TCP (Transmission
Control Protocol)
Transmission
Control Protocol uses a reliable delivery system to deliver layer 4 segments to
the destination. This would be analogous to using a certified, priority, or
next-day service with the Indian Speed Post;Service.
For
example, with a certified letter, the receiver must sign for it, indicating the
destination actually received the letter: proof of the delivery is provided. TCP operates under a
similar premise: it can detect whether or not the destination received a sent
segment. With the postal example, if the certified letter got lost, it would be
up to you to resend it; with TCP, you don’t have to worry about what was or
wasn’t received—TCP will take care of all the tracking and any necessary
resending of lost data for you.
TCP’s
main responsibility is to provide a reliable full-duplex, connection-oriented,
logical service between two devices.
TCP goes through a
three-way handshake to establish a session before data can be sent. Both the
source and destination can simultaneously send data across the session. It uses
windowing to implement flow control so that a source device doesn't overwhelm a
destination with too many segments. It supports data recovery, where any missed
or corrupted information can be re-sent by the source. Any packets that arrive
out of order, because the segments traveled different paths to reach the
destination, can easily be reordered, since segments use sequence numbers to
keep track of the ordering.
UDP (User Datagram
Protocol)
UDP uses a best-effort
delivery system, similar to how first class and lower postal services of the
Indian Postal Service work. With a first class letter (post card), you place
the destination address and put it in your mailbox, and hope that it arrives at
the destination.
With
this type of service, nothing guarantees that the letter will actually arrive
at the destination, but in most instances, it does. If, however, the letter
doesn’t arrive at the destination, it’s up to you, the letter writer, to resend
the letter: the post office isn’t going to perform this task for you.
UDP
operates under the same premise: it does not guarantee the delivery of the
transport layer segments. While TCP provides a reliable connection, UDP
provides an unreliable connection.
UDP doesn’t go through a
three-way handshake to set up a connection—it simply begins sending the data.
Likewise, UDP doesn’t check to see whether sent segments were received by a
destination; in other words, it doesn’t use an acknowledgment
Some commonly used
ports
|
Port Number
|
Service
|
|
80
|
HTTP
|
|
21
|
FTP
|
|
110
|
POP3
|
|
25
|
SMTP
|
|
23
|
Telnet
|
FTP (File Transfer
Protocol)
One
of the earliest uses of the Internet, long before Web browsing came along, was
transferring files between computers. The File
Transfer Protocol (FTP) is used to connect to remote computers,
list shared files, and either upload or download files between local and remote
computers.
FTP runs over TCP, which
provides a connection-oriented, guaranteed data-delivery service. FTP is a character-based
command interface, although many FTP applications have graphical interfaces. FTP is still used for file
transfer purposes, most commonly as a central FTP server with files available
for download. Web browsers can make FTP requests to download programs from
links selected on a Web page.
You
should become familiar with the basic commands available in an FTP session. To
begin a characterbased command session on a Windows computer, follow these
steps.
·
Open
a Command prompt window, type ftp at the prompt, and
press Enter.
·
This
will begin an FTP session on the local machine but will not initialize a
connection to another machine.
·
Without
a connection to another machine, you will not be able to do anything. To
connect, type open example.comor open
10.10.10.1, in which exmple.com or 10.10.10.1 is the name or IP address
of a host that is available as an FTP server. Most FTP servers require a logon
id and password, or they will accept anonymous connections. At this point you
will be prompted for a logon ID and password.
·
Once
you are connected, you can list the files on the remote server by typing dir.
·
If
you have create privileges on the remote server, you can create a new directory
by typing mkdir.
·
To
download a file, type get
filename.txt where filename.txt is the name of the
file you are downloading.
To upload a file, typeput filename.txt.
To upload a file, typeput filename.txt.
SFTP (Secure File
Transfer Protocol)
SSH
File Transfer Protocol or SFTP is a network protocol that provides file
transfer and manipulation functionality over any reliable data stream.
TFTP (Trivial File
Transfer Protocol)
TFTP
is used when a file transfer does not require an acknowledgment packet during
file transfer. TFTP is used often in router configuration. TFTP is similar in
operation to FTP. TFTP is also a command-line-based utility.
