Mechanism for performing LSP-Ping over MPLS tunnels
draft-ietf-mpls-lsp-ping-enhanced-dsmap-05

Abstract

This document describes methods for performing lsp-ping traceroute over mpls tunnels and for traceroute of stitched mpls LSPs. The techniques outlined in RFC 4379 are insufficient to perform traceroute FEC validation and path discovery for a LSP that goes over other mpls tunnels or for a stitched LSP. This document describes enhancements to the downstream-mapping TLV (defined in RFC 4379). These enhancements along with other procedures outlined in this document can be used to trace such LSPs.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”

This Internet-Draft will expire on November 27, 2010.

Copyright Notice

Copyright (c) 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.

Table of Contents

1.  Introduction
    1.1.  Conventions used in this document
2.  Motivation
3.  Packet format
    3.1.  Introduction
    3.2.  New Return Codes
        3.2.1.  Return code per downstream
        3.2.2.  Return code for stitched LSPs
    3.3.  Downstream Detailed Mapping TLV
        3.3.1.  Multipath data sub-TLV
        3.3.2.  Label stack sub-TLV
        3.3.3.  FEC Stack change sub-TLV
    3.4.  Deprecation of Downstream Mapping TLV
4.  Performing lsp-ping traceroute on tunnels
    4.1.  Transit node procedure
        4.1.1.  Addition of a new tunnel
        4.1.2.  Transition between tunnels
    4.2.  Ingress node procedure
        4.2.1.  Processing Downstream Detailed Mapping TLV
            4.2.1.1.  Stack Change sub-TLV not present
            4.2.1.2.  Stack Change sub-TLV(s) present
        4.2.2.  Modifications to handling to EGRESS_OK responses.
        4.2.3.  Handling of new return codes
    4.3.  Handling deprecated Downstream Mapping TLV
5.  Security Considerations
6.  IANA Considerations
7.  Acknowledgements
8.  References
    8.1.  Normative References
    8.2.  Informative References
§  Authors' Addresses

1.  Introduction

This documents describes methods for performing lsp-ping traceroute over mpls tunnels. The techniques outlined in [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.) outline a traceroute mechanism that includes FEC validation and ECMP path discovery. Those mechanisms are insufficient and do not provide details in case the FEC being traced traverses one or more mpls tunnels and in case where LSP stitching is in use. This document defines enhancements to the downstream-mapping TLV [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.) to make it more extensible and to enable retrieval of detailed information. Using the enhanced TLV format along with the existing definitions of [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.), this document describes procedures by which a traceroute request can correctly traverse mpls tunnels with proper FEC and label validations.

1.1.  Conventions used in this document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Motivation

A LSP-Ping traceroute may cross multiple mpls tunnels en-route the destination. Let us consider a simple case.

A          B          C           D           E
o -------- o -------- o --------- o --------- o
  \_____/  | \______/   \______/  | \______/
    LDP    |   RSVP       RSVP    |    LDP
           |                      |
            \____________________/
                     LDP

When a traceroute is initiated from router A, router B returns downstream mapping information for node C in the echo-response. The next echo request reaches router C with a LDP FEC. Node C is a pure RSVP node and does not run LDP. Node C will receive the packet with 2 labels but only 1 FEC in the Target FEC stack. Consequently, node C will be unable to perform FEC complete validation. It will let the trace continue by just providing next-hop information based on incoming label, and by looking up the forwarding state associated with that label. However, ignoring FEC validation defeats the purpose of control plane validatations. The echo request should contain sufficient information to allow node C to perform FEC validations to catch any misrouted echo-requests.

The above problem can be extended for a generic case of tunnel over tunnel or multiple tunnels (e.g. B-C can be a separate RSVP tunnel and C-D can be a separate RSVP tunnel). The problem of FEC validation for tunnels can be solved if the transit routers (router B in the above example) provide some hint or information to the ingress regarding the start of a new tunnel.

Stitched LSPs involve 2 or more LSP segments stitched together. The LSP segments can be signaled using the same or different signaling protocols. In order to perform an end-to-end trace of a stitched LSP, the ingress needs to know FEC information regarding each of the stitched LSP segments. For example, conside the figure below.

