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Versions: (draft-wei-rift-applicability) 00

RIFT WG                                                      Yuehua. Wei
Internet-Draft                                              Zheng. Zhang
Intended status: Informational                           ZTE Corporation
Expires: August 25, 2020                               Dmitry. Afanasiev
                                                                  Yandex
                                                            Tom. Verhaeg
                                              Interconnect Services B.V.
                                                     Jaroslaw. Kowalczyk
                                                           Orange Polska
                                                       February 22, 2020


                           RIFT Applicability
                    draft-ietf-rift-applicability-00

Abstract

   This document discusses the properties, applicability and operational
   considerations of RIFT in different network scenarios.  It intends to
   provide a rough guide how RIFT can be deployed to simplify routing
   operations in Clos topologies and their variations.

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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://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 August 25, 2020.

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem Statement of Routing in Modern IP Fabric Fat Tree
       Networks  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Applicability of RIFT to Clos IP Fabrics  . . . . . . . . . .   3
     3.1.  Overview of RIFT  . . . . . . . . . . . . . . . . . . . .   3
     3.2.  Applicable Topologies . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Horizontal Links  . . . . . . . . . . . . . . . . . .   6
       3.2.2.  Vertical Shortcuts  . . . . . . . . . . . . . . . . .   6
     3.3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   6
       3.3.1.  DC Fabrics  . . . . . . . . . . . . . . . . . . . . .   6
       3.3.2.  Metro Fabrics . . . . . . . . . . . . . . . . . . . .   7
       3.3.3.  Building Cabling  . . . . . . . . . . . . . . . . . .   7
       3.3.4.  Internal Router Switching Fabrics . . . . . . . . . .   7
       3.3.5.  CloudCO . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Deployment Considerations . . . . . . . . . . . . . . . . . .   9
     4.1.  South Reflection  . . . . . . . . . . . . . . . . . . . .  10
     4.2.  Suboptimal Routing on Link Failures . . . . . . . . . . .  10
     4.3.  Black-Holing on Link Failures . . . . . . . . . . . . . .  12
     4.4.  Zero Touch Provisioning (ZTP) . . . . . . . . . . . . . .  13
     4.5.  Miscabling Examples . . . . . . . . . . . . . . . . . . .  13
     4.6.  IPv4 over IPv6  . . . . . . . . . . . . . . . . . . . . .  16
     4.7.  In-Band Reachability of Nodes . . . . . . . . . . . . . .  17
       4.7.1.  Reachability of Leafs . . . . . . . . . . . . . . . .  17
       4.7.2.  Reachability of Spines  . . . . . . . . . . . . . . .  17
     4.8.  Dual Homing Servers . . . . . . . . . . . . . . . . . . .  17
     4.9.  Fabric With A Controller  . . . . . . . . . . . . . . . .  18
       4.9.1.  Controller Attached to ToFs . . . . . . . . . . . . .  19
       4.9.2.  Controller Attached to Leaf . . . . . . . . . . . . .  19
     4.10. Internet Connectivity Without Underlay  . . . . . . . . .  19
       4.10.1.  Internet Default on the Leafs  . . . . . . . . . . .  19
       4.10.2.  Internet Default on the ToFs . . . . . . . . . . . .  20
     4.11. Subnet Mismatch and Address Families  . . . . . . . . . .  20
     4.12. Anycast Considerations  . . . . . . . . . . . . . . . . .  20
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   6.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  21
   7.  Normative References  . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23






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1.  Introduction

   This document intends to explain the properties and applicability of
   RIFT [I-D.ietf-rift-rift] in different deployment scenarios and
   highlight the operational simplicity of the technology compared to
   traditional routing solutions.  It also documents special
   considerations when RIFT is used with or without overlays,
   controllers and corrects topology miscablings and/or node and link
   failures.

2.  Problem Statement of Routing in Modern IP Fabric Fat Tree Networks

   Clos and Fat-Tree topologies have gained prominence in today's
   networking, primarily as result of the paradigm shift towards a
   centralized data-center based architecture that is poised to deliver
   a majority of computation and storage services in the future.

   Today's current routing protocols were geared towards a network with
   an irregular topology and low degree of connectivity originally.
   When they are applied to Fat-Tree topologies:

   o  they tend to need extensive configuration or provisioning during
      bring up and re-dimensioning.

   o  spine and leaf nodes have the entire network topology and routing
      information, which is in fact, not needed on the leaf nodes during
      normal operation.

   o  significant Link State PDUs (LSPs) flooding duplication between
      spine nodes and leaf nodes occurs during network bring up and
      topology updates.  It consumes both spine and leaf nodes' CPU and
      link bandwidth resources and with that limits protocol
      scalability.

3.  Applicability of RIFT to Clos IP Fabrics

   Further content of this document assumes that the reader is familiar
   with the terms and concepts used in OSPF [RFC2328] and IS-IS
   [ISO10589-Second-Edition] link-state protocols and at least the
   sections of RIFT [I-D.ietf-rift-rift] outlining the requirement of
   routing in IP fabrics and RIFT protocol concepts.

