Webfinger
draft-jones-appsawg-webfinger-02

Network Working Group                                      Paul E. Jones
Internet Draft                                         Gonzalo Salgueiro
Intended status: Standards Track                           Cisco Systems
Expires: September 28, 2012                                 Joseph Smarr
                                                                  Google
                                                          March 28, 2012

                                 Webfinger
                   draft-jones-appsawg-webfinger-02.txt

Abstract

   This specification defines the Webfinger protocol.  Webfinger may be
   used to discover information about people on the Internet, such as a
   person's personal profile address, identity service, telephone
   number, or preferred avatar.  Webfinger may also be used to learn
   information about objects on the network, such as the amount of toner
   in a printer or the physical location of a server.

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 28, 2012.

Copyright Notice

   Copyright (c) 2012 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

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   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...................................................2
   2. Terminology....................................................3
   3. Example Uses of Webfinger......................................3
      3.1. Locating a User's Blog....................................3
      3.2. Retrieving a Person's Contact Information.................5
      3.3. Simplifying the Login Process.............................6
      3.4. Retrieving Device Information.............................7
   4. Webfinger Protocol.............................................8
      4.1. Performing a Webfinger Query..............................8
      4.2. The Web Host Metadata "resource" Parameter................9
   5. The "acct" URI................................................11
      5.1. Using the "acct" URI.....................................11
      5.2. Syntax of "acct" URI.....................................11
   6. The "acct" Link Relation......................................12
      6.1. Purpose for the "acct" Link Relation.....................12
      6.2. Example Message Exchange Using the "acct" Link Relation..13
   7. Cross-Origin Resource Sharing (CORS)..........................13
   8. Security Considerations.......................................14
   9. IANA Considerations...........................................14
      9.1. Registration of the "acct" URI scheme name...............15
      9.2. Registration of the "acct" Link Relation Type............15
   10. Acknowledgments..............................................16
   11. References...................................................16
      11.1. Normative References....................................16
      11.2. Informative References..................................17
   Author's Addresses...............................................18

1. Introduction

   There is a utility found on UNIX systems called "finger" [14] that
   allows a person to access information about another person.  The
   information being queried might be on a computer anywhere in the
   world.  The information returned via "finger" is simply a plain text
   file that contains unstructured information provided by the queried
   user.

   Webfinger borrows the concept of the legacy finger protocol, but
   introduces a very different approach to sharing information.  Rather
   than returning a simple unstructured text file, Webfinger uses
   structured documents that contain link relations.  These link
   relations point to information a user or entity on the Internet
   wishes to expose.  For a person, the kinds of information that might
   be exposed include a personal profile address, identity service,

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   telephone number, or preferred avatar.  Webfinger may also be used to
   learn information about objects on the network, such as 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., another user'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 identification 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].

   "SHOULD", "SHOULD NOT", "RECOMMENDED", and "NOT RECOMMENDED" are
   appropriate when valid exceptions to a general requirement are known
   to exist or appear to exist, and it is infeasible or impractical to
   enumerate all of them.  However, they should not be interpreted as
   permitting implementors to fail to implement the general requirement
   when such failure would result in interoperability failure.

   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) [2] and Web
   Linking [3], the attribute is a "rel" and the value is an "href".

3. Example Uses of Webfinger

   In this section, we describe just a few sample uses for Webfinger and
   show what the protocol looks like.  This is not an exhaustive list of
   possible uses and the entire section should be considered non-
   normative.  The list of potential use cases is virtually unlimited
   since a user can share any kind of machine-consumable information via
   Webfinger.

3.1. 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.

   Let's assume your email client discovers that blog automatically for
   you.  After receiving the message from Bob (bob@example.com), your
   email client performs the following steps behind the scenes.

