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COMS W4995-1 Lecture 6

COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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Page 1: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

COMS W4995-1

Lecture 6

Page 2: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Dynamic routing protocols II

1. Dynamic Routing Protocols: Link State Routing 2. Intra-Domain Routing Protocols: OSPF & BGP

Page 3: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Dynamic Routing Protocols

Link State Routing

Page 4: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

The Gang of Four

Link State Vectoring

EGP

IGP

BGP

RIPIS-IS

OSPF

Page 5: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Link State Routing

Based on Dijkstra’ s Shortest-Path-First algorithm.

Each router starts by knowing: Prefixes of its attached networks. Links to its neighbors.

Each router advertises to the entire network (flooding): Prefixes of its directly connected networks. Active links to its neighbors.

Each router learns: A complete topology of the network (routers, links).

Each router computes shortest path to each destination.

In a stable situation, all routers have the same graph, and compute the same paths.

Page 6: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Dijkstra’s Shortest Path Algorithm for a Graph

Input: Graph (N,E) with N the set of nodes and E the set of edges

cvw link cost (cvw = 1 if (v,w) E, cvv = 0)

s source node.

Output: Dn cost of the least-cost path from node s to node n

M = {s};

for each n M Dn = csn;

while (M all nodes) do Find w M for which Dw = min{Dj ; j M};Add w to M;for each neighbor n of w and n M

Dn = min[ Dn, Dw + cwn ];Update route;

end for end whileend for

Page 7: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Link state routing: graphical illustration

a

b

c d

3 1

62

a

36

b

c

a’s view:

a

b

c

3 1b’s view: c d2

d’s view:

Collecting all views yield a global & complete view of the network!

Global view:

a

b

c d

1

6

c’s view:

2

Page 8: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Operation of a Link State Routing protocol

ReceivedLSAs

IP Routing Table

Dijkstra’s

Algorithm

Link StateDatabase

LSAs are flooded to other interfaces

Page 9: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Link State Routing: Properties

Each node requires complete topology information

Link state information must be flooded to all nodes

Guaranteed to converge

Page 10: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Distance Vector vs. Link State Routing

With distance vector routing, each node has information only about the next hop:

Node A: to reach F go to B Node B: to reach F go to D Node D: to reach F go to E Node E: go directly to F

Distance vector routing makespoor routing decisions if directions are not completelycorrect (e.g., because a node is down).

If parts of the directions incorrect, the routing may be incorrect until the routing algorithms has re-converged.

AA BB CC

DD EE FF

Page 11: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Distance Vector vs. Link State Routing

In link state routing, each node has a complete map of the topology

If a node fails, each node can calculate the new route

Difficulty: All nodes need to have a consistent view of the network

AA BB CC

DD EE FF

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

A B C

D E F

Page 12: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

• 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

Distance Vector vs. Link State Routing

• 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

Page 13: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Dynamic Routing Protocols

Open Shortest Path First

Page 14: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

OSPF = Open Shortest Path First The OSPF routing protocol is the most important link state

routing protocol on the Internet (another link state routing protocol is IS-IS (intermediate system to intermediate system)

The complexity of OSPF is significant RIP (RFC 2453 ~ 40 pages) OSPF (RFC 2328 ~ 250 pages)

History: 1989: RFC 1131 OSPF Version 1 1991: RFC1247 OSPF Version 2 1994: RFC 1583 OSPF Version 2 (revised) 1997: RFC 2178 OSPF Version 2 (revised) 1998: RFC 2328 OSPF Version 2 (current version)

OSPF

Page 15: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Features of OSPF

Provides authentication of routing messages

Enables load balancing by allowing traffic to be split evenly across routes with equal cost

Type-of-Service routing allows to setup different routes dependent on the TOS field

Supports subnetting

Supports multicasting

Allows hierarchical routing

Page 16: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Hierarchical OSPF

Page 17: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Hierarchical OSPF

Two-level hierarchy: local area, backbone. Link-state advertisements only in area each nodes has detailed area topology; only know

direction (shortest path) to nets in other areas.

Area border routers: “summarize” distances to nets in own area, advertise to other Area Border routers.

Backbone routers: run OSPF routing limited to backbone.

