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Internet Engineering Task Force                     K. Fleischhauer, Ed.
Internet-Draft                                                O. Bonness
Intended status: Informational                       Deutsche Telekom AG
Expires: March 3, 2013                                   August 30, 2012

    On demand IPv4 address provisioning in Dual-Stack PPP deployment


   Today the Dual-Stack approach is the most straightforward and the
   most common way for introducing IPv6 into existing systems and
   networks.  However a typical drawback of implementing Dual-Stack is
   that each node will still require at least one IPv4 address.  Hence,
   solely deploying Dual-Stack does not provide a sufficient solution to
   the IPv4 address exhaustion problem.  Assuming a situation where most
   of the IP communication (e.g. always-on, VoIP etc.) can be provided
   via IPv6, the usage of public IPv4 addresses can significantly be
   reduced and the unused public IPv4 addresses can under certain
   circumstances be returned to the public IPv4 address pool of the
   service provider.  New Dual-Stack enabled services can be introduced
   without increasing the public IPv4 address demand, whereas IPv6 will
   be the preferred network layer protocol.  This document describes
   such a solution in a Dual-Stack PPP session network scenario and
   explains the protocol mechanisms which are used.

Status of this Memo

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   This Internet-Draft will expire on March 3, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the

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   document authors.  All rights reserved.

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   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

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Table of Contents

   1.  Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Problem Statement and Purpose of IPv4 address efficiency . . .  4
     2.1.  Illustrative service provider use case . . . . . . . . . .  5
     2.2.  Architecture and Communication in a PPP Dual-Stack
           environment  . . . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  The advantage of the dynamic IPv4 address assigning
           feature  . . . . . . . . . . . . . . . . . . . . . . . . .  9
   3.  Specification  . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  Definition of the participating elements and their
           functionalities  . . . . . . . . . . . . . . . . . . . . . 11
     3.2.  Assigning IPv4 address parameter on-demand after
           establishing PPP session with IPv6 connectivity  . . . . . 12
     3.3.  Releasing unused IPv4 address parameters . . . . . . . . . 13
     3.4.  Timer Considerations . . . . . . . . . . . . . . . . . . . 14
   4.  Potential for optimization . . . . . . . . . . . . . . . . . . 15
     4.1.  Avoiding unnecessary load on BRAS/NAS and AAA  . . . . . . 15
     4.2.  Reducing IPv4 traffic on external interfaces . . . . . . . 16
   5.  Impacts on user experience and operation . . . . . . . . . . . 16
     5.1.  Impacts on user experience and Happy Eyeballs
           implementations  . . . . . . . . . . . . . . . . . . . . . 16
     5.2.  Operational impacts  . . . . . . . . . . . . . . . . . . . 17
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  Normative Reference  . . . . . . . . . . . . . . . . . . . 19
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Appendix A.  Workplan  . . . . . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20

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1.  Abstract

   The Dual-Stack approach as defined in [RFC4213] provides the most
   straightforward and most common way for introducing IPv6 [RFC2460]
   into existing systems and networks.  However, an inherent drawback of
   usual Dual-Stack deployment scenarios according to [RFC4213] section
   2 is that network nodes will still require at least one IPv4
   [RFC0791] address.  A primary concern for most operators whose IPv6
   deployment strategy relies upon the deployment of Dual-Stack
   architectures is hence focused on the ability to rationalize the
   usage of its global IPv4 address blocks while encouraging the use of

   Assuming now a situation where most of the IP communication (e.g.
   always-on, VoIP, etc.) can be provided via IPv6, the usage of public
   IPv4 addresses can be reduced significantly and the operators need
   mechanisms and solutions in order to release unused IPv4 address
   resources of Dual-Stack nodes and reallocate them later on, on
   demand.  This document describes how such a solution can be deployed
   in a Dual-Stack PPP session scenario and details the protocol
   mechanisms of the solution which are also thought as contribution to
   [BBF-TR-242].  Furthermore it should be mentioned at this point that
   the sketched solution approach can also serve as general IPv4 sun
   setting approach for Dual-Stack PPP sessions, since it provides the
   possibility to return unused IPv4 addresses of Dual-Stack PPP
   sessions and transforming them into pure single stack IPv6 PPP

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119]

