SLM is a single-ended test that can be run on demand or proactively to determine in-loss, out-loss, and unacknowledged packets. The test uses a small amount of synthetic test traffic as a substitute for customer traffic. SLM is used between peer MEPs in point-to-point configurations. Only remote peer MEPs within the association and matching the unicast destination will respond to the SLM packet. SLM uses an optional TLV with a timestamp on the near-end and far-end MEPs for the combined loss and delay measurement.
Various sequence numbers and counters are used to determine loss in each direction. In order to properly use the information that is gathered, the following terms are defined:
count — number of probes that are sent if the last frame is not lost. If the last frame is lost, the count plus the number of unacknowledged packets equals the number of probes sent.
out-loss (far end) — packets lost on the way to the remote node from the test initiator
in-loss (near end) — packets lost on the way back from the remote node to the test initiator
unacknowledged — number of packets not responded to by the end of the test
An SLM test packet can be generated using the CLI or SNMP for an on-demand test and SAA for a proactive test. The on-demand test provides per-probe-specific loss indicators or individual probe information stored in the MIB. The test does not store data for later processing. An SAA scheduled or continuous test summarizes the per-probe data but does not maintain per-probe information, and any unacknowledged packets are recorded as in-loss packets.
SLM packets originate and terminate on the CSM card. The probe count for SLM has a configurable range of 1 to 100 with probe spacing between 1 s and 10 s. A single test therefore can be up to 1000 s in length. A node may only initiate and maintain a single active on-demand test at any given time. The results table maintains a maximum of one storage entry per remote MEP. Subsequent tests on the same peer overwrite the results for that peer. For this reason, operators should run the on-demand test and check the results before starting another test.
Proactive measurement functions are linked to SAA and provide scheduling, storage, and summarization capabilities. Scheduling can be continuous or periodic. Proactive measurement also allows for the interpretation and representation of data that may enhance the specification. As an example, an optional TLV allows for the measurement of both loss and delay/ jitter with a single test. The optional TLV is ignored by equipment that does not support it. In mixed-vendor environments, loss measurement is tracked but delay and jitter only report round-trip times.
The round-trip times in mixed-vendor environments include the remote node's processing time because only two timestamps are included in the packet. In an environment where both nodes support the optional TLV to include timestamps, unidirectional and round-trip times are reported. Because all four timestamps are included in the packet, the round-trip time in this case does not include remote node processing time.
The ETH-SL packet format contains a test-id that is internally generated and not configurable. The display summary for the on-demand test shows the test-id. A remote node processing the SLM frames could receive overlapping test-ids as a result of multiple MEPs measuring the loss at the same remote MEP. For this reason, the uniqueness of the test is based on the remote MEP-ID, test-id, and source MAC address of the packet.
All Ethernet adapter cards and ports in access mode support SLM Up and Down MEPs, and all Ethernet adapter cards and ports in network mode that support spoke or mesh SDPs support SLM Up and Down MEPs. SLM Down MEPs are also supported on Ethernet network interfaces. There is no coordination between various fault conditions that could impact loss measurement. This is also true for conditions where MEPs are placed in a shutdown state as a result of linkage to a redundancy scheme such as MC-LAG.
It is possible to have a valid configuration where multiple MEPs on the same remote node have the same MAC address. If this is the case, the first responder is used to measure packet loss. The second responder is dropped.
A configurable inactivity timer determines the length of time that an on-demand test is valid. The test is active as long as packets are received within the timeframe set by the inactivity timer, as defined by the test-id, remote MEP ID, and source MAC address. If there is a gap between the packets that exceeds the inactivity timer value, the responding node responds with a sequence number of 1. For the remote MEP, the previous test has expired and any new probes are now part of a new test. The inactivity timer default is 100 s with a range of 10 to 100 s.
The responding node is limited to 1000 concurrent SLM tests. A node that is already actively processing 1000 SLM tests will show as out-loss or unacknowledged packets on the node that initiated the test because the packets will be silently discarded at the responder. No log entries or alarms will be raised. These packets are ETH-CFM-based and the stated receive rate for ETH-CFM must not be exceeded for the platform.
Only the configuration is supported by the high availability function. There is no synchronization of data between active and standby. Any unwritten or active tests are lost during a switchover and the data cannot be recovered.