PDP 11/23 PLUS system for sale
ajp166 at bellatlantic.net
Wed Jun 15 18:36:35 CDT 2005
>Subject: RE: PDP 11/23 PLUS system for sale
> From: Paul Koning <pkoning at equallogic.com>
> Date: Wed, 15 Jun 2005 16:02:43 -0400
> To: cctalk at classiccmp.org
>>>>>> "Allison" == Allison <ajp166 at bellatlantic.net> writes:
> >>> From what I remember (very blurry now) DEQNAs were known to
> >>> corrupt
> >> data. That was very obvious on VAXclusters, which is why VMS
> >> eventually took them off the supported device list permanently.
> >> But it's an issue for any application (except, *maybe*, when
> >> running TCP since the TCP layer checksum may help -- or may not,
> >> it's not that strong...). That applies just as much for PDP11s.
> >> paul
> Allison> No they _could_ corrupt data, not they did all the time.
> Allison> The differnce was the error rate was not what DEC wanted for
> Allison> transactions. The VAX people put pressure to not have to
> Allison> test the data as LAVCs (Local Area VAX Clusters) were
> Allison> popular to a point and required a very high level of data
> Allison> (code!) integrety. The frequency of the failure was related
> Allison> to the traffic level on the local loop.
>"pressure not to have to test the data" -- hm. That's an interesting
>way of looking at. The way I would look at it is that ALL DEC network
>protocols put the responsibility for data integrity in the network
>devices; NONE of them had upper layer checksums the way TCP does.
>That's why DEC Ethernet bridges always had end to end CRC. And that's
>why DEC insisted on 32 bit CRC for Ethernet -- 16 bit CRC isn't good
>enough at those data rates. The expectation (and probably the
>expressly stated requirement, though I don't remember for sure) is
>that Ethernet NIC devices were required to deliver that level of data
>integrity to the host.
>So in fact the QNA was failing to deliver the data integrity that
>everyone expect it to deliver. The VAXcluster people were the most
>vocal and had the most pull, so they were the ones who actually had
>the power to say "we will not accept that device". But plenty of
>other people cheered when that happened. Certainly the network
>architecture group, responsible for standards, did.
That's about the story. One little bit was that doing a checksum
was really a CRC and the code to do that apparently was slow on uVAX.
They didn't want the impact of that in the OS driver when hardware
could do it far faster.
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