Quality of Service Methods for IP Networks
This article provides an analysis of the methods and protocols used to improve the Quality of Service (QoS) in Internet Protocol (IP) networks. The challenges of achieving a high level of QoS in IP networks are examined. The traditional methods for QoS are presented along with current and proposed methods for QoS in IP networks. The goal of the article is to educate the reader on the various methods of achieving QoS and to examine the best options for the future. As Internet bandwidth requirements grow and high quality IP applications such as real-time video and Voice over IP (VoIP) become widespread, QoS will be critical to the success of providing high quality Internet Protocol (IP) services.
Quality of Service
The Internet Engineering Task Force (IETF) defines Quality of Service (QoS) as a service agreement (or a guarantee) to provide a set of measurable networking service attributes, including end to end delay, delay variation (jitter), and available bandwidth. The International Telecommunication Union (ITU) defines QoS as the collective effect of service performance which determines the degree of customer satisfaction. Cisco defines QoS as the capacity of a network to provide better service to selected network traffic.
This article defines QoS as the capability of an IP network to fake email generator classify and prioritize traffic flows in order to ensure that the technical characteristics of packet loss, delay, error rate and jitter are met for each customer. Quality of Service (QoS) methods are based on the ability of an IP network to identify and classify traffic that is higher priority so that the technical requirements of the customer are met. QoS methods are based on having an adequate amount of bandwidth (i.e. low network utilization) to prevent traffic congestion and to permit the setup of priority traffic flows.
Traditional QoS Methods for IP Networks
In the early days of the Internet, the applications were low bandwidth and not real-time in nature, so a high quality of service was easy to maintain. The early Internet applications of email, File Transfer Protocol (FTP) and web browsing were low-speed bursty IP traffic so delay, latency and bit error rates were not critical. The early networks relied on the Transmission Control Protocol (TCP) to provide flow control, error checks and retransmission of data packets when necessary. TCP provides a best-effort level of QoS that is acceptable for email and web browsing. However, the TCP protocol alone will not provide an acceptable level of QoS for real-time high bandwidth applications such as video or VoIP. Another method used to improve the QoS in early TCP/IP networks was the First-In First-Out (FIFO) buffer. FIFO buffers provided a simple method to store packets when there was temporary network congestion, but FIFO buffers make no intelligent decision about the priority of traffic. This section will examine some of the traditional methods of QoS to include the Transmission Control Protocol (TCP), IP Routing Protocols, First-In First-Out (FIFO) buffers, the Real Time Protocol (RTP) and the Explicit Congestion Notification (ECN) protocol.
Transmission Control Protocol (TCP). The Transmission Control Protocol (TCP) is one of the most common protocols used in the Internet. Although it is not traditionally considered a QoS protocol, TCP can provide adequate QoS for best-effort Internet applications such as email and web browsing. The Transmission Control Protocol (TCP) is a connection oriented protocol that provides reliable data transport between host computers. The term connection-oriented means the two host computers using TCP must establish a connection with each other before they exchange data. TCP includes flow-control to control the transmission of data so that the receiver can limit how much data the sender transmits. TCP uses a sliding window protocol for flow control. TCP uses the sequence, acknowledge and window fields in the header for flow control. The window field identifies the number of bytes that can be sent without acknowledgements. The window size will slide up and down based on performance of the connection.
First-In, First-Out (FIFO) Buffers. FIFO buffers provide temporary queuing of data when there is network congestion. The shortcoming of FIFO queuing is that no intelligent decision is made on the priority of traffic. FIFO is still used in many networking devices, but is now considered a non-QoS method because FIFO is unable to meet the QoS standards of today’s IP networks.
Explicit Congestion Notification (ECN). The ECN protocol provides advance warning of network congestion so the routers can mark data packets being transmitted. With ECN, a bit is placed in the IP header to signal the transmitter that the network is congested. ECN will set a flag notifying the sender to decrease the window size to avoid having to retransmit packets. ECN is also not a true QoS protocol and is incapable of reserving resources or guaranteeing services.
Quality of Service Methods in IP Networks
Quality of Service Levels. The service levels of QoS refer to the actual capabilities of the network to provide end to end service. There are three levels of QoS for IP networks:
1. Best Effort Service. This level of service provides no guarantees of service and relies on basic TCP and FIFO functions to transmit data across the network.
2. Differentiated Service. This level prioritizes traffic and provides a statistical preference for higher priority traffic, but not a hard guarantee of service. Priority Queuing (PQ) is an example of differentiated service.
3. Guaranteed Service. This is the best level of QoS and provides a reservation of network resources for high priority traffic. The RSVP protocol is an example.
Classification of IP Traffic. To provide a high priority service for a type of data traffic, the data must first be identified and classified for service. If the data is marked, then IP precedence throughout the network can be used to provide a higher level of service. For data that is identified, but not marked, classification is on a ‘per-hop’ basis. On a per-hop basis means that classification of the data only pertains to a single device that uses a QoS method such as priority queuing.
Access Control Lists (ACL). Access control lists are used in IP networks to identify traffic for congestion management methods such as policy based routing. The ACL is a list of permissions on a router that determine the actions that the device will take with a given traffic flow.
Policy Based Routing (PBR). Policy Based Routing permits the classification of traffic based on extended access control lists and set IP precedence bits. PBR uses route-maps within a network to route traffic based on established policies. PBR can direct packets to take a different path than derived from routing protocols.