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Flooding Topology Minimum Degree Algorithm
draft-ietf-lsr-flooding-topo-min-degree-00

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Huaimo Chen , Mehmet Toy , Yi Yang , Aijun Wang , Xufeng Liu , Yanhe Fan , Lei Liu
Last updated 2020-11-25 (Latest revision 2020-06-06)
Replaces draft-chen-lsr-dynamic-flooding-algorithm
RFC stream Internet Engineering Task Force (IETF)
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draft-ietf-lsr-flooding-topo-min-degree-00
Network Working Group                                            H. Chen
Internet-Draft                                                 Futurewei
Intended status: Standards Track                                  M. Toy
Expires: December 8, 2020                                        Verizon
                                                                 Y. Yang
                                                                     IBM
                                                                 A. Wang
                                                           China Telecom
                                                                  X. Liu
                                                          Volta Networks
                                                                  Y. Fan
                                                            Casa Systems
                                                                  L. Liu
                                                                 Fujitsu
                                                            June 6, 2020

               Flooding Topology Minimum Degree Algorithm
               draft-ietf-lsr-flooding-topo-min-degree-00

Abstract

   This document proposes an algorithm for a node to compute a flooding
   topology, which is a subgraph of the complete topology per underline
   physical network.  When every node in an area automatically
   calculates a flooding topology by using a same algorithm and floods
   the link states using the flooding topology, the amount of flooding
   traffic in the network is greatly reduced.  This would reduce
   convergence time with a more stable and optimized routing
   environment.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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   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 December 8, 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
   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 . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Flooding Topology . . . . . . . . . . . . . . . . . . . . . .   3
     3.1.  Flooding Topology Construction  . . . . . . . . . . . . .   4
   4.  Algorithms to Compute Flooding Topology . . . . . . . . . . .   4
     4.1.  Algorithm with Considering Degree . . . . . . . . . . . .   5
     4.2.  Algorithm with Considering Others . . . . . . . . . . . .   6
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Appendix A.  FT Computation Details through Example . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   For some networks such as dense Data Center (DC) networks, the
   existing Link State (LS) flooding mechanism is not efficient and may
   have some issues.  The extra LS flooding consumes network bandwidth.
   Processing the extra LS flooding, including receiving, buffering and
   decoding the extra LSs, wastes memory space and processor time.  This

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   may cause scalability issues and affect the network convergence
   negatively.

   This document proposes an algorithm for a node to compute a flooding
   topology, which is a subgraph of the complete topology per underline
   physical network.  The physical network can be any network, including
   clos leaf spine network.  It can be used in the distributed mode of
   flooding topology computation for flooding reduction and the
   centralized mode, which are described in
   [I-D.ietf-lsr-dynamic-flooding].  When the distributed mode is
   selected, every node in an area automatically calculates a flooding
   topology by using a same algorithm and floods the link states using
   the flooding topology, the amount of flooding traffic in the network
   is greatly reduced.  This would reduce convergence time with a more
   stable and optimized routing environment.

   There may be multiple algorithms for computing a flooding topology.
   Users can select one they prefer, and smoothly switch from one to
   another.

2.  Terminology

   LSA:  A Link State Advertisement in OSPF.

   LSP:  A Link State Protocol Data Unit (PDU) in IS-IS.

   LS: A Link Sate, which is an LSA or LSP.

   FT: Flooding Topology.

   FTC:  Flooding Topology Computation.

3.  Flooding Topology

   For a given network topology, a flooding topology is a sub-graph or
   sub-network of the given network topology that has the same
   reachability to every node as the given network topology.  Thus all
   the nodes in the given network topology MUST be in the flooding
   topology.  All the nodes MUST be inter-connected directly or
   indirectly.  As a result, LS flooding will in most cases occur only
   on the flooding topology, that includes all nodes but a subset of
   links.  Note even though the flooding topology is a sub-graph of the
   original topology, any single LS MUST still be disseminated in the
   entire network.

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3.1.  Flooding Topology Construction

   Many different flooding topologies can be constructed for a given
   network topology.  For example, a chain connecting all the nodes in
   the given network topology is a flooding topology.  A circle
   connecting all the nodes is another flooding topology.  A tree
   connecting all the nodes is a flooding topology.  In addition, the
   tree plus the connections between some leaves of the tree and branch
   nodes of the tree is a flooding topology.

   The following parameters need to be considered for constructing a
   flooding topology:

   o  Degree: The degree of the flooding topology is the maximum degree
      among the degrees of the nodes on the flooding topology.  The
      degree of a node on the flooding topology is the number of
      connections on the flooding topology it has to other nodes.

   o  Number of links: The number of links on the flooding topology is a
      key factor for reducing the amount of LS flooding.  In general,
      the smaller the number of links, the less the amount of LS
      flooding.

   o  Diameter: The diameter of the flooding topology is the shortest
      distance between the two most distant nodes on the flooding
      topology.  It is a key factor for reducing the network convergence
      time.  The smaller the diameter, the less the convergence time.

   o  Redundancy: The redundancy of the flooding topology means a
      tolerance to the failures of some links and nodes on the flooding
      topology.  If the flooding topology is split by some failures, it
      is not tolerant to these failures.  In general, the larger the
      number of links on the flooding topology is, the more tolerant the
      flooding topology to failures.

   Note that the flooding topology constructed by a node is dynamic in
   nature, that means when the base topology (the entire topology graph)
   changes, the flooding topology (the sub-graph) MUST be re-computed/
   re-constructed to ensure that any node that is reachable on the base
   topology MUST also be reachable on the flooding topology.

4.  Algorithms to Compute Flooding Topology

   There are many algorithms to compute a flooding topology.  A simple
   and efficient one is briefed, which comprises:

   o  Selecting a node R0 with the smallest node ID;

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   o  Building a tree using R0 as root in breadth first; and then

   o  Connecting each node whose degree is one to another node to have a
      flooding topology.

4.1.  Algorithm with Considering Degree

   The algorithm is described below, where a variable MaxD with an
   initial value 3, data structures candidate queue Cq and flooding
   topology FT are used.  Cq and FT comprise elements of form (N, D,
   PHs), where N represents a Node, D is the Degree of node N, and PHs
   contains the Previous Hops of node N.  The detailed FT computation by
   the algorithm is illustrated in Appendix A through an example.

   The algorithm starts from node R0 as root with a maximum degree MaxD
   of value 3, a candidate queue Cq = {(R0, D = 0, PHs = { })}, and an
   empty flooding topology FT = { }.  Cq contains one element (R0, D =
   0, PHs = { }), where node R0 is the root, D = 0 indicates that the
   Degree (D for short) of R0 is 0 (i.e., the number of links on the
   flooding topology connected to R0 is 0), PHs = { } indicates that the
   Previous Hops (PHs for short) of R0 is empty.

   1.  Finding and removing the first element with node A in Cq that is
       not on FT and one PH's D in PHs < MaxD.

       If A is root R0, then add the element into FT

       otherwise  (i.e., A != R0 with one PH's D in PHs < MaxD.  Assume
          that PH is the first one in PHs whose D < MaxD), PH's D++, and
          add A with D = 1 and PHs = {PH} into FT.

       Note:  if no element in Cq satisfies the conditions, algorithm is
          restarted from R0, ++MaxD, Cq = {(R0,D=0,PHs={ })}, FT = { };

   2.  If all the nodes are on the FT, then goto step 4;

   3.  Suppose that node Xi (i = 1, 2,..., n) is connected to node A and
       not on FT, and X1, X2,..., Xn are in an increasing order by their
       IDs (i.e., X1's ID < X2's ID < ... < Xn's ID).  If Xi is not in
       Cq, then add it into the end of Cq with D = 0 and PHs = {A};
       otherwise (i.e., Xi is in Cq), add A into the end of Xi's PHs;
       Goto step 1.

   4.  For each node B on FT whose D is one (from minimum to maximum
       node ID), find a link L attached to B such that L's remote node R
       has minimum D and ID, add link L between B and R into FT and
       increase B's D and R's D by one.  Return FT.

