Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 145
IPv4 and IPv6 Header Differences

The first thing to notice about the IPv6 header is that it is a static 40 bytes in length. The length of the packet header is not variable in length. The checksum was removed. IP first ran over copper serial lines that tended to be noisy (static in voice speak). Checksums are all over the TCP/IP protocol suite and all over the access methods of FDDI, Ethernet, and Token Ring. The removal of the checksum also allowed for all systems that forwarded IP datagrams to speed up because they do not have to compute the checksum at every hop.

IPv4’s total-length field is replaced with a payload length. No significant changes here except that IPv6 is a static 40 bytes, so the payload length is truly a measurement of the payload and the IPv6 header is not included as part of the sum. This field is 16 bits in length, which allows for a maximum of 65,355 byte payload. However, IPv6 allows for a new concept known as jumbo datagrams (jumbograms), which allows for various network attachments such as I/O connections between high-speed computers that can process data segments higher than 64k (see RFC 2146).

One of the more interesting changes to IP with version 6 is the concept of concatenated headers. This is accomplished using the next header field on the IPv6 header. In IPv6, the protocol type field is set and that header would immediately follow. For example, if the payload was UDP then the protocol type is set to 17(decimal) and the UDP header would immediately follow.

The Time to Live (TTL) field is one of the more versatile fields in IP. It is used to prevent datagrams from constantly looping, keep packets on a local network, used in multicast datagrams to indicate scope (hearing range), and probably has many other private uses as well. In IPv6, this field is renamed to Hop Limit, because it is really used as a count-down-by-1 counter. The original intention of the field was to indicate a time (in seconds). It could be used, for example, by a router. If the router cannot forward the packet within the amount of time indicated in the TTL field, it should discard the datagram and generate an ICMP message. However, over time, most router delays were measured in milliseconds, not seconds. The accepted decrement of the field was set to 1, and therefore became a hop count and not an indication of time.