IPv6 Essentials, 3rd Edition

Chapter 3. The Structure of the IPv6 Protocol

This chapter explains the structure of the IPv6 header and compares it to the IPv4 header. It also discusses Extension headers, which are new in IPv6.

Understanding the structure of a protocol header and the type of information that comes with it is the best foundation for working with a protocol. This understanding helps you to identify how the protocol can best be configured and what the options are. It also helps you to identify possible sources of problems and issues when troubleshooting.

The header structure of an IPv6 packet is specified in RFC 2460. The header has a fixed length of 40 bytes. The two fields for Source and Destination addresses each use 16 bytes (128 bits), so there are only 8 bytes for general header information. The base IPv6 header is therefore much simpler and leaner than the IPv4 header, allowing for more efficient processing and, as we will see, more flexibility in extending the protocol to meet future needs.

General Header Structure

In IPv6, five fields from the IPv4 header have been removed:

• Header Length
• Identification
• Flags
• Fragment Offset
• Header Checksum

The Header Length field was removed because it is not needed in a header with a fixed length. In IPv4, the minimum header length is 20 bytes, but if options are added, it can be extended in 4-byte increments up to 60 bytes. Therefore, with IPv4, the information about the total length of the header is important. In IPv6, options are defined in Extension ...