Date: Wed, 25 Mar 1998 09:46:10 -0600 (CST) From: "Richard M. Neswold" <neswold@fnal.gov> To: pratap singh <v_pr@hotmail.com> Cc: freebsd-hackers@FreeBSD.ORG Subject: Re: ARP REQUEST question Message-ID: <Pine.BSF.3.96.980325092952.24258A-100000@spiv.fnal.gov> In-Reply-To: <19980325145544.10507.qmail@hotmail.com>
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On Wed, 25 Mar 1998, pratap singh wrote: > I perfectly know about the CRC carried in the ethernet frames. But if that > is the case the higher layer protocols still do the checksum like IP. the > reasons that I feel for not having the checksum in the ARP frame are: > 1> IP packets get routed and can be seen to ascend and descend protocol > stacks till they reach their destination. So even if the layer 2 > (ethernet) checksum is right, they may get corrupted during the ascend and > descend. And if the checksum is not provided by the IP itself, this error > could go unnoticed. If they get corrupted during the ascend or descend, you have a software bug in your TCP/IP stack. The reason IP adds its own checksum is because it can't make an assumption about what interface is being used. Although ethernet can reject a packet with a bad CRC, an RS-232C connection doesn't have this safeguard (unless you're using error correcting modems.) > 2> However same is not the case with ARP which in the protocol stack sat > with the hardware independent part of the Ethernet driver or any other > layer 2 driver (historically) and so the traversal of the protocol stack > was not necessary. And the local significance made the CRC just enough for > detecting errors. But what in case of network stacks of today where the > ARP is seen sitting together with IP or at the same level of IP. Someone > Please correct me If I am wrong. ARP is only used on LANs. To communicate on a LAN, you need a network card which can be ethernet, token ring, arcnet, etc. Each of these technologies has a way of validating incoming packets, so an ARP CRC is redundant. General IP packets can leave the local network through routers which can be RS-232C, radio broadcasts, satellite links, etc. which may not, themselves, have a way to detect transmission errors. Now it's my turn to be corrected if I'm wrong... Rich ------------------------------------------------------------------------ Richard Neswold, Accelerator Div./Controls Dept | neswold@fnal.gov Fermilab, PO Box 500, MS 347, Batavia, IL 60510 | voice (630) 840-3454 'finger neswold@aduxb.fnal.gov' for PGP key | fax (630) 840-3093 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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