Solved 1st term exam

You will find below the solved exam. Please have a careful look at the answered provided there and considering your own answers to the written exam, mark your own answers and email me the mark you think you have obtained. Consider only good or bad answers (that will be worth 2% or 0%). Do this as soon as possible (as your memory about your own answers will fade away).

You can get a full-page document view by clicking on this entry title. Do not hesitate contacting me if you find an error on any of the provided answers.
Solved 1st Term2011

Solution to 2nd assignment

Time is out. Here you have one solution to the second assignment. Whether you failed it or you are interested on learning how to do it, you can used this code as a reference implementation.

All submissions have been marked. You can see your mark on the attendance page, assignments tab. If you submit your assignment and the mark is missing, please send me an email.

Solution to 1st assignment

A sample solution.

Important reminder

Next Thursday, December 9th, there is a change in the regular class schedule and classes will proceed as if it were a Monday at ETSINF (please note this does not apply to other schools or studies).

We do not have lectures on Mondays but we do have lab sessions. Remember this not to miss anything.

Attendance rate is declining fast

If you look at the attendance record you'll see that I've added two rows in the bottom showing the total number of attendees on each session and a moving average too.

I can see there is a steep decline in the attendance to the class. Whether this is due to a problem that can be fixed or not I do not know. I just hope you're not given up the subject as still there is one month left. If there is something that needs to be changed in the classroom just talk to me.

Attendance to my office hours is very low too. I just want to remind you that I am here to help and that office hours are part of the instruction.

Sample micro web server

The sample code shows a very basic web server that handles requests one at a time. A text line is printed showing the file requests. Please note the contents to be served has to be in the same folder of the server binary.

BTW, web server listens on port 7777, but you may change that if you wish.

Click on this entry's title to see the code on pastie's page. Please note there is a download link on the upper right corner there.

Second assignment

Though you still have two weeks to turn in your first assignment, the second one is published too.

Do not worry because you have till December 20th, 2010.

This time it is a server what you have to do (a multi-threaded server). The good news is that the application protocol is very simple. After connecting to the server, the client may send one line of text containing an arithmetic expression (i.e. 1+2*3/4) and the server will respond with another text line containing the result of evaluating this expression (i.e. 2.5) or an error message if there is something wrong with the client expression.

Client will signal the server its willingness to quit by sending an empty line. Server will provide no other answer to the empty line but closing the connection.

Several clients can be using the server at the same time and all of them need to be served right away.

You can find here the Eval.java mentioned on the assignment text.

Another useful tool

Though we have mentioned traceroute command in the class. I want to bring your attention to a better alternative: mtr program and its windows version WinMTR.

What it does is a more in depth statistical analysis of the path from your computer to a certain destination. Contrary to traceroute, mtr command is kept running till you press "q" or Ctrl+C as it is constantly updating its measurements.

HTTP and your bike

While you may think HTTP is just a protocol for downloading HTML files, this idea is wrong. HTTP is used daily for transferring many different types of files, from HTML to several graphic formats (JPEG, PNG, GIF, etc) to Cascade Style Sheets (CSS) to JavaScript files and many others.

In this post you can study the way the city bicycle service (Valenbisi) works in order to provide an application the location of the different bike stations and the current state of each one. Here the body of each HTTP response is an XML file.

Armed with this knowledge, this site was created and, believe it or not, it turns out to be much more useful than the "official" one.

The software you don't want your friends to have

Not sure how long this trick will work, but it is a wake up call about Facebook security when it comes to users accessing over wifi networks. We'll be explaining HTTP protocol soon in the classroom, but for Wifi technology you'll have to wait till next term.

I reckon this tool will only work on WEP-protected wifi networks, but I'll be interested on your feedback if you get it working.

Now that "The social network" is on theaters it seems a good moment for this type of information :-)

Short Tutorial on Java Network Programming

I'll be using this material in the classroom to present the basics about Java Network Programming. You can download the PDF file by clicking on this entry title too.
Java Network Programming

It seems the guys at Scribd have some trouble with this document. If you see empty yellow boxes, please download PDF or watch the file on Scribd site (by clicking on Scribd logo) to be able to see the missing code.

Source code is available too.

