Help us improve your experience.

Let us know what you think.

Do you have time for a two-minute survey?

 

SIP Request Methods Supported in ScreenOS Devices

 

The SIP transaction model includes a number of request and response messages, each of which contains a method field that denotes the purpose of the message. The method types and response codes supported by ScreenOS are listed in Table 1.

Table 1: SIP Request Methods

Method Types/Response Codes

Description

Modified Fields

INVITE

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.

In NAT mode, the IP addresses in the Via, From, To, Call-ID, Contact, Route, and Record-Route header fields are modified.

ACK

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.

In NAT mode, the IP addresses in the Via, From, To, Call-ID, Contact, Route, and Record-Route header fields are modified.

OPTIONS

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.

NA

BYE

A user sends a BYE request to abandon a session. A BYE request from either user automatically terminates the session.

In NAT mode, the IP addresses in the Via, From, To, Call-ID, Contact, Route, and Record-Route header fields are modified.

CANCEL

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.

In NAT mode, the IP addresses in the Via, From, To, Call-ID, Contact, Route, and Record-Route header fields are modified.

REGISTER

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.

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.

Info

Used to communicate mid-session signaling information along the signaling path for the call.

In NAT mode, the IP addresses in the Via, From, To, Call-ID, Contact, Route, and Record-Route header fields are modified.

Subscribe

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.

The IP addresses in Via, From, To, Call-ID, Contact, Route, and Record-Route header fields are modified.

Notify

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.

The IP address in the Via, From, To, Call-ID, Contact, Route, and Record-Route header fields are modified.

Refer

Used to refer the recipient (identified by the Request-URI) to a third party by the contact information provided in the request.

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.

Update

Used to open pinhole for new or updated SDP information

The Via, From, To, Call-ID, Contact, Route, and Record-Route header fields are modified.

1xx, 202, 2xx, 3xx, 4xx, 5xx, 6xx Response Codes

Used to indicate the status of a transaction.