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Versions: 00 01 02 03 RFC 4925

Network Working Group                                    S. Dawkins, Ed.
Internet-Draft                                                    Huawei
Expires: November 23, 2006                                  May 22, 2006


                       Softwire Problem Statement
              draft-ietf-softwire-problem-statement-02.txt

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   This Internet-Draft will expire on November 23, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The Softwires Working Group is specifying the standardization of
   discovery, control and encapsulation methods for connecting IPv4
   networks across IPv6-only networks and IPv6 networks across IPv4-only
   networks in a way that will encourage multiple, inter-operable vendor
   implementations.  At the highest level, the Softwires Working Group
   is tasked to identify, and extend where necessary, standard protocols
   to support a selected set of "IPv4/IPv6" and "IPv6/IPv4" transition
   problems.  This document describes the specific problems ("Hubs and
   Spokes" and "Mesh") that will be solved as part of a solution phase



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   following the completion of this document, within a relatively tight
   "time-to-market" as requested by operators at IETF 63.  Some
   individual requirements (and non-requirements) are also identified in
   this document at times in order to better describe the specific scope
   for a given problem definition.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Hubs and Spokes Problem  . . . . . . . . . . . . . . . . . . .  7
     2.1.  Description  . . . . . . . . . . . . . . . . . . . . . . .  9
     2.2.  Non-upgradable CPE router  . . . . . . . . . . . . . . . . 10
     2.3.  Network Address Translation (NAT) and Port Address
           Translation (PAT)  . . . . . . . . . . . . . . . . . . . . 11
     2.4.  Static Prefix Delegation . . . . . . . . . . . . . . . . . 11
     2.5.  Softwire Initiator . . . . . . . . . . . . . . . . . . . . 12
     2.6.  Softwire Concentrator  . . . . . . . . . . . . . . . . . . 12
     2.7.  Softwire Concentrator Discovery  . . . . . . . . . . . . . 12
     2.8.  Scaling  . . . . . . . . . . . . . . . . . . . . . . . . . 13
     2.9.  Routing  . . . . . . . . . . . . . . . . . . . . . . . . . 13
     2.10. Multicast  . . . . . . . . . . . . . . . . . . . . . . . . 13
     2.11. Security . . . . . . . . . . . . . . . . . . . . . . . . . 13
       2.11.1.  Authentication, Authorization and Accounting  . . . . 13
       2.11.2.  Privacy, Integrity, and Replay protection . . . . . . 14
     2.12. Operations and Management (O&M)  . . . . . . . . . . . . . 14
     2.13. Encapsulations . . . . . . . . . . . . . . . . . . . . . . 14
   3.  Mesh Problem . . . . . . . . . . . . . . . . . . . . . . . . . 15
     3.1.  Mesh Description . . . . . . . . . . . . . . . . . . . . . 16
     3.2.  Scaling  . . . . . . . . . . . . . . . . . . . . . . . . . 16
     3.3.  Persistence, Discovery and Setup Time  . . . . . . . . . . 17
     3.4.  Address Family/SAF Reachability  . . . . . . . . . . . . . 17
     3.5.  Softwire Encapsulation . . . . . . . . . . . . . . . . . . 17
     3.6.  Security . . . . . . . . . . . . . . . . . . . . . . . . . 18
     3.7.  Operations and Management  . . . . . . . . . . . . . . . . 18
     3.8.  Address Family Encapsulations  . . . . . . . . . . . . . . 19
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   6.  Changes from -01 . . . . . . . . . . . . . . . . . . . . . . . 22
   7.  Changes from -00 . . . . . . . . . . . . . . . . . . . . . . . 23
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 24
     8.1.  Authors  . . . . . . . . . . . . . . . . . . . . . . . . . 24
     8.2.  Contributors . . . . . . . . . . . . . . . . . . . . . . . 25
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 27
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 27
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 27
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 29



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   Intellectual Property and Copyright Statements . . . . . . . . . . 30


















































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1.  Introduction

   The Softwires Working Group is specifying the standardization of
   discovery, control and encapsulation methods for connecting IPv4
   networks across IPv6-only networks and IPv6 networks across IPv4-only
   networks in a way that will encourage multiple, inter-operable vendor
   implementations.  This document is describing the scenarios that the
   Working Group is going to focus on leading toward defining solutions.
   A few generic assumptions are listed up front:

   o  Local Area Networks will often support both protocol families in
      order to accommodate both IPv4-only and IPv6-only applications, in
      addition to dual-stack applications.  Global reachability requires
      the establishment of softwire connectivity to transit across
      portions of the network that do not support both address families.
      Wide area networks that support one or both address families may
      be separated by transit networks that do not support all address
      families.  Softwire connectivity is necessary to establish global
      reachability of both address families.

   o  Softwires are to be used in IP-based networks to forward both
      unicast and multicast traffic.

   o  Softwires are assumed to be non-ephemeral in nature.

   o  Although Softwires are long-lived, the setup time of a softwire is
      expected to be a very small fraction of the total time required
      for startup of the Customer Premise Equipment (CPE)/Address Family
      Border Router (AFBR).

   o  The nodes that actually initiate softwires should support dual-
      stack (IPv4 and IPv6) functionality.

   o  The goal of this effort is to reuse or extending existing
      technology.  The 'time-to-market' requirement for solutions to the
      stated problems is very strict and existing, deployed technology
      must be very strongly considered in our solution selection.

