SDP 消息格式
在 SIP 協議詳解 中提到過一種Sip body消息體格式message/sdp,這一節來了解一下SDP消息格式。
- SDP（Session Description Protocol）是一種會話描述的數據格式
在 RFC4566 中有這樣一斷描述：
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.
翻譯過來大概是：
SDP（Session Description Protocol）是一種純粹的會話描述的格式。 SDP不是傳輸協議，信息傳輸上，其適當使用不用協議完成，包括：SAP，SIP，RTSP，HTTP，MIME協議擴展。 - SDP 通常用在會話初始化過程中，用來傳送會話參與者的能力列表，以協調會話雙方的各項參數。例如：媒體的種類爲 audio 或 video ；媒體的編碼方式(H.261 video、MPEG video 等) 。
SDP舉例：
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
[email protected] (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協議的結構如下：
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 | 含義 | 解釋 | 舉例 |
|---|---|---|---|
| v | protocol version | SDP的版本號 | v=0 |
| o | origion | 會話發起者的描述 | o={用戶名}{session id} {會話版本} {網絡類型}{地址類型}{地址} |
| s | Session Name | 會話名，在整個會話中有且只有一個”s=” | s=SDP Seminar |
| i | session information | 會話的文本描述信息 | 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 | [email protected] (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 | 含義 | 解釋 | 舉例 |
|---|---|---|---|
| t | Timing | specify the start and stop times for a session | t={開始時間} {結束時間} |
| r | Repeat Times | specify repeat times for a session | r={重複間隔}{活動時長}{啓動時間偏移} |
Media description
| Type | 含義 | 解釋 | 舉例 |
|---|---|---|---|
| m | media name and transport address | m={media}{port}{proto}{fmt} | m=video 49170/2 RTP/AVP 31 |