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Re: Questions about Internet Packet Losses

  • From: William Allen Simpson
  • Date: Tue Jan 14 00:49:17 1997

> From: Bob Metcalfe <metcalfe@infoworld.com>
> Is Merit's packet loss data (NetNow) credible?

The measurement needs more data points located inside other providers,
but is pretty accurate regarding links into and out of the major NAPs.

> Do packet losses in the
> Internet now average between 2% and 4% daily?

This is nothing new.  In fact, it used to be much worse.  Remember 1986?
1989?  1994?  We've had pretty serious loss rates at times.

> Are 30% packet losses common
> during peak periods?

I've personally measured 40% pretty regularly, and 80% at times.  But it
is much better now than a few years back.

> Is there any evidence that Internet packet losses are
> trending up or down?
>
Losses are on the rise again (past few months), but there was a downward
trend during 1996 for my own connections.  I specifically saw MCI and
Sprint put in faster links last year, and add more private interconnects.
Improved my life immensely.


> Were Merit's data correct, what would be the impact of 30% packet losses on
> opening up TCP connections?

TCP will keep working.   It's much harder on UDP, as the UDP
applications often aren't very good on recovery.  Lots of applications
use UDP that should have used TCP.  TCP is very robust.

> On TCP throughput, say through a 28.8Kbps
> modem?

Compared to what?  I remember using TCP pretty well over 300 bps and
1200 bps modems.  Even at 28.8 Kbps, the delay in the modem swamps the
delay of the net.

> On Web throughput, since so many TCP connections are involved?

The Web is actually a major source of the problem.  It was not very well
designed, protocol-wise.  It causes rapid transient congestion.

> On DNS look-ups?

Yes, DNS uses UDP, and fails a lot more often.  So, I just try again.

> On email transport?

SMTP uses TCP, and still works great.  Neither delay nor packet loss are
an issue.


> How big a problem is HTTP's opening of so many TCP connections?

It is a terrible problem.  There is no time for the congestion and round
trip estimation algorithms to kick in, as each connection is so short.

But, HTTP is being fixed.

> Does TCP
> need to operate differently than it does now when confronted routinely with
> 30% packet losses and quarter-second transit delays?

No.  It works fine.  BTW, 1/4 second delays are not a problem; as I said
earlier, this is actually better than we had when we developed the code,
2 second delays were typical in modems (576 byte payloads at 2400 bps).


> What is the proper
> response of an IP-based protocol, like TCP, as packet losses climb?  Try
> harder or back off or what?

Exponential backoff.  This is all well described by Van Jacobson nearly
a decade ago.  This kind of question is what makes folks think you
haven't done your homework, Bob.


> How robust are various widespread TCP/IP
> implementations in the face of 30% packet loss and quarter-second transit
> delays?
>
BSD 4.4 and derivatives work fine.  Newer implementations with Sack work
a bit better.

Karn's KA9Q stack (used in many small enterprise routers and some MS-DOS
hosts) is even more robust, as it was developed for amateur radio.  High
losses and high delay are typical in radio.

FTP Software's stack seems to perform very well, and they update it
regularly.

MacTCP is terrible.  And many early WWW servers and clients were Mac
based, so they have terrible TCP characteristics.  But, they still
actually worked!

But Apple replaced MacTCP last year with Open Transport, which works
much better.  Everyone should upgrade to MacOS 7.6.

I've also had some problems with Win'95, and actually removed it and
went back to Win 3.1.  Since I gave up on it, I never really got to the
bottom of why it was performing so badly.  As an ISP, we actually charge
more to setup folks with Win'95, while we charge nothing for Macs with
Open Transport.  Amazing difference in user support costs.

I've noticed that there are a rather a lot of old SunOS systems out
there.  They don't have modern versions of TCP/IP, with MTU Discovery
et cetera.  But they should have been upgraded years ago, and are
probably about to fall over anyway.


> Is the Internet's sometimes bogging down due mainly to packet losses or
> busy servers or what, or does the Internet not bog down?
>
Busy servers is the worst problem I've had in the past year.  Much worse
(more frequent) than the packet losses.


> What fraction of Internet traffic still goes through public exchange points
> and therefore sees these kinds of packet losses?  What fraction of Internet
> traffic originates and terminates within a single ISP?
>



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