Page 18: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Example Network

Router IDs can be selected independent of interface addresses, but usually chosen to be the smallest interface address

3

4 2

5

1

1

32

• Link costs are called Metric

• Metric is in the range [0 , 216]

• Metric can be asymmetric

10.1.1.0 / 24

.1 .2 .2

10.1.1.1

10.1.4.0 / 24

10.1.2.0 / 24

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.4

10.1.7.0 / 24

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/ 24

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10.1.5.0/24

10.1

.8.0

/ 24

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.3 .5

.2

.3

.5

.5

.4

.4

.6

.6

10.1.1.2 10.1.4.4 10.1.7.6

10.1.2.3 10.1.5.5

Page 19: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Link State Advertisement (LSA)

The LSA of router 10.1.1.1 is as follows:

Link State ID: 10.1.1.1 = Router ID

Advertising Router: 10.1.1.1 = Router ID Number of links: 3 = 2 links plus router itself

Description of Link 1: Link ID = 10.1.1.2, Metric = 4 Description of Link 2: Link ID = 10.1.2.2, Metric = 3 Description of Link 3: Link ID = 10.1.1.1, Metric = 0

10.1.1.0 / 24

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10.1.1.2 10.1.4.4 10.1.7.6

10.1.2.3 10.1.5.5

4

3 2

Page 20: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Network and Link State Database

Each router has a database which contains the LSAs from all other routers

LS Type Link StateID Adv. Router Checksum LS SeqNo LS Age

Router-LSA 10.1.1.1 10.1.1.1 0x9b47 0x80000006 0

Router-LSA 10.1.1.2 10.1.1.2 0x219e 0x80000007 1618

Router-LSA 10.1.2.3 10.1.2.3 0x6b53 0x80000003 1712

Router-LSA 10.1.4.4 10.1.4.4 0xe39a 0x8000003a 20

Router-LSA 10.1.5.5 10.1.5.5 0xd2a6 0x80000038 18

Router-LSA 10.1.7.6 10.1.7.6 0x05c3 0x80000005 1680

10.1.1.0 / 24

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10.1.1.1

10.1.4.0 / 24

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.4

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.6

10.1.1.2 10.1.4.4 10.1.7.6

10.1.2.3 10.1.5.5

Page 21: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Link State Database

The collection of all LSAs is called the link-state database

Each router has an identical link-state database Useful for debugging: Each router has a complete description of the

network

If neighboring routers discover each other for the first time, they will exchange their link-state databases

The link-state databases are synchronized using reliable flooding

Page 22: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

OSPF Packet Format

OSPF MessageIP header

Body of OSPF MessageOSPF MessageHeader

Message TypeSpecific Data

LSA LSALSA ...

LSAHeader

LSAData

...

Destination IP: neighbor’s IP address or 224.0.0.5 (ALLSPFRouters) or 224.0.0.6 (AllDRouters)

TTL: set to 1 (in most cases)

OSPF packets are not carried as UDP payload!OSPF has its own IP protocol number: 89

Page 23: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

OSPF Packet Format

source router IP address

authentication

authentication

32 bits

version type message length

Area ID

checksum authentication type

Body of OSPF MessageOSPF MessageHeader

2: current version is OSPF V2

Message types:1: Hello (tests reachability)2: Database description3: Link Status request4: Link state update5: Link state acknowledgement

ID of the Area from which the packet originated

Standard IP checksum taken over entire packet

0: no authentication1: Cleartext password2: MD5 checksum(added to end packet)

Authentication passwd = 1: 64 cleartext password Authentication passwd = 2: 0x0000 (16 bits)

KeyID (8 bits) Length of MD5 checksum (8 bits) Nondecreasing sequence number (32 bits)

Prevents replay attacks

Page 24: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

OSPF LSA Format

Link State ID

link sequence number

advertising router

Link Age Link Type

checksum length

Link ID

Link Data

Link Type Metric#TOS metrics

LSA

LSAHeader

LSAData

Link ID

Link Data

Link Type Metric#TOS metrics

LSA Header

Link 1

Link 2

Page 25: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Discovery of Neighbors

Routers multicasts OSPF Hello packets on all OSPF-enabled interfaces.

If two routers share a link, they can become neighbors, and establish an adjacency

After becoming a neighbor, routers exchange their link state databases

OSPF Hello

OSPF Hello: I heard 10.1.10.2

10.1.10.1 10.1.10.2

Scenario:Router 10.1.10.2 restarts

Page 26: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Neighbor discovery and database synchronization

OSPF Hello

OSPF Hello: I heard 10.1.10.2

Database Description: Sequence = X

10.1.10.1 10.1.10.2

Database Description: Sequence = X, 5 LSA headers = Router-LSA, 10.1.10.1, 0x80000006 Router-LSA, 10.1.10.2, 0x80000007 Router-LSA, 10.1.10.3, 0x80000003 Router-LSA, 10.1.10.4, 0x8000003a Router-LSA, 10.1.10.5, 0x80000038 Router-LSA, 10.1.10.6, 0x80000005

