6lo Working Group D. Dujovne
Internet-Draft Universidad Diego Portales
Intended status: Standards Track M. Richardson
Expires: March 19, 2020 Sandelman Software Works
September 16, 2019

IEEE802.15.4 Informational Element encapsulation of 6tisch Join and Enrollment Information


In TSCH mode of IEEE STD 802.15.4 opportunities for broadcasts are limited to specific times and specific channels. Nodes in a TSCH network typically frequently send Enhanced Beacon (EB) frames to announce the presence of the network. This document provides a mechanism by which small details critical for new nodes (pledges) and long sleeping nodes may be carried within the Enhanced Beacon.

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Table of Contents

1. Introduction

[RFC7554] describes the use of the time-slotted channel hopping (TSCH) mode of [ieee802154]. As further details in [RFC8180], an Enhanced Beacon is transmitted during a slot designated a broadcast slot.

1.1. Use of BCP 14 Terminology

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 [BCP14] [RFC2119] when, and only when, they appear in all capitals, as shown here.

Other terminology can be found in [I-D.ietf-6tisch-architecture] in section 2.1.

1.2. Layer-2 Synchronization

As explained in section 6 of [RFC8180], the Enhanced Beacon has a number of purposes: synchronization of ASN and Join Metric, timeslot template identifier, the channel hopping sequence identifier, TSCH SlotFrame and Link IE.

The Enhanced Beacon (EB) is used by nodes already part of a TSCH network to annouce its existance. Receiving an EB allows a Joining Node (pledge) to learn about the network and synchronize to it. The EB may also be used as a means for a node already part of the network to re-synchronize [RFC7554].

There are a limited number of timeslots designated as a broadcast slot by each router. These slots are rare, and with 10ms slots, with a slot-frame length of 100, there may be only 1 slot/s for the beacon.

1.3. Layer-3 synchronization IPv6 Router solicitations and advertisements

At layer 3, [RFC4861] defines a mechanism by which nodes learn about routers by listening for multicasted Router Advertisements (RA). If no RA is heard within a set time, then a Router Solicitation (RS) may be multicast, to which an RA will be received, usually unicast.

Although [RFC6775] reduces the amount of multicast necessary to do address resolution via Neighbor Solicitation messages, it still requires multicast of either RAs or RS. This is an expensive operation for two reasons: there are few multicast timeslots for unsolicited RAs; if a pledge node does not hear an RA, and decides to send a RS (consuming a broadcast aloha slot with unencrypted traffic), unicast RS may be sent in response.

This is a particularly acute issue for the join process for the following reasons:

  1. use of a multicast slot by even a non-malicious unauthenticated node for a Router Solicitation may overwhelm that time slot.
  2. it may require many seconds of on-time before a new pledge hears a Router Soliciation that it can use.
  3. a new pledge may listen to many Enhanced Beacons before it can pick an appropriate network and/or closest Join Assistant to attach to. If it must listen for a RS as well as find the Enhanced Beacon, then the process may take a very long time.

2. Protocol Definition

[RFC8137] creates a registry for new IETF IE subtypes. This document allocates a new subtype.

The new IE subtype structure is as follows. As explained in [RFC8137] the length of the Sub-Type Content can be calculated from the container, so no length information is necessary.

                     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
|   TBD-XXX     |R|P| res |  proxy prio |    rank priority      |
| pan priority  |                                               |
+---------------+                                               +
|                           Join Proxy lower-64                 |
+                        (present if P=1)                       +
|                                                               |
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               |                                               |
+-+-+-+-+-+-+-+-+                                               +
|                           network ID                          |
+                   variable length, up to 16 bytes             +
~                                                               ~
+                                                               +
|                                                               |
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               |

