Re: RF flag
Steve Deering <deering@pescadero.stanford.edu> Tue, 12 December 1989 01:02 UTC
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Date: Mon, 11 Dec 1989 16:32:00 -0000
From: Steve Deering <deering@pescadero.stanford.edu>
Subject: Re: RF flag
To: jnc@PTT.LCS.MIT.EDU
Cc: mtudwg
Message-Id: <89/12/11
In-Reply-To: jnc's message of Mon, 11 Dec 89 185346 EST
> ...the general feeling was that MTU discovery probably wasn't it. > So, that one's out... The second phrase does not follow from the first. I would interpret the first phrase as an invitation to convince the IESG that MTU discovery *is* what the bit should be used for. Or was the decision really, "It *shall* not be used for that purpose," and you were just trying to be gentle? > (although you could still have such a bit, albeit in an option > which you insert every so often). That's greatly inferior, as you know. If the header bit is unavailable, I'd be interested in exploring some of the approaches that Rich Fox has suggested. That is, let the transport protocol inform its peer that it is willing to receive fragmentation reports. The way of asking for reports may in fact be an IP option inserted by the transport protocol at connection-setup time (for connection or session oriented protocols, at least), and the way of informing the sender that fragmentation occurred may in fact be an ICMP message sent by the transport protocol. IP-layer MTU caching could still be used to share discovered MTU info between multiple transport connections, and to reduce the frequency of sending first-hop-MTU-size datagrams to probe for MTU increases. It would require minimal additional host state (one extra bit in the state already maintained for each connection/session/association/whatever), and, like the RF header bit scheme, would detect an MTU decrease in the shortest possible time (one RTT). Steve
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