Re: [TLS] I-D Action: draft-ietf-tls-tlsflags-00.txt

Benjamin Kaduk <bkaduk@akamai.com> Wed, 14 August 2019 01:12 UTC

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Date: Tue, 13 Aug 2019 20:12:00 -0500
From: Benjamin Kaduk <bkaduk@akamai.com>
To: Watson Ladd <watsonbladd@gmail.com>
Cc: Ilari Liusvaara <ilariliusvaara@welho.com>, TLS List <tls@ietf.org>
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Subject: Re: [TLS] I-D Action: draft-ietf-tls-tlsflags-00.txt
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On Tue, Aug 13, 2019 at 06:03:32PM -0700, Watson Ladd wrote:
> On Tue, Aug 13, 2019 at 6:00 PM Benjamin Kaduk <bkaduk@akamai.com>; wrote:
> >
> > On Mon, Aug 12, 2019 at 09:25:19PM +0300, Ilari Liusvaara wrote:
> > > On Mon, Aug 12, 2019 at 10:48:55AM -0700, internet-drafts@ietf.org wrote:
> > > >
> > > > A New Internet-Draft is available from the on-line Internet-Drafts directories.
> > > > This draft is a work item of the Transport Layer Security WG of the IETF.
> > > >
> > > >         Title           : A Flags Extension for TLS 1.3
> > > >         Author          : Yoav Nir
> > > >     Filename        : draft-ietf-tls-tlsflags-00.txt
> > > >     Pages           : 6
> > > >     Date            : 2019-08-12
> > > >
> > > >
> > > > The IETF datatracker status page for this draft is:
> > > > https://datatracker.ietf.org/doc/draft-ietf-tls-tlsflags/
> > > >
> > > > There are also htmlized versions available at:
> > > > https://tools.ietf.org/html/draft-ietf-tls-tlsflags-00
> > > > https://datatracker.ietf.org/doc/html/draft-ietf-tls-tlsflags-00
> > >
> > > Two things:
> > >
> > >
> > > 1) uint8 flags<0..31>;
> > >
> > > That adds an extra byte that is not technically necressary (because
> > > extensions have lengths anyway) and limits number of flags to 248
> > > (which might be enough).
> > >
> > > And I do not think the length of flags field can be 0 (if it would
> >
> > I think you need to send it in at least one protocol "response", to
> > confirm support for the extension, even if none of the flags offered
> > require confirmation/echo individually.
> 
> I'm not sure this is the case: if in the future we define flags, then
> what is the difference between not understanding any flag and not
> understanding the extension?

Nothing -- the difference is between understanding the "please frobnitz
my baddle" flag and not understanding it (or the extension, for that
matter).  If "please frobnitz my baddle" is defined such that it appears
in the ClientHello and if the server supports the extension, the server
has the option to send a Thwarp handshake message to the client at any
time post-handshake if the server detects its imminent demise, then the
client that observes "I didn't get a Thwarp" cannot distinguish between
"the server doesn't support the extension" and "the server supports the
extension but is unaware of an imminent demise".

Does that help?

-Ben