format.
应答者同意使用封装的RTP有效负载格式在包级别镜像来自报价者的媒体。
An agent sends an SDP offer that looks like:
代理发送的SDP报价如下所示：
v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
v=0
o=alice 2890844526 2890842807 IN IP4 host.atlanta.example.com
s=-
c=IN IP4 host.atlanta.example.com
t=0 0
m=audio 49170 RTP/AVP 0
a=loopback:rtp-media-loopback
a=loopback-source
a=rtpmap:0 pcmu/8000
The offerer is offering to source the media and expects the answerer to mirror the RTP stream at the media level.
报价人提供媒体来源，并希望应答人在媒体级别镜像RTP流。
An answering agent sends an SDP answer that looks like:
应答代理发送的SDP应答如下所示：
v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 0 RTP/AVP 0
a=rtpmap:0 pcmu/8000
v=0
o=bob 1234567890 1122334455 IN IP4 host.biloxi.example.com
s=-
c=IN IP4 host.biloxi.example.com
t=0 0
m=audio 0 RTP/AVP 0
a=rtpmap:0 pcmu/8000
Note in this case that the answerer did not indicate loopback support, although it could have and still used a port number of 0 to indicate that it does not wish to accept that media session.
请注意，在这种情况下，应答器没有表示支持环回，尽管它可能已经并且仍然使用端口号0表示它不希望接受该媒体会话。
Alternatively, the answering agent could have simply rejected the entire SDP offer through some higher-layer signaling protocol means (e.g., by rejecting the SIP INVITE request if the SDP offer was in the INVITE).
或者，应答代理可以通过一些更高层信令协议手段（例如，如果SDP提供在INVITE中，则通过拒绝SIP INVITE请求）简单地拒绝整个SDP提供。
The security considerations of [RFC3264] and [RFC3550] apply.
[RFC3264]和[RFC3550]的安全注意事项适用。
Given that media loopback may be automated without the end user's knowledge, the answerer of the media loopback should be aware of denial-of-service attacks. It is RECOMMENDED that session requests for media loopback be authenticated and the frequency of such sessions limited by the answerer.
由于媒体环回可能在最终用户不知情的情况下自动进行，因此媒体环回的应答者应该知道拒绝服务攻击。建议对媒体环回的会话请求进行身份验证，并由应答者限制此类会话的频率。
If the higher-layer signaling protocol were not authenticated, a malicious attacker could create a session between two parties the attacker wishes to target, with each party acting as the loopback mirror to the other, of the rtp-pkt-loopback type. A few RTP packets sent to either party would then infinitely loop among the two, as fast as they could process them, consuming their resources and network bandwidth.
如果高层信令协议未经身份验证，恶意攻击者可能会在攻击者希望攻击的双方之间创建一个会话，其中每一方充当另一方的环回镜像，属于rtp pkt环回类型。发送给任何一方的几个RTP数据包将在这两个数据包之间无限循环，尽可能快地处理它们，消耗它们的资源和网络带宽。
Furthermore, media loopback provides a means of attack indirection, whereby a malicious attacker creates a loopback session as the loopback source and uses the mirror to reflect the attacker's packets against a target -- perhaps a target the attacker could not reach directly, such as one behind a firewall, for example. Or, the attacker could initiate the session as the loopback mirror, in the hopes of making the peer generate media against another target.
此外，媒体环回提供了一种间接攻击手段，恶意攻击者通过该手段创建一个环回会话作为环回源，并使用镜像将攻击者的数据包反映到目标——可能是攻击者无法直接到达的目标，例如防火墙后的目标。或者，攻击者可以启动会话作为环回镜像，希望使对等方针对另一个目标生成媒体。
If end-user devices such as mobile phones answer loopback requests without authentication and without notifying the end user, then an attacker could cause the battery to drain, and possibly deny the end user normal phone service or cause network data usage fees. This could even occur naturally if a legitimate loopback session does not terminate properly and the end device does not have a timeout mechanism for such.
