Multicast IP datagrams are sent using the same "Send IP" operation
used to send unicast IP datagrams; an upper-layer protocol module
merely specifies an IP host group address, rather than an individual
IP address, as the destination. However, a number of extensions may
be necessary or desirable.
First, the service interface should provide a way for the upper-layer
protocol to specify the IP time-to-live of an outgoing multicast
datagram, if such a capability does not already exist. If the
upper-layer protocol chooses not to specify a time-to-live, it should
default to 1 for all multicast IP datagrams, so that an explicit
choice is required to multicast beyond a single network.
Second, for hosts that may be attached to more than one network, the
service interface should provide a way for the upper-layer protocol
to identify which network interface is be used for the multicast
transmission. Only one interface is used for the initial
transmission; multicast routers are responsible for forwarding to any
other networks, if necessary. If the upper-layer protocol chooses
not to identify an outgoing interface, a default interface should be
used, preferably under the control of system management.
Third (level 2 implementations only), for the case in which the host
is itself a member of a group to which a datagram is being sent, the
service interface should provide a way for the upper-layer protocol
to inhibit local delivery of the datagram; by default, a copy of the
datagram is looped back. This is a performance optimization for
upper-layer protocols that restrict the membership of a group to one
process per host (such as a routing protocol), or that handle
loopback of group communication at a higher layer (such as a
multicast transport protocol).
6.2. Extensions to the IP Module
To support the sending of multicast IP datagrams, the IP module must
be extended to recognize IP host group addresses when routing
outgoing datagrams. Most IP implementations include the following
logic:
if IP-destination is on the same local network,
send datagram locally to IP-destination
else
send datagram locally to GatewayTo( IP-destination )
To allow multicast transmissions, the routing logic must be changed
to:
if IP-destination is on the same local network
or IP-destination is a host group,
send datagram locally to IP-destination
else
send datagram locally to GatewayTo( IP-destination )
If the sending host is itself a member of the destination group, a
copy of the outgoing datagram must be looped-back for local delivery,
unless inhibited by the sender. (Level 2 implementations only.)
A host group address should not be placed in the source address field
or anywhere in a source routing option of an outgoing IP datagram.
No change to the local network service interface is required to
support the sending of multicast IP datagrams. The IP module merely
specifies an IP host group destination, rather than an individual IP
destination, when it invokes the existing "Send Local" operation.
6.4. Extensions to an Ethernet Local Network Module
The Ethernet directly supports the sending of local multicast packets
by allowing multicast addresses in the destination field of Ethernet
packets. All that is needed to support the sending of multicast IP
datagrams is a procedure for mapping IP host group addresses to
Ethernet multicast addresses.
An IP host group address is mapped to an Ethernet multicast address
by placing the low-order 23-bits of the IP address into the low-order
23 bits of the Ethernet multicast address 01-00-5E-00-00-00 (hex).
Because there are 28 significant bits in an IP host group address,
more than one host group address may map to the same Ethernet
multicast address.
6.5. Extensions to Local Network Modules other than Ethernet
Other networks that directly support multicasting, such as rings or
buses conforming to the IEEE 802.2 standard, may be handled the same
way as Ethernet for the purpose of sending multicast IP datagrams.
For a network that supports broadcast but not multicast, such as the
Experimental Ethernet, all IP host group addresses may be mapped to a
single local broadcast address (at the cost of increased overhead on
all local hosts). For a point-to-point link joining two hosts (or a
host and a multicast router), multicasts should be transmitted
exactly like unicasts. For a store-and-forward network like the
ARPANET or a public X.25 network, all IP host group addresses might
be mapped to the well-known local address of an IP multicast router;
a router on such a network would take responsibility for completing
multicast delivery within the network as well as among networks.
