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

[Joseph Touch <[e-mail]>]

Hi, Tom,

> On Aug 1, 2021, at 3:39 PM, Tom Herbert <[e-mail]> 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...

> 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."

QUIC over UDP with fragmentation wouldn’t be bothered by these limits at all; it could send a 1200B QUIC payload with up to 63KB of additional combined IP and UDP headers - not that it should, but it could.

By the time QUIC sees the packet, it would be both IP reassembled (at least 1500B) and, with UDP options, UDP reassembled (up to a total of 64K).

> 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."

See above.

> 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.

I think they missed something significant, even if just for IPv6 at 1500B.

Joe