Transport Layer

Introduction of Transport Layer

The Transport Layer Services

Elements of transport protocol

UDP

Principal of Reliable Data transfer

TCP

TCP Congestion Control

Congestion Control

Quality of Service

Performance
Introduction of Transport layer


The transport layer is the core of the Internet
model.The application layer programs interact with
each other using the services of the transport layer.
Lists the functiom of a transport layer.
Transport layer
packetizing
Connection Control
Addressing
Providing reliability
Transport layer functions :

This layer breaks message into packets.

It performance error recovery if the lower
layer are not adequately error free.

Function of flow control if not done

Function of multiplexing and demultiplexing
sessions together.
OBJECTIVES:
To define process-to-process communication at the transport layer and

compare it with host-to-host communication at the network layer.

To discuss the addressing mechanism at the transport layer, to discuss
port numbers, and to define the range of port numbers used for different
purposes.

To explain the packetizing issue at the transport layer: encapsulation and
decapsulation of messages.

To discuss multiplexing (many-to-one) and demultiplexing (one-to-many)
services provided by the transport layer.
The Transport Service
a)
b)
c)
Services Provided to the Upper Layers
Transport Service Primitives
Berkeley Sockets
Services Provided to the Upper Layers
The network, transport, and application layers.
Why the transport layer ?
1. The network layer exists on end hosts and routers in the
network. The end-user cannot control what is in the network. So
the end-user establishes another layer, only at end hosts, to
provide a transport service that is more reliable than the
underlying network service.
2. While the network layer deals with only a few transport
entities, the transport layer allows several concurrent applications
to use the transport service.
3. It provides a common interface to application writers,
regardless of the underlying network layer. In essence, an
application writer can write code once using the transport layer
primitive and use it on different networks (but with the same
transport layer).
Transport Service Primitives
The primitives for a simple transport service.
Transport Service Primitives
The nesting of TPDUs, packets, and frames.
Berkeley Sockets
The socket primitives for TCP.
Transport Protocol
(a) Environment of the data link layer.
(b) Environment of the transport layer.
Both data link layer and transport layer do error control, flow control,
sequencing. The differences are:
1. Storage capacity in subnet. Frames must arrive sequentially,
TPDUs can arrive in any sequence.
2. Frames are delivered to hosts, TPDUs need to be delivered to
users, so per user addressing and flow control within the hosts is
necessary.
Transport Layer

Introduction of Transport Layer

The Transport Layer Services

Elements of transport protocol

UDP

Principal of Reliable Data transfer

TCP

TCP Congestion Control

Congestion Control

Quality of Service

Performance
Elements of Transport Protocol
 Addressing
 Connection Establishment
 Multiplexing and Demultiplexing
 Flow Control and Buffering
 Crash Recovery

TSAP = transport service access point




Internet: IP address + local port
ATM: AAL-SAPs
Each station having only one transport entity,
so a transport entity identification is not
needed.
The Address should include a designation of
the type of transport protocol e.g. TCP,UDP.
Connection Establishment
How a user process in host 1
establishes a connection with a
time-of-day server in host 2.
How a user process in host 1 establishes a connection with
a time-of-day server in host 2.
The Connection establishment serves three
main purpose



It allows each end to assure that the other
exists.
It allows negotiation of optional parameter
like maximum segment size.
It triggers allocation of transport entity
resources like buffer space.
Establishing a connection

Tomlinson – three-way-handshake
Flow Control and Buffering



Flow control is implemented using modified
form of sliding window protocol.The window
size is variable and is controlled by the
receiver.
The receiver sends a credit allocation to the
sender.
If the receiver cannot guarantee that every
incoming TPDU wil be accepted, the sender
will have to buffer anyway.
Flow control and buffering

Buffer organization
Multiplexing and Demultiplexing
Multiplexing : many-to-one services provided
by
the transport layer
“The Dividing flows of Data from the application into
one or many packets”
 Demultiplexing : one-to-many services provided
by the transport layer
“ Allocating each communication flow a unique
identifier”

