Transcript COMPUTER NETWORKS

CS2302- COMPUTER NETWORKS
RAJALAKSHMI ENGINEERING
COLLEGE
DEPARTMENT OF INFORMATION
TECHNOLOGY
UNIT I
INTRODUCTION:
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A computer network is a group of interconnected
computers
A collection of computers and devices connected to
each other.
Allows computers to communicate with each other
and share resources and information.
Building a Network

To build a network
Identify the set of constraints and requirements
based on
Application programmer
Network designer
Network provider
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Requirements:
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Connectivity
 point to point or multiple access
 Links
physical medium
 Nodes,clouds
computer
Switched Network
 Circuit Switched
 Packet Switched
 Uses store and forward
 Establishes dedicated circuit
 More efficient in working

Routing
 Provides Systematic procedure for forwarding
messages
 Unicasting
 Multicasting
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Cost effective Resources sharing
How system resource is shared effectively by
multiple users
multiplexing
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Multiplexing methods
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STDM - Synchronous time division
multiplexing
FDM - Frequency division multiplexing
Network Architecture
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Provides a general, effective, fair, and robust
connectivity of computers
Provides a blueprint
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Types
OSI Architecture
 Internet Architecture
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OSI ARCHITECTURE

Open Systems Interconnection (OSI) model is a reference
model developed by ISO (International Organization for
Standardization) in 1984
OSI model defines the communications process into Layers
Provides a standards for communication in the
network
Primary architectural model for inter-computing and Inter
networking communications.
network communication protocols have a structure based
on OSI Model
OSI Architecture
Internet Architecture
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TCP/IP Architecture
Four Layer model
TCP,UDP,FTP,HTTP,SMTP Protocols used
Internet Protocol Graph
Direct Links: Outline
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Physical Layer
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Link Layer
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Link technologies
Encoding
Framing
Error Detection
Reliable Transmission (ARQ protocols)
Medium Access Control:
Existing protocols: Ethernet, Token Rings,
Wireless
Link Technologies
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Cables:
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Cat 5 twisted pair, 10-100Mbps, 100m
Thin-net coax, 10-100Mbps, 200m
Thick-net coax, 10-100Mbps, 500m
Fiber, 100Mbps-2.4Gbps, 2-40km
Leased Lines:
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Copper based: T1 (1.544Mbps), T3 (44.736Mbps)
Optical fiber: STS-1 (51.84Mbps), STS-N (N*51.84Mbps)
Link Technologies
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Last-Mile Links:
 POTS (56Kbps), ISDN (2*64Kbps)
 xDSL: ADSL (16-640Kbps, 1.554-8.448Mbps),
VDSL (12.96Mbps-55.2Mbps)
 CATV: 40Mbps downstream, 20Mbps upstream
Wireless Links: Cellular, Satellite, Wireless Local Loop
FRAMING

An efficient data transmission technique

It is a message forwarding system in which data
packets, called frames, are passed from one or many
start-points to one
Approaches

Byte oriented Protocol(PPP)
BISYNC
Binary Synchronous Communication
DDCMP
Digital Data Communication Message Protocol
Bit oriented Protocol(HDLC)
 Clock based Framing(SONET)

