Re: decreases in MTU

Steve Deering <> Sun, 04 March 1990 23:27 UTC

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Date: 4 Mar 1990 14:54-PST
From: Steve Deering <>
Subject: Re: decreases in MTU
To: sytek! (Rich Fox)
Cc: mtudwg
Message-Id: <90/03/04>
In-Reply-To: sytek!rfox's message of Sat, 3 Mar 90 132645 PST

>	In your new scheme how is the descrease/increase of the MTU handled?
>	It seems that this is a potential problem.


A sender detects a decrease of MTU whenever it tries to send a packet
larger than the new MTU with the DF bit set.  Such a packet triggers
a Can't Fragment message that tells the sender what the new MTU is.

Reception of an old-style Can't Fragment message that does not contain
a recommended MTU value causes the receiver to stop setting the DF bit
and to assume an MTU of 576; if the real MTU is less than 576, or if it
subsequently changes to a value less than 576, that will not be discovered
by the sender and fragmentation may occur.

A sender detects an increase of MTU by periodically restarting the
discovery process, i.e., by assuming that the MTU is MIN(first-hop-MTU,MSS)
and setting the DF bit on all packets.

Does that answer your question, or have I misunderstood?