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Center for Applied Internet Data Analysis > funding : atomized_routing : analysis : ruomei-communities.xml
Atoms and BGP Communities.
The results of a study by Ruomei Gao investigating the relationship between BGP policy atoms and BGP community attributes. We are interested in whether communities provide finer control of prefix routing than policy atoms, and to what extent communities further refine atoms. Ultimately, our goal is to find out to what extent non-AS path attributes (in general) further refine atoms. This study should be considered a preliminary step toward that goal.

Some Background Info:

Policy Atoms:

    Two prefixes are said to be path equivalent if we cannot find a BGP peer that sees them with different AS paths. An equivalence class of this relation is called a BGP policy atom. ["Analysis of RouteViews BGP data: policy atoms", Andre Broido and kc claffy]
    In this analysis policy atoms are derived from a RouteViews routing table snapshot on 2003, Jan, 15, in "show ip bgp" format. Note that RouteViews data in this format does not contain community information.

Grouping by Community:

    We group prefixes by community value in a somewhat similar way to the grouping of prefixes by AS path (atoms). One major difference is that BGP does not require routers to propagate community information: the community value can be dropped anywhere along the path.
    We derive the grouping by community value based on data in MRT format, also obtained from RouteViews. Fewer peers contribute to the MRT data than to the "show ip bgp" routing table. However the MRT data does contain community information.


    For the MRT data we use a RouteViews table dump of Jan 15, 2003 (the same date as "show ip bgp" data for atoms). The table dump is in MRT binary format and contributed by 22 peers.
    16 peers provide values of community attributes. These 16 BGP peers represent 12 ASes. 4 ASes are each represented by 2 BGP peers.

    There are three types of prefixes:

    • Global Prefixes:
    • These prefixes are carried by all 35 chosen participants from RouteViews. We use Global Prefixes to compute policy atoms.
    • MRT Prefixes:
    • These prefixes are carried by the 16 BGP peers mentioned above.
    • Global MRT Prefixes:
    • The intersection of Global Prefixes and MRT Prefixes.


    1. Grouping prefixes by community value per BGP peer.
    2. Analysis in this section is based on MRT prefixes.
              peer          # distinct community values      # prefixes carried
            42\                           101271 
             19/                           101271
          54                            116813
             5\                           117926
             5/                           117915
             5                            118959
         117                            101256
         22                            116949
        1113\                           117334
         1116/                           117344
         410                            117186
          78                            1230
           498                            117748
               110                            117546
          1382\                           117332
          1382/                           117332
          note: \  means BGP peers from the same AS.

      Conclusion: The number of groups that result when dividing the prefixes by community value varies from AS to AS.

    3. The difference of community values for BGP peers from the same AS.
    4.     Peer1          / Peer2         : #prefixes w/ diff    percentage of 
                                           community values     total # prefixes
                                                                carried by peers
   /   : 189                  0.18%
 / : 10                   0.008%
   /  : 85                   0.054%
   /  : 117919               100%

      We can see that the difference in community values between the first 3 pairs of peers is trivial, and might result from routing dynamics. (We didn't confirm this.)

      The last pair of peers comes from PSGnet. It seems that the operator uses the attribute of community for routing intensively (traffic engineering?).

      Conclusion: Community values do not significantly differ between the first 3 pairs of BGP peers from the same AS. We will do our evaluation on data from the last pair of peers respectively.

    5. Grouping by community and refinement of atoms.
    6. This section contains the grouping of prefixes by community value observed in RouteViews and the refinement of atoms by community values. For each item two computations are performed, each including a different PSGnet peer.
      1. for MRT prefixes (131836 prefixes)
      2.                       PSGnet           PSGnet
            grouping by      diff%
            community    :    12117            11345             6.80%
            aspath (atom):    28139            27556             2.12%
            commu+aspath :    30535            29863             2.25%
            increase %   :    8.51%            8.37%             1.67%
        Note: the last column "diff%" is the difference between two PSGnet peers over the smaller results. Here they are the results from the 2nd peer (, i.e (peer1-peer2)/peer2.
      3. for Global MRT prefixes (94868 prefixes)
      4.                       PSGnet           PSGnet
            grouping by      diff%
            community    :    10364            9628              7.64%
            aspath (atom):    23072            22423             2.89%
            commu+aspath :    25028            24276             3.10%
            increase %   :    8.48%            8.26%             2.66%
      Conclusion :
      • the refinement increased by community attribute is around 8~8.5%
      • the data from different peers of the same AS affect the final results, but not significantly.
  Last Modified: Wed Mar-27-2019 22:23:18 PDT
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