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Boost cloud network with intelligent hashing

Posted: 28 Mar 2013 ?? ?Print Version ?Bookmark and Share

Keywords:Data centres? Equal Cost Multipathing? hashing?

Data centres workloads are evolving at a fast pace, fuelled by high volumes of end users, application types, cluster nodes, and overall data movement in the cloud. In turn, the cross-sectional bandwidth of these cloud-scale data centre networks is quickly increasing, outpacing the increase in physical link speeds. Traditionally, both Equal Cost Multipathing (ECMP) and port-channel implementations attempt to distribute flows uniformly across physical links that form the logical path. Deciding which flows use which physical link has been traditionally based on a static hash of a fixed set of fields from the packet header. This static hashing scheme is not ideal for high and diverse cloud traffic patterns, causing network polarisation whereby multiple traffic flows may traverse and burden the same link while leaving other links underutilised. A range of flexible hashing enhancements has emerged to provide an alternative, improving network performance and overcoming limitations imposed by traditional hashing schemes.

New look at hashing mechanisms
Traditional load balancing systems split traffic bound through a logical fat link to multiple outgoing physical links. Typically, the physical link corresponding to a flow is ascertained by calculating a hash based on packet header fields and a subsequent modulo operation based on the number of physical links. A good load balancing system should be able to evenly split the traffic to the multiple outgoing links. In addition, packets belonging to the same flow should flow out in order to the end destination.

This static hashing scheme has worked well in carrier and enterprise networks, but is proving ineffective for data centres and cloud networks. Web, application, and database server applications running as virtual machines (VMs) that can reside in any server in any rack C coupled with the growing use of clustered applications (such as Hadoop) in modern data centres C has increased east-west traffic patterns in data centre networks. East-west traffic includes server-to-server, server-to-storage, and server rack-to-server rack. This is fundamentally changing the design of network topologies, from oversubscribed and tiered networks to fast, fat, and flat networks which demand new features in network switches.

Driven by the latest silicon advances, increased bandwidth and port densities of switch-on-a-chip systems, traditional 3-tier network designs are quickly being replaced by these fast, fat and flat networks C comprised of resilient and flexible CLOS topologies with very high cross-sectional bandwidth.

For designers, this transition has some important factors to consider. Data centres of massive scale, such as those using several thousand links, will experience frequent link failures resulting in network polarisation. Yet deployed networks must perform normally and deliver packets in order, even under such failure conditions. Further, as newer protocols and encapsulations are introduced as a means to improve data centre automation and network management, newer packet header fields redefine flows and how packets need to be treated. Along with the introduction of new network features, network operators must have the general ability to debug and trace packets. Lastly, with the increased adoption of cloud hosting services, security is more prominent and essential than ever in network operations. Stateful packet inspection and intrusion detection systems will certainly continue to gain importance.

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