TRANSPORT LAYER FULL REPORT
#1


Presented by:
Sakshi Garg
Kanchan

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TRANSPORT LAYER
Transport layer acts as a liaison between the upper-layer protocols and the lower-layer protocols. To make this separation possible, the transport layer is independent of the physical network. Examples: TCP, UDP, …
Transport layer concept
TRANSPORT LAYER *Duties *Connection *OSI Transport protocol *TCP *UDP *Traffic Shaping *Question Bank
Duties of the transport layer
End to end delivery
Addressing
Levels of Service Access Points
*Data page link level protocols need to know which two computers within a network are communicating.
MAC address
*Network level protocols need to know which two computers within an internet are communicating.
IP address
*Transport level protocols need to know which upper-layer protocols are communicating.
Port
Reliable delivery
Error control

●Mechanisms for error handling is present in data page link layer.
●Data page link layer guarantee error free delivery node-to node reliability not end to end.
●if error introduced as the packet is processed inside one of the routers , error will not be caught in data page link layer as it functions , no error being introduced b/w the beginning and end of the link
●transport layer thus , does its own checking end t0 end.
Sequence control
● On the sending end, transport layer is responsible for ensuring the data units received from upper layers are usable by the lower layers.
● On the receiving end, it is responsible for ensuring that the various pieces of the transmission are correctly assembled.
Segmentation:- Size of data received from the upper layer is too long for the data page link layer to handle , then transport protocol divides it into smaller, usable blocks. The dividing process is called segmentation.
Concatination:- size of the data units belonging to a single session are so small that several can fir together into a singla datagram or frame, the transport protocol combines them into a singla unit. The combining process is called concatenation
Sequence numbers:- * If a longer data has been segmented ,the number indicate the order of reassembly. * If a shorter data has been concatenated , the number indicate end to end each submit and allow them to separate accurately.
Loss control
All pieces of a transmission arrive at the destination .
Sequence numbers allow the receiver’s transport layer protocol to identify any missing segments and request redelivery.
Duplication Control
Sequence numbers allow receiver to identify and discard duplicate segments.
Flow Control
Flow control is from end to end
Uses sliding window protocol.
Window can vary in size to accommodate buffer company.
Sliding window protocol
Some points about sliding window protocols at the transport layer are as follows:-
The sender does not have to send a full window’s worth of data.
An acknowledgment can expand the size of the window based on the sequence number of the acknowledged data segment.
The size of the window can be increased or decreased by the receiver.
The receiver can send an acknowledgment at anytime.
To accommodate the variability in size, transport layer sliding window use three pointers.
Multiplexing
□Upward Multiplexing

