Augmenting RFC 4364 Technology to Provide Secure Layer L3VPNs over Public Infrastructure
draft-rosen-bess-secure-l3vpn-00

Internet Engineering Task Force                            E. Rosen, Ed.
Internet-Draft                                                 R. Bonica
Intended status: Informational                    Juniper Networks, Inc.
Expires: December 13, 2018                                 June 11, 2018

                   Augmenting RFC 4364 Technology to
         Provide Secure Layer L3VPNs over Public Infrastructure
                    draft-rosen-bess-secure-l3vpn-00

Abstract

   The Layer 3 Virtual Private Network (VPN) technology described in RFC
   4364 is focused on the scenario in which a network Service Provider
   (SP) maintains a secure backbone network and offers VPN service over
   that network to its customers.  Customers access the SP's network by
   attaching "Customer Edge" (CE) routers to "Provider Edge" (PE)
   routers, and exchanging cleartext IP packets.  PE routers generally
   serve multiple customers, and prevent unauthorized communication
   among customers.  Customer data sent across the backbone (from one PE
   to another) is encapsulated in MPLS, using an MPLS label to associate
   a given packet with a given customer.  The labeled packets are then
   sent across the backbone network in the clear, using MPLS transport.
   However, many customers want a VPN service that is secure enough to
   run over the public Internet, and which does not require them to send
   cleartext IP packets to a service provider.  Often they want to
   connect directly to edge nodes of the public Internet, which does not
   provide MPLS support.  Each customer may itself have multiple tenants
   who are not allowed to intercommunicate with each other freely.  In
   this case, the customer many need to provide a VPN service for the
   tenants.  This document describes a way in which this can be achieved
   using the technology of RFC 4364.  The functionality assigned therein
   to a PE router can be placed instead in Customer Premises Equipment.
   This functionality can be augmented by transmitting MPLS packets
   through IPsec Security Associations.  The BGP control plane sessions
   can also be protected by IPsec.  This allows a customer to use RFC
   4364 technology to provide VPN service to its internal departments,
   while sending only IPsec-protected packets to the Internet or other
   backbone network, and eliminating the need for MPLS transport in the
   backbone.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute

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   working documents as Internet-Drafts.  The list of current Internet-
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Review of L3VPN Concepts and Terminology  . . . . . . . .   3
     1.2.  Secured L3VPN . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Model of Operation  . . . . . . . . . . . . . . . . . . . . .   6
   3.  How the C-PEs Advertise Red Routes  . . . . . . . . . . . . .   9
     3.1.  Red and Black C-PE Loopback Addresses . . . . . . . . . .   9
     3.2.  Setting Up Red BGP Sessions Between C-PEs and RRs . . . .  10
     3.3.  Routes Transmited by the C-PE on Red BGP Sessions . . . .  12
     3.4.  Propagating Red Routes  . . . . . . . . . . . . . . . . .  13
   4.  VPN-IP Routes and Recursive Route Resolution  . . . . . . . .  13
   5.  MPLS-in-IPsec . . . . . . . . . . . . . . . . . . . . . . . .  15
   6.  Security Handle . . . . . . . . . . . . . . . . . . . . . . .  16
   7.  Data Plane Security Procedures  . . . . . . . . . . . . . . .  16
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
   10. Implementation Challenges . . . . . . . . . . . . . . . . . .  17
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  17
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     12.2.  Informational References . . . . . . . . . . . . . . . .  19

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

1.1.  Review of L3VPN Concepts and Terminology

   In conventional Virtual Private Networks (L3VPNs) based on the
   technology of [RFC4364], a Service Provider (SP) maintains a secure
   private network (known as the "SP backbone").  An SP maintains a
   number of "Provider Edge" (PE) routers to which customers may attach.
   A customer router that attaches to a PE router is known as a
   "Customer Edge" (CE) router.

   Multiple customers may connect to a single PE router.  Within a given
   PE, each customer is associated with a routing context of its own
   (known as a Virtual Routing and Forwarding table, or VRF).  A
   particular customer attaches to the PE via a set of one or more
   interfaces or Virtual LANs (VLANs) that are not shared with other
   customers.  (In the subsequent text, the term "interface" will
   include VLANs and other "virtual" interfaces.)  Each such interface
   is associated with a particular customer's VRF; thus such interfaces
   are known as "VRF interfaces".  These are the PE's customer-facing
   interfaces.  The VRF interfaces carry IP datagrams, either IPv4 or
   IPv6 or both.

