SDP协议解读

SDP(Session Description Protocol)，即会话描述协议，为会话通知、会话邀请和其它形式的多媒体会话初始化等目的提供了多媒体会话描述，在rfc2327中定义。

SDP至少需要包含:
- 会话名称及意图
- 会话有效时间
- 媒体流信息（类型、地址、端口、格式等）

协议格式

协议以key=value形式

v=0
o=102 2048 497 IN IP4 192.168.0.104
s=Talk
c=IN IP4 192.168.0.104
b=AS:380
t=0 0
a=rtcp-xr:rcvr-rtt=all:10000 stat-summary=loss,dup,jitt,TTL voip-metrics
m=audio 7076 RTP/AVP 120 111 110 0 8 101
a=rtpmap:120 SILK/16000
a=rtpmap:111 speex/16000
a=fmtp:111 vbr=on
a=rtpmap:110 speex/8000
a=fmtp:110 vbr=on
a=rtpmap:101 telephone-event/8000
a=fmtp:101 0-15

keys 包含Session，Time，Media

Session description

v= (protocol version)
- 没有次版本
o= (owner/creator and session identifier).
- o=username session-id version network—type address-type address
  - “IN” = Internet
  - “IP4” = ipv4
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)
- c=network-type address-type connection-address
b=* (bandwidth information)
- b=modifier:bandwidth-value(kpb/s)
One or more time descriptions (see below)
z=* (time zone adjustments)
k=* (encryption key)
a=* (zero or more session attribute lines)
- a=attribute:value …
Zero or more media descriptions (see below)
Time description
t= (time the session is active)
- t=start-time stop-time, 若为0 0，则视为永久会话
r=* (zero or more repeat times)
- r=repeat-interval active-duration list-of-offsets-from-start-time
  Media description
m= (media name and transport address)
- m=media port transport fmt-list
- a=rtpmap:payload-type encoding-name/clock-rate[/encoding-parameters]
i=* (media title)
c=* (connection information - optional if included at session-level)
b=* (bandwidth information)
k=* (encryption key)
a=* (zero or more media attribute lines)

协议格式

keys 包含Session，Time，Media

Session description

Time description

Media description