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``10`` (resp. ``11``) : if a node does not recognize this header, it must return a control packet (ICMP, see later) back to the source (resp. except if the destination was a multicast address)
This encoding allows the designers of protocol extensions to specify whether the option must be supported by all nodes on a path or not. Still, deploying such an extension can be difficult in practice.
Two `hop-by-hop` options have been defined. :rfc:`2675` specifies the jumbogram that enables IPv6 to support packets containing a payload larger than 65535 bytes. These jumbo packets have their `payload length` set to `0` and the jumbogram option contains the packet length as a 32 bits field. Such packets can only be sent from a source to a destination if all the routers on the path support this option. However, as of this writing it does not seem that the jumbogram option has been implemented. The router alert option defined in :rfc:`2711` is the second example of a `hop-by-hop` option. The packets that contain this option should be processed in a special way by intermediate routers. This option is used for IP packets that carry Resource Reservation Protocol (RSVP) messages, but this is outside the scope of this book.
The `Destinations Option` header uses the same format as the `Hop-by-Hop Options` header. It has some usages, e.g. to support mobile nodes :rfc:`6275`, but these are outside the scope of this document.
The `Fragment Options` header is more important. An important problem in the network layer is the ability to handle heterogeneous datalink layers. Most datalink layer technologies can only transmit and receive frames that are shorter than a given maximum frame size. Unfortunately, all datalink layer technologies use different maximum frames sizes.
Each datalink layer has its own characteristics and as indicated earlier, each datalink layer is characterized by a maximum frame size. From IP's point of view, a datalink layer interface is characterized by its `Maximum Transmission Unit (MTU)`. The MTU of an interface is the largest packet (including header) that it can send. The table below provides some common MTU sizes.
Datalink layer
MTU
Ethernet
1500 bytes
WiFi
2272 bytes
ATM (AAL5)
9180 bytes
802.15.4
102 or 81 bytes
Token Ring
4464 bytes
FDDI
4352 bytes
Although IPv6 can send 64 KBytes long packets, few datalink layer technologies that are used today are able to send a 64 KBytes packet inside a frame. Furthermore, as illustrated in the figure below, another problem is that a host may send a packet that would be too large for one of the datalink layers used by the intermediate routers.
The need for fragmentation and reassembly
To solve these problems, IPv6 includes a packet fragmentation and reassembly mechanism. In IPv4, fragmentation was performed by both the hosts and the intermediate routers. However, experience with IPv4 has shown that fragmenting packets in routers was costly [KM1995]_. For this reason, the developers of IPv6 have decided that routers would not fragment packets anymore. In IPv6, fragmentation is only performed by the source host. If a source has to send a packet which is larger than the MTU of the outgoing interface, the packet needs to be fragmented before being transmitted. In IPv6, each packet fragment is an IPv6 packet that includes the `Fragmentation` header. This header is included by the source in each packet fragment. The receiver uses them to reassemble the received fragments.
IPv6 fragmentation header
If a router receives a packet that is too long to be forwarded, the packet is dropped and the router returns an ICMPv6 message to inform the sender of the problem. The sender can then either fragment the packet or perform Path MTU discovery. In IPv6, packet fragmentation is performed only by the source by using IPv6 options.
In IPv6, fragmentation is performed exclusively by the source host and relies on the fragmentation header. This 64 bits header is composed of six fields :
a `Next Header` field that indicates the type of the header that follows the fragmentation header
two `Reserved` fields set to `0`.
the `Fragment Offset` is a 13-bit unsigned integer that contains the offset, in 8 bytes units, of the data following this header, relative to the start of the original packet.
the `More` flag, which is set to `0` in the last fragment of a packet and to `1` in all other fragments.
the 32-bit `Identification` field indicates to which original packet a fragment belongs. When a host sends fragmented packets, it should ensure that it does not reuse the same `identification` field for packets sent to the same destination during a period of `MSL` seconds. This is easier with the 32 bits `identification` used in the IPv6 fragmentation header, than with the 16 bits `identification` field of the IPv4 header.

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read-only
Source string location
../../protocols/ipv6.rst:407
String age
2 years ago
Source string age
2 years ago
Translation file
locale/pot/protocols/ipv6.pot, string 125