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Routing Area Working Group                                        B. Liu
Internet-Draft                                       ZTE Inc., ZTE Plaza
Intended status: Informational                                    Y. Sun
Expires: 3 December 2022                                        J. Cheng
                                                                Y. Zhang
                                             Beijing Jiaotong University
                                                           B. Khasnabish
                                                  Individual contributor
                                                             1 June 2022


   Generic Fault-Avoidance Routing Protocol for Data Center Networks
                       draft-sl-rtgwg-far-dcn-18

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 3 December 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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



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       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  . . . . . . . . . . . . . . . . . . . . . . . . . .  30
     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).  It cannot be used in general local
   networks whose topological structures are arbitrary, and whose scales



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   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




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   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

   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
   consuming too much network bandwidth and CPU(central processing unit)
   resources, and easily leads to packet loss and makes the challenge



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   (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.
   The SPF(Shortest Path First) algorithm itself does not thoroughly
   solve these problems.  On the contrary, the FAR protocol does not



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   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
   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



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   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.







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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



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   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.



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   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

    -------------------------------------------------------------------
    Destination| Mask| Next Hop| Interface| Failed Link No| Timestamp
    -------------------------------------------------------------------









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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.












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


                     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










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            |<----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







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


                    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.







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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.



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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.



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   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 sleep a short time (30 seconds) to wait for a router learning 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   |--   |
           |       +------------+   |  |    +------------+     |
           |                  ______|  |______                 |
           |                 |                |                |



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           |                \|/              \|/               |
           |           +------------+  +------------+          |
           |___________|  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




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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, FAR can support maximally 12 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 12.  For example, if the 1 built-in
   architectures are assigned 1, and other customized architectures are
   assigned 2 to 12.  1: Fat-tree 2: BCube 3: MatrixDCN.  4: xxx.
   ......  12: 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.








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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).



























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       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



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   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



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   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:




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   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




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   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.



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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.




















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   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.












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                         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.







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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

   [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).

   [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






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






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   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/
















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