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   First, it tries to get the host metadata [9] information for the
   domain example.com.  It does this by issuing the following HTTPS
   query to example.com:

     GET /.well-known/host-meta HTTP/1.1
     Host: example.com

   The server replies with an XRD [8] document:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/xrd+xml; charset=UTF-8

     <?xml version="1.0" encoding="UTF-8"?>
     <XRD xmlns="http://docs.oasis-open.org/ns/xri/xrd-1.0">
       <Link rel="lrdd"
             type="application/xrd+xml"
             template="https://example.com/lrdd/?uri={uri}"/>
     </XRD>

   The client then processes the received XRD in accordance with the Web
   Host Metadata [9] procedures.  The client will see the LRDD link
   relation and issue a query with the user's account URI [5].  (The
   Account URI is discussed in Section 4.2.)  The query might look like
   this:

     GET /lrdd/?uri=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/xrd+xml; charset=UTF-8

     <?xml version="1.0" encoding="UTF-8"?>
     <XRD xmlns="http://docs.oasis-open.org/ns/xri/xrd-1.0">
       <Expires>2012-03-13T20:56:11Z</Expires>
       <Subject>acct:bob@example.com</Subject>
       <Link rel="http://webfinger.net/rel/avatar"
             href="http://www.example.com/~bob/bob.jpg"/>
       <Link rel="http://webfinger.net/rel/profile-page"
             href="http://www.example.com/~bob/"/>
       <Link rel="http://packetizer.com/rel/blog"
             href="http://blogs.example.com/bob/"/>
     </XRD>

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   The email client might take note of the "blog" link relation in the
   above XRD 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.

3.2. Retrieving a Person's Contact Information

   Assume you have Alice in your address book, but her phone number
   appears to be invalid.  You could use Webfinger to find her current
   phone number and update your address book.

   Let's assume you have a web-based address book that you wish to
   update.  When you instruct the address book to pull Alice's current
   contact information, the address book might issue a query like this
   to get host metadata information for example.com:

     GET /.well-known/host-meta.json HTTP/1.1
     Host: example.com

   Note the address book is looking for a JSON [4] representation,
   whereas we used XML in the previous example.

   The server might reply with something like this:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "links" :
       [
         {
           "rel" : "lrdd",
           "type" : "application/json",
           "template" :
             "https://example.com/lrdd/?format=json&uri={uri}"
         }
       ]
     }

   The client processes the response as described in RFC 6415 [9].  It
   will process the LRDD link relation using Alice's account URI by
   issuing this query:

     GET /lrdd/?format=json&uri=acct%3Aalice%40example.com HTTP/1.1
     Host: example.com

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   The server might return a response like this:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "expires" : "2012-03-13T20:56:11Z",
       "subject" : "acct:alice@example.com",
       "links" :
       [
         {
           "rel" : "http://webfinger.net/rel/avatar",
           "href" : "http://example.com/~alice/alice.jpg"
         },
         {
           "rel" : "http://packetizer.com/rel/vcard",
           "href" : "http://example.com/~alice/alice.vcf"
         }
       ]
     }

   With this response, the address book might see the vcard [16] link
   relation and use that file to offer you updated contact information.

3.3. Simplifying the Login Process

   OpenID (http://www.openid.net) is great for allowing users to log
   into a web site, though one criticism is that it is challenging for
   users to remember the URI they are assigned.  Webfinger can help
   address this issue by allowing users to use user@domain-style
   addresses.  Using a user's account URI, a web site can perform a
   query to discover the associated OpenID identifier for a user.

   Let's assume Carol is trying to use OpenID to log into a blog.  The
   blog server might issue the following query to get the host metadata
   information:

     GET /.well-known/host-meta.json HTTP/1.1
     Host: example.com

   The response that comes back is similar to the previous example:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8
     {
       "expires" : "2012-03-13T20:56:11Z",
       "links" :

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       [
         {
           "rel" : "lrdd",
           "type" : "application/json",
           "template" :
             "https://example.com/lrdd/?format=json&uri={uri}"
         }
       ]
     }

   The blog server processes the response as described in RFC 6415.  It
   will process the LRDD link relation using Carol's account URI by
   issuing this query:

     GET /lrdd/?format=json&uri=acct%3Acarol%40example.com HTTP/1.1

   The server might return a response like this:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "subject" : "acct:carol@example.com",
       "links" :
       [
         {
           "rel" : "http://webfinger.net/rel/avatar",
           "href" : "http://example.com/~alice/alice.jpg"
         },
         {
           "rel" : "http://specs.openid.net/auth/2.0/provider",
           "href" : "https://openid.example.com/carol"
         }
       ]
     }

   At this point, the blog server knows that Carol's OpenID identifier
   is https://openid.example.com/carol and could then proceed with the
   login process as usual.