First assignment is out

This is our first assignment. It is about writing as small web server in Java language. Those of you not familiar with Java might use a different language upon request.

We will present the basic ideas of how to create networking programs in Java in a few days. Please note a sample server is also provided so you can now see exactly what is required.

For running the sample code you need to have a Java Virtual Machine installed on your system. Then at a terminal type: java EchoWebServer

And next type in your browser address bar: http://localhost:8080/anything

Every now and then ...

... we are told the transition to the new version of IP protocol is about to happen. This article says, once more, that old motto (it's been a decade of listening to the same message in slightly different ways).

We will be covering Internet Protocol in Chapter 3. But unless something earthbreaking happens next month, we will still be covering IP version 4 instead of the new version 6 (which is now twelve years old). Whatever it happens, please remember your computer (no matter what OS you use as far as it is recent) already supports both versions.

Labs schedule

You can get a copy of the laboratory sessions' schedule by clicking on this entry tittle.

Course presentation, 2010-2011

Click on this entry title to get the PDF file of subject's presentation slides.

September 2010 marks

Click on the title to go the mark's sheet.

Congratulations to Fernando and Isma that passed the subject.

Your exam is available for you to check it out.

Solved exams

Solved 2nd Term2010

Solved 1st Term2010

September 2010 exam

A few details about the September 13th, 2010 exam:

1. At 15:30 the 2nd term exam will be held.
2. At 18:00 the 1st term exam will be held.
3. Each part will last two hours.

June 2010 Final Marks

While still you can review your exam and ask for any correction that might be needed, current final marks have been published. Hopefully this will remove the anxiety of some of you about the final marks.

Please note that those with incomplete assessment (i.e. one term exam missing) will get no number in their mark. Instead it will be stated that not enough information is available so as to give any grade.

Those of you that have failed the subject, please remember there will be a new opportunity on September for both, first and second term exams. Also remember the only requirement is that the average mark of both terms has to be equal or greater than 50% to pass the subject. Whether you want to take one or two exams in September is your choice.

Marks are available

Both 1st and 2nd term exams have been marked. For many of you there is a small difference between your assessment and mine. There are a couple of cases where difference has been significant.

2nd Term June2010

I've made every effort to provide an accurate and correct marking, but errors may happen. Should you have any concern about your mark, please let me know.

Please note that the view of marked exams is available only for a limited time and that document download is disabled.

Solved 2nd term June 2010 exam

You can download the solved exam and mark yourself. Please email me your mark. Click this entry title to get the file.

First term exam has already been marked. Results are in the usual place.

Interesting IT security event in Valencia

Asegúr@IT 8 is a one-morning event to be held in Valencia. Speakers are knowledgeable people on the security field. Attendance to the event is free but the seats are limited.

BTW, this event is sponsored by Microsoft and a Passport/LiveID account is needed to register.

CodeJam2010

Maybe timing is not right, but if you want to test your programming skills against other talented people you may want to register for Google's programming contest CodeJam 2010. It's fun (if you like programming challenges) and you'll realize how much you need to learn.

I usually join the contest and I am kick out very soon. Java is among the programming languages accepted. There are many sample problems you can attempt to test your skills before the contest begins. Those of you making it to Round 2 can claim an extra 10% mark.

4th and last assignment is out

This time you have to write a simple "wrapper" code to allow the use of an interactive command-line program through a socket connection. While this can be achieved by piping your command to nc or sock commands, you are required to do this by yourself in Java language.

A brief explanation on how you can launch an external program and access its input and output is provided. You have till May 26th, 2010.

A sample implementation is provided too, but please note it won't work in a Windows computer (unless /usr/bin/bc command is found on your system).

New assignment is out

One of the problems we face when our computer is behind a NAT (or a firewall) is that incoming connections cannot reach our computer. While it is possible to fix this with some additional configuration, this assignment is about creating a piece of software that will help two users to be connected even when they are behind a NAT.

For testing purposes let's assume both users will use telnet application to have an interactive chat session (only two users per chat). Your meeting server will enable conversations between pairs of clients, but at any given moment several independent chat sessions may take place.

You can click on the title to get a copy of the assignment document.

Update: Please feel free to ask any question you might have about the assignment as a comment to this entry. This way other students may benefit from it being answered here to.

