Traffic Management for the Available Bit Rate (ABR) Service in ...
Traffic Management for the Available Bit Rate (ABR) Service in ... Traffic Management for the Available Bit Rate (ABR) Service in ...
the source to the switch. The feedback delay equals RTT for sources sending data after idle periods, because that is when these sources get their rst feedback for the new burst. The feedback delay equals SFD for sources which are already active, and new sources overload the switch. Inter-RM Cell Time (IRCT): The switches can give feedback inevery RM cell they see, and no faster. Hence, the time between successive RM cells determines the rate of feedback. This is true once there is a continuous ow of RM cells (the control loop is set up). The inter RM cell time (IRCT) is not constant, but a function of the source rate. Speci cally, it is the inverse of the rate of RM cells, which in turn is a small fraction (1/32) of the source's rate. The IRCT is large when the source rates are small. The IRCT and the switch averaging interval (see the next item) is usually the dominant factor in local area networks (LANs) in determining the time lag for feedback to be e ective (note that RTTs and SFDs are small in LANs). Further, in LANs, duetoasynchrony sources with smaller IRCTs get feedback faster. Switch Averaging Interval (AI): The switches usually measure certain quan- tities which are then used to calculate rate feedback. These quantities are called \metrics." For example, switches typically measure the ABR capac- ity to be shared among contending sources. Some switch algorithms may measure other quantities like the number of active ABR sources, the input rate and the individual rates of the sources. One important concern of the switch algorithm is to maintain the correlation of the measured quantities (\control") and the \feedback" given. Our algorithm, ERICA achieves this 51
y giving only one feedback per measurement (i.e., per averaging interval). As a result, the length of the averaging interval determines how often new feedback is given to sources. Shorter averaging intervals result in more feedback, but also more variation in feedback. Longer intervals impact the response time to load changes, but provides more stable feedback in the presence of asynchrony, and heterogeneous RTTs and FDs. The averaging interval length and the IRCT are the dominant factors in LANs (and for sources having short SFDs and RTTs). In general, the time required for convergence due to a change in load, assuming no further changes in load is depends upon the RTTs of the newly active con- nections, the feedback delays of already active connections, the length of the averaging interval and the inter-RM Cell time of all connections. ACR Retention by sources: It is possible that a source gets a high allocation, becomes idle and uses the (now stale) high allocation later when the network conditions have changed. This is called ACR Retention. When multiple sources use their stale allocations simultaneously, bu ers may over ow at the switch before feedback is e ective. While there are some source policies which can control this, the problem is not eliminated. The switches can gradually decrease a source's allocation (down to ICR) if it detects inactivity, but such policies can reduce the average response time and throughput of bursty sources over any time scale. There are several solutions to this problem (described in chapter 7, each with asetof side e ects. 52
- Page 27 and 28: C.3 Flow Chart of Bi-Directional Co
- Page 29 and 30: header information which limits the
- Page 31 and 32: switches since a standard does not
- Page 33 and 34: congestion control deals with the c
- Page 35 and 36: hand side of the equation is the su
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- Page 39 and 40: CHAPTER 2 THE ABR TRAFFIC MANAGEMEN
- Page 41 and 42: time (RTT). As we explain later, AB
- Page 43 and 44: feedback from nearby switches to re
- Page 45 and 46: Note that in-rate and out-of-rate d
- Page 47 and 48: Data cells also have an Explicit Fo
- Page 49 and 50: opportunity the source may nd that
- Page 51 and 52: Figure 2.7: Scheduling of forward R
- Page 53 and 54: as the timeout value) which reduces
- Page 55 and 56: It has been incorrectly believed th
- Page 57 and 58: NI CI Action 0 0 ACR Min(ER, ACR +
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- Page 61 and 62: Switch Rule 1: This rule speci es t
- Page 63 and 64: Observe that the ABR tra c manageme
- Page 65 and 66: complex method like per-VC queuing
- Page 67 and 68: graphs have a steady state with con
- Page 69 and 70: Figure 3.2: Operating point between
- Page 71 and 72: The following example illustrates t
- Page 73 and 74: mention this concept of fairness fo
- Page 75 and 76: control problem was also simpler. S
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- Page 83 and 84: The adaptive FCVC algorithm [63] co
- Page 85 and 86: np. Thus, long path VCs have fewer
- Page 87 and 88: networks like ATM can have complete
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y giv<strong>in</strong>g only one feedback per measurement (i.e., per averag<strong>in</strong>g <strong>in</strong>terval).<br />
As a result, <strong>the</strong> length of <strong>the</strong> averag<strong>in</strong>g <strong>in</strong>terval determ<strong>in</strong>es how often new<br />
feedback is given to sources. Shorter averag<strong>in</strong>g <strong>in</strong>tervals result <strong>in</strong> more<br />
feedback, but also more variation <strong>in</strong> feedback. Longer <strong>in</strong>tervals impact <strong>the</strong><br />
response time to load changes, but provides more stable feedback <strong>in</strong> <strong>the</strong><br />
presence of asynchrony, and heterogeneous RTTs and FDs. The averag<strong>in</strong>g<br />
<strong>in</strong>terval length and <strong>the</strong> IRCT are <strong>the</strong> dom<strong>in</strong>ant factors <strong>in</strong> LANs (and <strong>for</strong><br />
sources hav<strong>in</strong>g short SFDs and RTTs).<br />
In general, <strong>the</strong> time required <strong>for</strong> convergence due to a change <strong>in</strong> load, assum<strong>in</strong>g<br />
no fur<strong>the</strong>r changes <strong>in</strong> load is depends upon <strong>the</strong> RTTs of <strong>the</strong> newly active con-<br />
nections, <strong>the</strong> feedback delays of already active connections, <strong>the</strong> length of <strong>the</strong><br />
averag<strong>in</strong>g <strong>in</strong>terval and <strong>the</strong> <strong>in</strong>ter-RM Cell time of all connections.<br />
ACR Retention by sources: It is possible that a source gets a high allocation,<br />
becomes idle and uses <strong>the</strong> (now stale) high allocation later when <strong>the</strong> network<br />
conditions have changed. This is called ACR Retention. When multiple sources<br />
use <strong>the</strong>ir stale allocations simultaneously, bu ers may over ow at <strong>the</strong> switch<br />
be<strong>for</strong>e feedback is e ective. While <strong>the</strong>re are some source policies which can<br />
control this, <strong>the</strong> problem is not elim<strong>in</strong>ated. The switches can gradually decrease<br />
a source's allocation (down to ICR) if it detects <strong>in</strong>activity, but such policies can<br />
reduce <strong>the</strong> average response time and throughput of bursty sources over any<br />
time scale. There are several solutions to this problem (described <strong>in</strong> chapter 7,<br />
each with asetof side e ects.<br />
52
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