draft-przygienda-rift-dragonfly++.txt

Network Working Group                                 A. Przygienda, Ed.
Internet-Draft                                                   Juniper
Intended status: Experimental                             1 January 2024
Expires: 4 July 2024


                      RIFT in Dragonfly Topologies
                   draft-przygienda-rift-dragonfly-01

Abstract

   RIFT extensions for dragonfly topologies.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "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-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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 4 July 2024.

Copyright Notice

   Copyright (c) 2024 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 to this document.  Code Components



Przygienda                 Expires 4 July 2024                  [Page 1]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


   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
   2.  Glossary  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Horizontal Link Behavior at ToF Level . . . . . . . . . . . .   5
   4.  First Route Computation Change  . . . . . . . . . . . . . . .   6
     4.1.  Additional Bi-Sectional Bandwidth Route Computation
           Change  . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Dragonfly with Multi-Plane CLOS Fabrics . . . . . . . . .   6
   5.  Forwarding Considerations . . . . . . . . . . . . . . . . . .   9
   6.  Partitioning of inter Fabric Planes . . . . . . . . . . . . .   9
   7.  Specification . . . . . . . . . . . . . . . . . . . . . . . .   9
   8.  Summary Overview  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  10
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     12.1.  Informative References . . . . . . . . . . . . . . . . .  10
     12.2.  Normative References . . . . . . . . . . . . . . . . . .  10
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   RIFT today is standardized to deal with CLOS variant fabrics with
   some horizontal link exceptions.  Given that interconnecting multiple
   CLOS via a dragonfly variant is an interesting topology (whether it's
   a full mesh or some kind of non-completely meshed regular lattice)
   this document addresses the resulting changes necessary to base RIFT
   specification to support dragonfly interconnected CLOS fabrics.  The
   reader is advised that due to complexity of figures involved the
   ASCII version of the document leaves those out.
















Przygienda                 Expires 4 July 2024                  [Page 2]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


     +-----+   +-----+                              +-----+   +-----+
     | LA1 |   |     |                              |     |   |     |
     +-----+   +-----+                              +-----+   +-----+
        |   \ /   |                                    |   \ /   |
        |    X    |                                    |    X    |
        |   / \   |                                    |   / \   |
     +-----+   +-----+                              +-----+   +-----+
     | SA1 |   | SA2 |                              | SC1 |   | SC2 |
     +-----+   +-----+                              +-----+   +-----+
       | |       | |                                 | |        | |
       | +-------|-|----------------------+          | |        | |
       +---------|-|----------------------+----------+ |        | |
                 | |                      | +----------+        | |
                 | +----------------------|-|-------------------+ |
                 +--------------+ +-------|-|---------------------+
                                | |       | |
                              +-----+   +-----+
                              | SB2 |   | SB1 |
                              +-----+   +-----+
                                     \ /
                                      X
                                     / \
                              +-----+   +-----+
                              |     |   | LB1 |
                              +-----+   +-----+

                 Figure 1: Sparse Dragonfly of CLOS Fabrics

   To start with, Figure 1 visualizes three simple single plane fabrics
   interconnected via a DragonFly+ backbone.  The behavior of standard
   RIFT is better understood if we look at the homomorphic version of
   the same topology in Figure 2.  We can see that it is nothing else
   but a multi-plane CLOS with a lot of broken links for standard RIFT.
   The planes consist of S_x_1 and S_x_2 ToFs in each CLOS.  Given this,
   leaf LB1 should be connected to SA1 to be in the plane and since it
   is not, SA1 will deduct that leaf LB1 fell off the plane 1 and
   negatively disaggregate it.  Unfortunately the same is true for leaf
   LB1 from the view the SA2 in 2nd plane and it will negatively
   disaggregate it as well.  Hence, leaf LA1 will not have any
   possibility to forward to LB1 using standard RIFT computed
   forwarding.  This points us already to the first modification needed;
   we have to relax RIFT to forward through the horizontal links on ToFs
   and this will be the starting point of the next section.








