Introduction Overview The module presents a thorough overview of quality of service models and mechanisms as implemented in complex service provider and enterprise networks. It includes the following topics: n Introduction to IP Quality of Service n Integrated Services Model n Differentiated Services Model n Building Blocks of IP QoS Mechanisms n Enterprise Network Case Study n Service Provider Case Study Objectives Upon completion of this module, you will be able to perform the following tasks: n Describe the need for IP QoS n Describe the Integrated Services model n Describe the Differentiated Services model n Describe the building blocks of IP QoS mechanisms (classification, marking, metering, policing, shaping, dropping, forwarding, queuing) n List the IP QoS mechanisms available in the Cisco IOS n Describe what QoS features are supported by different IP QoS mechanisms 2 IP QoS Introduction Copyright 2001, Cisco Systems, Inc. Introduction to IP Quality of Service Objectives Upon completion of this lesson, you will be able to perform the following tasks: n Describe different types of applications and services that have special resource requirements n List the network components that affect the throughput, delay and jitter in IP networks n List the benefits of deploying QoS mechanisms in IP networks n List QoS mechanisms available in Cisco IOS n Describe typical enterprise and service provider networks and their QoS-related requirements Copyright 2001, Cisco Systems, Inc. IP QoS Introduction 3 © 2001, Cisco Systems, Inc. IP QoS Introduction-5 Why IP QoS? Why IP QoS? • Application X is slow! • Video broadcast occasionally stalls! • Phone calls over IP are no better than over satellite! • Phone calls have really bad voice quality! • ATM (the money-dispensing-type) are non- responsive! • The purpose of this module is to determine the following: n What is, or might be, missing in today’s IP networks? n What can IP Quality of Service (QoS) do to help solve the problem? A decade ago when the Internet was still in its early stages there was not much available. Most users were using Gopher to find information and FTP to retrieve it. The Internet was something new and exciting and no one was really bothered by the fact that it was slow. Today, however, the Internet is serving a large population of all walks of life. The Internet has also grown in its service offering. Users are using the Internet to view static or dynamic information, transmit voice and video, shop, play etc. Along with these new applications of the Internet come some demands on the service(s) it provides: n Some applications are slow n Video broadcast or conferencing may have bad picture quality or appear jerky n Voice sessions may have bad voice quality or periods of silence n Critical transactions may take too long (too many seconds) n Bulk transfers take too long (too many hours) This module focuses on most common quality-related problems people encounter in IP networks. 4 IP QoS Introduction Copyright 2001, Cisco Systems, Inc. © 2001, Cisco Systems, Inc. IP QoS Introduction-6 Because Because • Application X is slow! (not enough BANDWIDTH) • Video broadcast occasionally stalls! (DELAY temporarily increases – JITTER) • Phone calls over IP are no better than over satellite! (too much DELAY) • Phone calls have really bad voice quality! (too many phone calls – ADMISSION CONTROL) • ATM (the money-dispensing-type) are non responsive! (too many DROPs) • Quality of Service is usually identified by the following parameters: n Amount of bandwidth available to a certain application or user n Average delay experienced by IP packets on end-to-end or link basis n Jitter that affects applications that transmit packets at a certain fixed rate and expect to receive them at approximately the same rate (for example, voice and video) n Drops of packets when a link is congested can severely impact fragile applications n Admission control which prevents too many sessions from congesting links and causing degradation in quality of service (for example, voice sessions) Copyright 2001, Cisco Systems, Inc. IP QoS Introduction 5 © 2001, Cisco Systems, Inc. IP QoS Introduction-7 What Causes What Causes • Lack of bandwidth – multiple flows are contesting for a limited amount of bandwidth • Too much delay – packets have to traverse many network devices and links that add up to the overall delay • Variable delay – sometimes there is a lot of other traffic which results in more delay • Drops – packets have to be dropped when a link is congested If the network is empty any application should get enough bandwidth, acceptable low and fixed delay and not experience any drops. The reality, however, is that there are multiple users or applications using the network at the same time. 6 IP QoS Introduction Copyright 2001, Cisco Systems, Inc. © 2001, Cisco Systems, Inc. IP QoS Introduction-8 Available Bandwidth Available Bandwidth • Maximum available bandwidth equals the bandwidth of the weakest link • Multiple flows are contesting for the same bandwidth resulting in much less bandwidth being available to one single application. IP IP IP IP 10 Mbps 256 kbps 512 kbps 100 Mbps BW max = min(10M, 256k, 512k, 100M)=256kbps BW avail = BW max /Flows The example above illustrates an empty network with four hops between a server and a client. Each hop is using different media with a different bandwidth. The maximum available bandwidth is equal to the