[ntpwg] Section 7.5. NTP Extension Field Format

[David L. Mills]

Guys,

Following is a suggested replacement for Section 7.5. This preserves 
compatibility with symmetric key cryptography and Autokey, but allows up 
to 15 additional allications to use extenstion fields. As now, when a 
MAC is present it can be verified without knowledge of the extension 
field application and unknown applications are ignored.

7.5.  NTP Extension Field Format

In NTPv4 one or more extension fields can be inserted after the header 
and before the MAC.  Other than defining the field format, this document 
makes no use of the field contents.  An extension field has the format 
shown in Figure 14.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      xxx      | Class |  xxx  |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                            Value                              .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Padding (as needed)                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 14: Extension Field Format

All extension fields are zero-padded to a word (4 octets) boundary. The 
4-bit Class field defines the application using the extension field; the 
fields marked xxx are available to and defined by the application. 
Currently, only the Autokey class is defined with value zero.

The Length field is a 16-bit unsigned integer which indicates the length 
of the entire extension field in octets, including the Padding field. In 
order to correctly parse packets with and without extension fields and 
with and without MACs, the minimum extension field length when a MAC is 
present is 2 words. If a MAC is not present, the minimum field length is 
7 words. A maximum field length remains to be established.

When a MAC is present, it is validated before the extension fields are 
processed. If a MAC is not present, or if a MAC is present and valid, 
the extension fields are processed in order; however, if a particular 
class code is not understood, the extension field is discarded.

Dave