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7 layer OSI Model

The Transport Layer

Once the client and server are done with the above three-way handshake they are ready for data transfer. The data transfer mechanism is straightforward. Here you would need to understand the concept of segmentation. The data which is coming from the application layer would come in huge chunks of bytes; the transport layer segments this stream of bytes into segments and assigns a sequence number (in incrementing fashion) to each of these segments.

Once it's done with assigning a sequence number to the segment it passes it on to the networking layer which puts the network header and passes it on to the data link layer (which puts the link layer header). This datagram is then passed over the physical layer to other end point. The transport layer expects a positive ACK for every segment that it sends. The segment is retransmitted if an ACK for that segment is not received before timeout. Every time a segment is transmitted a transmit timer is started for that segment,if an ACK is not received before the timer expires then the segment is retransmitted. This mechanism ensures reliability of data transfer.

Let me explain another important feature of TCP while we are dealing with data transfer. Waiting for an ACK per segment causes a lot of delay while transferring large chunk of data, I am sure you agree, so the concept of WINDOWING was introduced. A window is defined as number of segments which can be aggregated for one ACK. In other words if window size is set to 10 it means 10 segments can be sent and if all the 10 are recieved then one ACK is sent back. So one ACK is sent to acknowledge 10 segments. When the ACK packet is recieved the next window is sent. So windowing provides for higher speed of transmission.

So who decides the window size in the first place? Well its the reciever who sets it because after all it is the reciever which has to allocate resources (memory/buffer) for the incoming packets so it gets to decide the window size to being with. The window size is sent by the reciever while sending the SYC+ACK packet during the three way handshaking stage. So the windowing per se is one-way through which TCP manages flow control.

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