SIP or Session Initiation Protocol is in an application layer protocol used to initiate, manage, terminate and control multimedia communication sessions such as voice and video calls across a network. It typically uses TCP or UDP port 5060 for non-encrypted sessions and port 5061 for encrypted ones.

Like HTTP, SIP uses the client-server architecture. A SIP User Agent Client sends the SIP request by initiating a call and the SIP User Agent Server receives the request by answering it and sending a response.

Voice and Video stream in SIP application is commonly carried over RTP (Real-time Transport Protocol) while parameter such as port number, protocols and codecs to use are defined using the SDP (Session Description Protocol)

Common SIP Requests:

• Invite - used to initiate session
• Ack - used to confirm an established session
• Bye - used to terminate session
• Options - SIP capability inquiry
• Register - used for account to IP address binding
• Cancel - used to cancel a pending invite
• Refer - used in call transfers
• Message - used for sending information
• Info/Update - used in DTMF or update configuration
• Subscribe/Notify - used in Busy Lamp Fields and MWI
• Prack - used for provisional acknowledgement

Common SIP Response Codes:

• 100 - Information
• 200 - OK
• 300 - for Call Redirection
• 400 - Client Error
• 404 - Not Found
• 500 - Server Failure
• 600 - Global Failure

Sample SIP Log

SIP REQUEST

REGISTER sip:192.168.1.1 SIP/2.0
Via: SIP/2.0/UDP 192.168.1.101:2051;branch=z9hG4bK-xq7i85k7bx0k;rport
From: sip:1001@192.168.1.1;tag=sin8ipdsx9
To: sip:1001@192.168.1.1
Call-ID: 3c26700d77a1-8u2cyoo4l3jh@snom360-000413237D2E
CSeq: 132639 REGISTER
Max-Forwards: 70
Contact: ;flow-id=1;q=1.0;+sip.instance="";audio;mobility="fixed";duplex="full";description="snom360";actor="principal";events="dialog";
methods="INVITE,ACK,CANCEL,BYE,REFER,OPTIONS,NOTIFY,SUBSCRIBE,PRACK,MESSAGE,INFO"
User-Agent: snom360/6.5.20
Supported: gruu
Allow-Events: dialog
X-Real-IP: 192.168.1.101
Expires: 3600
Content-Length: 0

SIP RESPONSE:

SIP/2.0 200 Ok
Via: SIP/2.0/UDP 192.168.1.101:2051;branch=z9hG4bK-xq7i85k7bx0k;rport=2051
From: sip:1001@192.168.1.1;tag=sin8ipdsx9
To: sip:1001@192.168.1.1;tag=c3d011ad45
Call-ID: 3c26700d77a1-8u2cyoo4l3jh@snom360-000413237D2E
CSeq: 132639 REGISTER
Contact: ;expires=31
Content-Length: 0

SOURCES: Wikipedia and RFC3261

NOTE TO TECH WRITER:
please do add the image found on the link below
http://10.1.0.70/wiki/index.php/Image:Sip-flow.JPG