Javascript disabled? Like other modern websites, the IETF Datatracker relies on Javascript. Please enable Javascript for full functionality.

Generic Fault-Avoidance Routing Protocol for Data Center Networks
draft-sl-rtgwg-far-dcn-21

Versions:

Document	Type	Active Internet-Draft (individual)
	Authors	Bin Liu , Yantao Sun , Jing Cheng , Yichen Zhang , Bhumip Khasnabish
	Last updated	2023-12-24
	RFC stream	Independent Submission
	Intended RFC status	Informational
	Formats	txt html htmlized pdf bibtex bibxml
Stream	ISE state	Response to Review Needed
	Consensus boilerplate	Unknown
	Document shepherd	Eliot Lear
IESG	IESG state	I-D Exists
	Telechat date	(None)
	Responsible AD	(None)
	Send notices to	rfc-ise@rfc-editor.org

Email authors IPR References Referenced by Nits Search email archive

draft-sl-rtgwg-far-dcn-21

Routing Area Working Group B. Liu
Internet-Draft ZTE Inc., ZTE Plaza
Intended status: Informational Y. Sun
Expires: 27 June 2024 J. Cheng
Y. Zhang
Beijing Jiaotong University
B. Khasnabish
Individual contributor
25 December 2023

Generic Fault-Avoidance Routing Protocol for Data Center Networks
draft-sl-rtgwg-far-dcn-21

Abstract

This document describes a generic routing method and protocol for a
regular data center network, named the Fault-Avoidance Routing (FAR)
protocol. The FAR protocol provides a generic routing method for all
types of regular topology network architectures that have been
proposed for large-scale cloud-based data centers over the past few
years. The FAR protocol is designed to leverage any regularity in
the topology and compute its routing table in a concise manner. Fat-
tree is taken as an example architecture to illustrate how the FAR
protocol can be applied in real operational scenarios.

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 27 June 2024.

Liu, et al. Expires 27 June 2024 [Page 1]
Internet-Draft FAR for DCN December 2023

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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Acronyms & Definitions . . . . . . . . . . . . . . . . . 4
2. Conventions used in this document . . . . . . . . . . . . . . 5
3. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
3.1. The Impact of Large-scale Networks on Route
Calculation . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Issues of Conventional Routing Methods in a Large-scale
Network with Giant Number Nodes of Routers . . . . . . . 6
3.3. Network Addressing Issues . . . . . . . . . . . . . . . . 8
3.4. Big Routing Table Issues . . . . . . . . . . . . . . . . 8
3.5. Adaptivity Issues for Routing Algorithms . . . . . . . . 9
3.6. Virtual Machine Migration Issues . . . . . . . . . . . . 9
4. The FAR Framework . . . . . . . . . . . . . . . . . . . . . . 9
5. Data Format . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Data Tables . . . . . . . . . . . . . . . . . . . . . . . 10
5.2. Messages . . . . . . . . . . . . . . . . . . . . . . . . 12
6. FAR Modules . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Neighbor and Link Detection Module(M1) . . . . . . . . . 15
6.2. Device Learning Module(M2) . . . . . . . . . . . . . . . 15
6.3. Invisible Neighbor and Link Failure Inferring
Module(M3) . . . . . . . . . . . . . . . . . . . . . . . 16
6.4. Link Failure Learning Module(M4) . . . . . . . . . . . . 16
6.5. BRT Building Module(M5) . . . . . . . . . . . . . . . . . 16
6.6. NRT Building Module(M6) . . . . . . . . . . . . . . . . . 17
6.7. Routing Table Lookup(M7) . . . . . . . . . . . . . . . . 17
7. How a FAR Router Works . . . . . . . . . . . . . . . . . . . 17
8. Compatible Architecture . . . . . . . . . . . . . . . . . . . 20
9. Topology identification and broadcast storm suppression . . . 20
10. Application Example . . . . . . . . . . . . . . . . . . . . . 21
10.1. BRT Building Procedure . . . . . . . . . . . . . . . . . 22
10.2. NRT Building Procedure . . . . . . . . . . . . . . . . . 23
10.2.1. Single Link Failure . . . . . . . . . . . . . . . . 23
10.2.2. A Group of Link Failures . . . . . . . . . . . . . . 24
10.2.3. Node Failures . . . . . . . . . . . . . . . . . . . 25
10.3. Routing Procedure . . . . . . . . . . . . . . . . . . . 25
10.4. FAR's Performance in Large-scale Networks . . . . . . . 26
10.4.1. The number of control messages required by FAR . . . 26

Liu, et al. Expires 27 June 2024 [Page 2]
Internet-Draft FAR for DCN December 2023

10.4.2. The Calculating Time of Routing Tables . . . . . . . 26
10.4.3. The Size of Routing Tables . . . . . . . . . . . . . 27
11. Implementations Examples . . . . . . . . . . . . . . . . . . 27
12. Security Considerations . . . . . . . . . . . . . . . . . . . 29
13. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 30
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 30
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
15.1. Normative References . . . . . . . . . . . . . . . . . . 30
15.2. Informative References . . . . . . . . . . . . . . . . . 30
16. Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . 31
16.1. Application Area of the Solution . . . . . . . . . . . . 31
16.2. Technical evolution roadmap . . . . . . . . . . . . . . 31
16.3. Updating roadmap . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31

1. Introduction

In recent years, with the rapid development of cloud computing
technologies, the widely deployed cloud services, such as Amazon EC2
and Google search, bring about huge challenges to data center
networking (DCN). Today's cloud-based data centers (DCs) require
large-scale networks with larger internal bandwidth and smaller
transfer delay. However, conventional networks cannot meet such
requirements due to limitations in their network architecture. In
order to satisfy the requirements of cloud computing services, many
new network architectures have been proposed for data centers, such
as Fat-tree, MatrixDCN[MatrixDCN], and BCube[BCube]. These new
architectures can support non-blocking large-scale datacenter
networks with more than tens of thousands of physical servers. All
of these architectures have regular topologies, which are common
features. The regular topology refers to the network topology
structure with obvious regularity and symmetry, which is conducive to
automatic configuration of the network, such as the Fat-tree network.
In a regular topology, each network node such as a switch or router
can be addressed by its location and through a node's address, the
node's connections to its neighbors in a network can be determined,
and furthermore, the route to the node from other nodes in the
network can be determined. So nodes can compute route entries
without learning topology. This document describes a generic routing
method and protocol, the Fault-Avoidance Routing (FAR) protocol, for
DCNs. This method leverages the regularity in the topologies of data
center networks to simplify routing learning and accelerate the query
of routing tables. This routing method has a better fault tolerance
and can be applied to any DCN with a regular topology. FAR is not a
routing protocol to replace generic routing protocols such as
OSPF(Open Shortest Path First)[RFC2328] and IS-IS(Intermediate System
to Intermediate System)[RFC1142][ISO10589]. It cannot be used in
general local networks whose topological structures are arbitrary,

Liu, et al. Expires 27 June 2024 [Page 3]
Internet-Draft FAR for DCN December 2023

and whose scales are also not very large. OSPF and IS-IS work very
well in such a network. But in a large-scale network with regular
topology, FAR has better performance. Compared with OSPF and IS-IS,
FAR has shorter time of network convergence and lower PDU(Protocol
Data Unit) overhead. Furthermore, FAR requires less computing and
storage resources, which lets FAR routers to run at a lower cost of
production than the generic routers. In addition, for each type of
network architecture, researchers designed a routing algorithm
according to the features of its topology. Because these routing
algorithms are different and lack compatibility with each other, it
is very difficult to develop a routing protocol for network routers
supporting multiple routing algorithms. FAR has better adaptability
than these specified routing methods.

