tcp: Don't "negotiate" MSS.
_NO_ OSes actually "negotiate" MSS.
"... This Maximum Segment Size (MSS) announcement (often mistakenly
called a negotiation) ..."
This negotiation behaviour was introduced 11 years ago by r159955
without any explaination about why FreeBSD had to "negotiate" MSS:
In syncache_respond() do not reply with a MSS that is larger than what the peer announced to us but make it at least tcp_minmss in size. Sponsored by: TCP/IP Optimization Fundraise 2005
The tcp_minmss behaviour is still kept.
Syncookie fix was prodded by tuexen, who also helped to test this
patch w/ packetdrill.
Reviewed by: tuexen, karels, bz (previous version)
MFC after: 2 week
Sponsored by: Microsoft
Differential Revision: https://reviews.freebsd.org/D12430