Session Initiation Protocol Tutorial Ronen Ben-Yossef VP of Products - RADCOM

Objectives What is SIP? Why Did the World Need SIP? What are the Differences between SIP, H.323 and MGCP? SIP Entities and their Functions Example of a Basic SIP Session.

What is SIP? Session Initiation Protocol (SIP) is an application layer control (signaling) protocol for creating, modifying and terminating sessions with one or more participants. SIP can invite both persons and "robots," such as a media storage service, to participate in a call. These sessions include multimedia such as Voice, Video Messaging and events. SIP is defined in RFC 3261 (preceded by RFC 2543).

What Else Is SIP Used For? SIP is a flexible protocol designed to be part of the overall IETF multimedia data and control architecture. Setup many types of sessions such as Push- To-Talk Over Cellular or IM Telephony over cable Carrier backbone VoIP sessions (SIP-T) between 2 PSTNs Voice for 3G wireless networks (3GPP)

What Doesn’t SIP Do? Basic SIP does not offer conference control services. SIP does not prescribe how a conference is to be managed. SIP does not reserve resources, but can convey to the invited system the information necessary to do this.

How Does SIP Compare with H.323? SIP offers similar functionality to H.323 SIP is protocol neutral: can use TCP/UDP or run on top of AAL5, IPX, X.25, SCTP etc. SIP architecture is modular hence extensible SIP is less complex SIP uses simpler encoding – Text vs. Binary. SIP supports calling-party number security SIP setup times are shorter SIP address is any URL while H.323 addressing is more confined Politics: SIP is IETF, H.323 is ITU-T

ITU H.323 H.225 H.245 RTP RTCP IETF SIP SDP SAP RTP RTCP RTSP MGCP RAS

MGCP Carrier Grade

MGCP At The Access

How is SIP different from MGCP? MGCP is a device control protocol. A slave (gateway (MG)) is controlled by a master (media gateway controller (MGC), call agent). SIP may be used between controllers, in a peer-to-peer relationship. To the SIP side, the MGC looks like a node with a large number of connections, but otherwise the same as a "native" SIP device. The MG is completely unaware that the call between MGCs is established via SIP. Only the MGC needs to understand both protocols.

Combined Architecture

Why Did The World Need SIP? H.323 –Uses the traditional approach of end-to-end call setup –Expands to accommodate other traditional services such as Video SIP –“Flexible” protocol –Designed to scale and stay simple –Allows integrated services Voice Video IM Presence Conferencing

Why Did The World Need SIP? VoIP–only applications are past the initial adoption phase Service providers are seeking to expand their offerings SIP is the most suitable tool to achieve that goal SIP is matured enough for prime time Example: adopted by cellular world for –3GPP Voice Core –Push-To-Talk over cellular –Packet Cable

SIP Entities SIP servers: –User Agent Client –User Agent Server –Proxy servers –Redirect servers –Registrars –Location server Application – specific servers e.g.: –IM server –POC server There are several different SIP entities, each with their own purpose.

User Agent Client SIP device or application that initiates SIP requests This is the SIP name for endpoint device

User Agent Server SIP device or application that terminates SIP requests UAS contacts the user when a SIP request is received. The UAS returns a response on behalf of the user. The response either accepts, rejects, or redirects the request. User Agent Server “Call for you?” “I’ll accept.” SIP Call Request Call Accepted

Proxy Server A proxy server is an intermediary program that acts as both a server and a client for the purpose of making requests on behalf of other clients. A proxy interprets a request, and if necessary, rewrites the request message before forwarding it. Helps routing requests to user’s current location Proxy Server Forwards SIP Requests. Responds on behalf Y and X. Implements provider’s Call-routing policies User Agent Y User Agent X

Redirect Server Redirects calls to new location. A redirect server is a server that accepts a SIP request, then returns new addresses to the client. Unlike a proxy server, a redirect server does not initiate its own SIP request. Unlike a user agent server, a redirect server does not accept calls. User Agent Redirect Server

Registrar Redirect Server Specialized UAS that Allows a user to register their presence. A registrar is a server that accepts REGISTER requests. A registrar is typically co- located with a proxy or redirect server. A registrar may offer location services. Proxy Server Register Server

Protocol Building Blocks Request Response Request Transaction Dialog Session Request Response Transaction

SIP Request Types INVITE ACK BYE CANCEL OPTIONS REGISTER MESSAGE SUBSCRIBE NOTIFY

SIP Response Codes CodePhrase 1xxProvisional 2xxSuccess 3xxRedirection 4xxClient Error 5xxServer Error 6xxGlobal Failure

SIP Transactions A SIP transaction consists of a SIP request with the appropriate response. To identify which SIP request a response is for, the following fields should be identical. –Call-ID - uniquely identifies a particular invitation. –Cseq - (command sequence) a unique number that is incremented for each command with the same call-ID. –To, From. –Tag (The Tag is only present when more than one Via header field is present). In RFC 3261 less fields are required, with the presence of “magic cookie”

INVITE The INVITE request asks a callee to establish a 2-party call or join a conference call. The INVITE includes a session description (using SDP). The session description enumerates media types & formats. If the callee agrees, the caller sends an ACK & returns a description listing the media it wishes to use. –For a multicast session, the callee should only return a session description if it is unable to receive the media indicated. You are cordially invited...

Sip Session Phone to Phone INVITE 1XX - Ringing ACK Conversation BYE 200OK

INVITE With a Proxy Server 200 OK End User End User Proxy Server Location Server User Agent Server INVITE RING 200 OK ACK mj or INVITE or ACK bulls.org 100 TRYING

Sip Session Via Proxy – RFC 3261 REGISTER (Address1) 200OK 100 Trying Conversation 200OK INVITE Register + Proxy Server REGISTER (Address2) 200OK INVITE 200Ok Invite (address2) ACK 200OK 200Ok Invite (address1) User decide

INVITE With a Redirect Server End User End User Redirect Server Location Server User Agent Server INVITE RING 200 OK mj INVITE ACK 302 MOVED ACK

Sip IM Session REGISTER - Online 200OK 200OK - Subscribe SUBSCRIBE (User1) Register + IM Server NOTIFY - Online 200OK - Notify REGISTER – Busy 200OK NOTIFY - Busy 200OK - Notify 200OK - Subscribe User1 User2 SUBSCRIBE (Expire=0) Delete Subscription

The Scenario Calling SIP PhoneCalled Proxy ServrCalled SIP PhoneCalling Proxy Servr

Invite Calling SIP PhoneCalled Proxy ServrCalled SIP PhoneCalling Proxy Servr INVITE Status 100 Trying

SIP Invite

Invite Calling SIP PhoneCalled Proxy ServrCalled SIP PhoneCalling Proxy Servr INVITE w/Token Status 100 Trying INVITE

Another Invite

Invite Calling SIP PhoneCalled Proxy ServrCalled SIP PhoneCalling Proxy Servr INVITE Status 180

SIP Invite with a Status Code

Provosional Response Calling SIP PhoneCalled Proxy ServrCalled SIP PhoneCalling Proxy Servr Status 180

SIP Status Codes

SIP OK Plus First RTP Calling SIP PhoneCalled Proxy ServrCalled SIP PhoneCalling Proxy Servr RTP 200 OK

SIP OK Plus First RTP Frame

SIP OK Messages Calling SIP PhoneCalled Proxy ServrCalled SIP PhoneCalling Proxy Servr 200 OK

SIP OK Messages

Calling SIP PhoneCalled Proxy ServrCalled SIP PhoneCalling Proxy Servr ACK Acknoledgement