Re: SMB's proposal: using an IP option for "Report Fragmentation" flag
mogul (Jeffrey Mogul) Tue, 28 November 1989 19:06 UTC
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From: mogul (Jeffrey Mogul)
Message-Id: <8911281906.AA13730@acetes.pa.dec.com>
Date: 28 Nov 1989 1106-PST (Tuesday)
To: Philippe Prindeville <philipp@gipsi.gipsi.fr>
Cc: MTU Discovery <mtudwg>
Subject: Re: SMB's proposal: using an IP option for "Report Fragmentation" flag
In-Reply-To: Philippe Prindeville <philipp@gipsi.gipsi.fr> /
Tue, 28 Nov 89 16:10:13 -0100. <8911281510.AA02552@gipsi.gipsi.fr>
This may sound a little heretical, but perhaps we are considering fragmentation "Not Harmful Enough"? Maybe fragmentation really should be looked upon as something quite severe and that *any* fragmentation should be reported via an ICMP message back to the source. This way, no options or header bits are used. We would have to allocate a new ICMP message type, though: reason being it must be something that hosts not yet equiped with this MTU discovery code can safely ignore (albiet at the cost of much degraded performance -- if the performance is bad enough, perhaps the users will bang on the management to upgrade to a more reasonable release). Anyway, just using Fragmentation Required But Not Permitted (or whatever, I don't have 791 handy) with a different Code would not be adequate. Chris and I examined a variant of this suggestion (inspired by Charles Lynn) in our paper. The problem is that if the receiver (of the fragments) sends ICMPs to a sender that is not going to pay attention to them, not only will this cause even more congestion but it will happen repeatedly (because the sender will persist in its behaviour). One could imagine some sort of throttle (only send a "Fragment Received" ICMP once per minute or something like that) but even in this case, I think it's a bad idea for the receiver to send totally unsolicited reports. There may be a few situations where fragmentation is in fact the correct strategy; my intuition is that these cases involve at least one low-bandwidth path (or else why would its MTU be so small) and asking that path to carry superfluous unsolicited traffic is a mistake. -Jeff
- Re: SMB's proposal: using an IP option for "Repor… Steve Deering
- Re: SMB's proposal: using an IP option for "Repor… Philippe Prindeville
- Re: SMB's proposal: using an IP option for "Repor… Jeffrey Mogul
- Re: Another proposal to think about Jeffrey Mogul
- Re: SMB's proposal: using an IP option for "Repor… Jeffrey Mogul
- Re: Another proposal to think about William Westfield
- Re: SMB's proposal: using an IP option for "Repor… Philippe Prindeville
- Re: yet another MTU discovery scheme Jeffrey Mogul
- Re: Minutes of MTU Discovery Working Group Meetin… Jeffrey Mogul
- Re: routing protocols will provide path-MTU Jeffrey Mogul
- Re: number of octets returned in ICMP error messa… Jeffrey Mogul