Question

Explain the purpose of QoS on a TCP/IP network. Define the basic purpose of IP precedence, TOS, Diffserv, and ECN functionality

Answer 1

Note This implementation of QoS is based on a set of evolving Internet standards and draft standards currently under development by the Internet Engineering Task Force (IETF) and its various working groups. This technology will become more consistent and well defined as these standardization efforts progress within the IETF. It is also important to note that QoS is an emerging Internet technology that is just beginning to be deployed within the Internet. There are many benefits of QoS at all stages of deployment. However, true end-to-end services can only be realized when QoS support exists all along a particular route.

The QoS support provides the following functions: • Differentiated services as defined in RFC 2474 Traffic policing • In-profile and out-of-profile packet marking • Traffic shaping • Metering • Integrated services for client and server applications as defined in RFC 1633 • RSVP signaling (RFC 2205) • Guaranteed service (RFC 2212) • Controlled-Load service (RFC 2211) • Policy-based networking • RAPI shared library for application

Quality of Service (QoS) is a family of evolving Internet standards that provides ways to give preferential treatment to certain types of IP traffic. With the proper support for QoS along a route, this can ameliorate the effects of variable queueing delays and congestion that contribute to poor network performance. The operating system provides host support for QoS to classify outbound traffic into distinct classes of service and to announce and establish resource reservations as requested by client applications. QoS can be used by an institution to deploy and enforce network policies governing the use of network bandwidth. With QoS, a host can: • Regulate the amount of traffic of a certain type injected into the network; • Mark selected packets according to some policy so that subsequent routers can deliver the indicated service; • Support services such as the virtual leased line service with proper QoS support along the route; and • Participate in the resource reservation requests from receivers and announce sender sessions available for resource reservation requests.

The Type of Service field in the IP header was originally defined in RFC 791. It defined a mechanism for assigning a priority to each IP packet as well as a mechanism to request specific treatment such as high throughput, high reliability or low latency. In practice, only the IP Precedence part of the field was ever used. At its simplest, the higher the value of the IP Precedence field, the higher the priority of the IP packet. Simple... In RFC 2474 the definition of this entire field was changed. It is now called the "DS" (Differentiated Services) field and the upper 6 bits contain a value called the "DSCP" (Differentiated Services Code Point). Since RFC 3168, the remaining two bits (the two least siginficant bits) are used for Explicit Congestion Notification.

The QoS subsystem consists of four components: QoS kernel extension (/usr/lib/drivers/qos) The QoS kernel extension resides in /usr/lib/drivers/gos and is loaded and unloaded using the cfgqos and ucfgqo s configuration methods. This kernel extension enables QoS support. Policy agent (/usr/sbin/policyd) The policy agent is a user-level daemon that resides in /usr/sbin/policyd. It provides support for policy management and interfaces with the QoS kernel extension to install, modify, and delete policy rules. Policy rules can be defined in the local configuration file(/etc/policyd.conf), retrieved from a central network policy server using LDAP, or both. RSVP agent (/usr/sbin/rsvpd) The RSVP agent is a user-level daemon that resides in /usr/sbin/rsvpd. It implements the RSVP signaling protocol semantics. RAPI shared library (/usr/lib/librapi.a) Applications can use the RSVP API (RAPI) to request enhanced quality of service as defined by the Integrated Services Internet QoS model. This library interacts with the local RSVP agent to propagate the QoS request along the path of the data flow using the RSVP protocol. This API is an open standard.

Differentiated services or DiffServ is a computer networking architecture that specifies a simple and scalable mechanism for classifying and managing network traffic and providing quality of service (QoS) on modern IP networks. DiffServ can, for example, be used to provide low-latency to critical network traffic such as voice or streaming media while providing simple best-effort service to non-critical services such as web traffic or file transfers.

The type of service (ToS) field is the second byte of the IPv4 header. It has had various purposes over the years, and has been defined in different ways by five RFCs. Prior to the redefinition, the ToS field could specify a datagram's priority and request a route for low-delay, high-throughput, or highly-reliable service. Based on these ToS values, a packet would be placed in a prioritized outgoing queue or take a route with appropriate latency, throughput, or reliability. In practice, the ToS field never saw widespread use outside of US Department of Defense networks. However, a great deal of experimental, research, and deployment work has focused on how to make use of these eight bits, resulting in the current DS field definition.

Explicit Congestion Notification (ECN) is an extension to the Internet Protocol and to the Transmission Control Protocol and is defined in RFC 3168) ECN allows end-to-end notification of network congestion without dropping packets. ECN is an optional feature that may be used between two ECN-enabled endpoints when the underlying network infrastructure also supports it. Conventionally, TCP/IP networks signal congestion by dropping packets. When ECN is successfully negotiated, an ECN-aware router may set a mark in the IP header instead of dropping a packet in order to signal impending congestion. The receiver of the packet echoes the congestion indication to the sender, which reduces its transmission rate as if it detected a dropped packet. detected sugestion indicapping a pack