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Internet Routing Instability

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Future legitimate announcements are accepted only. after a delay. Decrement counter with time ... But Duplicate Announcements (AADup) doubled ... – PowerPoint PPT presentation

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Title: Internet Routing Instability


1
Internet Routing Instability
Craig Labovitz G. Robert Malan Farnam Jahanian
Appeared SIGCOMM 97
Presenters Supranamaya Ranjan Mohammed
Ahamed
2
Internet Structure
  • Many small ISPs
  • at lowest level
  • Small number of
  • big ISPs at core

3
The Core of the Internet
Sprint
Verio
UUNet
rice.edu
  • Routing done using BGP at core
  • Inter-domain routing could be RIP/OSPF etc

4
BGP Overview
92.92.x.x
Sprint
128.42.x.x 196.29.x.x
92.92.x.x
Verio
100.100.x.x 196.29.x.x
UUNet
128.42.x.x
196.29.x.x
100.100.x.x
5
BGP Overview (contd.)
  • Path Vector protocol
  • Similar to Distance Vector routing
  • Loop detection done using AS_PATH field

R1
R2
Peering session (TCP)
  • Exchange full routing table at start
  • Updates sent incrementally

6
Key Point
The volume of BGP messages exchanged is
abnormally high
  • Most messages are redundant / unnecessary and do
    not
  • correspond to and topology or policy
    changes

7
Consequence Instability
  • Normal data packets handled by dedicated hardware
  • BGP packet processing consumes CPU time
  • Severe CPU processing overhead takes the router
    offline

Route Flap Storm
B
  • Router A temporarily fails

A
  • When A becomes alive B C
  • send full routing tables
  • B C failcascading effect

C
How do we avoid /lessen the impact of these
problems?
8
Route Dampening
  • Router does not accept frequent route updates to
    a
  • destination
  • Might signal that network has erratic
    connectivity
  • Increment counter for destination when route
    changes
  • Counter exceeds threshold stop accepting updates
  • Decrement counter with time

Problem
  • Future legitimate announcements are accepted
    only
  • after a delay

9
Prefix Aggregation/Super-netting
  • Core router advertises a less specific network
    prefix
  • Reduces size of routing tables exchanged

Problems
Prefix aggregation is not effective because
- Internet addresses largely non-hierarchically
assigned
- Domain renumbering not done when changing ISPs
- 25 of prefixes multi-homed
- Multi-homed prefixes should be exposed at the
core
10
Route Servers
  • O(N) peering sessions per
  • Router
  • 1 peering session per router

Route Server
In-spite of all these measures the BGP message
overhead is unexpectedly high
11
Evaluation Methodology
  • Data from Route Server at M.A.E west (D.C)
    peering point
  • Peering point for more than 60 major ISPs
  • Nine month log
  • Time series analysis of message exchange events

12
Observation Lots of redundant updates
  • Duplicate route with-drawls

Number of With-drawls
Unique
ISP
Ratio
A
23276
4344
5
F
86417
12435
7
I
2479023
14112
175
One Reason - Stateless BGP - No state
of previous with-drawls maintained
13
Observation Instability Proportional to Activity
After removing duplicate messages
Time of day
14
Evidence from Fine Grained Structure
7 days
24 hours
Power spectral density
Frequency (1/hour)
Conjecture BGP packets are competing with
data packets during high bandwidth activity.
15
Observation Instability size uncorrelated
  • ISPs serving more network prefixes
  • may not contribute more to instability

16
Observation Instability distributed over routes
75 median
Cumulative proportion
10
of announcements per prefixAS
  • 20 to 90 of routes change 10 times or less
  • No single route contributes significantly to
    instability

17
Observation Synchronized updates
  • Inter-arrival times of
  • updates shows periodicity
  • 30 s and 1 minute patterns
  • Some routers collect and send
  • Updates once every 30 s

Possible reasons
  • Routers get synchronized
  • Border router- Internal router interaction
    misconfigured??

18
End-to-end Perspective
Chinoy Dynamics of Internet routing
information (SIGCOMM 93)
Measurements on NSFNET showed - Processing
and forwarding latency of BDP update is 3
orders of magnitude more than the latency
incurred in forwarding data packets -
Will lead to packet drops during the intervening
period
Paxson End-to-End routing behavior in the
internet (SIGCOMM 96)
  • Routing loops introduce loops into other
    routers routing tables
  • An end-to-end route changes every 1.5 hours on
    an average

19
End-to-End perspective (Paxson)
Pathology type
Probability in 1995
Probability in 1996
same
Long-lived Routing loops
Short-lived Routing loops
same
Outagegt30s
0.96
2.2
Total
3.4
1.5
20
Summary and Conclusions
  • Redundant routing information flows in core
  • Instability distributed across autonomous systems

Possible reasons for instability
  • Stateless BGP updates
  • Misconfigured routers
  • Synchronization
  • Clocks driving the links not synchronized (link
    flaps)

21
Follow-up work impact
Origins of Internet Routing Instability-1999
  • Migration from stateless to stateful BGP
    decreased duplicate withdrawals
  • by an order of magnitude
  • But Duplicate Announcements (AADup) doubled
  • Reason Non-transitive attribute filtering not
    implemented

- BGP specification never
propagate non-transitive attributes..
- ASPATH is transitive attribute
- MED (Multi Exit Discriminator)
is NOT transitive
22
Propagating MEDs Causes Oscillations
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