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From: Mick <michaelkintzios@gmail.com>
To: gentoo-user@lists.gentoo.org
Subject: Re: [gentoo-user] PMTUD
Date: Sun, 1 Sep 2013 19:51:06 +0100
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References: <CAN0CFw1NOD-cfwwOZuU8geHbLP7kzgc9FRGa+3nRFp9sbBGowA@mail.gmail.com> <201309011753.17624.michaelkintzios@gmail.com> <CAN0CFw2ux87m6FRoFwbhyAyM_X_M5Nw_iHiGow5Von1gx_v0FA@mail.gmail.com>
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On Sunday 01 Sep 2013 18:54:45 Grant wrote:
> >> OK, does PMTUD lower the outgoing packet size on my system due to the
> >> hotel router's lower MTU or does the hotel router itself fragment my
> >> 1500 byte packets in order to send them out? Just curious.
> >=20
> > If you are sending out packets with the DF bit set no fragmentation will
> > take place - the packet is dropped and an appropriate message is
> > returned to sender. Otherwise the router will fragment them and send
> > them on to the recipient address.
>=20
> Shouldn't PMTUD change my MTU based on the hotel router's lower MTU?
Yes, it should. At the start of the connection the sender sends DF in the=
=20
header to find out what is the MRU that the network nodes will support. Th=
en=20
sends packets of the appropriate size so that they get through with no=20
fragmentation. This is the optimal scenario.
Now, imagine another scenario where some router/firewall/server does not se=
nd=20
back the correct ICMP packet with its required MRU, or even worse it sends=
=20
back a 1500 (full ethernet) size with DF set, or also drops fragments ... T=
his=20
reminds me of MSN IM which was a particularly bad implementation back when.
The sender may eventually try a smaller packet, after initially increasing =
the=20
time it waits for a response, and you could well get something through 30=20
seconds later, or even give up and time out.
If you are using Shorewall at your remote server I would expect it to behav=
e=20
properly and return the correct ICMP packet when it receives a DF. However=
, I=20
am not familiar with the Shorewall properties and settings, so if you suspe=
ct=20
this as the cause of your problem it would be better if you look into it=20
properly.
=2D-=20
Regards,
Mick
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