TCP (Transmission Control Protocol) is a transport-layer protocol that ensures reliable data transfer between devices over IP networks. One of the key mechanisms used by TCP to ensure reliable data transfer is the transmission timeout, also known as the Retransmission Timeout (RTO). The RTO is a timer that is started when a segment is sent by the sender, and it is used to determine when to retransmit a segment if no acknowledgment is received.
How TCP Transmission Timeout Works
When a segment is sent by the sender, the RTO is initialized to a default value, typically 3 seconds. The timer starts counting down from this value. If no acknowledgment is received for the segment before the timer expires, the segment is retransmitted. The timer is doubled after each retransmission, up to a maximum number of retransmissions, which is typically 5.
The RTO is adjusted on the fly to match the characteristics of the connection by using Smoothed Round Trip Time (SRTT) calculations. SRTT is a measure of the average round-trip time between the sender and receiver. The RTO is calculated as a function of SRTT, which ensures that the timer is adjusted to match the normal delay of the connection.
Factors Affecting TCP Transmission Timeout
Several factors can affect the TCP transmission timeout, including:
- Network congestion: If the network is congested, packets may be delayed or lost, causing the RTO to increase.
- Distance and latency: The distance between the sender and receiver can affect the RTO, as packets may take longer to travel.
- Network quality: The quality of the network can affect the RTO, with lower-quality networks resulting in longer timeouts.
- TCP implementation: Different TCP implementations may have different default RTO values and algorithms for adjusting the timer.
TCP Transmission Timeout in Practice
In practice, TCP transmission timeout can have a significant impact on network performance. For example:
- Retransmissions: If the RTO is too high, retransmissions may occur too frequently, leading to increased network congestion and decreased performance.
- Timeouts: If the RTO is too low, timeouts may occur too frequently, leading to packet loss and decreased performance.
- Network optimization: Optimizing the RTO can improve network performance by reducing retransmissions and timeouts.
Conclusion
In conclusion, TCP transmission timeout is a critical mechanism used by TCP to ensure reliable data transfer over IP networks. Understanding how TCP transmission timeout works and the factors that affect it is essential for optimizing network performance and troubleshooting network issues.
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In summary, there are many things that can cause the issue you're having, yes, including WAN provider issues, but in my experience, a lower than expected transmission rate, if some time interval average, may be just that, an average, not to be confused with peak possible. (Consider, have you ever done a large ping series, see a min/avg/max values? Notice often how they are often not all the same value? [British Prime Minister Benjamin Disraeli, who said, “There are three kinds of lies: lies, damned lies, and statistics.” - laugh])
Likewise, with interface drops, usually just egress queues being overfilled, which doesn't always require oversubscription of bandwidth! Consider, 9 gig ingress ports feeding a single 10g egress port. No way there can be drops, correct? Wrong (although unlikely). What if all 9 ports send their traffic at about the same exact time? The 10g port can only transmit one frame (even though 10x faster), so it must queue 8 frames. What if the queue is 7 or less frame capacity? Frames will be dropped!
Again, the prior example is unusual, but TCP in slow start will burst up to max RWIN before it settles to self clocking with return ACKs. Insufficient queue size for this will cause drops too.
