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Chapter 9: Transport Layer

  • 9.0 Introduction
  • 9.1 Subnetting an IPv4 Network
    • Describe the purpose of the transport layer in managing the transportation of data in end-to-end communication.
    • Describe characteristics of the TCP and UDP protocols, including port numbers and their uses.
  • 9.2 Addressing Schemes
    • Explain how TCP session establishment and termination processes facilitate reliable communication.
    • Explain how TCP protocol data units are transmitted and acknowledged to guarantee delivery.
    • Describe the UDP client processes to establish communication with a server.
    • Compare UDP and TCP.
  • 9.3 Summary

9.1 Transport Layer Protocols

Transportation of Data

  • Role of the Transport Layer
    • Responsible for establishing a temporary communication session between two applications and delivering data between them.
    • Provides Connection-oriented data stream support, Reliability, Flow control, Multiplexing
  • Transport Layer Responsibilities
    • Track individual conversations.
    • Segment Data and Reassemble Segments.
    • Identify the Applications.
  • Conversation Multiplexing
    • Segments data into small chunks.
    • Label data chunks according to the conversation.
  • Transport Layer Reliability
    • Two protocols provided: TCP and UDP.
    • TCP supports reliability while UDP doesn’t.
  • TCP
    • Supports packet delivery confirmation.
    • There are three basic operations that enable reliability with TCP:
      • Numbering and tracking data segments transmitted to a specific host from a specific application
      • Acknowledging received data
      • Retransmitting any unacknowledged data after a certain period of time
  • UDP
    • UDP provides the basic functions for delivering data segments between the appropriate applications, with very little overhead and data checking.
    • Perfect for applications that don’t require reliability.
  • The Right Transport Layer Protocol for the Right Application
    • TCP is better for databases, web browsers, email clients, etc.
    • UDP is better for live audio or video streaming, VoIP, etc.

TCP and UDP Overview

    • TCP Features
      • Establishing a session
      • Reliable delivery
      • Same-Order delivery
      • Flow control
    • TCP Header
      • TCP is a stateful protocol.
      • TCP adds 20 bytes of overhead in the segment header.

CCNA 1 v6.0 Study Material - Chapter 9: Transport Layer 8

    • UDP Features
      • Simple and fast.
    • UDP Header
      • UDP is a stateless protocol.
      • Reliability must be handled by the application.
      • The pieces of communication in UDP are called Datagrams.
      • UDP adds only 8 bytes of overhead.

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    • Multiple Separate Conversations
      • The transport layer separate sand manages multiple communications with different transport requirements.
      • Different applications are sending and receiving data over the network simultaneously.
      • Unique header values allow TCP and UDP to manage these multiple and simultaneous conversations by identifying these applications.
      • These unique identifiers are the port numbers.
    • Port Numbers
      • Usually seen in pairs: source port and destination port.
      • The source port is dynamically chosen by the sender.
      • The destination port is used to identify an application on the server (destination).

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    • Socket Pairs
      • The combination of the source IP address and source port number, or the destination IP address and destination port number, is known as a socket.
      • The socket is used to identify the server and service being requested by the client.
      • Two sockets combine to form a socket pair: (,
      • Sockets enable multiple processes running on a client and multiple connections to a server process to be distinguished from each other.
    • Port Number Groups
      • The IANA has created three port number groups:
      • Well-known ports (0 to 1023)
      • Registered Ports (1024 to 49151)
      • Private and/or Dynamic Ports (49152 to 65535)
    • The netstat Command
      • Netstat allows a user to see active connections in a host.
      • Netstat also displays the process using the connection.

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9.2 TCP and UDP

TCP Communication Process

  • TCP Server Processes
    • Each application process running on the server uses a port number.
    • An individual server cannot have two services assigned to the same port number within the same transport layer service.
    • An active server application assigned to a specific port is considered to be open.
    • Any incoming client request addressed to an open port is accepted and processed by the server application bound to that port.
    • There can be many ports open simultaneously on a server, one for each active server application.
  • TCP Connection Establishment
    • A TCP connection is established in three steps:
    • The initiating client requests a client-to-server communication session with the server.
    • The server acknowledges the client-to-server communication session and requests a server-to-client communication session.
    • The initiating client acknowledges the server-to-client communication session.
  • TCP Session Termination
    • The FIN TCP flag is used to terminate a TCP connection.
      • When the client has no more data to send in the stream, it sends a segment with the FIN flag set.
      • The server sends an ACK to acknowledge the receipt of the FIN to terminate the session from client to server.
      • The server sends a FIN to the client to terminate the server-to-client session.
      • The client responds with an ACK to acknowledge the FIN from the server.
      • When all segments have been acknowledged, the session is closed.
  • TCP Three-way Handshake Analysis
    • The three-way handshake:
      • Establishes that the destination device is present on the network.
      • Verifies that the destination device has an active service and is accepting requests on the destination port number that the initiating client intends to use
      • Informs the destination device that the source client intends to establish a communication session on that port number.

