An Overview of the IP Network Layer.ppt

1

An Overview of the IP Network Layer

Tim Griffin [email protected]

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

December 1, 2000

mailto:[email protected]

2

Goal

Today’s class should give you a basic understandingof how Internet Protocol (IP) data packets find their way from one end of the Internet to the other.

Understand connectivity in the IP world

This course will not say much about the applications that exploit this connectivity…

3

Outline• What is special about the IP network layer?

• What is an IP Routing Protocol?

• How is Addressing Implemented in IPv4?

• Routing inside an ISP. – RIP– OSPF

• Routing In the Global Internet – How do ISPs exchange routing information?– BGP

4

Networking Technologies

Circuit Based Packet Based

Virtual Circuits Connectionless

TDM TelephonySONET/SDH

Frame RelayATM

IP

X.25

CLNP (ISO)SNA (IBM)Appletalk IPX (Novell)

DWDM

Connection Oriented

(variable rate, store-and-forward)(constant rate)

5

Connection Oriented vs. Connectionless

Connection Oriented

Connectionless

Send and Pray

• Connection set up. Signaling reserves resources along the end-to-end path

• Traffic flows • Connection torn down and resources freed

6

Network Heterogeneity

SONET

ATM

IP

TCP

Circuits

Virtual Circuit

Connectionless

Virtual Connections

7

0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Version| IHL | Service Type | Total Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Identification |Flags| Fragment Offset | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Time to Live | Protocol | Header Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Source Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Destination Address | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options | Padding | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

IP Datagram

HEADER

DATA

1981, RFC 791

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

::|+|+|

::|+|+|

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

shaded fields little-used today

8

Famous Members of the IP Protocol Family

IP (and ICMP)

TCP UDP

FTP HTTPTelnet RTPSNMPDNS

9

IP is a Network Layer Protocol

Physical 1

Network

DataLink 1

Transport

Application

Session

Presentation

Network

Physical 1

DataLink 1

Physical 2

DataLink 2

Router

Physical 2

Network

DataLink 2

Transport

Application

Session

Presentation

Medium 1 Medium 2

Separate physical networks glued together into one logical network

10

Encapsulation Example

Ethernet Header

IP Header

TCP Header

HTTP Header

….

HTTP Data ….

An Ethernet segment transmitting HTTP data.

11

IP Hour Glass

IP

Networking Technologies

Networking Applications

Frame ATM

DWDMSONET

email

Webfile transfer

Ethernet

FDDI

Multimedia

X.25

HOST

Router

Link

Remote Access Voice

VPN

Minimalist network layer

TCP

e-stuff

IP is the mother of all disruptive technologies!

12

QoS

Quality of Service vs.Quantity of Service

13

Traditional Telco Network

The Brick. An internationally recognized symbol of dumbness

14

IP Network

The user community now owns the compute power and software.

15

How Dumb Can a Network Get?

• In the Internet, intelligence is in Hosts

• IP is connectionless, best effort.

• Routing protocols today provide only connectivity and supports only one type of service : best effort datagram

• Only real smarts left in the network layer : Dynamic Routing Protocols, which provide end-to-end connectivity

The technical part of the “Bell-head vs. Net-head” divide.

16

• “We reject kings, presidents, and voting. We believe in

rough consensus and running code.” --- Dave Clark

• Publishes “Requests for Comments” or RFCs

– Some are designated as Internet Standards

• Working Groups

– Internet Drafts

• http://www.ietf.org

Internet Engineering Task Force (IETF)

You must visit this site!

The cultural part of the “Bell-head vs. Net-head” divide.

