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History and Future of TCP/IP
• The U.S. Department of
Defense (DoD) created
the TCP/IP reference
model because it
wanted a network that
could survive any
conditions.
• Some of the layers in
the TCP/IP model have
the same name as
layers in the OSI model.
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Internet Layer
The purpose of the Internet layer is to send
packets from a network node and have them
arrive at the destination node independent of the
path taken.
Internet Protocol (IP)
Internet Control Message Protocol (ICMP)
Address Resolution (ARP)
Reverse Address Resolution Protocol (RARP)
Dynamic Host Configuration Protocol (DHCP)
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Internet Layer Protocols
• IP performs the following operations
– Defining a packet and an addressing scheme
– Transferring data between the Internet Layer
and the Network Access Layer
– Routing packets to remote hosts
• IP is sometimes referred to as an
unreliable protocol
– Provides connectionless, best-effort delivery
routing of packets
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The Transport Layer Functions
Five basic services:
• Segmenting upper-layer application data
• Establishing end-to-end operations
• Sending segments from one end host to
another end host
• Ensuring data reliability provided by
sequence numbers and acknowledgments
• Ensuring flow control provided by sliding
windows
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Reliable Data Transport
• Ensure that segments delivered will be
acknowledged to the sender
• Provide for retransmission of any
segments that are not acknowledged
• Put segments back into their correct
sequence at the destination
• Provide congestion avoidance and control
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Connectionless and Connection-
Oriented Protocols
• Connection-oriented protocol
– A protocol either that requires an exchange of
messages before data transfer begins or that
has a required pre-established correlation
between two endpoints
• Connectionless protocol
– A protocol that does not require an exchange
of messages and that does not require a pre-
established correlation between two
endpoints
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Three-way Handshake
• TCP is connection-oriented, so it requires
connection establishment before data transfer
begins
• For a connection to be established, two hosts
must synchronize on each other’s initial
sequence numbers (ISNs)
• Initial Sequence numbers are actually large
random numbers chosen by each host
• Connection establishment refers to the process
of initializing sequence and acknowledgement
fields and agreeing to the port numbers used.
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Windowing
• Flow-control mechanism requiring that source
device receive an acknowledgment from the
destination
• TCP uses expectational acknowledgments
(Forward Acknowledgment)
• Window size determines the amount of data can
transmit at one time before receiving an
acknowledgment
• Larger window sizes increase communication
efficiency.
• Window field implies the maximum number of
unacknowledged bytes allowed outstanding at any
instance in time.
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Window Size
• TCP window sizes are variable during the
lifetime of a connection.
• The window “Slides” up and down based on
network performance, so it is called sliding
window.
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Acknowledgment
• Sender keeps a record of each data
packet that it sends and expects an
acknowledgment.
• Sender starts a timer when it sends a
segment, and it retransmits if the timer
expires before an acknowledgment
(transmission rate should be slowed)
• Each Acknowledgement contains a
window advertisement that indicates the
number of bytes receiver can accept
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Segmentation, Reassembly, and
In-Order Delivery
• TCP on the receiving computer reassembles
data into its original form
• The data is put in the correct order
– If segments of a file are assembled out-of-
order, the file is useless
– TCP provides a guarantee of in-order delivery
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Segmentation, Reassembly, and
In-Order Delivery
• Due to IP routing, a TCP receiver can receive
data out of order
• If multiple routes exist between a source and a
destination, routers can load-balance over
several routes
• Packets can arrive out of order
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TCP and UDP Port Numbers
• Internet layer delivers data (packets) from one computer
to another, but it does not think about which application
sent the data or which application on the receiving
computer needs the data.
• For example, if you have five web-browser windows
open, the internet layer delivers the data to the
computer, but the transport layer works to ensure that
each browser gets the data destined for it and not one of
the others.
• TCP and UDP use port numbers to pass information to
the upper layers
• Port numbers use to keep track of different
conversations crossing the network at the same time
(Enables the receiving computer to know which
application to give the data to).
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Identifying Application Processes Using
Port Numbers
• In previous slide, the application was
assigned a dynamic port number by the
host computer
– A host typically dynamically allocates port
numbers of value 1024 (210
) through 65,535
(216
- 1).
– When a host starts a new application process,
it allocates a dynamic port number that is not
already in use by another process.
– By each process having its own port number,
a PC can have multiple conversations with
other PCs (sometimes called multiplexing).
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Identifying Application Processes Using
Port Numbers
• Connection to Servers: Well-Known Ports
– Most TCP/IP applications use a client/server model
for communications.
– Servers cannot use dynamic port numbers because
clients must know ahead of time what port numbers
servers use.
– Numbers below 1024 are considered well-known port
numbers.
– well-known port numbers are used by Servers, other
port numbers used by clients.
– Each client on the same host uses a different port
number, but a server uses a same port number for all
connections.
– Well-Know Port Numbers are controlled by Internet
Assigned Number Authority (IANA).
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TCP
• Connection Establishment and
Termination
• Reliable (Error recovery – consume more
bandwidth and use more processing
cycles)
• Divides outgoing messages into segments
• Reassembles messages at the destination
station
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TCP
• Flow control using sliding windows
• Multiplexing using port numbers
• TCP relies on IP for end-to-end delivery of
data
• At the receiving station, TCP reassembles
the segments into a complete message
using sequence numbers. TCP must
recover data that is damaged, lost or
delivered out of order.
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UDP
• Connectionless
• Unreliable ( No error recovery – use less
bandwidth and fewer processing cycle.)
• Does not reassemble incoming messages
• Uses no acknowledgments
• Provides no flow control
• Less overhead than TCP
•
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TCP Function Summary
Function Description
Multiplexing Function that allows receiving hosts to
decide the correct application for
which the data is destined, based on
the port number
Error recovery
(reliability)
Process of numbering and
acknowledging data with Sequence
and Acknowledgment header fields
Flow control using
windowing
Process that uses window sizes to
protect buffer space