Przygienda                 Expires 4 July 2024                  [Page 3]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


           Fabric A              Fabric B
     *********************  *********************
     *                   *  *                   *
     *                 +-*--*-----------+     +-*--------------+
     *       +---------|-*--*-+     +---|-----|-*----------|   |
     *       |         | *  * |     |   |     | *          |   |
     * +-----+   +-----+ *  * +-----+   +-----+ *    +-----+   +-----+
     * | SA2 |   | SA1 | *  * | SB2 |   | SB1 | *    |     |   |     |
     * +-----+   +-----+ *  * +-----+   +-----+ *    +-----+   +-----+
     * |  |  \   /  |  | *  *    |  \   /  |    *    |  |  \   /  |  |
     * +--|---\-/---|--|-*--*----|---\-/---|----*----+  |   \ /   |  |
     *    |    X    |  +-*--*----|----X----|----*-------|----X----|--+
     *    |   / \   |    *  *    |   / \   |    *       |   / \   |
     * +-----+   +-----+ *  * +-----+   +-----+ *    +-----+   +-----+
     * |     |   | LA1 | *  * |     |   | LB1 | *    |     |   |     |
     * +-----+   +-----+ *  * +-----+   +-----+ *    +-----+   +-----+
     *                   *  *                   *
     *                   *  *                   *
     *********************  *********************

    Figure 2: Homomorphic View of Sparse Dragonfly as a Multi-Plane CLOS

2.  Glossary

   The following terms are used in this document.

   DF+ capable ToF:
      ToF that provides DF+ extensions, both in recognizing the inter
      fabric links and computation procedures necessary to support
      those.  The resulting combination allows the use of RIFT with
      dragonfly topologies overall.

   Horizon:
      We define horizon as a concept differentiating between inter
      fabric links and southbound pointing standard RIFT intra fabric
      links on a ToF.  Both type of links need a different FIB to
      support alternate next hop when routing between fabrics.

   Inter Fabric Planes or IF-planes:
      Multi-Plane that spans multiple fabrics.

   Inter Fabric link or IF links:
      A horizontal ToF link between two fabrics.








Przygienda                 Expires 4 July 2024                  [Page 4]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


   Alternate Next Hop:
      A next hop through an IF interface that does not represent the
      shortest path through the inter fabric horizon but necessitates
      the receiving node to use the next hop on the shortest path to the
      destination fabric (direct next hop).

3.  Horizontal Link Behavior at ToF Level

   Dragonfly+, being basically, when seen a single fabric, a multi-plane
   CLOS with many broken links (which we will call inter fabric planes
   or IF planes to distinguish them from multi-plane within a fabric
   later) will somehow need to change the behavior of RIFT to allow
   forwarding via horizontal links at ToF level lest we end up inverting
   the fabric and force leaves to deal with transit traffic.  Moreover,
   the necessity to deal with new mis-cabling concepts leads us to
   change the solution framework and consider this configuration not as
   a single fabric but as a multi-fabric setup with dragonfly links
   building inter fabric planes now.  Additionally we will have to allow
   adjacencies on ToF horizontal links to another fabric and permit
   those to forward through such inter fabric planes while
   distinguishing such inter fabric (or IF) links from normal horizontal
   ToF "multi-plane ringing".  Hence in Figure 2 instead of the first
   assumption of a single fabric we break out fabric A and fabric B and
   consider the links SA2-SB2 and SA1-SB1 as two "inter fabric DF+"
   links, or in short, as already introduced, IF links.  And fortunately
   enough, IF links, just like all other horizontal ToF links, are
   considered northbound from both sides and northbound flooding rules
   apply, an ideal thing since with that ToFs will see full topology of
   their inter fabric plane.

   RIFT used in such DF+ configuration will require on ToF not only a
   DF+ capability flag but a fabric ID now which has to be distinct in
   each of the CLOS.  In case of non-DF+ mode a ToF will declare such
   links miscabled, once enabled to operate in DF+ it will mark those
   links as IF links.  Given `fabric_id` is an optional schema element a
   ToF operating in DF+ mode will reject all links to other ToFs without
   `fabric_id` value set or not indicating DF+ mode as mis-cabled to
   prevent a mixture of non-DF+ and DF+ ToFs in a setup.  On the other
   hand, a ToF indicating DF+ capability and showing matching fabric id
   is clearly a normal horizontal multi plane ring in the same fabric.











