Re: [Ntp] Tsvart last call review of draft-ietf-ntp-port-randomization-06

Fernando Gont <> Thu, 25 February 2021 09:20 UTC

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To: Brian Trammell <>,
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From: Fernando Gont <>
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Date: Thu, 25 Feb 2021 06:17:24 -0300
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Subject: Re: [Ntp] Tsvart last call review of draft-ietf-ntp-port-randomization-06
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Hi, Brian,

Thanks so much for you comments! In-line...

On 23/2/21 16:15, Brian Trammell via Datatracker wrote:
> This document is ready from a transport standpoint. It describes an
> already-implemented, relatively-straightforward application of an existing BCP
> to a well-understood protocol, and is clearly written. I especially appreciated
> the complete set of considerations, covering situations in which NTP port
> randomization could cause issues with certain deployment scenarios. The effect
> of routing on NTP is well studied (one such study is cited in the draft), and
> sections 3.3 and 3.4 discuss how this draft could interact with on-path
> modification of NTP traffic. Herein lies my only nit with the draft: it would
> be nice if 3.3 and 3.4 discussed how an NTP-speaking endpoint could detect and
> react to issues caused by misconfigured filtering or NAT.

You mean issues caused by packet filtering meant to mitigate DoS 
attacks? If the service port is used (src=dst=123), it'd be impossible 
(from the layer-4 header) to tell client or server traffic. So one would 
resort to dropping all traffic, or no traffic. BUt if port randomization 
is employed, client traffic will employ an ephemeral port and thus it 
becomes possible to distinguish client/server traffic by looking at the 
transport protocol port numbers -- i.e., port randomization helps in 
this scenario.

Similarly, in the case of mis-configured NATs (I'm assuming that you're 
referring to NATs not translating service ports) doing port 
randomization would actually allow for multiple clients to work across 
NATs, since there wouldn't be conflicting uses of the service port on 
the NAT internal realm.

i.e., in both cases, port randomization actually helps in these two 

Please do let me know if I've misinterpreted your comment.


Fernando Gont
SI6 Networks
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