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.
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.
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.
| 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.
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:
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.
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!!
$ 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.
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).
$ 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 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!!
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.
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.
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
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.
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
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.
Publish at Scribd or explore others:
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 ...
- Exam will be marked from 0 to 10 (being 10 the best mark possible).
- Attendance is already marked as a value from 0 to 1 (expressed as a %).
- Assignments have been marked too (0 to 2).
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.
TCP sockets programming
Here you can find a copy of the source code java files I wrote during last class.
This is the client.
And this is the server code:
Please note that I detailed during the class some DOs and DON'Ts that not necessarily show in my (quick and dirty) code.
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.
Second chapter
Here you have second chapter slides and second lab too.
I know some of you still need to get a valid user account and I'm sorry. While this is not fixed, please ask the lab teacher to pair you with another student with a valid user account so you can do the lab.
I know some of you still need to get a valid user account and I'm sorry. While this is not fixed, please ask the lab teacher to pair you with another student with a valid user account so you can do the lab.
Laboratory starts this week
Just in case you haven't noticed, lab sessions started October 13th. I'm afraid you may be looking for the English version of the lab I forgot to post :-(
1st chapter slides
As I mentioned today in the lecture, here you have the slides from Chapter 1. I have created a Google Docs spreadsheet that will contain all the relevant information about your performance (i.e. class attendance, assignments, etc).
Unfortunately today the service is not working nicely and at the moment I cannot even access that page. As soon as they are back on-line I'll post here the URL for that page.
It is advisable that you check your attendance record from time to time. Please report any error you may find as soon as possible.
Update: Attendance spreadsheet is back on-line.
Update2: Lab calendar is here.
Here we go again
A new course starts on Monday. Amazingly over 25 students are already enrolled on the group, this has to be an all times record for our subject. I want to welcome you all and I hope you'll enjoy the subject.
For the moment I'm only making the course syllabus available. All the slides and interesting materials will be added to this blog little by little. Feel free to go back in time too, but please be warned that links from years ago may no longer be working.
Applied knowledge to physical layer
Most of you are young enough not to have ever used a cassette tape as a digital storage system. But still you may feel curious about how it sounded and, even more important how it worked. This entry links to an article about using software to decode an old BASIC interpreter for Apple I computer.
Different techniques were used long time ago to create audio signals representing digital files that could be stored and recovered from a cassette using an inexpensive cassette player.
And we are back online!!
After one year of using the PoliformaT platform I've got enough. It might well be ok for some people but I'm not among them. I refuse to be a good well-trained monkey.
In what I think is the best interest of my students (and my own sanity) I'm recovering this blog as the vehicle to distribute the class material.
There are a number of reasons for this move:
As an starter I'm posting here the solved exams of last June 9th, 2008.
And the second semester's ...
Please note that older material that is linked from previous entries may not work as it was lost after a hard disk crash. If you absolutely need to get any of them send me an email and I'll recover it for you.
In what I think is the best interest of my students (and my own sanity) I'm recovering this blog as the vehicle to distribute the class material.
There are a number of reasons for this move:
- Students will no longer need to log-in to read any class related news.
- Visitors will now be able to see what's going on in the subject.
- Prospective students will learn what the subject is all about.
- I will be able to avoid to do a dance to upload a single comment or document. However I will have to add a comment with any document I add, which I find appropriated.
- Anyone could comment on the topics of the blog, whether they are enrolled in the subject or not.
- Availability will improve upon university's.
- I will be able to keep the English group of the subject apart from the noise of other languages' groups.
- Blogger's usability is miles away from the ugly and clunky interface of PoliformaT.
- We will be free to enjoy videos of any size (PoliformaT limits to 20 MB max which is a joke).
- Maybe the site will open up collaboration links with other universities.
- Site appearance could easily be changed (if I were not so lazy).
As an starter I'm posting here the solved exams of last June 9th, 2008.
And the second semester's ...
Please note that older material that is linked from previous entries may not work as it was lost after a hard disk crash. If you absolutely need to get any of them send me an email and I'll recover it for you.
Local storage is gone!
