univ. of tehranwireless ad hoc/sensor networks1 special topics on wireless ad-hoc networks...

Univ. of TehranWireless Ad Hoc/Sensor

Networks 1

Special TopicsSpecial Topics on on

Wireless Ad-hoc Wireless Ad-hoc NetworksNetworks

University of TehranDept. of EE and Computer Engineering

By:Dr. Nasser Yazdani & Farshad Lahouti

Lecture 5: Review of Review of Computer NetworkingComputer Networking

Univ. of Tehran Computer Network 2

A tour of networking Goal and objective

needs design requirement

Whirlwind tour of networking


Information, Computers and Networks

Information: anything that is represented in bits Form (can be represented) vs substance

(cannot) Properties:

Infinitely replicable Computers can “manipulate” information Networks create “access” to information

Potential of networking: move bits everywhere, cheaply, and with

desired performance characteristics Break the space barrier for information


Objective of Networking ? Direct or indirect access to every other

node in the network Connectivity is the magic needed to

communicate if you do not have a link. Must understand many connection

factors Traffic data rate Traffic pattern (bursty or constant bit rate) Traffic target (multipoint or single

destination, mobile or fixed) Application requirements, Delay sensitivity

Loss sensitivity.


Another view

Building a network to support diverse ranges of applications Distributed computing. Multimedia. Telecommunication. E-commerce, etc.

What kind of technology do we need? Hardware. Software.


What is a Network?

What is computer Network? Different views. Differences from other networks, Its

generality. What is requirements? Different

perspective: Network provider Network designer Application programmer


Design goals

Connectivity Scalability Simplicity

For designers. Most importantly for users.

Efficiency cost performance

Support for common user services.


Levels of Networking Communicating across a link, LANs.

Connecting together multiple links (Bridges)

Finding and routing data to nodes on Internet.

Communicating on the application level, matching application requirements


A First Step Creating a link between nodes Link: path followed by bits

Wired or wireless Broadcast or point-to-point (or both)

Node: any device connected to a link


Types of Links

Point-to-Point Multiple Access

…


Packet Transmission Modes

Unicast Transmission to single specific receiver

Broadcast Transmission to all network nodes

Multicast Transmission to specific subset of nodes

Anycast Transmission to one of a specific subset

of nodes


Switched Network

What are Switched Networks?

Switch: moves bits between links Packet switching Circuit switching


Back in the Old Days…


Then Came TDM…

Multiplex (mux) Demultiplex (demux)

• Synchronous time division multiplexing


TDM Logical Network View


Packet Switching (Internet)

Packets


Packet Switching Interleave packets from different

sources Efficient: resources used on demand

Statistical multiplexing General

Multiple types of applications Accommodates bursty traffic

Addition of queues


Statistical Multiplexing Gain

1 Mbps link; users require 0.1 Mbps when transmitting; users active only 10% of the time

Circuit switching: can support 10 users

Packet switching: with 35 users, probability that >=10 are transmitting at the same time < 0.0017


Characteristics of Packet Switching

Store and forward Packets are self contained units Can use alternate paths - reordering

Contention Congestion Delay


Internet[work]

Second Step: Internet[work]

A collection of interconnected networks

Host: network endpoints (computer, PDA, light switch, …)

Router: node that connects networks

Internet vs. internet


Challenge Many differences between networks

Address formats Performance – bandwidth/latency Packet size Loss rate/pattern handling Routing

How to translate between various network technologies


Third Step: How To Find Nodes?

internet

Computer 1 Computer 2


Naming Humans use readable host names

E.g. www.cmu.edu Globally unique (can correspond to

multiple hosts) Naming system translates to

physical address E.g. DNS translates name to IP Address

(e.g. 128.2.11.43) Address reflects location in network


Domain Name System

What’s the IP address for www.cmu.edu?

