3.3.3.1 Application layer protocols
FTP or file transfer protocol is used for the transmission of files between two devices. It uses TCP
to create a virtual connection for the control of information, then creates another connection
to be used for the delivery of data. The most commonly used ports are 20 and 21.
HTTP or hypertext transfer protocol is used to translate information into web pages. This
information is distributed in a manner similar to that used for electronic mail. The most
commonly used port is 80.
3.3.3.1 应用层协议
文件传输协议存在于两个设备文件传输过程中。它通过传输控制协议为控制信息设置虚拟连接，为数据传输设置连接。
最通用的接口是20和21。
超文本传输协议将信息转换成网页信息。这种信息传输方式和电子邮件类似。最通用的接口是80。
SMTP or simple mail transfer protocol is a mail service that is based on the FTP model. It
transfers electronic mail between two systems and provides notifications of incoming mail. The
most commonly used port is 25.
简单邮件传输协议是基于文本传输协议模型的邮政服务。它在两个系统之间传输电子邮件，提供往来邮件的通知。
最通用的接口是25。
DNS or domain name service provides a means to associate a domain name with an ip
address. The most commonly used port is 53.
域名服务器是一种将域名和IP地址相连的方式。最通用的接口是53。
TFTP or trivial file transfer protocol has the same functions as FTP but uses UDP instead of TCP.
(See Section 3.3.3.2 for details on the differences between UDP and TCP.) This gives it more
speed, but less security and trustworthiness. The most commonly used port is 69.
普通文件传输协议和文件传输协议功能相同，但用用户数据报协议代替了传输控制协议。这样速度变得更快，但是安全度
降低。最通用的接口是69。
3.3.3.2 Transport layer Protocols
There are two protocols which can be used by the transport layer to deliver information
segments.
TCP or transmission control protocol establishes a logical connection between the final points
of the network. It synchronizes and regulates the traffic with what is known as the "Three Way
Handshake". In the “Three Way Handshake,” the originating device sends an initial packet
called a SYN to the recipient device. The recipient device sends an acknowledgment
packet, called a SYN/ACK. The originating device then sends a packet called an ACK, which
is an acknowledgment of the acknowledgment（这句话真的很罗嗦）. At this point, both the originating device
and the recipient device have established that there is a connection between the two and
both are ready to send and receive data to and from each other.
UDP or user datagram protocol is a transport protocol which is not based on a connection. In
this case, the originating device sends packets without warning the recipient device to
expect these packets. It is then up to the recipient device to determine whether or not those
packets will be accepted. As a result, UDP is faster that TCP, but it cannot guarantee that a
packet will be accepted.
3.3.3.2 传输层协议
传输层传输数据片段遵守两个协议。
传输控制协议在网络的两个终端间建立逻辑连接。它同步并规范三向交握的通信。在“三向交握”中，发送器发送
一个名叫合成器的初始数据包给接收器。接收器发送一个确认数据报，称作SYN/ACK。发送器发送确认字符。这样，
发送器和接收器连接在一起，都准备发送和接受数据。
用户数据报协议在没有通知接收器之前发送数据包。然后由接收器决定是否接受这些数据。所以UDP的速度要比TCP
快，但是它不能保证数据被接收。
3.3.3.3 Internet layer Protocols
IP or internet protocol serves as a universal protocol to allow any two computers to
communicate through any network at any time. Like UDP, it is connectionless, because it does
not establish a connection with the remote computer. Instead, it is what is known as a best
effort service, in that it will do whatever is possible to ensure that it works correctly, but its
reliability is not guaranteed. The Internet Protocol determines the format for the packet
headers, including the IP addresses of both the originating and the recipient devices.
3.3.3.3 因特网层协议
因特网协议是任意两台电脑在任何时候进行连接的通用协议。和UDP相似，无连接，因为它不会和远程电脑建立连接。
被称为“尽量服务”，即会尽最大的努力确定工作正确，但也不是很可靠。因特网协议决定包标志的格式，包括发送器
和接收器的IP地址。