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Versions: (draft-miller-posh) 00 01 02 03 04 05 06 RFC 7711

XMPP Working Group                                             M. Miller
Internet-Draft                                       Cisco Systems, Inc.
Intended status: Standards Track                          P. Saint-Andre
Expires: April 13, 2015                                             &yet
                                                        October 10, 2014


                      PKIX over Secure HTTP (POSH)
                        draft-ietf-xmpp-posh-02

Abstract

   Experience has shown that it is extremely difficult to deploy proper
   PKIX certificates for TLS in multi-tenanted environments, since
   certification authorities will not issue certificates for hosted
   domains to hosting services, hosted domains do not want hosting
   services to hold their private keys, and hosting services wish to
   avoid liability for holding those keys.  As a result, domains hosted
   in multi-tenanted environments often deploy non-HTTP applications
   such as email and instant messaging using certificates that identify
   the hosting service, not the hosted domain.  Such deployments force
   end users and peer services to accept a certificate with an improper
   identifier, resulting in obvious security implications.  This
   document defines two methods that make it easier to deploy
   certificates for proper server identity checking in non-HTTP
   application protocols.  The first method enables the TLS client
   associated with a user agent or peer application server to obtain the
   end-entity certificate of a hosted domain over secure HTTP as an
   alternative to standard PKIX techniques.  The second method enables a
   hosted domain to securely delegate a non-HTTP application to a
   hosting service using redirects provided by HTTPS itself or by a
   pointer in a file served over HTTPS at the hosted domain.  While this
   approach was developed for use in the Extensible Messaging and
   Presence Protocol (XMPP) as a Domain Name Association prooftype, it
   can be applied to any non-HTTP application protocol.

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



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   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 April 13, 2015.

Copyright Notice

   Copyright (c) 2014 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Obtaining Verification Materials  . . . . . . . . . . . . . .   4
     3.1.  Source Domain Possesses PKIX Certificate Information  . .   5
     3.2.  Source Domain References PKIX Certificate . . . . . . . .   6
     3.3.  Performing Verification . . . . . . . . . . . . . . . . .   7
   4.  Secure Delegation . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Order of Operations . . . . . . . . . . . . . . . . . . . . .   8
   6.  Caching Results . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Alternates and Roll-over  . . . . . . . . . . . . . . . . . .  10
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Acknowledgements . . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   We start with a thought experiment.

   Imagine that you work on the operations team of a hosting company
   that provides the "foo" service (or email or instant messaging or
   social networking service) for ten thousand different customer
   organizations.  Each customer wants their service to be identified by



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   the customer's domain name (e.g., foo.example.com), not the hosting
   company's domain name (e.g., hosting.example.net).

   In order to properly secure each customer's "foo" service via
   Transport Layer Security (TLS) [RFC5246], you need to obtain PKIX
   certificates [RFC5280] containing identifiers such as
   foo.example.com, as explained in the "CertID" specification
   [RFC6125].  Unfortunately, you can't obtain such certificates
   because:

   o  Certification authorities won't issue such certificates to you
      because you work for the hosting company, not the customer
      organization.

   o  Customers won't obtain such certificates and then give them (plus
      the associated private keys) to you because their legal department
      is worried about liability.

   o  You don't want to install such certificates (plus the associated
      private keys) on your servers anyway because your legal department
      is worried about liability, too.

   Given your inability to deploy public keys / certificates containing
   the right identifiers, your back-up approach has always been to use a
   certificate containing hosting.example.net as the identifier.
   However, more and more customers and end users are complaining about
   warning messages in user agents and the inherent security issues
   involved with taking a "leap of faith" to accept the identity
   mismatch between the source domain (foo.example.com) and the
   delegated domain (hosting.example.net).

   This situation is both insecure and unsustainable.  You have
   investigated the possibility of using DNS Security [RFC4033] and DNS-
   Based Authentication of Named Entities (DANE) [RFC6698] to solve the
   problem.  However, your customers and your operations team have told
   you that it will be several years before they will be able to deploy
   DNSSEC and DANE for all of your customers (because of tooling
   updates, slow deployment of DNSSEC at some top-level domains, etc.).
   The product managers in your company are pushing you to find a method
   that can be deployed more quickly to overcome the lack of proper
   server identity checking for your hosted customers.

