Is Work is currently underway to try to provide a mechanism to guarantee that a Contact URI is globally routable. While in general, a Contact URI is glob- ally routable, in the presence of NATs, firewalls, and screening proxies, this may not be SO. Contributing to the problem is that a UA may not know if its Contact URI is globally routable or not. Current work is discussing the requirements, which, when agreed upon, can be translated into a SIP extension or convention to solve. 11.2.3 Service Examples This service examples document [8] shows how intelligent end points can implement a set of service commonly found in PBX switches and in service provider Centrex offerings. The set of features includes call hold, call park, call SIP: Understanding the Session Initiation Protocol pickup, auto recall, music on hold, attended and unattended transfer, and oth- ers. All of the flows use standard SIP operations and call control extensions. 11.3 SIP Instant Message and Presence Work In the SIMPLE working group, there has been much work in the area of instant message sessions, similar to that shown in Figure 4.10 in which an INVITE sets up an IM session between the end points. The current approach being developed uses a SIP-like protocol called Message Session Relay Protocol (MSRP) [9], which allows the IM session to be established either directly end- to-end or through a relay necessary for logging or NAT/firewall traversal. The SIMPLE working group is also working on documents to enable all aspects of a standard presence and IM client to be implemented using standard IETF protocols. While the basic ability is provided through the defined MESSAGE, SUBSCRIBE, and NOTIFY methods, tools to upload or publish presence information, and protocols for "buddy list" creation and manipulation are still needed. Other interesting work continues in the area of "rich" presence informa- tion delivery using SIP [10]. The current presence XML formats only list con- tacts and describe them as "open" or "closed," essentially on-line or off-l