COMS/CSEE 4140 Networking Laboratory Lecture 04 Salman Abdul Baset Spring 2008.

COMS/CSEE 4140 Networking Laboratory

Lecture 04

Salman Abdul BasetSpring 2008

2

Announcements Lab 3 (5-7) and prelab 4 due next week

before your slot. Grades for prelab 1 &2 and lab report 1 &

2 will be posted this week. Lab report format.

3

Previous Lecture More on CIDR

Hierarchical addressing, route aggregation Multi-homing

Internet Protocol (IP) Best effort datagram service. Fragmentation, TTL, protocol de-multiplexing

Internet Control Message Protocol (ICMP) Type/code, update routing tables

IP forwarding Forwarding and routing Configure IP address, default gw, static, dynamic

Router architecture Shared bus (1st generation), shared bus with line card

cache (2nd generation), switched architecture.

4

Agenda CIDR multi-homing and IP forwarding The Internet Routing protocols Routing Information Protocol (RIP)

5

CIDR Multi-homing

AOLAOL

Verizon

Verizon

Comp 1

Comp 1

My-ISP My-ISP

129.15/16

129.21/16129.0/8

Comp 2

Comp 2

208.20.16/8

208.20/16

6

CIDR Multi-homing

AOLAOL

Verizon

Verizon

Comp 1

Comp 1

My-ISP My-ISP

129.15/16

129.21/16129.0/8

Comp 2

Comp 2

208.20.16/8

208.20/16129.15/16

My-ISP requests Verizon to advertise its prefixes.

Q: I have a 128.59/16 network. Can I connect a 202.15.16.23 IP address?

Typically no. Announcements smaller than /24 not accepted.

7

Routing and Forwarding

Control plane: run routing protocols: (RIP, OSPF, BGP)

Data plane: forwarding packets from incoming to outgoing linkFIB: forwarding information base

RIB: routing information base

8

Routing and Forwarding Select the next-hop router.

Find the outgoing interface. Find the MAC address of the next-hop router. In Linux, you specify the IP address of the

next-hop router.

Longest-prefix first.

Default routing (implied by longest-prefix rule: default has prefix of length 0).

9

Longest Prefix Match

Network prefix / host

Network mask

Interface Next hop

10.0.2.0 255.255.255.0

eth1

10.30.0.0 255.255.0.0 eth2 10.30.0.1

10.30.4.0 255.255.255.0

eth3 10.30.4.4

0.0.0.0 0.0.0.0 eth1 10.0.2.1

Find route for 10.30.4.1

Host IP address: 10.0.2.41 netmask: 255.255.255.0

Bits mached

11

16

24

0

default gw

10

Agenda CIDR multi-homing and IP forwarding The Internet Routing protocols Routing Information Protocol (RIP)

11

A Definition (not the only one) On October 24, 1995, the FNC unanimously

passed a resolution defining the term Internet. •RESOLUTION: The Federal Networking Council (FNC) agrees that the

following language reflects our definition of the term "Internet". "Internet" refers to the global information system that --

•(i) is logically linked together by a globally unique address space based on the Internet Protocol (IP) or its subsequent extensions/follow-ons;

•(ii) is able to support communications using the Transmission Control Protocol/Internet Protocol (TCP/IP) suite or its subsequent extensions/follow-ons, and/or other IP-compatible protocols; and

•(iii) provides, uses or makes accessible, either publicly or privately, high level services layered on the communications and related infrastructure described herein.

12

Applications of the Internet Traditional core applications:

EmailNewsRemote loginFile transfer

The killer application:World-Wide Web (WWW)

New applications:VideoTelephony Mesh-upsP2P applicationsSocial networks

13

Brief History of the InternetMid 1960: Papers on “Packet Switching” emerge.End 1969s: ARPA sponsors the development of a packet-

switching network, called the ARPANET. First four nodes are UCLA, SRI, U.

Utah, UCSB. 1974: The TCP/IP protocols and model are being

proposed by Cerf/Kahn.1980: IPv4 is introduced1983: ARPANET adopts TCP/IP. At this time, the

ARPANET has200 routers.

1984: NSF funds a TCP/IP based backbone network. This backbone grows

into the NSFNET, which becomes the successor of the ARPANET.

