Introduction to Protocols
UDP, which is defined in RFC 768, is the brother of TCP. Like TCP, UDP uses IP as its transport protocol, but the big difference is that UDP does not guarantee delivery like TCP does. In a sense, UDP is a “fire and forget” protocol; it assumes that the data sent will reach its destination intact. In fact, the checking of whether data is delivered is left to upper-layer protocols.
File Transfer Protocol (FTP)
In addition to being popular as a mechanism for distributing files to the general public over networks such as the Internet, FTP is also popular with organizations that need to frequently exchange large files with other people or organizations. For example, the chapters in this book were sent between the author and Que Publishing using FTP. Such a system is necessary because the files we exchange are sometimes larger than can be easily accommodated using email.
Unlike some of the other protocols discussed in this chapter that perform tasks transparent to the user, FTP is an application layer service that is called upon frequently. Therefore, it can be useful to know some of the commands supported by FTP. If you are using a client such as CuteFTP, you might never need to use these commands, but they are useful to know in case you find yourself using a command-line FTP client. Table 4.3 lists some of the most commonly used FTP commands.
When computers were restricted to standalone systems, there was no need for mechanisms to speak between them. However, it wasn’t long before the requirement to attach computers for the aim of sharing files and printers became a necessity. Establishing communication between network devices needed quite a length of cabling; a way or a group of rules was required to ascertain how systems would communicate. Protocols give that methodology. it might be nice if one protocol facilitated communication between all devices, however this can be not the case. variety of protocols will be used on a network, every of that has its own options, advantages, and downsides. What protocol you select will have a big impact on the network’s functioning and performance. This section explores a number of the a lot of common protocols you'll be able to expect to figure with as a network administrator.
Internet Protocol (IP)
IP, that is outlined in RFC 791, is that the protocol used to move information from one node on a network to a different. IP is connectionless, which suggests that it doesn’t guarantee the delivery of data; it merely makes its best effort to try to to therefore. To ensure that transmissions sent via IP are completed, a higher-level protocol like TCP is needed.
In addition to providing best-effort delivery, IP conjointly performs fragmentation and reassembly tasks for network transmissions. Fragmentation is important as a result of the maximum transmission unit (MTU) size is restricted in IP. In alternative words, network transmissions that are too massive to traverse the network in an exceedingly single packet have to be broken into smaller chunks and reassembled at the opposite finish. Another perform of IP is addressing. IP addressing may be a complicated subject.
Transmission Control Protocol (TCP)
TCP, that is outlined in RFC 793, may be a connection-oriented protocol that uses IP as its transport protocol. Being connection-oriented means TCP establishes a mutually acknowledged session between 2 hosts before communication takes place. TCP provides reliability to IP communications. Specifically, TCP adds options like flow management, sequencing, and error detection and correction. For this reason, higher-level applications that require guaranteed delivery use TCP instead of its light-weight and connectionless brother, UDP.
How TCP Works
When TCP wants to open a connection with another host, it follows this procedure:
1. It sends a message called a SYN to the target host.
2. The target host opens a connection for the request and sends back an acknowledgment message called
2. The target host opens a connection for the request and sends back an acknowledgment message called
an ACK (or SYN ACK).
3. The host that originated the request sends back another acknowledgment, saying that it has received the
3. The host that originated the request sends back another acknowledgment, saying that it has received the
ACK message and that the session is ready to be used to transfer data.
User Datagram Protocol (UDP)
UDP, which is defined in RFC 768, is the brother of TCP. Like TCP, UDP uses IP as its transport protocol, but the big difference is that UDP does not guarantee delivery like TCP does. In a sense, UDP is a “fire and forget” protocol; it assumes that the data sent will reach its destination intact. In fact, the checking of whether data is delivered is left to upper-layer protocols.
