Re: Fibonaccing CID lengths

Christian Huitema <huitema@huitema.net> Sat, 25 May 2019 03:12 UTC

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To: Mikkel Fahnøe Jørgensen <mikkelfj@gmail.com>, David Schinazi <dschinazi.ietf@gmail.com>, Marten Seemann <martenseemann@gmail.com>, IETF QUIC WG <quic@ietf.org>, Kazuho Oku <kazuhooku@gmail.com>
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From: Christian Huitema <huitema@huitema.net>
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Date: Fri, 24 May 2019 20:11:51 -0700
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Subject: Re: Fibonaccing CID lengths
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On 5/24/2019 5:55 AM, Dmitri Tikhonov wrote:
> On Thu, May 23, 2019 at 08:18:00AM -0700, Mikkel Fahnøe Jørgensen wrote:
>> I’m all for the 8 bit lengths, but I don’t see how they prevent
>> ossification of the first two bits?
> I believe Marten meant 8-bit encoding for both DCID and SCID length,
> just like what we have now.  In the 4-bits-per-length scheme, it is
> likely that no single bit will be found to always have a particular
> value.  This would prevent ossification.

The current format of the long header encodes two lengths in one byte,
followed by the Dest CID and the Source CID. The proposal that got
consensus in London is to replace that by one byte of length of DCID,
DCID, one byte of SCID, SCID. The general agreement is that given the
usage of the long header, it is not necessary to compress the length field.

This would be a change in the invariants. Ossification of the format is
not really a concern. By definition, the invariants are supposed to
ossify. But there is an ossification risk for the actual length.

As far as V1 is concerned, we agreed to limit the actual length to less
than 64 bytes. The two most significant bits of the two length fields
will be zero, so there is a possibility that middle-boxes will use that
to detect whether a packet is using QUIC. The risk there is that this
would prevent future versions from using CID lengths larger than 64 bytes.

-- Christian Huitema