avishai wool lecture 10 - 1 introduction to systems programming lecture 10 networks

Avishai Woollecture 10 - 1

Introduction to Systems Programming Lecture 10

Networks


Protocols & Layering


First Principles

• Goal: transfer information, as a string of bits, across space

• Requires interpretation conventions

• Such a convention is called a protocol.


A telephone call:

• Each layer obeys strict interface rules

• Each layer “talks” to its counterpart only

The Layering Approach

idea

wording

sound

signals wire

idea

wording

sound

signals


The ISO/OSI Model • International Standards Organization: ISO

– (A bunch of people).

• Open Standards Interconnect: OSI – (A bunch of documents).

• A convention of organizing network protocols into layers

• Gives terminology and central roles to various layers


ISO/OSI Layers

MACHINE #1 MACHINE #2

DATA

|DATA

||DATA

|||DATA

||||DATA

|||||DATA

||||||DATA

Application

PresentationSession

Transport

Network

Data Link

Physical

DATA

|DATA

||DATA

|||DATA

||||DATA

|||||DATA

||||||DATA

Application

PresentationSession

Transport

Network

Data Link

Physical


Layer 1: Physical

• Unit of exchange: Bits

• Convert bits to signals, and signals to bits.

• Physical interfaces

• Power

• Frequency, amplitude, phase

• Coding

• Modulation and demodulation


Layer 2: Data Link

• Also called: MAC: Medium Access Control

• Unit of exchange: Frame– Usually over common (broadcast) media– Handles multiple access to shared PHY layer – Controls a Local-Area Network (LAN)

– Bit ordering– Error-correction codes (ECC)– May be unreliable due to noise


Layer 3: Network

• Unit of exchange: Packet / Datagram

• Controlling the operation of multiple networks

• Addressing

• Routing: how to get a packet to its destination

• Unreliable– Packets can arrive out of order or be dropped


Layer 4: Transport

• Transport: create a reliable message channel from an unreliable packet channel.

• Acknowledgements (ACKs) and retransmissions

• Sequencing and re-ordering

• Congestion

• Source and Destination port numbers

• Handshake


Layers 5-7

• Higher level protocols

• Less defined roles

• Not clear where “Application” ends and “Presentation” begins

• Session examples: SQL (database), NFS (files)

• Presentation examples: MPEG, MIDI


TCP/IP Layers

Each sending layer adds a “wrapper” to message, corresponding receiving layer “peels” it

1. physical

2. data-link

3. network

4. transport

5-7. application message

messageTCP header

messageTCP headerIP header

messageTCP headerIP headerethernet header


Layer 2

Layer 4

Layer 3


Local Area Networks

Layer 2


LAN Basics

• Bus/ring: twisted pair, coax, fiber, wireless (radio)

• Broadcast medium (Multiple Access): – Everyone hears everyone else– No routing, no switching, no buffering, no servers– One sender at a time

• Requires conflict resolution protocol


Ethernet


Ethernet: IEEE 802.3• Metcalfe & Boggs (1973) Xerox Digital 3Com• Speed: 10 Mbps / 100Mbps / 1000Mbps• Cabling: coax, twisted-pair• Secret of success: ingenious conflict resolution protocol

“thin ethernet” – coax – BNC Ethernet – twisted-pair – RJ45

Ethernet

Classic Ethernet Switched Ethernet

Computer

(a) (b)


802.3 Protocol Basics• Addresses: unique 48 bits, up to 1024 on a net

(MAC Address)

• Each vendor has a unique prefix and his own numbering system

• Devices have MAC address “burned” into them (not really…)

• Frame size 64-1518 bytes, containing:– destination (maybe everybody!), source, data,

checksum


Basic Idea• Bus architecture:

• TX of each connected to RX of all others (broadcast)

• Problem: simultaneous transmissions (collisions) overwrite and destroy each other


Ethernet sensing

• When a station wants to transmit:

1. Listen (sense) the wire. • If busy: wait

2. Start transmitting – and listen to own transmission on wire.

3. If (my sensed bits != my transmitted bits) another station must be transmitting too collision – start conflict resolution.


Ethernet Conflict ResolutionSuppose 2 or more stations • collide (transmit at same time) and • sense (know) they collided.

