RFC 6374, Packet Loss and Delay Measurement for MPLS Networks, provides a standard packet format and process for measuring delay of a unidirectional or MPLS-TP 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 header 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. The responder extracts the required PDU information to respond appropriately.
Launching MPLS DM tests are 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:
Channel Type: 0x000C (MPLS DM)
Flags: Query
Control Code: out-of-band (unidirectional LSP) and in-band (bidirectional LSP)
Querier Timestamp Format: IEEE 1588-2008 (1588v2) Precision Time Protocol truncated timestamp format
Session Identifier: The configured oam-pm>session>mpls>dm test-id test-id
DSCP: The configured oam-pm>session>mpls>dscp dscp-name (this value is not used to convey or influence the CoS setting in the MPLS TC field, on the querier or reflector. The profile {in | out} and fc fc-name must be used to influence CoS markings at launch, and the MPLS TC influences CoS handling and marking upon reception.
Timestamp 1: Set to the local transmit time in PTP format
Timestamp 2 and 3: set to 0
TVLs can also be included, based on the configuration.
Padding (Type 0)
Copy in Response: Padding is returned in the response when the oam-pm>session>mpls>dm>reflect-pad is configured.
Padding (Type 128)
Do not copy in Response: Padding is not returned in the response when the oam-pm>session>mpls>dm>pad-tlv-size is configured without the reflect-pad command. This is the typical configuration with unidirectional LSPs.
UDP Return (Type 131)
UDP Return object: The IP information used by the reflector to reach the querier for an out-of-band response, when the oam-pm>session>mpls>lsp is rsvp or rsvp-auto and the udp-return-object information is configured.
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 filed 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 using 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 are 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.
| 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 |
Not supported |
Because OAM-PM uses a common reporting model, unidirectional (forward and backward, and round trip) are always reported. With unidirectional LSPs, the T4 and T3 timestamps are zeroed but the backward and round trip directions are still reported. In the case of unidirectional LSPs, the backward and round trip values are of no significance for the measured MPLS network.
An MPLS DM test may measure the endpoints of the LSP when the TTL is set equal to or higher than the termination point distance. Midpoints along the path that support MPLS DM response functions can be queried by a test 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 MPLS DM Channel Type 0x000C from RFC 6374 function for the following:
Label Switched Path types
RSVP-TE and RSVP-TE Auto LSPs set out-of-band response request and require the configuration of the UDP-Return-Object.
MPLS-TP sets in-band response request.
Querier and Responder
Traffic class indicator set to on (1)
Querier Timestamp format PTP
Mandatory TLV 0 – Copy padding
Optional TLV 128 – Do not copy padding
Optional TLV 131 – UDP-Return-Object (RFC 7876)
The following functions are not supported:
Packet Loss measurement
Throughput management
Dyadic measurements
Loopback
Mandatory TLVs
TLV 1 – Return Address
TLV 2 – Session Query Interval
TLV 3 – Loopback Request
Optional TLVs
TLV 129 – Destination Address
TLV 130 – Source Address