3.1.  Overview of RIFT

   RIFT is a dynamic routing protocol for Clos and fat-tree network
   topologies.  It defines a link-state protocol when "pointing north"
   and path-vector protocol when "pointing south".




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   It floods flat link-state information northbound only so that each
   level obtains the full topology of levels south of it.  That
   information is never flooded East-West or back South again.  So a top
   tier node has full set of prefixes from the SPF calculation.

   In the southbound direction the protocol operates like a "fully
   summarizing, unidirectional" path vector protocol or rather a
   distance vector with implicit split horizon whereas the information
   propagates one hop south and is 're-advertised' by nodes at next
   lower level, normally just the default route.


                +-----------+          +-----------+
                |    ToF    |          |    ToF    |         LEVEL 2
      +         +-----+--+--+          +-+--+------+
      |         |     |  |  |          | |  |      |      ^
      +         |     |  |  +-------------------------+   |
      Distance  |  +-------------------+ |  |      |  |   |
      Vector    |  |  |  |               |  |      |  |   +
      South     |  |  |  |      +--------+  |      |  |   Link+State
      +         |  |  |  |      |           |      |  |   Flooding
      |         |  |  +-------------+       |      |  |   North
      v         |  |     |      |   |       |      |  |   +
              +-+--+-+   +------+   +-------+   +--+--+-+ |
              |SPINE |   |SPINE |   | SPINE |   | SPINE | |  LEVEL 1
      +       ++----++   ++---+-+   +--+--+-+   ++----+-+ |
      +        |    |     |   |        |  |      |    |   |     ^ N
      Distance |    +-------+ |        |  +--------+  |   |     |   E
      Vector   |          | | |        |         | |  |   |  +------>
      South    |  +-------+ | |        | +-------+ |  |   |     |
      +        |  |         | |        | |         |  |   |     +
      v       ++--++      +-+-++      ++-+-+     +-+--++  +
              |LEAF|      |LEAF|      |LEAF|     |LEAF |     LEVEL 0
              +----+      +----+      +----+     +-----+


                          Figure 1: Rift overview

   A middle tier node has only information necessary for its level,
   which are all destinations south of the node based on SPF
   calculation, default route and potential disaggregated routes.

   RIFT combines the advantage of both Link-State and Distance Vector:

   o  Fastest Possible Convergence

   o  Automatic Detection of Topology




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   o  Minimal Routes/Info on TORs

   o  High Degree of ECMP

   o  Fast De-commissioning of Nodes

   o  Maximum Propagation Speed with Flexible Prefixes in an Update

   And RIFT eliminates the disadvantages of Link-State or Distance
   Vector:

   o  Reduced and Balanced Flooding

   o  Automatic Neighbor Detection

   So there are two types of link state database which are "north
   representation" N-TIEs and "south representation" S-TIEs.  The N-TIEs
   contain a link state topology description of lower levels and S-TIEs
   carry simply default routes for the lower levels.

   There are a bunch of more advantages unique to RIFT listed below
   which could be understood if you read the details of RIFT
   [I-D.ietf-rift-rift].

   o  True ZTP

   o  Minimal Blast Radius on Failures

   o  Can Utilize All Paths Through Fabric Without Looping

   o  Automatic Disaggregation on Failures

   o  Simple Leaf Implementation that Can Scale Down to Servers

   o  Key-Value Store

   o  Horizontal Links Used for Protection Only

   o  Supports Non-Equal Cost Multipath and Can Replace MC-LAG

   o  Optimal Flooding Reduction and Load-Balancing

3.2.  Applicable Topologies

   Albeit RIFT is specified primarily for "proper" Clos or "fat-tree"
   structures, it already supports PoD concepts which are strictly
   speaking not found in original Clos concepts.




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   Further, the specification explains and supports operations of multi-
   plane Clos variants where the protocol relies on set of rings to
   allow the reconciliation of topology view of different planes as most
   desirable solution making proper disaggregation viable in case of
   failures.  This observations hold not only in case of RIFT but in the
   generic case of dynamic routing on Clos variants with multiple planes
   and failures in bi-sectional bandwidth, especially on the leafs.

3.2.1.  Horizontal Links

   RIFT is not limited to pure Clos divided into PoD and multi-planes
   but supports horizontal links below the top of fabric level.  Those
   links are used however only as routes of last resort northbound when
   a spine loses all northbound links or cannot compute a default route
   through them.

   A possible configuration is a "ring" of horizontal links at a level.
   In presence of such a "ring" in any level (except ToF level) neither
   N-SPF nor S-SPF will provide a "ring-based protection" scheme since
   such a computation would have to deal necessarily with breaking of
   "loops" in Dijkstra sense; an application for which RIFT is not
   intended.

   A full-mesh connectivity between nodes on the same level can be
   employed and that allows N-SPF to provide for any node loosing all
   its northbound adjacencies (as long as any of the other nodes in the
   level are northbound connected) to still participate in northbound
   forwarding.

