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Versions: 00 01 02 03 04 05 06                                          
VIPR WG                                                   M. Barnes, Ed.
Internet-Draft                                                   Polycom
Intended status:  Standards Track                            C. Jennings
Expires:  March 31, 2012                                           Cisco
                                                            J. Rosenberg
                                                       M. Petit-Huguenin
                                                      September 28, 2011

Verification Involving PSTN Reachability: Requirements and Architecture


   The Session Initiation Protocol (SIP) has seen widespread deployment
   within individual domains, typically supporting voice and video
   communications.  Though it was designed from the outset to support
   inter-domain federation over the public Internet, such federation has
   not materialized.  The primary reasons for this are the complexities
   of inter-domain phone number routing and concerns over security.
   This document reviews this problem space, outlines requirements, and
   then describes a new model and technique for inter-domain federation
   with SIP involving the Public Switched Telephone Network (PSTN),
   called Verification Involving PSTN Reachability (VIPR).  VIPR
   addresses the problems that have prevented inter-domain federation
   over the Internet.  It provides fully distributed inter-domain
   routing for phone numbers, authorized mappings from phone numbers to
   domains, a new technique for automated SIP anti-spam, and privacy of
   number ownership, all while preserving the trapezoidal model of SIP.

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."

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   This Internet-Draft will expire on March 31, 2012.

Copyright Notice

   Copyright (c) 2011 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.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Problem Statement  . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  The Phone Number Routing Problem . . . . . . . . . . . . .  5
     2.2.  The Open Pinhole Problem . . . . . . . . . . . . . . . . .  6
     2.3.  Quality of Service Problem . . . . . . . . . . . . . . . .  7
     2.4.  Troubleshooting Problem  . . . . . . . . . . . . . . . . .  7
   3.  Summary of Existing Solutions  . . . . . . . . . . . . . . . .  7
     3.1.  Domain Routing . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Public ENUM  . . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Private Federations  . . . . . . . . . . . . . . . . . . .  9
   4.  Key Requirements . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Executive Overview . . . . . . . . . . . . . . . . . . . . . . 10
     5.1.  Key Properties . . . . . . . . . . . . . . . . . . . . . . 10
     5.2.  Challenging Past Assumptions . . . . . . . . . . . . . . . 12
     5.3.  Technical Overview . . . . . . . . . . . . . . . . . . . . 13
       5.3.1.  Storage of Phone Numbers . . . . . . . . . . . . . . . 14
       5.3.2.  PSTN First Call  . . . . . . . . . . . . . . . . . . . 16
       5.3.3.  Validation and Caching . . . . . . . . . . . . . . . . 17
       5.3.4.  SIP Call . . . . . . . . . . . . . . . . . . . . . . . 20
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
     6.1.  Attacks on the DHT . . . . . . . . . . . . . . . . . . . . 22
     6.2.  Theft of Phone Numbers . . . . . . . . . . . . . . . . . . 22
     6.3.  Spam . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     6.4.  Eavesdropping  . . . . . . . . . . . . . . . . . . . . . . 24
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 24
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 24
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 24
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 25
   Appendix A.  Changes since last version  . . . . . . . . . . . . . 26
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26

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1.  Introduction

   The Session Initiation Protocol (SIP) was originally published as
   [RFC2543] in May of 1999.  This was followed by subsequent
   publication of [RFC3261], which brought the protocol to sufficient
   maturity to enable large scale market adoption.

   SIP has achieved large scale market adoption with hundreds of
   implementations, spanning consumer products, enterprise servers, and
   large scale carrier equipment.  It carries billions and billions of
   minutes of calls, and has become the standard for interconnection
   between products from different vendors.  If one measures success in
   deployment, then clearly SIP is a success.

   SIP was designed from the ground up to enable communications between
   users in different domains, all over the public Internet.  The
   intention was that real-time communications should be no different
   than email or the web, with the same any-to-any connectivity that has
   fueled the successes of those technologies.  However, when SIP is
   used between domains, it is typically through private federation
   agreements.  While such agreements are positive, they have typically
   been limited to voice, which has limited the use of video and the
   growth of advanced SIP features, thus preventing the innovation that
   SIP was expected to drive.  Thus, the any-to-any Internet federation
   model envisioned by SIP has not materialized at scale.

   This document introduces a new technology, called Verification
   Involving PSTN Reachability (VIPR), that breaks down the barriers
   that have prevented inter-domain voice, video and other multimedia
   services.  By stepping back and changing some of the most fundamental
   assumptions about federation, VIPR is able to address the key
   problems preventing its deployment.  VIPR focuses on incremental
   deployability.  At the same time, VIPR ensures that SIP's trapezoidal
   model of direct federation between domains without any intermediate
   processing beyond IP transport is realized.  That model is required
   in order to allow innovative new services to be deployed.

2.  Problem Statement

   The first question that must be asked is this - why haven't we seen
   widespread adoption of inter-domain SIP federation?  The reason for
   this is due to problems with the following - summarized in order of

   1.  Phone number routing

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   2.  Open pinhole
   3.  Quality of service
   4.  Troubleshooting

   The first two are the most significant.

2.1.  The Phone Number Routing Problem

   Inter-domain federation requires that the sending domain determine
   the address of the receiving domain, in the form of a DNS name
   (example.com) or one or more IP addresses that can be used to reach
   the domain.  In email and in the web, this is easy.  The identifiers
   used by those services - the email address and web URL respectively -
   embed the address of the receiving domain.  A simple DNS lookup is
   all that is required to route the connection.  SIP was designed to
   use the same email-style identifiers.

   However, most SIP deployments utilize phone numbers, and not email-
   style SIP URIs.  This is due to the huge installed base of users that
   continue to exist solely on the PSTN.  In order to be reached by
   users on the PSTN, and in order to reach them, users in SIP
   deployments need to be assigned a regular PSTN number.  Users in SIP
   deployments need to place that PSTN number on business cards, use it
   in their email signatures, and in general, give it out to their
   friends and colleagues, in order to be reached.  While those users
   could additionally have an email style SIP URI, the PSTN number
   serves as a single, global identifier that works for receiving calls
   from users on the PSTN as well as users within the same SIP domain.

