| 6TiSCH | Q. Wang, Ed. |
| Internet-Draft | Univ. of Sci. and Tech. Beijing |
| Intended status: Standards Track | X. Vilajosana |
| Expires: October 29, 2016 | Universitat Oberta de Catalunya |
| April 27, 2016 |
6top Protocol (6P)
draft-ietf-6tisch-6top-protocol-00
This document defines the 6top Protocol (6P), which enables distributed scheduling in 6TiSCH networks. 6P allows neighbor nodes in a 6TiSCH network to add/delete TSCH cells to one another. 6P is part of the 6TiSCH Operation Sublayer (6top), the next higher layer of the IEEE802.15.4 TSCH medium access control layer. The 6top Scheduling Function (SF) decides when to add/delete cells, and triggers 6P transactions. Several SFs can be defined, each identified by a different 6top Scheduling Function Identifier (SFID). This document lists the requirements for an SF, but leaves the definition of the SF out of scope. Different SFs are expected to be defined in future companion specifications.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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This document is an Internet Draft, so work-in-progress by nature. It contains the following work-in-progress elements:
All communication in a 6TiSCH network is orchestrated by a schedule [RFC7554]. This specification defines the 6top Protocol (6P), part of the 6TiSCH Operation Sublayer (6top) sublayer. 6P allow a node to communicate with a neighbor to add/remove a TSCH cell to one another. 6P hence enables distributed scheduling in a 6TiSCH network.
(A)
/ \
/ \
(B)-----(C)
| |
| |
(D) (E)
Figure 1: A simple 6TiSCH network.
For example, node C in Figure 1 monitors the communication cells to node A it has in its schedule.
This results in distributed schedule management in a 6TiSCH network.
The 6top Scheduling Function (SF) defines when to add/delete a cell to a neighbor. The SF functions as a (required) add-on to 6P. Different applications require different SFs, so the SF is left out of scope of this document. Different SFs are expected to be defined in future companion specifications. A node MAY implement multiple SFs and run them at the same time. The SFID field contained in all 6P messages allows a node to switch between SFs on a per-transaction basis.
Section 3 describes the 6TiSCH Operation Sublayer (6top). Section 4 defines the 6top Protocol (6P). Section 5 provides guidelines on how to design an SF.
As depicted in Figure 2, the 6TiSCH Operation Sublayer (6top) is the next higher layer to the IEEE802.15.4 TSCH medium access control layer [IEEE802154-2015].
.
| . |
| next higher layer |
+------------------------------------------+
| 6top |
+------------------------------------------+
| IEEE802.15.4 TSCH |
| . |
.
Figure 2: The 6top sublayer in the protocol stack.
The roles of the 6top sublayer are:
6top qualifies each cell in the schedule as either "hard" or "soft":
In the context of this specification, all the cells used by 6top are Soft Cells. Hard cells can be used for example when "hard-coding" a scheduling. This is done, for example, in the Minimal 6TiSCH Configuration [I-D.ietf-6tisch-minimal].
6top MAY be used alongside the Minimal 6TiSCH Configuration [I-D.ietf-6tisch-minimal]. In this case, it is RECOMMENDED to use 2 slotframes, as depicted in Figure 3:
Slotframe 0 SHOULD be of higher priority than Slotframe 1. 6top MAY support further slotframes; how to use more slotframes is out of the scope for this document.
| 0 1 2 3 4 | 0 1 2 3 4 |
+------------------------+------------------------+
Slotframe 0 | | | | | | | | | | |
5 slots long | EB | | | | | EB | | | | |
high priority | | | | | | | | | | |
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 |
+-------------------------------------------------+
Slotframe 1 | | | | | | | | | | |
10 slots long | |A->B| | | | | | |B->A| |
low priority | | | | | | | | | | |
+-------------------------------------------------+
Figure 3: 2-slotframe structure when using 6top alongside the Minimal 6TiSCH Configuration.
The 6top Protocol (6P) allows two neighbor nodes to communicate to add/delete cells to their TSCH schedule. Conceptually, two neighbor nodes "negotiate" the location of the cell(s) to add/delete.
We call "6top Transaction" a complete negotiation between two neighbor nodes. A transaction starts when a node wishes to add/remove one or more cells to one of its neighbors; it ends when the cell(s) have been added removed from the schedule of both neighbor, or when the transaction has failed.
A transaction can consist of 2 or 3 steps. It is the SF which determines whether to use 2-step or 3-step transactions. An SF MAY use both 2-step and 3-step transactions.
We reuse the topology in Figure 1 to illustrate 2-step and 3-step transactions.
