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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 5571

Softwires Working Group                                        B. Storer
Internet-Draft                                         C. Pignataro, Ed.
Intended status: Standards Track                           M. Dos Santos
Expires: December 30, 2007                                 Cisco Systems
                                                         B. Stevant, Ed.
                                                       GET/ENST Bretagne
                                                             J. Tremblay
                                                        Trellia Networks
                                                           June 28, 2007


         Softwires Hub & Spoke Deployment Framework with L2TPv2
               draft-ietf-softwire-hs-framework-l2tpv2-05

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   This Internet-Draft will expire on December 30, 2007.

Copyright Notice

   Copyright (C) The IETF Trust (2007).









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Abstract

   This document describes the framework of the Softwire "Hub and Spoke"
   solution with Layer 2 Tunneling Protocol (L2TPv2), and the
   implementation details specified in this document should be followed
   to achieve inter-operability among different vendor implementations.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5
     1.1.  Abbreviations  . . . . . . . . . . . . . . . . . . . . . .  6
     1.2.  Requirements Language  . . . . . . . . . . . . . . . . . .  6
     1.3.  Contributing Authors . . . . . . . . . . . . . . . . . . .  6
     1.4.  Considerations . . . . . . . . . . . . . . . . . . . . . .  6

   2.  Applicability of L2TPv2 for Softwire Requirements  . . . . . .  7
     2.1.  Network Address Translation (NAT and NAPT) . . . . . . . .  7
     2.2.  Scalability  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  Multicast  . . . . . . . . . . . . . . . . . . . . . . . .  7
     2.4.  Authentication, Authorization and Accounting . . . . . . .  7
     2.5.  Privacy, Integrity, and Replay Protection  . . . . . . . .  8
     2.6.  Operations and Management (OAM)  . . . . . . . . . . . . .  8
     2.7.  Encapsulations . . . . . . . . . . . . . . . . . . . . . .  8

   3.  Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 10
     3.1.  IPv6 over IPv4 Softwire with L2TPv2  . . . . . . . . . . . 10
       3.1.1.  Host CPE as Softwire Initiator . . . . . . . . . . . . 10
       3.1.2.  Router CPE as Softwire Initiator . . . . . . . . . . . 11
       3.1.3.  Host behind CPE as Softwire Initiator  . . . . . . . . 12
       3.1.4.  Router behind CPE as Softwire Initiator  . . . . . . . 13
     3.2.  IPv4 over IPv6 Softwire with L2TPv2  . . . . . . . . . . . 15
       3.2.1.  Host CPE as Softwire Initiator . . . . . . . . . . . . 15
       3.2.2.  Router CPE as Softwire Initiator . . . . . . . . . . . 15
       3.2.3.  Host behind CPE as Softwire Initiator  . . . . . . . . 16
       3.2.4.  Router behind CPE as Softwire Initiator  . . . . . . . 17

   4.  Standardisation Status . . . . . . . . . . . . . . . . . . . . 19
     4.1.  Softwire Transport Related . . . . . . . . . . . . . . . . 19
     4.2.  L2TPv2 . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.3.  Authentication Authorization Accounting  . . . . . . . . . 19
     4.4.  MIB  . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     4.5.  Softwire Payload Related . . . . . . . . . . . . . . . . . 20
       4.5.1.  For IPv6 Payloads  . . . . . . . . . . . . . . . . . . 20
       4.5.2.  For IPv4 Payloads  . . . . . . . . . . . . . . . . . . 20

   5.  Softwire Establishment . . . . . . . . . . . . . . . . . . . . 21
     5.1.  L2TPv2 Tunnel Setup  . . . . . . . . . . . . . . . . . . . 22



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       5.1.1.  Tunnel Establishment . . . . . . . . . . . . . . . . . 23
         5.1.1.1.  AVPs Required for Softwires  . . . . . . . . . . . 25
         5.1.1.2.  AVPs Optional for Softwires  . . . . . . . . . . . 26
         5.1.1.3.  AVPs not Relevant for Softwires  . . . . . . . . . 26
       5.1.2.  Tunnel Maintenance . . . . . . . . . . . . . . . . . . 26
       5.1.3.  Tunnel Teardown  . . . . . . . . . . . . . . . . . . . 27
     5.2.  PPP Connection . . . . . . . . . . . . . . . . . . . . . . 27
       5.2.1.  MTU  . . . . . . . . . . . . . . . . . . . . . . . . . 27
       5.2.2.  LCP  . . . . . . . . . . . . . . . . . . . . . . . . . 27
       5.2.3.  Authentication . . . . . . . . . . . . . . . . . . . . 27
       5.2.4.  IPCP . . . . . . . . . . . . . . . . . . . . . . . . . 27
         5.2.4.1.  IPV6CP . . . . . . . . . . . . . . . . . . . . . . 27
         5.2.4.2.  IPv4CP . . . . . . . . . . . . . . . . . . . . . . 28
     5.3.  Global IPv6 Address Assignement to Endpoints . . . . . . . 28
     5.4.  DHCP . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
       5.4.1.  DHCPv6 . . . . . . . . . . . . . . . . . . . . . . . . 29
       5.4.2.  DHCPv4 . . . . . . . . . . . . . . . . . . . . . . . . 29

   6.  Considerations about the Address Provisioning Model  . . . . . 30
     6.1.  Softwire Endpoints Addresses . . . . . . . . . . . . . . . 30
       6.1.1.  IPv6 . . . . . . . . . . . . . . . . . . . . . . . . . 30
       6.1.2.  IPv4 . . . . . . . . . . . . . . . . . . . . . . . . . 30
     6.2.  Delegated Prefixes . . . . . . . . . . . . . . . . . . . . 30
       6.2.1.  IPv6 Prefixes  . . . . . . . . . . . . . . . . . . . . 30
       6.2.2.  IPv4 Prefixes  . . . . . . . . . . . . . . . . . . . . 31
     6.3.  Possible scenarios . . . . . . . . . . . . . . . . . . . . 31
       6.3.1.  Scenarios for IPv6 . . . . . . . . . . . . . . . . . . 31
       6.3.2.  Scenarios for IPv4 . . . . . . . . . . . . . . . . . . 31

   7.  Considerations about Address Stability . . . . . . . . . . . . 33

   8.  Considerations about RADIUS Integration  . . . . . . . . . . . 34
     8.1.  Softwires Endpoints  . . . . . . . . . . . . . . . . . . . 34
       8.1.1.  IPv6 Softwires . . . . . . . . . . . . . . . . . . . . 34
       8.1.2.  IPv4 Softwires . . . . . . . . . . . . . . . . . . . . 34
     8.2.  Delegated Prefixes . . . . . . . . . . . . . . . . . . . . 34
       8.2.1.  IPv6 Prefixes  . . . . . . . . . . . . . . . . . . . . 35
       8.2.2.  IPv4 Prefixes  . . . . . . . . . . . . . . . . . . . . 35

   9.  Considerations for Maintenance and Statistics  . . . . . . . . 36
     9.1.  RADIUS Accounting  . . . . . . . . . . . . . . . . . . . . 36
     9.2.  MIBs . . . . . . . . . . . . . . . . . . . . . . . . . . . 36

   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 37

   11. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 38

   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 39



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   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 40
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 40
     13.2. Informative References . . . . . . . . . . . . . . . . . . 41

   Appendix A.  Revision History  . . . . . . . . . . . . . . . . . . 44

   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 49
   Intellectual Property and Copyright Statements . . . . . . . . . . 50











































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1.  Introduction

   The Softwires Working Group has selected Layer Two Tunneling Protocol
   version 2 (L2TPv2) as the phase 1 protocol to be deployed in the
   Softwires "Hubs and Spokes" solution space.  This document describes
   the framework for the L2TPv2 "Hubs and Spokes" solution, and the
   implementation details specified in this document should be followed
   to achieve interoperability among different vendor implementations.

   In the "Hubs and Spokes" solution space, a Softwire is established to
   provide the home network with IPv4 connectivity across an IPv6-only
   access network or IPv6 connectivity across an IPv4-only access
   network.  When L2TPv2 is used in the Softwire context, the voluntary
   tunneling model applies.  The Softwire Initiator (SI) at the home
   network takes the role of the L2TP Access Concentrator (LAC) client
   (initiating both the L2TP tunnel/session and PPP link) while the
   Softwire Concentrator (SC) at the ISP takes the role of the L2TP
   Network Server (LNS).  Since L2TPv2 compulsory tunneling model does
   not apply to Softwires, it should not be requested or honored.  This
   document identifies all the voluntary tunneling related L2TPv2
   attributes that apply to Softwires and specifies the handling
   mechanism for such attributes in order to avoid ambiguities in
   implementations.  This document also identifies the set of L2TPv2
   attributes specific to compulsory tunneling model that do not apply
   to Softwires and specifies the mechanism to ignore or nullify their
   effect within the Softwires context.

