mrouted - IP multicast routing daemon
/usr/etc/mrouted [ -p ] [ -c config_file ] [ -d [ debug_level ] ]
Mrouted 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.
Mrouted 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 also be used to limit its
In order to support multicasting among subnets that are separated by
(unicast) routers that do not support IP multicasting, mrouted includes
support for "tunnels", which are virtual point-to-point links between
pairs of mrouteds 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. The packets are encapsulated using the IP-in-IP protocol
(IP protocol number 4).
The tunnelling mechanism allows mrouted 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. Mrouted 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
Mrouted handles multicast routing only; there may or may not be unicast
routing software running on the same machine as mrouted. For example, an
Internet unicast firewall can function as a multicast router. 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.
If no "-d" option is given, or if the debug level is specified as 0,
mrouted 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 debug level, mrouted always writes warning and
error messages to the system log demon. Non-zero debug levels have the
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.
Upon startup, mrouted writes its pid to the file /etc/mrouted.pid .
Mrouted automatically configures itself to forward on all multicastcapable
interfaces, i.e., interfaces that have the IFF_MULTICAST flag set
(excluding the loopback "interface"), and it finds other mrouteds
directly reachable via those interfaces. To override the default
configuration, or to add tunnel links to other mrouteds, configuration
commands may be placed in /etc/mrouted.conf (or an alternative file,
specified by the "-c" option). There are four types of configuration
phyint <local-addr> [disable] [metric <m>]
[threshold <t>] [rate_limit <b>]
tunnel <local-addr> <remote-addr> [metric <m>]
[threshold <t>] [rate_limit <b>]
name <boundary-name> <scoped-addr>/<mask-len>
The file format is free-form; whitespace (including newlines) is not
significant. The boundary and altnet options may be specified as many
times as necessary.
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. The local IP address <local-addr> may be alternatively
replaced by the interface name (e.g., ec0). If a phyint is attached to
multiple IP subnets, describe each additional subnet with the altnet
keyword. 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 local
IP address <local-addr> may be replaced by the interface name (e.g.,
ec0). The remote IP address <remote-addr> 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 be used.
The cache_lifetime 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
The pruning <off/on> option is provided for mrouted to act as a nonpruning
router. It is also possible to start mrouted in a non-pruning
mode using the "-p" option on the command line. It is expected that a
router would be configured in this manner for test purposes only. The
default mode is pruning enabled.
You may assign names to boundaries to make configuration easier with the
name keyword. The boundary option on phyint or tunnel commands can
accept either a name or a boundary.
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 and you do not normally need to use a different
value. There is no reason to use a metric greater than 1 for any tunnel
that is the only path to a sub-cluster of tunnels and subnets. Metrics
should be kept as small as possible, because mrouted cannot route along
paths with a sum of metrics greater than 31.
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. You need to use this parameter only if you want to
prevent ``local'' multicast traffic from traversing a link. Suggested
32 for links that separate sites within an organization.
64 for links that separate communities or organizations, and are
attached to the Internet MBONE.
128 for links that separate continents on the MBONE.
For example, if the networks in sites A and B are connected by a tunnel
with a threshold of 8, then site-specific multicast datagrams should use
a TTL of 7 to avoid traversing the tunnel.
All mrouteds connected to a particular subnet or tunnel should use the
same metric and threshold for that subnet or tunnel to avoid connections
that only work in one direction.
The rate_limit option 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 5Mbps on physical
interfaces. Set it to zero (0) to mean unlimited.
The boundary option 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.
Here are some reasons why multicast datagrams may fail to traverse a
tunnel, even though mrouted is properly exchanging routes (i.e., shows
lots of entries in response to kill -USR1):
o programs use a TTL that is too small for the tunnel threshold (as
configured at the tunnel entry).
o there's a filtering router in the tunnel path that selectively discards
some IP datagrams such as IGMP packets or IP-over-IP encapsulated
multicast data packets.
o the two ends of tunnel are misconfigured, disagreeing on tunnel type
(encapsulating vs. source routing).
Mrouted will not initiate execution if it has fewer than two enabled
vifs, where a vif (virtual interface) is either a physical multicastcapable
interface or a tunnel. It will log a warning if all of its vifs
are tunnels; such an mrouted configuration would be better replaced by
more direct tunnels (i.e., eliminate the middle man).
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 18.104.22.168/16
name EE 22.214.171.124/16
# xpi0 is our gateway to compsci, don't forward our
# local groups to them
phyint xpi0 boundary EE
# xpi1 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
Mrouted responds to the following signals:
HUP restarts mrouted . The configuration file is reread every time this
signal is evoked.
INT terminates execution gracefully (i.e., by sending good-bye messages
to all neighboring routers).
USR1 dumps the internal routing tables to /var/tmp/mrouted.dump.
QUIT dumps the internal routing tables to stderr (only if mrouted was
invoked with a non-zero debug level).
/sbin/killall -USR1 mrouted
dumps the routing tables to /var/tmp/mrouted.dump, which looks like this:
Virtual Interface Table
Vif Local-Address Metric Thresh Flags
0 126.96.36.199 subnet: 36.2 1 1 querier
pkts in: 3456
pkts out: 2322323
1 188.8.131.52 subnet: 36.11 1 1 querier
pkts in: 345
pkts out: 3456
2 184.108.40.206 tunnel: 220.127.116.11 3 1
peers: 18.104.22.168 (2.2)
pkts in: 34545433
pkts out: 234342
3 22.214.171.124 tunnel: 126.96.36.199 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*
36.8 188.8.131.52 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 mrouted 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 no. 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.
Mrouted also maintains a copy of the kernel forwarding cache table.
Entries are created and deleted by mrouted.
The cache tables look like this:
Multicast Routing Cache Table (147 entries)
Origin Mcast-group CTmr Age Ptmr IVif Forwvifs
13.2.116/22 184.108.40.206 3m 2m - 0 1
138.96.48/21 220.127.116.11 5m 2m - 0 1
128.9.160/20 18.104.22.168 3m 2m - 0 1
198.106.194/24 22.214.171.124 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 are 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.
DVMRP 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.
Steve Deering, Ajit Thyagarajan, Bill Fenner
By default, mrouted and its kernel module is not loaded on new systems.
You may need to explicitly install the eoe.sw.ipgate subsystem in the
IRIX distribution to load it. After you install that subsystem, you will
need to do these commands as root:
# chkconfig mrouted on
to perform multicast routing.
If you configure your machine to run mrouted and it is connected to a
network having MBONE connectivity or if you create a tunnel to a MBONEconnected
machine, please make sure you have the latest mrouted software.
Consult ftp://ftp.sgi.com/sgi/ipmcast/README for information on
availability of updated mrouted software.
Older versions of mrouted (such as the version in IRIX 3.3 and 4.0.x)
encapsulate using IP source routing, which puts a heavy load on some
types of routers. This version does not support IP source route
mrouted is only capable of supporting 32 multicast interfaces.
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