Internet Routing Protocols Lecture 01

Internet Routing Protocols Lecture 01 Advanced Systems Topics Lent Term, 2008 Timothy G. Griffin Computer Lab Cambridge UK

Common View of the Telco Network

Brick

1

Common View of the IP Network (Layer 3)

IP routing is the little bit-o-smarts left in the IP network layer • Dynamic Routing protocols are used to implement and maintain connectivity in the Internet. • Which protocols are used? • How do they work? • How do they behave? • What are some of the fundamental tradeoffs in the design space of routing protocols?

2

Outline

• Lecture 1 : Routing vs. Forwarding. Internet routing architecture • Lecture 2: Intra-domain routing with “shortest paths”. Link-state vs. distancevector. • Lecture 3 : Inter-domain routing. The Border Gateway Protocol (BGP) • Lecture 4 : BGP continued • Lecture 5 : BGP dynamics • Lecture 6 : BGP routing anomalies

3

4

GARR-B

5

wiscnet.net UW-Superior

Rice Lake Rhinelander

UW-Stout

Stiles Jct.

Wausau

Marshfield

UW-River Falls UW-Eau Claire

Qwest and Other Provider(s)

(Su

Clintonville

) mmer '02

UW-Stevens Point UW-Green Bay (Summer

'02)

Fox Valley TC um

(Summer '03)

(S

UW-Oshkosh

m er '02

UW-La Crosse

)

La Crosse

Internet 2 & Qwest

Portage

Dodgeville

UW-Platteville

Genuity

UW-Madison (Summer '03)

UW-Milwaukee

UW-Whitewater

UW-Parkside

(Winter '02)

GO BUCKY! Gigabit Ethernet OC-12 (622Mbps) OC-3 (155Mbps) DS-3 (45Mbps) T1 (1.5Mbps)

Chicago ! ! ! ! ! !

Peering - Public and Private Commodity Internet Transit Internet2 Merit and Other State Networks National Education Network Regional Research Peers

(Win

ter

'02)

Chicago - 2 (Winter '02)

Chicago - 1

6

WorldCom (UUNet)

Internet Initiative Japan (IIJ)

7

Telstra international

AT&T IP Backbone

Anchorage, AK

Year end 2001

Seattle

Spokane

Portland

Portland Manchester

Worcester

Minneapolis

R

St. Paul Milwaukee

Glenvie w

Rochester

Madison Rolling Meadows

Des Moines

Sacramento

R

San San Francisco Francisco

R

Salt Lake City

Kansas City

Redwood City

Los Angeles

Albuquerque

Sherman Oaks

San Bernardino

LA-Airport Blvd Garden a

Chicago

Denver

Oklahom a City

Phil

Arlington Norfolk

Louisville

Nashville

Richmond

R

Rochelle Pk Hamilton Square

R

Baltimore Newark Bohemia

Florissant

Tulsa

Cambridge Framingham Providence Stamford Providence Bridgepor t New Brunswick White Plains Cedar Knolls

NYC

Wash. DC

Silver Springs

Camden, NJ

Freehold

R

NYCBdwy

Raleigh

Greensboro Charlotte

Little Rock Memphis

Phoenix

Gateway Node Ft. Worth

Birmingham Norcross

Columbia Dunwoody

Atlanta

Dallas

Backbone Node

R

R R

Harrisburg

Dayto Columbus n Cincinnati

St Louis

Anaheim

San Diego

R

Indianapolis

Hartford Wayne

Pittsburgh Cleveland Akron

South Bend

Albany

Syracuse

Buffalo

Detroit Plymouth

Chicago

Colorado Springs

Honolulu

Oak Brook

Omaha

Las Vegas

R San Jose

Oakland

Davenport

R

Grand Rapids Birmingham

Remote GSR Access Router

Jacksonville New Orleans

Austin

Remote Access Router

Orlando

Houston San Antonio

N X DS3 N X OC3 N X OC12 N X OC48 NX OC192

R

Note: Connectivity and nodes shown are targeted for deployment; actual deployment may vary. Maps should not be used to predict service availability.

