Independent Submission C. Filsfils, Ed.
Request for Comments: 8604 Cisco Systems, Inc.
Category: Informational S. Previdi
ISSN: 2070-1721 Huawei Technologies
G. Dawra, Ed.
LinkedIn
W. Henderickx
Nokia
D. Cooper
CenturyLink
June 2019
Interconnecting Millions of Endpoints with Segment Routing
Abstract
This document describes an application of Segment Routing to scale
the network to support hundreds of thousands of network nodes, and
tens of millions of physical underlay endpoints. This use case can
be applied to the interconnection of massive-scale Data Centers (DCs)
and/or large aggregation networks. Forwarding tables of midpoint and
leaf nodes only require a few tens of thousands of entries. This may
be achieved by the inherently scaleable nature of Segment Routing and
the design proposed in this document.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This is a contribution to the RFC Series, independently of any other
RFC stream. The RFC Editor has chosen to publish this document at
its discretion and makes no statement about its value for
implementation or deployment. Documents approved for publication by
the RFC Editor are not candidates for any level of Internet Standard;
see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8604.
Filsfils, et al. Informational [Page 1]RFC 8604 Large-Scale Segment Routing June 2019
Copyright Notice
Copyright (c) 2019 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
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to this document.
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. Reference Design ................................................3
4. Control Plane ...................................................5
5. Illustration of the Scale .......................................5
6. Design Options ..................................................6
6.1. Segment Routing Global Block (SRGB) Size ...................6
6.2. Redistribution of Routes for Agg Nodes .....................7
6.3. Sizing and Hierarchy .......................................7
6.4. Local Segments to Hosts/Servers ............................7
6.5. Compressed SRTE Policies ...................................7
7. Deployment Model ................................................8
8. Benefits ........................................................8
8.1. Simplified Operations ......................................8
8.2. Inter-domain SLAs ..........................................8
8.3. Scale ......................................................9
8.4. ECMP .......................................................9
9. IANA Considerations .............................................9
10. Manageability Considerations ...................................9
11. Security Considerations ........................................9
12. Informative References .........................................9
Acknowledgements ..................................................10
Contributors ......................................................10
Authors' Addresses ................................................11
Filsfils, et al. Informational [Page 2]RFC 8604 Large-Scale Segment Routing June 2019
1. Introduction
This document describes how Segment Routing (SR) can be used to
interconnect millions of endpoints.
2. Terminology
The following terms and abbreviations are used in this document:
Term Definition
-------------------------------------------------------------
Agg Aggregation
BGP Border Gateway Protocol
DC Data Center
DCI Data Center Interconnect
ECMP Equal-Cost Multipath
FIB Forwarding Information Base
LDP Label Distribution Protocol
LFIB Label Forwarding Information Base
MPLS Multiprotocol Label Switching
PCE Path Computation Element
PCEP Path Computation Element Communication Protocol
PW Pseudowire
SLA Service Level Agreement
SR Segment Routing
SRTE Policy Segment Routing Traffic Engineering Policy
TE Traffic Engineering
TI-LFA Topology Independent Loop-Free Alternate
3. Reference Design
The network diagram below illustrates the reference network topology
used in this document:
+-------+ +--------+ +--------+ +-------+ +-------+
A DCI1 Agg1 Agg3 DCI3 Z
| DC1 | | M1 | | C | | M2 | | DC2 |
| DCI2 Agg2 Agg4 DCI4 |
+-------+ +--------+ +--------+ +-------+ +-------+
Figure 1: Reference Topology
The following apply to the reference topology above:
o Independent ISIS-OSPF/SR instance in core (C) region.
o Independent ISIS-OSPF/SR instance in Metro1 (M1) region.
