Network Working Group D. Yon
Request for Comments: 4145 Tactical Software, LLC
Category: Standards Track G. Camarillo
Ericsson
September 2005
Network Working Group D. Yon
Request for Comments: 4145 Tactical Software, LLC
Category: Standards Track G. Camarillo
Ericsson
September 2005
TCP-Based Media Transport in the Session Description Protocol (SDP)
会话描述协议（SDP）中基于TCP的媒体传输
Status of this Memo
本备忘录的状况
This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.
本文件规定了互联网社区的互联网标准跟踪协议，并要求进行讨论和提出改进建议。有关本协议的标准化状态和状态，请参考当前版本的“互联网官方协议标准”（STD 1）。本备忘录的分发不受限制。
Copyright Notice
版权公告
Copyright (C) The Internet Society (2005).
版权所有（C）互联网协会（2005年）。
Abstract
摘要
This document describes how to express media transport over TCP using the Session Description Protocol (SDP). It defines the SDP 'TCP' protocol identifier, the SDP 'setup' attribute, which describes the connection setup procedure, and the SDP 'connection' attribute, which handles connection reestablishment.
本文档介绍如何使用会话描述协议（SDP）通过TCP表示媒体传输。它定义了SDP“TCP”协议标识符、描述连接设置过程的SDP“setup”属性和处理连接重建的SDP“connection”属性。
Table of Contents
目录
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 3
4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. The Setup Attribute in the Offer/Answer Model. . . . . . 4
5. The Connection Attribute . . . . . . . . . . . . . . . . . . . 5
5.1. Offerer Behaviour. . . . . . . . . . . . . . . . . . . . 6
5.2. Answerer Behaviour . . . . . . . . . . . . . . . . . . . 7
6. Connection Management . . . . . . . . . . . . . . . . . . . . 8
6.1. Connection Establishment . . . . . . . . . . . . . . . . 8
6.2. Connection Reestablishment . . . . . . . . . . . . . . . 8
6.3. Connection Termination . . . . . . . . . . . . . . . . . 8
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Passive/Active . . . . . . . . . . . . . . . . . . . . . 9
7.2. Actpass/Passive. . . . . . . . . . . . . . . . . . . . . 9
7.3. Existing Connection Reuse. . . . . . . . . . . . . . . . 10
7.4. Existing Connection Refusal. . . . . . . . . . . . . . . 10
8. Other Connection-Oriented Transport Protocols. . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . 13
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol Identifier . . . . . . . . . . . . . . . . . . . . . 3
4. Setup Attribute . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. The Setup Attribute in the Offer/Answer Model. . . . . . 4
5. The Connection Attribute . . . . . . . . . . . . . . . . . . . 5
5.1. Offerer Behaviour. . . . . . . . . . . . . . . . . . . . 6
5.2. Answerer Behaviour . . . . . . . . . . . . . . . . . . . 7
6. Connection Management . . . . . . . . . . . . . . . . . . . . 8
6.1. Connection Establishment . . . . . . . . . . . . . . . . 8
6.2. Connection Reestablishment . . . . . . . . . . . . . . . 8
6.3. Connection Termination . . . . . . . . . . . . . . . . . 8
7. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Passive/Active . . . . . . . . . . . . . . . . . . . . . 9
7.2. Actpass/Passive. . . . . . . . . . . . . . . . . . . . . 9
7.3. Existing Connection Reuse. . . . . . . . . . . . . . . . 10
7.4. Existing Connection Refusal. . . . . . . . . . . . . . . 10
8. Other Connection-Oriented Transport Protocols. . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
12.1. Normative References . . . . . . . . . . . . . . . . . . 13
12.2. Informative References . . . . . . . . . . . . . . . . . 13
The Session Description Protocol [4] provides a general-purpose format for describing multimedia sessions in announcements or invitations. SDP uses an entirely textual data format (the US-ASCII subset of UTF-8 [11]) to maximize portability among transports. SDP does not define a protocol; it defines the syntax to describe a multimedia session with sufficient information to participate in that session. Session descriptions may be sent using arbitrary existing application protocols for transport (e.g., SAP [9], SIP [10], RTSP [6], email, HTTP [8], etc.).
会话描述协议[4]提供了一种通用格式，用于描述公告或邀请中的多媒体会话。SDP使用完全文本数据格式（UTF-8[11]的US-ASCII子集）来最大限度地提高传输之间的可移植性。SDP没有定义协议；它定义了描述多媒体会话的语法，该会话具有足够的信息来参与该会话。会话描述可以使用用于传输的任意现有应用程序协议（例如，SAP[9]、SIP[10]、RTSP[6]、电子邮件、HTTP[8]等）发送。
SDP [4] defines two protocol identifiers: RTP/AVP and UDP, both of which represent unreliable, connectionless protocols. While these transports are appropriate choices for multimedia streams, there are applications for which TCP is more appropriate. This document defines a new protocol identifier, 'TCP', to describe TCP connections in SDP.