   The history of IPv4 and IPv6 transition strategies at the IETF is a
   very long and complex.  Here we list out some steps we have taken to
   further the effort and it has lead to the creation of this document
   and a few 'working rules' for us to accomplish our work:

   o  At the IETF 63 "LightWeight Reachability softWires" (LRW) BOF
      meeting, attendees from several operators requested a very tight
      timeframe for delivery of a solution, based on time-to-market
      considerations.  This problem statement is narrowly scoped to
      accommodate near-term deployment.



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   o  At the Paris Softwires interim meeting in October, 2005,
      participants divided the overall problem space into two separate
      "sub-problems" to solve based on network topology.  These two
      problems are referred to as "Hubs and Spokes" (described in
      section 3) and "Mesh" (described in Section 4).

   As stated, there are two scenarios that emerged when discussing the
   traversal of networks composed of differing address families.  The
   scenarios are quite common in today's network deployments.  The
   primary difference between "Spokes and Hubs" and "Mesh" is how many
   connections and associated routes are managed by each IPv4 or IPv6
   "island".  "Hubs and Spokes" is characterized with one connection and
   associated static default route, and "Mesh" is characterized by
   multiple connections and routing prefixes.  In general, the two can
   be categorized as host or LAN connectivity and network (or VPN)
   connectivity problems.  Looking at the history of multi-address
   family networking, the clear delineation of the two scenarios was
   never clearly illustrated but they are now the network norm, and both
   must be solved.  Later during the solution phase of the WG, these
   problems will be treated as related, but separate, problem spaces.
   Similar protocols and mechanisms will be used when possible, but
   different protocols and mechanisms may be selected when necessary to
   meet the requirements of each given problem space.

1.1.  Terminology

   Address Family (AF) - IPv4 or IPv6.  Presently defined values for
   this field are specified in
   http://www.iana.org/assignments/address-family-numbers.

   Address Family Border Router (AFBR) - The router that interconnects
   two networks that use different address families.

   Customer Premise Equipment (CPE) - Under the scope of this document,
   this refers to terminal and associated equipment and inside wiring
   located at a subscriber's premises and connected with a carrier's
   communication channel(s) at the demarcation point (" demarc ").  The
   demarc is a point established in a building or complex to separate
   customer equipment from telephone, cable or other service provider
   equipment.  CPE can be a host or router, depending on the specific
   characteristics of the access network.  The demarc point for IPv4 may
   or may not be the same as the demarc point for IPv6, thus there can
   be one CPE box acting for IPv4 and IPv6 or two separate ones, one for
   IPv4 and one for IPv6.

   Home gateway - Existing piece of equipment that connects the home
   network to the provider network.  Usually act as CPE for one or both
   address family.



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   Softwire (SW) - A "tunnel" that is created on the basis of a control
   protocol setup between softwire endpoints with shared point-to-point
   or multipoint-to-point state.  Softwires are generally dynamic in
   nature (they may be initiated and terminated on demand), but may be
   very long-lived.

   Softwire Concentrator (SC) - The node terminating the softwire in the
   service provider network.

   Softwire Initiator (SI) - The node initiating the softwire within the
   customer network.

   Softwire Transport Header AF (STH AF) - the address family of the
   outermost IP header of a softwire.

   Softwire Payload Header AF (SPH AF) - the address family of the IP
   headers being carried within a softwire.  Note that additional
   "levels" of IP headers may be present if (for example) a tunnel is
   carried over a softwire - the key attribute of SPH AF is that it is
   directly encapsulated within the softwire and the softwire endpoint
   will base forwarding decisions on the SPH AF when a packet is exiting
   the softwire.

   Subsequent Address Family (SAF) - Additional information about the
   type of the additional information about the type of the Network
   Layer Reachability Information (e.g. unicast or multicast).

























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2.  Hubs and Spokes Problem

   The "Hubs and Spokes" problem is named in reference to the airline
   industry where major companies have establised a relatively small
   number of well connected hubs and then serve smaller airports from
   those hubs.

   In some applications, manually configured tunnels (as described in
   [RFC4213] are sufficient as a transition mechanism.  For a variety of
   reasons (for example, use of dynamic IP addresses, and NAT
   traversal), other solutions are also necessary.

