Internet DRAFT - draft-agt-rtgwg-dragonfly-routing
draft-agt-rtgwg-dragonfly-routing
Routing Area Working Group D. Afanasiev
Internet-Draft
Intended status: Informational R. Glebov
Expires: 5 September 2024 Yandex
J. Tantsura
Nvidia
4 March 2024
Routing in Dragonfly+ Topologies
draft-agt-rtgwg-dragonfly-routing-01
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.t
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Network Design Requirements . . . . . . . . . . . . . . . . . 4
4. Dragonfly Topology . . . . . . . . . . . . . . . . . . . . . 4
4.1. Dragonfly Topology Overview . . . . . . . . . . . . . . . 4
4.2. Rouging and Paths in Dragonfly+ . . . . . . . . . . . . . 4
4.3. Topology Construction and Graph Wiring . . . . . . . . . 5
4.4. Adaptive Load Balancing . . . . . . . . . . . . . . . . . 5
5. Routing and Forwarding . . . . . . . . . . . . . . . . . . . 6
5.1. Forwarding . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Routing . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.3. Scalability and Optimizations . . . . . . . . . . . . . . 7
5.4. Failure handling and convergence . . . . . . . . . . . . 8
5.5. Asymmetry and traffic engineering . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 9
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 is 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 IP networks lack many mechanisms traditionally
available in HPC interconnection networks. Specifically , Dragonfly+
relies heavily on non-minimal routing and adaptive load balancing for
efficient use of available network capacity.
1.1. 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.
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2. Terminology
This section introduces the terminology used in this document.
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. Name is misleading 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 using global information
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ARN
Adaptive Routing Notification
3. 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.
4. Dragonfly Topology
Body text
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+ as 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
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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
One possible implementation is described in [WILKE2017]. TODO:
describe wiring scheme invariant under group rotation (consistent
renumbering of all groups by the same offset mod number of groups).
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
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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
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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. 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.
5.3. Scalability and Optimizations
TODO
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5.4. Failure handling and convergence
TODO
5.5. Asymmetry and traffic engineering
Body text
6. IANA Considerations
This memo includes no request to IANA.
7. Security Considerations
This document should not affect the security of the Internet.
8. References
8.1. Normative References
[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>.
8.2. Informative 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>.
[RFC7938] Bradner, S., "Use of BGP for Routing in Large-Scale Data
Centers", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March
1997, <https://www.rfc-editor.org/info/rfc7938>.
[KIM2008] Kim, J., Dally, W. J., Scott, S., and D. Abts,
"Technology-Driven, Highly-Scalable Dragonfly Topology",
2008, <https://doi.org/10.1109/ISCA.2008.19>.
[SPHINER2017]
Shpiner, A., Haramaty, Z., Eliad, S., Zdornov, V., Gafni,
B., and E. Zahavi, "Dragonfly+: Low Cost Topology for
Scaling Datacenters", February 2017,
<http://dx.doi.org/10.1109/HiPINEB.2017.11>.
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[FLAJSLIK2018]
Flajslik, M., Borch, E., and M. A. Parker, "Megafly: A
Topology for Exascale Systems", May 2018,
<https://doi.org/10.1007/978-3-319-92040-5_15>.
[WILKE2017]
J, W. J., Sebastien, R., and T. M. Yee, "Design space
exploration of the Dragonfly topology", 2017,
<https://www.researchgate.net/publication/320493515_Design
_Space_Exploration_of_the_Dragonfly_Topology>.
[SINGH2005]
Arjun, S., "Load-balanced routing in interconnection
networks", 2005,
<http://cva.stanford.edu/publications/2005/
thesis_arjuns.pdf>.
Authors' Addresses
Dmitry Afanasiev
Email: dmitry.afanasiev@gmail.com
Roman Glebov
Yandex
Email: kitaro630@yandex.ru
Jeff Tantsura
Nvidia
Email: jefftant.ietf@gmail.com
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