17

Hosts, Networks, and Routers

Network A

Network B Network CRouter

Host 1

Host 2

Host 7

Host 1

Host 12 Host 2

Unique IP Address = Network Number + Host Number

18

Actually, IP addresses Identify Interfaces

Network A

Network B Network C

Host 1

Host 2

Host 7

Host 1

Host 12 Host 2

Network C, Host 3

Network A, Host 3

Network B, Host 77

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

19

IP Forwarding Table

Destination Next Hop Interface

Net ANet BNet C, Host 3

Router 1DirectRouter 2Router 1

INT 7

INT 7INT 3INT 4

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

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

A physical interface

Net C

20

IP Forwarding Process

Forwarding Process

IP Forwarding Table Router

1. Remove a packet from an input queue

3. Match packet’s destination to a table entry

2. Check for sanity, decrement TTL field

4. Place packet on correct output queue

If queuesget full, just

drop packets!

If queuesget full, just

drop packets!

21

Route vs. Route • Route sometimes

means the end-to-

end path traversed

by data traffic

• Route often means

the “destination +

next hop” entry in an

IP forwarding table.

London

New York

22

Routing = Implementing End-to-End Paths with Next Hop Forwarding Tables

R

R

RA

B

C

D

R1

R2

R3

R4 R5

EDest. Nxt Hop

R4R3R3R4DirectR4

Dest. Nxt Hop

A B C D Edefault

R2R2DirectR5R5R2

Dest. Nxt Hop

A B C D Edefault

R1DirectR3R1R3R1

Default toupstreamrouter

A B C D Edefault

23

Something Fishy Here?

A

B

C

The next-hop forwarding paradigmdoes not allow router R to choosea route to A based on who originatedthe traffic, B or C.

R

24

Bad Things Can Happen

R1 R2

Dest. Nxt Hop

A:

R2:

Dest. Nxt Hop

A:

R1:

Loops are routingproblems, not forwarding problems

25






26

How are Forwarding Tables Populated?

Statically DynamicallyRouters exchange information using ROUTING PROTOCOLS that compute “best” routes

Administrator manually configurestable entries

In practice : a mix of these….

+ More control+ Not restricted to destination-based forwarding - Doesn’t scale- Slow to adapt to network failures

+ Can rapidly adapt to changes in network topology+ Can be made to scale well- Complex distributed algorithms- Consume CPU, Bandwidth, Memory- Debugging can be hell- Current protocols are destination-based

27

Dynamically route around network congestion? NO!

• IP traffic is very bursty

• Dynamic adjustments in routing typically operate more slowly than fluctuations in traffic load

• Attempt to adapt routing to account for load can lead to wild, unstable oscillations of routing system

28

Terminology Alert!

Static Routing

Dynamic Routing

Adaptive Routing

Possible Bell Head MeaningNet Head Meaning

Manual provisioningof routing table entries

Nailed up route

Automatically route around network failures.Alternate paths selecteddynamically.

Automatically route around network failures and congestion. Alternate paths often predetermined. May vary with time of day.

Automatically route around network failures, and congestion.

Use of these terms may vary ….

Net HeadTerm

????

29

Architecture of Routing Protocols

IGP

IGP IGP

EGPEGP

EGP

AS 701

AS 6431 AS 7018

Interior Gateway Protocols (IGP) : inside autonomous systems

Exterior Gateway Protocols (EGP) : between autonomous systems

OSPF, IS-IS,RIP, EIGRP, ... BGPMetric Based Policy Based

UUNet

AT&T Common BackboneAT&T Research

30

The Most Common Routing Protocols

Routing protocols exchange network reachability information between routers.

IP (and ICMP)

TCP UDP

BGP RIP

OSPF EIGRPIS-IS

Cisco proprietary

31

What is a Routing Process?

Routing Process

Protocol-Specific Routing Table

import informationfrom other routers

exportinformationto other routers

IP Forwarding Table Router

Manual configuration

OS kernel

32

Many routing processes can run on a single router

IP Forwarding TableOSPFDomain

RIPDomain

BGP

OS kernel

OSPF Process

OSPF Routing tables

RIP Process

RIP Routing tables

BGP Process

BGP Routing tables

33

Routing Configuration for IP Routers

• Configure List of Interfaces, with IP addresses and subnet masks

• List of Neighbors

• Link weights

• Define static routes

• Import routes from other routing domains

• Configure protocol-specific, vendor-specific parameters for each routing protocol