   The SI and SC must follow the L2TPv2 operations described in
   [RFC2661] when performing Softwire establishment, tear-down and OAM.
   With L2TPv2, a Softwire consists of an L2TPv2 Control Channel, a
   single Session, and the PPP link negotiated over the Session.  To
   establish the Softwire, the SI first initiates an L2TPv2 Control
   Channel to the SC which accepts the request and terminates the
   Control Channel.  L2TPv2 supports an optional mutual Control Channel
   authentication which allows both SI and SC to validate each other's
   identity at the initial phase of hand-shaking before proceeding with
   Control Channel establishment.  After the L2TPv2 Control Channel is
   established between the SI and SC, the SI initiates an L2TPv2 Session
   to the SC.  Then the PPP/IP link is negotiated over the L2TPv2
   Session between the SI and SC.  After the PPP/IP link is established,
   it acts as the Softwire between the SI and SC for tunneling IP
   traffic of one Address Family across the access network of another
   Address Family.

   During the life of the Softwire, both SI and SC send L2TPv2 keepalive
   HELLO messages to monitor the health of the Softwire and the peer
   LCCE, and to potentially refresh the NAT/NAPT translation entry at
   the CPE or at the other end of the access link.  Optionally, LCP ECHO



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   messages can be used as keepalives for the same purposes.  In the
   event of keepalive timeout or administrative shutdown of the
   Softwire, either SI or SC may tear down the Softwire by tearing down
   the L2TPv2 Control Channel and Session as specified in [RFC2661].

1.1.  Abbreviations

   LCCE   L2TP Control Connection Endpoint (See [RFC3931])

   SC     Softwire Concentrator, the node terminating the Softwire in
          the service provider network (See
          [I-D.ietf-softwire-problem-statement])

   SI     Softwire Initiator, the node initiating the Softwire within
          the customer network (See
          [I-D.ietf-softwire-problem-statement])

   STH    Softwire Transport Header, the outermost IP header of a
          softwire (See [I-D.ietf-softwire-problem-statement])

   SPH    Softwire Payload Header, the IP headers being carried within a
          softwire (See [I-D.ietf-softwire-problem-statement])

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

1.3.  Contributing Authors

   Following is the complete list of contributors to this document.

      Maria Alice Dos Santos
      Carlos Pignataro
      Bill Storer
      Cisco Systems
      Jean-Francois Tremblay
      Trellia Networks
      Laurent Toutain
      Bruno Stevant
      GET/ENST Bretagne

1.4.  Considerations

   Some sections of this document contain considerations that are not
   required for interoperability and correct operation of Softwire
   implementations.  These sections are marked as "Considerations".



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2.  Applicability of L2TPv2 for Softwire Requirements

   A list of Softwire "Hubs and Spokes" requirements have been
   identified by the Softwire Problem Statement
   [I-D.ietf-softwire-problem-statement].  The following section
   describes how L2TPv2 fulfills each of them.

2.1.  Network Address Translation (NAT and NAPT)

   A "Hubs and Spokes" Softwire must be able to traverse Network Address
   Translation and Network Address Port Translation (NAT and NAPT)
   devices [RFC3022] in case the scenario in question involves a non-
   upgradable pre-existing IPv4 home gateway performing NAT/NAPT or some
   carrier equipment at the other end of the access link performing NAT/
   NAPT.  The L2TPv2 Softwire (i.e., Control Channel and Session) is
   capable of NAT/NAPT traversal since L2TPv2 can run over UDP.

   Since L2TPv2 does not "autodetect" NAT/NAPT along the path, L2TPv2
   does not offer UDP bypass regardless of NAT/NAPT presence.  Both NAT/
   NAPT "autodetect" and UDP bypass are optional requirements.

2.2.  Scalability

   In the "Hubs and Spokes" model, a carrier must be able to scale the
   solution to millions of Softwire initiators by adding more hubs
   (i.e., Softwire concentrators).  L2TPv2 is a widely deployed protocol
   in broadband services, and its scalability has been proven in
   multiple large-scale IPv4 Virtual Private Network deployments which
   scale up to millions of subscribers each.

2.3.  Multicast

   Multicast protocols simply run over L2TPv2 Softwires transparently
   together with other regular IP traffic.

2.4.  Authentication, Authorization and Accounting

   L2TPv2 supports optional mutual Control Channel authentication and
   leverages the optional mutual PPP per-session authentication.  L2TPv2
   is well integrated with AAA solutions (such as RADIUS) for both
   authentication and authorization.  Most L2TPv2 implementations
   available in the market support logging of authentication and
   authorization events.

   L2TPv2 integration with RADIUS accounting (RADIUS Accounting
   extension for tunnel [RFC2867]) allows the collection and reporting
   of L2TPv2 Softwire usage statistics.




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2.5.  Privacy, Integrity, and Replay Protection

   Since L2TPv2 runs over IP/UDP in the Softwire context, IPSec ESP can
   be used in conjunction to provide per-packet authentication,
   integrity, replay protection and confidentiality for both L2TPv2
   control and data traffic [RFC3193] and [RFC3948].

   For Softwire deployments in which full payload security is not
   required, the L2TPv2 built-in Control Channel authentication and the
   inherited PPP authentication and PPP Encryption Control Protocol can
   be considered.

2.6.  Operations and Management (OAM)

   L2TPv2 supports an optional in-band keepalive mechanism which injects
   HELLO control messages after a specified period of time has elapsed
   since the last data or control message was received on a tunnel.  If
   the HELLO control message is not reliably delivered, then the Control
   Channel and its session will be torn down.  In the Softwire context,
   the L2TPv2 keepalive is used to monitor the connectivity status
   between the SI and SC and/or as a refresh mechanism for any NAT/NAPT
   translation entry along the access link.

   LCP ECHO offers a similar mechanism to monitor the connectivity
   status, as described in [RFC1661].  Softwires implementations SHOULD
   use L2TPv2 Hello keepalives and in addition MAY use PPP LCP Echo
   messages to ensure Dead End Detection and/or to refresh NAT/NAPT
   translation entries.  The combination of these two mechanisms can be
   used as an optimization.

   L2TPv2 MIB [RFC3371] supports the complete suite of management
   operations such as configuration of Control Channel and Session,
   polling of Control Channel and Session status and their traffic
   statistics and notifications of Control Channel and Session UP/DOWN
   events.

2.7.  Encapsulations

   L2TPv2 supports the following encapsulations:

   o  IPv6/PPP/L2TPv2/UDP/IPv4

   o  IPv4/PPP/L2TPv2/UDP/IPv6

   o  IPv4/PPP/L2TPv2/UDP/IPv4

   o  IPv6/PPP/L2TPv2/UDP/IPv6




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   Note that UDP bypass is not supported by L2TPv2 since L2TPv2 does not
   support "autodetect" of NAT/NAPT.

















































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3.  Deployment Scenarios

   For the "Hubs and Spokes" problem space, four scenarios have been
   identified.  In each of these four scenarios, different home
   equipment plays the role of the Softwire Initiator.  This section
   elaborates each scenario with L2TPv2 as the Softwire protocol and
   other possible protocols involved to complete the solution.  This
   section examines the four scenarios for both IPv6 over IPv4 and IPv4
   over IPv6 encapsulations.

3.1.  IPv6 over IPv4 Softwire with L2TPv2

   The following subsections cover IPv6 connectivity across an IPv4-only
   access network (STH) using a Softwire.

3.1.1.  Host CPE as Softwire Initiator

   The Softwire Initiator (SI) is the host CPE (directly connected to a
   modem), which is dual-stack.  There is no other gateway device.  The
   IPv4 traffic SHOULD NOT traverse the softwire.  See Figure 1.































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             IPv6 or dual-stack      IPv4-only      dual-stack
            |------------------||-----------------||----------|

      I                    SC                          SI
      N                  +-----+                   +----------+
      T                  |     |                   |  v4/v6   |
      E  <==[ IPv6  ]....|v4/v6|....[IPv4~only]....| host CPE |
      R     [network]    |     |    [ network ]    |          |
      N                  | LNS |                   |LAC Client|
      E                  +-----+                   +----------+
      T                          _ _ _ _ _ _ _ _ _
                               ()_ _ _ _ _ _ _ _ _() <-- IPv6 traffic
                             PPP o L2TPv2 o UDP o IPv4          (SPH)
                                      Softwire

                               <------------------>
                    IPV6CP: capable of /64 Intf-Id assignment or
                                 uniqueness check

                               |------------------>/64 prefix
                                        RA
                               |------------------>DNS, etc.
                                     DHCPv6/v4

                 Figure 1: Host CPE as Softwire Initiator

   In this scenario, IPV6CP negotiates IPv6 over PPP which also provides
   the capability for the ISP to assign the 64-bit Interface-Identifier
   to the host CPE or perform uniqueness validation for the two
   Interface-IDs at the two PPP ends [RFC2472].  After IPv6 over PPP is
   up, Neighbor Discovery runs over the IPv6 over PPP link, and the LNS
   can assign a 64-bit global prefix to the host CPE via Router
   Advertisement (RA) while other configuration options (such as DNS)
   can be conveyed to the host CPE via DHCPv6/v4.

3.1.2.  Router CPE as Softwire Initiator

   The Softwire Initiator (SI) is the router CPE, which is a dual-stack
   device.  The IPv4 traffic SHOULD NOT traverse the Softwire.  See
   Figure 2.











