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Network Working Group                                   C. Filsfils, Ed.
Internet-Draft                                               D. Cai, Ed.
Intended status: Informational                                S. Previdi
Expires: October 3, 2016                                        Cisco

                                                           W. Henderickx

                                                               R. Shakir

                                                               D. Cooper
                                                             F. Ferguson

                                                                  S. Lin

                                                              T. LaBerge

                                                             B. Decraene

                                                                L. Jalil

                                                             J. Tantsura

                                                          April 3, 2016

       Interconnecting Millions Of Endpoints With Segment Routing


   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 DC's and/or large
   aggregation networks.  Forwarding tables of midpoint and leaf nodes
   only require a few tens of thousands of entries.

Requirements Language

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   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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
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   Drafts is at http://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 May 1, 2016.

Copyright Notice

   Copyright (c) 2014 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
   (http://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  . . . . . . . . . . . . . . . . . . . . . . . . . 4
   2.  Reference Design  . . . . . . . . . . . . . . . . . . . . . . . 4
   3.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . . . 6
   4.  Illustration of the scale  . . . . . . . . . .. . . . . . . . . 7
   5.  Optional Designs  . . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Deployment Model . . . . . . . . . . . . . . . . . . . . . . . .9
   7.  Benefits  . . . . . . . . . . . . . . . . . . . . . . . . . . ..9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10

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   9.  Manageability Considerations . . . . . . . . . . . . . . . . . 10
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . . .10
   Appendix - Scale Example  . . . . . . . . . . . . . . . . . . . . .10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . ..12

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1  Introduction

   This document describes how SR can be used to interconnect 100s
   thousands of nodes and 10's of millions of applications/humans. This
   version of the document focuses on the MPLS/SR instantiation. No new
   protocol extensions are required.

1.1. Terminology

   Term              Definition
   -----------       ------------------------------------------------
   BGP               Border Gateway Protocol
   BGP-LS            Border Gateway Protocol - Link State
   DC                Data Center
   ECMP              Equal Cost MultiPathing
   FIB               Forwarding Information Base
   LDP               Label Distribution Protocol
   MPLS              Multi-Protocol Label Switching
   PCE               Path Computation Element
   PCEP              Path Computation Element Protocol
   PW                Pseudowire
   SR                Segment Routing
   SRTE              Segment Routing Traffic Engineering
   SRTE Policy       Segment Routing Traffic Engineering Policy
   TI-LFA            Topology Independent Loop Free Alternate

2  Reference Design

       +---------+ +---------+ +---------+
       A         X1a         X2a         C
       |   L1    | |    C    | |    L2   |
       B         X1b         X2b         D
       +---------+ +---------+ +---------+

   A  : PrefixSID 18001 is unique in L1
   B  : PrefixSID 18002 is unique in L1
   X1a: Anycast PrefixSID 16001 is unique across all the domains
        PrefixSID 16003 is unique across all the domains
   X1b: Anycast PrefixSID 16001 is unique across all the domains
        PrefixSID 16004 is unique across all the domains
   X2a: Anycast PrefixSID 16002 is unique across all the domains
        PrefixSID 16005 is unique across all the domains
   X2b: Anycast PrefixSID 16002 is unique across all the domains
        PrefixSID 16006 is unique across all the domains
   C  : PrefixSID 18001 is unique in L2

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   D  : PrefixSID 18002 is unique in L2

   We structure the network into leaf domains (L1, L2...) interconnected
   by a central core domain C. Each domain runs SR with its own
   independent routing protocol (e.g.: IS-IS, OSPF, BGP).

   A common SRGB of [16000-23999] is assumed (any other common block
   choice is possible) across all of the domains. We further assume that
   [16000-17999] is solely used to provide prefix segments in the C
   domain (any other choice is possible) while [18000, 23999] is reused
   to provide prefix segments in any leaf domain.

   For example, we see that A and C of the leaf domain L1 and L2
   respectively, receive the prefix segment 18001 while prefix segment
   16003 is allocated to node X1a in the C domain and is unique across
   the entire set of domains.

   Each leaf domain Lk connects to the domain C with 2 or more  nodes
   called Xka and Xkb. Each X node runs two independent SR routing
   protocols: one in the leaf domain and one in the core domain. Each X
   nodes is provided with two prefix segments allocated from the domain
   C: one uniquely identifies the node while the other (anycast prefix
   segment) identifies the pair number k of X nodes interconnecting the
   leaf domain k to the core domain.

   In our reference diagram, X1a has prefix segment 16003 and anycast
   prefix segment 16001 while X1b has prefix segment 16004 and anycast
   prefix segment 16001.

   No route is redistributed from a leaf domain to the core domain. All
   the routes (and their prefix SID's) of the X nodes are redistributed
   from the core domain into the leaf domains. No other route is
   redistributed from the core into the leaf domains. The FIB of an
   interior node within the C domain does not hold any entry for
   segments in the range [18000, 23999].  A node in a leaf domain only
   has FIB entries for all the segments in the local leaf domain and
   prefix segments towards all the X nodes in the network. For example,
   A of leaf L1 has a FIB entry for anycast segment 16002 which leads to
   the pair X2a and X2b and prefix segment 16005 which leads to X2a.

2.1 Examples

   We use the notation A.L1 to represent the node A of leaf domain L1.
   Leveraging the above design, any leaf node can be interconnected with
   any other leaf node.

   Intra-leaf, shortest-path: A.L1 uses the following SID list to reach

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   B.L1: {18002}
   Inter-leaf, shortest-path through any X: A.L1 uses the following SID
   list to reach D.L2 via any intermediate X: {16002, 18002}
   Inter-leaf, shortest-path through a specific X: A.L1 uses the
   following SID list to reach D.L2 via X2a: {16005, 18002}

   It is out of the scope of this document to describe how the SID lists
   are computed and programmed at the source nodes. As an example, a
   centralized controller could be the source of the Prefix SID
   allocation. The controller could continuously collect the state of
   each domain (e.g. BGP-LS). Upon any new service request (e.g.: from V
   to W), it could check whether W is in the same leaf domain of V. If
   so, a single SID would be required (dynamically learned via IGP-SR
   (IS-IS-SR, OSPF-SR) within the domain and would not be added by the
   controller). Otherwise, if V and W resides on separate domains, the
   SID of the X gateway to W's leaf domain would be inserted before W's
   SID by the controller.

3. Control-plane

   This section provides a high-level description of one example of an
   implemented control-plane. The example is for L2VPN PW service with
   certain SLA contract.

   The service orchestration programs A with a PW to a remote next-hop C
   with a given SLA contract (low-latency path, be disjoint from a
   specific core plane, be disjoint from a different PW service, etc.).

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

   The SR PCE is made of two components. A multi-domain topology and a
   compute block. The multi-domain topology is continuously refreshed
   from BGP-LS feeds from each domain. The compute block implements TE
   algorithms designed specifically for SR path expression. Upon
   receiving the PCEP request, the SR PCE computes the solution (e.g.
   {16002, 18001} and provides it to A.

   The SR PCE logs the request as a stateful query and hence recomputes
   another solution upon any multi-domain topology changes that
   invalidates the previous solution.

   A receives the PCEP reply with the solution. A installs the received
   SRTE policy in the dataplane. A automatically steers the PW on that
   SRTE policy.

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   It is out of the scope of this document to describe how the SRTE
   Policies are computed and programmed at the source nodes.

4. Illustration of the scale

   A review of a practical design use case can be used to determine the
   true scalability of this methodology in the real world. Such an
   example will demonstrate that over 50 million physical end points can
   be supported in as little as 12K of FIB space. Moreover, this example
   does not even suggest a maximum scale. Please refer to [Appendix] for

5  Optional Designs

   Section 2 describes the reference model of the design. However, there
   could be multiple different design options depends on the network
   scale, network node hardware capability, SLA requirement, etc.

