Intro to Networks ch 9: Transport Layer

Transport Layer Responsibilities

Tracking Individual Conversations Segmenting Data and Reassembling Segments Identifying the Applications

What OSI layer is responsible for establishing a temporary communication session between two applications and ensuring that transmitted data can be reassembled in proper sequence?

Transport

Segmenting Data

Transport layer protocols segment the data into blocks Encapsulate each piece of data Adds headers to each piece of data

Port 80

Hypertext Transfer Protocol HTTP

UDP User Datagram Protocol

simple, unreliable

Port Number Standards

IANA

What is an advantage of UDP over TCP?

Less overhead

Well Known Ports (0-123)

Reserved for services and applications: web browsers email clients remote access clients

What is the purpose of using a source port number in a TCP communication?

To keep track of multiple conversations between devices

What is the complete range of TCP and UDP well known ports?

0-1023

TCP Connection Establishment

1. Initiating client requests a client-to-server communication session with the server 2. The server acknowledges the client-to-server communication session and requests a server-to-client communication session 3. The initiating client acknowledges the server-to-client communication sessions 1.SYN 2.SYN ACK 3.ACK

TCP operations of Reliability

1. Numbering and tracking data segments transmitted to a specific host from a specific application 2. Acknowledging received data 3. Retransmitting any unacknowledged data after a certain period in time

TCP Session Termination

1. Segment is sent with the FIN flag set 2. Server sends an ACK to acknowledge the receipt of the FIN to terminate the session from client to server 3. The server sends a FIN to the client terminate the server-to-client session 4. The client responds with an ACK to acknowledge the FIN from the server 1.FIN 2.ACK 3.FIN 4.ACK

A PC is downloading a large file from a server. The TCP window is 1000 bytes. The server is sending the file using 100-byte segments. How many segments will the server send before it requires an acknowledgment from the PC?

10 segments

Urgent

16 bits Indicates if data is urgent

Window Size

16 bits Number of bytes that can be accepted at one time

Checksum

16 bits Used for error checking of segment header and data

Source Port and Destination Port

16 bits each Used to identify the application

Acknowledgement Number

32 bits Indicates the data has been received

Sequence Number

32 bits Used for data reassembly purposes

Header Length

4 bits "data offset" Indicated length of TCP segment header

Reserved

6 bits reserved for the future

Registered Ports (1024-49151)

Assigned for use with specific processes or applications

Why does HTTP use TCP as the transport layer protocol?

Because HTTP requires reliable delivery

Which flag in the TCP header is used in response to a received FIN in order to terminate connectivity between two network devices?

ACK

Which two flags in the TCP header are used in a TCP three way handshake to establish connectivity between two network devices?

ACK SYN

OSI Transport Layer

Accept data from the application layer and prepares it for addressing at the network layer

TCP Disadvantages

Additional overhead Delays in transmissions

Unreliable delivery

Any segments lost are not resent

What type of applications are best suited for using UDP?

Applications that are sensitive to delay

Which number or set of numbers represents a socket?

Combo of an IP address and port number 192.168.1.1:80

No Ordered Data Reconstruction

Data is reconstructed in the order that it is received

What type of information is included in the transport header?

Destination and source port number

Which two characteristics are associated with UDP sessions?

Destination devices receive traffic with minimal delay Received data is unacknowledged

No Flow Control

Does not inform the sender about resource availability

Source Port Number

Dynamically generated by the sending device to identify a conversation between two devices Allows several conversations to occur

Conversation

Each set of data flowing between a source application and a destination application

Flow Control

Ensures that the receiver is able to process the data received

Same Order Delivery

Ensures that the segments are reassembled in the proper order

Establishing a session

Ensures the application is ready to receive the data

TCP Three-Way Handshake

Establishes the destination device is present on the network Verifies the destination device has an active service and is accepting requests on the destination port number the client is intending to use Informs the destination device that the source client intends to establish a communication session on that port number

TCP services

Establishing a Session Reliable Delivery Same-Order Delivery Flow Control

OSI Transport Layer Responsible for

Establishing a temporary communication session between two applications and delivering data between them Without regard to: destination host type media path taken by data congestion on the link size of the network

Applications that Use TCP

FTP SMTP TelNet HTTP

Segmentation

Facilitates data transport by the lower network layers

When to use TCP

For applications such as: web browsers e-mail clients databases Any missing data could cause a corrupt communication

Which three application layer protocols use TCP>

HTTP SMTP FTP

UDP Client Processes

Initiated by a client application that requests data from a server process Dynamically selects a port number and uses this as the source port Destination port is usually well-known or registered port number assigned to the server processes

What does a client do when it has UDP datagrams to send?

It just sends the datagrams

Stateful protocol

Keeps track of the state of a communication session Begins with the session establishment Ends with the session termination

Which three fields are used in a UDP segment header?

Length Source Port Checksum

Applications that Use UDP

Live video and multimedia applications: VoIP IPTV Simple Request and Reply Applications: DNS DHCP Applications that handle reliability themselves: SNMP TFTP

Reliable delivery

Lost segments are re-sent so the data is received complete

Conversation Multiplexing

Multiple applications can use the same network at the same time

A host device needs to send a large video file across the network while providing data communication to other users. Which feature will allow different communication streams to occur at the same time, without having a single data stream using all available bandwidth?

Multiplexing

What are three responsibilities of the transport layer?

