IS-IS Optimal Distributed Flooding for Dense Topologies
draft-ietf-lsr-distoptflood-03
| Document | Type | Active Internet-Draft (lsr WG) | |
|---|---|---|---|
| Authors | Russ White , Shraddha Hegde , Tony Przygienda | ||
| Last updated | 2024-02-09 | ||
| Replaces | draft-white-lsr-distoptflood | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | (None) | ||
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
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draft-ietf-lsr-distoptflood-03
Network Working Group R. White
Internet-Draft Akamai
Intended status: Experimental S. Hegde
Expires: 12 August 2024 T. Przygienda
Juniper Networks
9 February 2024
IS-IS Optimal Distributed Flooding for Dense Topologies
draft-ietf-lsr-distoptflood-03
Abstract
In dense topologies (such as data center fabrics based on the Clos
and butterfly topologies, though not limited to those exclusively),
IGP flooding mechanisms designed originally for sparse topologies can
"overflood," or in other words generate too many identical copies of
topology and reachability information arriving at a given node from
other devices. This normally results in slower convergence times and
higher resource utilization to process and discard the superfluous
copies. The modifications to the flooding mechanism in the
Intermediate System to Intermediate System (IS-IS) link state
protocol described in this document reduce resource utilization
significantly, while increaseing convergence performance in dense
topologies. Beside reducing the extraneous copies it uses the dense
topologies to "load-balance" flooding across different possible paths
in the network to prevent build up of flooding hot-spots.
Note that a Clos fabric is used as the primary example of a dense
flooding topology throughout this document. However, the flooding
optimizations described in this document apply to any arbitrary
topology.
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."
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This Internet-Draft will expire on 12 August 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
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
1.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Contributors . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Experimental Evidence . . . . . . . . . . . . . . . . . . 3
1.4. Example Network . . . . . . . . . . . . . . . . . . . . . 3
2. Flooding Modifications . . . . . . . . . . . . . . . . . . . 5
2.1. Optimizing Flooding . . . . . . . . . . . . . . . . . . . 5
2.2. Optimization Process Details . . . . . . . . . . . . . . 6
2.3. Flooding Failures . . . . . . . . . . . . . . . . . . . . 8
2.4. Signaling . . . . . . . . . . . . . . . . . . . . . . . . 8
2.5. Additional Deployment Considerations . . . . . . . . . . 9
2.6. Flooding Example . . . . . . . . . . . . . . . . . . . . 9
2.7. A Note on Performance . . . . . . . . . . . . . . . . . . 9
3. Security Considerations . . . . . . . . . . . . . . . . . . . 9
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Normative References . . . . . . . . . . . . . . . . . . 10
4.2. Informative References . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
1.1. Goals
The goal of this draft is to solve one of the problems occurring when
operating a link state protocol in a densely meshed topology. Such
topologies with high average fanout, causes too many copies of
identical information to be flooded within the network. Analysis and
experiments show, for instance, that in a butterfly fabric of around
2'500 intermediate systems, each intermediate system will receive
over 40 copies of any changed LSP fragment. This not only wastes
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bandwidth and processor time, this dramatically slows convergence
speed under topological changes.
This document describes a set of modifications to the existing IS-IS
flooding mechanisms which will minimize the number of LSP fragments
received by individual intermediate systems. In its extreme version
the change leads to only one copy per intermediate system being
processed. The mechanisms described in this document are similar to
and based on those implemented in OSPF to support mobile ad-hoc
networks, as described in[RFC5449],[RFC5614]. These solutions have
been widely implemented and deployed.
1.2. Contributors
The following people have contributed to this draft and are mentioned
without any particular order: Abhishek Kumar, Nikos Triantafillis,
Ivan Pepelnjak, Christian Franke, Hannes Gredler, Les Ginsberg,
Naiming Shen, Uma Chunduri, Nick Russo, and Rodny Molina.
1.3. Experimental Evidence
Laboratory tests based on a well known open source codebase show that
modifications similar to the ones described in this draft reduce
flooding in a large scale emulated butterfly network topology
signficantly. Under unmodified flooding procedurs intermediate
systems receive, on average, 40 copies of any changed LSP fragment in
a 2'500 nodes butterfly network. With the changes described in this
document said systems received, on average, two copies of any changed
LSP fragment. In many cases, only a single copy of each changed LSP
was received and processed per node. In terms of performance,
overall convergence times were cut in roughly half.
