





Network Working Group                                              J. Wu
Request for Comments: 5565                                        Y. Cui
Category: Standards Track                            Tsinghua University
                                                                 C. Metz
                                                                E. Rosen
                                                     Cisco Systems, Inc.
                                                               June 2009


                        Softwire Mesh Framework

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (c) 2009 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents in effect on the date of
   publication of this document (http://trustee.ietf.org/license-info).
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.










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Abstract

   The Internet needs to be able to handle both IPv4 and IPv6 packets.
   However, it is expected that some constituent networks of the
   Internet will be "single-protocol" networks.  One kind of single-
   protocol network can parse only IPv4 packets and can process only
   IPv4 routing information; another kind can parse only IPv6 packets
   and can process only IPv6 routing information.  It is nevertheless
   required that either kind of single-protocol network be able to
   provide transit service for the "other" protocol.  This is done by
   passing the "other kind" of routing information from one edge of the
   single-protocol network to the other, and by tunneling the "other
   kind" of data packet from one edge to the other.  The tunnels are
   known as "softwires".  This framework document explains how the
   routing information and the data packets of one protocol are passed
   through a single-protocol network of the other protocol.  The
   document is careful to specify when this can be done with existing
   technology and when it requires the development of new or modified
   technology.

Table of Contents

   1. Introduction ....................................................3
   2. Specification of Requirements ...................................6
   3. Scenarios of Interest ...........................................7
      3.1. IPv6-over-IPv4 Scenario ....................................7
      3.2. IPv4-over-IPv6 Scenario ....................................9
   4. General Principles of the Solution .............................10
      4.1. E-IP and I-IP .............................................10
      4.2. Routing ...................................................10
      4.3. Tunneled Forwarding .......................................11
   5. Distribution of Inter-AFBR Routing Information .................11
   6. Softwire Signaling .............................................13
   7. Choosing to Forward through a Softwire .........................15
   8. Selecting a Tunneling Technology ...............................15
   9. Selecting the Softwire for a Given Packet ......................16
   10. Softwire OAM and MIBs .........................................17
      10.1. Operations and Maintenance (OAM) .........................17
      10.2. MIBs .....................................................18
   11. Softwire Multicast ............................................18
      11.1. One-to-One Mappings ......................................18
           11.1.1. Using PIM in the Core .............................19
           11.1.2. Using mLDP and Multicast MPLS in the Core .........20
      11.2. MVPN-Like Schemes ........................................21
   12. Inter-AS Considerations .......................................22
   13. Security Considerations .......................................23
      13.1. Problem Analysis .........................................23
      13.2. Non-Cryptographic Techniques .............................24



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      13.3. Cryptographic Techniques .................................26
   14. References ....................................................27
      14.1. Normative References .....................................27
      14.2. Informative References ...................................28
   15. Contributors ..................................................30
   16. Acknowledgments ...............................................30

1.  Introduction

   The routing information in any IP backbone network can be thought of
   as being in one of two categories: "internal routing information" or
   "external routing information".  The internal routing information
   consists of routes to the nodes that belong to the backbone, and to
   the interfaces of those nodes.  External routing information consists
   of routes to destinations beyond the backbone, especially
   destinations to which the backbone is not directly attached.  In
   general, BGP [RFC4271] is used to distribute external routing
   information, and an Interior Gateway Protocol (IGP) such as OSPF
   [RFC2328] or IS-IS [RFC1195] is used to distribute internal routing
   information.

   Often an IP backbone will provide transit routing services for
   packets that originate outside the backbone and whose destinations
   are outside the backbone.  These packets enter the backbone at one of
   its "edge routers".  They are routed through the backbone to another
   edge router, after which they leave the backbone and continue on
   their way.  The edge nodes of the backbone are often known as
   "Provider Edge" (PE) routers.  The term "ingress" (or "ingress PE")
   refers to the router at which a packet enters the backbone, and the
   term "egress" (or "egress PE") refers to the router at which it
   leaves the backbone.  Interior nodes are often known as "P routers".
   Routers that are outside the backbone but directly attached to it are
   known as "Customer Edge" (CE) routers.  (This terminology is taken
   from [RFC4364].)

   When a packet's destination is outside the backbone, the routing
   information that is needed within the backbone in order to route the
   packet to the proper egress is, by definition, external routing
   information.

   Traditionally, the external routing information has been distributed
   by BGP to all the routers in the backbone, not just to the edge
   routers (i.e., not just to the ingress and egress points).  Each of
   the interior nodes has been expected to look up the packet's
   destination address and route it towards the egress point.  This is
   known as "native forwarding":  the interior nodes look into each
   packet's header in order to match the information in the header with
   the external routing information.



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   It is, however, possible to provide transit services without
   requiring that all the backbone routers have the external routing
   information.  The routing information that BGP distributes to each
   ingress router specifies the egress router for each route.  The
   ingress router can therefore "tunnel" the packet directly to the
   egress router.  "Tunneling the packet" means putting on some sort of
   encapsulation header that will force the interior routers to forward
   the packet to the egress router.  The original packet is known as the
   "encapsulation payload".  The P routers do not look at the packet
   header of the payload but only at the encapsulation header.  Since
   the path to the egress router is part of the internal routing
   information of the backbone, the interior routers then do not need to
   know the external routing information.  This is known as "tunneled
   forwarding".  Of course, before the packet can leave the egress, it
   has to be decapsulated.

   The scenario where the P routers do not have external routes is
   sometimes known as a "BGP-free core".  That is something of a
   misnomer, though, since the crucial aspect of this scenario is not
   that the interior nodes don't run BGP, but that they don't maintain
   the external routing information.

   In recent years, we have seen this scenario deployed to support VPN
   services, as specified in [RFC4364].  An edge router maintains
   multiple independent routing/addressing spaces, one for each VPN to
   which it interfaces.  However, the routing information for the VPNs
   is not maintained by the interior routers.  In most of these
   scenarios, MPLS is used as the encapsulation mechanism for getting
   the packets from ingress to egress.  There are some deployments in
   which an IP-based encapsulation, such as L2TPv3 (Layer 2 Transport
   Protocol) [RFC3931] or GRE (Generic Routing Encapsulation) [RFC2784]
   is used.

   This same technique can also be useful when the external routing
   information consists not of VPN routes, but of "ordinary" Internet
   routes.  It can be used any time it is desired to keep external
   routing information out of a backbone's interior nodes, or in fact
   any time it is desired for any reason to avoid the native forwarding
   of certain kinds of packets.

   This framework focuses on two such scenarios.

      1. In this scenario, the backbone's interior nodes support only
         IPv6.  They do not maintain IPv4 routes at all, and are not
         expected to parse IPv4 packet headers.  Yet, it is desired to
         use such a backbone to provide transit services for IPv4
         packets.  Therefore, tunneled forwarding of IPv4 packets is




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         required.  Of course, the edge nodes must have the IPv4 routes,
         but the ingress must perform an encapsulation in order to get
         an IPv4 packet forwarded to the egress.

      2. This scenario is the reverse of scenario 1, i.e., the
         backbone's interior nodes support only IPv4, but it is desired
         to use the backbone for IPv6 transit.