It is 128.2.11.43

DNS server address manually configured into OS

Local DNS ServerComputer 1


Packet Routing/Delivery Each network technology has different

local delivery methods Address resolution provides delivery

information within network E.g., ARP maps IP addresses to Ethernet

addresses Local, works only on a particular network

Routing protocol provides path through an internetwork


Network:Address Resolution Protocol

Ethernet

Broadcast: who knows the Ethernet address for 128.2.11.43?

Ethernet

Broadcast: Yes, it is08-00-2c-19-dc-45


Internetwork: Datagram Routing

R

R

R

RRH

H

H

H

R

RH

R

Routers send packet to next closest point

H: Hosts

R: Routers


Routing Forwarding tables at each router

populated by routing protocols. Original Internet: manually updated Routing protocols update tables

based on “cost” Exchange tables with neighbors or

everyone Use neighbor leading to shortest path


Fourth Step: Application Demands

Reliability Corruption Lost packets

Flow and congestion control Fragmentation In-order delivery Etc…


What if the Data gets Corrupted?

InternetGET windex.htmlGET index.html

Solution: Add a checksum

Problem: Data Corruption

0,9 9 6,7,8 21 4,5 7 1,2,3 6X


What if the Data gets Lost?

InternetGET index.html

Problem: Lost Data

InternetGET index.html

Solution: Timeout and Retransmit

GET index.htmlGET index.html


What if Network is Overloaded?

Problem: Network Overload

Short bursts: buffer What if buffer overflows?

Packets dropped and retransmitted Sender adjusts rate until load = resources

Called “Congestion control”

Solution: Buffering and Congestion Control


Problem: Packet size

Solution: Fragment data across packets

What if the Data Doesn’t Fit?

On Ethernet, max IP packet is 1.5kbytes Typical web page is 10kbytes

GETindex.html

GET index.html


Solution: Add Sequence Numbers

Problem: Out of Order

What if the Data is Out of Order?

GETx.thindeml

GET x.thindeml

GET index.html

ml 4 inde 2 x.th 3 GET 1


Network Functionality Summary

Link Multiplexing Routing Addressing/naming (locating peers) Reliability Flow control Fragmentation Etc….


What is Layering? Modular approach to network

functionality The idea of divide and conquer Use abstraction to hide complexity. Example:

Link hardware

Host-to-host connectivity

Application-to-application channels

Application


Protocols Module in layered structure Set of rules governing communication

between network elements (applications, hosts, routers)

Protocols define: Interface to higher layers (API) Interface to peer

Format and order of messages Actions taken on receipt of a message


Protocols Building blocks of a network architecture Each protocol object has two different

interfaces service interface: operations on this protocol peer-to-peer interface: messages exchanged

with peer Term “protocol” is overloaded

specification of peer-to-peer interface module that implements this interface


Layering Characteristics Each layer relies on services from

layer below and exports services to layer above

Interface defines interaction Hides implementation - layers can

change without disturbing other layers (black box)


Layering

Host Host

Application

Transport

Network

Link

User A User B

Layering: technique to simplify complex systems


Layer Encapsulation

Get index.html

Connection ID

Source/Destination

Link Address

User A User B


Protocol Demultiplexing Multiple choices at each layer

FTP HTTP TFTPNV

TCP UDP

IP

NET1 NET2 NETn…

TCP/UDPIPIPX

Port Number

Network

Protocol Field

Type Field


E.g.: OSI Model: 7 Protocol Layers

Physical: how to transmit bits Data link: how to transmit frames Network: how to route packets Transport: how to send packets

end2end Session: how to tie flows together Presentation: byte ordering, security Application: everything else


OSI Layers and Locations

Switch RouterHost Host

Application

Transport

Network

Data Link

Presentation

Session

Physical


Physical Layer Dealing with Transmission/Receiving bits. Encoding digital data, 0 & 1, on the signal

NRZ, Manchester coding Framing Error Detection

CRC, checksum Error Correction- Reliable data Transmission

FEC- Forward Error Correction ARQ- Automatic Repeat Request, Stop & wait, ..


