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 −