Design and investigation of scalable multicast recursive protocols for wired and wireless ad hoc networks

On the other hand, Xcast has the following disadvantage:-

> Overhead. All the nodes in the neighbourhood that receive the Xcast packet need to
process the header to check if it is one of the next hops included in the packet. This is
a major overhead in terms of processing and delay.

> The huge increase in the packet header when the number of receivers increases. This
increase will cause consumes in bandwidth and delay in delivering packets.

As discussed above, Xcast is suitable for small groups because there is no need to maintain
information at intermediate nodes. Some modifications have been made to overcome Xcast
drawbacks; Xcast+ is one of these.

3.2.2 Explicit Multicast Extension (Xcast+)

Xcast+ [61] is a multicast routing protocol, that is an extension of Explicit Multicast Xcast
to achieve an efficient packet delivery of multicast packets. This is achieved by adding an
Internet Group Multicast Protocol (IGMP) [36] join at the receiver’s side and sending a
registration request message from the Designated Router (DR) received IGMP(S,G) join to
the sender, and by encoding the (DRs) instead of encoding the receivers themselves as
happened at Xcast.

In Xcast+, the receiver sends an IGMP(S,G) join, when the adjacent router -Designated
Router (DR) - receives this join message it initiates a registration request message to the
source. The source upon receiving the registration message replies by a registration reply
message to the DRs to acknowledge the receiving of the registration request message. The
source will keep track of the DRs addresses involved in the multicast session (S,G) in its
cache table. When the source needs to sent Xcast+ packets, the sender and all other
intermediate nodes follows the same process implemented in Xcast, except for encoding the
addresses of Designated Routers DRs in the data packet instead of the addresses of the
receivers. At the receiver’s side the Xcast+ packet is multicast to the receivers by DRs.

Figure 3.2 illustrates the Xcast+ process of delivering data packets. Receivers (A,B,C,D,E,F)
send a IGMP(S,G) join to the adjacent router; (A,B) to M1, (C) to M2 and (D,E,F) to M3.

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