[e2e] purpose of pseudo header in TCP checksum
jnc at mercury.lcs.mit.edu
Tue Feb 15 06:35:59 PST 2005
> 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
> 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.
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