Update2: As with other assignments, I'm providing you a sample binary for MServer for you to test.

1st term marks

Exams have been marked. Most of you will find your estimate was pretty accurate, but a few cases differ more than 10%, some underestimated and others overestimated their mark. It is ok. Those having no mark is because they did not show up for the exam.

We've won three new students that just showed up for the exam (so they have 0% attendance), for these people, welcome to the class, I hope to see you more often. It is likely some new Erasmus students will join us for the 2nd term too.

Exam average mark is 45% and term average mask is 50%. Both Juan and Anush are our top scorers this term, congratulations!

However, our top scorer in the exam was not any of them but Michał, who scored 80%, congratulations too!

The most difficult question seemed to be (17) how to print the IP address of a received datagram packet. Nobody realized getAddress method returns a InetAddress object and not an IP address dotted decimal string. For that, getHostAddress method was needed.

Those of you below of 40% this term really need to get up to speed to pass this subject. Whether you plan on just focus on the second term or repeat the first term exam too is up to you. But I strongly recommend you to work quite a bit harder not to fail the subject.

To see the list you can click this entry title or just go to the attendance record link where all the details are found.

Update: You can now check marked exams. Please email me if you find any marking error you want me to have a look at.

Solved exam

Click on the title to have access to the solved exam.

I sent it to all of you by email too.

Please have a look at it and mark your own answers (I guess you remember what you answered this morning) using a digital logic: answers are either right or wrong.

Please note each right answer scores 2%.

Once you have marked your exam I want you to send me an email with your name and mark.

Please do so as fast as possible (as your memory about what you answered today will fade away).

Exam Remainder

Please remember REDES' exam will be held in building 1G next Monday, January 25th, at 9:00. Max exam duration is two hours.

X-ray your network communication

While we've presented netstat as a tool to help you watch your connections, if you're running Windows you may want to hear about TCPview. From their website:

TCPView is a Windows program that will show you detailed listings of all TCP and UDP endpoints on your system, including the local and remote addresses and state of TCP connections. On Windows Server 2008, Vista, and XP, TCPView also reports the name of the process that owns the endpoint. TCPView provides a more informative and conveniently presented subset of the Netstat program that ships with Windows. The TCPView download includes Tcpvcon, a command-line version with the same functionality.

Please note that the tool can also be useful to track down suspicious communications happening on your computer. You can easily know where your computer is connecting to and which process is doing it.

Assignment#2 is marked

Apparently second assignment has been quite challenging as only five submissions were received. It's been marked and all of them have passed the assignment. Source code of any of the submissions can be obtained on the anti-plagiarism system report.

Torrents are good

Downloading large files using P2P technology can be quite convenient. As an example I've uploaded the ISO image of Graduation Ceremony DVD to Mininova. These guys are now prevented by a court order to distribute copyrighted contents. However there is no problem if the contents is public domain or posted by the rights holder. So here you have the torrent file.

If you did not graduate this year and therefore missed the ceremony, now you can have a look at it.

Class source code

Today we talked about Multi-thread servers and how we can create them easily in Java.

There are the three examples I wrote.

And let me correct myself: The first implementation of synchronized methods (using inc and dec methods for the simultaneous client counter) is as good as the last one (using just inc) as it is explained in here.

Excellence in Teaching Awards 2009

I've been granted an Excellence in Teaching award from the local government. Among other things lecturing in English (or other foreign languages) and having high-marks on student surveys of teaching were topics assessed.

Now it is time to look around and to thank all the people that made this possible: from my colleagues who share an interest on teaching to my students who keep on asking questions. From all of them I learn a lot every day.

I'm also grateful for my University and government that through various programs have supported different visits and stays at foreign universities.

And last, but not least, to my dear family and friends who have always been there for me.

For all of these I'm grateful.

Assignment#1 report

Not that the results are very bad but I want you to know that measures are taken to keep you all honest.

The following report checks the similarities between all of your submitted assignments.

Class problem

A 4-MByte file is transferred between two computers linked by a router. Each link is 100 Mbps and 100 meters long. Signal propagates at 85% the speed of light. TCP is used for the transfer with MSS=1460 bytes and Receive window is 2048 bytes. How long does it take for the transfer to complete?

