IP fragmentation
Encyclopedia
The Internet Protocol
(IP) implements datagram
fragmentation, so that packets may be formed that can pass through a link with a smaller maximum transmission unit
(MTU) than the original datagram size.
RFC 791 describes the procedure for IP fragmentation, and transmission and reassembly of
datagrams. RFC 815 describes a simplified reassembly algorithm.
The Identification field, and Fragment offset field along with Don't Fragment and More Fragment flags in the IP protocol header are used for fragmentation and reassembly of IP datagrams.
In a case where a router receives a protocol data unit
(PDU) larger than the next hop's MTU, it has two options if the transport is IPv4
. Drop the PDU and send an Internet Control Message Protocol
(ICMP) message which indicates the condition Packet too Big, or to fragment the IP packet and send it over the link with a smaller MTU. IPv6
hosts are required to determine the optimal Path MTU before sending packets.
If a receiving host receives a fragmented IP packet, it has to reassemble the datagram and pass it to the higher protocol layer. Reassembly is intended to happen in the receiving host but in practice it may be done by an intermediate router, for example, network address translation
may need to re-assemble fragments in order to translate data streams, e.g. the FTP control protocol, as described in RFC 2993.
IP fragmentation can cause excessive retransmissions when fragments encounter packet loss
and reliable protocols such as TCP must retransmit all of the fragments in order to recover from the loss of a single fragment. Thus, senders typically use two approaches to decide the size of IP datagrams to send over the network. The first is for the sending host to send an IP datagram of size equal to the MTU of the first hop of the source destination pair. The second is to run the path MTU discovery
algorithm, described in RFC 1191, to determine the path MTU between two IP hosts, so that IP fragmentation can be avoided.
. Though the header formats are different for IPv4 and IPv6, analogous fields are used for fragmentation, so the algorithm can be reused for fragmentation and reassembly.
In IPv4, hosts must make a best-effort attempt to reassemble fragmented IP datagrams with a total reassembled size of up to 576 bytes - equal to the minimum MTU for IPv4. They may also attempt to reassemble fragmented IP datagrams larger than 576 bytes, but they are also permitted to silently discard such larger datagrams. In IPv6, this minimum capability is increased to 1280 bytes - larger than the minimum MTU for IPv4.
Internet Protocol
The Internet Protocol is the principal communications protocol used for relaying datagrams across an internetwork using the Internet Protocol Suite...
(IP) implements datagram
Datagram
A datagram is a basic transfer unit associated with a packet-switched network in which the delivery, arrival time, and order are not guaranteed....
fragmentation, so that packets may be formed that can pass through a link with a smaller maximum transmission unit
Maximum transmission unit
In computer networking, the maximum transmission unit of a communications protocol of a layer is the size of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface...
(MTU) than the original datagram size.
RFC 791 describes the procedure for IP fragmentation, and transmission and reassembly of
datagrams. RFC 815 describes a simplified reassembly algorithm.
The Identification field, and Fragment offset field along with Don't Fragment and More Fragment flags in the IP protocol header are used for fragmentation and reassembly of IP datagrams.
In a case where a router receives a protocol data unit
Protocol data unit
In telecommunications, the term protocol data unit has the following meanings:#Information that is delivered as a unit among peer entities of a network and that may contain control information, address information, or data....
(PDU) larger than the next hop's MTU, it has two options if the transport is IPv4
IPv4
Internet Protocol version 4 is the fourth revision in the development of the Internet Protocol and the first version of the protocol to be widely deployed. Together with IPv6, it is at the core of standards-based internetworking methods of the Internet...
. Drop the PDU and send an Internet Control Message Protocol
Internet Control Message Protocol
The Internet Control Message Protocol is one of the core protocols of the Internet Protocol Suite. It is chiefly used by the operating systems of networked computers to send error messages indicating, for example, that a requested service is not available or that a host or router could not be...
(ICMP) message which indicates the condition Packet too Big, or to fragment the IP packet and send it over the link with a smaller MTU. IPv6
IPv6
Internet Protocol version 6 is a version of the Internet Protocol . It is designed to succeed the Internet Protocol version 4...
hosts are required to determine the optimal Path MTU before sending packets.
If a receiving host receives a fragmented IP packet, it has to reassemble the datagram and pass it to the higher protocol layer. Reassembly is intended to happen in the receiving host but in practice it may be done by an intermediate router, for example, network address translation
Network address translation
In computer networking, network address translation is the process of modifying IP address information in IP packet headers while in transit across a traffic routing device....
may need to re-assemble fragments in order to translate data streams, e.g. the FTP control protocol, as described in RFC 2993.
IP fragmentation can cause excessive retransmissions when fragments encounter packet loss
Packet loss
Packet loss occurs when one or more packets of data travelling across a computer network fail to reach their destination. Packet loss is distinguished as one of the three main error types encountered in digital communications; the other two being bit error and spurious packets caused due to noise.-...
and reliable protocols such as TCP must retransmit all of the fragments in order to recover from the loss of a single fragment. Thus, senders typically use two approaches to decide the size of IP datagrams to send over the network. The first is for the sending host to send an IP datagram of size equal to the MTU of the first hop of the source destination pair. The second is to run the path MTU discovery
Path MTU discovery
Path MTU Discovery is a standardized technique in computer networking for determining the maximum transmission unit size on the network path between two Internet Protocol hosts, usually with the goal of avoiding IP fragmentation...
algorithm, described in RFC 1191, to determine the path MTU between two IP hosts, so that IP fragmentation can be avoided.
IPv4 and IPv6 differences
The details of the fragmentation mechanism, as well as the overall architectural approach to fragmentation, are different between IPv4, the first official version of the Internet Protocol, and IPv6, the newer version. In IPv4, routers perform fragmentation, whereas in IPv6, routers do not fragment, but drop the packets that are larger than the MTUMaximum transmission unit
In computer networking, the maximum transmission unit of a communications protocol of a layer is the size of the largest protocol data unit that the layer can pass onwards. MTU parameters usually appear in association with a communications interface...
. Though the header formats are different for IPv4 and IPv6, analogous fields are used for fragmentation, so the algorithm can be reused for fragmentation and reassembly.
In IPv4, hosts must make a best-effort attempt to reassemble fragmented IP datagrams with a total reassembled size of up to 576 bytes - equal to the minimum MTU for IPv4. They may also attempt to reassemble fragmented IP datagrams larger than 576 bytes, but they are also permitted to silently discard such larger datagrams. In IPv6, this minimum capability is increased to 1280 bytes - larger than the minimum MTU for IPv4.
See also
- IPv4 fragmentation and reassembly
- IPv6 fragmentation
- IP fragmentation attacksIP fragmentation attacksIP fragmentation is the process of breaking up a single Internet Protocol datagram into multiple packets of smaller size. Every network link has a characteristic size of messages that may be transmitted, called the maximum transmission unit ....