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The Cisco IOS QoS software enables complex networks to control and predictably service a variety of networked applications and traffic types. Almost any network can take advantage of QoS for optimum efficiency, whether it is a small corporate network, an Internet service provider, or an enterprise network. The Cisco IOS QoS software provides these benefits:
The basic architecture introduces the three fundamental pieces for QoS implementation (see Figure 46-1):
Three basic levels of end-to-end QoS can be provided across a heterogeneous network, as shown in Figure 46-2:
Deciding which type of service is appropriate to deploy in the network depends on several factors:
One way network elements handle an overflow of arriving traffic is to use a queuing algorithm to sort the traffic, and then determine some method of prioritizing it onto an output link. Cisco IOS software includes the following queuing tools:
Each queuing algorithm was designed to solve a specific network traffic problem and has a particular effect on network performance, as described in the following sections.
PQ ensures that important traffic gets the fastest handling at each point where it is used. It was designed to give strict priority to important traffic. Priority queuing can flexibly prioritize according to network protocol (for example IP, IPX, or AppleTalk), incoming interface, packet size, source/destination address, and so on. In PQ, each packet is placed in one of four queues---high, medium, normal, or low---based on an assigned priority. Packets that are not classified by this priority-list mechanism fall into the normal queue; see Figure 46-3. During transmission, the algorithm gives higher-priority queues absolute preferential treatment over low-priority queues.
PQ is useful for making sure that mission-critical traffic traversing various WAN links gets priority treatment. For example, Cisco uses PQ to ensure that important Oracle-based sales reporting data gets to its destination ahead of other, less-critical traffic. PQ currently uses static configuration and thus does not automatically adapt to changing network requirements.
CQ was designed to allow various applications or organizations to share the network among applications with specific minimum bandwidth or latency requirements. In these environments, bandwidth must be shared proportionally between applications and users. You can use the Cisco CQ feature to provide guaranteed bandwidth at a potential congestion point, ensuring the specified traffic a fixed portion of available bandwidth and leaving the remaining bandwidth to other traffic. Custom queuing handles traffic by assigning a specified amount of queue space to each class of packets and then servicing the queues in a round-robin fashion (see Figure 46-4).
As an example, encapsulated Systems Network Architecture (SNA) requires a guaranteed minimum level of service. You could reserve half of available bandwidth for SNA data, and allow the remaining half to be used by other protocols such as IP and Internetwork Packet Exchange (IPX).
The queuing algorithm places the messages in one of 17 queues (queue 0 holds system messages such as keepalives, signaling, and so on), and is emptied with weighted priority. The router services queues 1 through 16 in round-robin order, dequeuing a configured byte count from each queue in each cycle. This feature ensures that no application (or specified group of applications) achieves more than a predetermined proportion of overall capacity when the line is under stress. Like PQ, CQ is statically configured and does not automatically adapt to changing network conditions.
WFQ ensures that queues do not starve for bandwidth, and that traffic gets predictable service. Low-volume traffic streams---which comprise the majority of traffic---receive preferential service, transmitting their entire offered loads in a timely fashion. High-volume traffic streams share the remaining capacity proportionally between them, as shown in Figure 46-5.
WFQ is designed to minimize configuration effort and automatically adapts to changing network traffic conditions. In fact, WFQ does such a good job for most applications that it has been made the default queuing mode on most serial interfaces configured to run at or below E1 speeds (2.048 Mbps).
WFQ is efficient in that it uses whatever bandwidth is available to forward traffic from lower-priority flows if no traffic from higher-priority flows is present. This is different from time-division multiplexing (TDM), which simply carves up the bandwidth and lets it go unused if no traffic is present for a particular traffic type. WFQ works with both of Cisco's primary QoS signaling techniques---IP precedence and Resource Reservation Protocol (RSVP), described later in this chapter---to help provide differentiated QoS as well as guaranteed services.
The WFQ algorithm also addresses the problem of round-trip delay variability. If multiple high-volume conversations are active, their transfer rates and interarrival periods are made much more predictable. WFQ greatly enhances algorithms such as the SNA Logical Link Control (LLC) and the Transmission Control Protocol (TCP) congestion control and slow-start features. The result is more predictable throughput and response time for each active flow, as shown in Figure 46-6.
