Path: utzoo!utgpu!water!watmath!clyde!att!pacbell!ames!pasteur!ucbvax!NDSUVM1.BITNET!NU021172 From: NU021172@NDSUVM1.BITNET (Marty Hoag) Newsgroups: comp.protocols.ibm Subject: Re: (none) Message-ID: <8809290241.AA07673@jade.berkeley.edu> Date: 28 Sep 88 16:37:26 GMT References:Sender: daemon@ucbvax.BERKELEY.EDU Reply-To: Marty Hoag Organization: The Internet Lines: 32 > >The one assumption TCP *does* make is that lower layers will not deliver >a corrupted datagram and claim it's correct. The TCP (and IP and UDP) >checksum(s) are a low-cost but extrememly useful last-ditch protection >of this assumption, and are well worth it. > PLEASE remember that IP is a connectionless UNRELIABLE delivery protocol. There is NO checking of the data in the packet - ONLY the header fields! TCP and UDP checksums are only needed if you care what data you will be getting... ;-) Other than the header fields of the IP packet, you are not guarenteed that the rest of the packet is correct or that the packet will even be delivered. The only reason IP checks the headers is to attempt to get them to the right place... =;-) This makes IP fairly simple to implement. I am sure the NFS implementation which turns off checksums should NOT do that if the packets go outside the local ethernet (unless there is some other way to ensure the integrity). In SNA there is a reliable connection provided at a lower level. Of course, many protocols do further checking on the data but it is quite a different arrangement than in IP. ----------- Marty Hoag ND Higher Education Computer Network US Mail: NDSU Computer Center Phone: (701)-237-8639 PO Box 5164 / UCCS Bitnet: NU021172@NDSUVM1 (NOTE 0 = ZERO) Fargo, ND 58105 ARPA: NU021172@VM1.NoDak.EDU UUCP: ...!psuvax1!NDSUVM1.BITNET!NU021172