Internet-Draft dragonfly-routing July 2023
Afanasiev, et al. Expires 11 January 2024 [Page]
Workgroup:
Routing Area Working Group
Internet-Draft:
draft-agt-rtgwg-dragonfly-routing-00
Published:
Intended Status:
Informational
Expires:
Authors:
D. Afanasiev
R. Glebov
Yandex
J. Tantsura
Nvidia

Routing in Dragonfly+ Topologies

Abstract

This document provides an overview of Dragonfly+ network topology and describes routing implementation for IP networks with Dragonfly+ topology with support for non-minimal routing.

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This Internet-Draft will expire on 11 January 2024.

1. Introduction

Dragonfly {KIM2008} is a high-scalability, low-diameter, cost-efficient network topology that provides high bandwidth and large path diversity. Dragonfly topology was originally designed for HPC and supercomputing systems and are now adopted in more and more supercomputing networks. Its properties also make it an interesting candidate for data center network topology, especially Dragonfly+ variant {SPHINER2017} with leaf-spine intra-group topology. But building IP networks with Dragonfly+ topology is a non-trivial problem because many mechanisms traditionally available in HPC interconnection networks are not available in IP networks. Specifically , Dragonfly+ relies heavily on non-minimal routing and adaptive load balancing for efficient use of available network capacity.

This document provides an overview of Dragonfly+ network topology and describes routing implementation for IP networks with Dragonfly+ topology with support for non-minimal routing. It also discusses load balancing in Dragonfly+.

2. Conventions and Definitions

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.

3. Terminology

Group: building block of Dragonfly network, collection of nodes connected by local links. In practical deployments, routers and associated end-points belonging to a group are assumed to be compactly colocated. Local (L) / intra-group link: link between routers in the same group. In Dragonfly+ group is a leaf-spine network (bipartite graph) so local links are always between leaf and spine. Global (G) / inter-group link: links between routers from different groups. Usually long and more expensive so it is desirable to minimize the number of global links. Path signature - sequence of letters corresponding to types of links in the path, e.g. LGLLGL. Local / intra-group network Global / inter-group network MIN: minimal routing VAL: randomized non-minimal routing (valiant load balanced) AR: adaptive routing. It is a misnomer because it has nothing to do with disseminating reachability information - it is a mapping mechanism that maps traffic to already known paths. UGAL: Universal Globally-Adaptive load-balanced UGAL-L: UGAL with using local queue information at current router node UGAL-G: UGAL with using global information ARN: Adaptive Routing Notification

4. Network Design Requirements

Network design requirements are largely the same as in {RFC7938}. The most notable difference is the extensive use of non-minimal paths. # Dragonfly Topology

4.1. Dragonfly Topology Overview

Dragonfly topology was introduced by Kim et al. {KIM2008}. It aims to decrease the cost and diameter of the network while providing good scalability. Dragonfly is a hierarchical topology that divides routers into groups connected by long (inter-group) links in a fully-connected global network. Each group essentially implements high-radix virtual router. Dragonfly is a direct topology, in which every router has a set of terminal connections leading to endpoints, and a set of topological connections leading to other routers, some from the same group and some from the other groups. While original Dragonfly uses fully-connected intra-group topology it doesn't prevent using other intra-group topologies. Different intra-group topologies produce different Dragonfly "flavors". Inter-group topology is always fully connected.

Dragonfly+ proposed in {SPHINER2017} relies on an extended group topology in which intra-group routers are connected as a bipartite graph (leaf-spine or Clos-like topology). Dragonfly+ is superior to conventional Dragonfly due to the significantly larger number of hosts which it is able to support. In addition, Dragonfly+ supports similar or better bisectional bandwidth for various traffic patterns and requires smaller number of buffers to avoid credit loop deadlocks in lossless networks. Dragonfly+ is a indirect topology where only leaf nodes are connect to endpoints.

TODO: spine sizing.

