terogeneous SIP user agents. In the case of full dual stack deployment, this objective can be considered as obsolete and service providers could judge appropriate to exclusively restrict their service offerings to IPv6 (i.e., Only IPv6 SIP support is considered as valid goal) to speed up the migration process. Nevertheless, this information is not available for "Virtual" service providers because they have no control of the underly- ing infrastructure. When enforcing this objective, the following requirements should be addressed: 1- Ability of heterogeneous customers to place SIP Calls: This means that an IPv4 and IPv6-enabled SIP user agents can place successful SIP calls between them even if there is a break of realm type. The deployed solution should act as a "broker," ensuring the call is successful. Furthermore, the solution adopted to implement this feature should satisfy the following constraints: 1-1- The nature of the call should be transparent for the call participants: This means that the complexity of establishing this type of SIP session should be maintained in the service side and that the SIP user agents believe that the call is issued by a homogenous party (i.e., a customer in the same IP realm type). C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security The customer should not be aware that the call is traversing heterogeneous realms. 1-2- The perceived Q0S of the call should not be severely altered compared to homogenous calls: This requirement expresses that the deployed solution should not have a negative impact on the end-to-end QoS observed by the call participants. In other words, the IP transport interworking mechanisms should be deployed in such a way the path of RTP flow is optimal and no significant delay is introduced due to protocol translation operations. This requirement is to be taken into account when deploying Intermediate nodes that will act as relays between IPv4 and IPv6 realms. Routing should be configured in order to select the shortest pat