INVITE
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A user sends an INVITE request to invite another user
to participate in a session. The body of an INVITE request may contain
the description of the session.
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In NAT mode, the IP addresses in the Via, From, To, Call-ID,
Contact, Route, and Record-Route header fields are modified.
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ACK
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The user from whom the INVITE originated sends an ACK
request to confirm reception of the final response to the INVITE request.
If the original INVITE request did not contain the session description,
the ACK request must include it.
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In NAT mode, the IP addresses in the Via, From, To, Call-ID,
Contact, Route, and Record-Route header fields are modified.
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OPTIONS
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A User Agent (UA) uses OPTIONS to obtain information
about the capabilities of the SIP proxy. A server responds with information
about what methods, session description protocols, and message encoding
it supports. In NAT mode, when the OPTIONS request is sent from a
UA outside NAT to a proxy inside NAT, the SIP ALG translates the address
in the Request-URI and the IP address in the To field to the appropriate
IP address of the internal client. When the UA is inside NAT and the
proxy is outside NAT, the SIP ALG translates the From, Via, and Call-ID
fields.
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NA
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BYE
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A user sends a BYE request to abandon a session. A BYE
request from either user automatically terminates the session.
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In NAT mode, the IP addresses in the Via, From, To,
Call-ID, Contact, Route, and Record-Route header fields are modified.
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CANCEL
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A user sends a CANCEL request to cancel a pending INVITE
request. A CANCEL request has no effect if the SIP server processing
the INVITE had sent a final response for the INVITE before it received
the CANCEL.
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In NAT mode, the IP addresses in the Via, From, To, Call-ID,
Contact, Route, and Record-Route header fields are modified.
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REGISTER
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A user sends a REGISTER request to a SIP registrar server
to inform it of the current location of the user. A SIP registrar
server records all the information it receives in REGISTER requests
and makes this information available to any SIP server attempting
to locate a user.
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In NAT mode, REGISTER requests are handled
as follows:
- REGISTER requests from an external client to an internal
registrar—When the SIP ALG receives the incoming REGISTER request
it translates the IP address, if any, in the Request-URI. Incoming
REGISTER messages are allowed only to a MIP or VIP address. No translation
is needed for the outgoing response.
- REGISTER requests from an internal client to an external
registrar—When the SIP ALG receives the outgoing REGISTER request
it translates the IP addresses in the To, From, Via, Call-ID, and
Contact: header fields. A backward translation is performed for the
incoming response.
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Info
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Used to communicate mid-session signaling information
along the signaling path for the call.
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In NAT mode, the IP addresses in the Via, From, To, Call-ID,
Contact, Route, and Record-Route header fields are modified.
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Subscribe
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Used to request current state and state updates from
a remote node. In NAT mode, the address in the Request-URI is changed
to a private IP address if the message is coming from the external
network into the internal network.
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The IP addresses in Via, From, To, Call-ID, Contact,
Route, and Record-Route header fields are modified.
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Notify
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Sent to inform subscribers of changes in state to which
the subscriber has a subscription. In NAT mode, the IP address in
the Request-URL header field is changed to a private IP address if
the message is coming from the external network into the internal
network.
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The IP address in the Via, From, To, Call-ID, Contact,
Route, and Record-Route header fields are modified.
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Refer
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Used to refer the recipient (identified by the Request-URI)
to a third party by the contact information provided in the request.
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In NAT mode, the IP address in the Request-URI is changed
to a private IP address if the message is coming from the external
network into the internal network. The IP addresses in the Via, From,
To, Call-ID, Contact, Route, and Record-Route header fields are modified.
For example, if user A in a private network refers user B in
a public network to user C, who is also in the private network, the
SIP ALG allocates a new IP address and port number for user C so that
user C can be contacted by user B. If user C is registered with a
registrar, however, its port mapping is stored in the ALG NAT table
and is reused to perform the translation.
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Update
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Used to open pinhole for new or updated SDP information
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The Via, From, To, Call-ID, Contact, Route, and Record-Route
header fields are modified.
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1xx, 202, 2xx, 3xx, 4xx, 5xx, 6xx Response Codes
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Used to indicate the status of a transaction.
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