The The Stable Paths ProblemStable Paths Problem As A As A Model Of BGP RoutingModel Of BGP Routing

NJIT April 24, 2002

Timothy G. Griffin AT&T Research

griffin@research.att.com

http://www.research.att.com/~griffin

Outline

Part I: The glue that holds the Internet together : interdomain routing with The Border Gateway Protocol (BGP)

Part II: A formal model of BGP routing policies

Joint work with Bruce Shepherd and Gordon Wilfong (Bell Labs)

Architecture of Dynamic Routing

AS 1

AS 2

BGP

EGP = Exterior Gateway Protocol

IGP = Interior Gateway Protocol

Metric based: OSPF, IS-IS, RIP, EIGRP (cisco)

Policy based: BGP

The Routing Domain of BGP is the entire Internet

OSPF

EIGRP

• Topology information is flooded within the routing domain

• Best end-to-end paths are computed locally at each router.

• Best end-to-end paths determine next-hops.

• Based on minimizing some notion of distance

• Works only if policy is shared and uniform

• Examples: OSPF, IS-IS

• Each router knows little about network topology

• Only best next-hops are chosen by each router for each destination network.

• Best end-to-end paths result from composition of all next-hop choices

• Does not require any notion of distance

• Does not require uniform policies at all routers

• Examples: RIP, BGP

Link State Vectoring

Technology of Distributed Routing

The Gang of Four

Link State Vectoring

EGP

IGP

BGP

RIPIS-IS

OSPF

6

Many Routing Processes Can Run on a Single Router

Forwarding Table

OSPFDomain

RIPDomain

BGP

OS kernel

OSPF Process

OSPF Routing tables

RIP Process

RIP Routing tables

BGP Process

BGP Routing tables

Forwarding Table Manager

AS Numbers (ASNs)ASNs are 16 bit values.

64512 through 65535 are “private”

• Yale: 29 • MIT: 3• Harvard: 11• Genuity: 1 • AT&T: 7018, 6341, 5074, … • UUNET: 701, 702, 284, 12199, …• Sprint: 1239, 1240, 6211, 6242, …• …

ASNs represent units of routing policy

Currently over 12,000 in use.

Autonomous Routing Domains Don’t Always Need BGP or an ASN

Qwest

Yale University

Nail up default routes 0.0.0.0/0pointing to Qwest

Nail up routes 130.132.0.0/16pointing to Yale

130.132.0.0/16

Static routing is the most common way of connecting anautonomous routing domain to the Internet. This helps explain why BGP is a mystery to many …

ASNs Can Be “Shared” (RFC 2270)

AS 701UUNet

ASN 7046 is assigned to UUNet. It is used byCustomers single homed to UUNet, but needing BGP for some reason (load balancing, etc..) [RFC 2270]

AS 7046Crestar Bank

AS 7046 NJIT

AS 7046HoodCollege

128.235.0.0/16

How Many ASNs are there?

Thanks to Geoff Huston. http://www.telstra.net/ops on June 23, 2001

AS Graphs Can Be Fun

The subgraph showing all ASes that have more than 100 neighbors in fullgraph of 11,158 nodes. July 6, 2001. Point of view: AT&T route-server

BGP Table Growth

Thanks to Geoff Huston. http://www.telstra.net/ops/bgptable.html on August 8, 2001

13

Nontransit vs. Transit ASes

ISP 1ISP 2

Nontransit ASmight be a corporateor campus network.Could be a “content provider”

NET ATraffic NEVER flows from ISP 1through NET A to ISP 2(At least not intentionally!)

IP traffic

Internet Serviceproviders (often)have transit networks

14

Selective Transit

NET BNET C

NET A provides transitbetween NET B and NET Cand between NET D and NET C

NET A

NET D

NET A DOES NOTprovide transitBetween NET D and NET B

Most transit networks transit in a selective manner…

IP traffic

Customers and Providers

Customer pays provider for access to the Internet

provider

customer

IP trafficprovider customer

The Peering Relationship

peer peer

customerprovider

Peers provide transit between their respective customers

Peers do not provide transit between peers

Peers (often) do not exchange $$$trafficallowed

traffic NOTallowed

Peering Provides Shortcuts

Peering also allows connectivity betweenthe customers of “Tier 1” providers.

peer peer

customerprovider

18

BGP-4• BGP = Border Gateway Protocol

• Is a Policy-Based routing protocol

• Is the de facto EGP of today’s global Internet

• Relatively simple protocol, but configuration is complex and the

entire world can see, and be impacted by, your mistakes.

