1. Chapter 4 OSI Transport Layer
Modified by Profs. Chen and Cappellino

2. 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.

3. 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 _

4. 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

5. 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.

6. Separating Multiple Communications
Example…
Consider a computer that is simultaneously receiving and sending 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 .

7. Controlling the Conversations
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 conversations
Reliable delivery
Ordered data reconstruction
Flow control
More to come on these…

8. Controlling the Conversations Cont…
Establishing a Session
Transport 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 Delivery
By __________________, 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…

9. 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 data
acknowledging received data
retransmitting 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

10. 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

11. Are considered reliable vs unreliable…
So what protocols
Are considered reliable vs unreliable…

12. 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 &
User Datagram Protocol (_______)
_____________________ 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…

13. 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.

14. Port Addressing cont…
Client application sends a request to a server application
Destination 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.

15. 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

16. The IANA assigns port numbers
Well Known Port Numbers (_________) - These numbers are ___________________________________.
HTTP (web server) POP3/SMTP ( 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.

17. TCP & UDP port summary…

18. 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.

19. 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 _________________________________

20. 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.

21. 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”

22. 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

23. 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 _____________________ ____________________________.

24. 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

25. 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

26. 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.

27. 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.

28. 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 ____________________________________

29. 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.

30. 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.

31. 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.

32. 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

33. UDP: Low Overhead vs. Reliability

34. 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

35. 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