   One possible approach that your team has investigated is to ask each
   customer to provide the public key / certificate for the "foo"
   service at a special HTTPS URL on their website
   ("https://foo.example.com/.well-known/posh.foo.json" is one
   possibility).  This could be a public key that you generate for the
   customer, but because the customer hosts it via HTTPS, any user agent



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   can find that public key and check it against the public key you
   provide during TLS negotiation for the "foo" service (as one added
   benefit, the customer never needs to hand you a private key).
   Alternatively, the customer can redirect requests for that special
   HTTPS URL to an HTTPS URL at your own website, thus making it
   explicit that they have delegated the "foo" service to you.

   The approach sketched out above, called POSH ("PKIX Over Secure
   HTTP"), is explained in the remainder of this document.  While this
   approach was developed for use in the Extensible Messaging and
   Presence Protocol (XMPP) as a prooftype for Domain Name Associations
   (DNA) [I-D.ietf-xmpp-dna], it can be applied to any non-HTTP
   application protocol.

2.  Terminology

   This document inherits security terminology from [RFC5280].  The
   terms "source domain", "derived domain", "reference identifier", and
   "presented identifier" are used as defined in the "CertID"
   specification [RFC6125].

   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].

3.  Obtaining Verification Materials

   Server identity checking (see [RFC6125]) involves three different
   aspects:

   1.  A proof of the TLS server's identity (in PKIX, this takes the
       form of a PKIX certificate [RFC5280]).

   2.  Rules for checking the certificate (which vary by application
       protocol, although [RFC6125] attempts to harmonize those rules).

   3.  The materials that a TLS client uses to verify the TLS server's
       identity or check the TLS server's proof (in PKIX, this takes the
       form of chaining the end-entity certificate back to a trusted
       root and performing all validity checks as described in
       [RFC5280], [RFC6125], and the relevant application protocol
       specification).

   When POSH is used, the first two aspects remain the same: the TLS
   server proves it identity by presenting a PKIX certificate [RFC5280]
   and the certificate is checked according to the rules defined in the
   appropriate application protocol specification (such as [RFC6120] for



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   XMPP).  However, the TLS client obtains the materials it will use to
   verify the server's proof by retrieving a JSON document [RFC7159]
   containing hashes of the PKIX certificate over HTTPS ([RFC7230] and
   [RFC2818]) from a well-known URI [RFC5785].

   The process for retrieving a PKIX certificate over secure HTTP is as
   follows.

   1.  The TLS client performs an HTTPS GET at the source domain to the
       path "/.well-known/posh.{servicedesc}.json".  The value of
       "{servicedesc}" is application-specific; see Section 8 of this
       document for more details.  For example, if the application
       protocol is some hypothetical "Foo" service, then "{servicedesc}"
       could be "foo"; thus if a Foo client were to use POSH to verify a
       Foo server for the domain "foo.example.com", the HTTPS GET
       request would be as follows:

       GET /.well-known/posh.foo.json HTTP/1.1
       Host: foo.example.com

   2.  The source domain HTTPS server responds in one of three ways:

       *  If it possesses PKIX certificate information for the requested
          path, it responds as detailed in Section 3.1.

       *  If it has a reference to where the PKIX certificate
          information can be obtained, it responds as detailed in
          Section 3.2.

       *  If it does not have any PKIX certificate information or a
          reference to such information for the requested path, it
          responds with an HTTP client error status code (e.g., 404).

3.1.  Source Domain Possesses PKIX Certificate Information

   If the source domain HTTPS server possesses the certificate
   information, it responds to the HTTPS GET with a success status code
   and the message body set to a JSON document [RFC7159]; the document
   is a JSON object which MUST have the following:

   o  A "fingerprints" field whose value is a JSON array of fingerprint
      descriptors.

   o  An "expires" field whose value is a JSON number specifying the
      number of seconds after which the TLS client ought to consider the
      key information to be stale (further explained under Section 6).





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   Each included fingerprint descriptor is a JSON object, where each
   member name is the textual name of a hash function (as listed in
   [HASH-NAMES]) and its associated value is the base 64 encoded
   fingerprint hash generated using the named hash function (where the
   encoding adheres to the definition in Section 4 of [RFC4648] and
   where the padding bits are set to zero).  Each fingerprint descriptor
   MUST possess at least one named hash function.

   The fingerprint hash for a given hash algorithm is generated by
   performing the named hash function over the DER encoding of the PKIX
   X.509 certifiate; for example, a "sha-1" fingerprint is generated by
   performing the SHA-1 hash function over the DER encoding of the PKIX
   certificate.

