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Introduccion a la capa de transporte, multiplexado/demultiplexado, puertos, sockets, modelo cliente/servidor, protocolos de transporte.
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Introduction tothe Transport
Layer
Transport level
Introduction to the Transport Layer
Telematics EngineeringUniversitat Politecnica de Catalunya (UPC)
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Introduction tothe Transport
Layer
Transport level
Outline
1 Transport level
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Introduction tothe Transport
Layer
Transport level
The Network Layer• The main goal of the network layer is homogenizing
different data link layer technologies.• IP provides a best effort service for delivering
datagrams.• Datagrams are packets that:
• Use IP addresses for identifying the source (NIC) andthe destination (NIC).
• A datagram with the same pair of IP addresses can takedifferent routes.
• Best effort:• IP networks do not guarantee a correct delivery of
datagrams: we may have incorrect or lost datagramsand disorders.
• It is said that IP does not offer Quality of Service (QoS).
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Introduction tothe Transport
Layer
Transport level
Transport Layer I
• The main goal of the transport layer is to implementcommunications between processes (applications).
• These communications are also called end-to-endcommunications.
• Introduces the concept of PORT for multiplexing anddemultiplexing.
transport
network
link
physical
P1
host 1
transport
network
link
physical
host 2
P2 P3 P4 P5 P6
InternetInternet
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Introduction tothe Transport
Layer
Transport level
Transport Layer II
• Ports are used to mux/demux different transportcommunications that can use the same IP source andIP destination addresses.
• Uses the network layer services.
• Responsible for providing an interface to the processesso they can send and receive data through the network.
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Introduction tothe Transport
Layer
Transport level
Multiplexing/Demultiplexing
• Multiplexing:• Multiplexing is
performed by thesender.
• The sender includes adestination portnumber.
• In this way, thereceiver candemultiplex and deliverthe data to the correctprocess. es is puttogether and deliveredto the transport layer.
• It must include thenecessary informationfor demultiplexing.
• Demultiplexing:• In reception.• Data from transport
layer is extracted anddelivered to the eachappropiate process.
• It uses the informationincluded by the senderto do so.
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Introduction tothe Transport
Layer
Transport level
The Port
• The final recipient of an IP packet is not the networkcard (with an associated @IP)
• Is a running process (even ICMP).• In a host there must be multiple processes running.• OSs identify the processes (i.e. Linux PID).• On the netwoks field a new identifier that does not
depend on OS is assigned to the processes: the port.• The port is an integer locally assigned to a running
process that requires communication with anotherprocess.
• In TCP and UDP, ports are identifiers of 16 bits (seeheaders).
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Introduction tothe Transport
Layer
Transport level
Sockets
• A process has a unique identifier in an IP networkcalled socket.
• Furthermore, a socket is defined by a network address(ie. IP), a protocol (ie. TCP) and a port number (ie 80).
• The socket is a way of uniquely identifying a process ona network.
• A connection between the two processes is identifiedby two sockets.
(Source IP, source port, destination IP, destination port, protocol)
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Introduction tothe Transport
Layer
Transport level
Client-Server Model
• Distributed communication system between multipleprocesses.
• Clients request services and Servers provide them.• Separates services by placing each one in the most
suitable platform.
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Introduction tothe Transport
Layer
Transport level
Client
• Is the one who initiates requests (active or master).• Waits and gets the replies from the servers.• Usually, it can connect to multiple servers at the same
time.• Typically it interacts directly with end users by a
graphical user interface.• Communication parameters:
• The local port is normally assigned dynamically.• The local network address is the output interface.• The port and remote network address must be known
(ie. web browsing).
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Introduction tothe Transport
Layer
Transport level
Server
• At first they wait for client requests, they play a passiverole (slave).
• After receiving a request, it is processed and then thereply is sent back to the client.
• Usually they accept connections from a large numberof clients (in some cases the maximum number ofrequests may be limited).
• Is not common that a server process interacts directlywith the end-user.
• “daemon” type application.• Communication parameters:
• Fixed port assignment (listen)
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Introduction tothe Transport
Layer
Transport level
Transport Protocols
• User Datagram Protocol (UDP):• Unreliable.• No connection-oriented.
• Transmission Control Protocol (TCP):• Reliable.• Connection-oriented.
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