Network Working Group H. Chen
Internet-Draft Futurewei
Intended status: Standards Track M. Toy
Expires: 4 July 2022 Verizon
Y. Yang
IBM
A. Wang
China Telecom
X. Liu
Volta Networks
Y. Fan
Casa Systems
L. Liu
Fujitsu
31 December 2021
Flooding Topology Minimum Degree Algorithm
draft-ietf-lsr-flooding-topo-min-degree-04
Abstract
This document proposes an algorithm for a node to compute a flooding
topology, which is a subgraph of the complete topology per underline
physical network. When every node in an area automatically
calculates a flooding topology by using a same algorithm and floods
the link states using the flooding topology, the amount of flooding
traffic in the network is greatly reduced. This would reduce
convergence time with a more stable and optimized routing
environment.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
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/.
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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 4 July 2022.
Copyright Notice
Copyright (c) 2021 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/
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Flooding Topology . . . . . . . . . . . . . . . . . . . . . . 3
3.1. Flooding Topology Construction . . . . . . . . . . . . . 4
4. Algorithms to Compute Flooding Topology . . . . . . . . . . . 4
4.1. Algorithm with Considering Degree . . . . . . . . . . . . 5
4.2. Algorithm with Considering Others . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
8.1. Normative References . . . . . . . . . . . . . . . . . . 7
8.2. Informative References . . . . . . . . . . . . . . . . . 7
Appendix A. FT Computation Details through Example . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
For some networks such as dense Data Center (DC) networks, the
existing Link State (LS) flooding mechanism is not efficient and may
have some issues. The extra LS flooding consumes network bandwidth.
Processing the extra LS flooding, including receiving, buffering and
decoding the extra LSs, wastes memory space and processor time. This
may cause scalability issues and affect the network convergence
negatively.
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This document proposes an algorithm for a node to compute a flooding
topology, which is a subgraph of the complete topology per underline
physical network. The physical network can be any network, including
clos leaf spine network. It can be used in the distributed mode of
flooding topology computation for flooding reduction and the
centralized mode, which are described in
[I-D.ietf-lsr-dynamic-flooding]. When the distributed mode is
selected, every node in an area automatically calculates a flooding
topology by using a same algorithm and floods the link states using
the flooding topology, the amount of flooding traffic in the network
is greatly reduced. This would reduce convergence time with a more
stable and optimized routing environment.
There may be multiple algorithms for computing a flooding topology.
Users can select one they prefer, and smoothly switch from one to
another.
2. Terminology
LSA: A Link State Advertisement in OSPF.
LSP: A Link State Protocol Data Unit (PDU) in IS-IS.
LS: A Link Sate, which is an LSA or LSP.
FT: Flooding Topology.
FTC: Flooding Topology Computation.
3. Flooding Topology
For a given network topology, a flooding topology is a sub-graph or
sub-network of the given network topology that has the same
reachability to every node as the given network topology. Thus all
the nodes in the given network topology MUST be in the flooding
topology. All the nodes MUST be inter-connected directly or
indirectly. As a result, LS flooding will in most cases occur only
on the flooding topology, that includes all nodes but a subset of
links. Note even though the flooding topology is a sub-graph of the
original topology, any single LS MUST still be disseminated in the
entire network.
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3.1. Flooding Topology Construction
Many different flooding topologies can be constructed for a given
network topology. For example, a chain connecting all the nodes in
the given network topology is a flooding topology. A circle
connecting all the nodes is another flooding topology. A tree
connecting all the nodes is a flooding topology. In addition, the
tree plus the connections between some leaves of the tree and branch
nodes of the tree is a flooding topology.
The following parameters need to be considered for constructing a
flooding topology:
* Degree: The degree of the flooding topology is the maximum degree
among the degrees of the nodes on the flooding topology. The
degree of a node on the flooding topology is the number of
connections on the flooding topology it has to other nodes.
* Number of links: The number of links on the flooding topology is a
key factor for reducing the amount of LS flooding. In general,
the smaller the number of links, the less the amount of LS
flooding.
* Diameter: The diameter of the flooding topology is the shortest
distance between the two most distant nodes on the flooding
topology. It is a key factor for reducing the network convergence
time. The smaller the diameter, the less the convergence time.
* Redundancy: The redundancy of the flooding topology means a
tolerance to the failures of some links and nodes on the flooding
topology. If the flooding topology is split by some failures, it
is not tolerant to these failures. In general, the larger the
number of links on the flooding topology is, the more tolerant the
flooding topology to failures.
