Endace DAG 4.5G2 Card, User Manual

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EDM 01-17 DAG 4.5G2/G4/GF Card User Guide
Setting
Captured
Packet Size
(cont.)
Variable/Fixed Length
The DAG 4.5 card is able to capture packets in two ways. They are:
•
Variable length capture (
varlen
)
•
Fixed length capture (
novarlen)
In variable length (
varlen
) mode the card will capture the whole packet,
providing its size is less than the
slen
value. Therefore to use this capture
mode effectively you should set the
slen
value to the largest number of bytes
that a captured packet is likely to contain.
Any packet that is larger than the
slen
value will be truncated to that size.
Any packet that is smaller than the
slen
value will be captured at its actual
size therefore producing a shorter record which save bandwidth and storage
space.
The example below shows a configuration for variable length full packet
capture:
dagconfig –d0 varlen slen=9600
In fixed length (
novarlen
) mode the card will capture all packets at the same
length. Any packet that is longer than the
slen
value will be truncated to that
size, in the same way as for
varlen
capture. However any packet that is
shorter than the
slen
value will be captured at its full size and then padded
out to the size of the
slen
value.
This means that in
novarlen
mode you should avoid large
slen
values
because short packets arriving will produce records with a large amount of
padding which wastes bandwidth and storage space.
The example below shows a configuration for fixed length packet capture that
will produce a 64-byte record:
Maximum allowable
slen value
Variable length capture
dagconfig –d0 novarlen slen=1536 First 1536 bytes will be captured.
Note: This is the default value.
Note: For Ethernet records a 64 byte record is made up of 46 bytes of
payload and 18 bytes of ERF header. For more information on Ethernet
records please refer to
Chapter 6: Data Formats later in this user guide.
Fixed length capture.
©2005-2006 20 Version 4: August 2006