Byte oriented Protocol(PPP)
BISYNC FRAME FORMAT
SYH
SYH
SOH
Header
STX
Body
ETX
PPP Frame Format
Flag
Address
Control
Protocol
Payload
Flag
CRC
DDCMP Frame Format
SYN
SYN
Class
Count
Header
Body
CRC
Bit Oriented Protocol(HDLC)
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Collection of Bits
1.HDLC
High-Level Data Link Control
2.Closed Based Framing(SONET)
Synchronous Optical Network
HDLC Frame Format
Beginning
sequence
Header
Body
CRC
Bit Stufffing
After 5 consecutive 1s insert 0
Next bit is 0 – stuffed removed
Next bit is 1 –end of frame or erorr
Ending
sequence
Closed Based Framing(SONET)
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STS-1 Frame
9 rows of 90 byte each
First 3 byte for overhead rest contains data
Payload bytes scrambled- exclusive OR
Supports Multiplexing
Payloads
9 rows
90 columuns
ERROR DETECTION
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Detecting Errors In Transmission
Electrical Interference, thermal noise
Approaches
Two Dimensional Parity
Internet Checksum Algorithm
Cyclic Redundancy Check
Two Dimensional Parity
7 bits of data
8 bits including parity
Number of 1s
even
odd
0000000 (0)
00000000
100000000
1010001 (3)
11010001
01010001
1101001 (4)
01101001
11101001
1111111 (7)
11111111
01111111
Transmission sent using even parity:
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A wants to transmit: 1001
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A computes parity bit value: 1^0^0^1 = 0
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A adds parity bit and sends: 10010
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B receives: 10010 B computes parity: 1^0^0^1^0 =
0
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B reports correct transmission after observing
expected even result.
Transmission sent using odd parity:
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A wants to transmit: 1001
A computes parity bit value: ~(1^0^0^1) = 1
A adds parity bit and sends: 10011
B receives: 10011
B computes overall parity: 1^0^0^1^1 = 1
B reports correct transmission after observing
expected odd result.
Reliable Transmission
Deliver Frames Reliably
Accomplished by Acknowledgements and Timeouts
ARQ-Automatic Repeat Request
Mechanism:
Stop and Wait
Sliding Window
Concurrent Logical Channels
Stop And Wait ARQ
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The source station transmits a single frame and then
waits for an acknowledgement (ACK).
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Data frames cannot be sent until the destination
station’s reply arrives at the source station.
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It discards the frame and sends a negative
acknowledgement (NAK) back to the sender
causes the source to retransmit the damaged frame in
case of error
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Acknowledgements & Timeouts
Sender
Receiver
Sender
Timeout
ACK
Timeout
Timeout
Fram
e
(a)
Timeout
Fram
e
Fram
e
ACK
Sender
Timeout
Receiver
Fram
e
ACK
(c)
Timeout
Sender
Timeout
Time
Fram
e
Receiver
Receiver
Fram
e
ACK
Fram
e
ACK
ACK
(b)
(d)
Stop & wait sequence numbers
Receiver
Sender
Receiver
Sender
Receiver
Timeout
Timeout
Timeout
Timeout
Sender
(c)
(d)
(e)
• Simple sequence numbers enable the client to discard
duplicate copies of the same frame
• Stop & wait allows one outstanding frame, requires two
distinct sequence numbers
Stop And Wait
Sliding Window
 bi-directional data transmission protocol used in the
data link layer (OSI model) as well as in TCP