WFQ is IP precedence aware; that is, it is able to detect higher-priority packets marked with precedence by the IP forwarder and can schedule them faster, providing superior response time for this traffic. The IP Precedence field has values between 0 (the default) and 7. As the precedence value increases, the algorithm allocates more bandwidth to that conversation to make sure that it is served more quickly when congestion occurs. WFQ assigns a weight to each flow, which determines the transmit order for queued packets. In this scheme, lower weights are served first. IP precedence serves as a divisor to this weighting factor. For instance, traffic with an IP Precedence field value of 7 gets a lower weight than traffic with an IP Precedence field value of 3, and thus has priority in the transmit order.
For example, if you have one flow at each precedence level on an interface, each flow will get precedence + 1 parts of the link, as follows:
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 = 36
and the flows will get 8/36, 7/36, 6/36, and 5/36 of the link, and so on. However, if you have 18 precedence - 1 flows and 1 of each of the others, the formula looks like this:
1 + 1 8 ¥ 2 + 3 + 4 + 5 + 6 + 7 + 8 = 36 - 2 + 18 ¥ 2 = 70
and the flows will get 8/70, 7/70, 6/70, 5/70, 4/70, 3/70, 2/70, and 1/70 of the link, and 18 of the flows will get approximately 2/70 of the link.
WFQ is also RSVP aware; RSVP uses WFQ to allocate buffer space and schedule packets, and guarantees bandwidth for reserved flows. Additionally, in a Frame Relay network, the presence of congestion is flagged by the forward explicit congestion notification (FECN) and backward explicit congestion notification (BECN) bits. WFQ weights are affected by Frame Relay discard eligible (DE), FECN, and BECN bits when the traffic is switched by the Frame Relay switching module. When congestion is flagged, the weights used by the algorithm are altered so that the conversation encountering the congestion transmits less frequently.
Congestion avoidance techniques monitor network traffic loads in an effort to anticipate and avoid congestion at common network bottlenecks, as opposed to congestion management techniques that operate to control congestion after it occurs. The primary Cisco IOS congestion avoidance tool is weighted random early detection.
WRED combines the capabilities of the RED algorithm with IP precedence. This combination provides for preferential traffic handling for higher-priority packets. It can selectively discard lower-priority traffic when the interface starts to get congested and provide differentiated performance characteristics for different classes of service. See Figure 46-7. WRED is also RSVP aware, and can provide an integrated services controlled-load QoS.
Cisco IOS software also provides distributed weighted random early detection (D-WRED), a high-speed version of WRED that runs on VIP-distributed processors. The D-WRED algorithm provides functionality beyond what WRED provides, such as minimum and maximum queue depth thresholds and drop capabilities for each class of service.
Cisco's QoS software solutions include two traffic shaping tools---generic traffic shaping (GTS) and Frame Relay traffic shaping (FRTS)---to manage traffic and congestion on the network.
GTS provides a mechanism to control the traffic flow on a particular interface. It reduces outbound traffic flow to avoid congestion by constraining specified traffic to a particular bit rate (also known as the token bucket approach), while queuing bursts of the specified traffic. Thus, traffic adhering to a particular profile can be shaped to meet downstream requirements, eliminating bottlenecks in topologies with data-rate mismatches. Figure 46-8 illustrates GTS.
GTS applies on a per-interface basis, can use access lists to select the traffic to shape, and works with a variety of Layer 2 technologies, including Frame Relay, ATM, Switched Multimegabit Data Service (SMDS), and Ethernet.
On a Frame Relay subinterface, GTS can be set up to adapt dynamically to available bandwidth by integrating BECN signals, or set up simply to shape to a prespecified rate. GTS can also be configured on an ATM/AIP (ATM Interface Processor) interface card to respond to RSVP signaled over statically configured ATM permanent virtual circuits (PVCs).
For example, you can configure rate enforcement---a peak rate configured to limit outbound traffic---to either the CIR or some other defined value, such as the excess information rate (EIR), on a per-virtual-circuit (VC) basis.
