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Fully Adaptive Routing Ethernet using BGP
draft-xu-idr-fare-00

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This is an older version of an Internet-Draft whose latest revision state is "Active".
Author Xiaohu Xu
Last updated 2024-07-04
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draft-xu-idr-fare-00
Network Working Group                                              X. Xu
Internet-Draft                                              China Mobile
Intended status: Standards Track                             1 July 2024
Expires: 2 January 2025

               Fully Adaptive Routing Ethernet using BGP
                          draft-xu-idr-fare-00

Abstract

   Large language models (LLMs) like ChatGPT have become increasingly
   popular in recent years due to their impressive performance in
   various natural language processing tasks.  These models are built by
   training deep neural networks on massive amounts of text data, often
   consisting of billions or even trillions of parameters.  However, the
   training process for these models can be extremely resource-
   intensive, requiring the deployment of thousands or even tens of
   thousands of GPUs in a single AI training cluster.  Therefore, three-
   stage or even five-stage CLOS networks are commonly adopted for AI
   networks.  The non-blocking nature of the network become increasingly
   critical for large-scale AI models.  Therefore, adaptive routing is
   necessary to dynamically load balance traffic to the same destination
   over multiple ECMP paths, based on network capacity and even
   congestion information along those paths.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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 2 January 2025.

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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
   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.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Path Bandwidth Extended Community . . . . . . . . . . . . . .   4
   4.  Solution Description  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Adaptive Routing in 3-stage CLOS  . . . . . . . . . . . .   5
     4.2.  Adaptive Routing in 5-stage CLOS  . . . . . . . . . . . .   6
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     8.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   Large language models (LLMs) like ChatGPT have become increasingly
   popular in recent years due to their impressive performance in
   various natural language processing tasks.  These models are built by
   training deep neural networks on massive amounts of text data, often
   consisting of billions or even trillions of parameters.  However, the
   training process for these models can be extremely resource-
   intensive, requiring the deployment of thousands or even tens of
   thousands of GPUs in a single AI training cluster.  Therefore, three-
   stage or even five-stage CLOS networks are commonly adopted for AI
   networks.  Furthermore, In rail-optimized CLOS topologies with
   standard GPU servers (HB domain of eight GPUs), the Nth GPUs of each
   server in a group of servers are connected to the Nth leaf switch,
   which provides higher bandwidth and non-blocking connectivity between
   the GPUs in the same rail.  In rail-optimized topology, most traffic
   between GPU servers would traverse the intra-rail networks rather
   than the inter-rail networks.

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   The non-blocking nature of the network, especially the network for
   intra-rail communication, become increasingly critical for large-
   scale AI models.  AI workloads tend to be extremely bandwidth-hungry
   and they usually generate a few elephant flows simultaneAously.  If
   the traditional hash-based ECMP load-balancing was used without any
   optimization, it's highly possible to cause serious congestion and
   high latency in the network once multiple elephant flows are routed
   to the same link.  Since the job completion time depends on worst-
   case performance, serious congestion will result in model training
   time longer than expected.  Therefore, adaptive routing is necessary
   to dynamically load balance traffic to the same destination over
   multiple ECMP paths, based on network capacity and even congestion
   information along those paths.  In other words, adaptive routing is a
   capacity-aware and even congestion-aware path selection algorithm.

   Furthermore, to reduce the congestion risk to the maximum extent, the
   routing should be more granular if possible.  Flow-granular adaptive
   routing still has a certain statistical possibility of congestion.
   Therefore, packet-granular adaptive routing is more desirable
   although packet spray would cause out-of-order delivery issue.  A
   flexible reordering mechanism must be put in place(e.g., egress ToRs
   or the receiving servers).  Recent optimizations for RoCE and newly
   invented transport protocols as alternatives to RoCE no longer
   require handling out-of-order delivery at the network layer.
   Instead, the message processing layer is used to address it.