A          B          C           D          E         F
o -------- o -------- o --------- o -------- o ------- o
  \_____/    \______/   \______/    \______/  \_______/
    LDP        LDP         BGP         RSVP      RSVP

Consider ingress (A) tracing end-to-end LSP A--F. When an echo request reaches router C, there is a FEC stack change happening at router C. With current lsp-ping mechanisms, there is no way to convey this information to A. Consequently, when the next echo request reaches router D, router D will know nothing about the LDP FEC that A is trying to trace.

Thus, the procedures outlined [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.) do not make it possible for the ingress node to:

1. Know that tunneling has occured
2. Trace the path of the tunnel
3. Trace the path of stitched LSPs

3.  Packet format

3.1.  Introduction

In many cases there has been a need to associate additional data in the lsping echo response. In most cases, the additional data needs to be associated on a per downstream neighbor basis. Currently, the echo response contains 1 downstream map TLV (DSMAP) per downstream neighbor. But the DSMAP format is not extensible and hence it's not possible to associate more information with a downstream neighbor. This draft defines a new extensible format for the DSMAP and provides mechanisms for solving the tunneled lsp-ping problem using the new format. In summary, the draft makes the following TLV changes:

• Addition of new Downstream Detailed Mapping TLV (DDMAP).
• Deprecation of existing Downstream Mapping TLV.
• Addition of Downstream FEC Stack Change Sub-TLV to DDMAP.

3.2.  New Return Codes

3.2.1.  Return code per downstream

A new Return Code is being defined "See DDM TLV for Return Code and Return SubCode" (Section 6.3 (New Return Codes)) to indicate that the Return Code is per Downstream Detailed Mapping TLV (Section 3.3 (Downstream Detailed Mapping TLV)). This Return Code MUST be used only in the message header and MUST be set only in the echo response message. If the Return Code is set in the echo request message, then it SHOULD be ignored. When this Return Code is set, each Downstream Detailed Mapping TLV MUST have an appropriate Return Code and Return SubCode. This Return Code is to be used when there are multiple downstreams for a given node (such as P2MP or ECMP), and the node wants to return a different Return Code/Return SubCode for each downstream.

3.2.2.  Return code for stitched LSPs

When a traceroute is being performed on stitched LSPs Figure 10 (Stitched LSPs), the stitching point SHOULD indicate the stitching action to the node performing the trace. This is done by setting the Return Code to "Label switched with FEC change" (Section 6.3 (New Return Codes)). If a node is performing FEC hiding, then it MAY choose to set the Return Code to a value other than "Label switched with FEC change". This Return Code MUST NOT be used if no Label Stack sub-TLVs (Section 3.3.3 (FEC Stack change sub-TLV)) are present in the Downstream Detailed Mapping TLV(s). This new Return Code MAY be used for hierarchical LSPs (for indicating start or end of an outer LSP).

3.3.  Downstream Detailed Mapping TLV

A new TLV has been added to the mandatory range of TLVs. The TLV type is pending IANA allocation.

     Type #   Value Field
     ------   ------------

     TBD      Downstream detailed mapping

The Downstream Detailed Mapping object is a TLV that MAY be included in an echo request message. Only one Downstream Detailed Mapping object may appear in an echo request. The presence of a Downstream Mapping object is a request that Downstream Detailed Mapping objects be included in the echo reply. If the replying router is the destination of the FEC, then a Downstream Detailed Mapping TLV SHOULD NOT be included in the echo reply. Otherwise the replying router SHOULD include a Downstream Detailed Mapping object for each interface over which this FEC could be forwarded.

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               MTU             | Address Type  |    DS Flags   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Downstream IP Address (4 or 16 octets)            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Downstream Interface Address (4 or 16 octets)         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Return Code  | Return SubCode|        Sub-tlv length         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                      List of Sub TLVs                         .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Downstream Detailed Mapping TLV format is derived from the Downstream Mapping TLV format. The key change is that variable length and optional fields have been coverted into sub-TLVs. The fields have the same use and meaning as in [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.). The newly added sub-TLVs and their fields are as described below.