6P supports both 2- and 3-step transactions; the SF determinisms which to use. Without loss of generality, this section illustrates 2-step transaction through an example.
Figure 4 is a sequence diagram to help understand the core principle of 6P (several elements are left out to simplify understanding). We assume the SF running on node A determines 2 extra cells need to be scheduled to node B. In this example, node A proposes the cells to use.
+----------+ +----------+
| Node A | | Node B |
+----+-----+ +-----+----+
| |
| 6P ADD Request |
| NumCells = 2 |
| CellList = [(1,2),(2,2),(3,5)] |
|-------------------------------------->|
| |
| 6P Response |
| Return Code = IANA_6TOP_RC_SUCCESS |
| CellList = [(2,2),(3,5)] |
|<--------------------------------------|
| |
Figure 4: A 2-step 6P transaction.
In this example, the 2-step transaction occurs as follows:
6P supports both 2- and 3-step transactions; the SF determinisms which to use. Without loss of generality, this section illustrates 3-step transaction through an example.
Figure 5 is a sequence diagram to help understand the core principle of 6P (several elements are left out to simplify understanding). We assume the SF running on node A determines 2 extra cells need to be scheduled to node B. In this example, node B proposes the cells to use.
+----------+ +----------+
| Node A | | Node B |
+----+-----+ +-----+----+
| |
| 6P ADD Request |
| NumCells = 2 |
| CellList = [] |
|-------------------------------------->|
| |
| 6P Response |
| Return Code = IANA_6TOP_RC_SUCCESS |
| CellList = [(1,2),(2,2),(3,5)] |
|<--------------------------------------|
| |
| 6P Confirmation |
| Return Code = IANA_6TOP_RC_SUCCESS |
| CellList = [(2,2),(3,5)] |
|-------------------------------------->|
| |
Figure 5: A 3-step 6P transaction.
In this example, the 3-step transaction occurs as follows:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload IE Length |GroupID|T| Sub-ID |6top IE Content
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Termination IE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
6P messages are carried as payload of IEEE802.15.4 Information Elements (IE) [IEEE802154-2015]. 6p messages travel over a single hop.
The 6top IE is an IETF IE with GroupID IANA_IETF_IE_GROUP_ID. The Sub-ID used by the 6top IE is IANA_6TOP_SUBIE_ID. The length of the 6top IE content is variable. The content of the 6top IE is specified in Section 4.2. The Payload Termination IE is defined by the IEEE802.15.4 standard [IEEE802154-2015]. TODO: IETF IE specified in Appendix A for now, but to be specified in a separate draft in the future.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Code | SFID | SeqNum | Other Fields...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
In all 6P messages, the 6top IE content has the following format:
Value Command ID Description +----------------------+--------------+---------------------------+ | IANA_6TOP_CMD_ADD | CMD_ADD | add one or more cells | +----------------------+------------------------------------------+ | IANA_6TOP_CMD_DELETE | CMD_DELETE | delete one or more cells | +----------------------+------------------------------------------+ | IANA_6TOP_CMD_COUNT | CMD_COUNT | count scheduled cells | +----------------------+------------------------------------------+ | IANA_6TOP_CMD_LIST | CMD_LIST | list the scheduled cells | +----------------------+------------------------------------------+ | IANA_6TOP_CMD_CLEAR | CMD_CLEAR | clear all cells | +----------------------+------------------------------------------+ | TODO-0xf | reserved | +----------------------+------------------------------------------+
Figure 6: 6P Command Identifiers
Figure 6 lists the 6P command identifiers.
Value Return Code Description +-----------------------+----------------------------------------+ | IANA_6TOP_RC_SUCCESS | RC_SUCCESS | operation succeeded | +-----------------------+----------------------------------------+ | IANA_6TOP_RC_VER_ERR | RC_VER_ERR | unsupported 6P version | +-----------------------+----------------------------------------+ | IANA_6TOP_RC_SFID_ERR | RC_SFID_ERR | unsupported SFID | +-----------------------+----------------------------------------+ | IANA_6TOP_RC_BUSY | RC_BUSY | handling previous request| +-----------------------+----------------------------------------+ | IANA_6TOP_RC_RESET | RC_RESET | abort 6P transaction | +-----------------------+----------------------------------------+ | IANA_6TOP_RC_ERR | RC_ERR | operation failed | +-----------------------+----------------------------------------+ | TODO-0xf | reserved | +-----------------------+----------------------------------------+
Figure 7: 6P Return Codes
Figure 7 lists the 6P Return Codes and their meaning.
