Common operational misconceptions

Tue Feb 21 03:08:53 UTC 2012

I, in fact, HAVE read the RFC.  

   The initial value for search_high SHOULD be the largest possible
   packet that might be supported by the flow.  This may be limited by
   the local interface MTU, by an explicit protocol mechanism such as
   the TCP MSS option, or by an intrinsic limit such as the size of a
   protocol length field.

So the initial probe would be the reported MSS of the remote system in this case 1460 which would fail.  I believe you might be getting confused by this paragraph:

   The general strategy is for the Packetization Layer to find an
   appropriate Path MTU by probing the path with progressively larger
   packets.  If a probe packet is successfully delivered, then the
   effective Path MTU is raised to the probe size.

And believing that as soon as a value is found that passes, the process stops.  That isn't the case.

   PLPMTUD uses a searching technique to find the Path MTU.  Each
   conclusive probe narrows the MTU search range, either by raising the
   lower limit on a successful probe or lowering the upper limit on a
   failed probe, converging toward the true Path MTU.  For most
   transport layers, the search should be stopped once the range is
   narrow enough that the benefit of a larger effective Path MTU is
   smaller than the search overhead of finding it.

The issue here is that the judgement of "once the range is narrow enough that the benefit of a larger effective Path MTU is smaller than the search overhead of finding it" and one might well argue that someone decided that the difference between 1400 and 1420 MSS (20 bytes) isn't worth the extra overhead of finding it those 20 bytes.  But in the case of a 7500 MTU and a 4500 MTU black hole link in between, it certainly does NOT go to 1400 and stay there.

> -----Original Message-----
> From: Masataka Ohta [mailto:mohta at necom830.hpcl.titech.ac.jp]
> Sent: Monday, February 20, 2012 6:43 PM
> To: George Bonser
> Cc: nanog at nanog.org
> Subject: Re: Common operational misconceptions
> 
> George Bonser wrote:
> 
> >> First, it sets eff_pmtu to 1400B. OK?
> >
> > Where did you get 1400 from?
> 
> Read the RFC. PERIOD.
> 
> 						Masataka Ohta