Subject: [dns-privacy] Tsvart last call review of draft-ietf-dprive-padding-policy-04

From: Magnus Westerlund <[email-masked]>
Date: Wed, 04 Apr 2018 02:30:34 -0700

Reviewer: Magnus Westerlund
Review result: Ready with Issues

I have reviewed this document as part of TSV-ART task to review documents with
potential transport related issues.

I note that the document in its final recommendation regarding block sizes do
consider MTU for reasonable size choices. What I am missing in Section 4 is the
discussion of MTU as impacting this. From my perspective, it appears reasonable
to: In Section 4.1 consider if the Block Size will interact with the MTU.
Especially for block sizes that are a small fraction of the MTU, unless the
block is chosen so that a multiple just fits the MTU, the block padding may
cause unnecessary fragmentation for UDP based delivery. Also chosing a block
size larger than the MTU of course forces one to always fragment.

In Section 4.2 I think depending on the negotiated size, the downside is that
it will commonly result in a consistent number of fragments reducing delivery
probability. I haven't digged into the negotiation part about maximum response
size. But, I assume that this is not necessarily chose based on MTU
constraints, but other limitations in the system.

Note that these comments only applies for datagram based transport without its
own fragmentation mechanism, e.g. UDP.

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From: Alexander Mayrhofer <[email-masked]>
Date: Fri, 13 Apr 2018 12:33:52 +0200

Magnus,

thanks for your review. I'm drafting appropriate text for the document
considering the interaction between MTU and Block Sizes. When you sad
"a small fraction of the MTU" above, i suppose you mean "a large
fraction of the MTU", because my understanding would be that
fragmentation is much more likely to occur when Block Size is set to
(e.g.)  MTU / 2  (large fraction) compared to when it was set to
(e.g.) MTU / 10 (small fraction).

I'm therefore adding

- for Maximal Length Padding:

"Depending on the negotiated size, this
strategy will commonly exceed the MTU, and then result in a consistent
number of fragments reducing delivery probability
when datagram based transport (such as UDP) is used."

- for Block Length Padding:

"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 Length
Padding may cause unneccessary fragmentation for UDP based delivery.
Also, chosing a block length larger than the MTU of course forces to
always fragment".

best,
Alex

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From: Alexander Mayrhofer <[email-masked]>
Date: Fri, 13 Apr 2018 12:33:52 +0200

Magnus,

I'm therefore adding

best,
Alex