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

Tuning
Expand Configuration and Management Manual — 529522-013
19 - 9
Packet Format
Packet Format
The Expand subsystem enforces a minimum value of 1024 bytes for the variable-
packet size (set with the PATHPACKETBYTES modifier). The default value for
PATHPACKETBYTES (1024 bytes) yields the same data-per-packet percentage as
nonextended packets with a frame size of 132 words.
Table 19-2 compares the data-per-packet percentages for nonextended packet
(L4EXTPACKETS_OFF modifier) and extended packet (L4EXTPACKETS_ON
modifier) header formats. An Expand network using a frame size of 132 words is
assumed.
Although the data-per-packet percentage is highest when the PATHPACKETBYTES
modifier is set to 4095 bytes, there are other effects of using a 4095-byte packet size
that you must consider. One of these considerations is the effect of the packet size on
a multi-line path. For more information on the packet size and multi-line paths, see
Multi-Line Paths on page 19-13.
Congestion Control
The Expand subsystem’s congestion control feature improves throughput in networks
that are subject to varying delays. The congestion control feature allows a connection
to reach an equilibrium point slowly and then pick up speed as additional bandwidth is
found. An additional mechanism allows quick error-recovery without wasting bandwidth
and is more efficient than the out-of-sequence timer recovery provided by Expand line-
handler processes that do not use the congestion control feature.
In networks with low delay variance and few or no errors, the congestion control
feature provides up to 8 percent improvement in response time with no significant
difference in processor utilization. In networks with high delay variance, the benefits of
congestion control are more significant, as recovery from lost packets because of
congestion is handled much more efficiently. In networks with high error or packet-loss
rates, congestion control provides a more efficient error-recovery mechanism;
however, congestion control cannot solve problems caused by noisy lines or poorly
configured bridges and routers.
Table 19-2. Data-Per-Packet Percentages
Packet
Header
Format
PATHPACKETBYTES
Modifier V
alue (Bytes)
Packet
Size
(Bytes)
Packet
Header
(Bytes)
Data
Portion
(Bytes)
Percentag
e
(Bytes)
Nonextended Any 256 16 240 93.75
Extended 0 256 64 192 75.00
Extended 1024 1024 64 960 93.75
Extended 4095 4095 64 4031 98.44
Note. The variable packet-size feature cannot be used with the L4EXTPACKETS_OFF
modifier
.