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Scalable and Accurate Identification of AS-Level Forwarding Paths

This research aims to improve the accuracy of IP-to-AS mappings for identifying AS-level forwarding paths in the Internet, crucial for network diagnosis and troubleshooting. By refining mappings using BGP tables and traceroute data, potential causes of discrepancies such as internet exchange points and sibling ASes can be addressed. The study involves identifying and resolving path discrepancies to enhance network analysis.

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Scalable and Accurate Identification of AS-Level Forwarding Paths

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  1. Scalable and Accurate Identification of AS-Level Forwarding Paths Z. Morley Mao University of Michigan, Ann Arbor Joint work with David Johnson, Jennifer Rexford, Jia Wang (AT&T-Research), and Randy Katz (UC Berkeley)

  2. Path packets traverse through the Internet IP Forwarding Path Internet IP traffic destination source • Why important? • Characterize end-to-end network paths • Discover the router-level Internet topology • Detect and diagnose reachability problems

  3. No response from router No name resolution Example Traceroute Output (Berkeley to CNN) Hop number, IP address, DNS name 1 169.229.62.1 2 169.229.59.225 3 128.32.255.169 4 128.32.0.249 5 128.32.0.66 6 209.247.159.109 7 * 8 64.159.1.46 9 209.247.9.170 10 66.185.138.33 11 * 12 66.185.136.17 13 64.236.16.52 inr-daedalus-0.CS.Berkeley.EDU soda-cr-1-1-soda-br-6-2 vlan242.inr-202-doecev.Berkeley.EDU gigE6-0-0.inr-666-doecev.Berkeley.EDU qsv-juniper--ucb-gw.calren2.net POS1-0.hsipaccess1.SanJose1.Level3.net ? ? pos8-0.hsa2.Atlanta2.Level3.net pop2-atm-P0-2.atdn.net ? pop1-atl-P4-0.atdn.net www4.cnn.com

  4. AS B AS C AS A AS D Autonomous System (AS) Autonomous System Forwarding Path Example: Pinpoint forwarding loop & responsible AS Internet IP traffic destination source

  5. d: path=[BC] d: path=[C] AS C AS A AS B Forwarding path: data traffic Border Gateway Protocol (BGP) Signaling path: control traffic BGP path may differ from forwarding AS path • Routing loops and deflections • Route aggregation and filtering • BGP misconfiguration Origin AS d: path=[A B C] d: path=[B C] prefix d

  6. AS25 AS25 AS25 AS25 AS11423 AS3356 AS3356 AS3356 AS3356 AS1668 AS1668 AS1668 AS5662 Berkeley Calren Level3 AOL CNN Map Traceroute Hops to ASes Traceroute output: (hop number, IP) 1 169.229.62.1 2 169.229.59.225 3 128.32.255.169 4 128.32.0.249 5 128.32.0.66 6 209.247.159.109 7 * 8 64.159.1.46 9 209.247.9.170 10 66.185.138.33 11 * 12 66.185.136.17 13 64.236.16.52 Need accurate IP-to-AS mappings (for network equipment).

  7. Possible Ways to Get IP-to-AS Mapping • Routing address registry • Voluntary public registry such as whois.radb.net • Used by prtraceroute and “NANOG traceroute” • Incomplete and quite out-of-date • Mergers, acquisitions, delegation to customers • Origin AS in BGP paths • Prefix=198.133.206.0/24, ASpath=[1239 2914 3130] • Public BGP routing tables such as RouteViews • Used to translate traceroute data to an AS graph • Incomplete and inaccurate… but usually right • Multiple Origin ASes (MOAS), no mapping, wrong mapping

  8. Refining Initial IP-to-AS Mapping • Start with initial IP-to-AS mapping • Mapping from BGP tables is usually correct • Good starting point for computing the mapping • Collect many BGP and traceroute paths • Signaling and forwarding AS path usually match • Good way to identify mistakes in IP-to-AS map • Successively refine the IP-to-AS mapping • Find add/change/delete that makes big difference • Validation: explain these “edits” by operational realities

  9. Traceroute paths from multiple locations For each location: Local BGP paths Traceroute AS paths • Compare (dynamic programming) • Edit IP-to-AS mappings to account for • known causes of mismatches • (e.g., IXP, sibling ASes) • (a single change explaining a large number of mismatches) Combine all locations: BGP and Traceroute Data Collection Initial mappings from origin AS of a large set of BGP tables (Ignoring unstable paths)

  10. Experimental Methodology 200,000 destinations: d0, d1, d2, d3, d4, … d200,000 For each di -Traceroute path -BGP path

  11. Measurement Data: Eight Vantage Points Sweep the routable IP address space • ~200,000 IP addresses • 160,000 prefixes • 15,000 destination ASes

  12. Assumptions • IP-to-AS mapping • Mappings from BGP tables are mostly correct. • Change slowly • BGP paths and forwarding paths mostlymatch. • 70% of the BGP path and traceroute path match

  13. Reasons BGP and Traceroute Paths Differ • IP-to-AS mapping is inaccurate (fix these!) • Internet eXchange Points (IXPs) • Sibling ASes owned by the same institution • Unannounced infrastructure addresses • Forwarding and signaling paths differ (study these!) • Forwarding loops and deflections • Route aggregation and filtering • Traceroute inaccuracies (don’t overreact to these!) • Forwarding path changing during measurement • Address assignment to border links between ASes • Outgoing link identified in “time exceeded” message

  14. Extra AS due to Internet eXchange Points • IXP: shared place where providers meet • E.g., Mae-East, Mae-West, PAIX • Large number of fan-in and fan-out ASes E A A E F B F B D G C G C Traceroute AS path BGP AS path Physical topology and BGP session graph do not always match.

  15. Extra AS due to Sibling ASes • Sibling: organizations with multiple ASes: • E.g., Sprint AS 1239 and AS 1791 • AS numbers equipment with addresses of another E A E A F B H D F B D G C G C Traceroute AS path BGP AS path Sibling ASes “belong together” as if they were one AS.

  16. A C A C A C B A C B C Weird Paths Due to Unannounced Addresses 12.0.0.0/8 A B C does not announce part of its address space in BGP(e.g., 12.1.2.0/24) C Fix the IP-to-AS map to associate 12.1.2.0/24 with C

  17. Optimization Framework • Start with initial IP-to-AS map A(x) • IP address x maps to A(x), a set of ASes • Compute traceroute IP to AS mapping • For each traceroute-BGP path pair • Dynamic programming to minimize mismatch • Iterative refinement • Modify A(x) depending on a small set of rules • Terminate when no further modifications

  18. Rules for Modifying the IP-to-AS Mapping • Computing match statistics across paths • Focusing on path pairs with at most two errors • Example rules • Create a mapping: A(x) is null • Assign to the AS y that appears in the most matchings • Replace a mapping: A(x) has one entry • If an AS ynot in A(x) accounts for > 55% of matchings • Delete from a mapping: A(x) has multiple entries • If an AS y in A(x) accounts for < 10% of matchings • Algorithm converges in less than ten iterations

  19. Optimization Results • Metric: Mismatch ratio • Percentage of traceroute-BGP path pairs with a mismatch • Modified 2.9% of original mappings Robustness

  20. Validatingthe Changes to the Mapping • AT&T’s tier-1 network (AS 7018) • Dump of configuration state from each of the routers • Explains 45 of 54 changes involving AS 7018 • E.g., customer numbered from AT&T addresses • E.g., Internet exchange point where AT&T connects • Whois query on prefix or AS • Look for “exchange point” or “Internet exchange” • Look for ASes with similar names (Sprintlink vs. Sprintlink3) • List of known Internet eXchange Points • Explains 24 of the MOAS inferences • Total of 38 IXPs contributed to mapping changes

