A Survey of Multipath Routing

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    A survey of multipath routing for traffic engineering

    Gyu Myoung Lee, Jin Seek Choi

    Information and Communications University (ICU)

    {gmlee, jin}@icu.ac.kr

    Abstract

    Traffic engineering broadly relates to optimization of the operational performance of a network. This

    survey discusses techniques like multi-path routing using traffic splitting, constraint-based routing, path-

    protection etc. that are used for traffic engineering. Multipath routing can be effectively used for

    maximum utilization of network resources. It gives the node a choice of next hops for the same destination.

    The various algorithms discussed give solutions for effectively calculating the multipaths and ways to

    minimize delay and increase throughput. Multipath routing is capable of aggregating the resources of

    multiple paths and reducing the blocking capabilities in QoS oriented networks, allowing data transfer at

    higher rate when compared to single path. It also increases the reliability of delivery. We surveyed the

    various multipath routing mechanisms for traffic engineering. Especially, these works can be applied to

    MPLS/GMPLS network, then enhance network performance through traffic engineering and meet the

    QoS requirements.

    1. Introduction

    The unprecedented growth of the Internet has lead to a growing challenge among the ISPs to provide a

    good quality of service, achieve operational efficiencies and differentiate their service offerings. ISPs are

    rapidly deploying more network infrastructure and resources to handle the emerging applications and

    growing number of users. Enhancing the performance of an operational network, at both the traffic and

    the resource levels, are major objectives of traffic engineering [1]. Traffic engineering (TE) is defined as

    that aspect of Internet network engineering dealing with the issue of performance evaluation and

    performance optimization of operational IP networks [1]. The goal of performance optimization of

    operational IP networks is accomplished by routing traffic in a way to utilize network resources

    efficiently and reliably. Traffic engineering has been used to imply a range of objectives, including load-

    balancing, constraint-based routing, multi-path routing, fast re -routing, protection switching etc.

    Many implementation strategies for Traffic Engineering and Quality of Service are deployed. Popular

    approaches such as virtual circuits and solutions based on MPLS use a sort of connection-oriented

    mechanisms. MPLS uses label switched paths between hosts to distribute the incoming traffic among

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    these paths. IETF calls for Integrated Services architecture with RSVP, but this is not a scalable solution

    because of the per-flow overhead associated. Progress towards aggregation of flows to reduce the state

    size and processing power at routers has been made. In Aggregated RSVP per-flow routing state hasgiven way to per source-destination routing state.

    The Internet today provides only a single path between any pair of hosts that fundamentally limits the

    throughput achievable between them. For example, dramatically faster large file transfer or higher frame-

    rate video would be possible if applications could expect that multiple transport level sessions would be

    mapped to different paths in the network, and they have control over splitting traffic between these

    sessions. Multipath routing can be effectively used for maximum utilization of network resources. It gives

    the node a choice of next hops for the same destination. The various algorithms discussed give solutions

    for effectively calculating the multipaths and ways to minimize delay and increase throughput. Multipath

    routing is capable of aggregating the resources of multiple paths and reducing the blocking capabilities in

    QoS oriented networks, allowing data transfer at higher rate when compared to single path. It also

    increases the reliability of delivery. We surveyed the various multipath routing mechanisms for traffic

    engineering. Especially, these works can be applied to MPLS/GMPLS network, then enhance network

    performance through traffic engineering and meet the QoS requirements.

    The organization of this paper is as follows. In section 2, the concept and fundamental scheme of

    multipath routing is explained. And then, we discuss the benefits of multipath routing. Section 3 presents

    the existing multipath routing algorithms. Then, in section 4, we discuss the multipath routing in

    traditional IP network. The multipath routing mechanisms applied to MPLS networks are discussed in

    section 5. In section 6, we propose the multipath routing method using GMPLS control plane in IP over

    optical networks. The conclusions and future work are given in section 7.

    2. Multipath routing fundamentals

    2.1. Multipath Routing

    Multipath routing aims to exploit the resources of the underlying physical network by providing

    multiple paths between source-destination pairs. Multipath routing has a potential to aggregate bandwidth

    on various paths, allowing a network to support data transfer rates higher than what is possible with any

    one path [2]. The work in the area of multi-path routing has focused mainly on extending intra-domain

    routing algorithms (both RIP and OSPF) for multipath support [3], [4], [5]. There are two aspects of a

    multi-path routing algorithm: computation of multiple loop-free paths and traffic splitting among these

    multiple paths. Extensive work has been done in both these areas. Distributed multi-path routing

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    algorithms can be viewed as an extension of hop-by-hop routing algorithms. Centralized multi-path

    algorithms can be used in the MPLS framework to influence establishment of LSPs. However, traffic

    splitting algorithms may be used to the traffic among multiple paths in both the cases.

    2.2. Multipath Routing scheme

    A host that provides multiple paths must first calculate the path sets between the source and destination.

    Two of the characteristics that can be used for determining a path set are path quantity and path

    independence. Path quantity is the number of available paths between nodes. The higher the number

    better are the chances for load distribution. Un iform path sets are preferable over high variance path sets

    i.e. a path set with every node having 5 paths is preferred than one with nodes having 1 path for some

    path sets and 9 paths for other path sets. The second characteristic of path sets is path independence,

    which is illustrated with Fig. 1. Consider a path set with 2 paths (a, b, c, d) and (a, f, c, d) and other path

    set with 2 paths as (a, b, c, d) and (a, f, e, d). The second set is independent when compared to the first set.

    a f e

    d

    b c

    a f e

    d

    b c

    Fig. 1. Illustration of multipath routing scheme

    So the second set would lead to better usage of resources and is less likely to be congested because at

    least one link in each path should be congested, whereas in the first set congestion at link (c, d) is

    reflected in both the path sets. Multipath sets with these attributes facilitate for higher performance.

    2.3. The benefits of multipath routing

    l Load balancing

    The main goal with load balancing is to make more use of available network resources in order to

    minimize the risk of traffic congestion. Hopefully this would lead to less delay and packet loss. It could

    however lead to additional propagation delay if the alternative routes are badly chosen. Some applications

    are very sensitive to delays (e.g. VoIP). Others are more sensitive to packet loss.

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    In SPF-algorithms, load balancing cannot be done over links with different assigned costs. When

    manually configuring a good load balancing, the traffic demand must be predictable to avoid

    unanticipated traffic congestions. Traffic or policy constraints are not taken into account in the staticsplitting of traffic. The administrator responsible for the load-sharing configuration will have to be

    attentive to changes in the traffic pattern. These changes could come from a change of routing policy in a

    peering network, a link failure, a change of topology or a sudden change of popularity for an application.

    As a result of this instability in traffic flow, the administrator will have to devote a lot of time tuning the

    configuration to achieve a stable network load balance.

    Load balancing requires an ability to control traffic flow precisely. In the traditional metric-based

    control, an administrator can change only link metrics, and changes of some link metrics may affect the

    overall traffic flow. To control traffic flow as the administrator wants, it is necessary to adjust the link

    metrics over a network; however, sometimes this adjustment is impossible. Therefore, explicit route

    based control, in which the administrator can control traffic as he wants, appears to be a far more

    promising choice.

    l Quality of Service

    Several architectures have been proposed for implementing Quality of Service. In the IETF s Integrated

    Services architecture reservations are made on per-flow basis, which is not a scalable solution because

    routers require large amount of memory to store routing and reservatio