   A given customer's VRF is automatically populated with, and only
   with, routes that lead out the local VRF interfaces, and routes that
   lead to remote VRF interfaces of the same customer.  Routes leading
   outside a customer's VPN are excluded from that customer's VRF unless
   explicitly allowed by policy.  Thus two customers can attach to the
   same PE even if they are not allow to communicate with each other
   through that PE.

   The PE at which a customer data packet enters the SP backbone network
   is known as the packet's "ingress PE".  The PE at which a customer
   data packet leaves the SP backbone is known as the packet's "egress
   PE".  Generally, the ingress PE pushes two MPLS labels onto each data
   packet.  The top label (sometimes known as the "transport label")
   directs the packet to its egress PE.  The second label (sometimes
   known as the "VPN label") is used at the egress PE to associate a
   given customer's packets with that customer's VRF at the egress PE.

   These labeled packets travel across the SP backbone "in the clear"
   (i.e., with no cryptographic protection to provide privacy,
   authentication, or integrity), as the SP backbone is presumed to be
   adequately secure.

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   The control plane protocol for this type of VPN is BGP.  A given
   customer's routes are distributed among the PEs to which that
   customer attaches by means of a BGP address family known as "VPN-IP"
   (either VPN-IPv4 or VPN-IPv6).  Distribution of these routes is
   controlled in such a way as to ensure that a given customer's routes,
   exported from one of that customer's VRFs, are imported only by other
   VRFs associated with the same customer.

1.2.  Secured L3VPN

   For security reasons, the L3VPN technology summarized in Section 1.1
   is not generally used in the following scenarios:

   o  Some or all of the customer sites need to be reached over the
      public Internet, rather than over a secure SP backbone network.

   o  The customer does not want to expose any of his data in cleartext
      to the SP, even if the SP backbone network is secure.

   o  The customer does not want to expose any of his routing control
      information in cleartext to the SP, and/or wishes to hide his
      internal IP addressing structure from the SP.

   In such situations, the customer needs to use cryptographic methods
   in order to ensure privacy, integrity, and authentication for the IP
   datagrams he sends over the backbone network; the cryptography must
   be applied before the datagrams are sent to the SP backbone network
   or Internet.  (It is presumed of course that the customer's own sites
   and systems have been secured to his satisfaction; how that is
   achieved is outside the scope of this document.)

   In these use cases, the customer may still want the benefits of the
   L3VPN service, e.g.:

   o  The customer may itself be providing a VPN service to multiple
      "tenants".  E.g.,

      *  The customer may be an enterprise or governmental agency that
         consists of multiple internal departments or organizations that
         are not allowed to communicate freely with each other, and that
         may even have independent IP address spaces.  We will use the
         term "tenant" to refer to such a department or organization.

      *  The customer may be a Data Center operator that is providing a
         virtual network to each of multiple Data Center tenants.

   o  In L3VPN, a CE router at one customer site does not have to be
      provisioned with the addresses of CE routers at other sites.

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      Rather, these are auto-discovered via BGP.  This sort of auto-
      discovery is just as valuable when the customer needs more
      security than is provided by conventional L3VPN.  Auto-discovery
      also allows some or all of the CE routers to be mobile, changing
      their IP addresses from time to time.

   It is possible to adapt the L3VPN technology to handle use cases
   where cryptographic methods must be applied before a packet is sent
   to an SP or to a backbone network .  This document describes a way in
   which this may be done.  We will refer to this adaptation as a
   "Secured L3VPN".  Section 2 outlines the way this adaptation works.
   Subsequent sections of this document specify the necessary procedures
   in more detail.

   Secured L3VPN makes use of IPsec technology.  This document does not
   discuss the details of IPsec.  A roadmap through the set of RFCs
   describing IPsec can be found in [RFC6071].  Of particular importance
   are [RFC4303] (IPsec Encapsulating Security Payload), [RFC7296]
   (Internet Key Exchange Protocol version 2), and [RFC8221]
   (Cryptographic Algorithm Implementation Requirements and Usage
   Guidance).

1.3.  Terminology

   In this document we shall use the following terminology:

   SP

      A network service provider (possibly an Internet service
      provider).

   customer

      An organization or other entity that obtains network service
      (private network service or Internet service) from an SP.

   tenant

      An organization or other entity that obtains VPN service from the
      customer.  For example, if the customer is a governmental agency,
      its tenants might be the various departments of the agency.  If
      the customer is an enterprise, its tenants might be the various
      organizations within the enterprise.  If the customer is a Data
      Center provider, its tenants might be organizations to which it
      sells Data Center services.