   There are several reasons why two identifiers are used when one will
   suffice.  The universality of PSTN numbers is the reason why most SIP
   deployments continue to use them - often exclusively.

   Another reason is that many SIP deployments utilize hardphones or
   telephony adaptors, and the user interfaces on these devices -
   patterned after existing phones - only allow phone-number based
   dialing.  Consequently, these users are only allocated PSTN numbers,
   and not email-style SIP URI.

   Finally, a large number of SIP deployments are in domains where the
   endpoints are not IP.  Rather, they are circuit based devices,
   connected to a SIP network through a gateway.  SIP is used within the
   core of the network, providing lower cost transit, or providing
   add-on services.  Clearly, in these deployments, only phone numbers
   are used.

   Consequently, to make inter-domain federation incrementally
   deployable and widely applicable, it needs to work with PSTN numbers

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   rather than email-style SIP URI.  Telephone numbers, unlike email
   addresses, do not provide any indication of the address of the domain
   which "owns" the phone number.  Indeed, the notion of phone number
   ownership is somewhat cloudy.  Numbers can be ported between
   carriers.  They can be assigned to a user or enterprise, and then
   later re-assigned to someone else.  Numbers are granted to users and
   enterprises through a complex delegation process involving the ITU,
   governments, and telecommunications carriers, often involving local
   regulations that vary from country to country.

   Therefore, in order to deploy inter-domain federation, domains are
   required to utilize some kind of mechanism to map phone numbers to
   the address of the domain to which calls should be routed.  Though
   several techniques have been developed to address this issue, none
   have achieved large-scale Internet deployments.

2.2.  The Open Pinhole Problem

   The inter-domain federation mechanism built into SIP borrows heavily
   from email.  Each domain runs a SIP server on an open port.  When one
   domain wishes to contact another, it looks up the domain name in the
   DNS, and connects to that server on the open port.  Here, "open"
   means that the server is reachable from anywhere on the public
   Internet, and is not blocked by firewalls.

   This simple design worked well in the early days of email.  However,
   the email system has now become plagued with spam, to the point of
   becoming useless.  Administrators of SIP domains fear - rightfully so
   - that if they make a SIP server available for anyone on the Internet
   to contact, it will open the floodgates for SIP spam, which is far
   more disruptive than email-based spam [RFC5039].  Administrators also
   worry - rightfully so - that an open server will create a back-door
   for denial-of-service and other attacks that can potentially disrupt
   their voice and video services.  Administrators are simply not
   willing to take that risk; rightly or wrongly, voice deployments
   demand higher uptimes and better levels of reliability than email,
   especially for enterprises.

   Fears around spam and denial-of-service attacks, when put together,
   form the "open pinhole problem" - that domains are not willing to
   enable SIP on an open port facing the Internet.

   To fix this, a new model for federation is needed - a model where
   these problems are addressed as part of the fundamental design, and
   not as an after-thought.

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2.3.  Quality of Service Problem

   The Internet does not provide any QoS guarantees.  All traffic is
   best effort.  This is not an issue for data transaction services,
   like web and email.  It is, however, a concern when using real-time
   services, such as voice and video.

   That said, there are a large number of existing SIP deployments that
   run over the Internet.  Though the lack of QoS is a concern, it has
   not proven a barrier to deployment.  We believe that, if the more
   fundamental issues - the phone number routing and open pinhole
   problems - can be addressed, the QoS problem will sort itself out.
   As such, we do not discuss this issue further here.

2.4.  Troubleshooting Problem

   The final problem that is stopping large scale inter-domain
   federation is the troubleshooting problem.  When connecting calls
   between domains, problems will happen.  Calls will get blocked.
   Calls will get misdelivered.  Features won't work.  There will be
   one-way media or no media at all.  The video won't start.  Call
   quality will be poor.

   These problems are common in SIP deployments, and they are tough to
   troubleshoot even within a single administrative domain.  When real-
   time services extend inter-domain, the problem becomes worse.  A new
   angle is introduced:  the first step is identifying who is at fault.

   Fortunately, work is underway to improve the ability for network
   administrators to diagnose SIP problems.  Common log formats
   [I-D.ietf-sipclf-format] and consistent session IDs
   [I-D.jones-ipmc-session-id-reqts], for example, can help troubleshoot
   interdomain calls.

   In addition to these, any new technology that facilitates inter-
   domain federation needs to have troubleshooting built-in, so that it
   is not a barrier to deployment.  Further consideration of necessary
   built-in techniques for troubleshooting is required for successful
   deployment of VIPR.

3.  Summary of Existing Solutions

   Given the value that inter-domain SIP federation brings, it is no
   surprise that many attempts have been made at solving it.  Indeed,
   these have all been deployed to varying degrees.  However, all of
   them have fundamental limitations that have inhibited widespread

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3.1.  Domain Routing

   The first solution that has been proposed for SIP inter-domain
   federation is built into SIP itself - domain routing.  In this
   technique, users utilize email-style SIP URI as identifiers.  By
   utilizing the DNS lookup mechanism defined in [RFC3263], SIP enables
   calls to be routed between domains in much the same way email is
   routed between domains.

   This technique works well in theory, but it has two limitations which
   have limited its deployment:

   1.  The majority of SIP deployments utilize phone numbers, often
       exclusively.  In such a case, domain routing cannot be used.
   2.  Domain federation brings with it the possibility (and strong
       likelihood) of the same levels of spam and DoS attacks that have
       plagued the email system.

   These issues have already been discussed above.

3.2.  Public ENUM

   Public ENUM, defined in [RFC6116] addresses the phone number routing
   problem by cleverly placing phone numbers into the public DNS.
   Clients can then perform a simple DNS lookup on a phone number, and
   retrieve a SIP URI which can be used to route to that phone number.