SDP[4]定义了两个协议标识符：RTP/AVP和UDP，它们都表示不可靠的无连接协议。虽然这些传输是多媒体流的合适选择，但也有TCP更适合的应用。本文档定义了一个新的协议标识符“TCP”，用于描述SDP中的TCP连接。
TCP introduces two new factors when describing a session: how and when should endpoints perform the TCP connection setup procedure. This document defines two new attributes to describe TCP connection setups: 'setup' and 'connection'.
TCP在描述会话时引入了两个新因素：端点应如何以及何时执行TCP连接设置过程。本文档定义了两个新属性来描述TCP连接设置：“设置”和“连接”。
In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 [3], and they indicate requirement levels for compliant implementations.
在本文件中，关键词“必须”、“不得”、“要求”、“应”、“不应”、“应”、“不应”、“建议”、“不建议”、“可”和“可选”将按照BCP 14、RFC 2119[3]中的描述进行解释，它们表示合规实施的要求级别。
The following is the ABNF for an 'm' line, as specified by RFC 2327 [4].
以下是RFC 2327[4]规定的“m”线的ABNF。
media-field = "m=" media space port ["/" integer]
space proto 1*(space fmt) CRLF
media-field = "m=" media space port ["/" integer]
space proto 1*(space fmt) CRLF
This document defines a new value for the proto field: 'TCP'.
此文档为proto字段定义了一个新值：“TCP”。
The 'TCP' protocol identifier is similar to the 'UDP' protocol identifier in that it only describes the transport protocol, and not the upper-layer protocol. An 'm' line that specifies 'TCP' MUST
“TCP”协议标识符与“UDP”协议标识符类似，因为它只描述传输协议，而不描述上层协议。指定“TCP”的“m”行必须
further qualify the application-layer protocol using an fmt identifier. Media described using an 'm' line containing the 'TCP' protocol identifier are carried using TCP [1].
使用fmt标识符进一步验证应用层协议。使用包含“TCP”协议标识符的“m”行描述的媒体使用TCP[1]进行传输。
The 'setup' attribute indicates which of the end points should initiate the TCP connection establishment (i.e., send the initial TCP SYN). The 'setup' attribute is charset-independent and can be a session-level or a media-level attribute. The following is the ABNF of the 'setup' attribute:
“setup”属性指示哪些端点应启动TCP连接建立（即发送初始TCP SYN）。“setup”属性与字符集无关，可以是会话级或媒体级属性。以下是“设置”属性的ABNF：
setup-attr = "a=setup:" role
role = "active" / "passive" / "actpass"
/ "holdconn"
setup-attr = "a=setup:" role
role = "active" / "passive" / "actpass"
/ "holdconn"
'active': The endpoint will initiate an outgoing connection.
“活动”：终结点将启动传出连接。
'passive': The endpoint will accept an incoming connection.
“被动”：端点将接受传入连接。
'actpass': The endpoint is willing to accept an incoming connection or to initiate an outgoing connection.
“actpass”：端点愿意接受传入连接或启动传出连接。
'holdconn': The endpoint does not want the connection to be established for the time being.
“holdconn”：终结点暂时不希望建立连接。
The offer/answer model, defined in RFC 3264 [5], provides endpoints with a means to obtain shared view of a session. Some session parameters are negotiated (e.g., codecs to use), while others are simply communicated from one endpoint to the other (e.g., IP addresses). The value of the 'setup' attribute falls into the first category. That is, both endpoints negotiate its value using the offer/answer model.
RFC 3264[5]中定义的提供/应答模型为端点提供了获取会话共享视图的方法。一些会话参数是协商的（例如，要使用的编解码器），而另一些只是从一个端点到另一个端点的通信（例如，IP地址）。“设置”属性的值属于第一类。也就是说，两个端点都使用提供/应答模型协商其价值。
The negotiation of the value of the 'setup' attribute takes places as follows. The offerer states which role or roles it is willing to perform; and the answerer, taking the offerer's willingness into consideration, chooses which roles both endpoints will actually perform during connection establishment. The following are the values that the 'setup' attribute can take in an offer/answer exchange:
“setup”属性值的协商如下所示。要约人陈述其愿意履行的一个或多个角色；而应答者，考虑到提供者的意愿，选择在连接建立期间两个端点实际执行的角色。以下是“设置”属性在报价/应答交换中可以接受的值：
Offer Answer
________________
active passive / holdconn
passive active / holdconn
actpass active / passive / holdconn
holdconn holdconn
Offer Answer
________________
active passive / holdconn
passive active / holdconn
actpass active / passive / holdconn
holdconn holdconn
The active endpoint SHOULD initiate a connection to the port number on the 'm' line of the other endpoint. The port number on its own 'm' line is irrelevant, and the opposite endpoint MUST NOT attempt to initiate a connection to the port number specified there. Nevertheless, since the 'm' line must contain a valid port number, the endpoint using the value 'active' SHOULD specify a port number of 9 (the discard port) on its 'm' line. The endpoint MUST NOT specify a port number of zero, except to denote an 'm' line that has been or is being refused.