You can also define priority and custom queuing at the VC or subinterface level. This allows for finer granularity in the prioritization and queuing of traffic and provides more control over the traffic flow on an individual VC. If you combine CQ with the per-VC queuing and rate enforcement capabilities, you enable Frame Relay VCs to carry multiple traffic types such as IP, SNA, and IPX, with bandwidth guaranteed for each traffic type.
FRTS can eliminate bottlenecks in Frame Relay networks with high-speed connections at the central site and low-speed connections at the branch sites. You can configure rate enforcement to limit the rate at which data is sent on the VC at the central site. You can also use rate enforcement with the existing data-link connection identifier (DLCI) prioritization feature to further improve performance in this situation. FRTS applies only to Frame Relay PVCs and switched virtual circuits (SVCs).
Using information contained in BECN-tagged packets received from the network, FRTS can also dynamically throttle traffic. With BECN-based throttling, packets are held in the router's buffers to reduce the data flow from the router into the Frame Relay network. The throttling is done on a per-VC basis and the transmission rate is adjusted based on the number of BECN-tagged packets received.
FRTS also provides a mechanism for sharing media by multiple VCs. Rate enforcement allows the transmission speed used by the router to be controlled by criteria other than line speed, such as the CIR or EIR. The rate enforcement feature can also be used to preallocate bandwidth to each VC, creating a virtual TDM network. Finally, with the Cisco's FRTS feature, you can integrate StrataCom ATM Foresight closed loop congestion control to actively adapt to downstream congestion conditions.
Currently, Cisco IOS software offers two link efficiency mechanisms---Link Fragmentation and Interleaving (LFI) and Real-Time Protocol Header Compression (RTP-HC)---which work with queuing and traffic shaping to improve the efficiency and predictability of the application service levels.
Interactive traffic (Telnet, voice over IP, and the like) is susceptible to increased latency and jitter when the network processes large packets (for example LAN-to-LAN FTP transfers traversing a WAN link), especially as they are queued on slower links. The Cisco IOS LFI feature reduces delay and jitter on slower-speed links by breaking up large datagrams and interleaving low-delay traffic packets with the resulting smaller packets (see Figure 46-9).
LFI was designed especially for lower-speed links in which serialization delay is significant. LFI requires that multilink Point-to-Point Protocol (PPP) be configured on the interface with interleaving turned on. A related IETF draft, titled Multiclass Extensions to Multilink PPP (MCML) implements almost the same function as LFI.
Real-Time Transport Protocol is a host-to-host protocol used for carrying newer multimedia application traffic, including packetized audio and video, over an IP network. Real-Time Transport Protocol provides end-to-end network transport functions intended for applications transmitting real-time requirements, such as audio, video, or simulation data over multicast or unicast network services. Real-Time Transport Protocol header compression increases efficiency for many of the newer voice over IP or multimedia applications that take advantage of Real-Time Transport Protocol, especially on slow links. Figure 46-10 illustrates Real-Time Transport Protocol header compression.
For compressed-payload audio applications, the RTP packet has a 40-byte header and typically a 20- to 150-byte payload. Given the size of the IP/UDP/Real-Time Transport Protocol header combination, it is inefficient to transmit an uncompressed header. Real-Time Transport Protocol header compression helps Real-Time Transport Protocol run more efficiently---especially over lower-speed links---by compressing the Real-Time Transport Protocol/UDP/IP header from 40 bytes to 2 to 5 bytes. This is especially beneficial for smaller packets (such as IP voice traffic) on slower links (385 kbps and below), where RTP header compression can reduce overhead and transmission delay significantly. Real-Time Transport Protocol header compression reduces line overhead for multimedia Real-Time Transport Protocol traffic with a corresponding reduction in delay, especially for traffic that uses short packets relative to header length.
RTP header compression is supported on serial lines using Frame Relay, High-Level Data Link Control (HDLC), or PPP encapsulation. It is also supported over ISDN interfaces. A related IETF draft, titled Compressed RTP (CRTP), defines essentially the same functionality.
True end-to-end QoS requires that every element in the network path---switch, router, firewall, host, client, and so on---deliver its part of QoS, and it all must be coordinated with QoS signaling. However, the challenge is finding a robust QoS signaling solution that can operate end-to-end over heterogeneous network infrastructures. Although many viable QoS signaling solutions provide QoS at some places in the infrastructure, they often have limited scope across the network, as shown in Figure 46-11.
Cisco IOS software takes advantage of the end-to-end nature of IP to meet this challenge by overlaying Layer 2 technology-specific QoS signaling solutions with the Layer 3 IP QoS signaling methods of RSVP and IP precedence.
This section focuses on IP precedence and RSVP because both of these methods take advantage of the end-to-end nature of IP. As the majority of applications converge on the use of IP as the primary networking protocol, IP precedence and RSVP provide a powerful combination for QoS signaling---IP precedence signals for differentiated QoS, and RSVP for guaranteed QoS.
IP precedence utilizes the three precedence bits in the IPv4 header's ToS (Type of Service) field to specify class of service for each packet, as shown in Figure 46-12. You can partition traffic in up to six classes of service using IP precedence (two others are reserved for internal network use). The queuing technologies throughout the network can then use this signal to provide the appropriate expedited handling.
Features such as policy-based routing and committed access rate (CAR) can be used to set precedence based on extended access-list classification. This allows considerable flexibility for precedence assignment, including assignment by application or user, or by destination and source subnet, and so on. Typically this functionality is deployed as close to the edge of the network (or administrative domain) as possible, so that each subsequent network element can provide service based on the determined policy.
IP precedence can also be set in the host or network client, and this signaling can be used optionally; however, this can be overridden by policy within the network. IP precedence enables service classes to be established using existing network queuing mechanisms (for example, WFQ or WRED), with no changes to existing applications or complicated network requirements. Note that this same approach is easily extended to IPv6 using its Priority field.
RSVP is an IETF Internet Standard (RFC 2205) protocol for allowing an application to dynamically reserve network bandwidth. RSVP enables applications to request a specific QoS for a data flow, as shown in Figure 46-13. Cisco's implementation also allows RSVP to be initiated within the network, using configured proxy RSVP. Network managers can thereby take advantage of the benefits of RSVP in the network, even for non-RSVP-enabled applications and hosts.
Hosts and routers use RSVP to deliver QoS requests to the routers along the paths of the data stream and to maintain router and host state to provide the requested service, usually bandwidth and latency. RSVP uses a mean data rate, the largest amount of data the router will keep in queue, and minimum QoS to determine bandwidth reservation.
WFQ or WRED acts as the workhorse for RSVP, setting up the packet classification and scheduling required for the reserved flows. Using WFQ, RSVP can deliver an integrated services guaranteed service. Using WRED, it can deliver a controlled load service. WFQ continues to provide its advantageous handling of nonreserved traffic by expediting interactive traffic and fairly sharing the remaining bandwidth between high-bandwidth flows, and WRED provides its commensurate advantages for non-RSVP flow traffic. RSVP can be deployed in existing networks with a software upgrade.
Cisco IOS software provides technologies that enable policy control, management, and accounting of the QoS techniques described in this chapter. The following sections provide an overview of these technologies.
At the infrastructure level, packet classification is a key capability for each policy technique that allows the appropriate packets traversing a network element or particular interface to be selected for QoS. These packets can then be marked for the appropriate IP precedence in some cases, or identified as an RSVP. Policy control also requires integration with underlying data link-layer network technologies or non-IP protocols.
SNA ToS in conjunction with data-link switching plus (DLSw+), allows mapping of traditional SNA class of service (CoS) into IP differentiated service. This feature takes advantage of both QoS signaling and pieces of the architecture. DLSW+ opens four TCP sessions and maps each SNA ToS traffic into a different session. Each session is marked by IP precedence. Cisco's congestion control technologies (CQ, PQ, and WFQ) act on these sessions to provide a bandwidth guarantee or other improved handling across an intranet, as shown in Figure 46-14. This provides a migration path for traditional SNA customers onto an IP-based intranet, while preserving the performance characteristics expected of SNA.