One
of the two primary differences between TFTP and FTP is speed and authentication. Because TFTP is used
without acknowledgment packets, it is usually faster than FTP. TFTP does not
provide user authentication like FTP and therefore the user must be logged on
to the client and the files on the remote computer must be writable. TFTP
supports only unidirectional data transfer (unlike FTP, which supports
bi-directional transfer). TFTP is operated over port 69.
SMTP (Simple Mail
Transfer Protocol)
SMTP
is a standard electronic-mail protocol that handles the sending of mail from
one SMTP to another SMTP server. To accomplish the transport, the SMTP server
has its own MX (mail exchanger) record in the DNS database that corresponds to
the domain for which it is configured to receive mail.
When
equipped for two-way communication, mail clients are configured with the
address of a POP3 server to receive mail and the address of an SMTP server to
send mail. The clients can configure server parameters in the properties sheets
of the mail client, basing the choices on an FQDN or an IP address.
SMTP
uses TCP for communication and operates on port 25. Simple Mail Transfer
Protocol (SMTP) is the application-layer protocol used for transmitting e-mail
messages. SMTP is capable of receiving e-mail messages, but it's limited in its
capabilities. The most common implementations of SMTP are in conjunction with
either POP3 or IMAP4. For example, users download an e-mail message from a POP3
server, and then transmit messages via an SMTP server
HTTP (Hypertext Transfer
Protocol)
HTTP
is often called the protocol of the Internet. HTTP received this designation
because most Internet traffic is based on HTTP. When a user requests a Web
resource, it is requested using HTTP. The following is a Web request:
http://www.example.com
When
a client enters this address into a Web browser, DNS is called to resolve the
Fully Qualified Domain Name (FQDN) to an IP address. When the address is
resolved, an HTTP get request is sent to the Web server. The Web server responds
with an HTTP send response. Such communication is done several times throughout
a single session to a Web site. HTTP uses TCP for communication between clients
and servers. HTTP operates on port 80.
HTTPS (Hypertext
Transfer Protocol Secure)
HTTP
is for Web sites using additional security features such as certificates. HTTPS
is used when Web transactions are required to be secure. HTTPS uses a
certificatebased technology such as VeriSign.
Certificate-based
transactions offer a mutual authentication between the client and the server.
Mutual authentication ensures the server of the client identity, and ensures
the client of the server identity. HTTPS, in addition to using
certificate-based authentication, encrypts all data packets sent during a session.
Because
of the encryption, confidential user information cannot be compromised. To use
HTTPS, a Web site must purchase a certificate from a third-party vendor such as
VeriSign, CertCo, United States Postal Service, or other certificate providers.
When the certificate is issued to a Web site from a third-party vendor, the Web
site is using trusted communication with the client. The communication is
trusted because the third party is not biased toward either the Web site or the
client. To view a certificate during a HTTPS session, simply double-click the
lock icon in the lower-right area of the Web browser. HTTPS operates on port
443 and uses TCP for communication.
POP3 / IMAP4 (Post
Office Protocol version 3 / Internet Message Access Protocol version 4)
Post
Office Protocol 3 (POP3) and Internet Message Access Protocol 4 (IMAP4) are two
application-layer protocols used for electronic messaging across the Internet.
POP3 is a protocol that involves both a server and a client. A POP3 server
receives an e-mail message and holds it for the user. A POP3 client application
periodically checks the mailbox on the server to download mail. POP3 does not
allow a client to send mail, only to receive it. POP3 transfers e-mail messages
over TCP port 110.
IMAP4
is an alternate e-mail protocol. IMAP4 works in the same way as POP3, in that
an e-mail message is held on a server and then downloaded to an e-mail client
application. Users can read their e-mail message locally in their e-mail client
application, but they can't send an e-mail message using IMAP4. When users
access e-mail messages via IMAP4, they have the option to view just the message
header, including its title and the sender's name, before downloading the body
of the message. Users can create, change, or delete folders on the server, as
well as search for messages and delete them from the server.
