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Versions: (draft-akiya-mpls-lsp-ping-reply-mode-simple) 00 01 02 03 04 05 RFC 7737

Internet Engineering Task Force                                 N. Akiya
Internet-Draft                                       Big Switch Networks
Updates: 7110 (if approved)                                   G. Swallow
Intended status: Standards Track                            C. Pignataro
Expires: March 16, 2016                                    Cisco Systems
                                                            L. Andersson
                                                                 M. Chen
                                                                  Huawei
                                                      September 13, 2015


  Label Switched Path (LSP) Ping/Traceroute Reply Mode Simplification
             draft-ietf-mpls-lsp-ping-reply-mode-simple-04

Abstract

   The Multiprotocol Label Switching (MPLS) Label Switched Path (LSP)
   Ping and Traceroute use the Reply Mode field to signal the method to
   be used in the MPLS echo reply.  This document updates the "Reply via
   Specified Path (5)" Reply Mode value to easily indicate the reverse
   LSP.  This document also adds an optional TLV which can carry ordered
   list of Reply Mode values.

   This document updates RFC7110.

Requirements Language

   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 RFC 2119 [RFC2119].

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 March 16, 2016.




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Copyright Notice

   Copyright (c) 2015 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Problem Statements  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Reply via Specified Path Update . . . . . . . . . . . . .   5
     3.2.  Reply Mode Order TLV  . . . . . . . . . . . . . . . . . .   6
   4.  Relations to Other LSP Ping/Trace Features  . . . . . . . . .   8
     4.1.  Backwards Compatibility with Reply via Specified Path (5)
           Reply Mode  . . . . . . . . . . . . . . . . . . . . . . .   8
     4.2.  Reply Path TLV  . . . . . . . . . . . . . . . . . . . . .   8
       4.2.1.  Example 1: Reply Mode Order TLV Usage with Reply Path
               TLV . . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.2.2.  Example 2: Reply Mode Order TLV Usage with Reply Path
               TLV . . . . . . . . . . . . . . . . . . . . . . . . .   9
     4.3.  Proxy LSP Ping  . . . . . . . . . . . . . . . . . . . . .  10
       4.3.1.  Proxy LSR Sending an MPLS Echo Request  . . . . . . .  10
       4.3.2.  Proxy LSR Sending an MPLS Proxy Ping Reply  . . . . .  10
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
     7.1.  New Reply Mode Order TLV  . . . . . . . . . . . . . . . .  11
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  11
   9.  Contributing Authors  . . . . . . . . . . . . . . . . . . . .  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  12
     10.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Appendix A.  Reply Mode Order TLV Beneficial Scenarios  . . . . .  12
     A.1.  Incorrect Forwarding Scenario . . . . . . . . . . . . . .  13
     A.2.  Non-Co-Routed Bidirectional LSP Scenario  . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15





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1.  Introduction

   The Multiprotocol Label Switching (MPLS) Label Switched Path (LSP)
   Ping, described in [RFC4379], allows an initiator LSR to encode
   instructions (Reply Mode) on how a responder LSR should send the
   response back to the initiator LSR.  [RFC7110] also allows the
   initiator LSR to encode a TLV (Reply Path TLV) which can instruct the
   responder LSR to use a specific LSP to send the response back to the
   initiator LSR.  Both approaches are powerful as they provide the
   ability for the initiator LSR to control the return path.

   However, it is becoming increasingly difficult for an initiator LSR
   to select a valid return path to encode in the MPLS LSP echo request
   packets.  If the initiator LSR does not select a valid return path,
   the MPLS LSP echo reply will not get back to the initiator LSR.  This
   results in a false failure of MPLS LSP Ping and Traceroute operation.
   In an effort to minimize such false failures, different
   implementations have chosen different default return path encoding
   for different LSP types and LSP operations.  The problem with
   implementations having different default return path encoding is that
   the MPLS echo reply will not work in many cases, and the default
   value may not be the preferred choice by the operators.

   This document describes:

   o  In Section 2, further description of the problems;

   o  In Section 3, a solution to minimize false failures while
      accommodating operator preferences;

   o  In Section 4, relationships to other LSP Ping/Traceroute features;

   o  In Appendix A, examples of scenarios where the mechanism described
      in this document provides benefits.

   This documents updates [RFC7110] by allowing the usage of the Reply
   Mode value 5 (Reply via Specified Path) without including the Reply
   Path TLV.