1995: NSF stops funding of NSFNET. The Internet is completely commercial.

14

A Brief Summary of the Evolution of the Internet

1945 1995

Memex Conceived

1945

WWWCreated

1989

MosaicCreated

1993

A Mathematical

Theory of Communication

1948

Packet Switching Invented

1964

SiliconChip1958

First Vast ComputerNetwork

Envisioned1962

ARPANET1969

TCP/IPCreated

1972

InternetNamed

and Goes

TCP/IP1984

HypertextInvented

1965

Age ofeCommerce

Begins1995

Copyright 2002, William F. Slater, III, Chicago, IL, USA

15

Internet History

Source:

16

Growth of the Internet

Source: Internet Software Consortium

17

Internet Infrastructure

local ISP

campusnetwork

corporatenetwork

IXP

RegionalNetwork

RegionalNetwork

local ISP

local ISP

IXP

IXP

Backbone Network

Backbone Network

RegionalNetwork

RegionalNetwork

Tier-1Tier-2

Tier-3

18

Internet Infrastructure The infrastructure of the Internet consists of a

federation of connected networks that are each independently managed (“autonomous system”) Note: Each “autonomous system may consist of multiple

IP networks Autonomous systems have a number (AS number)

Hierarchy of network service providers (NSPs) Tier-1: nation or worldwide network (US: less than 20) Tier-2: regional networks (in US: less than 100) Tier-3: local Internet service provider (in US: several

thousand)

19

Internet Infrastructure Location where a network (ISP, corporate

network, or regional network) gets access to the Internet is called a Point-of-Presence (POP).

Locations (Tier-1 or Tier-2) networks are connected for the purpose of exchanging traffic are called peering points. Public peering: Traffic is swapped in a specific

location, called Internet exchange points (IXPs) Private peering: Two networks establish a

direct link to each other.

20

Node degrees

Source: caida.org

21

Topology of a Tier-1 NSP

22

Organization of a single node in a Tier-1 network

Links to othernodes of thenetwork

....

Dial-up andleased linksto customers

.... Leased links

to customers

Peering points

Metropolitanarea networks

....

Com3

Com3

Com3

Com3

Bay Networks

Bay Networks

Modem Bank

SD

SD

Modem Bank

Modem Bank Modem Bank

Modem Bank

.... Leased links

to customers

.... Leased links

to customers

23

Who is Who on the Internet ?

IETF IRTF

IESG

Technical managementIAB

Internet standard

Approves RFC

IANAICANN

-IP addresses-port numbers-RIRs

[Get port number] RFC

-domain names

ISOCperformsIANA functions

24

Who is Who on the Internet ?

Internet Society (ISOC): Founded in 1992, an international nonprofit professional organization that provides administrative support for the Internet. Founded in 1992, ISOC is the organizational home for the standardization bodies of the Internet.

Internet Engineering Task Force (IETF): Forum that coordinates the development of new protocols and standards. Organized into working groups that are each devoted to a specific topic or protocol. Working groups document their work in reports, called Request For Comments (RFCs).

IRTF (Internet Research Task Force): The Internet Research Task Force is a composed of a number of focused, long-term and small Research Groups.

Internet Architecture Board (IAB): a technical advisory group of the Internet Society, provides oversight of the architecture for the protocols and the standardization process

The Internet Engineering Steering Group (IESG): The IESG is responsible for technical management of IETF activities and the Internet standards process. Standards. Composed of the Area Directors of the IETF working groups.

25

Internet Standardization Process Working groups present their working of the Internet

which are published as RFC (Request for Comments). RFCs are the basis for Internet standards.

Not all RFCs become Internet Standards ! (There are >5000 RFCs and less than 70 Internet standards)

A typical (but not only) way of standardization is: Internet Drafts RFC Proposed Standard Draft Standard (requires 2 working implementation) Internet Standard (declared by IAB)

26

Assigning Identifiers for the Internet Who gives the university the domain name “virginia.edu” Who assigns it the network prefix “128.143.0.0/16”? Who assigns port 80 as the default port for web servers?

The functions associated with the assignment of numbers is referred to as Internet Assigned Number Authority (IANA).