bandwidth of the slowest link. The calculation of the available bandwidth, however, is much more complex in cases where there are multiple flows traversing the network. The calculation of the available bandwidth in the illustration is a rough approximation. Copyright 2001, Cisco Systems, Inc. IP QoS Introduction 7 © 2001, Cisco Systems, Inc. IP QoS Introduction-9 End-to-end Delay End-to-end Delay • End-to-end delay equals a sum of all propagation, processing and queuing delays in the path • Propagation delay is fixed, processing and queuing delays are unpredictable in best-effort networks IP Propagation delay (P1) Processing and queuing delay (Q1) IP IP IP Propagation delay (P2) Processing and queuing delay (Q2) Propagation delay (P3) Processing and queuing delay (Q3) Delay = P1 + Q1 + P2 + Q2 + P3 + Q3 + P4 = X ms Propagation delay (P4) The figure illustrates the impact a network has on the end-to-end delay of packets going from one end to the other. Each hop in the network adds to the overall delay because of the following two factors: 1. Propagation (serialization) delay of the media that, for the most part, depends solely on the bandwidth. 2. Processing and queuing delays within a router, which can be caused by a wide variety of conditions. Ping (ICMP echoes and replies) can be used to measure the round-trip time of IP packets in a network. There are other tools available to periodically measure responsiveness of a network. 8 IP QoS Introduction Copyright 2001, Cisco Systems, Inc. © 2001, Cisco Systems, Inc. IP QoS Introduction-10 Processing and Queuing Delay Processing and Queuing Delay • Processing Delay is the time it takes for a router to take the packet from an input interface and put it into the output queue of the output interface. • Queuing Delay is the time a packets resides in the output queue of a router. • Propagation or Serialization Delay is the time it takes to transmit a packet. IP IPIPIP Forwarding Processing Delay Queuing Delay Propagation Delay bandwidth n Processing Delay is the time it takes for a router to take the packet from an input interface and put it into the output queue of the output interface. The processing delay depends on various factors, such as: – CPU speed – CPU utilization – IP switching mode – Router architecture – Configured features on both input and output interface n Queuing Delay is the time a packet resides in the output queue of a router. It depends on the number and sizes of packets already in the queue and on the bandwidth of the interface. It also depends on the queuing mechanism. n Propagation or Serialization Delay is the time it takes to transmit a packet. It usually only depends on the bandwidth of the interface. CSMA/CD media may add slightly more delay due to the increased probability of collisions when an interface is nearing congestion. Copyright 2001, Cisco Systems, Inc. IP QoS Introduction 9 © 2001, Cisco Systems, Inc. IP QoS Introduction-11 Packet Loss Packet Loss • Tail-drops occur when the output queue is full. These are the most common drops which happen when a link is congested. • There are also many other types of drops that are not as common and may require a hardware upgrade (input drop, ignore, overrun, no buffer, ). These drops are usually a result of router congestion. IP Forwarding IPIPIPIP Tail-drop The usual packet loss occurs when routers run out of buffer space for a particular interface (output queue). The figure illustrates a full output queue of an interface, which causes newly arriving packets to be dropped. The term used for such drops is simply “output drop” or “tail-drop” (packets are dropped at the tail of the queue). Routers might also drop packets for other (less common) reasons, for example: n Input queue drop - main CPU is congested and cannot process packets (the input queue is full) n Ignore - router ran out of buffer space n Overrun - CPU is congested and cannot assign a free buffer to the new packet n Frame errors (CRC, runt, giant)—hardware-detected error in a frame 10 IP QoS Introduction Copyright 2001, Cisco Systems, Inc. © 2001, Cisco Systems, Inc. IP QoS Introduction-12 How to Increase Available Bandwidth? How to Increase Available Bandwidth? • Upgrade the link. The best solution but also the most expensive. FIFO queuing IP TCP data Fancy queuing • Take some bandwidth from less important applications. Compress the Headers cTCP data • Compress the header of IP packets. Compress the Payload Compressed packet • Compress the payload of layer-2 frames. Priority Queuing (PQ) Custom Queuing (CQ) Modified Deficit Round Robin (MDRR) Class-based Weighted Fair Queing (CB-WFQ) Stacker Predictor TCP Header Compression RTP Header Compression There are several approaches to solving a problem of insufficient bandwidth: n The best approach is to increase the link capacity to accommodate all applications and users with some extra bandwidth to spare. This solution sounds simple enough but in the real world it brings a high cost in terms of the money and time it takes to implement. Very often there are also technological limitations to upgrading to a higher bandwidth. n Another option is to classify traffic into QoS classes and prioritize it according to importance (business-critical traffic should get enough bandwidth, voice should get enough bandwidth and prioritized forwarding and the least important traffic should get the remaining bandwidth). There are a wide variety of mechanisms available in the Cisco IOS that provide bandwidth guarantees, for example: – Priority or Custom Queuing – Modified Deficit Round Robin (on Cisco 12000 series routers) – Distributed ToS-based and QoS-group-based Weighted Fair Queuing (on Cisco 7x00 series routers) – Class-based Weighted Fair Queuing n Optimizing link usage by compressing the payload of frames (virtually) increases the link bandwidth. Compression, on the other hand, also increases delay due to complexity of compression algorithms. Using hardware compression can accelerate the compression of packet payloads. Stacker and Predictor are two compression algorithms available in Cisco IOS. n Another link efficiency mechanism is header compression. This mechanism is especially effective in networks where most packets carry small amounts of [...]