   In these scenarios, a backbone whose interior nodes support only one
   of the two address families is required to provide transit services
   for the other.  The backbone's edge routers must, of course, support
   both address families.  We use the term "Address Family Border
   Router" (AFBR) to refer to these PE routers.  The tunnels that are
   used for forwarding are referred to as "softwires".

   These two scenarios are known as the "Softwire Mesh Problem"
   [SW-PROB], and the framework specified in this document is therefore
   known as the "Softwire Mesh Framework".  In this framework, only the
   AFBRs need to support both address families.  The CE routers support
   only a single address family, and the P routers support only the
   other address family.

   It is possible to address these scenarios via a large variety of
   tunneling technologies.  This framework does not mandate the use of
   any particular tunneling technology.  In any given deployment, the
   choice of tunneling technology is a matter of policy.  The framework
   accommodates at least the use of MPLS ([RFC3031], [RFC3032]) -- both
   LDP-based (Label Distribution Protocol, [RFC5036]) and RSVP-TE-based
   (Resource Reservation Protocol - Traffic Engineering, [RFC3209]) --
   L2TPv3 [RFC3931], GRE [RFC2784], and IP-in-IP [RFC2003].  The
   framework will also accommodate the use of IPsec tunneling, when that
   is necessary in order to meet security requirements.

   It is expected that, in many deployments, the choice of tunneling
   technology will be made by a simple expression of policy, such as
   "always use IP-IP tunnels", or "always use LDP-based MPLS", or
   "always use L2TPv3".

   However, other deployments may have a mixture of routers, some of
   which support, say, both GRE and L2TPv3, but others of which support
   only one of those techniques.  It is desirable therefore to allow the
   network administration to create a small set of classes, and to
   configure each AFBR to be a member of one or more of these classes.
   Then the routers can advertise their class memberships to each other,
   and the encapsulation policies can be expressed as, e.g., "use L2TPv3
   to tunnel to routers in class X; use GRE to tunnel to routers in





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   class Y".  To support such policies, it is necessary for the AFBRs to
   be able to advertise their class memberships; a standard way of doing
   this must be developed.

   Policy may also require a certain class of traffic to receive a
   certain quality of service, and this may impact the choice of tunnel
   and/or tunneling technology used for packets in that class.  This
   needs to be accommodated by the Softwire Mesh Framework.

   The use of tunneled forwarding often requires that some sort of
   signaling protocol be used to set up and/or maintain the tunnels.
   Many of the tunneling technologies accommodated by this framework
   already have their own signaling protocols.  However, some do not,
   and in some cases the standard signaling protocol for a particular
   tunneling technology may not be appropriate (for one or another
   reason) in the scenarios of interest.  In such cases (and in such
   cases only), new signaling methodologies need to be defined and
   standardized.

   In this framework, the softwires do not form an overlay topology that
   is visible to routing; routing adjacencies are not maintained over
   the softwires, and routing control packets are not sent through the
   softwires.  Routing adjacencies among backbone nodes (including the
   edge nodes) are maintained via the native technology of the backbone.

   There is already a standard routing method for distributing external
   routing information among AFBRs, namely BGP.  However, in the
   scenarios of interest, we may be using IPv6-based BGP sessions to
   pass IPv4 routing information, and we may be using IPv4-based BGP
   sessions to pass IPv6 routing information.  Furthermore, when IPv4
   traffic is to be tunneled over an IPv6 backbone, it is necessary to
   encode the "BGP next hop" for an IPv4 route as an IPv6 address, and
   vice versa.  The method for encoding an IPv4 address as the next hop
   for an IPv6 route is specified in [V6NLRI-V4NH]; the method for
   encoding an IPv6 address as the next hop for an IPv4 route is
   specified in [V4NLRI-V6NH].

2.  Specification of Requirements

   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].









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3.  Scenarios of Interest

3.1.  IPv6-over-IPv4 Scenario

   In this scenario, the client networks run IPv6 but the backbone
   network runs IPv4.  This is illustrated in Figure 1.

                          +--------+   +--------+
                          |  IPv6  |   |  IPv6  |
                          | Client |   | Client |
                          | Network|   | Network|
                          +--------+   +--------+
                              |   \     /   |
                              |    \   /    |
                              |     \ /     |
                              |      X      |
                              |     / \     |
                              |    /   \    |
                              |   /     \   |
                          +--------+   +--------+
                          |  AFBR  |   |  AFBR  |
                       +--| IPv4/6 |---| IPv4/6 |--+
                       |  +--------+   +--------+  |
       +--------+      |                           |       +--------+
       |  IPv4  |      |                           |       |  IPv4  |
       | Client |      |                           |       | Client |
       | Network|------|            IPv4           |-------| Network|
       +--------+      |            only           |       +--------+
                       |                           |
                       |  +--------+   +--------+  |
                       +--|  AFBR  |---|  AFBR  |--+
                          | IPv4/6 |   | IPv4/6 |
                          +--------+   +--------+
                            |   \     /   |
                            |    \   /    |
                            |     \ /     |
                            |      X      |
                            |     / \     |
                            |    /   \    |
                            |   /     \   |
                         +--------+   +--------+
                         |  IPv6  |   |  IPv6  |
                         | Client |   | Client |
                         | Network|   | Network|
                         +--------+   +--------+

                     Figure 1: IPv6-over-IPv4 Scenario




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   The IPv4 transit core may or may not run MPLS.  If it does, MPLS may
   be used as part of the solution.

   While Figure 1 does not show any "backdoor" connections among the
   client networks, this framework assumes that there will be such
   connections.  That is, there is no assumption that the only path
   between two client networks is via the pictured transit-core network.
   Hence, the routing solution must be robust in any kind of topology.

   Many mechanisms for providing IPv6 connectivity across IPv4 networks
   have been devised over the past ten years.  A number of different
   tunneling mechanisms have been used, some provisioned manually, and
   others based on special addressing.  More recently, L3VPN (Layer 3
   Virtual Private Network) techniques from [RFC4364] have been extended
   to provide IPv6 connectivity, using MPLS in the AFBRs and,
   optionally, in the backbone [V6NLRI-V4NH].  The solution described in
   this framework can be thought of as a superset of [V6NLRI-V4NH], with
   a more generalized scheme for choosing the tunneling (softwire)
   technology.  In this framework, MPLS is allowed -- but not required
   -- even at the AFBRs.  As in [V6NLRI-V4NH], there is no manual
   provisioning of tunnels, and no special addressing is required.






























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3.2.  IPv4-over-IPv6 Scenario

   In this scenario, the client networks run IPv4 but the backbone
   network runs IPv6.  This is illustrated in Figure 2.

                          +--------+   +--------+
                          |  IPv4  |   |  IPv4  |
                          | Client |   | Client |
                          | Network|   | Network|
                          +--------+   +--------+
                              |   \     /   |
                              |    \   /    |
                              |     \ /     |
                              |      X      |
                              |     / \     |
                              |    /   \    |
                              |   /     \   |
                          +--------+   +--------+
                          |  AFBR  |   |  AFBR  |
                       +--| IPv4/6 |---| IPv4/6 |--+
                       |  +--------+   +--------+  |
       +--------+      |                           |       +--------+
       |  IPv6  |      |                           |       |  IPv6  |
       | Client |      |                           |       | Client |
       | Network|------|            IPv6           |-------| Network|
       +--------+      |            only           |       +--------+
                       |                           |
                       |  +--------+   +--------+  |
                       +--|  AFBR  |---|  AFBR  |--+
                          | IPv4/6 |   | IPv4/6 |
                          +--------+   +--------+
                            |   \     /   |
                            |    \   /    |
                            |     \ /     |
                            |      X      |
                            |     / \     |
                            |    /   \    |
                            |   /     \   |
                         +--------+   +--------+
                         |  IPv4  |   |  IPv4  |
                         | Client |   | Client |
                         | Network|   | Network|
                         +--------+   +--------+

                     Figure 2: IPv4-over-IPv6 Scenario

   The IPv6 transit core may or may not run MPLS.  If it does, MPLS may
   be used as part of the solution.



