Re: [TLS] WGLC for draft-ietf-tls-ticketrequests

Daniel Migault <daniel.migault@ericsson.com> Mon, 25 November 2019 17:05 UTC

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From: Daniel Migault <daniel.migault@ericsson.com>
Date: Mon, 25 Nov 2019 12:04:54 -0500
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To: David Schinazi <dschinazi.ietf@gmail.com>
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Subject: Re: [TLS] WGLC for draft-ietf-tls-ticketrequests
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On Wed, Nov 20, 2019 at 10:20 PM David Schinazi <dschinazi.ietf@gmail.com>;
wrote:

> Hi folks,
>
> I've chatted with Daniel and Chris offline, and I think there might
> have been some miscommunication here. Please allow me to
> rephrase what I think is going on, and please let me know if
> this accurately represents your views.
>
> Daniel has a IoT use-case where due to memory constraints,
> a client knows it can only handle a certain number of tickets,
> and therefore Daniel was wondering if it would make sense to
> make the requested number of tickets a required maximum
> (as in a RFC 2119 MUST). However, server operators on this
> thread have indicated that a MUST would get in the way of
> implementing this, due to STEK rotation for example. Daniel
> understands this, and is OK with the current mindset of the
> document (which is only a hint, not a MUST). Additionally,
> Daniel would prefer to see the document move forward.
>
> In order to try to address Daniel's comments, I attempted
> to rephrase the normative section to suggest in more
> stronger terms that servers really shouldn't be sending
> more than the client's request, but keeping it a SHOULD.
>
> Here is the PR with the rephrase:
> https://github.com/tlswg/draft-ietf-tls-ticketrequest/pull/10
>
> Here's a copy of the updated paragraph in the PR:
>    Clients can use TicketRequestContents.count to indicate the number of
>    tickets they would prefer to receive.  Servers SHOULD NOT send more
>    tickets than TicketRequestContents.count, as clients will most likely
>    discard any additional tickets.  Servers SHOULD additionally place a
>    limit on the number of tickets they are willing to send to save
>    resources.  Therefore, the number of NewSessionTicket messages sent
>    SHOULD be the minimum of the server's self-imposed limit and
>    TicketRequestContents.count.
>
> I apology for the late response. This addresses my MUST/SHOULD concern.
Thanks David.


> Regarding Viktor's suggestion, I personally believe it would increase the
> complexity of the proposal, and I don't see use-cases compelling enough
> to warrant that complexity. I would rather keep this proposal as simple as
> possible.
>

I particularly like the proposal from Viktor and - at least as I see it -
its associated simplicity. I believe it would be beneficial for the
document to include it.


>
> Thanks,
> David
>
>
> On Wed, Nov 20, 2019 at 2:48 PM Viktor Dukhovni <ietf-dane@dukhovni.org>;
> wrote:
>
>> On Wed, Nov 20, 2019 at 11:53:08AM +0800, Tommy Pauly wrote:
>>
>> > > Somebody should try to avoid ending up with N new tickets after
>> > > every connection, but in could well be the client.
>> >
>> > Yes, I think that can and ought to be the client. The client is really
>> the
>> > only party that can know how many tickets have been "consumed" by
>> potentially
>> > failed attempts to connect that the server didn't see in time or got
>> dropped.
>>
>> OK, in that case, the proposal, is that on resumption, if the proposed
>> extension is *absent*, then the server should generally send just one
>> rather
>> than any larger default.
>>
>> If the extension is present, it is up to the client to not blindly ask
>> for N
>> tickets each time, and generally ask for just one ticket on resumption,
>> once it
>> has sufficiently many tickets for the desired concurrency level, with each
>> parallel thread obtaining one replacement ticket its next connection.
>>
>> As discussed clients that prefer to reuse tickets, can ask for zero, and
>> get 1
>> only as-needed.  If the server supports reuse then all is well.
>>
>> If the server does not support and refuses reuse, then it will issue a new
>> ticket each time and may only accept it once, but the client may have a
>> single-slot cache.  If the client makes only one connection at a time,
>> this
>> also works, but if/when its handshakes with the server overlap (a narrow
>> window
>> of ~1RTT) and two connections attempt to resume with the same ticket, one
>> of
>> them will end up doing a full handshake, but only when the client and
>> server
>> applications have incompatible ticket reuse expectations.  Some friction
>> when the client and server are mismatched is not the end of the world.
>>
>> So on the whole I think the proposal still works with just a numeric
>> signal
>> (tweaked with 0xff == none and 0 == reuse), and the onus to "generally
>> send 1"
>> on resumption moved to the client (i.e. clients that don't solicit ticket
>> reuse
>> should request only 1 ticket once they have enough).
>>
>> --
>>     Viktor.
>>
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