   C-PE router:

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      A router that performs the functions of an L3VPN PE router
      ([RFC4364]), but that is operated and managed not by a network
      service provider, but rather by a customer of the network service
      provider.  The customer may use the C-PE to provide a Secured
      L3VPN service to one or more of its tenants.

   Red interface:

      A tenant-facing interface of a C-PE device, where the tenant in
      question is receiving Secured L3VPN service.

   Black interface:

      A C-PE interface that is not a red interface.  This may be a
      network-facing interface, or a tenant-facing interface to a tenant
      that is not receiving any L3VPN service.

   Red BGP session:

      A BGP session, protected by IPsec, between two C-PEs, or between a
      C-PE and a BGP Route Reflector.

   Black BGP session:

      A BGP session other than a red BGP session.

   Local red route:

      A route whose next hop interface is a local red interface.

   Remote red route:

      A route received, as a VPN-IPv4 or VPN-IPv6 route, over a red BGP
      session.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.  Model of Operation

   In a Secured L3VPN, the functions conventionally performed by an
   L3VPN PE router (as detailed in [RFC4364]) are instead performed by a
   router that is operated and managed by the customer, rather than by
   the SP.  Since such a router is part of the customer's network, but
   has the functionality of an L3VPN PE router, we will refer to it as a

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   "C-PE router".  The customer is responsible for ensuring that the
   C-PE itself is properly secured.  The C-PE provides L3VPN
   functionality to the customer's tenants.

   Each interface of a C-PE is either a "red interface" or a "black
   interface":

   o  A red interface is a tenant-facing interface that attaches to a
      tenant who is receiving Secured L3VPN service from the customer.

   o  A black interface is any interface that is not a red interface.
      Black interfaces may be network-facing interfaces (attached to an
      SP backbone), or may be tenant-facing interfaces attached to
      tenants that are not receiving any L3VPN from the customer.  We
      assume in this document that a C-PE that provides the Secured
      L3VPN service to one or more tenants does not provide a
      conventional (unsecured) L3VPN service to any of the tenants.

   A C-PE has one or more VRFs, one per tenant.  Each VRF is associated
   with a distinct set of red interfaces, the ones that lead to the
   network(s), VLAN(s), or virtual network(s) that is (are) specific to
   the given tenant.  Standard L3VPN techniques then prevent
   communication among the different tenants unless explicitly allowed
   by policy.  In simpler scenarios, the customer may have sites with
   only a single tenant.  The C-PEs at those sites require only a single
   VRF, and all the red interfaces will be associated with that VRF.

   The black interfaces of a C-PE can attach to an access router of the
   public internet, or to a conventional L3VPN PE router belonging to an
   SP, or to any other router that provides IP connectivity among the
   customer's C-PE routers.  (If a C-PE attaches to a conventional L3VPN
   PE router, then the C-PE appears to the conventional PE to be a CE
   router.)

   As in any L3VPN, the VRFs are populated with a combination of local
   routes and remote routes:

   o  The local routes in a given VRF are those routes whose "next hop
      interface" is a local red interface associated with that VRF.

   o  The remote routes in a given VRF are those routes learned via BGP
      from the customer's other C-PEs.  These routes may be learned
      directly via BGP sessions to those other C-PEs, or indirectly via
      one or more BGP Route Reflectors (RRs).

   In this document, we will use the term "red routes" to refer to
   routes within a VRF.  These are distinguished from the "black routes"
   that exist in a C-PE's global routing table.

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   A conventional PE router sends and receives MPLS packets over its
   network facing interfaces.  A C-PE, on the other hand, sends "MPLS-
   in-IPsec" packets (see [RFC4023] and Section 5 of this document) over
   its black interfaces.  Since an MPLS-in-IPsec packet is an IP
   datagram, there is no need for the backbone network to support MPLS
   transport.  IPsec is used to provide privacy, integrity and
   authentication for the packets sent by the C-PE to the backbone
   network.

   In a Secured L3VPN, protection of the control plane is just as
   important as is protection of the data plane.  It is therefore
   necessary to ensure that the BGP messages used to disseminate the red
   routes also have privacy, integrity, and authentication.  In order to
   ensure this, the BGP sessions used to disseminate information about
   red routes will be protected by IPsec.  We will refer to such BGP
   sessions as "red BGP sessions".  It is recommended to use IPsec
   Transport mode to protect these BGP sessions.  This means that a C-PE
   MUST NOT send or receive VPN-IP routes over any BGP session that is
   not protected by IPsec.  (A VPN-IP route is a route whose BGP Address
   Family (AFI) is 1 (IPv4) or 2 (IPv6) and whose Subsequent Address
   Family (SAFI) is 128, 129, or 5.)

