Network Working Group Paul E. Jones
Internet Draft Gonzalo Salgueiro
Intended status: Standards Track Cisco Systems
Expires: May 28, 2013 Joseph Smarr
Google
November 28, 2012
WebFinger
draft-ietf-appsawg-webfinger-05.txt
Abstract
This specification defines the WebFinger protocol, which can be used
to discover information about people or other entities on the
Internet using standard HTTP methods.
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
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on May 28, 2013.
Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction...................................................2
2. Terminology....................................................2
3. Overview.......................................................3
4. Example Use of WebFinger.......................................3
4.1. Locating a User's Blog....................................3
4.2. Identity Provider Discovery for OpenID Connect............5
4.3. Auto-Configuration of Email Clients.......................6
4.4. Retrieving Device Information.............................7
5. WebFinger Protocol.............................................8
5.1. Performing a WebFinger Query..............................8
5.2. The JSON Resource Descriptor (JRD) Document...............9
5.3. The "rel" Parameter.......................................9
5.4. WebFinger and URIs.......................................11
6. Cross-Origin Resource Sharing (CORS)..........................12
7. Access Control................................................12
8. Hosted WebFinger Services.....................................12
9. Security Considerations.......................................13
10. IANA Considerations..........................................14
11. Acknowledgments..............................................15
12. References...................................................15
12.1. Normative References....................................15
12.2. Informative References..................................16
Author's Addresses...............................................17
1. Introduction
WebFinger is used to discover information about people or other
entities on the Internet using standard HTTP [2] methods. The
WebFinger server returns a structured document that contains link
relations, properties, and/or other information that is suitable for
automated processing. For a person, the kinds of information that
might be shared via WebFinger include a personal profile address,
identity service, telephone number, or preferred avatar. For other
entities on the Internet, the server might return documents
containing link relations that allow a client to discover the amount
of toner in a printer or the physical location of a server.
Information returned via WebFinger might be for direct human
consumption (e.g., looking up someone's phone number), or it might be
used by systems to help carry out some operation (e.g., facilitate
logging into a web site by determining a user's identity service).
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
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WebFinger makes heavy use of "Link Relations". Briefly, a Link
Relation is an attribute and value pair used on the Internet wherein
the attribute identifies the type of link to which the associated
value refers. In Hypertext Transfer Protocol (HTTP) and Web Linking
[4], the attribute is a "rel" and the value is an "href". WebFinger
also uses the "rel" attribute, where the "rel" value is either a
single IANA-registered link relation type [12] or a URI [6].
3. Overview
WebFinger enables the discovery of information about users, devices,
and other entities that are associated with a host. Discovery
involves a single HTTP GET request to the well-known [3] "webfinger"
resource at the target host and receiving back a JavaScript Object
Notation (JSON) [5] Resource Descriptor (JRD) document [11]
containing link relations. The request MUST include the URI or IRI
[7] for the entity for which information is sought as a parameter
named "resource".
Use of WebFinger is illustrated in the examples in Section 4, then
described more formally in Section 5.
4. Example Use of WebFinger
This non-normative section shows a few sample uses of WebFinger.
4.1. Locating a User's Blog
Assume you receive an email from Bob and he refers to something he
posted on his blog, but you do not know where Bob's blog is located.
It would be simple to discover the address of Bob's blog if he makes
that information available via WebFinger.
Assume your email client can discover the blog for you. After
receiving the message from Bob (bob@example.com), you instruct your
email client to perform a WebFinger query. It does so by issuing the
following HTTPS query to example.com:
GET /.well-known/webfinger?
