sip header msg format

SIP Protocol

Protocol format and headers

Contents

• SIP Protocol generally• SIP addressing, SIP URIs• SIP responses• SIP methods• SIP message format

– Headers– Body

SIP Protocol generally

• SIP is a text based• Similar to HTTP• SIP transaction / Call Leg

– Client sends a Request– Server replies with one or more Responses– Identified by To, From and Call-ID headers

• UDP or TCP is used as a transmission protocol

SIP Protocol generally

• Three way handshake in INVITE

SIP Addressing

• SIP URIs are usedsip:user:password@host:port;url-parameters?headers

• Several SIP headers use URI format– Request line– From– To– Contact …

SIP Addressing (cont.)

• SIP URIs, url-parameters– user: used to determine if username is a

telephone number (user=phone)– method: indicates the method to be used– ttl: time to live– maddr: multicast address where request should

be directed• Headers, for example subject=Hey

SIP Addressing (cont.)

• tags: cryptographically random number, added to To and From headers – identifies a call-leg

SIP Responses

• 1xx – Informational• 2xx – Success• 3xx – Redirection• 4xx – Client Error – resubmit after

modification• 5xx – Server error – Message OK but cannot be

prosessed in this server• 6xx – Global Failure

SIP Responses, 1xx (Informational)

• 100 Trying– hop-by-hop response– ”100 Trying ” is never forwarded– generated by a Proxy server or an User agent

• 180 Ringing– Indicates that the INVITE has been received by the

called User agent– Ringtone can be conveyd in the message body from

UAS to UAC

SIP Responses, 2xx (Success)

• 200 OK– Currently only Success-class response defined

in SIP– Indicates acceptance of INVITE (includes

media properties of the called UAS)

SIP Responses, 3xx (Redirection)

• 300 Multiple Choices– Request is responded with multiple Contact choices,

which should be used in that order in re-request• 301 Moved Permanently

– Response contains a permanent Contact header of the called party – address can be saved and used later

• 302 Moved Temporarily– Response contains a temporary Contact header of the

requested party – should not be cached unless Expires header is present

SIP Responses, 3xx (Redirection)

• 305 Use Proxy– Response includes an address for a proxy, to

which the request should be sent• 380 Alternative Service

– Indicates the type of service that the called party would like – for example voice mail or redirect server

SIP Responses, 4xx (Client error)

• 400 Bad Request– Not understood by the server (missing required

header? or multiple INVITEs for same Call-ID)• 401 Unauthorized

– Indicates, that the user is required to perform Authentication

– Sent By ”User Agent” or ”Registrar”


• 404 Not Found– SIP URL can not be located by the server

• 407 Proxy Authentication Required– Indicates, that UAC must authenticate itself to

the proxy• 408 Request Timeout

– INVITE has not been forwarded before request has expired (Expires header)


• 480 Temporary unavailable– Called party is not available for some reason

• 482 Loop Detected– Request has been routed back to the proxy that

previously forwarded the request• 483 Too Many Hops

– Request has been forwarded the maximum number of times set by “Max-Forwards”

SIP Responses,5xx (Server error)

• 500 Server internal error• 501 Not Implemented• 503 Service unavailable

• Actual message was correct and may be sent to some other server

SIP Responses,6xx (Global error)

• Should not be sent anywhere – it fails everywhere

• 600 Busy everywhere• 606 Not Acceptable

SIP Methods• INVITE

– Invite to participate in a session or change parameters of an existing session

– Includes usually message body with media information of caller

• ACK– Acknowledge a received final response (200-699)– May contain message body with media information – Invite (A-B) – 200 OK (B-A) – ACK (A-B)

• BYE– Terminate an established media session

SIP Methods, example1

SIP Methods (cont.)

• REGISTER– register current locations to registrar server,

redirect/proxy requests to current location– remove existing registrations (Expires = 0)

• CANCEL– Request to cancel pending request– Also sent by forking proxies after succesful

response to one of the branched requests

SIP Methods, example3REGISTER sip:feanor.pc.lut.fi SIP/2.0Via: SIP/2.0/UDP 157.24.25.137:5060From: athamala <sip:[email protected]>To: athamala <sip:[email protected]>Call-ID: 123456789@aradanCSeq: 1 REGISTERContact: <sip:[email protected]>Content-Length: 0

SIP/2.0 200 OKVia: SIP/2.0/UDP feanor.pc.lut.fi:5060From: athamala <sip:[email protected]>To: athamala <sip:[email protected]>Call-ID: 123456789@aradanCSeq: 1 REGISTERContact: <sip:[email protected]>;expires=3600Content-Length: 0

UA REGISTRAR

UA REGISTRAR

SIP Methods (cont.)

