Internet DRAFT - draft-ietf-6tisch-enrollment-enhanced-beacon
draft-ietf-6tisch-enrollment-enhanced-beacon
6tisch Working Group D. Dujovne
Internet-Draft Universidad Diego Portales
Intended status: Standards Track M. Richardson
Expires: 24 August 2020 Sandelman Software Works
21 February 2020
IEEE 802.15.4 Information Element encapsulation of 6TiSCH Join and
Enrollment Information
draft-ietf-6tisch-enrollment-enhanced-beacon-14
Abstract
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.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on 24 August 2020.
Copyright Notice
Copyright (c) 2020 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
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Use of BCP 14 Terminology . . . . . . . . . . . . . . . . 2
1.2. Layer-2 Synchronization . . . . . . . . . . . . . . . . . 3
1.3. Layer-3 synchronization: IPv6 Router Solicitations and
Advertisements . . . . . . . . . . . . . . . . . . . . . 3
1.4. Layer-2 Selection . . . . . . . . . . . . . . . . . . . . 4
2. Protocol Definition . . . . . . . . . . . . . . . . . . . . . 5
3. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
[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.
1.1. 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 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
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Other terminology can be found in [I-D.ietf-6tisch-architecture] in
section 2.1.
1.2. Layer-2 Synchronization
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.
1.3. Layer-3 synchronization: IPv6 Router Solicitations and
Advertisements
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:
1. Use of a multicast slot by even a non-malicious unauthenticated
node for a Router Solicitation (RS) may overwhelm that time slot.
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2. It may require many seconds of on-time before a new pledge
receives a Router Advertisement (RA) that it can use.
3. A new pledge may have to receive many Enhanced Beacons (EB)
before it can pick an appropriate network and/or closest Join
Assistant to attach to. If it must remain in the receive state
for an RA as well as find the Enhanced Beacon (EB), then the
process may take dozens of seconds, even minutes for each
enrollment attempt that it needs to make.
1.4. Layer-2 Selection
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.
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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.
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 +
~ ~
+ +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+
Figure 1: IE subtype structure
res: reserved bits MUST be ignored upon receipt, and SHOULD be set
to 0 when sending.
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R: The Router Advertisement 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 hosts MUST send a unicast Router Solicitation message in
order to receive a RA with the Prefix Information Option.
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 might indicate 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 from 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. Ongoing work such as
[I-D.ietf-roll-enrollment-priority] documents one way to set this
field.
rank priority: The rank "priority" is set by the IPv6 LLN Router
(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 willingness,
and higher values indicate less willingness. 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. Pledges MUST ignore this value.
It helps enrolled devices only to compare connection points.
An attacker can use this value to determine which nodes are
potentially more interesting. Nodes which are less willingness to
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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 Acyclic 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 the Interface ID bits that should be used
for this node's 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. This is just a suggestion for a default algorithm: it may
be set in any convenience way that results in a non-identifing
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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 PANs MUST be
configured. Pledges that see the same networkID will not waste
time attempting to enroll multiple times with the same network
that 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 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 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.
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4. Privacy Considerations
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.
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.
7. References
7.1. Normative References
[BCP14] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[I-D.ietf-6tisch-minimal-security]
Vucinic, M., Simon, J., Pister, K., and M. Richardson,
"Constrained Join Protocol (CoJP) for 6TiSCH", Work in
Progress, Internet-Draft, draft-ietf-6tisch-minimal-
security-15, 10 December 2019, <http://www.ietf.org/
internet-drafts/draft-ietf-6tisch-minimal-security-
15.txt>.
[ieee802154]
IEEE standard for Information Technology, ., "IEEE Std.
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802.15.4, Part. 15.4: Wireless Medium Access Control (MAC)
and Physical Layer (PHY) Specifications for Low-Rate
Wireless Personal Area Networks", 2015,
<http://standards.ieee.org/findstds/
standard/802.15.4-2015.html>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC6775, November 2012,
<https://www.rfc-editor.org/info/rfc6775>.
[RFC8137] Kivinen, T. and P. Kinney, "IEEE 802.15.4 Information
Element for the IETF", RFC 8137, DOI 10.17487/RFC8137, May
2017, <https://www.rfc-editor.org/info/rfc8137>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
7.2. Informative References
[I-D.ietf-6lo-backbone-router]
Thubert, P., Perkins, C., and E. Levy-Abegnoli, "IPv6
Backbone Router", Work in Progress, Internet-Draft, draft-
ietf-6lo-backbone-router-17, 20 February 2020,
<http://www.ietf.org/internet-drafts/draft-ietf-6lo-
backbone-router-17.txt>.
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", Work in Progress, Internet-Draft,
draft-ietf-6tisch-architecture-28, 29 October 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-6tisch-
architecture-28.txt>.
[I-D.ietf-roll-enrollment-priority]
Richardson, M., "Enabling secure network enrollment in RPL
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networks", Work in Progress, Internet-Draft, draft-ietf-
roll-enrollment-priority-00, 16 September 2019,
<http://www.ietf.org/internet-drafts/draft-ietf-roll-
enrollment-priority-00.txt>.
[RFC4662] Roach, A. B., Campbell, B., and J. Rosenberg, "A Session
Initiation Protocol (SIP) Event Notification Extension for
Resource Lists", RFC 4662, DOI 10.17487/RFC4662, August
2006, <https://www.rfc-editor.org/info/rfc4662>.
[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/info/rfc7554>.
[RFC8180] Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
May 2017, <https://www.rfc-editor.org/info/rfc8180>.
Authors' Addresses
Diego Dujovne (editor)
Universidad Diego Portales
Escuela de Informatica y Telecomunicaciones, Av. Ejercito 441
Santiago, Region Metropolitana
Chile
Phone: +56 (2) 676-8121
Email: diego.dujovne@mail.udp.cl
Michael Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
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