internet topology n.
Download
Skip this Video
Loading SlideShow in 5 Seconds..
Internet Topology PowerPoint Presentation
Download Presentation
Internet Topology

Loading in 2 Seconds...

play fullscreen
1 / 38

Internet Topology - PowerPoint PPT Presentation


  • 111 Views
  • Uploaded on

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/. Returning the Midterm Exam. Exam scoring break down Range: 70-100 Average: 89

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

Internet Topology


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
internet topology

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/

returning the midterm exam
Returning the Midterm Exam
  • Exam scoring break down
    • Range: 70-100
    • Average: 89
    • Median: 92
  • See the course Web site
    • Exam
    • Answer key
goals of today s lecture
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
internet routing architecture
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
autonomous system numbers
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, …
as topology

4

3

5

2

6

7

1

AS Topology
  • Node: Autonomous System
  • Edge: Two ASes that connect to each other
what is an edge really
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

interdomain paths
Interdomain Paths

Path: 6, 5, 4, 3, 2, 1

4

3

5

2

6

7

1

Web server

Client

business relationships
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
customer provider relationship

advertisements

traffic

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

provider

customer

d

customer

peer peer relationship

advertisements

traffic

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

peer

peer

d

princeton example
Princeton Example
  • Internet: customer ofAT&T and USLEC
  • Research universities/labs: customer ofInternet2
  • Local residences: peer with Patriot Media
  • Local non-profits: provider for several non-profits

AT&T

Internet2

USLEC

Patriot

peer

as structure tier 1 providers
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
efficient early exit routing
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 B

Provider B

multiple

peering

points

Early-exit

routing

Provider A

Customer A

as structure other ases
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
characteristics of the as graph

Very few have degree >= 100

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

0.001

AS degree

1

10

100

1000

characteristics of as paths
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

d

s

3 AS hops, 7 router hops

intra as topology
Intra-AS Topology
  • Node: router
  • Edge: link
hub and spoke topology
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
princeton example1
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
simple alternatives to hub and spoke
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

backbone networks
Backbone Networks
  • Backbone networks
    • Multiple Points-of-Presence (PoPs)
    • Lots of communication between PoPs
    • Accommodate traffic demands and limit delay
points of presence pops
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

Inter-PoP

Intra-PoP

Other networks

where to locate nodes and links
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
customer connecting to a provider
Customer Connecting to a Provider

Provider

Provider

2 access links

1 access link

Provider

Provider

2 access PoPs

2 access routers

multi homing two or more providers
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

shared risks
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
learning the internet topology
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
inferring an intra as topology
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

challenges of intra as mapping
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
inferring the as level topology
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
map traceroute hops to ases

AS25

AS25

AS25

AS25

AS11423

AS3356

AS3356

AS3356

AS3356

AS1668

AS1668

AS1668

AS5662

Berkeley

Calren

Level3

AOL

CNN

Map Traceroute Hops to ASes

Traceroute output: (hop number, IP)

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

challenges of inter as mapping

d2

d1

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

Sprint

AT&T

???

Harvard

B-school

Harvard

inferring as relationships
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
paths you should never see invalid

two peer edges

transit through a customer

Paths You Should Never See (“Invalid”)

Customer-provider

Peer-peer

challenges of relationship inference
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
conclusions
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