活动端点应在另一个端点的“m”行上启动与端口号的连接。其自身“m”行上的端口号不相关，并且对方端点不得尝试启动与此处指定的端口号的连接。但是，由于“m”行必须包含有效的端口号，因此使用值“active”的端点应在其“m”行上指定端口号9（丢弃端口）。端点不得指定端口号为零，除非表示已被拒绝或正在被拒绝的“m”行。
The passive endpoint SHOULD be ready to accept a connection on the port number specified in the 'm' line.
被动终结点应准备好接受“m”行中指定的端口号上的连接。
A value of 'actpass' indicates that the offerer can either initiate a connection to the port number on the 'm' line in the answer, or accept a connection on the port number specified in the 'm' line in the offer. That is, the offerer has no preference as to whether it accepts or initiates the connection and, so, is letting the answerer choose.
“actpass”值表示报价人可以启动到应答中“m”行上的端口号的连接，或接受报价中“m”行中指定的端口号的连接。也就是说，对于是否接受或发起连接，发盘方没有偏好，因此，让应答方选择。
A value of 'holdconn' indicates that the connection should not be established for the time being.
值“holdconn”表示暂时不应建立连接。
The default value of the setup attribute in an offer/answer exchange is 'active' in the offer and 'passive' in the answer.
报价/应答交换中设置属性的默认值在报价中为“主动”，在应答中为“被动”。
The preceding description of the 'setup' attribute is placed in the context of using SDP to initiate a session. Still, SDP may be exchanged between endpoints at various stages of a session to accomplish tasks such as terminating a session, redirecting media to a new endpoint, or renegotiating the media parameters for a session. After the initial session has been established, it may be ambiguous whether a subsequent SDP exchange represents a confirmation that the endpoint is to continue using the current TCP connection unchanged, or is a request to make a new TCP connection. The media-level 'connection' attribute, which is charset-independent, is used to disambiguate these two scenarios. The following is the ABNF of the connection attribute:
前面对“setup”属性的描述放在使用SDP启动会话的上下文中。然而，SDP可以在会话的不同阶段在端点之间交换，以完成诸如终止会话、将媒体重定向到新端点或重新协商会话的媒体参数等任务。在建立初始会话后，后续SDP交换是表示确认端点将继续使用当前TCP连接，还是请求建立新的TCP连接，这可能是不明确的。媒体级别的“连接”属性与字符集无关，用于消除这两种情况的歧义。以下是连接属性的ABNF：
connection-attr = "a=connection:" conn-value
conn-value = "new" / "existing"
connection-attr = "a=connection:" conn-value
conn-value = "new" / "existing"
Offerers and answerers use the 'connection' attribute to decide whether a new transport connection needs to be established or, on the other hand, the existing TCP connection should still be used. When an offerer generates an 'm' line that uses TCP, it SHOULD provide a connection attribute for the 'm' line unless the application using the 'm' line has other means to deal with connection reestablishment.
报价人和应答人使用“connection”属性来决定是否需要建立新的传输连接，或者，另一方面，是否仍应使用现有的TCP连接。当报价人生成使用TCP的“m”线路时，应为“m”线路提供连接属性，除非使用“m”线路的应用程序有其他方法处理连接重建。
After the initial offer/answer exchange, any of the endpoints can generate a new offer to change some characteristics of the session (e.g., the direction attribute). If such an offerer wants to continue using the previously-established transport-layer connection for the 'm' line, the offerer MUST use a connection value of 'existing' for the 'm' line. If, on the other hand, the offerer wants to establish a new transport-layer connection for the 'm' line, it MUST use a connection value of 'new'.
在初始要约/应答交换之后，任何端点都可以生成新要约，以更改会话的某些特征（例如，方向属性）。如果该报价人希望继续使用先前为“m”线路建立的传输层连接，则报价人必须为“m”线路使用“现有”连接值。另一方面，如果报价人希望为“m”线路建立新的传输层连接，则必须使用连接值“new”。
Note that, according to the rules in this section, an offer that changes the transport address (IP address or port number) of an 'm' line will have a connection value of 'new'. Similarly, the 'connection' attribute in an initial offer (i.e., no transport connection has been established yet) takes the value of 'new'.
