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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