[150292] in North American Network Operators' Group
RE: Common operational misconceptions
daemon@ATHENA.MIT.EDU (George Bonser)
Mon Feb 20 22:09:56 2012
From: George Bonser <gbonser@seven.com>
To: Masataka Ohta <mohta@necom830.hpcl.titech.ac.jp>
Date: Tue, 21 Feb 2012 03:08:53 +0000
In-Reply-To: <4F4304B0.6020500@necom830.hpcl.titech.ac.jp>
Cc: "nanog@nanog.org" <nanog@nanog.org>
Errors-To: nanog-bounces+nanog.discuss=bloom-picayune.mit.edu@nanog.org
I, in fact, HAVE read the RFC. =20
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 th=
is 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 sto=
ps. 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 th=
at the benefit of a larger effective Path MTU is smaller than the search ov=
erhead of finding it" and one might well argue that someone decided that th=
e difference between 1400 and 1420 MSS (20 bytes) isn't worth the extra ove=
rhead of finding it those 20 bytes. But in the case of a 7500 MTU and a 45=
00 MTU black hole link in between, it certainly does NOT go to 1400 and sta=
y there.
> -----Original Message-----
> 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
>=20
> George Bonser wrote:
>=20
> >> First, it sets eff_pmtu to 1400B. OK?
> >
> > Where did you get 1400 from?
>=20
> Read the RFC. PERIOD.
>=20
> Masataka Ohta
