$Id: mrouted.8,v 1.1 1994/01/11 20:15:59 brezak Exp $
Mrouted forwards a multicast datagram along a shortest (reverse) path tree rooted at the subnet on which the datagram originates. It is a broadcast tree, which means it includes all subnets reachable by a cooperating set of mrouted routers. However, the datagram will not be forwarded onto leaf 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, mrouted includes support for "tunnels", which are virtual point-to-point links between pairs of mrouted s 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 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 encapsulate using IP source routing, which puts a heavy load on some types of routers. This version supports IP source route encapsulation only for backwards compatibility.
The tunnel 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 routing protocols.
Mrouted handles multicast routing only; there may or may not be a unicast router running on the same host 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.
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 following effects:
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 s directly reachable via those interfaces. To override the default configuration, or to add tunnel links to other mrouted s, 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 mrouted s, 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, because mrouted cannot route along paths with a sum of metrics greater than 31. When in doubt, the following metrics are recommended:
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. Suggested thresholds:
In general, all mrouted s 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 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 tunnels; such an mrouted configuration would be better replaced by more direct tunnels (i.e., eliminate the middle man).
Mrouted responds to the following signals:
The routing tables look like this:
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 1 36.11.0.1 subnet: 36.11 1 1 querier groups: 224.0.2.1 224.0.1.0 224.0.0.4 2 36.2.0.8 tunnel: 36.8.0.77 3 1 peers : 36.8.0.77 3 36.2.0.8 tunnel: 36.8.0.110 3 1 Multicast Routing Table Origin-Subnet From-Gateway Metric In-Vif Out-Vifs 36.2 1 0 1* 2 3* 36.8 36.8.0.77 4 2 0* 1* 3* 36.11 1 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.
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.