Multiplexing


Upward: reduce number of network connections to reduce cost
Downward: increase bandwidth to avoid per connection limits




If the host server & router are subject to crashes, the recovery
from these crashes makes some problem.
When the server crash while receiving data from client, the
outstanding TPDU is lost. TO recover the data, when the
server comes back up, its tables are reinitialized, so it no
longer knows precisely where it was.
The server send a broadcast TPDU to all the other host, Just
crashed and requesting that its clients inform it for the status
of all open connections.
Client can be in one of two states: TPDU outstanding or no
TPDU outstanding
Recovery from a layer N crash can only be done by layer N+1
and only if the higher layer retains enough status information.
Transport Layer

Introduction of Transport Layer

The Transport Layer Services

Elements of transport protocol

UDP

Principal of Reliable Data transfer

TCP

TCP Congestion Control

Congestion Control

Quality of Service

Performance


UDP is simple, datagram-oriented, transport
layer protocol. This protocol is used in place
of TCP
UDP is connectionless protocol provides no
reliability or flow control mechanisms.It also
has no error recovery procedures.
Internet Transport Protocols


Datagram messaging service (UDP)
 No-frills extension of “best-effort” IP
Reliable, in-order delivery (TCP)
 Connection set-up
Discarding of corrupted packets
 Retransmission of lost packets
 Flow control
 Congestion control (next lecture)
Other services not available
 Delay guarantees
 Bandwidth guarantees


UDP: User Datagram Protocol [RFC
768]



“no frills,” “bare bones”
Internet transport protocol
“best effort” service, UDP
segments may be:
 lost
 delivered out of order to
app
connectionless:
 no handshaking between
UDP sender, receiver
 each UDP segment
handled independently of
others
Why is there a UDP?




no connection
establishment (which can
add delay)
simple: no connection
state at sender, receiver
small segment header
no congestion control:
UDP can blast away as
fast as desired
Introduction to UDP
The UDP header.
Remote Procedure Call
Steps in making a remote procedure call. The
stubs are shaded.
Internet Checksum Example

Note
 When adding numbers, a carryout from the
most significant bit needs to be added to the
result

Example: add two 16-bit integers
1 1 1 1 0 0 1 1 0 0 1 1 0 0 1 1 0
1 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
wraparound 1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1
sum 1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 0
checksum 1 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1
Transport Layer

Introduction of Transport Layer

The Transport Layer Services

Elements of transport protocol

UDP

Principal of Reliable Data transfer

TCP

TCP Congestion Control

Congestion Control

Quality of Service

Performance


Reliable data transfer means, data received
without loss or corrupt. All the packets are
delivered in the order in which they were
sent.
The Complexity of the reliable data transfer
protocol determined by characteristics of
unreliable channel.
Principles of Reliable data
transfer



important in app., transport, link layers
top-10 list of important networking topics!
characteristics of unreliable channel will determine complexity
of reliable data transfer protocol (rdt)
Principles of Reliable data
transfer

important in app., transport, link layers
important networking topics!

characteristics of unreliable channel will determine complexity
of reliable data transfer protocol (rdt)
Principles of Reliable data
transfer



important in app., transport, link layers
important networking topics!
characteristics of unreliable channel will determine complexity
of reliable data transfer protocol (rdt)
Transport Layer
Reliable data transfer: getting started
rdt_send(): called from above,
(e.g., by app.). Passed data to
deliver to receiver upper layer
send
side
udt_send(): called by rdt,
to transfer packet over
unreliable channel to receiver
deliver_data(): called by
rdt to deliver data to upper
receive
side
rdt_rcv(): called when packet
arrives on rcv-side of channel
Reliable data transfer: getting started
We’ll:



incrementally develop sender, receiver sides of reliable
data transfer protocol (rdt)
consider only unidirectional data transfer
 but control info will flow on both directions!
use finite state machines (FSM) to specify sender, receiver
state: when in this
“state” next state
uniquely determined
by next event
event causing state transition
actions taken on state transition
state
1
event
actions
state
2
Transport Layer

