Chapter 4OSI Transport Layer

Modified by Profs. Chen and Cappellino

Objectives In this chapter, we examine the role of the Transport layer.

The Transport layer also encompasses these functions: Enables multiple applications to communicate over the network at

the same time on a single device Ensures that, if required, all the data is received reliably and in

order by the correct application Employs error handling mechanisms

Learning Objectives Upon completion of this chapter, you will be able to: Explain the need for the Transport layer. Identify the role of the Transport layer as it provides the end-to-end

transfer of data between applications. Describe the role of two TCP/IP Transport layer protocols: TCP and

UDP. Explain the key functions of the Transport layer, including reliability,

port addressing, and segmentation. Explain how TCP and UDP each handle key functions. Identify when it is appropriate to use TCP or UDP and provide

examples of applications that use each protocol.

Purpose of the Transport Layer Transport layer is the ____________________________

__________ that are responsible for ___________________ Provides for the ___________________________________ Its primary responsibilities to accomplish are:

_________ the individual communication between applications on the source and destination hosts

Any host may have ___________________________________ _________________________________

It is the responsibility of the ____________________________ ___________________________ between these applications.

_____________ data and managing each piece The Transport layer protocols describe services that ___________

received from the Application layer. Networks limit the amount of data that can be included in a single

PDU. Segmentation necessary so data blocks are of the ______________

Each piece of application data requires headers at the Transport layer to indicate which communication it is associated with

_

Purpose of the Transport Layer cont..

____________ the segments into application data At the receiving host, these individual pieces of data

must: Be ____ Be reassembled into a complete data stream that is useful to the

Application layer. __________________ the different applications

In order to pass data streams to the proper applications, the Transport layer must identify __________________.

Transport layer assigns an application an identifier called a __________________________

Each software process that needs to access the network is assigned a port number unique in that host.

Transport layer _____________________________ ________________________________ before delivering them to the appropriate application

Multiple Transport Layer protocols Because different applications have different

requirements, there are _________________ ______________________________

For some applications, segments must arrive in a very ______________ in order to be processed successfully.

In some cases, ______________________________ for any of it to be of use.

In other cases, an ___________________________ of data during transmission over the network.

The different Transport layer protocols have different rules to handle diverse data requirements.

__________________________ provide robust communication at the Transport layer

but also create ___________________________ and make larger demands on the network.

Separating Multiple Communications Example… Consider a computer that is

simultaneously receiving and sending e-mail and instant messages, viewing websites, and conducting a VoIP phone call.

Each of these applications is sending and receiving data over the network at the same time.

Transport layer ensures that data from the phone call is not directed to the web browser, and text from an instant message does not appear in an e-mail.

Controlling the Conversations Conversation ________________

There may be ________________ __________________________________

Each of these applications is assigned an address known as a port so that the Transport layer can determine with which application or service the data is identified.

In addition to the basic functions of data segmentation and reassembly, some protocols at the Transport layer provide:

Connection-oriented conversationsReliable deliveryOrdered data reconstructionFlow control

More to come on these…

Controlling the Conversations Cont…

Establishing a SessionTransport layer protocol can be considered a ______________

_____________ by _____________________________________ Prepare the applications to communicate with each other before

________________________________ Reliable Delivery

The Transport layer can _______________________________ ____________________ by having the source device _________ ___________________________________________

Same Order DeliveryBy __________________, Transport layer can ensure that segments

____________________________________ into the proper order. Flow Control

When Transport layer is aware that memory or bandwidth resources are overtaxed, it can ____________________________________ _____________________________________

Flow control can _____________________________ on the network and avoid the need for retransmission.

More to come on all of this…

Summarizing Reliable Communication Recall, there are different Transport protocols which

have been developed to meet the varying needs of applications.

A Transport layer protocol can implement a method to ensure reliable delivery of the data with the following operations:

tracking transmitted dataacknowledging received dataretransmitting any unacknowledged data

These ________________________________ _____________________________ due to the acknowledgement, tracking, and retransmission.

Control data contained within the Transport layer header assists in supporting the reliability operations

Determining the Need for Reliability At the Transport layer, there are protocols that specify

methods for either _____________________________ _______________________________

___________________________________, because there is no acknowledgement that the data is received at the destination.

Guaranteed (reliable) delivery example:databases, _________________________________________________________________________________ Additional network overhead necessary

Best-effort (unreliable) delivery example: ____________________________ stream _________________________________________________.

