Expand Configuration and Management Manual (H06.21+, J06.10+)

Tuning
Expand Configuration and Management Manual — 529522-013
19 - 6
Variable Packet Size
larger bulk transfers are much more expensive to form into small packets and route in
multihop networks.
Extended Packet Format
The extended packet format (L4EXTPACKETS_ON modifier) provides a means to
fragment packets in transit across the network. The extended packet format must be
enabled for the variable packet-size feature to function.
Although the extended packet format adds considerably more overhead than the
nonextended packet format—the extended packet header size is 64 bytes and the
nonextended packet header size is 16 bytes—the larger packet size more than
compensates for the increased overhead. The variable packet-size feature is designed
to increase the data-per-packet percentage so that only about 10 percent of available
bandwidth is used for non-user data. For more information on the extended and
nonextended packet formats, see Packet Format on page 19-9.
Latency and Multi-Line Paths
On multi-line paths, applications that send messages just below the size of the
configured variable packet size might experience higher latency because all data is
sent across only one line. For more information on the multi-line paths, see Multi-Line
Paths on page 19-13.
Expand-Over-IP and Expand-Over-ATM Connections
HP recommends that the variable packet size feature be enabled for Expand-over-IP
and Expand-over-ATM connections. For best performance, the variable packet size
should be set to 4095 bytes for Expand-over-IP and 8192 for Expand-over-ATM
connections.
Variable Packet-Size Configuration
The variable packet size is determined by the value assigned to the
PATHPACKETBYTES modifier. Because the Expand subsystem sends larger frames
than those configured by the FRAMESIZE modifier when the variable packet-size
feature is enabled, the value specified for the L2TIMEOUT modifier should be
compatible with the largest possible packet rather than the frame size. You can
calculate the value of the L2TIMEOUT modifier using this formula:
where txw is the T
XWINDOW modifier value, pathpacketbytes is the
PATHPACKETBYTES modifier value, lspd is the line speed in bits per second (actual,
not configured), and dl is the DELAY modifier value. The result of this formula is a
one-hundredth of a second value.
(((txw + 1) * pathpacketbytes * 8) / (lspd / 100)) + (2 * dl) + 10