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
  • Go-Back-N
  • Selective Repeat
• In a simple reliable protocol that uses both ACKs and NAKs, what can be added so that NAKs are not needed?
• Can a sender tell the difference between a lost packet and a lost ACK? If so, how? If not, why not?
• 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?
• Draw a diagram of connection teardown.
• Given some tcpdump 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?
  • is this a persistent connection?
  • what is the size of each HTTP request/response pair?
  • draw a diagram showing the TCP packets transferred.
• How is flow control enforced in 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
• How is congestion control enforced in TCP?
• What are the two phases of congestion control in TCP?
• How does the congestion window (cwnd) increase 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?

Network Layer

Overview

• Terms:
  • input port
  • output port
  • switching fabric
  • forwarding
  • routing
• 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 occur in a router?

IP

• Terms:
  • classful addressing
  • classless addressing (CIDR)
  • datagram
  • netmask
  • subnet
  • encapsulation
  • 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.
• Why do many TCP implementations try to avoid IP fragmentation by setting the MSS to 536 (and why does this value work)?
• What is a subnet? 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?
• 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.
• 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?
• Which routing algorithm does RIP use?
• What is the distance metric used in RIP?
• What is infinity in RIP? Why is this important?
• Which routing algorithm does OSPF use?
• 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 (ie, how many routing protocols does each type 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 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?
• What types of bit errors can the parity bit not detect?
• What are the two main components of CRC hardware?
• Given a bit stream, draw the waveform diagram using Manchester encoding.
• List three different LAN topologies.

General Link Layer

• Terms:
  • Ethernet / 802.3
  • frame (as opposed to segment or datagram)
  • collision
  • CSMA/CD
  • Manchester encoding
• What is the purpose of the link layer?
• What is a MAC address?
• How many bits are in a MAC address?
• How are MAC addresses assigned?
• 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?
• 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.

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?
• What protocol layers does a router implement?
• What protocol layers does a typical switch implement?
• What protocol layers does a typical hub implement?

Network Security

• What is the main difference between symmetric key and public key cryptography?
• Given the key to a Caesar cipher and ciphertext, decrypt the message.
• What is the difference between a Caesar cipher and a monoalphabetic cipher?
• Given a monoalphabetic cipher and a plaintext message, encrypt the message.
• What algorithm standard uses a symmetric key encryption algorithm?
• Describe how public key encryption and decryption work.
• What is the main public key encryption standard algorithm?
• Can public key encryption work in reverse? What could that be used for?
• How can Alice and Bob use public key encryption to communicate authenticated and secure messages?
• Given a data transfer scenario between Alice and Bob, show how Trudy could interfere.
• What is the purpose of a key distribution center (KDC)?
• What is the purpose of a certification authority (CA)?
• Given a list of firewall rules and a set of incoming packets, indicate which would be allowed entry to the network and which would be denied.

Wireless Networks

• Terms:
  • base station / access point
  • hidden terminal problem
  • CSMA/CA
  • RTS and CTS
• What are the two modes that wireless networks typically provide? How are they different?
• Why is wireless transmission more problematic than wired transmission?
• What are the three main IEEE standards for wireless?
• Explain why wireless networks can't use CSMA/CD. What do they use instead? What's the basic idea behind it?
• What network layers does an access point implement?
• Why does the default router's MAC address need to be included in the 802.11 frame going from a wireless node to the AP destined for the Internet?