Unit 4 - Transport Layer

KARPAGAM INSTITUTE OF TECHNOLOGY
DEPARTMENT OF COMPUTER SCIENCE AND ENGINEERING
Course Code with Name :CS8591/Computer
Networks
Staff Name / Designation : Mrs.C.Kalpana / AP
Department : CSE
Year/Semester : III/V
1
Karpagam Institute of Technology
4/5/2022

UNIT – IV
TRANSPORT LAYER
4/5/2022 Karpagam Institute of Technology 2
Introduction – Transport Layer Protocols –
Services – Port Numbers – User Datagram
Protocol – Transmission Control Protocol –
SCTP.

INTRODUCTION
 The transport layer is the fourth layer of the
OSI model and is the core of the Internet
model.
 It responds to service requests from the
session layer and issues service requests
to the network Layer.
 The transport layer provides transparent
transfer of data between hosts.
 It provides end-to-end control and
information transfer with the quality of service
needed by the application program.
 It is the first true end-to-end layer,
implemented in all End Systems (ES).

TRANSPORT LAYER
FUNCTIONS / SERVICES
 The transport layer is located between
the network layer and the application
layer.
 The transport layer is responsible for
providing services to the application
layer; it receives services from the
network layer.

TRANSPORT LAYER
FUNCTIONS / SERVICES
 The services that can be provided by
the transport layer are
1. Process-to-Process Communication
2. Addressing : Port Numbers
3. Encapsulation and Decapsulation
4. Multiplexing and Demultiplexing
5. Flow Control
6. Error Control
7. Congestion Control

Process-to-Process
Communication
 The Transport Layer is responsible for
delivering data to the appropriate
application process on the host
computers.
 This involves multiplexing of data from
different application processes, i.e.
forming data packets, and adding source
and destination port numbers in the
header of each Transport Layer data
packet.
 Together with the source and destination
IP address, the port numbers constitutes
a network socket, i.e. an identification
address of the process-to-process

Addressing: Port Numbers
 Ports are the essential ways to address multiple
entities in the same location.
 Using port addressing it is possible to use more
than one network-based application at the same
time.
 Three types of Port numbers are used :
Well-known ports - These are permanent port
numbers. They range between 0 to 1023.These
port numbers are used by Server Process.
Registered ports - The ports ranging from 1024 to
49,151 are not assigned or controlled.
Ephemeral ports (Dynamic Ports) – These are
temporary port numbers. They range between
49152–65535.These port numbers are used by
Client Process.

Encapsulation and
Decapsulation
 To send a message from one process to
another, the transport-layer protocol
encapsulates and decapsulates messages.
 Encapsulation happens at the sender site.
The transport layer receives the data and
adds the transport-layer header.
 Decapsulation happens at the receiver site.
When the message arrives at the
destination transport layer, the header is
dropped and the transport layer delivers the
message to the process running at the
application layer. 4/5/2022 Karpagam Institute of Technology 10

Multiplexing and
Demultiplexing
 Whenever an entity accepts items from
more than one source, this is referred to as
multiplexing (many to one).
 Whenever an entity delivers items to more
than one source, this is referred to as
demultiplexing (one to many).
 The transport layer at the source performs
multiplexing
 The transport layer at the destination
performs demultiplexing

Flow Control
 Flow Control is the process of managing
the rate of data transmission between
two nodes to prevent a fast sender from
overwhelming a slow receiver.
 It provides a mechanism for the receiver to
control the transmission speed, so that
the receiving node is not overwhelmed with
data from transmitting node.

Flow Control

Error Control
 Error control at the transport layer is
responsible for
1. Detecting and discarding corrupted
packets.
2. Keeping track of lost and discarded
packets and resending them.
3. Recognizing duplicate packets and
discarding them.
4. Buffering out-of-order packets until the
missing packets arrive.
 Error Control involves Error Detection and
Error Correction 4/5/2022 Karpagam Institute of Technology 14

Error Control

Congestion Control
 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 control refers to the mechanisms and
techniques that control the congestion and keep the
load below the capacity.
 Congestion Control refers to techniques and
mechanisms that can either prevent congestion,
before it happens, or remove congestion, after it has
happened
 Congestion control mechanisms are divided into two
categories,
1. Open loop - prevent the congestion before it
happens. 4/5/2022 Karpagam Institute of Technology 16

