\chapter{Multicast Routing}
\label{chap:multicast}

This section describes the usage and the internals of multicast
routing implementation in \ns.
We first describe 
\href{the user interface to enable multicast routing}{Section}{sec:mcast-api},
specify the multicast routing protocol to be used and the
various methods and configuration parameters specific to the
protocols currently supported in \ns.
We then describe in detail 
\href{the internals and the architecture of the
multicast routing implementation in \ns}{Section}{sec:mcast-internals}.

The procedures and functions described in this chapter can be found in
various files in the directories \nsf{tcl/mcast}, \nsf{tcl/ctr-mcast};
additional support routines
are in \nsf{mcast\_ctrl.\{cc,h\}},
\nsf{tcl/lib/ns-lib.tcl}, and \nsf{tcl/lib/ns-node.tcl}.

\section{Multicast API}
\label{sec:mcast-api}

Multicast forwarding requires enhancements
to the nodes and links in the topology.
Therefore, the user must specify multicast requirements
to the Simulator class before creating the topology.
This is done in one of two ways:
\begin{program}
        set ns [new Simulator -multicast on]
    {\rm or}
        set ns [new Simulator]
        $ns multicast
\end{program}                   %$
When multicast extensions are thus enabled, nodes will be created with
additional classifiers and replicators for multicast forwarding, and
links will contain elements to assign incoming interface labels to all
packets entering a node.

A multicast routing strategy is the mechanism by which
the multicast distribution tree is computed in the simulation.
\ns\ supports three multiast route computation strategies:
        centralised, dense mode(DM) or shared tree mode(ST).

The method \proc[]{mrtproto} in the Class Simulator specifies either
the route computation strategy, for centralised multicast routing, or
the specific detailed multicast routing protocol that should be used.

%%For detailed multicast routing, \proc[]{mrtproto} will accept, as
%%additional arguments, a list of nodes that will run an instance of
%%that routing protocol.  
%%Polly Huang Wed Oct 13 09:58:40 EDT 199: the above statement
%%is no longer supported.

The following are examples of valid
invocations of multicast routing in \ns:
\begin{program}
        set cmc [$ns mrtproto CtrMcast]    \; specify centralized multicast for all nodes;
        \; cmc is the handle for multicast protocol object;
        $ns mrtproto DM                   \; specify dense mode multicast for all nodes;
        $ns mrtproto ST                  \; specify shared tree mode to run on all nodes;
\end{program}
Notice in the above examples that CtrMcast returns a handle that can
be used for additional configuration of centralised multicast routing.
The other routing protocols will return a null string.  All the
nodes in the topology will run instances of the same protocol.

Multiple multicast routing protocols can be run at a node, but in this
case the user must specify which protocol owns which incoming
interface.  For this finer control \proc[]{mrtproto-iifs} is used.

New/unused multicast address are allocated using the procedure
\proc[]{allocaddr}.
%%The default configuration in \ns\ provides 32 bits each for node address and port address space.
%%The procedure
%%\proc[]{expandaddr} is now obsoleted.

The agents use the instance procedures
\proc[]{join-group} and \proc[]{leave-group}, in
the class Node to join and leave multicast groups. These procedures
take two mandatory arguments. The first argument identifies the
corresponding agent and second argument specifies the group address.

An example of a relatively simple multicast configuration is:
\begin{program}
        set ns [new Simulator {\bfseries{}-multicast on}] \; enable multicast routing;
        set group [{\bfseries{}Node allocaddr}]   \; allocate a multicast address;
        set node0 [$ns node]         \; create multicast capable nodes;
        set node1 [$ns node]
        $ns duplex-link $node0 $node1 1.5Mb 10ms DropTail

        set mproto DM          \; configure multicast protocol;
        set mrthandle [{\bfseries{}$ns mrtproto $mproto}] \; all nodes will contain multicast protocol agents;
        set udp [new Agent/UDP]         \; create a source agent at node 0;
        $ns attach-agent $node0 $udp 
        set src [new Application/Traffic/CBR]        
        $src attach-agent $udp
        {\bfseries{}$udp set dst_addr_ $group}
        {\bfseries{}$udp set dst_port_ 0}

        set rcvr [new Agent/LossMonitor]  \; create a receiver agent at node 1;
        $ns attach-agent $node1 $rcvr
        $ns at 0.3 "{\bfseries{}$node1 join-group $rcvr $group}" \; join the group at simulation time 0.3 (sec);
\end{program} %$

\subsection{Multicast Behavior Monitor Configuration}
\ns\ supports a multicast monitor module that can trace
user-defined packet activity.
The module counts the number of packets in transit periodically
and prints the results to specified files. \proc[]{attach} enables a 
monitor module to print output to a file. 
\proc[]{trace-topo} insets monitor modules into all links. 
\proc[]{filter} allows accounting on specified packet header, 
field in the header), and value for the field).  Calling \proc[]{filter}
repeatedly will result in an AND effect on the filtering condition.
\proc[]{print-trace} notifies the monitor module to begin dumping data.
\code{ptype()} is a global arrary that takes a packet type name (as seen in
\proc[]{trace-all} output) and maps it into the corresponding value.  
A simple configuration to filter cbr packets on a particular group is:

\begin{program}
        set mcastmonitor [new McastMonitor]
        set chan [open cbr.tr w] \; open trace file;
        $mmonitor attach $chan1  \; attach trace file to McastMoniotor object;
        $mcastmonitor set period_ 0.02         \; default 0.03 (sec);
        $mmonitor trace-topo   \; trace entire topology;
        $mmonitor filter Common ptype_ $ptype(cbr) \; filter on ptype_ in Common header;
        $mmonitor filter IP dst_ $group \; AND filter on dst_ address in IP header;
        $mmonitor print-trace  \; begin dumping periodic traces to specified files;

\end{program} %$

% SAMPLE OUTPUT?
The following sample output illustrates the output file format (time, count):
{\small
\begin{verbatim}
0.16 0
0.17999999999999999 0
0.19999999999999998 0
0.21999999999999997 6
0.23999999999999996 11
0.25999999999999995 12
0.27999999999999997 12
\end{verbatim}
}

\subsection{Protocol Specific configuration}

In this section, we briefly illustrate the
protocol specific configuration mechanisms
for all the protocols implemented in \ns.

\paragraph{Centralized Multicast}
The centralized multicast is a sparse mode implementation of multicast
similar to PIM-SM \cite{Deer94a:Architecture}.
A Rendezvous Point (RP) rooted shared tree is built
for a multicast group.  The actual sending of prune, join messages
etc. to set up state at the nodes is not simulated.  A centralized
computation agent is used to compute the forwarding trees and set up
multicast forwarding state, \tup{S, G} at the relevant nodes as new
receivers join a group.  Data packets from the senders to a group are
unicast to the RP.  Note that data packets from the senders are
unicast to the RP even if there are no receivers for the group.

The method of enabling centralised multicast routing in a simulation is:
\begin{program}
        set mproto CtrMcast    \; set multicast protocol;
        set mrthandle [$ns mrtproto $mproto] 
\end{program}
The command procedure \proc[]{mrtproto}
returns a handle to the multicast protocol object.
This handle can be used to control the RP and the boot-strap-router (BSR),
switch tree-types for a particular group,
from shared trees to source specific trees, and
recompute multicast routes.
\begin{program}
        $mrthandle set_c_rp $node0 $node1          \; set the RPs;
        $mrthandle set_c_bsr $node0:0 $node1:1     \; set the BSR, specified as list of node:priority;

        $mrthandle get_c_rp $node0 $group          \; get the current RP ???;
        $mrthandle get_c_bsr $node0                \; get the current BSR;

        $mrthandle switch-treetype $group         \; to source specific or shared tree;

        $mrthandle compute-mroutes       \; recompute routes. usually invoked automatically as needed;
\end{program}

Note that whenever network dynamics occur or unicast routing changes,
\proc[]{compute-mroutes} could be invoked to recompute the multicast routes.
The instantaneous re-computation feature of centralised algorithms
may result in causality violations during the transient
periods.