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          IPv6 or dual-stack      IPv4-only           dual-stack
         |------------------||-----------------||---------------------|

   I                    SC                          SI
   N                  +-----+                   +----------+
   T                  |     |                   |  v4/v6   |    +-----+
   E  <==[ IPv6  ]....|v4/v6|....[IPv4~only]....|   CPE    |----|v4/v6|
   R     [network]    |     |    [ network ]    |          |    | host|
   N                  | LNS |                   |LAC Client|    +-----+
   E                  +-----+                   +----------+
   T                          _ _ _ _ _ _ _ _ _
                            ()_ _ _ _ _ _ _ _ _() <-------- IPv6 traffic
                          PPP o L2TPv2 o UDP o IPv4                (SPH)
                                   Softwire

                            <------------------>
                    IPV6CP: capable of /64 Intf-Id assignment or
                                uniqueness check

                            |------------------>/64 prefix
                                     RA
                            |------------------>/48 prefix,
                                   DHCPv6       DNS, etc.

                                                     |------->/64 prefix
                                                         RA
                                                     |-------> DNS, etc.
                                                      DHCPv4/v6

                Figure 2: Router CPE as Softwire Initiator

   In this scenario, IPV6CP negotiates IPv6 over PPP which also provides
   the capability for the ISP to assign the 64-bit Interface-Identifier
   to the router CPE or perform uniqueness validation for the two
   Interface-IDs at the two PPP ends [RFC2472].  After IPv6 over PPP is
   up, Neighbor Discovery runs over the IPv6 over PPP link, and the LNS
   can assign a 64-bit global prefix to the router CPE via Router
   Advertisement (RA).  DHCPv6 can be used to perform IPv6 Prefix
   Delegation (e.g., delegating a 48-bit prefix to be used within the
   home network) and convey other configuration options (such as DNS) to
   the router CPE.

3.1.3.  Host behind CPE as Softwire Initiator

   The CPE is IPv4-only.  The Softwire Initiator (SI) is a dual-stack
   host (behind the IPv4-only CPE), which acts as an IPv6 host CPE.  The
   IPv4 traffic SHOULD NOT traverse the Softwire.  See Figure 3.




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          IPv6 or dual-stack            IPv4-only         dual-stack
         |------------------||----------------------------||----------|

    I                   SC                                    SI
    N                 +-----+                              +----------+
    T                 |     |                   +-------+  |   v4/v6  |
    E <==[ IPv6  ]....|v4/v6|....[IPv4~only]....|v4-only|--|   host   |
    R    [network]    |     |    [ network ]    |  CPE  |  |          |
    N                 | LNS |                   +-------+  |LAC Client|
    E                 +-----+                              +----------+
    T                         _ _ _ _ _ _ _ _ _ _ _ _ _ _
                            ()_ _ _ _ _ _ _ _ _ _ _ _ _ _() <--  IPv6
                               PPP o L2TPv2 o UDP o IPv4        traffic
                                        Softwire                 (SPH)

                            <------------------------------>
                     IPV6CP: capable of /64 Intf-Id assignment or
                                   uniqueness check

                            |------------------------------>/64 prefix
                                           RA
                            |------------------------------>DNS, etc.
                                        DHCPv4/v6

              Figure 3: Host behind CPE as Softwire Initiator

   In this scenario, IPV6CP negotiates IPv6 over PPP which also provides
   the capability for the ISP to assign the 64-bit Interface-Identifier
   to the host or perform uniqueness validation for the two Interface-
   IDs at the two PPP ends [RFC2472].  After IPv6 over PPP is up,
   Neighbor Discovery runs over the IPv6 over PPP link, and the LNS can
   assign a 64-bit global prefix to the host via Router Advertisement
   (RA) while other configuration options (such as DNS) can be conveyed
   to the host via DHCPv6/v4.

3.1.4.  Router behind CPE as Softwire Initiator

   The CPE is IPv4-only.  The Softwire Initiator (SI) is a dual-stack
   device (behind the IPv4-only CPE) acting as an IPv6 CPE router inside
   the home network.  The IPv4 traffic SHOULD NOT traverse the Softwire.
   See Figure 4.










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         IPv6 or dual-stack           IPv4-only            dual-stack
        |------------------||-------------------------||-------------|

   I                   SC                                 SI
   N                 +-----+                           +----------+
   T                 |     |               +-------+   |  v4/v6   |
   E <==[ IPv6  ]....|v4/v6|..[IPv4~only]..|v4-only|---|  router  |
   R    [network]    |     |  [ network ]  |  CPE  | | |          |
   N                 | LNS |               +-------+ | |LAC Client|
   E                 +-----+                         | +----------+
   T                                                 |
                                                     ---------+-----+
                                                              |v4/v6|
                                                              | host|
                             _ _ _ _ _ _ _ _ _ _ _ _ _        +-----+
                           ()_ _ _ _ _ _ _ _ _ _ _ _ _() <--  IPv6
                              PPP o L2TPv2 o UDP o IPv4      traffic
                                       Softwire               (SPH)

                           <--------------------------->
                  IPV6CP: capable of /64 Intf-Id assignment or
                                 uniqueness check

                           |--------------------------->/64 prefix
                                         RA
                           |--------------------------->/48 prefix,
                                       DHCPv6           DNS, etc.

                                                           |----> /64
                                                             RA   prefix
                                                           |----> DNS,
                                                       DHCPv4/v6  etc.

             Figure 4: Router behind CPE as Softwire Initiator

   In this scenario, IPV6CP negotiates IPv6 over PPP which also provides
   the capability for the ISP to assign the 64-bit Interface-Identifier
   to the v4/v6 router or perform uniqueness validation for the two
   Interface-IDs at the two PPP ends [RFC2472].  After IPv6 over PPP is
   up, Neighbor Discovery runs over the IPv6 over PPP link, and the LNS
   can assign a 64-bit global prefix to the v4/v6 router via Router
   Advertisement (RA).  DHCPv6 can be used to perform IPv6 Prefix
   Delegation (e.g., delegating a 48-bit prefix to be used within the
   home network) and convey other configuration options (such as DNS) to
   the v4/v6 router.






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3.2.  IPv4 over IPv6 Softwire with L2TPv2

   The following subsections cover IPv4 connectivity across an IPv6-only
   access network (STH) using a Softwire.

3.2.1.  Host CPE as Softwire Initiator

   The Softwire Initiator (SI) is the host CPE (directly connected to a
   modem), which is dual-stack.  There is no other gateway device.  The
   IPv6 traffic SHOULD NOT traverse the Softwire.  See Figure 5.


             IPv4 or dual-stack      IPv6-only      dual-stack
            |------------------||-----------------||---------|

       I                   SC                          SI
       N                 +-----+                   +----------+
       T                 |     |                   |  v4/v6   |
       E <==[ IPv4  ]....|v4/v6|....[IPv6~only]....| host CPE |
       R    [network]    |     |    [ network ]    |          |
       N                 | LNS |                   |LAC Client|
       E                 +-----+                   +----------+
       T                         _ _ _ _ _ _ _ _ _
                               ()_ _ _ _ _ _ _ _ _() <-- IPv4 traffic
                             PPP o L2TPv2 o UDP o IPv6          (SPH)
                                      Softwire

                               <------------------>
                       IPCP: capable of global IP assignment
                                    and DNS, etc.

                 Figure 5: Host CPE as Softwire Initiator

   In this scenario, IPCP negotiates IPv4 over PPP which also provides
   the capability for the ISP to assign a global IPv4 address to the
   host CPE.  A global IPv4 address can also be assigned via DHCP.
   Other configuration options (such as DNS) can be conveyed to the host
   CPE via IPCP [RFC1877] or DHCP.

3.2.2.  Router CPE as Softwire Initiator

   IPv4 connectivity across an IPv6-only access network (STH).  The
   Softwire Initiator (SI) is the router CPE, which is a dual-stack
   device.  The IPv6 traffic SHOULD NOT traverse the Softwire.  See
   Figure 6.






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         IPv4 or dual-stack      IPv6-only       dual-stack Home
        |------------------||-----------------||-------------------|

   I                   SC                          SI
   N                 +-----+                   +----------+
   T                 |     |                   |  v4/v6   |  +-----+
   E <==[ IPv4  ]....|v4/v6|....[IPv6~only]....|   CPE    |--|v4/v6|
   R    [network]    |     |    [ network ]    |          |  | host|
   N                 | LNS |                   |LAC Client|  +-----+
   E                 +-----+                   +----------+
   T                         _ _ _ _ _ _ _ _ _
                           ()_ _ _ _ _ _ _ _ _() <--------- IPv4 traffic
                         PPP o L2TPv2 o UDP o IPv6                 (SPH)
                                  Softwire

                           <------------------>
                   IPCP: capable of global IP assignment
                               and DNS, etc.

                           |------------------>
                         DHCPv4: prefix, mask, PD

                                                             private/
                                                    |------> global
                                                      DHCP   IP, DNS,
                                                             etc.

