Yes, our goals are much broader than VPN. You can think of ZeroTier as a virtual smart switch built on a P2P network; a "smart switch for Earth." We want to totally abstract away the physical network. A lot of ZT's complexity over WireGuard is about working around the underlying network (NAT-t, dual-stack support, dealing with timeouts and path failures, relay fallback, peer location and roaming, redundancy, etc.) as well as smart switch features and robustness against DOS attacks and similar things.
Nevertheless ZT's core code really isn't that big. The core protocol implementation is 23,000 lines of C++ code. This includes many very long comments documenting the protocol, etc., so total lines of actual code is probably more like 18-20k. It's comparable to an embedded TCP stack.
WireGuard does have some things in common with ZeroTier, such as the use of cryptography to identify endpoints and eliminate the hard-coding of endpoint addresses. This is one of many ways that IMHO cryptography actually simplifies networking. I really like the WireGuard design in general and I think it has a somewhat different use case from ZeroTier, namely fast long-lived provisioned links across WANs and insecure LANs. You could use ZT for that but this being in-kernel makes it likely faster. In any case I hate IPSec (over-engineered mess, terribly hard to configure) so any alternative to that is welcome.
(Edit: there are things you can do to speed up user-mode networking like zero-copy data paths, etc., so it's not guaranteed that this would always be faster. But that's another topic.)
Yes, our goals are much broader than VPN. You can think of ZeroTier as a virtual smart switch built on a P2P network; a "smart switch for Earth." We want to totally abstract away the physical network. A lot of ZT's complexity over WireGuard is about working around the underlying network (NAT-t, dual-stack support, dealing with timeouts and path failures, relay fallback, peer location and roaming, redundancy, etc.) as well as smart switch features and robustness against DOS attacks and similar things.
Nevertheless ZT's core code really isn't that big. The core protocol implementation is 23,000 lines of C++ code. This includes many very long comments documenting the protocol, etc., so total lines of actual code is probably more like 18-20k. It's comparable to an embedded TCP stack.
WireGuard does have some things in common with ZeroTier, such as the use of cryptography to identify endpoints and eliminate the hard-coding of endpoint addresses. This is one of many ways that IMHO cryptography actually simplifies networking. I really like the WireGuard design in general and I think it has a somewhat different use case from ZeroTier, namely fast long-lived provisioned links across WANs and insecure LANs. You could use ZT for that but this being in-kernel makes it likely faster. In any case I hate IPSec (over-engineered mess, terribly hard to configure) so any alternative to that is welcome.
(Edit: there are things you can do to speed up user-mode networking like zero-copy data paths, etc., so it's not guaranteed that this would always be faster. But that's another topic.)