resource=acct%3Abob%40example.com HTTP/1.1
Host: example.com
The server might then respond with a message like this:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
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"expires" : "2012-11-16T19:41:35Z",
"subject" : "acct:bob@example.com",
"aliases" :
[
"http://www.example.com/~bob/"
],
"properties" :
{
"http://example.com/rel/role/" : "employee"
},
"links" :
[
{
"rel" : "http://webfinger.net/rel/avatar",
"type" : "image/jpeg",
"href" : "http://www.example.com/~bob/bob.jpg"
},
{
"rel" : "http://webfinger.net/rel/profile-page",
"href" : "http://www.example.com/~bob/"
},
{
"rel" : "blog",
"type" : "text/html",
"href" : "http://blogs.example.com/bob/",
"titles" :
{
"en-us" : "The Magical World of Bob",
"fr" : "Le monde magique de Bob"
}
},
{
"rel" : "vcard",
"href" : "http://www.example.com/~bob/bob.vcf"
}
]
}
The email client would take note of the "blog" link relation in the
above JRD document that refers to Bob's blog. This URL would then be
presented to you so that you could then visit his blog. The email
client might also note that Bob has published an avatar link relation
and use that picture to represent Bob inside the email client.
Lastly, the client might consider the vcard [14] link relation in
order to update contact information for Bob.
In the above example, an "acct" URI [8] is used in the query, though
any valid alias for the user might also be used. An alias is a URI
that is different from the "subject" URI that identifies the same
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entity. In the above example, there is one "http" alias returned,
though there might have been more than one. Had the "http:" URI
shown as an alias been used to query for information about Bob, the
query would have appeared as:
GET /.well-known/webfinger?
resource=http%3A%2F%2Fwww.example.com%2F~bob%2F HTTP/1.1
Host: example.com
The response would have been substantially the same, with the subject
and alias information changed as necessary. Other information, such
as the expiration time might also change, but the set of link
relations and properties would be the same with either response.
4.2. Identity Provider Discovery for OpenID Connect
Suppose Carol wishes to authenticate with a web site she visits using
OpenID Connect [16]. She would provide the web site with her OpenID
Connect identifier, say carol@example.com. The visited web site
would perform a WebFinger query looking for the OpenID Connect
Provider. Since the site is interested in only one particular link
relation, the server might utilize the "rel" parameter as described
in section 5.3:
GET /.well-known/webfinger?
resource=acct%3Acarol%40example.com&
rel=http%3A%2F%2Fopenid.net%2Fspecs%2Fconnect%2F1.0%2Fissuer
HTTP/1.1
Host: example.com
The server might respond with a JSON Resource Descriptor document
like this:
{
"expires" : "2012-11-16T19:41:35Z",
"subject" : "acct:carol@example.com",
"aliases" :
[
"http://www.example.com/~carol/"
],
"properties" :
{
"http://example.com/rel/role/" : "employee"
},
"links" :
[
{
"rel" : "http://openid.net/specs/connect/1.0/issuer",
"href" : "https://openid.example.com/"
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}
]
}
Since the "rel" parameter only filters the link relations returned by
the server, other elements of the response, including any aliases or
properties, would be returned. Also, since support for the "rel"
parameter is optional, the client must not assume the "links" array
will contain only the requested link relation.
4.3. Auto-Configuration of Email Clients
WebFinger could be used to auto-provision an email client with basic
configuration data. Suppose that sue@example.com wants to configure
her email client. Her email client might issue the following query:
GET /.well-known/webfinger?
resource=mailto%3Asue%40example.com HTTP/1.1
Host: example.com
The response from the server would contain entries for the various
protocols, transport options, and security options. If there are
multiple options, the server might return a link relation that for
each of the valid options and the client or Sue might select which
option to choose. Since JRD documents list link relations in a
specific order, then the most-preferred choices could be presented
first. Consider this response:
{
"subject" : "mailto:sue@example.com",
"links" :
[
{
"rel" : "http://example.net/rel/smtp-server",
"properties" :
{
"host" : "smtp.example.com",
"port" : "587",
"login-required" : "yes",
"transport" : "starttls"
}
},
{
"rel" : "http://example.net/rel/imap-server",
"properties" :
{
"host" : "imap.example.com",
"port" : "993",
"transport" : "ssl"
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}
}
]
}
In this example, you can see that the WebFinger server advertises an
SMTP service and an IMAP service. In this example, the "href"
entries associated with the link relation are absent. This is valid
when there is no external reference that needs to be made.