如果最终用户设备（如移动电话）在未经身份验证且未通知最终用户的情况下响应环回请求，则攻击者可能导致电池耗尽，并可能拒绝最终用户的正常电话服务或导致网络数据使用费。如果合法的环回会话没有正确终止，并且终端设备没有此类会话的超时机制，甚至可能会自然发生这种情况。
For the reasons noted above, end-user devices SHOULD provide a means of indicating to the human user that the device is in a loopback session, even if it is an authenticated session. Devices that answer
出于上述原因，最终用户设备应提供一种向人类用户指示设备处于环回会话中的方法，即使它是经过身份验证的会话。回答问题的设备
or generate loopback sessions SHOULD either perform keepalive/refresh tests of the session state through some means or time out the session automatically.
或者生成环回会话应该通过某种方式执行会话状态的keepalive/refresh测试，或者自动超时会话。
The media loopback approach described in this document is a complete solution that would work under all scenarios. However, it is possible that the solution may not be lightweight enough for some implementations. In light of this concern, this section clarifies which features of the loopback proposal MUST be implemented for all implementations and which features MAY be deferred if the complete solution is not desired.
本文档中描述的媒体环回方法是一个完整的解决方案，适用于所有场景。但是，对于某些实现来说，解决方案可能不够轻量级。鉴于这一担忧，本节澄清了所有实施都必须实施环回方案的哪些功能，以及如果不需要完整的解决方案，哪些功能可能被推迟。
All implementations MUST at least support the rtp-pkt-loopback mode for loopback-type, with direct media loopback payload encoding. In addition, for the loopback role, all implementations of an SDP offerer MUST at least be able to act as a loopback source. These requirements are intended to provide a minimal level of interoperability between different implementations.
对于环回类型，所有实现必须至少支持rtp pkt环回模式，并使用直接媒体环回负载编码。此外，对于环回角色，SDP提供者的所有实现必须至少能够充当环回源。这些需求旨在提供不同实现之间的最低级别的互操作性。
This document defines three new media-level SDP attributes. IANA has registered the following attributes.
本文档定义了三个新的媒体级SDP属性。IANA已注册以下属性。
Contact name: Kaynam Hedayat Email address: kh274@cornell.edu Telephone number: +1-617-899-3279 Attribute name: loopback Type of attribute: Media level. Subject to charset: No. Purpose of attribute: The 'loopback' attribute is used to indicate the type of media loopback. Allowed attribute values: The parameters for 'loopback' may be one or more of "rtp-pkt-loopback" and "rtp-media-loopback". See Section 4 of RFC 6849 for syntax.
联系人姓名：Kaynam Hedayat电子邮件地址：kh274@cornell.edu电话号码：+1-617-899-3279属性名称：属性的环回类型：媒体级别。受制于字符集：否。属性用途：“环回”属性用于指示媒体环回的类型。允许的属性值：“环回”的参数可以是“rtp pkt环回”和“rtp媒体环回”中的一个或多个。语法见RFC 6849第4节。
Contact name: Kaynam Hedayat Email address: kh274@cornell.edu Telephone number: +1-617-899-3279 Attribute name: loopback-source Type of attribute: Media level. Subject to charset: No. Purpose of attribute: The 'loopback-source' attribute specifies that the sender is the media source and expects the receiver to act as a loopback mirror. Allowed attribute values: N/A
联系人姓名：Kaynam Hedayat电子邮件地址：kh274@cornell.edu电话号码：+1-617-899-3279属性名称：环回源属性类型：媒体级别。受制于字符集：否。属性的用途：“环回源”属性指定发送方是媒体源，并期望接收方充当环回镜像。允许的属性值：不适用
Contact name: Kaynam Hedayat Email address: kh274@cornell.edu Telephone number: +1-617-899-3279 Attribute name: loopback-mirror Type of attribute: Media level. Subject to charset: No. Purpose of attribute: The 'loopback-mirror' attribute specifies that the receiver will mirror (echo) all received media back to the sender of the RTP stream. Allowed attribute values: N/A
联系人姓名：Kaynam Hedayat电子邮件地址：kh274@cornell.edu电话号码：+1-617-899-3279属性名称：环回镜像属性类型：媒体级别。取决于字符集：否。属性用途：“环回镜像”属性指定接收方将所有接收到的媒体镜像（回显）回RTP流的发送方。允许的属性值：不适用
The IANA has registered the following media types.
