1、 TECHNICAL REPORT T1.TR.78-2003 Technical Report on Access Availability of Routers in IP-Based Networks Prepared by T1A1.2 Working Group on Network Survivability Performance Problem Solvers to the Telecommunications Industry A Word from ATIS and Committee T1 Established in February 1984, Committee T
2、1 develops technical standards, reports and requirements regarding interoperability of telecommunications networks at interfaces with end-user systems, carriers, information and enhanced-service providers, and customer premises equipment (CPE). Committee T1 is sponsored by ATIS and is accredited by
3、ANSI. T1.TR.78-2003 Published by Alliance for Telecommunications Industry Solutions 1200 G Street, NW, Suite 500 Washington, DC 20005 Committee T1 is sponsored by the Alliance for Telecommunications Industry Solutions (ATIS) and accredited by the American National Standards Institute (ANSI). Copyrig
4、ht 2003 by Alliance for Telecommunications Industry Solutions All rights reserved. No part of this publication may be reproduced in any form, in an electronic retrieval system or otherwise, without the prior written permission of the publisher. For information contact ATIS at 202.628.6380. ATIS is o
5、nline at . Printed in the United States of America. T1.TR.78-2003 Technical Report on Access Availability of Routers in IP-Based Networks Alliance for Telecommunications Industry Solutions Approved January 2003 Abstract This Technical Report (TR) introduces the concepts for use in assessing the avai
6、lability of access to IP-based telecommunications networks. The calculation is based on the access availability of IP routers in the network. The TR presents alternate methods for weighting the availability calculation in terms of customers, ports, and bandwidth. This Technical Report is intended as
7、 the first in a series of Technical Reports, on the reliability metrics for IP-based networks. The next report will include Backbone networks thereby permitting a complete network availability assessment. ii T1.TR.78-2003 Foreword This Technical Report (TR) provides a practical way of assessing the
8、availability of access to IP-based telecommunications networks. It is intended as a guide for network reliability practitioners. Future reports will address reliability metrics for IP backbone networks. This TR is intended for providers of IP-based telecommunications networks and services, and telec
9、ommunications equipment suppliers. Suggestions for enhancement of this report are welcome. These should be sent to the Alliance for Telecommunications Industry Solutions, Suite 500, 1200 G Street N.W., Washington, D.C. 20005. Working Group T1A1.2 on Network Survivability Performance, which developed
10、 this report, has the following officers and participants: O. Avellaneda, T1A1.2 Chair S. Makris, T1A1.2 Vice-Chair F. Kaudel, T1A1.2 Chief Editor J. Bennett, Y. Kogan, and P. Tarapore, T1A1.2 Editors Active Participants: O. Avellaneda J. Bennett D. Clark W. Chiles C. Dvorak F. Kaudel Y. Kogan J. La
11、nkford J. Lord A. McCain A. Nguyen R. Paterson J. Rupe A. Webster R. Wohlert iii T1.TR.78-2003 Table of Contents 0 EXECUTIVE SUMMARY .1 1 PURPOSE, SCOPE, APPLICATION, OUTLINE, AND DISCLAIMER.1 1.1 PURPOSE.1 1.2 SCOPE.1 1.3 APPLICATION 1 1.4 OUTLINE 1 2 INTRODUCTION2 3 RELATED WORK2 3.1 T1A1 TECHNICA
12、L SUBCOMMITTEE2 3.2 OTHER T1 TECHNICAL SUBCOMMITTEES.2 3.3 INTERNATIONAL STANDARDS WORK AT THE ITU-T .2 3.4 OTHER FORUMS AND COMMITTEES.3 4 ACCESS NETWORK TOPOLOGY3 5 IP ROUTER ACCESS AVAILABILITY CONCEPTS5 5.1 IP ROUTER ACCESS TOTAL AVAILABILITY5 5.2 IP ROUTER ACCESS AVAILABILITY 5 5.3 AVERAGE IP R
13、OUTER ACCESS AVAILABILITY FOR A NETWORK 6 6 CONCLUSIONS.7 7 FUTURE WORK AND REQUEST FOR USER/INDUSTRY FEEDBACK7 8 BIBLIOGRAPHY7 9 DEFINITIONS 8 10 ABBREVIATIONS AND ACRONYMS .8 APPENDIX A: EXAMPLES OF IP ROUTER ACCESS AVAILABILITY CONCEPTS .9 A.1 EXAMPLE OF IP ROUTER ACCESS TOTAL AVAILABILITY.9 A.2
14、EXAMPLES OF IP ROUTER ACCESS AVAILABILITY .9 A.3 EXAMPLES OF AVERAGE IP ROUTER ACCESS AVAILABILITY FOR A NETWORK .11 APPENDIX B: TECHNIQUES FOR SCALING AVAILABILITY.14 Table of Figures FIGURE 1 - ACCESS NETWORK ELEMENTS 4 Table of Tables TABLE 1 - EXAMPLES OF EQUIVALENCIES14 iv Technical Report on A
15、ccess Availability of Routers in IP-Based Networks 0 Executive Summary This Technical Report (TR) provides a practical way of assessing the availability of access to IP-based telecommunications networks. It is intended as a guide for network reliability practitioners. 1 Purpose, Scope, Application,
16、Outline, and Disclaimer 1.1 Purpose This TR was developed in the interest of relating useful availability measures for access to IP-based networks. In particular, there is no widely accepted way of relating IP component failures to service outages as experienced by users of IP services. This TR is i
17、ntended to provide initial guidance in this area, with the goal of stimulating greater industry collaboration on IP reliability issues. This TR is complementary to other T1A1 work on IP performance of networks and services. 1.2 Scope This TR provides a practical way of assessing the availability of
18、access to IP networks. The use of these measures is demonstrated through examples. This methodology is seen as being highly useful because it complements the traditional methods for reliability assessments. While highly practical, this method is one of several possible methods that could be used for
19、 assessing IP network reliability, and is not intended to preclude the use of other methodologies. This TR addresses only IP-based access networks (and not IP-based backbone networks). Other than total loss of service from equipment outages, it does not include consideration of defects (such as dela
20、y, packet loss, and jitter) that may prevent satisfactory delivery of service. Work is ongoing at Working Group T1A1.2 in these areas. This TR will be useful to the industry with the understanding that these issues still need to be addressed. 1.3 Application The guidance in this TR applies to IP acc
21、ess networks. Subsequent reports will address IP core networks and IP services and applications. This TR is intended as the first in a series. 1.4 Outline Clause 0 provides an executive summary. Clause 1 describes the Purpose, Scope, Application, and Outline. Clause 2 provides an Introduction. Relat
22、ed Work is described in Clause 3. A generic access network topology is described in Clause 4. Clause 5 defines availability concepts for access to IP networks. Clause 6 presents the Conclusions. Clause 7 describes Future Work. Clause 8 presents a Bibliography. Definitions are presented in Clause 9.
23、Clause 10 contains Acronyms and Abbreviations. Appendix A presents examples of calculating IP router access availability. Appendix B describes alternate equivalent forms for expressing availability. 1 T1.TR.78-2003 2 Introduction With the proliferation of technologies such as IP-based systems, there
24、 is an urgent need to be able to relate the overall quality requirements to the performance and reliability of the many underlying network and system elements. Yet to date there is no well-accepted method in the industry for relating failures in network elements to service-level outages, so this Tec
25、hnical Report (TR) is a start in this much-needed direction. Ultimately, all performance and reliability outages should be expressible in terms of their impacts on the users of a service. 3 Related Work 3.1 T1A1 Technical Subcommittee The T1A1 Technical Subcommittee comprises two Working Groups: T1A
26、1.2 Network Survivability Performance; and T1A1.3 Performance of Networks and Services. This TR is a product of the work underway in the T1A1.2 Working Group. This document supports T1.TR.70-2001, Reliability/Availability Framework for IP-based Networks and Services. Clause 8 of the latter document
27、deals with “Guides and Metrics” for assessing network reliability. The T1A1.2 Working Group (WG) is responsible for the following TRs: T1.TR.42-1995, Enhanced Analysis of FCC-Reportable Service Outages. T1.TR.55-1998, Reliability and Survivability Aspects of the Interactions between the Internet and
28、 the Public Telecommunications Network. T1.TR.68-2001, Enhanced Network Survivability Performance. 3.2 Other T1 Technical Subcommittees Other T1 technical subcommittees involved in network reliability work include: T1E1 Interfaces, Power, and Protection for Networks; T1M1 Internetwork Operations, Ad
29、ministration, Maintenance, and Provisioning; T1S1 Services, Architectures, and Signaling; and T1X1 Digital Hierarchy and Synchronization. 3.3 International Standards Work at the ITU-T Some of the ITU-T study groups involved in work related to network survivability performance are listed below: Study
30、 Group 2 Operational Aspects of Service Provision, Networks, and Performance (see ITU-T Recommendation E.436); 2 T1.TR.78-2003 Study Group 12 End-to-End Transmission Performance of Networks and Terminals (see ITU-T Recommendation I.350); Study Group 13 Multi-Protocol and IP-based Networks and their
31、Interworking; and Study Group 15 Optical and Other Transport Networks. 3.4 Other Forums and Committees Forums involved in network reliability include the following (see T1.TR.70-2001): IETF Internet Engineering Task Force; NRIC Network Reliability and Interoperability Council; NRSC Network Reliabili
32、ty Steering Committee; and OIF Optical Internetworking Forum. 4 Access Network Topology This clause describes a fairly general IP Access Network. This network comprises all elements responsible for delivering transactions from the Customer Premise Equipment (CPE) at a customer location into the IP n
33、etwork backbone. One common component to all these elements is the Customer Port. These ports are on the “drop side” of an Access Router, where facilities from a customers CPE terminate. A failure in any element in the Access Network may result in downtime for customer ports on the Access Routers. S
