





Internet Engineering Task Force (IETF)                      Q. Wang, Ed.
Request for Comments: 8480               Univ. of Sci. and Tech. Beijing
Category: Standards Track                                  X. Vilajosana
ISSN: 2070-1721                          Universitat Oberta de Catalunya
                                                             T. Watteyne
                                                          Analog Devices
                                                           November 2018


             6TiSCH Operation Sublayer (6top) Protocol (6P)

Abstract

   This document defines the "IPv6 over the TSCH mode of IEEE 802.15.4e"
   (6TiSCH) Operation Sublayer (6top) Protocol (6P), which enables
   distributed scheduling in 6TiSCH networks.  6P allows neighbor nodes
   to add/delete Time-Slotted Channel Hopping (TSCH) cells to/on one
   another.  6P is part of the 6TiSCH Operation Sublayer (6top), the
   layer just above the IEEE Std 802.15.4 TSCH Medium Access Control
   layer.  6top is composed of one or more Scheduling Functions (SFs)
   and the 6top Protocol defined in this document.  A 6top SF decides
   when to add/delete cells, and it triggers 6P Transactions.  The
   definition of SFs is out of scope for this document; however, this
   document provides the requirements for an SF.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8480.













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Copyright Notice

   Copyright (c) 2018 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
   (https://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. Introduction ....................................................3
      1.1. Requirements Language ......................................5
   2. 6TiSCH Operation Sublayer (6top) ................................5
      2.1. Hard/Soft Cells ............................................6
      2.2. Using 6P with the Minimal 6TiSCH Configuration .............6
   3. 6top Protocol (6P) ..............................................7
      3.1. 6P Transactions ............................................7
           3.1.1. 2-Step 6P Transaction ...............................8
           3.1.2. 3-Step 6P Transaction ..............................10
      3.2. Message Format ............................................12
           3.2.1. 6top Information Element (IE) ......................12
           3.2.2. Generic 6P Message Format ..........................12
           3.2.3. 6P CellOptions .....................................13
           3.2.4. 6P CellList ........................................16
      3.3. 6P Commands and Operations ................................17
           3.3.1. Adding Cells .......................................17
           3.3.2. Deleting Cells .....................................19
           3.3.3. Relocating Cells ...................................21
           3.3.4. Counting Cells .....................................27
           3.3.5. Listing Cells ......................................28
           3.3.6. Clearing the Schedule ..............................30
           3.3.7. Generic Signaling between SFs ......................31
      3.4. Protocol Functional Details ...............................31
           3.4.1. Version Checking ...................................31
           3.4.2. SFID Checking ......................................32
           3.4.3. Concurrent 6P Transactions .........................32
           3.4.4. 6P Timeout .........................................33
           3.4.5. Aborting a 6P Transaction ..........................33
           3.4.6. SeqNum Management ..................................33
           3.4.7. Handling Error Responses ...........................40
      3.5. Security ..................................................40



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RFC 8480                   6top Protocol (6P)              November 2018


   4. Requirements for 6top Scheduling Function (SF) Specifications ..41
      4.1. SF Identifier (SFID) ......................................41
      4.2. Requirements for an SF Specification ......................41
   5. Security Considerations ........................................42
   6. IANA Considerations ............................................43
      6.1. IETF IE Subtype 6P ........................................43
      6.2. 6TiSCH Parameters Subregistries ...........................43
           6.2.1. 6P Version Numbers .................................43
           6.2.2. 6P Message Types ...................................44
           6.2.3. 6P Command Identifiers .............................44
           6.2.4. 6P Return Codes ....................................45
           6.2.5. 6P Scheduling Function Identifiers .................46
           6.2.6. 6P CellOptions Bitmap ..............................47
   7. References .....................................................48
      7.1. Normative References ......................................48
      7.2. Informative References ....................................48
   Appendix A. Recommended Structure of an SF Specification ..........49
   Authors' Addresses ................................................50

1.  Introduction

   All communication in an "IPv6 over the TSCH mode of IEEE 802.15.4e"
   (6TiSCH) network is orchestrated by a schedule [RFC7554].  The
   schedule is composed of cells, each identified by a
   [slotOffset,channelOffset] (Section 3.2.4).  This specification
   defines the 6TiSCH Operation Sublayer (6top) Protocol (6P), which is
   terminated by 6top.  6P allows a node to communicate with a neighbor
   node to add/delete Time-Slotted Channel Hopping (TSCH) cells to/on
   one another.  This results in distributed schedule management in a
   6TiSCH network.  6top is composed of one or more Scheduling Functions
   (SFs) and the 6top Protocol defined in this document.  The definition
   of SFs is out of scope for this document; however, this document
   provides the requirements for an SF.


















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RFC 8480                   6top Protocol (6P)              November 2018


   The example network depicted in Figure 1 is used to describe the
   interaction between nodes.  We consider the canonical case where
   node "A" issues 6P Requests (also referred to as "commands" in this
   document) to node "B".  We use this example throughout this document:
   node A always represents the node that issues a 6P Request, and
   node B represents the node that receives this request.

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

                     Figure 1: A Simple 6TiSCH Network

   We consider that node A monitors the communication cells it has in
   its schedule to node B:

   o  If node A determines that the number of link-layer frames it is
      sending to node B per unit of time exceeds the capacity offered by
      the TSCH cells it has scheduled to node B, it triggers a 6P
      Transaction with node B to add one or more cells to the TSCH
      schedule of both nodes.

   o  If the traffic is lower than the capacity offered by the TSCH
      cells it has scheduled to node B, node A triggers a 6P Transaction
      with node B to delete one or more cells in the TSCH schedule of
      both nodes.

   o  Node A MAY also monitor statistics to determine whether collisions
      are happening on a particular cell to node B.  If this feature is
      enabled, node A communicates with node B to "relocate" this
      particular cell to a different [slotOffset,channelOffset] location
      in the TSCH schedule.

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

   The 6top SF defines when to add/delete a cell to/on a neighbor.
   Different applications require different SFs; this topic is out of
   scope for 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.  At least one SF MUST be running.  The
   SFID field contained in all 6P messages allows a node to invoke the
   appropriate SF on a per-6P Transaction basis.





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RFC 8480                   6top Protocol (6P)              November 2018


   Section 2 describes 6top.  Section 3 defines 6P.  Section 4 provides
   guidelines on how to define an SF.

1.1.  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
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  6TiSCH Operation Sublayer (6top)

   As depicted in Figure 2, 6top is the layer just above the IEEE Std
   802.15.4 TSCH Medium Access Control (MAC) layer [IEEE802154].  We use
   "802.15.4" as a short version of "IEEE Std 802.15.4" in this
   document.

                                   .
               |                   .                      |
               |             higher layers                |
               +------------------------------------------+
               |                 6top                     |
               +------------------------------------------+
               |          IEEE Std 802.15.4 TSCH          |
               |                   .                      |
                                   .

                   Figure 2: 6top in the Protocol Stack

   The roles of 6top are to:

   o  Terminate 6P, which allows neighbor nodes to communicate to
      add/delete cells to/on one another.

   o  Run one or multiple 6top SFs, which define the rules that decide
      when to add/delete cells.














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2.1.  Hard/Soft Cells

   Each cell in the schedule is 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 schedule [RFC8180].

2.2.  Using 6P with the Minimal 6TiSCH Configuration

   6P MAY be used alongside the minimal 6TiSCH configuration [RFC8180].
   In this case, it is RECOMMENDED to use two slotframes, as depicted in
   Figure 3:

   o  Slotframe 0 is used for traffic defined in the minimal 6TiSCH
      configuration.  In Figure 3, Slotframe 0 is five slots long, but
      it can be shorter or longer.

   o  6P allocates cells from Slotframe 1.  In Figure 3, Slotframe 1 is
      10 slots long, but it can be shorter or longer.

                    | 0    1    2    3    4  | 0    1    2    3    4  |
                    +------------------------+------------------------+
        Slotframe 0 |    |    |    |    |    |    |    |    |    |    |
       5 slots long | EB |    |    |    |    | EB |    |    |    |    |
   (Minimal 6TiSCH) |    |    |    |    |    |    |    |    |    |    |
                    +-------------------------------------------------+

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

        Figure 3: 2-Slotframe Structure when Using 6P alongside the
                       Minimal 6TiSCH Configuration

   The minimal 6TiSCH configuration cell SHOULD be allocated from a
   slotframe of higher priority than the slotframe used by 6P for
   dynamic cell allocation.  This way, dynamically allocated cells
   cannot "mask" the cells used by the minimal 6TiSCH configuration.
   6top MAY support additional slotframes; how to use additional
   slotframes is out of scope for this document.



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RFC 8480                   6top Protocol (6P)              November 2018


3.  6top Protocol (6P)

   6P enables two neighbor nodes to add/delete/relocate cells in their
   TSCH schedule.  Conceptually, two neighbor nodes "negotiate" the
   location of the cells to add, delete, or relocate in their TSCH
   schedule.

3.1.  6P Transactions

   We call "6P Transaction" a complete negotiation between two neighbor
   nodes.  A particular 6P Transaction is executed between two nodes as
   a result of an action triggered by one SF.  For a 6P Transaction to
   succeed, both nodes must use the same SF to handle the particular
   transaction.  A 6P Transaction starts when a node wishes to
   add/delete/relocate one or more cells with one of its neighbors.  A
   6P Transaction ends when (1) the cell(s) has been added/deleted/
   relocated in the schedule of both nodes or (2) the 6P Transaction has
   failed.

