Internet DRAFT - draft-white-openfabric


Network Working Group                                      R. White, Ed.
Internet-Draft                                             S. Zandi, Ed.
Intended status: Informational                                  LinkedIn
Expires: May 9, 2019                                    November 5, 2018

                      IS-IS Support for Openfabric


   Spine and leaf topologies are widely used in hyperscale and cloud
   scale networks.  In most of these networks, configuration is
   automated, but difficult, and topology information is extracted
   through broad based connections.  Policy is often integrated into the
   control plane, as well, making configuration, management, and
   troubleshooting difficult.  Openfabric is an adaptation of an
   existing, widely deployed link state protocol, Intermediate System to
   Intermediate System (IS-IS) that is designed to:

   o  Provide a full view of the topology from a single point in the
      network to simplify operations

   o  Minimize configuration of each Intermediate System (IS) (also
      called a router or switch) in the network

   o  Optimize the operation of IS-IS within a spine and leaf fabric to
      enable scaling

   This document begins with an overview of openfabric, including a
   description of what may be removed from IS-IS to enable scaling.  The
   document then describes an optimized adjacency formation process; an
   optimized flooding scheme; some thoughts on the operation of
   openfabric, metrics, and aggregation; and finally a description of
   the changes to the IS-IS protocol required for openfabric.

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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   and may be updated, replaced, or obsoleted by other documents at any

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   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 May 9, 2019.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Contributors  . . . . . . . . . . . . . . . . . . . . . .   3
     1.3.  Simplification  . . . . . . . . . . . . . . . . . . . . .   3
     1.4.  Additions and Requirements  . . . . . . . . . . . . . . .   4
     1.5.  Sample Network  . . . . . . . . . . . . . . . . . . . . .   4
   2.  Modified Adjacency Formation  . . . . . . . . . . . . . . . .   6
     2.1.  Level 2 Adjacencies Only  . . . . . . . . . . . . . . . .   6
     2.2.  Point-to-point Adjacencies  . . . . . . . . . . . . . . .   6
     2.3.  Three Way Handshake Support . . . . . . . . . . . . . . .   7
     2.4.  Adjacency Formation Optimization  . . . . . . . . . . . .   7
   3.  Advertisement of Reachability Information . . . . . . . . . .   8
   4.  Determining and Advertising Location on the Fabric  . . . . .   9
   5.  Flooding Optimization . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Flooding Failures . . . . . . . . . . . . . . . . . . . .  11
   6.  Other Optimizations . . . . . . . . . . . . . . . . . . . . .  12
     6.1.  Transit Link Reachability . . . . . . . . . . . . . . . .  12
     6.2.  Transiting T0 Intermediate Systems  . . . . . . . . . . .  12
   7.  Openfabric and Route Aggregation  . . . . . . . . . . . . . .  13
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  15
   Appendix A.  Flooding Optimization Operation  . . . . . . . . . .  17
   Appendix B.  Fabric Location Calculation  . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

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1.  Introduction

1.1.  Goals

   Spine and leaf fabrics are often used in large scale data centers; in
   this application, they are commonly called a fabric because of their
   regular structure and predictable forwarding and convergence
   properties.  This document describes modifications to the IS-IS
   protocol to enable it to run efficiently on a large scale spine and
   leaf fabric, openfabric.  The goals of this control plane are:

   o  Provide a full view of the topology from a single point in the
      network to simplify operations

   o  Minimize configuration of each IS in the network

   o  Optimize the operation of IS-IS within a spine and leaf fabric to
      enable scaling

1.2.  Contributors

   The following people have contributed to this draft: Nikos
   Triantafillis (reflected flooding optimization), Ivan Pepelnjak
   (fabric locality calculation modifications), Christian Franke (fabric
   localigy calculation modification), Hannes Gredler (do not reflood
   optimizations), Les Ginsberg (capabilities encoding, circuit local
   reflooding), Naiming Shen (capabilities encoding, circuit local
   reflooding), Uma Chunduri (failure mode suggestions, flooding), Nick
   Russo, and Rodny Molina.

