NAME
mrouted —
IP multicast routing
daemon
SYNOPSIS
mrouted |
[-c
config_file]
[-d
debug_level]
[-p] |
DESCRIPTION
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 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 using 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 (yet) support
hierarchical multicast routing.
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.
INVOCATION
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 daemon. Non-zero debug levels have the following effects:
- 1
- all syslog'ed messages are also printed to stderr.
- 2
- all level 1 messages plus notifications of
"significant" events are printed to stderr.
- 3
- 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
/var/run/mrouted.pid.
CONFIGURATION
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 other
mrouted configuration commands may be placed in
/etc/mrouted.conf (or an alternative file, specified by the
-c option). There are four types of configuration commands:
- phyint <local-addr> [disable] [metric <m>]
- [threshold <t>]
[rate_limit <b>]
-
- [boundary
(<boundary-name>|<scoped-addr>/<mask-len>)]
-
- [altnet
<network>/<mask-len>]
-
-
- tunnel <local-addr>
<remote-addr> [metric <m>]
-
- [threshold <t>]
[rate_limit <b>]
-
- [boundary
(<boundary-name>|<scoped-addr>/<mask-len>)]
-
- cache_lifetime <ct>
- pruning <off/on>
- 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 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.
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.
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
pruning option is provided for
mrouted to act as a non-pruning 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. 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.
In general, all
mrouted connected to a particular subnet or
tunnel should use the same metric and threshold for that subnet or tunnel.
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 0 (unlimited) on physical interfaces.
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.
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).
EXAMPLE CONFIGURATION
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
SIGNALS
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).
- TERM
- same as INT
- USR1
- dumps the internal routing tables to
/var/tmp/mrouted.dump.
- USR2
- dumps the internal cache tables to
/var/tmp/mrouted.cache.
- QUIT
- dumps the internal routing tables to stderr (only if
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.
FILES
- /etc/mrouted.conf
-
- /var/run/mrouted.pid
-
- /var/tmp/mrouted.dump
-
- /var/tmp/mrouted.cache
-
EXAMPLES
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
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 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.
SEE ALSO
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.
AUTHORS
Steve Deering, Ajit Thyagarajan, Bill Fenner