Database Description: Sequence = X+1, 1 LSA header= Router-LSA, 10.1.10.2, 0x80000005

Database Description: Sequence = X+1

Sends empty database description

Scenario:Router 10.1.10.2 restarts

Discovery of adjacency

Sends database description. (description only contains LSA headers)

Database description of 10.1.10.2

Acknowledges receipt of description

After neighbors are discovered the nodes exchange their databases

Page 27: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Regular LSA exchanges

10.1.10.2 explicitly requests each LSA from 10.1.10.1

10.1.10.1 sends requested LSAs

10.1.10.1 10.1.10.2

Link State Request packets, LSAs =

Router-LSA, 10.1.10.1,

Router-LSA, 10.1.10.2,

Router-LSA, 10.1.10.3,

Router-LSA, 10.1.10.4,

Router-LSA, 10.1.10.5,

Router-LSA, 10.1.10.6,

Link State Update Packet, LSAs =

Router-LSA, 10.1.10.1, 0x80000006

Router-LSA, 10.1.10.2, 0x80000007

Router-LSA, 10.1.10.3, 0x80000003

Router-LSA, 10.1.10.4, 0x8000003a

Router-LSA, 10.1.10.5, 0x80000038

Router-LSA, 10.1.10.6, 0x80000005

Page 28: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Dissemination of LSA-Update

A router sends and refloods LSA-Updates, whenever the topology or link cost changes. (If a received LSA does not contain new information, the router will not flood the packet)

Exception: Infrequently (every 30 minutes), a router will flood LSAs even if there are not new changes.

Acknowledgements of LSA-updates: explicit ACK, or implicit via reception of an LSA-Update

Question: If a new node comes up, it could build the database from regular LSA-Updates (rather than exchange of database description). What role do the database description packets play?

Page 29: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Dynamic Routing Protocols (Inter-domain)

Border Gateway Protocol

Page 30: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

BGP Quick View

BGP = Border Gateway Protocol . Currently in version 4, specified in RFC 1771. (~ 60 pages)

Note: In the context of BGP, a gateway is nothing else but an IP router that connects autonomous systems.

Interdomain routing protocol for routing between autonomous systems

Uses TCP to establish a BGP session and to send routing messages over the BGP session

BGP is a path vector protocol. Routing messages in BGP contain complete routes.

Network administrators can specify routing policies

Page 31: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

BGP Policy-based Routing

Each node is assigned an AS number (ASN)

BGP’s goal is to find any AS-path (not an optimal one). Since the internals of the AS are never revealed, finding an optimal path is not feasible.

Network administrator sets BGP’s policies to determine the best path to reach a destination network.

Page 32: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

How Many ASNs are there today?

Thanks to Geoff Huston. http://bgp.potaroo.net on October 9, 2005

20,570

14,588origin only (notransit)

Page 33: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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 …

ARDs versus ASes

Page 34: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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

Page 35: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

ARDs and ASes: Summary

Most ARDs have no ASN (statically routed at Internet edge)

Some unrelated ARDs share the same ASN (RFC 2270)

Some ARDs are implemented with multiple ASNs (example: Worldcom)

ASes are just an implementation detail of Inter-domain routing

Page 36: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

How many prefixes today?

Thanks to Geoff Huston. http://bgp.potaroo.net on October 9, 2005

221,002

33.3%

23%

IPv4 Address space covered

Page 37: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Policy-Based vs. Distance-Based Routing?

ISP1

ISP2

ISP3

Cust1

Cust2Cust3

Host 1

Host 2

Minimizing “hop count” can violate commercial relationships thatconstrain inter-domain routing.

YES

NO

Thanks to Tim Griffin http://www.cl.cam.ac.uk/users/tgg22

Page 38: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Customer versus Provider

Customer pays provider for access to the Internet

provider

customer

IP trafficprovider customer

Page 39: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Regional ISP1

Regional ISP2

Regional ISP3

Cust1Cust3 Cust2

National ISP1

National ISP2

YES

NO

Shortest path routing is not compatible with commercial relations

Why not minimize “AS hop Count”?

Page 40: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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

The “Peering” Relationship

Page 41: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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

peer peer

customerprovider

Peering Provides Shortcuts

Page 42: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Peering Wars

Reduces upstream transit costs Can increase end-to-end

performance May be the only way to connect

your customers to some part of the Internet (“Tier 1”)

You would rather have customers

Peers are usually your competition

Peering relationships may require periodic renegotiation

Peering struggles are by far the most contentious issues in the ISP world!

Peering agreements are often confidential.

