The DDMAP TLV, as defined in RFC 6424, provides users with the same features as the existing DSMAP TLV along with enhancements that allow LSP diagnostics to trace the details of LSP hierarchy. With the DDMAP TLV, all intermediate routers will appear in the LSP ping and traceroute lists, and intermediate nodes can append additional data with details about their relative functionality. The DDMAP TLV format is derived from the DSMAP TLV format with variable length and optional fields converted into sub-TLVs. Figure: DDMAP TLV shows the DDMAP TLV format.
The following process is used when sending or responding to an LSP ping or LSP traceroute packet using the DSMAP or DDMAP TLV.
When either the DSMAP TLV or the DDMAP TLV is received in an echo request message, the responder node includes the same type of TLV in the echo reply message with the correct downstream interface information and label stack information.
If an echo request message without a DSMAP or DDMAP TLV expires at a node that is not the egress for the target FEC stack, the responder node always includes the DSMAP TLV in the echo reply message. This can occur in the following cases:
The user issues an LSP trace from a sender node with a min-ttl value higher than 1 and a max-ttl value lower than the number of hops required to reach the egress of the target FEC stack. This is the sender node behavior when the global configuration or the per-test setting of the DSMAP/DDMAP is set to DSMAP.
The user issues an LSP ping from a sender node with a TTL value lower than the number of hops required to reach the egress of the target FEC stack. This is the sender node behavior when the global configuration of the DSMAP/DDMAP is set to DSMAP.
If the global configuration or the per-test setting of the DSMAP TLV is set to DDMAP, the sender node includes the DDMAP TLV with the downstream IP address field set to the all-routers multicast address as per Section 3.3 of RFC 4379. The responder node then bypasses the interface and label stack validation and replies with a DDMAP TLV with the correct downstream information for the target FEC stack.
A sender node never includes the DSMAP or DDMAP TLV in an LSP ping message.
The user can globally configure the downstream mapping TLV to be used for all LSP trace and LDP treetrace packets with the configure test-oam mpls-echo-request-downstream-map command. The configured global value becomes the default downstream mapping TLV for all newly created LSP trace and LDP treetrace tests. It has no effect on existing tests and can be overridden on a specific test by setting the downstream-map-tlv parameter in the lsp-trace or ldp-treetrace context.