CSSE 432 -- Computer Networks -- Spring 2005Midterm Exam Answers1. [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 1-3ii) End system 1-5iii) Server 1-5c) Name the transport protocol used by each of the following applications:i) Email -- TCPi) WWW -- TCPii) DNS -- UDPii) Internet Telephony -- UDPiii) FTP -- TCP2. [10 points] TCP Congestion Controla) 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 congestionwindow 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 valueTimeout -- set to 1 MSS3. [10 points] Checksums0 0 0 0 0 1 1 1 0 0 0 0 1 0 1 10 0 0 0 1 1 0 0 0 0 0 0 0 1 0 10 0 0 0 1 0 0 0 0 0 0 0 1 0 1 00 0 0 0 1 1 1 1 0 0 0 0 0 1 0 10 0 0 0 1 0 1 1 0 0 0 0 1 0 0 10 0 0 0 1 0 1 0 0 0 0 0 0 0 1 1-----------------------------------0 0 1 1 1 1 1 1 0 0 1 0 1 0 1 1 sum1 1 0 0 0 0 0 0 1 1 0 1 0 1 0 0 complement1CSSE 432 -- Computer Networks -- Spring 2005Midterm Exam Answers4. [25 points] Distance Vector ProtocolGiven the following network, compute the distance tables for the first 3 stages of the distance vector algorithm.Node Xto to toX Y Z X Y Z X Y ZfromX 0 7 2fromX 0 5 2fromX 0 5 2Y∞ ∞ ∞Y 7 0 3 Y 5 0 3Z∞ ∞ ∞Z 2 3 0 Z 2 3 0Node Yto to toX Y Z X Y Z X Y ZfromX∞ ∞ ∞fromX 0 7 2fromX 0 5 2Y 7 0 3 Y 5 0 3 Y 5 0 3Z∞ ∞ ∞Z 2 3 0 Z 2 3 0Node Zto to toX Y Z X Y Z X Y ZfromX∞ ∞ ∞fromX 0 7 2fromX 0 5 2Y∞ ∞ ∞Y 7 0 3 Y 5 0 3Z 2 3 0 Z 2 3 0 Z 2 3 02ZXY723CSSE 432 -- Computer Networks -- Spring 2005Midterm Exam Answers5. [40 points] Vocabulary11iterated query 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 ask2circuit switching a type of network where the resources along a path are reserved for the duration of the communication sessionfragmentation or segmentation breaking large packets into smaller segments in order to pass them onto another network12multiplicative decrease decreasing transmission rate quickly (halving it) when congestion is observed5Domain Name System (DNS) distributed database of mapping between host names and IP numbers8flow control a mechanism to prevent either side of a connection from overwhelmingthe other4cookie identifier that is associated with a user so that web server can keep track of state of session or previous history of user19service model the guarantees that a protocol layer provides23stop-and-wait waiting for an acknowledgment before sending the next packet6Dynamic Host Configuration Protocol (DHCP) get IP address dynamically from server25virtual circuit source-to-destination path behaves much like telephone circuit17query flooding method of propagating queries to other peers in a distributed P2P networkbootstrap node node in a P2P network that provides connection to other nodes when a new peer joins the networkPost Office Protocol 3 (POP3) protocol used between mail server and a user agent where the server keeps no state about the user between sessions10Internet Mail Access Protocol (IMAP) protocol used between mail server and a user agent where the server keeps track of the user's state between sessions20Simple Mail Transport Protocol (SMTP) protocol used between mail servers to exchange email9HyperText Transfer Protocol(HTTP) protocol used by web browsers and serversSelective Repeat(SR) protocols that only retransmit packets that were lost or corrupted15pipelining sending multiple packets without waiting for acknowledgments3conditional get server checks modification date of requested object; if client has latestversion then no object is sent7fast retransmit TCP retransmits a missing segment before its timer expires when 3 duplicate ACKs are received16propagation delay the time required to move a bit along a link18queuing delay the time that a packet spends in a queue waiting to be transmitted22socket a host-local; application-created; OS-controlled interface into which an application process can send and receive messages from another application process3CSSE 432 -- Computer Networks -- Spring 2005Midterm Exam AnswersHere are the terms that were not matched:1additive increase increasing the transmission rate slowly (adding a small amount) when congestion is not observed13Network Address Translation (NAT) mapping several host IP addresses to one IP address and new port numbers so that the local network can be hidden14out-of-band a method of signaling that uses a separate channel than the channel containing the data21slow start beginning of TCP transmission at a slow rate24User Datagram Protocol (UDP) unreliable data transfer protocol that provides faster response than
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