1. Session Initialization Protocol (SIP)
Presented by: Lei Luo
Computer & Information Science Department
University of Delaware
Nov. 18, 2004
Some slides are borrowed from Dorgham Sisalem’s SIP Tutorial

2. What Is SIP?
Alice’s Home Station
Alice
Phone Network
Bob

3. What is SIP? A signaling protocol Enable Internet endpoints Set up
Modify Multimedia Sessions
Tear down
Request/deliver presence and Instant messages
Enable Internet endpoints
Discover one another
Agree on a characterization of a session

4. What is NOT SIP? Not a transport protocol
Not a media transmission protocol
Not a QoS Reservation Protocol
Not an integrated communications system

5. Internet Multimedia Protocol Stack
SCTP
Source: Henning Schulzrinne,

6. What can SIP Do? User Location User Availability User Capability
Session Setup
Session Management

7. What Else Can SIP Do?
Request/deliver presence information and instant message sessions:
Publish and upload presence information
Request delivery of presence information
Notify presence and other events
Transport instant messages

8. Marconi
Tesla
INVITE
180 Ringing
200 OK
ACK
Media Session
BYE
200 OK
Simple SIP session establishment example

9. Proxy Server
Bob
Alice
INVITE
INVITE
180 Ringing
180 Ringing
200 OK
200 OK
ACK
Media Session
BYE
200 OK
SIP call example with proxy server

10. Alice
Registrar Server
REGISTER
Contact:
200 OK
SIP registration example

11. SUBSCRIBE
200 OK
NOTIFY
200 OK …
NOTIFY
200 OK
MESSAGE
200 OK
MESSAGE
200 OK
SIP presence and instant message example

12. How does SIP Work? Based on HTTP-like request/response model
Text-based protocol
Use the UTF-8 charset
SIP message
A request from a client to a server
A response from a server to a client

13. SIP User Agent (UA) SIP-enabled end-devices Maintain states on calls
Local and remote tags
Call-ID
Local and remote CSeq header fields
Contain client (UAC) & server applications (UAS)
Must support SDP for media description

14. SIP User Agent Examples
Cisco
Siemens
MIC

15. Presence Agent SIP devices Collect presence information
Receive subscription requests
Generate state notifications
Collect presence information
Need to authenticate a subscription request

16. SIP Gateway A special type of UA Terminate the signaling path
Interface a SIP network to a non-SIP network
Terminate the signaling path
May or may not terminate the media path
Support much more users than normal UA

17. SIP Networks with Gateways
Local/long distance
Telephone
switch
rtspd: media server
Quicktime
Single machine
RTSP
sipconf:
Conference server
RTSP clients
Department
PBX
sipum:
Unified
messaging
Internal
Telephone
Extn: 7040
713x
Proxy,
redirect,
registrar
server
SQL
database
Web server
SIP/PSTN Gateway
Web based
configuration
SNMP
(Network
Management)
Extn: 7134
Extn: 7136
H.323
SIP/H.323
Gateway
NetMeeting
Borrowed from Xiaotao Wu’s slides (Columbia University, 2002)

18. SIP Servers  SIP Workhorses
Logical Entities
Accept SIP requests
Respond to requests
Types of Servers
Proxy Server
Redirect Server
Registration Server

19. SIP Message Format Generic-message = Start-line *message-header CRLF
[ message-body ]
Start-line = Request-Line / Status-Line
Request-Line = Method SP Request-URI SP SIP-Version CRLF
Status-Line = SIP-Version SP Status-Code SP Reason-Phase CRLF
SP – blank space

20. SIP Methods Sets Basic Methods Extended Methods INVITE REGISTER BYE
ACK
CANCEL
OPTIONS
Extended Methods
REFER
SUBSCRIBE
NOTIFY
MESSAGE
etc.

21. SIP Response Codes Consistent with/extend HTTP/1.1 response codes
1xx – Informational
2xx – Success
3xx – Redirection
4xx – Client error
5xx – Server failure
6xx – Global Failure
600 – Busy Everywhere
603 – Decline
604 – Does Not Exist Anywhere
etc.
Borrowed from HTTP
 Created for SIP

22. SIP Registration Server
REGISTER sip:registrar.udel.edu
Via: SIP/2.0/UDP :5060;
branch=z9hG4bKus19
Max-Forwards: 70
To: Alice
From: Alice
Call-ID:
CSeq: 1 REGISTER
Contact:
Content-Length: 0
Alice
Registrar Server
SIP/ OK
Via: SIP/2.0/UDP :5060;
branch=z9hG4bKus19
To: Alice
From: Alice
Call-ID:
CSeq: 1 REGISTER
Contact:
Content-Length: 0

23. SIP Proxy Server Location server DNS server Outbound proxy server
INVITE SIP/2.0
Via: SIP/2.0/UDP proxy.udel.com:5060;
branch=z9hG4bK
Via: SIP/2.0/UDP :5060;
branch=z9hG4bKmp17a
To: Bob
From: Alice
Subject: Where are you exactly?
Contact:
Location server
DNS server
INVITE SIP/2.0
Via: SIP/2.0/UDP proxy.yahoo.com:5060;
branch=z9hG4bKtiop3
Via: SIP/2.0/UDP proxy.udel.com:5060;
branch=z9hG4bK
Via: SIP/2.0/UDP :5060;
branch=z9hG4bKmp17a
To: Bob
From: Alice
Subject: Where are you exactly?
Contact:
INVITE SIP/2.0
Via: SIP/2.0/UDP :5060;
branch=z9hG4bKmp17a
To: Bob
From: Alice
Subject: Where are you exactly?
Contact:
Outbound proxy server
Inbound proxy server
100 Trying
180 Ringing
200 OK
100 Trying
180 Ringing
200 OK
180 Ringing
200 OK
BYE
ACK
Media (RTP)
Alice
200 OK
Bob
SIP Trapezoid

24. SIP Proxy Server INVITE Bob INVITE Bob Bob’s Phone 486 Busy Here Alice
Proxy servers can make flexible “routing decisions” to decide where to send a request.
Bob’s voic

25. SIP Proxy Server Bob’s Cell Phone INVITE Bob 200 OK INVITE Bob
Bob’s Office Phone
INVITE Bob
CANCEL
Alice
200 OK
Bob’s Home Phone
INVITE Bob
CANCEL
In parallel search, a proxy issues several requests to possible user locations upon receiving an incoming request.

26. SIP Redirect Server ACK Redirect Server Alice Bob
SIP/ Moved Temporarily
Via: SIP/2.0/UDP :5060;
branch=z9hG4bKmp17a
To: Bob
From: Alice
Subject: Where are you exactly?
Contact:
INVITE SIP/2.0
Via: SIP/2.0/UDP :5060;
branch=z9hG4bKmp17a
To: Bob
From: Alice
Subject: Where are you exactly?
Contact:
ACK
Redirect Server
INVITE SIP/2.0
Via: SIP/2.0/UDP :5060;
branch=z9hG4bKmp17a
To: Bob
From: Alice
Subject: Where are you exactly?
Contact:
Alice
Bob

27. That’s It! We have learned a lot about SIP!! No Way!!!
Did we really learn a lot of stuff about SIP?
Then what are we gonna do?

28. Key References
J. Rosenberg, SIP: Session Initialization Protocol, RFC 3261, 2002
A. Johnston, SIP: Understanding the Session Initialization Protocol, 2nd edition
J. Kuthan and D. Sisalem, SIP Tutorial
SIP web site:

29. Questions?