5.1 Leaf and Core Domains Sizing
   The operator might choose to not redistribute the X routes into the
   leaf domains. In that case, one more segment is required in order to
   compose an end-to-end path. For example, to express an "inter-leaf,
   shortest-path through any X" path from A.L1 to D.L2, A.L1 uses
   {16001, 16002, 18002} instead of {16002, 18002}. This model gives the
   operator the ability to choose among a small number of larger leaf
   domains, a large number of small leaf domains or a mix of small and
   large domains.

5.2 Local Segments to Hosts/Servers
   Local segments can be programmed at any leaf node in order to
   identify locally-attached hosts (or VM's). For example, if D.L2 has
   bound a local segment 40001 to a local host DH1, then A uses the
   following SID list to reach that host: {16002, 18002, 40001}
   (assuming the reference design above). Such local segment could
   represent the NID (Network Interface Device) device in the context of
   the SP access network, or VM in the context of the DC network.

5.3 Sub-leaf Domains
   A third level of hierarchy called "Sub-Leaf" can be introduced for
   further scale.

        +---------+ +---------+ +---------+ +-------+
       A         X1a         X2a         Y21a      E
       |   L1    | |    C    | |    L2   | | SL21  |
       B         X1b         X2b         Y21b      F
       +---------+ +---------+ +---------+ +-------+

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   In the above diagram, a sub-leaf "SL21" has been added to the leaf
   domain L2. SL21 is connected to L2 via two (or more) Y nodes. The
   SRGB sub-space [18000, 23999] initially allocated for the leaf is
   splitted into two sub-spaces: [18000-19999] for the leaf allocation
   and [20000-23999] for the sub-leaf allocation.  Each Y node is
   allocated with a unique anycast prefix segment and a unique prefix
   segmemt within the leaf block. For example, Y21a receives anycast SID
   19021 and prefix SID 19211. Each node within a subleaf domain
   receives a unique prefix SID from that domain (e.g. E receives
   For example, to express an "inter-leaf, shortest-path, through any X,
   through any Y" path to E.L2.SL21, A.L1 uses {16002, 19021, 20001}.

   Alternatively, the operator may decide not to distribute any X route
   down into leaf domains, but instead, distribute Y gateways up to the
   C domain. In this case, A.L1 would express the "inter-leaf, shortest-
   path, through any X, through any Y" path to E.L2.SL21 with SID
   list:{16001, 19021, 20001}.

5.4 Traffic Engineering
   Traffic Engineering: Any leaf or core domain can use SR in order to
   traffic engineer its traffic locally within the domain.
   For example, a flow from A.L1 to X1a within L1 domain could be
   steered via B using the SR policy {18002, 16003}. Similarly a flow
   from X1a to X2a within the core domain could be steered via X2b with
   the SR policy {16006, 16005}.

   Similarly, a flow can be engineered across domains. For example, a
   flow from A.L1 to C.L2 could be steered via B then X1a then X2b then
   X2a then C using the SR policy {18002, 16003, 16006, 16005, 18001}.

   The SR policy at the source can be "compressed" (in terms of number
   of segments) by leveraging binding segments bound to SR policy. For
   example, assuming that the local binding segment 30000 is bound by A
   to the policy {18002, 16003} and that the local binding segment 30001
   is bound by X1a to the policy {16006, 16005}, then the previous
   inter-domain policy can also be expressed at A (or any node connected
   to A) as {30000, 30001, 18001}. Using a binding segment to refer to a
   remote SR policy provides other benefits such as decreasing  the need
   for the centralized controller in order to reflect a change from one
   domain to another.

