Making an AS Route Like a Single Node

1
Making an AS Route Like a Single Node

• Rui Zhang-Shen
• Princeton University
• rz_at_cs.princeton.edu

NANOG43, June 2, 2008
joint work with Jennifer Rexford and Yi Wang
2
Autonomous systems
The Internet (25k ASes)
An autonomous system
This talks focus BGP inside a single AS
3
Policy-based routing

• Each AS has well-defined policies (RPSL aut-num
  object), reflecting its
• Goals e.g., traffic engineering, security
• Obligations e.g., business contracts
• Todays practice
• Specify the AS-level policies in terms of BGP
  attributes
• Configure how each router selects and exports
  routes
• Problem Some policies are violated
• Even if routers are configured correctly
• Due to peculiarities of protocol and
  implementation
• Violations hard to detect, diagnose, or fix

4
Policies not jointly realizable today
Let customers use MED Use router ID tie break Use
hot-potato routing
No transit between peers Prefer shortest AS path
Export full customer routes to peers
Prefer shortest AS path Use hot-potato
routing Export consistently to peers
Can we satisfy all policies?
Use route reflectors Use hot-potato routing Let
customers use MED
Allow export communities Use hot-potato
routing Export consistently to peers
5
Current possible approaches

• Sacrifice correctness for flexibility
• Common practice today
• Sacrifice flexibility for correctness
• Disallow conflicting policies
• A costly way to achieve both
• Change the physical topology

6
Our contribution

• Atomic Routing Theory
• Mathematical model for policies and correctness
• Theoretical framework for exploring tradeoffs
• Flexibility and correctness
• Cost and flexibility
• Deriving minimum requirements for realizing
  policies
• Our proposal Atomic BGP
• Achieves both flexibility and correctness
  efficiently
• Requires only minor changes to the router
• Modify the decision process slightly
• Change iBGP route dissemination slightly

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Example I

• Let customers use MED
• Use router ID tie break
• Use hot-potato routing

r3
r1
My MED is ignored!
A
B
r3
r1
r2
MED1
MED0
Customer 2
Customer 1
MED-induced routing oscillation happened in the
real world.
d
8
Example I Solutions

• Disallow MED (sacrifice flexibility)
• May lose Customer 2

A
B
r3
r1
r2
Customer 2
Customer 1
d
9
Example I Solutions

• Disallow MED (sacrifice flexibility)
• Terminate MED links separately (costly)

A
B
C
r3
r1
r2
MED1
MED0
Customer 2
Customer 1
d
10
Example I Solutions

• Disallow MED (sacrifice flexibility)
• Terminate MED links separately (costly)
• Disseminate more routes (atomic BGP)

A
B
r3
r1
r2
MED1
MED0
Customer 2
Customer 1
d
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12
Example II

• No transit between peers
• Equally prefer customer and peer routes
• Export full customer routes to peers

Peer 1
Peer 2
r1
A
C
B
r2
d
Customer 1
http//www.merit.edu/mail.archives/nanog/1997-03/m
sg00250.html
13
Example II

• No transit between peers
• Equally prefer customer and peer routes
• Export full customer routes to peers

I cant reach d any more!
r1
Peer 1
Peer 2
r1
X
A
C
B
r2
d
Customer 2
Customer 1
http//www.merit.edu/mail.archives/nanog/1997-03/m
sg00250.html
14
Example II

• No transit between peers
• Equally prefer customer and peer routes
• Export full customer routes to peers

r1
I lost half of my users!
Peer 1
Peer 2
r1
X
d
A
C
B
r2
d
Customer 2
Customer 1
http//www.merit.edu/mail.archives/nanog/1997-03/m
sg00250.html
15
Example II Solutions

• No premium service (sacrifice flexibility)
• May lose Customer 2

Peer 1
Peer 2
r1
A
C
B
r2
d
Customer 2
Customer 1
16
Example II Solutions

• No premium service (sacrifice flexibility)
• Host peers and customers separately (costly)

Peer 1
Peer 2
r1
A
C
B
D
r2
d
Customer 2
Customer 1
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Example II Solutions

• No premium service (sacrifice flexibility)
• Host peers and customers separately (costly)
• Links export different routes (atomic BGP)

Peer 1
Peer 2
r1
A
C
B
r2
d
Customer 2
Customer 1
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Atomic Routing Theory

• An AS is atomic if it realizes all its policies
• Disseminate all routes within the AS realize any
  policies
• Disseminate one route realize policies with
  strict ranking

Atomic BGP
X
Policy Flexibility
Todays practice
X
Atomic Routing
Dissemination Overhead
BGP
X
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Atomic BGP Flexibility

• Realizes all policies based on comparing route
  attributes
• Supports todays common policies correctly
  (including MED)
• Supports any new attributes

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Atomic BGP Implementation

• Select routes for each neighbor independently
• Separate route selection process per neighbor
  class
• E.g., customers, peers, providers
• Separate forwarding table per neighbor class
• E.g., a different forwarding table on each
  linecard (VRF)
• Among set of best routes at the router,
  disseminate
• At least one route from each neighbor that uses
  MED
• At least one route
• E.g., use the new ADD-PATH feature
• Forward traffic according to the route assignment
• Tunneling from ingress link to egress link

ART shows these changes are necessary and
sufficient
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Atomic BGP offers

• Correct policy and simple configuration
• Define the AS-wide policies
• Configure each router with it
• No restrictions on the physical topology
• Minimum protocol overhead
• Dissemination of routes within the AS
• Storage for routing tables
• Incremental deployability
• Only modest changes to the routers
• Changes and benefits local to a single AS
• Incrementally deployable within an AS

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Backup Slides
25
Applications of ART

• When realizing a policy
• Find out potential policy violations
• Suggest minimum protocol changes
• When introducing new features to BGP
• Ensure no new violations are introduced
• When proposing new policy-based routing protocols
• Ensure the desired policies can be realized
• When analyzing multiple AS interactions
• Correctly model an AS as a single node

26
Example III

• Let customers use export communities
• Use hot-potato routing
• Export consistently to peers

Inconsistent export!
r1
r1
Customer
Peer
C
A
d
X
D
B
r2 (no export)
r2
27
Example III Solutions

• Disallow export communities
• Change how communities are handled

r1
r1
Customer
Peer
C
A
d
D
B
r2 (no export)
r1
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Example III Solutions

• Disallow export communities
• Change how communities are handled
• Allow a router to select multiple routes

r1
r1
Customer
Peer
C
A
d
D
B
r2 (no export)
r1, r2