09cs 6048 amrik singh sidhu

8/14/2019 09CS 6048 Amrik Singh Sidhu

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09CS 6048 Amrik Singh SidhuTransport Layer


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THE BASICSADDRESSING &CONNECTIONS

BUFFERS &FLOWMANAGEMENT

TCP

PREVIEW


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THE BASICS

Transport Layer


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Where are we?

Connection controlSequencingApplicationMultiplexing

Congestion controlFlow controlError control

Where are We?????


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Provides Efficient, Reliable, Cost Effective Service

Software/ hardware within the tpt layer that does the work is

called theTransport Entity.

Types of Services

Connection Oriented

Connection Less

Same type of services are provided by NW Layer. Why therepetition?

Services Provided to Transport Layer


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Users have no control over the subnet - cannot solve theproblem of poor service

Another layer on top of NW layer reqd to improve qualityof service.

Existence of tpt layer makes it possible for tpt service to bemore reliable than underlying NW service.

Isolate upper layers from technology, design andimperfections of the subnet

Requirement of a Tpt Layer


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The network, transport, and application layers.

Services Provided to the Upper Layers


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Provide application-to-application communication

Need extended addressing mechanism to identify applications

First end-to-end layer

Provides:

Reliability

Flow Control

Congestion Control

Transport interface allows application programs to establish, use,

and release connections.

Transport Protocols


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Explicit addressing scheme reqd for destinations

Connection establishment

Release of connections

Storage capacity of the subnet

Buffering and Flow control

Crash Recovery

Elements of Transport Protocols


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ADDRESSING

Transport Layer


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TSAP: Transport Service Access Point

Communication End Points

Define a tpt address to which processes can listen for

connection requests. End points (Also called Sockets) consist of an IP address

and a Local Port Number (16 bit in case of TCP)

NSAP : Network Service Access Point

32 bit IP address

TSAP = NSAP + Local Port Number

Addressing


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Addressing


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How do you know the destination TSAP ?

Well known ports are one way.

But to cater for lesser well known services the schemesused are :

Initial Connection RequestEvery machine runs a special processserver that acts as a proxy for lesser used services.

Name Server / Directory Server

Addressing


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Name Server/ Directory Server

Name/Directory Server listens to a well known port

User sets up a connection to the Name/Directory Server User sends a message specifying the service name.

Name Server sends back the TSAP address.

User then releases the connection with Name Server andestablishes new connection with desired service.

When a new service is created it registers itself with Name

Server giving its Service name and TSAP address.

AddressingAlternate Scheme


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CONNECTIONS

Transport Layer


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Clock based numbering scheme

Each host equipped with a clock

A clock is assumed to be running even if the host isdown

Lower order bits of the clock used as sequencenumber

Each connection starts numbering its TPDU with aunique sequence number

Connection Establishment


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Three-way Handshake Protocol for Connection Est.



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Three-way Handshake Protocol

Old CONNECTION REQUEST appearing out of nowhere.

Three Way Hanshake Protocol


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Three-way Handshake Protocol

Duplicate CR and duplicate ACK.

Three Way Hanshake Protocol


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Connection Release

Two types of connection release:

Symmetric- Each side closes connection independently

Asymmetric- Either side sends a DISCONNECTTPDU. Upon arrival the connection is released.

Asymmetric release is abrupt and may leadto loss of data

Connection Release


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Asymmetric Connection Release

Abrupt disconnection with loss of data.

Asymmetric Connection Release


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Symmetric Connection Release

Connection in each direction is closedindependently

Here host can continue to receive data even afterit has sent a Disconnection TPDU.

Symmetric Connection Release


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Connection Release

Consider a protocol as follows:

Host 1: I am done, are you done! Host 2: I am done too. Goodbye.

Connection is safely released.

Will this work?

Connection Release


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Connection Release

The two-army problem.

Connection Release


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Three-way Handshake ProtocolNormal case.

Connection Release - Solution


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Final ACK lost.



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Response lost.



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Response lost and subsequent DRs lost .



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Buffers & Flow Control

Transport Layer


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Flow Control and Buffering

A Sliding Window protocol required toavoid swamping of a slower receiverby a faster sender.

Requirement of Buffers

For an unreliable Network service buffering reqdat the sender end.

Buffering reqd at the receiver for a protocol

working on the basis of Selective repeat.

Flow Control & Buffering


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Size of the buffer a crucial issue.

Fixed size buffers

Buffer size = Size of the largest possible TPDU Buffer size < Size of the largest possible TPDU

Unused space

Chained fixed sizebuffers



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Variable size buffers - Better memory utilizationbut complicated buffer management

Unused space

Chained variablesize buffers



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Unused space

TPDU 1

TPDU 2

TPDU 3

Single Large circularbuffer per Connection



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BufferingTrade Offs

For low bandwidth bursty traffic, buffering should beconcentrated at the sender side.

For high bandwidth smooth traffic, buffering capacity at thereceiver side should be more

Sender and Receiver should dynamically adjust their bufferallocations as per changes in traffic pattern.

BufferingTrade Offs


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Buffering - Dynamic Allocation

Tpt layer protocol should thus allow sender torequest buffer space at receive end.

Sender requests for allocation of buffers usingcontrol TPDUs and receiver grants as many as itcan afford.

Sender decrements allocation on everytransmission and stops when allocation reached

zero.

