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Boosting TCP WAN performance

Opinion
Aug 19, 20043 mins
Networking

* Internap speeds up TCP ACKs over WAN

As we have observed frequently in this newsletter, there has been much innovation in the past few years to help you improve the performance of your networked applications over the WAN. Among the methods we’ve explored are compression, caching and traffic shaping. These functions can be activated using stand-alone appliances or capabilities embedded in routers.

One issue we haven’t discussed is the nature of TCP – the commonly used Layer-4 transport protocol – and how it tends to behave over the wide area. TCP is connection-oriented and reliable, because it has flow-control mechanisms in it.

However, the propagation delay caused by distance in wide-area networking can mean problems for TCP’s built-in flow-control mechanisms. Some vendors – most recently Internap Network Services – have attempted to mitigate the throughput ramifications associated with these issues by adjusting TCP’s feedback mechanism. Specifically, Internap, for one, now makes an appliance that speeds up the frequency at which TCP sends acknowledgements, or ACKs, to transmitting devices that packets have been received.

When it takes a long time for ACKs to be received, TCP assumes congestion on the link and adjusts the sending rate of the transmitting endpoint accordingly to ease congestion. However, when the ACK receive time is prolonged by propagation delay caused by distance, the transmission rate can be reduced unnecessarily, degrading throughput.

The Internap Flow Control Xcelerator (FCX), announced early this month, is an appliance that sits inline on the LAN side of the router at the WAN edge, Internap CTO Ali Marashi explains. In today’s implementation, you need a device at both ends of a link for application traffic to be TCP-accelerated, he says. The pair of appliances can reportedly accelerate TCP-based application performance by as much as 40 times.

Marashi, for example, says it’s not uncommon to see cross-country FTP traffic from California to Virginia on a 200M bit/sec link degrade to just 4M bit/sec throughput. The FCX can boost it back up to 160M bit/sec, he says.

Other companies have also attempted to twiddle with TCP to accelerate throughput over the WAN; most notably, those that make satellite modem and routing equipment (Encore Networks, V-One), where propagation delay from ground-to-satellite-to-ground wreaks particular havoc with TCP windowing.

Internap appliances are sold by per-site bandwidth speed; a one-time license key per site for a 1.5M-bit/sec connection lists at $10,000 (per end) plus a $1,500 annual service fee (per end).

Internap also sells route-control products and services via its 2003 acquisitions of appliance-maker netVmg and route-control product-maker and service provider Sockeye Networks.