mation from the called party. The modified SDP would point to the B2BUA itself, which would forward RTP media packets from the called party to the calling party and vice- versa. In this way, neither end point learns any identifying information about the other party during the session establishment. (Of course, the calling party needs to know the called party's URI in order for the call to take place.) Sometimes B2BUAs are employed to implement other SIP services. How- ever, they break the end-to-end nature of an Internet protocol such as SIP. Also, a B2BUA is a call-stateful single point of failure in a network, which means their use will reduce the reliability of SIP sessions over the Internet. The relayed media suf- fers from increased latency and increased probability of packet loss, which can reduce the quality of the media session. Geographic distribution of B2BUAs can reduce these effects, but the problem of selecting the best B2BUA for a particular session is a very difficult one since the source and destination IP address of the media are not known until the session is actually established (with a 200 OK). The most common form of B2BUA present in SIP networks is application layer gateways (ALG). Some firewalls have ALG functionality built in, which allows a firewall to permit SIP and media traffic while still maintaining a high level of security. The use of ALGs is described in Section 3.11. 3.4 SIP Gateways A SIP gateway is an application that interfaces a SIP network to a network utiliz- ing another signaling protocol. In terms of the SIP protocol, a gateway is just a SIP: Understanding the Session Initiation Protocol special type of user agent, where the user agent acts on behalf of another proto- col rather than a human. A gateway terminates the signaling path and can also terminate the media path, although this is not always the case. For example, a SIP to H.323 gateway terminates the SIP signaling path and converts the signal- ing to H.323, but the SIP user agent and H.323 terminal can exchange RTP 