3.2.2.  Vertical Shortcuts

   Through relaxations of the specified adjacency forming rules RIFT
   implementations can be extended to support vertical "shortcuts" as
   proposed by e.g.  [I-D.white-distoptflood].  The RIFT specification
   itself does not provide the exact details since the resulting
   solution suffers from either much larger blast radii with increased
   flooding volumes or in case of maximum aggregation routing bow-tie
   problems.

3.3.  Use Cases

3.3.1.  DC Fabrics

   RIFT is largely driven by demands and hence ideally suited for
   application in underlay of data center IP fabrics, vast majority of
   which seem to be currently (and for the foreseeable future) Clos
   architectures.  It significantly simplifies operation and deployment




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   of such fabrics as described in Section 4 for environments compared
   to extensive proprietary provisioning and operational solutions.

3.3.2.  Metro Fabrics

   The demand for bandwidth is increasing steadily, driven primarily by
   environments close to content producers (server farms connection via
   DC fabrics) but in proximity to content consumers as well.  Consumers
   are often clustered in metro areas with their own network
   architectures that can benefit from simplified, regular Clos
   structures and hence RIFT.

3.3.3.  Building Cabling

   Commercial edifices are often cabled in topologies that are either
   Clos or its isomorphic equivalents.  With many floors the Clos can
   grow rather high and with that present a challenge for traditional
   routing protocols (except BGP and by now largely phased-out PNNI)
   which do not support an arbitrary number of levels which RIFT does
   naturally.  Moreover, due to limited sizes of forwarding tables in
   active elements of building cabling the minimum FIB size RIFT
   maintains under normal conditions can prove particularly cost-
   effective in terms of hardware and operational costs.

3.3.4.  Internal Router Switching Fabrics

   It is common in high-speed communications switching and routing
   devices to use fabrics when a crossbar is not feasible due to cost,
   head-of-line blocking or size trade-offs.  Normally such fabrics are
   not self-healing or rely on 1:/+1 protection schemes but it is
   conceivable to use RIFT to operate Clos fabrics that can deal
   effectively with interconnections or subsystem failures in such
   module.  RIFT is neither IP specific and hence any link addressing
   connecting internal device subnets is conceivable.

3.3.5.  CloudCO

   The Cloud Central Office (CloudCO) is a new stage of telecom Central
   Office.  It takes the advantage of Software Defined Networking (SDN)
   and Network Function Virtualization (NFV) in conjunction with general
   purpose hardware to optimize current networks.  The following figure
   illustrates this architecture at a high level.  It describes a single
   instance or macro-node of cloud CO.  An Access I/O module faces a
   Cloud CO Access Node, and the CPEs behind it.  A Network I/O module
   is facing the core network.  The two I/O modules are interconnected
   by a leaf and spine fabric.  [TR-384]





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        +---------------------+           +----------------------+
        |         Spine       |           |     Spine            |
        |         Switch      |           |     Switch           |
        +------+---+------+-+-+           +--+-+-+-+-----+-------+
        |      |   |      | | |              | | | |     |       |
        |      |   |      | | +-------------------------------+  |
        |      |   |      | |                | | | |     |    |  |
        |      |   |      | +-------------------------+  |    |  |
        |      |   |      |                  | | | |  |  |    |  |
        |      |   +----------------------+  | | | |  |  |    |  |
        |      |          |               |  | | | |  |  |    |  |
        |  +---------------------------------+ | | |  |  |    |  |
        |  |   |          |               |    | | |  |  |    |  |
        |  |   |   +-----------------------------+ |  |  |    |  |
        |  |   |   |      |               |    |   |  |  |    |  |
        |  |   |   |      |   +--------------------+  |  |    |  |
        |  |   |   |      |   |           |    |      |  |    |  |
        +--+ +-+---+--+ +-+---+--+     +--+----+--+ +-+--+--+ +--+
        |L | | Leaf   | | Leaf   |     |  Leaf    | | Leaf  | |L |
        |S | | Switch | | Switch |     |  Switch  | | Switch| |S |
        ++-+ +-+-+-+--+ +-+-+-+--+     +--+-+--+--+ ++-+--+-+ +-++
         |     | | |      | | |           | |  |     | |  |     |
         |   +-+-+-+--+ +-+-+-+--+     +--+-+--+--+ ++-+--+-+   |
         |   |Compute | |Compute |     | Compute  | |Compute|   |
         |   |Node    | |Node    |     | Node     | |Node   |   |
         |   +--------+ +--------+     +----------+ +-------+   |
         |   || VAS5 || || vDHCP||     || vRouter|| ||VAS1 ||   |
         |   |--------| |--------|     |----------| |-------|   |
         |   |--------| |--------|     |----------| |-------|   |
         |   || VAS6 || || VAS3 ||     || v802.1x|| ||VAS2 ||   |
         |   |--------| |--------|     |----------| |-------|   |
         |   |--------| |--------|     |----------| |-------|   |
         |   || VAS7 || || VAS4 ||     ||  vIGMP || ||BAA  ||   |
         |   |--------| |--------|     |----------| |-------|   |
         |   +--------+ +--------+     +----------+ +-------+   |
         |                                                      |
        ++-----------+                                +---------++
        |Network I/O |                                |Access I/O|
        +------------+                                +----------+



               Figure 2: An example of CloudCO architecture

   The Spine-Leaf architectures deployed inside CloudCO meets the
   network requirements of adaptable, agile, scalable and dynamic.