   There are four variant cases of the Hubs and Spokes problem which are
   shown in the following figures.


                         +-------+  +------------+  +--------+
                         |       |  |Softwire    |  | IPv6   |
            +---------+  | IPv4  |--|concentrator|--| Network|=>Internet
            |v4/v6    |--|       |  +------------+  +--------+
            |Host CPE |  |       |
            +---------+  |Network|
                         +-------+
                       _ _ _ _ _ _ __
                     ()_ _ _ _ _ _ __()      IPv6 SPH
                         "softwire"
                     |--------------||-------------------------|
                        IPv4-only        IPv6 or dual-stack

   Case 1: IPv6 connectivity across an IPv4-only access network (STH).
   Softwire initiator 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.

   Figure 1: Case 1
















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                      +-------+  +-------------+  +--------+
                      |       |  | Softwire    |  |   v6   |
   +-----+  +------+  |  v4   |--| concentrator|--| Network|=>Internet
   |v4/v6|--|v4/v6 |--|       |  +-------------+  +--------+
   |Host |  |Router|  |Network|
   +-----+  |v4/v6 |  |       |
            |  CPE |  +-------+
            +------+
                    _ _ _ _ _ _ __
                  ()_ _ _ _ _ _ __()                          IPv6 SPH
                      "softwire"
   |--------------||--------------||-------------------------|
      Dual-stack       IPv4-only        IPv6 or dual-stack

   Case 2: IPv6 connectivity across an IPv4-only access network (STH).
   Softwire initiator is the router CPE, which is a dual-stack device.
   The IPv4 traffic should not traverse the softwire.

   Figure 2: Case 2


                       +-------+  +-------------+  +--------+
                       |       |  | Softwire    |  |   v6   |
   +------+  +------+  |  v4   |--| concentrator|--| Network|=>Internet
   |v4/v6 |--|v4    |--|       |  +-------------+  +--------+
   |Host  |  |Router|  |Network|
   |v6 CPE|  |v4 CPE|  |       |
   +------+  |      |  +-------+
             +------+
          _ _ _ _ _ _ _ _ _ _ _ _
        ()_ _ _ _ _ _ _ _ _ _ _ _()                           IPv6 SPH
                "softwire"
         |-----------------------||-------------------------|
                  IPv4 only           IPv6 or dual-stack

   Case 3: IPv6 connectivity across an IPv4-only access network (STH).
   The CPE is IPv4-only.  Softwire initiator is a host, which act as an
   IPv6 host CPE.  The IPv4 traffic should not traverse the softwire.

   Figure 3: Case 3











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   +-----+
   |v4/v6|                +-------+  +------------+  +-------+
   |Host |                |       |  |Softwire    |  |  v6   |
   +-----+      +------+  |  v4   |--|concentrator|--|Network|=>Internet
      |         |v4    |--|       |  +------------+  +-------+
      |---------|Router|  |Network|
      |         |v4 CPE|  +-------+
   +---------+  +------+
   |Softwire |
   |Initiator|
   |v6 Router|
   |   CPE   |
   +---------+
              _ _ _ _ _ _ _ _ _ _ _ _
            ()_ _ _ _ _ _ _ _ _ _ _ _()                       IPv6 SPH
                       "softwire"
   |--------||-----------------------||----------------------|
      Dual           IPv4 only             IPv6 or dual-stack
      stack

   Case 4: IPv6 connectivity across an IPv4-only access network (STH).
   The routing CPE is IPv4-only.  Softwire initiator is a device acting
   as an IPv6 CPE router inside the home network.  The IPv4 traffic
   should not traverse the softwire.

   Figure 4: Case 4

   The converse cases exist, replacing IPv4 by IPv6 and vice versa in
   the above figures.

2.1.  Description

   In this scenario, carriers (or large enterprise networks acting as
   carriers for their internal networks) have an infrastructure which in
   at least one device on any given path supports only one address
   family, with customers who wish to support applications bound to an
   address family that cannot be routed end-to-end.  The address family
   that must be "crossed" is called the Softwire Transport Header, or
   STH AF (which could be either IPv4 or IPv6).

   In order to support applications bound to another address family (the
   Softwire Payload Header Address Family, or SPH AF), it is necessary
   to establish a virtual dual-stack infrastructure (end-to-end),
   typically by means of automatically-established tunnels (Softwires).
   The traffic that can traverse the network via its native AF must not
   be forced to take the softwire path.  Only the traffic that otherwise
   would not be able to be forwarded due to the AF mismatch should be
   forwarded within the softwire.  The goal is to avoid overwhelming the



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   softwire concentrator (SC).

   A network operator may choose to enable a single address family in
   one or several parts of this infrastructure for policy reasons (i.e.,
   traffic on the network is dominant in one of the address families, a
   single address family is used to lower OAM cost, etc.) or for
   technical reasons (i.e., because one or more devices are not able to
   support both address families).

