Cisco Cisco Cisco The User Agent capabilities specification allows Session Initiation Protocol (SIP) User Agents to convey their capabilities and characteristics to other User Agents and to the registrar for its domain. This information is conveyed as parameters of the Contact header field. Amongst those capabilities are the type of User Agent that is available at a SIP Uniform Resource Identifier (URI). This document extends the User Agent capabilities specification to allow indication of Back-to-Back User Agent (B2BUA) types.