Internet DRAFT - draft-ietf-lake-reqs
draft-ietf-lake-reqs
Network Working Group M. Vucinic
Internet-Draft Inria
Intended status: Informational G. Selander
Expires: 10 December 2020 J. Mattsson
Ericsson AB
D. Garcia
Odin Solutions S.L.
8 June 2020
Requirements for a Lightweight AKE for OSCORE
draft-ietf-lake-reqs-04
Abstract
This document compiles the requirements for a lightweight
authenticated key exchange protocol for OSCORE. This draft has
completed a working group last call (WGLC) in the LAKE working group.
Post-WGLC, the requirements are considered sufficiently stable for
the working group to proceed with its work. It is not currently
planned to publish this draft as an RFC.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 10 December 2020.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Problem description . . . . . . . . . . . . . . . . . . . . . 3
2.1. AKE for OSCORE . . . . . . . . . . . . . . . . . . . . . 3
2.2. Credentials . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Initial Focus . . . . . . . . . . . . . . . . . . . . 6
2.3. Mutual Authentication . . . . . . . . . . . . . . . . . . 7
2.4. Confidentiality . . . . . . . . . . . . . . . . . . . . . 8
2.5. Cryptographic Agility and Negotiation Integrity . . . . . 8
2.6. Cryptographic Strength . . . . . . . . . . . . . . . . . 9
2.7. Identity Protection . . . . . . . . . . . . . . . . . . . 9
2.8. Auxiliary Data . . . . . . . . . . . . . . . . . . . . . 10
2.9. Extensibility . . . . . . . . . . . . . . . . . . . . . . 11
2.10. Availability . . . . . . . . . . . . . . . . . . . . . . 11
2.11. Lightweight . . . . . . . . . . . . . . . . . . . . . . . 11
2.11.1. LoRaWAN . . . . . . . . . . . . . . . . . . . . . . 13
2.11.2. 6TiSCH . . . . . . . . . . . . . . . . . . . . . . . 15
2.11.3. NB-IoT . . . . . . . . . . . . . . . . . . . . . . . 16
2.11.4. Discussion and Summary of Benchmarks . . . . . . . . 18
2.11.5. AKE frequency . . . . . . . . . . . . . . . . . . . 20
3. Security Considerations . . . . . . . . . . . . . . . . . . . 21
4. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 21
Informative References . . . . . . . . . . . . . . . . . . . . . 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 25
1. Introduction
OSCORE [RFC8613] is a lightweight communication security protocol
providing end-to-end security on application layer for constrained
IoT settings (cf. [RFC7228]). OSCORE lacks a matching authenticated
key exchange protocol (AKE). The intention with the LAKE WG
[LAKE-WG] is to create a simple yet secure AKE for implementation in
embedded devices supporting OSCORE.
To ensure that the AKE is efficient for the expected applications of
OSCORE, we list the relevant public specifications of technologies
where OSCORE is included:
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* The IETF 6TiSCH WG charter identifies the need to "secur[e] the
join process and mak[e] that fit within the constraints of high
latency, low throughput and small frame sizes that characterize
IEEE802.15.4 TSCH". OSCORE protects the join protocol as
described in 6TiSCH Minimal Security
[I-D.ietf-6tisch-minimal-security].
* The IETF LPWAN WG charter identifies the need to improve the
transport capabilities of LPWA networks such as NB-IoT and LoRa
whose "common traits include ... frame sizes ... [on] the order of
tens of bytes transmitted a few times per day at ultra-low
speeds". The application of OSCORE is described in
[I-D.ietf-lpwan-coap-static-context-hc].
* OMA Specworks LwM2M version 1.1 [LwM2M] defines bindings to two
challenging radio technologies where OSCORE is planned to be
deployed: LoRaWAN and NB-IoT.
* Open Connectivity Foundation (OCF) plans to use OSCORE for end-to-
end security of unicast messages [OCF].
This document compiles the requirements for the AKE for OSCORE. It
summarizes the security requirements that are expected from such an
AKE, as well as the main characteristics of the environments where
the solution is envisioned to be deployed. The solution will
presumably be useful in other scenarios as well since a low security
overhead improves the overall performance.
2. Problem description
2.1. AKE for OSCORE
The rationale for designing this protocol is that OSCORE is lacking a
matching AKE. OSCORE was designed for lightweight RESTful operations
for example by minimizing the overhead, and applying the protection
to the application layer, thereby limiting the data being encrypted
and integrity protected for the other endpoint. Moreover, OSCORE was
tailored for use with lightweight primitives that are likely to be
implemented in the device, specifically CoAP [RFC7252], CBOR
[RFC7049] and COSE [RFC8152]. The same properties should apply to
the AKE.
