Internet DRAFT - draft-sheffer-acme-star
draft-sheffer-acme-star
ACME Working Group Y. Sheffer
Internet-Draft Intuit
Intended status: Standards Track D. Lopez
Expires: November 28, 2017 O. Gonzalez de Dios
Telefonica I+D
T. Fossati
Nokia
May 27, 2017
Use of Short-Term, Automatically-Renewed (STAR) Certificates to Delegate
Authority over Web Sites
draft-sheffer-acme-star-02
Abstract
This memo proposes two mechanisms that work in concert to allow a
third party (e.g., a content delivery network) to terminate TLS
sessions on behalf of a domain name owner (e.g., a content provider).
The proposed mechanisms are:
1. An extension to the ACME protocol to enable the issuance of
short-term and automatically renewed certificates, and
2. A protocol that allows a domain name owner to delegate to a third
party control over a certificate that bears one or more names in
that domain.
It should be noted that these are in fact independent building blocks
that can be used separately to solve completely different problems.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 28, 2017.
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Copyright Notice
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Table of Contents
1. Introduction: A Solution for the HTTPS CDN Use Case . . . . . 3
1.1. Cloud Use Case . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Conventions used in this document . . . . . . . . . . . . 4
2. Protocol Flow . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Preconditions . . . . . . . . . . . . . . . . . . . . . . 5
2.2. Bootstrap . . . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Refresh . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.4. Termination . . . . . . . . . . . . . . . . . . . . . . . 8
3. Protocol Details . . . . . . . . . . . . . . . . . . . . . . 9
3.1. STAR API . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Creating a Registration . . . . . . . . . . . . . . . 9
3.1.2. Polling the Registration . . . . . . . . . . . . . . 10
3.2. ACME Authorization . . . . . . . . . . . . . . . . . . . 11
3.3. Transport Security for the STAR Protocol Leg . . . . . . 11
3.4. ACME Extensions between Proxy and Server . . . . . . . . 11
3.4.1. Extending the Order Resource . . . . . . . . . . . . 11
3.4.2. Canceling a Recurrent Order . . . . . . . . . . . . . 12
3.4.3. Indicating Support of Recurrent Orders . . . . . . . 12
3.5. Fetching the Certificates . . . . . . . . . . . . . . . . 12
4. CDNI Use Cases . . . . . . . . . . . . . . . . . . . . . . . 13
4.1. Multiple Parallel Delegates . . . . . . . . . . . . . . . 13
4.2. Chained Delegation . . . . . . . . . . . . . . . . . . . 13
5. Operational Considerations . . . . . . . . . . . . . . . . . 13
5.1. Certificate Transparency (CT) Logs . . . . . . . . . . . 13
6. Security Considerations . . . . . . . . . . . . . . . . . . . 14
6.1. STAR Protocol Authentication . . . . . . . . . . . . . . 14
6.2. Restricting CDNs to the Delegation Mechanism . . . . . . 14
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. Normative References . . . . . . . . . . . . . . . . . . 15
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8.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Document History . . . . . . . . . . . . . . . . . . 17
A.1. draft-sheffer-acme-star-02 . . . . . . . . . . . . . . . 17
A.2. draft-sheffer-acme-star-01 . . . . . . . . . . . . . . . 17
A.3. draft-sheffer-acme-star-00 . . . . . . . . . . . . . . . 17
A.4. draft-sheffer-acme-star-lurk-00 . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction: A Solution for the HTTPS CDN Use Case
A content provider (referred to in this document as Domain Name
Owner, DNO) has agreements in place with one or more Content Delivery
Networks (CDNs) that are contracted to serve its content over HTTPS.
The CDN terminates the HTTPS connection at one of its edge cache
servers and needs to present its clients (browsers, set-top-boxes) a
certificate whose name matches the authority of the URL that is
requested, i.e. that of the DNO. However, many DNOs balk at sharing
their long-term private keys with another organization and, equally,
CDN providers would rather not have to handle other parties' long-
term secrets. This problem has been discussed at the IETF under the
LURK (limited use of remote keys) title.
