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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Internet Layer (OSI Network Layer) Protocols            9  Internet Protocol (IP/IPv4, IPng/IPv6) and IP-Related Protocols (IP NAT, IPSec, Mobile IP)                 9  Internet Protocol Version 6 (IPv6) / IP Next Generation (IPng)                      9  IPv6 Datagram Encapsulation and Formatting

IPv6 Datagram Main Header Format 1 2 3 4 5 6 IPv6 Datagram Options

IPv6 Header Chaining Using the Next Header Field

The only field common to all extension header types is the Next Header field (which actually appears at the end of one header type, the ESP header). The 8-bit Next Header field is used to logically link all the headers in an IPv6 datagram as follows:

• The Next Header field in the main header contains a reference number for the first extension header type.
  
  
• The Next Header field in the first extension header contains the number of the second extension header type, if there is a second one. If there's a third, the second header's Next Header points to it, and so on.
  
  
• The Next Header field of the last extension header contains the protocol number of the encapsulated higher-layer protocol. In essence, this field points to the “next header” within the payload itself.

For example, suppose a datagram that encapsulates TCP has a Hop-By-Hop Options extension header and a Fragment extension header. Then, the Next Header fields of these headers would contain the following values:

• The main header would have a Next Header value of 0, indicating the Hop-By-Hop Options header.
  
  
• The Hop-By-Hop Options header would have a Next Header value of 44 (decimal), the value for the Fragment extension header.
  
  
• The Fragment header would have a Next Header value of 6.

This is illustrated in Figure 106.

IPv6 Datagram Main Header Format 1 2 3 4 5 6 IPv6 Datagram Options