Expand Configuration and Management Manual (G06.24+)

Tuning
Expand Configuration and Management Manual—523347-008
20-6
Variable Packet Size
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. Refer to Packet Format on page 20-9 for more
information about the extended and nonextended packet formats.
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 since all data is sent
across only one line. Refer to Multi-Line Paths on page 20-13 for more information
about multi-line paths.
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. Since 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 the following formula:
where txw is the TXWINDOW 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