Introduction of Transport Layer

The Transport Layer Services

Elements of transport protocol

UDP

Principal of Reliable Data transfer

TCP

TCP Congestion Control

Congestion Control

Quality of Service

Performance
The Internet Transport
Protocols: TCP
1.
2.
3.
4.
5.
6.
7.
8.
9.
Introduction of TCP
Features of TCP
The TCP Service Model
The TCP Protocol
The TCP Segment Header
TCP Connection Establishment
TCP Connection Release
TCP Connection Management Modeling
TCP Transmission Policy
Introduction of TCP




The Transmission Control Protocol (TCP) is a
critically important part of the TCP/IP suite.
The Transmission Control Protocol (TCP) is the
connection oriented protocol whereas User Data
Protocol (UDP) is connectionless protocol. Both are
internet protocols used in the transport layer.
The TCP provides reliable Transmission of data in IP
environment.
TCP offers reliability by providing connectionoriented , end-to-end reliable packet delivery
through an internetwork.
Features of TCP





TCP is a process-to-process protocal.
TCP uses port numbers.
It is connection oriented protocol.
It uses flow and error control mechanisms.
TCP is a reliable protocol.
TCP: Models
2581




(The 800 lbs gorilla in the transport stack! PAY ATTENTION!!)
point-to-point:
 one sender, one receiver
(not multicast)
reliable, in-order byte
steam:
 no “message
boundaries”
 In App layer, we need
delimiters.
pipelined:
 TCP congestion and flow
control set window size
send & receive buffers
socket
door
RFCs: 793, 1122, 1323, 2018,
application
writes data




application
reads data
TCP
send buffer
TCP
receive buffer
segment
full duplex data:
bi-directional data flow in same
connection

MSS: maximum segment
size
connection-oriented:

handshaking (exchange of
control msgs) init’s sender,
receiver state (e.g., buffer
size) before data exchange
flow controlled:

sender will not overwhelm
receiver
socket
door
TCP protocol



Three-way handshake to set up connections
Every byte has its own 32-bit sequence number
 Wrap around
 32-bit Acks; window size in bytes
Segment = unit of data exchange
 20-byte header + options + data
 Limits for size


Data from consecutive writes may be accumulated in a single
segment
 Fragmentation possible
Sliding window protocol


64Kbyte
MTU, agreed upon for each direction
TCP header








source & destination ports (16 bit)
sequence number (32 bit)
Acknowledgement number (32 bit)
Header length (4 bits) in 32-bit words
6 flags (1 bit)
window size (16 bit): number of bytes the sender is
allowed to send starting at byte acknowledged
checksum (16 bit)
urgent pointer (16 bit) : byte position of urgent data
TCP segment structure
URG: urgent data
(generally not used)
ACK: ACK #
valid
PSH: push data now
(generally not used)
RST, SYN, FIN:
connection estab
(setup, teardown
commands)
Internet
checksum
(as in UDP)
32 bits
source port #
dest port #
sequence number
acknowledgement number
head not
UA P R S F
len used
checksum
Receive window
Urg data pnter
Options (variable length)
application
data
(variable length)
counting
by “bytes”
of data
(not segments!)
# bytes
rcvr willing
to accept
Due to this
field we have a
variable length
header
Establishing a connection


Problem: delayed duplicates!
Scenario:

Correct bank transaction




connect
data transfer
disconnect
Problem: same packets are received in same
order a second time!
Recognized?
Establishing a connection

Connection establishment in a TCP session is
initialized through a three-way handshake.To
establish the connection, one side pasively waits for
an incoming connection by executing the LISTEN
and ACCEPT primitives, either specifying a specific
source.

connection identifier
 Never reused!


