I'm building a calendar server in .Net. I want the first version of the system to be functional and interoperable for any calendar client. The system is for my college and is my thesis, that's why I don't have enough time to implement all the protocols that these system should implement.
What protocols are REQUIRED in a calendar system server in order to be functional for the clients? Till now it implements RFC 5545 (iCalendar), I'm finishing the RFC 4791 (CalDAV) and some of the extension of WebDAV, after this I'm going to implement the RFC 3744 (ACL).
Should I implement the RFC 6638 (Scheduling Extensions to CalDAV), RFC 3253 (Versioning Extensions to WebDAV) or any other?
In the future I want to implement all these protocols but I have no time now.
Despite the "close votes", I think this is a valid question. There's a lot of standards out there, and a lot of dependencies. And you certainly don't need everything.
The truth is that you need only a subset, and almost no one implements the entire spec.
What's required for you depends on which clients and which features you want to support. So lets say that you want to support iCal, and Thunderbird.
Then at the very least you need large chunks of CalDAV (RFC4791). You don't need every REPORT, but at least calendar-multiget and calendar-query. The freebusy stuff is not used. But for the calendar-query report, there's a small subset of actual queries that clients do.
You need big parts of RFC3744. You can skip most of the REPORTs, but you need a principals system and access-control related WebDAV properties. You also don't need the ACL method. WebDAV ACL is primarily used for principals and reporting access information (but not altering it).
Nobody uses RFC3253 (versioning).
You probably need current-user-principal-URL (rfc5397).
You don't need scheduling (RFC6638). Without scheduling, clients will sync just fine.
Lastly, it's really useful to have support for WebDAV Sync (rfc6578). Clients should be able to live without it, but in reality they tend to misbehave. Without support for that spec, you can fall back on the proprietary ctag, which is widely supported. It's simpler, does the job, but is non-standard.
I would consider this answer a non-exhaustive list though. It's an overview to help you get started. If you have more specific questions about the specs I mentioned, comment here, I'm happy to further expand on this answer.