What to do to avoid livelock? • Goal: One station should get priority and transmit. • Solution: CSMA/CD

– Carrier Sense

– Multiple Access

– Collision Detection


Backoff

First idea: use randomization!• Each colliding station i picks its own random

delay di

• Waits di time units, and tries to transmit again

• How to pick the range of random values?• Too small stations will get same value and

collide again• Too large slows down the communication


Exponential backoff• If they know how many stations are colliding (n),

each could pick di randomly from, e.g., [1,...2n]. – If range is [1,...2n] then it is very likely that the

minimal di is chosen by only one station

• Second idea: guess number of colliding stations!• Start with guess n=2; if unsuccessful, double n.• This is exponential:

– range of random numbers is 2, 4, 8, 16, 32, …


802.3 CSMA/CD Algorithm

ready to send

transmitand sense

sense channel

wait MAX_DELAY *random[0, 2attempts]

done

new attempt

busy

free

no collision

collision detected

Exponential Backoff

Collision Detection

Carrier Sense


Main properties• If diameter = 1 km, max_delay < 5S.• Collisions possible in first 2 •max_delay time• Minimum packet lengths – ensure that collision

will be sensed• Extremely flexible!• Throughput degrades under heavy load

– optimal: 1/e 37%

• Non-deterministic service: real time connections?• No fixed priorities


Wireless LAN (WiFi)


Wireless LAN: IEEE 802.11• Uses wireless communication• Base station / PCMCIA cards for laptops

• Operates in unregulated 2.4GHz range• Has to deal with interference from microwave

ovens, new cordless phones, etc.• 802.11b: up to 11Mbps. 802.11g: 54Mbs• Range of 10-100 meters• Frequency-hopping (Spread-Spectrum)


Vendor solutions


Hobbyists build antennas from:


And:


Properties of IEEE 802.11• Home-brew antennas have excellent range: over

350m.• http://www.turnpoint.net/wireless/has.html

• Uses encryption: Wire-Equivalent Privacy (WEP)• But: Major security problems

– Encryption broken– Authentication broken– Data integrity broken– By default WEP is turned OFF…


Connecting networks to each other


Repeaters

Goal: enhance physical layer

physical

data-link

network

physical

data-link

network

physical physical

a2d-d2a


Bridges/Switches/Hubs

physical

data-link

network

physical

data-link

network

physical physical

filter

data-link data-link

• Goal: connect at the data-link layer• Can connect different physical layers (WiFi +

Ethernet)• Multiple ports• Filter packets: listen and learn location of sources


Internet Protocol (IP)

Layer 3


Properties of IP• Point-to-point, datagram service• Connect multiple LANs to each other• Addressing: 32-bit IP-addresses.

– Must be unique in entire network (whole Internet!)

• Get the packets to their destination (routing).• Connectionless

– each packet carries its source & destination IP addresses

– each packet routed independently through network

• Unreliable– packets can arrive out of order or be dropped entirely


IP addresses

• 32-bit IP address usually written in “dotted-decimal” notation, each number in 0-255– 132.66.32.10

• IP addresses are a valuable resource. Allocated by IANA (Internet Assigned Number Authority)– http://www.iana.org/

• Allocated hierarchically through delegation– ARIN : North America– RIPE : Europe, Middle east


Routers: IP traffic junctions• Goal: forward IP packets towards destination• Means: “sit” on more than one network (a few ports);

maintain a routing table; forward according to table

network A

network B

network C

router

Cdefault

B18.4.*

A10.0.4.3

A192.168.2.4destination next hop


Routers – more detail

• Routers connect two or more LANs:Multiple network interface cards (NIC)Each NIC has its own MAC addressEach NIC has its own IP address!

IP address mapped to NIC, not to computer


Routing to directly connected hosts

• Router decides what to do with packet based on its destination IP address

• Directly connected == destination on same LAN as one of the router’s NICs– Find out the destination MAC address– Encapsulate packet in Ethernet frame, – Send to destination via layer 2

Routers do NOT modify the destination IP address!


Routing to remote addresses

• If destination IP address not directly connected:

• Router checks routing table

• Finds IP address of next-hop router– Next-hop router has to be directly connected!

• Finds MAC address of next hop router

• Encapsulate packet & send as before


How to find a MAC address ?

• “ARP”: Address Resolution Protocol

• Layer “2.5”: above Ethernet but below IP

– Broadcast to Ethernet “who is IP a.b.c.d?”– Owner of that IP responds (or timeout)– Each station keeps a table of recent mappings

• Windows/Unix: type “arp –a” to see table


Every computer is a “router”

• Every machine running IP is a “router”– Only one NIC– Only one IP address– Usually only one next-hop router (default gateway)

for non-local traffic

• To see contents of routing table:– Windows (command prompt): “route print”– Unix: netstat –rn


Concepts for review• ISO 7-layer model• Physical layer• DataLink layer• Network layer• Transport layer• Ethernet• MAC Address• Exponential backoff• Wireless LAN (802.11)• Bridge / Hub / Switch

• Internet Protocol (IP)• IP Address• Router• Routing table• Next-hop router

avishai wool lecture 10 - 1 introduction to systems programming lecture 10 networks

Documents

avishai wool lecture

ethernet slide

demodulation slide

midi slide

noise slide

layer talks

systems programming

transport transport