SIP Session Initiation Protocol

(1)

Laurent Réveillère

Enseirb

Département Télécommunications

SIP

(2)

2

SIP [RAISIN 2007] / ENSEIRB

Overview

The Internet

This is a funny movie! I bet Laura

would love to watch it…

How could I tell her about it?

(3)

Functionality Provided by SIP



Session establishment, modification, and termination



Used to create brand new sessions



Used to invite new members to an existing session



Sessions are established only if



The callee accepts the call



Caller and callee agree on the media parameters that will be used



Independence of



The type of multimedia session handheld

»

Videoconferences, audio calls, shared whiteboards,

gaming sessions, …



The mechanism used to describe the session

»

Usually SDP for RTP streams carrying audio and video

(4)

4

Where to find SIP?

Voice over LAN / Broadband

Cable Telephony

IM, Web phones, Multimedia

IP PBX and IP

Centrex

SIP

Applications

Interconnect

(5)

SIP Philosophy



Internet Standard



IETF -

http://www.ietf.org



Based on Client-Server Model



Clients send requests, receive responses



Servers receive requests, send responses



Reuse Internet addressing (URIs, DNS, proxies)



Utilizes rich Internet features set



Reuse HTTP coding



Text based



Makes no assumptions about underlying protocol



TCP, UDP, X.25, frame relay, ATM, etc.

(6)

6

Where is SIP in the Protocol Zoo?

Source: Henning Schulzrinne

(7)

SIP Architecture

Location

Server

Feature

Server

Registrar

Server

Proxy

Server

SIP Components

Proxy

Server

(8)

8

SIP Architecture

Location

Server

Feature

Server

Registrar

Server

Proxy

Server

SIP Components

Proxy

Server

User Agent

(9)

User Agent (UA)



SIP entity that interacts with the user



Has an interface towards the user



May respond or forward invitations on user’s behalf

»

Does not necessary maintain interaction with the user



UAC (User Agent Client)



Caller application that initiates and sends SIP requests



UAS (User Agent Server)



Receives and responds to SIP requests on behalf of clients



Accepts, redirects or refuses calls

(10)

10

SIP Architecture

Location

Server

Feature

Server

Registrar

Server

Proxy

Server

SIP Components

Proxy

Server

User Agent

(11)

Registrar Server



Accept registrar requests



Usually co-located with a location or proxy server



Store current location of users



A user can register from different locations

SIP Registrar Server

at company.com

I am “Bob.Johnson”

Today I will be reachable at:

SIP Architecture

Location

Server

Feature

Server

Registrar

Server

Proxy

Server

SIP Components

Proxy

Server

User Agent

(13)

Location Server



Not really a SIP entity



General term used for a database (RFC 2543)



SIP servers use non-SIP protocols to query, update, and retrieve

records from a location server

»

Example: LDAP (Lightweight Directory Access Protocol)



Main purpose



Stores and returns information about a user

»

Current URI

»

IP address

»

User preferences

»

…



Usually accessed through a registrar server and/or a proxy

server

(14)

14

SIP Architecture

Location

Server

Feature

Server

Registrar

Server

Proxy

Server

SIP Components

Proxy

Server

User Agent

(15)

Proxy Server



Acts as both a server and a client



Make requests on behalf of other clients



Performs routing function



Determine to which hop signaling should be relayed

»

User agent, proxy, …



Interprets, rewrites or translates a request message before

forwarding it



Forking



Tries more than one location for a user



Parallel call

»

Trying all possible locations at the same time



Sequential search

(16)

16

Programming SIP

Location

Server

Feature

Server

Registrar

Server

Proxy

Server

SIP Components

Proxy

Server

User Agent

(17)

Programming SIP



A SIP based system can support rapid service creations



Example of services



Basic call features

»

Call waiting, call forwarding, call blocking, etc.



Click to call



Call center



Find me / follow me



…



Service logic options



Call Processing Language (CPL)



Session Processing Language (SPL)



SIP Common Gateway Interface (SIP-CGI)



SIP Java Servlet (SIP-Servlet)



SIP

Java API for Integrated Networks (SIP-JAIN)



SIP Express Router Language (SER)



Microsoft SIP Processing Language (MSPL)

(18)

Laurent Réveillère

Enseirb

Département Télécommunications

Session Initiation Protocol – Raisin 2007

(19)

Client/Server Transaction

Request Response 1 Response n

…

Client

_Server

Transaction

(20)

20

Requests and Responses



Similarities with HTTP



Text format (readability vs size)



Headers



Response code

»

Classes

»

Hierarchy



Body of requests and responses



Parameters (e.g., Session Description Protocol)

(21)

Session Description Protocol (SDP)



Format to describe a session (not a protocol)



Examples:



Name and purpose of the session



Media parameters of the underlying audio and video streams



IP addresses and port numbers



Session descriptions transferred via other protocols (SIP,

SAP, RTSP, HTTP…)

