Stable Internet Routing Without Global Coordination Jennifer Rexford Princeton University jrex Joint work with Lixin Gao,

Stable Internet Routing Without Stable Internet Routing Without Global CoordinationGlobal Coordination

Jennifer RexfordPrinceton University

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

Joint work with Lixin Gao, Michael Schapira, and Yi Wang

What is an Internet?What is an Internet?A “network of networks”

– Networks run by different institutions

Autonomous System (AS)– Collection of routers run by a single institution

ASes have different goals– Different views of which paths are good

Interdomain routing is what reconciles those views– To compute end-to-end paths through the Internet

Wonderful problem setting for game theory and mechanism design

An Open QuestionAn Open Question

Can we have all three? Under what conditions?

Evolvable Protocols(under-specified, programmable)

Autonomy(autonomous parties, with different economic objectives)

Global Properties (stability, scalability, reliability, security, managability, …)

?

Autonomous Systems (ASes)Autonomous Systems (ASes)

1

2

3

4

5

67

ClientWeb server

Path: 6, 5, 4, 3, 2, 1

Interdomain Routing: Border Gateway ProtocolInterdomain Routing: Border Gateway Protocol

ASes exchange info about who they can reach– Destination: block of IP addresses (an “IP prefix”)

– AS path: sequence of ASes along the path

Policies configured by the AS’s network operator– Path selection: which of the paths to use?

– Path export: which neighbors to tell?

1 2 3

d

“I can reach d”

“I can reach d via AS 1”

data traffic data traffic

Interdomain Routing Convergence ChallengesInterdomain Routing Convergence Challenges

Must scale– Address blocks: 300,000 and growing

– Autonomous Systems: around 35,000

Must support flexible policy– Path selection: which path your AS wants to use

– Path export: who can send packets through your AS

Must converge, and quickly– Routing convergence can take several minutes

– … and the system doesn’t necessarily converge at all!

Goal: Guaranteed convergence of the global routing system with purely local control.

Stable Paths Problem (SPP) ModelStable Paths Problem (SPP) Model

Model of routing policy– Each AS has a ranking of the permissible paths

Model of path selection– Pick the highest-ranked path consistent with neighbors

Flexibility is not free– Global system converges slowly, or not at all

– Depending on the way the ASes rank their paths

1 2 d1 d

2 3 d2 d

3 1 d3 d

1

3

2

d

Conflicting Policies Cause Convergence ProblemsConflicting Policies Cause Convergence Problems

0

1

23

1 2 01 0

2 3 02 0

3 1 03 0

Pick the highest-ranked path consistent with your neighbors’ choices.

Only choice!

Top choice!

Only choice!

Better choice!

Only choice!

Better choice!

Global Control is Not WorkableGlobal Control is Not Workable

Create a global Internet routing registry– Keeping the registry up-to-date would be difficult

Require each AS to publish its routing policies– ASes may be unwilling to reveal BGP policies

Check for conflicting policies, and resolve conflicts– Checking for convergence problems is NP-complete

– Link/router failure may result in an unstable system

Need a solution that does not require global coordination.

Think Globally, Act LocallyThink Globally, Act Locally

Key features of a good solution– Flexibility: allow diverse local policies for each AS

– Privacy: do not force ASes to divulge their policies

– Backwards-compatibility: no changes to BGP

– Guarantees: convergence even when system changes

Restrictions based on AS relationships– Path selection rules: which route you prefer

– Export policies: who you tell about your route

– AS graph structure: who is connected to who

Customer-Provider RelationshipCustomer-Provider Relationship

Customer pays provider for access to the Internet– Provider exports its customer’s routes to everybody

– Customer exports provider’s routes only to downstream customers

d

d

provider

customer

customer

provider

Traffic to the customer Traffic from the customer

advertisements

traffic

Peer-Peer RelationshipPeer-Peer Relationship

Peers exchange traffic between their customers – AS exports only customer routes to a peer

– AS exports a peer’s routes only to its customers

peerpeer

Traffic to/from the peer and its customers

d

advertisements

traffic

Hierarchical AS RelationshipsHierarchical AS Relationships

Provider-customer graph is a directed, acyclic graph– If u is a customer of v and v is a customer of w

– … then w is not a customer of u

u

v

w

Valid and Invalid PathsValid and Invalid Paths

2 3

1

d

4

5 6

7 8

Provider-Customer

Peer-Peer

Valid paths: “1 2 d” and “7 d”Invalid path: “5 8 d”Valid paths: “6 4 3 d” and “8 5 d”

Invalid paths: “6 5 d” and “1 4 3 d”