Przygienda                 Expires 4 July 2024                  [Page 5]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


4.  First Route Computation Change

   Now that we can detect IF links reliably we can also remove those
   from the computations used in negative disaggregation as first step.
   This will prevent ToFs in fabric A negatively disaggregating Fabric B
   prefixes, a desirable behavior.  Not being able to forward from
   Fabric A to fabric B is obviously a far less desirable behavior and
   hence a ToF in DF+ mode needs to extend its route computation by a
   special southbound DF+ computation where we use SPF taking in first
   step all IF links and the nodes behind them as candidates.  This
   computation will result in a "direct inter fabric forwarding
   database" containing amongst others shortest path to prefixes in
   fabric B or in other words, direct inter fabric next hops.  Section 5
   will expound further how that database is used.

4.1.  Additional Bi-Sectional Bandwidth Route Computation Change

   One of the DF+ properties is that it not only provides a direct path
   to a destination but guarantees that destinations are reachable via
   additional, alternate next hop to increase the bi-sectional
   bandwidth.  In our example SB1 forwarding to LA1 can take instead of
   SA1 directly a path through SC1 relying on it forwarding to SA1.  To
   support this we introduce an additional SPF computation which takes
   in first 2 iterations only IF links and generates a "indirect inter
   fabric forwarding database".  Section 5 will expound further how that
   database is used.

   Computing such alternate next hops will have the other beneficial
   effect of actually providing a backup path in case the direct IF
   plane link to another fabric becomes unavailable.

4.2.  Dragonfly with Multi-Plane CLOS Fabrics

   Most complex case of RIFT deployment would be a dragonfly topology of
   CLOS fabrics which are in themselves already multi-plane fabrics.  To
   present it as homomorphic graph Figure 3 is included.  The symmetry
   is obvious, we end up with the normal RIFT ringing within the fabric,
   e.g. r_A for fabric A and then for the inter fabric planes dragonfly
   is basically the according ringing itself, here IR_1 and IR_2.
   Observe that the northbound flooding occurring on all those links
   will present each ToF with the full topology of the dragonfly, a
   necessary condition for proper disaggregation and further
   reachability computations.  If the intra fabric ToF ringing should be
   avoided a tunnel between the ToFs within a fabric are necessary and
   may go all the way down to the leaves.  How such tunnels are
   provisioned is outside the specification here but it will necessitate
   basically flat distribution of the loopbacks of the ToFs across whole
   fabric via e.g. redistribution of some RIFT routes in northbound and



Przygienda                 Expires 4 July 2024                  [Page 6]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


   southbound direction or an equivalent scheme.


















































Przygienda                 Expires 4 July 2024                  [Page 7]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


                 Fabric A
               *******************************************
               *                                         *
               *  +-----+   +-----+   +-----+   +-----+  *
               *  |     |   |     |   |     |   |     |  *
               *  +-----+   +-----+   +-----+   +-----+  *
               *     |   \ /   |         |   \ /   |     *
               *     |    X    |         |    X    |     *
               *     |   / \   |         |   / \   |     *
               *  +-----+   +-----+   +-----+   +-----+  *
               *  |     |   |     |   |     |   |     |  *
               *  +-----+   +-----+   +-----+   +-----+  *
               *         \         \ /         /         *
               *          \         X         /          *
               *   +-------\---+   / \   +---/---------+ *
               *   |        +-----+   +-----+          | *
               *   |    +---| SA2 |   | SA1 |---+      | *
               *   |    |   +-----+   +-----+   |      | *
               *   |    |      |         |      | r_A  | *
               *   |    +------|---------|------+      | *
               *   |           |         |             | *
               ****|***********|*********|*************|**
                   |           |         |             |
              IR_2 |           |         |             | IR_1
                   |           |         |             |
               ****|***********|*********|*************|**
               *   |           |         |             | *
               *   |    +------|---------|------+      | *
               *   |    |      |         |      | r_B  | *
               *   |    |   +-----+   +-----+   |      | *
               *   |    +---| SB2 |   | SB1 |---+      | *
               *   |        +-----+   +-----+          | *
               *   +-------/---+   \ /   +---\---------+ *
               *          /         X         \          *
               *         /         / \         \         *
               *  +-----+   +-----+   +-----+   +-----+  *
               *  |     |   |     |   |     |   |     |  *
               *  +-----+   +-----+   +-----+   +-----+  *
               *     |   \ /   |         |   \ /   |     *
               *     |    X    |         |    X    |     *
               *     |   / \   |         |   / \   |     *
               *  +-----+   +-----+   +-----+   +-----+  *
               *  |     |   |     |   |     |   |     |  *
               *  +-----+   +-----+   +-----+   +-----+  *
               *                                         *
               *******************************************
                 Fabric B




