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BIER WG                                                         Z. Zhang
Internet-Draft                                                     B. Wu
Intended status: Informational                           ZTE Corporation
Expires: November 25, 2017                                  May 24, 2017


                    Analysis of BIER in NVO3 network
                  draft-zhangwu-bier-nvo3-analysis-00

Abstract

   This document analyses BIER deployment in NVO3 network.  The intent
   is to evaluate whether BIER could achieve some simplicity and
   efficiency.

Status of This Memo

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

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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on November 25, 2017.

Copyright Notice

   Copyright (c) 2017 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
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   described in the Simplified BSD License.





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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  BIER in NVO3 network  . . . . . . . . . . . . . . . . . . . .   2
     2.1.  Leaf devices as the edge BIER nodes . . . . . . . . . . .   4
     2.2.  NVE as edge BIER nodes  . . . . . . . . . . . . . . . . .   7
   3.  Other Consideration . . . . . . . . . . . . . . . . . . . . .   8
   4.  Normative References  . . . . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   As mentioned in [I-D.ietf-nvo3-mcast-framework], there is multicast
   requirement in NVO3 network, such as BUM packets for infrastructure
   multicast and other application-specific multicast.  Besides PIM,
   BIER is mentioned to be used as one of IP multicast underlay
   technology in section 3, which introduces several multicast
   mechanisms for NVO3 network.

   Bit Index Explicit Replication (BIER) [I-D.ietf-bier-architecture] is
   a new architecture for the forwarding of multicast data packets.  It
   provides optimal forwarding of multicast packets through a "multicast
   domain".  It does not require a protocol for explicitly building
   multicast distribution trees, nor does it require intermediate
   devices to maintain any per-flow state.  When a multicast data packet
   enters the BIER domain, the BIER ingress router determines the set of
   BIER egress routers to which the packet needs to be sent.  The BIER
   ingress router then encapsulates the packet in a BIER header.  The
   BIER header contains a bitstring in which each bit represents exactly
   one BIER egress router in the domain; to forward the packet to a
   given set of egress routers, the bits corresponding to those routers
   are set in the BIER header.  In this way, elimination of the per-flow
   state and the explicit tree-building protocols results in a
   considerable simplification.

   This document will give some basic analysis on how to deploy BIER in
   the NVO3 network to mitigate multicast states.

2.  BIER in NVO3 network












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       +--------+                                             +--------+
       | Tenant +--+                                     +----| Tenant |
       | System |  |                                    (')   | System |
       +--------+  |          ................         (   )  +--------+
                   |  +-+--+  .              .  +--+-+  (_)
                   |  | NVE|--.              .--| NVE|   |
                   +--|    |  .              .  |    |---+
                      +-+--+  .              .   +--+-+
                      /       .              .
                     /        .  L3 Overlay  .   +--+-++--------+
       +--------+   /         .    Network   .   | NVE|| Tenant |
       | Tenant +--+          .              .- -|    || System |
       | System |             .              .   +--+-++--------+
       +--------+             ................
                                     |
                                   +----+
                                   | NVE|
                                   |    |
                                   +----+
                                     |
                                     |
                           =====================
                             |               |
                         +--------+      +--------+
                         | Tenant |      | Tenant |
                         | System |      | System |
                         +--------+      +--------+
                  Figure 1: NVO3 Generic Reference Model

   As described in [RFC8014], figure 1 is a generic reference model of
   NVO3.  BIER can be used in this model to provide underlay multicast
   function.



















