Transcript NET 221 ch6-1 - NET 331 and net 221

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NET 221D:COMPUTER
NETWORKS FUNDAMENTALS
Behrouz A. Forouzan”
Data communications
and Networking
Lecture : Transport Layer:
Transport Layer
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


The transport layer in the TCP/IP suite is located between the
application layer and the network layer.
It provides services to the application layer and receives
services from the network layer.
The transport layer acts as a liaison between a client program
and a server program, a process-to-process connection.
Behrouz A. Forouzan” Data communications and Networking
Transport Layer
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
The transport layer is responsible for process-to-process delivery.

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A process is an application program running on a host.
The transport layer is responsible for:
 Service point or Port addressing
 Segmentation and reassembly


A message is divided into transmittable segments each segment
containing a sequence number.
Connection Control

Connection oriented or connectionless.
Flow control
 Error control

Behrouz A. Forouzan” Data communications and Networking
Process to Process Communication
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The first duty of a transport-layer protocol is to provide processto-process or (end to end delivery) communication.
A process is an application-layer entity (running program) that
uses the services of the transport layer.
Type of data deliveries
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Data Link Layer: Node to Node delivery
Network Layer: Host to host delivery
Transport Layer: Process to process delivery
Behrouz A. Forouzan” Data communications and Networking
Process to Process Communication
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
Network layer

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Communication at the computer level (host-to-host communication).
Deliver the message only to the destination computer.
Transport layer:

Message handed to the correct process on the host computer. shows the
domains of a network
Behrouz A. Forouzan” Data communications and Networking
Client-Server Communication
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
Most common way to achieve process-to-process
communication, is through the client-server paradigm.

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A process on the local host, called a client, needs services from a
process usually on the remote host, called a server.
A remote computer/server can run several server programs at the
same time, just as several local computers/clients can run one or
more client programs at the same time.
For communication, we must define
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The local host: Defined using IP address
Local process: Defined using identifiers called port numbers
Remote host:: Defined using IP address
Remote Process: Defined using identifiers called port numbers
In the TCP/IP protocol suite, the port numbers are integers between
0 and 65,535 (16 bits).
Computer networks / Andrew S. Tanenbaum-- 5th ed
IP addresses versus port numbers
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Behrouz A. Forouzan” Data communications and Networking
Port Addressing
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

Ephemeral port number
 Defined by the client program.
 Ephemeral (Short Lived) is used to describe these port
numbers because the life of a client is normally short.
Server port number
 The server process must also define itself with a port
number.
 Port number cannot be chosen randomly.
If the computer at the server site runs a server process and assigns
a random number as the port number, the process at the client site
that wants to access that server and use its services will not know
the port number.
 TCP/IP has uses universal port numbers for servers; these are
called well-known port numbers.
Behrouz A. Forouzan” Data communications and Networking

Socket address
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

A transport-layer protocol in the TCP suite at both ends needs the
following to establish connection
 IP address
 port number
The combination of an IP address and a port number is called a
socket address.
 The client socket address defines the client process uniquely.
 The server socket address defines the server process uniquely.
Behrouz A. Forouzan” Data communications and Networking
Encapsulation De-Capsulation/Multiplexing
De-multiplexing
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

