Subtitles for OSI Transport Layer (cisco ccna networking fundamentals chapter 4)

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applications such as email web browsers
and instant messaging clients enable
people to use computers and networks to
send messages and find information data
from each of these applications is
packaged transported and delivered to
the appropriate destination device the
transport layer of the OSI model is
responsible for the overall end-to-end
transfer of application data
this video presents the transport layer
mechanisms services and protocols that
support this into any connectivity the
primary responsibilities of the
transport layer include tracking the
individual communication between
applications on the source and
destination host segmenting and
reassembling the segments into streams
of application data and identifying the
different applications associated with
data in this graphic the user is working
with many applications simultaneously
these include email instant messaging
multiple web pages IP telephony and
streaming video each type of traffic is
represented by a different-colored arrow
the transport layer maintains these
communication streams by dividing the
applications data into small parts
called segments and interleaving these
segments across the network media to
identify each segment of data the
transport layer adds a header to each
segment that contains a unique number
these numbers are also used for
reassembling the segments at the
destination host protocols at the
transport layer enable
connection-oriented conversations by
establishing a session between source
and destination hosts before the data is
transmitted reliable delivery is assured
because the data is reset if it becomes
corrupted or is not received by the
destination ordered data reconstruction
means that data is delivered to the
application in the proper sequence flow
control ensures the sending and
receiving devices are communicating at a
mutually agreed-upon rate the two most
common transport layer protocols of the
tcp/ip protocol suite our transmission
control protocol or TCP and user
Datagram protocol also called UDP data
such as email and web pages require that
all of the data center arrive at the
destination in order to be useful any
missing data could cause a corrupt
communication that is either incomplete
or unreadable
therefore these applications must use a
transport layer protocol that is
reliable as is TCP other applications
such as IP telephony and streaming video
are more tolerant of the loss of small
amounts of data for example if one or
two segments of a video stream failed to
it would only create a momentary
disruption in the stream this may appear
as a distortion in the image but may not
even be noticeable to the user these
applications must use a transport layer
protocol that has low overhead and as
fast as is UDP
let's look at how a tcp conversation is
established TCP is a connection-oriented
protocol which means that when to host
communicate a connection is established
using a three-way handshake before data
is transmitted first the initiating
client sends a tcp segment containing an
initial sequence value which serves as a
request to begin a communication session
with the destination host next the
destination responds with the segment
containing an acknowledgement value
equal to the received sequence value
plus 1 this acknowledgement value
enables the source client to tie the
response back to the original segment
that it sent to the destination at the
same time the destination since its own
synchronization sequence value
finally the initiating client responds
with an acknowledgement value that is
equal to the sequence value it received
+1 after this process of connection
establishment data can be sent after the
communication is completed the sessions
are closed and the connection is
TCP is a reliable transport protocol
this seems like a good thing but the
overhead included in TCP makes it a poor
choice for applications such as IP
telephony and video which require
near-real-time delivery of packets UDP
is a simple protocol that provides only
basic transport layer functionality as
you can see the header information added
to upper layer data by UDP is smaller
than that added by tcp UDP is not
connection-oriented and does not provide
the sophisticated retransmission
sequencing and flow control mechanisms
that are provided by tcp this does not
mean that applications that use UDP are
always and reliable it simply means that
these functions are not provided by the
protocol and must be implemented
usually at the application layer if
required data networks and the internet
support the human network by supplying
seamless reliable communication between
people both locally and around the globe
you can see how important the transport
layer is to this communication as you
continue the study of networking you
will appreciate how understanding the
transport layer specifically tcp and UDP
contributes to designing supporting and
troubleshooting reliable and secure

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