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Internet Routing (COS 598A) Today: Intradomain Topology. Jennifer Rexford http://www.cs.princeton.edu/~jrex/teaching/spring2005 Tuesdays/Thursdays 11:00am-12:20pm. Outline. Router architecture Line cards Switching fabric Router processor Network topology From hub-and-spoke to backbones

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internet routing cos 598a today intradomain topology

Internet Routing (COS 598A)Today: Intradomain Topology

Jennifer Rexford

http://www.cs.princeton.edu/~jrex/teaching/spring2005

Tuesdays/Thursdays 11:00am-12:20pm

outline
Outline
  • Router architecture
    • Line cards
    • Switching fabric
    • Router processor
  • Network topology
    • From hub-and-spoke to backbones
    • Customer connecting to providers
  • Measuring the topology
    • Traceroute probes from many vantage points
    • Associating an IP address with an AS
  • Discussion of the papers
what is a router
What is a Router?
  • A computer with…
    • Multiple interfaces
    • Implementing routing protocols
    • Packet forwarding
  • Wide range of variations of routers
    • Small LinkSys device in a home network
    • Linux-based PC running router software
    • Million-dollar high-end routers with large chassis
  • … and links
    • Serial line
    • Ethernet
    • Packet-over-SONET
network components
Network Components

Links

Interfaces

Switches/routers

Ethernet card

Large router

Fibers

Wireless card

Coaxial Cable

Telephone

switch

inside a high end router
Inside a High-End Router

Processor

Switching

Fabric

Line card

Line card

Line card

Line card

Line card

Line card

router components line cards
Router Components: Line Cards
  • Interfacing
    • Physical link
    • Switching fabric
  • Packet handling
    • Buffer management
    • Link scheduling
    • Packet filtering (ACLs)
    • Packet forwarding (FIB)
    • Rate-limiting
    • Packet marking
    • Measurement

to/from link

Transmit

Receive

FIB

to/from switch

router components switching fabric
Router Components: 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
router components router processor
Router Components: 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
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
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
    • Need to accommodate diverse traffic demands
    • Need to limit propagation 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

deciding where to locate nodes and links
Deciding 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

motivation for measuring the topology
Motivation for Measuring the Topology
  • Business analysis
    • Comparisons with competitors
    • Selecting a provider or peer
  • Scientific curiosity
    • Treating data networks like an organism
    • Understand structure and evolution of Internet
  • Input to research studies
    • Network design, routing protocols, …
  • Interesting research problem in its own right
    • How to measure/infer the topology
where to get sources and destinations
Where to Get Sources and Destinations?
  • Source machines
    • Get accounts in many places
      • Good to have a lot of friends
    • Use an infrastructure like PlanetLab
      • Good to have friends who have lots of friends
    • Use public traceroute servers (nicely)
      • http://www.traceroute.org
  • Destination addresses
    • Walk through the IP address space
      • One (or a few) IP addresses per prefix
    • Learn destination prefixes from public BGP tables
      • http://www.route-views.org
traceroute measuring the forwarding path

Time

exceeded

TTL=1

TTL=2

Traceroute: Measuring the Forwarding Path
  • Time-To-Live field in IP packet header
    • Source sends a packet with a TTL of n
    • Each router along the path decrements the TTL
    • “TTL exceeded” sent when TTL reaches 0
  • Traceroute tool exploits this TTL behavior

destination

source

Send packets with TTL=1, 2, 3, … and record source of “time exceeded” message

example traceroute output berkeley to cnn

No response

from router

No name resolution

Example Traceroute Output (Berkeley to CNN)

Hop number, IP address, DNS name

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

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-P4-0.atdn.net

www4.cnn.com

problems with traceroute
Problems with Traceroute
  • Missing responses
    • Routers might not send “Time-Exceeded”
    • Firewalls may drop the probe packets
    • “Time-Exceeded” reply may be dropped
  • Misleading responses
    • Probes taken while the path is changing
    • Name not in DNS, or DNS entry misconfigured
  • Mapping IP addresses
    • Mapping interfaces to a common router
    • Mapping interface/router to Autonomous System
  • Angry operators who think this is an attack
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

Need accurate

IP-to-AS mappings

(for network equipment).

candidate ways to get ip to as mapping
Candidate Ways to Get IP-to-AS Mapping
  • Routing address registry
    • Voluntary public registry such as whois.radb.net
    • Used by prtraceroute and “NANOG traceroute”
    • Incomplete and quite out-of-date
      • Mergers, acquisitions, delegation to customers
  • Origin AS in BGP paths
    • Public BGP routing tables such as RouteViews
    • Used to translate traceroute data to an AS graph
    • Incomplete and inaccurate… but usually right
      • Multiple Origin ASes (MOAS), no mapping, wrong mapping
example bgp table show ip bgp at routeviews
Example: BGP Table (“show ip bgp” at RouteViews)

