| ID Number: DPRO-101188 |
Quality of Service Over IP Networks
20 November 2003 Kathleen M. Adams Kiran Bhalla Today's best-effort IP is good enough for non-real-time applications, such as e-mail, where packet loss is tolerable. However, enterprises require more predictable levels of service as IP gains ubiquity.
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Table of Contents List of Tables Table 1: Key QOS Parameters Table 2: Framework for Providing QOS Table 3: Queuing and Scheduling Algorithms Table 4: SLA Framework Table 5: Selected IP Services and SLAs

| Analysis
[Return to Table of Contents] Principles of Internet Protocol Internet Protocol (IP) has found increased ubiquity within the enterprise, thanks to the proliferation of the Internet and its applications, such as e-mail and the World Wide Web. Carriers have built their next-generation networks based on IP as they have migrated from legacy circuit-switched networking technology. IP is based on packet-switching technology where data messages are broken up into small packets, each wrapped in an electronic envelope that is labeled with its destination address. IP assumes little intelligence in the network, as packets are sent across a network of routers without the help of sending and receiving devices until they reach their final destination. As a result, an IP network is inherently dumb and behaves in a similar manner as a letter passing through several mail and sorting offices en route to its destination. IP is the transport vehicle used, and the routers direct the packages to the destination. Packages from the same source can arrive at the destination by traversing different routes. IP was initially conceived in the 1960s to help the U.S. military to communicate and withstand a nuclear attack. The terrorist attacks of September 2001 demonstrated the resiliency of IP. Crucial voice switches and network access points in and around the World Trade Center in New York City were destroyed, knocking out all voice services in the area. However, the Internet continued to function and provided one of the only means of communication following the attack. IP is a best-effort delivery mechanism. When the network becomes congested with a greater traffic load than the available bandwidth, routers discard IP packets they cannot process. IP was designed to support data applications that have less stringent requirements for bandwidth, jitter and delay. This simplicity is the ultimate reason IP (if unaltered) is unable to support a consistent level of service, particularly for time-sensitive applications. The increased proliferation of the Internet and its empowerment to the desktop has led to a new generation of enterprise applications that have been designed to run on an IP network to take advantage of its more efficient use of bandwidth. However, these enterprise applications are time-sensitive, typically demand more bandwidth and are less tolerant to delay. This represents the first set of challenges, since IP by its very nature has not been designed around providing a consistent level of service. IP is unique in that it is completely independent of infrastructure technology. IP can therefore function as a common denominator for infrastructure that will always be diversified and which can also handle any operating system. Benefits of IP IT network managers have been attracted to IP due to the potential of migrating all data and voice traffic onto a unified network. IP enables the separation of the network layer and the upper layers in such a way that any application can be supported by all infrastructures regardless of the network infrastructure, the local-area network or the metropolitan network. This enormous structure is controlled effectively on the basis of the interoperability principle, ensured by IP, which is resident in all Internet routers. This has had the potential of lowering the total cost of ownership by simplifying management and lowering operating costs by replacing the role of wide-area networks (WANs) using dedicated leased circuits. IP works well with non-real-time applications, such as e-mail and Web browsing, where jitter (service delay) and the occasional packet loss are tolerable to the application. However, the inconsistency in performance conferred by IP is inadequate to support real-time applications, such as enterprise resource planning (ERP) and videoconferencing. These applications require high levels of bandwidth with low latency (or where there is little tolerance for jitter and packet loss). Interactive real-time applications, such as IP telephony, place considerable demand on a "unified" IP network. The Challenge of IP The challenge for engineers and telecom operators has been to take advantage of the potentially lower cost that IP offers while delivering quality of service (QOS) for which a premium can be levied. This QOS can be defined as a series of measurements that ensures a consistent level of performance based on criteria such as availability, jitter, latency, packet loss and throughput. Detailed definitions of these QOS parameters are provided below. [Return to List of Tables]
Table 1
Key QOS Parameters
Configuration | Description
--- | ---
Source: Gartner.
