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Versions: (draft-kaliraj-idr-bgp-transport-vpn) 00 01 02 03 04 05 06

Network Working Group                                   K. Vairavakkalai
Internet-Draft                                           N. Venkataraman
Intended status: Standards Track                          B. Rajagopalan
Expires: June 1, 2021                             Juniper Networks, Inc.
                                                               G. Mishra
                                             Verizon Communications Inc.
                                                       November 28, 2020


                     BGP Classful Transport Planes
           draft-kaliraj-idr-bgp-classful-transport-planes-02

Abstract

   This document specifies a mechanism, referred to as "service
   mapping", to express association of overlay routes with underlay
   routes satisfying a certain SLA, using BGP.  The document describes a
   framework for classifying underlay routes into transport classes, and
   mapping service routes to specific transport class.  The transport
   class maps to a desired SLA.

   It specifies BGP protocol procedures that enable dissimination of
   such service mapping information that may span across administrative
   domains.  It makes it possible to advertise multiple tunnels to the
   same destination.

   A new BGP transport layer address family (SAFI 76) is defined for
   this purpose that uses RFC-4364 technology and follows RFC-8277 NLRI
   encoding.  This new address family is called "BGP Classful
   Transport", aka BGP-CT.

   It carries transport prefixes across tunnel domain boundaries (e.g.
   in Inter-AS Option-C networks), parallel to BGP-LU (SAFI 4) . It
   dissiminates "Transport class" information for the transport prefixes
   across the participating domains, which is not possible with BGP-LU.
   This makes the end-to-end network a "Transport Class" aware tunneled
   network.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].








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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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on June 1, 2021.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Transport Class . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  "Transport Class" Route Target Extended Community . . . . . .   7
   5.  Transport RIB . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Transport Routing Instance  . . . . . . . . . . . . . . . . .   8
   7.  Nexthop Resolution Scheme . . . . . . . . . . . . . . . . . .   8
   8.  BGP Classful Transport Family NLRI  . . . . . . . . . . . . .   9
   9.  Comparison with other families using RFC-8277 encoding  . . .  10
   10. Protocol Procedures . . . . . . . . . . . . . . . . . . . . .  11
   11. OAM considerations  . . . . . . . . . . . . . . . . . . . . .  14
   12. Illustration of procedures with example topology  . . . . . .  15
     12.1.  Topology . . . . . . . . . . . . . . . . . . . . . . . .  15
     12.2.  Service Layer route exchange . . . . . . . . . . . . . .  16



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     12.3.  Transport Layer route propagation  . . . . . . . . . . .  17
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     13.1.  New BGP SAFI . . . . . . . . . . . . . . . . . . . . . .  19
     13.2.  New Format for BGP Extended Community  . . . . . . . . .  19
       13.2.1.  Existing registries to be modified . . . . . . . . .  19
       13.2.2.  New registries to be created . . . . . . . . . . . .  20
     13.3.  MPLS OAM code points . . . . . . . . . . . . . . . . . .  21
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  21
   15. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     16.2.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   To facilitate service mapping, the tunnels in a network can be
   grouped by the purpose they serve into a "Transport Class".  The
   tunnels could be created using any signaling protocol, such as LDP,
   RSVP, BGP-LU or SPRING.  The tunnels could also use native IP or
   IPv6, as long as they can carry MPLS payload.  Tunnels may exist
   between different pair of end points.  Multiple tunnels may exist
   between the same pair of end points.

   Thus, a Transport Class consists of tunnels created by various
   protocols, that satisfy the properties of the class.  For example, a
   "Gold" transport class may consist of tunnels that traverse the
   shortest path with fast re-route protection, a "Silver" transport
   class may hold tunnels that traverse shortest paths without
   protection, a "To NbrAS Foo" transport class may hold tunnels that
   exit to neighboring AS Foo, and so on.

   The extensions specified in this document can be used to create a BGP
   transport tunnel that potentially spans domains, while preserving its
   Transport Class.  Examples of domain are Autonomous System (AS), or
   IGP area.  Within each domain, there is a second level underlay
   tunnel used by BGP to cross the domain.  The second level underlay
   tunnels could be hetrogeneous: Each domain may use a different type
   of tunnel (e.g.  MPLS, IP, GRE), or use a differnet signaling
   protocol.  A domain boundary is demarcated by a rewrite of BGP
   nexthop to 'self' while re-advertising tunnel routes in BGP.
   Examples of domain boundary are inter-AS links and inter-region ABRs.
   The path uses MPLS label-switching when crossing domain boundary and
   uses the native intra-AS tunnel of the desired transport class when
   traversing within a domain.

   Overlay routes carry sufficient indication of the Transport Classes
   they should be encapsulated over, in form of BGP community called the



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   "Mapping community".  Based on the mapping community, "route
   resolution" procedure on the ingress node selects from the
   corresponding Transport Class an appropriate tunnel whose destination
   matches (LPM) the nexthop of the overlay route.  If the overlay route
   is carried in BGP, the protocol nexthop (or, PNH) is generally
   carried as an attribute of the route.

   The PNH of the overlay route is also referred to as "service
   endpoint" (SEP).  The service endpoint may exist in the same domain
   as the service ingress node or lie in a different domain, adjacent or
   non-adjacent.  In the former case, reachability to the SEP is
   provided by an intra-domain tunneling protocol, and in the latter
   case, reachability to the SEP is via BGP transport families.

   In this architecture, the intra-domain transport protocols (e.g.
   RSVP, SRTE) are also "Transport Class aware", and they publish
   ingress routes in Transport RIB associated with the Transport Class,
   at the tunnel ingress node.  These routes are then redistributed into
   BGP-CT to be advertised to adjacent domains.

   This document describes mechanisms to:

      Model a "Transport Class" as "Transport RIB" on a router,
      consisting of tunnel ingress routes of a certain class.

      Enable service routes to resolve over an intended Transport Class
      by virtue of carrying the appropriate "Mapping community".  Which
      results in using the corresponding Transport RIB for finding
      nexthop reachability.

      Advertise tunnel ingress routes in a Transport RIB via BGP without
      any path hiding, using BGP VPN technology and Add-path.  Such that
      overlay routes in the receiving domains can also resolve over
      tunnels of associated Transport Class.

      Provide a way for co-operating domains to reconcile between
      independently administered extended community namespaces, and
      interoperate between different transport signaling protocols in
      each domain.

   In this document we focus mainly on MPLS LSPs as the intra-domain
   transport tunnels, but the mechanisms would work in similar manner
   for non-MPLS transport tunnels too.

   This document assumes MPLS forwarding when crossing domain
   boundaries, as that is the defacto standard in deployed networks
   today.  But mechanisms specified in this document can also support




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   different forwarding technologies (e.g.  SRv6).  A future document
   may describe such adaptations, it is out of scope of this document.

2.  Terminology

   LSP: Label Switched Path.

   TE : Traffic Engineering.

   SN : Service Node.

   BN : Border Node.

   TN : Transport Node, P-router.

   BGP-VPN : VPNs built using RFC4364 mechanisms.

   RT : Route-Target extended community.

   RD : Route-Distinguisher.

   PNH : Protocol-Nexthop address carried in a BGP Update message.

   SEP : Service End point, the PNH of a Service route.

   LPM : Longest Prefix Match.

   Service Family : BGP address family used for advertising routes for
   "data traffic", as opposed to tunnels.

   Transport Family : BGP address family used for advertising tunnels,
   which are in turn used by service routes for resolution.

   Transport Tunnel : A tunnel over which a service may place traffic.
   These tunnels can be GRE, UDP, LDP, RSVP, or SR-TE.

   Tunnel Domain : A domain of the network containing SN and BN, under a
   single administrative control that has a tunnel between SN and BN.
   An end-to-end tunnel spanning several adjacent tunnel domains can be
   created by "stitching" them together using labels.

   Transport Class : A group of transport tunnels offering the same type
   of service.

   Transport Class RT : A Route-Target extended community used to
   identify a specific Transport Class.





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   Transport RIB : At the SN and BN, a Transport Class has an associted
   Transport RIB that holds its tunnel routes.

   Transport Plane : An end to end plane comprising of transport tunnels
   belonging to same transport class.  Tunnels of same transport class
   are stitched together by BGP route readvertisements with nexthop-
   self, to span across domain boundaries using Label-Swap forwarding
   mechanism similar to Inter-AS option-b.

   Mapping Community : BGP Community/Extended-community on a service
   route, that maps it to resolve over a Transport Class.

3.  Transport Class

   A Transport Class is defined as a set of transport tunnels that share
   certain characteristics useful for underlay selection.

   On the wire, a transport class is represented as the Transport Class
   RT, which is a new Route-Target extended community.

   A Transport Class is configured at SN and BN, along with attributes
   like RD and Route-Target.  Creation of a Transport Class instantiates
   the associated Transport RIB and a Transport routing instance to
   contain them all.

   The operator may configure a SN/BN to classify a tunnel into an
   appropriate Transport Class, which causes the tunnel's ingress routes
   to be installed in the corresponding Transport RIB.  At a BN, these
   tunnel routes may then be advertised into BGP CT.

   Alternatively, a router receiving the transport routes in BGP with
   appropriate signaling information can associate those ingress routes
   to the appropriate Transport Class.  E.g. for Classful Transport
   family (SAFI 76) routes, the Transport Class RT indicates the
   Transport Class.  For BGP-LU family(SAFI 4) routes, import processing
   based on Communities or inter-AS source-peer may be used to place the
   route in the desired Transport Class.

   When the ingress route is received via SRTE [SRTE], which encodes the
   Transport Class as an integer 'Color' in the NLRI as
   "Color:Endpoint", the 'Color' is mapped to a Transport Class during
   import processing.  SRTE ingress route for 'Endpoint' is installed in
   that transport class.  The SRTE route when advertised out to BGP
   speakers will then be advertised in Classful Transport family with
   Transport Class RT and a new label.  The MPLS swap route thus
   installed for the new label will pop the label and deliver
   decapsulated traffic into the path determined by SRTE route.