It is used to keep a record of the frame sequences
sent
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respective acknowledgements received by both the
users.
Sliding Window: Sender
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Assign sequence number to each frame (SeqNum)
Maintain three state variables:
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send window size (SWS)
last acknowledgment received (LAR)
last frame sent (LFS)
Maintain invariant: LFS - LAR <= SWS
Advance LAR when ACK arrives
Buffer up to SWS frames  SWS
…
…
LAR
LFS
Sequence Number Space
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SeqNum field is finite; sequence numbers wrap around
Sequence number space must be larger then number of
outstanding frames
SWS <= MaxSeqNum-1 is not sufficient
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suppose 3-bit SeqNum field (0..7)
SWS=RWS=7
sender transmit frames 0..6
arrive successfully, but ACKs lost
sender retransmits 0..6
receiver expecting 7, 0..5, but receives the original
incarnation of 0..5
SWS < (MaxSeqNum+1)/2 is correct rule
Intuitively, SeqNum “slides” between two halves of sequence
number space
Sliding Window: Receiver
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Maintain three state variables
receive window size (RWS)
 largest frame acceptable (LFA)
 last frame received (LFR)
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 Maintain invariant: LFA RWS
- LFR <= RWS
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…
…
LFR
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LFA
Frame SeqNum arrives:
if LFR < SeqNum < = LFA
accept
 if SeqNum < = LFR or SeqNum > LFA
discarded
 Send cumulative ACKs – send ACK for largest frame
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such that all frames less than this have been received
UNIT II LAN Technology
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LAN (Local Area Network) refers to a group of
computers interconnected into a network
Objective:
they are able to communicate, exchange information
and share resources (e.g. printers, application
programs, database etc).
the same computer resources can be used by
multiple users in the network, regardless of the
physical location of the resources.
LAN Architecture
Describes the way in which the components in a Local
Area Network are connected
LAN Topologies:
Star
Ring
Bus
Tree
Star
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All stations are connected by cable (or wireless) to a
central point, such as hub or a switch.
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central node is operating in a broadcast fashion such as
a Hub
transmission of a frame from one station to the node is
retransmitted on all of the outgoing links.
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Ring
All nodes on the LAN are connected in a loop and their
Network Interface Cards (NIC) are working as repeaters.
No starting or ending point.
Each node will repeat any signal that is on the network
regardless its destination.
The destination station recognizes its address and copies
the frame into a local buffer.
The frame continues to circulate until it returns to the
source station, where it is removed.
Example:Token Ring (IEEE 802.5)
FDDI (IEEE 802.6) another protocol used in the
Bus
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All nodes on the LAN are connected by one linear cable,
which is called the shared medium.
Every node on this cable segment sees transmissions
from every other station on the same segment.
At each end of the bus is a terminator, which absorbs
any signal, removing it from the bus.
This medium cable apparently is the single point of
failure.
Example:Ethernet (IEEE 802.3)
Tree
 Is a logical extension of the bus topology.
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The transmission medium is a branching cable
no closed loops.
The tree layout begins at a point called the head-end
one or more cables start, and each of these may
have branches.
The branches in turn may have additional branches to
allow quite complex layouts.
Topologies
Token Ring
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All stations are connected in a ring and each station can directly
hear transmissions only from its immediate neighbor.
Permission to transmit is granted by a message (token) that
circulates around the ring.
Token Ring as defined in IEEE 802.5 is originated from the IBM
Token Ring LAN technologies.
Token-passing networks move a small frame, called a token
Possession of the token grants the right to transmit.
The information frame circulates the ring until it reaches the
intended destination station, which copies the information for
further processing.
The information frame continues to circle the ring and is finally
removed when it reaches the sending station.
The sending station can check the returning frame to see whether
the frame was seen and subsequently copied by the destination.
Ehernet
local-area network (LAN) covered by the
IEEE 802.3.
 two modes of operation:
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half-duplex
full-duplex modes.
Three basic elements :
1. the physical medium used to carry Ethernet
signals between computers,
2. a set of medium access control rules embedded
in each Ethernet interface that allow multiple
computers to fairly arbitrate access to the shared
Ethernet channel,
3. an Ethernet frame that consists of a
standardized set of bits used to carry data over the
system
IEEE 802.5 Format
Frame Format IEEE 802.5
IEEE 802.3 MAC Data Frame
Format
Wireless
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The process by which the radio waves are propagated
through air and transmits data
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Wireless technologies are differentiated by :
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Protocol
Connection type—Point-to-Point (P2P)
Spectrum—Licensed or unlicensed
Types
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Infrared Wireless Transmission
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Microwave Radio
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Tranmission of data signals using infrared-light
waves
sends data over long distances (regions,
states, countries) at up to 2 megabits per
second (AM/FM Radio)
Communications Satellites
 microwave relay stations in orbit around the earth.
