Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!bloom-beacon!husc6!genrad!decvax!jfcl.dec.com!frg
From: frg@jfcl.dec.com (Fred R. Goldstein)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: Re-fragmenting IP Datagrams
Message-ID: <453@jfcl.dec.com>
Date: 23 Feb 89 17:30:20 GMT
References: <8902211417.AA12976@etn-wlv.EATON.COM> <8902230119.AA22877@flora.wustl.edu>
Reply-To: frg@jfcl.nac.dec.com.UUCP (Fred R. Goldstein)
Organization: Digital Equipment Corporation
Lines: 34

In article <8902230119.AA22877@flora.wustl.edu> guru@FLORA.WUSTL.EDU (Gurudatta Parulkar) writes:
>
>    Fragmentation is addressed in RFC 791 section 2.4.  All Internet
>    hosts must be able to accept, at a minimum, a 576 octet packet.
>    Re-fragmentation of an IP datagram is permitted as long as the
>    "don't fragment" flag is not set. 
>
>I believe the specifications also require all networks to be able to
>forward a 576 octet datagram without fragmentation. I can understand
>logic behing this requirement as part of IP specifications. But I
>wonder what would happen when the first ATM network joins the internet
>with packet size of about 64 bytes. Most of the high speed packet
>switches being desinged plan to conform to the ATM standards.

There is a simple answer to that question.  ATM operates within
the (OSIRM) Physical layer, while IP operates within the SNICP role
of the Network layer.  As nonadjacent layers, they do not have to
get along.

ATM cells (not "packets") carry fragments of data link layer packets
((or "frames", but not really frames in this case).  A segmentation
and reassembly function takes place atop ATM to allow large packets
to be carried.  One such technique is proposed in the current 802.6
letter ballot.  ANother, called the ATM Adaptation Layer (AAL), has
been bandied about the Broadband ISDN group.  It is fatally flawed, 
as currently proposed, but it does support long packets.  Any number
of workable mechanisms (which would take the Data Link layer into
account) can be created for this purpose.

Or to look at it differently, T1 carrier with D4 channel banks
uses 1-byte fragments, position-interleaved.  ATM uses 64-byte
fragments, labeled.  But they're both down in mux-land, not in
the subnetwork role that IP has to contend with.
     fred (T1S1 member)