An Introduction to SIP Moshe Sambol

An Introduction to SIP

Moshe Sambol

Services Research Lab

November 18, 1998

November 18, 1998 GTE Laboratories SRL Presentation 2

The Session Initiation Protocol

Communications protocol under development by the IETF’s MMUSIC working group. Last call for comments is over, SIP is about to become a standard.

Text-based IP application-layer protocol (derived from HTTP) for Internet multi-party multi-media conferencing establishment and management.


SIP Basics

Separation of call data and connection control as opposed to PSTN: DTMF / pulses and on-

hook/off-hook signals carried in band with call.

Carries service and participant information provides natural support for advanced

telephony services.


SIP Basics continuedSimpler language than H.323 supports

flexibility, expandability, greater expression

Media and/or participants can be added to or removed from existing conferences

Supports personal mobility, degrees of security...


The SIP Call Model Session: A set of multimedia senders and receivers and

the data which flows between them. Conference: A multimedia session identified by a

session description. Composed of one or more calls.Call: All participants in a conference invited by a single

source.Call Leg: All communication between one caller and

callee within a call.Transaction: All messages within a call leg from a first

request until a final* response.


Actors in a SIP ConferenceUser Agent

Client application which initiates and accepts invitations; interacts with media applications.

SIP Proxy Server Intermediary which relays requests after possible lookup

and/or translation.

SIP Redirect Server Maps a request to a set of alternate addresses and returns

those to the client without initiating requests of its own.


SIP Actors, continued

Location Service Not defined by SIP but used by its servers to

locate other SIP servers.

No Gatekeeper Gatekeeper traditionally provides admission

control, address translation. These functions provided by other SIP actors.


SIP Operation in Proxy Mode

All illustrations courtesy of H.Schulzrinne


SIP Operation in Redirect Mode


SIP Requests

REGISTER Notify server of user location and availability

OPTIONS Determine user capabilities

INVITE CANCEL Call setup Transaction termination

BYE Call termination / transfer


SIP Responses

SIP Provides a rich set of response codes which make it well suited to support advanced telephony services:

1xx: Provisional responses (ringing, queued, etc.) 2xx: Final success responses 3xx: Redirect responses (moved perm/temp, mult. choices, etc.) 4xx: Client-Error (formatting, authorization, timeout, loop, etc.) 5xx: Server-Error 6xx: Global-Failure (busy, decline, does not exist, etc.)


An Example SIP MessageINVITE sip:[email protected] SIP/2.0

Via: SIP/2.0/UDP kton.bell-tel.com

From: A. Bell <sip:[email protected]>

To: T. Watson <sip:[email protected]>

Call-ID: [email protected]

Cseq: 1 INVITE

Subject: Mr. Watson, come here.

Content-Type: application/sdp

Content-Length: …

v=0

o=bell 53655765 2353687637 IN IP4 128.3.4.5

m=audio 3456 RTP/AVP 0 3 4 5


… Sample ResponseSIP/2.0 180 Ringing





Cseq: 1 INVITE

Content-Length: 0

-----------------------------------------------------------------------------

SIP/2.0 200 OK





Cseq: 1 INVITE

Contact: sip:[email protected]

Content-Type: …

m=audio 5004 RTP/AVP 0 3 …


Sample Services Multi-party call transfers generic addressing directed pick-up multiple pick-up hold music control third party call control camp-on click to dial back availability notification

SIP Supports Multicast UDP + TCP Privacy (authentication, encryption, path hiding)


Transfer Example: Telemarketing


State of SIP

SIP implementations are being worked on at: ObjectSoftware Nokia Research Lucent Mediatrix ISI HP Labs GMD Fokus Ericsson Dynamicsoft others?

My own graduate work was on...


Open IssuesBillingMappings to telephony APIsInterfacing to external media applicationsLocation services

Topics for Future PresentationsSIP vs. H.323 SDP

Security Richer examples!


Additional Info At:

Professor Henning Schulzrinne: http://www.cs.columbia.edu/~hgs/sip http://www.cs.columbia.edu/~hgs/internet/sdp.html

Working group mailing list: [email protected]

Struggling ex-graduate student: [email protected]

Documents

An Introduction to SIP Moshe Sambol