IANA used to be managed by Jon Postel at ISI

Since the 1990s, IP addresses and domain name allocation are delegated to independent organizations. Different organizations are responsible for allocating domain names and IP addresses

27

The IANA Function The functions associated with the assignment of

numbers in the Internet is referred to as Internet Assigned Number Authority (IANA).

IANA serves as a registry that keeps records of assigned numbers: IP addresses Protocol numbers Domain names (until 1992)

There is no charge for allocation.

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Regional Internet Registries (RIRs)

Registration and management of IP address is done by Regional Internet Registries (RIRs)

Where do RIRs get their addresses from: IANA maintains a high-level registry that distributes large blocks to RIRs

RIRs administer allocation of: IPv4 address blocks IPv6 address blocks Autonomous system (AS) numbers

There are currently 5 RIRs worldwide: APNIC (Asia/Pacific Region), ARIN (North America and Sub-Sahara Africa), LACNIC (Latin America and some Caribbean Islands) RIPE NCC (Europe, the Middle East, Central Asia) AfriNIC (African continent)

29

Transitioning of Domain Name Registration Until 1992: Domain name registration done as part of IANA 1992: InterNIC was created in a partnership between US

government and companies to organize and maintain the growing DNS registry and services. The company Network Solutions ran the administration of InterNIC. Until 1998, Network Solutions had a monopoly for domain names.

1995: InterNIC started harging for domain names ($100 for 2 years)

1997: President Clinton directs the Secretary of Commerce to privatize the management of the domain name system (DNS) in a manner that increases competition and facilitates international participation in its management.

1998: ICANN was created in response to a policy statement issued by the US Department of Commerce that called for the formation of a private sector not-for-profit Internet stakeholder to administer policy for the Internet name and address system. ICANN operates under a renewable 3-year contract with the US Department of the Commerce. ICANN accredits domain-name registrar for .com, .org., .net (and other

domain)

30

ICANN The Internet Corporation for Assigned Names and

Numbers (ICANN) is an internationally organized, non-profit corporation that has responsibility for Internet Protocol (IP) address space allocation, protocol identifier assignment, Top-Level Domain name system management, and root server system management functions. ICANN performs the IANA functions ICANN accredits domain-name registrar for .com, .org., .net

(and other domain)

Since ICANN performs the IANA functions, it is in charge for allocating all numbers. However, the main concern is the allocation of domain names.

ICANN role is to oversee the domain-name registration system's transition from government hands to private hands and to coordinate its decentralization and the integration into a global community.

31

Who is Who on the Internet ?

IETF IRTF

IESG

Technical managementIAB

Internet standard

Approves RFC

IANAICANN

-IP addresses-port numbers-RIRs

[Get port number] RFC

-domain names

ISOCperformsIANA functions

32

Agenda CIDR multi-homing and IP forwarding The Internet Routing protocols Routing Information Protocol (RIP)

33

Routing Protocols Recall: There are two parts to routing IP packets:

1. How to pass a packet from an input interface to the output interface of a router (forwarding) ? 2. How to find and setup a route (routing) ?

We already discussed the packet forwarding part. Longest prefix match

There are two approaches for calculating the routing tables: Static routing: manually modify the routes. Dynamic routing: routes are calculated by a routing protocol.

34

Routing Protocols vs. Routing Algorithms

Routing protocols establish routing tables at routers.

A routing protocol specifies: What messages are sent between routers. Under what conditions the messages are sent. How messages are processed to compute routing tables.

At the heart of any routing protocol is a routing algorithm that determines the path from a source to a destination.

35

Overview of Routing Protocols

Routing information protocol (RIP)

Distance vector

Interior gateway routing protocol (IGRP, Cisco proprietary)

Distance vector

Open shortest path first (OSPF) Link state

Intermediate System-to-Intermediate System (IS-IS)

Link state

Border gateway protocol (BGP) Path vector

Routing protocol Routing Algorithm

Intra-domain routing protocol

Inter-domain routing protocol

RIP, OSPF, IGRP, IS-IS BGP

Domain not to be confused with domain name. Here domain refers to administrative domain.

36

Intra-domain Routing vs. Inter-domain Routing Recall Internet is a network of networks.

