ACT header (the new attribute is written in bold): Contact= ("Contact"/"m") HCOLON (STAR / (contact-param * (COMMA contact-param) ) ) COMMA atypes ! For all attributes except atypes>>, refer to (Rosenberg 2002) atypes = "atypes" EQUAL 4/6/0 During the registration phase, a SIP user agent sends a REGISTER message with the new modified format of the CONTACT header, especially with the new atypes attribute. This tag may be positioned to 4 in order to indicate that the SIP user agent is IPv4-only (respectively 6 for IPv6-only and 0 for dual- stack). A given SIP user agent may restrict its SIP communications to its IPv4 interface or IPv6 ones or may use all available ones. Its local preference is conveyed in the atypes attribute. Within this context, an IPv6 interface is judged available only if the scope of this interface is global and not local to the link. After receiving the REGISTER message, the SIP Registrar Server stores, in addition to the AOR (Address Of Record) and the expire timer, the value of the atypes tag. This information may be refreshed by other upcoming REGISTER messages. Consequently, this new registration procedure replaces the ones defined in (Rosenberg 2002). Based on atypes tag value, the SIP Registrar Server classifies its customers to three categories: IPv4-only, IPv6- only and dual-stack ones. The following provides an example of a REGISTER message atypes tag: REGISTER sip:r.test.biz SIP/2.0 Via: SIP/2.0/UDP 192.168.25.5:5062; branch=z9hG4bK00e31d6ed Max-Forwards: 70 Content-Length: 0 To: DS <sip: DS@test.biz> From: DS <sip:DS@test.biz>; tag=ed3833bd768 Call-ID: 422fd431sdsd3878b7f1d8@test.biz CSeq: 425542 REGISTER Contact:DS<sip:DS@192.168.25.5> expires=90, atypes=0 © 2009 by Taylor & Francis Group, LLC SIP and IPv6 In order to route calls and to decide the need to invoke a SIP ALG or to alter SIP messages, which leads to a successful call between heterogeneous parties, a SIP proxy server may act as follows: - A first alternative is to question the registration database maintained by the re