CCNA2_M10_Intermediate_TCPIP (1130744)

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CCNA – Semester2Module 10Intermediate TCP/IPObjectives• Describe TCP functions and processes• Identify ports used for services and clientsTCP OperationTCP operation• IP makes no guarantees about delivery.• The transport layer is responsible for the reliable transport ofand regulation of data flow from source to destination.TCP Segment FormatTCP Synchronization• TCP is a connection-oriented protocol.• Prior to data transmission, the two communicatinghosts go through a synchronization process to establisha virtual connection.• This insures that both sides are ready for datatransmission and allows the devices to determine theinitial sequence numbers.• This process is known as a three-way handshake.3-way HandshakeDenial of service attacks• Denial of service (DoS) attacks are designed todeny services to legitimate hosts attempting toestablish connections.• In a DoS attack, the hacker initiates asynchronization but spoofs the source IPaddress.• Spoofing: replies to a non-existent, unreachableIP address and then is placed in a wait-statewhile waiting to receive the final ACK from theinitiator.SYN Flooding••One type of DoS is SYNflooding which exploits thenormal three-wayhandshake and causestargeted devices to ACK tofake source addresses thatwill not complete thehandshake.To defend against theseattacks, decrease theconnection timeout periodand increase the connectionqueue size.

Software alsocan detect these types ofattacks and initiatedefensive measures.Windowing & Sequencing Number• Flow control function of TCPregulates how much data issent during a giventransmission period.• Window size is negotiated anddetermines the amount of datain byte that can be transmittedat one time before receiving anacknowledgment from thedestination.• Sequencing numbers also actas reference numbers so thatthe receiver will know if it hasreceived all of the data.Sequencing numbersPositive ACK• With positive acknowledgment andretransmission (PAR), the source sends apacket, starts a timer, and waits for anacknowledgment before sending the nextpacket.• If the timer expires before the source receivesan acknowledgment, the source retransmits thepacket and starts the timer over again.UDP operation• UDP does not use windowing or acknowledgments soapplication layer protocols must provide error detection.Protocol Graph: TCP/IPOverview ofTransport Layer PortsMultiple conversations between hosts• A port number must be associated with the conversationbetween hosts to ensure that the packet reaches theappropriate service on the server.• The source and destination port numbers combine withthe network address to form a socket.• A pair of sockets, one on each host, forms a uniqueconnection.Port Number• Port numbers have the following assignedranges:– Below 255: reserved for public applications– From 255-1023:assigned to companies for marketableapplications– Above 1023: unregulated• Port numbers in the range of 0-1023 arecontrolled by the Internet Assigned NumbersAuthority (IANA).Telnet Port NumberPorts for services•Services running on hosts must have a port number assigned tothem so communication can occur.Ports for clients• Destination ports, or ports for services, arenormally defined using the well-known ports.Source ports set by the client are determineddynamically.• In general, a client determines the source portby randomly assigning a number above 1023.MAC addresses, IP addresses, and port numbers• Port numbers are located at the transport layerand are serviced by the network layer.• The network layer assigns the logical address(IP address) and is then serviced by the datalink layer which assigns the physical address(MAC address).Summary••••••TCP synchronization processDenial-of-service attacksWindowing and window sizeSequencing numbers and positive ACKMultiple conversations between hostsPorts for services and ports for clientsCCNA2 – Module10.

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