   6P messages exchanged between nodes A and B during a 6P Transaction
   SHOULD be exchanged on non-shared unicast cells ("dedicated" cells)
   between nodes A and B.  If no dedicated cells are scheduled between
   nodes A and B, shared cells MAY be used.

   Keeping consistency between the schedules of the two neighbor nodes
   is important.  A loss of consistency can cause loss of connectivity.
   One example is when node A has a transmit cell to node B but node B
   does not have the corresponding reception cell.  To verify
   consistency, neighbor nodes maintain a sequence number (SeqNum).
   Neighbor nodes exchange the SeqNum as part of each 6P Transaction to
   detect a possible inconsistency.  This mechanism is explained in
   Section 3.4.6.2.

   An implementation MUST include a mechanism to associate each
   scheduled cell with the SF that scheduled it.  This mechanism is
   implementation specific and is out of scope for this document.

   A 6P Transaction can consist of two or three steps.  A 2-step
   transaction is used when node A selects the cells to be allocated.  A
   3-step transaction is used when node B selects the cells to be
   allocated.  An SF MUST specify whether to use 2-step transactions,
   3-step transactions, or both.

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






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RFC 8480                   6top Protocol (6P)              November 2018


3.1.1.  2-Step 6P Transaction

   Figure 4 shows an example 2-step 6P Transaction.  In a 2-step
   transaction, node A selects the candidate cells.  Several elements
   are left out so that the diagram is easier to understand.

                +----------+                           +----------+
                |  Node A  |                           |  Node B  |
                +----+-----+                           +-----+----+
                     |                                       |
                     | 6P ADD Request                        |
                     |   Type         = REQUEST              |
                     |   Code         = ADD                  |
                     |   SeqNum       = 123                  |
      cells          |   NumCells     = 2                    |
      locked         |   CellList     = [(1,2),(2,2),(3,5)]  |
       +--           |-------------------------------------->|
       |             |                                L2 ACK |
       |  6P Timeout |<- - - - - - - - - - - - - - - - - - - |
       |        |    |                                       |
       |        |    | 6P Response                           |
       |        |    |   Type         = RESPONSE             |
       |        |    |   Code         = RC_SUCCESS           |
       |        |    |   SeqNum       = 123                  | cells
       |        |    |   CellList     = [(2,2),(3,5)]        | locked
       +->      X    |<--------------------------------------| --+
                     | L2 ACK                                |   |
                     | - - - - - - - - - - - - - - - - - - ->| <-+
                     |                                       |

                Figure 4: An Example 2-Step 6P Transaction

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

   1.  The SF running on node A determines that two extra cells need to
       be scheduled to node B.

   2.  The SF running on node A selects candidate cells for node B to
       choose from.  Node A MUST select at least as many candidate cells
       as the number of cells to add.  Here, node A selects three
       candidate cells.  Node A locks those candidate cells in its
       schedule until it receives a 6P Response.









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   3.  Node A sends a 6P ADD Request to node B, indicating that it
       wishes to add two cells (the "NumCells" value) and specifying the
       list of three candidate cells (the "CellList" value).  Each cell
       in the CellList is a [slotOffset,channelOffset] tuple.  This 6P
       ADD Request is link-layer acknowledged by node B (labeled "L2
       ACK" in Figure 4).

   4.  After having successfully sent the 6P ADD Request (i.e.,
       receiving the link-layer acknowledgment), node A starts a 6P
       Timeout to abort the 6P Transaction in the event that no response
       is received from node B.

   5.  The SF running on node B selects two out of the three cells from
       the CellList of the 6P ADD Request.  Node B locks those cells in
       its schedule until the transmission is successful (i.e., node B
       receives a link-layer ACK from node A).  Node B sends back a 6P
       Response to node A, indicating the cells it has selected.  The
       response is link-layer acknowledged by node A.

   6.  Upon completion of this 6P Transaction, two cells from node A to
       node B have been added to the TSCH schedule of both nodes A
       and B.

   7.  An inconsistency in the schedule can happen if the 6P Timeout
       expires when the 6P Response is in the air, if the last
       link-layer ACK for the 6P Response is lost, or if one of the
       nodes is power-cycled during the transaction.  6P provides an
       inconsistency detection mechanism to cope with such situations;
       see Section 3.4.6.2 for details.






















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RFC 8480                   6top Protocol (6P)              November 2018


3.1.2.  3-Step 6P Transaction

   Figure 5 shows an example 3-step 6P Transaction.  In a 3-step
   transaction, node B selects the candidate cells.  Several elements
   are left out so that the diagram is easier to understand.

            +----------+                           +----------+
            |  Node A  |                           |  Node B  |
            +----+-----+                           +-----+----+
                 |                                       |
                 | 6P ADD Request                        |
                 |   Type         = REQUEST              |
                 |   Code         = ADD                  |
                 |   SeqNum       = 178                  |
                 |   NumCells     = 2                    |
                 |   CellList     = []                   |
                 |-------------------------------------->|
                 |                                L2 ACK |
      6P Timeout |<- - - - - - - - - - - - - - - - - - - |
            |    |                                       |
            |    | 6P Response                           |
            |    |   Type         = RESPONSE             |
            |    |   Code         = RC_SUCCESS           |
            |    |   SeqNum       = 178                  |         cells
            |    |   CellList     = [(1,2),(2,2),(3,5)]  |        locked
            X    |<--------------------------------------|          --+
                 | L2 ACK                                |            |
                 | - - - - - - - - - - - - - - - - - - ->| 6P Timeout |
                 |                                       |    |       |
                 | 6P Confirmation                       |    |       |
                 |   Type         = CONFIRMATION         |    |       |
                 |   Code         = RC_SUCCESS           |    |       |
    cells        |   SeqNum       = 178                  |    |       |
    locked       |   CellList     = [(2,2),(3,5)]        |    |       |
     +--         |-------------------------------------->|    X    <--+
     |           |                                L2 ACK |
     +->         |<- - - - - - - - - - - - - - - - - - - |
                 |                                       |

                Figure 5: An Example 3-Step 6P Transaction











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

   1.  The SF running on node A determines that two extra cells need to
       be scheduled to node B.  The SF uses a 3-step transaction, so it
       does not select candidate cells.

   2.  Node A sends a 6P ADD Request to node B, indicating that it
       wishes to add two cells (the "NumCells" value), with an empty
       "CellList".  This 6P ADD Request is link-layer acknowledged by
       node B.

   3.  After having successfully sent the 6P ADD Request, node A starts
       a 6P Timeout to abort the transaction in the event that no 6P
       Response is received from node B.

   4.  The SF running on node B selects three candidate cells and locks
       them.  Node B sends back a 6P Response to node A, indicating the
       three cells it has selected.  The response is link-layer
       acknowledged by node A.

   5.  After having successfully sent the 6P Response, node B starts a
       6P Timeout to abort the transaction in the event that no 6P
       Confirmation is received from node A.

   6.  The SF running on node A selects two cells from the CellList
       field in the 6P Response and locks them.  Node A sends back a 6P
       Confirmation to node B, indicating the cells it selected.  The
       confirmation is link-layer acknowledged by node B.

   7.  Upon completion of the 6P Transaction, two cells from node A to
       node B have been added to the TSCH schedule of both nodes A
       and B.

   8.  An inconsistency in the schedule can happen if the 6P Timeout
       expires when the 6P Confirmation is in the air, if the last
       link-layer ACK for the 6P Confirmation is lost, or if one of the
       nodes is power-cycled during the transaction.  6P provides an
       inconsistency detection mechanism to cope with such situations;
       see Section 3.4.6.2 for details.












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RFC 8480                   6top Protocol (6P)              November 2018


3.2.  Message Format

3.2.1.  6top Information Element (IE)

   6P messages travel over a single hop.  6P messages are carried as
   payload of an 802.15.4 Payload Information Element (IE) [IEEE802154].
   The messages are encapsulated within the Payload IE header.  The
   Group ID is set to the IETF IE value defined in [RFC8137].  The
   content is encapsulated by a subtype ID, as defined in [RFC8137].

   Since 6P messages are carried in IEs, IEEE bit/byte ordering applies.
   Bits within each field in the "6top IE" subtype are numbered from 0
   (leftmost and least significant) to k-1 (rightmost and most
   significant), where the length of the field is k bits.  Fields that
   are longer than a single octet are copied to the packet in the order
   from the octet containing the lowest-numbered bits to the octet
   containing the highest-numbered bits (little endian).

   This document defines the 6top IE, a subtype of the IETF IE defined
   in [RFC8137], with subtype SUBID_6TOP.  The subtype content of the
   6top IE is defined in Section 3.2.2.  The length of the 6top IE
   content is variable.

3.2.2.  Generic 6P Message Format

   All 6P messages follow the generic format shown in Figure 6.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Other Fields...
     +-+-+-+-+-+-+-+-+-

                    Figure 6: Generic 6P Message Format

   6P Version (Version):  The version of 6P.  Only version 0 is defined
         in this document.  Future specifications may define subsequent
         versions of 6P.

   Type (T):  The type of message.  The message types are defined in
         Section 6.2.2.

   Reserved (R):  Reserved bits.  These two bits SHOULD be set to zero
         when sending the message and MUST be ignored upon reception.