4.4. Retrieving Device Information
As another example, suppose there are printers on the network and you
would like to check the current toner level for a particular printer
identified via the URI device:p1.example.com. While the "device" URI
scheme is not presently specified, we use it here for illustrative
purposes.
Following the procedures similar to those above, a query may be
issued to get link relations specific to this URI like this:
GET /.well-known/webfinger?resource=
device%3Ap1.example.com HTTP/1.1
Host: example.com
The link relations that are returned for a device may be quite
different than those for user accounts. Perhaps we may see a
response like this:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"subject" : "device:p1.example.com",
"links" :
[
{
"rel" : "http://example.com/rel/tipsi",
"href" : "http://192.168.1.5/npap/"
}
]
}
While this example is fictitious, you can imagine that perhaps the
Transport Independent, Printer/System Interface [15] may be enhanced
with a web interface that allows a device that understands the TIP/SI
web interface specification to query the printer for toner levels.
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5. WebFinger Protocol
WebFinger is a simple HTTP-based web service that utilizes the JSON
Resource Descriptor (JRD) document format and the Cross-Origin
Resource Sharing (CORS) [10] specification.
This specification defines URI parameters that are passed from the
client to the server when issuing a request. These parameters,
"resource" and "rel", and the parameter values are included in the
"query" component of the URI (see Section 3.4 of RFC 3986). To
construct the "query" component, the client performs the following
steps. First, each parameter value is percent-encoded as per Section
2.1 of RFC 3986. Next, the client constructs a string to be placed
in the query component by concatenating the name of the first
parameter together with an equal sign ("=") and the percent-encoded
parameter value. For any subsequent parameters, the client appends
an ampersand ("&") to the string, the name of the next parameter, an
equal sign, and percent-encoded parameter value. The client MUST NOT
insert any spaces while constructing the string. The order in which
the client places each parameter and its corresponding parameter
value is unspecified.
5.1. Performing a WebFinger Query
WebFinger clients issue queries to the well-known resource /.well-
known/webfinger. All queries MUST include the "resource" parameter
exactly once and set to the value of the URI for which information is
being sought. If the "resource" parameter is absent or malformed,
the WebFinger server MUST return a 400 status code.
Clients MUST first attempt a query the server using HTTPS and utilize
HTTP only if an HTTPS connection cannot be established. If the HTTPS
server has an invalid certificate or returns an HTTP status code
indicating some error, including a 4xx or 5xx, the client MUST NOT
use HTTP in attempt to complete the discovery.
WebFinger servers MUST return JRD documents as the default
representation for the resource. A client MAY include the "Accept"
header to indicate a desired representation, though no other
representation is defined in this specification. For the JRD
document, the media type is "application/json" [5].
If the client queries the WebFinger server and provides a URI for
which the server has no information, the server MUST return a 404
status code.
WebFinger servers can include cache validators in a response to
enable conditional requests by clients and/or expiration times as per
RFC 2616 section 13.
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5.2. The JSON Resource Descriptor (JRD) Document
The JSON Resource Descriptor (JRD) document is formally described in
Appendix A of [11]. There is a RECOMMENDED order of JRD elements.
Further, WebFinger requires some elements and some are optional. The
following list indicates the preferred order and comments on the
presence or absence of elements:
o "expires" (element) is optional
o "subject" (element) is required and MUST be the value of the
"resource" parameter
o "aliases" (array) is optional and absence or an empty array
are semantically the same
o "properties" (array) is optional and absence or an empty
array are semantically the same
o "links" (array) is optional and absence or an empty array are
semantically the same
Array elements within the "links" array are presented by the server
in order of preference.
The "links" array is comprised of several elements. As above, the
following list indicates the preferred order or elements within a
"links" array element and comments on the presence or absence of
elements within the array:
o "rel" (element) is required
o "type" (element) is optional
o "href" (element) is optional
o "template" (element) is forbidden
o "titles" (array) is optional and absence or an empty array
are semantically the same
o "properties" (array) is optional and absence or an empty
array are semantically the same
Clients MUST ignore any unknown or forbidden elements received in the
JRD document.
