Homework Of Computer Networking

CSE422 Section 002 – Computer Networking Fall 2018

Homework 2 – 50 points Sockets (10 points)

1. For a client-server application over TCP, why must the server program be executed before the client program?

2. For a client-server application over UDP, why may the client program be executed before the server program?

3. The UDP server shown in the course slides needed only one socket, whereas the TCP server needed two sockets. Why?

4. If the TCP server were to support N simultaneous connections, each from a different client host, how may sockets would the TCP server need?

5. You are creating an event logging service that will be handling event messages from multiple remote clients. This service can suffer delays in message delivery and even the loss of some event messages. Would you implement this using TCP or UDP? Why?

The HTTP GET message (10 Points) Consider the figure below, where a client is sending an HTTP GET message to a web server, gaia.cs.umass.edu.

Suppose the client-to-server HTTP GET message is the following:

GET /kurose_ross/interactive/quotation1.htm HTTP/1.1

Host: gaia.cs.umass.edu

Accept: text/plain, text/html, image/gif, image/jpeg, audio/basic,

audio/vnf.wave, video/mp4, video/wmv, application/*, */*

Accept-Language: en-us, en-gb;q=0.5, en;q=0.1, fr, fr-ch, zh, cs

If-Modified-Since: Wed, 10 Jan 2018 13:13:03 -0800

User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/535.11 (KHTML,

like Gecko) Chrome/17.0.963.56 Safari/535.11

Answer the following questions:

1. What is the name of the file that is being retrieved in this GET message?

2. What version of HTTP is the client running?

 

 

CSE422 Section 002 – Computer Networking Fall 2018

3. What formats of text, images, audio, and video does the client browser prefer to receive? [Note: for this and the following questions on browser media and language preferences, you will need to do a bit of additional reading on the Web. Here is a good place to start.]

4. What do the strings “application/*” and “*/*” signify in the Accept: header?

5. What languages is the browser indicating that it is willing to accept? [Note: you can look at your own browser preferences to get a listing of language codes.]

6. What is the meaning of the “relative quality factor,” q, associated with the various version of English? [Note: Here is a good place to start. See also [RFC 2616].]

7. What is the client’s preferred version of English? What is the browser’s least preferred version of English?

8. Does the browser sending the HTTP message prefer Swiss French over traditional French? Explain.

9. Does the client already have a (possibly out-of-date) copy of the requested file? Explain. If so, approximately how long ago did the client receive the file, assuming the GET request has just been issued?

10. What is the type of client browser and the client’s operating system? [Note: To answer this, you’ll need to understand the User Agent: header field. Here is a good place to start.]

The HTTP RESPONSE message (10 Points) Consider the figure below, where the server is sending a HTTP RESPONSE message back the client.

 

Suppose the server-to-client HTTP RESPONSE message is the following:

HTTP/1.1 200 OK

Date: Wed, 10 Jan 2018 21:23:35 +0000

Server: Apache/2.2.3 (CentOS)

Last-Modified: Wed, 10 Jan 2018 21:35:35 +0000

ETag:17dc6-a5c-bf716880.

Content-Length: 77385

Keep-Alive: timeout=41, max=92

Connection: Keep-alive

Content-type: image/html

Answer the following questions:

 

 

CSE422 Section 002 – Computer Networking Fall 2018

1. Is the response message using HTTP 1.0 or HTTP 1.1? Explain.

2. Was the server able to send the document successfully? Explain

3. At what date and time was this response sent?

4. When was the file last modified on the server?

5. How many bytes are there in the document being returned by the server?

6. What is the default mode of connection for HTTP protocol? Is the connection in the reply persistent or non-persistent? Explain.

7. What is the type of file being sent by the server in response?

8. Does the response message use separate keep-alive messages?

9. What is the name of the server and its version? List the advantages of the server used.

10. What is the timeout value for the response message?

DNS and HTTP delays (10 points) Before doing this question, you might want to review sections 2.2.1 and 2.2.2 on HTTP (in particular the text surrounding Figure 2.7) and the operation of the DNS (in particular the text surrounding Figure 2.19).

Suppose within your Web browser you click on a link to obtain a Web page. The IP address for the associated URL is not cached in your local host, so a DNS lookup is necessary to obtain the IP address. Suppose that four DNS servers are visited before your host receives the IP address from DNS. The first DNS server visited is the local DNS cache, with an RTT delay of RTT0 = 1 msecs. The second, third and fourth DNS servers contacted have RTTs of 31, 47, and 25 msecs, respectively. Initially, let’s suppose that the Web page associated with the link contains exactly one object, consisting of a small amount of HTML text. Suppose the RTT between the local host and the Web server containing the object is RTTHTTP = 14 msecs.

1. Assuming zero transmission time for the HTML object, how much time elapses from when the

client clicks on the link until the client receives the object?

 

 

CSE422 Section 002 – Computer Networking Fall 2018

2. Now suppose the HTML object references 7 very small objects on the same web server. Neglecting transmission times, how much time elapses from when the client clicks on the link until the base object and all 7 additional objects are received from web server at the client, assuming non-persistent HTTP and no parallel TCP connections?

3. Repeat 2. above but assume that the client is configured to support a maximum of 5 parallel TCP connections, with non-persistent HTTP.

4. Repeat 2. above but assume that the client is configured to support a maximum of 5 parallel TCP connections, with persistent HTTP.

5. What do you notice about the overall delays (taking into account both DNS and HTTP delays) that you computed in cases 2., 3. and 4. above?

Question: Reliable UDP data transfer (5 points) Is it possible for an application to enjoy reliable data transfer even when the applications run over UDP? If so, how?

Question: NAK-only vs ACK-only (5 points) Consider a reliable data transfer protocol that uses only negative acknowledgments. Suppose the sender sends data only infrequently. Would a NAK-only protocol be preferable to a protocol that uses ACKs? Why? Now suppose the sender has a lot of data to send and the end-to-end connection experiences few losses. In this case, would a NAK-only protocol be preferable to a protocol that uses ACKs? Why?