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1. It sets the MTU size to 1400, which is way too low and actually creates additional traffic and processing. Learn more about MTU.
2. The recommended RWIN usually is not an even multiple of the MSS (MTU minus 40 bytes) so it also creates additional traffic and processing. Learn more about RWIN.
3. In some cases it recommends an RWIN that is too large.
You should use the specific tweak FAQ associated with your level of service:
FastAccess Lite Tweak FAQ (MTU: 1492 RWIN: 63888)
FastAccess Ultra Tweak FAQ (MTU: 1492 RWIN: 63888)
Extreme 3.0 Tweak FAQ (MTU: 1492 RWIN: 63888)
FastAccess 6.0 Tweak FAQ (MTU: 1492 RWIN: 63888)
1)The 1400 byte MTU size was originally used as a stop gap measure back in the early part of the decade to compensate for an issue with specific network traffic that didn't handle packet fragmentation/reassembly well. That network issue was short lived and no longer exists. Additionally, changes made in the CPE helped alleviate certain fragmenting problems. 1400 bytes was also selected because some older applications, such as VPNs, did not automatically lower the MTU and would not work correctly.
2) Using an MTU of 1400 with BellSouth's current network and backbone creates additional packets and traffic. It sends and receives packets that are much smaller than optimum so more packets (and time) are required to send the same amount of data. With the additional packet comes additional overhead. So by recommending 1400 BellSouth has not solved any problems, but created more traffic on their network.
3) The Optimizer also may incorrectly recommend an RWIN that is too high and/or not an even multiple of the MSS. Routinely the Optimizer selects 65535 or higher for Extreme, not because it is the proper setting, but simply because it is the largest RWIN you can have without enabling windows scaling (which creates it's own problems). RWIN should always be an even multiple of the MSS (MTU minus 40 bytes). As recommended by BellSouth an acknowledgement must be sent to the sender right in the middle of receiving a packet. That packet is dumped and has to be resent after the sender gets the acknowledgement from the receiver. So, if they continue to recommend an MTU of 1400 they should at least make the RWIN 65280.
1360 X 48 = 65280.
4) They should universally be recommending an MTU of 1492 and a RWIN of 63888. The 1 in 10,000 customer that needs an MTU lower than 1492 should not be the yardstick for recommendations that create additional traffic for every customer.
Andy Houtz DSL