n proposed consists For further details on the path header filed, please refer to RFC 3327 [13]. C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security in having the edge proxies relaying all the SIP messages. When the edge prox- ies are not present, the SIP proxy/registrar itself must add a record route header field to the dialog forming request, since this specification needs to have mid-dialog requests routed over the same flow established through the registration. 12.5.3 Traversal for Media Flows: ICE Interactive Connectivity Establishment (ICE) [7] (developed by the Multipart Multimedia Session Control working group [8]) is used for NAT traversal of UDP-based multimedia sessions using an offer/answer model. ICE makes use of STUN and TURN and basically extends the Session Description Protocol (SDP) with the insertion of multiple network addresses and ports into SDP offers and answers. Consequently, ICE can be applied to all protocols making use of the SDP offer/answer mechanism, among which SIP is one of the most important. However, ICE is not intended for NAT traversal in SIP. On the con- trary, it assumes that SIP messages can be exchanged by UAs. Hence, in case NATs are present, UAs have to ensure SIP connectivity as discussed above. Figure 12.15 depicts a typical scenario for ICE-capable UAs. Before try- ing to establish a media session, each UA gathers all its relevant transport addresses in order to insert the MIN the SDP payloads as candidates. The first available candidate is obviously the one related to a physical or logical interface of the UA, which is called a host candidate. Multi-homed hosts will have multiple host candidates. Typically, each UA will request at least a pub- lic transport address to a TURN server, which is called a relayed candidate. A UA also tries to discover its external transport address using a STUN server. The resulting address is called a server reflexive candidate as it is a transport server proxy server Active signaling flow Active signa