ECE544: Communication Networks-II, Spring 2007

D. Raychaudhuri

Lecture 5

Includes teaching materials from L. Peterson

Today’s Lecture

• Routing metrics• Scalable IP routing• IPv6• Inter-domain routing (BGP)

Routing Metrics

Metric choices

• Static metrics (e.g., hop count)– good only if links are homogeneous– not the case in the Internet

• Static metrics do not take into account:– link delay– link capacity– link load (hard to measure)

Original ARPANET metric

• Cost proportional to queue size– instantaneous queue length as delay

estimator

• Problems:– did not take into account link speed– poor indicator of expected delay due to

rapid fluctuations– delay may be longer even if queue size is

small due to contention for other resources

New metric

• Delay = (depart time - arrival time) + transmission time + link propagation delay– (depart time - arrival time) captures queuing– transmission time captures link capacity– link propagation delay captures the physical

length of the link

• Measurements averaged over 10 seconds– Update sent if difference > threshold, or

every 50 seconds

Performance of new metric

• Works well for light to moderate load– static values dominate

• Oscillates under heavy load– queuing dominates

• Reason: there is no correlation between original and new values of delay after re-routing!

Specific problems

• Range is too wide– 9.6 Kbps highly loaded link can appear 127

times costlier than 56 Kbps lightly loaded link– can make a 127-hop path look better than 1-

hop

• No limit in reported delay variation• All nodes calculate routes simultaneously

– triggered by link update

Consequences

• Low network utilization (50% in example)

• Congestion can spread elsewhere• Routes could oscillate between short

and long paths• Large swings lead to frequent route

updates– more messages– frequent SPT re-calculation

Revised link metric

• Better metric: packet delay = f(queueing, transmission, propagation).

• When lightly loaded, transmission and propagation are good predictors

• When heavily loaded queueing delay is dominant and so transmission and propagation are bad predictors

Routing metric v.s. link utilization

0

30

60

140

75

50% 100%25% 75%

225New metric(routing units)

Utilization

9.6 satellite

9.6 terrestrial

56 terrestrial

56 satellite

90

Observations

• Cost of highly loaded link never more than 3*cost when idle

• Most expensive link is 7 * least expensive link

• High-speed satellite link is more attractive than low-speed terrestrial link

Routing dynamics

0

1.0

4.0

0.5

0.75

0.25

2.0 3.01.0 1.5 2.5 3.50.5

Link reported cost

UtilizationBoundedoscillation

Metric map

Network response

Routing dynamics

0

1.0

4.0

0.5

0.75

0.25

2.0 3.01.0 1.5 2.5 3.50.5

Reported cost

Utilization Metric map

Network response

Easing ina new link

Scalable IP Routing

How to Make Routing Scale

• Flat versus Hierarchical Addresses• Inefficient use of Hierarchical Address Space

– class C with 2 hosts (2/255 = 0.78% efficient)– class B with 256 hosts (256/65535 = 0.39%

efficient)

• Still Too Many Networks– routing tables do not scale– route propagation protocols do not scale

Internet StructureRecent Past

NSFNET backboneStanford

BARRNETregional

BerkeleyPARC

NCAR

UA

UNM

Westnetregional

UNL KU

ISU

MidNetregional…

Internet StructureToday

Backbone service provider

Peeringpoint

Peeringpoint

Large corporation

Large corporation

Smallcorporation

“Consumer ” ISP

“Consumer” ISP

“ Consumer” ISP

Subnetting• Add another level to address/routing

hierarchy: subnet• Subnet masks define variable partition of host

part• Subnets visible only within site

Network number Host number

Class B address

Subnet mask (255.255.255.0)

Subnetted address

111111111111111111111111 00000000

Network number Host IDSubnet ID

Subnet Example

Forwarding table at router R1Subnet Number Subnet Mask Next

Hop128.96.34.0 255.255.255.128

interface 0128.96.34.128 255.255.255.128

interface 1128.96.33.0 255.255.255.0 R2

Subnet mask: 255.255.255.128Subnet number: 128.96.34.0

128.96.34.15 128.96.34.1

H1R1

128.96.34.130Subnet mask: 255.255.255.128Subnet number: 128.96.34.128

128.96.34.129128.96.34.139

R2H2

128.96.33.1128.96.33.14

Subnet mask: 255.255.255.0Subnet number: 128.96.33.0

H3

Supernetting (CIDR)• Assign block of contiguous network numbers

to nearby networks• Called CIDR: Classless Inter-Domain Routing• Protocol uses a (length, value) pair length = # of bits in network prefix