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                  ...........................................
                  .                                 ------- .   -------
                  .            ................     | Leaf| .---| NVE |
                  .-------     .              .-----|     | .   |     |
        -------   .| Leaf|     .              .     ------- .   -------
        | NVE |---.|     |-----.              .             .
        |     |   .-------     .    Spine     .             .
        -------   .            .              .             .
                  .            .              .             .
                  .            .              .     ------- .
                  .            .              .-----| Leaf| .   -------
                  .            ................     |     | .---| NVE |
                  .                   |             ------- .   |     |
                  .                   |                     .   -------
                  .                 -------                 .
                  .                 | Leaf|                 .
                  .                 |     |                 .
                  .                 -------                 .
                  ...........................................
                                |              |
                                |              |
                             -------        -------
                             | NVE |        | NVE |
                             |     |        |     |
                             -------        -------
              Figure 2: Example topology of NVO3 underlay network

   The architecture of L3 underlay Network can be various.  As depicted
   in figure 2, the Clos topology mentioned in [RFC7938] is taken as an
   example to offer underlay L3 connectivity.  And it is assumed that
   the leaf and spine devices support BIER functionality including
   exchanging BFR-id and other information and building BIER forwarding
   table.

   In this document, two possible ways to handle multicast packets are
   identified.  One of the choices is placing BIER boundaries at leaf
   switches which are a general choice of implementing BIER.  The other
   way is putting the BIER boundary at NVE to achieve better efficiency.

2.1.  Leaf devices as the edge BIER nodes

   Leaf devices are taken as BIER edge node.  Each leaf device is
   allocated one unique BFR-id.  And leaf devices are the ingress BIER
   nodes and egress BIER nodes.  Both the leaf and spine devices
   exchange the BIER information by IGP/BGP extensions.  The according
   extensions are defined in [I-D.ietf-bier-ospf-bier-extensions],
   [I-D.ietf-bier-isis-extensions], and [I-D.ietf-bier-idr-extensions].




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   IGMP/MLD protocol is used between NVE and leaf device like the
   description in [I-D.ietf-nvo3-mcast-framework].  The BUM flows are
   encapsulated corresponding multicast group address.  BMLD
   [I-D.pfister-bier-mld] protocol is used among all the leaf devices to
   exchange multicast group information.  After BMLD protocol process is
   completed, each leaf devices knows the other leaf devices associated
   with specific multicast group address.  When one packet with
   multicast group address reaches leaf device, leaf device encapsulates
   the packet with BIER header which indicates all the destination leaf
   devices that belong to the same multicast group.

   After the BIER packet reaches the destination leaf devices through
   the spine network forwarding, the destination leaf device removes the
   BIER header of packet and forwards to corresponding NVEs.

   So the multicast state is eliminated because of removing of multicast
   tree in L3 underlay network.  NVE only needs to support IGMP/MLD
   protocol.  But one or several multicast group addresses for a tenant
   is still needed.
































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                                                             IGMP/MLD
        IGMP/MLD  ...........................................  |
            \     .    ................................     .  |
             \    .    .  BIER forwarding             .     .  |
              \   .    .                            ------- .  |-------
               \  .    .       ................     |Leaf2| .---| NVE2|
                \ .-------     .              .-----|     | .   |     |
        -------   .|Leaf1|     .              .     ------- .   -------
        | NVE1|---.|     |-----.              .       .     .
        |     |   .-------     .    Spine     .       .     .
        -------   .    .       .              .       .     .
                  .    .       .              .       .     .
                  .    .       .              .     ------- .
                  .    .       .              .-----|Leaf3| .   -------
                  .    .       ................     |     | .---| NVE3|
                  .    .              |             ------- . | |     |
                  .    .              |               .     . | -------
                  .    . BMLD       -------           .     . |
                  .    ............ |Leaf4|............     . IGMP/MLD
                  .                 |     |                 .
                  .                 -------                 .
                  ...........................................
                                |   IGMP/MLD   |
                                |              |
                             -------        -------
                             | NVE4|        | NVE5|
                             |     |        |     |
                             -------        -------
                             Figure 3: BIER in VNO3

   For example, as illustrated in Figure 3, NVE1 needs to send BUM flows
   to NVE2, NVE3, NVE4 and NVE5.  The NVE supports the basic IGMP/MLD
   snooping function.  In most of condition, there would have to be a
   separate group for each tenant, plus a separate group for each
   multicast address (used for multicast applications) within a tenant.
   NVE1 sends the packet to Leaf1.  According the BMLD exchange Leaf1
   encapsulate BIER header with the destination of Leaf2, Leaf3 and
   Leaf4.  After BIER forwarding the packet reaches Leaf2, Leaf3 and
   Leaf4, the leaf devices remove the BIER header and send it to NVE2,
   NVE3, NVE4 and NVE5.

