App. App. Master Informatique 1 st year 1 st term. ARes/ComNet Applications (6 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 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. 1 Applications (6 points). QUIT +OK Bye-bye. Connection closed by foreign host. 1. What is the user doing? What are the application layer protocols employed by the user? 2. Give the command line employed by the user to connect to the server. The following presents the messages exchanged between a user and a server: Trying Connected to mail.etu1.plateforme.lan. Escape character is ^]. +OK Hello there. USER etudiant +OK Password required. PASS mmqos +OK logged in. LIST +OK POP3 clients that break here, they violate STD RETR 4 +OK 714 octets follow. Return-Path: <etudiant@mail.etu1.plateforme.lan> X-Original-To: etudiant@mail.etu1.plateforme.lan Delivered-To: etudiant@mail.etu1.plateforme.lan Received: from [ ] (unknown [ ]) by mail.etu1.plateforme.lan (Postfix) with ESMTP id D5BEB1C467 for <etudiant@mail.etu1.plateforme.lan>; Thu, 16 Oct :04: (UTC) Subject: =?ISO ?Q?=E7a?= marche! From: etudiant sur la VM3 <etudiant@mail.etu1.plateforme.lan> To: etudiant@mail.etu1.plateforme.lan Content-Type: text/plain Date: Thu, 16 Oct :24: Message-ID: < camel@1vm1.etu1.plateforme.lan> Mime-Version: 1.0 X-Mailer: Evolution Content-Transfer-Encoding: 7bit 3. What is purpose of the LIST command? What does mean in the response? 4. What does the user want to do when typing RETR 4? 5. In the message retrieved by the user, indicate four header lines that have been added by the mail server. 6. The subject of the message entered by the user has been re-encoded by the application. What is the original characterencoding scheme of the subject? What is the encoding scheme used for re-encoding? What is the format of the re-encoded text? 7. What is the content of the message? 8. If the user wants to delete this message, write down the necessary command. Bravo!!! 1/12 Version X en-vA-b 2/12 Version X en-vA-b

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 A B You must write your answers in the frames provided for that purpose Transport layer (7 points) To illustrate a part of an exchange of TCP segment between host A and B, we use the following formalism: A sloping arrow from A s timeline to B s timeline (or going the other direction) represents a segment (a TCP packet), with the number of bytes of application-layer data being shown in the pentagon attached to the arrow. Each segment is annotated by a label attached by a thin line, containing the following information: the sequence number of the segment () the acknowledgment number of the segment () the value of the advertised Receive Window (RWIN). Thus, in the example below, we have: A segment of 1200 bytes of data going from A to B. Its sequence number is 9000, its acknowledgment number is 8300 and its advertised Receive Window is A second segment, zero size, is emitted from B to A before receiving the previous one. Its sequence number is 8300, its acknowledgment number is 7000 and the advertised Receive Window is A B Explain the probable reason for the temporary cessation of transmission of segments from A to B around the middle of this exchange (justify your answer). Fill in the blanks for sequence numbers, acknowledgement numbers, and Receive Window sizes in the following portion of a TCP exchange that has already begun. 3/12 Version X en-vA-b 4/12 Version X en-vA-b

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 (7 points) 1. 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 (page 7) to help in the analysis c d1 c3 dc 54 ed e e T...^..N..E. 00c fb d b8 dc 14 c ]...p 00d f e c t...ns1.p 00e0 c f e p...t...ns2 00f0 c0 70 c0 b p...4..H c0 b0 00 1c P...4..& c0 c2 ( c d f e3 4a e3.<...?..h..j" c0 c h.p e 4d de e d NM.5.y.( c &.( c 65 6d 6f 6e c0 0c de.fr ca 00 2f c 65 6d 6f 6e..7../ e 32 2d d d de.fr d d.cdx.cedexis e c0 2c e 00.net.., b wac.3604.edgec The frame you have just decoded was sent in response to another frame. Below, write out that frame in hexadecimal format, suing two hexadecimal digits per box. (If you cannot deduce the value in a box, leave it blank. Byte num Which action from the user may have initiated this message exchange (justify)? 00a e c0 56 c astcdn.v.g... 00b0 09 1a 00 0a c0 70 c0 8a...gp1.wac.p.. 5/12 Version X en-vA-b 6/12 Version X en-vA-b

4 Don t hand back this sheet! Don t hand back this sheet! Appendix 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 TCP Segment Layout <-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 = negotiate MSS 3 = adaptation of window size 4 = selective acknowledgment 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 A querry: <-----N-Bytes-----><2-Bytes><2-Bytes> Name Type Classe An Answer/AutoritativeNS/AdditionalRecord: <M-Bytes>< 2B. >< 2B. ><4-Bytes>< 2B. ><--D-Bytes---> Name Type Classe T.T.L. Size Data * Name : each domain name is preceded by one byte showing the number of following ASCII characters (if the value is < 63 else the value 0xC0 show a reference to the offset value of the next byte (from the beginning of the message) Ending by 0x00. * A few types: 1 = A (IPv4 Address) 12 = PTR (Name Pointer) 2 = NS (Name Server) 15 = MX (Mail Server) 5 = CNAME (Alias) 28 = AAAA (IPv6 Address) 6 = SOA (Start Of Authority)... * Classe: 1 = Internet * T.T.L.: Cache Validity (seconds) * Size: Data length in Bytes * Data: - Name (Character string as for a query) for NS - Priority (2 bytes) and Name for MX - Address (4 bytes for A, 16 bytes for AAAA)... 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 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 Type Code Datagram Checksum Variable... Original Datagram + 8 Bytes A few ICMP types: 0 = Echo request 3 = Destination Unreachable 5 = Redirection 8 = Echo response 11 = Time exceed DNS Messages Layout < 2B.>< 2B.><2B.><2B.><2B.><2B.>< qb.>< ab.>< nb.>< ib.> Ident Flags NbQu NbAn NbNS NbAR Quer. Ans. AutNS. AddR * Ident. = Request Identifier * Flags = DNS parameters (don t explain) * NbQu = Number of Queries * NbAn = Number of Answers * NbNS = Number of Autoritative Name Server * NbAR = Number of Additional Reccords 7/12 Version X en-vA-b 8/12 Version X en-vA-b

5 Don t hand back this sheet! Don t hand back this sheet! 9/12 Version X en-vA-b 10/12 Version X en-vA-b

6 Don t hand back this sheet! Don t hand back this sheet! 11/12 Version X en-vA-b 12/12 Version X en-vA-b