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

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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 detailed in [RFC8180], an Enhanced Beacon (EB) is transmitted during a slot designated as 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 Interface-ID                   |
+                        (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.¶

In most cases, every node sending a beacon will set this flag, and in a typical mesh, this will be every single node. When this bit is not set, it indicates that this node may be under provisioned, or may have no additional slots for additional nodes. This could make this node more interesting to an attacker.¶

P:

If the Proxy Address P-flag is set, then the Join Proxy Interface ID bit field is present. Otherwise, it is not provided.¶

This bit only indicates if another part of the structure is present, and has little security or privacy impact.¶

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.¶

Lower values in this field indicate that the transmitter may have more capacity to handle unencrypted traffic.¶

A higher value may indicate that the transmitter is low on neighbor cache entries, or other resources.¶

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 {?RFC6550} parent within a particular network ID.¶

Lower values indicate more willing, and higher values indicate less willing. This value is calculated by each 6LR according to algorithms specific to the routing metrics used by the RPL ({?RFC6550}).¶

The exact process is a subject of significant research work.¶

It will typically be calculated from the 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 to help enrolled devices only to compare different connection points.¶

An attacker can use this value to determine which nodes are potentially more interesting. Nodes which are less willing to be parents likely have more traffic, and an attacker could use this information to determine which nodes would be more interesting to attack or disrupt.¶

pan priority:

The pan priority is a value set by the Destination-Oriented Directed Acycling Graph (DODAG) root (see {?RFC6550}, typically, the 6LBR) to indicate the relative priority of this LLN compared to those with different PANIDs that the operator might control.¶

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 when resuming from a long sleep. 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. Lower values indicate a higher willingness to accept new nodes.¶

An attacker can use this value, along with the observed PANID in the Beacon to determine which PANIDs have more network resources, and may have more interesting traffic.¶

Join Proxy Interface ID:

If the P bit is set, then 64 bits (8 bytes) of address are present. This field provides the Interface ID (IID) of the Link-Local address of the Join Proxy. The associated prefix is well-known as fe80::/64. If this field is not present, then IID is derived from the layer-2 address of the sender as per SLAAC ({?RFC4662}).¶

This field communicates an Interface ID bits that should be used for this nodes' layer-3 address, if it should not be derived from the layer-2 address. Communication with the Join Proxy occurs in the clear, this field avoids the need for an additional service discovery process for the case where the L3 address is not derived from the L2 address. An attacker will see both L2 and L3 addresses, so this field provides no new information.¶

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 truncated SHA256 hash of the prefix (/64) of the network. This will be done by the RPL DODAG root and communicated by the RPL Configuration Option payloads, so it is not calculated more than once. That is just a suggestion for a default algorithm: it may be set in any convenience way that results in a non-identifing 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 so that pledges that attempt to enroll do not waste time attempting multiple times with the same network that has multiple attachment points.¶

If the network ID is derived as suggested, then it will an opaque, seemingly random value, and will reveal nothing in of itself. An attacker can match this value across many transmissions to map the extent of a network beyond what the PANID might already provide.¶

3. Security Considerations

All of the contents of this Information Element are transmitted in the clear. The content of the Enhanced Beacon is not encrypted. This is a restriction in the cryptographic architecture of the 802.15.4 mechanism. In order to decrypt or do integrity checking of layer-2 frames in TSCH, the TSCH Absolute Slot Number (ASN) is needed. The Enhanced Beacon provides the ASN to new (and long-sleeping) nodes.¶

The sensitivity of each field is describe within the description of each field.¶

The Enhanced Beacon 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, and will be forced to temporarily take the contents on faith. After enrollment, a newly enrolled node will be able to return to the beacon and validate it.¶

In addition to the enrollment and join information described in this document, the Enhanced Beacon contains a description of the TSCH schedule to be used by the transmitter of this packet. The schedule can provide an attacker with a list of channels and frequencies on which communication will occur. Knowledge of this can help an attacker to more efficiently jam communications, although there is future work being considered to make some of the schedule less visible. Encrypting the schedule does not prevent an attacker from jamming, but rather increases the energy cost of doing that jamming.¶

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 (which is usually derived from the LLN prefix) directly provides some privacy for the the addresses used within the network, as 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

IANA is asked to assign a new number TBD-XXX from Registry "IEEE Std 802.15.4 IETF IE Subtype IDs" as defined by [RFC8137].¶

This entry should be called 6tisch-Join-Info, and should refer to this document.¶

       Value   Subtype-ID          Reference
       ----    ----------          -----------
       TBD-XXX 6tisch-Join-Inbfo   [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