3.4. Retrieving Device Information

   While the examples thus far have been focused on information about
   humans, Webfinger does not limit queries to only those that use the
   account URI scheme.  Let's suppose there are devices on the network
   like printers and you would like to check the current toner level for
   a particular printer identified via the URI like

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   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 /lrdd/?format=json&uri=device%3Ap1.example.com HTTP/1.1
     Host: example.com

   The link relations that are returned may be quite different than
   those for human users.  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" : "tipsi",
           "href" : "http://192.168.1.5/npap/"
         }
       ]
     }

   While this example is entirely fictitious, you can imagine that
   perhaps the Transport Independent, Printer/System Interface [18] 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.

4. Webfinger Protocol

   Webfinger does not actually introduce a new protocol, per se.
   Rather, it builds upon the existing Web Host Metadata [9]
   specification and leverages the Cross-Origin Resource Sharing (CORS)
   [7] specification.

4.1. Performing a Webfinger Query

   The first step a client must perform in executing a Webfinger query
   is to query for the host metadata using HTTPS or HTTP.  The
   procedures are defined in the Web Host Metadata [9] specification.

   Webfinger clients MUST locate the LRDD link relation, if present, and
   perform a query for that link relation, if present.  All other link
   templates found must be processed to form a complete resource

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   descriptor.  The processing rules in Section 4.2 of RFC 6415 MUST be
   followed.

   Webfinger servers MUST accept requests for both XRD [8] and JRD [9]
   documents.  The default representation returned by the server MUST be
   an XRD document, but a JRD document MUST be returned if the client
   explicitly requests it by using /.well-known/host-meta.json or
   includes an Accept header in the HTTP request with a type of
   "application/json" [4].

   If the client requests a JRD document when querying for host
   metadata, the Webfinger server can assume that the client will want a
   JRD documents when querying the LRDD resource.  As such, when the
   Webfinger server returns a JRD document containing host metadata it
   should include a URI for an LRDD resource that can return a JRD
   document and MAY include a URI for an LRDD resource that will return
   an XRD document.

   If the client queries the LRDD resource and provides a URI for which
   the server has no information, the server MUST return a 404 status
   code.  Likewise, any query to a URI in the resource descriptor that
   is unknown to the server should result in the server returning a 404
   status code.

4.2. The Web Host Metadata "resource" Parameter

   In addition to the normal processing logic for processing host
   metadata information, Webfinger defines the "resource" parameter for
   querying for host metadata and returning all of the link relations
   from LRDD and other resource-specific link templates in a single
   query.  This resource essentially pushes the work to the server to
   form a complete resource descriptor for the specified resource.

   Note that support for the "resource" parameter is optional, but
   strongly RECOMMENDED for improved performance.  If a server does not
   implement the "resource" parameter, then the server's host metadata
   processing logic remains unchanged from RFC 6415.

   To utilize the host-meta "resource" parameter, a Webfinger client
   issues a request to /.well-known/host-meta or /.well-known/host-
   meta.json as usual, but then appends a "resource" parameter as shown
   in this example:

     GET /.well-known/host-meta.json?resource=\
                            acct%3Abob%40example.com HTTP/1.1
     Host: example.com

   Note that the "\" character shown above is to indicate that the line
   breaks at this point and continues on the next line.  This was shown

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   only to avoid line wrapping in this document and is not a part of the
   HTTP protocol.