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4.  Deployment Considerations

   RIFT presents the opportunity for organizations building and
   operating IP fabrics to simplify their operation and deployments
   while achieving many desirable properties of a dynamic routing on
   such a substrate:

   o  RIFT design follows minimum blast radius and minimum necessary
      epistemological scope philosophy which leads to very good scaling
      properties while delivering maximum reactiveness.

   o  RIFT allows for extensive Zero Touch Provisioning within the
      protocol.  In its most extreme version RIFT does not rely on any
      specific addressing and for IP fabric can operate using IPv6 ND
      [RFC4861] only.

   o  RIFT has provisions to detect common IP fabric mis-cabling
      scenarios.

   o  RIFT negotiates automatically BFD per link allowing this way for
      IP and micro-BFD [RFC7130] to replace LAGs which do hide bandwidth
      imbalances in case of constituent failures.  Further automatic
      link validation techniques similar to [RFC5357] could be supported
      as well.

   o  RIFT inherently solves many difficult problems associated with the
      use of traditional routing topologies with dense meshes and high
      degrees of ECMP by including automatic bandwidth balancing, flood
      reduction and automatic disaggregation on failures while providing
      maximum aggregation of prefixes in default scenarios.

   o  RIFT reduces FIB size towards the bottom of the IP fabric where
      most nodes reside and allows with that for cheaper hardware on the
      edges and introduction of modern IP fabric architectures that
      encompass e.g. server multi-homing.

   o  RIFT provides valley-free routing and with that is loop free.
      This allows the use of any such valley-free path in bi-sectional
      fabric bandwidth between two destination irrespective of their
      metrics which can be used to balance load on the fabric in
      different ways.

   o  RIFT includes a key-value distribution mechanism which allows for
      many future applications such as automatic provisioning of basic
      overlay services or automatic key roll-overs over whole fabrics.

   o  RIFT is designed for minimum delay in case of prefix mobility on
      the fabric.



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   o  Many further operational and design points collected over many
      years of routing protocol deployments have been incorporated in
      RIFT such as fast flooding rates, protection of information
      lifetimes and operationally easily recognizable remote ends of
      links and node names.

4.1.  South Reflection

   South reflection is a mechanism that South Node TIEs are "reflected"
   back up north to allow nodes in same level without E-W links to "see"
   each other.

   For example, Spine111\Spine112\Spine121\Spine122 reflects Node S-TIEs
   from ToF21 to ToF22 separately.  Respectively,
   Spine111\Spine112\Spine121\Spine122 reflects Node S-TIEs from ToF22
   to ToF21 separately.  So ToF22 and ToF21 see each other's node
   information as level 2 nodes.

   In an equivalent fashion, as the result of the south reflection
   between Spine121-Leaf121-Spine122 and Spine121-Leaf122-Spine122,
   Spine121 and Spine 122 knows each other at level 1.

4.2.  Suboptimal Routing on Link Failures




























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                  +--------+          +--------+
                  | ToF21  |          |  ToF22 |                LEVEL 2
                  ++--+-+-++          ++-+--+-++
                   |  | | |            | |  | +
                   |  | | |            | |  | linkTS8
     +-------------+  | +-+linkTS3+-+  | |  | +--------------+
     |                |   |         |  | |  +                |
     |    +----------------------------+ |  linkTS7          |
     |    |           |   |         +    +  +                |
     |    |           |   +-------+linkTS4+------------+     |
     |    |           |             +    +  |          |     |
     |    |           |     +------------+--+          |     |
     |    |           |     |       |  linkTS6         |     |
   +-+----++         ++-----++     ++------+          ++-----++
   |Spin111|         |Spin112|     |Spin121|          |Spin122| LEVEL 1
   +-+---+-+         ++----+-+     +-+---+-+          ++---+--+
     |   |            |    |         |   |             |   |
     |   +--------------+  |         +   ++XX+linkSL6+---+ +
     |                | |  |      linkSL5              | | linkSL8
     |   +------------+ |  |         +   +---+linkSL7+-+ | +
     |   |              |  |         |   |               | |
   +-+---+-+         +--+--+-+     +-+---+-+          +--+-+--+
   |Leaf111|         |Leaf112|     |Leaf121|          |Leaf122| LEVEL 0
   +-+-----+         ++------+     +-----+-+          +-+-----+
     +                +                  +              +
   Prefix111         Prefix112     Prefix121          Prefix122


          Figure 3: Suboptimal routing upon link failure use case

   As shown in Figure 3, as the result of the south reflection between
   Spine121-Leaf121-Spine122 and Spine121-Leaf122-Spine122, Spine121 and
   Spine 122 knows each other at level 1.

   Without disaggregation mechanism, when linkSL6 fails, the packet from
   leaf121 to prefix122 will probably go up through linkSL5 to linkTS3
   then go down through linkTS4 to linkSL8 to Leaf122 or go up through
   linkSL5 to linkTS6 then go down through linkTS4 and linkSL8 to
   Leaf122 based on pure default route.  It's the case of suboptimal
   routing or bow-tieing.