                Figure 6: Router CPE as Softwire Initiator

   In this scenario, IPCP negotiates IPv4 over PPP which also provides
   the capability for the ISP to assign a global IPv4 address to the
   router CPE.  A global IPv4 address can also be assigned via DHCP.
   Other configuration options (such as DNS) can be conveyed to the
   router CPE via IPCP [RFC1877] or DHCP.  For IPv4 Prefix Delegation
   for the home network, DHCP [I-D.ietf-dhc-subnet-alloc] can be used.

3.2.3.  Host behind CPE as Softwire Initiator

   IPv4 connectivity across an IPv6-only access network (STH).  The CPE
   is IPv6-only.  The Softwire Initiator (SI) is a dual-stack host
   (behind the IPv6 CPE), which acts as an IPv4 host CPE.  The IPv6
   traffic SHOULD NOT traverse the Softwire.  See Figure 7.









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          IPv4 or dual-stack            IPv6-only           dual-stack
         |------------------||----------------------------||----------|

    I                   SC                                      SI
    N                 +-----+                              +----------+
    T                 |     |                   +-------+  |   v4/v6  |
    E <==[ IPv4  ]....|v4/v6|....[IPv6~only]....|v6-only|--|   host   |
    R    [network]    |     |    [ network ]    |  CPE  |  |          |
    N                 | LNS |                   +-------+  |LAC Client|
    E                 +-----+                              +----------+
    T                         _ _ _ _ _ _ _ _ _ _ _ _ _ _
                            ()_ _ _ _ _ _ _ _ _ _ _ _ _ _() <--  IPv4
                               PPP o L2TPv2 o UDP o IPv6        traffic
                                        Softwire                 (SPH)

                            <------------------------------>
                        IPCP: capable of global IP assignment
                                    and DNS, etc.

              Figure 7: Host behind CPE as Softwire Initiator

   In this scenario, IPCP negotiates IPv4 over PPP which also provides
   the capability for the ISP to assign a global IPv4 address to the
   host.  A global IPv4 address can also be assigned via DHCP.  Other
   configuration options (such as DNS) can be conveyed to the host CPE
   via IPCP [RFC1877] or DHCP.

3.2.4.  Router behind CPE as Softwire Initiator

   IPv4 connectivity across an IPv6-only access network (STH).  The CPE
   is IPv6-only.  The Softwire Initiator (SI) is a dual-stack device
   (behind the IPv6-only CPE) acting as an IPv4 CPE router inside the
   home network.  The IPv6 traffic SHOULD NOT traverse the Softwire.
   See Figure 8.

















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         IPv4 or dual-stack          IPv6-only           dual-stack
        |------------------||-------------------------||------------|

   I                   SC                                 SI
   N                 +-----+                           +----------+
   T                 |     |               +-------+   |  v4/v6   |
   E <==[ IPv4  ]....|v4/v6|..[IPv6~only]..|v6-only|---|  router  |
   R    [network]    |     |  [ network ]  |  CPE  | | |          |
   N                 | LNS |               +-------+ | |LAC Client|
   E                 +-----+                         | +----------+
   T                                                 |
                                                     --------+-----+
                                                             |v4/v6|
                                                             | host|
                             _ _ _ _ _ _ _ _ _ _ _ _ _       +-----+
                           ()_ _ _ _ _ _ _ _ _ _ _ _ _() <---  IPv4
                             PPP o L2TPv2 o UDP o IPv4        traffic
                                      Softwire                 (SPH)

                           <--------------------------->
                   IPCP: assigns global IP address and DNS, etc.

                           |--------------------------->
                              DHCPv4: prefix, mask, PD

                                                                private/
                                                         |----> global
                                                          DHCP  IP, DNS,
                                                                etc.

             Figure 8: Router behind CPE as Softwire Initiator

   In this scenario, IPCP negotiates IPv4 over PPP which also provides
   the capability for the ISP to assign a global IPv4 address to the
   v4/v6 router.  A global IPv4 address can also be assigned via DHCP.
   Other configuration options (such as DNS) can be conveyed to the
   v4/v6 router via IPCP [RFC1877] or DHCP.  For IPv4 Prefix Delegation
   for the home network, DHCP [I-D.ietf-dhc-subnet-alloc] can be used.













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4.  Standardisation Status

   This section groups various Internet standards documents and other
   publications used in Softwires.

4.1.  Softwire Transport Related

   RFC 3193   "Securing L2TP using IPsec" [RFC3193].

   RFC 3948   "UDP Encapsulation of IPsec ESP Packets" [RFC3948].

              *  IPSec supports both IPv4 and IPv6 transports.

4.2.  L2TPv2

   RFC 2661   "Layer Two Tunneling Protocol "L2TP"" [RFC2661].

              *  For both IPv4 and IPv6 payloads (SPH), support is
                 complete.

              *  For both IPv4 and IPv6 transports (STH), support is
                 complete.

4.3.  Authentication Authorization Accounting

   RFC 2865   "Remote Authentication Dial In User Service (RADIUS)"
              [RFC2865].

   RFC 2867   "RADIUS Accounting Modifications for Tunnel Protocol
              Support" [RFC2867].

   RFC 2868   "RADIUS Attributes for Tunnel Protocol Support" [RFC2868].

   RFC 3162   "RADIUS and IPv6" [RFC3162].

4.4.  MIB

   RFC 1471   "The Definitions of Managed Objects for the Link Control
              Protocol of the Point-to-Point Protocol" [RFC1471].

   RFC 1473   "The Definitions of Managed Objects for the IP Network
              Control Protocol of the Point-to-Point Protocol"
              [RFC1473].

   RFC 3371   "Layer Two Tunneling Protocol "L2TP" Management
              Information Base" [RFC3371].





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   RFC 4087   "IP Tunnel MIB" [RFC4087].

              *  Both IPv4 and IPv6 transports are supported.

4.5.  Softwire Payload Related

4.5.1.  For IPv6 Payloads

   RFC 2461   "Neighbor Discovery for IP Version 6 (IPv6)" [RFC2461].

   RFC 2472   "IP Version 6 over PPP" [RFC2472].

              *  See also [I-D.ietf-ipv6-over-ppp-v2].

   RFC 3315   "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)"
              [RFC3315].

   RFC 3646   "DNS Configuration options for Dynamic Host Configuration
              Protocol for IPv6 (DHCPv6)" [RFC3646].

4.5.2.  For IPv4 Payloads

   RFC 1332   "The PPP Internet Protocol Control Protocol (IPCP)"
              [RFC1332].

   RFC 1661   "The Point-to-Point Protocol (PPP)" [RFC1661].

   RFC 1877   "PPP Internet Protocol Control Protocol Extensions for
              Name Server Addresses" [RFC1877].

   DHCP Subnet Allocation  "Subnet Allocation Option".

              *  Work in progress, see [I-D.ietf-dhc-subnet-alloc].


















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5.  Softwire Establishment

   A Softwire is established in three distinct steps (see Figure 9).
   First an L2TPv2 tunnel with a single session is established from the
   SI to the SC.  Second a PPP session is established over the L2TPv2
   session and the SI obtains an address.  Third the SI optionally gets
   other information through DHCP such as a delegated prefix and DNS
   servers.


      SC                                 SI
       |                                 |
       |<-------------L2TPv2------------>| Step 1
       |                                 | L2TPv2 Tunnel establishment
       |                                 |
       |<-------------PPP--------------->| Step 2
       |<----End Point Configuration---->| PPP and End Point
       |                                 | configuration
       |                                 |
       |<------Router Configuration----->| Step 3
       |                                 | Additional configuration
       |                                 | (optional)

            Figure 9: Steps for the Establishment of a Softwire

   In the following diagram (see Figure 10), each of the three steps
   required to establish a Softwire is described in detail.
























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         SC                                 SI
          |                                 |
          |                                 | Step 1
          |<------------SCCRQ---------------| -
          |-------------SCCRP-------------->| |
          |<------------SCCCN---------------| |
          |<------------ICRQ----------------| | L2TPv2
          |-------------ICRP--------------->| |
          |<------------ICCN----------------| -
          |                                 |
          |                                 | Step 2
          |<-----Configuration-Request------| -
          |------Configuration-Request----->| | PPP
          |<-------Configuration-Ack--------| | LCP
          |--------Configuration-Ack------->| -
          |                                 |
          |-----------Challenge------------>| - PPP Authentication
          |<----------Response--------------| | (Optional - CHAP)
          |------------Success------------->| -
          |                                 |
          |<-----Configuration-Request------| -
          |------Configuration-Request----->| | PPP NCP
          |<-------Configuration-Ack--------| | (IPV6CP or IPCP)
          |--------Configuration-Ack------->| -
          |                                 |
          |<------Router-Solicitation-------| - Neighbor Discovery
          |-------Router-Advertisement----->| | (IPv6 only)
          |                                 | -
          |                                 |
          |                                 | Step3
          |<-----------Solicit--------------| -
          |-----------Advertise------------>| | DHCP
          |<---------- Request--------------| | (Optional)
          |-------------Reply-------------->| -

       Figure 10: Detailed Steps in the Establishment of a Softwire

5.1.  L2TPv2 Tunnel Setup

   L2TPv2 [RFC2661] was originally designed to provide private network
   access to end users connected to a public network.  In the L2TPv2
   model, the end user makes a connection to an L2TP Access Concentrator
   (LAC).  The LAC then initiates an L2TPv2 tunnel to an L2TP Network
   Server (LNS).  The LNS then transfers end user traffic between the
   L2TPv2 tunnel and the private network.

