tailieunhanh - The Illustrated Network- P33

The Illustrated Network- P33:In this chapter, you will learn about the protocol stack used on the global public Internet and how these protocols have been evolving in today’s world. We’ll review some key basic defi nitions and see the network used to illustrate all of the examples in this book, as well as the packet content, the role that hosts and routers play on the network, and how graphic user and command line interfaces (GUI and CLI, respectively) both are used to interact with devices. | CHAPTER 11 Transmission Control Protocol 289 Each device chooses a random initial sequence number to begin counting every byte in the stream sent. How can the two devices agree on both sequence number values in about only three messages Each segment contains a separate sequence number field and acknowledgment field. In Figure the client chooses an initial sequence number ISN in the first SYN sent to the server. The server ACKs the ISN by adding one to the proposed ISN ACKs always inform the sender of the next byte expected and sending it in the SYN sent to the client to propose its own ISN. The client s ISN could be rejected if for example the number is the same as used for the previous connection but that is not considered here. Usually the ACK from the client both acknowledges the ISN from the server with server s ISN 1 in the acknowledgment field and the connection is established with both sides agreeing on ISN. Note that no information is sent in the three-way handshake it should be held until the connection is established. This three-way handshake is the universal mechanism for opening a TCP connection. Oddly the RFC does not insist that connections begin this way especially with regard to setting other control bits in the TCP header there are three others in addition to SYN and ACK and FIN . Because TCP really expects some control bits to be used during connection establishment and release and others only during data transfer hackers can cause a lot of damage simply by messing around with wild combinations of the six control bits especially SYN ACK FIN which asks for uses and releases a connection all at the same time. For example forging a SYN within the window of an existing SYN would cause a reset. For this reason developers have become more rigorous in their interpretation of RFC 793. Data Transfer Sending data in the SYN segment is allowed in transaction TCP but this is not typical. Any data included are accepted but are not processed until after .