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.

• • 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.

• • 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).

• • 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: (192.168.1.5:1099, 192.168.1.7:80).
    • 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.

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.

• • 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.

• • 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.

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.

TCP or UDP

• 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

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