From CS 455 Spring 2013

CS455-S13: Final Exam Study Guide

Notes:

• Regarding terms, you should know the definitions and concepts well enough to be able to answer "compare and contrast", "define the term", "fill-in-the-blank", and "definition matching" questions.
• This is not meant to be an exhaustive list of possible questions for the exam. It is meant to give you a general idea of the possible types of questions that might be asked and an idea of the topics I think are most important.
• You should also study homework questions and problems that we've discussed during lectures.
• The final exam is comprehensive, so make sure to study the midterm study guide as well.
• The format of the final exam will be similar to the mid-term, but longer. You will have 3 hours to complete the final exam.

Transport Layer

Transport Layer Overview

• Terms:
  • multiplexing
  • demultiplexing
  • segment
  • socket
• Does UDP offer reliable transfer?
• Why would you want to use UDP?
• What items does TCP use for demultiplexing (ie, how does TCP figure out what process a segment should be delivered to)? What about UDP?
• What is the size of the UDP header?

Reliable Transfer and Pipelined Protocols

• Terms:
  • ACK
  • NAK
  • Go-Back-N
  • Selective Repeat
• In a simple reliable protocol that uses both ACKs and NAKs (such as RDT 2.0), what can be added so that NAKs are not needed?
• Can a simple reliable protocol sender tell the difference between a lost packet and a lost ACK? If so, how? If not, why not?
• Why doesn't the simple reliable protocol RDT 3.0 retransmit packets upon the receipt of a duplicate ACK and upon the expiration of the timeout timer?
• What is the advantage of using a sliding window protocol over a stop-and-wait?
• What is the speedup for a sliding window protocol with window size w over a stop-and-wait protocol?
• Does the window size always limit performance? If not, when does it not limit performance?
• How many sequence numbers are needed to correctly and reliably transmit with a given window size when using Selective Repeat?
• Assume we are using a sliding window protocol with a window size W, where W > 2. What packets are retransmitted if packet 2 is lost using Go-Back-N? Using selective repeat? Draw a diagram showing the correct operation using each protocol.
• How is the receiver buffer used for Selective Repeat?

TCP

• Terms:
  • flow control
  • sequence number
  • ACK
  • MSS
  • delayed ACKs
  • cumulative ACKs
  • piggybacking
• How many bytes is the TCP header without options?
• What's the maximum TCP sequence number?
• Why does TCP perform RTT estimation?
• Draw a diagram of connection setup (including proper sequence numbers and ACK numbers).
• How is a TCP segment designated as an ACK?
• Why is a 3-way handshake needed (explicitly, why are there 3 packets needed)?
• Draw a diagram of connection teardown.
• Why should delayed ACKs be used?
• If delayed ACKs are used, what conditions would trigger an ACK being sent immediately?
• Given some tcpdump/Wireshark output of an HTTP connection, answer the following questions:
  • what is the initial sequence number from client to server?
  • what is the initial sequence number from server to client?
  • what is the size of each HTTP request/response pair?
  • draw a diagram showing the TCP packets transferred.
• What is the impact of the receive buffer on TCP performance?
• How is flow control enforced in TCP?
• Given an RTT and link speed, compute the appropriate TCP receiver window size.
• Given a receiver window size and RTT, compute the maximum throughput achievable with TCP.
• In which TCP state is all data transferred?

TCP Congestion Control

• Terms:
  • congestion control
  • MAX-MIN fairness
  • congestion
  • congestion collapse
  • congestion window
  • slow-start
  • congestion avoidance
  • fast retransmit
  • fast recovery
• What are some of the costs (not causes) of congestion?
• What is the maximum rate that a TCP sender can transmit at (in terms of the window size and RTT)?
• What does "additive-increase, multiplicative-decrease (AIMD)" mean?
• How is congestion control enforced in TCP?
• What are the two phases of congestion control in TCP?
• How does the congestion window (cwnd) increase on the receipt of an ACK during slow-start? During congestion avoidance?
• How does the congestion window (cwnd) increase every RTT, or round, during slow-start? During congestion avoidance?
• What controls when the transition between slow-start and congestion avoidance occurs?
• In the basic TCP Tahoe, what happens to cwnd and the slow-start threshold when loss is detected?
• Given a graph of cwnd vs. time, identify the time periods when TCP is in slow-start and when it is in congestion avoidance.
• Why is congestion control in TCP often referred to as having a "sawtooth" pattern?
• What are 2 ways to detect loss in TCP Reno?
• What is the difference between TCP Tahoe and TCP Reno?
• What is the average throughput for a TCP connection in congestion avoidance, given W and RTT?

Network Layer

Overview

• Terms:
  • input port
  • output port
  • switching fabric
  • forwarding
  • routing
  • virtual circuit
• What are the two main functions of the network layer?
• How do virtual circuits differ from datagram networks?
• In which, virtual circuits or datagram networks, are the end systems "smart", but the network "dumb"?
• How is longest prefix matching used in the network layer?
• What is the difference between forwarding and routing?
• How does the routing algorithm interact with the forwarding table?
• What are the four main parts of every router?
• Where can queuing occur in a router? What is the cause of the queuing?
• Where does queuing due to network congestion typically occur in a router?

