RFC 5906, "Network Time Protocol Version 4: Autokey Specification", June 2010

Errata ID: 4026
Status: Verified
Type: Editorial
Publication Format(s) : TEXT
Reported By: Tal Mizrahi
Date Reported: 2014-06-25
Verifier Name: Brian Haberman
Date Verified: 2014-07-01

Section 10 says:

One or more extension fields follow the NTP packet header and the
   last followed by the MAC.  The extension field parser initializes a
   pointer to the first octet beyond the NTP packet header and
   calculates the number of octets remaining to the end of the packet.
   If the remaining length is 20 (128-bit digest plus 4-octet key ID) or
   22 (160-bit digest plus 4-octet key ID), the remaining data are the
   MAC and parsing is complete.  If the remaining length is greater than
   22, an extension field is present.  If the remaining length is less
   than 8 or not a multiple of 4, a format error has occurred and the
   packet is discarded; otherwise, the parser increments the pointer by
   the extension field length and then uses the same rules as above to
   determine whether a MAC is present or another extension field.

It should say:

One or more extension fields follow the NTP packet header and the
   last followed by the MAC.  The extension field parser initializes a
   pointer to the first octet beyond the NTP packet header and
   calculates the number of octets remaining to the end of the packet.
   If the remaining length is 20 (128-bit digest plus 4-octet key ID) or
   24 (160-bit digest plus 4-octet key ID), the remaining data are the
   MAC and parsing is complete.  If the remaining length is greater than
   24, an extension field is present.  If the remaining length is less
   than 8 or not a multiple of 4, a format error has occurred and the
   packet is discarded; otherwise, the parser increments the pointer by
   the extension field length and then uses the same rules as above to
   determine whether a MAC is present or another extension field.

Notes:

The original text stated that a MAC is present if the remaining length is 20 octets or 22 octets. This was a typo; the correct statement is that a MAC is present if the remaining length is 20 octets or *24 octets*.

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