IP multicast routing daemon
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
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 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,
mrouted includes support for "tunnels",
which are virtual point-to-point links between pairs of
mrouted 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
mrouted 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
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.
mrouted handles multicast routing only;
there may or may not be unicast routing software running on the same machine
mrouted. With the use of tunnels, it is not
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 default mrouted.conf.
- By default,
mrouteddetaches from the invoking terminal. If this option is specified,
mroutedremains attached to the invoking terminal and responsive to signals from that terminal. If
-dis given with no argument, the debug level defaults to 2. Regardless of the debug level,
mroutedalways writes warning and error messages to the system log daemon. Debug levels have the following effects:
- Detach from the invoking terminal.
- All 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.
mroutedin 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.
mrouted 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
mrouted, configuration commands may be placed
in /etc/mrouted.conf. There are five types of
nameboundary-name | scoped-addr/mask-len
boundaryboundary-name | scoped-addr/mask-len] [
boundaryboundary-name | scoped-addr/mask-len] [
The file format is free-form: whitespace (including newlines) is
not significant. The
boundary option can accept
either a name or a boundary; the
altnet options may be specified as many times as
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 to 300.
name option assigns names to
boundaries to make configuration easier.
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 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.
pruning option is provided for
mrouted to act as a non-pruning router.
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.
le0). 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.
boundary 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.
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, because
route along paths with a sum of metrics greater than 31.
rate_limit 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 physical interfaces.
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.
In general, all
mrouted connected to a
particular subnet or tunnel should use the same metric and threshold for
that subnet or tunnel.
mrouted 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
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 school.
# # mrouted.conf example # # Name our boundaries to make it easier. name LOCAL 22.214.171.124/16 name EE 126.96.36.199/16 # # 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 rate_limit 0 # # 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
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 /var/tmp/mrouted.dump.
- Dumps the internal cache tables to /var/tmp/mrouted.cache.
- Dumps the internal routing tables to stderr (only if
mroutedwas invoked with a non-zero debug level).
The routing tables look like this:
Virtual Interface Table Vif Local-Address Metric Thresh Flags 0 188.8.131.52 subnet: 36.2 1 1 querier groups: 184.108.40.206 220.127.116.11 pkts in: 3456 pkts out: 2322323 1 18.104.22.168 subnet: 36.11 1 1 querier groups: 22.214.171.124 126.96.36.199 188.8.131.52 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) boundaries: 239.0.1 : 239.1.2 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
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 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.
mrouted 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 13.2.116/22 184.108.40.206 3m 2m - 0 1 >220.127.116.11 >18.104.22.168 138.96.48/21 22.214.171.124 5m 2m - 0 1 >126.96.36.199 128.9.160/20 188.8.131.52 3m 2m - 0 1 >184.108.40.206 198.106.194/24 220.127.116.11 9m 28s 9m 0P >18.104.22.168
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.
map-mbone(8), mrinfo(8), mtrace(8)
S. Deering, Multicast Routing in Internetworks and Extended LANs, Proceedings of the ACM SIGCOMM '88 Conference.
Steve Deering, Ajit Thyagarajan, Bill Fenner