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   While Figure 2 does not show any "backdoor" connections among the
   client networks, this framework assumes that there will be such
   connections.  That is, there is no assumption that the only path
   between two client networks is via the pictured transit-core network.
   Hence, the routing solution must be robust in any kind of topology.

   While the issue of IPv6-over-IPv4 has received considerable attention
   in the past, the scenario of IPv4-over-IPv6 has not.  Yet, it is a
   significant emerging requirement, as a number of service providers
   are building IPv6 backbone networks and do not wish to provide native
   IPv4 support in their core routers.  These service providers have a
   large legacy of IPv4 networks and applications that need to operate
   across their IPv6 backbone.  Solutions for this do not exist yet
   because it had always been assumed that the backbone networks of the
   foreseeable future would be dual stack.

4.  General Principles of the Solution

   This section gives a very brief overview of the procedures.  The
   subsequent sections provide more detail.

4.1.  E-IP and I-IP

   In the following sections, we use the term "I-IP" (Internal IP) to
   refer to the form of IP (i.e., either IPv4 or IPv6) that is supported
   by the transit network.  We use the term "E-IP" (External IP) to
   refer to the form of IP that is supported by the client networks.
   In the scenarios of interest, E-IP is IPv4 if and only if I-IP is
   IPv6, and E-IP is IPv6 if and only if I-IP is IPv4.

   We assume that the P routers support only I-IP.  That is, they are
   expected to have only I-IP routing information, and they are not
   expected to be able to parse E-IP headers.  We similarly assume that
   the CE routers support only E-IP.

   The AFBRs handle both I-IP and E-IP.  However, only I-IP is used on
   AFBR's "core-facing interfaces", and E-IP is only used on its client-
   facing interfaces.

4.2.  Routing

   The P routers and the AFBRs of the transit network participate in an
   IGP for the purposes of distributing I-IP routing information.

   The AFBRs use Internal BGP (IBGP) to exchange E-IP routing
   information with each other.  Either there is a full mesh of IBGP
   connections among the AFBRs, or else some or all of the AFBRs are
   clients of a BGP Route Reflector.  Although these IBGP connections



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   are used to pass E-IP routing information (i.e., the Network Layer
   Reachability Information (NLRI) of the BGP updates is in the E-IP
   address family), the IBGP connections run over I-IP, and the BGP next
   hop for each E-IP NLRI is in the I-IP address family.

4.3.  Tunneled Forwarding

   When an ingress AFBR receives an E-IP packet from a client-facing
   interface, it looks up the packet's destination IP address.  In the
   scenarios of interest, the best match for that address will be a BGP-
   distributed route whose next hop is the I-IP address of another AFBR,
   the egress AFBR.

   The ingress AFBR must forward the packet through a tunnel (i.e,
   through a softwire) to the egress AFBR.  This is done by
   encapsulating the packet, using an encapsulation header that the P
   routers can process and that will cause the P routers to send the
   packet to the egress AFBR.  The egress AFBR then extracts the
   payload, i.e., the original E-IP packet, and forwards it further by
   looking up its IP destination address.

   Several kinds of tunneling technologies are supported.  Some of those
   technologies require explicit AFBR-to-AFBR signaling before the
   tunnel can be used, others do not.

   Transmitting a packet through a softwire always requires that an
   encapsulation header be added to the original packet.  The resulting
   packet is therefore always longer than the encapsulation payload.  As
   an operational matter, the Maximum Transmission Unit (MTU) of the
   softwire's path SHOULD be large enough so that (a) no packet will
   need to be fragmented before being encapsulated, and (b) no
   encapsulated packet will need to be fragmented while it is being
   forwarded along a softwire.  A general discussion of MTU issues in
   the context of tunneled forwarding may be found in [RFC4459].

5.  Distribution of Inter-AFBR Routing Information

   AFBRs peer with routers in the client networks to exchange routing
   information for the E-IP family.

   AFBRs use BGP to distribute the E-IP routing information to each
   other.  This can be done by an AFBR-AFBR mesh of IBGP sessions, but
   more likely is done through a BGP Route Reflector, i.e., where each
   AFBR has an IBGP session to one or two Route Reflectors rather than
   to other AFBRs.






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   The BGP sessions between the AFBRs, or between the AFBRs and the
   Route Reflector, will run on top of the I-IP address family.  That
   is, if the transit core supports only IPv6, the IBGP sessions used to
   distribute IPv4 routing information from the client networks will run
   over IPv6; if the transit core supports only IPv4, the IBGP sessions
   used to distribute IPv6 routing information from the client networks
   will run over IPv4.  The BGP sessions thus use the native networking
   layer of the core; BGP messages are NOT tunneled through softwires or
   through any other mechanism.

   In BGP, a routing update associates an address prefix (or more
   generally, NLRI) with the address of a BGP next hop (NH).  The NLRI
   is associated with a particular address family.  The NH address is
   also associated with a particular address family, which may be the
   same as or different than the address family associated with the
   NLRI.  Generally, the NH address belongs to the address family that
   is used to communicate with the BGP speaker to whom the NH address
   belongs.

   Since routing updates that contain information about E-IP address
   prefixes are carried over BGP sessions that use I-IP transport, and
   since the BGP messages are not tunneled, a BGP update providing
   information about an E-IP address prefix will need to specify a next
   hop address in the I-IP family.

   Due to a variety of historical circumstances, when the NLRI and the
   NH in a given BGP update are of different address families, it is not
   always obvious how the NH should be encoded.  There is a different
   encoding procedure for each pair of address families.

   In the case where the NLRI is in the IPv6 address family, and the NH
   is in the IPv4 address family, [V6NLRI-V4NH] explains how to encode
   the NH.

   In the case where the NLRI is in the IPv4 address family, and the NH
   is in the IPv6 address family, [V4NLRI-V6NH] explains how to encode
   the NH.

   If a BGP speaker sends an update for an NLRI in the E-IP family, and
   the update is being sent over a BGP session that is running on top of
   the I-IP network layer, and the BGP speaker is advertising itself as
   the NH for that NLRI, then the BGP speaker MUST, unless explicitly
   overridden by policy, specify the NH address in the I-IP family.  The
   address family of the NH MUST NOT be changed by a Route Reflector.

   In some cases (e.g., when [V4NLRI-V6NH] is used), one cannot follow
   this rule unless one's BGP peers have advertised a particular BGP
   capability.  This leads to the following softwire deployment



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   restriction: if a BGP capability is defined for the case in which an
   E-IP NLRI has an I-IP NH, all the AFBRs in a given transit core MUST
   advertise that capability.

