Internet Engineering Task Force (IETF) H. Kaplan
Request for Comments: 7092 Oracle
Category: Informational V. Pascual
ISSN: 2070-1721 Quobis
December 2013
Internet Engineering Task Force (IETF) H. Kaplan
Request for Comments: 7092 Oracle
Category: Informational V. Pascual
ISSN: 2070-1721 Quobis
December 2013
A Taxonomy of Session Initiation Protocol (SIP) Back-to-Back User Agents
会话启动协议（SIP）背靠背用户代理的分类
Abstract
摘要
In many SIP deployments, SIP entities exist in the SIP signaling path between the originating and final terminating endpoints, which go beyond the definition of a SIP proxy, performing functions not defined in Standards Track RFCs. The only term for such devices provided in RFC 3261 is for a Back-to-Back User Agent (B2BUA), which is defined as the logical concatenation of a SIP User Agent Server (UAS) and User Agent Client (UAC).
在许多SIP部署中，SIP实体存在于发起和最终终止端点之间的SIP信令路径中，这超出了SIP代理的定义，执行标准跟踪RFC中未定义的功能。RFC 3261中提供的此类设备的唯一术语是背靠背用户代理（B2BUA），其定义为SIP用户代理服务器（UAS）和用户代理客户端（UAC）的逻辑连接。
There are numerous types of SIP B2BUAs performing different roles in different ways; for example, IP Private Branch Exchanges (IPBXs), Session Border Controllers (SBCs), and Application Servers (ASs). This document identifies several common B2BUA roles in order to provide taxonomy other documents can use and reference.
有许多类型的SIP B2BUA以不同的方式执行不同的角色；例如，IP专用分支交换机（IPBX）、会话边界控制器（SBC）和应用程序服务器（ASs）。本文档确定了几个常见的B2BUA角色，以便提供其他文档可以使用和参考的分类法。
Status of This Memo
关于下段备忘
This document is not an Internet Standards Track specification; it is published for informational purposes.
本文件不是互联网标准跟踪规范；它是为了提供信息而发布的。
This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741.
本文件是互联网工程任务组（IETF）的产品。它代表了IETF社区的共识。它已经接受了公众审查，并已被互联网工程指导小组（IESG）批准出版。并非IESG批准的所有文件都适用于任何级别的互联网标准；见RFC 5741第2节。
Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7092.
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/rfc7092.
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。
Table of Contents
目录
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. B2BUA Role Types . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Signaling Plane B2BUA Roles . . . . . . . . . . . . . . . 4
3.1.1. Proxy-B2BUA . . . . . . . . . . . . . . . . . . . . . 4
3.1.2. Signaling-only . . . . . . . . . . . . . . . . . . . 4
3.1.3. SDP-Modifying Signaling-only . . . . . . . . . . . . 5
3.2. Signaling/Media Plane B2BUA Roles . . . . . . . . . . . . 5
3.2.1. Media Relay . . . . . . . . . . . . . . . . . . . . . 5
3.2.2. Media Aware . . . . . . . . . . . . . . . . . . . . . 6
3.2.3. Media Termination . . . . . . . . . . . . . . . . . . 6
4. Mapping SIP Device Types to B2BUA Roles . . . . . . . . . . . 6
4.1. SIP PBXs and Softswitches . . . . . . . . . . . . . . . . 6
4.2. Application Servers . . . . . . . . . . . . . . . . . . . 7
4.3. Session Border Controllers . . . . . . . . . . . . . . . 7
4.4. Transcoders . . . . . . . . . . . . . . . . . . . . . . . 7
4.5. Conference Servers . . . . . . . . . . . . . . . . . . . 8
4.6. P-CSCF and IBCF Functions . . . . . . . . . . . . . . . . 8
4.7. S-CSCF Function . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Informative References . . . . . . . . . . . . . . . . . . . 9
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. B2BUA Role Types . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Signaling Plane B2BUA Roles . . . . . . . . . . . . . . . 4
3.1.1. Proxy-B2BUA . . . . . . . . . . . . . . . . . . . . . 4
3.1.2. Signaling-only . . . . . . . . . . . . . . . . . . . 4
3.1.3. SDP-Modifying Signaling-only . . . . . . . . . . . . 5
3.2. Signaling/Media Plane B2BUA Roles . . . . . . . . . . . . 5
