SIP-SDP-2

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Lecture No.9

SIP - requests and responses,header fields and extensions

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Methods

INVITE :Establishes a session

ACK :Confirms an INVITE request, 3-way hand-shaking

BYE :Ends a established session

CANCEL :Cancels establishing of a session

REGISTER :Communicates user location (host name, IP)

OPTIONS :Communicates information about thecapabilities of the calling and receiving SIP phones

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Miroslav Voznak, lecture on SIP 3

Methods extension

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Responses 1xx informational responses, such as 180, which means

ringing 200 - 699 final response

2xx success responses, such as 200 OK

3xx redirection responses

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Responses

4xx request failures 5xx server errors

6xx global failures

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TransactionTransaction

a request and all responses

old type hash To,From,RURI, CSeq

new parameter branchinVia

z9hG4bK is beginning of astring branch(new way oftransaction determination)

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Dialog

Dialog

Call-ID, From tag, To tag

CSeq identifies atransaction in dialog

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Registration

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Registration- bundling of user URI and device URI (From and Contact)

- Expires is a time period of registration (Expires=0 , cancellation)

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Redirection

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SIP message

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Session Description Protocolv=0o=ja 987504994 987504994 IN IP4 1.2.3.4s=Hovor 1c=IN IP4 1.2.3.4t=3196493794 3196497394m=audio 10000 RTP/AVP 0 22a=rtpmap:0 aplication/g711a=rtpmap:22 aplication/g723.1

SDP

v = Version number (ignored by SIP)o = Session Origin used by SIP

s = Subjectc = Connection Data (IN =internet, IP4 = IPv4, IP Address)t = Time (ignored by SIP)m = Media (type, port, RTP/AVP Profile)a = Attribute (profile, codec, sampling rate)

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INVITEINVITE sip:[email protected] SIP/2.0

Via: SIP/2.0/UDP pc33.atlanta.com:5060;branch=z9hG4bK776asdhdsTo: Bob

From: Alice ;tag=1928301774

Call-ID: a84b4c76e66710

CSeq: 314159 INVITE

Contact:

Content-Type: application/sdpContent-Length: 142

(SDP part)

the first line contains :

method name: INVITE

Request-URI (next hope is sip:[email protected])

SIP version number: SIP/2.0

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SIP viaINVITE sip:[email protected] SIP/2.0

Via: SIP/2.0/UDP proxy.stockholm.se:5060;branch=82.1

Via: SIP/2.0/UDP pc33.atlanta.com:5060;branch=z9hG4bK776asdhds

To: Bob



CSeq: 314159 INVITE

Contact:

Content-Type: application/sdp

Content-Length: 142

(SDP part)

Via field:

insterted records point to the used route, Proxy serversinsert next records

response should use the same route as a request

branch enables to detect the loops

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Dialog (Call-leg)INVITE sip:[email protected] SIP/2.0


To: Bob From: Alice ;tag=1928301774


CSeq: 314159 INVITE

Contact:


Content-Length: 142

(SDP part)

Dialog the same information during the same call-leg are in therecords:

To , From a Call-ID To a From specifies logical address of endpoints of connection

Call-ID unique identifier during the dialog

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CSeqINVITE sip:[email protected] SIP/2.0


To: Bob



CSeq: 314159 INVITE

Contact:


Content-Length: 142

(SDP part)

CSeq

every new request increments CSeq repeated request has the sameCSeq

all responses regarding the one request has the same CSeq

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ContactINVITE sip:[email protected] SIP/2.0


To: Bob



CSeq: 314159 INVITE

Contact:


Content-Length: 142

Contact

contains SIP URI for direct communication, if SIP Proxy

doesnt insertRecord-Route field than nexttransctions can be routed directly

field Contact is also stated in response 200 OK

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Content type, LengthINVITE sip:[email protected] SIP/2.0

Via: SIP/2.0/UDP pc33.atlanta.com:5060;branch=z9hG4bK776asdhdsTo: Bob



CSeq: 314159 INVITE

Contact:


Content-Type describes the type of body (SDP)

Content-Length includes the size of content (in bytes), thevalue 0means without content

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Max ForwardsINVITE sip:[email protected] SIP/2.0


Max-Forwards: 30To: Bob



CSeq: 314159 INVITE

Contact:


Max-Forwards

decreases with every forwarded request at the Proxy

if its value is 0 than is sent 483 Too Many Hops

stateless Proxy detects a loop by field Max-Forwards

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INVITE and response 200 OK

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Tagging

in the case of forking there would be difficult to resolve whoanswered

UAS adds a tag into the response

To: sip:[email protected];tag=a48s

next request contains tag inserted into field To

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Response routing the reguests are routed according to Request URI (RURI)

and the responses according to field Via

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Forking

the first 200 OK

confirmed by ACK

the rest obtains487 CancelledINVITE

R d R ti

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Record Routing- Record route field is inserted by SIP Proxy not only intothe header of method (request) but into the final responseas well- a next transaction is routed via the elements stated in therecord route

SIP t id

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SIP trapezoid

- Outbound Proxy has to find a SIP Proxy which is able toserve the request

- SIP Proxy serves one or more domains (multidomain Proxy)- destination SIP Proxy can be find via DNS (SRV record in

DNS resolves serving SIP Proxy) or by static mapping

ENUM

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ENUM

- SIP addressing is based on SIP URI

- the number (E.164) can be resolved via DNS whereinNAPTR record contains a regular expression which is appliedto ENUM query and the result of this operation is SIP URI (ora list of URIs)

- SIP URI contains a host (users domain), via SRV recordis resolved serving SIP Proxy

- IP address of SIP proxy is resolved via A record in DNS

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Call Setup

Termination of established session

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Termination of established session- Bye

d bli hi i

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and establishing session Cancel

- two requests INVITE and CANCEL open two transactions

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SIP and PSTN Interworking

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Presence

UA gets status and presence

information which correspondsto users of server

Users are subscribed to users

presence

NOTIFY is sent on an event (achange of the state)

SUBCRIBE creates a dialogwherein the NOTIFY is sent ona change of the state

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Subscribe a Notify

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Instant Messaging

Method MESSAGE isused for IM

text is in body ofmessage

request Messagedoesnt create a dialog

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NAT Issue: clients behind NAT insert private IP

into signaling in SIP fields Via, From,Contact and SDP c-line (contact)

NAT

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NAT Solution: an intervention on application layer

ALG (Application Layer Gateway), ALG workstogether with NAT and makes rewriting inapplication layer

NAT s pport

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NAT support STUN, TURN, ICE help on the Client Side

(these protocols help to find out public IP)

Session Border Controller (SBC) help on

the Provider Side, clients behind NATcommunicate through a border device

SIP Proxy with s Media Relay support (RTPProxy) in order to ensure signaling and mediarouting on the same IP

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Thank you for your attention

[email protected]
mailto:[email protected]:[email protected]:[email protected]:[email protected]

SIP-SDP-2

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