   If an AFBR has multiple IP addresses, the network administrators
   usually have considerable flexibility in choosing which one the AFBR
   uses to identify itself as the next hop in a BGP update.  However, if
   the AFBR expects to receive packets through a softwire of a
   particular tunneling technology, and if the AFBR is known to that
   tunneling technology via a specific IP address, then that same IP
   address must be used to identify the AFBR in the next hop field of
   the BGP updates.  For example, if L2TPv3 tunneling is used, then the
   IP address that the AFBR uses when engaging in L2TPv3 signaling must
   be the same as the IP address it uses to identify itself in the next
   hop field of a BGP update.

   In [V6NLRI-V4NH], IPv6 routing information is distributed using the
   labeled IPv6 address family.  This allows the egress AFBR to
   associate an MPLS label with each IPv6 address prefix.  If an ingress
   AFBR forwards packets through a softwire that can carry MPLS packets,
   each data packet can carry the MPLS label corresponding to the IPv6
   route that it matched.  This may be useful at the egress AFBR, for
   demultiplexing and/or enhanced performance.  It is also possible to
   do the same for the IPv4 address family, i.e., to use the labeled
   IPv4 address family instead of the IPv4 address family.  The use of
   the labeled IP address families in this manner is OPTIONAL.

6.  Softwire Signaling

   A mesh of inter-AFBR softwires spanning the transit core must be in
   place before packets can flow between client networks.  Given N dual-
   stack AFBRs, this requires N^2 "point-to-point IP" or "label switched
   path" (LSP) tunnels.  While in theory these could be configured
   manually, that would result in a very undesirable O(N^2) provisioning
   problem.  Therefore, manual configuration of point-to-point tunnels
   is not considered part of this framework.

   Because the transit core is providing layer 3 transit services,
   point-to-point tunnels are not required by this framework;
   multipoint-to-point tunnels are all that is needed.  In a multipoint-
   to-point tunnel, when a packet emerges from the tunnel there is no
   way to tell which router put the packet into the tunnel.  This models
   the native IP forwarding paradigm, wherein the egress router cannot
   determine a given packet's ingress router.  Of course, point-to-point
   tunnels might be required for some reason beyond the basic
   requirements described in this document.  For example, Quality of





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   Service (QoS) or security considerations might require the use of
   point-to-point tunnels.  So point-to-point tunnels are allowed, but
   not required, by this framework.

   If it is desired to use a particular tunneling technology for the
   softwires, and if that technology has its own "native" signaling
   methodology, the presumption is that the native signaling will be
   used.  This would certainly apply to MPLS-based softwires, where LDP
   or RSVP-TE would be used.  An IPsec-based softwire would use standard
   IKEv2 (Internet Key Exchange) [RFC4306] and IPsec [RFC4301]
   signaling, as that is necessary in order to guarantee the softwire's
   security properties.

   A GRE-based softwire might or might not require signaling, depending
   on whether various optional GRE header fields are to be used.  GRE
   does not have any "native" signaling, so for those cases, a signaling
   procedure needs to be developed to support softwires.

   Another possible softwire technology is L2TPv3.  While L2TPv3 does
   have its own native signaling, that signaling sets up point-to-point
   tunnels.  For the purpose of softwires, it is better to use L2TPv3 in
   a multipoint-to-point mode, and this requires a different kind of
   signaling.

   The signaling to be used for GRE and L2TPv3 to cover these scenarios
   is BGP-based, and is described in [RFC5512].

   If IP-IP tunneling is used, or if GRE tunneling is used without
   options, no signaling is required, as the only information needed by
   the ingress AFBR to create the encapsulation header is the IP address
   of the egress AFBR, and that is distributed by BGP.

   When the encapsulation IP header is constructed, there may be fields
   in the IP whose value is determined neither by whatever signaling has
   been done nor by the distributed routing information.  The values of
   these fields are determined by policy in the ingress AFBR.  Examples
   of such fields may be the TTL (Time to Live) field, the DSCP
   (Diffserv Service Classes) bits, etc.

   It is desirable for all necessary softwires to be fully set up before
   the arrival of any packets that need to go through the softwires.
   That is, the softwires should be "always on".  From the perspective
   of any particular AFBR, the softwire endpoints are always BGP next
   hops of routes that the AFBR has installed.  This suggests that any
   necessary softwire signaling should either be done as part of normal
   system startup (as would happen, e.g., with LDP-based MPLS) or else





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   be triggered by the reception of BGP routing information (such as is
   described in [RFC5512]); it is also helpful if distribution of the
   routing information that serves as the trigger is prioritized.

7.  Choosing to Forward through a Softwire

   The decision to forward through a softwire, instead of to forward
   natively, is made by the ingress AFBR.  This decision is a matter of
   policy.

   In many cases, the policy will be very simple.  Some useful policies
   are:

     - If routing says that an E-IP packet has to be sent out a core-
       facing interface to an I-IP core, then send the packet through a
       softwire.

     - If routing says that an E-IP packet has to be sent out an
       interface that only supports I-IP packets, then send the E-IP
       packet through a softwire.

     - If routing says that the BGP next hop address for an E-IP packet
       is an I-IP address, then send the E-IP packet through a softwire.

     - If the route that is the best match for a particular packet's
       destination address is a BGP-distributed route, then send the
       packet through a softwire (i.e., tunnel all BGP-routed packets).

   More complicated policies are also possible, but a consideration of
   those policies is outside the scope of this document.

8. Selecting a Tunneling Technology

   The choice of tunneling technology is a matter of policy configured
   at the ingress AFBR.

   It is envisioned that, in most cases, the policy will be a very
   simple one, and will be the same at all the AFBRs of a given transit
   core -- e.g., "always use LDP-based MPLS" or "always use L2TPv3".

   However, other deployments may have a mixture of routers, some of
   which support, say, both GRE and L2TPv3, but others of which support
   only one of those techniques.  It is desirable therefore to allow the
   network administration to create a small set of classes and to
   configure each AFBR to be a member of one or more of these classes.
   Then the routers can advertise their class memberships to each other,
   and the encapsulation policies can be expressed as, e.g., "use L2TPv3
   to talk to routers in class X; use GRE to talk to routers in class



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   Y".  To support such policies, it is necessary for the AFBRs to be
   able to advertise their class memberships.  [RFC5512] specifies a way
   in which an AFBR may advertise, to other AFBRS, various
   characteristics that may be relevant to the policy (e.g., "I belong
   to class Y").  In many cases, these characteristics can be
   represented by arbitrarily selected communities or extended
   communities, and the policies at the ingress can be expressed in
   terms of these classes (i.e., communities).

   Policy may also require a certain class of traffic to receive a
   certain quality of service, and this may impact the choice of tunnel
   and/or tunneling technology used for packets in that class.  This
   framework allows a variety of tunneling technologies to be used for
   instantiating softwires.  The choice of tunneling technology is a
   matter of policy, as discussed in Section 1.

   While in many cases the policy will be unconditional, e.g., "always
   use L2TPv3 for softwires", in other cases the policy may specify that
   the choice is conditional upon information about the softwire remote
   endpoint, e.g., "use L2TPv3 to talk to routers in class X; use GRE to
   talk to routers in class Y".  It is desirable therefore to allow the
   network administration to create a small set of classes, and to
   configure each AFBR to be a member of one or more of these classes.
   If each such class is represented as a community or extended
   community, then [RFC5512] specifies a method that AFBRs can use to
   advertise their class memberships to each other.