• 1989 : BGP-1 [RFC 1105]– Replacement for EGP (1984, RFC 904)

• 1990 : BGP-2 [RFC 1163]

• 1991 : BGP-3 [RFC 1267]

• 1995 : BGP-4 [RFC 1771] – Support for Classless Interdomain Routing (CIDR)

19

BGP Operations (Simplified)

Establish session on TCP port 179

Exchange all active routes

Exchange incremental updates

AS1

AS2

While connection is ALIVE exchangeroute UPDATE messages

BGP session

20

Four Types of BGP Messages

• Open : Establish a peering session.

• Keep Alive : Handshake at regular intervals.

• Notification : Shuts down a peering session.

• Update : Announcing new routes or withdrawing previously announced routes.

announcement = prefix + attributes values

BGP Attributes

Value Code Reference----- --------------------------------- --------- 1 ORIGIN [RFC1771] 2 AS_PATH [RFC1771] 3 NEXT_HOP [RFC1771] 4 MULTI_EXIT_DISC [RFC1771] 5 LOCAL_PREF [RFC1771] 6 ATOMIC_AGGREGATE [RFC1771] 7 AGGREGATOR [RFC1771] 8 COMMUNITY [RFC1997] 9 ORIGINATOR_ID [RFC2796] 10 CLUSTER_LIST [RFC2796] 11 DPA [Chen] 12 ADVERTISER [RFC1863] 13 RCID_PATH / CLUSTER_ID [RFC1863] 14 MP_REACH_NLRI [RFC2283] 15 MP_UNREACH_NLRI [RFC2283] 16 EXTENDED COMMUNITIES [Rosen] ... 255 reserved for development

From IANA: http://www.iana.org/assignments/bgp-parameters

Mostimportantattributes

Not all attributesneed to be present inevery announcement

Attributes are Used to Select Best Routes

192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

192.0.2.0/24pick me!

Given multipleroutes to the sameprefix, a BGP speakermust pick at mostone best route

(Note: it could reject them all!)

23

BGP Route Processing

Best Route Selection

Apply Import Policies

Best Route Table

Apply Export Policies

Install forwardingEntries for bestRoutes.

ReceiveBGPUpdates

BestRoutes

TransmitBGP Updates

Apply Policy =filter routes & tweak attributes

Based onAttributeValues

IP Forwarding Table

Apply Policy =filter routes & tweak attributes

Open ended programming.Constrained only by vendor configuration language

Route Selection Summary

Highest Local Preference

Shortest ASPATH

Lowest MED

i-BGP < e-BGP

Lowest IGP cost to BGP egress

Lowest router ID

traffic engineering

Enforce relationships

Throw up hands andbreak ties

Tweak Tweak Tweak

• For inbound traffic– Filter outbound routes– Tweak attributes on

outbound routes in the hope of influencing your neighbor’s best route selection

• For outbound traffic– Filter inbound routes– Tweak attributes on

inbound routes to influence best route selection

outboundroutes

inboundroutes

inboundtraffic

outboundtraffic

In general, an AS has morecontrol over outbound traffic

26

ASPATH Attribute

AS7018135.207.0.0/16AS Path = 6341

AS 1239Sprint

AS 1755Ebone

AT&T

AS 3549Global Crossing

135.207.0.0/16AS Path = 7018 6341

135.207.0.0/16AS Path = 3549 7018 6341

AS 6341

135.207.0.0/16

AT&T Research

Prefix Originated

AS 12654RIPE NCCRIS project

AS 1129Global Access

135.207.0.0/16AS Path = 7018 6341

135.207.0.0/16AS Path = 1239 7018 6341

135.207.0.0/16AS Path = 1755 1239 7018 6341

135.207.0.0/16AS Path = 1129 1755 1239 7018 6341

AS Graphs Do Not Show Topology!

