Information Technology Reference
In-Depth Information
fIgure 8.1
TCP/IP model
FTP
TFTP
Application Layer
TCP
UDP
Transport Layer
IP
Internet Layer
Ethernet
To ken Ring
Link Layer
How TCP/IP Layers Communicate
When an application like FTP is called upon, the application moves down the layers and
TCP is retrieved. TCP then connects itself to the IP protocol and gets released onto the
network through the Link layer (see Figure 8.2). This is a connection-oriented protocol
because TCP is the protocol that guarantees delivery.
When an application like TFTP gets called, the application moves down the layers and
UDP is retrieved. UDP then connects itself to the IP protocol and gets released onto the
network through the Link layer. This is a connectionless protocol because UDP does not
have guaranteed delivery.
Understanding Port Numbers
TCP and UDP rely on port numbers assigned by the Internet Assigned Numbers Authority
(IANA) to forward packets to the appropriate application process. Port numbers are 16-bit
integers that are part of a message header. They identify the application software process
with which the packet should be associated. For example, let's say that a client has a copy
of Internet Explorer and a copy of Mail open at the same time. Both applications are
sending TCP requests across the Internet to retrieve web pages and email, respectively. How
does the computer know which return packets to forward to Internet Explorer and which
packets to forward to Mail?
 
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