D-Link DFL-2500 - Manual (Page 110)

4.5. Multicast Routing
4.5.1. Overview
Certain types of Internet interactions, such as conferencing and video broadcasts, require a single
client or host to send the same packet to multiple receivers. This could be achieved through the
sender duplicating the packet with different receiving IP addresses or by a broadcast of the packet
across the Internet. These solutions waste large amounts of sender resources or network bandwidth
and are therefore not satisfactory. An appropriate solution should also be able to scale to large
numbers of receivers.
Multicast Routing solves the problem by the network routers themselves, replicating and forwarding
packets via the optimum route to all members of a group. The IETF standards that enable Multicast
Routing are:
1. Class D of the IP address space which is reserved for multicast traffic. Each multicast IP
address represent an arbitrary group of recipients.
2. The Internet Group Membership Protocol (IGMP) allows a receiver to tell the network that it is
a member of a particular multicast group.
3. Protocol Independent Multicast (PIM) is a group of routing protocols for deciding the optimal
path for multicast packets.
Multicast routing operates on the principle that an interested receiver joins a group for a multicast by
using the IGMP protocol. PIM routers can then duplicate and forward packets to all members of
such a multicast group, thus creating a distribution tree for packet flow. Rather than aquiring new
network information, PIM uses the routing information from existing protocols, such as OSPF, to
decide the optimal path.
A key mechanism in the Multicast Routing process is that of Reverse Path Forwarding. For unicast
traffic a router is concerned only with a packet's destination. With multicast, the router is also
concerned with a packets source since it forwards the packet on paths which are known to be
downstream, away from the packet's source. This approach is adopted to avoid loops in the
distribution tree.
By default multicast packets are routed by NetDefendOS to the core interface. SAT Mutliplex rules
are set up in the IP rule set in order to perform forwarding to the correct interfaces. This is
demonstrated in the examples which follow.
Note
For multicast to function with an Ethernet interface on any D-Link Firewall, that
interface must have multicast handling set to On or Auto. For further details on this
see Section 3.3.2, “Ethernet”.
4.5.2. Multicast Forwarding using the SAT Multiplex Rule
The SAT Multiplex rule is used to achieve duplication and forwarding of packets through more than
one interface. This feature implements multicast forwarding in NetDefendOS, where a multicast
packet is sent through several interfaces. Note that, since this rule overrides the normal routing
tables, packets that should be duplicated by the multiplex rule needs to be routed to the core
interface.
By default, the multicast IP range 224.0.0.0/4 is always routed to core and does not have to be
manually added to the routing tables. Each specified output interface can individually be configured
with static address translation of the destination address. The Interface field in the Interface/Net
Tuple dialog may be left empty if the IPAddress field is set. In this case, the output interface will
be determined by a route lookup on the specified IP address.
4.5. Multicast Routing Chapter 4. Routing
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