   With disaggregation mechanism, when linkSL6 fails, Spine122 will
   detect the failure according to the reflected node S-TIE from
   Spine121.  Based on the disaggregation algorithm provided by RIFT,
   Spine122 will explicitly advertise prefix122 in Disaggregated Prefix
   S-TIE PrefixesElement(prefix122, cost 1).  The packet from leaf121 to
   prefix122 will only be sent to linkSL7 following a longest-prefix




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   match to prefix 122 directly then go down through linkSL8 to Leaf122
   .

4.3.  Black-Holing on Link Failures

                   +--------+          +--------+
                   | ToF 21 |          | ToF 22 |                LEVEL 2
                   ++-+--+-++          ++-+--+-++
                    | |  | |            | |  | |
                    | |  | |            | |  | linkTS8
     +--------------+ |  +--linkTS3-X+  | |  | +--------------+
     linkTS1          |    |         |  | |  |                |
     |    +-----------------------------+ |  linkTS7          |
     |    |           |    |         |    |  |                |
     |    |      linkTS2   +--------linkTS4-X-----------+     |
     |    |           |              |    |  |          |     |
     |   linkTS5      +-+    +---------------+          |     |
     |    |             |    |       |  linkTS6         |     |
   +-+----++          +-+-----+     ++----+-+          ++-----++
   |Spin111|          |Spin112|     |Spin121|          |Spin122| LEVEL 1
   +-+---+-+          ++----+-+     +-+---+-+          ++---+--+
     |   |             |    |         |   |             |   |
     |   +---------------+  |         |   +----linkSL6----+ |
     linkSL1           | |  |      linkSL5              | | linkSL8
     |   +---linkSL3---+ |  |         |   +----linkSL7--+ | |
     |   |               |  |         |   |               | |
   +-+---+-+          +--+--+-+     +-+---+-+          +--+-+--+
   |Leaf111|          |Leaf112|     |Leaf121|          |Leaf122| LEVEL 0
   +-+-----+          ++------+     +-----+-+          +-+-----+
     +                 +                  +              +
   Prefix111          Prefix112     Prefix121          Prefix122

             Figure 4: Black-holing upon link failure use case

   This scenario illustrates a case when double link failure occurs and
   with that black-holing can happen.

   Without disaggregation mechanism, when linkTS3 and linkTS4 both fail,
   the packet from leaf111 to prefix122 would suffer 50% black-holing
   based on pure default route.  The packet supposed to go up through
   linkSL1 to linkTS1 then go down through linkTS3 or linkTS4 will be
   dropped.  The packet supposed to go up through linkSL3 to linkTS2
   then go down through linkTS3 or linkTS4 will be dropped as well.
   It's the case of black-holing.

   With disaggregation mechanism, when linkTS3 and linkTS4 both fail,
   ToF22 will detect the failure according to the reflected node S-TIE
   of ToF21 from Spine111\Spine112\Spine121\Spine122.  Based on the



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   disaggregation algorithm provided by RITF, ToF22 will explicitly
   originate an S-TIE with prefix 121 and prefix 122, that is flooded to
   spines 111, 112, 121 and 122.

   The packet from leaf111 to prefix122 will not be routed to linkTS1 or
   linkTS2.  The packet from leaf111 to prefix122 will only be routed to
   linkTS5 or linkTS7 following a longest-prefix match to prefix122.

4.4.  Zero Touch Provisioning (ZTP)

   Each RIFT node may operate in zero touch provisioning (ZTP) mode.  It
   has no configuration (unless it is a Top-of-Fabric at the top of the
   topology or it is desired to confine it to leaf role w/o leaf-2-leaf
   procedures).  In such case RIFT will fully configure the node's level
   after it is attached to the topology.

   The most import component for ZTP is the automatic level derivation
   procedure.  All the Top-of-Fabric nodes are explicitly marked with
   TOP_OF_FABRIC flag which are initial 'seeds' needed for other ZTP
   nodes to derive their level in the topology.  The derivation of the
   level of each node happens then based on LIEs received from its
   neighbors whereas each node (with possibly exceptions of configured
   leafs) tries to attach at the highest possible point in the fabric.
   This guarantees that even if the diffusion front reaches a node from
   "below" faster than from "above", it will greedily abandon already
   negotiated level derived from nodes topologically below it and
   properly peer with nodes above.