• Use CIDR bit mask to identify block size• All routers must understand CIDR addressing• Routers can aggregate routes with a single

advertisement -> use longest prefix match

Supernetting (CIDR)• Routers can aggregate routes with a single

advertisement -> use longest prefix match• Hex/length notation for CIDR address:

– C4.50.0.0/12 denotes a netmask with 12 leading 1 bits, i.e. FF.F0.0.0

• Routing table uses “longest prefix match”– 171.69 (16 bit prefix) = port #1– 171.69.10 (24 bit prefix) = port #2– then DA=171.69.10.5 matches port #1– and DA = 171.69.20.3 matches port#2

Chapter 4, Figure 26

Border gateway(advertises path to

11000000000001)

Regional network

Corporation X(11000000000001000001)

Corporation Y(11000000000001000000)

Route Aggregation with CIDR

IP Version 6• Features

– 128-bit addresses (classless)– multicast– real-time service– authentication and security – autoconfiguration – end-to-end fragmentation– protocol extensions

• Header– 40-byte “base” header– extension headers (fixed order, mostly fixed length)

• fragmentation• source routing• authentication and security• other options

IP ServiceIP Service IPv4 SolutionIPv4 Solution IPv6 SolutionIPv6 Solution

Mobile IP with Direct Routing

Mobile IP with Direct Routing

DHCPDHCP

Mobile IPMobile IP

IGMP/PIM/Multicast BGP

IGMP/PIM/Multicast BGP

IP MulticastIP Multicast MLD/PIM/Multicast BGP,Scope IdentifierMLD/PIM/Multicast

BGP,Scope Identifier

MobilityMobility

AutoconfigurationAutoconfigurationServerless,

Reconfiguration, DHCPServerless,

Reconfiguration, DHCP

IPv6 Technology Scope

32-bit, Network Address Translation

32-bit, Network Address Translation

128-bit, MultipleScopes

128-bit, MultipleScopes

Addressing RangeAddressing Range

Quality-of-ServiceQuality-of-Service Differentiated Service, Integrated Service

Differentiated Service, Integrated Service

Differentiated Service, Integrated Service

Differentiated Service, Integrated Service

SecuritySecurity IPSec Mandated, works End-to-EndIPSec Mandated,

works End-to-EndIPSecIPSec

IPv4 & IPv6 Header Comparison

Version IHLType of Service

Total Length

Identification FlagsFragment

Offset

Time to Live

Protocol Header Checksum

Source Address

Destination Address

Options Padding

Version Traffic Class Flow Label

Payload LengthNext

HeaderHop Limit

Source Address

Destination Address

IPv4 HeaderIPv4 Header IPv6 HeaderHeader

- field’s name kept from IPv4 to IPv6

- fields not kept in IPv6

- Name & position changed in IPv6

- New field in IPv6Leg

end

27

IPv6 Addressing • IPv6 Addressing rules are covered by multiples

RFC’s–Architecture defined by RFC 2373

• Address Types are :–Unicast : One to One (Global, Link local, Site local, Compatible)–Anycast : One to Nearest (Allocated from Unicast)–Multicast : One to Many–Reserved

• A single interface may be assigned multiple IPv6 addresses of any type (unicast, anycast, multicast)

–No Broadcast Address -> Use Multicast

IPv6 Address Representation

• 16-bit fields in case insensitive colon hexadecimal representation

• 2031:0000:130F:0000:0000:09C0:876A:130B

• Leading zeros in a field are optional:• 2031:0:130F:0:0:9C0:876A:130B

• Successive fields of 0 represented as ::, but only once in an address:

• 2031:0:130F::9C0:876A:130B• 2031::130F::9C0:876A:130B• 0:0:0:0:0:0:0:1 => ::1• 0:0:0:0:0:0:0:0 => ::

• IPv4-compatible address representation• 0:0:0:0:0:0:192.168.30.1 = ::192.168.30.1 = ::C0A8:1E01

29

IPv6 Addressing

• Prefix Format (PF) Allocation–PF = 0000 0000 : Reserved–PF = 001 : Aggregatable Global Unicast Address–PF = 1111 1110 10 : Link Local Use Addresses (FE80::/10)–PF = 1111 1110 11 : Site Local Use Addresses (FEC)::/10)–PF = 1111 1111 : Multicast Addresses (FF00::/8)–Other values are currently Unassigned (approx. 7/8th of total)

• All Prefix Formats have to support EUI-64 bits Interface ID setting

–But Multicast

Aggregatable Global Unicast Addresses

• Aggregatable Global Unicast addresses are:–Addresses for generic use of IPv6–Structured as a hierarchy to keep the aggregation