Return code

The Return Code and Return SubCode are set to zero by the sender. The receiver can set it to one of the values specified in the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Parameters" registry, "Return Codes" sub-registry. The notation <RSC> refers to the Return SubCode. This field is filled in with the stack-depth for those codes that specify that. For all other codes, the Return SubCode MUST be set to zero.

If the receiver sets a non-zero value of the Return Code field in the Downstream Detailed Mapping TLV, then the receiver MUST also set the Return Code field in the echo response header to "See DDM TLV for Return Code and Return SubCode" (Section 6.3 (New Return Codes)). An exception to this is if the receiver is a bud node and is replying as both an egress and a transit node with a Return Code of 3 ("Replying router is an egress for the FEC") in the echo response header.

If the Return Code of the echo response message is not set to either "See DDM TLV for Return Code and Return SubCode" (Section 6.3 (New Return Codes)) or "Replying router is an egress for the FEC", then the Return Code and Return SubCode specified in the Downstream Detailed Mapping TLV SHOULD be ignored.

Sub-tlv length

Total length in bytes of the sub-TLVs associated with this TLV.

     Sub-Type    Value Field
     ---------   ------------
     TBD         Multipath data
     TBD         Label stack
     TBD         FEC Stack change

3.3.1.  Multipath data sub-TLV

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Multipath Type |       Multipath Length        |Reserved (MBZ) |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      |                  (Multipath Information)                      |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The multipath data sub-TLV includes information multipath information. The TLV fields and their usage is as defined in [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.).

3.3.2.  Label stack sub-TLV

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Label stack sub-TLV contains the set of labels in the label stack as it would have appeared if this router were forwarding the packet through this interface. Any Implicit Null labels are explicitly included. The number of labels present in the sub-TLV is determined based on the sub-TLV data length. Labels are treated as numbers, i.e., they are right justified in the field. The label format and protocol type are as defined in [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.). When the Downstream Detailed Mapping TLV in sent in the echo response, this sub-TLV MUST be included.

3.3.3.  FEC Stack change sub-TLV

A router SHOULD include the the FEC Stack change sub-TLV when the downstream node in the echo response has a different FEC stack than the FEC stack received in the echo request. One or more FEC Stack change sub-TLVs MAY be present in the Downstream Detailed Mapping TLV. The format is as below.

0                   1                   2                   3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Operation Type | Address type  | FEC-tlv length|  Reserved     |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|           Remote Peer Address (0, 4 or 16 octets)             |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.                                                               .
.                         FEC TLV                               .
.                                                               .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Operation Type

The operation type specifies the action associated with the FEC stack change. The following operation types are defined.

     Type #     Operation
     ------     ---------
     1          Push
     2          Pop
     3          Swap

A FEC Stack change sub-TLV containing a PUSH operation MUST NOT be followed by a FEC Stack change sub-TLV containing a POP operation. One or more POP operations MAY be followed by one or more PUSH operations. One FEC Stack change sub-TLV MUST be included per FEC stack change. For example, if 2 labels are going to be pushed, then 1 FEC Stack change sub-TLV MUST be included for each FEC. A FEC splice operation (an operation where 1 FEC ends and another FEC starts, see Figure 10 (Stitched LSPs)) SHOULD be performed by including a POP type FEC Stack change sub-TLV followed by a PUSH type FEC Stack change sub-TLV.

A Downstream detailed mapping TLV containing only 1 FEC change sub-TLV with Pop operation is equivalent to EGRESS_OK (Return code 3, [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.)) for the outermost FEC in the FEC stack. The ingress router performing the lsp trace MUST treat such a case as an EGRESS_OK for the outermost FEC.

The Downstream Detailed Mapping TLV MUST contain at most 1 FEC Stack change sub-TLV of type Swap. In a FEC Stack change sub-TLV with Swap operation, the address-type MAY be set to Unspecified and the FEC-tlv length MAY be set to 0.

FEC tlv Length

Length in bytes of the FEC TLV.