Availability | The reliability of the connection to a network such as the Internet.
Latency | The duration between a node sending data and its receipt by an incoming node. In routers, latency can also be viewed as the duration interval as data packets are received and retransmitted. Delay can occur, depending on the transmission path and any potential congestion routes along the way.
Jitter | As the Internet is provided on a "best-effort" basis, data packets may not arrive in the same sequence due to latency. This largely affects time-sensitive applications, such as videoconferencing, which may appear shaky. Often, jitter may occur because of poor network design.
Packet Loss | As data packets traverse across the network and they encounter latency and jitter, routers or nodes may drop packets as they encounter congestion.
Throughput | Measures the average or peak rate at which IP packets traverse through a network.
The main stumbling block to QOS is that IP is a connectionless technology that cannot alone guarantee bandwidth. It should be noted that QOS itself does not increase bandwidth, but aims to manage this more effectively to ensure consistent levels of service for a wide range of applications. There are several ways that QOS can be provisioned, and a framework is provided below. [Return to List of Tables]
Table 2
Framework for Providing QOS
Layer | Transmission Control Protocol (TCP)/IP Function | Mechanisms
--- | --- | ---
Source: Gartner.
Network (Internet) Layer | Handles the movement of packets through the network and manages the routing of the packets from node to node. | Multiprotocol Label Switching (MPLS) to bring connection-like characteristics in a connectionless environment
Transport Layer | The software segments the data message to be transmitted into small packets and adds address labels. There are two transport protocols, User Datagram Protocol (UDP) and TCP. TCP provides the added function of verifying the correct and complete delivery of data. It detects lost data and triggers retransmission. | Differentiated Services (DiffServ) to classify and prioritize important traffic, such as mission-critical applications
Application Layer | Manages the details related to specific applications. The application program elects the kind of transport needed and passes it to the transport level. | Uniform Resource Locater (URL) forwarding, load serving, mirroring
[Return to Table of Contents] Network managers have argued that by facilitating sufficient bandwidth, applications should receive the resources they require, minimizing delays and queues. Sufficient high-capacity links can be provided by Gigabit Ethernet or even Asynchronous Transfer Mode (ATM). On its own rights, ATM supports QOS through its support of class of service, running at 622 Mbps. However, such assumptions may be too simplistic, since data traffic on an enterprise basis is growing exponentially, and applications such as videoconferencing are inherently greedy and swallow up any "excess" bandwidth. While sufficient bandwidth can be provided within an enterprise, the same does not hold true outside it. Data traffic is inherently difficult to map, since it is largely unpredictable — even in a modern mixed network of data and voice where bottlenecks and congestion could build up. Overprovisioning across an entire network becomes cost-prohibitive and an inefficient use of resources. Operators may be driven to increase capital expenditure to add more bandwidth. However, they may not necessarily see the same growth in their service revenue. Throwing bandwidth at the problem does not solve the problem of prioritizing certain traffic applications over others, since all traffic is treated the same. [Return to Table of Contents] Other than throwing bandwidth at the problem, an alternative technique is traffic engineering, where data traffic can be differentiated and served appropriately to provide consistent levels of service. There are two models that allow traffic engineering: Resource reservation-based — Resources for traffic are reserved specifically to an application's request before the data is transmitted. A router provides differentiated services based on the QOS request. Prioritization-based — Network traffic is classified into a set of classes. Network nodes provide preferential treatment to those classes deemed as a priority. Resource Reservation Protocol RSVP is a reservation-based model that aims to reserve resources such as bandwidth and buffer space so that applications can secure their needs. It emulates a circuit connection in IP's connectionless environment and is the most complex of all QOS techniques. The protocol relies on end devices to request their priority so that resources can be reserved for flows to facilitate the requested QOS. A videoconferencing application can request the desired bandwidth, latency and jitter that it requires. Each router along the path to the end device attempts to honor the RSVP request by maintaining connection state information. If this cannot be honored, service is denied and packets are dropped. However, RSVP has had immense