The AS graphmay look like this. Reality may be closer to this…

BGP was designed to throw away information!

AS Graphs Depend on Point of View

This explains why there is no UUNET (701) Sprint (1239) link on previous slide!

peer peer

customerprovider

54

2

1 3

6

54

2

6

1 3

54 6

1 3

54

2

6

1 32

In fairness: could you do this “right” and still scale?

Exporting internalstate would dramatically increase global instability and amount of routingstate

Shorter Doesn’t Always Mean Shorter

AS 4

AS 3

AS 2

AS 1

Mr. BGP says that path 4 1 is better than path 3 2 1

Duh!

30

Shedding Inbound Traffic with ASPATH Padding Hack

Padding will (usually) force inbound traffic from AS 1to take primary link

AS 1

192.0.2.0/24ASPATH = 2 2 2

customerAS 2

provider

192.0.2.0/24

backupprimary

192.0.2.0/24ASPATH = 2

31

Padding May Not Shut Off All Traffic

AS 1

192.0.2.0/24ASPATH = 2 2 2 2 2 2 2 2 2 2 2 2 2 2

customerAS 2

provider

192.0.2.0/24

192.0.2.0/24ASPATH = 2

AS 3provider

AS 3 will sendtraffic on “backup”link because it prefers customer routes and localpreference is considered before ASPATH length!

Padding in this way is oftenused as a form of loadbalancing

backupprimary

32

COMMUNITY Attribute to the Rescue!

AS 1

customerAS 2

provider

192.0.2.0/24

192.0.2.0/24ASPATH = 2

AS 3provider

backupprimary

192.0.2.0/24ASPATH = 2 COMMUNITY = 3:70

Customer import policy at AS 3:If 3:90 in COMMUNITY then set local preference to 90If 3:80 in COMMUNITY then set local preference to 80If 3:70 in COMMUNITY then set local preference to 70

AS 3: normal customer local pref is 100,peer local pref is 90

33

Hot Potato Routing: Go for the Closest Egress Point

192.44.78.0/24

15 56 IGP distances

egress 1 egress 2

This Router has two BGP routes to 192.44.78.0/24.

Hot potato: get traffic off of your network as Soon as possible. Go for egress 1!

34

Getting Burned by the Hot Potato

15 56

172865High bandwidth

Provider backbone

Low bandwidthcustomer backbone

Heavy Content Web Farm

Many customers want their provider to carry the bits!

tiny http request

huge http reply

SFF NYC

San Diego

35

Cold Potato Routing with MEDs(Multi-Exit Discriminator Attribute)

15 56

172865

Heavy Content Web Farm

192.44.78.0/24

192.44.78.0/24MED = 15

192.44.78.0/24MED = 56

This means that MEDs must be considered BEFOREIGP distance!

Prefer lower MED values

Note1 : some providers will not listen to MEDs

Note2 : MEDs need not be tied to IGP distance

Policies Can Interact Strangely(“Route Pinning” Example)

backup

Disaster strikes primary linkand the backup takes over

Primary link is restored but sometraffic remains pinned to backup

1 2

3 4

Install backup link using community

customer

News at 11:00h

• BGP is not guaranteed to converge on a stable routing. Policy interactions could lead to “livelock” protocol oscillations. See “Persistent Route Oscillations in Inter-domain Routing” by K. Varadhan,

R. Govindan, and D. Estrin. ISI report, 1996 • Corollary: BGP is not guaranteed to recover

from network failures.

PART II

What Problem is BGP solving?

Underlying problem

Shortest Paths

Distributed means of computing a solution.

X?

RIP, OSPF, IS-IS

BGP

• aid in the design of policy analysis algorithms and heuristics• aid in the analysis and design of BGP and extensions• help explain some BGP routing anomalies • provide a fun way of thinking about the protocol

X could

Can we model BGP?

