Prefix Lengths

Prefix lengths are relevant to router vendors because of the need to estimate lookup overhead required for a full route table. Each segment of the address space carries a different average lookup cost and sees different average traffic patterns due to characteristics of address allocation and use.

These graphs are based on a 7 minute OC3mon packet trace collected on March 12, 1998 from FIX West, and a route table from the University of Oregon Route Views project[Meyer97] collected on the same day. At this time, the route table included routes from the following peers:

    ANS           (Cleveland)     206.157.77.11   through AS1673
    CERFnet       (San Diego)     134.24.127.3    through AS1740
    DIGEX         (MAE-EAST)      192.41.177.192  through AS2548
    ESnet         (GA)            134.55.24.6     through AS293
    Europe        (RIPE)          193.0.0.242     through AS3333
    IAGnet        (Chicago)       131.103.0.2     through AS1225
    IIJ           (Japan)         202.232.1.8     through AS2497
    JINX          (Johannesburg)  196.7.106.152   through AS2905
    LINX          (London)        194.68.130.254  through AS5459
    MCI           (San Francisco) 204.70.4.89     through AS3561
    PIPEX         (London)        158.43.133.48   through AS1849
    Sprint        (Stockton)      144.228.240.93  through AS1239
    vBNS          (Hayward)       204.147.128.137 through AS145
    Verio         (MAE-WEST)      205.238.48.3    through AS2914
    blackrose.org (Ann Arbor)     204.212.44.128  through AS234

Note that these results are directly comparable to the CAIDA paper presented at Inet 98[Claffy98a]. The results are quite similar to figures 11a, 11b, and 12 in that paper despite the fact that the data came from a public exchange point rather than inside a private backbone.

The first two graphs show the distribution of packets and bytes seen relative to the length of the network mask of the network that either generated or received the data. The green line in the first graph superimposes the number of routes for each prefix length in our composite routing table. The largest number of routes have a 24 bit mask (corresponding to the old `class C' networks), but the largest amount of traffic is sourced from and destined to networks with a 16 bit route prefix mask (corresponding to the old `class B' networks).

figure 1: Distributions of traffic volume as a function of prefix length of source and destination IP addresses.
From a 20-minute trace of IP packets from a wide-area exchange point (FIX-West)

figure 2: Distributions of packets as a function of prefix length of source and destination IP addresses. Taken from a 20-minute trace of IP packets from a wide-area exchange point (FIX-West).

This last graph shows that the longer prefixes are net producers of network traffic compared to the shorter prefixes, as indicated by the larger mean packet size and thus greater average payload size. This suggests the presence of more (content) servers on long prefix networks than on short prefix ones.

figure 3: Mean sizes of packets as a function of the prefix lengths of their source and destination addresses. Based on a 20-minute IP packet trace from a wide-area exchange point (FIX-West).

references

1. [Claffy98a]. k claffy, G. Miller, and K. Thompson, the nature of the beast: recent traffic measurements from an Internet backbone. 23 April 1998 (INET '98 presentation).

2. [Meyer97], University of Oregon Route Views Project, http://www.routeviews.org. Advanced Network Technology Center, David Meyer (now at Cisco Systems).

Thanks to Hans-Werner Braun and NLANR/MOAT for supplying the packet trace data, and David Meyer for maintaining the Route Views project.

Sean McCreary and kc, 10/26/98

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