Administrative autonomy Internet = network of networks each network admin. may want to control routing in its

own network

Scale: with 550 million destinations: can’t store all destination IP addresses in routing tables!

perhaps yes but then … routing table exchange would swamp links.

37

Autonomous Systems

aggregate routers into regions, “autonomous systems” (AS) or domain

routers in the same AS run the same routing protocol. “intra-AS” or intra-domain routing protocol routers in different AS can run different intra-AS routing protocol

Ethernet

Router

Ethernet

Ethernet

RouterRouter

Ethernet

Ethernet

EthernetRouterRouter

Router

AutonomousSystem 2

AutonomousSystem 1

38

Autonomous Systems An autonomous system is a region of the Internet that is

administered by a single entity.

Examples of autonomous regions are: Columbia campus network AT&T backbone network Regional Internet Service Provider

Routing is done differently within an autonomous system (intradomain routing) and between autonomous system (interdomain routing).

RIP, OSPF, IGRP, and IS-IS are intra-domain routing protocols.

BGP is the only inter-domain routing protocol.

39

Components of a Routing Algorithm A procedure for sending and receiving

reachability information about network to other routers.

A procedures for reacting to and advertising topology changes.

A procedure for calculating optimal routes. Routes are calculated using a shortest path algorithm:

Goal: Given a network were each link is assigned a cost. Find the path with the least cost between two networks with minimum cost.

40

Approaches to Shortest Path Routing There are two basic routing algorithms found on the Internet. 1. Distance Vector Routing Each node knows the distance (=cost) to its directly connected neighbors. A node sends periodically a list of routing updates to its neighbors. If all nodes update their distances, the routing tables eventually converge. New nodes advertise themselves to their neighbors.

2. Link State Routing Each node knows the distance to its neighbors. The distance information (=link state) is broadcast to all nodes in the

network. Each node calculates the routing tables independently after receiving the

broadcast.

41

Distance Vector Routing Variations of Bellman-Ford algorithm.

Each router starts by knowing: Prefixes of its attached networks (“zero” distance). Its next hop routers (how to find them?)

Each router advertises only to its neighbors: All prefixes it knows about. Its distance from them.

Each router learns: All prefixes its neighbors know about. Their distance from them.

Each router figures out, for each destination prefix: The “distance” (how far away it is). The “vector” (the next hop router).

42

Distance Vector Routing Properties DV computes the shortest path

“Routing by rumor” Each router believes what its neighbors tell it.

In steady-state, each router has the “shortest” (smallest metric) path to the destination.

Convergence time is (on the average) proportional to the diameter of the network.

Any link change affects the entire network.

43

Distance Vector Algorithm A decentralized algorithm

A router knows physically-connected neighbors and link costs to neighbors

A router does not have a global view of the network

Path computation is iterative and mutually dependent. A router sends its known distances to each destination

(distance vector) to its neighbors. A router updates the distance to a destination from all its

neighbors’ distance vectors A router sends its updated distance vector to its

neighbors. The process repeats until all routers’ distance vectors do

not change (this condition is called convergence).

44

Bellman-Ford AlgorithmBellman-Ford EquationDefinedx(y) := cost of the least-cost path from x to y

Then dx(y) = minv{c(x,v) + dv(y) }, where min is

taken over all neighbors of node x

45

Distance Vector Algorithm: Initialization Let Dx(y) be the estimate of least cost from x

to y

Initialization: Each node x knows the cost to each neighbor:

c(x,v). For each neighbor v of x, Dx(v) = c(x,v)

Dx(y) to other nodes are initialized as infinity.

Each node x maintains a distance vector (DV): Dx = [Dx(y): y in N ]

46

Distance Vector Algorithm: Updates Each node x sends its distance vector to its neighbors,

either periodically, or triggered by a change in its DV.

When a node x receives a new DV estimate from a neighbor v, it updates its own DV using B-F equation: If c(x,v) + Dv(y) < Dx(y) then

Dx(y) = c(x,v) + Dv(y) Sets the next hop to reach the destination y to the neighbor v Notify neighbors of the change

The estimate Dx(y) will converge to the actual least cost dx(y)

47

Distance vector algorithm: an example

1 1

1

1 1

1

1

1

Time = 0

48

Distance vector algorithm: an example

Time = 1

49

Distance vector algorithm: an example

Time = 2 (End)

50

How to map the abstract graph to the physical network

Nodes (e.g., v, w, n) are routers, identified by IP addresses, e.g. 10.0.0.1

Nodes are connected by either a directed link or a broadcast link (Ethernet)

Destinations are IP networks, represented by the network prefixes, e.g., 10.0.0.0/16 Net(v,n) is the network directly connected to router v and n.

