#transportlayer

If You Understand The Name Of " Transport Layer Protocols " You'll Understand Its Role Easily. Transport Layer Protocols Are Used To Transport The Information From One Place To Another Place, From One Device To Another Device, From One Computer To Another Computer, From Source To Destination.  

Before Knowing About Transport Layer Protocols,We Have To Know What Actually Transport Layer Is? Transport Layer Is Fourth Layer. Every Layer In OSI Model Is Responsible For Transferring Information From Source To Destination(one computer to another computer).

But Only Transport Layer Is Called As Heart Of OSI Model, Why?

Transport Layer Decided If Data Transmission Should Be Parallel Or Single Path. Multiplexing(Combining The Data), Splitting(Dividing The Data) Are Done By This Layer. It Provides Additional Quality Of Service. Transport Layer Receives Data From Session Layer And Sends To Network Layer, It Receives Data Session Layer And Converts The Data Into Smaller Units And It Passes It To The Network Layer. Takes Large Data From Session Layer And Splits The Data Into Small Units And Transmits To Network Layer. It Is Converting The Data To Smaller Units. It Is Responsible For End To End Delivery Or Port To Port Or Process To Process Delivery.  

Services Provided By Transport Layer

Connection-Oriented Connectionless Service Connection-Oriented Can Be Done In Three Methods. Establishing The Connection Between Two Devices(Computers) After Establishing Data Has To Be Transferred After Transferring The Data It Has To Be Disconnected It Operated In Three Phases. In Connectionless Service It Has Only One Phase. Just Data Transfer Very Fast Data Transfer Takes Place In Connectionless Service   Transport Layer Is Operated By Operating System Application Layer, Presentation Layer, Session Layer, Transport Layer Comes Under Software. Transport Layer Is Operated Based On The Operating System.   TCP And UDP Are The Protocols Of Transport Layer Which Are Majorly Used TCP Means Transmission Control Protocol, UDP Means User Datagram Protocol.  

TCP(Transmission Control Protocol)

Data Comes To Transport Layer From Application Layer Through Presentation And Session Layers Transport Layer Add's Header To The Data Got To It From Application Layer. It Can Be TCP Or UDP If Transport Layer Add's TCP Header To The Data Received By It. The Advantages Of Add's TCP Header To The Data Got To Transport Layer Is Byte Streaming Connection-Oriented Full Duplex Piggybacking Error Control Flow Control  Congestion Control These Are The Mandatory Fields In TCP Byte Streaming  Transport Layer Receives Data From Application Layer Continuously And Ther Is Not Limitation For Data. Transport layer Converts The Data Into Bytes Or It Converts Into Segments. The Important Functionality of TCP Is Converting The Received Data Into Segments. Segment Means Collection Of Bytes. Bytes Means Collection Of Bits. The Bits Are Received By The Transport Layer And Bits Are Converted Into Bytes And Bytes Into Segments. TCP Creates Segments And It Sends Furthur. Connection-Oriented TCP Is Working Like A Connection Oriented. The Main Advantage Of Connection Oriented Is Reliability. It Means The Toal Data From Sender To Destination Should Reach Without Any Loss. If There Is Loss TCP Retransmits The Data. TCP Sends Request To The Sender And It Receives The Request From The Receiver Saying That I'm Ready To Receive The Data And The TCP Sends The Data. To Establish Connection TCP Sends, Receives And Sends Request To Receiver. The Advantage Of This Is Reliability(Total Data Should Be Sent And Received By Sender And Receiver). Full Duplex Full Duplex Means Sender And Receiver Can Send And Receive Data To Each Other After Establishing Connection Between Them By TCP. Both Can Transmit And Receive Data At The Same Time. This Is Called Full Duplex. Piggybacking The Receiver Sends Data To The Sender By Attaching The Acknowledgement Of Already Received Data. If Sender Sends 10 Packets To The Receiver, Receiver Don't Need To Send The Acknowledgement For All Each And Every Packet, It Can Send A Acknowledge Which Say's "I Have Received  All The 10 Packets" And Data Attached To To It. The Load Reduces On the Network Because Of Piggybacking In TCP. Error Control TCP Also Provides Error Control. Error Control Means " If The Sender Sends Data To A Receiver If There Is A Change In The Data Because Of Some Noise or Disturbance In The In The Transmission, The Receiver Can Detect The Error(change in data). So TCP Support Error Control. Flow Control Flow Control Means Whenever Sender Sends The Data To The Receiver, Sender Must Maintain Some Flow Control. The Sender Knows The Reciever's Buffer(Storage Space) Because Of TCP in Transport Layer. Congestion Control Congestion Control Means, Whenever Sender  Send The Data To The Receiver, If The Receiver Capacity Is 1000 Byte, The Sender Can Send Only up to To 1000 Byte. The Data From Sender To Receiver Goes Through The Network, TCP Takes Cares Of The Network Also, TCP Wont Allow Network To Go Into Congestion. It Takes Care Of Receiver Capacity As Well As The Route In The Network Where The Data Flows.  

