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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 Encapsulation and Formatting 1 2 IPv6 Datagram Main Header Format

The method by which IPv6 encapsulates data received from higher-layer protocols for transmission across the internetwork is basically the same as that used by IPv4. The data received from the transport or higher layers is made the payload of an IPv6 datagram, which has one or more headers that control the delivery of the message. These headers provide information to routers to enable them to move the datagram across the network, and also to hosts so they can tell which datagrams they are intended to receive.

While the basic use of datagrams hasn't changed since IPv4, many modifications were made to their structure and format when IPv6 was created. This was done partly out of necessity: IPv6 addresses are different than IPv4 addresses and IP addresses go in the datagram header. The increase in the size of IP addresses from 32 bits to 128 bits adds a whopping extra 192 bits, or 24 bytes of information to the header. This in turn led to an effort to remove fields that weren't strictly necessary, to compensate for the necessary increase in size. However, changes were also made to IPv6 datagrams to add features to them and to make them better suit the needs of modern internetworking.

The following is a list of the most significant overall changes to datagrams in IPv6:

• Multiple Header Structure: Rather than a single header that contains all fields for the datagram (possibly including options), the IPv6 datagram supports a “main” header and then extension headers for additional information when needed.
  
  
• Streamlined Header Format: Several fields have been removed from the main header to reduce its size and increase efficiency. Only the fields that are truly required for pretty much all datagrams remain in the main header; others are put into extension headers and used as needed. Some were removed because they were no longer needed, such as the Internet Header Length field; the IPv6 header is of fixed length. This is discussed more thoroughly in the topic on the main IPv6 header.
  
  
• Renamed Fields: Some fields have been renamed to better reflect their actual use in modern networks.
  
  
• Greater Flexibility: The extension headers allow a great deal of extra information to accompany datagrams when needed. Options are also supported in IPv6.
  
  
• Elimination of Checksum Calculation: In IPv6, a checksum is no longer computed on the header. This saves both the calculation time spent by every device that packages IP datagrams (hosts and routers) and the space the checksum field took up in the IPv4 header.
  
  
• Improved Quality of Service Support: A new field, the Flow Label, is defined to help support the prioritization of traffic.

IPv6 Datagram Encapsulation and Formatting 1 2 IPv6 Datagram Main Header Format