After the last storm the computer that used to act as this blog storage server gave up the ghost.
That means that the copies of laboratory exercises and course slides are gone too. Together with many of the documents referenced in this blog. I am not planning on restoring either the contents or the server anytime soon.
However, I will be posting each new document on the "private" PoliformaT site.
That means that the copies of laboratory exercises and course slides are gone too. Together with many of the documents referenced in this blog. I am not planning on restoring either the contents or the server anytime soon.
However, I will be posting each new document on the "private" PoliformaT site.
I have to let this blog go
I want to welcome all the new students. This blog has been the home for the English language class of the subject RED (code 5662). The format was simple (and sometimes even Internet Explorer rendered it properly) and you needed no silly authorization to download anything. However, the UPV in its infinite wisdom has decided to curtail the use of external tools, like this blog, in favor of their locally cooked version of Sakai they have named PoliformaT.
It seems that faculty not using this tool will be considered as doing no work on cyberspace. There is some money involved, don't get me wrong, we have a job to do and some money to make. By putting the carrot on the use of PoliformaT they are insuring all of us will end up using that platform whether it fits our (or your) needs or not.
For our English group it has been a constant cause of trouble the fact that students join us a bit later than the beginning of the term. Because of the involved paperwork, they use to come to the class sooner than they have got a username and password. Under the university instituted systems (so called microwebs and now PoliformaT) they have no access to course syllabus or slides. To address this problem I have been using for two years this blog (the fact blogger interface was cooler and simpler than the local alternatives did help too).
So from here I want to apology in advance for the possible trouble you are facing with this new move at the UPV. While technically I cannot claim this is censorship I would say they are giving as the same choice that Ford Motor Company gave to T-model customers: "You can choose any color you like as long as it is black".
It seems that faculty not using this tool will be considered as doing no work on cyberspace. There is some money involved, don't get me wrong, we have a job to do and some money to make. By putting the carrot on the use of PoliformaT they are insuring all of us will end up using that platform whether it fits our (or your) needs or not.
For our English group it has been a constant cause of trouble the fact that students join us a bit later than the beginning of the term. Because of the involved paperwork, they use to come to the class sooner than they have got a username and password. Under the university instituted systems (so called microwebs and now PoliformaT) they have no access to course syllabus or slides. To address this problem I have been using for two years this blog (the fact blogger interface was cooler and simpler than the local alternatives did help too).
So from here I want to apology in advance for the possible trouble you are facing with this new move at the UPV. While technically I cannot claim this is censorship I would say they are giving as the same choice that Ford Motor Company gave to T-model customers: "You can choose any color you like as long as it is black".
September marks
!!! I am glad to inform you that after marking the exams all of you have passed the subject. Details can be found on the usual place.
Next September 11th
Those of you who haven't pass the subject yet do not forget exam will be held September 11th, at 4:00pm.
Course Marks available
On our usual place in the "exams" tab.
Results are quite disappointing as several people did not show up for the exam. Others did but failed the subject by not scoring good enough. Most of you did pass, for those of you, congratulations.
Those who did not pass the subject, please think whether you could have done better by turning in any of the assignments, showing up for the lectures more often, doing the lab exam and, perhaps, studying a bit more.
Anyway, you are welcome to review your exam so you see for yourself what was wrong. Drop me an email for a reviewing appointment.
Solved exam is now available.
Results are quite disappointing as several people did not show up for the exam. Others did but failed the subject by not scoring good enough. Most of you did pass, for those of you, congratulations.
Those who did not pass the subject, please think whether you could have done better by turning in any of the assignments, showing up for the lectures more often, doing the lab exam and, perhaps, studying a bit more.
Anyway, you are welcome to review your exam so you see for yourself what was wrong. Drop me an email for a reviewing appointment.
Solved exam is now available.
Graded assignments
All the assignments turned in have been graded. Please go to the attendance/marks page to check yours.
Weekend reading
I mentioned today in the class I will post this article on one of the many network security battles. I guess the printer friendly version will remove most of the annoying advertising from the web page (but I was not sure the server would be happy about my referencing of a printer friendly page).
