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TCP Operation
TCP operation
IP makes no guarantees about delivery.
The transport layer is responsible for the reliable transport of
and regulation ofdata flow from source to destination.
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TCP Segment Format
TCP Synchronization
TCP is a connection-oriented protocol.
Prior to data transmission, the two communicating
hosts go through a synchronization process to establish
a virtual connection.
This insures that both sides are ready for data
transmission and allows the devices to determine the
initial sequence numbers.
This process is known as a three-way handshake.
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3-way Handshake
Denial 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.
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SYN Flooding
One type of DoS is SYNflooding which exploits thenormal three-wayhandshake and causes
targeted 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 initiate
defensive 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.
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Sequencing numbers
Positive ACK
With positive acknowledgment and
retransmission (PAR), the source sends a
packet, starts a timer, and waits for an
acknowledgment before sending the next
packet.
If the timer expires before the source receivesan acknowledgment, the source retransmits the
packet and starts the timer over again.
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UDP operation
UDP does not use windowing or acknowledgments so
application layer protocols must provide error detection.
Protocol Graph: TCP/IP
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Overview of
Transport Layer Ports
Multiple conversations between hosts
A port number must be associated with the conversation
between hosts to ensure that the packet reaches the
appropriate service on the server.
The source and destination port numbers combine with
the network address to form a socket.
A pair of sockets, one on each host, forms a unique
connection.
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Port Number
Port numbers have the following assigned
ranges:
Below 255: reserved for public applications From 255-1023:assigned to companies for marketable
applications
Above 1023: unregulated
Port numbers in the range of 0-1023 are
controlled by the Internet Assigned Numbers
Authority (IANA).
Telnet Port Number
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Ports for services
Services running on hosts must have a port number assigned to
them so communication can occur.
Ports for clients
Destination ports, or ports for services, are
normally defined using the well-known ports.
Source ports set by the client are determined
dynamically.
In general, a client determines the source port
by randomly assigning a numberabove 1023.
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MAC addresses, IP addresses, and port numbers
Port numbers are located at the transport layer
and are serviced by the network layer.
The network layer assigns the logical address(IP address) and is then serviced by the data
link layer which assigns the physical address
(MAC address).
Summary
TCP synchronization process
Denial-of-service attacks
Windowing and window size
Sequencing numbers and positive ACK
Multiple conversations between hosts Ports for services and ports for clients
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CCNA2 Module10