Perle Systems P840 User Manual download pdf (Page 50)

Filtering

Transport Control Protocol / Internet Protocol (TCP/IP)

The previous example showed how to filter all Ethernet frames that contained an IP protocol packet. However, IP is

used as the Network-layer protocol for more than 40 different Transport-layer protocols, TCP being only one of

them. Therefore, with the mask that was used as noted in the previous IP example, all Transport layer protocols that

used IP would also be filtered. This may not be desirable in all cases.

For this example, the discrimination of the Transport Layer used within an IP packet will be demonstrated. This

requires an AND function, since we want to filter data that both is IP and contains TCP information.

Within the IP frame, there is a single octet field that may be used to indicate the protocol of the Transport layer, or

the protocol of the data in the IP packet. If TCP were the protocol within the IP packet, this octet, or 8-bit byte,

would be equal to 6.

The location of this field, remembering that the start of the Ethernet frame is always the base reference, is octet 23.

Filter only TCP/IP

To filter only those packets that are TCP/IP, the mask would therefore be : 12-0800&23-06

The 12-0800 is the IP expression and the 23-06 will represent TCP in an IP frame. The “&” is the logical AND

operator, so the expression requires that the frame be both an IP and TCP.

Filter all IP without TCP traffic

To filter all IP packets that do not contain TCP traffic, the mask would be : 12-0800&~(23-06)

Filter all except TCP/IP

To filter all other packets except TCP/IP packets, the mask would be: ~(12-0800&23-06)

Local Area Transport (LAT)

The Local Area Transport (LAT) protocol is used exclusively by DEC for terminal access between DEC hosts and

terminal servers located on an Ethernet network.

This example is similar to the Internet Protocol example described previously.

The protocol type field value that is used for LAT frames is equal to 6004.

Filter all LAT

Therefore, to filter all LAT frames, the filter mask would be : 12-6004

Filter all but LAT

To filter all frames but LAT frames, the filter mask would be: ~(12-6004)

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Perle Systems P840 User Manual Page 50