2.  Problem Statements

   It is becoming increasingly difficult for implementations to
   automatically supply a workable return path encoding for all MPLS LSP
   Ping and Traceroute operations across all LSP types.  There are
   several factors which are contributing to this complication.






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   o  Some LSPs have a control-channel, and some do not.  Some LSPs have
      a reverse LSP, and some do not.  Some LSPs have IP reachability in
      the reverse direction, and some do not.

   o  LSRs on some LSPs can have different available return path(s).
      Available return path(s) can depend on whether the responder LSR
      is a transit LSR or an egress LSR.  In case of a bi-directional
      LSP, available return path(s) on transit LSRs can also depend on
      whether the LSP is completely co-routed, partially co-routed or
      associated (i.e., the LSPs in the two directions are not co-
      routed).

   o  MPLS echo request packets may incorrectly terminate on an
      unintended target, which can have different available return
      path(s) than the intended target.

   o  The MPLS LSP Ping operation is expected to terminate on an egress
      LSR.  However, the MPLS LSP Ping operation with specific TTL
      values and MPLS LSP Traceroute operation can terminate on both
      transit LSR(s) and the egress LSR.

   Except for the case where the responder LSR does not have an IP route
   back to the initiator LSR, it is possible to use the "Reply via an
   IPv4/IPv6 UDP packet (2)" Reply Mode value in all cases.  However,
   some operators are preferring control-channel and reverse LSP as
   default return path if they are available, which is not always the
   case.

   When specific return path encoding is supplied by users or
   applications, then there are no issues in choosing the return path
   encoding.  When specific return path encoding is not supplied by
   users or applications, then implementations use extra logic to
   compute, and sometimes guess, the default return path encodings.  If
   a responder LSR receives an MPLS echo request containing return path
   instructions which cannot be accommodated due to unavailability, then
   the responder LSR often drops such packets.  This failure mode
   results in the initiator LSR not receiving the intended MPLS LSP echo
   reply packets.  The scenario described here is a potentially
   acceptable result in some failure cases, like a broken LSP, where the
   MPLS echo request terminated on an unintended target.  However, if
   the initiator LSR does not receive an MPLS echo replay, even after
   the responder LSR receives the MPLS echo request and is able to
   verify the request, information is sent back to the user(s) which is
   considered a false failure.

   Many operators prefer particular return path(s) over others return
   path(s) for specific LSP types.  To accommodate operator preferred
   paths, implementations may default to operator preferred return paths



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   for particular operations, or allow a default return path to be
   configured.  It would not be considered beneficial to use a preferred
   return path for an intended target LSR if there is previous knowledge
   at the initiator LSR that the return path is not available.  Using a
   unavailable preferred return path would undesirably result in the
   initiator LSR not receiving the MPLS echo return packets.  It would
   be considered beneficial, for given operations, if the sender of the
   MPLS echo request would be able to determined return path
   availability before the operation is initiated.

   This document updates the "Reply via Specified Path (5)" Reply Mode
   to easily indicate the reverse the reverse LSP, and adds one optional
   TLV to describe an ordered list of reply modes.  Based on operational
   needs, the TLV can describe multiple Reply Mode values in a preferred
   order to allow the responder LSR to use the first available Reply
   Mode from the list.  This eliminates the need for the initiator LSR
   to compute, or sometimes guess, the default return path encoding.
   This new mode of operation would resulted in a simplification to
   implementations across the various vendors and improve both usability
   and operational needs.

3.  Solution

   This document updates the "Reply via Specified Path (5)" Reply Mode
   to easily indicate the reverse LSP.  This document also adds an
   optional TLV which can carry an ordered list of reply modes.

3.1.  Reply via Specified Path Update

   Some LSP types are capable of having a related LSP in reverse
   direction, through signaling or other association mechanisms.
   Examples of such LSP types are bidirectional Resource ReserVation
   Protocol-Traffic Engineering (RSVP-TE) LSPs [RFC3473] and MPLS
   Transport Profile (MPLS-TP) LSPs ([RFC5960]).  This document uses the
   term "Reverse LSP" to refer to the LSP in the reverse direction of
   such LSP types.  Note that this document restricts the scope of
   "Reverse LSP" applicability to those reverse LSPs which are capable
   and allowed to carry the IP encapsulated MPLS echo reply.

