Egress Validation in Label Switched Path Ping and Traceroute Mechanisms
draft-ietf-mpls-egress-tlv-for-nil-fec-12

Routing area                                               D. Rathi, Ed.
Internet-Draft                                                     Nokia
Intended status: Standards Track                           S. Hegde, Ed.
Expires: 2 September 2024                          Juniper Networks Inc.
                                                                K. Arora
                                                  Individual Contributor
                                                                  Z. Ali
                                                               N. Nainar
                                                     Cisco Systems, Inc.
                                                            1 March 2024

Egress Validation in Label Switched Path Ping and Traceroute Mechanisms
               draft-ietf-mpls-egress-tlv-for-nil-fec-12

Abstract

   The MPLS ping and traceroute mechanism as described in RFC 8029 and
   related extensions for Segment Routing(SR) as defined in RFC 8287 is
   very useful to validate the control plane and data plane
   synchronization.  In some environments, only some intermediate or
   transit nodes may have been upgraded to support these validation
   procedures.  A simple MPLS ping and traceroute mechanism allows
   traversing any path without validating the control plane state.  RFC
   8029 supports this mechanism with Nil Forwarding Equivalence Class
   (FEC).  The procedures described in RFC 8029 mostly apply when the
   Nil FEC is used as an intermediate FEC in the label stack.  When all
   labels in the label stack are represented using Nil FEC, it poses
   some challenges.

   This document introduces a new Type-Length-Value (TLV) as an
   extension to exisiting Nil FEC.  It describes MPLS ping and
   traceroute procedures using Nil FEC with this extension to overcome
   these challenges.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

Status of This Memo

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

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem with Nil FEC  . . . . . . . . . . . . . . . . . . . .   4
   3.  Egress TLV  . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Procedure . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Sending Egress TLV in MPLS Echo Request . . . . . . . . .   5
       4.1.1.  Ping Mode . . . . . . . . . . . . . . . . . . . . . .   6
       4.1.2.  Traceroute Mode . . . . . . . . . . . . . . . . . . .   6
       4.1.3.  Detailed Example  . . . . . . . . . . . . . . . . . .   6
     4.2.  Receiving Egress TLV in MPLS Echo Request . . . . . . . .   7
   5.  Backward Compatibility  . . . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
     6.1.  New TLV . . . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  New Return code . . . . . . . . . . . . . . . . . . . . .   9
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   8.  Implementation Status . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Juniper Networks  . . . . . . . . . . . . . . . . . . . .  10
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10
     10.2.  Informative References . . . . . . . . . . . . . . . . .  11

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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Segment routing supports the creation of explicit paths using Adj-
   SIDs, Node-SIDs, and Anycast-SIDs defined in [RFC8402].  In certain
   usecases, the TE paths are built using mechanisms described in
   [RFC9256] by stacking the labels that represent the nodes and links
   in the explicit path.  Controllers are often deployed to construct
   paths across multi-domain networks.  In such deployments, the head-
   end routers may have a database of its own domain and may not be
   aware of the FEC associated with labels that are used by the
   controller to build paths across multiple domains.  A very useful
   Operations, Administration, and Maintenance (OAM) requirement is to
   be able to ping and trace these paths.

   [RFC8029] describes a simple and efficient mechanism to detect data-
   plane failures in MPLS Label Switched Paths (LSPs).  It defines a
   probe message called an "MPLS echo request" and a response message
   called an "MPLS echo reply" for returning the result of the probe.
   SR related extensions to Echo Request/Echo Reply are specified in
   [RFC8287].  [RFC8029] provides mechanisms to primarily validate the
   data plane and secondarily to verify the data plane against the
   control plane.  It also provides the ability to traverse Equal cost
   Mutiple Paths (ECMP) and validate each of the ECMP paths.  The use of
   Target FEC requires all nodes in the network to have implemented the
   validation procedures.  All intermediate nodes may not have been
   upgraded to support validation procedures.  In such cases, it is
   useful to have the ability to traverse the paths in ping/traceroute
   mode without having to obtain the FEC for each label.  A simple MPLS
   Echo Request/Echo Reply mechanism allows for traversing the SR Policy
   path without validating the control plane state.  [RFC8029] supports
   this mechanism with FECs like Nil FEC and Generic FEC.