   To enable adaptive routing, no matter whether flow-granular or
   packet-granular adaptive routing, it is necessary to propagate
   network topology information, including link capacity and/or even
   available link capacity (i.e., link capacity minus link load) across
   the CLOS network.  Therefore, it seems straightforward to use link-
   state protocols such as OSPF or ISIS as the underlay routing protocol
   in the CLOS network, instead of BGP, for propagating link capacity
   information and/or even available link capacity information.  How to
   leverage OSPF or ISIS to achieve adaptive routing has been described
   in [I-D.xu-lsr-fare].  However, some data center network operators
   have been used to the use of BGP as the underlay routing protocol of
   data center networks [RFC7938].  Therefore, there is a need to
   leverage BGP to achieve adaptive routing as well.

   [I-D.ietf-idr-link-bandwidth] has specified a way to perform weighted
   ECMP based on link bandwidths conveyed in the non-transitive link
   bandwith extended community.  However, it is impractical to enable
   adaptive routing by directly using the non-transitive link bandwidth
   extended community due to the following constraints as mentioned in
   [I-D.ietf-idr-link-bandwidth].

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   "No more than one link bandwidth extended community SHALL be attached
   to a route.  Additionally, if a route is received with link bandwidth
   extended community and the BGP speaker sets itself as next-hop while
   announcing that route to other peers, the link bandwidth extended
   community should be removed.  The extended community is optional non-
   transitive."

   Hence, this document defines a new extended community referred to as
   Path Bandwidth Extended Community and describes how to use this newly
   defined path bandwidth extended community to achieve adaptive
   routing.

   Note that while adaptive routing especially at the packet-granular
   level can help reduce congestion between switches in the network,
   thereby achieving a non-blocking fabric, it does not address the
   incast congestion issue which is commonly experienced in last-hop
   switches that are connected to the receivers in many-to-one
   communication patterns.  Therefore, a congestion control mechanism is
   always necessary between the sending and receiving servers to
   mitigate such congestion.

2.  Terminology

   This memo makes use of the terms defined in [RFC4360].

3.  Path Bandwidth Extended Community

   The Path Bandwidth Extended Community is used to indicate the minimum
   bandwith of the path towards the destination.  It is an new IPv4
   Address Specific Extended Community that can be transitive or non-
   transitive.

   The value of the high-order octet of this extended type is either
   0x01 or 0x41.  The low-order octet of this extended type is TBD.

   The Value field consists of two sub-fields:

      Global Administrator sub-field: This sub-field contains the router
      ID of the advertising router that appends the path bandwidth
      extended community or updates the path bandwidth value of the
      existing path bandwidth extended community.

      Local Administrator sub-field: This sub-field contains the path
      bandwidth value in IEEE floating point format with units of
      Gigabytes per second (GB/s).

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4.  Solution Description

4.1.  Adaptive Routing in 3-stage CLOS

       +----+ +----+ +----+ +----+
       | S1 | | S2 | | S3 | | S4 |  (Spine)
       +----+ +----+ +----+ +----+

       +----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+
       | L1 | | L2 | | L3 | | L4 | | L5 | | L6 | | L7 | | L8 |  (Leaf)
       +----+ +----+ +----+ +----+ +----+ +----+ +----+ +----+

                                  Figure 1

   (Note that the diagram above does not include the connections between
   nodes.  However, it can be assumed that leaf nodes are connected to
   every spine node in their CLOS topology.)

   In a three-stage CLOS network as shown in Figure 1, also known as a
   leaf-spine network, each leaf node would establish eBGP sessions with
   all spine nodes.

   All nodes are enabled for adaptive routing.

   When a leaf node, such as L1, advertises the route to a specific IP
   prefix that it originates, it will attach a transitive path bandwidth
   extended community filled with a maximum bandwidth value.

