Path mtu discovery

I use tracepath to probe pmtu to some host:

#tracepath belmag.vpn.clink.ru
1: luna 0.124ms pmtu 1500
1: msk-ar-9-vl110.masterhost.ru (217.16.18.3) 0.750ms
2: ge000-263.RT502-001.msk.retn.net (81.222.8.57) asymm 4
1.096ms
3: MSK15-Gi11.175.transtelecom.net (217.150.45.138) asymm 12
58.718ms
4: YuzhUralTTK-gw.transtelecom.net (217.150.39.149) asymm 15
90.892ms
5: gw-SvyazTelecom-magn.suttk.ru (62.165.50.130) asymm 15
90.179ms
6: gw-stknet.svtk.ru (194.242.120.254) asymm 14
91.413ms
7: distcore-tline.svtk.ru (194.242.120.189) asymm 14
91.504ms
8: rzv-tline.svtk.ru (194.242.120.245) asymm 14
91.644ms
9: 194.242.123.226 (194.242.123.226) asymm 15
93.340ms
10: 194.242.123.226 (194.242.123.226) asymm 15
92.888ms pmtu 1492
11: 194.42.113.64.clink.ru (194.42.113.64) asymm 16
178.320ms reached
Resume: pmtu 1492 hops 11 back 16


mtu on hop #10 is 1492. If I rerun tracepath, starting mtu to the host
should be 1492, but it is 1500 again:


luna:~# tracepath belmag.vpn.clink.ru
1: luna 0.137ms pmtu 1500
1: msk-ar-9-vl110.masterhost.ru (217.16.18.3) 0.735ms
2: ge000-263.RT502-001.msk.retn.net (81.222.8.57) asymm 4
1.078ms
3: MSK15-Gi11.175.transtelecom.net (217.150.45.138) asymm 12
58.638ms
4: YuzhUralTTK-gw.transtelecom.net (217.150.39.149) asymm 15
91.413ms
5: gw-SvyazTelecom-magn.suttk.ru (62.165.50.130) asymm 15
90.845ms
6: gw-stknet.svtk.ru (194.242.120.254) asymm 14
91.378ms
7: distcore-tline.svtk.ru (194.242.120.189) asymm 14
91.320ms
8: rzv-tline.svtk.ru (194.242.120.245) asymm 14
91.770ms
9: 194.242.123.226 (194.242.123.226) asymm 15
93.676ms
10: 194.242.123.226 (194.242.123.226) asymm 15
94.496ms pmtu 1492
11: 194.42.113.64.clink.ru (194.42.113.64) asymm 16
161.152ms reached
Resume: pmtu 1492 hops 11 back 16


So mtu is not saved. Why?

On 17 Mar 2007, in the Usenet newsgroup comp.os.linux.networking, in article
<(E-Mail Removed). com>,
(E-Mail Removed) wrote:

>I use tracepath to probe pmtu to some host:

OK that's a user application.

>mtu on hop #10 is 1492. If I rerun tracepath, starting mtu to the host
>should be 1492, but it is 1500 again:

Ummm, no. 'tracepath' provides data about a path. It has no means to
change the routing setup on the computer, and thus will always start with
what-ever is the default for this computer interface.

You may want to read RFC1191 which is the reference on PMTU, as well as
RFC1435 and RFC2923.

1191 Path MTU discovery. J.C. Mogul, S.E. Deering. November 1990.
(Format: TXT=47936 bytes) (Obsoletes RFC1063) (Status: DRAFT
STANDARD)

1435 IESG Advice from Experience with Path MTU Discovery. S. Knowles.
March 1993. (Format: TXT=2708 bytes) (Status: INFORMATIONAL)

2923 TCP Problems with Path MTU Discovery. K. Lahey. September 2000.
(Format: TXT=30976 bytes) (Status: INFORMATIONAL)

A key is in the Abstract paragraph of 1191, which reads

This memo describes a technique for dynamically discovering the
maximum transmission unit (MTU) of an arbitrary internet path. It
specifies a small change to the way routers generate one type of ICMP
message. For a path that passes through a router that has not been
so changed, this technique might not discover the correct Path MTU,
but it will always choose a Path MTU as accurate as, and in many
cases more accurate than, the Path MTU that would be chosen by
current practice.

First sentence - "dynamically discovering". For most networking setups,
the PMTU is discovered on a connection basis, and is only used while
that specific connection exists (example - fetching a web page, or a
file from a FTP server). The next time you connect to the same host, you
will again run a PMTUD, because the route may have changed.

>So mtu is not saved. Why?

1. PMTU is dynamic, and may change over time.
2. tracepath is a _user_ application, and has no control over the network
setup on your computer. Thus, even if tracepath remembered the PMTU,
this would have no effect on any _other_ application, never mind the
kernel's routing table.

Old guy

>
> >So mtu is not saved. Why?
>
> 1. PMTU is dynamic, and may change over time.
> 2. tracepath is a _user_ application, and has no control over the network
> setup on your computer. Thus, even if tracepath remembered the PMTU,
> this would have no effect on any _other_ application, never mind the
> kernel's routing table.
>
> Old guy

1. Discovered mtu is saved in memory for 10 minutes. This time the mtu
is constant, not dynamic. But in my situation, mtu is not saved by OS.
2. tracepath shows us pmtu along some ip-route-path. MTU is remembered
not by tracepath, but by OS. tracepath asks OS to discover mtu.