2.  Problem Statement and Purpose of IPv4 address efficiency

   The Broadband Forum describes in [BBF-TR-187] a target IPv4/IPv6
   Dual-Stack Architecture.  TR-187 builds on the capabilities of
   existing protocols such as Point-to-Point Protocol (PPP) [RFC1661]
   and Layer 2 Tunnelling Protocol (L2TP) [RFC2661] to provide IPv6
   service in addition to today's IPv4 service.  These protocols allow
   the parallel usage of IPv4 and IPv6 within a single PPP respectively
   L2TP session.  Usually in such a scenario the service provider
   assigns both, a global IPv4 address and also IPv6 address/prefix
   parameter, to the CPE deployed in the customer's premises for the
   whole duration of the PPP session.  Because of the potential parallel
   usage of IPv4 and IPv6 within such a Dual-Stack PPP scenario a public

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   IPv4 address is always provisioned, also in the (future) case where
   it is assumed that most (or even all) of the communication is running
   on top of IPv6.  This document extends the sketched Dual-Stack
   deployment scenario for PPP and L2TPv2 with a mechanism that allows a
   temporary assignment and a release of an unused IPv4 address.  This
   approach covers also situations where the IPv4 address may only be
   provided on-demand later on, after initiating the Dual-Stack PPP
   session with an IPv6 context only.  For a service provider using this
   mechanism it is assumed that a valuable increase of IPv4 address
   efficiency due to a time based sharing of complete IPv4 addresses can
   be achieved.

   Basically, the mechanism is also applicable to cable and mobile
   networks.  The corresponding DOCSIS and 3GPP standards may be adapted
   as a follow-on work to this draft later on.

2.1.  Illustrative service provider use case

   In order to illustrate the applicability and usefulness of the
   proposed "On demand IPv4 address provisioning" mechanism an
   illustrative network operator use case is provided in this section.
   Let's assume a network access and service provider which is offering
   Dual-Stack services via a single PPP connection to its customers,
   hence assuming a PPP encapsulation scheme.  Independently of the
   nature and the number of services subscribed by the customer,
   (Single, Play, Double Play etc.), all customers should be produced
   and provisioned in the same way in order to keep the network
   operation costs and the network complexity as low as possible.  Let's
   assume furthermore that the above mentioned network access and
   service provider has already migrated its VoIP service to IPv6, so
   that all Single play VoIP customers only need IPv6 connectivity and
   have no need for an IPv4 context within their Dual-Stack PPP session.
   However, the standard Dual-Stack PPP connection set-up today assumes
   the triggering of the IPCP negotiation phase, as well as an IPv6CP
   negotiation independently of the real need for IPv4 and/or IPv6
   connectivity, so that after a successful Dual-Stack PPP connection
   establishment the PPP client site is provisioned with a complete set
   of IPv6 and IPv4 connection parameters.  As a consequence in our
   example, the whole Single Play VoIP customer base of the network
   access and service provider has also been provisioned with public
   IPv4 addresses, although these customers will never need IPv4
   Internet connectivity during the whole lifetime of their PPP session.
   Hence a huge amount of not required and therefore unused IPv4
   addresses has been wasted, that should be better kept in the provider
   address pools and delegated to other customers that really need IPv4
   connectivity.  In order to allow a more dynamic and on-demand
   provisioning of IPv4 parameters within Dual-Stack PPP sessions, a new
   mechanism is needed, that requests and also releases IPv4 addresses

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   on-demand when they are really needed during the PPP session
   lifetime.  Such a mechanism is proposed and described within this

   (An additional advantage of such an on-demand IPv4 address releasing
   and provisioning mechanism consists in the fact that a straight-
   forward to operate and dynamic change in the customer profiles (e.g.
   upgrade of Single Play customers to Double Play services and vice
   versa) becomes possible with only minor changes to the customer
   service profile in the AAA platform of the service provider - no
   changes in the CPE or BRAS/NAS port configuration are needed.
   Besides that, this dynamic on-demand IPv4 address provisioning and
   releasing approach allows it to share one public IPv4 address in a
   timely sequential fashion between a bunch of customers.)

   The following sections describe the basic network architecture and
   the "On demand IPv4 address provisioning" mechanisms in more details.

2.2.  Architecture and Communication in a PPP Dual-Stack environment

   Assuming a Dual-Stack network access via PPP, terminal devices can
   communicate via IPv4 and/or IPv6 transport, depending on their own
   and their IP communication partner capabilities.  The actual usage of
   IPv4 or IPv6 or both protocols depends on the capabilities of

   o  the IP communication endpoints (e.g. protocol stack, applications,
      configuration of the preferences etc.),

   o  the network deployment itself (e.g. access network based on PPP,
      backbone network, Internet) and also on

   o  the used communication services (like e.g.  VoIP over IPv6).