PORT NUMBERS
 A transport-layer protocol usually has several
responsibilities.
 One is to create a process-to-process
communication.
 Processes are programs that run on hosts. It could
be either server or client.
 A process on the local host, called a client, needs
services from a process usually on the remote host,
called a server.
 Processes are assigned a unique 16-bit port number
on that host.
 Port numbers provide end-to-end addresses at the
transport layer
 They also provide multiplexing and demultiplexing at
this layer. 4/5/2022 Karpagam Institute of Technology 17

PORT NUMBERS

 ICANN (Internet Corporation for Assigned
Names and Numbers) has divided the port
numbers into three ranges:
◦ Well-known ports
◦ Registered
◦ Ephemeral ports (Dynamic Ports)

WELL-KNOWN PORTS
 These are permanent port numbers used by
the servers.
 They range between 0 to 1023.
 This port number cannot be chosen
randomly.
 These port numbers are universal port
numbers for servers.
 Every client process knows the well-known
port number of the corresponding server
process.
 For example, while the daytime client
process, a well-known client program, can
use an ephemeral (temporary) port number,
52,000, to identify itself, the daytime server
process must use the well-known

WELL-KNOWN PORTS

EPHEMERAL PORTS
(DYNAMIC PORTS)
 The client program defines itself with a port
number, called the ephemeral port number.
 The word ephemeral means “short-lived” and
is used because the life of a client is normally
short.
 An ephemeral port number is recommended
to be greater than 1023.
 These port number ranges from 49,152 to
65,535
 They are neither controlled nor registered.
They can be used as temporary or private
port numbers.

REGISTERED PORTS
 The ports ranging from 1024 to 49,151
are not assigned or controlled.

TRANSPORT LAYER
PROTOCOLS
 Three protocols are associated with the
Transport layer.
 They are
(1) UDP –User Datagram Protocol
(2) TCP – Transmission Control Protocol
(3) SCTP - Stream Control Transmission
Protocol
 Each protocol provides a different type of
service and should be used appropriately.

 UDP - UDP is an unreliable connectionless
transport-layer protocol used for its simplicity and
efficiency in applications where error control can be
provided by the application-layer process.
 TCP - TCP is a reliable connection-oriented
protocol that can be used in any application where
reliability is important.
 SCTP - SCTP is a new transport-layer protocol
designed to combine some features of UDP and
TCP in an effort to create a better protocol for
multimedia communication.

USER DATAGRAM PROTOCOL (UDP)
 User Datagram Protocol (UDP) is a connectionless,
unreliable transport protocol.
 UDP adds process-to-process communication to best-
effort service provided by IP.
 UDP is a very simple protocol using a minimum of
overhead.
 UDP is a simple demultiplexer, which allows multiple
processes on each host to communicate.
 UDP does not provide flow control , reliable or
ordered delivery.
 UDP can be used to send small message where
reliability is not expected.
 Sending a small message using UDP takes much less
interaction between the sender and receiver.
 UDP allow processes to indirectly identify each other
using an abstract locator called port or mailbox

UDP PORTS
 Processes (server/client) are identified by an abstract
locator known as port.
 Server accepts message at well known port.
 Some well-known UDP ports are
7–Echo, 53–DNS, 111–RPC, 161–SNMP, etc.
 < port, host > pair is used as key for demultiplexing.
 Ports are implemented as a message queue.
 When a message arrives, UDP appends it to end of the
queue.
 When queue is full, the message is discarded.
 When a message is read, it is removed from the queue.
 When an application process wants to receive a
message, one is removed from the front of the queue.
 If the queue is empty, the process blocks until a
message becomes available.

UDP DATAGRAM (PACKET) FORMAT
 UDP packets are known as user datagrams
.
 These user datagrams, have a fixed-size
header of 8 bytes made of four fields, each
of 2 bytes (16 bits).

Source Port Number
 Port number used by process on source host with 16
bits long.
 If the source host is client (sending request) then the
port number is an temporary one requested by the
process and chosen by UDP.
 If the source is server (sending response) then it is
well known port number.
Destination Port Number
 Port number used by process on Destination host with 16 bits
long.
 If the destination host is the server (a client sending request)
then the port number is a well known port number.
 If the destination host is client (a server sending response) then
port number is an temporary one copied by server from the
request packet.

Length
 This field denotes the total length of the UDP
Packet (Header plus data)
 The total length of any UDP datagram can be
from 0 to 65,535 bytes.
Checksum
 UDP computes its checksum over the UDP
header, the contents of the message body, and
something called the pseudoheader.
 The pseudoheader consists of three fields from
the IP header—protocol number, source IP
address, destination IP address plus the UDP
length field.
Data
 Data field defines tha actual payload to be
transmitted.
 Its size is variable.

UDP SERVICES
Process-to-Process Communication
 UDP provides process-to-process
communication using socket
addresses, a combination of IP
addresses and port numbers.

UDP SERVICES
Connectionless Services
 UDP provides a connectionless service.
 There is no connection establishment and no
connection termination .
 Each user datagram sent by UDP is an
independent datagram.
 There is no relationship between the different
user datagrams even if they are coming from
the same source process and going to the
same destination program.
 The user datagrams are not numbered.
 Each user datagram can travel on a different
path.

UDP SERVICES
Flow Control
 UDP is a very simple protocol.
 There is no flow control, and hence no
window mechanism.
 The receiver may overflow with incoming
messages.
 The lack of flow control means that the
process using UDP should provide for this
service, if needed.

UDP SERVICES
Error Control
 There is no error control mechanism in UDP
except for the checksum.
 This means that the 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.
 The lack of error control means that the
process using UDP should provide for
this service, if needed.

UDP SERVICES
Checksum
 UDP checksum calculation includes three
sections: a pseudoheader, the UDP header,
and the data coming from the application
layer.
 The pseudoheader is the part of the header
in which the user datagram is to be
encapsulated with some fields filled with 0s.

UDP SERVICES
Optional Inclusion of Checksum
 The sender of a UDP packet can choose
not to calculate the checksum.
 In this case, the checksum field is filled with
all 0s before being sent.
 In the situation where the sender decides to
calculate the checksum, but it happens that
the result is all 0s, the checksum is
changed to all 1s before the packet is sent.
 In other words, the sender complements
the sum two times.

UDP SERVICES
Congestion Control
 Since UDP is a connectionless protocol, it
does not provide congestion control.
 UDP assumes that the packets sent are
small and sporadic(occasionally or at
irregular intervals) and cannot create
congestion in the network.
 This assumption may or may not be true,
when UDP is used for interactive real-time
transfer of audio and video.

UDP SERVICES
Encapsulation and Decapsulation
 To send a message from one process
to another, the UDP protocol
encapsulates and decapsulates
messages.

UDP SERVICES
Queuing
 In UDP, queues are associated with
ports.
 At the client site, when a process starts,
it requests a port number from the
operating system.
 Some implementations create both an
incoming and an outgoing queue
associated with each process.
 Other implementations create only an
incoming queue associated with each
process.

UDP SERVICES
Multiplexing and Demultiplexing
 In a host running a transport 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.

APPLICATIONS OF UDP
 UDP is used for management processes such as
SNMP.
 UDP is used for route updating protocols such as
RIP.
 UDP is a suitable transport protocol for
multicasting. Multicasting capability is embedded in
the UDP software
 UDP is suitable for a process with internal flow and
error control mechanisms such as Trivial File
Transfer Protocol (TFTP).
 UDP is suitable for a process that requires simple
request-response communication with little concern
for flow and error control.
 UDP is normally used for interactive real-time
applications that cannot tolerate uneven delay

The Transport Services
 The transport protocol should provide
to higher-level protocols.
 The transport entity that provides
services to transport service users,
which might be an application
process.

TRANSMISSION CONTROL
PROTOCOL (TCP)
 TCP is a reliable, connection-oriented, byte-stream
protocol.
 TCP guarantees the reliable, in-order delivery of a
stream of bytes. It is a full-duplex protocol, meaning that
each TCP connection supports a pair of byte streams,
one flowing in each direction.
 TCP includes a flow-control mechanism for each of
these byte streams that allow the receiver to limit how
much data the sender can transmit at a given time.
 TCP supports a demultiplexing mechanism that allows
multiple application programs on any given host to
simultaneously carry on a conversation with their peers.
 TCP also implements congestion-control mechanism.
The idea of this mechanism is to prevent sender from
overloading the network.
 Flow control is an end to end issue, whereas
congestion control is concerned with how host and
network interact. 4/5/2022 Karpagam Institute of Technology 44

