6 DS réseau du 21 juin 2004
As you know very well, there is a strong incitation to make you
fluent in English. The executive board of our school has make it clear
that every teacher must participate to this effort. So, even
given my difficulties with the barbaric micro$oft native language,
I decided to write this exam text in English.
All documents allowed (including computers, technical books, ...).
Exam duration : 2 hours.
The notation should be of 6 points for the first exercice on Internet
Protocols, 9 points for the Ethernet stuff, 5 points for the exercice
previously given by email and finaly 2 points for the RFC summary that
you had to publish in the TWiKi tool.
Traduction :
Comme il ne semble pas légal de vous imposer un texte
en anglais (faudrait quand même savoir ce que l'on veut) je vous traduirai
les passages les plus importants. Pour cette introduction cela se résume à :
tous documents autorisés, épreuve d'une durée de 2h.
6.1 About Protocols for Internet Services
Remember that an Internet application protocol is a communication standard
that a client uses to communicate with an Internet server. As an example,
consider a web browser : this Internet client must have a way to get the web
page the user has specified by typing an URL or clicking on a link. It does so
by connecting itself to the web server which name is given in the first
part of the URL. The default port for the connection is 80 for a web server
unless another one is given in the URL. After the connection is established
the client will ask the server for the page (specified in the second part
of the URL) using a special syntax which is known as the HTTP protocol.
Traduction :
Juste un rappel sans rapport avec les questions.
Let us give you the context of this exercice. We are in the office of
some system and network administrator (called "sysadmin" in the sequel
of this text). Some user tells the sysadmin that he suspects that the
mail server is down. The sysadmin use ssh to connect to the user
machine and start verifying that the sending mail process is functionnal
from this machine.
Traduction :
Un administrateur système se connecte sur la machine d'un utilisateur
pour vérifier que le serveur de messagerie est bien fonctionnel.
-
The data that may help you for the verification are that the user
machine name is iamthebest, the name of the mail server (the SMTP server)
is mail.someplace.com, the user email address is
wizeman@someplace.com and the sysadmin email address
is punchingball@someplace.com.
Traduction :
Utilisez la commande nc pour vérifier qu'il est
possible d'envoyer un message à partir de la machine iamthebest, en
utilisant le serveur SMTP mail.someplace.com, l'adresse d'expéditeur
wizeman@someplace.com et l'adresse de destinataire
punchingball@someplace.com.
- The SMTP server is OK, but the user says that he was also unable
to read emails. So help the sysadmin to test that email can be retrieved from
the IMAP server imap.someplace.com for user wizeman with the
password imustnotforgetit.
Traduction :
Utilisez la commande nc pour vérifier qu'il est
possible de lire un message sur le serveur IMAP
imap.someplace.com à partir de la machine iamthebest en
utilisant l'identifiant wizeman et le mot de passe
imustnotforgetit.
6.2 Ethernet Case Simulation
The aim of this exercice is to simulate the behaviour of a legacy Ethernet
network (the classical stuff: 10Mb/s, shared bus). Remember that the time
needed to send an Ethernet packet of n bytes on such a network is
n× 8/10,000,000 seconds. Moreover, if the distance between
two stations is d meters, the time needed for the propagation of a bit
from the first to the other is d/200,000,000 (we assume that
information is propagated along a copper cable at the speed of 200,000km/s).
Traduction :
Pour cet exercice de simulation de réseau Ethernet, il
vous est rappelé qu'un paquet Ethernet de n octets est transmis en
n× 8/10,000,000 secondes et qu'un bit se transmet à
200,000km/s sur un câble en cuivre.
The simulation will take place in a small network build with an unique
500 meters long Ethernet bus. There is a station at each end of the
cable and a third located 100 meters from the first one and 400
meters from the second one. The simulation begin at time t=0 with
the emission of an Ethernet packet of 192 bytes from station 3.
Traduction :
La simulation se fera dans le cadre du réseau représenté
par ce schéma :
La simulation débute par l'emission d'un paquet Ethernet de 192 octets
par la station 3.
-
At which time will station 3 end its transmission ? Do
not forget to take into account the fact that an Ethernet packet is sent
after a preamble of 64 bit times (Donnez la date de fin de transmission
pour la station 3, n'oubliez pas qu'un préambule d'une longueur équivalent à
8 octets est envoyé sur le câble avant tout paquet Ethernet).
- Will S1 and S2 be aware that the medium is free at the same time
(Les deux stations S1 et S2 sauront-elle que le bus est libre en même
temps) ?
Give the date(s) at which the stations know that the Ethernet bus is free
(Donnez la date à laquelle les stations savent que le bus est libre).
- Now, let us assume that the network interfaces of the stations S1
and S2 received, respectively at t=54µ s for S1 and t=134.6µ s for S2,
a packet to be transmited (On suppose que S1 et S2 ont reçu à t=54µ s
et à t=134.6µ s des paquets à transmettre). Both packets have a length of
64 bytes (Ces paquets font 64 octets). When will these packets be
sent on the network (Quand ces paquets seront-ils envoyés sur le réseau) ?
Take into account that, in the Ethernet protocol, an inter-frame space of
9.6µ s must be respected before assuming that the medium is free (Notez
qu'un delai inter-packet de 9.6µ s doit être respecté).
- What happens to the two packets sent by S1 and S2 as stated above
(Qu'arrive-t-il à ces deux paquets) ? Give the exact date at which this
event begin - beware, you will have to solve an equation to obtain
the beginning time (Donnez la date de début de cet événement).
- When will the collision episode end for the stations S1 and S2
(Quand l'évemenent de la question précédente sera-t-il terminé pour
S1 et S2) ? Give the exact dates (Donnez les dates précises).
Remember that 4 padding bytes are send by a station after a Collision
Detection (Notez que 4 octets de bourrage sont envoyés par une
station après la détection d'une collision).
- Let us assume that station S1 has a quite broken random generator
that only gives zeros and that S2 has the same type of random
generator which only gives ones (Supposons que le générateur aléatoire de
S1 ne tire que des 0 et que celui de S2 ne tire que des 1). Give the date
at which the packets of S1 and S2 will effectively be sent (Donnez les
dates réelles d'envoi des paquets de S1 et S2).
6.3 Free Time Exercice on FTP Protocol
Since this exercice has already be given in french by email I think it is
not too unfair to give only its English translation
(Questions libres posées par messagerie électronique).
The context of these
questions is the one of the laboratory classes
The questions are :
-
It is possible to establish a FTP connection from your virtual machine
to real one ? In this connection context, is it possible to make some
transfers in active mode ? Why ? In the case of a negative answer suggest
a modification of the virtual machine configuration to allow such a transfer.
- Is it possible to establish a FTP connection from your virtual machine
to the school FTP server ? In this connection context, is it possible to make
some transfers in active mode ? Why ? In the case of a negative answer suggest
a modification of the virtual machine configuration to allow such a transfer.