An early version of mechanisms described in this document has been
implemented in the FR Routing open source routing stack as part of
`fabricd` daemon.
1.4. Example Network
Following spine and leaf fabric will be used in further description
of the introduced modifications.
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+====+ +====+ +====+ +====+ +====+ +====+
| 1A | | 1B | | 1C | | 1D | | 1E | | 1F | (T0)
+====+ +====+ +====+ +====+ +====+ +====+
+====+ +====+ +====+ +====+ +====+ +====+
| 2A | | 2B | | 2C | | 2D | | 2E | | 2F | (T1)
+====+ +====+ +====+ +====+ +====+ +====+
+====+ +====+ +====+ +====+ +====+ +====+
| 3A | | 3B | | 3C | | 3D | | 3E | | 3F | (T2)
+====+ +====+ +====+ +====+ +====+ +====+
+====+ +====+ +====+ +====+ +====+ +====+
| 4A | | 4B | | 4C | | 4D | | 4E | | 4F | (T1)
+====+ +====+ +====+ +====+ +====+ +====+
+====+ +====+ +====+ +====+ +====+ +====+
| 5A | | 5B | | 5C | | 5D | | 5E | | 5F | (T0)
+====+ +====+ +====+ +====+ +====+ +====+
Figure 1
The above picture does not contain the connections between devices
for readability purposes. The reader should assume that each device
in a given layer is connected to every device in the layer above it
in a butterfly network fashion. For instance:
* 5A is connected to 4A, 4B, 4C, 4D, 4E, and 4F
* 5B is connected to 4A, 4B, 4C, 4D, 4E, and 4F
* 4A is connected to 3A, 3B, 3C, 3D, 3E, 3F, 5A, 5B, 5C, 5D, 5E, and
5F
* 4B is connected to 3A, 3B, 3C, 3D, 3E, 3F, 5A, 5B, 5C, 5D, 5E, and
5F
* etc.
The tiers or stages of the fabric are marked for easier reference.
Alternate representation of this topology is a "folded Clos" with T2
being the "top of the fabric" and T0 representing the leaves.
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2. Flooding Modifications
This section describes detailed modifications to the IS-IS flooding
process to reduce flooding load in a densely meshed topology. It
does at the same time distribute the reduced flooding across the
whole topology to prevent hot-spots.
2.1. Optimizing Flooding
The simplest way to conceive of the solution presented here is in two
stages:
* Stage 1: Forward Optimization
- Find the group of intermediate systems that will all flood to
the same set of neighbors as the local IS
- Decide (deterministically) which subset of the intermediate
systems within this group should re-flood any received LSPs
* Stage 2: Reverse Optimization
- Find neighbors on the shortest path towards the origin of the
change
- Do not flood towards these neighbors
The first stage is best explained through an illustration. In the
network above, if 5A transmits a modified Link State Protocol Data
Unit (LSP) to 4A-4F, each of 4A-4F nodes will, in turn, flood this
modified LSP to 3A (for instance). With this, 3A will receive 6
copies of the modified LSP, while only one copy is necessary for the
intermediate systems shown to converge on the same view of the
topology. If 4A-4F could determine that all of them will all flood
identical copies of the modified LSP to 3A, it would be possible for
all of them except one to decide not to flood the changed LSP to 3A.
The technique used in this draft to determine such flooding group is
for each intermediate system to calculate a special SPT (shortest-
path spanning tree) from the point of view of the transmitting
neighbor. As next step, by setting the metric of all links to 1 and
truncating the SPT to two hops, the local IS can find the group of
neighbors it will flood any changed LSP towards and the set of
intermediate systems (not necessarily neighbors) which will also
flood to this same set of neighbors. If every intermediate system in
the flooding set performs this same calculation, they will all obtain
the same flooding group.
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Once such a flooding group is determined, the members of the flooding
group will each (independently) choose which of the members should
re-flood the received information. A common hash function is used
across a set of shared variables so each member of the group comes to
the same conclusion as to the designated flooding nodes. The group
member which is in such a way `selected` to flood the changed LSP
does so normally; the remaining group members suppress the flooding
of the LSP initially.
Note that there is no signaling between the intermediate systems
running this flooding reduction mechanism for the solution to work.
