[ntpwg] Section 7.5. NTP Extension Field Format

[Danny Mayer]

I strongly object to this change. The extension-private data belongs 
inside the extension and placing them in the extension type field makes 
it more difficult to deploy additional extensions.

Danny

David L. Mills wrote:
> 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
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