FAR consists of three components, i.e., link state learning unit,
routing table building unit and routing table querying unit. In the
link state learning unit, FAR exchanges link failures among routers
to establish a consistent knowledge of the entire network. In this
stage, the regularity in topology is exploited to infer failed links
and routers. In the routing table building unit, FAR builds up two
routing tables, i.e., a basic routing table (BRT) and a negative
routing table (NRT), for each router according to the network
topology and link states. In the last component, routers forward
incoming packets by looking up the two routing tables. The matched
entries in BRT minus the matched entries in NRT are the final route
entries to be used to forward an incoming packet.

This document describes a protocol developed by ZTE and Beijing
Jiaotong University. It is just presented here to record the work
and to make it available for use in later IETF work if desirable.

The remainder of this draft is organized as follows. The problem to
be addressed by FAR is described in Section 3. The framework of FAR
routing protocol is described in Section 4. Section 5 and 6
introduce FAR's data format FAR and modules in detail. Section 7
describe how FAR works by finite state machine (FSM). In Section 8,
we discussed how FAR works with variable network architectures.
Section 9 takes Fat-tree network as an example to illuminate how FAR
works.

1.1. Acronyms & Definitions

DCN - Data Center Network

FAR - Fault-Avoidance Routing

BRT - Basic Routing Table

Liu, et al. Expires 27 June 2024 [Page 4]
Internet-Draft FAR for DCN December 2023

NRT - Negative Routing Table

NDT - Neighbor Devices Table

ADT - All Devices Table

LFT - Link Failure Table

DA - Device Announcement

LFA - Link Failure Announcement

DLR - Device and Link Request

IP - Internet Protocol

SDN - Software Defined Network

VM - Virtual Machine

2. Conventions used in this document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. In this document, these words will appear
with that interpretation only when in ALL CAPS. Lower case uses of
these words are not to be interpreted as carrying special
significance.

3. Problem Statement

The problem to be addressed by FAR as proposed in this draft is
described in this section. The expansion of Cloud data center
networks has brought significant challenges to the existing routing
technologies. FAR mainly solves a series of routing problems faced
by large-scale data center networks.

3.1. The Impact of Large-scale Networks on Route Calculation

In a large-scale cloud data center network, there may be thousands of
routers. Running OSPF or IS-IS in such network will encounter these
two challenges: (1) Network convergence time would be too long, which
will cause a longer time to elapse for creating and updating the
routes. The response time to network failures may be excessively
long; (2) High resource consumption. Since a large number of routing
protocol packets need to be sent, it causes the routing information

Liu, et al. Expires 27 June 2024 [Page 5]
Internet-Draft FAR for DCN December 2023

consuming too much network bandwidth and CPU(central processing unit)
resources, and easily leads to packet loss and makes the challenge
(1) more prominent. In order to solve these two challenges, a common
practice is to partition a large network into some small areas, where
the route calculation runs independently within different areas.
However, nowadays the cloud data centers typically require very large
internal bandwidth. To meet this requirement, a large number of
parallel equivalent links are deployed in a network, such as a Fat-
tree network. Partitioning such a network will affect the
utilization of routing algorithm on equivalent multi-path and reduce
internal network bandwidth requirements. In the FAR routing
calculation process, a Basic Routing Table (BRT) is built on local
network topology leveraging the regularity of the network topologies.
In addition to BRT, FAR also builds a Negative Routing Table (NRT).
FAR gradually builds NRT in the process of learning network link
failure information, which does not require learning a complete
network fault information. FAR does not need to wait for the
completion of the network convergence in the process of building
these two tables. Therefore, it avoids the problem of excessive
network convergence overheads in the route calculation process. In
addition, FAR only needs to exchange a small amount of link change
information between routers, and hence consumes less network
bandwidth.

3.2. Issues of Conventional Routing Methods in a Large-scale Network
with Giant Number Nodes of Routers

There are many real world scenario where tens of thousands of
nodes(or much more nodes) need to be deployed in a flat area, such as
infiniband routing and switching system, high-performance computer
network, and many IDC(Internet Data Center) networks in China. The
similar problems have been existed long ago. People have solved the
problems through similar solutions, such as the traditional regular
topology-based RFC3619[RFC3619] protocol, the routing protocols of
infiniband routing and switching system, and high-performance
computer network routing protocol. Infiniband defines a switch-based
network to interconnect processing nodes and the I/O nodes.
Infiniband can support very large scale networks, use the regularity
in topology to simplify its routing algorithm, which is just the same
to what we do in FAR. Why OSPF and IS-IS do not work well in a
large-scale network with giant number nodes of routers? As we know,
the OSPF protocol uses multiple databases, more topological exchange
information (as seen in the following example) and complicated
algorithm. It requires routers to consume more memory and CPU
processing capability. But the processing rate of CPU on the
protocol message per second is very limited. When the network
expands, CPU will quickly approach its processing limits, and at this
time OSPF can not continue to expand the scale of the management.

Liu, et al. Expires 27 June 2024 [Page 6]
Internet-Draft FAR for DCN December 2023

The SPF(Shortest Path First) algorithm itself does not thoroughly
solve these problems. On the contrary, the FAR protocol does not
need to calculate SPF, which saves calculation time and resources, so
FAR does not have the convergence time delay and the additional CPU
overheads, which SPF requires. Because in the initial stage, FAR
already knows the regular information of the whole network topology
and does not need to periodically do SPF operation. One of the
examples of "more topological exchange information": In the OSPF
protocol, LSA(Link-State Advertisement) floods every 1800 seconds.
Especially in the larger network, the occupation of CPU and band
bandwidth will soon reach the router’s performance bottleneck. In
order to reduce these adverse effects, OSPF introduced the concept of
Area, which still has not solved the problem thoroughly. By dividing
the OSPF Area into several areas, the routers in the same area do not
need to know the topological details outside their area. (In
comparison with FAR, after OSPF introducing the concept of Area, the
equivalent paths cannot be selected in the whole network scope) OSPF
can achieve the following results by Area : 1) Routers only need to
maintain the same link state databases as other routers within the
same Area, without the necessity of maintaining the same link state
database as all routers in the whole OSPF domain. 2) The reduction
of the link state databases means dealing with relatively fewer LSA,
which reduces the CPU consumption of routers; 3) The large number of
LSAs flood only within the same Area. But, its negative effect is
that the smaller number of routers which can be managed in each OSPF
area. On the contrary, because FAR does not have the above
disadvantages, FAR can also manage large-scale network even without
dividing Areas. The aging time of OSPF is set in order to adapt to
routing transformation and protocol message exchange happened
frequently in the irregular topology. Its negative effect is: when
the network does not change, the LSA needs to be refreshed every 1800
seconds to reset the aging time. In the regular topology, as the
routings are fixed, it does not need the complex protocol message
exchange and aging rules to reflect the routing changes, as long as
LFA mechanism in the FAR is enough. Compared with the LSVR(Link
State Vector Routing) protocol, the LSVR protocol has no special
requirements for the network topology structure, however, the FAR
draft is applicable to the regular topology network architecture and
simplifies unnecessary processing. It is a solution proposed to
greatly improve the routing efficiency of the regular network
topology. The FAR solution is more efficient than the general
methods such as LSVR in regular topology. Therefore, in FAR, we can
omit many unnecessary processing and the packet exchange. The
benefits are fast convergence speed and much larger network scale
than other dynamic routing protocol. Now there are some successful
implementations of simplified routings in the regular topology in the
HPC(High Performance Computing) environment. Conclusion: As FAR
needs few routing entries and the topology is regular, the database

