Subject: Re: Question on Martini AAL-5 SDU mode

From: Eric Rosen <[email-masked]>
Date: Wed, 14 Aug 2002 12:37:45 -0400

John> Those NIC cards several companies sold in the 90s went somewhere. 

Next to the FDDI cards, I think. 

John> And even when they went into a  server instead of a desktop, it is the
John> same problem. 

No,  you'd  need  to  argue  that  both the  clients  and  the  servers  are
ATM-attached, and in a way that uses ATM as the network layer rather than as
a data link.

John> we know  that IP  provides a checksum  that will ultimately  catch any
John> errors. 

TCP; IP only checksums the IP  header.  Actually, even the TCP checksum does
not provide  application level reliability.  If  you really want  to be sure
that your file  has transferred correctly, you need a  checksum of the file,
not checksums of the TCP segments into which the file has been split.

John> I agree that replacing the protection mechanism with another
John> is a valid approach.  But it must provide equivalent protection. 

John> Ethernet over ATM does this because AAL5 provides a CRC that
John> goes between two Ethernet/ATM interworking devices.

If one  regards the purpose  of the ethernet  CRC as demonstrating  that the
packet  has not  been unintentionally  altered  between the  source and  the
destination (as indicated  by the MAC address fields), then  the only way to
achieve this purpose  is to carry the original CRC  unchanged from source to
destination. If  on the other  hand, one regards  the purpose of the  CRC as
demonstrating that the packet has  not been unintentionally altered while on
the  ethernet medium,  then its  purpose has  been achieved  as soon  as the
packet leaves  the ethernet medium.   In either case,  I don't see  that the
existence  of the  ethernet CRC  implies a  requirement for  an AAL5  CRC in
ethernet/ATM interworking.

Theoretical considerations aside, we do all  seem to agree that the SDU mode
specification should simply but clearly state the facts.

---

From: Jim Harford <[email-masked]>
Date: Wed, 14 Aug 2002 13:24:25 -0400

Lloyd,
In case you have not heard, there's this technology called ADSL that uses 
ATM.  I think several million lines have been installed in North America 
and a decent percentage of those are NIC cards that are installed into 
desktop PCs.

Jim Harford

At 05:16 PM 8/14/2002 +0100, you wrote:
>On Wed, 14 Aug 2002, Rutemiller, John wrote:
>
> > Guess again. Those NIC cards several companies sold in the 90s went
> > somewhere.
>
>dumpsters, mostly.
>
>
> > If you consider only the case of IP over ATM, then even though AAL5
> > does provide an end-to-end service (between routers), we can do without
> > it because we know that IP provides a checksum that will ultimately
> > catch any errors.
> >
> > But we can't make that assumption with AAL5 in general.
> >
> > Which is why I agree with other comments that clearly stating the
> > limitations of SDU mode and its applicability is a way around the
> > problem.
> >
> > If anyone ever wants to use SDU mode generically, then PWE3 will
> > need to provide a CRC to replace the AAL5 CRC. Or use only PDU mode.
>
>No. PWE3 needs to do nothing, since SDU mode can be used generically
>in an ATM network without PWE3, with exactly the same problems.
>
>L.
>
><http://www.ee.surrey.ac.uk/Personal/L.Wood/><[email-masked]>
>
>
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