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Session Initiation Protocol (SIP)

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A simple, powerful protocol that allows users to initiate ... Members: AT&T, British Telecom, Cisco, dynamicsoft, Ericsson, & Sun. New extensions being added ... – PowerPoint PPT presentation

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Title: Session Initiation Protocol (SIP)


1
Session Initiation Protocol (SIP)
  • Stephen Jones
  • 1/25/01

Organization G061 Project 51MSR81G
2
What is SIP?
  • A simple, powerful protocol that allows users to
    initiate interactive sessions (audio/video/VoIP)
  • An alternative approach to the H.323 protocol
  • Advantages
  • Flexible
  • Scalable
  • Easy to implement
  • Interoperable
  • Extensible

3
Where did SIP come from?
  • Began as a component of the Mbone, a multicast
    network that provides dynamic content across the
    Internet.
  • Needed to be interoperable with other protocols,
    including Session Description Protocol (SDP),
    Real-Time Streaming Protocol (RTSP), and Session
    Announcement Protocol (SAP)
  • Interest in VoIP provided impetus to complete
    specification in 1998.
  • Became an official IETF standard, RFC 2543, in
    March 1999.
  • Tri-annual bake-offs ensure compatibility
    between different implementations

4
What does SIP do?
  • Handles the establishment of user sessions
  • Does not define the actual session information
  • Locate users in a very dynamic environment
  • Delivers a description of the session
  • Uses common MIME extensions
  • Can negotiate a common format
  • Allows users to change/control/terminate sessions

5
Example 1
SIP Signaling
IP Network
Media Stream
User 1
User 2
  • User 1 requests a session, passing
    characteristics to other users
  • User 2 accepts invitation, modifying session as
    necessary
  • Session begins
  • Either user signals the other if changes are
    necessary, or when terminating the session

6
Network Entities
  • User Agent
  • User Agent Client (sends requests)
  • User Agent Server (returns responses)
  • Usually combined on a users client machine
  • Redirect Server
  • Accepts SIP requests, translates them to new
    addresses, and returns these values
  • Proxy Server
  • Acts like a proxy server for web accesses
  • Registrar
  • Accepts REGISTER requests. Typically combined
    with a Proxy or Redirect server

7
Example 2
Deb ltsipdeb_at_machine1.mitre.orggt
1. INVITE
2. Resp. 302
3. ACK
ltsipserver.mitre.orggt
4. INVITE
5. Resp. 200
Steve ltsipsrjones_at_machine2.mitre.orggt
  • Steve registers with the local SIP server (Not
    shown)
  • Deb invites Steveltsipsrjones_at_mitre.orggt (Found
    using DNS SRV Resource Records)
  • Server responds with a redirect message
  • machine1 ACKs server response and resends to
    machine2
  • Steve receives request and replies with OK to
    join session

8
Points about Example 2
  • SIP URLs similar to email addresses that use
    mailto
  • sipsrjones_at_mitre.org
  • sip1-781-271-2517_at_mitre.org userphone
  • SIP is a request - response protocol
  • Small set of well-defined Request messages
    (INVITE, REGISTER, CANCEL, ACK, OPTIONS, BYE)
  • Responses include status code that may define
    whether further interactions required
  • Allows protocol to use either UDP or TCP
  • Handles timing and retransmit issues
  • Users can be addressed by a common name
  • No matter which machine the recipient is on, they
    can be reached at their address
  • Internet scalable

9
Message Structure
  • Structure of SIP messages are text-based and
    based on HTTP
  • start line
    message-header line
  • message-header line
  • message-header line ltCRLFgt
    message-body