Let's See The TCP Header

TCP Header Should Be 20-60 Byte. 1 Byte = 8 Bits,20 Bytes = 20*8=160 Bits Header Must Be 160Bits(20Bytes), This Is Minimum. Header Must Be 480 Bits (60Bytes), This Is Maximum.   Less Than 20bytes & More Than 60Bytes Of Data In Header Cant Be Sent. TCP Knows And TCP Header First Field Consists Of Source Port And Destination Port See The Following Diagram. Port=16 Bit,Port =16 Bit   Port Numbers Port = 16Bit 16 Bit= 216  216 = 0 - 65535(Numbers)          (0-1023) Are Well Known Port Numbers. In Those 0-1023 Are Well Known Port Numbers Used By Standard Applications Like Facebook Gmail. Like HTTP Uses 18 As Port Number, SMTP Uses 25 As Port Number, FTP Uses 21 As Number After Range 0-1023, From 1024 Some Are Registered And Some Are Normal. If You Open Your Browser "Google Chrome", Your Operating System Gives A Port Number And That Number Will Not Come Under Well Known Port Numbers, That Is Normal Port Number Which Will Up to 65535. If From Your Machine From A Application In Your Browser Sends A Request To A Server, If You Are Trying To Access Web Server, Your Request Will Go to Port Number 80 In The Server Applications. Web pages Transfer From One Port To Another Port By Using HTTP Protocol (80).  

Sequence Numbers

In TCP Every Byte Is Counted, For Every Byte There Is A Sequence Number In The Segment. TCP Uses Random Sequence Numbers For Segements.  

Acknowledgment Number

Receiver Sends Acknowledgement, The Acknowledgement Number Will Be The Next Expected Byte Number. If You Send "10th Data Byte " Receiver Sends Acknowledgement Saying " I Have Got 10th Byte , I Need 11th Byte" That Means Acknowledgement Number Will Be " 11".  

Header Length (HLEN) 

Header Is Length Is For 4 Bit Number. Example Let's Think " I Have A Number 1000 " 1000 is 8 My Header Is Of 8 Bytes But TCP Header Should Be Between 20-60 Bytes.   Here We Use Scale Of 4 It Means Any Number Which Comes Under HLEN Should Be Multiplies With 4 i.e 8*4=32 Byte  

6 (Not Used)

6 Bits Are Reserved For Future Use.  

Flags (URG, ACK,PSH,RST,SYN,FIN) 6 Bit

URG FLAG - = URGENT, If Some Data Out Of Total Data Is Urgent, In This Flag The Urgent Data Number Will Be Mentioned,(1Bit). ACK FLAG - If Receiver Is Sending An Acknowledgement, I.e 1 Bit PSH FLAG - PUSH, Urgent Data Will Be Pushed First, I.e 1 Bit RST FLAG - RESET, Sometimes It Is Necessary To Reset The Connection, So This RESET FLAG Is Used To RESET Connection. SYN FLAG - Used For Synchronisation, After Establishing Connection Sender Sends SYN FLAG Which Is The First Byte Sending To Receiver After Establishing The Connection, Sender Sends It By Putting SYN (1). FIN FLAG - FINISH,It The Sender Wants To Terminate The Connection Or To Say Receiver Than The Data Is Completed, Sender Puts 1 For FIN Flag,So That Receiver Understands That Sender Is Going To Terminate The Connection Because Of Completion Of Data Transmission.  

WINDOW SIZE (16 BIT)

Sender And Receiver Say's To Each That About Their Window Size, So That Each Other Knows The Window Size And Send Data According To That Since TCP Is Full Duplex. The Capacity Of The Window Is Mentioned Here By Both, Sender And Receiver And Which Should Be 16 Bit.  

CHECKSUM (16 BIT)

CHECKSUM is 16 Bit That Is Used For Error Control.  

URGENT POINTER (16 BIT)

In Urgent Flag (1), It Is Mentioned That SomeData Is Urgent, But Where Is That Data? What Is The Range Of The Data? What Is The Capacity Of That Data? Urgent Pointer Says The Range Of The Urgent Data. Example - (1000-1020) is urgent out of 1050  

Options And Padding (40 Bytes)

EXTRA BIt, Extra Data If Receiver Want To Send To The Sender. Then Receiver Send Through The Options And Padding. The Major Point Used Here Is MSS, Maximum Segment Size. Here The Segment Size In The Window Of The Browser Of A Computer Is Is Sent To Say That My Maximum Segment Size In My Window Is So And So. Example My Total Window Size Is 10,000 Byte My Maximum Segment Size Is 1,000 Byte   MSS By Sending This Sender Knows The Maximum Segment Size Of Receiver.  

UDP(USER DATAGRAM PROTOCOL)

UDP Is Connectionless Protocol (Unreliable) It Is Simpler And Faster Than TCP UDP Will Not Establish Connection Between Source And Destination. In UDP The Sender Will Not Demand Acknowledgement For The Data Sent To Reciever.   UDP Headers Are All 8 Bytes, While TCP Headers Can Be 20-60 Bytes.UDP Provides Delivery Of Segments Using IP(Internet Protocol)   Diagram Of UDP Header