02 Layers

IT 605 Computer Networks Networking Layers Prof . Anirudha Sahoo KReSIT IIT Bombay

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.1

Network Architecture (OSI)

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.2

TCP/IP Layers • Physical: – transmitting bits over a communication channel

• Data Link: – transforming the raw physical layer into a ‘link’ for the higher layer

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.3

TCP/IP Layers • Network: – addressing and routing of packets

• Transport: – end-to-end connection characteristics

• Application: – “application” protocols

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.4

Physical Layer • Moves bits from one end to other • Bandwidth, latency, S/N ratio

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.5

Bandwidth • Amount of data that can be transmitted per unit time – expressed in cycles per second, or Hertz (Hz) for analog devices – expressed in bits per second (bps) for digital devices – KB = 2^10 bytes; Mbps = 10^6 bps

• Link v/s End-to-End Introduction

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

1.6

Bandwidth v/s bit width

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.7

Latency (delay) • Time it takes to send message from point A to point B – Latency = Propagation + Transmit + Queue – Propagation = Distance / SpeedOfLight – Transmit = Size / Bandwidth

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.8

Latency • Queuing not relevant for direct links • Bandwidth not relevant if Size = 1 bit • Process-to-process latency includes software overhead • Software overhead can dominate when Distance is small • RTT: round-trip time Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.9

Delay X Bandwidth Product • Relative importance of bandwidth and delay • Small message: 1ms vs 100ms dominates 1Mbps vs 100Mbps • Large message: 1Mbps vs 100Mbps dominates 1ms vs 100ms

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.10

Delay X Bandwidth Product

• 100ms RTT and 45Mbps Bandwidth = 560 KB of data Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.11

Application requirements • Best-effort: FTP • Bandwidth guarantees: Video – burst versus peak rate

• Delay guarantees: voice – jitter: variance in latency (interpacket gap)

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.12

Physical Layer – Media dependent components • Copper: Coaxial/Twisted Pair – Typically upto 100 Mbps

• Fibre: Single/Multi Mode – Can transmit in Gigabits/second

• Satellite: – Channels of 64 kbps, 128 kbps

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.13

Physical Layer – Media independent • Connectors: Interface between equipment and link • Control, clock and ground signals • Protocols: – RS 232 (20 kbps, 10 ft) – RS 449 (2 Mbps, 60 ft)

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.14

Data Link Layer Controls a single physical link Service interface to network layer

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.15

Data link functions • Grouping of bits into frames • Dealing with transmission errors • Regulating the flow of frames – so that slow receivers are not swamped by fast senders

• Regulating multiple access to the medium Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.16

Data Link Layer – Logical link control (LLC) • Framing (start and stop) • Error Detection • Error Correction • Optimal Use of Links (Sliding Window Protocol) – Examples: HDLC, LAP-B, LAP-D

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.17

Data Link Layer – Medium access control (MAC) • Multiple Access Protocols • Channel Allocation • Contention, Reservation, Round-robin • Examples: Ethernet (IEEE 802.3), Token Ring (802.5)

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.18

Example: Ethernet (IEEE 802.3) • Ethernet Address (48 bits) – Example: 08:00:0D:01:74:71

• Ethernet Frame Format

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.19

Ethernet medium access: CSMA/CD • Carrier Sense: – Wait for medium to become free

• Multiple Access: – Persistent: Transmit immediately – Non-Persistent: Wait for some (random) time, before trying!

• Collision Detection Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.20

Collision detection and resolution • Listen while transmitting – Minimum transmission time is needed to detect collision

• Upon collision detection – Backoff and retry – Binary exponential backoff

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.21

Network Layer • Need for network layer – All machines are not Ethernet!

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.22

Network layer goals • Hide type of subnet (Ethernet, Token Ring, FDDI ... ) • Hide topology of subnets • Uniform addressing scheme • Routing

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.23

Network layer functions • Internetworking – uniform addressing scheme

• Routing – choice of appropriate paths from source to destination

• Congestion Control – avoid overload on links/routers Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.24

Network layer services • Provides an end to end logical link – Provides packet forwarding, segmentation and reassembly

• end system needs to only format a packet with the specified header and destination address

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.25

Connection Oriented service • Network layer at sender must set up a connection to its peer at the receiver • Negotiation about parameters, quality, and costing are possible • Avoids having to choose routes on a per packet basis

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.26

Connectionless service • Network layer at sender simply puts the packet on the outgoing link without connection setup • Intermediate nodes use routing tables to deliver the packet to destination • Avoids connection setup delays

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.27

Example: Internet Protocol (IP) • Provides connectionless packet delivery and “best-effort” quality of service • No assurance that the packet will reach intended destination

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.28

IP addresses • Logical address at network layer • 32 bit address space – Network number, Host number

• Machines on the same "network" have same network number • One address per interface

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.29

IP address notation • Dotted decimal notation – 144.16.111.2 (Class B) – 202.54.44.120 (Class C) – Special Conventions • All 0s -- this host • All 1s -- limited broadcast (localnet)