10
Example messages
ltsipdeb_at_machine1.mitre.orggt
ltsipsrjones_at_machine2.mitre.orggt
INVITE sipsrjones_at_machine2.mitre.org Via
SIP/2.0/UDP machine1.mitre.org From
ltsipdeb_at_machine1.mitre.orggt To
ltsipsrjones_at_machine2.mitre.orggt Call-ID
123456_at_machine1.mitre.org CSeq 1 INVITE Subject
Big meeting Content-Length 125 Content-Type
application/sdp (SDP message body)
SIP/2.0 180 Ringing Via SIP/2.0/UDP
machine1.mitre.org From ltsipdeb_at_machine1.mitre.o
rggt To ltsipsrjones_at_machine2.mitre.orggt Call-ID
123456_at_machine1.mitre.org Cseq 1
INVITE Content-Length 0
SIP/2.0 200 OK ...
11
Sample Implementation - A Conferencing Service
  • User is automatically registered upon starting
    tool. They will be found through an extension to
    DNS which will forward sip calls to the
    appropriate SIP server
  • Active tools are selected for each area (audio,
    video, VoIP phone, etc) by clicking the
    appropriate button, which cycles through choices
    (e.g. desktop chat tool or Palm chat tool)
  • A prompt (with an appropriate text message) will
    be displayed when someone attempts to call
  • The Call button will allow for quick lookup of
    users (LDAP) and tools users wish to use
  • When iconified, the tool goes into User Busy
    mode

12
Services That SIP Can Provide
  • Call Hold
  • Consultation Hold
  • Unattended Transfer
  • Call forward on Busy/No Answer/Unconditional
  • 3-Way Conference
  • Find-Me
  • Incoming/Outgoing Call Screening
  • Call Waiting

13
Dynamic Capabilities Provided By SIP
  • Call Processing Language (CPL)
  • XML scripts that may be created by end users and
    uploaded to SIP servers for configurable call
    setup
  • SIP CGI
  • Provide information in the same manner as HTTP
    CGI
  • SIP Servlets
  • Again work the same as the HTTP version
  • Can be hosted in end user devices as well as
    servers

14
SIP vs. H.323
  • SIP (IETF)
  • A piece of a communications system
  • Flexible - can be combined with other protocols
  • Designed as a WAN protocol
  • Many concepts derived from HTTP
  • Extremely easy to implement
  • Fully integrated with other Internet protocols
    (email, web)
  • H.323 (ITU)
  • A complete, integrated suite of protocols for
    communications
  • Provides all protocols it requires
  • Designed as a LAN protocol
  • Many components derived from other ITU protocols
  • Very complicated to implement

15
Interoperability with Other Services
  • Internet-PSTN telephony
  • Use of phone numbers allows VoIP phones and
    standard PSTN phones to contact each other
  • Gateways already exist to do this
  • SIP-H.323 Internetworking
  • Internet Draft has been prepared to address this
    capability
  • Lucent PacketStar IP
  • SIP-WAP interoperability
  • Proxy server can pass information between
    networks
  • SIP-Instant Messaging/Presence Awareness
  • Extensions to SIP have been proposed to allow
    IMPP interoperability
  • SIP-SOAP (Simple Object Access Protocol)
  • Internet Draft has been prepared

16
Potential Problems for SIP
  • Shortage of IPv4 addresses and growing use of
    Network Address Translators (NAT)
  • SIP carries the IP addresses and ports of the
    session to be established
  • NATs may cause these sessions to be un-routable
  • This effects other protocols as well (H.323)
  • Same problem can exist with Firewalls
  • Relatively new

17
Future Directions
  • Will see more implementations of the protocol
  • Bake-offs have increased in participation
    significantly
  • SIP initiative within JAIN
  • Members ATT, British Telecom, Cisco,
    dynamicsoft, Ericsson, Sun
  • New extensions being added
  • RFC 2543 currently being revised
  • New methods, headers, and responses have been
    proposed
  • Investigating management of SIP resources through
    SNMP
  • Exploring QoS issues in determining call setup

18
Resources
  • Hennig Schulzrinnes page http//www.cs.columbia.
    edu/hgs/sip
  • Sip Forum http//www.sipforum.org
  • IETF charter http//www.ietf.org/html.charters
    /sip-charter.html
  • The SIP Center http//www.sipcenter.com
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