   Note that if RRs are used, the RRs must be as secure as the C-PEs.
   This likely means that they are managed by the customer and located
   at sites regarded by the customer as adequately secure.

   Thus in a Secured L3VPN, red routes are propagated only among trusted
   systems, and always in red BGP sessions.  The propagation of red
   routes on red BGP sessions is controlled by attaching Route Targets
   to those routes, as with any [RFC4364]-based technology.

   As in any L3VPN, BGP uses the VPN-IPv4 and/or VPN-IPv6 address
   families when disseminating information about VPN routes from one VRF
   to another.  Each such route carries an MPLS label that is to be
   pushed on the label stack of any tenant packet for which the address
   prefix in the route's NLRI is the best match to the packet's IP
   destination address.

   In Secured L3VPNs, these routes MUST also carry a Tunnel
   Encapsulation attribute ([tunnel_encaps]) specifying an "MPLS-in-
   IPsec" tunnel type (see Section 5, as well as Sections 3 and 8.1 of
   [RFC4023]).  This indicates that before a tenant's MPLS packet is
   sent to the backbone network, it must be encapsulated in IP and then
   sent on an IPsec Transport Mode Security Association (SA).

   A C-PE may have unprotected (black) BGP sessions, e.g., to gather
   public Internet routes.  However, the black BGP sessions MUST NOT be

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   enabled for the VPN-IP AFI/SAFIs.  This prevents any routes learned
   over the black BGP sessions from being imported into the VRFs.

   As we shall see in Section 3.3, there are also some IP routes (as
   distinguished from VPN-IP routes) that MUST NOT be transmitted on
   black BGP sessions, and that MUST be ignored if received on black BGP
   sessions.  These are known as the "red loopback routes".

   The procedures of this document result in a network overlay whose
   control plane consists of red BGP sessions, and whose data plane
   consists of MPLS-in-IPsec Security Associations.  This allows an SP's
   customer to provide Secured L3VPN service to its tenants.

   When RRs are used, C-PEs "register" with the RRs by setting up BGP
   sessions to them, running the BGP sessions through IPsec SAs.  The
   necessary authentication of the C-PE is provided during the course of
   setting up the IPsec SA.  One C-PE learns of another other C-PE's
   presence when the RR propagates routes from the latter C-PE to the
   former.

   The procedures specified in this document result in one MPLS-in-IPsec
   SA between a given pair of C-PEs.  This one SA will carry the traffic
   of all the tenants that are attached to both C-PEs.  That should
   provide adequate security, as the tenants' data is already exposed to
   the C-PEs.  If for some reason it is desired to have a distinct SA
   for each tenant, a method of doing so is mentioned in Section 4.

3.  How the C-PEs Advertise Red Routes

3.1.  Red and Black C-PE Loopback Addresses

   To support the Secured L3VPN control plane, each C-PE MUST have two
   loopback addresses.  One of these will be known as its "red
   loopback", the other as its "black loopback".

   The black loopbacks MUST be addresses that are globally routable.
   That is, they are public addresses.  (Strictly speaking, the black
   loopback only needs to be routable in any network that might be used
   to carry traffic between two C-PEs.  But we will assume that traffic
   between two C-PEs might need to traverse the public Internet.)
   Typically a C-PE's black loopback will be in the address space
   administered by the network service provider to which the C-PE
   attaches.  The service provider may assign it dynamically, or it may
   be assigned statically and configured in the C-PE by the customer.

   In addition to having a globally routable black loopback, a C-PE will
   of course have globally routable interface addresses for each of its
   black interfaces.

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   Interface addresses of the red interfaces SHOULD NOT be globally
   routable.

   If the C-PE attaches to multiple service providers, the black
   loopback is likely to be be a provider-independent address.  However,
   it MUST be routable in the backbone network of both providers, and
   most likely will need to be globally routable.

   The C-PE may have one or more (black) BGP sessions with service
   provider peers, in which case it may advertise the black loopback;
   the next hop field of such an advertisement would be the interface
   address of the interface over which that BGP session runs.