Multiplexing multiple streams from many users or applications on the same network Meeting the reliability requirements of applications, if any Identifying the applications and services on the client and server that should handle transmitted data

Stateless Protocol

Neither the client or server is obligated to keep track of the state of the communication session Application must handle reliability

Connectionless

No session establishment

Error checking

Performed on the data segment to check if the segment was changed during transmission

Remote Authentication Dial-In User Service (RADIUS)

Provides authentication, authorization, accounting services to manage users

TCP/IP

Provides two transport layer protocols: Transmission Control Protocol User Datagram Protocol Uses these to enable hosts to communicate and transfer data

Applications that Can Use Both UDP and TCP

SNMP DNS

UDP Header

Source Port (16) Destination Port (16) Length (16) Checksum (16)

TCP segment Header

Source Port and Destination Port Sequence Number Acknowledgement Number Header Length Reserved Control bits Window Size Checksum Urgent

What is the purpose of the TCP sliding window?

To request that a source decrease the rate at which it transmits data

A client needs to terminate a TCP communication session with a server. What are the steps?

Step 1. Client sends FIN Step 2. Server sends ACK Step 3. Server sends FIN Step 4. Client sends ACK

What transport layer feature is used to guarantee session establishment?

TCP 3 way handshake

Port 20

TCP File Tranfer Protocol (data)

Port 21

TCP File Transfer Protocol (control)

Port 22

TCP Secure Shell SSH

Port 25

TCP Simple Mail Transfer Protocol SMTP

Port 23

TCP Telnet

What happens if the first packet of a TFTP transfer is lost?

The TFTP application will retry the request if a reply is not received.

What does the value of the window specify?

The amount of data that can be sent before an acknowledgement is required

Which factor determines window size?

The amount of data the destination can process at one time

Destination Port

The client places a destination port number in the segment to tell the destination server which service is being requested

Which action is performed by a client when establishing communication with a server via the use of UDP at the transport layer?

The client randomly selects a source port number

Socket

The combination of the source IP address and source port number OR The combination of the destination IP address and destination port number used to identify the server and service being requested by the client

SACK Selective Acknowledgement

The destination can acknowledge bytes in discontinuous segments and the host only needs to retransmit the missing data

A host device is receiving live streaming video. How does the device account for video data that is lost during transmission?

The device will continue receiving the streaming video, but there may be a momentary disruption.

What happens if part of an FTP message is not delivered to the destination?

The part of the FTP message that was lost is resent (FTP uses TCP)

Sliding windows

The process of the destination sending acknowledgements as it processes bytes received and the continual adjustment of the source's send window

Flags

The six bits in the Control Bits field of the TCP segment Header A bit set to on or off

Network congestion has resulted in the source learning of the loss of TCP segments that were sent to the destination. What is one way that the TCP protocol addresses this?

The source decreases the amount of data that it transmits before it receives an acknowledgement from the destination

What does TCP do if the sending source detects network congestion on the path to the destination

The source will decrease the amount of data that it sends before it must receive acknowledgements from the destination.

Initial Sequence Number ISN

The starting value of the bytes for the session that is transmitted to receiving application As data is transmitted the sequence number is incremented by the number of bytes transmitted Enables each segment to be identified and acknowledged

Identifying Applications

The transport layer assigns each application an identifier called a port number

Reassembling Segments

The transport layer reconstructs the data into a system useful to the application layer

What is a characteristic of TCP server process?

There can be many ports open simultaneously on a server, one for each active server application.

UDP Datagram Reassembly

UDP reassembles data in the order it was received and forwards it to the application The application must identify the proper sequence and determine how the data is processed

What is a characteristic of UDP?

UDP reassembles the received datagrams in the order they were received

Port 53

UDP, TCP Domain Name Service DNS

Control Bits

URG: Urgent pointer field significant ACK: Acknowledgement field significant PSH: Push function RST: Reset the connection SYN: Synchronize sequence numbers FIN: No more data from sender

UDP Features

Unreliable Delivery No Ordered Data Reconstruction Connectionless No Flow Control

Header

Used for reassembly Track the data stream

Which two services or protocols use the preferred UDP protocol for fast transmission and low overhead?

VoiP DNS

When to use UDP

When an application can tolerate some data loss during transmission but delays in the transmission are unacceptable eg: streaming live audio, live video, VoIP

When is UDP preferred to TCP?

When an application can tolerate some loss of data during transmission

Congestion Avoidance

When there is congestion, re-transmission must be controlled or it can contribute to the congestion

What kind of port must be requested from IANA in order to be used with a specific application?

a registered port

Dynamic or Private Ports (49152-65535)

aka ephemeral ports Assigned dynamically by a client's OS when a connection to a service is initiated

UDP Unreliability

best-effort no acknowledgement that the data is received

netstat

command to list protocols in use, local addresses and port numbers, foreign address and port numbers, connection state By default, will attempt to resolve IP addresses to domain names and port numbers to well known applications

netstat -n

display IP addresses and Port numbers in their numerical form

TCP Transmission Control Protocol

reliable ensures all data arrives at destination

What information is used by TCP to reassemble and reorder received segments?

sequence numbers

What is a socket?

the combination of a source IP address and port number or a destination IP address and port number

During a TCP session, a destination device sends an acknowledgment number to the source device. What does the acknowledgment number represent?

the next byte that the destination expects to receive