Visara CNA-8000 Installation - Manual (Page 69)

CNA-8000 Installation and Configuration
707158-003 5-5
Window Size Selection
The Transmit and Receive Windows Size parameters are part of a static sliding Window
algorithm used by LLC for controlling packet flow. There is a separate set of Transmit
and Receive Window settings for each end of an LLC circuit. Coordinating the values
used is recommended.
The Transmit Window represents the maximum number of outstanding packets (packets
without a response or acknowledgement) a platform can send before having to stop and
wait for a response to one of the packets. Setting the Transmit Window too small may
result in a platform having to stop and wait for a response often, causing poor response
times. Setting the value too high, may result in having to retransmit several packets,
whenever a packet gets clobbered or lost. (If a negative acknowledgement is received for
a packet, then that packet and subsequent packets that have already been sent must be
resent.)
The Receive Window represents how many packets in a single conversation can be
received before an acknowledgement must be queued to be sent. If a Receive Window
size is set too large, the receiver may be waiting to receive additional packets that are not
forthcoming, resulting in unnecessary delays in sending acknowledgements.
As a general rule, you want to keep the Transmit Window set to a higher value than the
Receive Window. This allows the transmitter to keep sending packets, and with the lower
Receive Window at the session partner, the partner will send acknowledgements back
sooner than required. As the transmitter receives acknowledgements, the number of
outstanding unacknowledged packets will decrement allowing for even more packets to
be sent. The longer the path from transmitter to receiver, the larger difference you may
want between Transmit and Receive Windows. (This is the sliding window.) This works
well for LAN environments, but may not for WAN environments.
In WAN environments packets can be distorted or lost or delayed to the point where an
earlier packet arrives after a later packet. In cases where there is an apparent sequence
number error, the normal recovery is for the LLC circuit to be taken down and brought
back up. This can result in abnormal termination of a session. Keeping the Transmit and
Receive Windows set to 1 ensures that there are no sequence number errors.
The rules of thumb for setting the Window sizes are as follows:
1) For a LAN environment use a Transmit Window size of 2 + an additional
value of 1 for each hop (bridge or router that is crossed) between the
transmitter and receiver, up to the maximum value.
2) Keep the Receive Window set to the common default of 1.
3) For a WAN environment with frequent congestion or unreliable delivery, use
a Transmit Window of 1 and a Receive Window of 1.