Dave, in a email message to the ntpwg list on July 14, you wrote to Todd
Glassey and said:
"A careful read of the specification should give you comfort that the
parsing rules do distinguish between an extension field and a MAC. In
particular, the rules permit one or more extension fields without a MAC.
The only requirement is that a MAC, if present, must be the last field."
Which is it?
Personally, I would love for the extension field to be independent of
the MAC field.
On 12/12/11 13:53, David L. Mills wrote:
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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blu
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violent psychopath who knows where you live. - Martin Golding
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