This document proposes a solution to the above problem that involves
the use of short-term certificates with a DNO's name on them, and a
scheme for handling the naming delegation from the DNO to the CDN.
The generated short-term credentials are automatically renewed by an
ACME Certification Authority (CA) [I-D.ietf-acme-acme] and routinely
rotated by the CDN on its edge cache servers. The DNO can end the
delegation at any time by simply instructing the CA to stop the
automatic renewal and let the certificate expire shortly thereafter.
Using short-term certificates makes revocation cheap and effective
[Topalovic] [I-D.iab-web-pki-problems] in case of key compromise or
of termination of the delegation; seamless certificate issuance and
renewal enable the level of workflow automation that is expected in
today's cloud environments. Also, compared to other keyless-TLS
solutions [I-D.cairns-tls-session-key-interface]
[I-D.erb-lurk-rsalg], the proposed approach doesn't suffer from
scalability issues or increase in connection setup latency, while
requiring virtually no changes to existing COTS caching software used
by the CDN.
1.1. Cloud Use Case
A similar use case is that of cloud infrastructure components, such
as load balancers and Web Application Firewalls (WAF). These
components are typically provisioned with the DNO's certificate, and
similarly to the CDN use case, many organizations would prefer to
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manage the private key only on their own cloud-based or on-premise
hosts, often on Hardware Security Modules (HSMs).
Here again, the STAR solution allows the DNO to delegate authority
over the domain to the cloud provider, with the ability to revoke
this authority at any time.
1.2. Terminology
DNO Domain Name Owner, the owner of a domain that needs to be
delegated.
NDC Name Delegation Consumer, the entity to which the domain name is
delegated for a limited time. This is often a CDN (in fact,
readers may note the similarity of the two acronyms).
CDN Content Delivery Network, a widely distributed network that
serves the domain's web content to a wide audience at high
performance.
STAR Short-Term, Automatically Renewed X.509 certificates.
ACME The IETF Automated Certificate Management Environment, a
certificate management protocol.
CA A Certificate Authority that implements the ACME protocol.
1.3. Conventions used in this document
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].
2. Protocol Flow
The protocol flow can be split into two: a STAR interface, used by
NDC and DNO to agree on the name delegation, and the extended ACME
interface, used by DNO to obtain the short-term and automatically
renewed certificate from the CA, which is eventually consumed by the
NDC. The latter is also used to terminate the delegation, if so
needed.
The following subsections describe the preconditions (Section 2.1),
and the three main phases of the protocol:
o Bootstrap: the NDC requests from the DNO the delegation of a
specific name and in turn DNO asks an ACME CA to create the
corresponding short-term and auto-renewed (STAR) certificate
(Section 2.2);
o Auto-renewal: the ACME CA periodically re-issues the short-term
certificate and posts it to a public URL (Section 2.3);
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o Termination: the DNO (indirectly) stops name delegation by
explicitly requesting the ACME CA to discontinue the automatic
renewal of the certificate (Section 2.4).
This diagram presents the entities involved in the protocol and their
interactions during the different phases.
+-----------------+
| STAR Proxy |
| (DNO) |
Bootstrap +-----------------+ Bootstrap
+---------->+ STAR | ACME +-----------+
| | Server | Client | Terminate |
| +--------+--------+ |
| v
+--------+ +--------+
| STAR | Refresh | ACME |
| Client +------------------------------->| Server |
| (NDC) | | (CA) |
+--------+ +--------+
2.1. Preconditions
The protocol assumes the following preconditions are met:
o A mutually authenticated channel between NDC and DNO pre-exists.
This is called "STAR channel" and all STAR protocol exchanges
between NDC and DNO are run over it. It provides the guarantee
that requests and responses are authentic.
o NDC and DNO have agreed on a "CSR template" to use, including at a
minimum:
- Subject name (e.g., "somesite.example.com"),
- Validity (e.g., 24 to 72 hours),
- Requested algorithms,
- Key length,
- Key usage.