BufferingDynamic Allocation


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Receiver piggybacks ACKs and fresh bufferallocation onto reverse traffic.

Here, buffering is decoupled from ACKs resulting

in variable size sliding window protocol.

Control TPDUs may be lost resulting in deadlock.

Thus, control TDPUs sent periodically givingbuffer status.

BufferingDynamic Allocation


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Dynamic Buffer Allocation

PROTOCOLS

Transport Layer


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Dynamic Buffer Allocation

TCP

Transport Layer


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Transmission Control Protocol

TCP Protocol specifically designed to:

providea reliable end-to-end byte streamoveran unreliable internetwork

dynamically adapt to properties of

Internetwork be robust in face of many kinds of failures



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At sender side, TCP entity:

accepts user data streams from local processes breaks them into pieces not exceeding 64 KB

hands over to IP layer as separate datagrams.

At receiver side,TCP entity reconstructs theoriginal byte stream from received IPdatagrams.



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As underlying subnet is unreliable,TCP is responsible for:

Maintenance of timers

Retransmission on delivery failure

Reassemble segments in propersequence.



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TCP Service Model

TCP service is obtained by having both Sender andReceiver create end points calledsockets .

Each socket has a socket number consisting of

IP address of host

16 bit number local to the host calledport.

Connections are identified by socket identifiers atboth ends.

TCP Service Model


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TCP Service Model

A socket may be used for multiple connections at the sametime.

All TCP connections arefull duplex.

TCP does not support multicasting and broadcasting.

TCP connection is a byte stream, not a message stream.Message boundaries are not preserved end to end.

TCP Service Model


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TCP Service Model

Four 512-bytesegments sent asseparate IPdatagrams.

The 2048 bytes ofdata delivered to the

application in a singleREAD CALL.

TCP Service Model


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TCP Service Model

When an application passes data to TCP, it may:

send it immediately or buffer it at its own discretion.

If application wants the data to be sent immediately, it can use thePUSH flag, which tells the TCP not to delay the transmission.

Eg.Carriage return after typing Command on Command Line

TCP Service Model


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TCP Service Model

Urgent DataEg.-Control-C

Control info along with Urgent flag causes the sending TCP entityto stop accumulating data and transmit everything immediately.

When the urgent data is received at destination, the receivingapplication is interrupted to read the data stream to find theurgent data.

TCP Service Model


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Every byte on a TCP connection has its own32 bit sequence number.

Separate 32 bit sequence numbers are used foracks and the window mechanism.

Sending and receiving TCP entities exchangedata in terms ofsegments.

The TCP Protocol


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A segment consists of a fixed 20 byte header + an optionalpart + zero or more data bytes.

Size of segments decided by TCP

Data from several writes can be put into one segment or

Can split data of one write over multiple segments

Limits to segment size

Data + Header must fit in IP datagram payload (64 KB)

Dependent on Maximum Transfer Unit (MTU) of the network

Max payload =64k-20(TCP header)-20(IP header) =64k-40bytes

The TCP Protocol


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TCP Segment Header


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Pseudoheader in TCP ChecksumPsuedo Header in TCP Checksum

It is a pseudo-IP-header, only used for

the checksum calculation


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Four TCP segments required

One FIN and one ACK in each direction

ACK to others FIN and own FIN may be containedin the same segment.

To avoid Two Army Problem timers are used.

TCP Connection Release


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Window management in TCP

TCP Transmission Policy


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When the window size is 0 the sender may not sendnormal segments with the following exceptions

Urgent data may be sent to kill a process on the remote end Sender may send a 1 byte segment to make the receiver

announce the next byte expected and window size.

TCP provides this option to prevent a deadlock.

Flow control

Sliding window with selective repeat is resorted to.



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Improvements on Receiver side

Wait for data. Send the ACK and windowsize advertisement together

Improvements on Transmitter side

Nagles Algorithm



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Send the first byte and buffer all the rest till first byteis ack

Then send the rest and again buffer till the previousbatch is ack.

Additionally allows to send when enough data tricklesdown to fill half the window size or max segment size.

Nagles algorithm may not be desirable in some

situations. Eg mouse movements on remote desktops


Nagles Algorithm


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Clarks Solution- Prevent receiver from advertising a window size of one byte. Instead

wait till the window is half empty or the window size = full segment size.


Silly Window Syndrome


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Congestion causing factors

Receiver capacity - Limitedbuffer size of the receiver.

Sender transmits only as perwindow size advertised by receiver

Network capacity - Internalcongestion within the NW

TCP Congestion Control


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Two windows maintained by the sender, eachreflecting number of bytes it can transmit. These are:

Window granted by receiver and

The Congestion window.

Effective sending window is the lesser of the two.


Slow Start Algorithm


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At connection est time the congestion window is set tomax segment size in use on the connection.

The congestion window grows exponentially witheach ack, till either a time out occurs or the receiverswindow size is reached.


Slow Start Algorithm


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Uses a third parameter called threshold.

Initially set to a high value of 64KB

When time out occurs, it is set to half of the current congestion window and thecongestion window is set to max segment size.

Slow start is then used

Exponential growth stops at the threshold and the congestion window increaseslinearly (one max segment size per ACK) till the receiver window size is reached.


Internet Congestion algorithm


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09cs 6048 amrik singh sidhu

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