Ethernet Frame Delay Measurements Overview

ITU-T Y.1731 Frame Delay Measurement Feature

The IEEE 802.3-2005 standard for Ethernet Operations, Administration, and Maintenance (OAM) defines a set of link fault management mechanisms to detect and report link faults on a single point-to-point Ethernet LAN.

• IEEE Standard 802.1ag, also known as “Connectivity Fault Management (CFM).”
• ITU-T Recommendation Y.1731, which uses different terminology than IEEE 802.1ag and defines Ethernet service OAM features for fault monitoring, diagnostics, and performance monitoring.

These capabilities allow operators to offer binding service-level agreements (SLAs) and generate new revenues from rate- and performance-guaranteed service packages that are tailored to the specific needs of their customers.

ACX Series routers support proactive and on-demand modes.

Note: ACX5048 and ACX5096 routers supports only software-based time stamping for delay measurement.

Ethernet Frame Delay Measurement

Two key objectives of OAM functionality are to measure quality-of-service attributes such as frame delay and frame delay variation (also known as “frame jitter”). Such measurements can enable you to identify network problems before customers are impacted by network defects.

Junos OS supports Ethernet frame delay measurement between MEPs configured on Ethernet physical or logical interfaces on ACX Series routers. Ethernet frame delay measurement provides fine control to operators for triggering delay measurement on a given service and can be used to monitor SLAs. Ethernet frame delay measurement also collects other useful information, such as worst and best case delays, average delay, and average delay variation. The Junos OS implementation of Ethernet frame delay measurement (ETH-DM) is fully compliant with the ITU-T Recommendation Y.1731, OAM Functions and Mechanisms for Ethernet-based Networks. The recommendation defines OAM mechanisms for operating and maintaining the network at the Ethernet service layer, which is called the "ETH layer" in ITU-T terminology.

Note: ACX Series routers do not support one-way Ethernet frame delay measurement.

Two-Way Ethernet Frame Delay Measurement

In two-way ETH-DM mode, frame delay and frame delay variation values are based on the time difference between when the initiator MEP transmits a request frame and receives a reply frame from the responder MEP, subtracting the time elapsed at the responder MEP.

DMM Transmission

When you start a two-way frame delay measurement, the router sends delay measurement message (DMM) frames— frames that carry the PDU for a two-way ETH-DM request—from the initiator MEP to the responder MEP at the rate and for the number of frames you specify. The router marks each DMM frame as drop-ineligible and inserts a timestamp of the transmission time into the frame.

DMR Transmission

When an MEP receives a DMM frame, the responder MEP responds with a delay measurement reply (DMR) frame, which carries ETH-DM reply information and a copy of the timestamp contained in the DMM frame.

DMR Reception

1. TI_TxDMM
2. TR_RxDMM
3. TR_TxDMR
4. TI_RxDMR

• [TI_RxDMR – TI_TxDMM] – [TR_TxDMR – TR_RxDMM]

The calculation show that frame delay is the difference between the time at which the initiator MEP sends a DMM frame and the time at which the initiator MEP receives the associated DMR frame from the responder MEP, minus the time elapsed at the responder MEP.

The delay variation is the difference between the current and previous delay values.

Two-Way ETH-DM Statistics

The router that contains the initiator MEP stores each set of two-way delay statistics in the ETH-DM database. The ETH-DM database collects up to 100 sets of statistics for any given CFM session (pair of peer MEPs). You can access these statistics at any time by displaying the ETH-DM database contents.

Two-Way ETH-DM Frame Counts

• For an initiator MEP, the router counts the number DMM frames transmitted, the number of valid DMR frames received, and the number of invalid DMR frames received.
• For a responder MEP, the router counts the number of DMR frames sent.

Each router stores ETH-DM frame counts in the CFM database. The CFM database stores CFM session statistics and, for interfaces that support ETH-DM, any ETH-DM frame counts. You can access the frame counts at any time by displaying CFM database information for Ethernet interfaces assigned to MEPs or for MEPs in CFM sessions.

Note: For a given two-way Ethernet frame delay measurement, frame delay and frame delay variation values are available only at the router that contains the initiator MEP.

Restrictions for Ethernet Frame Delay Measurement

• Ethernet frame delay measurements can be triggered only when the distributed periodic packet management daemon (ppm) is enabled. To enable distributed ppm, include the delegate-server-processing CLI statement at the [edit protocols oam ethernet connectivity-fault-management performance-monitoring] hierarchy.  For more information about this limitation, see Guidelines for Configuring Routers to Support an ETH-DM Session and Ensuring That Distributed ppm Is Not Disabled.
• You can monitor only one session at a time to the same remote MEP or MAC address. For more information about starting an ETH-DM session, see Starting an ETH-DM Session.
• ETH-DM statistics are collected at only one of the two peer routers in the ETH-DM session. For a one-way ETH-DM session, you can display frame ETH-DM statistics at the receiver MEP only, using ETH-DM-specific show commands. For a two-way ETH-DM session, you can display frame delay statistics at the initiator MEP only, using the same ETH-DM-specific show commands. For more information, see Managing ETH-DM Statistics and ETH-DM Frame Counts.
• ETH-DM frame counts are collected at both MEPs and are stored in the respective CFM databases.
• Accuracy of frame delay statistics is compromised when the system is changing (such as from reconfiguration). We recommend performing Ethernet frame delay measurements on a stable system.
• One-way Ethernet frame delay measurement is not supported.