   Unfortunately, public ENUM requires that the entries placed into the
   DNS be populated following a chain of responsibility that mirrors the
   ownership of the numbers themselves.  This means that, in order for a
   number to be placed into the DNS, authorization to do so must start
   with the ITU, and from there, move to the country, telecom regulator,
   and ultimately the end user.  The number of layers of bureaucracy
   required to accomplish this is non-trivial.  In addition, the telecom
   operators - which would be partly responsible for populating the
   numbers into the DNS - have little incentive to do so.  As a
   consequence, public ENUM is largely empty, and is likely to remain so
   for the foreseeable future.

   Instead, ENUM has morphed into a technique for federation amongst
   closed peering partners, called private ENUM or infrastructure ENUM
   [RFC5067].  While there is value in this technology, it does not
   enable the open federation that public ENUM was designed to solve.

   It is clear from the legacy of ENUM deployments, that any kind of
   phone number routing solution should not rely on government or
   telecom processes for population of the databases.

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3.3.  Private Federations

   Private federations are a cooperative formed amongst a small number
   of participating domains.  The cooperative agrees to use a common
   technique for federation, and through it, is able to connect to each
   other.  There are many such federations in use today.

   Some of these federations rely on a central database, typically run
   by the federation provider, that can be queried by participating
   domains.  The database contains mappings from phone numbers to
   domains, and is populated by each of the participating domains, often
   manually.  Each domain implements an agreed-upon query interface that
   can be used to access the database when a number is called.
   Sometimes ENUM is used for this interface (called private ENUM),
   other times, a SIP redirection is used.  Some federations also
   utilize private IP networks in order to address QoS problems.  "SIP
   trunking" - a service being offered by many telecom operators as a
   SIP-based PRI replacement - is a form of private federation.

   Private federations work, but they have one major limitation:  scale.
   As the number of participating domains grows, several problems arise.
   Firstly, the size of the databases become unruly.  Secondly, the
   correctness of the database becomes an issue, since the odds of
   misconfigured numbers (either intentionally or accidentally)
   increases.  As the membership grows further, the odds increase that
   "bad" domains will be let in, introducing a source of spam and
   further problems.  The owner of the federation can - and often does -
   assume responsibility for this, and can attempt to identify and shut
   down misbehaving participants.  Indeed, as the size of the
   federations grow, the owner of the federation needs to spend
   increasing levels of capital on maintaining it.  This, in turn,
   requires them to charge money for membership, and this can be a
   barrier to entry.

4.  Key Requirements

   From the discussion on the problems of inter-domain federation and
   the solutions that have been attempted so far, several key
   requirements emerge:

   REQ-1:  The solution must allow for federation between any number of
   REQ-2:  The solution must enable users in one domain to identify
      users in another domain through the use of their existing E.164
      based phone numbers.

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   REQ-3:  The solution must work with deployments that utilize any kind
      of endpoint, including non-IP phones connected through gateways,
      IP softphones and hardphones.
   REQ-4:  The solution must not require any change in user behavior.
      The devices and techniques that users have been using previously
      to make inter-domain calls must continue to work, but now result
      in inter-domain IP federation.
   REQ-5:  The solution must work worldwide, for any domain anywhere.
   REQ-6:  The solution must not require any new services from any kind
      of centralized provider.  A domain should be able, of its own
      free-will and accord, to deploy equipment and connect to the
   REQ-7:  The solution must not require any prior arrangement between
      domains in order to facilitate federation between those domains.
      Federation must occur opportunistically - connections established
      when they can be.
   REQ-8:  The solution must work for domains of any size - starting at
      a single phone to the largest telecom operator with tens of
      millions of numbers.
   REQ-9:  The solution must have built-in mechanisms for preventing
      spam and DoS attacks.  These mechanisms must be fully automated.
   REQ-10:  The solution must not require any processing whatsoever by
      SIP or RTP intermediaries.  It must be possible for a direct SIP
      connection to be established between participating domains.
   REQ-11:  The solution should provide a mechanism for removal of
      cached routes in cases of failures of VIPR calls using the
      specific route.
   REQ-12:  The solution should support the handover of a call across
      domains between different SIP providers.

5.  Executive Overview

   Verification Involving PSTN Reachability (VIPR) is a new technology
   that is aimed at solving the problems that have prevented large-scale
   Internet-based SIP federation of voice and video.  VIPR solves these
   problems by creating a hybrid of three technologies - the PSTN
   itself, a Peer to Peer (P2P) network, and SIP.  By combining all
   three, VIPR enables an incrementally deployable solution to

5.1.  Key Properties

   VIPR has several important properties that enable it to solve the
   federation problem:

