multicast.tex

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central CtrMcastComp agent to compute and install multicast routes for
the entire topology.  Each CtrMcast agent processes membership dynamic
commands, and redirects the CtrMcastComp agent to recompute the
appropriate routes.
\begin{alist}
\proc[]{join-group} &
	registers the new member with the \code{CtrMCastComp} agent, and
	invokes that agent to recompute routes for that member. \\
\proc[]{leave-group} & is the inverse of \proc[]{join-group}. \\
\proc[]{handle-cache-miss} &
	 called when no proper forwarding entry is found
	 for a particular packet source.
	In case of centralized multicast,
	it means a new source has started sending data packets.
	Thus, the CtrMcast agent registers this new source with the
	\code{CtrMcastComp} agent.
	If there are any members in that group, compute the new multicast tree.
	If the group is in RPT (shared tree) mode, then
	\begin{enumerate} 
	\item create an encapsulation agent at the source;
	\item a corresponding decapsulation agent is created at the RP;
	\item the two agents are connected by unicast; and
	\item the \tup{S,G} entry points its outgoing interface to the
	      encapsulation agent.
	\end{enumerate}
\end{alist}

\code{CtrMcastComp} is the centralised multicast route computation agent.
\begin{alist}
\proc[]{reset-mroutes} & 
	resets all multicast forwarding entries.\\
\proc[]{compute-mroutes} & 
	(re)computes all multicast forwarding entries.\\
\proc[source, group]{compute-tree} & 
	computes a multicast tree for one source to reach all the
	receivers in a specific group.\\ 
\proc[source, group, member]{compute-branch} & 
	is executed when a receiver joins a multicast group.  It could
        also be invoked by \proc[]{compute-tree} when it itself is
        recomputing the multicast tree, and has to reparent all
        receivers.  The algorithm starts at the receiver, recursively
        finding successive next hops, until it either reaches the
        source or RP, or it reaches a node that is already a part of
        the relevant multicast tree.  During the process, several new
        replicators and an outgoing interface will be installed.\\
\proc[source, group, member]{prune-branch} & 
	is similar to \proc[]{compute-branch} except the outgoing
        interface is disabled; if the outgoing interface list is empty
        at that node, it will walk up the multicast tree, pruning at
        each of the intermediate nodes, until it reaches a node that
        has a non-empty outgoing interface list for the particular
        multicast tree.
\end{alist}

\subsubsection{Dense Mode}
\begin{alist}
\proc[group]{join-group} & 
	sends graft messages upstream if \tup{S,G} does not contain
        any active outgoing slots (\ie, no downstream receivers).
        If the next hop towards the source is a LAN, icrements a
        counter of receivers for a particular group for the LAN\\
\proc[group]{leave-group} & 
	decrements LAN counters. \\
\proc[srcID group iface]{handle-cache-miss} & 
	depending on the value of \code{CacheMissMode} calls either
        \code{handle-cache-miss-pimdm} or
        \code{handle-cache-miss-dvmrp}. \\
\proc[srcID group iface]{handle-cache-miss-pimdm} &   
	if the packet was received on the correct iif (from the node
        that is the next hop towards the source), fan out the packet
        on all oifs except the oif that leads back to the
        next--hop--neighbor and oifs that are LANs for which this node
        is not a forwarder. Otherwise, if the interface was incorrect,
        send a prune back.\\
\proc[srcID group iface]{handle-cache-miss-dvmrp} &
	fans out the packet only to nodes for which this node is a
	next hop towards the source (parent).\\
\proc[replicator source group iface]{drop} & 
	sends a prune message back to the previous hop.\\
\proc[from source group iface]{recv-prune} & 
	resets the prune timer if the interface had been pruned
        previously; otherwise, it starts the prune timer and disables
        the interface; furthermore, if the outgoing interface list
        becomes empty, it propagates the prune message upstream.\\
\proc[from source group iface]{recv-graft} & 
	cancels any existing prune timer, andre-enables the pruned
        interface.  If the outgoing interface list was previously
        empty, it forwards the graft upstream.\\
\proc[srcID group iface]{handle-wrong-iif} & 
        This is invoked when the multicast classifier drops a packet
        because it arrived on the wrong interface, and invoked
        \proc[]{new-group}.  This routine is invoked by
        \proc[]{mrtObject~instproc~new-group}.  When invoked, it sends
        a prune message back to the source.\\
\end{alist}

\subsection{The internal variables}
\begin{alist}
\textbf{Class mrtObject}\hfill & \\
\code{protocols\_} &
	An array of handles of protocol instances active at the node
	at which this protocol operates indexed by incoming
	interface. \\
\code{mask-wkgroups} &
	Class variable---defines the mask used to identify well known
	groups. \\
\code{wkgroups} &
	Class array variable---array of allocated well known groups
	addresses, indexed by the group name.  \code{wkgroups}(Allocd)
	is a special variable indicating the highest currently
	allocated well known group. \\[3ex]

\textbf{McastProtocol}\hfill & \\
\code{status\_} &
	takes values ``up'' or ``down'', to indicate the status of
	execution of the protocol instance. \\
\code{type\_} &
	contains the type (class name) of protocol executed by this
	instance, \eg, DM, or ST. \\

\textbf{Simulator}\hfill & \\
\code{multiSim\_} &
	1 if multicast simulation is enabled, 0 otherwise.\\
\code{MrtHandle\_} &
	handle to the centralised multicast simulation object.\\[3ex]