TCP SERVICES
 Process-to-Process Communication
◦ TCP provides process-to-process communication
using port numbers.
 Stream Delivery Service
◦ TCP is a stream-oriented protocol.
◦ TCP allows the sending process to deliver data
as a stream of bytes and allows the receiving
process to obtain data as a stream of bytes.
◦ TCP creates an environment in which the two
processes seem to be connected by an
imaginary “tube” that carries their bytes across
the Internet.
◦ The sending process produces (writes to) the
stream and the receiving process consumes
(reads from) it. 4/5/2022 Karpagam Institute of Technology 45

Stream Delivery Service

Full-Duplex Communication
◦ TCP offers full-duplex service, where data
can flow in both directions at the same
time.
◦ Each TCP endpoint then has its own
sending and receiving buffer, and
segments move in both directions.
Multiplexing and Demultiplexing
 TCP performs multiplexing at the
sender and demultiplexing at the
receiver.

 Connection-Oriented Service
◦ TCP is a connection-oriented protocol.
◦ A connection needs to be established for
each pair of processes.
◦ When a process at site A wants to send to
and receive data from another process at
site B, the following three phases occur:
 The two TCP’s establish a logical
connection between them.
 Data are exchanged in both directions.
 The connection is terminated.

Reliable Service
◦ TCP is a reliable transport protocol.
◦ It uses an acknowledgment mechanism to
check the safe and sound arrival of data.

TCP SEGMENT
◦ A packet in TCP is called a segment.
◦ Data unit exchanged between TCP peers
are called segments.
◦ A TCP segment encapsulates the data
received from the application layer.
◦ The TCP segment is encapsulated in an
IP datagram, which in turn is
encapsulated in a frame at the data-link
layer.


TCP SEGMENT

TCP SEGMENT
◦ TCP is a byte-oriented protocol, which means that the
sender writes bytes into a TCP connection and the
receiver reads bytes out of the TCP connection.
◦ TCP does not, itself, transmit individual bytes over the
Internet.
◦ TCP on the source host buffers enough bytes from
the sending process to fill a reasonably sized packet
and then sends this packet to its peer on the
destination host.
◦ TCP on the destination host then empties the
contents of the packet into a receive buffer, and the
receiving process reads from this buffer at its leisure.
◦ TCP connection supports byte streams flowing in
both directions.
◦ The packets exchanged between TCP peers are
called segments, since each one carries a segment of
the byte stream.

TCP PACKET FORMAT
◦ Each TCP segment contains the header
plus the data.
◦ The segment consists of a header of 20 to
60 bytes, followed by data from the
application program.
◦ The header is 20 bytes if there are no
options and up to 60 bytes if it contains
options.

TCP PACKET FORMAT

TCP CONNECTION
MANAGEMENT
◦ TCP is connection-oriented.
◦ A connection-oriented transport protocol
establishes a logical path between the
source and destination.
◦ All of the segments belonging to a
message are then sent over this logical
path.
◦ In TCP, connection-oriented transmission
requires three phases: Connection
Establishment, Data Transfer and
Connection Termination.

Connection Establishment
 While opening a TCP connection the
two nodes(client and server) want to
agree on a set of parameters.
 The parameters are the starting
sequence numbers that is to be used
for their respective byte streams.
 Connection establishment in TCP is a
three-way handshaking.

three-way handshaking

Connection Establishment
 Client sends a SYN segment to the
server containing its initial sequence
number (Flags
 = SYN, SequenceNum = x)
 Server responds with a segment that
acknowledges client’s segment and
specifies its initial sequence number
(Flags = SYN + ACK, ACK = x + 1
SequenceNum = y).
 Finally, client responds with a segment
that acknowledges server’s sequence
number 4/5/2022 Karpagam Institute of Technology 58

Data Transfer
◦ After connection is established,
bidirectional data transfer can take place.
◦ The client and server can send data and
acknowledgments in both directions.
◦ The data traveling in the same direction
as an acknowledgment are carried on the
same segment.
◦ The acknowledgment is piggybacked with
the data.

Connection Termination
 Connection termination or teardown
can be done in two ways :
 Three-way Close and Half-Close
 Three-way Close—Both client and
server close simultaneously.

