CCNA Exp1 - Chapter04 - OSI Transport Layer

Chapter 4 - OSI Transport Layer

CCNA Exploration version 4.0CCNA Exploration version 4.0

Overview

Explain the role of Transport Layer protocols and services in supporting communications across data networks

Analyze the application and operation of TCP mechanisms that support reliability

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Analyze the application and operation of TCP mechanisms that support reassembly and manage data loss.

Analyze the operation of UDP to support communicate between two processes on end devices

Transport Layer Role and Services

Explain the purpose of the Transport layer Enables multiple applications

to communicate over the network at the same time on a single device

Ensures that, if required, all


Ensures that, if required, all the data is received reliably and in order by the correct application.

Employs error handlingmechanisms.


Major functions of the transport layer and the role it plays in data networks Tracking the individual communication between applications on

the source and destination hosts. Segmenting data and managing each piece. Reassembling the segments into streams of application data. Identifying the different applications.


Identifying the different applications.

Separating Multiple Communications Computer connected

to a network that is simultaneously receiving and sending e-mail and instant messages, viewing websites, and conducting a VoIP phone call. Each of these applications is


these applications is sending and receiving data over the network at the same time. However, 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.

Segmentation

Without segmentation, only one application, the streaming video for example, would be able to receive data. You could not


You could not receive e-mails, chat on instant messenger, or view web pages while also viewing the video.

Controlling the Conversation

Segmentation and Reassembly -Most networks have a limitation on the amount of data that can be included in a single PDU. The Transport layer divides application

Conversation Multiplexing -There may be many applications or services running on each host in the network. Each of these applications or services is assigned an


divides application data into blocks of data that are an appropriate size. At the destination, the Transport layer reassembles the data before sending it to the destination application or service.

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

Controlling the Conversation




At the Transport layer the three basic operations of reliability are: Tracking transmitted data Acknowledging received data Retransmitting any unacknowledged data


Transmission Control Protocol (TCP) TCP is a connection-oriented

protocol, described in RFC 793. TCP incurs additional overhead to gain functions. Additional functions specified by TCP are the same order delivery, reliable delivery, and


delivery, reliable delivery, andflow control. Each TCP segment has 20 bytes of overhead in the header encapsulating the Application layer data, whereas each UDP segment only has 8 bytes of overhead

UDP is a simple, connectionless protocol, described in RFC 768. It has the advantage of providing for low overhead data delivery, datagrams are sent as "best effort" by this Transport layer protocol.

Web, email, file transfer

DNS, Video streaming, VoIP

TCP Header format:


TCP Header format: Port number


Source Port 16 bits. Destination Port 16 bits.

TCP Header format: Sequence


Sequence Number: 32 bits The sequence number of the

first data octet in this segment (except when SYN is present).

TCP Header format: Acknowledgment


Acknowledgment Number: 32 bits This field contains the value of the

next sequence number the sender of the segment is expecting to receive.

TCP Header format: Code bits


Control Bits: 8 bits ACK: Acknowledgment field significant RST: Reset the connection SYN: Synchronize sequence numbers FIN: No more data from sender

TCP Header format: Window


Window: 16 bits The number of data octets beginning

with the one indicated in the acknowledgment field which the sender of this segment is willing to accept.

User Datagram Protocol (UDP)


UDP is a simple protocol that exchanges datagrams, without acknowledgments or guaranteed delivery. Error processing and retransmission must be handled by higher layer protocols

UDP is designed for applications that do not need to put sequences of segments together

UDP header fields

Source port Number of the calling port

Destination port Number of the called port

Length Number of bytes including header and data

Checksum Calculated checksum of the header and data fields

Data Upper-layer protocol data


Data Upper-layer protocol data

The protocols that use UDP include:

- TFTP (Trivial File Transfer Protocol). - SNMP (Simple Network Management Protocol). - DHCP (Dynamic Host Control Protocol). - DNS (Domain Name System).


Identify how a port number is represented and describe the role port numbers play in the TCP and UDP protocols.


Port and TCP port


Reserved for services and applications. They are commonly used for applications such as HTTP (web server) POP3/SMTP (e-mail server) and Telnet

These port numbers are assigned to user processes or applications. When not used for a server resource, these ports may also be used dynamically selected by a client as its source port.

These are usually assigned dynamically to client applications when initiating a connection. It is not very common for a client to connect to a service using a Dynamic or Private Port

UDP port


TCP and UDP Common Port


TCP and UDP port numbers


Both TCP and UDP use port (socket) numbers to pass information to the upper layers. Port numbers are used to keep track of different conversations crossing the network at the same time.

Netstat command



Describe the role of segments in the transport layer and the two principle ways segments can be marked for reassembly.


Application and Operation of TCP Mechanisms

Trace the steps that show how the TCP reliability mechanism works as part of a session



Describe the role of port numbers in establishing TCP sessions and directing segments to server process


3 way hand shake



Trace the steps in the handshake in the establishment of TCP sessions



Trace the steps in the handshake in the termination of TCP sessions


Managing TCP Sessions

TCP uses sequence numbers to reconstruct the data stream with segments placed in the correct order.

Sequence number incremented by the number of bytes that have been transmitted.



Trace the steps used by the TCP protocol in which sequence numbers and acknowledgement numbers are used to manage exchanges in a conversation



Describe the retransmission remedy for lost data employed by TCP (refer to 4.3.3.1)



TCP uses window size and acknowledgement to manage data loss and congestion during a session.


Flow Control

When datagrams arrive too quickly for a host or gateway to process and the transport function can issue a not ready indicator to the sender to stop sending data.


sender to stop sending data. When the receiver can handle additional data, the receiver sends a readytransport indicator. When this indicator is received, the sender can resume the segment transmission.

UDP Protocol

UDP is a simple protocol that provides the basic Transport layer functions. It much lower overhead than TCP, does not provide the sophisticated retransmission, sequencing, and flow control mechanisms.

Some UDP applications: Domain Name System (DNS) Simple Network Management Protocol (SNMP)


Simple Network Management Protocol (SNMP) Dynamic Host Configuration Protocol (DHCP) Routing Information Protocol (RIP) Trivial File Transfer Protocol (TFTP) Online games

UDP Protocol

Describe in detail the process specified by the UDP protocol to reassemble PDUs at the destination device


UDP Protocol

Trace the steps as the UDP protocol and port numbers are utilized in client-server communication.


Labs

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Summary


Documents

CCNA Exp1 - Chapter04 - OSI Transport Layer