Costs (e.g. c(v,n)) are associated with network interfaces. Router1(config)# router rip Router1(config-router)# offset-list 0 out 10 Ethernet0/0 Router1(config-router)# offset-list 0 out 10 Ethernet0/1

n

v

w

Net

Net(v,w)

Net(v,n)

c(v,w)

c(v,n)

51

Distance Vector Algorithm: Routing Table

Dest

n

v

w

D (v,Net)n

costvia(next hop)

Net

RoutingTable of node v

Net

Net(v,w)c(v,w)

Net(v,n)c(v,n)

Net(v,w): Network address of the network between v and w The network can be a link, but could also be a LAN

c(v,w): cost to transmit on the interface to network Net(v,w)

52

Distance Vector Algorithm: Messages

Dest

D (v,Net)n

costvia(next hop)

Net

RoutingTable of node v

• Nodes send messages to their neighbors which contain routing table entries

• A message has the format: [Net , D(v,Net)] means“My cost to go to Net is D (v,Net)”

vv nn[Net , D(v,Net)]

53

Distance Vector Algorithm: Sending Updates

Dest

D (v,Net 2)n

costvia(next hop)

Net 2

RoutingTable of node v

D (v,Net 1)mNet 1

D (v,Net N)wNet N

Periodically, each node v sends the content of its routing table to its neighbors:

n

v wm

[Net N,D(v,Net N)]

[Net 1,D(v,Net 1)]

[Net N,D(v,Net N)]

[Net 1,D(v,Net 1)]

[Net N,D(v,Net N)]

[Net 1,D(v,Net 1)]

54

Initiating Routing Table I

Destc (v,w)

Net(v,w)

0m

costvia(next hop)

Net(v,m)

RoutingTablec(v,m)

Net(v,m)

c(v,n)Net(v,n) 0wNet(v,w)

0nNet(v,n)n

v wm

Suppose a new node v becomes active. The cost to access directly connected

networks is zero: D (v, Net(v,m)) = 0 D (v, Net(v,w)) = 0 D (v, Net(v,n)) = 0

55

Initiating Routing Table IIDest

0m

costvia(next hop)

Net(v,m)

RoutingTable

0wNet(v,w)

0nNet(v,n)

New node v sends the routing table entry to all its neighbors:

n

v wm

[w,0]

[n,0 ] [n,0 ]

[m,0]

[m,0]

[w,0]

n

v wm

[Net(v,w),0]

[Net(v,n),0] [Net(v,n),0]

[Net(v,m),0]

[Net(v,w),0]

[Net(v,m),0]

n

v wm

[Net(v,w),0]

[Net(v,n),0] [Net(v,n),0]

[Net(v,m),0]

[Net(v,w),0]

[Net(v,m),0]

56

n

v wm

[Net N,D(n,Net N)]

[Net 1,D(n,Net 1)]

[Net N,D(m,Net N)]

[Net 1,D(m,Net 1)]

[Net N,D(w,Net N)]

[Net 1,D(w,Net 1)]

Initiating Routing Table III Node v receives the routing tables from

other nodes and builds up its routing table

57

Updating Routing Tables I

c(v,m)Net(v,m)

n

v wmNet

[Net,D(m,Net)]

• Suppose node v receives a message from node m: [Net,D(m,Net)]

if ( D(m,Net) + c (v,m) < D (v,Net) ) {Dnew (v,Net) := D (m,Net) + c (v,m);Update routing table;send message [Net, Dnew (v,Net)] to all neighbors

}

Node v updates its routing table and sends out further messages if the message reduces the cost of a route:

58

Updating Routing Tables II

c(v,m)Net(v,m)

n

v wmNet

[Net,D(m,Net)]

• Before receiving the message:Dest

D(v,Net)??

costvia(next hop)

Net

RoutingTable

c(v,m)Net(v,m)

n

v wmNet

[Net,D new (v,Net)]

[Net,D new (v,Net)]

Dest

m

costvia(next hop)

Net

RoutingTable

D new (v,Net)

• Suppose D (m,Net) + c (v,m) < D (v,Net):

59

Example

Router A Router B Router C Router D

10.0.2.0/24 10.0.3.0/24 10.0.4.0/24 10.0.5.0/2410.0.1.0/24

.1.2.2.2.2 .1.1.1

Assume: - link cost is 1, i.e., c(v,w) = 1 - all updates, updates occur simultaneously - Initially, each router only knows the cost of connected interfaces

t=0:10.0.1.0 - 010.0.2.0 - 0

Net via cost

t=0:10.0.2.0 - 010.0.3.0 - 0

Net via cost

t=0:10.0.3.0 - 010.0.4.0 - 0

Net via cost

t=0:10.0.4.0 - 010.0.5.0 - 0

Net via cost

t=1:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 1

t=2:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 110.0.4.0 10.0.2.2 2

t=2:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 110.0.5.0 10.0.3.2 2

t=1:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 1

t=2:10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1

t=1:10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1

t=2:10.0.2.0 10.0.4.1 210.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0

t=1:10.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0

60

Example

Router A Router B Router C Router D

10.0.2.0/24 10.0.3.0/24 10.0.4.0/24 10.0.5.0/2410.0.1.0/24

.1.2.2.2.2 .1.1.1

t=3:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 110.0.4.0 10.0.2.2 210.0.5.0 10.0.2.2 3

Net via cost

t=3:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 110.0.5.0 10.0.3.2 2

Net via cost

t=3:10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1

Net via cost

t=3:10.0.1.0 10.0.4.1 310.0.2.0 10.0.4.1 210.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0

Net via cost

Now, routing tables have converged !

t=2:10.0.1.0 - 010.0.2.0 - 0 10.0.3.0 10.0.2.2 110.0.4.0 10.0.2.2 2

t=2:10.0.1.0 10.0.2.1 1 10.0.2.0 - 010.0.3.0 - 010.0.4.0 10.0.3.2 110.0.5.0 10.0.3.2 2

t=2:10.0.1.0 10.0.3.1 2 10.0.2.0 10.0.3.1 1 10.0.3.0 - 010.0.4.0 - 010.0.5.0 10.0.4.2 1

t=2:10.0.2.0 10.0.4.1 210.0.3.0 10.0.4.1 110.0.4.0 - 010.0.5.0 - 0

61

Characteristics of Distance Vector Routing Periodic Updates: Updates to the routing tables are

sent at the end of a certain time period. A typical value is 90 seconds.

Triggered Updates: If a metric changes on a link, a router immediately sends out an update without waiting for the end of the update period.

Full Routing Table Update: Most distance vector routing protocol send their neighbors the entire routing table (not only entries which change).

Route invalidation timers: Routing table entries are invalid if they are not refreshed. A typical value is to invalidate an entry if no update is received after 3-6 update periods.

62

The Count-to-Infinity ProblemAA BB CC1 1

A's Routing Table B's Routing Table

C

to costvia(next hop)

2B C

to costvia(next hop)

1C

now link B-C goes down

C 2 C oo

C oo-C 2B

C oo C 3

C 3AC oo-

C 4 C oo

C oo-C 4B

63

Count-to-Infinity The reason for the count-to-infinity

problem is that each node only has a “next-hop-view”

For example, in the first step, A did not realize that its route (with cost 2) to C went through node B

How can the count-to-infinity problem be solved?

64

Count-to-Infinity The reason for the count-to-infinity problem is that

each node only has a “next-hop-view”. For example, in the first step, A did not realize that

its route (with cost 2) to C went through node B.

How can the Count-to-Infinity problem be solved? Solution 1: Always advertise the entire path in an

update message (Path vectors). If routing tables are large, the routing messages

require substantial bandwidth. BGP uses this solution.

65

Count-to-Infinity The reason for the count-to-infinity problem is that

each node only has a “next-hop-view” For example, in the first step, A did not realize that

its route (with cost 2) to C went through node B

How can the Count-to-Infinity problem be solved? Solution 2: Never advertise the cost to a neighbor

if this neighbor is the next hop on the current path (Split Horizon) Example: A would not send the first routing update to B,

since B is the next hop on A’s current route to C Split Horizon does not solve count-to-infinity in all cases!