Expressed in low-level configuration files

34






35

IPv4 Implementation of Addresses

Thirty Two Bits: 0 8 1

624

11111111 00010001 10000111 00000000

255 013417

255.17.134.0

Dotted Quad notation for “human readability”

36

IP Addresses come in two parts

11111111 00010001 10000111 00000000

Network Number Host Number

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

37

Classful Addresses

0nnnnnnn

10nnnnnn nnnnnnnn

nnnnnnnn nnnnnnnn110nnnnn

hhhhhhhh hhhhhhhh hhhhhhhh

hhhhhhhh hhhhhhhh

hhhhhhhh

n = network address bit h = host identifier bit

Class A

Class C

Class B

1981, RFC 791 (definition of IPv4)

38

The Classful Address Space

Class Networks Hosts Share of IPaddress space

A 127 16,777,214 1/2

B 16,384 65,534 1/4

C 2,097,152 254 1/8

Leads to very inefficient allocation of addresses …

39

Flat Network Addressing

This router needsat least12 table entries

Exports at least 12 network addresses

Z

Y

X

WT

UV

SQ

R

P

N

40

Hierarchical Network Addressing

Network Z

This Router needsonly 3 table entries for networks Z-T, Z-S, and Z-U

Z

T

S

UNetwork Z-U-X

This is called Aggregation

Network Z-TExports onlyone AddressM P

Q W

WP

SX

MF

KJ

41

Best Match Forwarding

Destination Address : Network X-W-Y-V, Host 12

Lookup in IP forwarding table isno longer based on exact match of network prefix.

Destination Next Hop

X

X-W-Y

X-W

R2

R7

R5

Match

Best Match

Match

42

The IPv4 hack

IP Address : 12.4.0.0 IP Mask: 255.254.0.0

00001100 00000100 00000000 00000000

11111111 11111110 00000000 00000000

Address

Mask

for hosts Network Prefix

Use two 32 bit numbers to represent a network. Network number = IP address + Mask

Usually written as 12.4.0.0/15

43

Which IP addresses are covered?

00001100 00000100 00000000 00000000

11111111 11111110 00000000 0000000012.4.0.0/15

00001100 00000101 00001001 00010000

00001100 00000111 00001001 00010000

12.5.9.16

12.7.9.16

IP address 12.5.9.16 is covered by network 12.4.0.0/15

IP address 12.7.9.16 is not

44

Mask may vary with location

12.0.0.0/8

12.0.0.0/16

12.254.0.0/16

12.1.0.0/16

12.2.0.0/16

12.3.0.0/16

:::

12.253.0.0/16

12.3.0.0/2412.3.1.0/24

::

12.3.254.0/24

12.253.0.0/1912.253.32.0/1912.253.64.0/19

12.253.96.0/1912.253.128.0/1912.253.160.0/1912.253.192.0/19

:::

This allows all of these (sub)networksto be aggregated into one entry in an IP forwarding table

45

Private Address Space

10.0.0.0/8 (10.0.0.0 --- 10.255.255.255)172.16.0.0/12 (172.16.0.0 --- 172.31.255.255)192.168.0.0/16 (192.168.0.0 --- 192.168.255.255)

RFC 1918

Private Addresses + Network Address Translation (NAT) + Firewalls =No need for IPv6

An ongoing debate

NATs are not compatible with some VPN technologiessuch as IPSec andtunneling. Can breakother applications.

46

What do routing protocols exchange?

Network numbers

Network number = IP address + Mask

Reachability information

How to reach a given network

+depends onprotocol

47






48

Basic Model: Finding Shortest Paths

A

D E

C

B

100

10020

20

80

80

20

20 10

10

10

10

10

10

Source

50

30

20 30

Find paths from a sourceto all other nodes that minimizes sum of arcmetrics

Arc metrics arestatically provisioned

IP routing protocols DO NOT route aroundnetwork congestion!

49

Arcs, Nodes,Arc Weights

Directed Graph data structures

+Algorithm

Dijkstra’s or Bellman-Ford

=

Solution to shortest path problem

How can shortest path computation be decentralized and performed by many cooperating routers?