Filsfils, et al. Informational [Page 3]RFC 8604 Large-Scale Segment Routing June 2019
o Independent ISIS-OSPF/SR instance in Metro2 (M2) region.
o BGP/SR in DC1.
o BGP/SR in DC2.
o Agg routes (Agg1, Agg2, Agg3, Agg4) are redistributed from C to M
(M1 and M2) and from M to DC domains.
o No other route is advertised or redistributed between regions.
o The same homogeneous Segment Routing Global Block (SRGB) is used
throughout the domains (e.g., 16000-23999).
o Unique SRGB sub-ranges are allocated to each metro (M) and core
(C) domain:
* The 16000-16999 range is allocated to the core (C)
domain/region.
* The 17000-17999 range is allocated to the M1 domain/region.
* The 18000-18999 range is allocated to the M2 domain/region.
* Specifically, the Agg1 router has Segment Identifier (SID)
16001 allocated, and the Agg2 router has SID 16002 allocated.
* Specifically, the Agg3 router has SID 16003 allocated, and the
anycast SID for Agg3 and Agg4 is 16006.
* Specifically, the DCI3 router has SID 18003 allocated, and the
anycast SID for DCI3 and DCI4 is 18006.
* Specifically, at the Agg1 router, the binding SID 4001 leads to
DCI pair (DCI3, DCI4) via a specific low-latency path {16002,
16003, 18006}.
o The same SRGB sub-range is reused within each DC (DC1 and DC2)
region for each DC (e.g., 20000-23999). Specifically, nodes A
and Z both have SID 20001 allocated to them.
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4. Control Plane
This section provides a high-level description of how a control plane
could be implemented using protocol components already defined in
other RFCs.
The mechanism through which SRTE Policies are defined, computed, and
programmed in the source nodes is outside the scope of this document.
Typically, a controller or a service orchestration system programs
node A with a PW to a remote next-hop node Z with a given SLA
contract (e.g., low-latency path, disjointness from a specific core
plane, disjointness from a different PW service).
Node A automatically detects that node Z is not reachable. It then
automatically sends a PCEP request to an SR PCE for an SRTE policy
that provides reachability information for node Z with the
requested SLA.
The SR PCE [RFC4655] is made of two components: a multi-domain
topology and a computation engine. The multi-domain topology is
continuously refreshed through BGP - Link State (BGP-LS) feeds
[RFC7752] from each domain. The computation engine is designed to
implement TE algorithms and provide output in SR Path format. Upon
receiving the PCEP request [RFC5440], the SR PCE computes the
requested path. The path is expressed through a list of segments
(e.g., {16003, 18006, 20001}) and provided to node A.
The SR PCE logs the request as a stateful query and hence is able to
recompute the path at each network topology change.
Node A receives the PCEP reply with the path (expressed as a segment
list). Node A installs the received SRTE policy in the data plane.
Node A then automatically steers the PW into that SRTE policy.
5. Illustration of the Scale
According to the reference topology shown in Figure 1, the following
assumptions are made:
o There is one core domain, and there are 100 leaf (metro) domains.
o The core domain includes 200 nodes.
o Two nodes connect each leaf (metro) domain. Each node connecting
a leaf domain has a SID allocated. Each pair of nodes connecting
a leaf domain also has a common anycast SID. This yields up to
300 prefix segments in total.
Filsfils, et al. Informational [Page 5]RFC 8604 Large-Scale Segment Routing June 2019
o A core node connects only one leaf domain.
o Each leaf domain has 6,000 leaf-node segments. Each leaf node has
500 endpoints attached and thus 500 adjacency segments. This
yields a total of 3 million endpoints for a leaf domain.
Based on the above, the network scaling numbers are as follows:
o 6,000 leaf-node segments multiplied by 100 leaf domains:
600,000 nodes.
o 600,000 nodes multiplied by 500 endpoints: 300 million endpoints.
The node scaling numbers are as follows:
o Leaf-node segment scale: 6,000 leaf-node segments + 300 core-node
segments + 500 adjacency segments = 6,800 segments.
o Core-node segment scale: 6,000 leaf-domain segments +
300 core-domain segments = 6,300 segments.
