Locations and Times
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/
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
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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
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Landmarks
• Fixed vantage points
–Known geographic locations
–Probe the client IP address
V2
V1
V3
Delay vector: [D1, D2, D3]
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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
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
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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)
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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
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Network Time Protocol (NTP)
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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)
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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
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Clock Strata
• 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, …
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Clock Synchronization
• Client polls NTP server
T2
T1
Server
Client
T3
T4
• Client computes
– Round-trip delay
d = (T4 -T1 )-(T3 -T2 )
– Offset
q = 1 [(T2 -T1 )+(T3 -T4 )]
2
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Reducing Errors
• Client regularly polls three or more NTP servers
• Applying filters and analyzing statistics
– Most accurate offset is at the lowest delay
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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
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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/
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