• See draft-ietf-ipngwg-addr-arch-v3-07

Interface IDGlobal Routing Prefix SLA

001

64 bits3 45 bits 16 bits

Provider Site Host

Address Allocation

• The allocation process is under reviewed by the Registries: –IANA allocates 2001::/16 to registries–Each registry gets a /23 prefix from IANA–Formely, all ISP were getting a /35–With the new proposal, Registry allocates a /36 (immediate allocation) or /32 (initial allocation) prefix to an IPv6 ISP–Policy is that an ISP allocates a /48 prefix to each end customer–ftp://ftp.cs.duke.edu/pub/narten/ietf/global-ipv6-assign-2002-04-25.txt

2001 0410

ISP prefix

Site prefix

LAN prefix

/32 /48 /64

Registry

/23

Bootstrap process - RFC2450

Interface ID

Hierarchical Addressing & Aggregation

–Larger address space enables:•Aggregation of prefixes announced in the global routing table.•Efficient and scalable routing.

ISP

2001:0410::/32

Customerno 2

IPv6 Internet

2001::/16

2001:0410:0002:/48

2001:0410:0001:/48

Customerno 1

Only announces the /32 prefix

• Link-local addresses for use during auto-configuration and when no routers are present:

• Site-local addresses for independence from Global Reachability, similar to IPv4 private address space

Link-Local & Site-Local Unicast Addresses

1111 1110 10 0 interface ID

1111 1110 11 0 interface IDSLA*

Multicast Addresses (RFC 2375)

• low-order flag indicates permanent / transient group; three other flags reserved

• scope field: 1 - node local–2 - link-local–5 - site-local–8 - organization-local–B - community-local–E - global–(all other values reserved)

4 112 bits8

group IDscopeflags11111111

4

35

more on IPv6 Addressing

80 bits 32 bits16 bits

IPv4 Address00000000……………………………0000

IPv6 Addresses with Embedded IPv4 AddressesIPv6 Addresses with Embedded IPv4 Addresses

80 bits 32 bits16 bits

IPv4 AddressFFFF0000……………………………0000

IPv4 mapped IPv6 addressIPv4 mapped IPv6 address

IPv6 Addressing Examples

LAN: 3ffe:b00:c18:1::/64

Ethernet0

MAC address: 0060.3e47.1530

router# show ipv6 interface Ethernet0Ethernet0 is up, line protocol is up IPv6 is enabled, link-local address is FE80::260:3EFF:FE47:1530Global unicast address(es): 2001:410:213:1:260:3EFF:FE47:1530, subnet is 2001:410:213:1::/64 Joined group address(es): FF02::1:FF47:1530 FF02::1 FF02::2 MTU is 1500 bytes

interface Ethernet0 ipv6 address 2001:410:213:1::/64 eui-64

BGP Overview

BGP-4: Border Gateway Protocol

• AS (Autonomous System) Types– stub AS: has a single connection to one other AS

• carries local traffic only– multihomed AS: has connections to more than one

AS• refuses to carry transit traffic

– transit AS: has connections to more than one AS• carries both transit and local traffic

• Each AS has:– one or more border routers– one BGP speaker that advertises:

• local networks• other reachable networks (transit AS only)• gives path information

Example

1 2

3

1.11.2

2.1 2.2

3.1 3.2

2.2.1

44.1 4.2

5

5.1 5.2

EGP

IGP

EGPEGP

IGP

IGP

IGPIGP

EGPEGP

Path Suboptimality

1 2

3

1.11.2

2.1 2.2

3.1 3.2

2.2.1

3 hop red pathvs2 hop green path

Choices• Link state or distance vector?

– no universal metric - policy decisions

• Problems with distance-vector:– Bellman-Ford algorithm may not converge

• Problems with link state:– metric used by routers not the same - loops– LS database too large - entire Internet– may expose policies to other AS’s

Solution: Path Vectors• Each routing update carries the

entire path• Loops are detected as follows:

– when AS gets route check if AS already in path– if yes, reject route– if no, add self and advertise route further

• Advantage:– metrics are local - AS chooses path, protocol

ensures no loops

Problems

• Routing table size– need an entry for all paths to all networks

• Required memory= O(N + M*A) * K)– N: number of networks– M: mean AS distance– A: number of AS’s– K: number of BGP peers

• Problem reduced with CIDR

Routing table size

networks(NLRI)

mean ASdistance

# of AS’s BGPpeers/net

memory

2,100 5 59 3 27,000

4,000 10 100 6 108,000

10,000 15 300 10 490,000

100,000 20 3,000 20 1,040,000

                                                                                                                        

