Socket
wikipedia Network socket
A network socket is an internal endpoint for sending or receiving data within a node on a computer network. Concretely, it is a representation of this endpoint in networking software (protocol stack), such as an entry in a table (listing communication protocol, destination, status, etc.), and is a form of system resource.
The term socket is analogous to physical female connectors, communication between two nodes through a channel being visualized as a cable with two male connectors plugging into sockets at each node. Similarly, the term port (another term for a female connector) is used for external endpoints at a node, and the term socket is also used for an internal endpoint of local inter-process communication (IPC) (not over a network). However, the analogy is strained, as network communication need not be one-to-one or have a dedicated communication channel.
NOTE: 术语套接字类似于物理母连接器，通过信道将两个节点之间的通信视为电缆，其中两个公连接器插入每个节点的插座。 类似地，术语端口（女性连接器的另一个术语）用于节点处的外部端点，术语套接字也用于本地进程间通信（IPC）的内部端点（不通过网络）。 然而，类比是紧张的，因为网络通信不需要是一对一的或具有专用的通信信道。
Use
A process can refer to a socket using a socket descriptor, a type of handle. A process first requests that the protocol stack create a socket, and the stack returns a descriptor to the process so it can identify the socket. The process then passes the descriptor back to the protocol stack when it wishes to send or receive data using this socket.
Unlike ports, sockets are specific to one node; they are local resources and cannot be referred to directly by other nodes. Further, sockets are not necessarily associated with a persistent connection (channel) for communication between two nodes, nor is there necessarily some single other endpoint. For example, a datagram socket can be used for connectionless communication, and a multicast socket can be used to send to multiple nodes. However, in practice for internet communication, sockets are generally used to connect to a specific endpoint and often with a persistent connection.
NOTE : socket的行为由它的protocol来指定；
Socket addresses
In practice, socket usually refers to a socket in an Internet Protocol (IP) network (where a socket may be called an Internet socket), in particular for the Transmission Control Protocol (TCP), which is a protocol for one-to-one connections. In this context, sockets are assumed to be associated with a specific socket address, namely the IP address and a port number for the local node, and there is a corresponding socket address at the foreign node (other node), which itself has an associated socket, used by the foreign process. Associating a socket with a socket address is called binding.
NOTE : 是否是所有的socket都需要***binding***？不是的，参见《networking-code.md》，其中展示了基本的TCP和UDP socket的编程范式；
Note that while a local process can communicate with a foreign process by sending or receiving data to or from a foreign socket address, it does not have access to the foreign socket itself, nor can it use the foreign socket descriptor, as these are both internal to the foreign node. For example, in a connection between 10.20.30.40:4444 and 50.60.70.80:8888 (local IP address:local port, foreign IP address:foreign port), there will also be an associated socket at each end, corresponding to the internal representation of the connection by the protocol stack on that node. These are referred to locally by numerical socket descriptors, say 317 at one side and 922 at the other. A process on node 10.20.30.40 can request to communicate with node 50.60.70.80 on port 8888 (request that the protocol stack create a socket to communicate with that destination), and once it has created a socket and received a socket descriptor (317), it can communicate via this socket by using the descriptor (317). The protocol stack will then forward data to and from node 50.60.70.80 on port 8888. However, a process on node 10.20.30.40 cannot request to communicate based on the foreign socket descriptor, (e.g. "socket 922" or "socket 922 on node 50.60.70.80") as these are internal to the foreign node and are not usable by the protocol stack on node 10.20.30.40.
Socket pairs
Communicating local and remote sockets are called socket pairs. Each socket pair is described by a unique 4-tuple consisting of source and destination IP addresses and port numbers, i.e. of local and remote socket addresses.[8][9] As seen in the discussion above, in the TCP case, each unique socket pair 4-tuple is assigned a socket number, while in the UDP case, each unique local socket address is assigned a socket number.
wikipedia Berkeley sockets
Berkeley sockets is an application programming interface (API) for Internet sockets and Unix domain sockets, used for inter-process communication (IPC). It is commonly implemented as a library of linkable modules. It originated with the 4.2BSD Unix operating system, released in 1983.
A socket is an abstract representation (handle) for the local endpoint of a network communication path. The Berkeley sockets API represents it as a file descriptor (file handle) in the Unix philosophy that provides a common interface for input and output to streams of data.
Berkeley sockets evolved with little modification from a de facto standard into a component of the POSIX specification. The term POSIX sockets is essentially synonymous with Berkeley sockets, but they are also known as BSD sockets, acknowledging the first implementation in the Berkeley Software Distribution.
Linux name convention
对于network socket IO，Linux使用的是:
1、read、recv
2、write、send