To
perform these functions, users must have continued access to the IMAP server
while they are working with e-mail messages. With IMAP4, an e-mail message is
copied from the server to the e-mail client. When a user deletes a message in
the e-mail client, the message remains on the server until it is deleted on the
server. POP3 works differently in that an e-mail message is downloaded and not
maintained on the server, unless configured otherwise. Therefore, the
difference between POP3 and IMAP4 is that IMAP4 acts like a remote file server,
while POP3 acts in a store-and-forward manner in its default configuration.
(You can configure POP3 clients to leave copies of messages on the server, if
you prefer.)
Both
Microsoft and Netscape Web browsers have incorporated POP3. In addition, the
Eudora and Microsoft Outlook Express e-mail client applications support both
POP3 and IMAP4.
Telnet
Short
for Telecommunication Network, a virtual terminal protocol allowing a user
logged on to one TCP/IP host to access other hosts on the network. Many people
use remote control applications to access computers at their workplace from
outside the network. In remote control, a session appears in which the user is
able to manage the files on the remote computer, although the session appears
to be functioning locally. Telnet is an early version of a remote control
application.
Telnet
is very basic; it offers solely character-based access to another computer. If
you want to see a person's graphical desktop, you would need a different type
of protocol, such as Remote Desktop Protocol (RDP), Independent Computing
Architecture (ICA), or X Windows. Telnet acts as a user command with an
underlying Transmission Control Protocol/Internet Protocol (TCP/IP) protocol
that handles the establishment, maintenance, and termination of a remote
session. The difference between using Telnet and a protocol such as File
Transfer Protocol (FTP), is that Telnet logs you directly on to the remote
host, and you see a window into that session on your local computer. A typical
Telnet command might be as follows:
telnet example.com
Because
this particular host is invalid, this command will have no result. However, if
it were a valid host the remote computer would ask you to log on with a user ID
and password. A correct ID and password would allow you to log on and execute
Telnet commands.
You
can often use Telnet to manage equipment that lacks a monitor. For example,
most routers have Telnet enabled so that the administrator can log in and
manage the router. Telnet also provides a quick check to make certain that
network connectivity is functioning. Because Telnet sits at the application
layer, if it can connect to a remote host, you can be certain that network
connectivity between the two hosts is operational, as well as all lower-layer
protocols.
SSH (Secure Shell)
is
a program for logging in to and executing commands on a remote machine. It
provides secure encrypted communications between two untrusted hosts over an
insecure network. X11 connections and arbitrary TCP/IP ports can also be
forwarded over the secure channel. When SSH connects and logs in to a specified
computer, the user must prove his/her identity to the remote machine which is
transmitted across the connection using one of three forms of data encryption.
This process makes SSH impervious to Internet eavesdroppers who might otherwise
steal account information.
ICMP (Internet Control
Message Protocol)
ICMP
provides network diagnostic functions and error reporting. One of the most used
IP commands is the Packet Internet Grouper (PING) command. When a host PINGS
another client, it sends an ICMP ECHO request, and the receiving host responds
with an ICMP ECHO REPLY. PING checks network connectivity on clients and
routers. ICMP also provides a little network help for routers. When a router is
being overloaded with route requests, the router sends a source quench message
to all clients on the network, instructing them to slow their data requests to
the router.
ARP / RARP (Address
Resolution Protocol / Reverse Address Resolution Protocol)
The
Address Resolution Protocol (ARP) is an Internet layer protocol that helps
TCP/IP network components find other devices in the same broadcast domain. ARP
uses a local broadcast (255.255.255.255) at layer 3 and FF:FF:FF:FF:FF:FF at
layer 2 to discover neighboring devices. Basically stated, you have the IP
address you want to reach, but you need a physical (MAC) address to send the
frame to the destination at layer 2.
ARP
resolves an IP address of a destination to the MAC address of the destination
on the same data link layer medium, such as Ethernet. Remember that for two
devices to talk to each other in Ethernet (as with most layer 2 technologies),
the data link layer uses a physical address (MAC) to differentiate the machines
on the segment. When Ethernet devices talk to each other at the data link
layer, they need to know each other’s MAC addresses.
