Re: [Ntp] Is the real MTU still 576?
Watson Ladd <watsonbladd@gmail.com> Mon, 15 June 2020 16:04 UTC
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From: Watson Ladd <watsonbladd@gmail.com>
Date: Mon, 15 Jun 2020 12:04:11 -0400
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To: Daniel Franke <dfoxfranke@gmail.com>
Cc: Hal Murray <hmurray@megapathdsl.net>, NTP WG <ntp@ietf.org>
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Subject: Re: [Ntp] Is the real MTU still 576?
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On Mon, Jun 15, 2020 at 10:42 AM Daniel Franke <dfoxfranke@gmail.com> wrote: > > Support for 1500 bytes or more at the link layer is pretty much universal, but that includes IP and UDP headers so for the application you have less than that. Those headers may contain options so you can't assume they'll be some particular size, and you may have multiple layers of them due to VPN tunneling, leaving an MTU of less than 1500 bytes for the innermost layer. What's ultimately left over for the application payload is the "segment size", rather than the "transmission unit". > > The only "correct" option if you want to go beyond the minimum MTU is to use path MTU discovery, but for a stateless protocol like NTP that's not really practical, at least not for servers. IPv6 mandates a minimum MTU of 1280 bytes. Again that's an MTU, not an MSS, but nonetheless it's very rare that a 1280-byte segment would get fragmented, no matter whether it's going over IPv4 or IPv6. I think "MSS is probably at least 1280" makes a good rule of thumb whenever PMTUD isn't practical. We have some data backing this up: https://blog.cloudflare.com/increasing-ipv6-mtu/. 98% of IPv4 and 93.23% of IPv6 hosts advertised 1340 MSS in SYN. 1280 is likely to be much closer to 99% of IPv6, but sadly we didn't get that measurement. Sincerely, Watson Ladd -- "Man is born free, but everywhere he is in chains". --Rousseau.
- [Ntp] Is the real MTU still 576? Hal Murray
- Re: [Ntp] Is the real MTU still 576? Steven Sommars
- Re: [Ntp] Is the real MTU still 576? Daniel Franke
- Re: [Ntp] Is the real MTU still 576? Watson Ladd
- Re: [Ntp] Is the real MTU still 576? Erik Kline