   This framework also allows for policies of arbitrary complexity,
   which may depend on characteristics or attributes of individual
   address prefixes as well as on QoS or security considerations.
   However, the specification of such policies is not within the scope
   of this document.

9.  Selecting the Softwire for a Given Packet

   Suppose it has been decided to send a given packet through a
   softwire.  Routing provides the address, in the address family of the
   transport network, of the BGP next hop.  The packet MUST be sent
   through a softwire whose remote endpoint address is the same as the
   BGP next hop address.

   Sending a packet through a softwire is a matter of first
   encapsulating the packet with an encapsulation header that can be
   processed by the transit network and then transmitting towards the
   softwire's remote endpoint address.






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   In many cases, once one knows the remote endpoint address, one has
   all the information one needs in order to form the encapsulation
   header.  This will be the case if the tunnel technology instantiating
   the softwire is, e.g., LDP-based MPLS, IP-in-IP, or GRE without
   optional header fields.

   If the tunnel technology being used is L2TPv3 or GRE with optional
   header fields, additional information from the remote endpoint is
   needed in order to form the encapsulation header.  The procedures for
   sending and receiving this information are described in [RFC5512].

   If the tunnel technology being used is RSVP-TE-based MPLS or IPsec,
   the native signaling procedures of those technologies will need to be
   used.

   If the packet being sent through the softwire matches a route in the
   labeled IPv4 or labeled IPv6 address families, it should be sent
   through the softwire as an MPLS packet with the corresponding label.
   Note that most of the tunneling technologies mentioned in this
   document are capable of carrying MPLS packets, so this does not
   presuppose support for MPLS in the core routers.

10.  Softwire OAM and MIBs

10.1.  Operations and Maintenance (OAM)

   Softwires are essentially tunnels connecting routers.  If they
   disappear or degrade in performance, then connectivity through those
   tunnels will be impacted.  There are several techniques available to
   monitor the status of the tunnel endpoints (AFBRs) as well as the
   tunnels themselves.  These techniques allow operations such as
   softwire path tracing, remote softwire endpoint pinging, and remote
   softwire endpoint liveness failure detection.

   Examples of techniques applicable to softwire OAM include:

     o BGP/TCP timeouts between AFBRs

     o ICMP or LSP echo request and reply addressed to a particular AFBR

     o BFD (Bidirectional Forwarding Detection) [BFD] packet exchange
       between AFBR routers

   Another possibility for softwire OAM is to build something similar to
   [RFC4378] or, in other words, to create and generate softwire echo
   request/reply packets.  The echo request sent to a well-known UDP
   port would contain the egress AFBR IP address and the softwire
   identifier as the payload (similar to the MPLS Forwarding Equivalence



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   Class contained in the LSP echo request).  The softwire echo packet
   would be encapsulated with the encapsulation header and forwarded
   across the same path (inband) as that of the softwire itself.

   This mechanism can also be automated to periodically verify remote
   softwire endpoint reachability, with the loss of reachability being
   signaled to the softwire application on the local AFBR, thus enabling
   suitable actions to be taken.  Consideration must be given to the
   trade-offs between the scalability of such mechanisms versus the time
   required for detection of loss of endpoint reachability for such
   automated mechanisms.

   In general, a framework for softwire OAM can, for a large part, be
   based on the [RFC4176] framework.

10.2.  MIBs

   Specific MIBs do exist to manage elements of the Softwire Mesh
   Framework.  However, there will be a need to either extend these MIBs
   or create new ones that reflect the functional elements that can be
   SNMP-managed within the softwire network.

11.  Softwire Multicast

   A set of client networks, running E-IP, that are connected to a
   provider's I-IP transit core may wish to run IP multicast
   applications.  Extending IP multicast connectivity across the transit
   core can be done in a number of ways, each with a different set of
   characteristics.  Most (though not all) of the possibilities are
   either slight variations of the procedures defined for L3VPNs in
   [L3VPN-MCAST].

   We will focus on supporting those multicast features and protocols
   that are typically used across inter-provider boundaries.  Support is
   provided for PIM-SM (Protocol Independent Multicast - Sparse Mode)
   and PIM-SSM (PIM Source-Specific Mode).  Support for BIDIR-PIM
   (Bidirectional PIM), BSR (Bootstrap Router Mechanism for PIM), and
   AutoRP (Automatic Rendezvous Point Determination) is not provided as
   these features are not typically used across inter-provider
   boundaries.

11.1.  One-to-One Mappings

   In the "one-to-one mapping" scheme, each client multicast tree is
   extended through the transit core so that for each client tree there
   is exactly one tree through the core.





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   The one-to-one scheme is not used in [L3VPN-MCAST] because it
   requires an amount of state in the core routers that is proportional
   to the number of client multicast trees passing through the core.  In
   the VPN context, this is considered undesirable because the amount of
   state is unbounded and out of the control of the service provider.
   However, the one-to-one scheme models the typical "Internet
   multicast" scenario where the client network and the transit core are
   both IPv4 or both IPv6.  If it scales satisfactorily for that case,
   it should also scale satisfactorily for the case where the client
   network and the transit core support different versions of IP.

11.1.1.  Using PIM in the Core

   When an AFBR receives an E-IP PIM control message from one of its
   CEs, it translates it from E-IP to I-IP, and forwards it towards the
   source of the tree.  Since the routers in the transit core will not
   generally have a route to the source of the tree, the AFBR must
   include an "RPF (Reverse Path Forwarding) Vector" [RFC5496] in the
   PIM message.

   Suppose an AFBR A receives an E-IP PIM Join/Prune message from a CE
   for either an (S,G) tree or a (*,G) tree.  The AFBR would have to
   "translate" the PIM message into an I-IP PIM message.  It would then
   send it to the neighbor that is the next hop along the route to the
   root of the (S,G) or (*,G) tree.  In the case of an (S,G) tree, the
   root of the tree is S; in the case of a (*,G) tree, the root of the
   tree is the Rendezvous Point (RP) for the group G.

   Note that the address of the root of the tree will be an E-IP
   address.  Since the routers within the transit core (other than the
   AFBRs) do not have routes to E-IP addresses, A must put an RPF Vector
   [RFC5496] in the PIM Join/Prune message that it sends to its upstream
   neighbor.  The RPF Vector will identify, as an I-IP address, the AFBR
   B that is the egress point in the transit network along the route to
   the root of the multicast tree.  AFBR B is AFBR A's BGP next hop for
   the route to the root of the tree.  The RPF Vector allows the core
   routers to forward PIM Join/Prune messages upstream towards the root
   of the tree, even though they do not maintain E-IP routes.

   In order to translate an E-IP PIM message into an I-IP PIM message,
   the AFBR A must translate the address of S (in the case of an (S,G)
   group) or the address of G's RP from the E-IP address family to the
   I-IP address family, and the AFBR B must translate them back.

   In the case where E-IP is IPv4 and I-IP is IPv6, it may be possible
   to do this translation algorithmically.  A can translate the IPv4 S
   into the corresponding IPv4-mapped IPv6 address [RFC4291], and then B
   can translate it back.  At the time of this writing, there is no such



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   thing as an IPv4-mapped IPv6 multicast address, but if such a thing
   were to be standardized, then A could also translate the IPv4 G into
   IPv6, and B could translate it back.  The precise circumstances under
   which these translations are to be done would be a matter of policy.

