Post on 19-Dec-2015
transcript
TIE Breaking: Tunable Interdomain Egress Selection
Renata TeixeiraLaboratoire d’Informatique de Paris 6
Université Pierre et Marie Curie
with Tim Griffin (Cambridge), Mauricio Resende (AT&T), and Jennifer Rexford (Princeton)
2
Internet as a Communication Infrastructure
InternetHighly-sensitive to transient and persistent performance problems
3
Two-Tier Routing Architecture
InternetUCSD
Sprint
AT&T Verio
AOL
User
Web Server
UCSD
AT&T Verio
AOL
Interdomain routing (BGP)Selects AS-level path
based on policies
Intradomain routing (IGP)Most common: OSPF, IS-IS
Selects shortest path from ingress to egress based on link weights
4
UCSDSprint
AT&T Verio
AOL
Selecting Among Multiple Egresses Today
User
Web Server
UCSD
AT&T Verio
Hot-potato routingBGP selects closest egress by
comparing IGP distances
1 130
1 1525
NY
SF
LAB’s IGP distance
d(B,NY): 2d(B,SF): 31d(B,LA): 26
B
5
However, Hot-Potato Routing is…
Too disruptive Small changes inside can lead to big disruptions
A B
C
DG
EF4
5
11
39
34
108
68
A Bdst
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
AB
C
DG
EF4
5
39
34
108
8
A Bdst
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Egress Selection Mechanismsau
tom
atic
ada
ptat
ion
robustness to internal changes
hot-potato routing
fixed ranking
Explore trade-off
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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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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)
m (e) = (e) . d(i,e) + (e)
TIE: Tunable Interdomain Egress Selection
mi(dst,e) = i(dst,e) . d(i,e) + i(dst,e)
weighted intradomain
distance
constant
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Routers
ManagementSystem
Using TIE
Run optimization
, Configure routers
Path computationusing mi(dst,e)
Forwarding table
Administrator defines policy
Upon and changeor routing change
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Configuring TIE to Minimize Sensitivity
Simulation Phase
Optimization Phase
Network topology
Set of egress routers per prefix
Set of failuressystem of
inequalities
Management System
configure routers withvalues i(dst,e) and i(dst,e)
that minimize sensitivity
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At design time: mC(dst,A) < mC(dst,B)
9.C(dst,A) + C(dst,A) < 10.C(dst,B) + C(dst,B)
Simulation Phase
AB
C
dst
911
2010
20.C(dst,A) + C(dst,A) > 10.C(dst,B) + C(dst,B)11.C(dst,A) + C(dst,A) < 10.C(dst,B) + C(dst,B)
Output of simulation phase
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
Integer programming
Objective function: min (+)
1
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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
fraction prefixes affected
CC
DF
of
(no
de,
fail
ure
) p
airs
15% of egress changes can be avoided without harming delay
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Delay under Node FailuresC
CD
F o
f (
no
de,
des
tin
atio
n,f
ailu
re)
tup
les
ratio of delay after failure to design time delay
Under threshold, TIE has longer delay than hot-potato
It is better than fixed ranking for 60% of tuples
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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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UCSDSprint
AT&T Verio
AOL
Multiple Interdomain Egresses
User
Web Server
UCSD
AT&T Verio
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
NY
SF
LA
AOL
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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
A B
C
DG
EF4
5
11
39
34
108
68
A Bdst
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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