Lab#3 Sockets

This week will start the Java Network Programming labs (first out of five) in the lab. You can get the non self-explanatory guide from here.


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.

Do you want to learn Node.js?

If you like Javascript, now you can run it outside of your browser. Not sure it is a great thing, but you may see that you can put your HTTP knowledge to use as you'll be able to follow the video.

Computer mysteries

I do not know why the domain upvnet.upv.es was not solved during the class but as soon as I got back to my office it worked as shown below:



$ nslookup -type=MX upvnet.upv.es
Server: 158.42.249.8
Address: 158.42.249.8#53


upvnet.upv.es mail exchanger = 10 albali.cc.upv.es.
upvnet.upv.es mail exchanger = 20 mxv.cc.upv.es.
upvnet.upv.es mail exchanger = 30 mx2.cc.upv.es.
upvnet.upv.es mail exchanger = 50 vega.cc.upv.es.
upvnet.upv.es mail exchanger = 10 mx4.cc.upv.es.

You can see there are five different servers to be used with the precedence indicated by the numbers, the lower  the number the more is preferred.

Please read sections 3.1 to 3.3 before next lecture.

Second lab

It's available here. It is about SMTP, POP3, FTP and DNS application protocols.

Lecture#7 previous reading

I forgot [again] to mention it at the end of the class: It's Kurose section 2.5 about the Domain Name System (DNS), a basic building block of almost all Internet services.

Lab#1

I've just got the first lab text. You can get it from the usual place

I'll explain some of it during the Lab.

We'll use this link for exercises #6 and #7 though.

Lecture#6 previous reading

Last class I forgot to tell you what to read for the next lecture. Here it is:

• Kurose2010, section 2.4
• RFC 2045 (MIME) section 6.8 (Base-64)
• http://es.wikipedia.org/wiki/Multipurpose_Internet_Mail_Extensions

Don't forget to post a comment if there is something you want me to elaborate in the next lecture. Don't leave for the last five minutes of the class to make your interesting questions.

Getting ready for the first lab

Next week we will start the Labs. In order to get ready for Lab#1 you need to get familiar with Wireshark application. You can read this document that explains how it works. As it is free software, you should have no problem downloading and installing it in your home computer, no matter what operating system you use there is a version available. I want you to capture the traffic to port 53 while you visit one or more web pages and to send me an email detailing the source and destination IP addresses and source and destiantion ports of the captured traffic.

Next week reading

Just as a reminder, please read section 2.2 of the book by October 14th lecture. Anything you find difficult or requiring more explanation, please add it as a comment to this entry.

Next lecture will explain how the World Wide Web service works and the different components it is made of, including the application protocol HTTP.

More exercises to do at home

These are a few more exercises to do at home. Please submit your solutions by email before October 14th class.

Transmission time

Last lecture left us with a confusing view of what transmission time really means. A bit of searching around did not help me a lot. The problem is that different people interpret the term in different ways:

1. Same as transmission delay = L /  R (the one I meant)
2. The amount of time it takes a bit to go from the start of the link to its destination node (first bit transmission time + propagation delay)
3. Transmission time is the time it takes a message to reach its destination from is source. It is the time between the first bit leaving the sender and the last bit arriving the receiver. The first bit leaves earlier and arrives earlier; the last bit leaves later and arrives later (packet transmission time + propagation delay).

Just to avoid trouble the best thing to do use to use the term transmission delay instead. Simple, isn't it?


Update: After discussing the problem with a friend of mine, he decided to make a move and fixed the wikipedia entry. Now it is also shown as meaning "1".

Class dynamics

This subject covers a wide variety of concepts. In the past our lectures have tried to cover them all. Now we are trying a different approach. That is why it is important that you read the section of the book mentioned at the end of each class before the next lecture. Next class you should have read section 2.1 of Kurose's book.

What we need to establish is a way for you to let me know, before the lecture, which topics of the reading you have had trouble understanding.

Please post as a comment any topic you want me to cover in more detail in the next lecture.