wdiff draft-white-lsr-distoptflood-00.txt draft-white-lsr-distoptflood-01.txt

Network Working Group R. White
Internet-Draft S. Hegde
Intended status: Informational T. Przygienda
Expires: 3 September 2022 Juniper Networks
Expires: June 2, 2021 S. Zandi
LinkedIn
November 29, 2020
2 March 2022

IS-IS Optimal Distributed Flooding for Dense Topologies
draft-white-lsr-distoptflood-00
draft-white-lsr-distoptflood-01

Abstract

In dense topologies, such topologies (such as data center fabrics based on the Clos
and butterfly fabric topologies, butterfly, though not limited to these topologies), flooding
mechanisms designed for sparse topologies, when used in these dense topologies, topologies can
"overflood," flood excessively, or
carry too many copies of topology and reachability 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 to a
minimum, while increaseing increasing convergence performance in dense
topologies.

Note that a Clos fabric is used as the primary example of a desne dense
flooding topology throughout this document. However, the flooding
optimizations described in this document apply to any dense 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
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on June 2, 2021. 3 September 2022.

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Please review these documents carefully, as they describe your rights
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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 . . . . . . . . . . . . . . . . . . . . 6
3. Use of Flooding Leaders and 7
2.4. Flooding Mechanism Advertisements 6
4. Security Considerations Example . . . . . . . . . . . . . . . . . . . 6
5. References . 7
2.5. A Note on Performance . . . . . . . . . . . . . . . . . . 8
3. Security Considerations . . . . . . . 7
5.1. Normative . . . . . . . . . . . . 8
4. References . . . . . . . . . . . . . . . . . . 7
5.2. Informative . . . . . . . 8
4.1. Normative References . . . . . . . . . . . . . . . . . . 8
Appendix A. Flooding Optimization Operation
4.2. Informative References . . . . . . . . . . . . . . . . . 10
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 11

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 dense mesh topology.
The problem with such topologies is the connectivity density, which
causes too much information to be flooded (or too much repeated state
to be flooded). Analysis and experiment show, for instance, that in
a butterfly fabric of around 2500 intermediate systems, each
intermediate system will receive 40+ copies of any changed LSP
fragment. This not only wastes bandwidth and processor time, this
dramatically slows convergence speed.

This document describes a set of modifications to existing IS-IS
flooding mechanisms which minimize the number of LSP framgents
received by individual intermediate systems, potentially to one copy
per intermediate system. The mechanism described here chooses
different flooding intermediates using a hash across the system ID to
prevent single failures from having a large impact on flooding, and
to spread the processing load of flooding across many systems and
prevent bottlenecks.

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 Mechanisms similar to these
have been widely deployed implemented and tested. 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, and
Rodny Molina.

1.3. Experience

The modifications described in this draft have been implemented in
the FR Routing open source routing stack, and hence are available for
testing and modification. The implementation is part of the
openfabric daemon, which can be conditionally compiled from isisd.
Note openfabricd has further modifications are not described in this
document.

Lab testing shows these 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 these modifications, intermediate
systems recieve, on average, two copies of any changed LSP fragment.
In many cases, each intermediate system receives one copy of each
changed LSP. In terms of performance, the modifications described
here reduce convergence times by around 50%. A network that converges
in about 30-40 seconds without these modifications converged in 15-20
seconds with these modifications. 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.

+----+ +----+ +----+ +----+ +----+ +----+
| 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:

* 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 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.

2. Flooding Modifications

Flooding is perhaps the most challenging scaling issue for a link
state protocol running on a dense, large scale fabric. This section
describes modifications to the IS-IS flooding process to reduce
flooding load on a dense or mesh topology.

2.1. Optimizing Flooding

To reduce the flooding

The simplest way to conceive of link state information the solution presented here is in two
stages:

* Stage 1: Forward Optimization

- Find the form group of intermediate systems that will all flood to
the same set of neighbors as the local IS

- Decide (deterministially) which of the intermediate systems
within this group should 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 Units (LSPs),
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 following tables are required modified
LSP, while only one copy is necessary for the intermediate systems
shown to compute converge on a set of reflooders:

o Neighbor List (NL) list: The set single view of neighbors

o Neighbor's Neighbors (NN) list: The set the topology. If 4A-4F could
determine they will all flood identical copies of neighbor's neighbors;
this can be calculated by running SPF truncated the modified LSP to two hops

o Do Not Reflood (DNR) list: The set
3A, it is possible for all of neighbors who should have
LSPs (or fragments) who should them except one to decide not reflood LSPs

o Reflood (RF) list: The set of neighbors who should to flood LSPs (or
fragments)
the changed LSP to their adjacent neighbors 3A.