Distribute pathcomputation.

Keep only local link data.

Distribute all link data.

Perform pathcomputations locally.

RIP, EIGRP,BGP OSPF, IS-IS,

IDPR

Technology : Distance Vector vs. Link State Protocols

Distance Vector Link State

50

RIP• RIP = Routing Information Protocol

• Does not scale well, designed for small LANs

• Is a “distance vector protocol”

• Very simple, easy to configure, easy to

implement

• Is most widely used routing protocol

51

RIP History • Developed at Xerox PARC in early 1980s

• Reimplemented in Berkeley UNIX

• 1988 : Standardized in RFC 1058

• 1994 : RIP-2, RFC 1723

– Support CIDR addressing

– Authentication

• 1997 : RIPng for IPv6, RFC 2080

52

RIP Routing Table Destination Next Hop Metric

Net ANet BNet C, Host 3

Router 1DirectRouter 2Router 1Default

3

50

A destination is either a network, a host, or a “gateway of last resort”

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

Measures how many“hops away” is the destination

0

53

Basic RIP Protocol Periodically exchange list of destinations and metrics with all neighboring routers

Dest. Metric

ABC

305

Dest. Metric

AEF

120

RIP routers exchangetheir entire “distancevector” every 30 seconds

54

Basic RIP Protocol (cont.)

c

Neighbor N

Is Dest. A in my RIP Table?

Receive

Dest. Metric

A m

From Neighbor N

Dest. Metric

A m + c

Nxt Hop

N

Add

to RIP table

Is m + c less than current metric for Dest. A?

NO

Yes

YesDest. Metric

A m + c

Nxt Hop

N

Replace current entry with

in RIP table

Is N my next hop for Dest. A andm + c is not the current metric?

NO

Yes

Trust your neighbor...

55

OSPF• OSPF =Open Shortest Path First

• Developed to address shortcomings of RIP– has rapid, loop-free convergence – does not count to infinity

• Link metrics between 0 and 65,535, no limit on path metric

• Is a “link state protocol”

• Has reputation for being complex

• Scales well

• Defined in RFCs 1247 (1991), 1583 (1994), 2178 (1997), 2328 (1998).

56

Link State Database

A

D E

C

B100

10020

20

80

80

20

20 1010

10

10

1010

ME

Each Router has a database representing the entire networkthat is constructed from the local knowledge at each router

B

D

A

C

80100

20

57

Building OSPF Routing Table

Compute locally using Link State Database!

A

D E

C

B100

10020

20

80

80

20

20 1010

10

10

1010

ME

50

30

20 30

Dijkstra

Dest. Nxt Hop Metric

B D 50

C D

D

D

D

E

3020

30

58

That’s Easy!

Not so fast!

RIP RFC 1058 : 33 pages

OSPF RFC 2328 : 244 pages

Much of this complexity is related to thesynchronization ofthe distributed, replicated link statedatabase.Plus network modeling ….

Distance Vector vs. Link State….

59

Area 1

Area 2

Area 3

Scalability: OSPF Areas

Area 0

Area K...

LS database unique within an area

Special OSPFprotocol to exchange routesbetween areas. This is a “distancevector” protocol!

• Decentralize administration

• Reduce memory usage per router

• Reduce bandwidth used by flooding

60

Traffic engineering is hard with current protocols

• Link metrics attract or repel all traffic

• Results in congestion and unused capacity

• Destination based, next-hop forwarding

paradigm has very coarse granularity

• Routing and forwarding are distinct, but still

very tightly coupled

MPLS to the rescue??

61






62

Interdomain routing = routing between autonomous systems

AS 701AS 1239

AS 7018

UUNet

AT&T Common Backbone

Sprint

Fidelity Investments

AT&T Research

207.104.168.0/24

AS 6431

AS 11040192.223.184.0/21

134.244.0.0/16

63

Why not just use OSPF?