In order to be suitable for OSCORE, at the end of the AKE protocol
run the two parties must agree on (see Section 3.2 of [RFC8613]):
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* A shared secret (OSCORE Master Secret) with Perfect Forward
Secrecy (PFS, see Section 2.4) and a good amount of randomness.
(The term "good amount of randomness" is borrowed from [HKDF] to
signify not necessarily uniformly distributed randomness.)
* OSCORE Sender IDs of peer endpoints, arbitrarily short.
- Sender IDs are expected to be unique for a given Master Secret,
more precisely the quartet (Master Secret, Master Salt, ID
Context, Sender ID) must be unique, see Section 3.3. of
[RFC8613].
* COSE algorithms to use with OSCORE
COSE provides the crypto primitives for OSCORE. The AKE shall
specify how it provides COSE algorithms to OSCORE. It is strongly
recommended that COSE is reused by the AKE, for identification of
credentials and algorithms, as extension point for new schemes, and
to avoid duplicated implementation of crypto wrapper.
The AKE cannot rely on messages being exchanged in both directions
after the AKE has completed, because CoAP/OSCORE requests may not
have a response [RFC7967]. Furthermore, there is no assumption of
dependence between CoAP client/server and AKE initiator/responder
roles, and an OSCORE context may be used with CoAP client and server
roles interchanged as is done, for example, in [LwM2M].
Moreover, the AKE must support transport over CoAP. When transported
over CoAP, the AKE must support the traversal of CoAP intermediaries,
as required by the 6TiSCH network formation setting
[I-D.ietf-6tisch-minimal-security].
Since the AKE messages most commonly will be encapsulated in CoAP,
the AKE must not duplicate functionality provided by CoAP, or at
least not duplicate functionality in such a way that it adds non-
negligible extra costs in terms of code size, code maintenance, etc.
It is therefore assumed that the AKE is being transported in a
protocol that provides reliable transport, that can preserve packet
ordering and handle message duplication [RFC7252], that can perform
fragmentation [RFC7959] and protect against denial of service attacks
as provided by the CoAP Echo option [I-D.ietf-core-echo-request-tag].
The AKE may use other transport than CoAP. In this case the
underlying layers must correspondingly handle message loss,
reordering, message duplication, fragmentation, and denial of service
protection.
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2.2. Credentials
IoT deployments differ from one another in terms of what credentials
can be supported. Currently many systems use pre-shared keys (PSKs)
provisioned out of band, for various reasons. PSKs are sometimes
used in a first deployment because of their perceived simplicity.
The use of PSKs allows for protection of communication without major
additional security processing, and also enables the use of symmetric
crypto algorithms only, reducing the implementation and computational
effort in the endpoints.
However, PSK-based provisioning has inherent weaknesses. There has
been reports of massive breaches of PSK provisioning systems
[massive-breach], and as many systems use PSKs without Perfect
Forward Secrecy (PFS, see Section 2.4) they are vulnerable to passive
pervasive monitoring. The security of these systems can be improved
by adding PFS through an AKE authenticated by the provisioned PSK.
Shared keys can alternatively be established in the endpoints using
an AKE protocol authenticated with asymmetric public keys instead of
symmetric secret keys. Raw public keys (RPK) can be provisioned with
the same scheme as PSKs, which allows for a more relaxed trust model
since RPKs need not be secret. The corresponding private keys are
assumed to be provisioned to the party being authenticated beforehand
(e.g. in factory or generated on-board).
As a third option, by using a public key infrastructure and running
an asymmetric key AKE with public key certificates instead of RPKs,
key provisioning can be omitted, leading to a more automated ("zero-
touch") bootstrapping procedure. The root CA keys are assumed to be
provisioned beforehand. Public key certificates are important for
several IoT settings, e.g., facility management with a large number
of devices from many different manufacturers.
These steps provide an example of a migration path in limited scoped
steps from simple to more robust security bootstrapping and
provisioning schemes where each step improves the overall security
and/or simplicity of deployment of the IoT system, although not all
steps are necessarily feasible for the most constrained settings.
In order to allow for these different schemes, the AKE must support
PSK- (shared between two nodes), RPK- and certificate-based
authentication. These are also the schemes for which CoAP is
designed (see Section 9 of [RFC7252]).
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Multiple public key authentication credential types may need to be
supported for RPK and certificate-based authentication. In case of a
Diffie-Hellman key exchange both the use of signature based public
keys (for compatibility with existing ecosystem) and static DH public
keys (for reduced message size) is expected.
