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Understanding the SIP Protocol
November 2009 248
contains a number of header fields. Header fields are named
attributes that provide additional information about a message. The
ones present in an INVITE include the following:
Unique identifier for the call
Destination address
Caller A's address
Information about the type of session that Caller A wishes to
establish with Caller
B.
The INVITE (message F1 in Figure 20) might look like this:
atlanta.com . . . biloxi.com
. proxy proxy .
. .
A's . . . . . . . . . . . . . . . . . . . . . . b's
softphone SIP Phone
| | | |
| INVITE F1 | | |
|--------------->| INVITE F2 | |
| 100 Trying F3 |--------------->| INVITE F4 |
|<---------------| 100 Trying F5 |--------------->|
| |<-------------- | 180 Ringing F6 |
| | 180 Ringing F7 |<---------------|
| 180 Ringing F8 |<---------------| 200 OK F9 |
|<---------------| 200 OK F10 |<---------------|
| 200 OK F11 |<---------------| |
|<---------------| | |
| ACK F12 |
|------------------------------------------------->|
| Media Session |
|<================================================>|
| BYE F13 |
|<-------------------------------------------------|
| 200 OK F14 |
|------------------------------------------------->|
| |
Figure 20. SIP Session Setup Example With Sip Trapezoid