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.30

IP address issues • Inefficient: wasted addresses • Inflexible: fixed interpretation • Not scalable: – Number of networks is growing – Not enough network numbers

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.31

IP addressing schemes • Sub-netting: Create sub networks within an address space • CIDR: Variable interpretations for the network number • Ipv6: 128 bit address space

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.32

Address Resolution Protocol (ARP) [RFC 1010] • ARP request - Broadcast ``who is a.b.c.d?'' • ARP reply - Target alone fills • ARP cache “arp –a”

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.33

IP forwarding at a Host • Destination on my net? • If yes, – use ARP and deliver directly

• If not, – give to default gateway

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.34

IP forwarding at a Gateway • Am I the destination IP? • If yes, – deliver packet to higher layer

• If not, – which interface to forward on? – consult Routing Tables to decide

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.35

Network Layer – Building routing tables • Routing Information Protocol (RIP) [RFC 2453] • Open Shortest Path First (OSPF) [RFC 2328] • Border Gateway Protocol (BGP) [RFC 1771]

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.36

Example: RIP

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.37

Network Layer – Congestion Control • Congestion: – Performance degradation due to too many packets present in the subnet

• Causes: – Packets from several input lines needing the same output line – Bursty traffic, slow processors – Insufficient bandwidth/buffering

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.38

Congestion control strategies • Packet discarding • Flow control at higher layers Choke the input • Allocate resources in advance

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.39

Transport Layer • Lowest end-to-end service • Main Issues: – Reliable end-to-end delivery – Flow control – Congestion control

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.40

User Datagram Protocol (UDP) • Datagram oriented • Doesn't guarantee any reliability • Useful for Applications such as voice and video, where – retransmission should be avoided – the loss of a few packets does Prof . Aniruddha sahoo, KReSIT, IIT Bombay Introduction not greatly affect performance

1.41

Transmission Control Protocol (TCP) • Guaranteed service protocol – ensures that a packet has been received by the destination by using acknowledgements and retransmission – applications need to establish a TCP connection prior to transfer

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.42

TCP features • Connection Oriented • Reliable • Byte Stream • Flow Control • Congestion Control

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.43

Different Network Devices

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.44

Interconnection devices (switch)

Basic idea: Transfer data from input to output Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.45

Generic Switch

Latency: Time a switch takes to figure out where to forward a data unit Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.46

Repeater • Amplifies the signal received on input and transmits it on output • Enables the extension of a signal over a distance • More widely deployed in wireless networks Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.47

Repeaters

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.48

Modem • Accepts a serial stream of bits as input and produces a modulated carrier as output (or vice versa) • Typically between (digital) computer and (analog) telephones • ISDN adapter: 128 Kbps v/s 56 Kbps Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.49

Modems

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.50

Hub • Connect nodes/segments of a LAN. – Contains multiple ports. – When a packet arrives at one port, it is copied to the other ports so that all segments of the LAN can see all packets. – A multiple port repeater

• Intelligent/Manageable Hub Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.51

Hubs

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.52

Switch • Reads destination address of each packet and forwards appropriately – Does not re-broadcast every packet to every port

• Layer 3 switches (IP switches) also perform routing functions – “Route once, switch many” – Initially packets are routed – Cache entry for next hop for source and dest pair is made Prof . – Aniruddha sahoo, KReSIT, IIT Bombay Introduction Packets are then switched using the cache1.53

Switches

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.54

Bridge • Connects two LANs or two segments of the same LAN – The two LANs may be alike or dissimilar – e.g., a bridge can connect an Ethernet with a Token-Ring LAN

• May be needed due to the physical distance between two LANs • May be because providing so many hosts may not be possible on one LAN. • Provides security against promiscuous mode Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.55

Bridge • Examines each message on a LAN – “ignores” same LAN destinations – forwards ones for interconnected LANs

• messages are accepted only by the intended destination

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.56

Bridges

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.57

Router • Connects at least two networks – decides routes for packets, based on destination address and network topology

• Exchanges information with other routers – to learn network topology

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.58

Router • Maintains a table of – available routes and their conditions

• uses table along with path cost algorithms to – determine the best route for a given packet

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.59

Routers

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.60

Gateway • A network point that acts as an entrance to another network • Often associated with a router • Often acts as a proxy server and a firewall server

Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.61

Satellites • Geo-Stationary: 35,680 km – Footprint (30% of Earth's surface)

• Functions: – Accept uplink signals – Translate signals to downlink frequency – Transmit downlink signals Prof . Aniruddha sahoo, KReSIT, IIT Bombay

Introduction

1.62

Satellites • Characteristics – Transmission cost independent of distance – Propagation delay (0.28 seconds!) – Broadcast medium (Security problem!)

• Master Earth Station • Transponders – Listens to some portion of the spectrum, amplifies the incoming Prof . Aniruddha sahoo, KReSIT, IIT Bombay Introduction signal and rebroadcast at another

1.63