Re: [tcpinc] Adam Roach's Yes on draft-ietf-tcpinc-tcpcrypt-09: (with COMMENT)

Daniel B Giffin <dbg@scs.stanford.edu> Fri, 17 November 2017 03:14 UTC

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Date: Thu, 16 Nov 2017 19:14:49 -0800
From: Daniel B Giffin <dbg@scs.stanford.edu>
To: Adam Roach <adam@nostrum.com>
Cc: Kyle Rose <krose@krose.org>, "Mirja Kuehlewind (IETF)" <ietf@kuehlewind.net>, The IESG <iesg@ietf.org>, tcpinc <tcpinc@ietf.org>, tcpinc-chairs@ietf.org, draft-ietf-tcpinc-tcpcrypt@ietf.org
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Subject: Re: [tcpinc] Adam Roach's Yes on draft-ietf-tcpinc-tcpcrypt-09: (with COMMENT)
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Adam Roach wrote:
> On 11/9/17 08:16, Kyle Rose wrote:
> > On Thu, Nov 9, 2017 at 10:11 AM, Adam Roach <adam@nostrum.com> wrote:
> > > On 11/8/17 19:45, Mirja Kuehlewind (IETF) wrote:
> > > > That’s not true. This is to cover the case where the packet got corrupted on
> > > > the path, thus hopefully the retransmission will decrypt correctly.
> > > So, to be clear, you're talking about packet corruption that happens to
> > > produce a valid checksum, right? If that's the reasoning here, the authors
> > > probably want to include that rationale in the document.
> > Mirja's is my interpretation, as well.
> > 
> > Off-path attackers wouldn't be able to sent segments with the right
> > sequence number with high probability, so it's unlikely that this is a
> > DoS vector; but giving implementations the option of simply dropping
> > segments for which the authenticity check fails is not likely to cause
> > recurring timeout problems for correct implementations.
> 
> Yes, and you're giving implementors options here without really explaining
> why -- which means they're probably just going to pick one randomly. Adding
> text that gives some notion about why one might choose one option over the
> other would allow them to make an informed choice rather than a random one.

That makes sense.  How about the following text instead?
(It is very similar to how we treat spurious FIN in the
following section, 3.7.)

  [...] The output of this operation is either a plaintext
  value `P` or the special symbol FAIL.  In the latter case,
  the implementation SHOULD abort the connection and raise
  an error condition distinct from the end-of-file
  condition.  But if none of the TCP segment(s) containing
  the frame have been acknowledged and retransmission could
  potentially result in a valid frame, an implementation MAY
  instead drop these segments.

This doesn't speculate on how the corruption came about (I
agree it's difficult to find a reasonable case), but it
gives a reasonable default via SHOULD and also leaves a
little room for implementations with some sort of clever
anti-DOS mitigation.

daniel

> 
> Again, this remains a suggestion and not a blocking comment. You can feel
> free to proceed with the document as-is.
> 
> /a