   Upon receiving the above advertisement, a spine node, such as S1,
   SHOULD determine the minimum value between the bandwidth of the link
   towards the advertising node (e.g., L1) and the value of the path
   bandwidth extended community carried in the received route, and then
   update the path bandwidth extended community with the above minimum
   value before readvertising that route to remote eBGP peers.  Once S1
   receives multiple equal-cost routes for a given prefix from multiple
   leaf nodes (e.g., L1 and L2 in the server multi-homing scenario), for
   each route, it SHOULD determine the minimum value between the
   bandwidth of the link towards the advertising node and the value of
   the path bandwidth extended community carried in the received route,
   and then use that minimum bandwidth value as a weight value for that
   route when performing weighted ECMP.  When readvertising the route
   for that prefix to remote eBGP peers further, the path bandwidth
   extended community would be updated with the sum of the minimum
   bandwidth value of each route.

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   When a leaf node, such as L8, receives multiple equal-cost routes for
   that prefix from spine nodes (e.g., S1, S2, S3 and S4), for each
   route, it will determine the minimum value between the bandwidth of
   the link towards the advertising node and the value of the path
   bandwidth extended community carried in the received route, and then
   use that minimum bandwidth value as a weight value for that route
   when performing weighted ECMP.

   Note that the weighted ECMP according to path bandwidth SHOULD NOT be
   performed unless all equal-cost routes for a given prefix carry the
   path bandwidth extended community.

4.2.  Adaptive Routing in 5-stage CLOS

     =========================================
     # +----+ +----+ +----+ +----+           #
     # | L1 | | L2 | | L3 | | L4 | (Leaf)    #
     # +----+ +----+ +----+ +----+           #
     #                                PoD-1  #
     # +----+ +----+ +----+ +----+           #
     # | S1 | | S2 | | S3 | | S4 | (Spine)   #
     # +----+ +----+ +----+ +----+           #
     =========================================

     ===============================     ===============================
     # +----+ +----+ +----+ +----+ #     # +----+ +----+ +----+ +----+ #
     # |SS1 | |SS2 | |SS3 | |SS4 | #     # |SS1 | |SS2 | |SS3 | |SS4 | #
     # +----+ +----+ +----+ +----+ #     # +----+ +----+ +----+ +----+ #
     #   (Super-Spine@Plane-1)     #     #   (Super-Spine@Plane-4)     #
     #============================== ... ===============================

     =========================================
     # +----+ +----+ +----+ +----+           #
     # | S1 | | S2 | | S3 | | S4 | (Spine)   #
     # +----+ +----+ +----+ +----+           #
     #                                PoD-8  #
     # +----+ +----+ +----+ +----+           #
     # | L1 | | L2 | | L3 | | L4 | (Leaf)    #
     # +----+ +----+ +----+ +----+           #
     =========================================

                                Figure 2

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   (Note that the diagram above does not include the connections between
   nodes.  However, it can be assumed that the leaf nodes in a given PoD
   are connected to every spine node in that PoD.  Similarly, each spine
   node (e.g., S1) is connected to all super-spine nodes in the
   corresponding PoD-interconnect plane (e.g., Plane-1).)

   For a five-stage CLOS network as illustrated in Figure 2, each leaf
   node would establish eBGP sessions with all spine nodes of the same
   PoD while each spine node would establish eBGP sessions with all
   super-spine nodes in the corresponding PoD-interconnect plane.

   In rail-optimized topology, Intra-rail communication with high
   bandwidth requirements would be restricted to a single PoD.  Inter-
   rail communication with relatively lower bandwidth requirements need
   to travel across PoDs through PoD-interconnect planes.  Therefore,
   enabling adaptive routing only in PoD networks is sufficient.  It's
   optional to perform adaptive routing for cross-PoD traffic.

   When a leaf node, such as L1 in PoD-1, advertises the route for a
   specific IP prefix that it originates, it will attach a transitive
   path bandwidth extended community filled with a maximum bandwidth
   value.

