Chapter 9. UDP/TCP Analysis

This chapter contains the following recipes:

  • Configuring TCP and UDP preferences for troubleshooting
  • TCP connection problems
  • TCP retransmissions – where they come from and why
  • Duplicate ACKs and fast retransmissions
  • TCP out-of-order packet events
  • TCP Zero Window, Window Full, Window Change, and other Window indicators
  • TCP resets and why they happen

Introduction

The goal of Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) is to pass information between end applications, for example, from a web client to a web server, mail client to a mail server, and so on. This is done by providing identification to end applications and forwarding packets between them. These identifications are called port numbers, and a port number with its IP address is called a socket. In the following diagram you can see what happens when you open a connection from your browser to a web server. The web server listens on port 80 and you will open a connection, for example, from port 1024.

So, the server is listening to requests on port 80 and will send responses to you on port 1024.

Introduction

While TCP is a reliable, connection-oriented protocol, UDP does not support connectivity and reliability, but simply transfers datagrams between two end processes.

Tip

There is an additional layer-4 protocol, which is called SCTP (Stream Control Transmission Protocol). This protocol can be considered as an improved version of TCP, and mostly used in a service provider's networks. SCTP is not included in the scope of this book.

In this chapter, we will focus on TCP, its behavior, various problems, and how to use Wireshark in order to isolate and solve them.

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