请注意，根据本节中的规则，更改“m”线路的传输地址（IP地址或端口号）的报价将具有“new”连接值。类似地，初始报价中的“连接”属性（即，尚未建立传输连接）的值为“新建”。
The 'connection' value resulting from an offer/answer exchange is the 'connection' value in the answer. If the 'connection' value in the answer is 'new', the end-points SHOULD establish a new connection. If the connection value in the answer is 'existing', the end-points SHOULD continue using the exiting connection.
由报价/应答交换产生的“连接”值是应答中的“连接”值。如果答案中的“连接”值为“新”，则端点应建立新连接。如果答案中的连接值为“现有”，则端点应继续使用现有连接。
Taking into consideration the rules in Section 5.2, the following are the values that the 'connection' attribute can take in an offer/answer exchange:
考虑到第5.2节中的规则，“连接”属性在报价/应答交换中可以采用以下值：
Offer Answer
________________
new new
existing existing / new
Offer Answer
________________
new new
existing existing / new
If the connection value resulting from an offer/answer exchange is 'existing', the end-points continue using the existing connection. Consequently, the port numbers, IP addresses, and 'setup' attributes negotiated in the offer/answer exchange are ignored because there is no need to establish a new connection.
如果提供/应答交换产生的连接值为“现有”，则端点将继续使用现有连接。因此，在提供/应答交换中协商的端口号、IP地址和“设置”属性将被忽略，因为不需要建立新连接。
The previous rule implies that an offerer generating an offer with a connection value of 'existing' and a setup value of 'passive' needs to be ready (i.e., needs to allocate resources) to receive a connection request from the answerer just in case the answerer chooses a connection value of 'new' for the answer. However, if the answerer uses a connection value of 'existing' in the answer, the offerer would need to deallocate the previously allocated resources that were never used because no connection request was received.
前面的规则意味着，生成连接值为“现有”且设置值为“被动”的报价的报价人需要准备好（即，需要分配资源）从应答人接收连接请求，以防应答人选择连接值为“新”的答案。但是，如果应答者在应答中使用连接值'existing'，则报价人需要解除分配以前分配的资源，这些资源从未使用过，因为没有收到连接请求。
To avoid allocating resources unnecessarily, offerers using a connection value of 'existing' in their offers may choose to use a setup value of 'holdconn'. Nevertheless, offerers using this strategy should be aware that if the answerer chooses a connection value of 'new', a new offer/answer exchange (typically initiated by the previous offerer) with setup value different than 'holdconn' will be needed to establish the new connection. This may, of course, cause delays in the application using the TCP connection.
为避免不必要地分配资源，在其报价中使用“现有”连接值的报价人可以选择使用“holdconn”设置值。然而，使用此策略的报价人应注意，如果应答人选择的连接值为“new”，则需要一个设置值不同于“holdconn”的新报价/应答交换（通常由前报价人发起），以建立新连接。当然，这可能会导致使用TCP连接的应用程序延迟。
The default value of the connection attribute in both offers and answers is 'new'.
offers和answers中connection属性的默认值均为“new”。
The connection value for an 'm' line is negotiated using the offer/ answer model. The resulting connection value after an offer/answer exchange is the connection value in the answer. If the connection value in the offer is 'new', the answerer MUST also use a value of 'new' in the answer. If the connection value in the offer is 'existing', the answerer uses a value of 'existing' in the answer if it wishes to continue using the existing connection and a value of 'new' if it wants a new connection to be established.
“m”线路的连接值使用提供/应答模型协商。交换报价/应答后得到的连接值就是应答中的连接值。如果报价中的连接值为“新”，则应答者还必须在回答中使用“新”值。如果报价中的连接值为“现有”，则回答者如果希望继续使用现有连接，则在回答中使用“现有”值，如果希望建立新连接，则使用“新”值。
In some scenarios where third party call control [12] is used, an endpoint may receive an initial offer with a connection value of 'existing'. Following the previous rules, such an answerer would use a connection value of 'new' in the answer.
在使用第三方呼叫控制[12]的某些场景中，端点可能会收到连接值为“现有”的初始报价。按照前面的规则，这样的回答者会在回答中使用连接值“new”。
If the connection value for an 'm' line resulting from an offer/ answer exchange is 'new', the endpoints SHOULD establish a new TCP connection as indicated by the 'setup' attribute. If a previous TCP connection is still up, the endpoints SHOULD close it as soon as the offer/answer exchange is completed. It is up to the application to ensure proper data synchronization between the two TCP connections.
