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Versions: 00 draft-ietf-6tisch-6top-protocol

6TiSCH                                                      Q. Wang, Ed.
Internet-Draft                           Univ. of Sci. and Tech. Beijing
Intended status: Informational                             X. Vilajosana
Expires: September 21, 2016              Universitat Oberta de Catalunya
                                                          March 20, 2016


                           6top Protocol (6P)
                   draft-wang-6tisch-6top-protocol-00

Abstract

   This document defines the 6top Protocol (6P), which enables
   distributed scheduling in 6TiSCH networks.  6P allows neighbor nodes
   in a 6TiSCH network to add/delete TSCH cells to one another.  6P is
   part of the 6TiSCH Operation Sublayer (6top), the next higher layer
   of the IEEE802.15.4 TSCH medium access control layer.  The 6top
   Scheduling Function (SF) decides when to add/delete cells, and
   triggers 6P transactions.  Several SFs can be defined, each
   identified by a different 6top Scheduling Function Identifier (SFID).
   This document lists the requirements for an SF, but leaves the
   definition of the SF out of scope.  Different SFs are expected to be
   defined in future companion specifications.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 21, 2016.




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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  TEMPORARY EDITORIAL NOTES . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  6TiSCH Operation Sublayer (6top)  . . . . . . . . . . . . . .   5
     3.1.  Hard/Soft Cells . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Using 6top with the Minimal 6TiSCH Configuration  . . . .   5
   4.  6top Protocol (6P)  . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  6top Transaction  . . . . . . . . . . . . . . . . . . . .   6
       4.1.1.  2-step 6top Transaction . . . . . . . . . . . . . . .   6
       4.1.2.  3-step 6top Transaction . . . . . . . . . . . . . . .   7
     4.2.  Message Format  . . . . . . . . . . . . . . . . . . . . .   8
       4.2.1.  6top Information Element  . . . . . . . . . . . . . .   8
       4.2.2.  General Message Format  . . . . . . . . . . . . . . .   9
       4.2.3.  6P Command Identifiers  . . . . . . . . . . . . . . .   9
       4.2.4.  6P Return Codes . . . . . . . . . . . . . . . . . . .  10
       4.2.5.  6P Cell Format  . . . . . . . . . . . . . . . . . . .  10
       4.2.6.  6P ADD Request Format . . . . . . . . . . . . . . . .  11
       4.2.7.  6P DELETE Request Format  . . . . . . . . . . . . . .  11
       4.2.8.  6P COUNT Request Format . . . . . . . . . . . . . . .  12
       4.2.9.  6P LIST Request Format  . . . . . . . . . . . . . . .  12
       4.2.10. 6P CLEAR Request Format . . . . . . . . . . . . . . .  12
       4.2.11. 6P Response Format  . . . . . . . . . . . . . . . . .  12
       4.2.12. 6P Confirmation Format  . . . . . . . . . . . . . . .  13
     4.3.  Protocol Behavior . . . . . . . . . . . . . . . . . . . .  13
       4.3.1.  Version Checking  . . . . . . . . . . . . . . . . . .  13
       4.3.2.  SFID Checking . . . . . . . . . . . . . . . . . . . .  13
       4.3.3.  Concurrent 6P Transactions  . . . . . . . . . . . . .  13
       4.3.4.  Timeout . . . . . . . . . . . . . . . . . . . . . . .  14
       4.3.5.  SeqNum Mismatch . . . . . . . . . . . . . . . . . . .  14
       4.3.6.  Adding cells  . . . . . . . . . . . . . . . . . . . .  14
       4.3.7.  Aborting a 6P Transaction . . . . . . . . . . . . . .  15
       4.3.8.  Deleting cells  . . . . . . . . . . . . . . . . . . .  15



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       4.3.9.  Handling error responses  . . . . . . . . . . . . . .  15
     4.4.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  16
   5.  Guidelines for 6top Scheduling Functions (SF) . . . . . . . .  16
     5.1.  SF Identifier (SFID)  . . . . . . . . . . . . . . . . . .  16
     5.2.  Requirements for an SF  . . . . . . . . . . . . . . . . .  16
     5.3.  Recommended Structure of an SF Specification  . . . . . .  17
   6.  Implementation Status . . . . . . . . . . . . . . . . . . . .  17
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  18
   8.  IANA Consideration  . . . . . . . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  19
   Appendix A.  [TEMPORARY] IETF IE  . . . . . . . . . . . . . . . .  20
   Appendix B.  [TEMPORARY] IEEE Liaison Considerations  . . . . . .  20
   Appendix C.  [TEMPORARY] Terms for the Terminology Draft  . . . .  20
   Appendix D.  [TEMPORARY] Changelog  . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  TEMPORARY EDITORIAL NOTES

   This document is an Internet Draft, so work-in-progress by nature.
   It contains the following work-in-progress elements:

   o  "TODO" statements are elements which have not yet been written by
      the authors for some reason (lack of time, ongoing discussions
      with no clear consensus yet, etc).  The statement does indicate
      that the text will be written.
   o  "TEMPORARY" Appendices are there to capture current ongoing
      discussions or the changelog of the document.  These appendices
      will be removed in the final text.
   o  "IANA_" identifiers are placeholders for numbers assigned by IANA.
      These placeholders are to be replaced by the actual values they
      represent after their assignment by IANA.
   o  This section will be removed in the final text.