5.3. The "rel" Parameter
WebFinger defines the "rel" parameter to request only a subset of the
information that would otherwise be returned without the "rel"
parameter. When the "rel" parameter is used, only the link relations
that match the link relations provided via "rel" are included in the
array of links returned in the JSON Resource Descriptor document.
All other information normally present in a resource descriptor is
present in the resource descriptor, even when "rel" is employed.
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The "rel" parameter MAY be transmitted to the server multiple times
in order to request multiple types of link relations.
The purpose of the "rel" parameter is to return a subset of
resource's link relations. Use of the parameter might reduce
processing requirements on either the client or server, and it might
also reduce the bandwidth required to convey the partial resource
descriptor, especially if there are numerous link relation values to
convey for a given resource.
Support for the "rel" parameter is OPTIONAL, but RECOMMENDED on the
server. Should the server not support the "rel" parameter, it MUST
ignore it and process the request as if any "rel" parameter values
were not present.
The following example presents the same example as found in section
4.1, but uses the "rel" parameter in order to select two link
relations:
GET /.well-known/webfinger?
resource=acct%3Abob%40example.com&
rel=http%3A%2F%2Fwebfinger.net%2Frel%2Fprofile-page&
rel=vcard HTTP/1.1
Host: example.com
In this example, the client requests the link relations of type
"http://webfinger.net/rel/profile-page" and "vcard". The server then
responds with a message like this:
HTTP/1.1 200 OK
Access-Control-Allow-Origin: *
Content-Type: application/json; charset=UTF-8
{
"expires" : "2012-11-16T19:41:35Z",
"subject" : "acct:bob@example.com",
"aliases" :
[
"http://www.example.com/~bob/"
],
"properties" :
{
"http://example.com/rel/role/" : "employee"
},
"links" :
[
{
"rel" : "http://webfinger.net/rel/profile-page",
"href" : "http://www.example.com/~bob/"
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},
{
"rel" : "vcard",
"href" : "http://www.example.com/~bob/bob.vcf"
}
]
}
As you can see, the server returned only the link relations requested
by the client, but also included the other parts of the JSON resource
Descriptor document.
In the event that a client requests links for link relations that are
not defined for the specified resource, a resource descriptor MUST be
returned. In the returned JRD, the "links" array MAY be absent,
empty, or contain only links that did match a provided "rel" value.
The server MUST NOT return a 404 status code when a particular link
relation specified via "rel" is not defined for the resource, as a
404 status code is reserved for indicating that the resource itself
(e.g., either /.well-known/webfinger or the resource indicated via
the "resource" parameter) does not exist.
5.4. WebFinger and URIs
WebFinger requests can include a parameter specifying the URI of an
account, device, or other entity. WebFinger is agnostic regarding
the scheme of such a URI: it could be an "acct" URI [7], an "http" or
"https" URI, a "mailto" URI, or some other scheme.
For resources associated with a user account at a host, use of the
"acct" URI scheme is RECOMMENDED, since it explicitly identifies an
account accessible via WebFinger. Further, the "acct" URI scheme is
not associated with other protocols as, by way of example, the
"mailto" URI scheme is associated with email. Since not every host
offers email service, using the "mailto" URI scheme [9] is not ideal
for identifying user accounts on all hosts. That said, use of the
"mailto" URI scheme would be ideal for use with WebFinger to discover
mail server configuration information for a user.
A host MAY utilize one or more URIs that serve as aliases for the
user's account, such as URIs that use the "http" URI scheme [2]. A
WebFinger server MUST return substantially the same response to both
an "acct" URI and any alias URI for the account, including the same
set of link relations and properties. The only elements in the
response that MAY be different include "subject", "expires", and
"aliases". In addition, the server SHOULD include the entire list
aliases for the user's account in the JRD returned when querying the
LRDD resource or when utilizing the "resource" parameter.
