Chapter 3 transport layer review quétion năm 2024

314CHAPTER 3• TRANSPORT LAYER congestion-control mechanism that could be used in another transport protocol such as DCCP (indeed one of the two application-selectable protocols available in DCCP is TFRC). The goal of TFRC is to smooth out the “saw tooth” behavior (see Fig- ure 3.53) in TCP congestion control, while maintaining a long-term sending rate that is “reasonably” close to that of TCP. With a smoother sending rate than TCP, TFRC is well-suited for multimedia applications such as IP telephony or streaming media where such a smooth rate is important. TFRC is an “equation-based” protocol that uses the measured packet loss rate as input to an equation [Padhye 2000] that esti- mates what TCP’s throughput would be if a TCP session experiences that loss rate. This rate is then taken as TFRC’s target sending rate. Only the future will tell whether DCCP, SCTP, QUIC, or TFRC will see wide- spread deployment. While these protocols clearly provide enhanced capabilities over TCP and UDP, TCP and UDP have proven themselves “good enough” over the years. Whether “better” wins out over “good enough” will depend on a complex mix of technical, social, and business considerations. In Chapter 1, we said that a computer network can be partitioned into the “net- work edge” and the “network core.” The network edge covers everything that hap- pens in the end systems. Having now covered the application layer and the transport layer, our discussion of the network edge is complete. It is time to explore the net- work core! This journey begins in the next two chapters, where we’ll study the net- work layer, and continues into Chapter 6, where we’ll study the link layer. Homework Problems and Questions Chapter 3 Review Questions SECTIONS 3.1–3.3

R1.Suppose the network layer provides the following service. The network layer in the source host accepts a segment of maximum size 1,200 bytes and a destination host address from the transport layer. The network layer then guarantees to deliver the segment to the transport layer at the destination host. Suppose many network application processes can be running at the destination host. a. Design the simplest possible transport-layer protocol that will get applica- tion data to the desired process at the destination host. Assume the operat- ing system in the destination host has assigned a 4-byte port number to each running application process. b. Modify this protocol so that it provides a “return address” to the destina- tion process. c. In your protocols, does the transport layer “have to do anything” in the core of the computer network?

Chapter 3: Transport Layer

3.1 Transport Layer Services

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.

3.2 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

3.3 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.

5. Application

4. Transport

3. Network

2. Data Link

1. Physical (bits)