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1 ATM Traffic Management ATG s Communications & Networking Technology Guide Series This guide has been sponsored by

2 Table of Contents The Challenge: Efficiency with Service Integrity ATM Service Categories QoS Parameters The Traffic Contract Traffic Management Functions Switch Architecture Conclusions Acronyms About the Editor Gerald P. Ryan is the founder of Connections Telecommunications Inc., a Massachusetts-based company specializing in consulting, education and software tools which address Wide Area Network issues. Mr. Ryan has developed and taught numerous courses in network analysis and design for carriers, government agencies and private industry. Connections has provided consulting support in the areas of WAN network design, negotiation with carriers for contract pricing and services, technology acquisition, customized software development for network administration, billing and auditing of telecommunications expenses, project management, and RFP generation. Mr. Ryan is a member of the Networld+Interop program committee. N.E.T. 800 Saginaw Drive Redwood City, CA U.S.A. tel fax net@net.com This book is the property of The Applied Technologies Group and is made available upon these terms and conditions. The Applied Technologies Group reserves all rights herein. Reproduction in whole or in part of this book is only permitted with the written consent of The Applied Technologies Group. This report shall be treated at all times as a proprietary document for internal use only. This book may not be duplicated in any way, except in the form of brief excerpts or quotations for the purpose of review. In addition, the information contained herein may not be duplicated in other books, databases or any other medium. Making copies of this book, or any portion for any purpose other than your own, is a violation of United States Copyright Laws. The information contained in this report is believed to be reliable but cannot be guaranteed to be complete or correct. Copyright 1997 by The Applied Technologies Group, One Apple Hill, Suite 216, Natick, MA 01760, Tel: (508) , Fax: (508) info@techguide.com Web Site:

3 The Challenge: Efficiency with Service Integrity ATM technology consolidates diverse traffic types in a single multiservice network. By statistically multiplexing traffic streams, an ATM multiservice network allows multiple circuits to share the same links, network resources, and bandwidth. Each circuit in an ATM network has an apparent bandwidth capacity that is equal to its peak rate. But all circuits normally do not peak simultaneously. Individual circuit bandwidth requirements fluctuate, producing peaks and valleys. As a result, the sum of all peak rates of circuits can be greater than the total bandwidth capacity of the link. ATM combines a large number of circuits, so that the peaks and valleys average out, producing a relatively consistent aggregate bandwidth demand. The advantages of this traffic consolidation include a cost-effective use of bandwidth, a simplification of network operations, lower equipment costs, and the opportunity to expand service offerings. But different types of voice, video, and data traffic have different service requirements. Migration of existing end-user services to ATM requires the transparent replacement of existing networks. And transparent replacement demands service integrity the appropriate Quality of Service (QoS) for various traffic types. As a result, the challenge facing an ATM network is to achieve efficiency while maintaining service integrity. Traffic management is the key to meeting that challenge. What is Traffic Management? Traffic management is a set of network actions that monitor and control the flow of traffic. It provides each traffic stream its desired level of bandwidth and control over cell loss, cell delay, and delay variation. Traffic management supports differentiated service levels. It prevents different streams from interfering with one another and keeps each stream from consuming more than its contracted share of network resources. And traffic management protects the network and its end systems from congestion. Contention ATM switches deal with contention among asynchronous, bursty data streams and even among constant bit rate streams. When traffic streams from multiple input ports are destined for one output port, the result is contention. Traffic streams contend for such limited resources as buffer space and bandwidth. In many architectures, contention occurs within the switch fabric as well as the output port. Serious contention, if not quickly resolved, can cause switch overload. Prolonged system overload then escalates into congestion as higher-layer applications request retransmission of lost packets. Then the congested network cannot guarantee QoS levels, and the performance of higher-layer applications is seriously degraded. To quickly resolve contention and provide each connection with its negotiated QoS, an ATM switch must deploy advanced traffic-management functions. 2 ATM Traffic Management Technology Guide 3