The technique used in this draft to ensure synchronization

NL determine the flooding group is set
for each intermediate system to contain calculate a special SPT from the
point of view of the transmitting neighbor. By setting the metric of
all neighbors, links to 1 and sorted deterministically (for
instance, from truncating the highest IS identifier SPT to two hops, the lowest). All local IS can
find the group of neighbors it will flood any changed LSP towards and
the set of intermediate systems within a single fabric SHOULD use (not necessarily neighbors) which
will also flood to this same set of neighbors. If every intermediate
system in the flooding set performs this same
mechanism for sorting calculation, they will
all obtain the NL list. NN same flooding group.

Once this flooding group is determined, the members of the flooding
group will each (independently) choose which of the members should
flood. Each member of the flooding group calculates this
independently of all the other members, using a common hash across a
set of shared variables so each member of the group comes to contain all
neighbor's neighbors, or all 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 that are two hops
away, as determined by performing running
this flooding reduction mechanism. Each IS calculates a special,
truncated SPF. The DNR SPT separately, and RF
tables determines which IS should flood any
changed LSPs independently. Because these calculations are initially empty. To begin, performed
using a shared view of the following steps are taken
to reduce network, however (based on the size common link
state database) and a shared sorting hash, each member of NN 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 NL:

o Remove all 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 systems system will determine whether it should reflood as
described below, when a modified LSP arrives from NL a Transmitting
Neighbor (TN), by:

Step 1: Build the Two-Hop List (THL) and NN Remote Neighbor's List (RNL)
by:

* Set all link metrics to 1

* Calculate an SPT truncated to 2 hops from the perspective of TN

* For each IS that is two hops (has a metric of two in the truncated
SPT) from TN:

- If the IS is on the shortest path to towards the IS that originated originator of the LSP

Then, for every IS in NL:

o
modified LSP, skip

- If the current entry in NL IS is connected to any entries in NN:

* Move not on the IS shortest path towards the originator of
the modified LSP, add it to RF THL

* Remove the intermediate systems connected Add each IS that is one hop away from TN to the IS RNL
Step 2: Sort RNL by system IDs, from NN

o Else move the IS least to DNR

The calculation terminates when the NL is empty.

When flooding, LSPs transmitted to adjacent neighbors greatest.

Step 3: Calculate a number, N, by adding each byte in the LSP
originator's System-ID and then taking MOD on the RF list
will number of
neighbors. N MUST be transmitted normally. Adjacent intermediate systems on less than the number of members of RNL.

Step 4: Starting with the Nth member of RNL:

* If THL is empty, exit

* If this
list will member of RNL is the local calculating IS, this IS MUST
reflood received LSPs into the next stage modified LSP; exit

* Remove all members of THL connected to (adjacent to) this member
of RNL

* Move to the topology,
ensuring database synchronization. LSPs transmitted next member of RNL, wrapping to adjacent
neighbors on the DNR list, however, MUST be transmitted using a
circuit scope PDU as described in [RFC7356].

2.2. 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 situations,
any IS receiving faliures, an
intermediate system which does not reflood an LSP transmitted using DNR SHOULD:

o (or fragment)
should:

* Set a short timer; the default should be less than one second

* When the timer expires, send a Complete Partial Sequence Number Packet
(CSNP)
(PSNP) to all neighbors

* Process any Partial Sequence Number Packets (PSNPs) as required to
resynchronize

o If

* Log/notify if a resynchronization is required, notify required

2.4. Flooding Example

Assume, in the network operator
through above, 5A floods some modified LSP towards 4A-
4F. To determine whether 4A should flood this LSP to 3A-3F:

* 5A is TN; 4A calculates a network management system

3. Use of Flooding Leaders 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 Flooding Mechanism Advertisements

[I-D.ietf-lsr-dynamic-flooding], section 5.1.1, describes the
election of a flooding domain leader, 5F

* 4A builds RNL, which can advertise contains 4A,4B,4C,4D,4E and 4F, sorting it by
the kind of
flooding reduction mechanism used in system ID

* 4A computes hash on the flooding domain.
Implementations of received LSP-ID to get N; assume N is 1 in
this draft MAY implement case

* Since 4A is the election and
advertisement Nth member of a flooding domain leader as described R-NL and there are members in section
5.1.1 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 [I-D.ietf-lsr-dynamic-flooding]. If calculation is so much higher
than the election cost of a flooding domain leader that this optimization is implemented, implementations SHOULD also
advertise counter-
productive. The description provided here is designed for clarity
rather than optimal calculation, however. Many of the flooding mechanism using calculations
can be performed in advance and stored, rather than being performed
for each LSP and each neighbor. Optimized versions of the IS-IS Dynamic Flooding
Sub-TLV process
described here have been implemented, and do result in section 5.1.2 of
[I-D.ietf-lsr-dynamic-flooding], using Algorithm number (TBD).

4. 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.

4. References

5.1.

