http://www.rfc-editor.org/rfc/rfc4092.txt
RFC 4092: Usage of the Session Description Protocol (SDP) Alternative Network Address Types (ANAT) Semantics in the Session Initiation Protocol (SIP), G. Camarillo, J. Rosenberg2005-06-01T23:00:00-00:00This document describes how to use the Alternative Network Address Types (ANAT) semantics of the Session Description Protocol (SDP) grouping framework in SIP. In particular, we define the sdp-anat SIP option-tag. This SIP option-tag ensures that SDP session descriptions that use ANAT are only handled by SIP entities with ANAT support. To justify the need for such a SIP option-tag, we describe what could possibly happen if an ANAT-unaware SIP entity tried to handle media lines grouped with ANAT. [STANDARDS-TRACK]