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Internet Routing 11/11/2009 Admin. Assignment 3 Recap Distance vector protocol Link state protocol Outline Routing in the Internet overview Routing in the Internet The Global Internet consists of Autonomous Systems (AS) interconnected with each other

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Internet routing l.jpg

Internet Routing

11/11/2009


Admin l.jpg
Admin.

  • Assignment 3


Recap l.jpg
Recap

  • Distance vector protocol

  • Link state protocol


Slide4 l.jpg

Outline

  • Routing in the Internet

    • overview


Slide5 l.jpg

Routing in the Internet

  • The Global Internet consists of Autonomous Systems (AS) interconnected with each other

    • An AS is identified by an AS Number (ASN), e.g. Yale ASN is 29

    • try %whois -h whois.arin.net “a Yale“




Slide8 l.jpg

Routing with AS

  • Internet routing is divided into intra-AS routing and inter-AS routing

  • Intra-AS

    • A protocol running insides an AS is called an Interior Gateway Protocol (IGP)

      • RIP: Routing Information Protocol

      • E/IGRP: Interior Gateway Routing Protocol (Cisco)

      • OSPF: Open Shortest Path First

      • IS-IS: very similar to OSPF (or should we say OSPF is very similar to IS-IS?)

  • Inter-AS

    • a protocol runs among AS’s is also called an Exterior Gateway Protocol (EGP)

    • for global connectivity, a single interdomain routing protocol


Intra as and inter as example l.jpg

b

c

a

a

C

b

B

b

c

a

d

A

A.a

A.c

C.b

B.a

Intra-AS and Inter-AS: Example

border (exterior gateway) routers

interior (gateway) routers


Slide10 l.jpg

AS C

(RIP intra

AS B

(OSPF intra

AS A

(OSPF intra

routing)

routing)

routing)

a

b

Routing in the Internet: Example

Gateway routers participate in intradomain to learn internal routes.

gateway routers of same AS share learned externalroutes using iBGP.

i

e

eBGP

iBGP

g

Gateway routers of diff. AS’s exchange routes using eBGP

f

d

h

c


Slide11 l.jpg

RIP process

OSPF process

BGP process

RIP routing table

BGP routing table

Many Routing Processes on a Single Router

BGP

OSPF Routing table

OS kernel

RIP

domain

OSPF

domain

Forwarding Table Manager

Forwarding Table


Intra as and inter as routing l.jpg

inter-AS

routing

between

A and B

b

c

a

a

C

b

B

b

a

c

d

Host

h1

A

A.a

A.c

C.b

B.a

Intra-AS and Inter-AS Routing

border (exterior gateway) routers

Host

h2

intra-AS routing

within AS B

Intra-AS routing

within AS A

interior (gateway) routers


Slide13 l.jpg

Why Partition into Intra- and Inter-AS Routing?

  • This partition allows ASes flexibility to choose their own intra-AS routing protocols

    • autonomy

  • By aggregating many destinations inside an AS into a single destination in interdomain routing, it improves scalability

    • the partition is a type of hierarchical routing

    • hierarchical routing improves scalability: only a small number of routers are involved with outside


Yale internet connectivity l.jpg

Internet2

AT&T

Qwest

Yale Internet Connectivity

Yale

default routes 0.0.0.0/0

pointing to provider.

132.130.0.0/16

128.36.0.0/16


Slide15 l.jpg

Outline

  • Routing in the Internet

    • overview

    • BGP



Slide17 l.jpg

Internet Interdomain Routing: BGP

  • BGP (Border Gateway Protocol):the de facto standard

  • Path Vector protocol:

    • similar to Distance Vector protocol

    • a border gateway sends to a neighbor entire path (i.e., a sequence of ASes) to a destination, e.g.,

      • gateway X sends to neighbor N its path to dest. Z:

        path (X,Z) = X,Y1,Y2,Y3,…,Z

    • if N selects path(X, Z) advertised by X, then: path (N,Z) = N, path (X,Z)

Z

N

X


Slide18 l.jpg

BGP Operations (Simplified)

Establish session on

TCP port 179

AS1

BGP session

Exchange all

active routes

AS2

while (connection is ALIVE) exchange UPDATE messageselect best available route

if route changes, export to neigh.

Exchange incremental

updates


Bgp messages l.jpg
BGP Messages

  • Four types of messages

    • OPEN: opens TCP connection to peer and authenticates sender

    • UPDATE: advertises new path (or withdraws old)

    • KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request

    • NOTIFICATION: reports errors in previous msg; also used to close connection


Slide20 l.jpg

Why Path Vector?

