ETE405 :: Lecture 9 Chapter 9. Quality of Service Dr. Mashiur Rahman
Quality of Service
Quality of service (QoS) is an often-used and misused term that has a variety of meanings. In this course, QoS refers to both class of service (CoS) and type of service (ToS). The basic goal of CoS and ToS is to achieve the bandwidth and latency needed for a particular application.
A CoS enables a network administrator to group different packet flows, each having distinct latency and bandwidth requirements. A ToS is a field in an Internet Protocol (IP) header that enables CoS to take place. Currently, a ToS field uses three bits, which allow for eight packet-flow groupings, or CoSs (0-7). New Requests For Comments (RFCs) will enable six bits in a ToS field to allow for more CoSs.
It is important to note that the tools for implementing these services are not as important as the end result achieved. In other words, do not focus on one QoS tool to solve all your QoS problems. Instead, look at the network as a whole to determine which tools, if any, belong in which portions of your network.
QoS Network Toolkit
End-to-End Delay Budget
A VoIP phone call can be equivalent to any other large expense you would plan for. Therefore, it is important to know which parts of the budget you cannot change and which parts you might be able to control, as shown in Figure
The International Telecommunication Union Telecommunication Standardization Sector (ITU-T) G.114 recommendation suggests no more than 150 milliseconds (ms) of end-to-end delay to maintain "good" voice quality.
Bandwidth Limitations The first issue of major concern when designing a VoIP network is bandwidth constraints. Depending upon which codec you use and how many voice samples you want per packet, the amount of bandwidth per call can increase drastically. For an explanation of packet sizes and bandwidth consumed, see Table After reviewing this table, you might be asking yourself why 24 kbps of bandwidth is consumed when you're using an 8-kbps codec. This occurs due to a phenomenon called " The IP Tax." G.729 using two 10-ms samples consumes 20 bytes per frame, which works out to 8 kbps. The packet headers that include IP, RTP, and User Datagram Protocol (UDP) add 40 bytes to each frame. This "IP Tax" header is twice the amount of the payload.
cRTP To reduce the large percentage of bandwidth consumed by a G.729 voice call, you can use cRTP. cRTP enables you to compress the 40-byte IP/RTP/UDP header to 2 to 4 bytes most of the time
cRTP uses some of the same techniques as Transmission Control Protocol (TCP) header compression. In TCP header compression, the first factor-of-two reduction in data rate occurs because half of the bytes in the IP and TCP headers remain constant over the life of the connection.
You should not use cRTP on high-speed interfaces, as the disadvantages of doing so outweigh the advantages. "High-speed network" is a relative term: Usually anything higher than T1 or E1 speed does not need cRTP, but in some networks 512 kbps can qualify as a high-speed connection.
As with any compression, the CPU incurs extra processing duties to compress the packet. This increases the amount of CPU utilization on the router. Therefore, you must weigh the advantages (lower bandwidth requirements) against the disadvantages (higher CPU utilization). A router with higher CPU utilization can experience problems running other tasks. As such, it is usually a good rule of thumb to keep CPU utilization at less than 60 to 70 percent to keep your network running smoothly.
As in the tollbooth line, in queuing the concept of first in, first out (FIFO) exists, which means that if you are the first to get in the line, you are the first to get out of the line.
Today's networks, with their variety of applications, protocols, and users, require a way to classify different traffic. Going back to the tollbooth example, a special "lane" is necessary to enable some cars to get bumped up in line.
Queuing tool: that enable a network administrator to specify what type of traffic is "special" or important and to queue the traffic based on that information instead of when a packet arrives. Queuing techniques is known as WFQ
Weighted Fair Queuing
FIFO queuing places all packets it receives in one queue and transmits them as bandwidth becomes available. WFQ, on the other hand, uses multiple queues to separate flows and gives equal amounts of bandwidth to each flow. This prevents one application, such as File Transfer Protocol (FTP), from consuming all available bandwidth.
WFQ ensures that queues do not starve for bandwidth and that traffic gets predictable service. Low-volume data streams receive preferential service, transmitting their entire offered loads in a timely fashion. High-volume traffic streams share the remaining capacity, obtaining equal or proportional bandwidth.
Fair queuing enables low-bandwidth applications, which make up most of the traffic, to have as much bandwidth as needed, relegating higher-bandwidth traffic to share the remaining traffic in a fair manner. Fair queuing offers reduced jitter and enables efficient sharing of available bandwidth between all applications.
Custom queuing (CQ) enables users to specify a percentage of available bandwidth to a particular protocol. You can define up to 16 output queues as well as one additional queue for system messages (such as keep alives).
The router determines how many bytes from each queue should be transmitted, based on the speed of the interface as well as the configured traffic percentage. In other words, another traffic type can use unused bandwidth from queue A until queue A requires its full percentage.
CQ Caveats: CQ requires knowledge of port types and traffic types. This equates to a large amount of administrative overhead. But after the administrative overhead is complete, CQ offers a highly granular approach to queuing, which is what some customers prefer.
PQ enables the network administrator to configure four traffic priorities—high, normal, medium, and low. Inbound traffic is assigned to one of the four output queues. Traffic in the high-priority queue is serviced until the queue is empty; then, packets in the next priority queue are transmitted.
PQ Caveats: PQ enables a network administrator to "starve" applications. An improperly configured PQ can service one queue and completely disregard all other queues. This can, in effect, force some applications to stop working. As long as the system administrator realizes this caveat, PQ can be the proper alternative for some customers.
To achieve your intended packet delivery, you must know how to properly weight WFQ. This section focuses on different weighting techniques and ways you can use them in various networks to achieve the amount of QoS you require.
Resource Reservation Protocol (RSVP)
IP Real-Time Transport Protocol Reserve (IP RTP Reserve)
IP RTP Priority
IP Header and ToS Field IP Precedence refers to the three bits in the ToS field in an IP header, as shown in Figure.
IP Precedence Caveats
IP Precedence has no built-in mechanism for refusing incorrect IP Precedence settings. The network administrator needs to take precautions to ensure that the IP Precedence settings in the network remain as they were originally planned.
The following example shows the problems that can occur when IP Precedence is not carefully configured.
Company B uses WFQ with VoIP on all its WAN links and uses IP Precedence to prioritize traffic on the network. Company B uses a precedence setting of 5 for VoIP and a precedence setting of 4 for Systems Network Architecture (SNA) traffic. All other traffic is assumed to have a precedence setting of 0 (the lowest precedence).
With policy-based routing, you can configure a defined policy for traffic flows and not have to rely completely on routing protocols to determine traffic forwarding and routing. Policy routing also enables you to set the IP Precedence field so that the network can utilize different classes of service.
You can base policies on IP addresses, port numbers, protocols, or the size of packets. You can use one of these descriptors to create a simple policy, or you can use all of them to create a complicated policy.
All packets received on an interface with policy-based routing enabled are passed through enhanced packet filters known as route maps. The route maps dictate where the packets are forwarded.
Generic Traffic Shaping (GTS)
Frame Relay Traffic Shaping (FRTS)
Committed Access Rate (CAR)
Generic Traffic Shaping (GTS) GTS applies on a per-interface basis and can use access lists to select the traffic to shape. It works with a variety of Layer 2 technologies, including Frame Relay, ATM, Switched Multimegabit Data Service (SMDS), and Ethernet.
This chapter covered a broad topic called QoS. The concept of receiving a packet at the instant the sender wants it to be received is complex. Indeed, building a network to run VoIP is complex. Proper QoS is the most important step in ensuring good voice quality.