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   Code:  The Code field contains a 6P command identifier when the 6P
         message has a Type value of REQUEST.  Section 6.2.3 lists the
         6P command identifiers.  The Code field contains a 6P return
         code when the 6P message has a Type value of RESPONSE or
         CONFIRMATION.  Section 6.2.4 lists the 6P return codes.  The
         same return codes are used in both 6P Response and 6P
         Confirmation messages.

   6top Scheduling Function Identifier (SFID):  The identifier of the SF
         to use to handle this message.  The SFID is defined in
         Section 4.1.

   SeqNum:  The sequence number associated with the 6P Transaction.
         Used to match the 6P Request, 6P Response, and 6P Confirmation
         of the same 6P Transaction.  The value of SeqNum MUST be
         different for each new 6P Request issued to the same neighbor
         and using the same SF.  The SeqNum is also used to ensure
         consistency between the schedules of the two neighbors.
         Section 3.4.6 details how the SeqNum is managed.

   Other Fields:  The list of other fields and how they are used are
         detailed in Section 3.3.

   6P Request, 6P Response, and 6P Confirmation messages for a given
   transaction MUST share the same Version, SFID, and SeqNum values.

   Future versions of the 6P message SHOULD maintain the format of the
   6P Version, Type, and Code fields for backward compatibility.

3.2.3.  6P CellOptions

   An 8-bit 6P CellOptions bitmap is present in the following 6P
   Requests: ADD, DELETE, COUNT, LIST, and RELOCATE.  The format and
   meaning of this field MAY be redefined by the SF; the routine that
   parses this field is therefore associated with a specific SF.

   o  In the 6P ADD Request, the 6P CellOptions bitmap is used to
      specify what type of cell to add.

   o  In the 6P DELETE Request, the 6P CellOptions bitmap is used to
      specify what type of cell to delete.

   o  In the 6P RELOCATE Request, the 6P CellOptions bitmap is used to
      specify what type of cell to relocate.

   o  In the 6P COUNT and LIST Requests, the 6P CellOptions bitmap is
      used as a selector of a particular type of cells.




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RFC 8480                   6top Protocol (6P)              November 2018


   The content of the 6P CellOptions bitmap applies to all elements in
   the CellList field.  The possible values of the 6P CellOptions are as
   follows:

   o  TX = 1 (resp. 0) refers to macTxType = TRUE (resp. FALSE) in the
      macLinkTable of 802.15.4 [IEEE802154].

   o  RX = 1 (resp. 0) refers to macRxType = TRUE (resp. FALSE) in the
      macLinkTable of 802.15.4.

   o  S = 1 (resp. 0) refers to macSharedType = TRUE (resp. FALSE) in
      the macLinkTable of 802.15.4.

   Section 6.2.6 provides the format of the 6P CellOptions bitmap; this
   format applies unless redefined by the SF.  Figure 7 shows the
   meaning of the 6P CellOptions bitmap for the 6P ADD, DELETE, and
   RELOCATE Requests (unless redefined by the SF).  Figure 8 shows the
   meaning of the 6P CellOptions bitmap for the 6P COUNT and LIST
   Requests (unless redefined by the SF).

    Note: Here, we assume that node A issues the 6P command to node B.
   +-------------+-----------------------------------------------------+
   | CellOptions | The type of cells B adds/deletes/relocates to its   |
   | Value       | schedule when receiving a 6P ADD/DELETE/RELOCATE    |
   |             | Request from A                                      |
   +-------------+-----------------------------------------------------+
   |TX=0,RX=0,S=0| Invalid combination.  RC_ERR is returned            |
   +-------------+-----------------------------------------------------+
   |TX=1,RX=0,S=0| Add/delete/relocate RX cells at B (TX cells at A)   |
   +-------------+-----------------------------------------------------+
   |TX=0,RX=1,S=0| Add/delete/relocate TX cells at B (RX cells at A)   |
   +-------------+-----------------------------------------------------+
   |TX=1,RX=1,S=0| Add/delete/relocate TX|RX cells at B (and at A)     |
   +-------------+-----------------------------------------------------+
   |TX=0,RX=0,S=1| Invalid combination.  RC_ERR is returned            |
   +-------------+-----------------------------------------------------+
   |TX=1,RX=0,S=1| Add/delete/relocate RX|SHARED cells at B            |
   |             | (TX|SHARED cells at A)                              |
   +-------------+-----------------------------------------------------+
   |TX=0,RX=1,S=1| Add/delete/relocate TX|SHARED cells at B            |
   |             | (RX|SHARED cells at A)                              |
   +-------------+-----------------------------------------------------+
   |TX=1,RX=1,S=1| Add/delete/relocate TX|RX|SHARED cells at B         |
   |             | (and at A)                                          |
   +-------------+-----------------------------------------------------+

          Figure 7: Meaning of the 6P CellOptions Bitmap for the
                   6P ADD, DELETE, and RELOCATE Requests



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    Note: Here, we assume that node A issues the 6P command to node B.
   +-------------+-----------------------------------------------------+
   | CellOptions | The type of cells B selects from its schedule when  |
   | Value       | receiving a 6P COUNT or LIST Request from A,        |
   |             | from all the cells B has scheduled with A           |
   +-------------+-----------------------------------------------------+
   |TX=0,RX=0,S=0| All cells                                           |
   +-------------+-----------------------------------------------------+
   |TX=1,RX=0,S=0| All cells marked as RX only                         |
   +-------------+-----------------------------------------------------+
   |TX=0,RX=1,S=0| All cells marked as TX only                         |
   +-------------+-----------------------------------------------------+
   |TX=1,RX=1,S=0| All cells marked as TX and RX only                  |
   +-------------+-----------------------------------------------------+
   |TX=0,RX=0,S=1| All cells marked as SHARED (regardless of TX, RX)   |
   +-------------+-----------------------------------------------------+
   |TX=1,RX=0,S=1| All cells marked as RX and SHARED only              |
   +-------------+-----------------------------------------------------+
   |TX=0,RX=1,S=1| All cells marked as TX and SHARED only              |
   +-------------+-----------------------------------------------------+
   |TX=1,RX=1,S=1| All cells marked as TX, RX, and SHARED              |
   +-------------+-----------------------------------------------------+

          Figure 8: Meaning of the 6P CellOptions Bitmap for the
                        6P COUNT and LIST Requests

   The CellOptions constitute an opaque set of bits, sent unmodified to
   the SF.  The SF MAY redefine the format and meaning of the
   CellOptions field.






















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3.2.4.  6P CellList

   A CellList field MAY be present in a 6P ADD Request, a 6P DELETE
   Request, a 6P RELOCATE Request, a 6P Response, or a 6P Confirmation.
   It is composed of a concatenation of zero or more 6P Cells as defined
   in Figure 9.  The content of the CellOptions field specifies the
   options associated with all cells in the CellList.  This necessarily
   means that the same options are associated with all cells in the
   CellList.

   A 6P Cell is a 4-byte field; its default format is:

                          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         |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 9: 6P Cell Format

      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 length of the CellList field is implicit and is determined by the
   IE Length field of the Payload IE header as defined in 802.15.4.  The
   SF MAY redefine the format of the CellList field; the routine that
   parses this field is therefore associated with a specific SF.






















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RFC 8480                   6top Protocol (6P)              November 2018


3.3.  6P Commands and Operations

3.3.1.  Adding Cells

   Cells are added by using the 6P ADD command.  The Type field (T) is
   set to REQUEST.  The Code field is set to ADD.  Figure 10 defines the
   format of a 6P ADD Request.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Metadata            |  CellOptions  |   NumCells    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | CellList ...
     +-+-+-+-+-+-+-+-+-

                     Figure 10: 6P ADD Request Format

   Metadata:  Used as extra signaling to the SF.  The contents of the
         Metadata field are an opaque set of bytes passed unmodified to
         the SF.  The meaning of this field depends on the SF and is out
         of scope for this document.  For example, Metadata can specify
         in which slotframe to add the cells.

   CellOptions:  Indicates the options to associate with the cells to be
         added.  If more than one cell is added (NumCells > 1), the same
         options are associated with each one.  This necessarily means
         that if node A needs to add multiple cells with different
         options it needs to initiate multiple 6P ADD Transactions.

   NumCells:  The number of additional cells node A wants to schedule to
         node B.

   CellList:  A list of zero or multiple candidate cells.  Its length is
         implicit and is determined by the Length field of the Payload
         IE header.













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   Figure 11 defines the format of a 6P ADD Response and Confirmation.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | CellList ...
     +-+-+-+-+-+-+-+-+-

            Figure 11: 6P ADD Response and Confirmation Format

   CellList:  A list of zero or more 6P Cells.

   Consider the topology in Figure 1; in this case, the SF on node A
   decides to add NumCells cells to node B.

   Node A's SF selects NumCandidate cells from its schedule.  These are
   cells that are candidates to be scheduled with node B.  The
   CellOptions field specifies the type of these cells.  NumCandidate
   MUST be greater than or equal to NumCells.  How many cells node A
   selects (NumCandidate) and how that selection is done are specified
   in the SF and are out of scope for this document.  Node A sends a 6P
   ADD Request to node B that contains the CellOptions, the value of
   NumCells, and a selection of NumCandidate cells in the CellList.  If
   the NumCandidate cells do not fit in a single packet, this operation
   MUST be split into multiple independent 6P ADD Requests, each for a
   subset of the number of cells that eventually need to be added.  In
   the case of a 3-step transaction, the SF is responsible for ensuring
   that the returned Candidate CellList fits into the 6P Response.

   Upon receiving the request, node B checks to see whether the
   CellOptions are set to a valid value as noted by Figure 7.  If this
   is not the case, a Response with code RC_ERR is returned.  If the
   number of cells in the received CellList in node B is smaller than
   NumCells, node B MUST return a 6P Response with the RC_ERR_CELLLIST
   code.  Otherwise, node B's SF verifies which of the cells in the
   CellList it can install in node B's schedule, following the specified
   CellOptions field.  How that selection is done is specified in the SF
   and is out of scope for this document.  The verification can succeed
   (NumCells cells from the CellList can be used), fail (none of the
   cells from the CellList can be used), or partially succeed (fewer
   than NumCells cells from the CellList can be used).  In all cases,
   node B MUST send a 6P Response that includes a return code set to
   RC_SUCCESS and that specifies the list of cells that were scheduled
   following the CellOptions field.  That list can contain NumCells
   elements (succeed), 0 elements (fail), or between 0 and NumCells
   elements (partially succeed).



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   Upon receiving the response, node A adds the cells specified in the
   CellList according to the CellOptions field.

3.3.2.  Deleting Cells

   Cells are deleted by using the 6P DELETE command.  The Type field (T)
   is set to REQUEST.  The Code field is set to DELETE.  Figure 12
   defines the format of a 6P DELETE Request.

                          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| T | R |     Code      |     SFID      |    SeqNum     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Metadata            |  CellOptions  |   NumCells    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | CellList ...
     +-+-+-+-+-+-+-+-+-

                    Figure 12: 6P DELETE Request Format

   Metadata:  Same usage as for the 6P ADD command; see Section 3.3.1.
         Its format is the same as that in the 6P ADD command, but its
         content could be different.

   CellOptions:  Indicates the options that need to be associated with
         the cells to delete.  Only cells matching the CellOptions can
         be deleted.

   NumCells:  The number of cells from the specified CellList the sender
         wants to delete from the schedule of both sender and receiver.

   CellList:  A list of zero or more 6P Cells.  Its length is determined
         by the Length field of the Payload IE header.

















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RFC 8480                   6top Protocol (6P)              November 2018


   Figure 13 defines the format of a 6P DELETE Response and
   Confirmation.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | CellList ...
     +-+-+-+-+-+-+-+-+-

           Figure 13: 6P DELETE Response and Confirmation Format

   CellList:  A list of zero or more 6P 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 already be scheduled between the
      two nodes and MUST match the CellOptions field.  If node A puts
      cells in its CellList that are not already scheduled between the
      two nodes and match the CellOptions field, node B MUST reply with
      a RC_ERR_CELLLIST return code.

   o  The CellList in a 6P Request (2-step transaction) or 6P Response
      (3-step transaction) MUST be empty, contain exactly NumCells
      cells, or contain more than NumCells cells.  The case where the
      CellList is not empty but contains fewer than NumCells cells is
      not supported; the RC_ERR_CELLLIST code MUST be returned when the
      CellList contains fewer than NumCells cells.  If the CellList is
      empty, the SF on the receiving node MUST choose NumCells cells
      scheduled to the sender matching the CellOptions field and delete
      them.  If the CellList contains more than NumCells cells, the SF
      on the receiving node chooses exactly NumCells cells from the
      CellList to delete.













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RFC 8480                   6top Protocol (6P)              November 2018


3.3.3.  Relocating Cells

   Cell relocation consists of moving a cell to a different
   [slotOffset,channelOffset] location in the schedule.  The Type field
   (T) is set to REQUEST.  The Code field is set to RELOCATE.  Figure 14
   defines the format of a 6P RELOCATE Request.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Metadata            |  CellOptions  |   NumCells    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Relocation CellList          ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-
     | Candidate CellList           ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-

                   Figure 14: 6P RELOCATE Request Format

   Metadata:  Same usage as for the 6P ADD command; see Section 3.3.1.

   CellOptions:  Indicates the options that need to be associated with
         cells to be relocated.

   NumCells:  The number of cells to relocate.  MUST be greater than or
         equal to 1.

   Relocation CellList:  The list of NumCells 6P Cells to relocate.

   Candidate CellList:  A list of NumCandidate candidate cells for
         node B to pick from.  NumCandidate MUST be 0, equal to
         NumCells, or greater than NumCells.  Its length is determined
         by the Length field of the Payload IE header.

   In a 2-step 6P RELOCATE Transaction, node A specifies both (1) the
   cells it needs to relocate and (2) the list of candidate cells to
   relocate to.  The Relocation CellList MUST contain exactly NumCells
   entries.  The Candidate CellList MUST contain at least NumCells
   entries (NumCandidate >= NumCells).

   In a 3-step 6P RELOCATE Transaction, node A specifies only the cells
   it needs to relocate -- not the list of candidate cells to relocate
   to.  The Candidate CellList MUST therefore be empty.






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   Figure 15 defines the format of a 6P RELOCATE Response and
   Confirmation.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | CellList ...
     +-+-+-+-+-+-+-+-+-

          Figure 15: 6P RELOCATE Response and Confirmation Format

   CellList:  A list of zero or more 6P Cells.

   Node A's SF wants to relocate NumCells cells.  Node A creates a 6P
   RELOCATE Request and indicates the cells it wants to relocate in the
   Relocation CellList.  It also selects NumCandidate cells from its
   schedule as candidate cells to relocate the cells to, and it puts
   them in the Candidate CellList.  The CellOptions field specifies the
   type of the cell(s) to relocate.  NumCandidate MUST be greater than
   or equal to NumCells.  How many cells it selects (NumCandidate) and
   how that selection is done are specified in the SF and are out of
   scope for this document.  Node A sends the 6P RELOCATE Request to
   node B.

   Upon receiving the request, node B checks to see if the length of the
   Candidate CellList is greater than or equal to NumCells.  Node B's SF
   verifies that all the cells in the Relocation CellList are scheduled
   with node A and are associated with the options specified in the
   CellOptions field.  If either check fails, node B MUST send a 6P
   Response to node A with return code RC_ERR_CELLLIST.  If both checks
   pass, node B's SF verifies which of the cells in the Candidate
   CellList it can install in its schedule.  How that selection is done
   is specified in the SF and is out of scope for this document.  That
   verification for the Candidate CellList can succeed (NumCells cells
   from the Candidate CellList can be used), fail (none of the cells
   from the Candidate CellList can be used), or partially succeed (fewer
   than NumCells cells from the Candidate CellList can be used).  In all
   cases, node B MUST send a 6P Response that includes a return code set
   to RC_SUCCESS and that specifies the list of cells that will be
   rescheduled following the CellOptions field.  That list can contain
   NumCells elements (succeed), 0 elements (fail), or between 0 and
   NumCells elements (partially succeed).  If N < NumCells cells appear
   in the CellList, this means that the first N cells in the Relocation
   CellList have been relocated and the remainder have not.





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   Upon receiving the response with code RC_SUCCESS, node A relocates
   the cells specified in the Relocation CellList of its RELOCATE
   Request to the new locations specified in the CellList of the 6P
   Response, in the same order.  If the received return code is
   RC_ERR_CELLLIST, the transaction is aborted and no cell is relocated.
   In the case of a 2-step transaction, node B relocates the selected
   cells upon receiving the link-layer ACK for the 6P Response.  In the
   case of a 3-step transaction, node B relocates the selected cells
   upon receiving the 6P Confirmation.

   The SF SHOULD NOT relocate all cells between two nodes at the same
   time, as this might result in the schedules of both nodes diverging
   significantly.

   Figure 16 shows an example of a successful 2-step 6P RELOCATE
   Transaction.

            +----------+                           +----------+
            |  Node A  |                           |  Node B  |
            +----+-----+                           +-----+----+
                 |                                       |
                 | 6P RELOCATE Request                   |
                 |   Type         = REQUEST              |
                 |   Code         = RELOCATE             |
                 |   SeqNum       = 11                   |
                 |   NumCells     = 2                    |
                 |   R.CellList   = [(1,2),(2,2)]        |
                 |   C.CellList   = [(3,3),(4,3),(5,3)]  |
                 |-------------------------------------->| B prepares
                 |                                L2 ACK | to relocate
                 |<- - - - - - - - - - - - - - - - - - - | (1,2)->(5,3)
                 |                                       | and
                 |                                       | (2,2)->(3,3)
                 | 6P Response                           |
                 |   Code         = RC_SUCCESS           |
                 |   SeqNum       = 11                   |
                 |   CellList     = [(5,3),(3,3)]        |
     A relocates |<--------------------------------------|
    (1,2)->(5,3) | L2 ACK                                |
             and | - - - - - - - - - - - - - - - - - - ->| B relocates
    (2,2)->(3,3) |                                       | (1,2)->(5,3)
                 |                                       | and
                 |                                       | (2,2)->(3,3)

     Figure 16: Example of a Successful 2-Step 6P RELOCATE Transaction






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   Figure 17 shows an example of a partially successful 2-step 6P
   RELOCATE Transaction.