2.  Introduction

   All communication in a 6TiSCH network is orchestrated by a schedule
   [RFC7554].  This specification defines the 6top Protocol (6P), part
   of the 6TiSCH Operation Sublayer (6top) sublayer.  6P allow a node to
   communicate with a neighbor to add/remove a TSCH cell to one another.
   6P hence enables distributed scheduling in a 6TiSCH network.









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                                    (A)
                                    / \
                                   /   \
                                (B)-----(C)
                                 |       |
                                 |       |
                                (D)     (E)

                    Figure 1: A simple 6TiSCH network.

   For example, node C in Figure 1 monitors the communication cells to
   node A it has in its schedule.

   o  If node C determines the number of frames it is sending to A per
      unit of time is larger than the capacity offered by the TSCH cells
      it has scheduled to A, it triggers a 6P transaction with node A to
      add one or more cells to A in the TSCH schedule.
   o  If the traffic is lower than the capacity, node C triggers a 6P
      transaction with node A to delete one or more cells to A in the
      TSCH schedule.
   o  Node C might also monitor statistics to determine whether
      collisions are happening on a particular cell to node A.  If this
      feature is enabled, node C communicates with node A to add a new
      cell and delete the cell which suffered from collisions.  This
      conceptually results in "relocating" the cell which suffered from
      collisions to a different slotOffset/channelOffset location in the
      TSCH schedule.  The mechanism to handle cell relocation is out of
      the scope of this document and might be defined in a future
      document.

   This results in distributed schedule management in a 6TiSCH network.

   The 6top Scheduling Function (SF) defines when to add/delete a cell
   to a neighbor.  The SF functions as a (required) add-on to 6P.
   Different applications require different SFs, so the SF is left out
   of scope of this document.  Different SFs are expected to be defined
   in future companion specifications.  A node MAY implement multiple
   SFs and run them at the same time.  The SFID field contained in all
   6P messages allows a node to switch between SFs on a per-transaction
   basis.

   Section 3 describes the 6TiSCH Operation Sublayer (6top).  Section 4
   defines the 6top Protocol (6P).  Section 5 provides guidelines on how
   to design an SF.







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3.  6TiSCH Operation Sublayer (6top)

   As depicted in Figure 2, the 6TiSCH Operation Sublayer (6top) is the
   next higher layer to the IEEE802.15.4 TSCH medium access control
   layer [IEEE802154-2015].

                                   .
               |                   .                      |
               |            next higher layer             |
               +------------------------------------------+
               |                 6top                     |
               +------------------------------------------+
               |           IEEE802.15.4 TSCH              |
               |                   .                      |
                                   .

            Figure 2: The 6top sublayer in the protocol stack.

   The roles of the 6top sublayer are:

   o  Implement and terminate the 6top Protocol (6P), which allows
      neighbor nodes to communicate to add/delete cells to one another.
   o  Run one or more 6top Scheduling Function (SF), which define the
      algorithm to decide when to add/delete cells.

3.1.  Hard/Soft Cells

   6top qualifies each cell in the schedule as either "hard" or "soft":

   o  a Soft Cell can be read, added, deleted or updated by 6top.
   o  a Hard Cell is read-only for 6top.

   In the context of this specification, all the cells used by 6top are
   Soft Cells.  Hard cells can be used for example when "hard-coding" a
   scheduling.  This is done, for example, in the Minimal 6TiSCH
   Configuration [I-D.ietf-6tisch-minimal].

3.2.  Using 6top with the Minimal 6TiSCH Configuration

   6top MAY be used alongside the Minimal 6TiSCH Configuration
   [I-D.ietf-6tisch-minimal].  In this case, it is RECOMMENDED to use 2
   slotframes, as depicted in Figure 3:

   o  Slotframe 0 is used for traffic defined in the Minimal 6TiSCH
      Configuration.  In Figure 3, this slotframe is 5 slots long, but
      it can be of any length.
   o  Slotframe 1 is used by 6top to allocate cells from.  In Figure 3,
      this slotframe is 10 slots long, but it can be of any length.



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   Slotframe 0 SHOULD be of higher priority than Slotframe 1.  6top MAY
   support further slotframes; how to use more slotframes is out of the
   scope for this document.

                   | 0    1    2    3    4  | 0    1    2    3    4  |
                   +------------------------+------------------------+
       Slotframe 0 |    |    |    |    |    |    |    |    |    |    |
      5 slots long | EB |    |    |    |    | EB |    |    |    |    |
     high priority |    |    |    |    |    |    |    |    |    |    |
                   +-------------------------------------------------+

                   | 0    1    2    3    4    5    6    7    8    9  |
                   +-------------------------------------------------+
       Slotframe 1 |    |    |    |    |    |    |    |    |    |    |
     10 slots long |    |A->B|    |    |    |    |    |    |B->A|    |
      low priority |    |    |    |    |    |    |    |    |    |    |
                   +-------------------------------------------------+

   Figure 3: 2-slotframe structure when using 6top alongside the Minimal
                           6TiSCH Configuration.

