Path: utzoo!utgpu!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!bloom-beacon!think!barmar
From: barmar@think.COM (Barry Margolin)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Trailer implementation techniques
Message-ID: <31039@think.UUCP>
Date: 11 Nov 88 18:11:46 GMT
References: <5380@medusa.cs.purdue.edu>
Sender: news@think.UUCP
Reply-To: barmar@kulla.think.com.UUCP (Barry Margolin)
Organization: Thinking Machines Corporation, Cambridge MA, USA
Lines: 33

In article <5380@medusa.cs.purdue.edu> narten@cs.purdue.EDU (Thomas Narten) writes:
[Summary of RFC-893 ommitted.]
>  This may involve copying the header to the beginning of the
>packet, but most importantly, the *data* part does not move.

That is the crux of my question.  How do you copy the header to the
beginning of the packet without moving the data to make room?  As I
said, remapping page tables is not an option, since the Ethernet
driver is implemented in a process running in virtual mode on
Symbolics machines (packets are just Lisp byte arrays).

>The relevent trailer question is not "how does one implement
>trailers", but "why implement them at all".

That issue is not important to me.  The point is, some of our machines
like to send trailer packets, and we lose when they try to do it to
Symbolics machines.  The Suns and 4.3bsd machine use trailer
negotiation, but Ultrix requires us to specify "-trailers" in the
ifconfig command in /etc/rc.local, and sometimes this gets omitted.
Sometimes I'll realize the problem right away, but other times people
go for days without figuring it out.  I see no reason why the Lispms
shouldn't be able to receive anything the Vaxen like to send (within
reason).  I have NO plans to teach the Lispms how to SEND trailers,
though.

I see this as an instance of "be liberal in what you accept,
conservative in what you send" rule.

Barry Margolin
Thinking Machines Corp.

barmar@think.com
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