Re: [tsvwg] UDP options and header-data split (zero copy)

Tom Herbert <tom@herbertland.com> Mon, 02 August 2021 01:45 UTC

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From: Tom Herbert <tom@herbertland.com>
Date: Sun, 01 Aug 2021 18:45:19 -0700
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To: Joseph Touch <touch@strayalpha.com>
Cc: Michael Tuexen <michael.tuexen@lurchi.franken.de>, tsvwg <tsvwg@ietf.org>
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Subject: Re: [tsvwg] UDP options and header-data split (zero copy)
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On Sun, Aug 1, 2021, 6:13 PM Joseph Touch <touch@strayalpha.com> wrote:

>
>
> On Aug 1, 2021, at 5:41 PM, Tom Herbert <tom@herbertland.com> wrote:
>
>
>
> On Sun, Aug 1, 2021, 3:48 PM Joseph Touch <touch@strayalpha.com> wrote:
>
>> Hi, Tom,
>>
>> > On Aug 1, 2021, at 3:39 PM, Tom Herbert <tom@herbertland.com> wrote:
>> >
>> > There is also RFC9000:
>> >
>> > "QUIC assumes a minimum IP packet size of at least 1280 bytes.  This
>> > is the IPv6 minimum size [IPv6] and is also supported by most modern
>> > IPv4 networks.
>>
>> Hmm. Seems like they completely overlooked IP source fragmentation
>> support and 1500B reassembly...
>>
>
> No IP fragmentation in QUIC. DF bit is always set.
>
>
> RFC 9000 talks about DF being set for IPv4, but that’s the default for
> IPv6 (no on-path fragmentation).
>
> It does say "no fragmentation at the IP layer”, which presumably also
> prohibits IP source fragmentation, though it’s not at all clear why, other
> than to avoid black-holing through NAT. DF being set in IPv4 is critical to
> RFC1191 path MTU discovery, but that does not prohibit use of source
> fragmentation as long as DF is set.
>

Path MTU discovery doesn't work if the source fragments. For instance, if a
host fragments a 1500 byte packet into three packets of 500 bytes then
we're probing the path with 500 bytes not 1500 which is what the transport
protocol expects.

>
> However, that does not prevent use of UDP fragmentation - which would both
> be opaque to avoiding IP fragmentation and QUIC, as well as would traverse
> NATs because of the replicated UDP header in UDP fragments.
>

UDP fragmentation similarly breaks pmtud.


> Joe
>
>
>
>