Network Next Hop Metric LocPrf Weight Path

* 3.0.0.0/8 205.215.45.50 0 4006 701 80 i

* 167.142.3.6 0 5056 701 80 i

* 157.22.9.7 0 715 1 701 80 i

* 195.219.96.239 0 8297 6453 701 80 i

* 195.211.29.254 0 5409 6667 6427 3356 701 80 i

*>12.127.0.249 0 7018 701 80 i

* 213.200.87.254 929 0 3257 701 80 i

* 9.184.112.0/20 205.215.45.50 0 4006 6461 3786 i

* 195.66.225.254 0 5459 6461 3786 i

*>203.62.248.4 0 1221 3786 i

* 167.142.3.6 0 5056 6461 6461 3786 i

* 195.219.96.239 0 8297 6461 3786 i

* 195.211.29.254 0 5409 6461 3786 i

AS 80 is General Electric, AS 701 is UUNET, AS 7018 is AT&T

AS 3786 is DACOM (Korea), AS 1221 is Telstra

refining initial ip to as mapping
Refining Initial IP-to-AS Mapping
  • Start with initial IP-to-AS mapping
    • Mapping from BGP tables is usually correct
    • Good starting point for computing the mapping
  • Collect many BGP and traceroute paths
    • Signaling and forwarding AS path usually match
    • Good way to identify mistakes in IP-to-AS map
  • Successively refine the IP-to-AS mapping
    • Find add/change/delete that makes big difference
    • Base these “edits” on operational realities

http://www.cs.princeton.edu/~jrex/papers/sigcomm03.pdf

http://www.cs.princeton.edu/~jrex/papers/infocom04.pdf

extra as due to internet exchange points
Extra AS due to Internet eXchange Points
  • IXP: shared place where providers meet
    • E.g., Mae-East, Mae-West, PAIX
    • Large number of fan-in and fan-out ASes

E

A

A

E

F

B

F

B

D

G

C

G

C

Traceroute AS path

BGP AS path

Ignore extra traceroute AS hop with high fan-in and fan-out

extra as due to sibling ases
Extra AS due to Sibling ASes
  • Sibling: organizations with multiple ASes:
    • E.g., Sprint AS 1239 and AS 1791
    • AS numbers equipment with addresses of another

E

A

E

A

F

B

H

D

F

B

D

G

C

G

C

Traceroute AS path

BGP AS path

Merge sibling ASes “belong together” as if they were one AS.

unannounced infrastructure addresses

A C A C

A C

B A C

B C

Unannounced Infrastructure Addresses

12.0.0.0/8

A

B

C does not announce part of

its address space in BGP(e.g., 12.1.2.0/24)

C

Fix the IP-to-AS map to associate 12.1.2.0/24 with C

improving the ip to as mapping
Improving the IP-to-AS Mapping
  • Algorithm for modifying the IP-to-AS map
    • Small number of rules for modifying the map
    • Making small changes that make a big difference
  • Results of the algorithm
    • Changes about 2.9% of mappings
    • Much better agreement (95%) with BGP AS paths
  • Validation
    • AT&T router configuration data
    • Whois queries to verify sibling ASes
    • List of known Internet eXchange Points
exploring the remaining mismatches

D

D

D

D

E

E

B

B

C

C

C

B

Exploring the Remaining Mismatches
  • Route aggregation
    • Traceroute AS path longer in 20% of mismatches
    • Different paths for destinations in same prefix
  • Interface numbering at AS boundaries
    • Boundary links numbered from one AS
    • Verified cases where AT&T (AS 7018) is involved

BGP path: B C

Traceroute path: B C D

BGP path: B C D

Traceroute path: B D

discussion of the two papers
Discussion of the Two Papers
  • Measuring ISP topologies with RocketFuel
    • Measure judiciously
    • First view of ISP topologies
    • PoP structure, inter-PoP graphs, peering, …
    • Good? Bad? What areas for future work?
  • First-principles of router-level topology
    • Explain the high variability in router degree
    • Technological limits on switching capacity
    • Many low-speed links at edge, few large in core
    • High variability at edge due to economics
    • Good? Bad? What areas for future work?
some project ideas
Some Project Ideas
  • Accuracy of router-level mapping
    • Apply traceroute to map out the Abilene network
    • Use PlanetLab nodes for many vantage points
    • Verify against the actual topology of the network
  • Influence of inaccuracy in router-level maps
    • Characterize the types of inaccuracy that arise
    • Determine the influence on key graph metrics
    • Identify ways to limit the effects of inaccuracy
  • Design better router support for measurement
    • To support topology discovery, troubleshooting, …
    • Be cognizant of need to be efficient, not used for attacks, not reveal too-sensitive information, etc.
reading for thursday as level topology
Reading for Thursday: AS-Level Topology
  • Two papers, and one video
    • “Toward capturing representative AS-level Internet topologies”
    • “Interconnection, peering, and settlements”
    • NANOG video on evolution of Internet peering
  • One-page review of first paper (hard-copy)
    • Brief summary of the paper
    • Reasons to accept the paper
    • Reasons to reject the paper
    • Three suggestions for future research directions
  • Optional reading
    • Should computer scientists experiment more?