Assume only data and TCP headers are transmitted. Assume TCP acknowledgments transmission and queuing delay are negligible. Assume TCP uses delayed acknowledgments. Assume no transmission errors happen and no congestion control is used. Assume TCP connection already established.

For solution OpenOffice's spreadsheet click on this entry title.


Or...


No thanks, I prefer to shot myself on the foot than to use OpenOffice.

Solution to assignment#1

As you have asked me for a solution I'm providing you mine.

You can see that as soon as a client does not provide expected command it is kicked out.

This server won't accept multiple messages sent over the same connection (nor it was required).

An alternative implementation using a finite state machine for the implementation is shown in this other solution.

Second assignment is out

Second assignment is about coding a simple web proxy. No local storage (no cache handling). Just a simple iterative server but with a twist: As you need to read from two streams (request and response) you have to figure out the best way of doing it.

Only GET method is permitted, so you can safely reject any other method coming from the browser (to keep things as simple as possible).

You can click on this entry title to get the assignment text. Second assignment due date is December 23rd, 2009.

Haven't read RFC 821 yet?

Please note that some of you are doing assumptions for assignment#1 that do not match the protocol specification. I want to bring your attention to page 4 of RFC 821 where MAIL FROM and RCPT TO commands syntax is shown. Please note there is not a space between the colon and mailbox name. Do not confuse that with what appears inside the message header (From: and To: headers, where a space is found).

Click on the image on the left to get a better view of it.

Issues with Assignment#1

Apparently there are some difficult statements on the assignment text (given the submissions I'm getting). Let me repeat some of them here (pasted from original document, boldface added here):

1. Please include a MANDATORY comment on the source code with your name.
2. Please send me the java source code file by email to the email account shown here with
   EXACTLY the subject: Assignment#1
3. Each time your server receives a message it will write it down to a text file named "received.txt".
4. Main class name MUST BE SmtpServer.

In case you have not noticed it, I was using UPPER CASE to remark that these were IMPORTANT information.

Class code

Thrusday's class source code below:

• Nine
  
• Eight
  
• Seven
  
• Six
• Five
• Four
• Three
• Two
  

How to write to a text file in Java?

I would suggest you to use PrintWriter class to write to a text file the same way you'll print to a socket. In the below example object out is created so later on out.println("whatever") can be invoked.

PrintWriter out = new PrintWriter(new FileOutputStream("received.txt"));

Friday's code

You can click on the title to get the code I typed in the class today.
 

Office hours change

I've changed office hours due to agenda conflicts. You can see below the new office hours.

Lab#2

This week lab#2 starts. You can get the lab text from this page file repository (or by clicking on this message's title). On the other hand, it has been a subtle change about lab's local storage: Now your home directory is set to your own private network storage. So, contrary to what the lab instructs you to do, you can keep the lab folder if you want.

This lab is about using sockets from the command line interface (CLI). FTP protocol is also presented here.

 

Java Network Programming

There are many good sources to learn from. But I think the best advice I can give you is that network programming is like riding a bike: you learn by doing it.

While on the campus network you can access a huge collection of Safari Books Online. A relevant one is this one.

But among many online resources, the Java Tutorial is worth trying. They even have a Sockets section you may enjoy reading.

Please remember that Java is free and open. You can easily download JDK to your system, so you can both compile and run any examples. I encourage to do so.

First assignment

I've published the first assignment on our file repository. If lazy, just click on this entry title.

You have till Nov 20th, 2009 to turn it in. Remember that while optional, assignments will count to your final mark.

Those from foreign universities can use a programming language different than Java.

I'm providing you a sample server binary that, hopefully, will work. You can run it by typing

java SmtpServer

Update: There is been already one submission.

Lab Blues

I failed to mention that there is a self-assessment test at the end of each lab session. Unfortunately these exams are only available in Spanish. The good news is that you can safely skip them. It is ok if you do them too. It is ok if you provide random answers too.

The real assessment for the lab exercises will happen in the written exams, where 30% of the mark will come from questions about the labs. Please note other students (not the English one) have a different policy regarding the lab tests (these being mandatory). Please note the teacher in the lab may not know you're from the English group.

