You forced me to go back to my old documents to see where this is discussed. The idea of using IPv4 fragmentation as a lossless path MTU determination mechanism appears in RFC5320. The SEAL header indeed includes an "R" bit telling the receiver when a fragmentation report response is desired.
As an experimental, RFC5320 did not go into an implementation diversity survey; only linux. Listening to hearsay that MTU-sized fragments cannot normally be expected is one of the reasons we abandoned the SEAL approach. But, if you have done a survey and concluded that the vast majority of implementations result in MTU-sized fragments then good for you. I still think though that some kind of RF bit is needed. In the same way that ICMP "fragmentation needed" is only sent when DF is 1, ICMP "fragmentation report" should only be sent when RF is 1. Or, maybe you want to send ICMP "fragmentation needed" regardless of the state of the DF bit? I think applications that rely on fragmentation will not want to receive ICMP fragmentation reports over links that might have long delays and/or low data rate capacities. The application knows it is invoking fragmentation; it does not want for the network to be sending it useless reports along the reverse path. Hence the RF bit. Thanks - Fred > -----Original Message----- > From: Manoj Nayak [mailto:[email protected]] > Sent: Wednesday, November 20, 2019 8:15 PM > To: Templin (US), Fred L <[email protected]>; [email protected] > Cc: [email protected] > Subject: Re: [Int-area] New Version Notification for > draft-bonica-intarea-lossless-pmtud-00.txt > > Hello Fred, > > Yes, it is possible that the largest fragment received is *much* smaller than > the PMTU. However, a survey of > popular operating systems reveals that the largest fragment does reflect the > PMTU. > > If we are really worried about this problem, the sender can ignore the ICMP > message when the MTU is smaller than > the “smallest believable value” (e.g., 1500 bytes). > > There was another query, if it is worth doing Lossless PMTUD for ipv4. > IPv4 will be with us for a long time. Lossless PMTUD doesn’t take much > effort. So why not ... > > Regards > Manoj Nayak > > On 14/11/19, 8:15 PM, "Templin (US), Fred L" <[email protected]> > wrote: > > Manoj, > > > As per section 4.2.2.7 from rfc 1812, > > > > “There are several fragmentation techniques in common use in the > > Internet. One involves splitting the IP datagram into IP > > fragments with the first being MTU sized, and the others being > > approximately the same size, smaller than the MTU. “ > > > > In both of the above cases, idea in our draft works. > > No it doesn't work. The text you quoted says that some techniques can > cause all fragments to be "smaller than the MTU". It doesn't say how much > smaller, so we must assume that it could be significantly smaller such > that > the maximum fragment size bears no relation to the path MTU. Therefore, > in the general sense, it just doesn't work. > > We have been through this before. It is in my expired drafts. > > Fred > > > -----Original Message----- > > From: Int-area [mailto:[email protected]] On Behalf Of Manoj > Nayak > > Sent: Wednesday, November 13, 2019 8:35 PM > > To: [email protected] > > Cc: [email protected] > > Subject: Re: [Int-area] New Version Notification for > draft-bonica-intarea-lossless-pmtud-00.txt > > > > Hello Joe, > > > > Please find my reply. > > > > >>- why does this doc assume the max ICMP is 576? > > >> we?re still talking IPv4 here; it?s still 68 (that?s why only > 64 bits of the orig payload are guaranteed) > > >> (yes, your note in the end of sec 1 is relevant, but given > v4-in-v4 tunneling, it?s possible that > > >> paths might be smaller than the 576 assumption) > > > > We use an unused field in first 8 bytes of ICMP error/reply message. > How the idea would be > > affected if minimum packet size is 68 bytes or 576 bytes. As per my > understanding, > > existing ICMP error/reply message works in v4-in-v4 tunnelling, so it > would continue to > > work with the idea proposed in our draft. we won’t let the ICMP message > exceed a reasonable size. > > in our implementation, that will be 576. > > > > > > > > >>- why would this approach find the largest fragment through a system? > > >> rfc1812 talks about various strategies, one of which is ?equal > sized?, which might never find > > >> the max the way you propose > > > > > > As per section 4.2.2.7 from rfc 1812, > > > > “There are several fragmentation techniques in common use in the > > Internet. One involves splitting the IP datagram into IP > > fragments with the first being MTU sized, and the others being > > approximately the same size, smaller than the MTU. “ > > > > In both of the above cases, idea in our draft works. In our > implementation, > > We can assume the first fragment to be largest fragment. This first > fragment remains > > Largest fragment unless until one more fragment is found to be greater > than the first fragment. > > > > For example: > > > > While assembling the fragments, > > > > I=0; > > Largest Fragment = packet-I; > > For (, I < n , ++I) > > If ( packet-I > Largest Fragment) > > Largest Fragment = packet-I; > > > > Hopefully I did not miss anything. > > > > Regards > > Manoj Nayak > > > > ------------------------------ > > > > Message: 3 > > Date: Tue, 29 Oct 2019 21:43:33 -0700 > > From: Joe Touch <[email protected]> > > To: Ron Bonica <[email protected]> > > Cc: "[email protected]" <[email protected]> > > Subject: Re: [Int-area] New Version Notification for > > draft-bonica-intarea-lossless-pmtud-00.txt > > Message-ID: <[email protected]> > > Content-Type: text/plain; charset=utf-8 > > > > Hi, Ron, > > > > A few things come to mind. The first one, IMO, renders the rest > somewhat less important. > > > > Joe > > > > ------------- > > > > - this approach applies only to IPv4; not sure it?s worth trying to > optimize for only that case > > (it requires on-path fragmentation permitted) > > > > - this approach relies on ICMPs, so it?s as robust (or, more to the > point, not) as PMTUD > > if ICMPs can find the reverse path from the dest, why wouldn?t > the routers? > > i.e., isn?t the problem with ICMPs not just routers not sending > them but firewalls BLOCKING them? > > (i.e., if ICMPs would work here, PMTU would have worked, > rendering this unnecessary) > > > > - why does this doc assume the max ICMP is 576? > > we?re still talking IPv4 here; it?s still 68 (that?s why only > 64 bits of the orig payload are guaranteed) > > (yes, your note in the end of sec 1 is relevant, but given > v4-in-v4 tunneling, it?s possible that paths might be smaller than the > > 576 assumption) > > > > - why would this approach find the largest fragment through a > system? > > rfc1812 talks about various strategies, one of which is ?equal > sized?, which might never find the max the way you propose > > > > > > > On Oct 29, 2019, at 9:53 AM, Ron Bonica > <[email protected]> wrote: > > > > > > Folks, > > > > > > Please review and comment. > > > > > > Ron > > > > > > > > > > > > Juniper Business Use Only > > > > > > -----Original Message----- > > > From: [email protected] <[email protected]> > > > Sent: Tuesday, October 29, 2019 11:48 AM > > > To: Ron Bonica <[email protected]>; Hakan Alpan > <[email protected]>; Radon Rosborough <[email protected]>; > Bradely > > Newton <[email protected]>; Miles President <[email protected]>; Manoj > Nayak <[email protected]> > > > Subject: New Version Notification for > draft-bonica-intarea-lossless-pmtud-00.txt > > > > > > > > > A new version of I-D, draft-bonica-intarea-lossless-pmtud-00.txt > > > has been successfully submitted by Ron Bonica and posted to the > IETF repository. > > > > > > Name: draft-bonica-intarea-lossless-pmtud > > > Revision: 00 > > > Title: Lossless Path MTU Discovery (PMTUD) > > > Document date: 2019-10-29 > > > Group: Individual Submission > > > Pages: 8 > > > URL: > https://urldefense.com/v3/__https://tools.ietf.org/html/draft-bonica-intarea-lossless-pmtud- > > > 00__;!8WoA6RjC81c!XifH7EcqHRKXHyGSwB3ojXm6YmKn_vYWCjgM-VDTPTEzP-khJGlb9MqM8x-YTELKJ64$ > > > > > > > > > > > > Abstract: > > > This document describes alternative IPv4 PMTUD procedures that > do not > > > prevent IP fragmentation and do no rely on the network's > ability to > > > deliver ICMP Destination Unreachable messages to the source > node. > > > This document also defines a new ICMP message. IPv4 nodes emit > this > > > new message when they reassemble a fragmented packet. > > > > > > > > > > > > > > > Please note that it may take a couple of minutes from the time of > submission until the htmlized version and diff are available at > > tools.ietf.org. > > > > > > The IETF Secretariat > > > _______________________________________________ > > > Int-area mailing list > > > [email protected] > > > > https://urldefense.com/v3/__https://www.ietf.org/mailman/listinfo/int- > > > area__;!8WoA6RjC81c!XifH7EcqHRKXHyGSwB3ojXm6YmKn_vYWCjgM-VDTPTEzP-khJGlb9MqM8x-YOVqXLNE$ > > > > > > > > > > _______________________________________________ > > Int-area mailing list > > [email protected] > > > https://urldefense.com/v3/__https://www.ietf.org/mailman/listinfo/int-area__;!8WoA6RjC81c!Tp5Pvame6pgAtmlsi- > 9_todREdZAH9ervNS0qugjDf4SKKWoRMSMvJ_yFYKUwTwaXCI$ > _______________________________________________ Int-area mailing list [email protected] https://www.ietf.org/mailman/listinfo/int-area
