Subject: [dns-privacy] Benjamin Kaduk's No Objection on draft-ietf-dprive-padding-policy-05: (with COMMENT)

From: Benjamin Kaduk <[email-masked]>
Date: Wed, 20 Jun 2018 14:38:27 -0700

Benjamin Kaduk has entered the following ballot position for
draft-ietf-dprive-padding-policy-05: No Objection

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COMMENT:
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At risk of triggering suggestions that there is an echo in the room: This
document is targetting Experimental status.  Is there a way to know when
the experiment has ended and/or what the conclusion is?

I know this document does not claim to be exhaustive, but I wonder
if there was any consideration for a "random multiple fixed block length
padding", where a fixed block length is used, but the padded message does
not always use the smallest multiple of the block length that will fit the
message.  That is, sometimes there is an extra block length or three of
padding after the "normal" padding to get to the block length.  (This
strategy is quite closly related to Random Block Length Padding, where the
candidate block lengths are multiples of the single "fixed" block length,
but I cannot convince myself that the two are completely identical.)

Section 4.1

   The Block Size will interact with the MTU size.  Especially for
   length values that are a large fraction of the MTU, unless the block
   length is chosen so that a multiple just fits into the MTU, Block
   Padding may cause unneccessary fragmentation for UDP based delivery.
   Also, chosing a block length larger than the MTU of course always
   forces to always fragment.

This paragraph is (modulo two words) a duplication of a previous paragrpah
in this section; I think this one (the penultimate paragraph) should be
removed.

Section 7

   No matter how carefully a client selects their Padding policy, this
   effort can be jeopardized if the server chooses to apply an
   ineffective Padding policy to the corresponding response packets.
   Therefore, a client applying Padding may want to choose a DNS server
   which does apply at least an equally effective Padding policy on
   responses.

Is there any way for the client to determine the behavior of DNS servers in
this matter other than trial-and-error?  Perhaps some additional text would
be helpful.

   [...] Counter-measures
   against such other side channels could include injecting artificial
   "cover traffic" into the stream of DNS messages, or delaying DNS
   responses by a certain amount of jitter.  Such strategies are out of
   scope of this document.  Additionally, there is neither enough
   theoretic analysis nor experimental data available to recommend any
   such countermeasures.

(This seems very closly aligned with Eric's DISCUSS.)  My understanding is
that in general, random jitter is not actually very helpful in this regard.
So I'm curious to hear a summary of the WG discussions on this strategy.