There's actually a lot of combinations of (domain, type, protocol) that are available. It is not always the case that the protocol implies the type. In IP land

There's actually a lot of combinations of (domain, type, protocol) that are available. It is not always the case that the protocol implies the type. In IP land (domains AF_INET and AF_INET6), we have the well known UDP and TCP protocols, of course. UDP is always datagram (SOCK_DGRAM) and TCP is always stream (SOCK_STREAM). Besides datagram-only ICMP, there's also SCTP, which lets you choose between stream and sequential-packet (SOCK_SEQPACKET) types. A sequential-packet socket provides in-order delivery of packet-sized messages, so it sits somewhere between datagram and stream in terms of features. In AF_UNIX land, there are no protocols (the protocol field is always 0), but all 3 of the aforementioned types are available. You just have to pick the same type on both sides. Footnotes: SCTP is not widely usable because Windows doesn't natively support it and many routers will drop or reject it instead of forwarding it. Also, AF_UNIX is now supported on Windows, but only with SOCK_STREAM type.