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									          Internet Topology
             COS 461: Computer Networks
       Spring 2006 (MW 1:30-2:50 in Friend 109)


                     Jennifer Rexford
         Teaching Assistant: Mike Wawrzoniak
http://www.cs.princeton.edu/courses/archive/spring06/cos461/
                                                               1
Returning the Midterm Exam
• Exam scoring break down
  –Range: 70-100
  –Average: 89
  –Median: 92

• See the course Web site
  –Exam
  –Answer key




                             2
Goals of Today’s Lecture
• Internet’s two-tiered topology
  – Autonomous Systems, and connections between them
  – Routers, and the links between them

• AS-level topology
  – Autonomous System (AS) numbers
  – Business relationships between ASes

• Router-level topology
  – Points of Presence (PoPs)
  – Backbone and enterprise network topologies

• Inferring network topologies
  – By measuring paths from many vantage points
                                                       3
Internet Routing Architecture
• Divided into Autonomous Systems
  – Distinct regions of administrative control
  – Routers/links managed by a single “institution”
  – Service provider, company, university, …

• Hierarchy of Autonomous Systems
  – Large, tier-1 provider with a nationwide backbone
  – Medium-sized regional provider with smaller backbone
  – Small network run by a single company or university

• Interaction between Autonomous Systems
  – Internal topology is not shared between ASes
  – … but, neighboring ASes interact to coordinate routing   4
Autonomous System Numbers
          AS Numbers are 16 bit values.
         Currently just over 20,000 in use.
 •   Level 3: 1
 •   MIT: 3
 •   Harvard: 11
 •   Yale: 29
 •   Princeton: 88
 •   AT&T: 7018, 6341, 5074, …
 •   UUNET: 701, 702, 284, 12199, …
 •   Sprint: 1239, 1240, 6211, 6242, …
 •   …
                                              5
AS Topology
• Node: Autonomous System
• Edge: Two ASes that connect to each other


                                4
          3

                                              5

      2
                            7                     6

  1
                                                      6
What is an Edge, Really?
• Edge in the AS graph
  – At least one connection between two ASes
  – Some destinations reached from one AS via the other
                                         d
      d

      AS 1                            AS 1


                                  Exchange Point


    AS 2                   AS 2                 AS 3
                                                          7
Interdomain Paths
 Path: 6, 5, 4, 3, 2, 1

                              4

           3

                                  5


       2
                          7            6


  1
                                  Web server
      Client                                   8
Business Relationships
• Neighboring ASes have business contracts
  –How much traffic to carry
  –Which destinations to reach
  –How much money to pay

• Common business relationships
  –Customer-provider
     E.g., Princeton is a customer of AT&T
     E.g., MIT is a customer of Level 3
  –Peer-peer
     E.g., Princeton is a peer of Patriot Media
     E.g., AT&T is a peer of Sprint
                                                  9
Customer-Provider Relationship
• Customer needs to be reachable from everyone
  – Provider tells all neighbors how to reach the customer

• Customer does not want to provide transit service
  – Customer does not let its providers route through it
  Traffic to the customer           Traffic from the customer

                                             d     provider
                advertisements

               provider
 traffic
                                                    customer
           d   customer
                                                                10
Peer-Peer Relationship
• Peers exchange traffic between customers
  – AS exports only customer routes to a peer
  – AS exports a peer’s routes only to its customers
  – Often the relationship is settlement-free (i.e., no $$$)

       Traffic to/from the peer and its customers


                    advertisements

          peer                          peer
                        traffic


           d                                                   11
Princeton Example
• Internet: customer of AT&T and USLEC
• Research universities/labs: customer of Internet2
• Local residences: peer with Patriot Media
• Local non-profits: provider for several non-profits


     AT&T            USLEC         Internet2


                                 peer          Patriot
                                                         12
AS Structure: Tier-1 Providers
• Tier-1 provider
  – Has no upstream provider of its own
  – Typically has a national or international backbone
  – UUNET, Sprint, AT&T, Level 3, …
• Top of the Internet hierarchy of 12-20 ASes
  – Full peer-peer connections between tier-1 providers




                                                          13
Efficient Early-Exit Routing
                                     • Diverse peering locations
  Customer B
                                         – Both costs, and middle

                                     • Comparable capacity at all
                                       peering points
               Provider B
                                         – Can handle even load

                                     • Consistent routes
                                         – Same destinations advertised
multiple
                                           at all points
peering
points                      Early-exit   – Same AS path length for a
                            routing        destination at all points


