[Docs] [txt|pdf|xml|html] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits] [IPR]
Versions: (draft-pritikin-anima-bootstrapping-keyinfra)
00 01 02 03 04 05 06 07 08 09 10 11
12 13 14 15 16 17 18 19 20
ANIMA WG M. Pritikin
Internet-Draft Cisco
Intended status: Informational M. Richardson
Expires: September 14, 2017 SSW
M. Behringer
S. Bjarnason
Cisco
K. Watsen
Juniper Networks
March 13, 2017
Bootstrapping Remote Secure Key Infrastructures (BRSKI)
draft-ietf-anima-bootstrapping-keyinfra-05
Abstract
This document specifies automated bootstrapping of a remote secure
key infrastructure (BRSKI) using vendor installed X.509 certificate,
in combination with a vendor's authorizing service, both online the
Internet, and offline. Bootstrapping a new device can occur using a
routable address and a cloud service, or using only link-local
connectivity, or on limited/disconnected networks. Support for lower
security models, including devices with minimal identity, is
described for legacy reasons but not encouraged. Bootstrapping is
complete when the cryptographic identity of the new key
infrastructure is successfully deployed to the device but the
established secure connection can be used to deploy a locally issued
certificate to the device as well.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 14, 2017.
Pritikin, et al. Expires September 14, 2017 [Page 1]
Internet-Draft BRSKI March 2017
Copyright Notice
Copyright (c) 2017 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
1.1. Secure Imprinting without Vouchers . . . . . . . . . . . 5
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 6
1.3. Scope of solution . . . . . . . . . . . . . . . . . . . . 8
2. Architectural Overview . . . . . . . . . . . . . . . . . . . 9
2.1. Secure Imprinting without Vouchers . . . . . . . . . . . 11
2.2. Secure Imprinting using Vouchers . . . . . . . . . . . . 12
2.3. Initial Device Identifier . . . . . . . . . . . . . . . . 12
3. Functional Overview . . . . . . . . . . . . . . . . . . . . . 13
3.1. Behavior of a Pledge . . . . . . . . . . . . . . . . . . 15
3.1.1. Discovery . . . . . . . . . . . . . . . . . . . . . . 17
3.1.2. Identity . . . . . . . . . . . . . . . . . . . . . . 18
3.1.3. Request Join . . . . . . . . . . . . . . . . . . . . 18
3.1.4. Imprint . . . . . . . . . . . . . . . . . . . . . . . 19
3.1.5. Lack of realtime clock . . . . . . . . . . . . . . . 19
3.1.6. Enrollment . . . . . . . . . . . . . . . . . . . . . 20
3.1.7. Being Managed . . . . . . . . . . . . . . . . . . . . 20
3.2. Behavior of a Join Proxy . . . . . . . . . . . . . . . . 21
3.2.1. CoAP connection to Registrar . . . . . . . . . . . . 22
3.2.2. HTTPS proxy connection to Registrar . . . . . . . . . 22
3.3. Behavior of the Registrar . . . . . . . . . . . . . . . . 22
3.3.1. Pledge Authentication . . . . . . . . . . . . . . . . 23
3.3.2. Pledge Authorization . . . . . . . . . . . . . . . . 24
3.3.3. Claiming the New Entity . . . . . . . . . . . . . . . 24
3.3.4. Log Verification . . . . . . . . . . . . . . . . . . 25
3.4. Behavior of the MASA Service . . . . . . . . . . . . . . 26
3.5. Leveraging the new key infrastructure / next steps . . . 26
3.5.1. Network boundaries . . . . . . . . . . . . . . . . . 26
3.6. Interactions with Network Access Control . . . . . . . . 27
4. Domain Operator Activities . . . . . . . . . . . . . . . . . 27
4.1. Instantiating the Domain Certification Authority . . . . 27
Pritikin, et al. Expires September 14, 2017 [Page 2]
Internet-Draft BRSKI March 2017
4.2. Instantiating the Registrar . . . . . . . . . . . . . . . 27
4.3. Accepting New Entities . . . . . . . . . . . . . . . . . 28
4.4. Automatic Enrollment of Devices . . . . . . . . . . . . . 29
4.5. Secure Network Operations . . . . . . . . . . . . . . . . 29
5. Proxy Discovery Protocol Details . . . . . . . . . . . . . . 29
6. Registrar Discovery Protocol Details . . . . . . . . . . . . 29
7. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 30
7.1. Request Voucher from the Registrar . . . . . . . . . . . 34
7.2. Request Voucher from MASA . . . . . . . . . . . . . . . . 35
7.3. Voucher Response . . . . . . . . . . . . . . . . . . . . 36
7.3.1. Completing authentication of Provisional TLS
connection . . . . . . . . . . . . . . . . . . . . . 37
7.4. Voucher Status Telemetry . . . . . . . . . . . . . . . . 38
7.5. MASA authorization log Request . . . . . . . . . . . . . 39
7.6. MASA authorization log Response . . . . . . . . . . . . . 39
7.7. EST Integration for PKI bootstrapping . . . . . . . . . . 40
7.7.1. EST Distribution of CA Certificates . . . . . . . . . 41
7.7.2. EST CSR Attributes . . . . . . . . . . . . . . . . . 41
7.7.3. EST Client Certificate Request . . . . . . . . . . . 42
7.7.4. Enrollment Status Telemetry . . . . . . . . . . . . . 42
7.7.5. EST over CoAP . . . . . . . . . . . . . . . . . . . . 43
8. Reduced security operational modes . . . . . . . . . . . . . 43
8.1. Trust Model . . . . . . . . . . . . . . . . . . . . . . . 43
8.2. New Entity security reductions . . . . . . . . . . . . . 44
8.3. Registrar security reductions . . . . . . . . . . . . . . 44
8.4. MASA security reductions . . . . . . . . . . . . . . . . 45
9. Security Considerations . . . . . . . . . . . . . . . . . . . 46
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 48
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 48
11.1. Normative References . . . . . . . . . . . . . . . . . . 48
11.2. Informative References . . . . . . . . . . . . . . . . . 49
Appendix A. IPv4 operations . . . . . . . . . . . . . . . . . . 51
A.1. IPv4 Link Local addresses . . . . . . . . . . . . . . . . 51
A.2. Use of DHCPv4 . . . . . . . . . . . . . . . . . . . . . . 51
Appendix B. mDNS / DNSSD proxy discovery options . . . . . . . . 51
Appendix C. IPIP Join Proxy mechanism . . . . . . . . . . . . . 52
C.1. Multiple Join networks on the Join Proxy side . . . . . . 53
C.2. Automatic configuration of tunnels on Registrar . . . . . 53
C.3. Proxy Neighbor Discovery by Join Proxy . . . . . . . . . 53
C.4. Use of connected sockets; or IP_PKTINFO for CoAP on
Registrar . . . . . . . . . . . . . . . . . . . . . . . . 54
C.5. Use of socket extension rather than virtual interface . . 54
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 54
Pritikin, et al. Expires September 14, 2017 [Page 3]
Internet-Draft BRSKI March 2017
1. Introduction
To literally "pull yourself up by the bootstraps" is an impossible
action. Similarly the secure establishment of a key infrastructure
without external help is also an impossibility. Today it is commonly
accepted that the initial connections between nodes are insecure,
until key distribution is complete, or that domain-specific keying
material is pre-provisioned on each new device in a costly and non-
scalable manner. These existing mechanisms are known as non-secured
'Trust on First Use' (TOFU) [RFC7435], 'resurrecting duckling'
[Stajano99theresurrecting] or 'pre-staging'.
This document describes a zero-touch approach to bootstrapping that
secures the initial distribution of key material between an
unconfigured and untouched device called a "Pledge" and the
"Registrar" device that is a member of an established network domain.
The bootstrapping process provides a foundation to securely answer
the following questions:
o Registrar authenticating the Pledge: "Who is this device? What is
its identity?"
o Registrar authorization the Pledge: "Is it mine? Do I want it?
What are the chances it has been compromised?"
o Pledge authenticating the Registrar/Domain: "What is this domain's
identity?"
o Pledge authorization the Registrar: "Should I join it?"
This document details protocols and messages to the endpoints to
answer the above questions. The Registrar actions derive from Pledge
identity, third party cloud service communications, and local access
control lists. The Pledge actions derive from a cryptographically
protected "voucher" message delivered through the Registrar.
Multiple forms of "vouchers" are described to support a variety of
use cases.
The syntactic details of vouchers are described in detail in
[I-D.ietf-anima-voucher]. This document details automated protocol
mechanisms to obtain vouchers.
The result of bootstrapping is that a security association between
the Pledge and the Registrar is established. A method of leveraging
this association to optimize PKI enrollment is described.
Pritikin, et al. Expires September 14, 2017 [Page 4]
Internet-Draft BRSKI March 2017
The described system is agile enough to support bootstrapping
alternative key infrastructures, such as a symmetric key solutions,
but no such system is described.
1.1. Secure Imprinting without Vouchers
There are pre-existing methods available for establishing initial
trust. For example the enrollment protocol EST [RFC7030] details a
set of non-autonomic bootstrapping methods such as:
o using the Implicit Trust Anchor database (not an autonomic
solution because the URL must be securely distributed),
o engaging a human user to authorize the CA certificate using out-
of-band data (not an autonomic solution because the human user is
involved),
o using a configured Explicit TA database (not an autonomic solution
because the distribution of an explicit TA database is not
autonomic),
o and using a Certificate-Less TLS mutual authentication method (not
an autonomic solution because the distribution of symmetric key
material is not autonomic).
These "touch" methods do not meet the requirements for zero-touch.
There are "call home" technologies where the Pledge first establishes
a connection to a well known vendor service using a common client-
server authentication model. After mutual authentication appropriate
credentials to authenticate the target domain are transfered to the
Pledge. This creates serveral problems and limitations:
o the pledge requires realtime connectivity to the vendor service,
o the domain identity is exposed to the vendor service (this is a
privacy concern),
o the vendor is responsible for making the authorization decisions
(this is a liability concern),
BRSKI addresses these issues by introducting an authorization layer
via "vouchers". The additional complexity provides for significant
flexibility.
Pritikin, et al. Expires September 14, 2017 [Page 5]
Internet-Draft BRSKI March 2017
1.2. 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
[RFC2119].
The following terms are defined for clarity:
DomainID: The domain identity is the 160-bit SHA-1 hash of the BIT
STRING of the subjectPublicKey of the domain trust anchor that is
stored by the Domain CA. This is consistent with the
Certification Authority subject key identifier (Section 4.2.1.2
[RFC5280]) of the Domain CA's self signed root certificate. (A
string value bound to the Domain CA's self signed root certificate
subject and issuer fields is often colloquially used as a
humanized identity value but during protocol discussions the more
exact term as defined here is used).
drop ship: The physical distribution of equipment containing the
"factory default" configuration to a final destination. In zero-
touch scenarios there is no staging or pre-configuration during
drop-ship.
imprint: The process where a device obtains the cryptographic key
material to identify and trust future interactions with a network.
This term is taken from Konrad Lorenz's work in biology with new
ducklings: during a critical period, the duckling would assume
that anything that looks like a mother duck is in fact their
mother. An equivalent for a device is to obtain the fingerprint
of the network's root certification authority certificate. A
device that imprints on an attacker suffers a similar fate to a
duckling that imprints on a hungry wolf. Securely imprinting is a
primary focus of this document.[imprinting]. The analogy to
Lorenz's work was first noted in [Stajano99theresurrecting].
enrollment: The process where a device presents key material to a
network and acquires a network specific identity. For example
when a certificate signing request is presented to a certification
authority and a certificate is obtained in response.
Pledge: The prospective device, which has an identity installed by a
third-party (e.g., vendor, manufacturer or integrator).
Voucher A signed statement from the MASA service that indicates to a
Pledge the cryptographic identity of the Registrar it should
trust. There are different types of vouchers depending on how
Pritikin, et al. Expires September 14, 2017 [Page 6]
Internet-Draft BRSKI March 2017
that trust asserted. Multiple voucher types are defined in
[I-D.ietf-anima-voucher]
Domain: The set of entities that trust a common key infrastructure
trust anchor. This includes the Proxy, Registrar, Domain
Certificate Authority, Management components and any existing
entity that is already a member of the domain.