   For example, let us assume that something changes within the core
   domain such that the path followed by the policy 30001 at X1a
   changes. The SR policy associated with 30001 is updated at X1a
   without any change at A. The binding segment 30001 remains "stable"
   from the viewpoint of L1 leaf domain. Updating a remote domain
   becomes necessary only when the headend of the binding segment

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   becomes unavailable  (X1a becomes unavailable) or when the policy
   attached to the binding segment is no longer achievable. An example
   could be: upon a double and independent failure, a policy avoiding
   some resources (e.g. another plane of the backbone) might no longer
   be possible). In only these cases, the policy at A needs to be
   changed. It is out of the scope of this document to describe how the
   SID lists are computed in order to realize a specific traffic-
   engineering objective, with or without the use of binding SID. For
   example, an application could request a specific treatment via a
   north-bound API to a centralized controller. The centralized
   controller might collect the topology of all the domains. It might
   also translate the application requirement into an end-to-end path
   through the domains. Finally, it might then translate that end-to-end
   path in a list of segments. It might create intermediate per-domain
   policies (e.g. using PCEP provisioning) and learn their associated
   binding segments (e.g. PCEP or BGP-LS) and return to the application
   the resulting SID list where some of the SID's are binding segments.

6 Deployment Model

   It is expected that this design be deployed as a greenfield but as
   well in interworking (brownfield) with seamless-mpls design (draft-

7 Benefit

   There are some notable benefits that this proposal can bring to the
   large scale network deployment:

   ECMP: each policy (intra or inter-domain, with or without TE) is
   expressed as a list of segments. As each segment is optimized for
   ECMP, therefore 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 L1 leaf domain to Core and 16002
   load-shares across any gateway from Core to L2 leaf domain.

   Sub-50msec FRR: Topology-Independent FRR using SR [draft-francois-
   spring-segment-routing-ti-lfa-01] ensures sub-50msec upon any link or
   node failure, in any topology.

   Simple and better node redundancy: furthermore the use of anycast
   segment provides for an additional high-availability mechanism (e.g.:
   flows directed to 16001 can either go via X1a or X1b).

   No new protocol extensions are required to support this.

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8.  IANA Considerations


9.  Manageability Considerations


10.  Security Considerations


11.  Acknowledgements

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

12.  References

12.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

12.2.  Informative References
   [draft-ietf-mpls-seamless-mpls] Leymann, et al., "Seamless MPLS
              Architecture",  draft-ietf-mpls-seamless-mpls-07, (work in
              progress), July 2015

   [draft-francois-spring-segment-routing-ti-lfa-01] Pierre Francois, et
              al., "Topology Independent Fast Reroute using Segment
              Routing", draft-francois-spring-segment-routing-ti-lfa-01,
              (work in progress), April 2015

Appendix - Scale Example

   A review of a practical design use case can be used to determine the
   true scalability of this methodology in the real world. This example
   does not suggest a maximum scale, but it provides a simple framework
   that can be deployed on emerging silicon products from a variety of
   sources. In doing so, it also demonstrates huge scale potential on
   the order of 50 million end-points.

   The design

   The practical design is based on the reference topology above, but

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   differs in that each data center layer is enumerated for addressing
   and the SRGB is necessarily expanded beyond the default (more on that
   follows). Otherwise, the topology is the same.  For the sake of
   brevity, the design is described only at a high level here.

   For terminology's sake, each leaf domain in the original topology is
   shown below as a data center (DC). Further, the X nodes above are
   referred to here as data center routers (DCR's).  For example:

          +---------+ +---------+ +---------+
          |         DCR         DCR         |
          |   DC1   | |   Core  | |   DC2   |
          |         DCR         DCR         |
          +---------+ +---------+ +---------+

   The design leverages well-worn physical deployment practices (such as
   Clos-based fabric principles and relatively low oversuscription) in
   conjunction with emerging silicon options supporting 25G/50G/100G

   Each data center is composed of three layers of switching: Top-of-
   Rack (ToR), leaf, and spine.  Sixteen ToR's and eight leaf devices
   combine to create clusters supporting 1,536 physical servers each
   (connected at 25G).  All switching devices in the cluster are based
   on 32x100G switches (leveraging 25G breakout where required).

