Comments on PMTU-Draft-RFC
Nuggehalli Pradeep <pradeep@orville.nas.nasa.gov> Sat, 27 January 1990 03:00 UTC
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Date: Fri, 26 Jan 1990 18:12:43 -0800
From: Nuggehalli Pradeep <pradeep@orville.nas.nasa.gov>
Message-Id: <9001270212.AA00653@orville.nas.nasa.gov>
To: mtudwg
Subject: Comments on PMTU-Draft-RFC
Comments on the PMTU Discovery Draft-RFC: 1. > ... ... ... ... ... ... Thus, if the option > arrives at the destination with its next-hop field non- > zero, then all gateways along that path have updated the > option and the minimum-MTU-so-far value is indeed the PMTU; Not true! (or else I'm missing something here!) It should read: "If the option arrives at the destination with its next-hop field *same as destination address*, then all routers along that path have updated the option and the minimum-MTU-so-far value is indeed the PMTU." The destination host should check whether or not the next-hop field has the same address as the destination address (not check for zero/non-zero). This applies to all references in the document where the destination checks for zero/non-zero. Consider the case where the last-hop router, or the n, n+1, ...thru... the last-hop router are non-cooperating. Then the option would arrive at the destination host with its next-hop field non-zero (and not the same as destination address). 2. Next-Hop-Address: I don't see the full justification of using this field. The only thing it helps us in detecting is the case of two consecutive non-cooperating routers with a low-MTU link when probing. I am inclined to think that this case would not be very common. Even so, this case would later be detected by the backup fragment-report mechanism. Even without this field, isolated non-cooperating routers would not cause a problem because the neighboring routers would handle the case. But this is only an opinion and I leave it to the group to decide on this. 3. IP PMTU-Query Option: The length of the Min-MTU field of 2 bytes is sufficient for almost all currently available networks, as it allows a max value of 64 KBytes. But, for example, UltraNet is already using an MTU value of 32 KB, and I think that new network technologies would soon reach and exceed the limit of 64 KB for the MTU. So, could we consider increasing the length of the Min-MTU field in the option to 3 or perhaps 4 bytes. Possible option format: ------------------------------------------------------------ | Type | Length | Reserved (for future use) | ------------------------------------------------------------ | Min-MTU | ------------------------------------------------------------ | Next-Hop | ------------------------------------------------------------ 4. > ... ... ... and the size of a fragment-0 are > considered accurate indications of the current PMTU of the > path, ... ... ... Not true! because the fragments of a datagram could take different paths, and some of those fragments could get re-fragmented, and we don't know which of these paths the subsequent datagrams will take. Perhaps we could use the size of the largest fragment, but this would still be a guess. So, I prefer to use the size of fragment-0. Thus, the size of fragment-0 is only a best-guess. Overall, the new hybrid scheme seems to be a pretty good scheme. --- Pradeep
- Comments on PMTU-Draft-RFC Nuggehalli Pradeep
- Re: Comments on PMTU-Draft-RFC Keith Mc Cloghrie
- Re: Comments on PMTU-Draft-RFC Steve Deering
- Re: Comments on PMTU-Draft-RFC Jeffrey Mogul
- Re: Comments on PMTU-Draft-RFC Philippe Prindeville
- Re: Comments on PMTU-Draft-RFC Philippe Prindeville