   The following example illustrates the usage described above.

   Example Content Response

   HTTP/1.1 200 OK
   Content-Type: application/json
   Content-Length: 134

   {
     "fingerprints": [
       {
         "sha-1":"UpjRI/A3afKE8/AIeTZ5o1dECTY=",
         "sha-256":"4/mggdlVx8A3pvHAWW5sD+qJyMtUHgiRuPjVC48N0XQ="
       }
     ],
     "expires": 604800
   }


   The "expires" value is a hint regarding the expiration of the keying
   materials.  It MUST be a non-negative integer.  If no "expires" field
   is included or its value is equal to 0, a TLS client SHOULD consider
   these verification materials invalid.  See Section 6 for how to
   reconcile this "expires" field with the reference's "expires" field.

3.2.  Source Domain References PKIX Certificate

   If the source domain HTTPS server has a reference to the certificate
   information, it responds to the HTTPS GET with a success status code
   and message body set to a JSON document.  The document is a JSON
   object which MUST contain the following:






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   o  A "url" field whose value is a JSON string specifying the HTTPS
      URL where TLS clients can obtain the actual certificate
      information.

   o  An "expires" field whose value is a JSON number specifying the
      number of seconds after which the TLS client ought to consider the
      delegation to be stale (further explained under Section 6).

   Example Reference Response

   HTTP/1.1 200 Ok
   Content-Type: application/json
   Content-Length: 79

   {
     "url":"https://hosting.example.net/.well-known/posh.foo.json",
     "expires":86400
   }

   The client performs an HTTPS GET for the URL specified in the "url"
   field value.  The HTTPS server for the URL to which the client has
   been redirected responds to the request with a JSON document
   containing fingerprints as described in Section 3.1.  The content
   retrieved from the "url" location MUST NOT itself be a reference
   (i.e., containing a "url" field instead of a "fingerprints" field),
   in order to prevent circular delegations.

      Note: The JSON document returned by the source domain HTTPS server
      MUST contain either a reference or a fingerprints document, but
      MUST NOT contain both.

      Note: See Section 9 for discussion about HTTPS redirects.

   The "expires" value is a hint regarding the expiration of the source
   domain's delegation of service to the delegated domain.  It MUST be a
   non-negative integer.  If no "expires" field is included or its value
   is equal to 0, a TLS client SHOULD consider the delegation invalid.
   See Section 6 for guidelines about reconciling this "expires" field
   with the "expires" field of the fingerprints document.

3.3.  Performing Verification

   The TLS client compares the PKIX information obtained from the TLS
   server against each fingerprint descriptor object in the POSH
   results, until a match is found or the collection of POSH
   verification materials is exhausted.  If none of the fingerprint
   descriptor objects match the TLS server PKIX information, the TLS
   client SHOULD reject the connection (however, the TLS client might



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   still accept the connection if other verification schemes are
   successful).

4.  Secure Delegation

   The delegation from the source domain to the delegated domain can be
   considered secure if the certificate offered by the TLS server
   matches the POSH certificate, regardless of how the POSH certificate
   is obtained.

5.  Order of Operations

   In order for the TLS client to perform verification of reference
   identifiers without potentially compromising data, POSH processes
   MUST be complete before any application-level data is exchanged for
   the source domain.  The TLS client SHOULD perform all POSH retrievals
   before opening any socket connections to the application protocol
   server.  For application protocols that use DNS SRV (including
   queries for TLSA records in concert with SRV records as described in
   [I-D.ietf-dane-srv]), the POSH processes ideally ought to be done in
   parallel with resolving the SRV records and the addresses of any
   targets, similar to the "happy eyeballs" approach for IPv4 and IPv6
   [RFC6555].

