Special Topics on Wireless Adhoc Networks

1
Special Topics on Wireless Ad-hoc Networks
Lecture 1 Review of Computer Networking

• University of Tehran
• Dept. of EE and Computer Engineering
• By
• Dr. Nasser Yazdani Farshad Lahouti

2
A tour of networking

• Goal and objective
• needs
• design
• requirement
• Whirlwind tour of networking

3
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

4
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.

5
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.

6
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

7
Design goals

• Connectivity
• Scalability
• Simplicity
• For designers.
• Most importantly for users.
• Efficiency
• cost
• performance
• Support for common user services.

8
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

9
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

10
Types of Links
Point-to-Point
Multiple Access

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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

12
What are Switched Networks?

• Switch moves bits between links
• Packet switching
• Circuit switching

Switched Network
13
Back in the Old Days
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Then Came TDM

• Synchronous time division multiplexing

Multiplex (mux)
Demultiplex (demux)
15
TDM Logical Network View
16
Packet Switching (Internet)
Packets
17
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

18
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
  gt10 are transmitting at the same time lt 0.0017

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Characteristics of Packet Switching

• Store and forward
• Packets are self contained units
• Can use alternate paths - reordering
• Contention
• Congestion
• Delay

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Second Step Internetwork

• A collection of interconnected networks
• Host network endpoints (computer, PDA, light
  switch, )
• Router node that connects networks
• Internet vs. internet

Internetwork
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Challenge

• Many differences between networks
• Address formats
• Performance bandwidth/latency
• Packet size
• Loss rate/pattern handling
• Routing
• How to translate between various network
  technologies

22
Third Step How To Find Nodes?
internet
Computer 1
Computer 2
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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

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Domain Name System
Whats the IP address for www.cmu.edu?
It is 128.2.11.43
Local DNS Server
Computer 1
DNS server address manually configured into OS
25
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

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NetworkAddress Resolution Protocol
Broadcast who knows the Ethernet address for
128.2.11.43?
Ethernet
Broadcast Yes, it is 08-00-2c-19-dc-45
Ethernet
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Internetwork Datagram Routing
Routers send packet to next closest point
H
R
H
R
H
R
R
R
R
H
R
H Hosts R Routers
R
H
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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

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Fourth Step Application Demands

• Reliability
• Corruption
• Lost packets
• Flow and congestion control
• Fragmentation
• In-order delivery
• Etc

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What if the Data gets Corrupted?
Problem Data Corruption
GET windex.html
GET index.html
Internet
Solution Add a checksum
X
0,9
9
6,7,8
21
4,5
7
1,2,3
6
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What if the Data gets Lost?
Problem Lost Data
GET index.html
Internet
Solution Timeout and Retransmit
GET index.html
GET index.html
Internet
GET index.html
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What if Network is Overloaded?
Problem Network Overload
Solution Buffering and Congestion Control

• Short bursts buffer
• What if buffer overflows?
• Packets dropped and retransmitted
• Sender adjusts rate until load resources
• Called Congestion control

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What if the Data Doesnt Fit?
Problem Packet size

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

Solution Fragment data across packets
GET
inde
x.ht
ml
GET index.html
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What if the Data is Out of Order?
Problem Out of Order
GET
x.th
inde
ml
GET x.thindeml
Solution Add Sequence Numbers
ml
4
inde
2
x.th
3
GET
1
GET index.html
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Network Functionality Summary

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

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What is Layering?

• Modular approach to network functionality
• The idea of divide and conquer
• Use abstraction to hide complexity.
• Example

Application
Application-to-application channels
Host-to-host connectivity
Link hardware
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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

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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

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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)

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Layering
User A
User B
Application
Transport
Network
Link
Host
Host
Layering technique to simplify complex systems
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Layer Encapsulation
User A
User B
Get index.html
Connection ID
Source/Destination
Link Address
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Protocol Demultiplexing

• Multiple choices at each layer

FTP
HTTP
TFTP
NV
TCP
UDP
TCP/UDP
IP
Network
IP
IPX
Port Number
Protocol Field
Type Field
NET1
NET2
NETn

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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

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OSI Layers and Locations
Application
Presentation
Session
Transport
Network
Data Link
Physical
Switch
Host
Router
Host
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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, ..

