Joemon M Jose (with Ioannis Arapakis & Ioannis Konstas) Department of Computing Science.
1 Internet Topology COS 461: Computer Networks Spring 2007 (MW 1:30-2:50 in Friend 004) Jennifer...
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Transcript of 1 Internet Topology COS 461: Computer Networks Spring 2007 (MW 1:30-2:50 in Friend 004) Jennifer...
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Internet Topology
COS 461: Computer Networks
Spring 2007 (MW 1:30-2:50 in Friend 004)
Jennifer Rexford
Teaching Assistant: Ioannis Avramopoulos http://www.cs.princeton.edu/courses/archive/spring07/cos461/
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Returning the Midterm Exam
• Exam scoring break down–Average: 90–High: 99–Median: low 90s
• See the course Web site–Exam–Answer key
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Goals of Today’s Lecture
• IP routers– Interface cards– Switching fabric– Route processor
• Router-level topology with a single network– Points of Presence (PoPs)– Backbone and enterprise network topologies
• AS-level topology of the Internet– Autonomous System (AS) numbers– Tier-1 ISPs, regional providers, and stub ASes– Business relationships between ASes
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IP Routers
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Inside a High-End Router
SwitchingFabric
Processor
Line card
Line card
Line card
Line card
Line card
Line card
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Router Physical Layout
Juniper T series
Cisco 12000
Crossbar
Linecards
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Line Cards (Interface Cards, Adaptors)
• Interfacing – Physical link– Switching fabric
• Packet handling– Packet forwarding– Decrement time-to-live– Buffer management– Link scheduling– Packet filtering– Rate limiting– Packet marking– Measurement
to/from link
to/from switch
lookup
Rec
eive
Transm
it
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Switching Fabric
• Deliver packet inside the router– From incoming interface to outgoing interface– A small network in and of itself
• Must operate very quickly– Multiple packets going to same outgoing interface– Switch scheduling to match inputs to outputs
• Implementation techniques– Bus, crossbar, interconnection network, …– Running at a faster speed (e.g., 2X) than links– Dividing variable-length packets into cells
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Packet Switching
R1Link 1
Link 2
Link 3
Link 4
Link 1, ingress Link 1, egress
Link 2, ingress Link 2, egress
Link 3, ingress Link 3, egress
Link 4, ingress Link 4, egress
ChooseEgress
ChooseEgress
ChooseEgress
ChooseEgress
“4”
“4”
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Router Processor
• So-called “Loopback” interface–IP address of the CPU on the router
• Control-plane software–Implementation of the routing protocols–Creation of forwarding table for the line cards
• Interface to network administrators–Command-line interface for configuration–Transmission of measurement statistics
• Handling of special data packets–Packets with IP options enabled–Packets with expired Time-To-Live field
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Error Reporting
• Examples of errors a router may see– Router doesn’t know where to forward a packet– Packet’s time-to-live field expires
• Router doesn’t really need to respond– Best effort means never having to say you’re sorry– So, IP could conceivably just silently drop packets
• But, silent failures are really hard to diagnose– IP includes basic feedback about network problems– Internet Control Message Protocol (ICMP)
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Internet Control Message Protocol
• ICMP runs on top of IP– In parallel to TCP and UDP– Though still viewed as an integral part of IP
• Diagnostics– Triggered when an IP packet encounters a problem
E.g., time exceeded or destination unreachable
– ICMP packet sent back to the source IP address Includes the error information (e.g., type and code) … and an excerpt of the original data packet for identification
– Source host receives the ICMP packet And inspects the except of the packet (e.g., protocol and ports) … to identify which socket should receive the error
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Example: Time Exceeded
host DNS... host host DNS...
router routerrouter
host
1.2.3.7
8.9.10.11
5.6.7.156
• Host sends an IP packet– Each router decrements the time-to-live field
• If time-to-live field reaches 0– Router generates an ICMP message– Sends a “time exceeded” message back to the source
Time exceeded
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Ping: Echo and Reply
• ICMP includes a simple “echo” function– Sending node sends an ICMP “echo” message– Receiving node sends an ICMP “echo reply”
• Ping tool– Tests the connectivity with a remote host– … by sending regularly spaced echo commands– … and measuring the delay until receiving the reply
• Pinging a host– “ping www.cs.princeton.edu” or “ping 12.157.34.212”– Used to test if a machine is reachable and alive– (However, some nodes have ICMP disabled… )
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Router-level topology of a network
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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 (“NFL cities”)
• Placing links between PoPs–Already fiber in the ground–Needed to limit propagation delay–Needed to handle the traffic load
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AS-level topology of the Internet
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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
67
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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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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Princeton Example
• Internet: customer of AT&T and USLEC
• Research universities/labs: customer of Internet2
• Local non-profits: provider for several non-profits
AT&T USLEC Internet2
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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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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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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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Conclusions
• Internet topology–Network inside a router–Network inside an AS–Network connecting ASes
• Coming up: two-tiered routing system–Intradomain routing–Interdomain routing
• Reading– 4.2, 4.3.3, 4.3.4