如果由提供/应答交换产生的“m”行的连接值为“新建”，则端点应按照“设置”属性的指示建立新的TCP连接。如果以前的TCP连接仍处于运行状态，则端点应在提供/应答交换完成后立即将其关闭。由应用程序来确保两个TCP连接之间的正确数据同步。
If the connection value for an 'm' line resulting from an offer/ answer exchange is 'existing', the endpoints SHOULD continue using the existing TCP connection.
如果由提供/应答交换产生的“m”行的连接值为“现有”，则端点应继续使用现有TCP连接。
This section addresses connection establishment, connection reestablishment, and connection termination.
本节介绍连接建立、连接重建和连接终止。
An endpoint that according to an offer/answer exchange is supposed to initiate a new TCP connection SHOULD initiate it as soon as it is able to, even if the endpoint does not intend to immediately begin sending media to the remote endpoint. This allows media to flow from the remote endpoint if needed.
根据提供/应答交换应启动新TCP连接的端点应尽快启动新TCP连接，即使该端点不打算立即开始向远程端点发送媒体。这允许媒体在需要时从远程端点流出。
Note that some endpoints need to wait for some event to happen before being able to establish the connection. For example, a wireless terminal may need to set up a radio bearer before being able to initiate a TCP connection.
请注意，某些端点需要等待某个事件发生后才能建立连接。例如，无线终端在能够发起TCP连接之前可能需要设置无线电承载。
If an endpoint determines that the TCP for an 'm' line has been closed and should be reestablished, it SHOULD perform a new offer/ answer exchange using a connection value of 'new' for this 'm' line.
如果端点确定“m”线路的TCP已关闭且应重新建立，则应使用此“m”线路的连接值“new”执行新的提供/应答交换。
Note that the SDP direction attribute (e.g., 'a=sendonly') deals with the media sent over the TCP connection, but has no impact on the TCP connection itself.
请注意，SDP方向属性（例如，“a=sendonly”）处理通过TCP连接发送的媒体，但对TCP连接本身没有影响。
Typically, endpoints do not close the TCP connection until the session has expired, been explicitly terminated, or a new connection value has been provided for the 'm' line. Additionally, specific applications can describe further scenarios where an end-point may close a given TCP connection (e.g., whenever a connection is in the half-close state). As soon as an end-point notices that it needs to terminate a TCP connection, it SHOULD do so.
通常，在会话过期、显式终止或为“m”行提供了新的连接值之前，端点不会关闭TCP连接。此外，特定应用程序可以描述端点可能关闭给定TCP连接的进一步场景（例如，当连接处于半关闭状态时）。一旦端点注意到它需要终止TCP连接，它就应该这样做。
In any case, individual applications may provide further considerations on how to achieve a graceful connection termination. For example, a file application using TCP to receive a FIN from the remote endpoint may need to finish the ongoing transmission of a file before sending its own FIN.
在任何情况下，单个应用程序都可以提供关于如何实现优雅连接终止的进一步考虑。例如，使用TCP从远程端点接收FIN的文件应用程序可能需要在发送自己的FIN之前完成正在进行的文件传输。
The following examples show the most common usage of the 'setup' attribute combined with TCP-based media descriptions. For the purpose of brevity, the main portion of the session description is omitted in the examples, which only show 'm' lines and their attributes (including 'c' lines).
以下示例显示了“设置”属性与基于TCP的媒体描述相结合的最常见用法。为简洁起见，示例中省略了会话描述的主要部分，仅显示“m”行及其属性（包括“c”行）。
An offerer at 192.0.2.2 signals its availability for a T.38 fax session at port 54111:
192.0.2.2的报价人在端口54111处发出T.38传真会话可用性信号：
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive
a=connection:new
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive
a=connection:new
An answerer at 192.0.2.1 receiving this offer responds with the following answer:
192.0.2.1收到本报价的回复者回复如下：
m=image 9 TCP t38
c=IN IP4 192.0.2.1
a=setup:active
a=connection:new
m=image 9 TCP t38
c=IN IP4 192.0.2.1
a=setup:active
a=connection:new
The endpoint at 192.0.2.1 then initiates the TCP connection to port 54111 at 192.0.2.2.
192.0.2.1处的端点随后启动到192.0.2.2处端口54111的TCP连接。
In another example, an offerer at 192.0.2.2 signals its availability for a T.38 fax session at TCP port 54111. Additionally, this offerer is also willing to set up the media stream by initiating the TCP connection:
在另一个示例中，192.0.2.2处的报价人在TCP端口54111处发出T.38传真会话可用性的信号。此外，该报价人还愿意通过启动TCP连接来设置媒体流：
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:actpass
a=connection:new
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:actpass
a=connection:new
The endpoint at 192.0.2.1 responds with the following description:
192.0.2.1处的端点响应如下描述：
m=image 54321 TCP t38
c=IN IP4 192.0.2.1
a=setup:passive
a=connection:new
m=image 54321 TCP t38
c=IN IP4 192.0.2.1
a=setup:passive
a=connection:new
This will cause the offerer (at 192.0.2.2) to initiate a connection to port 54321 at 192.0.2.1.
这将导致报价人（192.0.2.2处）在192.0.2.1处启动到端口54321的连接。
Subsequent to the exchange in Section 7.2, another offer/answer exchange is initiated in the opposite direction. The endpoint at 192.0.2.1 wishes to continue using the existing connection:
继第7.2节中的交换之后，另一个报价/应答交换在相反方向启动。192.0.2.1处的端点希望继续使用现有连接：
m=image 54321 TCP t38
c=IN IP4 192.0.2.1
a=setup:passive
a=connection:existing
m=image 54321 TCP t38
c=IN IP4 192.0.2.1
a=setup:passive
a=connection:existing
The endpoint at 192.0.2.2 also wishes to use the existing connection and responds with the following description:
192.0.2.2处的端点也希望使用现有连接，并响应以下描述：
m=image 9 TCP t38
c=IN IP4 192.0.2.2
a=setup:active
a=connection:existing
m=image 9 TCP t38
c=IN IP4 192.0.2.2
a=setup:active
a=connection:existing
The existing connection from 192.0.2.2 to 192.0.2.1 will be reused.
从192.0.2.2到192.0.2.1的现有连接将被重用。
Note that the endpoint at 192.0.2.2 uses 'setup:active' in response to the offer of 'setup:passive', and uses port 9 because it is active.
请注意，192.0.2.2处的端点使用“setup:active”响应“setup:passive”的提供，并使用端口9，因为它是活动的。
Subsequent to the exchange in Section 7.3, another offer/answer exchange is initiated by the endpoint at 192.0.2.2, again wishing to reuse the existing connection:
继第7.3节中的交换之后，端点在192.0.2.2发起另一个提供/应答交换，再次希望重用现有连接：
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive
a=connection:existing
m=image 54111 TCP t38
c=IN IP4 192.0.2.2
a=setup:passive
a=connection:existing
However, this time the answerer is unaware of the old connection and thus wishes to establish a new one. (This could be the result of a transfer via third-party call control.) It is unable to act in the 'passive' mode and thus responds as 'active':
然而，这一次回答者不知道旧的联系，因此希望建立一个新的联系。（这可能是通过第三方呼叫控制进行传输的结果。）它无法在“被动”模式下工作，因此响应为“主动”：
m=image 9 TCP t38
c=IN IP4 192.0.2.3
a=setup:active
a=connection:new
m=image 9 TCP t38
c=IN IP4 192.0.2.3
a=setup:active
a=connection:new
The endpoint at 192.0.2.3 then initiates the TCP connection to port 54111 at 192.0.2.2, and the endpoint at 192.0.2.2 closes the old connection.
然后，192.0.2.3处的端点启动到192.0.2.2处端口54111的TCP连接，192.0.2.2处的端点关闭旧连接。
Note that the endpoint at 192.0.2.2, while using a connection value of 'existing', has used a setup value of 'passive'. Had it not done this and instead used a setup value of 'holdconn' (probably to avoid allocating resources as described in Section 5.1), a new offer/answer exchange would have been needed in order to establish the new connection.
请注意，192.0.2.2处的端点在使用“现有”连接值时，使用了“被动”设置值。如果它没有这样做，而是使用设置值“holdconn”（可能是为了避免分配第5.1节所述的资源），则需要新的报价/应答交换来建立新的连接。
This document specifies how to describe TCP-based media streams using SDP. Still, some of the attributes defined here could possibly be used to describe media streams based on other connection-oriented transport protocols as well. This section provides advice to authors of specifications of SDP extensions that deal with connection-oriented transport protocols other than TCP.
本文档指定如何使用SDP描述基于TCP的媒体流。尽管如此，这里定义的一些属性也可能用于描述基于其他面向连接的传输协议的媒体流。本节向SDP扩展规范的作者提供建议，SDP扩展规范处理TCP以外的面向连接的传输协议。
It is recommended that documents defining new SDP protocol identifiers that involve extra protocol layers between TCP and the media itself (e.g., TLS [7] over TCP) start with the string 'TCP/' (e.g., 'TCP/TLS').
建议定义新SDP协议标识符的文档以字符串“TCP/”（例如，“TCP/TLS”）开头，这些标识符涉及TCP和媒体本身之间的额外协议层（例如，TCP上的TLS[7]）。
The 'setup' and the 'connection' attributes are specified in Section 4 and Section 5 respectively. While both attributes are applicable to 'm' lines that use the 'TCP' protocol identifier, they are general enough to be reused in 'm' lines with other connection-oriented transport protocols. Therefore, it is recommended that the 'setup' and 'connection' attributes are reused, as long as it is possible, for new proto values associated with connection-oriented transport protocols.
