Question

Host $\mathrm{A}$ and $\mathrm{B}$ are directly connected with a $100\mathrm{Mbps}$ link. There is one TCP connection between the two hosts, and Host $\mathrm{A}$ is sending to Host $\mathrm{B}$ an enormous file over this connection. Host A can send its application data into its TCP socket at a rate as high as $120\mathrm{Mbps}$ but Host B can read out of its TCP receive buffer at a maximum rate of $50\mathrm{Mbps}$. Describe the effect of TCP flow control.

Short answer

TCP flow control will adjust Host A's sending rate to match Host B's reading capacity of 50 Mbps, preventing buffer overflow.

Step-by-step solution

Understand the Scenario

In this scenario, Host A can send data at a rate of 120 Mbps, and Host B can receive data at a maximum rate of 50 Mbps. However, the connection between them is a 100 Mbps link. The issues of concern are the rate at which Host A sends data into the network and the maximum rate at which Host B can process this data.

Identify the Bottleneck

The bottleneck in this scenario is Host B's receiving capacity since it can only read at 50 Mbps, which is slower than the link speed (100 Mbps) and Host A's sending rate (120 Mbps). TCP flow control will need to address this disparity to prevent overwhelming Host B's buffer.

Explain TCP Flow Control

TCP flow control uses a mechanism called the "receive window" to manage the flow of data so that Host B can process it without overflowing its buffer. Host B will advertise this receive window size to Host A, indicating how much data it can handle at any given time.

Match Sending Rate to Receiving Capacity

Since Host B can only handle 50 Mbps, the receive window will restrict Host A from sending more data than this rate, effectively slowing down the data transmission to a rate that Host B can accommodate. This prevents data loss and buffer overflow at Host B.

Key concepts

TCP receive window

In a TCP connection, the receive window is a critical component that determines how much data a receiver is ready to accept at any point in time. The receive window acts as a buffer, providing a space where incoming data is temporarily stored before the receiving computer, known as Host B in our scenario, processes it.
Host B informs Host A of the size of this receive window during their communication. Suppose the window size is set to have a capacity of 50 Mbps. In that case, Host A will know that it should not send more than 50 Mbps worth of data, because that's the limit of what Host B can handle without being overwhelmed. The dynamic nature of TCP's flow control approach allows for adjustments, depending on the receiver’s current processing ability.
When the receive window is optimized, it prevents data loss and ensures a smooth flow of information throughout the network, maintaining efficiency and stability. Host A continues to monitor the window size to avoid surpassing the receiving end's capacity, ensuring data is sent at a manageable pace.

data transmission rate

Data transmission rate refers to the speed at which data is sent from one host to another over a network connection. In the given scenario, Host A has the capability to send data at a maximum of 120 Mbps. However, the effective transmission rate is often constrained by multiple factors.

• The receiving capacity of the other host (in this case Host B, which can handle only 50 Mbps).
• The bandwidth of the linking network (a 100 Mbps link in our example).

These constraints mean the real data transmission rate can never exceed the capacity of the slowest link in the entire chain, which could be either the network link or the receiving host's capabilities.
Due to TCP flow control and the receive window, the data transmission rate is automatically adjusted to fit the receiver’s ability to handle the data. This results in a fluid and balanced exchange that equals Host B's maximum reception rate of 50 Mbps, ensuring efficient network usage without any loss of data.

network bottleneck

A network bottleneck occurs when the capacity of one component – like a receiver or link – limits the flow of data in the network. It's analogous to water getting slowed down when it flows through a narrow part of a pipe. In the scenario involving Hosts A and B, the bottleneck is found at Host B, which can process data at only 50 Mbps, less than both the transmission rate of Host A (120 Mbps) and the capacity of the linkage (100 Mbps).
This mismatch leads to a potential backlog of data waiting to be processed, akin to cars stacked in a traffic jam. The bottleneck can cause inefficiencies, as Host A would have to keep waiting until Host B is ready to receive data again.
To resolve a bottleneck, TCP's flow control (through the receive window) adapts the rate at which data is sent, aligning it with what the bottlenecked receiver can manage. This prevents congestion and ensures that the network remains as efficient as possible.

buffer overflow prevention

Buffer overflow is a common issue in computer networks, which happens when a buffer – an allocated temporary space for data – becomes filled beyond its capacity. This occurs when data is sent at a quicker pace than it can be processed or removed from the buffer.
In our scenario, if TCP did not use flow control mechanisms like the receive window, Host B's buffer could quickly become overloaded as Host A sends data at a higher rate (120 Mbps) than Host B can process (50 Mbps). Once the buffer is full, any further incoming data would be lost, leading to inefficiencies and potential data corruption.
TCP's receive window effectively prevents buffer overflow by restricting the data transmission to match Host B's capacity. This ensures buffers are safely managed, with each piece of incoming data carefully accounted for and stored. Proper buffer management maintains data integrity and keeps the network running smoothly, avoiding the costly repercussions of data loss.