   [RFC7110] has defined the Reply Mode "Reply via Specified Path (5)"
   which allows the initiator LSR to instruct the responder LSR to send
   the MPLS echo reply message on the reverse LSP.  However, the
   instruction also requires the initiator LSR to include the "Reply
   Path TLV" with the B bit (Bidirectional bit) set in the Flags field.
   Additionally, [RFC7110] defines the usage of the "Reply via Specified
   Path (5)" Reply Mode without inclusion of the "Reply Path TLV" to be
   invalid.




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   This document updates the "Reply via Specified Path (5)" Reply Mode
   as follows:

   o  The usage of the "Reply via Specified Path (5)" without inclusion
      of a "Reply Path TLV" is no longer invalid.

   o  The usage of the "Reply via Specified Path (5)" without inclusion
      of a "Reply Path TLV" implies the reverse LSP.  In other words,
      the usage of the "Reply via Specified Path (5)" without inclusion
      of a "Reply Path TLV" has the same semantics as the usage of the
      "Reply via Specified Path (5)" with inclusion of a "Reply Path
      TLV" with the B bit set in the Flags field.

   Note that the reverse LSP is in relation to the last FEC specified in
   the Target FEC Stack TLV.

3.2.  Reply Mode Order TLV

   This document also introduces a new optional TLV to describe a list
   of Reply Mode values.  The new TLV will contain one or more Reply
   Mode value(s) in preferred order.  The first Reply Mode value is the
   most preferred and the last Reply Mode value is the least preferred.
   Following rules apply when using Reply Mode Order TLV.

   1.  The Reply Mode Order TLV MUST NOT be included in any MPLS echo
       reply.  If the initiator LSR receives an MPLS echo reply with the
       Reply Mode Order TLV, the initiator LSR MUST ignore the whole
       Reply Mode Order TLV and MUST only use the value from the Reply
       Mode field of the received MPLS echo reply.  It may be beneficial
       for implementations to provide counters and/or loggings, with
       appropriate log dampening, to record this error case.

   2.  The Reply Mode Order TLV MAY be included in MPLS echo request.

   3.  The Reply Mode field of an MPLS echo request MUST be set to a
       valid value even when supplying the Reply Mode Order TLV.  The
       initiator LSR SHOULD set the Reply Mode field of an MPLS echo
       request to a value that corresponds to a return path which most
       likely to be available, in case the responder LSR does not
       understand the Reply Mode Order TLV.

   4.  If a responder LSR understands the Reply Mode Order TLV but the
       TLV is not valid (due to conditions described in the items 6, 7,
       8 and 9 immediately below), then the responder LSR MUST ignore
       the whole Reply Mode Order TLV and MUST only use the value from
       the Reply Mode field of the received MPLS echo request.  It may
       be beneficial for implementations to provide counters and/or




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       loggings, with appropriate log dampening, to record this error
       case.

   5.  If a responder LSR understands the Reply Mode Order TLV and the
       TLV is valid, then the responder LSR MUST consider the Reply Mode
       values described in the TLV and MUST NOT use the value described
       in the Reply Mode field of the received MPLS echo request.  In
       other words, a valid Reply Mode Order TLV overrides the value
       specified in the Reply Mode field of the received MPLS echo
       request.

   6.  Reply Mode Order TLV MUST contain at least one Reply Mode value.

   7.  A Reply Mode value, except for Reply Mode value 5 (Reply via
       Specified Path), MUST NOT be repeated (i.e., MUST NOT appear
       multiple times) in the Reply Mode Order TLV.

   8.  The Reply Mode value 5 (Reply via Specified Path) MAY be included
       more than once in the Reply Mode Order TLV.  However, in such
       case a Reply Path TLV MUST be included for all instances of the
       Reply Mode value 5 included in the Reply Mode Order TLV.  In
       other words, 3 instances of the Reply Mode value 5 in the Reply
       Mode Order TLV will require 3 instances of the Reply Path TLVs.

   9.  The Reply Mode value 1 (Do not reply) MUST NOT be used in the
       Reply Mode Order TLV.

   The responder LSR SHOULD select the first available return path in
   this TLV.  The Reply Mode value corresponding to the selected return
   path MUST be set in Reply Mode field of the MPLS echo reply to
   communicate back to the initiator LSR which return path was chosen.