4 ATM Service Categories An ATM network can provide Virtual Path (VP) or Virtual Channel (VC) connections with distinct levels of service. The ATM Forum Traffic Management Specification 4.0 defines five service categories: CBR VBR-rt VBR-nrt UBR ABR CBR resembles a leased-line service. It is suited to connections requesting a static amount of bandwidth that is continuously available during the connection lifetime. This bandwidth amount is characterized by a Peak Cell Rate (PCR) value. Typical applications for CBR are videoconferencing, interactive audio (telephony), audio/video distribution (television, distance learning, pay-per-view), and audio/video retrieval (video-on-demand, audio library). VBR-rt is intended for real-time applications that is, applications requiring tightly constrained delay and delay variation, but not necessarily a fixed transmission rate. VBR-rt connections are characterized in terms of a Peak Cell Rate (PCR), Sustainable Cell Rate (SCR), and Maximum Burst Size (MBS). VBR-rt can be used by native ATM voice with bandwidth compression and silence suppression. VBR-rt is also suitable for some types of multimedia communications. VBR-nrt is appropriate for non-real-time bursty applications that require service guarantees from the network. Like VBR-rt, VBR-nrt connections are characterized in terms of a PCR, SCR, and MBS. Typical applications for this service are data transfers for trans- action-processing applications such as airline reservation, banking transactions, and process monitoring. Frame relay traffic can also use VBR-nrt service. UBR is intended for non-real-time, bursty applications that are tolerant of delay and loss. UBR service does not specify service guarantees and is sometimes referred to as the best effort service. It can be used for text/data/image transfers, remote terminal (telecommuting), , store-and-forward networks, LAN interconnection, LAN emulation, supercomputer applications, remote procedure call, distributed file services, and computer process swapping/paging. ABR is used by applications that can tolerate a Minimum Cell Rate (MCR) but are able to adapt to feedback from the network to take advantage of available bandwidth. On the establishment of an ABR connection, the end-system specifies both a PCR and an MCR. A flow-control mechanism, which supports several types of feedback, fairly allocates the available bandwidth among ABR connections. Application examples for ABR are similar to those for UBR. QoS Parameters In addition to one of five ATM service categories, each ATM connection may request a QoS parameter. The negotiation of these QoS parameters is a function of provisioning (for PVCs) or signaling (for SVCs). There are three QoS parameters: Cell Loss Ratio (CLR) Maximum Cell Transfer Delay (maxctd) Cell Delay Variation (CDV) 4 ATM Traffic Management Technology Guide 5

5 CLR is the ratio of lost cells to total transmitted cells. Cells may be lost due to an ATM switch malfunction, but cells are usually lost because they are explicitly discarded by the ATM switch. An ATM switch will discard cells that belong to noncompliant traffic flows at the network ingress, or in response to network congestion. The method by which an ATM switch discards cells in the face of congestion is critical to a network s performance. A switch may discard cells in such a way that the congestion is not resolved or is increased. A discard algorithm based on efficiency and fairness provides optimal traffic performance. CTD is defined as the elapsed time between a cell exit event at the source and the corresponding cell entry event at the destination. CTD is the sum of the total inter-atm node transmission delay and the total ATM node processing delay between source and destination. End systems using CBR or VBR-rt service indicate their CTD requirement by negotiating with the network a maxctd. CDV is sometimes called cell jitter. It refers to the fact that while cells may be sent into a network evenly spaced, a variety of factors may contribute to cell clumping and gaps in the cell stream. CDV is mostly an issue for end systems running voice, video, and multimedia applications over CBR and VBR-rt connections. If the network cannot properly control CDV, some of these real-time services may have their communications distorted. An end system using CBR or VBR-rt service indicates its end-to-end cell delay variation requirement by negotiating a peak-to-peak CDV with the network. The Traffic Contract An ATM traffic contract specifies the characteristics of a connection, as defined by a Connection Traffic Descriptor and a QoS. The traffic contract is an agreement between an ATM end-system and the ATM network. As long as the end-system sends traffic across the UNI in conformance with the Connection Traffic Descriptor, the network will enforce the negotiated QoS. The Connection Traffic Descriptor includes two key elements: the Cell Delay Variation Tolerance (CDVT) and the Source Traffic Descriptor. The Source Traffic Descriptor is a set of parameters that describes the connection s expected bandwidth utilization. These parameters are: Peak Cell Rate (PCR), Sustainable Cell Rate (SCR), Maximum Burst Size (MBS), and Minimum Cell Rate (MCR). With these parameters, an ATM end-system describes its average and peak bandwidth requirements and its maximum packet size. The network determines whether it can establish the connection and still meet the required QoS level for all connections including the new one. This calculation is referred to as Connection Admission Control (CAC). Conformance is determined by a Usage Parameter Control (UPC) function at the ingress (input edge) to the network. 6 ATM Traffic Management Technology Guide 7

