ssl impact on web server

Question

Many of us have web and application servers that use plain TCP.

Some of us have web and other servers that use a secure layer such as SSL.

My understanding of SSL is that the handshaking is very computationally intensive, and the encryption of an ongoing connection is (relatively) cheap.

My assumption for you to correct: an average hosting box (and info on what is average at cloud hosting would be cool too) might easy be expected to be able to saturate its network connections with AES-encrypted packets, but have difficulty doing a thousand RSA handshakes per second. Client authentication with certificates is substantially more expensive for the server than anonymous clients too.

What kind of rules of thumb for the number of session setups per second for SSL are there?

Answer 1

Why not just measure? It will give you real numbers on the exact software and hardware that you are using. You'll also be able to measure the impact of changes in the server infrastructure (adding more boxes, SSL accelerators, tweaking parameters, what have you).

You are correct that you would be hard pressed to get to a thousand SSL handshakes per second on a single box. In fact, I'd say it's probably impossible. A few dozen per second, not a problem. A thousand, not without a lot of $$$.

It's also likely that you don't really need 1000 handshakes per second. That's quite a lot, and you'd already need quite a lot of traffic to need something like that: See this: What do I need in SSL TPS Performance?

Remember that normally you won't be doing new SSL handshakes all the time. Browsers do the handshake once, and keep the connection open over a number of requests and/or page views, so your needs for handshakes per second may be much lower than you think.

Answer 2

As Ville said there is no real option then to try it out on your configuration. But don't underestimated the symmetric encryption of data after establishing a link. It might be less expensive but if you are going to download a lot of data over the encrypted channel than it might cost a lot more than the initial negotiation.

So for this you have to build a common scenario for the usage of your site and then stress test.