34、uch failures prevent delivery of customer transactions to the backbone. There are five (5) element types in a typical IP Access Network topology (see Figure 1) whose failure can cause downtime for customer ports: Access Routers that form an edge on an ISP backbone network. Customer port downtime can
35、 be caused by a failure in a router component, such as a failed interface card, or from a total router failure. Facilities and supporting elements, such as cross-connects, which link routers at a customers CPE to ISP Access Router customer ports. Note that a customer may also use redundancy (double
36、homing) by buying access to more than one Access Router of the same ISP or even different ISPs. However, this type of redundancy is not counted in the availability calculation because customers pay separately for each access port. Facilities and supporting elements, such as cross-connects, which lin
37、k Access Routers to Backbone Routers. To increase the availability of the Access Network, an ISP usually provides redundancy by connecting each Access Router to two Backbone Routers at the same access node using two independent sets of uplinks1(Figure 1 depicts a typical access node with several Acc
38、ess Routers and two Backbone Routers). This permits customer traffic to enter the backbone in the following failure scenarios: o A failed uplink; o A failed card supporting an uplink; and o A failed Backbone Router at the access node. 1An uplink is a facility connecting any router to a backbone rout
39、er. 3 T1.TR.78-2003 Backbone Routers linked to Access Routers. As shown in Figure 1, if both Backbone Routers at an access node fail (a rare event), then all Access Routers at this node lose connection to the backbone. Facilities linking Backbone Routers at an access node to backbone routers at othe
40、r backbone nodes. Such facility failures decrease the available bandwidth from Access Routers to the backbone. Note that if all Backbone Router uplinks at an access node fail (a rare event), then all Access Routers at this node lose connection to the backbone. KEY AR: Access Router BR: Backbone Rout
41、er C: Customer Port Figure 1 - Access Network Elements As mentioned above, the common denominator for all failure types is the customer port on the Access Router. Any one of these failures may result in the “unavailability” of some of these Access Router customer ports. Customer port “unavailability
42、” can be caused by a direct port failure or from the failure of other elements described above (example: both Backbone Routers failed at the access node as shown in Figure 1). From the perspective of network access, the Access Router (see Figure 1) is the key element of the Access Network. These rou
43、ters receive IP packets from a customer location and route them into the 4 T1.TR.78-2003 selected network Backbone Router. Thus, any failure in the Access Network may result in downtime for some, or all, of the customer ports in the Access Router over the failure duration (usually measured in hours)
44、. For example, if a facility (or a facility supporting element such as a cross-connect) connecting the customer office location to the Access Router fails, then the corresponding Access Router port where the failed facility terminates, is considered to be unavailable over the failure duration. Claus
45、e 5 defines availability concepts for access to these routers. 5 IP Router Access Availability Concepts 5.1 IP Router Access Total Availability Given: A router is monitored for outages in a time period from time t1to time t2; In that time period, the router is in-service for a length of time T; N ou
46、tages are reported in that time period; and The duration D is recorded for each outage; then the availability of the router in time period t1, t2 is: Total access availability of routerTD1N1ii= . The total availability of the router is a measure of the fraction of the in-service time that the router
47、 was not impacted by an outage. 5.2 IP Router Access Availability The definition given above treats each outage as a total outage, one that denies access to all services for all customers. Router access availability may be defined with greater precision if each outage duration is weighted by the fra
48、ction of router service affected by the outage. Given: A router is monitored for outages in a time period from time t1to time t2; In that time period, the router is in-service for a length of time T; N outages are reported in that time period; The duration D is recorded for each outage; and The frac
49、tion f of the router access impacted by the outage is recorded for each outage; then the access availability of the router in time period t1, t2 is: Access availability of routerTDf1N1iii= . 5 T1.TR.78-2003 The fraction f of a router impacted by an outage may be examined in many ways. Three of the most intuitive techniques for calculating the fraction are based on: Customers: f = routerthebyservedcustomersofNumberoutagethebyimpactedcustomersofNumber Ports: f = routertheonserviceinportsacc