Note that the flooding topology constructed by a node is dynamic in
nature, that means when the base topology (the entire topology graph)
changes, the flooding topology (the sub-graph) MUST be re-computed/
re-constructed to ensure that any node that is reachable on the base
topology MUST also be reachable on the flooding topology.
4. Algorithms to Compute Flooding Topology
There are many algorithms to compute a flooding topology. A simple
and efficient one is briefed, which comprises:
* Selecting a node R0 with the smallest node ID;
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* Building a tree using R0 as root in breadth first; and then
* Connecting each node whose degree is one to another node to have a
flooding topology.
4.1. Algorithm with Considering Degree
The algorithm is described below, where a variable MaxD with an
initial value 3, data structures candidate queue Cq and flooding
topology FT are used. Cq and FT comprise elements of form (N, D,
PHs), where N represents a Node, D is the Degree of node N, and PHs
contains the Previous Hops of node N. The detailed FT computation by
the algorithm is illustrated in Appendix A through an example.
The algorithm starts from node R0 as root with
* a maximum degree MaxD of value 3;
* an initial flooding topology FT = {(R0, D=0, PHs={ })}, where node
R0 is the root, D = 0 indicates that the Degree (D for short) of
R0 is 0 (i.e., the number of links on the flooding topology
connected to R0 is 0), PHs = { } indicates that the Previous Hops
(PHs for short) of R0 is empty;
* an initial candidate queue Cq = {(R1,D=0, PHs={R0}), (R2,D=0,
PHs={R0}), ..., (Rm,D=0, PHs={R0})}, where each of nodes R1 to Rm
is connected to R0, its Degree D = 0 and Previous Hops PHs ={R0},
R1 to Rm are in increasing order by their IDs.
1. Finding and removing the first element with node A from Cq that
is not on FT and one PH's D in PHs < MaxD, and add the element
with A into FT; Set A's D to one, increase A's PH's D by one. If
no element in Cq satisfies the conditions, algorithm is restarted
with ++MaxD, the initial FT and Cq.
2. If all the nodes are on the FT, then goto step 4;
3. Suppose that node Xi (i = 1, 2,..., n) is connected to node A and
not on FT, and X1, X2,..., Xn are in an increasing order by their
IDs (i.e., X1's ID < X2's ID < ... < Xn's ID). If Xi is not in
Cq, then add it into the end of Cq with D = 0 and PHs = {A};
otherwise (i.e., Xi is in Cq), add A into the end of Xi's PHs;
Goto step 1.
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4. For each node B on FT whose D is one (from minimum to maximum
node ID), find a link L attached to B such that L's remote node R
has minimum D and ID and R'D < MaxD, add link L between B and R
into FT and increase B's D and R's D by one. Return FT if every
node on FT has its D > 1; otherwise, algorithm is restarted with
++MaxD, the initial FT and Cq.
4.2. Algorithm with Considering Others
There may be some constraints on some nodes in a network. For
example, in a spine-and-leaf network, there may be a constraint on
the degree of every leaf node on the flooding topology, which is that
the degree of every leaf node is not greater than a given number
ConMaxD of value 2. For each of the other nodes such as the spine
nodes, there is no such constraint, that is that ConMaxD is a huge
number for each of these nodes.
Step 1 of the algorithm described above is updated below to consider
this constraint. In addition to checking constraint PH's D < MaxD,
step 1 checks another constraint PH's D < PH's ConMaxD.
1. Finding and removing the first element with node A from Cq that
is not on FT and one PH's D in PHs < MaxD and PH's D < PH's
ConMaxD, and add the element with A into FT; Set A's D to one,
increase A's PH's D by one. If no element in Cq satisfies the
conditions, algorithm is restarted with ++MaxD, the initial FT
and Cq.
Similarly, step 4 of the algorithm described above is updated to
consider this constraint. In addition to checking constraint R's D <
MaxD, step 4 checks another constraint R's D < R's ConMaxD.
5. Security Considerations
This document does not introduce any new security issue.
