Transcript Network layer

NETE0510
Network and Protocol
Architecture
Supakorn Kungpisdan
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Outline
Requirements
Network Architecture
Performance
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Links, Nodes, and Clouds
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Switched Network
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Switched Network (cont’d)
 Circuit-switched network: telephone system
 Establish a dedicated circuit across a sequence of links
 Packet-switched network: data network
 Store-and-forward
 Packet or message
 Efficiency of circuit-switched VS packet-switched
networks
 Cloud: any type of network e.g. point-to-point, multiple
access, switched
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Internetwork
A set of independence
networks are interconnected to
form an internetwork
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Internetwork (cont’d)
 Internet VS internet
 Router or gateway:
 a node connecting to two or more networks
 Address:
 a byte string that identifies a node; used to distinguish a node
from others
 Routing
 A process of determining systematically how to forward
messages toward the destination node based on its address
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Unicast, Multicast, Broadcast
 Unicast: a source node sends a message to a single
destination node
 Broadcast: a source node sends a message to all the
nodes on the network
 Multicast: a source node sends a message to some
subset of nodes
 Network:
 two or more nodes connected by a physical link, or
 Two or more networks connected by a node
 A large message is divided into packets
 Why?
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Cost-effective Resource Sharing 
Efficiency
 How do all the hosts that want to communicate at the
same time share the network?
 Multiplexing : a system resource is shared among multiple
users
 Analogous to time sharing computer: CPU is shared among
multiple job
 Multiplexing Techniques
 Synchronous Time-Division Multiplexing (STDM)
 Frequency-Division Multiplexing (FDM)
 Same concept as TV transmission
 Statistical Multiplexing
 Share physical link only when more than one node transmit data
at the same time
 Transmit data on demand rater than during a predetermined
time slot
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Multiplexing
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Switch Multiplexing Packets
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Switch Multiplexing Packets (cont’d)
 Switch makes decision on a packet-by-packet basis
 FIFO
 Round robin  STDM
 Quality of Service (QoS)
 Congestion
 Switch receives packets faster than the share link can
accommodate  need a buffer
 Running out of buffer  packet loss
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Additional Benefits of Statistical
Multiplexing
 Cost effective for multiple users to share network
resources
Define the packet as the granularity with which the links
of the network are allocated to different flows
Decide the flow with per packet basis
Fairly allocating capacity to different flows
Dealing with congestion when it occurs
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Support for Common Services
 Network supports application-level processes to
communicate with each other
 Viewed as logical “channel”
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Support for Common Services (cont’d)
 What functionality the channels should provide to
application programs?
Delivery guarantee?
In-order delivery?
Secure from eavesdropping?
Etc.
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Identifying Common Communication
Patterns
 Two general types of channels
Request/reply channel
 Used in file transfer and digital library apps
 Need security/privacy protection
Message stream channel
 Used in video-on-demand and videoconferencing apps
 No 100% delivery guarantee, but in-order
 Unicast/multicast/broadcast
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Reliability
 3 classes of failures
 Bit errors or burst errors
 Occurred from outside forces e.g. lightning strikes, power surges, and
microwave ovens
 Rare 1/106-107 bits on copper-based cable and 1/1012-1014 bits on optical
fiber
 Packet errors
 Packet loss because there are bit errors
 Congestion
 Software error e.g. forward packet to the wrong link
 Node and link errors
 Physical link is cut, computer crashes by software, power failure
 Need time to fix
 Need to understand application’s requirements and recognize
limitations of underlying technology
 Semantic gap: the gap between that application expects and what
the underlying technology can provide
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Outline
Requirements
Network Architecture
Performance
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Network Architecture
 A network must provide general, cost-effective,
fair, and robust connectivity among a large
number of computers
 Network architecture: a general blueprint that
guide design and implementation of networks
OSI and Internet (TCP/IP) architecture
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Layering and Protocols
 When the system gets complex, abstraction is needed
 Abstraction leads to layering
 Start by services offered by the underlying hardware and
then add a sequence of layers of services
 The services provided at the higher layers are
implemented in terms of the ones provided by the low
layers
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Layering
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Protocols
 Protocols: abstract objectives that make up the layers of
a network system
 Protocol provides a communication service that higherlevel objects use to exchange messages
 Each protocol defines two different interfaces:
 Service interface to other objects on the same computer
 Peer interface to another computer
 Indirect communications: protocol in each layer passes a
message to lower layer-protocol which in turn deliver the
message to its peer
 Multiple protocols provide a different communication service
 Protocol graph: a suite of protocol that make up a network
system
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Protocols (cont’d)
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Example of Protocol Graph
Request/Reply
Message Stream
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Encapsulation
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OSI Architecture
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OSI Architecture (cont’d)
 Physical layer: handle the transmission of raw bits over a
communications link
 Data-link layer: collect a stream of bits into a large aggregate called
a frame
 Network layer: handle routing among nodes within a packetswitched network.
