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

Expand Modifiers
Expand Configuration and Management Manual — 529522-013
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L4EXTPACKETS_OFF/L4EXTPACKETS_ON
To calculate the size of the congestion window, use the following formula:
<Congestion_window_size_in_Bytes> = bandwidth * delay
Where,
<Congestion_window_size_in_Bytes> is the maximum amount of data
on the network circuit in bits. (bandwidth delay product)
bandwidth is the capacity of the data link in bits per second
delay is the end-to-end delay in seconds (round trip time).
You can use the bandwidth delay product (BDP) to calculate the maximum amount of
data that can be in transit in the network. It is used to tune systems to the type of
network being used. If given the actual data link speed and delay on the network, the
network capacity can be calculated. Conversely, If you want to limit the amount of data
sent, it can be used to calculate the maximum value to limit or clamp the window.
Example
Consider a data link speed of 50 megabits per second and a delay of 200 milliseconds.
Therefore, the maximum congestion window capacity is calculated as:
50,000,000 bits per second X 0.200 seconds = 10,000,000 bits
This means, the path can have 10,000,000 bits (or 1,250,000 byte) outstanding before
an ack is required. When this window capacity is in use, the data link is fully utilized.
To set a limit on the bandwidth to be used, use the cwnd (L4CWNDCLAMP) clamp. To
calculate the cwnd clamp value, choose the desired maximum amount of bandwidth
the path should use and then, using the BDP formula, calculate the cwnd clamp size.
For example, to limit the path to use only 30 megabits of the 50 megabit link, the
calculation is as follows:
30,000,000 bits per second X 0.200 seconds = 6,000,000 bits
And convert the cwnd clamp value to bytes. Applying Expand's window scale factor
gives a clamp value of 23437.
6,000,000 / 8 = 750,000 bytes
75,000 bytes / 32 = 23437 (L4CWNDCLAMP)
L4EXTPACKETS_OFF/L4EXTPACKETS_ON
Default: L4EXTPACKETS_ON
Units: Not applicable
Range: ON or OFF
These path modifiers are applicable to all Expand line types. The
L4EXTPACKETS_ON modifier enables an extended packet header format of 64 bytes
Note. The congestion control feature must be enabled through the modifier
L4CONGCTRL_ON to use the L4CWNDCLAMP modifier.