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4.  "Transport Class" Route Target Extended Community

   This document defines a new type of Route Target, called "Transport
   Class" Route Target Extended Community.

   "Transport Class" Route Target extended community is a transitive
   extended community EXT-COMM [RFC4360] of extended-type, with a new
   Format (Type high = 0xa) and SubType as 0x2 (Route Target).

   This new Route Target Format has the following encoding:




    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Type= 0xa   | SubType= 0x02 |            Reserved           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Transport Class ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

            "Transport Class" Route Target Extended Community

    Type: 2 octets

       Type field contains value 0xa.

    SubType: 2 octets

       Subtype field contain 0x2. This indicates 'Route Target'.

    Transport Class ID: 4 octets

       The least significant 32-bits of the value field contain the
       "Transport Class" identifier, which is a 32-bit integer.

    The remaining 2 octets after SubType field are Reserved, they MUST
    be set to zero by originator, and ignored, left unaltered by
    receiver.

   The "Transport class" Route Target Extended community follows the
   mechanisms for VPN route import, export as specified in BGP-VPN
   [RFC4364], and Route Target Contrain mechanisms as specified in VPN-
   RTC [RFC4684]






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   A BGP speaker that implements RT Constraint VPN-RTC [RFC4684] MUST
   apply the RT Constraint procedures to the "Transport class" Route
   Target Extended community as-well.

   The Transport Class Route Target Extended community is carried on
   Classful Transport family routes, and allows associating them with
   appropriate Transport RIBs at receiving BGP speakers.

   Use of the Transport Class Route Target Extended community with a new
   Type code avoids conflicts with any VPN Route Target assignments
   already in use for service families.

5.  Transport RIB

   A Transport RIB is a routing-only RIB that is not installed in
   forwarding path.  However, the routes in this RIB are used to resolve
   reachability of overlay routes' PNH.  Transport RIB is created when
   the Transport Class it represents is configured.

   Overlay routes that want to use a specific Transport Class confine
   the scope of nexthop resolution to the set of routes contained in the
   corresponding Transport RIB.  This Transport RIB is the "Routing
   Table" referred in Section 9.1.2.1 RFC4271 [1]

   Routes in a Transport RIB are exported out in 'Classful Transport'
   address family.

6.  Transport Routing Instance

   A BGP VPN routing instance that is a container for the Transport RIB.
   It imports, and exports routes in this RIB with Transport Class RT.
   Tunnel destination addresses in this routing instance's context come
   from the "provider namespace".  This is different from user VRFs for
   e.g., which contain prefixes in "customer namespace"

   The Transport Routing instance uses the RD and RT configured for the
   Transport Class.

7.  Nexthop Resolution Scheme

   An implementation may provide an option for the service route to
   resolve over less preferred Transport Classes, should the resolution
   over preferred, or "primary" Transport Class fail.

   To accomplish this, the set of service routes may be associated with
   a user-configured "resolution scheme", which consists of the primary
   Transport Class, and optionally, an ordered list of fallback
   Transport Classes.



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   A community called as "Mapping Community" is configured for a
   "resolution scheme".  A Mapping community maps to exactly one
   resolution scheme.  A resolution scheme comprises of one primary
   transport class and optionally one or more fallback transport
   classes.

   A BGP route is associated with a resolution scheme during import
   processing.  The first community on the route that matches a mapping
   community of a locally configured resolution scheme is considered the
   effective mapping community for the route.  The resolution scheme
   thus found is used when resolving the route's PNH.  If a route
   contains more than one mapping community, it indicates that the route
   considers these multiple mapping communities as equivalent.  So the
   first community that maps to a resolution scheme is chosen.

   A transport route received in BGP Classful Transport family SHOULD
   use a resolution scheme that contains the primary Transport Class
   without any fallback to best effort tunnels.  The primary Transport
   Class is identified by the Transport Class RT carried on the route.
   Thus Transport Class RT serves as the Mapping Community for Classful
   Transport routes.

   A service route received in a BGP service family MAY map to a
   resolution scheme that contains the primary Transport Class
   identified by the mapping community on the route, and a fallback to
   best effort tunnels transport class.  The primary Transport Class is
   identified by the Mapping community carried on the route.  For e.g.
   the Extended Color community may serve as the Mapping Community for
   service routes.  Color:0:<n> MAY map to a resolution scheme that has
   primary transport class <n>, and a fallback to best-effort transport
   class.

8.  BGP Classful Transport Family NLRI

   The Classful Transport family will use the existing AFI of IPv4 or
   IPv6, and a new SAFI 76 "Classful Transport" that will apply to both
   IPv4 and IPv6 AFIs.

   The "Classful Transport" SAFI NLRI itself is encoded as specified in
   https://tools.ietf.org/html/rfc8277#section-2 [RFC8277].

   When AFI is IPv4 the "Prefix" portion of Classful Transport family
   NLRI consists of an 8-byte RD followed by an IPv4 prefix.  When AFI
   is IPv6 the "Prefix" consists of an 8-byte RD followed by an IPv6
   prefix.