3.2.1. Media Relay . . . . . . . . . . . . . . . . . . . . . 5
3.2.2. Media Aware . . . . . . . . . . . . . . . . . . . . . 6
3.2.3. Media Termination . . . . . . . . . . . . . . . . . . 6
4. Mapping SIP Device Types to B2BUA Roles . . . . . . . . . . . 6
4.1. SIP PBXs and Softswitches . . . . . . . . . . . . . . . . 6
4.2. Application Servers . . . . . . . . . . . . . . . . . . . 7
4.3. Session Border Controllers . . . . . . . . . . . . . . . 7
4.4. Transcoders . . . . . . . . . . . . . . . . . . . . . . . 7
4.5. Conference Servers . . . . . . . . . . . . . . . . . . . 8
4.6. P-CSCF and IBCF Functions . . . . . . . . . . . . . . . . 8
4.7. S-CSCF Function . . . . . . . . . . . . . . . . . . . . . 8
5. Security Considerations . . . . . . . . . . . . . . . . . . . 8
6. Informative References . . . . . . . . . . . . . . . . . . . 9
In current SIP deployments, there are numerous forms of Back-to-Back User Agents (B2BUAs), operating at various levels of transparency and for various purposes, with widely varying behavior from a SIP perspective. Some act as pure SIP proxies and only change to the role of B2BUA in order to generate BYEs to terminate dead sessions. Some are full User Agent stacks with only high-level event and application logic binding the User Agent Server (UAS) and User Agent Client (UAC) sides. Some B2BUAs operate only in the SIP signaling plane, while others participate in the media plane as well.
在当前的SIP部署中，有多种形式的背对背用户代理（B2BUA），它们以不同的透明度和不同的目的运行，从SIP的角度来看，它们的行为千差万别。有些充当纯SIP代理，仅更改为B2BUA的角色，以便生成“是”以终止死会话。有些是完整的用户代理堆栈，只有绑定用户代理服务器（UAS）和用户代理客户端（UAC）的高级事件和应用程序逻辑。一些B2BUA仅在SIP信令平面中操作，而其他B2BUA也参与媒体平面。
As more SIP domains are deployed and interconnected, the probability of a single SIP session crossing multiple B2BUAs at both the signaling and media planes increases significantly.
随着更多SIP域的部署和互连，单个SIP会话在信令和媒体平面上跨越多个B2BUA的概率显著增加。
This document provides a taxonomy of several common B2BUA roles so that other documents may refer to them using their given names without redefining them in each document.
本文档提供了几种常见B2BUA角色的分类，以便其他文档可以使用它们的给定名称引用它们，而无需在每个文档中重新定义它们。
The following terms are defined in [RFC3261], Section 6.
[RFC3261]第6节定义了以下术语。
B2BUA: a SIP Back-to-Back User Agent, which is the logical combination of a User Agent Server (UAS) and User Agent Client (UAC).
B2BUA:SIP背靠背用户代理，是用户代理服务器（UAS）和用户代理客户端（UAC）的逻辑组合。
UAS: a SIP User Agent Server.
UAS：SIP用户代理服务器。
UAC: a SIP User Agent Client.
UAC：SIP用户代理客户端。
Within the context of this document, the classification refers to a B2BUA role, not a particular system type. A given system type may change its role in the middle of a SIP session, for example, when a stateful proxy tears down a session by generating BYEs or when an SBC [RFC5853] performs transcoding or REFER termination.
在本文档的上下文中，分类指的是B2BUA角色，而不是特定的系统类型。给定的系统类型可以在SIP会话的中间改变其作用，例如，当状态代理通过生成BYES或SBC [RCF5853]执行代码转换或引用终止时撕裂会话。
Furthermore, this document defines "B2BUA" following the definition provided in [RFC3261], which is the logical concatenation of a UAS and UAC. A typical centralized conference server, for example, is not a B2BUA because it is the target UAS of multiple UACs, whereby the UACs individually and independently initiate separate SIP sessions to the central conference server. Likewise, a third-party call control transcoder, as described in Section 3.1 of [RFC5369], is
此外，本文件根据[RFC3261]中的定义定义了“B2BUA”，即UAS和UAC的逻辑连接。例如，典型的集中式会议服务器不是B2BUA，因为它是多个UAC的目标UAS，因此UAC单独和独立地向中央会议服务器发起单独的SIP会话。同样，第三方呼叫控制转码器（如[RFC5369]第3.1节所述）也适用
not a B2BUA, whereas an inline (conference bridge) transcoder based on [RFC5370] is a B2BUA.