6. IANA Considerations
Under Registry Name: "IGP Algorithm Type For Computing Flooding
Topology" under an existing "Interior Gateway Protocol (IGP)
Parameters" IANA registries (refer to Section 7.3. IGP
[I-D.ietf-lsr-dynamic-flooding]), IANA is requested to assign one
value of IGP Algorithm Type For Computing Flooding Topology as
follows:
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+==========+========================================+=============+
|Type Value| Type Name | reference |
+==========+========================================+=============+
| 1 | Breadth First Minimum Degree Algorithm |This document|
+----------+----------------------------------------+-------------+
| 2 | Breadth First Leaf Constraint Algorithm|This document|
+----------+----------------------------------------+-------------+
7. Acknowledgements
The authors would like to thank Dean Cheng, Acee Lindem, Zhibo Hu,
Robin Li, Stephane Litkowski and Alvaro Retana for their valuable
suggestions and comments on this draft.
8. References
8.1. 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", Work in Progress,
Internet-Draft, draft-ietf-lsr-dynamic-flooding-10, 7
December 2021, .
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, .
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[RFC2328] Moy, J., "OSPF Version 2", STD 54, RFC 2328,
DOI 10.17487/RFC2328, April 1998,
.
8.2. Informative References
[I-D.ietf-rtgwg-spf-uloop-pb-statement]
Litkowski, S., Decraene, B., and M. Horneffer, "Impact of
Shortest Path First (SPF) Trigger and Delay Strategies on
IGP Micro-loops", Work in Progress, Internet-Draft, draft-
ietf-rtgwg-spf-uloop-pb-statement-10, 16 January 2019,
.
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[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
.
Appendix A. FT Computation Details through Example
This section presents the details on FT computation by the algorithm
through an example. The detailed procedure of computing a FT for a
network of five nodes with full mess connections is illustrated.
Suppose that the network has five nodes R0, R1, R2, R3 and R4; R0's
ID < R1's ID < R2's ID < R3's ID < R4's ID. The algorithm starts
with MaxD = 3, FT = {(R0, D=0, PH={})}, Cq = { (R1,0,{R0}),
(R2,0,{R0}), (R3,0,{R0}), (R4,0,{R0}) }.
1. //remove first element (R1,D=0,PHs={R0}) from Cq, R0's D=0 < MaxD
Cq = { (R2,0,{R0}), (R3,0,{R0}), (R4,0,{R0}) };
// add (R1,1,{R0}) into FT, increase PH R0's D by one
FT = { (R0,1, { }), (R1,1, {R0}) }; // Link R1--R0 on FT
^^^ ^^^^^^^^^^^^
// for Ri connected to R1 (in Cq) not on FT, append R1 to Ri's PHs
Cq = { (R2,0, {R0,R1}), (R3,0, {R0,R1}), (R4,0,{R0,R1}) }.
^^ ^^ ^^
R0 ==== Link on FT
__//== O ---\__
__// / \ \__ link R1--R0 added to FT
__// / \ \__
__// / \ \__
// / \ \
R1 O--\_--------/---------\--------_/--O R4
\ \____ / \ ____/ /
\ \/ \/ /
\ /\_____ ____/\ /
\ / ___\__/___ \ /
\ / / \ \ /
\/____/ \___\/
R2 O --------------------- O R3
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2. // remove the first element (R2,0, {R0,R1}) from Cq, R0's D=1 < MaxD
Cq = { (R3,0, {R0,R1}), (R4,0,{R0,R1}) }
// add (R2,1,{R0}) into FT, increase R0's D by one
FT = { (R0,2,{ }), (R1,1,{R0}), (R2,1,{R0}) } //Link R2--R0 on FT
^^^ ^^^^^^^^^^^
// for Ri connected to R2 (in Cq) not on FT, append R2 to Ri's PHs
Cq = { (R3,0, {R0,R1,R2}), (R4,0,{R0,R1,R2}) }
^^ ^^
R0 ==== Link on FT
__//== O ---\__
__// // \ \__ link R2--R0 added to FT
__// // \ \__
__// // \ \__
// // \ \
R1 O--\_-------//---------\--------_/--O R4
\ \____ // \ ____/ /
\ \/ \/ /
\ //\_____ ____/\ /
\ // ___\__/___ \ /
\ // / \ \ /
\/____/ \___\/
R2 O --------------------- O R3
3. //remove the 1st element (R3,0,{R0,R1,R2}) from Cq, R0's D=2 < MaxD
Cq = { (R4,0,{R0,R1,R2}) }
// add (R3,1,{R0}) into FT, increase R0's D by one
FT = { (R0,3,{}), (R1,1,{R0}), (R2,1,{R0}), (R3,1,{R0}) }
^^^ ^^^^^^^^^^^
// for Ri connected to R3 (in Cq) not on FT, append R3 to Ri's PHs
Cq = { (R4,0,{R0,R1,R2,R3}) }.
