This parameter defines the maximum amount of incoming data (in bytes) that can be buffered by the receiving system before the transmitting host must wait for an acknowledgement.
It makes sense to set an RWIN value that is an even multiple of your MSS setting, as this increases the number of full-sized TCP segments transmitted, which naturally improves throughput (particularly for large transfers).
Letting Windows set RWIN
If you allow Windows® to set RWIN it defaults to 8192 bytes, which it then automatically rounds up to the nearest multiple of the MSS value used for the current connection. If this isn’t at least 4 times the MSS value it is adjusted to 4 * MSS, with a maximum limit of 64K. If your MaxMTU also uses the Windows® default of 1500, RWIN is rounded up to 8760 (1460 * 6).
Setting a fixed value of 4*MSS
Setting this to a lower value can significantly improve performance. On most systems, setting exactly 4 times your MSS has proved to deliver optimum throughput. This is also the recommended setting.
Setting RWIN manually
Performance can vary with different configurations and connection types, however. Many users report better performance with RWIN set to 6, 8 or even 10 times the MSS value.
Be warned, however, that setting RWIN manually can also decrease performance. The reason for this is because it takes control away from Windows®. If the MaxMTU is then negotiated to a different value RWIN will no longer be a whole integer multiple. For manual setting to be effective you must take steps to ensure that your MaxMTU will not be negotiated to a lower value.
DSL and other broadband connections
Much of the above may not apply for DSL and other broadband connections. You probably shouldn’t try to adjust the settings manually for these systems unless you have the necessary background information and experience.