EndtoEnd Routing Behavior in the Internet

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CS 6390 Advanced Computer Networks Dr. Ravi
Prakash
End-to-End Routing Behavior in the
Internet Internet Routing Instability
Presented by
Carlos Flores Gaurav Jain
May 31st. 2000
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Topics of Presentation
I. Introduction II. Routing Behavior in the
Internet III. Routing Instability IV. Conclusions
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Introduction

• Purpose of Studies
• Analyze the routing behavior in the Internet
  for pathological conditions, routing stability
  and routing symmetry for end-to-end measurements.
• Analyze BGP routing messages to examine
  Internet routing instability.

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Routing Behavior
Main questions What pathologies and
failures occur in routing? Stable or unstable
routes? Symmetric or Asymmetric routes?
Terms ASs - Autonomous Systems. Set of routers
and hosts unified by a single administrative
authority. BGP - Border Gateway Protocol.
Protocol used for transmission among different
ASs. Flapping - Frequent change of routes
between ASs.
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Methodology
Number of Internet sites 37 Tools -
Traceroute - NPD (Network Probe Deamon) -
npd_control program. Time D1 dataset -
collected Nov - Dec 94. D2 dataset - collected
Nov - Dec 95. Size D1 - 6991
Measurements. D2 - 37,097 Measurements.
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Routing Pathologies
1) Routing loops A) Forwarding Loops Packets
forwarded by a router return to the router. B)
Information Loops Router acts on connectivity
info. derived by information it itself
propagated earlier. C) Traceroute Loops
measurement reports the same sequence of routers
multiple times. Results D1 - 10 traceroute
loops (0.13) D2 - 50 traceroute loops
(0.16) Loops Duration 1) lt 3 hours 2) gt half
day
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Routing Pathologies
2) Erroneous Routing D1 - 1 Packet routed to
Israel instead of London! No safe assumption can
be made of correct routing. 3) Connectivity
Altered Midstream Results Routes lost or
altered D1 - 10 traces D2 - 155
traces Conclusion Recovery time bimodal 1)
lt 1 second 2) Order of 1 minute.
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Routing Pathologies
4) Fluttering Rapid oscillating routing. D2 -
Very little fluttering observed. Problems -
Unstable Network paths - Occur in one
direction (asymmetry) - Roundtrip time
difficult to estimate. Advantages Balance
network load. 5) Infrastructure failure. host
unreachable deep inside the network. Results D1
- 99.8 Availability D2 - 99.5 Availability
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Routing Pathologies
6) Unreachable due to too many hops. Hop
count not always proportional to geographic
distance A) End-to-end route 1500 Km 3
hops. B) End-to-end route 3 Km 11 hops.
Operational diameter of the Internet grown beyond
default value of 30 hops. Longer initial
value of TTL needed.
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Routing Pathologies
7) Temporary Outages. Sequence of consecutive
traceroute packets lost.
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Routing Pathologies
8) Time of day patterns. Temporary outages
D2 - Minimum 0.4. Outages between 0100 -
0200 hrs. Maximum 8.0. Outages between
1500 - 1600 hrs. Infrastructure
failure Minimum 1.2. 0900 - 1000
hrs. Maximum 9.3. 1500 - 1600 hrs.
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Routing Pathologies
Summary
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Routing Symmetry

• Goal Assess the degree to which routes are
  symmetric or asymmetric.
• Effects of network asymmetries
• Complicate network measurements, troubleshooting,
  accounting and routers anticipatory flow state.
• Sources
• Link asymmetric costs (bandwidth, payment
  scheme).
• Configuration errors, inconsistencies.
• hot potato, cold potato routing.

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Routing Symmetry
Analysis
D2 49 of measurements showed an asymmetric path
visiting at least one different path. Size of
asymmetries - Majority of asymmetries confined
to a single hop (only one city or AS different).
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End-to-End Routing Stability
Objective
Do routes change often or are routes stable over
time? Views of routing stability A)
Prevalence likeliness of observing the same
route in the future. B) Persistence How long
a route will remain the same. Routes level of
granularity - Internet granularity (host
granurality) - City granularity - ASs
granularity
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End-to-End Routing Stability

• Routing Prevalence
• - Host granularity For half of virtual paths
  measured, same route observed 82 or more of the
  time.
• Internet paths strongly dominated by a single
  route.
• City granularity 97
• AS granularity 100
• Internet paths very strongly dominated by same
  set of cities and same ASs, but significant
  site-to-site variation.

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End-to-End Routing Stability
Routing Persistence How long a route is likely
to endure before changing? Rapid Route
Alternation No high-frequency routing
oscillation for measurements of less than 1
hour. Medium Scale Route Alternation Observation
of virtual paths spaced 1 hour apart not likely
to suffer a route change. Large scale Route
Alternation 90 chance of observing a route with
a duration of at least a week.
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End-to-End Routing Stability

• Summary of routing persistence
• Route changes occur over a wide range of time
  scales (seconds to days)
• 2/3 of Internet paths have stable routes lasting
  from days to weeks.

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Internet Routing Instability

• Analysis based on data collected from BGP routing
  messages (interdomain routing).
• What is Routing Instability?
• Rapid change of network reachability and topology
  information.
• Origins
• Router configuration errors.
• Physical and data link problems.
• Software bugs.

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Internet Routing Instability

• Effects
• Increase packet loss.
• Delays in time for network convergence.
• Resource overhead (memory, CPU) within Internet
  Infrastructure.
• Terminology
• Prefixes Destination IP addresses blocks.
• ASPATH List of ASs numbers in a particular
  route.

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Internet Routing Instability

• Routing forms
• Announcements.
• Withdrawals.
• Types of interdomain routing updates
• Forwarding instability.
• Routing policy fluctuation.
• Redundant pathological updates.
• Instability Forwarding Instability Routing
  policy fluctuation.

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Internet Routing Instability
Methodology
Time of study 9 months. 1996 Data Logged
BGP routing messages at 5 major U.S.
Network exchange points. Purpose -
Analyze the BGP data in attempt to characterize
and understand the origins and operational
impact of routing instability.
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Internet Routing Instability
Analysis of pathological routing information
Update categories A Announcement W
Withdrawal - WADiff route withdrawn and
replaced with an alternative route. - AADiff
route implicitly withdrawn and replaced by a
preferred alternative path. - WADup route
explicitly withdrawn and then reannounced as
reachable. - AADup route implicitly withdrawn
and replaced with a duplicate of original. -
WWDup repeated transmission of BGP
withdrawals for a prefix currently unreachable.
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Internet Routing Instability
Analysis of pathological routing information
Update categories
Instability
Pathological behavior
WWDup
AADiff WADiff
WADup
5
95
AADup
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Internet Routing Instability
Results
1) BGP updates dominated by WWDup. 2) AADup and
WADup consistently dominate the remaining
categories. 3) Only a small portion of BGP
updates contribute to AADiff and WADiff.
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Internet Routing Instability
Results

• All pathological routing incidents caused by
  small service providers.
• Some WWDups caused by a vendors router
  implementation decision.
• Instability AADiff WADiff WADups.
• Trends
• Peaks of updates in the afternoons.
• Little instability in the weekend.
• Routing instability closely related to
  bandwidth usage and packet loss.

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Internet Routing Instability
Results
Plot of time of day vs. no. of updates --gt bell
shaped curve (peak afternoon). Weekends --gt
less instability Rigorous approach to identify
instability frequency - peak at 24 hrs. and 7
days. In a day, periodicity observed at 30 s.
and 60 s. NO SINGLE ROUTE DOMINATES
INSTABILITY. NO SINGLE AS DOMINATES INSTABILITY.
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Internet Routing Instability
Possible origins of routing pathologies
Stateless BGP implementations. Each
withdrawal induces some short lived pathological
network oscillation. Oscillations due to
misconfigured CSUs. Jittered timer to coalesce
multiple routing updates. Unjittered timers in
periodic message model. Improper configuration
of the interaction between interior gateway
protocols and BGP.
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Internet Routing Instability
Results...
99 of routing information is pathological
(redundant) and many not reflect real network
topological changes. Although redundant
updates are quickly discarded by routers, they
consume router resources and high rates of them
(300 updates per second) can crash a router.
Forwarding instability highly present 3-10
of routes have 1 or more WADiff per day. 5-20
of routes have 1 or more AADiff per day.
10-50 1 or more WADup per day.
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Conclusions

• No typical Internet site or path.
• Likelihood of encounter a major routing pathology
  more than doubled from 1994-1995.
• Internet paths heavily dominated by a prevalent
  route, but routes persistence show wide variation
  of time (seconds to days).
• 2/3 of Internet routes have routes persisting
  from days or weeks.

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Conclusions

• Internet routing instability still poorly
  understood.
• By 1995, half of virtual paths differ by gt1 city
  in a two way path.
• How can we make it better?