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Application layer

9 important questions on Application layer

List five nonproprietary Internet applications and the application-layer protocols they use.

Web: HTTP, File transfer: FTP, Remote login: Telnet, Network News: NNTP, Email: SMTP

What is the difference between network architecture and application architecture?

Network architecture refers to the organization of the communication process into layers (e.g., the five-layer Internet architecture). Application architecture, on the other hand, is designed by an application developer and dictates the broad structure of the application (e.g., client-server or P2P).

For a communication session between a pair of processes, which process is the client and which is the server?

No. In a P2P file-sharing application, the peer that is receiving a file is typically the client and the peer that is sending the file is typically the server.

List the four broad classes of services that a transport protocol can provide. For each of the service classes, indicate if either UDP or TCP (or both) provides such a service.

a) Reliable data transfer: TCP provides a reliable byte-stream, UDP does not.
b) Guaranteed throughput value: Neither
c) Guaranteed timed delivery: Neither
d) Confidentiality (encrypted): Neither

Recall that TCP can be enhanced with SSL to provide process-to-process security services, including encryption. Does SSL operate at the transport layer or the application layer? If the application developer wants TCP to be enhanced with SSL, what does the developer have to do?

SSL operates at the application layer. The SSL socket takes unencrypted data from the application layer, encrypts it and then passes it to the TCP socket. If the application developer wants TCP to be enhanced with SSL, she has to include the SSL code in the application.

Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?

The applications associated with those protocols require that all application data be received in the correct order and without gaps. TCP provides this service whereas UDP does not.

Consider an e-commerce site that wants to keep a purchase record for each of its customers. Describe how this can be done with cookies.

When the user first visits the site, the server creates a unique identification number, creates an entry in its back-end database, and returns this identification number as a cookie number. This cookie number is stored on the user’s host and is managed by the browser. During each subsequent visit (and purchase), the browser sends the cookie number back to the site. Thus the site knows when this user (more precisely, this browser) is visiting the site.

Describe how Web caching can reduce the delay in receiving a requested object. Will Web caching reduce the delay for all objects requested by a user or for only some of the objects? Why?

Web caching can bring the desired content ‘closer’ to the user, perhaps to the same LAN to which the user’s host is connected. Web caching can reduce the delay for all objects, even objects that are not cached, since caching reduces the traffic on links.

Why is it said that FTP sends control information “out-of-band”?

FTP uses two parallel TCP connections, one connection for sending control information (such as a request to transfer a file) and another connection for actually transferring the file. Because the control information is not sent over the same connection that the file is sent over, FTP sends control information out of band.

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Application layer

9 important questions on Application layer

List five nonproprietary Internet applications and the application-layer protocols they use.

What is the difference between network architecture and application architecture?

For a communication session between a pair of processes, which process is the client and which is the server?

List the four broad classes of services that a transport protocol can provide. For each of the service classes, indicate if either UDP or TCP (or both) provides such a service.

Recall that TCP can be enhanced with SSL to provide process-to-process security services, including encryption. Does SSL operate at the transport layer or the application layer? If the application developer wants TCP to be enhanced with SSL, what does the developer have to do?

Why do HTTP, FTP, SMTP, and POP3 run on top of TCP rather than on UDP?

Consider an e-commerce site that wants to keep a purchase record for each of its customers. Describe how this can be done with cookies.

Describe how Web caching can reduce the delay in receiving a requested object. Will Web caching reduce the delay for all objects requested by a user or for only some of the objects? Why?

Why is it said that FTP sends control information “out-of-band”?

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