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Versions: 00 01 02 03 04 05                                             
Networking Working Group                                     L. Ginsberg
Internet-Draft                                                 P. Psenak
Intended status: Informational                                 A. Lindem
Expires: May 24, 2020                                      Cisco Systems
                                                       November 21, 2019

                  IS-IS Flooding Scale Considerations


   Link State PDU flooding rates in use are much slower than what modern
   networks can support.  The use of IS-IS at larger scale requires
   faster flooding rates to achieve desired convergence goals.  This
   document discusses issues associated with increasing flooding rates
   and some recommended practices which allow faster flooding rates to
   be used safely.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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-
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 24, 2020.

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Copyright Notice

   Copyright (c) 2019 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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   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.  Historical Behavior . . . . . . . . . . . . . . . . . . . . .   3
   3.  Flooding Rate and Convergence . . . . . . . . . . . . . . . .   4
     3.1.  Flow Control  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Bandwidth Utilization . . . . . . . . . . . . . . . . . .   6
     3.3.  Packet Prioritization on Receive  . . . . . . . . . . . .   7
   4.  Minimizing LSP Generation . . . . . . . . . . . . . . . . . .   7
   5.  Redundant Flooding  . . . . . . . . . . . . . . . . . . . . .   9
   6.  Use of Jumbo Frames . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Link state IGPs such as Intermediate-System-to-Intermediate-System
   (IS-IS) depend upon having consistent Link State Databases (LSDB) on
   all Intermediate Systems (ISs) in the network in order to provide
   correct forwarding of data packets.  When topology changes occur,
   new/updated Link State PDUs (LSPs) are propagated network-wide.  The
   speed of propagation is a key contributor to convergence time.

   Historically, flooding rates have been conservative - on the order of
   10s of LSPs/second.  This derives from guidance in the base
   specification [ISO10589] and early deployments when both CPU speeds
   and interface speeds were much slower than they are today and the
   scale of an IS-IS area was smaller than it may be today.

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   As IS-IS is deployed in greater scale (larger number of nodes in an
   area and larger number of neighbors/node) the impact of the historic
   flooding rates becomes more significant.  Consider the bringup or
   failure of a node with 1000 neighbors.  This will result in a minimum
   of 1000 LSP updates.  At a typical LSP flooding rate used in many
   deployments today (33 LSPs/second) it would take 30+ seconds simply
   to send the updated LSPs to a given neighbor.  Depending on the
   diameter of the network, achieving a consistent LSDB on all nodes in
   the network could easily take a minute (or more).

   Increasing LSP flooding rate therefore becomes an essential element
   of supporting greater network scale.

   The remainder of this document discusses various aspects of protocol
   operation and how they are impacted by increased flooding rate.
   Where appropriate, best practices are defined which enhance an
   implementation's ability to support faster flooding rates.

2.  Historical Behavior

   The base specification for IS-IS [ISO10589] was first published in
   1992 and updated in 2002.  The update made no changes in regards to
   suggested timer values.  Convergence targets at the time were on the
   order of seconds and the timer values specified reflect that.  Here
   are some examples:

     minimumLSPGenerationInterval - This is the minimum time interval
        between generation of Link State PDUs. A source Intermediate
        system shall wait at least this long before re-generating one
        of its own Link State PDUs.
     The recommended value was 30 seconds.

     minimumLSPTransmissionInterval - This is the amount of time an
        Intermediate system shall wait before further propagating
        another Link State PDU from the same source system.
     The recommended value was 5 seconds.

     partialSNPInterval - This is the amount of time between periodic
        action for transmission of  Partial Sequence Number PDUs.
        It shall be less than minimumLSPTransmission-Interval.
     The recommend value was 2 seconds.

