TCP/IP Tutorial and Technical Overview - IBM Redbooks

address. In this situation, multicast packets are filtered by lower-layer network
interface hardware.
6.1.2 Multicasting between network segments
Multicast traffic is not limited to a single physical network. However, there are
inherent dangers when multicasting between networks. If the environment
contains multiple routers, specific precautions must be taken to ensure multicast
packets do not continuously loop through the network. It is simple to create a
multicast routing loop. To address this, multicast routing protocols have been
developed to deliver packets while simultaneously avoiding routing loops and
excess transmissions.
There are two requirements to multicast data across multiple networks:
► Determining multicast participants: A mechanism for determining if a
multicast datagram needs to be forwarded on a specific network. This
mechanism is defined in RFC 3376, Internet Group Management Protocol
(IGMP), Version 3.
► Determining multicast scope: A mechanism for determining the scope of a
transmission. Unlike unicast addresses, multicast addresses can extend
through the entire Internet.
The TTL field in a multicast datagram can be used to determine the scope of
a transmission. Like other datagrams, each multicast datagram has a Time
To Live (TTL) field. The value contained in this field is decremented at each
hop. When a host or multicast router receives a datagram, packet processing
depends on both the TTL value and the destination IP address:
– TTL = 0: A multicast datagram received with a TTL value of zero is
restricted to the source host.
– TTL = 1: A multicast datagram with a TTL value of one reaches all hosts
on the subnet that are members of the group. Multicast routers decrement
the value to zero. However unlike unicast datagrams, no ICMP Time
Exceeded error message is returned to the source host. Datagram
expiration is a standard occurrence in multicast environments.
– TTL = 2 (or more): A multicast datagram with this TTL value reaches all
hosts on the subnet that are members of the group. The action performed
by multicast routers depends on the specific group address:
240 TCP/IP Tutorial and Technical Overview
224.0.0.0 - 224.0.0.255: This range of addresses is intended for
single-hop multicast applications. Multicast routers will not forward
datagrams with destination addresses in this range.