第4节和第5节分别规定了“设置”和“连接”属性。虽然这两个属性都适用于使用“TCP”协议标识符的“m”线路，但它们足够通用，可以与其他面向连接的传输协议一起在“m”线路中重用。因此，建议尽可能对与面向连接的传输协议相关联的新proto值重用“设置”和“连接”属性。
Section 6 deals with TCP connection management. It should be noted that while in TCP both end-points need to close a connection, other connection-oriented transport protocols may not have the concept of half-close connections. In such a case, a connection would be terminated as soon as one of the end-points closed it, making it unnecessary for the other end-point to perform any further action to terminate the connection. So, specifications dealing with such transport protocols may need to specify slightly different procedures regarding connection termination.
第6节讨论TCP连接管理。应该注意的是，在TCP中，两个端点都需要关闭一个连接，而其他面向连接的传输协议可能没有半关闭连接的概念。在这种情况下，一个连接将在其中一个端点关闭后立即终止，使得另一个端点无需执行任何进一步的操作来终止连接。因此，处理此类传输协议的规范可能需要指定关于连接终止的稍微不同的过程。
See RFC 2327 [4] for security and other considerations specific to the Session Description Protocol in general.
请参阅RFC 2327[4]，了解会话描述协议的安全性和其他注意事项。
An attacker may attempt to modify the values of the connection and setup attributes in order to have endpoints reestablish connections unnecessarily or to keep them from establishing a connection. So, it is strongly RECOMMENDED that integrity protection be applied to the SDP session descriptions. For session descriptions carried in SIP [10], S/MIME is the natural choice to provide such end-to-end integrity protection, as described in RFC 3261 [10]. Other applications MAY use a different form of integrity protection.
攻击者可能试图修改连接和设置属性的值，以使端点不必要地重新建立连接或阻止它们建立连接。因此，强烈建议对SDP会话描述应用完整性保护。对于SIP[10]中的会话描述，S/MIME是提供端到端完整性保护的自然选择，如RFC 3261[10]中所述。其他应用程序可能使用不同形式的完整性保护。
This document defines two session- and media-level SDP attributes: setup and connection. Their formats are defined in Section 4 and Section 5, respectively. These two attributes should be registered by the IANA under "Session Description Protocol (SDP) Parameters" under "att-field (both session and media level)".
本文档定义了两个会话级和媒体级SDP属性：设置和连接。其格式分别在第4节和第5节中定义。IANA应在“att字段（会话和媒体级别）”下的“会话描述协议（SDP）参数”下注册这两个属性。
This document defines a proto value: TCP. Its format is defined in Section 3. This proto value should be registered by the IANA under "Session Description Protocol (SDP) Parameters" under "proto".
本文档定义了一个proto值：TCP。其格式见第3节。IANA应在“proto”下的“会话描述协议（SDP）参数”下注册此proto值。
The SDP specification, RFC2327, states that specifications defining new proto values, like the TCP proto value defined in this RFC, must define the rules by which their media format (fmt) namespace is managed. For the TCP protocol, new formats SHOULD have an associated MIME registration. Use of an existing MIME subtype for the format is encouraged. If no MIME subtype exists, it is RECOMMENDED that a suitable one is registered through the IETF process [2] by production of, or reference to, a standards-track RFC that defines the transport protocol for the format.
SDP规范RFC2327规定，定义新协议值的规范（如本RFC中定义的TCP协议值）必须定义管理其媒体格式（fmt）命名空间的规则。对于TCP协议，新格式应具有关联的MIME注册。鼓励对该格式使用现有的MIME子类型。如果不存在MIME子类型，建议通过IETF过程[2]生成或引用定义格式传输协议的标准跟踪RFC来注册合适的MIME子类型。
Jonathan Rosenberg, Rohan Mahy, Anders Kristensen, Joerg Ott, Paul Kyzivat, Robert Fairlie-Cuninghame, Colin Perkins, and Christer Holmberg provided valuable insights and contributions.
Jonathan Rosenberg、Rohan Mahy、Anders Kristensen、Joerg Ott、Paul Kyzivat、Robert Fairlie Cuninghame、Colin Perkins和Christer Holmberg提供了宝贵的见解和贡献。
[1] Postel, J., "Transmission Control Protocol", STD 7, RFC 793, September 1981.
[1] 《传输控制协议》，标准7，RFC 793，1981年9月。
[2] Freed, N., Klensin, J., and J. Postel, "Multipurpose Internet Mail Extensions (MIME) Part Four: Registration Procedures", BCP 13, RFC 2048, November 1996.