6 Traffic Management Functions A number of ATM traffic management functions, defined by the ITU-T and the ATM Forum, support the standard service categories. These functions provide a framework for monitoring and controlling traffic and congestion. They include Connection Admission Control (CAC), User Parameter Control/Network Parameter Control (UPC/NPC), traffic shaping, frame discard (EPD/PPD), multiple classes of service and priority control, buffer management, and route management. Figure 1 shows how the functions work together to achieve the goals of ATM traffic management. Inbound Data UPC / NPC Switch Fabric Per-VC Traffic Shaping Management of Multiple QoS Classes Priority Control Cell Buffer Management of Shared Buffer Per-VC Cell discard Per-VC Frame discard Figure 1 ATM Traffic Management Functions Connection Admission Control (CAC) Outbound Data ATM is a connection-oriented technology. The establishment of virtual circuits allows the network to provision the necessary resources for the transfer of information. At the time of SVC or PVC establishment, the ATM network uses the Connection Admission Control (CAC) function to determine the acceptance or rejection of a connection request. The CAC accepts a connection request only if sufficient resources are available at each network element and if the new connection will not affect the QoS of existing connections. The CAC considers the following factors: Source Traffic Descriptor and QoS of requested connections Traffic contracts of connections currently supported Previously allocated bandwidth at port level (logical and physical) Shared buffer and output queue occupancy A user-specific overbooking parameter Policing The policing function makes sure that that a connection complies with the parameters of its traffic contract so that QoS guarantees to other connections are not jeopardized. Noncompliant cells at the network edge are denied admission to the network, or they are admitted and tagged as noncompliant. Policing is also referred to as User Parameter Control (UPC) and Network Parameter Control (NPC). 8 ATM Traffic Management Technology Guide 9

7 Frame 1 (ATM cells) To determine compliance with a traffic contract, UPC/NPC implements a leaky bucket algorithm such as the Generic Cell Rate Algorithm (GCRA). The leaky bucket refers to a leak at a certain rate corresponding to a cell rate parameter. GCRA has two parameters: The Increment parameter corresponds to the inverse of the compliant rate (the fill rate of the bucket) The Limit parameter corresponds to the number of cells that can burst at a higher rate (the size of the bucket) Traffic Shaping Traffic contract violations Frame 2 (ATM cells) 155 Mbps 10 Mbps Shaded areas indicate cells that could be discarded by network policing function Figure 2 Traffic Policing Traffic shaping refers to the value-added processing of a connection s cell stream to ensure that it conforms to an idealized definition of traffic flow or to a traffic contract. Traffic is shaped to eliminate bursty peaks in a flow or excessive cell jitter. By smoothing the cell stream of each connection, traffic shaping delivers a more predictable traffic profile, leading to fairness, lower cell loss, and less stress on network resources. There are some cases where traffic shaping may mean the difference between an application working well or not working at all. This would be true for an ATM end-system that outputs cells at the full line rate, instead of limiting output to the peak cell rate as defined by the traffic contract. ATM, however, requires more than just segmenting frames or packets into cells; it requires traffic shaping to ensure that the interval between cells conforms to the peak cell rate, which may be substantially lower than the full line rate. Assume that an ATM end system supporting Ethernet LANs wishes to access an ATM network. Since the peak bandwidth of any port is 10 Mbps, the user provisions PVCs with a peak cell rate of 10 Mbps per second. The service is installed, and everything works fine. Then as more customers are added to the network and traffic grows, the carrier upgrades the network by enabling UPC traffic policing in order to ensure that the QoS of all virtual connections can be met. Now the shortcomings of the ATM access device are exposed, since it fails to limit its transmissions to the peak cell rate. For very short packets, the tolerance built into the policing algorithm allows the cells to enter the network. But for longer packets, cells are discarded by the policing function. Retransmissions are of no avail, since the policing function will again discard cells in excess of the peak cell rate. All of a sudden, applications that used to work have failed. And for most networks, there is no notification or reporting of cell discard due to policing, so the cause of the failure may be difficult to determine. Traffic shaping ensures compliance Frame 1 (cells) Frame 2 (cells) 155 Mbps 10 Mbps The cells which constitute each frame are spaced in time to comply with PCR of traffic contract Figure 3 Traffic Shaping lowers the PCR 10 ATM Traffic Management Technology Guide 11

8 There are only two remedies to the failure caused by the enabling of traffic policing: One, the user may renegotiate the traffic contract for a peak cell rate at the full line rate of 155 Mbps for all virtual connections (increasing the cost of the service by an order of magnitude). Second, the user may implement traffic shaping, probably by installing a ATM switch capable of traffic shaping between the Ethernet product and the network. Traffic shaping at the egress of a network reduces CDV across the UNI to the ATM end system. This allows users the flexibility to deploy less sophisticated ATM devices for CBR and VBR-rt services that may not be able to tolerate higher cell delay variances. In doing so, users will not be subject to the quality degradation that results from buffer overruns or cell loss for real-time ATM services. In addition to performance benefits, traffic shaping provides economic benefits to ATM WANs. For example, an ATM end system may specify a Source Traffic Descriptor that results in a lower tariff service while achieving the same throughput as a higher tariff that would have resulted without traffic shaping. Or a network operator (private or public) can engineer a higher level of trunk utilization while supporting the same level of traffic due to traffic shaping, thereby lowering the cost of the network. Shaping allows inter-switch trunks to operate at a higher level of utilization while maintaining a fixed CLR. Conversely, the network could maintain the same amount of bandwidth and with traffic shaping improve the overall CLR. The reason is that peak traffic bursts will periodically occur due to the statistical nature of ATM cell streams converging onto common PNNI links. If these bursts pass over a PNNI link without being shaped into compliant cell streams, they can cause congestion and cell discard at next hop in the network. Frame Discard A cell discarded by an ATM switch can cause the loss of an entire packet. End-to-end error recovery may then be required through packet retransmission by a higher-layer protocol. Mild congestion could escalate to severe congestion. To prevent this escalation, an ATM switch can discard cells on a frame basis, using Early Packet Discard (EPD) and Partial Packet Discard (PPD) mechanisms. EPD discards an entire incoming packet after detecting that congestion is about to occur. If congestion goes undetected or the EPD action is not drastic enough to mitigate the congestive event, PPD discards all of the remaining cells (with the exception of the End-of-Packet cell) of a packet that has been partially discarded. EPD and PPD together can significantly reduce partial packet loss and improve overall packet throughput, especially for TCP/IP applications using UBR service. Even better TCP/IP performance can be achieved with the addition of per-vc discard. EPD/PPD schemes that discard packets indiscriminately may suffer a global synchronization problem. This phenomenon is when TCP/IP packets going through a congested switch are discarded at the same time. Related end systems react to packet loss in a synchronous fashion, causing congestion and reduced throughput. Staggering or randomizing EPD/PPD actions on VCs can avoid global synchronization. 12 ATM Traffic Management Technology Guide 13

