The Illustrated Network- P22: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 6 IPv4 and IPv6 Headers 179 The reassembly time-out value must have a value low enough to make the recovery process delay of the transport layer reasonable. The transport layer contains session connection information that will detect a missing packet in a sequence of segments the contents of the packets and TCP always requests missing information to be resent. Too long a value for the reassembly timer makes this retransmission process very inefficient. Too short a value leads to needlessly discarded packets. In most TCP IP implementations the reassembly timer is set by the software vendor and cannot be changed. This is yet another reason to avoid fragmentation. Reassembly deadlock used to be a problem as well. When memory was a scarce commodity in hosts all available local buffer memory could end up holding partially assembled fragments. An arriving fragment could not be accepted even if it completed a set and the system eventually hung. However in these days of cheap and plentiful memory this rarely happens. Limitations of IPv4 The limitations of IPv4 are often cast solely in terms of address space. As important as that is it is only part of the story. Address space is not the only IPv4 limitation. Some others follow The fragmentation fields are present in every IPv4 packet. Fragmentation is always done with a performance penalty and is best avoided. Yet the fields involved all 6 bytes worth and more than 25 of the basic 20-byte IPv4 header must be present in each and every packet. IPv4 Options were seldom used and limited in scope. The IPv4 Type of Service field was never used as intended. The IPv4 Time To Live field was also never used as intended. The 8-bit IPv4 Type field limited IPv4 packet content to 256 possibilities. All of these factors contributed to the structure of the IPv6 packet header. The IPv6 Header Structure Let s go back to our Windows devices and capture some IPv6 packets. Then we can examine those headers and compare them to IPv4 .
