Sequence numbers enable flow control by providing a mechanism for the receiver to acknowledge received data and for the sender to manage the rate at which it transmits data, preventing the receiver from being overwhelmed. The sender uses sequence numbers to track the order of data packets, and the receiver uses them to identify lost or out-of-order packets.
Here's a breakdown of how sequence numbers establish flow control:
1. Sequence Number Assignment:
- The sender assigns a unique sequence number to each data packet it transmits. This sequence number is typically included in the header of the packet.
- Sequence numbers are typically sequential, allowing for easy identification of missing packets.
2. Transmission and Acknowledgement:
- The sender transmits a window of frames, which is a defined number of packets.
- The receiver, upon successfully receiving packets, sends acknowledgements (ACKs) back to the sender. These ACKs include the sequence number of the next expected packet. This implicitly acknowledges all packets up to (but not including) that sequence number.
3. Window Management:
- The sender maintains a "window" of sequence numbers representing the data it is allowed to transmit without receiving acknowledgment.
- When the sender receives an ACK, it advances its window, allowing it to transmit more data. This sliding window mechanism is crucial for flow control.
4. Handling Lost or Out-of-Order Packets:
- If the receiver detects a gap in the sequence numbers (i.e., a missing packet), it can either:
- Send a negative acknowledgement (NACK) indicating the missing sequence number.
- Send duplicate acknowledgements for the last correctly received packet. Multiple duplicate ACKs are often interpreted as a signal that a packet has been lost.
- The sender, upon receiving a NACK or detecting duplicate ACKs, retransmits the missing packet(s).
5. Flow Control Mechanism:
- If the receiver is becoming overwhelmed, it can delay sending acknowledgements or advertise a smaller receive window to the sender. This signals to the sender to slow down its transmission rate.
- Conversely, if the receiver has ample buffer space, it can advertise a larger receive window, allowing the sender to transmit more data more quickly.
Example: Sliding Window Protocol
Consider a sender with a window size of 3 and sequence numbers starting from 0.
- The sender transmits packets with sequence numbers 0, 1, and 2.
- The receiver successfully receives packets 0 and 1.
- The receiver sends an ACK with the sequence number 2, indicating it expects the next packet to be 2.
- Packet 2 is lost in transit.
- The receiver sends another ACK with sequence number 2 (duplicate ACK).
- The sender, upon receiving the duplicate ACK, retransmits packet 2.
- The receiver successfully receives packet 2.
- The receiver sends an ACK with sequence number 3.
- The sender advances its window and can now transmit packets with sequence numbers 3, 4, and 5.
Benefits of Using Sequence Numbers for Flow Control:
- Reliable Data Delivery: Guarantees that data is delivered in the correct order and without loss.
- Efficient Use of Bandwidth: Allows the sender to transmit data at the optimal rate based on the receiver's capacity.
- Prevents Congestion: Helps to prevent the receiver from being overwhelmed, which can lead to packet loss and network congestion.
In summary, establishing flow control using sequence numbers involves assigning unique identifiers to data packets, using acknowledgements to track received data, and implementing a sliding window mechanism to manage the transmission rate and ensure reliable data delivery.
