DirectAccess Design
The inherent design of DirectAccess is a major limiting factor for performance. DirectAccess uses a complex and heavy communication channel, with multiple layers of encapsulation, encryption, and translation. Fundamentally it is IPsec encrypted IPv6 traffic, encapsulated in HTTP, and then encrypted with Transport Layer Security (TLS) and routed over IPv4. It is then decrypted, decapsulated, decrypted again, then converted back to IPv4. The high protocol overhead incurred with multiple layers of encapsulation, encryption, and translation result in increased packet fragmentation, which further reduces performance.
DirectAccess Performance
Even under the best circumstances, DirectAccess performance is limited by many other factors, most notably the quality of the network connection between the client and the server. DirectAccess performs reasonably well over high bandwidth, low latency connections. However, network performance drops precipitously as latency increases and packet loss is encountered. This is to be expected given the design of the solution.
Intermediary Devices
It is not uncommon to find intermediary devices like firewalls, intrusion detection systems, malware scanners, and other security inspection devices limit the performance of DirectAccess clients. In addition, many security appliances have bandwidth caps enforced in software for licensing restrictions. Further, incorrect configuration of inline edge devices can contribute to increased fragmentation, which leads to poor performance as well.
Slow Downloads over IP-HTTPS
Many people report that download speeds seem to be artificially capped at 355Kbps. While this seems to be a display bug in the UI, there is plenty of evidence to indicate that, in some scenarios, DirectAccess is incapable of high throughput even over high-quality connections. Some who have deployed DirectAccess and VPN on the same server have reported that download speeds are only limited when using DirectAccess over IP-HTTPS and not with VPN using Secure Socket Tunneling Protocol (SSTP), which also uses TLS. This has led many to speculate that the issue is either a bug or a design flaw in the IP-HTTPS tunnel interface itself.
TCP Window Scaling Issues
In some of the network traces I’ve analyzed I’ve seen evidence that seems to support this theory. For example, a network trace taken when downloading a file over DirectAccess with IP-HTTPS showed the TCP window never scaled beyond 64K, which would seriously impede performance. Interestingly this doesn’t seem to happy when the client uploads files over IP-HTTPS. Clearly something unusual is happening.
Microsoft KB Article
Microsoft recently released a vaguely-worded KB article that appears to lend credence to some of these findings. The article seems to acknowledge the fact there are known issues with DirectAccess performance, but it lacks any specific details as to what the root cause is. Instead, it simply advises migrating to Windows 10 Always On VPN.
Summary
DirectAccess IP-HTTPS performance issues don’t appear to affect everyone, and the problem only seems to apply to file downloads and not to other types of traffic. However, there is mounting evidence of a systemic issue with DirectAccess performance especially over IP-HTTPS. Customers are advised to closely evaluate their uses cases for DirectAccess and if remote clients are frequently required to download large files over a DirectAccess connection, an alternative method of file transfer might be required. Optionally customers can consider evaluating alternative remote access solutions that offer better performance such as Windows 10 Always On VPN or third-party solutions such as NetMotion Mobility.
Additional Resources
Always On VPN and the Future of DirectAccess
What’s the Difference Between DirectAccess and Always On VPN?
NetMotion Mobility as an Alternative to Microsoft DirectAccess