   Generic IPv4 and IPv6 FECs are used when the protocol that is
   advertising the label is unknown.  The information that is carried in
   Generic FEC is the IPv4 or IPv6 prefix and prefix length.  Thus
   Generic FEC types perform an additional control plane validation.
   However, the details of Generic FEC and validation procedures are not
   very detailed in the [RFC8029].  The use-case mostly specifies inter-
   AS VPNs as the motivation.  Certain aspects of SR such as anycast
   SIDs require clear guidelines on how the validation procedure should
   work.  Also, Generic FEC may not be widely supported and if transit
   routers are not upgraded to support validation of Generic FEC,
   traceroute may fail.  On other hand, Nil FEC consists of the label
   and there is no other associated FEC information.  Nil FEC is used to
   traverse the path without validation for cases where the FEC is not
   defined or routers are not upgraded to support the FECs.  Thus, it

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   can be used to check any combination of segments on any data path.
   The procedures described in [RFC8029] are mostly applicable when the
   Nil FEC is used where the Nil FEC is an intermediate FEC in the label
   stack.  When all labels in the label-stack are represented using Nil
   FEC, it poses some challenges.

   Section 2 discusses the problems associated with using Nil FEC in an
   MPLS ping/traceroute procedure and Section 3 and Section 4 discuss
   simple extensions needed to solve the problem.

2.  Problem with Nil FEC

   The purpose of Nil FEC as described in [RFC8029] is to ensure hiding
   of transit tunnel information and in some cases to avoid false
   negatives when the FEC information is unknown.

   This document uses a Nil FEC to represent the complete label stack in
   an MPLS Echo Request message in ping and traceroute mode.  A single
   Nil FEC is used in the MPLS Echo Request message irrespective of the
   number of segments in the label-stack.  As described in sec 4.4.1 of
   [RFC8029], "If the outermost FEC of the Target FEC stack is the Nil
   FEC, then the node MUST skip the Target FEC validation completely."
   When a router in the label-stack path receives an MPLS Echo Request
   message, there is no definite way to decide on whether it is the
   intended egress router since Nil FEC does not carry any information
   and no validation is performed by the router.  So there is high
   possibility that the packet may be mis-forwarded to an incorrect
   destination but the MPLS Echo Reply might still return success.

   To avoid this problem, there is a need to add additional information
   in the MPLS Echo Request message in ping and treaceroute mode along
   with Nil FEC to do minimal validation on the egress/destination
   router and send proper information on success and failure to the
   ingress router.  This additional information should help to report
   transit router information to the ingress/initiator router that can
   be used by an offline application to validate the traceroute path.

   Thus the addition of egress information in the MPLS Echo Request
   message in ping and traceroute mode will help in validating Nil-FEC
   on each receiving router on the label-stack path to ensure the
   correct destination.  It can be used to check any combination of
   segments on any path without upgrading transit nodes.  The code point
   used for Egress TLV is from the range 32768-65535 and can can be
   silently dropped if not recognised as per [RFC8029] and as per
   clarifications from [RFC9041]

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3.  Egress TLV

   The Egress TLV MAY be included in an MPLS Echo Request message.  It
   is an optional TLV and if it is present it MUST appear before the
   FEC-stack TLV in the MPLS Echo Request packet.  This TLV can only be
   used in LSP ping/traceroute requests generated by the head-end node
   of an LSP or SR policy for which verification is performed.  In case
   multiple Nil FECs are present in Target FEC Stack TLV, Egress TLV
   MUST be added corresponding to the ultimate egress of the label-
   stack.  It can be used for any kind of path with Egress TLV added
   corresponding to the endpoint of the path.  Explicit Path can be
   created using Node-SID, Adj-SID, Binding-SID etc.  Prefix field of
   Egress TLV MUST be derived from path egress/destination.  The format
   is as specified 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |      Type = 32771 (Egress TLV)  |          Length             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                      Prefix (4 or 16 octets)                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 1: Egress TLV

   Type : 32771 (Section 6.1)

   Length : variable based on IPV4/IPV6 prefix.  Length excludes the
   length of the Type and Length fields.  Length will be 4 octets for
   IPv4 and 16 octets for IPv6.

   Prefix : This field carries the valid IPv4 prefix of length 4 octets
   or valid IPv6 Prefix of length 16 octets.  It can be obtained from
   the egress of Nil FEC corresponding to the last label in the label-
   stack or SR policy endpoint field
   [I.D-ietf-idr-segment-routing-te-policy].

4.  Procedure

   This section describes aspects of LSP ping and traceroute operations
   that require further considerations beyond [RFC8029].

4.1.  Sending Egress TLV in MPLS Echo Request

   As stated earlier, when the sender node builds an Echo Request with
   target FEC Stack TLV, Egress TLV when present, MUST appear before
   Target FEC-stack TLV in the MPLS Echo Request packet.

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4.1.1.  Ping Mode

   When sender node builds an Echo Request with target FEC Stack TLV
   that contains a single NiL FEC corresponding to the last segment of
   the SR Policy path, the sender node MUST add an Egress TLV with the
   prefix obtained from SR policy endpoint field
   [I.D-ietf-idr-segment-routing-te-policy] to indicate the egress for
   this Nil FEC in the Echo Request packet.  The Label value in the Nil
   FEC MAY be set to zero when single Nil FEC is added for multiple
   labels in the label stack.  In case the endpoint is not specified or
   is equal to 0, the sender MUST use the prefix corresponding to the
   last segment of the SR Policy as prefix for Egress TLV.  Some
   specific cases on how to derive the prefix field in the Egress TLV
   are listed below:

      a.  If the last SID in the policy is an Adj-SID, the prefix
      represents the node at the remote end of the corresponding
      adjacency

      b.  If the last SID in the policy is a Binding SID, the prefix
      represents the last node of the path represented by the Binding
      SID.

4.1.2.  Traceroute Mode

   When sender node builds an Echo Request with target FEC Stack TLV
   that contains NiL FEC corresponding to last segment of the segment-
   list of the SR Policy, the sender node MUST add an Egress TLV with
   the prefix obtained from the SR policy endpoint field
   [I.D-ietf-idr-segment-routing-te-policy] to indicate the egress for
   this Nil FEC in the Echo Request packet.

   Although there is no requirement to do so, an implementation MAY send
   multiple Nil FEC if that makes the it easier for the implementation.
   In case the headend sends multiple Nil FECs the last one MUST
   correspond to the Egress TLV.  The Label value in the Nil FEC MAY be
   set to zero for the last Nil FEC.  In case the endpoint is not
   specified or is equal to 0 ( as in case of color-only SR Policy),the
   sender MUST use the the prefix corresponding to the last segment
   endpoint of the SR Policy path i.e. ultimate egress as the prefix for
   Egress TLV.

4.1.3.  Detailed Example

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                     ----R3----
                    /  (1003)  \
         (1001)    /            \(1005)     (1007)
           R1----R2(1002)        R5----R6----R7(prefix X)
                   \            /     (1006)
                    \   (1004) /
                     ----R4----

             Figure 2: Egress TLV processing on sample topology

   Consider the SR Policy configured with label-stack as 1002, 1004 ,
   1007 and end point/destination as prefix X on ingress router R1 to
   reach egress router R7.  Segment 1007 belongs to R7, which has the
   prefix X locally configured on it.

   In ping Echo Request, with target FEC Stack TLV that contains a
   single NiL FEC corresponding to 1007, should add Egress TLV for
   endpoint/destination prefix X with type as Egress TLV, length depends
   on if X is IPv4 or IPv6 address and prefix as X.

