1.4 Performance

Introduction to Network Performance

In this section, we will explore the issue of network performance and focus on understanding delay and its components.

Components of Delay

• Delay can occur at routers within the network.

• Four components of delay at a router:

• Processing delay (microseconds or less)

• Queuing delay (depends on congestion level)

• Transmission delay (number of bits divided by transmission rate)

• Propagation delay (time for a bit to travel through media)

Understanding Transmission and Propagation Delay

• Transmission delay is the time it takes for all bits in a packet to be pushed into an outgoing link.

• Propagation delay is the time it takes for a bit to travel from the sending side to the receiving side of a link.

• An analogy with caravans and tollbooths helps understand transmission and propagation delays.

Quantifying Queuing Delay

• Traffic intensity is the ratio of arrival rates of bits to transmission rate.

• When traffic intensity is low, there is seldom a queue. When it exceeds one, delays become infinite.

• Graph shows average queuing delay as a function of traffic intensity.

Conclusion

Throughout this course, we will use real examples from the live internet to illustrate concepts related to network performance.

Understanding Traceroute

This section explains how traceroute works and its purpose in measuring packet delay along a network path.

How Traceroute Works

• Traceroute sends three packets from the sender to the first hop router.

• The first hop router replies with a message for each packet, allowing the sender to measure round trip time (RTT).

• Traceroute displays the RTT measurements for the first hop router.

• It then sends three packets to the second hop router, measures and displays the RTT.

• This process continues until reaching the final destination.

Example of a Trace Route

• A trace route from gaia.cs.umass.edu in Massachusetts to eurocom.fr in France is examined.

• The measured RTT delays are displayed for each hop router along the path.

• Delays can vary due to propagation delays across long distances or congestion levels.

Packet Loss and Congestion Control

• Packet loss can occur when router buffers fill up during high congestion scenarios.

• Techniques exist for hosts to detect and recover from packet loss and control their sending rate.

Understanding Throughput

This section discusses throughput as a performance metric in networking and how it is affected by transmission capacity along a network path.

Definition of Throughput

• Throughput is the rate at which bits are sent from a sender to a receiver, measured in bits per second (bps).

• It can be instantaneous or averaged over different time intervals.

Analogy of Water Flowing through Pipes

• Sender represents fluid flowing into pipes at a certain rate.

• Each transmission link has its own capacity to carry fluid at a specific rate.

• The overall end-to-end throughput is determined by the capacity of each pipe along the path.

Impact of Pipe Capacity on Throughput

• In scenarios with multiple hops, the throughput is limited by the capacity of the thinnest pipe (bottleneck link).

• Examples demonstrate how different pipe capacities affect the overall throughput.

Interaction of Flows and Achievable Throughput

• Multiple flows sharing a network can impact each other's achievable throughput.

• The per connection throughput is determined by the minimum capacity among all pipes in the path.

Summary of Network Performance Metrics

This section provides a summary of network performance metrics discussed, including delay components, traceroute measurements, packet loss, congestion control, and throughput limitations.

Delay Components

• Four components contribute to delay: processing delay, queuing delay, transmission delay, and propagation delay.

• Traceroute measurements help visualize real delays in the internet.

Packet Loss and Congestion Control

• Packet loss occurs when router buffers fill up during high congestion scenarios.

• Techniques exist for hosts to detect and recover from packet loss and control their sending rate.

Throughput Limitations

• Throughput is the rate at which bits are sent from sender to receiver.

• It is influenced by transmission capacities along the network path.

• The bottleneck link determines the overall end-to-end throughput.

• Multiple flows sharing a network can impact each other's achievable throughput.