   When processing this request, the Webfinger server MUST

       *  Return a 404 status code if the URI provided in the resource
          parameter is unknown to the server; and

       *  Set the "Subject" returned in the response to the value of the
          "resource" parameter if the URI provided in the resource
          parameter is known to the server

   The Webfinger client can verify support for the "resource" parameter
   by checking the value of the Subject returned in the response.  If
   the Subject matches the value of the "resource" parameter, then the
   "resource" parameter is supported by the server.

   For illustrative purposes, the following is an example usage of the
   "resource" parameter that aligns with the example in Section 1.1.1 of
   RFC 6415.  The Webfinger client would issue this request:

     GET /.well-known/host-meta.json?resource=\
                            http%3A%2F%2Fexample.com%2Fxy HTTP/1.1
     Host: example.com

   The Webfinger server would reply with this response:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "subject" : "http://example.com/xy",
       "properties" :
       {
         "http://spec.example.net/color" : "red"
       },
       "links" :
       [
         {
           "rel" : "hub",
           "href" : "http://example.com/hub"
         },
         {
           "rel" : "hub",
           "href" : "http://example.com/another/hub"
         },
         {
           "rel" : "author",

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           "href" : "http://example.com/john"
         },
         {
           "rel" : "author",
           "template" : "http://example.com/author?\
                              q=http%3A%2F%2Fexample.com%2Fxy"
         }
       ]
     }

5. The "acct" URI

   The Web Host Metadata specification [9] allows for any kind of
   resource to be queried, as does Webfinger.  However, a specific type
   of resource is needed in order to query information about a human
   user.

   Webfinger uses the "acct" URI to refer to a human user's account on
   the Internet.  While other URI scheme MAY be used to query for
   information related to a human user, other schemes are not explicitly
   defined for that purpose.

5.1. Using the "acct" URI

   The "acct" URI takes a familiar form in looking like an email
   address.  However, the account URI is not an email address and should
   not be mistaken for one.  Quite often, the account URI minus the
   "acct:" scheme prefix may be exactly the same as the user's email
   address.

   A user MUST NOT be required to enter the "acct" URI scheme name along
   with his account identifier into any Webfinger client.  Rather, the
   Webfinger client MUST accept identifiers that are void of the "acct:"
   portion of the identifier.  Composing a properly formatted "acct" URI
   is the responsibility of the Webfinger client.

   A user MAY provide a fully-specified "acct" URI.

5.2. Syntax of "acct" URI

   The "acct" URI syntax is defined here in Augmented Backus-Naur Form
   (ABNF) [6] and borrows syntax elements from RFC 3986 [5]:

       acctURI      =  "acct:" userpart "@" domainpart
       userpart     =  1*( unreserved / pct-encoded )
       domainpart   =  domainlabel 1*( "." domainlabel)
       domainlabel  =  alphanum / alphanum *( alphanum / "-" ) alphanum
       alphanum     =  ALPHA / DIGIT

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   The "acct" URI scheme allows any character from the Unicode [11]
   character set encoded as a UTF-8 [19] string that is then percent-
   encoded as necessary into valid ASCII [20].  Characters in the
   domainpart must be encoded to support internationalized domain names
   (IDNs) [12].

   Characters in the userpart or domainpart that are not unreserved must
   be percent-encoded when used in a protocol or document that only
   supports or requires ASCII.  When carried in a document (e.g., XRD or
   JRD) or protocol that supports the Unicode character set (e.g., UTF-8
   or UTF-16 [21]), the URI strings may appear in the protocol or
   document's native encoding without percent-encoding.  Such usage of a
   URI is commonly referred to as an Internationalized Resource
   Identifier (IRI).  Conversion between an IRI and URI is described in
   Section 3 of RFC 3987 [13].

6. The "acct" Link Relation

6.1. Purpose for the "acct" Link Relation

   Users of some services might have an "acct" URI that looks
   significantly different from his or her email address, perhaps using
   an entirely different domain name.  It is also possible for a user
   have multiple accounts that a user wants to advertise and that a
   Webfinger client may want to query.  To address both of these needs,
   this specification defines the "acct" link relation.

