MPLS PM

RFC 6374, Packet Loss and Delay Measurement for MPLS Networks, provides a standard packet format and process for measuring delay of a unidirectional or bidirectional label switched path (LSP) using the General Associated Channel (G-ACh), channel type 0x000C. Unidirectional LSPs, such as RSVP-TE, require an additional TLV to return a response to the querier (the launch point). RFC 7876, UDP Return Path for Packet Loss and Delay Measurement for MPLS Networks, defines the source IP information to include in the UDP Path Return TLV so the responding node can reach the querier using an IP network. The MPLS DM PDU does not natively include any IP source information. With MPLS TP there is no requirement for the TLV defined in RFC 7876.

The function of MPLS delay measurement is similar regardless of LSP type. The querier sends the MPLS DM query message toward the responder, transported in an MPLS LSP. he responder extracts the required PDU information to response appropriately.

Launching MPLS DM tests is configured in the config>oam-pm>session session-name test-family mpls context. Basic architectural OAM PM components are required to be completed along with the MPLS specific configuration. The test PDU includes the following PDU settings;

For the base PDU:

TVLs can also be included, based on the configuration.

The maximum pad size of 257 is a result of the structure of the defined TLV. The length field is one byte, limiting the overall value to 255 bytes.

The reflector processes the inbound MPLS DM PDU and respond back to the querier based on the received information, using the response flag setting. Specific to the timestamp, the responder responds to the Query Timestamp Format, filling in the Timestamp 2 and Timestamp 3 values.

When the response arrives back at the querier, the delay metrics are computed. The common OAM-PM computation model and naming is used to simplify and rationalize the different technologies that leverage the OAM-PM infrastructure. The common methodology reports unidirectional and round trip delay metrics for Frame Delay (FD), InterFrame Delay Variation (IFDV), and Frame Delay Range (FDR). The term, "frame" is not indicative of the underlying technology being measured. It represents a normal cross technology naming with applicability to the appropriate naming for the measured technology. The common normal naming requires a mapping to the supported delay measurements included in RFC 6374.

Table: Normalized naming mapping
Description RFC 6374 OAM-PM

A to B Delay

Forward

Forward

B to A Delay

Reverse

Backward

Two Way Delay (regardless of processing delays within the remote endpoint B)

Channel

Round-Trip

Two Way Delay (includes processing delay at the remote endpoint B)

Round-Trip

—

Because OAM-PM uses a common reporting model, unidirectional (forward and backward), round-trip is always reported. With unidirectional measurements, the T4 and T3 timestamps are zeroed but the round-trip and backward direction are still reported. With unidirectional measurements, the backward and round trip values are not of any significance.

An MPLS DM test may measure the endpoints of the LSP when the TTL is set to or higher than the termination point. Midpoints along the path that support MPLS DM response functions can be targeted by a test by setting a TTL to expire along the path. The MPLS DM launch and reflection, including mid-path transit nodes, capability is disabled by default. To launch and reflect MPLS DM test packets config>test-oam>mpls-dm must be enabled.

The SR OS implementation supports the following MPLS DM Channel Type 0x000C from RFC 6374 function:

The following functions are not supported:

Parent topic: OAM fault and performance tools and protocols