Q: Can you set RWIN too high?
A: YES!

Here is the issue:

• Large RWIN values (e.g. 150K) can results in 100+ outstanding TCP segments
• *Culmultive* ACKs used by default in many TCP stacks
• Drop just one segment (e.g. #50 of 100) and 50-100 have to be retransmitted
• Even modest error rates (<1%) result in using 33% of bandwidth for retransmissions
• *Selective* ACKs (SACKs) minimize this issue, but they may not be enabled

Conclusion:

• Determine if you have SACK enabled
• If yes, then you can be aggressive (RWIN=64KB+)
• If not, evaluate your error rate
• If low (<.01%) then you can still be aggressive (RWIN=64KB+)
• If high (>.01%) then you should be conservative (RWIN=8-16KB)

Note: RWIN values >64KB are seldom necessary, so don't set it higher unless your bandwidth*delay warrants.

For more details on this subject see:

RFC 1072 - TCP Extensions for Long-Delay Paths

RFC 2018 - TCP Selective Acknowledgment Options