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SIP: A Flexible Signaling Protocol for Multimedia Communication

Learn about the Session Initiation Protocol (SIP), its history, architecture, entities, deployment, security, services, and future prospects. Compare SIP with H.323, analyze market trends, and draw conclusions.

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SIP: A Flexible Signaling Protocol for Multimedia Communication

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  1. SIP Chapter 5

  2. SIP History • 1980s – first packet multimedia experiments • 1992 – first IETF audio-cast • 1996 – first SIP related IETF drafts Session Invitation Protocol • Simple Conference Invitation Protocol MMUSIC IETF WG • 1999 – RFC 2543 • 2002 – RFC 3261 • Today - over 30 IETF RFCs related to SIP, many Internet Drafts and Working Groups

  3. What is SIP? • Application-layer signalling protocol • Easy to understand • Creation, modification and termination of multimedia communication sessions • Negotiation of session's parameters • Re-negotiation during communication session • User mobility • Ability to allow supplementary services • Extensibility

  4. SIP And VoIP Architecture

  5. VoIP Protocols • Transport protocols • TCP – Transmission Control Protocol • UDP – User Datagram Protocol • SCTP – Stream Control Transmission Protocol • TLS – Transport Layer Security Protocol • Media transport and control protocols • RTP – Real-time Transport Protocol (RFC1889) • RTCP – Real-time Control Protocol (RFC3605) • SRTP – Secure Real-time Transport Protocol (RFC3711) • Signalling protocol • H323 – Set of standards to transmit voice, video over IP. • SIP – Session Initiation Protocol (RFC3261) • Session negotiation • SDP – Session Description Protocol (RFC2327)

  6. SIP Design • Text based protocol in a format similar to HTTP • Client-server communication • Transaction oriented: request-response sequences • Independent of transport layer protocol • Request meaning is specified by method type • Session capability negotiation • Allow alpha-numeric addresses in URL format • (email-like address) as well as E.164 numbers • Use of domain names to locate servers • PSTN number translation

  7. SIP Entities • User Agent (UA) • User Agent Client (UAC) – initiates a SIP request • User Agent Server (UAS) – handles and eventually sends a response to a request • Proxy server – routing of SIP requests • Registrar server – registration of user's contact addresses • Location server – providing of user location details • Redirect server – return callee's addresses to caller • Application server – providing advanced services for users

  8. SIP Deployment Architecture

  9. SIP Request Syntax

  10. SIP Reply Syntax

  11. SIP Media Session

  12. SIP Registration

  13. SIP Re-Direction

  14. SIP Proxy

  15. SIP Security • Ensure privacy, service protection, proper accounting and billing • HTTP digest authentication schema • Challenge-response architecture • Basic authentication deprecated • Transport Layer Security for SIP entity id and traffic encryption • IPSec secure channels between SIP servers • S/MIME (Secure/Multipurpose Internet Mail Extensions) - for end to end encryption.

  16. SIP Services • Voice conversations • Advanced call features: call redirect, call forwarding, call barring, black/white lists • Easy to manage and use caller's preferences and callee's capabilities • Parallel and serial search of users • Audio conferences, video and instant messaging sessions, gaming • Presence and service location • System provisioning • Extensible and programmable environment

  17. Future of SIP • Deployed all over the world • Europe, USA, Asia • Replacement for H.323 and adopted as signalling protocol in 3GPP • Continuous extension development within IETF • Widest used protocol by newest ITSP • Devices and applications from most famous providers: CISCO, Avaya, Microsoft and very good representation in Open Source world

  18. Comparison of SIP and H.323 • Complexity • Extensibility • Scalability • Services • Security Mechanisms used in SIP and H.323 • Market Analysis • Conclusions

  19. Definition – H.323 • ITU H.323 series of recommendations (“Packet Based Multimedia Communications Systems”) defines protocols and procedures for multimedia communications on the Internet. • It is an umbrella standard that provides a well-defined system architecture and implementation guidelines. • It includes • H.245 for control • H.225.0 for connection establishment • H.332 for large conferences • H.450(.1,.2,.3) for supplementary services • H.235 for security • H.246 for interoperability with circuit-switched services.

  20. Definition – SIP • The Session Initiation Protocol (SIP), developed by MMUSIC working group of the IETF, is a signaling protocol for establishing real-time calls and conferences over IP networks. • It resembles HTTP and SMTP. • It uses SDP for media description. • It is not as strictly defined as a complete system like H.323. Therefore, it is flexible and can be adapted to a number of implementations. • It allows for the use of established protocols from other applications, such as HTTP and HTML.

