HP-UX C SIP Stack Programmer's Guide (Novembery 2007)

Session Initiation Protocol (SIP) 5

Messages

MESSAGE PARTS SIP messages are composed of the following three parts:

 Start line

 Headers

 Message body

TART LINE Every SIP message begins with a Start Line. The Start Line conveys the

message type (method type in requests, and response code in responses) and the

protocol version. The Start Line may be either a Request-line (requests) or a

Status-line (responses), as follows:

 The Request-line includes a Request-URI, which indicates the

user or service to which this request is being addressed.

 The Status-line holds the numeric Status-code and its associated

textual phrase.

EADERS SIP header fields convey message attributes that provide additional information

about the message. They are similar in syntax and semantics to HTTP header

fields (in fact, some headers are borrowed from HTTP) and thus always take the

format:

Headers can span multiple lines. Some SIP headers such as Via, Contact, Route

and Record-Route can appear multiple times in a message or, alternatively, can

take multiple comma-separated values in a single header occurrence.

ESSAGE BODY A message body is used to describe the session to be initiated (for example, in a

multimedia session this may include audio and video codec types and sampling

rates), 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.

603 Decline

604 Does not exist anywhere

606 Not acceptable

Table 1-2 Response Codes (Continued)

Number Meaning