   Obviously, this translation procedure does not generalize to the case
   where the client multicast is IPv6 but the core is IPv4.  To handle
   that case, one needs additional signaling between the two AFBRs.
   Each downstream AFBR needs to signal the upstream AFBR that it needs
   a multicast tunnel for (S,G).  The upstream AFBR must then assign a
   multicast address G' to the tunnel and inform the downstream of the
   P-G value to use.  The downstream AFBR then uses PIM/IPv4 to join the
   (S',G') tree, where S' is the IPv4 address of the upstream ASBR
   (Autonomous System Border Router).

   The (S',G') trees should be SSM trees.

   This procedure can be used to support client multicasts of either
   IPv4 or IPv6 over a transit core of the opposite protocol.  However,
   it only works when the client multicasts are SSM, since it provides
   no method for mapping a client "prune a source off the (*,G) tree"
   operation into an operation on the (S',G') tree.  This method also
   requires additional signaling.  The BGP-based signaling of
   [L3VPN-MCAST-BGP] is one signaling method that could be used.  Other
   signaling methods could be defined as well.

11.1.2.  Using mLDP and Multicast MPLS in the Core

   LDP extensions for point-to-multipoint and multipoint-to-multipoint
   LSPs are specified in [MLDP]; we will use the term "mLDP" to refer to
   those LDP extensions.  If the transit core implements mLDP and
   supports multicast MPLS, then client Source-Specific Multicast (SSM)
   trees can be mapped one-to-one onto P2MP (Point-to-Multipoint) LSPs.

   When an AFBR A receives an E-IP PIM Join/Prune message for (S,G) from
   one of its CEs, where G is an SSM group, it would use mLDP to join a
   P2MP LSP.  The root of the P2MP LSP would be the AFBR B that is A's
   BGP next hop on the route to S.  In mLDP, a P2MP LSP is uniquely
   identified by a combination of its root and an "FEC (Forwarding
   Equivalence Class) identifier".  The original (S,G) can be
   algorithmically encoded into the FEC identifier so that all AFBRs
   that need to join the P2MP LSP for (S,G) will generate the same FEC
   identifier.  When the root of the P2MP LSP (AFBR B) receives such an
   mLDP message, it extracts the original (S,G) from the FEC identifier,
   creates an "ordinary" E-IP PIM Join/Prune message, and sends it to
   the CE that is its next hop on the route to S.





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   The method of encoding the (S,G) into the FEC identifier needs to be
   standardized.  The encoding must be self-identifying so that a node
   that is the root of a P2MP LSP can determine whether a FEC identifier
   is the result of having encoded a PIM (S,G).

   The appropriate state machinery must be standardized so that PIM
   events at the AFBRs result in the proper mLDP events.  For example,
   if at some point an AFBR determines (via PIM procedures) that it no
   longer has any downstream receivers for (S,G), the AFBR should invoke
   the proper mLDP procedures to prune itself off the corresponding P2MP
   LSP.

   Note that this method cannot be used when the G is a Sparse Mode
   group.  The reason this method cannot be used is that mLDP does not
   have any function corresponding to the PIM "prune this source off the
   shared tree" function.  So if a P2MP LSP were mapped one-to-one with
   a P2MP LSP, duplicate traffic could end up traversing the transit
   core (i.e., traffic from S might travel down both the shared tree and
   S's source tree).  Alternatively, one could devise an AFBR-to-AFBR
   protocol to prune sources off the P2MP LSP at the root of the LSP.
   It is recommended, though, that client SM multicast groups be
   supported by other methods, such as those discussed below.

   Client-side bidirectional multicast groups set up by PIM-bidir could
   be mapped using the above technique to MP2MP (Multipoint-to-
   Multipoint) LSPs set up by mLDP [MLDP].  We do not consider this
   further, as inter-provider bidirectional groups are not in use
   anywhere.

11.2.  MVPN-Like Schemes

   The "MVPN (Multicast VPN)-like schemes" are those described in
   [L3VPN-MCAST] and its companion documents (such as
   [L3VPN-MCAST-BGP]).  To apply those schemes to the softwire
   environment, it is necessary only to treat all the AFBRs of a given
   transit core as if they were all, for multicast purposes, PE routers
   attached to the same VPN.

   The MVPN-like schemes do not require a one-to-one mapping between
   client multicast trees and transit-core multicast trees.  In the MVPN
   environment, it is a requirement that the number of trees in the core
   scales less than linearly with the number of client trees.  This
   requirement may not hold in the softwire scenarios.

   The MVPN-like schemes can support SM, SSM, and Bidir groups.  They
   provide a number of options for the control plane:





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     - LAN-like

       Use a set of multicast trees in the core to emulate a LAN (Local
       Area Network) and run the client-side PIM protocol over that
       "LAN".  The "LAN" can consist of a single Bidir tree containing
       all the AFBRs or a set of SSM trees, one rooted at each AFBR and
       containing all the other AFBRs as receivers.

     - NBMA (Non-Broadcast Multiple Access), using BGP

       The client-side PIM signaling can be translated into BGP-based
       signaling, with a BGP Route Reflector mediating the signaling.

   These two basic options admit of many variations; a comprehensive
   discussion is in [L3VPN-MCAST].

   For the data plane, there are also a number of options:

     - All multicast data sent over the emulated LAN.  This particular
       option is not very attractive, though, for the softwire
       scenarios, as every AFBR would have to receive every client
       multicast packet.

     - Every multicast group mapped to a tree that is considered
       appropriate for that group, in the sense of causing the traffic
       of that group to go to "too many" AFBRs that don't need to
       receive it.

   Again, a comprehensive discussion of the issues can be found in
   [L3VPN-MCAST].

12.  Inter-AS Considerations

   We have so far only considered the case where a "transit core"
   consists of a single Autonomous System (AS).  If the transit core
   consists of multiple ASes, then it may be necessary to use softwires
   whose endpoints are AFBRs attached to different Autonomous Systems.
   In this case, the AFBR at the remote endpoint of a softwire is not
   the BGP next hop for packets that need to be sent on the softwire.
   Since the procedures described above require the address of a remote
   softwire endpoint to be the same as the address of the BGP next hop,
   those procedures do not work as specified when the transit core
   consists of multiple ASes.

   There are several ways to deal with this situation.

      1. Don't do it; require that there be AFBRs at the edge of each AS
         so that a transit core does not extend more than one AS.



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      2. Use multi-hop EBGP to allow AFBRs to send BGP routes to each
         other, even if the ABFRs are not in the same or in neighboring
         ASes.

      3. Ensure that an ASBR that is not an AFBR does not change the
         next hop field of the routes for which encapsulation is needed.

   In the latter two cases, BGP recursive next hop resolution needs to
   be done, and encapsulations may need to be "stacked" (i.e., multiple
   layers of encapsulation may need to be used).

   For instance, consider packet P with destination IP address D.
   Suppose it arrives at ingress AFBR A1 and that the route that is the
   best match for D has BGP next hop B1.  So A1 will encapsulate the
   packet for delivery to B1.  If B1 is not within A1's AS, A1 will need
   to look up the route to B1 and then find the BGP next hop, call it
   B2, of that route.  If the interior routers of A1's AS do not have
   routes to B1, then A1 needs to encapsulate the packet a second time,
   this time for delivery to B2.

