Packet Filter Solution - 3Com LANPLEX 6000 User Manual

14-12 C 14: C
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REATING AND SING ACKET

Packet Filter Solution

The solution 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 writing 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 either on TCP/IP or on XNS. In both cases, the broad-
cast packets have socket values that are greater than 0x076c and less than
0x0898. The socket value is located 24 bytes into the packet in the case of IP
datagrams, and 30 bytes into the packet in the case of XNS datagrams.
The above information can be used to create pseudocode that simplifies
the process of writing the actual filter. The pseudocode may be written in
outline form, as shown below:
1 Determine if the packet has a broadcast address. (This is done through the
packet filter path assignment.)
2 Determine if the packet is an XNS datagram.
3 Check socket values and discard the packet if:
a The socket value is greater than or equal to 0x76c
b The socket value is less than 0x898
4 Determine if the packet is an IP datagram.
5 Check socket values and discard the packet if:
a The socket value is greater than or equal to 0x76c
b The socket value is less than 0x898
6 End the filter.
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ILTERS