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

Chapter 143
IPv6 Header

Notice the differences between IPv4 and IPv6 headers. IPv6 seems to be missing a few options, but they are there; they just cannot be seen (yet!). In fact, the only field that seems to not have changed or moved positions is the VERS field. This field was to play a great role. It was going to be used as the delineating factor to determine if a received IP packet was based on IPv4 or IPv6. In other words, the EtherType field of an Ethernet packet would remain as 0800(h) and the version field of the header would determine the processing of a received IP datagram. This changed and IPv6 has its own Type (for Ethernet packets) field: 86DD(h) (and SAP in IEEE 802 networks).

The internet protocol (version 4) uses four key mechanisms to provide its service: Type of Service (TOS), Time to Live (TTL), Options, Fragmentation, Protocol and Header Checksum. However, in looking at the slide, these fields are missing.

These mechanisms were previously discussed, but the options field is further described here. The Options provide for control functions needed or useful in some situations but unnecessary for the most common communications. The options include provisions for timestamps, security, and special routing (strict and loose source route—nothing to do with Token Ring). However, over the years, it was noticed that these options fields were not being used by the majority of Internet hosts for various reasons. First, IP datagrams that contain options cannot be simply forwarded; they require special attention. They are placed in another queue and the router operates on this queue separately from the received datagram queue. Second, if the options fields were not used very often, many implementers of routers did not optimize their software to operate on datagrams that included special options. This generally resulted in a performance penalty on the router.

IPv6 Header

So, why have them? Well, their functions are used in some cases. IP multicasting, for example, uses the loose source route option when incorporating the tunneling mechanism for DVMRP (refer to Part V, “Multicasting,” to understand more IPv6 decided to allow for it to be extensible, so IPv6 implements the concept of an extension header.