I am sorry I did not mention that before but please remember last week we did not have lectures of RED.

Fiber optics Nobel prize

It is said that success has many fathers.

Charles Kao, one of the fathers of fiber optic technology has been awarded this year's Nobel prize in Physics. The prize is shared with two other notable physicists.

This gives you an idea of how important fiber optic technology is.

Odd week

As you know next Friday is a national holiday. Next Thursday, however, it is not a holiday. But, in order to balance the number of weekdays next Thursday will be like a Monday regarding to lectures' schedule.

So to make a long story short: This week we won't have RED lectures.

September marks

Just in case you've not searched for them on the usual place, here you have September marks. They will become official by the end of this week.

Sorry, former link was not public, it should be fixed by now.

Some ideas if you bring your laptop to the lectures

A former student, Ismael, mentioned that some students found interesting to use software for adding comments on PDF slide files. Apparently PDF-Xchange Viewer (for Windows) and Skim (for OS X) can do the job (for free).

Of course this may only be interesting for those of you that are bringing a laptop to the class. But, time-proven technology, like paper and pen, can be very useful tool too.

 

Welcome to the 2010 class of RED

We'll use this blog for posting interesting data about the subject, slides, laboratory exercises, solved exams, assignments and marks.

Contrary to PoliformaT this blog is open to anyone, whether you are enrolled in the subject or not (or not yet).

Feel free to use the comments on any topic to ask questions or to make any comment you think it is appropriated.

Please note the "File Repository" link on the top right side will lead you to a Google site with (hopefully) all the required files related to the course.

Let's keep in touch

While I do know some of you are going to stay around for a while, others are already back to your home country. Because many of you are using the university provided email, contact will be lost as soon as your account has expired.

You still can reach me on Facebook or Linkedin networks, and I think it is a good idea to keep the link on. Sometimes you may need a recommendation letter or discuss any technical issue. Other times I may ask for some traveling advice. Who knows.

So farewell and see you online.

Challenge your programming skills

The new edition of Google's CodeJam contest is about to start. While you won't get any points on this subject, it can be fun and you can also get some cash!

Summer reading

I've found this article on Tom's hardware website quite interesting to read.

Spicing up your marks

After some changes on the marking due to your requests, marks have been adjusted so all the available range of the pass mark is covered. As a result some of the marks actually crossed the fail/pass border and those of you affected will be pleasantly surprised. Still some of you did not make it and you will have to come back in September for another exam. Please note the exam "algorithm" will be the same as in June: You can take 1st term, 2nd term or both exams.

REDES-2008_09

Publish at Scribd or explore others:

Do you want to check your exam?

Please note you'll need to scroll down to find your exam. They are not stored in any special order, so be patient.

1st term exam

and

2nd term exam

are available for you to see them on-line (no download is allowed).

Exams will only be available for a limited time.

Update: Question 21 of 2nd term exam has been re-examined and now Dijkstra is also considered a valid answer. Marks have been modified accordingly for all of you.

June 2009 2nd and 1st term solved exams

As we did before, please double check the solved exam and with the answers you remember you provided and then, mark your own exam and email the mark to me.

Thanks.

Jun2009A Solved

The 1st term exam solution is below. As with the other exam, please mark yours and email it.

Jun2009B Solved

A few more solved exams

If you took the time to explore this blog's content backwards you've realized some of the older links are dead. But fear not, some of that content is brought back to life here. These are a few solved exams more for you to look at.

Don't forget next Monday we'll have the exam.

Either you are attending to one term exam or two, see you there.
 

Breaking bad

Some years ago we've got a situation at UPV. This paper details some of the events that happened. I reckon both students involved are still at UPV.
Paper

Publish at Scribd or explore others: Technology Business & Law computer breakin intrusion

Exam details

Next June 2009 exam will consist of two parts. Those of you only interested on the second term part have to come at 10:00. 2nd term exam will last two hours.

After the 2nd term exam a 1st term exam will be held (same place but fewer rooms).

Please note that it is mandatory you take now the 2nd term exam but the 1st term one is optional. If you are happy with the mark you already have got in the 1st term then you do not need to repeat that. Your final mark will average the mark of both semesters "no strings attached".

Both exams will follow a similar format than the ones on the blog (sorry but old links may not work).
 