Maintain state in hosts
Satisfactory solutions
TCP Connection
Establishment
6-31
(a) TCP connection establishment in the normal case.
(b) Call collision.
TCP Connection Management
Modeling
The states used in the TCP connection
management finite state machine.
TCP Connection Management
Modeling (2)
TCP connection
management finite state
machine. The heavy solid
line is the normal path for a
client. The heavy dashed
line is the normal path for a
server. The light lines are
unusual events. Each
transition is labeled by the
event causing it and the
action resulting from it,
separated by a slash.
TCP transmission policy


Window size decoupled from Acks (ex. next slides)
Window = 0  no packets except for
Urgent data
 1 byte segment to send Ack & window size
Incoming user data may be buffered
 May improve performance: less segments to send
Ways to improve performance:
 Delay acks and window updates for 500 msec
 Nagle’s algorithm
 Silly window syndrome



Transport Layer

Introduction of Transport Layer

The Transport Layer Services

Elements of transport protocol

UDP

Principal of Reliable Data transfer

TCP

TCP Congestion Control

Congestion Control

Quality of Service

Performance
Principles of Congestion
Control
Congestion:


informally: “too many sources sending too much
data too fast for network to handle”
different from flow control!
=


end-to-end issue!
manifestations:
 lost packets (buffer overflow at routers)
 long delays (queue-ing in router buffers)
a top-10 problem!
Approaches towards congestion
control
Two broad approaches towards congestion
control:
Network-assisted congestion
end-to-end congestion
control:
control:
 routers provide feedback to
 no explicit feedback
end systems
 single bit indicating
from network
congestion (SNA, ATM)
 congestion inferred
 explicit rate sender should
from end-system
send at
observed loss, delay
 approach taken by TCP
TCP Congestion Control


How to detect congestion?
: Rare
for wired networks
Packet loss
Timeout caused by packet loss: reasons


Transmission errors
Packed discarded at congested router
Hydraulic example illustrating two limitations
for sender!
TCP congestion control
TCP Congestion Control


How to detect congestion?
: Rare
Timeout caused by packet loss: reasons


Transmission errors
loss  router
congestion
Packed discarded atPacket
congested
Approach: 2 windows for sender
Minimum of

Receiver window
Congestion window
TCP Congestion Control


end-end control (no network assistance)
transmission rate limited by congestion window size,
Congwin, over segments:
Congwin

w segments, each with MSS bytes sent in one
RTT:
w * MSS
throughput =
RTT
Bytes/sec
TCP timer management

How long should the timeout interval be?
 Data link: expected delay predictable

Transport: different environment; impact of
 Host
Network (routers, lines)
unpredictable


Consequences
 Too small: unnecessary retransmissions
Too large: poor performance
Solution: adjust timeout interval based on continuous
measurements of network performance


TCP timer management
Data link layer
Transport layer
Transport Layer

Introduction of Transport Layer

The Transport Layer Services

Elements of transport protocol

UDP

Principal of Reliable Data transfer

TCP

TCP Congestion Control

Congestion Control

Quality of Service

Performance
Congestion Control
1.
2.
3.
4.
5.
6.
Congestion Control
Additive Increase,Multiplicative Decrease
Control(AIMD)
Slow start Method
Causes of Congestion
General Principles of Congestion Control
Congestion Prevention Policies
1. Congestion Control

TCP uses a form of end to end flow control. Both
are sender and the receiver agree on a common
window size for packet flow.The window size
represents the number of bytes that the source
can send at a time.

