Having just covered some history of the Enterprise WAN to help give you a feel of WAN economics and reliability, I’d now like to cover the factors that affect application performance over the WAN, as prelude to understanding the consequences of WAN service history on the evolution of WAN technologies.
We’re looking here at the various networking-specific factors that affect application performance on the WAN. I.e. we're not attempting to cover important application performance factors such as server capacity, or computational complexity, storage performance, or even data center LAN design that are independent of the wide area network. As a guide, then, we’re focusing exclusively on the issues that cause application performance over a WAN to be so much worse than performance of the same application run entirely on a campus LAN.
RELATED: HP Converged Cloud: the missing (WAN) link
WAN Virtualization technology: 'RAID for WANs'
In future posts, we’ll look more at which networking and computing techniques – WAN Optimization, WAN Virtualization, and distributed replicated/synchronized file service, which are all part of the Next-generation Enterprise WAN (NEW) architecture, as well as other techniques – address each of these factors. Before doing any kind of technology deployment to “make the network work better,” it’s useful to understand what factors are impacting network application performance and performance predictability.
For those of you familiar with what WAN Optimization products like those from Riverbed or Silver Peak or Cisco's WAAS group do, a lot of what’s in these next couple columns will be old hat; hopefully, you’ll still find at least some of the detail beneficial.
My first contention is that WAN-specific application performance is driven entirely by three factors: latency, packet loss and bandwidth. While networking geeks might say “duh!,” some people familiar with WAN issues might say “huh? That’s not right.” These people will argue that there are other factors as well beyond those three. I will show here and in my next column that these other factors – which in fact are very important – matter precisely because of the impact of loss, high latency and/or limited bandwidth. Said another way, a WAN with near zero packet loss, unlimited bandwidth and with constant latency less than a millisecond is basically a LAN – at least in terms of application performance! And while much of the time you will see near zero packet loss on any given WAN, and for many applications bandwidth is rarely if ever an issue, the speed of light alone is guaranteed to make wide area network latency a lot greater than LAN latency. Further, as most of you know, bandwidth and packet loss frequently are issues on WANs, given how the TCP/IP protocol works.
Some definitions (from Wikipedia) to get us started:
“Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured."
"Network latency in a packet-switched network is measured either one-way (the time from the source sending a packet to the destination receiving it), or round-trip (the one-way latency from source to destination plus the one-way latency from the destination back to the source). Round-trip latency [a.k.a. RTT - Round Trip Time] is more often quoted, because it can be measured from a single point.”
"Packet loss occurs when one or more packets of data traveling across a computer network fail to reach their destination.”
“Bandwidth is a measure of available or consumed data communication resources expressed in bit/s or multiples of it (Kbps, Mbps etc.).”
Bandwidth is an obvious limiting factor on transfers of large amounts of data, of course. While it is most definitely not the only reason for poor application performance on the WAN, and in some cases has little or nothing to do with application performance, just as in the LAN, having more bandwidth will make many applications run better and more predictably, and in particular will make the network manager’s life easier. And of course even if for a single application bandwidth isn’t an issue, when multiple users are sharing the same link, it can quickly become an issue. In fact, as we’ll see next time, given the nature of TCP, bandwidth is almost guaranteed to be a performance issue some of the time.
In our next column, we’ll get into the reasons for latency and packet loss problems on WANs, as well as the three other major factors affecting WAN application performance, which themselves are hugely impacted by latency and packet loss.
A twenty-five year data networking veteran, Andy founded Talari Networks, a pioneer in WAN Virtualization technology, and served as its first CEO. Andy is the author of an upcoming book on Next-generation Enterprise WANs.