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IS-IS Optimal Distributed Flooding for Dense Topologies

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Authors Russ White , Shraddha Hegde , Tony Przygienda
Last updated 2023-01-06
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Network Working Group                                           R. White
Internet-Draft                                                  S. Hegde
Intended status: Experimental                              T. Przygienda
Expires: 10 July 2023                                   Juniper Networks
                                                          6 January 2023

        IS-IS Optimal Distributed Flooding for Dense Topologies


   In dense topologies (such as data center fabrics based on the Clos
   and butterfly topologies, though not limited to these), IGP flooding
   mechanisms designed for sparse topologies can "overflood," or carry
   too many copies of topology and reachability information to fabric
   devices.  This results in slower convergence times and higher
   resource utilization.  The modifications to the flooding mechanism in
   the Intermediate System to Intermediate System (IS-IS) link state
   protocol described in this document reduce resource utilization
   significantly, while increasing convergence performance in dense

   Note that a Clos fabric is used as the primary example of a dense
   flooding topology throughout this document.  However, the flooding
   optimizations described in this document apply to any topology.

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

   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 10 July 2023.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (
   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 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Contributors  . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Experience  . . . . . . . . . . . . . . . . . . . . . . .   3
     1.4.  Sample Network  . . . . . . . . . . . . . . . . . . . . .   3
   2.  Flooding Modifications  . . . . . . . . . . . . . . . . . . .   5
     2.1.  Optimizing Flooding . . . . . . . . . . . . . . . . . . .   5
     2.2.  Optimization Process  . . . . . . . . . . . . . . . . . .   6
     2.3.  Flooding Failures . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Flooding Example  . . . . . . . . . . . . . . . . . . . .   8
     2.5.  A Note on Performance . . . . . . . . . . . . . . . . . .   8
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     4.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

1.  Introduction

1.1.  Goals

   The goal of this draft is to solve one specific set of problems
   involved in operating a link state protocol in a densely meshed
   topology.  The problem with such topologies is the connectivity
   density, which causes too many copies of identical information to be
   flooded.  Analysis and experiment show, for instance, that in a
   butterfly fabric of around 2500 intermediate systems, each
   intermediate system will receive more than 40 copies of any changed
   LSP fragment.  This not only wastes bandwidth and processor time,
   this dramatically slows convergence speed.

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   This document describes a set of modifications to existing IS-IS
   flooding mechanisms which minimize the number of LSP fragments
   received by individual intermediate systems, in its extreme version
   to one copy per intermediate system.  The mechanisms described in
   this document are similar to those implemented in OSPF to support
   mobile ad-hoc networks, as described in [RFC5449], [RFC5614], and
   [RFC7182].  These mechanisms have been widely implemented and

1.2.  Contributors

   The following people have contributed to this draft: Abhishek Kumar,
   Nikos Triantafillis, Ivan Pepelnjak, Christian Franke, Hannes
   Gredler, Les Ginsberg, Naiming Shen, Uma Chunduri, Nick Russo, Shawn
   Zandi, and Rodny Molina.

1.3.  Experience

   Laboratory tests show modifications similar to these reduce flooding
   in a large scale emulated butterfly network topology; without these
   modifications, intermediate systems receive, on average, 40 copies of
   any changed LSP fragment.  With the modifications described in this
   document intermediate systems recieve, on average, two copies of any
   changed LSP fragment.  In many cases, each intermediate system
   receives only a single copy of each changed LSP.  In terms of
   performance, the modifications described here cut convergence times
   in half.  Processor load times were not checked, as this was an
   emulated environment.

