[e2e] purpose of pseudo header in TCP checksum

[Michael Welzl]

> In IPv4, the IP header checksum does cover the SA/DA - it covers
> everything in the IP header, including the hop count. In IPv6, there is
> no header checksum, of course.

...which was a bad decision IMO, because it breaks the
"IP over everything" model and will now lead to problems
with mechanisms such as UDP Lite, where link layers are
now required check the headers (but ONLY the headers)
of things above. Yuck.

Cheers,
Michael