\paragraph{Dense Mode}
The Dense Mode protocol (\code{DM.tcl}) is an implementation of a
dense--mode--like protocol.  Depending on the value of DM class
variable \code{CacheMissMode} it can run in one of two modes.  If
\code{CacheMissMode} is set to \code{pimdm} (default), PIM-DM-like
forwarding rules will be used.  Alternatively, \code{CacheMissMode}
can be set to \code{dvmrp} (loosely based on DVMRP \cite{rfc1075}).
The main difference between these two modes is that DVMRP maintains
parent--child relationships among nodes to reduce the number of links
over which data packets are broadcast.  The implementation works on
point-to-point links as well as LANs and adapts to the network
dynamics (links going up and down).

Any node that receives data for a particular group for which it has no
downstream receivers, send a prune upstream.  A prune message causes
the upstream node to initiate prune state at that node.  The prune
state prevents that node from sending data for that group downstream
to the node that sent the original prune message while the state is
active.  The time duration for which a prune state is active is
configured through the DM class variable, \code{PruneTimeout}.  A
typical DM configuration is shown below:
\begin{program}
        DM set PruneTimeout 0.3           \; default 0.5 (sec);
        DM set CacheMissMode dvmrp        \; default pimdm;
        $ns mrtproto DM
\end{program} %$

\paragraph{Shared Tree Mode}
Simplified sparse mode \code{ST.tcl} is a version of a shared--tree
multicast protocol.  Class variable array \code{RP\_} indexed by group
determines which node is the RP for a particular group.  For example:
\begin{program}
        ST set RP_($group) $node0
        $ns mrtproto ST
\end{program}
At the time the multicast simulation is started, the protocol will
create and install encapsulator objects at nodes that have multicast
senders, decapsulator objects at RPs and connect them.  To join a
group, a node sends a graft message towards the RP of the group.  To
leave a group, it sends a prune message.  The protocol currently does
not support dynamic changes and LANs.

\paragraph{Bi-directional Shared Tree Mode}
\code{BST.tcl} is an experimental version of a bi--directional shared
tree protocol.  As in shared tree mode, RPs must be configured
manually by using the class array \code{RP\_}.  The protocol currently
does not support dynamic changes and LANs.

\section{Internals of Multicast Routing}
\label{sec:mcast-internals}

We describe the internals in three parts: first the classes to
implement and support multicast routing; second, the specific protocol
implementation details; and finally, provide a list of the variables
that are used in the implementations.

\subsection{The classes}
The main classes in the implementation are the
\clsref{mrtObject}{../ns-2/tcl/mcast/McastProto.tcl} and the
\clsref{McastProtocol}{../ns-2/tcl/mcast/McastProto.tcl}.  There are
also extensions to the base classes: Simulator, Node, Classifier,
\etc.  We describe these classes and extensions in this subsection.
The specific protocol implementations also use adjunct data structures
for specific tasks, such as timer mechanisms by detailed dense mode,
encapsulation/decapsulation agents for centralised multicast \etc.; we
defer the description of these objects to the section on the
description of the particular protocol itself.

\paragraph{mrtObject class}
There is one mrtObject (aka Arbiter) object per multicast capable
node.  This object supports the ability for a node to run multiple
multicast routing protocols by maintaining an array of multicast
protocols indexed by the incoming interface.  Thus, if there are
several multicast protocols at a node, each interface is owned by just
one protocol.  Therefore, this object supports the ability for a node
to run multiple multicast routing protocols.  The node uses the
arbiter to perform protocol actions, either to a specific protocol
instance active at that node, or to all protocol instances at that
node.
\begin{alist}
{\let\[=[\let\]=]
\proc[instance, \[iiflist\]]{addproto}} &
        adds the handle for a protocol instance to its array of
        protocols.  The second optional argument is the incoming
        interface list controlled by the protocol.  If this argument
        is an empty list or not specified, the protocol is assumed to
        run on all interfaces (just one protocol). \\
\proc[protocol]{getType} &
        returns the handle to the protocol instance active at that
        node that matches the specified type (first and only
        argument).  This function is often used to locate a protocol's
        peer at another node.  An empty string is returned if the
        protocol of the given type could not be found. \\
\proc[op, args]{all-mprotos} &
        internal routine to execute ``\code{op}'' with ``\code{args}''
        on all protocol instances active at that node. \\
\proc[]{start} & \\
\proc[]{stop} &
        start/stop execution of all protocols. \\
\proc[dummy]{notify} &
        is called when a topology change occurs. The dummy argument is
        currently not used.\\
{\let\[=[\let\]=]
\proc[file-handle, \[grp\], \[src\]]{dump-mroutes}} &
        dump multicast routes to specified file-handle. \\
\proc[G, S]{join-group} &
        signals all protocol instances to join \tup{S, G}. \\
\proc[G, S]{leave-group} &
        signals all protocol instances to leave \tup{S, G}. \\
\proc[code, s, g, iif]{upcall} &
        signalled by node on forwarding errors in classifier;
        this routine in turn signals the protocol instance that owns
        the incoming interface (\code{iif}) by invoking the
        appropriate handle function for that particular code.\\ 
\proc[rep, s, g, iif]{drop} &
        Called on packet drop, possibly to prune an interface. \\
\end{alist}