To further minimize the bandwidth consumption it is required to
support transporting certificates and raw public keys by reference
rather than by value. Considering the wide variety of deployments,
the AKE must support different schemes for transporting and
identifying credentials. While there are many existing mechanisms
for doing so, ranging from PSK to raw public key by reference to
x5chain of in-band certificates [I-D.ietf-cose-x509], what is
appropriate for a given deployment will depend on the nature of that
deployment. In order to provide a clear initial effort,
Section 2.2.1 lists a set of credential types of immediate relevance;
the mechanism for selecting credential scheme is presumed to enable
future extensibility if needed.
The use of RPKs may be appropriate for the authentication of the AKE
initiator but not for the AKE responder. The AKE must support
different credentials for authentication in different directions of
the AKE run, e.g. certificate-based authentication for the initiating
endpoint and RPK-based authentication for the responding endpoint.
Assuming that both signature public keys and static DH public keys
are in use, then also the case of mixed credentials need to be
supported with one endpoint using a static DH public key and the
other using a signature public key. The AKE shall support
negotiation of public key credential mix and that both initiator and
responder can verify the variant that was executed.
2.2.1. Initial Focus
As illustrated above, the setting is much more diverse in terms of
credentials and trust anchors than that of the unconstrained web. In
order to deliver a timely result, there is a need to initially focus
on what is considered most important at the time of writing: RPK (by
reference and value) and certificate by reference. Information about
validity of a certificate may be omitted from the AKE if available
over unconstrained links. The case of transporting certificate
validation information over the AKE may be specified in the initial
phase if there is a lightweight solution that matches existing
standards and tools.
A subsequent extension beyond the initial focus may be inevitable to
maintain a homogenous deployment without having to implement a mix of
AKE protocols, for example, to support the migration path described
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above. The AKE needs to make clear the scope of cases analysed in
the initial phase, and that a new analysis is required for additional
cases.
The initial scope as described in this subsection does not cover all
credentials as detailed previously in Section 2.2: an AKE which is
extensible but does not include PSK ECDHE would be conformant with
the requirements for the initial scope. A solution to the
requirements for the initial scope is intended to be a deliverable of
the LAKE WG.
2.3. Mutual Authentication
The AKE must provide mutual authentication during the protocol run.
At the end of the AKE protocol, each endpoint shall have freshly
authenticated the other's credential. In particular, both endpoints
must agree on a fresh session identifier, and the roles and
credentials of both endpoints.
Since the protocol may be initiated by different endpoints, it shall
not be necessary to determine beforehand which endpoint takes the
role of initiator of the AKE.
The mutual authentication guarantees of the AKE shall at least
guarantee the following properties:
* The AKE shall provide Key Compromise Impersonation (KCI)
resistance [KCI].
* The AKE shall protect against identity misbinding attacks
[Misbinding]. Note that the identity may be directly related to a
public key such as for example the public key itself, a hash of
the public key, or data unrelated to a key.
* The AKE shall protect against reflection attacks, but need not
protect against attacks when more than two parties legitimately
share keys (cf. the Selfie attack on TLS 1.3 [Selfie]) as that
setting is out of scope.
Replayed messages shall not affect the security of an AKE session.
As often is the case, it is expected that an AKE fulfilling these
goals would have at least three flights of messages (with each flight
potentially consisting of one or more messages, depending on the AKE
design and the mapping to OSCORE).
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2.4. Confidentiality
The shared secret established by the AKE must be known only to the
two authenticated endpoints.
A passive network attacker should never learn any session keys, even
if it knows both endpoints' long-term keys.
An active attacker who has compromised the initiator or responder
credential shall still not be able to compute past session keys
(Perfect Forward Secrecy, PFS). These properties can be achieved,
e.g., with an ephemeral Diffie-Hellman key exchange.
PFS may also be achieved in other ways, for example, using hash-based
ratcheting or with a nonce exchange followed by appropriately derived
new session keys provided that state can be kept in the form of a
session counter. Note that OSCORE specifies a method for session key
update involving a nonce exchange (see Appendix B in [RFC8613]).
The AKE shall provide a mechanism to use the output of one handshake
to optimize future handshakes, e.g., by generating keying material
which can be used to authenticate a future handshake, thus avoiding
the need for public key authentication in that handshake.
The AKE should give recommendations for frequency of re-keying
potentially dependent on the amount of data.
To mitigate against bad random number generators the AKE shall
provide recommendations for randomness, for example to use
[I-D.irtf-cfrg-randomness-improvements].
2.5. Cryptographic Agility and Negotiation Integrity
Motivated by long deployment lifetimes, the AKE is required to
support cryptographic agility, including the modularity of COSE
crypto algorithms and negotiation of preferred crypto algorithms for
OSCORE and the AKE.