   In traceroute Echo Request, with target FEC Stack TLV that contains a
   single NiL FEC corresponding to the complete label-stack (1002, 1004,
   1007) or multiple Nil-FEC corresponding to each label in the label-
   stack, should add single Egress TLV for endpoint/destination prefix X
   with type as Egress TLV, length depends on if X is IPv4 or IPv6
   address and prefix as X.  In case X is not present or is set to 0 (
   as in the case of color-only SR Policy), sender should use the
   endpoint of segment 1007 as a prefix for Egress TLV.

4.2.  Receiving Egress TLV in MPLS Echo Request

   No change in the processing for Nil FEC as defined in [RFC8029] in
   Target FEC stack TLV Node that receives an MPLS echo request.  The
   presence of Egress TLV does not affect the validation of Target FEC
   Stack sub-TLV at FEC-stack-depth if it is different than Nil FEC.

   Additional processing is done for the Egress TLV on the receiver node
   as follows:

   1.  If the Label-stack-depth is greater than 0 and the Target FEC
   Stack sub-TLV at FEC-stack-depth is Nil FEC, set Best-return-code to
   8 ("Label switched at stack-depth") and Best-return-subcode to Label-
   stack-depth to report transit switching in MPLS Echo Reply message.

   2.  If the Label-stack-depth is 0 and the Target FEC Stack sub-TLV at
   FEC-stack-depth is Nil FEC then do the lookup for an exact match of
   the Egress TLV prefix to any of the locally configured interfaces or
   loopback addresses.

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   2a.  If the Egress TLV prefix lookup succeeds, set Best-return-code
   to 36 ("Replying router is an egress for the prefix in Egress TLV for
   the FEC at stack depth RSC") (Section 6.2) egress ok in MPLS Echo
   Reply message.

   2b.  If the Egress TLV prefix lookup fails, set the Best-return-code
   to 10, "Mapping for this FEC is not the given label at stack-depth
   RSC"

   3.In cases where multiple Nil FECs are sent from ingress, one each
   corresponding to the labels in the label stack along with Egress
   TLV,when the packet reaches egress, the number of labels in the
   received packet (Size of stack-R) becomes zero or a label with
   Bottom-of-Stack bit set to 1 is processed, all Nil FEC sub-TLVs MUST
   be removed and the Egress TLV MUST be validated.

5.  Backward Compatibility

   The extensions defined in this document is backward compatible with
   procedures described in [RFC8029].  A Router that does not support
   Egress TLV, will ignore it and use current the Nil-FEC procedures
   described in [RFC8029].

   When the egress node in the path does not support the extensions
   defined in this document egress validation will not be done and Best-
   return-code as 3 ("Replying router is an egress for the FEC at stack-
   depth") and Best-return- subcode set to stack-depth to will be set in
   the MPLS Echo Reply message.

   When the transit node in the path does not support the extensions
   defined in this document Best-return-code as 8 ("Label switched at
   stack-depth") and Best-return-subcode as Label-stack-depth to report
   transit switching will be set in the MPLS Echo Reply message.

6.  IANA Considerations

   The code points in section Section 6.1 and Section 6.2 have been
   assigned by [IANA] by early allocation on 2023-10-05 and 2021-11-08
   respectively.

6.1.  New TLV

   [IANA] is requested to update the early allocation for Egress TLV in
   the "Multi-Protocol Label Switching (MPLS) Label Switched Paths
   (LSPs) Ping Parameters" in the "TLVs" sub-registry to reference this
   document when published as an RFC.

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           +=======+=============+============================+
           | Value | Description | Reference                  |
           +=======+=============+============================+
           | 32771 |  Egress TLV | Section 3 of this document |
           +-------+-------------+----------------------------+

                        Table 1: TLVs Sub-Registry

6.2.  New Return code

   [IANA] is requested to update the early allocation of Return Code for
   "Replying router is an egress for the prefix in Egress TLV" in the
   "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs)
   Ping Parameters" in "Return Codes" sub-registry to reference this
   document when published as an RFC.

