Network Applications: Architectures, Processes, Sockets, and Transport Requirements

Network Applications

A network application consists of programs running on different end systems and communicating over a network. A web application, for example, has a browser program on the user's host and a server program on a web-server host.

Two common architectures are client-server and peer-to-peer (P2P). In client-server applications, an always-running server handles client requests at an address clients can reach. Examples include the Web, FTP, Telnet, and email. One server can become overloaded, so large services operate multiple data centers with many servers that require continuous power and maintenance: search engines such as Google, Bing, and Baidu; retailers such as Amazon, eBay, and Alibaba; email services such as Gmail and Yahoo!; and social platforms such as Facebook, Instagram, Twitter, and WeChat. In P2P applications, peers need not send all data through a dedicated server. Bandwidth-intensive examples include peer-assisted downloads and BitTorrent file sharing.

Communicating Processes

Processes on one end system can use interprocess communication. Processes on different hosts, possibly running different operating systems, exchange messages over the network. A process sends and receives through a socket interface.

Delivery to a remote process requires an IP address and a port number. The IP address identifies the host; the port identifies the receiving application's endpoint on that host. Because multiple applications can run on one host, a host address is not enough. Familiar assignments include port 80 for an HTTP web server and port 25 for SMTP mail transfer.

An application chooses a transport protocol when using a socket. Consider four service requirements:

1. Reliable data transfer: Overflow, corruption, or other problems can cause loss. Email, file transfer, and financial applications require complete data. Some audio and video applications tolerate limited loss as a small disturbance rather than a fatal failure.

2. Throughput: Some applications need a particular delivery rate; others can adapt to the available rate.

3. Timing: Internet calls and multiplayer games are sensitive to delay. Long pauses make conversation unnatural or interaction unresponsive.

4. Security: Encryption at the sender and decryption at the receiver can protect content against interception.

TCP / UDP

TCP (Transmission Control Protocol) provides:

• Connection-oriented service: Before application data, the endpoints exchange transport control information to establish a connection between their sockets. The connection is closed when communication ends.

• Reliable data transfer: Data accepted for delivery is presented in order, without duplicates; failures can still terminate a connection rather than guaranteeing eventual delivery under every circumstance.

• Congestion control.

UDP (User Datagram Protocol) provides:

• Connectionless communication.

• Unreliable data transfer: A datagram may fail to arrive or may arrive out of order.

• No built-in congestion control.