week13.md

Socket

A socket is one endpoint of a two-way communication link between two programs running on the network. A socket is bound to a port number so that the TCP layer can identify the application that data is destined to be sent to.

Normally, a server runs on a specific computer and has a socket that is bound to a specific port number. The server just waits, listening to the socket for a client to make a connection request.

On the client-side: The client knows the hostname of the machine on which the server is running and the port number on which the server is listening. To make a connection request, the client tries to rendezvous with the server on the server's machine and port. The client also needs to identify itself to the server so it binds to a local port number that it will use during this connection. This is usually assigned by the system.

A client's connection request If everything goes well, the server accepts the connection. Upon acceptance, the server gets a new socket bound to the same local port and also has its remote endpoint set to the address and port of the client. It needs a new socket so that it can continue to listen to the original socket for connection requests while tending to the needs of the connected client.

The connection is made On the client side, if the connection is accepted, a socket is successfully created and the client can use the socket to communicate with the server.

The client and server can now communicate by writing to or reading from their sockets.

An endpoint is a combination of an IP address and a port number. Every TCP connection can be uniquely identified by its two endpoints. That way you can have multiple connections between your host and the server.

The java.net package in the Java platform provides a class, Socket, that implements one side of a two-way connection between your Java program and another program on the network. The Socket class sits on top of a platform-dependent implementation, hiding the details of any particular system from your Java program. By using the java.net.Socket class instead of relying on native code, your Java programs can communicate over the network in a platform-independent fashion.

Additionally, java.net includes the ServerSocket class, which implements a socket that servers can use to listen for and accept connections to clients

Socket mysocket = getSocket(type = "TCP")
connect(mysocket, address = "1.2.3.4", port = "80")
send(mysocket, "Hello, world!")
close(mysocket)

Several types of Internet sockets are available:

• Datagram sockets Connectionless sockets, which use User Datagram Protocol (UDP). Each packet sent or received on a datagram socket is individually addressed and routed. Order and reliability are not guaranteed with datagram sockets, so multiple packets sent from one machine or process to another may arrive in any order or might not arrive at all. Special configuration may be required to send broadcasts on a datagram socket. In order to receive broadcast packets, a datagram socket should not be bound to a specific address, though in some implementations, broadcast packets may also be received when a datagram socket is bound to a specific address.
• Stream sockets Connection-oriented sockets, which use Transmission Control Protocol (TCP), Stream Control Transmission Protocol (SCTP) or Datagram Congestion Control Protocol (DCCP). A stream socket provides a sequenced and unique flow of error-free data without record boundaries, with well -defined mechanisms for creating and destroying connections and reporting errors. A stream socket transmits data reliably, in order, and with out-of-band capabilities. On the Internet, stream sockets are typically implemented using TCP so that applications can run across any networks using TCP/IP protocol.
• Raw sockets Allow direct sending and receiving of IP packets without any protocol-specific transport layer formatting. With other types of sockets, the payload is automatically encapsulated according to the chosen transport layer protocol (e.g. TCP, UDP), and the socket user is unaware of the The existence of protocol headers that are broadcast with the payload. When reading from a raw socket, the headers are usually included. When transmitting packets from a raw socket, the automatic addition of a header is optional. Most socket application programming interfaces (APIs), for example, those based on Berkeley sockets, support raw sockets. Windows XP was released in 2001 with raw socket support implemented in the Winsock interface, but three years later, Microsoft limited Winsock's raw socket support because of security concerns. Raw sockets are used in security-related applications like Nmap. One use case for raw sockets is the implementation of new transport-layer protocols in user space. Raw sockets are typically available in network equipment, and used for routing protocols such as the Internet Group Management Protocol (IGMP) and Open Shortest Path First (OSPF), and in the Internet Control Message Protocol (ICMP) used, among other things, by the ping utility. Other socket types are implemented over other transport protocols, such as Systems Network Architecture and Unix domain sockets for internal inter-process communication.

References : https://docs.oracle.com/javase/tutorial/networking/sockets/definition.html#:~:text=A%20socket%20is%20one%20endpoint,address%20and%20a%20port%20number. https://en.wikipedia.org/wiki/Network_socket