           +----------+                           +----------+
           |  Node A  |                           |  Node B  |
           +----+-----+                           +-----+----+
                |                                       |
                | 6P RELOCATE Request                   |
                |   Type         = REQUEST              |
                |   Code         = RELOCATE             |
                |   SeqNum       = 199                  |
                |   NumCells     = 2                    |
                |   R.CellList   = [(1,2),(2,2)]        |
                |   C.CellList   = [(3,3),(4,3),(5,3)]  | B prepares
                |-------------------------------------->| to relocate
                |                                L2 ACK | (1,2)->(4,3)
                |<- - - - - - - - - - - - - - - - - - - | but cannot
                |                                       | relocate (2,2)
                | 6P Response                           |
                |   Type         = RESPONSE             |
                |   Code         = RC_SUCCESS           |
                |   SeqNum       = 199                  |
                |   CellList     = [(4,3)]              |
    A relocates |<--------------------------------------|
   (1,2)->(4,3) | L2 ACK                                |
                | - - - - - - - - - - - - - - - - - - ->| B relocates
                |                                       | (1,2)->(4,3)
                |                                       |
                |                                       |

          Figure 17: Example of a Partially Successful 2-Step 6P
                           RELOCATE Transaction



















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   Figure 18 shows an example of a failed 2-step 6P RELOCATE
   Transaction.

           +----------+                           +----------+
           |  Node A  |                           |  Node B  |
           +----+-----+                           +-----+----+
                |                                       |
                | 6P RELOCATE Request                   |
                |   Type         = REQUEST              |
                |   Code         = RELOCATE             |
                |   SeqNum       = 53                   |
                |   NumCells     = 2                    |
                |   R.CellList   = [(1,2),(2,2)]        |
                |   C.CellList   = [(3,3),(4,3),(5,3)]  |
                |-------------------------------------->| B cannot
                |                                L2 ACK | relocate
                |<- - - - - - - - - - - - - - - - - - - | (1,2)
                |                                       | or (2,2)
                | 6P Response                           |
                |   Type         = RESPONSE             |
                |   Code         = RC_SUCCESS           |
                |   SeqNum       = 53                   |
                |   CellList     = []                   |
                |<--------------------------------------| B does not
                | L2 ACK                                | relocate
     A does not | - - - - - - - - - - - - - - - - - - ->|
       relocate |                                       |
                |                                       |

         Figure 18: Failed 2-Step 6P RELOCATE Transaction Example





















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   Figure 19 shows an example of a successful 3-step 6P RELOCATE
   Transaction.

           +----------+                           +----------+
           |  Node A  |                           |  Node B  |
           +----+-----+                           +-----+----+
                |                                       |
                | 6P RELOCATE Request                   |
                |   Type         = REQUEST              |
                |   Code         = RELOCATE             |
                |   SeqNum       = 11                   |
                |   NumCells     = 2                    |
                |   R.CellList   = [(1,2),(2,2)]        |
                |   C.CellList   = []                   |
                |-------------------------------------->|
                |                                L2 ACK |
                |<- - - - - - - - - - - - - - - - - - - | B identifies
                |                                       | candidate
                |                                       | cells
                | 6P Response                           | (3,3),
                |   Code         = RC_SUCCESS           | (4,3), and
                |   SeqNum       = 11                   | (5,3)
                |   CellList     = [(3,3),(4,3),(5,3)]  |
     A prepares |<--------------------------------------|
    to relocate | L2 ACK                                |
   (1,2)->(5,3) | - - - - - - - - - - - - - - - - - - ->|
            and |                                       |
   (2,2)->(3,3) | 6P Confirmation                       |
                |   Code         = RC_SUCCESS           |
                |   SeqNum       = 11                   |
                |   CellList     = [(5,3),(3,3)]        |
                |-------------------------------------->| B relocates
                |                                L2 ACK | (1,2)->(5,3)
    A relocates |<- - - - - - - - - - - - - - - - - - - | and
   (1,2)->(5,3) |                                       | (2,2)->(3,3)
            and |                                       |
   (2,2)->(3,3) |                                       |
                |                                       |

     Figure 19: Example of a Successful 3-Step 6P RELOCATE Transaction











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RFC 8480                   6top Protocol (6P)              November 2018


3.3.4.  Counting Cells

   To retrieve the number of scheduled cells node A has with B, node A
   issues a 6P COUNT command.  The Type field (T) is set to REQUEST.
   The Code field is set to COUNT.  Figure 20 defines the format of a 6P
   COUNT Request.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Metadata            |  CellOptions  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 20: 6P COUNT Request Format

   Metadata:  Same usage as for the 6P ADD command; see Section 3.3.1.
         Its format is the same as that in the 6P ADD command, but its
         content could be different.

   CellOptions:  Specifies which type of cell to be counted.

   Figure 21 defines the format of a 6P COUNT Response.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           NumCells            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 21: 6P COUNT Response Format

   NumCells:  The number of cells that correspond to the fields of the
         request.

   Node A issues a COUNT command to node B, specifying some cell
   options.  Upon receiving the 6P COUNT Request, node B goes through
   its schedule and counts the number of cells scheduled with node A in
   its own schedule that match the cell options in the CellOptions field
   of the request.  Section 3.2.3 details the use of the CellOptions
   field.

   Node B issues a 6P Response to node A with return code RC_SUCCESS and
   with NumCells containing the number of cells that match the request.




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RFC 8480                   6top Protocol (6P)              November 2018


3.3.5.  Listing Cells

   To retrieve a list of scheduled cells node A has with node B, node A
   issues a 6P LIST command.  The Type field (T) is set to REQUEST.  The
   Code field is set to LIST.  Figure 22 defines the format of a 6P LIST
   Request.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Metadata            |  CellOptions  |   Reserved    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Offset              |          MaxNumCells          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 22: 6P LIST Request Format

   Metadata:  Same usage as for the 6P ADD command; see Section 3.3.1.
         Its format is the same as that in the 6P ADD command, but its
         content could be different.

   CellOptions:  Specifies which type of cell to be listed.

   Reserved:  Reserved bits.  These bits SHOULD be set to zero when
         sending the message and MUST be ignored upon reception.

   Offset:  The offset of the first scheduled cell that is requested.
         The mechanism assumes that cells are ordered according to a
         rule defined in the SF.  The rule MUST always order the cells
         in the same way.

   MaxNumCells:  The maximum number of cells to be listed.  Node B MAY
         return fewer than MaxNumCells cells -- for example, if
         MaxNumCells cells do not fit in the frame.















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   Figure 23 defines the format of a 6P LIST Response.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | CellList ...
     +-+-+-+-+-+-+-+-+-

                    Figure 23: 6P LIST Response Format

   CellList:  A list of zero or more 6P Cells.

   When receiving a LIST command, node B returns the cells scheduled
   with A in its schedule that match the CellOptions field as specified
   in Section 3.2.3.

   When node B receives a LIST Request, the returned CellList in the 6P
   Response contains between 0 and MaxNumCells cells, starting from the
   specified offset.  Node B SHOULD include as many cells as will fit in
   the frame.  If the response contains the last cell, node B MUST set
   the Code field in the response to RC_EOL ("End of List", as per
   Figure 38 in Section 6.2.4), indicating to node A that there are no
   more cells that match the request.  Node B MUST return at least one
   cell, unless the specified offset is beyond the end of B's cell list
   in its schedule.  If node B has fewer than Offset cells that match
   the request, node B returns an empty CellList and a Code field set
   to RC_EOL.






















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3.3.6.  Clearing the Schedule

   To clear the schedule between nodes A and B (for example, after a
   schedule inconsistency is detected), node A issues a CLEAR command.
   The Type field (T) is set to REQUEST.  The Code field is set to
   CLEAR.  Figure 24 defines the format of a 6P CLEAR Request.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Metadata            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 24: 6P CLEAR Request Format

   Metadata:  Same usage as for the 6P ADD command; see Section 3.3.1.
         Its format is the same as that in the 6P ADD command, but its
         content could be different.

   Figure 25 defines the format of a 6P CLEAR Response.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 25: 6P CLEAR Response Format

   When a 6P CLEAR command is issued from node A to node B, both nodes A
   and B MUST remove all the cells scheduled between them.  That is,
   node A MUST remove all the cells scheduled with node B, and node B
   MUST remove all the cells scheduled with node A.  In a 6P CLEAR
   command, the SeqNum MUST NOT be checked.  In particular, even if the
   request contains a SeqNum value that would normally cause node B to
   detect a schedule inconsistency, the transaction MUST NOT be aborted.
   Upon 6P CLEAR completion, the value of SeqNum MUST be reset to 0.

   The return code sent in response to a 6P CLEAR command SHOULD be
   RC_SUCCESS unless the operation cannot be executed.  When the CLEAR
   operation cannot be executed, the return code MUST be set to
   RC_RESET.







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3.3.7.  Generic Signaling between SFs

   The 6P SIGNAL message allows the SF implementations on two neighbor
   nodes to exchange generic commands.  The payload in a received SIGNAL
   message is an opaque set of bytes passed unmodified to the SF.  The
   length of the payload is determined by the Length field of the
   Payload IE header.  How the generic SIGNAL command is used is
   specified by the SF and is outside the scope of this document.  The
   Type field (T) is set to REQUEST.  The Code field is set to SIGNAL.
   Figure 26 defines the format of a 6P SIGNAL Request.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Metadata            |  payload ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                    Figure 26: 6P SIGNAL Request Format

   Metadata:  Same usage as for the 6P ADD command; see Section 3.3.1.
         Its format is the same as that in the 6P ADD command, but its
         content could be different.