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   Works With Numbers:  VIPR enables federation for existing PSTN
      numbers.  It does not require users or administrators to know or
      configure email-style identifiers.  It does not require the
      allocation of new numbers.  It does not require a change in user
      behaviors.  Whatever way users were dialing numbers yesterday,
      works with VIPR tomorrow.
   Works with Existing Endpoints:  VIPR does not require any changes to
      endpoints.  Consequently, it works with existing SIP endpoints, or
      with non-IP endpoints connected through gateways.
   Verified Mappings:  The biggest issue in mapping from a phone number
      to a domain or IP address, is determining whether the mapping is
      correct.  Does that domain really own the given phone number?
      While solutions like ENUM have solved this problem by relying on
      centralized delegations of authorization, VIPR provides a secure
      mapping in a fully distributed way.  VIPR guarantees that phone
      calls cannot be misrouted or numbers stolen.
   Worldwide:  VIPR works worldwide.  Any domain that is connected to
      both the PSTN and the Internet can participate.  It doesn't matter
      whether the domain is in Africa, the Americas, or Australia.
      Since VIPR does not depend on availability of any regional
      services beyond IP and PSTN access - both of which are already
      available globally - VIPR itself is globally available.
   Unlimited Scale:  VIPR has nearly infinite scale.  Any number of
      domains can participate.
   Self-Scale:  VIPR self-scales.  This means that the amount of
      computation, memory, and bandwidth that a domain must deploy
      scales in direct proportion to the size of their own user base.
   Self-Learning:  VIPR is completely automated.  A domain never, ever
      has to configure any information about another domain.  It never
      has to provision IP addresses, domain names, certificates, phone
      number prefixes or routing rules.  Without any prior coordination,
      VIPR enables one domain to connect to a different domain.
   Automated Anti-Spam  VIPR comes with a built-in mechanism for
      preventing SIP spam.  This mechanism is new, and specific to SIP.
      In this way, it is fundamentally different from existing SIP anti-
      spam techniques which borrow from email [RFC5039].  This new
      technique is fully automated, and requires no configuration by
      administrators and no participation from end users.  Though it is
      not a 100% solution to the problem, it brings substantial economic
      and legal ammunition to the table to act as a good deterrent for a
      long while.
   Feature Velocity:  VIPR enables direct SIP connections between two
      domains seeking to federate.  There are no SIP intermediaries of
      any sort between the two.  This means that domains have no
      dependencies on intermediaries for deployment of new features.

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   Designed for the Modern Internet:  VIPR is built to run on the modern
      Internet.  It assumes the worst from everyone.  It assumes limited
      connectivity.  It assumes network failures.  It assumes there are
      attackers seeking to eavesdrop calls.  Security is built-in and
      cannot be disabled.
   Reliable:  VIPR is reliable.  Through its hybridization of the PSTN
      and the Internet, it makes sure that calls always go through.
      Indeed, to route a call between domains A and B, VIPR never
      depends on a server or service anywhere outside of domains A and B
      (besides vanilla PSTN and IP access) being operational.

   Given the assumptions that have traditionally been made about how
   federation has to work, these properties are impossible to realize.
   It is only by stepping back, and rethinking these fundamental
   assumptions, that a solution can be found.

5.2.  Challenging Past Assumptions

   Two unstated assumptions of SIP federation are challenged by VIPR.

   The first assumption that federation solutions have made is this:
      The purpose of SIP federation is to eliminate the PSTN, and
      consequently, we cannot assume the PSTN itself as part of the
   Though unstated, this assumption has clearly been part of the design
   of existing solutions.  SIP federation based on email-style URIs, as
   defined in RFC 3261, doesn't utilize or make mention of the PSTN.
   Solutions like ENUM, or private registries, do not utilize or make
   mention of the PSTN.  In one sense, it's obvious that they shouldn't
   - after all, the purpose is to replace the PSTN.  However, such an
   approach ignores an incremental solution - a solution which utilizes
   the PSTN itself to solve the hard problems in SIP federation.

   After all, the PSTN has accomplished a great deal.  It reaches
   worldwide.  It provides a global numbering translation service that
   maps phone numbers to circuits.  It is highly reliable, and provides
   QoS.  It has been built up over decades to achieve these goals.  This
   begs the question - can we build upon the capabilities already
   provided by the PSTN, and use them to solve the problems that plague
   SIP federation?  Indeed, the answer is yes once another assumption is

   This second assumption is:
      A federation solution must be the same as the final target
      federation architecture, and not just a step towards it.
   Though unstated, this assumption has also been true.  SIP's email-
   style federation was a pure 'target architecture' - the place we want
   to get to.  ENUM was the same - a worldwide global DNS database with

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   everyone's phone numbers providing open connectivity.

   Historically, technologies are more successful when they are
   incrementally deployable.  Indeed, in many cases, a target
   architecture is unrealizable because there is no obvious way to get
   there.  As such, the focus needs to be on the next incremental step
   and that step in turn creates the technological and market pressures
   that will drive the next step.  In the end, the target may not be the
   perfect solution originally envisioned, but we've at least arrived.

   As such, VIPR is very much focused on incremental deployability.  It
   is not the end of the federation story, it is the beginning.  It
   discards the notion of perfect IP federation for a solution that
   federates most, but not all calls, by relying on the PSTN to fill in
   the gaps.

5.3.  Technical Overview

   A high level view of the VIPR architecture is shown in Figure 1.  A
   more detailed description of the functional components of this
   architecture is provided in draft-petithuguenin-vipr-framework.

   In the VIPR architectural model, call agents Alice and Bob are shown
   in two different domains - example.com and example.net respectively.
   There are two additional domains, example.org and example.edu, with
   all four domains federated using VIPR technology.  Each domain is
   connected to both the public Internet and to the traditional PSTN.
   For simplicity, the connection for the Call Agents in example.org and
   example.edu to the PSTN is not indicated in the diagram as that
   interface is not relevant to the subsequent examples.

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                         +-------+    +-------+
                         |  Call |    |  Call |
          example.org    | Agent |    | Agent |  example.edu
                         |       |    |       |
                         +-------+    +-------+
                             \           /
                              \         /
                               \       /
                                \     /
                           |//          \\|
                          |    Internet    |
             +-------+     |\\          //|    +-------+
             |  Call |------ \\ _______//------|  Call |
     //\\    | Agent |                         | Agent |    //\\
     \  /    |       |                         |       |    \  /
      \/  ---|       |      +-----------+      |       |---- \/
      Alice  |       |======|           |======|       |    Bob
             +-------+      |    PSTN   |      +-------+
             example.com    |           |    example.net

                     Figure 1: High Level Architecture

   For purposes of explanation, it is easiest to think of each domain as
   having a single call agent which participates in the federation
   solution.  The functionality is decomposed into several sub-
   components, and this is discussed in more detail below.  The call
   agent is connected to one or more phones in the domain, and is
   responsible for routing calls, handling features, and processing call
   state.  The call agent is stateful, and is aware of when calls start
   and stop.

