bob@acornrc.UUCP (Bob Weissman) (02/13/88)
In "rc.local" files distributed with new systems, I always see
"-trailers" in "ifconfig" statements. Why is this? Aren't
trailers a Good Thing?
We're talking about Vaxen & Suns here, running 4.2 and 4.3 BSD.
--
Bob Weissman
Internet: bob@acornrc.UUCP
UUCP: ...!{ ames | decwrl | oliveb | apple }!acornrc!bob
Arpanet: bob%acornrc.UUCP@AMES.ARPAchris@trantor.umd.edu (Chris Torek) (02/13/88)
In article <599@acornrc.UUCP> bob@acornrc.UUCP (Bob Weissman) writes: >In "rc.local" files distributed with new systems, I always see >"-trailers" in "ifconfig" statements. Why is this? Aren't >trailers a Good Thing? Easy: No; or rather, not in 4.2BSD, not unless you talk only to hosts that also understand trailers. 4.3BSD attempts to negotiate trailers when ARPing, hence trailers can be enabled under 4.3BSD without losing portability. Unfortunately, a legal ARP implementation can confuse the negotiation code in pre-Tahoe-release 4.3BSD. Other than that, trailers are fine. -- In-Real-Life: Chris Torek, Univ of MD Computer Science, +1 301 454 7163 (hiding out on trantor.umd.edu until mimsy is reassembled in its new home) Domain: chris@mimsy.umd.edu Path: not easily reachable
ron@topaz.rutgers.edu (Ron Natalie) (02/13/88)
The reason they are turned off is that they were nonstandard and not all systems can handle them (this has changed, see later). They are a good idea in paged UNIX implemenations which are resigned to do at least one copy of data anyhow. They are not a good idea for other implementations, including most gateways as they do not copy the IP packet at all. They expect the IP header to be in the proper place and just diddle the local net headers and pass it to the output network driver. Trailers are now a standard and leaving the trailers flag turned on in later (standard conforming) systems should be OK. The system now checks while ARPing if the remote system wants to do trailers and uses them when the remote system likes, and avoids them otherwise. -Ron USER: We're having a problem with our gateway. ME: Really? What is it? USER: We just put up a Sun and now the gateway is printing Ethernet errors. ME: Hmm, sounds serious. What kind of errors? Interlan transmit register errors? USER: No, it was something about trailers. ME: Oh, was it by any chance "Trailers make me barf!"? USER: Oh yes, that's it!
kurt@hi.unm.edu (Kurt Zeilenga) (02/14/88)
Bob Weissman writes: >In "rc.local" files distributed with new systems, I always see >"-trailers" in "ifconfig" statements. Why is this? Aren't >trailers a Good Thing? > >We're talking about Vaxen & Suns here, running 4.2 and 4.3 BSD. Since there are many systems that can NOT run trailers at this time, it is VERY wise to only run trailers on systems that do trailer arbitration. 4.3 BSD based systems do this, 4.2 BSD based don't. Even if every existing system ran trailers, I would only enable them on 4.3 based systems. You never know what the guy down the hall is going to put on the network.... >Bob Weissman -- Kurt (zeilenga@hc.dspo.gov)