Download Chapter 12: The Transport layer

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Chapter 12:
The Transport Layer
Purpose of the transport layer
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The phrase "quality of
service" is often used to
describe the purpose of
Layer 4 - the transport
layer.
The transport layer
regulates information
flow to ensure end-toend connectivity between
host applications reliably
and accurately
The end-to-end control,
provided by sliding
windows, and reliability
in sequencing numbers
and acknowledgments
are primary duties of
Layer 4.
Transport layer segments and reassembles
data into a data stream. TCP is one of the
transport layer protocols used with IP.
Transport Layer PDU is segment.
Transport Layer Protocols
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The TCP/IP protocol of the OSI
model Layer 4 (transport layer)
has two protocols - TCP and UDP:
TCP
(Transmission Control Protocol)
supplies a virtual circuit between enduser applications. These are its
characteristics:
connection-oriented
reliable
divides outgoing messages into
segments
reassembles messages at the
destination station
re-sends anything not received
reassembles messages from incoming
segments.
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UDP
(User Datagram Protocol)
transports data unreliably between
hosts. Following are the
characteristics of UDP:
connectionless
unreliable
transmit messages (called user
datagrams)
provides no software checking for
message delivery (unreliable)
does not reassemble incoming
messages
uses no acknowledgments
provides no flow control
Comparing TCP and IP
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TCP/IP is a combination of
two individual protocols:
TCP and IP
IP is a Layer 3 protocol - a
connectionless service that
provides best-effort delivery
across a network.
TCP is a Layer 4 protocol - a
connection-oriented service
that provides flow control as
well as reliability.
Pairing the protocols enables
them to provide a wider
range of services. Together,
they represent the entire
suite.
TCP/IP is the Layer 3 and
Layer 4 protocol on which the
Internet is based.
TCP and UDP Segments Format
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UDP
TCP
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Following are the definitions of the fields in the TCP segment:
source port -- number of the calling port
destination port -- number of the called port
sequence number -- number used to ensure correct sequencing of the arriving data
acknowledgment number - next expected TCP octet
HLEN -- number of 32-bit words in the header
reserved -- set to zero
code bits -- control functions (such as setup and termination of a session)
window-- number of octets that the sender is willing to accept
checksum -- calculated checksum of the header and data fields
urgent pointer -- indicates the end of the urgent data
option-one option -- maximum TCP segment size
data -- upper-layer protocol data
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TCP/IP Protocols
Port Numbers
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Port numbers have the
following assigned
ranges:
Numbers below 255 - for
public applications
Numbers from 255-1023 assigned to companies for
marketable applications
Numbers above 1023 are unregulated
Both TCP and UDP use port (or socket) numbers to pass information to
the upper layers.
Port numbers are used to keep track of different conversations that cross
the network at the same time.
Application software developers have agreed to use the well-known port
numbers that are defined in RFC1700.
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Connection-oriented Session
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Connection oriented services involve three phases:
In the connection establishment phase, a single path between the
source and destination is determined. Resources are typically reserved at
this time to ensure a consistent grade of service.
During the data transfer phase, data is transmitted sequentially over
the established path, arriving at the destination in the order in which it
was sent.
The connection termination phase consists of terminating the
connection between the source and destination when it is no longer
needed.
TCP establishes a connection
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In order for data transfer to
begin, one user of the
transport layer must establish
a connection-oriented session
with its peer system.
Protocol software modules in
the two operating systems
communicate by sending
messages across the network
to verify that the transfer is
authorized and that both
sides are ready.
After all synchronization has
occurred, a connection is
established, and data transfer
begins.
Three way-handshake
The first handshake requests synchronization.
The second and third handshakes acknowledge the
initial synchronization request, and synchronize the
connection parameters in the opposite direction.
The final handshake segment sends an
acknowledgement to the destination that both sides
agree that a connection has been established. As soon
as the connection has been established, data transfer
begins.
TCP sends data with flow control
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While data transfer is in
progress, congestion can occur
for two different reasons.
First, a high-speed computer
might generate traffic faster than
a network can transfer it.
Second, if many computers send
datagrams simultaneously to a
single destination, that
destination can experience
congestion.
When datagrams arrive too
quickly for a host or gateway to
process, they are temporarily
stored in memory. If the traffic
continues, the host or gateway
eventually exhausts its memory
and discards any additional
datagrams that arrive.
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Instead of allowing data to be lost, the
transport function can issue a "not ready"
indicator to the sender. This indicator acts
like a stop sign and signals the sender to
stop sending data. When the receiver is able
to accept additional data, it sends a "ready"
transport indicator, which is like a go signal.
When the sending device receives this
indicator, it resumes segment transmission.
TCP achieves reliability with windowing
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Reliable connectionoriented data
transfer means that
data packets arrive
in the same order in
which they are sent
The number of data
packets a sender is
allowed to transmit
without having
received an
acknowledgment is
known as a window.
TCP acknowledgment technique
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Reliable delivery guarantees that
a stream of data that is sent
from one device will be delivered
through a data link to another
device without duplication or
data loss.
Positive acknowledgment with
retransmission is one process
that guarantees reliable delivery
of data streams.It requires a
recipient to send an
acknowledgment message to the
sender whenever it receives
data.
The sender keeps a record of
each data packet that it sends
and then waits for the
acknowledgment before sending
the next data packet. The sender
also starts a timer whenever it
sends a segment, and
retransmits the segment if the
timer expires before the
acknowledgment arrives.
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The End