App. App. Master Informatique 1 st year 1 st term. ARes/ComNet Applications (7 points) Anonymous ID: stick number HERE

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1 Master Informatique 1 st year 1 st term Anonymous ID: stick number HERE Master Informatique 1 st year 1 st term App ARes/ComNet Midterm exam : Version A in English Duration: 2h00 Allowed: One handwritten A4 sheet (front and back) Forbidden: All other documents, pocket calculators, mobile phones, etc. App 3. Hosts A and B are located in the same subnetwork of the same LAN, with a 1Gbps Ethernet infrastructure. They are now separated by a firewall that enables all ports from A to B, but that only enables ports 23 and 6000 from B to A. Thefirewall adds a 10ms delay in each direction. How to modify commande.txt? The exam consists of 3 sheets recto/verso, including the questions and spaces in which to write your answers. You must hand back only these sheets at the end of the exam. To ensure anonymity, you must not write your name on the exam. Be absolutely sure that the anonymous ID number assigned to you by the exam supervisors appears on each sheet. You must write your answers in the frames provided for that purpose. 4. We consider the same configuration as in Question 3. What is the latency for host A to download file config.txt (we assume its size 300 bytes, and that the transmission time is negligible)? 1 Applications (7 points) We consider a host A running Unix on which the classical text-mode FTP client ftp is installed, and a host B running Unix on which a FTP server with default options is installed. To get the IP address of host B when it is not in its local DNS, host A must access five DNS servers (three of them have a RTT of 100ms, the other have a RTT of 80ms). To get the IP address of host A when it is not in its local DNS, host B must access six DNS servers (two of them have a RTT of 120ms, the other have a RTT of 100ms). Local DNS have LAN hosts resolution informations and is used as cache for other DNS resolutions. Local DNS access delay is negligible. Hosts A and B feature an 1Gbps Ethernet card. Throughout the section, we assume a TCP segment is 500 bytes. Host A is to regularly download file config.txt from host B, so we execute the following command on host A: 5. Hosts A and B are now located at two di erent internet hosting services. Host A is protected by a firewall configured as in Question 3. The throughput between the two internet hosting services is 100Mbps, and the RTT is 120ms. How to modify commande.txt? ftp B < commande.txt where commande.txt contains: pipo toto get config.txt 1. The < Unix shell operator redirects the contents of the argument file to the standard input of the executed program. Explain the steps following the execution of the command on Machine A. 6. We consider the same configuration as in Question 5. What is the latency for A to download file config.txt (we assume its size 300 bytes, and that the transmission time is negligible)? 2. Hosts A and B are located in the same subnetwork of the same LAN, with a 1Gbps Ethernet infrastructure, and a symmetric 20ms RTT. What is the latency for A to download file config.txt (we assume its size 300 bytes, and that the transmission time is negligible)? 7. We consider the same configuration as in Question 5. The file config.txt is now 3000 bytes long. If we assume a negligible transmission time, what is the evolution of the latency compared to Question 6? 1/12 Version X en-vA-a 2/12 Version X en-vA-a

2 Master Informatique 1 st year 1 st term Anonymous ID: stick number HERE Master Informatique 1 st year 1 st term Trp ARes/ComNet Midterm exam : Version A in English Duration: 2h00 Allowed: One handwritten A4 sheet (front and back) Forbidden: All other documents, pocket calculators, mobile phones, etc. Trp The exam consists of 3 sheets recto/verso, including the questions and spaces in which to write your answers. You must hand back only these sheets at the end of the exam. To ensure anonymity, you must not write your name on the exam. Be absolutely sure that the anonymous ID number assigned to you by the exam supervisors appears on each sheet. You must write your answers in the frames provided for that purpose. 2 Transport layer (7 points) 1. The initial sequence number of TCP connection always start by 0? (True or False) Justify your answer. 7. The following figure shows the congestion window (CWND) evolves over time for a TCP Reno. The x-axis represents the round trip times (RTT) and the y-axis represents the congestion window size in Kbytes. The initial SSthresh (Slow Start threshold) is 16 kbytes. 2. HTTP/1.0 uses the same TCP connection to transfer all the objects in the web page? (True or False) Justify your answer. 3. What is the maximum TCP options length? Justify your answer. 4. TCP window scale option is used when the bu er may be larger than 64 Kbytes. What could be the e ective receiver window size when the advertised receiver window size is 1000 and window scale option (scale factor) is 8? 5. In 2012, a researcher proposed recommendations to make TCP faster. One is to decrease the initial retransmission time out (RTO) from 3s to 1s. For a network with average RTT of 1.5s, will you use this recommendation or not? Justify. (a) What is the value SSthresh at A? (b) What causes the reduction of the congestion window after B? (c) What is the value SSthresh at C? (d) What causes the reduction of the congestion window after D? (e) What is the value SSthresh at E? (f) What causes the reduction of the congestion window after F? 6. A company developed a new client-server application using UDP as a transport layer. After while they found the direct use of UDP is unsuitable for their application due to high packet losses. Then they propose to use a stop and wait protocol over UDP to achieve reliable transfer and as a proof of concept, they use the following network to test the application. The application sends packets with same size 1000 bytes. The RTT between the two hosts is 10ms and the link capacity (C) is 10 Mbps. (a) What is the maximum throughput of the new application over this networks if we ignore the overhead from headers and there is no packet loss? (b) Propose a solution to improve the application performance. 3/12 Version X en-vA-a 4/12 Version X en-vA-a