   The last two points are mainly left to the responsibility of the
   network and service providers.  The approach, sketched in this
   document, is based on the operational scenario that the customer
   starts a Dual-Stack PPP session in "IPv6-only" mode first and "adds"
   IPv4 later on only in the case that applications or services
   explicitly require IPv4 connectivity.  When IPv4 connectivity is not
   needed during the whole duration of PPP network connectivity then a
   continuous provisioning of a global IPv4 address to the customer
   device (e.g. end system, CPE etc.) is not necessary.  Therefore
   mechanisms are needed to provision and release public IPv4 addresses
   for Dual-Stack PPP sessions dynamically and on-demand.

   The goal of the solution sketched in this document, is to limit and
   decrease the public IPv4 address pool size of the PPP network access
   provider and hence to better rationalize the usage of the remaining

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   IPv4 address blocks.  Assuming that always-on services are reachable
   via IPv6, a Dual-Stack-capable PPP connected customer side device
   should in any case request IPv4 address parameters only on demand,
   when the need for establishing IPv4 connectivity has been detected
   and there is a need to forward IPv4 traffic towards the PPP WAN
   interface (e.g. of a CPE).  Following this above sketched network
   scenario it is sufficient, when initially only IPv6 address
   parameters are provisioned to the PPP customer endpoint (e.g., end
   systems, CPE).

   This means as a consequence that a customer device does not initially
   start a complete Dual-Stack PPP session but an IPv6-only PPP session.
   The IPv4 part of the complete Dual-Stack is initiated later on only
   in the case that IPv4 connectivity is explicitly requested.

   Figure 1 below illustrates the network architecture of a PPP Dual-
   Stack environment for providing Internet access to residential

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                                 |       +------------+        |
                                 |       |  external  |        |
                                 |       |  Address   |        |
                                 |       |    Pool    |        |
                                 |       | Management |        |
                                 |       +------------+        |
                                 |      service | provider     |
                                 |          AAA | area         |
   +-------+                     +--------------|--------------+
   |Private|__                                  |
   |  Host |_ \IPv4                             |
   |   1   | \ \    +-------------+             |
   +-------+  \ \   |    CPE      |  IPv4       |
               \ \  |    App      |  over       |
            IPv6\ \_+------+------+  PPP   +---------+  IPv4  +--------+
                 \__| CPE  |  CPE |--------|BRAS/NAS |--------| Public |
                  __|Router|  PPP |        |  (PPP   |        |  Host  |
                 / _|      | Peer |--------|  Peer)  |--------|   n    |
            IPv6/ / +------+------+  IPv6  +---------+  IPv6  +--------+
               / /                   over
   +-------+  / /                     PPP
   |Private|_/ /
   |  Host |__/IPv4
   |    n  |

       Private Internet                     Public Internet

                   Figure 1: PPP Dual-Stack architecture

   This abstract network topology consists of 3 major components:

   1.  Private Internet (aka.  Customer LAN)

   2.  Public Internet (including access and service provider network)

   3.  Service Provider AAA area

   The focus of this draft is mainly directed to the access network of
   the service provider as part of the Public Internet, where in our
   scenario PPP is used between the CPE and the provider Network Access
   Server (BRAS, NAS) in order to provide public Internet access to the

   The Service Provider's AAA area is a network which consists of
   several systems that interact with the Network Access Servers and
   provide AAA functionalities.  Such Service Provider AAA

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   functionalities also include management of the public IPv4 and public
   IPv6 address and prefix pools inside the BRAS/NAS and can also be
   integrated directly into the BRAS/NAS.

2.3.  The advantage of the dynamic IPv4 address assigning feature

   The dynamic IPv4 address assigning approach, sketched in this
   document, is based on the operational approach that the customer CPE
   initiates a PPP session based on IPv6 and adds IPv4 later on only if
   certain IPv4 applications or services explicitly require IPv4
   connectivity.  A particular public IPv4 address can therefore be
   assigned consecutively to different customers for the lifetime of
   their IPv4 PPP connection and has not to be bound to a single
   customer for the whole lifetime of the Dual-Stack PPP session.  This
   consecutive assignment of public IPv4 addresses allows from a
   provider perspective a less complex IPv4-to-IPv6 migration in
   comparison to other IPv4-to-IPv6 migration approaches that are based
   on Carrier Grade NATs in service provider network (like e.g.  Dual-
   Stack lite (like e.g.  Dual-Stack lite [RFC6333]) or shared IPv4
   addresses, since no additional network devices have to be deployed
   and operated and the complete solution is based on simple extensions
   to already existing infrastructure components and processes.  The
   customer will be provisioned with a public IPv4 address only in the
   case when global IPv4 connectivity is really needed and will not be
   provisioned with an IPv4 address by default when the Dual-Stack PPP
   session is initiated.  Furthermore, a provisioned IPv4 address can be
   released (e.g., after a certain time interval) in case the CPE
   detects that there is no need any more for global IPv4 connectivity.
   In other words, when global IPv4 connectivity is not needed during
   the lifetime of the Dual-Stack PPP session then a (continuous)
   provisioning of a public IPv4 address to the CPE is not necessary and
   the provisioning of a public IPv4 address can be done on-demand and

   Hence, one of the main achievements of this mechanism is to limit and
   decrease the pool size for public IPv4 addresses at the service
   provider site.

