SIP (Session Initiation Protocol) is the backbone of all Lync Platforms, Lync was built on the use of SIP, SIP was also extended to accommodates the additional functionalities provided by Lync.
In this article I will be talking about some Lync Protocols, explain the anatomy of the protocol , how to read it and understand it, and how you can use
This knowledge in troubleshooting scenarios.
SDP (Session Description Protocol), as you can see from the name, it’s used to describe the session.
Consider it as a negotiation protocol, it’s used so that each endpoint in the session can tell each other what methods they can talk (media codecs), the IP and ports and authentication
o= originator / creator of the session
s= session name
c= connection information
a= media attribute lines
m= media description
it provide the version of the SDP protocol used for now it should always show 0.
This field provide the GUID of the creator of the session and its value usually look like (consist of 5 parts)
o= LoginID SessionID SessionVersion Network type/address type OriginatorIP
o= lyncdude 29336 0 IN IP4 192.168.1.10
Login ID [lyncdude] = the username or sometime the device type like “phone”.
Session ID  = this is used for the purpose of VOIP, should be the UDP/TCP that the originator expect to receive RTP
Session Version  = usually get a value of the creating function in this case 0 is for voice
Network Type / address type [IN IP4] = IN for “internet” and IP4 is for “IPv4”
Originator IP [192.168.1.10] = the IP address of the creator of the session.
This provide the name of the session in a textual form, two thing to know here
- There must be ONLY one name per session
- This field CANNOT be empty.
This field shows the network type and the IP address type that the other end should contact.
c= IN IP4 192.168.1.10
You will notice is a part of the O= line
This field shows the start and the stop time of the session, one session might have multiple t= lines. Each line will specifies an additional
Period of time for which the session will be active.
This line extend the capability of the SDP, and you can think of it as the what media codec can the creator understand.
So usually it look something like this:
Its listing the each different media codec and the rate used.
Here is a table with some of the available media codecs and relevant rates (RTP A/V profile):
|Codec name||Rate (Hz)||PT|
Also this line is used for exchange of the ICE candidate information.
This line shows the order in which the media codec should be negotiated.
m=audio 4680 RTP/AVP 114 9 112 111 0
Previous line explaining that the media type is “audio” using the port “4680” and codec “RTAudio” and the available rates are 114, 9, 112,…etc.
SDP for IM session
So this is a screen shot of my environment when starting an IM session
SDP for Audio Session
This is a screen shot from my environment for an audio session you can see it listing all available media attributes
so if you can read those last two screen shots and understand half of them, you are on the correct way.