         +=======+================================+=============+
         | Value |          Description           | Reference   |
         +=======+================================+=============+
         | 36    |  Replying router is an egress  | Section 4.2 |
         |       |  for the prefix in Egress TLV  | of this     |
         |       | for the FEC at stack depth RSC | document    |
         +-------+--------------------------------+-------------+

                    Table 2: Return code Sub-Registry

7.  Security Considerations

   This document defines additional MPLS LSP ping TLVs and follows the
   mechanisms defined in [RFC8029].  All the security considerations
   defined in [RFC8287] will be applicable for this document and, in
   addition, they do not impose any additional security challenges to be
   considered.

8.  Implementation Status

   This section is to be removed before publishing as an RFC.

   RFC-Editor: Please clean up the references cited by this section
   before publication.

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort

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   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.

8.1.  Juniper Networks

   Organization: Juniper Networks

   Implementation: JUNOS

   Description: Implementation for sending and validating Egress TLV

   Maturity Level: Released

   Coverage: Full

   Contact: shraddha@juniper.net

9.  Acknowledgements

   Authors would like to thank Stewart Bryant, Greg Mirsky, Alexander
   Vainshtein, Sanga Mitra Rajgopal, Adrian Farrel for their careful
   review and comments.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8029]  Kompella, K., Swallow, G., Pignataro, C., Ed., Kumar, N.,
              Aldrin, S., and M. Chen, "Detecting Multiprotocol Label
              Switched (MPLS) Data-Plane Failures", RFC 8029,
              DOI 10.17487/RFC8029, March 2017,
              <https://www.rfc-editor.org/info/rfc8029>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8287]  Kumar, N., Ed., Pignataro, C., Ed., Swallow, G., Akiya,
              N., Kini, S., and M. Chen, "Label Switched Path (LSP)
              Ping/Traceroute for Segment Routing (SR) IGP-Prefix and

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              IGP-Adjacency Segment Identifiers (SIDs) with MPLS Data
              Planes", RFC 8287, DOI 10.17487/RFC8287, December 2017,
              <https://www.rfc-editor.org/info/rfc8287>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC9041]  Andersson, L., Chen, M., Pignataro, C., and T. Saad,
              "Updating the MPLS Label Switched Paths (LSPs) Ping
              Parameters IANA Registry", RFC 9041, DOI 10.17487/RFC9041,
              July 2021, <https://www.rfc-editor.org/info/rfc9041>.

   [RFC9256]  Filsfils, C., Talaulikar, K., Ed., Bogdanov, A., Mattes,
              P., and D. Voyer, "Segment Routing Policy Architecture",
              RFC 9256, DOI 10.17487/RFC9256, July 2020,
              <https://www.rfc-editor.org/info/rfc9256>.

10.2.  Informative References

   [I.D-ietf-idr-segment-routing-te-policy]
              Filsfils, C., Ed., Previdi, S., Ed., Talaulikar, K.,
              Mattes, P., Rosen, E., Jain, D., and S. Lin, "Advertising
              Segment Routing Policies in BGP",  draft-ietf-idr-segment-
              routing-te-policy-20,  work in progress, May 2020,
              <https://datatracker.ietf.org/doc/html/draft-ietf-idr-
              segment-routing-te-policy-20>.

   [IANA]     IANA, "Multiprotocol Label Switching (MPLS) Label Switched
              Paths (LSPs) Ping Parameters",
              <http://www.iana.org/assignments/mpls-lsp-ping-
              parameters>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/info/rfc7942>.

Authors' Addresses

   Deepti N. Rathi (editor)
   Nokia
   Manyata Embassy Business Park
   Bangalore 560045
   Karnataka
   India
   Email: deepti.nirmalkumarji_rathi@nokia.com

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   Shraddha Hegde (editor)
   Juniper Networks Inc.
   Exora Business Park
   Bangalore 560103
   KA
   India
   Email: shraddha@juniper.net

   Kapil Arora
   Individual Contributor
   Email: kapil.it@gmail.com

   Zafar Ali
   Cisco Systems, Inc.
   Email: zali@cisco.com

   Nagendra Kumar Nainar
   Cisco Systems, Inc.
   Email: naikumar@cisco.com

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