Wisdom of Helios

Basics of Session Initiation Protocol (SIP) (part-3)

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The previous posts were –

Basics of Session Initiation Protocol (part – 1)

Basics of Session Initiation Protocol  (part-2)

SIP Message Parts:

A SIP Message usually has 3 parts-

Start/First Line.



Start Line—Conveys the message type(request or response).

Start Line  = Method for Request or Response Code for Response + Protocol version.

A Request’s Start Line(Request Line) uses the following format:

<Request method><URI><Protocol version>

URI indicates the user/service to which the request is addressed. This address/URI can be re-written by the Proxy Servers.

An Example of Request Line is –

REGISTER sip:arstechnica.com SIP/2.0

A Response’s Start Line (Status Line) uses the following format:

<Protocol  version><Response Code><Reason phrase>

The reason phrase could be any text describing the nature of the response.

An Example of Status Line is –

SIP/2.0 200 OK


Header—Conveys the message attributes and modifies the meaning of the message. Very similar to the HTTP Headers.

All headers maintains the format-


Headers can span multiple lines. Some SIP headers such as Via, Contact, Route and Request-Route can appear multiple times in a message or, alternatively, can take multiple comma-separated values in a single header occurrence.

An Example of Header is –

Contact: sip:gilad@voxisoft.com;Expires=2000

Contact is the header name, sip:gilad@voxisoft.com is the value, and expires=2000 is a parameter. Other parameters may appear separated by semicolons.

Body(Content)— Describes the Session to be initiated (for example, in a multimedia session this may include audio and video codec types, sampling rates etc.), or alternatively it may be used to contain opaque textual or binary data of any type which relates in some way to the session. Message bodies can appear both in request and in response messages. SIP makes a clear distinction between signaling information, conveyed in the SIP Start Line and headers, and the session description information, which is outside the scope of SIP.

Possible body types include:

ü   SDP—Session Description Protocol (SDP).

ü   Multipurpose Internet Mail Extensions (MIME).

ü  Others—to be defined in the IETF and in specific

Other posts related to SIP –

Basics of Session Initiation Protocol  (part-4)

Basics of Session Initiation Protocol  (part-5)

Author: Munir

I'm good for nothing

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