   Each C-PE of a given customer MUST be provisioned with a red loopback
   that is unique among the set of C-PEs of that customer.  The red
   loopback MUST NOT be a routable address in the public Internet or in
   the backbone networks of any service provider to which any of the
   C-PEs is attached.  If a C-PE has a (black) BGP session with a
   service provider peer, it MUST NOT advertise a route to its red
   loopback over that session.  That is, any IP route to a red loopback
   is considered to be a red route, and MUST NOT be advertised or
   received on a black BGP session.  These "red loopback routes" can
   thus be considered to be "red routes", even though they are IP rather
   than VPN-IP routes.

3.2.  Setting Up Red BGP Sessions Between C-PEs and RRs

   The customer is expected to have two or more BGP Route Reflectors
   (red RRs).  The red RRs are presumed to be secure; making them so is
   the responsibility of the customer.  As with the C-PEs, each red RR
   has a black loopback and a red loopback.  If the RR is not also a
   C-PE, it will have only black interfaces, each of course with a
   globally routable interface address.

   A customer's red RRs will form BGP sessions with that customer's
   C-PEs.  These BGP sessions MUST be protected by IPsec.  The use of
   IPsec transport mode is RECOMMENDED.  If the RR's red loopback is an
   IPv4 address, it may be used as the RR's BGP Identifier (see
   [RFC4271]).

   When a C-PE device comes up, it attempts to set up an IPsec-protected
   BGP session with the red RRs.  This requires first setting up an
   IPsec SA with each red RR, and then using IPsec Transport Mode to
   protect the BGP session.

   If the C-PE's red loopback is an IPv4 address, the C-PE's BGP
   Identifier (see [RFC4271]) may be the red loopback.

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   The endpoint addresses of the IPsec SA are the black loopbacks of the
   endpoint systems.

   Therefore, in order to initiate a BGP session to a red RR, a C-PE
   must be provisioned to know a publicly routable address (i.e., the
   black loopback) of the RR.  A C-PE must also be provisioned with
   whatever additional information is needed in order to set up an IPsec
   SA with each of the red RRs.  Each C-PE will attempt to continuously
   maintain live BGP sessions (protected by IPsec) with each red RR.
   Note that the source and destination IP address fields of the IP
   datagrams carrying the IPsec-encapsulated BGP messages will be
   publicly routable adresses.

   In some scenarios, it may be desirable to provision each red RR with
   the publicly routable address and pre-shared secret of every C-PE.
   This makes it easy for the C-PEs to authenticate themselves to the
   RR, but requires each RR to be reprovisioned every time a new C-PE is
   added to the network.

   In other scenarios, it may be considered desirable to allow the RRs
   to auto-discover the C-PEs, without the need for any per-C-PE pre-
   provisioning of the RRs.  In this case, a certificate-based
   authentication method [reference?] can be used when setting up the
   IPsec SAs that carry the BGP sessions.

   In either type of scenario, the C-PE SHOULD NOT be assumed to have a
   fixed black loopback address or fixed black interface addresses;
   rather, it SHOULD be assumed that a C-PE might be a mobile device
   whose globally routable addresses change from time to time.

   If a customer's C-PEs support multiple VPNs (for multiple tenants),
   that customer's red RRs will receive and disseminate the VPN-IP
   routes of all those VPNs.

   Note that according to the above procedures, the C-PEs will only have
   red BGP sessions to the red RRs; the C-PEs will not have BGP sessions
   to each other.  Thus it is not necessary for the C-PEs to know of
   each other in advance.  Of course, if a particular customer deems it
   desirable for the C-PEs to have red BGP sessions to each other, each
   C-PE can be provisioned with a publicly routable address of each
   other C-PE, along with any additional information needed to set up an
   IPsec SA to each other C-PE.

   It is RECOMMENDED that, for the purpose of setting up the red BGP
   sessions, all the RRs and C-PEs be considered to be in the same
   Autonomous System (AS).  Then the red BGP sessions will all be IBGP
   sessions, and the next hop field of a red route will not be modified
   as the route is propagated.  Note that if an implementation allows a

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   given router to be attached to two different ASes, this does not
   require that all the C-PEs and red RRs attach to the Internet via the
   same AS.  The "red overlay" may appear to be within a single AS, but
   the "black underlay" need not be within a single AS.

   If it is necessary to use an EBGP session between a C-PE and an RR,
   the RR SHOULD have a configured policy to leave the next hop
   unchanged when propagating red VPN-IP routes on an EBGP session.  See
   Section 3.4.

   In some scenarios, the C-PEs may set up red BGP sessions to
   Autonomous System Border Routers (ASBRs), rather than to RRs.  This
   is transparent to the C-PE.