●The transport layer can send several transmissions bound for the same destination along the same path.
● It is useful when the underlying networks have high throughput.
□ Downward Multiplexing
● It is useful when the underlying networks have low or slow capacity(e.g., X.25’s three bit sequence code).
Connection Establishment
Connection Establishment requires three actions in what is called a THREE-WAY HANDSHAKE:
The computer requesting the connection sends a connection request packet to the intended receiver.
The responding Computer returns a confirmation packet to the requesting computer.
The requesting computer returns a packet acknowledging the confirmation.
Connection Establishment
Connection Termination
Connection termination also requires three actions in what is called a THREE-WAY HANDSHAKE:
The computer requesting the connection sends a dis connection request packet to the intended receiver.
The responding Computer returns a confirmation packet to the requesting computer.
The requesting computer returns a packet acknowledging the confirmation.
Connection Termination
OSI Transport Protocol
TRANSPORT CLASSES
OSI model defines five types of transport classes:
1.TPO- Simple class
2. TP1- Basic error recovery class
3. TP2- Multiplexing class
4. TP3- Error Recovery and multiplexing class
5. TP4- Error Detection and recovery class
Which class is used depends on the type of service required by the upper layers. The transport layer tries to match these requests to the available networking services: * TP0 and TP2 – perfect network layers, loss of packets or data is zero. *TP1 and TP3- residual-error network, some percentage of errors are never corrected. *TP4- used with unreliable network layers, TP4 provides reliable, fully-duplex, connection oriented services similar to TCP in the TCP/IP model
Transport Protocol Data Unit(TDPU)
Length-occupies the first byte and indicates the total number of bytes(excluding the length bit itself).
Fixed Parameters- contains parameters or control fields, has five parts(code, source reference, destination reference, sequence number, and credit allocation).
* code-identifies the data unit e.g DT for data, CR for connection request etc.
The following code are recognized by the ISO and ITU-T CR: Connection Request CC: Connection Confirm DR: Disconnect Request DC: Disconnect Confirm DT: Data ED: Expedited Data AK: Data Acknowledge EA: Expedited Data acknowledge RJ: Reject ER: Error
* Source and Destination Reference- contains the addresses of the original sender and the ultimate destination of the packet. * Sequence Number * Credit allocation- It enables a receiver to tell the sender how many more data units may be sent. * Variable Parameters-contains parameters that occur infrequently. Are mostly for management(e.g.- testing of router). * Data-contains regular data or expedited data coming from upper layers.
The OSI Transport Protocol
Connection-Oriented and Connectionless Service
The OSI model supports both COTS and CLTS.
Connection-oriented model is more commonly used.
TCP-Transmission Control Protocol
Connection-oriented, port to port protocol.
Commonly used protocol on the internet.
Offers error correction.
There is a "guaranteed delivery.“ due to "flow control."
Flow control determines when data needs to be re-sent, and stops the flow of data until previous packets are successfully transferred.
This works because if a packet of data is sent, a collision may occur. When this happens, the client re-requests the packet from the server until the whole packet is complete and is identical to its original. 
Frame Structure
Source port-address of application program in the source computer. •Destination port-address of application program in the destination computer. •Sequence number •Acknowledgment number •Data offset(Header length-HLEN)-indicates 32 bit words in the TCP header. 4 bit can define up to 15.therefore six of the header is 60 bytes. •Reserved- a six bit field is reserved for future use. •Control bits- URG-urgent bit, when set ,validates the urgent pointer field., indicates that the data in the bit are urgent.
ACK-ACK bit when set, validates the acknowledgment number field. PSH-(Push) used to inform the sender that a high throughput is needed. RST-(Reset) used to reset the connection when there is a confusion in sequence numbers. SYN-used for sequence number synchronization in three types of segments- connection request, connection confirmation, connection acknowledgment. FIN-used in connection termination in three types of segments-termination request, termination confirmation, termination acknowledgment.
•Window size-defines the size of the sliding window. •Checksum-used in error detection. • Urgent pointer-its value is valid only if the URG bit in the control field is set. •Options and padding- used to convey additional information to the receiver or alignment purposes.
UDP-User Datagram Protocol
Connection less, end to end transport level protocol.
commonly used protocol on the Internet.
Provides less security and reliability.
ever used to send important data such as webpages, database information, etc
used for streaming audio and video. Streaming media such as Windows Media audio files (.WMA) , Real Player (.RM), and others use UDP because it offers speed!
The reason UDP is faster than TCP is because there is no form of flow control or error correction. The data sent over the Internet is affected by collisions, and errors will be present.
UDP is only concerned with speed. This is the main reason why streaming media is not high quality.
Frame structure
Difference b/w TCP and UDP
Traffic Shaping
●also known as "packet shaping“
● is an attempt to control computer network traffic in order to
-optimize or guarantee performance,
- improve latency, and/or
- increase usable bandwidth for some kinds of packets by delaying other kinds of packets that meet certain criteria.  
●provides a means to control the volume of traffic being sent into a network in a specified period (bandwidth throttling), or the maximum rate at which the traffic is sent (rate limiting ), or more complex criteria such as GCRA.
●This control can be accomplished in many ways and for many reasons; however traffic shaping is always achieved by delaying packets.
●Traffic shaping is commonly applied at the network edges to control traffic entering the network, but can also be applied by the traffic source (for example, computer or network card[2]) or by an element in the network.
 ●Traffic policing is the distinct but related practice of packet dropping and packet marking.[3]
Address resolution
Address resolution is used for finding or resolving the MAC address from IP address. For a LAN each device has its own physical or station address as its identification. This address is imprinted on the NIC.
When a router or a host needs to find the MAC address of another host or a network the sequence of even taking place is as follows:
1.The router or host A who wants to find the MAC address of some other router, sends an ARP request packet. This Packet consist of IP and MAC of the sender A and the IP address of the receiver B.
2.This request packet is broadcast over the network as shown below:
Every host and router on the network receives and the process the ARP request packet. But only the intended receiver (B) recognizes its IP address in the request packet and sends back an ARP response packet. The ARP response packet contains the IP and Physical addresses of the receiver(B). This packet is delivered only to A using A’s physical address in the ARP request packet. This is shown in fig.
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