It seems that another one bites the dust in terms of getting their encryption system hacked (not that I am interested or worried as I do not have either content or player for that format).
It seems that another one bites the dust in terms of getting their encryption system hacked (not that I am interested or worried as I do not have either content or player for that format).
Fourth assignment
It's only one month left so here is the last assignment for you. Please note its due date is May 28th.
Some people's comments about IPv6
Slashdot was publishing a note about the use of IPv6 in space. Some people found this an opportunity to voice their opinion about IPv6.
All you wanted to know about bit error rate but were afraid to ask
One important detail about BER (bit error ratio) is that it is an average value. If you use it as such there is no problem. But for if you have link whose BER=10^-4 and you transmit 10000 bits over that channel it is likely you won't get any error at all. The reason is that BER is an statistic but not a RULE.
If you want to learn more abot what this means, read on this interesting but yet accessible document.
If you want to learn more abot what this means, read on this interesting but yet accessible document.
Computer Networks by Andrew S. Tanembaum
It seems the book is also available for campus-wide access in electronic format (part of the O'Reilly Safari Books on-line package) so as far as you are part of the campus network either connected locally or by a VPN from your home you should be able to read the recomended chapter 4.
Green third assignment
It seems the current trend is all for recycling and reducing the waste we create. I am joining this effort now by presenting you a fully recycled third assignment about IP fragmentation.
Please note that the due date is wrong (recycling sometimes has some problems). The correct due date is April 6th.
Please note that the due date is wrong (recycling sometimes has some problems). The correct due date is April 6th.
Solved 1st term exam
Already available, just follow this link.
Marks will be available in a few days in this same site as an update of this post.
Update: Exam marks are available in the same link as the attendance pages. Please note that solved exam contained a couple of errors that have been now fixed, just download a fresh version.
Second Update: It seems that some of you have trouble doing the math to know your current score, so now you have that numbers on the mark's page. I repeat what I explained the first day of class and it is detailed in Chapter0 slides.
The same logic will be applied to the second term marking and then, both marks will be averaged to obtain your final grade.
Please remember the pass mark is 50%, so if you score less than this you will have to work harder on the second term to pass the subject.
Lab exam is still being marked so it has not yet been accounted for.
Third Update: Now the lab mark has been included. Go and check yours!
Marks will be available in a few days in this same site as an update of this post.
Update: Exam marks are available in the same link as the attendance pages. Please note that solved exam contained a couple of errors that have been now fixed, just download a fresh version.
Second Update: It seems that some of you have trouble doing the math to know your current score, so now you have that numbers on the mark's page. I repeat what I explained the first day of class and it is detailed in Chapter0 slides.
mark(%) = attendance*10/classes + (9-assignments)*exam/10 + assignments*10 + labThe underlying logic is that exam only accounts for 70% of the final mark if you pass two assignments, 80% if you pass one assignment or 90% if you did not pass (or did not turn in) any assignment. Also note that failing on an assignment has not any negative effect on your score.
The same logic will be applied to the second term marking and then, both marks will be averaged to obtain your final grade.
Please remember the pass mark is 50%, so if you score less than this you will have to work harder on the second term to pass the subject.
Lab exam is still being marked so it has not yet been accounted for.
Third Update: Now the lab mark has been included. Go and check yours!
1st term exam
To be held January 30th, classroom B-5 at 10:00am laboratory exam.
For those of you who missed one or more laboratory sessions the exam will start at 10:30am. Duration two and a half hours.
For those of you who missed one or more laboratory sessions the exam will start at 10:30am. Duration two and a half hours.
Second assignment solution
It might not be the cleanest approach but I try to keep the line count low. You may download this server code that performs as required on the assignment#2 text.
What you may find weird is the fact that a timeout message is printed every 10 seconds when an upload is not taking place (therefore block number is zero). If you do not like this you can add an "if" sentence not to print that unless block number is larger than zero, but for me it serves as a sign that the server is up and running even when a file is not being received.