The NDC is required to use this template for every CSR created
under the same delegation.
o DNO has registered through the ACME interface exposed by the
Certificate Authority (CA) using the usual ACME registration
procedure. In ACME terms, the DNO has an Account on the server
and is ready to issue Orders.
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2.2. Bootstrap
The NDC (STAR Client) generates a key-pair, wraps it into a
Certificate Signing Request (CSR) according to the agreed upon CSR
template, and sends it to the DNO (STAR Proxy) over the pre-
established STAR channel. The DNO uses the NDC identity provided on
the STAR channel to look up the CSR template that applies to the
requesting NDC and decides whether or not to accept the request.
Assuming everything is in order, it then "forwards" the NDC request
to the ACME CA by means of the usual ACME application procedure.
Specifically, the DNO, in its role as an ACME client, requests the CA
to issue a STAR certificate, i.e., one that:
o Has a short validity (e.g., 24 to 72 hours);
o Is automatically renewed by the CA for a certain period of time;
o Is downloadable from a (highly available) public link without
requiring any special authorization.
Other than that, the ACME protocol flows as normal between DNO and
CA, in particular DNO is responsible for satisfying the requested
ACME challenges until the CA is willing to issue the requested
certificate. Per normal ACME processing, the DNO is given back an
Order ID for the issued STAR certificate to be used in subsequent
interaction with the CA (e.g., if the certificate needs to be
terminated.)
Concurrently, a response is sent back to the NDC with an endpoint to
poll for completion of the certificate generation process.
The bootstrap phase ends when the DNO obtains the OK from the ACME CA
and posts the certificate's URL to the "completion endpoint" where
the NDC can retrieve it.
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...........................
STAR : STAR Proxy / : ACME/STAR
Client : ACME Client : Server
| : | | : |
| : | | ACME registration |
+-------. : | |<--------------------->|
| | : | | STAR capabilities |
| generate CSR : | | : |
| | : | | : |
|<------' : | | : |
| : | | : |
| Request new : | | : |
+---------------------->| | : |
| cert for CSR : | | : |
| : +-------. | : |
| : | | | : |
| : | Verify CSR | : |
| : | | | : |
| : +<------' | : |
| Accepted, poll at | | : |
|<----------------------+ | : |
| "completion URL" |- - - - - - - >| Application for |
| : | +---------------------->|
| : | | STAR certificate |
| : | | : |
| GET "completion URL" | | : Challenge |
|<--------------------->| |<--------------------->|
| in progress : | | : Response |
| : | | : |
| : | | Finalize/Certificate |
| : | |<----------------------+
| GET "completion URL" |< - - - - - - -| : + Order Id |
+---------------------->| | : |
| : | | : |
| 200, certificate URL | | : |
|<----------------------+ | : |
| and other metadata | | : |
| : | | : |
`.........................'
Figure 1: Bootstrap
2.3. Refresh
The CA automatically re-issues the certificate (using the same CSR)
before it expires and publishes it to the URL that the NDC has come
to know at the end of the bootstrap phase. The NDC downloads and
installs it. This process goes on until either:
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o DNO terminates the delegation, or
o Automatic renewal expires.
STAR ACME/STAR
Client Server
| Retrieve cert | [...]
|<--------------------->| |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|<--------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| Retrieve cert | |
|<--------------------->| 72 hours
| | |
| +------. /
| | | /
| | Automatic renewal :
| | | \
| |<-----' \
| | |
| [...] | [...]
Figure 2: Auto renewal
2.4. Termination
The DNO may request early termination of the STAR certificate by
including the Order ID in a certificate termination request to the
ACME interface, defined below. After the CA receives and verifies
the request, it shall:
o Cancel the automatic renewal process for the STAR certificate;
o Change the certificate publication resource to return an error
indicating the termination of the delegation to external clients,
including the NDC.
Note that it is not necessary to explicitly revoke the short-term
certificate.