9 Multiple Classes of Service and Priority Control Multiple, differentiated services in an ATM network provide the ability to optimize the trade-off between application requirements and service capabilities. At every cell opportunity, each output port in an ATM switch has to decide which cell in the buffer to transmit. To provide differentiated QoS levels, an ATM switch can establish multiple queues at an output port. Typically, a strict priority queuing discipline is adopted at an output port where cells of a higher traffic class get non-preemptive priority over cells of a lesser traffic class. Under a strict priority discipline, the lower-class traffic can suffer from a bandwidth starvation problem. If many higher-class connections transmit at their PCR simultaneously with the aggregate traffic load exceeding the available bandwidth of the output queue, the lower-class queues can not transmit any cells. This causes time-outs in the higherlayer applications, which must retransmit the entire packet, thereby worsening the congestion problem. To overcome the bandwidth starvation problem, each QoS class can be assigned a minimum guaranteed bandwidth, as illustrated in the example of Figure 4. Even if the cell tagging option is adopted at UPC/NPC, the excessive tagged traffic (CLP=1) at higher priority would not affect the QoS of the lowerpriority traffic. The minimum bandwidth guarantee required for each traffic class can be assigned based on a projected, long-term average load. Alternatively, the minimum bandwidth guarantee can be adjusted as connections are set up and torn down to meet the bandwidth demands of the connections in each class. Upper class Lower class Figure 4 Non-preemptive Priority Discipline with Minimum Bandwidth Guarantee The example of Figure 4 describes how the sum of minimum bandwidth guarantees of all service queues is less than the total available bandwidth. Provisioning a common pool of bandwidth can accommodate the difference between instant traffic demand and the projected long-term average traffic mix. Managing this common pool of bandwidth on a priority basis provides QoS differentiation among higher and lower traffic classes. Buffer Management CBR 20% VBR-rt (1) 5% VBR-rt (2) 5% VBR-nrt (1) 15% VBR-nrt (2) 15% ABR 20% UBR 10% When cells traveling to an output port exceed the port s transmission capacity, output port buffers provide temporary storage. Efficient management of output port buffers can maintain the appropriate QoS for each traffic type through prioritization and intelligent buffer allocation. Bandwidth and buffer resources must be made available on a per-connection basis at all times to guaranteed-qos service classes (CBR, VBR-nrt, and VBRrt). In contrast, for best-effort service classes (ABR and UBR), all the connections contend for a common pool of buffer and for the remaining bandwidth left by PRIORITY 14 ATM Traffic Management Technology Guide 15

10 guaranteed-qos service classes. Making sure these common resources are shared fairly is critical to the effectiveness of the best-effort service offering. The ways in which ATM switches handle besteffort traffic can be categorized into three queuemanagement schemes: FIFO queuing, FIFO queuing with per-vc accounting, and per-vc queuing. In the FIFO queuing scheme, cells in the same service category are handled on a first-come-first-serve basis. A problem with this scheme is that individual connections of the same class of service are not given fair treatment. FIFO queuing with per-vc accounting, on the other hand, detects congestion and discards cells on a per-vc basis. Per-VC queuing offers separate queues for each connection. By implementing either FIFO with per-vc accounting schemes or per-vc queuing schemes, efficient traffic isolation and fairness can be achieved. However, most current implementations fail to deliver efficiency of buffer utilization at the same time. Typically, a per-vc static discard threshold is used in these implementations to avoid buffer monopoly by a particular connection. The discard threshold is fixed, regardless of total buffer occupancy or the amount of service provided to the individual connection. Consequently, either fairness is achieved with poor buffer utilization or high buffer utilization is achieved through heavy overbooking, which causes unfair buffer sharing. Per-VC Buffering Limit Buffer cell Discard cell A per-vc adaptive discard scheme provides the optimal balance between fairness and buffer utilization. The discard decision of each cell is made by dynamically comparing the current total usage of the perclass-of-service shared buffer against a programmed per-connection buffering limit. Buffering limit Instantaneous number of cells buffered of this connection Impossible With an adaptive discard curve, the discard decision adapts to instantaneous buffer occupancy, and each VC gets a larger buffering limit when the buffer is only lightly utilized. As a result, a connection s instant buffering demand can be better met without compromising the fairness among connections. Route Management Buffer cell Instantaneous buffer occupancy Instantaneous decision point Discard cell Per-COS buffer occupancy Figure 6 Adaptive Discard Curve To route a connection through an ATM network, ATM switches maintain a detailed and frequently updated topology of the network, which includes information on connectivity, bandwidth, and congestion. Neighboring ATM switches communicate with each other using the ATM Forum PNNI protocol. Upon receipt of a call request, the routing algorithm supplies the end-to-end route through the network that best meets the requested QoS. Occupancy of the buffer shared by best-effort service classes Figure 5 Static Discard Threshold 16 ATM Traffic Management Technology Guide 17

