On AS-Level Path Inference Jia Wang (AT&T Labs Research) Joint work with Z. Morley Mao (University...
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Transcript of On AS-Level Path Inference Jia Wang (AT&T Labs Research) Joint work with Z. Morley Mao (University...
On AS-Level Path Inference
Jia Wang (AT&T Labs Research) Joint work withZ. Morley Mao (University of Michigan, Ann Arbor)Lili Qiu (University of Texas, Austin) Yin Zhang (University of Texas, Austin)
On AS-Level Path Inference 2June 10, 2005
Discover end-to-end forwarding path between two hosts
Berkeley
Internet
CNN
Calren
Level3
GNN
Qwest Sprint UUnet
University company
AT&T
business
ISP ISP ISP
ISP ISP ISP ISP
ISP
Berkeley
Calren
Level3 Qwest Sprint UUnet
University company
AT&T
business
ISP ISP ISP
ISP ISP ISP ISP
ISP
Berkeley
Calren
Level3 Qwest Sprint UUnet
University company
AT&T
business
ISP ISP ISP
ISP ISP ISP ISP
ISP
On AS-Level Path Inference 3June 10, 2005
Motivation
Network diagnoses Performance optimization Overlay network Content distribution Network modeling
On AS-Level Path Inference 4June 10, 2005
Example – overlay routing
SourceDestination
?
?? ?
154
2
3
Internet
On AS-Level Path Inference 5June 10, 2005
Outline
Related work Routescope Evaluation Improvements
AS relationship inferenceFirst AS hop inference
Conclusion
On AS-Level Path Inference 6June 10, 2005
Related work
Forwarding path discoveryWith direct access to the source
Router-level: traceroute AS-level: [Mao Sigcomm2003] [Mao Infocom2004]
Without direct access to the source None!
On AS-Level Path Inference 7June 10, 2005
Challenges
Asymmetric routing Over 60% of AS paths asymmetric
Complicated routing policies Not shortest path routing Commercial relationship between ASes determines
how traffic flow though the Internet
Multi-homing Very common
On AS-Level Path Inference 8June 10, 2005
Routescope
Key observation: relationships among ASes play important role in determining feasible forwarding paths
Approach: Infer AS-level paths by finding the shortest policy path in an AS graph obtained from BGP tables collected from multiple vantage points
On AS-Level Path Inference 9June 10, 2005
Assumptions
Explicit AS relationships Peer-peer Provider-customer
Shortest AS policy path preferred “Valley-free” rule
Uniform routing policy within an AS AS destination based uniform routing Stability
These assumptions are mostly correct.
On AS-Level Path Inference 10June 10, 2005
AS relationships translate into BGP export rules Export to a provider or a peer
Allowed: its routes and routes of its customers and siblings
Disallowed: routes learned from other providers or peers
Export to a customer or a siblingAllowed: its routes, the routes of its customers
and siblings, and routes learned from its providers and peers
On AS-Level Path Inference 11June 10, 2005
“Valley-free” rule
After traversing a provider-customer or peer-peer edge, cannot traverse a customer-provider or peer-peer edge
Invalid path: >= 2 peer links downhill-uphilldownhill-peerpeer-uphill
On AS-Level Path Inference 12June 10, 2005
Example of valley-free paths
XX
[1 2 3], [1 2 6 3] are valley-free
[1 4 3], [1 4 5 3] are not valley free
On AS-Level Path Inference 13June 10, 2005
AS path inference algorithm
Compose the AS graph based on BGP tables Infer AS relationship Classify edges based on AS relationship
Customer-provider (UP) link Provider-customer (DOWN) link Peer-peer (FLAT) link
Compute shortest policy path conforming the “valley-free” rule using modified Dijkstra’s algorithm
Infer the first AS hop if multiple paths returned
On AS-Level Path Inference 14June 10, 2005
Evaluation
Based on existing AS relationship inference algorithms Gao: based on the degree of ASes along the path SARK: consider AS hierarchy properties BPP: formulate as 2SAT problem and develop
heuristics that yield minimum of invalid paths Compare AS-level paths
Extracted from a large number of BGP tables Among 125 public BGP gateways
On AS-Level Path Inference 15June 10, 2005
Paths in BGP tables
BGP table
AlgorithmUnique paths
MatchMatch length
Exact match
Shorter Longer
AS7018 (tier-1)
Gao
18085
77% 80% 33% 18% 2%
SARK 67% 79% 34% 15% 4%
BPP 84% 85% 37% 15% 0%
AS2152 (tier-2)
Gao
11990
62% 65% 10% 34% 1%
SARK 48% 57% 29% 40% 3%
BPP 67% 67% 12% 33% 0%
AS8121 (tier-3)
Gao
15757
16% 27% 3% 69% 4%
SARK 14% 23% 3% 72% 4%
BPP 18% 30% 3% 66% 5%
On AS-Level Path Inference 16June 10, 2005
Paths between BGP gateways
BGP gateway
AlgorithmUnique paths
MatchMatch length
Exact match
Shorter Longer
All
Gao
2457
30% 51% 21% 15% 35%
SARK 38% 61% 24% 20% 19%
BPP 18% 29% 15% 5% 66%
US
Gao
1907
24% 43% 16% 18% 40%
SARK 40% 57% 24% 24% 19%
BPP 22% 42% 18% 10% 48%
BPP yields most accurate AS path inference than GAO and SARK
On AS-Level Path Inference 17June 10, 2005
Possible causes of mismatches
Inaccuracy in AS relationship inferenceEspecially in non-North American regions
Multihoming
On AS-Level Path Inference 18June 10, 2005
Inaccurate AS relationship inference 19%~66% of inferred paths are longer than
actual paths Significant inconsistency among AS relationship
inference results
Common peer-peer Common provider-customer
Gao vs SARK 229 (3.63%, 36.12%) 41730 (89.43%, 94.68%)
Gao vs BPP 5959 (94.51%, 48.42%) 39606 (84.87%, 97.74%)
SARK vs BPP 334 (52.68%, 2.71%) 33752 (85.66%, 93.17%)
Solution: infer more accurate AS relationships
On AS-Level Path Inference 19June 10, 2005
A new AS relationship inferencealgorithm Problem formulation: integer programming
Each edge e in the direct graph G = (V,E) Relation(e) = 1 (customer-provider), 2 (peer-peer), or 3 (provider-
customer) Constraints
If r is reverse edge of e, relation(e)+relation(r) = 4. Every path in use is valley-free, i.e., for (e1,e2) on a path, relation(e1)
= 1 relation(e2) = 3. For any (src,dst), if there is a path P that is shorter than actual path,
then P is not valley free, i.e., (e1,e2) on P s.t. relation(e1) ≠ 1 relation(e1) ≠ 3.
Novelty: derive additional constraints that violate valley free constraints
Solution: improved random walk algorithm [Selman et al. 1993] Handle non-binary variables Repeatedly remove stub ASes with out-degree of 0
On AS-Level Path Inference 20June 10, 2005
AS path inference with accurate AS relationship
Unique paths
MatchMatch length
Exact match
Shorter Longer
AS7018 (tier-1) 18085 82% 83% 35% 17% 0%
AS2152 (tier-2) 11990 64% 64% 10% 35% 0%
AS8121 (tier-3) 15757 16% 27% 3% 69% 4%
All BGP gateways 2457 70% 73% 30% 22% 4%
US BGP gateways 1907 60% 62% 27% 34% 4%
The accuracy is among the best of other three in BGP table experiments and is much higher than alternatives in BGP gateway experiments.
On AS-Level Path Inference 21June 10, 2005
Multihoming
Over half of the mismatches occur at the very first hop AS
If first hop is known, over 15% of mismatches can be eliminated
Solution: infer the first hop AS
AS S
AS T2
AS T1 AS D
AS C
Inferred path
Actual path
On AS-Level Path Inference 22June 10, 2005
First hop inference
Gather candidate first hop ASes from S by launch traceroute to S from multiple vantage points
Identify the transition point T that is likely to be on the path from S to D by testinghop_count(S,T) + hop_count(T,D) = hop_count(S,D)
SourceDestination
AS S
AS T2
AS T1 AS D
AS C
Transition point T1
T2 Assume having access to D
On AS-Level Path Inference 23June 10, 2005
Hop count inference
Hop_count(S,T) ≈ hop_count(T,S) Hop_count(H,D): H = S or T
Send ping packet to H Guess the initial TTL value TTL0 set by H Get TTL value TTL1 in ICMP response packet received from H Hop_count(H,D) = TTL0 - TTL1 + 1
Common value for TTL0 32 (Win95/98/Me) 64 (Linux, Compaq Tru64) 128 (Win NT/2000/XP) 255 (most UNIX systems)
On AS-Level Path Inference 24June 10, 2005
Improvement with known first AS hop
Unique paths Match length Improvement
AS7018 (tier-1) 18085 86% 3%
AS2152 (tier-2) 11990 76% 12%
AS8121 (tier-3) 15757 48% 21%
All BGP gateways 1907 70% 8%
US BGP gateways 2457 88% 15%
On AS-Level Path Inference 25June 10, 2005
Possible causes of inaccuracy
Complicated AS relationships: 15% paths Two consecutive FLAT links DOWN link followed by a FLAT link FLAT link followed by UP link Dual transit/peering relationship
Routing policies Shortest path vs. customer routes Inconsistent advertisement to different peering locations BGP tie-breaking rules
AS prepending > 28% ASes
On AS-Level Path Inference 26June 10, 2005
Conclusion
Routescope: AS-level path inference tool without access to the source
Two enhancements AS relationship inference First hop inference
Accuracy: up to 88% inferred paths have the same length as the actual paths
New metric for evaluating AS relationship inference Evaluate existing AS relationship inference algorithms