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STAR STAR ACME/STAR
Client Proxy Server
| | |
| | Terminate Order ID |
| +---------------------->|
| | +-------.
| | | |
| | | End auto renewal
| | | Remove cert link
| | | etc.
| | | |
| | Done |<------'
| |<----------------------+
| | |
| |
| Retrieve cert |
+---------------------------------------------->|
| Error: terminated |
|<----------------------------------------------+
| |
Figure 3: Termination
3. Protocol Details
This section describes the protocol's details. We start with the
STAR API between the STAR Client and the STAR Proxy. Then we
describe a few extensions to the ACME protocol running between the
STAR Proxy and the ACME Server.
3.1. STAR API
This API allows the STAR Client to request a STAR certificate via the
STAR Proxy, using a previously agreed-upon CSR template.
The API consists of a single resource, "registration". A new
Registration is created with a POST request, and the Registration
instance is polled to obtain its details.
3.1.1. Creating a Registration
To create a registration, use:
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POST /star/registration
Host: star-proxy.example.net
Content-Type: application/json
{
"csr": "...", // CSR in PEM format
"lifetime": 365 // requested registration lifetime in days,
// between 1 and 1095
}
Upon success, the call returns the new Registration resource.
HTTP/1.1 201 Created
Location: https://star-proxy.example.net/star/registration/567
3.1.2. Polling the Registration
The returned Registration can be polled until the information is
available from the ACME server.
GET /star/registration/567
Host: star-proxy.example.net
In responding to poll requests while the validation is still in
progress, the server MUST return a 200 (OK) response and MAY include
a Retry-After header field to suggest a polling interval to the
client. The Retry-After value MUST be expressed in seconds. If the
Retry-After header is present, in order to avoid surprising
interactions with heuristic expiration times, a max-age Cache-Control
SHOULD also be present and set to a value slightly smaller than the
Retry-After value.
HTTP/1.1 200 OK
Retry-After: 10
Cache-Control: max-age=9
{
"status": "pending"
}
When the operation is successfully completed, the ACME Proxy returns:
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HTTP/1.1 200 OK
Expires: Sun, 09 Sep 2018 14:09:00 GMT
{
"status": "valid", // or "failed"
"lifetime": 365, // lifetime of the registration in days,
// possibly less than requested
"certificates": "https://acme-server.example.org/certificates/A51A3"
}
The Expires header applies to the Registration resource itself, and
may be as small as a few minutes. It is unrelated to the Order's
lifetime which is measured in days or longer. The "certificates"
attribute contains a URL of the certificate pull endpoint, see
Section 3.5.
If the registration fails for any reason, the server returns a "200
OK" response, with the status as "failed" and a "reason" attribute
containing a human readable error message.
3.2. ACME Authorization
The DNO MUST restrict the authorizations it requests from the ACME
server to only those that cannot be spoofed by a malicious DNC. In
most cases the DNC will have strong control of HTTP content under the
delegated domain, and therefore HTTPS-based authorization MUST NOT be
used. See also Section 6.2.
3.3. Transport Security for the STAR Protocol Leg
Traffic between the STAR Client and the STAR Proxy MUST be protected
with HTTPS. For interoperability, all implementations MUST support
HTTP Basic Authentication [RFC7617]. However some deployments MAY
prefer mutually- authenticated HTTPS or two-legged OAUTH.
3.4. ACME Extensions between Proxy and Server
This protocol extends the ACME protocol, to allow for recurrent
orders.
3.4.1. Extending the Order Resource
The Order resource is extended with the following attributes:
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{
"recurrent": true,
"recurrent-total-lifetime": 365, // requested lifetime of the
// recurrent registration, in days
"recurrent-certificate-validity": 7
// requested validity of each certificate, in days
}
These attributes are included in a POST message when creating the
order, as part of the "payload" encoded object. They are returned
when the order has been created, and the ACME server MAY adjust them
at will, according to its local policy.
