SIP Messages
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SIPxNano IP-PBX Getting Started Guide
A • Session Initiation Protocol (SIP)
Methods
SIP call control methods identify the type of request that is being made. The following table lists SIP methods
and their functions.
Table 7. SIP Methods and Functions
Method
Function
INVITE
Invites a target to participate in a session; estab-
lishes a connection. Also used to change call state
or capabilities, such as the codec used.
ACK
Confirms receipt of a final response to an
INVITE request.
BYE
Indicates that either the originator or the target
wishes to end the call; terminates a connection or
declines an invitation.
REFER
Indicates that the recipient should contact a third
party using provided contact information; ini-
tiates a transfer.
CANCEL
Cancels a pending request; does not affect a com-
pleted request.
REGISTER
Registers a user’s address with a SIP location
server; resolves a public address to a specific
address. Not related to specific session.
OPTIONS
Solicits information about features supported by
SIP servers such as supported methods and media
capabilities.
NOTIFY
Provides information about a state change; not
related to a specific session. Used for message
waiting communications with a voicemail server,
to indicate the outcome of transfers, and for con-
figuration.
SUBSCRIBE
Indicates the desire for NOTIFY (state change)
requests. Used for message waiting communica-
tions with a voicemail server and for configura-
tion.