Reliability and Flow Control

    • TCP Reliability – Ordered Delivery
      • TCP segments use sequence numbers to uniquely identify and acknowledge each segment, keep track of segment order, and indicate how to reassemble and reorder received segments.
      • An initial sequence number (ISN) is randomly chosen during the TCP session setup. The ISN is then incremented by the number of transmitted bytes.
      • The receiving TCP process buffers the segment data until all data is received and reassembled.
      • Segments received out of order are held for later processing.
      • The data is delivered to the application layer only when it has been completely received and reassembled.

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    • TCP Flow Control – Window Size and Acknowledgments
      • TCP provides mechanisms for flow control.
      • Flow control ensures the TCP endpoints can receive and process data reliably.
      • TCP handles flow control by adjusting the rate of data flow between source and destination for a given session.
      • TCP flow control function relies on a 16-bit TCP header field called the Window size. The window size is the number of bytes that the destination device of a TCP session can accept and process at one time.
      • TCP source and destination agree on the initial window size when the TCP session is established
      • TCP endpoints can adjust the window size during a session if necessary.

CCNA 1 v6.0 Study Material - Chapter 9: Transport Layer 13

    • TCP Flow Control – Congestion Avoidance
      • Network congestion usually results in discarded packets.
      • Undelivered TCP segments trigger re-transmission. TCP segment retransmission can make the congestion even worse.
      • The source can estimate a certain level of network congestion by looking at the rate at which TCP segments are sent but not acknowledged.
      • The source can reduce the number of bytes it sends before receiving an acknowledgement upon congestion detection.
      • The source reduces the number of unacknowledged bytes it sends and not the window size, which is determined by the destination.
      • The destination is usually unaware of the network congestion and sees no need to suggest a new window size.

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UDP Communication

  • UDP Low Overhead Vs. Reliability
    • UDP has much lower overhead than TCP.
    • UDP is not connection-oriented and does not offer the sophisticated retransmission, sequencing, and flow control mechanisms.
    • Applications running UDP can still use reliability, but it must be implemented in the application layer.
    • However, UDP is not inferior.
  • UDP Datagram Reassembly
    • UDP simply reassembles the data in the order in which it was received.
    • The application must identify the proper sequence, if necessary.
  • UDP Server Processes and Requests
    • UDP-based server applications are also assigned well-known or registered port numbers.
    • Requests received on a specific port are forwarded to the proper application based on port numbers.
  • UDP Client Processes
    • UDP client-server communication is also initiated by a client application.
    • The UDP client process dynamically selects a port number and uses this as the source port.
    • The destination port is usually the well-known or registered port number assigned to the server process.
    • The same source-destination pair of ports is used in the header of all datagrams used in the transaction.
    • Data returning to the client from the server uses a flipped source and destination port numbers in the datagram header.


  • Applications that Use TCP
    • TCP handles all transport layer related tasks.
    • This frees the application from having to manage any of these tasks.
    • Applications can simply send the data stream to the transport layer and use the services of TCP.
  • Applications that Use UDP
    • Live video and multimedia applications – Can tolerate some data loss, but require little or no delay. Examples include VoIP and live streaming video.
    • Simple request and reply applications – Applications with simple transactions where a host sends a request and may or may not receive a reply. Examples include DNS and DHCP.
    • Applications that handle reliability themselves – Unidirectional communications where flow control, error detection, acknowledgements, and error recovery is not required or can be handled by the application. Examples include SNMP and TFTP.

9.3 Summary


  • Implement an IPv4 addressing scheme to enable end-to-end connectivity in a small to medium-sized business network.
  • Given a set of requirements, implement a VLSM addressing scheme to provide connectivity to end users in a small to medium-sized network.
  • Explain design considerations for implementing IPv6 in a business network.

Chapter 9 New Terms and Commands

  • Segmentation
  • Multiplexing
  • Transmission Control Protocol (TCP)
  • Flow Control
  • User Datagram Protocol (UDP)
  • Port Addressing
  • Socket
  • Three-way Handshake
  • Initial Sequence Number (ISN)
  • Sequence Numbers
  • Window Size
  • Flow Control – Congestion Avoidance