WAN optimisation is not as simple as the vendors - including his own company - would like it to sound, admits Jeff Aaron, Silver Peak Systems' product marketing director.

"WAN optimisation will show up other problems and bottlenecks in your network," he warns. "It is a fairly complicated thing to deploy - you have to deal with asymmetric routes, high availability, firewall policies and so on. You have to understand networking and the effect on your network."

There can also be human factors involved, as well as technological ones, with the need to avoid demarcation disputes or treading on high-profile toes.

"It isn't always easy to reconfigure a network for application performance," he says. "There may be other groups involved for example, such as the security team, throttling flows via a firewall."

And in particular, he says that while WAN optimisers can cut latency times and increase bandwidth utilisation, they may have less success with lossy lines and with networks that cannot guarantee in-order packet reception.

"If the network has problems, such as packet loss on a WAN, application optimisation and data reduction won't help as much," he warns. "TCP fixes packet loss by retransmission, but on big traffic flows, such as replication or backup between data centres, that makes the problem worse. And real-time applications and UDP can't retransmit."

Fixing MPLS

He claims in addition that a poorly-engineered MPLS network can result in a high proportion of out-of-order packets, for instance, and without correction that can translate to network utilisation as low as 10 percent.

"But even a well-engineered MPLS will have 0.5 percent or so loss, because it is over-subscribed and routers are always going to service smaller packets first, so packet ordering is always going to be a problem," he adds. "A lot of people lump out-of-order and lost packets together, but if you have the technology to reorder, it's not lost."

Aaron says that all of this is why Silver Peak recently added or upgraded three key features in its WAN appliance firmware, drawing on experience from other areas of telecoms to improve its performance on high-capacity connections.

The first one he highlights is Forward Error Correction (FEC), which he says has been in the box since Silver Peak first shipped products, two years ago, but has now been uprated from 155Mbit/s to enable it to fix packet loss on flows up to 500Mbit/s, including UDP traffic - he says Silver Peak's high-end NX devices can process 500Mbit/s with all acceleration features enabled.

"FEC has been standard in the service provider world, so we've taken it and adapted it," he adds. "It does add an overhead but it's dynamic, so if there's no packet loss, there's no FEC packets. On a high-loss route, maybe one packet in five is FEC."

The second is Packet Order Correction, or POC, which is basically an extra-large cache that enables the device to reconstruct a flow even though its packets arrive in the wrong order - thanks to a meshed network perhaps.

"POC isn't new either - you can do it in a router or on a host, but not at high volumes, as that needs a dedicated device," Aaron claims. "It's for high-bandwidth applications, and that's our target. While the other vendors talk about PC client software and so on, we have a different user-base."

He adds, "You could fix out-of-order packets in a branch device, but for larger offices you need a dedicated devices. POC and FEC work for anything that runs over IP, not just TCP, so that's FCIP, replication and so on. They could be network-level functions, but haven't been built into routers today."

Of course, with Juniper and others inviting third parties to write applications to run on their routers, that could change.

Now with go-faster stripes...

The third feature Aaron highlights is striping, a technique which splits one large flow into multiple smaller ones and then reconstitutes it at the receiving end. He says this can be used to overcome inherent or deliberately applied network limitations which would otherwise boost smaller flows at the expense of large ones.

"Striping can be used in conjunction with congestion management and QOS. It's useful if your router is doing weighted fair queuing, say," he says. "We are also adding statistics on packet loss and out-of-order packets."

Aaron claims there is something of a divide emerging between those WAN optimisers who target the branch office and are now moving into remote PC connectivity as well, with client software for laptops - such as Riverbed and Packeteer, who already have PC software, and Expand and Juniper who are understood to be developing it.

He argues that while that's a perfectly valid business model, it's a different model from the one he says Silver Peak is building, which targets the high-volume links between primary and secondary data centres, for instance, or between a company's regional headquarters around the globe.

And he's happy to admit that there are some customers where his company's gear is installed alongside another supplier, because the customer found that Silver Peak's NX handled the fat pipes better, but preferred the other equipment for its branch office links.

To underline this, he adds: "The first thing we encourage is to test it on your biggest pipe. If the customer isn't willing to test on a 30Mbit/s link or faster, we won't do the evaluation."