Domain CA: The domain Certification Authority (CA) provides
certification functionalities to the domain. At a minimum it
provides certification functionalities to a Registrar and stores
the trust anchor that defines the domain. Optionally, it
certifies all elements.
Join Registrar (and Coordinator): A representative of the domain
that is configured, perhaps autonomically, to decide whether a new
device is allowed to join the domain. The administrator of the
domain interfaces with a Join Registrar (and Coordinator) to
control this process. Typically a Join Registrar is "inside" its
domain. For simplicity this document often refers to this as just
"Registrar". The term JRC is used in common with other bootstrap
mechanisms.
Join Proxy: A domain entity that helps the pledge join the domain.
A Proxy facilitates communication for devices that find themselves
in an environment where they are not provided connectivity until
after they are validated as members of the domain. The pledge is
unaware that they are communicating with a proxy rather than
directly with a Registrar.
MASA Service: A third-party Manufacturer Authorized Signing
Authority (MASA) service on the global Internet. The MASA signs
vouchers. It also provides a repository for audit log information
of privacy protected bootstrapping events. It does not track
ownership.
Ownership Tracker: An Ownership Tracker service on the global
internet. The Ownership Tracker uses business processes to
accurately track ownership of all devices shipped against domains
that have purchased them. Although optional this component allows
vendors to provide additional value in cases where their sales and
distribution channels allow for accurately tracking of such
ownership. Ownership tracking information is indicated in
vouchers as described in [I-D.ietf-anima-voucher]
IDevID: An Initial Device Identity X.509 certificate installed by
the vendor on new equipment.
Pritikin, et al. Expires September 14, 2017 [Page 7]
Internet-Draft BRSKI March 2017
TOFU: Trust on First Use. Used similarly to [RFC7435]. This is
where a Pledge device makes no security decisions but rather
simply trusts the first Registrar it is contacted by. This is
also known as the "resurrecting duckling" model.
1.3. Scope of solution
Questions have been posed as to whether this solution is suitable in
general for Internet of Things (IoT) networks. This depends on the
capabilities of the devices in question. The terminology of
[RFC7228] is best used to describe the boundaries.
The solution described in this document is aimed in general at non-
constrained (i.e. class 2+) devices operating on a non-Challenged
network. The entire solution as described here is not intended to be
useable as-is by constrained devices operating on challenged networks
(such as 802.15.4 LLNs).
There are a number of optional mechanisms in BRSKI. These mechanisms
are not mandatory to implement for the core applicability to ANIMA.
These mechanisms have been moved out of the main flow of the document
to appendices to emphasis that they are not considered normative,
mandatory to implement, while making it easier for another document
to normatively reference them.
In many target applications, the systems involved are large router
platforms with multi-gigabit inter-connections, mounted in controlled
access data centers. But this solution is not exclusive to the
large, it is intended to scale to thousands of devices located in
hostile environments, such as ISP provided CPE devices which are
drop-shipped to the end user. The situation where an order is
fulfilled from distributed warehouse from a common stock and shipped
directly to the target location at the request of the domain owner is
explicitly supported. That stock ("SKU") could be provided to a
number of potential domain owners, and the eventual domain owner will
not know a-priori which device will go to which location.
The bootstrapping process can take minutes to complete depending on
the network infrastructure and device processing speed. The network
communication itself is not optimized for speed; for privacy reasons,
the discovery process allows for the Pledge to avoid announcing it's
presence through broadcasting. This protocol is not intended for low
latency handoffs. In networks requiring such things, the pledge
SHOULD already have been enrolled.
Specifically, there are protocol aspects described here which might
result in congestion collapse or energy-exhaustion of intermediate
battery powered routers in an LLN. Those types of networks SHOULD
Pritikin, et al. Expires September 14, 2017 [Page 8]
Internet-Draft BRSKI March 2017
NOT use this solution. These limitations are predominately related
to the large credential and key sizes required for device
authentication. Defining symmetric key techniques that meet the
operational requirements is out-of-scope but the underlying protocol
operations (TLS handshake and signing structures) have sufficient
algorithm agility to support such techniques when defined.
The imprint protocol described here could, however, be used by non-
energy constrained devices joining a non-constrained network (for
instance, smart light bulbs are usually mains powered, and speak
802.11). It could also be used by non-constrained devices across a
non-energy constrained, but challenged network (such as 802.15.4).
This document presumes that network access control has either already
occurred, is not required, or is integrated by the proxy and
registrar in such a way that the device itself does not need to be
aware of the details. Although the use of an X.509 Initial Device
Identity is consistant with IEEE 802.1AR [IDevID], and allows for
alignment with 802.1X network access control methods, its use here is
for Pledge authentication rather than network access control.
Some aspects are in scope for constrained devices on challenged
networks: the certificate contents, and the process by which the four
questions above are resolved is in scope. It is simply the actual
on-the-wire imprint protocol which is likely inappropriate.
2. Architectural Overview
The logical elements of the bootstrapping framework are described in
this section. Figure 1 provides a simplified overview of the
components. Each component is logical and may be combined with other
components as necessary.
Pritikin, et al. Expires September 14, 2017 [Page 9]
Internet-Draft BRSKI March 2017
.
.+------------------------+
+--------------Drop Ship-------------->.| Vendor Service |
| .+------------------------+
| .| M anufacturer| |
| .| A uthorized |Ownership|
| .| S igning |Tracker |
| .| A uthority | |
| .+--------------+---------+
| .............. ^
V |
+-------+ ............................................|...
| | . | .
| | . +------------+ +-----------+ | .
| | . | | | | | .
|Pledge | . | Circuit | | Domain <-------+ .
| | . | Proxy | | Registrar | .
| <--------> <-------> | .
| | . | | | | .
| | . +------------+ +-----+-----+ .
|IDevID | . | .
| | . +-----------------+----------+ .
| | . | Domain Certification | .
| | . | Authority | .
+-------+ . | Management and etc | .
. +----------------------------+ .
. .
................................................
"Domain" components
Figure 1
We assume a multi-vendor network. In such an environment there could
be a Vendor Service for each vendor that supports devices following
this document's specification, or an integrator could provide a
generic service authorized by multiple vendors. It is unlikely that
an integrator could provide Ownership Tracking services for multiple
vendors due to the required sales channel integrations necessary to
track ownership.
The domain is the managed network infrastructure the Pledge is
managed by. The a domain provides initial device connectivity
minimally sufficient for bootstrapping through the Circuit Proxy.
The Domain registrar makes authorization decisions and handles
connectivity to the vendor services and authenticates the Pledge.
Optional cryptographic credential and configuration management
systems are expected.
Pritikin, et al. Expires September 14, 2017 [Page 10]
Internet-Draft BRSKI March 2017
This document describes a secure zero-touch approach to bootstrapping
a remote key infrastructure. Secure bootstrapping requires
mitigating the threat of an attacker domain establishing a management
role over the pledge device. In a "trust on first use" model, where
this threat is ignored, the attacker has an opportunity to install a
persistent malware component. This document uses Vouchers to address
the threat while maintaining a significant level of flexibility.
2.1. Secure Imprinting without Vouchers
There are pre-existing methods available for establishing initial
trust. For example the enrollment protocol EST [RFC7030] details a
set of non-autonomic bootstrapping methods such as:
o using the Implicit Trust Anchor database (not an autonomic
solution because the URL must be securely distributed),
o engaging a human user to authorize the CA certificate using out-
of-band data (not an autonomic solution because the human user is
involved),
o using a configured Explicit TA database (not an autonomic solution
because the distribution of an explicit TA database is not
autonomic),
o and using a Certificate-Less TLS mutual authentication method (not
an autonomic solution because the distribution of symmetric key
material is not autonomic).
These "touch" methods do not meet the requirements for zero-touch.
There are "call home" technologies where the Pledge first establishes
a connection to a well known vendor service using a common client-
server authentication model. After mutual authentication appropriate
credentials to authenticate the target domain are transfered to the
Pledge. This creates serveral problems and limitations:
o the pledge requires realtime connectivity to the vendor service,
o the domain identity is exposed to the vendor service (this is a
privacy concern),
o the vendor is responsible for making the authorization decisions
(this is a liability concern),
BRSKI addresses these issues by introducting an authorization layer
via "vouchers". The additional complexity provides for significant
flexibility.
Pritikin, et al. Expires September 14, 2017 [Page 11]
Internet-Draft BRSKI March 2017
2.2. Secure Imprinting using Vouchers
A voucher is a cryptographically protected statement to the Pledge
device authorizing a zero-touch imprint on the Registrar domain.
The format and cryptographic mechanism of vouchers is described in
detail in [I-D.ietf-anima-voucher].
Vouchers provide a flexible mechanism to secure imprinting: the
Pledge device only imprints when a voucher can be validated. At the
lowest security levels the MASA server can indiscriminately issue
vouchers. At the highest security levels issuance of vouchers can be
integrated with complex sales channel integrations that are beyond
the scope of this document. This provides the flexability for a
number of use cases via a single common protocol mechanism on the
Pledge and Registrar devices that are to be widely deployed in the
field. The MASA vendor services have the flexibility to leverage
either the currently defined claim mechanisms or to experiment with
higher or lower security levels.
2.3. Initial Device Identifier
Pledge authentication is via an X.509 certificate installed during
the manufacturing process. This Initial Device Identifier provides a
basis for authenticating the Pledge during subsequent protocol
exchanges and informing the Registrar of the MASA URI. There is no
requirement for a common root PKI hierarchy. Each device vendor can
generate their own root certificate.
The following previously defined fields are in the X.509 IDevID
certificate:
o The subject field's DN encoding MUST include the "serialNumber"
attribute with the device's unique serial number.
o The subject alt field's encoding SHOULD include the a non-critical
version of the RFC4108 defined HardwareModuleName.
The following newly defined field SHOULD be in the X.509 IDevID
certificate: An X.509 non-critical certificate extension that
contains a single Uniform Resource Identifier (URI) that points to an
on-line Manufacturer Authorized Signing Authority. The URI is
represented as described in Section 7.4 of [RFC5280].
Any Internationalized Resource Identifiers (IRIs) MUST be mapped to
URIs as specified in Section 3.1 of [RFC3987] before they are placed
in the certificate extension.
Pritikin, et al. Expires September 14, 2017 [Page 12]
Internet-Draft BRSKI March 2017
The semantics of the URI are defined in Section 7 of this document.
The new extension is identified as follows:
<CODE BEGINS>
MASAURLExtnModule-2016 { iso(1) identified-organization(3) dod(6)
internet(1) security(5) mechanisms(5) pkix(7)
id-mod(0) id-mod-MASAURLExtn2016(TBD) }
DEFINITIONS IMPLICIT TAGS ::= BEGIN
-- EXPORTS ALL --
IMPORTS
EXTENSION
FROM PKIX-CommonTypes-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkixCommon-02(57) }
id-pe
FROM PKIX1Explicit-2009
{ iso(1) identified-organization(3) dod(6) internet(1)
security(5) mechanisms(5) pkix(7) id-mod(0)
id-mod-pkix1-explicit-02(51) } ;
MASACertExtensions EXTENSION ::= { ext-MASAURL, ... }
ext-MASAURL EXTENSION ::= { SYNTAX MASAURLSyntax
IDENTIFIED BY id-pe-masa-url }
id-pe-masa-url OBJECT IDENTIFIER ::= { id-pe TBD }
MASAURLSyntax ::= IA5String
END
<CODE ENDS>
The choice of id-pe is based on guidance found in Section 4.2.2 of
[RFC5280], "These extensions may be used to direct applications to
on-line information about the issuer or the subject". The MASA URL
is precisely that: online information about the particular subject.
3. Functional Overview
Entities behave in an autonomic fashion. They discover each other
and autonomically bootstrap into a key infrastructure delineating the
autonomic domain. See [RFC7575] for more information.
Pritikin, et al. Expires September 14, 2017 [Page 13]
Internet-Draft BRSKI March 2017
This section details the state machine and operational flow for each
of the main three entities. The pledge, the domain (primarily a
Registrar) and the MASA service.