problems with scalability as it can lead to degradation of services as applications (end devices) request their own priority. The protocol requires forwarding decisions to be made about routing at each intermediate node along the RSVP path. These decisions require RSVP-controlled packets to carry the "abstract handle," or session ID, for the TCP/IP session to which they belong. Each intermediate router has to maintain a dynamically updated table containing abstract handles and reservation information set up by RSVP exchanges. When a router receives a packet belonging to an RSVP session, it looks up the packet's abstract handle in its local table to know what to do with the packet. This adds significant complexity to the network in its implementation. As such, RSVP is just a signaling protocol, as it is not concerned with how routers implement the reservation request either with traffic classification or scheduling. RSVP is geared toward supporting real-time application, such as voice and videoconferencing. The protocol is very good at securing the resources necessary to transmit traffic and can also be mixed with other protocols, such as DiffServ and MPLS. While RSVP is implemented today on some switches and routers, it is considered more of an enterprise protocol, since its complexity can impair performance on backbone networks. RSVP requires all routers to support it, and this can prove challenging on the public Internet. DiffServ DiffServ is a prioritization-based Layer-3 protocol that addresses QOS requirements in the connectionless environment that IP provides. It aims to divide traffic into classes and treat the classes differently, especially when there is a shortage of resources, such as bandwidth, because it prioritizes important traffic at the expense of lower-priority traffic. The protocol works by rewriting the type of service (TOS) field found in the header of Internet Protocol version 4 (IPv4) packets and Internet Protocol version 6 (IPv6) octets. The protocol renames it the DiffServ (DS) byte to define class-of-service parameters. The protocol by itself does not guarantee QOS, but the priority scheme translates into higher throughput for high-priority classes by classifying traffic into aggregate traffic flows to receive better or worse treatment relative to each other. Since the TOS field is a standard element of IP headers, the protocol can easily be implemented, since it operates on the Layer 3 of the Open Systems Interconnection (OSI) model and can be run without modification on Layer 2 infrastructure that supports IP. DiffServ can be largely implemented on established equipment that supports firmware or software upgrades. Both DiffServ routers and non-DiffServ routers can be mixed in the same environment. DiffServ allows traffic to be treated in a certain manner, allowing forwarding decisions determined in the DS byte and manifested in the per-hop behavior (PHB). The default setup in DiffServ is best-effort, using first in, first out methodology. However, the Default Behavior (DB) setup does not distinguish between high-priority and low-priority packets, as all packets are treated equally. There are numerous PHBs; however, the Internet Engineering Task Force (IETF) has defined only two PHBs for standardization: Expedited Forwarding (EF) attempts to handle traffic in short queues to minimize delay, latency and packet loss. EF PHB is defined by the IETF as the DiffServ's "aggregate maximum arrival rate is less than the aggregate's minimum departure rate." EF ensures an almost point-to-point connection or a virtual leased-line service. Assured Forwarding (AF) PHB attempts to ensure that IP packets arrive in sequence. AF works by defining four classes of traffic receiving bandwidth and buffer space. In case of congestion, drop precedence will determine the relative merit of the packet being dropped. The protocol attempts to protect packets with a lower precedence value in favor of packets with a higher precedence value. DiffServ relies on traffic conditioners residing at the network edge to indicate each packet's requirements — using primitives or priority sets. These primitives and policy sets contain information about the DS byte that relates to the classification, policing, marking and shaping. As a result, DiffServ comes with a performance cost. Policy primitives allow the traffic conditioners to monitor the behavior of incoming packets and classify them into aggregates or drop them or re-mark them in favor of different PHBs. Traffic is classed into queues to match the PHB desired. Detailed definitions of these queues are highlighted in the Table "Queuing and Scheduling Algorithms." [Return to List of Tables]
Table 3
Queuing and Scheduling Algorithms
Configuration | Description
--- | ---
Source: Gartner.
First In, First Out (FIFO) | As the term suggests, packets arriving first are dealt with in a simple-queue first come, first-served basis. However, higher-priority packets are given no preferential treatment and are queued in the order that they are received. FIFO is often the default setup in networks configured for DiffServ.
