WAAS TCP windows scaling in WAN

[snarayanaraju]

Hello - I recently read from Cisco WAAS configuration guide that WAAS uses windows scaling technique as a part of TFO optimization that allows the receiver of a TCP packet to advertise that its TCP receive window can exceed 64 KB. The receive window size determines the amount of space that the receiver has available for unacknowledged data. By default, TCP headers limit the receive window size to 64 KB, but Windows scaling allows the TCP header to specify receive windows of up to 1 GB.

But on WAN interfaces the MTU would be maximum 1500 Bytes or 1360 Bytes (GRE Tunnel). How does WAAS makes the TCP window size optimization when the WAN does not allow the MTU beyond 1500 Bytes.

Thanks in advance

SAIRAM

[Joseph W. Doherty]

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Posting

About the same as a 64KB receive window works with a 1500 MTU.

MTU is a L2/L3 consideration.  RWIN is a L4/TCP consideration.

I.e. it doesn't matter that the MTU cannot exceed 1500 bytes.

[snarayanaraju]

Thank you Joseph for the response

Can i understand this way?. MTU means L2/L3 size should not exceed 1500 Bytes. But Data size (which is encapsulated by L2/L3 headers) can be 64000 Bytes?

SAIRAM

[Joseph W. Doherty]

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages wha2tsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

If I remember correctly, a TCP segment can be up to 64KB, although usually the sending host will size the segment so it will fit within a single packet.

BTW, don't confuse TCP segment size with RWIN (although the latter must be large enough to contain the former).