Address Type

The Address Type indicates the remote peer's address type. The Address Type is set to one of the following values. The peer address length is determined based on the address type. The address type MAY be different from the address type included in the Downstream Detailed Mapping TLV. This can happen in case the LSP goes over a tunnel of a different address family. The address type MAY be set to Unspecified if the peer-address is either unavailable or the transit router does not wish it provide it for security or administrative reasons.

     Type #   Address Type   Address length
     ------   ------------   --------------

     0        Unspecified    0
     1        IPv4           4
     2        IPv6           16

Remote peer address

The remote peer address specifies the remote peer which is the next-hop for the FEC being currently traced. E.g. In the LDP over RSVP case Figure 1 (LDP over RSVP tunnel), router B would respond back with the address of router D as the remote peer address for the LDP FEC being traced. This allows the ingress node to provide helpful information regarding FEC peers. If the operation type is PUSH, the remote peer address is the address of the peer from which the FEC was learned. If the operation type is POP, the remote peer address MAY be set to Unspecified. For upstream assigned labels [RFC5331] (Aggarwal, R., Rekhter, Y., and E. Rosen, “MPLS Upstream Label Assignment and Context-Specific Label Space,” August 2008.), an operation type of POP will have a remote peer address (the upstream node that assigned the label) and this SHOULD be included in the FEC Stack change sub-TLV.

FEC TLV

The FEC TLV is present only when FEC-tlv length field is non-zero. The FEC TLV specifies the FEC associated with the FEC stack change operation. This TLV MAY be included when the operation type is POP. It SHOULD be included when the operation type is PUSH. The FEC TLV contains exactly 1 FEC from the list of FECs specified in [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.). A NIL FEC MAY be associated with a PUSH operation if the responding router wishes to hide the details of the FEC being pushed.

3.4.  Deprecation of Downstream Mapping TLV

The Downstream Mapping TLV has been deprecated. LSP-ping procedures should now use the Downstream Detailed Mapping TLV. Detailed procedures regarding interoperability between the deprecated TLV and the new tlv are specified in Section 4.3 (Handling deprecated Downstream Mapping TLV).

4.  Performing lsp-ping traceroute on tunnels

This section describes the procedures to be followed by an ingress node and transit nodes when performing lsp-ping traceroute over mpls tunnels.

4.1.  Transit node procedure

4.1.1.  Addition of a new tunnel

A transit node (Figure 1 (LDP over RSVP tunnel)) knows when the FEC being traced is going to enter a tunnel at that node. Thus, it knows about the new outer FEC. All transit nodes that are the origination point of a new tunnel SHOULD add the a FEC Stack change sub-TLV (Section 3.3.3 (FEC Stack change sub-TLV)) to the Downstream Detailed Mapping TLV (Figure 4 (Downstream Detailed Mapping TLV)) in the echo-response. The transit node SHOULD add 1 FEC Stack change sub-TLV of operation type PUSH, per new tunnel being originated at the transit node.

A transit node that sends a Downstream FEC Stack change sub-TLV in the echo response SHOULD fill the address of the remote peer; which is the peer of the current LSP being traced. If the transit node does not know the address of the remote peer, it MAY leave it as unspecified.

If the transit node wishes to hide the nature of the tunnel from the ingress of the echo-request, then it MAY not want to send details about the new tunnel FEC to the ingress. In such a case, the transit node SHOULD use the NIL FEC. The echo response would then contain a FEC Stack change sub-TLV with operation type PUSH and a NIL FEC. The value of the label in the NIL FEC MUST be set to zero. The remote peer address type MUST be set to Unspecified. The transit node SHOULD add 1 FEC Stack change sub-TLV of operation type PUSH, per new tunnel being originated at the transit node.