(22)

22

SDP Syntax



A set of lines of the form

Type = value



Type: One character



value: Format depends on type



Two sections



Session-level information

»

First line – SDP version: v=0

»

Applies to the whole session

»

Examples: originator or session name



Media-level information (if present)

»

First line – media type: m

»

Applies to a particular media stream

(23)

Example

v=0

o=bob 2890844526 28900842807 IN IP4 131.160.1.112

s=SIP seminar

i=A seminar on the Session Initiation Protocol

u=http://www.cs.columbia.edu/sip

c=IN IP4 224.2.17.12/127

t=2873397496 2873404696

a=recvonly

m=audio 49170 RTP/AVP 0

a=rtpmap:0 PCMU/8000

m=video 51372 RTP/AVP 31

a=rtpmap:31 H261/9000

m=video 53 RTP/AVP 32

a=rtpmap:32 MPV/9000



“o”: creator of the session and his IP addr



“s”: name of the session



“i”: general information about the session



“u”: URI for more information



“e”: e-mail address of the contact person



“c”: multicast address of the session

“t”: when the session is active



“m

”=<media type> <port #> <transp. prot.> <media fmt>



“a”: information about media format



Pars: clock rate, number of channels…



Introduction of new attributes

E.g., a=volume:8



Examples

(24)

24

SIP Responses

Range

Response class

100-199

Informational (provisional)

200-299

Success

300-399

Redirection

400-499

Client error

500-599

Server error

600-699

Global Failure

(25)

Protocol Operations/Methods



OPTIONS



Querying capabilities

»

Supported methods (future extensions)

»

Supported SDP

»

Supported message body encoding



REGISTER



Register current location



Multiple locations can be registered for a user



INVITE



ACK



BYE

(26)

26

Session Invitation (INVITE)

(1)INVITE (4) ACK (2) 180 Ringing (3) 200 OK

Conversation

Bob

Laura

(27)

Three-way Handshake (1)

(1)INVITE (4) ACK (2) 180 Ringing

Conversation



Response delay



Server



Client still there



Successful session



Billing



Parallel requests

(28)

28

Three-way Handshake (2) – Unreliable Protocol (UDP)

(1)INVITE

?

(2)INVITE (3)INVITE (4)INVITE

Bob

gives up

(5) 200 OK

(29)

End of Session (BYE)

(1)INVITE

(4) ACK (2) 180 Ringing

Conversation



Abandon by the issuing

participant

(3) 200 OK

(5) BYE (6) 200 OK

(30)

30

Session Cancellation (CANCEL)

(1)INVITE (4) 200 OK (2) 180 Ringing (3) CANCEL (5) 487 Transaction Cancelled (6) ACK



Interpretation



If session not established

»

Cancel



If session established

»

No effect



Three-way handshake

even for cancellation



Usage context

(31)

Forking Proxy and Cancellation

(1)INVITE (7) ACK

Conversation

(2)INVITE (3)INVITE (4)INVITE (5) 200 OK (6) 200 OK (8) CANCEL (9) 200 OK (10) 487 Cancelled (11) ACK (12) CANCEL (13) 200 OK (14) 487 Cancelled (15) ACK

Forking

(32)

32

Format of SIP messages



Status line



Protocol version



Status code



Reason phrase

Request-line

Header

Header-name : Header-value

…

Header

Message body (optional)

Empty Line

Status line

Header

Header-name : Header-value

…

Header

Message body (optional)



Request-line



Method



Request-URI (next hop)



Protocol version

Empty Line

(33)

The SIP Headers

Accept

Content-encoding

Max-forwards

Route

Accept-encoding

Content-language

MIME-version

Server

Accept-language

Content-length

Organization

Subject

Alert-info

Content-type

Priority

Supported

Allow

Cseq

Proxy-authenticate

Timestamp

Also

Date

Proxy-authorization

To

Authorization

Encryption

Proxy-require

Unsupported

Call-ID

Error-info

Record-route

User-agent

Call-info

Expires

Require

Via

Contact

From

Response-key

Warning

(34)

34

To Header

(1) INVITE SIP:[email protected] SIP/2.0 To:SIP:[email protected] (2) INVITE SIP:[email protected] SIP/2.0 To:SIP:[email protected]

Laura

P2

131.6.2.1 university.com

(3) INVITE SIP:[email protected] SIP/2.0 To:SIP:[email protected]



Recipient of the request



Also the public address



Unchanged throughout the session (not changed by proxies)



Different from the Request-URI, which is



The next hop in the signaling path

(35)

Call-ID Header

(1)INVITE

(3) ACK

Conversation



signaling relationship

among 2 or more users



Unique identifier of an

invitation and subsequent

related transactions



Association mechanism of

messages for servers

(2) 200 OK (7) BYE (8) 200 OK Call-ID: [email protected] Call-ID: [email protected] Call-ID: [email protected] (4)INVITE (6) ACK (5) 200 OK Call-ID: [email protected] Call-ID: [email protected] Call-ID: [email protected] Call-ID: [email protected] Call-ID: [email protected]