   There are several obstacles that may preclude support for both
   address families:

   a) One or more devices (routers or some other media-specific
   aggregation point device) being used across the infrastructure (core,
   access) that supports only one address family.  Typically the reasons
   for this situation include a lack of vendor support for one of the
   address families, the (perceived) cost of upgrading them, the
   (perceived) complexity of running both address families natively,
   operation/management reasons to avoid upgrades (perhaps temporarily),
   or economic reasons (such as a commercially insignificant amount of
   traffic with the non-supported address family).

   b) The home gateway (CPE router or other equipment at the demarc
   point), cannot be easily upgraded to support both address families.
   Typically the reason for this is the lack of vendor support for one
   of the address families, commercial or operational reasons for not
   carrying out the upgrade (i.e., operational changes and/or cost may
   need to be supported by the carrier for all the customers, which can
   turn into millions of units), or customer reluctance to change/
   upgrade CPE router (cost, "not broken, so don't change it").

2.2.  Non-upgradable CPE router

   Residential and small-office CPE equipment may be limited to support
   only one address family.  Often, they are owned by a customer or
   carrier who is unwilling or unable to upgrade them to run in dual
   stack mode (as shown in Figure 3 and Figure 4).

   When the CPE router cannot run in dual-stack mode a softwire will
   have to be established by a node located behind that CPE router.
   This can be accomplished either by a regular host in the home running
   softwire software (Figure 1 or Figure 3) or by a dedicated piece of
   hardware acting as the "IPv6 router" (Figure 4).  Such a device is
   fairly simple in design and only requires one physical network
   interface.  Again, only the traffic of the mismatched AF will be
   forwarded via the softwire.  Traffic that can otherwise be forwarded
   without a softwire should not be encapsulated.




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2.3.  Network Address Translation (NAT) and Port Address Translation
      (PAT)

   A typical case of non-upgradable CPE router is a pre-existing IPv4/
   NAT home gateway, so the softwires solution must support NAT
   traversal.

   If the NAT is not in the home gateway, but in carrier equipment
   located at the other end of the access link (typically in an carrier
   POP), support for NAT traversal is still required.

   Establishing a Softwire through NAT or PAT must be supported without
   an explicit requirement to "autodetect" NAT or PAT presence during
   softwire setup.  Simply enabling NAT traversal could be sufficient to
   meet this requirement.

   Although the tunneling protocol must be able to traverse NATs,
   tunneling protocols may have an optional capability to bypass UDP
   encapsulation if not traversing a NAT.

2.4.  Static Prefix Delegation

   An important characteristic of this problem in IPv4 networks is that
   the carrier-facing CPE IP address is typically dynamically assigned.
   Also, if the softwire has to be established from a node behind a CPE
   router, that node IP address can also be dynamically assigned.  In
   cases where static IP addresses are unavailable, dynamic addresses
   are a problem for some Internet accessible services.  Solutions like
   external dynamic DNS and dynamic NAT port forwarding have been
   deployed, but it would be simpler if, in IPv6 networks, a static
   prefix was delegated to the customer, even in the case of single node
   network.  That prefix would allow for the registration of stable
   addresses in the DNS and also enough room to use either RFC3041
   privacy extension or cryptographically generated addresses (CGA)
   [RFC3972].  The softwire protocol does not need to define a new
   method for prefix delegation however DHCPv6 prefix delegation must be
   able to run over a softwire.  Note also that the IP addresses of the
   softwire link itself do not need to be stable, as, even if a single
   PC is attached behind it, a /64 prefix will be delegated.

   Link local addresses allocated at both ends of the tunnel are enough
   for packet forwarding, but for management purpose like traceroute,
   global addresses can be allocated using existing protocols such as
   Neighbor Discovery or DHCPv6.

   Similarly, in the case of an IPv4 softwire, the address could be
   provided by means of DHCP.  In the case of an IPv4 softwire, a
   mechanism should be available in order to delegate an IPv4 prefix.



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2.5.  Softwire Initiator

   In the Hubs and Spokes problem, softwires are always initiated by the
   customer side.  Thus, the node hosting the end of the softwire within
   the customer network is called the softwire initiator.  It can run on
   any dual-stack node.  As noted earlier, this can be the CPE access
   device, another dedicated CPE router behind the original CPE access
   device or simply any kind of node (host, appliance, sensor, etc.).

   The softwire initiator does not have to be always the same node
   and/or always have been delegated the same IP address.  In
   particular, softwires should work in the nomadic case (e.g. a user
   opening up his laptop in various Wi-Fi hot-spots), since the softwire
   initiator could potentially obtain an IP address of one address
   family outside its original carrier network and still want to obtain
   the other address family addresses from its original carrier.

