Transport Layer Introduction of Transport Layer The Transport Layer Services Elements of transport protocol UDP Principal of Reliable Data transfer

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) Services Provided to the Upper Layers

b) Transport Service Primitives

c) 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




UDP


TCP


Congestion Control

Quality of Service

Performance

Elements of Transport ProtocolElements 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




UDP


TCP


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 01 1 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1

1 1 0 1 1 1 0 1 1 1 0 1 1 1 0 1 1

1 1 0 1 1 1 0 1 1 1 0 1 1 1 1 0 01 0 1 0 0 0 1 0 0 0 1 0 0 0 0 1 1

wraparound

sumchecksum

Transport Layer




UDP


TCP


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!


Transport Layer

Principles of Reliable data transfer important in app., transport, link layers important networking topics!


Reliable data transfer: getting started

sendside

receiveside

rdt_send(): called from above, (e.g., by app.). Passed data to deliver to receiver upper layer

udt_send(): called by rdt,to transfer packet over unreliable channel to

receiver

rdt_rcv(): called when packet arrives on rcv-side of channel

deliver_data(): called by rdt to deliver data to

upper

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

state1

state2

event causing state transitionactions taken on state transition

state: when in this “state” next state

uniquely determined by

next event

eventactions

Transport Layer




UDP


TCP


Congestion Control

Quality of Service

Performance

The Internet Transport Protocols: TCP

1. Introduction of TCP2. Features of TCP3. The TCP Service Model4. The TCP Protocol5. The TCP Segment Header6. TCP Connection Establishment7. TCP Connection Release8. TCP Connection Management Modeling9. 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 connection-oriented , 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 RFCs: 793, 1122, 1323, 2018, 2581

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

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

socketdoor

T C Psend buffer

T C Preceive buffer

socketdoor

segm ent

applicationwrites data

applicationreads data

(The 800 lbs gorilla in the transport stack! PAY ATTENTION!!)

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

64Kbyte MTU, agreed upon for each direction

Data from consecutive writes may be accumulated in a single segment

Fragmentation possible Sliding window protocol

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

source port # dest port #

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberReceive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG: urgent data (generally not used)

ACK: ACK #valid

PSH: push data now(generally not used)

RST, SYN, FIN:connection estab(setup, teardown

commands)

# bytes rcvr willingto accept

countingby “bytes” of data(not segments!)

Internetchecksum

(as in UDP)

Due to this field we have a variable length header


Problem: delayed duplicates! Scenario:

Correct bank transaction connect data transfer disconnect

Problem: same packets are received in same order a second time!

Recognized?


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

(a) TCP connection establishment in the normal case.(b) Call collision.

6-31

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




UDP


TCP


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

end-to-end congestion control:

no explicit feedback from network

congestion inferred from end-system observed loss, delay

approach taken by TCP

Network-assisted congestion control:

routers provide feedback to end systems single bit indicating

congestion (SNA, ATM) explicit rate sender should

send at

Two broad approaches towards congestion control:


How to detect congestion?

Timeout caused by packet loss: reasons Transmission errors Packed discarded at congested router

: Rare

Packet loss

Hydraulic example illustrating two limitations for sender!

for wired networks

TCP congestion control


How to detect congestion? Timeout caused by packet loss: reasons

Transmission errors Packed discarded at congested router

: Rare

Packet loss congestion

Approach: 2 windows for sender

Receiver window

Congestion window Minimum of

TCP Congestion Control end-end control (no network assistance) transmission rate limited by congestion window size, Congwin,

over segments:

w segments, each with MSS bytes sent in one RTT:

throughput = w * MSS

RTT Bytes/sec

Congwin

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




UDP


TCP


Congestion Control

Quality of Service

Performance

Congestion Control

1. Congestion Control

2. Additive Increase,Multiplicative Decrease Control(AIMD)

3. Slow start Method

4. Causes of Congestion

5. General Principles of Congestion Control

6. 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 methods1. Additive increase2. Slow start3. Retransmit

2.TCP congestion control: additive increase, multiplicative decrease

8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow

Approach: increase transmission rate (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

timecong

estio

n w

indo

w s

ize

Saw toothbehavior: probing

for bandwidth

TCP Congestion Control: details

sender limits transmission: LastByteSent-LastByteAcked

CongWin Roughly,

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

rate = CongWin

RTT Bytes/sec

3. Slow Start Method

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 used1. Just after a TCP connection is set up.2. When a source is blocked, waiting for a

timeout.

TCP Slow Start (more)

When connection begins, increase rate exponentially until first loss event: double CongWin every

RTT done by incrementing

CongWin for every ACK received

Summary: initial rate is slow but ramps up exponentially fast

Host A

one segment

RTT

Host B

time

two segments

four segments

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

2. ACK policy

3. Retransmission policy

4. Out of order caching policy

Network layer

1. Routing algorithm

2. Packet queueing service policy

3. Packet Lifetime management policy

4. Packet discard policy

Transport layer policies

1. Flow control policy

2. Acknowledgment policy

3. Retransmission policy

4. Out of order caching policy

5. Timeout determination.

Transport Layer




UDP


TCP


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




UDP


TCP


Congestion Control

Quality of Service

Performance

Performance

1. Bandwidth

2. Throughput

3. Letency

4. Bandwidth- Delay Product

5. 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.

Documents

Transport Layer Introduction of Transport Layer The Transport Layer Services Elements of transport protocol UDP Principal of Reliable Data transfer