Game

(36)

36

Contact Header

(1)INVITE

(5) ACK

Conversation



Where the user can be

reached



Enabling server(s) to be

skipped

(6) BYE (7) 200 OK (2)INVITE (3) 200 OK (4) 200 OK sip:[email protected] SIP/2.0 Contact: sip:[email protected]

Bob

SIP proxy

Laura

sip:[email protected] SIP/2.0 Contact: sip:[email protected]

Contact: sip:[email protected]

(37)

Cseq Header

(1)INVITE

(3) ACK



Command sequence (Cseq)



An integer: request order within

a session (Call-ID)



Method name: to distinguish

OK response from CANCEL

and INVITE



Request/response matching



New Cseq only generated by

UA for 2 methods



INVITE



BYE



Cseq generated independently

between sender receiver

CANCEL’s Cseq = INVITE’s

(2) 200 OK (4)INVITE (6) ACK (5) 200 OK (7) BYE (8) 200 OK Cseq: 1 INVITE Cseq: 1 INVITE Cseq: 1 ACK Cseq: 2 INVITE Cseq: 2 INVITE Cseq: 2 ACK Cseq: 3 BYE Cseq: 3 BYE

Re-invite

(38)

38

Responses for INVITEs and CANCELs Differ Thanks to Cseq

(1)INVITE

(3)200 OK



The response OK is either for



INVITE



CANCEL



Method name of the Cseq

enables to make the distinction

(2) CANCEL (4)487 Transaction cancelled (5) ACK Cseq: 1 INVITE Cseq: 1 CANCEL Cseq: 1 CANCEL Cseq: 1 INVITE Cseq: 1 ACK

(39)

Via Header

Conversation

(1) INVITE SIP:[email protected] SIP/2.0 Via:SIP/2.0/UDP Workst 100.univ.com:5060

(2) INVITE SIP:[email protected] SIP/2.0 Via:SIP/2.0/UDP 131.6.2.1:5060 Via:SIP/2.0/UDP Workst 100.univ.com:5060

(3) 200 OK

Via:SIP/2.0/UDP 131.6.2.1:5060 Via:SIP/2.0/UDP Workst 100.univ.com:5060 (4) 200 OK

Via:SIP/2.0/UDP Workst 100.univ.com:5060

(5) ACK SIP:[email protected] SIP/2.0 Via:SIP/2.0/UDP Workst 100.univ.com:5060

(6) BYE SIP:[email protected] SIP/2.0 Via:SIP/2.0/UDP Workst 100.univ.com:5060

(7) OK SIP:[email protected] SIP/2.0

Via:SIP/2.0/UDP Workst 100.univ.com:5060

Laura

SIP Proxy

Bob



Via headers store all proxies that handle the request



They contain the signaling path



Used to detect loop



Also used to route the response (opposite of request route)

(40)

40

Record-Route and Route Headers (1)

(1)INVITE

Conversation



Headers used by proxies



Opposite of Contact header

(aimed to short-circuit proxies)



Proxies stay in signaling

path



Security: filtering incoming

messages



Service provision:

session-related service needs (e.g.,

duration)

(7) BYE SIP:[email protected] SIP/2.0

(2)INVITE SIP:[email protected] SIP/2

(3) 200 OK (4) 200 OK

SIP:[email protected] SIP/2

Laura

SIP proxy

131.6.1.99

Bob

131.6.1.2 Record-Route:<SIP:[email protected]; maddr=131.6.1.99 Route:<SIP:[email protected]> Contact:<SIP:[email protected]> Record-Route:<SIP:[email protected]; maddr=131.6.1.99 Contact:<SIP:[email protected]> Record-Route:<SIP:[email protected]; maddr=131.6.1.99

(6) ACK SIP:[email protected] SIP/2

(8) BYE SIP:[email protected] SIP/2.0

(10) 200 OK

(9) 200 OK (5) ACK SIP:[email protected] SIP/2

(41)

Record-Route and Route Headers (2)

Laura

P1

P2

P3

Bob

Record-Route:P1 (1)INVITE (2)INVITE Record-Route:P1 (3)INVITE Record-Route:P3 Record-Route:P1 (4)INVITE Contact:Bob Record-Route:P3 Record-Route:P1 (5)200 OK Contact:Bob Record-Route:P3 Record-Route:P1 (6)200 OK Contact:Bob Record-Route:P3 Record-Route:P1 (7)200 OK Contact:Bob Record-Route:P3 Record-Route:P1 (8)200 OK Route:P3 Route:Bob (9) ACK Route:Bob (10) ACK (11) ACK

Conversation

Route:P3 Route:Bob (12) BYE Route:Bob (13) BYE (14) BYE (15) 200 OK (16) 200 OK