   IPv4 provider can also periodically change the IPv4 address allocated
   to the gateway.  The softwire initiator has to discover in a
   reasonable period of time that the tunnel is down and restart tunnel
   establishment.  This re-establishment should not change the IPv6
   prefix and other parameters allocated to the site.

2.6.  Softwire Concentrator

   On the carrier side, softwires are terminated on a softwire
   concentrator.  An carrier may deploy several softwire concentrators
   (for example one per POP) for scalability reasons.  A softwire
   concentrator is in practice a dual-stack router connected to the
   dual-stack core of the carrier or directly to the upstream providers.
   Softwire concentrators are not nomadic and have stable IP addresses.
   It may be the case that one of the address families is not natively
   supported, even if this is not optimal, in the softwire concentrator,
   but instead by means of tunnels to the upstreams (or other networks).

   Softwire concentrator functionality will be based on existing
   standards for tunneling, prefixes and addresses allocation,
   management.  The working group must define a Softwires Concentrator
   architecture and interaction between these protocols and recommend
   profiles.  These recommendations must take into account the
   distributed nature of the Softwires Concentrator in the provider
   network and the impact on core IPv6 networks (for instance: prefix
   aggregation).

2.7.  Softwire Concentrator Discovery

   The softwire initiator must know the DNS name or IP address of the
   softwire concentrator.  An automated discovery phase may be used to



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   return the IP address(s), or name(s) of the concentrator.
   Alternatively, this information may be configured by the user, or or
   by the provider of the softwire initiator in advance.  The details of
   this discovery problem are outside the scope of this document,
   however previous work could be taken in consideration.  Examples
   include: [I-D.durand-naptr-service-discovery], [I-D.ietf-v6ops-ipsec-
   tunnels], and [I-D.palet-v6ops-tun-auto-disc].

2.8.  Scaling

   In a 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).  DNS redirection and/or local anycast
   addresses among other choices, coupled with the (to-be-determined)
   softwire concentrator discovery solution will enable sharing the load
   among concentrators.

2.9.  Routing

   As customer networks are typically attached via a single link to
   their carrier, the minimum routing requirement is a default route for
   each of the address families.

2.10.  Multicast

   Existing multicast solutions can be used over the softwire.
   Typically, such solution would be either Multicast Listener
   Discovery/Internet Group Membership Protocol proxy [I-D.ietf-magma-
   igmp-proxy] or Protocol-Independent Multicast.

2.11.  Security

2.11.1.  Authentication, Authorization and Accounting

   The softwire protocol must support customer authentication in the
   control plane, in order to authorize access to the service, and
   provide adequate logging of activity (accounting).  However, an
   carrier may decide to turn it off in some circumstances, for
   instance, when the customer is already authenticated by some other
   means, such as closed networks, cellular networks, etc., in order to
   avoid unnecessary overhead.

   The protocol should offer mutual authentication in scenarios where
   the initiator requires identity proof from the concentrator.

   The softwire solution, at least for "Hubs and Spokes", must be
   integrable with commonly deployed AAA solutions (although extensions
   to those AAA solutions may be needed).



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2.11.2.  Privacy, Integrity, and Replay protection

   The softwire Control and/or Data plane must be able to provide full
   payload security (such as IPsec or SSL) when desired.  This
   additional protection must be separable from the tunneling aspect of
   the softwire mechanism itself.  For IPsec, default profiles must be
   defined. [draft-ietf-v6ops-ipsec-tunnels] provides guidelines on
   this.

2.12.  Operations and Management (O&M)

   As it is assumed that the softwire may have to go across NAT or PAT,
   a keepalive mechanism must be defined.  Such a mechanism is also
   useful for dead peer detection.  However in some circumstances (i.e.,
   narrowband access, billing per traffic, etc.) the keepalive mechanism
   may consume unnecessary bandwidth, so turning it on or off, and
   modifying the periodicity, must be supported administrative options.

   Other needed O&M features include:

   - Logging

   - Usage accounting

   - End-point failure detection (the detection mechanism must operate
   within the tunnel)

   - Path failure detection (the detection mechanism must operate
   outside the tunnel)

2.13.  Encapsulations

   IPv6/IPv4, IPv6/UDP/IPv4 and IPv4/IPv6 are on the critical path for
   "Hubs and Spokes" softwires.  Other encapsulations, like IPv6/IPv6 or
   IPv4/IPv4, are nice to have but not on the critical path.  There is
   no intention to place limits on additional encapsulations beyond
   those explicitly mentioned in this specification.














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3.  Mesh Problem

   The "Mesh" problem is named in reference to typical routing problems
   in which there are more than one paths to a destination and a routing
   protocol is needed to select the best path.  It is also extremely
   similar to the problems that the L3VPN Working Group is tackling in
   which reduced, private and/or overlapping virtual routing and
   forwarding tables are announced.  The key difference is that the
   reachability that must be announced across the transit core will
   include more than VPN address family routes.

