TIE Breaking: Tunable Interdomain Egress Selection

1
TIE Breaking Tunable Interdomain Egress Selection

• Renata Teixeira
• Laboratoire dInformatique de Paris 6
• Université Pierre et Marie Curie
• with Tim Griffin (Cambridge), Mauricio Resende
  (ATT), and Jennifer Rexford (Princeton)

2
Internet as a Communication Infrastructure
Highly-sensitive to transient and persistent
performance problems
3
Two-Tier Routing Architecture
4
Selecting Among Multiple Egresses Today
NY
B
SF
LA
Bs IGP distance d(B,NY) 2 d(B,SF) 31 d(B,LA)
26
Hot-potato routing BGP selects closest egress
by comparing IGP distances
5
However, Hot-Potato Routing is

• Too disruptive
• Small changes inside can lead to big disruptions

dst
9
B
B
A
A
D
4
3
8
10
3
4
G
11
E
F
8
6
5
C

• Consequences
• Transient forwarding instability
• Traffic shift (largest traffic variations)
• BGP updates to other domains

6
However, Hot-Potato Routing is

• Too disruptive
• Small changes inside can lead to big disruptions
• Too restrictive
• Egress selection mechanism dictates a policy
• Too convoluted
• IGP metrics determine BGP egress selection
• IGP paths and egress selection are coupled

7
Maybe a Fixed Ranking?

• Goal No disruptions because of internal changes
• Solution
• Each router has a fixed ranking of egresses
• Select the highest-ranked egress for each
  destination
• Use tunnels from ingress to egress
• Disadvantage
• Sometimes changing egresses would be useful

dst
8
Egress Selection Mechanisms
automatic adaptation
robustness to internal changes
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Metrics for Ranking Egresses

• Egress selection mechanisms are based on a metric
  (m) that each ingress router (i) uses to rank
  each egress router (e) for a destination
• Hot-potato routing
• m is the intradomain distance (d(i,e))
• Fixed ranking
• m is a constant

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Goals of New Metric

• Configurable
• Implement a wide-range of egress selection
  policies
• Simple computation
• Compute on-line, in real-time
• Based on information already available in routers
  (distance)
• Easy to optimize
• Expressive for a management system to optimize
• Fine control
• Each ingress can implement its own ranking policy
  for each destination

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TIE Tunable Interdomain Egress Selection
m (e) ? (e) . d(i,e) ? (e)
mi(dst,e) ?i(dst,e) . d(i,e) ?i(dst,e)

• Decouples egress selection from internal paths
• Egress selection is done by tuning ? and ?
• Allow a wide variety of egress selection policies
• Hot-potato ? 1, ? 0
• Fixed ranking ? 0, ? constant rank
• Requirements
• Small change in router decision logic
• Use of tunnels (as with fixed ranking)

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Using TIE
Administrator defines policy
Run optimization
?, ?
Configure routers
Routers
Upon ? and ? change or routing change
Path computation using mi(dst,e)
Forwarding table
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Configuring TIE to Minimize Sensitivity
Network topology
Management System
Set of egress routers per prefix
Simulation Phase
Set of failures
system of inequalities
Optimization Phase
configure routers with values ?i(dst,e) and
?i(dst,e) that minimize sensitivity
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Simulation Phase
dst
B
A
20
10
11
9
C
11.?C(dst,A) ?C(dst,A) lt 10.?C(dst,B)
?C(dst,B)
20.?C(dst,A) ?C(dst,A) gt 10.?C(dst,B)
?C(dst,B)
?C(dst,A)1, ?C(dst,A)1, ?C(dst,B) 2, ?C(dst,B)
0
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Optimization Phase

• One system of inequalities per (node, prefix)
  pair
• (num egresses 1) x (num failures 1)
• Practical requirements for setting parameters
• Finite-precision parameter values
• Limiting the number of unique values
• Robustness to unplanned events
• Running time
• 37 seconds (Abilene network) and 12 minutes (ISP
  network)
• 196MHz MIPS R10000 processor on an SGI Challenge

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Evaluation of TIE on Operational Networks

• Topology and egress sets
• Abilene network (U.S. research network)
• Set link weight with geographic distance
• Configuration of TIE
• Considering single-link failures
• Threshold of delay ratio 2
• ? ?1,4 and 93 of ?i(dst,e)1
• ? ? 0,1,3251 and 90 of ?i(dst,e)0
• Evaluation against hot-potato and fixed ranking
• Simulate single-node failures
• Measure routing sensitivity and delay

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Sensitivity to Node Failures
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Delay under Node Failures
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Conclusion

• TIE mechanism for selecting egresses
• Decouples interdomain and intradomain routing
• Designed for being easy to optimize
• Small change to router implementation
• Operators can optimize TIE for other policies
• Traffic engineering
• Robust traffic engineering
• Planning for maintenance

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More details

• http//rp.lip6.fr/teixeira

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Multiple Interdomain Egresses
NY
SF
AOL
LA
Multiple egresses for a destination are
common! ISPs usually peer in multiple locations
and customers buy multiple connections to one or
more ISPs for reliability and performance
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Why Hot-Potato Routing?

• Independent and consistent egress decision
• Forward packet to neighbors that have selected
  same (closest) egress
• Minimize resource consumption
• Limits consumption of bandwidth by sending
  traffic to next domain as early as possible

dst
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Summary ofBGP Decision Process

• BGP decision process
• Ignore if exit point unreachable
• Highest local preference
• Lowest AS path length
• Lowest origin type
• Lowest MED (with same next hop AS)
• Lowest IGP cost to next hop
• Lowest router ID of BGP speaker

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Other Policies

• Traffic engineering
• Configure TIE parameters to select egresses to
  obtain optimal link utilization
• Solution Path-based multi-commodity flow
• Robust traffic engineering
• Combine minimizing sensitivity with traffic
  engineering problem
• Preparing for maintenance

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Traffic Engineering with TIE

• Problem definition
• Balance utilization of internal links
• Configure TIE parameters to select egresses to
  obtain optimal link utilization
• No need to set intradomain link weights
• Solution
• Path-based multicommodity flow
• No need to tweak routing protocols
• Avoid routing convergence

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Example Policy Minimizing Sensitivity

• Problem definition
• Minimize sensitivity to equipment failures
• No delay more than twice design time delay
• Would be a simple change to routers
• If distance is more than twice original distance
• Change to closest egress
• Else
• Keep using old egress point
• But cannot change routers for all possible goals

We can do this with TIE just by setting ? and ?