Tai,
All I can say is read my message again. Doing without the MAC is a very
special case and unintended by the specification.
Dave
Tal Mizrahi wrote:
Hi Dave,
So you are saying that according to the current spec it is possible in
some configurations to have an extension field without the existence
of a MAC?
Tal.
From: David L. Mills [mailto:[email protected]]
Sent: Monday, December 12, 2011 6:01 AM
To: Tal Mizrahi
Cc: [email protected]; [email protected]
Subject: Re: [ntpwg] [TICTOC] NTP Extension Field without Authentication
Tal ,
It's a little more complicated than it seems. The parsing rules assume
that a message digest is always present if an extension field is
present. The NT$ packet header, extension fields and MAC are multiples
of 32-bit words. The minimum MAC length is 5 words and maximum length
is 6 words. The minimum extension field length is 2 words. If the
remaining number of words during the parse is less than 7, the
remainder is the MAC. If not, an extension field is present. The
parser updates the parser pointer folloowing the extension field and
tries again.
Thus, if there are at least 7 words remaining and the extension field
eats up all those words, the MAC could be assumed absent . This is a
rather hokey design, but would in principle work.
Dave
Tal Mizrahi wrote:
Hi,
Revisiting an issue that was raised a few months ago and is yet to be
resolved:
RFC 5905 defines an extension field. The RFC states that a MAC must be
present when there is an extension field.
Obviously, it would be beneficial for various purposes to allow
Extension Fields independent of whether the MAC is present.
Some people thought this is a mistake in the spec, and that it should
be included in the errata. Others thought that Extension Fields
without MAC are something new that needs to be defined in a new document.
This was discussed in IETF 81, and then revisited in the ad-hoc
meeting in October, but no conclusion was reached.
It would be great to hear the opinion of the WG and the chairs about
how to proceed with this.
Thanks,
Tal.
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