'\"COPYRIGHT 1989 by The Board of Trustees of Leland Stanford Junior University.
mrouted \- IP multicast routing daemon
is an implementation of the Distance-Vector Multicast Routing
Protocol (DVMRP), an earlier version of which is specified in RFC-1075.
It maintains topological knowledge via a distance-vector routing protocol
(like RIP, described in RFC-1058), upon which it implements a multicast
forwarding algorithm called Truncated Reverse Path Broadcasting (TRPB).
forwards a multicast datagram along a shortest (reverse) path tree
rooted at the subnet on which the datagram originates. It is a
tree, which means it includes
subnets reachable by a cooperating set of
routers. However, the datagram will not be forwarded onto
subnets of the tree if those subnets do not have members of the destination
group. Furthermore, the IP time-to-live of a multicast datagram may prevent
it from being forwarded along the entire tree.
In order to support multicasting among subnets that are separated by (unicast)
routers that do not support IP multicasting,
"tunnels", which are virtual point-to-point links between pairs of
located anywhere in an internet. IP multicast packets are encapsulated for
transmission through tunnels, so that they look like normal unicast datagrams
to intervening routers and subnets. The encapsulation
is inserted on entry to a tunnel, and stripped out
By default, the packets are encapsulated using the IP-in-IP protocol
encapsulate using IP source routing, which puts a heavy load on some
This version supports IP source route encapsulation only for backwards
The tunnel mechanism allows
to establish a virtual internet, for
the purpose of multicasting only, which is independent of the physical
internet, and which may span multiple Autonomous Systems. This capability
is intended for experimental support of internet multicasting only, pending
widespread support for multicast routing by the regular (unicast) routers.
suffers from the well-known scaling problems of any distance-vector
routing protocol, and does not (yet) support hierarchical multicast routing
or inter-operation with other multicast routing protocols.
handles multicast routing only; there may or may not be a unicast
router running on the same host as
With the use of tunnels, it
to have access to more than one physical subnet
in order to perform multicast forwarding.
If no "\-d" option is given, or if the debug level is specified as 0,
detaches from the invoking terminal. Otherwise, it remains attached to the
invoking terminal and responsive to signals from that terminal. If "\-d" is
given with no argument, the debug level defaults to 2. Regardless of the
always writes warning and error messages to the system
log demon. Non-zero debug levels have the following effects:
all syslog'ed messages are also printed to stderr.
all level 1 messages plus notifications of "significant"
events are printed to stderr.
all level 2 messages plus notifications of all packet
arrivals and departures are printed to stderr.
automatically configures itself to forward on all multicast-capable
interfaces, i.e., interfaces that have the IFF_MULTICAST flag set (excluding
the loopback "interface"), and it finds other
via those interfaces. To override the default configuration, or to add
configuration commands may be placed in
/etc/mrouted.conf (or an alternative file, specified by the "\-c" option).
There are two types of configuration command:
phyint <local-addr> [disable] [metric <m>] [threshold <t>]
tunnel <local-addr> <remote-addr> [metric <m>] [threshold <t>] [srcrt]
The phyint command can be used to disable multicast routing on the physical
interface identified by local IP address <local-addr>, or to associate a
non-default metric or threshold with the specified physical interface.
Phyint commands must precede tunnel commands.
The tunnel command can be used to establish a tunnel link between local
IP address <local-addr> and remote IP address <remote-addr>, and to associate
a non-default metric or threshold with that tunnel. The tunnel must be set
up in the mrouted.conf files of both ends before it will be used.
For backwards compatibility with older
the srcrt keyword specifies
encapsulation using IP source routing.
The metric is the "cost" associated with sending a datagram on the given
interface or tunnel; it may be used to influence the choice of routes.
The metric defaults to 1. Metrics should be kept as small as possible,
cannot route along paths with a sum of metrics greater
than 31. When in doubt, the following metrics are recommended:
LAN, or tunnel across a single LAN
serial link, or tunnel across a single serial link
The threshold is the minimum IP time-to-live required for a multicast datagram
to be forwarded to the given interface or tunnel. It is used to control the
scope of multicast datagrams. (The TTL of forwarded packets is only compared
to the threshold, it is not decremented by the threshold. Every multicast
router decrements the TTL by 1.) The default threshold is 1.
for links that separate sites,
for links that separate regions,
for links that separate continents.
connected to a particular subnet or tunnel should
use the same metric and threshold for that subnet or tunnel.
will not initiate execution if it has fewer than two enabled vifs,
where a vif (virtual interface) is either a physical multicast-capable
interface or a tunnel. It will log a warning if all of its vifs are
configuration would be better replaced by more
direct tunnels (i.e., eliminate the middle man).
responds to the following signals:
terminates execution gracefully (i.e., by sending
good-bye messages to all neighboring routers).
dumps the internal routing tables to /usr/tmp/mrouted.dump.
dumps the internal routing tables to stderr (only if
was invoked with a non-zero debug level).
The routing tables look like this:
Vif Local-Address Metric Thresh Flags
0 36.2.0.8 subnet: 36.2 1 1 querier
1 36.11.0.1 subnet: 36.11 1 1 querier
2 36.2.0.8 tunnel: 36.8.0.77 3 1
3 36.2.0.8 tunnel: 36.8.0.110 3 1
Origin-Subnet From-Gateway Metric In-Vif Out-Vifs
36.8 36.8.0.77 4 2 0* 1* 3*
In this example, there are four vifs connecting to two subnets and two
tunnels. The vif 3 tunnel is not in use (no peer address). The vif 0 and
vif 1 subnets have some groups present; tunnels never have any groups. This
is the one responsible for sending periodic group
membership queries on the vif 0 and vif 1 subnets, as indicated by the
Associated with each subnet from which a multicast datagram can originate
is the address of the previous hop gateway (unless the subnet is directly-
connected), the metric of the path back to the origin, the incoming vif for
multicasts from that origin, and a list of outgoing vifs. "*" means that
the outgoing vif is connected to a leaf of the broadcast tree rooted at the
origin, and a multicast datagram from that origin will be forwarded on that
outgoing vif only if there are members of the destination group on that leaf.
TRPB is described, along with other multicast routing algorithms, in the
paper "Multicast Routing in Internetworks and Extended LANs" by S. Deering,
in the Proceedings of the ACM SIGCOMM '88 Conference.
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