Each IS calculates the special, truncated SPT separately, and
determines which IS should flood any changed LSPs independently based
on a common hash function. Because these calculations are performed
using a shared view of the network, however (based on the common link
state database) and such a shared hash function, each member of the
flooding group will make the same decision under converged
conditions. In the transitory state of nodes having potentially
different view of topologies the flooding may either overflood or in
worse case not flood enough for which we introduce a 'quick-patching'
mechanism later but ultimately will converge due to periodic CSNP
origination per normal protocol operation.
The second stage is simpler, consisting of a single rule: do not
flood modified LSPs along the shortest path towards the origin of the
modified LSP. This rule relies on the observation that any IS
between the origin of the modified LSP and the local IS should
receive the modified LSP from some other IS closer to the source of
the modified LSP. It is worth to observe that if all the nodes that
should be designated to flood within a peer group are pruned by the
second stage the receiving node is at the `tail-end` of the flooding
chain and no further flooding will be necessary. Also, per normal
protocol procedures flooding to the node from which the LSP has been
received will not be performed.
2.2. Optimization Process Details
This section provides normative description of the specification.
Any node implementing this solution MUST exhibit external behavior
that conforms to the algorithms provided.
Each intermediate system will determine whether it should re-flood
LSPs as described below. When a modified LSP arrives from a
Transmitting Neighbor (TN), the result of the following algorithm
obtains the necessary decision:
Step 1: Build the Two-Hop List (THL) and Remote Neighbor's List (RNL)
by:
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A) Set all link metrics to 1
B) Calculate an SPT truncated to 2 hops from the perspective of TN
C) For each IS that is two hops away (has a metric of two in the
truncated SPT) from TN:
i. If the IS is the LSP originator, skip
ii. If the IS is a neighbor of the LSP originator, skip
iii. If the IS is on the shortest path from the TN towards
towards the originator of the modified LSP, skip
iv. If the IS is *not* on the shortest path from the TN towards
the originator of the modified LSP, add it to THL
D) Add each IS that is one hop away from TN to the RNL
Step 2: Sort nodes in RNL by system IDs, from the least value to the
greatest.
Step 3: Calculate a number, H, by adding each byte in LSP-ID under
consideration. RNum is the number of nodes in the RNL.
Consequently, set N to the H MOD of RNum (N=H MOD RNum). With that N
will be less than the number of members of RNL. (footnote 1: this
allows for some balancing of LSPs coming from same system ID).
Step 4: Starting with the Nth member of RNL: where N is the index
into the members in RNL, with index starting from zero (Index zero
assigned to the IS with lowest system-id):
A) If THL is empty, exit
B) If this member of RNL is the local calculating IS, it MUST
reflood the modified LSP; exit
C) Remove all members of THL connected to (adjacent to) this member
of RNL
D) Move to the next member of RNL, wrapping to the beginning of RNL
if necessary
Note 1: This description is leaning towards clarity rather than
optimal performance when implemented.
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Note 2: An implementation in a node MAY choose independently of
others to provide a configurable parameter to allow for more than one
node in RNL to reflood, e.g. it may reflood even if it's only the
member that would be chosen from the RNL if a double coverage of THL
is required. The modifications to the algorithm are simple enough to
not require further text.
2.3. Flooding Failures
It is possible that during initial convergence or in some failure
modes the flooding will be incomplete due to the optimizations
outlined. Specifically, if a reflooder fails, or is somehow
disconnected from all the links across which it should be reflooding,
an LSP could be only partially distributed through the topology. To
speed up convergence under such partition failures (observe that
periodic CSNPs will under any circumstances converge the topology
though at a slower pace), an intermediate system which does not
reflood a specific LSP (or fragment) SHOULD:
A) Set a short, configurable timer which should be significantly
shorter than CSNP interval used.
B) When the timer expires, send Partial Sequence Number Packet
(PSNP) of all LSPs that have *not* been reflooded during the
timer runtime to all neighbors unless an up-to-date PSNP or CSNP
has been already received from the neighbor.
C) Per normal protocol procedures process any Partial Sequence
Number Packets (PSNPs) received that indicate that neighbors
still have older versions of the LSP will lead to the usual
synchronization of the databases that are out of sync due to
optimized flooding.
D) If such resynchronizations above a configurable threshold are
required (i.e. PSNPs are sent to the neighbors and are answered
with requests), an implementation SHOULD notify the network
operator via the according mechanism about the condition.
