SIP Transactions
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SIPxNano IP-PBX Getting Started Guide
A • Session Initiation Protocol (SIP)
SIP Transactions
A SIP transaction consists of a set of related SIP messages: usually, a request such as an INVITE, zero or more
provisional responses (1xx response code), and a final response (2xx or greater). For example, the set of mes-
sages sent by a callee phone during call setup to indicate trying, ringing, and Ok make up a SIP transaction.
See
page 90
for an example of this message flow.
The message header To, From, Call-ID, and Cseq fields have the same values in every message in a SIP transac-
tion.
SIP Sessions
SIP sessions encompass all messages sent between two SIP endpoints. That is, all SIP messages sent between
two phones, beginning with call setup and ending with call teardown, make up a SIP session.
The headers of all messages involved in a SIP session have the same values in the To, From, and Call-ID fields.
However, the addresses in the To and From fields switch to reflect the endpoint that originated the message.
SIP Calls
A SIP call consists of one or more SIP sessions. An example of a call that encompasses multiple sessions is a
conference call.
All SIP messages in a call will have the same values in all Call-ID message header fields. However, a user can set
an alias as the Caller ID (see “Caller ID” below) for SIP INVITE requests.
Caller ID
A user can set an alias for outbound calls so that the alias will appear as the Caller ID on the recipient’s phone.
To set an alias for the caller ID, modify the From header in the SIP INVITE request.
An external number can also be specified on a gateway so that all outbound calls through the gateway will have
the same caller ID. This only affects calls that do not have a user alias specified.
Caller ID can be defined per user, on a user group level, or per gateway. Caller ID can also be blocked on a per
user or per gateway basis (CLIR).