[2] Freed，N.，Klensin，J.，和J.Postel，“多用途互联网邮件扩展（MIME）第四部分：注册程序”，BCP 13，RFC 2048，1996年11月。
[3] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[3] Bradner，S.，“RFC中用于表示需求水平的关键词”，BCP 14，RFC 2119，1997年3月。
[4] Handley, M. and V. Jacobson, "SDP: Session Description Protocol", RFC 2327, April 1998.
[4] Handley，M.和V.Jacobson，“SDP：会话描述协议”，RFC 2327，1998年4月。
[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with Session Description Protocol (SDP)", RFC 3264, June 2002.
[5] 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。
[6] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time Streaming Protocol (RTSP)", RFC 2326, April 1998.
[6] Schulzrinne，H.，Rao，A.，和R.Lanphier，“实时流协议（RTSP）”，RFC2326，1998年4月。
[7] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC 2246, January 1999.
[7] Dierks，T.和C.Allen，“TLS协议1.0版”，RFC 2246，1999年1月。
[8] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[8] 菲尔丁，R.，盖蒂斯，J.，莫卧儿，J.，弗莱斯蒂克，H.，马斯特，L.，利奇，P.，和T.伯纳斯李，“超文本传输协议——HTTP/1.1”，RFC2616，1999年6月。
[9] Handley, M., Perkins, C., and E. Whelan, "Session Announcement Protocol", RFC 2974, October 2000.
[9] Handley，M.，Perkins，C.，和E.Whelan，“会话公告协议”，RFC 29742000年10月。
[10] 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.
[10] 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。
[11] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, November 2003.
[11] Yergeau，F.，“UTF-8，ISO 10646的转换格式”，STD 63，RFC 3629，2003年11月。
[12] Rosenberg, J., Peterson, J., Schulzrinne, H., and G. Camarillo, "Best Current Practices for Third Party Call Control (3pcc) in the Session Initiation Protocol (SIP)", BCP 85, RFC 3725, April 2004.
[12] Rosenberg，J.，Peterson，J.，Schulzrinne，H.，和G.Camarillo，“会话启动协议（SIP）中第三方呼叫控制（3pcc）的最佳当前实践”，BCP 85，RFC 37252004年4月。
Authors' Addresses
作者地址
David Yon Tactical Software, LLC 1750 Elm St., Suite 803 Manchester, NH 03104 USA
美国新罕布什尔州曼彻斯特Elm街1750号803室David Yon战术软件有限责任公司，邮编：03104
EMail: yon-comedia@rfdsoftware.com
EMail: yon-comedia@rfdsoftware.com
Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420 Finland
Gonzalo Camarillo Ericsson Hirsalantie 11 Jorvas 02420芬兰
EMail: Gonzalo.Camarillo@ericsson.com
EMail: Gonzalo.Camarillo@ericsson.com
Full Copyright Statement
完整版权声明
Copyright (C) The Internet Society (2005).
版权所有（C）互联网协会（2005年）。
This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights.
本文件受BCP 78中包含的权利、许可和限制的约束，除其中规定外，作者保留其所有权利。
This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
本文件及其包含的信息是按“原样”提供的，贡献者、他/她所代表或赞助的组织（如有）、互联网协会和互联网工程任务组不承担任何明示或暗示的担保，包括但不限于任何保证，即使用本文中的信息不会侵犯任何权利，或对适销性或特定用途适用性的任何默示保证。
Intellectual Property
知识产权
The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. Information on the procedures with respect to rights in RFC documents can be found in BCP 78 and BCP 79.
IETF对可能声称与本文件所述技术的实施或使用有关的任何知识产权或其他权利的有效性或范围，或此类权利下的任何许可可能或可能不可用的程度，不采取任何立场；它也不表示它已作出任何独立努力来确定任何此类权利。有关RFC文件中权利的程序信息，请参见BCP 78和BCP 79。
Copies of IPR disclosures made to the IETF Secretariat and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF on-line IPR repository at http://www.ietf.org/ipr.
向IETF秘书处披露的知识产权副本和任何许可证保证，或本规范实施者或用户试图获得使用此类专有权利的一般许可证或许可的结果，可从IETF在线知识产权存储库获取，网址为http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietf-ipr@ietf.org.
IETF邀请任何相关方提请其注意任何版权、专利或专利申请，或其他可能涵盖实施本标准所需技术的专有权利。请将信息发送至IETF的IETF-ipr@ietf.org.
Acknowledgement
确认
Funding for the RFC Editor function is currently provided by the Internet Society.
RFC编辑功能的资金目前由互联网协会提供。