11 Switch Architecture Switch architecture is vital to the success of traffic management. Most switch fabric architectures can not move cells from all input ports to a single output port simultaneously. Consequently, excessive cells have to be buffered at the input or inside the switch fabric. Such architectures suffer either Head of Line (HOL) blocking at the input port, as shown in Figure 7, or extensive flow-control overhead if flow-control feedback from output port is used, as shown in Figure 8. At each queuing point introduced, whether it is at the input or inside the switch fabric, the potential for congestion escalation is introduced. A better way to solve the problem is to eliminate all unnecessary points of contention. Reducing contention points also reduces the total amount of buffer required, resulting in a higher performance-tocost ratio. Such architectures utilize a contentionless switch fabric and pure output buffering. A singlebuffer-point switch architecture minimizes delay and cell loss by avoiding bottlenecks in the switch fabric. Blocked because HOL cell lost contention even if no cell in other I/P queue is destined for the same O/P queue HOL cell wins the I/P queue contention Blocking Switch Fabric I/P queues O/P queues Figure 7 HOL Blocking for Input Buffering Switch At I/P port, separate queue is provisioned for different O/P port Feedback flow control I/P queues Blocking Switch Fabric O/P queues Figure 8 Input Buffering Switch with Back Pressure 18 ATM Traffic Management Technology Guide 19

12 Conclusions Acronyms The promise of ATM includes greater bandwidth and switching efficiency through statistical multiplexing of all traffic types in a shared network. One of ATM's main challenges, however, is to achieve statistical gain while maintaining guaranteed service levels. High gain and high service become competing objectives. The key to accomplishing these two competing objectives, and to delivering on the promise of ATM, is advanced traffic management. On one hand, advanced traffic management supports the most efficient use of bandwidth and switching resources. On the other, advanced traffic management assures clearly differentiated services for different classes of traffic with fairness among users within each class. Vital to the success of ATM service provisioning, advanced traffic management requires a switch design with the following features: Successful management of contention Adequate buffering (tens of thousands of cells per output port) shared across multiple output ports and across all service classes Adaptive discard thresholds that respond to changes in the traffic mix Per-VC discard threshold adjustment to provide fairness and isolation Many queues to provide differentiation Frame discard mechanisms such as EPD and PPD Traffic shaping to avoid contagious congestion ABR ATM CAC CBR CDV CDVT CLP CLR EPD GCRA HOL maxctd MBS PCR PPD PVC SCR SVC UBR UPC VBR-nrt VBR-rt VC WAN Available Bit Rate Asynchronous Transfer Mode Connection Admission Control Constant Bit Rate Cell Delay Variation Cell Delay Variation Tolerance Cell Loss Priority Cell Loss Ratio Early Packet Discard Generic Cell Rate Algorithm Head of Line maximum Cell Transfer Delay Maximum Burst Size Peak Cell Rate Partial Packet Discard Permanent Virtual Circuit Sustainable Cell Rate Switched Virtual Circuit Unspecified Bit Rate Usage Parameter Control (also called Policing) Non-Real-Time VBR Real-Time Variable Bit Rate Virtual Circuit Wide Area Network 20 ATM Traffic Management Acronyms 21

13 NOTES NOTES 22 ATM Traffic Management Notes 23

14 Visit ATG s Web Site to read, download, and print all the Technology Guides in this series. N.E.T. is a leading worldwide supplier of multiservice wide area networks (WANs) that integrate voice, data, image, and video traffic. N.E.T. multiservice networks offer ATM, frame relay, LAN internetworking, ISDN switching, and advanced voice-compression capabilities to create highly reliable, cost-effective solutions that simplify network operations, control, and management. N.E.T. has introduced a family of ATM products to enable carriers, network service providers, Internet service providers, and large enterprises to expand service offerings as a result of new levels of availability combined with the benefits of multiservice networking in the ATM environment. Join us at our Web site for more information. The significant problems we face cannot be solved by the same level of thinking that created them. Albert Einstein The N.E.T. logo is a registered trademark of Network Equipment Technologies, Inc.

15 This Technology Guide is one of a series of guides, published by ATG, designed to put complex communications and networking technology concepts into practical and understandable terms. Each guide provides objective, non-biased information to assist in the internal education, evaluation and decision making process. This Technology Guide, as well as the other Communications and Networking Technology Guides in the series, are available on ATG s Web Site. Produced and Published by One Apple Hill, Suite 216, Natick, MA Tel: (508) Fax: (508) info@techguide.com