   In the Softwire "Hub and Spoke" model, the Softwire Initiator (SI)
   assumes the role of the LAC and the Softwire Concentrator assumes the



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   role of the LNS.

   In the Softwire model, an L2TPv2 packet MUST be carried over UDP.
   The underlying version of the IP protocol may be IPv4 or IPv6,
   depending on the Softwires scenario.

   In the following sections, the term "Tunnel" follows the definition
   from [RFC2661], namely: "The Tunnel consists of a Control Connection
   and zero or more L2TP Sessions".

5.1.1.  Tunnel Establishment

   Figure 11 describes the messages exchanged and AVPs used to establish
   a tunnel between an SI (LAC) and an SC (LNS).  The messages and AVPs
   described here are only a subset of those defined in [RFC2661].  This
   is because Softwires uses only a subset of the L2TPv2 functionality.
   The subset of L2TP Control Connection Management AVPs that is
   applicable to Softwires is grouped into Mandatory AVPs and Optional
   AVPs (see Figure 11).  Note that in the Softwires environment, the SI
   always initiates the tunnel.  L2TPv2 attributes SHOULD NOT be hidden.































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                          SCCRQ
                          Mandatory AVP
                             Message Type
                             Protocol Version
                             Host Name
                             Framing Capabilities
                             Assigned Tunnel ID
                          Optional AVP:
                             Receive Window Size
                             Firmware Revision
                             Vendor Name
                             Challenge

                          SCCRP
                          Mandatory AVP:
                             Message Type
                             Protocol Version
                             Framing Capabilities
                             Host Name
                             Assigned Tunnel ID
                          Optional AVP:
                             Firmware Revision
                             Vendor Name
                             Receive Window Size
                             Challenge Response
                             Challenge

                          SCCCN
                          Mandatory AVP:
                             Message Type
                          Optional AVP:
                             Challenge Response

                Figure 11: Control Connection Establishment

   In L2TPv2, the tunnel between an LAC and LNS may carry the data of
   multiple users.  Each of these user's is represented by an L2TPv2
   session within the tunnel.  In the Softwires environment, the tunnel
   carries the information of a single user.  So, there is only one
   L2TPv2 session per tunnel.  Figure 12 describes the messages
   exchanged and the AVPs used to establish a session between an SI
   (LAC) and an SC (LNS).  The messages and AVPs described here are only
   a subset of those defined in [RFC2661].  This is because Softwires
   uses only a subset of the L2TPv2 functionality.  The subset of L2TP
   Call Management AVPs that is applicable to Softwires is grouped into
   Mandatory AVPs and Optional AVPs (see Figure 12).  Note that in the
   Softwires environment, the SI always initiates the session.  No
   outgoing or analog calls are permitted.  L2TPv2 attributes SHOULD NOT



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   be hidden.


                           ICRQ
                           Mandatory AVP:
                              Message Type
                              Assigned Session ID
                              Call Serial Number

                           ICRP
                           Mandatory AVP:
                              Message Type
                              Assigned Session ID

                           ICCN
                           Mandatory AVP:
                              Message Type
                              (Tx) Connect Speed
                              Framing Type

                     Figure 12: Session Establishment

5.1.1.1.  AVPs Required for Softwires

   This section prescribes specific values for AVPs used in the
   Softwires context that are Mandatory.

   Host Name AVP

      This AVP is mandatory and is present in SCCRQ and SCCRP messages.
      This AVP may be used to authenticate users, in which case it would
      contain a user identification.  If this AVP is not used to
      authenticate users, it may be used for logging purposes.

   Framing Capabilities AVP

      Synchronous bit SHOULD be set to 1 and Asynchronous bit to 1.
      This AVP SHOULD be ignored by the receiver.

   Framing Type AVP

      Synchronous bit SHOULD be set to 1 and Asynchronous bit to 0.
      This AVP SHOULD be ignored by the receiver.

   (Tx) Connect Speed

      (Tx) Connect Speed is a mandatory AVP but is not meaningful in the
      Softwires context.  It SHOULD be left to 0 and ignored by the



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      receiver.

   Message Type AVP, Protocol Version AVP, Assigned Tunnel ID AVP, Call
   Serial Number AVP, and Assigned Session ID AVP

      As defined in [RFC2661]

5.1.1.2.  AVPs Optional for Softwires

   This section prescribes specific values for AVPs used in the
   Softwires context that are Optional.

   Challenge AVP and Challenge Response AVP

      These AVPs are not required, but are necessary to implement tunnel
      authentication.  Since tunnel authentication happens at the
      beginning of L2TPv2 tunnel creation, it can be helpful in
      preventing DoS attacks.

   Receive Window Size AVP, Firmware Revision AVP, and Vendor Name AVP

      As defined in [RFC2661]

5.1.1.3.  AVPs not Relevant for Softwires

   L2TPv2 specifies numerous AVPs that, while allowed for a given
   command, are irrelevant to a Softwires.  Softwires implementations
   SHOULD NOT send these AVPs.  However, they SHOULD ignore them when
   they are received.  This will make it easier to create Softwires
   applications on top of existing L2TPv2 implementations.

5.1.2.  Tunnel Maintenance

   Periodically, the SI MUST transmit a message to the SC to maintain
   NAT/NAPT contexts and detect tunnel failure.  The L2TPv2 HELLO
   message provides a simple, low overhead method of doing this.

   The default values specified in [RFC2661] for L2TPv2 HELLO messages
   could result in a dead end detection time of 83 seconds.  Although
   these retransmission timers and counters SHOULD be configurable (see
   Section 5.8 of [RFC2661]), these values may not be adapted for all
   situations, where a quicker dead end detection is required, or where
   NAT/NAPT context needs to be refreshed more frequently.  In such
   cases, the SI MAY use, in combination with L2TPv2 HELLO, LCP ECHO
   messages (Echo-Request and Echo-Reply codes) described in [RFC1661].
   When used, LCP ECHO messages SHOULD have a re-emission timer lower
   than the value for L2TPv2 HELLO hello messages.  The default value
   recommended in Section 6.5 of [RFC2661] for the HELLO message



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   retransmission interval is 60 seconds.  When used, a set of suggested
   values (included here only for guidance) for the LCP ECHO message
   request interval is a default of 30 seconds, a minimum of 10 seconds,
   and a maximum of the lesser of the configured L2TPv2 HELLO
   retransmisison interval and 60 seconds.

5.1.3.  Tunnel Teardown

   Either the SI or SC can teardown the session and tunnel.  This is
   done as specified in [RFC2661].  There is no action specific to
   Softwires in this case.

5.2.  PPP Connection

5.2.1.  MTU

   The MTU of the PPP link SHOULD be the link MTU minus the size of the
   IP, UDP, L2TPv2, and PPP headers together.  On an IPv4 link with an
   MTU equal to 1500 bytes, this could tipically mean a PPP MTU of 1460
   bytes.  This may vary according to the size of the L2TP header, as
   defined by the leading bits of the L2TP message header (see
   [RFC2661]).  Additionally, see [RFC4623] for a detailed discussion of
   fragmentation issues.

5.2.2.  LCP

   Once the L2TPv2 session is established, the SI and SC initiate the
   PPP connection by negotiating LCP as described in [RFC1661].  The
   Address-and-Control-Field-Compression configuration option (ACFC)
   [RFC1661] SHOULD be rejected.

5.2.3.  Authentication

   After completing LCP negotiation, the SI and SC may optionally
   perform authentication.  If authentication is chosen, CHAP [RFC1994]
   authentication MUST be supported by both the Softwire Initiator and
   Softwire Concentrator.  Other authentication methods such as MS-
   CHAPv1 [RFC2433], and EAP [RFC3748] MAY be supported.

   A detailed discussion of Softwires security is contained in
   [I-D.ietf-softwire-security-requirements].

5.2.4.  IPCP

5.2.4.1.  IPV6CP

   In the IPv6 over IPv4 scenarios (see Section 3.1), after the
   authentication phase, the Softwire Initiator MUST negotiate IPV6CP as



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   defined in [RFC2472].  IPV6CP provides a way to negotiate a unique
   64-bit Interface-Identifier to be used for the address
   autoconfiguration at the local end of the link.

5.2.4.2.  IPv4CP

   In the IPv4 over IPv6 scenarios (see Section 3.2), a Softwire
   Initiator MUST negotiate IPCP [RFC1332].  The SI uses IPCP to obtain
   an IPv4 address from the SC.  IPCP may also be used to obtain DNS
   information as described in [RFC1877].