   Most relevant to a discussion of LSP flooding rate is the recommended
   interval between the transmission of two different LSPs on a given

   For broadcast interfaces, [ISO10589] defined:

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     minimumBroadcastLSPTransmissionInterval - the minimum interval
        between PDU arrivals which can be processed by the slowest
        Intermediate System on the LAN.
     The default value was defined as 33 milliseconds.
     NOTE: It was permitted to send multiple LSPs "back-to-back"
     as a burst, but this was limited to 10 LSPs in a one second

   Although this value was specific to LAN interfaces, this has commonly
   been applied by implementations to all interfaces though that was not
   the original intent of the base specification.  In fact
   Section states:

     On point-to-point links the peak rate of arrival is limited only
     by the speed of the data link and the other traffic flowing on
     that link.

   Although modern implementations have not strictly adhered to the 33
   millisecond interval, it is commonplace for implementations to limit
   flooding rate to an order of magnitude similar to the 33 ms value.

   In the past 20 years, significant work on achieving faster
   convergence - more specifically sub-second convergence - has resulted
   in implementations modifying a number of the above timers in order to
   support faster signaling of topology changes.  For example,
   minimumLSPGenerationInterval has been modified to support millisecond
   intervals - often with a backoff algorithm applied to prevent LSP
   generation storms in the event of a series of rapid oscillations.

   However, flooding rate has not been fundamentally altered.

3.  Flooding Rate and Convergence

   Convergence involves a number of sequential operations.

   First the topology change needs to be detected.  This is a local
   activity occurring only on the node or nodes directly connected to
   the topology change.  The directly connected node(s) then must
   advertise the topology change by updating their LSPs and flooding the
   changed LSPs.  Routers then must process the updated LSDB and
   recalculate paths to affected destinations.  The updated paths must
   then be installed in the forwarding plane.

   Only when all of the steps are completed on all nodes in the network
   has the network completed convergence.

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   As the convergence requirement is consistency of LSDBs on all nodes
   in the network, it is fundamental to understand that the goal of
   flooding is to update the LSDB on all nodes in the network "as fast
   as possible".  Controling the rate of flooding per interface is done
   to address some practical limitations which include:

   o  Fairness to other data and control traffic on the same interface

   o  Limitations on the processing rate of incoming control traffic

   However, intentionally using different flooding rates on different
   interfaces increases the possibility of longer periods of LSDB
   inconsistency which in turn delays network wide convergence.

   Many implementations provide knobs to control the rate of LSP
   flooding on a per interface basis.  To the extent that this serves as
   a flow control mechanism, this may reduce the number of dropped LSPs
   during high activity bursts and thereby reduce the number of LSP
   retransmissions required.  As LSP retransimssion timers are typically
   long (multiple seconds), this may result in shorter convergence times
   than if the LSP burst was uncontrolled.  But if the performance
   characteristics of routers in the network are such that some routers
   consistently accept and process fewer LSPs/second than other routers,
   convergence will be degraded.  Tuning LSP transmission timers on a
   per interface basis will never provide optimal convergence.
   Consistent flooding rates should be used on all interfaces.

3.1.  Flow Control

   In large scale deployments where an increased flooding rate is being
   used, it becomes more likely that a burst of LSPs may temporarily
   overwhelm a receiver.  Normal operation of the Update Process will
   recover from this, but it may well make sense to employ some form of
   flow control.  This will not serve to optimize convergence, but it
   can serve to reduce the number of LSP retransmissions.  As
   retransmissions are deliberately done at a slow rate, the result of
   flow control will be to provide a shorter recovery time from a
   transient condition which causes a node to be unable to handle the
   targeted rate of LSP transmission.  Inability to handle LSP reception
   at the targeted flooding rate should be viewed as an error condition
   which should be reported.  If this condition persists, it indicates
   that the network is provisioned in a way which does not support
   optimal convergence.  Steps need to be taken to resolve this issue.
   Such steps could involve upgrading the routers that demonstrate this
   condition consistently, altering the configuration on the problematic
   routers or altering the position of the problematic routers in the
   network so as to reduce the overall load on those routers, or
   reducing the LSP transmission rate network-wide.

