Preventing the unnecessary propagation of BGP withdraws

Transcription

1 Preventing the unnecessary propagation of BGP withdraws V. Van den Schrieck, P. François, C. Pelsser, O.Bonaventure Networking 2009, May 13th

2 Agenda Introduction Presentation and evaluation of ibgp-caused withdraws Preventing ibgp-caused withdraws Advertising more than one path in ibgp (IETF) Our proposal : Advertising the availability of alternate paths in ibgp 2

3 Agenda Introduction Presentation and evaluation of ibgp-caused withdraws Preventing ibgp-caused withdraws Advertising more than one path in ibgp (IETF) Our proposal : Advertising the availability of alternate paths in ibgp 3

4 BGP, the glue of the Internet Border Gateway Protocol - Interdomain routing protocol used in the Internet Exchange paths to destinations between Autonomous Systems (AS) Each router advertises its best path to its BGP peers Selection process : Several rules A path is described by the AS Path, i.e. the sequence of AS that leads to the path destination 4

5 BGP P 5

6 BGP P 5

7 BGP P : 1 P : 1 P 5

8 BGP P 5

9 BGP P : 3 1 P : 2 1 P 5

10 BGP P 5

11 BGP P : P : P : P 5

12 BGP P : > > P P :: P : > P : P 5

13 ibgp vs ebgp An Autonomous System has several routers All routers of an AS need to learn the paths to all destination advertised to the AS BGP sessions between routers of the AS : ibgp - Specific ibgp rule : Do not advertise an ibgpreceived path on an ibgp session - Exception : Route Reflectors BGP sessions between routers of different AS : ebgp 6

14 ibgp vs ebgp ebgp session ibgp session 7

15 Agenda Introduction Presentation and evaluation of ibgpcaused withdraws Preventing ibgp-caused withdraws Advertising more than one path in ibgp (IETF) Our proposal : Advertising the availability of alternate paths in ibgp 8

16 BGP Churn Churn is one of today s BGP issues Churn : High number of unnecessary BGP messages Unnecessary : Do not impact the final routing state Consequences : Load on router s CPU Transient losses of reachability 9

17 BGP Churn Average daily number of BGP updates :

18 Origin of BGP churn Path exploration during BGP convergence Flapping prefixes Bad ibgp propagation Routers without path diversity could send unnecessary BGP withdraws in case of failure Even tough alternate paths exist in the AS 11

19 Bad ibgp propagation P : paths ebgp session ibgp session 1 path > P :

20 Origins of bad ibgp propagation Only best path announced on ibgp sessions One path preferred over all others => alternate paths are never propagated Path quality (AS path length) BGP policies (Local preference) Complex ibgp organizations Paths hidden by Route Reflectors 13

21 Withdraw Propagation when alternate path exists P : ebgp session ibgp session P : > P :

22 Withdraw Propagation when alternate path exists P : ebgp session ibgp session P : > P :

23 Withdraw Propagation when alternate path exists P : ebgp session ibgp session P : W(P) > P :

24 Withdraw Propagation when alternate path exists P : ebgp session ibgp session P : > P :

25 Withdraw Propagation when alternate path exists P : ebgp session ibgp session W(P) P : W(P) > P :

26 Withdraw Propagation when alternate path exists P : ebgp session ibgp session W(P) P : W(P) > P :

27 Withdraw Propagation when alternate path exists P : ebgp session ibgp session P : > P :

28 Withdraw Propagation when alternate path exists > P : ebgp session ibgp session P : > P :

29 Withdraw Propagation when alternate path exists > P : W(P) ebgp session ibgp session P : P : > P :

30 Withdraw Propagation when alternate path exists > P : ebgp session ibgp session P : 2 1 > P : 2 > P : P : > P :

31 Withdraw Propagation when alternate path exists > P : P : 2 1 > P : 2 > P : > P : P : P : ebgp session ibgp session 14

32 Withdraw Propagation when alternate path exists > P : P : 2 1 > P : 2 > P : P : > P : P : ebgp session ibgp session 14

33 Quantification of unnecessary ibgp-caused Withdraws RouteViews Dataset : BGP feeds of parallel sessions (2 sessions with the same AS) AS X Withdraw defined as ibgp-caused if the destination is withdrawn on one session, but stay stable on the other during 30 seconds before and after Withdraw is unnecessary because an alternate path exists in the AS W (D) RouteView Collector 15

34 Detecting unnecessary ibgp-caused withdraws > P : 2 > P : 2 P : 3 4 RouteView Collector 16

35 Detecting unnecessary ibgp-caused withdraws > P : 2 > P : 2 P : 3 4 RouteView Collector 16

36 Detecting unnecessary ibgp-caused withdraws > P : 2 > P : 2 P : 3 4 RouteView Collector 16

37 Detecting unnecessary ibgp-caused withdraws > P : 2 W(P) > P : 2 P : 3 4 W(P) RouteView Collector 16

38 Detecting unnecessary ibgp-caused withdraws > P : 2 W(P) > P : 2 P : 3 4 W(P) Withdraw detected => start timer RouteView Collector 16

