So as a general rule, if a ???dialog??? is needed between two devices, the message body
will contain requirements for a session. Think of a session as a ???connection??? between
the calling party and the called party. If there is no ???connection??? needed and the intent
is to simply deliver content (such as a text message), then the message body can be used
to contain the actual content to be delivered.
If the message is a response, the first line is referred to as the status line. The status
line provides a status code indicating the status of the request. The status code falls
into one of six categories, as detailed in another section of this chapter. Below is an
example of a standard response message.
SIP/2.0 180 RINGING
VIA: SIP/2.0/UDP 128.10.10.1 raleighproxy.com:5060; branch=z9hG4bK63x2f
FROM: Deby Russell
;tag=9hz34567sl
TO: Travis Russell
MAX-FORWARDS: 70
CALL-ID: 82167534@126.18.27.0
CSEQ: 1 INVITE
CONTACT: Deby Russell
CONTENT-TYPE: application/SDP
CONTENT-LENGTH: 154
Note in this example that the first line indicates the status of the request. In this
example the provisional response 180 RINGING is given to indicate that the network
is attempting to alert the called party. The status line is then followed by the various
headers as necessary to support the response and any other dialog between the two
entities.
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