Net­work­ing evolves, get­ting eas­ier and more flex­i­ble

New pro­to­cols SDN and Open­flow look like promis­ing ways to cut costs and in­crease au­toma­tion

InformationWeek - - Feature - By Greg Ferro

Soft­ware De­fined Net­work­ing and the Open­flow pro­to­col could change the way net­works op­er­ate, mak­ing them more flex­i­ble and cheaper.

SDN takes the de­ci­sion-mak­ing about how net­work traf­fic should flow away from in­di­vid­ual switches and routers and shifts it to a cen­tral­ized con­troller or set of con­trollers. Th­ese con­trollers use soft­ware to build a pic­ture of ex­ist­ing path­ways through the net­work and tell net­work de­vices which path­ways to use. Open­flow is a pro­to­col that im­ple­ments SDN, de­scrib­ing how a con­troller com­mu­ni­cates with other net­work de­vices.

SDN po­ten­tially pro­vides two ma­jor ben­e­fits. It could let IT more eas­ily and quickly make changes to net­work con­fig­u­ra­tions. It also could lower hard­ware costs if IT can re­place ex­pen­sive, in­tel­li­gent switches and routers with fast but dumb com­mod­ity de­vices. How­ever, SDN and Open­flow have yet to be widely adopted, and net­work­ing pros may be re­luc­tant to swap re­li­able, wellun­der­stood ar­chi­tec­tures for a new one. There also are ex­ist­ing pro­to­cols that are de­signed to ad­dress some of the same is­sues as SDN and Open­flow without dis­rupt­ing the net­work­ing model.

Here’s a look at SDN and Open­flow’s po­ten­tial ben­e­fits and pit­falls, and how they com­pare with ex­ist­ing net­work­ing prac­tices and pro­to­cols.


Net­work­ing equip­ment typ­i­cally has three planes of op­er­a­tion: man­age­ment, control, and for­ward­ing. The man­age­ment plane han­dles func­tions such as de­vice man­age­ment, firmware up­dates, SNMP, and ex­ter­nal con­fig­u­ra­tion via the com­mand line. The for­ward­ing plane (some­times called the data plane) gov­erns packet and frame for­ward­ing of data pay­loads through the net­work de­vice. The control plane con­sists of rout­ing and switch­ing pro­to­cols.

While SDN and Open­Flow prom­ise a more gran­u­lar and open sys­tem, it re­mains to be seen whether they can re­place ex­ist­ing man­age­ment tools

In a typ­i­cal op­er­a­tion, the control plane uses rout­ing pro­to­cols to build the for­ward­ing ta­ble used by the for­ward­ing plane. This for­ward­ing ta­ble is de­liv­ered to the for­ward­ing plane by the man­age­ment plane as part of the de­vice op­er­at­ing sys­tem. When an Eth­er­net frame ar­rives on the switch in­ter­face, the for­ward­ing plane sends it to an out­put port.

SDN and Open­Flow aim to re­place or sup­ple­ment this model by pro­vid­ing a prepared for­ward­ing ta­ble from a cen­tral­ized con­troller (see di­a­gram, above). A con­troller in an SDN ar­chi­tec­ture is a soft­ware ap­pli­ca­tion that per­forms four func­tions. First, it presents a net­work man­age­ment in­ter­face to the IT ad­min­is­tra­tor. Then it maps out the net­work’s sta­tus and cur­rent control plane. Next, it takes a given con­fig­u­ra­tion from an ad­min­is­tra­tor and log­i­cally ren­ders it into Open­Flow en­tries. Fi­nally, it sends those en­tries to the ap­pro­pri­ate net­work de­vices, which add them to their for­ward­ing ta­bles, cre­at­ing a new path through the net­work.

To­day, an ad­min­is­tra­tor might have to re­con­fig­ure dozens of net­work de­vices to make changes to net­work paths. In the­ory, SDN and Open­Flow would let the ad­min­is­tra­tor pro­vide the de­sired pa­ram­e­ters to a few con­trollers, which then re­con­fig­ure the ap­pro­pri­ate de­vices. This would sim­plify the ad­min­is­tra­tor’s job and make the net­work bet­ter able to re­spond to de­mands, such as pri­or­i­tiz­ing one type of ap­pli­ca­tion over an­other.

In addition, in a con­ven­tional net­work, each ven­dor uses a dif­fer­ent in­ter­face to con­fig­ure its own de­vices. Thus, while the stan­dards-based Border Gate­way Pro­to­col works the same on de­vices from dif­fer­ent ven­dors, each ven­dor’s con­fig­u­ra­tion in­ter­face can be very dif­fer­ent, mak­ing the man­age­ment plane of mul­ti­ple ven­dors hard to op­er­ate. Open­Flow has a stan­dard­ized pro­to­col and API be­tween the con­trollers and the switches, so com­mu­ni­ca­tion is

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