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.

Data:

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.

---

Results:

1. Grouping prefixes by community value per BGP peer.
Analysis in this section is based on MRT prefixes.

        peer          # distinct community values      # prefixes carried
    129.250.0.11            42\                           101271 
    129.250.0.6             19/                           101271
    144.228.241.81          54                            116813
    147.28.255.1             5\                           117926
    147.28.255.2             5/                           117915
    199.74.221.1             5                            118959
    204.42.253.253         117                            101256
    206.186.255.223         22                            116949
    207.246.129.14        1113\                           117334
    207.246.129.6         1116/                           117344
    208.51.113.254         410                            117186
    213.140.32.184          78                            1230
    217.75.96.60           498                            117748
    4.0.4.90               110                            117546
    64.200.199.3          1382\                           117332
    64.200.199.4          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.

2. The difference of community values for BGP peers from the same AS.

    Peer1          / Peer2         : #prefixes w/ diff    percentage of 
                                     community values     total # prefixes
                                                          carried by peers
    -------------------------------------------------------------------------
    129.250.0.11   / 129.250.0.6   : 189                  0.18%
    207.246.129.14 / 207.246.129.6 : 10                   0.008%
    64.200.199.3   / 64.200.199.4  : 85                   0.054%
    147.28.255.1   / 147.28.255.2  : 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.

3. Grouping by community and refinement of atoms.
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)

                      PSGnet           PSGnet
    grouping by       147.28.255.1     147.28.255.2      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 (147.28.255.2), i.e (peer1-peer2)/peer2.
2. for Global MRT prefixes (94868 prefixes)

                      PSGnet           PSGnet
    grouping by       147.28.255.1     147.28.255.2      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.