Net Optics iTap 10 GigaBit Port Aggregator - Manual (Page 7)

iTap 10 GigaBit Port Aggregator
3
User-defined Maximum Packet Size
The user can set a maximum packet size of 64 to 12,000 bytes for each network port.
If any oversized packets are received (the packet length exceeds the programmed
value), they are counted and discarded. This function enables the iTap Port Aggregator
to pass jumbo packets, but discard packets that are oversized as defined by the user.
Timestamping
When the timestamping feature is enabled, the iTap Port Aggregator replaces
each packet's CRC with a timestamp before it is sent to the monitoring tool. The
timestamp records the exact time that the packet arrived, and which Network Port
it arrived from, so events can be correlated with other data.
The timestamp is a 32-bit value. A 30-bit timer is contained in the lower part of the
timestamp. The timer increments every 6.4 nanoseconds (156 Mhz). The two most
significant bits indicate the Network Port: 00 means Port A and 10 means Port B.
Optionally, a new CRC can be regenerated and appended to the packet so the
monitoring tool does not see CRC errors due to the changed field. (See following
section, CRC Regeneration.)
Packet Slicing
Sometimes it is desirable for monitoring tools to see only packet headers and not the
packet payload. For example, when executing a lawful intercept, it may be illegal for
the monitoring tool to read the packet payload. Another reason for passing only the
packet headers is to reduce the bandwidth requirement on the monitor ports.
The iTap Port Aggregator supports this requirement by providing a packet slicing
function. When packet slicing is enabled, a user‑configurable number of bytes at the
beginning of each packet are copied to the monitor ports, and the rest of the packet
is discarded. (Of course, the full packets are always passed between Network Ports
A and B.) Optionally, a new CRC can be regenerated and appended to the packet so
the monitoring tool does not see CRC errors due to the truncated packets. (See
CRC
Regeneration
on the following page.)