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

Jim Schaad <ietf@augustcellars.com> Mon, 19 February 2018 16:32 UTC

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From: Jim Schaad <ietf@augustcellars.com>
To: 'Martin Thomson' <martin.thomson@gmail.com>, tls@ietf.org
CC: draft-ietf-tls-record-limit@ietf.org
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Date: Mon, 19 Feb 2018 08:31:53 -0800
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Subject: Re: [TLS] Mail regarding draft-ietf-tls-record-limit
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Martin,

I think that the wording I would prefer would be along the lines of 

A server MUST NOT error on the value of the extension when a higher TLS version is requested.  The server MUST use the minimum of the requested value and the maximum value for the TLS version negotiated.  A server MAY error if a the value of the extension is exceeded for the version of TLS requested.

> -----Original Message-----
> From: Martin Thomson [mailto:martin.thomson@gmail.com]
> Sent: Monday, February 19, 2018 2:15 AM
> To: Jim Schaad <ietf@augustcellars.com>; <tls@ietf.org> <tls@ietf.org>
> Cc: draft-ietf-tls-record-limit@ietf.org
> Subject: Re: Mail regarding draft-ietf-tls-record-limit
> 
> (+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?
> 
> 
> 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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