ENVS 1200 Lecture 33: ENVS 1200 Lecture 33 Notes
13 May 2018
School
Department
Course
Professor
ENVS 1200 Lecture 33 Notes – Next steps—TCP and the transport layer
Introduction
• DNS clients normally select the first IP address on the list
• Therefore the requests are distributed approximately evenly among the different sites.
• This has the additional advantage of equalizing the traffic load over different parts of
the Internet.
• Once the HTTP application has obtained the IP address, it is ready to pass the request to
the transport layer for transmission through the network.
• HTTP issues a request asking TCP to open a connection to the desired website.
• Sockets are used for this purpose.
• Sockets are equivalent to an open door through which messages can pass.
• The use of sockets makes it possible to create multiple open connections through which
data can flow simultaneously without losing track of which is which.
• Obviously, a Web server must be able to handle many requests simultaneously.
• The HTTP request includes the IP address of the site and the port number of the server
application at the destination, presumably port 80 in this case.
• As we already discussed in the previous chapter, TCP sends a packet (through the usual
network layers) to TCP at the Web site, requesting a connection
• This results in a brief back-and-forth series of requests and acknowledgments known as
handshaking.
• If the negotiation is successful, a connection is opened.
• This connection is logically a full-duplex connection, both because Web page requests
from the browser application at one end node will result in file transfers from the Web
server at the other, and because TCP requires an acknowledgment packet be sent in
return for every packet received.
• The format of a TCP packet
• The same format is used at both ends of the connection.