   See [RFC5449], [RFC5614], and [RFC7182] for similar solutions in the
   Mobile Ad Hoc Networking (MANET) solution space.

1.3.  Simplification

   In building any scalable system, it is often best to begin by
   removing what is not needed.  In this spirit, openfabric
   implementations MAY remove the following from IS-IS:

   o  External metrics.  There is no need for external metrics in large
      scale spine and leaf fabrics; it is assumed that metrics will be
      properly configured by the operator to account for the correct
      order of route preference at any route redistribution point.

   o  Tags and traffic engineering processing.  Openfabric is only
      designed to provide topology and reachability information.  It is
      not designed to provide for traffic engineering, route preference
      through tags, or other policy mechanisms.  It is assumed that all

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      routing policy will be provided through an overlay system which
      communicates directly with each IS in the fabric, such as PCEP
      [RFC5440] or I2RS [RFC7921].  Traffic engineering is assumed to be
      provided through Segment Routing (SR)

1.4.  Additions and Requirements

   To create a scalable link state fabric, openfabric includes the

   o  A slightly modified adjacency formation process.

   o  Mechanisms for determining which tier within a spine and leaf
      fabric in which the IS is located.

   o  A mechanism that reduces flooding to the minimum possible, while
      still ensuring complete database synchronization among the
      intermediate systems within the fabric.

   Three general requirements are placed here; more specific
   requirements are considered in the following sections.  Openfabric

   o  MUST support [RFC5301] and enable hostname advertisement by
      default if a hostname is configured on the intermediate system.

   o  SHOULD support [RFC6232], purge originator identification for IS-

   o  MUST NOT be mixed with standard IS-IS implementations in
      operational deployments.  Openfabric and standard IS-IS
      implementations SHOULD be treated as two separate protocols.

1.5.  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

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

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

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

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

   o  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 the fabric itself.
   Note there are no "cross links," or "east west" links in the
   illustrated fabric.  The fabric locality detection mechanism
   described here will not work if there are cross links running east/

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   west through the fabric.  Locality detection may be possible in such
   a fabric; this is an area for further study.

2.  Modified Adjacency Formation

   Because Openfabric operates in a tightly controlled data center
   environment, various modifications can be made to the IS-IS neighbor
   formation process to increase efficencicy and simplify the protocol.
   Specifically, Openfabric implementations SHOULD support [RFC3719],
   section 4, hello padding for IS-IS.  Variable hello padding SHOULD
   NOT be used, as data center fabrics are built using high speed links
   on which padded hellos will have little performance impact.  Further
   modifications to the neighbor formation process are considered in the
   following sections.

2.1.  Level 2 Adjacencies Only

   Openfabric is designed to work in a single flooding domain over a
   single data center fabric at the scale of thousands of routers with
   hundreds of thousands of routes (so a moderate scale in router and
   route count terms).  Because of the way Openfabric optimizes
   operation in this environment, it is not necessary nor desirable to
   build multiple flooding domains.  For instance, the flooding
   optimizations described later this document require a full view of
   the topology, as does any proposed overlay to inject policy into the
   forwarding plane.  In light of this, the following changes SHOULD BE
   to IS-IS implemetations to support Openfabric:

   o  IIH PDU 17 (level 2 point-to-point circuit hello) should be the
      only IIH PDU type transmitted (see section 9.7 of ISO 10589)

   o  In IIH PDU 17 (level 2 point-to-point circuit hello), the Circuit
      Type field should be set to 2 (see section 9.7 of ISO 10589)

   o  Support for IIH PDU 15 (level 1 broadcast hello) should be removed
      (see section 9.5 of ISO 10589)

   o  Support for IIH PDU 16 (level 2 broadcast hello) should be removed
      (see section 9.6 of ISO 10589)

2.2.  Point-to-point Adjacencies

   Data center network fabrics only contain point-to-point links;
   because of this, there is no reason to support any broadcast link
   types, nor to support the Designated Intermediate System processing,
   including pseudonode creation.  In light ot his, processing related
   to sections 7.2.3 (broadcast networks), 7.3.8 (generation of level 1
   pseudonode LSPs), 7.3.10 (generation of level 2 pseudonode LSPs), and

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   section 8.4.5 (LAN designated intermediate systems) in [ISO10589]
   SHOULD BE removed.