proxy priority
this field indicates the willingness to act as join proxy. Lower value indicates willing to act as a Join Proxy as described in [I-D.ietf-6tisch-minimal-security]. Values range 0 (most willing) to 0x7e (least willing). A priority of 0x7f indicates that the announcer should never be considered as a viable enrollment proxy. Only unenrolled pledges look at this value.
pan priority
the pan priority is a value set by the DODAG root to indicate the relative priority of this LLN compared to those with different PANIDs. This value may be used as part of the enrollment priority, but typically is used by devices which have already enrolled, and need to determine which PAN to pick. Unenrolled pledges MAY consider this value when selecting a PAN to join. Enrolled devices MAY consider this value when looking for an eligible parent device.
rank priority
the rank "priority" is set by the 6LR which sent the beacon and is an indication of how willing this 6LR is to serve as an RPL parent within a particular network ID. This is a local value to be determined in other work. It might be calculated from RPL rank, and it may include some modifications based upon current number of children, or number of neighbor cache entries available. This value MUST be ignored by pledges, it is for enrolled devices only.
the Router Advertisement R-flag is set if the sending node will act as a Router for host-only nodes that need addressing via unicast Router Solicitation messages.
if the Proxy Address P-flag is set, then the lower 64-bits of the Join Proxy's Link Layer address follows the network ID. If the Proxy Address bit is not set, then the Link Layer address of the Join Proxy is identical to the Layer-2 8-byte address used to originate this enhanced beacon. In either case, the layer-2 address of any IPv6 traffic to the originator of this beacon may use the layer-2 address which was used to originate the beacon.
join-proxy interface ID
if the P bit is set, then 64 bits (8 bytes) of address are present. This field provides the suffix of the Link-Local address of the Join Proxy. The associated prefix is well-known as fe80::/64.
network ID
this is an variable length field, up to 16-bytes in size that uniquely identifies this network, potentially among many networks that are operating in the same frequencies in overlapping physical space. The length of this field can be calculated as being whatever is left in the Information Element.

In a 6tisch network, where RPL [RFC6550] is used as the mesh routing protocol, the network ID can be constructed from a SHA256 hash of the prefix (/64) of the network. That is just a suggestion for a default value. In some LLNs where multiple PANIDs may lead to the same management device (the JRC), then a common value that is the same across all PANs MUST be configured.

3. Security Considerations

All of the contents of this Information Element are sent in the clear. The containing Enhanced Beacon is not encrypted.

The Enhanced Beagon is authenticated at the layer-2 level using 802.15.4 mechanisms using the network-wide keying material. Nodes which are enrolled will have the network-wide keying material and can validate the beacon.

Pledges which have not yet enrolled are unable to authenticate the beacons.

4. Privacy Considerations

The use of a network ID may reveal information about the network. The use of a SHA256 hash of the DODAGID, rather than using the DODAGID directly provides some cover the addresses used within the network. The DODAGID is usually the IPv6 address of the root of the RPL mesh.

An interloper with a radio sniffer would be able to use the network ID to map out the extend of the mesh network.

5. IANA Considerations

Allocate a new number TBD-XXX from Registry IETF IE Sub-type ID. This entry should be called 6tisch-Join-Info, and should refer to this document.

6. Acknowledgements

Thomas Watteyne provided extensive editorial comments on the document.

7. References

7.1. Normative References

[BCP14] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017.
[I-D.ietf-6tisch-minimal-security] Vucinic, M., Simon, J., Pister, K. and M. Richardson, "Minimal Security Framework for 6TiSCH", Internet-Draft draft-ietf-6tisch-minimal-security-12, July 2019.
[ieee802154] IEEE standard for Information Technology, ., "IEEE Std. 802.15.4, Part. 15.4: Wireless Medium Access Control (MAC) and Physical Layer (PHY) Specifications for Low-Rate Wireless Personal Area Networks", n.d..
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC4861] Narten, T., Nordmark, E., Simpson, W. and H. Soliman, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, DOI 10.17487/RFC4861, September 2007.
[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E. and C. Bormann, "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 6775, DOI 10.17487/RFC6775, November 2012.
[RFC8137] Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information Element for the IETF", RFC 8137, DOI 10.17487/RFC8137, May 2017.

7.2. Informative References

[I-D.ietf-6tisch-architecture] Thubert, P., "An Architecture for IPv6 over the TSCH mode of IEEE 802.15.4", Internet-Draft draft-ietf-6tisch-architecture-26, August 2019.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP. and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/RFC6550, March 2012.
[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.
[RFC8180] Vilajosana, X., 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.

Authors' Addresses

Diego Dujovne (editor) Universidad Diego Portales Escuela de Informatica y Telecomunicaciones, Av. Ejercito 441 Santiago, Region Metropolitana, Chile Phone: +56 (2) 676-8121 EMail: diego.dujovne@mail.udp.cl
Michael Richardson Sandelman Software Works EMail: mcr+ietf@sandelman.ca