|Internet-Draft||Error Performance Measurement||March 2021|
|Mirsky, et al.||Expires 27 September 2021||[Page]|
- Network Working Group
- Intended Status:
- Standards Track
Error Performance Measurement in Packet-switched Networks
This document describes the use of the error performance metric to characterize a packet-switched network's conformance to the pre-defined set of performance objectives. In this document, metrics that characterize error performance in a packet-switched network (PSN) are defined, as well as methods to measure and calculate them. Also, the requirements for an active Operation, Administration, and Maintenance protocol to support the error performance measurement in PSN are discussed, and potential candidate protocols are analyzed. All metrics and measurement methods are equally applicable to underlay and overlay networks.¶
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Operations, Administration, and Maintenance (OAM) is a collection of methods to detect, characterize, localize failures in a network, and monitor the network's performance using various measurement methods. Traditionally, the former set of OAM tools identified as Fault Management (FM) OAM. The latter - Performance Monitoring (PM) OAM. Some OAM protocols can be used for both groups of tasks, while some serve one particular group. But regardless of how many OAM protocols are in use, network operators and network users are faced with multiple metrics that characterize the network conditions. This document describes a new component of packet-switched network (PSN) OAM.¶
Error performance measurement (EPM) is a part of an OAM toolset that provides an operator with information related to network measurements for a uni-directional or a bidirectional connection between two systems. In current technology, EPM has been defined only for data communication methods that have a constant bit-rate transmission [ITU.G.826] and not for PSN, where transmissions are statistically random. As a statistically multiplexed network in a PSN, a receiver node does not expect a packet to arrive from a sender node at a specific moment, less from a particular sender. That is what differentiates PSN from networks built on a constant bit-rate transmission, where a stream of bits between two nodes is always present, whether it represents data or not. That provides the receiver with a predictable number of measurements in a series of measurement intervals. In PSN, on-path OAM methods, i.e., measurement methods that use data flow, cannot provide such predictability and thus be used for EPM. In PSN, EPM needs to use active OAM methods, per definition in [RFC7799]. This document identifies metrics that characterize PSN error performance and methods to measure and calculate them. Also, the requirements for an active OAM protocol to support EPM in PSN are discussed, and potential candidate protocols are analyzed.¶
OAM Operations, Administration, and Maintenance¶
EP Error Performance¶
EPM Error Performance Measurement¶
ES Errored Second¶
ESR Errored Second Ratio¶
SES Severely Errored Second¶
SESR Severely Errored Second Ratio¶
EFS Error-Free Second¶
PSN Packet-switched Network¶
FM Fault Management¶
PM Performance Monitoring¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
When analyzing the error performance of a path between two nodes, we need to select a time interval as the unit of EPM. In [ITU.G.826], a time interval of one second is used. It is reasonable to use the same time interval for EPM for PSNs. Further, for the purpose of EPM, each time interval, i.e., second, is classified either as Errored Second (ES), Severely Errored Second (SES), or Error-Free Second (EFS). These are defined as follows:¶
- An ES is a time interval during which at least one of the performance parameters degraded below its optimal level threshold or a defect was detected.¶
- An SES is a time interval during which at least one the performance parameters degraded below its critical threshold or a defect was detected.¶
- Consequently, an EFS is a time interval during which all performance objectives are at or above their respective optimal levels, and no defect has been detected.¶
The definition of a state of a defect in the network is also necessary for understanding the EPM. In this document, the defect is interpreted as the state of inability to communicate between a particular set of nodes. It is important to note that it is being defined as a state, and thus, it has conditions that define entry into it and exit out of it. Also, the state of defect exists only in connection to the particular group of nodes in the network, not the network as a domain.¶