Strict Priority Scheduling | This allows the class of traffic to pass only if there are no more priority packets queued. However, this could mean that a class could be denied and starved of bandwidth.
Fair Queuing Round Robin | A simple round-robin scheme. However, with fair queuing, large packet streams that exceed a certain time frame will often be demoted to the back of the queue. The scheme does not provision extra bandwidth for large-packet streams.
Weighted Fair Queuing (WFQ) | Packet classes are given a weight that determines how much bandwidth can be used for the class.
Class-Based Queuing (CBQ) | Utilizes several queues corresponding to different classes of traffic. The queue can be used to service traffic in multiple ways.
Hierarchical CBQ | Packets are broken down in a hierarchical tree, allowing data packets to borrow bandwidth from sister branches when capacity is used up.
Marking primitives allows traffic conditioners to specify the PHB by marking the DS field at each packet. Shaping primitives allow the traffic conditioners to enforce conformance to pre-configured traffic properties by smoothing burst transmissions to avoid congestion. Each class is allocated a data rate, committed-excess burst rate and a maximum-burst rate. Packets may be discarded if they exceed the buffer size. By using different classification, policing, shaping and scheduling rules, several classes of services can be provided. Despite the relative progress of IETF's DiffServ working group, the protocol is relatively immature and is several years away from being widely available. The working group is looking to resolve issues over security, especially theft-of-service and denial-of-service attacks that have gained increasing prominence given the terrorist attacks of September 2001. [Return to Table of Contents] MPLS MPLS is entrenched in the IP switching and tag switching originally developed by Cisco in the mid-1990s to bring circuit switch concepts to IP's connectionless routing environment. In conventional networks, a carrier's Interior Gateway Protocol (IGP) determines a packet route. This routing protocol, operating in every router, calculates the shortest distance across the network based on the lowest hop count. Every router along a path must examine every packet header and look up the 32-bit IP address to determine where to send the packet until it arrives at its ultimate destination. This process is invariably inefficient, as each router must maintain forwarding tables and process each packet that comes its way. The service is unpredictable because routers may only choose the shortest route, which may not necessarily be the fastest route. MPLS does not itself invoke QOS. It is more of an enabling technology, since this is not the main purpose of the protocol — only three bits are reserved for specifying QOS. The protocol works at the edge of the network with routers known as label switch routers (LSRs) that assign a 20-bit label to the header of IP packets. Each path on the network is assigned a label switched path (LSP) that corresponds to an explicit path across the network, allowing packets to traverse a pre-established path that may be more efficient. Every possible label can be stored in a simple table, allowing fast direct lookups, helping to reduce router overhead and increasing processing speed. MPLS helps to eliminate latency caused by traffic, as it encounters multiple routers throughout the network where message headers are read and re-encrypted before forwarding decisions are made. Without labels, processor-intensive algorithms must be used to reduce address table size, because there are more than four billion possible addresses for IP. MPLS introduces connection-like behavior in a connectionless environment that greatly helps to reduce operating costs, automate traffic engineering and provide IP network QOS. This helps provide more deterministic, predictable levels of service that can be used in conjunction with offers of tiered levels of service. The protocol behaves in a similar way to ATM, although it is frame-based rather than cell-based. MPLS is largely suited to the backbone networks, where traffic can be aggregated. The protocol does not scale well in an enterprise network, but since it functions on Layer 2 of the OSI model, it is mutually exclusive and can be used in conjunction with other protocols, such as DiffServ and RSVP, to offer tiered levels of service. Since MPLS is platform-neutral, it can be mapped directly on any Layer 2 connection, such as ATM, frame relay circuit, Synchronous Digital Hierarchy (SDH)/Synchronous Optical Network (SONET) and an optical wavelength. In this way, labels simplify the integration of cell switches and routers by providing a set of addressing, routing and management procedures common to both Layer 2 and Layer 3 equipment. For example, MPLS can be mapped on an ATM virtual circuit on a Virtual Path Identifier (VPI)/Virtual Circuit Identifier (VCI) field on an ATM cell header leveraging ATM's strong ability to support class of service and QOS, helping ISPs optimize their investments in ATM switches, IP routers and other internetworking equipment. Due to its architecture, MPLS scales better to backbone networks rather than an enterprise