INSTITUT NATIONAL DES TELECOMMUNICATIONS. MSc Computer and Communication Networks MSc Information Technology Final Examination

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1 INSTITUT NATIONAL DES TELECOMMUNICATIONS Course: Computer Networking (CCN112) Duration: 2 hours MSc Computer and Communication Networks MSc Information Technology Final Examination Date: 13 February 2007 Coordinator: Pascal HENNEQUIN Lectures notes and paper docs are authorized, Internet access and electronic docs are forbidden. Indicative timing and grading: Part Total time 40min 40min 30min 5min 1min 116min grade 6p. 5p. 7p. -1p. to 3p. 0p. -1p. to 21p. 1 HDLC Trace Consider the following trace given by a protocol analyzer capturing HDLC frames on a point-to-point Data Link between 2 Hosts A and B. Each line describes one frame and gives: a frame index: capture order direction for the frame between A and B Numb. Way Type N(S) N(R) P or F 1 B A I A B I A B I B A I B A RR 7 6 A B RR 0 7 B A I A B I B A REJ 7 10 A B I B A I 0 0 P 12 A B I A B RR 1 F 14 B A RR 1 HDLC frame type: I=Information, RR=Receive Ready, REJ=Reject N(S) counter: Number of Information to Send. N(R) counter: Number of next Information to Receive. P/F bit indication: Poll/Final

2 1.1 Accurately build the chronological schema of the interchange, and find out important events (frame crossing, frame lost, timeout) which occur during the communication. 1.2 Describe incidents which occur during the interchange, and explain how they are solved by HDLC recovery mechanisms. 1.3 What could be said about the Window sizes (each way) here? minimum size, maximum size? 1.4 Assume that this trace fits with an X25 Communication: layer 3 protocol uses X25 Packets. How many X25 Packets occur each way during this interchange? Explain how to count these packets. 1.1 Solution PA1 PA2 PB1 A I 5,6 I 6,6 I 6,5 I 7,6 B PA1 PA2 PB1 PB2 PB2 RR 7 RR 0 PA3 I 7,0 I 0,7 PB3 PA4 I 0,0 REJ 7 Reprise Tempo T1 I 7,0 PA3 I 0,0 P I 0,0 PB3 RR1 F RR 1 PA4 : Packet Send Request / Packet delivery Indication From A to B, I7 is lost, detection on next frame I0 and recovery by a REJ. From B to A, I0 is lost, retransmission on timeout with bit P/F indication Counters are modulo 8. This implies a maximum window size of 7. Host A send I5 and I6 before receiving Ack for I5, so Window is at least 2. From B to A, 2 frames (I6, I7) sent before Ack for the first one, so Window is at least From Encapsulation principle, X25 Packets are Information field in Info Frame. So 4 packets from A to B in frames I5,I6,I7 and I0. 3 packets from B to A in frames I6,I7 and I0.

3 2 Layered Architecture 2.1 Explain the principles of Layered Network Architectures as OSI model and TCP/IP. Especially define the concepts of Layer, Protocol, layer Interface or Service. 2.2 Describe the encapsulation function and explain how this function operates to ensure the operational running of layered architectures. 2.3 Explain the difference between connection-oriented and connectionless protocol. Illustrate with two layer 3 protocols and two layer 4 protocols. 2.4 Give the name and the main purpose of each layer in the OSI model 2.5 Compare the Internet layered model to the OSI reference model 2.1 Solution... 3 Fast Questions, quickies 3.1 Two IPv4 Networks are defined using CIDR notation by: Net1 = /18 and Net2 = /20. Which one is the biggest (contains more host addresses) and how many time bigger? Net1 is 2 (20 18) bits less for network part, 2 bits more for host part. So, Net1 is 4 times bigger than Net2 3.2 Give the IPv4 Network Mask for Net1 = /18. Mask = =2Bytes+2bits and bin( )=128+64= Give the CIDR notation for Addr= , NetMask = hints: = 224 Net = / = =bin( ), so mask is 2 Bytes + 3 bits 3.4 The HDLC protocol uses fixed value for retransmission timer. TCP uses an adaptive retransmission algorithm. Explain why, and how it relates to protocol layer. Transmission Delay are fixed in Layer 2, and highly variable at layer 4 (different connnections and also during the same connection) 3.5 Define the term Anycast Wikipedia says : each destination address identifies a set of receiver endpoints, but only one of them is chosen at any given time to receive information from any given sender.. In unicast, destination is a unique host and not a set of host. In multicast, all destinations receive a copy of the packet. 3.6 How are organized the data in a LDAP directory? A tree

