[ntpwg] MAC field (was RE: Stronger symmetric NTP authentication)

[John Smith]

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Hi,=0A=0AI tried searching the archives and i couldnt find any solution, an=
d hence my mail. =0A=0AI am trying to follow the discussion thats taking pl=
ace on the mailing list and i have a few doubts.=0A=0A(a) How does a receiv=
er know that there is a MAC present? As per my understanding, if authentica=
tion is enabled then the receiver keeps parsing the packet till it finds th=
at only 20 bytes are remaining. Anything between this and the standard NTP =
header is then the extension field. Once we are left with just 20 bytes, we=
 take the first 4 bytes as the Key ID and the remaining 16 bytes as the MD5=
 digest. Am i correct till here?=0A=0A(b) Now, when we say that NTP provide=
s provision for 20 octets of digest length, then how would be encode the Ke=
y ID in the MAC field?=0A=0A(c) What if we have an extension field thats of=
 20 bytes. How can we disambiguate between that extension field and a MAC f=
ield?=0A=0AWarm Regards,=0AJohn=0A=0A=0A=0A