   Assume that all four domains have a 'fresh' installation of VIPR, and
   that domain example.net 'owns' +1 408 555 5xxx, a block of 1000
   numbers allocated by its PSTN provider.

   The VIPR mechanism can be broken into four basic steps:  storage of
   phone numbers, PSTN first call, validation and caching, and
   subsequent SIP call(s).

5.3.1.  Storage of Phone Numbers

   The first step is that the call agents form a single, worldwide P2P
   network, using a VIPR specific usage

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   [I-D.petithuguenin-vipr-reload-usage] of RELOAD
   [I-D.ietf-p2psip-base] with a variant of the Chord algorithm.  This
   P2P network forms a distributed hash table (DHT) running amongst all
   participating domains.  A distributed hash table is like a simple
   database, allowing storage of key-value pairs, and lookup of objects
   by key.  Unlike a normal hash table, which resides in the memory of a
   single computer, a distributed hash table is spread across all of the
   servers which make up the P2P network.  In this case, it is spread
   across all of the domains participating in the VIPR federation.

   The neat trick solved by the variant of the Chord algorithm (and by
   other DHT algorithms), is an answer to the following:  given that the
   desired operation is to read or write an object with key K, which
   node in the DHT is the box that currently stores the object with that
   key?  The P2P SIP variant of the Chord algorithm provides a clever
   algorithm which routes read and write operations through nodes in the
   DHT until they eventually arrive at the right place.  With Chord,
   this will take no more than log2N hops, where N is the number of
   nodes in the DHT.  Consequently, for a DHT with 1024 nodes, 10 hops
   are required in the worst case.  For 2048, 11 hops.  And so on.  The
   logarithmic factor allows DHTs to achieve incredible scale and to
   provide enormous storage summed across all of the nodes that make up
   the DHT.

   This logarithmic hopping behavior also means that each node in the
   DHT does not need to establish a TCP/TLS connection to every other
   node.  Rather, connections are established to a smaller subset - just
   log(N) of the nodes.

   In DHTs, each participating entity is identified by a Node-ID.  The
   Node-ID is a 128 bit number, assigned randomly to each entity.  They
   have no inherent semantic meaning; they are not like domain names or
   IP addresses.

   In the case of VIPR, each call agent is identified by one or more
   Node-IDs.  For purposes of discussion, consider the case where the
   call agent has just one Node-ID.  Each participating domain,
   including example.net in our example, uses the DHT to store a mapping
   from each phone number that it owns, to the domain's Node-ID.  In the
   case of example.net, it would store 1000 entries into the DHT, each
   one being a mapping from one of its phone numbers, to the domain's
   Node-ID.  Furthermore, when the mappings are stored, the mapping is
   actually from the SHA-1 hash of the phone number, to the Node-ID of
   the call agent which claims ownership of that number.

   For example, if the Node-ID of the call agent in domain example.net
   is 0x1234 (a shorter 16 bit value to simplify discussion), the
   entries stored into the DHT by example.net would be:

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      Key             |    Value
   SHA1(+14085555000)  |   0x1234
   SHA1(+14085555001)  |   0x1234
   SHA1(+14085555002)  |   0x1234
   SHA1(+14085555999)  |   0x1234

                          Figure 2: DHT Contents

   It is important to note that the DHT does not contain phone numbers
   (it contains hashes of them), nor does it contain IP addresses or
   domain names.  Instead, it is a mapping from the hash of a phone
   number (in E.164 format) to a Node-ID.

   example.net will store this mapping when it starts up, or when a new
   number is provisioned.  The information is refreshed periodically by
   example.net.  The actual server on which these mappings are stored
   depends on the variant of the Chord algorithm.  Typically, the
   entries will be uniformly distributed amongst all of the call agents
   participating in the network.

5.3.2.  PSTN First Call

   At some point, a user (Alice) in example.com makes a call to +1 408
   555 5432, which is her colleague Bob. Even though both sides have
   VIPR, the call takes place over the plain old PSTN, per Figure 3.
   Alice talks to Bob for a bit, and they hang up.

             +-------+                         +-------+
             |  Call |                         |  Call |
     //\\    | Agent |                         | Agent |    //\\
     \  /    |       |                         |       |    \  /
      \/  ---|       |      +-----------+      |       |---- \/
      Alice  |       |<=======<========>======>|       |    Bob
             +-------+      |    PSTN   |      +-------+
             example.com    |           |    example.net

                         Figure 3: PSTN First Call

   At a random point in time after the call has completed, the call
   agent in example.com "wakes up" and says to itself, "that's
   interesting, someone in my domain called +1 408 555 5432, and it went

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   over the PSTN.  I wonder if that number is reachable over IP
   instead?".  To make this determination, it hashes the called phone
   number, and looks it up in the DHT.  It is important to note that
   this lookup is not at the time of an actual phone call - this lookup
   process happens outside of any phone call, and is a background

   The query for +1 408 555 5432 will traverse the DHT, and eventually
   arrive at the node that is responsible for storing the mapping for
   that number.  Typically, that node will not be example.net, but
   rather one of the other nodes in the network (e.g., example.org).  In
   many cases, the called number will not find a matching mapping in the
   DHT.  This happens when the number that was dialed is not owned by a
   domain participating in VIPR.  When that happens, example.com takes
   no further action.  Next time there is another call to the same
   number, it will repeat the process and check once more whether the
   dialed number is in the DHT.

   In this case, there is a match in the DHT, and example.com learns the
   Node-ID of example.net.  It then proceeds to the validation step per
   Section 5.3.3.  It is also possible that there are multiple matches
   in the DHT.  This can happen if another domain - example.edu for
   example - also claims ownership of that number.  When there are
   multiple matching results, example.com learns all of them, and
   performs the validation step with each.

5.3.3.  Validation and Caching

   Why not just store the domain in the DHT, instead of the Node-ID?  If
   the domain was stored in the DHT, once example.com performed the
   lookup, it would immediately learn that the number maps to
   example.net, and could then make a direct SIP call next time.