   A similar effect in limiting and decreasing the IPv4 address demand
   can also be reached by using separate PPP sessions for IPv4 and IPv6.
   But in that case the following problems occur:

   o  For each additional PPP session additional AAA parameters have to
      be created and handled which leads to an extension of AAA domains
      and more complex processes.

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   o  Each additional PPP session will require additional resources on
      the PPP endpoints (e.g. for handling additional customer
      credentials) also in devices that act as PPP intermediate agents.

   o  Accounting and controlling of traffic classes on an access line or
      customer base will be impeded or at least complicated.

   Because of these reasons the introduction of an additional PPP
   session for IPv6 as additional network layer protocol on an access
   line with an additional PPP session is not recommended.

3.  Specification

   As defined in RFC 2661 [RFC2661] PPP and L2TP provide the following
   main functionalities:

   1.  A method for encapsulating datagrams over serial links.

   2.  A Link Control Protocol (LCP) for establishing, configuring, and
       testing the data-link connection.

   3.  (Optional) Authentication Protocol for one or both peers.

   4.  A family of Network Control Protocols (NCPs) for establishing and
       configuring different network-layer protocols.

   For provisioning of IPv4 or IPv6 communication parameters (e.g.
   addresses, DNS resolver) as network-layer protocols only the NCPs
   Internet Protocol (Version 4) Control Protocol (IPCP) RFC 1661
   [RFC1661] and Internet Protocol (Version 6) Control Protocol (IPv6CP)
   RFC 2472 [RFC2472] are used.  Whereas IPCP is responsible for
   configuring, enabling, and disabling the IPv4 protocol modules on
   both ends of the point-to-point link, IPv6CP is responsible for
   configuring, enabling, and disabling the IPv6 protocol modules on
   both ends of the point-to-point link.  Once one of the two network-
   layer protocols has been configured, datagrams belonging to this
   network-layer protocol can be sent over the PPP link.  Both NCP
   protocol mechanisms act independently of each other (see also
   requirement WLL-3 in [RFC6204]) and can be used to establish and
   pull-down IPv4 and IPv6 connection contexts within a Dual-Stack PPP
   session independently.

   As an example, an implementation that wishes to close a dedicated NCP
   connection (e.g., IPCP or IPv6CP) SHOULD transmit a Terminate-Request
   to the peer.  Upon reception of a Terminate-Request, a Terminate-Ack
   MUST be transmitted to the sender of the Terminate-Request.  The PPP
   session itself and the other NCP connection inside the PPP session

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   will remain existent.  Only in the case that both NCP connections are
   closed, the Dual-Stack PPP session will be terminated.

3.1.  Definition of the participating elements and their functionalities

   This chapter identifies the network elements that are involved in the
   message flows to enable the on-demand IPv4 address provisioning
   functionality and describes their functionalities related to this

   o  Customer Edge Router (CER a.k.a.  CPE) / End System

   Within the context of this document the CPE/End System is any device
   implementing a Dual-Stack PPP stack and acting as a PPP client with
   respect to the PPP server (e.g.  BRAS/NAS) in the service provider
   network in order to achieve connectivity to the service provider
   network.  In the case of a Customer Edge Router (CPE) this is a node
   (e.g. intended for home or small office usage) which forwards IPv4
   and IPv6 packets that are not explicitly addressed to itself between
   the Local Area Network and WAN interface.  The CPE itself can be
   abstracted into three functional blocks, one that carries the PPP
   session (e.g. a standalone DSL modem), one that is operating simply
   as a local router which includes the NAPT44 function and any IPV6
   PD/ND, DHCPv6 and DHCP for both stacks and one which includes the
   local CPE functionalities (e.g., DNS forwarder/cache, VoIP SIP
   agent).  The PPP interface of this device is also called WAN (Wide
   Area Network) interface [RFC6204].  In the case of IPv4 an additional
   Network Address Translation (NAT) functionality is implemented on the
   router part.  So within the Local Area Network private IPv4 addresses
   can be used as defined in [RFC1918].  Therefore the demand for global
   IPv4 connectivity of such a Customer Edge Router will be triggered
   either by local applications on the CPE or by receiving IPv4 packets
   on its customer network facing interfaces that are addressed to the
   public Internet.

