1DT066 Distributed Information System Chapter 4 Network Layer

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1DT066Distributed Information System

Chapter 4Network Layer

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

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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 IPv6

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

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analogy:

routing: process of planning trip from source to dest

forwarding: process of getting through single interchange

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1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing and forwarding


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Datagram format IPv4 addressing ICMP IPv6

NETWORK LAYER CONNECTION AND CONNECTION-LESS SERVICE datagram network provides network-layer

connectionless service VC network provides network-layer

connection service

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VIRTUAL CIRCUITS

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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

FORWARDING TABLE

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12 22 32

1 23

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!

VIRTUAL CIRCUITS: SIGNALING PROTOCOLS

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used in ATM, frame-relay, X.25 not used in today’s Internet

application

transportnetworkdata linkphysical

application


1. Initiate call 2. incoming call

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

DATAGRAM NETWORKSN

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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

application


application


1. Send data 2. Receive data

FORWARDING TABLE

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Destination Address Range Link Interface

11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000 through 2

11001000 00010111 00011111 11111111

otherwise 3

4 billion possible entries

LONGEST PREFIX MATCHING

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Prefix Match Link Interface 11001000 00010111 00010 0 11001000 00010111 00011000 1 11001000 00010111 00011 2 otherwise 3

DA: 11001000 00010111 00011000 10101010

Examples

DA: 11001000 00010111 00010110 10100001 Which interface?

Which interface?


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ROUTER ARCHITECTURE OVERVIEW

Two key router functions: run routing algorithms/protocol (RIP, OSPF, BGP) forwarding datagrams from incoming to outgoing link

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THE INTERNET NETWORK LAYERN

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Host, router network layer functions:

forwardingtable

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


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IP DATAGRAM FORMAT

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ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifier

header checksum

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

offsetupper layer

32 bit destination IP address

Options (if any)

IP FRAGMENTATION & REASSEMBLY

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

frame. 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


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IP ADDRESSING: INTRODUCTION

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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

SUBNETS

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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

SUBNETS

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

223.1.1.0/24223.1.2.0/24

223.1.3.0/24

Subnet mask: /24

SUBNETS

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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

IP ADDRESSING: CIDRCIDR: 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

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11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

IP ADDRESSES: HOW TO GET ONE?

Q: How does a host get IP address?

hard-coded by system admin in a file Windows: 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”

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DHCP: DYNAMIC HOST CONFIGURATION PROTOCOL

Goal: allow host to dynamically obtain its IP address from network server when it joins network Allows reuse of addresses

4-30

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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

BE

DHCP server

arriving DHCP client needsaddress in thisnetwork

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

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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

HIERARCHICAL ADDRESSING: ROUTE AGGREGATION

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with 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:

NAT: NETWORK ADDRESS TRANSLATION

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10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers


Motivation: local network uses just one IP address as far as outside world is concerned: range of addresses not needed from ISP: just one IP

address for all devices can change addresses of devices in local network without

notifying outside world can change ISP without changing addresses of devices in

local network devices inside local net not explicitly addressable, visible

by outside world (a security plus).

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10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

1

10.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr

138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345

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S: 138.76.29.7, 5001D: 128.119.40.186, 80

2

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001

3

3: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345


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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

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IPV6 HEADER (CONT)

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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

CHAPTER 4: SUMMARY

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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 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

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1DT066 Distributed Information System Chapter 4 Network Layer