Interconnecting Millions Of Endpoints With Segment Routing
draft-filsfils-spring-large-scale-interconnect-13

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Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                                S. Previdi
Intended status: Informational                       Cisco Systems, Inc.
Expires: September 6, 2019                                 G. Dawra, Ed.
                                                                LinkedIn
                                                           W. Henderickx
                                                                   Nokia
                                                               D. Cooper
                                                                 Level 3
                                                           March 5, 2019

       Interconnecting Millions Of Endpoints With Segment Routing
           draft-filsfils-spring-large-scale-interconnect-13

Abstract

   This document describes an application of Segment Routing to scale
   the network to support hundreds of thousands of network nodes, and
   tens of millions of physical underlay endpoints.  This use-case can
   be applied to the interconnection of massive-scale DCs and/or large
   aggregation networks.  Forwarding tables of midpoint and leaf nodes
   only require a few tens of thousands of entries.  This may be
   achieved by inherert scaleable nature of Segment Routing and designed
   proposed in this document.

Status of This Memo

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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Reference Design  . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Illustration of the scale . . . . . . . . . . . . . . . . . .   5
   6.  Design Options  . . . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Segment Routing Global Block(SRGB) Size . . . . . . . . .   6
     6.2.  Redistribution of Agg nodes routes  . . . . . . . . . . .   6
     6.3.  Sizing and hierarchy  . . . . . . . . . . . . . . . . . .   6
     6.4.  Local Segments to Hosts/Servers . . . . . . . . . . . . .   7
     6.5.  Compressed SRTE policies  . . . . . . . . . . . . . . . .   7
   7.  Deployment Model  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Simplified operations . . . . . . . . . . . . . . . . . .   8
     8.2.  Inter-domain SLA  . . . . . . . . . . . . . . . . . . . .   8
     8.3.  Scale . . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.4.  ECMP  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   10. Manageability Considerations  . . . . . . . . . . . . . . . .   9
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   13. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .   9
   14. Informative References  . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   This document describes how SR can be used to interconnect millions
   of endpoints.The following terminology is used in this document:

2.  Terminology

   The following terms and abbreviations are used in this document:

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   Term        Definition
   ---------------------------------------------------------
   Agg         Aggregation
   BGP         Border Gateway Protocol
   DC          Data Center
   DCI         Data Center Interconnect
   ECMP        Equal Cost MultiPathing
   FIB         Forwarding Information Base
   LDP         Label Distribution Protocol
   LFIB        Label Forwarding Information Base
   MPLS        Multi-Protocol Label Switching
   PCE         Path Computation Element
   PCEP        Path Computation Element Protocol
   PW          Pseudowire
   SLA         Service level Agreement
   SR          Segment Routing
   SRTE Policy Segment Routing Traffic Engineering Policy
   TE          Traffic Engineering
   TI-LFA      Topology Independent - Loop Free Alternative

3.  Reference Design

   The network diagram here below describes the reference network
   topology used in this document:

   +-------+ +--------+ +--------+ +-------+ +-------+
   A       DCI1       Agg1       Agg3      DCI3      Z
   |  DC1  | |   M1   | |   C    | |   M2  | |  DC2  |
   |       DCI2       Agg2       Agg4      DCI4      |
   +-------+ +--------+ +--------+ +-------+ +-------+

                       Figure 1: Reference Topology

   The following applies to the reference topology above:

      Independent ISIS-OSPF/SR instance in core (C) region.

      Independent ISIS-OSPF/SR instance in Metro1 (M1) region.

      Independent ISIS-OSPF/SR instance in Metro2 (M2) region.

      BGP/SR in DC1.

      BGP/SR in DC2.

      Agg routes (Agg1, Agg2, Agg3, Agg4) are redistributed from C to M
      (M1 and M2) and from M to DC domains.

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      No other route is advertised or redistributed between regions.

      The same homogeneous SRGB is used throughout the domains (e.g.
      16000-23999).

      Unique SRGB sub-ranges are allocated to each metro (M) and core
      (C) domains:

         16000-16999 range is allocated to the core (C) domain/region.

         17000-17999 range is allocated to the M1 domain/region.

         18000-18999 range is allocated to the M2 domain/region.

         Specifically, Agg1 router has SID 16001 allocated and Agg2
         router has SID 16002 allocated.

         Specifically, Agg3 router has SID 16003 allocated and the
         anycast SID for Agg3 and Agg4 is 16006.

         Specifically, DCI3 router has SID 18003 allocated and the
         anycast SID for DCI3 and DCI4 is 18006.

         Specifically, at Agg1 router Binding SID 4001 leads to DCI Pair
         DCI3, DCI4 via specific low-latency path {16002, 16003, 18006}.

      The same SRGB sub-range is re-used within each DC (DC1 and DC2)
      region. for each DC: e.g. 20000-23999.  Specifically, nodes A and
      Z both have SID 20001 allocated to them.