   In the case of an end system, this is a node that intends to
   communicate with other nodes by sending IPv4 and/or IPv6 packets.  On
   an end system, the IPv4 connectivity demand can only be triggered by
   local protocols and own applications.  However, in both cases (CPE or
   end system) an IPv4_idle_timer is implemented on the upstream (WAN)
   interface in order to detect IPv4 packets passing the WAN interface
   (incoming/ outgoing) and to measure the related IPv4 idle time when
   no IPv4 packet has been sent or received.

   o  Network Access Server (NAS a.k.a.  BRAS)/Layer 2 Network Server

   The Network Access Server (NAS) (a.k.a.  Broadband Remote Access

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   Server BRAS) is a device providing local Dual- Stack PPP connectivity
   to the Service Provider access network and acting as a PPP server to
   the PPP client on the Customer Edge Router or customer end system.
   Within a RFC 2661 architecture the PPP server within the service
   provider network is the L2TP Network Server (LNS).  The IPv4 address
   pool management can be provided locally on the BRAS/NAS/LNS or
   remotely.  In the case of a local address pool management no
   additional information exchange to an external address pool
   management system is needed in order to assign or release IPv4
   addresses.  In the case of an external address pool management an
   information exchange between the BRAS/NAS/LNS and the address pool
   management system is required.

   o  External Address Pool Management

   External Address Pool Management is used in the case when no local
   Address Pool Management system is implemented in the BRAS/NAS/LNS.
   In this case it is necessary that the BRAS/NAS/LNS communicates with
   an External Address Pool Management System for signaling assignment
   or release of IPv4 addresses.  RADIUS as specified in [RFC2865] or
   DIAMETER as specified in [RFC3588] can be used as protocol between
   BRAS/NAS/LNS and the External Address Pool Management System.

3.2.  Assigning IPv4 address parameter on-demand after establishing PPP
      session with IPv6 connectivity

   A PPP client implementation wishing to establish a PPP connection
   MUST transmit a NCP Configure-Request to the PPP server.  If every
   Configuration Option received in a NCP Configure-Request is
   recognizable and all values are acceptable, then the PPP server
   implementation MUST transmit a NCP Configure-Ack to the initiator of
   the NCP Configure-Request.

   Applied to the above sketched Dual-Stack PPP session use case the
   configuration and enabling of the IPv6 protocol module will be done
   immediately after a successful LCP data link configuration (and maybe
   successful authentication phase) of the PPP session.  Assuming that
   this IPv6CP configuration exchange has been successfully completed,
   the PPP session is now established and operational containing an
   IPv6-only network layer connection.

   Separately from that, the IPv4 protocol module can (later on and
   dynamically on-demand) be configured and enabled using IPCP.  However
   this SHALL only be done in the case that an IPv4 connectivity demand
   has been detected on the PPP customer end system or CPE (PPP client).
   Therefore the BRAS/NAS MUST not initiate the negotiation of IPCP.

   The following diagram illustrates the corresponding IPCP (and

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   accounting) message exchange that is needed to configure the IPv4
   protocol modules of an existing (Dual-Stack) PPP session on-demand.

         CPE/End System                BRAS/NAS           ext.  Address
            (PPP Peer)                (PPP Peer)         Pool management
                                                         (if necessary)
                |                         |                      |
           1. ->|                         |                      |
           2.   |-IPCP-Configure-Request->|                      |
           3.   |                         |----Access-Request--->|
           4.   |                         |<---Access-Accept-----|
           5.   |<-IPCP-Configure-Request-|                      |
           6.   |---IPCP-Configure-Ack--->|                      |
           7.   |<--IPCP-Configure-Nack---|                      |
           8.   |-IPCP-Configure-Request->|                      |
           9.   |<---IPCP-Configure-Ack---|                      |
           10.  |                         |--Accounting-Request->|
           11.  |                         |<---Accounting-Resp.--|

