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Re: PMTU-D: remember, your load balancer is broken

  • From: Valdis.Kletnieks
  • Date: Wed Jun 14 11:10:19 2000

On Tue, 13 Jun 2000 22:36:08 MDT, Marc Slemko said:
> I shouldn't get started here.  I have trouble buying into HP's
> way of doing things (I was only aware that HPUX did this; but it seems
> that AIX does too...).  If you run a high traffic DNS server

AIX started supporting PMTU-D for both TCP and UDP in 4.2.1.  The gotcha
was it being on by default in 4.3.3.

> on an AIX box without disabling this "feature" then you must just
> be spewing ICMP echo requests.  It could add up to more bytes
> than your DNS responses...

Well, as I said, it was done in error, and yes, the bytes for the ICMP
*were* running almost as high as the actual NTP traffic... 

The surprising part is that it was broken for close to 3 months before
somebody noticed (yesterday, just a few hours before this discussion started,
in fact).

As noted, PMTU-D for TCP is a lot lighter weight, and has an actual chance
of winning sometimes. Does anybody know of a UDP-based application that is
able to *do* anything with PMTU-D?  A co-worker had heard of research at
PSC that dealt with TCP-friendly multicast, but that was all we could think of...

> And, obviously, ICMP pings don't work too well much of the time
> anyway.  And I'm concerned about the possibility of some nasty DoS
> potential by exploiting this.  I haven't looked into this in depth, and
> it depends on how it handles cache replacement, etc.

> Except that, technically, you are not permitted to just blindly send 
> segments of such size.  Well, you can but systems in the middle don't 
> have to handle them.  No?

Hmm.. either I did a bad job of explaining or I haven't had enough caffiene
to parse what you said.  Given that you also suggest going to a 1460 MSS,
I suspect that we're actually violently in agreement here.

Now if I can remember why I chose 1396 for a default MSS.... ;)

> It is also a concern that, in my experience, many of the links with
> MTUs <1500 are also the links with greater packet loss, etc. so 
> you really don't want fragmentation on them.

The worst part here is that I suspect that most of these links (just on
sheer numbers of shipped product) are the aformentioned Win98 576-MTU.

However, in this case, the fragmentation happens in a terminal server on
the last hop, and hopefully the case of a terminal server running out of
queueing buffers and having to drop one of the 2 remaining fragments of
a 1500->576 split after sending the first one is pretty rare....

I seem to remember that the *original* motivation for slow-start and
all that was Van Jacobson's observation that the most common cause of
a TCP retransmit was that an *entire* packet had been silently dropped
due to queueing congestion, and could thus be treated identical to
an ICMP Source Quench.

Has this changed?  Has "fragmentation" become a Great Evil, rather than
an annoyance that some links have to deal with?

> I think it is simply that we the net is in a state of somewhat
> amazing homogoney right now.  I don't think this will continue,
> but who knows.  I do think that PMTU-D is an important feature, and 
> people should be encouraged to leave it enabled wherever possible,
> so that one day if networks do change to make it more useful in the
> general case, it will be there...

At least for TCP.  I'm still unconvinced for UDP ;)

				Valdis Kletnieks
				Operating Systems Analyst
				Virginia Tech

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