ULE Extension headers - should we recommend an ordering?

[Gorry Fairhurst <gorry@erg.abdn.ac.uk>]

Discussion in the security thread raised several issues that could have 
been simpler if there were some "guidance on the normal ordering".

Here is a draft idea that defines 4 levels of headers.  Security I could 
come in at level 2, if I understand correctly. This could even be 
RECOMMENDED ordering in the ULE Extension header draft? Would this be good?

The authors would be interested in thoughts about whether this is 
useful, or whether it is better to leave the topic of ordering 
completely to the implementor, and not offer guidance....

Thoughts on the sample text below?

Gorry (& co-author Bernhard)

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3.4 Recommended ordering of Extension Headers

There is a natural ordering of extension headers, which is determined by 
the fields that they are associated with. A suitable ordering for many 
applications is presented in the list below (from first to last header 
within an SNDU). This does not imply that all types of Extensions should 
be present in a single SNDU. The presented ordering may serve as a 
guideline for optimisation of Receiver processing.

Please view in a fixed-width font such as Monaco or Courier.

+-+----------------------------------+-------------------------------+
| |Fields related to Extension Header| Example Extension Headers     |
+-+----------------------------------+-------------------------------+
|1| Link framing and transmission    | Timestamp Extension           |
+-+----------------------------------+-------------------------------+
|2| Entire remaining SNDU Payload    | Encryption Extension          |
+-+----------------------------------+-------------------------------+
|3| Group of encapsulated PDUs       | PDU-Concat or TS-Concat       |
+-+----------------------------------+-------------------------------+
|4| Specific to encapsulated PDU     | IEEE-defined type             |
| |                                  | Test or MAC bridging Extension|
+----+----------------------------------+-------------------------------+


Table 1: Recommended ordering of Extension Headers