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Re: [TLS] Mail regarding draft-ietf-tls-record-limit

Benjamin Kaduk <bkaduk@akamai.com> Mon, 19 February 2018 15:50 UTC

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To: Martin Thomson <martin.thomson@gmail.com>, Jim Schaad <ietf@augustcellars.com>, "<tls@ietf.org>" <tls@ietf.org>
Cc: draft-ietf-tls-record-limit@ietf.org
References: <008b01d3a94e$1cc174e0$56445ea0$@augustcellars.com> <CABkgnnUMKhnQS59Xt2=WaJp9Ywbzej6_vZ1u9R1dHE02ocir3Q@mail.gmail.com>
From: Benjamin Kaduk <bkaduk@akamai.com>
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Date: Mon, 19 Feb 2018 09:50:35 -0600
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Subject: Re: [TLS] Mail regarding draft-ietf-tls-record-limit
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On 02/19/2018 04:14 AM, Martin Thomson wrote:
> (+tls@)
>
> This is a good question Jim and one that I thought through during
> implementation, but failed to capture in the doc.
>
> Basically, there is no way to validate the extension if the client
> includes an unknown version of TLS or an extension that it doesn't
> understand.  A client can know because it should understand the
> protocol as negotiated.
>
> The text currently says "an endpoint MUST NOT send a value higher than
> the protocol-defined maximum record size unless explicitly allowed by
> such a future version or extension"  I think that we should add "A
> server MUST NOT enforce this restriction; a client might advertise a
> higher limit that is enabled by an extension or version the server
> does not understand."
>
> Does that make sense?

Mostly.  I am not sure that that exact phrasing is great, though, as we
do want the server to enforce the restriction *as written*, i.e., taking
into account that the client may know better than the server if a newer
version and/or extension is in use.  So, maybe something like "Receiving
a larger value is not necessarily cause for a server to abort the
handshake, given this possibility".

-Ben

>
> On Mon, Feb 19, 2018 at 5:51 PM, Jim Schaad <ietf@augustcellars.com> wrote:
>> I was looking at this document relative to a specific question for Kathleen,
>> and I had one thing that I would like you to look at and see if you think it
>> is clear enough.
>>
>> I have a server that is TLS 1.2, a client that is TLS 1.2 & 1.3.   It sends
>> a hello w/ and extension value of 2^14+1.  It is not completely clear to me
>> that the server should accept this as a legal value and compute the min of
>> it and the maximum 1.2 value as the value to use when sending messages to
>> the client rather than producing an error message because the value is too
>> large.
>>
>> Jim
>>
>>
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