3Com 4007, Implementation Manual, Page: 336 / 684

336 C
HAPTER
15: P
ACKET
F
ILTERING
Port Group Filter See “Using Port Groups in Custom Packet Filters” for a port group filter
example.
Limits to Filter Size
A packet filter program is stored in a preprocessed format to minimize the
space that is required by the packet filter definition. Comments are
stripped. When assigned to a port, the packet filter is converted from the
stored format to a run-time format to optimize the performance of the
filter. Each module is limited to a maximum of 16 packet filter programs.
The maximum length of a packet filter definition is 4096 bytes.
Storage Rules for
Preprocessed
Packet Filters
Each module provides a maximum of 2048 bytes of nonvolatile storage
for
preprocessed packet filter programs. In the preprocessed stored
format:
■
A single packet filter program is limited to 254 bytes.
■
Each instruction in the packet filter program requires 1 byte for the
opcode and size, plus additional bytes for any explicit operands.
■
Module overhead is 22 bytes, plus a per-packet-filter overhead of
13 bytes. For example, assume a packet filter program requires 200
bytes for storing the instructions in the program. If this packet filter is
the only one loaded, the nonvolatile memory required is 22 bytes (for
module overhead) plus 13 bytes (for packet filter overhead) plus
200 bytes (for the program itself) — a total of 235 bytes.
Run-time Storage of
Packet Filters
For
run-time
storage of packet filter programs, each module provides a
maximum of 8192 bytes. There is no explicit system or per-packet-filter
overhead; however, performance considerations can result in unused
areas of the run-time storage.
The run-time format is approximately eight times the size of the stored
format. Thus a 200-byte packet filter program in stored format expands
to approximately 1600 bytes in the run-time format. A single packet filter
program cannot exceed 2048 bytes in the run-time format.