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4.2.  Algorithm with Considering Others

   There may be some constraints on some nodes in a network.  For
   example, in a spine-and-leaf network, there may be a constraint on
   the degree of every leaf node on the flooding topology, which is that
   the degree of every leaf node is not greater than a given number
   ConMaxD of value 2.  For each of the other nodes such as the spine
   nodes, there is no such constraint, that is that ConMaxD is a huge
   number for each of these nodes.

   Step 1 of the algorithm described above is updated below to consider
   this constraint.  In addition to checking constraint PH's D < MaxD,
   step 1 checks another constraint PH's D < PH's ConMaxD.

   1.  Finding and removing the first element with node A in Cq that is
       not on FT and one PH's D in PHs < MaxD and PH's D < PH's ConMaxD.

       If A is root R0, then add the element into FT

       otherwise  (i.e., A != R0 with one PH's D in PHs < MaxD and PH's
          D < PH's ConMaxD.  Assume that PH is the first one in PHs
          whose D < MaxD and PH's D < PH's ConMaxD), PH's D++, and add A
          with D = 1 and PHs = {PH} into FT.

       Note:  if no element in Cq satisfies the conditions, algorithm is
          restarted from R0, ++MaxD, Cq = {(R0,D=0,PHs={ })}, FT = { };

5.  Security Considerations

   This document does not introduce any new security issue.

6.  IANA Considerations

   Under Registry Name: "IGP Algorithm Type For Computing Flooding
   Topology" under an existing "Interior Gateway Protocol (IGP)
   Parameters" IANA registries (refer to Section 7.3.  IGP
   [I-D.ietf-lsr-dynamic-flooding]), IANA is requested to assign one
   value of IGP Algorithm Type For Computing Flooding Topology as
   follows:

   +==========+========================================+=============+
   |Type Value|               Type Name                | reference   |
   +==========+========================================+=============+
   |    1     | Breadth First Minimum Degree Algorithm |This document|
   +----------+----------------------------------------+-------------+
   |    2     | Breadth First Leaf Constraint Algorithm|This document|
   +----------+----------------------------------------+-------------+

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7.  Acknowledgements

   The authors would like to thank Dean Cheng, Acee Lindem, Zhibo Hu,
   Robin Li, Stephane Litkowski and Alvaro Retana for their valuable
   suggestions and comments on this draft.

8.  References

8.1.  Normative References

   [I-D.ietf-lsr-dynamic-flooding]
              Li, T., Psenak, P., Ginsberg, L., Chen, H., Przygienda,
              T., Cooper, D., Jalil, L., and S. Dontula, "Dynamic
              Flooding on Dense Graphs", draft-ietf-lsr-dynamic-
              flooding-06 (work in progress), May 2020.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <https://www.rfc-editor.org/info/rfc1195>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

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

8.2.  Informative References

   [I-D.ietf-rtgwg-spf-uloop-pb-statement]
              Litkowski, S., Decraene, B., and M. Horneffer, "Link State
              protocols SPF trigger and delay algorithm impact on IGP
              micro-loops", draft-ietf-rtgwg-spf-uloop-pb-statement-10
              (work in progress), January 2019.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

Appendix A.  FT Computation Details through Example

   This section presents the details on FT computation by the algorithm
   through an example.  The detailed procedure of computing a FT for a
   network of five nodes with full mess connections is illustrated.
   Suppose that the network has five nodes R0, R1, R2, R3 and R4; R0's

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   ID < R1's ID < R2's ID < R3's ID < R4's ID.  The algorithm starts
   with Cq = {(R0, D=0, PH={})}, FT = {}, MaxD = 3.

  0. // remove the first element containing root R0 from Cq
     Cq = { };
     // add the element into FT
     FT = { (R0,D=0,PHs={ }) }; // root R0 on FT
     // for each Ri connected to R0 (not in Cq), add it to the end of Cq
     Cq = { (R1,D=0,PHs={R0}), (R2,D=0,PHs={R0}), (R3,D=0,PHs={R0}),
            ^^^^^^^^^^^^^^^^^  (R4,D=0,PHs={R0}) }

