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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&TResearch), and Randy Katz (UC Berkeley) 1 IP Forwarding Path Path packets traverse through the Internet Internet IP traffic source Why important? destination Characterize end-to-end network paths Discover the router-level Internet topology Detect and diagnose reachability problems 2 Example Traceroute Output (Berkeley to CNN) Hop number, IP address, DNS name No response from router 1 169.229.62.1 inr-daedalus-0.CS.Berkeley.EDU 2 169.229.59.225 soda-cr-1-1-soda-br-6-2 3 128.32.255.169 vlan242.inr-202-doecev.Berkeley.EDU 4 128.32.0.249 gigE6-0-0.inr-666-doecev.Berkeley.EDU 5 128.32.0.66 qsv-juniper--ucb-gw.calren2.net 6 209.247.159.109 POS1-0.hsipaccess1.SanJose1.Level3.net 7 * ? 8 64.159.1.46 ? 9 209.247.9.170 pos8-0.hsa2.Atlanta2.Level3.net No name resolution 10 66.185.138.33 pop2-atm-P0-2.atdn.net 11 * ? 12 66.185.136.17 pop1-atl-P4-0.atdn.net 13 64.236.16.52 www4.cnn.com 3 Autonomous System Forwarding Path Example: Pinpoint forwarding loop & responsible AS IP traffic Internet destination source Autonomous System (AS) 4 Border Gateway Protocol (BGP) Signaling path: control traffic d: path=[A B C] d: path=[B C] d: path=[BC] d: path=[C] Forwarding path: data traffic Origin AS prefix d BGP path may differ from forwarding AS path Routing loops and deflections Route aggregation and filtering BGP misconfiguration 5 Map Traceroute Hops to ASes Traceroute output: (hop number, IP) 1 169.229.62.1 AS25 2 169.229.59.225 AS25 Berkeley 3 128.32.255.169 AS25 4 128.32.0.249 AS25 5 128.32.0.66 AS11423 Calren 6 209.247.159.109 AS3356 7 * AS3356 8 64.159.1.46 AS3356 9 209.247.9.170 AS3356 10 66.185.138.33 AS1668 11 * AS1668 12 66.185.136.17 AS1668 13 64.236.16.52 AS5662 CNN Level3 AOL Need accurate IP-to-AS mappings (for network equipment). 6 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Voluntary public registry such as whois.radb.net Used by prtraceroute and “NANOG traceroute” Incomplete and quite out-of-date Mergers, acquisitions, delegation to customers Origin AS in BGP paths Prefix=198.133.206.0/24, ASpath=[1239 2914 3130] Public BGP routing tables such as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 7 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 8 BGP and Traceroute Data Collection Initial mappings from origin AS of a large set of BGP tables (Ignoring unstable paths) For each location: Combine all locations: Local BGP paths Traceroute paths from multiple locations 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) 9 Experimental Methodology 200,000 destinations: d0, d1, d2, d3, d4, … d200,000 For each di -Traceroute path -BGP path 10 Measurement Data: Eight Vantage Points Sweep the routable IP address space ~200,000 IP addresses 160,000 prefixes 15,000 destination ASes Organization Location Upstream Provider AT&T Research NJ, US UUNET, AT&T UC Berkeley CA, US Qwest, Level3, Internet 2 PSG home network WA, US Sprint, Verio Univ of Washington WA, US Verio, Cable&Wireless ArosNet UT, US UUNET Nortel ON, Canada AT&T Canada Vineyard.NET MA, US UUNET, Sprint, Level3 Peak Web Hosting CA, US 11 Level 3, Global Crossing, Teleglobe Assumptions IP-to-AS mapping Mappings from BGP tables are mostly correct. Change slowly BGP paths and forwarding paths mostly match. 70% of the BGP path and traceroute path match 12 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 13 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 A B C D E A E F B F G C G Traceroute AS path BGP AS path Physical topology and BGP session graph do not always match. 14 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 A B C H D E A F B G C Traceroute AS path E D F G BGP AS path Sibling ASes “belong together” as if they were one15 AS. 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) ACAC C AC BAC BC 16 Fix the IP-to-AS map to associate 12.1.2.0/24 with C 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 17 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 y not 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 18 Optimization Results Metric: Mismatch ratio Percentage of traceroute-BGP path pairs with a mismatch Modified 2.9% of original mappings Mismatch ratio Full initial Mapping Heuristically optimized mapping Omit 10% initial mapping 5.23% 3.08% 6.57% Omit 4 probing sources Omit probing destinations (one probe per unique BGP path) 6.34% 7.12%19 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” 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 20 Validation: Exploring the Remaining Mismatches B C D D C BGP path: B C Traceroute path: B C D E E Route aggregation Traceroute AS path longer in 20% of mismatches Different paths for destinations in same prefix Interface numbering at AS boundaries B B C D D BGP path: B C D Traceroute path: B D Boundary links numbered from one AS Verified cases where AT&T (AS 7018) is involved 21 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 22 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 23 anomalies Comparison of IP-to-AS Mappings Comparing BGP and Traceroute AS paths for various IP-to-AS mappings Whois Match Mismatch Ratio 47% 53% 0.88 BGP origins 85% 15% 5.8 Refined mapping 95% 5% 18 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 24 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 25 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 TTL=1 source TTL=2 Time exceeded destination 26 Send packets with TTL=1, 2, 3, … and record source of “time exceeded” message Matching Function and Unavoidable Error Matching function m for BGP/traceroute pair Traceroute path: t1, t2, …, tn of n IP addresses BGP path: b1, b2, …, bl of l AS numbers Matching: associate IP hop ti with AS hop bm(i) t: 1 2 b: A 3 4 5 B 6 7 8 C 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 27 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 28 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 29