Internet DRAFT - draft-xu-lsr-isis-flooding-reduction-in-msdc
draft-xu-lsr-isis-flooding-reduction-in-msdc
Network Working Group X. Xu
Internet-Draft China Mobile
Intended status: Standards Track L. Fang
Expires: 3 August 2024 eBay
J. Tantsura
Nvidia
S. Ma
Google
31 January 2024
IS-IS Flooding Reduction in MSDC
draft-xu-lsr-isis-flooding-reduction-in-msdc-05
Abstract
IS-IS is a commonly used routing protocol in MSDC (Massively Scalable
Data Center) networks where CLOS is the most popular topology. In a
CLOS topology, each IS-IS router would receive multiple copies of the
same LSP (Link State Packet) from multiple IS-IS neighbors.
Moreover, two IS-IS neighbors may send each other the same LSP
simultaneously. The unnecessary link-state information flooding
results in a large waste of resources for IS-IS routers, as there are
too many neighbors for each router. To address this scaling problem,
this document introduces some extensions to the IS-IS protocol.
These extensions aim to significantly reduce the IS-IS flooding
within MSDC networks, which can greatly improve the scalability of
such networks.
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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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 3 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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Modifications to Current IS-IS Behaviors . . . . . . . . . . 5
3.1. IS-IS Routers as Non-DIS . . . . . . . . . . . . . . . . 5
3.2. Controllers as DIS . . . . . . . . . . . . . . . . . . . 6
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
IS-IS is a commonly used routing protocol in MSDC (Massively Scalable
Data Center) networks where CLOS is the most popular topology. In a
CLOS topology, each IS-IS router would receive multiple copies of the
same LSP (Link State Packet) from multiple IS-IS neighbors.
Moreover, two IS-IS neighbors may send each other the same LSP
simultaneously. The unnecessary link-state information flooding
results in a large waste of resources for IS-IS routers, as there are
too many neighbors for each router.
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As a result, some MSDC operators had to opt for BGP as the routing
protocol [RFC7938]. However, with the introduction of high-
performance Ethernet networks, which are widely used in AI and high-
performance computing (HPC), it has become essential to have
visibility of the whole network topology and even the link capacity
and load information for global load-balancing. Therefore, for
large-scale AI and HPC Ethernet networks, link-state routing
protocols like IS-IS should be reconsidered as the routing protocol.
However, it is crucial to address the scaling issue associated with
link-state routing protocols as mentioned earlier.
This document presents an effective solution to the scaling issue
mentioned above. Instead of transmitting link-state information
between neighboring IS-IS routers with the MSDC network fabric, link-
state information originating from each IS-IS router will be gathered
by centralized controllers. These controllers will then distribute
the collected link-state information to all IS-IS routers within the
MSDC. As illustrated in Figure 1, all IS-IS routers in an MDSC
network fabric will be linked to one or more centralized controllers
through a dedicated Local Area Network (LAN). This LAN is
specifically intended for link-state information collection and
distribution. For redundancy purposes, there should be at least two
link-state collection and distribution LANs.
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+----------+ +----------+
|Controller| |Controller|
+----+-----+ +-----+----+
|DIS |Candidate DIS
| |
| |
---+---------+---+----------+-----------+---+---------+-LS Collection&Distribution LAN
| | | | |
|Non-DIS |Non-DIS |Non-DIS |Non-DIS |Non-DIS
| | | | |
| +---+--+ | +---+--+ |
| |Router| | |Router| |
| *------*- | /*---/--* |
| / \ -- | // / \ |
| / \ -- | // / \ |
| / \ --|// / \ |
| / \ /*- / \ |
| / \ // | -- / \ |
| / \ // | -- / \ |
| / /X | -- \ |
| / // \ | / -- \ |
| / // \ | / -- \ |
| / // \ | / -- \ |
| / // \ | / -- \ |
| / // \ | / -- \ |
| / // \ | / -- \ |
+-+- //* +\\+-/-+ +---\-++
|Router| |Router| |Router|
+------+ +------+ +------+
Figure 1
In the MSDC network, the IS-IS routers do not need to exchange any
IS-IS Protocol Datagram Units (PDUs) other than Hello packets among
them. This is due to the presence of a controller that acts as an
IS-IS Designated Intermediate System (DIS) for the link-state
collection and distribution LAN. To obtain the complete topology
information of the MSDC network, these IS-IS routers exchange the
link-state information with the controller, which is elected as IS-IS
DIS for the link-state collection and distribution LAN.
To further reduce the flooding of the multicast IS-IS PDUs over the
link-state collection and distribution LAN, IS-IS routers will not
send multicast IS-IS Hello packets over that LAN. Instead, they will
wait for IS-IS Hello packets from the controller that has been
elected as IS-IS DIS initially. Once an IS-IS DIS has been
discovered, the routers will start sending IS-IS Hello packets
directly to the IS-IS DIS at regular intervals as unicasts.
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Consequently, IS-IS routers would only form an adjacency with the IS-
IS DIS over that LAN. Additionally, IS-IS routers will send IS-IS
PDUs to the IS-IS DIS as unicasts. However, the IS-IS DIS will
continue to send IS-IS PDUs as before. These changes to the current
IS-IS router behaviors will significantly reduce IS-IS flooding and
improve the scalability of MSDC networks.
2. Terminology
This memo makes use of the terms defined in [RFC1195].
3. Modifications to Current IS-IS Behaviors
3.1. IS-IS Routers as Non-DIS
IS-IS routers exchange Hello packets bidirectionally. After that,
they originate Link State PDUs (LSPs) accordingly. However, these
self-originated LSPs don't need to be directly exchanged between the
routers. They only need to be sent to the IS-IS DIS for the link-
state collection and distribution LAN. It is important to note that
IS-IS routers should not be elected as IS-IS DIS for the link-state
collection and distribution LAN (this can be done by setting the DIS
Priority of those IS-IS routers to zero).
To further minimize the number of multicast IS-IS PDUs transmitted
over the link-state collection and distribution LAN, IS-IS routers
should send IS-IS PDUs as unicasts. Specifically, IS-IS routers must
send unicast IS-IS Hello packets periodically to the controller
elected as IS-IS DIS. This means that IS-IS routers will not send
any IS-IS Hello packet over the link-state collection and
distribution LAN until they have identified an IS-IS DIS for the
link-state collection and distribution LAN. As a result, IS-IS
routers will not discover each other over the link-state collection
and distribution LAN, and will not establish adjacencies with each
other. Moreover, IS-IS routers should send all types of IS-IS PDUs
to the IS-IS DIS as unicasts as well.
To prevent data traffic from being forwarded across the link-state
collection and distribution LAN, the interfaces of all IS-IS routers
to the LAN must be set to the maximum cost value.
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3.2. Controllers as DIS
When a controller is elected as the IS-IS DIS, it would send IS-IS
PDUs as multicasts or unicasts as normal. Additionally, it is
required to accept and process those unicast IS-IS PDUs originated
from other IS-IS routers. Upon receiving any new LSP from a given
IS-IS router, the DIS must flood it immediately to the link-state
collection and distribution LAN. This serves two purposes: 1) to
acknowledge the receipt of that LSP implicitly, and 2) to synchronize
that LSP to all other IS-IS routers.
To reduce the frequency of advertising the Complete Sequence Number
PDU (CSNP) on the DIS for the link-state collection and distribution
LAN, it is recommended that IS-IS routers send an explicit
acknowledgement with a Partial Sequence Number PDU (PSNP) upon
receiving a new LSP from that DIS.
4. Acknowledgements
The authors would like to thank Peter Lothberg and Erik Auerswald for
their valuable comments and suggestions on this document.
5. IANA Considerations
TBD.
6. Security Considerations
TBD.
7. References
7.1. Normative References
[RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
dual environments", RFC 1195, DOI 10.17487/RFC1195,
December 1990, <https://www.rfc-editor.org/info/rfc1195>.
[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>.
7.2. Informative References
[RFC4136] Pillay-Esnault, P., "OSPF Refresh and Flooding Reduction
in Stable Topologies", RFC 4136, DOI 10.17487/RFC4136,
July 2005, <https://www.rfc-editor.org/info/rfc4136>.
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[RFC7938] Lapukhov, P., Premji, A., and J. Mitchell, Ed., "Use of
BGP for Routing in Large-Scale Data Centers", RFC 7938,
DOI 10.17487/RFC7938, August 2016,
<https://www.rfc-editor.org/info/rfc7938>.
Authors' Addresses
Xiaohu Xu
China Mobile
Email: xuxiaohu_ietf@hotmail.com
Luyuan Fang
eBay
Email: luyuanf@gmail.com
Jeff Tantsura
Nvidia
Email: jefftant.ietf@gmail.com
Shaowen Ma
Google
Email: shaowen@google.com
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