46
Stop and Wait

• Simplest ARQ protocol
• Send a packet, stop and wait until
  acknowledgement arrives
• Use sequence number to recognize repeat

Sender
Receiver
Time
Timeout
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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

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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

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Goals of MAC Protocols
MAC Protocols arbitrate access to a common shared
channel among a population of nodes
Goals 1. Fair among users 2. High
efficiency 3. Low delay 4. Fault tolerant 5-
Simple
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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

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Examples of MAC Protocols
Packet-Switched Radio Network
Simple Random
Aloha
Carrier Sense Multiple Access/Collision Detection
Ethernet (IEEE 802.3)
Token Passing
Token Ring (IEEE 802.5) Fiber Distributed Data
Interface (FDDI)
Complex Deterministic
Wireless
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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.

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Goals

• Connect existing networks
• initially ARPANET and ARPA packet radio network
• Survivability
• ensure communication service even in the presence
  of network and router failures
• Support multiple types of services
• Must accommodate a variety of networks
• Allow distributed management
• Allow host attachment with a low level of effort
• Allow resource accountability

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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

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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

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IP Layering (Principle 8)

• Relatively simple

Application
Transport
Network
Link
Router
Host
Router
Host
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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

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Common View of the Telco Network
Brick
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Common View of the IP Network
60
Needs some intelligence!
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IP Internet

• Concatenation of Networks or networks of
  Networks.
• R is routers and H is hosts.

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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 format

Contains all information for routing!
63
IP Address Classes (Some are Obsolete)
Network ID
Host ID
8
16
32
24
Class A
Network ID
Host ID
0
Class B
10
Class C
110
Class D
Multicast Addresses
1110
Class E
Reserved for experiments
1111
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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.

65
Original IP Route Lookup

• Address classes
• A 0 7 bit network 24 bit host (16M each)
• B 10 14 bit network 16 bit host (64K)
• C 110 21 bit network 8 bit host (255)
• We need to keep only network address, 221entries.
• Address would specify prefix for forwarding table
• Simple lookup

66
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

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What is Routing?
68
What is Routing?
69
What is Routing?
70
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

71
Factors Affecting Routing

• Routing algorithms view the network as a graph
• Problem find the lowest cost path between two
  nodes
• Factors
• Static topology
• Dynamic load
• Policy

A
6
1
3
2
F
1
E
B
4
1
9
D
C
72
Internet Routing

• Internet organized as a two level hierarchy
• First level autonomous systems (ASs)
• AS region of network under a single
  administrative domain
• ASs run an intra-domain routing protocols
• Distance Vector, e.g., Routing Information
  Protocol (RIP)
• Link State, e.g., Open Shortest Path First (OSPF)
• Between ASs runs inter-domain routing protocols,
  e.g., Border Gateway Routing (BGP)
• De facto standard today, BGP-4

73
Example
Interior router
BGP router
AS-1
AS-3
AS-2
74
Transport Layer

• First end-to-end layer
• End-to-end state
• May provide reliability, flow and congestion
  control

75
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.

