On a single computer, the different layers in the TCP/IP model
work together to accomplish the ultimate goal, which is to give the end user the
data that he needs. HTTP is concerned about getting a web page to the user. TCP
is concerned about segmentation, error recovery, and other things, on behalf of
HTTP. So, as you read in Chapter 9,
TCP provides service to the application layerin this case, HTTP.
Similarly, IP (network layer) provides a service to the layer
above it, namely TCP (transport layer): the service of end-to-end delivery of
packets. TCP wants to send each segment to the other computer, but TCP does not
define details about how the segment is delivered. Instead, TCP just asks IP for
help. The IP software on Hannah puts an IP header around the segment. That
header includes the correct source and destination IP address, which allows the
IP protocol to deliver the packet to the web server.
Figure 10-3 shows what
is inside the packet; however, just as a shipper does not care about what's in a
package, IP does not care about the TCP header, the HTTP header, or the end user
data. IP just needs to know the destination address so that it can forward the
packet toward the destination. So, from IP's perspective, the IP packet looks
like Figure 10-4.
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