SIP ALG Overview

Session Initiation Protocol (SIP) is an Internet Engineering Task Force (IETF) standard protocol for initiating, modifying, and terminating multimedia sessions over the Internet. Such sessions might include conferencing, telephony, or multimedia, with features such as instant messaging and application-level mobility in network environments.

Juniper Networks security devices support SIP as a service and can screen SIP traffic, allowing and denying it based on a policy that you configure. SIP is a predefined service in ScreenOS and uses port 5060 as the destination port.

SIP’s primary function is to distribute session-description information and, during the session, to negotiate and modify the parameters of the session. SIP is also used to terminate a multimedia session.

Session-description information is included in INVITE and ACK messages and indicates the multimedia type of the session, for example, voice or video. Although SIP can use different description protocols to describe the session, the Juniper Networks SIP ALG supports only Session Description Protocol (SDP).

SDP provides information that a system can use to join a multimedia session. SDP might include information such as IP addresses, port numbers, times, and dates. Note that the IP address and port number in the SDP header (the “c=” and “m=” fields, respectively) are the address and port where the client wants to receive the media streams and not the IP address and port number from which the SIP request originates (although they can be the same). See SDP Session Description Overview for more information.

SIP messages consist of requests from a client to a server and responses to the requests from a server to a client with the purpose of establishing a session (or a call). A User Agent (UA) is an application that runs at the endpoints of the call and consists of two parts: the User Agent Client (UAC), which sends SIP requests on behalf of the user; and a User Agent Server (UAS), which listens to the responses and notifies the user when they arrive. Examples of UAs are SIP proxy servers and phones.

Related Documentation