cscu9yh / itnp2b – telecoms systems•note: indeed sdp is a data format rather than a protocol. dr...

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CSCU9YH / ITNP2B –Telecoms Systems

SIP I Components

SIP

• SIP = Session Initiation Protocol• Establishes, modifies, terminates multimedia sessions– Add parties– Add, remove, change media

• SIP is independent of the type of multimedia session handled

• SIP is also independent of the mechanisms used to describe that session

Dr Mario Kolberg / Prof Evan Magill

CSCU9YH/ITNP2B Telecommunications Systems and Services

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Example



Would you like to join this session?

Session description

I would like to talk to

Laura and Carol.

Session Descriptions: SDP• SDP = Session Description Protocol• Specifies how the information necessary to describe a

s session should be encoded• No transport mechanisms• No parameter negotiation• Simply some information about a session• Text-based• Type = Value• Session-level and media-level information



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SDP

• Media to use (codec, sampling rate)• Media destination (IP address and port number)• Session name and purpose• Times the session is active• Contact information• Note: indeed SDP is a data format rather than a

protocol.



SDP example• v=0• o=Bob 28908044538 289080890 IN IP4 193.175.132.118• s=SIP lecture• i=The first lecture on SIP• u=http://www.cs.stir.ac.uk• [email protected]• c=IN IP4 126.16.69.4• t=28908044900 28908045000• a=recvonly• m=audio 49170 RTP/AVP 0• a=rtpmap:0 PCMU/8000• m=video 51372 RTP/AVP 31• a=rtpmap:31 H261/90000



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Session Establishment Progress



Bob Laura

I will call Laura

Would you like to join a voice session?

Laura is being alerted. Here phone is ringing.

Laura is willing to join the session. She has picked up the phone.

User Mobility• Where should the session invitation been delivered too?• User needs to be located.• User may be at different IP addresses (PCs)• SIP URLs• Users need to register• Proxy or Redirect mode



eagle.cs.stir.ac.uk

SIP server atcs.stir.ac.uk

I am ”Tip” I will be reachable at [email protected]

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SIP entities

• User Agents• Redirect Servers• Proxy Servers• Registrars

• Looking at each one in turn …



User Agents

• SIP entity which interacts with the user• User interface– Special computer program– SIP phone– SIP for appliances

• Not all devices need to interact with the user directly (answering machines)

• Sessions are delivered to UA• User Agent Server, User Agent Client



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Forking Proxy Server



SIP server atcompany.com

[email protected]

[email protected]

iptel.org

(1) Invitation for SIP: [email protected]

(4) Invitation forSIP: [email protected]


(3) I’m busy

(5) I’m available

Registrars and Location Servers• Registrars– SIP server accepting registrations– Usually co-located with a redirect server or a proxy server

• Location Server– Strictly, are not SIP entities– Stores and returns possible locations for users– Make use of Registrars or other databases– Most registrars upload information to a location server

upon receipt– Communication is not SIP



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Proxy and location servers



SIP server (registrar and proxy) at [email protected] [email protected]


(2) Where is reachableSIP: [email protected]

Location server

(3) Try [email protected]


Overall Architecture



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Client – Server Transactions

• SIP is based on HTTP• Request/response protocol– Client ® generates requests– Server ® receives requests, returns responses

• User Agent Client, User Agent Server• Request + Response = Transaction



SIP Responses

• Upon receiving a request, a server issues one or more responses

• Responses have a code indicating status of transaction (integers 100 – 699)

• Range Response class100 – 199 informational200 – 299 success300 – 399 redirection400 – 499 client error500 – 599 server error600 – 699 global failure



Provisional responses

Final responses

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SIP Responses

• Responses carry a reason phrase (string)– Example: 180 – user is being alerted – Ringing

• Computer ignores the reason phrase– 200 OK– 100 Trying – 181 Call is being forwarded – 302 moved temporarily – 305 use proxy – 402 payment required – 486 busy here – 501 not implemented – 603 busy everywhere



SIP Requests• Core of SIP defines 6 types of requests• Requests contain a field “method” denoting

purpose– INVITE– ACK– OPTIONS– BYE– CANCEL– REGISTER

• Requests and responses may contain SIP bodies– Payload, e.g. session description (SDP)



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INVITE



Bob Laura

I will call

Laura

(1) INVITE

(2) 180 Ringing

(3) 200 OKv=0o=Bob 2890844526 2890842807 IN IP4 131.160.1.112s=I want to talk to youc=IN IP4 131.160.1.112t=0 0 m=audio 49170 RTP/AVP 0

v=0o=Bob 2890844526 2890842807 IN IP4 138.85.27.10s=I want to talk to youc=IN IP4 138.85.27.10t=0 0 m=audio 20000 RTP/AVP 0

SIP Example



Bob(1) INVITE

SIP Proxy(2) INVITE

(3) 180 Ringing(4) 180 Ringing

(7) ACK

Conversation

Mario

(5) 200 OK(6) 200 OK

(8) BYE

(9) 200 OK

saturn.company.comproxy.company.comeagle.cs.stir.ac.uk

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Characteristics of SIP

• Part of IETF toolkit• Distinguishes between session establishment

and session description• Intelligence in the end system• Interoperability• Scalability



cscu9yh / itnp2b – telecoms systems•note: indeed sdp is a data format rather than a protocol. dr...

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