IANA已注册了以下媒体类型。
To: ietf-types@iana.org
致：ietf-types@iana.org
Subject: Registration of media type audio/encaprtp
主题：媒体类型音频/encaprtp的注册
Type name: audio
类型名称：音频
Subtype name: encaprtp
子类型名称：encaprtp
Required parameters:
所需参数：
rate: RTP timestamp clock rate, which is equal to the sampling rate. This is specified by the loopback source and reflected by the mirror.
速率：RTP时间戳时钟速率，等于采样速率。这由环回源指定，并由镜像反射。
Optional parameters: N/A
可选参数：不适用
Encoding considerations: This media type is framed.
编码注意事项：此媒体类型为框架。
Security considerations: See Section 12 of RFC 6849.
安全注意事项：见RFC 6849第12节。
Interoperability considerations: N/A
互操作性注意事项：不适用
Published specification: RFC 6849.
已发布规范：RFC6849。
Applications that use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP service.
使用此媒体类型的应用程序：希望监视和确保到给定VoIP服务边缘的传输质量的应用程序。
Additional information: N/A
其他信息：不适用
Contact: the authors of RFC 6849.
联系人：RFC6849的作者。
Intended usage: LIMITED USE
预期用途：有限用途
Restrictions on usage: This media type depends on RTP framing and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time.
使用限制：此媒体类型取决于RTP帧，因此仅定义为通过RTP传输。此时未定义其他帧协议内的传输。
Author: Kaynam Hedayat.
作者：Kaynam Hedayat。
Change controller: IETF PAYLOAD working group delegated from the IESG.
变更控制员：IESG授权的IETF有效载荷工作组。
To: ietf-types@iana.org
致：ietf-types@iana.org
Subject: Registration of media type video/encaprtp
主题：媒体类型视频/encaprtp的注册
Type name: video
类型名称：视频
Subtype name: encaprtp
子类型名称：encaprtp
Required parameters:
所需参数：
rate: RTP timestamp clock rate, which is equal to the sampling rate. This is specified by the loopback source and reflected by the mirror.
速率：RTP时间戳时钟速率，等于采样速率。这由环回源指定，并由镜像反射。
Optional parameters: N/A
可选参数：不适用
Encoding considerations: This media type is framed.
编码注意事项：此媒体类型为框架。
Security considerations: See Section 12 of RFC 6849.
安全注意事项：见RFC 6849第12节。
Interoperability considerations: N/A
互操作性注意事项：不适用
Published specification: RFC 6849.
已发布规范：RFC6849。
Applications that use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given Video Over IP service.
使用此媒体类型的应用程序：希望监视并确保传输到给定IP视频服务边缘的质量的应用程序。
Additional information: N/A
其他信息：不适用
Contact: the authors of RFC 6849.
联系人：RFC6849的作者。
Intended usage: LIMITED USE
预期用途：有限用途
Restrictions on usage: This media type depends on RTP framing and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time.
使用限制：此媒体类型取决于RTP帧，因此仅定义为通过RTP传输。此时未定义其他帧协议内的传输。
Author: Kaynam Hedayat.
作者：Kaynam Hedayat。
Change controller: IETF PAYLOAD working group delegated from the IESG.