* The protocol shall support both pre-shared key and asymmetric key
authentication. PAKE, post-quantum and "hybrid" (simultaneously
more than one) key exchange is out of scope, but may be supported
in a later version.
* The protocol shall allow negotiation of elliptic curves for
Diffie-Hellman operations and signature-based authentication.
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* The AKE shall support negotiation of all COSE algorithms
[IANA-COSE-Algorithms] to be used in OSCORE. The AKE shall
support negotiation of algorithms used in the AKE. It is strongly
recommended that the AKE algorithms are identified using
[IANA-COSE-Algorithms] to reduce unnecessary complexity of a
combined OSCORE/AKE implementation.
* A successful negotiation shall result in the most preferred
algorithms of one of the parties which are supported by the other.
* The AKE may choose different sets of symmetric crypto algorithms
(AEAD, MAC, etc.) for AKE and for OSCORE. In particular, the
length of the MAC for the AKE may be required to be larger than
for OSCORE.
The AKE negotiation must provide strong integrity guarantees against
active attackers. At the end of the AKE protocol, both endpoints
must agree on both the crypto algorithms that were proposed and those
that were chosen. In particular, the protocol must protect against
downgrade attacks.
2.6. Cryptographic Strength
The AKE shall establish a key with a target security level
[keylength] of >= 127 bits. This level was chosen to include X25519
and applies to the strength of authentication, the established keys,
and the protection for the negotiation of all cryptographic
parameters.
2.7. Identity Protection
In general, it is necessary to transport identities as part of the
AKE run in order to provide authentication of an entity not
identified beforehand. In the case of constrained devices, the
identity may contain sensitive information on the manufacturer of the
device, the batch, default firmware version, etc. Protecting
identifying information from passive and active attacks is important
from a privacy point of view, but needs to be balanced with the other
requirements, including security and lightweightness.
In the case of public key identities, the AKE is required to protect
the identity of one of the peers against active attackers and the
identity of the other peer against passive attackers. SIGMA-I and
SIGMA-R differ in this respect. SIGMA-I protects the identity of the
initiator against active attackers and the identity of the responder
against passive attackers. For SIGMA-R, the properties of the roles
are reversed at the cost of an additional flight.
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It is not required to protect the PSK identifier, and it may thus be
sent in the first flight. Protection of PSK identifier in many cases
require extra flights of the AKE.
Other identifying information may also need to be transported in
plain text, for example, identifiers to allow correlation between AKE
messages, and cipher suites. Mechanisms to encrypt these kind of
parameters, such as using pre-configured public keys typically adds
to message overhead.
2.8. Auxiliary Data
In order to reduce round trips and the number of flights, and in some
cases also streamline processing, certain security features may be
integrated into the AKE by transporting "auxiliary data" together
with the AKE messages.
One example is the transport of third-party authorization information
from initiator to responder or vice versa. Such a scheme could
enable the party receiving the authorization information to make a
decision about whether the party being authenticated is also
authorized before the protocol is completed, and if not then
discontinue the protocol before it is complete, thereby saving time,
message processing and data transmission.
Another, orthogonal, example is the embedding of a certificate
enrolment request or a newly issued certificate in the AKE.
For example, the auxiliary data in the first two messages of the AKE
may transport authorization related information as in
[I-D.selander-ace-ake-authz] followed by a Certificate Signing
Request (CSR) in the auxiliary data of the third message.
The AKE must support the transport of such auxiliary data together
with the protocol messages. The auxiliary data field must not
contain data that violates the AKE security properties. The
auxiliary data field must only be used with security analysed
protocols.
The auxiliary data may contain privacy sensitive information. The
auxiliary data must be protected to the same level as AKE data in the
same flight. For example, for a SIGMA-I AKE it is expected that the
3 flights will provide the following protection of the auxiliary
data:
* Auxiliary data in the first flight is unprotected
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* Auxiliary data in the second flight is confidentiality protected
against passive attackers and integrity protected against active
attackers
* Auxiliary data in the third flight is confidentiality and
integrity protected against active attackers
2.9. Extensibility
It is desirable that the AKE supports some kind of extensibility, in
particular, the ability to later include new AKE modes such as PAKE
support. COSE provides an extension mechanism for new algorithms,
new certificate formats, ways to identify credentials, etc.
The main objective with this work is to create a simple yet secure
AKE. The AKE should avoid having multiple ways to express the same
thing. If the underlying encodings offered by CBOR offer multiple
possibility the AKE should be strongly opinionated, and clearly
specify which one will be used.
While remaining extensible, the AKE should avoid optional mechanisms
which introduce code paths that are less well tested.