3.3.  Routes Transmited by the C-PE on Red BGP Sessions

   A C-PE MUST transmit its local VPN-IP routes on the red BGP sessions,
   and only on the red BGP sessions.  The next hop of each local VPN-IP
   route MUST be set to the red loopback of the C-PE.  The choice to
   transmit a particular VPN-IP route on a particular session may of
   course be influenced by the route's Route Targets.

   A C-PE MUST NOT transmit its local VPN-IP routes on black BGP
   sessions.  VPN-IP routes MUST NOT be accepted from black BGP
   sessions.

   In all other respects, the handling of VPN-IP routes is done by
   normal L3VPN procedures.

   Each C-PE MUST also transmit the following IP (IPv4 or IPv6) route on
   the red BGP sessions.  We refer to this route as the C-PE's "red
   loopback route":

   o  The address prefix field of the route's Network Layer Reachability
      Information (NLRI) contains the C-PE's red loopback as a host
      address.

   o  The Next Hop of the route is the C-PE's black loopback.

   o  The route carries a Tunnel Encapsulation Attribute [tunnel_encaps]
      with the the following parameters:

      *  Tunnel Type = "MPLS-in-IPsec" (see Section 5.)

      *  Remote Endpoint = the C-PE's black loopback

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      *  A "Security Handle" (see Section 6).  This provides any
         information needed by another C-PE to set up an MPLS-in-IPsec
         Security Association with the advertising C-PE.

   A C-PE MUST NOT transmit, on any black BGP session, an IP route whose
   NLRI contains its red loopback.

   A given C-PE's red loopback route must be propagated to all other the
   C-PEs belonging to the same customer.  Therefore, such routes SHOULD
   NOT carry Route Targets.

3.4.  Propagating Red Routes

   A route that is received over a red BGP session may need to be
   propagated to other red BGP sessions.  A route that is received over
   a red BGP session MUST NOT be propagated over a black BGP session.
   Similarly, a route that is received over a black BGP session MUST NOT
   be propagated over a red BGP session.

   When a route is propagated from one red BGP session to another, its
   next hop SHOULD be left unchanged.  As specified in Section 4, this
   will ensure that a data packet sent on the path advertised by that
   route are sent on an IPsec SA between its ingress C-PE and its egress
   C-PE.  Changing the next hop will change the IPsec SA endpoint.

   This may be useful in certain deployments.  For instance, the path
   from an ingress C-PE to an egress C-PE may traverse several ASBRS.
   If these ASBRs are secure, it may be desirable to set up a sequence
   of IPsec SAs, (e.g., C-PE1--ASBR1, ASBR1--ASBR2, ASBR2--C-PE2)
   instead of using a single IPsec SA between C-PE1 and C-PE2.  If this
   is not the intention, the red BGP sessions MUST leave the next hop
   unchanged, even if those sessions are EBGP sessions.

   In all other respects, propagation of red routes is governed by the
   normal procedures for propagating routes.  If the route carries one
   or more Route Targets, these may affect its propagation.  However,
   note that propagation of a route between a red BGP session and a
   black BGP session MUST NOT be done, irrespective of the Route
   Targets.

4.  VPN-IP Routes and Recursive Route Resolution

   Suppose a C-PE, say C-PE1, receives a packet, say packet P, on one of
   its local red interfaces.  Suppose that packet P is addressed to a
   system that is reachable via one of the red interfaces at another
   C-PE, say C-PE2.  C-PE1 looks up packet P's destination address in
   the VRF associated with P's incoming interface.  The matching route
   will be a "Labeled VPN-IP route" [RFC4364] originated by C-PE2, and

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   disseminated to C-PE1 over a red BGP session.  Per Section 3.3, the
   next hop of that route will be C-PE2's red loopback.

   The labeled VPN-IP route matched by packet P's destination address
   will contain an MPLS label, the "VPN label".  C-PE1 pushes the VPN
   label onto packet P's MPLS label stack.  Then C-PE1 needs to
   determine how to transmit the resulting MPLS packet to the next hop
   of the VPN-IP route.  The next hop of the labeled VPN-IP route will
   be the red loopback address C-PE2.  So C-PE1 looks for the route to
   that red loopback address.  This will be the red loopback route
   (i.e., the red IP route, see Section 3.3) originated by C-PE2.

   C-PE2's red loopback will then be recursively resolved by means of
   C-PE2's red loopback route.  By virtue of the Tunnel Encapsulation
   attribute carried by that route, C-PE1 will realize that to send
   packet P, it must set up an MPLS-in-IPsec SA (see Section 5, as well
   as Sections 3 and 8.1 of [RFC4023]) with C-PE2.  Per the route's
   Tunnel Encapsulation attribute, the remote endpoint of this IPsec SA
   will be C-PE2's black loopback, and the Security Handle in the Tunnel
   Encapsulation attribute will carry any other information needed to
   set up the Security Association.

