Networking Basics CCNA 1
Chapter 11
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The TCP/IP Transport Layer
• TCP/IP transport layer includes several
protocols
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Transmission Control Protocol (TCP)
User Datagram Protocol (UDP)
TCP has more functions
UDP has less overhead
• Main goal of the transport layer:
– Provide the service of taking data from one
application process on one computer and delivering
that data to the correct application process on another
computer
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The TCP/IP Transport Layer
• Comparing transport and internet layers:
– Internet layer delivers packets from one
computer to another, but is not concerned
with which application sent the data or which
application on the receiving computer needs
the data
– If multiple instances of an application are
running on the destination computer, the
transport layer works to ensure the proper
instance gets the data and not the others
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The TCP/IP Transport Layer
Comparing TCP and UDP
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The TCP/IP Transport Layer
Flow Control and Windowing
• When a host sends data using TCP, the
receiving host can control how fast each TCP
sender sends the data over time – this is called
flow control
• Flow control is used for many reasons
– Receiver needs time to process the data
– Receiver has finite amount of memory, so if data
keeps arriving, it could run out of memory
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The TCP/IP Transport Layer
Flow Control Through Dynamic Sliding Windows
• Receiver tells sender how many bytes the
sending host can send before it receives an
acknowledgement – a value called a window
• When the sending host has sent an entire
window’s worth of data, it must wait for an
acknowledgement, slowing its rate of sending
data
• The window size can be increased to allow a
faster transfer rate
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The TCP/IP Transport Layer
Dynamic Windowing
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The TCP/IP Transport Layer
Flow Control Through Withholding
Acknowledgements
• After a sending host has sent one window worth
of bytes, it must wait to send more
• The receiver can wait to send
acknowledgements, which prevents the sender
from sending more data
• This process of withholding acknowledgements
is sometimes known as start/stop flow control
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The TCP/IP Transport Layer
Withholding Acknowledgements
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The TCP/IP Transport Layer
Establishing and Terminating TCP Connections
• TCP is a connection-oriented protocol
– Allows it to set initial values to the window and to
initialize sequence number values
• TCP uses a process called a three-way
handshake to create a new TCP connection
– Uses three TCP segments that use two of the TCP
flags in the TCP header
• The synchronize (SYN) flag
• The acknowledge (ACK) flag
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The TCP/IP Transport Layer
Three-Way TCP Connection Establishment
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The TCP/IP Transport Layer
TCP Error Recovery (Reliability)
• TCP is a reliable protocol (at least in a
networking perspective)
– Performs error recovery
– All data eventually gets to the destination,
even if some is initially lost in transport
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The TCP/IP Transport Layer
TCP Acknowledgements with no Error Recovery
Needed
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The TCP/IP Transport Layer
TCP Error Recovery (Reliability)
• TCP uses two header fields to signal to other
computer as to whether a segment was received
– Sequence number
• Keeps track of all the bytes sent over a TCP connection
– Acknowledgement number
• Lets the sending device know the number of the segment the
receiving device expects next
• Practice of stating the next byte expected to be received is
called a forward acknowledgement or expectational
acknowledgement
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The TCP/IP Transport Layer
TCP Error Recovery
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The TCP/IP Transport Layer
TCP Error Recovery (Reliability)
• If a segment is lost or destroyed during
transmission, the receiving PC requests
that it be retransmitted
• If an acknowledgement is not received by
the sending PC within a certain time (a
timer has been set), the sending PC
resends the segment
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The TCP/IP Transport Layer
Segmentation, Reassembly, and In-Order Delivery
• TCP segmentation refers to the process of
accepting a large chunk of data from the
application protocol and breaking it into pieces
that are small enough for transmission
– The size of the segment is limited
– Ethernet limits frames to 1500 data bytes in the data
field of a frame
• IP and TCP headers are each 20 bytes long
• Maximum data portion is therefore 1460 bytes
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The TCP/IP Transport Layer
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-oforder, the file is useless
– TCP provides a guarantee of in-order delivery
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The TCP/IP Transport Layer
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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The TCP/IP Transport Layer
TCP Providing In-Order Delivery
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The TCP/IP Transport Layer
TCP and UDP Header Reference
• TCP and UDP use a header to hold information
for performing tasks
– TCP needs ACK and SYN flags
• First two fields in TCP and UDP are identical
– Both use port numbers to identify application
processes
• TCP has a longer header (20 bytes vs. 8 bytes)
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The TCP/IP Transport Layer
TCP and UDP Headers
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The TCP/IP Transport Layer
TCP Header Fields
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The TCP/IP Transport Layer
TCP Header Fields
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The TCP/IP Transport Layer
UDP Header Fields
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The TCP/IP Transport Layer
Identifying Application Processes Using Port
Numbers
• Both TCP and UDP identify the specific
application process that sends the data and the
application process that needs to receive the
data
– To make this determination, TCP and UDP use port
numbers
– Each application uses a different local port number
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The TCP/IP Transport Layer
Using Port Numbers to Identify the Correct
Application Process
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The TCP/IP Transport Layer
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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The TCP/IP Transport Layer
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
– Servers must wait and listen on certain port
numbers for client requests
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The TCP/IP Transport Layer
Identifying Application Processes Using
Port Numbers
• Connection to Servers: Well-Known Ports
(continued)
– For servers to work well, TCP/IP defines one
or more well-known ports, each reserved for
use by a specific application protocol
• For example, HTTP connects to a server listening
on port 80
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The TCP/IP Transport Layer
Client Connecting to Well-Known Port of a Web
Server (80)
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The TCP/IP Transport Layer
Popular Applications and Their Well-Known Port Numbers
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The TCP/IP Transport Layer
Identifying Application Processes Using Port
Numbers
• Comparing Well-Known, Dynamic, and
Registered Ports
– IANA assigns the values for well-known ports and for
registered port numbers
• Registered ports are assigned to servers that the average
end-user can start, such as instant-messaging applications
• Well-known ports are started by IT staff for applications such
as web servers, email, FTP servers
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The TCP/IP Transport Layer
Uses of Port Numbers
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The TCP/IP Transport Layer
Identifying Application Processes Using
Port Numbers
• Comparing Well-Known, Dynamic, and
Registered Ports (continued)
– Both TCP and UDP use port numbers
– To see the port numbers used on a
computers running a Microsoft OS, use the
netstat –an command
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