mailing list archives
RE: Common operational misconceptions
From: George Bonser <gbonser () seven com>
Date: Tue, 21 Feb 2012 03:08:53 +0000
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.
From: Masataka Ohta [mailto:mohta () necom830 hpcl titech ac jp]
Sent: Monday, February 20, 2012 6:43 PM
To: George Bonser
Cc: nanog () 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.