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

[Tom Herbert <[e-mail]>]

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.  Assuming the minimum IP header size of 40 bytes for
IPv6 and 20 bytes for IPv4 and a UDP header size of 8 bytes, this
results in a maximum datagram size of 1232 bytes for IPv6 and 1252
bytes for IPv4.  Thus, modern IPv4 and all IPv6 network paths are
expected to be able to support QUIC."

and

"This requirement to support a UDP payload of 1200 bytes limits the
space available for IPv6 extension headers to 32  bytes or IPv4
options to 52 bytes if the path only supports the IPv6 minimum MTU of
1280 bytes.  This affects Initial packets and path validation."

If UDP options were used with QUIC, these limits would be applicable
to UDP options as well, i.e. 32 bytes of UDP options for IPv6, 52
bytes for IPv4 at least for initial packets and path validation.


On Sun, Aug 1, 2021 at 3:22 PM Michael Tuexen
<[e-mail]> wrote:
>
> > On 2. Aug 2021, at 00:14, Joe Touch <[e-mail]> wrote:
> >
> > Right, but for UDP the receive MTU is the one we care about.
> Ahh, OK.
>
> Best regards
> Michael
> >
> > Joe
> >
> >> On Aug 1, 2021, at 3:05 PM, Michael Tuexen <[e-mail]> wrote:
> >>
> >> 
> >>>
> >>> On 1. Aug 2021, at 23:43, Tom Herbert <[e-mail]> wrote:
> >>>
> >>>> On Sun, Aug 1, 2021 at 2:24 PM Joseph Touch <[e-mail]> wrote:
> >>>>
> >>>>
> >>>>
> >>>>> On Aug 1, 2021, at 2:19 PM, Tom Herbert <[e-mail]> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On Sun, Aug 1, 2021, 12:42 PM Joseph Touch <[e-mail]> wrote:
> >>>>>
> >>>>>
> >>>>> ...
> >>>>> Had we limited the option length as a few suggested when this work started, we would not have FRAG.
> >>>>>
> >>>>> We don’t know what others are, but we also don’t know that the first frag will have hundreds of bytes of available space either.
> >>>>
> >>>>
> >>>> Actually, we do know that. The minimum MTU in IPv6 is 1280 and the minimum MTU for IPv4 is 576.
> >>>>
> >>>>
> >>>> The min MTU for IPv4 is 68..
> >>>
> >>> Per RFC791 it's 576 bytes.
> >> RFC 791 makes a requirement on the receiver, not on the links.
> >>
> >> Page 25 of RFC 791 reads:
> >>
> >>   Every internet module must be able to forward a datagram of 68
> >>   octets without further fragmentation.  This is because an internet
> >>   header may be up to 60 octets, and the minimum fragment is 8 octets.
> >>
> >> Therefore, the min MTU for IPv4 is 68.
> >>
> >> Best regards
> >> Michael
> >>>
> >>>>
> >>>> If someone we're so inclined they could fill up the first fragment packet with nothing but options and start the payload in the second. That means you'll have at least 520 bytes for options.
> >>>>
> >>>>
> >>>> That includes BOTH per-fragment and per-reassembled datagram options.
> >>>>
> >>>> But all this is academic since there's no use case other than DoS attack that would need anything close to that much space.
> >>>>
> >>>>
> >>>> That was true until FRAG too.
> >>>>
> >>>> Again, this is a new decision - to limit the option space.
> >>>
> >>> I don't know what that means.
> >>>
> >>>>
> >>>> Joe
> >>>
> >>
> >
>