Observations from Router-level Traces Lisa Amini IBM T. J. Watson Research Center Joint with Henning Schulzrinne, Aurel Lazar Columbia University

Context Near-term issues facing service providers for Web-facing applications Mirrored servers where to place servers? which network access providers? how many connection points? how many servers to deploy? where to direct clients? End-to-end routing behavior/delay vs. connectivity AS level focus

Beyond Connectivity Inter-domain (AS) routing tables Aggregation Policy-based routing Oregon Route Server Stub Networks Transit Networks A C E D B Autonomous Systems

Traceroute Router-level end-to-end probing Traceroute.org website AS lookup via Routing Arbiter Database (RADB) Round trip time (RTT) Loose source-routing Artifacts

D1 Dataset 189 sites Random pairings Forward and reverse paths Poisson arrivals Mean time between probes 10 minutes 220,551 measurements 5 days 1/3 US sites, 2/3 non-US (31 countries) [ Zhang, Duffield, Paxson, Shenker]

Router-level Path Length mean=16.45, std. dev.=4.39 Number of Routes Router Hops

AS Path Length AS Hops Number of Routes mean=6.47, std. dev.=2.33

AS Degree Frequency Faloutsos (or higher) correlation coefficient 98% of nodes represented AS degree Frequency Rank 0.97 correlation coefficient required discarding 31% of nodes

Which is correct? Routing view traceroute target traceroute source

AS Frequency AS frequency Rank 0.97 correlation coefficient required discarding 10% of nodes

Response Time Prediction Does path length predict delay? Probe Round Trip Time Router Hops mean RTT=263ms Correlation coefficient= 0.31 Correlation coefficient =0.27 AS Hops

AS Properties Can we predict delay based on AS path properties? Ranked each AS according to: AS edge degree AS frequency Calculated mean RTT per path length Grouped by top 5, 6-10, AS’s

Influence of Path Properties High edge degree AS? Number of Router Hops / Path Average RTT

High Frequency AS? Number of Router Hops / Path Average RTT

High Edge Degree AS? High edge degree AS in 2 AS hops? Number of Router Hops / Path Average RTT

Backbone AS? Number of Router Hops / Path Average RTT [ 7/2001]

Backbone AS in 2 AS hops? Number of Router Hops / Path Average RTT

AS Affinity? Number of Router Hops / Path Average RTT

Route Asymmetry Paxson, 1995 Path(A,B)  Path(B,A) Issues Ping triangulation Congestion Avoidance Internet mapping AB

AS Hop Differences Number of AS hop differences Number of Routes paired, unique routes 57% routes were AS-path asymmetric Compare with 30% based on 1995 data 74% asymmetric from first AS hop

Traceroute Issues AS assignment [Chang, Jamin, Willinger] AS1 AS7 AS6AS5 AS4 AS3 AS2

BGP AS Path Can we predict forward and reverse end-to-end metrics from BGP AS_PATH? Looking Glass Probing 92 Sites 8372 unique path measurements 2202 fully paired (BGP forward and reverse, traceroute forward and reverse)

AS Hop Differences Number of AS hop differences Number of Routes 47% of forward paths correctly predicted by reverse traceroute 49% of forward paths correctly predicted by reverse BGP AS_PATH 69% of forward paths correctly predicted by forward BGP AS_PATH 34% asymmetry between forward/reverse BGP AS_PATH

Summary BGP routing tables provide complete view from single location. Aggregation Filtering End-to-end probing from points throughout network can provide insights beyond connectivity Limited view of connectivity Traceroute issues with noise, node assignment BGP AS_PATH inaccurate as path predictor

Discard Criteria Origin traceroute server not responding Incomplete traceroute output Internal use only node address (e.g., 10.x.x.x, x.x x.x ranges) Route did note terminate in target AS Intermediate node did not respond to ICMP echo No matching reverse probe for same time period

What can we conclude? Results are for RTT only Negative results for Path Length AS degree, AS frequency, Backbone, hops from origin High delay as result of transient conditions (congestion)