Tampa

R

Ft. Lauderdale

W. Palm Beach Ft. Lauderdale Ojus Miami San Juan PR

Rev. 6-4-01

8

Architecture of Dynamic Routing IGP

BGP

AS 1 IGP = Interior Gateway Protocol. Metric based. OSPF, IS-IS, RIP, EIGRP (cisco)

IGP

AS 2

EGP = Exterior Gateway Protocol. Policy Based. Only one: BGP The Routing Domain of BGP is the entire Internet

How many ASN are used today? http://bgp.potaroo.net

Jan 28. 2008

9

Routers Talking to Routers (The “control plane”) Routing info

Routing info

• Routing computation is distributed among routers within a routing domain • Computation of best next hop based on routing information is the most CPU/memory intensive task on a router • Routing messages are usually not routed, but exchanged via layer 2 between physically adjacent routers (internal BGP and multi-hop external BGP are exceptions)

Technology of Distributed Routing Link State • 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

Vectoring • 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 nexthop choices • Does not require any notion of distance • Does not require uniform policies at all routers • Examples: RIP, BGP

10

The Gang of Four Link State

IGP

OSPF IS-IS

Vectoring

RIP BGP

EGP

The standard model data received

data sent

Application

Application

Presentation

Presentation

Session

Session

Transport

Transport

Network

Network

DataLink

DataLink

Physical

Physical

22

11

All Hail the IP Datagram! H E A D E R D A T A

0 1 2 3 01234567890123456789012345678901 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL | Service Type | Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ : : | + | + | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

... up to 65,515 octets of data ...

shaded fields little-used today

1981, RFC 791

: : | + | + | 23

IP Hour Glass Networking Applications Remote Access Voice e-stuff email file transfer Multimedia TCP

Web

VPN

IP

Minimalist network layer

Frame X.25

SONET

ATM DWDM

Ethernet FDDI

Networking Technologies 24

12

Best Effort, Connectionless, Connectivity IP traffic 135.207.49.8

192.0.2.153

The is the fundamental service provided by Internet Service Providers (ISPs) All other IP services depend on connectivity: DNS, email, VPNs, Web Hosting, …

IP is a Network Layer Protocol

Application

Separate physical networks glued together into one Application logical network

Router

Presentation

Presentation

Session

Session

Transport

Transport Network

Network

Network

DataLink 1

DataLink 1 DataLink 2

DataLink 2

Physical 1

Physical 1 Physical 2

Physical 2

Medium 1

Medium 2

26

13

Hosts, Networks, and Routers Host 7 Host 1 Network A Host 2 Host 1 Router

Network C

Network B

Unique IP Address = Network Number + Host Number Host 12

Host 2

27

Actually, IP addresses Identify Interfaces Host 7 Host 1 Network C, Host 3

Network A

Host 1

Host 2

Network A, Host 3 Network B, Host 77

Network B

Host 12

Host 2

Network C

Machines can have more than one IP address. 28 All routers do!

14

IP Forwarding Table Destination Net A Net B Net C, Host 3 Net C

A destination is usually a network. May also be a host, or a “gateway of last resort” (default)

Next Hop

Interface

Router 1 Direct Router 2 Router 1

INT 7 INT 4 INT 3 INT 7

The next hop is either a directly connected network or a router on a directly connected network

A physical interface

29

IP Forwarding Process 1. Remove a packet from an input queue

2. Check for sanity, decrement TTL field

4. Place packet on correct output queue

Forwarding Process If queues get full, just drop packets!

3. Match packet’s destination to a table entry

If queues get full, just drop packets!

