Our company is looking at hosting our software with an ASP model of distribution and we're looking at firewall hardware to protect our data center. My question is what is the difference between the number of concurrent sessions (which can be in the millions) and concurrent IPsec/VPN sessions (which is in the low thousands to ten thousand). We may need to serve up to 45,000 connections via a VPN. So we're looking for hardware that will support that.
"Concurrent sessions" is more often called "concurrent connections", and refers to the number state-tracked (and possibly NAT translated) connections that the device can keep in memory at a given time; typically meaning TCP since it has state, but tracking is also needed for NAT-translated UDP and ICMP communication.
Sessions are different than connections. One connection can handle multiple sessions. For example, if we open a webpage google.com our PC uses an ephemeral port. A random port like 24563 as a source and destination port 443 to connect to the Google and this is one session again if we click on duplicate it connects the same destination port 443 with other ephemeral port and this is one session but the connection is same, so in one connection it is handling two sessions.