Peer Don’t Peer

Page 43: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

BGP = RFC 1771

+ “optional” extensionsRFC 1997 (communities) RFC 2439 (damping) RFC 2796 (reflection) RFC3065 (confederation) …

+ routing policy configurationlanguages (vendor-specific)

+ Current Best Practices in management of Interdomain Routing

BGP was not DESIGNED. It EVOLVED.

The Border Gateway Protocol (BGP)

Page 44: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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

Page 45: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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

Page 46: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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

ASPATH Attribute

Page 47: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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

Exporting internalstate would dramatically increase global instability and amount of routingstate

AS 4

AS 3

AS 2

AS 1

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

Duh!

Shorter Doesn’t Always Mean Shorter

Page 48: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Thanks to Han Zheng

Routing Example 1

Page 49: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Thanks to Han Zheng

Routing Example 2

Page 50: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Tweak Tweak Tweak (TE)

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 more control over outbound traffic

Page 51: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Forces outbound traffic to take primary link, unless link is down.

AS 1

primary link backup link

Set Local Pref = 100for all routes from AS 1 AS 65000

Set Local Pref = 50for all routes from AS 1

Backup Links with Local Preference (Outbound Traffic)

Page 52: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Forces outbound traffic to take primary link, unless link is down.

AS 1

primary link backup link

Set Local Pref = 100for all routes from AS 1

AS 2

Set Local Pref = 50for all routes from AS 3

AS 3provider provider

Multihomed Backups (Outbound Traffic)

Page 53: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Prepending will (usually) force inbound traffic from AS 1to take primary linkAS 1

192.0.2.0/24ASPATH = 2 2 2

customerAS 2

provider

192.0.2.0/24

backupprimary

192.0.2.0/24ASPATH = 2

Yes, this is a Glorious Hack …

Shedding Inbound Traffic with ASPATH Prepending

Page 54: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

AS 1

192.0.2.0/24ASPATH = 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

… But Padding Does Not Always Work

Page 55: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

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

COMMUNITY Attribute to the Rescue!

Page 56: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

BGP Issues - What is a BGP Wedgie?

BGP policies make sense locally Interaction of local policies allows

multiple stable routings Some routings are consistent with

intended policies, and some are not If an unintended routing is

installed (BGP is “wedged”), then manual intervention is needed to change to an intended routing

When an unintended routing is installed, no single group of network operators has enough knowledge to debug the problem

¾ wedgie

Full wedgie

Page 57: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

Dynamic Routing Protocols: Summary

Dynamic routing protocols: RIP, OSPF, BGP

RIP uses distance vector algorithm, and converges slow (the count-to-infinity problem)

OSPF uses link state algorithm, and converges fast. But it is more complicated than RIP.

Both RIP and OSPF finds lowest-cost path.

BGP uses path vector algorithm, and its path selection algorithm is complicated, and is influenced by policies.

BGP has its own problems see WIDGI by Tim Griffin

Page 58: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

More Readings (Optional)

BGP Wedgies: Bad Routing Policy Interactions that Cannot be Debugged

JI’s Intro to interdomain routing.

"Interdomain Setting of PlanetLab Nodes." PlanetLab Meeting, May 14, 2004.

Understanding the Border Gateway Protocol (BGP) ICNP 2002 Tutorial Session

Page 59: COMS W4995-1 COMS W4995-1 Lecture 6. Dynamic routing protocols II 1.Dynamic Routing Protocols: Link State Routing 2.Intra-Domain Routing Protocols: OSPF

References

[VGE1996, VGE2000] Persistent Route Oscillations in Inter-Domain Routing. Kannan Varadhan, Ramesh Govindan, and Deborah Estrin. Computer Networks, Jan. 2000. (Also USC Tech Report, Feb. 1996)

[GW1999] An Analysis of BGP Convergence Properties. Timothy G. Griffin, Gordon Wilfong. SIGCOMM 1999

[GSW1999] Policy Disputes in Path Vector Protocols. Timothy G. Griffin, F. Bruce Shepherd, Gordon Wilfong. ICNP 1999

[GW2001] A Safe Path Vector Protocol. Timothy G. Griffin, Gordon Wilfong. INFOCOM 2001

[GR2000] Stable Internet Routing without Global Coordination. Lixin Gao, Jennifer Rexford. SIGMETRICS 2000

[GGR2001] Inherently safe backup routing with BGP. Lixin Gao, Timothy G. Griffin, Jennifer Rexford. INFOCOM 2001

– [GW2002a] On the Correctness of IBGP Configurations. Griffin and Wilfong.SIGCOMM 2002.

– [GW2002b] An Analysis of the MED oscillation Problem. Griffin and Wilfong. ICNP 2002.