What is the work of the fourth layer Transport layers IOS?

We use different protocols in this layer, such as TCP and UDP.

TCP is a connection oriented protocol.

UDP is a connection less protocol.

Ports are also assign in this layer. Ports are very important part of transmission. I call ports as service identifier.

In simple words,In transport layer, first thing it does is, it decides HOW TO SEND your data meaning should the date be sent reliably (if the data gets dropped in the way then it will be retransmitted = TCP) or unreliably (if the data gets dropped then its dropped for good, no retransmission = UDP)  and all this will depend on the application layer meaning which application layer service (protocol eg: HTTP DHCP DNS etc ) your using, because  all the protocols of application layer have already been define that how they will carry out the transmission i.e. reliably or unreliably.For example: If your using a HTTP protocol or service on the application layer  then all the will be transmitted using the TCP protocol on the L4 in other words it will create a connection known as connection-oriented meaning if the data gets droppped then it will be retransmitted.Hope that answers your questionAssalam-o-Alaikum Wr Wb

In simple words – Transport layer takes care of only ‘what’ part and doesn’t bother about ‘how’ or which path to take, where to go, how much payload can physical line take etc...

 What needs to be transported? It is aware of only destination port. Not even complete address. It takes care of segmentation at start and re assembly at destination in reliable or non-reliable mode. In addition it takes care of some other simple tasks like flow control etc...

If I compare it with IP layer, IP layer is more smart in terms of ‘how’ packet will be transported which path or route it will take, what is destination address and where to go, how much underlying physical layer can take load based on that further fragmentation is needed or not.

Both Transport and internet Protocol layers have their own significance and loop holes and based on their duties they are logically separate protocols.

The transport layer is the layer in the open system interconnection (OSI) model responsible for end-to-end communication over a network. It provides logical communication between application processes running on different hosts within a layered architecture of protocols and other network components.      The transport layer is also responsible for the management of error correction,providing quality and reliability to the end user. This layer enables the host to send and receive error corrected data, packets or messages over a network and is the network component that allows multiplexing

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. It relieves the higher layer protocols from any concern with the transfer of data between them and their peers. The size and complexity of a transport protocol depends on the type of service it can get from the network layer. For a reliable network layer with virtual circuit capability, a minimal transport layer is required. If the network layer is unreliable and/or only supports datagrams, the transport protocol should include extensive error detection and recovery. The transport layer provides:

• Message segmentation: accepts a message from the (session) layer above it, splits the message into smaller units (if not already small enough), and passes the smaller units down to the network layer. The transport layer at the destination station reassembles the message.
• Message acknowledgment: provides reliable end-to-end message delivery with acknowledgments.
• Message traffic control: tells the transmitting station to "back-off" when no message buffers are available.
• Session multiplexing: multiplexes several message streams, or sessions onto one logical link and keeps track of which messages belong to which sessions (see session layer).

Typically, the transport layer can accept relatively large messages, but there are strict message size limits imposed by the network (or lower) layer. Consequently, the transport layer must break up the messages into smaller units, or frames, prepending a header to each frame. The transport layer header information must then include control information, such as message start and message end flags, to enable the transport layer on the other end to recognize message boundaries. In addition, if the lower layers do not maintain sequence, the transport header must contain sequence information to enable the transport layer on the receiving end to get the pieces back together in the right order before handing the received message up to the layer above.

The Transport layer provides services for both "connection-mode" transmissions and for "connectionless-mode" transmissions. For connection-mode transmissions, a transmission may be sent or arrive in the form of packets that need to be reconstructed into a complete message at the other end.

in osi model tansport layer work on end to end data transportation . It assure you data is securly reach the destination . it has responsibility to send the data on the from host to destination .

and it follows the tcp/ip

which gives you the reliability .

The Transport layer or layer4 provide both services : " connxion mode "  like TCP and " Connexionless mode " like UDP , also it provide a lot of services like :

 Connection-oriented communication , Congestion avoidance , Flow control,  Byte orientation....

The transport layer ensures that messages are delivered error-free, in sequence, and with no losses or duplications. The transport layer provides convenient services such as connection-oriented data stream support, reliability, flow control, and multiplexing.

A transport layer is the interface between the application layer and the complex hardware of the network. It is designed to allow peer entities on the source and destination hosts to carry on conversations.

take the data segments and put the transport header which contain:

1- destination port number

2-source port number

3-transmission protocols TCP or UDP 

and send it as data packet to the next layer (network layer)