   Since an account may make a reference to one or more different
   accounts, Webfinger clients MUST take steps to avoid loops wherein
   two accounts, directly or indirectly, refer the client to each other.

   There are no limits on the number of "acct" link relations that might
   be returned in a Webfinger query.

   An "acct" link relation used within the context of a Webfinger query
   for a user's account MUST NOT return "acct" link relations for
   another individual.

   The "acct" link relation also makes it possible to use the link
   relation in HTML documents or in HTTP headers as described in the Web
   Linking specification [3].  This would allow, by way of example, for
   a user to advertise his or her account identifier in a blog, article,
   or other content located on a server that is unrelated to his user
   account.  Since there may be multiple contributors to an article,
   there may be more than one "acct" link relation in an HTML document
   or in HTTP headers. It is RECOMMENDED that no more than one "acct"
   link relation is advertised per author of a given web page, as a
   client may otherwise not understand that the multiple link relations
   are for the same person; references to other accounts should be done

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   from within a user's account, as described in the preceding
   paragraphs.

6.2. Example Message Exchange Using the "acct" Link Relation

   Consider the following non-normative example.

   Suppose Alice receives an email from bob@example.net. While Bob's
   email identifier might be in the example.net domain, he holds his
   account with an "acct" URI in the example.com domain.  His email
   provider may provide Webfinger services to enable redirecting Alice
   when she queries for acct:bob@example.net.

   Suppose Alice's client issues the following request:

     GET /.well-known/host-meta.json?resource=\
                            acct%3Abob%40example.net HTTP/1.1
     Host: example.net

   The response that Alice's client receives back might be:

     HTTP/1.1 200 OK
     Access-Control-Allow-Origin: *
     Content-Type: application/json; charset=UTF-8

     {
       "subject" : "acct:bob@example.net",
       "links" :
       [
         {
           "rel" : "acct",
           "href" : "acct:bob@example.com"
         },
         {
           "rel" : "acct",
           "href" : "acct:bob@example.org"
         }
       ]
     }

   Alice's Webfinger client could then perform queries against the URIs
   acct:bob@example.com and acct:bob@example.org in order to get the
   information Alice is seeking.

7. Cross-Origin Resource Sharing (CORS)

   Webfinger is most useful when it is accessible without restrictions
   on the Internet, and that includes web browsers.  Therefore,
   Webfinger servers MUST support Cross-Origin Resource Sharing (CORS)

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   [7].  Specifically, all queries to /.well-known/host-meta, /.well-
   known/host-meta.json, and to the LRDD URI must include the following
   HTTP header in the response:

      Access-Control-Allow-Origin: *

8. Security Considerations

   All of the security considerations applicable to Web Host Metadata
   [9] and Cross-Origin Resource Sharing [7] 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 should verify that the certificate used on an HTTPS
   connection is valid.

   When using HTTP to request an XRD document, Webfinger clients SHOULD
   verify the XRD document's signature, if present, to ensure that the
   XRD document has not been modified.  Webfinger servers SHOULD include
   a signature for XRD documents.

   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.
   If one does not wish to share certain information with the world, do
   not allow that information to be accessible through Webfinger.

   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.

9. IANA Considerations

   RFC Editor: Please replace QQQQ in the following two sub-sections
   with a reference to this RFC.

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9.1. Registration of the "acct" URI scheme name

   This specification requests IANA to register the "acct" URI scheme in
   the "Permanent URI Schemes" sub-registry in the "Uniform Resource
   Identifier (URI) Schemes" IANA registry [17].  This registration
   follows the URI Scheme Registration Template detailed in Section 5.4
   of RFC 4395 [15].