network. MPLS, like DiffServ, has made tremendous progress in standardization, but there are a number of issues that are still in draft stages. There are several dozen MPLS standards and drafts undergoing further work. However, the MPLS forum has made progress working on MPLS's use in the backbone to support virtual private networks (VPNs). A more recent development of an MPLS user-to-network interface (UNI) allows for the first-time traffic management tools to control not only the core network but also the access network where bottlenecks are more likely to develop. [Return to Table of Contents] Businesses that require QOS should obtain stringent service-level agreements (SLAs) from their providers that detail bandwidth provision and include measures for due diligence in case service conditions are not met. It is therefore critical that QOS measurements are regularly monitored and measured. Nevertheless, customers are still responsible for deciding how their applications will share the bandwidth. While QOS protocols, such as RSVP, DiffServ and MPLS, can be independently configured to deliver some form of QOS, it does inevitably mean that some applications will receive better network services than others. If applications are allowed to decide how much bandwidth they should receive, there is an incentive to steal. Hence, IT managers must adopt policing to enforce the rules. This can be done with policy software that allows IT managers to define QOS policies for devices, interfaces and other properties across a network that itself lacks intelligence. Policy management can control the behavior of various agents within a network to provide the access rights of multiple users to resources. By using a central directory, such as Microsoft's and Cisco's Directory Enabled Networks, policy management can keep track of the class to which a sender and recipient belong. While it is crucial to design a robust network with careful network planning, a central management system helps to enforce these rules, reduce administrative costs and monitor whether SLAs are being met. [Return to Table of Contents] There has been a surge in the number of operators offering Internet and IP services in Europe and North America as a result of deregulation. The dot-com era provided a catalyst for the telecom industry, as investors poured billions of dollars into next-generation carriers. Each new carrier employed the latest dense wave division multiplexer (DWDM) kit to exponentially increase capacity. Each new network added more capacity in aggregate than all previous networks combined. With so much surplus capacity, a price war led to a collapse in operating margins, forcing many into bankruptcy. Surviving carriers have attempted to differentiate their offerings away from basic transport services to offer more value-added services. Overcoming IP's variability and unpredictability has been crucial as a precursor to offering valued-added services. By offering IP QOS, carriers can facilitate differentiated levels of service with tiered pricing levels, reflecting the premium of a guaranteed service level. As a result, a number of carriers have implemented MPLS in their backbone networks and DiffServ at the edge due to their relative scaling, although MCI has implemented DiffServ in access networks on its Advantage converged networking. Carriers have been using ATM and frame relay in conjunction with IP, which facilitates class of service and prioritization, for a number of years. They support legacy traffic, such as voice, that IP cannot fully provision at this stage. However, ATM and frame relay are relatively more expensive to implement, particularly within an enterprise WAN. [Return to List of Tables]
Table 4
SLA Framework
Phase | Service-Level Prerequisite
--- | ---
Source: Gartner.
Phase 1 | • Within confines of carriers' own networks
Phase 2 | • Between autonomous interworking networks • Sophisticated billing for recharge for network packets/prioritization and different levels of service
Phase 3 | • True end-to-end service-level guarantees • Network Prioritization Management in place for application-layer QOS
At present, carriers are offering SLAs within the confines of their own networks (Phase 1). Very few carriers have engaged in offering SLAs across multiple autonomous interworking networks. However, carriers have begun to offer more stringent SLAs for managed services where the carrier also takes responsibility for customer premises equipment (CPE). Carriers will need to make the transition to accommodate sophisticated billing to differentiate between the service levels offered for network interconnection. [Return to List of Tables]
Table 5
Selected IP Services and SLAs
IP Services | Typical IP Solutions | SLAs | Features
--- | --- | --- | ---
AT&T | • AT&T Network-Based IP-VPNs offering class of service (COS) transport • AT&T Enhanced Virtual Private Network (EVPN) with voice over IP (VoIP) • AT&T Managed Internet Service (including support of VoIP) | Guarantees on-site availability, backbone availability, backbone latency, backbone reliability, and on-time service restoration. Site latency and site reliability SLAs are available on Private IP networks. | Four COS standards offered with voice given premium COS 1.