4.5.  Miscabling Examples






















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        +----------------+              +-----------------+
        |     ToF21      |       +------+      ToF22      |   LEVEL 2
        +-------+----+---+       |      +----+---+--------+
        |       |    |   |       |      |    |   |        |
        |       |    |   +----------------------------+   |
        |   +---------------------------+    |   |    |   |
        |   |   |    |           |           |   |    |   |
        |   |   |    |   +-----------------------+    |   |
        |   |   +------------------------+   |        |   |
        |   |        |   |       |       |   |        |   |
      +-+---+-+    +-+---+-+     |     +-+---+-+    +-+---+-+
      |Spin111|    |Spin112|     |     |Spin121|    |Spin122| LEVEL 1
      +-+---+-+    ++----+-+     |     +-+---+-+    ++----+-+
        |   |       |    |       |       |   |       |    |
        |   +---------+  |     link-M    |   +---------+  |
        |           | |  |       |       |           | |  |
        |   +-------+ |  |       |       |   +-------+ |  |
        |   |         |  |       |       |   |         |  |
      +-+---+-+    +--+--+-+     |     +-+---+-+    +--+--+-+
      |Leaf111|    |Leaf112+-----+     |Leaf121|    |Leaf122| LEVEL 0
      +-------+    +-------+           +-------+    +-------+

                Figure 5: A single plane miscabling example

   Figure Figure 5 shows a single plane miscabling example.  It's a
   perfect fat-tree fabric except link-M connecting Leaf112 to ToF22.

   The RIFT control protocol can discover the physical links
   automatically and be able to detect cabling that violates fat-tree
   topology constraints.  It react accordingly to such mis-cabling
   attempts, at a minimum preventing adjacencies between nodes from
   being formed and traffic from being forwarded on those mis-cabled
   links.  Leaf112 will in such scenario use link-M to derive its level
   (unless it is leaf) and can report links to spines 111 and 112 as
   miscabled unless the implementations allows horizontal links.

   Figure Figure 6 shows a multiple plane miscabling example.  Since
   Leaf112 and Spine121 belong to two different PoDs, the adjacency
   between Leaf112 and Spine121 can not be formed. link-W would be
   detected and prevented.











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      +-------+    +-------+           +-------+    +-------+
      |ToF  A1|    |ToF  A2|           |ToF  B1|    |ToF  B2| LEVEL 2
      +-------+    +-------+           +-------+    +-------+
      |       |    |       |           |       |    |       |
      |       |    |       +-----------------+ |    |       |
      |       +--------------------------+   | |    |       |
      |            |                   | |   | |    |       |
      |     +------+                   | |   | +------+     |
      |     |        +-----------------+ |   |      | |     |
      |     |        |   +--------------------------+ |     |
      |  A  |        | B |               | A |        |  B  |
      +-----+-+    +-+---+-+           +-+---+-+    +-+-----+
      |Spin111|    |Spin112|      +----+Spin121|    |Spin122| LEVEL 1
      +-+---+-+    ++----+-+      |    +-+---+-+    ++----+-+
        |   |       |    |        |      |   |       |    |
        |   +---------+  |        |      |   +---------+  |
        |           | |  |      link-W   |           | |  |
        |   +-------+ |  |        |      |   +-------+ |  |
        |   |         |  |        |      |   |         |  |
      +-+---+-+    +--+--+-+      |    +-+---+-+    +--+--+-+
      |Leaf111|    |Leaf112+------+    |Leaf121|    |Leaf122| LEVEL 0
      +-------+    +-------+           +-------+    +-------+

     +--------PoD#1----------+       +---------PoD#2---------+

               Figure 6: A multiple plane miscabling example

   RIFT provides an optional level determination procedure in its Zero
   Touch Provisioning mode.  Nodes in the fabric without their level
   configured determine it automatically.  This can have possibly
   counter-intuitive consequences however.  One extreme failure scenario
   is depicted in Figure 7 and it shows that if all northbound links of
   spine11 fail at the same time, spine11 negotiates a lower level than
   Leaf11 and Leaf12.

   To prevent such scenario where leafs are expected to act as switches,
   LEAF_ONLY flag can be set for Leaf111 and Leaf112.  Since level -1 is
   invalid, Spine11 would not derive a valid level from the topology in
   Figure 7.  It will be isolated from the whole fabric and it would be
   up to the leafs to declare the links towards such spine as miscabled.











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           +-------+    +-------+        +-------+    +-------+
           |ToF  A1|    |ToF  A2|        |ToF  A1|    |ToF  A2|
           +-------+    +-------+        +-------+    +-------+
           |       |    |       |                |            |
           |    +-------+       |                |            |
           +    +  |            |  ====>         |            |
           X    X  +------+     |                +------+     |
           +    +         |     |                       |     |
           +----+--+    +-+-----+                     +-+-----+
           |Spine11|    |Spine12|                     |Spine12|
           +-+---+-+    ++----+-+                     ++----+-+
             |   |       |    |                        |    |
             |   +---------+  |                        |    |
             |           | |  |                        |    |
             |   +-------+ |  |                +-------+    |
             |   |         |  |                |            |
           +-+---+-+    +--+--+-+        +-----+-+    +-----+-+
           |Leaf111|    |Leaf112|        |Leaf111|    |Leaf112|
           +-------+    +-------+        +-+-----+    +-+-----+
                                           |            |
                                           |   +--------+
                                           |   |
                                         +-+---+-+
                                         |Spine11|
                                         +-------+

                          Figure 7: Fallen spine

4.6.  IPv4 over IPv6

   RIFT allows advertising IPv4 prefixes over IPv6 RIFT network.  IPv6
   AF configures via the usual ND mechanisms and then V4 can use V6
   nexthops analogous to RFC5549.  It is expected that the whole fabric
   supports the same type of forwarding of address families on all the
   links.  RIFT provides an indication whether a node is v4 forwarding
   capable and implementations are possible where different routing
   tables are computed per address family as long as the computation
   remains loop-free.