Liu, et al. Expires 27 June 2024 [Page 7]
Internet-Draft FAR for DCN December 2023

does not need to be updated regularly. Without the need for aging,
there is no need for CPU and bandwidth overhead brought by LSA flood
every 30 minutes, so the expansion of the network has no obvious
effect on the performance of FAR, which is contrary to OSPF.
Comparison of convergence time: The settings of OSPF spf_delay and
spf_hold_time can affect the change of convergence time. The
convergence time of the network with 2480 nodes is about 15-20
seconds; while the FAR does not need to calculate the SPF, so there
is no such convergence time. These issues still exist in rapid
convergence technology of OSPF ,ISIS (such as I-SPF, Incremental SPF)
and LSVR. The convergence speed and network scale constraint each
other. FAR does not have the above problems, and the convergence
time is almost negligible. Can FRR(Fast Reroute) solve these
problems? IP FRR has some limitations. The establishment of IP FRR
backup scheme will not affect the original topology and traffic
forwarding which are established by protocol, however, we can not get
the information of whereabouts and status when the traffic is
switched to an alternate next hop.

3.3. Network Addressing Issues

Routers are typically configured with multiple network interfaces,
each connected to a subnet. OSPF and other routing algorithms
require that each interface of a router must be configured with an IP
address. A large-scale data center network may contain thousands of
routers and each router has dozens of network interfaces, thus, there
are tens of thousands of IP addresses needed to be configured in a
data center. It will be very complex to configure and manage a large
number of network interfaces and will be difficult to troubleshoot
network problems, then network maintenance will be costly and error-
prone. In FAR, the device position information is encoded in the IP
address of the router. Each router only needs to be assigned a
unique IP address according its location, which greatly solves
complex network addressing issues in large-scale networks.

3.4. Big Routing Table Issues

There are a large number of subnets in the large-scale data center
network. A router may build a routing entry for each subnet, and
therefore the size of routing tables on each router may be very
large. It will increase a router's cost and reduce the querying
speed of the routing table. FAR uses two measures to reduce the size
of its routing tables: a)It builds a BRT on the regularity of the
network topologies; b)It introduces a new routing table, i.e., a NRT.
In this way FAR can reduce the size of routing tables to only a few
dozen routing entries.

Liu, et al. Expires 27 June 2024 [Page 8]
Internet-Draft FAR for DCN December 2023

3.5. Adaptivity Issues for Routing Algorithms

To implement efficient routing in large-scale datacenters, besides
FAR, some other routing methods are proposed for some specific
network architectures, such as Fat-tree and BCube. These routing
methods are different(from both design and implementation viewpoints)
and not compatible with the conventional routing methods, which
brings big troubles to network equipment providers to develop new
routers supporting various new routing methods. FAR is a generic
routing method. With slight modification, FAR method can be applied
to most of regular datacenter networks. Furthermore, the structure
of routing tables and querying a routing table in FAR are the same as
conventional routing method. If FAR is adopted, the workload of
developing a new type of router will be significantly decreased.

3.6. Virtual Machine Migration Issues

Supporting VM migration is very important for cloud-based datacenter
networks. However, in order to support layer-3 routing, routing
methods including OSPF and FAR require limiting VM migration within a
subnet. For this paradox, the mainstream methods still utilize
layer-3 routing on routers or switches, transmit packets encapsulated
by IPinIP or MACinIP between hosts by tunnels passing through network
to the destination access switch, and then extract original packet
out and send it to the destination host. By utilizing the
aforementioned methods, FAR can be applied to Fat-tree, MatrixDCN or
BCube networks for supporting VM migration in entire network.

4. The FAR Framework

FAR requires that a DCN has a regular topology, and network devices,
including routers, switches, and servers, are assigned IP addresses
according to their locations in the network. In other word, we can
locate a device in the network according to its IP address. FAR is a
distributed routing method. In order to support FAR, each router
needs to have a routing module that implements the FAR algorithm.
FAR algorithm is composed of three parts, i.e., link-state learning,
routing table building and routing table querying, as shown in Fig.
1. 1:Neighbor and Link Detection Module(M1) 2:Device Learning
Module(M2) 3:Invisible Neighbor and Link Failure Inferring Module(M3)
4:Link Failure Learning Module(M4) 5:BRT Building Module(M5) 6:NRT
Building Module(M6) 7:Routing Table Lookup(M7) The meanings of M1-M7
are explained in detail in section 6. Link-state learning is
responsible for a router to detect the states of its connected links
and learn the states of all the other links in the entire network.
The second part builds two routing tables, a basic routing table
(BRT) and an negative routing table (NRT), according to the learned
link states in the first part. The third part queries the BRT and

Liu, et al. Expires 27 June 2024 [Page 9]
Internet-Draft FAR for DCN December 2023

the NRT to decide a next forwarding hop for the received (ingress)
packets.

Link-state Learning |Routing Table | Routing Table
| Building | Querying
| |
+--------+ /---------------\ | +--------+ |
|2 Device|<--| 1 Neighbor & |----->| 5 BRT \ | Packets
|Learning| | Link Detection| | |Building|\ | |
+--------+ \---------------/ | +--------+ \ | \|/
| | | \ |+--------------+
| | | /|| 7 Querying |
\|/ \|/ | / || Routing Table|
+-----------------------+| / |+--------------+
|3 Invisible Neighbor & || / |
|Link Failure Inferring || +--------- |
+-----------------------+|/| 6 NRT | |
| / |Building| |
\|/ /| +--------+ |
+--------------+ / | |
|4 Link Failure| / | |
| Learning | | |
+--------------+ | |
| |

Figure 1: The FAR framework

5. Data Format

5.1. Data Tables

Some data tables are maintained on each router in FAR. They are:
Neighbor Device Table (NDT): To store neighbor routers and related
links. All Devices Table (ADT): To store all routers in the entire
network. Link Failures Table (LFT): To store all link failures in
the entire network. Basic Routing Table (BRT): To store the
candidate routes. Negative Routing Table(NRT): To store the avoiding
routes. Device ID: The ID of a neighbor router. Device IP: The IP
address of a neighbor router. Port ID: The port ID that a neighbor
router is attached to. Link State: The state of the link between a
router and its neighbor router. There are two states: Up and Down.
Update Time: The time of updating the entry. Device ID: The ID of a
neighbor router. Device IP: The IP address of a neighbor router.
Type: The type of a neighbor router. State: The state of a neighbor
router. There are two states: Up and Down. Update Time: The time of
updating the entry. No: The entry number. Router 1 IP: The IP
address of one router that a failed link connects to. Router 2 IP:
The IP address of another router that a failed link connects to.