... application areas 28 IP QoS Introduction Copyright © 2001, Cisco Systems, Inc IntServ Support in IOS • RSVP and Weighted Fair Queuing supported since ’95 • RSVP signaling for VoIP calls supported on all VoIP platforms • IOS supports hop-by-hop and pass-through RSVP • RSVP-to-DSCP (DiffServ Code Point) mapping (RSVP proxy) in 12.1T © 2001, Cisco Systems, Inc IP QoS Introduction-29 Both RSVP and WFQ... Copyright © 2001, Cisco Systems, Inc IP QoS Introduction 17 How can QoS be Applied? • Best effort – no QoS is applied to packets (default behavior) • Integrated Services model – applications signal to the network that they require special QoS • Differentiated Services model – the network recognizes classes that requires special QoS © 2001, Cisco Systems, Inc IP QoS Introduction-17 By investigating the history... Queuing (PQ) Custom Queuing (CQ) Strict Priority MDRR IP RTP prioritization Class-based Low-latency Queuing (CB-LLQ) • Upgrade the link The best solution but also the most expensive • Forward the important packets first • Compress the payload of layer-2 frames (it takes time) • Compress the header of IP packets © 2001, Cisco Systems, Inc IP QoS Introduction-13 Assuming that a router is powerful enough to... Decouple service and application in use • No application modification • No hop-by-hop signaling • Interoperability with non-DS-compliant nodes • Incremental deployment © 2001, Cisco Systems, Inc IP QoS Introduction-37 The DiffServ model describes services and allows for more user-defined services to be used in a DiffServ-enabled network Services are provided to classes A class can be identified as... packets to select a per-hop behavior • Per-hop Behavior (PHB) is realized using a particular QoS mechanism • Provisioning is used to allocate resources to traffic classes © 2001, Cisco Systems, Inc IP QoS Introduction-38 A traffic aggregate is a collection of all flows that require the same service A service is implemented using different QoS mechanisms (a QoS mechanism implements a per-hop behavior) The... resources for their VoIP sessions There are an increasing number of applications that use RSVP to request QoS guarantees from a network Copyright © 2001, Cisco Systems, Inc IP QoS Introduction 25 IntServ Implementation Options RSVP 1) Explicit RSVP on each network node Class of Service or Best Effort 2) RSVP ‘pass -through’ and CoS transport - map RSVP to CoS at network edge - pass -through RSVP request... to CoS at network edge - pass -through RSVP request to egress 3) RSVP at network edges and ‘pass -through’ with - best-effort forwarding in the core (if there is enough bandwidth in the core) © 2001, Cisco Systems, Inc IP QoS Introduction-26 The figure illustrates three options available when implementing QoS mechanisms via RSVP in a network 1 The first option is to simply enable RSVP on all interfaces... Transport IntServ End-to-End RSVP All Routers • WFQ applied per flow based on RSVP requests © 2001, Cisco Systems, Inc IP QoS Introduction-27 In the first scenario, each router in the network processes RSVP messages and keeps track of the special resource needs for each individual RSVP flow Weighted Fair Queuing (WFQ) can be used in the backbone to provide resource allocation on a flow-by-flow basis One... delay-sensitive packets by forwarding them ahead of other packets There are a wide variety of queuing mechanisms available in Cisco IOS that have pre-emptive queuing capabilities, for example: – – Strict-priority or Alternate Priority queuing within the Modified Deficit Round Robin (on Cisco 12000 series routers) – IP RTP prioritization – IP QoS Introduction Custom Queuing – 12 Priority Queuing Class-based... low-end platforms and on high-end platforms that are typically used to concentrate customer networks Newer RSVP mechanisms include: n Mapping of RSVP to DSCP (the Differentiated Services model with the details of the DiffServ Code point is covered in the next lesson) n Mapping of RSVP to ATM SVCs (this technology is covered in the IP QoS IP over ATM” module) Copyright © 2001, Cisco Systems, Inc IP QoS . networks and their QoS- related requirements Copyright 2001, Cisco Systems, Inc. IP QoS Introduction 3 © 2001, Cisco Systems, Inc. IP QoS Introduction-5 Why IP QoS? Why IP QoS? • Application. Systems, Inc. IP QoS Introduction 7 © 2001, Cisco Systems, Inc. IP QoS Introduction-9 End-to-end Delay End-to-end Delay • End-to-end delay equals a sum of all propagation, processing and queuing. 18 IP QoS Introduction Copyright 2001, Cisco Systems, Inc. © 2001, Cisco Systems, Inc. IP QoS Introduction-17 How can QoS be Applied? How can QoS be Applied? • Best effort – no QoS is