Kacper Kokot [off-list ref] wrote:

Padding TCP options with NOPs is optional, so it is legal to send an
MSS option that is not aligned to a word boundary and therefore not
aligned for checksum calculation. The current TCPMSS target is not
robust to this: when the MSS option is unaligned it produces an
invalid checksum, and the packet is dropped.

Is this an actual, real world bug?  This code is 20+ years old, all that
this hints at is that they are always aligned in reality?

(Not disputing theoretical problem).