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   Attributes on a Classful Transport route include the Transport Class
   Route-Target extended community, which is used to leak the route into
   the right Transport RIBs on SNs and BNs in the network.

9.  Comparison with other families using RFC-8277 encoding

   SAFI 128 (Inet-VPN) is a RF8277 encoded family that carries service
   prefixes in the NLRI, where the prefixes come from the customer
   namespaces, and are contexualized into separate user virtual service
   RIBs called VRFs, using RFC4364 procedures.

   SAFI 4 (BGP-LU) is a RFC8277 encoded family that carries transport
   prefixes in the NLRI, where the prefixes come from the provider
   namespace.

   SAFI 76 (Classful Transport) is a RFC8277 encoded family that carries
   transport prefixes in the NLRI, where the prefixes come from the
   provider namespace, but are contexualized into separate Transport
   RIBs, using RFC4364 procedures.

   It is worth noting that SAFI 128 has been used to carry transport
   prefixes in "L3VPN Inter-AS Carrier's carrier" scenario, where BGP-
   LU/LDP prefixes in CsC VRF are advertised in SAFI 128 towards the
   remote-end baby carrier.

   In this document a new AFI/SAFI is used instead of reusing SAFI 128
   to carry these transport routes, because it is operationally
   advantageous to segregate transport and service prefixes into
   separate address families, RIBs.  E.g.  It allows to safely enable
   "per-prefix" label allocation scheme for Classful Transport prefixes
   without affecting SAFI 128 service prefixes which may have huge
   scale. "per prefix" label allocation scheme keeps the routing churn
   local during topology changes.  A new family also facilitates having
   a different readvertisement path of the transport family routes in a
   network than the service route readvertisement path. viz. Service
   routes (Inet-VPN) are exchanged over an EBGP multihop session between
   Autonomous systems with nexthop unchanged; whereas Classful Transport
   routes are readvertised over EBGP single hop sessions with "nexthop-
   self" rewrite over inter-AS links.

   The Classful Transport family is similar in vein to BGP-LU, in that
   it carries transport prefixes.  The only difference is, it also
   carries in Route Target an indication of which Transport Class the
   transport prefix belongs to, and uses RD to disambiguate multiple
   instances of the same transport prefix in a BGP Update.






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10.  Protocol Procedures

   This section summarizes the procedures followed by various nodes
   speaking Classful Transport family

   Preparing the network for deploying Classful Transport planes

      Operator decides on the Transport Classes that exist in the
      network, and allocates a Route-Target to identify each Transport
      Class.

      Operator configures Transport Classes on the SNs and BNs in the
      network with unique Route-Distinguishers and Route-Targets.

      Implementations may provide automatic generation and assignment of
      RD, RT values for a transport routing instance; they MAY also
      provide a way to manually override the automatic mechanism, in
      order to deal with any conflicts that may arise with existing RD,
      RT values in the different network domains participating in a
      deployment.

   Origination of Classful Transport route:

      At the ingress node of the tunnel's home domain, the tunneling
      protocols install routes in the Transport RIB associated with the
      Transport Class the tunnel belongs to.  The ingress node then
      advertises this tunnel route into BGP as a Classful Transport
      route with NLRI RD:TunnelEndpoint, attaching a 'Transport Class'
      Route Target that identifies the Transport Class.

      Alternatively, the egress node of the tunnel i.e. the tunnel
      endpoint can originate the same BGP Classful Transport route, with
      NLRI RD:TunnelEndpoint and PNH TunnelEndpoint, which will resolve
      over the tunnel route at the ingress node.  When the tunnel is up,
      the Classful Transport BGP route will become usable and get re-
      advertised.

      Unique RD SHOULD be used by the originator of a Classful Transport
      route to disambiguate the multiple BGP advertisements for a
      transport end point.

   Ingress node receiving Classful Transport route

      On receiving a BGP Classful Transport route with a PNH that is not
      directly connected, e.g. an IBGP-route, a mapping community on the
      route (the Transport Class RT) indicates which Transport Class
      this route maps to.  The routes in the associated Transport RIB
      are used to resolve the received PNH.  If there does not exist a



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      route in the Transport RIB matching the PNH, the Classful
      Transport route is considered unusable, and MUST NOT be re-
      advertised further.

   Border node readvertising Classful Transport route with nexthop self:

      The BN allocates an MPLS label to advertise upstream in Classful
      Transport NLRI.  The BN also installs an MPLS swap-route for that
      label that swaps the incoming label with a label received from the
      downstream BGP speaker, or pops the incoming label.  And then
      pushes received traffic to the transport tunnel or direct
      interface that the Classful Transport route's PNH resolved over.

   Border node receiving Classful Transport route on EBGP :

      If the route is received with PNH that is known to be directly
      connected, e.g.  EBGP single-hop peering address, the directly
      connected interface is checked for MPLS forwarding capability.  No
      other nexthop resolution process is performed, as the inter-AS
      link can be used for any Transport Class.

      If the inter-AS links should honor Transport Class, then the BN
      SHOULD follow procedures of an Ingress node described above, and
      perform nexthop resolution process.  The interface routes SHOULD
      be installed in the Transport RIB belonging to the associated
      Transport Class.

