Slide -1- February, 2006 Interdomain Routing Gordon Wilfong Distinguished Member of Technical Staff...

Slide -1-February, 2006

Interdomain Routing

Gordon WilfongDistinguished Member of Technical Staff

Algorithms Research DepartmentMathematical and Algorithmic Sciences Center

Bell Laboratories

February, 2006


Current Internet Routing

Autonomous Systems (ASes)– Subnetwork under single administrative control

• Each AS has its own economic incentives to:

– cooperate so as to achieve connectivity and to abide by economic contracts (SLAs)

– to hide information about its internal network

Routing between ASes achieved by protocol where– AS selectively announces to neighbors its chosen route to

destinations

– AS selects highest ranked route announced to it


Border Gateway Protocol (BGP)

“I routedirectly to d”

AS1 AS2

“The route I use to d goes through AS0”

AS0

d

“The route I use to d goes throughAS1 then AS0”

AS3

Data packets


Routing Protocol Requirements

Protocol must scale

– Routing table entries (destinations): ~150,000 and increasing

– Autonomous Systems: ~20,000 and increasing

Administrators require expressiveness for routing policies

– Local AS policy (based on SLAs, traffic engineering etc) determines

• route selection (effects where AS sends packets)

• route announcements (effects how/if packets from other ASes traverse it)


External BGP (E-BGP) is the mode of BGP that propagates routes between autonomous systems.

E-BGP


Stable Paths Problem

1

3

2

3 2 03 1 03 2 1 0

1 01 2 0

0

2 1 02 0

1 0210

310

0


1 2 01 0

Independent policies can lead to nonconvergence

0

1

23 2 3 02 0

3 1 03 0

Bad Gadget

Each AS chooses“favorite” availableroute.

0


May Be Multiple Solutions

First solution

1

0

2

1 2 01 0

1

0

2

1

0

2

2 1 02 0

1 2 01 0

2 1 02 0

1 2 01 0

2 1 02 0

Second solutionDISAGREE


What can be done?

Modify protocol

– Design a protocol that has provably good properties. e.g., [Griffin, W.]

Constrain topology and/or policy

– Prove good protocol behavior assuming constrained configurations. e.g., [Gao, Rexford]

• These constraints give rise to interesting “valley-free” routing problems (from customer to local provider to regional provider to national provider and then down chain again to another customer). e.g. [Erlebach et al]


Possible Research Directions

Complexity issues

– What is the complexity of deciding if a configuration is “safe” (i.e., will it always converge)?

• NP-complete to determine if there is a solution it can converge to [Griffin, W.]

Extending model

– Model assumes destinations can be considered independently

• Aggregation has not been taken into consideration

Trade-offs between protocol expressivity and safety

– Formal model to study such trade-offs


Internal BGP (I-BGP) is the protocol used to propagate external routes within an autonomous system.

I-BGP


Fully Meshed

RR

RR

RR

RR

RR

RR

A router only announcesits own routes


Route Reflectors

RR

RR

RR

RR

RR

RR

Route Reflectors must be fullymeshed

Route Reflectors pass along updatesto client routers


Route Selection Summary

Highest Local Preference

Shortest ASPATH

Lowest MED (if same next AS)

I-BGP < E-BGP

Lowest IGP cost to BGP egress

Lowest router ID

Traffic engineering

Enforce relationships

Throw up hands andbreak ties


What Are MEDs?

MED=2MED=0

AS1

AS2

CP

Client

“Hot Potato Routing” vs.“Cold Potato Routing”

MED=1

AS3


Persistent Route Oscillations

135.104.54.4

5040

120

1

MED=1

MED=2MED=1

r1

r2

r3

A B


Oscillations in I-BGP

135.104.54.4

1

115 5 5

RR1 RR2 RR3

C1 C2 C3

P1 P2 P3I-BGP connection

Physical link

AS0


Deflections

135.104.54.4

3

1

1

P

Q

RR

A B

Packets can be diverted out of a network unexpectedly.