Act Locally, Prove GloballyAct Locally, Prove Globally

Route export– Do not export routes learned from a peer or provider

– … to another peer or provider

Global topology– Provider-customer relationship graph is acyclic

– E.g., my customer’s customer is not my provider

Route selection– Prefer routes through customers

– … over routes through peers and providers

Guaranteed to converge to unique, stable solution

Our Our LocalLocal Path Selection Rules Path Selection Rules

Classify routes based on next-hop AS– Customer routes, peer routes, and provider routes

Rank routes based on classification– Prefer customer routes over peer and provider routes

Allow any ranking of routes within a class– E.g., can rank one customer route higher than another

– Gives network operators the flexibility they need

Consistent with traffic engineering practices– Customers pay for service, and providers are paid

– Peer relationship contingent on balanced traffic load

Solving the Convergence ProblemSolving the Convergence Problem

Result– Safety: guaranteed convergence to unique stable solution

– Inherent safety: holds under failures and policy changes

Definitions– System state: current best route at each AS

– Activating AS: re-do decision based on neighbor choices

Sketch of (constructive) proof– Find an activation sequence that leads to a stable state

– Any “fair” sequence (eventually) includes this sequence

Rough Sketch of the ProofRough Sketch of the Proof

Two phases– Walking up the customer-provider hierarchy

– Walking down the provider-customer hierarchy

2 3

1

d

4

5 6

7 8

Provider-Customer

Peer-Peer

Economic Incentives Affect Protocol BehaviorEconomic Incentives Affect Protocol Behavior

ASes already follow our rules, so system is stable– High-level argument

» Export and topology assumptions are reasonable

» Path selection rule matches with financial incentives

– Empirical results» BGP routes for popular destinations are stable for ~10 days

» Most instability from failure/recovery of a few destinations

ASes should follow our rules to make system stable– Need to encourage operators to obey these guidelines

– … and provide ways to verify the network configuration

– Need to consider more complex relationships and graphs

Playing One Condition Off Against AnotherPlaying One Condition Off Against Another

All three conditions are important– Path ranking, export policy, and graph structure

Allowing more flexibility in ranking routes– Allow same preference for peer and customer routes

– Never choose a peer route over a shorter customer route

… at the expense of stricter AS graph assumptions– Hierarchical provider-customer relationship (as before)

– No private peering with (direct or indirect) providers

Peer-peer

Extension to Backup RelationshipsExtension to Backup Relationships

Backups: more liberal export policies, and different ranking– The motivation is increased reliability

– …but ironically it may cause routing instability!

Generalize rule: prefer routes with fewest backup links– Need to maintain a count of the # of backup links in the path

backup pathprimaryprovider

backupproviderfailure

Backup Provider

backup pathfailure

peer

provider

Peer-Peer Backup [RFC 1998]

Results Hold Under More Complex ScenariosResults Hold Under More Complex Scenarios

Complex AS relationships– AS pair with different relationship for different prefixes

– AS pair with both a backup and a peer relationships

– AS providing transit service between two peer ASes

Stability under changing AS relationships– Customer-provider to/from peer-peer

– Customer-provider to/from provider-customer

Extensions of the WorkExtensions of the Work Influence of AS relationships on BGP convergence

– Algebraic framework and design principles for policy languages

– Fundamental limits on relaxing the assumptions Application of the idea to internal BGP inside an AS

– Sufficient conditions for iBGP convergence inside an AS

– “What-if” tool for traffic engineering inside an AS AS-level analysis of the Internet topology

– Inference of AS relationships and policies from routing data

– Characterization of AS-level topology and growth Practical applications of knowing AS relationships

– Analyzing your competitors’ business relationships

– Identifying BGP routes that violate export conditions

A Case For Customized Route SelectionA Case For Customized Route Selection

ISPs usually have multiple paths to the destinationDifferent paths have different propertiesDifferent neighbors may prefer different routes

Bank

VoIPprovider

School

Most secureShortest latency

Lowest cost

Neighbor-Specific Route SelectionNeighbor-Specific Route Selection

A node has a ranking function per neighbor

i

j is node i’s ranking function for neighbor node j.

Stability Conditions for NS-BGP Stability Conditions for NS-BGP

Surprisingly, NS-BGP improves stability!– Neighbor-specific selection is more flexible

– Yet, requires less restrictive stability conditions

“Prefer customer” assumption is not needed– Choose any “permissible” route per neighbor

That is, need just two assumptions– No cycle of provider-customer relationships

– Do not export routes learned from one peer/provider to other peers/providers

Why Do Weaker Conditions Work?Why Do Weaker Conditions Work?