IP

• Terms:
  • classful addressing
  • classless addressing (CIDR)
  • datagram
  • netmask
  • subnet
  • fragmentation
  • DHCP
  • NAT
• How many bytes are in the IP header (not including options)?
• How many bits are in an IP address?
• Is IP connection-oriented or connectionless?
• What causes datagram fragmentation?
• Where is fragmentation and reassembly done (at endpoints, at routers, at both endpoints and routers)?
• Given a large datagram that will be fragmented and the MTU, give the values of the flag, offset, and size IP header fields.
• Many TCP implementations try to avoid IP fragmentation by setting the MSS to 536. Why does this value work?
• Given a network diagram and IP addresses, identify the subnets.
• Given a CIDR network address, how many valid host IP addresses are available? What is the range of those addresses?
• With CIDR network addresses, which IP addresses are reserved and what are they reserved for?
• Given a range of addresses, determine the CIDR network address.
• Given an IP address and a netmask, determine the CIDR network address.
• Given a private network and an outside website to visit, show the possible contents of the NAT translation table.

Routing Algorithms

• Terms:
  • link-state
  • distance-vector
  • count-to-infinity
  • poisoned reverse
• What is the goal of a routing algorithm?
• Give an example of a global routing algorithm and of a decentralized routing algorithm.
• Given a network, compute the link-state algorithm table.
• Given a network, complete the distance table for the distance-vector algorithm.
• Which of the routing algorithms can form routing loops?
• What causes the "count-to-infinity" problem in the distance vector algorithm?
• What mechanism can avoid the "count-to-infinity" problem in the distance-vector algorithm? What's the main idea behind it?
• What are the main differences between the link-state algorithm and the distance-vector algorithm?

Internet Routing

• Terms:
  • autonomous system (AS)
  • gateway, or border, router
  • inter-AS
  • intra-AS
  • hierarchical routing
• Which routing protocol uses the link-state algorithm?
• Which routing protocol uses the distance-vector algorithm?
• What is the distance metric used in RIP?
• What is infinity in RIP? Why is this important?
• What is the difference between intra-AS and inter-AS routing?
• Given a network diagram, circle the border routers.
• How are border routers different from interior routers?
• How many routing protocols does a border router run?
• How many routing protocols does an interior router run?
• What routing protocol do all border routers in the Internet run?
• Where are routing protocols such as RIP and OSPF used?
• Why is hierarchical routing used in the Internet?

Link Layer

Transmission / Error Detection

• What are the three ways we discussed to detect bit errors? What is the simplest? What is the most complex?
• Given a bit stream, what is the parity bit to achieve odd parity?
• Given a bit stream, what are the two-dimensional parity bits to achieve even parity?
• Given a bit stream and two-dimensional parity bits, detect the bit error.
• What types of bit errors can the single parity bit not detect?
• What networking layer uses checksum? CRC? Why?

General Link Layer

• Terms:
  • frame (as opposed to segment or datagram)
  • multiple access protocol
  • collision
  • TDMA
  • FDMA
  • ALOHA
  • slotted ALOHA
  • CSMA
  • CSMA/CD
• What is the main purpose of the link layer?
• What are the three main types of MAC protocols?
• What are two channel partitioning protocols?
• What are three random access protocols?
• What is the difference between pure ALOHA and slotted ALOHA? Which is more efficient? Why?
• What is an example of a 'taking turns' protocol?

Ethernet

• Terms:
  • ARP
  • Ethernet / 802.3
  • VLAN
  • trunk port
• What is a MAC address?
• How many bits are in a MAC address?
• How are MAC addresses assigned?
• How can a host find the MAC address for an interface if it knows its IP address?
• Why does a host need to know another host's MAC address?
• Given a network with IP and MAC addresses provided, show the IP and MAC level source and destination addresses for a packet traveling through the network.
• Given a bit stream, draw the waveform diagram using Manchester encoding (as discussed in class, not in the book).
• What is the purpose of the preamble in the Ethernet frame?
• What happens if there is a collision when using CSMA/CD?
• Why is there a minimum size for Ethernet frames? How would this change if the link speed doubled?
• What is the purpose of a VLAN?
• Are the switch ports in the same VLAN required to be contiguous (i.e., all next to each other)?

Router / Hub / Switch

• Terms
  • router
  • hub
  • switch
• Can a machine on a LAN connected with a hub snoop packets from other machines on the LAN?
• Can a machine on a LAN connected with a switch snoop packets from other machines on the LAN?
• Given a network with switches, show the contents of the switch table as self-learning operates.
• What protocol layers does a router implement?
• What protocol layers does a typical switch implement?
• What protocol layers does a typical hub implement?

A Day in the Life

• Explain the steps required for a laptop connecting to the campus network to be able to download a web page.

Presentations

Don't forget to review what presentations were given during the last few weeks of class.