In addition, the mrtObject class supports the concept of well known
groups, \ie, those groups that do not require explicit protocol support.
Two well known groups, \code{ALL_ROUTERS} and \code{ALL_PIM_ROUTERS}
are predefined in \ns.

The \clsref{mrtObject}{../ns-2/tcl/mcast/McastProto.tcl} defines
two class procedures to set and get information about these well known groups.
\begin{alist}
\proc[name]{registerWellKnownGroups} & 
        assigns \code{name} a well known group address. \\
\proc[name]{getWellKnownGroup} &
        returns the address allocated to well known group, \code{name}.
        If \code{name} is not registered as a well known group,
        then it returns the address for \code{ALL\_ROUTERS}.
\end{alist}

\paragraph{McastProtocol class}
This is the base class for the implementation of all the multicast protocols.
It contains basic multicast functions:
\begin{alist}
\proc[]{start}, \proc[]{stop} &
        Set the \code{status\_} of execution of this protocol instance. \\
\proc[]{getStatus} &
        return the status of execution of this protocol instance. \\
\proc[]{getType} &
        return the type of protocol executed by this instance. \\
\proc[code args]{upcall} &
        invoked when the node classifier signals an error, either due to 
        a cache-miss or a wrong-iif for incoming packet.  This routine
        invokes the protocol specific handle, \proc{handle-\tup{code}} with
        specified \code{args} to handle the signal. \\
\end{alist}

A few words about interfaces.  Multicast implementation in \ns\
assumes duplex links i.e. if there is a simplex link from node~1 to
node~2, there must be a reverse simplex link from node~2 to node~1.
To be able to tell from which link a packet was received, multicast
simulator configures links with an interface labeller at the end of
each link, which labels packets with a particular and unique label
(id).  Thus, ``incoming interface'' is referred to this label and is a
number greater or equal to zero.  Incoming interface value can be
negative (-1) for a special case when the packet was sent by a local
to the given node agent.

In contrast, an ``outgoing interface'' refers to an object handler,
usually a head of a link which can be installed at a replicator.  This
destinction is important: \textit{incoming interface is a numeric label to
a packet, while outgoing interface is a handler to an object that is
able to receive packets, e.g. head of a link.}
 
\subsection{Extensions to other classes in \ns}
In \href{the earlier chapter describing nodes in
\ns}{Chapter}{chap:nodes}, we described the internal structure of the
node in \ns.  To briefly recap that description, the node entry for a
multicast node is the
\code{switch\_}.  It looks at the highest bit to decide if the
destination is a multicast or unicast packet.  Multicast packets are
forwarded to a multicast classifier which maintains a list of
replicators; there is one replicator per \tup{source, group} tuple.
Replicators copy the incoming packet and forward to all outgoing
interfaces.

