Re: QUIC Version Negotiation Extension

David Schinazi <> Mon, 04 November 2019 22:56 UTC

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From: David Schinazi <>
Date: Mon, 4 Nov 2019 14:55:57 -0800
Message-ID: <>
Subject: Re: QUIC Version Negotiation Extension
To: =?UTF-8?Q?Mikkel_Fahn=C3=B8e_J=C3=B8rgensen?= <>
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On Mon, Nov 4, 2019 at 2:35 PM Mikkel Fahnøe Jørgensen <>;

> Response also inline
> On Mon, Nov 4, 2019 at 1:47 PM Mikkel Fahnøe Jørgensen <>;
> wrote:
>> (reposting on proper topic)
>> Some random observations reading through the document
>> - Is the order relevant in the receiver version list?
> Yes all lists of versions are ordered.
> I might have missed it, but otherwise I think this should be mentioned
> explicitly.
Isn't it mentioned in the definitions of these fields?

> - It is tempting to just hash the received version list, but that requires
>> agreeing on an algorithm, unless the algorithm is stated to be specific to
>> that version, which is complicated.
> Which list are you referring to? For all the ones in the draft, the peer
> needs access to all the elements so I don't think a hash would help.
> I am referring the received list. The client receives the list. The server
> (modulo redeployment/routing) knows its own list and does not need to read
> it, it just needs proof that the response matches what it initially sent.
> This could be done with a hash. (If I understand correctly). But maybe not
> because the list is filtered by clients input so that would be many hashes
> to track.
I don't know what the received list is, there's nothing with that name in
the draft.

> - VERSION_NEGOTIATION_ERROR vs drop - I’m not sure it is a good idea to
>> close the connection. The initials are public so it is possible to inject
>> false versions. There are probably many other similar attacks we don’t
>> bother with, but still …
> Once we're this far in the handshake, we cannot recover from this error.
> Dropping the packet will only make things worse.
> It is that a given across all versions, without mentioning it explicitly
> here? This links to the at most one roundtrip.
That's a given across all current versions. New versions can override the
text in this spec.

> - Downgrade - I’m a bit worried about state management and server
>> redeployments. A server could reject a valid packet because an Initial was
>> routed to a new server. (Reading further, I see this is addressed). This is
>> probably a pragmatic solution, but it has an assumption about eventual
>> global coordination. I suspect something could be done here with tokens or
>> CID routing, but it is not trivial.
> Sounds interesting. Do you have a concrete proposal?
> Not off hand, but the idea is that the server encodes some context in a
> token, that an upgraded server can understand such that it can either
> impersonate the old version or reject the handshake. As for CID routing: (I
> don’t recall if new initials with new version must carry same original CID,
> and this might be version specific) - but if the a second Initial routes
> the same place, similar to 0-RTT, you can limit the impact of server
> deployment coordination to the one or the few servers affected by that CID.
> If a version negotiation packet decides a new CID via the destination CID
> field, a server farm is also able to route traffic to a segment with a
> predictable version scope - for example by never updating some servers on
> even hours and always redirect vneg to a subcluster that is not upgrading
> and which therefor has a predictable version scope.
The current design of connection IDs does not guarantee consistent routing
on client-generated server connection IDs, so I don't think we can rely on

> - Security Considerations - perhaps it is worth noting the transport
>> parameters need additional protection beyond the Initial packet protection?
>> This follows from TLS, but if TLS is not being used, this can version
>> negotiation even if other parts of the protocol version is not sensitive to
>> this in that particular version.
> draft-ietf-quic-transport section 7 requires transport parameters to be
> authenticated.
> but that is for QUIC v1. QUIC v3 might be for constrained devices that do
> not use TLS and which do not migrate and which only has two non-hardcoded
> transport parameters which are verified in 1-RTT in order to achieve a
> light handshake. What would vneg do then? Notably such a version would
> depend heavily on vneg because it really needs the one version that works
> with its hardware accelerator, or whatever.
I think QUICv3 should also provide this property, otherwise it'll be likely
to be unsafe. If a future version doesn't have authenticated transport
parameters then we'll have to create a separate downgrade prevention
mechanism. But we can cross that bridge when we get there.

> - Improve discussion of Previously Attempted Version. While the
>> requirements are readable, the purpose of doing this check is less obvious.
>> Presumably this deals with downgrade attacks, but more explanation would be
>> appreciated.
> The downgrade prevention section discusses the purpose of this field. What
> details do you think we should add?
> Generally QUIC does not argue why something is done, although sometimes
> providing motivation (like you do here - downgrade prevention), but I’d
> still like to better understand why this prevents a downgrade and why a
> downgrade is such a bad thing, and how such an attack could happen, and why
> comparing the previous version would prevent that from happening. Because
> this is such a core element if the entire document.
It sounds like we could be clearer. Would you be able to send us a PR?