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 Extension Header Order

Each extension header appears only once in any datagram (with one exception; see below). Also, extension headers are only examined by the final recipients of the datagram, not intermediate devices (again with one exception, which we will get to momentarily). RFC 2460 specifies that when multiple headers appear, they should be in the following order after the main header and before the higher-layer encapsulated header in the IPv6 datagram payload:

1. Hop-By-Hop Options
   
   
2. Destination Options (for options to be processed by the destination as well as devices specified in a Routing header)
   
   
3. Routing
   
   
4. Fragmentation
   
   
5. Authentication Header
   
   
6. Encapsulating Security Payload
   
   
7. Destination Options (for options processed only by the final destination)

Now let's look at those exceptions. The only header that can appear twice is Destination Options. Normally, it appears as the last header. However, a Destination Options header may exist that contain options that must be examined by a list of devices specified in a source route, in addition to the destination. In this case, the Destination Options header for these options is placed before the Routing header. A second such header containing options only for the final destination may also appear.

The only header normally examined by all intermediate devices is the Hop-By-Hop Options extension header. It is used specifically to convey management information to all routers in a route. The Hop-By-Hop Options extension header must appear as the first extension header if present. Since it is the only one that must be read by every router (which represents a performance drain on routers) it is given “top billing” to make it easier and faster to find and process.

Finally, note that all extension headers must be a multiple of 8 bytes in length for alignment purposes. Also, remember that the Next Header value for a particular extension header appears in the Next Header field of the preceding header, not the header itself.

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