   Upon receiving the above route advertisement, a spine node, such as
   S1 in PoD-1, will determine the minimum value between the bandwidth
   of the link towards the advertising node (e.g., L1 in PoD-1) and the
   value of the path bandwidth extended community carried in the route,
   and then update the path bandwidth extended community with the above
   minimum value before readvertising that route to remote eBGP peers.
   Once S1 in PoD-1 receives multiple equal-cost routes for a given
   prefix from multiple leaf nodes (e.g., L1 and L2 in PoD-1 in the
   server multi-homing scenario), for each route, it will determine the
   minimum value between the bandwidth of the link towards the
   advertising node and the bandwidth value of the path bandwidth
   extended community carried in the route, and then use that minimum
   bandwidth value as a weight value for that route when performing
   weighted ECMP.  When readvertising the route for that prefix to
   remote eBGP peers, the path bandwidth extended community would be
   updated with the sum of the minimum bandwidth value of each route.

   When a given super-spine node, such as SS1 in Plane-1, receives the
   route for that prefix from S1 in PoD-1, it will not update the
   transtive path bandwidth extended community when readvertising that
   route.  It COULD optionally attach another path bandwidth extended
   community which is non-transitive to indicate the bandwith of the
   link towards the advertising router.

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   When a given spine node in another PoD, such as S1 in PoD-8, receives
   multiple equal-cost routes for a given prefix from super-spine nodes
   in Plane-1 (e.g., SS1, SS2, SS3 and SS4 in Plane-1), it will not
   update the value of the transitive path bandwidth extended community
   when readvertising that route towards remote peers (Note that the
   transitive path bandwidth extended community of those multiple equal-
   cost routes carry the same value that was set by S1 in PoD-1).
   Meanwhile, once each route contains a non-transitive path bandwidth
   extended community, for each route, it will determine the minimum
   value between the bandwidth of the link towards the advertising node
   and the bandwidth value of the non-transitive path bandwidth extended
   community carried in the route, and then use that minimum bandwidth
   value as a weight value for that route when performing weighted ECMP.

   When a leaf node, such as L8 in PoD-8, receives multiple equal-cost
   routes for that prefix from multiple spine nodes (e.g., S1, S2, S3
   and S4 in PoD-8), for each route, it will determine the minimum value
   between the bandwidth of the link towards the advertising node and
   the value of the path bandwidth extended community carried in the
   route, and then use that minimum bandwidth value as a weight value
   for that route when performing weighted ECMP.

   Note that the weighted ECMP according to path bandwidth SHOULD NOT be
   performed unless all equal-cost routes for a given prefix carry the
   path bandwidth extended community.

5.  Acknowledgements

   TBD.

6.  IANA Considerations

   TBD.

7.  Security Considerations

   TBD.

8.  References

8.1.  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>.

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   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, DOI 10.17487/RFC4360,
              February 2006, <https://www.rfc-editor.org/info/rfc4360>.

8.2.  Informative References

   [I-D.ietf-idr-link-bandwidth]
              Mohapatra, P. and R. Fernando, "BGP Link Bandwidth
              Extended Community", Work in Progress, Internet-Draft,
              draft-ietf-idr-link-bandwidth-07, 5 March 2018,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-
              link-bandwidth-07>.

   [I-D.xu-lsr-fare]
              Xu, X., He, Z., Wang, J., Huang, H., Zhang, Q., Wu, H.,
              Liu, Y., Xia, Y., Wang, P., and S. Hegde, "Fully Adaptive
              Routing Ethernet", Work in Progress, Internet-Draft,
              draft-xu-lsr-fare-02, 25 February 2024,
              <https://datatracker.ietf.org/doc/html/draft-xu-lsr-fare-
              02>.

   [RFC7938]  Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
              BGP for Routing in Large-Scale Data Centers", RFC 7938,
              DOI 10.17487/RFC7938, August 2016,
              <https://www.rfc-editor.org/info/rfc7938>.

Author's Address

   Xiaohu Xu
   China Mobile
   Email: xuxiaohu_ietf@hotmail.com

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