   A mechanism similar to the one described in this document has been
   implemented in the FR Routing open source routing stack as part of

1.4.  Sample Network

   The following spine and leaf fabric will be used to describe these

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   +----+ +----+ +----+ +----+ +----+ +----+
   | 1A | | 1B | | 1C | | 1D | | 1E | | 1F | (T0)
   +----+ +----+ +----+ +----+ +----+ +----+

   +----+ +----+ +----+ +----+ +----+ +----+
   | 2A | | 2B | | 2C | | 2D | | 2E | | 2F | (T1)
   +----+ +----+ +----+ +----+ +----+ +----+

   +----+ +----+ +----+ +----+ +----+ +----+
   | 3A | | 3B | | 3C | | 3D | | 3E | | 3F | (T2)
   +----+ +----+ +----+ +----+ +----+ +----+

   +----+ +----+ +----+ +----+ +----+ +----+
   | 4A | | 4B | | 4C | | 4D | | 4E | | 4F | (T1)
   +----+ +----+ +----+ +----+ +----+ +----+

   +----+ +----+ +----+ +----+ +----+ +----+
   | 5A | | 5B | | 5C | | 5D | | 5E | | 5F | (T0)
   +----+ +----+ +----+ +----+ +----+ +----+

                                  Figure 1

   To reduce confusion (spine and leaf fabrics are difficult to draw in
   plain text art), this diagram does not contain the connections
   between devices.  The reader should assume that each device in a
   given layer is connected to every device in the layer above it.  For

   *  5A is connected to 4A, 4B, 4C, 4D, 4E, and 4F

   *  5B is connected to 4A, 4B, 4C, 4D, 4E, and 4F

   *  4A is connected to 3A, 3B, 3C, 3D, 3E, 3F, 5A, 5B, 5C, 5D, 5E, and

   *  4B is connected to 3A, 3B, 3C, 3D, 3E, 3F, 5A, 5B, 5C, 5D, 5E, and

   *  etc.

   The tiers or stages of the fabric are also marked for easier
   reference.  T0 is assumed to be connected to application servers, or
   rather they are Top of Rack (ToR) intermediate systems.  The
   remaining tiers, T1 and T2, are connected only to other devices in
   the fabric itself.  A common alternate representation of this
   topology is drawn "folded" with T2, the "top of fabric," shown on
   top, while T1 is shown below, and T0 below T1.

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2.  Flooding Modifications

   Flooding is perhaps the most challenging scaling issue for a link
   state protocol running on a dense, large scale topology.  This
   section describes detailed modifications to the IS-IS flooding
   process to reduce flooding load in a densely meshed topology.

2.1.  Optimizing Flooding

   The simplest way to conceive of the solution presented here is in two

   *  Stage 1: Forward Optimization

      -  Find the group of intermediate systems that will all flood to
         the same set of neighbors as the local IS

      -  Decide (deterministically) which subset of the intermediate
         systems within this group should re-flood any received LSPs

   *  Stage 2: Reverse Optimization

      -  Find neighbors on the shortest path towards the origin of the

      -  Do not flood towards these neighbors

   The first stage is best explained through an illustration.  In the
   network above, if 5A transmits a modified Link State Protocol Data
   Unit (LSP) to 4A-4F, each of 4A-4F will, in turn, flood this modified
   LSP to 3A (for instance). 3A will receive 6 copies of the modified
   LSP, while only one copy is necessary for the intermediate systems
   shown to converge on a single view of the topology.  If 4A-4F could
   determine they will all flood identical copies of the modified LSP to
   3A, it is possible for all of them except one to decide not to flood
   the changed LSP to 3A.

   The technique used in this draft to determine the flooding group is
   for each intermediate system to calculate a special Shortest-path
   Spanning Tree (SPT) from the point of view of the transmitting
   neighbor.  By setting the metric of all links to 1 and truncating the
   SPT to two hops, the local IS can find the group of neighbors it will
   flood any changed LSP towards and the set of intermediate systems
   (not necessarily neighbors) which will also flood to this same set of
   neighbors.  If every intermediate system in the flooding set performs
   this same calculation, they will all obtain the same flooding group.

