ACE Working Group S. Gerdes
Internet-Draft Universitaet Bremen TZI
Intended status: Informational L. Seitz
Expires: September 2, 2016 SICS Swedish ICT AB
G. Selander
Ericsson
C. Bormann, Ed.
Universitaet Bremen TZI
March 01, 2016
An architecture for authorization in constrained environments
draft-ietf-ace-actors-03
Abstract
Constrained-node networks are networks where some nodes have severe
constraints on code size, state memory, processing capabilities, user
interface, power and communication bandwidth (RFC 7228).
This document provides terminology, and identifies the elements that
an architecture needs to address, providing a problem statement, for
authentication and authorization in these networks.
Status of This Memo
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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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. Architecture and High-level Problem Statement . . . . . . . . 6
2.1. Elements of an Architecture . . . . . . . . . . . . . . . 6
2.2. Architecture Variants . . . . . . . . . . . . . . . . . . 8
2.3. Information Flows . . . . . . . . . . . . . . . . . . . . 11
3. Security Objectives . . . . . . . . . . . . . . . . . . . . . 12
3.1. End-to-End Security Objectives in Multi-Hop Scenarios . . 13
4. Authentication and Authorization . . . . . . . . . . . . . . 14
5. Actors and their Tasks . . . . . . . . . . . . . . . . . . . 16
5.1. Constrained Level Actors . . . . . . . . . . . . . . . . 16
5.2. Principal Level Actors . . . . . . . . . . . . . . . . . 17
5.3. Less-Constrained Level Actors . . . . . . . . . . . . . . 18
6. Kinds of Protocols . . . . . . . . . . . . . . . . . . . . . 18
6.1. Constrained Level Protocols . . . . . . . . . . . . . . . 19
6.1.1. Cross Level Support Protocols . . . . . . . . . . . . 19
6.2. Less-Constrained Level Protocols . . . . . . . . . . . . 19
7. Elements of a Solution . . . . . . . . . . . . . . . . . . . 20
7.1. Authorization . . . . . . . . . . . . . . . . . . . . . . 20
7.2. Authentication . . . . . . . . . . . . . . . . . . . . . 20
7.3. Communication Security . . . . . . . . . . . . . . . . . 21
7.4. Cryptographic Keys . . . . . . . . . . . . . . . . . . . 22
8. Assumptions and Requirements . . . . . . . . . . . . . . . . 22
8.1. Architecture . . . . . . . . . . . . . . . . . . . . . . 22
8.2. Constrained Devices . . . . . . . . . . . . . . . . . . . 23
8.3. Authentication . . . . . . . . . . . . . . . . . . . . . 24
8.4. Server-side Authorization . . . . . . . . . . . . . . . . 24
8.5. Client-side Authorization Information . . . . . . . . . . 25
8.6. Server-side Authorization Information . . . . . . . . . . 25
8.7. Resource Access . . . . . . . . . . . . . . . . . . . . . 26
8.8. Keys and Cipher Suites . . . . . . . . . . . . . . . . . 26
8.9. Network Considerations . . . . . . . . . . . . . . . . . 26
8.10. Legacy Considerations . . . . . . . . . . . . . . . . . . 27
9. Security Considerations . . . . . . . . . . . . . . . . . . . 27
9.1. Physical Attacks on Sensor and Actuator Networks . . . . 27
9.2. Time Measurements . . . . . . . . . . . . . . . . . . . . 29
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 29
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
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12. Informative References . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
Constrained nodes are small devices with limited abilities which in
many cases are made to fulfill a specific simple task. They have
limited hardware resources such as processing power, memory, non-
volatile storage and transmission capacity and additionally in most
cases do not have user interfaces and displays. Due to these
constraints, commonly used security protocols are not always easily
applicable.
Constrained nodes are expected to be integrated in all aspects of
everyday life and thus will be entrusted with vast amounts of data.
Without appropriate security mechanisms attackers might gain control
over things relevant to our lives. Authentication and authorization
mechanisms are therefore prerequisites for a secure Internet of
Things.
Authorization is about who can do what to which objects.
Authentication specifically addresses the who, but is often specific
to the authorization that is required (for example, it may be
sufficient to authenticate the age of an actor, so no identifier is
needed or even desired). Authentication often involves credentials,
only some of which need to be long-lived and generic; others may be
directed towards specific authorizations (but still possibly long-
lived). Authorization then makes use of these credentials, as well
as other information (such as the time of day). This means that the
application-induced complexity of authenticated authorization can
often be moved back and forth between these two aspects.
In some cases authentication and authorization can be addressed by
static configuration provisioned during manufacturing or deployment
by means of fixed trust anchors and static access control lists.
This is particularly applicable to siloed, fixed-purpose deployments.
However, as the need for flexible access to assets already deployed
increases, the legitimate set of authorized entities as well as their
specific privileges cannot be conclusively defined during deployment,
without any need for change during the lifetime of the device.
Moreover, several use cases illustrate the need for fine-grained
access control policies, for which for instance a basic access
control list concept may not be sufficiently powerful [RFC7744].
The limitations of the constrained nodes ask for security mechanisms
which take the special characteristics of constrained environments
into account; not all constituents may be able to perform all
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necessary tasks by themselves. In order to meet the security
requirements in constrained scenarios, the necessary tasks need to be
assigned to logical functional entities.
In order to be able to achieve complex security objectives between
actors some of which are hosted on simple ("constrained") devices,
some of the actors will make use of help from other, less constrained
actors. (This offloading is not specific to networks with
constrained nodes, but their constrainedness as the main motivation
is.)
We therefore group the logical functional entities by whether they
can be assigned to a constrained device ("constrained level") or need
higher function platforms ("less-constrained level"); the latter does
not necessarily mean high-function, "server" or "cloud" platforms.
Note that assigning a logical functional entity to the constrained
level does not mean that the specific implementation needs to be
constrained, only that it _can_ be.
This document provides some terminology, and identifies the elements
an architecture needs to address, representing the relationships
between the logical functional entities involved; on this basis, a
problem description for authentication and authorization in
constrained-node networks is provided.
1.1. Terminology
Readers are required to be familiar with the terms and concepts
defined in [RFC4949], including "authentication", "authorization",
"confidentiality", "(data) integrity", "message authentication code",
and "verify".
REST terms including "resource", "representation", etc. are to be
understood as used in HTTP [RFC7231] and CoAP [RFC7252]; the latter
also defines additional terms such as "endpoint".
Terminology for constrained environments including "constrained
device", "constrained-node network", "class 1", etc. is defined in
[RFC7228].
In addition, this document uses the following terminology:
Resource (R): an item of interest which is represented through an
interface. It might contain sensor or actuator values or other
information. (Intended to coincide with the definitions of
[RFC7252] and [RFC7231].)
Constrained node: a constrained device in the sense of [RFC7228].
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Actor: A logical functional entity that performs one or more tasks.
Multiple actors may be present within a single device or a single
piece of software.
Resource Server (RS): An entity which hosts and represents a
Resource. (Used here to discuss the server that provides a
resource that is the end, not the means, of the authenticated
authorization process - i.e., not CAS or AS.)
Client (C): An entity which attempts to access a resource on a RS.
(Used to discuss the client whose access to a resource is the end,
not the means, of the authenticated authorization process.)
Principal: (Used in its English sense here, and specifically as:) An
individual that is either RqP or RO or both.
Resource Owner (RO): The principal that is in charge of the resource
and controls its access permissions.
Requesting Party (RqP): The principal that is in charge of the
Client and controls the requests a Client makes and its acceptance
of responses.
Authorization Server (AS): An entity that prepares and endorses
authentication and authorization data for a Resource Server.
Client Authorization Server (CAS): An entity that prepares and
endorses authentication and authorization data for a Client.
Authorization Manager: An entity that prepares and endorses
authentication and authorization data for a constrained node.
Used in constructions such as "a constrained node's authorization
manager" to denote AS for RS and CAS for C.
Authenticated Authorization: The confluence of mechanisms for
authentication and authorization, ensuring that authorization is
applied to and made available for authenticated entities and that
entities providing authentication services are authorized to do so
for the specific authorization process at hand.
Note that other authorization architectures such as OAuth [RFC6749]
or UMA [I-D.hardjono-oauth-umacore] focus on the authorization
problems on the RS side, in particular what accesses to resources the
RS is to allow. In this document the term authorization includes
this aspect, but is also used for the client-side aspect of
authorization, i.e., more generally allowing RqPs to decide what
interactions clients may perform with other endpoints.
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2. Architecture and High-level Problem Statement
This document deals with how to control and protect resource-based
interaction between potentially constrained endpoints. The following
setting is assumed as a high-level problem statement:
o An endpoint may host functionality of one or more actors.
o C in one endpoint requests to access R on a RS in another
endpoint.
o A priori, the endpoints do not necessarily have a pre-existing
security relationship to each other.
o Either of the endpoints, or both, may be constrained.
2.1. Elements of an Architecture
Without loss of generality, we focus on the C functionality in one
endpoint, which we therefore also call C, accessing the RS
functionality in another endpoint, which we therefore also call RS.
The constrained level and its security objectives are detailed in
Section 5.1.
-------------- --------------
| ------- | | ------- |
| | C | ------ requests resource -----> | RS | |
| ------- <----- provides resource ------ ------- |
| Endpoint | | Endpoint |
-------------- --------------
Figure 1: Constrained Level
The authorization decisions at the endpoints are made on behalf of
the principals that control the endpoints. To reuse OAuth and UMA
terminology, the present document calls the principal that is
controlling C the Requesting Party (RqP), and calls the principal
that is controlling RS the Resource Owner (RO). Each principal makes
authorization decisions (possibly encapsulating them into security
policies) which the endpoint it controls then enforces.
The specific security objectives will vary, but for any specific
version of this scenario will include one or more of:
o Objectives of type 1: No entity not authorized by the RO has
access to (or otherwise gains knowledge of) R.
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o Objectives of type 2: C is exchanging information with (sending a
request to, accepting a response from) a resource only where it
can ascertain that RqP has authorized the exchange with R.
Objectives of type 1 require performing authorization on the Resource
Server side while objectives of type 2 require performing
authorization on the Client side.
More on the security objectives of the principal level in
Section 5.2.
------- -------
| RqP | | RO | Principal Level
------- -------
| |
in charge of in charge of
| |
V V
------- -------
| C | -- requests resource --> | RS | Constrained Level
------- <-- provides resource-- -------
Figure 2: Constrained Level and Principal Level
The use cases defined in [RFC7744] demonstrate that constrained
devices are often used for scenarios where their principals are not
present at the time of the communication, are not able to communicate
directly with the device because of a lack of user interfaces or
displays, or may prefer the device to communicate autonomously.
Moreover, constrained endpoints may need support with tasks requiring
heavy processing, large memory or storage, or interfacing to humans,
such as management of security policies defined by a principal. The
principal, in turn, requires some agent maintaining the policies
governing how its endpoints will interact.
For these reasons, another level of nodes is introduced in the
architecture, the less-constrained level. Using OAuth terminology,
AS acts on behalf of the RO to control and support the RS in handling
access requests, employing a pre-existing security relationship with
RS. We complement this with CAS acting on behalf of RqP to control
and support the C in making resource requests and acting on the
responses received, employing a pre-existing security relationship
with C. To further relieve the constrained level, authorization (and
related authentication) mechanisms may be employed between CAS and AS
(Section 6.2). (Again, both CAS and AS are conceptual entities
controlled by their respective principals. Many of these entities,
often acting for different principals, can be combined into a single
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server implementation; this of course requires proper segregation of
the control information provided by each principal.)
------- -------
| RqP | | RO | Principal Level
------- -------
| |
controls controls
| |
V V
-------- -------
| CAS | <- AuthN and AuthZ -> | AS | Less-Constrained Level
-------- -------
| |
controls and supports controls and supports
authentication authentication
and authorization and authorization
| |
V V
------- -------
| C | -- requests resource --> | RS | Constrained Level
------- <-- provides resource-- -------
Figure 3: Overall architecture
Figure 3 shows all three levels considered in this document. Note
that the vertical arrows point down to illustrate exerting control
and providing support; this is complemented by information flows that
often are bidirectional. Note also that not all entities need to be
ready to communicate at any point in time; for instance, RqP may have
provided enough information to CAS that CAS can autonomously
negotiate access to RS with AS for C based on this information.
2.2. Architecture Variants
The elements of the architecture described above are architectural.
In a specific scenario, several elements can share a single device or
even be combined in a single piece of software. If C is located on a
more powerful device, it can be combined with CAS:
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------- --------
| RqP | | RO | Principal Level
------- --------
| |
in charge of in charge of
| |
V V
------------ --------
| CAS + C | <- AuthN and AuthZ -> | AS | Less-Constrained Level
------------ --------
^ |
\__ |
\___ authentication
\___ and authorization
requests resource/ \___ support
provides resource \___ |
\___ |
V V
-------
| RS | Constrained Level
-------
Figure 4: Combined C and CAS
If RS is located on a more powerful device, it can be combined with
AS:
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------- -------
| RqP | | RO | Principal Level
------- -------
| |
in charge of in charge of
| |
V V
---------- -----------
| CAS | <- AuthN and AuthZ -> | RS + AS | Less-Constrained Level
---------- -----------
| ^
authentication ___/
and authorization ___/
support ___/ request resource / provides resource
| ___/
V ___/
------- /
| C | <-
-------
Figure 5: Combined AS and RS
If C and RS have the same principal, CAS and AS can be combined.
------------
| RqP = RO | Principal Level
------------
|
in charge of
|
V
--------------
| CAS + AS | Less-Constrained Level
--------------
/ \
/ \
authentication authentication
and authorization and authorization
support support
/ \
V V
------- -------
| C | -- requests resource --> | RS | Constrained Level
------- <-- provides resource -- -------
Figure 6: CAS combined with AS
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2.3. Information Flows
We now formulate the problem statement in terms of the information
flows the architecture focuses on.
The interaction with the nodes on the principal level, RO and RqP, is
not involving constrained nodes and therefore can employ an existing
mechanism. The less-constrained nodes, CAS and AS, support the
constrained nodes, C and RS, with control information, for example
permissions of clients, conditions on resources, attributes of client
and resource servers, keys and credentials. This control information
may be rather different for C and RS, reflecting the intrinsic
asymmetry with C initiating the request for access to a resource, and
RS acting on a received request, and C finally acting on the received
response.
The potential information flows are shown in Figure 7. The direction
of the vertical arrows expresses the exertion of control; actual
information flow is bidirectional.
The message flow may pass unprotected paths and thus need to be
protected, potentially beyond a single REST hop (Section 3.1):
------- -------
| CAS | | AS |
------- -------
a ^ | b a = requests for control info a ^ | b
| | b = control information | |
| v | v
------- -------
| C | ------ request -------------------> | RS |
| | <----- response ------------------- | |
------- -------
Figure 7: Information flows that need to be protected
o We assume that the necessary keys/credentials for protecting the
control information between the potentially constrained nodes and
their associated less-constrained nodes are pre-established, for
example as part of the commissioning procedure.
o Any necessary keys/credentials for protecting the interaction
between the potentially constrained nodes will need to be
established and maintained as part of a solution.
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In terms of the elements of the architecture laid out above, this
document's problem statement for authorization in constrained
environments can then be summarized as follows:
o The interaction between potentially constrained endpoints is
controlled by control information provided by less-constrained
nodes on behalf of the principals of the endpoints.
o The interaction between the endpoints needs to be secured, as well
as the establishment of the necessary keys for securing the
interaction, potentially end-to-end through intermediary nodes.
o The mechanism for transferring control information needs to be
secured, potentially end-to-end through intermediary nodes. Pre-
established keying material may need to be employed for
establishing the keys used to protect these information flows.
(Note that other aspects relevant to secure constrained node
communication such as secure bootstrap or group communication are not
specifically addressed by the present document.)
3. Security Objectives
The security objectives that are addressed by an authorization
solution include confidentiality and integrity. Additionally,
allowing only selected entities limits the burden on system
resources, thus helping to achieve availability. Misconfigured or
wrongly designed authorization solutions can result in availability
breaches (denial of service): Users might no longer be able to use
data and services as they are supposed to.
Authentication mechanisms can achieve additional security objectives
such as accountability and third-party verifiability. These
additional objectives are not directly related to authorization and
thus are not in scope of this draft, but may nevertheless be
relevant. Accountability and third-party verifiability may require
authentication on a device level, if it is necessary to determine
which device performed an action. In other cases it may be more
important to find out who is responsible for the device's actions.
See also Section 4 for more discussion about authentication and
authorization.
The security objectives and their relative importance differ for the
various constrained environment applications and use cases [RFC7744].
In many cases, one participating party has different security
objectives than another. To achieve a security objective of one
party, another party may be required to provide a service. For
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example, if RqP requires the integrity of representations of a
resource R that RS is hosting, both C and RS need to partake in
integrity-protecting the transmitted data. Moreover, RS needs to
protect any write access to this resource as well as to relevant
other resources (such as configuration information, firmware update
resources) to prevent unauthorized users from manipulating R.
3.1. End-to-End Security Objectives in Multi-Hop Scenarios
In many cases, the information flows described in Section 2.3 cross
multiple client-server pairings but still need to be protected end-
to-end. For example, AS may not be connected to RS (or may not want
to exercise such a connection), relying on C for transferring
authorization information. As the authorization information is
related to the permissions granted to C, C must not be in a position
to manipulate this information, which therefore requires integrity
protection on the way between AS and RS.
As another example, resource representations sent between endpoints
may be stored in intermediary nodes, such as caching proxies or pub-
sub brokers. Where these intermediaries cannot be relied on to
fulfill the security objectives of the endpoints, these will need to
protect the exchanges beyond a single client-server exchange.
Note that there may also be cases of intermediary nodes that very
much partake in the security objectives to be achieved. The question
what are the pairs of endpoints between which the communication needs
end-to-end protection (and which aspect of protection) is defined by
the specific use case. Two examples of intermediary nodes executing
security functionality:
o To enable a trustworthy publication service, a pub-sub broker may
be untrusted with the plaintext content of a publication
(confidentiality), but required to verify that the publication is
performed by claimed publisher and is not a replay of an old
publication (authenticity/integrity).
o To comply with requirements of transparency, a gateway may be
allowed to read, verify (authenticity) but not modify (integrity)
a resource representation which therefore also is end-to-end
integrity protected from the server towards a client behind the
gateway.
In order to support the required communication and application
security, keying material needs to be established between the
relevant nodes in the architecture.
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4. Authentication and Authorization
Server-side authorization solutions aim at protecting the access to
items of interest, for instance hardware or software resources or
data: They enable the resource owner to control who can access it and
how.
To determine if an entity is authorized to access a resource, an
authentication mechanism is needed. According to the Internet
Security Glossary [RFC4949], authentication is "the process of
verifying a claim that a system entity or system resource has a
certain attribute value." Examples for attribute values are the ID
of a device, the type of the device or the name of its owner.
The security objectives the authorization mechanism aims at can only
be achieved if the authentication and the authorization mechanism
work together correctly. We speak of authenticated authorization to
refer to the required synthesis of mechanism for authentication and
authorization.
Where used for authorization, the set of authenticated attributes
must be meaningful for this purpose, i.e., authorization decisions
must be possible based on these attributes. If the authorization
policy assigns permissions to an individual entity, the set of
authenticated attributes must be suitable to uniquely identify this
entity.
In scenarios where devices are communicating autonomously there is
often less need to uniquely identify an individual device: For a
principal, the fact that a device belongs to a certain company or
that it has a specific type (such as a light bulb) or location may be
more important than that it has a unique identifier.
(As a special case for the authorization of read access to a
resource, RS may simply make an encrypted representation available to
anyone [OSCAR]. In this case, controlling read access to that
resource can be reduced to controlling read access to the key;
partially removing access also requires a timely update of the key
for RS and all participants still authorized.)
Principals (RqP and RO) need to decide about the required level of
granularity for the authorization. For example, we distinguish
device authorization from owner authorization, and flat authorization
from unrestricted authorization. In the first case different access
permissions are granted to individual devices while in the second
case individual owners are authorized. If flat authorization is
used, all authenticated entities are implicitly authorized and have
the same access permissions. Unrestricted authorization for an item
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of interest means that no authorization mechanism is used for
accessing this resource (not even by authentication) and all entities
are able to access the item as they see fit (note that an
authorization mechanism may still be used to arrive at the decision
to employ unrestricted authorization).
More fine-grained authorization does not necessarily provide more
security but can be more flexible. Principals need to consider that
an entity should only be granted the permissions it really needs
(principle of least privilege), to ensure the confidentiality and
integrity of resources.
Client-side authorization solutions aim at protecting the client from
disclosing information to or ingesting information from resource
servers RqP does not want it to interact with in the given way.
Again, flat authorization (the server can be authenticated) may be
sufficient, or more fine-grained authorization may be required. The
client-side authorization also pertains to the level of protection
required for the exchanges with the server (e.g., confidentiality).
In the browser web, client-side authorization is often left to the
human user; a constrained client may not have that available all the
time but still needs to implement the wishes of the principal
controlling it, the RqP.
For all cases where an authorization solution is needed (all but
unrestricted authorization), the enforcing party needs to be able to
authenticate the party that is to be authorized. Authentication is
therefore required for messages that contain (or otherwise update)
representations of an accessed item. More precisely: The enforcing
party needs to make sure that the receiver of a message containing a
representation is authorized to receive it, both in the case of a
client sending a representation to a server and vice versa. In
addition, it needs to ensure that the actual sender of a message
containing a representation is indeed the one authorized to send this
message, again for both the client-to-server and server-to-client
case. To achieve this, integrity protection of these messages is
required: Authenticity cannot be assured if it is possible for an
attacker to modify the message during transmission.
In some cases, only one side (client or server side) requires the
integrity and / or confidentiality of a resource value. Principals
may decide to omit authentication (unrestricted authorization), or
use flat authorization (just employing an authentication mechanism).
However, as indicated in Section 3, the security objectives of both
sides must be considered, which can often only be achieved when the
other side can be relied on to perform some security service.
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5. Actors and their Tasks
This and the following section look at the resulting architecture
from two different perspectives: This section provides a more
detailed description of the various "actors" in the architecture, the
logical functional entities performing the tasks required. The
following section then will focus on the protocols run between these
functional entities.
For the purposes of this document, an actor consists of a set of
tasks and additionally has a security domain (client domain or server
domain) and a level (constrained, principal, less-constrained).
Tasks are assigned to actors according to their security domain and
required level.
Note that actors are a concept to understand the security
requirements for constrained devices. The architecture of an actual
solution might differ as long as the security requirements that
derive from the relationship between the identified actors are
considered. Several actors might share a single device or even be
combined in a single piece of software. Interfaces between actors
may be realized as protocols or be internal to such a piece of
software.
A more detailed discussion of the tasks the actors have to perform in
order to achieve specific security objectives is provided in
[I-D.gerdes-ace-tasks].
5.1. Constrained Level Actors
As described in the problem statement (see Section 2), either C or RS
or both of them may be located on a constrained node. We therefore
define that C and RS must be able to perform their tasks even if they
are located on a constrained node. Thus, C and RS are considered to
be Constrained Level Actors.
C performs the following tasks:
o Communicate in a secure way (provide for confidentiality and
integrity of messages), including access requests.
o Validate that the RqP ("client-side") authorization information
allows C to communicate with RS as a server for R (i.e., from C's
point of view, RS is authorized as a server for the specific
access to R).
RS performs the following tasks:
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o Communicate in a secure way (provide for confidentiality and
integrity of messages), including responses to access requests.
o Validate that the RO ("server-side") authorization information
allows RS to grant C access to the requested resource as requested
(i.e., from RS' point of view, C is authorized as a client for the
specific access to R).
R is an item of interest such as a sensor or actuator value. R is
considered to be part of RS and not a separate actor. The device on
which RS is located might contain several resources controlled by
different ROs. For simplicity of exposition, these resources are
described as if they had separate RS.
As C and RS do not necessarily know each other they might belong to
different security domains.
(See Figure 8.)
------- --------
| C | -- requests resource ---> | RS | Constrained Level
------- <-- provides resource--- --------
Figure 8: Constrained Level Actors
5.2. Principal Level Actors
Our objective is that C and RS are under control of principals in the
physical world, the Requesting Party (RqP) and the Resource Owner
(RO) respectively. The principals decide about the security policies
of their respective endpoints and belong to the same security domain.
RqP is in charge of C, i.e. RqP specifies security policies for C,
such as with whom C is allowed to communicate. By definition, C and
RqP belong to the same security domain.
RqP must fulfill the following task:
o Configure for C authorization information for sources for R.
RO is in charge of R and RS. RO specifies authorization policies for
R and decides with whom RS is allowed to communicate. By definition,
R, RS and RO belong to the same security domain.
RO must fulfill the following task:
o Configure for RS authorization information for accessing R.
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(See Figure 2.)
5.3. Less-Constrained Level Actors
Constrained level actors can only fulfill a limited number of tasks
and may not have network connectivity all the time. To relieve them
from having to manage keys for numerous endpoints and conducting
computationally intensive tasks, another complexity level for actors
is introduced. An actor on the less-constrained level belongs to the
same security domain as its respective constrained level actor. They
also have the same principal.
The Client Authorization Server (CAS) belongs to the same security
domain as C and RqP. CAS acts on behalf of RqP. It assists C in
authenticating RS and determining if RS is an authorized server for
R. CAS can do that because for C, CAS is the authority for claims
about RS.
CAS performs the following tasks:
o Validate on the client side that an entity has certain attributes.
o Obtain authorization information about an entity from C's
principal (RqP) and provide it to C.
o Negotiate means for secure communication to communicate with C.
The Authorization Server (AS) belongs to the same security domain as
R, RS and RO. AS acts on behalf of RO. It supports RS by
authenticating C and determining C's permissions on R. AS can do
that because for RS, AS is the authority for claims about C.
AS performs the following tasks:
o Validate on the server side that an entity has certain attributes.
o Obtain authorization information about an entity from RS'
principal (RO) and provide it to RS.
o Negotiate means for secure communication to communicate with RS.
6. Kinds of Protocols
Devices on the less-constrained level potentially are more powerful
than constrained level devices in terms of processing power, memory,
non-volatile storage. This results in different characteristics for
the protocols used on these levels.
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6.1. Constrained Level Protocols
A protocol is considered to be on the constrained level if it is used
between the actors C and RS which are considered to be constrained
(see Section 5.1). C and RS might not belong to the same security
domain. Therefore, constrained level protocols need to work between
different security domains.
Commonly used Internet protocols can not in every case be applied to
constrained environments. In some cases, tweaking and profiling is
required. In other cases it is beneficial to define new protocols
which were designed with the special characteristics of constrained
environments in mind.
On the constrained level, protocols need to address the specific
requirements of constrained environments. Examples for protocols
that consider these requirements is the transfer protocol CoAP
(Constrained Application Protocol) [RFC7252] and the Datagram
Transport Layer Security Protocol (DTLS) [RFC6347] which can be used
for channel security.
Constrained devices have only limited storage space and thus cannot
store large numbers of keys. This is especially important because
constrained networks are expected to consist of thousands of nodes.
Protocols on the constrained level should keep this limitation in
mind.
6.1.1. Cross Level Support Protocols
Protocols which operate between a constrained device on one side and
the corresponding less-constrained device on the other are considered
to be (cross level) support protocols. Protocols used between C and
CAS or RS and AS are therefore support protocols.
Support protocols must consider the limitations of their constrained
endpoint and therefore belong to the constrained level protocols.
6.2. Less-Constrained Level Protocols
A protocol is considered to be on the less-constrained level if it is
used between the actors CAS and AS. CAS and AS might belong to
different security domains.
On the less-constrained level, HTTP [RFC7230] and Transport Layer
Security (TLS) [RFC5246] can be used alongside or instead of CoAP and
DTLS. Moreover, existing security solutions for authentication and
authorization such as the OAuth web authorization framework [RFC6749]
and Kerberos [RFC4120] can likely be used without modifications and
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there are no limitations for the use of a Public Key Infrastructure
(PKI).
7. Elements of a Solution
Without anticipating specific solutions, the following considerations
may be helpful in discussing them.
7.1. Authorization
The core problem we are trying to solve is authorization. The
following problems related to authorization need to be addressed:
o AS needs to transfer authorization information to RS and CAS needs
to transfer authorization information to C.
o The transferred authorization information needs to follow a
defined format and encoding, which must be efficient for
constrained devices, considering size of authorization information
and parser complexity.
o C and RS need to be able to verify the authenticity of the
authorization information they receive. Here as well, there is a
trade-off between processing complexity and deployment complexity.
o The RS needs to enforce the authorization decisions of the AS,
while C needs to abide with the authorization decisions of the
CAS. The authorization information might require additional
policy evaluation (such as matching against local access control
lists, evaluating local conditions). The required "policy
evaluation" at the constrained actors needs to be adapted to the
capabilities of the devices implementing them.
o Finally, as is indicated in the previous bullet, for a particular
authorization decision there may be different kinds of
authorization information needed, and these pieces of information
may be transferred to C and RS at different times and in different
ways prior to or during the client request.
7.2. Authentication
The following problems need to be addressed, when considering
authentication:
o RS needs to authenticate AS, and C needs to authenticate CAS, to
ensure that the authorization information and related data comes
from the correct source.
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o CAS and AS may need to authenticate each other, both to perform
the required business logic and to ensure that CAS gets security
information related to the resources from the right source.
o In some use cases RS needs to authenticate some property of C, in
order to map it to the relevant authorization information. In
other use cases, authentication and authorization of C may be
implicit, for example by encrypting the resource representation
the RS only providing access to those who possess the key to
decrypt.
o C may need to authenticate RS, in order to ensure that it is
interacting with the right resources. Alternatively C may just
verify the integrity of a received resource representation.
o CAS and AS need to authenticate their communication partner (C or
RS), in order to ensure it serves the correct device.
7.3. Communication Security
There are different alternatives to provide communication security,
and the problem here is to choose the optimal one for each scenario.
We list the available alternatives:
o Session-based security at transport layer such as DTLS [RFC6347]
offers security, including integrity and confidentiality
protection, for the whole application layer exchange. However,
DTLS may not provide end-to-end security over multiple hops.
Another problem with DTLS is the cost of the handshake protocol,
which may be too expensive for constrained devices especially in
terms of memory and power consumption for message transmissions.
o An alternative is object security at application layer, for
instance using [I-D.selander-ace-object-security]. Secure objects
can be stored or cached in network nodes and provide security for
a more flexible communication model such as publish/subscribe
(compare e.g. CoRE Mirror Server [I-D.koster-core-coap-pubsub]).
A problem with object security is that it can not provide
confidentiality for the message headers.
o Hybrid solutions using both session-based and object security are
also possible. An example of a hybrid is where authorization
information and cryptographic keys are provided by AS in the
format of secure data objects, but where the resource access is
protected by session-based security.
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7.4. Cryptographic Keys
With respect to cryptographic keys, we see the following problems
that need to be addressed:
Symmetric vs Asymmetric Keys
We need keys both for protection of resource access and for
protection of transport of authentication and authorization
information. Do we want to support solutions based on asymmetric
keys or symmetric keys in both cases? There are classes of
devices that can easily perform symmetric cryptography, but
consume considerably more time/battery for asymmetric operations.
On the other hand asymmetric cryptography has benefits such as in
terms of deployment.
Key Establishment
How are the corresponding cryptographic keys established?
Considering Section 7.1 there must be a mapping between these keys
and the authorization information, at least in the sense that AS
must be able to specify a unique client identifier which RS can
verify (using an associated key). One of the use cases of
[RFC7744] describes spontaneous change of access policies - such
as giving a hitherto unknown client the right to temporarily
unlock your house door. In this case C is not previously known to
RS and a key must be provisioned by AS.
Revocation and Expiration
How are keys replaced and how is a key that has been compromised
revoked in a manner that reaches all affected parties, also
keeping in mind scenarios with intermittent connectivity?
8. Assumptions and Requirements
In this section we list a set of candidate assumptions and
requirements to make the problem description in the previous sections
more concise and precise.
8.1. Architecture
The architecture consists of at least the following types of nodes:
o RS hosting resources, and responding to access requests
o C requesting access to resources
o AS supporting the access request/response procedure by providing
authorization information to RS
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* AS may support this by aiding RS in authenticating C, or
providing cryptographic keys or credentials to C and/or RS to
secure the request/response procedure.
o CAS supporting the access request/response procedure by providing
authorization information to C
* CAS may support this by aiding C in authenticating RS,
forwarding information between AS and C (possibly ultimately
for RS), or providing cryptographic keys or credentials to C
and/or RS to secure the request/response procedure.
o The architecture allows for intermediary nodes between any pair of
C, RS, AS, and CAS, such as forward or reverse proxies in the CoRE
architecture. (Solutions may or may not support all
combinations.)
* The architecture does not make a choice between session based
security and data object security.
8.2. Constrained Devices
o C and/or RS may be constrained in terms of power, processing,
communication bandwidth, memory and storage space, and moreover:
* unable to manage complex authorization policies
* unable to manage a large number of secure connections
* without user interface
* without constant network connectivity
* unable to precisely measure time
* required to save on wireless communication due to high power
consumption
o CAS and AS are not assumed to be constrained devices.
o All devices under consideration can process symmetric cryptography
without incurring an excessive performance penalty.
* We assume the use of a standardized symmetric key algorithm,
such as AES.
* Except for the most constrained devices we assume the use of a
standardized cryptographic hash function such as SHA-256 (which
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can be used with the HMAC construction for integrity
protection).
o Public key cryptography requires additional resources (such as
RAM, ROM, power, specialized hardware).
o A DTLS handshake involves significant computation, communication,
and memory overheads in the context of constrained devices.
* The RAM requirements of DTLS handshakes with public key
cryptography are prohibitive for certain constrained devices.
* Certificate-based DTLS handshakes require significant volumes
of communication, RAM (message buffers) and computation.
o A solution will need to consider support for a simple scheme for
expiring authentication and authorization information on devices
which are unable to measure time (cf. section Section 9.2).
8.3. Authentication
o RS needs to authenticate AS to ensure that the authorization
information and related data comes from the correct source.
o Similarly, C needs to authenticate CAS to ensure that the
authorization information and related data comes from the correct
source.
o Depending on use case and authorization requirements, C, RS, CAS,
or AS may need to authenticate messages from each other.
8.4. Server-side Authorization
o RS enforces authorization for access to a resource based on
credentials presented by C, the requested resource, the REST
method, and local context in RS at the time of the request, or on
any subset of this information.
o The credentials presented by C may have been provided by CAS.
o The underlying authorization decision is taken either by AS or RS.
o The authorization decision is enforced by RS.
* RS needs to have authorization information in order to verify
that C is allowed to access the resource as requested.
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* RS needs to make sure that it provides resource access only to
authorized clients.
o Apart from authorization for access to a resource, authorization
may also be required for access to information about a resource
(for instance, resource descriptions).
o The solution may need to be able to support the delegation of
access rights.
8.5. Client-side Authorization Information
o C enforces client-side authorization by protecting its requests to
RS and by authenticating results from RS, making use of decisions
and policies as well as keying material provided by CAS.
8.6. Server-side Authorization Information
o Authorization information is transferred from AS to RS using
Agent, Push or Pull mechanisms [RFC2904].
o RS needs to authenticate that the authorization information is
coming from AS (integrity).
o The authorization information may also be encrypted end-to-end
between AS and RS (confidentiality).
o The architecture supports the case where RS may not be able to
communicate with AS at the time of the request from C.
o RS may store or cache authorization information.
o Authorization information may be pre-configured in RS.
o Authorization information stored or cached in RS needs to be
possible to change. The change of such information needs to be
subject to authorization.
o Authorization policies stored on RS may be handled as a resource,
i.e. information located at a particular URI, accessed with
RESTful methods, and the access being subject to the same
authorization mechanics. AS may have special privileges when
requesting access to the authorization policy resources on RS.
o There may be mechanisms for C to look up the AS which provides
authorization information about a particular resource.
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8.7. Resource Access
o Resources are accessed in a RESTful manner using methods such as
GET, PUT, POST, DELETE.
o By default, the resource request needs to be integrity protected
and may be encrypted end-to-end from C to RS. It needs to be
possible for RS to detect a replayed request.
o By default, the response to a request needs to be integrity
protected and may be encrypted end-to-end from RS to C. It needs
to be possible for C to detect a replayed response.
o RS needs to be able to verify that the request comes from an
authorized client.
o C needs to be able to verify that the response to a request comes
from the intended RS.
o There may be resources whose access need not be protected (e.g.
for discovery of the responsible AS).
8.8. Keys and Cipher Suites
o A constrained node and its authorization manager (i.e., RS and AS,
and C and CAS) have established cryptographic keys. For example,
they share a secret key or each have the other's public key.
o The transfer of authorization information is protected with
symmetric and/or asymmetric keys.
o The access request/response can be protected with symmetric and/or
asymmetric keys.
o There must be a mechanism for RS to establish the necessary key(s)
to verify and decrypt the request and to protect the response.
o There must be a mechanism for C to establish the necessary key(s)
to protect the request and to verify and decrypt the response.
o There must be a mechanism for C to obtain the supported cipher
suites of a RS.
8.9. Network Considerations
o A solution will need to consider network overload due to avoidable
communication of a constrained node with its authorization manager
(C with CAS, RS with AS).
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o A solution will need to consider network overload by compact
authorization information representation.
o A solution may want to optimize the case where authorization
information does not change often.
o A solution may consider support for an efficient mechanism for
providing authorization information to multiple RSs, for example
when multiple entities need to be configured or change state.
8.10. Legacy Considerations
o A solution may consider interworking with existing infrastructure.
o A solution may consider supporting authorization of access to
legacy devices.
9. Security Considerations
This document discusses authorization-related tasks for constrained
environments and describes how these tasks can be mapped to actors in
the architecture.
The entire document is about security. Security considerations
applicable to authentication and authorization in RESTful
environments are provided in e.g. OAuth 2.0 [RFC6749].
In this section we focus on specific security aspects related to
authorization in constrained-node networks. Section 11.6 of
[RFC7252], "Constrained node considerations", discusses implications
of specific constraints on the security mechanisms employed. A wider
view of security in constrained-node networks is provided in
[I-D.garcia-core-security].
9.1. Physical Attacks on Sensor and Actuator Networks
The focus of this work is on constrained-node networks consisting of
connected sensors and actuators. The main function of such devices
is to interact with the physical world by gathering information or
performing an action. We now discuss attacks performed with physical
access to such devices.
The main threats to sensors and actuator networks are:
o Unauthorized access to data to and from sensors and actuators,
including eavesdropping and manipulation of data.
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o Denial-of-service making the sensor/actuator unable to perform its
intended task correctly.
A number of attacks can be made with physical access to a device
including probing attacks, timing attacks, power attacks, etc.
However, with physical access to a sensor or actuator device it is
possible to directly perform attacks equivalent of eavesdropping,
manipulating data or denial of service. For example:
o Instead of eavesdropping the sensor data or attacking the
authorization system to gain access to the data, the attacker
could make its own measurements on the physical object.
o Instead of manipulating the sensor data the attacker could change
the physical object which the sensor is measuring, thereby
changing the payload data which is being sent.
o Instead of manipulating data for an actuator or attacking the
authorization system, the attacker could perform an unauthorized
action directly on the physical object.
o A denial-of-service attack could be performed physically on the
object or device.
All these attacks are possible by having physical access to the
device, since the assets are related to the physical world.
Moreover, this kind of attacks are in many cases straightforward
(requires no special competence or tools, low cost given physical
access, etc.)
As a conclusion, if an attacker has full physical access to a
sensor or actuator device, then much of the security functionality
elaborated in this draft is not effective to protect the asset
during the physical attack.
Since it does not make sense to design a solution for a situation
that cannot be protected against we assume there is no need to
protect assets which are exposed during a physical attack. In
other words, either an attacker does not have physical access to
the sensor or actuator device, or if it has, the attack shall only
have effect during the period of physical attack, and shall be
limited in extent to the physical control the attacker exerts
(e.g., must not affect the security of other devices.)
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9.2. Time Measurements
Measuring time with certain accuracy is important to achieve certain
security properties, for example to determine whether a public key
certificate, access token or some other assertion is valid.
Dynamic authorization in itself requires the ability to handle expiry
or revocation of authorization decisions or to distinguish new
authorization decisions from old.
For certain categories of devices we can assume that there is an
internal clock which is sufficiently accurate to handle the time
measurement requirements. If RS can connect directly to AS it could
get updated in terms of time as well as revocation information.
If RS continuously measures time but can't connect to AS or other
trusted source, time drift may have to be accepted and it may not be
able to manage revocation. However, it may still be able to handle
short lived access rights within some margins, by measuring the time
since arrival of authorization information or request.
Some categories of devices in scope may be unable measure time with
any accuracy (e.g. because of sleep cycles). This category of
devices is not suitable for the use cases which require measuring
validity of assertions and authorizations in terms of absolute time.
10. IANA Considerations
This document has no actions for IANA.
11. Acknowledgements
The authors would like to thank Olaf Bergmann, Robert Cragie, Samuel
Erdtman, Klaus Hartke, Sandeep Kumar, John Mattson, Corinna Schmitt,
Mohit Sethi, Abhinav Somaraju, Hannes Tschofenig, Vlasios Tsiatsis
and Erik Wahlstroem for contributing to the discussion, giving
helpful input and commenting on previous forms of this draft. The
authors would also like to specifically acknowledge input provided by
Hummen and others [HUM14delegation].
12. Informative References
[HUM14delegation]
Hummen, R., Shafagh, H., Raza, S., Voigt, T., and K.
Wehrle, "Delegation-based Authentication and Authorization
for the IP-based Internet of Things", 11th IEEE
International Conference on Sensing, Communication, and
Networking (SECON'14), June 30 - July 3, 2014.
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[I-D.garcia-core-security]
Garcia-Morchon, O., Kumar, S., Keoh, S., Hummen, R., and
R. Struik, "Security Considerations in the IP-based
Internet of Things", draft-garcia-core-security-06 (work
in progress), September 2013.
[I-D.gerdes-ace-tasks]
Gerdes, S., "Authorization-Related Tasks in Constrained
Environments", draft-gerdes-ace-tasks-00 (work in
progress), September 2015.
[I-D.hardjono-oauth-umacore]
Hardjono, T., Maler, E., Machulak, M., and D. Catalano,
"User-Managed Access (UMA) Profile of OAuth 2.0", draft-
hardjono-oauth-umacore-14 (work in progress), January
2016.
[I-D.koster-core-coap-pubsub]
Koster, M., Keranen, A., and J. Jimenez, "Publish-
Subscribe Broker for the Constrained Application Protocol
(CoAP)", draft-koster-core-coap-pubsub-04 (work in
progress), November 2015.
[I-D.selander-ace-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security of CoAP (OSCOAP)", draft-selander-ace-
object-security-03 (work in progress), October 2015.
[OSCAR] Vucinic, M., Tourancheau, B., Rousseau, F., Duda, A.,
Damon, L., and R. Guizzetti, "OSCAR: Object Security
Architecture for the Internet of Things", CoRR vol.
abs/1404.7799, 2014.
[RFC2904] Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
Gross, G., de Bruijn, B., de Laat, C., Holdrege, M., and
D. Spence, "AAA Authorization Framework", RFC 2904, DOI
10.17487/RFC2904, August 2000,
<http://www.rfc-editor.org/info/rfc2904>.
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
DOI 10.17487/RFC4120, July 2005,
<http://www.rfc-editor.org/info/rfc4120>.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", FYI
36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
<http://www.rfc-editor.org/info/rfc4949>.
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[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>.
[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>.
[RFC6749] Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
RFC 6749, DOI 10.17487/RFC6749, October 2012,
<http://www.rfc-editor.org/info/rfc6749>.
[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>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", RFC
7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI
10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
[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>.
[RFC7744] Seitz, L., Ed., Gerdes, S., Ed., Selander, G., Mani, M.,
and S. Kumar, "Use Cases for Authentication and
Authorization in Constrained Environments", RFC 7744, DOI
10.17487/RFC7744, January 2016,
<http://www.rfc-editor.org/info/rfc7744>.
Authors' Addresses
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Stefanie Gerdes
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63906
Email: gerdes@tzi.org
Ludwig Seitz
SICS Swedish ICT AB
Scheelevaegen 17
Lund 223 70
Sweden
Email: ludwig@sics.se
Goeran Selander
Ericsson
Faroegatan 6
Kista 164 80
Sweden
Email: goran.selander@ericsson.com
Carsten Bormann (editor)
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
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