Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!ukma!david From: david@ms.uky.edu (David Herron -- One of the vertebrae) Newsgroups: comp.protocols.tcp-ip Subject: A 'horror story' for the books Message-ID: <10208@s.ms.uky.edu> Date: 4 Sep 88 15:03:56 GMT Organization: U of Ky, Math. Sciences, Lexington KY Lines: 116 Ok, maybe this one should have been obvious. But if it hadn't been for Doug Kingston giving me a short list of things to check over I would've been a *lot* longer in finding the problem. (*Thanks*!) The short description is that a number of applications (ftp, smtp, etc) stopped working between some of our machines after switching our vaxen over to Ultrix v2.2 (from MtXinu 4.3bsd + NFS -- a step backward if you ask me, but it's a looong story). A question to the mmdf mailing list (at the time I knew only that smtp didn't work) elicited a reply from Doug that he thought it was more likely TCP/IP differences and to check things like trailers and MSS ... A check found that sure 'nuff we had some machines with trailers and some without. Switching off trailers on the interface made the applications work again. I've got a couple of questions for the assembled experts: 1. Why did things continue to sort-of work between the conflicting machines? I haven't looked at the code yet, but my understanding of the rfc is that ALL packets will be trailer-ified when going out a trailer link (or on 4.3bsd, out a trailer link AND when the host in question negotiated trailer use). If ALL the packets were trailer-ified then the hosts would be seeing data where they were expecting header data and get all confused. 2. Why does Sun not recommend trailers? Do they use a different page size than vaxen? Or is it -- in general it's not good to use trailers on machines other than vaxen or it's not good to use trailers in a mixed environment? 3. Is there any financial aid and/or cheaper rates for a student who wants to attend Interop '88? The following is the long version. It's the report which I wrote up for all the networking people on campus. - Date: Thu, 1 Sep 88 17:11:33 EDT - From: David Herron E-Mail Hack - To: uk-net-people@ms.uky.edu - Subject: trailers - Message-ID: <8809011711.aa06222@g.e.ms.uky.edu> - - Oooo boy, the tiny things that'll cause problems ... - - We've had a confusing problem over here since converting to Ultrix, - that some of the programs would work to/from Ultrix machines and other - times they wouldn't. Like, an outgoing smtp connection would work fine - until it sent out that trailing '.' whereupon it would hang. - - Some asking around led to a suggestion to check trailers, MSS (Max - Segment Size) and a few other options. Some checking around in the - code of the affected programs revealed no non-portable code which - Ultrix broke. Ultrix was, fortunately, enough alike (still) BSD that - things worked as they did under BSD. Albeit with an older technology - of TCP/IP. Eventually I ended up at the trailers suggestion. - - What's a trailer? Well, all it says in the manual page is some - mumbling about changing the layout of IP packets to reduce the amount - of copying that's involved. They are documented in RFC893, and related - rfc's are 984 & 894 which cover the details of doing IP across ethernet - like mediums. - - The trailer idea is to fix the size of the data portion of the IP - packet at some multiple of the page size of your machine. Since the - idea was originally developed at UCB for 4.2, the size is 512 bytes or - some multiple (The page size on a Vax). The information which would - normally be at the head of the packet (IP header information like - to/from addresses, packet size & etc) are moved to the end and are now - called 'trailers'. There is also two other things added to the trailer; - a protocol type field and a trailer length field. - - Unfortunately they didn't do anything intelligent originally like - negotiate use of trailers on a per host basis. Instead trailers - are either on or off on a per interface basis, and is done at - boot time when ifconfig is run. UCB's next version did do - negotiation as part of ARP but in the meantime the 4.2 version - of TCP/IP became part of many systems, many of which we have - here on our ethernet. - - Looking at the various manual pages I have access to: - - 4.3bsd negotiable per host (default=trailers) - WIN/TCP non-negotiable (default=trailers) - sun v3.4 non-negotiable. also 'not recommended' - because it's host dependant. (default=trailers) - ultrix 2.2 non-negotiable (default=trailers) - - Some of our machines had trailers turned off and some had them turned - on. Brian had thought it wasn't important because it was negotiated - and turned them on ... oh well. - - One thing I'm not sure about is why things sort-of worked ... between - two non-negotiating hosts which disagreed over the trailer issue there - shouldn't have been *any* communication, because they disagree over - where the 'header' information is to be kept. Probably there is something - else going on as well, but I'm not sure what. - - For now we've turned off trailers on all of our machines. Would the rest - of you look into your configurations and tell me which ones can do trailers - to begin with, and which ones can negotiate it. (The negotiation is part - of the ARP protocol). This is another of those TCP/IP options which needs - to be agreed upon across our whole ethernet. er.. Well ... if someone were - to have an IP gateway between their net and the campus net, they would be - able to do what they want on their net. - - Maybe we want to run with trailers on everywhere. But we need to make - sure that it makes sense for all the machines... - -- - <---- David Herron -- The E-Mail guy - <---- ska: David le casse\*' {rutgers,uunet}!ukma!david, david@UKMA.BITNET - <---- Problem: how to get people to call ...; Solution: Completely reconfigure - <---- your mail system then leave for a weeks vacation when 90% done. -- <---- David Herron -- The E-Mail guy <---- ska: David le casse\*' {rutgers,uunet}!ukma!david, david@UKMA.BITNET <---- Problem: how to get people to call ...; Solution: Completely reconfigure <---- your mail system then leave for a weeks vacation when 90% done.