Internet Routing COS 598A Today: Interdomain Topology

1
Internet Routing (COS 598A)Today Interdomain
Topology

• Jennifer Rexford
• http//www.cs.princeton.edu/jrex/teaching/spring2
  005
• Tuesdays/Thursdays 1100am-1220pm

2
Outline

• Interdomain topology
• AS graph
• Inferring the topology from routing data
• Structure of the AS graph
• AS relationships
• Common pair-wise relationships
• Inferring the relationships from routing data
• Characteristics of the relationship graph
• Peering policies
• Example of AOLs peering agreement

3
What is the AS Graph?

• Node Autonomous System
• Edge Two ASes that speak BGP to each other

4
What is an Edge, Really?

• Edge in the AS graph
• At least one BGP session between two ASes
• Some destinations reached from one AS via the
  other

d
d
AS 1
AS 1
Exchange Point
AS 2
AS 3
AS 2
5
How Do You Know a Node or Edge Exists?

• Consult the Whois database?
• Tells which ASes have been allocated
• But, might be out-of-date on who owns it
• and often doesnt say who the neighbors are
• See a path that uses the node/edge
• Collect measurements of AS paths
• Extract all of the nodes and edges
• E.g., AS path 7018 1 88 implies
• Nodes 7018, 1, and 88
• Edges (7018, 1) and (1, 88)

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Interdomain Routing Border Gateway Protocol

• ASes exchange info about who they can reach
• IP prefix block of destination IP addresses
• AS path sequence of ASes along the path
• Example a BGP route in AS 7018 shows
• Prefix 12.34.158.0/24 has path 7018, 1, 88

12.34.158.0/24 path (7018,1,88)
12.34.158.0/24 path (88)
88
1
7018
data traffic
data traffic
12.34.158.5
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Where to Get BGP Routes Public Servers
4
7018
1221
701
3786
7
80
9.184.112.0/20
3.0.0.0/8
BGP sessions
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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 gt
12.127.0.249
0 7018 701 80 i
213.200.87.254
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 gt 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 ATT AS 3786 is DACOM (Korea), AS 1221 is
Telstra
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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
0.1
CCDF
0.01
0.001
AS degree
1
10
100
1000
10
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
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Problem of Missing Edges

• Limited collection of paths
• Some edges might never be traversed
• Especially low in the AS hierarchy
• and backup links
• Example paths from two tier-1 ISPs miss an edge

Sprint
ATT
???
Harvard B-school
Harvard
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Problem of Missing Nodes

• Route aggregation
• AS advertises a larger address block
• Smaller address block not seen everywhere
• Can cause an AS not to appear in BGP table
• Cs table has paths C, C D, and C E
• Bs table has only path C for 12.0.0.0/8

C 12.0.0.0/8
D 12.1.0.0/16
D
A
B
C
E
E 12.2.0.0/16
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Research Questions

• Incomplete data
• How to get more data?
• How much does missing data affect answers?
• What kinds of questions can be answered safely?

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AS Relationships
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Interdomain Routing Policies

• Two main decisions
• Path selection which of the paths to use?
• Path export which neighbors to tell?
• Both driven by business relationships, e.g.,
• Customer pays provider for Internet access
• Peers find it mutually advantageous to cooperate

1
data traffic
data traffic
12.34.158.5
16
Customer-Provider Relationship

• Customer needs to be reachable from everyone
• Provider exports routes learned from customer to
  everyone
• Customer does not want to provide transit service
• Customer does not export from one provider to
  another

Traffic to the customer
Traffic from the customer
d
provider
provider
customer
d
customer
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Peer-Peer Relationship

• Peers exchange traffic between customers
• AS exports only customer routes to a peer
• AS exports a peers routes only to its customers

Traffic to/from the peer and its customers
peer
peer
d
18
Paths You Should Never See (Invalid)
Customer-provider
Peer-peer
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Other Kinds of Relationships

• Siblings
• Same company
• Mutual transit service
• Like one bigger AS
• Mergers, acquisitions,
• Backup
• Used only when failure
• Second provider
• Backup peering
• Geography-specific
• Customer in U.S.
• Peer in Europe

A
E
B
F
D
H
C
G
primary
backup
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AS Relationships Matter

• Scientific understanding
• Understanding Internet structure and evolution
• Understanding why certain paths are used for
  traffic
• Placement of Web servers
• Want to be close to most customer networks
• Business decisions
• Selecting new peer or provider, or renegotiating
  relations
• Security policies
• Knowing which BGP routes look suspicious
• Analyzing BGP convergence
• Relationships have a big impact here (more later!)