   The format of the TLV is as follows:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Reply Mode Order TLV Type     |          Length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     ~ Reply mode 1  | Reply mode 2  | Reply mode 3  | Reply mode 4  ~
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 1 Reply Mode Order TLV

   This is a variable length optional TLV.  Each Reply Mode field is 1
   octet.






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4.  Relations to Other LSP Ping/Trace Features

4.1.  Backwards Compatibility with Reply via Specified Path (5) Reply
      Mode

   [RFC7110] has introduced the "Reply via Specified path (5)" Reply
   Mode, and defined that usage of the "Reply via Specified path (5)"
   Reply Mode without also including the "Reply Path TLV" to be invalid.
   This document relaxes this semantic by defining the use of the "Reply
   via Specified path (5)" Reply Mode without the "Reply via Specified
   path (5)" to indicate the reverse LSP.  In other words, what was
   considered invalid is now valid.  If the initiator LSR, which sent an
   MPLS echo request message with the "Reply via Specified path (5)"
   Reply Mode but without including the "Reply Path TLV", receives back
   an MPLS echo reply message with the return code being "Malformed echo
   request received", then the initiator LSR SHOULD assume that the
   responder LSR does not support the mechanism defined in this
   document.

4.2.  Reply Path TLV

   A "Reply Path TLV" [RFC7110] is defined to identify a single return
   path.  When the initiator LSR wants to use the Reply Mode Order TLV
   to describe multiple return paths, then the initiator SHOULD include
   multiple "Reply via Specified Path (5)" Reply mode values and
   multiple corresponding "Reply Path TLV" objects (one "Reply Path TLV"
   corresponding to a "Reply via Specified Path (5)" Reply mode, and one
   "Reply Path TLV" identifies a return path).

   As described in Section 3.1, it's valid to use the "Reply via
   Specified Path (5)" Reply mode without inclusion a "Reply Path TLV".
   For the Reply Mode Order TLV, it's also valid to include a "Reply via
   Specified Path (5)" Reply mode value without a corresponding "Reply
   Path TLV", this implies that the reverse LSP is the preferred return
   path.  When multiple consecutive "Reply via Specified Path (5)" Reply
   mode values are included but less corresponding "Reply Path TLV"
   objects exist, the responder LSR SHOULD think that the former "Reply
   via Specified Path (5)" Reply mode values have corresponding "Reply
   Path TLV", the latter "Reply via Specified Path (5)" Reply mode
   values have no corresponding "Reply Path TLV".  For example, if the
   Reply Mode Order TLV carrying Reply Modes {5, 5, 5} and only two
   Reply Path TLVs carrying FEC X and FEC Y respectively.  The reply
   path order is as follows:

   1.  Reply via Specified Path (FEC X)

   2.  Reply via Specified Path (FEC Y)




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   3.  Reply via Specified Path (Reverse LSP)

4.2.1.  Example 1: Reply Mode Order TLV Usage with Reply Path TLV

   If the initiator LSR was interested in encoding following return
   paths:

   1.  Reply via application level control channel

   2.  FEC X

   3.  FEC Y

   4.  Reply via an IPv4/IPv6 UDP packet

   Then the MPLS echo request message is to carry:

   o  The Reply Mode Order TLV carrying Reply Modes {4, 5, 5, 2}

   o  One Reply Path TLV carrying FEC X

   o  One Reply Path TLV carrying FEC Y

   Described encoding of the Reply Mode Order TLV and the Reply Path TLV
   in the MPLS echo request message will result in the responder LSR to
   prefer "Reply via application level control channel (4)", followed by
   FEC X, FEC Y and then "Reply via an IPv4/IPv6 UDP packet (2)".

4.2.2.  Example 2: Reply Mode Order TLV Usage with Reply Path TLV

   If the initiator LSR was interested in encoding following return
   paths:

   1.  Reverse LSP

   2.  Reply via an IPv4/IPv6 UDP packet

   3.  FEC X

   4.  FEC Y

   Then the MPLS echo request message is to carry:

   o  The Reply Mode Order TLV carrying Reply Modes {5, 2, 5, 5}

   o  One Reply Path TLV with the B bit set.

   o  One Reply Path TLV carrying FEC X



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   o  One Reply Path TLV carrying FEC Y

   Described encoding of the Reply Mode Order TLV and the Reply Path TLV
   in the MPLS echo request message will result in the responder LSR to
   prefer the reverse LSP, followed by "Reply via an IPv4/IPv6 UDP
   packet (2)", FEC X and then FEC Y.