不是B2BUA，而基于[RFC5370]的内联（会议桥）转码器是B2BUA。
A signaling plane B2BUA is one that operates only on the SIP message protocol layer and only with SIP messages and headers but not with the media itself in any way. This implies that it does not modify the Session Description Protocol (SDP) bodies, although it may save them and/or operate on other MIME body types. This category is further subdivided into specific roles as described in this section.
信令平面B2BUA是仅在SIP消息协议层上操作并且仅与SIP消息和报头一起操作而不以任何方式与媒体本身一起操作的平面。这意味着它不会修改会话描述协议（SDP）主体，尽管它可以保存它们和/或对其他MIME主体类型进行操作。该类别进一步细分为本节所述的特定角色。
It should be noted that there is a large variety of modifications made by "signaling plane B2BUAs".
应当注意，“信令平面B2BUAs”进行了多种修改。
A Proxy-B2BUA is one that appears, from a SIP perspective, to be a SIP proxy based on [RFC3261] and its extensions, except that it maintains a sufficient dialog state to generate in-dialog SIP messages on its own and does so in specific cases. The most common example of this is a SIP proxy that can generate BYE requests to tear down a dead session.
从SIP的角度来看，代理B2BUA是一个基于[RFC3261]及其扩展的SIP代理，除了它保持足够的对话状态以自行生成对话内SIP消息，并且在特定情况下这样做。最常见的例子是SIP代理，它可以生成BYE请求来中断一个死会话。
A Proxy-B2BUA does not modify the received header fields such as To, From, or Contact, and it only modifies the Via and Record-Route header fields following the rules in [RFC3261] and its extensions. If a Proxy-B2BUA can generate in-dialog messages, then it will also need to modify the CSeq header after it has generated its own. A Proxy-B2BUA neither modifies nor inspects MIME bodies (including SDP), does not have any awareness of media, will forward any method type, etc.
代理B2BUA不修改接收到的报头字段，如To、From或Contact，它只按照[RFC3261]及其扩展中的规则修改Via和记录路由报头字段。如果代理B2BUA可以在对话框中生成消息，那么它还需要在生成自己的消息后修改CSeq头。代理B2BUA既不修改也不检查MIME主体（包括SDP），对媒体没有任何感知，将转发任何方法类型，等等。
A Signaling-only B2BUA is one that operates at the SIP layer but in ways beyond those of [RFC3261] proxies, even for normally forwarded requests. For example, such a B2BUA might replace the Contact URI, modify or remove all Via and Record-Route headers, modify the To and From header fields, modify or inspect specific MIME bodies, etc. No SIP header field is guaranteed to be copied from the received request on the UAS side to the generated request on the UAC side.
仅信令B2BUA是在SIP层上运行的，但其方式超出[RFC3261]代理的方式，即使对于正常转发的请求也是如此。例如，这样的B2BUA可能会替换联系人URI、修改或删除所有Via和记录路由头、修改To和From头字段、修改或检查特定MIME主体等。不保证将SIP头字段从UAS端接收的请求复制到UAC端生成的请求。
An example of such a B2BUA would be some form of an Application Server and a PBX, such as a server that locally processes REFER requests and generates new INVITEs on behalf of the REFER's target. Another example would be a privacy service proxy [RFC3323] performing the 'header' privacy function.
这种B2BUA的一个示例是某种形式的应用服务器和PBX，例如本地处理REFER请求并代表REFER的目标生成新邀请的服务器。另一个例子是执行“头”隐私功能的隐私服务代理[RFC3323]。
An SDP-Modifying Signaling-only B2BUA is one that operates in the signaling plane only and is not in the media path, but it does modify SDP bodies and is thus aware of and understands SDP syntax and semantics. In some cases, some Application Servers and PBXs act in this role, for example, to remove certain codec choices or merge two media endpoints into one SDP offer.
仅修改信令的SDP B2BUA是仅在信令平面中操作且不在媒体路径中的SDP，但它确实修改SDP主体，因此知道并理解SDP语法和语义。在某些情况下，某些应用程序服务器和PBX扮演此角色，例如，删除某些编解码器选择或将两个媒体端点合并到一个SDP产品中。
These B2BUAs don't do anything that changes the path that the media takes (in particular, they don't insert themselves on the media path), but they may make SDP changes that affect what is sent on the media plane.
