The links below will take you to knowledge checks where you'll be presented with a set of questions relating to material covered in the Chapter per section.
You may be interested in other supplemental material [online lectures, powerpoint slides, review questions, Wireshark labs] for our book, available here.
This page replaces the earlier knowledge check pages and includes a number of new questions. We're actively adding new questions here in the summer of 2021. If you've got any comments or suggestions - let us know at kurose@cs.umass.edu
We gratefully acknowledge the programming and problem design work of John Broderick [UMass '21], which has really helped to substantially improve this site.
Network+ Guide to Networks, Seventh Edition Chapter 3, Solutions Review Questions
1.Which protocol’s header would a Layer 4 device read and process?
- IP B. TCP C. ARP D. HTTP Answer: B. TCP
2.What number does a host use to identify the application involved in a transmission?
3.What field in a TCP segment is used to determine if an arriving data unit exactly matches the data unit sent by the source?
4.At which OSI layer does IP operate?
B.Transport layer C.Network layer D.Data Link layer Answer: C. Network layer
5.Which OSI layer is responsible for directing data from one LAN to another?
Recently submitted questions
The purpose of transport layers and protocols is to provide logical communication between apps running on different servers. The transport protocols available are TCP and/or UDP.
TCP [Transport Control Protocol] is reliable and provides in order delivery with congestion control, flow control and connection setup.
UDP [User Datagram protocol] is unreliable and provides unordered delivery.
3 Multiplexing and demultiplexing
Multiplexing is a method by which multiple analogue or digital signals are combined into one signal over a shared medium. The aim is to share a scarce resource. For example, in telecommunications, several telephone calls may be carried using one wire.
Demultiplexing is the reverse of multiplexing. It is the process of reconverting a signal containing multiple analogue or digital signal streams back into the original/unrelated signals.
Connectionless demultiplexing: IP datagrams with the same destination port number with different source IP addresses will be directed to the same socket at destination.
Connection-oriented demultiplexing: TCP socket is identified by 4-tuple: - Source IP address - Source port number - Destination IP address - Destination port number
Transport layer Vs Network layer: - Transport Layer: Logical communication between processes; relies on and enhances network layer services - Network layer: Logical communication between hosts via IP addresses
3 Connectionless transport: UDP
UDP: User Datagram Protocol is a communication protocol used for time sensitive transmissions like video playback or DNS lookups. It speeds up communication by not formally establishing a connection before data is transferred. This can cause packet loss and is not as secure as TCP.
- Application
- Transport
- Network
- Data Link
- Physical [bits]
UDP benefits: - No connection establishment, much faster - Simple: no connection state at sender or receiver - Small header size - No congestion control: UDP is fast
UDP Checksum checks the segment contents for errors
3 Principles of reliable data transfer
Characteristics of an unreliable channel will determine complexity of reliable data transfer protocol [rdt].
Rdt 1: Reliable transfer over a reliable channel: no bit errors or packet loss
Rdt 2 Channel with bit errors: underlying channel may flip bits in packet: use checksum to detect bit errors. To recover from errors, use: - Acknowledgements [ACKs]: receiver explicitly tells sender that packet was received - Negative acknowledgements [NAKs]: receiver explicitly tells sender that packet had errors
New mechanisms in rdt2: error detection, feedback: ACK/NAK from receiver to sender.
Rdt3: Channels with errors and loss: sender waits a reasonable time for ACK. Retransmit packet if ACK is not received. If packet or ACK is delayed and not lost; retransmission will be a duplicate, but the sequence number will handle this.
Performance of RDT3 is not great
Pipelining: sender allows multiple “in-flight” yet-to-be-
TCP Flow Control:
Receiver controls sender, so the sender will not overflow the receiver’s buffer by transmitting too much data.
The receiver advertises how much free space they have by including “rwnd” value in the TCP header of receiver-sender segments.
“RCVBuffer” size is set via socket options – the default is 4096 bytes. Many operating systems auto adjust Rcvbuffer.
The sender limits the amount of unpacked [“in-flight”] data to reveiver’s rwnd value.
This guarantees the receive buffer will not overflow.
Sequence numbers: Byte stream “number” of first byte in segments data.
Acknowledgements: Sequence number of next byte expected from other side. Cumulative ACK
Question: How does a receiver handle out of order segments? Answer: TCP spec doesn’t specify, it is up to the implementor.
TCP Reliable data Transfer: TCP creates rdt service on top of an IP’s unreliable service [pipelined segments, cumulative ACKS, single retransmission timer].
Retransmissions are triggered by: timeout events and duplicate ACKS.
RRT = Round trip time
Typical value of alpha is 0.
Timeout interval is the estimatedRTT plus a “safety margin”:
TCP fast retransmit: If a sender receives 3 ACKs for some data, resend the unacked segment with smallest sequence # It is likely that the unacked segment is lost, so don’t wait for the timeout.
Connection Management Before exchanging data, both sender and receiver must agree to establish a connection and agree on connection parameters.
A two-way handshake may not work If there is an unreliable channel or
retransmitted messages due to loss.
TCP: Closing a connection Client and server each close their side of the connection. A TCP segment is sent with FIN bit = 1.
3 Principles of congestion Control
Congestion is basically when too many sources are sending too much data at a fast rate, and the network can’t handle it.
Causes of congestion: - Two senders, two receivers - One router - Output link capacity R - As traffic increases, delay increases, packet loss