Hi Rob,
By default, when the ACE sends a probe, it expects a response within a time period of 10 seconds. For example, for an HTTP probe, the timeout period is the number of seconds to receive an HTTP reply for a GET or HEAD request. If the server fails to respond to the probe, the ACE marks the server as failed.
Here is where the "Server reply timeout (no reply)" comes into play, and it is due to the server not replaying back to the ACE once the content request was made (sequence: SYN,SYN/ACK,ACK,GETorHEAD,..........no reply), this is the difference between Server open timeout (no SYN ACK).
Server open timeout (no SYN ACK), here the ACE is just opening the connection and doing the TCP synchronization.
In the logic you explain, yes, we could say that if there is no SYN/ACK, there is no reply from the server. But, in ACE language, if there is no SYN/ACK, well we have an error to know that the problem is during the 3-way-handshake, reason could be that the port is not open in the server or in the firewall if we have one in between, etc..., and if there is a reply timeout, this is to know, that the issue might be related with the server having trouble replying back to the ACE, and possible reasons could be that the server is overloaded and unable to reply within the 10 seconds, there is a lot of congestion in the network, etc.....
So as you can see, this help to differentiate the possible troubleshooting that we might need to apply depending on the error message.
Hope this help.
Rod.