  21. Definition - Functional Entities Terminal Terminal Gatekeeper Terminal H.323 Zone PSTN MCU Gateway H.323/SIP gateway H.323 network UA (softphone) SIP telephony gateway SIP network Proxy/ Registrar Enterprise network Redirect server SIP Realm UA (IP phone)

  22. H.323 Rather complex protocol Defines hundreds of elements Uses binary representation for its messages → therefore it requires special code generators to parse Uses several protocol components →therefore, many services require interaction between many of them → this also complicates firewall traversal SIP Simpler protocol Defines only 37 headers Encodes its messages as text, similar to HTTP → this allows simple parsing and generation Uses a single request that contains all necessary information Comparison - Complexity Source: schulzrinne and Rosenberg

  23. H.323 Provides extensibility generally by use of nonstandardParam fields → this allows for different vendors to develop their own extensions Extensions are limited only to those places where a non-standard parameter has been added It has no mechanisms for allowing terminals to exchange information about which extensions each supports. SIP Built in a rich set of extensibility and compatibility functions Numerical error codes are hierarchically organized → this allows for additional features to be added by defining semantics for the error codes in a class, while achieving compatibility Uses textual encoding which is self describing → this enables developers to determine usage from the name Comparison - Extensibility Source: schulzrinne and Rosenberg

  24. H.323 Large Number of Domains It provides no easy way to perform loop detection in complex multi-domain searches. Server Processing The complexity of signaling makes it less scalable. Conference Sizes Three distinct mechanisms exits to support different conference sizes. Comparison - Scalability SIP • Large Number of Domains • It uses a loop detection algorithm which can be performed in a stateless manner. • Server Processing • Simple signaling mechanism makes it more scalable. • Conference Sizes • It scales all different conference sizes. Source: schulzrinne and Rosenberg

  25. Comparison - Services • H.323 and SIP offer roughly equivalent call control services. • H.323 provides a much richer set of functionality for capabilities exchange services. • SIP provides rich support for personal mobility services. • H.323 supports various conference control services. Sip does not provide conference control, rather it relies on other protocols for this service. Source: schulzrinne and Rosenberg

  26. H.323/H.235 Two mechanisms that provide Authentication or/and Integrity are: Annex D - Baseline Security Profile Hop-by-hop processing Password based security Shared Secret-Key Digest (Hashing) Algorithm Annex E - Signature Security Profile Signature Profile – Public Key Infrastructure (PKI) Certificate Based Security Scalable - applicable for “Global” IP Telephony Hop-by-Hop and End-to-End security Digest Algorithms (Source: Radvision PPT) SIP End-to-end mechanisms Basic authentication Digest authentication S/MIME Hop-by-hop mechanisms Transport Layer Security (TLS) IP Security (IPSec) The SIPS URI schema (source: Ben Campbell presentation) Security Mechanisms

  27. SIP Authentication SIP Server SIP Client REQUEST Generate the Nonce value CHALLENGE Nonce, realm Compute response = F(nonce, Username, password, realm) F= MD5 REQUEST Nonce, realm, Username, response Authenticate: compute F(nonce, username, password, realm) And compare with response

  28. Chart 1 summarizes the technology supported by the 77 products. (source: Wind River White Paper) Chart 2 summarizes the technology supported by VoIP Service Providers. (source: Wind River White Paper) Market Analysis

  29. Interoperability Source: Ho et al.

  30. Conclusion • If SIP is better, why is H.323 important? • Huge installed base and backward compatibility is important. • However, newer products may not need H.323. • In videoconferencing world, H.323 is still a dominant player. • Most VoIP products support H.323 and SIP together. But this has the potential to increase the cost, size and power requirements of the products. • An all-SIP network is simple and cleaner to run/manage but we will see H.323/SIP for a long time. • Security mechanisms (authentication, privacy, authorization, integrity, non-repudiation) may well decide their fate.

  31. References • www.ietf.org drafts and RFCs (3261, 2543) for SIP • ITU-T and H.323 specifications. • SIP Vs. H.323:A Business Analysis, white paper from WindRiver. • SIP versus H.323, iptel.org/info/trends/sip.html • H.323 versus SIP: A Comparison, packetizer analysis at http://www.packetizer.com/iptel/h323_vs_sip/ • A Comparison of SIP and H.323 for Internet Telephony • Henning Schulzrinne and Jonathan RosenbergNetwork and Operating System Support for Digital Audio and Video (NOSSDAV), (Cambridge, England), July 1998. • For our work on SIP/H.323 security, see http://middleware.internet2.edu/video/

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