   The main reason this doesn't work is security.  The information in
   the DHT is completely untrusted.  There is nothing so far that
   enables example.com to know that example.net does, in fact, own the
   phone number in question.  Indeed, if multiple domains make a claim
   on the number, it has no way to know which one (if any) actually owns

   To address this critical problem, VIPR requires a mechanism called
   phone number validation.  Phone number validation is a key concept in
   VIPR.  There are several models for this validation as detailed in
   [I-D.petithuguenin-vipr-pvp].  The essential idea is that example.com
   will connect to the example.net server, by asking the DHT to form a
   connection to example.net's Node-ID.  Once connected, example.com
   demands proof of ownership of the phone number.  This proof comes in
   the form of demonstrated knowledge of the previous PSTN call.  When a

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   call was placed from example.com to +1 408 555 5432, the details of
   that call - including its caller ID, start time, and stop time,
   create a form of shared secret - information that is only known to
   entities that participated in the call.  Thus, to obtain proof that
   example.net really owns the number in question, example.com will
   demand a knowledge proof - that example.net is aware of the details
   of the call.  A consequence of this is that the following property is

      A domain can only call a specific number over SIP, if it had
      previously called that exact same number over the PSTN.

   This property is key in fighting spam and denial-of-service attacks.
   Because calling numbers on the PSTN costs money - especially
   international calls - VIPR creates a financial disincentive for
   spammers.  For a spammer to ring every phone in a domain with a SIP
   call, it must have previously called every number in the domain with
   a PSTN call, and had a successfully completed call to each and every
   one of them.  [I-D.petithuguenin-vipr-sip-antispam] provides an
   overview and further details on the security mechanisms for VIPR for
   mitigation of SPAM.

   There are a great many details required for this validation protocol
   to be secured.  For example, the mechanism needs to handle the fact
   that call start and stop times won't exactly match on both sides.  It
   needs to deal with the fact that many calls start on the top of the
   hour.  It needs to deal with the fact that caller ID is not often
   delivered, and when it is delivered, is not reliable.  It needs to
   deal with the fact that example.com may in fact be the attacker,
   trying to use the validation protocol to extract the shared secret
   from example.net.  All of this is, in fact, handled by the protocol.
   The protocol is based on the Secure Remote Password for TLS
   Authentication (SRP-TLS) [RFC5054], and is described more fully in

   Towards the end of the validation process, domains example.com and
   example.net had determined that each was, in fact in possession of
   the shared secret information about the prior PSTN call.  However,
   neither side has any information about the domain names of the other

   At the end of the validation process, both example.com and
   example.net have been able to ascertain that the other side did in
   fact participate in the previous PSTN call.  At that point,
   example.com sends its domain name to example.net as shown in
   Figure 4.

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                         +-------+    +-------+
                         |  Call |    |  Call |
          example.org    | Agent |    | Agent |  example.edu
                         |       |    |       |
                         +-------+    +-------+
                            \             /
   +----------------------+  \           /
   | Hi, I am example.com.|   \         /
   | How do I reach you?  |    \       /
   +--------------\-------+  //-------\\
                   \        //         \\
                ^          |    Internet  |        |
                |          |              |        v
             +-------+     |\\          //|    +-------+
             |  Call |------ \\ _______//------|  Call |
     //\\    | Agent |                         | Agent |    //\\
     \  /    |       |                         |       |    \  /
      \/  ---|       |                         |       |---- \/
      Alice  |       |                         |       |    Bob
             +-------+                         +-------+
             example.com                      example.net

                    Figure 4: Ticket Validation Step 1

   Next, the example.net domain generates the ticket.  The ticket has
   three fundamental parts to it:

   1.  The phone number that was just validated - in this case, +1 408
       555 5432.
   2.  The domain name that the originating side claims it has -
       example.com in this case.
   3.  A signature generated by example.net, using a key known to itself
       only, over the other two pieces of information.

   Then, example.net sends to example.com - all over a secured channel -
   a SIP URI to use for routing calls to this number, and a ticket, as
   shown in Figure 5.  The ticket is a cryptographic object, opaque to
   example.com, but used by example.net to allow incoming SIP calls.  It
   is similar in concept to kerberos tickets - it is a grant of access.
   In this case, it is a grant of access for example.com to call +1 408
   555 5432, and only +1 408 555 5432.

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                         +-------+    +-------+
                         |  Call |    |  Call |
          example.org    | Agent |    | Agent |  example.edu
                         |       |    |       |
                         +-------+    +-------+
                            \             /
                             \           /    +------------------------+
                              \         /     | Here is your ticket    |
                               \       /      | & SIP URI to reach Bob |
                             //-------\\      +----/-------------------+
                            //         \\         /
                |          |    Internet  |        ^
                v          |              |        |
             +-------+     |\\          //|    +-------+
             |  Call |------ \\ _______//------|  Call |
     //\\    | Agent |                         | Agent |    //\\
     \  /    |       |                         |       |    \  /
      \/  ---|       |                         |       |---- \/
      Alice  |       |                         |       |    Bob
             +-------+                         +-------+
             example.com                       example.net

                    Figure 5: Ticket Validation Step 2

   The example.com call agent receives the SIP URI and ticket, and
   stores both of them in an internal cache.  This cache builds up
   slowly over time, containing the phone number, SIP URI, and ticket,
   for those numbers which are called by example.com and validated using
   VIPR.  Because the cache entries are only built for numbers which
   have actually been called by users in the enterprise, the size of the
   cache self-scales.  A call agent supporting only ten users will build
   up a cache proportional to the volume of numbers called by ten
   people, whereas a call agent supporting ten thousand users will build
   up a cache which is typically a thousand times larger.

   This cache, containing the phone number, SIP URI and ticket will be
   accessed later when Alice (or another caller from the same call
   agent) makes another call to Bob, as detailed in Section 5.3.4.