\paragraph{Class Node}
Node support for multicast is realized in two primary ways: it serves
as a focal point for access to the multicast protocols, in the areas
of address allocation, control and management, and group membership
dynamics; and secondly, it provides primitives to access and control
interfaces on links incident on that node.
\begin{alist}
\proc[]{expandaddr}, & \\
\proc[]{allocaddr} &
        Class procedures for address management.
        \proc[]{expandaddr} is now obsoleted.
        \proc[]{allocaddr} allocates the next available multicast
        address.\\[2ex]
\proc[]{start-mcast}, & \\
\proc[]{stop-mcast} &
        To start and stop multicast routing at that node. \\
\proc[]{notify-mcast} &
        \proc[]{notify-mcast} signals the mrtObject at that node to
        recompute multicastroutes following a topology change or
        unicast route update from a neighbour.  \\[2ex]
\proc[]{getArbiter} &
        returns a handle to mrtObject operating at that node. \\
\proc[file-handle]{dump-routes} &
        to dump the multicast forwarding tables at that node. \\[2ex]
\proc[s g iif code]{new-group} &
        When a multicast data packet is received, and the multicast
        classifier cannot find the slot corresponding to that data
        packet, it invokes \proc[]{Node~nstproc~new-group} to
        establish the appropriate entry.  The code indicates the
        reason for not finding the slot.  Currently there are two
        possibilities, cache-miss and wrong-iif.  This procedure
        notifies the arbiter instance to establish the new group. \\
\proc[a g]{join-group} &
        An \code{agent} at a node that joins a particular group invokes
        ``\code{node join-group <agent> <group>}''.  The
        node signals the mrtObject to join the particular \code{group},
        and adds \code{agent} to its list of agents at that
        \code{group}.  It then adds \code{agent} to all replicators
        associated with \code{group}. \\
\proc[a g]{leave-group} &
        \code{Node~instproc~leave-group} reverses the process
        described earlier.  It disables the outgoing interfaces to the
        receiver agents for all the replicators of the group, deletes
        the receiver agents from the local \code{Agents\_} list; it
        then invokes the arbiter instance's
        \proc[]{leave-group}.\\[2ex]
\proc[s g iif oiflist]{add-mfc} &
        \code{Node~instproc~add-mfc} adds a \textit{multicast forwarding cache}
        entry for a particular \tup{source, group, iif}.
        The mechanism is:
        \begin{itemize}
        \item create a new replicator (if one does not already exist),
        \item update the \code{replicator\_} instance variable array at the node,
        \item add all outgoing interfaces and local agents to the
            appropriate replicator,
        \item invoke the multicast classifier's \proc[]{add-rep}
            to create a slot for the replicator in the multicast
            classifier.
        \end{itemize} \\
\proc[s g oiflist]{del-mfc} &
        disables each oif in \code{oiflist} from the replicator for \tup{s, g}.\\
\end{alist}

The list of primitives accessible at the node to control its interfaces are listed below.
\begin{alist}
\proc[ifid link]{add-iif}, & \\
\proc[link if]{add-oif} &
        Invoked during link creation to prep the node about its 
        incoming interface label and outgoing interface object. \\

\proc[]{get-all-oifs} &
        Returns all oifs for this node. \\
\proc[]{get-all-iifs} &
        Returns all iifs for this node. \\

\proc[ifid]{iif2link} &
        Returns the link object labelled with given interface
        label. \\
\proc[link]{link2iif} &
        Returns the incoming interface label for the given
        \code{link}. \\

\proc[oif]{oif2link} &
        Returns the link object corresponding to the given outgoing
        interface. \\
\proc[link]{link2oif} &
        Returns the outgoing interface for the \code{link} (\ns\
        object that is incident to the node).\\

\proc[src]{rpf-nbr} &
        Returns a handle to the neighbour node that is its next hop to the 
        specified \code{src}.\\

\proc[s g]{getReps} &
        Returns a handle to the replicator that matches \tup{s, g}.
        Either argument can be a wildcard (*). \\
\proc[s g]{getReps-raw} &
        As above, but returns a list of \tup{key, handle} pairs. \\
\proc[s g]{clearReps} &
        Removes all replicators associated with \tup{s, g}. \\[2ex]
\end{alist}