A representative flow is shown in Figure 2:
+--------+ +---------+ +------------+ +------------+
| Pledge | | Circuit | | Domain | | Vendor |
| | | Proxy | | Registrar | | Service |
| | | | | | | (Internet |
+--------+ +---------+ +------------+ +------------+
| | | |
|<-RFC3927 IPv4 adr | Appendix A | |
or|<-RFC4862 IPv6 adr | | |
| | | |
|-------------------->| | |
| optional: mDNS query| Appendix B | |
| RFC6763/RFC6762 | | |
| | | |
|<--------------------| | |
| GRASP M_FLOOD | | |
| periodic broadcast| | |
| | | |
|<------------------->C<----------------->| |
| TLS via the Circuit Proxy | |
|<--Registrar TLS server authentication---| |
[PROVISIONAL accept of server cert] | |
P---X.509 client authentication---------->| |
P | | |
P---Request Voucher (include nonce)------>| |
P | | |
P | /---> | |
P | | [accept device?] |
P | | [contact Vendor] |
P | | |--Pledge ID-------->|
P | | |--Domain ID-------->|
P | | |--optional:nonce--->|
P | | | [extract DomainID]
P | | | |
P | optional: | [update audit log]
P | |can | |
P | |occur | |
P | |in | |
P | |advance | |
P | | | |
P | | |<-device audit log--|
P | | |<- voucher ---------|
P | \----> | |
Pritikin, et al. Expires September 14, 2017 [Page 14]
Internet-Draft BRSKI March 2017
P | | |
P | [verify audit log and voucher] |
P | | |
P<------voucher---------------------------| |
[verify voucher ] | | |
[verify provisional cert ]| | |
| | | |
|---------------------------------------->| |
| Continue with RFC7030 enrollment | |
| using now bidirectionally authenticated | |
| TLS session. | | |
| | | |
| | | |
| | | |
Figure 2
3.1. Behavior of a Pledge
A pledge that has not yet been bootstrapped attempts to find a local
domain and join it. A pledge MUST NOT automatically initiate
bootstrapping if it has already been configured or is in the process
of being configured.
States of a pledge are as follows:
Pritikin, et al. Expires September 14, 2017 [Page 15]
Internet-Draft BRSKI March 2017
+--------------+
| Start |
| |
+------+-------+
|
+------v-------+
| Discover |
+------------> |
| +------+-------+
| |
| +------v-------+
| | Identity |
^------------+ |
| rejected +------+-------+
| |
| +------v-------+
| | Request |
| | Join |
| +------+-------+
| |
| +------v-------+
| | Imprint | Optional
^------------+ <--+Manual input (Appendix C)
| Bad Vendor +------+-------+
| response |
| +------v-------+
| | Enroll |
^------------+ |
| Enroll +------+-------+
| Failure |
| +------v-------+
| | Being |
^------------+ Managed |
Factory +--------------+
reset
Figure 3
State descriptions for the pledge are as follows:
1. Discover a communication channel to a Registrar.
2. Identify itself. This is done by presenting an X.509 IDevID
credential to the discovered Registrar (via the Proxy) in a TLS
handshake. (The Registrar credentials are only provisionally
accepted at this time).
Pritikin, et al. Expires September 14, 2017 [Page 16]
Internet-Draft BRSKI March 2017
3. Requests to Join the discovered Registrar. A unique nonce can be
included ensuring that any responses can be associated with this
particular bootstrapping attempt.
4. Imprint on the Registrar. This requires verification of the
vendor service provided voucher. A voucher contains sufficient
information for the Pledge to complete authentication of a
Registrar. (It enables the Pledge to finish authentication of
the Registrar TLS server certificate).
5. Enroll. By accepting the domain specific information from a
Registrar, and by obtaining a domain certificate from a Registrar
using a standard enrollment protocol, e.g. Enrollment over
Secure Transport (EST) [RFC7030].
6. The Pledge is now a member of, and can be managed by, the domain
and will only repeat the discovery aspects of bootstrapping if it
is returned to factory default settings.
The following sections describe each of these steps in more detail.
3.1.1. Discovery
The result of discovery is a logical communication with a Registrar,
through a Proxy. The Proxy is transparent to the Pledge but is
always assumed to exist.
To discover the Registrar the Pledge performs the following actions:
a. MUST: Obtains a local address using IPv6 methods as described in
[RFC4862] IPv6 Stateless Address AutoConfiguration. [RFC7217] is
encouraged. IPv4 methods are described in Appendix A
b. MUST: Listen for GRASP M_FLOOD ([I-D.ietf-anima-grasp])
announcements of the objective: "ACP+Proxy". See section
Section 5 for the details of the the objective. The Pledge may
listen concurrently for other sources of information, see
Appendix B.
Once a proxy is discovered the Pledge communicates with a Registrar
through the proxy using the bootstrapping protocol defined in
Section 7.
Each discovery method attempted SHOULD exponentially back-off
attempts (to a maximum of one hour) to avoid overloading the network
infrastructure with discovery. The back-off timer for each method
MUST be independent of other methods. Methods SHOULD be run in
parallel to avoid head of queue problems. Once a connection to a
Pritikin, et al. Expires September 14, 2017 [Page 17]
Internet-Draft BRSKI March 2017
Registrar is established (e.g. establishment of a TLS session key)
there are expectations of more timely responses, see Section 7.1.
Once all discovered services are attempted the device SHOULD return
to listening for GRASP M_FLOOD. It should periodically retry the
vendor specific mechanisms. The Pledge MAY prioritize selection
order as appropriate for the anticipated environment.
3.1.2. Identity
The Pledge identifies itself during the communication protocol
handshake. If the client identity is rejected (that is, the TLS
handshake does not complete) the Pledge repeats the Identity process
using the next proxy or discovery method available.
The bootstrapping protocol server is not initially authenticated.
Thus the connection is provisional and all data received is untrusted
until sufficiently validated even though it is over a TLS connection.
This is aligned with the existing provisional mode of EST [RFC7030]
during s4.1.1 "Bootstrap Distribution of CA Certificates". See
Section 7.3 for more information about when the TLS connection
authentication is completed.
All security associations established are between the new device and
the Bootstrapping server regardless of proxy operations.
3.1.2.1. Concurrent attempts to join
The Pledge MAY attempt multiple mechanisms concurrently, but if it
does so, it MUST wait in the provisional state until all mechanisms
have either succeeded or failed, and then MUST proceed with the
highest priority mechanism which has succeed. To proceed beyond this
point, specifically, to provide a nonce, could result in the MASA
gratuitously auditing a connection.
3.1.3. Request Join
The Pledge POSTs a request to join the domain to the Bootstrapping
server. This request contains a Pledge generated nonce and informs
the Bootstrapping server which imprint methods the Pledge will
accept.
The nonce ensures the Pledge can verify that responses are specific
to this bootstrapping attempt. This minimizes the use of global time
and provides a substantial benefit for devices without a valid clock.
Pritikin, et al. Expires September 14, 2017 [Page 18]
Internet-Draft BRSKI March 2017
3.1.3.1. Redirects during the Join Process
EST [RFC7030] describes situations where the bootstrapping server MAY
redirect the client to an alternate server via a 3xx status code.
Such redirects MAY be accepted if the pledge has used the methods
described in Appendix B, in combination with an implicit trust
anchor. Redirects during the provisional period are otherwise
unstrusted, and MUST cause a failure.
3.1.4. Imprint
The Pledge validates the voucher and accepts the Registrar ID. The
provisional TLS connection is validated using the Registrar ID from
the voucher.
3.1.5. Lack of realtime clock
Many devices when bootstrapping do not have knowledge of the current
time. Mechanisms like Network Time Protocols can not be secured
until bootstrapping is complete. Therefore bootstrapping is defined
in a method that does not require knowledge of the current time.
Unfortunately there are moments during bootstrapping when
certificates are verified, such as during the TLS handshake, where
validity periods are confirmed. This paradoxical "catch-22" is
resolved by the Pledge maintaining a concept of the current "window"
of presumed time validity that is continually refined throughout the
bootstrapping process as follows:
o Initially the Pledge does not know the current time.
o During Pledge authentiation by the Registrar a realtime clock can
be used by the Registrar. This bullet expands on a closely
related issue regarding Pledge lifetimes. RFC5280 indicates that
long lived Pledge certifiates "SHOULD be assigned the
GeneralizedTime value of 99991231235959Z" [RFC7030] so the
Registrar MUST support such lifetimes and SHOULD support ignoring
Pledge lifetimes if they did not follow the RFC5280
recommendations.
o The Pledge authenticates the voucher presented to it. During this
authentication the Pledge ignores certificate lifetimes (by
necessity because it does not have a clock). The voucher itself
SHOULD contain the nonce included in the original request which
proves the voucher is fresh.
o Once the voucher is accepted the validity period of the
domainCAcert in the voucher (see Section 7.3) now serves as a
Pritikin, et al. Expires September 14, 2017 [Page 19]
Internet-Draft BRSKI March 2017
valid time window. Any subsequent certificate validity periods
checked during RFC5280 path validation MUST occur within this
window.
o When accepting an enrollment certificate the validity period
within the new certificate is assumed to be valid by the Pledge.
The Pledge is now willing to use this credential for client
authentication.
3.1.6. Enrollment
As the final step of bootstrapping a Registrar helps to issue a
domain specific credential to the Pledge. For simplicity in this
document, a Registrar primarily facilitates issuing a credential by
acting as an RFC5280 Registration Authority for the Domain
Certification Authority.
Enrollment proceeds as described in [RFC7030]. Authentication of the
EST server is done using the Voucher rather than the methods defined
in EST.
Once the Voucher is received, as specified in this document, the
client has sufficient information to leverage the existing
communication channel with a Registrar to continue an EST RFC7030
enrollment. Enrollment picks up at RFC7030 section 4.1.1.
bootstrapping where the Voucher provides the "out-of-band" CA
certificate fingerprint (in this case the full CA certificate) such
that the client can now complete the TLS server authentication. At
this point the client continues with EST enrollment operations
including "CA Certificates Request", "CSR Attributes" and "Client
Certificate Request" or "Server-Side Key Generation".
For the purposes of creating the ANIMA Autonomic Control Plane, the
contents of the new certificate MUST be carefully specified.
[I-D.ietf-anima-autonomic-control-plane] section 5.1.1 contains
details. The Registrar MUST provide the the correct ACP information
to populate the subjectAltName / rfc822Name field in the "CSR
Attributes" step.
3.1.7. Being Managed
Functionality to provide generic "configuration" information is
supported. The parsing of this data and any subsequent use of the
data, for example communications with a Network Management System is
out of scope but is expected to occur after bootstrapping enrollment
is complete. This ensures that all communications with management
systems which can divulge local security information (e.g. network
Pritikin, et al. Expires September 14, 2017 [Page 20]
Internet-Draft BRSKI March 2017
topology or raw key material) is secured using the local credentials
issued during enrollment.
The Pledge uses bootstrapping to join only one domain. Management by
multiple domains is out-of-scope of bootstrapping. After the device
has successfully joined a domain and is being managed it is plausible
that the domain can insert credentials for other domains depending on
the device capabilities.
See Section 3.5.
3.2. Behavior of a Join Proxy
The role of the Proxy is to facilitate communications. The Proxy
forwards packets between the Pledge and a Registrar that has been
configured on the Proxy.
The Proxy does not terminate the TLS handshake.
A Proxy is always assumed even if it is directly integrated into a
Registrar. (In a completely autonomic network, the Registrar MUST
provide proxy functionality so that it can be discovered, and the
network can grow concentrically around the Registrar)
As a result of the Proxy Discovery process in section Section 3.1.1,
the port number exposed by the proxy does not need to be well known,
or require an IANA allocation.
If the Proxy joins an Autonomic Control Plane
([I-D.ietf-anima-autonomic-control-plane]) it SHOULD use Autonomic
Control Plane secured GRASP ([I-D.ietf-anima-grasp]) to discovery the
Registrar address and port. As part of the discovery process, the
proxy mechanism (Circuit Proxy vs IPIP encapsulation) is agreed to
between the Registrar and Join Proxy.