Improve your Spanish while learning cryptography

Manuel Lucena is a professor from Jaen University who has a great book on cryptographic algorithms and who is kind enough to provide us a free PDF version.

There is only a catch on this offer: it's only available in Spanish. What a good opportunity for practicing your newly acquired skills!!

Contrary to what happened in the classroom, the book provides a detailed view of many cryptographic algorithms, including DES, IDEA, AES, RSA, ElGamal, DSA, ... etc

Update: For those of you that still prefer a book written in English you can download this one. Though a bit aged it is still a very solid book.
 

Put your crypto skills to work

For many of you [non-Spaniards] this might be a more difficult challenge as Spanish knowledge can help. Can you discover what is the hidden part of this message?

You are welcome to bring the solution to the class and I'll pay you a coffee.

For more challenging proposals you can go to this contest.
 

Last two chapters

Chapter 6 and Chapter 8 are the last chapters of our course. There is not much time left and we will skip some of the sections, please stay tuned to get the details about what is left out.

Lab#10

This is the last lab of the course. Click on the title to get the file.

More info here.

Easter Fun

Now that we're approaching the end of the course you might be eager to see the whole thing in action. There is no better way to learn anything that when you're facing the task of building a working system. And I mean to build all of it!

Most of the time you work on a personal computer. Not the hardware or the operating system are part of what you have to build. In an extreme case you might be rebuilding the kernel of the operating system you're running.

If you like hardware and to experiment I propose you to get one Arduino board and a LAN adapter for it. I've used in the past the ones I'm referencing and I've found that seller the cheapest option at the time.

To get things running you'll need to download the software library for the LAN card plus the Arduino IDE appropriated for your computer (OS X, Linux or Windows).

Arduino is an easy-to-use low-cost "open hardware" microcontroller board around ATmega168 chip. USB bus server to both power the board and download developed code to it. Many extensions are possible and the LAN card adds a simple Microchip LAN controller to Arduino. All the source code you see is all what's available: no operating system below, no hidden strings, just the bare source of the library.

Lab#9

Click on the title.

Error checking (kind of)

If you want to know the character (for error control purposes) of a Spanish identification document, just perform the modulo 23 operation over the DNI number. Then use the next table. That's it.

0	T	12	N
1	R	13	J
2	W	14	Z
3	A	15	S
4	G	16	Q
5	M	17	V
6	Y	18	H
7	F	19	L
8	P	20	C
9	D	21	K
10	X	22	E
11	B

And if you want to get a better idea about how CRC works you can visit this old post. There is a detailed Wikipedia article too.

Chapter #5 slides

Oops, I've almost forgot to post it here. You can click on the title to get the file.

Lab#8

Just click on the title.

IPv6 survey result

It was published recently on NetworkWorld site. ISP's are not seeing a big business drive for IPv6.

Sample implementation of assignment#3

You'll need it if you're planning to do assignment#4 but you did not do #3. Source code is available here. It consists of a main class and three additional classes one for each server (peer, query and file server).

Better late than never

After some questions I've learned that I mention nowhere what the expected command-line parameters order for #3 and #4 is. Three are the parameters you need: port number, shared folder and initial peer, like this:

$ java P2P 7123 ./ 158.42.53.17:7777
          |   |        |
        port  |        |
            folder     |
                   initial peer

I just thought this afternoon that a test tool might come in handy for you to test your assignment #3 servers. So here you have such a tool. Please remember a full implementation of a peer was also available (as another way to test that your server works nicely).

The test program is intended to be run in the same computer as #3, so you only need to provide the port information.

$ java P2Tester 7777
>>> CONNECT OK
158.42.53.17:7777
158.42.53.17:8899
1.1.1.1:1234

P2P.class
QueryServer.class
ListManagement.class
Peers.class
FileServer.class

>>> READ FROM FILE (first ten bytes): FileServer.class
ca fe ba be 00 00 00 32 00 72

Please note the output above shows the expected behaviour where you have three sections. On the top it is the peer list, after connect ok message that tells you the three server ports are open (p, p+1 and p+2). Next you have the list of filenames (only if your code reacts this way to an empty line query, something mine does) and in the third place you have the first 10 data bytes of the filename that appears last on your file list (FileServer.class).

