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Routing Area Working Group                                       Bin Liu
Internet-Draft                                       ZTE Inc., ZTE Plaza
Intended status: Informational                                Yantao Sun
Expires: November 16, 2017                                    Jing Cheng
                                                            Yichen Zhang
                                             Beijing Jiaotong University
                                                       Bhumip Khasnabish
                                                             ZTE TX Inc.
                                                            May 15, 2017


   Generic Fault-avoidance Routing Protocol for Data Center Networks
                       draft-sl-rtgwg-far-dcn-08

Abstract

   This draft proposes a generic routing method and protocol for a
   regular data center network, named as fault-avoidance routing (FAR)
   protocol.  FAR protocol provides a generic routing method for all
   types of network architectures that are proposed for large-scale
   cloud-based data centers over the past few years.  FAR protocol is
   well designed to fully leverage the regularity in the topology and
   compute its routing table in a simplistic manner.  Fat-tree is taken
   as an example architecture to illustrate how 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 http://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 November 16, 2017.

Copyright Notice

   Copyright (c) 2017 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
   (http://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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified 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.  Dilemma 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  . . . . . . . .   8
     3.6.  Virtual Machine Migration Issues  . . . . . . . . . . . .   9
   4.  The FAR Framework . . . . . . . . . . . . . . . . . . . . . .   9
   5.  Data Format . . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Data Tables . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Messages  . . . . . . . . . . . . . . . . . . . . . . . .  13
   6.  FAR Modules . . . . . . . . . . . . . . . . . . . . . . . . .  17
     6.1.  Neighbor and Link Detection Module(M1)  . . . . . . . . .  17
     6.2.  Device Learning Module(M2)  . . . . . . . . . . . . . . .  17
     6.3.  Invisible Neighbor and Link Failure Inferring Module(M3)   18
     6.4.  Link Failure Learning Module(M4)  . . . . . . . . . . . .  18
     6.5.  BRT Building Module(M5) . . . . . . . . . . . . . . . . .  18
     6.6.  NRT Building Module(M6) . . . . . . . . . . . . . . . . .  19
     6.7.  Routing Table Lookup(M7)  . . . . . . . . . . . . . . . .  19
   7.  How a FAR Router Works  . . . . . . . . . . . . . . . . . . .  19
   8.  Compatible Architecture . . . . . . . . . . . . . . . . . . .  22
   9.  Application Example . . . . . . . . . . . . . . . . . . . . .  22
     9.1.  BRT Building Procedure  . . . . . . . . . . . . . . . . .  24
     9.2.  NRT Building Procedure  . . . . . . . . . . . . . . . . .  25
       9.2.1.  Single Link Failure . . . . . . . . . . . . . . . . .  25
       9.2.2.  A Group of Link Failures  . . . . . . . . . . . . . .  26
       9.2.3.  Node Failures . . . . . . . . . . . . . . . . . . . .  27
     9.3.  Routing Procedure . . . . . . . . . . . . . . . . . . . .  27
     9.4.  FAR's Performance in Large-scale Networks . . . . . . . .  29
       9.4.1.  The number of control messages required by FAR  . . .  29
       9.4.2.  The Calculating Time of Routing Tables  . . . . . . .  29
       9.4.3.  The Size of Routing Tables  . . . . . . . . . . . . .  30



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   10. Security Considerations . . . . . . . . . . . . . . . . . . .  30
   11. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . .  30
   12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  31
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  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, and 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 a common feature that is with a
   regular topology.  Here a regular topology is not a mathematical or
   definite conception, which means a non-arbitrary network fabric, or
   an inerratic network fabric.  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 draft proposes a generic routing method and protocol, 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 and IS-IS.  It cannot be used in general local networks
   whose topological structures are arbitrary, and whose scales are also
   not very large.  OSPF works very well in such a network.  But in a
   large-scale network with regular topology, FAR has a better
   performance.  Compared with OSPF and IS-IS, FAR has shorter time of
   network convergence and lower PDU overhead.  Furthermore, FAR
   requires less computing and storage resources, which let FAR routers
   to run at a lower cost of production than the generic routers.




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   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.

   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

   NRT - Negative Routing Table

   NDT - Neighbor Devices Table

   ADT - All Devices Table

   LFT - Link Failure Table

   DA - Device Announcement




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   LFA - Link Failure Announcement

   DLR - Device and Link Request

   IP - Internet Protocol

   UDP - User Datagram Protocol

   VM - Virtual Machine

2.  Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL
   NOT","SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in
   this document are to be interpreted as described in RFC-2119
   [RFC2119].  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 RFC-2119 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 and other routing protocols in such network
   will encounter these two challenges: a) 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; b) a large number of routing
   protocol packets need to be sent.  The routing information consumes
   too much network bandwidth and CPU resources, which easily leads to
   packet loss and makes the problem (a) more prominent.

   In order to solve these problems, 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.



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   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.  Dilemma 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 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 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 other conventional routing methods do not work well in a
   large-scale network with giant number nodes of routers?

   As everyone knows, 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 algorithm itself does not thoroughly solve these problems.

   On the contrary, 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":



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   In the OSPF protocol, LSA 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.