     Provider A

                                                                      14
                        Customer A
AS Structure: Other ASes
• Tier-2 providers
  – Provide transit service to downstream customers
  – … but, need at least one provider of their own
  – Typically have national or regional scope
  – E.g., Minnesota Regional Network
  – Includes a few thousand of the ASes

• Stub ASes
  – Do not provide transit service to others
  – Connect to one or more upstream providers
  – Includes vast majority (e.g., 85-90%) of the ASes


                                                        15
Characteristics of the AS Graph
• AS graph structure
   – High variability in node degree (“power law”)
   – A few very highly-connected ASes
   – Many ASes have only a few connections

           1        All ASes have 1 or more neighbors
          0.1
 CCDF




         0.01                      Very few have degree >= 100

        0.001
                1     10     100    1000   AS degree       16
 Characteristics of AS Paths
  • AS path may be longer than shortest AS path
   • Router path may be longer than shortest path
 2 AS hops,
8 router hops




  s                                                 d


                                                        17
                3 AS hops, 7 router hops
Intra-AS Topology
• Node: router
• Edge: link




                    18
Hub-and-Spoke Topology
• Single hub node
  –Common in enterprise networks
  –Main location and satellite sites
  –Simple design and trivial routing

• Problems
  –Single point of failure
  –Bandwidth limitations
  –High delay between sites
  –Costs to backhaul to hub

                                       19
Princeton Example
• Hub-and-spoke
  –Four hub routers and many spokes

• Hub routers
  –Outside world (e.g., AT&T, USLEC, …)
  –Dorms
  –Academic and administrative buildings
  –Servers




                                           20
Simple Alternatives to Hub-and-Spoke
• Dual hub-and-spoke
  – Higher reliability
  – Higher cost
  – Good building block




• Levels of hierarchy
  – Reduce backhaul cost
  – Aggregate the bandwidth
  – Shorter site-to-site delay
                                 …
                                       21
Backbone Networks
• Backbone networks
  –Multiple Points-of-Presence (PoPs)
  –Lots of communication between PoPs
  –Accommodate traffic demands and limit delay




                                                 22
Abilene Internet2 Backbone




                             23
Points-of-Presence (PoPs)
• Inter-PoP links
  –Long distances         Inter-PoP
  –High bandwidth                     Intra-PoP

• Intra-PoP links
  –Short cables between
   racks or floors
  –Aggregated bandwidth

• Links to other networks     Other networks
  –Wide range of media and
   bandwidth
                                                  24
Where to Locate Nodes and Links
• Placing Points-of-Presence (PoPs)
  –Large population of potential customers
  –Other providers or exchange points
  –Cost and availability of real-estate
  –Mostly in major metropolitan areas

• Placing links between PoPs
  –Already fiber in the ground
  –Needed to limit propagation delay
  –Needed to handle the traffic load

                                             25
Customer Connecting to a Provider
     Provider               Provider


          1 access link           2 access links




    Provider                Provider


         2 access routers         2 access PoPs
                                             26
Multi-Homing: Two or More Providers
• Motivations for multi-homing
  –Extra reliability, survive single ISP failure
  –Financial leverage through competition
  –Better performance by selecting better path
  –Gaming the 95th-percentile billing model

          Provider 1            Provider 2




                                                   27
Shared Risks
• Co-location facilities (“co-lo hotels”)
  – Places ISPs meet to connect to each other
  – … and co-locate their routers, and share space & power
  – E.g., 32 Avenue of the Americas in NYC

• Shared links
  – Fiber is sometimes leased by one institution to another
  – Multiple fibers run through the same conduits
  – … and run through the same tunnels, bridges, etc.

• Difficult to identify and accounts for these risks
  – Not visible in network-layer measurements
  – E.g., traceroute does not tell you links in the same ditch
                                                                 28
Learning the Internet Topology
• Internet does not have any central management
  – No public record of the AS-level topology
  – No public record of the intra-AS topologies

• Some public topologies are available
  – Maps on public Web sites
  – E.g., Abilene Internet2 backbone

• Otherwise, you have to infer the topology
  – Measure many paths from many vantage points
  – Extract the nodes and edges from the paths
  – Infer the relationships between neighboring ASes