这些B2BUA不会改变介质的路径（特别是，它们不会将自身插入介质路径），但它们可能会进行SDP更改，从而影响介质平面上发送的内容。
A signaling/media plane B2BUA is one that operates at both the SIP and media planes and not only on SIP messages but also on SDP and potentially the Real-time Transport Protocol (RTP) / the Real-Time Control Protocol (RTCP) [RFC3550] or other media. Such a B2BUA may or may not replace the Contact URI, modify or remove all Via and Record-Route headers, modify the To and From header fields, etc. No SIP header field is guaranteed to be copied from the received request on the UAS side to the generated request on the UAC side, and SDP will also be modified.
信令/媒体平面B2BUA是在SIP和媒体平面上并且不仅在SIP消息上而且在SDP和潜在的实时传输协议（RTP）/实时控制协议（RTCP）[RFC3550]或其他媒体上操作的平面。这样的B2BUA可以也可以不替换联系人URI、修改或删除所有Via和记录路由头、修改To和From头字段等。不保证将SIP头字段从UAS端接收的请求复制到UAC端生成的请求，并且还将修改SDP。
An example of such a B2BUA would be a Session Border Controller (SBC) performing the functions defined in [RFC5853], a B2BUA transcoder as defined in [RFC5370], a rich-ringtone Application Server, or a recording system. Another example would be a privacy service proxy [RFC3323] performing the 'session' privacy function.
此类B2BUA的一个示例是执行[RFC5853]中定义的功能的会话边界控制器（SBC）、如[RFC5370]中定义的B2BUA转码器、富铃声应用服务器或记录系统。另一个例子是执行“会话”隐私功能的隐私服务代理[RFC3323]。
Note that a signaling/media plane B2BUA need not be instantiated in a single physical system, but it may be decomposed into separate signaling and media functions.
注意，信令/媒体平面B2BUA不需要在单个物理系统中被实例化，但是它可以被分解为单独的信令和媒体功能。
The signaling/media plane B2BUA category is further subdivided into specific roles as described in this section.
信令/媒体平面B2BUA类别进一步细分为本节所述的特定角色。
A B2BUA that performs a media-relay role is one that terminates the media plane at the IP and transport (UDP/TCP) layers on its UAS and UAC sides, but neither modifies nor restricts which forms of payload are carried within the packets. Rather, the payload is transparently copied from one side to the other. Such a role may or may not support only UDP, only TCP, both UDP and TCP, as well as other transport types. It may also involve policing the IP packets to fit
执行媒体中继角色的B2BUA在其UAS和UAC侧的IP和传输（UDP/TCP）层终止媒体平面，但既不修改也不限制包中承载的有效负载形式。相反，有效载荷是从一侧透明地复制到另一侧的。这种角色可能只支持UDP、TCP、UDP和TCP以及其他传输类型，也可能不支持。它还可能涉及管理IP数据包以适应
within a bandwidth limit or converting from IPv4 to IPv6, or vice versa. This is typically similar to NAT behavior, except a NAT operating in both directions on both the source and destination information; it is often found as the default behavior in SBCs.
在带宽限制内，或从IPv4转换为IPv6，或从IPv4转换为IPv6。这通常类似于NAT行为，除了NAT在源信息和目标信息的两个方向上运行；它通常被认为是SBC中的默认行为。
A B2BUA that performs a media-aware role is similar to a media relay except that it inspects and potentially modifies the payload carried in UDP or TCP (as it could be RTP or RTCP [RFC3550]), but it does not at a codec or higher layer. An example of such a role is a Secure Real-time Transport Protocol (SRTP) [RFC3711] terminator, which does not need to care about the RTP payload but does care about the RTP header; or, a device that monitors RTCP for QoS information; or, a device that multiplexes/demultiplexes RTP and RTCP packets on the same 5-tuple.
执行媒体感知角色的B2BUA与媒体中继类似，只是它检查并可能修改UDP或TCP中承载的有效负载（可能是RTP或RTCP[RFC3550]），但不在编解码器或更高的层。这种角色的一个示例是安全实时传输协议（SRTP）[RFC3711]终止器，它不需要关心RTP有效载荷，但关心RTP报头；或者，监控RTCP的QoS信息的设备；或者，在同一个5元组上复用/解复用RTP和RTCP数据包的设备。
A B2BUA that performs a media-termination role is one that operates at the media payload layer, such as RTP/RTCP codec or the Message Session Relay Protocol (MSRP) message layer and higher. Such a role may only terminate/generate specific RTP media, such as dual-tone multi-frequency (DTMF) packets [RFC4733], or it may convert between media codecs or act as a Back-to-Back MSRP [RFC4975] agent. This is the role performed by transcoders, conference servers based on [RFC5366], etc.
