LSP ping and LSP trace operation

The following operations are supported with both lsp-ping and lsp-trace.

• The lsp-ping and lsp-trace features model the tested segment list as a NIL FEC target FEC stack.

• Both an IPv4 SR policy (endpoint is an IPv4 address) and IPv6 SR policy (endpoint is an IPv6 address) can potentially contain a mix of IPv4 and IPv6 (node, adjacency, or adjacency set) SIDs in the same segment list or across segment lists of the same policy. While this is not a typical use of the SR policy, it is nonetheless allowed in the IETF standard and supported in SR OS. As a result, the downstream interface and node address information returned in the DSMAP or DDMAP TLV can have a different IP family across the path of the SR policy.

  Also, the IPv4 or IPv6 endpoint address can be null (0.0.0.0 or 0::0). This has no impact on the OAM capability.

• Unlike a SR-TE LSP path, the type of each segment (node, adjacency, or adjacency set) in the SID list may not be known to the sender node, except for the top SID that is validated by the SR policy database and which uses this segment type to resolve the outgoing interface or interfaces and outgoing label or labels to forward the packet out.

• The NIL FEC type is used to represent each SID in the segment list, including the top SID. The NIL FEC is defined RFC 8029 and has three main applications:

  • Allow the sender node to insert a FEC stack sub-TLV into the target FEC TLV when the FEC type is not known to the sender node (case of SIDs of the SR policy except the top SID) or if there is no explicit FEC associated with the label (case of a label of a static LSP or a MPLS forwarding policy). This is the application applicable to the SR policy.

    Although the sender node knows the FEC type for the top SID in the segment list of a SR policy, the NIL FEC is used for consistency. However, the sender node does all the processing required to look up the top SID as per the procedures of any other explicit FEC type.

  • Allow the sender node to insert a FEC stack sub-TLV into the target FEC stack sub-TLV if a special purpose label (for example, Router Alert) is inserted in the packet's label stack to maintain the correct 1-to-1 mapping of the packet's stacked labels to the hierarchy of FEC elements in the target FEC stack TLV processing at the responder node.

    SR OS does not support this application in a sender node role but can process the NIL FEC if received by a third-party implementation.

  • Allow the responder node to hide from the sender node a FEC element that it is pushing or stitching to by adding a NIL FEC TLV with a PUSH or a POP and PUSH (equivalent to a SWAP) operation into the FEC stack change sub-TLV.

    SR OS does not support this application in a sender node role but can process the NIL FEC if received by a third-party implementation.

• In the case of lsp-ping, the sender node builds a target FEC Stack TLV which contains a single NIL FEC element corresponding to the last segment of the tested segment list of the SR policy.

• In the case of lsp-trace, the sender node builds a target FEC Stack TLV which contains a NIL FEC element for each SID in the segment list.

• To support the processing of the NIL FEC in the context of the SR policy and the applications in RFC 8029, SR OS in a receiver node role performs the following operations:

  1. Look-up the label of the NIL FEC in the SR database to match against the SID of a resolved node, a resolved adjacency, a resolved adjacency SET or a binding SID.

  2. If a match exists, continue processing of the NIL FEC.

  3. Otherwise, look up the label of the NIL FEC in the Label Manager.

  4. If a match exists, then process the FEC as per the POP or SWAP operation provided by the lookup and following the NIL FEC procedures in RFC 8029.

  5. Otherwise, fail the validation and send a return code of 3 < Replying router has no mapping for the FEC at stack-depth <RSC>> in the MPLS echo reply message. The sender node fails the probe at this point.

• A SID label associated with a NIL FEC and which is popped at an LSR, acting in a receiver node role, is first looked up. If the label is valid, then the processing results in a return code of 3 <Replying router is an egress for the FEC at stack-depth <RSC>>.

  A label is valid if the LSR validates it in its Segment Routing (SR) database. Because the LSR does not know the actual FEC type and FEC value, it successfully validates it if the SR database indicates a programmed POP operation with that label for a node SID exists.

• A SID label associated with a NIL FEC and which is swapped at an LSR, acting in a receiver node role, is first looked up. If the label is valid, then the processing results in the return code of 8 Label switched at stack-depth <RSC> as per RFC 8029.

  A label is valid if the LSR validates it in its Segment Routing (SR) database. Because the LSR does not know the actual FEC type and FEC value, it successfully validates it if the SR database indicates a programmed SWAP operation with that label for either a node SID, an adjacency SID, an adjacency SET SID, or a binding SID exists.

  The swap operation corresponds to swapping the incoming label to an implicit-null label toward the downstream router in the case of an adjacency and toward a set of downstream routers in the case of an adjacency set.

  The swap operation corresponds to swapping the incoming label to one or more labels toward a set of downstream routers in the case of a node SID and a binding SID.

• The lsp-trace command is supported with the inclusion of the DSMAP TLV, the DDMAP TLV or none of them by the sender node in the ECHO request message. The responder node returns in the DSMAP or DDMAP TLV the downstream interface information along with the egress label and protocol ID that corresponds to the looked up node SID, adjacency SID, adjacency SET SID, or binding SID.

• When the Target FEC Stack TLV contains more than one NIL FEC element, the responder node that is the termination of a FEC element indicates the FEC POP operation implicitly by replying with a return code of 3 <Replying router is an egress for the FEC at stack-depth <RSC>>. When the sender node gets this reply, the sender node adjusts the Target FEC Stack TLV by stripping the top FEC before sending the next probe for the same TTL value. When the responder node receives the next echo request message with the same TTL value from the sender node, the responder node processes the next FEC element in the stack.

• The responder node performs validation of the top FEC in the target FEC stack TLV provided that the depth of the incoming label stack in the packet's header is strictly higher than the depth of the target FEC stack TLV.

• The ttl value in lsp-ping context can be set to a value lower than 255 and the responder node replies if the NIL FEC element in the Target FEC Stack TLV corresponds to a node SID resolved at that node. The responder node, however, fails the validation if the NIL FEC element in Target FEC Stack TLV corresponds to adjacency of a remote node. The return code in the echo reply message can be one of: rc=4(NoFECMapping), and rc=10(DSRtrUnmatchLabel).

• The min-ttl and max-ttl commands in lsp-trace context can be set to values other than default. The min-ttl can, however, properly trace the partial path of a SR policy only if there is not segment termination before the node that corresponds to the min-ttl value. Otherwise, the validation fails and returns an error as the responder node receives a Target FEC Stack depth that is higher than incoming label stack size. The return code in the echo reply message can be one of: rc=4(NoFECMapping), rc=5(DSMappingMismatched), and rc=10(DSRtrUnmatchLabel).

  This is true when the downstream-map-tlv option is set to any of ddmap, dsmap, or none values.