Liu, et al. Expires 27 June 2024 [Page 10]
Internet-Draft FAR for DCN December 2023

Timestamp: It identifies when the entry is created. Destination: A
destination network Mask: The subnet mask of a destination network.
Next Hop: The IP address of a next hop for a destination. Interface:
The interface related to a next hop. Update Time: The time of
updating the entry. Destination: A destination network. Mask: The
subnet mask of a destination network. Next Hop: The IP address of a
next hop that should be avoided for a destination. Interface: The
interface related to a next hop that should be avoided. Failed Link
No: A group of failed link numbers divided by "/", for example 1/2/3.
Timestamp: The time of updating the entry. The format of NDT

----------------------------------------------------------
Device ID | Device IP | Port ID | Link State | Update Time
----------------------------------------------------------

The format of ADT

--------------------------------------------------
Device ID | Device IP | Type | State | Update Time
--------------------------------------------------

The format of LFT

--------------------------------------------
No | Router 1 IP | Router 2 IP | Timestamp
--------------------------------------------

The format of BRT

-------------------------------------------------------
Destination | Mask | Next Hop | Interface | Update Time
-------------------------------------------------------

The format of NRT

Liu, et al. Expires 27 June 2024 [Page 11]
Internet-Draft FAR for DCN December 2023

5.2. Messages

Some protocol messages are exchanged between routers in FAR. Hello
Message: This message is exchanged between neighbor routers to learn
adjacency. Device Announcement (DA): Synchronize the knowledge of
routers between routers. Link Failure Announcement (LFA):
Synchronize link failures between routers. Device and Link Request
(DLR): When a router starts, it requests the knowledge of routers and
links from its neighbors by a DLR message. A FAR Message is directly
encapsulated in an IP packet. The protocol field of IP header
indicates an IP packet is an FAR message. The four types of FAR
messages have same format of packet header, called FAR header (as
shown in Figure 2). Version: FAR version Message Type: The type of
FAR message. Packet Length: The packet length of the total FAR
message. Checksum: The checksum of an entire FAR message.
AuType:Authentication type. 0: no authentication, 1: Plaintext
Authentication, 2: MD5 Authentication. Authentication:
Authentication information. 0: undefined, 1: Key, 2: key ID, MD5 data
length and packet number. MD5 data is appended to the backend of the
packet. AuType and Authentication can refer to the definition of
OSPF packet. For Hello messages, the Message Type in FAR header is
set to 1.Besides FAR header, a Hello message(Fig. 3) requires the
following fields: Router IP: The router IP address. HelloInterval:
The interval of sending Hello messages to neighbor routers.
RouterDeadInterval: The interval to set a neighbor router dead(out-
of-service). If in the interval time, a router doesn't receive a
Hello message from its neighbor router, the neighbor router is
treated as dead. Neighbor Router IP: The IP address of a neighbor
router. All the neighbor router's addresses should be included in a
Hello message. For DA messages(Fig. 4), the Message Type in FAR
header is set to 2. Besides FAR header, a DA message includes IP
addresses of all the announced routers. For LFA messages(Fig. 5),
the Message Type in FAR header is set to 3. Besides FAR header, a
LFA message includes all the announced link failures. Left IP: The
IP address of the left endpoint router of a link. Right IP: The IP
address of the right endpoint router of a link. State: Link state.
0: Up, 1: down For DLR messages(Fig. 6), the Message Type in FAR
header is set to 1.Except for FAR header, DLR has no additional
fields.

Liu, et al. Expires 27 June 2024 [Page 12]
Internet-Draft FAR for DCN December 2023

Figure 2: The format of FAR header

|<--- 1 --->| <--- 1 --->|<--------- 2 ---------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+-------------------------------------------------+
| Router IP |
+------------------------+------------------------+
| HelloInterval | HelloDeadInterval |
+------------------------+------------------------+
| Neighbor Router IP |
+-------------------------------------------------+
| ... |
+-------------------------------------------------+

Figure 3: The Format of Hello Messages

Liu, et al. Expires 27 June 2024 [Page 13]
Internet-Draft FAR for DCN December 2023

|<----1---->| <----1---->|<----------2----------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+------------------------+------------------------+
| Router1 IP |
+-------------------------------------------------+
| ... |
+-------------------------------------------------+

Figure 4: The Format of DA Messages

|<----1---->| <----1---->|<----------2----------->|
+-----------+------------+------------------------+
| Version |Message Type| Message Length |
+-----------+------------+------------------------+
| Checksum | AuType |
+------------------------+------------------------+
| Authentication |
+-------------------------------------------------+
| Authentication |
+-------------------------------------------------+
| Timestamp |
+------------------------+------------------------+
| Left IP |
+-------------------------------------------------+
| Right IP |
+------------------------+------------------------+
| State |
+-------------------------------------------------+
| ... |
+-------------------------------------------------+

Figure 5: The Format of LFA Messages

Liu, et al. Expires 27 June 2024 [Page 14]
Internet-Draft FAR for DCN December 2023

Figure 6: The Format of DLR Messages

6. FAR Modules

6.1. Neighbor and Link Detection Module(M1)

M1 is responsible for sending and receiving Hello messages, and
detecting directly-connected links and neighbor routers. Each Hello
message is encapsulated in an IP packet. M1 sends Hello messages
periodically to all the active router ports and receives Hello
messages from its neighbor routers. M1 detects neighbor routers and
directly-connected links according to received Hello Messages and
stores these neighbors and links into a Neighbor Devices Table (NDT).
Additionally, M1 also stores neighbor routers into an All Devices
Table (ADT).

6.2. Device Learning Module(M2)

M2 is responsible for sending, receiving, and forwarding device
announcement (DA) messages, learning all the routers in the whole
network, and deducing faulted routers. When a router starts, it
sends a DA message announcing itself to its neighbors and a DLR
message requesting the knowledge of routers and links from its
neighbors. If M2 module of a router receives a DA message, it checks
whether the router encapsulated in the message is in an ADT. If the
router is not in the ADT, M2 puts this router into the ADT and
forwards this DA message to all the active ports except for the
incoming one, otherwise, M2 discards this message directly. If M2
module of a router receives a DLR message, it replies a DA message
that encapsulates all of the learned routers.

Liu, et al. Expires 27 June 2024 [Page 15]
Internet-Draft FAR for DCN December 2023

6.3. Invisible Neighbor and Link Failure Inferring Module(M3)

M3 is responsible for inferring invisible neighbors of the current
router by means of the ADT. If the link between a router A and its
neighbor B breaks, which results in that M1 module of A cannot detect
the existence of B, then B is an invisible neighbor of A. Since a
device's location is coded into its IP address, it can be judged
whether two routers are adjacent, according to their IP addresses.
Based on this idea, M3 infers all of the invisible neighbors of the
current router and the related link failures. The results are stored
into an NDT. Moreover, link failures also are added into a link-
failure table (LFT). LFT stores all of the failed links in the
entire network.

6.4. Link Failure Learning Module(M4)

M4 is responsible for sending, receiving and forwarding link failure
announcement (LFA) and learning all the link failures in the whole
network. M4 broadcasts each newly inferred link failure to all the
routers in the network. Each link failure is encapsulated in a LFA
message and one link failure is broadcasted only once. If a router
receives a DLR request from its neighbor, it will reply a LFA message
that encapsulates all the learned link failures through M4 module.
If M4 receives a LFA message, it checks whether the link failure
encapsulated in the message is in a LFT by comparing two link ends
and timestamp. If the link failure is not in the LFT or timestamp is
different, M4 puts this link failure into the LFT (or update
timestamp only) and forwards this LFA message to all the active ports
except for the incoming one, otherwise, M4 discards this message
directly. There is a special case a router will rebroadcast a link
failure. If a router receives a data packet and must forward the
packet going ahead to destination through a failed link, it means
some previous router should avoid this failed link according to its
NRT but it doesn't. In this case, maybe the previous router missed
the LFA message of the link failure due to some uncertain reasons.
So the forwarding router rebroadcasts the LFA message.

6.5. BRT Building Module(M5)

M5 is responsible for building a BRT for the current router. By
leveraging the regularity in topology, M5 can calculate the routing
paths for any destination without the knowledge of the topology of
whole network, and then build the BRT based on an NDT. Since the IP
addresses of network devices are continuous, M5 only creates one
route entry for a group of destination addresses that have the same
network prefix by means of route aggregation technology. Usually,
the size of a BRT is very small. The detail of how to build a BRT is
described in section 5.