The AKE should avoid mechanisms where an initiator takes a guess at
the policy, and when it receives a negative response, must guess,
based upon what it has tried, what to do next.
2.10. Availability
Jamming attacks, cutting cables etc. leading to long term loss of
availability may not be possible to mitigate, but an attacker
temporarily injecting messages or disturbing the communication shall
not have a similar impact.
2.11. Lightweight
We target an AKE which is efficiently deployable in 6TiSCH multi-hop
networks, LoRaWAN networks and NB-IoT networks. (For an overview of
low-power wide area networks, see e.g. [RFC8376].) The desire is to
optimize the AKE to be 'as lightweight as reasonably achievable' in
these environments, where 'lightweight' refers to:
* resource consumption, measured by bytes on the wire, wall-clock
time and number of round trips to complete, or power consumption
* the amount of new code required on end systems which already have
an OSCORE stack
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These properties need to be considered in the context of the use of
an existing CoAP/OSCORE stack in the targeted networks and
technologies. Some properties are difficult to evaluate for a given
protocol, for example, because they depend on the radio conditions or
other simultaneous network traffic. Additionally, these properties
are not independent. Therefore the properties listed here should be
taken as input for identifying plausible protocol metrics that can be
more easily measured and compared between protocols.
Per 'bytes on the wire', it is desirable for the AKE messages to fit
into the MTU size of these protocols; and if not possible, within as
few frames as possible, since using multiple MTUs can have
significant costs in terms of time and power. Note that the MTU size
depends on radio technology and its characteristics, including data
rates, number of hops, etc. Example benchmarks are given further
down in this section.
Per 'time', it is desirable for the AKE message exchange(s) to
complete in a reasonable amount of time, both for a single
uncongested exchange and when multiple exchanges are running in an
interleaved fashion, like e.g. in a "network formation" setting when
multiple devices connect for the first time. This latency may not be
a linear function depending on congestion and the specific radio
technology used. As these are relatively low data rate networks, the
latency contribution due to computation is in general not expected to
be dominant.
Per 'round-trips', it is desirable that the number of completed
request/response message exchanges required before the initiating
endpoint can start sending protected traffic data is as small as
possible, since this reduces completion time. See Section 2.11.4 for
a discussion about the trade-off between message size and number of
flights.
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Per 'power', it is desirable for the transmission of AKE messages and
crypto to draw as little power as possible. The best mechanism for
doing so differs across radio technologies. For example, NB-IoT uses
licensed spectrum and thus can transmit at higher power to improve
coverage, making the transmitted byte count relatively more important
than for other radio technologies. In other cases, the radio
transmitter will be active for a full MTU frame regardless of how
much of the frame is occupied by message content, which makes the
byte count less sensitive for the power consumption as long as it
fits into the MTU frame. The power consumption thus increases with
AKE message size and the largest impact is on average under poor
network conditions. Note that listening for messages to receive can
in many cases be a large contribution to the power consumption, for
which there are separate techniques to handle, e.g., time slots,
discontinuous reception, etc. but this is not considered in scope of
the AKE design.
Per 'new code', it is desirable to introduce as little new code as
possible onto OSCORE-enabled devices to support this new AKE. These
devices have on the order of 10s of kB of memory and 100 kB of
storage on which an embedded OS; a COAP stack; CORE and AKE
libraries; and target applications would run. It is expected that
the majority of this space is available for actual application logic,
as opposed to the support libraries. In a typical OSCORE
implementation COSE encrypt and signature structures will be
available, as will support for COSE algorithms relevant for IoT
enabling the same algorithms as is used for OSCORE (e.g. COSE
algorithm no. 10 = CCM* used by 6TiSCH). The use of those, or CBOR
or CoAP, would not add to the footprint.
While the large variety of settings and capabilities of the devices
and networks makes it challenging to produce exact values of some
these dimensions, there are some key benchmarks that are tractable
for security protocol engineering and which have a significant
impact.
2.11.1. LoRaWAN
Reflecting deployment reality as of now, we focus on the European
regulation as described in ETSI EN 300 220. LoRaWAN employs
unlicensed radio frequency bands in the 868 MHz ISM band. For
LoRaWAN the most relevant metric is the Time-on-Air, which determines
the period before the next communication can occur and also which can
be used as an indicator to calculate energy consumption. LoRaWAN is
legally required to use a duty cycle with values such as 0.1%, 1% and
10% depending on the sub-band that is being used, leading to a
payload split into fragments interleaved with unavailable times. For
Europe, the duty cycle is 1% (or smaller). Although there are
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exceptions from the use of duty cycle, the use of an AKE for
providing end-to-end security on application layer needs to comply
with the duty cycle.
