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Scalable and Accurate Identification of AS-Level Forwarding Paths

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). Path packets traverse through the Internet. IP Forwarding Path.

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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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