• INFO– used to send call signalling information to

another user agent– it doesn’t change anything – for example in SIP<->PSTN to transfer DTMF

signals• OPTIONS

– Query server or user agent about its capabilities

SIP Methods (cont.)

• SUBSCRIBE– To query presence information of other user

• NOTIFY– To inform other users about state of presentity

• MESSAGE– To send instant message body

SIP Methods (cont.)

• PRACK– To acknowledge informational (1xx) messages– Not very widely used

• COMET– Allows UAS to indicate, that QOS

preconditions have been met• REFER

– To initiate a third-party call control

SIP Message format

1.a) Request URL in Request (contains next hop in the path)

– INVITE [email protected] SIP/2.01.b) Status line in responses

– SIP/2.0 200 OK or similar2) Headers (From, To, Via, ...)3) Empty line4) Message body

SIP Message, request exampleINVITE sip:[email protected] SIP/2.0Via: SIP/2.0/UDP 157.24.25.137:5060From: Arto <sip:[email protected]>To: Jari <sip:[email protected]>Call-ID: 123456789@aradanCSeq: 1 INVITEContact: <sip:[email protected]>Content-Type: application/sdpContent-Length: 147

v=0 o=athamala 2890844526 2890844526 IN IP4 aradan.pc.lut.fis=Session SDP c=IN IP4 157.24.25.137 t=0 0 m=audio 49172 RTP/AVP 0 a=rtpmap:0 PCMU/8000

SIP Message, response exampleSIP/2.0 200 OKVia: SIP/2.0/UDP 157.24.25.137:5060From: Arto <sip:[email protected]>To: Jari <sip:[email protected]>;tag=314152 Call-ID: 123456789@aradanCSeq: 1 INVITEContact: <sip:[email protected]>Content-Type: application/sdp Content-Length: 149

v=0 o=jarsku 2890844527 2890844527 IN IP4 haldir.pc.lut.fis=Session SDPc=IN IP4 157.24.25.139 t=0 0 m=audio 3456 RTP/AVP 0 a=rtpmap:0 PCMU/8000

SIP Message format, headers

• From– Contains the initiator of the request (SIP URI),

required in all SIP messages• To

– Contains the recipient of the request (SIP URI),required in all SIP messages


• Call-ID– Unique identification code for Call, mandatory

in all SIP requests and responses– Call-ID: [email protected]

• CSeq– Sequence number for requests, UAS decide if

the request is a retransmission (same CSeq) or new request

– Cseq: 1 INVITE


• Contact– URL, which identifies the requested resource or the

request originator, mandatory in “INVITE” and “200 OK” responses to invitations.

– Contact: “Arto Hamalainen” <sip:[email protected]>

• Via– To store addresses of all the proxies which handle the

request, helps in loop detection and routing, required– Via: SIP/2.0/UDP feanor.pc.lut.fi:5060

SIP Message format, via processing


• WWW-Authenticate / Proxy-Authenticate– in ”401 Unauthorized” or ”407 Proxy

authentication required”• Authorization / Proxy Authorization

– in reply to 401 or 407, includes user credentials• Content-Length• Content-Type (for example application/sdp)


• Record-Route– Record Route header is used by proxies to force

that future requests in the same call will be forwarded through this route

• Route– In future requests Record-route is copied as a

Route –header which forces request to go through the proxy which inserted Record-route in the previous request

SIP Message format, body

• Some SIP messages have also message body. It can be for example– SDP/XML media description– Instant message data (in MESSAGE)

• Body type and length explained in Content-Type and Content-Length headers

SIP Message format, body (SDP)

...Content-Type: application/sdpContent-Length: 147

v=0o=athamala 2890844526 2890844526 IN IP4 here.com s=Session SDP c=IN IP4 100.101.102.103 t=0 0 m=audio 49172 RTP/AVP 0 a=rtpmap:0 PCMU/8000

SIP Protocol format

• SIP is text-based signalling protocol• Client sends Requests– Server responds

with a Response• Request methods: INVITE, REGISTER,

CANCEL, BYE, INFO, OPTIONS, ACK• Response codes: 1xx-6xx

SIP Protocol format

• Headers: From, To, Via, Call-ID, CSeq, Contact, ...

• Payload usually SDP Description, can be something else (XML, ...)

sip header msg format

Documents

sip headers

sip url

request server replies

proxy server

bad request

hops request

sip protocolprotocol

headerscontents sip