 

      

      

      

      

     

      

      

    

BGP Example• Speaker for AS2 advertises reachability to P and Q

– network 128.96, 192.4.153, 192.4.32, and 192.4.3, can be reached directly from AS2

• Speaker for backbone advertises– networks 128.96, 192.4.153, 192.4.32, and 192.4.3 can

be reached along the path (AS1, AS2).• Speaker can cancel previously advertised paths

Backbone network(AS 1)

Regional provider A(AS 2)

Regional provider B(AS 3)

Customer P(AS 4)

Customer Q(AS 5)

Customer R(AS 6)

Customer S(AS 7)

128.96192.4.153

192.4.32192.4.3

192.12.69

192.4.54192.4.23

Interior BGP peers• IGP cannot propagate all the

information required by BGP• External routers in an AS use

interior BGP (IBGP) connections to communicate

• External routers agree on routes and inform IGP

IBGP

                                                                                                                        

 

      

      

      

      

     

      

      

    

Interconnecting BGP peers

• BGP uses TCP to connect peers• Advantages:

– BGP much simpler– no need for periodic refresh– incremental updates

• Disadvantages– congestion control on a routing

protocol?

Hop-by-hop model

• BGP advertises to neighbors only those routes that it uses– consistent with the hop-by-hop

Internet paradigm– e.g., AS1 cannot tell AS2 to route to

other AS’s in a manner different than what AS2 has chosen (need source routing for that)

Policy with BGP

• BGP provides capability for enforcing various policies

• Policies are not part of BGP: they are provided to BGP as configuration information

• BGP enforces policies by choosing paths from multiple alternatives and controlling advertisement to other AS’s

Examples of BGP policies

• A multihomed AS refuses to act as transit– limit path advertisement

• A multihomed AS can become transit for some AS’s– only advertise paths to some AS’s

• An AS can favor or disfavor certain AS’s for traffic transit from itself

                                                                                                                        

 

      

      

      

      

     

      

      

    

BGP-4

• Latest version of BGP• BGP-4 supports CIDR

Routing information bases (RIB)

• Routes are stored in RIBs• Adj-RIBs-In: routing info that has been

learned from other routers (unprocessed routing info)

• Loc-RIB: local routing information selected from Adj-RIBs-In (routes selected locally)

• Adj-RIBs-Out: info to be advertised to peers (routes to be advertised)

BGP common header

Length Type

0 1 2 3

Marker (security and message delineation)

Types: OPEN, UPDATE, NOTIFICATION, KEEPALIVE

Optional parameters <type, length, value>

BGP OPEN message

Length Type: update

0 1 2 3

Marker (security and message delineation)

versionMy autonomous system Hold time

BGP identifierParameter length

My AS: id assigned to that ASHold timer: max interval between KEEPALIVE or UPDATE messagesBGP ID: address of one interface (same for all messages)

BGP UPDATE message

Length Type: open

0 1 2 3

Marker (security and message delineation)

Withdrawn....routes len Withdrawn routes (variable)

Path attribute len Path attributes (variable)

Network layer reachability information (NLRI) (variable)

UPDATE message reports information on a SINGLE path,but can report multiple withdrawn routes

...

NLRI

• Network Level Reachability Information– list of IP address prefixes encoded as

follows:

Length (1 byte) Prefix (variable)

Data

BGP NOTIFICATION message

Length Type: NOTIFICATION

0 1 2 3

Marker (security and message delineation)

Error code

Error sub-code

Used for error notification

BGP KEEPALIVE message

Length Type: KEEPALIVE

0 1 2 3

Marker (security and message delineation)

Sent periodically to peers to ensure connectivityIf hold_time is zero, messages are not sent

Policy routing

T Z XY

U V

Assume Y forbids T’s trafficT cannot reach X, but X can reach T!

Options

• Advertise all paths:– Path 1: through T can reach 197.8.0.0/23– Path 2: through T can reach 197.8.2.0/24– Path 3: through T can reach 197.8.3.0/24

• But this does not reduce routing tables! We would like to advertise:– Path 1: through T can reach 197.8.0.0/22

Sources

• RFC1771: main BGP RFC• RFC1772-3-4: application, experiences,

and analysis of BGP• RFC1965: AS confederations for BGP• Christian Huitema’s book “Routing in the

Internet”, chapters 8 and 9.• http://www.academ.com/nanog/

feb1997/BGPTutorial/sld022.htm (Cisco tutorial)

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Today’s Homework• Peterson & Davie, Chap 4

4.31 (3rd Ed)4.334.404.45

Download and browse IPv6 and BGP RFC’s