执行媒体终止角色的B2BUA是在媒体有效负载层操作的B2BUA，例如RTP/RTCP编解码器或消息会话中继协议（MSRP）消息层及更高级别。这样的角色可以仅终止/生成特定的RTP媒体，例如双音多频（DTMF）分组[RFC4733]，或者可以在媒体编解码器之间转换，或者充当背靠背MSRP[RFC4975]代理。这是由转码器、基于[RFC5366]的会议服务器等执行的角色。
Although the B2BUA roles defined previously do not define system types, as discussed in Section 3, some discussion of what common system types perform which defined roles may be appropriate. This section provides such a 'mapping' for general cases to aid in understanding of the roles.
尽管前面定义的B2BUA角色没有定义系统类型，如第3节中所述，但是一些关于哪些通用系统类型执行哪些定义的角色可能是合适的讨论。本节提供了一般案例的“映射”，以帮助理解角色。
SIP-enabled Private Branch Exchanges (SIP PBXs) and softswitches are market category terms and are not specified in any standard. In general, they can perform every role described in this document at any given time based on their architecture or local policy. Some are based on architectures that make them the equivalent of a SIP UAS and UAC connected with a logical Primary Rate Interface (PRI) loopback; others are built as a SIP proxy core with optional modules to "do more".
支持SIP的专用分支交换机（SIP PBX）和软交换是市场类别术语，没有在任何标准中指定。一般来说，他们可以在任何给定时间根据其体系结构或本地策略执行本文档中描述的每个角色。一些基于的架构使其相当于SIP UAS和UAC，通过逻辑主速率接口（PRI）环回连接；其他的则构建为SIP代理核心，带有可选的模块来“做更多”。
Application Servers are a broad marketing term and are not specified in any standard in general, although the Third Generation Partnership Project (3GPP) and other organizations specify some specific Application Server functions and behaviors. Common examples of Application Server functions are message-waiting indicators (MWIs), Find Me/Follow Me services, privacy services, call center Automatic Call Distributor (ACD) services, call screening, and Voice Call Continuity (VCC) services. Some only operate in the signaling plane in either Proxy-B2BUA or Signaling-only B2BUA roles; others operate as full Media-termination B2BUAs, such as when providing Interactive Voice Recognition (IVR), rich ringtones, or integrated voicemail services.
应用服务器是一个广泛的营销术语，一般来说没有在任何标准中指定，尽管第三代合作伙伴计划（3GPP）和其他组织指定了一些特定的应用服务器功能和行为。应用服务器功能的常见示例有消息等待指示器（MWI）、查找我/跟踪我服务、隐私服务、呼叫中心自动呼叫分发（ACD）服务、呼叫屏蔽和语音呼叫连续性（VCC）服务。一些仅在代理B2BUA或仅发信号B2BUA角色的信令平面中操作；另一些则作为全媒体终端B2BUA运行，例如在提供交互式语音识别（IVR）、富格铃声或集成语音邮件服务时。
Session Border Controllers (SBCs) are a market category term and are not specified in any standard. Some of the common functions performed by SBCs are described in [RFC5853], but in general, they can perform every role described in this document at any given time based on local policy. By default, most SBCs are either Media-relay or Media-aware B2BUAs and replace the Contact URI; remove the Via and Record-Route headers; modify Call-ID, To, From, and various other headers; and modify SDP. Some SBCs remove all headers, all bodies, and reject all method types unless explicitly allowed by local policy; other SBCs pass all such elements through unless explicitly forbidden by local policy.