Second assignment marks
Ramazan, Anna, Manuel, Javier and Edgar have turned assignment#2 in. All of them have passed, though there are some comments, read on.
Some of them had the creative idea that a timeout event can stop the server. Let me tell you that "the show must go on" whatever it happens to the server. So you need to treat (catch) any exception that might cause the server to stop working.
On the other hand, I asked explicitly for the received files to be named in a certain way. Again some of you did not follow this part of the specification.
Edgar made the important observation that the performance when transmitting with this system is quite poor. And he also pointed out the cause: The use of a stop and wait protocol where the round trip time compared with the transmission time is quite significant.
Finally, to all of you that for any reason did not send me your work I want to encourage you to do it next time. I really think that assignment work can help you to improve your practical knowledge of network programming. Think of it like a little challenge and try not to follow the advice on this page.
Some of them had the creative idea that a timeout event can stop the server. Let me tell you that "the show must go on" whatever it happens to the server. So you need to treat (catch) any exception that might cause the server to stop working.
On the other hand, I asked explicitly for the received files to be named in a certain way. Again some of you did not follow this part of the specification.
Edgar made the important observation that the performance when transmitting with this system is quite poor. And he also pointed out the cause: The use of a stop and wait protocol where the round trip time compared with the transmission time is quite significant.
Finally, to all of you that for any reason did not send me your work I want to encourage you to do it next time. I really think that assignment work can help you to improve your practical knowledge of network programming. Think of it like a little challenge and try not to follow the advice on this page.
Class attendance and assignments records
By following this link you can check a copy of the records I keep about both attendance and assignments so you can double check them. If you detect an error please contact me ASAP to get it fixed.
Please bookmark it on your browser so you can return to it easily.
Please bookmark it on your browser so you can return to it easily.
Second assignment is out
As promised here you have the second assignment work that deals with UDP network programming. As in the first assignment, it is a client/server application. Again you have to program a small server that receives files from the client and writes them down to disk.
I have chosen an "invented" protocol as there are lots of Java code out there for many standard protocols (including TFTP). The good news is that I tried to keep it as simple as possible. The bad news is that my skills at writing the protocol specification are not on par with a good RFC. As usual, feel free to ask me any question you may have about the assignment.
Although I know you can copy and paste the client's source code from the PDF file, you can also find the source code file here.
First assignment results
I want you to know that Javier, Edgar, Aitor, Ana, Ramazan and Manuel turned in their work. Unfortunately, one of them failed to meet the problem requirements but the rest pass the test with flying colors.
Congratulations, not bad for your first networking problem.
Those of you that did not turn in anything please remember there are still three other assignments to come.
Network programming is like riding a bike: None of you learnt it by reading a manual, did you?
The many uses this blog
I was reviewing last year's posts and I realized that many of the posts are applicable now as we are going through the same chapters at the same time. So instead of reposting the same entries I invite you to check last year's entries along the whole course and there you'll find useful information, interesting links and related material.
Please note that assignments are different.
Class attendance record
These are the attendance numbers so far (out of 13 classes).
Please note that some of you are joining the class quite late. I understand that you may have other subjects too, but please note you'll need some effort to catch up with the class.
|
Please note that some of you are joining the class quite late. I understand that you may have other subjects too, but please note you'll need some effort to catch up with the class.
Sample IM server application
I've created a server application like the one you have to write for assignment #1.
You'll find it running on port 7777 on the IP address 158.42.53.17. If for some reason it is not accepting your connections, please let me now, as a power outage might have happened on my side and manual restart would be needed.
Please note that the messages about a new client entering the server or a client leaving the server are not required on your code. I've just added them to give users a bit more of information.
You'll find it running on port 7777 on the IP address 158.42.53.17. If for some reason it is not accepting your connections, please let me now, as a power outage might have happened on my side and manual restart would be needed.
Please note that the messages about a new client entering the server or a client leaving the server are not required on your code. I've just added them to give users a bit more of information.
News and reminders
Since Nov 16th we will be back to the "Seminari Nord" room. I've just posted the text of the first assignment, and I want to remind you that --although it is optional work-- doing it right will be worth 5 out of 100 points.