   Note that the remote endpoint of the IPsec SA is determined by the
   Tunnel Encapsulation attribute of the red loopback route, rather than
   by the next hop field of that route.  This ensures that the SA is
   made to the proper endpoint, even if the next hop field of the red
   loopback route was modified while the route was propagated.

   IMPORTANT: A VPN-IP route MUST NOT be recursively resolved by an IP
   route that was received over a black BGP session.  If a VPN-IP
   route's next hop resolves to a route received over a black BGP
   session, the existence of the latter route MUST be regarded as the
   result of an attempt to spoof the location of the egress C-PE.  That
   is, the latter route MUST be considered to be a spoofed route.  Note
   that it may not be possible to detect this spoofing attack until the
   attempt is made to recursively resolve the VPN-IP route.
   Implementors should take special care to ensure that their
   implementations are not vulnerable to this sort of spoofing attack.
   IF an implementation cannot detect this sort of attack during the
   recursive route resolution process, then the C-PE MUST NOT have any
   black BGP sessions.

   When packet P is transmitted, it is transmitted through an MPLS-in-
   IPsec SA.  Thus the only information that appears in the clear is the
   IP header needed to get the packet across the network.  The IP source
   and destination addresses of that packet will be the black loopbacks
   of C-PE1 and C-PE2 respectively.  The red loopback addresses do not
   appear in the packets at all, and no part of the payload packet

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   (neither the VPN label nor the IP datagram following the VPN label)
   appears in the clear.

   The MPLS-in-IPsec SA between C-PE1 and C-PE2 may be initiated by
   C-PE1 as soon as it receives a red loopback route originated by
   C-PE2.  Alternatively, the initiation of the setup of the Security
   Association may be delayed until the SA is actually needed for
   transmitting packets.

   These procedures will result in a single IPsec SA between a pair of
   C-PEs, with the data of multiple tenants carried on that single SA.
   If for some reason it is considered preferable to have an SA per
   tenant, the following procedures can be used:

   o  On each C-PE, provision a distinct red loopback for each tenant.

   o  Each C-PE will originate a red loopback route for each red
      loopback.

   o  Each red loopback route will have its own Tunnel Encapsulation
      attribute.  The respective Security Handle sub-TLVs (if present)
      MUST be distinct.

5.  MPLS-in-IPsec

   Packets traveling from one C-PE to another travel through "MPLS-in-
   IPsec" tunnels.  To transmit an MPLS packet through an MPLS-in-IPsec
   tunnel, one does the following:

   o  Encapsulate the MPLS packet in IP, as specified in Section 3 of
      [RFC4023].

   o  Use an IPsec transport mode Security Association to send the MPLS-
      in-IP packet from one C-PE to the other.  This is specified in
      Section 8.1 of [RFC4023].

   The result of encapsulating MPLS in IP and then transmitting the
   MPLS-in-IP packet on an IPsec transport mode Security Association is
   known as an MPLS-in-IPsec packet.

   On the wire, an MPLS-in-IPsec packet consists of a cleartext IP
   header followed by a payload.  The IP source and destination
   addresses of an MPLS-in-IPsec packet will be the black loopbacks of
   the source and destination C-PEs.  The payload will be an MPLS
   packet.  If the IPsec Security Association is providing privacy,
   authentication, and integrity, the payload is protected from
   inspection or alteration.

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   When the packet arrives at the destination C-PE, any necessary
   decryption is done, and packet appears to be an MPLS-in-IP packet
   addressed to the black address of the destination C-PE.  The IP
   encapsulation is removed, yielding an MPLS packet.  Per the usual
   L3VPN procedures, the label at the top of the MPLS label stack will
   be used to govern the further disposition of the packet.  However, if
   a packet received over a black interface was not received though an
   IPsec SA, the packet MUST NOT be sent out any VRF interface.

   MPLS-in-IP packets received in the clear (i.e., not received over an
   IPsec SA) MUST be discarded.

   Note that this section is not intended to describe an implementation
   strategy.

6.  Security Handle

   This document defines a new BGP Tunnel Encapsulation attribute sub-
   TLV, the "Security Handle".  This sub-TLV has a one-octet length
   field.  It is intended for use in the Tunnel Encapsulation attribute
   carried by the red loopback routes.  Its use is deployment specific.