2.3.  Three Way Handshake Support

   It is important that two way connectivity be established before
   synchronizing the link state database, or routing through a link in a
   data center fabric.  To reject optical failures that cause a one way
   connection between two routers, fabricDC must support the three way
   handshake mechanism described in [RFC5303].

2.4.  Adjacency Formation Optimization

   While adjacency formation is not considered particularly burdensome
   in IS-IS, it may still be useful to reduce the amount of state
   transferred across the network when connecting a new IS to the
   fabric.  In its simplest form, the process is:

   o  An IS connected to the fabric will send hellos on all links.

   o  The IS will only complete the three-way handshake with one newly
      discovered neighbor; this would normally be the first neighbor
      which sends the newly connected intermediate system's ID back in
      the three-way handshake process.

   o  The IS will complete its database exchange with this one newly
      adjacent neighbor.

   o  Once this process is completed, the IS will continue processing
      the remaining neighbors as normal.

   o  If synchronization is not achieved within twice the dead timer on
      the local interface, the newly connected IS will repeat this
      process with the second neighbor with which it forms a three-way

   This process allows each IS newly added to the fabric to exchange a
   full table once; a very minimal amount of information will be
   transferred with the remaining neighbors to reach full

   Any such optimization is bound to present a tradeoff between several
   factors; the mechanism described here increases the amount of time
   required to form adjacencies slightly in order to reduce the total
   state carried across the network.  An alternative mechanism could
   provide a better balance of the amount of information carried across
   the network for initial synchronization and the time required to
   synchronize a new IS.  For instance, an IS could choose to

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   synchronize its database with two or three adjacent intermediate
   systems, which could speed the synchronization process up at the cost
   of carrying additional data on the network.  A locally determined
   balance between the speed of synchronization and the amount of data
   carried on the network can be acheived by adjusting the number of
   adjacent intermediate systems the newly attached IS synchronizes

3.  Advertisement of Reachability Information

   IS-IS describes the topology in two different sets of TLVs; the first
   describes the set of neighbors connected to an IS, the second
   describes the set of reachable destination connected to an IS.  There
   are two different forms of both of these descriptions, one of which
   carries what are widely called narrow metrics, the other of which
   carries what are widely called wide metrics.  In a tightly controlled
   data center fabric implementation, such as the ones Openfabric is
   designed to support, no IS that supports narrow metrics will ever be
   deployed or supported; hence there is no reason to support any metric
   type other than wide metrics.

   o  The Level 2 Link State PDU (type 20 in section 9.9 of [ISO10589])
      and the scoped flooding PDU (type 10 in section 3.1 of [RFC7356])
      SHOULD BE the only PDU types used to carry link state information
      in a Openfabric implementation

   o  Processing related to the Level 1 Link State PDU (type 18) MAY BE
      removed from Openfabric implementations (see section 9.8 of

   o  Neighbor reachability MUST BE carried in TLV type 22 (see section
      3 of [RFC5305])

   o  IPv4 reachability SHOULD BE carried in TLV type 135 (see section 4
      of [RFC5305]), or TLV type 235 for multitopology implementations
      (see [RFC5120])

   o  IPv6 reachability SHOULD BE carried in TLV type 236 (see
      [RFC5308]), or TLV type 237 for multitopology implemenations (see

   o  Processing related to the neighbor reachability TLV (type 2, see
      sections 9.8 and 9.9 of [ISO10589]) SHOULD BE removed

   o  Processing related to the narrow metric IP reachability TLV (types
      128 and 130) SHOULD BE removed

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   Further, if segment routing support is desired, Openfabric MAY
   support the Prefix Segment Identifier sub-TLV and other TLVs as
   required in [I-D.ietf-isis-segment-routing-extensions].