4.1.2.  Transition between tunnels

A          B          C           D          E         F
o -------- o -------- o --------- o -------- o ------- o
  \_____/    \______/   \______/    \______/  \_______/
    LDP        LDP         BGP         RSVP      RSVP

In the above figure, we have 3 seperate LSP segments stitched at C and D. Node C SHOULD include 2 FEC Stack change sub-TLVs. One with a POP operation for the LDP FEC and one with the PUSH operation for the BGP FEC. Similarly, node D SHOULD include 2 FEC Stack change sub-TLVs, one with a POP operation for the BGP FEC and one with a PUSH operation for the RSVP FEC. Nodes C and D SHOULD set the Return Code to "Label switched with FEC change" (Section 6.3 (New Return Codes)) to indicate change in FEC being traced.

If node C wishes to perform FEC hiding, it SHOULD respond back with 2 FEC Stack change sub-TLVs. One POP followed by 1 PUSH. The POP operation MAY either exclude the FEC TLV (by setting FEC TLV length to 0) or set the FEC TLV to contain the LDP FEC. The PUSH operation SHOULD have the FEC TLV containing the NIL FEC. The Return Code SHOULD be set to "Label switched with FEC change".

If node C performs FEC hiding and node D also performs FEC hiding, then node D MAY choose to not send any FEC Stack change sub-TLVs in the echo response since the number of labels has not changed (for the downstream of node D) and the FEC type also has not changed (NIL FEC). In such a case, node D MUST NOT set the Return Code to "Label switched with FEC change". If node D performs FEC hiding, then node F will respond as EGRESS_OK for the NIL FEC. The ingress (node A) will know that EGRESS_OK corresponds to the end-to-end LSP.

A          B          C           D           E           F
o -------- o -------- o --------- o --------- o --------- o
  \_____/  |\____________________/            |\_______/
    LDP    |\       RSVP-A                    |    LDP
           | \_______________________________/|
           |       RSVP-B                     |
            \________________________________/
                            LDP

In the above figure, the following sequence of FEC Stack change sub-TLVs will be performed

Node B:

Respond with 2 FEC Stack change sub-TLVs: PUSH RSVP-B, PUSH RSVP-A.

Node D:

Respond with a Return Code of EGRESS_OK when RSVP-A is top of FEC stack. Downstream information for node E when echo request contains RSVP-B as top of FEC stack and an appropriate Return Code.

If node B is performing tunnel hiding, then:

Node B:

Respond with 2 FEC Stack change sub-TLVs: PUSH NIL-FEC, PUSH NIL-FEC.

Node D:

Respond with either Return Code of EGRESS_OK (if D can co-relate that the NIL-FEC corresponds to RSVP-A which is terminating at D) or respond with FEC Stack change sub-TLV: POP (since D knows that number of labels towards next-hop is decreasing).

A          B          C        D        E       F       G
o -------- o -------- o ------ o ------ o ----- o ----- o
     LDP       LDP        BGP   \  RSVP    RSVP /  LDP
                                 \_____________/
                                      LDP

In the above case, node D will send 3 FEC Stack change sub-TLVs. One POP (for the BGP FEC) followed by 2 PUSHes (one for LDP and one for RSVP). Nodes C and D SHOULD set the Return Code to "Label switched with FEC change" (Section 6.3 (New Return Codes)) to indicate change in FEC being traced.

4.2.  Ingress node procedure

It is the responsibility of an ingress node to understand tunnel within tunnel semantics and lsp stitching semantics when performing a lsp traceroute. This section describes the ingress node procedure based on the kind of response an ingress node receives from a transit node.

4.2.1.  Processing Downstream Detailed Mapping TLV

Downstream Detailed Mapping TLV should be processed in procedures similar to those of Downstream Mapping TLV, defined in Section 4.4 of [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.)

4.2.1.1.  Stack Change sub-TLV not present

This would be the default behavior as described in [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.). The ingress node MUST perform echo response processing as per the procedures in [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.).

4.2.1.2.  Stack Change sub-TLV(s) present

If one or more FEC Stack change sub-TLVs (Section 3.3.3 (FEC Stack change sub-TLV)) are received in the echo response, the ingress node SHOULD process them and perform some validation.