Small loss may create a momentary disruption in the stream. Imposing overhead to ensure reliability for this application could

___________________________________________ possibly causing unacceptable delay

So what protocols

Are considered reliable vs unreliable…

Common Transport layer protocols: TCP and UDP Transmission Control Protocol (___)

A _________________________________ ______________________________

_______________________________________________________________Each TCP segment has ___ bytes of overhead

in the header encapsulating the Application layer data,

Two applications that use TCP are web browsers & e-mail

User Datagram Protocol (_______)A _________________________________

_____________________ data delivery_________________________________- ___________________ in UDP are called

________________________________. Each UDP segment has ___ bytes of overhead. Applications that use UDP are DNS & video

streaming and VoIP

More to come on TCP and UDP…

Port Addressing: Identifying the Conversations

Recall: the header of both _________ ____________________ that can uniquely identify various applications.

The ___________________________ ____________________________.

The source port number is the number for the communication associated with the ______________________________.

The destination port number is the number for this communication associated with the _________________________________

Server processes have _________ numbers assigned to them

They are ___________________ that are ___________________ by clients.

___________________________ ____________ for each conversation.

Port Addressing cont… Client application sends a request to a server

applicationDestination port contained in the header is the

port number that is assigned to the service daemon running on the remote host.

Generally a commonly known predefined port number

Example, when a web browser application makes a request to a web server, the browser uses TCP and port number 80

___________________________________________________________________________

Can not conflict with other ports in use on the system

Client can choose any port number higher than 1024

This port number ________________________ ________________________________

The __________________________________ ______________________________ so that when a response is returned, it can be forwarded to the correct application.

Socket

The ______________________________ _________________________________ is called a socket number

uniquely identifies a particular process running on a specific host device.

A socket pair, consisting of the source and destination IP addresses and port numbers, is also unique and identifies the conversation between the two hosts

The IANA assigns port numbers Well Known Port Numbers (_________) - These numbers

are ___________________________________. HTTP (web server) POP3/SMTP (e-mail server) and Telnet.

Registered Port Numbers (__________) - These port numbers are ____________________________________.

These processes are primarily individual applications that a user has chosen to install.

When not used for a server resource, these ports may also be used dynamically selected by a client as its source port.

Dynamic or Private Port Numbers (_____________) - Also known as Ephemeral Ports, these are ______________ ____________________________________________________________________________________

Some applications may ________________________. For example, the low overhead of UDP enables DNS to serve many client

requests very quickly. Sometimes, however, sending the requested information may require the

reliability of TCP.

TCP & UDP port summary…

Port Addressing: _______ command Netstat is a __________

_______________________________________________________________________________ on a networked host.

Netstat lists: the protocol in use, the local address and port number,

the foreign address and port number,

the state of the connection.

TCP/UDP Server Processes It is common for a server ____________________

______________________, such as a web server and an FTP server, ____________________

_______________ process running on the server is configured to use a _______________________

An individual server cannot have two services assigned to the same port number within the same Transport layer services.

When an active server application is assigned to a specific port, that _____________________________________

Any incoming client request addressed to the correct socket is accepted and the data is passed to the server application.

One way to improve security on a server is to _____ _____________________________________________________________ and applications that should be _________________________________

TCP- Reliable protocol The reliability of TCP communication is performed using

__________________________________________Before a host using TCP sends data to another host, the Transport

layer initiates a process to create a connection with the destination. More to come…

After a session has been established – using a 3 way handshake- the _____________________________ to the source for the segments that it receives.

As the source ______________________________, it _________ ____________________________________________________

If the source does not receive an acknowledgement within a predetermined amount of time, it ______________ that data to the destination.

TCP Segment Header Fields

Individual sections of a segment called ______

We’ll review some fields on the following slide…

NOTE: Some texts refer to this field as the “control field”

Some Fields in Detail… ___________________________ – random numeric TCP session value

of the device opening the connection ____________________________________ – TCP session value of the

protocol or application on the remote site _____________________________________ -- number used to ensure

correct sequencing of the arriving data Allows the _______________________________________ in the order in

which they were transmitted. ____________________ - next TCP octet expected by the receiver _________________ -- number of bytes that the sender can be send

before waiting for an acknowledgement and the receiver can accept before having to send an acknowledgement back

to the sender ____________________________ upper-layer protocol data

Six One-bit fields… These are ________________

that make up the “________” field in our diagram

Fields _______________ used to manage the TCP processes URG - Urgent pointer field is valid

(when this bit set) ACK - Acknowledgement field valid PSH - Push the data RST - Reset the connection SYN - Synchronize sequence

numbers FIN – Terminate the connection

Each one-bit field has only two values: ___________________ Setting of

_____________________ ____________________________.

Why used a three-way handshake?

______________________ that the destination device is present

_________________ that the destination device has an active service and is accepting requests

_______________________ the destination device that the source client intends to establish a communication session on that port number

Three-way Handshake Steps in TCP connection

establishment are:1. The initiating client sends a segment containing an _____________, which serves as a request to the server to begin a communications session.

