HTTP 1.1 is backward compatible with 1.0, and I'm pretty sure 0.9 is. I think with SSH and SSL they basically have an (if version) statement at the top level,

HTTP 1.1 is backward compatible with 1.0, and I'm pretty sure 0.9 is. I think with SSH and SSL they basically have an (if version) statement at the top level, and then enter a completely different codebase for each version. They can get away with this because there are so few implementations. With HTTP you had more graceful upgrade. They made "mistakes" but were able to accomodate it within the existing structure.