2.11.1.1. Bytes on the wire
LoRaWAN has a variable MTU depending on the Spreading Factor (SF).
The higher the spreading factor, the higher distances can be achieved
and/or better reception. If the coverage and distance allows it,
with SF7 - corresponding to higher data rates - the maximum payload
is 222 bytes. For a SF12 - and low data rates - the maximum payload
is 51 bytes on data link layer.
The size and number of packets impact the Time-on-Air (ToA). The
benchmark used here is based on SF12 and a packet size of 51 bytes
[LoRaWAN]. The use of larger packets depend on good radio conditions
which are not always present. Some libraries/providers only support
51-bytes packet size.
2.11.1.2. Time
The time it takes to send a message over the air in LoRaWAN can be
calculated as a function of the different parameters of the
communication. These are the Spreading Factor (SF), the message
size, the channel, bandwidth, coding rate, etc. An important feature
of LoRaWAN is the duty cycle limitation due to the use of the ISM
band. The duty cycle is evaluated in a 1-hour sliding window. It is
legal for a device to transmit a burst for a total of up to 36
seconds ToA on a 1%-duty-cyle sub-band, but the device must then
pause the transmission for the rest of the hour [lorawan-duty-cycle].
In order to avoid extreme waiting times, the AKE needs to complete
before the duty cycle limit is exhausted, also taking into account
potential retransmissions and allowing additional air time for lower
level MAC frames and application data. As a challenging but
realistic example we assume each message is retransmitted 2 times and
allow a factor 2-3 for additional air time. With these assumptions
it is required with a ToA of 4-6 seconds for the uplink protocol
messages to ensure that the entire burst stays within the 36 seconds
duty cycle.
It should be noted that some libraries/providers enforce the duty
cycle limitation through a stop-and-wait operation, which restricts
the number of bytes to the size of the packets after which duty cycle
waiting times are incurred.
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2.11.1.3. Round trips and number of flights
Considering the duty cycle of LoRaWAN and associated unavailable
times, the round trips and number of LoRaWAN packets needs to be
reduced as much as possible.
2.11.1.4. Power
The calculation of the power consumption in LoRaWAN is dependent on
several factors, such as the spreading factor used and the length of
the messages sent, both having a clear dependency with the time it
takes to transmit the messages. The communication model (inherent to
the different LoRaWAN classes of devices) also has an impact on the
energy consumption, but overall the Time-on-Air is an important
indication of the performance.
2.11.2. 6TiSCH
6TiSCH operates in the 2.4 GHz unlicensed frequency band and uses
hybrid Time Division/Frequency Division multiple access (TDMA/FDMA).
Nodes in a 6TiSCH network form a mesh. The basic unit of
communication, a cell, is uniquely defined by its time and frequency
offset in the communication schedule matrix. Cells can be assigned
for communication to a pair of nodes in the mesh and so be collision-
free, or shared by multiple nodes, for example during network
formation. In case of shared cells, some collision-resolution scheme
such as slotted-Aloha is employed. Nodes exchange frames which are
at most 127-bytes long, including the link-layer headers. To
preserve energy, the schedule is typically computed in such a way
that nodes switch on their radio below 1% of the time ("radio duty
cycle"). A 6TiSCH mesh can be several hops deep. In typical use
cases considered by the 6TiSCH working group, a network that is 2-4
hops deep is commonplace; a network which is more than 8 hops deep is
not common.
2.11.2.1. Bytes on the wire
Increasing the number of bytes on the wire in a protocol message has
an important effect on the 6TiSCH network in case the fragmentation
is triggered. More fragments contribute to congestion of shared
cells (and concomitant error rates) in a non-linear way.
The available size for key exchange messages depends on the topology
of the network, whether the message is traveling uplink or downlink,
and other stack parameters. A key performance indicator for a 6TiSCH
network is "network formation", i.e. the time it takes from switching
on all devices, until the last device has executed the AKE and
securely joined. As a benchmark, given the size limit on the frames
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and taking into account the different headers (including link-layer
security), for a 6TiSCH network 5 hops deep, the maximum CoAP payload
size to avoid fragmentation is 47/45 bytes (uplink/downlink)
[AKE-for-6TiSCH].
2.11.2.2. Time
Given the slotted nature of 6TiSCH, the number of bytes in a frame
has insignificant impact on latency, but the number of frames has.
The relevant metric for studying AKE is the network formation time,
which implies parallel AKE runs among nodes that are attempting to
join the network. Network formation time directly affects the time
installers need to spend on site at deployment time.
2.11.2.3. Round trips and number of flights
Given the mesh nature of the 6TiSCH network, and given that each
message may travel several hops before reaching its destination, it
is highly desirable to minimize the number of round trips to reduce
latency.
