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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.
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.
- 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.
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:
Results:
- Grouping prefixes by community value per BGP peer. Analysis in this section is based on MRT prefixes.
- The difference of community values for BGP peers from the same AS.
- 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.
- for MRT prefixes (131836 prefixes)
- for Global MRT prefixes (94868 prefixes)
- 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.
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.
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.
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.
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%