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   Once this flooding group is determined, the members of the flooding
   group will each (independently) choose which of the members should
   re-flood the received information.  Each member of the flooding group
   calculates this independently of all the other members, but a common
   hash MUST be used across a set of shared variables so each member of
   the group comes to the same conclusion.  The group member which is
   selected to flood the changed LSP does so normally; the remaining
   group members do not flood the LSP.

   Note there is no signaling between the intermediate systems running
   this flooding reduction mechanism.  Each IS calculates the special,
   truncated SPT separately, and determines which IS should flood any
   changed LSPs independently based on a common hash function.  Because
   these calculations are performed using a shared view of the network,
   however (based on the common link state database) and a shared hash
   function, each member of the flooding group will make the same

   The second stage is simpler, consisting of a single rule: do not
   flood modified LSPs along the shortest path towards the origin of the
   modified LSP.  This rule relies on the observation that any IS
   between the origin of the modified LSP and the local IS should
   receive the modified LSP from some other IS closer to the source of
   the modified LSP.

2.2.  Optimization Process

   Each intermediate system will determine whether it should re-flood
   LSPs as described below.  When a modified LSP arrives from a
   Transmitting Neighbor (TN), the result of the following algorithm
   obtains the necessary decision:

   Step 1: Build the Two-Hop List (THL) and Remote Neighbor's List (RNL)

   *  Set all link metrics to 1

   *  Calculate an SPT truncated to 2 hops from the perspective of TN

   *  For each IS that is two hops (has a metric of two in the truncated
      SPT) from TN:

      -  If the IS is on the shortest path towards the originator of the
         modified LSP, skip

      -  If the IS is the neighbor of the LSP originator, skip

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      -  If the IS is not on the shortest path towards the originator of
         the modified LSP, add it to THL

   *  Add each IS that is one hop away from TN to the RNL

   Step 2: Sort RNL by system IDs, from the least to the greatest.

   Step 3: Calculate a number, N, by adding each byte in LSP-ID (without
   the fragment ID) and fragment ID MOD 2 (allowing for some balancing
   of LSPs coming from same system ID without introducing excessive
   amount of state in an implementation) and then taking MOD on the
   number of neighbors.  N MUST be less than the number of members of

   Step 4: Starting with the Nth member of RNL:

   *  If THL is empty, exit

   *  If this member of RNL is the local calculating IS, this IS MUST
      reflood the modified LSP; exit

   *  Remove all members of THL connected to (adjacent to) this member
      of RNL

   *  Move to the next member of RNL, wrapping to the beginning of RNL
      if necessary

   Note: This description is geared to clarity rather than optimal

2.3.  Flooding Failures

   It is possible in some failure modes for flooding to be incomplete
   because of the flooding optimizations outlined.  Specifically, if a
   reflooder fails, or is somehow disconnected from all the links across
   which it should be reflooding, it is possible an LSP is only
   partially flooded through the fabric.  To prevent such partition
   failures, an intermediate system which does not reflood an LSP (or
   fragment) should:

   *  Set a short timer; the default should be one second

   *  When the timer expires, send Partial Sequence Number Packet (PSNP)
      of all LSPs that have not been reflooded during the timer runtime
      to all neighbors unless an up-to-date PSNP or CSNP has been
      already received from the neighbor

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   *  Process any Partial Sequence Number Packets (PSNPs) received that
      indicate that neighbors still have older versions of the LSP per
      normal protocol procedures to resynchronize

   *  If resynchronization above a configurable threshold is required,
      an implementation SHOULD notify the network operator

2.4.  Flooding Example

   Assume, in the network above, 5A floods some modified LSP towards 4A-
   4F.  To determine whether 4A should flood this LSP to 3A-3F:

   *  5A is TN; 4A calculates a truncated SPT from 5A's perspective with
      all link metrics set to 1

   *  4A builds THL, which contains 3A, 3B, 3C, 3D, 3E, 3F, 5B, 5C, 5D,
      5E and 5F

   *  4A builds RNL, which contains 4A,4B,4C,4D,4E and 4F, sorting it by
      the system ID

   *  4A computes hash on the received LSP-ID to get N; assume N is 1 in
      this case

   *  Since 4A is the Nth member of R-NL and there are members in N-NL,
      4A must reflood; the loop exits

2.5.  A Note on Performance

   The calculations described here are complex, which might lead the
   reader to conclude that the cost of calculation is so much higher
   than the cost of flooding that this optimization is counter-
   productive.  The description provided here is designed for clarity
   rather than optimal calculation, however.  Many of the calculations
   can be performed in advance and stored, rather than being performed
   for each LSP and each neighbor.  Optimized versions of the process
   described here have been implemented, and do result in strong
   convergence speed gains.

3.  Security Considerations

   This document outlines modifications to the IS-IS protocol for
   operation on high density network topologies.  Implementations SHOULD
   implement IS-IS cryptographic authentication, as described in
   [RFC5304], and should enable other security measures in accordance
   with best common practices for the IS-IS protocol.

4.  References

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4.1.  Normative References

              Li, T., Przygienda, T., Psenak, P., Ginsberg, L., Chen,
              H., Cooper, D., Jalil, L., Dontula, S., and G. S. Mishra,
              "Dynamic Flooding on Dense Graphs", Work in Progress,
              Internet-Draft, draft-ietf-lsr-dynamic-flooding-11, 7 June
              2022, <

   [ISO10589] ISO, "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)", ISO/
              IEC 10589:2002, Second Edition, November 2002.

   [RFC2119]  Bradner, S. and RFC Publisher, "Key words for use in RFCs
              to Indicate Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC2629]  Rose, M. and RFC Publisher, "Writing I-Ds and RFCs using
              XML", RFC 2629, DOI 10.17487/RFC2629, June 1999,

   [RFC5120]  Przygienda, T., Shen, N., Sheth, N., and RFC Publisher,
              "M-ISIS: Multi Topology (MT) Routing in Intermediate
              System to Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,

   [RFC5301]  McPherson, D., Shen, N., and RFC Publisher, "Dynamic
              Hostname Exchange Mechanism for IS-IS", RFC 5301,
              DOI 10.17487/RFC5301, October 2008,

   [RFC5303]  Katz, D., Saluja, R., Eastlake 3rd, D., and RFC Publisher,
              "Three-Way Handshake for IS-IS Point-to-Point
              Adjacencies", RFC 5303, DOI 10.17487/RFC5303, October
              2008, <>.

   [RFC5305]  Li, T., Smit, H., and RFC Publisher, "IS-IS Extensions for
              Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305,
              October 2008, <>.

   [RFC5308]  Hopps, C. and RFC Publisher, "Routing IPv6 with IS-IS",
              RFC 5308, DOI 10.17487/RFC5308, October 2008,

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   [RFC5309]  Shen, N., Ed., Zinin, A., Ed., and RFC Publisher, "Point-
              to-Point Operation over LAN in Link State Routing
              Protocols", RFC 5309, DOI 10.17487/RFC5309, October 2008,

   [RFC5311]  McPherson, D., Ed., Ginsberg, L., Previdi, S., Shand, M.,
              and RFC Publisher, "Simplified Extension of Link State PDU
              (LSP) Space for IS-IS", RFC 5311, DOI 10.17487/RFC5311,
              February 2009, <>.