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 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 6P Cell is an element which is present in several messages. It is a 4-byte field, its RECOMMENDED format is:
The CellList is an opaque set of bytes, sent unmodified to the SF. The SF MAY redefine the format of the CellList field.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Code | SFID | SeqNum | NumCells |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Metadata | CellList ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
The 6P DELETE Request has the exact same format as the 6P ADD Request, except for the code which is set to IANA_6TOP_CMD_DELETE.
1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Code | SFID | SeqNum | Metadata
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Metadata |
+-+-+-+-+-+-+-+-+
The 6P LIST Request has the exact same format as the 6P COUNT Request, except for the code which is set to IANA_6TOP_CMD_LIST.
The 6P CLEAR Request has the exact same format as the 6P COUNT Request, except for the code which is set to IANA_6TOP_CMD_CLEAR.
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Version| Code | SFID | SeqNum | Other Fields...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
A 6P Confirmation is only used in a 3-step transaction, as the third step. A 6P Confirmation Message has the exact same format as a 6P Response Message. It is only the fact that it appears as the third step in a 3-step transaction that distinguishes it from a 6P Response. In particular, the same Return Codes are used in both 6P Response and 6P Confirmation messages.
For illustration, we assume we use the topology in Figure 1, and that node A negotiates to add/delete cells to node B.
All messages contain a Version field. If multiple Versions of the 6P protocol have been defined (in future specifications for Version values different than IANA_6TOP_6P_VERSION), a node MAY implement multiple protocol versions at the same time. When receiving a 6P message with a Version number it does not implement, a node MUST reply with a 6P Response and a return code of IANA_6TOP_RC_VER_ERR. The Version field in the 6P Response MUST be the same as the Version field in the corresponding 6P Request.
All messages contain a SFID field. If multiple SFs has been defined, a node MAY support multiple SFs at the same time. When receiving a 6P message with an unsupported SFID, a node MUST reply with a 6P Response and a return code of IANA_6TOP_RC_SFID_ERR. The Version field in the 6P Response MUST be the same as the Version field in the corresponding 6P Request.
Only a single 6P Transaction between two neighbors, in a given direction, can take place at the same time. That is, a node MUST NOT issue a new 6P Request to a given neighbor before having received the 6P Response for a previous request to that neighbor. The only exception to this rule is when the previous 6P Transaction has timed out. If a node receives a 6P Request from a given neighbor before having sent the 6P Response to the previous 6P Request from that neighbor, it MUST send back a 6P Response with a return code of IANA_6TOP_RC_ERR.
A node MAY support concurrent 6P Transactions from different neighbors. In this case, in Figure 1, node C can have a different ongoing 6P Transaction with nodes B and E. In case a node does not have enough resources to handle concurrent 6P Transactions from different neighbors, when it receives a 6P Request from a neighbor while already handling a different request from a different neighbor, it MUST reply to that second request with a 6P Response with return code IANA_6TOP_RC_BUSY.
A timeout happens when the node sending the 6P Request has not received the 6P Response. The value of the timeout is coupled with how the cells between the nodes are scheduled. The SF determines the value of the timeout. The value of the timeout is out of scope of this document.
When a node receives a 6P Response with SeqNum value different from the SeqNum value in the 6P Request, it MUST drop the packet and consider the 6P Transaction as having failed.
We assume the topology in Figure 1 where the SF on node C decides to add NumCell cells to node A.
Node C's SF selects NumCandidate>=NumCell cells from its schedule as candidate transmit cells to node A. NumCandidate MUST be larger or equal to NumCell. How many cells it selects (NumCandidate) and how that selection is done is specified in the SF and out of scope of this document. Node C sends a 6P ADD Request to node A which contains the value of NumCells and the NumCandidate cells in the CellList.
Upon receiving the request, node A's SF verifies which of the cells in the CellList it can add as receive cells from node C in its own schedule. How that selection is done is specified in the SF and out of scope of this document. That verification can succeed (NumCell cells from the CellList can be used), fail (none of the cells from the CellList can be used) or partially succeed (less than NumCell cells from the CellList can be used). In all cases, node A MUST send a 6P Response with return code set to IANA_6TOP_RC_SUCCESS, and which specifies the list of cells that were scheduled as receive cells from C. That can contain 0 elements (when the verification failed), NumCell elements (succeeded) or between 0 and NumCell elements (partially succeeded).
Upon receiving the response, node C adds the cells specified in the CellList as transmit cells to node A.
In case the receiver of a 6top request fails during a 6P Transaction and is unable to complete it, it SHOULD reply to that request with a 6P Response with return code IANA_6TOP_RC_RESET. Upon receiving this 6top reply, the initiator of the 6P Transaction MUST consider the 6P Transaction as failed.
