Ebook Computer networks: A systems approach (5th edition) – Part 2
➤ Gửi thông báo lỗi ⚠️ Báo cáo tài liệu vi phạmNội dung chi tiết: Ebook Computer networks: A systems approach (5th edition) – Part 2
Ebook Computer networks: A systems approach (5th edition) – Part 2
Congestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 rs of the network protocol hierarchy to understand how data can be transferred among processes across heterogeneous networks. We now turn to a problem that spans the entire protocol stack how to effectively and fairly allocate resources among a collection of competing users. The resources being shar Ebook Computer networks: A systems approach (5th edition) – Part 2 ed include the bandwidth of the links and the buffers on the routers or switches where packets are queued awaiting transmission. Packets contend at aEbook Computer networks: A systems approach (5th edition) – Part 2
router for the use of a link, with each contending packet placed in a queue waiting Its turn to be transmitted over the link. WhenPROBLEM: ALLOCATING Congestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 the network is said to be congested. Most networks provide a congestion-control mechanism to deal with just such a situation.Congestion control and resource allocation are two sides of the same coin. On the one hand, if the network takes an active role in allocating resources—for example, scheduling Ebook Computer networks: A systems approach (5th edition) – Part 2 which virtual circuit gets to use a given physical link during a certainComputer Networks: A Systems Approach. DOI: IO.IO1&'B978 o 13.38305». 1.00006Ebook Computer networks: A systems approach (5th edition) – Part 2
-5 Copyright o 2012 EHevier. Inc All rights reserved.479480 CHAPTER 6 Congestion control and resource allocationperiod of time—then congestion may be Congestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 n question are distributed throughout the network; multiple links connecting a series of routers need to be scheduled. On the other hand, you can always let packet sources send as much data as they want and then recover from congestion should it occur. This is the easier approach, but it can be disr Ebook Computer networks: A systems approach (5th edition) – Part 2 uptive because many packets may be discarded by the network before conges tion can be controlled. Furthermore, it is precisely at those times when theEbook Computer networks: A systems approach (5th edition) – Part 2
network is congested—that is. resources have become scarce relative to demand—that the need for resource allocation among competing users is most keeCongestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 sm is still needed to recover from it. Whether you call such a mixed solution congestion control or resource allocation does not really matter. In some sense, it is both.Congestion control and resource allocation involve both hosts and network elements such as routers. In network elements, various q Ebook Computer networks: A systems approach (5th edition) – Part 2 ueuing disciplines can be used to control the order in which packets get transmitted and which packets get dropped. The queuing discipline can also seEbook Computer networks: A systems approach (5th edition) – Part 2
gregate traffic to keep one user's packets from unduly affecting another user's packets. At the end hosts, the congestioncontrol mechanism paces how fCongestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 eliminate the congestion.This chapter starts with an overview of congestion control and resource allocation. We then discuss different queuing disciplines that can be implemented on the routers inside the network, followed by a description of the congestion-control algorithm provided by TCP on the h Ebook Computer networks: A systems approach (5th edition) – Part 2 osts. The fourth section explores various techniques involving both routers and hosts that aim to avoid congestion before it becomes a problem. FinallEbook Computer networks: A systems approach (5th edition) – Part 2
y, we examine the broad area of quality of service. We consider the needs of applications to receive different levels of resource allocation in the neCongestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 esource allocation and congestion control are complex issues that have been the subject of much study ever since the first network was designed. They are still active areas of research. One factor that makes these issues complex is that they are not isolated to one single level of a protocol6.11ssue Ebook Computer networks: A systems approach (5th edition) – Part 2 s in resource allocation481hierarchy. Resource allocation is partially implemented in the routers, switches, and links inside the network and partiallEbook Computer networks: A systems approach (5th edition) – Part 2
y in the transport protocol running on the end hosts. End systems may use signalling protocols to convey their resource requirements to network nodes,Congestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 can be understood, as well as to give the relevant details about a representative sample of mechanisms.We should clarify our terminology before going any further. By resource allocation, we mean the process by which network elements try to meet the competing demands that applications have for networ Ebook Computer networks: A systems approach (5th edition) – Part 2 k resources— primarily link bandwidth and buffer space in routers or switches. Of course, it will often not be possible to meet all the demands, meaniEbook Computer networks: A systems approach (5th edition) – Part 2
ng that some users or applications may receive fewer network resources than they want. Part of the resource allocation problem is deciding when to sayCongestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 ongestion is generally bad for everyone, the first order of business is making congestion subside, or preventing ir in the first place. This might bo achieved simply by persuading a few hosts to stop sending, thus improving the situation for everyone else. I lowever. it is more common for congestion Ebook Computer networks: A systems approach (5th edition) – Part 2 -control mechanisms ro have some aspect of fairness—that is, they try to share the pain among all users, rather than causing great pain to a few. ThusEbook Computer networks: A systems approach (5th edition) – Part 2
, we see that many congestion-control mechanisms have some sort of resource allocation built into them.It is also important to understand the differenCongestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 eceiver. Congestion control, by contrast, is intended to keep a set of senders from sending loo much data into the network because of lack of resources at some point. These two concepts are often confused; as we will see. they also share some mechanisms.6.1.1 Network ModelWe begin by defining three Ebook Computer networks: A systems approach (5th edition) – Part 2 salient features of the network architecture. For the most part, this is a summary of material presented in the previous chapters that is relevant toEbook Computer networks: A systems approach (5th edition) – Part 2
the problem of resource allocation.482 CHAPTER 6 Congestion control and resource allocationPacket-Switched NetworkWe consider resource allocation in aCongestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 were designed for use on the Internet, and therefore were originally defined in terms of routers rather than switches, we use the term router throughout our discussion, rhe problem is essentially the same, whether on a network or an internetwork.hl such an environment, a given source may have more t Ebook Computer networks: A systems approach (5th edition) – Part 2 han enough capacity on the immediate outgoing link to send a packet, but somewhere in the middle of a network its packets encounter a link that is beiEbook Computer networks: A systems approach (5th edition) – Part 2
ng used by many different traffic sources. I igure 6.1 illustrates this situation— two high-speed links are feeding a low-speed link. This is in contrCongestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 ngly whether or not to send a packet. We have already seen the algorithms used to allocate bandwidth on shared-access networks (Chapter 2). These access-control algorithms are, in some sense, analogous to congestion-control algorithms in a switched network.Note that congestion control is a different Ebook Computer networks: A systems approach (5th edition) – Part 2 problem than routing. While it is true that a congested link could be assigned a large edge weight by the routing protocol, and, as a consequence, roEbook Computer networks: A systems approach (5th edition) – Part 2
uters would route around it, 'routing around’’ a congested link does not generally solve the congestion problem. To see this, we need look no further Congestion Control and Resource jAllocationXI ho hand that hath made you fair hath made you good. -William ShakespeareBy now we have seen enough layer Ebook Computer networks: A systems approach (5th edition) – Part 2 treme example, it is common to have a certain router that it is not possible to route around.1 This router can become congested, and there is nothing the routing mechanism can do about it. This congested router is sometimes called the bottleneck router.VJConnectionless Howsl;or much of our discussio Ebook Computer networks: A systems approach (5th edition) – Part 2 n, we assume that the network is essentially connectionless, with any connection-oriented service implemented in the transport protocol that is runninEbook Computer networks: A systems approach (5th edition) – Part 2
g on the end hosts. (We explain the qualification “essentially" in a moment.) This is precisely the model of the‘It is also worth noting that the compGọi ngay
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