   Figure 27 defines the format of a 6P SIGNAL Response.

                          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| T | R |     Code      |     SFID      |     SeqNum    |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | payload ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 27: 6P SIGNAL Response Format

3.4.  Protocol Functional Details

3.4.1.  Version Checking

   All messages contain a Version field.  If multiple protocol versions
   of 6P have been defined (in future specifications for Version values
   different from 0), a node MAY implement multiple protocol versions at
   the same time.  When a node receives a 6P message with a version
   number it does not implement, the node MUST reply with a 6P Response
   with a return code field set to RC_ERR_VERSION.  The format of this
   6P Response message MUST be compliant with version 0 and MUST be



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   supported by all future versions of the protocol.  This ensures that
   when node B sends a 6P Response to node A indicating that it does not
   implement the 6P version in the 6P Request, node A can successfully
   parse that response.

   When a node supports a version number received in a 6P Request
   message, the Version field in the 6P Response MUST be the same as the
   Version field in the corresponding 6P Request.  Similarly, in a
   3-step transaction, the Version field in the 6P Confirmation MUST
   match that of the 6P Request and 6P Response of the same transaction.

3.4.2.  SFID Checking

   All messages contain an SFID field.  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 with a return code of
   RC_ERR_SFID.  The SFID field in the 6P Response MUST be the same as
   the SFID field in the corresponding 6P Request.  In a 3-step
   transaction, the SFID field in the 6P Confirmation MUST match that of
   the 6P Request and the 6P Response of the same transaction.

3.4.3.  Concurrent 6P Transactions

   Only a single 6P Transaction at a time in a given direction can take
   place between two neighbors.  That is, a node MUST NOT issue a new 6P
   Request to a given neighbor before the previous 6P Transaction it
   initiated has finished (or possibly 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 RC_RESET (as per Figure 38 in
   Section 6.2.4) and discard this ongoing second transaction.  A node
   receiving a RC_RESET code MUST abort the second transaction and treat
   it as though it never happened (i.e., reverting changes to the
   schedule or SeqNum done by this transaction).

   Nodes A and B MAY support having two transactions going on at the
   same time, one in each direction.  Similarly, a node MAY support
   concurrent 6P Transactions with different neighbors.  In this case,
   the cells involved in an ongoing 6P Transaction MUST be "locked"
   until the transaction finishes.  For example, in Figure 1, node C can
   have a different ongoing 6P Transaction with nodes B and R.  If a
   node does not have enough resources to handle concurrent 6P
   Transactions from different neighbors, it MUST reply with a 6P
   Response with return code RC_ERR_BUSY (as per Figure 38 in
   Section 6.2.4).  If the requested cells are locked, it MUST reply to
   that request with a 6P Response with return code RC_ERR_LOCKED (as
   per Figure 38).  The node receiving RC_ERR_BUSY or RC_ERR_LOCKED MAY
   implement a retry mechanism as defined by the SF.



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3.4.4.  6P Timeout

   A timeout occurs when the node that successfully sent a 6P Request
   does not receive the corresponding 6P Response within an amount of
   time specified by the SF.  In a 3-step transaction, a timeout also
   occurs when a node sending the 6P Response does not receive a 6P
   Confirmation.  When a timeout occurs, the transaction MUST be
   canceled at the node where the timeout occurs.  The value of the 6P
   Timeout should be greater than the longest possible time it takes to
   receive the 6P Response or Confirmation.  The value of the 6P Timeout
   hence depends on the number of cells scheduled between the neighbor
   nodes, the maximum number of link-layer retransmissions, etc.  The SF
   MUST determine the value of the timeout.  The value of the timeout is
   out of scope for this document.

3.4.5.  Aborting a 6P Transaction

   If the receiver of a 6P 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 RC_RESET.  Upon receiving this 6P Response,
   the initiator of the 6P Transaction MUST consider the 6P Transaction
   as having failed.

   Similarly, in the case of a 3-step transaction, when the receiver of
   a 6P Response fails during the 6P Transaction and is unable to
   complete it, it MUST reply to that 6P Response with a 6P Confirmation
   with return code RC_RESET.  Upon receiving this 6P Confirmation, the
   sender of the 6P Response MUST consider the 6P Transaction as having
   failed.

3.4.6.  SeqNum Management

   The SeqNum is the field in the 6top IE header used to match Request,
   Response, and Confirmation messages for a given transaction.  The
   SeqNum is used to detect and handle duplicate commands
   (Section 3.4.6.1) and inconsistent schedules (Section 3.4.6.2).  Each
   node remembers the last used SeqNum for each neighbor.  That is, a
   node stores as many SeqNum values as it has neighbors.  In the case
   of supporting multiple SFs at a time, a SeqNum value is maintained
   per SF and per neighbor.  In the remainder of this section, we
   describe the use of SeqNum between two neighbors; the same happens
   for each other neighbor, independently.

   When a node resets, or after a CLEAR Transaction, it MUST reset
   SeqNum to 0.  The 6P Response and 6P Confirmation for a transaction
   MUST use the same SeqNum value as that in the request.  After every
   transaction, the SeqNum MUST be incremented by exactly 1.




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   Specifically, if node A receives the link-layer acknowledgment for
   its 6P Request, it will increment the SeqNum by exactly 1 after the
   6P Transaction ends.  This ensures that, for the next 6P Transaction
   where it sends a 6P Request, the 6P Request will have a different
   SeqNum.

   Similarly, node B increments the SeqNum by exactly 1 after having
   received the link-layer acknowledgment for the 6P Response (2-step 6P
   Transaction) or after having sent the link-layer acknowledgment for
   the 6P Confirmation (3-step 6P Transaction).

   When node B receives a 6P Request from node A with SeqNum equal to 0,
   it checks the stored SeqNum for A.  If A is a new neighbor, the
   stored SeqNum in B will be 0.  The transaction can continue.  If the
   stored SeqNum for A in B is different than 0, a potential
   inconsistency is detected.  In this case, B MUST return RC_ERR_SEQNUM
   with SeqNum=0.  The SF of node A MAY decide what to do next, as
   described in Section 3.4.6.2.

   The SeqNum MUST be implemented as a lollipop counter: it rolls over
   from 0xFF to 0x01 (not to 0x00).  This is used to detect a neighbor
   reset.  Figure 28 lists the possible values of the SeqNum.

               +-----------+------------------------------+
               |   Value   | Meaning                      |
               +-----------+------------------------------+
               |      0x00 | Clear, or after device reset |
               | 0x01-0xFF | Lollipop counter values      |
               +-----------+------------------------------+

                 Figure 28: Possible Values of the SeqNum

3.4.6.1.  Detecting and Handling Duplicate 6P Messages

   All 6P commands are link-layer acknowledged.  A duplicate message
   means that a node receives a second 6P Request, Response, or
   Confirmation.  This happens when the link-layer acknowledgment is not
   received and a link-layer retransmission happens.  Duplicate messages
   are normal and unavoidable.












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   Figure 29 shows an example 2-step transaction in which node A
   receives a duplicate 6P Response.

           +----------+                           +----------+
           |  Node A  |                           |  Node B  |
           +----+-----+                           +-----+----+
                |                                       |
                | 6P Request (SeqNum=456)               |
                |-------------------------------------->|
                |                                L2 ACK |
                |<- - - - - - - - - - - - - - - - - - - |
                |                                       |
                | 6P Response  (SeqNum=456)             |
                |<--------------------------------------|
                | L2 ACK                                |
                | - - - - - - - - - - -X                | no ACK:
                |                                       | link-layer
                | 6P Response  (SeqNum=456)             | retransmit
      duplicate |<--------------------------------------|
    6P Response | L2 ACK                                |
       received | - - - - - - - - - - - - - - - - - - ->|
                |                                       |

                  Figure 29: Example Duplicate 6P Message



























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   Figure 30 shows an example 3-step transaction in which node A
   receives an out-of-order duplicate 6P Response after having sent a 6P
   Confirmation.

           +----------+                           +----------+
           |  Node A  |                           |  Node B  |
           +----+-----+                           +-----+----+
                |                                       |
                | 6P Request  (SeqNum=123)              |
                |-------------------------------------->|
                |                                L2 ACK |
                |<- - - - - - - - - - - - - - - - - - - |
                |                                       |
                | 6P Response  (SeqNum=123)             |
                |<--------------------------------------|
                | L2 ACK                                |
                | - - - - - - - - - - -X                | no ACK:
                |                                       | link-layer
                | 6P Confirmation  (SeqNum=123)         | retransmit
                |-------------------------------------->|    |
                |                                L2 ACK |    |
                |<- - - - - - - - - - - - - - - - - - - |  frame
                |                                       |  queued
                | 6P Response  (SeqNum=123)             |    |
      duplicate |<--------------------------------------| <--+
   out-of-order | L2 ACK                                |
    6P Response | - - - - - - - - - - - - - - - - - - ->|
       received |                                       |

           Figure 30: Example Out-of-Order Duplicate 6P Message

   A node detects a duplicate 6P message when it has the same SeqNum and
   type as the last frame received from the same neighbor.  When
   receiving a duplicate 6P message, a node MUST send a link-layer
   acknowledgment but MUST silently ignore the 6P message at 6top.

