HBH tree construction has the ability to solve the asymmetric problem found in multicast
routing protocols. HBH uses two tables; MCT and MFT, these two table are the same tables
used at REUNITE [65] protocol with the difference that each table entry at HBH contains the
address of the next branching node instead of the address of the receiver itself. The data
packet received by the branching node has a unicast destination address equals to the HBH
branching node address. Each multicast group is identified by a unique pair of addresses
(S,G), where S is the source node address and G is the group class-D IP address allocated by
the source.
3.2.4.1 Tree construction in HBH
HBH uses three types of messages: Join Tree and Fusion. Join message is sent towards the
source by a receiver which prefers to join the multicast group, this message is sent
periodically in order to refresh the MFT entry in the branching node where this receiver is
connected. Tree message is sent periodically by the source to finish and refresh the
construction of multicast tree and to find possibly different path to the receiver which sent
the join message. Fusion message is sent by a potential branching node to construct the
distribution of the tree along with tree message.
Each router at HBH constructed tree maintains either MCT or MFT tables. Non-branching
routers maintain a single MCT entry which contains the source and the receiver addresses
which the later sends the join message through this router. Each branching router maintains a
MFT table; which contains entries to the addresses of the receivers and/or the addresses of
the next branching router side by side to the source address that is the first branching node in
the network. The first join message send by the receiver goes directly to the source and does
not intercepted by any intermediate router. Each entry on both tables is associated by two
timers, (t1,t2). With expiry of t1 the table entry becomes stale, i.e used for data forwarding
but no tree messages is sent to downstream nodes; and when t2 expires the entry is
considered as destroyed. The basic ideas are: the join message is sent to the source and never
intercepted by any intermediate nodes; the tree message is periodically send by the source;
these are combined with fusion messages sent by potentials nodes to construct and refine the
tree structure.
41
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