5.3.4.  SIP Call

   At some point in the future, another call is made to +1 408 555 5432.
   The caller could be Alice, or it could be any other user attached to
   the same call agent.  This time, the call agent notes that it has a

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   cached route for the number in question, along with a SIP URI that
   can be used to reach that route.  It also has a ticket.  It is
   important to note that there may be multiple routes for a given
   number.  For example, both an Enterprise and Service Provider may
   register the same number in the RELOAD distributed database.  It may
   also be possible to fork a call using the multiple routes.  [Editor's
   note:  this requires further discussion as to whether we want to
   allow this functionality.]

   The example.com call agent attempts to contact the SIP URI by
   establishing a TCP/TLS connection to the SIP URI it learned.  If a
   connection cannot be made and there are no other cached routes (e.g.,
   at the next call agent in the path) for the number in question, the
   call agent proceeds with the call over the PSTN.  This ensures that,
   in the event of an Internet failure or server failure, the call can
   still proceed.  Assuming the connection is established, the
   example.com call agent sends a SIP INVITE to the terminating call
   agent, over this newly formed secure connection.  The SIP INVITE
   request also contains the ticket, placed into a new SIP header field
   in the message.

   When the SIP INVITE arrives at the example.net call agent, the call
   agent can extract the ticket from the new SIP header field.  This
   ticket is an object, opaque to example.com, that was previously
   generated by the example.net call agent as described in Section 5.3.3
   . example.net first verifies the signature over the ticket.  Remember
   that the example.net agent is the one that generated the ticket in
   the first place; as such, it is in possession of the key required to
   validate the signature.  Once validated, it performs two checks:

   1.  It compares the phone number in the call setup request (the
       Request URI) against the phone number stored in the ticket.
   2.  It compares the domain name of the calling domain, learned from
       the certificates in the mutual TLS exchange, against the domain
       name stored in the ticket.

   If both match, the example.net call agent knows that the calling
   party is in fact the domain they claimed previously, and that they
   had in fact gone through the validation process successfully for the
   number in question.  At this time, the call is now completed per
   normal SIP processing.

6.  Security Considerations

   Security is incredibly important for VIPR.  This section provides an
   overview of some of the key threats and how they are handled.

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6.1.  Attacks on the DHT

   Attackers could attempt to disrupt service through a variety of
   attacks on the DHT.

   Firstly, it must be noted that the DHT is never used at call setup
   time.  It is accessed as a background task, solely to learn NEW
   numbers and routes that are not already known.  If, by some tragedy,
   an attacker destroyed the P2P network completely, it would not cause
   a single call to fail.  Furthermore, it would not cause calls to
   revert to the PSTN - calls to routes learned previously would still
   go over the IP network.  The only impact to such a devastating
   attack, is that a domain could not learn *new* routes to new numbers,
   until the DHT is restored to service.  This service failure is hard
   for users and administrators to even notice.

   That said, VIPR prevents many of these attacks.  The DHT itself is
   secured using TLS - its usage is mandatory.  Quota mechanisms are put
   into place that prevent an attacker from storing large amounts of
   data in the DHT.  Other attacks are prevented by mechanisms defined
   by RELOAD itself, and are not VIPR specific.

6.2.  Theft of Phone Numbers

   The key security threat that VIPR is trying to address is the theft
   of phone numbers.  In particular, a malicious domain could store,
   into the DHT, phone numbers that it does not own, in an attempt to
   steal calls targeted to those numbers.  This attack is prevented by
   the core validation mechanism, which performs a proof of knowledge
   check to verify ownership of numbers.

   An attacker could try to claim numbers it doesn't own, which are
   claimed legitimately by other domains in the VIPR network.  This
   attack is prevented as well.  Each domain storing information into
   the DHT can never overwrite information stored by another domain.  As
   a consequence, if two domains claim the same number, two records are
   stored in the DHT.  An originating domain will validate against both,
   and only one will validate - the real owner.

   An attacker could actually own a phone number, use it for a while,
   validate with it, and build up a cache of routes at other domains.
   Then, it gives back the phone number to the PSTN provider, who
   allocates it to someone else.  However, the attacker still claims
   ownership of the number, even though they no longer have it.  This
   attack is prevented by expiring the learned routes after a while.
   Typically, operators do not re-assign a number for a few months, to
   allow out-of-service messages to be played to people that still have
   the old number.  Thus, the TTL for cached routes is set to match the

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   duration that carriers typically hold numbers.

   An attacker could advertise a lot of numbers, most of which are
   correct, some of which are not.  VIPR prevents this by requiring each
   number to be validated individually.

   An attacker could make a call so they know the call details of the
   call they made and use this to forge a validation for that call.
   They could then try to convince other users, which would have to be
   in the same domain as the attacker, to trust this validation.  This
   is mitigated by not sharing validations inside of domains where the
   users that can originate call from that domain are not trusted by the

6.3.  Spam

   Another serious concern is that attackers may try to launch SIP spam
   (also known as SPIT) calls into a domain.  As described in
   Section 5.3.3, VIPR prevents this by requiring that a domain make a
   PSTN call to a number before it will allow a SIP call to be accepted
   to that same number.  This provides a financial disincentive to
   spammers.  The current relatively high cost of international calling,
   and the presence of national do-not-call regulations, have prevented
   spam on the PSTN to a large degree.  VIPR applies those same
   protections to SIP connections.

   VIPR still lowers the cost of communications, but it does so by
   amortizing that savings over a large number of calls.  The costs of
   communications remain high for infrequent calls to many numbers, and
   become low for frequent calls to a smaller set of numbers.  Since the
   former is more interesting to spammers, VIPR gears its cost
   incentives away from the spammers, and towards domains which
   collaborate frequently.