^^
R0 ==== Link on FT
__//== O ---\__
__// // \\ \__ link R3--R0 added to FT
__// // \\ \__
__// // \\ \__
// // \\ \
R1 O--\_-------//---------\\-------_/--O R4
\ \____ // \\ ____/ /
\ \/ \/ /
\ //\_____ ____/\\ /
\ // ___\__/___ \\ /
\ // / \ \\ /
\/____/ \___\/
R2 O --------------------- O R3
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4. //remove the 1st element (R4,0,{R0,R1,R2,R3}) from Cq,R1's D=1 < MaxD
Cq = { }
// add (R4,1,{R1}) into FT, increase R1's D by one
FT = {(R0,3,{}), (R1,2,{R0}), (R2,1,{R0}), (R3,1,{R0}), (R4,1,{R1})}
^^^ ^^^^^^^^^^^
R0 ==== Link on FT
__//== O ---\__
__// // \\ \__ link R4--R1 added to FT
__// // \\ \__
__// // \\ \__
// // \\ \
R1 O==\_=======//=========\\=======_/==O R4
\ \____ // \\ ____/ /
\ \/ \/ /
\ //\_____ ____/\\ /
\ // ___\__/___ \\ /
\ // / \ \\ /
\/____/ \___\/
R2 O --------------------- O R3
All nodes are on FT now. In the following, for each node on FT whose
D = 1 (from minimum to maximum ID), link L attached to it and not on
FT is found such that L's remote node has minimum D and ID. L is
added into FT.
5. // On FT, get node R2 with smallest ID whose D=1
FT = {(R0,3,{}),(R1,2,{R0}),(R2,1,{R0}),(R3,1,{R0}), (R4,1,{R1})}
// Add link R2--R3 to FT, ^^^^^^^^^^^
// where R2--R3 is not on FT, R3's D=1 is minimum first and then
// R3's ID is minimum (R3 and R4 tie for D), R2's D++ and R3's D++
FT = {(R0,3,{}),(R1,2,{R0}),(R2,2,{R0,R3}),(R3,2,{R0}),(R4,1,{R1})}
^^^ ^^ ^^^
R0 ==== Link on FT
__//== O ---\__
__// // \\ \__ link R2--R3 added to FT
__// // \\ \__
__// // \\ \__
// // \\ \
R1 O==\_=======//=========\\=======_/==O R4
\ \____ // \\ ____/ /
\ \/ \/ /
\ //\_____ ____/\\ /
\ // ___\__/___ \\ /
\ // / \ \\ /
\/____/ \___\/
R2 O ===================== O R3
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6. // On FT, get node R4 with smallest ID whose D=1
FT = {(R0,3,{}),(R1,2,{R0}),(R2,2,{R0,R3}),(R3,2,{R0}),(R4,1,{R1})}
// Add link R4--R2 to FT, where ^^^^^^^^^^^
// R4--R2 is not on FT, R2's D=2 is minimum first and then R2's ID is
// minimum (R2 and R3 tie for D), increase R2's D and R4's D by one
FT = {(R0,3,{}),(R1,2,{R0}),(R2,3,{R0,R3}),(R3,2,{R0}),(R4,2,{R1,R2})}
^^^ ^^^ ^^
R0 ==== Link on FT
__//== O ---\__
__// // \\ \__ link R4--R2 added to FT
__// // \\ \__
__// // \\ \__
// // \\ \
R1 O==\_=======//=========\\=======//==O R4
\ \____ // \\ ____// /
\ \/ \// /
\ //\_____ ___//\\ /
\ // ___\__//__ \\ /
\ // // \ \\ /
\/ _// \___\/
R2 O ==//================= O R3
FT is computed, which has Degree of 3 and Diameter of 2.
Authors' Addresses
Huaimo Chen
Futurewei
Boston,
United States of America
Email: huaimo.chen@futurewei.com
Mehmet Toy
Verizon
United States of America
Email: mehmet.toy@verizon.com
Yi Yang
IBM
Cary, NC
United States of America
Email: yyietf@gmail.com
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Aijun Wang
China Telecom
Beiqijia Town, Changping District
Beijing
102209
China
Email: wangaj3@chinatelecom.cn
Xufeng Liu
Volta Networks
McLean, VA
United States of America
Email: xufeng.liu.ietf@gmail.com
Yanhe Fan
Casa Systems
United States of America
Email: yfan@casa-systems.com
Lei Liu
Fujitsu
United States of America
Email: liulei.kddi@gmail.com
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