3.4.2. Canceling a Recurrent Order
An important property of the recurrent Order is that it can be
cancelled by the domain name owner, with no need for certificate
revocation. We use the DELETE message to cancel the Order:
DELETE /acme/order/1 HTTP/1.1
Host: acme-server.example.org
Which returns:
HTTP/1.1 202 Deleted
The server MUST NOT issue any additional certificates for this Order,
beyond the certificate that is available for collection at the time
of deletion.
3.4.3. Indicating Support of Recurrent Orders
ACME supports sending arbitrary extensions when creating an Order,
and as a result, there is no need to explicitly indicate support of
this extension. The Proxy MUST verify that the "recurrent" attribute
was understood, as indicated by the "recurrent" attribute included in
the created Order. Since the standard ACME protocol does not allow
to explicitly cancel a pending Order (the DELETE operation above is
an extension), a Proxy that encounters an non-supporting server will
probably let the Order expire instead of following through with the
authorization process.
3.5. Fetching the Certificates
The certificate is fetched from the certificate endpoint, as per
[I-D.ietf-acme-acme], Sec. 7.4.2 "Downloading the Certificate". The
server MUST include an Expires header that indicates expiry of the
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specific certificate. When the certificate expires, the client MAY
assume that a newer certificate is already in place.
A certificate MUST be replaced by its successor at the latest 24
hours before its "Not After" time.
4. CDNI Use Cases
Members of the IETF CDNI (Content Delivery Network Interconnection)
working group are interested in delegating authority over web content
to CDNs. Their requirements are described in a draft
[I-D.fieau-cdni-https-delegation] that compares several solutions.
This section discusses two particular requirements in the context of
the STAR protocol.
4.1. Multiple Parallel Delegates
In some cases the DNO would like to delegate authority over a web
site to multiple CDNs. This could happen if the DNO has agreements
in place with different regional CDNs for different geographical
regions. STAR enables this use case naturally, since each CDN can
authenticate separately to the DNO specifying its CSR, and the DNO is
free to allow or deny each certificate request according to its own
policy.
4.2. Chained Delegation
In other cases, a content owner (DNO) delegates some domains to a
large CDN (CDN1), which in turn delegates to a smaller regional CDN,
CDN2. The DNO has a contractual relationship with CDN1, and CDN1 has
a similar relationship with CDN2. However DNO may not even know
about CDN2.
The STAR protocol does not prevent this use case, although there is
no special support for it. CDN1 can forward requests from CDN2 to
DNO, and forward responses back to CDN2. Whether such proxying is
allowed is governed by policy and contracts between the parties.
5. Operational Considerations
5.1. Certificate Transparency (CT) Logs
TBD: larger logs and how to deal with them.
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6. Security Considerations
6.1. STAR Protocol Authentication
The STAR protocol allows its client to obtain certificates bearing
the DNO's identity. Therefore strong client authentication is
mandatory.
When multiple NDCs may connect to the same DNO, the STAR protocol's
authentication must allow the DNO to distinguish between different
NDCs. Among other benefits, this allows to DNO to cancel a STAR
registration for one of its clients instead of all of them.
6.2. Restricting CDNs to the Delegation Mechanism
Currently there are no standard methods for the DNO to ensure that
the CDN cannot issue a certificate through mechanisms other than the
one described here, for the URLs under the CDN's control. For
example, regardless of the STAR solution, a rogue CDN employee can
use the ACME protocol (or proprietary mechanisms used by various CAs)
to create a fake certificate for the DNO's content because ACME
authorizes its requests using information that may be under the
adversary's control.
The best solution currently being worked on would consist of several
related configuration steps:
o Make sure that the CDN cannot modify the DNS records for the
domain. Typically this would mean that the content owner
establishes a CNAME resource record from a subdomain into a CDN-
managed domain.
o Restrict certificate issuance for the domain to specific CAs that
comply with ACME. This assumes universal deployment of CAA
[RFC6844] by CAs, which is not the case yet. We note that the CA/
Browser Forum has recently decided to require CAA checking
[CAB-CAA].
o Deploy ACME-specific methods to restrict issuance to a specific
authorization key which is controlled by the content owner
[I-D.landau-acme-caa], and/or to specific ACME authorization
methods.
