Burst Management - How Bursting Works

What is Bursting

There's a lot of talk about network traffic "bursting" and it can be very confusing. I pulled up some documents about the burstiness of internet traffic and it made my head spin. Complex equations with scatter charts. Crazy stuff. A new way to explain it is badly needed.

When you think about bursts, think about how you use your web browser every day. You download a page, and then you read it. So you're using a lot of bandwidth for a second or two, and then you're using none for 30 seconds, perhaps more. When you check your email, it's the same thing. Depending on what service you use, you either get 1 email at a time to your browser or your phone, or you get all of the new messages at regular intervals. Again, you're using a lot of bandwidth for a few seconds, and then none for as long as it takes you to read your email and to compose a new message.

Bursts vs Downloads

If everyone on the internet just used browsers and email, there would be no need to talk about Traffic Management. But that's not the case. For every casual user who only turns their computer on for 15 minutes a day to read/respond to a few emails, there's another user who has all kinds of streaming music, videos, or filesharing apps It's this second type of user that requires management.


Let's take an easy to understand case. Let's talk NetFlix. When you select a movie from NetFlix (or buy or rent one from Apple TV), the first thing that it does is download it and store it on some local medium. Depending on your connection, it can take a long time to download a movie. I get almost 30Mb/s on my cable connection, and it takes about 20 minutes to download an HD movie. On a DSL connection, it may take 5-10 hours.

So this creates a problem. A user downloading a few web pages at full wire speed for 15-20 seconds likely won't impact your network, but a download at full wire speed for even 5 minutes, let alone 20-30 will almost certainly degrade service.

Something needs to be done. Let's create some goals.

The Goals

- Goal #1: We want our customers or users to have a great experience when browsing.

- Goal #2: We don't want it to take an hour to download mail, or small files.

- Goal #3: We want to lower overall traffic when big downloads are occurring.

Pretty simple. Now what's a solution?

A Solution

With the ET/BWMGR, the solution is the Burst Manager.. Instead of assigning a single bandwidth limit to a user's IP, there is an additional Burst Limit, which allows for more bandwidth for a short period of time - the definition of a Burst. A related setting is Burst Max, which is the maximum number of seconds a user is allowed to download at the Burst Limit before their connection is slowed to the regular bandwidth limit.

Lets look as some drawings.

Burst limit

The first graph shows traffic with no shaping at all, with intermittent usage.

The second shows the traffic shaped to a single bandwidth level that is expected to provide an acceptable experience for browsing and other bursty but low-bandwidth tasks.

The third graph shows the use of a Burst Level. Let's say we have the following settings:

Burst limit example

These settings allow for 5Mb/s when bursting, with a 2Mb/s regular limit. Bursting is allowed for a maximum of 10 seconds. Matching that to the 3rd graph above, we can see that when a download is started, the user can get 5Mb/s for the Burst Max period (10s), after which they can only get 2Mb/s for the duration of the download. Once the download stops and requested usage drops below the 2Mb/s normal limit, bursts will again be allowed.

That's it. A very simple construct that solves a simple problem without the use of scatter charts or complex calculus.

Advanced Usage

Worthy of note is that the burst trigger in this example is set to "Self", and the Burst Threshold is higher than the Burst Limit, which effectively ignores the trigger. As a result, only the Burst Max setting will control whether or not bursting is allowed. See the ET/BWMGR User Guide for other ways to use Burst Limits that are responsive to network conditions, using external triggers to control if bursting is allowed.

Next: Bandwidth Management FAQ