gement in con- ferencing vary depending on the communication model, i.e., the use of unicast or multicast. 6.2.4.1 Unicast-Based Mobility A tightly coupled conference scenario is likely to operate in unicast mode. This does not only hold for SIP signaling relations, but may also apply for media distribution in a central mixing or conference bridge scheme. On handover, any mobile member changing within a given IP subnet may just continue operation on upper layers, any disturbance being bound to disconnection times on the network access layer. In cases where movement occurs between providers or internal subnets, e.g., triggered by a vertical handover between different access technologies, the mobile conference member changes its IP address and needs to procure conference session persistence through additional means. Session continuation can be accomplished by SIP mobility management as described in Chapter 4 and Chapter 17. Alternatively, transparent handover management may be operated on the transport layer, provided (mobile) SCTP is in use. The most general approach to network layer mobility resides in IPv4/v6 protocol extensions, which have been the focus of IETF work over recent years (see [39] for an excellent discussion). In conferencing situations, special care is needed whenever the mobile node performs internal relaying, as its disconnection will cause data delay or damage for downstream members. Such threat of service degradation is obvious for a peer-managed focus or mixer in tightly coupled conferences, but may result C 2009 by Taylor & Francis Group, LLC SIP Handbook: Services, Technologies, and Security from routing and forwarding functions in distributed settings or application layer multicast, as well. 6.2.4.2 Mobile Multicast Multicast mobility management must be assured for mobile user agents when- ever native multicast routing serves for media or signaling distribution. IP multicasting is of particular importance to mobile environments, where users commonly share frequency bands of limited capaci