4.  6top Protocol (6P)

   The 6top Protocol (6P) allows two neighbor nodes to communicate to
   add/delete cells to their TSCH schedule.  Conceptually, two neighbor
   nodes "negotiate" the location of the cell(s) to add/delete.

4.1.  6top Transaction

   We call "6top Transaction" a complete negotiation between two
   neighbor nodes.  A transaction starts when a node wishes to add/
   remove one or more cells to one of its neighbors; it ends when the
   cell(s) have been added removed from the schedule of both neighbor,
   or when the transaction has failed.

   A transaction can consist of 2 or 3 steps.  It is the SF which
   determines whether to use 2-step or 3-step transactions.  An SF MAY
   use both 2-step and 3-step transactions.

   We reuse the topology in Figure 1 to illustrate 2-step and 3-step
   transactions.

4.1.1.  2-step 6top Transaction

   6P supports both 2- and 3-step transactions; the SF determinisms
   which to use.  Without loss of generality, this section illustrates
   2-step transaction through an example.




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   Figure 4 is a sequence diagram to help understand the core principle
   of 6P (several elements are left out to simplify understanding).  We
   assume the SF running on node A determines 2 extra cells need to be
   scheduled to node B.  In this example, node A proposes the cells to
   use.

      +----------+                           +----------+
      |  Node A  |                           |  Node B  |
      +----+-----+                           +-----+----+
           |                                       |
           | 6P ADD Request                        |
           |   NumCells     = 2                    |
           |   CellList     = [(1,2),(2,2),(3,5)]  |
           |-------------------------------------->|
           |                                       |
           | 6P Response                           |
           |   Return Code  = IANA_6TOP_RC_SUCCESS |
           |   CellList     = [(2,2),(3,5)]        |
           |<--------------------------------------|
           |                                       |

                    Figure 4: A 2-step 6P transaction.

   In this example, the 2-step transaction occurs as follows:

   1.  The SF running on node A selects 3 candidate cells.
   2.  Node A sends a 6P ADD Request to node B, indicating it wishes to
       add 2 cells (the "NumCells" value), and specifying the list of 3
       candidate (the "CellList" value).  Each cell in the CellList is a
       (slotOffset,channelOffset) tuple.
   3.  The SF running on node B selects 2 of the 3 cells in the CellList
       of the 6P ADD Request.  Node B sends back a 6P Response to node
       A, indicating the cells it selected.
   4.  The result of this 6P transaction is that 2 cells from A to B
       have been added to the TSCH schedule of both nodes A and B.

4.1.2.  3-step 6top Transaction

   6P supports both 2- and 3-step transactions; the SF determinisms
   which to use.  Without loss of generality, this section illustrates
   3-step transaction through an example.

   Figure 5 is a sequence diagram to help understand the core principle
   of 6P (several elements are left out to simplify understanding).  We
   assume the SF running on node A determines 2 extra cells need to be
   scheduled to node B.  In this example, node B proposes the cells to
   use.




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      +----------+                           +----------+
      |  Node A  |                           |  Node B  |
      +----+-----+                           +-----+----+
           |                                       |
           | 6P ADD Request                        |
           |   NumCells     = 2                    |
           |   CellList     = []                   |
           |-------------------------------------->|
           |                                       |
           | 6P Response                           |
           |   Return Code  = IANA_6TOP_RC_SUCCESS |
           |   CellList     = [(1,2),(2,2),(3,5)]  |
           |<--------------------------------------|
           |                                       |
           | 6P Confirmation                       |
           |   Return Code  = IANA_6TOP_RC_SUCCESS |
           |   CellList     = [(2,2),(3,5)]        |
           |-------------------------------------->|
           |                                       |

                    Figure 5: A 3-step 6P transaction.

   In this example, the 3-step transaction occurs as follows:

   1.  The SF running on node A determines 2 extra cells need to be
       scheduled to node B, but does not select candidate cells.
   2.  Node A sends a 6P ADD Request to node B, indicating it wishes to
       add 2 cells (the "NumCells" value), with en empty "CellList".
   3.  The SF running on node B selects 3 candidate cells.  Node B sends
       back a 6P Response to node A, indicating the 3 cells it selected.
   4.  The SF running on node B selects 2 cells.  Node A sends back a 6P
       Confirmation to node B, indicating the cells it selected.
   5.  The result of this 6P transaction is that 2 cells from A to B
       have been added to the TSCH schedule of both nodes A and B.

4.2.  Message Format

4.2.1.  6top Information Element

   6P messages are carried as payload of IEEE802.15.4 Information
   Elements (IE) [IEEE802154-2015].  6p messages travel over a single
   hop.









