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 (built-in) 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 route tunneling.
The tunneling 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 support hierarchical multicast routing.
A more common practise today is to set up GRE tunnels between multicast capable routers and limit mrouted to run on a select number of interfaces listed in the configuration.
mrouted handles multicast routing only; there may or may not be unicast routing software running on the same machine as mrouted 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.
Available subystems:
To override the default settings, for example to to add tunnel links to other DVMRP routers, configuration commands may be placed in /etc/mrouted.conf There are five types of commands:
[boundary
boundary-name | scoped-addr / mask-len
]
[igmpv1 | igmpv2 | igmpv3
]
[disable | enable
]
[metric <1-31>
]
[rate-limit kbps
]
[threshold ttl
]
[boundary
boundary-name | scoped-addr / mask-len
]
[metric <1-31>
]
[rate-limit kbps
]
[threshold ttl
]
The file format is free-form: whitespace (including newlines) is not significant. The boundary option to all commands can accept either a name or a network boundary; the boundary and altnet options may be specified as many times as necessary.
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 to 300.
The name option assigns names to boundaries to make configuration easier.
The igmp-query-interval setting controls the IGMP query interval used when this router is elected querier on a LAN. It can be set to any value in the range 1-1024, inclusive. However, it is not recommended to go below 10 sec. Default: 125.
The igmp-robustness setting controls many aspects of IGMP timers. It can be any value in the range 2-10, inclusive. The query-response-interval used below is hard-coded to 10 sec in :
The phyint command can be used to disable multicast routing (or enable if mrouted is started with all interfaces disabled) 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 an interface is attached to multiple IP subnets, describe each additional subnet with the altnet keyword. mrouted supports all IGMP versions and defaults to use IGMP v3. Use igmpv2 or igmpv1 to force compatibility modes on the phyint
NOTE: All phyint commands must precede tunnel commands.
The pruning command is provided for mrouted to act as a non-pruning router. This is no longer supported and the configuration option is only kept for compatibility reasons.
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. 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.
In general, all DVMRP routers connected to a particular subnet or tunnel should use the same metric and threshold for that subnet or tunnel.
#
# mrouted.conf example
#
# Name our boundaries to make it easier.
name LOCAL 239.255.0.0/16
name EE 239.254.0.0/16
# 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
For convenience, mrouted writes its process ID to /var/run/mrouted.pid when it has completed its start up and is ready to receive signals.
Virtual Interface Table
Vif Local-Address Metric Thresh Flags
0 36.2.0.8 subnet: 36.2 1 1 querier
groups: 224.0.2.1
224.0.0.4
pkts in: 3456
pkts out: 2322323
1 36.11.0.1 subnet: 36.11 1 1 querier
groups: 224.0.2.1
224.0.1.0
224.0.0.4
pkts in: 345
pkts out: 3456
2 36.2.0.8 tunnel: 36.8.0.77 3 1
peers: 36.8.0.77 (2.2)
boundaries: 239.0.1
: 239.1.2
pkts in: 34545433
pkts out: 234342
3 36.2.0.8 tunnel: 36.6.8.23 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 36.8.0.77 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 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 mrouted
The cache tables look like this:
Multicast Routing Cache Table (147 entries) Origin Mcast-group CTmr Age Ptmr IVif Forwvifs 13.2.116/22 224.2.127.255 3m 2m - 0 1 >13.2.116.19 13.2.116.196 138.96.48/21 224.2.127.255 5m 2m - 0 1 >138.96.48.108 128.9.160/20 224.2.127.255 3m 2m - 0 1 >128.9.160.45 198.106.194/24 224.2.135.190 9m 28s 9m 0P >198.106.194.22
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.