2.11.2.4. Power
From the power consumption point of view, it is more favorable to
send a small number of large frames than a larger number of short
frames.
2.11.3. NB-IoT
3GPP has specified Narrow-Band IoT (NB-IoT) for support of infrequent
data transmission via user plane and via control plane. NB-IoT is
built on cellular licensed spectrum at low data rates for the purpose
of supporting:
* operations in extreme coverage conditions,
* device battery life of 10 years or more,
* low device complexity and cost, and
* a high system capacity of millions of connected devices per square
kilometer.
NB-IoT achieves these design objectives by:
* Reduced baseband processing, memory and RF enabling low complexity
device implementation.
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* A lightweight setup minimizing control signaling overhead to
optimize power consumption.
* In-band, guard-band, and stand-alone deployment enabling efficient
use of spectrum and network infrastructure.
2.11.3.1. Bytes on the wire
The number of bytes on the wire in a protocol message has a direct
effect on the performance for NB-IoT. In contrast to LoRaWAN and
6TiSCH, the NB-IoT radio bearers are not characterized by a fixed
sized PDU. Concatenation, segmentation and reassembly are part of
the service provided by the NB-IoT radio layer. As a consequence,
the byte count has a measurable impact on time and energy consumption
for running the AKE.
2.11.3.2. Time
Coverage significantly impacts the available bit rate and thereby the
time for transmitting a message, and there is also a difference
between downlink and uplink transmissions (see Section 2.11.3.4).
The transmission time for a message is essentially proportional to
the number of bytes.
Since NB-IoT is operating in licensed spectrum, in contrast to e.g.
LoRaWAN, the packets on the radio interface can be transmitted back-
to-back, so the time before sending OSCORE protected data is limited
by the number of round trips/flights of the AKE and not by a duty
cycle.
2.11.3.3. Round trips and number of flights
As indicated in Section 2.11.3.2, the number of frames and round-
trips is one limiting factor for protocol completion time.
2.11.3.4. Power
Since NB-IoT is operating in licensed spectrum, the device is allowed
to transmit at a relatively high power, which has a large impact on
the energy consumption.
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The benchmark for NB-IoT energy consumption is based on the same
computational model as was used by 3GPP in the design of this radio
layer [NB-IoT-battery-life-evaluation]. The device power consumption
is assumed to be 500mW for transmission and 80mW for reception.
Power consumption for "light sleep" (~ 3mW) and "deep sleep" (~
0.015mW) are negligible in comparison. The bitrates (uplink/
downlink) are assumed to be 28/170 kbps for good coverage and
0,37/2,5 kbps for bad coverage.
The results [AKE-for-NB-IoT] show a high per-byte energy consumption
for uplink transmissions, in particular in bad coverage. Given that
the application decides about the device being initiator or responder
in the AKE, the protocol cannot be tailored for a particular message
being uplink or downlink. To perform well in both kind of
applications the overall number of bytes of the protocol needs to be
as low as possible.
2.11.4. Discussion and Summary of Benchmarks
The difference between uplink and downlink performance must not be
engineered into the protocol since it cannot be assumed that a
particular protocol message will be sent uplink or downlink.
For NB-IoT the byte count on the wire has a measurable impact on time
and energy consumption for running the AKE, so the number of bytes in
the messages needs to be as low as possible.
While "as small protocol messages as possible" does not lend itself
to a sharp boundary threshold, "as few flights as possible" does and
is relevant in all settings above.
The penalty is high for not fitting into the frame sizes of 6TiSCH
and LoRaWAN networks. Fragmentation is not defined within these
technologies so requires fragmentation scheme on a higher layer in
the stack. With fragmentation increases the number of frames per
message, each with its associated overhead in terms of power
consumption and latency. Additionally the probability for errors
increases, which leads to retransmissions of frames or entire
messages that in turn increases the power consumption and latency.
There are trade-offs between "few messages" and "few frames"; if
overhead is spread out over more messages such that each message fits
into a particular frame this may reduce the overall power
consumption. For example, with a frame size of 50 bytes, two 60-byte
messages will fragment into 4 frames in total, whereas three 40-byte
messages fragment into 3 frames in total. On the other hand, a
smaller message has less probability to collide with other messages
and incur retransmission.
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While it may be possible to engineer such a solution for a particular
radio technology and AKE protocol, optimizing for a specific scenario
may not be optimal for other settings. It is expected that specific
scenarios are evaluated in the design phase to ensure that the AKE is
fit for purpose. But in order to start the design work some general
criteria for the AKE performance need to be formulated that takes
into account the differences in the expected deployments.