Please note the file is not read from the filesystem but sending a file download request to your server listening on port P+2.

Also note that peer 1.1.1.1:1234 might not appear on the peer list till you run test program twice.

Do not use an empty folder for your P2P server: share some files!!

Interesting video

Last assignment is out

Fourth and last assignment completes the P2P software you started building on #3. Therefore you'll need to add the #4 code to the one you wrote for #3. If you did not do third assignment but you'd like to attempt the fourth, please contact me for a reference implementation for third assignment (once #3 due time has expired).

This last assignment takes care of the user interface of the peer to peer program, that allows the user to search for files on the network of peers and to download a file from a peer. It also takes care of maintaining a "live" list of active peers (pruning it when a peer is found not responsive).

Due date is April 30th, 2009.

Assume command line information is correct.

Lab #7 available only in Spanish

Just click on the title to get the file.

IPv6 addresses problem

IPv6 is proposed as an almost unlimited source of addresses to overcome the IPv4 address shortage some people have suggested. As IPv6 addresses are 128-bit long, 2^128 (aprox 3.4 x 10^38 ) addresses are possible. This should be large enough.

To put things into perspective consider the following problem: How many IPv6 addresses could you put into each squared centimeter of the planet? (Assume the Earth is a regular sphere of 40.000 Km of perimeter).

And a second part ... compare the above number of IPv6 addresses per square centimeter to the total address space of IPv4.

Fragmentation exercise

A 4000 byte-long datagram has to be transmitted over a network whose MTU is 512 bytes. Determine the number of fragments and, for each one of them, write down the fragment offset field contents and the number of data bytes of each fragment.

Solution was presented as a spreadsheet in the class. Here you have a copy. You can export it to your favorite spreadsheet software too (to see the formulas).

Third assignment announcement

Third and forth assignments will be based on a peer-to-peer protocol invented for the occasion. The so-called "P+2" protocol has been created to be one of the simplest P2P protocols (but not any simpler). You need to read and to understand the protocol specification. Protocol was already presented in the class but if there is general call I will repeat the explanation.

This assignment covers three different servers needed to create a full implementation. Assignment's servers can be tested to work using either telnet, netcat (nc) or sock commands. Java language is mandatory for Spanish students but the rest may use the language of your choice. A working implementation of a peer is available too (just a binary version).

You can download assignment text from here. Or you can just go to the file repository (linked on the top right corner, under the new stuff title).

There is a sample peer running too. You may not need it yet for this assignment. Anyway, it is available at 158.42.53.17:7777

You are welcome to ask me for help. Please remember you have to write code by yourself. As assignments are marked individually, it is not something you can develop jointly with a classmate. If two or more assignments are copied from each other all the involved parties will fail the assignment. Due date is March 13th.

p_2

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Second term starts

Chapter 4 slides are already available. For those of you joining us for the second term please note you should read the syllabus that contains important details about how the subject works.

Next week we'll start with the lab again. Lab#6 is out too.
 

Marks are out

Not all of you have sent me your own grading of your exam. The reason I was asking you to do that is not because I can save some work this way but because I want you to have a look at your own answers. Most of your marks are quite close to what I've got, at least from the set of students who sent them to me.

You can check your mark on-line. There a few more students who have never attended lectures that will appear at the end of the list of names. This marks are not final as you still have a choice to check your exam. If you want to do that, please tell me so next week in the classroom (or send me an e-mail).

You can see we have a tie on the top performers. I want to congratulate both: Rasmus and Juliusz. Keep up the good work!

Solved 1st term exam Feb 2009

Feb2009 Solved

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You know what you have answered. Now you know the right answers. So please check them out, use a binary logic (an answer is either right or wrong, and it scores or not) and calculate your mark and email it to me.

My mark after the exam ...

1. Exam will be marked from 0 to 10 (being 10 the best mark possible).
2. Attendance is already marked as a value from 0 to 1 (expressed as a %).
3. Assignments have been marked too (0 to 2).

First term mark (again from 0 to 10) we'll be calculated as:

1st-term = attendance + (9-assignments)*exam/10 + assignments

Same process will hold for the second term. Course mark will be obtained by averaging both terms. There is not a minimum average you need to obtain on each term, but course average needs to be equal or higher than 5 points to pass the course.

