I stumble upon both abbreviations in the same context and it seems they are used interchangeably. But I'd like clarification if this is actually the case or if there are differences between the two.
The answer may be as simple as “the two are the same” but it would be very nice to see some proof. I failed to find anything.
Best Answer
From the diffs of draft-ietf-ippm-tcp-throughput-tm:
This diff seems to show all the RWIN abberviations being replaced with RWND, below is an example.
The latest version of this document, RFC 6349, doesn't have RWIN anywhere in it. Looks like they mean the same thing to me.