In the above calculations, the link-adjacency segments are not taken
into account. These are local segments and, typically, less than 100
per node.
It has to be noted that, depending on leaf-node FIB capabilities,
leaf domains could be split into multiple smaller domains. In the
above example, the leaf domains could be split into six smaller
domains so that each leaf node only needs to learn 1,000 leaf-node
segments + 300 core-node segments + 500 adjacency segments, yielding
a total of 1,800 segments.
6. Design Options
This section describes multiple design options to illustrate scale as
described in the previous section.
6.1. Segment Routing Global Block (SRGB) Size
In the simplified illustrations in this document, we picked a small
homogeneous SRGB range of 16000-23999. In practice, a large-scale
design would use a bigger range, such as 16000-80000 or even larger.
A larger range provides allocations for various TE applications
within a given domain.
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6.2. Redistribution of Routes for Agg Nodes
The operator might choose to not redistribute the routes for Agg
nodes into the Metro/DC domains. In that case, more segments are
required in order to express an inter-domain path.
For example, node A would use an SRTE Policy {DCI1, Agg1, Agg3,
DCI3, Z} in order to reach Z instead of {Agg3, DCI3, Z} in the
reference design.
6.3. Sizing and Hierarchy
The operator is free to choose among a small number of larger leaf
domains, a large number of small leaf domains, or a mix of small and
large core/leaf domains.
The operator is free to use a two-tier (Core/Metro) or three-tier
(Core/Metro/DC) design.
6.4. Local Segments to Hosts/Servers
Local segments can be programmed at any leaf node (e.g., node Z) in
order to identify locally attached hosts (or Virtual Machines (VMs)).
For example, if node Z has bound a local segment 40001 to a local
host ZH1, then node A uses the following SRTE Policy in order to
reach that host: {16006, 18006, 20001, 40001}. Such a local segment
could represent the NID (Network Interface Device) in the context of
the service provider access network, or a VM in the context of the DC
network.
6.5. Compressed SRTE Policies
As an example and according to Section 3, we assume that node A can
reach node Z (e.g., with a low-latency SLA contract) via the SRTE
policy that consists of the path Agg1, Agg2, Agg3, DCI3/4(anycast),
Z. The path is represented by the segment list {16001, 16002, 16003,
18006, 20001}.
It is clear that the control-plane solution can install an SRTE
Policy {16002, 16003, 18006} at Agg1, collect the binding SID
allocated by Agg1 to that policy (e.g., 4001), and hence program
node A with the compressed SRTE Policy {16001, 4001, 20001}.
From node A, 16001 leads to Agg1. Once at Agg1, 4001 leads to the
DCI pair (DCI3, DCI4) via a specific low-latency path {16002, 16003,
18006}. Once at that DCI pair, 20001 leads to Z.
Filsfils, et al. Informational [Page 7]RFC 8604 Large-Scale Segment Routing June 2019
Binding SIDs allocated to "intermediate" SRTE Policies achieve the
compression of end-to-end SRTE Policies.
The segment list {16001, 4001, 20001} expresses the same path as
{16001, 16002, 16003, 18006, 20001} but with two less segments.
The binding SID also provides for inherent churn protection.
When the core topology changes, the control plane can update the
low-latency SRTE Policy from Agg1 to the DCI pair to DC2 without
updating the SRTE Policy from A to Z.
7. Deployment Model
It is expected that this design will be used in "green field"
deployments as well as interworking ("brown field") deployments with
an MPLS design across multiple domains.
8. Benefits
The design options illustrated in this document allow
interconnections on a very large scale. Millions of endpoints across
different domains can be interconnected.
8.1. Simplified Operations
Two control-plane protocols not needed in this design are LDP and
RSVP-TE. No new protocol has been introduced. The design leverages
the core IP protocols ISIS, OSPF, BGP, and PCEP with straightforward
SR extensions.
