How much overhead does SSL impose?
I know there's no single hard-and-fast answer, but is there a generic order-of-magnitude estimate approximation for the encryption overhead of SSL versus unencrypted socket communication? I'm talking only about the comm processing and wire time, not counting application-level processing.
Update
There is a question about HTTPS versus HTTP, but I'm interested in looking lower in the stack.
(I replaced the phrase "order of magnitude" to avoid confusion; I was using it as informal jargon rather than in the formal CompSci sense. Of course if I had meant it formally, as a true geek I would have been thinking binary rather than decimal! ;-)
Update
Per request in comment, assume we're talking about good-sized messages (range of 1k-10k) over persistent connections. So connection set-up and packet overhead are not significant issues.
Answer
Order of magnitude: zero.
In other words, you won't see your throughput cut in half, or anything like it, when you add TLS. Answers to the "duplicate" question focus heavily on application performance, and how that compares to SSL overhead. This question specifically excludes application processing, and seeks to compare non-SSL to SSL only. While it makes sense to take a global view of performance when optimizing, that is not what this question is asking.
The main overhead of SSL is the handshake. That's where the expensive asymmetric cryptography happens. After negotiation, relatively efficient symmetric ciphers are used. That's why it can be very helpful to enable SSL sessions for your HTTPS service, where many connections are made. For a long-lived connection, this "end-effect" isn't as significant, and sessions aren't as useful.
Here's an interesting anecdote. When Google switched Gmail to use HTTPS, no additional resources were required; no network hardware, no new hosts. It only increased CPU load by about 1%.