变更控制员：IESG授权的IETF有效载荷工作组。
To: ietf-types@iana.org
致：ietf-types@iana.org
Subject: Registration of media type text/encaprtp
主题：媒体类型文本/encaprtp的注册
Type name: text
类型名称：text
Subtype name: encaprtp
子类型名称：encaprtp
Required parameters:
所需参数：
rate: RTP timestamp clock rate, which is equal to the sampling rate. This is specified by the loopback source and reflected by the mirror.
速率：RTP时间戳时钟速率，等于采样速率。这由环回源指定，并由镜像反射。
Optional parameters: N/A
可选参数：不适用
Encoding considerations: This media type is framed.
编码注意事项：此媒体类型为框架。
Security considerations: See Section 12 of RFC 6849.
安全注意事项：见RFC 6849第12节。
Interoperability considerations: N/A
互操作性注意事项：不适用
Published specification: RFC 6849.
已发布规范：RFC6849。
Applications that use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given real-time text service.
使用此媒体类型的应用程序：希望监视并确保传输到给定实时文本服务边缘的质量的应用程序。
Additional information: N/A
其他信息：不适用
Contact: the authors of RFC 6849.
联系人：RFC6849的作者。
Intended usage: LIMITED USE
预期用途：有限用途
Restrictions on usage: This media type depends on RTP framing and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time.
使用限制：此媒体类型取决于RTP帧，因此仅定义为通过RTP传输。此时未定义其他帧协议内的传输。
Author: Kaynam Hedayat.
作者：Kaynam Hedayat。
Change controller: IETF PAYLOAD working group delegated from the IESG.
变更控制员：IESG授权的IETF有效载荷工作组。
To: ietf-types@iana.org
致：ietf-types@iana.org
Subject: Registration of media type application/encaprtp
主题：媒体类型申请注册/encaprtp
Type name: application
类型名称：应用程序
Subtype name: encaprtp
子类型名称：encaprtp
Required parameters:
所需参数：
rate: RTP timestamp clock rate, which is equal to the sampling rate. This is specified by the loopback source and reflected by the mirror.
速率：RTP时间戳时钟速率，等于采样速率。这由环回源指定，并由镜像反射。
Optional parameters: N/A
可选参数：不适用
Encoding considerations: This media type is framed.
编码注意事项：此媒体类型为框架。
Security considerations: See Section 12 of RFC 6849.
安全注意事项：见RFC 6849第12节。
Interoperability considerations: N/A
互操作性注意事项：不适用
Published specification: RFC 6849.
已发布规范：RFC6849。
Applications that use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given real-time application service.
使用此媒体类型的应用程序：希望监视并确保传输到给定实时应用程序服务边缘的质量的应用程序。
Additional information: N/A
其他信息：不适用
Contact: the authors of RFC 6849.
联系人：RFC6849的作者。
Intended usage: LIMITED USE
预期用途：有限用途
Restrictions on usage: This media type depends on RTP framing and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time.
使用限制：此媒体类型取决于RTP帧，因此仅定义为通过RTP传输。此时未定义其他帧协议内的传输。
Author: Kaynam Hedayat.
作者：Kaynam Hedayat。
Change controller: IETF PAYLOAD working group delegated from the IESG.
变更控制员：IESG授权的IETF有效载荷工作组。
To: ietf-types@iana.org
致：ietf-types@iana.org
Subject: Registration of media type audio/rtploopback
主题：注册媒体类型音频/rtploopback
Type name: audio
类型名称：音频
Subtype name: rtploopback
子类型名称：rtploopback
Required parameters:
所需参数：
rate: RTP timestamp clock rate, which is equal to the sampling rate. This is specified by the loopback source and reflected by the mirror.
速率：RTP时间戳时钟速率，等于采样速率。这由环回源指定，并由镜像反射。
Optional parameters: N/A
可选参数：不适用
Encoding considerations: This media type is framed.