•Scale– The Internet is very large

•Policy– My good route might be your bad

route

64

An ASN represents a unit of Routing Policy 834 Canada Long Distance Services1740 CerfNet1838 CerfNet2386 Data Communications Services2685 AGNS (IBM Network)2713 Jens Corporation2740 Canada Long Distance Services2751Paradyne 2767 Canada Long Distance Services2768 Canada Long Distance Services2855 Concert2915 Jens Corporation 3300 AT&T Unisource3448 AT&T Wireless3668 Network Systems4465 Easy Link Services4466 Easy Link Services4467 Easy Link Services 4468 Easy Link Services4661 Easy Link Asia-Pacific5074 Dial Platform (BMGS) 5075 FDDI Access Ring (BMGS) 5400 Concert5727 Asia-Pacific PVCs5728 WorldNet Dial Platform5729 WorldNet Services5730 FDDI Access Ring5731 WorldNet Services6269 AT&T-Internet26290 ALASCOM-DIAL6308 ALASCOM-MIS6431 AT&T Labs

6478 Worldnet Services 6537 CANADA LONG DISTANCE SERVICES 6742 AT&T London UK 6905 European Internal 6934 Columbus 7018 WorldNet Backbone 7170 Government Markets 7329 CMS 7948 GEOPLEX 8030 WorldNet 8031 WorldNet 8032 WorldNet 8033 WorldNet 8034 WorldNet 8035 WorldNet10456 Government Markets11698 AT&T-PWSS13979 AT&T-IPFR14005 AT&T Y2K ICC14470 ATT-PWSS-215259 Solutions15290 Canada IES17224 Enhanced Network Services17225 Enhanced Network Services17226 Enhanced Network Services17227 Enhanced Network Services17228 Enhanced Network Services17229 Enhanced Network Services17230 Enhanced Network Services17231 Enhanced Network Services17232 Enhanced Network Services17233 Enhanced Network Services

64 ASNs known to beassigned to AT&T(list may not be complete!)

33 (in green)seen in BGProuting tables on11/25/2000(by RouteDB)

65

How many ASNs are there?

From http://www.telstra.net/ops/bgptable.html on 11/26/2000

66

Sample prefixes from announced to the CBB

These are called “networks” or “prefixes”or “CIDR blocks” or “network blocks” or“routes”. Yes, it is confusing.

• 207.104.168.0/24 : from AT&T Labs (covers 256 IP addresses in range 207.104.168.0 to 207.104.168.255)

• 192.223.184.0/21 : from Fidelity Investments• 134.244.0.0/16 : from UUnet

67

How Many Prefixes are There?

From http://www.telstra.net/ops/bgptable.html on 11/26/2000

68

How many hosts are there?

For more information, see http://www.isc.org/ds

69

Policy : Transit vs. Nontransit

AS 701

AS144

AS 701

A nontransit AS allows only traffic originating from AS or traffic with destination within AS

A transit AS allows traffic with neither source nor destination within AS to flow across the network

IP traffic

UUnet

Bell Labs

AT&T CBB

70

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

71

Why not minimize “AS hop count”?

Regional ISP1

Regional ISP2

Regional ISP3

Cust2Cust3 Cust3

National ISP1

National ISP2

YES

NO

72

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)

73

BGP Operations Simplified

Establish Peering on TCP port 179

Peers Exchange All Routes

Exchange Incremental Updates

AS1

AS2

While connection is ALIVE exchangeroute UPDATE messages

BGP

74

Two Types of BGP Neighbor Relationships

• External Neighbor (eBGP) in a different Autonomous Systems

• Internal Neighbor (iBGP) in the same Autonomous System

AS1

AS2

eBGP

iBGPPhysical Connection

Logical (TCP) Connection

75

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 = Network prefix + attributes

76

BGP Attributes • Local Preference : Used by IBGP to rank routes within

an autonomous system• AS path : list of ASNs the announcement traversed• Next Hop : IP address of external neighbor’s interface• Origin : How was the route originated? • Multi Exit Discriminator : preference of egress points• Community : arbitrary colors for routes• Atomic Aggregate :indicates if info has been lost• Aggregator : which ASN lost the information• Originator ID : for ibgp route reflectors• ….

