The 4.3 Reno-derived TCPs (the majority) are fairly robust.

Yeah, but I doubt that they're in the majority. Recent stats indicate that about 9% of the packets out there are using an MSS of 512. Another 1% are using an MSS of 256. Only 6% are using 536, and 7% are using 1460. This indicates to me that there are a whole lot of broken boxes out there still.

I'm wondering why. The easiest way to implement TCP stack always was to pull the pieces off BSD code. Even oldest MS-DOS TCPs (like Phil Karn's KA9Q) are essentially that. --vadim