4 3.7 What is the use of DN in the LDAP protocol? Give an example of DN. Indexing directory entries, Explain the relationship between SASL and LDAP. SASL is used in LDAP (or in other applications) to managed strong authentication and authorization of directory access (read and/or write) 3.9 Explain shortly (1 or 2 lines by iteration!) the iterative resolution process for a DNS query on the name step 1 : Ask about to any Authoritative DNS server for zone.. The answer is the list of Authoritative servers for zone.it. step 2 : Ask to any.it. server and obtain server list for zone CCN.IT. step 3 : Ask to any CCN.IT. server and obtain the final response You can illustrate the answer using the following active DNS Resource Records: FQDN TTL CLASS TYPE DATA IN NS A.ROOT-SERVERS.NET IN NS B.ROOT-SERVERS.NET IN NS C.ROOT-SERVERS.NET IN NS D.ROOT-SERVERS.NET IN NS E.ROOT-SERVERS.NET IN NS F.ROOT-SERVERS.NET IN NS G.ROOT-SERVERS.NET IN NS H.ROOT-SERVERS.NET IN NS I.ROOT-SERVERS.NET IN NS J.ROOT-SERVERS.NET IN NS K.ROOT-SERVERS.NET IN NS L.ROOT-SERVERS.NET IN NS M.ROOT-SERVERS.NET. CCN.FR IN NS ns1.amen.fr. CCN.FR IN NS ns2.amen.fr. CCN.IT IN MX 5 mailserver.ccn.it. CCN.IT IN NS dns2.geniusnet.it. CCN.IT IN NS dns3.nic.it. IT IN NS DNS.NIC.IT. IT IN NS J.DNS.IT. IT IN NS M.DNS.IT. IT IN NS NAMESERVER.CNR.IT. IT IN NS NS-IT.RIPE.NET. IT IN NS R.DNS.IT. IT IN NS S.DNS.IT IN A IN A Using these DNS RR data : step1 : choose 1 from the 13 wellknown root servers (Records. NS ), i.e H.ROOT-SERVERS.NET.. Ask about The answer is the set of the 7 IT. NS records

5 step2 : choose 1 from the 7.IT. DNS Server, i.e NS-IT.RIPE.NET.. Ask about and obtain the set of the 2 CCN.IT. NS records step 3 : Ask to dns2.geniusnet.it. or to dns3.nic.it. and obtain the final response (A record) IPv4 address for is PostScriptum: site is call Center Network in Italia, site is Centre Cardiologique du Nord in france 4 Faster Questions 4.1 Receiving an HDLC frame with a wrong FCS (Frame Check Sequence), the HDLC protocol must: (a) send back a REJect frame (b) forget received frame (c) make a computation using the FCS to recover the transmission error the best to do is nothing (forget received frame) 4.2 X25 Data packets contain the Destination Address false. Addresses occur only during connection establishment. In data packet only a Virtual channel number identifies the connection 4.3 IP packets contain the Source and the Destination Addresses True. Connectionless protocol implies Addresses in all packets 4.4 The IPv4 TTL field (a) disappears in IPv6 to optimize router performance (b) is just renamed in IPv6 with the same behaviour (c) is extended in IPv6 to take into account the growing of the Internet loop prevention during routing is still important in IPv6, so TTL is just renamed 4.5 The IPv4 Checksum field (a) disappears in IPv6 to optimize router performance (b) is just renamed in IPv6 with the same behaviour (c) is extended in IPv6 to offer some security features Checksum is not so usefull at network layer and disappears in IPv6

6 4.6 An Intermediate IP router must manage the fragmentation mechanism for IPv4 and for IPv6 (b) True for IPv4, False for IPv6 (c) False for IPv4, True for IPv6 (d) False for IPv4 and for IPv6 fragmentation could be hop by hop in IPv4, but is required to be end-to-end in IPv6 4.7 LDAP can replace DNS False. DNS is a globally unique directory. LDAP is a collection of independant local directories. 4.8 LDAP can replace NIS (aka Yellow Pages) True. Local Information Database in the 2 cases 5 Not a Question 5.1 DNS means (a) Don t forget your Name on examination Sheets! (b) Domain Name System, but doesn t matter here! Quick solutions from Pascal Hennequin

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