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6. Cross-Origin Resource Sharing (CORS)
WebFinger might not be useable by code running in web browsers due to
"Same-Origin" policies. Therefore, WebFinger servers MUST support
Cross-Origin Resource Sharing (CORS) [10] and SHOULD do so by
including the following HTTP header in responses:
Access-Control-Allow-Origin: *
Enterprise WebFinger servers that wish to restrict access to
information from external entities MAY use a more restrictive Access-
Control-Allow-Origin header.
7. Access Control
As with all web resources, access to the /.well-known/webfinger
resource MAY require authentication. Further, failure to provide
required credentials MAY result in the server forbidding access or
providing a different response than had the client authenticated with
the server.
Likewise, a server MAY provide different responses to different
clients based on other factors, such as whether the client is inside
or outside a corporate network. As a concrete example, a query
performed on the internal corporate network might return link
relations to employee pictures, whereas link relations for employee
pictures might not be provided to external entities.
Further, link relations provided in a WebFinger server response MAY
point to web resources that impose access restrictions. For example,
the aforementioned corporate server may provide both internal and
external entities with URIs to employee pictures, but further
authentication might be required in order for the client to access
the picture resources if the request comes from outside the corporate
network.
The decisions made with respect to what set of link relations a
WebFinger server provides to one client versus another and what
resources require further authentication, as well as the specific
authentication mechanisms employed, are outside the scope of this
document.
8. Hosted WebFinger Services
As with most services provided on the Internet, it is possible for a
domain owner to utilize "hosted" WebFinger services. By way of
example, a domain owner might control most aspects of their domain,
but use a third-party hosting service for email. In the case of
email, mail servers for a domain are identified by MX records. An MX
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record points to the mail server to which mail for the domain should
be delivered. It does not matter to the sending mail server whether
those MX records point to a server in the destination domain or a
different domain.
Likewise, a domain owner might utilize the services of a third party
to provide WebFinger services on behalf of its users. Just as a
domain owner was required to insert MX records into DNS to allow for
hosted email serves, the domain owner is required to redirect HTTP(S)
queries to its domain to allow for hosted WebFinger services.
When a query is issued to /.well-known/webfinger, the web server MUST
return a 301, 302, or 307 response status code that includes a
Location header pointing to the location of the hosted WebFinger
service URL. The WebFinger service URL does not need to point to
/.well-known/* on the hosting service provider server. WebFinger
clients MUST follow all 301, 302, or 307 redirection requests.
As an example, assume that example.com's WebFinger services are
hosted by example.net. Suppose a client issues a query for
acct:alice@example.com like this:
GET /.well-known/webfinger?
resource=acct%3Aalice%40example.com HTTP/1.1
Host: example.com
The server might respond with this:
HTTP/1.1 307 Temporary Redirect
Location: http://wf.example.net/example.com/webfinger?
resource=acct%3Aalice%40example.com HTTP/1.1
The client MUST follow the redirection, re-issuing the request to the
URL provided in the Location header.
9. Security Considerations
All of the security considerations applicable to Web Host Metadata
[11] and Cross-Origin Resource Sharing [10] are also applicable to
this specification. Of particular importance is the recommended use
of HTTPS to ensure that information is not modified during transit.
Clients MUST verify that the certificate used on an HTTPS connection
is valid.
Service providers and users should be aware that placing information
on the Internet accessible through WebFinger means that any user can
access that information. While WebFinger can be an extremely useful
tool for allowing quick and easy access to one's avatar, blog, or
other personal information, users should understand the risks, too.
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If one does not wish to share certain information with the world, do
not allow that information to be freely accessible through WebFinger
and do not use any service supporting WebFinger. Further, WebFinger
servers MUST NOT be used to provide any personal information to any
party unless explicitly or implicitly authorized by the person whose
information is being shared. Implicit authorization can be determined
by the user's voluntary utilization of a service as defined by that
service's relevant terms of use or published privacy policy.
