Installation Manual

TSUNAMI MULTIPOINT
2-7
Figure 2-6: SU Filters in IP Routing Mode
Proxy: to ARP or not to ARP ....
In addition to enforcement of the access modes discussed above, one of the main
functions of the SU is to serve as a proxy server for the local devices. Without this
function many of our network applications will fail. Before we go into detailed
discussions, however, we need to first understand the basics in IP networking.
When a device such as a PC, router, SU or Base Station Unit wants to transmit an IP
packet, it needs to first decide if the destination IP address resides within its own
subnet. If the answer is yes, the sending device needs to search for the corresponding
Ethernet address associated with the destination IP address by broadcasting an ARP
request to its local network. A device with a matching IP address or one which is
interested in receiving packets for that IP address will send back an ARP response,
which contains the destination Ethernet address. Once the destination Ethernet
address is known, the sending device can then deliver the packet directly to that
Ethernet address. If the destination IP address resides in a different subnet, then the
sending device will not even bother to issue an ARP request but will instead forward
the packet to a default gateway for delivery.
Now back to the PMP discussion. In the PMP network, it is quite common to have
multiple SUs share the same subnet address. In that case, when a PC connected to a
SU wants to send packets to a PC connected to another SU, the originating PC will
issue an ARP request to explore the Ethernet address of the destination PC, since
both of them reside in the same subnet. Unfortunately, the destination PC will not be
able to receive the ARP request because it is not physically connected to the
originating PC. This then forces the SU connected to the originating PC to issue a
proxy response on behalf of the destination PC. The SU will issue a proxy response
only when it knows via initial learning that the destination PC is not connected to its
local network.
Access Filter
Programmable
Hash Filter
Subscriber Unit (SU)
Local traffic
ARP Table