The REGISTER function is used in a SIP system primarily to associate a temporary contact address with an address-of-record. This contact is generally in the form of a URI, such as Contact: <sip:alice@pc33.atlanta.com> and is generally dynamic and associated with the IP address or hostname of the SIP UA. The problem is that network topology may have one or more SIP proxies between the UA and the registrar, such that any request travelling from the user's home network to the registered UA must traverse these proxies. The REGISTER method does not give us a mechanism to discover and record this sequence of proxies in the registrar for future use. This document defines an extension header field, 'Path' which provides such a mechanism