Stop and Wait

Time

Packet

ACKTim

eout

Simplest ARQ protocol

Send a packet, stop and wait until acknowledgement arrives

Use sequence number to recognize repeat

Sender Receiver


How to Keep the Pipe Full?How to Keep the Pipe Full? Send multiple packets

Number of pkts in flight = window

How large a window is needed Round trip delay * bandwidth =

capacity of pipe Reliable, unordered delivery

Several parallel stop & waits Send new packet after each ack

After Nack Go back N Resent the Nacked packet only


Data Link Layer Three or more machines are physically

connected and communicating. Problems:

How to connect them? Topology Sharing links

How to address each machine? Addressing How to regulate accessing to the media?

MAC (Media Access method or protocol) Collision!

Different LAN technology Ethernet ~85 Token Ring ~12


Goals of MAC Protocols

MAC Protocols arbitrate access to acommon shared channel among a

population of nodes

Goals:1. Fair among users2. High efficiency3. Low delay4. Fault tolerant5- Simple


Multiplexing/Media Access/Sharing

Partition the channel and give everybody a time/freq slot FDMA CDMA: (Code Division Multiple Access)

Different codes TDMA

Taking turn Token Ring

Random Access ALOHA CSMA, etc

Reservation Based access or Centralized arbiter


Examples of MAC Protocols

Packet-Switched Radio NetworkAloha

Carrier Sense Multiple Access/Collision Detection

Ethernet (IEEE 802.3)Token Passing

Token Ring (IEEE 802.5)Fiber Distributed Data Interface (FDDI)

Sim

ple

Ran

dom

Com

ple

x

Dete

rmin

isti

c

Wireless


Internetworking Communication between networks. Problems & Challenges

Different Networking technologies (Heterogeneity).

So many Networks (Scaling). Some terminologies:

“internetworking” refer to an arbitrary collection of connected networks.

“Internet” the global internetwork. “Network” either directly connected or

switched network using any LAN technology such as Ethernet, Token ring, ATM, etc.


Goals0 Connect existing networks

initially ARPANET and ARPA packet radio network

1. Survivability- ensure communication service even in the

presence of network and router failures

2. Support multiple types of services3. Must accommodate a variety of networks4. Allow distributed management5. Allow host attachment with a low level of effort6. Allow resource accountability


Gateway Alternatives Translation

Difficulty in dealing with different features supported by networks

Scales poorly with number of network types (N2 conversions)

Standardization “IP over everything” (Design Principle 1) Minimal assumptions about network Hourglass design


Survivability Continue to operate even in the presence of

network failures (e.g., link and router failures) As long as the network is not partitioned, two endpoint

should be able to communicate, moreover, any other failure (excepting network partition) should be transparent to endpoints

Decision: maintain state only at end-points (fate-sharing)

Eliminate the problem of handling state inconsistency and performing state restoration when router fails

Internet: stateless network architecture


IP Layering (Principle 8) Relatively simple

Router RouterHost Host

Application

Transport

Network

Link


End-to-End Argument (Principle 2)

Deals with where to place functionality Inside the network (in switching elements) At the edges

Argument There are functions that can only be correctly

implemented by the endpoints do not try to completely implement these at them elsewhere

Can provide a partial form as performance enhancement

Guideline not a law


Common View of the Telco Network

Brick


Common View of the IP Network


Needs some intelligence!

routing


IP Internet Concatenation of Networks or

“networks of Networks”. “R” is routers and “H” is hosts.

R2

R1

H4

H5

H3H2H1

Network 2 (Ethernet)

Network 1 (Ethernet)

H6

Network 3 (FDDI)

Network 4(point-to-point)

H7 R3 H8


Service Model Connectionless (datagram-based) Best-effort delivery (unreliable service)

packets are lost. No recover from lost. packets are delivered out of order duplicate copies of a packet are delivered packets can be delayed for a long time

Datagram formatVersion HLen TOS Length

Ident Flags Offset

TTL Protocol Checksum

SourceAddr

DestinationAddr

Options (variable) Pad(variable)

0 4 8 16 19 31

Data

Contains all information for routing!


