SCOPE PURPOSE Service availability is a critical component in designing IP-based networks. It is also a key measure in Service Level Agreements (SLA) between service providers and customers. Hence@ it is important that the concept of availability is well-defined and understood@ and that the capabilities for accurate estimation are readily available and implemented by service providers. This document provides a definition of a commonly accepted availability metric for IP-based networks and services that can be correlated to network element outages. Future documents will discuss this metric within the context of MPLS-based transport networks. Current network trends indicate a significant increase in traffic of different types over IP networks. Traffic types include real-time Voice over IP (VoIP) and video services@ data services@ ??Enterprise Customer?? services over ??Virtual Private Networks?? (VPN)@ IPTV services@ as well as traditional Internet services such as e-mail and Web browsing. In designing SLAs with critical customers for key services such as VPN and video@ service providers typically agree to a specified level of availability within the bounds of a pre-determined traffic volume for the service in question. Under such conditions@ the predominant and overwhelming contribution to the unavailability of the specified service is a failure or outage in the network. A hardware or software outage may result in downtime for one or more network elements@ thus resulting in service disruption with negative impacts on service availability. This standard describes a metric that quantifies the impact on service availability of an underlying network element outage. Currently@ Network Management System (NMS) tools offer limited capabilities to collect necessary data for estimating this metric. The purpose of this metric is to encourage development of outage measurement capabilities/techniques for metric estimation by equipment vendors. The scope of this document is restricted to estimating service availability impacts within a single IP network domain. It is also restricted to service availability impacts caused by network element outages; it does not include service unavailability periods outside of such conditions (e.g.@ service degradation as a result of packet loss or jitter). It is recognized that communications services can span several network domains@ possibly of differing technologies (e.g.@ 3GPP Access-IP Backbone-PSTN Termination). Once agreements have been reached on service availability metrics and estimation methodologies@ discussions on extending availability on an end-to-end basis over all domains and technologies can commence. Finally@ it is recognized that evolving technologies such as MPLS are further transforming IP-based networks. This document serves as the first in a series of documents that will also examine availability estimation within the context of MPLS-based networks and services.