4.  Determining and Advertising Location on the Fabric

   The tier to which a IS is connected is useful to enable
   autoconfiguration of intermediate systems connected to the fabric and
   to reduce flooding.  Once the tier of an intermediate system within
   the fabric has been determined, it MUST be advertised using the 4 bit
   Tier field described in section 3.3 of
   [I-D.shen-isis-spine-leaf-ext].  This section describes a method of
   calculating the tier number, assuming the tier numbers rise in value
   from the edge of the fabric.

   This method begins with two of the T0 intermediate systems
   advertising their location in the fabric.  This information can
   either be obtained through:

   o  Two T0 intermediate systems are manually configured to advertise
      0x00 in their IS reachability tier sub-TLV, indicating they are at
      the edge of the fabric (a ToR IS).

   o  The T0 intermediate systems detect they are T0 through the
      presence connected hosts (i.e. through a request for address
      assignment or some other means).  If such detection is used, and
      the IS determines it is located at T0, it should advertise 0x00 in
      its IS reachability tier sub-TLV.

   If the first method is used, the two T0 routers MUST be "maximally
   separated" on the fabric.  They must be a maximal number of hops
   apart, or rather thay MUST NOT be connected to the same T1 device as
   their "upstream" towards the superspines in a 5 ary fabric.

   The second method above SHOULD be used with care, as it may not be
   secure, and it may not work in all data center environments.  For
   instance, if a host is mistakenly (or intentionally, as a form of
   attack) attached to a spine IS, or a request for address assignment
   is transmitted to a spine IS during the bootup phase of the device or
   fabric, it is possible to cause a spine IS to advertise itself as a
   T0.  Unless the autodetection of the T0 devices is secured, the
   manual mechanism SHOULD BE used (configuring at least one T0 device

   Given the correct configuration of two T0 devices, maximally spaced
   on the fabric, the remaining intermediate systems calculate their
   tier number as follows:

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   o  The local IS calculates an SPT (using SPF) setting the cost of
      every link to 1; this effectively calculates a topology only view
      of the network, without considering any configured link costs

   o  Ensure that at least two T0 are in the calculated SPT; otherwise

   o  Find the furthest T0; call this node A and set LD to the cost; the
      "farthest T0" is the T0 with the largest metric, or the farthest
      distance from the local calculating node

   o  Calculate an SPT (using SPF) from the perspective of A (above)
      setting the cost of every link to 1

   o  Find the furthest IS in A's SPT; call this node B and set RD to
      the cost from A to B

   o  Calculate the tier number of the local IS by subtracting LD from

   In the example network, assume 5A and 1C are manually configured as a
   T0, and are advertising their tier numbers.  From here:

   o  From 1A the path to 5A is 4 hops; this is LD

   o  Run SPF from the perspective of 5A with all link metrics set to 1

   o  From 5A the path length to 1C is 4; this is RD

   o  RD - LD is 0 at 1A, so 1A is T0, or a ToR

   This process will work for any spine and leaf fabric without "cross

5.  Flooding Optimization

   Flooding is perhaps the most challenging scaling issue for a link
   state protocol running on a dense, large scale fabric.  To reduce the
   flooding of link state information in the form of Link State Protocol
   Data Units (LSPs), Openfabric takes advantage of information already
   available in the link state protocol, the list of the local
   intermediate system's neighbor's neighbors, and the fabric locality
   computed above.  The following tables are required to compute a set
   of reflooders:

   o  Neighbor List (NL) list: The set of neighbors

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   o  Neighbor's Neighbors (NN) list: The set of neighbor's neighbors;
      this can be calculated by running SPF truncated to two hops

   o  Do Not Reflood (DNR) list: The set of neighbors who should have
      LSPs (or fragments) who should not reflood LSPs

   o  Reflood (RF) list: The set of neighbors who should flood LSPs (or
      fragments) to their adjacent neighbors to ensure synchronization

