1 BGP Policy Atoms Yehuda Afek Omer Ben-Shalom Anat Bremler-Barr Tel-Aviv University

2 Outline Atom definition Atom calculation methods Atom stability Correlation to BGP update Atom creation points Using Atoms

3 BGP ‘reminder’ The internet is a group of Autonomous systems (ASs) BGP is the routing protocol between AS Distance vector by AS hop count Same for any prefix destination AS path built dynamically Per prefix attributes to affect policy

4 multiple AS path to prefixes in the same AS BGP tables may contain different BGP AS paths to different prefixes on the same destination AS PrefixAS path / / /

5 Why different AS path ? AS path same for all prefix by default Human intervention (Policy) By changing attributes per prefix By selectively blocking updates Finding out where policy was set is not easy !

PrefixAS pathPreference / / AS 56 AS 2 AS 1 AS 34 Network /21 Network / filter here ? Ingress ? Egress ? filter here ? Ingress ? Egress ? AS 34 BGP table:

7 Atom definition - local Atom definition by Andre Broido and kc claffy a group of prefixes sharing the same BGP AS path attributes globally For single router – groups of prefixes with same AS path in the BGP table

View of a single router in AS 9 – two atoms

View of a single router in AS 8 – two atoms

10 Atom definition - global An Atom is defined globally if ANY backbone router agrees all ‘global’ atom member prefixes belong in it’s same ‘local’ atom An Atom is likely the result of policy, fault should not make a distinction.

Joint view of AS Three atoms

12 Scale comparison Atoms are much closer to AS in scope !!! EntityCount Announced Prefixes > 110K (now ~115K) Atoms> 20K AS> 12K (now ~14K)

13 AS, Atom and Prefix size

14 Practical Atom definition Defined in theory by all BGP tables Can be calculated well by sample (8) of BGP views taken in a ‘snapshot’ Analog to viewing a very complex ‘3D’ graph from 8 spatial locations Beware of ‘same angle’

15 Practical Atom calculation Get ‘snapshot’ of many BGP tables Create an ‘AS PATH’ set for each prefix Each distinct set defines an atom

16 Alternate Calculation Is the snapshot method valid ? Uses distributed snapshot No guarantee of synchronization Filter out prefixes not assured converged Search for stable prefix cliques during 4 hours -> Atoms Results close to ‘snapshot’ method (2-3%)

17 Atom Stability Atoms calculated stable to 3% of prefixes in 8 hour period Much more stable then the prefix AS path attribute. Still Less stable then expected Policy set Manually or by script but by design

18 BGP updates BGP groups prefixes in same update if they share same attributes TIME: 09/01/01 12:23:27 TYPE: BGP4MP/MESSAGE/Update FROM: AS3549 TO: AS12654 ORIGIN: IGP ASPATH: NEXT HOP: COMMUNITY : 3549: :9840 ANNOUNCE = =24

19 Correlating Atoms to updates Fault should affect full atoms and be in one update All attributes are the same 75% of updates include single, full atom 86% contain prefixes from one atom only Full AS prefix set appears in just 20% of updates

20 Atom split locations Where do atoms get created ? Policy normally enforced in source AS or neighbor How can we calculate the ‘split’ location ?

22 Atom split example AtomPrefixesAS path set # / / / / /24 R1: 12 R2: 12 R3: 12 All prefixes belong to AS 12 (len 1)

23 Atom split example AtomPrefixesAS path set # / /24 R1: 3 12 R2: 4 12 R # / / /24 R1: 3 12 R2: 4 12 R New atom due to different path at R3 (len 2)

24 Atom split example AtomPrefixesAS path set # / /24 R1: R2: R # /24R1: R2: R # / /24 R1: R2: R New atom by different path at R1 (len 3)

25 Atom split example AtomPrefixesAS path set # / /24 R1: R2: R3: # /24R1: R2: R3: # / /24 R2: R1: R3: Longer paths will not provide extra resolution

27 Atom Split location 85% of atoms are created between origin AS and neighbor AS Consistent with Atoms resulting from policy.

28 Application of Atoms 1. Differentiate Fault from Policy 2. Tweak update hold timers: Panelize partial Atom updates Reward full updates 3. Reduce scope of tags used in MPLS Set on a per prefix basis Could be reduced by as much as 1:5

29 Application of Atoms 4.Reduce load of active measurements Use Atoms sampling and not prefix sampling Results are not very favorable 5.Compress BGP updates Gain limited to 66% of prefix section 6.Shows limit on route table reduction while keeping information Renumbering to CIDR by atoms gain 50% only

30 Administration and distribution of Atom structure 1.Central body (CAIDA ?) ‘Perfect’ results How to distribute ? 2.Origin AS tag by extended community Knowledge propagation within BGP How to automate and not introduce more administration overhead 3.Use ‘local’ versions of Atoms No distribution Reduces potential benefits