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 datagram forwarding algorithm called Reverse Path Multicasting.
forwards a multicast datagram along a
shortest (reverse) path tree rooted at the subnet on which the datagram
originates. The multicast delivery tree may be thought of as a broadcast
delivery tree that has been pruned back so that it does not extend beyond
those subnetworks that have members of the destination group. Hence, datagrams
are not forwarded along those branches which have no listeners of the
multicast group. The IP time-to-live of a multicast datagram can be used to
limit the range of multicast datagrams.
In order to support multicasting among subnets that are separated by (unicast)
routers that do not support IP multicasting,
includes support for "tunnels",
which are virtual point-to-point links between pairs of
daemons 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 added on entry to a tunnel, and stripped off on
exit from a tunnel. By default, the packets are encapsulated using the
IP-in-IP protocol (IP protocol number 4). Older versions of
tunnel use IP source routing, which puts
a heavy load on some types of routers. This version does not support IP source
The tunnelling mechanism allows mrouted
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. mrouted
the well-known scaling problems of any distance-vector routing protocol, and
does not (yet) support hierarchical multicast routing.
handles multicast routing only; there may
or may not be unicast routing software running on the same machine as
. With the use of tunnels, it is not
necessary for mrouted
to have access to more than
one physical subnet in order to perform multicast forwarding.
The options are as follows:
- Specify an alternative configuration file, instead of the
- By default, mrouted detaches
from the invoking terminal. If this option is specified,
mrouted 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 debug level,
mrouted always writes warning and error
messages to the system log daemon. Debug levels have the following
- Detach from the invoking terminal.
syslog(3) 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.
- Start mrouted in a non-pruning
mode. It is expected that a router would be configured in this manner for
test purposes only. The default mode is pruning enabled.
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 mrouted
directly reachable via those
interfaces. To override the default configuration, or to add tunnel links to
, configuration commands may be
placed in /etc/mrouted.conf
. There are five types
of configuration commands:
The file format is free-form: whitespace (including newlines) is not
significant. The boundary
option can accept
either a name or a boundary; the boundary
options may be specified as many times as
is a value that determines the
amount of time that a cached multicast route stays in kernel before timing
out. The value of this entry should lie between 300 (5 min) and 86400 (1 day).
It defaults to 300.
option assigns names to boundaries to make
command can be used to disable multicast
routing on the physical interface identified by local IP address
, or to associate a non-default
metric or threshold with the specified physical interface. The local IP
may be replaced by the
interface name (e.g. le0). If a phyint is attached to multiple IP subnets,
describe each additional subnet with the altnet
keyword. Phyint commands must precede tunnel commands.
option is provided for
to act as a non-pruning router.
command can be used to establish a
tunnel link between local IP address
and remote IP address
, and to associate a non-default
metric or threshold with that tunnel. The local IP address
may be replaced by the interface
name (e.g. le0). The remote IP address
may be replaced by a host name,
if and only if the host name has a single IP address associated with it. The
tunnel must be set up in the mrouted.conf files of both routers before it can
allows an interface to be configured as an
administrative boundary for the specified scoped address. Packets belonging to
this address will not be forwarded on a scoped interface. The boundary option
accepts either a name or a boundary spec.
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, because mrouted
route along paths with a sum of metrics greater than 31.
allows the network administrator to
specify a certain bandwidth in Kbits/second which would be allocated to
multicast traffic. It defaults to 500Kbps on tunnels, and 0 (unlimited) on
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.