4.  Control Plane

   This section provides a high-level description of a how a control
   plane could be implemented using protocol components already defined
   in other RFCs.

   The mechanism through which SRTE Policies are defined, computed and
   programmed in the source nodes, are outside the scope of this
   document.

   Typically, a controller or a service orchestration system programs
   node A with a pseudowire (PW) to a remote next-hop Z with a given SLA
   contract (e.g. low-latency path, be disjoint from a specific core
   plane, be disjoint from a different PW service, etc.).

   Node A automatically detects that it does not have reachability to Z.
   It then automatically sends a PCEP request to an SR PCE for an SRTE
   policy that provides reachability to Z with the requested SLA.

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   The SR PCE [RFC4655] is made of two components.  A multi-domain
   topology and a computation engine.  The multi-domain topology is
   continuously refreshed through BGP-LS [RFC7752] feeds from each
   domain.  The computing engine is desined to implemet Traffic
   Engineering (TE) algorithms and provide output in SR Path format.
   Upon receiving the PCEP [RFC5440] request, the SR PCE computes the
   requested path.  The path is expressed through a list of segments
   (e.g. {16003, 18006, 20001} and provided to node A.

   The SR PCE logs the request as a stateful query and hence is capable
   to recompute the path at each network topology change.

   Node A receives the PCEP reply with the path (expressed as a segment
   list).  Node A installs the received SRTE policy in the dataplane.
   Node A then automatically steers the PW into that SRTE policy.

5.  Illustration of the scale

   According to the reference topology described in Figure 1 the
   following assumptions are made:

      There's 1 core domain and 100 leaf (metro) domains.

      The core domain includes 200 nodes.

      Two nodes connect each leaf (metro) domain.  Each node connecting
      a leaf domain has a SID allocated.  Each pair of nodes connecting
      a leaf domain also has a common anycast SID.  This brings up to
      300 prefix segments in total.

      A core node connects only one leaf domain.

      Each leaf domain has 6000 leaf node segments.  Each leaf-node has
      500 endpoints attached, thus 500 adjacency segments.  In total, it
      is 3 millions endpoints for a leaf domain.

   Based on the above, the network scaling numbers are as follows:

      6,000 leaf node segments multiplied by 100 leaf domains: 600,000
      nodes.

      600,000 nodes multiplied by 500 endpoints: 300 millions of
      endpoints.

   The node scaling numbers are as follows:

      Leaf node segment scale: 6,000 leaf node segments + 300 core node
      segments + 500 adjacency segments = 6,800 segments

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      Core node segment scale: 6,000 leaf domain segments + 300 core
      domain segments = 6,300 segments

   In the above calculations, the link adjacency segments are not taken
   into account.  These are local segments and, typically, less than 100
   per node.

   It has to be noted that, depending on leaf node FIB capabilities,
   leaf domains could be split into multiple smaller domains.  In the
   above example, the leaf domains could be split into 6 smaller domains
   so that each leaf node only need to learn 1000 leaf node segments +
   300 core node segments + 500 adjacency segments which gives a total
   of 1,800 segments.

6.  Design Options

   This section describes multiple design options to the illustration of
   previous section.

6.1.  Segment Routing Global Block(SRGB) Size

   In the simplified illustrations of this document, we picked a small
   homogeneous SRGB range of 16000-23999.  In practice, a large-scale
   design would use a bigger range such as 16000-80000, or even larger.
   Larger range provides allocations for various Traffic Engineering
   applications within a given domain

6.2.  Redistribution of Agg nodes routes

   The operator might choose to not redistribute the Agg nodes routes
   into the Metro/DC domains.  In that case, more segments are required
   in order to express an inter-domain path.

   For example, node A would use an SRTE Policy {DCI1, Agg1, Agg3, DCI3,
   Z} in order to reach Z instead of {Agg3, DCI3, Z} in the reference
   design.

6.3.  Sizing and hierarchy

   The operator is free to choose among a small number of larger leaf
   domains, a large number of small leaf domains or a mix of small and
   large core/leaf domains.

   The operator is free to use a 2-tier design (Core/Metro) or a 3-tier
   (Core/Metro/DC).

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6.4.  Local Segments to Hosts/Servers

   Local segments can be programmed at any leaf node (e.g. node Z) in
   order to identify locally-attached hosts (or VM's).  For example, if
   node Z has bound a local segment 40001 to a local host ZH1, then node
   A uses the following SRTE Policy in order to reach that host: {16006,
   18006, 20001, 40001}. Such local segment could represent the NID
   (Network Interface Device) in the context of the SP access network,
   or VM in the context of the DC network.

6.5.  Compressed SRTE policies

   As an example and according to Section 3, we assume node A can reach
   node Z (e.g., with a low-latency SLA contract) via the SRTE policy
   consisting of the path: Agg1, Agg2, Agg3, DCI3/4(anycast), Z.  The
   path is represented by the segment list: {16001, 16002, 16003, 18006,
   20001}.