  21. D D D D E E B B C C C B Validation: Exploring the Remaining Mismatches BGP path: B C Traceroute path: B C D • Route aggregation • Traceroute AS path longer in 20% of mismatches • Different paths for destinations in same prefix • Interface numbering at AS boundaries • Boundary links numbered from one AS • Verified cases where AT&T (AS 7018) is involved BGP path: B C D Traceroute path: B D

  22. Contributions • Problem formulation • AS-level traceroute tool for troubleshooting • Compute an accurate IP-to-AS mapping • Optimization approach • Compute matchings using dynamic programming • Improve mapping through iterative refinement • Measurement methodology • Traceroute and BGP paths from many locations • Validation of our results • Changes to the IP-to-AS mappings • Remaining mismatches between traceroute and BGP

  23. Future Work on AS Traceroute • Lower measurement overhead • Avoid traceroute probes that would discover similar paths • Work with BGP routing tables rather than live feeds • Limiting the effects of traceroute inaccuracies • Catch routing changes through repeat experiments • Use router-level graphs to detect AS boundaries • Detect routers using outgoing link in “time exceeded” • Public AS traceroute tool • Periodic data collection and computation of IP-to-AS mapping • Software to apply mapping to traceroute output • Network troubleshooting • Analyze valid differences between forwarding and signaling paths • Use the AS traceroute tool to detect and characterize anomalies

  24. Whois: unmapped hops cause half of mismatches BGP tables: mostly match, as our algorithm assumes Refined mapping: change 2.9% of original mapping Robust to reducing # of probes and introducing noise Comparison of IP-to-AS Mappings Comparing BGP and Traceroute AS paths for various IP-to-AS mappings

  25. Systematic optimization • Dynamic-programming and iterative improvement • Initial IP-to-AS mapping derived from BGP routing tables • Identify a small number of modifications that significantly improve the match rate. • 95% match ratio, less than 3% changes, very robust

  26. Time exceeded TTL=1 TTL=2 Traceroute: Measuring the Forwarding Path • Time-To-Live field in IP packet header • Source sends a packet with a TTL of n • Each router along the path decrements the TTL • “TTL exceeded” sent when TTL reaches 0 • Traceroute tool exploits this TTL behavior destination source Send packets with TTL=1, 2, 3, … and record source of “time exceeded” message

  27. Matching Function and Unavoidable Error • Matching function m for BGP/traceroute pair • Traceroute path: t1, t2, …, tnof n IP addresses • BGP path: b1, b2, …, blof l AS numbers • Matching: associateIP hop tiwith AS hop bm(i) • Find the matching m that minimizes error • Number of traceroute hops with bm(i) not in A(ti) • Dynamic programming algorithm to find best m t: 1 2 34 56 7 8 b: ABC

  28. Initial Analysis of BGP and Traceroute Paths • Traceroute paths: initial mapping A from BGP • Unmapped hops: match no ASes (1-3% of paths) • MOAS hops: match any AS in the set (10-13% of paths) • “*” hops: match any AS (7-9% of paths) • BGP paths: discard 1% of prefixes with AS paths • Routing changes based on BGP updates • Private AS numbers (e.g., 65100) • Empty AS paths (local destinations) • Apparent AS-level loops from misconfiguration • AS_SET instead of AS sequence

  29. Validating the Changes to the Mapping • AT&T’s tier-1 network (AS 7018) • Dump of configuration state from each of the routers • Explains 45 of 54 changes involving AS 7018 • E.g., customer numbered from AT&T addresses • E.g., Internet exchange point where AT&T connects • Whois query on prefix or AS • Look for “exchange point” or “Internet exchange” • Explains 24 of the changes to the mappings • Look for ASes with similar names (Sprintlink vs. Sprintlink3) • Explains many of the changes to the mappings • List of known Internet eXchange Points • Explains 24 of the MOAS inferences • Total of 38 IXPs contributed to mapping changes

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