The FEC stack changes are associated with a downstream neighbor and along a particular path of the LSP. Consequently, the ingress will need to maintain a FEC-stack per path being traced (in case of multipath). All changes to the FEC stack resulting from the processing of FEC Stack change sub-TLV(s) should be applied only for the path along a given downstream neighbor. The following algorithm should be followed for processing FEC Stack change sub-TLVs.

 push_seen = FALSE
 fec_stack_depth = current-depth-of-fec-stack-being-traced
 saved_fec_stack = current_fec_stack

 while (sub-tlv = get_next_sub_tlv(downstream_detailed_map_tlv))

     if (sub-tlv == NULL) break

     if (sub-tlv.type == FEC-Stack-Change) {

         if (sub-tlv.operation == POP) {
             if (push_seen) {
                 Drop the echo response
                 current_fec_stack = saved_fec_stack
                 return
             }

             if (fec_stack_depth == 0) {
                 Drop the echo response
                 current_fec_stack = saved_fec_stack
                 return
             }

             Pop FEC from FEC stack being traced
             fec_stack_depth--;
         }

         if (sub-tlv.operation == PUSH) {
             push_seen = 1
             Push FEC on FEC stack being traced
             fec_stack_depth++;
         }
      }
  }


  if (fec_stack_depth == 0) {
      Drop the echo response
      current_fec_stack = saved_fec_stack
      return
  }

The next echo request along the same path should use the modified FEC stack obtained after processing the FEC Stack change sub-TLVs. A non-NIL FEC guarantees that the next echo request along the same path will have the Downstream Detailed Mapping TLV validated for IP address, Interface address and label stack mismatches.

If the top of the FEC stack is a NIL FEC and the echo response does not contain any FEC Stack change sub-TLV, then it does not necessarily mean that the LSP has not started traversing a different tunnel. It could be that the LSP associated with the NIL FEC terminated at a transit node and at the same time a new LSP started at the same transit node. The NIL FEC would now be associated with the new LSP (and the ingress has no way of knowing this). Thus, it is not possible to build an accurate hierarchical LSP topology if a traceroute contains NIL FECs.

4.2.2.  Modifications to handling to EGRESS_OK responses.

The procedures above allow the addition of new FECs to the original FEC being traced. Consequently, the EGRESS_OK response from a downstream node may not necessarily be for the FEC being traced. It could be for one of the new FECs that was added. On receipt of an EGRESS_OK response, the ingress should check if the depth of Target FEC sent to the node that just responded, was the same as the depth of the FEC that was being traced. If it was not, then it should pop the an entry from the Target FEC stack and resend the request with the same TTL (as previously sent). The process of popping a FEC is to be repeated until either the ingress receives a non-EGRESS_OK response or until all the additional FECs added to the FEC stack have already been popped. Using EGRESS_OK responses, an ingress can build a map of the hierarchical LSP structure traversed by a given FEC.

4.2.3.  Handling of new return codes

When the echo response Return Code is "Label switched with FEC change" (Section 3.2.2 (Return code for stitched LSPs)), the ingress node SHOULD manipulate the FEC stack as per the FEC Stack change sub-TLVs contained in the detailed downstream mapping TLV. A transit node can use this Return Code for stitched LSPs and for hierarchical LSPs. In case of ECMP or P2MP, there could be multiple paths and detailed downstream mapping TLVs with different return codes (Section 3.2.1 (Return code per downstream)). The ingress node should build the topology based off the Return Code per ECMP path/P2MP branch.

4.3.  Handling deprecated Downstream Mapping TLV

The Downstream Mapping TLV has been deprecated. Applications should now use the Downstream Detailed Mapping TLV. The following procedures SHOULD be used for backward compatibility with routers that do not support the Downstream Detailed Mapping TLV.