IP Address Classes (Some are Obsolete)

Network ID Host ID

Network ID Host ID

8 16

Class A32

0

Class B 10

Class C 110

Multicast AddressesClass D 1110

Reserved for experimentsClass E 1111

24


Forwarding vs. Routing Forwarding: the process of moving

packets from input to output The forwarding table Lookup. How to populate the lookup table?

Routing: process by which the forwarding table is built and maintained One or more routing protocols Procedures (algorithms) to convert routing

info to forwarding table.


CIDR Revisited Supernets

Assign adjacent net addresses to same org

Classless routing (CIDR) How does this help routing table?

Combine routing table entries whenever all nodes with same prefix share same hop


What is Routing?R3

A

B

C

R1

R2

R4 D

E

FR5

R5F

R3E

R3D

Next Hop

Destination

D


What is Routing?R3

A

B

C

R1

R2

R4 D

E

FR5

R5F

R3E

R3D

Next Hop

Destination

D

16 3241

Data

Options (if any)

Destination Address

Source Address

Header ChecksumProtocolTTL

Fragment OffsetFlags

Fragment ID

Total Packet LengthT.ServiceHLen

Ver

20

byte

s


What is Routing?

A

B

C

R1

R2

R3

R4 D

E

FR5


How do we set up Routing Tables?

Graph theory to compute “shortest path” Switches = nodes Links = edges Delay, hops = cost

Need to adapt to changes in topology


Factors Affecting Routing

4

3

6

21

9

1

1

Routing algorithms view the network as a graph

Problem: find the lowest cost path between two nodes

Factors Static topology Dynamic load Policy

D

A

FE

B

C


Internet Routing Internet organized as a two level hierarchy

First level – autonomous systems (AS’s) AS – region of network under a single

administrative domain AS’s run an intra-domain routing protocols

Distance Vector, e.g., Routing Information Protocol (RIP) Link State, e.g., Open Shortest Path First (OSPF)

Between AS’s runs inter-domain routing protocols, e.g., Border Gateway Routing (BGP)

De facto standard today, BGP-4


Example

AS-1

AS-2

AS-3

Interior router

BGP router


Transport Layer First end-to-end layer End-to-end state May provide reliability, flow and

congestion control


Why End-to-End Protocols?

Underlying best-effort network drop messages re-orders messages delivers duplicate copies of a given message limits messages to some finite size delivers messages after an arbitrarily long delay multiple application processes on each host Different speed of sender and receiver (Flow

control) Congestion in the network (Congestion controls)

Initially, there was no end to end protocol.


User Datagram Protocol (UDP)

Minimal Transport Service: Port addressing: for application multiplexing Error detection (Checksum): formerly optional Connectionless end-to-end datagram service

No flow control. No error recovery (no acks) Used by SNMP, DNS, TFTP, RTP, RPC, etc

SourcePort

DestPort

Check-sum

Length

16 16 16 Size in bits16


TCP Communication abstraction:

Connection oriented, Point to point Reliable

Error Detection and correction Ordered Byte-stream

Application writes bytes TCP sends segments Application reads bytes

Full duplex, two way connection Flow and congestion controlled

Protocol implemented entirely at the ends Fate sharing


What’s Different From Link Layers?

Logical link vs. physical link Must establish connection

Variable RTT May vary within a connection

Reordering packets How long can packets live max segment

lifetime Can’t expect endpoints to exactly match link

Buffer space availability Packets in transmission, delay X bandwidth

Transmission rate Don’t directly know transmission rate


TCP Header

Source port Destination port

Sequence number

Acknowledgement

Advertised windowHdrLen Flags0

Checksum Urgent pointer

Options (variable)