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                            +-----+        +-----+
                 +---+---+  | ToF |        | ToF |
                     ^      +--+--+        +-----+
                     |      |  |           |     |
                     |      |  +-------------+   |
                     |      |     +--------+ |   |
                     |      |     |          |   |
                    V6      +-----+        +-+---+
                 Forwarding |SPINE|        |SPINE|
                     |      +--+--+        +-----+
                     |      |  |           |     |
                     |      |  +-------------+   |
                     |      |     +--------+ |   |
                     |      |     |          |   |
                     v      +-----+        +-+---+
                 +---+---+  |LEAF |        | LEAF|
                            +--+--+        +--+--+
                               |              |
                  IPv4 prefixes|              |IPv4 prefixes
                               |              |
                           +---+----+     +---+----+
                           |   V4   |     |   V4   |
                           | subnet |     | subnet |
                           +--------+     +--------+

                         Figure 8: IPv4 over IPv6

4.7.  In-Band Reachability of Nodes

4.7.1.  Reachability of Leafs

   TODO

4.7.2.  Reachability of Spines

   TODO

4.8.  Dual Homing Servers

   Each RIFT node may operate in zero touch provisioning (ZTP) mode.  It
   has no configuration (unless it is a Top-of-Fabric at the top of the
   topology or the must operate in the topology as leaf and/or support
   leaf-2-leaf procedures) and it will fully configure itself after
   being attached to the topology.







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                     +---+         +---+         +---+
                     |ToF|         |ToF|         |ToF|
                     +---+         +---+         +---+
                     |   |         |   |         |   |
                     |   +----------------+      |   |
                     |             |   |  |      |   |
                     |          +----------------+   |
                     |          |  |   |  |          |
                     +----------+--+   +--+----------+
                     | Spine|ToR1  |   | Spine|ToR2  |
                     +--+------+---+   +--+-------+--+
                 +---+  |      |   |   |  |       |  +---+
                 |      |      |   |   |  |       |      |
                 |   +-----------------+  |       |      |
                 |   |  |   +-------------+       |      |
                 +   |  +   |  |   |-----------------+   |
                 X   |  X   |  +--------x-----+   |  X   |
                 +   |  +   |                 |   |  +   |
                 +---+  +---+                 +---+  +---+
                 |   |  |   |                 |   |  |   |
                 +---+  +---+  ...............+---+  +---+
                 SV(1) SV(2)                 SV(n+1) SV(n)

                       Figure 9: Dual-homing servers

   In the single plane, the worst condition is disaggregation of every
   other servers at the same level.  Suppose the links from ToR1 to all
   the leaves become not available.  All the servers' routes are
   disaggregated and the FIB of the servers will be expanded with n-1
   more spicific routes.

   Sometimes, pleople may prefer to disaggregate from ToR to servers
   from start on, i.e. the servers have couple tens of routes in FIB
   from start on beside default routes to avoid breakages at rack level.
   Full disaggregation of the fabric could be achieved by configuration
   supported by RIFT.

4.9.  Fabric With A Controller

   There are many different ways to deploy the controller.  One
   possibility is attaching a controller to the RIFT domain from ToF and
   another possibility is attaching a controller from the leaf.









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                                       +------------+
                                       | Controller |
                                       ++----------++
                                        |          |
                                        |          |
                                   +----++        ++----+
                  ----------       | ToF |        | ToF |
                       |           +--+--+        +-----+
                       |           |  |           |     |
                       |           |  +-------------+   |
                       |           |     +--------+ |   |
                       |           |     |          |   |
                                   +-----+        +-+---+
                  RIFT domain      |SPINE|        |SPINE|
                                   +--+--+        +-----+
                       |           |  |           |     |
                       |           |  +-------------+   |
                       |           |     +--------+ |   |
                       |           |     |          |   |
                       |           +-----+        +-+---+
                  ----------       |LEAF |        | LEAF|
                                   +-----+        +-----+

                    Figure 10: Fabric with a controller

4.9.1.  Controller Attached to ToFs

   If a controller is attaching to the RIFT domain from ToF, it usually
   uses dual-homing connections.  The loopback prefix of the controller
   should be advertised down by the ToF and spine to leaves.  If the
   controller loses link to ToF, make sure the ToF withdraw the prefix
   of the controller(use different mechanisms).

4.9.2.  Controller Attached to Leaf

   If the controller is attaching from a leaf to the fabric, no special
   provisions are needed.

4.10.  Internet Connectivity Without Underlay

4.10.1.  Internet Default on the Leafs

   TODO








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4.10.2.  Internet Default on the ToFs

   TODO

4.11.  Subnet Mismatch and Address Families

                 +--------+                     +--------+
                 |        |  LIE          LIE   |        |
                 |   A    | +---->       <----+ |   B    |
                 |        +---------------------+        |
                 +--------+                     +--------+
                    X/24                           Y/24

                        Figure 11: subnet mismatch

   LIEs are exchanged over all links running RIFT to perform Link
   (Neighbor) Discovery.  A node MUST NOT originate LIEs on an address
   family if it does not process received LIEs on that family.  LIEs on
   same link are considered part of the same negotiation independent on
   the address family they arrive on.  An implementation MUST be ready
   to accept TIEs on all addresses it used as source of LIE frames.

