3Com CoreBuilder 3500, Implementation Manual

Page: 247 / 592

Long Custom Filter Example 247
Long Custom Filter
Example
The following solution shows a complex packet filter built from three
simple packet filters. Each of the shorter, simpler packet filters can be
used on its own to accomplish its own task. Combined, these filters
create a solution for a larger filtering problem.
Filtering Problem Your network contains market data feed servers that receive time-critical
financial data needed for trading floor applications. At the center of the
trading floor networks is a system that is being used to switch Ethernet
traffic and to concentrate the market data feed servers onto the FDDI
departmental backbone.
The difficulty is that the market data feed servers transmit data to users
with broadcast packets that are forwarded to all stations on all segments
attached to the system. Not all of the segments attached to the system
have stations that require these broadcast updates. To optimize the
performance of these Ethernet segments, you need to filter the
broadcasts.
Packet Filter Solution The solution described here is to create a highly sophisticated packet filter
that prevents only the broadcast packets from the market data servers
from being forwarded onto the segments that are not part of an active
trading floor.
Before you write the packet filter, it is important to understand the
functions that the filter must provide. The broadcast packets that are
transmitted by the servers are based on either TCP/IP or XNS protocol. In
both cases, the broadcast packets have socket values that are greater
than 0x076c and less than 0x0898. The socket value is located 24 bytes
into the packet in IP datagrams and 30 bytes into the packet in XNS
datagrams.