   It is important to note that VIPR does not completely address the
   spam problem.  A large spamming clearing house organization could
   actually incur the costs of launching the PSTN calls to numbers, and
   then, in turn, act as a conduit allowing other spammers to launch
   their calls to those numbers for a fee.  The clearinghouse would
   actually need to transit the signaling traffic (or, divulge the
   private keys to their domain name), which would incur some cost.  As
   such, while this is not an impossible situation, the barrier is set
   reasonably high to start with - high enough that it is likely to
   deter spammers until it becomes a highly attractive target, at which
   point other mechanisms can be brought to bear.  This is, again, an
   example of the incremental deployability philosophy of VIPR.

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6.4.  Eavesdropping

   Another class of attacks involves outsiders attempting to listen in
   on the calls that run over the Internet, or obtain information about
   the call through observation of signaling.

   All of these attacks are prevented by requiring the usage of SIP over
   TLS and SRTP.  These are mandatory to use.

7.  IANA Considerations

   This specification does not require any actions from IANA.

8.  Acknowledgements

   Thanks for review comments from Ken Fischer, Rob Maidhof, Michael
   Procter, and others.  Thanks to Theo Zourzouvillys for pointing out
   the 5th thief of phone numbers attack.

9.  References

9.1.  Normative References

              Jennings, C., Lowekamp, B., Rescorla, E., Baset, S., and
              H. Schulzrinne, "REsource LOcation And Discovery (RELOAD)
              Base Protocol", draft-ietf-p2psip-base-18 (work in
              progress), August 2011.

              Rosenberg, J., Jennings, C., and M. Petit-Huguenin, "A
              Usage of Resource Location and Discovery (RELOAD) for
              Public Switched Telephone Network (PSTN) Verification",
              draft-petithuguenin-vipr-reload-usage-02 (work in
              progress), July 2011.

              Rosenberg, J., Jennings, C., and M. Petit-Huguenin,
              "Session Initiation Protocol (SIP) Extensions for Blocking
              VoIP Spam Using PSTN Validation",
              draft-petithuguenin-vipr-sip-antispam-02 (work in
              progress), July 2011.

              Jennings, C., Rosenberg, J., and M. Petit-Huguenin,

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              "Verification Involving PSTN Reachability: The ViPR Access
              Protocol (VAP)", draft-jennings-vipr-vap-01 (work in
              progress), July 2011.

              Rosenberg, J., Jennings, C., and M. Petit-Huguenin, "The
              Public Switched Telephone Network (PSTN) Validation
              Protocol (PVP)", draft-petithuguenin-vipr-pvp-01 (work in
              progress), June 2011.

9.2.  Informative References

   [RFC2543]  Handley, M., Schulzrinne, H., Schooler, E., and J.
              Rosenberg, "SIP: Session Initiation Protocol", RFC 2543,
              March 1999.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3263]  Rosenberg, J. and H. Schulzrinne, "Session Initiation
              Protocol (SIP): Locating SIP Servers", RFC 3263,
              June 2002.

   [RFC5039]  Rosenberg, J. and C. Jennings, "The Session Initiation
              Protocol (SIP) and Spam", RFC 5039, January 2008.

   [RFC6116]  Bradner, S., Conroy, L., and K. Fujiwara, "The E.164 to
              Uniform Resource Identifiers (URI) Dynamic Delegation
              Discovery System (DDDS) Application (ENUM)", RFC 6116,
              March 2011.

   [RFC5067]  Lind, S. and P. Pfautz, "Infrastructure ENUM
              Requirements", RFC 5067, November 2007.

   [RFC5054]  Taylor, D., Wu, T., Mavrogiannopoulos, N., and T. Perrin,
              "Using the Secure Remote Password (SRP) Protocol for TLS
              Authentication", RFC 5054, November 2007.

              Salgueiro, G. and V. Gurbani, "Format for the Session
              Initiation Protocol (SIP) Common Log Format (CLF)",
              draft-ietf-sipclf-format-02 (work in progress),
              September 2011.

              Jones, P., Salgueiro, G., Polk, J., R, P., Liess, L.,

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              Jesske, R., Loreto, S., and H. Kaplan, "Requirements for
              an End-to-End Session Identification in IP-Based
              Multimedia Communication Networks",
              draft-jones-ipmc-session-id-reqts-00 (work in progress),
              May 2011.

Appendix A.  Changes since last version

   This section must be removed before publication as an RFC.

   Modifications between jennings-02 and jennings-01:

   1.  Sections 6,7,8 moved to new VIPR framework document.
   2.  Editorial changes.
   3.  Clarifications to re-enforce that the primary objective is not
       PSTN bypass but rather to enable enhanced services such as video
       between domains.  Changed "VoIP" to "SIP" since the focus is not
       specifically voice.
   4.  Added reference for new framework document.
   5.  Section 5.3:  Added references to other documents as appropriate
       - e.g., -pvp, -spam, etc.
   6.  Moved validation diagrams and text (from 5.3.4) into Validation
       and caching section (5.3.3).
   7.  Condensed discussion of spam in section 5.3.3 and updated SPAM
       section in security section.

   Modifications between jennings-01 and rosenberg-04:

   o  Not specified.

   Modifications between rosenberg-04 and rosenberg-03

   o  Nits.
   o  Shorter I-Ds references.
   o  Changed phone numbers to follow E.123 presentation.
   o  Expanded P2P initialisms.
   o  Uses +1 408 555 prefix for phone numbers in examples.

Barnes, Ed., et al.      Expires March 31, 2012                [Page 26]

Internet-Draft                VIPR Overview               September 2011

Authors' Addresses

   Mary Barnes

   Email:  mary.ietf.barnes@gmail.com

   Cullen Jennings
   170 West Tasman Drive
   MS: SJC-21/2
   San Jose, CA  95134

   Phone:  +1 408 421-9990
   Email:  fluffy@cisco.com

   Jonathan Rosenberg
   Monmouth, NJ

   Email:  jdrosen@jdrosen.net
   URI:    http://www.jdrosen.net

   Marc Petit-Huguenin

   Email:  marc@stonyfish.com

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