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Measuring the Autonomous System Path Through the Internet Jennifer Rexford Internet and Networking...
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Transcript of Measuring the Autonomous System Path Through the Internet Jennifer Rexford Internet and Networking...
Measuring the Autonomous System Measuring the Autonomous System Path Through the InternetPath Through the Internet
Jennifer Rexford
Internet and Networking SystemsAT&T Labs - Research; Florham Park, NJ
http://www.research.att.com/~jrex
Joint work with Z. Morley Mao, David Johnson, Jia Wang, and Randy Katz
IP Forwarding PathIP Forwarding Path
Path packets traverse through the Internet
Why important? Characterize end-to-end network
paths
Discover the router-level Internet topology
Detect and diagnose reachability problems
IP traffic
Internet
sourcedestination
Traceroute: Measuring the Forwarding PathTraceroute: 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
source destination
TTL=1
Time exceeded
TTL=2
Send packets with TTL=1, 2, 3, … and record source of “time exceeded” message
Example Traceroute Output (Berkeley to CNN)Example Traceroute Output (Berkeley to CNN)
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
Hop number, IP address, DNS nameinr-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
No responsefrom router
No name resolution
AS A
AS BAS C
AS DAutonomous System (AS)
Autonomous System Forwarding PathAutonomous System Forwarding Path
Example: Pinpoint forwarding loop & responsible AS
IP trafficInternet
sourcedestination
Border Gateway Protocol (BGP)Border Gateway Protocol (BGP)
BGP path may differ from forwarding AS path– Routing loops and deflections
– Route aggregation and filtering
– BGP misconfiguration
AS A AS B AS Cprefix d
Signaling path: control traffic
d: path=[C]
Forwarding path: data traffic
d: path=[BC]Origin AS
Map Traceroute Hops to ASesMap Traceroute Hops to ASes
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
Traceroute output: (hop number, IP)AS25
AS25
AS25
AS25
AS11423
AS3356
AS3356
AS3356
AS3356
AS1668
AS1668
AS1668
AS5662
Berkeley
CNN
Calren
Level3
AOL
Need accurate IP-to-AS mappings(for network equipment).
Candidate Ways to Get IP-to-AS MappingCandidate 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– 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
Refining Initial IP-to-AS MappingRefining 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
– Base these “edits” on operational realities
Extra AS due to Internet eXchange PointsExtra 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
F
G
Traceroute AS path BGP AS path
Physical topology and BGP session graph do not always match.
B
C
F
G
A E
Extra AS due to Sibling ASesExtra 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
Traceroute AS path BGP AS path
A
B
C
D
E
F
G
H
A
B
C
D
E
F
G
Sibling ASes “belong together” as if they were one AS.
Weird Paths Due to Unannounced AddressesWeird Paths Due to Unannounced Addresses
A B
C
A C
A C A C
B A C B C
C does not announce part ofits address space in BGP
(e.g., 12.1.2.0/24)
12.0.0.0/8
Fix the IP-to-AS map to associate 12.1.2.0/24 with C
Reasons BGP and Traceroute Paths May DifferReasons BGP and Traceroute Paths May Differ
IP-to-AS mapping is inaccurate (fix these!)– Internet eXchange Points (IXPs)
– Sibling ASes owned by the same institution
– Unannounced infrastructure addressesForwarding and signaling paths differ (study these!)
– Forwarding loops and deflections
– Route aggregation and filteringTraceroute 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
Optimization FrameworkOptimization Framework
Start with initial IP-to-AS map A(x)– IP address x maps to A(x), a set of ASes
Iterative refinement– Apply A(x) to the hops in each traceroute path
– Compare the traceroute hops to the BGP AS path
– Compute mismatch statistics for each entry x
– Modify A(x) depending on a small set of rules
Terminate when no further modifications
Matching Function and Unavoidable ErrorMatching 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)
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: 7 13 6 5 8 3 10 2
b: A B C
Rules for Modifying the IP-to-AS MappingRules 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
Measurement Data: Eight Vantage PointsMeasurement Data: Eight Vantage Points
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 Level 3, Global Crossing, Teleglobe
Sweep the routable IP address space– ~200,000 IP addresses
– 160,000 prefixes
– 15,000 destination ASes
Initial Analysis of BGP and Traceroute PathsInitial 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
Comparison of IP-to-AS MappingsComparison of IP-to-AS Mappings
Whois: unmapped hops cause half of mismatchesBGP tables: mostly match, as our algorithm assumesRefined mapping: change 2.9% of original mapping
– Robust to reducing # of probes and introducing noise
Whois BGPorigins
Refined mapping
Match 47% 85% 95%
Mismatch 53% 15% 5%
Ratio 0.88 5.8 18
Comparing BGP and Traceroute AS paths for various IP-to-AS mappings
Validating the Changes to the MappingValidating 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
Exploring the Remaining MismatchesExploring the Remaining Mismatches
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 CTraceroute path: B C DB CC
DD
EE
B CB D DBGP path: B C DTraceroute path: B D
ContributionsContributions
Problem formulation– AS-level traceroute tool for troubleshooting
– Compute an accurate IP-to-AS mappingOptimization approach
– Compute matchings using dynamic programming
– Improve mapping through iterative refinementMeasurement methodology
– Traceroute and BGP paths from many locationsValidation of our results
– Changes to the IP-to-AS mappings
– Remaining mismatches between traceroute and BGP
Future Work on AS TracerouteFuture 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