MRCPv2协议详解：媒体资源控制协议

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"MRCP协议英文原文，用于深入理解Media Resource Control Protocol (MRCP)协议，适合相关开发和运维人员阅读。" Media Resource Control Protocol (MRCP) Version 2 是一种控制协议，允许客户端主机控制网络服务器上的媒体服务资源，如语音合成器、识别器、验证器和标识器。MRCPv2并不是一个独立的协议，它依赖于其他协议如Session Initiation Protocol (SIP)来协调MRCPv2客户端和服务器之间的交互，并管理它们之间的会话。Session Description Protocol (SDP)则用于描述、发现和交换能力信息。 SIP在MRCPv2中起到关键作用，它负责建立和管理客户端与服务器之间的会话。SDP则用来描述媒体源或接收端与媒体服务器之间的媒体会话及相关的参数。一旦这些准备工作完成，MRCPv2控制会话就在上面建立的控制连接上进行，使客户端能够对语音资源服务器上的媒体处理资源进行控制。该文档是Internet Engineering Task Force (IETF)的标准草案，由D. Burnett和S. Shanmugham编写，属于标准轨道文档，旨在定义和标准化MRCPv2的使用。文档中的内容详细阐述了MRCPv2的工作原理、交互流程以及与其他协议（如SIP和SDP）的协作机制。对于初次接触MRCPv2的人来说，直接阅读原文可能会有一定难度，因此建议具备一定基础的开发和运维人员查阅。MRCPv2的应用场景主要集中在电信网络通信领域，尤其是在语音识别和合成技术中，对于实现高效、准确的语音交互有着重要作用。 MRCPv2是构建智能语音应用的关键组件，它提供了一种标准化的方式来控制和管理网络上的媒体资源，从而实现诸如自动语音识别（ASR）、文本转语音（TTS）等功能。通过深入理解和应用MRCPv2，开发者可以构建更加灵活和强大的语音服务系统，而运维人员则能更好地管理和优化语音服务的性能。

RFC 6787 MRCPv2 November 2012

exchange carried by this SIP transaction contains one or more

additional control "m=" lines for the new resources to be allocated

to the session. The server, on seeing the new "m=" line, allocates

the resources (if they are available) and responds with a

corresponding control "m=" line in the SDP answer carried in the SIP

response. If the new resources are not available, the re-INVITE

receives an error message, and existing media processing going on

before the re-INVITE will continue as it was before. It is not

possible to allocate more than one resource of each type. If a

client requests more than one resource of any type, the server MUST

behave as if the resources of that type (beyond the first one) are

not available.

MRCPv2 clients and servers using TCP as a transport protocol MUST use

the procedures specified in RFC 4145 [RFC4145] for setting up the TCP

connection, with the considerations described hereby. Similarly,

MRCPv2 clients and servers using TCP/TLS as a transport protocol MUST

use the procedures specified in RFC 4572 [RFC4572] for setting up the

TLS connection, with the considerations described hereby. The

a=setup attribute, as described in RFC 4145 [RFC4145], MUST be

"active" for the offer from the client and MUST be "passive" for the

answer from the MRCPv2 server. The a=connection attribute MUST have

a value of "new" on the very first control "m=" line offer from the

client to an MRCPv2 server. Subsequent control "m=" line offers from

the client to the MRCP server MAY contain "new" or "existing",

depending on whether the client wants to set up a new connection or

share an existing connection, respectively. If the client specifies

a value of "new", the server MUST respond with a value of "new". If

the client specifies a value of "existing", the server MUST respond.

The legal values in the response are "existing" if the server prefers

to share an existing connection or "new" if not. In the latter case,

the client MUST initiate a new transport connection.

When the client wants to deallocate the resource from this session,

it issues a new SDP offer, according to RFC 3264 [RFC3264], where the

control "m=" line port MUST be set to 0. This SDP offer is sent in a

SIP re-INVITE request. This deallocates the associated MRCPv2

identifier and resource. The server MUST NOT close the TCP or TLS

connection if it is currently being shared among multiple MRCP

channels. When all MRCP channels that may be sharing the connection

are released and/or the associated SIP dialog is terminated, the

client or server terminates the connection.

When the client wants to tear down the whole session and all its

resources, it MUST issue a SIP BYE request to close the SIP session.

This will deallocate all the control channels and resources allocated

under the session.

Burnett & Shanmugham Standards Track [Page 16]

RFC 6787 MRCPv2 November 2012

All servers MUST support TLS. Servers MAY use TCP without TLS in

controlled environments (e.g., not in the public Internet) where both

nodes are inside a protected perimeter, for example, preventing

access to the MRCP server from remote nodes outside the controlled

perimeter. It is up to the client, through the SDP offer, to choose

which transport it wants to use for an MRCPv2 session. Aside from

the exceptions given above, when using TCP, the "m=" lines MUST

conform to RFC4145 [RFC4145], which describes the usage of SDP for

connection-oriented transport. When using TLS, the SDP "m=" line for

the control stream MUST conform to Connection-Oriented Media

(COMEDIA) over TLS [RFC4572], which specifies the usage of SDP for

establishing a secure connection-oriented transport over TLS.

4.3. SIP Session Example

This first example shows the power of using SIP to route to the

appropriate resource. In the example, note the use of a request to a

domain’s speech server service in the INVITE to

mresources@example.com. The SIP routing machinery in the domain

locates the actual server, mresources@server.example.com, which gets

returned in the 200 OK. Note that "cmid" is defined in Section 4.4.

This example exchange adds a resource control channel for a

synthesizer. Since a synthesizer also generates an audio stream,

this interaction also creates a receive-only Real-Time Protocol (RTP)

[RFC3550] media session for the server to send audio to. The SIP

dialog with the media source/sink is independent of MRCP and is not

shown.

C->S: INVITE sip:mresources@example.com SIP/2.0

Via:SIP/2.0/TCP client.atlanta.example.com:5060;

branch=z9hG4bK74bf1

Max-Forwards:6

To:MediaServer <sip:mresources@example.com>

From:sarvi <sip:sarvi@example.com>;tag=1928301774

Call-ID:a84b4c76e66710

CSeq:314161 INVITE

Contact:<sip:sarvi@client.example.com>

Content-Type:application/sdp

Content-Length:...

v=0

o=sarvi 2890844526 2890844526 IN IP4 192.0.2.12

s=-

c=IN IP4 192.0.2.12

t=0 0

m=application 9 TCP/MRCPv2 1

a=setup:active

Burnett & Shanmugham Standards Track [Page 17]

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