Date: Sun, 27 Jun 93 04:30:10 PDT
From: Advanced Amateur Radio Networking Group <tcp-group@ucsd.edu>
Errors-To: TCP-Group-Errors@UCSD.Edu
Reply-To: TCP-Group@UCSD.Edu
Precedence: Bulk
Subject: TCP-Group Digest V93 #166
To: tcp-group-digest


TCP-Group Digest            Sun, 27 Jun 93       Volume 93 : Issue  166

Today's Topics:
                         Datagram vs Virtual

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Date: Sat, 26 Jun 1993 13:32:52 -0500 (CDT)
From: Mr. Sampson <ssampson@sabea-oc.af.mil>
Subject: Datagram vs Virtual
To: J.R.Jagger@sheffield-hallam.ac.uk

Jon Jagger <J.R.Jagger@sheffield-hallam.ac.uk> says:
> "AX.25 v2.1 provides. One large IP datagram is fragmented into a
> series of paclen sized AX.25 segments, one per I-frame"
>
> I thought that the paclen parameter did not apply in datagram mode.
> Before I read these two I was sure this was what happened.

First there's no such thing as version 2.1.  This is the figment of someones
imagination. Version 2.0 (October 1984) is the only official specification.
The way I read the sentence was that if you have an IP datagram (not an AX.25
datagram) that you want to send over AX.25 connected mode (VC), then it will
be fragmented into paclen packets.  And it will.  If you're using connected
mode and Netrom, then the channel MTU should be reduced 20 bytes to cover the
Netrom header (196 MTU max I think: 256 - 40 - 20).

I think the words "IP datagram" in the sentence should probably be removed and
replaced with "IP Frame" or something.  The words "IP Datagram" are common
because that's normally what they are.  But then Hams try and pass these over
Virtual Ciruits such as Netrom or Rose.  Oklahoma Hams for example, use NOS
and NET as Netrom replacements or personal mailbox's, so Datagrams are never
used at the link level, and performance sucks.  I'm sure the rest of the world
uses NOS the same way.  At least in my travels, I've never seen a NOS with a
datagram backbone and a VC user port.  They are all limited to very small MTU's
based on the Netrom limitations.  They like it that way because it's the way
God would have wanted it (ie, a religious issue) :-)
---
Steve, N5OWK
"Moslems are like Indians,
    they need little reservations to hold survivors,
     and an army willing to slaughter the rest"

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End of TCP-Group Digest V93 #166
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