SDP 消息格式

在 SIP 协议详解 中提到过一种Sip body消息体格式message/sdp,这一节来了解一下SDP消息格式。

• SDP（Session Description Protocol）是一种会话描述的数据格式
  在 RFC4566 中有这样一断描述：
  SDP is purely a format for session description -- it does not incorporate a transport protocol, and it is intended to use different transport protocols as appropriate, including the SAP(Session Announcement Protocol) ,SIP(Session Initiation Protocol) , RTSP(Real Time Streaming Protocol) ,electronic mail using the MIME extensions, and the Hypertext Transport Protocol.
  翻译过来大概是：
  SDP（Session Description Protocol）是一种纯粹的会话描述的格式。 SDP不是传输协议，信息传输上，其适当使用不用协议完成，包括：SAP，SIP，RTSP，HTTP，MIME协议扩展。
• SDP 通常用在会话初始化过程中，用来传送会话参与者的能力列表，以协调会话双方的各项参数。例如：媒体的种类为 audio 或 video ；媒体的编码方式(H.261 video、MPEG video 等) 。

SDP举例：

Session description
   v=0
   o=jdoe 2890844526 2890842807 IN IP4 10.47.16.5
   s=SDP Seminar
   i=A Seminar on the session description protocol
   u=http://www.example.com/seminars/sdp.pdf
   e=j.doe@example.com (Jane Doe)
   c=IN IP4 224.2.17.12/127
   t=2873397496 2873404696
   a=recvonly
   m=audio 49170 RTP/AVP 0
   m=video 51372 RTP/AVP 99
   a=rtpmap:99 h263-1998/90000

Time description
   t=3034423619 3042462419
   r=604800 3600 0 90000

Media description
   m=video 49170/2 RTP/AVP 31

SDP协议的结构如下：

Session description
   v=  (protocol version)
   o=  (originator and session identifier)
   s=  (session name)
   i=* (session information)
   u=* (URI of description)
   e=* (email address)
   p=* (phone number)
   c=* (connection information -- not required if included in
        all media)
   b=* (zero or more bandwidth information lines)
   One or more time descriptions ("t=" and "r=" lines; see below)
   z=* (time zone adjustments)
   k=* (encryption key)
   a=* (zero or more session attribute lines)
   Zero or more media descriptions

Time description
   t=  (time the session is active)
   r=* (zero or more repeat times)

Media description, if present
   m=  (media name and transport address)
   i=* (media title)
   c=* (connection information -- optional if included at
        session level)
   b=* (zero or more bandwidth information lines)
   k=* (encryption key)
   a=* (zero or more media attribute lines)

Session description

Type	含义	解释	举例
v	protocol version	SDP的版本号	v=0
o	origion	会话发起者的描述	o={用户名}{session id} {会话版本} {网络类型}{地址类型}{地址}
s	Session Name	会话名，在整个会话中有且只有一个”s=”	s=SDP Seminar
i	session information	会话的文本描述信息	i=A Seminar on the session description protocol
u	URI	The URI should be a pointer to additional information about the session.	u=http://www.example.com/seminars/sdp.pdf
e	Email Address and Phone Number	specify contact information for the person responsible for the conference	e=j.doe@example.com (Jane Doe)
c	Connection Data	the connection type and address	c=IN IP4 224.2.17.12/127
b	Bandwidth	the proposed bandwidth to be used by thesession or media	c=IN IP4 224.2.17.12/127

Time description

Type	含义	解释	举例
t	Timing	specify the start and stop times for a session	t={开始时间} {结束时间}
r	Repeat Times	specify repeat times for a session	r={重复间隔}{活动时长}{启动时间偏移}

Media description

Type	含义	解释	举例
m	media name and transport address	m={media}{port}{proto}{fmt}	m=video 49170/2 RTP/AVP 31

参考

RFC4566：https://tools.ietf.org/html/rfc4566

SDP协议介绍：https://www.cnblogs.com/idignew/p/7249056.html