Reconsidering fragmentation and reassembly
Girish P. Chandranmenon, George Varghese · 1998
Transmissionlinks often have different maximum packet sizes.Thus most network protocols allow large packets to be fragmented in order to be carried over a link with a small maximum packet size.The fragments are then reassembled either at the next hop or at the destination to recreate the original packet.However, both the old and the new versions of the Internet Protocol discourage fragmentation and reassembly.This is because the loss of a fragment can lead to the loss of a packet, and because reassembly implementations were perceived to be inefficient.In this paper, we reconsider the underlying principles of fragmentation and reassembly and introduce:(1) a more discriminating and general form of reassembly that allows reassembly to be done at any point in the network(2) a scheme to reduce the degradation in performance caused when fragments are lost and, (3) an efficient reassembly algorithm based on an expected case optimization that can process a fragment in 34 instructions.We use the Internet protocol suite to describe and evaluate specific modifications.We show that we can considerably improve end-to-end (i.e., TCP) performance using our mechanisms by effectively increasing link packet sizes beyond the required minimum.Experiments over a two hop path show an improvement of 42% (in TCP throughput) using hop by hop reassembly across a 1500 byte link.