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Resource Utilization

``Poor choices of fragment sizes can greatly increase the cost of delivering a datagram. Additional bandwidth is used for the additional header information, intermediate gateways must expend computational resources to make additional routing decisions and the receiving host must reassemble the datagram.'' [KM87]

Kent and Mogul are making three separate points here regarding the effect of fragmentation on the network or between two endpoints:

1.
Bandwidth is wasted on additional headers.
2.
Computation is wasted by intermediate routers who must fragment a datagram.
3.
Computation is wasted by the recipient who must reassemble fragments into the original datagram.

In the context of preventing fragmentation by intermediate networks, Kent and Mogul were unquestionably justified in their concerns. By focusing on the effects of arbitrary intermediate fragmentation, they promoted the use of pathMTU discovery, which is employed in most (although not all) modern operating systems, precisely for reasons such as these. See http://www.psc.edu/networking/perf_tune.html for a survey of pathMTU discovery support in various operating systems.

While agreeing with the validity of any of these points at the time they were written, it is useful to reexamine them in light of modern networks and in the context of endpoint fragmentation rather than intermediate fragmentation.



 
next up previous
Next: Bandwidth Up: Fragmentation Made Friendly Previous: Fragmentation Considered Harmful

2000-07-01