Packet Fragmentation on the UCSD-CERF Link

This page describes packet fragmentation on the UCSD-CERF link between Wed May 17 14:40:39 PDT 2000 to Fri Jun 2 20:42:48 PDT 2000.

Introduction Packet fragmentation occurs when a packet too large for the subsequent link reaches a network interface and is broken up into a series of fragments the size of the MTU of the ensuing link. Packet fragmentation has a negative impact on network performance for several reasons. First, a router has to perform the fragmentation - an expensive operation. Second, all the routers in the path between the router performing the fragmentation and the destination have to carry additional packets with the requisite additional headers.
About This Trace The data in the following section comes from a trace which contained only fragmented traffic. The trace was taken on the UCSD-CERF link from Wed May 17 14:40:39 PDT 2000 to Fri Jun 2 20:42:48 PDT 2000. 50,375,114 packets containing 50,945,519,628 bytes of data composing 17,718,285 fragment series were captured in this interval. The graphs shown here represent all observed fragments -- both complete fragment series -- series in which all of the fragments were observed, and series with missing fragments. There are several reasons a series would have missing fragments, including some fragments taking different paths to the destination and consequently not traveling along the UCSD-CERF link, and fragments being dropped somewhere along the path between the source and our monitor box.
The number of bytes per fragment series corresponds to the size of the original packet, plus an additional header for each fragment. This graph has several interesting features. There is a concentration of smaller packets, which is to be expected since most machines don't try to send out packets larger than 1500 bytes. Also of note are the curve starting at around 10000 bytes, the second curve shadowing it starting around 12000 bytes, and thrid, similarly shaped curve of much smaller magnitude starting at around 30000 bytes.	Top Ten Not Shown on graph: Bytes/Series Complete Trace Bytes/Series Complete Trace 1540 8854427 170244 1 1689 2436828 957784 1 3826 846891 871624 1 2616 726561 202360 1 1694 704239 8867997 1 3916 530681 185100 1 1639 470480 153980 1 3080 387543 78000 1 3912 367885 81285 1 1572 232685 491164 1 76500 1 569364 1 83157 1 276900 1 352584 1 368280 1 79965 1

The curve evident in this graph appears to be almost the same as the curve in the bytes/series graph. Coincidence? Probably not.	Top Ten Not Shown on Graph: Fragments/Series Complete Trace Fragments/Series Complete Trace 2 14849154 51 1 3 2647790 52 1 4 505019 58 1 1 285740 60 1 6 94016 64 1 21 75508 91 1 5 71692 97 1 13 66403 100 1 14 65815 210 1 12 58701 294 1 374 1 475 1 575 1 588 1 1445 1 2195 1 7699 1 8511 1 9255 1 13845 1 17628 1 18414 1 24557 1 28467 1 43580 1 47888 1 305793 1

No fragments larger than 1500 bytes were observed. This is to be expected, since most links have an MTU of 1500 bytes. There is a random, "background" of sizes that spans from 44 to 1500 bytes from machines set to fragment traffic into infrequently used sizes of packets. There are many 1500 and 1484 byte packets, typical of ethernet and other link MTUs. There are also many 572 byte first fragments, a common PPP MTU.	Top Ten First Fragment Size Complete Trace 1484 9286933 4348 3 1500 9131863 0 313807 1356 166105 300 74812 572 23715 1492 14325 124 13697 1196 8878 1468 4482 44 3354

As with the first fragment sizes, the last fragment sizes have a wide 'background' range, which would be expected to be roughly half of the average of all of the fragments sizes.	Top Ten Not Shown On Graph: Last Fragment Size Complete Trace Last Fragment Size Complete Trace 56 9013120 189 2447439 826 869408 1116 756657 194 707683 0 629266 916 544779 139 473642 912 385459 40 281446

The number of milliseconds for complete packet transmission has been shown to be independent of the number of packets in the series. This transmission time may be indicative of the line rate of the slowest router to handle the packet series after it is fragmented, the speed of the slowest link through which the packet series traveled, or the time difference between different paths taken by various fragments within a series.	Milliseconds Complete Trace 0.5890131 764707 0.5919933 743547 0.5869865 741478 0.5899668 624079 0.5910397 611737 0.5860329 500940 0.5940199 498140 0.5840063 398748 0.5849600 397713 0.5949736 393271

Introduction

About This Trace