To send a message from one process to another, the transport-layer protocol
encapsulates and de-capsulates messages.
Multiplexing (many to one)
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Whenever an entity accepts items from more than one source.
The transport layer at the source performs multiplexing
De-multiplexing (one to many).
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Whenever an entity delivers items to more than one source.
A. Forouzan”
Data communications and Networking
The transport layer at the destinationBehrouz
performs
de-multiplexing.
Flow Control
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If the items are produced faster than they can be consumed, the consumer
can be overwhelmed and may need to discard some items.
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Pushing: sender delivers items whenever they are produced⎯ without a prior
request from the consumer.
Pulling: producer delivers the items after the consumer has requested them
Two cases of flow control at the transport layer:
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
From the sending transport layer to the sending application layer and
From the receiving transport layer to the sending transport layer
Behrouz A. Forouzan” Data communications and Networking
Congestion control
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
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Congestion control refers to the mechanisms and techniques that
control the congestion and keep the load below the capacity.
Congestion in a network may occur if the load on the network—the
number of packets sent to the network—is greater than the capacity
of the network—the number of packets a network can handle.
Congestion in a network or internetwork occurs because routers and
switches have queues—
buffers that hold the packets before and after processing. A router,
for example, has an input queue and an output queue for each
interface.
If a router cannot process the packets at the same rate at which they
arrive, the queues become overloaded and congestion occurs.
Congestion at the transport layer is actually the result of congestion at the
network layer, which manifests itself at the transport layer.
Congestion at the transport layer can be implemented if there is no
congestion control at the network layer.
Behrouz A. Forouzan” Data communications and Networking
Error Control
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
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Network layer (IP) is unreliable.
Transport layer should be reliable if the application requires
reliability.
Reliability can be achieved by adding error control services
to the transport layer. Error control at the transport layer is
responsible for
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Detecting and discarding corrupted packets.
Keeping track of lost and discarded packets and resending them.
Recognizing duplicate packets and discarding them.
Buffering out-of-order packets until the missing packets arrive.
Error control, unlike flow control, involves only the sending
and receiving transport layers.

We assume that the message chunks exchanged between the
application and transport layers are error free.
Behrouz A. Forouzan” Data communications and Networking
Transport Layer Protocol
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
The transport layer in the TCP/IP suite is located between
the application layer and the network layer.

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It provides services to the application layer and receives services
from the network layer.
Following are the transport protocols in the Internet/TCP/IP
Protocol Suite.
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UDP (User data gram protocol)
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
TCP (Transmission Control protocol)

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Unreliable connectionless transport-layer protocol used for its
simplicity and efficiency in applications where error control can be
provided by the application-layer process.
Reliable connection-oriented protocol that can be used in any
application where reliability is important.
SCTP

Combines the features of TCP and UDP.
Behrouz A. Forouzan” Data communications and Networking
UDP
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
User Datagram Protocol (UDP)
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Connectionless, unreliable transport protocol.
It does not add anything to the services of IP except for providing
process-to-process communication instead of host-to-host communication.
UDP is a very simple protocol using a minimum of overhead.
UDP packets called user datagrams,
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Fixed-size header of 8 bytes made of four fields, each of 2 bytes (16
bits).
The first two fields define the source and destination port numbers.
Behrouz A. Forouzan” Data communications and Networking
UDP Packets-User datagram
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 The
third field defines the total length of the user
datagram, header plus data.
 The 16 bits can define a total length of 0 to 65,535 bytes.
 The total length needs to be less because a UDP user
datagram is stored in an IP datagram with the total length
of 65,535 bytes.
 The last field can carry the optional checksum
Behrouz A. Forouzan” Data communications and Networking
UDP Services
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
Process-to-Process Communication
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Using socket addresses, a combination of IP addresses and
port numbers.
Connectionless Services
Independent datagram, No relationship between the
different user datagrams even if they are coming from the
same source process and going to the same destination
program, Datagrams are not numbered.
 There is no connection establishment and no connection
termination.
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Each user datagram can travel on a different path.
Flow Control

There is no flow control, and
hence
no window
mechanism.
Behrouz
A. Forouzan”
Data communications
and Networking
UDP Services
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
Error Control
 No
error control mechanism in UDP except for the
checksum.
 Sender does not know if a message has been lost or
duplicated.
 When the receiver detects an error through the
checksum, the user datagram is silently discarded.

Checksum
 UDP
checksum calculation includes three sections.
A
pseudo-header
 The UDP header
 And the data coming from the application layer.
Behrouz A. Forouzan” Data communications and Networking
UDP-Checksum
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Checksum includes the pseudoheader to ensure that the datagram
is not delivered to the wrong host in
case the IP address is corrupted.
The protocol field is added to ensure
that the packet belongs to UDP, and
not to TCP.

