资源描述
Click to edit Master title style,Click to edit Master text styles,Second Level,Third Level,Fourth Level,Fifth Level,115,Congestion Controland Traffic Management in High Speed Networks,Carey Williamson,University of Calgary,Introduction,The goal of congestion control is to regulate traffic flow in the network in order to avoid saturating or overloading intermediate nodes in the network,Congestion: Effects,Congestion is undesirable because it can cause:,Increased delay, due to queueing within the network,Packet loss, due to buffer overflow,Reduced throughput, due to packet loss and retransmission,Analogy: “rush hour” traffic,Congestion: Causes,The basic cause of congestion is that the,input traffic demands exceed the capacity of the network,In typical packet switching networks, this can occur quite easily when:,- output links are slower than inputs,- multiple traffic sources competing for same output link at the same time,Buffering: A Solution?,Buffering in switches can help alleviate,short term,or,transient,congestion problems,but,.,Under sustained overload, buffers will still fill up, and packets will be lost,only,defers,the congestion problem,More buffers means more queuing delay,beyond a certain point, more buffering makes the congestion problem,worse, because of increased delay and retransmission,Motivation,The congestion control problem is even,more acute,in high speed networks,Faster link speeds mean that congestion can happen,faster,than before,e.g., 64 kilobyte buffer, 64 kbps: 8.2 seconds, 10 Mbps: 52 milliseconds, 1 Gbps: 0.52 milliseconds,Motivation (Contd),Buffer requirements,increase with link speeds,e.g., to store 1 second worth of traffic, 64 kbps: 8 kilobytes, 10 Mbps: 1.25 Mbytes, 1 Gbps: 125 Mbytes,Motivation (Contd),Heterogeneity,of link speeds,- just because you add new high speed links to a network doesnt mean that the old low speed links go away,- interconnecting high speed and lower speed networks creates congestion problems at the point of interconnect,Motivation (Contd),Traffic is,bursty,- high peak-to-mean ratio, peak rates,- e.g., data traffic: 10-to-1, 1-10 Mbps,- e.g., video traffic: 20-to-1, 5-100 Mbps,- can statistically multiplex several channels, but if too many are active at the same time, congestion is inevitable,Motivation (Contd),Reaction time,is bounded by the propagation delay,- in a high-speed wide-area network, the,delay x bandwidth product,is HUGE!,- d x b tells you how many bits fit in the “pipe” between you and the receiver,- by the time you realize that network is congested, you may have already sent another Mbit or more of data!,Reactive versus Preventive,There are two fundamental approaches to congestion control:,reactive,approaches and,preventive,approaches,Reactive:,feedback-based,attempt to detect congestion, or the onset of congestion, and take action to resolve the problem before things get worse,Preventive:,reservation-based,prevent congestion from ever happening in the first place, by reserving resources,Reactive versus Preventive (Contd),Most of the Internet approaches are reactive schemes,TCP Slow Start,Random-Early-Detection (RED) Gateways,Source Quench,The large d x b product means that many of these approaches are,not,applicable to high speed networks,Most ATM congestion control strategies are preventive, reservation-based,Congestion Control in ATM,When people discuss congestion control in the context of high speed ATM networks, they usually distinguish between,call-level,controls and,cell-level,controls,Call-Level Control,An example of the call-level approach to congestion control is call admission control (to be discussed later this semester),Tries to prevent congestion by not allowing new calls or connections into the network unless the network has sufficient capacity to support them,Call-Level Control (Contd),At time of call setup (connection establishment) you request the resources that you need for the duration of the call (e.g., bandwidth, buffers),If available, your call proceeds,If not, your call is blocked,E.g., telephone network, busy signal,Call-Level Control (Contd),Tradeoff: aggressive vs conservative,Want to accept enough calls to have reasonably high network utilization, but dont want to accept so many calls that you have a high probability of network congestion (which might compromise the QOS requirements that you are trying to meet),Call-Level Control (Contd),Problems:,Can be unfair,- denial of service, long access delay,Hard to specify resource requirements and QOS parameters precisely,- may not know, or may cheat,- congestion can still occur,Cell-Level Control,Also called,input rate control,Control the input rate of traffic sources to prevent, reduce, or control the level of congestion,Many possible mechanisms:,Traffic shaping, traffic policing, UPC,Leaky bucket (token bucket),Cell tagging (colouring), cell discarding,Cell scheduling disciplines,Congestion Control in ATM,There is actually a complete spectrum of traffic control functions, ranging from the very short-term (e.g., traffic shaping, cell discarding) to the very long-term (e.g., network provisioning),See Gilbert et al 1991,ATM Traffic Control Schemes,Time Scale,ATM Traffic Control Schemes,Time Scale,Short,Term,usec,ATM Traffic Control Schemes,Time Scale,Short,Term,usec,Long,Term,Months, years,ATM Traffic Control Schemes,Time Scale,Cell Time,Usage Parameter Control,Priority Control,Traffic Shaping,Cell Discarding,Propagation,Delay Time,Explicit Congestion Notification,Fast Reservation Protocol,Node to Node Flow Control,Call,Duration,Admission Control,Routing, Load Balancing,Long,Term,Resource Provisioning,ATM Traffic Control Schemes,Time Scale,Cell Time,Usage Parameter Control,Priority Control,Traffic Shaping,Cell Discarding,ATM Traffic Control Schemes,Time Scale,Propagation,Delay Time,Explicit Congestion Notification,Fast Reservation Protocol,Node to Node Flow Control,ATM Traffic Control Schemes,Time Scale,Call,Duration,Admission Control,Routing, Load Balancing,ATM Traffic Control Schemes,Time Scale,Long,Term,Resource Provisioning,ATM Traffic Control Schemes,Time Scale,Cell Time,Usage Parameter Control,Priority Control,Traffic Shaping,Cell Discarding,Propagation,Delay Time,Explicit Congestion Notification,Fast Reservation Protocol,Node to Node Flow Control,Call,Duration,Admission Control,Routing, Load Balancing,Long,Term,Resource Provisioning,ATM Traffic Control Schemes,Preventive controls:,Resource provisioning,Connection admission control,Call routing and load balancing,Usage parameter control,Priority control,Traffic shaping,Fast reservation protocol,ATM Traffic Control Schemes,Reactive controls:,Adaptive admission control,Call routing and load balancing,Adaptive usage parameter control,Explicit congestion notification,(forward or backward),Node to node flow control,Selective cell discarding,Leaky Bucket,One of the cell-level control mechanisms that has been proposed is the,leaky bucket,(a.k.a. token bucket),Has been proposed as a traffic policing mechanism for Usage Parameter Control (UPC), to check conformance of a source to its traffic descriptor,Can also be used as a traffic shaper,Leaky Bucket (Contd),Think of a bucket (pail) with a small hole in the bottom,You fill the bucket with water,Water drips out the bottom at a nice constant rate: drip, drip, drip.,Leaky Bucket (Contd),Leaky Bucket (Contd),Bucket,Leaky Bucket (Contd),Bucket,Empty,Leaky Bucket (Contd),Bucket,Hole,Leaky Bucket (Contd),Bucket,Water,Hole,Leaky Bucket (Contd),Leaky Bucket (Contd),Drip,Leaky Bucket (Contd),Leaky Bucket (Contd),Leaky Bucket (Contd),Leaky Bucket (Contd),Constant,rate stream of,drips, all nicely,spaced, periodic,Leaky Bucket (Contd),Constant,rate stream of,drips, all nicely,spaced, periodic,Storage area,for drips,waiting to go,Leaky Bucket (Contd),A leaky bucket flow control mechanism is then a software realization of this very simple idea,Packets (cells) waiting for transmission arrive according to some (perhaps unknown) arrival distribution,Tokens arrive periodically (deterministically),Cell must have a token to enter network,Leaky Bucket (Contd),Incoming Cells,(generated by,traffic source,with rate X),Incoming Tokens,at rate r tokens/sec,+,To,Network,Leaky Bucket (Contd),Incoming Cells,Incoming Tokens,at rate r tokens/sec,+,To,Network,1,2,3,4,5,Leaky Bucket (Contd),Incoming Cells,Incoming Tokens,+,To,Network,1,2,3,4,5,Leaky Bucket (Contd),Incoming Cells,Incoming Tokens,+,To,Network,1,2,3,4,5,Leaky Bucket (Contd),Incoming Cells,Incoming Tokens,+,To,Network,1,2,3,4,5,Leaky Bucket (Contd),Incoming Cells,Incoming Tokens,+,To,Network,1,2,3,4,5,Leaky Bucket (Contd),Incoming Cells,Incoming Tokens,+,To,Network,X,1,2,3,4,5,Leaky Bucket (Contd),Incoming Cells,Incoming Tokens,+,To,Network,X,X,1,2,3,4,5,Leaky Bucket (Contd),Incoming Cells,Incoming Tokens,+,To,Network,X,X,1,2,3,Leaky Bucket (Contd),Cells must obtain tokens in order to proceed into the network,If no token available, then cell is discarded,Constrains the rate at which cells enter the network