   Avoiding path-hiding through Route Reflectors

      When multiple BNs exist that advertise a RDn:PEn prefix to RRs,
      the RRs may hide all but one of the BNs, unless ADDPATH [RFC7911]
      is used for the Classful Transport family.  This is similar to
      L3VPN option-B scenarios.  Hence ADDPATH SHOULD be used for
      Classful Transport family, to avoid path-hiding through RRs.

   Ingress node receiving service route with mapping community

      Service routes received with mapping community resolve using
      Transport RIBs determined by the resolution scheme.  If the
      resolution process does not find an usable Classful Transport
      route or tunnel route in any of the Transport RIBs, the service
      route MUST be considered unusable for forwarding purpose.

   Coordinating between domains using different community namespaces.

      Domains not agreeing on RT, RD, Mapping-community values because
      of independently administered community namespaces may deploy
      mechanisms to map and rewrite the Route-target values on domain



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      boundaries, using per ASBR import policies.  This is no different
      than any other BGP VPN family.  Mechanisms employed in inter-AS
      VPN deployments may be used with the Classful Transport family
      also.

      The resolution schemes SHOULD allow association with multiple
      mapping communities.  This helps with renumbering, network
      mergers, or transitions.

      Though RD can also be rewritten on domain boundaries, deploying
      unique RDs is strongly RECOMMENDED, because it helps in trouble
      shooting by uniquely identifying originator of a route, and avoids
      path-hiding.

      This document defines a new format of Route-Target extended-
      community to carry Transport Class, this avoids collision with
      regular Route Target namespace used by service routes.

   Constrained distribution of PNHs to SNs.

      This section describes how the number of Protocol Nexthops
      advertised to a SN can be constrained using BGP Classsful
      Transport and VPN RTC [RFC4684]

      An egress SN MAY advertise BGP CT route for RD:eSN with two Route
      Targets: transport-target:0:<TC> and a RT carrying <eSN>:<TC>.
      Where TC is the Transport Class identifier, and eSN is the IP-
      address used by SN as BGP nexthop in it's service route
      advertisements.

      transport-target:0:<TC> is the new type of route target (Transport
      Class RT) defined in this document.  It is carried in BGP extended
      community attribute (BGP attribute code 16).

      The RT carrying <eSN>:<TC> MAY be an IP-address specific regular
      RT (BGP attribute code 16), IPv6-address specific RT (BGP
      attribute code 25), or a Wide-communities based RT (BGP attribute
      code 34) as described in RTC-Ext [RTC-Ext]

      An ingress SN MAY import BGP CT routes with Route Target carrying:
      <eSN>:<TC>.  The ingress SN MAY learn the eSN values either by
      configuration, or it MAY discover them from the BGP nexthop field
      in the BGP VPN service routes received from eSN.  A BGP ingress SN
      receiving a BGP service route with nexthop of eSN SHOULD generate
      a RTC/Extended-RTC route for Route Target prefix <Origin
      ASN>:<eSN>/<prefix-length> in order to learn BGP CT transport
      routes to reach eSN.  This allows constrained distribution of the
      transport routes to the PNHs actually required by iSN.



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      A BN in the core of the network SHOULD import BGP CT routes with
      Transport Class Route Target: 0:TC.  Because it is interested in
      transport routes to all eSN nodes.

11.  OAM considerations

   Standard MPLS OAM procedures specified in [RFC8029] also apply to BGP
   Classful Transport.

   The 'Target FEC Stack' sub-TLV for IPv4 Classful Transport has a Sub-
   Type of [TBD], and a length of 13.  The Value field consists of the
   RD advertised with the Classful Transport prefix, the IPv4 prefix
   (with trailing 0 bits to make 32 bits in all), and a prefix length,
   encoded as follows:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Route Distinguisher                      |
       |                          (8 octets)                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         IPv4 prefix                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Prefix Length |                 Must Be Zero                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 1: Classful Transport IPv4 FEC

   The 'Target FEC Stack' sub-TLV for IPv6 Classful Transport has a Sub-
   Type of [TBD], and a length of 25.  The Value field consists of the
   RD advertised with the Classful Transport prefix, the IPv6 prefix
   (with trailing 0 bits to make 128 bits in all), and a prefix length,
   encoded as follows:


















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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Route Distinguisher                      |
       |                          (8 octets)                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                         IPv6 prefix                           |
       |                                                               |
       |                                                               |
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Prefix Length |                 Must Be Zero                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 2: Classful Transport IPv6 FEC

12.  Illustration of procedures with example topology

12.1.  Topology

                   [RR26]      [RR27]                       [RR16]
                    |            |                             |
                    |            |                             |
                    |+-[ABR23]--+|+--[ASBR21]---[ASBR13]-+|+--[PE11]--+
                    ||          |||          `  /        |||          |
   [CE41]--[PE25]--[P28]       [P29]          `/        [P15]     [CE31]
                    |           | |           /`         | |          |
                    |           | |          /  `        | |          |
                    |           | |         /    `       | |          |
                    +--[ABR24]--+ +--[ASBR22]---[ASBR14]-+ +--[PE12]--+


          |                |                  |                    |
          +                +                  +                    +
       CE |     region-1   |   region-2       |                    |CE
      AS4              ...AS2...                       AS1          AS3

   41.41.41.41  ------------ Traffic Direction ---------->   31.31.31.31

   This example shows a provider network that comprises of two
   Autonomous systems, AS1, AS2.  They are serving customers AS3, AS4
   respectively.  Traffic direction being described is CE41 to CE31.
   CE31 may request a specific SLA, e.g.  Gold for this traffic, when
   traversing these provider networks.