The value of the protocol field for
UDP is 17.

If this value is changed during
transmission, the checksum calculation
at the receiver will detect it and UDP
drops the packet.
Behrouz A. Forouzan” Data communications and Networking
UDP Services
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
Congestion Control
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
Encapsulation and Decapsulation
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No congestion control
UDP does not create additional traffic in an error-prone network.
Therefore, in some cases, lack of error control in UDP can be
considered an advantage when congestion is a big issue.
To send a message from one process to another, the UDP protocol
encapsulates and decapsulates messages.
Multiplexing and Demultiplexing

In a host running a TCP/IP protocol suite, there is only one UDP
but possibly several processes that may want to use the services
of UDP. To handle this situation, UDP multiplexes and
demultiplexes.
Behrouz A. Forouzan” Data communications and Networking
Transmission control protocol (TCP)
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
Transmission Control Protocol (TCP)

A connection-oriented, reliable protocol.
 TCP
explicitly defines connection establishment, data
transfer, and connection teardown phases to provide
connection oriented service.
 TCP
uses checksum (for error detection), retransmission
of lost or corrupted packets, cumulative and selective
acknowledgments, and timers.
Behrouz A. Forouzan” Data communications and Networking
TCP Services
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
Process-to-Process Communication
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Provides process-to-process communication using port numbers
Stream Delivery Service
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
In TCP the sending process delivers data as a stream of bytes
and allows the receiving process to obtain data as a stream of
bytes.
In TCP two processes seem to be connected by an imaginary
“tube” that carries their bytes across the Internet.
Behrouz A. Forouzan” Data communications and Networking
TCP Services
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
Full-Duplex Communication
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Multiplexing and Demultiplexing
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Offers full-duplex service, where data can flow in both directions at the same
time.
TCP performs multiplexing at the sender and demultiplexing at the receiver.
Connection-Oriented Service

TCP is a connection-oriented protocol. When a process at site A wants to send to
and receive data from another process at site B, the following three phases
occur:
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The two TCP’s establish a logical connection between them.
Data are exchanged in both directions.
The connection is terminated.
This is a logical connection, not a physical connection.
Reliable Service

TCP is a reliable transport protocol. It uses an acknowledgment mechanism to
check the safe and sound arrival of data.
Behrouz A. Forouzan” Data communications and Networking
TCP Packets/Segments
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
A packet in TCP is called a segment.
Format
Behrouz A. Forouzan” Data communications and Networking
Format
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Header: The segment consists of a header of 20 to 60 bytes,
followed by data from the application program.
Source port address. This is a 16-bit field that defines the port
number of the application program in the host that is sending the
segment.
Destination port address. This is a 16-bit field that defines the port
number of the application program in the host that is receiving the
segment.
Sequence number. This 32-bit field defines the number assigned to
the first byte of data contained in this segment.
Acknowledgment number. This 32-bit field defines the byte number
that the receiver of the segment is expecting to receive from the
other party.
Header length. This 4-bit field indicates the number of 4-byte words
in the TCP header.
Behrouz A. Forouzan” Data communications and Networking
Format
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

Window size. This field defines the window size of the
sending TCP in bytes.
Checksum.
This 16-bit field contains the checksum.
 The calculation of the checksum for TCP follows the same
procedure as the one described for UDP.
 The use of the checksum in the UDP datagram is optional,
whereas the use of the checksum for TCP is mandatory.
 The pseudoheader serves the same purpose as in UDP.


