[6tisch] follow up LIST cmd 6P

[Xavier Vilajosana <[e-mail]>]

Hi,

according to our discussion during the last 6TiSCH meeting I want to follow
up Tero's proposal for the LIST cmd.

here you will find his initial proposal
https://www.ietf.org/mail-archive/web/6tisch/current/msg04719.html

As a summary, the proposal enables 6P list to return a best effort set of
cells, this is as many as can be fitted in a packet below a maximum. This
makes the list command robust to the presence of other IEs in the packet
(which sometimes may be present and some times don't and hence would make
two identical requests to succeed or fail. With this approach LIST is
independent of the remaining space in the packet)

I follow up the discussion by proposing these changes to 6P.

1- change numCells field to numCells_max as follow:

                        1                   2
      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Version|  Code |    SFID       | SeqNum|GAB|GBA|   Metadata
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Metadata    |            Offset             |   numCells_max
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     |
     +-+-+-+-+-+-+-+-+

where

numCells_max:  The maximum number of requested cells.  Less cells
         than numCells_max can be returned if they do not fit in the
         packet.

Then I propose the following text

4.3.10.  Listing Cells

   When a node A issues a LIST_AB or LIST_BA command, it specifies:

   o  Through the "Offset" field, the offset of the first cell to be
      present in the returned list.  The cell ordering policy is defined
      by the SF.
   o  Through the "numCells_max" field, the maximum number of cells to
      be present in the reponse.  If numCells_max cannot be fitted in the
      packet less cells will be returned.

   When receiving a LIST_AB command, node B returns the cells that are
   scheduled from A to B in its schedule (i.e. receive cells from node
   A).  When receiving a LIST_BA command, node B returns the cells that
   are scheduled from B to A in its schedule (i.e. transmit cells to
   node A).  The RECOMMENDED format of each 6P Cell is defined in
   Section 4.2.5.  The SF MAY redefine the format of the CellList field.

   Depending on how many cells node B has in its schedule which match the
   LIST_AB or LIST_BA request, the cellList returned in the 6P Response
   contains between 0 and numCells_max cells:

   o  If node B has more than Offset+numCells cells, the cellList it
      returns contains exactly numCells cells as long as the cellList
      fits in the packet.  Otherwise it returns a cellList containing
      the maximum number of cells that fit in the packet.
   o  If node B has N cells, where Offset<N and N<Offset+numCells_max
      cells, the cellList it returns contains exactly N-Offset cells.
      If that number of cells do not fit in the packet, the cellList
      containing the maximum number of cells that fit in the packet is
      returned.
   o  If node B has less than Offset cells, the cellList it returns is
      empty.


Please, raise any pros and cons you may find in this approach.