Question

Under what circumstances may coarse-grained timeouts still occur in TCP even when the fast retransmit mechanism is being used?

Short answer

Coarse-grained timeouts can occur due to high latency, network congestion, multiple packet losses beyond duplicate ACK threshold, and ACK compression.

Step-by-step solution

Understanding Fast Retransmit in TCP

Fast retransmit is a mechanism in TCP that allows for the retransmission of a lost packet without waiting for a timeout. It occurs when the sender receives a certain number of duplicate acknowledgments (usually three), indicating that a packet has likely been lost.

Coarse-Grained Timeouts in TCP

Coarse-grained timeouts are those that occur over longer intervals, typically when the sender does not receive any acknowledgments for a packet within a certain timeframe, forcing a retransmission.

Latency and Network Congestion

High latency and network congestion can delay acknowledgments. If ACKs are delayed longer than the expected interval for triggering fast retransmit, a coarse-grained timeout may still occur.

Packet Loss Beyond Duplicate ACK Threshold

If multiple packets are lost in a sequence such that the number of duplicate ACKs does not reach the threshold, fast retransmit will not be triggered, leading to a coarse-grained timeout.

ACK Compression

ACK compression could occur when ACK packets are held up in the network and released in quick succession, leading to the sender not receiving the required number of duplicate ACKs in time. This causes coarse-grained timeouts.

Key concepts

Fast Retransmit

Fast retransmit is a critical mechanism in TCP that helps in quickly handling lost packets. When a sender notices a certain number of duplicate acknowledgments, often three, it assumes a packet is lost and retransmits it without waiting for a longer timeout period. This mechanism aids in maintaining the flow of data and minimizing delays in communication.

• It's faster than waiting for a traditional timeout.
• It relies on 'duplicate ACKs' as an indicator of packet loss.

Imagine you're sending a series of packets numbered 1, 2, 3, and 3 doesn't make it to the receiver. The receiver, noticing the absence of packet 3, will keep sending an acknowledgment for packet 2 (duplicate ACKs). When the sender receives three of these duplicate ACKs, it retransmits packet 3 immediately, effectively speeding up the recovery process.

Coarse-Grained Timeouts

Coarse-grained timeouts occur when the expected acknowledgment for a sent packet doesn't arrive within a certain longer timeframe. These timeouts are typically broader and less precise, often leading to longer delays before a retransmission happens.

• They are less efficient than fast retransmit.
• Typically used as a fallback mechanism.

For instance, if a sender sends packet 1 and doesn't receive an acknowledgment for it within the timeout period, it will retransmit packet 1. This timeout and retransmission process can be lengthy, introducing delays in the communication process.

Duplicate Acknowledgments

Duplicate Acknowledgments (ACKs) are crucial signals in TCP for detecting packet loss. When a receiver gets an out-of-order packet, it sends an ACK for the last in-order packet it received. If the sender gets multiple duplicate ACKs for the same packet, it knows there might have been a packet loss.

• Typically, three duplicate ACKs are needed to trigger fast retransmit.
• It helps in quickly identifying and retransmitting lost packets.

For example, if packets 1, 2, 4, 5 are received, the receiver will send ACKs for packet 2 until it receives packet 3. The sender, noticing three ACKs for packet 2, realizes packet 3 was lost and retransmits it, instead of waiting for a timeout.

Network Latency

Network latency refers to the time it takes for data to travel from the sender to the receiver and back again. High latency can delay acknowledgments, significantly affecting TCP's efficiency. When the latency is high, acknowledgments might not arrive promptly, causing the sender to fall back on coarse-grained timeouts.

• High latency lengthens acknowledgment times.
• It can trigger coarse-grained timeouts, reducing efficiency.

For instance, if the sender has a high RTT (Round-Trip Time), it might not receive ACKs soon enough for fast retransmit to engage, leading to delays and inefficiencies.

Packet Loss

Packet loss occurs when packets of data fail to reach their intended destination. Packet loss can trigger both fast retransmit and coarse-grained timeouts, depending on the detection method.

• Fast retransmit quickly recovers from packet loss using duplicate ACKs.
• When duplicate ACKs aren't sufficient, coarse-grained timeouts handle retransmission.

For example, in a congested network, multiple packets might be lost. If the number of duplicate ACKs isn’t enough to trigger fast retransmit, the sender will have to wait for a coarse-grained timeout before retransmitting, leading to delays.