IP Forwarding Table Router 30

15

IPv4 Addresses are 32 Bit Values 11111111 00010001 10000111 00000000 255

17

134

0

255.17.134.0 Dotted quad notation IPv6 addresses have 128 bits 31

IP Addresses come in two parts 11111111 00010001 10000111 00000000 Network Number

Host Number

Where is this dividing line? Well, that depends .... 32

16

Classful Addresses Class A

0nnnnnnn hhhhhhhh hhhhhhhh hhhhhhhh

Class B

10nnnnnn nnnnnnnn hhhhhhhh hhhhhhhh

Class C

110nnnnn nnnnnnnn nnnnnnnn hhhhhhhh n = network address bit

h = host identifier bit

Leads to a rigid, flat, inefficient use of address space …

33

RFC 1519: Classless Inter-Domain Routing (CIDR) Use two 32 bit numbers to represent a network. Network number = IP address + Mask

IP Address : 12.4.0.0 Address

Mask

IP Mask: 255.254.0.0

00001100 00000100 00000000 00000000 11111111 11111110 00000000 00000000 Network Prefix

for hosts

Usually written as 12.4.0.0/15

34

17

Which IP Addresses are Covered by a Prefix?

12.5.9.16 is covered by prefix 12.4.0.0/15 12.5.9.16

12.4.0.0/15

00001100 00000101 00001001 00010000 00001100 00000100 00000000 00000000 11111111 11111110 00000000 00000000

12.7.9.16

00001100 00000111 00001001 00010000

12.7.9.16 is not covered by prefix 12.4.0.0/15

CIDR allows Hierarchy in Addressing 12.0.0.0/16 12.1.0.0/16 12.2.0.0/16 12.3.0.0/16 12.0.0.0/8

: : : 12.253.0.0/16 12.254.0.0/16

12.3.0.0/24 12.3.1.0/24

: :

: : :

12.3.254.0/24 12.253.0.0/19 12.253.32.0/19 12.253.64.0/19 12.253.96.0/19 12.253.128.0/19 12.253.160.0/19 12.253.192.0/19 36

18

Classless Forwarding

Destination =12.5.9.16 ------------------------------payload

OK better

Prefix

Next Hop

Interface

0.0.0.0/0

10.14.11.33

ATM 5/0/9

12.0.0.0/8

10.14.22.19

ATM 5/0/8

even better

12.4.0.0/15 10.1.3.77

Ethernet 0/1/3

best!

12.5.8.0/23 attached

Serial 1/0/7

IP Forwarding Table

How Are Forwarding Tables Populated to implement Routing? Statically Administrator manually configures forwarding table entries + More control + Not restricted to destination-based forwarding - Doesn’t scale - Slow to adapt to network failures

Dynamically Routers exchange network reachability information using ROUTING PROTOCOLS. Routers use this to compute best routes + Can rapidly adapt to changes in network topology + Can be made to scale well - Complex distributed algorithms - Consume CPU, Bandwidth, Memory - Debugging can be difficult - Current protocols are destination-based

In practice : a mix of these. Static routing mostly at the “edge”

38

19

Routing vs. Forwarding Net Default to upstream router

A

A B C D E default

B R R2 R

R4

D

R3 R

R1

Nxt Hop R1 Direct R3 R1 R3 R1

C

R5 Net

E

Forwarding: determine next hop Routing: establish end-to-end paths

A B C D E default

Net

Nxt Hop

A B C D E default

R2 R2 Direct R5 R5 R2

Nxt Hop Forwarding always works

R4 R3 R3 R4 Direct R4

Routing can be badly broken 39

Happy Packets: The Internet Does Not Exist Only to Populated Routing Tables

BGP

RIP Process

BGP Process

RIP Routing tables

BGP Routing tables

OSPF Process OSPF Routing tables

RIP Domain

OSPF Domain

Forwarding Table Manager

Forwarding Table 40

20

Before We Go Any Further … IP ROUTING PROTOCOLS DO NOT DYNAMICALLY ROUTE AROUND NETWORK CONGESTION • IP traffic can be very bursty • Dynamic adjustments in routing typically operate more slowly than fluctuations in traffic load • Dynamically adapting routing to account for traffic load can lead to wild, unstable oscillations of routing system

Next Lecture: Shortest Path Routing B

Dest. Nxt Hop 100

B

C

C D

C D

E

D

55

20

100

20

35

A

35

35

C

20

20 20

10 20

20 D

10

10

30 E

10

This is what IS-IS, OSPF, and RIP do, more or less.

42

21