Przygienda                 Expires 4 July 2024                  [Page 8]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


      Figure 3: Multi-Plane CLOS Fabrics Connected in Sparse Dragonfly

5.  Forwarding Considerations

   Since RIFT is being extended with the concept of "indirect inter
   fabric next hop" and IP packets do not carry any marking as the path
   they have taken indiscriminate forwarding using non-shortest paths at
   ToF level may loop in inter fabric case.  To prevent this the ToFs
   have to maintain the concept of a "split horizon" on the arriving
   traffic.  Any traffic arriving at the ToF that is targeted at the
   prefix within its fabric can be forwarded without any further
   considerations.  On the other hand, traffic arriving at a inter
   fabric link MUST use a FIB which does not contain the indirect inter
   fabric next hops and hence the FIB used to forward traffic on the IF
   inter faces MUST NOT include the results of indirect next hop
   computation.  The solution will naturally limit any non-shortest
   inter fabric path in ToF case to maximally one alternate next hop.
   Observe that per inter face specific FIB is nothing particularly
   special, any technology supporting VPN or trunking today is already
   capable of provisioning inter face specific forwarding behavior.

6.  Partitioning of inter Fabric Planes

   A special case where a plane within a remote fabric breaks down is
   not noticeable in another fabric and hence the traffic can black hole
   since we do suppress the IF links during negative disaggregation
   normally.  To detect the condition reliably a ToF has to compute the
   inter fabric view of all the other ToFs in its own fabric while
   including IF links and consider the resulting difference as "inter
   fabric negative disaggregation".  This is possible but at scale can
   present significant computational load and is left therefore as
   optional behavior.  Additionally, even when the fabric is a single
   plane fabric it must be then ringed at ToF level since otherwise the
   ToFs do not see the inter fabric planes they are not part of as an IF
   ring.

   The same computation will deal with an even stranger case of a double
   failure on the IF links where a ToF becomes completely separated from
   the other fabrics.  It will detect this and initiate negative
   disaggregation for the according prefixes.

7.  Specification

   Precise schema changes and computation algorithms are to be provided
   in future version of the draft in detail.  Basically the LIEs and
   Node TIEs need to be extended by fabric_id and DF+ mode indication
   and computations described conceptually in former chapters tightly
   specified.



Przygienda                 Expires 4 July 2024                  [Page 9]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


8.  Summary Overview

   A final Figure 4 is provided to map things back to the usual
   dragonfly sparse topology and show the concepts in action.

   We see three fabrics, each of them multi-plane (though mixes are
   absolutely possible as long the number of ToFs connected to dragonfly
   are kept the same).  The fat links represent the "IF horizon", i.e.
   any traffic coming from those links cannot use alternate next hops to
   the destination.  In this example traffic from LA11 going through
   PA11 and SA2 towards LC11 is given two choices of next hops, either
   SC2 or SB2.  Now that it entered the IF horizon in case SB2 receives
   it no further alternate next hops will be used but traffic will be
   handed off to SC2 which applies the same rule and in this case
   actually forwards the traffic into the fabric.

                      [[ Overview of the solution,
                         Refer to PDF for Picture ]]

      Figure 4: Multi-Plane CLOS Fabrics Connected in Sparse Dragonfly

9.  IANA Considerations

   This document requests allocation for the following RIFT codepoints.

   TBD

10.  Security Considerations

   TBD

11.  Acknowledgements

   Dmitry Afanasiev's ideas around his work with BGP and dragonfly
   started interesting discussions, and he provided the crucial split
   horizon forwarding idea.  Jeff Tantsura encouraged the work from its
   initial conception.  Many thanks to Benson Muite for ASCII figures.

12.  References

12.1.  Informative References

12.2.  Normative References

   [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>.



Przygienda                 Expires 4 July 2024                 [Page 10]

Internet-Draft       draft-przygienda-rift-dragonfly        January 2024


   [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>.

Author's Address

   Tony Przygienda (editor)
   Juniper
   1137 Innovation Way
   Sunnyvale, CA
   United States of America
   Email: prz@juniper.net







































Przygienda                 Expires 4 July 2024                 [Page 11]