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   When flow control is necessary, it can be implemented in a
   straightforward manner based on current transmission queue length.
   Presented below is one possible algorithm for point-to-point

   MaxLSPTx = maximum # LSPs transmitted/second/interface
   Umax = Maximum Unacknowledged LSP/Interface
   Usafe = Safe level of Unacknowledged LSP/Interface
   U(i) = # of unacknowledged LSPs previously transmitted/interface
   LSPTx(i) = max # LSPs transmitted/second for a given interface

   Initialize LSPTx(i) = MaxLSPTx for all interfaces

   On a given interface when U(i) >= Umax
     Generate a log
     Only retransmissions of unacknowledged LSPs are performed
     Temporarily set LSPTx(i) = MaxLSPTx/2

     For each second U(i) >= Usafe
     further reduce LSPTx(i) = LSPTx(i)/2

     When U(i) <= Usafe
      restore LSPTx(i) = MaxLSPTx
      New LSPs may be transmitted

3.2.  Bandwidth Utilization

   Routing protocol traffic has to share bandwidth on a link with other
   control traffic and data traffic.  During periods of instability,
   routing protocol traffic will increase, but it is still desirable
   that the maximum bandwidth consumption by routing protocol traffic be
   modest.  This needs to be considered when setting IS-IS flooding

   If we assume a maximum size of 1492 bytes for an LSP, here are some
   rough estimates of bandwidth consumption at different flooding rates:

   | LSPs/second  | 100 Mb Link    | 1 Gb Link   |
   |   100        |    1.2 %       |  0.1 %      |
   |   500        |    6.1 %       |  0.6 %      |
   |  1000        |    12.1 %      |  1.2 %      |

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3.3.  Packet Prioritization on Receive

   There are three classes of PDUs sent by IS-IS:

   o  Hellos

   o  LSPs

   o  Complete Sequence Number PDUs (CSNPs) and Partial Sequence Number
      PDUs (PSNPs)

   Implementations today may prioritize the reception of Hellos over
   LSPs and SNPs in order to prevent a burst of LSP updates from
   triggering an adjacency timeout which in turn would require
   additional LSPs to be updated.

   SNPs serve to acknowledge or trigger the transmission of specified
   LSPs.  On a point-to-point link, PSNPs acknowledge the receipt of one
   or more LSPs.  Because PSNPs (like all IS-IS PDUs) use TLVs in the
   body, it is possible to acknowledge multiple LSPs using a single
   PSNP.  For this reason, [ISO10589] specifies a delay
   (partialSNPInterval) before sending a PSNP so that the number of
   PSNPs required to be sent is reduced.  On receipt of a PSNP, the set
   of LSPs acknowledged by that PSNP can be marked so that they do not
   need to be retransmitted.

   If a PSNP is dropped on receive, this has a significant impact as the
   set of LSPs advertised in the PSNP cannot be marked as acknowledged
   and this results in needless retransmissions which may further delay
   transmission of other LSPs which have yet to be transmitted.  It may
   also make it more likely that a receiver becomes overwhelmed by LSP

   It is therefore recommended that implementations prioritize the
   receipt of SNPs over LSPs.

4.  Minimizing LSP Generation

   In IS-IS the unit of flooding is an LSP.  Each router may generate a
   set of LSPs at each supported level.  Each LSP in the set has an LSP
   number - which is a value from 0-N where N = 255 for the base
   protocol.  (N has been extended to 65535 by [RFC7356].)  Each LSP
   carries network information using defined Type/Length/Value (TLV)
   tuples.  For example, some TLVs carry neighbor information and some
   TLVs carry reachable prefix information.  [ISO10589] strongly
   recommends preserving the association of a given advertisement (such
   as a neighbor) with a specific LSP whenever possible.  This minimizes
   the number of LSPs which need to be regenerated when a topology

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   change occurs.  This recommendation becomes even more important as
   the scale of the network increases.

