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Ethernet Frame Delay Measurements on Switches

In many cases, a service provider could be subject to penalties imposed by regulation, statute, or contract if network performance is not within the bounds established for the service. One key performance objective is delay, along with its close relative, delay variation (often called jitter). Some applications (such as bulk file transfer) will function just as well with high delays across the network and high delay variations, while other applications (such as voice) can function only with low and stable delays. Many networks invoke protocols or features available at Layer 3 (the packet layer) or higher to measure network delays and jitter link by link. However, when the network consists of many Ethernet links, there are few protocols and features available at Layer 2 (the frame layer) that allow routers and switches to measure frame delay and jitter. This is where the ability to configure and monitor Ethernet frame delay is helpful.

This topic includes:

Ethernet Frame Delay Measurements

You can perform Ethernet frame delay measurements (referred to as ETH-DM in Ethernet specifications) on Juniper Networks EX Series Ethernet Switches. This feature allows you to configure on-demand Operation, Administration, and Maintenance (OAM) statements for the measurement of frame delay and frame delay variation (jitter). You can configure Ethernet frame delay measurement in either one-way or two-way (round-trip) mode to gather frame delay statistics simultaneously from multiple sessions. 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. It supports software-assisted timestamping in the receive direction for delay measurements. It also provides runtime display of delay statistics when two-way delay measurement is triggered. Ethernet frame delay measurement records the last 100 samples collected per remote maintenance association end point (MEP) or per connectivity fault management (CFM) session. You can retrieve the history at any time using simple commands. You can clear all Ethernet frame delay measurement statistics and PDU counters. Ethernet frame delay measurement is fully compliant with the ITU-T Y.1731 (OAM Functions and Mechanisms for Ethernet-based Networks) specification.

Ethernet frame delay measurement uses the IEEE 802.1ag CFM infrastructure.

Generally, Ethernet frame delay measurements are made in a peer fashion from one MEP or CFM session to another. However, these measurements are not made to maintenance association intermediate points (MIPs).

For a complete description of Ethernet frame delay measurement, see the ITU-T Y.1731 Ethernet Service OAM topics in the Junos OS Network Interfaces Library for Routing Devices.

Types of Ethernet Frame Delay Measurements

There are two types of Ethernet frame delay measurements:

  • One-way

  • Two-way (round-trip)

For one-way Ethernet frame delay measurement, either MEP can send a request to begin a one-way delay measurement to its peer MEP. However, the statistics are collected only at the receiver MEP. This feature requires the clocks at the transmitting and receiving MEPs to be synchronized. If these clocks fall out of synchronization, only one-way delay variation and average delay variation values are computed correctly (and will, therefore, be valid). Use the show commands at the receiver MEP to display one-way delay statistics.

For two-way (round-trip) Ethernet frame delay measurement, either MEP can send a request to begin a two-way delay measurement to its peer MEP, which responds with timestamp information. Run-time statistics are collected and displayed at the initiator MEP. The clocks do not need to be synchronized at the transmitting and receiving MEPs. Junos OS supports timestamps in delay measurement reply (DMR) frames to increase the accuracy of delay calculations.

Use the show commands at the initiator MEP to display two-way delay statistics, and at the receiver MEP to display one-way delay statistics.

You can create an iterator profile to periodically transmit SLA measurement packets in the form of ITU-Y.1731-compliant frames for delay measurement or loss measurement.

Limitations

The following are some limitations with regard to using Ethernet frame delay measurement:

  • Ethernet frame delay measurements are available only when distributed periodic packet management (PPM) is enabled.

  • The statistics collected are lost after a graceful Routing Engine switchover (GRES).

  • You can monitor only one session to the same remote MEP or MAC address.

  • Accuracy is compromised when the system configuration changes (such as from reconfiguration). We recommend performing Ethernet frame delay measurements on a stable system.