File Transfer Protocol (FTP)
As its name suggests, FTP provides for the uploading and downloading of files from a remote host running FTP server software. As well as uploading and downloading files, FTP allows you to view the contents of folders on an FTP server and rename and delete files and directories if you have the necessary permissions. FTP, which is defined in RFC 959, uses TCP as a transport protocol to guarantee delivery of packets.
In addition to being popular as a mechanism for distributing files to the general public over networks such as the Internet, FTP is also popular with organizations that need to frequently exchange large files with other people or organizations. For example, the chapters in this book were sent between the author and Que Publishing using FTP. Such a system is necessary because the files we exchange are sometimes larger than can be easily accommodated using email.
Unlike some of the other protocols discussed in this chapter that perform tasks transparent to the user, FTP is an application layer service that is called upon frequently. Therefore, it can be useful to know some of the commands supported by FTP. If you are using a client such as CuteFTP, you might never need to use these commands, but they are useful to know in case you find yourself using a command-line FTP client. Table 4.3 lists some of the most commonly used FTP commands.
Command
|
Description
|
Ls
|
Lists the files in the current
directory on the remote system.
|
cd
|
Changes the working directory on the
remote host.
|
lcd
|
Changes the working directory on the
local host.
|
put
|
Uploads a single file to the remote
host.
|
get
|
Downloads
a single file from the remote host.
|
mput
|
Uploads
multiple files to the remote host.
|
mget
|
Switches transfers into binary mode.
|
binary
|
Switches transfers into binary mode.
|
ascii
|
Switches transfers into ASCII mode (the
default).
|
Secure File Transfer Protocol (SFTP)
One of the big problems associated with FTP is that it is considered insecure. Even though simple authentication methods are associated with FTP, it is still susceptible to relatively simple hacking approaches. In addition, FTP transmits data between sender and receiver in an unencrypted format. By using a packet sniffer, a hacker could easily copy packets from the network and read the contents. In today’s high-security computing environments, a more robust solution is needed. That solution is the Secure File Transfer Protocol, which, based on Secure Shell (SSH) technology, provides robust authentication between sender and receiver. It also provides encryption capabilities, which means that even if packets are copied from the network, their contents remain hidden from prying eyes. SFTP is implemented through client and server software available for all commonly used computing platforms.
Trivial File Transfer Protocol (TFTP)
A variation on FTP is TFTP, which is also a file transfer mechanism. However, TFTP does not have the security capability or the level of functionality that FTP has. TFTP, which is defined in RFC 1350, is most often associated with simple downloads, such as those associated with transferring firmware to a device such as a router and booting diskless workstations.
Simple Mail Transfer Protocol (SMTP)
SMTP, in RFC 821, is a which is defined in RFC 821, is a protocol that defines how mail messages are sent between hosts. SMTP uses TCP connections to guarantee error-free delivery of messages. SMTP is not overly sophisticated, and it requires that the destination host always be available. For this reason, mail systems spool incoming mail so that users can read it later. How the user then reads the mail depends on how the client accesses the SMTP server.
Hypertext Transfer Protocol (HTTP)
HTTP, which is defined in RFC 2068, is the protocol that allows text, graphics, multimedia, and other material to be downloaded from an HTTP server. HTTP defines what actions can be requested by clients and how servers should answer those requests. In a practical implementation, HTTP clients (that is, web browsers) make requests in an HTTP format to servers running HTTP server applications (that is, web servers). Files created in a special language such as Hypertext Markup Language (HTML) are returned to the client, and the connection is closed.
Hypertext Transfer Protocol Secure (HTTPS)
One of the downsides of using HTTP is that HTTP requests are sent in clear text. For some applications, such as e-commerce, this method of exchanging information is unsuitable—a more secure method is needed. The solution is HTTPS. HTTPS uses a system known as Secure Socket Layer (SSL), which encrypts the information sent between the client and host.