UNIT III Packet Switching
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Is a network communications method
Groups all transmitted data, irrespective of content, type,
or structure into suitably-sized blocks, called packets.
Optimize utilization of available link capacity
Increase the robustness of communication.
When traversing network adapters, switches and other
network nodes
packets are buffered and queued, resulting in variable
delay and throughput, depending on the traffic
Types
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Connectionless
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each packet is labeled with a connection ID rather
than an address.
Example:Datagram packet switching
connection-oriented
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each packet is labeled with a destination
address
Example:X.25 vs. Frame Relay
Star Topology
Source Routing
0 Switch 1
3
0
1
2 Switch 2
2
3 0 1
3
3
1
1
2
1 3 0
0
Host A
0 1 3
1
0 Switch 3
3
2
Host B
Virtual Circuit Switching
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Explicit connection setup (and tear-down)
phase
Subsequence packets follow same circuit
Sometimes called connection-oriented
model
0 Switch 1
3
1
2
5
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Analogy:
phone call
Each switch
maintains a
VC table
3
11
2 Switch 2
1
0
Host A
7
1
0 Switch 3
3
4
2
Host B
Datagram Switching
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No connection setup phase
Each packet forwarded independently
Sometimes called connectionless
model
Host D
Analogy:
postal system
Each switch
maintains a
forwarding
(routing) table
Host E
0 Switch 1
3
Host C
Host F
1
2 Switch 2
2
3
1
0
Host A
Host G
1
0 Switch 3 Host B
3
2
Host H
Virtual Circuit Model
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Typically wait full RTT for connection setup before
sending first data packet.
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While the connection request contains the full
address for destination
each data packet contains only a small identifier,
making the per-packet header overhead small.
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If a switch or a link in a connection fails, the
connection is broken and a new one needs to be
established.
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Connection setup provides an opportunity to
reserve resources.
Datagram Model
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There is no round trip delay waiting for connection
setup; a host can send data as soon as it is ready.
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Source host has no way of knowing if the network
is capable of delivering a packet or if the
destination host is even up.
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Since packets are treated independently, it is
possible to route around link and node failures.
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Since every packet must carry the full address of
the destination, the overhead per packet is higher
than for the connection-oriented model.
Bridges and Extended LANs
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LANs have physical limitations (e.g., 2500m)
Connect two or more LANs with a bridge
 accept and forward strategy
 level 2 connection (does not add packet header)
A
B
C
Port 1
Bridge
Port 2
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Ethernet Switch = Bridge on Steroids
X
Y
Z
Spanning Tree Algorithm
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Problem: loops
A
B
B3
C
B5
D
B2
B7
E
K
F
B1
G
H
B6
B4
I
J
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Bridges run a distributed spanning tree
algorithm
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select which bridges actively forward
developed by Radia Perlman
now IEEE 802.1 specification
Algorithm Details
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Bridges exchange configuration messages
 id for bridge sending the message
 id for what the sending bridge believes to be
root bridge
 distance (hops) from sending bridge to root
bridge
Each bridge records current best configuration
message for each port
Initially, each bridge believes it is the root
Algorithm Details
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Bridges exchange configuration messages
 id for bridge sending the message
 id for what the sending bridge believes to be
root bridge
 distance (hops) from sending bridge to root
bridge
Each bridge records current best configuration
message for each port
Initially, each bridge believes it is the root
Internetworking
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An internetwork is a collection of individual networks,
connected by intermediate networking devices, that
functions as a single large network.
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different kinds of network technologies that can be
interconnected by routers and other networking
devices to create an internetwork
Types
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Local-area networks (LANs)enabled multiple users in a
relatively small geographical area to exchange files and
messages, as well as access shared resources such as
file servers and printers.
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Wide-area networks (WANs) interconnect LANs with
geographically dispersed users to create connectivity.
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technologies used for connecting LANs include T1, T3,
ATM, ISDN, ADSL, Frame Relay, radio links, and others.
ETH
IPV4 Packet Header
Version HLen
TOS
Ident
TTL
Length
Flags
Offset
Protocol
Checksum
SourceAddr
Destination Addr
Options(variable)
Pad(variable)
Data
Datagram Delivery
Packet Format
IPV4 Packet header
Fragmentation and Reassembly
Fragmentation and Reassembly
Fragmentation and Reassembly
(RARP)Reverse Address Resolution
Protocol
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(RARP) is a Link layer networking protocol
RARP is described in internet EngineeringTask ForceETF)
publication RFC 903
It has been rendered obsolete by the Bootstrap Protocol
(BOOTP) and the modern Dynamic Host Configuration
Protocol(DHCP)
BOOTP configuration server assigns an IP address to
each client from a pool of addresses.
BOOTP uses the User Datagram Protocol (UDP)
Routing
 is the process of selecting paths in a network along
which to send network traffic.

