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IP fragmentation

 
Computer Desktop Encyclopedia: IP fragmentation
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Breaking an IP datagram (packet) into pieces in order to be sent across a transmission link with a frame size smaller than the datagram. Performed in a router, the header of the original IP packet is replicated with minor changes to each of the fragments. If one of the fragments is dropped, the original datagram must be fragmented again and retransmitted.

IP Fragmentation Is Often Avoided

IP fragments are problematic. Some firewalls and content switches may not allow them, and they add overhead to routers, which are designed for high-speed packet forwarding, not fragment creation and reassembly. To avoid IP fragmentation, Maximum Segment Size (MSS) messages are sent between two end points to find the maximum frame size that can be transmitted intact.

In a network with multiple data links, the router uses Path Maximum Transmission Unit Discovery (PMTUD) to send error messages back to the sender whenever a link with a smaller frame size is encountered. The host then reduces its TCP packet size and retransmits. PMTUD does not work with UDP, only TCP.

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Wikipedia: IP fragmentation
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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.[1] RFC 815 describes a simplified reassembly algorithm.[2]

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 requires re-calculating checksums across entire datagrams, and so routers supporting this will often reassemble packets in the process.

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.[3] 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,[4] described in RFC 1191, to determine the path MTU between two IP hosts, so that IP fragmentation can be avoided.

Contents

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 MTU. 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.

See also

References

  1. ^ RFC 791, Internet Protocol, Information Sciences Institute (September 1981)
  2. ^ RFC 815, IP Datagram Reassembly Algorithms, David D. Clark (July 1982)
  3. ^ Christopher A. Kent, Jeffrey C. Mogul. "Fragmentation Considered Harmful". http://citeseer.ist.psu.edu/335647.html. 
  4. ^ RFC 1191, Path MTU Discovery (November 1990)

External links


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