Understanding socket basics

Question

I've been reading up on basic network programming, but am having a difficult time finding a straight-forward explanation for what exactly and socket is, and how it relates to either the OSI or TCP/IP stack.

1. Can someone explain to me what a socket is? Is it a programmer- or API-defined data structure, or is it a hardware device on a network card?

2. What layers of the mentioned network models deal with "raw" sockets? Transport layer? Network layer?

3. In terms of the data they pass between them, are socket text-based or binary?

4. Is there an alternative to sockets-based network programming? Or do all networked applications use some form of socket?

If I can get this much I should have a pretty clear understanding of everything else I'm reading. Thanks for any help!

Answer 1

Short answers:

1. Socket is an abstraction of an IP connection endpoint - so if you think of it as an API structure, you are not very far off. Please do read http://en.wikipedia.org/wiki/Internet_socket
2. Internet layer i.e. IP Protocol. In practice you usually use explicitly sockets that bind to a certain transport layer parameters (datagram/UDP or stream/TCP)
3. Sockets send data, in network byte order - whether it is text or binary, depends on the upper layer protocol.
4. Theoretically, probably yes - but in practice all IP traffic is done using 'sockets'