Path: utzoo!attcan!uunet!pyrdc!pyrnj!rutgers!aramis.rutgers.edu!athos.rutgers.edu!hedrick From: hedrick@athos.rutgers.edu (Charles Hedrick) Newsgroups: comp.protocols.ibm Subject: Re: SNA reliability above DLC Message-ID:Date: 24 Sep 88 00:25:22 GMT References: <8809220906.AA11914@jade.berkeley.edu> Organization: Rutgers Univ., New Brunswick, N.J. Lines: 28 Richard Stevens asks whether it's enough to do checksums at the data link level. He says SNA only checks those, but doesn't do anything at a higher level. This issue came up a few months ago in connection with Sun's NFS. NFS turns off checksumming of UDP packets. It depends upon the Ethernet link-level checksums. That is actually fairly good as long as you are operating on a local Ethernet. The problem comes when you start doing NFS through gateways and in our case even serial lines. We got corrupted files due to a hardware problem in a gateway. I don't think we have seen any problems due to corruption by the serial line, but I wouldn't be surprised if we did at some point. A site I talked to about some DECnet problems (another protocol that depends upon link-level reliability) was having trouble due to a very noisy serial line. To be fair to DEC, the line wasn't under the control of a DEC router, and was using HDLC rather than DEC's DDCMP. On the other hand, the checksums being used by the HDLC chip were as strong as (actually I think identical to) those used by DDCMP. I think the consensus in the IP community is that if you care about your data, you do end to end checksums. Also, you probably don't want to depend upon the checksums to correct errors in high error rate situations. You want to clean up your lines, and depend upon the checksums to protect you against occasional glitches (and to tell you when a line needs fixing). You can get away depending on the link-level checksums if you go through only a few links, and you maintain them yourself (so you know when they are being flaky). But it's dangerous as a general practice. This is a major item of contention between Sun and certain customers, since at least as of SunOS 3.x there wasn't even a way to turn on checksumming for NFS. I don't know whether this is "Fixed In 4.0 (TM)".