Thus, traditional mainframe-based, mission-critical applications can take advantage of evolving IP intranets and extranets without sacrificing the QoS capabilities historically provided by SNA networking.
Cisco IOS PBR allows you to classify traffic based on extended access list criteria, set IP precedence bits, and even route to specific traffic-engineered paths that may be required to allow a specific QoS through the network. By setting precedence levels on incoming traffic and using them in combination with the queuing tools described earlier in this chapter, you can create differentiated service. These tools provide powerful, simple, and flexible options for implementing QoS policies in your network.
You can also set up PBR as a way to route packets based on configured policies. Some applications or traffic can benefit from QoS-specific routing-transferring stock records to a corporate office (for example, on a higher-bandwidth, higher-cost link for a short time), while transmitting routine application data such as e-mail over a lower-bandwidth, lower-cost link. PBR can be used to direct packets to take different paths than the path derived from the routing protocols. It provides a more flexible mechanism for routing packets, complementing the existing mechanisms provided by routing protocols.
Similar in some ways to PBR, the CAR feature allows you to classify and police traffic on an incoming interface. It also allows specification of policies for handling traffic that exceeds a certain bandwidth allocation. CAR looks at traffic received on an interface, or a subset of that traffic selected by access list criteria, compares its rate to that of a configured token bucket, and then takes action based on the result (for example, drop or rewrite IP precedence).
One of the most promising uses for IP networks is to allow sharing of voice traffic with the traditional data and LAN-to-LAN traffic. Typically, this can help reduce transmission costs by reducing the number of network connections, sharing existing connections and infrastructure, and so on.
Cisco has a wide range of voice networking products and technologies, including a number of voice over IP (VoIP) solutions. To provide the required voice quality, however, QoS capability must be added to the traditional data-only network. Cisco IOS software QoS features give VoIP traffic the service it needs, while providing the traditional data traffic with the service it needs as well.
Figure 46-15 shows a business that has chosen to reduce some of its voice costs by combining voice traffic onto its existing IP network. Voice traffic at each office is digitized on voice modules on 3600 processors. This traffic is then routed via H.323 Gatekeeper, which also requests specific QoS for the voice traffic. In this case, IP precedence is set to high for the voice traffic. WFQ is enabled on all the router interfaces for this network. WFQ automatically expedites the forwarding of high-precedence voice traffic out each interface, reducing delay and jitter for this traffic.
Because the IP network was originally handling LAN-to-LAN traffic, many data-grams traversing the network are large 1500-byte packets. On slow links (below T1/E1 speeds), voice packets may be forced to wait behind one of these large packets, adding tens or even hundreds of milliseconds to the delay. LFI is used in conjunction with WFQ to break up these "jumbograms" and interleave the voice traffic to reduce this delay as well as jitter.
One of the most significant challenges for IP-based networks, which have traditionally provided only best-effort service, has been to provide some type of service guarantees for different types of traffic. This has been a particular challenge for streaming video applications, which often require a significant amount of reserved bandwidth to be useful.
In the network shown in Figure 46-16, RSVP is used in conjunction with ATM PVCs to provide guaranteed bandwidth to a mesh of locations. RSVP is configured from within Cisco IOS to provide paths from the router networks, at the edges, and through the ATM core. Simulation traffic then uses these guaranteed paths to meet the constraints of geographically distributed real-time simulation. Video-enabled machines at the various sites also use this network to do live videoconferencing.
In this instance, OC-3 ATM links are configured with multiple 3 Mbps PVCs connecting to various remote sites. RSVP ensures that QoS from this PVC is extended to the appropriate application across the local routed network. In the future, Cisco IOS will extend this RSVP capability to dynamically set up ATM SVCs. This will reduce configuration complexity and add a great degree of automatic configuration.
In a continued evolution toward end-to-end services, Cisco is expanding QoS interworking to operate more seamlessly across heterogeneous link-layer technologies, and working closely with host platform partners to ensure interoperation between networks and end systems.
Posted: Thu Jun 17 16:30:35 PDT 1999
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