   NL is set to contain all neighbors, and sorted deterministically (for
   instance, from the highest IS identifier to the lowest).  All
   intermediate systems within a single fabric SHOULD use the same
   mechanism for sorting the NL list.  NN is set to contain all
   neighbor's neighbors, or all intermediate systems that are two hops
   away, as determined by performing a truncated SPF.  The DNR and RF
   tables are initially empty.  To begin, the following steps are taken
   to reduce the size of NN and NL:

   o  Move any IS in NL with its tier (or fabric location) set to T0 to

   o  Remove all intermediate systems from NL and NN that in the
      shortest path to the IS that originated the LSP

   Then, for every IS in NL:

   o  If the current entry in NL is connected to any entries in NN:

      *  Move the IS to RF

      *  Remove the intermediate systems connected to the IS from NN

   o  Else move the IS to DNR

   The calculation terminates when the NL is empty.

   When flooding, LSPs transmitted to adjacent neighbors on the RF list
   will be transmitted normally.  Adjacent intermediate systems on this
   list will reflood received LSPs into the next stage of the topology,
   ensuring database synchronization.  LSPs transmitted to adjacent
   neighbors on the DNR list, however, MUST be transmitted using a
   circuit scope PDU as described in [RFC7356].

5.1.  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

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   which it should be reflooding, it is possible an LSP is only
   partially flooded through the fabric.  To prevent such situations,
   any IS receiving an LSP transmitted using DNR SHOULD:

   o  Set a short timer; the default should be less than one second

   o  When the timer expires, send a Complete Sequence Number Packet
      (CSNP) to all neighbors

   o  Process any Partial Sequence Number Packets (PSNPs) as required to

   o  If a resynchronization is required, notify the network operator
      through a network management system

6.  Other Optimizations

6.1.  Transit Link Reachability

   In order to reduce the amount of control plane state carried on large
   scale spine and leaf fabrics, openfabric implementations SHOULD NOT
   advertise reachability for transit links.  These links MAY remain
   unnumbered, as IS-IS does not require layer 3 IP addresses to
   operate.  Each IS SHOULD be configured with a single loopback
   address, which is assigned an IPv6 address, to provide reachability
   to intermediate systems which make up the fabric.

   [RFC3277] SHOULD be supported on devices supporting openfabric with
   unnumbered interface in order to support traceability and network

6.2.  Transiting T0 Intermediate Systems

   In data center fabrics, ToR intermediate systems SHOULD NOT be used
   to transit between two T1 (or above) spine intermediate systems.  The
   simplest way to prevent this is to set the overload bit [RFC3277] for
   all the LSPs originated from T0 intermediate systems.  However, this
   solution would have the unfortunate side effect of causing all
   reachability beyond any T0 IS to have the same metric, and many
   implementations treat a set overload bit as a metric of 0xFFFF in
   calculating the Shortest Path Tree (SPT).  This document proposes an
   alternate solution which preserves the leaf node metric, while still
   avoiding transiting T0 intermediate systems.

   Specifically, all T0 intermediate systems SHOULD advertise their
   metric to reach any T1 adjacent neighbor with a cost of 0XFFE.  T1
   intermediate systems, on the other hand, will advertise T0
   intermediate systems with the actual interface cost used to reach the

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   T0 IS.  Hence, links connecting T0 and T1 intermediate systems will
   be advertised with an asymmetric cost that discourages transiting T0
   intermediate systems, while leaving reachability to the destinations
   attached to T0 devices the same.

