ProxySG OS versions 126.96.36.199 and later has an RFC compliant TCP Keep-alive timer of 2 hours. According to RFC1122, section 188.8.131.52:
Keep-alive packets MUST only be sent when no data or acknowledgement packets have been received for the connection within an interval. This interval MUST be configurable and MUST default to no less than two hours.
However, this was not the implementation in SGOS 4.x. In version 4.x, the keep-alive timer was set to 2 minutes (120 seconds). Because of this implementation and the long history of SGOS 4, some Blue Coat customers may have implementations which rely on the shorter keep-alive timer.
Though many applications do not rely on the TCP keep-alive timer and have their own application-based keep-alive messages, some applications such as FTP do use the TCP timer.
For example, an FTP connection will have a control connection over which commands are sent and a separate data connection over which files are sent. A slow connection will have a lot of activity on the data connection, but none on the control connection after it is established. If the control connection times out, the file transfer may fail.