TCP uses datagram sockets or ports to establish a host-to-host communication. The standard body called Internet Assigned Numbers Authority (IANA) designates well-known ports to indicate certain services, such as port 80 for HTTP (web) and port 25 for SMTP (mail). The server in the client-server model typically listens on one of these well-known ports in order to receive communication requests from the client. The TCP connection is managed by the operating system by the socket that represents the local endpoint for connection.

The protocol operation consist of a state machine, where the machine needs to keep track of when it is listening for incoming connection, during communication session, as well as releasing resources once the connection is closed. Each TCP connection goes through a series of states such as Listen, SYN-SENT, SYN-RECEIVED, ESTABLISHED, FIN-WAIT, CLOSE-WAIT, CLOSING, LAST-ACK, TIME-WAIT, and CLOSED.

The biggest difference between TCP and User Datagram Protocol (UDP), which is its close cousin at the same layer, is that it transmits data in an ordered and reliable fashion. The fact that the operation guarantees delivery often referred to TCP as a connection-oriented protocol. It does this by first establishing a three-way handshake to synchronize the sequence number between the transmitter and the receiver, SYN, SYN-ACK, and ACK.

The acknowledgement is used to keep track of subsequent segments in the conversation. Finally at the end of the conversation, one side will send a FIN message, the other side will ACK the FIN message as well as send a FIN message of its own. The FIN initiator will then ACK the FIN message that it received.

As many of us who have troubleshot a TCP connection can tell you, the operation can get quite complex. One can certainly appreciate that most of the time, the operation just happens silently in the background.

A whole book can be written about the TCP protocol; in fact, many excellent books have been written on the protocol.

Network Address Translation (NAT) is typically used for translating a range of private IPv4 addresses to publicly routable IPv4 addresses. But it can also mean a translation between IPv4 to IPv6, such as at a carrier edge when they use IPv6 inside of the network that needs to be translated to IPv4, when the packet leaves the network. Sometimes, NAT6to6 is used as well for security reasons.

Security is a continuous process that integrates all the aspects of networking, including automation and Python. This book aims at using Python to help you manage the network; security will be addressed as part of the chapter such as using SSHv2 over telnet. We will also look at how we can use Python and other tool chains to gain visibility in the network.

In my opinion, IP routing is about having the intermediate devices between the two endpoint transmit the packets between them based the IP header. For every communication on the internet, the packet will traverse through various intermediate devices. As mentioned, the intermediate devices consist of routers, switches, optical gears, and various other gears that do not examine beyond the network and transport layer. In a road trip analogy, you might travel in the United States from the city of San Diego in California to the city of Seattle in Washington. The IP source address is analogous to San Diego and the destination IP address can be thought of as Seattle. On your road trip, you will stop by many different intermediate spots, such as Los Angeles, San Francisco, and Portland; these can be thought of as the routers and switches between the source and destination.

Why was this important? In a way, this book is about managing and optimizing these intermediate devices. In the age of mega datacenters that span sizes of multiple American football fields, the need for efficient, agile, cost effective way to manage the network becomes a major point of competitive advantage for companies. In the future chapters, we will dive into how we can use Python programming to effectively manage the network.