Internet DRAFT - draft-white-lsr-distoptflood
draft-white-lsr-distoptflood
Network Working Group R. White
Internet-Draft S. Hegde
Intended status: Informational T. Przygienda
Expires: January 9, 2023 Juniper Networks
July 8, 2022
IS-IS Optimal Distributed Flooding for Dense Topologies
draft-white-lsr-distoptflood-03
Abstract
In dense topologies (such as data center fabrics based on the Clos
and butterfly topologies, though not limited to these), IGP flooding
mechanisms designed for sparse topologies can "overflood," or carry
too many copies of topology and reachability information to fabric
devices. This results in slower convergence times and higher
resource utilization. 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 increasing convergence performance in dense
topologies.
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 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."
This Internet-Draft will expire on January 9, 2023.
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Copyright Notice
Copyright (c) 2022 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Contributors . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Experience . . . . . . . . . . . . . . . . . . . . . . . 3
1.4. Sample Network . . . . . . . . . . . . . . . . . . . . . 3
2. Flooding Modifications . . . . . . . . . . . . . . . . . . . 5
2.1. Optimizing Flooding . . . . . . . . . . . . . . . . . . . 5
2.2. Optimization Process . . . . . . . . . . . . . . . . . . 6
2.3. Flooding Failures . . . . . . . . . . . . . . . . . . . . 7
2.4. Flooding Example . . . . . . . . . . . . . . . . . . . . 8
2.5. A Note on Performance . . . . . . . . . . . . . . . . . . 8
3. Security Considerations . . . . . . . . . . . . . . . . . . . 9
4. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Normative References . . . . . . . . . . . . . . . . . . 9
4.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
1.1. Goals
The goal of this draft is to solve one specific set of problems
involved in operating a link state protocol in a densely meshed
topology. The problem with such topologies is the connectivity
density, which causes too many copies of identical information to be
flooded. Analysis and experiment show, for instance, that in a
butterfly fabric of around 2500 intermediate systems, each
intermediate system will receive more than 40 copies of any changed
LSP fragment. This not only wastes bandwidth and processor time,
this dramatically slows convergence speed.
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This document describes a set of modifications to existing IS-IS
flooding mechanisms which minimize the number of LSP fragments
received by individual intermediate systems, in its extreme version
to one copy per intermediate system. The mechanisms described in
this document are similar to those implemented in OSPF to support
mobile ad-hoc networks, as described in [RFC5449], [RFC5614], and
[RFC7182]. These mechanisms have been widely implemented and
deployed.
1.2. Contributors
The following people have contributed to this draft: Abhishek Kumar,
Nikos Triantafillis, Ivan Pepelnjak, Christian Franke, Hannes
Gredler, Les Ginsberg, Naiming Shen, Uma Chunduri, Nick Russo, Shawn
Zandi, and Rodny Molina.
1.3. Experience
Laboratory tests show modifications similar to these reduce flooding
in a large scale emulated butterfly network topology; without these
modifications, intermediate systems receive, on average, 40 copies of
any changed LSP fragment. With the modifications described in this
document intermediate systems recieve, on average, two copies of any
changed LSP fragment. In many cases, each intermediate system
receives only a single copy of each changed LSP. In terms of
performance, the modifications described here cut convergence times
in half. Processor load times were not checked, as this was an
emulated environment.
A mechanism similar to the one described in this document has been
implemented in the FR Routing open source routing stack as part of
fabricd.
1.4. Sample Network
The following spine and leaf fabric will be used to describe these
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
To reduce confusion (spine and leaf fabrics are difficult to draw in
plain text art), this diagram does not contain the connections
between devices. The reader should assume that each device in a
given layer is connected to every device in the layer above it. For
instance:
o 5A is connected to 4A, 4B, 4C, 4D, 4E, and 4F
o 5B is connected to 4A, 4B, 4C, 4D, 4E, and 4F
o 4A is connected to 3A, 3B, 3C, 3D, 3E, 3F, 5A, 5B, 5C, 5D, 5E, and
5F
o 4B is connected to 3A, 3B, 3C, 3D, 3E, 3F, 5A, 5B, 5C, 5D, 5E, and
5F
o etc.
The tiers or stages of the fabric are also marked for easier
reference. T0 is assumed to be connected to application servers, or
rather they are Top of Rack (ToR) intermediate systems. The
remaining tiers, T1 and T2, are connected only to other devices in
the fabric itself. A common alternate representation of this
topology is drawn "folded" with T2, the "top of fabric," shown on
top, while T1 is shown below, and T0 below T1.
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2. Flooding Modifications
Flooding is perhaps the most challenging scaling issue for a link
state protocol running on a dense, large scale topology. This
section describes detailed modifications to the IS-IS flooding
process to reduce flooding load in a densely meshed topology.
2.1. Optimizing Flooding
The simplest way to conceive of the solution presented here is in two
stages:
o Stage 1: Forward Optimization
o
* 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
o Stage 2: Reverse Optimization
o
* 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 will, in turn, flood this modified
LSP to 3A (for instance). 3A will receive 6 copies of the modified
LSP, while only one copy is necessary for the intermediate systems
shown to converge on a single view of the topology. If 4A-4F could
determine they will all flood identical copies of the modified LSP to
3A, it is 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 the flooding group is
for each intermediate system to calculate a special Shortest-path
Spanning Tree (SPT) from the point of view of the transmitting
neighbor. 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
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neighbors. If every intermediate system in the flooding set performs
this same calculation, they will all obtain the same flooding group.
Once this flooding group is determined, the members of the flooding
group will each (independently) choose which of the members should
re-flood the received information. Each member of the flooding group
calculates this independently of all the other members, but a common
hash MUST be used across a set of shared variables so each member of
the group comes to the same conclusion. The group member which is
selected to flood the changed LSP does so normally; the remaining
group members do not flood the LSP.
Note there is no signaling between the intermediate systems running
this flooding reduction mechanism. 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 a shared hash
function, each member of the flooding group will make the same
decision.
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.
2.2. Optimization Process
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:
o Set all link metrics to 1
o Calculate an SPT truncated to 2 hops from the perspective of TN
o For each IS that is two hops (has a metric of two in the truncated
SPT) from TN:
o
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* If the IS is on the shortest path towards the originator of the
modified LSP, skip
* If the IS is the neighbor of the LSP originator, skip
* If the IS is not on the shortest path towards the originator of
the modified LSP, add it to THL
o Add each IS that is one hop away from TN to the RNL
Step 2: Sort RNL by system IDs, from the least to the greatest.
Step 3: Calculate a number, N, by adding each byte in LSP-ID (without
the fragment ID) and fragment ID MOD 2 (allowing for some balancing
of LSPs coming from same system ID without introducing excessive
amount of state in an implementation) and then taking MOD on the
number of neighbors. N MUST be less than the number of members of
RNL.
Step 4: Starting with the Nth member of RNL:
o If THL is empty, exit
o If this member of RNL is the local calculating IS, this IS MUST
reflood the modified LSP; exit
o Remove all members of THL connected to (adjacent to) this member
of RNL
o Move to the next member of RNL, wrapping to the beginning of RNL
if necessary
Note: This description is geared to clarity rather than optimal
performance.
2.3. Flooding Failures
It is possible in some failure modes for flooding to be incomplete
because of the flooding optimizations outlined. Specifically, if a
reflooder fails, or is somehow disconnected from all the links across
which it should be reflooding, it is possible an LSP is only
partially flooded through the fabric. To prevent such partition
failures, an intermediate system which does not reflood an LSP (or
fragment) should:
o Set a short timer; the default should be one second
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o 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
o Process any Partial Sequence Number Packets (PSNPs) received that
indicate that neighbors still have older versions of the LSP per
normal protocol procedures to resynchronize
o If resynchronization above a configurable threshold is required,
an implementation SHOULD notify the network operator
2.4. Flooding Example
Assume, in the network above, 5A floods some modified LSP towards 4A-
4F. To determine whether 4A should flood this LSP to 3A-3F:
o 5A is TN; 4A calculates a truncated SPT from 5A's perspective with
all link metrics set to 1
o 4A builds THL, which contains 3A, 3B, 3C, 3D, 3E, 3F, 5B, 5C, 5D,
5E and 5F
o 4A builds RNL, which contains 4A,4B,4C,4D,4E and 4F, sorting it by
the system ID
o 4A computes hash on the received LSP-ID to get N; assume N is 1 in
this case
o Since 4A is the Nth member of R-NL and there are members in N-NL,
4A must reflood; the loop exits
2.5. A Note on Performance
The calculations described here are 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. The description provided here is designed for clarity
rather than optimal calculation, however. Many of the calculations
can be performed in advance and stored, rather than being performed
for each LSP and each neighbor. Optimized versions of the process
described here have been implemented, and do result in strong
convergence speed gains.
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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.
4. References
4.1. Normative References
[I-D.ietf-lsr-dynamic-flooding]
Li, T., Przygienda, T., Psenak, P., Ginsberg, L., Chen,
H., Cooper, D., Jalil, L., Dontula, S., and G. S. Mishra,
"Dynamic Flooding on Dense Graphs", draft-ietf-lsr-
dynamic-flooding-11 (work in progress), June 2022.
[ISO10589]
International Organization for Standardization,
"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, Nov 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>.
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[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>.
[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
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[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", draft-ietf-isis-segment-routing-
extensions-25 (work in progress), May 2019.
[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>.
[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>.
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[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>.
[RFC7182] Herberg, U., Clausen, T., and C. Dearlove, "Integrity
Check Value and Timestamp TLV Definitions for Mobile Ad
Hoc Networks (MANETs)", RFC 7182, DOI 10.17487/RFC7182,
April 2014, <https://www.rfc-editor.org/info/rfc7182>.
[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
Juniper Networks
Email: russ@riw.us
Shraddha Hegde
Juniper Networks
Email: shraddha@juniper.net
Tony Przygienda
Juniper Networks
Email: prz@juniper.net
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