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                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Payload IE Length   |GroupID|T|    Sub-ID     |6top IE Content
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Payload Termination IE        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   The 6top IE is an IETF IE with GroupID IANA_IETF_IE_GROUP_ID.  The
   Sub-ID used by the 6top IE is IANA_6TOP_SUBIE_ID.  The length of the
   6top IE content is variable.  The content of the 6top IE is specified
   in Section 4.2.  The Payload Termination IE is defined by the
   IEEE802.15.4 standard [IEEE802154-2015].  TODO: IETF IE specified in
   Appendix A for now, but to be specified in a separate draft in the
   future.

4.2.2.  General Message Format

   In all 6P messages, the 6top IE content has the following format:

                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Version| Code  |     SFID      |    SeqNum     | Other Fields...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version (6P Version):  The version of the 6P protocol.  Only version
         IANA_6TOP_6P_VERSION is defined in this document.  Future
         specifications MIGHT define further version of the 6P protocol.
   Code: Command to carry out or response code.  The list of command
         identifiers and return codes is defined only for version
         IANA_6TOP_6P_VERSION in this document.
   SFID (6top Scheduling Function Identifier):  The identifier of the SF
         to use to handle this message.  The SFID is defined in
         Section 5.1.
   SeqNum:  An identifier of the packet, used to match request and
         response.  The value of SeqNum MUST increment by exactly one at
         each new 6P request issued to the same neighbor.
   Other Fields:  The list of other fields depends on the value of the
         code field, as detailed below.

4.2.3.  6P Command Identifiers

   Figure 6 lists the 6P command identifiers.






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    Value                   Command ID     Description
   +----------------------+--------------+---------------------------+
   | IANA_6TOP_CMD_ADD    | CMD_ADD      | add one or more cells     |
   +----------------------+------------------------------------------+
   | IANA_6TOP_CMD_DELETE | CMD_DELETE   | delete one or more cells  |
   +----------------------+------------------------------------------+
   | IANA_6TOP_CMD_COUNT  | CMD_COUNT    | count scheduled cells     |
   +----------------------+------------------------------------------+
   | IANA_6TOP_CMD_LIST   | CMD_LIST     | list the scheduled cells  |
   +----------------------+------------------------------------------+
   | IANA_6TOP_CMD_CLEAR  | CMD_CLEAR    | clear all cells           |
   +----------------------+------------------------------------------+
   | TODO-0xf             | reserved                                 |
   +----------------------+------------------------------------------+

                     Figure 6: 6P Command Identifiers

4.2.4.  6P Return Codes

   Figure 7 lists the 6P Return Codes and their meaning.

    Value                    Return Code      Description
   +-----------------------+----------------------------------------+
   | IANA_6TOP_RC_SUCCESS  | RC_SUCCESS  | operation succeeded      |
   +-----------------------+----------------------------------------+
   | IANA_6TOP_RC_VER_ERR  | RC_VER_ERR  | unsupported 6P version   |
   +-----------------------+----------------------------------------+
   | IANA_6TOP_RC_SFID_ERR | RC_SFID_ERR | unsupported SFID         |
   +-----------------------+----------------------------------------+
   | IANA_6TOP_RC_BUSY     | RC_BUSY     | handling previous request|
   +-----------------------+----------------------------------------+
   | IANA_6TOP_RC_RESET    | RC_RESET    | abort 6P transaction     |
   +-----------------------+----------------------------------------+
   | IANA_6TOP_RC_ERR      | RC_ERR      | operation failed         |
   +-----------------------+----------------------------------------+
   | TODO-0xf              | reserved                               |
   +-----------------------+----------------------------------------+

                         Figure 7: 6P Return Codes

4.2.5.  6P Cell Format

   The 6P Cell is an element which is present in several messages.  It
   is a 4-byte field, its RECOMMENDED format is:







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                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |          slotOffset           |         channelOffset         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   slotOffset:  The slot offset of the cell.
   channelOffset:  The channel offset of the cell.

   The CellList is an opaque set of bytes, sent unmodified to the SF.
   The SF MAY redefine the format of the CellList field.

4.2.6.  6P ADD Request Format

                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Version|  Code |    SFID       |     SeqNum    |   NumCells    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Metadata            | CellList ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version:  Set to IANA_6TOP_6P_VERSION.
   Code: Set to IANA_6TOP_CMD_ADD for a 6P ADD Request.
   SFID: Identifier of the SF to be used by the receiver to handle the
         message.
   SeqNum:  Packet identifier to match 6P Request and 6P Response.
   NumCells:  The number of additional TX cells the sender wants to
         schedule to the receiver.
   Metadata:  Metadata used as extra signaling to the SF.  One example
         use can be to specify which slotframe to schedule the cells to.
         The contents of the Metadata field is an opaque set of bytes,
         and passed unmodified to the SF.  The meaning of this field
         depends on the SF, and is hence out of scope of this document.
   CellList:  A list of 0, 1 or multiple 6P Cells.  The RECOMMENDED
         format of each 6P Cell is defined in Section 4.2.5.  The
         CellList is an opaque set of bytes, sent unmodified to the SF.
         The SF MAY redefine the format of the CellList field.