The definitions of ES, SES, and EFS allow for characterization of the communication between two nodes relative to the level of required and acceptable performance and when performance degrades below the acceptable level. The former condition in this document referred to as network availability. The latter - network unavailability. Based on the definitions, SES is the one-second of network unavailability while ES and EFS present an interval of network availability. But since the conditions of network are everchanging periods of network availability and unavailability need to be defined with duration larger than one-second interval to reduce the number of state changes while correctly reflecting the network condition. The method to determine the state of the network in terms of EPM OAM is described below:¶
- If ten consecutive SES intervals been detected, then the EPM state of the network determined as unavailability and the beginning of that period of unavailability state is at the start of the first SES in the sequence of the consecutive SES intervals.¶
- Similarly, ten consecutive non-SES intervals, i.e., either ES or EFS, indicate that the network is in the availability period, i.e., available. The start of that period is at the beginning of the first non-SES interval.¶
- Resulting from these two definitions, a sequence of less than ten consecutive SES or non-SES intervals does not change the EPM state of the network. For example, if the EPM state is determined as unavailability, a sequence of seven EFS intervals is not viewed as an availability period.¶
Determining the period in which the path is currently EP-wise is helpful. But because switching between periods requires ten consecutive one-second intervals, conditions that last shorter intervals may not be adequately reflected. Two additional EP OAM metrics can be used, and they are defined as follows:¶
- errored second ratio (ESR) is the ratio of ES to the total number of seconds in a time of the availability periods during a fixed measurement interval.¶
- severely errored second ratio (SESR) - is the ratio of SES to the total number of seconds in a time of the availability periods during a fixed measurement interval.¶
Digital communication methods characterized as the constant-bit rate digital paths and connections allow measurement of the error performance without using an active OAM. That is possible because a predictable flow of digital signals is expected at an egress system. That is not the case for packet-switched networks that are based on the principle of statistical multiplexing flows. The latter usually improves the utilization of the communication network's resources, but it also makes the flow unpredictable for the egress system. For that reason, an active OAM has to be used in measuring the error performance in a network. A combination of OAM protocols can provide the necessary for EPM functionality. For example, Bidirectional Forwarding Detection (BFD) [RFC5880] can be used to monitor the continuity of a path between the ingress and egress systems. And STAMP [RFC8762] can be used to measure and calculate performance metrics that are used as Service Level Objectives. But using two protocols and correlating the state of the network from them adds to the complexity in network operation.¶
The Integrated OAM, described in [I-D.mmm-rtgwg-integrated-oam], combines lightweight FM OAM with the comprehensive set of performance measurement methods. PM component of the Integrated OAM is based on [RFC6374] that supports, among other measurement methods, one-way and two-way packet loss and packet delay measurements.¶
- Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <https://www.rfc-editor.org/info/rfc2119>.
- Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <https://www.rfc-editor.org/info/rfc8174>.
- Mirsky, G., Min, X., and G. Mishra, "Integrated Operation, Administration, and Maintenance", Work in Progress, Internet-Draft, draft-mmm-rtgwg-integrated-oam-00, , <https://tools.ietf.org/html/draft-mmm-rtgwg-integrated-oam-00>.
- ITU-T, "End-to-end error performance parameters and objectives for international, constant bit-rate digital paths and connections", ITU-T G.826, .
- Katz, D. and D. Ward, "Bidirectional Forwarding Detection (BFD)", RFC 5880, DOI 10.17487/RFC5880, , <https://www.rfc-editor.org/info/rfc5880>.
- Frost, D. and S. Bryant, "Packet Loss and Delay Measurement for MPLS Networks", RFC 6374, DOI 10.17487/RFC6374, , <https://www.rfc-editor.org/info/rfc6374>.
- Morton, A., "Active and Passive Metrics and Methods (with Hybrid Types In-Between)", RFC 7799, DOI 10.17487/RFC7799, , <https://www.rfc-editor.org/info/rfc7799>.
- Mirsky, G., Jun, G., Nydell, H., and R. Foote, "Simple Two-Way Active Measurement Protocol", RFC 8762, DOI 10.17487/RFC8762, , <https://www.rfc-editor.org/info/rfc8762>.