The behavior for deleting cells is equivalent to that of adding cells except that:
A return code with a name starting with "RC_ERR" in Figure 7 indicates an error. When a node receives a 6P Response with such an error, it MUST consider the 6P Transaction failed. In particular, if this was a response to a 6P ADD/DELETE Request, the node MUST NOT add/delete any of the cells involved in this 6P Transaction. Similarly, a node sending a 6P Response with an "RC_ERR" return code MUST NOT add/delete any cells as part of that 6P Transaction. The SF defines what to do after an error has occurred. Defining what to do after an error has occurred is out of scope of this document.
6P messages are secured through link-layer security. When link-layer security is enabled, the 6P messages MUST be secured. This is possible because 6P messages are carried as Payload IE.
Range Meaning
+-----------+-------------+
| 0x00-0xef | managed |
+-----------+--------------
| 0xf0-0xfe | unmanaged |
+-----------+-------------+
| 0xff | reserved |
+-----------+-------------+
Figure 8: SFID range.
Each SF has an identifier. The identifier is encoded as a 1-byte field. The identifier space is divided in the following ranges.
The specification for an SF
The following section structure for a SF document is RECOMMENDED:
This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in [RFC6982]. The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist.
According to [RFC6982], "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".
TODO: explicit risks
6P messages are carried inside IEEE802.15.4 Payload Information Elements (IEs). Those Payload IEs are encrypted and authenticated at the link layer through CCM*. 6P benefits from the same level of security as any other Payload IE. The 6P protocol does not define its own security mechanisms. A key management solution is out of scope for this document. The 6P protocol will benefit for the key management solution used in the network.
TODO: write out this section as soon as the discussion with the IEEE about a possible IETF IE ID has concluded.
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [IEEE802154-2015] | IEEE standard for Information Technology, "IEEE Std 802.15.4-2015 - IEEE Standard for Low-Rate Wireless Personal Area Networks (WPANs)", October 2015. |
| [RFC7554] | Watteyne, T., 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. |
| [RFC6982] | Sheffer, Y. and A. Farrel, "Improving Awareness of Running Code: The Implementation Status Section", RFC 6982, DOI 10.17487/RFC6982, July 2013. |
| [I-D.ietf-6tisch-minimal] | Vilajosana, X. and K. Pister, "Minimal 6TiSCH Configuration", Internet-Draft draft-ietf-6tisch-minimal-15, February 2016. |
| [I-D.ietf-6tisch-terminology] | Palattella, M., Thubert, P., Watteyne, T. and Q. Wang, "Terminology in IPv6 over the TSCH mode of IEEE 802.15.4e", Internet-Draft draft-ietf-6tisch-terminology-07, March 2016. |
| [OpenWSN] | Watteyne, T., Vilajosana, X., Kerkez, B., Chraim, F., Weekly, K., Wang, Q., Glaser, S. and K. Pister, "OpenWSN: a Standards-Based Low-Power Wireless Development Environment", Transactions on Emerging Telecommunications Technologies , August 2012. |
This section contains a proposal for the specification of an IETF IE. If this proposal is supported by the 6TiSCH WG, the authors of this draft recommend for the specification of the IETF IE to be its own draft, possibly developed in the 6TiSCH WG. The reason for having it a separated document is that the scope of the IETF IE is wider that the 6P protocol defined in this document.
The proposal is to use an IETF IE, a IEEE802.15.4 Payload Information Element with the Group ID set to IANA_IETF_IE_GROUP_ID. The value of IANA_IETF_IE_GROUP_ID is defined by the IEEE, communicated to the IETF, and noted by IANA. The format of the IETF IE is exactly the same as the format of an MLME Information Element, as specified in [IEEE802154-2015], Section 5.2.4.5. The difference is that the space of Sub-IDs is managed by the IETF/IANA. The Sub-ID used by 6top commands is IANA_6TOP_SUBIE_ID with value 0x00.
Other options are being discussed between the IETF 6TiSCH WG and the IEEE 6TiSCH IG, and listed in https://www.ietf.org/mail-archive/web/6tisch/current/msg04469.html. These options concern the way 6P Messages are transported as IEEE802.15.4 IEs, and do not impact the format of those messages.
If the specification described in this document is supported by the 6TiSCH WG, the authors of this document ask the 6TiSCH WG chairs to liaise with the IEEE to request a Payload Information Element Group ID to be assigned to the IETF (Group ID IANA_IETF_IE_GROUP_ID described in Appendix A).
Terms introduced by this document, and which needs to be added to [I-D.ietf-6tisch-terminology]: