chapter 4 network layer - student.cs.uwaterloo.cacs456/s08/week4.pdf · typically a tcp or udp...

1

Chapter 4Network Layer

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following:� If you use these slides (e.g., in a class) in substantially unaltered form, that you mention their source (after all, we’d like people to use our book!)� If you post any slides in substantially unaltered form on a www site, that you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2007J.F Kurose and K.W. Ross, All Rights Reserved

Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.

2

Chapter 4: Network Layer

Chapter goals:❒ understand principles behind network layer services:❍ network layer service models

❍ forwarding versus routing

❍ how a router works

❍ routing (path selection)

❍ dealing with scale

❍ advanced topics: IPv6, mobility

❒ instantiation, implementation in the Internet

3


❒ 4. 1 Introduction

❒ 4.2 Virtual circuit and datagram networks

❒ 4.3 What’s inside a router

❒ 4.4 IP: Internet Protocol❍ Datagram format

❍ IPv4 addressing

❍ ICMP

❍ IPv6

❒ 4.5 Routing algorithms❍ Link state

❍ Distance Vector

❍ Hierarchical routing

❒ 4.6 Routing in the Internet

❍ RIP

❍ OSPF

❍ BGP

❒ 4.7 Broadcast and multicast routing

4

Network layer

❒ transport segment from sending to receiving host

❒ on sending side encapsulates segments into datagrams

❒ on rcving side, delivers segments to transport layer

❒ network layer protocols in every host, router

❒ router examines header fields in all IP datagrams passing through it

applicationtransportnetworkdata linkphysical


networkdata linkphysical network

data linkphysical

networkdata linkphysical







networkdata linkphysicalnetwork

data linkphysical

5

Two Key Network-Layer Functions

❒ forwarding: move packets from router’s input to appropriate router output

❒ routing: determine route taken by packets from source to dest.

❍ routing algorithms

analogy:

❒ routing: process of planning trip from source to dest

❒ forwarding: process of getting through single interchange

6

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding

7

Connection setup

❒ 3rd important function in some network architectures:

❍ATM, frame relay, X.25

❒ before datagrams flow, two end hosts andintervening routers establish virtual connection

❍ routers get involved

8

Network service model

Q: What service model for “channel” transporting datagrams from sender to receiver?

Example services for individual datagrams:

❒ guaranteed delivery

❒ guaranteed delivery with less than 40 msec delay

Example services for a flow of datagrams:

❒ in-order datagram delivery

❒ guaranteed minimum bandwidth to flow

❒ restrictions on changes in inter-packet spacing

9






❍ IPv4 addressing

❍ ICMP

❍ IPv6


❍ Distance Vector



❍ RIP

❍ OSPF

❍ BGP


10

Network layer connection and connection-less service

❒ datagram network provides network-layer connectionless service

❒ VC network provides network-layer connection service

❒ Specifically:

❍ service: host-to-host

❍ no choice: network provides one or the other

❍ implementation: in network core

11

Virtual circuits

❒ call setup, teardown for each call before data can flow

❒ each packet carries VC identifier (not destination host address)

❒ every router on source-dest path maintains “state” for each passing connection

❒ link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)

“source-to-dest path behaves much like telephone circuit”

❍ performance-wise

❍ network actions along source-to-dest path

12

VC implementation

a VC consists of:

1. path from source to destination

2. VC numbers, one number for each link along path

3. entries in forwarding tables in routers along path

❒ packet belonging to VC carries VC number (rather than dest address)

❒ VC number can be changed on each link.

❍ New VC number comes from forwarding table

13

Forwarding table

12 22 32

12

3

VC number

interfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …

Forwarding table innorthwest router:

Routers maintain connection state information!

14

Virtual circuits: signaling protocols

❒ used to setup, maintain teardown VC

❒ used in ATM, frame-relay, X.25

❒ not used in today’s Internet



1. Initiate call 2. incoming call

3. Accept call4. Call connected5. Data flow begins 6. Receive data

15

Datagram networks

❒ no call setup at network layer

❒ routers: no state about end-to-end connections❍ no network-level concept of “connection”

❒ packets forwarded using destination host address❍ packets between same source-dest pair may take different

paths



1. Send data 2. Receive data

16

Forwarding table

Destination Address Range Link Interface

11001000 00010111 00010000 00000000through 0

11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000through 1

11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000through 2

11001000 00010111 00011111 11111111

otherwise 3

4 billion possible entries

17

Longest prefix matching

Prefix Match Link Interface11001000 00010111 00010 0 11001000 00010111 00011000 111001000 00010111 00011 2

otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?

18

Datagram or VC network: why?

Internet (datagram)❒ data exchange among

computers

❍ “elastic” service, no strict timing req.

❒ “smart” end systems (computers)

❍ can adapt, perform control, error recovery

❍ simple inside network, complexity at “edge”

❒ many link types

❍ different characteristics

❍ uniform service difficult

ATM (VC)❒ evolved from telephony

❒ human conversation:

❍ strict timing, reliability requirements

❍ need for guaranteed service

❒ “dumb” end systems

❍ telephones

❍ complexity inside network

19






❍ IPv4 addressing

❍ ICMP

❍ IPv6


❍ Distance Vector



❍ RIP

❍ OSPF

❍ BGP


20

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols•path selection•RIP, OSPF, BGP

IP protocol•addressing conventions•datagram format•packet handling conventions

ICMP protocol•error reporting•router “signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

21

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

headerchecksum

time tolive

32 bit source IP address

IP protocol versionnumber

header length(bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgsfragment

offsetupperlayer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

❒ 20 bytes of TCP

❒ 20 bytes of IP

❒ = 40 bytes + app layer overhead

22

IP Fragmentation & Reassembly

❒ network links have MTU (max.transfer size) - largest possible link-level frame.

❍ different link types, different MTUs

❒ large IP datagram divided (“fragmented”) within net

❍ one datagram becomes several datagrams

❍ “reassembled” only at final destination

❍ IP header bits used to identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

23

IP Fragmentation and Reassembly

ID=x

offset=0

fragflag=0

length=4000

ID=x

offset=0

fragflag=1

length=1500

ID=x

offset=185

fragflag=1

length=1500

ID=x

offset=370

fragflag=0

length=1040

One large datagram becomesseveral smaller datagrams

Example

❒ 4000 byte datagram

❒ MTU = 1500 bytes

1480 bytes in data field

offset =1480/8

24






❍ IPv4 addressing

❍ ICMP

❍ IPv6


❍ Distance Vector



❍ RIP

❍ OSPF

❍ BGP


25

IP Addressing: introduction

❒ IP address: 32-bit identifier for host, router interface

❒ interface: connection between host/router and physical link

❍ router’s typically have multiple interfaces

❍ host typically has one interface

❍ IP addresses associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

26

Subnets

❒ IP address:❍ subnet part (high order

bits)

❍ host part (low order bits)

❒ What’s a subnet ?❍ device interfaces with

same subnet part of IP address

❍ can physically reach each other without intervening router

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet

27

Subnets 223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Recipe

❒ To determine the subnets, detach each interface from its host or router, creating islands of isolated networks. Each isolated network is called a subnet.

Subnet mask: /24

28

Subnets

How many? 223.1.1.1

223.1.1.3

223.1.1.4

223.1.2.2223.1.2.1

223.1.2.6

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.2

223.1.7.0

223.1.7.1223.1.8.0223.1.8.1

223.1.9.1

223.1.9.2

29

IP addressing: CIDR

CIDR: Classless InterDomain Routing❍ subnet portion of address of arbitrary length

❍ address format: a.b.c.d/x, where x is # bits in subnet portion of address

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

30

IP addresses: how to get one?

Q: How does host get IP address?

❒ hard-coded by system admin in a file

❍ Wintel: control-panel->network->configuration->tcp/ip->properties

❍ UNIX: /etc/rc.config

❒ DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server

❍ “plug-and-play”

31

IP addresses: how to get one?

Q: How does network get subnet part of IP addr?

A: gets allocated portion of its provider ISP’s address space

ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20

Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23

... ….. …. ….

Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

32

Hierarchical addressing: route aggregation

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

.

.

.

.

.

.

Hierarchical addressing allows efficient advertisement of routing information:

33

Hierarchical addressing: more specific routes

ISPs-R-Us has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us“Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

.

.

.

.

.

.

34

IP addressing: the last word...

Q: How does an ISP get block of addresses?

A: ICANN: Internet Corporation for Assigned

Names and Numbers

❍ allocates addresses

❍ manages DNS

❍ assigns domain names, resolves disputes

35

Chapter 5: Link Layer

❒ 5.1 Introduction and services

❒ 5.2 Error detection and correction

❒ 5.3Multiple access protocols

❒ 5.4 Link-Layer Addressing (ARP)

❒ 5.5 Ethernet

❒ 5.6 Link-layer switches

❒ 5.7 PPP

❒ 5.8 Link Virtualization: ATM, MPLS

36

ARP: Address Resolution Protocol

❒ Each IP node (host, router) on LAN has ARP table

❒ ARP table: IP/MAC address mappings for some LAN nodes< IP address; MAC address; TTL>

❍ TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min)

Question: how to determineMAC address of Bknowing B’s IP address?

1A-2F-BB-76-09-AD

58-23-D7-FA-20-B0

0C-C4-11-6F-E3-98

71-65-F7-2B-08-53

LAN

137.196.7.23

137.196.7.78

137.196.7.14

137.196.7.88

37

ARP protocol: Same LAN (network)

❒ A wants to send datagram to B, and B’s MAC address not in A’s ARP table.

❒ A broadcasts ARP query packet, containing B's IP address

❍ dest MAC address = FF-FF-FF-FF-FF-FF

❍ all machines on LAN receive ARP query

❒ B receives ARP packet, replies to A with its (B's) MAC address

❍ frame sent to A’s MAC address (unicast)

❒ A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out)

❍ soft state: information that times out (goes away) unless refreshed

❒ ARP is “plug-and-play”:❍ nodes create their ARP

tables without intervention from net administrator

38

Addressing: routing to another LAN

R

1A-23-F9-CD-06-9B

222.222.222.220111.111.111.110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

A74-29-9C-E8-FF-55

222.222.222.221

88-B2-2F-54-1A-0F

B222.222.222.222

49-BD-D2-C7-56-2A

walkthrough: send datagram from A to B via R

assume A knows B’s IP address

❒ two ARP tables in router R, one for each IP network (LAN)

39

❒ A creates IP datagram with source A, destination B

❒ A uses ARP to get R’s MAC address for 111.111.111.110

❒ A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram

❒ A’s NIC sends frame

❒ R’s NIC receives frame

❒ R removes IP datagram from Ethernet frame, sees its destined to B

❒ R uses ARP to get B’s MAC address

❒ R creates frame containing A-to-B IP datagram sends to B

R

1A-23-F9-CD-06-9B

222.222.222.220

111.111.111.110

E6-E9-00-17-BB-4B

CC-49-DE-D0-AB-7D

111.111.111.112

111.111.111.111

A74-29-9C-E8-FF-55

222.222.222.221

88-B2-2F-54-1A-0F

B222.222.222.222

49-BD-D2-C7-56-2A

This is a really importantexample – make sure youunderstand!

40






❍ IPv4 addressing (DHCP)

❍ ICMP

❍ IPv6


❍ Distance Vector



❍ RIP

❍ OSPF

❍ BGP


41

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when joining network

❍ support for mobile users joining network

❍ host holds address only while connected and “on”(allowing address reuse)

❍ renew address already in use

❒ DHCP overview:

❍ 1. host broadcasts “DHCP discover” msg

❍ 2. DHCP server responds with “DHCP offer” msg

❍ 3. host requests IP address: “DHCP request” msg

❍ 4. DHCP server sends address: “DHCP ack” msg

42

DHCP client-server scenario

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

A

B

E

DHCP server

arriving DHCP client needsaddress in this(223.1.2/24) network

43

DHCP client-server scenario

DHCP server: 223.1.2.5 arrivingclient

time

DHCP discover

src : 0.0.0.0, 68 dest.: 255.255.255.255,67yiaddr: 0.0.0.0transaction ID: 654

DHCP offer

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 654Lifetime: 3600 secs

DHCP request

src: 0.0.0.0, 68 dest:: 255.255.255.255, 67yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

DHCP ACK

src: 223.1.2.5, 67 dest: 255.255.255.255, 68yiaddrr: 223.1.2.4transaction ID: 655Lifetime: 3600 secs

44






❍ IPv4 addressing

❍ ICMP

❍ IPv6


❍ Distance Vector



❍ RIP

❍ OSPF

❍ BGP


45

ICMP: Internet Control Message Protocol

❒ used by hosts & routers to communicate network-level information

❍ error reporting: unreachable host, network, port, protocol

❍ echo request/reply (used by ping)

❒ network-layer “above” IP:

❍ ICMP msgs carried in IP datagrams

❒ ICMP message: type, code plus first 8 bytes of IP datagram causing error

Type Code description0 0 echo reply (ping)3 0 dest. network unreachable3 1 dest host unreachable3 2 dest protocol unreachable3 3 dest port unreachable3 6 dest network unknown3 7 dest host unknown4 0 source quench (congestion

control - not used)8 0 echo request (ping)9 0 route advertisement10 0 router discovery11 0 TTL expired12 0 bad IP header

46

Traceroute and ICMP

❒ Source sends series of UDP segments to dest

❍ First has TTL =1

❍ Second has TTL=2, etc.

❍ Unlikely port number

❒ When nth datagram arrives to nth router:

❍ Router discards datagram

❍ And sends to source an ICMP message (type 11, code 0)

❍ Message includes name of router& IP address

❒ When ICMP message arrives, source calculates RTT

❒ Traceroute does this 3 times

Stopping criterion

❒ UDP segment eventually arrives at destination host

❒ Destination returns ICMP “port unreachable” packet (type 3, code 3)

❒ When source gets this ICMP, stops.

47






❍ IPv4 addressing

❍ ICMP

❍ IPv6


❍ Distance Vector



❍ RIP

❍ OSPF

❍ BGP


48

IPv6

❒ Initial motivation: 32-bit address space soon to be completely allocated.

❒ Additional motivation:

❍ header format helps speed processing/forwarding

❍ header changes to facilitate QoS

IPv6 datagram format:

❍ fixed-length 40 byte header

❍ no fragmentation allowed

49

IPv6 Header (Cont)

Priority: identify priority among datagrams in flowFlow Label: identify datagrams in same “flow.”

(concept of“flow” not well defined).Next header: identify upper layer protocol for data

50

Other Changes from IPv4

❒ Checksum: removed entirely to reduce processing time at each hop

❒ Options: allowed, but outside of header, indicated by “Next Header” field

❒ ICMPv6: new version of ICMP

❍ additional message types, e.g. “Packet Too Big”

❍ multicast group management functions

51

Transition From IPv4 To IPv6

❒ Not all routers can be upgraded simultaneous

❍ no “flag days”

❍ How will the network operate with mixed IPv4 and IPv6 routers?

❒ Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers

52

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6IPv4 IPv4

53

TunnelingA B E F

IPv6 IPv6 IPv6 IPv6

tunnelLogical view:

Physical view:A B E F

IPv6 IPv6 IPv6 IPv6

C D

IPv4 IPv4

Flow: XSrc: ADest: F

data


data


data

Src:BDest: E


data

Src:BDest: E

A-to-B:IPv6

E-to-F:IPv6

B-to-C:IPv6 inside

IPv4

B-to-C:IPv6 inside

IPv4

54

Gateway

❒ Interaction between IPv6 clients and IPv4 servers – or vice versa?

❍ tunneling does not help

❒ Solution: special “IPv4” address range in IPv6

addresses

❒ Need protocol gateway to convert between

IPv4 and IPv6

❒ More complicated than tunneling

chapter 4 network layer - student.cs.uwaterloo.cacs456/s08/week4.pdf · typically a tcp or udp...

Documents