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CSSE 432 -- Computer Networks -- Spring 2005 Midterm Exam Answers 1. [15 points] Internet Protocols: a) What is the difference between a stop-and-wait protocol and a sliding window protocol? A stop-and-wait protocol waits for each sent packet to be acknolwedged before sending the next packet. In a sliding window protocol a limited number of messages are sent before receiving any acknowledgements. b) Which layers in the Internet protocol stack are processed by each of the following: i) Router 1-3 ii) End system 1-5 iii) Server 1-5 c) Name the transport protocol used by each of the following applications: i) Email -- TCP i) WWW -- TCP ii) DNS -- UDP ii) Internet Telephony -- UDP iii) FTP -- TCP 2. [10 points] TCP Congestion Control a) Name all the types of events that cause the TCP congestion window to increase in value. Indicate how much does it increases for each type of event. New ACK -- if the congestion window is currently below the threshold, then increase by 1 MSS for each successful ACK (exponential growth), if congestion window is at or above the threshold, then increase linearly (1 MSS for each RTT) b) Name all the types of events that cause the TCP congestion window to decrease in value. Indicate how much it decreases for each type of event. Triple duplicate ACKs -- half of its current value Timeout -- set to 1 MSS 3. [10 points] Checksums 0000011100001011 0000110000000101 0000100000001010 0000111100000101 0000101100001001 0000101000000011 ----------------------------------0 0 1 1 1 1 1 1 0 0 1 0 1 0 1 1 sum 1 1 0 0 0 0 0 0 1 1 0 1 0 1 0 0 complement 1 CSSE 432 -- Computer Networks -- Spring 2005 Midterm Exam Answers 4. [25 points] Distance Vector Protocol Given the following network, compute the distance tables for the first 3 stages of the distance vector algorithm. X 2 7 Z 3 Y ∞ ∞ ∞ ∞ X X 0 Y 7 Z 2 to Y Z 5 2 0 3 3 0 X X 0 Y 5 Z 2 to Y Z 7 2 0 3 3 0 X X 0 Y 7 Z 2 to Y Z 7 2 0 3 3 0 from X X 0 Y ∞ Z ∞ to Y Z 7 2 from from Node X X X 0 Y 5 Z 2 to Y Z 5 2 0 3 3 0 X X 0 Y 5 Z 2 to Y Z 5 2 0 3 3 0 X X 0 Y 5 Z 2 to Y Z 5 2 0 3 3 0 X ∞ Y 7 Z ∞ Z ∞ ∞ 0 3 from from X to Y ∞ ∞ from Node Y X ∞ Y ∞ Z 2 Z ∞ ∞ ∞ ∞ 3 0 from from X to Y 2 from Node Z CSSE 432 -- Computer Networks -- Spring 2005 Midterm Exam Answers 5. [40 points] Vocabulary 11 iterated query 2 circuit switching 8 fragmentation or segmentation multiplicative decrease Domain Name System (DNS) flow control 4 cookie 19 23 6 service model stop-and-wait Dynamic Host Configuration Protocol (DHCP) virtual circuit query flooding 12 5 25 17 bootstrap node 15 3 Post Office Protocol 3 (POP3) Internet Mail Access Protocol (IMAP) Simple Mail Transport Protocol (SMTP) HyperText Transfer Protocol (HTTP) Selective Repeat (SR) pipelining conditional get 7 fast retransmit 16 propagation delay queuing delay socket 10 20 9 18 22 a method of handling unresolved queries for DNS information where the contacted server does not try to resolve the name but passes back the name of another server to ask a type of network where the resources along a path are reserved for the duration of the communication session breaking large packets into smaller segments in order to pass them onto another network decreasing transmission rate quickly (halving it) when congestion is observed distributed database of mapping between host names and IP numbers a mechanism to prevent either side of a connection from overwhelming the other identifier that is associated with a user so that web server can keep track of state of session or previous history of user the guarantees that a protocol layer provides waiting for an acknowledgment before sending the next packet get IP address dynamically from server source-to-destination path behaves much like telephone circuit method of propagating queries to other peers in a distributed P2P network node in a P2P network that provides connection to other nodes when a new peer joins the network protocol used between mail server and a user agent where the server keeps no state about the user between sessions protocol used between mail server and a user agent where the server keeps track of the user's state between sessions protocol used between mail servers to exchange email protocol used by web browsers and servers protocols that only retransmit packets that were lost or corrupted sending multiple packets without waiting for acknowledgments server checks modification date of requested object; if client has latest version then no object is sent TCP retransmits a missing segment before its timer expires when 3 duplicate ACKs are received the time required to move a bit along a link the time that a packet spends in a queue waiting to be transmitted a host-local; application-created; OS-controlled interface into which an application process can send and receive messages from another application process 3 CSSE 432 -- Computer Networks -- Spring 2005 Midterm Exam Answers Here are the terms that were not matched: 1 additive increase 13 Network Address Translation (NAT) 14 out-of-band 21 slow start 24 User Datagram Protocol (UDP) increasing the transmission rate slowly (adding a small amount) when congestion is not observed mapping several host IP addresses to one IP address and new port numbers so that the local network can be hidden a method of signaling that uses a separate channel than the channel containing the data beginning of TCP transmission at a slow rate unreliable data transfer protocol that provides faster response than TCP 4