Routing is performed for many kinds of networks,
including the telephone network electronic data
networks (such as the Internet), and transportation
networks.
Components
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determining optimal routing paths and transporting
information groups (typically called packets) through
an internetwork.
In the context of the routing process, the latter of
these is referred to as packet switching.
Although
packet
switching
is
relatively
straightforward, path determination can be very
complex.
Distance Vector:
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Distance Vector routing protocols are based on Bellman and
Ford algorithms.
Distance Vector routing protocols are less scalable such as
RIP supports 16 hops and IGRP has a maximum of 100
hops.
Distance Vector are classful routing protocols which means
that there is no support of Variable Length Subnet Mask
(VLSM) and Classless Inter Domain Routing (CIDR).
Distance Vector routing protocols uses hop count and
composite metric.
Distance Vector routing protocols support discontiguous
subnets.
Link State:
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Link State routing protocols are based on Dijkstra
algorithms.
Link State routing protocols are very much scalable
supports infinite hops.
Link State routing protocols are classless which means
that they support VLSM and CIDR.
Cost is the metric of the Link State routing protocols.
Link State routing protocols support contiguous
subnets.
UNIT IV Reliable Byte Stream
TCP Overview
End to end issues
Segment format
Connection establishment
TCP sliding window
Stream control Transmission Protocol
Simple demultiplexor
TCP Congestion Control
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Determines the network capacity
Adjust the number of packets that can have safely in
transit
Acks to pace the transmission of packets
TCP is self clocking
Avoids congestion
Maxwindow=MIN(CongestionWindow,AdvertisedWindo
w)
EffectiveWindow=MaxWindow-(LastByteSentLastByteAcked)
Caused By
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the shortage of buffer space.
slow links.
slow processors
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Possible solutions
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End-to-end versus link-by-link control
Rate-Based versus Credit-Based control
The rate-based traffic-flow technique constantly
Integrated congestion control
Integrated congestion control
Principles of Congestion Control
Congestion:
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informally: “too many sources sending too much
data too fast for network to handle”
different from flow control!
manifestations:
 lost packets (buffer overflow at routers)
 long delays (queueing in router buffers)
a top-10 problem!
Scenario 1: Queuing Delays
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two senders,
two receivers
one router,
infinite buffers
no
retransmission
Host A
Host B
lout
lin : original data
unlimited shared
output link buffers
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large delays
when
congested
maximum
achievable
throughput
Scenario 2: Retransmits
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one router, finite buffers
sender retransmission of lost packet
Host A
Host B
lin : original
data
l'in : original data, plus
retransmitted data
finite shared output
link buffers
lout
Scenario 3: Congestion Near Receiver
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four senders
multihop paths
timeout/retransmit
Host A
l
Q: what happens as
in
and l increase ?
in
lin : original data
l'in : original data, plus
retransmitted data
finite shared output
link buffers
Host B
lout
Approaches towards congestion control
Two broad approaches towards congestion control:
End-end congestion
control:
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no explicit feedback from
network
congestion inferred from
end-system observed loss,
delay
approach taken by TCP
Network-assisted
congestion control:

routers provide feedback
to end systems
 single bit indicating
congestion (SNA,
DECbit, TCP/IP ECN,
ATM)
 explicit rate sender
should send at
TCP Congestion Control
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
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
end-end control (no network How does sender
assistance)
perceive congestion?
sender limits transmission:
 loss event = timeout
LastByteSent-LastByteAcked
or 3 duplicate acks
 CongWin
 TCP sender reduces
rate (CongWin) after
Roughly,
loss event
CongWin
rate =
Bytes/sec
RTT
three mechanisms:
CongWin is dynamic, function
of perceived network
congestion



AIMD
slow start
conservative after
timeout events
TCP AIMD
multiplicative
decrease: cut
CongWin in half after
loss event
congestion
window
additive increase:
increase CongWin by
1 MSS every RTT in
the absence of loss
events: probing
24 Kbytes
16 Kbytes
8 Kbytes
time
Long-lived TCP connection
TCP Slow Start

When connection
begins, CongWin = 1
MSS



Example: MSS = 500
bytes & RTT = 200
msec
initial rate = 20 kbps
available bandwidth
may be >> MSS/RTT

desirable to quickly
ramp up to respectable
rate

When connection
begins, increase rate
exponentially fast until
first loss event
TCP Slow Start (more)

When connection begins,
increase rate exponentially
until first loss event:
 double CongWin every
RTT
 done by incrementing
CongWin for every ACK
received
Summary: initial rate is
slow but ramps up
exponentially fast
Host A
Host B
RTT

time
Refinement (more)
Q: When should the
exponential
increase switch to
linear?
A: When CongWin
gets to 1/2 of its
value before
timeout.
Implementation:


Variable Threshold
At loss event, Threshold is
set to 1/2 of CongWin just
before loss event
TCP sender congestion control
Event
State
TCP Sender Action
Commentary
ACK receipt
for
previously
unacked
data
Slow Start
(SS)
CongWin = CongWin +
MSS,
If (CongWin > Threshold)
set state to “Congestion
Avoidance”
Resulting in a doubling
of CongWin every RTT
ACK receipt
for
previously
unacked
data
Congestio
n
Avoidance
(CA)
CongWin = CongWin+MSS
* (MSS/CongWin)
Additive increase,
resulting in increase of
CongWin by 1 MSS
every RTT
Loss event
detected by
triple
duplicate
ACK
SS or CA
Threshold = CongWin/2,
CongWin = Threshold,
Set state to “Congestion
Avoidance”
Fast recovery,
implementing
multiplicative decrease.
CongWin will not drop
below 1 MSS.
Timeout
SS or CA
Threshold = CongWin/2,
CongWin = 1 MSS,
Set state to “Slow Start”
Enter slow start
Duplicate
ACK
SS or CA
Increment duplicate ACK
count for segment being
acked
CongWin and Threshold
not changed
Congestion Avoidance Mechanisms




Helps to avoid congestion
Additional functionality into the router to assist in
anticipation of congestion
to control congestion once it happens
to repeatedly increase load in an effort to find t
he point at which congestion occurs, and then b
ack off
Mechanisms

router-centric: DECbit and RED Gateways

host-centric: TCP Vegas
DECbit

DECbit



Add binary congestion bit to each packet header
Router
 monitors average queue length over last busy+
idle cycle

set congestion bit if average queue length gre
ater than 1 when packet arrives

attempts to balance throughput against delay
DECbit

End Hosts
destination echos bit back to source

source records how many packets resulted in set bit

if less than 50% of last window's worth had bit set,
then increase CongestionWindow by 1 packet

if 50% or more of last window's worth had bit set, t
hen decrease CongestionWindow by 0.875 times

Random Early Detection (RED)

Notification is implicit
 just drop the packet (TCP will timeout)


could make explicit by marking the packet
Early random drop
 rather than wait for queue to become full, dro
p each arriving packet with some drop probabi
lity whenever the queue length exceeds some
drop level
Random Early Detection (RED)

RED: fills in the details
 compute average queue length

AvgLen=(1Weight)*AvgLen+Weight*SampleLen

0 < Weight < 1 (usually 0.002)

SampleLen is queue length each time a pac
ket arrives
Random Early Detection (RED
Random Early Detection (RED)

two queue length thresholds
if AvgLen ? MinThreshold then

enqueue the packet

if MinThreshold < AvgLen < MaxThreshold

calculate probability P

if MaxThreshold ? AvgLen

drop arriving packet

UNIT V Domain Name Service


is a hierarchical naming system for computers, services in the
Internet
is an IETF-standard name service.

enables client computers on your network to register and
resolve DNS domain names.

