SIP Chapter 5

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 – RFC 3261 Today - over 30 IETF RFCs related to SIP, many Internet Drafts and Working Groups

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

SIP And VoIP Architecture

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)

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 ( -like address) as well as E.164 numbers Use of domain names to locate servers PSTN number translation

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

SIP Deployment Architecture

SIP Request Syntax

SIP Reply Syntax

SIP Media Session

SIP Registration

SIP Re-Direction

SIP Proxy

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.

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

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 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 MCU Gateway PSTN H.323 Zone Enterprise network UA (softphone) UA (IP phone) Proxy/ Registrar Redirect server SIP telephony gateway H.323/SIP gateway SIP Realm SIP network H.323 network

22 Comparison - Complexity 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 Source: schulzrinne and Rosenberg

23 Comparison - Extensibility 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 Source: schulzrinne and Rosenberg

24 Comparison - Scalability 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. Source: schulzrinne and Rosenberg 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.

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 Security Mechanisms 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)

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

28 Market Analysis 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)

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 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 A Comparison of SIP and H.323 for Internet Telephony Henning Schulzrinne and Jonathan Rosenberg Network and Operating System Support for Digital Audio and Video (NOSSDAV), (Cambridge, England), July For our work on SIP/H.323 security, see