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Versions: 00 01                                                         
routing                                                         Z. Zhang
Internet-Draft                                          Juniper Networks
Intended status: Standards Track                        October 25, 2021
Expires: April 28, 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 April 28, 2022.

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   Copyright (c) 2021 IETF Trust and the persons identified as the
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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 . . . . . . . . . . . . . . . . . . .   6
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   6
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   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

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   Segmented Next-Hop Extended Community (referred to as Segmentation
   EC) can be set accordingly when it re-advertise the x-PMSI routes.

   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.

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

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   subject to the same export policy, which can be used to control the
   modification of attributes for the purpose of segmentation.

   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

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   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
   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

   This section applies 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 due to implementation

   If a segmentation point is a bud node for a segmented x-PMSI tunnel
   with the above mentioned limitation, it SHOULD request an additional
   copy to be sent by the upstream RSVP neighbor if the upstream segment
   is a RSVP-TE P2MP tunnel, or by the upstream PE/RBR when the upstream
   segment is an IR or mLDP tunnel.

   The RSVP-TE P2MP case is outside the scope of this document (though
   there are known implementations).  For the IR/mLDP case, it is done
   by including a Tunnel Encapsulation Attribute (TEA) [RFC9012] in the
   Leaf A-D route in response to the x-PMSI route for the upstream
   segment.  Note that the leaf A-D route is sent for this purpose even
   if the Leaf Information Required (LIR) flag is not set in the x-PMSI
   route (e.g. for mLDP tunnel).

   The TEA includes one tunnel of a desired type (e.g.  MPLS or Any
   Encapsulation [I-D.ietf-bess-bgp-multicast-controller]) that is used
   for the upstream PE/RBR to send the additional copy to this bud node.
   The tunnel MUST include a Tunnel Egress Endpoint sub-TLV set to a
   loal address on the bud node, and MUST include a Tree Label Stack
   sub-TLV that includes a single label.  The node MUST 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).

   When the upstream PE/RBR decodes the TEA in the Leaf A-D route in
   response to an x-PMSI A-D route that it (re-)advertises (even if it
   did set the LIR flag in the x-PMSI A-D route), it SHOULD send an
   extra copy via unicast tunneling with the label encoded in the Tree
   Label Stack sub-TLV.  If the extra copy is not sent the downstream
   bud node segmentation point will not be able to send traffic to its
   local receivers.

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3.  Security Considerations

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

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.  Acknowledgements

   The author thanks Sanoj Vivekanandan for his review and comments.

6.  References

6.1.  Normative References

              Zhang, Z., Raszuk, R., Pacella, D., and A. Gulko,
              "Controller Based BGP Multicast Signaling", draft-ietf-
              bess-bgp-multicast-controller-07 (work in progress), July

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

   [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,

   [RFC9012]  Patel, K., Van de Velde, G., Sangli, S., and J. Scudder,
              "The BGP Tunnel Encapsulation Attribute", RFC 9012,
              DOI 10.17487/RFC9012, April 2021,

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6.2.  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-11 (work in progress),
              October 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>.

Author's Address

   Zhaohui Zhang
   Juniper Networks

   Email: zzhang@juniper.net

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