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Fragmentation Made Friendly

Patricia Gilfeather and Todd Underwood

31 January, 2000

Abstract:

Fragmentation and reassembly of datagrams too large to be transmitted on the network was built into the TCP/IP protocol in order to allow the internetwork to successfully transmit data across very different kinds of physical and link-layer networks. Early in the days of the internet, fragmentation and reassembly were abandoned in favor of sending datagrams small enough to avoid fragmentation. This adoption was due to inefficiencies in fragmentation and reassembly that were well-stated in Kent and Mogul's 1987 paper, Fragmentation Considered Harmful [KM87]. Our tests conclude that currently fragmentation rarely occurs on the Internet due to widespread adoption of Kent and Mogul's pathMTU discovery technique in which the Maximum Transmission Unit (MTU) of an entire path is discovered and used for that destination. The pathMTU is defined as the minimum MTU for each hop along a network path [MD90] However, with the advent of gigabit Ethernet and other high-speed, reliable networks, the state of network technology has changed drastically since 1987. Where the network was the performance bottleneck in network systems in 1987, the memory access speed of the processors, and interrupt processing latency is quite frequently the bottleneck today. We reconsider fragmentation and reassembly in light of current technology and discuss a use for fragmentation that relieves memory accesses in the endpoints of a network transaction and allows for the possibility of greater overall performance in today's high-speed networks.

The IP protocol has become the most commonly used network protocol. Its adoption has been widespread for over a decade now. However, the world in which and on which IP works has changed dramatically over that time. This paper studies one portion of the IP protocol, fragmentation, in depth and with an eye to the future of IP fragmentation.

The IP protocol supports network traffic over heterogeneous networks by allowing for variable size MTUs (maximum transmission units) at the link layer. The maximum transmission unit is the largest size frame supported by the link layer of a network segment. The IP protocol adapts to various sized MTUs using a technique called fragmentation. Fragmentation occurs when the IP layer receives a datagram that is too large for the outgoing link to which it should be sent. The IP layer breaks the datagram up into fragments smaller than the MTU of the outbound link and sends the fragments out. When all of the fragments of a datagram are received at the final destination or endpoint, the IP layer of the receiver reassembles them and passes the complete datagram up the protocol stack to the appropriate protocol (usally TCP or UDP).



 
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Next: Fragmentation Considered Harmful

2000-07-01