STATE TRANSITION DIAGRAM
 To keep track of all the different events
happening during connection
establishment, connection termination,
and data transfer, TCP is specified as the
finite state machine (FSM).
 The transition from one state to another
is shown using directed lines.
 States involved in opening and closing a
connection is shown above and below
ESTABLISHED state respectively.
 States Involved in TCP :

TCP FLOW CONTROL

Flow Control in TCP
 Size of send and receive buffer is MaxSendBuffer
and MaxRcvBuffer respectively.
 Sending TCP prevents overflowing of send buffer
by maintaining
LastByteWritten − LastByteAcked ≤
MaxSendBuﬀer
 Receiving TCP avoids overflowing its receive buffer
by maintaining
LastByteRcvd − LastByteRead ≤ MaxRcvBuffer
 Receiver throttles the sender by having
AdvertisedWindow based on free space available
for buffering.
 AdvertisedWindow = MaxRcvBuffer −
((NextByteExpected − 1) – LastByteRead)

 Sending TCP adheres to AdvertisedWindow by
computing EffectiveWindow that
 limits how much data it should send.
 EffectiveWindow = AdvertisedWindow −
(LastByteSent − LastByteAcked)
 When data arrives, LastByteRcvd moves to its
right and AdvertisedWindow shrinks.
 Receiver acknowledges only, if preceding bytes
have arrived.
 AdvertisedWindow expands when data is read by
the application. o If data is read as fast as it
arrives then
 AdvertisedWindow = MaxRcvBuffer
 o If data is read slowly, it eventually leads to a
AdvertisedWindow of size 0.
 AdvertisedWindow field is designed to allow
sender to keep the pipe full.

TCP TRANSMISSION
 TCP has three mechanism to trigger
the transmission of a segment.
 They are
◦ Maximum Segment Size (MSS) - Silly
Window Syndrome
◦ Timeout - Nagle’s Algorithm

TCP CONGESTION CONTROL
 TCP Congestion Control mechanisms
are:
 Additive Increase / Multiplicative
Decrease (AIMD)
 Slow Start
 Fast Retransmit and Fast Recovery

Additive Increase /
Multiplicative Decrease (AIMD)

Slow Start

Fast Retransmit And Fast
Recovery

TCP CONGESTION AVOIDANCE
 Congestion avoidance mechanisms prevent
congestion before it actually occurs.
 These mechanisms predict when congestion
is about to happen and then to reduce the
rate at which hosts send data just before
packets start being discarded.
 TCP creates loss of packets in order to
determine bandwidth of the connection.
 Routers help the end nodes by intimating
when congestion is likely to occur.
 Congestion-avoidance mechanisms are:
◦ DEC bit - Destination Experiencing Congestion
Bit
◦ RED - Random Early Detection

The Transport Services
 The following categories of service are
useful for describing the transport
service.
Type of service
Quality of service
Data transfer
User interface
Connection management
Expedited delivery
Status reporting
Security

STREAM CONTROL
TRANSMISSION PROTOCOL
(SCTP)
 Stream Control Transmission Protocol
(SCTP) is a reliable, message-oriented
transport layer protocol.
 SCTP has mixed features of TCP and
UDP.
 SCTP maintains the message
boundaries and detects the lost data,
duplicate data as well as out-of-order
data.
 SCTP provides the Congestion control
as well as Flow control.
 SCTP is especially designed for internet
applications as well as multimedia
communication. 4/5/2022 Karpagam Institute of Technology 77

SCTP SERVICES
Process-to-Process Communication
 SCTP provides process-to-process
communication.
Multiple Streams
 SCTP allows multistream service in each
connection, which is called association
in SCTP terminology.
 If one of the streams is blocked, the
other streams can still deliver their data.

Multihoming
 An SCTP association supports
multihoming service.
 The sending and receiving host can
define multiple IP addresses in each
end for an association.
 In this fault-tolerant approach, when
one path fails, another interface can
be used for data delivery without
interruption.

Multihoming

Full-Duplex Communication
 SCTP offers full-duplex service, where data can
flow in both directions at the same time. Each
SCTP then has a sending and receiving buffer
and packets are sent in both directions.
Connection-Oriented Service
 SCTP is a connection-oriented protocol.
 In SCTP, a connection is called an association.
 If a client wants to send and receive message
from server , the steps are :
 Step1: The two SCTPs establish the connection
with each other.
 Step2: Once the connection is established, the
data gets exchanged in both the directions.
 Step3: Finally, the association is terminated.

 Reliable Service
 SCTP is a reliable transport protocol.
 It uses an acknowledgment
mechanism to check the safe and
sound arrival of data.

