Re: Proposal: Run QUIC over DTLS

Kazuho Oku <kazuhooku@gmail.com> Tue, 06 March 2018 04:32 UTC

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From: Kazuho Oku <kazuhooku@gmail.com>
Date: Tue, 6 Mar 2018 13:32:28 +0900
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Subject: Re: Proposal: Run QUIC over DTLS
To: Eric Rescorla <ekr@rtfm.com>
Cc: IETF QUIC WG <quic@ietf.org>
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2018-03-06 8:05 GMT+09:00 Eric Rescorla <ekr@rtfm.com>;:
> Hi folks,
>
> Sorry to be the one randomizing things again, but the asymmetric
> conn-id thing went well, so here goes....
>
> TL;DR.
> I'd like to discuss refactoring things to run QUIC over DTLS.

EKR, Thank you for writing this. This is a very interesting proposal!

I can see the point that the QUIC specification will become more
straightforward if we adopt the DTLS-based approach, while, as an
implementor of TLS 1.3 and QUIC, I do not think that the total cost of
maintaining crypto and transport would change a lot.

One thing that I like about the proposed approach in particular is the
negotiation. As your draft points out, current approach has the
overhead of requiring additional round-trip when the server's
preferred version differs from that of the client.

It makes sense to negotiate the QUIC version using the negotiation
scheme available in (D)TLS.

That said, I feel uneasy about building QUIC **on top of** DTLS.

My understanding is that the intent of the working group is to build a
transport. We have spent maybe about a half of our time discussing
things that need to be implemented below the crypto (e.g., connection
ID, stateless reset, path migration, etc.), while the other half being
above the crypto (e.g., streams and multiplexing).

I think that that reflects the nature of what transport is, and that
the two need to be discussed together. In other words, I am afraid
that moving half of the design to TLSWG while retaining the other half
in QUICWG is likely to lead to the need for much more discussion
(would we holding interims for two workgroups?) as well as making the
decision process more complex.

In my view, that increases the risk of QUIC standardization process
being either delayed or ultimately failing.

Therefore, I am sorry to say that I am negative to the proposal. I
would feel safer if the proposal is adjusted to retain the sandboxing
approach (i.e. define the layers below and above the crypto layers in
QUIC WG while switching to DTLS).

However, I also believe that the issues of the current approach that
have been pointed out by the proposal can be adjusted without
switching to DTLS, and I would prefer doing so unless we unanimously
agree to switch to DTLS.

PS. If we decide to adopt the proposal, I am happy to implement DTLS
in picotls. It would not take a lot of time to do so.

>
> DETAILS
> When we originally designed the interaction between TLS and QUIC,
> there seemed like a lot of advantages to embedding the crypto
> handshake on stream 0, in particular the ability to share a common
> reliability and congestion mechanism. However, as we've gotten further
> along in design and implementation, it's also become clear that it's
> archictecturally kind of crufty and this creates a bunch of problems,
> including:
>
>   * Stream 0 is unencrypted at the beginning of the connection, but
>     encrypted after the handshake completes, and you still need
>     to service it.
>
>   * Retransmission of stream 0 frames from lost packets needs special
>     handling to avoid accidentally encrypting them.
>
>   * Stream 0 is not subject to flow control; it can exceed limits and
>     goes into negative credit after the handshake completes.
>
>   * There are complicated rules about which packets can ACK other
>     packets, as both cleartext and ciphertext ACKs are possible.
>
>   * Very tight coupling between the crypto stack and the transport
>     stack, especially in terms of knowing where you are in the
>     crypto state machine.
>
> I've been looking at an alternative design in which we instead adopt a
> more natural layering of putting QUIC on top of DTLS. The basic
> intuition is that you do a DTLS handshake and just put QUIC frames
> directly in DTLS records (rather than QUIC packets). This
> significantly reduces the degree of entanglement between the two
> components and removes the corner cases above, as well as just
> generally being a more conventional architecture. Of course, no design
> is perfect, but on balance, I think this is a cleaner structure.
>
> I have a draft for this at:
> https://datatracker.ietf.org/doc/draft-rescorla-quic-over-dtls/
>
> And a partial implementation of it in Minq at:
>
> Mint: https://github.com/ekr/mint/tree/dtls_for_quic
> Minq: https://github.com/ekr/minq/tree/quic_over_dtls
>
>
> I can't speak for anyone else's implementation, but at least in my
> case, the result was considerable simplification.
>
> It's natural at this point to say that this is coming late in the
> process after we have a lot invested in the current design, as well as
> to worry that it will delay the process. That's not my intention, and
> as I say in the draft, many of the issues we have struggled over
> (headers especially) can be directly ported into this architecture (or
> perhaps just reused with QUIC-over-DTLS while letting ordinary DTLS do
> its thing) and this change would allow us to sidestep issued we are
> still fighting with, so on balance I believe we can keep the schedule
> impact contained.
>
> We are designing a protocol that will be used long into the future, so
> having the right architecture is especially important. Our goal has
> always been to guide this effort by implementation experience and we
> are learning about the deficiencies of the Stream 0 design as we go
> down our current path. If the primary concern to this proposal is
> schedule we should have an explicit discussion about those relative
> priorities in the context of the pros and cons of the proposal.
>
> The hackathon would be a good opportunity to have a face to face chat
> about this in addition to on-list discussion.
>
> Thanks in advance for taking a look,
> -Ekr
>
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-- 
Kazuho Oku