2.4. Signaling
A node deploying this algorithm SHOULD advertise algorithm value
<TBD> in the IS-IS Dynamic Flooding sub-TLV of the Router Capability
TLV (242) [RFC7981] as specified in [I-D.ietf-lsr-dynamic-flooding].
It bares repeating again that in case the hashing algorithm a node
uses is different from this draft a different algorithm number must
be assigned and used.
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2.5. Additional Deployment Considerations
A node deploying this algorithm on point-to-point links MUST send
CSNPs on such links. This does not represent a dramatic change given
most deployed implementations today already exhibit this behavior to
prevent possible slow synchronization of IS-IS database across such
links and to provide additional periodic consistency guarantees.
2.6. Flooding Example
Assume, in the network specified, that 5A floods some modified LSP
towards 4A-4F and we only use a single node to reflood. To determine
whether 4A should flood this LSP to 3A-3F:
* 5A is TN; 4A calculates a truncated SPT from 5A's perspective with
all link metrics set to 1
* 4A builds THL, which contains 3A, 3B, 3C, 3D, 3E, 3F, 5B, 5C, 5D,
5E and 5F
* 4A builds RNL, which contains 4A,4B,4C,4D,4E and 4F, sorting it by
the system ID
* 4A computes hash on the received LSP-ID to get N; assume N is 1 in
this case
* Since 4A is the 1st member of RNL and there are members in THL, 4A
must reflood; the loop exits
2.7. A Note on Performance
The calculations described here seem complex, which might lead the
reader to conclude that the cost of calculation is so much higher
than the cost of flooding that this optimization is counter-
productive. First, The description provided here is designed for
clarity rather than optimal calculation. Second, many of the
involved calculations can be easily performed in advance and stored,
rather than being performed for each LSP occurence and each neighbor.
Optimized versions of the process described here have been
implemented, and do result in strong convergence speed gains.
3. Security Considerations
This document outlines modifications to the IS-IS protocol for
operation on high density network topologies. Implementations SHOULD
implement IS-IS cryptographic authentication, as described in
[RFC5304], and should enable other security measures in accordance
with best common practices for the IS-IS protocol.
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4. References
4.1. Normative References
[I-D.ietf-lsr-dynamic-flooding]
Li, T., Psenak, P., Chen, H., Jalil, L., and S. Dontula,
"Dynamic Flooding on Dense Graphs", Work in Progress,
Internet-Draft, draft-ietf-lsr-dynamic-flooding-15, 6
February 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-lsr-dynamic-flooding-15>.
[ISO10589] ISO, "Intermediate system to Intermediate system intra-
domain routeing information exchange protocol for use in
conjunction with the protocol for providing the
connectionless-mode Network Service (ISO 8473)", ISO/
IEC 10589:2002, Second Edition, November 2002.
[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>.
[RFC2629] Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
DOI 10.17487/RFC2629, June 1999,
<https://www.rfc-editor.org/info/rfc2629>.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120,
DOI 10.17487/RFC5120, February 2008,
<https://www.rfc-editor.org/info/rfc5120>.
[RFC5301] McPherson, D. and N. Shen, "Dynamic Hostname Exchange
Mechanism for IS-IS", RFC 5301, DOI 10.17487/RFC5301,
October 2008, <https://www.rfc-editor.org/info/rfc5301>.
[RFC5303] Katz, D., Saluja, R., and D. Eastlake 3rd, "Three-Way
Handshake for IS-IS Point-to-Point Adjacencies", RFC 5303,
DOI 10.17487/RFC5303, October 2008,
<https://www.rfc-editor.org/info/rfc5303>.
[RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
Engineering", RFC 5305, DOI 10.17487/RFC5305, October
2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5308] Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
DOI 10.17487/RFC5308, October 2008,
<https://www.rfc-editor.org/info/rfc5308>.
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[RFC5309] Shen, N., Ed. and A. Zinin, Ed., "Point-to-Point Operation
over LAN in Link State Routing Protocols", RFC 5309,
DOI 10.17487/RFC5309, October 2008,
<https://www.rfc-editor.org/info/rfc5309>.
[RFC5311] McPherson, D., Ed., Ginsberg, L., Previdi, S., and M.
Shand, "Simplified Extension of Link State PDU (LSP) Space
for IS-IS", RFC 5311, DOI 10.17487/RFC5311, February 2009,
<https://www.rfc-editor.org/info/rfc5311>.
