services. Also, end-point resident services are only avail- able when the end point is connected to the Internet. A terminating service such as a call forwarding service implemented in an end point will fail if the end point has temporarily lost its Internet connection. For this and other reasons, some services are implemented in the network using SIP proxy servers. For these SIP: Understanding the Session Initiation Protocol services, service mobility for a user agent means that the same set of proxies are used to route incoming and outgoing requests when mobile. Due to the nature of the Internet, in general, there is no reason why a user agent cannot use the same proxies when connected to the Internet at a different point. That is, a user agent that is normally in the United States that is config- ured to use a set of proxies in the United States can still use those proxies when roaming in Europe, for example. Perhaps the SIP hops will have a slightly higher latency due to more router hops and a call setup request may take a second or two longer to complete. However, this has no impact on the quality of the media session as the media always flows directly between the two user agents and does not traverse the SIP proxy servers. As a result, SIP can easily support service mobility over the Internet. However, there are some cases in which this service mobility approach will not work. For example, if a local proxy server must be traversed in order to facilitate firewall or NAT traversal, or for some other security reason, then a user agent may have to use a different first hop proxy when roaming. In this case, service mobility is still possible provided that: 1. The roaming user agent is able to discover the necessary local proxy. 2. Both incoming and outgoing requests are routed through the home proxy in addition to any local proxies. The first requirement is met by the DHCP extension to SIP [5], which allows a user agent to learn of a local proxy server at the same time it learns its IP address and other IP confi