7.  Openfabric and Route Aggregation

   While schemes may be designed so reachability information can be
   aggregated in Openfabric deployments, this is not a recommended

8.  Security Considerations

   This document outlines modifications to the IS-IS protocol for
   operation on large scale data center fabrics.  While it does add new
   TLVs, and some local processing changes, it does not add any new
   security vulnerabilities to the operation of IS-IS.  However,
   openfabric 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.

   If T0 intermediate systems are auto-detected using information
   outside Openfabric, it is possible to attack the calucations used for
   flooding reduction and auto-configuration of intermediate systems.
   For instance, if a request for an address pool is used as an
   indicator of an attached host, and hence receiving such a request
   causes an intermediate system to advertise itself as T0, it is
   possible for an attacker (or a simple mistake) to cause auto-
   configuration to fail.  Any such auto-detection mechanims SHOULD BE
   secured using appropriate techniques, as described by any protocols
   or mechanisms used.

9.  References

9.1.  Normative References

              Shen, N., Ginsberg, L., and S. Thyamagundalu, "IS-IS
              Routing for Spine-Leaf Topology", draft-shen-isis-spine-
              leaf-ext-07 (work in progress), October 2018.

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              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,

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

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "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. and N. Shen, "Dynamic Hostname Exchange
              Mechanism for IS-IS", RFC 5301, DOI 10.17487/RFC5301,
              October 2008, <>.

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

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

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

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

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

   [RFC5316]  Chen, M., Zhang, R., and X. Duan, "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., and Y. Yang, "IS-IS Flooding
              Scope Link State PDUs (LSPs)", RFC 7356,
              DOI 10.17487/RFC7356, September 2014,

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

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

9.2.  Informative References

              Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
              Gredler, H., Litkowski, S., Decraene, B., and J. Tantsura,
              "IS-IS Extensions for Segment Routing", draft-ietf-isis-
              segment-routing-extensions-19 (work in progress), July

              Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", draft-ietf-spring-segment-routing-15 (work
              in progress), January 2018.

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

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

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

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

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

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

   [RFC5614]  Ogier, R. and P. Spagnolo, "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. and M. Chandra, Ed., "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., and JR. Rivers, "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., and J. Dong, "Purge
              Originator Identification TLV for IS-IS", RFC 6232,
              DOI 10.17487/RFC6232, May 2011,

   [RFC7182]  Herberg, U., Clausen, T., and C. Dearlove, "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., and T.
              Nadeau, "An Architecture for the Interface to the Routing
              System", RFC 7921, DOI 10.17487/RFC7921, June 2016,

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Appendix A.  Flooding Optimization Operation

   Recent testing has shown that flooding is largely a "non-issue" in
   terms of scaling when using high speed links connecting intermediate
   systems with reasonable processing power and memory.  However,
   testing has also shown that flooding will impact convergence speed
   even in such environments, and flooding optimization has a major
   impact on the performance of a link state protocol in resource
   constrained environments.  Some thoughts on flooding optimization in
   general, and the flooding optimization contained in this document,

   There are two general classes of flooding optimization available for
   link state protocols.  The first class of optimization relies on a
   centralized service or server to gather the link state information
   and redistribute it back into the intermediate systems making up the
   fabric.  Such solutions are attractive in many, but not all,
   environments; hence these systems compliment, rather than compete
   with, the system described here.  Systems relying on a service or
   server necessarily also rely on connectivity to that service or
   server, either through an out-of-band network or connectivity through
   the fabric itself.  Because of this, these mechanisms do not apply to
   all deployments; some deployments require underlying reachability
   regardless of connectivity to an outside service or server.

   The second possibility is to create a fully distributed system that
   floods the minimal amount of information possible to every
   intermediate system.  The system described in this draft is an
   example of such a system.  Again, there are many ways to accomplish
   this goal, but simplicity is a primary goal of the system described
   in this draft.