Liu, et al. Expires 27 June 2024 [Page 16]
Internet-Draft FAR for DCN December 2023

6.6. NRT Building Module(M6)

M6 is responsible for building a NRT for the current router. Because
M5 builds a BRT without considering link failures in network, the
routing paths calculated by the BRT cannot avoid failed links. To
solve this problem, a NRT is used to exclude the routing paths that
include some failed links from the paths calculated by a BRT. M6
calculate the routing paths that include failed links and stored them
into the NRT. The details of how to build a NRT is described in
section 5.

6.7. Routing Table Lookup(M7)

M7 is responsible for querying routing tables and selecting the next
hop for forwarding the packets. Firstly, M7 takes the destination
address of a forwarding packet as a criterion to look up route
entries in a BRT based on longest prefix match. All of the matched
entries are composed of a candidate hops list. Secondly, M7 look up
negative route entries in a NRT taking the destination address of the
forwarding packet as criteria. This lookup is not limited to the
longest prefix match, any entry that matches the criteria would be
selected and composed of an avoiding hops list. Thirdly, the
candidate hops minus avoiding hops are composed of an applicable hops
list. At last, M7 sends the forwarding packet to any one of the
applicable hops. If the applicable list is empty, the forwarding
packet will be dropped.

7. How a FAR Router Works

Figure 7 shows how a FAR router works by its FSM. 1)When a router
starts up, it starts a Hello thread and then starts ND (neighbor
detection) timer (3 seconds). Next the router goes into ND (neighbor
detection) state. 2)In the ND state, if a router received a Hello
message, then it performs a Hello-message processing and goes back to
the ND state. 3)When the ND timer is over, a router goes into ND-FIN
(neighbor detection finished) state. 4)A router starts the LFD (link
failure detection) thread and DFD (device failure detection) state,
and sends DA message and DLR message to all of its active ports.
Then the router goes into Listen state. 5) If a router receives a
Hello message, then goes into HELLO-RECV state. 6) If a router
receives a DLR message, then goes into DLR-RECV state. 7) If a
router receives a DA message, then goes into DA-RECV state. 8) If a
router receives a LFA message, then goes into LFA-RECV state. 9) A
router performs the Hello-message processing. After that, it goes
back to Listen state. 10) A router performs the DLR-message
processing. After that, it goes back to Listen state. 11) A router
performs the DA-message processing. After that, it goes back to
Listen state. 12) A router performs the LFA-message processing.

Liu, et al. Expires 27 June 2024 [Page 17]
Internet-Draft FAR for DCN December 2023

After that, it goes back to Listen state. 13) Hello thread produces
and sends Hello messages to all its ports periodically. 14) LFD
thread calls link-failure-detection processing to check link failures
in all links periodically 15) DA thread produces and sends DA
messages periodically (30 minutes). 16) When DFD thread starts up,
it sleeps a short time (30 seconds) to wait for a router to learn all
the active routers in the network. Then the thread calls the device-
failure-detection processing to check device failures periodically
(30 minutes).

/---------------\
| Start |
| |
\---------------/
|
+--------+ |1
| | |
| \|/ \|/
| +--------------------+
|2 | ND |
| | |
| +--------------------+
| | |
| | |3
+--------+ |
\|/
+--------------+
| ND-FIN |
| |
+--------------+
|
|4
|
\|/
________10_______\+--------------+/_______11________
| /| |\ |
| | Listen | |
| ____9_______\| |/_______12___ |
| | /| |\ | |
| | +--------------+ | |
| | 5/ | | \8 | |
| | |/_ | | _\| | |
| | +------------+ 6| |7 +------------+ | |
| --| HELLO-RECV | | | | LFA-RECV |-- |
| +------------+ | | +------------+ |
| ______| |______ |
| | | |

Liu, et al. Expires 27 June 2024 [Page 18]
Internet-Draft FAR for DCN December 2023

| \|/ \|/ |
| +------------+ +------------+ |
|___________| DLR-RECV | | DA-RECV |__________|
+------------+ +------------+

_________
| |
| |
\|/ |
+--------------+ |13
| Hello Thread | |
+--------------+ |
| |
|_________|

_________
| |
| |
\|/ |
+--------------+ |14
| LFD Thread | |
+--------------+ |
| |
|_________|

_________
| |
| |
\|/ |
+--------------+ |15
| DA Thread | |
+--------------+ |
| |
|_________|

_________
| |
| |
\|/ |
+--------------+ |16
| DFD Thread | |
+--------------+ |
| |
|_________|

Figure 7: The Finite State Machine of FAR Router

Liu, et al. Expires 27 June 2024 [Page 19]
Internet-Draft FAR for DCN December 2023

8. Compatible Architecture

As a generic routing protocol, FAR can be run in various DCNs with
regular topology. Up to now, we have implemented the FAR protocol
for 4 types of DCN, including Fat-tree, BCube, MatrixDCN and Diamond.
For different network architectures, most processing of FAR is same
besides calculation of routing tables. BRT routing tables are
calculated based on Hello messages and NRT routing tables are
calculated based on LFA messages in FAR. To extend FAR to support a
new network architecture, only processing of Hello and LFA messages
need providing to build BRT and NRT routing tables. In this
protocol, the topology type number is represented by four bits, the
maximum number of topology types supported is 16, which is generally
sufficient.So FAR can support maximally 16 network architectures and
at least support 1 built-in network architecture, such as Fat-tree,
BCube and MatrixDCN,etc. Each network architecture is assigned a
unique number from 1 to 16. For example, if the 1 built-in
architectures are assigned 1, and other customized architectures are
assigned 2 to 16. 1: Fat-tree 2: BCube 3: MatrixDCN. 4: xxx.
...... 16: xxx.

9. Topology identification and broadcast storm suppression

In this design, the initial topology discovery process is not a
mandatory option for a FAR routing protocol. The recommended
solution here is to use a pre-configured configuration file, which
contains topology parameters of the current system, each node device
as long as according to these configuration parameters will be able
to know the topology information. In this way, we do not have to
deal with complex topology discovery processes, nor do we need to
calculate the shortest path, because the optimal path can be
calculated from the parameters. This protocol also allows the
formation of configuration files to be submitted to the topology
discovery protocol, allowing for a variety of different
implementation options. Regarding the flood suppression processing
of broadcast packets, it has been considered in the previous content.
Since the hello packets is only transmitted between the two nodes, it
cannot be spread out. The link error message is only sent to the
CPU, and are not forwarded to the nodes in layer 2 broadcasting.
Moreover, each node will discard the repeated error messages when the
node receives them. In this way, the broadcast storm can be
suppressed. If a link is unstable and repeatedly up or down, the
system will not send new messages after sending notifications, and
the system will not oscillate repeatedly. The topology is updated
only when the link is later detected to be stable for a long time.