TCP has three congestion control methods
Additive increase
Slow start
Retransmit
1.
2.
3.
2.TCP congestion control: additive
increase, multiplicative decrease
 Approach: increase transmission rate (window size),
Saw tooth
behavior: probing
for bandwidth
congestion window size
probing for usable bandwidth, until loss occurs
 additive increase: increase CongWin by 1 MSS
every RTT until loss detected
 multiplicative decrease: cut CongWin in half after
loss
congestion
window
24 Kbytes
16 Kbytes
8 Kbytes
time
time
TCP Congestion Control:
details

sender limits transmission:
LastByteSent-LastByteAcked
 CongWin

Roughly,
rate =

CongWin
Bytes/sec
RTT
CongWin is dynamic, function of
perceived network congestion
How does sender perceive
congestion?
 loss event = timeout or 3
duplicate acks
 TCP sender reduces rate
(CongWin) after loss event
three mechanisms:
 AIMD
 slow start
 conservative after timeout
events
3. Slow Start Method


1.
2.
Slow start method increase the congestion
window size nonlinearly and in most cases
exponentially, as compared to the linear
increase in additive increase.
The slow start method is normally used
Just after a TCP connection is set up.
When a source is blocked, waiting for a
timeout.
TCP Slow Start (more)
When connection
begins, increase rate
exponentially until first
loss event:



Host A
Host B
RTT

double CongWin every
RTT
done by incrementing
CongWin for every ACK
received
Summary: initial rate is
slow but ramps up
exponentially fast
time
4. Causes of Congestion


When too many packets rushing to a node or
part of network , the network performance
degrades, and this situation is called as
Congestion.
Congestion Control is a procces of
maintaining the number of packets in a
network below a certain level at which
performance falls off.Congestion Control
makes sure that subnet is able to carry the
offered traffic.
5. General Principal of
Congestion Control



Two Group : (1) OPEN loop and
(2) CLOSED loop
Open loop solutions attempt to solve the
problem by good design, to make sure it does
not occur in the first place.
Closed loop solutions are based on the
concept of a feedback loop. One part is a
Pass this informance to places where action
can be taken.
6. Congestion Prevention
Policies

Data link layer
1.
Flow control policy
ACK policy
Retransmission policy
Out of order caching
policy
2.
3.
4.

Network layer
1.
Routing algorithm
Packet queueing
service policy
Packet Lifetime
management policy
Packet discard policy
2.
3.
4.

1.
2.
3.
4.
5.
Transport layer policies
Flow control policy
Acknowledgment policy
Retransmission policy
Out of order caching policy
Timeout determination.
Transport Layer

Introduction of Transport Layer

The Transport Layer Services

Elements of transport protocol

UDP

Principal of Reliable Data transfer

TCP

TCP Congestion Control

Congestion Control

Quality of Service

Performance
1. Quality of Service



In any multimedia application audio/video
packets are delay sensitive but by internet all
packets are treated equally
Analyzing varying network scenarios principal
of Quality of Services (Qos) needed for
multimedia applications are derived.
Principal 1: Packet marking allows a router to
distinguish among packets belonging to
different classes of traffic.

Pricipal 2 :
A degree of isolation is desirable among
traffic flows, so that one flow is not adversely
by another misbehaving flow.

Principal 3 :
A call admission process is needed where
flows declare their QoS requirement.
2. Policing
3. Integrated Services

Framework to provide guaranteed QoS to
individual application sessions.

Call step process
1. Traffic specification of desired QoS
2. Signalling for call setup
3. Pre element call admission.
Transport Layer

Introduction of Transport Layer

The Transport Layer Services

Elements of transport protocol

UDP

Principal of Reliable Data transfer

TCP

TCP Congestion Control

Congestion Control

Quality of Service

Performance
Performance
1.
2.
3.
4.
5.
Bandwidth
Throughput
Letency
Bandwidth- Delay Product
Jitter



Bandwidth : Characteristic of network.
Measured : Hertz and Bits per Second
Throughput
“ An Actual measurement of how fast data can be transmitted where
as bandwidth is a potential measurement of link”
Letency
“ It is time required for amessage to completely arrive at the
destination from source”
Components : propagation time, transmission time, quequing time
and processing delay.

Bandwidth – Delay Product
The bandwidth-delay product defines the
number of bits that can fill the link

Jitter
It is a parameter related to delay. Jitter is
introduced since different packets of data
encounter different delay.