5.3.  Global IPv6 Address Assignement to Endpoints

   In several scenarios defined in Section 3, Global IPv6 addresses are
   expected to be allocated to Softwires end points.  Since IPV6CP only
   provide Link-Local addresses (see [RFC2472]), IPv6 Neighbor Discovery
   [RFC2462] or DHCPv6 [RFC3315] SHOULD be used to configure these
   addresses.

   The Softwire Initiator of an IPv6 Softwire MUST send a Router
   Solicitation message to the Softwire Concentrator after IPV6CP is
   completed.  The Softwire Concentrator MUST answer with a Router
   Advertisement.  This message MUST contains the global IPv6 prefix of
   the PPP link if Neighbor discovery is used to configure addresses of
   Softwire end points.

   If DHCPv6 is available for address delegation, the M bits of the
   Router Advertisement SHOULD be set.  The Softwire Initiator MUST then
   send a DHCPv6 Request to configure the address of the Softwire
   endpoint.

   Duplicate Address Detection ([I-D.ietf-ipv6-2461bis]) MUST be
   performed on the Softwire in both cases.

5.4.  DHCP

   The Softwire Initiator MAY use DHCP to get additional information
   such as delegated prefix and DNS servers.  If the SI supports DHCP,
   it SHOULD send a Solicit message to verify if more information is
   available.

   When delegating an IPv4 prefix to the SI, the SC SHOULD inject a
   route for this prefix in the IPv4 routing table in order to forward
   the traffic to the relevant Softwire.







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5.4.1.  DHCPv6

   If an SI establishing an IPv6 Softwire acts as a router (i.e., in the
   scenarios in Section 3.1.2 and Section 3.1.4) it MUST include the
   IA_PD option [RFC3633] in the DHCPv6 Solicit message [RFC3315] in
   order to request an IPv6 prefix.

   When delegating an IPv6 prefix to the SI, the SC SHOULD inject a
   route for this prefix in the IPv4 routing table in order to forward
   the traffic to the relevant Softwire.

5.4.2.  DHCPv4

   An SI establishing an IPv4 Softwire MAY send a DHCP request
   containing the Subnet Allocation option [I-D.ietf-dhc-subnet-alloc].
   This practice is not common but may be used to connect IPv4 subnets
   using Softwires, as defined in Section 3.2.2 and Section 3.2.4.

   One Subnet-Request suboption MUST be configured with the 'h' bit set
   to '1', as the SI is expected to perform the DHCP server function.
   The 'i' bit of the Subnet-Request suboption SHOULD be set to '0' the
   first time a prefix is requested and to '1' on subsequent requests,
   if a prefix has been allocated.  The Prefix length suboption SHOULD
   be 0 by default.  If the SI is configured to support only specific
   prefix lengths, it SHOULD specify the longest (smallest) prefix
   length it supports.

   If the SI was previously assigned a prefix from that same SC, it
   SHOULD include the Subnet Information suboption with the prefix it
   was previously assigned.  The 'c' and 's' bits of the suboption
   SHOULD be set to '0'.




















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6.  Considerations about the Address Provisioning Model

   This section describes how a Softwire Concentrator may manage
   delegated addresses for Softwire endpoints and for subnets behind the
   Softwire Initiator.  One common practice is to aggregate endpoints
   addresses and delegated prefixes into one prefix routed to the SC.
   The main benefit is to ease the routing scheme by isolating on the SC
   succeeding route injections (when delegating new prefixes for SI).

6.1.  Softwire Endpoints Addresses

6.1.1.  IPv6

   A Softwire Concentrator should provide globally routable addresses to
   Softwire endpoints.  Other types of addresses such as Unique Local
   Addresses [RFC4193] may be used to address Softwire end points in a
   private network with no global connectivity.  A single /64 should be
   assigned to the Softwire to address both Softwire endpoints.

   Global or ULA addresses must be assigned to endpoints when the
   scenario "Host CPE as Softwire Initiator" (described in
   Section 3.1.1) is considered to be deployed.  For other scenarios,
   this may be optional and link local addresses should be used.

6.1.2.  IPv4

   A Softwire Concentrator may provide either globally routable or
   private IPv4 addresses.  When using IPv4 private addresses [RFC1918]
   on the endpoints, it is not recommended to delegate an IPv4 private
   prefix to the SI, as it can lead to a nested-NAT situations.

   The endpoints of the PPP link use host addresses (i.e., /32),
   negotiated using IPCP.

6.2.  Delegated Prefixes

6.2.1.  IPv6 Prefixes

   Delegated IPv6 should be of global scope if the IPv6 addresses
   assigned to endpoints are global.  Using ULA addresses is not
   recommended when the subnet is connected to the global IPv6 Internet.
   When using ULA IPv6 address for endpoint, the delegated IPv6 prefix
   may be either of Global or ULA scope.

   Delegated IPv6 prefixes are between /48 and /64 in length.  When an
   SI receives a prefix shorter than 64, it can assign different /64
   prefixes to each of its interfaces.  An SI receiving a single /64 is
   expected to perform bridging if more than one interface are available



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   (wired and wireless for example).

6.2.2.  IPv4 Prefixes

   Delegated IPv4 prefixes should be routable within the address space
   used by assigned IPv4 addresses.  Delegate non-routable IPv4 prefixes
   (i.e. private IPv4 prefix over public IPv4 addresses or another class
   of private IPv4 addresses) is not recommended as a practice for
   provisioning and address translation should be considered in these
   cases.  The prefix length is between /8 and /30.

6.3.  Possible scenarios

   This section summarizes the differents scenarios for address
   provisioning with the considerations given in the previous sections.

6.3.1.  Scenarios for IPv6

   This table describes the possible combination of IPv6 address scope
   for endpoints and delegated prefixes.

   +------------------+-----------------------+------------------------+
   | Endpoint IPv6    | Delegated Global IPv6 | Delegated ULA IPv6     |
   | Address          | Prefix                | Prefix                 |
   +------------------+-----------------------+------------------------+
   | Link Local       | Possible              | Possible               |
   |                  |                       |                        |
   | ULA              | Possible              | Possible               |
   |                  |                       |                        |
   | Global           | Possible              | Possible, but Not      |
   |                  |                       | Recommended            |
   +------------------+-----------------------+------------------------+

                        Table 1: Scenarios for IPv6

6.3.2.  Scenarios for IPv4

   This table describes the possible combination of IPv4 address scope
   for endpoints and delegated prefixes.












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   +-------------+-----------------+-----------------------------------+
   | Endpoint    | Delegated       | Delegated Private IPv4 Prefix     |
   | IPv4        | Public IPv4     |                                   |
   | Address     | Prefix          |                                   |
   +-------------+-----------------+-----------------------------------+
   | Private     | Possible        | Possible, but Not Recommended     |
   | IPv4        |                 | when using NAT (cf.               |
   |             |                 | Section 6.1.2)                    |
   |             |                 |                                   |
   | Public IPv4 | Possible        | Possible, but NAT usage is        |
   |             |                 | recommended (cf. Section 6.2.2)   |
   +-------------+-----------------+-----------------------------------+

                        Table 2: Scenarios for IPv4





































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7.  Considerations about Address Stability

   A Softwire can provide stable addresses even if the underlying
   addressing scheme changes, by opposition to automatic tunneling.  A
   Softwire Concentrator should always provide the same address and
   prefix to a reconnecting user.  However, if the goal of the Softwire
   service is to provide a temporary address for a roaming user, it may
   be provisioned to provide only a temporary address.

   The address and prefix are expected to change when reconnecting to a
   different Softwire Concentrator.  However an organization providing a
   Softwire service may provide the same address and prefix across
   different Softwire Concentrators at the cost of a more fragmented
   routing table.  The routing fragmentation issue may be limited if the
   prefixes are aggregated in a location topologically close to the SC.
   This would be the case for example if several SCs are put in parallel
   for load-balancing purpose.


































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8.  Considerations about RADIUS Integration

   The Softwire Concentrator is expected to act as a client to a AAA
   server, for example a RADIUS server.  During the PPP authentication
   phase, the RADIUS server may return additional informations in the
   form of attributes in the Access Accept message.

   The Softwire Concentrator may include the Tunnel-Type and Tunnel-
   Medium-Type attributes [RFC2868] in the Access Request messages to
   provide a hint of the type of Softwire being configured.

8.1.  Softwires Endpoints

8.1.1.  IPv6 Softwires

   If the RADIUS server includes a Framed-Interface-Id attribute
   [RFC3162], the Softwire Concentrator must send it to the Softwire
   Initiator in the Interface-Identifier field of its IPV6CP
   Configuration Request message.

   If the Framed-IPv6-Prefix attribute is mentioned [RFC3162], that
   prefix must be used in the router advertisements sent to the SI.  If
   Framed-IPv6-Prefix is not present but Framed-IPv6-Pool is, the SC
   must choose a prefix with that pool to send RAs.

   If none of the attributes above are included but the AAA server
   returns the Tunnel-Client-Endpoint and Tunnel-Server-Endpoint
   attributes [RFC2868] with the correct address family, these must be
   used in the IPV6CP Interface-Identifier and for the router
   advertisements.