You do the four scheduled assignments right and you already have 20% of the subject.
You come to all the classes and you have 10% more.
Still not enough to pass the subject (you need 50%) but closer ...
Just a word of warning: Assignments are personal work, not team work. Your code will be checked against all the other submitters. If a copy is found all the involved parties will score 0% no questions asked. It is in your best interest to keep your work for yourself.
As with any other doubts you may have about the subject, I'm available during office hours for helping you solving them.
You do the four scheduled assignments right and you already have 20% of the subject.
You come to all the classes and you have 10% more.
Still not enough to pass the subject (you need 50%) but closer ...
Just a word of warning: Assignments are personal work, not team work. Your code will be checked against all the other submitters. If a copy is found all the involved parties will score 0% no questions asked. It is in your best interest to keep your work for yourself.
As with any other doubts you may have about the subject, I'm available during office hours for helping you solving them.
Programming Assignment #1
Instant Messaging Server
Your job is to write a Java application that will act as a the server for an Instant Messaging application. Clients will connect to the server [using a TCP connection] to talk to each other. Clients will type messages that are composed of one or more lines of text. Each time a user types a new line of text at the keyboard, that line will be transmitted to the server.
Each time the server receives a new line from any client, it will send a copy to all the clients but the one who sent it (i.e. to prevent a confusing double echo on the sending-client side).
Any client that wants to abandon the service MUST type a line with the text QUIT. Whenever the server receives such a line of text it will interpret its meaning, closing the socket this client was using. From them on, no attempt to send any more data to that client will happen (note: trying to send data to an already closed socket would trigger an exception).
Still, a client could leave the service by just closing the connection without the mandatory QUIT command. You do not need to manage that case though.
Your Code
Your code has to conduct the following tasks:
Accept any number of simultaneous clients, but it is safe to assume no more than one hundred simultaneous clients.
Retransmit any text line, including empty lines, to any active client but the one typing that text line.
Not to retransmit those lines which only contents is the QUIT command. Any combination of upper and lower case characters should be accepted and recognized (i.e: both QuiT and quit should work too).
The server program prints “NEW CLIENT” whenever a new client connects to the server and it prints “CLIENT GONE” when any client quits the system.
Some tips
Server listens on port 7777
You may use several telnet windows as clients of your server (for testing purposes) by just typing;
telnet localhost 7777
Due date
Your Java source code has to be submitted by email (address available on the front page) by Nov 30th.
If you have any doubt about the assignment I strongly suggest you to stop by my office during office hours (posted on the web).
News
I mentioned in the class this sample server program.
Next class will be held on Aula Informática, on the ground level, see the plane (click on the image to get a larger view).
A bird's eye view of the Internet
There are several projects aimed to provide such a view. This is just one of them.
A tree is built connecting lower level networks. As you can see a huge hierarchical view is shown, and that is just part of how it was the Internet some time ago as it is evolving on a daily basis.
Changing location
During the two first weeks of November we will have our classes in the "Aula Informática" so all of you can try for yourself with a networked computer how things really work. This will be from November the 2nd till November 10th.
FYI: For Your Information
Any submission to the IETF intended by the Contributor for publication as all or part of an IETF Internet-Draft or RFC and any statement made within the context of an IETF activity is considered an "IETF Contribution". Such statements include oral statements in IETF sessions, as well as written and electronic communications made at any time or place, which are addressed to:
All IETF Contributions are subject to the rules of RFC 3978 and RFC 3979.
Statements made outside of an IETF session, mailing list or other function, that are clearly not intended to be input to an IETF activity, group or function, are not IETF Contributions in the context of this notice.
Please consult RFC 3978 for details.
- the IETF plenary session,
- any IETF working group or portion thereof,
- the IESG, or any member thereof on behalf of the IESG,
- the IAB or any member thereof on behalf of the IAB,
- any IETF mailing list, including the IETF list itself, any working group or design team list, or any other list functioning under IETF auspices,
- the RFC Editor or the Internet-Drafts function
All IETF Contributions are subject to the rules of RFC 3978 and RFC 3979.