4.2.7.  6P DELETE Request Format

   The 6P DELETE Request has the exact same format as the 6P ADD
   Request, except for the code which is set to IANA_6TOP_CMD_DELETE.








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4.2.8.  6P COUNT Request Format

                          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    |   Metadata
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         Metadata    |
     +-+-+-+-+-+-+-+-+

   Version:  Set to IANA_6TOP_6P_VERSION.
   Code: Set to IANA_6TOP_CMD_COUNT for a 6P COUNT Request.
   SFID: Identifier of the SF to be used by the receiver to handle the
         message.
   SeqNum:  Packet identifier to match request and response.
   Metadata:  Metadata used as extra signaling to the SF.  One example
         use can be to specify which slotframe to schedule the cells to.
         The contents of the Metadata field is an opaque set of bytes,
         and passed unmodified to the SF.  The meaning of this field
         depends on the SF, and is hence out of scope of this document.

4.2.9.  6P LIST Request Format

   The 6P LIST Request has the exact same format as the 6P COUNT
   Request, except for the code which is set to IANA_6TOP_CMD_LIST.

4.2.10.  6P CLEAR Request Format

   The 6P CLEAR Request has the exact same format as the 6P COUNT
   Request, except for the code which is set to IANA_6TOP_CMD_CLEAR.

4.2.11.  6P Response Format

                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |Version|  Code |    SFID       |     SeqNum    | Other Fields...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version:  Set to IANA_6TOP_6P_VERSION.
   SFID:  Identifier of the SF to be used by the receiver to handle the
      message.  The response MUST contain the same SFID value as the
      value in the SFID field of the 6P Request is responds to.
   Code:  One of the 6P Return Codes listed in Section 4.2.4.
   SeqNum:  Packet identifier to match request and response.  The
      response MUST contain the same SeqNum value as the value in the
      SeqNum field of the 6P Request is responds to.
   Other Fields:  The fields depends on what command the request is for:



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      Response to an ADD, DELETE or LIST command:  A list of 0, 1 or
         multiple 6P Cells.  The format of a 6P Cell is defined in
         Section 4.2.5.
      Response to COUNT command:  The number of cells scheduled from the
         requesting node to the receiver node by the 6P protocol,
         encoded as a 2-octet unsigned integer.
      Response to CLEAR command:  No other fields are present in the
         response.

4.2.12.  6P Confirmation Format

   A 6P Confirmation is only used in a 3-step transaction, as the third
   step.  A 6P Confirmation Message has the exact same format as a 6P
   Response Message.  It is only the fact that it appears as the third
   step in a 3-step transaction that distinguishes it from a 6P
   Response.  In particular, the same Return Codes are used in both 6P
   Response and 6P Confirmation messages.

4.3.  Protocol Behavior

   For illustration, we assume we use the topology in Figure 1, and that
   node A negotiates to add/delete cells to node B.

4.3.1.  Version Checking

   All messages contain a Version field.  If multiple Versions of the 6P
   protocol have been defined (in future specifications for Version
   values different than IANA_6TOP_6P_VERSION), a node MAY implement
   multiple protocol versions at the same time.  When receiving a 6P
   message with a Version number it does not implement, a node MUST
   reply with a 6P Response and a return code of IANA_6TOP_RC_VER_ERR.
   The Version field in the 6P Response MUST be the same as the Version
   field in the corresponding 6P Request.

4.3.2.  SFID Checking

   All messages contain a SFID field.  If multiple SFs has been defined,
   a node MAY support multiple SFs at the same time.  When receiving a
   6P message with an unsupported SFID, a node MUST reply with a 6P
   Response and a return code of IANA_6TOP_RC_SFID_ERR.  The Version
   field in the 6P Response MUST be the same as the Version field in the
   corresponding 6P Request.

4.3.3.  Concurrent 6P Transactions

   Only a single 6P Transaction between two neighbors, in a given
   direction, can take place at the same time.  That is, a node MUST NOT
   issue a new 6P Request to a given neighbor before having received the



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   6P Response for a previous request to that neighbor.  The only
   exception to this rule is when the previous 6P Transaction has timed
   out.  If a node receives a 6P Request from a given neighbor before
   having sent the 6P Response to the previous 6P Request from that
   neighbor, it MUST send back a 6P Response with a return code of
   IANA_6TOP_RC_ERR.

   A node MAY support concurrent 6P Transactions from different
   neighbors.  In this case, in Figure 1, node C can have a different
   ongoing 6P Transaction with nodes B and E.  In case a node does not
   have enough resources to handle concurrent 6P Transactions from
   different neighbors, when it receives a 6P Request from a neighbor
   while already handling a different request from a different neighbor,
   it MUST reply to that second request with a 6P Response with return
   code IANA_6TOP_RC_BUSY.

4.3.4.  Timeout

   A timeout happens when the node sending the 6P Request has not
   received the 6P Response.  The value of the timeout is coupled with
   how the cells between the nodes are scheduled.  The SF determines the
   value of the timeout.  The value of the timeout is out of scope of
   this document.