• The Downstream Mapping TLV and the Downstream Detailed Mapping TLV MUST never be sent together in the same echo request or in the same echo response.
• If the echo request contains a Downstream Detailed Mapping TLV and the corresponding echo response contains an Return Code of 2 (one or more of the TLVs was not understood), then the sender of the echo request MAY resend the echo request with the Downstream Mapping TLV (instead of the Downstream Detailed Mapping TLV). In cases where a detailed response is needed, the sender can choose to ignore the router that does not support the Downstream Detailed Mapping TLV.
• If the echo request contains a Downstream Mapping TLV, then a Downstream Detailed Mapping TLV MUST NOT be sent in the echo response. This is to handle the case that the sender of the echo request does not support the new TLV.
• If echo request forwarding is in use; such that the echo request is processed at an intermediate router and then forwarded on; then the intermediate router is responsible for making sure that the TLVs being used among the ingress, intermediate and destination are consistent. The intermediate router MUST NOT forward an echo request or an echo response containing a Downstream Detailed Mapping TLV if it itself does not support that TLV.

5.  Security Considerations

Tracing inside a tunnel might have some security implications. There are different ways to prevent tracing tunnel details.

1. If one wants to prevent tracing inside a tunnel, one can hide the outer MPLS tunnel by not propagating the MPLS TTL into the outer tunnel (at the start of the outer tunnel). By doing this, lsp-ping packets will not expire in the outer tunnel and the outer tunnel will not get traced. TTL hiding can be imposed on a per LSP basis, as need be.
2. If one doesn't wish to expose the details of the new outer LSP, then the NIL FEC can be used to hide those details. Using the NIL FEC ensures that the trace progresses without false negatives and all transit nodes (of the new outer tunnel) perform some minimal validations on the received echo requests.

In inter-AS (autonomous system) scenarios, information regarding the LSP FEC change(s) SHOULD NOT be passed across domains. A NIL FEC MAY be used to make the trace go through without false positives. An ASBR (autonomous system border router) may choose to intercept all echo requests and echo responses and change them to hide FEC information from other domains. Detailed operation regarding the same is outside the scope of this document. Passing of FEC stack change information between domains MAY be done if the two AS domains belong to the same provider/organization.

Other security considerations, as discussed in [RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.) are also applicable to this document.

6.  IANA Considerations

6.1.  New TLV

IANA is requested to assign TLV type value to the following TLV from the "Multiprotocol Label Switching Architecture (MPLS) Label Switched Paths (LSPs) Parameters - TLVs" registry, "TLVs and sub-TLVs" sub-registry.

Downstream Detailed Mapping TLV (See Section 3.3 (Downstream Detailed Mapping TLV)). Suggested value: 20.

6.2.  New Sub-TLV types and associated registry

IANA is requested to create a new registry for the Sub-Type field of Downstream Detailed Mapping TLV. The valid range for this is 0-65535. Assignments in the range 0-16383 and 32768-49161 are made via Standards Action as defined in [RFC3692] (Narten, T., “Assigning Experimental and Testing Numbers Considered Useful,” January 2004.); assignments in the range 16384-31743 and 49162-64511 are made via Specification Required ([RFC4379] (Kompella, K. and G. Swallow, “Detecting Multi-Protocol Label Switched (MPLS) Data Plane Failures,” February 2006.)); values in the range 31744-32767 and 64512-65535 are for Vendor Private Use, and MUST NOT be allocated. If a sub-TLV has a Type that falls in the range for Vendor Private Use, the Length MUST be at least 4, and the first four octets MUST be that vendor's SMI Enterprise Code, in network octet order. The rest of the Value field is private to the vendor.

It is requested that IANA assign sub-TLV types from this new registry to the following sub-TLVs (See Figure 5 (Downstream Detailed Mapping Sub-TLV List)).

Multipath data sub-TLV: Suggested value: 1

Label stack sub-TLV: Suggested value: 2

FEC Stack change sub-TLV: Suggested value: 3

6.3.  New Return Codes

IANA is requested to assign new Return Code values from the "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs) Parameters" registry, "Return Codes" sub-registry as follows using a Standards Action value.

    Value    Meaning
    -----    -------
    TBD      See DDM TLV for Return Code and Return SubCode
    TBD      Label switched with FEC change

Suggested values: 14 and 15 respectively

7.  Acknowledgements

The authors would like to thank Yakov Rekhter and Adrian Farrel for their suggestions on the draft.

8.  References

8.1. Normative References

8.2. Informative References

Authors' Addresses