Internet-Draft | IE for ICMPv6 | January 2020 |
Dujovne & Richardson | Expires 25 July 2020 | [Page] |
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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[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.¶
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 [RFC2119] [RFC8174] 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.¶
As explained in section 6 of [RFC8180], 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 [RFC7554].¶
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.¶
At layer 3, [RFC4861] 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 [RFC6775] 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:¶
This document defines a new IETF IE subtype to provide join and enrollment information to prospective pledges in a more efficient way.¶
[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.¶
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.¶
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. Encrypting the schedule does not prevent an attacker from jamming, but rather increases the energy cost of doing that jamming.¶
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.¶
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.¶
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.¶