Re: Space for Packet Metadata

Mikkel Fahnøe Jørgensen <mikkelfj@gmail.com> Wed, 28 February 2018 16:04 UTC

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Subject: Re: Space for Packet Metadata
To: Nick Banks <nibanks@microsoft.com>, Michael Tuexen <michael.tuexen@lurchi.franken.de>
Cc: IETF QUIC WG <quic@ietf.org>, "Eggert, Lars" <lars@netapp.com>
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https://blogs.cisco.com/enterprise/ipv6-mtu-gotchas-and-other-icmp-issues

IPV6 UDP overhead is 48 bytes
https://en.wikipedia.org/wiki/User_Datagram_Protocol

Minimum expected PMTU is 1280 bytes (and required by QUIC)
Local MTU expected 1536 bytes

1536 - 48 = 1488
1488 - 1280 = 208 bytes

So there is already 208 bytes reserved for tunnelling etc. Which is fine.
Exactly what I was asking for.


On 28 February 2018 at 16.55.05, Michael Tuexen (
michael.tuexen@lurchi.franken.de) wrote:

> On Feb 28, 2018, at 4:30 PM, Nick Banks <nibanks@microsoft.com> wrote:
>
> According to spec, QUIC requires at least 1280 (from 9.4):
>
> "QUIC depends on the network path supporting a MTU of at least 1280
octets. This is the IPv6 minimum MTU and therefore also supported by most
modern IPv4 networks. An endpoint MUST NOT reduce its MTU below this
number, even if it receives signals that indicate a smaller limit might
exist.'
When discussing safe MTU values for WebRTC, 1280 for IPv6 was acceptable,
but not for IPv4. After a discussion,
the suggested value for IPv4 was 1200. See
https://tools.ietf.org/html/draft-ietf-rtcweb-data-channel-13#section-5

Not sure what changed in between...

Best regards
Michael
>
> - Nick
>
> -----Original Message-----
> From: QUIC <quic-bounces@ietf.org> On Behalf Of Eggert, Lars
> Sent: Wednesday, February 28, 2018 7:12 AM
> To: Mikkel Fahnøe Jørgensen <mikkelfj@gmail.com>
> Cc: IETF QUIC WG <quic@ietf.org>
> Subject: Re: Space for Packet Metadata
>
> On 2018-2-28, at 16:03, Mikkel Fahnøe Jørgensen <mikkelfj@gmail.com>
wrote:
>> Yes, but can it go below 1200? It seems the handshake aims to squeeze up
near the minimum guaranteed (required) PMTU and QUIC implementations would
probably not expect to maintain a connection that drops below PMTU 1200.
>
> The minimum IPv4 MTU is 576, so I'd expect QUIC stacks to probe as least
that low.
>
> Lars