to be the rate negotiated at the time of call setup,Shapes traffic, reduces burstiness,Buffered,Leaky Bucket,Arriving cells that find a token waiting can proceed directly into the network,Arriving cells that find no token ready must,wait in queue,for a token,Cells that arrive to a full queue are lost,Tokens that arrive to a full token pool are simply discarded,Buffered Leaky Bucket,Incoming Cells,Incoming Tokens,at rate r tokens/sec,+,To,Network,Queue of,at most B,waiting cells,Pool of at most M,waiting tokens,Buffered Leaky Bucket (Contd),Incoming cell rate: X,Token rate: r,If X r, then cells wait in buffer until tokens are available,Output traffic is r cells/sec, nicely paced,If X r, then tokens always ready,Output traffic is X ( r),Use for traffic shaping or UPC,Buffered Leaky Bucket (Contd),A station can “save up” at most M tokens,Limits the maximum burst size in the network,Can send at most M cells back to back,B can be set to balance the tradeoff between cell loss and cell delay,Leaky Bucket UPC,The token rate r is set based on the rate declared at the time of call setup,Makes sure that each source obeys rate that was used when the call admission decision was made (i.e., descriptor),Can use “single leaky bucket” to police just the peak cell rate (PCR),Can use “dual leaky bucket” to police both PCR and SCR,Variations,There are several different variations of the basic leaky bucket concept described in the literature, such as the virtual leaky bucket, spacer, others,Basic idea: rather than strictly enforcing rates, allow senders to occasionally exceed their prescribed rate, as long as they mark or,tag their extra cells,Cell Marking Scheme,Uses leaky bucket to regulate cell transmissions as before, but rather than having cells wait for tokens when there are no tokens ready, the station can transmit the cell and mark it as a violation cell (i.e., cell colouring),Green,(CLP = 0) for cells that obey rate,Red,(CLP = 1) for cells that dont,Cell Colouring (Contd),If the network detects congestion at any point, then it does not hesitate to throw away,red,cells (CLP = 1), but always gives preference to,green,cells,Green,cells must get through,Red,cells get through only if there is spare capacity in the network,“No harm in trying!” principle,Selective Cell Discard (SCD),A cell-level control mechanism in ATM switches called,selective cell discard,can be implemented quite easily using a,CLP threshold,on each queue/buffer,Below the threshold, can accept both,green,and,red,cells,Beyond the threshold, can only accept,green,cells,Selective Cell Discard (Contd),Buffer in an ATM switch,Selective Cell Discard (Contd),Buffer in an ATM switch,Selective Cell Discard (Contd),Buffer in an ATM switch,Some cells,waiting to go,Selective Cell Discard (Contd),CLP,Threshold,Buffer in an ATM switch,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Selective Cell Discard (Contd),CLP,Threshold,Explicit Congestion Notification,There are some proposals to use reactive congestion control approaches for end-to-end flow control in ATM,One of the mechanisms proposed is called Explicit Forward Congestion Notification (EFCN) (or EFCI, for Explicit Forward Congestion Indication),EFCI: Basic Operation,Switches can detect the onset of congestion (e.g., buffers filling up),Switches set a control bit in cell headers to indicate this congestion condition,Sources react by reducing the volume of traffic that they are sending through that switch,Suitable for VBR or ABR traffic,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,Buffer,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,Occupied,Unoccupied,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,Data,Cell,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,!,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,Ack,Cell,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,!,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI: Basic Operation (Contd),Traffic,Source,Traffic,Sink,Switch,Switch,Switch,EFCI Threshold,EFCI Issues,How to set EFCI threshold,What should sources do when EFCI signal is seen,What should sources do when no EFCI signal is seen,Forward versus backward notification,Effect of feedback delay,Delay x bandwidth product,Summary,Congestion control in high speed ATM networks is a difficult problem,Lots of good ideas of how to do it, but no real standard (yet?),Will likely require a combination of schemes at different time scales and for different classes of traffic,Lots more remains to be done,
展开阅读全文