Piggybacking in Computer Networks

Piggybacking is a technique where the receiver delays sending an acknowledgment (ACK) and attaches it to its next outgoing data packet. This reduces the number of separate control frames, improving network efficiency.

• Sliding window protocols allow the sender to transmit multiple packets before receiving acknowledgments, increasing throughput.
• Both sender and receiver maintain finite buffers to hold outgoing and incoming packets.
• Unacknowledged packets are retransmitted after a timeout.

How To Increase Network Efficiency?

Efficiency can also be improved by making use of the Full-duplex transmission mode. Full Duplex transmission is a two-way directional communication simultaneously which means that it can communicate in both directions, just like we are using two half-duplex transmission nodes. It provides better performance than simple transmission modes and half-duplex transmission modes.

There are two ways through which we can achieve full-duplex transmission:

1. Two Separate Channels: One channel for sending data and another for receiving. This wastes bandwidth in the reverse direction.
2. Piggybacking: A better approach where data and acknowledgments share the same channel. For example, if Host A sends data to Host B, the acknowledgment from B can be attached to the next outgoing data frame. The receiver checks the header to distinguish between data and acknowledgment.

When a data frame arrives, the receiver can delay sending an ACK and attach it to its next outgoing data frame, reducing the number of separate messages and improving network efficiency.

Working of Piggybacking

As we can see in the figure, we can see with piggybacking, a single message (ACK + DATA)  over the wire in place of two separate messages. Piggybacking improves the efficiency of the bidirectional protocols.

• If a host has both data and an acknowledgment, it sends a single frame containing both.
• If only an acknowledgment exists, the host waits for some time to see if data is ready; otherwise, it sends the ACK separately..
• If only data is left, the last acknowledgment is included in the data frame.

Advantages of Piggybacking

1. The major advantage of piggybacking is the better use of available channel bandwidth. This happens because an acknowledgment frame does not need to be sent separately.
2. Usage cost reduction.
3. Improves latency of data transfer.

Disadvantages of Piggybacking

1. The disadvantage of piggybacking is the additional complexity.
2. If the data link layer waits long before transmitting the acknowledgment (blocks the ACK for some time), the frame will rebroadcast.