3.4.6.2.  Detecting and Handling a Schedule Inconsistency

   A schedule inconsistency happens when the schedules of nodes A and B
   are inconsistent -- for example, when node A has a transmit cell to
   node B, but node B does not have the corresponding receive cell and
   therefore isn't listening to node A on that cell.  A schedule
   inconsistency results in loss of connectivity.

   The SeqNum field, which is present in each 6P message, is used to
   detect an inconsistency.  The SeqNum field increments by 1 in each
   message, as detailed in Section 3.4.6.  A node computes the expected




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   SeqNum field for the next 6P Transaction.  If a node receives a 6P
   Request with a SeqNum value that is not the expected value, it has
   detected an inconsistency.

   There are two cases in which a schedule inconsistency happens.

   The first case is when a node loses state -- for example, when it is
   power-cycled (turned off, then on).  In that case, its SeqNum value
   is reset to 0.  Since the SeqNum is a lollipop counter, its neighbor
   detects an inconsistency in the next 6P Transaction.  This is
   illustrated in Figures 31 and 32.

           +----------+                           +----------+
           |  Node A  |                           |  Node B  |
           +----+-----+                           +-----+----+
      SeqNum=87 |                                       | SeqNum=87
                |                                       |
                | 6P Request  (SeqNum=87)               |
                |-------------------------------------->|
                |                                L2 ACK |
                |<- - - - - - - - - - - - - - - - - - - |
                |                                       |
                | 6P Response  (SeqNum=87)              |
                |<--------------------------------------|
                | L2 ACK                                |
                | - - - - - - - - - - - - - - - - - - ->|
                |                                     ==== power-cycle
                |                                       |
      SeqNum=88 |                                       | SeqNum=0
                |                                       |
                | 6P Request (SeqNum=88)                |
                |-------------------------------------->| Inconsistency
                |                                L2 ACK | detected
                |<- - - - - - - - - - - - - - - - - - - |
                |                                       |
                | 6P Response (SeqNum=0, RC_ERR_SEQNUM) |
                |<--------------------------------------|
                | L2 ACK                                |
                | - - - - - - - - - - - - - - - - - - ->|

         Figure 31: Example of Inconsistency Because Node B Resets
                           (Detected by Node B)









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            +----------+                           +----------+
            |  Node A  |                           |  Node B  |
            +----+-----+                           +-----+----+
       SeqNum=97 |                                       | SeqNum=97
                 |                                       |
                 | 6P Request  (SeqNum=97)               |
                 |-------------------------------------->|
                 |                                L2 ACK |
                 |<- - - - - - - - - - - - - - - - - - - |
                 |                                       |
                 | 6P Response  (SeqNum=97)              |
                 |<--------------------------------------|
                 | L2 ACK                                |
                 | - - - - - - - - - - - - - - - - - - ->|
                 |                                     ==== power-cycle
                 |                                       |
       SeqNum=98 |                                       | SeqNum=0
                 |                                       |
                 | 6P Request (SeqNum=0)                 |
   Inconsistency |<--------------------------------------|
        detected | L2 ACK                                |
                 |- - - - - - - - - - - - - - - - - - - >|
                 |                                       |
                 | 6P Response (SeqNum=0, RC_ERR_SEQNUM) |
                 |-------------------------------------->|
                 | L2 ACK                                |
                 |<- - - - - - - - - - - - - - - - - - - |

         Figure 32: Example of Inconsistency Because Node B Resets
                           (Detected by Node A)





















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   The second case is when the maximum number of link-layer
   retransmissions is reached on the 6P Response of a 2-step transaction
   (or, equivalently, on a 6P Confirmation of a 3-step transaction).
   This is illustrated in Figure 33.

          +----------+                           +----------+
          |  Node A  |                           |  Node B  |
          +----+-----+                           +-----+----+
     SeqNum=87 |                                       | SeqNum=87
               |                                       |
               | 6P Request  (SeqNum=87)               |
               |-------------------------------------->|
               |                                L2 ACK |
               |<- - - - - - - - - - - - - - - - - - - |
               |                                       |
               | 6P Response  (SeqNum=87)              |
               |<--------------------------------------|
               | L2 ACK                                |
               | - - - - - - - - X                     |
     SeqNum=88 |                                       | no ACK:
               | 6P Response  (SeqNum=87)              | retrans. 1
   (duplicate) |<--------------------------------------|
               | L2 ACK                                |
               | - - - - - - - - X                     |
               |                                       | no ACK:
               | 6P Response  (SeqNum=87)              | retrans. 2
   (duplicate) |<--------------------------------------|
               | L2 ACK                                |
               | - - - - - - - - X                     |
               |                                       | max. retrans.:
               |                                       | inconsistency
               |                                       | detected

      Figure 33: Example Inconsistency Because of Maximum Link-Layer
                    Retransmissions (where Maximum = 2)

   In both cases, node B detects the inconsistency.

   If the inconsistency is detected during a 6P Transaction (Figure 31),
   the node that has detected it MUST send back a 6P Response or 6P
   Confirmation with an error code of RC_ERR_SEQNUM.  In this 6P
   Response or 6P Confirmation, the SeqNum field MUST be set to the
   value of the sender of the message (0 in the example in Figure 31).








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   The SF of the node that has detected the inconsistency MUST define
   how to handle the inconsistency.  Three possible ways to do this are
   as follows:

   o  Issue a 6P CLEAR Request to clear the schedule, and then rebuild.

   o  Issue a 6P LIST Request to retrieve the schedule.

   o  Internally "roll back" the schedule.

   How to handle an inconsistency is out of scope for this document.
   The SF defines how to handle an inconsistency.

3.4.7.  Handling Error Responses

   A return code marked as Yes in the "Is Error?" column in Figure 38
   (Section 6.2.4) indicates an error.  When a node receives a 6P
   Response or 6P Confirmation with an error, it MUST consider the 6P
   Transaction as having failed.  In particular, if this was a response
   to a 6P ADD, DELETE, or RELOCATE Request, the node MUST NOT add,
   delete, or relocate any of the cells involved in this 6P Transaction.
   Similarly, a node sending a 6P Response or a 6P Confirmation with an
   error code MUST NOT add, delete, or relocate any cells as part of
   that 6P Transaction.  If a node receives an unrecognized return code,
   the 6P Transaction MUST be considered as having failed.  In
   particular, in a 3-step 6P Transaction, when receiving a 6P Response
   with a return code that it does not recognize, the requester (node A)
   MUST send a 6P Confirmation to the responder (node B) with return
   code RC_ERR and consider the transaction failed.  Upon reception of a
   6P Confirmation with return code RC_ERR, the responder MUST consider
   the transaction failed as well.  Defining what to do after an error
   has occurred is out of scope for this document.  The SF defines what
   to do after an error has occurred.

3.5.  Security

   6P messages MUST be secured through link-layer security.  This is
   possible because 6P messages are carried as Payload IEs.













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4.  Requirements for 6top Scheduling Function (SF) Specifications

4.1.  SF Identifier (SFID)

   Each SF has a 1-byte identifier.  Section 6.2.5 defines the rules for
   applying for an SFID.

4.2.  Requirements for an SF Specification

   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/on 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 the 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 the rule for ordering cells.

   o  MUST specify a meaning for the Metadata field in the 6P ADD
      Request.

   o  MUST specify the SF behavior of a node when it boots.

   o  MUST specify how to handle a schedule inconsistency.

   o  MUST specify what to do after an error has occurred (the node
      either sent a 6P Response with an error code or received one).

   o  MUST specify the list of statistics to gather.  Example statistics
      include the number of transmitted frames to each neighbor.  If the
      SF does not require that statistics be gathered, the SF
      specification MUST explicitly say so.

   o  SHOULD clearly state the application domain the SF is created for.

   o  SHOULD contain examples that highlight normal and error scenarios.

   o  SHOULD contain a list of current implementations, at least during
      the Internet-Draft (I-D) state of the document, per [RFC7942].



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   o  SHOULD contain a performance evaluation of the scheme, possibly
      through references to external documents.

   o  SHOULD define the format of the SIGNAL command payload and
      its use.

   o  MAY redefine the format of the CellList field.

   o  MAY redefine the format of the CellOptions field.

   o  MAY redefine the meaning of the CellOptions field.

5.  Security Considerations

   6P messages are carried inside 802.15.4 Payload Information Elements
   (IEs).  Those Payload IEs are encrypted and authenticated at the link
   layer through CCM* [CCM-Star] ("CCM" stands for "Cipher block
   Chaining -- Message authentication code").  6P benefits from the same
   level of security as any other Payload IE.  6P does not define its
   own security mechanisms.  In particular, although a key management
   solution is out of scope for this document, 6P will benefit from the
   key management solution used in the network.  This is relevant, as
   security attacks such as forgery and misattribution attacks become
   more damaging when a single key is shared amongst a group of more
   than two participants.

   6P does not provide protection against DoS attacks.  Example attacks
   include not sending confirmation messages in 3-step transactions and
   sending incorrectly formatted requests.  These cases SHOULD be
   handled by an appropriate policy, such as rate-limiting or
   time-limited blacklisting of the attacker after several attempts.
   The effect on the overall network is mostly localized to the two
   nodes in question, as communication happens in dedicated cells.


