                   +----------+            +----------+
                   |differing |            |differing |
                   |AF as core|            |AF as core|
                   |access    |            |access    |
                   |island    |            |island    |
                   +----------+            +----------+
                       |                    |
                       |                    |
                   Dual-Stack           Dual-Stack
                     "AFBR"               "AFBR"
                       |                    |
                       |                    |
                   +----------------------------+
                   |                            |
                   |                            |
   +-------+       |                            |       +-------+
   |same AF|       |       Single AF only       |       |same AF|
   |as core|-------|        transit core        |-------|as core|
   |access |       |                            |       |access |
   |network|       |                            |       |network|
   +-------+       |                            |       +-------+
                   |                            |
                   +----------------------------+
                      |   /              \    |
                      |  /                \   |
                    Dual-Stack          Dual-Stack
                     "AFBR"              "AFBR"
                      | |                   |
                      | |                   |
                   +--------+            +--------+
                   |dual    |            |dual    |
                   |stack   |            |stack   |
                   |access  |            |access  |
                   |island  |            |island  |
                   +--------+            +--------+

   Figure 5: Mesh Topology



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3.1.  Mesh Description

   In this problem, carriers (or large enterprise networks acting as
   carrier for their internal resources) may be required to establish
   connectivity to 'islands' of networks of one address family type
   across a transit core of a differing address family type.  For an
   example, see Figure 5.  To provide reachability across the transit
   core, dual-stack devices are installed that act as "Address Family
   Border Routers."  These AFBRs can be performing peering across
   autonomous systems or, performing as Provider Edge routers (PE) in
   VPN parlance within an autonomous system.  With respect to deployment
   considerations, the islands do not have to be upgraded at the time of
   deploying the transit core and interwork as if there was no awareness
   of the AFBR.

   The AFBRs are the only devices in the carrier's network that must be
   able to perform dual-stack operations and setup and encapsulate
   softwires in a mesh to the other islands.  They then pass
   reachability information as appropriate according to policy.  They
   may be multiply connected to the transit network and thus, have to be
   able to exchange appropriate information and make a routing selection
   choice as to the best exit point.  Note that this creates multipoint-
   to-point reachability using a point-to-point logical overlay of
   softwire connectivity.

   It should also be noted that the mesh problem can be considered as a
   derivative of L3VPN, where the core provides transit in one address
   family and the islands are connected via L3VPN of another address
   family.  This analogy only holds true if the islands can to be
   represented as VPNs.  In general, the diagrams frequently used for
   L3VPNs is very similar.  The key point is that the reachability
   information that is to be exchanged must not be limited to VPNs or
   any single AF or SAF or combination of AF/SAF.  The solution must be
   generic enough to carry any AF or SAF.

   In the future a tunnel concentrator may be a different device than
   the AFBR that is announcing reachability.  In that future phase, the
   AFBR may need to announce a third party tunnel concentrator.

3.2.  Scaling

   In the mesh problem, the number of AFBRs is on the order of the
   number of islands though it should be clear that a single AFBR could
   handle many islands if the islands have distinct routing and
   forwarding tables.  A primary issue in the Mesh problem is that the
   size of the routing tables exchanged between the islands is of the
   order of the 'full Internet' (with respect to the island's native
   Address Family) plus any VPNs.  These tables plus the routing tables



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   associated with the transit core (and VPNs of the same AF as the
   transit core) must be stored on the AFBRs.  The number of peering
   points of an AFBR will be on the order of the number of Autonomous
   System Border Routers (ASBRs), which are assumed to be multiply
   peered to the transit core (multi-homed) for reliability.  An island
   can also have multiple AFBRs for reliability as well.  Both the
   island or the transit core may contain route reflectors or
   hierarchical routing with impunity.

   An AFBR should be able to pass route filters of data or routing
   tables it does not wish to receive.  Peering AFBRs must adhere to the
   route or route table filters and not send reachability information.
   Other attributes that can be sent from one AFBR to the other may
   include "no export" or similar mechanisms to prevent subsequent
   reannouncements of reachability information.  The scaling of the
   information to be exchanged is on the order of similar data exchanged
   for L3VPNs.

3.3.  Persistence, Discovery and Setup Time

   Discovery of the AFBRs and softwire encapsulation could be
   accomplished by the routing protocol during capability advertisement.
   An alternative is that the endpoints could be passed in new data
   formats or attributes, within a routing protocol.

   The duration of the softwire for inter-island reachability is
   considered to be as long as the duration of the peering session.
   Thus, dynamicity is very low.  The setup time should be on the order
   of the same duration to setup L3VPNs.

3.4.  Address Family/SAF Reachability

   It has been reported that the softwires to connect the islands will
   need to be able to perform IPv4/IPv6, IPv6/IPv4 and be able to
   exchange multicast and VPN routing tables.  The islands will need to
   be able to perform multicast routing and if the transit core does not
   provide native multicast services, the "classic" multicast solutions
   can be used over the softwires.  If native multicast services are
   enabled, further work may need to be accomplished to optimize the
   multicast forwarding path, receiver transmission load or receiver
   load.

3.5.  Softwire Encapsulation

   In the strictest sense, the softwire encapsulation has to be dual
   stack.  There is no requirement that only one encapsulation technique
   must be used.  It could be possible to have more than one available
   at each AFBR.  The AFBR must be able to prioritize which



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   encapsulation technique it will use if there is more than one
   available.

   The encapsulations used to traverse the transit core must be enabled
   to handle a choice of methods.  Common choices that should create a
   minimal set would include: L2TPv3, IP in IP, MPLS, IPsec, GRE.  The
   choice of encapsulation must not be subject to either an island or
   peer-wise limitation.  Different AF/SAF combinations must be able to
   be encapsulated differently according to the requirements of the
   network deployment.  For example, IPv4 unicast may be encapsulated in
   MPLS while IPv4 VPNs may be encapsulated in IPsec or L2TPv3.  This
   flexibility should not cause multiple peering sessions although it is
   not precluded that this may be the desired network deployment.  There
   must be a scheme in which preferencing the encapsulation to be used
   is exchanged between peers.  Also, once the softwire encapsulation is
   established a minimal amount of information must be passed with
   reachability information to connect the AF/SAF reachability to
   softwire.  The linking of reachability information should not be
   passed on a per route basis.

3.6.  Security

   In contrast with the hubs and spokes problem, routers are advertising
   route for relatively large network islands, not individual users, so
   fine-grained authentication is not necessary.  However the solution
   should support security of the softwire mechanism itself or the
   softwire data plane or both.

   In the softwire initialization mechanism, the softwire solution must
   support authentication, but an carrier may decide to turn it off in
   some circumstances.  This means that if a routing protocol is used to
   advertise the softwire encapsulation, it must also support
   authentication.

   In the data plane, the softwire solution must support IPsec and an
   IPsec profile must be defined. (see recommendations in [I-D.bellovin-
   useipsec]).

   The verification of the reachability information exchanged and issues
   surrounding the security of routing protocols themselves is outside
   the scope of the specification.

3.7.  Operations and Management

   There have been no reports of NATs between the AFBRs (in the transit
   core) so a NAT detection solution is not needed.

   Other O&M needed features include:



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   - Usage accounting

   - End-point failure detection (must be encapsulated within the tunnel
   in the transmitting direction)

   - Path failure detection

   Upon failure of a softwire, all reachability information must be
   withdrawn or a backup path used immediately.

3.8.  Address Family Encapsulations

   IPv6/IPv4, IPv4/IPv6 and overlapping address space as defined in the
   L3VPN working group (including overlapping RFC-1918 private address
   space) are on the critical path for "Mesh" softwires.  Other
   encapsulations, like IPv6/IPv6, IPv4/IPv4 or IP-only LAN Service
   (IPLS) as defined in the L2VPN working group, are nice to have but
   not on the critical path.There is no intention to place limits on
   additional encapsulations beyond those explicitly mentioned in this
   specification.































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4.  Security Considerations

   Security considerations specific to the "Hubs and Spokes" and "Mesh"
   models appear in those sections of the document.

   As with any tunneling protocol, using this protocol may introduce a
   security issue by circumventing a site security policy implemented as
   ingress filtering, since these filters will only be applied to STH AF
   IP headers.










































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5.  IANA Considerations

   There are no IANA actions requested in this specification.
















































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6.  Changes from -01

   1.  Detailed mailing list comments from Jordi Palet Marinez
       (2006/03/07).

   2.  Detailed mailing list comments from Pekka Savola (2006/05/03).













































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7.  Changes from -00

   1.   Individual-draft authors moved to Authors section, and added an
        acknowledgements section.

   2.   Detailed mailing list comments from Alain Baudot (2005/12/20).

   3.   Detailed mailing list comments from Pekka Savola (2005/12/22).

   4.   Detailed mailing list comments from Laurent Toutain
        (2005/12/26).

   5.   Detailed mailing list comments from Francis Dupont (editorial)
        (2005/12/29).

   6.   Detailed mailing list comments from Francis Dupont (non-
        editorial) (2005/12/29).

   7.   Detailed mailing list comments from Tom Pusateri (2005/12/29).

   8.   Detailed mailing list comments from Tom Alain Durant
        (2005/12/30).

   9.   Changed all occurances of "HGW" to "CPE" and added definitio

   10.  Removed all occurances of "TEP" (which seemed to be a synonym
        for concentrator anyway).

   11.  Changed all occurances of "ISP" to be "operator".

   12.  Removed all RFC 2119 language from the specification (since it's
        a problem statement).

   13.  Further linguistic clarificatons and edits (2006/01 and 02)

   14.  Remove Compare and Contrast section after discussion w/ authors
        (2006/02/19)














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8.  Acknowledgements

8.1.  Authors

   These are the principal authors for this document.

