Measuring ISP topologies with Rocketfuel

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Measuring ISP topologies with Rocketfuel

• Ratul Mahajan
• Neil Spring
• David Wetherall
• University of Washington
• ACM SIGCOMM 2002

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Motivation

• To understand Internet structure and design.
• How ISP router-level topologies are designed.
• Cant get the real maps.
• Backbone maps often available in marketing form.
• Severely lacking in router-level detail.

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ISP topologies for research

• Could extract from a Whole-Internet mapeg.
  Skitter, Mercator, Lumeta.
• Papers Philosophy
• By focusing on an ISP, can get better precision.
• ISPs publish enough information to reconstruct
  maps.
• End goal is more accurate maps for research.

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Terminology

• Each POP is a physical location where the ISP
  houses a collection of routers.
• The ISP backbone connects these POPs, and the
  routers attached to inter-POP links are called
  backbone or core routers.
• Within every POP, access routers provide an
  intermediate layer between the ISP backbone and
  routers in neighboring networks.

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Points of Presence and Backbone
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Rocketfuels Backbone Map
They arent telling us everything
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Rocketfuel Methodology

• ISPs release helpful information
• BGP - which prefixes are served
• Traceroute - what the paths are
• DNS - where routers are and what they do
• Build detailed maps
• Backbone
• POPs
• Peering links

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Traceroutes

• Publicly available traceroute servers
• Challenge To build accurate ISP maps using few
  measurements
• Brute Force Method
• 784 vantage points to 120,000 allocated prefixed
  in BGP table
• Queried every 1.5 minutes 125 days to complete a
  map.

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Directed probing

• Capitalize on routing information
• Identify traceroutes which transit the ISP
  network

Example AS 7 Dependent Prefixes
4.5.0.0/16 Insiders 4.5.0.0/16 Up/down traces
AS 11 to 1.2.3.0/24
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Path Reductions
T1 and T2 enter the ISP at the same point on the
way to the same destination
Paths to P1 and P2 leave the ISP at the same point
Next-hop AS Reduction
Ingress Reduction
Egress Reduction
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Reduction Effectiveness

• Brute force 90-150 million traceroutes required
• BGP directed probes 0.2-15 million traceroutes
  required
• Executed after path reduction 8-300 thousand
  traceroutes required

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Location and Role Discovery

• Where is this router located?
• use DNS names
• S1-bb11-nyc-3-0.sprintlink.net is a Sprint
  router in New York City
• use connectivity information
• if a router connects only to router in
  Seatles, it is in Seattle
• What role does this router play in the topology?
• only backbone routers connect to other
  cities
• use DNS names
• s1-gw2-sea-3-1.sprintlink.net is a Sprint
  gateway router

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Alias resolution problem
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Alias resolution solution

• Send a packet to each interface to solicit
  responses.
• Previous work - responses have the same source
  Routers often set source address to outgoing
  interface
• New approach
• responses have nearby IP identifiers
• IP ID is commonly set from a counter.
• Alias resolution optimization
• Sort by DNS name - find aliases quickly
• Cluster by return TTL - rule out many addresses
• ALLY found 2.8 times as many

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IP ID method

• xltyltz, z-x small ? likely aliases
• If x-ygt200
• ? Aliases are disqualified, third packet is not
  sent

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ISP MAPS
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AT T
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Sprint
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Level 3
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Telstra
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POP Structure
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Completeness

• Validation with ISPs
• Good to excellent
• Hesitant to reveal customer data
• Scanning IP addresses
• Comparison with Routeviews
• Number of BGP adjacencies
• Worst case 70
• Skitter
• Seven times as many links and routers

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Impact of reductions

• Ingress and Egress reductions

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Next HOP ASs

• Specially beneficial for Insiders

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Analysis

• POP Sizes
• All skewed
• Most routers present in ten largest POPs
• Sprint 60 POPs less than 20 of Sprint routers

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Router Degree Distribution

• Small range in data
• Layer 2 switches unaccounted

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Peering Structure

• Advantage here Where and how many places do two
  ISPs connect
• Highly skewed for all ISPs