66

RIP - Routing Information Protocol A simple intradomain protocol Straightforward implementation of distance

vector routing. Each router advertises its distance vector every

30 seconds (or whenever its routing table changes) to all of its neighbors

RIP always uses 1 as link metric. Maximum hop count is 15, with “16” equal to “” Routes are timeout (set to 16) after 3 minutes if

they are not updated.

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RIP - History Late 1960s : Distance Vector protocols were used in

the ARPANET Mid-1970s: XNS (Xerox Network system) routing

protocol is the precursor of RIP in IP (and Novell’s IPX RIP and Apple’s routing protocol)

1982 Release of routed for BSD Unix 1988 RIPv1 (RFC 1058)

- classful routing 1993 RIPv2 (RFC 1388)

- adds subnet masks with each route entry

- allows classless routing 1998 Current version of RIPv2 (RFC 2453)

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RIPv1 Packet FormatIP header UDP header RIP Message

Command Version Set to 00...0

32-bit address

Unused (Set to 00...0)

address family Set to 00.00

Unused (Set to 00...0)

metric (1-16)

one

rout

e en

try(2

0 by

tes)

Up to 24 more routes (each 20 bytes)

32 bits

One RIP message can have up to 25 route entries

1: request2: response

2: for IP0…0: request full rou-ting table

Address of destination

Cost (measured in hops)

1: RIPv1

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RIPv2 RIPv2 is an extends RIPv1:

Subnet masks are carried in the route information

Authentication of routing messages Route information carries next-hop address Exploits IP multicasting

Extensions of RIPv2 are carried in unused fields of RIPv1 messages

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RIPv2 Packet FormatIP header UDP header RIP Message

Command Version Set to 00...0

32-bit address

Unused (Set to 00...0)

address family Set to 00.00

Unused (Set to 00...0)

metric (1-16)

one

rout

e en

try(2

0 by

tes)

Up to 24 more routes (each 20 bytes)

32 bits

One RIP message can have up to 25 route entries

1: request2: response

2: for IP0…0: request full rou-ting table

Address of destination

Cost (measured in hops)

2: RIPv2

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RIPv2 Packet FormatIP header UDP header RIPv2 Message

Command Version Set to 00.00

IP address

Subnet Mask

address family route tag

Next-Hop IP address

metric (1-16)

one

rout

e en

try(2

0 by

tes)

Up to 24 more routes (each 20 bytes)

32 bits

Used to carry information from other routing protocols (e.g., autonomous system number)

Identifies a better next-hop address on the same subnet than the advertising router, if one exists (otherwise 0….0)

2: RIPv2

Subnet mask for IP address

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RIP Messages This is the operation of RIP in routed.

Dedicated port for RIP is UDP port 520.

Two types of messages: Request messages

used to ask neighboring nodes for an update Response messages

contains an update

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Routing with RIP Initialization: Send a request packet (command = 1,

address family=0..0) on all interfaces: RIPv1 uses broadcast if possible, RIPv2 uses multicast address 224.0.0.9, if possible

requesting routing tables from neighboring routers Request received: Routers that receive above request send

their entire routing table Response received: Update the routing table

Regular routing updates: Every 30 seconds, send all or part of the routing tables to every neighbor in an response message

Triggered Updates: Whenever the metric for a route change, send entire routing table.

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RIP Security Issue: Sending bogus routing updates to a router RIPv1: No protection RIPv2: Simple authentication scheme

IP header UDP header RIPv2 Message

Command Version Set to 00.00

Password (Bytes 0 - 3)

Password (Bytes 4 - 7)

0xffff Authentication Type

Password (Bytes 8- 11)

Password (Bytes 12 - 15) Auth

etic

atio

nUp to 24 more routes (each 20 bytes)

32 bits

2: plaintext password

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RIP Problems RIP takes a long time to stabilize

Even for a small network, it takes several minutes until the routing tables have settled after a change

RIP has all the problems of distance vector algorithms, e.g., count-to-Infinity RIP uses split horizon to avoid count-to-infinity

The maximum path in RIP is 15 hops