   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 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 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 SFP, so there
   is no such convergence time.




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   These issues still exist in rapid convergence technology of OSPF and
   ISIS (such as I-SPF).  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 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.

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)



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   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.













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               Link-state Learning    |Routing Table  | Routing Table
                                      |  Building
                                      |               |   Querying
    +--------+   /---------------\    | +--------+    |    Packets
    |2 Device|<--| 1 Neighbor &  |----->| 5 BRT  \    |
    |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

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

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.




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   Negative Routing Table(NRT): To store the avoiding routes.

   The format of NDT

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


   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.

   The format of ADT

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


   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.

   The format of LFT

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


   No: The entry number.





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   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.

   Timestamp: It identifies when the entry is created.

   The format of BRT

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


   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.

   The format of NRT

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


   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.





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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
   protocol of IP for FAR should be assigned by IANA.

   The four types of FAR messages have same format of packet header,
   called FAR header (as shown in Figure 2).


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


                    Figure 2: The format of FAR header

   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.




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   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.



            |<--- 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

   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.



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   Neighbor Router IP: The IP address of a neighbor router.  All the
   neighbor router's addresses should be included in a Hello message.



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


                    Figure 4: The Format of DA Messages

   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.























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            |<----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

   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



















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


                   Figure 6: The Format of DLR Messages

   For DLR messages(Fig. 6), the Message Type in FAR header is set to
   1.Except for FAR header, DLR has no additional fields.


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 a UDP 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 a 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 a 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,



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   the size of a BRT is very small.  The detail of how to build a BRT is
   described in section 5.

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.




                            /---------------\
                            |     Start     |
                            |               |
                            \---------------/
                                    |
                     +--------+     |1
                     |        |     |
                     |       \|/   \|/
                     |    +--------------------+
                     |2   |         ND         |
                     |    |                    |



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                     |    +--------------------+
                     |        |      |
                     |        |      |3
                     +--------+      |
                                    \|/
                              +--------------+
                              |    ND-FIN    |
                              |              |
                              +--------------+
                                     |
                                     |4
                                     |
                                    \|/
            ________10_______\+--------------+/_______11________
           |                 /|              |\                |
           |                  |    Listen    |                 |
           |     ____9_______\|              |/_______12___    |
           |    |            /|              |\            |   |
           |    |             +--------------+             |   |
           |    |               5/  |  |    \8             |   |
           |    |              |/_  |  |    _\|            |   |
           |    |  +------------+  6|  |7   +------------+ |   |
           |     --| HELLO-RECV |   |  |    | LFA-RECV   |--   |
           |       +------------+   |  |    +------------+     |
           |                  ______|  |______                 |
           |                 |                |                |
           |                \|/              \|/               |
           |           +------------+  +------------+          |
           |___________|  DLR-RECV  |  |  DA-RECV   |__________|
                       +------------+  +------------+


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

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



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                         +--------------+  |
                                 |         |
                                 |_________|

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

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

             Figure 7: The Finite State Machine of FAR Router

   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.



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   12) A router performs the LFA-message processing.  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).

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.  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



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   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.


       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

   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



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   (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).


9.1.  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 an edge switch, such as 10.1.1.1, 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

   The BRT of an aggregation switch, such as 10.1.0.1, is composed of
   the following entries:


   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

   The BRT of acore switch, such as 10.0.1.1, is composed of the
   following entries:








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   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

9.2.  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.

9.2.1.  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:


   Destination/Mask            Next hop
   10.1.2.0/255.255.255.0      10.1.0.1

   o Destinations lie in the same subnet with A, sources lie in another
   pod.  In this case, the link failure will 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




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   through the link between A and B, 10.3.1.1 need add a route entry to
   its NRT:


   Destination/Mask            Next hop
   10.1.2.0/255.255.255.0      10.3.0.1

   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
   10.1.0.0/255.255.0.0        10.0.1.2

9.2.2.  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



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   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:


   Destination/Mask            Next hop
   10.1.0.0/255.255.0.0        10.3.0.1


9.2.3.  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.

9.3.  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:




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   Destination/Mask            Next hop
   10.0.0.0/255.0.0.0          10.3.0.1

   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:


   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


   So the candidate hops = {10.0.1.1, 10.0.1.2}

   2) Calculate avoiding hops


   Destination/Mask           Next hop
   10.1.0.0/255.255.0.0       10.0.1.2


   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




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   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.

9.4.  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.

9.4.1.  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

9.4.2.  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



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   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.

9.4.3.  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.

10.  Security Considerations

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

11.  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.






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12.  Acknowledgments

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

13.  References

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March
   1997.

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

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


   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








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   Yichen Zhang
   Beijing Jiaotong University
   No.3 Shang Yuan Cun, Hai Dian District
   Beijing  100044
   China

   Email: snowfall_dan@sina.com


   Bhumip Khasnabish
   ZTE TX Inc.
   55 Madison Avenue, Suite 160
   Morristown, New Jersey    07960
   USA

   Phone: +001-781-752-8003
   Email: vumip1@gmail.com, bhumip.khasnabish@ztetx.com
   URI:   http://tinyurl.com/bhumip/

































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