Internet-Draft | IE for ICMPv6 | February 2020 |
Dujovne & Richardson | Expires 24 August 2020 | [Page] |
In TSCH mode of IEEE STD 802.15.4, opportunities for broadcasts are limited to specific times and specific channels. Routers in a Time-Slotted Channel Hopping (TSCH) network transmit Enhanced Beacon (EB) frames to announce the presence of the network. This document provides a mechanism by which additional information critical for new nodes (pledges) and long sleeping nodes may be carried within the Enhanced Beacon in order to conserve use of broadcast opportunities.¶
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[RFC7554] describes the use of the Time-Slotted Channel Hopping (TSCH) mode of [ieee802154].¶
In TSCH mode of IEEE STD 802.15.4, opportunities for broadcasts are limited to specific times and specific channels. Routers in a Time-Slotted Channel Hopping (TSCH) network transmit Enhanced Beacon (EB) frames during broadcast slots in order to announce the time and channel schedule.¶
This document defines a new IETF Information Element (IE) subtype to place into the Enhanced Beacon (EB) to provide join and enrollment information to prospective pledges in a more efficient way.¶
The following sub-sections explain the problem being solved, which justify carrying the join and enrollement information in the EB.¶
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 the Absolute Slot Number (ASN) and Join Metric, carrying the timeslot template identifier, carrying the channel hopping sequence identifier, and indicating the TSCH SlotFrame.¶
An EB announces the existence of a TSCH network, and of the nodes already joined to that network. 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 are a limited number of timeslots designated as broadcast slots 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 per second for broadcasts, which needs to be used for the beacon. Additional broadcasts for Router Advertisements (RA), or Neighbor Discovery (ND) 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 received within a set time, then a Router Solicitation (RS) may be transmitted as a 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 RSes. This is an expensive operation for two reasons: there are few multicast timeslots for unsolicited RAs; and if a pledge node does not receive an RA, and decides to transmit an RS, a broadcast aloha slot (see [RFC7554] section A.5) is consumed with unencrypted traffic. [RFC6775] already allows for a unicast reply to such an RS.¶
This is a particularly acute issue for the join process for the following reasons:¶
In a complex Low-power and Lossy Networks (LLN), multiple LLNs may be connected together by backbone routers ( technology such as [I-D.ietf-6lo-backbone-router]), resulting in an area that is serviced by multiple distinct Layer-2 instances. These are called Personal Area Networks (PAN). Each instance will have a separate Layer-2 security profile, and will be distinguished by a different PANID. The PANID is part of the [ieee802154] layer-2 header: it is a 16-bit value which is chosen to be unique, and it contributes context to the layer-2 security mechanisms. The PANID provides a context similar to the ESSID does in 802.11 networking, and can be conceived of in a similar fashion as the 802.3 ethernet VLAN tag in that it provides context for all layer-2 addresses.¶
A device which is already enrolled in a network may find after a long sleep that it needs to resynchronize to the Layer 2 network. The enrollment keys that it has will be specific to a PANID, but it may have more than one set of keys. Such a device may wish to connect to a PAN that is experiencing less congestion, or which has a shalower ([RFC6550]) Routing Protocol for LLNs (RPL) tree. It may even observe PANs for which it does not have keys, but which is believes it may have credentials that would allow it to join.¶
In order to identify which PANs are part of the same backbone network, the network ID is introduced in this extension. PANs that are part of the same backbone will be configured to use the same network ID. For [RFC6550] RPL networks, configuration of the network ID can be done with an configuration option, which is the subject of future work.¶
In order to provide some input to the choice of which PAN to use, the PAN priority field has been added. This lists the relative priority for the PAN among different PANs. Every Enhanced Beacon from a given PAN will likely have the same PAN priority. Determination of the the PAN priority is the subject of future work; but it is expected that it will be calculated by an algorithm in the 6LBR, possibly involving communication between 6LBRs over the backbone network.¶
The [RFC6550] parent selection process can only operate within a single PAN, because it depends upon receiving RPL DIO messages from all available parents. As part of the PAN selection process, the device may wish to know how deep in the LLN mesh it will be if it joins a particular PAN, and the rank priority field provides an estimation of what the rank of each announcer is. Once the device synchronizes to a particular PAN's TSCH schedule then it may receive DIOs that are richer in their diversity than this value. How this value will be used in practice is the subject of future research, and the interpretation of this value of the structure is considered experimental.¶
[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 + ~ ~ + + | | + +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | +-+-+-+-+-+-+-+-+
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 described 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.¶
The use of a network ID may reveal information about the network. The use of a SHA256 hash of the DODAGID (see [RFC6550]), rather than using the DODAGID itself 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.¶
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]¶
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.¶