4.3.5.  SeqNum Mismatch

   When a node receives a 6P Response with SeqNum value different from
   the SeqNum value in the 6P Request, it MUST drop the packet and
   consider the 6P Transaction as having failed.

4.3.6.  Adding cells

   We assume the topology in Figure 1 where the SF on node C decides to
   add NumCell cells to node A.

   Node C's SF selects NumCandidate>=NumCell cells from its schedule as
   candidate transmit cells to node A.  NumCandidate MUST be larger or
   equal to NumCell.  How many cells it selects (NumCandidate) and how
   that selection is done is specified in the SF and out of scope of
   this document.  Node C sends a 6P ADD Request to node A which
   contains the value of NumCells and the NumCandidate cells in the
   CellList.

   Upon receiving the request, node A's SF verifies which of the cells
   in the CellList it can add as receive cells from node C in its own
   schedule.  How that selection is done is specified in the SF and out
   of scope of this document.  That verification can succeed (NumCell
   cells from the CellList can be used), fail (none of the cells from



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   the CellList can be used) or partially succeed (less than NumCell
   cells from the CellList can be used).  In all cases, node A MUST send
   a 6P Response with return code set to IANA_6TOP_RC_SUCCESS, and which
   specifies the list of cells that were scheduled as receive cells from
   C.  That can contain 0 elements (when the verification failed),
   NumCell elements (succeeded) or between 0 and NumCell elements
   (partially succeeded).

   Upon receiving the response, node C adds the cells specified in the
   CellList as transmit cells to node A.

4.3.7.  Aborting a 6P Transaction

   In case the receiver of a 6top request fails during a 6P Transaction
   and is unable to complete it, it SHOULD reply to that request with a
   6P Response with return code IANA_6TOP_RC_RESET.  Upon receiving this
   6top reply, the initiator of the 6P Transaction MUST consider the 6P
   Transaction as failed.

4.3.8.  Deleting cells

   The behavior for deleting cells is equivalent to that of adding cells
   except that:

   o  The nodes delete the cells they agree upon rather than adding
      them.
   o  All cells in the CellList MUST be already scheduled between the
      two nodes.
   o  If the CellList in the 6P Request is empty, the SF on the
      receiving node is free to delete any cell from the sender.
   o  The CellList MUST either be equal, contain exactly NumCell cells,
      or more than NumCell cells.  The case where the CellList is not
      empty but contains less than NumCell cells is not supported.

4.3.9.  Handling error responses

   A return code with a name starting with "RC_ERR" in Figure 7
   indicates an error.  When a node receives a 6P Response with such an
   error, it MUST consider the 6P Transaction failed.  In particular, if
   this was a response to a 6P ADD/DELETE Request, the node MUST NOT
   add/delete any of the cells involved in this 6P Transaction.
   Similarly, a node sending a 6P Response with an "RC_ERR" return code
   MUST NOT add/delete any cells as part of that 6P Transaction.  The SF
   defines what to do after an error has occurred.  Defining what to do
   after an error has occurred is out of scope of this document.






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4.4.  Security

   6P messages are secured through link-layer security.  When link-layer
   security is enabled, the 6P messages MUST be secured.  This is
   possible because 6P messages are carried as Payload IE.

5.  Guidelines for 6top Scheduling Functions (SF)

5.1.  SF Identifier (SFID)

   Each SF has an identifier.  The identifier is encoded as a 1-byte
   field.  The identifier space is divided in the following ranges.

                          Range      Meaning
                        +-----------+-------------+
                        | 0x00-0xef | managed     |
                        +-----------+--------------
                        | 0xf0-0xfe | unmanaged   |
                        +-----------+-------------+
                        | 0xff      | reserved    |
                        +-----------+-------------+

                           Figure 8: SFID range.

   SF identifiers in the managed space MUST be managed by IANA.

5.2.  Requirements for an SF

   The specification for an SF

   o  MUST specify an identifier for that SF.
   o  MUST specify the rule for a node to decide when to add/delete one
      or more cells to a neighbor.
   o  MUST specify the rule for a Transaction source to select cells to
      add to the CellList field in the 6P ADD Request.
   o  MUST specify the rule for a Transaction destination to select
      cells from CellList to add to its schedule.
   o  MUST specify a value for the 6P Timeout, or a rule/equation to
      calculate it.
   o  MUST specify a meaning for the "Metadata" field in the 6P ADD
      Request.
   o  MUST specify the behavior of a node when it boots.
   o  MUST specify what to do after an error has occurred (either the
      node sent a 6P Response with an error code, or received one).
   o  MUST specify the list of statistics to gather.  An example
      statistic if the number of transmitted frames to each neighbor.
      In case the SF requires no statistics to be gathered, the specific
      of the SF MUST explicitly state so.



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   o  SHOULD clearly state the application domain the SF is created for.
   o  SHOULD contain examples which highlight normal and error
      scenarios.
   o  SHOULD contain a list of current implementations, at least during
      the I-D state of the document, per [RFC6982].
   o  SHOULD contain a performance evaluation of the scheme, possibly
      through references to external documents.
   o  MAY redefine the format of the CellList field.

