New MAE-EAST

Kent W. England kwe at 6SigmaNets.com
Sat Nov 8 14:58:56 UTC 1997


At 06:17 PM 11/5/97 -0800, Al Roethlisberger wrote:
>
>Perhaps he is referring to latencies that some beleive is incurred as ATM
>'packet shredding' when applied to typical data distributions encountered on
>the Internet that fall between the 53byte ATM cell size and any even
>multiple thereof?

I'm going to rant a little. Sorry Al, but it was you repeating something
allegedly BAD about ATM that once ATM promoters used to say was GOOD, well
it's just too funny and too ironic to pass up.

One of the advantages of ATM as touted by ATM bigots in the early days was
the advantage of "cell interleaving". When two "packets" meet at an
intermediate ATM node, their cells interleave as they are switched through.
This reduces the per-hop latency of an ATM network over a frame network on
the order of microseconds for large packets. An idiotic marketing-initiated
"advantage" that I used to make fun of when ATM marketers would trot it out.

Now you tell me that ATM segmentation probably increases latency because
the modulo 48 byte payload causes the extra padding bytes on some packets
to "take a long time" to be forwarded? On the order of picoseconds. An
idiotic "what else can we think of that's wrong with ATM"
engineering-initiated disadvantage.

And if we could remember what we were actually talking about -- an ATM
switch for an exchange point and not an ATM network -- we can see that none
of this matters, except to show how we know that ATM is Just Bad and we
would never do that.
>
>Some reports that I have seen show a direct disavantage for data where a
>large portion of 64byte TCP ACKS, etc. are inefficiently split among two
>53byte ATM cells, wasting a considerable amount of 'available' bandwidth.
>i.e. one 64byte packet is SARd into two 53byte ATM cells, wasting 42bytes of
>space.  If a large portion of Internet traffic followed this model, ATM may
>not be a good solution.
>

The TCP ACKs are 40 bytes long and if you aren't trying to solve too many
problems at once, you can use an encapsulation that will fit a 40 byte TCP
ACK in a single cell. There isn't a way to stuff a 64 byte packet into a 48
byte payload. Is that a problem!? Only if you have a lot of 64 byte
datagrams, which you don't, because the ACKs are 40 bytes long. I have
actually looked at some Internet traffic distributions to see how big a
problem this isn't.

There is no point agreeing with the Big Backbone Network Engineers that the
MAEs suck. It is in their best interest that the MAEs suck, the CIX is
crippled, you aren't bugging them to plug into a high perf exchange, and
that you, the little ISP, go out of business soon. THEY have private
interconnects which you can't join. Find a co-lo where you can
cross-connect without being robbed or build your own NAP, just don't use
DEC-designed Gigaswitches and FDDI. Use full duplex 100 Mbps Ethernet
switch or find an old Fore switch cheap.

--Kent

Kent W. England                                         President and CEO
Six Sigma Networks                        Experienced Internet Consulting
1655 Landquist Drive, Suite 100                   Voice/Fax: 760.632.8400
Encinitas, CA  92024                            mailto:kwe at 6SigmaNets.com
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