Liu, et al. Expires 27 June 2024 [Page 20]
Internet-Draft FAR for DCN December 2023

10. Application Example

In this section, we take a Fat-tree network(Fig. 7) as an example to
describe how to apply FAR routing. Since M1 to M4 are very simple,
we only introduce how the modules M5, M6, and M7 work in a Fat-tree
network. A Fat-tree network is composed of 4 layers. The top layer
is core layer, and the other layers are aggregation layer, edge layer
and server layer. There are k pods, each one containing two layers
of k/2 switches. Each k-port switch in the edge layer is directly
connected to k/2 hosts. The remaining k/2 ports are connected to k/2
of the k-port switches in the aggregation layer. There are (k/2)2
k-port core switches. Each core switch has one port connected to
each of the k pods. Aggregation switches are given addresses of the
form 10.pod.0.switch, where pod denotes the pod number, and switch
denotes the position of that switch in the upper pod (in [1, k/2]).
Edge switches are given addresses of the form 10.pod.switch.1, where
pod denotes the pod number, and switch denotes the position of that
switch in the lower pod (in [1, k/2]). The core switches are given
addresses of the form 10.0.j.i, where j and i denote that switch's
coordinates in the (k/2)2 core switch grid (each in[1, (k/2)],
starting from top-left). The address of a host follows the pod
switch to which it is connected to; hosts have addresses of the form:
10.pod.switch.ID, where ID is the host's position in that subnet (in
[2, k/2+1], starting from left to the right).

Liu, et al. Expires 27 June 2024 [Page 21]
Internet-Draft FAR for DCN December 2023

10.0.1.1 10.0.1.2 10.0.2.1 10.0.2.2
+--+ +--+ +--+ +--+
| | | | | | | |
+--+,_ +--+', +--+ ,,+--+
|`,`',`-., / | \ `. .'` - .-'``.` /|
| . `', `'., / | ' ' ,-`,' |`. .` ' |
| \ `', `-., .` / | `, .` ,' |
| `, `'. `'-,_ .'` ,' | ', / |
| . `'. `-.,-` / | \
| \ `'., .` `'., ` | `.
| `, .'`, ,`'., | ',
| . ,-` '., - `'-,_ | `.
| \ .` ,'., `|., .
| .'` / `-, | `'., `.
10.1.0.1 ,-` . .' 10.3.0.1 10.3.0.2`'., ',
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
| | | | | | | | | | | | | | | |
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
| \/ | | \/ | | \/ | | \/ |
+--+/\+--+ +--+/\+--+ +--+/\+--+ +--+/\+--+
| | | | | | | | | | | | | | | |
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
/| 10.1.2.1 /| |\ 10 3.1.3 |\ /| | |
/ | | \ / | | \ / | | \ / | | |
/ | | \ / | | \ / | | \ / | | |
/ | | \ / | | \ / | | \ / | | |
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++
10.1.2.2 10.3.1.3

Figure 8: Fat-tree Network

10.1. BRT Building Procedure

By leveraging the topology's regularity, every switch clearly knows
how it forwards a packet. When a packet arrives at an edge switch,
if the destination of the packet lies in the same subnet with the
switch, then the switch directly forwards the packet to the
destination server through layer-2 switching. Otherwise, the switch
forwards the packet to any of aggregation switches in the same pod.
When a packet arrives at an aggregation switch, if the destination of
the packet lies in the same pod, the switch forwards the packet to
the corresponding edge switch. Otherwise, the switch forwards the
packet to any of core switches that it is connected to. If a core
switch receives a packet, it forwards the packet to the corresponding
aggregation switch that lies in the destination pod. The forwarding
policy discussed above is easily expressed through a BRT. The BRT of

Liu, et al. Expires 27 June 2024 [Page 22]
Internet-Draft FAR for DCN December 2023

an edge switch, such as 10.1.1.1, is composed of the following
entries: The BRT of an aggregation switch, such as 10.1.0.1, is
composed of the following entries: The BRT of a core switch, such as
10.0.1.1, is composed of the following entries:

Destination/Mask Next hop
10.0.0.0/255.0.0.0 10.1.0.1
10.0.0.0/255.0.0.0 10.1.0.2

Destination/Mask Next hop
10.1.1.0/255 255.255.0 10.1.1.1
10.1.2.0/255.255.255.0 10.1.2.1
10.0.0.0/255.0.0.0 10.0.1.1
10.0.0.0/255.0.0.0 10.0.1.2

Destination/Mask Next hop
10.1.0.0/255 255.0.0 10.1.0.1
10.2.0.0/255.255.0.0 10.2.0.1
10.3.0.0/255.255.0.0 10.3.0.1
10.4.0.0/255.255.0.0 10.4.0.1

10.2. NRT Building Procedure

The route entries in an NRT are related with link and node failures.
We summarize all types of cases into three (3) catalogs.

10.2.1. Single Link Failure

In Fat-tree, Links can be classified as 3 types by their locations:
1) servers to edge switches; 2) edge to aggregation switches; 3)
aggregation to core switches. Link failures between servers to edge
switches only affect the communication of the corresponding servers
and don't affect the routing tables of any switch, so we only discuss
the second and third type of links failures. Edge to Aggregation
Switches Suppose that the link between an edge switch, such as
10.1.2.1 (A), and an aggregation switch, such as 10.1.0.1(B),fails.
This link failure may affect 3 types of communications. o Sources
lie in the same subnet with A, and destinations do not. In this
case, the link failure will only affect the routing tables of A. As
this link is attached to A directly, A only needs to delete the route
entries whose next hop is B in its BRT and add no entries to its NRT
when A's M6 module detect the link failure. o Destinations lie in
the same subnet with A, and sources lie in another subnet of the same
pod. In this case, the link failure will affect the routing tables
of all the edge switches in the same pod except for A. When an edge
switch, such as 10.1.1.1, learns the link failure, it will add a
route entry to its NRT: o Destinations lie in the same subnet with A,
sources lie in another pod. In this case, the link failure will

Liu, et al. Expires 27 June 2024 [Page 23]
Internet-Draft FAR for DCN December 2023

affect the routing tables of all the edge switches in the other pods.
When an edge switch in one other pod, such as 10.3.1.1, learns the
link failure, because all the routings that pass through 10.3.0.1 to
A will certainly pass through the link between A and B, 10.3.1.1 need
add a route entry to its NRT: Aggregation to Core Switches Suppose
that the link between an aggregation switch, such as 10.1.0.1 (A),
and a core switch, such as 10.0.1.2(B), fails. This link failure may
affect 2 types of communications. o Sources lie in the same pod (pod
1) with A, and destinations lie in the other pods. In this case, the
link failure will only affect the routing tables of A. As this link
is attached to A directly, A only need to delete the route entries
whose next hop is B in its BRT and add no entries to its NRT when A's
M6 module detect the link failure. o Destinations lie in the same
pod (pod 1) with A, and sources lie in another pod. In this case,
the link failure will affect the routing tables of all the
aggregation switches in other pods except for pod 1. When an
aggregation switch in one other pod, such as 10.3.0.1, learns the
link failure, because all the routings that pass through 10.0.1.2 to
the pod 1 where A lies will certainly pass through the link between A
and B, 10.3.0.1 need add a route entry to its NRT:

Destination/Mask Next hop
10.1.2.0/255.255.255.0 10.1.0.1

Destination/Mask Next hop
10.1.2.0/255.255.255.0 10.3.0.1

Destination/Mask Next hop
10.1.0.0/255.255.0.0 10.0.1.2

10.2.2. A Group of Link Failures

If all the uplinks of an aggregation switch fail, then this switch
cannot forward packets, which will affect the routing of every edge
switches. Suppose that all the uplinks of the node A (10.1.0.1)
fail, it will affect two types of communications. o Sources lie in
the same pod (pod 1) with A, and destinations lie in the other pods.
In this case, the link failures will affect the routing of the edge
switches in the Pod of A. To avoid the node A, each edge switch
should remove the route entry "10.0.0.0/255.0.0.0 10.1.0.1" in which
the next hop is the node A. o Destinations lie in the same pod (pod
1) with A, and sources lie in other pods. In this case, the link
failures will affect the routing of edge switches in other pods. For
example, if the edge switch 10.3.1.1 communicates with some node in
the pod of A, it should avoid the node 10.3.0.1, because any
communication through 10.3.0.1 to the pod of A will pass through the
node A. So a route entry should be added to 10.3.1.1:

Liu, et al. Expires 27 June 2024 [Page 24]
Internet-Draft FAR for DCN December 2023

Destination/Mask Next hop
10.1.0.0/255.255.0.0 10.3.0.1

10.2.3. Node Failures

At last, we discuss the effect of node failures to a NRT. There are
3 types of node failures: the failure of edge, aggregation and core
switches. o An edge switch fails. The failure doesn't affect the
routing table of any switch. o A core switch fails. Only when all
the core switches connected to the same aggregation switch fail, they
will affect the routing of other switches. This case is equal to the
case that all the uplinks of an aggregation switch fail, so the
process of link failures can cover it. o An aggregation switch
fails. This case is similar to the case that all the uplinks of an
aggregation switch fail. It affects the routing of edge switches in
other pods, but doesn't affect the routing of edge switches in pod of
the failed switch. The process of this failure is same to the second
case in section 6.2.2.