5.3.  Recommended Structure of an SF Specification

   The following section structure for a SF document is RECOMMENDED:

   o  Introduction
   o  Scheduling Function Identifier
   o  Rules for Adding/Deleting Cells
   o  Rules for CellList
   o  6P Timeout Value
   o  Meaning of the Metadata Field
   o  Node Behavior at Boot
   o  6P Error Handling
   o  Examples
   o  Implementation Status
   o  Security Considerations
   o  IANA Considerations

6.  Implementation Status

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC6982].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

   According to [RFC6982], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".




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   ETSI 6TiSCH #2 plugtests:  6P was one of two protocols addressed
      during the ETSI 6TiSCH #2 plugtests organized on 2-4 February 2016
      in Paris, France.  14 entities participated in this event,
      verifying the compliance and interoperability of their
      implementation of 6P.  This event happened under NDA, so neither
      the name of the entities nor the test results are public.  This
      event is, however, a clear indication of the maturity of 6P, and
      the interest it generates.  More information about the event at
      http://www.etsi.org/news-events/events/1022-6TiSCH-2-plugtests.
   OpenWSN:  6P is implemented in the OpenWSN project [OpenWSN] under a
      BSD open-source license.  The authors of this document are
      collaborating with the OpenWSN community to gather feedback about
      the status and performance of the protocols described in this
      document.  Results from that discussion will appear in this
      section in future revision of this specification.  More
      information about this implementation at http://www.openwsn.org/.
   Wireshark Dissector:  A Wireshark dissector for 6P is implemented
      under a BSD open-source license.  It is not yet merged into the
      main Wireshark build, but can be downloaded at https://github.com/
      openwsn-berkeley/dissectors/.

7.  Security Considerations

   TODO: explicit risks

   6P messages are carried inside IEEE802.15.4 Payload Information
   Elements (IEs).  Those Payload IEs are encrypted and authenticated at
   the link layer through CCM*.  6P benefits from the same level of
   security as any other Payload IE.  The 6P protocol does not define
   its own security mechanisms.  A key management solution is out of
   scope for this document.  The 6P protocol will benefit for the key
   management solution used in the network.

8.  IANA Consideration

   TODO: write out this section as soon as the discussion with the IEEE
   about a possible IETF IE ID has concluded.

   o  TODO: IANA_IETF_IE_GROUP_ID
   o  TODO: IANA_6TOP_SUBIE_ID
   o  TODO: IANA_6TOP_6P_VERSION
   o  TODO: IANA_6TOP_CMD_ADD
   o  TODO: IANA_6TOP_CMD_DELETE
   o  TODO: IANA_6TOP_CMD_LIST
   o  TODO: IANA_6TOP_CMD_COUNT
   o  TODO: IANA_6TOP_CMD_CLEAR
   o  TODO: IANA_6TOP_RC_SUCCESS
   o  TODO: IANA_6TOP_RC_VER_ERR



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   o  TODO: IANA_6TOP_RC_SFID_ERR
   o  TODO: IANA_6TOP_RC_BUSY
   o  TODO: IANA_6TOP_RC_RESET
   o  TODO: IANA_6TOP_RC_ERR

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [IEEE802154-2015]
              IEEE standard for Information Technology, "IEEE Std
              802.15.4-2015 - IEEE Standard for Low-Rate Wireless
              Personal Area Networks (WPANs)", October 2015.

9.2.  Informative References

   [RFC7554]  Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
              IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
              Internet of Things (IoT): Problem Statement", RFC 7554,
              DOI 10.17487/RFC7554, May 2015,
              <http://www.rfc-editor.org/info/rfc7554>.

   [RFC6982]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", RFC 6982,
              DOI 10.17487/RFC6982, July 2013,
              <http://www.rfc-editor.org/info/rfc6982>.

   [I-D.ietf-6tisch-minimal]
              Vilajosana, X. and K. Pister, "Minimal 6TiSCH
              Configuration", draft-ietf-6tisch-minimal-15 (work in
              progress), February 2016.

   [I-D.ietf-6tisch-terminology]
              Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
              "Terminology in IPv6 over the TSCH mode of IEEE
              802.15.4e", draft-ietf-6tisch-terminology-06 (work in
              progress), November 2015.

   [OpenWSN]  Watteyne, T., Vilajosana, X., Kerkez, B., Chraim, F.,
              Weekly, K., Wang, Q., Glaser, S., and K. Pister, "OpenWSN:
              a Standards-Based Low-Power Wireless Development
              Environment", Transactions on Emerging Telecommunications
              Technologies , August 2012.



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Appendix A.  [TEMPORARY] IETF IE

   This section contains a proposal for the specification of an IETF IE.
   If this proposal is supported by the 6TiSCH WG, the authors of this
   draft recommend for the specification of the IETF IE to be its own
   draft, possibly developed in the 6TiSCH WG.  The reason for having it
   a separated document is that the scope of the IETF IE is wider that
   the 6P protocol defined in this document.

