chapter 28 -- network perforance (qos and diffserv)

(rev. 01/28/2010)

Notes On Chapter Twenty-Eight -- Network Performance (QoS and DiffServ)

28.1 Introduction
- Quantitative Measures of Networks
- How Technologies Can Provide Priority for Some Traffic
28.2 Measures of Performance
- Refer to Figure 28.1 on page 472.
- Major measures of network performance:
  - Latency (delay): time required for data to travel between two points on a network.
  - Throughput (capacity): how many bits per second can a channel deliver?
  - Jitter (variability): the variance of delay (how much change in delay?
28.3 Latency or Delay
- Refer to Figure 28.2 on page 472.
- Types of delay:
  - Propagation Delay: delay in the medium
  - Access Delay: access-to-medium delay - e.g. due to contention for shared media.
  - Switching Delay: time to forward a packet - usually not significant
  - Queuing Delay: in switch or router, time waiting to be selected for transmission - accounts for most delay in the Internet
  - Server Delay: elapsed time on server between arrival of request transmission of first response - often users attribute this to network delay.
28.4 Throughput, Capacity and Goodput
- Effective Throughput is what users actually care about.
- Effective Throughput varies with the application in use.
28.5 Understanding Throughput and Delay
- The terms bandwidth, speed and throughput are often used interchangeably. This is an unfortunate abuse of language.
- Throughput is a measure of capacity, not speed. It's a matter of how many bits per second of information can flow past a given point in a communication channel.
28.6 Jitter
- Suppose that an audio performance is sampled every 125 microseconds, the result placed in a packet, and transmitted to a receiver R.
- If there's no jitter in the channel, and if each packet can be played back as soon as it arrives at its destination, the playback rate will exactly match the sampling rate, and the playback will sound as it should. Note that it doesn't matter how large or small the delay is - only that the delay is constant.
- If everything is as above, except that there's significant jitter, then some packets won't be played back at the right times, relative to other packets. So the playback won't sound right.
- Through the use of real-time protocols it is possible for the receiver to compensate for some jitter, but only if the jitter does not become too great.
28.7 The Relationship between Delay and Throughput
- 28.7.1 Utilization as an Estimate of Delay
  - In practice a network operated at nearly maximum capacity will suffer from severe delay.
  - Generally the amount of delay on a network can be estimated as:
```
	            D₀
          D  =  -----------
	          (1 - U)
```
    Where D₀ is the delay when the network is idle, and where U is the fraction (between 0 and 1) of the network throughput currently being utilized.
  - In other words, the delay on a network tends to be inversely proportional to the unused channel capacity.
  - In still other words: the speed of bits or packets moving along a network tends to be proportionate to the unused channel capacity.
  - Therefore, if we want to insure that a network will operate at close to minimum delay, we must insure that it does not operate very close to maximum utilization.
  - Therefore, when throughput at maximum allowable utilization is not adequate, it's probably time to get technology with higher throughput - e.g. get a 100 Mbps Ethernet if operating the 10 Mbps Ethernet at 65% does not yield adequate throughput. Don't try to operate the 10Mps network at 95% of capacity.
- 28.7.2 Delay-Throughput Product
  - The number, NB, of bits in a communication channel between points A and B is:
```
              NB  =  D X T
```
    Where D is the delay between A and B, and T is the current rate of flow through the channel, in bits per second.
28.8 Measuring Delay, Throughput and Jitter
- To measure throughput and jitter is relatively simple.
- Measuring delay is more difficult because of the need to synchronize the clocks of the sender and receiver.
- Performance measurement in a network is also hampered by the rapidity with which traffic conditions can change, and by the fact that the measurement procedures themselves can affect performance.
28.9 Passive Measurement, Small Packets and NetFlow
- Passive measurement refers to monitoring without injecting additional traffic into the network. (Active monitoring does inject traffic.)
- For example, a router performs passive monitoring if it increments a counter each time it processes a packet.
- Passive monitoring tends not to greatly affect the performance it attempt to measure.
- There is an IETF standard called NetFlow for passive network measurement.
28.10 Quality of Service (QoS)
- Network Provisioning is designing a network to provide desired levels of service.
- Quality of Service (QoS) is defined in terms of constraints on delay, throughput and/or jitter.
28.11 Fine-Grain and Coarse-Grain QoS
- Refer to Figure 28.3 on page 482.
- QoS may be provided in a fine-grain manner. For example, each TCP connection might have its own QoS requirements.
- QoS may be provided in a course-grained manner. For example a client might have to pick one broad class of service that all its traffic must use.
- 28.11.1 Fine-Grain QoS and Flows
  - Refer to Figure 28.4 on page 483.
  - Telephone companies that did early work on QoS felt that fine-grain provisioning would be needed to support voice communication on a packet network.
- 28.11.2 Coarse-Grain QoS and Classes of Service
  - The current thinking is that fine-grain QoS provisioning is not practical or necessary.
28.12 Implementation of QoS
- Refer to Figure 28.5 on page 484.
- The Four Steps of QoS Implementation (performed by a router or switch)
  - Classification and Policing: Router assigns flow identifier to arriving packet, drops packets if customer sends faster than agreed.
  - Forwarding Computation: This takes the flow identifier into consideration.
  - Output Queuing: The idea is to have separate queues for separate QoS requirements.
  - Traffic Scheduling: The scheduler services the queues according to a scheme that assures the QoS requirements are met.
28.13 Internet QoS Technologies
- Refer to Figure 28.6 on page 486.
- RSVP and COPS: relates to fine-grain - seldom used
- DiffServ - seldom used
- MPLS - involves using encapsulation to route. say, voice data along special paths.