\paragraph{Class Link and SimpleLink}
This class provides methods to check the type of link, and the label it 
affixes on individual packets that traverse it.
There is one additional method to actually place the interface objects on this link.
These methods are:
\begin{alist}
\proc[]{if-label?} & 
        returns the interface label affixed by this link to packets
        that traverse it. \\
% \proc[]{enable-mcast} & 
%       Internal procedure called by the SimpleLink constructor to add
%       appropriate objects and state for multicast.  By default, (and
%       for the point-to-point link case) it places a NetworkInterface
%       object at the end of the link, and signals the nodes on
%       incident on the link about this link.\\
\end{alist}

\paragraph{Class NetworkInterface}
This is a simple connector that is placed on each link.  It affixes
its label id to each packet that traverses it.  The packet id is used
by the destination node incident on that link to identify the link by
which the packet reached it.  The label id is configured by the Link
constructor.  This object is an internal object, and is not designed
to be manipulated by user level simulation scripts.  The object only
supports two methods:
\begin{alist}
\proc[ifid]{label} & 
        assigns \code{ifid} that this object will affix to each packet. \\
\proc[]{label} & 
        returns the label that this object affixes to each packet.\\
\end{alist}
The global class variable, \code{ifacenum\_}, specifies the next
available \code{ifid} number.

\paragraph{Class Multicast Classifier}
\code{Classifier/Multicast} maintains a \emph{multicast forwarding
cache}.  There is one multicast classifier per node. The node stores a
reference to this classifier in its instance variable
\code{multiclassifier\_}. When this classifier receives a packet, it
looks at the \tup{source, group} information in the packet headers,
and the interface that the packet arrived from (the incoming interface
or iif); does a lookup in the MFC and identifies the slot that should
be used to forward that packet.  The slot will point to the
appropriate replicator.

However, if the classifier does not have an entry for this
\tup{source, group}, or the iif for this entry is different, it will
invoke an upcall \proc[]{new-group} for the classifier, with one of
two codes to identify the problem:

\begin{itemize}
        \item \code{cache-miss} indicates that the classifier did not
        find any \tup{source, group} entries;

        \item \code{wrong-iif} indicates that the classifier found
        \tup{source, group} entries, but none matching the interface
        that this packet arrived on.
\end{itemize}
These upcalls to TCL give it a chance to correct the situation:
install an appropriate MFC--entry (for \code{cache-miss}) or change
the incoming interface for the existing MFC--entry (for
\code{wrong-iif}).  The \emph{return value} of the upcall determines
what classifier will do with the packet.  If the return value is
``1'', it will assume that TCL upcall has appropriately modified MFC
will try to classify packet (lookup MFC) for the second time.  If the
return value is ``0'', no further lookups will be done, and the packet
will be thus dropped.

\proc[]{add-rep} creates a slot in the classifier
and adds a replicator for \tup{source, group, iif} to that slot.

\paragraph{Class Replicator}
When a replicator receives a packet, it copies the packet to all of
its slots.  Each slot points to an outgoing interface for a particular
\tup{source, group}.

If no slot is found, the C++ replicator invokes the class instance
procedure \proc[]{drop} to trigger protocol specific actions.  We will
describe the protocol specific actions in the next section, when we
describe the internal procedures of each of the multicast routing
protocols.

There are instance procedures to control the elements in each slot:
\begin{alist}
\proc[oif]{insert} & inserting a new outgoing interface
                        to the next available slot.\\
\proc[oif]{disable} & disable the slot pointing to the specified oif.\\
\proc[oif]{enable} &  enable the slot pointing to the specified oif.\\
\proc[]{is-active} & returns true if the replicator has at least one active slot.\\
\proc[oif]{exists} & returns true if the slot pointing to the specified oif is active.\\
\proc[source, group, oldiif, newiif]{change-iface} & modified the iif entry for the particular replicator.\\
\end{alist}

\subsection{Protocol Internals}
\label{sec:mcastproto-internals}

We now describe the implementation of the different multicast routing
protocol agents.

\subsubsection{Centralized Multicast}
\code{CtrMcast} is inherits from \code{McastProtocol}.
One CtrMcast agent needs to be created for each node.  There is a
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 IS OBSOLETE NOW
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
