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TCP uses a host-centric, feedback based, windows based resource allocation model. How might TCP have been designed to use instead the following models? (a) Host-centric, feedback based, and rate based (b) Router-centric and feedback based

Short Answer

Expert verified
For a rate-based model, adjust transmission rate based on feedback instead of window size. For a router-centric model, routers provide feedback to manage congestion.

Step by step solution

01

Understand the original TCP model

TCP (Transmission Control Protocol) uses a host-centric, feedback-based, windows-based resource allocation model. This model adjusts the window size (amount of data sent before needing an acknowledgment) based on network feedback (such as packet loss).
02

Analyze a host-centric, feedback-based, rate-based model

In this model, the host still initiates control and adjusts based on feedback from the network. However, instead of adjusting the window size, it adjusts the data transmission rate. Rate-based control refers to explicitly managing how fast data packets are sent, often measured in bits per second (bps).
03

Design changes for host-centric, feedback-based, rate-based model

TCP would need mechanisms for measuring and managing transmission rate. A feedback loop would monitor the network performance (e.g., acknowledgments, delays, packet loss) and adjust the rate accordingly. Formula adjustments could be included to decrease the rate on encountering congestion signals and gradually increase it when conditions improve.
04

Analyze a router-centric, feedback-based model

In a router-centric model, routers play a significant role in resource allocation. Feedback from routers would dictate strategies for congestion control rather than relying on host-based mechanisms.
05

Design changes for router-centric, feedback-based model

Routers would need to monitor traffic flow and provide explicit feedback to hosts about network conditions. This feedback could include signaling congestion points or available bandwidth, prompting hosts to adjust their transmission accordingly.
06

Compare and contrast the models

Host-centric models offer simpler implementation with the control within hosts, whereas router-centric models may provide more precise congestion management but require more complexity in router feedback mechanisms.

Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Host-Centric Model
TCP traditionally uses a host-centric model for congestion control. In this approach, the host (sender) is responsible for adjusting the transmission characteristics. The host listens to feedback from the network, such as acknowledgments (ACKs), delays, or packets loss indicators. This feedback helps the host determine the state of the network and adjust its sending behavior.

In this model, the host might increase the sent data window size when the network seems to handle it well, or it might decrease it in the face of congestion signs. The main advantage of a host-centric approach is its relative simplicity since the complexity of congestion control is managed at the host level, not distributed across the network.
Rate-Based Control
Rate-based control is an alternative to window-based control. Instead of adjusting the window size, the transmission rate of data packets is directly controlled. Transmission rate is typically measured in bits per second (bps).

For instance:
  • If congestion is detected, the host reduces the transmission rate to lessen the load on the network.
  • When favorable conditions are identified, the transmission rate is increased to better utilize the available bandwidth.
This method requires precise measurements of the network’s conditions and adjustments to how quickly data is sent. It's efficient for systems where managing transmission rates directly can yield better performance readjustments during congestion.
Router-Centric Model
A router-centric model places routers at the core of congestion control strategies. In this model, routers actively monitor network conditions and provide explicit feedback to hosts about congestion issues and available bandwidth.

This involves:
  • Routers detecting congestion and sending signals to hosts about current network conditions.
  • Hosts adjusting their transmission rates or data window sizes based on the feedback from routers.
This model can potentially manage congestion more precisely because routers have greater visibility and control over the network state. However, it can also introduce complexity in router design and operation as they need to handle feedback mechanisms.
Feedback Mechanisms
Feedback mechanisms are essential in any congestion control model. They form the communication channel between different network components to ensure data flows smoothly without overwhelming the network.

In a host-centric model, feedback primarily comes from ACKs or signals like packet loss or delays. In a router-centric model, routers provide explicit feedback about the network state. Effective feedback mechanisms help adjust transmission behavior to preempt congestion or recover from it quickly.

Considerations include:
  • Timeliness: Immediate and rapid feedback ensures swift adjustment.
  • Accuracy: Precise network condition reports help in making smarter adjustments.
The type and sophistication of feedback greatly influence the quality of congestion control.
Resource Allocation
Resource allocation in TCP is about efficiently using network resources (bandwidth, buffer space) to maximize data flow while preventing congestion. Different models approach this in varied ways:
  • Host-Centric/Window-Based: Adjusts the amount of data a host can send before needing an ACK, a less direct but simpler resource management technique.
  • Rate-Based: Precisely regulates the rate of data transmission, providing fine control of bandwidth utilization.
  • Router-Centric: Routers balance and allocate resources based on real-time network conditions, potentially optimizing flow better but requiring more complex router logic.
Effective resource allocation ensures high throughput and low latency while avoiding packet loss and congestion collapse.

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Most popular questions from this chapter

Suppose two hosts \(\mathrm{A}\) and \(\mathrm{B}\) are connected via a router \(\mathrm{R}\). The \(\mathrm{A}-\mathrm{R}\) link has infinite bandwidth; the \(R-B\) link can send one packet per second. \(R\) 's queue is infinite. Load is to be measured as the number of packets per second sent from A to B. Sketch the throughput-versus-load and delay-versus-load graphs, or if a graph cannot be drawn, explain why. Would another way to measure load be more appropriate?

Consider a simple congestion-control algorithm that uses linear increase and multiplicative decrease but not slow start, that works in units of packets rather than bytes, and that starts each connection with a congestion window equal to one packet. Give a detailed sketch of this algorithm. Assume the delay is latency only, and that when a group of packets is sent, only a single ACK is returned. Plot the congestion window as a function of round-trip times for the situation in which the following packets are lost: \(9,25,30,38\), and 50 . For simplicity, assume a perfect timeout mechanism that detects a lost packet exactly 1 RTT after it is transmitted.

You are an Internet service provider; your client hosts connect directly to your routers. You know some hosts are using experimental TCPs and suspect some may be using a "greedy" TCP with no congestion control. What measurements might you make at your router to establish that a client was not using slow start at all? If a client used slow start on startup but not after a timeout, could you detect that?

Discuss the relative advantages and disadvantages of marking a packet (as in the DECbit mechanism) versus dropping a packet (as in RED gateways).

Suppose TCP is used over a lossy link that loses on average one segment in four. Assume the bandwidth \(x\) delay window size is considerably larger than four segments. (a) What happens when we start a connection? Do we ever get to the linearincrease phase of congestion avoidance? (b) Without using an explicit feedback mechanism from the routers, would TCP have any way to distinguish such link losses from congestion losses, at least over the short term? (c) Suppose TCP senders did reliably get explicit congestion indications from routers. Assuming links as above were common, would it be feasible to support window sizes much larger than four segments? What would TCP have to do?

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