39 Detecting unnecessary ibgp-caused withdraws > P : 2 > P : 2 > P : 3 4 Withdraw detected RouteView Collector 16

40 Detecting unnecessary ibgp-caused withdraws > P : 2 P : 3 4 > P : 2 > P : 3 4 P : Withdraw detected RouteView Collector 16

41 Detecting unnecessary ibgp-caused withdraws P : 2 > P : 2 > P : 3 4 > P : 2 > P : 3 4 Withdraw detected RouteView Collector 16

42 Detecting unnecessary ibgp-caused withdraws P : 2 > P : 2 > P : 3 4 > P : 2 > P : 3 4 P : Withdraw detected RouteView Collector 16

43 Detecting unnecessary ibgp-caused withdraws P : 2 > P : 2 > P : 3 4 > P : 2 > P : 3 4 P : Withdraw detected Timer expired No withdraw from MATCH RouteView Collector 16

44 Quantification of ibgp-caused withdraws on RouteViews sessions Number of withdraws Days 17

45 Agenda Introduction Presentation and evaluation of ibgp-caused withdraws Preventing ibgp-caused withdraws Advertising more than one path in ibgp (IETF) Our proposal : Advertising the availability of alternate paths in ibgp 18

46 Advertising multiple paths in ibgp P : ebgp session ibgp session P : > P :

47 Advertising multiple paths in ibgp P : ebgp session ibgp session P : 2 1 P : P : > P :

48 Advertising multiple paths in ibgp P : ebgp session ibgp session P : P : > P :

49 Advertising multiple paths in ibgp P : ebgp session ibgp session P : P : > P :

50 Advertising multiple paths in ibgp P : ebgp session ibgp session P : P : W(P) > P :

51 Advertising multiple paths in ibgp P : ebgp session ibgp session W(P) P : > P : P : > P :

52 Advertising multiple paths in ibgp > P : ebgp session ibgp session W(P) P : > P : P : > P :

53 Advertising multiple paths in ibgp Very promising solution, but : Need to modify the BGP protocol to support advertisement of multiple paths for a prefix All the routers of an AS need to be upgraded accordingly 20

54 Agenda Introduction Presentation and evaluation of ibgp-caused withdraws Preventing ibgp-caused withdraws Advertising more than one path in ibgp (IETF) Our proposal : Advertising the availability of alternate paths in ibgp 21

55 How to prevent ibgp-caused withdraws? (2) Instead of advertising alternate paths, advertise the availability of alternate paths BGP Community attribute used to tag this information to the path : DIVERSITY_AVAILABLE Lighter solution than the previous one, can be incrementally deployed No modification to BGP protocol DIVERSITY_AVAILABLE BGP community is simply not considered by legacy routers 22

56 Advertising the availability of alternate paths Principle : A router knowing an alternate path - Tags the community DIVERSITY_AVAILABLE to the primary path - Then, it re-advertises the primary path to all its ibgp peers 23

57 Advertising the availability of P : alternate paths P : ebgp session ibgp session > P :

58 Advertising the availability of P : alternate paths P : DIV_AV P : ebgp session ibgp session > P :

59 Advertising the availability of P : alternate paths P : DIV_AV P : ebgp session ibgp session - DIV_AV > P :

60 Advertising the availability of alternate paths When the primary path fails : Routers without an alternate path check the community If present : Wait for the alternate path before sending withdraws on ebgp Otherwise : send the withdraw on ebgp sessions immediately 25

61 Advertising the availability of P : alternate paths P : ebgp session ibgp session - DIV_AV > P :

62 Advertising the availability of P : alternate paths P : ebgp session ibgp session - DIV_AV > P :

63 Advertising the availability of P : alternate paths P : ebgp session ibgp session - DIV_AV W(P) > P :

64 Advertising the availability of P : alternate paths P : ebgp session ibgp session - DIV_AV > P :

65 Advertising the availability of P : alternate paths P : ebgp session ibgp session - DIV_AV No ebgp Withdraw Wait alternate path > P :

66 Advertising the availability of P : DIV_AV W(P) alternate paths P : No ebgp Withdraw Wait alternate path ebgp session ibgp session > P :

67 Advertising the availability of > P : alternate paths P : ebgp session ibgp session - DIV_AV No ebgp Withdraw Wait alternate path > P :

68 Advertising the availability of > P : P : DIV_AV alternate paths W(P) P : No ebgp Withdraw Wait alternate path ebgp session ibgp session > P :

69 Advertising the availability of > P : P : P : DIV_AV DIV_AV > P : alternate paths W(P) P : No ebgp Withdraw Wait alternate path ebgp session ibgp session > P :

70 Advertising the availability of > P : alternate paths P : ebgp session ibgp session P : DIV_AV DIV_AV > P : P : > P :

71 Advertising the availability of > P : alternate paths P : ebgp session ibgp session P : DIV_AV DIV_AV > P : P : > P :

72 Policy-equivalent paths In practice : due to policies, alternate paths can still trigger ebgp withdraws Use two communities instead of one 27

73 Conclusion We highlighted the contribution of ibgp-caused withdraws to BGP churn Two solutions to prevent this unnecessary withdraw propagation : Add-Paths (IETF) Our proposal : Diversity communities Work in progress : Evaluate the reduction of churn resulting from using our solution Simulations on generated Internet topologies 28

74 Questions? More information on : 29