Spring 2019 Exam Flashcards
List the 5-layer in the TCP/IP Internet reference model, give a brief explanation of each.
5 Application : Interacts with transport layer to send and receive data.
4 Transport : protocols ensure that packets arrive in sequence and without error; Provides end to end communication
3 Network (or Internet) : Accepts and delivers packets to network; Handles the communication from one computer to another
2 Data Link : Responsible for accepting IP packets and transmitting them over a specific
network
1 Physical : Physical Hardware, communication across the physical medium; encoding 1’s
and 0’s on the wire with electricity or light pulses, radiowaves, etc.
Assuming TCP/IP reference model is adopted, what layers of a protocol stack are used on a router? How about a host?
For router : Network layer / Internet layer
For host : Application layer, Transport layer, Internet layer and Network layer
List and describe the three primary measures of network performance.
1 Latency : The time required to transfer data across a network.
2 Jitter : The changes in delay that occur and the duration of the changes.
3 Throughput : The amount of data that can be transferred per unit time.
Please give at least two technologies for (a) home Internet access and (b) Internet access in the Enterprise scenario, respectively.
(a) Home Internet access
DSL
Dialup modem attached with the telephone
Wireless LAN
Fiber optics coaxial cable
(b) Enterprise scenario
Wireless LAN
100 Mbps Wired Ethernet
Why do we need hierarchical routing in Internet? What is an autonomous system (AS)?
- To avoid packet collision
- To have fair channel allocation
- To have channel reservation based on scheduling
- To have overhead of cluster information all over the network
Autonomous system is a set of internet routable IP prefixes belonging to a network or a collection of networks that are all managed, controlled and supervised by a single entity.
Suppose there are ten users sharing a 2Mbps link, and user 1 suddenly generate one thousand 2000-bit packets, while other users remain quiescent and do not generate packet.
(6 pts) (2.1) Assume under Time Division Multiplexing (TDM) circuit switching with 10 slots per frame and user 1 is allocated one time slot. How long does it take to transmit the one thousand 2000-bit packets (2 million bits) data for user 1?
time = 2 x 10^6 / (2 x 10^6 / 10) = 10 sec
How about using packet switching, how long it will take to transmit the one thousand 2000-bit packets (2 million bits) data for user 1?
time = 2 x 10^6 / 2 x 10^6 = 1 sec
Briefly explain the difference between circuit-switching and packet-switching. Please explain what scheme is Internet based?
Circuit Switching
- Reserves channel before data transfer
- Not efficient
- Packets does not have to wait at switch
- Suitable for realtime services
- Idle resources cannot be used by other users in the network
- Connection oriented
- Supports less user simultaniously
- Allocates dedicated transmission rate.
Packet Switching (internet based)
- It does not reserves channels before data transfer
- Efficient
- The packet has to wait at the switch if data rate is more than the link capacity
- Not suitable for realtime services
- Idle resources can be used by other users in the network
- Connectionless
- Supports more users simultaneously
8 Allocates variable transmission rate on demand
What are the two key functions of the network layer?
- To determine how data is sent to the receiving device
2. It is responsible for packet addressing, forwarding and routing
Please briefly explain the router architecture.
Router architecture is designed so that routers are equipped to perform two main functions: process routable protocols and use routing protocols to determine best path.
(a) The components of a router
- Input Ports: at the router the incoming
physical ink is terminated. The link layer
functions are performed by the router,
that is required to inter-operate with the
data link layer present at the other side.
It also performs a lookup and
forwarding function so that a datagram
forwarded into the switching fabric of the
router emerges at the appropriate
output port. - Switching fabric: The switching fabric
connects the router’s input ports to its
output ports. This switching fabric is
completely contained with the router -
a network inside of a network router!
3. Output ports: Stores datagrams that needs to be forwarded through switching fabric and then transmits the datagram on the outgoing link. It performs reverse data link and physical layer funcitionality as the input port.
4 Routing processor: Executes the routing protocols, maintain routing tables and performs network management functions within the router.
(b) the function of each block in the router input ports and output ports.
Functionality of input port:-
The input port’s line termination function and data link processing implement the physical and data link layers associated with an individual input link to the router. The lookup/forwarding function of the input port is central to the switching function of the router.
Functionality of output port:-
Output port processing, takes the datagrams that have been stored in the output port’s memory and transmits them over the outgoing link. The data link protocol processing and line termination are the send-side link- and physical layer functionality that interact with the input port on the other end of the outgoing link. The queueing and buffer management functionality are needed when the switch fabric delivers packets to the output port at a rate that exceeds the output link rate; we’ll cover output port queueing below.
What is NAT? Please Explain the difference of Private IP address vs. Public IP address
NAT = Network Address Translation
technology provides transparent IP-level access to the internet for a host that has a private, non routable IP address
Private IP 1. The scope is local 2 Available on LAN only 3 It is free 4 It is used to communicate insde of the network 5 Address range: 10.0.0.0 - 10.255.255.255, 172.16.0.0 - 172.31.255.255 192.168.0.0 - 192.168.255.255
Public IP
1. The scope is global
2. It is used to get Internet service
3 It is not free
4 It is used to communicate to outside of the network
5 Rest IP addresses are public (excluding private IP addresses)
The Transmission Control Protocol uses a method called congestion control to regulate the traffic entering the network. The behavior of TCP congestion control can be represented as a graph in which the x-axis indicates the time, and the y-axis indicates congestion window size. Please use the graph shown below to the answer the following questions. Note that the graph does not explicitly show timeouts, but you should be able to figure out when timeouts happened based on the events shown (assume TCP Reno is used).
Slow Start: give some reason why slow start is used, and explain why it does a better job than congestion avoidance for that function. Identify the intervals of time when TCP slow start is operating.
Slow start is used as a congestion avoidance scheme in TCP in which TCP increases its window size as ACKs arrive. It does a better job than congestion avoidance for that function because it is designed to avoid sending more data than the network is capable of transmitting.
Slow start at 1-6, 7-11, 23-26
Congestion Avoidance: identify the intervals of time when TCP congestion avoidance is operating. Why should congestion avoidance be used instead of slow start during these intervals?
Congestion avoidance intervals: 11-23
When there is no congestion in TCP, then with increase in rounds there is a linear increment in the window size.