   In some use cases, there could be a big amount of leaf switch, and it
   is impossible to encapsulate destination BFR-ids in one same BIER
   header because of the limitation of BitStringLength, the packet
   should be sent more than once to reach destination.  In case the BFR-
   id of Leaf2 is 28, the BFR-id of Leaf3 is 78; the BitStringLength
   used in BIER encapsulation is 64, it is impossible to encapsulate the
   two BFR-ids in one BIER header.  In this situation, one solution is



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   Leaf1 send two copies of packet to Leaf2 and Leaf3.  SI in BIER
   header defined in [I-D.ietf-bier-architecture] is be used to
   distinguish these two copies to deliver to the final destination.
   The other solution is increase the forwarding BitStringLength to 128.
   From the above analysis, BIER is quite applicable in the scenario
   with limited size of leaf switches.  But in a large scale of NVO3
   underlay network, there is some limitation due to the
   BitStringLength.

2.2.  NVE as edge BIER nodes

   In last section, IGMP/MLD is still needed to run between NVE and leaf
   devices.  And the multicast groups for Tenants and specific multicast
   applications are needed.

   If NVE is the edge BIER node, IGMP/MLD protocol does not need to run
   between NVE and leaf device.  NVE encapsulates the BIER header with
   the BFR-ids of destination NVEs straightly and send it to leaf
   devices.  Leaf and spine devices forward the BIER packet to
   destination NVEs.  Destination NVEs remove the BIER header and
   forwarding according to the inner encapsulation.

   In this situation, leaf device does not need to be allocated BFR-id.
   Every NVE should be allocated with one unique BFR-id.  The BFR-id
   information should be exchanged within the L3 underlay network.  If
   NVE supports the IGP/BGP BIER extension, NVE takes part in the BIER
   information exchange.  The forwarding plane will be established
   easily.  But in some situations NVE does not support IGP or BGP; NVE
   can not take part in the BIER information exchange.  So the BFR-id of
   NVE should be advertised by some other method.

   Besides multicast state elimination in L3 underlay network, IGMP/MLD
   does not need to run between NVE and leaf devices.  And BMLD does not
   need to run among all the leaf devices.  This function makes the BIER
   deployment more simply.
















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                  ...........................................
                  .    ................................     .
                  .    .  BIER forwarding             .     .
                  .    .                            ------- .   -------
                  .    .       ................     |Leaf2| .---| NVE2|
                  .-------     .              .-----|     | .   |     |
        -------   .|Leaf1|     .              .     ------- .   -------
        | NVE1|---.|     |-----.              .       .     .
        |     |   .-------     .    Spine     .       .     .
        -------   .    .       .              .       .     .
                  .    .       .              .       .     .
                  .    .       .              .     ------- .
                  .    .       .              .-----|Leaf3| .   -------
                  .    .       ................     |     | .---| NVE3|
                  .    .              |             ------- .   |     |
                  .    .              |               .     .   -------
                  .    .            -------           .     .
                  .    ............ |Leaf4|............     .
                  .                 |     |                 .
                  .                 -------                 .
                  ...........................................
                                |              |
                                |              |
                             -------        -------
                             | NVE4|        | NVE5|
                             |     |        |     |
                             -------        -------
                             Figure 4: BIER in VNO3

   The same example for figure 3, NVE1 should send BUM packet to NVE2,
   NVE3, NVE4 and NVE5.  NVE1 encapsulates BIER header with NVE2, NVE3,
   NVE4 and NVE5 as destination and send it to Leaf1.  Leaf1 and spine
   devices forward the packet to NVE2, NVE3, NVE4 and NVE5 according to
   the BIER header.  The NVEs remove the BIER header and forward it.