8.1.2.  IPv4 Softwires

   If the Framed-IP-Address attribute [RFC2865] is present, the Softwire
   Concentrator must provide that address to the Softwire Initiator
   during IPCP address negotiation.  That is, when the Softwire
   Initiator requests an IP address from the Softwire Concentrator, the
   address provided should be the Framed-IP-Address.

   If the Framed-IP-Address attribute is not present and the Tunnel-
   Client-Endpoint and Tunnel-Server-Endpoint attributes [RFC2868] are
   present and of the correct address family, these should be used in
   the IPCP IP-Address configuration option.

8.2.  Delegated Prefixes






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8.2.1.  IPv6 Prefixes

   If the attribute Delegated-IPv6-Prefix [RFC4818] is present in the
   RADIUS Access Accept message, it must be used by the Softwire
   Concentrator for the delegation of the IPv6 prefix.  Since the prefix
   delegation is performed by DHCPv6 and the attribute is linked to a
   username, the SC must associate the DHCP Unique Identifier (DUID) of
   a DHCPv6 request to the tunnel it came from and its user.

   Interaction between RADIUS, PPP and DHCPv6 server may follow the
   mechanism proposed in [I-D.ietf-dhc-v6-relay-radius].  During the
   Softwire authentication phase, PPP collects the RADIUS attributes for
   the user such as Delegated-IPv6-Prefix.  A specific DHCPv6 relay is
   assigned to the Softwire.  The DHCPv6 relay fills in these attributes
   in the Relay agent RADIUS Attribute Option (RRAO) DHCPv6 option,
   before forwarding the DHCPv6 requests to the DHCPv6 server.

8.2.2.  IPv4 Prefixes

   The combination of the Framed-IP-Address and Framed-IP-Netmask
   attributes [RFC2865] may be used by the Softwire Concentrator to
   delegate an IPv4 prefix to the Softwire Initiator.





























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9.  Considerations for Maintenance and Statistics

9.1.  RADIUS Accounting

   RADIUS Accounting for L2TP and PPP are documented (see Section 4.3).

   When deploying Softwires solutions, operators may experience
   difficulties to differentiate the address family of the traffic
   reported in accounting information from RADIUS.  This problem and
   some potential solutions are described in
   [I-D.stevant-softwire-accounting].

9.2.  MIBs

   MIB support for L2TPv2 and PPP are documented (see Section 4.4).
   Also see [RFC4293].



































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10.  Security Considerations

   A detailed discussion of Softwires security is contained in
   [I-D.ietf-softwire-security-requirements].

   The L2TPv2 Softwires solution provides the following tools for
   security:

   o  IPsec [RFC3193] provides the highest level of security.

   o  PPP CHAP [RFC1994] provides basic user authentication.

   o  L2TP Tunnel Authentication [RFC2661] provides authentication at
      tunnel setup.  It may be used to limit DoS attacks by
      authenticating the tunnel before L2TP session and PPP resources
      are allocated.



































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11.  IANA Considerations

   This document creates no new requirements on IANA namespaces
   [RFC2434].















































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12.  Acknowledgements

   The authors would like to acknowledge the following contributors who
   provided helpful input on this document: Florent Parent, Jordi Palet
   Martinez, Ole Troan, Shin Miyakawa, Carl Williams, Mark Townsley, and
   Francis Dupont.

   The authors would also like to acknowledge the participants in the
   Softwires interim meetings held in Hong Kong, China and Barcelona,
   Spain.  The minutes for the interim meeting at the China University -
   Hong Kong (February 23-24, 2006) are at
   <http://www3.ietf.org/proceedings/06mar/minutes/isoftwire.html>.  The
   minutes for the interim meeting at Polytechnic University of
   Catalonia - Barcelona (September 14-15, 2006) are reachable at <http:
   //bgp.nu/~dward/softwires/InterimmeetingBarcelonaSeptember2006.htm>.




































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13.  References

13.1.  Normative References

   [RFC1332]  McGregor, G., "The PPP Internet Protocol Control Protocol
              (IPCP)", RFC 1332, May 1992.

   [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.

   [RFC1994]  Simpson, W., "PPP Challenge Handshake Authentication
              Protocol (CHAP)", RFC 1994, August 1996.

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

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, 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,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, June 2000.

   [RFC3162]  Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",
              RFC 3162, August 2001.

   [RFC3193]  Patel, B., Aboba, B., Dixon, W., Zorn, G., and S. Booth,
              "Securing L2TP using IPsec", RFC 3193, November 2001.

   [RFC3315]  Droms, R., Bound, J., Volz, B., Lemon, T., Perkins, C.,
              and M. Carney, "Dynamic Host Configuration Protocol for
              IPv6 (DHCPv6)", RFC 3315, July 2003.




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   [RFC3371]  Caves, E., Calhoun, P., and R. Wheeler, "Layer Two
              Tunneling Protocol "L2TP" Management Information Base",
              RFC 3371, August 2002.

   [RFC3633]  Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic
              Host Configuration Protocol (DHCP) version 6", RFC 3633,
              December 2003.

   [RFC3948]  Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
              Stenberg, "UDP Encapsulation of IPsec ESP Packets",
              RFC 3948, January 2005.

   [RFC4818]  Salowey, J. and R. Droms, "RADIUS Delegated-IPv6-Prefix
              Attribute", RFC 4818, April 2007.

13.2.  Informative References

   [I-D.ietf-dhc-subnet-alloc]
              Johnson, R., "Subnet Allocation Option",
              draft-ietf-dhc-subnet-alloc-05 (work in progress),
              June 2007.

   [I-D.ietf-dhc-v6-relay-radius]
              Lau, W., "DHCPv6 Relay agent RADIUS Attribute Option",
              draft-ietf-dhc-v6-relay-radius-02 (work in progress),
              February 2006.

   [I-D.ietf-ipv6-2461bis]
              Narten, T., "Neighbor Discovery for IP version 6 (IPv6)",
              draft-ietf-ipv6-2461bis-11 (work in progress), March 2007.

   [I-D.ietf-ipv6-over-ppp-v2]
              Varada, S., "IP Version 6 over PPP",
              draft-ietf-ipv6-over-ppp-v2-03 (work in progress),
              May 2007.

   [I-D.ietf-softwire-problem-statement]
              Dawkins, S., "Softwire Problem Statement",
              draft-ietf-softwire-problem-statement-03 (work in
              progress), March 2007.

   [I-D.ietf-softwire-security-requirements]
              Yamamoto, S., "Softwire Security Analysis and
              Requirements",
              draft-ietf-softwire-security-requirements-02 (work in
              progress), March 2007.

   [I-D.stevant-softwire-accounting]



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              Stevant, B., "Accounting on Softwires",
              draft-stevant-softwire-accounting-01 (work in progress),
              October 2006.

   [RFC1471]  Kastenholz, F., "The Definitions of Managed Objects for
              the Link Control Protocol of the Point-to-Point Protocol",
              RFC 1471, June 1993.

   [RFC1473]  Kastenholz, F., "The Definitions of Managed Objects for
              the IP Network Control Protocol of the Point-to-Point
              Protocol", RFC 1473, June 1993.

   [RFC1877]  Cobb, S. and F. Baker, "PPP Internet Protocol Control
              Protocol Extensions for Name Server Addresses", RFC 1877,
              December 1995.

   [RFC2433]  Zorn, G. and S. Cobb, "Microsoft PPP CHAP Extensions",
              RFC 2433, October 1998.

   [RFC2461]  Narten, T., Nordmark, E., and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461,
              December 1998.

   [RFC2867]  Zorn, G., Aboba, B., and D. Mitton, "RADIUS Accounting
              Modifications for Tunnel Protocol Support", RFC 2867,
              June 2000.

   [RFC2868]  Zorn, G., Leifer, D., Rubens, A., Shriver, J., Holdrege,
              M., and I. Goyret, "RADIUS Attributes for Tunnel Protocol
              Support", RFC 2868, June 2000.

   [RFC3022]  Srisuresh, P. and K. Egevang, "Traditional IP Network
              Address Translator (Traditional NAT)", RFC 3022,
              January 2001.

   [RFC3646]  Droms, R., "DNS Configuration options for Dynamic Host
              Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
              December 2003.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)",
              RFC 3748, June 2004.

   [RFC3931]  Lau, J., Townsley, M., and I. Goyret, "Layer Two Tunneling
              Protocol - Version 3 (L2TPv3)", RFC 3931, March 2005.

   [RFC4087]  Thaler, D., "IP Tunnel MIB", RFC 4087, June 2005.




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   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4293]  Routhier, S., "Management Information Base for the
              Internet Protocol (IP)", RFC 4293, April 2006.

   [RFC4623]  Malis, A. and M. Townsley, "Pseudowire Emulation Edge-to-
              Edge (PWE3) Fragmentation and Reassembly", RFC 4623,
              August 2006.










































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Appendix A.  Revision History

   [Note to RFC Editor: please remove this entire appendix, and the
   corresponding entries in the table of contents, prior to
   publication.]

   Changes between -04 and -05:

   o  Replaced "documentation" with "logging purposes" in
      Section 5.1.1.1.

   o  Added suggested values (only as guidance) for Keepalives in
      Section 5.1.2.