8.2. Inter-domain SLAs
Fast reroute and resiliency are provided by TI-LFA with sub-50-ms
fast reroute upon failure of a link, node, or Shared Risk Link Group
(SRLG). TI-LFA is described in [SR-TI-LFA].
The use of anycast SIDs also provides improved availability and
resiliency.
Inter-domain SLAs can be delivered (e.g., latency vs. cost-optimized
paths, disjointness from backbone planes, disjointness from other
services, disjointness between primary and backup paths).
Existing inter-domain solutions do not provide any support for SLA
contracts. They just provide best-effort reachability across
domains.
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8.3. Scale
In addition to having eliminated the need for LDP and RSVP-TE,
per-service midpoint states have also been removed from the network.
8.4. ECMP
Each policy (intra-domain or inter-domain, with or without TE) is
expressed as a list of segments. Since each segment is optimized for
ECMP, the entire policy is optimized for ECMP. The benefit of an
anycast prefix segment optimized for ECMP should also be considered
(e.g., 16001 load-shares across any gateway from the M1 leaf domain
to the Core and 16002 load-shares across any gateway from the Core to
the M1 leaf domain).
9. IANA Considerations
This document has no IANA actions.
10. Manageability Considerations
This document describes an application of SR over the MPLS data
plane. SR does not introduce any changes in the MPLS data plane.
The manageability considerations described in [RFC8402] apply to the
MPLS data plane when used with SR.
11. Security Considerations
This document does not introduce additional security requirements and
mechanisms other than those described in [RFC8402].
12. Informative References
[RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
Computation Element (PCE)-Based Architecture", RFC 4655,
DOI 10.17487/RFC4655, August 2006,
<https://www.rfc-editor.org/info/rfc4655>.
[RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
Element (PCE) Communication Protocol (PCEP)", RFC 5440,
DOI 10.17487/RFC5440, March 2009,
<https://www.rfc-editor.org/info/rfc5440>.
[RFC7752] Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
S. Ray, "North-Bound Distribution of Link-State and
Traffic Engineering (TE) Information Using BGP", RFC 7752,
DOI 10.17487/RFC7752, March 2016,
<https://www.rfc-editor.org/info/rfc7752>.
Filsfils, et al. Informational [Page 9]RFC 8604 Large-Scale Segment Routing June 2019
[RFC8402] Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
Decraene, B., Litkowski, S., and R. Shakir, "Segment
Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
July 2018, <https://www.rfc-editor.org/info/rfc8402>.
[SR-TI-LFA]
Litkowski, S., Bashandy, A., Filsfils, C.,
Decraene, B., Francois, P., Voyer, D., Clad, F., and
P. Camarillo, "Topology Independent Fast Reroute
using Segment Routing", Work in Progress,
draft-ietf-rtgwg-segment-routing-ti-lfa-01, March 2019.
Acknowledgements
We would like to thank Giles Heron, Alexander Preusche, Steve
Braaten, and Francis Ferguson for their contributions to the content
of this document.
Contributors
The following people substantially contributed to the editing of this
document:
Dennis Cai
Individual
Tim Laberge
Individual
Steven Lin
Google Inc.
Bruno Decraene
Orange
Luay Jalil
Verizon
Jeff Tantsura
Individual
Rob Shakir
Google Inc.
Filsfils, et al. Informational [Page 10]RFC 8604 Large-Scale Segment Routing June 2019
Authors' Addresses
Clarence Filsfils (editor)
Cisco Systems, Inc.
Brussels
Belgium
Email: cfilsfil@cisco.com
Stefano Previdi
Huawei Technologies
Email: stefano@previdi.net
Gaurav Dawra (editor)
LinkedIn
United States of America
Email: gdawra.ietf@gmail.com
Wim Henderickx
Nokia
Copernicuslaan 50
Antwerp 2018
Belgium
Email: wim.henderickx@nokia.com
Dave Cooper
CenturyLink
Email: Dave.Cooper@centurylink.com
Filsfils, et al. Informational [Page 11]