  • Path vector prevents counting-to-infinity problem

  • Path vector allows an AS to define local policies on the ASes of a given path


Bgp routing decision process l.jpg
BGP Routing Decision Process

route selection policy: rank paths

select best path

routing cache

export policy: which paths export to which neighbors

export path to neighbors


Bgp route selection export policies l.jpg

Internet2

AT&T

Qwest

BGP Route Selection/Export Policies

  • Route selection

    • algorithm to select the best currently available route from routing cache (routing information base)

    • typical (Cisco) route selection policy

      • highest local pref

      • shortest AS path length

      • prefer eBGP over iBGP

  • Route export policy:

    • since the export of a path to a neighbor is an indication that the AS is willing to transport traffic for the neighbor, an AS may not export some routes to some neighbors (more later)

Yale


Routing example l.jpg

AS B

(OSPF intra

AS D

AS A

(OSPF)

routing)

Routing: Example

d

E

d->a2: I can reach

hosts in D; my path: D

Export to E: i->e: I can reach hosts in D; path: IBCD

a2

a2->a1: I can reach

hosts in D; path: D

a1->i: I can reach hosts in D; my path: AD

a1

No Exportto F

i

F

AS C

choose BCD using i2

i2->i: I can reach hosts in D; path: BCD

b->i: I can reach hosts

in D; my path: BCD

i2

b

b->i2: I can reach hosts

in D; my path: BCD

AS I



Outline l.jpg
Outline

  • Admin.

  • BGP

    • Overview

    • BGP instability and stability condition


Slide26 l.jpg

2 1 0

2 0

2

4

0

preferred

3 2 0

3 0

1 3 0

1 0

3

3

1

less

preferred

BGP Policy Interactions

The BAD GADGET example:

- 0 is the destination

- the route selection policy of each AS is to prefer its counter clock-wise neighbor

Policy interaction causes routing instability !


Understanding instability p graph l.jpg

Nodes in P-graph are feasible paths

A directed edge from path N1P1 to P1

intuition: to let N1 choose N1P1, P1 must be chosen and exported to N1

A directed edge from a lower ranked path to a higher ranked path

intuition: the higher ranked path should be considered first

210

20

2

320

30

130

10

0

3

1

2 1 0

3 2 0

1 3 0

3 0

2 0

1 0

Understanding Instability: P-Graph

P-graph


Partial order graph and convergence l.jpg

If the P-graph has no loop, then BGP policy converges.

intuition: choose the node (i.e., a path) from the partial order graph with no out-going edge, choose the path, remove the path and all other lowered ranked paths of the same node; remove nodes if suffix removed; continue

Example: suppose we swap the order of 30 and 320

210

20

2

30

320

130

10

0

3

1

2 1 0

3 0

1 3 0

3 2 0

2 0

1 0

Partial Order Graph and Convergence


Slide29 l.jpg

Partial Order Graph and BGP Convergence

Preview: A reason we do not often see instability in the Internet is that: the current Internet ISP economy implies no loop in P-graph !


Slide30 l.jpg

Customer provider relationship

a provider is an AS that connects the customer to the rest of the Internet

customer pays the provider for the transit service

e.g., Yale is a customer of AT&T and QWEST

Peer-to-peer relationship

mutually agree to exchange traffic between their respective customers

there is no payment between peers

peer

peer

Internet Economy: Two Types of Business Relationship

provider

provider

customer

provider to

customer


Implication of business relationship on policies l.jpg
Implication of Business Relationship on Policies

  • Route selection (ranking) policy:

    • the typical route selection policy is to prefer customers over peers/providers to reach a destination, i.e., Customer > pEer/Provider


Slide32 l.jpg

provider

provider

provider

customer

provider

peer

customer

customer

customer

peer

peer

peer

Typical Export Policies

case 1: routes learned from customer

routes learned from acustomer are sent toall other neighbors

case 2: routes learned from provider

case 3: routes learned from peer

routes learned from a peer are sent only to customers

routes learned from a provider are sentonly to customers


Slide33 l.jpg

A advertises to C paths to P1 and P2

A advertises path to C, but not P2

A advertises path to C, but not P1

IP traffic

Typical Export -> No-Valley Routing

P1

P2

A learns

paths toC, P1, P2

A

C

Suppose P1 and P2 are providers of A; A is a provider of C


Yale internet connectivity34 l.jpg

Internet2

AT&T

Qwest

Yale Internet Connectivity

Yale

default routes 0.0.0.0/0

pointing to provider.

132.130.0.0/16

128.36.0.0/16


Typical export policies imply patterns of routes l.jpg
Typical Export Policies Imply Patterns of Routes

  • Assume a BGP path SABCD to destination AS D. Consider the business relationship between each pair:

  • Three types of business relationships:

    • PC (provider-customer)

    • CP (customer-provider)

    • PP (peer-peer)

S A B C D