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When a packet is received, the headers are removed. The
Ethernet Network Interface Card (NIC) removes the Ethernet
header and checks the checksum. It then looks at the type
code. If the type code is for IP, the packet is given to IP. IP
then removes the IP header and looks at its protocol field. If
the protocol field is TCP, the packet is sent to TCP. TCP
then looks at the sequence number and uses this number
and other data from the headers to reassemble the data into
the original file.
TCP and UDP Well-Known Ports
Application protocols run 'on top of' TCP/IP. When an
application wants to send data or a message, it gives the
data to TCP. Because TCP and IP take care of the
networking details, the application can look at the network
connection as a simple data stream.
To transfer a file across a network using the File Transfer
Protocol (FTP), a connection must first be established. The
computer requesting the file transfer must connect
specifically to the FTP server on the computer that has the
file.
This is accomplished using sockets. A socket is a pair of
TCP port numbers used to establish a connection from one
9033691-01
Figure 2-12. Ethernet Packet Header
VH-2402-L3 Management Guide 155
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