   [RFC5316]  Chen, M., Zhang, R., Duan, X., and RFC Publisher, "ISIS
              Extensions in Support of Inter-Autonomous System (AS) MPLS
              and GMPLS Traffic Engineering", RFC 5316,
              DOI 10.17487/RFC5316, December 2008,

   [RFC7356]  Ginsberg, L., Previdi, S., Yang, Y., and RFC Publisher,
              "IS-IS Flooding Scope Link State PDUs (LSPs)", RFC 7356,
              DOI 10.17487/RFC7356, September 2014,

   [RFC7981]  Ginsberg, L., Previdi, S., Chen, M., and RFC Publisher,
              "IS-IS Extensions for Advertising Router Information",
              RFC 7981, DOI 10.17487/RFC7981, October 2016,

   [RFC8174]  Leiba, B. and RFC Publisher, "Ambiguity of Uppercase vs
              Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,

4.2.  Informative References

              Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
              Gredler, H., and B. Decraene, "IS-IS Extensions for
              Segment Routing", Work in Progress, Internet-Draft, draft-
              ietf-isis-segment-routing-extensions-25, 19 May 2019,

   [RFC3277]  McPherson, D. and RFC Publisher, "Intermediate System to
              Intermediate System (IS-IS) Transient Blackhole
              Avoidance", RFC 3277, DOI 10.17487/RFC3277, April 2002,

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   [RFC3719]  Parker, J., Ed. and RFC Publisher, "Recommendations for
              Interoperable Networks using Intermediate System to
              Intermediate System (IS-IS)", RFC 3719,
              DOI 10.17487/RFC3719, February 2004,

   [RFC4271]  Rekhter, Y., Ed., Li, T., Ed., Hares, S., Ed., and RFC
              Publisher, "A Border Gateway Protocol 4 (BGP-4)",
              RFC 4271, DOI 10.17487/RFC4271, January 2006,

   [RFC5304]  Li, T., Atkinson, R., and RFC Publisher, "IS-IS
              Cryptographic Authentication", RFC 5304,
              DOI 10.17487/RFC5304, October 2008,

   [RFC5440]  Vasseur, JP., Ed., Le Roux, JL., Ed., and RFC Publisher,
              "Path Computation Element (PCE) Communication Protocol
              (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009,

   [RFC5449]  Baccelli, E., Jacquet, P., Nguyen, D., Clausen, T., and
              RFC Publisher, "OSPF Multipoint Relay (MPR) Extension for
              Ad Hoc Networks", RFC 5449, DOI 10.17487/RFC5449, February
              2009, <>.

   [RFC5614]  Ogier, R., Spagnolo, P., and RFC Publisher, "Mobile Ad Hoc
              Network (MANET) Extension of OSPF Using Connected
              Dominating Set (CDS) Flooding", RFC 5614,
              DOI 10.17487/RFC5614, August 2009,

   [RFC5820]  Roy, A., Ed., Chandra, M., Ed., and RFC Publisher,
              "Extensions to OSPF to Support Mobile Ad Hoc Networking",
              RFC 5820, DOI 10.17487/RFC5820, March 2010,

   [RFC5837]  Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
              N., Rivers, JR., and RFC Publisher, "Extending ICMP for
              Interface and Next-Hop Identification", RFC 5837,
              DOI 10.17487/RFC5837, April 2010,

   [RFC6232]  Wei, F., Qin, Y., Li, Z., Li, T., Dong, J., and RFC
              Publisher, "Purge Originator Identification TLV for IS-
              IS", RFC 6232, DOI 10.17487/RFC6232, May 2011,

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   [RFC7182]  Herberg, U., Clausen, T., Dearlove, C., and RFC Publisher,
              "Integrity Check Value and Timestamp TLV Definitions for
              Mobile Ad Hoc Networks (MANETs)", RFC 7182,
              DOI 10.17487/RFC7182, April 2014,

   [RFC7921]  Atlas, A., Halpern, J., Hares, S., Ward, D., Nadeau, T.,
              and RFC Publisher, "An Architecture for the Interface to
              the Routing System", RFC 7921, DOI 10.17487/RFC7921, June
              2016, <>.

Authors' Addresses

   Russ White
   Juniper Networks

   Shraddha Hegde
   Juniper Networks

   Tony Przygienda
   Juniper Networks

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