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Inferring AS Relationships

• Top down how routes are selected
• AS relationships define routing policy
• Routing policy determines the routes you see
• Bottom up how policies can be inferred
• Routing data are available from public sources
• The chosen routes tell you about the policy
• Example seeing path A B C tells you
• B permits A to transit through B to reach C
• (A,B) and (B,C) cannot both be peering links
• A and C are not both upstream providers of B

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Type-of-Relationship Problem

• Given the inputs
• AS graph G(V,E) with vertices V and edges E
• Set of paths P on the graph G
• Find a solution that
• Labels each edge with an AS relationship
• Minimizes the number of invalid paths in P
• Rich area of research work
• http//www-unix.ecs.umass.edu/lgao/ton.ps
• http//www.cs.princeton.edu/jrex/papers/infocom02
  .pdf
• http//www.cs.berkeley.edu/sagarwal/research/BGP-
  hierarchy/
• lots of scope for algorithms-oriented research
  project

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AS Relationship Graph (2002)

• Lowest level Stubs
• Leaf nodes no peers or downstream customers
• 8898 of the 10915 ASes (82.5 of ASes)
• Ex UC Berkeley (25), Princeton (88), ATT Labs
  (6431), and INRIA (1300)
• Next lowest level Regional ISPs
• Leaf nodes after successive pruning of leaf nodes
• 971 ASes of the 10915 ASes (8.9 of ASes)
• Ex PacBell (5676), US West (6223), and UUNET
  Canada (815)
• Remaining 1046 ASes Core

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AS Relationship Graph (2002), Continued

• Dense core Tier-1 providers
• (Nearly) fully-connected nodes with no providers
• Around 15-20 ASes in a near-clique
• Ex Sprint, UUNET, ATT, Verio, Level3, CW,
• Transit core
• ASes that peer with the dense core and each other
• 129 ASes, including top providers in Europe and
  Asia
• Ex UUNET Europe, KDDI, and Singapore Telecom
• Outer core
• All of the remaining ASes in the core
• 897 ASes, including large regional and national
  ISPs
• Ex Turkish Telecom and Minnesota Regional Network

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Node Degree is Not Enough

• Node degree ignores relationships
• A stub AS may have many upstream providers
• A core AS may have a small number of peers
• Some ASes have customers that dont have AS
  numbers

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Ideas for Class Projects

• AS relationship inference
• New algorithms for inferring AS relationships
• Longitudinal study of AS relationship graph
• Influence of policies on measuring AS graph
• AS peering policies
• Analysis of incentives to peer or not
• Study of how one AS can game another
• Analysis of whether regulation is necessary to
  keep large ASes from locking out smaller ones
• Alternate settlement models
• Associating prices with routes?
• Source-based routing with third-party control?

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Peering Policies

• Contracts that outline
• Operational requirements on peer network
• Backbone and peering capacity requirements
• Number and geographic diversity of peering points
• Volume and ratio of traffic between two peers
• Routing-policy requirements
• AOLs Settlement-Free Interconnection Policy
• http//www.atdn.net/settlement_free_int.shtml

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AOLs Settlement-Free Interconnection Policy

• Operational requirements on a peer network
• Handle a single-node outage w/o traffic impact
• Single AS number
• Network Operations Center staffed at all times
• Backbone capacity
• At least 10 gigabits/sec between 8 or more cities
• Minimum peering link speed of 622 megabits/sec
• Peering locations (in U.S.)
• At least four locations
• Must include D.C. area, middle of country, Bay
  area, and NYC or Atlanta

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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 B
Provider B
multiple peering points
Early-exit routing
Provider A
Customer A
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AOL Routing Requirements

• Consistent advertisements
• All customer routes
• At all peering points
• With the same AS path length
• Address blocks
• Routes aggregated as much as possible
• No address blocks smaller than /24
• Address blocks are registered (e.g., with ARIN)
• No default routing
• Only send traffic to destinations AOL advertises

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For Next Tuesdays Class

• Adapting routing inside an AS to the traffic
• The Revised ARPANET Routing Metric (1989)
• Traffic Engineering With Traditional IP Routing
  Protocols (survey paper, 2002)
• Written one-page review of first paper
• Brief summary of the paper
• Reasons to accept the paper
• Reasons to reject the paper
• Suggestions for future research directions
• Just to skim
• RFC 3272 Overview and Principles of Internet
  Traffic Engineering