Data

Flags: SYNFINRESETPUSHURGACK


TCP Flow Control TCP is a sliding window protocol

For window size n, can send up to n bytes without receiving an acknowledgement

When the data is acknowledged then the window slides forward

Each packet advertises a window size Indicates number of bytes the receiver has

space for Original TCP always sent entire window

Congestion control now limits this


TCP Congestion Control Underlying design principle: packet

conservation, Make load udaptable At equilibrium, inject packet into network only

when one is removed Reaching equilibrium

Slow start Eliminates spurious retransmissions

Accurate RTO estimation Fast retransmit

Adapting to resource availability Congestion avoidance


TCP Congestion Control Basics Keep a congestion window, cwnd

Denotes how much network is able to absorb

Sender’s maximum window: Min (advertised window, cwnd)

Sender’s actual window: Max window - unacknowledged segments

If we have large actual window, should we send data in one shot? No, use acks to clock sending new data


Self-clocking

PrPb

Ar

Ab

ReceiverSender

As


Slow Start How do we get this clocking behavior

to start? Initialize cwnd = 1 Upon receipt of every ack, cwnd = cwnd

+ 1 Implications

Window actually increases to W in RTT * log2(W)

Can overshoot window and cause packet loss


Slow Start Example

1

One RTT

One pkt time

0R

2

1R

3

4

2R

567

83R

91011

1213

1415

1

2 3

4 5 6 7


Congestion Window

Time

CongestionWindow

Slow start with each time out


Congestion Avoidance Loss implies congestion – why?

Not necessarily true on all link types If loss occurs when cwnd = W

Network can handle 0.5W ~ W segments Set cwnd to 0.5W (multiplicative

decrease) Upon receiving ACK

Increase cwnd by 1/cwnd Results in additive increase


Return to Slow Start If packet is lost we lose our self

clocking as well Need to implement slow-start and

congestion avoidance together When timeout occurs set ssthresh to

0.5w If cwnd < ssthresh, use slow start Else use congestion avoidance


Fast Retransmit Don’t wait for window

to drain Resend a segment

after 3 duplicate ACKs remember a duplicate

ACK means that an out-of sequence segment was received

Notes: duplicate ACKs due to

packet reordering why reordering?

window may be too small to get duplicate ACKs

ACK 2

segment 1cwnd = 1

cwnd = 2 segment 2segment 3

ACK 4cwnd = 4 segment 4

segment 5segment 6segment 7

ACK 3

3 duplicateACKs

ACK 4

ACK 4

ACK 4


Fast Recovery Each duplicate ack notifies sender that

single packet has cleared network When < cwnd packets are outstanding

Allow new packets out with each new duplicate acknowledgement

Behavior Sender is idle for some time – waiting for ½

cwnd worth of dupacks Transmits at original rate after wait

Ack clocking rate is same as before loss


Fast Recovery


Fast Recovery

Time

Sequence NoSent for each dupack after

W/2 dupacks arrive


Is Layering Harmful? Sometimes..

Layer N may duplicate lower level functionality (e.g., error recovery)

Layers may need same info (timestamp, MTU)

Strict adherence to layering may hurt performance


Performance Metrics Bandwidth (throughput)

data transmitted per time unit link versus end-to-end notation

KB = 210 bytes Mbps = 106 bits per second

Latency (delay) time to send message from point A to point B one-way versus round-trip time (RTT) components

Latency = Propagation + Transmit + QueuingQueuing time can be a dominant factor


LatencyLatency (Queuing Delay)

Host A

Host B

R1

R2

R3

TRANSP1

TRANSP2

TRANSP3

TRANSP4

PROP1

PROP2

PROP3

PROP4

( )i i ii

TRANSP PROP Q Actual end to end latency

Q2

The egress link might not be free, packets may be queued in a buffer. If the network is busy, packets might have to wait a long time.

How can we determine the queuing delay?


Queues and Queuing Delay

Cross traffic causes congestion and

variable queuing delay.


A router queue

A(t), D(t)

Model of router queue

Q(t)

( ) : [0, ].

:

( ) : [0, ].

1 :

The arrival process. The number of arrivals in interval

The average rate of new arrivals in packets/ second.