The aforementioned word of caution is perhaps worth emphasizing again
with respect to dynamic information one might wish to share, such as
the current location of a user. WebFinger can be a powerful tool
used to assemble information about a person all in one place, but
service providers and users should be mindful of the nature of that
information shared and the fact that it might be available for the
entire world to see. Sharing location information, for example,
would potentially put a person in danger from any individual who
might seek to inflict harm on that person.
The easy access to user information via WebFinger was a design goal
of the protocol, not a limitation. If one wishes to limit access to
information available via WebFinger, such as a WebFinger server for
use inside a corporate network, the network administrator must take
measures necessary to limit access from outside the network. Using
standard methods for securing web resources, network administrators
do have the ability to control access to resources that might return
sensitive information. Further, WebFinger servers can be employed in
such a way as to require authentication and prevent disclosure of
information to unauthorized entities.
Finally, a WebFinger server has no means of ensuring that information
provided by a user is accurate. Likewise, neither the server nor the
client can be absolutely guaranteed that information has not been
manipulated either at the server or along the communication path
between the client and server. Use of HTTPS helps to address some
concerns with manipulation of information along the communication
path, but it clearly cannot address issues where the server provided
incorrect information, either due to being provided false information
or due to malicious behavior on the part of the server administrator.
As with any information service available on the Internet, users
should wary of information received from untrusted sources.
10. IANA Considerations
This specification registers the "webfinger" well-known URI in the
Well-Known URI Registry as defined by [3].
URI suffix: webfinger
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Change controller: IETF
Specification document(s): RFC QQQ
Related information: The JSON Resource Descriptor (JRD) documents
obtained via the WebFinger web service are described in RFC 6415
Appendix A and RFC QQQ.
[RFC EDITOR: Please replace "QQQ" references in this section with the
number for this RFC.]
11. Acknowledgments
The authors would like to acknowledge Eran Hammer-Lahav, Blaine Cook,
Brad Fitzpatrick, Laurent-Walter Goix, Joe Clarke, Michael B. Jones,
and Peter Saint-Andre for their invaluable input.
12. References
12.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[3] Nottingham, M., Hammer-Lahav, E., "Defining Well-Known Uniform
Resource Identifiers (URIs)", RFC 5785, April 2010.
[4] Nottingham, M., "Web Linking", RFC 5988, October 2010.
[5] Crockford, D., "The application/json Media Type for
JavaScript Object Notation (JSON)", RFC 4627, July 2006.
[6] Berners-Lee, T., Fielding, R., and Masinter, L., "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
January 2005.
[7] Duerst, M., "Internationalized Resource Identifiers (IRIs)",
RFC 3987, January 2005.
[8] Saint-Andre, P., "The 'acct' URI Scheme", draft-ietf-appsawg-
acct-uri-01, October 2012.
[9] Duerst, M., Masinter, L., and J. Zawinski, "The 'mailto' URI
Scheme", RFC 6068, October 2010.
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[10] Van Kesteren, A., "Cross-Origin Resource Sharing", W3C CORS
http://www.w3.org/TR/cors/, July 2010.
[11] Hammer-Lahav, E. and Cook, B., "Web Host Metadata", RFC 6415,
October 2011.
12.2. Informative References
[12] IANA, "Link Relations", http://www.iana.org/assignments/link-
relations/.
[13] Zimmerman, D., "The Finger User Information Protocol", RFC
1288, December 1991.
[14] Perreault, S., "vCard Format Specification", RFC 6350, August
2011.
[15] "Transport Independent, Printer/System Interface", IEEE Std
1284.1-1997, 1997.
[16] Sakimura, N., Bradley, J., Jones, M., de Medeiros, B.,
Mortimore, C., and E. Jay, "OpenID Connect Messages 1.0", June
2012, http://openid.net/specs/openid-connect-messages-1_0.html.
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Author's Addresses
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Phone: +1 919 476 2048
Email: paulej@packetizer.com
IM: xmpp:paulej@packetizer.com
Gonzalo Salgueiro
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Phone: +1 919 392 3266
Email: gsalguei@cisco.com
IM: xmpp:gsalguei@cisco.com
Joseph Smarr
Google
Email: jsmarr@google.com
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