TCP/IP

The suite of communication protocols that is used to transfer data over the Internet. These communication protocols are most often referred to as TCP/IP, which is an abbreviation that refers to the two main protocols involved - Transmission Control Protocol (TCP) and Internet Protocol (IP).

TCP/IP uses:

• IP address: An IP address identifies the location of a computer on a network.
• Ports: A port is a location on the recipient computer, where data is received.

While an IP address tells you where to find a particular computer, it doesn’t tell you specifically where on that computer a particular connection should be made - that’s what port numbers are for.

可以将IP address理解成，去邮局邮寄信件，得知道双方的地址
Port可以理解成，一个飞机场，飞机场有很多登机口

Some port numbers you should know:

• Port 80: The port number most commonly used for HTTP requests. For example, when a client makes a request to a web server, this request is usually sent through port 80.
• Port 5432: The port number used by most database systems; default port for Postgres.

A IP address has many ports.

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Connections and Sessions in TCP/IP

• TCP/IP is connection-based, meaning all communications between parties are arranged over a connection. A connection is established before any data transmission begins.
• Over TCP/IP, we will always need to establish a connection between clients and servers in order to enable communications. Deliveries over the connection are error-checked: if packets arrive damaged or lost, then they are reset (known as retransmission).
• Connecting starts a session (会话). Ending the connection ends the session.
• In a database session, many transactions can occur during a given session. Each transaction does work to commit changes to the database (updating, inserting, or deleting records).

UDP Protocol:
The internet also offers the UDP protocol. UDP stands for User Datagram Protocol. UDP is much simpler than TCP: hosts on the network send data (in units called datagrams) without any connections needing to be established.

TCP vs UDP
If TCP is like building highways between houses before sending packages between them, then UDP is much like sending over a carrier pigeon from one house to another in order to deliver packages: you don’t know whether the pigeon will head in the right way, drop your package along the way, or encounter an issue mid-travel. On the other hand, there is less overhead to use UDP than managing a connection over TCP / building a highway.

When spend is more important than reliability, especially when applications need to stream very small amounts of information quickly (smaller packages of information means less issues with reliability), then UDP is preferred.

A lot of real time streaming applications, (e.g. live TV streaming, Voice over IP (VoIP) prefer UDP over TCP. Since UDP does not need to retransmit lost datagrams, nor does it do any connection setup, there are fewer delays over UDP than TCP. TCP’s continuous connection is more reliable but has more latency.