SIP ALG Request Methods Overview

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.

JUNOS software with enhanced services supports the following method types and response codes:

• 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.
• 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.
• OPTIONS—Used by the User Agent (UA) 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.
• BYE—A user sends a BYE request to abandon a session. A BYE request from either user automatically terminates the session.
• CANCEL—A user can send 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.
• 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.
• Info—Used to communicate mid-session signaling information along the signaling path for the call.
• Subscribe—Used to request current state and state updates from a remote node.
• Notify—Sent to inform subscribers of changes in state to which the subscriber has a subscription.
• Refer—Used to refer the recipient (identified by the Request-URI) to a third party by the contact information provided in the request.

  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. Header fields are modified.