nd about 6-7 round-trip times (RTT). In contrast, SIP reuses many HTTP header fields, encoding rules, error codes, and authentication mechanisms to simplify multimedia conferencing over the Internet. Defined in 736 pages, H.323 defines hundreds of elements; defined in only 128 pages, SIP contains 37 headers. One can implement a SIP call with only the headers To, From, Call-ID, and CSeq, and the request types INVITE, ACK, and BYE. While H.323 represents its messages in binary, SIP (like HTTP) encodes messages as text. Moreover, SIP allows for HTTP code reuse SO that existing HTTP parsers can be quickly modified for its use. H.323 was not designed to perform loop detection in multi-domain searches. It requires gatekeepers to maintain information about a session (referred to as state) for that session's entire duration. For service providers, this is impractical. SIP leverages a loop detection algorithm that is similar to that in BGP that allows a gatekeeper to forget about an established session by using UDP. SIP also allows for stateful sessions over TCP. Therefore, SIP scales much more efficiently. In addition, unlike H.323, SIP does not require a central control point that must remain in the session to provide functionality that the session requires. Members in a SIP session can communicate via multicast, unicast, or a combination of the two. SIP can be used to initiate sessions, or to invite members (or to add media) to sessions that have been advertised and established by multicast protocols such as SAP, Email, news groups, web pages, or directories (e.g., LDAP). SIP does not require a session initiator to participate in any session that she (or it) initiates. SIP transparently supports name mapping and redirection services, and thereby allows the implementation of ISDN and Intelligent Network telephony subscriber services. SIP is an enabling technology for personal mobility, or the ability of end users to originate and receive calls and access subscribed telecommunication services on any terminal in any locat