Now click the ⫹ sign next to Frame there will be a frame number next to the
Defining Networks with the OSI Model |
39
CERTIFICATION READY How do you define the
upper layers of the OSI model?
3.1
In the following exercises, you will do the following:
• Define the transport layer by showing connections in the command prompt and
describing ports.
• Define the session layer by logging into Web sites and other servers, as well as logging
on and off of Microsoft networks and email programs.
• Define the presentation layer by showing encryption in Windows and within Web
sites.
• Define the application layer by capturing web server packets and analyzing them.
Layer 4 governs the transmission of messages through the communications subnetwork. Two common TCPIP protocols that are utilized on this layer include the Transmission Control
Protocol TCP, which is a connection-oriented protocol, and the User Datagram Protocol UDP, which is connectionless. An example of an application that uses TCP is a web brows-
er, and an example of an application that uses UDP is streaming media. When you download a web page, you don’t want to lose any packets of information because graphics would appear
broken, certain text wouldn’t read correctly, and so on. By using TCP, we ensure that data gets to its final destination. If a packet is lost along the way, it will be resent until the destination
computer acknowledges delivery or ends the session. But with streaming media, we are either watching or listening in real time. So, if a packet is lost, we don’t really care, because that time
frame of the video or music has already passed. Once the packet is lost, we really don’t want it back. Of course, if the packet loss becomes too severe, the streaming media will become
incomprehensible.
Connection-oriented also known as CO mode communications require that both devices or computers involved in the communication establish an end-to-end logical connection
before data can be sent between the two. These connection-oriented systems are often con- sidered reliable network services. If an individual packet is not delivered in a timely manner,
it is resent; this can be done because the sending computer established the connection at the beginning of the session and knows where to resend the packet.
In connectionless communications CL mode, no end-to-end connection is necessary before data is sent. Every packet that is sent has the destination address located in its header. This
is sufficient to move independent packets, such as in the previously mentioned streaming media. But if a packet is lost, it cannot be resent, because the sending computer never
established a logical connection and doesn’t know which logical connection to use to send the failed packet.
Layer 4 also takes care of the ports that a computer uses for data transmission. Ports act as logical communications endpoints for computers. There are a total of 65,536 ports, numbering
between 0 and 65,535. They are defined by the Internet Assigned Numbers Authority or IANA and divided into categories as shown in Table 2-1.
■
Defining the Upper OSI Layers
The upper OSI layers are layers 4 through 7—the transport, session, presentation, and application layers. It is this portion of the OSI model that deals with protocols such as
HTTP, FTP, and mail protocols. Compression, encryption, and session creation are also classified by these layers.
THE BOTTOM LINE
40 |
Lesson 2
Table 2-1 IANA port categories
P
ORT
R
ANGE
C
ATEGORY
T
YPE
D
ESCRIPTION
0–1023 Well-known ports
This range defines commonly used protocols e.g., FTP utilizes port 21 to accept client connections.
1024–49,151 Registered ports
Ports used by vendors for proprietary applications. These must be registered with the IANA e.g.,
Microsoft registered 3389 for use with the Remote Desktop Protocol.
49,152–65,535 Dynamic and
These ports can be used by applications, but private ports
they cannot be registered by vendors.
Port numbers correspond to specific applications; for example, port 80 is used by web browsers via the HTTP protocol. It is important to understand the difference between inbound and
outbound ports:
• Inbound ports: These are used when another computer wants to connect to a service
or application running on your computer. Servers primarily use inbound ports so that they can accept incoming connections and serve data. IP addresses and port num-
bers are combined together, for example, a server’s IPport 66.249.91.104:80 is the IP address 66.249.91.104 with port number 80 open in order to accept incoming web page
requests.
• Outbound ports: These are used when your computer wants to connect to a service or
application running on another computer. Client computers primarily use outbound ports, and these are assigned dynamically by the operating system.
There are a lot of ports and corresponding protocols you should know. Although you don’t need to know all 65,536 ports, Table 2-2 highlights some of the basic ones that you should
memorize.
Table 2-2 Ports and associated protocols
P
ORT
N
UMBER
A
SSOCIATED
P
ROTOCOL
F
ULL
N
AME
21 FTP File
Transfer Protocol
22 SSH Secure
Shell 23 Telnet
Terminal Network
25 SMTP
Simple Mail Transfer Protocol 53
DNS Domain Name System
80 HTTP Hypertext
Transfer Protocol
88 Kerberos Kerberos
110 POP3
Post Office Protocol Version 3 119
NNTP Network News Transfer Protocol
137–139 NetBIOS
NetBIOS Name, Datagram, and Session Services, respectively
143 IMAP Internet
Access Message
Protocol
Defining Networks with the OSI Model |
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DEFINE THE TRANSPORT LAYER
GET READY. Let’s take a look at ports and the transport layer in action by performing the following steps: