Flow control protocols : Sliding Window Protocol and Selective Repeat ARQ

Flow control protocols

Flow control protocols are mechanisms used in computer networks to manage the flow of data between sender and receiver, ensuring that data is transmitted at an appropriate rate and preventing overwhelm or congestion of the receiving device. These protocols help regulate the flow of data to match the processing capabilities of the receiving device, thus optimizing network performance and reliability. There are two main types of flow control protocols:

1. Stop-and-Wait Flow Control: In stop-and-wait flow control, the sender transmits a single data packet and then waits for an acknowledgment (ACK) from the receiver before sending the next packet. This process ensures that the receiver can process each packet before more data is sent, preventing data overflow and ensuring reliable transmission. If the sender does not receive an ACK within a specified timeout period, it retransmits the packet.
2. Sliding Window Flow Control: Sliding window flow control allows the sender to transmit multiple data packets without waiting for individual acknowledgments from the receiver. The sender maintains a sliding window of acceptable sequence numbers, indicating the range of packets that can be sent without acknowledgment. As the receiver receives and acknowledges packets, the window slides forward, allowing the sender to transmit more packets. This approach improves network efficiency by maximizing the use of available bandwidth while still ensuring reliable transmission.

Flow control protocols play a crucial role in managing data transmission in various networking technologies, including Ethernet, TCP/IP, and wireless networks. By regulating the flow of data between sender and receiver, these protocols help prevent data loss, minimize network congestion, and ensure efficient use of network resources. Different flow control mechanisms may be employed depending on the specific requirements and characteristics of the network environment.

Sliding Window Protocol

The sliding window protocol is a flow control mechanism used in network communication to allow the sender to transmit multiple data packets before waiting for acknowledgments from the receiver. It enables efficient utilization of the network bandwidth by keeping the transmission pipeline full while ensuring that the sender does not overwhelm the receiver.

Working:

1. Sender’s Perspective:
   • The sender maintains a sending window, which is a contiguous range of sequence numbers representing the packets it can transmit.
   • Initially, the sending window is empty. As the sender receives data from the higher layers, it places the data packets into the sending window and starts transmitting them.
   • After transmitting a packet, the sender advances the window by one position, allowing it to transmit the next packet in sequence.
2. Receiver’s Perspective:
   • The receiver maintains a receiving window, which represents the range of sequence numbers it expects to receive.
   • Upon receiving a packet, the receiver checks whether the packet falls within its receiving window. If it does, the receiver accepts the packet and sends an acknowledgment (ACK) to the sender.
   • If the received packet falls outside the receiving window, it is discarded, and no acknowledgment is sent.
3. Acknowledgment:
   • The sender waits for acknowledgments from the receiver before advancing the sending window. If an acknowledgment is not received within a specified timeout period, the sender retransmits the unacknowledged packets.
4. Flow Control:
   • The size of the sending window determines the maximum number of packets that can be in transit at any given time. By adjusting the size of the window dynamically based on network conditions, the sliding window protocol optimizes network performance and throughput.

Selective Repeat ARQ

Selective Repeat ARQ is an enhanced version of the Automatic Repeat request (ARQ) protocol used for error recovery in network communication. Unlike other ARQ techniques, Selective Repeat ARQ retransmits only the corrupted or lost packets, rather than retransmitting the entire window.

Working:

1. Sender’s Perspective:
   • The sender divides the data stream into fixed-size packets and assigns a unique sequence number to each packet.
   • Upon transmitting a packet, the sender waits for an acknowledgment (ACK) from the receiver. If an ACK is not received within a timeout period, the sender assumes that the packet was lost or corrupted and retransmits only the missing packet.
2. Receiver’s Perspective:
   • The receiver accepts and buffers the incoming packets in sequential order. It sends an ACK for each correctly received packet.
   • If the receiver detects an error in a received packet, it discards the packet and does not send an ACK for that packet.
3. Acknowledgment:
   • The sender maintains a timer for each transmitted packet. If an ACK is not received within the timeout period for a particular packet, the sender retransmits that packet.
   • Upon receiving a duplicate packet or out-of-order packet, the receiver sends a selective acknowledgment (SACK) to inform the sender of the missing packets.
4. Retransmission:
   • Upon receiving a SACK from the receiver, the sender retransmits only the missing packets, rather than retransmitting the entire window. This selective retransmission improves efficiency and reduces network overhead.

Both the sliding window protocol and Selective Repeat ARQ are essential techniques used in modern network communication to ensure reliable and efficient data transfer over unreliable channels. They play a crucial role in optimizing network performance, throughput, and reliability in various networking scenarios.