BT | IP network access services through to managed solutions, equipment supply and support services | SLAs include mean and committed data transfer rates, as BT believes absolute SLAs are not possible due to the nature of packet networks. | Voice is managed as another form of data, but BT has designed rules on the committed data rates that guarantee voice performance.
Cable and Wireless | IP convergence solution includes IP Virtual Private Network with QOS (IP-VPN QOS) for the WAN and IP-LAN. | SLAs on per class of service (including standard) on latency, jitter and packet loss. SLAs on availability (for example, bandwidth). SLAs on delivery times (per site or per upgrade). | Provides native multicast over MPLS.
Equant | • Voice over IP-VPN (works with both traditional TDM PBXs and IP PBXs) • IP Telephony Solutions (IP PBXs, IP phone, network connectivity over a dedicated COS, off-net traffic) • IP Video Solutions | Router availability: Up to 100 percent (according to the region). Transit Delay (on a route per route basis) Jitter <10 ms Packet Loss down to 0.1 percent Voice Call Completion Ratio >99.5 percent On top of these SLAs, Equant commits on: — Provisioning lead time — Mean time to restore — Change lead time | Offers fully managed end-to-end solution with SLAs on all service elements. Equant deploys a fully private and dedicated network with QOS guarantees all along the voice call path. Equant offers voice, IP telephony and video solutions in more than 86 countries. Equant offers a dedicated COS for video (on top of the voice COS) on a fully convergent network (no overlay network).
Infonet | Private Internet/IP-VPN Secure VoIP Dedicated Internet Access with Network Signaling System (DIA w/NSS)/IP VPN Internet IP-VPN Offnet Network Security Services: — Internet Protocol security (IPsec) encryption — Network Firewall — Premise Firewall Internet Gateway Service DialXpress/Dial IP-VPN | Typical measurements of quality by class: — Site availability — Backbone network delay — Backbone network jitter — Backbone network throughput — Service provisioning (installation and changes) — Problem management (proactive monitoring, update intervals, escalation procedures) | Application Defined Network approach: Six years of experience with frame Customized Networking Options (CNOs). Network Analysis Program (NAP): Patent pending modeling tool for applications over the WAN to optimize application performance for best use of COS. Profiling of most known applications: Baan, SAP, Oracle, PeopleSoft, Lotus Notes, Citrix. Four COSs (IP/RealTime, IP/Interactive, IP/LAN-to-LAN, IP/Access) with six port types.
MCI | MCI Advantage (converged network offering) | 99.9 percent uptime (no more than 0.1 percent of downtime) based on trouble ticket time. Network latency service levels on MCI North American Network is average round-trip transmissions of 55 milliseconds or less between MCI-designated interregional transit backbone network routers ("Hub Routers") in North America and Europe. Packet delivery of 99.5 percent or greater between MCI-designated Hub Routers in North America and Europe. | MCI is implementing DiffServ in its access networks to prioritize voice. Uses Session Initiation Protocol (SIP) handsets, allowing enterprises not to invest in new telephony systems.
[Return to Table of Contents] IP networks are best-effort networks supporting non-real-time applications. Key technologies, such as MPLS and DiffServ, should help IP deliver QOS and real-time communication. This allows enterprises to converge all their communications onto a single network to reduce total cost of ownership. However, this strategy is not without risk, given the volatility in the telecom marketplace today and the dependence on any single provider for all communication services. Many of the QOS standards in place today are still in their nascent stages. It will be several years before vendors and industry associations work through the immensely complex interoperability of standards and protocols.

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