The departure process. The number of departures in interval

Th

A t t

D t t

( )

e average time to service each packet.

: The number of packets in the queue at time . Q t t

Buffer Server


A router queue (cont)

A(t), D(t)

Model of router queue

Q(t)

Buffer Server

Usually buffer size is finiteState of the system depends on :1. Packet arrival process, (Poisson, deterministic, etc)2. Packet length distribution3. The service discipline (FCFS, LCFS, priority, etc)4. # of Server, service process


A simple deterministic modelbytes or “fluid”

A(t)

D(t)Cumulative number of

departed bits up until time t.

time

Service process

Cumulativenumber of bits

Cumulative number of bits that arrived up until time

t.

A(t)

D(t)

Q(t)

Properties of A(t), D(t): A(t), D(t) are non-decreasing A(t) >= D(t)


D(t)

A(t)

time

Q(t)

d(t)

•Queue occupancy: Q(t) = A(t) - D(t).•Queuing delay, d(t), is the time spent in the queue by a bit that arrived at time t, and if the queue is served first-come-first-served (FCFS or FIFO)

Simple deterministic model

Cumulativenumber of bits


Time evolution of a queuePackets

A(t), D(t)

Model of FIFO router queue

Q(t)

time

Packet Arrivals:

Departures:

Q(t)

1


Little’s Result

Where:

is the average number of customers in the system

(the number in the queue + the number in service),

is the arrival rate, in customers per second, and

is the average time that a

L

L

d

d

customer waits in the

system (time in queue + time in service).

Result holds so long as no customers are lost/ dropped.


The Poisson process

( )( )

!.

Poisson process is a simple arrival process in which:

1. Probability of arrivals in an interval of seconds is:

2. The expected number of arrivals in interval is:

3. Successive

kt

k

k t

tP t e

kt t

interarrival times are independent of each other

(i.e. arrivals are not bursty).

Arrival process is PoissonQueuing system is M/M/1, Poisson arrival, Exponential service,with 1 server.• Arrival process is momeryless or arrival of packets are independent of each others •Prob. of one arrival in Δt is λ Δt + o(Δt)


The Poisson process (cont) Poisson process is a probability distribution

function.Σp(k) = 1 for all k=0, 1, …

How many arrivals in t second? It is the expected value:Σkp(k) = λt

What is interarrival time, r, between two arrivalf(r) = λe-λr

This is the same the service time.f(r) = μe- μr


The Poisson process Why use the Poisson process?

It is the continuous time equivalent of a series of coin tosses. It is known to model well systems in which a large number of

independent events are aggregated together. e.g. Arrival of new phone calls to a telephone switch Decay of nuclear particles “Shot noise” in an electrical circuit

It makes the math easy. Be warned

Network traffic is very bursty! Packet arrivals are not Poisson. But it models quite well the arrival of new flows.


An M/M/1 queue

A(t) is a Poisson process with rate , and the time to serve each packet is exponentially distributed with rate , then:

We assume the system is in steady state, or stationary, with none time varying values.

Pn is the probability that there are n customer in the queue including the one in the service.

ρ= ration of load on capacity, is utilization or traffic intensity.

A(t), D(t)



An M/M/1 queue (cont)

Prob. that the system move from state n-1 to n is with no departure, and probability that it moves from state n to n-1 is In order the system to be in stationary state the probability of departure and moving state should be equal.PnPn-1Pn+1

0 21 n-1 n n+1

….


An M/M/1 queue (cont)

Considering the rate of interring and leaving the surface gives us .PnPn+1 => Pn+1= Pn => Pn = nP0

What is the value of P0?

ΣnPn=> P0Σnn==> P0 =–

Pn =(–n

0 21 n-1 n n+1

….


An M/M/1 queue

If A(t) is a Poisson process with rate , and the time to serve each packet is exponentially distributed with rate , then:

A(t), D(t)


1;Average delay, and so f rom Little's Result: d L d

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