SCTP PACKET FORMAT

SCTP ASSOCIATION
 SCTP is a connection-oriented
protocol.
 A connection in SCTP is called an
association to emphasize
multihoming.
 SCTP Associations consists of three
phases:
 Association Establishment
 Data Transfer
 Association Termination

SCTP FLOW CONTROL
 Flow control in SCTP is similar to that
in TCP.
 Current SCTP implementations use a
byte-oriented window for flow control.

Receiver Site
 The receiver has one buffer (queue) and
three variables.
 The queue holds the received data chunks
that have not yet been read by the process.
 The first variable holds the last TSN received,
cumTSN.
 The second variable holds the available
buffer size; winsize.
 The third variable holds the last accumulative
acknowledgment, lastACK.
 The following figure shows the queue and
variables at the receiver site.

Receiver Site

Receiver Site
 When the site receives a data chunk, it stores it
at the end of the buffer (queue) and subtracts the
size of the chunk from winSize.
 The TSN number of the chunk is stored in the
cumTSN variable.
 When the process reads a chunk, it removes it
from the queue and adds the size of the removed
chunk to winSize (recycling).
 When the receiver decides to send a SACK, it
checks the value of lastAck; if it is less than
cumTSN, it sends a SACK with a cumulative
TSN number equal to the cumTSN.
 It also includes the value of winSize as the
advertised window size.

Sender Site
 The sender has one buffer (queue) and three variables:
curTSN, rwnd, and inTransit.
 We assume each chunk is 100 bytes long. The buffer
holds the chunks produced by the process that either
have been sent or are ready to be sent.
 The first variable, curTSN, refers to the next chunk to
be sent.
 All chunks in the queue with a TSN less than this value
have been sent, but not acknowledged; they are
outstanding.
 The second variable, rwnd, holds the last value
advertised by the receiver (in bytes).
 The third variable, inTransit, holds the number of bytes
in transit, bytes sent but not yet acknowledged.
 The following figure shows the queue and variables at
the sender site.

Sender Site

Sender Site
 A chunk pointed to by curTSN can be sent if the size of
the data is less than or equal to the quantity rwnd -
inTransit.
 After sending the chunk, the value of curTSN is
incremented by 1 and now points to the next chunk to
be sent.
 The value of inTransit is incremented by the size of the
data in the transmitted chunk.
 When a SACK is received, the chunks with a TSN less
than or equal to the cumulative TSN in the SACK are
removed from the queue and discarded. The sender
does not have to worry about them anymore.
 The value of inTransit is reduced by the total size of the
discarded chunks.
 The value of rwnd is updated with the value of the
advertised window in the SACK.

SCTP ERROR CONTROL
 SCTP is a reliable transport layer
protocol.
 It uses a SACK chunk to report the
state of the receiver buffer to the
sender.
 Each implementation uses a different
set of entities and timers for the
receiver and sender sites.

Receiver Site
 The receiver stores all chunks that have
arrived in its queue including the out-of-
order ones. However, it leaves spaces
for any missing chunks.
 It discards duplicate messages, but
keeps track of them for reports to the
sender.

The following figure shows a typical
design for the receiver site and the state
of the receiving queue at a particular
point in time.

Receiver Site

Receiver Site
 The available window size is 1000 bytes.
 The last acknowledgment sent was for data chunk
20.
 Chunks 21 to 23 have been received in order.
 The first out-of-order block contains chunks 26 to
28.
 The second out-of-order block contains chunks 31
to 34.
 A variable holds the value of cumTSN.
 An array of variables keeps track of the beginning
and the end of each block that is out of order.
 An array of variables holds the duplicate chunks
received.
 There is no need for storing duplicate chunks in the
queue and they will be discarded.

Sender Site

 The sending queue holds chunks 23 to 40.
 The chunks 23 to 36 have already been sent, but
not acknowledged; they are outstanding chunks.
 The curTSN points to the next chunk to be sent
(37).
 We assume that each chunk is 100 bytes, which
means that 1400 bytes of data (chunks 23 to 36)
is in transit.
 The sender at this moment has a retransmission
queue.
 When a packet is sent, a retransmission timer
starts for that packet (all data chunks in that
packet).
 Some implementations use one single timer for
the entire association, but other implementations
use one timer for each packet.

SCTP CONGESTION
CONTROL
 SCTP is a transport-layer protocol with
packets subject to congestion in the
network.
 The SCTP designers have used the
same strategies for congestion control
as those used in TCP.

THANK YOU

Unit 4 - Transport Layer

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Unit 4 - Transport Layer