        Figure 2: Message flow for assigning IPv4 address parameter

   In the above diagram, the CPE/End System is triggered (1) to set up
   IPv4 connectivity via an already existing PPP session.  The CPE/End
   System detects that there is no context (incl. a public IPv4 address)
   for its WAN interface available and starts the negotiation of the
   required IPv4 address and protocol parameters by sending an IPCP
   Configure-Request to the BRAS/NAS (2).  The BRAS/NAS will request the
   corresponding IPv4 connectivity parameters (e.g.  IPv4 address, DNS
   resolver address) from a local (e.g. within the BRAS/NAS) or remote
   database representing the Address Pool Management System(e.g. via
   RADIUS/DIAMETER) (3, 4).  After this the PPP peers use the standard
   IPCP procedures to finalize the IPv4 address parameter negotiation
   (5, 6, 7, 8, 9).  After a successful provisioning of the IPv4 address
   parameter the CPE/End system has full global IPv4 connectivity and
   can proceed with the IPv4 communication (in parallel to IPv6).  In
   case of an external Address Pool Management, the BRAS/NAS will send
   an Accounting-Request message (10) to the external Address Pool
   Management System in order to signal the successful negotiation of
   the IPv4 address parameter.  The external Address Pool Management
   System will answer with an Accounting-Response (11) message.

3.3.  Releasing unused IPv4 address parameters

   A PPP client implementation according to this draft wishing to close
   a dedicated NCP connection (e.g., IPCP or IPv6CP) SHOULD transmit a
   Terminate-Request to the peer.  Upon reception of a NCP Terminate-
   Request, a Terminate-Ack MUST be transmitted to the sender of the

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   In the PPP Dual-Stack session scenario discussed here, the generation
   of the Terminate-Request message for the IPCP part of the PPP Dual-
   Stack session MUST be triggered by an IPv4 traffic idle timer within
   the PPP client when no IPv4 traffic has been detected on the upstream
   interface for a time interval longer than Initial_IPv4_Idle_Time.  As
   long as there is still an ongoing IPv6 connection within the PPP
   session, the PPP session MUST be kept open.  Equivalently, when no
   IPv6 connectivity is detected the IPv6CP session can be terminated
   again by sending an IPv6CP Terminate-Request and accepting this by a
   Terminate-Ack.  Afterwards the link layer connectivity and hence the
   whole PPP connection can be terminated by exchanging the LCP
   Terminate-Request and Terminate-Ack messages.

         CPE/End System                BRAS/NAS           ext.  Address
            (PPP Peer)                (PPP Peer)         Pool Management
                |                         |                      |
           1. ->|                         |                      |
           2.   |--IPCP-Termin.-Request-->|                      |
           3.   |<----IPCP-Termin.-Ack.---|                      |
           4.   |                         |-Interim-Acc.-Requ.-->|
           5.   |                         |<---Accounting-Resp.--|

        Figure 3: Message flow for releasing IPv4 address parameter

   The termination of an IPCP connection within a Dual-Stack PPP session
   is illustrated in figure 3 above.

   For this sample message flow it is assumed that there is still an
   IPv6CP connection active inside the Dual-Stack PPP session.  After
   the expiration of the IPv4 traffic idle timer (1) the CPE/End system
   sends an IPCP terminate request to the peer (2).  The request will be
   answered with an Terminate-Ack message (3).  The IPv4 address can be
   returned to the local address pool (e.g. within the BRAS/NAS) or to
   the remote IPv4 address pool by sending Interim-Accounting messages
   (4, 5) (e.g. via RADIUS/DIAMETER).

3.4.  Timer Considerations


   The IPv4_Idle_Timer on the upstream interface of the PPP client has
   to be started immediately after a successful establishment of the
   IPCP session within the PPP connection and MUST count down starting
   from the Initial_IPv4_Idle_Time value to 0.  When the upstream
   interface of the PPP client discovers incoming / outgoing IPv4

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   traffic then the IPv4_Idle_Time MUST be reset to the
   Initial_IPv4_Idle_Timer value.  When the IPv4_Idle_Timer reaches the
   value 0 sending a Terminate-Request message MUST be triggered by a
   the PPP client (e.g., end system, CPE).  The Initial_IPv4_Idle_Time
   value MUST be configurable to adopt the mechanism due to the needs of
   the applications which are using IPv4 and with respect to an
   optimization of the IPv4 address saving potential.

4.  Potential for optimization

   The efficiency of the "On demand IPv4 address provisioning" mechanism
   can be measured in the ratio of IPCP/RADIUS/DIAMETER signalling
   traffic to the amount of the saved global IPv4 addresses.  Hence
   different options to optimize the efficiency of the proposed solution
   are possible, by suppressing unnecessary signalling load and blocking
   forbidden IPv4 connectivity requests.

4.1.  Avoiding unnecessary load on BRAS/NAS and AAA

   Unnecessary signaling load between PPP peers as well as between BRAS/
   NAS and external Address Pool Management can for instance occur when
   a IPv6-only customer requests IPv4 address parameters.  This can be
   prevented by restricting the usage of a Dual-Stack CPE for IPv6-only
   customers to IPv6 only and/or by administratively refusing the IPCP
   configure requests of such an IPv6-only customer inside the BRAS/NAS.

