Spin bit as a negotiated option

[<[e-mail]>]

On 10/03/18 09:58, Brian Trammell (IETF) wrote:
> Backing off the MUST for now for such situations is IMO a good tradeoff,
> though, especially since we only need fractions of a percent of deployment to
> start seeing useful signal for baseline/anomaly measurement of large
> aggregates.

If the consensus is that we must allow for such situations, then there are two 
possibilities:

  (a) weak spec language (MAY WISH TO or similar) => many implementations will 
simply drop it

  (b) negotiated option where the negotiation mechanism is mandatory

In the vein of (b), Christian suggested offline to introduce negotiation to 
allow for experimentation of the remaining two reserved bits. Then may be we can 
synthesize both ideas by the following proposal:

  - in the first few exchanges of the 5-tuple, use the three bits for option 
negotiation

  - then use them as defined by the selected option

Example encodings:

  000 : nothing
  001 : spin bit alone : S00
  010 : spin bit + VEC : SVV
  ... : other extensions

The negotiation mechanism allows both endpoints to force 000.
And since it is in the clear first byte, it allows on-path observers to identify 
the option without resorting to heuristics; this helps in the case of a small 
support ratio.




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