Statements made outside of an IETF session, mailing list or other function, that are clearly not intended to be input to an IETF activity, group or function, are not IETF Contributions in the context of this notice.
Please consult RFC 3978 for details.
Welcome to all of you
I've just returned from a trip to a Finish university and I've discovered there are thrity eight people enrolled on the subject this year. This is definitely our best score ever and I just hope most of you will actually stick to the English group (I'm afraid that some of you might have ended in this group because other groups are full).
I'll see you all on Thursday,
Miguel
I'll see you all on Thursday,
Miguel
September news
I am glad to inform you that Rami passed the subject.
Antonio: I'm glad to see you again enrolled in the subject. You are welcome to come to the class too, work permiting.
Antonio: I'm glad to see you again enrolled in the subject. You are welcome to come to the class too, work permiting.
Still reading this blog?
Well, maybe you failed the subject, sorry.
On the other hand, if you are planning joining us next course, please be welcome and feel free stoping by my office (I'll be out of town week 38th though).
There is just another opportunity for you to take an exam next September 14th, Thursday, at 16:00.
Best of luck (not that I think it has lots to do with the outcome),
Dr. Sanchez
On the other hand, if you are planning joining us next course, please be welcome and feel free stoping by my office (I'll be out of town week 38th though).
There is just another opportunity for you to take an exam next September 14th, Thursday, at 16:00.
Best of luck (not that I think it has lots to do with the outcome),
Dr. Sanchez
Solved Exam
It is here.
Please contact me if you want to have a look at your exam. Marks will be final in a few days.
Please contact me if you want to have a look at your exam. Marks will be final in a few days.
What not to do this summer
This list contains some of the IP ranges that you better avoid if you do not want a free trip to Guantanamo or another sunny place with the latest fashion in orange suits.
Subject's marks
Several students did not show up at the exams and they have no marks:
The rest of you, brave people, did get the following results:
Congratulations on your results. Unfortunately those below the 50% course mark have failed the subject, please remember we will have another exam in September covering THE WHOLE SUBJECT (assignments or lab exam are not available in September).
Your exam is available for you to check your results (just email me for a revision slot). In a few days I'll post the 2nd term exam answered.
ANTONIO, ALBERTO, JOSE LUIS and RAMON
The rest of you, brave people, did get the following results:
NAME EXAM LAB ASSIG. ATT. TOTAL COURSE_MARK
-------------- ---- --- ------ ---- ------- -----------
DIEGO 44 35 1 20 53.05 63.25
JORGE 62 70 0 0 64.60 67.29
TOMASZ 70 70 2 17 83.39 82.85
IGNACIO 62 45 1 17 71.49 79.05
LUISMANUEL 22 0 0 2 20.67 28.73
VLADISLAV 82 55 1 21 90.23 86.24
RAMI 38 0 0 2 36.37 21.15
Congratulations on your results. Unfortunately those below the 50% course mark have failed the subject, please remember we will have another exam in September covering THE WHOLE SUBJECT (assignments or lab exam are not available in September).
Your exam is available for you to check your results (just email me for a revision slot). In a few days I'll post the 2nd term exam answered.
Wold Cup telnet fun
I've just learned that some people are using the old and aged telnet application for new uses:
So try any of these live-streams based on telnet:
_ajjaa
_Q???4Qf
_,...,_ ) a/]QQb
.'@/~~~\@'. jQQba
//~~\___/~~\\ _, .?QQ#[ _
|@\__/@@@\__/@| ]m _.7 "asLaas_a/
|@/ \@@@/ \@| , ,\J#L -!4Wba
\\__/~~~\__// [aL[ \ \jmm jP
'.@\___/@.' ,b#'"[ \jmmmmm _P.
`"""""` a##' "4P#mmm# _ya
_P !4####m ?]aa/
/' aaJ#U###m# 4QP'
' aa,/4!44! '
jf _'jQQQQyb7b /
'. '.QQQQ4QQPb )?
QQQ'QQP?' jg/ f
_yQP']QQb aa
a#W?'..QQQQ?)? ?'
