Templin, Fred L wrote: > Iljitsch, > > See below: > >> -----Original Message----- >> From: Iljitsch van Beijnum [mailto:[EMAIL PROTECTED] >> Sent: Friday, August 03, 2007 2:57 PM >> To: Templin, Fred L >> Cc: Joe Touch; Joe Macker; Fred Baker; Gorry Fairhurst; >> Stephen Casner; Internet Area >> Subject: Re: Mucking with IP ID >> >> [CCing int area again] >> >> On 3-aug-2007, at 2:13, Templin, Fred L wrote: >> >>>> I don't think "unless RFC4821" is reasonable either; >> perhaps "unless >>>> they make measures to react to silent failure, by RFC4821, or >>>> some other >>>> means, e.g., application timeout and retry with other MTUs". >>> I like the spirit of your text, but the problem I have with >>> it is that it doesn't say how the application can discern loss >>> due to an MTU restriction from loss due to, e.g., congestion. >> I strongly disagree here. PMTUD isn't something that should >> happen in applications. It's easy to get it wrong, and >> applications tend to stick around for a LONG time. > > I believe all Joe was saying was that an application that can > discern MTU-related loss (with or without explicit indications > from the network), and has a way to reduce the size of the > messages it sends, should probably do so.
Yes, but I agree with Iljitsch's point as well. This doesn't address legacy apps, and apps want transparent networking services. >> Applications that use UDP or other transports that can't implement >> RFC 4821 should clear the DF bit in IPv4 so that fragmentation can >> happen as needed. Anything else will cause breakage. > > Any application can clear DF in the packets it sends and > hope that the final destination has a large enough buffer to > reassemble any fragmentation that occurs in the network. But, > it would be well advised to try to ascertain the reassembly > buffer size first - unless it has some way of knowing or > guessing this in advance. It currently does: 576 for IPv4. That doesn't stop some UDP apps from exceeding that - but that's incorrect app behavior. There ought to be a way to find out what the reassy size is - though I don't recall one (short of SNMP). Joe
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