   AS2 is further divided into two regions.  So there are three tunnel
   domains in provider space.  AS1 uses ISIS Flex-Algo intra-domain
   tunnels, whereas AS2 uses RSVP intra-domain tunnels.



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   The network has two Transport classes: Gold with transport-class id
   100, Bronze with transport-class id 200.  These transport classes are
   provisioned at the PEs and the Border nodes (ABRs, ASBRs) in the
   network.

   Following tunnels exist for gold transport class.

      PE25_to_ABR23_gold - RSVP tunnel

      PE25_to_ABR24_gold - RSVP tunnel

      ABR23_to_ASBR22_gold - RSVP tunnel

      ASBR13_to_PE11_gold - ISIS FlexAlgo tunnel

      ASBR14_to_PE11_gold - ISIS FlexAlgo tunnel

   Following tunnels exist for bronze transport class.

      PE25_to_ABR23_bronze - RSVP tunnel

      ABR23_to_ASBR21_bronze - RSVP tunnel

      ABR23_to_ASBR22_bronze - RSVP tunnel

      ABR24_to_ASBR21_bronze - RSVP tunnel

      ASBR13_to_PE12_bronze - ISIS FlexAlgo tunnel

      ASBR14_to_PE11_bronze - ISIS FlexAlgo tunnel

   These tunnels are either provisioned or auto-discovered to belong to
   transport class 100 or 200.

12.2.  Service Layer route exchange

   Service nodes PE11, PE12 negotiate service families (SAFI 1, 128) on
   the BGP session with RR16.  Service helpers RR16, RR26 have multihop
   EBGP session to exchange service routes between the two AS.
   Similarly PE25 negotiates service families with RR26.

   Forwarding happens using service routes at service nodes PE25, PE11,
   PE12 only.  Routes received from CEs are not present in any other
   nodes' FIB in the network.

   CE31 advertises a route for example prefix 31.31.31.31 with nexthop
   self to PE11, PE12.  CE31 can attach a mapping community Color:0:100




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   on this route, to indicate its request for Gold SLA.  Or, PE11 can
   attach the same using locally configured policies.

   The 31.31.31.31 route is readvertised by PE11 with nexthop self
   (1.1.1.1) to RR16 with the mapping community Color:0:100 attached.
   This route reaches PE25 via RR16, RR26 with the nexthop unchanged, as
   PE11.  Now PE25 can resolve the PNH 1.1.1.1 using transport routes
   received in BGP-CT or BGP-LU.

   The IP FIB at PE25 will have a route for 31.31.31.31 with a nexthop
   thus found, that points to a gold tunnel in ingress domain.

12.3.  Transport Layer route propagation

   ASBR13 negotiates BGP-CT family with transport ASBRs ASBR21, ASBR22.
   They negotiate BGP-CT family with RR27 in region 2.  ABR23, ABR24
   negotiate BGP-CT family with RR27 in region 2 and RR26 in region 1.
   PE25 receives BGP-CT routes from RR26.  BGP-LU family is also
   negotiated on these sessions alongside BGP-CT family.  BGP-LU carries
   "best effort" transport class routes, BGP-CT carries gold, bronze
   transport class routes.

   ASBR13 is provisioned with transport class 100, RD value 1.1.1.3:10
   and a transport route target 0:100.  And a Transport class 200 with
   RD value 1.1.1.3:20, and transport route target 0:200.

   Similarly, these transoprt classes are also configured on ASBRs, ABRs
   and PEs, with same transport route target, but unique RDs.

   Ingress route for ASBR13_to_PE11_gold is advertised by ASBR13 in BGP-
   CT family with a NLRI containing RD prefix 1.1.1.3:10:1.1.1.1 with
   Label L1 and a route target extended community transport-
   target:0:100.  MPLS swap route is installed at ASBR13 for L1 with a
   nexthop pointing to ASBR13_to_PE11_gold tunnel.

   Ingress route for ASBR13_to_PE11_bronze is advertised by ASBR13 in
   BGP-CT family with a NLRI containing RD prefix 1.1.1.3:20:1.1.1.1
   with Label L2 and a route target extended community transport-
   target:0:200.  MPLS swap route is installed at ASBR13 for label L2
   with a nexthop pointing to ASBR13_to_PE11_bronze tunnel

   ASBR21 receives BGP-CT route 1.1.1.3:10:1.1.1.1 over the single hop
   EBGP sesion, and readvertises with nexthop self (loopback adderss
   2.2.2.1) to RR27, advertising a new label L3.  MPLS swap route is
   installed for label L3 at ASBR21 to swap to received label L1 and
   forwards to ASBR13.  RR27 readvertises this BGP-CT route to ABR23,
   ABR24.