3 Master Informatique 1 st year 1 st term Anonymous ID: stick number HERE Master Informatique 1 st year 1 st term Frm ARes/ComNet Midterm exam : Version A in English Duration: 2h00 Allowed: One handwritten A4 sheet (front and back) Forbidden: All other documents, pocket calculators, mobile phones, etc. Frm The exam consists of 3 sheets recto/verso, including the questions and spaces in which to write your answers. You must hand back only these sheets at the end of the exam. To ensure anonymity, you must not write your name on the exam. Be absolutely sure that the anonymous ID number assigned to you by the exam supervisors appears on each sheet. You must write your answers in the places provided for that purpose. 3 Multi-protocol analysis (6 points) Decode the following frame. Draw and write directly on it. Carefully delimit each protocol field and provide an accurate interpretation of its value. You may refer to the Appendix 1 (page 7) to help in the analysis.. 5. The frame you previously decoded belongs to an exchange that began with three successive messages: The frame 1 recalled below is the one you just analyze. This frame is followed by the frame 2 then the frame 3. Knowing the frame 1 and frame 3, filltheframe 2 with 2 hexadecimal digits per box (write XX in boxes whose value can not be deduced). Assume that the same options as those of the frame 1 are present (with their updated values if needed). frame e7 8c a9 46 b c bf de ff c0 a8 00 c0 6a ba a7 8f cd a a a b a de 3f frame e7 8c a9 46 b F...E c bf de ff c0 a8 00 c0 6a ba.<.%...j a7 8f cd a a0 02 pt...p.6a a b a r.c...!h 0040 de 3f ?... Num. octet Is the studied frame complete? Justify. frame e7 8c a9 46 b What are the observable protocols? Justify bf df 06 c0 a8 00 c0 6a ba a7 8f cd b 9a f What are the noticeable addresses (shown in conventional format)? e5 f5 1a a de c 0040 ec What are the options? 5/12 Version X en-vA-a 6/12 Version X en-vA-a

4 Don t hand back this sheet! Don t hand back this sheet! Appendix 1 Ethernet Frame Layout without preamble/crc: bits bits--+16b Destination Source Type Data address address A few types: 0x0800 = DoD Internet (IPv4) 0x0806 = ARP 0x86DD = Internet Protocol Version 6 (IPv6) IPv4 Paquet Layout <-4b-> <--8bits--->< bits > Ver IHL TOS Total Length Identifier Fl FO TTL Protocol Header Checksum Source Address Destination Address... Options... Data Ver = IP Version IHL = IP Header Length (32 bits words) TOS = Type of service Total Length of the IP paquet (in bytes) Fl (3 left bits) = fragmentation flags [Reserved Don t Fragment Next Fragment] FO (remaining 13 bits) = Fragment Offset * value to multiply by 8 bytes TTL = Time To Live (hop count) A few protocol fields: 1 = ICMP 33 = DCCP 2 = IGMP 41 = IPv6 Encapsulation 6 = TCP 89 = OSPF 8 = EGP 132 = SCTP 17 = UDP... ICMP Datagram Layout Type Code Datagram Checksum Variable... Original Datagram + 8 Bytes A few ICMP types: TCP Segment Layout 0 = Echo response 3 = Destination Unreachable 5 = Redirection 8 = Echo request 11 = Time exceed <-4b-> <-6bits->< bits > Source Port Destination Port Sequence Number Acknowledgement Number THL Flag Windows Size Segment Checksum Urgence Pointer... Options... Data THL = TCP header length on 4 bits (32 bits words) Flags on 6 bits (from most significant bit to less) = < URG, ACK, PSH, RST, SYN, FIN > Options = series of options encoded as: 1 byte 00 = end of options (if needed) 1 byte 01 = no operation L bytes with TLV fields (T one byte of type: 2 = Maximum segment size 3 = Window scale 4 = TCP SACK Permitted 8 = Timestamps... L one byte for the total size of the option V the value of the option on L-2 bytes) UDP Datagram Layout Source Port Destination Port Longueur UDP Datagram Checksum... Data A few ports associated services ftp-data 20/tcp domain 53/udp ftp 21/tcp tftp 69/udp ssh 22/tcp snmp 161/udp telnet 23/tcp snmp-trap 162/udp smtp 25/tcp www 80/tcp... 7/12 Version X en-vA-a 8/12 Version X en-vA-a

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