Skip Links

Tests Show IPv6 Performance Issues Lurk

IPv6 performance problems must be fixed before we run out of IPv4 addresses

By Sevcik and Wetzel on Mon, 07/12/10 - 9:42am.

As the world reluctantly yet inevitably adopts IPv6, performance problems are inescapable--and early results of research by the University of Pennsylvania and Comcast illustrate the magnitude and shifting nature of the performance challenges ahead. UPenn and Comcast are collaborating to measure the rate of IPv6 adoption as well as the performance of IPv6-enabled websites. The upshot is that IPv6-enabled sites are exceedingly rare, and their performance is significantly slower than their IPV4-enabled counterparts.

Here's how the UPenn and Comcast testing works. Test software gets the top 2+M Internet web sites as ranked by Alexa. It does a DNS lookup for each site, and identifies sites that return an IPv4 as well as an IPv6 address for the same DNS name. After identifying sites accessible via IPv4 and IPv6, the test software then downloads the base page from each site using both protocols, and compares the two download times.

As of this writing, the testing has found that a mere 0.15% of the world's top 1M websites are IPv6 enabled, which means that more than 99% of the Internet's content is unreachable via native IPv6. Surprisingly, IPv6 adoption rates are miniscule even among the world's busiest websites. In fact, among the top ten sites as ranked by Alexa, only Google has an IPv6 host.

On the performance front, the following figure from UPenn's IPv6 Adoption Monitor website shows that IPv6 pages nearly always take longer to download than equivalent IPv4-enabled pages (click here to access the latest data feed). Although IPv6 versus IPv4 performance is highly variable, IPv6 is clearly much slower than IPv4. The figure shows bytes per millisecond normalized to the size of the file downloaded. Dots appearing above X-axis represent sites where IPv6 performance is slower than IPv4 performance, and you can see that all but five of the dots are above the X-axis.

Those of you who read our blog IPV4 to IPV6 Migration, the 'Real' Y2K Problem in which we explained why it is important for websites to offer IPv6 connectivity for performance reasons may be shaking your heads and asking: "First you tell me that I have to support IPv6 to improve my IPv6-connected user's experience, and now you tell me that IPv6 performance is a lot slower than IPv4--what's with that?"

Let's put it this way--transitions of this magnitude are never smooth sailing, and things will get worse before they get better.

What accounts for these performance differences? Cisco says that they've done research that shows their router performance is nearly identical running IPv4 and IPv6--so if we take them at their word, their router software is not to blame. Here's what we think is a lineup of likely culprits:

1) Many carriers don't yet provide end-to-end IPv6 connectivity. If traffic flows through a carrier that doesn't support IPv6, then IPv6 traffic is tunneled through IPv4. The tunnel encapsulation and de-encapsulation is probably done in a router slow path or in a separate piece of equipment (via software), which likely degrades performance.

2) Content Distribution Networks (CDNs) may deliver IPv4 content from nearby, while IPv6 content is fetched from the source location. Content providers may not yet be buying IPv6-based CDN distribution service since there is relatively little IPv6 demand today (or they have not figured this out yet!)

3) Networks are not yet tuned to optimize IPv6 performance. Remember that for decades carriers have been tuning their networks for IPv4 connectivity to achieve the performance levels we see today on the Internet. Similar effort must be expended for IPv6 to incrementally improve performance over time.

All would be well and good if we had another 20 years to phase in IPv6. But time is not our friend. Current predictions are that we will run out of IPv4 addresses a year from now as Intec NetCore's "IPv4 Exhaustion Meter" below shows, and if we don't address the performance problems by then, IPv6-connected users will be justifiably outraged at the poor performance they will experience. Network service providers and content providers need to get cracking to identify and fix IPv6 performance problems wherever they may lurk!

Note: For more information about the rate of IPv6 adoption, visit