   As shown in the above figure, without further checks adjacency of
   node A and B may form, but the forwarding between node A and node B
   may fail because subnet X mismatches with subnet Y.

   To prevent this a RIFT implementation should check for subnet
   mismatch just like e.g.  ISIS does.  This can lead to scenarios where
   an adjacency, despite exchange of LIEs in both address families may
   end up having an adjacency in a single AF only.  This is a
   consideration especially in Section 4.6 scenarios.

4.12.  Anycast Considerations


















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                                 + traffic
                                 |
                                 v
                          +------+------+
                          |     ToF     |
                          +---+-----+---+
                          |   |     |   |
             +------------+   |     |   +------------+
             |                |     |                |
         +---+---+    +-------+     +-------+    +---+---+
         |       |    |       |     |       |    |       |
         |Spine11|    |Spine12|     |Spine21|    |Spine22| LEVEL 1
         +-+---+-+    ++----+-+     +-+---+-+    ++----+-+
           |   |       |    |         |   |       |    |
           |   +---------+  |         |   +---------+  |
           |           | |  |         |           | |  |
           |   +-------+ |  |         |   +-------+ |  |
           |   |         |  |         |   |         |  |
         +-+---+-+    +--+--+-+     +-+---+-+    +--+--+-+
         |       |    |       |     |       |    |       |
         |Leaf111|    |Leaf112|     |Leaf121|    |Leaf122| LEVEL 0
         +-+-----+    ++------+     +-----+-+    +-----+-+
           +           +                  +      ^     |
         PrefixA      PrefixB         PrefixA    | PrefixC
                                                 |
                                                 + traffic

                            Figure 12: Anycast

   If the traffic comes from ToF to Leaf111 or Leaf121 which has anycast
   prefix PrefixA.  RIFT can deal with this case well.  But if the
   traffic comes from Leaf122, it will always get to Leaf121 and never
   get to Leaf111.  If the intension is that the traffic should been
   offloaded to Leaf111, then use policy guided prefixes [PGP
   reference].

5.  Acknowledgements

6.  Contributors

   The following people (listed in alphabetical order) contributed
   significantly to the content of this document and should be
   considered co-authors:

   Tony Przygienda

   Juniper Networks




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   1194 N.  Mathilda Ave

   Sunnyvale, CA 94089

   US

   Email: prz@juniper.net

7.  Normative References

   [I-D.ietf-rift-rift]
              Przygienda, T., Sharma, A., Thubert, P., Rijsman, B., and
              D. Afanasiev, "RIFT: Routing in Fat Trees", draft-ietf-
              rift-rift-10 (work in progress), January 2020.

   [I-D.white-distoptflood]
              White, R., Hegde, S., and S. Zandi, "IS-IS Optimal
              Distributed Flooding for Dense Topologies", draft-white-
              distoptflood-01 (work in progress), September 2019.

   [ISO10589-Second-Edition]
              International Organization for Standardization,
              "Intermediate system to Intermediate system intra-domain
              routeing information exchange protocol for use in
              conjunction with the protocol for providing the
              connectionless-mode Network Service (ISO 8473)", Nov 2002.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC5357]  Hedayat, K., Krzanowski, R., Morton, A., Yum, K., and J.
              Babiarz, "A Two-Way Active Measurement Protocol (TWAMP)",
              RFC 5357, DOI 10.17487/RFC5357, October 2008,
              <https://www.rfc-editor.org/info/rfc5357>.

   [RFC7130]  Bhatia, M., Ed., Chen, M., Ed., Boutros, S., Ed.,
              Binderberger, M., Ed., and J. Haas, Ed., "Bidirectional
              Forwarding Detection (BFD) on Link Aggregation Group (LAG)
              Interfaces", RFC 7130, DOI 10.17487/RFC7130, February
              2014, <https://www.rfc-editor.org/info/rfc7130>.





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   [TR-384]   Broadband Forum Technical Report, "TR-384 Cloud Central
              Office Reference Architectural Framework", Jan 2018.

Authors' Addresses

   Yuehua Wei
   ZTE Corporation
   No.50, Software Avenue
   Nanjing  210012
   P. R. China

   Email: wei.yuehua@zte.com.cn


   Zheng Zhang
   ZTE Corporation
   No.50, Software Avenue
   Nanjing  210012
   P. R. China

   Email: zzhang_ietf@hotmail.com


   Dmitry Afanasiev
   Yandex

   Email: fl0w@yandex-team.ru


   Tom Verhaeg
   Interconnect Services B.V.

   Email: t.verhaeg@interconnect.nl


   Jaroslaw Kowalczyk
   Orange Polska

   Email: jaroslaw.kowalczyk2@orange.com












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