For the IPIP encapsulation methods, the port announced by the Proxy
MUST be the same as on the registrar in order for the proxy to remain
stateless.
In order to permit the proxy functionality to be implemented on the
maximum variety of devices the chosen mechanism SHOULD use the
minimum amount of state on the proxy device. While many devices in
the ANIMA target space will be rather large routers, the proxy
function is likely to be implemented in the control plane CPU of such
a device, with available capabilities for the proxy function similar
to many class 2 IoT devices.
Pritikin, et al. Expires September 14, 2017 [Page 21]
Internet-Draft BRSKI March 2017
The document [I-D.richardson-anima-state-for-joinrouter] provides a
more extensive analysis of the alternative proxy methods.
3.2.1. CoAP connection to Registrar
The CoAP mechanism was depreciated.
3.2.2. HTTPS proxy connection to Registrar
The proxy SHOULD also provide one of: an IPIP encapsulation of HTTP
traffic on TCP port TBD to the registrar, or a TCP circuit proxy that
connects the Pledge to a Registrar.
When the Proxy provides a circuit proxy to a Registrar the Registrar
MUST accept HTTPS connections.
When the Proxy provides a stateless IPIP encapsulation to a
Registrar, then the Registrar will have to perform IPIP
decapsulation, remembering the originating outer IPIP source address
in order to qualify the inner link-local address. This is a kind of
encapsulation and processing which is similar in many ways to how
mobile IP works.
Being able to connect a TCP (HTTP) or UDP (CoAP) socket to a link-
local address with an encapsulated IPIP header requires API
extensions beyond [RFC3542] for UDP use, and requires a form of
connection latching (see section 4.1 of [RFC5386] and all of
[RFC5660], except that a simple IPIP tunnel is used rather than an
IPsec tunnel).
3.3. Behavior of the Registrar
A Registrar listens for Pledges and determines if they can join the
domain. A Registrar obtains a Voucher from the MASA service and
delivers them to the Pledge as well as facilitating enrollment with
the domain PKI.
A Registrar is typically configured manually. When the Registrar
joins an Autonomic Control Plane
([I-D.ietf-anima-autonomic-control-plane]) it MUST respond to GRASP
([I-D.ietf-anima-grasp]) M_DISCOVERY message. See Section 6
Registrar behavior is as follows:
Pritikin, et al. Expires September 14, 2017 [Page 22]
Internet-Draft BRSKI March 2017
Contacted by Pledge
+
|
+-------v----------+
| Entity | fail?
| Authentication +---------+
+-------+----------+ |
| |
+-------v----------+ |
| Entity | fail? |
| Authorization +--------->
+-------+----------+ |
| |
+-------v----------+ |
| Claiming the | fail? |
| Entity +--------->
+-------+----------+ |
| |
+-------v----------+ |
| Log Verification | fail? |
| +--------->
+-------+----------+ |
| |
+-------v----------+ +----v-------+
| Forward | | |
| Voucher | | Reject |
| to the Pledge | | Device |
| | | |
+------------------+ +------------+
Figure 4
3.3.1. Pledge Authentication
The applicable authentication methods detailed in EST [RFC7030] are:
o the use of an X.509 IDevID credential during the TLS client
authentication,
o or the use of a secret that is transmitted out of band between the
Pledge and a Registrar (this use case is not autonomic).
In order to validate the X.509 IDevID credential a Registrar
maintains a database of vendor trust anchors (e.g. vendor root
certificates or keyIdentifiers for vendor root public keys). For
user interface purposes this database can be mapped to colloquial
vendor names. Registrars can be shipped with the trust anchors of a
significant number of third-party vendors within the target market.
Pritikin, et al. Expires September 14, 2017 [Page 23]
Internet-Draft BRSKI March 2017
3.3.2. Pledge Authorization
In a fully automated network all devices must be securely identified
and authorized to join the domain.
A Registrar accepts or declines a request to join the domain, based
on the authenticated identity presented. Automated acceptance
criteria include:
o allow any device of a specific type (as determined by the X.509
IDevID),
o allow any device from a specific vendor (as determined by the
X.509 IDevID),
o allow a specific device from a vendor (as determined by the X.509
IDevID) against a domain white list. (The mechanism for checking
a shared white list potentially used by multiple Registrars is out
of scope).
To look the Pledge up in a domain white list a consistent method for
extracting device identity from the X.509 certificate is required.
RFC6125 describes Domain-Based Application Service identity but here
we require Vendor Device-Based identity. The subject field's DN
encoding MUST include the "serialNumber" attribute with the device's
unique serial number. In the language of RFC6125 this provides for a
SERIALNUM-ID category of identifier that can be included in a
certificate and therefore that can also be used for matching
purposes. The SERIALNUM-ID whitelist is collated according to vendor
trust anchor since serial numbers are not globally unique.
The Registrar MUST use the vendor provided MASA service to verify
that the device's history log does not include unexpected Registrars.
If a device had previously registered with another domain, a
Registrar of that domain would show in the log.
The authorization performed during BRSKI MAY be used for EST
enrollment requests by proceeding with EST enrollment using the
authenticated and authorized TLS connection. This minimizes the
number of cryptographic and protocol operations necessary to complete
bootstraping of the local key infrastructure.
3.3.3. Claiming the New Entity
Claiming an entity establishes an audit log at the MASA server and
provides a Registrar with proof, in the form of the Voucher, that the
log entry has been inserted. As indicated in Section 3.1.4 a Pledge
will only proceed with bootstrapping if a Voucher has been received.
Pritikin, et al. Expires September 14, 2017 [Page 24]
Internet-Draft BRSKI March 2017
The Pledge therefore enforces that bootstrapping only occurs if the
claim has been logged. There is no requirement for the vendor to
definitively know that the device is owned by the Registrar.
The Registrar obtains the MASA URI via static configuration or by
extracting it from the X.509 IDevID credential. See Section 2.3.
During initial bootstrapping the Pledge provides a nonce specific to
the particular bootstrapping attempt. The Registrar SHOULD include
this nonce when claiming the Pledge from the MASA service. Claims
from an unauthenticated Registrar are only serviced by the MASA
resource if a nonce is provided.
The Registrar can claim a Pledge that is not online by forming the
request using the entities unique identifier and not including a
nonce in the claim request. Vouchers obtained in this way do not
have a lifetime and they provide a permanent method for the domain to
claim the device. Evidence of such a claim is provided in the audit
log entries available to any future Registrar. Such claims reduce
the ability for future domains to secure bootstrapping and therefore
the Registrar MUST be authenticated by the MASA service although no
requirement is implied that the MASA associates this authentication
with ownership.
An Ownership Voucher requires the vendor to definitively know that a
device is owned by a specific domain. The method used to "claim"
this are out-of-scope. A MASA ignores or reports failures when an
attempt is made to claim a device that has a an Ownership Voucher.
3.3.4. Log Verification
A Registrar requests the log information for the Pledge from the MASA
service. The log is verified to confirm that the following is true
to the satisfaction of a Registrar's configured policy:
o Any nonceless entries in the log are associated with domainIDs
recognized by the registrar.
o Any nonce'd entries are older than when the domain is known to
have physical possession of the Pledge or that the domainIDs are
recognized by the registrar.
If any of these criteria are unacceptable to a Registrar the entity
is rejected. A Registrar MAY be configured to ignore the history of
the device but it is RECOMMENDED that this only be configured if
hardware assisted NEA [RFC5209] is supported.
Pritikin, et al. Expires September 14, 2017 [Page 25]
Internet-Draft BRSKI March 2017
This document specifies a simple log format as provided by the MASA
service to the registar. This format could be improved by
distributed consensus technologies that integrate vouchers with a
technologies such as block-chain or hash trees or the like. Doing so
is out of the scope of this document but are anticipated improvements
for future work.
3.4. Behavior of the MASA Service
The Manufacturer Authorized Signing Authority service is directly
provided by the manufacturer, or can be provided by a third party the
manufacturer authorizes. It is a cloud resource. The MASA service
provides the following functionalities to Registrars:
Issue Vouchers: In response to Registrar requests the MASA service
issues vouchers. Depending on the MASA policy the Registrar claim
of device ownership is either accepted or verified using out-of-
scope methods (that are expected to improve over time).
Log Vouchers Issued: When a voucher is issued the act of issuing it
includes updating the certifiable logs. Future work to enhance
and distribute these logs is out-of-scope but expected over time.
Provide Logs: As a baseline implementation of the certified logging
mechanism the MASA is repsonsible for reporting logged
information. The current method involves trusting the MASA.
Other logging methods where the MASA is less trusted are expected
to be developed over time.
3.5. Leveraging the new key infrastructure / next steps
As the devices have a common trust anchor, device identity can be
securely established, making it possible to automatically deploy
services across the domain in a secure manner.
Examples of services:
o Device management.
o Routing authentication.
o Service discovery.
3.5.1. Network boundaries
When a device has joined the domain, it can validate the domain
membership of other devices. This makes it possible to create trust
boundaries where domain members have higher level of trusted than
Pritikin, et al. Expires September 14, 2017 [Page 26]
Internet-Draft BRSKI March 2017
external devices. Using the autonomic User Interface, specific
devices can be grouped into to sub domains and specific trust levels
can be implemented between those.
3.6. Interactions with Network Access Control
The assumption is that Network Access Control (NAC) completes using
the Pledge 's X.509 IDevID credentials and results in the device
having sufficient connectivity to discovery and communicate with the
proxy. Any additional connectivity or quarantine behavior by the NAC
infrastructure is out-of-scope. After the devices has completed
bootstrapping the mechanism to trigger NAC to re-authenticate the
device and provide updated network privileges is also out-of-scope.
This achieves the goal of a bootstrap architecture that can integrate
with NAC but does not require NAC within the network where it wasn't
previously required. Future optimizations can be achieved by
integrating the bootstrapping protocol directly into an initial EAP
exchange.
4. Domain Operator Activities
This section describes how an operator interacts with a domain that
supports the bootstrapping as described in this document.
4.1. Instantiating the Domain Certification Authority
This is a one time step by the domain administrator. This is an "off
the shelf" CA with the exception that it is designed to work as an
integrated part of the security solution. This precludes the use of
3rd party certification authority services that do not provide
support for delegation of certificate issuance decisions to a domain
managed Registration Authority.
4.2. Instantiating the Registrar
This is a one time step by the domain administrator. One or more
devices in the domain are configured take on a Registrar function.
A device can be configured to act as a Registrar or a device can
auto-select itself to take on this function, using a detection
mechanism to resolve potential conflicts and setup communication with
the Domain Certification Authority. Automated Registrar selection is
outside scope for this document.
Pritikin, et al. Expires September 14, 2017 [Page 27]
Internet-Draft BRSKI March 2017
4.3. Accepting New Entities
For each Pledge the Registrar is informed of the unique identifier
(e.g. serial number) along with the manufacturer's identifying
information (e.g. manufacturer root certificate). This can happen in
different ways:
1. Default acceptance: In the simplest case, the new device asserts
its unique identity to a Registrar. The registrar accepts all
devices without authorization checks. This mode does not provide
security against intruders and is not recommended.
2. Per device acceptance: The new device asserts its unique identity
to a Registrar. A non-technical human validates the identity,
for example by comparing the identity displayed by the registrar
(for example using a smartphone app) with the identity shown on
the packaging of the device. Acceptance may be triggered by a
click on a smartphone app "accept this device", or by other forms
of pairing. See also [I-D.behringer-homenet-trust-bootstrap] for
how the approach could work in a homenet.
3. Whitelist acceptance: In larger networks, neither of the previous
approaches is acceptable. Default acceptance is not secure, and
a manual per device methods do not scale. Here, the registrar is
provided a priori with a list of identifiers of devices that
belong to the network. This list can be extracted from an
inventory database, or sales records. If a device is detected
that is not on the list of known devices, it can still be
manually accepted using the per device acceptance methods.
4. Automated Whitelist: an automated process that builds the
necessary whitelists and inserts them into the larger network
domain infrastructure is plausible. Once set up, no human
intervention is required in this process. Defining the exact
mechanisms for this is out of scope although the registrar
authorization checks is identified as the logical integration
point of any future work in this area.