4.2. Rouging and Paths in Dragonfly+

In Dragonfly and Dragonfly+ topologies there exists at least one direct global link between every pair of groups. Minimal intergroup routes traverse a single global link. The capacity of minimal routes between each pair of groups is lower than the aggregate link capacity of hosts in a group. Therefore, conventional minimal routing is not enough to obtain maximal throughput and efficiently support various traffic patters. {KIM2008} introduces the concept of non-minimal adaptive routing. For Dragonfly+ we can define three priority levels of inter-group routes. We use notations of ”L” and ”G” below to express where the route traverses local or global link, respectively. 1. High priority: Minimal route (LGL) - a shortest distance route which passes through two spine routers using a single global link. 2. Medium priority: Intermediate spine route (LGGL) - a route which traverses an intermediate group, using its spine router, passing exactly three spine routers using two global links. 3. Low priority: Intermediate leaf route (LGLLGL) - a route which traverses an intermediate group using its two spine routers and a leaf router, passing exactly four spine routers using two global links.

LGLLGL routes normally appear only when some spines are not connected to at least one spine in every other group - in this case non-minimal routes through intermediate group might need to use different ingress and egress spines in the intermediate group.

TODO: discuss imbalance, density and LGLLGL routes {WILKE2017}

4.3. Topology Construction and Graph Wiring

Possible implementation is described in {WILKE2017}.

4.4. Adaptive Load Balancing

While routing and forwarding setup described in this document allows to propagate reachability information and install forwarding state required for Dragonfly+ topologies, including non-minimal paths, it's not enough to efficiently use Dragonfly network capacity, especially in presence of LGLLGL paths. Efficient traffic to paths mapping in Dragonfly network can not be described by static mechanisms because ideally we would like to - fill paths starting from high priority - try to move flows from congested paths as a possible reaction to congestion.

This requires dynamic adaptive load balancing and coupling between adaptive load balancing and congestion control. Adaptive load balancing MUST be able to work without complete knowledge of network link utilization and queue state since such state can significantly change over the period of several RTTs and collecting and distributing global network utilization information often enough in any network of practically interesting size in infeasible.

Adaptive routing can also work as a complementary failure handling mechanism with much faster reaction time than routing convergence.

TODO: separate document describing possible adaptive load balancing implementation using existing mechanisms.

5. Routing and Forwarding

This section describes routing design supporting non-minimal paths. It uses only existing mechanisms - VRFs, route leaking and EBGP as a routing protocol. EBGP is chosen for scalability and flexibility - routing policies and communities allow to implement additional logic and precisely control propagation of routing updates.

Routing design is based on following principles: - intra-group traffic MUST use minimal routing as group in Dragonfly+ is just a leaf-spine network - path can contain at most one transit group - transit spine(s) MUST use shortest path forwarding to avoid forwarding loops - LGLLGL paths require traffic reflection via leaves in the transit group but only appear if number of uplinks per spine is less than number of remote groups

5.1. Forwarding

To achieve desired forwarding behavior several VRFs are configured on every spine: - local VRF in each group containing local links - core VRF containing all global links

Additional VRF serving as a virtual link is configured if network is using LGLLGL paths - reflect VRF in each group containing local links. Since both local VRF and reflect VRF include leaf-spine links some form of VRF multiplexing over leaf-spine links is required when LGLLGL paths are used.

Local VRF: - imports minimal and non-minmal paths from the core VRF and installs them

Core VRF - imports locally originated paths from local VRF in each group - imports transit paths from reflect VRF

Reflect VRF - imports minimal paths from core VRF

5.2. Routing

Each group is in a separate AS.

Communities, routing policies and update propagation: - When a announcing a route originated in the local group towards other groups add community C1 - When propagating announce with community C1 add community C2 - Do not propagate updates with community C2 - Import routes with C1 and C2 into local VRFs - Import routes with C1 only into reflect VRFs, add community C3 - Import routes with C3 from reflect VRFs into core VRF

During import into local VRFs prepend ASPATH: - 2 times for routes with C1 only - 1 time for routes with C2
- do not prepend for routes with C3 As result paths with C1, C2 and C3 will all have has the same ASPATH length in local VRFs and will be eligible for ECMP.

7. IANA Considerations

This document does not request any IANA allocations.

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, , <https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174]
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/rfc/rfc8174>.

8.2. Informative References

[RFC7938]
"Load-balanced routing in interconnection networks", , <http://cva.stanford.edu/publications/2005/thesis_arjuns.pdf>.

Authors' Addresses

Dmitry Afanasiev
Roman Glebov
Yandex
Jeff Tantsura
Nvidia