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   ASBR22 receives BGP-CT route 1.1.1.3:10:1.1.1.1 over the single hop
   EBGP sesion, and readvertises with nexthop self (loopback adderss
   2.2.2.2) to RR27, advertising a new label L4.  MPLS swap route is
   installed for label L4 at ASBR21 to swap to received label L2 and
   forwards to ASBR13.  RR27 readvertises this BGP-CT route to ABR23,
   ABR24.

   Addpath is enabled for BGP-CT family on the sessions between RR27 and
   ASBRs, ABRs.  Such that routes for 1.1.1.3:10:1.1.1.1 with the
   nexthops ASBR21 and ASBR22 are reflected to ABR23, ABR24 without any
   path hiding.  Thus giving ABR23 visibiity of all available paths for
   gold SLA.

   ABR23 receives the route with nexthop 2.2.2.1, label L3 from RR27.
   The route target "transport-target:0:100" on this route acts as
   mapping community, and instructs ABR23 to strictly resolve the
   nexthop using transport class 100 routes only.  ABR23 is unable to
   find a route for 2.2.2.1 with transport class 100.  Thus it considers
   this route unusable and does not propagate it further.  This prunes
   ASBR21 from gold SLA tunneled path.

   ABR23 also receives the route with nexthop 2.2.2.2, label L4 from
   RR27.  The route target "transport-target:0:100" on this route acts
   as mapping community, and instructs ABR23 to strictly resolve the
   nexthop using transport class 100 routes only.  ABR23 successfully
   resolves the nexthop to point to ABR23_to_ASBR22_gold tunnel.  ABR23
   readvertises this route with nexthop self (loopback address 2.2.2.3)
   and a new label L5 to RR26.  Swap route for L5 is installed by ABR23
   to swap to label L4, and forward into ABR23_to_ASBR22_gold tunnel.

   RR27 reflects the route from ABR23 to PE25.  PE25 receives the BGP-CT
   route for prefix 1.1.1.3:10:1.1.1.1 with label L5, nexthop 2.2.2.3
   and transport-target:0:100 from RR26.  And it similarly resolves the
   nexthop 2.2.2.3 over transport class 100, pushing labels associated
   with PE25_to_ABR23_gold tunnel.

   In this manner, the tunnels ASBR13_to_PE11_gold, ABR23_to_ASBR22_gold
   and PE25_to_ABR23_gold are stitched together by MPLS swap routes to
   form an inter-domain BGP-CT LSP satisfying gold SLA end to end.  MPLS
   swap routes are installed at ASBR13, ASBR21 and ABR23, when
   propagating the PE11 BGP-CT route with nexthop self towards PE25.

   When PE25 receives service routes with nexthop 1.1.1.1 and mapping
   community Color:0:100, it resolves over this BGP-CT route
   1.1.1.3:10:1.1.1.1.  Thus pushing label L5, and pushing as top label
   the labels associated with PE25_to_ABR23_gold tunnel.





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13.  IANA Considerations

   This document makes following requests of IANA.

13.1.  New BGP SAFI

   New BGP SAFI code for "Classful Transport".  Value 76.

   This will be used to create new AFI,SAFI pairs for IPv4, IPv6
   Classful Transport families. viz:

   o  "Inet, Classful Transport".  AFI/SAFI = "1/76" for carrying IPv4
      Classful Transport prefixes.

   o  "Inet6, Classful Transport".  AFI/SAFI = "2/76" for carrying IPv6
      Classful Transport prefixes.

13.2.  New Format for BGP Extended Community

   Please assign a new Format (Type high = 0xa) of extended community
   EXT-COMM [RFC4360] called "Transport Class" from the following
   registries:

      the "BGP Transitive Extended Community Types" registry, and

      the "BGP Non-Transitive Extended Community Types" registry.

   Please assign the same low-order six bits for both allocations.

   This document uses this new Format with subtype 0x2 (route target),
   as a transitive extended community.

   The Route Target thus formed is called "Transport Class" route target
   extended community.

   Taking reference of RFC7153 [RFC7153] , following requests are made:

13.2.1.  Existing registries to be modified

13.2.1.1.  Registries for the "Type" Field

13.2.1.1.1.  Transitive Types

   This registry contains values of the high-order octet (the "Type"
   field) of a Transitive Extended Community.






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   Registry Name: BGP Transitive Extended Community Types

         TYPE VALUE       NAME
   +      0x0a             Transitive Transport Class Extended
   +                       Community (Sub-Types are defined in the
   +                       "Transitive Transport Class Extended
   +                       Community Sub-Types" registry)


13.2.1.1.2.  Non-Transitive Types

   This registry contains values of the high-order octet (the "Type"
   field) of a Non-transitive Extended Community.

   Registry Name: BGP Non-Transitive Extended Community Types

        TYPE VALUE       NAME

   +     0x4a             Non-Transitive Transport Class Extended
   +                      Community (Sub-Types are defined in the
   +                      "Non-Transitive Transport Class Extended
   +                      Community Sub-Types" registry)

13.2.2.  New registries to be created

13.2.2.1.  Transitive "Transport Class" Extended Community Sub-Types
           Registry

    This registry contains values of the second octet (the "Sub-Type"
    field) of an extended community when the value of the first octet
    (the "Type" field) is 0x07.

      Registry Name: Transitive Transport Class Extended
                     Community Sub-Types

         RANGE              REGISTRATION PROCEDURE

         0x00-0xBF          First Come First Served
         0xC0-0xFF          IETF Review

         SUB-TYPE VALUE     NAME

         0x02               Route Target








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13.2.2.2.  Non-Transitive "Transport Class" Extended Community Sub-Types
           Registry

   This registry contains values of the second octet (the "Sub-Type"
   field) of an extended community when the value of the first octet
   (the "Type" field) is 0x47.

      Registry Name: Non-Transitive Transport Class Extended
                     Community Sub-Types

         RANGE              REGISTRATION PROCEDURE

         0x00-0xBF          First Come First Served
         0xC0-0xFF          IETF Review

         SUB-TYPE VALUE     NAME

         0x02               Route Target

13.3.  MPLS OAM code points

   The following two code points are sought for Target FEC Stack sub-
   TLVs:

   o  IPv4 BGP Classful Transport

   o  IPv6 BGP Classful Transport

14.  Security Considerations

   Mechanisms described in this document carry Transport routes in a new
   BGP address family.  That minimizes possibility of these routes
   leaking outside the expected domain or mixing with service routes.

   When redistributing between SAFI 4 and SAFI 76 Classful Transport
   routes, there is a possibility of SAFI 4 routes mixing with SAFI 1
   service routes.  To avoid such scenarios, it is RECOMMENDED that
   implementations support keeping SAFI 4 routes in a separate transport
   RIB, distinct from service RIB that contain SAFI 1 service routes.

15.  Acknowledgements

   The authors thank Jeff Haas, John Scudder, Navaneetha Krishnan, Ravi
   M R, Chandrasekar Ramachandran, Shradha Hegde, Richard Roberts,
   Krzysztof Szarkowicz, John E Drake, Srihari Sangli, Vijay Kestur,
   Santosh Kolenchery, Robert Raszuk for the valuable discussions.





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   The decision to not reuse SAFI 128 and create a new address-family to
   carry these transport-routes was based on suggestion made by Richard
   Roberts and Krzysztof Szarkowicz.

16.  References

16.1.  Normative References

   [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>.

   [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>.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <https://www.rfc-editor.org/info/rfc4360>.

   [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>.

   [RFC4684]  Marques, P., Bonica, R., Fang, L., Martini, L., Raszuk,
              R., Patel, K., and J. Guichard, "Constrained Route
              Distribution for Border Gateway Protocol/MultiProtocol
              Label Switching (BGP/MPLS) Internet Protocol (IP) Virtual
              Private Networks (VPNs)", RFC 4684, DOI 10.17487/RFC4684,
              November 2006, <https://www.rfc-editor.org/info/rfc4684>.

   [RFC7153]  Rosen, E. and Y. Rekhter, "IANA Registries for BGP
              Extended Communities", RFC 7153, DOI 10.17487/RFC7153,
              March 2014, <https://www.rfc-editor.org/info/rfc7153>.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <https://www.rfc-editor.org/info/rfc7911>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.




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   [RFC8277]  Rosen, E., "Using BGP to Bind MPLS Labels to Address
              Prefixes", RFC 8277, DOI 10.17487/RFC8277, October 2017,
              <https://www.rfc-editor.org/info/rfc8277>.

   [RTC-Ext]  Zhang, Z., Ed., "Route Target Constrain Extension", 07
              2020, <https://tools.ietf.org/html/draft-zzhang-idr-bgp-
              rt-constrains-extension-00#section-2>.

   [SRTE]     Previdi, S., Ed., "Advertising Segment Routing Policies in
              BGP", 11 2019, <https://tools.ietf.org/html/draft-ietf-
              idr-segment-routing-te-policy-08>.

16.2.  URIs

   [1] https://www.rfc-editor.org/rfc/rfc4271#section-9.1.2.1

Authors' Addresses

   Kaliraj Vairavakkalai
   Juniper Networks, Inc.
   1133 Innovation Way,
   Sunnyvale, CA  94089
   US

   Email: kaliraj@juniper.net


   Natrajan Venkataraman
   Juniper Networks, Inc.
   1133 Innovation Way,
   Sunnyvale, CA  94089
   US

   Email: natv@juniper.net


   Balaji Rajagopalan
   Juniper Networks, Inc.
   Electra, Exora Business Park~Marathahalli - Sarjapur Outer
             Ring Road,
   Bangalore, KA  560103
   India

   Email: balajir@juniper.net







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   Gyan Mishra
   Verizon Communications Inc.
   13101 Columbia Pike
   Silver Spring, MD  20904
   USA

   Email: gyan.s.mishra@verizon.com












































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