None of these approaches require the network to have permanent
Internet connectivity. Even when the Internet based MASA service is
used, it is possible to pre-fetch the required information from the
MASA a priori, for example at time of purchase such that devices can
enroll later. This supports use cases where the domain network may
be entirely isolated during device deployment.
Additional policy can be stored for future authorization decisions.
For example an expected deployment time window or that a certain
Proxy must be used.
Pritikin, et al. Expires September 14, 2017 [Page 28]
Internet-Draft BRSKI March 2017
4.4. Automatic Enrollment of Devices
The approach outlined in this document provides a secure zero-touch
method to enroll new devices without any pre-staged configuration.
New devices communicate with already enrolled devices of the domain,
which proxy between the new device and a Registrar. As a result of
this completely automatic operation, all devices obtain a domain
based certificate.
4.5. Secure Network Operations
The certificate installed in the previous step can be used for all
subsequent operations. For example, to determine the boundaries of
the domain: If a neighbor has a certificate from the same trust
anchor it can be assumed "inside" the same organization; if not, as
outside. See also Section 3.5.1. The certificate can also be used
to securely establish a connection between devices and central
control functions. Also autonomic transactions can use the domain
certificates to authenticate and/or encrypt direct interactions
between devices. The usage of the domain certificates is outside
scope for this document.
5. Proxy Discovery Protocol Details
The proxy uses the GRASP M_FLOOD mechanism to announce itself. This
announcement is done with the same message as the ACP announcement
detailed in [I-D.ietf-anima-autonomic-control-plane].
proxy-objective = ["Proxy", [ O_IPv6_LOCATOR, ipv6-address,
transport-proto, port-number ] ]
ipv6-address - the v6 LL of the proxy
transport-proto - 6, for TCP 17 for UDP
port-number - the TCP or UDP port number to find the proxy
Figure 5
6. Registrar Discovery Protocol Details
The registrar responds to discovery messages from the proxy (or GRASP
caches between them) as follows: (XXX changed from M_DISCOVERY)
objective = ["AN_registrar", F_DISC, 255 ]
discovery-message = [M_NEG_SYN, session-id, initiator, objective]
Figure 6: Registrar Discovery
Pritikin, et al. Expires September 14, 2017 [Page 29]
Internet-Draft BRSKI March 2017
The response from the registrar (or cache) will be a M_RESPONSE with
the following parameters:
response-message = [M_RESPONSE, session-id, initiator, ttl,
(+locator-option // divert-option), ?objective)]
initiator = ACP address of Registrar
locator1 = [O_IPv6_LOCATOR, fd45:1345::6789, 6, 443]
locator2 = [O_IPv6_LOCATOR, fd45:1345::6789, 17, 5683]
locator3 = [O_IPv6_LOCATOR, fe80::1234, 41, nil]
Figure 7: Registrar Response
The set of locators is to be interpreted as follows. A protocol of 6
indicates that TCP proxying on the indicated port is desired. A
protocol of 17 indicates that UDP proxying on the indicated port is
desired. In each case, the traffic SHOULD be proxied to the same
port at the ULA address provided.
A protocol of 41 indicates that packets may be IPIP proxy'ed. The
address in the locator In the case of that IPIP proxying is used,
then the provided link-local address MUST be advertised on the local
link using proxy neighbour discovery. The Join Proxy MAY limit
forwarded traffic to the protocol (6 and 17) and port numbers
indicated by locator1 and locator2. The address to which the IPIP
traffic should be sent is the initiator address (an ACP address of
the Registrar), not the address given in the locator.
All Registrar MUST accept TCP / UDP traffic on the ports given at the
ACP address of the Registrar. If the Registrar supports IPIP
tunnelling, it MUST also accept traffic encapsulated with IPIP.
Registrars MUST accept HTTPS/EST traffic on the ports indicated.
Registrars MAY accept DTLS/CoAP/EST traffic in addition.
7. Protocol Details
A bootstrapping protocol could be implemented as an independent
protocol from EST, but for simplicity and to reduce the number of TLS
connections and crypto operations required on the Pledge, it is
described specifically as extensions to EST. These extensions MUST
be supported by the Registrar EST server within the same .well-known
URI tree as the existing EST URIs as described in EST [RFC7030]
section 3.2.2.
A MASA URI is therefore "https:// authority "./well-known/est". The
portion contained in the IDevID extension is only
"https://example.com" since everything after that is well known.
Pritikin, et al. Expires September 14, 2017 [Page 30]
Internet-Draft BRSKI March 2017
Establishment of the TLS connection for bootstrapping is as specified
for EST [RFC7030]. In particular server identity and client identity
are as described in EST [RFC7030] section 3.3. In EST [RFC7030]
provisional server authentication for bootstrapping is described in
section 4.1.1 wherein EST clients can "engage a human user to
authorize the CA certificate using out-of-band data such as a CA
certificate" or wherein a human user configures the URI of the EST
server for Implicit TA based authentication. This documented
establishes automated methods of authorizing the CA certificate using
in-band vouchers.
If the Pledge uses a well known URI for contacting a well known
Registrar the EST Implicit Trust Anchor database is used to
authenticate the well known URI. In this case the connection is not
provisional and RFC6125 methods can be used to authenticate the
Registrar
The Pledge establishes a TLS connection with the Registrar through
the circuit proxy (see Section 3.2) but the TLS connection is with
the Registar; so for this section the "Pledge" is the TLS client and
the "Registrar" is the TLS server.
The extensions for the Pledge client are as follows:
o The Pledge provisionally accept the EST server certificate during
the TLS handshake as detailed in Section 7.3.1.
o The Pledge requests and validates the Voucher as described below.
At this point the Pledge has sufficient information to validate
domain credentials.
o The Pledge calls the EST defined /cacerts method to obtain the
current CA certificate. These are validated using the Voucher.
o The Pledge completes bootstrapping as detailed in EST section
4.1.1.
In order to obtain a Voucher and associated logs a Registrar contacts
the MASA service Service using REST calls:
Pritikin, et al. Expires September 14, 2017 [Page 31]
Internet-Draft BRSKI March 2017
+-----------+ +----------+ +-----------+ +----------+
| New | | Circuit | | | | |
| Entity | | Proxy | | Registrar | | Vendor |
| | | | | | | |
++----------+ +--+-------+ +-----+-----+ +--------+-+
| | | |
| | | |
| TLS hello | TLS hello | |
Establish +---------------C---------------> |
TLS | | | |
connection | | Server Cert | |
<---------------C---------------+ |
| Client Cert | | |
+---------------C---------------> |
| | | |
HTTP REST | POST /requestvoucher | |
Data +--------------------nonce------> |
| . | /requestvoucher|
| . +---------------->
| <----------------+
| | /requestlog |
| +---------------->
| voucher <----------------+
<-------------------------------+ |
| (optional config information) | |
| . | |
| . | |
Figure 8
In some use cases the Registrar may need to contact the Vendor in
advanced, for example when the target network is air-gapped. The
nonceless request format is provided for this and the resulting flow
is slightly different. The security differences associated with not
knowing the nonce are discussed below:
Pritikin, et al. Expires September 14, 2017 [Page 32]
Internet-Draft BRSKI March 2017
+-----------+ +----------+ +-----------+ +----------+
| New | | Circuit | | | | |
| Entity | | Proxy | | Registrar | | Vendor |
| | | | | | | |
++----------+ +--+-------+ +-----+-----+ +--------+-+
| | | |
| | | |
| | | /requestvoucher|
| | (nonce +---------------->
| | unknown) <----------------+
| | | /requestlog |
| | +---------------->
| | <----------------+
| TLS hello | TLS hello | |
Establish +---------------C---------------> |
TLS | | | |
connection | | Server Cert | |
<---------------C---------------+ |
| Client Cert | | |
| | | |
HTTP REST | POST /requestvoucher | |
Data +----------------------nonce----> (discard |
| voucher | nonce) |
<-------------------------------+ |
| (optional config information) | |
| . | |
| . | |
Figure 9
The extensions for a Registrar server are as follows:
o The Registrar requests and validates the Voucher from the vendor
authorized MASA service.
o The Registrar forwards the Voucher to the Pledge when requested.
o The Registar performs log verifications in addition to local
authorization checks before accepting the Pledge device.
The provisional TLS connection introduces security risks that are
addressed as follows:
If the Registrar provides a redirect response the Pledge MUST follow
the redirect but the connection remains provisional. The Pledge MUST
only follow a single redirection.
Pritikin, et al. Expires September 14, 2017 [Page 33]
Internet-Draft BRSKI March 2017
The Registar MAY respond with an HTTP 202 ("the request has been
accepted for processing, but the processing has not been completed")
as described in EST [RFC7030] section 4.2.3 wherein the client "MUST
wait at least the specified 'retry-after' time before repeating the
same request". The Pledge is RECOMMENDED to provide local feed
(blinked LED etc) during this wait cycle if mechanisms for this are
available. To prevent an attacker Registrar from significantly
delaying bootstrapping the Pledge MUST limit the 'retry-after' time
to 60 seconds. To avoid waiting on a single erroneous Registrar the
Pledge MUST drop the connection after 5 seconds and proceed to other
discovered Registrars. Ideally the Pledge could keep track of the
appropriate retry-after value for any number of outstanding
Registrars but this would involve a large state table on the Pledge.
Instead the Pledge MAY ignore the exact retry-after value in favor of
a single hard coded value that takes effect between discovery
(Section 3.1.1) attempts. A Registrar that is unable to complete the
transaction the first time due to timing reasons will have future
chances.
7.1. Request Voucher from the Registrar
When the Pledge bootstraps it makes a request for a Voucher from a
Registrar.
This is done with an HTTPS POST using the operation path value of
"/requestvoucher".
The request format is JSON object containing a 64bit nonce generated
by the client for each request. This nonce MUST be a
cryptographically strong random or pseudo-random number that can not
be easily predicted. The nonce MUST NOT be reused for multiple
attempts to join a network domain. The nonce assures the Pledge that
the Voucher response is associated with this bootstrapping attempt
and is not a replay.
Request media type: application/voucherrequest
Request format: a JSON file with the following:
{
"version":"1",
"nonce":"<64bit nonce value>",
}
[[EDNOTE: Even if the nonce was signed it would provide no defense
against rogue registrars; although it would assure the MASA that a
certified Pledge exists. To protect against rogue registrars a nonce
component generated by the MASA (a new round trip) would be
Pritikin, et al. Expires September 14, 2017 [Page 34]
Internet-Draft BRSKI March 2017
required). Instead this is addressed by requiring MASA & Registrar
authentications but it is worth exploring additional protections.
This to be explored more at IETF96.]]
The Registrar validates the client identity as described in EST
[RFC7030] section 3.3.2. The registrar performs authorization as
detailed in Section 3.3.2. If authorization is successful the
Registrar obtains an Voucher from the MASA service (see Section 5.2).
The received Voucher is forwarded to the Pledge.
7.2. Request Voucher from MASA
A Registrar requests a Voucher from the MASA service using a REST
interface. For simplicity this is defined as an optional EST message
between a Registrar and an EST server running on the MASA service
although the Registrar is not required to make use of any other EST
functionality when communicating with the MASA service. (The MASA
service MUST properly reject any EST functionality requests it does
not wish to service; a requirement that holds for any REST
interface).
This is done with an HTTP POST using the operation path value of
"/requestvoucher".
Request media type: application/voucherrequest+cms
The request format is a JSON object optionally containing the nonce
value (as obtained from the bootstrap request) and the X.509 IDevID
extracted serial number (the full certificate is not needed and no
proof-of-possession information for the device identity is included).
The AuthorityKeyIdentifier value from the certificate is included to
ensure a statistically unique identity. The Pledge's serial number
is extracted from the X.509 IDevID. See Section 2.3.