[RFC5316] Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
Support of Inter-Autonomous System (AS) MPLS and GMPLS
Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316,
December 2008, <https://www.rfc-editor.org/info/rfc5316>.
[RFC7356] Ginsberg, L., Previdi, S., and Y. Yang, "IS-IS Flooding
Scope Link State PDUs (LSPs)", RFC 7356,
DOI 10.17487/RFC7356, September 2014,
<https://www.rfc-editor.org/info/rfc7356>.
[RFC7981] Ginsberg, L., Previdi, S., and M. Chen, "IS-IS Extensions
for Advertising Router Information", RFC 7981,
DOI 10.17487/RFC7981, October 2016,
<https://www.rfc-editor.org/info/rfc7981>.
[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>.
4.2. Informative References
[I-D.ietf-isis-segment-routing-extensions]
Previdi, S., Ginsberg, L., Filsfils, C., Bashandy, A.,
Gredler, H., and B. Decraene, "IS-IS Extensions for
Segment Routing", Work in Progress, Internet-Draft, draft-
ietf-isis-segment-routing-extensions-25, 19 May 2019,
<https://datatracker.ietf.org/doc/html/draft-ietf-isis-
segment-routing-extensions-25>.
[RFC3277] McPherson, D., "Intermediate System to Intermediate System
(IS-IS) Transient Blackhole Avoidance", RFC 3277,
DOI 10.17487/RFC3277, April 2002,
<https://www.rfc-editor.org/info/rfc3277>.
[RFC3719] Parker, J., Ed., "Recommendations for Interoperable
Networks using Intermediate System to Intermediate System
(IS-IS)", RFC 3719, DOI 10.17487/RFC3719, February 2004,
<https://www.rfc-editor.org/info/rfc3719>.
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[RFC4271] Rekhter, Y., Ed., Li, T., Ed., and S. Hares, Ed., "A
Border Gateway Protocol 4 (BGP-4)", RFC 4271,
DOI 10.17487/RFC4271, January 2006,
<https://www.rfc-editor.org/info/rfc4271>.
[RFC5304] Li, T. and R. Atkinson, "IS-IS Cryptographic
Authentication", RFC 5304, DOI 10.17487/RFC5304, October
2008, <https://www.rfc-editor.org/info/rfc5304>.
[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>.
[RFC5449] Baccelli, E., Jacquet, P., Nguyen, D., and T. Clausen,
"OSPF Multipoint Relay (MPR) Extension for Ad Hoc
Networks", RFC 5449, DOI 10.17487/RFC5449, February 2009,
<https://www.rfc-editor.org/info/rfc5449>.
[RFC5614] Ogier, R. and P. Spagnolo, "Mobile Ad Hoc Network (MANET)
Extension of OSPF Using Connected Dominating Set (CDS)
Flooding", RFC 5614, DOI 10.17487/RFC5614, August 2009,
<https://www.rfc-editor.org/info/rfc5614>.
[RFC5820] Roy, A., Ed. and M. Chandra, Ed., "Extensions to OSPF to
Support Mobile Ad Hoc Networking", RFC 5820,
DOI 10.17487/RFC5820, March 2010,
<https://www.rfc-editor.org/info/rfc5820>.
[RFC5837] Atlas, A., Ed., Bonica, R., Ed., Pignataro, C., Ed., Shen,
N., and JR. Rivers, "Extending ICMP for Interface and
Next-Hop Identification", RFC 5837, DOI 10.17487/RFC5837,
April 2010, <https://www.rfc-editor.org/info/rfc5837>.
[RFC6232] Wei, F., Qin, Y., Li, Z., Li, T., and J. Dong, "Purge
Originator Identification TLV for IS-IS", RFC 6232,
DOI 10.17487/RFC6232, May 2011,
<https://www.rfc-editor.org/info/rfc6232>.
[RFC7921] Atlas, A., Halpern, J., Hares, S., Ward, D., and T.
Nadeau, "An Architecture for the Interface to the Routing
System", RFC 7921, DOI 10.17487/RFC7921, June 2016,
<https://www.rfc-editor.org/info/rfc7921>.
Authors' Addresses
Russ White
Akamai
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Email: russ@riw.us
Shraddha Hegde
Juniper Networks
Email: shraddha@juniper.net
Tony Przygienda
Juniper Networks
Email: prz@juniper.net
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