RFC 7925 - Transport Layer Security (TLS) / Datagram Transport L
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Internet Engineering Task Force (IETF) H. Tschofenig, Ed.
Request for Comments: 7925 ARM Ltd.
Category: Standards Track T. Fossati
ISSN: 2070-1721 Nokia
July 2016
Transport Layer Security (TLS) /
Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things
Abstract
A common design pattern in Internet of Things (IoT) deployments is
the use of a constrained device that collects data via sensors or
controls actuators for use in home automation, industrial control
systems, smart cities, and other IoT deployments.
This document defines a Transport Layer Security (TLS) and Datagram
Transport Layer Security (DTLS) 1.2 profile that offers
communications security for this data exchange thereby preventing
eavesdropping, tampering, and message forgery. The lack of
communication security is a common vulnerability in IoT products that
can easily be solved by using these well-researched and widely
deployed Internet security protocols.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7925.
Copyright Notice
Copyright (c) 2016 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
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. TLS and DTLS . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Communication Models . . . . . . . . . . . . . . . . . . 6
3.3. The Ciphersuite Concept . . . . . . . . . . . . . . . . . 20
4. Credential Types . . . . . . . . . . . . . . . . . . . . . . 21
4.1. Preconditions . . . . . . . . . . . . . . . . . . . . . . 21
4.2. Pre-Shared Secret . . . . . . . . . . . . . . . . . . . . 23
4.3. Raw Public Key . . . . . . . . . . . . . . . . . . . . . 25
4.4. Certificates . . . . . . . . . . . . . . . . . . . . . . 27
5. Signature Algorithm Extension . . . . . . . . . . . . . . . . 32
6. Error Handling . . . . . . . . . . . . . . . . . . . . . . . 32
7. Session Resumption . . . . . . . . . . . . . . . . . . . . . 34
8. Compression . . . . . . . . . . . . . . . . . . . . . . . . . 35
9. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . . 35
10. Keep-Alive . . . . . . . . . . . . . . . . . . . . . . . . . 36
11. Timeouts . . . . . . . . . . . . . . . . . . . . . . . . . . 38
12. Random Number Generation . . . . . . . . . . . . . . . . . . 39
13. Truncated MAC and Encrypt-then-MAC Extension . . . . . . . . 40
14. Server Name Indication (SNI) . . . . . . . . . . . . . . . . 40
15. Maximum Fragment Length Negotiation . . . . . . . . . . . . . 41
16. Session Hash . . . . . . . . . . . . . . . . . . . . . . . . 41
17. Renegotiation Attacks . . . . . . . . . . . . . . . . . . . . 42
18. Downgrading Attacks . . . . . . . . . . . . . . . . . . . . . 42
19. Crypto Agility . . . . . . . . . . . . . . . . . . . . . . . 43
20. Key Length Recommendations . . . . . . . . . . . . . . . . . 44
21. False Start . . . . . . . . . . . . . . . . . . . . . . . . . 45
22. Privacy Considerations . . . . . . . . . . . . . . . . . . . 45
23. Security Considerations . . . . . . . . . . . . . . . . . . . 46
24. References . . . . . . . . . . . . . . . . . . . . . . . . . 47
24.1. Normative References . . . . . . . . . . . . . . . . . . 47
24.2. Informative References . . . . . . . . . . . . . . . . . 48
Appendix A. Conveying DTLS over SMS . . . . . . . . . . . . . . 56
A.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 56
A.2. Message Segmentation and Reassembly . . . . . . . . . . . 57
A.3. Multiplexing Security Associations . . . . . . . . . . . 57
A.4. Timeout . . . . . . . . . . . . . . . . . . . . . . . . . 58
Appendix B. DTLS Record Layer Per-Packet Overhead . . . . . . . 59
Appendix C. DTLS Fragmentation . . . . . . . . . . . . . . . . . 60
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 60
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 61
1. Introduction
An engineer developing an Internet of Things (IoT) device needs to
investigate the security threats and decide about the security
services that can be used to mitigate these threats.
Enabling IoT devices to exchange data often requires authentication
of the two endpoints and the ability to provide integrity and
confidentiality protection of exchanged data. While these security
services can be provided at different layers in the protocol stack,
the use of Transport Layer Security (TLS) / Datagram Transport Layer
Security (DTLS) has been very popular with many application
protocols, and it is likely to be useful for IoT scenarios as well.
Fitting Internet protocols into constrained devices can be difficult,
but thanks to the standardization efforts, new profiles and protocols
are available, such as the Constrained Application Protocol (CoAP)
[RFC7252]. CoAP messages are mainly carried over UDP/DTLS, but other
transports can be utilized, such as SMS (as described in Appendix A)
or TCP (as currently being proposed with [COAP-TCP-TLS]).
While the main goal for this document is to protect CoAP messages
using DTLS 1.2 [RFC6347], the information contained in the following
sections is not limited to CoAP nor to DTLS itself.
Instead, this document defines a profile of DTLS 1.2 [RFC6347] and
TLS 1.2 [RFC5246] that offers communication security services for IoT
applications and is reasonably implementable on many constrained
devices. Profile thereby means that available configuration options
and protocol extensions are utilized to best support the IoT
environment. This document does not alter TLS/DTLS specifications
and does not introduce any new TLS/DTLS extension.
The main target audience for this document is the embedded system
developer configuring and using a TLS/DTLS stack. This document may,
however, also help those developing or selecting a suitable TLS/DTLS
stack for an IoT product. If you are familiar with (D)TLS, then skip
ahead to Section 4.
2. 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 RFC
2119 [RFC2119].
This specification refers to TLS as well as DTLS and particularly to
version 1.2, which is the most recent version at the time of writing.
We refer to TLS/DTLS whenever the text is applicable to both versions
of the protocol and to TLS or DTLS when there are differences between
the two protocols. Note that TLS 1.3 is being developed, but it is
not expected that this profile will "just work" due to the
significant changes being done to TLS for version 1.3.
Note that "client" and "server" in this document refer to TLS/DTLS
roles, where the client initiates the handshake. This does not
restrict the interaction pattern of the protocols on top of DTLS
since the record layer allows bidirectional communication. This
aspect is further described in Section 3.2.
RFC 7228 [RFC7228] introduces the notion of constrained-node
networks, which are made of small devices with severe constraints on
power, memory, and processing resources. The terms constrained
devices and IoT devices are used interchangeably.
The terms "certification authority" (CA) and "distinguished name"
(DN) are taken from [RFC5280]. The terms "trust anchor" and "trust
anchor store" are defined in [RFC6024] as:
A trust anchor represents an authoritative entity via a public key
and associated data. The public key is used to verify digital
signatures, and the associated data is used to constrain the types
of information for which the trust anchor is authoritative.
A trust anchor store is a set of one or more trust anchors stored
in a device.... A device may have more than one trust anchor
store, each of which may be used by one or more applications.
3. Overview
3.1. TLS and DTLS
The TLS protocol [RFC5246] provides authenticated, confidentiality-
and integrity-protected communication between two endpoints. The
protocol is composed of two layers: the Record Protocol and the
handshaking protocols. At the lowest level, layered on top of a
reliable transport protocol (e.g., TCP), is the Record Protocol. It
provides connection security by using symmetric cryptography for
confidentiality, data origin authentication, and integrity
protection. The Record Protocol is used for encapsulation of various
higher-level protocols. The handshaking protocols consist of three
subprotocols -- namely, the handshake protocol, the change cipher
spec protocol, and the alert protocol. The handshake protocol allows
the server and client to authenticate each other and to negotiate an
encryption algorithm and cryptographic keys before the application
protocol transmits or receives data.
The design of DTLS [RFC6347] is intentionally very similar to TLS.
However, since DTLS operates on top of an unreliable datagram
transport, it must explicitly cope with the absence of reliable and
ordered delivery assumptions made by TLS. RFC 6347 explains these
differences in great detail. As a short summary, for those not
familiar with DTLS, the differences are:
o An explicit sequence number and an epoch field is included in the
Record Protocol. Section 4.1 of RFC 6347 explains the processing
rules for these two new fields. The value used to compute the
Message Authentication Code (MAC) is the 64-bit value formed by
concatenating the epoch and the sequence number.
o Stream ciphers must not be used with DTLS. The only stream cipher
defined for TLS 1.2 is RC4, and due to cryptographic weaknesses,
it is not recommended anymore even for use with TLS [RFC7465].
Note that the term "stream cipher" is a technical term in the TLS
specification. Section 4.7 of RFC 5246 defines stream ciphers in
TLS as follows: "In stream cipher encryption, the plaintext is
exclusive-ORed with an identical amount of output generated from a
cryptographically secure keyed pseudorandom number generator."
o The TLS handshake protocol has been enhanced to include a
stateless cookie exchange for Denial-of-Service (DoS) resistance.
For this purpose, a new handshake message, the HelloVerifyRequest,
was added to DTLS. This handshake message is sent by the server
and includes a stateless cookie, which is returned in a
ClientHello message back to the server. Although the exchange is
optional for the server to execute, a client implementation has to
be prepared to respond to it. Furthermore, the handshake message
format has been extended to deal with message loss, reordering,
and fragmentation.
3.2. Communication Models
This document describes a profile of DTLS and, to be useful, it has
to make assumptions about the envisioned communication architecture.
Two communication architectures (and consequently two profiles) are
described in this document.
3.2.1. Constrained TLS/DTLS Clients
The communication architecture shown in Figure 1 assumes a unicast
communication interaction with an IoT device utilizing a constrained
TLS/DTLS client interacting with one or multiple TLS/DTLS servers.
Before a client can initiate the TLS/DTLS handshake, it needs to know
the IP address of that server and what credentials to use.
Application-layer protocols, such as CoAP, which is conveyed on top
of DTLS, may be configured with URIs of the endpoints to which CoAP
needs to register and publish data. This configuration information
(including non-confidential credentials, like certificates) may be
conveyed to clients as part of a firmware/software package or via a
configuration protocol. The following credential types are supported
by this profile:
o For authentication based on the Pre-Shared Key (PSK) (see
Section 4.2), this includes the paired "PSK identity" and shared
secret to be used with each server.
o For authentication based on the raw public key (see Section 4.3),
this includes either the server's public key or the hash of the
server's public key.
o For certificate-based authentication (see Section 4.4), this
includes a pre-populated trust anchor store that allows the DTLS
stack to perform path validation for the certificate obtained
during the handshake with the server.
Figure 1 shows example configuration information stored at the
constrained client for use with respective servers.
This document focuses on the description of the DTLS client-side
functionality but, quite naturally, the equivalent server-side
support has to be available.
+////////////////////////////////////+
| Configuration |
|////////////////////////////////////|
| Server A --> PSK Identity, PSK |
| |
| Server B --> Public Key (Server B),|
| Public/Private Key |
| (for Client) |
| |
| Server C --> Public/Private Key |
| (for Client) |
| Trust Anchor Store |
+------------------------------------+
oo
oooooo
o
+-----------+
|Constrained|
|TLS/DTLS |
|Client |-
+-----------+ \
\ ,-------.
,' `. +------+
/ IP-Based \ |Server|
( Network ) | A |
\ / +------+
`. ,'
'---+---' +------+
| |Server|
| | B |
| +------+
|
| +------+
+----------------->|Server|
| C |
+------+
Figure 1: Constrained Client Profile
3.2.1.1. Examples of Constrained Client Exchanges
3.2.1.1.1. Network Access Authentication Example
Reuse is a recurring theme when considering constrained environments
and is behind a lot of the directions taken in developments for
constrained environments. The corollary of reuse is to not add
functionality if it can be avoided. An example relevant to the use
of TLS is network access authentication, which takes place when a
device connects to a network and needs to go through an
authentication and access control procedure before it is allowed to
communicate with other devices or connect to the Internet.
