MPLS–TP supports mechanisms for on demand CC/CV as well as route tracing for LSPs and PWs. These are required to enable an operator to test the initial configuration of a transport path, or to assist with fault isolation and diagnosis. On demand CC/CV and route tracing for MPLS-TP is based on LSP-Ping and is described in RFC6426. Three possible encapsulations are specified in that RFC:
IP encapsulation, using the same label stack as RFC 8029, or encapsulated in the IPv4 G-ACh channel with a GAL/ACH
non-IP encapsulation with GAL/ACH for LSPs and ACH for PWs
In IP-encapsulation, LSP-Ping packets are sent over the MPLS LSP for which OAM is being performed and contain an IP/UDP packet within them. The On-demand CV echo response message is sent on the reverse path of the LSP, and the reply contains IP/UDP headers followed by the On-demand CV payload.
In non-IP environments, LSP ping can be encapsulated with no IP/UDP headers in a G-ACh and use a source address TLV to identify the source node, using forward and reverse LSP or PW associated channels on the same LSP or PW for the echo request and reply packets. In this case, no IP/UDP headers are included in the LSP-Ping packets.
The routers support the following encapsulations:
IP encapsulation with ACH for PWs (as per VCCV type 1)
IP encapsulation without ACH for LSPs using labeled encapsulation
non-IP encapsulation with ACH for both PWs and LSPs
LSP Ping and VCCV Ping for MPLS-TP use two new FEC sub-types in the target FEC stack to identify the static LSP or static PW being checked. These are the Static LSP FEC sub-type, which has the same format as the LSP identifier described above, and the Static PW FEC sub-type,. These are used in-place of the currently defined target FEC stack sub-TLVs.
In addition, MPLS-TP uses a source/destination TLV to carry the MPLS-TP global-id and node-id of the target node for the LSP ping packet, and the source node of the LSP ping packet.
LSP Ping and VCCV-Ping for MPLS-TP can only be launched by the LER or T-PE. The replying node therefore sets the TTL of the LSP label or PW label in the reply packet to 255 to ensure that it reaches the node that launched the LSP ping or VCCV Ping request.
RFC 8029 specifies four address types for the downstream mapping TLV for use with IP numbered and unnumbered interfaces, as listed in Table: Downstream mapping (RFC 8029) :
| Type # | Address type | K Octets | Reference |
|---|---|---|---|
1 |
IPv4 Numbered |
16 |
RFC 8029 |
2 |
IPv4 Unnumbered |
16 |
|
3 |
IPv6 Numbered |
40 |
|
4 |
IPv6 Unnumbered |
28 |
RFC 6426 adds address type 5 for use with Non IP interfaces, including MPLS-TP interfaces. In addition, this RFC specifies that type 5 must be used when non-IP ACH encapsulation is used for LSP Trace.
It is possible to send and respond to a DSMAP/DDMAP TLV in the LSP Trace packet for numbered IP interfaces as per RFC8029. In this case, the echo request message contains a downstream mapping TLV with address type 1 (IPv4 address) and the IPv4 address in the DDMAP/DSMAP TLV is taken to be the IP address of the IP interface that the LSP uses. The LSP trace packet therefore contains a DSMAP TLV in addition to the MPLS-TP static LSP TLV in the target FEC stack.
DSMAP/DDMAP is not supported for pseudo wires.