                                 R0
                          __/--- O ---\__
                       __/      / \      \__
                    __/        /   \        \__
                 __/          /     \          \__
                /            /       \            \
            R1 O--\_--------/---------\--------_/--O R4
                \   \____  /           \  ____/   /
                 \       \/             \/       /
                  \      /\_____    ____/\      /
                   \    /    ___\__/___   \    /
                    \  /    /          \   \  /
                     \/____/            \___\/
                  R2  O -------------------- O R3

   1. //remove first element (R1,D=0,PHs={R0}) from Cq, R0's D=0 < MaxD
      Cq = { (R2,0,{R0}), (R3,0,{R0}), (R4,0,{R0}) };
      // add (R1,1,{R0}) into FT, increase PH R0's D by one
      FT = { (R0,1, { }), (R1,1, {R0}) }; // Link R1--R0 on FT
                ^^^       ^^^^^^^^^^^^
      // for Ri connected to R1 (in Cq) not on FT, append R1 to Ri's PHs
      Cq = { (R2,0, {R0,R1}), (R3,0, {R0,R1}), (R4,0,{R0,R1}) }.
                        ^^               ^^              ^^
                                   R0            ==== Link on FT
                            __//== O ---\__
                         __//     / \      \__   link R1--R0 added to FT
                      __//       /   \        \__
                   __//         /     \          \__
                  //           /       \            \
              R1 O--\_--------/---------\--------_/--O R4
                  \   \____  /           \  ____/   /
                   \       \/             \/       /
                    \      /\_____    ____/\      /
                     \    /    ___\__/___   \    /
                      \  /    /          \   \  /
                       \/____/            \___\/
                    R2 O --------------------- O R3

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 2. // remove the first element (R2,0, {R0,R1}) from Cq, R0's D=1 < MaxD
    Cq = { (R3,0, {R0,R1}), (R4,0,{R0,R1}) }
    // add (R2,1,{R0}) into FT, increase R0's D by one
    FT = { (R0,2,{ }), (R1,1,{R0}), (R2,1,{R0}) } //Link R2--R0 on FT
              ^^^                   ^^^^^^^^^^^
    // for Ri connected to R2 (in Cq) not on FT, append R2 to Ri's PHs
    Cq = { (R3,0, {R0,R1,R2}), (R4,0,{R0,R1,R2}) }
                         ^^                 ^^
                                 R0           ==== Link on FT
                          __//== O ---\__
                       __//    // \      \__  link R2--R0 added to FT
                    __//      //   \        \__
                 __//        //     \          \__
                //          //       \            \
            R1 O--\_-------//---------\--------_/--O R4
                \   \____ //           \  ____/   /
                 \       \/             \/       /
                  \     //\_____    ____/\      /
                   \   //    ___\__/___   \    /
                    \ //    /          \   \  /
                     \/____/            \___\/
                  R2 O --------------------- O R3

  3. //remove the 1st element (R3,0,{R0,R1,R2}) from Cq, R0's D=2 < MaxD
     Cq = { (R4,0,{R0,R1,R2}) }
     // add (R3,1,{R0}) into FT, increase R0's D by one
     FT = { (R0,3,{}), (R1,1,{R0}), (R2,1,{R0}), (R3,1,{R0}) }
               ^^^                               ^^^^^^^^^^^
     // for Ri connected to R3 (in Cq) not on FT, append R3 to Ri's PHs
     Cq = { (R4,0,{R0,R1,R2,R3}) }.
                            ^^
                                  R0           ==== Link on FT
                           __//== O ---\__
                        __//    // \\     \__  link R3--R0 added to FT
                     __//      //   \\       \__
                  __//        //     \\         \__
                 //          //       \\           \
             R1 O--\_-------//---------\\-------_/--O R4
                 \   \____ //           \\ ____/   /
                  \       \/             \/       /
                   \     //\_____    ____/\\     /
                    \   //    ___\__/___   \\   /
                     \ //    /          \   \\ /
                      \/____/            \___\/
                   R2 O --------------------- O R3