会话边界控制器（SBC）是一个市场类别术语，未在任何标准中指定。[RFC5853]中描述了SBC执行的一些常见功能，但一般来说，它们可以根据本地策略在任何给定时间执行本文档中描述的每个角色。默认情况下，大多数SBC是媒体中继或媒体感知B2BUA，并替换联系人URI；移除通孔并记录路由标头；修改调用ID、To、From和各种其他标头；并修改SDP。除非本地策略明确允许，否则某些SBC会删除所有头文件、所有正文并拒绝所有方法类型；除非当地政策明确禁止，否则其他SBC将通过所有此类要素。
Transcoders perform the function of transcoding one audio or video media codec type to another, such as G.711 to G.729. As such, they perform the Media-termination role, although they may only terminate media in specific cases of codec mismatch between the two ends. Although [RFC5369] and [RFC5370] define two types of SIP transcoders, in practice, a popular variant of the inline conference bridge model [RFC5370] is to behave as a SIP B2BUA without using the resource-list mechanism but rather simply routing a normal INVITE request through a B2BUA with a built-in transcoder. SIP transcoder architectures are based on everything from SIP media servers and SBCs to looped-back Time Division Multiplexing (TDM) gateways, and thus run the gamut from replacing only specific headers/bodies and SDP content needed to perform their function to replacing almost all SIP headers and SDP content. Some transcoders save and remove SDP offers in INVITEs from the UAC, and wait for an offer in the response from the UAS, similar
转码器执行将一种音频或视频媒体编解码器类型转码为另一种的功能，例如G.711到G.729。因此，它们执行媒体终止角色，尽管它们可能仅在两端编解码器不匹配的特定情况下终止媒体。尽管[RFC5369]和[RFC5370]定义了两种类型的SIP转码器，但在实践中，内联会议桥模型[RFC5370]的一个流行变体是在不使用资源列表机制的情况下充当SIP B2BUA，而只是通过带有内置转码器的B2BUA路由正常INVITE请求。SIP转码器体系结构基于从SIP媒体服务器和SBC到环回时分多路复用（TDM）网关的所有内容，因此运行范围从仅替换执行其功能所需的特定头/体和SDP内容到替换几乎所有SIP头和SDP内容。一些转码器保存和删除UAC邀请中的SDP报价，并等待UAS响应中的报价，类似
to a Third Party Call Control (3PCC) model; others just insert additional codecs in SDP offers and only transcode if the inserted codec(s) is selected in the answer.
第三方呼叫控制（3PCC）模型；其他人只是在SDP产品中插入额外的编解码器，并且只有在答案中选择了插入的编解码器时才进行转码。
In general, conference servers do not fall under the term "B2BUA" as defined by this document, since typically they involve multiple SIP UACs initiating independent SIP sessions to the single conference UAS. However, a conference server supporting [RFC5366], whereby the received INVITE triggers the conference focus UAS to initiate multiple INVITEs as a UAC, would be in a Media-termination B2BUA role when performing that function.
一般来说，会议服务器不属于本文档定义的术语“B2BUA”，因为它们通常涉及多个SIP UAC，向单个会议UAS发起独立的SIP会话。然而，支持[RFC5366]的会议服务器在执行该功能时将处于媒体终止B2BUA角色，由此接收到的邀请触发会议焦点UAS以作为UAC发起多个邀请。
The Proxy-Call Session Control Function (P-CSCF) and the Interconnection Border Control Function (IBCF) are defined by 3GPP [IMS] standards, and when coupled with the IP Multimedia Subsystems (IMS) media plane gateways (IMS Access Gateway (AGW), Transition Gateway (TrGW), etc.), they typically form a logical Media-relay or Media-aware B2BUA role.
代理呼叫会话控制功能（P-CSCF）和互连边界控制功能（IBCF）由3GPP[IMS]标准定义，并且当与IP多媒体子系统（IMS）媒体平面网关（IMS接入网关（AGW）、转换网关（TrGW）等）耦合时，它们通常形成逻辑媒体中继或媒体感知B2BUA角色。
The Serving-Call Session Control Function (S-CSCF) is defined by 3GPP [IMS] standards and typically follows a Proxy-B2BUA role.
服务呼叫会话控制功能（S-CSCF）由3GPP[IMS]标准定义，并且通常遵循代理-B2BUA角色。
Security risks are specific to each type of B2BUA, so little can be said in general. Of course, adding extra systems in the communication path creates extra points of attack, reduces or eliminates the ability to perform end-to-end encryption, decreases the privacy of SIP communications, and complicates trust models. Thus, every B2BUA design requires particular attention to security analysis.