{
"version":"1",
"nonce":"<64bit nonce value>",
"IDevIDAuthorityKeyIdentifier":"<base64 encoded keyIdentifier">,
"DevIDSerialNumber":"<id-at-serialNumber or base64 encoded
hardwareModuleName hwSerialNum>",
}
A Registrar MAY exclude the nonce from the request. Doing so allows
the Registrar to request a Voucher when the Pledge is not online, or
when the target bootstrapping environment is not on the same network
as the MASA server (this requires the Registrar to learn the
appropriate DevIDSerialNumber field from the physical device labeling
Pritikin, et al. Expires September 14, 2017 [Page 35]
Internet-Draft BRSKI March 2017
or from the sales channel -- how this occurs is out-of-scope of this
document). If a nonce is not provided the MASA server MUST
authenticate the Registrar as described in EST [RFC7030] section
3.3.2 to reduce the risk of DDoS attacks. The MASA performs
authorization as detailed in Section 3.3.2.
As described in [I-D.ietf-anima-voucher] vouchers are normally short
lived to avoid revocation issues. If the request is for a previous
(expired) voucher using the same Registrar (as determined by
domainID) and the MASA has not been informed that the claim is no
longer valid - the request for a renewed voucher SHOULD be
automatically authorized. If authorization is successful the MASA
responds with a [I-D.ietf-anima-voucher] voucher. The MASA SHOULD
check for revocation of the Registrar certificate. The maximum
lifetime of the voucher issued SHOULD NOT exceed the lifetime of the
Registrar's revocation validation (for example if the Registrar
revocation status is indicated in a CRL that is valid for two weeks
then that is an appropriate lifetime for the voucher).
The voucher request is encapsulated in a [RFC5652] Signed-data that
is signed by the Registrar. The entire certificate chain, up to and
including the Domain CA, MUST be included in the CertificateSet
structure. The MASA service checks the internal consistency of the
CMS but does not authenticate the domain identity information. The
domain is not know to the MASA server in advance and a shared trust
anchor is not implied. The MASA server MUST verify that the CMS is
signed by a Registrar certificate (by checking for the cmc-idRA
field) that was issued by a the root certificate included in the CMS.
This ensures that the Registrar making the claim is an authorized
Registrar of the unauthenticated domain.
The root certificate is extracted and used to populate the Voucher.
The domain ID (e.g. hash of the public key of the domain) is
extracted from the root certificate and is used to update the audit
log.
7.3. Voucher Response
The voucher response to requests from the device and requests from a
Registrar are in the same format. A Registrar either caches prior
MASA responses or dynamically requests a new Voucher based on local
policy.
If the the join operation is successful, the server response MUST
contain an HTTP 200 response code. The server MUST answer with a
suitable 4xx or 5xx HTTP [RFC2616] error code when a problem occurs.
The response data from the MASA server MUST be a plaintext human-
Pritikin, et al. Expires September 14, 2017 [Page 36]
Internet-Draft BRSKI March 2017
readable error message containing explanatory information describing
why the request was rejected.
Response media type: application/voucher+cms
The syntactic details of vouchers are described in detail in
[I-D.ietf-anima-voucher]. For example, the voucher consists of:
{
"version":"1",
"nonce":"<64bit nonce value>",
"IDevIDAuthorityKeyIdentifier":"<base64 encoded keyIdentifier>",
"DevIDSerialNumber":"<id-at-serialNumber>",
"domainCAcert":"<the base64 encoded domain CA's certificate>"
}
The Voucher response is encapsulated in a [RFC5652] Signed-data that
is signed by the MASA server. The Pledge verifies this signed
message using the manufacturer installed trust anchor associated with
the X.509 IDevID. [[EDNOTE: As detailed in netconf-zerotouch this
might be a distinct trust anchor rather than re-using the trust
anchor for the IDevID. This concept will need to be detailed in this
document as well.]]
The 'domainCAcert' element of this message contains the domain CA's
public key. This is specific to bootstrapping a public key
infrastructure. To support bootstrapping other key infrastructures
additional domain identity types might be defined in the future.
Clients MUST be prepared to ignore additional fields they do not
recognize. Clients MUST be prepared to parse and fail gracefully
from an Voucher response that does not contain a 'domainCAcert' field
at all.
To minimize the size of the Voucher response message the domainCAcert
is not a complete distribution of the EST section 4.1.3 CA
Certificate Response. The Pledge installs the domainCAcert trust
anchor. As indicated in Section 3.1.2 the newly installed trust
anchor is used as an EST RFC7030 Explicit Trust Anchor. The Pledge
MUST use the domainCAcert trust anchor to immediately validate the
currently provisional TLS connection to a Registrar.
7.3.1. Completing authentication of Provisional TLS connection
If a Registrar's credential can not be verified using the
domainCAcert trust anchor the TLS connection is immediately discarded
and the Pledge abandons attempts to bootstrap with this discovered
registrar.
Pritikin, et al. Expires September 14, 2017 [Page 37]
Internet-Draft BRSKI March 2017
The following behaviors on a Registrar and Pledge are in addition to
normal PKIX operations:
o The EST server MUST use a certificate that chains to the
domainCAcert. This means that when the EST server obtains renewed
credentials the credentials included in the Section 7.2 request
match the chain used in the current provisional TLS connection.
o The Pledge PKIX path validation of a Registrar validity period
information is as described in Section 3.1.5.
Because the domainCAcert trust anchor is installed as an Explicit
Trust Anchor it can be used to authenticate any dynamically
discovered EST server that contain the id-kp-cmcRA extended key usage
extension as detailed in EST RFC7030 section 3.6.1; but to reduce
system complexity the Pledge SHOULD avoid additional discovery
operations. Instead the Pledge SHOULD communicate directly with the
Registrar as the EST server to complete PKI local certificate
enrollment. Additionally the Pledge SHOULD use the existing TLS
connection to proceed with EST enrollment, thus reducing the total
amount of cryptographic and round trip operations required during
bootstrapping. [[EDNOTE: It is reasonable to mandate that the
existing TLS connection be re-used? e.g. MUST >> SHOULD?]]
7.4. Voucher Status Telemetry
For automated bootstrapping of devices the adminstrative elements
providing bootstrapping also provide indications to the system
administrators concerning device lifecycle status. To facilitate
this those elements need telemetry information concerning the
device's status.
To indicate Pledge status regarding the Voucher the client SHOULD
post a status message.
The posted data media type: application/json
The client HTTP POSTs the following to the server at the EST well
known URI /voucher_status. The Status field indicates if the Voucher
was acceptable. If it was not acceptable the Reason string indicates
why. In the failure case this message is being sent to an
unauthenticated, potentially malicious Registrar and therefore the
Reason string SHOULD NOT provide information beneficial to an
attacker. The operational benefit of this telemetry information is
balanced against the operational costs of not recording that an
Voucher was ignored by a client the registar expected to continue
joining the domain.
Pritikin, et al. Expires September 14, 2017 [Page 38]
Internet-Draft BRSKI March 2017
{
"version":"1",
"Status":FALSE /* TRUE=Success, FALSE=Fail"
"Reason":"Informative human readable message"
}
The server SHOULD respond with an HTTP 200 but MAY simply fail with
an HTTP 404 error. The client ignores any response. Within the
server logs the server SHOULD capture this telemetry information.
7.5. MASA authorization log Request
A registrar requests the MASA authorization log from the MASA service
using this EST extension.
This is done with an HTTP GET using the operation path value of
"/requestauditlog".
The client MUST HTTP POSTs the same Voucher Request as for requesting
a Voucher. It is posted to the /requestauditlog URI instead. The
IDevIDAuthorityKeyIdentifier and DevIDSerialNumber informs the MASA
server which log is requested so the appropriate log can be prepared
for the response. Using the same media type and message minimizes
cryptographic and message operations although it results in
additional network traffic. The relying MASA server implementation
MAY leverage internal state to associate this request with the
original, and by now already validated, voucher request so as to
avoid an extra crypto validation.
Request media type: application/voucherrequest+cms
7.6. MASA authorization log Response
A log data file is returned consisting of all log entries. For
example:
Pritikin, et al. Expires September 14, 2017 [Page 39]
Internet-Draft BRSKI March 2017
{
"version":"1",
"events":[
{
"date":"<date/time of the entry>",
"domainID":"<domainID as extracted from the domain CA certificate
within the CMS of the audit voucher request>",
"nonce":"<any nonce if supplied (or the exact string 'NULL')>"
},
{
"date":"<date/time of the entry>",
"domainID":"<domainID as extracted from the domain CA certificate
within the CMS of the audit voucher request>",
"nonce":"<any nonce if supplied (or the exact string 'NULL')>"
}
]
}
Distribution of a large log is less than ideal. This structure can
be optimized as follows: All nonce-less entries for the same domainID
MAY be condensed into the single most recent nonceless entry.
A Registrar uses this log information to make an informed decision
regarding the continued bootstrapping of the Pledge. For example if
the log includes unexpected domainIDs this is indicative of
problematic imprints by the Pledge. If the log includes nonce-less
entries this is indicative of the permanent ability for the indicated
domain to trigger a reset of the device and take over management of
it. Equipment that is purchased pre-owned can be expected to have an
extensive history.
Log entries containing the Domain's ID can be compared against local
history logs in search of discrepancies.
7.7. EST Integration for PKI bootstrapping
The prior sections describe EST extensions necessary to enable fully
automated bootstrapping. Although the Voucher request/response
structure members IDevIDAuthorityKeyIdentifier and DevIDSerialNumber
are specific to PKI bootstrapping these are the only PKI specific
aspects of the extensions and future work might replace them with
non-PKI structures.
The prior sections provide functionality for the Pledge to obtain a
trust anchor representative of the Domain. The following section
describe using EST to obtain a locally issued PKI certificate. The
Pledge SHOULD leverage the discovered Registrar to proceed with
certificate enrollment and, if they do, MUST implement the EST
Pritikin, et al. Expires September 14, 2017 [Page 40]
Internet-Draft BRSKI March 2017
options described in this section. The Pledge MAY perform
alternative enrollment methods including discovering an alternate EST
server, or proceed to use its X.509 IDevID credential indefinitely.
7.7.1. EST Distribution of CA Certificates
The Pledge MUST request the full EST Distribution of CA Certificates
message. See RFC7030, section 4.1.
This ensures that the Pledge has the complete set of current CA
certificates beyond the domainCAcert (see Section 7.3 for a
discussion of the limitations). Although these restrictions are
acceptable for a Registrar integrated with initial bootstrapping they
are not appropriate for ongoing PKIX end entity certificate
validation.
7.7.2. EST CSR Attributes
Automated bootstrapping occurs without local administrative
configuration of the Pledge. In some deployments its plausible that
the Pledge generates a certificate request containing only identity
information known to the Pledge (essentially the X.509 IDevID
information) and ultimately receives a certificate containing domain
specific identity information. Conceptually the CA has complete
control over all fields issued in the end entity certificate.
Realistically this is operationally difficult with the current status
of PKI certificate authority deployments where the CSR is submitted
to the CA via a number of non-standard protocols.
To alleviate operational difficulty the Pledge MUST request the EST
"CSR Attributes" from the EST server. This allows the local
infrastructure to inform the Pledge of the proper fields to include
in the generated CSR.
[[EDNOTE: The following is specific to anima purposes and should be
moved to an appropriate anima document so as to keep bootstrapping as
generic as possible: What we want are a 'domain name' stored in [TBD]
and an 'ACP IPv6 address' stored in the iPAddress field as specified
in RFC5208 s4.2.1.6. ref ACP draft where certificate verification
[TBD]. These should go into the subjectaltname in the [TBD]
fields.]]. If the hardwareModuleName in the X.509 IDevID is
populated then it SHOULD by default be propagated to the LDevID along
with the hwSerialNum. The registar SHOULD support local policy
concerning this functionality. [[EDNOTE: extensive use of EST CSR
Attributes might need an new OID definition]].]]
The Registar MUST also confirm the resulting CSR is formatted as
indicated before forwarding the request to a CA. If the Registar is
Pritikin, et al. Expires September 14, 2017 [Page 41]
Internet-Draft BRSKI March 2017
communicating with the CA using a protocol like full CMC which
provides mechanisms to override the CSR attributes, then these
mechanisms MAY be used even if the client ignores CSR Attribute
guidance.
7.7.3. EST Client Certificate Request
The Pledge MUST request a new client certificate. See RFC7030,
section 4.2.
