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From: kiernan@sham.berkeley.edu (Mike Kiernan)
Newsgroups: comp.protocols.tcp-ip
Subject: Pseudo headers in TCP/UDP checksum -- Why?
Message-ID: <10010@pasteur.Berkeley.EDU>
Date: 17 Feb 89 09:14:18 GMT
Sender: news@pasteur.Berkeley.EDU
Reply-To: kiernan@sham.berkeley.edu (Mike Kiernan)
Organization: Experimental Computing Facility (XCF), UC Berkeley
Lines: 17

What is the historical reasoning behind including a pseudo header
(IP src/dst addr, proto #, TCP/UDP len) in the TCP/UDP checksums?

The RFCs state:
	This gives the TCP/UDP protection against misrouted segments.

Misrouted segments from the IP level???  I can't buy this...

People have suggested that it is because TCP might be layered on top
of something else (that doesn't checksum its header), but that doesn't
seem likely (adding a header checksum layer makes more sense).

What is the real reason behind the pseudo header?
Do misrouted packets really get through the IP level to the TCP/UDP level?

Mike Kiernan
kiernan@xcf.berkeley.edu