AS0


Routing Loops

135.104.54.4

5

11

1

RR1RR2

C1 C2

Badly configurednetworks can alsoexperience routingloops

AS0


A Real-world MED Oscillation Example

• A functioning network breaks into a state of persistent route oscillations when a BGP session goes down• First thought to be a hardware problem• Analysis shows that route oscillations caused by the use of the MED attribute• The example was actually predicted by the theoretical analysis [Griffin, W.]


Initial State

AS 4

A B

E F

AS 3

AS 1

AS 2

reflector reflector

I-BGP

E-BGP

(1)(2)

C

MEDs

IGP12

D

21

(2)

70000

Only AS 2 sends MEDs to AS 4


Initial Routing

AS 4

A B

E F

AS 3

AS 1

AS 2

reflector reflector

I-BGP

E-BGP

(1)

C

IGP12

D

21

(2)MEDs (2)

70000

D prefers AS 2path due to router IDtie breaking


B Changes Its Route

AS 4

A B

E F

AS 3

AS 1

AS 2

reflector reflector

I-BGP

E-BGP

(1)

C

IGP12

D

21

sessi

on down!

The AS 4 AS 3 BGP Session is dropped

(2)MEDs (2)

70000


A Changes Its Route

AS 4

A B

E F

AS 3

AS 1

AS 2

reflector reflector

I-BGP

E-BGP

(1)

C

IGP12

D

21

sessi

on down!

(2)MEDs (2)

The MED 1 route from B beats the MED 2 routesthat A sees from its clients….

70000

CALL THISSTATE ZERO


C & D Change Routes

AS 4

A B

E F

AS 3

AS 1

AS 2

reflector reflector

I-BGP

E-BGP

(1)

C

IGP12

D

21

sessi

on down!

(2)MEDs (2)

The MED 1 route from A knocks both MED 2 routesout of the picture for C & D …

70000


A Changes Route Again

AS 4

A B

E F

AS 3

AS 1

AS 2

reflector reflector

I-BGP

E-BGP

(1)

C

IGP12

D

21

sessi

on down!

(2)MEDs (2)

A now sees the route from D through AS 3, and it is closer IGP-wise than the route from B…

70000


C&D Return to Initial Routes

AS 4

A B

E F

AS 3

AS 1

AS 2

reflector reflector

I-BGP

E-BGP

(1)

C

IGP12

D

21

sessi

on down!

(2)MEDs (2)

C & D no longer see MED 1 route from A, so they return to the eBGP routes with MED 2…

70000


Back to State Zero!

AS 4

A B

E F

AS 3

AS 1

AS 2

reflector reflector

I-BGP

E-BGP

(1)

C

IGP12

D

21

sessi

on down!

(2)MEDs (2)

A switches back to MED 1 route through B.

70000


What Can Be Done?

Possible Approaches:

Use only configurations that guarantee no problems

No modification to BGP required

Previous example shows this might be difficult

Prevent problems for any configuration

Modification to BGP required, e.g. [Basu et al]

Minimize overhead (number of messages, memory requirements, …)


• BGP is extremely flexible, allowing operators to easily make obscure errors that are difficult to find and correct.

• Policy based routing is difficult to get right.

• Defining a formal model of path vector protocols helps to understand what features can cause problems and what can be done to avoid errors.

Conclusions


• BGP is the routing protocol in today’s internet

• What about the future?• new Internet architectures• new architectures within an AS (e.g., Routing Control Platform [J. Rexford, et al])

• In any case, a formal theoretical approach is essential to getting it right.

What about the future?


Extras


Persistent Route Oscillations

135.104.54.4

5040

120

1

MED=1

MED=2MED=1

r1

r2

r3

A B

r2r1r3

r1r3

r1r2r3

r3r2

r1r2r3

r1r3

r2r13

r1r3

r2r1rr3

r3r2

Slide -1- February, 2006 Interdomain Routing Gordon Wilfong Distinguished Member of Technical Staff...

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