This solution is recommended in general, even if an alternative to
the mechanism described here is used.
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7. Acknowledgments
This work is partially supported by the European Commission under
Horizon 2020 grant agreement no. 688421 Measurement and Architecture
for a Middleboxed Internet (MAMI). This support does not imply
endorsement.
8. References
8.1. Normative References
[I-D.ietf-acme-acme]
Barnes, R., Hoffman-Andrews, J., and J. Kasten, "Automatic
Certificate Management Environment (ACME)", draft-ietf-
acme-acme-06 (work in progress), March 2017.
[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>.
[RFC7617] Reschke, J., "The 'Basic' HTTP Authentication Scheme",
RFC 7617, DOI 10.17487/RFC7617, September 2015,
<http://www.rfc-editor.org/info/rfc7617>.
8.2. Informative References
[CAB-CAA] CA/Browser Forum, "Ballot 187 - Make CAA Checking
Mandatory", March 2017, <https://cabforum.org/2017/03/08/
ballot-187-make-caa-checking-mandatory/>.
[I-D.cairns-tls-session-key-interface]
Cairns, K., Mattsson, J., Skog, R., and D. Migault,
"Session Key Interface (SKI) for TLS and DTLS", draft-
cairns-tls-session-key-interface-01 (work in progress),
October 2015.
[I-D.erb-lurk-rsalg]
Erb, S. and R. Salz, "A PFS-preserving protocol for LURK",
draft-erb-lurk-rsalg-01 (work in progress), May 2016.
[I-D.fieau-cdni-https-delegation]
Fieau, F., Emile, S., and S. Mishra, "HTTPS delegation in
CDNI", draft-fieau-cdni-https-delegation-01 (work in
progress), March 2017.
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[I-D.iab-web-pki-problems]
Housley, R. and K. O'Donoghue, "Improving the Public Key
Infrastructure (PKI) for the World Wide Web", draft-iab-
web-pki-problems-05 (work in progress), October 2016.
[I-D.landau-acme-caa]
Landau, H., "CA Account URI Binding for CAA Records",
draft-landau-acme-caa-01 (work in progress), October 2016.
[RFC6844] Hallam-Baker, P. and R. Stradling, "DNS Certification
Authority Authorization (CAA) Resource Record", RFC 6844,
DOI 10.17487/RFC6844, January 2013,
<http://www.rfc-editor.org/info/rfc6844>.
[Topalovic]
Topalovic, E., Saeta, B., Huang, L., Jackson, C., and D.
Boneh, "Towards Short-Lived Certificates", 2012,
<http://www.w2spconf.com/2012/papers/w2sp12-final9.pdf>.
Sheffer, et al. Expires November 28, 2017 [Page 16]
Internet-Draft ACME STAR May 2017
Appendix A. Document History
[[Note to RFC Editor: please remove before publication.]]
A.1. draft-sheffer-acme-star-02
o Using a more generic term for the delegation client, NDC.
o Added an additional use case: public cloud services.
o More detail on ACME authorization.
A.2. draft-sheffer-acme-star-01
o A terminology section.
o Some cleanup.
A.3. draft-sheffer-acme-star-00
o Renamed draft to prevent confusion with other work in this space.
o Added an initial STAR protocol: a REST API.
o Discussion of CDNI use cases.
A.4. draft-sheffer-acme-star-lurk-00
o Initial version.
Authors' Addresses
Yaron Sheffer
Intuit
EMail: yaronf.ietf@gmail.com
Diego Lopez
Telefonica I+D
EMail: diego.r.lopez@telefonica.com
Oscar Gonzalez de Dios
Telefonica I+D
EMail: oscar.gonzalezdedios@telefonica.com
Sheffer, et al. Expires November 28, 2017 [Page 17]
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Thomas Fossati
Nokia
EMail: thomas.fossati@nokia.com
Sheffer, et al. Expires November 28, 2017 [Page 18]
