Nortel Networks NN10029-111 Back-to-Back User Agent service UserA, Agent Client Internal Protocol

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—changing IP addresses in the contact header so that the SIP Application Module remains on the signaling path

—modifying the Session Description Protocol (SDP) using values supplied by the RTP Media Portal to control media endpoints

—providing advanced screening capabilities

The architecture of a BBUA service consists of two user agent clients linked back-to-back through a proprietary interface.

The BBUA is guaranteed to be on the signaling path of all future requests and responses because it is an endpoint relative to the SIP network components. This is important for services such as billing, which need to be aware of all events that take place on a session. The BBUA in the network also provides a barrier for clients that are not fully SIP compliant and entry and exit points for traffic travelling to and from the public network, including agents behind an enterprise firewall. See Figure 1, “Back-to-Back User Agent service.”

Figure 1 Back-to-Back User Agent service

userA	User	User	userB
	Agent	Agent
	Client	Client
	Internal Protocol

Routing in a SIP network is based on the same hop-by-hop principle as routing e-mail within the Internet. The next hop for a SIP request is determined by a proxy using the domain or the host part of a SIP URL (user@domain). The terminating proxy determines whether the domain sent in the SIP URL is one of the domains managed by the SIP proxy. Otherwise, the SIP request is forwarded to another Proxy based on the location lookup performed by the

SIP Application Module. The SIP Application Module supports routing using table lookup in the SIP database or using the Domain Name Server (DNS) to find a route.

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