Post Office Protocol Version 3/Internet Message Access Protocol Version 4
(POP3/IMAP4)
Both POP3, which is defined in RFC 1939, and IMAP4, the latest version of which is defined in RFC 1731, are mechanisms for downloading, or pulling, email from a server. They are necessary because, although the mail is transported around the network via SMTP, users cannot always read it immediately, so it
must be stored in a central location. From this location, it needs to be downloaded, which is what POP and IMAP allow you to do. POP and IMAP are popular, and many people now access email through applications such as Microsoft Outlook, Netscape Communicator, and Eudora, which are POP and IMAP clients.
Telnet
Secure Shell (SSH)
Created by students at the Helsinki University of Technology, Secure Shell (SSH) is a secure alternative to Telnet. SSH provides security by encrypting data as it travels between systems. This makes it difficult for hackers using packet sniffers and other traffic-detection systems. It also provides more robust authentication systems than Telnet. Two versions of SSH are available—SSH1 and SSH2. Of the two, SSH2 is considered more secure. Also note that the two versions are incompatible. So, if you are using an SSH client program, the server implementation of SSH that you are connecting to must be the same version.
A variation on FTP is TFTP, which is also a file transfer mechanism. However, TFTP does not have the security capability or the level of functionality that FTP has. TFTP, which is defined in RFC 1350, is most often associated with simple downloads, such as those associated with transferring firmware to a device such as a router and booting diskless workstations.
Simple Mail Transfer Protocol (SMTP)
SMTP, in RFC 821, is a which is defined in RFC 821, is a protocol that defines how mail messages are sent between hosts. SMTP uses TCP connections to guarantee error-free delivery of messages. SMTP is not overly sophisticated, and it requires that the destination host always be available. For this reason, mail systems spool incoming mail so that users can read it later. How the user then reads the mail depends on how the client accesses the SMTP server.
Hypertext Transfer Protocol (HTTP)
HTTP, which is defined in RFC 2068, is the protocol that allows text, graphics, multimedia, and other material to be downloaded from an HTTP server. HTTP defines what actions can be requested by clients and how servers should answer those requests. In a practical implementation, HTTP clients (that is, web browsers) make requests in an HTTP format to servers running HTTP server applications (that is, web servers). Files created in a special language such as Hypertext Markup Language (HTML) are returned to the client, and the connection is closed.
Hypertext Transfer Protocol Secure (HTTPS)
One of the downsides of using HTTP is that HTTP requests are sent in clear text. For some applications, such as e-commerce, this method of exchanging information is unsuitable—a more secure method is needed. The solution is HTTPS. HTTPS uses a system known as Secure Socket Layer (SSL), which encrypts the information sent between the client and host.
Post Office Protocol Version 3/Internet Message Access Protocol Version 4
(POP3/IMAP4)
Both POP3, which is defined in RFC 1939, and IMAP4, the latest version of which is defined in RFC 1731, are mechanisms for downloading, or pulling, email from a server. They are necessary because, although the mail is transported around the network via SMTP, users cannot always read it immediately, so it
must be stored in a central location. From this location, it needs to be downloaded, which is what POP and IMAP allow you to do. POP and IMAP are popular, and many people now access email through applications such as Microsoft Outlook, Netscape Communicator, and Eudora, which are POP and IMAP clients.
Telnet
Telnet, which is defined in RFC 854, is a virtual terminal protocol. It allows sessions to be opened on a remote host, and then commands can be executed on that remote host. For many years, Telnet was the method by which clients accessed multiuser systems such as mainframes and minicomputers. It also was the connection method of choice for UNIX systems. Today, Telnet is still commonly used to access routers and other managed network devices.
Secure Shell (SSH)
Created by students at the Helsinki University of Technology, Secure Shell (SSH) is a secure alternative to Telnet. SSH provides security by encrypting data as it travels between systems. This makes it difficult for hackers using packet sniffers and other traffic-detection systems. It also provides more robust authentication systems than Telnet. Two versions of SSH are available—SSH1 and SSH2. Of the two, SSH2 is considered more secure. Also note that the two versions are incompatible. So, if you are using an SSH client program, the server implementation of SSH that you are connecting to must be the same version.
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