Frame-Delay Measurement

ExtremeXOS software supports:

By default, DMM is not enabled. You must explicitly enable the DMM transmission for a CFM segment, either as continuous or on-demand mode.

A network interface is considered attached to a subnetwork. The term segmentis used to refer to such a subnetwork, whether it be an Ethernet LAN, a ring, a WAN link, or even an SDH virtual circuit.

Frame-Delay measurement is done between two specific end points within an administrative domain. Frame delay and frame delay variation measurements are performed in a maintenance association end point (MEP) by sending and receiving periodic frames with ETH-DM information to and from the peer end point during the diagnostic interval.

When a CFM segment is enabled to generate frames with ETH-DM information, it periodically sends frames with ETH-DM information to its peer in the same maintenance association (MA) and expects to receive frames with ETH-DM information from its peer in the same MA.

The following list offers specific configuration information that is required by a peer to support ETH-DM:
  • Maintenance domain (MD) level—The MD level at which the peer exists.
  • Priority—The priority of the frames with ETH-DM information.
  • Drop eligibility—Frames with ETH-DM information that are always marked as drop ineligible.
  • Transmission rate.
  • Total transmit interval.

A node transmits frames with ETH-DM information with the following information element:

  • TxTimeStampf: Timestamp at the transmission time of the ETH-DM frame.
  • RxTimeStampb: Timestamp at which the switch receives the DMR back.

    Whenever a valid DMM frame is received by the peer, a DMR frame is generated and transmitted to the requesting node.

  • A DMM frame with a valid MD level and a destination MAC address equal to the receiving node‘s MAC address is considered to be a valid DMM frame. Every field in the DMM frame is copied to the DMR frame with the following exceptions:
    • The source and destination MAC addressed are swapped.
    • The OpCode field is changed from DMM to DMR.

The switch makes two-way frame delay variation measurements based on its ability to calculate the difference between two subsequent two-way frame delay measurements.

To allow a more precise two-way frame delay measurement, the peer replying to the frame with ETH-DM request information may include two additional timestamps in the ETH-DM reply information:
  • RxTimeStampf—Timestamp at the time of receiving a frame with ETH-DM request information
  • TxTimeStampb—Timestamp at the time of transmitting a frame with ETH-DM reply information
Here the frame delay is calculated by the peer that receives the DMR as follows:
  • Frame Delay = (RxTimeStampb - TxTimeStampf) - (TxTimeStampb - RxTimeStampf)
The following figure describes the DMM and DMR message flows between two end points.
Click to expand in new window
Two-Way Frame Delay and Frame Delay Variance Measurement
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The PDUs used to measure frame delay and frame delay variation are the DMM and the DMR PDUs where DMM is initiated from a node as a request to its peer and DMR is the reply from the peer.

If you try to enable the transmission for a CFM segment whose configuration is not complete, the trigger is rejected and an error message similar to the following is given:

 ERROR: CFM Configuration is not complete for segment "s1" to start transmission 
Note

Note

A CFM segment without a domain and an association is considered to be an incomplete segment.

Upon enabling the transmission from a CFM segment, the segment transmits DMM frames, one at each transmit-interval which is configured through the CLI. If the user enables on-demand transmission, the segment transmits "X" number of DMMs and moves back to the disabled state, where "X" is the number of frames specified by the user through the CLI.

For continuous transmission, the segment continues to transmit DMM frames until stopped by the user. This transmission continues even after reboot for both continuous and on-demand mode. For on-demand transmission, the segment, which was enabled to transmit "X" number of frames, and is still transmitting, starts transmitting again "X" number of frames after reboot, or failover, or process restart. The old statistics are not preserved for both continuous and on-demand mode for all the above three scenarios.

Upon transmitting a DMM, the segment is expected to get a reply from the destination within the specified time. If a reply is received after that time, that reply will be considered as a delayed one.

If a reply is not received within the transmit-interval, that is, between two subsequent DMM transmissions, then that frame is considered as lost. Once the percentage of the sum of lost and delayed frames reaches the alarm threshold, an alarm is generated and the segment is moved to the alarming state. This state is maintained until the percentage of valid replies reaches the clear threshold. These alarm and clear states are maintained for a specified window, which holds a set of recent frames and their corresponding delays.

Various times are recorded at the segment level during the transmission of DMM frames.
  • Start time—Time at which the segment started the current transmission.
  • Min delay time—Time at which the minimum delay occurred in the current transmission window.
  • Max delay time—Time at which the maximum delay occurred in the current transmission window.
  • Alarm time—The recent alarm time, if any, during the current transmission.

The mean delay and delay variance for the current window is also measured whenever the user polls the segment statistics.