76
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

16
16
16
Size in bits
16
77
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

78
Whats 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
• Cant expect endpoints to exactly match link
• Buffer space availability
• Packets in transmission, delay X bandwidth
• Transmission rate
• Dont directly know transmission rate

79
TCP Header
Source port
Destination port
Sequence number
Flags
SYN FIN RESET PUSH URG ACK
Acknowledgement
Advertised window
HdrLen
Flags
0
Checksum
Urgent pointer
Options (variable)
Data
80
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

81
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

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TCP Congestion Control Basics

• Keep a congestion window, cwnd
• Denotes how much network is able to absorb
• Senders maximum window
• Min (advertised window, cwnd)
• Senders 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

83
Self-clocking
Pr
Pb
Sender
Receiver
Ab
As
Ar
84
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

85
Slow Start Example
86
Congestion Window
Slow start with each time out
Congestion Window
Time
87
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

88
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 lt ssthresh, use slow start
• Else use congestion avoidance

89
Fast Retransmit

• Dont 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
cwnd 2
segment 2
segment 3
ACK 3
ACK 4
cwnd 4
segment 4
segment 5
segment 6
segment 7
ACK 4
ACK 4
3 duplicate ACKs
ACK 4
90
Fast Recovery

• Each duplicate ack notifies sender that single
  packet has cleared network
• When lt 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

91
Fast Recovery
92
Fast Recovery
Sent for each dupack after W/2 dupacks arrive
Sequence No
Time
93
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

94
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 Queuing
• Queuing time can be a dominant factor

95
Latency (Queuing Delay)
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.
TRANSP1
Host A
Q2
TRANSP2
How can we determine the queuing delay?
R1
PROP1
TRANSP3
R2
PROP2
TRANSP4
R3
PROP3
Host B
PROP4
96
Queues and Queuing Delay
Cross traffic causes congestion and variable
queuing delay.
97
A router queue
Model of router queue
Buffer
Server
A(t), l
D(t)
m
Q(t)
98
A router queue (cont)

• Usually buffer size is finite
• State of the system depends on
• Packet arrival process, (Poisson, deterministic,
  etc)
• Packet length distribution
• The service discipline (FCFS, LCFS, priority,
  etc)
• of Server, service process

99
A simple deterministic modelbytes or fluid
Cumulative number of bits that arrived up until
time t.
A(t)
A(t)
Cumulative number of bits
D(t)
Q(t)
m
Service process
m
time
D(t)

• Properties of A(t), D(t)
• A(t), D(t) are non-decreasing
• A(t) gt D(t)

Cumulative number of departed bits up until time
t.
100
Simple deterministic model
Cumulative number of bits
d(t)
A(t)
Q(t)
D(t)
time

• 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)

101
Time evolution of a queuePackets
Model of FIFO router queue
A(t), l
D(t)
m
Q(t)
Packet Arrivals
time
Departures
Q(t)
102
Littles Result
103
The Poisson process

• Arrival process is Poisson
• Queuing 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 ?t is ? ?t o(?t)

104
The Poisson process (cont)

• Poisson process is a probability distribution
  function.
• Sp(k) 1 for all k0, 1,
• How many arrivals in t second? It is the expected
  value
• Skp(k) ?t
• What is interarrival time, r, between two arrival
• f(r) ?e-?r
• This is the same the service time.
• f(r) µe- µr

105
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.

106
An M/M/1 queue

• A(t) is a Poisson process with rate l, and the
  time to serve each packet is exponentially
  distributed with rate m, 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.
• ? l/m , ration of load on capacity, is
  utilization or traffic intensity.

107
An M/M/1 queue (cont)
l
l
l
l
l
l
.
n1
n
n-1
2
1
0
m
m
m
m
m
m
m

• Prob. that the system move from state n-1 to n is
  l , with no departure, and probability that it
  moves from state n to n-1 is m. In order the
  system to be in stationary state the probability
  of departure and moving state should be equal.
• (l m)Pn lPn-1 mPn1

108
An M/M/1 queue (cont)
l
l
l
l
l
.
n1
n
n-1
2
1
0
m
m
m
m
m
m

• Considering the rate of interring and leaving the
  surface gives us .
• Pn mPn1 gt Pn1 rPn gt Pn rnP0
• What is the value of P0?
• SnPn 1 gt P0Snrn 1 gt P0 1 r
• Pn (1 r) rn

109
An M/M/1 queue
Model of FIFO router queue
A(t), l
D(t)
m

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