Question

What mechanism is available for TCP to ensure that the receiver of a TCP stream will receive data in a pace that it can handle? Your explanation must include an example of how it works.

Answer 1

Flow control basically means that tcp will ensure that a sender is not overwhelming a receiver by sending packets faster than it can consume.

How it works:

This is normally happen, when we send the data.

Sender Receiver Transport Layer Transport Layer Network Layer Network Layer Link Layer Link Layer

Figure.1

The sender application write data to socket the transport layer will wrap this data in a segment and hand it to network layer, that will somehow route this packet to the receiving node.

On the other side of communication, the network layer will deliver this piece of data to transport layer, that will make it available to receiver application as an exact copy of data sent, meaning if will not deliver packets out of order and will wait for a retransmission in case it notices a gap in the byte stream.

TCP stores the data it needs to send in the send buffer, and the data it receives in the receive buffer. When the application is ready, it will then read data from the receive buffer.

Flow Control is all about making sure we don’t send more packets when the receive buffer is already full, as the receiver wouldn’t be able to handle them and would need to drop these packets.

To control the amount of data that TCP can send, the receiver will advertise its Receive Window (rwnd), that is, the spare room in the receive buffer.

Data in the buffer Receive Window - - TCP Receive Buffer

Figure.2

Every time TCP receives a packet, it needs to send an ack message to the sender, acknowledging it received that packet correctly, and with this ack message it sends the value of the current receive window, so the sender knows if it can keep sending data.

The sliding window

TCP uses a sliding window protocol to control the number of bytes in flight it can have. In other words, the number of bytes that were sent but not yet acked. Figure.3 shows the sliding window in TCP. The window spans a portion of the buffer containing bytes received from the process. The bytes inside the window are the bytes that can be in transit; they can be sent without worrying about acknowledgment. The imaginary window has two walls: one left and one right. The window is opened, closed, or shrunk. These three activities, as we will see, are in the control of the receiver (and depend on congestion in the network), not the sender. The sender must obey the commands of the receiver in this matter. Opening a window means moving the right wall to the right. This allows more new bytes in the buffer that are eligible for sending. Closing the window means moving the left wall to the right. This means that some bytes have been acknowledged and the sender need not worry about them anymore. Sluinking the window means moving the right wall to the left. This is strongly discouraged and not allowed in some implementations because it means revoking the eligibility of some bytes for sending. This is a problem if the sender has already sent these bytes. Note that the left wall cannot move to the left because this would revoke some ofthe previously sent acknowledgments.

Window size =minimum (rwnd, cwnd) Shrinking ... In-I n In+l 1m-ll mm .. Sliding window | Closing | Opening

Figure.3

The size ofthe window at one end is determined by the lesser of two values: receiver window (rwnd) or congestion window (cwnd). The receiver window is the value advertised by the opposite end in a segment containing acknowledgment. It is the number of bytes the other end can accept before its buffer overflows and data are discarded.

Example :

What is the value of the receiver window (rwnd) for host A if the receiver, host B, has a buffer size of 5000 bytes and 1000 bytes of received and unprocessed data?

Solution The value of rwnd =5000 - 1000 =4000. Host B can receive only 4000 bytes of data before overflowing its buffer. Host B advertises this value in its next segment to A.