   The system described here divides the work into two different parts;
   forward and reverse optimization.  The forward optimization begins by
   finding the set of intermediate systems two hops away from the
   flooding device, and choosing a subset of connected neighbors that
   will successfully reach this entire set of intermediate systems, as
   shown in the diagram below.

   A     B    C--+
   |     |    |  |
   +--D--+    E  H
      |       |  |

                                 Figure 2

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   If F is flooding some piece of information, then it will find the
   entire set of intermediate systems within two hops by discovering its
   neighbors and their neighbors from the local LSDB.  This will include
   A, B, C, D, and E--but not G.  From this set, F can determine that D
   can reach A and B, while a single flood to either E or H will reach
   C.  Hence F can flood to D and either E or H to reach C.  F can
   choose to flood to D and E normally.  Because H still needs to
   receive this new LSP (or fragment!), but does not need to reflood to
   C, F can send the LSP using link local signaling.  In this case, H
   will receive and process the new LSP, but not reflood it.

   Rather than carrying the information necessary through hello
   extensions, as is done in [RFC5820], the neighbors are allowed to
   complete initial synchronization, and then a truncated shortest path
   tree is built to determine the "two hop neighborhood."  This has the
   advantage of using mechanisms already used in IS-IS, rather than
   adding new processes.  The risk with this process is any LSPs flooded
   through the network before this initial calculation takes place will
   be suboptimal.  This "two hop neighborhood" process has been used in
   OSPF deployments for a number of years, and has proven stable in

   Rather than setting a timer for reflooding, the implementation
   described here uses IS-IS' ability to describe the entire database
   using a CSNP to ensure flooding is successful.  This adds some small
   amount of overhead, so there is some balance between optimal flooding
   and ensuring flooding is complete.

   The reverse optimization is simpler.  It relies on the observation
   that any intermediate system between the local IS and the origin of
   the LSP, other than in the case of floods removing an LSP from the
   shared LSDB, should have already received a copy of the LSP.  For
   instance, if F originates an LSP in the figure above, and E refloods
   the LSP to C, C does not need to reflood back to F if F is on its
   shortest path tree towards F.  It is obvious this is not a "perfect"
   optimization.  A perfect optimization would block flooding back along
   a directed acyclic graph towards the originator.  Using the SPT,
   however, is a quick way to reduce flooding without performing more

   The combination of these two optimizations have been seen, in
   testing, to reduce the number of copies any IS receives from the tens
   to precisely one.

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Appendix B.  Fabric Location Calculation

   Determining the location of a device in a symmetric topology is quite
   challenging.  The authors of this draft worked through a number of
   possible solutions to this problem, each of which was found to either
   not work in some topology, or was found to be liable to unacceptable
   errors.  For instance:

   o  Method 1:

      *  Caculate the maximum distance through the fabric, and the
         distance from one of those points to the local intermediate

      *  This works in a five stage Clos spine and leaf, but not in a
         three stage, nor in some other five stage spine and leaf
         fabrics, such as the common butterfly or Benes fabric

   o  Method 2:

      *  Manually mark one edge leaf node in the fabric as T0

      *  Calculate maximum distance through the fabric from this point

      *  Calculate local position based on this maximum distance the
         distance to the single marked device

      *  This works in three and five stage Clod fabrics, but does not
         work from every location in other spine and leaf fabrics, such
         as the common butterfly or Benes fabric

   In the end, marking two devices located as far from one another
   topologically as possible provides the anchor points necessary to
   calculate the total distance through the fabric, and then from those
   points to the location of the calculating device.

   The information obtained in this way can also be combined with other
   forms of location calculation, such as whether a device requesting an
   address through some mechanism is attached to the local device, or
   other indications of fabric locality.  It generally true that having
   more than one method to determine fabric location will be better than
   any single method to account for errors, failures, and other problems
   that can arise with any mechanism.

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Authors' Addresses

   Russ White (editor)


   Shawn Zandi (editor)


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