   The former case is more or less a business and customer relationship
   related issue which needs no engineering concepts.

   This case can be solved by answering an IPCP Configure Request
   message from a IPv6-only customer with a LCP reject message as
   described in chapter 5.7 of [RFC1661].  The field Rejected-Protocol
   of the LCP reject message contains the value 0x8021 for IPCP and the
   Rejected-Information field contains a copy of the IPCP packet which
   is being rejected.  Due to [RFC1661] upon reception of a Protocol-
   Reject, the implementation of the IPv4 capable CPE of the IPv6-only
   customer MUST immediately stop sending packets of the indicated
   protocol at the earliest opportunity.  So the transmission of
   unnecessary IPCP and RADIUS messages during the running PPP session
   can be prevented.

   Another opportunity to reduce IPCP signaling load and the
   corresponding signalling overhead between BRAS/NAS and external
   Address Pool Management is the definition of default IPv4 traffic
   idle timer values for always-on applications that are sending
   periodic messages (see chapter 3.3).  The value of this IPv4 traffic
   idle timer should be chosen a few seconds larger than the interval

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   between periodic messages of always-on applications.  Such an
   approach avoids problems for these applications when IPv4 is used and
   optimizes IPv4 address release and address assign message exchange.
   Very short and periodic IPv4 address renewal cycles can be avoided by
   such an approach.

4.2.  Reducing IPv4 traffic on external interfaces

   The easiest way to reduce IPv4 traffic demand (and hence the need for
   public IPv4 addresses) is to shift applications from usage of IPv4 to
   IPv6.  In using the Dual-Stack approach which is a prerequisite of
   the mechanism described in this draft, no differences regarding the
   service level of both protocols are expected.  Each service can be
   provided with the same quality level independently of the chosen
   version of the Internet Protocol.

   But regarding applications on end systems the Internet access
   provider has only very limited influence.  However for applications
   and services running on the CPE itself (e.g.  VoIP User Agent) the
   internet access provider should be able to define and require their
   IPv6 readiness.

   An additional point is the preferred usage of IPv6 on all external
   (WAN) interfaces in the case when the CPE acts as a relay and caches
   on behalf of certain protocols (e.g.  DNS).  When on a LAN interface
   a request message for such a protocol is received via IPv4 and a
   relaying to the external WAN interface is needed IPv6 should be the
   preferred network protocol.  Such a requirement has already been
   defined for relaying/caching devices in [BBF-TR-124-i2] (section
   LAN.DNSv6, item 6).

5.  Impacts on user experience and operation

5.1.  Impacts on user experience and Happy Eyeballs implementations

   In order to mitigate delays in end-to-end establishment in unstable
   Dual-Stack environments I [RFC6555] describes a mechanism to optimize
   the communication establishment for connection-oriented transports
   (e.g., TCP, SCTP).  The IPv6 connectivity can be impaired for
   instance due connection failure to the IPv6 Internet, broken 6to4 or
   Teredo tunnels, or broken IPv6 peering.  After making a connection
   attempt on the preferred address family (e.g.  IPv6) and failing to
   establish a connection within a certain time period, a "Happy
   Eyeballs" implementation will decide to initiate a second connection
   attempt in parallel using the same or the other address family.  It
   is recommended that the non-winning connections be abandoned, even
   though they could -- in some cases -- be put to reasonable use.  In

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   the case of IPv6 connectivity problems a Dual-Stack host will hence
   use IPv4; in the case of IPv4 connectivity problems a Dual-Stack host
   will use IPv6 for reaching a certain destination.

   In a Dual-Stack environment according to this document it is assumed
   that the IPv6 connectivity (at least in the access network) is not
   impaired.  Nevertheless it is possible that the network path between
   access area and IPv6 destination is broken.  In this case a fast
   fall-back to IPv4 is needed.  In a Dual-Stack environment are,
   according to this draft, in general 3 states regarding IPv4 and IPv6
   connectivity of interest:

   1.  Neither IPv4 nor IPv6 connectivity is given (PPP link is dead),

   2.  Only IPv6 connectivity is established and

   3.  IPv4 and IPv6 connectivity is established.

   In the first case the "Happy Eyeball" scenario is not relevant.

