[e2e] purpose of pseudo header in TCP checksum

[Noel Chiappa]

    > From: "Alok" <alokdube at hotpop.com>

    > I dont understand what "This gives the TCP protection against
    > misrouted segments" means, do intermediate routing nodes look at
    > TCP checksum?

What it means is that a software/hardware bug just about anywhere in the
network might mean that a packet intended for host X arrives at host Y
(e.g. by overwriting the destination address X with Y). If the SA/DA were
not included in the TCP checksum, such a packet would pass the TCP
checksum test.

    > Is it because that the SA/DA may be corrupted as the IP header
    > checksum does not run on SA/DA ?

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.

	Noel