7.7.4. Enrollment Status Telemetry
For automated bootstrapping of devices the adminstrative elements
providing bootstrapping also provide indications to the system
administrators concerning device lifecycle status. This might
include information concerning attempted bootstrapping messages seen
by the client, MASA provides logs and status of credential
enrollment. The EST protocol assumes an end user and therefore does
not include a final success indication back to the server. This is
insufficient for automated use cases.
To indicate successful enrollment the client SHOULD re-negotiate the
EST TLS session using the newly obtained credentials. This occurs by
the client initiating a new TLS ClientHello message on the existing
TLS connection. The client MAY simply close the old TLS session and
start a new one. The server MUST support either model.
In the case of a FAIL the Reason string indicates why the most recent
enrollment failed. The SubjectKeyIdentifier field MUST be included
if the enrollment attempt was for a keypair that is locally known to
the client. If EST /serverkeygen was used and failed then the field
is ommited from the status telemetry.
In the case of a SUCCESS the Reason string is ommitted. The
SubjectKeyIdentifier is included so that the server can record the
successful certificate distribution.
Status media type: application/json
The client HTTP POSTs the following to the server at the new EST well
known URI /enrollstatus.
Pritikin, et al. Expires September 14, 2017 [Page 42]
Internet-Draft BRSKI March 2017
{
"version":"1",
"Status":TRUE /* TRUE=Success, FALSE=Fail"
"Reason":"Informative human readable message"
"SubjectKeyIdentifier":"<base64 encoded subjectkeyidentifier for the
enrollment that failed>"
}
The server SHOULD respond with an HTTP 200 but MAY simply fail with
an HTTP 404 error.
Within the server logs the server MUST capture if this message was
recieved over an TLS session with a matching client certificate.
This allows for clients that wish to minimize their crypto operations
to simply POST this response without renegotiating the TLS session -
at the cost of the server not being able to accurately verify that
enrollment was truly successful.
7.7.5. EST over CoAP
[[EDNOTE: In order to support smaller devices the above section on
Proxy behavior introduces mandatory to implement support for CoAP
support by the Proxy. This implies similar support by the Pledge and
Registrar and means that the EST protocol operation encapsulation
into CoAP needs to be described. EST is HTTP based and "CoaP is
designed to easily interface with HTTP for integration" [RFC7252].
Use of CoAP implies Datagram TLS (DTLS) wherever this document
describes TLS handshake specifics. A complexity is that the large
message sizes necessary for bootstrapping will require support for
[draft-ietf-core-block].]]
8. Reduced security operational modes
A common requirement of bootstrapping is to support less secure
operational modes for support specific use cases. The following
sections detail specific ways that the Pledge, Registrar and MASA can
be configured to run in a less secure mode for the indicated reasons.
8.1. Trust Model
+--------+ +---------+ +------------+ +------------+
| New | | Circuit | | Domain | | Vendor |
| Entity | | Proxy | | Registrar | | Service |
| | | | | | | (Internet |
+--------+ +---------+ +------------+ +------------+
Figure 10
Pritikin, et al. Expires September 14, 2017 [Page 43]
Internet-Draft BRSKI March 2017
Pledge: The Pledge could be compromised and providing an attack
vector for malware. The entity is trusted to only imprint using
secure methods described in this document. Additional endpoint
assessment techniques are RECOMMENDED but are out-of-scope of this
document.
Proxy: Provides proxy functionalities but is not involved in
security considerations.
Registrar: When interacting with a MASA server a Registrar makes all
decisions. When Ownership Vouchers are involved a Registrar is
only a conduit and all security decisions are made on the vendor
service.
Vendor Service, MASA: This form of vendor service is trusted to
accurately log all claim attempts and to provide authoritative log
information to Registrars. The MASA does not know which devices
are associated with which domains. These claims could be
strengthened by using cryptographic log techniques to provide
append only, cryptographic assured, publicly auditable logs.
Current text provides only for a trusted vendor.
Vendor Service, Ownership Validation: This form of vendor service is
trusted to accurately know which device is owned by which domain.
8.2. New Entity security reductions
The Pledge MAY support "trust on first use" on physical interfaces
but MUST NOT support "trust on first use" on network interfaces.
This is because "trust on first use" permanently degrades the
security for all other use cases.
The Pledge MAY have an operational mode where it skips Voucher
validation one time. For example if a physical button is depressed
during the bootstrapping operation. This can be useful if the vendor
service is unavailable. This behavior SHOULD be available via local
configuration or physical presence methods to ensure new entities can
always be deployed even when autonomic methods fail. This allows for
unsecured imprint.
It is RECOMMENDED that this only be available if hardware assisted
NEA [RFC5209] is supported.
8.3. Registrar security reductions
A Registrar can choose to accept devices using less secure methods.
These methods are acceptable when low security models are needed, as
Pritikin, et al. Expires September 14, 2017 [Page 44]
Internet-Draft BRSKI March 2017
the security decisions are being made by the local administrator, but
they MUST NOT be the default behavior:
1. A registrar MAY choose to accept all devices, or all devices of a
particular type, at the administrator's discretion. This could
occur when informing all Registrars of unique identifiers of new
entities might be operationally difficult.
2. A registrar MAY choose to accept devices that claim a unique
identity without the benefit of authenticating that claimed
identity. This could occur when the Pledge does not include an
X.509 IDevID factory installed credential. New Entities without
an X.509 IDevID credential MAY form the Section 7.1 request using
the Section 7.2 format to ensure the Pledge's serial number
information is provided to the Registar (this includes the
IDevIDAuthorityKeyIdentifier value which would be statically
configured on the Pledge). The Pledge MAY refused to provide a
TLS client certificate (as one is not available). The Pledge
SHOULD support HTTP-based or certificate-less TLS authentication
as described in EST RFC7030 section 3.3.2. A Registrar MUST NOT
accept unauthenticated New Entities unless it has been configured
to do so by an administrator that has verified that only expected
new entities can communicate with a Registrar (presumably via a
physically secured perimeter).
3. A Registrar MAY request nonce-less Vouchers from the MASA service
(by not including a nonce in the request). These Vouchers can
then be transmitted to the Registrar and stored until they are
needed during bootstrapping operations. This is for use cases
where target network is protected by an air gap and therefore can
not contact the MASA service during Pledge deployment.
4. A registrar MAY ignore unrecognized nonce-less log entries. This
could occur when used equipment is purchased with a valid history
being deployed in air gap networks that required permanent
Vouchers.
8.4. MASA security reductions
Lower security modes chosen by the MASA service effect all device
deployments unless bound to the specific device identities. In which
case these modes can be provided as additional features for specific
customers. The MASA service can choose to run in less secure modes
by:
1. Not enforcing that a nonce is in the Voucher. This results in
distribution of Voucher that never expires and in effect makes
the Domain an always trusted entity to the Pledge during any
Pritikin, et al. Expires September 14, 2017 [Page 45]
Internet-Draft BRSKI March 2017
subsequent bootstrapping attempts. That this occurred is
captured in the log information so that the Domain registrar can
make appropriate security decisions when a Pledge joins the
Domain. This is useful to support use cases where Registrars
might not be online during actual device deployment. Because
this results in long lived Voucher and does not require the proof
that the device is online this is only accepted when the
Registrar is authenticated by the MASA server and authorized to
provide this functionality. The MASA server is RECOMMENDED to
use this functionality only in concert with an enhanced level of
ownership tracking (out-of-scope). If the Pledge device is known
to have a real-time-clock that is set from the factory use of a
voucher validity period is RECOMMENDED.
2. Not verifying ownership before responding with an Voucher. This
is expected to be a common operational model because doing so
relieves the vendor providing MASA services from having to
tracking ownership during shipping and supply chain and allows
for a very low overhead MASA service. A Registrar uses the audit
log information as a defense in depth strategy to ensure that
this does not occur unexpectedly (for example when purchasing new
equipment the Registrar would throw an error if any audit log
information is reported).
9. Security Considerations
There are uses cases where the MASA could be unavailable or
uncooperative to the Registrar. They include planned and unplanned
network partitions, changes to MASA policy, or other instances where
MASA policy rejects a claim. These introduce an operational risk to
the Registrar owner that MASA/vendor behavior might limit the ability
to re-boostrap a Pledge device. For example this might be an issue
during disaster recovery. This risk can be mitigated by Registrars
that request and maintain long term copies of "nonceless" Vouchers.
In that way they are guaranteed to be able to repeat bootstrapping
for their devices.
The issuance of nonceless vouchers themselves create a security
concern. If the Registrar of a previous domain can intercept
protocol communications then it can use a previously issued nonceless
voucher to establish management control of a pledge device even after
having sold it. This risk is mitigated by recording the issuance of
such vouchers in the MASA audit log that is verified by the
subsequent Registrar. This reduces the resale value of the equipment
because future owners will detect the lowered security inherent in
the existence of a nonceless voucher that would be trusted by their
Pledge. This accurately reflects a balance between partition
resistant recovery and security of future bootstrapping. Registrars
Pritikin, et al. Expires September 14, 2017 [Page 46]
Internet-Draft BRSKI March 2017
take the Pledge's audit history into account when applying policy to
new devices.
The MASA server is exposed to DoS attacks wherein attackers claim an
unbounded number of devices. Ensuring a Registrar is representative
of a valid vendor customer, even without validating ownership of
specific Pledge devices, helps to mitigate this. Inserting a
cryptographic proof-of-possession step to the protocol operations is
a possible area of future work. One method that would not introduce
additional round-trips would be for the Registrar to share the Plege-
Registrar TLS handshake with the MASA service when requesting a
voucher. Doing so would allow the MASA service to verify that the
Registrar's Server Certificate was signed by the Pledge's Certificate
Verify message (which covers the entire handshake).
It is possible for an attacker to request a voucher from the MASA
service directly after the real Registrar obtains an audit log. If
the attacker could also force the bootstrapping protocol to reset
there is a theoretical opportunity for the attacker to use their
voucher to take control of the Pledge but then proceed to enroll with
the target domain. Possible prevention mechanisms include:
o Per device rate limits on the MASA service ensure such timing
attacks are difficult.
o The Registrar can repeat the request for audit log information at
some time after bootstrapping is complete.
To facilitate logging and administrative oversight the Pledge reports
on Voucher parsing status to the Registrar. In the case of a failure
this information is informative to a potentially malicious Registar
but this is RECOMMENDED anyway because of the operational benefits of
an informed administrator in cases where the failure is indicative of
a problem.
To facilitate truely limited clients EST RFC7030 section 3.3.2
requirements that the client MUST support a client authentication
model have been reduced in Section 8 to a statement that clients only
"SHOULD" support such a model. This reflects current (poor)
practices that are NOT RECOMMENDED.
During the provisional period of the connection all HTTP header and
content data MUST treated as untrusted data. HTTP libraries are
regularly exposed to non-secured HTTP traffic.
Pritikin, et al. Expires September 14, 2017 [Page 47]
Internet-Draft BRSKI March 2017
10. Acknowledgements
We would like to thank the various reviewers for their input, in
particular Brian Carpenter, Toerless Eckert, Fuyu Eleven, Eliot Lear,
Sergey Kasatkin, Markus Stenberg, and Peter van der Stok
11. References
11.1. Normative References
[I-D.ietf-anima-autonomic-control-plane]
Behringer, M., Eckert, T., and S. Bjarnason, "An Autonomic
Control Plane", draft-ietf-anima-autonomic-control-
plane-05 (work in progress), January 2017.
[I-D.ietf-anima-voucher]
Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
"Voucher and Voucher Revocation Profiles for Bootstrapping
Protocols", draft-ietf-anima-voucher-00 (work in
progress), January 2017.
[IDevID] IEEE Standard, , "IEEE 802.1AR Secure Device Identifier",
December 2009, <http://standards.ieee.org/findstds/
standard/802.1AR-2009.html>.
[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>.
[RFC3542] Stevens, W., Thomas, M., Nordmark, E., and T. Jinmei,
"Advanced Sockets Application Program Interface (API) for
IPv6", RFC 3542, DOI 10.17487/RFC3542, May 2003,
<http://www.rfc-editor.org/info/rfc3542>.