"##' _jQQP'
.j? [ jQQ'
aJ jmaaX#L???
? am'
_QjQQQ/
)QQQP?
4QQQ/
So try any of these live-streams based on telnet:
telnet ascii-wm.net 2006
telnet diego.ascii-wm.net 2006
telnet pinguin.eikon2.fs.ei.tum.de 2006
Warning: Exam ahead
As a reminder: Written exam will be held on June 23rd at 4:00pm. You either have to do the second term exam if you passed the first term or a final exam covering first and second terms if you didn't.
If you have attended to all the lab sessions please remember you are eligible for a written laboratory test, to be held afterwards (same day). Any score you get there will only add to your total score (it is my way of telling you should do it if you are eligible).
Those who fail the subject will have an exam in September covering all the subject.
Remember the final grade will include the marks from you class attendance and the assignments you turned in.
Till the exam, I'll be available to address any of your questions on my office hours (Mon & Tue 9:00 - 12:00).
Encryption ... rap?
Rapper MC Plus+ has created a song titled "Alice and Bob" based on the improbable theme of computer cryptography. It is freely available online. I'm not sure this will become a huge hit but, who knows?
New software threats
As I mention in the last class, I read this post where new types of malware activity are commented. While I am not afraid this will have a wide effect it is a new twist as to how some people can turn out a benefit from creating this particular type of sofware.
Last assignment is out
This last assignment is about finding the best route in a graph. This is part of what a routing algorithm does. Actually, a routing algorithm will search for all the best routes to all the destinations.
Moore's law of networking?
Apparently, there is a Metcalfe's law about the value of a network I have never heard before. All it all, it does not seem to make everybody happy. I guess Metcalfe will be better recalled because of Ethernet technology.
Getting to know an autonomous system
In this link you can learn more about this ISP's set of addresses. Comsat Colombia is the local branch of an international ISP that offers satellite Internet access. In the report all the BGP-related information is shown, so you can learn about their addressing space and the other AS they are routing through.
For those of you that feel happier with a closer ISP. Here you have the ONO-CableEuropa AS details.
Third assignment
As we are studying IP fragmentation, it seems appropriated to get some hands-on experience on the topic. This is what this third assignment is all about.
Please remember this is just an optional programming exercise dealing with computer network's concepts. But if you decide to go for it you'll get the double benefit of learning plus scoring a bit towards your final grade.
Please remember this is just an optional programming exercise dealing with computer network's concepts. But if you decide to go for it you'll get the double benefit of learning plus scoring a bit towards your final grade.
Back to work
Exams' period is over. We've got an interesting second term where we'll cover from IP network-layer to LANs and network security. For those of you new in the subject I want you to know that we'll have a couple programming assignments this term. For you all remember that attendance to the lectures while not mandatory is recorded and it will be marked (sometimes you can even learn something new).See you in class next Feb, 16th.
Exam results
You may want to get a copy of the exam with all the answers too. It's already here.
EXAM LAB.TEST ATTENDANCE TOTAL
Jorge 63% 55% 1 70%
Luis Manuel 38% 7 37%
Ignacio 73% 55% 27 87%
Alberto 41% 6 39%
Diego 59% 40% 22 73%
Johan 57% 20% 19 65%
Tomasz 67% 80% 20 82%
Vladislav 63% 100% 22 82%
Solved lab test is also available.
The column "total" includes your assignments too (if any).
Marks are calculated as follows:
TOTAL = EXAM * (9 - #ASSIGN) / 10 + #ASSIGN * 10 + ATTENDANCE * 10 / 27 + LAB.TEST / 10
Fall term is over
Lectures are interrupted till the beginning of Spring term. I guess we'll keep you busy with exams. Please remember there is still one week to turn in assignment #2.
My best wishes with the exams.
Johan, we'll miss you.
Maybe some new students will join us next term.
My best wishes with the exams.
Johan, we'll miss you.
Maybe some new students will join us next term.
Some help for Lab #5
I've just learned about this site. It contains interesting information about almost everything on the course. I'm not sure how good the book is but, never reject any extra help.