RARP
is sort of the reverse of an ARP. In an ARP, the device knows the layer 3
address, but not the data link layer address. With a RARP, the device doesn’t
have an IP address and wants to acquire one. The only address that this device
has is a MAC address. Common protocols that use RARP are BOOTP and DHCP
NTP (Network Time
Protocol)
The
Network Time Protocol is used to synchronize the time of a computer client or
server to another server or reference time source, such as a radio or satellite
receiver or modem. It provides accuracy's typically within a millisecond on
LANs and up to a few tens of milliseconds on WANs.
SNMP
SNMP
is a two-way network management protocol. SNMP consists of two components, the
SNMP Agent, and the SNMP Management Console. The SNMP Management Console is the
server side for SNMP. The management console sends requests to the SNMP Agents
as get commands that call for information about the client.
The
SNMP Agent responds to the Management Console’s get request with a trap
message. The trap message has the requested information for the Management
Console to evaluate. Security can be provided in many ways with SNMP; however,
the most common form of security for SNMP is the use of community names,
associations that link SNMP Agents to their Management Consoles:
·
Agents,
by default, respond only to Management Consoles that are part of the same
community name.
·
If
an SNMP Agent receives a request from a Management Console that is not part of
the same community name, then the request for information is denied.
Because
SNMP is an industry-standard protocol, heterogeneous environments are common.
Many vendors provide versions of SNMP Management Consoles. Hewlett Packard, for
example provides HP Open View (one of the most popular Management Consoles on
the market); Microsoft provides SNMP Server with the Windows NT and 2000
Resource Kits and Systems Management Server. SNMP Management Consoles request
information according to a Management Information Base (MIB) format. An MIB is
a numeric value that specifies the type of request, and to which layer of the
OSI model the request is being sent.
SCP (Secure Copy
Protocol)
Secure
Copy or SCP is a means of securely transferring computer files between a local
and a remote host or between two remote hosts, using the Secure Shell (SSH)
protocol. The protocol itself does not provide authentication and security; it
expects the underlying protocol, SSH, to secure this.
The
SCP protocol implements file transfers only. It does so by connecting to the
host using SSH and there executes an SCP server (scp). The SCP server program
is typically the very same program as the SCP client.
LDAP (Lightweight
Directory Access Protocol)
Lightweight
Directory Access Protocol, or LDAP, is a networking protocol for querying and
modifying directory services running over TCP/IP.
A
directory is a set of information with similar attributes organized in a
logical and hierarchical manner. The most common example is the telephone
directory, which consists of a series of names organized alphabetically, with
an address and phone number attached.
An
LDAP directory often reflects various political, geographic, and/or
organizational boundaries, depending on the model chosen. LDAP deployments
today tend to use Domain Name System (DNS) names for structuring the topmost
levels of the hierarchy. Deeper inside the directory might appear entries
representing people, organizational units, printers, documents, groups of
people or anything else which represents a given tree entry.
IGMP (Internet Group
Multicast Protocol)
The
Internet Group Management Protocol is a communications protocol used to manage
the membership of Internet Protocol multicast groups. IGMP is used by IP hosts
and adjacent multicast routers to establish multicast group memberships. It is
an integral part of the IP multicast specification, like ICMP for unicast
connections. IGMP can be used for online video and gaming, and allows more
efficient use of resources when supporting these uses.
LPR (Line Printer
Remote)
The
Line Printer Daemon protocol/Line Printer Remote protocol (or LPD, LPR) also
known as the Berkeley printing system, is a set of programs that provide
printer spooling and network print server functionality for Unix-like systems.
The
most common implementations of LPD are the official BSD UNIX operating system
and the LPRng project. The Common Unix Printing System (or CUPS), which is more
common on modern Linux distributions, borrows heavily from LPD.
A
printer that supports LPD/LPR is sometimes referred to as a "TCP/IP
printer" (TCP/IP is used to establish connections between printers and
workstations on a network), although that term seems equally applicable to a
printer that supports CUPS.
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