   The following diagram illustrates the possession flow:

   Client                     Domain                     Server
   ------                     ------                     ------
     |                          |                          |
     |      Request POSH        |                          |
     |------------------------->|                          |
     |                          |                          |
     | Return POSH fingerprints |                          |
     |<-------------------------|                          |
     |                          |                          |
     |                  Service TLS Handshake              |
     |<===================================================>|
     |                          |                          |
     |                     Service Data                    |
     |<===================================================>|
     |                          |                          |

               Figure 1: Order of Events for Possession Flow

   While the following diagram illustrates the reference flow:






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   Client                     Domain                     Server
   ------                     ------                     ------
     |                          |                          |
     |      Request POSH        |                          |
     |------------------------->|                          |
     |                          |                          |
     |     Return POSH url      |                          |
     |<-------------------------|                          |
     |                          |                          |
     |                      Request POSH                   |
     |---------------------------------------------------->|
     |                          |                          |
     |                Return POSH fingerprints             |
     |<----------------------------------------------------|
     |                          |                          |
     |                 Service TLS Handshake               |
     |<===================================================>|
     |                          |                          |
     |                     Service Data                    |
     |<===================================================>|
     |                          |                          |

               Figure 2: Order of Events for Reference Flow

6.  Caching Results

   The TLS client MUST NOT cache results (reference or fingerprints)
   indefinitely.  If the source domain returns a reference, the TLS
   client MUST use the lower of the two "expires" values when
   determining how long to cache results (i.e., if the reference
   "expires" value is lower than the fingerprints "expires" value, honor
   the reference "expires" value).  Once the TLS client considers the
   results stale, it needs to perform the entire POSH process again
   starting with the HTTPS GET to the source domain.  The TLS client MAY
   use a lower value than any provided in the "expires" field(s), or not
   cache results at all.

   The TLS client SHOULD NOT rely on HTTP caching mechanisms, instead
   using the expiration hints provided in the POSH reference document or
   fingerprints documents.  To that end, the HTTPS servers for source
   domains and derived domains SHOULD specify a 'Cache-Control' header
   indicating a very short duration (e.g., max-age=60) or "no-cache" to
   indicate that the response (redirect, reference, or content) is not
   appropriate to cache at the HTTP level.







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7.  Alternates and Roll-over

   To indicate alternate PKIX certificates (such as when an existing
   certificate will soon expire), the returned fingerprints document MAY
   contain multiple fingerprint descriptors.  The fingerprints SHOULD be
   ordered with the most relevant certificate first as determined by the
   application service operator (e.g., the renewed certificate),
   followed by the next most relevant certificate (e.g., the certificate
   soonest to expire).  Here is an example:

   {
     "fingerprints": [
       {
         "sha-1":"UpjRI/A3afKE8/AIeTZ5o1dECTY=",
         "sha-256":"4/mggdlVx8A3pvHAWW5sD+qJyMtUHgiRuPjVC48N0XQ"
       },
       {
         "sha-1":"T29tGO9d7kxbfWnUaac8+5+ICLM=",
         "sha-256":"otyLADSKjRDjVpj8X7/hmCAD5C7Qe+PedcmYV7cUncE="
       }
     ],
     "expires": 806400
   }

8.  IANA Considerations

   This document registers a well-known URI [RFC5785] for protocols that
   use POSH.  The completed template follows.



      URI suffix:  posh.

      Change controller:  IETF

      Specification document:  [[ this document ]]

      Related information:  Because the "posh." string is merely a
         prefix, protocols that use POSH need to register particular
         URIs that are prefixed with the "posh." string.

   Note that the registered URI is "posh." (with a trailing dot).  This
   is merely a prefix to be placed at the front of well-known URIs
   [RFC5785] registered by protocols that use POSH, which themselves are
   responsible for the relevant registrations with the IANA.  The URIs
   registered by such protocols SHOULD match the URI template [RFC6570]
   path "/.well-known/posh.{servicedesc}.json"; that is, begin with




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   "posh." and end with ".json" (indicating a media type of application/
   json [RFC7159]).

   For POSH-using protocols that rely on DNS SRV records [RFC2782], the
   "{servicedesc}" part of the well-known URI SHOULD be
   "{service}.{proto}", where the "{service}" is the DNS SRV "Service"
   prepended by the underscore character "_" and the "{proto}" is the
   DNS SRV "Proto" also prepended by the underscore character "_".  As
   an example, the well-known URI for XMPP server-to-server connections
   would be "posh._xmpp-server._tcp.json" since XMPP [RFC6120] registers
   a service name of "xmpp-server" and uses TCP as the underlying
   transport protocol.

   For other POSH-using protocols, the "{servicedesc}" part of the well-
   known URI can be any unique string or identifier for the protocol,
   which might be a service name registered with the IANA in accordance
   with [RFC6335] or which might be an unregistered name.  As an
   example, the well-known URI for the mythical "Foo" service could be
   "posh.foo.json".