In general, all mrouted
connected to a particular
subnet or tunnel should use the same metric and threshold for that subnet or
will not initiate execution if it has fewer
than two enabled virtual interfaces (vifs), where a vif is either a physical
multicast-capable interface or a tunnel. It will log a warning if all of its
vifs are tunnels; such an mrouted
would be better replaced by more direct tunnels (i.e. eliminate the middle
This is an example configuration for a mythical multicast router at a big
# mrouted.conf example
# Name our boundaries to make it easier.
name LOCAL 126.96.36.199/16
name EE 188.8.131.52/16
# le1 is our gateway to compsci, don't forward our
# local groups to them.
phyint le1 boundary EE
# le2 is our interface on the classroom net, it has four
# different length subnets on it.
# Note that you can use either an ip address or an
# interface name
phyint 172.16.12.38 boundary EE altnet 172.16.15.0/26
altnet 172.16.15.128/26 altnet 172.16.48.0/24
# atm0 is our ATM interface, which doesn't properly
# support multicasting.
phyint atm0 disable
# This is an internal tunnel to another EE subnet.
# Remove the default tunnel rate limit, since this
# tunnel is over Ethernets.
tunnel 192.168.5.4 192.168.55.101 metric 1 threshold 1
# This is our tunnel to the outside world.
# Careful with those boundaries, Eugene.
tunnel 192.168.5.4 10.11.12.13 metric 1 threshold 32
boundary LOCAL boundary EE
responds to the following signals:
- Restarts mrouted. The
configuration file is reread every time this signal is evoked.
- Terminates execution gracefully (i.e. by sending good-bye
messages to all neighboring routers).
- The same as INT.
- Dumps the internal routing tables to
- Dumps the internal cache tables to
- Dumps the internal routing tables to stderr (only if
mrouted was invoked with a non-zero debug
The routing tables look like this:
Virtual Interface Table
Vif Local-Address Metric Thresh Flags
0 184.108.40.206 subnet: 36.2 1 1 querier
pkts in: 3456
pkts out: 2322323
1 220.127.116.11 subnet: 36.11 1 1 querier
pkts in: 345
pkts out: 3456
2 18.104.22.168 tunnel: 22.214.171.124 3 1
peers: 126.96.36.199 (2.2)
pkts in: 34545433
pkts out: 234342
3 188.8.131.52 tunnel: 184.108.40.206 3 16
Multicast Routing Table (1136 entries)
Origin-Subnet From-Gateway Metric Tmr In-Vif Out-Vifs
36.2 1 45 0 1* 2 3*
36.8 220.127.116.11 4 15 2 0* 1* 3*
36.11 1 20 1 0* 2 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 instance of
is the one responsible for sending
periodic group membership queries on the vif 0 and vif 1 subnets, as indicated
by the "querier" flags. The list of boundaries indicate the scoped
addresses on that interface. A count of the number of incoming and outgoing
packets is also shown at each interface.
Associated with each subnet from which a multicast datagram can originate is the
address of the previous hop router (unless the subnet is directly- connected),
the metric of the path back to the origin, the amount of time since we last
received an update for this subnet, 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.
also maintains a copy of the kernel
forwarding cache table. Entries are created and deleted by
The cache tables look like this:
Multicast Routing Cache Table (147 entries)
Origin Mcast-group CTmr Age Ptmr IVif Forwvifs
13.2.116/22 18.104.22.168 3m 2m - 0 1
138.96.48/21 22.214.171.124 5m 2m - 0 1
128.9.160/20 126.96.36.199 3m 2m - 0 1
198.106.194/24 188.8.131.52 9m 28s 9m 0P
Each entry is characterized by the origin subnet number and mask and the
destination multicast group. The 'CTmr' field indicates the lifetime of the
entry. The entry is deleted from the cache table when the timer decrements to
zero. The 'Age' field is the time since this cache entry was originally
created. Since cache entries get refreshed if traffic is flowing, routing
entries can grow very old. The 'Ptmr' field is simply a dash if no prune was
sent upstream, or the amount of time until the upstream prune will time out.
The 'Ivif' field indicates the incoming vif for multicast packets from that
origin. Each router also maintains a record of the number of prunes received
from neighboring routers for a particular source and group. If there are no
members of a multicast group on any downward link of the multicast tree for a
subnet, a prune message is sent to the upstream router. They are indicated by
a "P" after the vif number. The Forwvifs field shows the interfaces
along which datagrams belonging to the source-group are forwarded. A
"p" indicates that no datagrams are being forwarded along that
interface. An unlisted interface is a leaf subnet with no members of the
particular group on that subnet. A "b" on an interface indicates
that it is a boundary interface, i.e. traffic will not be forwarded on the
scoped address on that interface. An additional line with a
‘>’ as the first character is printed for each source on the
subnet. Note that there can be many sources in one subnet.
Multicast Routing in Internetworks and Extended LANs,
Proceedings of the ACM SIGCOMM '88 Conference.