   Consider the following example;

   Node A has 11 neighbors currently in the UP state and is advertising
   them in three LSPs with content as follows:

   A.00-00 contains the following advertisements
      Neighbor 1
      Neighbor 2
      Neighbor 3
      Neighbor 4
      Neighbor 5
   A.00-01 contains the following advertisements:
      Neighbor 6
      Neighbor 7
      Neighbor 8
      Neighbor 9
      Neighbor 10
   A.00-02 contains the following advertisements
      Neighbor 11

   Imagine that the adjacency to Neighbor 3 goes down.  There are (at
   least) two ways that A could update its LSPs.

   Method 1: A removes the neighbor advertisement for neighbor 3 from
   A.00-00 and sends an update for that LSP.  LSPs #1 and #2 are
   unchanged and so do not have to be flooded.

   Method 2: A attempts to reduce the number of LSPs currently active
   and updates the content as follows:

   A.00-00 contains the following advertisements
      Neighbor 1
      Neighbor 2
      Neighbor 4
      Neighbor 5
      Neighbor 6
   A.00-01 contains the following advertisements:
      Neighbor 7
      Neighbor 8
      Neighbor 9
      Neighbor 10
      Neighbor 11
   A.00-02 becomes empty

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   Niode A now has to flood all three LSPs.  LSPs #0 and #1 are
   reflooded because their content has changed.  LSP #2 is purged.

   In a large scale network, the impact of using Method #2 becomes
   significant and introduces conditions where a much larger number of
   LSPs need to be flooded than is the case with Method #1.

   In order to operate at scale, implementations need to follow the
   guidance in [ISO10589] and use Method #1 whenever possible.

5.  Redundant Flooding

   Default operation of the Update Process is to flood on all
   interfaces.  In cases where a network is highly meshed, this can
   result in a significant amount of redundant flooding.  Nodes will
   receive multiple copies of each updated LSP.

   There are defined mechanisms which can greatly reduce the redundant
   flooding.  These include:

   o  Mesh Groups ( [RFC2973] )

   o  Dynamic Flooding ( [I-D.ietf-lsr-dynamic-flooding] )

6.  Use of Jumbo Frames

   The maximum size of an LSP (LSPBufferSize) is a parameter that needs
   to be set consistently network wide.  This is because IS-IS does not
   support fragmentation of its PDUs - so in order for network wide
   flooding of an LSP to be successful all routers must restrict their
   LSP size to a size which all interfaces on which IS-IS operates can
   support without fragmentation.

   In networks where all interfaces on which IS-IS operates support
   large frames, LSPBufferSize may be set to a larger value than the
   default (1492).  This allows more routing information to be encoded
   in a single LSP, which means that fewer LSPs are required to make up
   the LSDB and therefore the number of LSPs which need to be flooded
   can be reduced in some scenarios (e.g., node or interface bringup).

7.  IANA Considerations

   This document requires no actions by IANA.

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8.  Security Considerations

   Security concerns for IS-IS are addressed in [ISO10589, [RFC5304],
   and [RFC5310].

9.  Acknowledgements

   Thanks to Bruno Decraene for his careful review and insightful

10.  References

10.1.  Normative References

              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)", ISO/
              IEC 10589:2002, Second Edition, Nov 2002.

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

   [RFC2973]  Balay, R., Katz, D., and J. Parker, "IS-IS Mesh Groups",
              RFC 2973, DOI 10.17487/RFC2973, October 2000,

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

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10.2.  Informative References

              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-03 (work in progress), June 2019.

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

Authors' Addresses

   Les Ginsberg
   Cisco Systems
   821 Alder Drive
   Milpitas, CA  95035

   Email: ginsberg@cisco.com

   Peter Psenak
   Cisco Systems
   Apollo Business Center Mlynske nivy 43
   Bratislava  821 09

   Email: ppsenak@cisco.com

   Acee Lindem
   Cisco Systems
   301 Midenhall Way
   Cary, NC  27513

   Email: acee@cisco.com

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