   Changes between -03 and -04:

   o  Added missing references to [RFC4087], [RFC2461], and [RFC3646],
      moved [RFC4623] to Informative.

   o  Rephrasing in Section 6.2.2.  Added pointers Section 6.1.2 and
      Section 6.2.2 in Table 2.

   o  Added citations (and corresponding references) to [RFC1471] and
      [RFC1473] in Section 4.4, since Section 9.2 explicitly mentions:
      "MIB support for L2TPv2 and PPP are documented."

   o  Fixed some RFC2119 keywords in Section 3.1.1, Section 3.1.2,
      Section 3.1.3, Section 3.1.4, Section 3.2.1, Section 3.2.2,
      Section 3.2.3, Section 3.2.4, Section 5.1.1.3, Section 5.4.2, and
      Section 8.1.1.

   o  Added [RFC2868] to Section 4.3, and added missing citations to
      Section 4.

   o  Added missing "Optional AVP" to Figure 11.

   o  Updated the text in Section 6.2.2.

   o  Added some clarifying sentences in Section 5.1.1, and completed
      Section 5.1.1.1 and Section 5.1.1.2.

   o  Added an Informative reference to [RFC3022] for NAT/NAPT.

   o  Corrected reference to [RFC1661] in Section 5.2.2, removed
      reference and citation to RFC1662.

   o  Updated references, Delegated-IPv6-Prefix became [RFC4818] moved
      to Normative.



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   o  Added new address and email for J.F. Tremblay.

   o  Added an acknowledgement to the participants, and pointer to the
      minutes, for the Barcelona interim meeting.

   o  Moved the Softwire Problem Statement reference
      [I-D.ietf-softwire-problem-statement] to Informative.

   o  Some additional purely editorial changes.

   Changes between -02 and -03:

   o  Boiler changes in support of RFC 4748.

   o  Added text about L2TPv2 HELLO and LCP ECHO usage in Section 1,
      Section 2.6 and Section 5.1.2.

   o  Moved some downref to Informative ([RFC1877], [RFC2433],
      [RFC2867], [RFC2868], and [RFC3748]), and moved CHAP reference to
      Normative ([RFC1994]).

   o  Removed the mention and citation for PAP authentication.

   Changes between -01 and -02:

   o  Renamed all "Best Current Practices" sections as
      "Recommendations".  See for example Section 1.4.

   o  Moved Provisioning in Section 6.  Removed intro text to new
      Section 7.

   o  Removed all normative language from Section 6, Section 7,
      Section 8, and Section 9.

   o  Removed empty sections "Implementation Status", and "Open Issues".

   o  Fixed "Phase 0" in Section 1.

   o  Small changes to Section 3.1.

   o  Change L2TP -> L2TPv2 in some sections, including Section 6.

   o  Small additions and typo fixes in Section 5.1.1.1 and
      Section 5.1.1.2.

   o  Retitled Section 8 and Section 8.1, changed AAA -> RADIUS.





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   o  New paragraph in Section 9.1.

   o  New paragraph in Section 8.2.1, including a pointer to
      [I-D.ietf-dhc-v6-relay-radius].

   o  Moved last paragraph to start of Section 10.

   o  Moved some references from Normative to Informative.

   o  Label the steps in Figure 9 and Figure 10.

   o  Reword paragraph of Section 5.1.

   o  Describe more messages than flows in Figure 11 and Figure 12.

   o  Add text about Session Establishment between Figure 11 and
      Figure 12.

   o  Section 5.1.1.1 and Section 5.1.1.2: Updates on Hostname AVP,
      Framing Capabilities AVP, Framing Type AVP, Connect Speed AVP and
      Challenge and Challenge Response AVPs.

   o  Retitled Section 5.1.1.3.

   o  Updates on the use of L2TP HELLO versus LCP, delay for Dead-End
      peer detection, and allow LCP.

   o  Rewording in Section 5.1.3.

   o  Section 5.2.1: Add a pointer to [RFC4623] and small updates.

   o  Clarifications on PFC and ACFC, remove Figure 13.

   o  Section 5.2.3: make references to RFCs for PAP, CHAP, etc.

   o  Rewordings in Section 5.2.4.1 and Section 5.2.4.2.

   o  Added Informative references to [RFC4623], [RFC1661], [RFC2433],
      and [RFC3748].

   o  Renamed the title and added more details on Section 5.3 and IPv6
      ND, including a pointer to [I-D.ietf-ipv6-2461bis].

   o  Added text in Section 5.4 about IPv4 PD is non-trivial and non
      commonly done.

   o  Added B. Stevant as Editor.




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   o  Clarification in Section 5.2.4.1 and Section 5.2.4.2 regarding the
      scope of the MUST (to the specific scenarios).

   o  Removed considerations about reverse DNS from Section 6, agreed on
      Barcelona.

   o  Clarified the NAT case in Section 6.1.2

   o  Added first paragraph in Section 6.2.1 regarding delegated IPv6
      prefixes.

   o  Added new Section 6.3, Section 6.3.1 and Section 6.3.2 summarizing
      the scenarios for address allocation.

   Changes between -00 and -01:

   o  Changed alignment in all figures to be centered, and fixed
      Figure 9 reference.

   o  Section 1.4: Added new section with "Best Current Practices"
      definition.

   o  Marked the following sections as "Best Current Practices":
      Section 6, Section 8, and Section 9.

   o  Section 6.1.1, last paragraph: Removed sentence on IPv6 link
      address on the PPP link.  Mailing list comment from Florent
      Parent, 13-Jul-2006.

   o  Section 6.1.2, last paragraph: Changed IPv4 PPP link address to
      use host addresses (/32) negotiated with IPCP instead of /30.
      Mailing list comment from Bill Storer, 5-Jul-2006.

   o  Section 5, Figure 10: Correction, s/SCCSN/SCCCN/; added missing
      ICRP, and changed direction of ICCN; typo correction s/IPV6P/
      IPV6CP/.  Mailing list comment from Bill Storer, 5-Jul-2006.

   o  Section 5, Figure 10: Marked CHAP as "Optional - CHAP".

   o  Section 5.1, Section 5.1.1, and Section 5.1.3: Minor typographical
      error correction and rewording of some sentences for grammar.

   o  Section 5.1.1.1, Host Name AVP: Removed "for debugging purposes"
      and added that MAY be used to authenticate users.

   o  Section 5.1.1.1, Framing Capabilities AVP: Swapped SHOULD and
      MUST.  Mailing list comment from Bill Storer, 5-Jul-2006, text
      from Laurent Toutain.



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   o  Section 5.1.1.1, (Tx) Connect Speed: Correction: "but is *not*
      meaningful".

   o  Section 5.1.1.2, Challenge and Challenge Response AVPs: Removed
      the last sentence of the first paragraph.  Mailing list comment
      from Bill Storer, 5-Jul-2006, text from Laurent Toutain.

   o  Section 5.1.2: Rewrote paragraph to use L2TPv2 HELLO messages and
      not LCP Echo Request and Reply messages to detect tunnel failure,
      as redundant in Softwires.  Mailing list comment from Florent
      Parent, 10-Jul-2006, text from Bill Storer.

   o  Section 5.2.1, first paragraph: Fixed PPP MTU calculation.
      Mailing list comment from Florent Parent, 10-Jul-2006.

   o  Section 5.2.4.2: Rewrote to generalize the address assignment
      failure, to be an all-zeroes address or a protocol reject in
      response to the IPCP CONFREQ.  Mailing list comment from Bill
      Storer, 5-Jul-2006, text from JF Tremblay.

   o  Section 8, second paragraph: s/Tunnel-Medium /Tunnel-Type /.
      Mailing list comment from Bill Storer, 5-Jul-2006.

   o  Section 8.1.2: Added usage of Framed-IP-Address attribute, and if
      not present then use the Tunnel-Client-Endpoint and Tunnel-Server-
      Endpoint attributes.  Mailing list comment from Bill Storer,
      5-Jul-2006, text from JF Tremblay and Bill Storer.

   o  Completed Section 8.2.2, Section 9.1, Section 9.2, and Section 10.

   Revision -00:

   o  Initial revision.


















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Authors' Addresses

   Bill Storer
   Cisco Systems
   170 W Tasman Dr
   San Jose, CA  95134
   USA

   Email: bstorer@cisco.com


   Carlos Pignataro (editor)
   Cisco Systems
   7025 Kit Creek Road
   PO Box 14987
   Research Triangle Park, NC  27709
   USA

   Email: cpignata@cisco.com


   Maria Alice Dos Santos
   Cisco Systems
   170 W Tasman Dr
   San Jose, CA  95134
   USA

   Email: mariados@cisco.com


   Bruno Stevant (editor)
   GET/ENST Bretagne
   2 rue de la Chataigneraie CS17607
   Cesson Sevigne,   35576
   France

   Email: bruno.stevant@enst-bretagne.fr


   Jean-Francois Tremblay
   Trellia Networks
   100, Alexis-Nihon, Suite 770
   Montreal, QC  H4M 2P3
   Canada

   Email: jf@jftremblay.com





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Full Copyright Statement

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