計(jì)算機(jī)網(wǎng)絡(luò)楊獻(xiàn)春教學(xué)講稿2004dcc

1、Data Communications Data Communications and Networkingand Networking 2022-3-9Department of Computer Science and Technology, Nanjing University2擁塞? ? 擁塞擁塞產(chǎn)生于通過(guò)網(wǎng)絡(luò)傳輸?shù)姆纸M數(shù)量接近網(wǎng)絡(luò)的最大分組處理能力之時(shí) 擁塞控制擁塞控制旨在保持分組數(shù)量不超越致使性能急劇下降的水平 數(shù)據(jù)網(wǎng)本質(zhì)上是一個(gè)由隊(duì)列組成的網(wǎng)絡(luò) 一般而言，達(dá)到80%利用率是一條警戒線 隊(duì)列的有限長(zhǎng)度意味著數(shù)據(jù)可能因溢出而丟失2022-3-9Department of Comput

2、er Science and Technology, Nanjing University3What is Congestion?What is Congestion? Congestion occurs when the number of packets being transmitted through the network approaches the packet handling capacity of the network Congestion control aims to keep number of packets below level at which perfor

3、mance falls off dramatically Data network is a network of queues Generally 80% utilization is critical Finite queues mean data may be lostQueues at a NodeQueues at a NodeAny given node has a number of I/O ports: One or more to other nodes Zero or more to end systemsWhy?2022-3-9Department of Computer

4、 Science and Technology, Nanjing University512.1 12.1 擁塞對(duì)網(wǎng)絡(luò)性能的影響擁塞對(duì)網(wǎng)絡(luò)性能的影響 到達(dá)的分組存放在輸入緩存中 考察每個(gè)分組并作出路由選擇判決 將分組移至輸出緩存 排隊(duì)輸出的分組盡快地發(fā)出 統(tǒng)計(jì)時(shí)分復(fù)用 若分組到達(dá)過(guò)快以至來(lái)不及選路, 或者說(shuō)輸入快于輸出, 緩沖會(huì)溢滿 兩種通用策略 丟棄分組 使用流量控制2022-3-9Department of Computer Science and Technology, Nanjing University612.1 12.1 Effects of CongestionEffects o

5、f Congestion Packets arriving are stored at input buffers Examines each packet and makes routing decision Packet moves to output buffer Packets queued for output are transmitted as fast as possible Statistical time division multiplexing If packets arrive too fast to be routed, or to be output, buffe

6、rs will fill Two general strategies Can discard packets Can use flow controlInteraction of QueuesInteraction of Queues Can propagate congestion through network2022-3-9Department of Computer Science and Technology, Nanjing University812.1.1 12.1.1 Ideal Ideal PerformancePerformance理想性能理想性能10.80.5網(wǎng)絡(luò)傳播

7、時(shí)延網(wǎng)絡(luò)傳播時(shí)延+節(jié)點(diǎn)處理時(shí)延節(jié)點(diǎn)處理時(shí)延+節(jié)點(diǎn)排隊(duì)時(shí)延節(jié)點(diǎn)排隊(duì)時(shí)延 ThroughputPower Delay2022-3-9Department of Computer Science and Technology, Nanjing University912.1.2 12.1.2 實(shí)際性能實(shí)際性能 理想情況是假定緩存空間無(wú)限且無(wú)額外開(kāi)銷 實(shí)際應(yīng)用中緩存是有限的 在交換擁塞控制報(bào)文時(shí)產(chǎn)生額外開(kāi)銷2022-3-9Department of Computer Science and Technology, Nanjing University1012.1.2 12.1.2 Practical

8、PerformancePractical Performance Ideal assumes infinite buffers and no overhead Buffers are finite Overheads occur in exchanging congestion control messages2022-3-9Department of Computer Science and Technology, Nanjing University11Effects of Effects of Congestion -Congestion -No ControlNo Control 負(fù)荷

9、較輕時(shí) 隨負(fù)荷增長(zhǎng)，網(wǎng)絡(luò)吞吐率及網(wǎng)絡(luò)利用率增加 進(jìn)入中度擁塞（A點(diǎn)） 隨負(fù)荷增長(zhǎng)，網(wǎng)絡(luò)吞吐率增長(zhǎng)減慢 負(fù)載不均勻?qū)е虏糠止?jié)點(diǎn)丟棄分組 平衡負(fù)載的選路分組增加開(kāi)銷，降低數(shù)據(jù)通信量 進(jìn)入嚴(yán)重?fù)砣˙點(diǎn)） 隨負(fù)荷增長(zhǎng)，網(wǎng)絡(luò)吞吐率反而下降 有限緩存丟棄分組導(dǎo)致重傳 應(yīng)答延滯使已交付分組重傳12.2 12.2 Congestion ControlCongestion Control 擁塞控制擁塞控制Mechanisms for Congestion ControlMechanisms for Congestion Control2022-3-9Department of Computer Science

10、 and Technology, Nanjing University1312.2.1 12.2.1 反壓反壓 倘若節(jié)點(diǎn)擁塞，可能減緩或阻停來(lái)自其它節(jié)點(diǎn)的分組流量 可能引起其它節(jié)點(diǎn)不得不對(duì)分組進(jìn)入速率施加控制 流量限制傳播回源節(jié)點(diǎn) 可以對(duì)產(chǎn)生大部分通信量的邏輯連接給予限制 用于允許逐跳擁塞控制的面向連接網(wǎng)絡(luò) (如 X. 25) ATM和幀中繼中不采用 因其無(wú)逐跳限制流量的功能 近年僅為IP開(kāi)發(fā)出基于流量的機(jī)制 IPv6分組中的逐跳選項(xiàng)首部（15.4.5），所攜信息必須被沿途每個(gè)路由器檢查2022-3-9Department of Computer Science and Technology,

11、 Nanjing University1412.2.1 12.2.1 BackpressureBackpressure If node becomes congested it can slow down or halt flow of packets from other nodes May mean that other nodes have to apply control on incoming packet rates Propagates back to source Can restrict to logical connections generating most traff

12、ic Used in connection oriented that allow hop by hop congestion control (e.g. X.25) Not used in ATM nor frame relay Only recently developed for IP2022-3-9Department of Computer Science and Technology, Nanjing University1512.2.2 12.2.2 阻流分組阻流分組 控制分組 由發(fā)生擁塞的節(jié)點(diǎn)生成 送至源節(jié)點(diǎn) 例如， ICMP 源站抑制 來(lái)自路由器或目的站 源站削減流量直至再無(wú)

13、源站抑制報(bào)文 為每個(gè)丟棄的分組或預(yù)見(jiàn)的擁塞發(fā)送抑制報(bào)文 相當(dāng)粗糙的機(jī)制 預(yù)測(cè)擁塞發(fā)送的抑制報(bào)文所指的數(shù)據(jù)報(bào)可能正確交付2022-3-9Department of Computer Science and Technology, Nanjing University1612.2.2 12.2.2 Choke PacketChoke Packet Control packet Generated at congested node Sent to source node e.g. ICMP source quench From router or destination Source cuts b

14、ack until no more source quench message Sent for every discarded packet, or anticipated Rather crude mechanism2022-3-9Department of Computer Science and Technology, Nanjing University1712.2.3 12.2.3 隱式擁塞信令隱式擁塞信令 傳輸時(shí)延會(huì)隨擁塞而增加 分組可能被丟棄 隨著擁塞的隱含指示，源站能夠檢測(cè)到這些 在無(wú)連接（數(shù)據(jù)報(bào)）網(wǎng)絡(luò)中是有用的 例如，基于 IP的因特網(wǎng) (TCP 包含擁塞控制和流量控制 見(jiàn)

15、第 17章) 用于幀中繼 LAPF控制協(xié)議2022-3-9Department of Computer Science and Technology, Nanjing University1812.2.3 12.2.3 Implicit Congestion Implicit Congestion SignalingSignaling Transmission delay may increase with congestion Packet may be discarded Source can detect these as implicit indications of congesti

16、on Useful on connectionless (datagram) networks e.g. IP based (TCP includes congestion and flow control - see chapter 17) Used in frame relay LAPF2022-3-9Department of Computer Science and Technology, Nanjing University1912.2.4 12.2.4 顯式擁塞信令顯式擁塞信令 網(wǎng)絡(luò)會(huì)對(duì)漸增的擁塞向端系統(tǒng)告警 端系統(tǒng)采取步驟減輕供給網(wǎng)絡(luò)的負(fù)載 反向 在所需分組的相反方向上采取擁塞避

17、免措施 前向 在所需分組的相同方向上采取擁塞避免措施2022-3-9Department of Computer Science and Technology, Nanjing University2012.2.4 12.2.4 Explicit Congestion Explicit Congestion SignalingSignaling Network alerts end systems of increasing congestion End systems take steps to reduce offered load Backwards Congestion avoidan

18、ce in opposite direction to packet required Forwards Congestion avoidance in same direction as packet required2022-3-9Department of Computer Science and Technology, Nanjing University21顯式信令種類顯式信令種類 二進(jìn)制 在分組中置位一個(gè)比特來(lái)指示擁塞 基于信用值 指示源站可以發(fā)送多少分組 常見(jiàn)于端到端流量控制 基于速率 提供明確的數(shù)據(jù)率上限 例如， ATM2022-3-9Department of Compute

19、r Science and Technology, Nanjing University22Categories of Explicit SignalingCategories of Explicit Signaling Binary A bit set in a packet indicates congestion Credit based Indicates how many packets source may send Common for end to end flow control Rate based Supply explicit data rate limit e.g.

20、ATM2022-3-9Department of Computer Science and Technology, Nanjing University2312.3 12.3 通信量管理通信量管理 公平性 最后到達(dá)最先丟棄策略并不保證對(duì)不同流量體現(xiàn)公平 為每個(gè)邏輯連接或每對(duì)源-目的站維持緩存大小相同的一個(gè)隊(duì)列，通信量負(fù)荷最大的隊(duì)列應(yīng)更頻繁丟棄分組 服務(wù)質(zhì)量(QoS) 不同的連接可以希望不同的對(duì)待 ISA和DS 見(jiàn)16.2和16.4 預(yù)約 例如， ATM 用戶與網(wǎng)絡(luò)之間的通信量合約 RSVP協(xié)議 見(jiàn)16.32022-3-9Department of Computer Science and Te

21、chnology, Nanjing University2412.3 12.3 Traffic ManagementTraffic Management Fairness Quality of service May want different treatment for different connections Reservations e.g. ATM Traffic contract between user and network RSVP protocol - see Chapter 162022-3-9Department of Computer Science and Tec

22、hnology, Nanjing University2512.4 12.4 分組交換網(wǎng)中的擁塞控制分組交換網(wǎng)中的擁塞控制 發(fā)送控制分組到某些或全部源節(jié)點(diǎn) 在擁塞期間需要額外的通信量 依賴路由選擇信息 可能會(huì)反應(yīng)過(guò)快 端到端探測(cè)分組 增加網(wǎng)絡(luò)開(kāi)銷 當(dāng)分組穿越節(jié)點(diǎn)時(shí)向其添加擁塞信息 或者反向或者前向2022-3-9Department of Computer Science and Technology, Nanjing University2612.4 12.4 Congestion Control in Packet Congestion Control in Packet Switched

23、 NetworksSwitched Networks Send control packet to some or all source nodes Requires additional traffic during congestion Rely on routing information May react too quickly End to end probe packets Adds to overhead Add congestion info to packets as they cross nodes Either backwards or forwards2022-3-9

24、Department of Computer Science and Technology, Nanjing University27虛電路或幀中繼連接上的通信量虛電路或幀中繼連接上的通信量 通信量本質(zhì)上是突發(fā)性的 接收系統(tǒng)期望以突發(fā)方式接收從各條連接上進(jìn)入的通信量 網(wǎng)絡(luò)不必在出口節(jié)點(diǎn)復(fù)制進(jìn)入通信量完全相同的時(shí)序模式 可使用簡(jiǎn)單的STDM在一個(gè)物理UNI之上容納多條邏輯連接2022-3-9Department of Computer Science and Technology, Nanjing University28Traffic on Virtual Circuits or Frame

25、Traffic on Virtual Circuits or Frame Realy ConnectionsRealy Connections Traffic is bursty in nature Receiving system expects to receive incoming traffic on each connection in a bursty fashion Network does not need to replicate the exact timing pattern of incoming traffic at exit node Simple STDM can

26、 be used to accommodate multiple logical connections over physical UNI2022-3-9Department of Computer Science and Technology, Nanjing University2912.5 ATM通信量管理通信量管理 ATM通信量特點(diǎn) 速度高、信元長(zhǎng)度小、額外開(kāi)銷比特有限 仍在演變 主要論題 通信量及擁塞控制需求 反應(yīng)遲滯 / 速度影響 信元時(shí)延偏差 通信量與擁塞控制框架 通信量管理和擁塞控制技巧2022-3-9Department of Computer Science and Te

27、chnology, Nanjing University3012.5 12.5 ATM Traffic ManagementATM Traffic Management High speed, small cell size, limited overhead bits Still evolving Key Topics Requirements Latency / Speed Effects Cell Delay Variation Traffic and Congestion Control Framework Traffic Management & Congestion Con

28、trol Techniques2022-3-9Department of Computer Science and Technology, Nanjing University3112.5.1 ATM通信量及擁塞控制的需求通信量及擁塞控制的需求 多數(shù)通信量并不服從流量控制 因信元傳輸時(shí)間比之傳播時(shí)延大幅減小顯得反饋遲緩 廣泛的應(yīng)用要求 差異很大的通信量模式 不同的網(wǎng)絡(luò)服務(wù) 高速交換與傳輸增加了易變性2022-3-9Department of Computer Science and Technology, Nanjing University3212.5.1 12.5.1 Requiremen

29、ts for ATM Traffic and Requirements for ATM Traffic and Congestion ControlCongestion Control Majority of traffic not amenable to flow control Feedback slow due to reduced transmission time compared with propagation delay Wide range of application demands Different traffic patterns Different network

30、services High speed switching and transmission increases volatility2022-3-9Department of Computer Science and Technology, Nanjing University3312.5.2 12.5.2 反應(yīng)遲滯反應(yīng)遲滯 / 速度影響速度影響 ATM 150Mbps 插入單個(gè)信元大約需2.8x10-6秒 跑完網(wǎng)絡(luò)全程的時(shí)間取決于傳播時(shí)延和交換時(shí)延 假定以2/3光速傳播 倘若源站和目的站處于美國(guó)東西兩端, 傳播時(shí)間約 48x10-3 秒 采用隱式擁塞控制, 在丟棄信元通知報(bào)文到達(dá)源站之時(shí)為

31、止, 已經(jīng)傳輸了7.2x106比特 參見(jiàn)教材第313頁(yè)倒數(shù)第4行的計(jì)算（英文版p.396） 因此這對(duì)ATM不是一個(gè)好策略 當(dāng)源站對(duì)擁塞指示反應(yīng)過(guò)來(lái)時(shí)，網(wǎng)絡(luò)擁塞得一塌糊涂2022-3-9Department of Computer Science and Technology, Nanjing University3412.5.2 12.5.2 Latency/Speed EffectsLatency/Speed Effects ATM 150Mbps 2.8x10-6 seconds to insert single cell Time to traverse network depends

32、on propagation delay, switching delay Assume propagation at two-thirds speed of light If source and destination on opposite sides of USA, propagation time 48x10-3 seconds Given implicit congestion control, by the time dropped cell notification has reached source, 7.2x106 bits have been transmitted S

33、o, this is not a good strategy for ATM2022-3-9Department of Computer Science and Technology, Nanjing University3512.5.3 12.5.3 信元時(shí)延偏差信元時(shí)延偏差 對(duì)于ATM話音/視頻, 數(shù)據(jù)是一信元流 穿越網(wǎng)絡(luò)的時(shí)延必須短小 交付速率必須恒定 運(yùn)送中總是存在一些偏差 信元交付給應(yīng)用之前增加一點(diǎn)可調(diào)時(shí)延，使之能夠?yàn)閼?yīng)用維持恒定的比特率2022-3-9Department of Computer Science and Technology, Nanjing University3

34、612.5.3 12.5.3 Cell Delay VariationCell Delay Variation For ATM voice/video, data is a stream of cells Delay across network must be short Rate of delivery must be constant There will always be some variation in transit Delay cell delivery to application so that constant bit rate can be maintained to

35、 applicationTime Re-assembly of CBR CellsTime Re-assembly of CBR CellsV(i)=V(i-1)-ti-(ti-1+d)2022-3-9Department of Computer Science and Technology, Nanjing University38網(wǎng)絡(luò)對(duì)信元時(shí)延偏差的影響網(wǎng)絡(luò)對(duì)信元時(shí)延偏差的影響 分組交換的網(wǎng)絡(luò) 排隊(duì)時(shí)延 路由選擇判決時(shí)間 幀中繼 同上，但程度較輕 ATM 比幀中繼還小 ATM協(xié)議就是為使交換節(jié)點(diǎn)處理開(kāi)銷最小化而設(shè)計(jì)的 ATM交換機(jī)有著極高的吞吐率 唯一值得注意的時(shí)延偏差來(lái)自擁塞 必須不接受

36、引起擁塞的負(fù)荷 丟棄引起網(wǎng)絡(luò)擁塞的信元2022-3-9Department of Computer Science and Technology, Nanjing University39Network Contribution to Network Contribution to Cell Delay VariationCell Delay Variation Packet switched networks Queuing delays Routing decision time Frame relay As above but to lesser extent ATM Less than

37、 frame relay ATM protocol designed to minimize processing overheads at switches ATM switches have very high throughput Only noticeable delay is from congestion Must not accept load that causes congestion2022-3-9Department of Computer Science and Technology, Nanjing University40UNI上的信元時(shí)延偏差上的信元時(shí)延偏差 應(yīng)用

38、程序產(chǎn)生固定速率的數(shù)據(jù) ATM的三層處理仍會(huì)引起時(shí)延偏差 交織的信元來(lái)自不同連接 運(yùn)行維護(hù)管理（OAM）信元也必須與用戶信元相交織 若使用SDH幀, 這些幀的額外開(kāi)銷比特插入物理層 無(wú)法預(yù)測(cè)這些時(shí)延2022-3-9Department of Computer Science and Technology, Nanjing University41Cell Delay Variation At The UNICell Delay Variation At The UNI Application produces data at fixed rate Processing at three lay

39、ers of ATM causes delay Interleaving cells from different connections Operation and maintenance cell interleaving If using synchronous digital hierarchy frames, these are inserted at physical layer Can not predict these delaysOrigins of Cell Delay VariationOrigins of Cell Delay Variation2022-3-9Depa

40、rtment of Computer Science and Technology, Nanjing University4312.5.4 12.5.4 通信量與擁塞控制框架通信量與擁塞控制框架 ATM層通信量控制與擁塞控制應(yīng)該為所有可預(yù)見(jiàn)的網(wǎng)絡(luò)服務(wù)支持QoS類別 應(yīng)該不依賴于與具體網(wǎng)絡(luò)相關(guān)的 AAL協(xié)議，也不依賴于高層的具體應(yīng)用協(xié)議 應(yīng)該使網(wǎng)絡(luò)以及端到端系統(tǒng)的復(fù)雜性最小化2022-3-9Department of Computer Science and Technology, Nanjing University4412.5.4 12.5.4 Traffic and Congestion

41、Traffic and Congestion Control FrameworkControl Framework ATM layer traffic and congestion control should support QoS classes for all foreseeable network services Should not rely on AAL protocols that are network specific, nor higher level application specific protocols Should minimize network and e

42、nd to end system complexity2022-3-9Department of Computer Science and Technology, Nanjing University45通信量控制策略本質(zhì)通信量控制策略本質(zhì) 確定是否可以容納一條給定的新連接 與用戶設(shè)備協(xié)商需要支持性能參數(shù) 實(shí)際上用戶與網(wǎng)絡(luò)達(dá)成一份通信量合約 網(wǎng)絡(luò)答應(yīng)在該連接上以某種性能級(jí)別支持通信量 用戶承諾不超越通信量參數(shù)限制2022-3-9Department of Computer Science and Technology, Nanjing University46Nature of Strateg

43、yNature of Strategy Determine whether a given new connection can be accommodated Agree performance parameters with subscriber In effect, subscriber and network enter into a traffic contract Network agrees to support traffic at a certain level of performance on this connection Subscriber agrees not e

44、xceed traffic parameter limits2022-3-9Department of Computer Science and Technology, Nanjing University47考慮的時(shí)序考慮的時(shí)序 信元插入時(shí)間 往返傳播時(shí)間 連接持續(xù)時(shí)間 長(zhǎng)期2022-3-9Department of Computer Science and Technology, Nanjing University48Timings ConsideredTimings Considered Cell insertion time Round trip propagation time C

45、onnection duration Long term2022-3-9Department of Computer Science and Technology, Nanjing University492022-3-9Department of Computer Science and Technology, Nanjing University5012.5.5 12.5.5 通信量管理與擁塞控制技巧通信量管理與擁塞控制技巧 使用VP管理資源 連接許可控制 使用參數(shù)控制 選擇性信元丟棄 通信量整形2022-3-9Department of Computer Science and Tech

46、nology, Nanjing University5112.5.5 12.5.5 Traffic Management and Traffic Management and Congestion Control TechniquesCongestion Control Techniques Resource management using virtual paths Connection admission control Usage parameter control Selective cell discard Traffic shaping2022-3-9Department of

47、Computer Science and Technology, Nanjing University52利用利用VPVP進(jìn)行資源管理進(jìn)行資源管理 根據(jù)服務(wù)特性區(qū)別不同的通信流量 用戶到用戶應(yīng)用 用戶到網(wǎng)絡(luò)應(yīng)用 網(wǎng)絡(luò)到網(wǎng)絡(luò)應(yīng)用 網(wǎng)絡(luò)資源管理涉及的QoS參數(shù) 信元丟失率 信元傳送時(shí)延 信元時(shí)延偏差2022-3-9Department of Computer Science and Technology, Nanjing University53Resource Management Using Virtual Paths Separate traffic flow according to se

48、rvice characteristics User to user application User to network application Network to network application Concern with: Cell loss ratio Cell transfer delay Cell delay variationConfiguration of VCCs and VPCsConfiguration of VCCs and VPCs2022-3-9Department of Computer Science and Technology, Nanjing U

49、niversity55在在VPCVPC內(nèi)部分配內(nèi)部分配VCCVCC VPC內(nèi)的所有VCC應(yīng)該體驗(yàn)類似的網(wǎng)絡(luò)性能 分配容量的選項(xiàng) 聚集峰值需求 統(tǒng)計(jì)復(fù)用2022-3-9Department of Computer Science and Technology, Nanjing University56Allocating VCCs within VPCAllocating VCCs within VPC All VCCs within VPC should experience similar network performance Options for allocation: Aggrega

50、te peak demand Statistical multiplexing2022-3-9Department of Computer Science and Technology, Nanjing University57連接許可控制連接許可控制 防止網(wǎng)絡(luò)過(guò)載的第一道防線 用戶通過(guò)選擇一種QoS為新連接(VCC或VPC)規(guī)定通信量 僅當(dāng)網(wǎng)絡(luò)能夠滿足該需求時(shí)才認(rèn)可該連接 通信量合約 最大信元速率 信元時(shí)延偏差 持續(xù)信元速率 突發(fā)容限2022-3-9Department of Computer Science and Technology, Nanjing University58Conne

51、ction Admission Control First line of defence User specifies traffic characteristics for new connection (VCC or VPC) by selecting a QoS Network accepts connection only if it can meet the demand Traffic contract Peak cell rate Cell delay variation Sustainable cell rate Burst tolerance2022-3-9Departme

52、nt of Computer Science and Technology, Nanjing University592022-3-9Department of Computer Science and Technology, Nanjing University602022-3-9Department of Computer Science and Technology, Nanjing University61使用參數(shù)控制使用參數(shù)控制 監(jiān)控連接確保通信量遵從合約 防止網(wǎng)絡(luò)資源因某條連接而超載 在 VCC和 VPC兩級(jí)上實(shí)現(xiàn) 包含兩個(gè)獨(dú)立的功能 最大信元速率及相關(guān)的信元時(shí)延偏差的控制 持續(xù)信

53、元速率及相關(guān)的突發(fā)容限 丟棄不遵從通信量合約的信元 被稱為通信量管制2022-3-9Department of Computer Science and Technology, Nanjing University62Usage Parameter Control Monitor connection to ensure traffic conforms to contract Protection of network resources from overload by one connection Done on VCC and VPC Peak cell rate and cell d

54、elay variation Sustainable cell rate and burst tolerance Discard cells that do not conform to traffic contract Called traffic policing2022-3-9Department of Computer Science and Technology, Nanjing University63信元丟棄策略信元丟棄策略 加標(biāo)簽方法及相關(guān)的通信量合約 選擇性信元丟棄CLP的應(yīng)用作為課后作業(yè)，仔細(xì)研讀教材第321頁(yè)2022-3-9Department of Computer S

55、cience and Technology, Nanjing University64通信量整形通信量整形 通信量管制的輔助策略 使通信量平穩(wěn)并減少信元堆積 令牌桶2022-3-9Department of Computer Science and Technology, Nanjing University65Traffic Shaping Smooth out traffic flow and reduce cell clumping Token bucketToken BucketToken Bucket例例 一個(gè)ATM網(wǎng)絡(luò)使用令牌桶方案的通信量管制。每5ms放入桶內(nèi)的一個(gè)新的令牌。試問(wèn)

56、最大的可持續(xù)的凈數(shù)據(jù)率（不包含信元首部）是多少？解答解答 已知每5ms放入信令桶的一個(gè)新的令牌， 則1秒鐘可放入2105個(gè)令牌，即每秒可傳送2105個(gè)信元。 每個(gè)信元不包括首部有48個(gè)八比特組，即 848384個(gè)比特。 因此，最大的可持續(xù)的凈數(shù)據(jù)率 R 3842105 768105 76.8Mbps令牌桶計(jì)算題2022-3-9Department of Computer Science and Technology, Nanjing University6812.6 12.6 ATM-ABRATM-ABR通信量管理通信量管理 應(yīng)用于CBR、VBR的上述方法稱為開(kāi)環(huán)控制 某些應(yīng)用未很好定義通信量

57、特性 Web, FTP, RPC, NFS 盡最大努力 允許這些應(yīng)用分享未用容量 如果擁塞構(gòu)成，信元被丟棄 閉環(huán)控制 多個(gè)ABR連接共享可用容量 該共享容量在最小信元速率(MCR)與最大信元速率(PCR) 之間變化 因?yàn)榉答伿笰RB流量受到可用容量的限制 在反饋時(shí)延期間緩沖吸收超出的通信量 信元丟失率低閱讀ABR服務(wù)主要特性 P.3232022-3-9Department of Computer Science and Technology, Nanjing University6912.6 ATM-ABR Traffic Management Some applications (Web,

58、file transfer) do not have well defined traffic characteristics Best efforts Allow these applications to share unused capacity If congestion builds, cells are dropped Closed loop control ABR connections share available capacity Share varies between minimum cell rate (MCR) and peak cell rate (PCR) AR

59、B flow limited to available capacity by feedback Buffers absorb excess traffic during feedback delay Low cell loss2022-3-9Department of Computer Science and Technology, Nanjing University7012.6.1 12.6.1 反饋機(jī)制反饋機(jī)制 傳輸速率特性 允許的信元速率 ACR 最小的信元速率 MCR 最大信元速率 PCR 初始信元速率 ICR 以ACR=ICR開(kāi)始 根據(jù)來(lái)自網(wǎng)絡(luò)的反饋調(diào)整ACR 資源管理信元 擁塞

60、指示比特 CI 不增加比特 NI 顯式信元速率字段 ER2022-3-9Department of Computer Science and Technology, Nanjing University7112.6.1 Feedback Mechanisms Transmission rate characteristics: Allowed cell rate Minimum cell rate Peak cell rate Initial cell rate Start with ACR=ICR Adjust ACR based on feedback from network Resource