   It is clear that the control-plane solution can install an SRTE
   Policy {16002, 16003, 18006} at Agg1, collect the Binding SID
   allocated by Agg1 to that policy (e.g. 4001) and hence program node A
   with the compressed SRTE Policy {16001, 4001, 20001}.

   From node A, 16001 leads to Agg1.  Once at Agg1, 4001 leads to the
   DCI pair (DCI3, DCI4) via a specific low-latency path {16002, 16003,
   18006}. Once at that DCI pair, 20001 leads to Z.

   Binding SID's allocated to "intermediate" SRTE Policies allow to
   compress end-to-end SRTE Policies.

   The segment list {16001, 4001, 20001} expresses the same path as
   {16001, 16002, 16003, 18006, 20001} but with 2 less segments.

   The Binding SID also provides for an inherent churn protection.

   When the core topology changes, the control-plane can update the low-
   latency SRTE Policy from Agg1 to the DCI pair to DC2 without updating
   the SRTE Policy from A to Z.

7.  Deployment Model

   It is expected that this design be deployed as a green field but as
   well in interworking (brown field) with MPLS design across multiple
   domains.

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8.  Benefits

   The design options illustrated in this document allow the
   interconnection on a very large scale.  Millions of endpoints across
   different domains can be interconnected.

8.1.  Simplified operations

   Two protocols are not needed in this design: LDP and RSVP-TE.  No new
   protocol has been introduced.  The design leverages the core IP
   protocols: ISIS, OSPF, BGP, PCEP with straightforward SR extensions.

8.2.  Inter-domain SLA

   Fast reroute and resiliency is provided by TI-LFA with sub-50msec FRR
   upon Link/Node/SRLG failure.  TI-LFA is described in
   [I-D.bashandy-rtgwg-segment-routing-ti-lfa].

   The use of anycast SIDs also provides an improvement in availability
   and resiliency.

   Inter-domain SLA's can be delivered, e.g., latency vs. cost optimized
   path, disjointness from backbone planes, disjointness from other
   services, disjointness between primary and backup paths.

   Existing inter-domain solutions do not provide any support for SLA
   contracts.  They just provide a best-effort reachability across
   domains.

8.3.  Scale

   In addition to having eliminated two control plane protocols, per-
   service midpoint states have also been removed from the network.

8.4.  ECMP

   Each policy (intra or inter-domain, with or without TE) is expressed
   as a list of segments.  Since each segment is optimized for ECMP,
   then the entire policy is optimized for ECMP.  The ECMP gain of
   anycast prefix segment should also be considered (e.g. 16001 load-
   shares across any gateway from M1 leaf domain to Core and 16002 load-
   shares across any gateway from Core to M1 leaf domain).

9.  IANA Considerations

   This document does not make any IANA request.

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10.  Manageability Considerations

   This document describes an application of Segment Routing over the
   MPLS data plane.  Segment Routing does not introduce any change in
   the MPLS data plane.  Manageability considerations described in
   [RFC8402] apply to the MPLS data plane when used with Segment
   Routing.

11.  Security Considerations

   This document does not introduce additional security requirements and
   mechanisms other than the ones described in [RFC8402].

12.  Acknowledgements

   We would like to thank Giles Heron, Alexander Preusche, Steve Braaten
   and Francis Ferguson for their contribution to the content of this
   document.

13.  Contributors

   The following people have substantially contributed to the editing of
   this document:

   Dennis Cai
   Individual

   Tim Laberge
   Individual

   Steven Lin
   Google Inc.

   Steven Lin
   Google Inc.

   Bruno Decraene
   Orange

   Luay Jalil
   Verizon

   Jeff Tantsura
   Individual

   Rob Shakir
   Google

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14.  Informative References

   [I-D.bashandy-rtgwg-segment-routing-ti-lfa]
              Bashandy, A., Filsfils, C., Decraene, B., Litkowski, S.,
              Francois, P., daniel.voyer@bell.ca, d., Clad, F., and P.
              Camarillo, "Topology Independent Fast Reroute using
              Segment Routing", draft-bashandy-rtgwg-segment-routing-ti-
              lfa-05 (work in progress), October 2018.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <https://www.rfc-editor.org/info/rfc5440>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

Authors' Addresses

   Clarence Filsfils (editor)
   Cisco Systems, Inc.
   Brussels
   Belgium

   Email: cfilsfil@cisco.com

   Stefano Previdi
   Cisco Systems, Inc.
   Via Del Serafico, 200
   Rome  00142
   Italy

   Email: stefano@previdi.net

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   Gaurav Dawra (editor)
   LinkedIn
   USA

   Email: gdawra.ietf@gmail.com

   Wim Henderickx
   Nokia
   Copernicuslaan 50
   Antwerp  2018
   Belgium

   Email: wim.henderickx@nokia.com

   Dave Cooper
   Level 3

   Email: Dave.Cooper@Level3.com

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