Separate dynamic and static semantics

SPVP = Simple Path Vector Protocol = a distributed algorithm for solving SPP

BGP

SPVP

Booo Hooo, Many, many complications...

BGP Policies

Stable Paths Problem (SPP)

staticsemantics

dynamicsemantics

1

An instance of the Stable Paths Problem (SPP)

2 5 5 2 1 0

0

2 1 02 0

1 3 01 0

3 0

4 2 04 3 0

3

4

2

1

• A graph of nodes and edges, • Node 0, called the origin, • For each non-zero node, a set

or permitted paths to the origin. This set always contains the “null path”.

• A ranking of permitted paths at each node. Null path is always least preferred. (Not shown in diagram)

When modeling BGP : nodes represent BGP speaking routers, and 0 represents a node originating some address block

most preferred…least preferred

Yes, the translation gets messy!

5 5 2 1 0

1

A Solution to a Stable Paths Problem

2

0

2 1 02 0

1 3 01 0

3 0

4 2 04 3 0

3

4

2

1

• node u’s assigned path is either the null path or is a path uwP, where wP is assigned to node w and {u,w} is an edge in the graph,

• each node is assigned the highest ranked path among those consistent with the paths assigned to its neighbors.

A Solution need not represent a shortest path tree, or a spanning tree.

A solution is an assignment of permitted paths to each node such that

An SPP may have 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

Multiple solutions can result in “Route Triggering”

1

02

3

1 01 2 3 0

2 3 02 1 0

3 2 1 03 0

1

02

3

1

02

3

Remove primary link Restore primary link

1 01 2 3 0

2 3 02 1 0

3 2 1 03 0

primary link

backup link

BAD GADGET : No Solution

2

0

31

2 1 02 0

1 3 01 0

3 2 03 0

4

3

Persistent Route Oscillations in Inter-Domain Routing. Kannan Varadhan, Ramesh Govindan, and Deborah Estrin. Computer Networks, Jan. 2000

SURPRISE : Beware of Backup Policies

2

0

31

2 1 02 0

1 3 01 0

3 4 2 03 0

4

4 04 2 04 3 0

Becomes a BAD GADGET if link (4, 0) goes down.

BGP is not robust : it is not guaranteed to recover from network failures.

PRECARIOUS

1

0

2

1 2 01 0

2 1 02 0

3

4

5 6

5 3 1 05 6 3 1 2 05 3 1 2 0

6 3 1 06 4 3 1 2 06 3 1 2 0

4 3 1 04 5 3 1 2 04 3 1 2 0

3 1 03 1 2 0

As with DISAGREE, this part has two distinct solutions

This part has a solution only when node 1 is assigned the direct path (1 0).

Has a solution, but can get “trapped”

Solving an SPP

Just enumerate all path assignmentsAnd check stability of each….

Exponential complexity

But, in worst case you (probably) can’t do any better…

Use 3-SAT…Variables V = {X1, X2, …, Xn}

Clauses C1 = X17 or ~X23 or ~X3, C2 = ~X2 or X3 or ~X12 …. Cm = X6 or ~X7 or X18

Question Is there an variable assignment A : V {true, false} such that each clause C1, … ,Cm is true?

3-SAT is NP-complete

Modeling assignment to variable X

X

0

X

X = trueX = false

X

0X

X

0X

X X 0

X 0

X X 0

X 0

SPP Solvability is NP-complete

BAD GADGET

0

X5 X5X7 X7 X3 X3

CX7 or X5 or X3

C X7 0C X5 0C X3 0

SPVP protocol

process spvp[u] { receive P from w { rib-in(uw) := u P if rib(u) != best(u) { rib(u) := best(u)

foreach v in peers(u) { send rib(u) to v } } } }

Pick the best path available at any given time…

SPVP wanders around assignment space

= assignment = solution

Distributed algorithms to solve SPP?

• OSPF-like :– Distribute topology, path ranks– Solve SPP locally– Exponential worst case– How can loops be avoided when multiple solutions

exist?

• RIP-like:– Pick the best path from the set of your neighbor’s paths,

tell your neighbors when you change your mind– Can diverge– Not guaranteed to find a solution, even when one exists– Even when converges, no bound on convergence time

This is BGP…

A sufficient condition for sanity

Static (SPP)

solvable

Dynamic (SPVP)

unique solution

safe (can’t diverge)

predictable restoration

If an instance of SPP has an acyclic dispute digraph, then

all sub-problems uniquely solvable

robust with respect to link/node failures

Dispute Digraph

…(u v)P…(u v)Q...

…Q…P...

u v 0P

Q

Q (u v)P

Gives the dispute arc

Dispute Digraph (cont.)

…(u,v)P...

…P...

u v 0P

P (u,v)P

Gives the transmission arc

Dispute Digraph Example

21

0

43

1 3 01 0

2 1 02 0

4 2 0

4 3 0

3 4 2 0

3 0

3 4 2 0

2 1 0

2 0 1 0

3 0

4 3 0 1 3 0

4 2 0

BAD GADGET II

CYCLE

What is to be done?

StaticApproach

Inter-AScoordination

Automated Analysis of Routing Policies

Dynamic Approach

Extend BGP with a dynamic means of detecting and suppressingpolicy-based oscillations?

These approaches are complementary

Some Applications SPP Theory

• A Safe Path Vector Protocol. Timothy G. Griffin, Gordon Wilfong. INFOCOM 2001– Dynamic solution for SPVP based on histories

(dynamically constructed dispute cycles). • Inherently safe backup routing with BGP. Lixin Gao,

Timothy G. Griffin, Jennifer Rexford. INFOCOM 2001• Show that if customer/provider peer/peer model is

followed, then all is well, – Show that this can be exteded with complex

backup policies and remain safe. – Analysis of “cold potato” routing problems (MED

oscillation). Griffin and Wilfong. Work in progress– MED requires a modification to SPP model

– Analysis of Internal BGP (IBGP) configuration. Griffin and Wilfong. Work in progress.

A Few Research Topics

• Dynamic Behavior of BGP – Convergence time, message overhead

• BGP Security– S-BGP defined, but not deployed. Is it a

good solution.– Need an “interdomain trust model”

• Beyond BGP? – When will it break? What will replace it?

Selected Papers on BGP Sanity

• Persistent Route Oscillations in Inter-Domain Routing. Kannan Varadhan, Ramesh Govindan, and Deborah Estrin. Computer Networks, Jan. 2000. (Also USC Tech Report, Feb. 1996)

– Shows that BGP is not guaranteed to converge• An Architecture for Stable, Analyzable Internet Routing. Ramesh Govindan, Cengiz Alaettinoglu,

George Eddy, David Kessens, Satish Kumar, and WeeSan Lee. IEEE Network Magazine, Jan-Feb 1999.

– Use RPSL to specify policies. Store them in registries. Use registry for conguration generation and analysis.

• An Analysis of BGP Convergence Properties. Timothy G. Griffin, Gordon Wilfong. SIGCOMM 1999

– Model BGP, shows static analysis of divergence in policies is NP complete• Policy Disputes in Path Vector Protocols. Timothy G. Griffin, F. Bruce Shepherd, Gordon

Wilfong. ICNP 1999– Define Stable Paths Problem and develop sufficient condition for “sanity”

• A Safe Path Vector Protocol. Timothy G. Griffin, Gordon Wilfong. INFOCOM 2001– Dynamic solution for SPVP based on histories

• Stable Internet Routing without Global Coordination. Lixin Gao, Jennifer Rexford. SIGMETRICS 2000

– Show that if certain guidelines are followed, then all is well. • Inherently safe backup routing with BGP. Lixin Gao, Timothy G. Griffin, Jennifer Rexford.

INFOCOM 2001– Use SPP to study complex backup policies

Pointers

• Links on Interdomain routing and BGP:

• http://www.research.att.com/~griffin/interdomain.html

• SIGCOMM 2001 Tutorial on BGP:

• http://www.research.att.com/~griffin/sigcomm2001_bgp_tutorial/abstract.html

• Papers on BGP theory:

• http://www.research.att.com/~griffin/bgpresearch.html

griffin@research.att.com

Thank You!