10.3. Routing Procedure

FAR decides a routing by looking up its BRT and NRT. We illuminate
the routing procedure by an example. In this example, we suppose
that the link between 10.3.1.1 and 10.3.0.2 and the link between
10.1.2.1 and 10.1.0.2 have failed. Then we look into the routing
procedure of a communication from 10.3.1.3 (source) to 10.1.2.2
(destination). Step 1: The source 10.3.1.3 sends packets to its
default router 10.3.1.1 Step 2: The routing of 10.3.1.1. 1)
Calculate candidate hops 10.3.1.1 looks up its BRT and gets the
following matched entries: So the candidate hops = {10.3.0.1} 2)
Calculate avoiding hops Its NRT is empty, so the set of avoiding hop
is empty too. 3) Calculate applicable hops The applicable hops are
candidate hops minus avoiding hops, so: The applicable hops =
{10.3.0.1} 4) Forward packets to 10.3.0.1 Step 3: The routing of
10.3.0.1 1) Calculate candidate hops. 10.3. 0.1 looks up its BRT and
gets the following matched entries: So the candidate hops =
{10.0.1.1, 10.0.1.2} 2) Calculate avoiding hops So the avoiding hops
= {10.0.1.2} 3) Calculate applicable hops The applicable hops are
candidate hops minus avoiding hops, so: The applicable hops =
{10.0.1.1} 4) Forward packets to 10.0.1.1 Step 4: 10.0.1.1 forwards
packets to 10.1.0.1 by looking up its routing tables. Step 5:
10.1.0.1 forwards packets to 10.1.2.1 by looking up its routing
tables. Step 6:10.1.2.1 forwards packets to the destination 10.1.2.2
by layer-2 switching.

Destination/Mask Next hop
10.0.0.0/255.0.0.0 10.3.0.1

Liu, et al. Expires 27 June 2024 [Page 25]
Internet-Draft FAR for DCN December 2023

Destination/Mask Next hop
10.1.0.0/255.255.0.0 10.0.1.1
10.1.0.0/255.255.0.0 10.0.1.2

Destination/Mask Next hop
10.1.0.0/255.255.0.0 10.0.1.2

10.4. FAR's Performance in Large-scale Networks

FAR has good performance to support large-scale networks. In this
section, we take a Fat-tree network composed of 2,880 48-port
switches and 27,648 servers as an example to show FAR's performance.

10.4.1. The number of control messages required by FAR

FAR exchanges a few messages between routers and only consumes a
little network bandwidth. Tab. 1 shows the required messages in the
example Fat-tree network. Table 1:Required messages in a Fat-tree
network.
_____________________________________________________________________
Message Type| Scope | size(bytes) | Rate | Bandwidth
---------------------------------------------------------------------
Hello |adjacent switches|less than 48|10 messages/sec|less than 4
kbps
---------------------------------------------------------------------
DLR |adjacent switches| less than 48 | (1) |48bytes
---------------------------------------------------------------------
DA |entire network| less than 48 | (2) |1.106M
---------------------------------------------------------------------
LFA |entire network| less than 48 | (3) |48 bytes
_____________________________________________________________________
(1)Produce one when a router starts (2)The number of switches(2,880)
in a period (3)Produce one when a link fails or recovers

10.4.2. The Calculating Time of Routing Tables

A BRT is calculated according to the states of its neighbor routers
and attached links. An NRT is calculated according to device and
link failures in the entire network. So FAR does not calculate
network topology and has no problem of network convergence, which
greatly reduces the calculating time of routing tables. The
detection and spread time of link failures is very short in FAR.
Detection time is up to the interval of sending Hello message. In
FAR, the interval is set to 100ms, and a link failure will be
detected in 200ms. The spread time between any pair of routers is
less than 200ms.If a link fails in a data center network, FAR can
detect it, spread it to all the routers, and calculate routing tables
in no more than 500ms.

Liu, et al. Expires 27 June 2024 [Page 26]
Internet-Draft FAR for DCN December 2023

10.4.3. The Size of Routing Tables

For the test Fat-tree network, the sizes of BRTs and NRTs are shown
in Tab. 2. Table 2: The size of routing tables in FAR
_____________________________________________________________________
Routing Table| Core Switch | Aggregation Switch | Edge Switch |
---------------------------------------------------------------------
BRT | 48 | 48 | 24
---------------------------------------------------------------------
NRT | 0 | 14 | 333
_____________________________________________________________________
The BRT's size at a switch is determined by the number of its
neighbor switches. In the example network, a core switch has 48
neighbor switches (aggregation switch), so it has 48 entries in its
BRT.Only aggregation and edge switches have NRTs. The NRT size at a
switch is related to the number of link failures in the network.
Suppose that there are 1000 link failures in the example network, the
number of failed links is 1.2% of total links, which is a very high
failure ratio. We suppose that link failures are uniformly
distributed in the entire network. The NRT size at an edge switch is
about 333 and the NRT size of an aggregation switch is about 14in
average.

11. Implementations Examples

In the FAR draft scenario, Fat-Tree topology has only three layers of
routers. To expand the network scale is achieved through horizontal
expansion: increase the number of core switches, and increase the
number of aggregation switches and edge switches in pod.

For example, the following two scenarios.

Liu, et al. Expires 27 June 2024 [Page 27]
Internet-Draft FAR for DCN December 2023

10.0.1.1 10.0.2.1 10.0.3.1 10.0.24.1
+--+ +--+ +--+ +--+
| |24devices | |24devices | |24devices | | 24devices
+--+,_ +--+', +--+ ,,+--+
|`,`',`-., / | \ `. .'` - .-'``.` /|
| . `', `'., / | ' ' ,-`,' |`. .` ' |
| \ `', `-., .` / | `, .` ,' |
| `, `'. `'-,_ .'` ,' | ', / |
| . `'. `-.,-` / | \
| \ `'., .` `'., ` | `.
| `, .'`, ,`'., | ',
| . ,-` '., - `'-,_ | `.
| \ .` ,'., `|., .
| .'` / `-, | `'., `.
10.1.0.1 ,-` . .' `'., ',
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
| |..| | | |..| | | |..| | | |..| |A total of 1152 aggregation switches
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
| \/ | | \/ | | \/ | | \/ |
+--+/\+--+ +--+/\+--+ +--+/\+--+ +--+/\+--+
| |..| | | |..| | | |..| | | |..| |A total of 1152 access switches
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
/| 10.1.24.1 /| |\ |\ /| | |
/ | | \ / | | \ / | | \ / | | |
/ | | \ / | | \ / | | \ / | | |
/ | | \ / | | \ / | | \ / | | |
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++

Figure 9: 48 pods, each of which has 24 aggregation switches and
24 access switches

In the Fat-tree network of Figure 9, there are a total of 48 pods,
each of which has 24 aggregation switches and 24 access switches, and
each access switch is connected to 24 servers. 576 core switches,
1152 aggregation switches, and 1152 access switches are required, for
a total of 2880 switches, which can accommodate 27,648 servers.