     URI scheme name: acct

     Status: Permanent

     URI scheme syntax: see Section 4.1 of RFC QQQQ

     URI scheme semantics: see Section 4.1 of RFC QQQQ

     Encoding considerations: The "acct" URI scheme allows any character
     from the Unicode character set encoded as a UTF-8 string that is
     then percent-encoded as necessary to result in an internal
     representation in US-ASCII [10]

     Applications/protocols that use this URI scheme name: Webfinger

     Security considerations: see Section 7 of RFC QQQQ

     Contact: Gonzalo Salgueiro <gsalguei@cisco.com>

     Author/Change controller: IETF <ietf@ietf.org>

     References: See Section 10 of RFC QQQQ

9.2. Registration of the "acct" Link Relation Type

     Relation Name: acct

     Description: A link relation that refers to a user's Webfinger
     account identifier.

     Reference: RFC QQQQ

     Notes:

     Application Data:

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10. Acknowledgments

   The authors would like to acknowledge Eran Hammer-Lahav, Blaine Cook,
   Brad Fitzpatrick, Laurent-Walter Goix, and Joe Clarke for their
   invaluable input.

11. References

11.1. 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., "Web Linking", RFC 5988, October 2010.

   [4]   Crockford, D., "The application/json Media Type for
         JavaScript Object Notation (JSON)", RFC 4627, July 2006.

   [5]   Berners-Lee, T., Fielding, R., and Masinter, L., "Uniform
         Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986,
         January 2005.

   [6]   Crocker, D. and P. Overell, "Augmented BNF for Syntax
         Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [7]   Van Kesteren, A., "Cross-Origin Resource Sharing", W3C CORS
         http://www.w3.org/TR/cors/, July 2010.

   [8]   Hammer-Lahav, E. and W. Norris, "Extensible Resource Descriptor
         (XRD) Version 1.0", http://docs.oasis-
         open.org/xri/xrd/v1.0/xrd-1.0.html.

   [9]   Hammer-Lahav, E. and Cook, B., "Web Host Metadata", RFC 6415,
         October 2011.

   [10]  American National Standards Institute, "Coded Character Set -
         7-bit American Standard Code for Information Interchange", ANSI
         X3.4, 1986.

   [11]  The Unicode Consortium. The Unicode Standard, Version 6.1.0,
         (Mountain View, CA: The Unicode Consortium, 2012. ISBN 978-1-
         936213-02-3) http://www.unicode.org/versions/Unicode6.1.0/.

   [12]  Klensin, J., "Internationalized Domain Names in Applications
         (IDNA): Protocol", RFC 5891, August 2010.

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   [13]  Duerst, M., "Internationalized Resource Identifiers (IRIs)",
         RFC 3987, January 2005.

11.2. Informative References

   [14]  Zimmerman, D., "The Finger User Information Protocol", RFC
         1288, December 1991.

   [15]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
         Registration Procedures for New URI Schemes", BCP 35, RFC 4395,
         February 2006.

   [16]  Perreault, S., "vCard Format Specification", RFC 6350, August
         2011.

   [17]  Internet Assigned Numbers Authority (IANA) Registry, "Uniform
         Resource Identifier (URI) Schemes",
         <http://www.iana.org/assignments/uri-schemes.html>.

   [18]  "Transport Independent, Printer/System Interface", IEEE Std
         1284.1-1997, 1997.

   [19]  Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC
         2279, November 2003.

   [20]  Information Systems -- Coded Character Sets 7-Bit American
         National Standard Code for Information Interchange (7-Bit
         ASCII), ANSI X3.4-1986, December 30, 1986.

   [21]  Hoffman, P., Yergeau, F., "UTF-16, an encoding of ISO 10646",
         RFC 2781, February 2000.

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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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Change Log (To Be Deleted Before Publication)
=============================================

-02 Draft

* Minor editorial changes

* Added <Expires/> to the XML example to highlight that this element
  exists, since some may not be aware

* Changed some of the link relation values, particularly for those that
  are not yet standardized

* Added a note about "device:" not being standard

* Overhauled the "acct" link relation text, breaking the normative and
  non-normative pieces apart

* Added additional text to the security considerations section related
  to dynamic information (e.g., geographic information)

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