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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)

IPv6 Datagram Options 1 2 3 4 IPv6 Datagram Delivery and Routing

The job of the Internet Protocol is to convey messages across an internet of connected networks. When datagrams are sent between hosts on distant networks they are carried along their journey by routers, one hop at a time, over many physical network links. On each step of this journey the datagram is encoded in a data link layer frame for transmission.

In order for a datagram to be successfully carried along a route, its size must be small enough to fit within the lower-layer frame at each step on the way. The term maximum transmission unit (MTU) describes the size limit for any given physical network. If a datagram is too large for the MTU of a network, it must be broken into pieces, a process called fragmentation, and then the pieces reassembled at the destination device. This has been a requirement since IPv4, and I explain the concepts and issues related to datagram size, MTUs, fragmentation and reassembly in detail in a section devoted to these matters in IPv4.

All of these issues apply to sending datagrams in IPv6 as much as they did in IPv4. However, as in other areas of the protocol, some important details of how fragmentation and reassembly are done have changed. These changes were made to improve the efficiency of the routing process, and also to reflect the realities of current networking technologies: most can handle average IP datagrams without needing fragmentation.

The most important differences between IPv4 and IPv6 with respect to datagram size, MTU, fragmentation and reassembly are:

• Increased Default MTU: In IPv4, the minimum MTU that routers and physical links were required to handle was 576 bytes. In IPv6, all links must handle a datagram size of at least 1280 bytes. This more-than-doubling in size improves efficiency by increasing the ratio of maximum payload to header length, and reduces the frequency with which fragmentation is required.
  
  
• Elimination of En Route Fragmentation: In IPv4, datagrams may be fragmented by either the source device, or by routers during delivery. In IPv6, only the source node can fragment; routers do not. The source must therefore fragment to the size of the smallest MTU on the route before transmission. This has both advantages and disadvantages, as we will see. Reassembly is of course still done only by the destination, as in IPv4.
  
  
• MTU Size Error Feedback: Since routers cannot fragment datagrams, they must drop them if they are forced to try to send a too-large datagram over a physical link. A feedback process has been defined using ICMPv6 that lets routers tell source devices that they are using datagrams that are too large for the route.
  
  
• Path MTU Discovery: Since source devices must decide on the correct size of fragments, it is helpful if they have a mechanism for determining what this should be. This capability is provided through a special technique called Path MTU Discovery, which was originally defined for IPv4 but has been refined for IPv6.
  
  
• Movement of Fragmentation Header Fields: To reflect the decreased importance of fragmentation in IPv4, the permanent fields related to the process that were in the IPv4 header have been farmed out to a Fragment extension header, included only when needed.

IPv6 Datagram Options 1 2 3 4 IPv6 Datagram Delivery and Routing