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Re: [ntpwg] I-D Action: draft-ietf-ntp-checksum-trailer-02.txt

Tal Mizrahi <talmi@marvell.com> Sun, 30 August 2015 06:48 UTC

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From: Tal Mizrahi <talmi@marvell.com>
To: Miroslav Lichvar <mlichvar@redhat.com>, "ntpwg@lists.ntp.org" <ntpwg@lists.ntp.org>
Thread-Topic: [ntpwg] I-D Action: draft-ietf-ntp-checksum-trailer-02.txt
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Date: Sun, 30 Aug 2015 06:21:48 +0000
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Subject: Re: [ntpwg] I-D Action: draft-ietf-ntp-checksum-trailer-02.txt
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Hi Miroslav,

Thanks for the comments.
Please see my responses below.

Tal.


>It says the new extension field can be used in packets without MAC and also
>in packets with MAC, even though it says it's not very useful in the later case.
>But it's not clear to me how the checksum complement can be calculated
>when a MAC is present. When the value of the complement is changed to not
>invalidate the UDP checksum (e.g. after updating a timestamp), the MAC
>needs to be updated too as it's calculated from all data including the
>timestamp and the complement.
>But then the UDP checksum won't match, the complement needs to be
>updated again, and we have a cyclic dependency.
>

That is a good point. The checksum complement cannot be updated in the presence of a MAC, and I will update the draft to explain this point.


>As for interoperability with existing implementations, shouldn't be a server
>allowed to use the extension only when the client request had it? There are
>NTP implementations that ignore any replies that are not exactly 48 bytes.

I understand there are implementations that behave this way, but I could not find such a requirement in RFC 5905 - please correct me if I am wrong.
I am not sure we want the *current draft* to be interoperable with unspec'ed behavior. If we believe this is the correct behavior, we should probably start by updating draft-ietf-ntp-extension-field.
I would be interested to hear if other people believe this is the right direction. 

>
>BTW, is anyone working on the "timestamp correction" extension field that is
>mentioned in this draft? I'd be very interested in that.

We (at Marvell) have implemented the hardware aspect of this feature, i.e., timestamping an NTP message + updating its checksum complement. 
 
>
>--
>Miroslav Lichvar
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