4.1. Normative References

[I-D.ietf-lsr-dynamic-flooding]
Li, T., Przygienda, T., Psenak, P., Ginsberg, L., Chen,
H., Przygienda,
T., Cooper, D., Jalil, L., Dontula, S., and G. S. Mishra,
"Dynamic Flooding on Dense Graphs", draft-ietf-lsr-
dynamic-flooding-07 (work Work in progress), June 2020. Progress,
Internet-Draft, draft-ietf-lsr-dynamic-flooding-10, 7
December 2021, <https://www.ietf.org/archive/id/draft-
ietf-lsr-dynamic-flooding-10.txt>.

[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 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>.

[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>.

5.2.

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", draft-ietf-isis-segment-routing-
extensions-25 (work Work in progress), Progress, Internet-Draft, draft-
ietf-isis-segment-routing-extensions-25, 19 May 2019. 2019,
<https://www.ietf.org/archive/id/draft-ietf-isis-segment-
routing-extensions-25.txt>.

[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>.

[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>.

Appendix A. Flooding Optimization Operation

Recent testing has shown that flooding is largely a "non-issue" in
terms of scaling when using high speed links connecting intermediate
systems with reasonable processing power and memory. However,
testing has also shown that flooding will impact convergence speed
even in such environments, and flooding optimization has a major
impact on the performance of a link state protocol in resource
constrained environments. Some thoughts on flooding optimization in
general, and the flooding optimization contained in this document,
follow.

There are two general classes of flooding optimization available for
link state protocols. The first class of optimization relies on a
centralized service or server to gather the link state information
and redistribute it back into the intermediate systems making up the
fabric. Such solutions are attractive in many, but not all,
environments; hence these systems compliment, rather than compete
with, the system described here. Systems relying on a service or
server necessarily also rely on connectivity to that service or
server, either through an out-of-band network or connectivity through
the fabric itself. Because of this, these mechanisms do not apply to
all deployments; some deployments require underlying reachability
regardless of connectivity to an outside service or server.

The second possibility is to create a fully distributed system that
floods the minimal amount of information possible to every
intermediate system. The system described in this draft is an
example of such a system. Again, there are many ways to accomplish
this goal, but simplicity is a primary goal of the system described
in this draft.

The system described here divides the work into two different parts;
forward and reverse optimization. The forward optimization begins by
finding the set of intermediate systems two hops away from the
flooding device, and choosing a subset of connected neighbors that
will successfully reach this entire set of intermediate systems, as
shown in the diagram below.

G
|
A B C--+
| | | |
+--D--+ E H
| | |
+----F--+--+

Figure 2

If F is flooding some piece of information, then it will find the
entire set of intermediate systems within two hops by discovering its
neighbors and their neighbors from the local LSDB. This will include
A, B, C, D, and E--but not G. From this set, F can determine that D
can reach A and B, while a single flood to either E or H will reach
C. Hence F can flood to D and either E or H to reach C. F can
choose to flood to D and E normally. Because H still needs to
receive this new LSP (or fragment!), but does not need to reflood to
C, F can send the LSP using link local signaling. In this case, H
will receive and process the new LSP, but not reflood it.

Rather than carrying the information necessary through hello
extensions, as is done in [RFC5820], the neighbors are allowed to
complete initial synchronization, and then a truncated shortest path
tree is built to determine the "two hop neighborhood." This has the
advantage of using mechanisms already used in IS-IS, rather than
adding new processes. The risk with this process is any LSPs flooded
through the network before this initial calculation takes place will
be suboptimal. This "two hop neighborhood" process has been used in
OSPF deployments for a number of years, and has proven stable in
practice.

Rather than setting a timer for reflooding, the implementation
described here uses IS-IS' ability to describe the entire database
using a CSNP to ensure flooding is successful. This adds some small
amount of overhead, so there is some balance between optimal flooding
and ensuring flooding is complete.

The reverse optimization is simpler. It relies on the observation
that any intermediate system between the local IS and the origin of
the LSP, other than in the case of floods removing an LSP from the
shared LSDB, should have already received a copy of the LSP. For
instance, if F originates an LSP in the figure above, and E refloods
the LSP to C, C does not need to reflood back to F if F is on its
shortest path tree towards F. It is obvious this is not a "perfect"
optimization. A perfect optimization would block flooding back along
a directed acyclic graph towards the originator. Using the SPT,
however, is a quick way to reduce flooding without performing more
calculations.

The combination of these two optimizations have been seen, in
testing, to reduce the number of copies any IS receives from the tens
to precisely one.

Authors' Addresses

Russ White
Juniper Networks
Email: russ@riw.us
Shraddha Hegde
Juniper Networks
Email: shraddha@juniper.net

Shawn Zandi
LinkedIn

Tony Przygienda
Juniper Networks
Email: szandi@linkedin.com prz@juniper.net