[RFC3927] Cheshire, S., Aboba, B., and E. Guttman, "Dynamic
Configuration of IPv4 Link-Local Addresses", RFC 3927,
DOI 10.17487/RFC3927, May 2005,
<http://www.rfc-editor.org/info/rfc3927>.
[RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
Address Autoconfiguration", RFC 4862,
DOI 10.17487/RFC4862, September 2007,
<http://www.rfc-editor.org/info/rfc4862>.
Pritikin, et al. Expires September 14, 2017 [Page 48]
Internet-Draft BRSKI March 2017
[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>.
[RFC5386] Williams, N. and M. Richardson, "Better-Than-Nothing
Security: An Unauthenticated Mode of IPsec", RFC 5386,
DOI 10.17487/RFC5386, November 2008,
<http://www.rfc-editor.org/info/rfc5386>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<http://www.rfc-editor.org/info/rfc5652>.
[RFC5660] Williams, N., "IPsec Channels: Connection Latching",
RFC 5660, DOI 10.17487/RFC5660, October 2009,
<http://www.rfc-editor.org/info/rfc5660>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<http://www.rfc-editor.org/info/rfc6762>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<http://www.rfc-editor.org/info/rfc6763>.
[RFC7030] Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
"Enrollment over Secure Transport", RFC 7030,
DOI 10.17487/RFC7030, October 2013,
<http://www.rfc-editor.org/info/rfc7030>.
[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>.
11.2. Informative References
[I-D.behringer-homenet-trust-bootstrap]
Behringer, M., Pritikin, M., and S. Bjarnason,
"Bootstrapping Trust on a Homenet", draft-behringer-
homenet-trust-bootstrap-02 (work in progress), February
2014.
Pritikin, et al. Expires September 14, 2017 [Page 49]
Internet-Draft BRSKI March 2017
[I-D.ietf-anima-grasp]
Bormann, C., Carpenter, B., and B. Liu, "A Generic
Autonomic Signaling Protocol (GRASP)", draft-ietf-anima-
grasp-09 (work in progress), December 2016.
[I-D.ietf-netconf-zerotouch]
Watsen, K. and M. Abrahamsson, "Zero Touch Provisioning
for NETCONF or RESTCONF based Management", draft-ietf-
netconf-zerotouch-12 (work in progress), January 2017.
[I-D.lear-mud-framework]
Lear, E., "Manufacturer Usage Description Framework",
draft-lear-mud-framework-00 (work in progress), January
2016.
[I-D.richardson-anima-state-for-joinrouter]
Richardson, M., "Considerations for stateful vs stateless
join router in ANIMA bootstrap", draft-richardson-anima-
state-for-joinrouter-01 (work in progress), July 2016.
[imprinting]
Wikipedia, , "Wikipedia article: Imprinting", July 2015,
<https://en.wikipedia.org/wiki/Imprinting_(psychology)>.
[RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in
IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473,
December 1998, <http://www.rfc-editor.org/info/rfc2473>.
[RFC7217] Gont, F., "A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address
Autoconfiguration (SLAAC)", RFC 7217,
DOI 10.17487/RFC7217, April 2014,
<http://www.rfc-editor.org/info/rfc7217>.
[RFC7435] Dukhovni, V., "Opportunistic Security: Some Protection
Most of the Time", RFC 7435, DOI 10.17487/RFC7435,
December 2014, <http://www.rfc-editor.org/info/rfc7435>.
[RFC7575] Behringer, M., Pritikin, M., Bjarnason, S., Clemm, A.,
Carpenter, B., Jiang, S., and L. Ciavaglia, "Autonomic
Networking: Definitions and Design Goals", RFC 7575,
DOI 10.17487/RFC7575, June 2015,
<http://www.rfc-editor.org/info/rfc7575>.
Pritikin, et al. Expires September 14, 2017 [Page 50]
Internet-Draft BRSKI March 2017
[Stajano99theresurrecting]
Stajano, F. and R. Anderson, "The resurrecting duckling:
security issues for ad-hoc wireless networks", 1999,
<https://www.cl.cam.ac.uk/~fms27/papers/1999-StajanoAnd-
duckling.pdf>.
Appendix A. IPv4 operations
A.1. IPv4 Link Local addresses
Instead of an IPv6 link-local address, an IPv4 address may be
generated using [RFC3927] Dynamic Configuration of IPv4 Link-Local
Addresses.
In the case that an IPv4 Local-Local address is formed, then the
bootstrap process would continue as in the IPv6 case by looking for a
(circuit) proxy.
A.2. Use of DHCPv4
The Plege MAY obtain an IP address via DHCP [RFC2131]. The DHCP
provided parameters for the Domain Name System can be used to perform
DNS operations if all local discovery attempts fail.
Appendix B. mDNS / DNSSD proxy discovery options
The Pledge MAY perform DNS-based Service Discovery [RFC6763] over
Multicast DNS [RFC6762] searching for the service
"_bootstrapks._tcp.local.".
To prevent unaccceptable levels of network traffic the congestion
avoidance mechanisms specified in [RFC6762] section 7 MUST be
followed. The Pledge SHOULD listen for an unsolicited broadcast
response as described in [RFC6762]. This allows devices to avoid
announcing their presence via mDNS broadcasts and instead silently
join a network by watching for periodic unsolicited broadcast
responses.
Performs DNS-based Service Discovery [RFC6763] over normal DNS
operations. The service searched for is
"_bootstrapks._tcp.example.com". In this case the domain
"example.com" is discovered as described in [RFC6763] section 11.
This method is only available if the host has received a useable IPv4
address via DHCPv4 as suggested in Appendix A.
If no local bootstrapks service is located using the GRASP
mechanisms, or the above mentioned DNS-based Service Discovery
methods the Pledge MAY contact a well known vendor provided
Pritikin, et al. Expires September 14, 2017 [Page 51]
Internet-Draft BRSKI March 2017
bootstrapping server by performing a DNS lookup using a well known
URI such as "bootstrapks.vendor-example.com". The details of the URI
are vendor specific. Vendors that leverage this method on the Pledge
are responsible for providing the bootstrapks service.
The current DNS services returned during each query is maintained
until bootstrapping is completed. If bootstrapping fails and the
Pledge returns to the Discovery state it picks up where it left off
and continues attempting bootstrapping. For example if the first
Multicast DNS _bootstrapks._tcp.local response doesn't work then the
second and third responses are tried. If these fail the Pledge moves
on to normal DNS-based Service Discovery.
Appendix C. IPIP Join Proxy mechanism
The Circuit Proxy mechanism suffers from requiring a state on the
Join Proxy for each connection that is relayed. The Circuit Proxy
can be considered a kind of Algorithm Gateway [FIND-good-REF].
An alternative to proxying at the TCP layer is to selectively forward
at the IP layer. This moves all per-connection to the Join
Registrar. The IPIP tunnel statelessly forwards packets. This
section provides some explanation of some of the details of the
Registrar discovery procotol which are not important to Circuit
Proxy, and some implementation advice.
The IPIP tunnel is described in [RFC2473]. Each such tunnel is
considered a unidirectional construct, but two tunnels may be
associated to form a bidirectional mechanism. An IPIP tunnel is
setup as follows. The outer addresses are an ACP address of the Join
Proxy, and the ACP address of the Join Registrar. The inner
addresses seen in the tunnel are the link-local addresses of the
network on which the join activity is occuring.
One way to look at this construct is to consider that the Registrar
is extending attaching an interface to the network on which the Join
Proxy is physically present. The Registrar then interacts as if it
were present on that network using link-local (fe80::) addresses.
The Join node is unaware that the traffic is being proxied through a
tunnel, and does not need any special routing.
There are a number of considerations with this mechanism which
require cause some minor amounts of complexity. Note that due to the
tunnels, the Registrar sees multiple connections to a fe80::/10
network on not just physical interfaces, but on each of the virtual
interfaces represending the tunnels.
Pritikin, et al. Expires September 14, 2017 [Page 52]
Internet-Draft BRSKI March 2017
C.1. Multiple Join networks on the Join Proxy side
The Join Proxy will in the general case be a routing device with
multiple interfaces. Even a device as simple as a wifi access point
may have wired, and multiple frequencies of wireless interfaces,
potentially with multiple ESSIDs.
Each of these interfaces on the Join Proxy may be seperate L3 routing
domains, and therefore will have a unique set of link-local
addresses. An IPIP packet being returned by the Registrar needs to
be forwarded to the correct interface, so the Join Proxy needs an
additional key to distinguish which network the packet should be
returned to.
The simplest way to get this additional key is to allocate an
additional ACP address; one address for each network on which join
traffic is occuring. The Join Proxy SHOULD do a GRASP M_NEG_SYN for
each interface which they wish to relay traffic, as this allows the
Registrar to do any static tunnel configuration that may be required.
C.2. Automatic configuration of tunnels on Registrar
The Join Proxy is expected to do a GRASP negotiation with the proxy
for each Join Interface that it needs to relay traffic from. This is
to permit Registrars to configure the appropriate virtual interfaces
before join traffic arrives.
A Registrar serving a large number of interfaces may not wish to
allocate resources to every interface at all times, but can instead
dynamically allocate interfaces. It can do this by monitoring IPIP
traffic that arrives on it's ACP interface, and when packets arrive
from new Join Proxys, it can dynamically configure virtual
interfaces.
A more sophisticated Registrar willing to modify the behaviour of
it's TCP and UDP stack could note the IPIP traffic origination in the
socket control block and make information available to the TCP layer
(for HTTPS connections), or to the the application (for CoAP
connections) via a proprietary extension to the socket API.
C.3. Proxy Neighbor Discovery by Join Proxy
The Join Proxy MUST answer neighbor discovery messages for the
address given by the Registrar as being it's link-local address. The
Join Proxy must also advertise this address as the address to which
to connect to when advertising it's existence.
Pritikin, et al. Expires September 14, 2017 [Page 53]
Internet-Draft BRSKI March 2017
This proxy neighbor discovery means that the pledge will create TCP
and UDP connections to the correct Registrar address. This matters
as the TCP and UDP pseudo-header checksum includes the destination
address, and for the proxy to remain completely stateless, it must
not be necessary for the checksum to be updated.
C.4. Use of connected sockets; or IP_PKTINFO for CoAP on Registrar
TCP connections on the registrar SHOULD properly capture the ifindex
of the incoming connection into the socket structure. This is normal
IPv6 socket API processing. The outgoing responses will go out on
the same (virtual) interface by ifindex.
When using UDP sockets with CoAP, the application will have to pay
attention to the incoming ifindex on the socket. Access to this
information is available using the IP_PKTINFO auxiliary extension
which is a standard part of the IPv6 sockets API.
A registrar application could, after receipt of an initial CoAP
message from the Pledge, create a connected UDP socket (including the
ifindex information). The kernel would then take care of accurate
demultiplexing upon receive, and subsequent transmission to the
correct interface.
C.5. Use of socket extension rather than virtual interface
Some operating systems on which a Registrar need be implemented may
find need for a virtual interface per Join Proxy to be problematic.
There are other mechanism which can make be done.
If the IPIP decapsulator can mark the (SYN) packet inside the kernel
with the address of the Join Proxy sending the traffic, then an
interface per Join Proxy may not be needed. The outgoing path need
just pay attention to this extra information and add an appropriate
IPIP header on outgoing. A CoAP over UDP mechanism may need to
expose this extra information to the application as the UDP sockets
are often not connected, and the application will need to specify the
outgoing path on each packet send.
Such an additional socket mechanism has not been standardized.
Terminating L2TP connections over IPsec transport mode suffers from
the same challenges.
Authors' Addresses
Pritikin, et al. Expires September 14, 2017 [Page 54]
Internet-Draft BRSKI March 2017
Max Pritikin
Cisco
Email: pritikin@cisco.com
Michael C. Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
URI: http://www.sandelman.ca/
Michael H. Behringer
Cisco
Email: mbehring@cisco.com
Steinthor Bjarnason
Cisco
Email: sbjarnas@cisco.com
Kent Watsen
Juniper Networks
Email: kwatsen@juniper.net
Pritikin, et al. Expires September 14, 2017 [Page 55]
Html markup produced by rfcmarkup 1.129c, available from
https://tools.ietf.org/tools/rfcmarkup/