Multicast is essentially the ability to send one IP packet to multiple receivers. Multicast is often used for audio and video conferencing systems.
You often hear about the Mbone in reference to Multicast. The Mbone is essentially a ``virtual backbone'' which exists in the Internet itself. If you want to send and/or receive Multicast, you need to be ``on the Mbone.''
One way is to ask someone who manages your network. If your network manager doesn't know, or looks at you funny, then you are very likely NOT on the Mbone
Another way is to use the mtrace program, which can be found
Xerox PARC FTP site. Mtrace is similar to traceroute. It will
tell you about the multicast path between your site and another. For example:
> mtrace mbone.ucar.edu
mtrace: WARNING: no multicast group specified, so no statistics printed
Mtrace from 184.108.40.206 to 220.127.116.11 via group 18.104.22.168
Querying full reverse path... * switching to hop-by-hop:
0 oceana-ether.nlanr.net (22.214.171.124)
-1 avidya-ether.nlanr.net (126.96.36.199) DVMRP thresh^ 1
-2 mbone.sdsc.edu (188.8.131.52) DVMRP thresh^ 1
-3 * nccosc-mbone.dren.net (184.108.40.206) DVMRP thresh^ 48
-4 * * FIXW-MBONE.NSN.NASA.GOV (220.127.116.11) PIM/Special thresh^ 64
-5 dec3800-2-fddi-0.SanFrancisco.mci.net (18.104.22.168) DVMRP thresh^ 64
-6 dec3800-2-fddi-0.Denver.mci.net (22.214.171.124) DVMRP thresh^ 1
-7 mbone.ucar.edu (126.96.36.199) DVMRP thresh^ 64
-8 mbone.ucar.edu (188.8.131.52)
Round trip time 196 ms; total ttl of 68 required.
If you think you need to be on the Mbone, this is how you can join.
Short answer: No, probably not.
Reasons why you SHOULD use Multicast:
Reasons why you SHOULD NOT use Multicast:
We only recommend people to use Multicast ICP over network infrastructure which they have close control over. In other words, only use Multicast over your local area network, or maybe your wide area network if you are an ISP. We think it is probably a bad idea to use Multicast ICP over congested links or commodity backbones.
To configure Squid to send ICP queries to a Multicast address, you
need to create another neighbour cache entry specified as multicast.
cache_host 184.108.40.206 multicast 3128 3130 ttl=64
220.127.116.11 is a sample multicast group address.
multicast indicates that this
is a special type of neighbour. The HTTP-port argument (3128)
is ignored for multicast peers, but the ICP-port (3130) is
very important. The final argument, ttl=64
specifies the multicast TTL value for queries sent to this
It is probably a good
idea to increment the minimum TTL by a few to provide a margin
for error and changing conditions.
You must also specify which of your neighbours will respond
to your multicast queries, since it would
be a bad idea to implicitly trust any ICP reply from an unknown
address. Note that ICP replies are sent back to unicast
addresses; they are NOT multicast, so Squid has no indication
whether a reply is from a regular query or a multicast
query. To configure your multicast group neighbours, use the
cache_host directive and the multicast-responder
cache_host cache1 sibling 3128 3130 multicast-responder
cache_host cache2 sibling 3128 3130 multicast-responder
Here all fields are relevant. The ICP port number (3130)
must be the same as in the cache_host line defining the
multicast peer above. The third field must either be
parent or sibling to indicate how Squid should treat replies.
With the multicast-responder flag set for a peer,
Squid will NOT send ICP queries to it directly (i.e. unicast).
The Multicast TTL (which is specified on the cache_host line
of your multicast group) determines how ``far'' your ICP queries
will go. In the Mbone, there is a certain TTL threshold defined
for each network interface or tunnel. A multicast packet's TTL must
be larger than the defined TTL for that packet to be forwarded across
that link. For example, the mrouted manual page recommends:
32 for links that separate sites within an organization.
64 for links that separate communities or organizations, and are
attached to the Internet MBONE.
128 for links that separate continents on the MBONE.
A good way to determine the TTL you need is to run mtrace as shown above and look at the last line. It will show you the minimum TTL required to reach the other host.
If you set you TTL too high, then your ICP messages may travel ``too far'' and will be subject to eavesdropping by others. If you're only using multicast on your LAN, as we suggest, then your TTL will be quite small, for example ttl=4.
You must tell Squid to join a multicast group address with the
mcast_groups directive. For example:
Of course, all members of your Multicast ICP group will need to use the
exact same multicast group address.
NOTE: Choose a multicast group address with care! If two organizations happen to choose the same multicast address, then they may find that their groups ``overlap'' at some point. This will be especially true if one of the querying caches uses a large TTL value. There are two ways to reduce the risk of group overlap:
Using a unique address is a good idea, but not without some potential problems. If you choose an address randomly, how do you know that someone else will not also randomly choose the same address? NLANR has been assigned a block of multicast addresses by the IANA for use in situations such as this. If you would like to be assigned one of these addresses, please write to us. However, note that NLANR or IANA have no authority to prevent anyone from using an address assigned to you.
Limiting the scope of your multicast messages is probably a better solution. They can be limited with the TTL value discussed above, or with some newer techniques known as administratively scoped addresses. Here you can configure well-defined boundaries for the traffic to a specific address. There are some Internet Draft documents describing this: