Characterizing Residential Broadband Networks

1. Characterizing Residential Broadband Networks Marcel Dischinger†, Andreas Haeberlen†‡, Krishna P. Gummadi†, Stefan Saroiu* †MPI-SWS, ‡Rice University, * University of Toronto

2. Why study residential broadband networks? • Used by millions of users to connect to the Internet • Rapidly growing user base • Used for many different workloads: • Music / movie downloads, VoIP, online games • Yet, researchers know little about the characteristics of deployed cable and DSL networks • Such as provisioned bandwidths, queueing delays, or loss rates

3. Why do we know so little? • Commercial ISPs have no incentives to reveal information about their network deployments • Researchers lack access to broadband networks • Testbeds composed of academic nodes • PlanetLab only has two DSL nodes • Prior studies were limited in scale • Largest study so far had 47 broadband nodes [PAM’04] • Prior studies depended on access to the broadband hosts Challenge: Can we measure hosts without access to them?

4. Finding broadband hosts to measure • Identified IP addresses of broadband hosts using reverse-DNS lookups • E.g., BellSouth’s DNS names follow the schemeadsl-*.bellsouth.net • Sent TCP ACK and ICMP PING probes to the broadband IPs • 1000s of hosts from 100s of DSL/cable ISPs responded

5. We focused on 11 major ISPs from North America and Europe • DSL • Cable

6. How do we measure the broadband hosts? Broadband link Measurement hosts • We measured from well-connected hosts in University networks • TCP ACK / ICMP PING probes sent at 10Mbps for a short duration • Probes saturate the bottleneck, which is often the broadband link • TCP ACK probes saturate just downstream direction • ICMP PING probes saturate both directions • We analyzed probe responses to infer various characteristics Internet Broadband host Last-hop router

7. Broadband host Last-hop router Are broadband links the bottleneck? • Broadband links are the bandwidth bottlenecks along the measured path • More validation results in the paper

8. Rest of the talk • Allocated link bandwidths • Packet latencies • Packet loss

9. Outline • Allocated link bandwidths • Do broadband providers allocate advertised link bandwidths? • How do the downstream and upstream bandwidths compare? • Are broadband bandwidths stable over the short-term? • Are broadband bandwidths stable over diurnal time-scales? • Is there evidence for traffic shaping? • Packet latencies • Packet loss

10. Do ISPs allocate advertised link bandwidths? • DSL ISPs allocate advertised bandwidths • Some Cable ISPs do not offer discrete bandwidths PacBell BellSouth Rogers Road Runner

11. What is the ratio of downstream to upstream bandwidths? • Upstream bandwidths are significantly lower than downstream • Broadband networks are provisioned for client-server workloads Ameritech PacBell Road Runner Comcast

12. Are link bandwidths stable over the short-term? • DSL bandwidths are relatively stable, while cable are not • Hard for protocols like TCP to adapt to highly variable cable BWs Unstable (Rogers cable host) Stable (PacBell DSL host)

13. Outline • Allocated link bandwidths • Packet latencies • How large are broadband queueing delays? • Queues should be proportional to the end-to-end RTT • Recent research recommends even shorter queues [SIGCOMM’04] • How do cable’s time-slotted policies affect transmission delays? • Do broadband links have large propagation delays? • Packet loss

14. How large are downstream queueing delays? PacBell • Downstream queues are significantly larger than avg. path RTT BellSouth Comcast Road Runner US coast-to-coast delay Transatlantic delay

15. How large are upstream queueing delays? PacBell BellSouth • Upstream queues are extremely large • Packets can experience latencies in the order of seconds BellSouth Comcast Road Runner

16. Why are large queues worrisome? • Large queues avoid losses at the cost of latency • Good for web workloads • But, bad for popular emerging workloads • Interactive traffic like VoIP and online games • Multimedia downloads like music and movies • Low latency vs. maximum bandwidth • TCP does not fully drain large queues after a loss event

17. Outline • Allocated link bandwidths • Packet latencies • Packet loss • Do ISPs deploy active queue management (AQM)? • Tail-drop queue • Active queue management techniques, such as Random Early Detect (RED) • Do broadband links see high packet loss?

18. Do ISPs deploy active queue management? • 25% of DSL hosts have AQM deployed in the upstream Active queue management (probably RED) (SWBell) Threshold Tail-drop (PacBell)

19. Conclusion • We presented the first large-scale study of broadband networks • Measured their bandwidth, latency, and loss characteristics • Broadband networks are very different from academic networks • Cable networks have unstable bandwidths • Large queues can cause latencies in the order of seconds • Broadband links have low loss rates, show deployment of AQM • Our findings have important implications for network operators and systems designers

20. Thank you! For more information, please contact me at: marcel.dischinger@mpi-sws.mpg.de