To know more about threads synchronization

Assignment#2 presented you an scenario where multiple threads handle one client each. However, given that each thread has to send a text line to each any other thread's client, a synchronization problem may happen.

If all you use to test server's code is a few telnet clients running on your computer, it is difficult you see any problem (as only one client is typing at a time). If you're interested on researching this topic further, you may use IMTester.java code. This program will throw one hundred simultaneous clients to your server, each one sending on hundred text lines and then the mandatory quit command.

You may well see errors like this "Exception in thread "Thread-85" java.util.ConcurrentModificationException" on the server program. It basically means that an object was modified while others were reading from it (not a good thing). While I was not interested on raising this topic of synchronization, some of you pointed it out, so I cannot avoid explaining a bit more.

The main problem happens because the list of clients may change while one (or more) thread is iterating through it. If the client list changes while messages are being sent to other clients it is possible that either a client is missing the message or that a message is attempted to be delivered to client who is gone.

The solution is pretty simple but not obvious, and most of you failed to provide a proper one (not that it was a requirement though). I'm including an slightly modified version of my sample implementation for you that addresses the synchronization problem. The basic idea is that looping through client list and removing a client from the list are performed exclusively.

Update: A Vector object should have been used instead of ArrayList (as Vector is Thread safe and ArrayList is not).

Assignment #2 is marked

You should have got an email answer about your work. At any rate, marks are on the usual place (assignments tab). Those with a 0 mark still can try to fix it by Thursday 15th.

If any of you is missing a mark, please let me know.



You can have a look at my implementation.

December 18th

Today's class was used to explain the basics of a P2P protocol that will be used for assignments #3 and #4. Same explanation is here.

Please note the text of the assignments will not be provided until the beginning of the second term lectures after the exams break.

Not sure how useful it will be ...

It's been suggested that having all the information in one place might be interesting. So I am putting everything together on a Google site (as an excuse to learn a bit about that feature).

First term last lab

You're required to write a concurrent web server. Here you have the source code of the lab (which appears written on the lab document). You're not supposed to waste your time re-typing it.

Second assignment is out

Now that assignment#1 is due I'm publishing assignment#2 text.

This second assignment is about writing a multithreaded server application for an instant messaging server. Not that you're going to program the next Messenger server (at least not yet) but I hope it'll be fun.

Please remember this assignment is due on January 12th, 2009.

Classroom source code

One_java

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New chapter and lab#4

We move on to chapter 3 and lab#4 has already started too.

UDP sockets programming

UDPEchoServer Java

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Third lab

Some programming will be required for the third lab.

TCP sockets programming

Here you can find a copy of the source code java files I wrote during last class.

This is the client.

import java.net.Socket;
import java.io.*;
import java.util.Scanner;

class TestTCP {
    public static void main(String args[]) throws  java.net.UnknownHostException, java.io.IOException ,  java.lang.InterruptedException{
        Socket s = new Socket ( "www.upv.es", 80 );
        // Thread.sleep(100000);
        PrintWriter output = new PrintWriter( s.getOutputStream(), true);
        output.println("GET / HTTP/1.0");
        output.println("");
        Scanner input = new Scanner( s.getInputStream());
        while(true) System.out.println(input.nextLine());
    }
}

And this is the server code:

import java.net.*;
import java.io.*;
import java.util.Scanner;

class Server {

    public static void main(String args[]) throws IOException, InterruptedException{
        ServerSocket ss = new ServerSocket(8080);
        while(true) {
            Socket s = ss.accept();    
            Scanner input = new Scanner( s.getInputStream());
            PrintWriter output = new PrintWriter( s.getOutputStream(), true);
            System.out.println(input.nextLine()); // reads a line from the socket (client)
            output.println("Hello client!");
            s.close();
        }
    }
}

Please note that I detailed during the class some DOs and DON'Ts that not necessarily show in my (quick and dirty) code.

First assignment is out

You have till December 1st to turn in this first assignment. You may need to have a look to SMTP protocol specification (RFC 821). Please note that this RFC has been obsoleted by newer versions. However, for the purpose of this assignment, the original version of SMTP specs are ok.