Our customer has over 1,000 users using thin client terminals at multiple offices around the UK. They use Office 365 with Outlook mailboxes hosted on office.com.
Users started to experience a severe problem where Outlook would appear to hang when starting. The problem was reported to happen between 06:00 and 11:00 each day, with a peak around 09:00 on Mondays and Tuesdays.
The key components are as follows:
- Client PCs and Xen servers using Microsoft Outlook
- Bluecoat Proxy servers
- Fortinet Internet firewalls
- Internet connection
- Hosted e-mail service (office.com)
The Xen servers, proxy servers and firewalls are based in two corporate data centres.
We analysed several examples of the problem and found the following behaviour:
- When the system worked normally, the client would connect to office.com, exchange data for a few seconds and then close the connection
- When the problem occurred, the session settled into a pattern where data packets were only received from office.com once per minute:
- This behaviour continued for over 15 minutes (until the client closed the application)
- There was no significant difference between the delays seen on the inside and outside of the proxy servers – the proxy servers were not responsible for any of the delay
The Outlook hang was caused by Outlook connections being throttled:
We packaged up all the diagnostic evidence and our customer presented it to Microsoft.