                                                       29
Inferring an Intra-AS Topology
• Run traceroute from many vantage points
  – Learn the paths running through an AS
  – Extract the hops within the AS of interest
        1 169.229.62.1     inr-daedalus-0.CS.Berkeley.EDU
        2 169.229.59.225   soda-cr-1-1-soda-br-6-2
        3 128.32.255.169   vlan242.inr-202-doecev.Berkeley.EDU
        4 128.32.0.249     gigE6-0-0.inr-666-doecev.Berkeley.EDU
        5 128.32.0.66      qsv-juniper--ucb-gw.calren2.net
        6 209.247.159.109 POS1-0.hsipaccess1.SanJose1.Level3.net
        7 209.247.9.170    pos8-0.hsa2.Atlanta2.Level3.net
        8 66.185.138.33    pop2-atm-P0-2.atdn.net
 AOL    9 66.185.142.97    Pop1-atl-P3-0.atdn.net
       10 66.185.136.17    pop1-atl-P4-0.atdn.net
       11 64.236.16.52     www4.cnn.com                            30
Challenges of Intra-AS Mapping
• Firewalls at the network edge
  – Cannot typically map inside another stub AS
  – … because the probe packets will be blocked by firewall
  – So, typically used only to study service providers

• Identifying the hops within a particular AS
  – Relies on addressing and DNS naming conventions
  – Difficult to identify the boundaries between ASes

• Seeing enough of the edges
  – Need to measure from a large number of vantage points
  – And, hope that the topology and routing doesn’t change

                                                          31
Inferring the AS-Level Topology
• Collect AS paths from many vantage points
  – Learn a large number of AS paths
  – Extract the nodes and the edges from the path

• Example: AS path “1 7018 88” implies
  – Nodes: 1, 7018, and 88
  – Edges: (1, 7018) and (7018, 88)

• Ways to collect AS paths from many places
  – Mapping traceroute data to the AS level
  – Measurements of the interdomain routing protocol



                                                       32
Map Traceroute Hops to ASes
         Traceroute output: (hop number, IP)
          1 169.229.62.1     AS25
          2 169.229.59.225 AS25       Berkeley
          3 128.32.255.169 AS25
          4 128.32.0.249     AS25
          5 128.32.0.66      AS11423 Calren
          6 209.247.159.109 AS3356
          7 *                AS3356
                                       Level3
          8 64.159.1.46      AS3356
          9 209.247.9.170    AS3356
          10 66.185.138.33   AS1668
          11 *               AS1668    AOL
          12 66.185.136.17   AS1668
          13 64.236.16.52    AS5662 CNN
                                                 33
Challenges of Inter-AS Mapping
• Mapping traceroute hops to ASes is hard
  – Need an accurate registry of IP address ownership
  – Whois data are notoriously out of date

• Collecting diverse interdomain data is hard
  – Public repositories like RouteViews and RIPE-RIS
  – Covers hundreds to thousands of vantage points
  – Especially hard to see peer-peer edges

              AT&T                  Sprint


                          ???     Harvard
              Harvard                                   34
    d1                            B-school       d2
Inferring AS Relationships
• Key idea
  – The business relationships determine the routing policies
  – The routing policies determine the paths that are chosen
  – So, look at the chosen paths and infer the policies

• Example: AS path “1 7018 88” implies
  – AS 7018 allows AS 1 to reach AS 88
  – AT&T allows Level 3 to reach Princeton
  – Each “triple” tells something about transit service

• Collect and analyze AS path data
  – Identify which ASes can transit through the other
  – … and which other ASes they are able to reach this way
                                                           35
Paths You Should Never See (“Invalid”)

       Customer-provider

        Peer-peer


                               two peer edges




transit through a customer                 36
Challenges of Relationship Inference
• Incomplete measurement data
  – Hard to get a complete view of the AS graph
  – Especially hard to see peer-peer edges low in hierarchy

• Real relationships are sometime more complex
  – Peer is one part of the world, customer in another
  – Other kinds of relationships (e.g., backup and sibling)
  – Special relationships for certain destination prefixes



• Still, inference work has proven very useful
  – Qualitative view of Internet topology and relationships
                                                              37
Conclusions
• Two-tiered Internet topology
 –AS-level topology
 –Intra-AS topology
• Inferring network topologies
 –By measuring paths from many vantage points
• Next class
 –Vivek Pai guest lecture
     See reading assignment on the course Web site
 –Mike Wawrzoniak talking about assignment #2
     Start the assignment so you can ask questions
• Next week
 –Intradomain and interdomain routing                38

								
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