Figure 2 shows the network access architecture with the IoT device
initiating the communication to an access point in the network using
the procedures defined for a specific physical layer. Since
credentials may be managed and stored centrally, in the
Authentication, Authorization, and Accounting (AAA) server, the
security protocol exchange may need to be relayed via the
Authenticator, i.e., functionality running on the access point to the
AAA server. The authentication and key exchange protocol itself is
encapsulated within a container, the Extensible Authentication
Protocol (EAP) [RFC3748], and messages are conveyed back and forth
between the EAP endpoints, namely the EAP peer located on the IoT
device and the EAP server located on the AAA server or the access
point. To route EAP messages from the access point, acting as a AAA
client, to the AAA server requires an adequate protocol mechanism,
namely RADIUS [RFC2865] or Diameter [RFC6733].
More details about the concepts and a description about the
terminology can be found in RFC 5247 [RFC5247].
+--------------+
|Authentication|
|Authorization |
|Accounting |
|Server |
|(EAP Server) |
| |
+-^----------^-+
* EAP o RADIUS/
* o Diameter
--v----------v--
/// \\\
// \\
| Federation |
| Substrate |
\\ //
\\\ ///
--^----------^--
* EAP o RADIUS/
* o Diameter
+-------------+ +-v----------v--+
| | EAP/EAP Method | |
| Internet of |<***************************>| Access Point |
| Things | |(Authenticator)|
| Device | EAP Lower Layer and |(AAA Client) |
| (EAP Peer) | Secure Association Protocol | |
| |<--------------------------->| |
| | | |
| | Physical Layer | |
| |<===========================>| |
+-------------+ +---------------+
Legend:
<****>: Device-to-AAA-Server Exchange
<---->: Device-to-Authenticator Exchange
<oooo>: AAA-Client-to-AAA-Server Exchange
<====>: Physical layer like IEEE 802.11/802.15.4
Figure 2: Network Access Architecture
One standardized EAP method is EAP-TLS, defined in RFC 5216
[RFC5216], which reuses the TLS-based protocol exchange and
encapsulates it inside the EAP payload. In terms of reuse, this
allows many components of the TLS protocol to be shared between the
network access security functionality and the TLS functionality
needed for securing application-layer traffic. In the EAP-TLS
exchange shown in Figure 3, the IoT device as the EAP peer acts as a
TLS client.
Authenticating Peer Authenticator
------------------- -------------
<- EAP-Request/
Identity
EAP-Response/
Identity (MyID) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS Start)
EAP-Response/
EAP-Type=EAP-TLS
(TLS client_hello)->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS server_hello,
TLS certificate,
[TLS server_key_exchange,]
TLS certificate_request,
TLS server_hello_done)
EAP-Response/
EAP-Type=EAP-TLS
(TLS certificate,
TLS client_key_exchange,
TLS certificate_verify,
TLS change_cipher_spec,
TLS finished) ->
<- EAP-Request/
EAP-Type=EAP-TLS
(TLS change_cipher_spec,
TLS finished)
EAP-Response/
EAP-Type=EAP-TLS ->
<- EAP-Success
Figure 3: EAP-TLS Exchange
The guidance in this document also applies to the use of EAP-TLS for
network access authentication. An IoT device using a network access
authentication solution based on TLS can reuse most parts of the code
for the use of DTLS/TLS at the application layer, thereby saving a
significant amount of flash memory. Note, however, that the
credentials used for network access authentication and those used for
application-layer security are very likely different.
3.2.1.1.2. CoAP-Based Data Exchange Example
When a constrained client uploads sensor data to a server
infrastructure, it may use CoAP by pushing the data via a POST
message to a preconfigured endpoint on the server. In certain
circumstances, this might be too limiting and additional
functionality is needed, as shown in Figures 4 and 5, where the IoT
device itself runs a CoAP server hosting the resource that is made
accessible to other entities. Despite running a CoAP server on the
IoT device, it is still the DTLS client on the IoT device that
initiates the interaction with the non-constrained resource server in
our scenario.
Figure 4 shows a sensor starting a DTLS exchange with a resource
directory and uses CoAP to register available resources in Figure 5.
[CoRE-RD] defines the resource directory (RD) as a web entity that
stores information about web resources and implements
Representational State Transfer (REST) interfaces for registration
and lookup of those resources. Note that the described exchange is
borrowed from the Open Mobile Alliance (OMA) Lightweight
Machine-to-Machine (LWM2M) specification [LWM2M] that uses RD but
adds proxy functionality.
The initial DTLS interaction between the sensor, acting as a DTLS
client, and the resource directory, acting as a DTLS server, will be
a full DTLS handshake. Once this handshake is complete, both parties
have established the DTLS record layer. Subsequently, the CoAP
client can securely register at the resource directory.
After some time (assuming that the client regularly refreshes its
registration), the resource directory receives a request from an
application to retrieve the temperature information from the sensor.
This request is relayed by the resource directory to the sensor using
a GET message exchange. The already established DTLS record layer
can be used to secure the message exchange.
Resource
Sensor Directory
------ ---------
+---
|
| ClientHello -------->
| #client_certificate_type#
F| #server_certificate_type#
U|
L| <------- HelloVerifyRequest
L|
| ClientHello -------->
D| #client_certificate_type#
T| #server_certificate_type#
L|
S| ServerHello
| #client_certificate_type#
H| #server_certificate_type#
A| Certificate
N| ServerKeyExchange
D| CertificateRequest
S| <-------- ServerHelloDone
H|
A| Certificate
K| ClientKeyExchange
E| CertificateVerify
| [ChangeCipherSpec]
| Finished -------->
|
| [ChangeCipherSpec]
| <-------- Finished
+---
Note: Extensions marked with "#" were introduced with
RFC 7250.
Figure 4: DTLS/CoAP Exchange Using Resource Directory:
Part 1 -- DTLS Handshake
Figure 5 shows the DTLS-secured communication between the sensor and
the resource directory using CoAP.
Resource
Sensor Directory
------ ---------
[[==============DTLS-Secured Communication===================]]
+--- ///+
C| \ D
O| Req: POST coap://rd.example.com/rd?ep=node1 \ T
A| Payload: \ L
P| </temp>;ct=41; \ S
| rt="temperature-c";if="sensor", \
R| </light>;ct=41; \ R
D| rt="light-lux";if="sensor" \ E
| --------> \ C
R| \ O
E| \ R
G| Res: 2.01 Created \ D
| <-------- Location: /rd/4521 \
| \ L
+--- \ A
\ Y
* \ E
* (time passes) \ R
* \
+--- \ P
C| \ R
O| Req: GET coaps://sensor.example.com/temp \ O
A| <-------- \ T
P| \ E
| Res: 2.05 Content \ C
G| Payload: \ T
E| 25.5 --------> \ E
T| \ D
+--- ///+
Figure 5: DTLS/CoAP Exchange Using Resource Directory:
Part 2 -- CoAP/RD Exchange
Note that the CoAP GET message transmitted from the resource server
is protected using the previously established DTLS record layer.
3.2.2. Constrained TLS/DTLS Servers
Section 3.2.1 illustrates a deployment model where the TLS/DTLS
client is constrained and efforts need to be taken to improve memory
utilization, bandwidth consumption, reduce performance impacts, etc.
In this section, we assume a scenario where constrained devices run
TLS/DTLS servers to secure access to application-layer services
running on top of CoAP, HTTP, or other protocols. Figure 6
illustrates a possible deployment whereby a number of constrained
servers are waiting for regular clients to access their resources.
The entire process is likely, but not necessarily, controlled by a
third party, the authentication and authorization server. This
authentication and authorization server is responsible for holding
authorization policies that govern the access to resources and
distribution of keying material.
+////////////////////////////////////+
| Configuration |
|////////////////////////////////////|
| Credentials |
| Client A -> Public Key |
| Server S1 -> Symmetric Key |
| Server S2 -> Certificate |
| Server S3 -> Public Key |
| Trust Anchor Store |
| Access Control Lists |
| Resource X: Client A / GET |
| Resource Y: Client A / PUT |
+------------------------------------+
oo
oooooo
o
+---------------+ +-----------+
|Authentication | +-------->|TLS/DTLS |
|& Authorization| | |Client A |
|Server | | +-----------+
+---------------+ ++
^ | +-----------+
\ | |Constrained|
\ ,-------. | Server S1 |
,' `. +-----------+
/ Local \
( Network )
\ / +-----------+
`. ,' |Constrained|
'---+---' | Server S2 |
| +-----------+
|
| +-----------+
+-----------------> |Constrained|
| Server S3 |
+-----------+
Figure 6: Constrained Server Profile
A deployment with constrained servers has to overcome several
challenges. Below we explain how these challenges can be solved with
CoAP, as an example. Other protocols may offer similar capabilities.
While the requirements for the TLS/DTLS protocol profile change only
slightly when run on a constrained server (in comparison to running
it on a constrained client), several other ecosystem factors will
impact deployment.
There are several challenges that need to be addressed:
Discovery and Reachability:
A client must first and foremost discover the server before
initiating a connection to it. Once it has been discovered,
reachability to the device needs to be maintained.
In CoAP, the discovery of resources offered by servers is
accomplished by sending a unicast or multicast CoAP GET to a well-
known URI. The Constrained RESTful Environments (CoRE) Link
Format specification [RFC6690] describes the use case (see
Section 1.2.1) and reserves the URI (see Section 7.1). Section 7
of the CoAP specification [RFC7252] describes the discovery
procedure. [RFC7390] describes the use case for discovering CoAP
servers using multicast (see Section 3.3) and specifies the
protocol processing rules for CoAP group communications (see
Section 2.7).
The use of RD [CoRE-RD] is yet another possibility for discovering
registered servers and their resources. Since RD is usually not a
proxy, clients can discover links registered with the RD and then
access them directly.
Authentication:
The next challenge concerns the provisioning of authentication
credentials to the clients as well as servers. In Section 3.2.1,
we assume that credentials (and other configuration information)
are provisioned to the device, and that those can be used with the
authorization servers. Of course, this leads to a very static
relationship between the clients and their server-side
infrastructure but poses fewer challenges from a deployment point
of view, as described in Section 2 of [RFC7452]. In any case,
engineers and product designers have to determine how the relevant
credentials are distributed to the respective parties. For
example, shared secrets may need to be provisioned to clients and
the constrained servers for subsequent use of TLS/DTLS PSK. In
other deployments, certificates, private keys, and trust anchors
for use with certificate-based authentication may need to be
utilized.
Practical solutions use either pairing (also called imprinting) or
a trusted third party. With pairing, two devices execute a
special protocol exchange that is unauthenticated to establish a
shared key (for example, using an unauthenticated Diffie-Hellman
(DH) exchange). To avoid man-in-the-middle attacks, an
out-of-band channel is used to verify that nobody has tampered
with the exchanged protocol messages. This out-of-band channel
can come in many forms, including:
* Human involvement by comparing hashed keys, entering passkeys,
and scanning QR codes
* The use of alternative wireless communication channels (e.g.,
infrared communication in addition to Wi-Fi)
* Proximity-based information
More details about these different pairing/imprinting techniques
can be found in the Smart Object Security Workshop report
[RFC7397] and various position papers submitted on that topic,
such as [ImprintingSurvey]. The use of a trusted third party
follows a different approach and is subject to ongoing
standardization efforts in the Authentication and Authorization
for Constrained Environments (ACE) working group [ACE-WG].
Authorization
The last challenge is the ability for the constrained server to
make an authorization decision when clients access protected
resources. Pre-provisioning access control information to
constrained servers may be one option but works only in a small
scale, less dynamic environment. For a finer-grained and more
dynamic access control solution, the reader is referred to the
ongoing work in the IETF ACE working group.
Figure 7 shows an example interaction whereby a device, a thermostat
in our case, searches in the local network for discoverable resources
and accesses those. The thermostat starts the procedure using a
link-local discovery message using the "All CoAP Nodes" multicast
address by utilizing the link format per RFC 6690 [RFC6690]. The
IPv6 multicast address used for CoAP link-local discovery is
FF02::FD. As a result, a temperature sensor and a fan respond.
These responses allow the thermostat to subsequently read temperature
information from the temperature sensor with a CoAP GET request
issued to the previously learned endpoint. In this example we assume
that accessing the temperature sensor readings and controlling the
fan requires authentication and authorization of the thermostat and
TLS is used to authenticate both endpoints and to secure the
communication.
Temperature
Thermostat Sensor Fan
---------- --------- ---
Discovery
-------------------->
GET coap://[FF02::FD]/.well-known/core
CoAP 2.05 Content
<-------------------------------
</3303/0/5700>;rt="temperature";
if="sensor"
CoAP 2.05 Content
<--------------------------------------------------
</fan>;rt="fan";if="actuation"
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
\ Protocol steps to obtain access token or keying /
\ material for access to the temperature sensor and fan. /
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
Read Sensor Data
(authenticated/authorized)
------------------------------->
GET /3303/0/5700
CoAP 2.05 Content
<-------------------------------
22.5 C
Configure Actuator
(authenticated/authorized)
------------------------------------------------->
PUT /fan?on-off=true
CoAP 2.04 Changed
<-------------------------------------------------
Figure 7: Local Discovery and Resource Access
3.3. The Ciphersuite Concept
TLS (and consequently DTLS) support ciphersuites, and an IANA
registry [IANA-TLS] was created to register the suites. A
ciphersuite (and the specification that defines it) contains the
following information:
o Authentication and key exchange algorithm (e.g., PSK)
o Cipher and key length (e.g., Advanced Encryption Standard (AES)
with 128-bit keys [AES])
o Mode of operation (e.g., Counter with CBC-MAC (CCM) mode for AES)
[RFC3610]
o Hash algorithm for integrity protection, such as the Secure Hash
Algorithm (SHA) in combination with Keyed-Hashing for Message
Authentication (HMAC) (see [RFC2104] and [RFC6234])
o Hash algorithm for use with pseudorandom functions (e.g., HMAC
with the SHA-256)
o Misc information (e.g., length of authentication tags)
o Information whether the ciphersuite is suitable for DTLS or only
for TLS
The TLS ciphersuite TLS_PSK_WITH_AES_128_CCM_8, for example, uses a
pre-shared authentication and key exchange algorithm. [RFC6655]
defines this ciphersuite. It uses the AES encryption algorithm,
which is a block cipher. Since the AES algorithm supports different
key lengths (such as 128, 192, and 256 bits), this information has to
be specified as well, and the selected ciphersuite supports 128-bit
keys. A block cipher encrypts plaintext in fixed-size blocks, and
AES operates on a block size of 128 bits. For messages exceeding 128
bits, the message is partitioned into 128-bit blocks, and the AES
cipher is applied to these input blocks with appropriate chaining,
which is called mode of operation.
TLS 1.2 introduced Authenticated Encryption with Associated Data
(AEAD) ciphersuites (see [RFC5116] and [RFC6655]). AEAD is a class
of block cipher modes that encrypt (parts of) the message and
authenticate the message simultaneously. AES-CCM [RFC6655] is an
example of such a mode.
Some AEAD ciphersuites have shorter authentication tags (i.e.,
message authentication codes) and are therefore more suitable for
networks with low bandwidth where small message size matters. The
TLS_PSK_WITH_AES_128_CCM_8 ciphersuite that ends in "_8" has an
8-octet authentication tag, while the regular CCM ciphersuites have,
at the time of writing, 16-octet authentication tags. The design of
CCM and the security properties are described in [CCM].
TLS 1.2 also replaced the combination of MD5/SHA-1 hash functions in
the TLS pseudorandom function (PRF) used in earlier versions of TLS
with ciphersuite-specified PRFs. For this reason, authors of more
recent TLS 1.2 ciphersuite specifications explicitly indicate the MAC
algorithm and the hash functions used with the TLS PRF.
4. Credential Types
The mandatory-to-implement functionality will depend on the
credential type used with IoT devices. The subsections below
describe the implications of three different credential types, namely
pre-shared secrets, raw public keys, and certificates.
4.1. Preconditions
All exchanges described in the subsequent sections assume that some
information has been distributed before the TLS/DTLS interaction
starts. The credentials are used to authenticate the client to the
server, and vice versa. What information items have to be
distributed depends on the chosen credential types. In all cases,
the IoT device needs to know what algorithms to prefer, particularly
if there are multiple algorithm choices available as part of the
implemented ciphersuites, as well as information about the other
communication endpoint (for example, in the form of a URI) a
particular credential has to be used with.
Pre-Shared Secrets: In this case, a shared secret together with an
identifier needs to be made available to the device as well as to
the other communication party.
Raw Public Keys: A public key together with a private key are stored
on the device and typically associated with some identifier. To
authenticate the other communication party, the appropriate
credential has to be known. If the other end uses raw public keys
as well, then their public key needs to be provisioned (out of
band) to the device.
Certificates: The use of certificates requires the device to store
the public key (as part of the certificate) as well as the private
key. The certificate will contain the identifier of the device as
well as various other attributes. Both communication parties are
assumed to be in possession of a trust anchor store that contains
CA certificates and, in case of certificate pinning, end-entity
certificates. Similar to the other credentials, the IoT device
needs information about which entity to use which certificate
with. Without a trust anchor store on the IoT device, it will not
be possible to perform certificate validation.
We call the above-listed information "device credentials" and these
device credentials may be provisioned to the device already during
the manufacturing time or later in the process, depending on the
envisioned business and deployment model. These initial credentials
are often called "root of trust". Whatever process is chosen for
generating these initial device credentials, it MUST be ensured that
a different key pair is provisioned for each device and installed in
as secure a manner as possible. For example, it is preferable to
generate public/private keys on the IoT device itself rather than
generating them outside the device. Since an IoT device is likely to
interact with various other parties, the initial device credential
may only be used with some dedicated entities, and configuring
further configuration and credentials to the device is left to a
separate interaction. An example of a dedicated protocol used to
distribute credentials, access control lists, and configure
information is the LWM2M protocol [LWM2M].
For all the credentials listed above, there is a chance that those
may need to be replaced or deleted. While separate protocols have
been developed to check the status of these credentials and to manage
these credentials, such as the Trust Anchor Management Protocol
(TAMP) [RFC5934], their usage is, however, not envisioned in the IoT
context so far. IoT devices are assumed to have a software update
mechanism built-in, and it will allow updates of low-level device
information, including credentials and configuration parameters.
This document does, however, not mandate a specific software/firmware
update protocol.
With all credentials used as input to TLS/DTLS authentication, it is
important that these credentials have been generated with care. When
using a pre-shared secret, a critical consideration is using
sufficient entropy during the key generation, as discussed in
[RFC4086]. Deriving a shared secret from a password, some device
identifiers, or other low-entropy sources is not secure. A low-
entropy secret, or password, is subject to dictionary attacks.
Attention also has to be paid when generating public/private key
pairs since the lack of randomness can result in the same key pair
being used in many devices. This topic is also discussed in
Section 12 since keys are generated during the TLS/DTLS exchange
itself as well, and the same considerations apply.
4.2. Pre-Shared Secret
The use of pre-shared secrets is one of the most basic techniques for
TLS/DTLS since it is both computationally efficient and bandwidth
conserving. Authentication based on pre-shared secrets was
introduced to TLS in RFC 4279 [RFC4279].
Figure 8 illustrates the DTLS exchange including the cookie exchange.
While the server is not required to initiate a cookie exchange with
every handshake, the client is required to implement and to react on
it when challenged, as defined in RFC 6347 [RFC6347]. The cookie
exchange allows the server to react to flooding attacks.
Client Server
------ ------
ClientHello -------->
<-------- HelloVerifyRequest
(contains cookie)
ClientHello -------->
(with cookie)
ServerHello
*ServerKeyExchange
<-------- ServerHelloDone
ClientKeyExchange
ChangeCipherSpec
Finished -------->
ChangeCipherSpec
<-------- Finished
Application Data <-------> Application Data
Legend:
* indicates an optional message payload
Figure 8: DTLS PSK Authentication Including the Cookie Exchange
Note that [RFC4279] used the term "PSK identity" to refer to the
identifier used to refer to the appropriate secret. While
"identifier" would be more appropriate in this context, we reuse the
terminology defined in RFC 4279 to avoid confusion. RFC 4279 does
not mandate the use of any particular type of PSK identity, and the
client and server have to agree on the identities and keys to be
used. The UTF-8 encoding of identities described in Section 5.1 of
RFC 4279 aims to improve interoperability for those cases where the
identity is configured by a human using some management interface
provided by a web browser. However, many IoT devices do not have a
user interface, and most of their credentials are bound to the device
rather than to the user. Furthermore, credentials are often
provisioned into hardware modules or provisioned alongside with
firmware. As such, the encoding considerations are not applicable to
this usage environment. For use with this profile, the PSK
identities SHOULD NOT assume a structured format (such as domain
names, distinguished names, or IP addresses), and a byte-by-byte
comparison operation MUST be used by the server for any operation
related to the PSK identity. These types of identifiers are called
"absolute" per RFC 6943 [RFC6943].
Protocol-wise, the client indicates which key it uses by including a
"PSK identity" in the ClientKeyExchange message. As described in
Section 3.2, clients may have multiple pre-shared keys with a single
server, for example, in a hosting context. The TLS Server Name
Indication (SNI) extension allows the client to convey the name of
the server it is contacting. A server implementation needs to guide
the selection based on a received SNI value from the client.
RFC 4279 requires TLS implementations supporting PSK ciphersuites to
support arbitrary PSK identities up to 128 octets in length and
arbitrary PSKs up to 64 octets in length. This is a useful
assumption for TLS stacks used in the desktop and mobile environments
where management interfaces are used to provision identities and
keys. Implementations in compliance with this profile MAY use PSK
identities up to 128 octets in length and arbitrary PSKs up to 64
octets in length. The use of shorter PSK identities is RECOMMENDED.
"The Constrained Application Protocol (CoAP)" [RFC7252] currently
specifies TLS_PSK_WITH_AES_128_CCM_8 as the mandatory-to-implement
ciphersuite for use with shared secrets. This ciphersuite uses the
AES algorithm with 128 bit keys and CCM as the mode of operation.
The label "_8" indicates that an 8-octet authentication tag is used.
Note that the shorted authentication tag increases the chance that an
adversary with no knowledge of the secret key can present a message
with a MAC that will pass the verification procedure. The likelihood
of accepting forged data is explained in Section 5.3.5 of
[SP800-107-rev1] and depends on the lengths of the authentication tag
and allowed numbers of MAC verifications using a given key.
This ciphersuite makes use of the default TLS 1.2 PRF, which uses an
HMAC with the SHA-256 hash function. Note: Starting with TLS 1.2
(and consequently DTLS 1.2), ciphersuites have to specify the PRF.
RFC 5246 states that "New cipher suites MUST explicitly specify a PRF
and, in general, SHOULD use the TLS PRF with SHA-256 or a stronger
standard hash function." The ciphersuites recommended in this
document use the SHA-256 construct defined in Section 5 of RFC 5246.
A device compliant with the profile in this section MUST implement
TLS_PSK_WITH_AES_128_CCM_8 and follow the guidance from this section.
4.3. Raw Public Key
The use of raw public keys with TLS/DTLS, as defined in [RFC7250], is
the first entry point into public key cryptography without having to
pay the price of certificates and a public key infrastructure (PKI).
The specification reuses the existing Certificate message to convey
the raw public key encoded in the SubjectPublicKeyInfo structure. To
indicate support, two new extensions had been defined, as shown in
Figure 9, namely the server_certificate_type and the
client_certificate_type.
Client Server
------ ------
ClientHello -------->
#client_certificate_type#
#server_certificate_type#
ServerHello
#client_certificate_type#
#server_certificate_type#
Certificate
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Note: Extensions marked with "#" were introduced with
RFC 7250.
Figure 9: DTLS Raw Public Key Exchange
The CoAP-recommended ciphersuite for use with this credential type is
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251]. This AES-CCM TLS
ciphersuite based on elliptic curve cryptography (ECC) uses the
Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) as the key
establishment mechanism and an Elliptic Curve Digital Signature
Algorithm (ECDSA) for authentication. The named DH groups
[FFDHE-TLS] are not applicable to this profile since it relies on the
ECC-based counterparts. This ciphersuite makes use of the AEAD
capability in DTLS 1.2 and utilizes an 8-octet authentication tag.
The use of a DH key exchange provides perfect forward secrecy (PFS).
More details about PFS can be found in Section 9.
[RFC6090] provides valuable information for implementing ECC
algorithms, particularly for choosing methods that have been
available in the literature for a long time (i.e., 20 years and
more).
A device compliant with the profile in this section MUST implement
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and follow the guidance from this
section.
4.4. Certificates
The use of mutual certificate-based authentication is shown in
Figure 10, which makes use of the "cached_info" extension [RFC7924].
Support of the "cached_info" extension is REQUIRED. Caching
certificate chains allows the client to reduce the communication
overhead significantly, otherwise the server would provide the end-
entity certificate and the certificate chain with every full DTLS
handshake.
Client Server
------ ------
ClientHello -------->
*cached_info*
ServerHello
*cached_info*
Certificate
ServerKeyExchange
CertificateRequest
<-------- ServerHelloDone
Certificate
ClientKeyExchange
CertificateVerify
[ChangeCipherSpec]
Finished -------->
[ChangeCipherSpec]
<-------- Finished
Note: Extensions marked with "*" were introduced with
RFC 7924.
Figure 10: DTLS Mutual Certificate-Based Authentication
TLS/DTLS offers a lot of choices when selecting ECC-based
ciphersuites. This document restricts the use to named curves
defined in RFC 4492 [RFC4492]. At the time of writing, the
recommended curve is secp256r1, and the use of uncompressed points
follows the recommendation in CoAP. Note that standardization for
Curve25519 (for ECDHE) is ongoing (see [RFC7748]), and support for
this curve will likely be required in the future.
A device compliant with the profile in this section MUST implement
TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and follow the guidance from this
section.
4.4.1. Certificates Used by Servers
The algorithm for verifying the service identity, as described in RFC
6125 [RFC6125], is essential for ensuring proper security when
certificates are used. As a summary, the algorithm contains the
following steps:
1. The client constructs a list of acceptable reference identifiers
based on the source domain and, optionally, the type of service
to which the client is connecting.
2. The server provides its identifiers in the form of a PKIX
certificate.
3. The client checks each of its reference identifiers against the
presented identifiers for the purpose of finding a match.
4. When checking a reference identifier against a presented
identifier, the client matches the source domain of the
identifiers and, optionally, their application service type.
For various terms used in the algorithm shown above, consult RFC
6125. It is important to highlight that comparing the reference
identifier against the presented identifier obtained from the
certificate is required to ensure the client is communicating with
the intended server.
It is worth noting that the algorithm description and the text in RFC
6125 assumes that fully qualified DNS domain names are used. If a
server node is provisioned with a fully qualified DNS domain, then
the server certificate MUST contain the fully qualified DNS domain
name or "FQDN" as dNSName [RFC5280]. For CoAP, the coaps URI scheme
is described in Section 6.2 of [RFC7252]. This FQDN is stored in the
SubjectAltName or in the leftmost Common Name (CN) component of the
subject name, as explained in Section 9.1.3.3 of [RFC7252], and used
by the client to match it against the FQDN used during the lookup
process, as described in [RFC6125]. For other protocols, the
appropriate URI scheme specification has to be consulted.
The following recommendation is provided:
1. Certificates MUST NOT use DNS domain names in the CN of
certificates and instead use the subjectAltName attribute, as
described in the previous paragraph.
2. Certificates MUST NOT contain domain names with wildcard
characters.
3. Certificates MUST NOT contain multiple names (e.g., more than one
dNSName field).
Note that there will be servers that are not provisioned for use with
DNS domain names, for example, IoT devices that offer resources to
nearby devices in a local area network, as shown in Figure 7. When
such constrained servers are used, then the use of certificates as
described in Section 4.4.2 is applicable. Note that the SNI
extension cannot be used in this case since SNI does not offer the
ability to convey a 64-bit Extended Unique Identifier (EUI-64)
[EUI64]. Note that this document does not recommend use of IP
addresses in certificates nor does it discuss the implications of
placing IP addresses in certificates.
4.4.2. Certificates Used by Clients
For client certificates, the identifier used in the SubjectAltName or
in the leftmost CN component of subject name MUST be an EUI-64.
4.4.3. Certificate Revocation
For certificate revocation, neither the Online Certificate Status
Protocol (OCSP) nor Certificate Revocation Lists (CRLs) are used.
Instead, this profile relies on a software update mechanism to
provision information about revoked certificates. While multiple
OCSP stapling [RFC6961] has recently been introduced as a mechanism
to piggyback OCSP request/responses inside the DTLS/TLS handshake (to
avoid the cost of a separate protocol handshake), further
investigations are needed to determine its suitability for the IoT
environment.
As stated earlier in this section, modifications to the trust anchor
store depends on a software update mechanism as well. There are
limitations to the use of a software update mechanism because of the
potential inability to change certain types of keys, such as those
provisioned during manufacturing. For this reason, manufacturer-
provisioned credentials are typically employed only to obtain further
certificates (for example, via a key distribution server) for use
with servers the IoT device is finally communicating with.
4.4.4. Certificate Content
All certificate elements listed in Table 1 MUST be implemented by
clients and servers claiming support for certificate-based
authentication. No other certificate elements are used by this
specification.
When using certificates, IoT devices MUST provide support for a
server certificate chain of at least 3, not including the trust
anchor, and MAY reject connections from servers offering chains
longer than 3. IoT devices MAY have client certificate chains of any
length. Obviously, longer chains require more digital signature
verification operations to perform and lead to larger certificate
messages in the TLS handshake.
Table 1 provides a summary of the elements in a certificate for use
with this profile.
+----------------------+--------------------------------------------+
| Element | Notes |
+----------------------+--------------------------------------------+
| version | This profile uses X.509 v3 certificates |
| | [RFC5280]. |
| | |
| serialNumber | Positive integer unique per certificate. |
| | |
| signature | This field contains the signature |
| | algorithm, and this profile uses ecdsa- |
| | with-SHA256 or stronger [RFC5758]. |
| | |
| issuer | Contains the DN of the issuing CA. |
| | |
| validity | Values expressed as UTC time in notBefore |
| | and notAfter fields. No validity period |
| | mandated. |
| | |
| subject | See rules outlined in this section. |
| | |
| subjectPublicKeyInfo | The SubjectPublicKeyInfo structure |
| | indicates the algorithm and any associated |
| | parameters for the ECC public key. This |
| | profile uses the id-ecPublicKey algorithm |
| | identifier for ECDSA signature keys, as |
| | defined and specified in [RFC5480]. |
| | |
| signatureAlgorithm | The ECDSA signature algorithm with ecdsa- |
| | with-SHA256 or stronger. |
| | |
| signatureValue | Bit string containing the digital |
| | signature. |
| | |
| Extension: | See rules outlined in this section. |
| subjectAltName | |
| | |
| Extension: | Indicates whether the subject of the |
| BasicConstraints | certificate is a CA and the maximum depth |
| | of valid certification paths that include |
| | this certificate. This extension is used |
| | for CA certs only, and then the value of |
| | the "cA" field is set to TRUE. The |
| | default is FALSE. |
| | |
| Extension: Key Usage | The KeyUsage field MAY have the following |
| | values in the context of this profile: |
| | digitalSignature or keyAgreement, |
| | keyCertSign for verifying signatures on |
| | public key certificates. |
| | |
| Extension: Extended | The ExtKeyUsageSyntax field MAY have the |
| Key Usage | following values in context of this |
| | profile: id-kp-serverAuth for server |
| | authentication, id-kp-clientAuth for |
| | client authentication, id-kp-codeSigning |
| | for code signing (for software update |
| | mechanism), and id-kp-OCSPSigning for |
| | future OCSP usage in TLS. |
+----------------------+--------------------------------------------+
Table 1: Certificate Content
There are various cryptographic algorithms available to sign digital
certificates; those algorithms include RSA, the Digital Signature
Algorithm (DSA), and ECDSA. As Table 1 shows, certificates are
signed using ECDSA in this profile. This is not only true for the
end-entity certificates but also for all other certificates in the
chain, including CA certificates. This profiling reduces the amount
of flash memory needed on an IoT device to store the code of several
algorithm implementations due to the smaller number of options.
Further details about X.509 certificates can be found in
Section 9.1.3.3 of [RFC7252].
4.4.5. Client Certificate URLs
RFC 6066 [RFC6066] allows the sending of client-side certificates to
be avoided and uses URLs instead. This reduces the over-the-air
transmission. Note that the TLS "cached_info" extension does not
provide any help with caching client certificates.
TLS/DTLS clients MUST implement support for client certificate URLs
for those environments where client-side certificates are used and
the server-side is not constrained. For constrained servers this
functionality is NOT RECOMMENDED since it forces the server to
execute an additional protocol exchange, potentially using a protocol
it does not even support. The use of this extension also increases
the risk of a DoS attack against the constrained server due to the
additional workload.
4.4.6. Trusted CA Indication
RFC 6066 [RFC6066] allows clients to indicate what trust anchor they
support. With certificate-based authentication, a DTLS server
conveys its end-entity certificate to the client during the DTLS
handshake. Since the server does not necessarily know what trust
anchors the client has stored, to facilitate certification path
construction and validation, it includes intermediate CA certs in the
certificate payload.
Today, in most IoT deployments there is a fairly static relationship
between the IoT device (and the software running on them) and the
server-side infrastructure. For these deployments where IoT devices
interact with a fixed, preconfigured set of servers, this extension
is NOT RECOMMENDED.
In cases where clients interact with dynamically discovered TLS/DTLS
servers, for example, in the use cases described in Section 3.2.2,
the use of this extension is RECOMMENDED.
5. Signature Algorithm Extension
The "signature_algorithms" extension, defined in Section 7.4.1.4.1 of
RFC 5246 [RFC5246], allows the client to indicate to the server which
signature/hash algorithm pairs may be used in digital signatures.
The client MUST send this extension to select the use of SHA-256,
otherwise if this extension is absent, RFC 5246 defaults to SHA-1 /
ECDSA for the ECDH_ECDSA and the ECDHE_ECDSA key exchange algorithms.
The "signature_algorithms" extension is not applicable to the PSK-
based ciphersuite described in Section 4.2.
6. Error Handling
TLS/DTLS uses the alert protocol to convey errors and specifies a
long list of error types. However, not all error messages defined in
the TLS/DTLS specification are applicable to this profile. In
general, there are two categories of errors (as defined in
Section 7.2 of RFC 5246), namely fatal errors and warnings. Alert
messages with a level of "fatal" result in the immediate termination
of the connection. If possible, developers should try to develop
strategies to react to those fatal errors, such as restarting the
handshake or informing the user using the (often limited) user
interface. Warnings may be ignored by the application since many IoT
devices will have either limited ways to log errors or no ability at
all. In any case, implementers have to carefully evaluate the impact
of errors and ways to remedy the situation since a commonly used
approach for delegating decision making to users is difficult (or
impossible) to accomplish in a timely fashion.
All error messages marked as RESERVED are only supported for
backwards compatibility with the Secure Socket Layer (SSL) and MUST
NOT be used with this profile. Those include
decryption_failed_RESERVED, no_certificate_RESERVED, and
export_restriction_RESERVED.
A number of the error messages MUST only be used for certificate-
based ciphersuites. Hence, the following error messages MUST NOT be
used with PSK and raw public key authentication:
o bad_certificate,
o unsupported_certificate,
o certificate_revoked,
o certificate_expired,
o certificate_unknown,
o unknown_ca, and
o access_denied.
Since this profile does not make use of compression at the TLS layer,
the decompression_failure error message MUST NOT be used either.
RFC 4279 introduced the new alert message "unknown_psk_identity" for
PSK ciphersuites. As stated in Section 2 of RFC 4279, the
decrypt_error error message may also be used instead. For this
profile, the TLS server MUST return the decrypt_error error message
instead of the unknown_psk_identity since the two mechanisms exist
and provide the same functionality.
Furthermore, the following errors should not occur with devices and
servers supporting this specification, but implementations MUST be
prepared to process these errors to deal with servers that are not
compliant to the profiles in this document:
protocol_version: While this document focuses only on one version of
the TLS/DTLS protocol, namely version 1.2, ongoing work on TLS/
DTLS 1.3 is in progress at the time of writing.
insufficient_security: This error message indicates that the server
requires ciphers to be more secure. This document specifies only
one ciphersuite per profile, but it is likely that additional
ciphersuites will get added over time.
user_canceled: Many IoT devices are unattended and hence this error
message is unlikely to occur.
7. Session Resumption
Session resumption is a feature of the core TLS/DTLS specifications
that allows a client to continue with an earlier established session
state. The resulting exchange is shown in Figure 11. In addition,
the server may choose not to do a cookie exchange when a session is
resumed. Still, clients have to be prepared to do a cookie exchange
with every handshake. The cookie exchange is not shown in the
figure.
Client Server
------ ------
ClientHello -------->
ServerHello
[ChangeCipherSpec]
<-------- Finished
[ChangeCipherSpec]
Finished -------->
Application Data <-------> Application Data
Figure 11: DTLS Session Resumption
Constrained clients MUST implement session resumption to improve the
performance of the handshake. This will lead to a reduced number of
message exchanges, lower computational overhead (since only symmetric
cryptography is used during a session resumption exchange), and
session resumption requires less bandwidth.
For cases where the server is constrained (but not the client), the
client MUST implement RFC 5077 [RFC5077]. Note that the constrained
server refers to a device that has limitations in terms of RAM and
flash memory, which place restrictions on the amount of TLS/DTLS
security state information that can be stored on such a device. RFC
5077 specifies a version of TLS/DTLS session resumption that does not
require per-session state information to be maintained by the
constrained server. This is accomplished by using a ticket-based
approach.
If both the client and the server are constrained devices, both
devices SHOULD implement RFC 5077 and MUST implement basic session
resumption. Clients that do not want to use session resumption are
always able to send a ClientHello message with an empty session_id to
revert to a full handshake.
8. Compression
Section 3.3 of [RFC7525] recommends disabling TLS-/DTLS-level
compression due to attacks, such as CRIME [CRIME]. For IoT
applications, compression at the TLS/DTLS layer is not needed since
application-layer protocols are highly optimized, and the compression
algorithms at the DTLS layer increases code size and complexity.
TLS/DTLS layer compression is NOT RECOMMENDED by this TLS/DTLS
profile.
9. Perfect Forward Secrecy
PFS is a property that preserves the confidentiality of past protocol
interactions even in situations where the long-term secret is
compromised.
The PSK ciphersuite recommended in Section 4.2 does not offer this
property since it does not utilize a DH exchange. New ciphersuites
that support PFS for PSK-based authentication, such as proposed in
[PSK-AES-CCM-TLS], might become available as a standardized
ciphersuite in the (near) future. The recommended PSK-based
ciphersuite offers excellent performance, a very small memory
footprint, and has the lowest on the wire overhead at the expense of
not using any public cryptography. For deployments where public key
cryptography is acceptable, the use of raw public keys might offer a
middle ground between the PSK ciphersuite in terms of out-of-band
validation and the functionality offered by asymmetric cryptography.
Physical attacks create additional opportunities to gain access to
the crypto material stored on IoT devices. A PFS ciphersuite
prevents an attacker from obtaining the communication content
exchanged prior to a successful long-term key compromise; however, an
implementation that (for performance or energy efficiency reasons)
has been reusing the same ephemeral DH keys over multiple different
sessions partially defeats PFS, thus increasing the damage extent.
For this reason, implementations SHOULD NOT reuse ephemeral DH keys
over multiple protocol exchanges.
The impact of the disclosure of past communication interactions and
the desire to increase the cost for pervasive monitoring (as demanded
by [RFC7258]) has to be taken into account when selecting a
ciphersuite that does not support the PFS property.
Client implementations claiming support of this profile MUST
implement the ciphersuites listed in Section 4 according to the
selected credential type.
10. Keep-Alive
Application-layer communication may create state at the endpoints,
and this state may expire at some time. For this reason,
applications define ways to refresh state, if necessary. While the
application-layer exchanges are largely outside the scope of the
underlying TLS/DTLS exchange, similar state considerations also play
a role at the level of TLS/DTLS. While TLS/DTLS also creates state
in the form of a security context (see the security parameter
described in Appendix A.6 in RFC 5246) at the client and the server,
this state information does not expire. However, network
intermediaries may also allocate state and require this state to be
kept alive. Failure to keep state alive at a stateful packet
filtering firewall or at a NAT may result in the inability for one
node to reach the other since packets will get blocked by these
middleboxes. Periodic keep-alive messages exchanged between the TLS/
DTLS client and server keep state at these middleboxes alive.
According to measurements described in [HomeGateway], there is some
variance in state management practices used in residential gateways,
but the timeouts are heavily impacted by the choice of the transport-
layer protocol: timeouts for UDP are typically much shorter than
those for TCP.
RFC 6520 [RFC6520] defines a heartbeat mechanism to test whether the
other peer is still alive. As an additional feature, the same
mechanism can also be used to perform Path Maximum Transmission Unit
(MTU) Discovery.
A recommendation about the use of RFC 6520 depends on the type of
message exchange an IoT device performs and the number of messages
the application needs to exchange as part of their application
functionality. There are three types of exchanges that need to be
analyzed:
Client-Initiated, One-Shot Messages
This is a common communication pattern where IoT devices upload
data to a server on the Internet on an irregular basis. The
communication may be triggered by specific events, such as opening
a door.
The DTLS handshake may need to be restarted (ideally using session
resumption, if possible) in case of an IP address change.
In this case, there is no use for a keep-alive extension for this
scenario.
Client-Initiated, Regular Data Uploads
This is a variation of the previous case whereby data gets
uploaded on a regular basis, for example, based on frequent
temperature readings. If neither NAT bindings nor IP address
changes occurred, then the record layer will not notice any
changes. For the case where the IP address and port number
changes, it is necessary to recreate the record layer using
session resumption.
In this scenario, there is no use for a keep-alive extension. It
is also very likely that the device will enter a sleep cycle in
between data transmissions to keep power consumption low.
Server-Initiated Messages
In the two previous scenarios, the client initiates the protocol
interaction and maintains it. Since messages to the client may
get blocked by middleboxes, the initial connection setup is
triggered by the client and then kept alive by the server.
For this message exchange pattern, the use of DTLS heartbeat
messages is quite useful but may have to be coordinated with
application exchanges (for example, when the CoAP resource
directory is used) to avoid redundant keep-alive message
exchanges. The MTU discovery mechanism, which is also part of
[RFC6520], is less likely to be relevant since for many IoT
deployments, the most constrained link is the wireless interface
between the IoT device and the network itself (rather than some
links along the end-to-end path). Only in more complex network
topologies, such as multi-hop mesh networks, path MTU discovery
might be appropriate. It also has to be noted that DTLS itself
already provides a basic path discovery mechanism (see
Section 4.1.1.1 of RFC 6347) by using the fragmentation capability
of the handshake protocol.
For server-initiated messages, the heartbeat extension is
RECOMMENDED.
11. Timeouts
A variety of wired and wireless technologies are available to connect
devices to the Internet. Many of the low-power radio technologies,
such as IEEE 802.15.4 or Bluetooth Smart, only support small frame
sizes (e.g., 127 bytes in case of IEEE 802.15.4 as explained in
[RFC4919]). Other radio technologies, such as the Global System for
Mobile Communications (GSM) using the short messaging service (SMS),
have similar constraints in terms of payload sizes, such as 140 bytes
without the optional segmentation and reassembly scheme known as
"Concatenated SMS", but show higher latency.
The DTLS handshake protocol adds a fragmentation and reassembly
mechanism to the TLS handshake protocol since each DTLS record must
fit within a single transport layer datagram, as described in
Section 4.2.3 of [RFC6347]. Since handshake messages are potentially
bigger than the maximum record size, the mechanism fragments a
handshake message over a number of DTLS records, each of which can be
transmitted separately.
To deal with the unreliable message delivery provided by UDP, DTLS
adds timeouts and "per-flight" retransmissions, as described in
Section 4.2.4 of [RFC6347]. Although the timeout values are
implementation specific, recommendations are provided in
Section 4.2.4.1 of [RFC6347], with an initial timer value of 1 second
and double the value at each retransmission, up to no less than 60
seconds.
TLS protocol steps can take longer due to higher processing time on
the constrained side. On the other hand, the way DTLS handles
retransmission, which is per-flight instead of per-segment, tends to
interact poorly with low-bandwidth networks.
For these reasons, it's essential that the probability of a spurious
retransmit is minimized and, on timeout, the sending endpoint does
not react too aggressively. The latter is particularly relevant when
the Wireless Sensor Network (WSN) is temporarily congested: if lost
packets are reinjected too quickly, congestion worsens.
An initial timer value of 9 seconds with exponential back off up to
no less then 60 seconds is therefore RECOMMENDED.
This value is chosen big enough to absorb large latency variance due
to either slow computation on constrained endpoints or intrinsic
network characteristics (e.g., GSM-SMS), as well as to produce a low
number of retransmission events and relax the pacing between them.
Its worst case wait time is the same as using 1s timeout (i.e., 63s),
while triggering less than half of the retransmissions (2 instead of
5).
In order to minimize the wake time during DTLS handshake, sleepy
nodes might decide to select a lower threshold and, consequently, a
smaller initial timeout value. If this is the case, the
implementation MUST keep into account the considerations about
network stability described in this section.
12. Random Number Generation
The TLS/DTLS protocol requires random numbers to be available during
the protocol run. For example, during the ClientHello and the
ServerHello exchange, the client and the server exchange random
numbers. Also, the use of the DH exchange requires random numbers
during the key pair generation.
It is important to note that sources contributing to the randomness
pool on laptops or desktop PCs are not available on many IoT devices,
such as mouse movement, timing of keystrokes, air turbulence on the
movement of hard drive heads, etc. Other sources have to be found or
dedicated hardware has to be added.
Lacking sources of randomness in an embedded system may lead to the
same keys generated again and again.
The ClientHello and the ServerHello messages contain the "Random"
structure, which has two components: gmt_unix_time and a sequence of
28 random bytes. gmt_unix_time holds the current time and date in
standard UNIX 32-bit format (seconds since the midnight starting Jan
1, 1970, GMT). Since many IoT devices do not have access to an
accurate clock, it is RECOMMENDED that the receiver of a ClientHello
or ServerHello does not assume that the value in
"Random.gmt_unix_time" is an accurate representation of the current
time and instead treats it as an opaque random string.
When TLS is used with certificate-based authentication, the
availability of time information is needed to check the validity of a
certificate. Higher-layer protocols may provide secure time
information. The gmt_unix_time component of the ServerHello is not
used for this purpose.
IoT devices using TLS/DTLS must offer ways to generate quality random
numbers. There are various implementation choices for integrating a
hardware-based random number generator into a product: an
implementation inside the microcontroller itself is one option, but
dedicated crypto chips are also reasonable choices. The best choice
will depend on various factors outside the scope of this document.
Guidelines and requirements for random number generation can be found
in RFC 4086 [RFC4086] and in the NIST Special Publication 800-90a
[SP800-90A].
Chip manufacturers are highly encouraged to provide sufficient
documentation of their design for random number generators so that
customers can have confidence about the quality of the generated
random numbers. The confidence can be increased by providing
information about the procedures that have been used to verify the
randomness of numbers generated by the hardware modules. For
example, NIST Special Publication 800-22b [SP800-22b] describes
statistical tests that can be used to verify random number
generators.
13. Truncated MAC and Encrypt-then-MAC Extension
The truncated MAC extension was introduced in RFC 6066 [RFC6066] with
the goal to reduce the size of the MAC used at the record layer.
This extension was developed for TLS ciphersuites that used older
modes of operation where the MAC and the encryption operation were
performed independently.
The recommended ciphersuites in this document use the newer AEAD
construct, namely the CCM mode with 8-octet authentication tags, and
are therefore not applicable to the truncated MAC extension.
RFC 7366 [RFC7366] introduced the encrypt-then-MAC extension (instead
of the previously used MAC-then-encrypt) since the MAC-then-encrypt
mechanism has been the subject of a number of security
vulnerabilities. RFC 7366 is, however, also not applicable to the
AEAD ciphers recommended in this document.
Implementations conformant to this specification MUST use AEAD
ciphers. RFC 7366 ("encrypt-then-MAC") and RFC 6066 ("truncated MAC
extension") are not applicable to this specification and MUST NOT be
used.
14. Server Name Indication (SNI)
The SNI extension [RFC6066] defines a mechanism for a client to tell
a TLS/DTLS server the name of the server it wants to contact. This
is a useful extension for many hosting environments where multiple
virtual servers are run on a single IP address.
Implementing the Server Name Indication extension is REQUIRED unless
it is known that a TLS/DTLS client does not interact with a server in
a hosting environment.
15. Maximum Fragment Length Negotiation
This RFC 6066 extension lowers the maximum fragment length support
needed for the record layer from 2^14 bytes to 2^9 bytes.
This is a very useful extension that allows the client to indicate to
the server how much maximum memory buffers it uses for incoming
messages. Ultimately, the main benefit of this extension is to allow
client implementations to lower their RAM requirements since the
client does not need to accept packets of large size (such as 16K
packets as required by plain TLS/DTLS).
Client implementations MUST support this extension.
16. Session Hash
In order to begin connection protection, the Record Protocol requires
specification of a suite of algorithms, a master secret, and the
client and server random values. The algorithm for computing the
master secret is defined in Section 8.1 of RFC 5246, but it only
includes a small number of parameters exchanged during the handshake
and does not include parameters like the client and server
identities. This can be utilized by an attacker to mount a
man-in-the-middle attack since the master secret is not guaranteed to
be unique across sessions, as discovered in the "triple handshake"
attack [Triple-HS].
[RFC7627] defines a TLS extension that binds the master secret to a
log of the full handshake that computes it, thus preventing such
attacks.
Client implementations SHOULD implement this extension even though
the ciphersuites recommended by this profile are not vulnerable to
this attack. For DH-based ciphersuites, the keying material is
contributed by both parties and in case of the pre-shared secret key
ciphersuite, both parties need to be in possession of the shared
secret to ensure that the handshake completes successfully. It is,
however, possible that some application-layer protocols will tunnel
other authentication protocols on top of DTLS making this attack
relevant again.
17. Renegotiation Attacks
TLS/DTLS allows a client and a server that already have a TLS/DTLS
connection to negotiate new parameters, generate new keys, etc., by
using the renegotiation feature. Renegotiation happens in the
existing connection, with the new handshake packets being encrypted
along with application data. Upon completion of the renegotiation
procedure, the new channel replaces the old channel.
As described in RFC 5746 [RFC5746], there is no cryptographic binding
between the two handshakes, although the new handshake is carried out
using the cryptographic parameters established by the original
handshake.
To prevent the renegotiation attack [RFC5746], this specification
REQUIRES the TLS renegotiation feature to be disabled. Clients MUST
respond to server-initiated renegotiation attempts with an alert
message (no_renegotiation), and clients MUST NOT initiate them.
18. Downgrading Attacks
When a client sends a ClientHello with a version higher than the
highest version known to the server, the server is supposed to reply
with ServerHello.version equal to the highest version known to the
server, and then the handshake can proceed. This behavior is known
as version tolerance. Version intolerance is when the server (or a
middlebox) breaks the handshake when it sees a ClientHello.version
higher than what it knows about. This is the behavior that leads
some clients to rerun the handshake with a lower version. As a
result, a potential security vulnerability is introduced when a
system is running an old TLS/SSL version (e.g., because of the need
to integrate with legacy systems). In the worst case, this allows an
attacker to downgrade the protocol handshake to SSL 3.0. SSL 3.0 is
so broken that there is no secure cipher available for it (see
[RFC7568]).
The above-described downgrade vulnerability is solved by the TLS
Fallback Signaling Cipher Suite Value (SCSV) [RFC7507] extension.
However, the solution is not applicable to implementations conforming
to this profile since the version negotiation MUST use TLS/DTLS
version 1.2 (or higher). More specifically, this implies:
o Clients MUST NOT send a TLS/DTLS version lower than version 1.2 in
the ClientHello.
o Clients MUST NOT retry a failed negotiation offering a TLS/DTLS
version lower than 1.2.
o Servers MUST fail the handshake by sending a protocol_version
fatal alert if a TLS/DTLS version >= 1.2 cannot be negotiated.
Note that the aborted connection is non-resumable.
19. Crypto Agility
This document recommends that software and chip manufacturers
implement AES and the CCM mode of operation. This document
references the CoAP-recommended ciphersuite choices, which have been
selected based on implementation and deployment experience from the
IoT community. Over time, the preference for algorithms will,
however, change. Not all components of a ciphersuite are likely to
change at the same speed. Changes are more likely expected for
ciphers, the mode of operation, and the hash algorithms. The
recommended key lengths have to be adjusted over time as well. Some
deployment environments will also be impacted by local regulation,
which might dictate a certain algorithm and key size combination.
Ongoing discussions regarding the choice of specific ECC curves will
also likely impact implementations. Note that this document does not
recommend or mandate a specific ECC curve.
The following recommendations can be made to chip manufacturers:
o Make any AES hardware-based crypto implementation accessible to
developers working on security implementations at higher layers in
the protocol stack. Sometimes hardware implementations are added
to microcontrollers to offer support for functionality needed at
the link layer and are only available to the on-chip link-layer
protocol implementation. Such a setup does not allow application
developers to reuse the hardware-based AES implementation.
o Provide flexibility for the use of the crypto function with future
extensibility in mind. For example, making an AES-CCM
implementation available to developers is a first step but such an
implementation may not be usable due to parameter differences
between an AES-CCM implementation. AES-CCM in IEEE 802.15.4 and
Bluetooth Smart use a nonce length of 13 octets while DTLS uses a
nonce length of 12 octets. Hardware implementations of AES-CCM
for IEEE 802.15.4 and Bluetooth Smart are therefore not reusable
by a DTLS stack.
o Offer access to building blocks in addition (or as an alternative)
to the complete functionality. For example, a chip manufacturer
who gives developers access to the AES crypto function can use it
to build an efficient AES-GCM implementation. Another example is
to make a special instruction available that increases the speed
of speed-up carryless multiplications.
As a recommendation for developers and product architects, we suggest
that sufficient headroom is provided to allow an upgrade to a newer
cryptographic algorithm over the lifetime of the product. As an
example, while AES-CCM is recommended throughout this specification,
future products might use the ChaCha20 cipher in combination with the
Poly1305 authenticator [RFC7539]. The assumption is made that a
robust software update mechanism is offered.
20. Key Length Recommendations
RFC 4492 [RFC4492] gives approximate comparable key sizes for
symmetric- and asymmetric-key cryptosystems based on the best-known
algorithms for attacking them. While other publications suggest
slightly different numbers, such as [Keylength], the approximate
relationship still holds true. Figure 12 illustrates the comparable
key sizes in bits.
Symmetric | ECC | DH/DSA/RSA
------------+---------+-------------
80 | 163 | 1024
112 | 233 | 2048
128 | 283 | 3072
192 | 409 | 7680
256 | 571 | 15360
Figure 12: Comparable Key Sizes (in Bits) Based on RFC 4492
At the time of writing, the key size recommendations for use with
TLS-based ciphers found in [RFC7525] recommend DH key lengths of at
least 2048 bits, which corresponds to a 112-bit symmetric key and a
233-bit ECC key. These recommendations are roughly in line with
those from other organizations, such as the National Institute of
Standards and Technology (NIST) or the European Network and
Information Security Agency (ENISA). The authors of
[ENISA-Report2013] add that a 80-bit symmetric key is sufficient for
legacy applications for the coming years, but a 128-bit symmetric key
is the minimum requirement for new systems being deployed. The
authors further note that one needs to also take into account the
length of time data needs to be kept secure for. The use of 80-bit
symmetric keys for transactional data may be acceptable for the near
future while one has to insist on 128-bit symmetric keys for long-
lived data.
Note that the recommendations for 112-bit symmetric keys are chosen
conservatively under the assumption that IoT devices have a long
expected lifetime (such as 10+ years) and that this key length
recommendation refers to the long-term keys used for device
authentication. Keys, which are provisioned dynamically, for the
protection of transactional data (such as ephemeral DH keys used in
various TLS/DTLS ciphersuites) may be shorter considering the
sensitivity of the exchanged data.
21. False Start
A full TLS handshake as specified in [RFC5246] requires two full
protocol rounds (four flights) before the handshake is complete and
the protocol parties may begin to send application data.
An abbreviated handshake (resuming an earlier TLS session) is
complete after three flights, thus adding just one round-trip time if
the client sends application data first.
If the conditions outlined in [TLS-FALSESTART] are met, application
data can be transmitted when the sender has sent its own
"ChangeCipherSpec" and "Finished" messages. This achieves an
improvement of one round-trip time for full handshakes if the client
sends application data first and for abbreviated handshakes if the
server sends application data first.
The conditions for using the TLS False Start mechanism are met by the
public-key-based ciphersuites in this document. In summary, the
conditions are:
o Modern symmetric ciphers with an effective key length of 128 bits,
such as AES-128-CCM
o Client certificate types, such as ecdsa_sign
o Key exchange methods, such as ECDHE_ECDSA
Based on the improvement over a full round-trip for the full TLS/DTLS
exchange, this specification RECOMMENDS the use of the False Start
mechanism when clients send application data first.
22. Privacy Considerations
The DTLS handshake exchange conveys various identifiers, which can be
observed by an on-path eavesdropper. For example, the DTLS PSK
exchange reveals the PSK identity, the supported extensions, the
session ID, algorithm parameters, etc. When session resumption is
used, then individual TLS sessions can be correlated by an on-path
adversary. With many IoT deployments, it is likely that keying
material and their identifiers are persistent over a longer period of
time due to the cost of updating software on these devices.
User participation poses a challenge in many IoT deployments since
many of the IoT devices operate unattended, even though they are
initially provisioned by a human. The ability to control data
sharing and to configure preferences will have to be provided at a
system level rather than at the level of the DTLS exchange itself,
which is the scope of this document. Quite naturally, the use of
DTLS with mutual authentication will allow a TLS server to collect
authentication information about the IoT device (likely over a long
period of time). While this strong form of authentication will
prevent misattribution, it also allows strong identification.
Device-related data collection (e.g., sensor recordings) associated
with other data types will prove to be truly useful, but this extra
data might include personal information about the owner of the device
or data about the environment it senses. Consequently, the data
stored on the server side will be vulnerable to stored data
compromise. For the communication between the client and the server,
this specification prevents eavesdroppers from gaining access to the
communication content. While the PSK-based ciphersuite does not
provide PFS, the asymmetric versions do. This prevents an adversary
from obtaining past communication content when access to a long-term
secret has been gained. Note that no extra effort to make traffic
analysis more difficult is provided by the recommendations made in
this document.
Note that the absence or presence of communication itself might
reveal information to an adversary. For example, a presence sensor
may initiate messaging when a person enters a building. While TLS/
DTLS would offer confidentiality protection of the transmitted
information, it does not help to conceal all communication patterns.
Furthermore, the IP header, which is not protected by TLS/DTLS,
additionally reveals information about the other communication
endpoint. For applications where such privacy concerns exist,
additional safeguards are required, such as injecting dummy traffic
and onion routing. A detailed treatment of such solutions is outside
the scope of this document and requires a system-level view.
23. Security Considerations
This entire document is about security.
We would also like to point out that designing a software update
mechanism into an IoT system is crucial to ensure that both
functionality can be enhanced and that potential vulnerabilities can
be fixed. This software update mechanism is important for changing
configuration information, for example, trust anchors and other
keying-related information. Such a suitable software update
mechanism is available with the LWM2M protocol published by the OMA
[LWM2M].
24. References
24.1. Normative References
[EUI64] IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)",
Registration Authority,
<https://standards.ieee.org/regauth/
oui/tutorials/EUI64.html>.
[GSM-SMS] ETSI, "3rd Generation Partnership Project; Technical
Specification Group Core Network and Terminals; Technical
realization of the Short Message Service (SMS) (Release
13)", 3GPP TS 23.040 V13.1.0, March 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC4279] Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
Ciphersuites for Transport Layer Security (TLS)",
RFC 4279, DOI 10.17487/RFC4279, December 2005,
<http://www.rfc-editor.org/info/rfc4279>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5746] Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
"Transport Layer Security (TLS) Renegotiation Indication
Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
<http://www.rfc-editor.org/info/rfc5746>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <http://www.rfc-editor.org/info/rfc6347>.
[RFC6520] Seggelmann, R., Tuexen, M., and M. Williams, "Transport
Layer Security (TLS) and Datagram Transport Layer Security
(DTLS) Heartbeat Extension", RFC 6520,
DOI 10.17487/RFC6520, February 2012,
<http://www.rfc-editor.org/info/rfc6520>.
[RFC7250] Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
Weiler, S., and T. Kivinen, "Using Raw Public Keys in
Transport Layer Security (TLS) and Datagram Transport
Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
June 2014, <http://www.rfc-editor.org/info/rfc7250>.
[RFC7251] McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
<http://www.rfc-editor.org/info/rfc7251>.
[RFC7627] Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
Langley, A., and M. Ray, "Transport Layer Security (TLS)
Session Hash and Extended Master Secret Extension",
RFC 7627, DOI 10.17487/RFC7627, September 2015,
<http://www.rfc-editor.org/info/rfc7627>.
[RFC7924] Santesson, S. and H. Tschofenig, "Transport Layer Security
(TLS) Cached Information Extension", RFC 7924,
DOI 10.17487/RFC7924, July 2016,
<http://www.rfc-editor.org/info/rfc7924>.
[WAP-WDP] Open Mobile Alliance, "Wireless Datagram Protocol",
Wireless Application Protocol, WAP-259-WDP, June 2001.
24.2. Informative References
[ACE-WG] IETF, "Authentication and Authorization for Constrained
Environments (ACE) Working Group",
<https://datatracker.ietf.org/wg/ace/charter>.
[AES] National Institute of Standards and Technology, "Advanced
Encryption Standard (AES)", NIST FIPS PUB 197, November
2001, <http://csrc.nist.gov/publications/fips/fips197/
fips-197.pdf>.
[CCM] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation: The
CCM Mode for Authentication and Confidentiality", NIST
Special Publication 800-38C, May 2004,
<http://csrc.nist.gov/publications/nistpubs/800-38C/
SP800-38C_updated-July20_2007.pdf>.
[COAP-TCP-TLS]
Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets", Work
in Progress, draft-ietf-core-coap-tcp-tls-03, July 2016.
[CoRE-RD] Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", Work in Progress, draft-ietf-core-
resource-directory-08, July 2016.
[CRIME] Wikipedia, "CRIME", May 2016, <https://en.wikipedia.org/w/
index.php?title=CRIME&oldid=721665716>.
[ENISA-Report2013]
ENISA, "Algorithms, Key Sizes and Parameters Report -
2013", October 2013, <https://www.enisa.europa.eu/
activities/identity-and-trust/library/deliverables/
algorithms-key-sizes-and-parameters-report>.
[FFDHE-TLS]
Gillmor, D., "Negotiated Finite Field Diffie-Hellman
Ephemeral Parameters for TLS", Work in Progress,
draft-ietf-tls-negotiated-ff-dhe-10, June 2015.
[HomeGateway]
Eggert, L., Hatoen, S., Kojo, M., Nyrhinen, A., Sarolahti,
P., and S. Strowes, "An Experimental Study of Home Gateway
Characteristics", In Proceedings of the 10th ACM SIGCOMM
conference on Internet measurement,
DOI 10.1145/1879141.1879174, 2010,
<http://conferences.sigcomm.org/imc/2010/papers/p260.pdf>.
[IANA-TLS] IANA, "Transport Layer Security (TLS) Parameters",
<https://www.iana.org/assignments/tls-parameters>.
[ImprintingSurvey]
Chilton, E., "A Brief Survey of Imprinting Options for
Constrained Devices", March 2012,
<http://www.lix.polytechnique.fr/hipercom/
SmartObjectSecurity/papers/EricRescorla.pdf>.
[Keylength]
Giry, D., "Cryptographic Key Length Recommendations",
September 2015, <http://www.keylength.com>.
[LWM2M] Open Mobile Alliance, "Lightweight Machine-to-Machine
Requirements", Candidate Version 1.0, OMA-RD-
LightweightM2M-V1_0-20131210-C, December 2013,
<http://openmobilealliance.org/about-oma/work-program/
m2m-enablers>.
[PSK-AES-CCM-TLS]
Schmertmann, L. and C. Bormann, "ECDHE-PSK AES-CCM Cipher
Suites with Forward Secrecy for Transport Layer Security
(TLS)", Work in Progress, draft-schmertmann-dice-ccm-
psk-pfs-01, August 2014.
[RFC1981] McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
1996, <http://www.rfc-editor.org/info/rfc1981>.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
DOI 10.17487/RFC2104, February 1997,
<http://www.rfc-editor.org/info/rfc2104>.
[RFC2865] Rigney, C., Willens, S., Rubens, A., and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, DOI 10.17487/RFC2865, June 2000,
<http://www.rfc-editor.org/info/rfc2865>.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
2003, <http://www.rfc-editor.org/info/rfc3610>.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, Ed., "Extensible Authentication Protocol
(EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
<http://www.rfc-editor.org/info/rfc3748>.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
DOI 10.17487/RFC4086, June 2005,
<http://www.rfc-editor.org/info/rfc4086>.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492,
DOI 10.17487/RFC4492, May 2006,
<http://www.rfc-editor.org/info/rfc4492>.
[RFC4821] Mathis, M. and J. Heffner, "Packetization Layer Path MTU
Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
<http://www.rfc-editor.org/info/rfc4821>.
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals",
RFC 4919, DOI 10.17487/RFC4919, August 2007,
<http://www.rfc-editor.org/info/rfc4919>.
[RFC5077] Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
"Transport Layer Security (TLS) Session Resumption without
Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
January 2008, <http://www.rfc-editor.org/info/rfc5077>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.
[RFC5216] Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
March 2008, <http://www.rfc-editor.org/info/rfc5216>.
[RFC5247] Aboba, B., Simon, D., and P. Eronen, "Extensible
Authentication Protocol (EAP) Key Management Framework",
RFC 5247, DOI 10.17487/RFC5247, August 2008,
<http://www.rfc-editor.org/info/rfc5247>.
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
DOI 10.17487/RFC5288, August 2008,
<http://www.rfc-editor.org/info/rfc5288>.
[RFC5480] Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
"Elliptic Curve Cryptography Subject Public Key
Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
<http://www.rfc-editor.org/info/rfc5480>.
[RFC5758] Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
Polk, "Internet X.509 Public Key Infrastructure:
Additional Algorithms and Identifiers for DSA and ECDSA",
RFC 5758, DOI 10.17487/RFC5758, January 2010,
<http://www.rfc-editor.org/info/rfc5758>.
[RFC5934] Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
Management Protocol (TAMP)", RFC 5934,
DOI 10.17487/RFC5934, August 2010,
<http://www.rfc-editor.org/info/rfc5934>.
[RFC6024] Reddy, R. and C. Wallace, "Trust Anchor Management
Requirements", RFC 6024, DOI 10.17487/RFC6024, October
2010, <http://www.rfc-editor.org/info/rfc6024>.
[RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
Curve Cryptography Algorithms", RFC 6090,
DOI 10.17487/RFC6090, February 2011,
<http://www.rfc-editor.org/info/rfc6090>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<http://www.rfc-editor.org/info/rfc6234>.
[RFC6655] McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
Transport Layer Security (TLS)", RFC 6655,
DOI 10.17487/RFC6655, July 2012,
<http://www.rfc-editor.org/info/rfc6655>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<http://www.rfc-editor.org/info/rfc6690>.
[RFC6733] Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
Ed., "Diameter Base Protocol", RFC 6733,
DOI 10.17487/RFC6733, October 2012,
<http://www.rfc-editor.org/info/rfc6733>.
[RFC6943] Thaler, D., Ed., "Issues in Identifier Comparison for
Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
2013, <http://www.rfc-editor.org/info/rfc6943>.
[RFC6961] Pettersen, Y., "The Transport Layer Security (TLS)
Multiple Certificate Status Request Extension", RFC 6961,
DOI 10.17487/RFC6961, June 2013,
<http://www.rfc-editor.org/info/rfc6961>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7258] Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
2014, <http://www.rfc-editor.org/info/rfc7258>.
[RFC7366] Gutmann, P., "Encrypt-then-MAC for Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
<http://www.rfc-editor.org/info/rfc7366>.
[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<http://www.rfc-editor.org/info/rfc7390>.
[RFC7397] Gilger, J. and H. Tschofenig, "Report from the Smart
Object Security Workshop", RFC 7397, DOI 10.17487/RFC7397,
December 2014, <http://www.rfc-editor.org/info/rfc7397>.
[RFC7400] Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
IPv6 over Low-Power Wireless Personal Area Networks
(6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
2014, <http://www.rfc-editor.org/info/rfc7400>.
[RFC7452] Tschofenig, H., Arkko, J., Thaler, D., and D. McPherson,
"Architectural Considerations in Smart Object Networking",
RFC 7452, DOI 10.17487/RFC7452, March 2015,
<http://www.rfc-editor.org/info/rfc7452>.
[RFC7465] Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
DOI 10.17487/RFC7465, February 2015,
<http://www.rfc-editor.org/info/rfc7465>.
[RFC7507] Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
Suite Value (SCSV) for Preventing Protocol Downgrade
Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
<http://www.rfc-editor.org/info/rfc7507>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
2015, <http://www.rfc-editor.org/info/rfc7525>.
[RFC7539] Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
<http://www.rfc-editor.org/info/rfc7539>.
[RFC7568] Barnes, R., Thomson, M., Pironti, A., and A. Langley,
"Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
DOI 10.17487/RFC7568, June 2015,
<http://www.rfc-editor.org/info/rfc7568>.
[RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
for Security", RFC 7748, DOI 10.17487/RFC7748, January
2016, <http://www.rfc-editor.org/info/rfc7748>.
[SP800-107-rev1]
National Institute of Standards and Technology,
"Recommendation for Applications Using Approved Hash
Algorithms", NIST Special Publication 800-107, Revision 1,
DOI 10.6028/NIST.SP.800-107r1, August 2012,
<http://csrc.nist.gov/publications/nistpubs/800-107-rev1/
sp800-107-rev1.pdf>.
[SP800-22b]
National Institute of Standards and Technology, "A
Statistical Test Suite for Random and Pseudorandom Number
Generators for Cryptographic Applications", NIST Special
Publication 800-22, Revision 1a, April 2010,
<http://csrc.nist.gov/publications/nistpubs/800-22-rev1a/
SP800-22rev1a.pdf>.
[SP800-90A]
National Institute of Standards and Technology,
"Recommendation for Random Number Generation Using
Deterministic Random Bit Generators", NIST Special
Publication 800-90A Revision 1,
DOI 10.6028/NIST.SP.800-90Ar1, June 2015,
<http://csrc.nist.gov/publications/drafts/800-90/
sp800-90a_r1_draft_november2014_ver.pdf>.
[TLS-FALSESTART]
Langley, A., Modadugu, N., and B. Moeller, "Transport
Layer Security (TLS) False Start", Work in Progress,
draft-ietf-tls-falsestart-02, May 2016.
[Triple-HS]
Bhargavan, K., Delignat-Lavaud, C., Pironti, A., and P.
Yves Strub, "Triple Handshakes and Cookie Cutters:
Breaking and Fixing Authentication over TLS", In
Proceedings of the IEEE Symposium on Security and Privacy,
Pages 98-113, DOI 10.1109/SP.2014.14, 2014.
Appendix A. Conveying DTLS over SMS
This section is normative for the use of DTLS over SMS. Timer
recommendations are already outlined in Section 11 and also
applicable to the transport of DTLS over SMS.
This section requires readers to be familiar with the terminology and
concepts described in [GSM-SMS] and [WAP-WDP].
The remainder of this section assumes Mobile Stations are capable of
producing and consuming Transport Protocol Data Units (TPDUs) encoded
as 8-bit binary data.
A.1. Overview
DTLS adds an additional round-trip to the TLS [RFC5246] handshake to
serve as a return-routability test for protection against certain
types of DoS attacks. Thus, a full-blown DTLS handshake comprises up
to 6 "flights" (i.e., logical message exchanges), each of which is
then mapped on to one or more DTLS records using the segmentation and
reassembly (SaR) scheme described in Section 4.2.3 of [RFC6347]. The
overhead for said scheme is 6 bytes per handshake message which,
given a realistic 10+ messages handshake, would amount to around 60
bytes across the whole handshake sequence.
Note that the DTLS SaR scheme is defined for handshake messages only.
In fact, DTLS records are never fragmented and MUST fit within a
single transport layer datagram.
SMS provides an optional segmentation and reassembly scheme as well,
known as Concatenated short messages (see Section 9.2.3.24.1 of
[GSM-SMS]). However, since the SaR scheme in DTLS cannot be
circumvented, the Concatenated short messages mechanism SHOULD NOT be
used during handshake to avoid redundant overhead. Before starting
the handshake phase (either actively or passively), the DTLS
implementation MUST be explicitly configured with the Path MTU (PMTU)
of the SMS transport in order to correctly instrument its SaR
function. The PMTU SHALL be 133 bytes if multiplexing based on the
Wireless Datagram Protocol (WDP) is used (see Appendix A.3); 140
bytes otherwise.
It is RECOMMENDED that the established security context over the
longest possible period be used (possibly until a Closure Alert
message is received or after a very long inactivity timeout) to avoid
the expensive re-establishment of the security association.
A.2. Message Segmentation and Reassembly
The content of an SMS message is carried in the TP-UserData field,
and its size may be up to 140 bytes. As already mentioned in
Appendix A.1, longer (i.e., up to 34170 bytes) messages can be sent
using Concatenated SMS.
This scheme consumes 6-7 bytes (depending on whether the short or
long segmentation format is used) of the TP-UserData field, thus
reducing the space available for the actual content of the SMS
message to 133-134 bytes per TPDU.
Though in principle a PMTU value higher than 140 bytes could be used,
which may look like an appealing option given its more efficient use
of the transport, there are disadvantages to consider. First, there
is an additional overhead of 7 bytes per TPDU to be paid to the SaR
function (which is in addition to the overhead introduced by the DTLS
SaR mechanism. Second, some networks only partially support the
Concatenated SMS function, and others do not support it at all.
For these reasons, the Concatenated short messages mechanism SHOULD
NOT be used, and it is RECOMMENDED to leave the same PMTU settings
used during the handshake phase, i.e., 133 bytes if WDP-based
multiplexing is enabled; 140 bytes otherwise.
Note that, after the DTLS handshake has completed, any fragmentation
and reassembly logic that pertains the application layer (e.g.,
segmenting CoAP messages into DTLS records and reassembling them
after the crypto operations have been successfully performed) needs
to be handled by the application that uses the established DTLS
tunnel.
A.3. Multiplexing Security Associations
Unlike IPsec Encapsulating Security Payload (ESP) / Authentication
Header (AH), DTLS records do not contain any association identifiers.
Applications must arrange to multiplex between associations on the
same endpoint which, when using UDP/IP, is usually done with the
host/port number.
If the DTLS server allows more than one client to be active at any
given time, then the Wireless Application Protocol (WAP) User
Datagram Protocol [WAP-WDP] can be used to achieve multiplexing of
the different security associations. (The use of WDP provides the
additional benefit that upper-layer protocols can operate
independently of the underlying wireless network, hence achieving
application-agnostic transport handover.)
The total overhead cost for encoding the WDP source and destination
ports is either 5 or 7 bytes out of the total available for the SMS
content depending on if 1-byte or 2-byte port identifiers are used,
as shown in Figures 13 and 14.
0 1 2 3 4
+--------+--------+--------+--------+--------+
| ... | 0x04 | 2 | ... | ... |
+--------+--------+--------+--------+--------+
UDH IEI IE Dest Source
Length Length Port Port
Legend:
UDH = user data header
IEI = information element identifier
Figure 13: Application Port Addressing Scheme (8-Bit Address)
0 1 2 3 4 5 6
+--------+--------+--------+--------+--------+--------+--------+
| ... | 0x05 | 4 | ... | ... |
+--------+--------+--------+--------+--------+--------+--------+
UDH IEI IE Dest Source
Length Length Port Port
Figure 14: Application Port Addressing Scheme (16-Bit Address)
The receiving side of the communication gets the source address from
the originator address (TP-OA) field of the SMS-DELIVER TPDU. This
way, a unique 4-tuple identifying the security association can be
reconstructed at both ends. (When replying to its DTLS peer, the
sender will swap the TP-OA and destination address (TP-DA) parameters
and the source and destination ports in the WDP.)
A.4. Timeout
If SMS-STATUS-REPORT messages are enabled, their receipt is not to be
interpreted as the signal that the specific handshake message has
been acted upon by the receiving party. Therefore, it MUST NOT be
taken into account by the DTLS timeout and retransmission function.
Handshake messages MUST carry a validity period (TP-VP parameter in a
SMS-SUBMIT TPDU) that is not less than the current value of the
retransmission timeout. In order to avoid persisting messages in the
network that will be discarded by the receiving party, handshake
messages SHOULD carry a validity period that is the same as, or just
slightly higher than, the current value of the retransmission
timeout.
Appendix B. DTLS Record Layer Per-Packet Overhead
Figure 15 shows the overhead for the DTLS record layer for protecting
data traffic when AES-128-CCM with an 8-octet Integrity Check Value
(ICV) is used.
DTLS Record Layer Header................13 bytes
Nonce (Explicit).........................8 bytes
ICV..................................... 8 bytes
------------------------------------------------
Overhead................................29 bytes
------------------------------------------------
Figure 15: AES-128-CCM-8 DTLS Record Layer Per-Packet Overhead
The DTLS record layer header has 13 octets and consists of:
o 1-octet content type field,
o 2-octet version field,
o 2-octet epoch field,
o 6-octet sequence number, and
o 2-octet length field.
The "nonce" input to the AEAD algorithm is exactly that of [RFC5288],
i.e., 12 bytes long. It consists of two values, namely a 4-octet
salt and an 8-octet nonce_explicit:
The salt is the "implicit" part and is not sent in the packet.
Instead, the salt is generated as part of the handshake process.
The nonce_explicit value is 8 octets long and it is chosen by the
sender and carried in each TLS record. RFC 6655 [RFC6655] allows
the nonce_explicit to be a sequence number or something else.
This document makes this use more restrictive for use with DTLS:
the 64-bit none_explicit value MUST be the 16-bit epoch
concatenated with the 48-bit seq_num. The sequence number
component of the nonce_explicit field at the AES-CCM layer is an
exact copy of the sequence number in the record layer header
field. This leads to a duplication of 8-bytes per record.
To avoid this 8-byte duplication, RFC 7400 [RFC7400] provides help
with the use of the generic header compression technique for IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs). Note
that this header compression technique is not available when DTLS
is exchanged over transports that do not use IPv6 or 6LoWPAN, such
as the SMS transport described in Appendix A of this document.
Appendix C. DTLS Fragmentation
Section 4.2.3 of [RFC6347] advises DTLS implementations to not
produce overlapping fragments. However, it requires receivers to be
able to cope with them. The need for the latter requisite is
explained in Section 4.1.1.1 of [RFC6347]: accurate PMTU estimation
may be traded for shorter handshake completion time.
In many cases, the cost of handling fragment overlaps has proved to
be unaffordable for constrained implementations, particularly because
of the increased complexity in buffer management.
In order to reduce the likelihood of producing different fragment
sizes and consequent overlaps within the same handshake, this
document RECOMMENDs:
o clients (handshake initiators) to use reliable PMTU information
for the intended destination; and
o servers to mirror the fragment size selected by their clients.
The PMTU information comes from either a "fresh enough" discovery
performed by the client [RFC1981] [RFC4821] or some other reliable
out-of-band channel.
Acknowledgments
Thanks to Derek Atkins, Paul Bakker, Olaf Bergmann, Carsten Bormann,
Ben Campbell, Brian Carpenter, Robert Cragie, Spencer Dawkins, Russ
Housley, Rene Hummen, Jayaraghavendran K, Sye Loong Keoh, Matthias
Kovatsch, Sandeep Kumar, Barry Leiba, Simon Lemay, Alexey Melnikov,
Gabriel Montenegro, Manuel Pegourie-Gonnard, Akbar Rahman, Eric
Rescorla, Michael Richardson, Ludwig Seitz, Zach Shelby, Michael
StJohns, Rene Struik, Tina Tsou, and Sean Turner for their helpful
comments and discussions that have shaped the document.
A big thanks also to Klaus Hartke, who wrote the initial draft
version of this document.
Finally, we would like to thank our area director (Stephen Farrell)
and our working group chairs (Zach Shelby and Dorothy Gellert) for
their support.
Authors' Addresses
Hannes Tschofenig (editor)
ARM Ltd.
110 Fulbourn Rd
Cambridge CB1 9NJ
United Kingdom
Email: Hannes.tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Thomas Fossati
Nokia
3 Ely Road
Milton, Cambridge CB24 6DD
United Kingdom
Email: thomas.fossati@nokia.com
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