Question

Consider the effect of using slow start on a line with a 10 -msec round-trip time and no congestion. The receive window is \(24 \mathrm{~KB}\) and the maximum segment size is \(2 \mathrm{~KB}\). How long does it take before the first full window can be sent?

Short answer

It takes 40 msec to send the first full window.

Step-by-step solution

Identify the Requirements

The problem involves calculating the time it takes to send a full window using TCP's slow start mechanism. The given values are: round-trip time (RTT) of 10 ms, receive window of 24 KB, and maximum segment size (MSS) of 2 KB.

Calculate the Number of Segments in a Full Window

A full window is equal to the receive window. To calculate the number of segments in a full window, divide the receive window by the MSS. Thus, the number of segments is \( \frac{24 \text{ KB}}{2 \text{ KB}} = 12 \).

Understand the Slow Start Mechanism

Slow start works by starting with a congestion window size equal to 1 MSS and doubling the congestion window size every RTT until it reaches the receive window size or encounters congestion. This means initially 1 segment is sent, then 2, then 4, etc.

Determine the Rounds Needed to Reach a Full Window

Starting with 1 segment, the progression of the segments is 1, 2, 4, 8, and then 16. This takes four RTTs to reach at least 12 segments, as the progression until then covers sizes 1, 3 (1+2), 7 (1+2+4), 15 (1+2+4+8). Hence, four rounds are needed.

Calculate the Time for Each Round

Given that each round is 10 msec in duration (the RTT), and we need four rounds to reach a congestion window size of 15 MSS, the total time taken is \( 4 \times 10 \text{ msec} = 40 \text{ msec} \).

Key concepts

round-trip time calculation

Round-trip time (RTT) is a key concept in networking that represents the time it takes for a signal to go from a source to a destination and back again. It's an essential factor when considering the efficiency of data transmissions. In the context of TCP, the RTT directly influences how quickly data can be sent and acknowledged.
RTT can be affected by several factors, including the geographical distance between the sender and receiver, as well as the network's current load. For accurate data transmissions, it is crucial to measure RTT frequently, often using timestamps.
In this exercise, a 10-msec RTT indicates a quick communication link, allowing for speedy exchange of data packets. The effectiveness of TCP's slow start, as described in the problem, relies heavily on this RTT measurement to estimate how much data can be sent without overwhelming the network's capacity.

congestion window

The congestion window (cwnd) is another critical component of TCP's congestion control strategy. It defines the number of packets, specifically segments, that a source can send into the network before it must wait for an acknowledgment. This bandwidth-throttling mechanism helps prevent network congestion by ensuring data is transmitted only as fast as the network can handle.
In the slow start phase, the congestion window begins at one maximum segment size (MSS). It then grows exponentially with each successive RTT. With each doubling, the congestion window aims to reach the size of the receive window, or until congestion is detected. The exponential increase allows for a rapid discovery of the available network capacity.
From the exercise, the congestion window evolves through stages: 1 MSS, 2 MSS, then 4 MSS, and so forth, doubling until it optimizes data flow efficiency and meets network conditions.

maximum segment size

Maximum segment size (MSS) represents the largest amount of data, measured in bytes, that can be sent in a single packet over the network. It is determined during the TCP handshake process and is crucial for optimizing network efficiency.
An MSS of 2 KB in the provided exercise implies that each TCP segment sent to the network can carry 2 kilobytes of data. This size is chosen based on the maximum transmission unit (MTU) of the underlying network links to avoid fragmentation and associated delays.
By dividing the receive window by the MSS, we can calculate how many segments fit into the receive window. In this scenario, a receive window of 24 KB divided by an MSS of 2 KB yields 12 segments. This calculation helps determine the number of complete packets that can be sent before waiting for an acknowledgment in the TCP slow start process.

receive window

The receive window is a crucial element of the TCP protocol that helps manage data flow between sender and receiver. It signals to the sender how much data the receiver is prepared to accept without overflowing its buffer.
A larger receive window allows the sender to transmit more data before needing an acknowledgment, improving throughput. In our example, a 24 KB receive window means the receiver can handle up to 24 kilobytes of unacknowledged data.
This buffer size is instrumental in adjusting the rate of data flow. Aspects such as available memory and network conditions can influence the size of the receive window. For efficient data exchange, balancing the size of the receive window with the congestion window is vital to maintain a smooth and uninterrupted flow of data.

TCP mechanism

TCP (Transmission Control Protocol) is one of the core protocols of the Internet protocol suite. It ensures reliable, ordered, and error-checked delivery of data between applications running on hosts communicating over an IP network. This reliability is achieved through several key mechanisms:

• Connection Establishment: TCP utilizes a three-way handshake to establish connections, ensuring both the sender and receiver are ready to begin communication.
• Flow Control: Uses the receive window to prevent sending too much data and overwhelming the recipient.
• Congestion Control: Techniques like slow start and congestion window size adjustments manage the flow of packets into the network to prevent congestion.
• Data Integrity: Provides checksums and sequence numbers, ensuring data arrives correctly and in order.

In the context of this exercise, TCP's slow start mechanism plays a significant role in regulating the initial packet transmission rates. It helps gradually probe the network's capacity, minimizing the risk of congestion and ensuring data is sent efficiently and reliably.