names are used to find and access resources offered by other
computers on your network or other networks, such as the
Internet.
three main components of DNS:

Domain name space and associated resource
records (RRs)

DNS Name Servers

DNS Resolvers
Domain name space for the Internet.
Domain Names
Email

Electronic mail abbreviated as e-mail or email

is method of creating, transmitting, or storing primarily
text-based human communications with digital
communications systems

based on a store-and-forward model in which e-mail
computer server systems, accept, forward, or store
messages on behalf of users
SMTP(Simple Mail Transfer
Protocol)

is an Internet standard for electronic mail
transmission

is a TCP/IP protocol used in sending and receiving email

to send and receive mail messages to send and
receive mail messages
SMTP(Simple Mail Transfer Protocol)
SMTP(Simple Mail Transfer Protocol)
MIME




Multipurpose Internet Mail Extensions
SMTP is ASCII based
allows multi part messages containing content of various
types combined into one message
Types



GIF graphics files
PostScript files
MIME messages can contain

text, images, audio, video, and other applicationspecific data.
format of messages




textual message bodies in character sets other than USASCII,
an extensible set of different formats for non-textual
message bodies,
multi-part message bodies, and
textual header information in character sets other than USASCII.
HTTP




is an application-level protocol for distributed,
collaborative, hypermedia information systems.
It is a generic, stateless, protocol which can be
used for many tasks such as name servers and
distributed object management systems, through
extension of its request methods, error codes and
headers [47].
typing and negotiation of data representation
allows systems to be built independently of the
data being transferred.
SNMP


to monitor network-attached devices for conditions
that warrant administrative attention
SNMP basic components






Managed devices
Agents
Network-management stations (NMSs)
Managed devices
Agents
Network-management stations (NMSs)
Email Features




Email is Fast
Email is Inexpensive
Email is Easy to Filter
Transmission is Secure and Reliable

1.Fast - Messages can be sent anywhere
around the world
in an instant
2.cheap - Transmission usually costs nothing,
or at the
most, very little
3.simple - Easy to use, after initial set-up
4.efficient - Sending to a group can be done
in one step
5.versatile - Pictures, powerpoints or other
files can be sent too
World Wide Web
Hypertext and Hypermedia
Browser Architecture
Static Document/HTML
Dynamic Document/CGI
Active Document/Java
Distributed services
Hypertext
Browser architecture
Categories of Web documents
Static document
Boldface tags
Effect of boldface tags
Beginning and ending tags
Common tags
Beginning
Tag
Ending
Tag
Meaning
Skeletal Tags
<HTML>
</HTML>
Defines an HTML document
<HEAD>
</HEAD>
Defines the head of the document
<BODY>
</BODY>
Defines the body of the document
Title and Header Tags
<TITLE>
</TITLE>
Defines the title of the document
<Hn>
</Hn>
Defines the title of the document
Common tags (continued)
Beginning
Tag
Ending
Tag
Meaning
Text Formatting Tags
<B>
</B>
Boldface
<I>
</I>
Italic
<U>
</U>
Underlined
<SUB>
</SUB>
Subscript
<SUP>
</SUP>
Superscript
Data Flow Tag
<CENTER>
</CENTER>
<BR>
</BR>
Centered
Line break
Common tags (continued)
Beginning
Tag
Ending
Tag
Meaning
List Tags
<OL>
</OL>
Ordered list
<UL>
</UL>
Unordered list
<LI>
</LI>
An item in a list
Image Tag
<IMG>
Defines an image
Hyperlink Tag
<A>
</A>
Defines an address (hyperlink)
Executable Contents
<APPLET>
</APPLET>
The document is an applet
Dynamic document
Active document
Skeleton of an applet
Instantiation of the object defined by an applet
Creation and compilation
HTML document carrying an applet
File Transfer
Connections
Communication
File Transfer
User Interface
Anonymous
Note:
FTP uses the services of TCP. It needs
two TCP connections. The well-known
port 21 is used for the control
connection, and the well-known port
20 is used for the data connection.
FTP
Using the control connection
Using the data connection
File transfer