13.  Security Considerations

13.1.  Problem Analysis

   In the Softwire Mesh Framework, the data packets that are
   encapsulated are E-IP data packets that are traveling through the
   Internet.  These data packets (the softwire "payload") may or may not
   need such security features as authentication, integrity,
   confidentiality, or replay protection.  However, the security needs
   of the payload packets are independent of whether or not those
   packets are traversing softwires.  The fact that a particular payload
   packet is traveling through a softwire does not in any way affect its
   security needs.

   Thus, the only security issues we need to consider are those that
   affect the I-IP encapsulation headers, rather than those that affect
   the E-IP payload.

   Since the encapsulation headers determine the routing of packets
   traveling through softwires, they must appear "in the clear".

   In the Softwire Mesh Framework, for each receiving endpoint of a
   tunnel, there are one or more "valid" transmitting endpoints, where
   the valid transmitting endpoints are those that are authorized to
   tunnel packets to the receiving endpoint.  If the encapsulation
   header has no guarantee of authentication or integrity, then it is
   possible to have spoofing attacks, in which unauthorized nodes send




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   encapsulated packets to the receiving endpoint, giving the receiving
   endpoint the invalid impression the encapsulated packets have really
   traveled through the softwire.  Replay attacks are also possible.

   The effect of such attacks is somewhat limited, though.  The
   receiving endpoint of a softwire decapsulates the payload and does
   further routing based on the IP destination address of the payload.
   Since the payload packets are traveling through the Internet, they
   have addresses from the globally unique address space (rather than,
   e.g., from a private address space of some sort).  Therefore, these
   attacks cannot cause payload packets to be delivered to an address
   other than the one appearing in the destination IP address field of
   the payload packet.

   However, attacks of this sort can result in policy violations.  The
   authorized transmitting endpoint(s) of a softwire may be following a
   policy according to which only certain payload packets get sent
   through the softwire.  If unauthorized nodes are able to encapsulate
   the payload packets so that they arrive at the receiving endpoint
   looking as if they arrived from authorized nodes, then the properly
   authorized policies have been side-stepped.

   Attacks of the sort we are considering can also be used in denial-
   of-service attacks on the receiving tunnel endpoints.  However, such
   attacks cannot be prevented by use of cryptographic
   authentication/integrity techniques, as the need to do cryptography
   on spoofed packets only makes the denial-of-service problem worse.
   (The assumption is that the cryptography mechanisms are likely to be
   more costly than the decapsulation/forwarding mechanisms.  So if one
   tries to eliminate a flooding attack on the decapsulation/forwarding
   mechanisms by discarding packets that do not pass a cryptographic
   integrity test, one ends up just trading one kind of attack for
   another.)

   This section is largely based on the security considerations section
   of RFC 4023, which also deals with encapsulations and tunnels.

13.2.  Non-Cryptographic Techniques

   If a tunnel lies entirely within a single administrative domain,
   then, to a certain extent, there are certain non-cryptographic
   techniques one can use to prevent spoofed packets from reaching a
   tunnel's receiving endpoint.  For example, when the tunnel
   encapsulation is IP-based:







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     - The receiving endpoints of the tunnels can be given a distinct
       set of addresses, and those addresses can be made known to the
       border routers.  The border routers can then filter out packets,
       destined to those addresses, that arrive from outside the domain.

     - The transmitting endpoints of the tunnels can be given a distinct
       set of addresses, and those addresses can be made known to the
       border routers and to the receiving endpoints of the tunnels.
       The border routers can filter out all packets arriving from
       outside the domain with source addresses that are in this set,
       and the receiving endpoints can discard all packets that appear
       to be part of a softwire, but whose source addresses are not in
       this set.

   If an MPLS-based encapsulation is used, the border routers can refuse
   to accept MPLS packets from outside the domain, or they can refuse to
   accept such MPLS packets whenever the top label corresponds to the
   address of a tunnel receiving endpoint.

   These techniques assume that, within a domain, the network is secure
   enough to prevent the introduction of spoofed packets from within the
   domain itself.  That may not always be the case.  Also, these
   techniques can be difficult or impossible to use effectively for
   tunnels that are not in the same administrative domain.

   A different technique is to have the encapsulation header contain a
   cleartext password.  The 64-bit "cookie" of L2TPv3 [RFC3931] is
   sometimes used in this way.  This can be useful within an
   administrative domain if it is regarded as infeasible for an attacker
   to spy on packets that originate in the domain and that do not leave
   the domain.  An attacker would then not be able to discover the
   password.  An attacker could, of course, try to guess the password,
   but if the password is an arbitrary 64-bit binary sequence, brute
   force attacks that run through all the possible passwords would be
   infeasible.  This technique may be easier to manage than ingress
   filtering is, and may be just as effective if the assumptions hold.
   Like ingress filtering, though, it may not be applicable for tunnels
   that cross domain boundaries.

   Therefore, it is necessary to also consider the use of cryptographic
   techniques for setting up the tunnels and for passing data through
   them.









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13.3.  Cryptographic Techniques

   If the path between the two endpoints of a tunnel is not adequately
   secure, then:

     - If a control protocol is used to set up the tunnels (e.g., to
       inform one tunnel endpoint of the IP address of the other), the
       control protocol MUST have an authentication mechanism, and this
       MUST be used when the tunnel is set up.  If the tunnel is set up
       automatically as the result of, for example, information
       distributed by BGP, then the use of BGP's MD5-based
       authentication mechanism [RFC2385] is satisfactory.

     - Data transmission through the tunnel should be secured with
       IPsec.  In the remainder of this section, we specify the way
       IPsec may be used, and the implementation requirements we mention
       are meant to be applicable whenever IPsec is being used.

   We consider only the case where IPsec is used together with an IP-
   based tunneling mechanism.  Use of IPsec with an MPLS-based tunneling
   mechanism is for further study.

   If it is deemed necessary to use tunnels that are protected by IPsec,
   the tunnel type SHOULD be negotiated by the tunnel endpoints using
   the procedures specified in [RFC5566].  That document allows the use
   of IPsec tunnel mode but also allows one to treat the tunnel head and
   the tunnel tail as the endpoints of a Security Association, and to
   use IPsec transport mode.

   In order to use IPsec transport mode, encapsulated packets should be
   viewed as originating at the tunnel head and as being destined for
   the tunnel tail.  A single IP address of the tunnel head will be used
   as the source IP address, and a single IP address of the tunnel tail
   will be used as the destination IP address.  This technique can be
   used to carry MPLS packets through an IPsec Security Association, by
   first encapsulating the MPLS packets in MPLS-in-IP or MPLS-in-GRE
   [RFC4023] and then applying IPsec transport mode.

   When IPsec is used to secure softwires, IPsec MUST provide
   authentication and integrity.  Thus, the implementation MUST support
   either ESP (IP Encapsulating Security Payload) with null encryption
   [RFC4303] or else AH (IP Authentication Header) [RFC4302].  ESP with
   encryption MAY be supported.  If ESP is used, the tunnel tail MUST
   check that the source IP address of any packet received on a given SA
   (IPsec Security Association) is the one expected, as specified in
   Section 5.2, step 4, of [RFC4301].





