1. Jennifer Rexford Fall 2014 (TTh 3:00-4:20 in CS 105) COS 561: Advanced Computer Networks http://www.cs.princeton.edu/courses/archive/fall14/cos561/ Locations and Times

2. Announcements Course projects –Presentations will be in January –… not on the last day of class –… as a “poster session” So, –Written report: Dean’s Date –Poster session: A few days after 2

3. Locating a Host Geographic location –Actual physical location (e.g., latitude and longitude) –Location-based search results, advertising, content, … –Blocking access based on client location –Deciding whether traffic can be wiretapped Best or closest server in a set –Server with the lowest round-trip time to the client –E.g., the shortest network path –To select the best server replica The two are similar, but not the same 3

4. Landmarks Fixed vantage points –Known geographic locations –Probe the client IP address 4 V1 V2 V3 Delay vector: [D1, D2, D3]

5. Latency != Distance Indirect routes –E.g., Princeton town to Princeton campus –Violations of the triangle inequality Asymmetric routing –A-to-B is not the reverse of the B-to-A path –Errors when dividing round-trip-time in half Network congestion –Queuing delay, particularly with deep buffers Last-mile latency –High transmission delays on slow access link –Inflates the latency measurement 5

6. IP2Geo Paper GeoTrack –Infer location from DNS names –Of the host or routers in the traceroute path –E.g., airport codes like sjc2-cw- oc3.sjc.above.net GeoPing –Hosts with similar delays are close to each other –Store delay vectors for hosts of known locations –For new target, find the nearest neighbor in delay space GeoCluster –Group IP addresses by IP prefix –Infer location of other IP addresses in the cluster –Identify possible errors (outliers) in IP geolocation 6

7. Octant Paper (NSDI’07) Construct a system of constraints –And solve to locate the host Include a “height” constraint for each node –To capture the latency on the access link Place more weight on low latency measurements –Less likely to involve an indirect route Use intermediate routers as additional landmarks –By extracting router location from the DNS name Both positive and negative constraints –Positive: zip codes from Whois data, others in prefix –Negative: uninhabitable regions (e.g., oceans) 7

8. Street-Level Geolocation (NSDI’11) Main insights –Many organizations host their web sites locally –Many web sites list the organization’s postal address Example: Princeton CS –Princeton CS is hosted in the Princeton CS building –Princeton CS address at 35 Olden St., Princeton, NJ Combining the information –Infer a coarse-grain region using delay measurements –Identify the zip codes (and Web sites) in the region –Traceroute to the Web sites within the zip code –Find last common hop in the path to Web site and target –Select the location of the “closest” of the Web sites 8

9. Network Time Protocol (NTP) 9

10. What is Time in NTP Coordinated Universal Time (UTC) –A successor to Greenwich Mean Time (GMT) –Time zones are expressed as an offset Time relative to an epoch –Starting January 1, 1900 64-bit representation –32 bits for seconds since the epoch –32 bits for fractions of a second –(Will roll over in 2036) 10

11. Computer Clocks Time stored in a counter –Incremented periodically based on interrupts Generating clock ticks –Quartz crystal oscillator –Common clock hardware is not very accurate –E.g., 0.001% error amounts to almost 1 sec per day –(My laptop is off by 34 parts per million: /var/db/ntp.drift) Terminology –Clock skew: difference between two clocks at one time –Clock drift: change in reference time per period of time  Normal to drift about 1 second every 11-12 days 11

12. Clock Strata 12 Multi-level hierarchy of time sources –Level based on distance from reference clock Stratum 0 –High-precision timekeeping devices –Atomic clocks, GPS clocks, other radio clocks Stratum 1 –Computers synchronized to attached stratum 0 devices –Stratum 1 computers can peer for sanity checking Stratum 2 –Synchronized over the network to stratum 1 computers –And so on, for stratum 3, 4, 5, …

13. Clock Synchronization Client polls NTP server Client computes –Round-trip delay –Offset 13 T1T1 T3T3 T2T2 T4T4 Server Client

14. Reducing Errors Client regularly polls three or more NTP servers Applying filters and analyzing statistics –Most accurate offset is at the lowest delay 14

15. Correcting the Clock Time should be a continuous, steady stream –Should not have abrupt changes in the clock –Should not take the clock backwards Gradually adjust the client clock –E.g., 0.5 msec per second Accuracy of NTP –Often within tens of msec over the public Internet –Better than 1 msec within a local area network 15

16. Finding an NTP Server NTP pool project –Default time server for major Linux distributions –Hundreds of publicly accessible NTP servers –http://www.pool.ntp.org/en/http://www.pool.ntp.org/en/ 16