IEEE 802.15.4 Information Element encapsulation of 6TiSCH Join and Enrollment Information

Status of This Memo

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

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

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

This Internet-Draft will expire on 19 July 2020.¶

Copyright Notice

Copyright (c) 2020 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

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

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             +
~                                                               ~
+                                                               +
|                                                               |
+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|               |
+-+-+-+-+-+-+-+-+

R

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.¶

P

if the Proxy Address P-flag is set, then the lower 64-bits of the Join Proxy's link-local 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 destination layer-2 address of this beacon may use the layer-2 address which was used to originate the beacon.¶

proxy priority (proxy prio)

this field indicates the willingness of the sender to act as join proxy. Lower value indicates greater willingness to act as a Join Proxy as described in [I-D.ietf-6tisch-minimal-security]. Values range 0x00 (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.¶

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.¶

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.¶

Join Proxy lower-64

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 a 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 extent of the mesh network.¶

5. IANA Considerations

Allocate a new number TBD-XXX from Registry IETF IE Sub-type ID, as defined by [RFC8137]. 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. Carles Gomez Montenegro generated a detailed review of the document at WGLC. Tim Evens provided a number of useful editorial suggestions.¶

Authors' Addresses