编码注意事项：此媒体类型为框架。
Security considerations: See Section 12 of RFC 6849.
安全注意事项：见RFC 6849第12节。
Interoperability considerations: N/A
互操作性注意事项：不适用
Published specification: RFC 6849.
已发布规范：RFC6849。
Applications that use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given VoIP service.
使用此媒体类型的应用程序：希望监视和确保到给定VoIP服务边缘的传输质量的应用程序。
Additional information: N/A
其他信息：不适用
Contact: the authors of RFC 6849.
联系人：RFC6849的作者。
Intended usage: LIMITED USE
预期用途：有限用途
Restrictions on usage: This media type depends on RTP framing and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time.
使用限制：此媒体类型取决于RTP帧，因此仅定义为通过RTP传输。此时未定义其他帧协议内的传输。
Author: Kaynam Hedayat.
作者：Kaynam Hedayat。
Change controller: IETF PAYLOAD working group delegated from the IESG.
变更控制员：IESG授权的IETF有效载荷工作组。
To: ietf-types@iana.org
致：ietf-types@iana.org
Subject: Registration of media type video/rtploopback
主题：媒体类型视频/rtploopback的注册
Type name: video
类型名称：视频
Subtype name: rtploopback
子类型名称：rtploopback
Required parameters:
所需参数：
rate: RTP timestamp clock rate, which is equal to the sampling rate. This is specified by the loopback source and reflected by the mirror.
速率：RTP时间戳时钟速率，等于采样速率。这由环回源指定，并由镜像反射。
Optional parameters: N/A
可选参数：不适用
Encoding considerations: This media type is framed.
编码注意事项：此媒体类型为框架。
Security considerations: See Section 12 of RFC 6849.
安全注意事项：见RFC 6849第12节。
Interoperability considerations: N/A
互操作性注意事项：不适用
Published specification: RFC 6849.
已发布规范：RFC6849。
Applications that use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given Video Over IP service.
使用此媒体类型的应用程序：希望监视并确保传输到给定IP视频服务边缘的质量的应用程序。
Additional information: N/A
其他信息：不适用
Contact: the authors of RFC 6849.
联系人：RFC6849的作者。
Intended usage: LIMITED USE
预期用途：有限用途
Restrictions on usage: This media type depends on RTP framing and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time.
使用限制：此媒体类型取决于RTP帧，因此仅定义为通过RTP传输。此时未定义其他帧协议内的传输。
Author: Kaynam Hedayat.
作者：Kaynam Hedayat。
Change controller: IETF PAYLOAD working group delegated from the IESG.
变更控制员：IESG授权的IETF有效载荷工作组。
To: ietf-types@iana.org
致：ietf-types@iana.org
Subject: Registration of media type text/rtploopback
主题：注册媒体类型text/rtploopback
Type name: text
类型名称：text
Subtype name: rtploopback
子类型名称：rtploopback
Required parameters:
所需参数：
rate: RTP timestamp clock rate, which is equal to the sampling rate. This is specified by the loopback source and reflected by the mirror.
速率：RTP时间戳时钟速率，等于采样速率。这由环回源指定，并由镜像反射。
Optional parameters: N/A
可选参数：不适用
Encoding considerations: This media type is framed.
编码注意事项：此媒体类型为框架。
Security considerations: See Section 12 of RFC 6849.
安全注意事项：见RFC 6849第12节。
Interoperability considerations: N/A
互操作性注意事项：不适用
Published specification: RFC 6849.
已发布规范：RFC6849。
Applications that use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given real-time text service.
使用此媒体类型的应用程序：希望监视并确保传输到给定实时文本服务边缘的质量的应用程序。
Additional information: N/A
其他信息：不适用
Contact: the authors of RFC 6849.
联系人：RFC6849的作者。
Intended usage: LIMITED USE
预期用途：有限用途
Restrictions on usage: This media type depends on RTP framing and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time.
使用限制：此媒体类型取决于RTP帧，因此仅定义为通过RTP传输。此时未定义其他帧协议内的传输。
Author: Kaynam Hedayat.
作者：Kaynam Hedayat。
Change controller: IETF PAYLOAD working group delegated from the IESG.
变更控制员：IESG授权的IETF有效载荷工作组。
To: ietf-types@iana.org
致：ietf-types@iana.org
Subject: Registration of media type application/rtploopback
主题：注册媒体类型应用程序/rtploopback
Type name: application
类型名称：应用程序
Subtype name: rtploopback
子类型名称：rtploopback
Required parameters:
所需参数：
rate: RTP timestamp clock rate, which is equal to the sampling rate. This is specified by the loopback source and reflected by the mirror.
速率：RTP时间戳时钟速率，等于采样速率。这由环回源指定，并由镜像反射。
Optional parameters: N/A
可选参数：不适用
Encoding considerations: This media type is framed.
编码注意事项：此媒体类型为框架。
Security considerations: See Section 12 of RFC 6849.
安全注意事项：见RFC 6849第12节。
Interoperability considerations: N/A
互操作性注意事项：不适用
Published specification: RFC 6849.
已发布规范：RFC6849。
Applications that use this media type: Applications wishing to monitor and ensure the quality of transport to the edge of a given real-time application service.
使用此媒体类型的应用程序：希望监视并确保传输到给定实时应用程序服务边缘的质量的应用程序。
Additional information: N/A
其他信息：不适用
Contact: the authors of RFC 6849.
联系人：RFC6849的作者。
Intended usage: LIMITED USE
预期用途：有限用途
Restrictions on usage: This media type depends on RTP framing and hence is only defined for transfer via RTP. Transfer within other framing protocols is not defined at this time.
使用限制：此媒体类型取决于RTP帧，因此仅定义为通过RTP传输。此时未定义其他帧协议内的传输。
Author: Kaynam Hedayat.
作者：Kaynam Hedayat。
Change controller: IETF PAYLOAD working group delegated from the IESG.
变更控制员：IESG授权的IETF有效载荷工作组。
This document's editor would like to thank the original authors of the document: Kaynam Hedayat, Nagarjuna Venna, Paul E. Jones, Arjun Roychowdhury, Chelliah SivaChelvan, and Nathan Stratton. The editor has made fairly insignificant changes in the end. Also, we'd like to thank Magnus Westerlund, Miguel Garcia, Muthu Arul Mozhi Perumal, Jeff Bernstein, Paul Kyzivat, Dave Oran, Flemming Andreasen, Gunnar Hellstrom, Emil Ivov, and Dan Wing for their feedback, comments, and suggestions.
本文件的编辑要感谢文件的原始作者：Kaynam Hedayat、Nagarjuna Venna、Paul E.Jones、Arjun Roychowdhury、Chelliah SivaChelvan和Nathan Stratton。编辑最后做了一些微不足道的改动。此外，我们还要感谢马格努斯·韦斯特隆德、米格尔·加西亚、穆图·阿鲁尔·莫齐·佩鲁马尔、杰夫·伯恩斯坦、保罗·基齐瓦特、戴夫·奥兰、弗莱明·安德烈森、古纳尔·赫尔斯特罗姆、埃米尔·伊沃夫和丹·温的反馈、评论和建议。
[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。
[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。
[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。
[RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and Video Conferences with Minimal Control", STD 65, RFC 3551, July 2003.
[RFC3551]Schulzrinne，H.和S.Casner，“具有最小控制的音频和视频会议的RTP配置文件”，STD 65，RFC 3551，2003年7月。
[RFC3611] Friedman, T., Ed., Caceres, R., Ed., and A. Clark, Ed., "RTP Control Protocol Extended Reports (RTCP XR)", RFC 3611, November 2003.
[RFC3611]Friedman，T.，Ed.，Caceres，R.，Ed.，和A.Clark，Ed.，“RTP控制协议扩展报告（RTCP XR）”，RFC 36112003年11月。
[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。
[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。
[RFC4961] Wing, D., "Symmetric RTP / RTP Control Protocol (RTCP)", BCP 131, RFC 4961, July 2007.
[RFC4961]Wing，D，“对称RTP/RTP控制协议（RTCP）”，BCP 131，RFC 49612007年7月。
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5234]Crocker，D.，Ed.，和P.Overell，“语法规范的扩充BNF:ABNF”，STD 68，RFC 5234，2008年1月。
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment (ICE): A Protocol for Network Address Translator (NAT) Traversal for Offer/Answer Protocols", RFC 5245, April 2010.
[RFC5245]Rosenberg，J.，“交互式连接建立（ICE）：提供/应答协议的网络地址转换器（NAT）遍历协议”，RFC 52452010年4月。
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing, "Session Traversal Utilities for NAT (STUN)", RFC 5389, October 2008.
[RFC5389]Rosenberg，J.，Mahy，R.，Matthews，P.，和D.Wing，“NAT的会话遍历实用程序（STUN）”，RFC 5389，2008年10月。
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using Relays around NAT (TURN): Relay Extensions to Session Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC5766]Mahy，R.，Matthews，P.，和J.Rosenberg，“使用NAT周围的中继进行遍历（TURN）：NAT（STUN）会话遍历实用程序的中继扩展”，RFC 5766，2010年4月。
[RFC6263] Marjou, X. and A. Sollaud, "Application Mechanism for Keeping Alive the NAT Mappings Associated with RTP / RTP Control Protocol (RTCP) Flows", RFC 6263, June 2011.
[RFC6263]Marjou，X.和A.Sollaud，“与RTP/RTP控制协议（RTCP）流相关的NAT映射保持活动的应用机制”，RFC 6263，2011年6月。
Authors' Addresses
作者地址
Hadriel Kaplan (editor) Acme Packet 100 Crosby Drive Bedford, MA 01730 US EMail: hkaplan@acmepacket.com URI: http://www.acmepacket.com
Hadriel Kaplan（编辑）Acme Packet 100 Crosby Drive Bedford，马萨诸塞州01730美国电子邮件：hkaplan@acmepacket.comURI:http://www.acmepacket.com
Kaynam Hedayat EXFO 285 Mill Road Chelmsford, MA 01824 US EMail: kh274@cornell.edu URI: http://www.exfo.com/
Kaynam Hedayat EXFO马萨诸塞州切姆斯福德米尔路285号01824美国电子邮件：kh274@cornell.eduURI:http://www.exfo.com/
Nagarjuna Venna Saperix c/o DogPatch Labs One Cambridge Center, 6th Floor Cambridge, MA 02142 US EMail: vnagarjuna@saperix.com URI: http://www.saperix.com/
Nagarjuna Venna Saperix c/o DogPatch Labs One Cambridge Center，马萨诸塞州坎布里奇市6楼美国电子邮件：vnagarjuna@saperix.comURI:http://www.saperix.com/
Paul E. Jones Cisco Systems, Inc. 7025 Kit Creek Rd. Research Triangle Park, NC 27709 US EMail: paulej@packetizer.com URI: http://www.cisco.com/
Paul E.Jones Cisco Systems，Inc.地址：北卡罗来纳州三角研究公园Kit Creek路7025号，邮编：27709美国电子邮件：paulej@packetizer.comURI:http://www.cisco.com/
Nathan Stratton BlinkMind, Inc. 2027 Briarchester Dr. Katy, TX 77450 US EMail: nathan@robotics.net URI: http://www.robotics.net/
Nathan Stratton BlinkMind，Inc.2027 Briarchester Katy博士，德克萨斯州77450美国电子邮件：nathan@robotics.netURI:http://www.robotics.net/