IP Multicasting Issues
By Chris Koeritz
This document describes IP multicast and compares it
with directed IP broadcast. It is intended as a brief
rather than a technical explanation. For more information than is
available here, you might want to check out this
Directed IP Broadcast
The diagram below shows the essential points of a
broadcast. The sender transmits some data (denoted by the little
envelopes) out to the three subnets depicted. The data must
be sent three times, one for each subnet. On the receiving
every machine on the subnet will be sent the data, whether it wants it
The diagram below shows the essential points of a
The sender transmits some data out to a multicast address. The
only needs to be sent once, and the routers will take care of the
The middle subnet has no hosts registered for this data and so will not
see the data at all. On the other two subnets, only the hosts
have specifically registered for the data will see it; the other
on the subnet will not. Note that the routers manage the delivery
list; the sender need not know who the recipients are (and it will not
know the recipients unless a higher level protocol is providing the
An Assortment of Multicast Issues
Multicast Group Addresses
- IP multicast groups have a group address.
- This is similar to an IP address but in a restricted range of
- These are called "Class D" addresses.
- The range from 126.96.36.199 through 188.8.131.52 are reserved for
- You can be officially assigned an IP multicast address by the
backbone) group. This indicates that you are the only authorized
user of that particular address.
- Multicast addresses can also be dynamically assigned. A
process will provide a randomly chosen IP multicast group
This choice of address must be communicated to all programs that
the broadcasts, potentially during a login process at a server.
- IP multicast only supports sending packets of data, not arbitrary
- Multicast is also intrinsically unreliable.
- There are other higher level protocols that make multicast
none have been chosen as standards yet. (For example, PGM:
- IP multicast groups are called "open" because anyone can send
to a group.
- The scope of a multicast send is governed by the Time To Live
in the data packet.
- TTL is a number that dictates how many router hops the packet is
- The classification scheme for TTLs is as follows:
Comparison With Standard TCP/IP
- 0: restricted to same
other machines can see the data.
- 1: restricted to same
just 'this' subnet will see the data.
- 32: restricted to same site; as
by network topology and firewalls.
- 64: restricted to same region;
- 128: restricted to same continent.
- Sending multicasts is easy. This is just a matter of
IP multicast address, and specifying a port number on that
This is essentially the same as sending a regular UDP (user datagram
- Receiving multicasts is not too hard either. One joins a
group by registering with the IP multicast address. The address
correspond to a specific machine, but to a virtual group defined by the
IP address and the port number.
- Once one has joined a multicast group, a TCP/IP socket can be
Socket usage is the same as for any other kind of TCP/IP
Now we will receive any data that is sent to the multicast group at our
Comparison With Directed IP Broadcast
What are the drawbacks of using multicast?
Why is multicast a potentially superior choice for data distribution?
- Multicast is a newer protocol and needs support from a multicast
- Multicast transmissions devolve to a directed broadcast when
no multicast routers, so even when customers use a multicast enabled
they might actually be getting broadcasts on their lan.
- Customers with older routers can still be supported with our
- Multicast is an unreliable protocol.
- There are no guarantees for delivery of transmissions.
- Reliable multicast exists in several higher level protocols,
these are standard yet. These are also not supported by any
yet, so they are implemented in software rather than hardware.
- However, directed broadcast is also unreliable. Software
can provide a
reliable broadcast by supporting a retransmission process.
- Multicast is wide open.
- With directed broadcasts, the network administrator can
however desired and know that traffic is not going off of the LAN/WAN.
- Multicast group membership is open to machines across the
anyone can listen to multicast traffic if they know the address.
- To enforce multicast security / visibility, one must either
at the software level (by restricting the "time to live" on multicast
which could still expose the traffic to unwanted listeners) or at the
level (by modifying the router's multicast properties).
- With directed broadcast, one must know the recipients for the
of their subnets. In IP Multicast, one does not need to know the
recipients ahead of time.
- In a directed broadcast, every machine on an entire subnet sees
With multicast, only the machines that have registered will see the
The machines that are not included in a group will not see any traffic,
whereas in a directed broadcast, each machine must discard the packets
it was not interested in; thus multicast is less of a drain on