There are benefits in terms of fewer flights/round trips for NB-IoT
(Section 2.11.3.3) and 6TiSCH (Section 2.11.2.3). An AKE protocol
complying with the requirements of this memo is expected to have at
least 3 messages. With a 3-message AKE, the initiator is able to
derive the OSCORE security context after receiving message 2,
rendering the AKE essentially one round trip before traffic data can
be exchanged, which is ideal.
If the AKE has 3 messages then optimal performance for 6TiSCH is when
each message fits into as few frames as possible, ideally 1 frame per
message.
For LoRaWAN, optimal performance is determined by the duty cycle
which puts a limit to ToA or, for certain libraries/providers, the
number of packets (see Section 2.11.1.2). If the AKE has 3 messages
and each message fits into a 51 byte packet then this is optimal for
the latter case. The same assumption incurs a ToA for uplink
messages in the interval of 4-6 seconds at SF12 both for a device-
initiated and infrastructure-initiated AKE, which complies with the
challenging example stated in Section 2.11.1.2.
One avenue to good performance is therefore to target message sizes
which avoids fragmentation or with as few fragments as possible. For
the LoRaWAN benchmark, the limit for fragmentation is 51 bytes at
link layer. For the 6TiSCH benchmark, messages less than or equal to
45 bytes at CoAP payload layer need not be fragmented.
For the initial focus cases (Section 2.2.1), i.e. RPK (by reference
and value) and certificate by reference, it is required that the AKE
shall perform optimally with respect to the available criteria for
the radio technologies.
To determine with certainty what are the minimal number of fragments
for an AKE under different assumptions requires to design and analyse
the AKE, which is clearly beyond the requirements phase. However, by
means of an example we have reason to believe that an AKE with 3
messages can be designed to support RPK by reference in 3 fragments.
Thus the ideal number of fragments is expected for RPK by reference.
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While such performance may not be possible for the other initial
focus cases, it is expected that if one of the peers send RPK by
value or certificate by reference, then one additional fragment is
sufficient, thus in total a maximum of 5 fragments. Alternatively,
for the LoRaWAN challenge (Section 2.11.1.2), it is expected that the
duty cycle for a burst can be complied with for RPK by value and
certificate by reference, assuming that each message only needs to be
retransmitted at most once (i.e. good AKE performance for RPK by
value and certificate by reference in not too poor radio
environments).
2.11.5. AKE frequency
One question that has been asked in the context of lightweightness
is: - How often is the AKE executed? While it may be impossible to
give a precise answer there are other perspectives to this question.
1. For some use cases, already one execution of the AKE is heavy,
for example, because
* there are a number of parallel executions of the AKE which
loads down the network, such as in a network formation
setting, or
* the duty cycle makes the completion time long for even one run
of the protocol.
2. If a device reboots it may not be able to recover the security
context, e.g. due to lack of persistent storage, and is required
to establish a new security context for which an AKE is
preferred. Reboot frequency may be difficult to predict in
general.
3. To limit the impact of a key compromise, BSI, NIST and ANSSI and
other organizations recommend in other contexts frequent renewal
of keys by means of Diffie-Hellman key exchange. This may be a
symmetric key authenticated key exchange, where the symmetric key
is obtained from a previous asymmetric key based run of the AKE.
To summarize, even if it we are unable to give precise numbers for
AKE frequency, a lightweight AKE:
* reduces the time for network formation and AKE runs in challenging
radio technologies,
* allows devices to quickly re-establish security in case of
reboots, and
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* enables support for recommendations of frequent key renewal.
3. Security Considerations
This document compiles the requirements for an AKE and provides some
related security considerations.
The AKE must provide the security properties expected of IETF
protocols, e.g., providing mutual authentication, confidentiality,
and negotiation integrity as is further detailed in the requirements.
4. Privacy Considerations
In the privacy properties for the AKE, the transport over CoAP needs
to be considered.
5. IANA Considerations
None.
Acknowledgments
The authors want to thank Richard Barnes, Dominique Barthel, Karthik
Bhargavan, Stephen Farrell, Ivaylo Petrov, Eric Rescorla, Michael
Richardson, Jesus Sanchez-Gomez, Claes Tidestav, Hannes Tschofenig
and Christopher Wood for providing valuable input.
Informative References
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
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<https://www.rfc-editor.org/info/rfc7959>.
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[RFC7967] Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
Bose, "Constrained Application Protocol (CoAP) Option for
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[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
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[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
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[RFC8376] Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
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Authors' Addresses
Malisa Vucinic
Inria
Email: malisa.vucinic@inria.fr
Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
John Preuss Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com
Dan Garcia-Carrillo
Odin Solutions S.L.
Email: dgarcia@odins.es
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