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

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

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 BCP 14 when, and only when, they appear in all capitals, as shown here. Other terminology can be found in in section 2.1.

Layer-2 Synchronization As explained in section 6 of , the Enhanced Beacon (EB) has a number of purposes: synchronization of ASN and Join Metric, carrying timeslot template identifier, carrying the channel hopping sequence identifier, and indicating the TSCH SlotFrame. The EB is used by nodes already part of a TSCH network to annouce its existence. 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 . There is a limited number of timeslots designated as a broadcast slot by each router in the network. Considering 10ms slots and a slot-frame length of 100, these slots are rare and could result in only 1 slot/s for a broadcast, which needs to be used for the beacon. Additional broadcasts for Router Advertisements, or Neighbor Discovery could even more scarce.

Layer-3 synchronization: IPv6 Router Solicitations and Advertisements At layer 3, defines a mechanism by which nodes learn about routers by receiving multicast Router Advertisements (RA). If no RA is heard within a set time, then a Router Solicitation (RS) may be sent as multicast, to which an RA will be received, usually unicast. Although reduces the amount of multicast necessary to do address resolution via Neighbor Solicitation (NS) messages, it still requires multicast of either RAs or RS. This is an expensive operation for two reasons: First, there are few multicast timeslots for unsolicited RAs; and second, if a pledge node does not hear an RA, and decides to send a RS, a broadcast aloha slot is consumed with unencrypted traffic. In this case, a unicast RS may be sent in response. This is a particularly acute issue for the join process for the following reasons:

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

This document defines a new IETF IE subtype to provide join and enrollment information to prospective pledges in a more efficient way.

Protocol Definition 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 the length of the Sub-Type Content can be calculated from the container, so no length information is necessary.

IE subtype structure

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 . 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 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.

Security Considerations All of the contents of this Information Element are sent in the clear. The containing Enhanced Beacon is not encrypted. This is a restriction in the cryptographic architecture of the TSCH 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 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, providing them with a trusted 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.

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.

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

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.