Re: [tsvwg] UDP-Options: UDP has two ???maximums???
Joseph Touch <touch@strayalpha.com> Sun, 04 April 2021 04:25 UTC
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From: Joseph Touch <touch@strayalpha.com>
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Date: Sat, 03 Apr 2021 21:25:00 -0700
Cc: Gorry Fairhurst <gorry@erg.abdn.ac.uk>, "tsvwg@ietf.org" <tsvwg@ietf.org>
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To: Paul Vixie <paul@redbarn.org>
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Subject: Re: [tsvwg] UDP-Options: UDP has two ???maximums???
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> On Apr 3, 2021, at 8:31 PM, Paul Vixie <paul@redbarn.org> wrote: > > On Sat, Apr 03, 2021 at 06:49:46PM -0700, Joseph Touch wrote: >>> On Apr 3, 2021, at 6:29 PM, Paul Vixie <paul@redbarn.org> wrote: >>> >>> you're implicitly positing a situation wherein a DNS speaker could know >>> that the far endpoint knew about UDP options and could reassemble, and >>> that the local endpoint (kernel) knows about UDP options and could >>> fragment, and that using UDP Fragmentation would be seen as a better >>> choice than leaving out optional data or signalling a need to retry >>> with TCP. >> >> Yes. Note though that the fragmentation in UDP can be used safely; >> legacy endpoints just see (at most) packets with zero data. > > i wasn't considering it unsafe. some kind of initiation signal would > be required though, or else the initiator would see a small set of > apparently empty UDP payloads coming back, and wonder why. The same could happen for nearly any new variant of DNS, too. >>> i worry about microbursts. ... >>> >>> what we've learned from NFS, and high volume authoriative UDP DNS, >>> is that the network doesn't love minimum-spaced back-to-back packets, >>> and that if an 8KiB NFS result gets chopped into ~1500B chunks, tail >>> drop is likely. this is the biggest source of operator pain from IP >>> fragmentation, fwiw. >> >> Although I appreciate this concern, TCP does the same kind of bursts -- > > TCP has a congestion window that commonly keeps burst size within "range”. The window isn’t based on the ability to burst an entire window’s worth of messages back-to-back. It represents an entire round trip's worth of transmissions; when the source goes idle, the next active period can burst up to that entire window. >> I had thought we knew about this long enough that vendors didn???t use >> tail drop; they should have been doing AQM or at least something akin >> to RED. > > in routers and endpoints, yes. in switches, no. to a switch (multiport > bridge), the problem is felt too late and too near to the copper. in a fan-in > topology there can be too many gozinta for the gozouta, and this doesn't even > depend on link-layer flow control or whether it works, just ten gallons of > water trying to fit into a five gallon hat. If you’re speaking of Ethernet, there is link layer flow control, e.g., congestion notification messages or explicit on/off flow control intended to push this feedback to routers and endpoints. > …. >>> further digression: the framer of messages (like TCP, or DNS, or NFS) >>> ought to know the PMTU, which is why PMTUD was originally a non-optional >>> feature of IPv6 until we learned that ICMPv6 was dangerous as hell and >>> threw out PMTUD, thus leaving us with the pessimal and never-expected- >>> to-be-used 1280 and 1232 numbers. if we can get PLPMTUD then we can make >>> IPv6 better than IPv4 in terms of header amortization rather than (as it >>> currently is) worse. >> >> Agreed; that???s aided in UDP with options as per Gorry???s draft. > > if we implement PLPMTUD/UDP for DNS, we're going to have a decades-long > period during which the far end doesn't understand UDP options, s/UDP options/{TCP, QUIC, etc.}/ Yes, all new mechanisms take time. > ... > the original IPng PMTUD model whereby the endpoint's routing table would > remember a discovered MTU for each endpoint was a shinier city than this > on a better hill. i hope to see this outcome in the PLPMTUD world, PLPMTUD intends to cache MTUs per-endpoint exactly the same way that PMTUD did. > so that > (for example) TCP, QUIC, NFS, and UDP can all set their MSS accordingly, > without each service having to do its own discovery work per endpoint. Sure, but someone has to figure it out for others to use it. That’s why we’re working on a mechanism native to UDP - so it doesn’t rely on TCP or QUIC connections that precede its use. Joe
- Re: [tsvwg] UDP-Options: UDP has two ”maximums” Gorry Fairhurst
- Re: [tsvwg] UDP-Options: UDP has two ”maximums” Joseph Touch
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Paul Vixie
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Paul Vixie
- Re: [tsvwg] UDP-Options: UDP has two ”maximums” Gorry Fairhurst
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Gorry Fairhurst
- Re: [tsvwg] UDP-Options: UDP has two ”maximums” Joseph Touch
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Paul Vixie
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Joseph Touch
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Paul Vixie
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Joseph Touch
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Paul Vixie
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Joseph Touch
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Paul Vixie
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Joseph Touch
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Jeremy Harris
- Re: [tsvwg] UDP-Options: UDP has two ???maximums?… Paul Vixie