Routing System Stability draft-dimitri-grow-rss-01.txt

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Routing System Stability draft-dimitri-grow-rss
-01.txt

• dimitri.papadimitriou_at_alcatel-lucent.be
• jim.lowe_at_alcatel-lucent.be
• IETF71 - Philadelphia

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Motivation Scope

• Understanding dynamics of the Internet routing
  system to
• i) ensure its robustness/stability
• ii) improve mechanisms of BGP routing protocol
• Scope
• program of WG activity for identifying,
  documenting and analyzing the dynamic properties
  of the Internet and its routing system

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Dependencies (1)

• Investigations on Internet RS dynamics
• gt investigations on routing engine / system
  resource consumption (memory CPU)
• System resource consumption depends on
• size of the routing space
• ? routing entries gt ? memory
• ? routing entries gt ? processing and searching
  (lookup)
• number of peering adjacencies between routers
• ? peering adj. gt ? dynamics associated with
  routing information updates exchanged
• gt increasing memory requirements

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Dependencies (2)

• Current routing engines potentially support up to
  O(1M) routing table entries instabilities
  resulting
• i) from routing protocol behavior
• ii) routing protocol information exchanges
• iii) changes in network topology that may
  adversely affect the network's ability to remain
  in a useable state for extended periods of time

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Objectives and Tasks

• Objective identify root cause and document
  occurrences of Internet RS stability phenomena
  (using data from operational networks)
• Tasks
• 1. Methodology to process and interpret routing
  table data
• 2. Identification of set of stability criteria
  and development of methods for using them to
  provide a better understanding of the routing
  system's stability
• 3. Investigate how routing protocol behavior and
  network dynamics mutually influence each other

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Stability Criteria

• Routing system (RS) stability
• characterized by its response (in terms of
  processing routing information) to inputs of
  finite amplitude
• Input classification
• internal system events e.g. routing protocol
  config. changes,
• external system events e.g. routing information
  updates
• note sometimes loosely referred to as routing
  instabilities
• Stable vs Unstable
• Stable/marginally stable RS returns to its
  initial/new equilibrium state, when disturbed by
  external and/or internal event
• Unstable RS remains in an unending condition of
  transition from one state to another when
  disturbed by external and/or internal event

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Stability Criteria

• Definition of stability implies
• Define system being examined
• Routing system and associated events, such as
  input events, outputs, and related arrival rates
• Convergence metric
• Metric to define the convergence characteristics
  of the system
• Stability metric
• Degree of system stability that indicates how
  close the system is to being unstable

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Stability Criteria

• Convergence and stability metrics dependency
• Number of routing entries
• each entry R toward prefix D has associated
  attribute set A consisting of AS-Path, MED, and
  Local Preference, etc.
• Number of CPU cycles, C, required to process a
  routing entry, and its associated memory space, M
• Input events and their arrival rates
• Output events associated with the processing of
  each input event.

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Formulation

• RT(n) routing table at some time n
• At time n1, RT(n1) RTo(n) deltaRT(n1)
• RTo(n) set of routes that experience no change
  between n and n1
• deltaRT(n1) accounts for all route changes
  (additions, deletions, and changes to previously
  existing routes) between n and n1
• At time n1, deltaRT(n1) RTc(n1) RTn(n1)
  
• RTc(n1) set of routes at time n that experience
  some change at time n1
• Rtn(n1) set of new routes observed at time n1
  that were not present at time n

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Derivation of Stability Metric

• Algorithm for calculating a stability metric
• Stability, marginal stability, and unstability
  quantification in the context of RT
• deltaRT(n1) magnitude of RT change at time
  n1
• Stability condition deltaRT(n1) lt a, as t -gt
  infinity
• a is small, positive number
• Marginal stability condition a lt deltaRT(n1)
  lt b, as t -gt infinity
• b is small, positive number, b greater than a
• Unstability condition deltaRT(n1) gt beta as t
  -gt infinity.
• Note
• no distinctions for new routes or changed routes,
  or for the source of system disturbances
• a and b to be set based on some sort of
  operational criteria (a.o. dependent on the
  observation sampling frequency)

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RT Stability Metric

• To compute deltaRT(n1) gt compute stability
  metric for an individual route
• single route, rti(n1), component of RT(n1)
• rti(n1) destination, path attributes
• deltarti(n1) change in rti stability metric,
  from tn to tn1
• fi stability metric associated with route rti
• fi initial value 0
• At time n1
• if rti(n1) ! rti(n) / the route has changed
  /
• then fi(n1) fi(n) 1
• else / the route did not change /
• if fi(n) 0 then fi(n1) 0
• else fi(n1) f(n) - 1

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RT Stability Metric

• Using stability metric definition for individual
  route, compute stability metric for entire
  routing table (RT)
• Stability metric of RT, at time tn1
  deltaRT(n1)
• deltaRT(n1) is normalized
• Perfect stability deltaRT(n1) 0 (minimum
  value)
• Complete instability deltaRT(n1) 1 (maximum
  value)

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RT Stability Metric

• for i 1 to number of routes in RT(n1)
• if rti(n1) is a new route then
  deltarti(n1)0
• else / rti(n1) is an existing route /
• if fi(n)0 and fi(n1)0 then
  deltarti(n1)0
• else / a change occurred to the route /
• if fi(n1)gtfi(n) then deltarti(n1)fi(n)/
  fi(n1)
• else deltarti(n1)fi(n1)/fi(n)
• end if
• end if
• end if
• end i loop
• deltaRT(n1) Sum(deltarti(n1))
• total number of routes in RT(n1)

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Examples

• Example 1
• fi(n) 0, 1, 2, 1, 0, 0 and fi(n1) 1, 2,
  1, 0, 0, 0
• deltaRT(n1) (0/1 1/2 1/2 0/1 0 0)
  / 6 0.1667 (rather stable)
• Example 2
• fi(n) 0, 0, 0, 0, 0, 0 and fi(n1) 1, 1,
  1, 1, 1, 1
• deltaRT(n1) (0/1 0/1 0/1 0/1 0/1
  0/1) / 6 0 (still stable, too early to judge)
• Example 3
• fi(n) 56, 20, 63, 64, 0, 5 and fi(n1)
  57, 19, 64, 65, 0, 4
• deltaRT(n1) (56/57 19/20 63/64 64/65
  0 4/5) / 6 0.783 (very unstable)

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Relevance to GROW

• BGP operational issues related to routing table
  growth rates and the dynamic properties of the
  routing system.
• Advisory role to the IDR working group to provide
  commentary on whether BGP is addressing relevant
  operational needs and, where appropriate, suggest
  course corrections
• gt Effort positioned at central place in the BGP
  investigation process (beneficial for other WGs)
• Note effort goes together with obtaining routing
  table data from the field

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Concluding remarks

• We think interesting topic for investigation in
  GROW and beneficial to other WGs
• Analysis/measurement of Internet RS/RT stability
• Unified approach to cross-validation of
  techniques looking at improving path exploration
  effects on RS
• Validation of the metric with real data on its
  way
• Several data repositories available but only
  daily variation (ideally, smaller sampling time)
  
• Ex. daily report on BGP activity for AS 65000
• Additional operational data (routing entries)
  would be appreciated need to capture variety of
  timescales
• Reason different dynamic behaviour will be
  observable on different timescales