SIP - Headers

A header is a component of a SIP message that conveys information about the message. It is structured as a sequence of header fields.

SIP header fields in most cases follow the same rules as HTTP header fields. Header fields are defined as Header: field, where Header is used to represent the header field name, and field is the set of tokens that contains the information. Each field consists of a fieldname followed by a colon (":") and the field-value (i.e., field-name: field-value).

SIP Headers - Compact Form

Many common SIP header fields have a compact form where the header field name is denoted by a single lower case character. Some examples are given below −

Header	Compact Form
To	T
Via	V
Call-ID	I
Contact	M
From	F
Subject	S
Content-Length	I

SIP Header Format

The following image shows the structure of a typical SIP header.

Headers are categorized as follows depending on their usage in SIP −

Request and Response
Request Only
Response Only
Message Body