   The BitStringLength limitation also remains in this solution.  And
   the situation may be more seriously because of the larger number of
   NVEs than leaf devices.  If the BFR-id of NVE is not allocated
   reasonably, in the worst situation, the forwarding efficiency is the
   same with the source replication described in
   [I-D.ietf-nvo3-mcast-framework] section 3.2.

3.  Other Consideration

   As illustrated in [RFC7938], there could be hundred thousand servers
   connected by underlay network in some NVO3 network.  So there are
   more than thousands of NVEs and leaf devices in the network.  Using
   BIER as multicast underlay protocol make significant advantage



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   because of the elimination of multicast state stored in the underlay
   network.  But the BitStringLength limitation is one problem.

   In order to achieve the optimization of BIER, the BFR-ids allocation
   should be more reasonable.  The BFR-id of NVE/leaf device that is
   belong to one same VN could be allocated adjacent as much as
   possible.  So the encapsulation of BIER header can be more efficient.

   Along with the number of BFR-id increasing for NVE/leaf devices,
   there are thousands BIER forwarding items in the L3 underlay network.
   The forwarding efficiency in the L3 underlay network should also be
   considered.

4.  Normative References

   [I-D.ietf-bier-architecture]
              Wijnands, I., Rosen, E., Dolganow, A., Przygienda, T., and
              S. Aldrin, "Multicast using Bit Index Explicit
              Replication", draft-ietf-bier-architecture-06 (work in
              progress), April 2017.

   [I-D.ietf-bier-idr-extensions]
              Xu, X., Chen, M., Patel, K., Wijnands, I., and T.
              Przygienda, "BGP Extensions for BIER", draft-ietf-bier-
              idr-extensions-02 (work in progress), January 2017.

   [I-D.ietf-bier-isis-extensions]
              Ginsberg, L., Przygienda, T., Aldrin, S., and Z. Zhang,
              "BIER support via ISIS", draft-ietf-bier-isis-
              extensions-04 (work in progress), March 2017.

   [I-D.ietf-bier-ospf-bier-extensions]
              Psenak, P., Kumar, N., Wijnands, I., Dolganow, A.,
              Przygienda, T., Zhang, Z., and S. Aldrin, "OSPF Extensions
              for BIER", draft-ietf-bier-ospf-bier-extensions-05 (work
              in progress), March 2017.

   [I-D.ietf-nvo3-mcast-framework]
              Ghanwani, A., Dunbar, L., McBride, M., Bannai, V., and R.
              Krishnan, "A Framework for Multicast in Network
              Virtualization Overlays", draft-ietf-nvo3-mcast-
              framework-08 (work in progress), May 2017.

   [I-D.pfister-bier-mld]
              Pfister, P., Wijnands, I., Venaas, S., Wang, C., Zhang,
              Z., and M. Stenberg, "BIER Ingress Multicast Flow Overlay
              using Multicast Listener Discovery Protocols", draft-
              pfister-bier-mld-03 (work in progress), March 2017.



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   [RFC7938]  Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
              BGP for Routing in Large-Scale Data Centers", RFC 7938,
              DOI 10.17487/RFC7938, August 2016,
              <http://www.rfc-editor.org/info/rfc7938>.

   [RFC8014]  Black, D., Hudson, J., Kreeger, L., Lasserre, M., and T.
              Narten, "An Architecture for Data-Center Network
              Virtualization over Layer 3 (NVO3)", RFC 8014,
              DOI 10.17487/RFC8014, December 2016,
              <http://www.rfc-editor.org/info/rfc8014>.

Authors' Addresses

   Zheng(Sandy) Zhang
   ZTE Corporation
   No. 50 Software Ave, Yuhuatai Distinct
   Nanjing
   China

   Email: zhang.zheng@zte.com.cn


   Bo Wu
   ZTE Corporation
   No. 50 Software Ave, Yuhuatai Distinct
   Nanjing
   China

   Email: wu.bo@zte.com.cn






















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