   As an example, in some deployments, this sub-TLV might be used to
   carry the IPsec Security Parameters Index (SPI).  When setting up an
   SA to the originator of a particular Tunnel Encapsulation attribute,
   the SPI would be used as part of the SA setup procedure.

   In deployments where the C-PEs auto-discover each other through RRs,
   and authenticate via certificate-based mechanisms, the Security
   Handle may not be needed at all.  If a given deployment does not make
   use of the Security Handle, the sub-TLV SHOULD be omitted from the
   Tunnel Encapsulation attribue.

7.  Data Plane Security Procedures

   If a C-PE receives data over one of its local red interfaces, it may
   forward the data out another of its local red interfaces, as long as
   those two interfaces are associated with the same VRF, or if there is
   policy allowing communication ("extranet") between those two
   interfaces.

   However, data received by a C-PE over one of its red interfaces MUST
   NOT be forwarded out a black interface, unless that data is being
   sent over the black interface through an IPsec SA.

   Similarly, data received by a C-PE over one of its black interfaces
   MUST NOT be forwarded out a red interface unless the data arrived
   through an IPsec SA.

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   Typically an IPsec implementation has procedures to prevent
   unauthorized red-to-black or black-to-red forwarding.  However, the
   conventional procedures are based on filtering of IP addresses, and
   hence do not apply directly if MPLS-in-IPsec is used.  Implementors
   should take care to ensure that unauthorized red-to-black or black-
   to-red forwarding is prohibited.

8.  Acknowledgments

   The authors wish to thank John Scudder for his ideas and
   contributions to this work.

9.  IANA Considerations

   IANA is requested to create a new entry in the "BGP Tunnel
   Encapsulation Attribute Sub-TLVs" registry, "Security Handle".  This
   sub-TLV is defined in Section 6 to have a one-octet length field.
   Thus it needs to be assigned a codepoint in the range 0-127
   inclusive.

10.  Implementation Challenges

   This document specifies an architecture for Secured L3VPNs, but a
   successful implementation faces a number of challenges.

   This document specifies a recursive route resolution process that
   makes use of the Tunnel Encapsulation attribute.  This is a new
   feature.

   This document specifies that during recurisve route resolution,
   resolution of a red route via a route received over a black BGP
   session must be prohibited.  This is a new feature that may present
   challenges.

   Ultimately, success will require a highly scalable IPsec
   implementation, that can set up SAs dynamically based on information
   disseminated by BGP.  This presents a number of implementation
   challenges.

11.  Security Considerations

12.  References

12.1.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4023]  Worster, T., Rekhter, Y., and E. Rosen, Ed.,
              "Encapsulating MPLS in IP or Generic Routing Encapsulation
              (GRE)", RFC 4023, DOI 10.17487/RFC4023, March 2005,
              <https://www.rfc-editor.org/info/rfc4023>.

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
              Border Gateway Protocol 4 (BGP-4)", RFC 4271,
              DOI 10.17487/RFC4271, January 2006,
              <https://www.rfc-editor.org/info/rfc4271>.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, DOI 10.17487/RFC4303, December 2005,
              <https://www.rfc-editor.org/info/rfc4303>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8221]  Wouters, P., Migault, D., Mattsson, J., Nir, Y., and T.
              Kivinen, "Cryptographic Algorithm Implementation
              Requirements and Usage Guidance for Encapsulating Security
              Payload (ESP) and Authentication Header (AH)", RFC 8221,
              DOI 10.17487/RFC8221, October 2017,
              <https://www.rfc-editor.org/info/rfc8221>.

   [tunnel_encaps]
              Rosen, E., Patel, K., and G. Van de Velde, "The BGP Tunnel
              Encapsulation Attribute VPN", internet-draft draft-ietf-
              idr-tunnel-encaps-09, February 2018.

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12.2.  Informational References

   [RFC6071]  Frankel, S. and S. Krishnan, "IP Security (IPsec) and
              Internet Key Exchange (IKE) Document Roadmap", RFC 6071,
              DOI 10.17487/RFC6071, February 2011,
              <https://www.rfc-editor.org/info/rfc6071>.

Authors' Addresses

   Eric C. Rosen (editor)
   Juniper Networks, Inc.
   10 Technology Park Drive
   Westford, Massachusetts  01886
   United States

   Email: erosen@juniper.net

   Ron Bonica
   Juniper Networks, Inc.
   2251 Corporate Park Drive
   Herndon, Virginia  20171
   United States

   Email: rbonica@juniper.net

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