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 athttps://www.rfc-editor.org/info/rfc9032.¶

Copyright Notice

Copyright (c) 2021 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[IEEE.802.15.4].¶

In TSCH mode of IEEE Std 802.15.4, opportunities for broadcasts are limited tospecific times and specific channels.Routers in a TSCH networktransmit Enhanced Beacon (EB) frames during broadcast slots in order toannounce the time and channel schedule.¶

This document defines a new IETF Information Element (IE) subtype to placeinto the EB to provide join and enrollment information to prospectivepledges in a more efficient way.¶

The following subsections explain the problem being solved, whichjustifies carrying the join and enrollment information in the EB.¶

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 subtype content can be calculated from thecontainer, 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+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|       2       |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             +~                                                               ~+                                                               +|                                                               |+               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+|               |+-+-+-+-+-+-+-+-+

res:

Reserved bitsMUST be ignored upon receipt andSHOULD be set to 0 when sending.¶

R:

The RA R-flag is set if the sending node will act as a router for host-only nodes relying on stateless address auto-configuration (SLAAC) to get their global IPv6 address.Those hostsMUST send a unicast RS message in order to receive an RA with the Prefix Information Option.¶

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

P:

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

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

proxy prio (proxy priority):

This field indicates the willingness of the sender to act as a Join Proxy.Lower value indicates greater willingness to act as a Join Proxy as described in[RFC9031].Values range from 0x00 (most willing) to 0x7e (least willing).A priority of 0x7f indicates that the announcer should never be considered as a viable Join Proxy.Only unenrolled pledges look at this value.¶

Lower values in this field indicate that the transmitter may have morecapacity to handle unencrypted traffic.A higher value may indicate that the transmitter is low on neighbor cache entries or other resources.Ongoing work such as[NETWORK-ENROLLMENT] documents one way to set this field.¶

rank priority:

The rank priority is set by the IPv6 LLN Router (6LR) that sent the beacon and is anindication of how willing this 6LR is to serve as a RPL parent[RFC6550] within aparticular network ID.Lower values indicate more willingness, and higher values indicate less willingness.This value is calculated by each 6LR according to algorithms specific to therouting 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 modificationsbased upon current number of children or the number of neighbor cache entriesavailable.PledgesMUST ignore this value.It helps enrolled devices only to compare connection points.¶

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

PAN priority:

The PAN priority is a value set by the Destination-Oriented DirectedAcyclic Graph (DODAG) root (see[RFC6550], typically the 6LBR) to indicate the relativepriority of this LLN compared to those with different PANIDs that theoperator might control.This value may be used as part of the enrollment priority, but typically it is used by devicesthat have already enrolled and need to determine which PAN to pick whenresuming from a long sleep.Unenrolled pledgesMAY consider this value when selecting a PAN to join.Enrolled devicesMAY consider this value when looking for an eligible parentdevice.Lower values indicate more willingness to accept new nodes.¶

An attacker can use this value, along with the observed PANID in the EB to determine which PANIDs have more network resources, and may have moreinteresting 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 notpresent, then IID is derived from the Layer 2 address of the sender perSLAAC[RFC4862].¶

This field communicates the IID bits that should be used for this node'sLayer 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 Layer 3address is not derived from the Layer 2 address.An attacker will see both Layer 2 and Layer 3 addresses, so this field provides no new information.¶

network ID:

This is a variable length field, up to 16-bytes in size that uniquely identifiesthis network, potentially among many networks that are operating in the samefrequencies in overlapping physical space. The length of this field can becalculated as being whatever is left in the IE.¶

In a 6TiSCH network, where RPL[RFC6550] is used as the mesh routing protocol, thenetwork ID can be constructed from a truncated SHA-256 hash of the prefix (/64) of thenetwork. This will be done by the RPL DODAG root and communicated by the RPLConfiguration Option payloads, so it is not calculated more than once.This is just a suggestion for a default algorithm: it may be set in anyconvenient way that results in a non-identifying value.In some LLNs where multiple PANIDs may lead to the same management device(the Join Registrar/Coordinator (JRC)), then a common value that is the same across all the PANsMUST beconfigured.Pledges that see the same network ID will not waste timeattempting to enroll multiple times with the same network when the network has multiple attachment points.¶

If the network ID is derived as suggested, then it will be an opaque,seemingly random value and will not directly reveal any information about the network.An attacker can match this value across many transmissions to map the extentof a network beyond what the PANID might already provide.¶

3.Security Considerations

All of the contents of this IE are transmitted in the clear.The content of the EB 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, theTSCH ASN is needed.The EB provides the ASN to new (and long-sleeping) nodes.¶

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

The EB is authenticated at the Layer 2 level using 802.15.4mechanisms using the network-wide keying material. Nodes that are enrolledwill have the network-wide keying material and can validate the beacon.¶

Pledges that have not yet enrolled are unable to authenticate the beaconsand will be forced to temporarily take the contents on faith.After enrollment, a newly enrolled node will be able to return to the beacon andvalidate it.¶

In addition to the enrollment and join information described in thisdocument, the EB contains a description of the TSCH schedule tobe used by the transmitter of this packet.The schedule can provide an attacker with a list of channels and frequencieson which communication will occur.Knowledge of this can help an attacker to more efficiently jamcommunications, although there is future work being considered to make someof the schedule less visible.Encrypting the schedule does not prevent an attacker from jamming, but ratherincreases 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 SHA-256 hash of the DODAGID (see[RFC6550]), rather than using the DODAGID itselfdirectly provides some privacy for 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 mapout the extent of the mesh network.¶

5.IANA Considerations

IANA has assigned the following value in the "IEEE Std 802.15.4 IETF IE Subtype IDs" registry, as defined by[RFC8137].¶

Acknowledgments

Thomas Watteyne provided extensive editorial comments on the document.Carles Gomez Montenegro generated a detailed review of the document at Working Group Last Call.Tim Evens provided a number of useful editorial suggestions.¶

Authors' Addresses