Internet Topology
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Transcript of Internet Topology
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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/
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Returning the Midterm Exam
• Exam scoring break down–Range: 70-100–Average: 89–Median: 92
• See the course Web site–Exam–Answer key
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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
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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
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Autonomous System NumbersAS Numbers are 16 bit values.
• 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, …• …
Currently just over 20,000 in use.
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AS Topology
• Node: Autonomous System
• Edge: Two ASes that connect to each other
1
2
3
4
5
67
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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
AS 1
AS 2
d
Exchange Point
AS 1
AS 2
d
AS 3
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Interdomain Paths
1
2
3
4
5
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ClientWeb server
Path: 6, 5, 4, 3, 2, 1
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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
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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
d
d
provider
customer
customer
provider
Traffic to the customer Traffic from the customer
advertisements
traffic
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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 $$$)
peerpeer
Traffic to/from the peer and its customers
d
advertisements
traffic
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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
Patriotpeer
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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
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Efficient Early-Exit Routing
• Diverse peering locations– Both costs, and middle
• Comparable capacity at all peering points– Can handle even load
• Consistent routes– Same destinations advertised
at all points– Same AS path length for a
destination at all points
Customer A
Customer B
multiplepeeringpoints
Provider A
Provider B
Early-exit routing
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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
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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 10 100 1000
CC
DF
1
0.1
0.01
0.001
AS degree
All ASes have 1 or more neighbors
Very few have degree >= 100
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Characteristics of AS Paths
• AS path may be longer than shortest AS path
• Router path may be longer than shortest path
s d
3 AS hops, 7 router hops
2 AS hops, 8 router hops
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Intra-AS Topology
• Node: router
• Edge: link
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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
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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
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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 …
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Backbone Networks
• Backbone networks–Multiple Points-of-Presence (PoPs)–Lots of communication between PoPs–Accommodate traffic demands and limit delay
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Abilene Internet2 Backbone
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Points-of-Presence (PoPs)
• Inter-PoP links–Long distances–High bandwidth
• Intra-PoP links–Short cables between
racks or floors–Aggregated bandwidth
• Links to other networks–Wide range of media and
bandwidth
Intra-PoP
Other networks
Inter-PoP
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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
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Customer Connecting to a Provider
Provider Provider
1 access link 2 access links
Provider
2 access routers
Provider
2 access PoPs
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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
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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
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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
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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
2 169.229.59.225
3 128.32.255.169
4 128.32.0.249
5 128.32.0.66
6 209.247.159.109
7 209.247.9.170
8 66.185.138.33
9 66.185.142.97
10 66.185.136.17
11 64.236.16.52
inr-daedalus-0.CS.Berkeley.EDU
soda-cr-1-1-soda-br-6-2
vlan242.inr-202-doecev.Berkeley.EDU
gigE6-0-0.inr-666-doecev.Berkeley.EDU
qsv-juniper--ucb-gw.calren2.net
POS1-0.hsipaccess1.SanJose1.Level3.net
pos8-0.hsa2.Atlanta2.Level3.net
pop2-atm-P0-2.atdn.net
Pop1-atl-P3-0.atdn.net
pop1-atl-P4-0.atdn.net
www4.cnn.com
AOL
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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
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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
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Map Traceroute Hops to ASes
1 169.229.62.1
2 169.229.59.225
3 128.32.255.169
4 128.32.0.249
5 128.32.0.66
6 209.247.159.109
7 *
8 64.159.1.46
9 209.247.9.170
10 66.185.138.33
11 *
12 66.185.136.17
13 64.236.16.52
Traceroute output: (hop number, IP)AS25
AS25
AS25
AS25
AS11423
AS3356
AS3356
AS3356
AS3356
AS1668
AS1668
AS1668
AS5662
Berkeley
CNN
Calren
Level3
AOL
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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 HarvardB-schoold1 d2
???
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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
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Paths You Should Never See (“Invalid”)
Customer-provider
Peer-peer
two peer edges
transit through a customer
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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
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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