Re: How to use an IP-header bit for Path MTU discovery.

Steve Deering <> Sun, 25 February 1990 07:14 UTC

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Date: 24 Feb 1990 22:59-PST
From: Steve Deering <>
Subject: Re: How to use an IP-header bit for Path MTU discovery.
To: mogul (Jeffrey Mogul)
Cc: mtudwg
Message-Id: <90/02/24>
In-Reply-To: mogul's message of 19 Feb 1990 1324-PST (Monday)

In response to John Wobus's clever new proposal, Jeff says:

>	Basically, if one is forced to think of how many rational
>	possibilities there are for the sender's segment size, one
>	can easily come up with 2: "576" and the first-hop-net MTU.
>	Coming up with a third choice is only necessary if we believe
>	that it is too far from optimal to be stuck with these two
>	choices.

Another scenario that *may* become important is FDDI-backbone-FDDI,
where the backbone has a 1500 byte MTU.  You'd rather not end up using
576 bytes as the path-MTU.

Also, the 576 rule still incurs fragmentation on those subnets whose MTU
is less than 576.  If the goal is to eliminate *all* fragmentation,
the simple "two-sizes" rule won't do it (unless you mandate intra-subnet
fragmentation and reassembly for subnets with MTU less than 576).