RIP : “distance vector”, BGP : “path vector”

77

AS Path Attribute

AS7018

135.104.0.0/16AS Path = 144

135.104.0.0/16AS Path = 144

AS701Alternet (Uunet)

AS702Alternet (Uunet)

135.104.0.0/16AS Path = 701 144

135.104.0.0/16AS Path = 702 701 144

AS1849Uunet UK

AT&T CBB

AS5413GXN

135.104.0.0/16AS Path = 7018 144

AS5459LINX

135.104.0.0/16AS Path = 5413 7018 144

135.104.0.0/16AS Path = 5459 5413 7018 144

AS144

135.104.0.0/16

Bell Labs

Route Originated

78

AS Path Attribute (cont.)

BGP at AS YYY will never accept a route whose AS Path contains YYY. This avoids interdomain routing loops.

AS702UUnet

10.22.0.0/16AS Path = 1 333 702 877

Don’t Accept!

79

Next Hop Attribute

Every time a route announcement crosses an AS boundary, the Next Hop attribute is changed to the IP address of the border router that announced the route.

AS144Bell Labs

135.104.0.0/16Next Hop = 11.0.0.1

AS701UUnet

AS702UUnet

135.104.0.0/16Next Hop = 11.0.0.1

11.0.0.1

135.104.0.0/16Next Hop = 33.0.0.3

33.0.0.3

80

Local Preference Attribute

AT&T CBB

AS 7018

Primary T3 lineBackup T1 line

Forces outboundtraffic to takeprimary link, unless link is down.

Set Local Pref = 100for all routes from 7018

Customer

Set Local Pref = 50for all routes from 7018

81

Local Preference AttributeUsed only in iBGP to prefer a point of exit

Higher LocalPreference Valuesare more preferred

AS 1

AS 2

AS 4

AS 3

Frank’s Internet Barn

Frank’s Customer

Customer of Frank’s Customer

13.13.0.0/16AS Path = 4 1Loc pref = 80

13.13.0.0/16AS Path = 2 1Loc pref = 100

13.13.0.0/16AS Path = 3 1Loc pref = 90

Frank’s Upstream Provider

Frank’s Local Competition

13.13.0.0/16

82

Multi Exit Discriminator

AT&T CBB

AS 7018

Set MED to metric of IGP at each border router

192.44.78.0/24

35 56

192.44.78.0/24MED = 56

192.44.78.0/24MED = 35

Used only in eBGP to suggestpreferred points of entry to a BGPneighbor

Lower MED valuesare more preferred

83

BGP Route Processing

Best Route Selection

Apply Import Policies

BGP 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

84

BGP Best Route Selection Process

Can select at most one route to any given prefix

• Prefer routes with highest local preference• Then prefer routes with shortest AS-paths• Then prefer routes with lowest origin (IGP < EGP <

INCOMPLETE)• Then prefer lowest MEDs (if routes are from same neighbor

ASN) • Then prefer EBGP learned routes over IBGP learned routes• Then prefer routes with lowest IGP cost to BGP next-hop• Then break ties by selecting route with lowest BGP next-

hop

This is somewhat simplified ….

85

BGP Routing Tables

Network Next Hop Metric LocPrf Path* 63.108.216.0/21 137.39.23.173 81 701 701 10409 i* i 12.123.210.15 0 100 10409 10409 10409 i*>i 12.123.210.15 0 100 10409 10409 10409 i* i63.109.3.0/24 12.123.9.240 0 82 1239 16886 i*>i 12.123.9.240 0 82 1239 16886 i* 63.109.64.0/24 137.39.23.173 81 701 8143 i*>i 12.123.21.242 0 82 3561 8143 i* i 12.123.21.242 0 82 3561 8143 i* i63.109.65.0/24 12.123.21.242 0 82 3561 8143 ?*>i 12.123.21.242 0 82 3561 8143 ?* 137.39.23.173 81 701 8143 ?