每种类型的B2BUA都有特定的安全风险，因此一般来说几乎没有什么可以说的。当然，在通信路径中添加额外的系统会产生额外的攻击点，降低或消除执行端到端加密的能力，降低SIP通信的保密性，并使信任模型复杂化。因此，每个B2BUA设计都需要特别注意安全性分析。
A few general points can be made:
可以提出以下几点：
1. The B2BUA processing of SDP and media packets is an impediment to the deployment of end-to-end SRTP and reduces the ability to deploy new, stronger forms of SRTP key exchange.
1. SDP和媒体数据包的B2BUA处理妨碍了端到端SRTP的部署，并降低了部署新的、更强大的SRTP密钥交换形式的能力。
2. The ability for B2BUAs to modify any SIP header field value and body impacts the ability to deploy SIP identity and message integrity.
2. B2BUAs修改任何SIP头字段值和正文的能力会影响部署SIP标识和消息完整性的能力。
3. The management and configuration mechanisms of B2BUAs are a tempting point of attack and must be strongly defended.
3. B2BUA的管理和配置机制是一个诱人的攻击点，必须进行有力的防御。
Further security considerations related to the functionality described in this document are addressed in the relevant references.
与本文档中描述的功能相关的其他安全注意事项在相关参考文献中进行了说明。
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A., Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP: Session Initiation Protocol", RFC 3261, June 2002.
[RFC3261]Rosenberg，J.，Schulzrinne，H.，Camarillo，G.，Johnston，A.，Peterson，J.，Sparks，R.，Handley，M.，和E.Schooler，“SIP：会话启动协议”，RFC 3261，2002年6月。
[RFC3323] Peterson, J., "A Privacy Mechanism for the Session Initiation Protocol (SIP)", RFC 3323, November 2002.
[RFC3323]Peterson，J.，“会话启动协议（SIP）的隐私机制”，RFC3323，2002年11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。
[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。
[RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF Digits, Telephony Tones, and Telephony Signals", RFC 4733, December 2006.
[RFC4733]Schulzrinne，H.和T.Taylor，“DTMF数字、电话音和电话信号的RTP有效载荷”，RFC 47332006年12月。
[RFC4975] Campbell, B., Mahy, R., and C. Jennings, "The Message Session Relay Protocol (MSRP)", RFC 4975, September 2007.
[RFC4975]Campbell，B.，Mahy，R.，和C.Jennings，“消息会话中继协议（MSRP）”，RFC 49752007年9月。
[RFC5366] Camarillo, G. and A. Johnston, "Conference Establishment Using Request-Contained Lists in the Session Initiation Protocol (SIP)", RFC 5366, October 2008.
[RFC5366]Camarillo，G.和A.Johnston，“使用会话启动协议（SIP）中包含的请求列表建立会议”，RFC 5366，2008年10月。
[RFC5369] Camarillo, G., "Framework for Transcoding with the Session Initiation Protocol (SIP)", RFC 5369, October 2008.
[RFC5369]Camarillo，G.“会话启动协议（SIP）转码框架”，RFC 5369，2008年10月。
[RFC5370] Camarillo, G., "The Session Initiation Protocol (SIP) Conference Bridge Transcoding Model", RFC 5370, October 2008.
[RFC5370]Camarillo，G.“会话启动协议（SIP）会议桥转码模型”，RFC 5370，2008年10月。
[RFC5853] Hautakorpi, J., Camarillo, G., Penfield, R., Hawrylyshen, A., and M. Bhatia, "Requirements from Session Initiation Protocol (SIP) Session Border Control (SBC) Deployments", RFC 5853, April 2010.
[RFC5853]Hautakorpi，J.，Camarillo，G.，Penfield，R.，Hawrylyshen，A.，和M.Bhatia，“会话启动协议（SIP）会话边界控制（SBC）部署的要求”，RFC 58532010年4月。
[IMS] 3GPP, "IP Multimedia Subsystem (IMS); Stage 2, 3GPP TS 23.228", Version 12.2.0, September 2013.
[IMS]3GPP，“IP多媒体子系统（IMS）；第2阶段，3GPP TS 23.228”，版本12.2.0，2013年9月。
Authors' Addresses
作者地址
Hadriel Kaplan Oracle
哈德里尔·卡普兰神谕
EMail: hadriel.kaplan@oracle.com
EMail: hadriel.kaplan@oracle.com
Victor Pascual Quobis
维克多·帕斯夸尔·库比斯
EMail: victor.pascual@quobis.com
EMail: victor.pascual@quobis.com