I recomend you to read and understand the goals of Lab #5 before going to the Lab. It might be more complicated that previous ones . You might need all session's time to complete the proposed exercises.
I recomend you to read and understand the goals of Lab #5 before going to the Lab. It might be more complicated that previous ones . You might need all session's time to complete the proposed exercises.
Assignment #2 is out
It can be obtained from here and it is due by January, 27th.
Please remember there are just a few days left to turn in assignment #1 (due by Dec, 13th).
Web-based nslookup
Easier than the nslookup command-line tool and independent of your operating system, this site gives you answers to any of your DNS records curiosity.
One example of securing POP3 with SSL
This example comes from Google's Gmail POP configuration.
Note the checked "Use secure connection (SSL)" on the left image (you will need to click on it to make it readable).
With it, any eavesdropper will see just random bytes instead of a nicely crafted ASCII dialog. So capturing username and password or the nature of the exchanged messages is now quite difficult.
Spanish Research Network Infrastructure
This one is the network UPV is connected to. As you can see (click on the image for a better view) Valencia is connected using several 2.5Gbps channels: Not bad!!
MIME and Transfer-Encoding
Common encoding schemes used by MIME include base64 and quoted-printable. Please note the goal here is to keep the text transmitted using SMTP protocol as 7-bit ASCII code regardless the original message contents. This transfer encoding will be undone on the receiving end to restore the original message. This way, images, sounds and any other binary contents can be sent as SMTP email.
Java Sockets Source Code
I've posted some of the Java source code we used in the classroom lately.
As we mentioned in the class, the conversion between bytes and characters depends on the encoding scheme. Most current Operating Systems no longer use ASCII but other schemes like UTF-8 to encode text-file contents. That is why we prefer the use of *Reader and *Writer classes instead of just the byte streams. (Why is encoding important?).
As we mentioned in the class, the conversion between bytes and characters depends on the encoding scheme. Most current Operating Systems no longer use ASCII but other schemes like UTF-8 to encode text-file contents. That is why we prefer the use of *Reader and *Writer classes instead of just the byte streams. (Why is encoding important?).
Java Sockets' Programming
HTTP result codes
Detailed list of HTTP result codes. But if you need any more detail you can check the current up to date RFC-2616. Please note that Postscript or PDF versions of the document are available in the same page in case you do not feel comfortable with the ASCII-only version.
First assignment is out
You may want to wait some days before starting to work on it as some topics have yet to covered in the lectures. But your first assignment is available. I want you to remember these are optional exercises you may or may not do. But if you do the assignment right (right meaning that it works), then you get a 0.45 points per assignment to be added to your final mark. If you are in doubt of what that means I suggest you to check the assessment details presented on chapter 0 slides.
I do not want to repartition my PC hard drive
You've noticed by now that our lab exercises are mostly Linux-based. This is for a reason: All the software we use at the lab is freely available. But it is very likely you have a Windows-only PC. Fear no more, you may use Knoppix (fantastic Linux distribution that boots up from a CD or DVD). It contains all that you need to complete most of the exercises without installing anything on your PC, just boot the system off the CD media. You can download the latest version from this FTP. As it is an on-campus mirror you should expect ultra-fast download. Just burn the ISO and that's all. You'll find a complete desktop, development tools, Java-support, audio and video players, and some games too. And you can't beat the price. Feel free to share it.
Having fun with the network
This applet shows propagation and transmission delays using animation.
This other applet shows queueing delay and packet loss at a network router.
Message segmentation is shown in this applet.
Some network uses are not fun anymore.
This other applet shows queueing delay and packet loss at a network router.
Message segmentation is shown in this applet.
Some network uses are not fun anymore.
Computer Networks' Lab is ready
Next Monday October 24th first Lab session will be held at the Computer Networks Lab (2nd Floor). Because of our delayed start, Lab exercise number one is skipped and we will start with exercise number two. Text of Lab exercises will be available on the Problems and Exercises book (still being printed). For the time being, you can find Lab exercises one and two in here.
Subscribe to:
Posts (Atom)
Posts