An AS always tells its neighbor a route– If it has any route that is permissible for that neighbor

0

1

23

1 2 01 0

2 3 02 0

3 1 03 0

Deploying NS-BGPDeploying NS-BGP

An AS can deploy NS-BGP alone– Without upgrading their routers

– Without coordinating with all their neighbors

Three aspects to the solution– Disseminating extra BGP routes

– Customized route selection

– Directing traffic from ingress to egress

Can be done exploiting existing mechanisms– Designed for Virtual Private Networks (VPNs)

Disseminating Extra BGP RoutesDisseminating Extra BGP Routes

Advertising more than one BGP route– Route distinguisher feature for VPNs

– Multiple internal BGP sessions

– ADD-PATHs extensions to internal BGP

Customized Route SelectionCustomized Route Selection

Multiple virtual routing and forwarding tables– Cisco: Virtual Routing and Forwarding (VRF)

– Juniper: Virtual Router

D: (red path): R6D: (blue path): R7

R3’s forwarding table (FIB) entries

Directing Traffic from Ingress to EgressDirecting Traffic from Ingress to Egress

Tunnels from ingress to egress– IP-in-IP tunneling

– MPLS

?

Customized Route SelectionCustomized Route Selection

Customized route selection as a service– Select a different best route for different neighbors

Different menu options– Cheapest route (e.g., “prefer customer”)

– Best performing routes, or most secure routes

– Routes that avoid undesirable ASes (e.g., censorship)

Nice practical features of NS-BGP– An individual AS can deploy NS-BGP alone

– … without compromising global stability!

ConclusionsConclusions

Avoiding convergence problems– Hierarchical of provider-customer relationships

– Export policies based on commercial relationships

– (Path ranking based on AS relationships)

Salient features– No global coordination (locally implementable)

– No changes to BGP protocol or decision process

– Guaranteed convergence, even under failures

– Guidelines consistent with financial incentives

References Related to This TalkReferences Related to This Talk

“The stable paths problem and interdomain routing”– Tim Griffin, Bruce Shepherd, and Gordon Wilfong

– http://portal.acm.org/citation.cfm?id=508332

“Stable Internet routing without global coordination”– Lixin Gao and Jennifer Rexford

– http://www.cs.princeton.edu/~jrex/papers/sigmetrics00.long.pdf

“Inherently Safe Backup Routing with BGP”– Lixin Gao, Tim Griffin, and Jennifer Rexford

– http://www.cs.princeton.edu/~jrex/papers/infocom01.pdf

“Neighbor-Specific BGP: More flexible routing policies while improving global stability”– Yi Wang, Michael Schapira, and Jennifer Rexford

– http://www.cs.princeton.edu/~jrex/papers/nsbgp_sigmetrics09.pdf

Other Related Research PapersOther Related Research Papers Inherently Safe Backup Routing with BGP

– http://www.cs.princeton.edu/~jrex/papers/infocom01.pdf

Design Principles of Policy Languages for Path Vector Protocols– http://conferences.sigcomm.org/sigcomm/2003/papers/p61-griffin.pdf

Implications of Autonomy for the Expressiveness of Policy Routing– http://conferences.sigcomm.org/sigcomm/2005/paper-FeaBal.pdf

Meta-routing– http://conferences.sigcomm.org/sigcomm/2005/paper-GriSob.pdf

An Algebraic Theory of Interdomain Routing– http://portal.acm.org/citation.cfm?id=1103561

Searching for Stability In Interdomain Routing– http://www.cs.yale.edu/homes/schapira/PID808559.pdf

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Related Papers With Game TheoryRelated Papers With Game Theory Interdomain Routing and Games

– http://www.cs.huji.ac.il/~mikesch/routing_games-full.pdf Rationality and Traffic Attraction: Incentives for Honest Path Announcements in BGP

– http://ccr.sigcomm.org/online/?q=node/395 Incentive-Compatible Interdomain Routing

– http://cs-www.cs.yale.edu/homes/jf/FRS.pdf Mechanism Design for Policy Routing

– http://cs-www.cs.yale.edu/homes/jf/FSS.pdf The Complexity of Game Dynamics: BGP Oscillations, Sink Equlibria, and Beyond

– http://www.cs.berkeley.edu/~alexf/papers/fp08.pdf Specification Faithfulness in Networks with Rational Nodes

– http://www.eecs.harvard.edu/econcs/pubs/podc04.pdf Distributed Algorithmic Mechanism Design

– http://cs-www.cs.yale.edu/homes/jf/AGTchapter14.pdf Partially Optimal Routing

– http://www.stanford.edu/~rjohari/pubs/por.pdf

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Background on Interdomain EconomicsBackground on Interdomain Economics

http://drpeering.net/a/Home.html http://www.fcc.gov/Bureaus/OPP/working_papers/oppwp32.pdf http://www.potaroo.net/papers/1999-6-peer/peering.pdf http://www.cisco.com/en/US/about/ac123/ac147/ac174/ac201/

about_cisco_ipj_archive_article09186a00800c83a5.html http://www.cisco.com/en/US/about/ac123/ac147/ac174/ac200/

about_cisco_ipj_archive_article09186a00800c8900.html http://www.vjolt.net/vol3/issue/vol3_art8.html