\textbf{Node}\hfill & \\
\code{switch\_} & 
	handle for classifier that looks at the high bit of the
	destination address in each packet to determine whether it is
	a multicast packet (bit = 1) or a unicast packet (bit = 0).\\
\code{multiclassifier\_} & 
	handle to classifier that performs the \tup{s, g, iif} match. \\
\code{replicator\_} & 
	array indexed by \tup{s, g} of handles that replicate a
	multicast packet on to the required links. \\
\code{Agents\_} & 
	array indexed by multicast group of the list of agents at the
	local node that listen to the specific group. \\
\code{outLink\_} & 
	Cached list of outgoing interfaces at this node.\\
\code{inLink\_} &
	Cached list of incoming interfaces at this node.\\

\textbf{Link} and \textbf{SimpleLink}\hfill & \\
\code{iif\_} & 
	handle for the NetworkInterface object placed on this link.\\
\code{head\_} & 
	first object on the link, a no-op connector.  However, this
	object contains the instance variable, \code{link\_}, that
	points to the container Link object.\\

\textbf{NetworkInterface}\hfill & \\
\code{ifacenum\_} & 
	Class variable---holds the next available interface
	number.\\
\end{alist}


\section{Commands at a glance}
\label{sec:mcastcommand}

Following is a list of commands used for multicast simulations:
\begin{flushleft}
\code{set ns [new Simulator -mcast on]}\\
This turns the multicast flag on for the the given simulation, at the time of
creation of the simulator object.


\code{ns_ multicast}\\
This like the command above turns the multicast flag on.


\code{ns_ multicast?}\\
This returns true if multicast flag has been turned on for the simulation
and returns false if multicast is not turned on.


\code{$ns_ mrtproto <mproto> <optional:nodelist>}\\
This command specifies the type of multicast protocol <mproto> to be used
like DM, CtrMcast etc. As an additional argument, the list of nodes <nodelist>
that will run an instance of detailed routing protocol (other than
centralised mcast) can also be passed.


\code{$ns_ mrtproto-iifs <mproto> <node> <iifs>}\\
This command allows a finer control than mrtproto. Since multiple mcast
protocols can be run at a node, this command specifies which mcast protocol
<mproto> to run at which of the incoming interfaces given by <iifs> in the <node>.


\code{Node allocaddr}\\
This returns a new/unused multicast address that may be used to assign a multicast
address to a group.


\code{Node expandaddr}\\
This command expands the address space from 16 bits to 30 bits. However this
command has been replaced by \code{"ns_ set-address-format-expanded"}.


\code{$node_ join-group <agent> <grp>}\\
This command is used when the <agent> at the node joins a particular group <grp>.


\code{$node_ leave-group <agent> <grp>}\\
This is used when the <agent> at the nodes decides to leave the group <grp>.

Internal methods:\\

\code{$ns_ run-mcast}\\
This command starts multicast routing at all nodes. 


\code{$ns_ clear-mcast}\\
This stopd mcast routing at all nodes.


\code{$node_ enable-mcast <sim>}\\
This allows special mcast supporting mechanisms (like a mcast classifier) to
be added to the mcast-enabled node. <sim> is the a handle to the simulator
object.

In addition to the internal methods listed here there are other methods specific to
protocols like centralized mcast (CtrMcast), dense mode (DM), shared tree
mode (ST) or bi-directional shared tree mode (BST), Node and Link class
methods and NetworkInterface and Multicast classifier methods specific to
multicast routing. All mcast related files may be found under
\ns/tcl/mcast directory. 
\begin{description}

\item[Centralised Multicast] A handle to the CtrMcastComp object is
returned when the protocol is specified as `CtrMcast' in mrtproto. 
Ctrmcast methods are: \\

\code{$ctrmcastcomp switch-treetype group-addr}\\
Switch from the Rendezvous Point rooted shared tree to source-specific
trees for the group specified by group-addr. Note that this method cannot
be used to switch from source-specific trees back to a shared tree for a
multicast group. 

\code{$ctrmcastcomp set_c_rp <node-list>}\\
This sets the RPs.

\code{$ctrmcastcomp set_c_bsr <node0:0> <node1:1>}\\
This sets the BSR, specified as list of node:priority.

\code{$ctrmcastcomp get_c_rp <node> <group>}\\
Returns the RP for the group as seen by the node node for the multicast
group with address group-addr. Note that different nodes may see different
RPs for the group if the network is partitioned as the nodes might be in
different partitions. 

\code{$ctrmcastcomp get_c_bsr <node>}\\
Returns the current BSR for the group.

\code{$ctrmcastcomp compute-mroutes}\\
This recomputes multicast routes in the event of network dynamics or a
change in unicast routes.


\item[Dense Mode]
The dense mode (DM) protocol can be run as PIM-DM (default) or DVMRP
depending on the class variable \code{CacheMissMode}. There are no methods
specific to this mcast protocol object. Class variables are:
 \begin{description}
   \item[PruneTimeout] Timeout value for prune state at nodes. defaults to
0.5sec.
   \item[CacheMissMode] Used to set PIM-DM or DVMRP type forwarding rules.
 \end{description}


\item[Shared Tree]
There are no methods for this class. Variables are:
\begin{description}
\item[RP\_] RP\_ indexed by group determines which node is the RP for a
particular group.
\end{description}


\item[Bidirectional Shared Tree]
There are no methods for this class. Variable is same as that of Shared
Tree described above.

\end{description}

\end{flushleft}

\endinput