 Transport layer: implement a process-to-process channel
 Session layer: provide a name space used to tie together the
potential different transport streams
 Presentation layer: concern with the format of data exchanged
between peers
 Application layer: include network applications
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OSI Model Analogy
 Create document (paper + pen, pencil, etc,
used for separate rooms)
 Translate, arrange format (dictionary,
translator)
 Doorman, enter and leave the room
 Check document condition and bring
document to each room (port number)
living room (80), dining room (21), art
studio (23)
 Postal address (IP address) front door ,
post office
 How to deliver document  trucks, ships,
planes (ID card = MAC address))
 Street, ocean, air
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Ethernet and the OSI Model
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Network Layer Devices in Data Flow
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Internet (TCP/IP) Architecture
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TCP/IP Layers
no official model but a working one
Application layer
Host-to-host, or transport layer
Internet layer
Network access layer
Physical layer
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Physical Layer
 concerned with physical interface between
computer and network
 concerned with issues like:
characteristics of transmission medium
signal levels
data rates
other related matters
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Network Access Layer
 exchange of data between an end system and
attached network
 concerned with issues like :
destination address provision
invoking specific services like priority
access to & routing data across a network link between
two attached systems
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Internet Layer (IP)
 routing functions across multiple networks
 for systems attached to different networks
 using IP protocol
 implemented in end systems and routers
 routers connect two networks and relays data
between them
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Transport Layer (TCP)
 common layer shared by all applications
 provides reliable delivery of data
 in same order as sent
 commonly uses TCP
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Application Layer
 provide support for user applications
 need a separate module for each type of
application
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OSI v TCP/IP
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Operation of TCP and IP
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Addressing Requirements
 two levels of addressing required
 each host on a subnet needs a unique global
network address
its IP address
 each application on a (multi-tasking) host needs
a unique address within the host
known as a port
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Operation of TCP/IP
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Transmission Control Protocol (TCP)
 usual transport layer is (TCP)
 provides a reliable connection for transfer of
data between applications
 a TCP segment is the basic protocol unit
 TCP tracks segments between entities for
duration of each connection
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TCP Header
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User Datagram Protocol (UDP)
 an alternative to TCP
 no guaranteed delivery
 no preservation of sequence
 no protection against duplication
 minimum overhead
 adds port addressing to IP
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UDP Header
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IP Header
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IPv6 Header
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TCP/IP Applications
 have a number of standard TCP/IP applications
such as
Simple Mail Transfer Protocol (SMTP)
File Transfer Protocol (FTP)
Telnet
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Some TCP/IP Protocols
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Features of Internet Architecture
 Does not imply strict layering
 Free to bypass the defined transport layers and directly use IP or
one of the underlying networks
 Hourglass shape
 IP serves as the focal point of the architecture – common
method for exchanging packets among a wide collection of
networks
 (According to IETF) If someone propose a new protocol
to be included in the architecture, they must produce
both a protocol specification and representative
implementation of the specification
 Ensure that the protocols can be efficiently implemented
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Outline
Requirements
Network Architecture
Performance
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Bandwidth
 Bandwidth: the number of bits that can be transmitted
over the network in a certain period of time
 Bandwidth of a single physical link
 Bandwidth of a logical process-to-process channel
 At the physical level, transmitting 1 bit of data on a 1Mbps link takes 1 µs
 For logical process-to-process channels, bandwidth is
also influenced by other factors
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Latency
 Latency: time taken a message to travel from one end of
a network to the other
 E.g. transcontinental network has a latency of 24 ms.
 Round-trip Time (RTT): time taken to send a message
from one end of a network to the other and back
 Components of latency:
 Speed-of-light propagation delay:
 3 x 108 m/s in a vacuum, 2.3 x 108 m/s in a cable, 2 x 108 m/s in
a fiber
 Transmission delay: time taken to transmit a unit of data
 Queuing delay
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Latency (cont’d)
 TotalLatency = Propagation + Transmit + Queue
 Propagation = Distance/SpeedOfLight
 Transmit = Size/Bandwidth
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Delay X Bandwidth Product
 A channel where latency is the length of the pipe and the
bandwidth is diameter of the pipe
 Then the product gives the volume of the pipe  the number
of bits it holds
 E.g. a transcontinental channel with a one-way latency
of 50 ms and a bandwidth of 45 Mbps is able to hold
 50 x 103 s x 45 x 106 bps = 2.25 x 106 bits or approx 280 KB
 Important when constructing high performance
networks because it tells how many bits the sender
must transmit before the first bit arrives at the receiver.
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Delay X Bandwidth Product (cont’d)
 The sender sends 2 delay X bandwidth of data before
hearing from the receiver
 The bits are said to be “in flight”
 If the receiver tells the sender to stop transmitting, it will
takes up to a delay X bandwidth before the sender can
respond.
 Takes 5.5 x 106 bits (671 KB) of data
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Questions?
Next Lecture
Introduction to Transmission
Technologies
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