2. The server responds with a segment containing an __________________ ____________________________________________________________

The value is one greater than the sequence number because the ACK is always the next expected byte

3. Initiating client responds with an __________________________________________________________

Connection established

TCP Termination To terminate a single conversation supported by

TCP, ________ exchanges are needed to end the entire session:

1. When the ___________ (A) has no more data to send in the stream, it sends a segment with the _______________________

2. The __________ (B) sends an ________ to acknowledge the receipt of the FIN to terminate the session from client to server.

3. The __________ (B) sends a _____________ to the client, to _______ the server to client session.

4. The ________ (A) responds with an ___________ to acknowledge the FIN from the server.

It is also possible to terminate the connection by a three-way handshake.

If the server has no more data to send the client, it can reply to the client’s FIN with both the FIN and ACK flags set, combining steps #2 and #3 in the above graphic into one step.

TCP Segment Reassembly Segments may _______________________ and must be

__________________________ ______________________________________________

This ISN represents the starting value for the bytes that will be transmitted to the receiving application.

As data is transmitted, the _____________________ _________________________________________

Enables __________________________ and any missing segments can be identified.

The receiving TCP process places the data from a segment into a ____________________________

Any segments that arrive with noncontiguous sequence numbers are __________________________________________

Then, when the segments with the missing bytes arrive, these segments are processed.

TCP Acknowledgement with Windowing

The segment _______________________ ________________________ are used to _____ _________ of the bytes of data in the segments.

The sequence number is the relative number of bytes that have been transmitted in this session plus 1

The ______________ is expected to send a segment that uses a sequence number that is equal to the _______________________________________

The source is informed that the destination has received all bytes in this data up to, but not including, the byte indicated by the acknowledgement number.

This is called ____________________________________

Let’s look at an example… The host on the left is sending 10 bytes of

data and a sequence number equal to 1 in the header.

Host on the right receives the segment and determines that the sequence number is 1 and that it has 10 bytes of data.

The host then sends an acknowledgement #11 back to the host on the left to acknowledge the receipt of this data.

11 indicates to the receiving host that the next byte of data it expects to receive is byte number 11.

When the sending host on the left receives this acknowledgement, it can now send the next segment starting with byte number 11.

TCP Retransmission When TCP at the source host has _______________

_________________________, it will _____________ ____________________________ that it received and ______________________ from that point forward.

A destination host service generally only _____________ data for _________________________________

If one or more segments are missing, _____________________ _____________________________________________

For example, if segments with sequence numbers 1500 to 3000 and 3400 to 3500 were received, the acknowledgement number would be 3001.

Because segments numbers 3001 to 3399 have not been received.

Hosts today may also employ an optional feature called ________________________________________

If both hosts support Selective Acknowledgements, it is possible for the _______________________________________________ and the host only need to retransmit the missing data.

TCP Flow Control using variable window sizes

Flow control assists the reliability of TCP transmission by _____________________________________________ between the two services.

Recall: Window Size field specifies the amount of data that can be transmitted before an acknowledgement must be received.

The ____________________________ that the network and destination device can support ______________

_______________________________ is key During the delay in receiving the acknowledgement, the

sender will not be sending any additional segments for this session.

As delay increases, the effective transmission rate of the data for this session decreases.

If the receiver indicates a window size of 0, then the sender cannot send any more bytes until the receiver sends a packet with a window size greater than 0.

TCP Flow Control using variable window sizes

______________________ to require that received segments be ________________________ during network congestion

If the destination needs to slow down the rate of communication it can send a ______________________________ _____________________________

After periods of transmission with no data losses or constrained resources, the receiver will begin to increase the window field.

Window size will _______________ __________________ to obtain optimum window size for each TCP session.

Window size can be a factor in determining the overall efficiency of the network

Original window size

UDP: Low Overhead vs. Reliability

UDP – Low Overhead vs. Reliability

UDP is a ________________________________________ __________________________________________

It is _____________________________________ Will not establish a connection prior to sending data

It does ________________________________________________. If above functions are needed, must be implemented elsewhere

Less overhead results in _________________________ What are some Application layer protocols that use UDP? If applications used TCP, they may experience large delays

while TCP detects data loss and retransmits data. These delays would be more detrimental to the application than

small data losses. They are unnecessary for other applications

UDP Datagram Reassembly UDP is said to be ________________________

When an application has data to send, __________________ What is another – specific- name for the UDP segment? When multiple datagrams are sent to a destination, they

may take different paths and arrive in the wrong order. UDP does not keep track of sequence numbers.UDP has no way to reorder the datagrams into their transmission

order. _______________________________________________

____________________________________Application layer determines how the data should be processed