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4. //remove the 1st element (R4,0,{R0,R1,R2,R3}) from Cq,R1's D=1 < MaxD
   Cq = { }
   // add (R4,1,{R1}) into FT, increase R1's D by one
   FT = {(R0,3,{}), (R1,2,{R0}), (R2,1,{R0}), (R3,1,{R0}), (R4,1,{R1})}
                       ^^^                                 ^^^^^^^^^^^
                                R0           ==== Link on FT
                         __//== O ---\__
                      __//    // \\     \__  link R4--R1 added to FT
                   __//      //   \\       \__
                __//        //     \\         \__
               //          //       \\           \
           R1 O==\_=======//=========\\=======_/==O R4
               \   \____ //           \\ ____/   /
                \       \/             \/       /
                 \     //\_____    ____/\\     /
                  \   //    ___\__/___   \\   /
                   \ //    /          \   \\ /
                    \/____/            \___\/
                 R2 O --------------------- O R3

   All nodes are on FT now.  In the following, for each node on FT whose
   D = 1 (from minimum to maximum ID), link L attached to it and not on
   FT is found such that L's remote node has minimum D and ID.  L is
   added into FT.

  5. // On FT, get node R2 with smallest ID whose D=1
     FT = {(R0,3,{}),(R1,2,{R0}),(R2,1,{R0}),(R3,1,{R0}), (R4,1,{R1})}
     // Add link R2--R3 to FT,   ^^^^^^^^^^^
     // where R2--R3 is not on FT, R3's D=1 is minimum first and then
     // R3's ID is minimum (R3 and R4 tie for D), R2's D++ and R3's D++
     FT = {(R0,3,{}),(R1,2,{R0}),(R2,2,{R0,R3}),(R3,2,{R0}),(R4,1,{R1})}
                                    ^^^    ^^      ^^^
                                  R0           ==== Link on FT
                           __//== O ---\__
                        __//    // \\     \__  link R2--R3 added to FT
                     __//      //   \\       \__
                  __//        //     \\         \__
                 //          //       \\           \
             R1 O==\_=======//=========\\=======_/==O R4
                 \   \____ //           \\ ____/   /
                  \       \/             \/       /
                   \     //\_____    ____/\\     /
                    \   //    ___\__/___   \\   /
                     \ //    /          \   \\ /
                      \/____/            \___\/
                   R2 O ===================== O R3

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6. // On FT, get node R4 with smallest ID whose D=1
   FT = {(R0,3,{}),(R1,2,{R0}),(R2,2,{R0,R3}),(R3,2,{R0}),(R4,1,{R1})}
   // Add link R4--R2 to FT, where                        ^^^^^^^^^^^
   // R4--R2 is not on FT, R2's D=2 is minimum first and then R2's ID is
   // minimum (R2 and R3 tie for D), increase R2's D and R4's D by one
   FT = {(R0,3,{}),(R1,2,{R0}),(R2,3,{R0,R3}),(R3,2,{R0}),(R4,2,{R1,R2})}
                                  ^^^                        ^^^    ^^
                                R0           ==== Link on FT
                         __//== O ---\__
                      __//    // \\     \__  link R4--R2 added to FT
                   __//      //   \\       \__
                __//        //     \\         \__
               //          //       \\           \
           R1 O==\_=======//=========\\=======//==O R4
               \   \____ //           \\ ____//  /
                \       \/             \//      /
                 \     //\_____    ___//\\     /
                  \   //    ___\__//__   \\   /
                   \ //    //         \   \\ /
                    \/   _//           \___\/
                 R2 O ==//================= O R3

   FT is computed, which has Degree of 3 and Diameter of 2.

Authors' Addresses

   Huaimo Chen
   Futurewei
   Boston
   USA

   Email: huaimo.chen@futurewei.com

   Mehmet Toy
   Verizon
   USA

   Email: mehmet.toy@verizon.com

   Yi Yang
   IBM
   Cary, NC
   United States of America

   Email: yyietf@gmail.com

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   Aijun Wang
   China Telecom
   Beiqijia Town, Changping District
   Beijing  102209
   China

   Email: wangaj3@chinatelecom.cn

   Xufeng Liu
   Volta Networks
   McLean, VA
   USA

   Email: xufeng.liu.ietf@gmail.com

   Yanhe Fan
   Casa Systems
   USA

   Email: yfan@casa-systems.com

   Lei Liu
   Fujitsu
   USA

   Email: liulei.kddi@gmail.com

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