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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 Overview and General Structure 1 2 3 IPv6 Datagram Extension Headers

Key Changes to Main Header Between IPv4 and IPv6

To summarize, the IPv6 main header compares to the IPv4 header as follows:

• Unchanged Fields: Three fields are used the same way and retain the same name (though they have different content and/or size): Version, Source Address and Destination Address.
  
  
• Renamed Fields: Two fields are used the same way but renamed: Traffic Class and Hop Limit.
  
  
• Modified Fields: Two fields are used in a way similar way to their IPv4 predecessors but are slightly different in meaning and also renamed: Payload Length and Next Header.
  
  
• Added Fields: There is one new field: Flow Label.
  
  
• Removed Fields: To cut down on header length and unnecessary work, five IPv4 header fields are removed from the IPv6 header:
  • Internet Header Length: No longer needed, as the main IPv6 header is fixed in length at 40 bytes.
    
    
  • Identification, Flags, Fragment Offset: These are used for fragmentation, which is done less in IPv6 than IPv4, so these fields are now found only when needed in the Fragmentation extension header.
    
    
  • Header Checksum: The decision was made to eliminate header checksum calculations in IPv6. It was viewed as redundant with higher-layer error-checking and data link layer CRC calculations. This saves processing time for routers and 2 bytes in the datagram header.

In addition, while options were formerly considered part of the main header in IPv4, they are separate in IPv6.

IPv6 Datagram Overview and General Structure 1 2 3 IPv6 Datagram Extension Headers