   The proposal is to use an IETF IE, a IEEE802.15.4 Payload Information
   Element with the Group ID set to IANA_IETF_IE_GROUP_ID.  The value of
   IANA_IETF_IE_GROUP_ID is defined by the IEEE, communicated to the
   IETF, and noted by IANA.  The format of the IETF IE is exactly the
   same as the format of an MLME Information Element, as specified in
   [IEEE802154-2015], Section 5.2.4.5.  The difference is that the space
   of Sub-IDs is managed by the IETF/IANA.  The Sub-ID used by 6top
   commands is IANA_6TOP_SUBIE_ID with value 0x00.

   Other options are being discussed between the IETF 6TiSCH WG and the
   IEEE 6TiSCH IG, and listed in https://www.ietf.org/mail-
   archive/web/6tisch/current/msg04469.html.  These options concern the
   way 6P Messages are transported as IEEE802.15.4 IEs, and do not
   impact the format of those messages.

Appendix B.  [TEMPORARY] IEEE Liaison Considerations

   If the specification described in this document is supported by the
   6TiSCH WG, the authors of this document ask the 6TiSCH WG chairs to
   liaise with the IEEE to request a Payload Information Element Group
   ID to be assigned to the IETF (Group ID IANA_IETF_IE_GROUP_ID
   described in Appendix A).

Appendix C.  [TEMPORARY] Terms for the Terminology Draft

   Terms introduced by this document, and which needs to be added to
   [I-D.ietf-6tisch-terminology]:

   6top:       The "6TiSCH Operation Sublayer" (6top) is the next
               highest layer of the IEEE802.15.4 TSCH medium access
               control layer.  It implements and terminates the "6top
               Protocol" (6P), and contains one or more "6top Scheduling
               Function" (SF).  It is defined in TODO_LINK_draft-wang-
               6tisch-6top-protocol.
   SF:         The "6top Scheduling Function" (SF) is the policy inside
               the "6TiSCH Operation Sublayer" (6top) which decides when
               to add/remove cells.  It is defined in TODO_LINK_draft-
               wang-6tisch-6top-protocol.




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   SFID:       The "6top Scheduling Function Identifier" (SFID) is a
               1-byte field identifying a SF.  It is defined in
               TODO_LINK_draft-wang-6tisch-6top-protocol.
   6P:         The "6top Protocol" (6P) allows neighbor nodes to
               communicate to add/delete cells to one another in their
               TSCH schedule.  It is defined in TODO_LINK_draft-wang-
               6tisch-6top-protocol.
   6P Transaction:  Part of the "6top Protocol" (6P), we call "6top
               Transaction" a complete negotiation between two neighbor
               nodes.  A transaction starts when a node wishes to add/
               remove one or more cells to one of its neighbors; it ends
               when the cell(s) have been added removed from the
               schedule of both neighbor, or when the transaction has
               failed.  It is defined in TODO_LINK_draft-wang-6tisch-
               6top-protocol.

Appendix D.  [TEMPORARY] Changelog

   o  draft-wang-6tisch-6top-protocol-00

      *  Editorial overhaul: fixing typos, increasing readability,
         clarifying figures.
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/47
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/54
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/55
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/49
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/53
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/44
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/48
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/43
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/52
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/45
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/51
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/50
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/46



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      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/41
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/42
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/39
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/40
   o  draft-wang-6tisch-6top-sublayer-05

      *  Specifies format of IE
      *  Adds token in messages to match request and response
   o  draft-wang-6tisch-6top-sublayer-04

      *  Renames IANA_6TOP_IE_GROUP_ID to IANA_IETF_IE_GROUP_ID.
      *  Renames IANA_CMD and IANA_RC to IANA_6TOP_CMD and IANA_6TOP_RC.
      *  Proposes IANA_6TOP_SUBIE_ID with value 0x00 for the 6top sub-
         IE.
   o  draft-wang-6tisch-6top-sublayer-03

      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/32/missing-command-list
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/31/missing-command-count
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/30/missing-command-clear
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/37/6top-atomic-transaction-6p-transaction
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/35/separate-opcode-from-rc
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/36/add-length-field-in-ie
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/27/differentiate-rc_err_busy-and
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/29/missing-rc-rc_reset
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/28/the-sf-must-specify-the-behavior-of-a-mote
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/26/remove-including-their-number
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-protocol/
         issues/34/6of-sf
      *  https://bitbucket.org/6tisch/draft-wang-6tisch-6top-
         protocol/issues/33/add-a-figure-showing-the-negociation
   o  draft-wang-6tisch-6top-sublayer-02

      *  introduces the 6P protocol and the notion of 6top Transaction.
      *  introduces the concept of 6OF and its 6OFID.



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Authors' Addresses

   Qin Wang (editor)
   Univ. of Sci. and Tech. Beijing
   30 Xueyuan Road
   Beijing, Hebei  100083
   China

   Phone: +86 (10) 6233 4781
   Email: wangqin@ies.ustb.edu.cn


   Xavier Vilajosana
   Universitat Oberta de Catalunya
   156 Rambla Poblenou
   Barcelona, Catalonia  08018
   Spain

   Phone: +34 (646) 633 681
   Email: xvilajosana@uoc.edu































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