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   Since the softwires are set up dynamically as a byproduct of passing
   routing information, key distribution MUST be done automatically by
   means of IKEv2 [RFC4306].  If a PKI (Public Key Infrastructure) is
   not available, the IPsec Tunnel Authenticator sub-TLV described in
   [RFC5566] MUST be used and validated before setting up an SA.

   The selectors associated with the SA are the source and destination
   addresses of the encapsulation header, along with the IP protocol
   number representing the encapsulation protocol being used.

14.  References

14.1.  Normative References

   [RFC2003]      Perkins, C., "IP Encapsulation within IP", RFC 2003,
                  October 1996.

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

   [RFC2784]      Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
                  Traina, "Generic Routing Encapsulation (GRE)", RFC
                  2784, March 2000.

   [RFC3031]      Rosen, E., Viswanathan, A., and R. Callon,
                  "Multiprotocol Label Switching Architecture", RFC
                  3031, January 2001.

   [RFC3032]      Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
                  Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
                  Encoding", RFC 3032, January 2001.

   [RFC3209]      Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
                  V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
                  LSP Tunnels", RFC 3209, December 2001.

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

   [RFC4023]      Worster, T., Rekhter, Y., and E. Rosen, Ed.,
                  "Encapsulating MPLS in IP or Generic Routing
                  Encapsulation (GRE)", RFC 4023, March 2005.

   [RFC5512]      Mohapatra, P. and E. Rosen, "The BGP Encapsulation
                  Subsequent Address Family Identifier (SAFI) and the
                  BGP Tunnel Encapsulation Attribute", RFC 5512, April
                  2009.



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   [RFC5566]      Berger, L., White, R. and E. Rosen, "BGP IPsec Tunnel
                  Encapsulation Attribute", RFC 5566, June 2009.

   [V4NLRI-V6NH]  Le Faucheur, F. and E. Rosen, "Advertising IPv4
                  Network Layer Reachability Information with an IPv6
                  Next Hop", RFC 5549, May 2009.

   [V6NLRI-V4NH]  De Clercq, J., Ooms, D., Prevost, S., and F. Le
                  Faucheur, "Connecting IPv6 Islands over IPv4 MPLS
                  Using IPv6 Provider Edge Routers (6PE)", RFC 4798,
                  February 2007.

14.2.  Informative References

   [BFD]          Katz, D. and D. Ward, "Bidirectional Forwarding
                  Detection", Work in Progress, February 2009.

   [L3VPN-MCAST]  Rosen, E., Ed., and R. Aggarwal, Ed., "Multicast in
                  MPLS/BGP IP VPNs", Work in Progress, March 2009.

   [L3VPN-MCAST-BGP]
                  Aggarwal, R., Rosen, E., Morin, T. and Y. Rekhter,
                  "BGP Encodings and Procedures for Multicast in
                  MPLS/BGP IP VPNs", Work in Progress, April 2009.

   [MLDP]         Minei, I., Ed., Kompella, K., Wijnands, IJ., Ed., and
                  B. Thomas, "Label Distribution Protocol Extensions for
                  Point-to-Multipoint and Multipoint-to-Multipoint Label
                  Switched Paths", Work in Progress, April 2009.

   [RFC1195]      Callon, R., "Use of OSI IS-IS for routing in TCP/IP
                  and dual environments", RFC 1195, December 1990.

   [RFC2328]      Moy, J., "OSPF Version 2", STD 54, RFC 2328, April
                  1998.

   [RFC2385]      Heffernan, A., "Protection of BGP Sessions via the TCP
                  MD5 Signature Option", RFC 2385, August 1998.

   [RFC4176]      El Mghazli, Y., Ed., Nadeau, T., Boucadair, M., Chan,
                  K., and A. Gonguet, "Framework for Layer 3 Virtual
                  Private Networks (L3VPN) Operations and Management",
                  RFC 4176, October 2005.

   [RFC4271]      Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
                  Border Gateway Protocol 4 (BGP-4)", RFC 4271, January
                  2006.




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   [RFC4291]      Hinden, R. and S. Deering, "IP Version 6 Addressing
                  Architecture", RFC 4291, February 2006.

   [RFC4301]      Kent, S. and K. Seo, "Security Architecture for the
                  Internet Protocol", RFC 4301, December 2005.

   [RFC4302]      Kent, S., "IP Authentication Header", RFC 4302,
                  December 2005.

   [RFC4303]      Kent, S., "IP Encapsulating Security Payload (ESP)",
                  RFC 4303, December 2005.

   [RFC4306]      Kaufman, C., Ed., "Internet Key Exchange (IKEv2)
                  Protocol", RFC 4306, December 2005.

   [RFC4364]      Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
                  Networks (VPNs)", RFC 4364, February 2006.

   [RFC4378]      Allan, D., Ed., and T. Nadeau, Ed., "A Framework for
                  Multi-Protocol Label Switching (MPLS) Operations and
                  Management (OAM)", RFC 4378, February 2006.

   [RFC4459]      Savola, P., "MTU and Fragmentation Issues with In-
                  the-Network Tunneling", RFC 4459, April 2006.

   [RFC5036]      Andersson, L., Ed., Minei, I., Ed., and B. Thomas,
                  Ed., "LDP Specification", RFC 5036, October 2007.

   [RFC5496]      Wijnands, IJ., Boers, A., and E. Rosen, "The Reverse
                  Path Forwarding (RPF) Vector TLV", RFC 5496, March
                  2009.

   [SW-PROB]      Li, X., Ed., Dawkins, S., Ed., Ward, D., Ed., and A.
                  Durand, Ed., "Softwire Problem Statement", RFC 4925,
                  July 2007.
















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15.  Contributors

   Xing Li
   Tsinghua University
   Department of Electronic Engineering, Tsinghua University
   Beijing  100084
   P.R.China

   Phone: +86-10-6278-5983
   EMail: xing@cernet.edu.cn


   Simon Barber
   Cisco Systems, Inc.
   250 Longwater Avenue
   Reading, ENGLAND, RG2 6GB
   United Kingdom

   EMail: sbarber@cisco.com


   Pradosh Mohapatra
   Cisco Systems, Inc.
   3700 Cisco Way
   San Jose, CA  95134
   USA

   EMail: pmohapat@cisco.com


   John Scudder
   Juniper Networks
   1194 North Mathilda Avenue
   Sunnyvale, CA  94089
   USA

   EMail: jgs@juniper.net

16.  Acknowledgments

   David Ward, Chris Cassar, Gargi Nalawade, Ruchi Kapoor, Pranav Mehta,
   Mingwei Xu, and Ke Xu provided useful input into this document.









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Authors' Addresses

   Jianping Wu
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R.China

   Phone: +86-10-6278-5983
   EMail: jianping@cernet.edu.cn


   Yong Cui
   Tsinghua University
   Department of Computer Science, Tsinghua University
   Beijing  100084
   P.R.China

   Phone: +86-10-6278-5822
   EMail: yong@csnet1.cs.tsinghua.edu.cn


   Chris Metz
   Cisco Systems, Inc.
   3700 Cisco Way
   San Jose, CA  95134
   USA

   EMail: chmetz@cisco.com


   Eric C. Rosen
   Cisco Systems, Inc.
   1414 Massachusetts Avenue
   Boxborough, MA  01719
   USA

   EMail: erosen@cisco.com













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