Re: Authentication in draft-kuhn-quic-bdpframe-extension

[Nicolas Kuhn <[e-mail]>]

Dear all,

Thank you for your interest in this work !

I would tend to agree with Lucas and think we should consider scenarios 
where BDP frames would be used with TLS resumption and I do not see the 
need for proposing another trust mechanism; But there may be scenarios I 
do not see ?

More comments inline.

Kind regards,

Nico

On 11/3/23 16:44, Lucas Pardue wrote:
> Hi folks,
>
> I'm still trying to come up to speed on this spec. But when I've 
> thought about it a little, its seemed very natural to associate the 
> BDP frame (contents) with the TLS session. We already have a lot of 
> text about TLS session resumption in QUIC. It feels like there is 
> already a template design with HTTP/3 - a server sends SETTINGS to 
> tell a client something unique about the active QUIC connection. RFC 
> 9114 section 7.2.4.2 [1]states
>
> > When a 0-RTT QUIC connection is being used, the initial value of 
> each server setting is the value used in the previous session. 
> ClientsSHOULDstore the settings the server provided in the HTTP/3 
> connection where resumption information was provided, but theyMAYopt 
> not to store settings in certain cases (e.g., if the session ticket is 
> received before the SETTINGS frame). A clientMUSTcomply with stored 
> settings -- or default values if no values are stored -- when 
> attempting 0-RTT. Once a server has provided new settings, 
> clientsMUSTcomply with those values.¶ 
> <https://www.rfc-editor.org/rfc/rfc9114.html#section-7.2.4.2-6>
>
> So with a bit of massaging, if we can link BDP frame to session 
> resumption. we know that it is based on a previous trust relationship.
>
> Is there any scenario where BDP frame would want to be used without 
> TLS resumption?
[NK] I agree.
>
> Cheers
> Lucas
>
> [1] - https://www.rfc-editor.org/rfc/rfc9114.html#section-7.2.4.2-6
>
> On Thu, Nov 2, 2023 at 6:17 PM Gorry Fairhurst <[e-mail]> 
> wrote:
>
>     On 02/11/2023 16:43, Q Misell wrote:
>>     Hi all,
>>
>>     I've been working with Gorry (and others) on actually
>>     implementing the BDP frame extension, and further refining the
>>     draft based on experience from implementation.
>     Q, I think I can help a little, see below, but I think there are
>     good questions here.
>
[NK] If the draft is not clear enough on these relevant questions, we 
ought to make things clearer.
>
>>
>>     One thing that came up that I'd like to ask the WG's opinion on
>>     is that of authentication of the BDP frame, and when it should be
>>     sent in the exchange. I've had a few thoughts on this, it'd be
>>     great to hear what others think of them, or what other
>>     suggestions people might have.
>>
>>     First, my thoughts on authentication. Do the CC parameters need
>>     to be authenticated at all? I would say "yes" as a client sending
>>     some unauthenticated CC parameters could cause a DoS of the
>>     server (or any other node along the path) by trying to send far
>>     too much data at once.
>     The reason for the secure hash around the contents of the BDP
>     Frame is to allow a server to know the CC params had not been
>     modified. Of course you caould ask what sort of information
>     contributes to that hash, to make the server confident that it can
>     accept CC params from the client and believe that these have not
>     been modifed? That could be important?
>
[NK] The client should not be able to transmit unauthenticated CC 
parameters that are not checked / known by the server. In the current 
spec, the client can only send data previously received by the server. 
Malicious clients could try to cause a DoS on the server but that would 
not be specific to BDP Frame but to 0-RTT in general.
>
>>     Should the CC parameters be encrypted? Probably not, as a client
>>     which is aware of a major decrease in available capacity could
>>     compare the new link capacity to its stored CC parameters and
>>     decide not to send them. If they're encrypted the client can't
>>     inspect what CC parameters the server thinks the link will have.
>
>     Perhaps the ID ought to be clearer. The QUIC Session is of course
>     encrypted and authenticated, so, in this respect, the BDP Frame is
>     protected in transit along the path using TLS.
>
>     The current proposal is not to additionally encrypt the CC params
>     *within* the BDP, so that a client could read these and utlise as
>     it sees fit. This still needs to authenticate the entire set of
>     params, so that the server could trust them.
>
>     The params include an endpoint token used by a server  to
>     represent the remote endpoint - we could have used the client IP
>     source address for this if the client had an invariant public IP 
>     source address. That's not so common with IPv6 or the use of IPv4
>     NAPT - so the server has to find a way to represent it's view of
>     the client as the endpoint token. There could be possibilities to
>     do this quite differently.
>
>>
>>     How should they be authenticated? There are a few options I can
>>     see here, and I'm unsure which is best:
>>     (1) Authenticated with the TLS certificate
>>     (2) Authenticated with some other asymmetric key
>>     (3) Authenticated using some symmetric key known only to the server
>>     (4) Same as 3 but with a key identifier
>>
>>     Options 1 and 2 allow the client to verify the authentication
>>     over the CC parameters, but this doesn't seem to be of much use
>>     to me. Option 1 additionally sets a time limit on use of stored
>>     CC parameters, as the TLS certificate will eventually expire.
>>     This doesn't seem to me to be much of an issue. A new connection
>>     far into the future (say 1-2 months) would almost certainly have
>>     different CC parameters anyway.
>>
>>     Option 3 seems the best to me. It would allow one key to be
>>     shared across an array of anycast servers, without sharing other
>>     keying material that might be used to protect more sensitive
>>     parts of the connection. Option 4 additionally expands on this by
>>     allowing key rotation without immediately invalidating all
>>     current stored CC parameters.
>     So, if this is about how to construct the secure hash, irt seems
>     like an interesting topic to find out more, I'd agree.
>
[NK] We may not specify how to compute the secure hash but that could be 
interesting discussions if you think the draft needs to be more specific 
on this. IMHO the client does not need to know how the secure hash is 
compute and thus not sure we need interoperability.
>
>>
>>     When should the BDP frame be sent? There are two places I can see
>>     BDP frames being useful to send:
>>     (1) After initial frames but before crypto frames
>>     (2) After crypto frames and before application data
>>
>>     Option 1 allows for the previously calculated CC parameters to be
>>     used for the sometimes quite large TLS handshake, but also
>>     precludes options 1 and 2 for authentication. Option 2 allows for
>>     greater flexibility in authentication, and also makes the BDP
>>     frame encrypted in transit. I'm unsure what the privacy
>>     implications of an unencrypted BDP frame are, so if anyone can
>>     come up with a reason CC data shouldn't be observable to an
>>     intermediary that would be greatly appreciated.
>     :-)
>
[NK] Do we need to specify this in the draft or should this be let to 
implementers to define the most relevant approach (w.r.t. frame 
scheduling to format QUIC packets).
>
>>
>>     Looking forward to hearing your thoughts.
>
[NK] Thank you for your comments !
>
>>     Cheers,
>>     Q Misell
>     Gorry
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