      Xing Li
      CERNET
      Room 225 Main Building, Tsinghua University
      Beijing 100084
      China

      Phone: +86 10 62785983
      Fax:   +86 10 62785933
      Email: xing@cernet.edu.cn




      Alain Durand
      Comcast
      1500 Market st
      Philadelphia
      PA 19102 USA

      Email: Alain_Durand@cable.comcast.com




      Shin Miyakawa
      NTT Communications
      3-20-2 TOC 21F, Nishi-shinjuku, Shinjuku
      Tokyo
      Japan

      Phone: +81-3-6800-3262
      Fax:   +81-3-5365-2990
      Email: miyakawa@nttv6.jp












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      Jordi Palet Martinez
      Consulintel
      San Jose Artesano, 1
      Alcobendas - Madrid
      E-28108 - Spain

      Phone: +34 91 151 81 99
      Fax:   +34 91 151 81 98
      Email: jordi.palet@consulintel.es




      Florent Parent
      Hexago
      2875 boul. Laurier, suite 300
      Sainte-Foy, QC  G1V 2M2
      Canada

      Phone: +1 418 266 5533
      Email: Florent.Parent@hexago.com




      David Ward
      Cisco Systems
      170 W. Tasman Dr.
      San Jose, CA 95134
      USA

      Phone: +1-408-526-4000
      Email: dward@cisco.com



8.2.  Contributors

   The authors would like to acknowledge the following contributors who
   provided helpful inputs on earlier versions of this document: Alain
   Baudot, Hui Deng, Francis Dupont, Rob Evans, Ed Koehler Jr, Erik
   Nordmark, Soohong Daniel Park, Tom Pusateri, Pekka Savola, Bruno
   Stevant, Laurent Totain, Bill Storer, Maria (Alice) Dos Santos, Yong
   Cui, Chris Metz, Simon Barber, Skip Booth, Scott Wainner and Carl
   Williams.

   The authors would also like to acknowledge the participants in the
   Softwires interim meeting in Paris, France (October 11-12, 2005)



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   (minutes are at
   http://bgp.nu/~dward/softwires/InterimMeetingMinutes.htm).

   The authors would also like to express a special acknowledgement and
   thanks to Mark Townsley.  Without his leadership, persistence,
   editing skills and thorough suggestions for the document; we would
   have not have been successful.

   Tunnel-based transition mechanisms have been under discussion in the
   IETF for more than a decade.  Initial work related to softwire can be
   found in RFC3053.  The earlier "V6 Tunnel Configuration" BOF problem
   statement [I-D.palet-v6tc-goals-tunneling] includes a reasonable
   pointer to prior work.






































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9.  References

9.1.  Normative References

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

   [RFC2663]  Srisuresh, P. and M. Holdrege, "IP Network Address
              Translator (NAT) Terminology and Considerations",
              RFC 2663, August 1999.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              January 2001.

   [RFC3053]  Durand, A., Fasano, P., Guardini, I., and D. Lento, "IPv6
              Tunnel Broker", RFC 3053, January 2001.

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

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

9.2.  Informative References

   [I-D.bellovin-useipsec]
              S, "Guidelines for Mandating the Use of IPsec,
              draft-bellovin-useipsec-04", September 2005.

   [I-D.durand-naptr-service-discovery]
              A, ""Service Discovery using NAPTR records in DNS",
              draft-durand-naptr-service-discovery-00", October 2004.

   [I-D.ietf-v6ops-ipsec-tunnels]
              P, ""Using IPsec to Secure IPv6-in-IPv4 Tunnels",
              draft-ietf-v6ops-ipsec-tunnels-01", August 2005.

   [I-D.palet-v6ops-solution-tun-auto-disc]
              J, ""IPv6 Tunnel End-point Automatic Discovery Mechanism",
              draft-palet-v6ops-solution-tun-auto-disc-01",
              October 2004.

   [I-D.palet-v6ops-tun-auto-disc]
              J and M, ""Analysis of IPv6 Tunnel End-point Discovery
              Mechanisms", draft-palet-v6ops-tun-auto-disc-03",
              January 2005.




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   [I-D.palet-v6tc-goals-tunneling]
              J, ""Goals for Tunneling Configuration",
              draft-palet-v6tc-goals-tunneling-00", February 2005.
















































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Author's Address

   Spencer Dawkins (editor)
   Huawei Technologies (USA)
   1700 Alma Drive, Suite 100
   Plano, TX  75075
   US

   Phone: +1 972 509 0309
   Fax:   +1 469 229 5397
   Email: spencer@mcsr-labs.org








































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