Liu, et al. Expires 27 June 2024 [Page 28]
Internet-Draft FAR for DCN December 2023

96 10 gigabit core switches
+--+ +--+ +--+ +--+
| |4devices | |4devices | |4devices | |4devices
+--+,_ +--+', +--+ ,,+--+
|`,`',`-., / | \ `. .'` - .-'``.` /|
| . `', `'., / | ' ' ,-`,' |`. .` ' |
| \ `', `-., .` / | `, .` ,' |
| `, `'. `'-,_ .'` ,' | ', / |
| . `'. `-.,-` / | \
| \ `'., .` `'., ` | `.
| `, .'`, ,`'., | ',
| . ,-` '., - `'-,_ | `.
| \ .` ,'., `|., .
| .'` / `-, | `'., `.
10.1.0.1 ,-` . .' `'., ',
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
| |..| | | |..| | | |..| | | |..| |A total of 192 10 gigabit aggregation switches
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
| \/ | | \/ | | \/ | | \/ |
+--+/\+--+ +--+/\+--+ +--+/\+--+ +--+/\+--+
| |..| | | |..| | | |..| | | |..| |A total of 1152 access switches
+--+ +--+ +--+ +--+ +--+ +--+ +--+ +--+
/| 10.1.4.1 /| |\ |\ /| | |
/ | | \ / | | \ / | | \ / | | |
/ | | \ / | | \ / | | \ / | | |
/ | | \ / | | \ / | | \ / | | |
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++
++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++ ++

Figure 10: 48 pods. Each pod has 4 10G aggregation switches and
24 Gigabit access switches

In the Fat-Tree network of Figure 10, there are a total of 48 pods.
Each pod has 4 10G aggregation switches and 24 Gigabit access
switches. Each access switch is connected to 40 servers. Requires
96 core switches, 192 aggregation switches, and 1152 access switches,
for a total of 1,440 switches, which can accommodate 46,080 servers.

12. Security Considerations

The security considerations will be discussed in a future version of
this document.

Liu, et al. Expires 27 June 2024 [Page 29]
Internet-Draft FAR for DCN December 2023

13. Conclusions

This draft introduces FAR protocol, a generic routing method and
protocol, for data centers that have a regular topology. It uses two
routing tables, a BRT and an NRT, to store the normal routing paths
and the forbidden (to-be-avoided) routing paths, respectively. This
makes the FAR protocol very simple and efficient. The sizes of these
two tables are very small. Usually, a BRT has only several tens of
entries and an NRT has only several or about a dozen entries.

14. Acknowledgments

This document is supported by ZTE Enterprise-University-Research
Joint Project.

15. References

15.1. Normative References

[ISO10589] ISO, "Intermediate System to Intermediate System Intra-
Domain Routing Information Exchange Protocol for use in Conjunction
with the Protocol for Providing the Connectionless-mode Network
Service (ISO 8473)", ISO 10589:1992.

[RFC1142] D. Oran, "OSI IS-IS Intra-domain Routing Protocol", RFC
1142, February 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] J. Moy, "OSPF Version 2", BCP 14, RFC2328, April 1998.

[RFC3619] SHAH, S.; YIP, M. RFC3619: Extreme Networks' Ethernet
Automatic Protection Switching (EAPS) Version 1. 2003.

15.2. Informative References

[FAT-TREE] M. Al-Fares, A. Loukissas, and A. Vahdat."A
Scalable,Commodity, Data Center Network Architecture",In ACM SIGCOMM
2008.

[BCube] Guo, C., Lu, G., Li, D., Wu, H., Zhang, X., Shi, Y., ... Lu,
S. (2009, August). BCube: a high performance, server-centric network
architecture for modular data centers. In Proceedings of the ACM
SIGCOMM 2009 conference on Data communication (pp. 63-74).

Liu, et al. Expires 27 June 2024 [Page 30]
Internet-Draft FAR for DCN December 2023

[MatrixDCN] Sun, Y., Chen, M., Peng, L., Hassan, M. M., Alelaiwi, A.
(2016). MatrixDCN: a high performance network architecture for
large‐scale cloud data centers. Wireless Communications and Mobile
Computing, 16(8), 942-959.

16. Appendix

16.1. Application Area of the Solution

According to the horizontal expansion mode of the above scenarios,
the whole Fat-Tree network does not need to be expanded to 4 layers
(4 order Fat-Tree) even if it is expanded. Using a standard three-
tier Fat-tree network, we can scale the network to meet all the
problems of commercial network applications. This scheme is suitable
for non-SDN(Software Defined Network,SDN) distributed Fat-Tree
network architecture.

16.2. Technical evolution roadmap

In this draft, we should design different rules for FAR switches in
different regular networks to calculate routing tables, which limits
FAR's extensibility. Fortunately, the latest SDN technology make it
is easy to update the control plane of switches, since all the
function of control plane are centralized to a controller in SDN. We
are designing the next generation routing scheme for regular networks
based on SDN. In the new scheme, we design a regular ToPology
Description Language (TPDL) to descript a regular network. In TPDL,
the distance between different type of node groups is defined by a
group of distance formulas and the number of formulas is finite and
fixed without increasing by the scale of a network. And then,
switches learn the topology of a network by taking advantage of TPDL
and generate flow table entries to forward packets without help of
the SDN controller. If no entry is found for a forwarding packet,
switches transport the packet to the SDN controller, and the
controller recalculates a new routing path using A* algorithm by
taking TPDL’s distance formulas as a heuristic function and dispatch
flow table entries down to related switches on the path.

16.3. Updating roadmap

In the next version, we will continue to advance the remaining issues
such as protocol fields, section 3.2 simplification, etc.

Authors' Addresses

Bin Liu
ZTE Inc., ZTE Plaza
No.19 East Huayuan Road,Hai Dian District

Liu, et al. Expires 27 June 2024 [Page 31]
Internet-Draft FAR for DCN December 2023

Beijing
100191
China
Phone: +86 -010-59932039
Email: 13683610386@139.com

Yantao Sun
Beijing Jiaotong University
No.3 Shang Yuan Cun, Hai Dian District
Beijing
100044
China
Email: ytsun@bjtu.edu.cn

Jing Cheng
Beijing Jiaotong University
No.3 Shang Yuan Cun, Hai Dian District
Beijing
100044
China
Email: yourney.j@gmail.com

Yichen Zhang
Beijing Jiaotong University
No.3 Shang Yuan Cun, Hai Dian District
Beijing
100044
China
Email: snowfall_dan@sina.com

Bhumip Khasnabish
Individual contributor
55 Madison Avenue, Suite 160
Morristown, New Jersey , 07960
United States of America
Phone: +001-781-752-8003
Email: vumip1@gmail.com
URI: http://tinyurl.com/bhumip/

Liu, et al. Expires 27 June 2024 [Page 32]

Generic Fault-Avoidance Routing Protocol for Data Center Networks draft-sl-rtgwg-far-dcn-21

Generic Fault-Avoidance Routing Protocol for Data Center Networks
draft-sl-rtgwg-far-dcn-21