For the TCP pseudoheader, the value for the protocol field is 6.
Behrouz A. Forouzan” Data communications and Networking
Format
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
Urgent pointer. This 16-bit field, which is valid only if the
urgent flag is set, is used when the segment contains urgent
data.
Behrouz A. Forouzan” Data communications and Networking
Encapsulation
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TCP
header
Application-layer data
IP
header
Frame
header
TCP payload
IP payload
Data-link layer payload
TCP Connection
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TCP is connection-oriented.
It establishes a virtual path between the source and destination.
All of the segments belonging to a message are then sent over this
virtual path.
You may wonder how TCP, which uses the services of IP, a
connectionless protocol, can be connection-oriented.
The point is that a TCP connection is virtual, not physical.
TCP operates at a higher level. TCP uses the services of IP to deliver
individual segments to the receiver, but it controls the connection
itself. If a segment is lost or corrupted, it is retransmitted.
A SYN segment cannot carry data, but it consumes one sequence number.
. A SYN + ACK segment cannot carry data, but does consume one sequence

number.
 An ACK segment, if carrying no data, consumes no sequence number

Behrouz A. Forouzan” Data communications and Networking
Connection establishment using three
way handshake Figure 15.9
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TCP/IP Protocol Suite
Data Transfer
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Behrouz A. Forouzan” Data communications and Networking
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Connection termination using three-way
handshake
The FIN segment consumes one sequence number if it does not carry data.
The FIN + ACK segment consumes one sequence number if it does not carry
data.
Behrouz A. Forouzan” Data communications and Networking
Congestion Control
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

In the Internet, TCP plays the main role in controlling
congestion, as well as the main role in reliable transport.
TCP uses the following techniques for congestion control

A congestion window
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TCP congestion window size is the number of bytes the sender may have in
the network at any time.
Congestion policy that avoid congestion and detect and alleviate
congestion after it has occurred.
The congestion window is maintained in addition to the flow control
window, which specifies the number of bytes that the receiver can
buffer.
Both windows are tracked in parallel, and the number of bytes that
may be sent is the smaller of the two windows.
Thus, the effective window is the smaller of what the sender thinks is
all right and what the receiver thinks is all right.
TCP will stop sending data if either the congestion or the flow control
window is temporarily full.
Behrouz A. Forouzan” Data communications and Networking
Congestion Control
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
Congestion Window
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The use of flow control strategy at the transport layer guarantees that
the receive window is never overflowed with the received bytes (no end
congestion).
Intermediate buffers, buffers in the routers can become congested.
TCP needs to define policies that accelerate the data transmission when
there is no congestion and decelerate the transmission when congestion is
detected.
To control the number of segments to transmit, TCP uses a variable called
a congestion window, cwnd, whose size is controlled by the congestion
situation in the network.
The size of the window is the minimum of the cwnd variable and the rwnd
variable.
Actual window size = minimum (rwnd, cwnd)
Behrouz A. Forouzan” Data communications and Networking
Congestion Policy
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
Congestion Policies
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Slow Start Algorithm
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TCP’s general policy for handling congestion is based on
three algorithms: slow start, congestion avoidance, and fast
recovery.
In the slow start algorithm, the size of the congestion window
increases exponentially until it reaches a threshold.
Congestion Avoidance Algorithm

In the congestion avoidance algorithm the size of the
congestion window increases additively until congestion is
detected.
Behrouz A. Forouzan” Data communications and Networking
Slow Start Algorithm
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
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When a connection is established, the
sender initializes the congestion window to
a small initial value of at most four
segments
The sender then sends the initial window.
The packets will take a round-trip time to
be acknowledged.
For each segment that is acknowledged
before the retransmission timer goes off,
the sender adds one segment’s worth of
bytes to the congestion window.
As that segment has been acknowledged,
there is now one less segment in the
network.
The upshot is that every acknowledged
segment allows two more segments to be
sent.
The congestion window is doubling every
round trip time.
Behrouz A. Forouzan” Data communications and Networking
This algorithm is called slow start
Congestion avoidance
algorithm/Additive Increase
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


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If we continue with the slow-start
algorithm, the size of the congestion
window increases exponentially.
To avoid congestion before it happens, we
must slow down this exponential growth.
TCP defines another algorithm called
congestion avoidance, which increases the
cwnd additively instead of exponentially.
In the congestion-avoidance algorithm, the
size of the congestion window increases
additively until congestion is detected.
Behrouz A. Forouzan” Data communications and Networking