| MROUTED(8) | MidnightBSD System Manager's Manual | MROUTED(8) |
mrouted — IP
multicast routing daemon
mrouted |
[-c config_file]
[-d [debug_level]]
[-p] |
The mrouted utility 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.
The mrouted utility 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 multicast routers
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). Older
versions of mrouted tunneled using IP source
routing, which puts a heavy load on some types of routers. This version does
not support IP source route tunnelling.
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. The mrouted utility
suffers from the well-known scaling problems of any distance-vector routing
protocol, and does not (yet) support hierarchical multicast routing.
The mrouted utility 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.
The following options are available:
-c
config_file-d
[debug_level]-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. Regardless of the debug level,
mrouted always writes warning and error messages
to the system log daemon. The -debug-level
argument is a comma-separated list of any of the following:
Upon startup, mrouted writes its pid
to the file /var/run/mrouted.pid.
The mrouted utility 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 DVMRP routers directly reachable
via those interfaces. To override the default configuration, or to add
tunnel links to other multicast routers, configuration commands may be
placed in /etc/mrouted.conf (or an alternative file,
specified by the -c option).
The file format is free-form; whitespace (including newlines) is
not significant. The file begins with commands that apply to
mrouted's overall operation or set defaults.
mrouted <3.3 and all currently known versions
of cisco's IOS) do not use the DVMRP generation ID to determine that a
neighbor has rebooted. Prunes sent towards these neighbors should be kept
short, in order to shorten the time to recover from a reboot. For use in
this situation, the prune_lifetime keyword may be specified on an
interface as described below.mrouted utility uses a DVMRP optimization to
prevent having to keep individual routing tables for each neighbor; part
of this optimization is that mrouted assumes that
it is the forwarder for each of its attached subnets on startup. This can
cause duplicates for a short period (approximately one full route report
interval), since both the router that just started up and the proper
forwarder will be forwarding traffic. This behavior can be turned off with
the noflood keyword; mrouted will not assume that
it is the forwarder on startup. Turning on noflood can cause black holes
on restart, which will generally last approximately one full route report
interval. The noflood keyword can also be specified on individual
interfaces.The second section of the configuration file, which may optionally be empty, describes options that apply to physical interfaces.
mrouted discovers all locally attached multicast
capable interfaces and forwards on all of them.mrouted into IGMPv1 mode. All
routers on the phyint must use the same version of IGMP.mrouted to ignore other routers on this
interface. mrouted will never send or accept
neighbor probes or route reports on this interface.In addition, the common vif commands described later may all be used on a phyint.
The third section of the configuration file, also optional, describes the configuration of any DVMRP tunnels this router might have.
Be careful that the unicast route to the remote address goes
out the interface specified by the
local-addr|ifname argument. Some UNIX kernels
rewrite the source address of mrouted's packets
on their way out to contain the address of the transmission interface.
This is best assured via a static host route.
The common vif commands described below may all be used on tunnels or phyints.
In general, all multicast routers connected to a particular subnet or tunnel should use the same metric and threshold for that subnet or tunnel.
mrouted refuses to peer with DVMRP
neighbors that do not claim to support pruning. This option allows such
peerings on this interface.accept and deny
commands allow rudimentary route filtering. The
accept command causes
mrouted to accept only the listed routes on the
configured interface; the deny command causes
mrouted to accept all but the listed routes. Only
one of accept or deny
commands may be used on a given interface.
The list of routes follows the accept
or deny keyword. If the keyword
exact follows a route, then only that route is
matched; otherwise, that route and any more specific route is matched.
For example, deny 0/0 denys all routes, while
deny 0/0 exact denys only the default route. The
default route may also be specified with the
default keyword.
The bidir keyword enables bidirectional
route filtering; the filter will be applied to routes on both output and
input. Without the bidir keyword,
accept and deny filters
are only applied on input. Poison reverse routes are never filtered
out.
The mrouted utility 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).
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 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
The mrouted utility responds to the
following signals:
mrouted. The configuration file is reread
every time this signal is evoked.mrouted was invoked with a non-zero debug
level).For convenience in sending signals,
mrouted writes its pid to
/var/run/mrouted.pid upon startup.
The routing tables look like this:
Virtual Interface Table
Vif Local-Address Metric Thresh Flags
0 36.2.0.8 subnet: 36.2/16 1 1 querier
groups: 224.0.2.1
224.0.0.4
pkts in: 3456
pkts out: 2322323
1 36.11.0.1 subnet: 36.11/16 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 (3.255)
boundaries: 239.0.1/24
: 239.1.2/24
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 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.
The mrouted utility 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 (or refreshed, if traffic is flowing) 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. An additional line with a "<" as the first character is printed describing any prunes received from downstream dependent neighbors for this subnet and group.
map-mbone(8), mrinfo(8), mtrace(8)
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.
| May 8, 1995 | midnightbsd-3.1 |