   The spine layer interconnects 272 clusters using a total of 128
   288x100G modular switches.  To leave each data center, 16 DCR devices
   (using 256 ports of 100G) provide connectivity to 16 core devices
   (also using 256 ports of 100G). From a server's perspective, north-
   south oversubscription in this model is 51:1 and east-west
   oversubscription is 9:1.

   All told, the data center (including its portion of the core) is
   composed of 6,688 networking devices:

      4,352 ToR's (16 ToR x 272 clusters)   2,176 Leaf nodes (8 leaf
   nodes x 272 clusters)   128 spine switches   16 DCR's   16 Core

   For the sake of Segment Routing scale calculations, each of these
   nodes will advertise its node SID.  The Leaf domain FIB size is
   6,672, consisting of the spine, leaf, and ToR switching layers as
   well as the DCR layer.

   The core domain consists of the DCR's and the Core routers (32

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   nodes).  Finally, each data center site will advertise one anycast
   SID into the core domain, so the per-site SID count in the core
   domain is 33.


   Each data center site has the following scale characteristics:

      417,792 physical server ports (at 25G per port)   6,672 SID
   entries in the Leaf domain   33 SID entries in the Core domain

   Each device in the Leaf domain must be aware of all other devices in
   the data center, as well as all SID's advertised on the core.
   Therefore, the FIB in these devices must support:

     7K segments for local connectivity (from the SRGB)  <300 adjacency
   segments for large switches (locally installed)  Up to 104 adjacency
   segments on small switches (locally installed)  4K Core segments
   (from the SRGB, to be covered shortly)

   This represents a total FIB requirement of about 12K (rounded up).
   Most of this addressing involves global addressing. This means that
   the SRGB must be expanded beyond its 8K default configuration (values
   16000 - 23999).  For this design, the SRGB is expanded to 12K (16000
   - 27999) and apportioned in the following way:

      16000 - 19999 (4K) Core and inter-site connectivity (truly global)
     20000 - 27999 (8K) Leaf domain (reused per DC site)

   The core domain size of 4K is somewhat arbitrary and was chosen to
   demonstrate huge scale without requiring a large FIB.  Assuming every
   data center site consumes 33 SID's in the Core domain space, a 4K
   Core domain results in the following system scalability:

      120 data center sites (4K / 33)   50M servers at 25G (417,792 *

   It should be appreciated that this example covers infrastructure
   only.  Further, design options are too numerous to describe
   succinctly. Therefore, subjects such as an application-based
   addressing architecture, multi-tenancy, service chaining, and so
   forth are not accounted for. Nonetheless, deploying 50M endpoints
   using a maximum FIB of 12K should generally provide super-granular
   traffic steering and SDN services on low-cost hardware.

Authors' Addresses

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        Clarence Filsfils (editor)
        Cisco Systems, Inc.
        Email: cfilsfil@cisco.com

        Dennis Cai (editor)
        Cisco Systems, Inc.
        170, West Tasman Drive
        San Jose, CA  95134
        Email: dcai@cisco.com

        Stefano Previdi
        Cisco Systems, Inc.
        Via Del Serafico, 200
        Rome  00142
        Email: sprevidi@cisco.com

        Wim Henderickx
        Email: wim.henderickx@alcatel-lucent.com

        Rob Shakir
        Email: rob.shakir@bt.com

        Dave Cooper
        Level 3
        Email: Dave.Cooper@Level3.com

        Francis Ferguson
        Level 3
        Email: Francis.Ferguson@level3.com

        Tim LaBerge
        Email: tlaberge@cisco.com

        Steven Lin
        Email: slin@microsoft.com

        Bruno Decraene
        Email: bruno.decraene@orange.com

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        Luay Jalil
        400 International Pkwy
        Richardson, TX 75081
        Email: luay.jalil@verizon.com

        Jeff Tantsura

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