kern/56869: TCP Encapsulation of IPsec Packets from rfc8229

[andrew.cagney%gmail.com@localhost]

>Number:         56869
>Category:       kern
>Synopsis:       TCP Encapsulation of IPsec Packets from rfc8229
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    kern-bug-people
>State:          open
>Class:          change-request
>Submitter-Id:   net
>Arrival-Date:   Tue Jun 07 19:00:01 +0000 2022
>Originator:     Andrew Cagney
>Release:        trunk
>Organization:
>Environment:
NetBSD arm64 9.2_STABLE NetBSD 9.2_STABLE (GENERIC64)
>Description:
The RFC is https://datatracker.ietf.org/doc/html/rfc8229

As a starting point for discussions, on linux the theory is

Initiator:
  opens connection
  writes the magic IKETCP
  flips to encap mode
  any further reads/writes are encapsulated by the kernel

Responder:
  accepts connection
  reads magic IKETCP string
  flips to encap mode
  any further reads/writes are encapsulated by the kernel

where flipping to encap mode uses the call:
  if (setsockopt(fd, IPPROTO_TCP, TCP_ULP, "espintcp", sizeof("espintcp"))) {
(there's also some SPD fiddling but I'm not sure that is relevant)

>How-To-Repeat:

>Fix: