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Versions: 00 01                                                         
routing                                                         Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                      September 15, 2021
Expires: March 19, 2022

              MVPN Inter/Intra-region Tunnel Segmentation


   RFC7524 specifies procedures for Inter-Area Point-to-Multipoint
   Segmented Label Switched Paths (aka MVPN tunnel segmentation).  This
   document updates RFC7524 by extending the inter-area segmentation
   concept to inter-region and intra-region segmentation where a region
   is no longer tied to an IGP area.

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
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   This Internet-Draft will expire on March 19, 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Tunnel Segmentation . . . . . . . . . . . . . . . . . . .   2
     1.2.  Intra-region Segmentation . . . . . . . . . . . . . . . .   3
     1.3.  Bud Node Support  . . . . . . . . . . . . . . . . . . . .   3
   2.  Specifications  . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Inter-region Segmentation . . . . . . . . . . . . . . . .   3
     2.2.  Intra-region Segmentation . . . . . . . . . . . . . . . .   4
     2.3.  Bud Node Support  . . . . . . . . . . . . . . . . . . . .   5
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

1.1.  Tunnel Segmentation

   [RFC6514] specifies (among other things) inter-AS MVPN tunnel
   segmentation procedures and [RFC7524] specifies inter-area MVPN
   tunnel segmentation procedures.  The procedures for inter-AS and
   inter-area are similar in that the segmentation points - ASBRs or
   ABRs - change the PMSI Tunnel Attribute (PTA) attached to I/S-PMSI
   routes to specify the type and identification of tunnel to be used in
   the next AS/area, when they re-advertise the PMSI routes into the
   next AS/area.

   This change of tunnel at the segmentation points and stitching of
   upstream and downstream tunnel segments not only allows different
   tunnel technology/instance to be used in different AS/area, but also
   limits the replication of traffic to only PEs and segmentation points
   in the local AS/area, instead of to all PEs.

   The inter-area segmentation points are route reflectors and when they
   re-advertise the x-PMSI routes to different downstream areas they may
   use different BGP neighbor groups so that different tunnel type/
   identification can be encoded in PTA for different downstream areas.
   If the ABR is also responsible for reflecting the routes to PEs in
   the same area, the ABR does not modify the PTA (because of that those
   local PEs are also put into a different neighbor group).

   As a result, a segmentation point will likely have different neighbor
   groups (one group for each area) so that the PTA and Inter-Area P2MP
   Segmented Next-Hop Extended Community (referred to as Segmentation
   EC) can be set accordingly when it re-advertise the x-PMSI routes.

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   The provisioning of a RR with these different neighbor groups for
   segmentation purpose can actually be done on any router (as a
   segmentation point) - not necessarily on an ABR.  As a result, the
   procedures in RFC7524, while specified for inter-area, can be
   extended inter-regiona as well - the segmentation points can be any
   border routers between arbitrarily defined "regions".

   This concept is already described in Section 6 of
   [I-D.ietf-bess-evpn-bum-procedure-updates], but specified formally in
   this document for MVPN.

1.2.  Intra-region Segmentation

   Even with the inter-area segmentation extended to inter-region, when
   a regional border router (RBR) reflects routes to PEs in the same
   region, it does not modify the PTA or Segmentation EC.  But if the
   RBR also modifies the two attributes when reflecting routes to the
   local PEs, tunnel segmentation is achieved even intra-region - both
   the upstream and downstream tunnel segments are in the same region.

   This Intra-region Segmentation is one way to achieve Assisted
   Replication in MVPN: a PE sends traffic to assisting replicators who
   will then relay traffic to other PEs (even in the same region).

1.3.  Bud Node Support

   A segmentation point may have both local receivers off a VRF and
   downstream receivers off a remote PE for traffic arriving on an
   upstream segment.  This segmentation point is referred to a bud node,
   just like that a node can be both a transit and leaf node for a P2MP

   Depending on implementation, a bud node may need to receive two
   copies of a packet, one for local delivery and one for remote
   delivery.  If so, the bud node may request the upstream PE or
   segmentation point to send two copies.  This is done by specifying an
   additional label in the PTA.

2.  Specifications

2.1.  Inter-region Segmentation

   The procedures in RFC7524 are extended to beyond IGP area-based.  A
   provider network can be arranged into "regions" connected by
   "Regional Border Routers" (RBRs).  On a segmentation point a region
   MAY be defined as a BGP neighbor group - all peers in the group are
   subject to the same export policy, which can be used to control the
   modification of attributes for the purpose of segmentation.

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   RFC7524 procedures apply as is, though "area" is replaced with
   "region" and "Area Border Router" (ABR) is replaced with "Regional
   Border Router" (RBR).

   The concept of Per-region Aggregation, as explained in Section 6.1 of
   [I-D.ietf-bess-evpn-bum-procedure-updates], is also applicable to
   MVPN.  A future revision of this document will specify details of
   Per-region Aggregation for MVPN.

2.2.  Intra-region Segmentation

   The following procedures are applicable for intra-region
   segmentation.  One use of intra-region segmentation is for Assisted
   Replication where PE-PE traffic goes through a relay point (assisting

   If it is known that the local PEs are only peered with the RBRs (as
   RRs and segmentation points), the PEs and RBRs follow the procedures
   in RFC7524.  In addition, the local RBRs modify the PTA and
   Segmentation EC even when they re-advertise x-PMSI routes to PEs in
   the ingress region, thus achieving Intra-region Segmentation.

   Otherwise (i.e., if a local PE may import BGP-MVPN routes directly
   unless with the modified procedures specified below), the following
   modified procedures apply:

   o  When an ingress PE advertises an x-PMSI route, it attaches an
      Extended Community (EC) derived from the Route Target for the VPN
      (RT-VPN) [I-D.zzhang-idr-rt-derived-community] but not the RT-VPN
      itself.  Call this EC as EC-VPN.  The route also carry a
      Segmentation EC as specified in RFC7524.

   o  When the local RBRs (as RRs and segmentation points) receive this
      route, it replaces the EC-VPN with the corresponding RT-VPN (the
      EC-VPN and RT-VPN can be derived from each other), and then re-
      advertise the route to its peers, with the Segmentation EC and PTA
      modified as specified in RFC7524.  The modification applies even
      when re-advertising to peers in the same ingress region.

   This is to ensure that local egress PEs will only import the routes
   re-advertised by the RBRs after the modification of PTA and
   Segmentation EC.

   Additionally, if there are intermediate RRs between the ingress PE
   and local RBRs, and Route Target Constrain [RFC4684] is in use, the
   ingress PE MUST also attach a Route Target (referred to as RT-RBR)
   and the local RBRs MUST be provisioned to import routes with RT-RBR
   (otherwise the intermediate RRs will not re-advertise the routes

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   towards the RBRs because the routes carry only EC-VPN but not RT-
   VPN).  The local RBRs MUST remove the RT-RBR when they re-advertise
   the routes.

2.3.  Bud Node Support

   If a segmentation point is a bud node for a segmented x-PMSI tunnel,
   it MAY specify an additional label in the PTA of the Leaf A-D routes
   in response to the x-PMSI route received from its upstream node (an
   ingress PE or an upstream segmentation point).  If so, the node
   SHOULD program a label forwarding entry to pop the label and forward
   packet based on IP lookup in a VRF identified by the label (while the
   tunnel label for the upstream segment or the label in the PTA of the
   x-PMSI/Leaf route for the upstream segment is used to stitch the
   upstream and downstream segments together).  This is done only if the
   segmentation point can not both route traffic arriving on the
   upstream segment to local receivers and label switch the traffic to
   downstream segments.

   This is done by extending the PTA with an Extension TLV.  After
   encoding the PTA per Section 5 of [RFC6514] or other specifications
   for other tunnel types, an Extension TLV is encoded, with its total
   length added to the PTA's attribute length [RFC4271].  Upon receiving
   a PTA, after decoding the PTA per Section 5 of [RFC6514] or other
   specifications for other tunnel types if there is additional data
   according to the PTA's attribute length, the remaining data is
   interpreted as an Extension TLV.

   The Extension TLV includes sub-TLVs, each with a one-octet type, a
   one-octet length, and a variable-length value field.  This document
   defines an Additional Label sub-TLV to specify the additional label
   with which the bud node segmentation point wants its upstream node to
   send the extra copy.

   When the upstream node decodes the extra label in the PTA attached to
   the Leaf A-D route in response to an x-PMSI A-D route that it
   (re-)advertises, it SHOULD send an extra copy via unicast tunneling
   with the additional label.  If the extra copy is not sent the
   downstream bud node segmentation point will not be able to send
   traffic to its local receivers.

3.  Security Considerations

   No additional security considerations are needed beyond what are
   discussed in RFC7524.

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4.  IANA Considerations

   This document requests the IANA to create a "PMSI Tunnel Attribute
   Extension sub-TLV Type Registry".  Allocation from the registry is
   First Come First Serve, with an initial allocation for "Additional

5.  References

5.1.  Normative References

              Zhang, Z., "Extended Communities Derived from Route
              Targets", draft-zzhang-idr-rt-derived-community-01 (work
              in progress), March 2021.

   [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,

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, DOI 10.17487/RFC6514, February 2012,

   [RFC7524]  Rekhter, Y., Rosen, E., Aggarwal, R., Morin, T.,
              Grosclaude, I., Leymann, N., and S. Saad, "Inter-Area
              Point-to-Multipoint (P2MP) Segmented Label Switched Paths
              (LSPs)", RFC 7524, DOI 10.17487/RFC7524, May 2015,

5.2.  Informative References

              Zhang, Z., Lin, W., Rabadan, J., Patel, K., and A.
              Sajassi, "Updates on EVPN BUM Procedures", draft-ietf-
              bess-evpn-bum-procedure-updates-09 (work in progress),
              June 2021.

   [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>.

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Author's Address

   Zhaohui Zhang
   Juniper Networks

   Email: zzhang@juniper.net

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