   Note: As explained in [RFC5785], the IANA registration policy
   [RFC5226] for well-known URIs is Specification Required.

9.  Security Considerations

   This document supplements but does not supersede the security
   considerations provided in specifications for application protocols
   that decide to use POSH (e.g., [RFC6120] and [RFC6125] for XMPP).
   Specifically, the security of requests and responses sent via HTTPS
   depends on checking the identity of the HTTP server in accordance
   with [RFC2818].  Additionally, the security of POSH can benefit from
   other HTTP hardening protocols, such as HSTS [RFC6797] and key
   pinning [I-D.ietf-websec-key-pinning], especially if the TLS client
   shares some information with a common HTTPS implementation (e.g.,
   platform-default web browser).

   Note well that POSH is used by a TLS client to obtain the public key
   of a TLS server to which it might connect for a particular
   application protocol such as IMAP or XMPP.  POSH does not enable a
   hosted domain to transfer private keys to a hosting service via
   HTTPS.  POSH also does not enable a TLS server to engage in
   certificate enrollment with a certification authority via HTTPS, as
   is done in Enrollment over Secure Transport [RFC7030].

   A web server at the source domain might redirect an HTTPS request to
   another URL.  The location provided in the redirect response MUST
   specify an HTTPS URL.  Source domains SHOULD use only temporary
   redirect mechanisms, such as HTTP status codes 302 (Found) and 307



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   (Temporary Redirect).  Clients MAY treat any redirect as temporary,
   ignoring the specific semantics for 301 (Moved Permanently) and 308
   (Permanent Redirect) [RFC7238].  To protect against circular
   references, clients MUST NOT follow an infinite number of redirects.
   It is RECOMMENDED that clients follow no more than 10 redirects,
   although applications or implementations can require that fewer
   redirects be followed.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [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, May 2008.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785, April
              2010.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

   [RFC7159]  Bray, T., "The JavaScript Object Notation (JSON) Data
              Interchange Format", RFC 7159, March 2014.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014.







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10.2.  Informative References

   [I-D.ietf-dane-srv]
              Finch, T., Miller, M., and P. Saint-Andre, "Using DNS-
              Based Authentication of Named Entities (DANE) TLSA Records
              with SRV Records", draft-ietf-dane-srv-06 (work in
              progress), June 2014.

   [I-D.ietf-websec-key-pinning]
              Evans, C., Palmer, C., and R. Sleevi, "Public Key Pinning
              Extension for HTTP", draft-ietf-websec-key-pinning-14
              (work in progress), June 2014.

   [I-D.ietf-xmpp-dna]
              Saint-Andre, P. and M. Miller, "Domain Name Associations
              (DNA) in the Extensible Messaging and Presence Protocol
              (XMPP)", draft-ietf-xmpp-dna-05 (work in progress),
              February 2014.

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC6120]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Core", RFC 6120, March 2011.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165, RFC
              6335, August 2011.

   [RFC6555]  Wing, D. and A. Yourtchenko, "Happy Eyeballs: Success with
              Dual-Stack Hosts", RFC 6555, April 2012.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570, March 2012.






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Internet-Draft                    POSH                      October 2014


   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

   [RFC6797]  Hodges, J., Jackson, C., and A. Barth, "HTTP Strict
              Transport Security (HSTS)", RFC 6797, November 2012.

   [RFC7030]  Pritikin, M., Yee, P., and D. Harkins, "Enrollment over
              Secure Transport", RFC 7030, October 2013.

   [RFC7238]  Reschke, J., "The Hypertext Transfer Protocol Status Code
              308 (Permanent Redirect)", RFC 7238, June 2014.

   [HASH-NAMES]
              "Hash Function Textual Names",
              <http://www.iana.org/assignments/hash-function-text-names/
              hash-function-text-names.xhtml>.

Appendix A.  Acknowledgements

   Many thanks to Thijs Alkemade, Philipp Hancke, Joe Hildebrand, and
   Tobias Markmann for their implementation feedback.  Thanks also to
   Dave Cridland, Chris Newton, Max Pritikin, and Joe Salowey for their
   input on the specification.

Authors' Addresses

   Matthew Miller
   Cisco Systems, Inc.
   1899 Wynkoop Street, Suite 600
   Denver, CO  80202
   USA

   Email: mamille2@cisco.com


   Peter Saint-Andre
   &yet
   P.O. Box 787
   Parker, CO  80134
   USA

   Email: peter@andyet.com








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