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6.  IANA Considerations

6.1.  IETF IE Subtype 6P

   This document adds the following number to the "IEEE Std 802.15.4
   IETF IE Subtype IDs" registry defined by [RFC8137]:

                    +--------+------------+-----------+
                    | Value  | Subtype ID | Reference |
                    +--------+------------+-----------+
                    |   1    | SUBID_6TOP | RFC 8480  |
                    +---------------------+-----------+

                   Figure 34: IETF IE Subtype SUBID_6TOP

6.2.  6TiSCH Parameters Subregistries

   This section defines subregistries within the "IPv6 Over the TSCH
   Mode of IEEE 802.15.4e (6TiSCH)" parameters registry, hereafter
   referred to as the "6TiSCH parameters" registry.  Each subregistry is
   described in a subsection.

6.2.1.  6P Version Numbers

   The name of the subregistry is "6P Version Numbers".

   The following note is included in this registry: "In the 6top
   Protocol (6P) [RFC8480], there is a field to identify the version of
   the protocol.  This field is 4 bits in size."

   Each entry in the subregistry must include the version in the
   range 0-15 and a reference to the 6P version's documentation.

   The initial entry in this subregistry is as follows:

                          +---------+-----------+
                          | Version | Reference |
                          +---------+-----------+
                          |       0 | RFC 8480  |
                          +---------+-----------+

                    Figure 35: 6P Version Number Entry

   All other version numbers are Unassigned.

   The IANA policy for future additions to this subregistry is "IETF
   Review" or "IESG Approval" as described in [RFC8126].




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6.2.2.  6P Message Types

   The name of the subregistry is "6P Message Types".

   The following note is included in this registry: "In version 0 of the
   6top Protocol (6P) [RFC8480], there is a field to identify the type
   of message.  This field is 2 bits in size."

   Each entry in the subregistry must include the message type in the
   range b00-b11, the corresponding name, and a reference to the 6P
   message type's documentation.

   Initial entries in this subregistry are as follows:

                   +------+--------------+-----------+
                   | Type | Name         | Reference |
                   +------+--------------+-----------+
                   | b00  | REQUEST      | RFC 8480  |
                   | b01  | RESPONSE     | RFC 8480  |
                   | b10  | CONFIRMATION | RFC 8480  |
                   +------+--------------+-----------+

                        Figure 36: 6P Message Types

   All other message types are Unassigned.

   The IANA policy for future additions to this subregistry is "IETF
   Review" or "IESG Approval" as described in [RFC8126].

6.2.3.  6P Command Identifiers

   The name of the subregistry is "6P Command Identifiers".

   The following note is included in this registry: "In version 0 of the
   6top Protocol (6P) [RFC8480], there is a Code field that is 8 bits in
   size.  In a 6P Request, the value of this Code field is used to
   identify the command."

   Each entry in the subregistry must include an identifier in the
   range 0-255, the corresponding name, and a reference to the 6P
   command identifier's documentation.










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   Initial entries in this subregistry are as follows:

                  +------------+------------+-----------+
                  | Identifier | Name       | Reference |
                  +------------+------------+-----------+
                  |          0 | Reserved   | RFC 8480  |
                  |          1 | ADD        | RFC 8480  |
                  |          2 | DELETE     | RFC 8480  |
                  |          3 | RELOCATE   | RFC 8480  |
                  |          4 | COUNT      | RFC 8480  |
                  |          5 | LIST       | RFC 8480  |
                  |          6 | SIGNAL     | RFC 8480  |
                  |          7 | CLEAR      | RFC 8480  |
                  |      8-254 | Unassigned |           |
                  |        255 | Reserved   | RFC 8480  |
                  +------------+------------+-----------+

                     Figure 37: 6P Command Identifiers

   The IANA policy for future additions to this subregistry is "IETF
   Review" or "IESG Approval" as described in [RFC8126].

6.2.4.  6P Return Codes

   The name of the subregistry is "6P Return Codes".

   The following note is included in this registry: "In version 0 of the
   6top Protocol (6P) [RFC8480], there is a Code field that is 8 bits in
   size.  In a 6P Response or 6P Confirmation, the value of this Code
   field is used to identify the return code."

   Each entry in the subregistry must include a return code in the
   range 0-255, the corresponding name, the corresponding description,
   and a reference to the 6P return code's documentation.  If the return
   code corresponds to a Response error, the "Is Error?" entry must
   indicate "Yes".  Otherwise, "No" must be used.















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   Initial entries in this subregistry are as follows:

     +------+-----------------+---------------------------+-----------+
     | Code | Name            | Description               | Is Error? |
     +------+-----------------+---------------------------+-----------+
     |    0 | RC_SUCCESS      | operation succeeded       |        No |
     |    1 | RC_EOL          | end of list               |        No |
     |    2 | RC_ERR          | generic error             |       Yes |
     |    3 | RC_RESET        | critical error, reset     |       Yes |
     |    4 | RC_ERR_VERSION  | unsupported 6P version    |       Yes |
     |    5 | RC_ERR_SFID     | unsupported SFID          |       Yes |
     |    6 | RC_ERR_SEQNUM   | schedule inconsistency    |       Yes |
     |    7 | RC_ERR_CELLLIST | cellList error            |       Yes |
     |    8 | RC_ERR_BUSY     | busy                      |       Yes |
     |    9 | RC_ERR_LOCKED   | cells are locked          |       Yes |
     +------+-----------------+---------------------------+-----------+

                        Figure 38: 6P Return Codes

   All other message types are Unassigned.

   The IANA policy for future additions to this subregistry is "IETF
   Review" or "IESG Approval" as described in [RFC8126].

6.2.5.  6P Scheduling Function Identifiers

   The name of the subregistry is "6P Scheduling Function Identifiers".

   The following note is included in this registry: "In version 0 of the
   6top Protocol (6P) [RFC8480], there is a field to identify the
   Scheduling Function to handle the message.  This field is 8 bits
   in size."

   Each entry in the subregistry must include an SFID in the
   range 0-255, the corresponding name, and a reference to the 6P
   Scheduling Function's documentation.

   There are currently no entries in this subregistry.

   +------+---------------------------------+--------------------------+
   | SFID | Name                            | Reference                |
   +------+---------------------------------+--------------------------+
   | 0-255| Unassigned                      |                          |
   +------+---------------------------------+--------------------------+

                   Figure 39: SF Identifier (SFID) Entry

   All message types are Unassigned.



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   The IANA policy for future additions to this subregistry depends on
   the value of the SFID, as shown in Figure 40.  These specifications
   must follow the guidelines of Section 4.

                +-----------+------------------------------+
                |     Range | Registration Procedures      |
                +-----------+------------------------------+
                |     0-127 | IETF Review or IESG Approval |
                |   128-255 | Expert Review                |
                +-----------+------------------------------+

          Figure 40: SF Identifier (SFID): Registration Procedure

6.2.6.  6P CellOptions Bitmap

   The name of the subregistry is "6P CellOptions Bitmap".

   The following note is included in this registry: "In version 0 of the
   6top Protocol (6P) [RFC8480], there is an optional CellOptions field
   that is 8 bits in size."

   Each entry in the subregistry must include a bit position in the
   range 0-7, the corresponding name, and a reference to the bit's
   documentation.

   Initial entries in this subregistry are as follows:

                    +-----+---------------+-----------+
                    | bit | Name          | Reference |
                    +-----+---------------+-----------+
                    |   0 | TX (Transmit) | RFC 8480  |
                    |   1 | RX (Receive)  | RFC 8480  |
                    |   2 | SHARED        | RFC 8480  |
                    | 3-7 | Reserved      |           |
                    +-----+---------------+-----------+

                     Figure 41: 6P CellOptions Bitmap

   All other message types are Unassigned.

   The IANA policy for future additions to this subregistry is "IETF
   Review" or "IESG Approval" as described in [RFC8126].









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7.  References

7.1.  Normative References

   [IEEE802154]
              IEEE, "IEEE Standard for Low-Rate Wireless Networks",
              IEEE 802.15.4, DOI 10.1109/IEEESTD.2016.7460875.

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

   [RFC8137]  Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
              Element for the IETF", RFC 8137, DOI 10.17487/RFC8137,
              May 2017, <https://www.rfc-editor.org/info/rfc8137>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,
              <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [CCM-Star] Struik, R., "Formal Specification of the CCM* Mode of
              Operation", IEEE P802.15-4/0537r2, September 2005.

   [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,
              <https://www.rfc-editor.org/info/rfc7554>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8180]  Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
              IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
              Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
              May 2017, <https://www.rfc-editor.org/info/rfc8180>.




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Appendix A.  Recommended Structure of an SF Specification

   The following section structure for an SF document is RECOMMENDED:

   o  Introduction

   o  RFC 2119 Requirements Language (if applicable)

   o  Scheduling Function Identifier

   o  Rules for Adding/Deleting Cells

   o  Rules for CellList

   o  6P Timeout Value

   o  Rule for Ordering Cells

   o  Meaning of the Metadata Field

   o  Node Behavior at Boot

   o  Schedule Inconsistency Handling

   o  6P Error Handling

   o  Examples

   o  Implementation Status

   o  Security Considerations

   o  IANA Considerations

   o  Normative References (if applicable)

   o  Informative References (if applicable)














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Authors' Addresses

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

   Email: wangqin@ies.ustb.edu.cn


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

   Email: xvilajosana@uoc.edu


   Thomas Watteyne
   Analog Devices
   32990 Alvarado-Niles Road, Suite 910
   Union City, CA  94587
   United States of America

   Email: thomas.watteyne@analog.com
























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