   In the second case a fast IPv4 fall-back has to be realized by
   triggering and using the mechanism described in chapter 3.2.
   Depending on the architecture scenario (IP address pool management
   inside or outside the BRAS/NAS) and the CPE and BRAS/NAS performance
   capabilities a delay of about hundred milliseconds for establishing
   the IPCP session has to be considered.  In the case that meanwhile
   the communication is not established via IPv6 this will be done via
   IPv4.  If the "Happy Eyeball" algorithm caches connection
   establishment successes/failures, this additional IPCP establishment
   delay could lead to wrong assumptions regarding the quality of the
   IPv6 and IPv4 connectivity.  However, in following connection
   attempts using "Happy Eyeball" this can be corrected, because IPv4
   connectivity is already established and no additional delay will be

   The third case corresponds to a native Dual-Stack architecture, so no
   additional considerations are needed.

5.2.  Operational impacts

   As described above the used mechanisms for dynamically assigning /
   releasing IPv4 addresses do not need new PPP, IPCP, IPv6CP or RADIUS
   protocol elements.  Therefore it can be assumed that an
   implementation of the proposed mechanisms on the distinct network
   elements can be realized easily.  Nevertheless depending on the
   service provider IPv6 migration strategy and schedule it is possible
   that this mechanism is not everywhere in a PPP service provider
   deployment active or passive supported.  When a service provider

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   allows the customer the usage of CPEs of their own choice it is
   possible that an IPv4 address releasing CPE will be connected to a
   non compatible BRAS/NAS in the service provider network.  In this
   case the message flow initiated from the CPE could lead to IPv4
   connectivity problems.  In order to avoiding this, a CPE
   implementation according to this draft MAY provide capabilities to
   switch on/off the above described functionality in order to fall back
   to a support of an IPv6-only or a "standard" Dual-Stack service.

6.  Acknowledgements

   The author and contributors also wish to acknowledge the assistance
   and feedback of the following individuals or groups.

   Tina Tsou

   Alain Durand

   Sven Schmidtke

   Dan Wing

   Vernon Schryer

   Mark Townsley

   Wesley George

   Joel M. Halpern

   Christian Jaquenet

7.  IANA Considerations

   This memo includes no request to IANA.


   All drafts are required to have an IANA considerations section (see
   Guidelines for Writing an IANA Considerations Section in RFCs
   [RFC5226] for a guide).  If the draft does not require IANA to do
   anything, the section contains an explicit statement that this is the
   case (as above).  If there are no requirements for IANA, the section
   will be removed during conversion into an RFC by the RFC Editor.

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8.  Security Considerations


   All drafts are required to have a security considerations section.
   See RFC 3552 [RFC3552] for a guide.

9.  References

9.1.  Normative Reference

              Broadbandforum, "Functional Requirements for Broadband
              Residential Gateway Devices (Issue 2)", May 2010.

              Broadbandforum, "Technical Report TR187 IPv6 over PPP
              Broadband Access (Issue 1)", May 2010.

              Broadbandforum, "Draft TR242 IPv6 Transition Mechanisms
              for Broadband Networks".

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC1661]  Simpson, W., "The Point-to-Point Protocol (PPP)", STD 51,
              RFC 1661, July 1994.

   [RFC1918]  Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
              E. Lear, "Address Allocation for Private Internets",
              BCP 5, RFC 1918, February 1996.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2472]  Haskin, D. and E. Allen, "IP Version 6 over PPP",
              RFC 2472, December 1998.

   [RFC2661]  Townsley, W., Valencia, A., Rubens, A., Pall, G., Zorn,
              G., and B. Palter, "Layer Two Tunneling Protocol "L2TP"",
              RFC 2661, August 1999.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,

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              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC3588]  Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
              Arkko, "Diameter Base Protocol", RFC 3588, September 2003.

   [RFC4213]  Nordmark, E. and R. Gilligan, "Basic Transition Mechanisms
              for IPv6 Hosts and Routers", RFC 4213, October 2005.

   [RFC6204]  Singh, H., Beebee, W., Donley, C., Stark, B., and O.
              Troan, "Basic Requirements for IPv6 Customer Edge
              Routers", RFC 6204, April 2011.

   [RFC6333]  Durand, A., Droms, R., Woodyatt, J., and Y. Lee, "Dual-
              Stack Lite Broadband Deployments Following IPv4
              Exhaustion", RFC 6333, August 2011.

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

9.2.  Informative References

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              July 2003.

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

Appendix A.  Workplan

Authors' Addresses

   Karsten Fleischhauer (editor)
   Deutsche Telekom AG
   Heinrich-Hertz-Strasse 3-7
   64295 Darmstadt

   Phone: +49 6151 58 12831
   Email: k.fleischhauer@telekom.de

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   Olaf Bonness
   Deutsche Telekom AG
   Winterfeldtstr. 21-27
   10781 Berlin

   Phone: +49 30 835358826
   Email: olaf.bonness@telekom.de

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