Server/Application State Protocol v1
RFC 4678

RIFT WG                                                 Yuehua. Wei, Ed.
Internet-Draft                                              Zheng. Zhang
Intended status: Informational                           ZTE Corporation
Expires: 15 June 2022                                  Dmitry. Afanasiev
                                                                  Yandex
                                                              P. Thubert
                                                           Cisco Systems
                                                     Jaroslaw. Kowalczyk
                                                           Orange Polska
                                                        12 December 2021

                           RIFT Applicability
                    draft-ietf-rift-applicability-09

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
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   This Internet-Draft will expire on 15 June 2022.

Copyright Notice

   Copyright (c) 2021 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 carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Problem Statement of Routing in Modern IP Fabric Fat Tree
           Networks  . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Applicability of RIFT to Clos IP Fabrics  . . . . . . . . . .   5
     4.1.  Overview of RIFT  . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Applicable Topologies . . . . . . . . . . . . . . . . . .   8
       4.2.1.  Horizontal Links  . . . . . . . . . . . . . . . . . .   8
       4.2.2.  Vertical Shortcuts  . . . . . . . . . . . . . . . . .   9
       4.2.3.  Generalizing to any Directed Acyclic Graph  . . . . .   9
       4.2.4.  Reachability of Internal Nodes in the Fabric  . . . .  11
     4.3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  11
       4.3.1.  Data Center Topologies  . . . . . . . . . . . . . . .  11
       4.3.2.  Metro Fabrics . . . . . . . . . . . . . . . . . . . .  12
       4.3.3.  Building Cabling  . . . . . . . . . . . . . . . . . .  13
       4.3.4.  Internal Router Switching Fabrics . . . . . . . . . .  13
       4.3.5.  CloudCO . . . . . . . . . . . . . . . . . . . . . . .  13
   5.  Operational Considerations  . . . . . . . . . . . . . . . . .  15
     5.1.  South Reflection  . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Suboptimal Routing on Link Failures . . . . . . . . . . .  16
     5.3.  Black-Holing on Link Failures . . . . . . . . . . . . . .  18
     5.4.  Zero Touch Provisioning (ZTP) . . . . . . . . . . . . . .  19
     5.5.  Mis-cabling Examples  . . . . . . . . . . . . . . . . . .  20
     5.6.  Positive vs. Negative Disaggregation  . . . . . . . . . .  22
     5.7.  Mobile Edge and Anycast . . . . . . . . . . . . . . . . .  24
     5.8.  IPv4 over IPv6  . . . . . . . . . . . . . . . . . . . . .  26
     5.9.  In-Band Reachability of Nodes . . . . . . . . . . . . . .  26
     5.10. Dual Homing Servers . . . . . . . . . . . . . . . . . . .  28
     5.11. Fabric With A Controller  . . . . . . . . . . . . . . . .  28
       5.11.1.  Controller Attached to ToFs  . . . . . . . . . . . .  29
       5.11.2.  Controller Attached to Leaf  . . . . . . . . . . . .  29
     5.12. Internet Connectivity Within Underlay . . . . . . . . . .  29
       5.12.1.  Internet Default on the Leaf . . . . . . . . . . . .  30
       5.12.2.  Internet Default on the ToFs . . . . . . . . . . . .  30
     5.13. Subnet Mismatch and Address Families  . . . . . . . . . .  30
     5.14. Anycast Considerations  . . . . . . . . . . . . . . . . .  30
     5.15. IoT Applicability . . . . . . . . . . . . . . . . . . . .  31
     5.16. Key Management  . . . . . . . . . . . . . . . . . . . . .  32
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  32
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
   8.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  33

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   9.  Normative References  . . . . . . . . . . . . . . . . . . . .  33
   10. Informative References  . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

1.  Introduction

   This document discusses the properties and applicability of "Routing
   in Fat Trees" [RIFT] in different deployment scenarios and highlights
   the operational simplicity of the technology compared to traditional
   routing solutions.  It also documents special considerations when
   RIFT is used with or without overlays and/or controllers, and how
   RIFT identifies topology mis-cablings and reroutes around node and
   link failures.

2.  Terminology

   Clos/Fat Tree:

   This document uses the terms Clos and Fat Tree interchangeably
   whereas it always refers to a folded spine-and-leaf topology with
   possibly multiple Points of Delivery (PoDs) and one or multiple Top
   of Fabric (ToF) planes.

   Directed Acyclic Graph (DAG):

   A finite directed graph with no directed cycles (loops).  If links in
   a Clos are considered as either being all directed towards the top or
   vice versa, each of such two graphs is a DAG.

   Disaggregation:

   Process in which a node decides to advertise more specific prefixes
   Southwards, either positively to attract the corresponding traffic,
   or negatively to repel it.  Disaggregation is performed to prevent
   black-holing and suboptimal routing to the more specific prefixes.

   TIE:

   This is an acronym for a "Topology Information Element".  TIEs are
   exchanged between RIFT nodes to describe parts of a network such as
   links and address prefixes.  A TIE has always a direction and a type.
   North TIEs (sometimes abbreviated as N-TIEs) are used when dealing
   with TIEs in the northbound representation and South-TIEs (sometimes
   abbreviated as S-TIEs) for the southbound equivalent.  TIEs have
   different types such as node and prefix TIEs.

   Node TIE:

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   This stands as acronym for a "Node Topology Information Element",
   which contains all adjacencies the node discovered and information
   about the node itself.  Node TIE should NOT be confused with a North
   TIE since "node" defines the type of TIE rather than its direction.
   Consequently North Node TIEs and South Node TIEs exist.

   Prefix TIE:

   This is an acronym for a "Prefix Topology Information Element" and it
   contains all prefixes directly attached to this node in case of a
   North TIE and in case of South TIE the necessary default routes and
   disaggregated routes the node advertises southbound.

   South Reflection:

   Often abbreviated just as "reflection", it defines a mechanism where
   South Node TIEs are "reflected" from the level south back up north to
   allow nodes in the same level without East- West links to "see" each
   other's node Topology Information Elements (TIEs).

   LIE:

   This is an acronym for a "Link Information Element" exchanged on all
   the system's links running RIFT to form ThreeWay adjacencies and
   carry information used to perform Zero Touch Provisioning (ZTP) of
   levels.

   Shortest-Path First (SPF):

   A well-known graph algorithm attributed to Dijkstra that establishes
   a tree of shortest paths from a source to destinations on the graph.
   SPF acronym is used due to its familiarity as general term for the
   node reachability calculations that RIFT can employ to ultimately
   calculate routes of which Dijkstra algorithm is a possible one.

   North SPF (N-SPF):

   A reachability calculation that is progressing northbound, as example
   SPF that is using South Node TIEs only.  Normally it progresses a
   single hop only and installs default routes.

   South SPF (S-SPF):

   A reachability calculation that is progressing southbound, as example
   SPF that is using North Node TIEs only.