• CBB tables often have 100K –200K routes.• Use “whois” queries to associate an ASN with “owner” (for example,

http://www.arin.net/whois/arinwhois.html)• 701 =Uunet, 10409 = allegro.net, 1239 = Sprint, 16886 = LVMH group, 8143 = Publicom, 3561

= Cable & Wireless

show ip bgp

http://www.arin.net/whois/arinwhois.html

86

BGP Communities

Community Attribute is 32 bits

By convention, first 16 bits is ASN of owner of community definition

communitynumber

Very powerful BECAUSE it is meaningless

Two reserved communities

no_export (0xFFFFFF01) don’t export out of AS

no_advertise (0xFFFFFF02)don’t pass to BGP peers

A route can belong to more than one community

RFC 1997 (August 1996)

groups routes into logical collections

87

Implementing “Backup” Policies with Communities

AS1221Telstra

AS4000Sprint Internat.

AS4805Global One

AS2764Connect Net

Connect Net wants to provide backup Internet access for Global One in case its connection to Sprint is lost.

1996, RFC 1998

88

First, Without Communities ...

AS1221Telstra


AS4805Global One

AS2764Connect Net

Local Pref = 80

Local Pref = 70

Connect Net and Global One would have to convince Telstra to do some custom configuration its Local Pref value for some routes…..

89

With Communities ….

{1221,70} = I’ll set local pref to 70 (cust. provided backup){1221,80} = I’ll set local pref to 80 (other ISP routes){1221,90} = I’ll set local pref to 90 (my customer backup){1221,100} = I’ll set local pref to 100 (my customers)


AS4805Global One

AS2764Connect Net

AS1221Telstra

Telstra’s communities:

90

Global One Using Telstra’s Communities

AS1221Telstra


AS4805Global One

AS2764Connect Net

Tag these routes with community {1221, 70}

Tag these routes with community {1221, 80}

All I have to do is pass along AS4805’sroutes to AS1221

91

iBGP Peers Must be Fully MeshedeBGP update

iBGP updates

iBGP peers do not announce routes received via iBGP

• N border routers means

N(N-1)/2 peering sessions

-- this does not scale

• Currently three solutions :

– Break an AS up into smaller

Autonomous Systems

– Route Reflectors

– Confederations

92

Route Reflectors

RR

RR

RR

RR

RR

RR

Route Reflectors must be fullymeshed

Route Reflectors pass along updatesto client routers

93

Route Flapping

Routes that go “up” and “down” are said to be flapping

Flapping routes are often penalized, and if penalty goes over a threshold, they are Suppressed for some period of time.

94

Recommended Reading • IDR : http://www.ietf.org/html.charters/idr-charter.html

• Internet Routing Architectures. Bassam Halabi. . The

BGP Bible! Get second edition (2000).

• BGP4: Inter-domain Routing in the Internet. John W.

Stewart, III. 1999. Good high-level introduction.

• Routing in the Internet. Christian Huitema. 2000.

• OSPF -- Anatomy of an Internet Routing Protocol. John

T. Moy. 1998.

• Managing IP Networks with Cisco Routers. Scott M.

Ballew. 1997.

95

Links

www.research.att.com/~griffin/IPcourses.html

www.research.att.com/~griffin/interdomain.html

These slides :

Links on interdomain routing:

96

What I Didn’t talk about…

IP Layer

TCP UDP

FTP HTTPTelnet RTPSNMPDNS

… Applications …

97

Selected R&D work on IP Network Management

• Bravo : backbone routing analysis, visualization, and optimization

• Tap : collection and analysis of traffic flow data• Metrica : SNMP polling • Netdb : router configuration management • Peermon : monitoring CBB from the outside • Routdb : monitoring CBB routes from the inside• Topology server : building live network maps • Falcon : fault correlation, visualization • Wipm : active delay measurement

An Overview of the IP Network Layer.ppt

Documents

Transcript of An Overview of the IP Network Layer.ppt