4.3.  Proxy LSP Ping

   The mechanism defined in this document will work with Proxy LSP Ping
   defined by [RFC7555].  The MPLS proxy ping request message can carry
   a Reply Mode value in the header and one or more Reply Mode values in
   the Reply Mode Order TLV.  It is RECOMMENDED that the Reply Mode 2
   (Reply via an IPv4/IPv6 UDP packet) be used in the Reply Mode field
   of the MPLS proxy ping request message.

4.3.1.  Proxy LSR Sending an MPLS Echo Request

   If the proxy LSR is sending an MPLS echo request, then the proxy LSR
   MUST copy the following elements from the MPLS proxy ping request
   message to the MPLS echo request message.

   o  The Reply Mode field.

   o  The Reply Mode Order TLV.

   o  The Reply Path TLV(s).  If there are more than one Reply Path
      TLVs, then order of them MUST be preserved when copying.

4.3.2.  Proxy LSR Sending an MPLS Proxy Ping Reply

   If the proxy LSR is sending an MPLS proxy ping reply, then it is
   RECOMMENDED that the Reply Mode Order TLV is ignored and the Reply
   Mode field in the MPLS proxy ping request message is used.

5.  Security Considerations

   Beyond those specified in [RFC4379] and [RFC7110], there are no
   further security measures required.

6.  Manageability Considerations

   Section 2 described the problems which increases the complexity with
   respect to operations and implementations.  In order to simplify
   operations and to allow for the LSP Ping/Traceroute to function
   efficiently whilst preserving the code simplicity, it is RECOMMENDED
   that implementations allow devices to have configuration options to
   set operator preferred Reply Modes.  For example:



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   o  For those operators who are more interested in MPLS echo reply
      packets reaching back to the initiator LSR:

      1.  Reply via an IPv4/IPv6 UDP packet (2)

      2.  Reply via application level control channel (4)

      3.  Reply via Specified Path (5)

   o  For those operators who are more interested in MPLS echo reply
      packets testing the paths related to the forward LSP:

      1.  Reply via Specified Path (5)

      2.  Reply via application level control channel (4)

      3.  Reply via an IPv4/IPv6 UDP packet (2)

7.  IANA Considerations

7.1.  New Reply Mode Order TLV

   IANA is requested to assign a new TLV type value from the "TLVs" sub-
   registry within the "Multiprotocol Label Switching Architecture
   (MPLS)" registry, for the "Reply Mode Order TLV".

   The new TLV Type value should be assigned from the range
   (32768-49161) specified in [RFC4379] section 3 that allows the TLV
   type to be silently dropped if not recognized.

     Type   Meaning                            Reference
     ----   -------                            ---------
     TBD1   Reply Mode Order TLV               this document

8.  Acknowledgements

   Authors would like to thank Santiago Alvarez and Faisal Iqbal for
   discussions which motivated creation of this document.  Authors would
   also like to thank Sam Aldrin, Curtis Villamizar, Ross Callon,
   Jeffrey Zhang, Jeremy Whittaker, Mustapha Alissaoui, Qin Wu, Jie Dong
   and Adrian Farrel for providing valuable comments to influence the
   contents of the draft.  Authors would also like to thank Dan Frost,
   Tom Taylor, Victor Kuarsingh and Deborah Brungard for reviewing the
   document and providing useful comments.







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9.  Contributing Authors

   Shaleen Saxena
   Brocade
   Email: ssaxena@brocade.com

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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              DOI 10.17487/RFC4379, February 2006,
              <http://www.rfc-editor.org/info/rfc4379>.

   [RFC7110]  Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
              "Return Path Specified Label Switched Path (LSP) Ping",
              RFC 7110, DOI 10.17487/RFC7110, January 2014,
              <http://www.rfc-editor.org/info/rfc7110>.

10.2.  Informative References

   [RFC3473]  Berger, L., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Signaling Resource ReserVation Protocol-
              Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
              DOI 10.17487/RFC3473, January 2003,
              <http://www.rfc-editor.org/info/rfc3473>.

   [RFC5960]  Frost, D., Ed., Bryant, S., Ed., and M. Bocci, Ed., "MPLS
              Transport Profile Data Plane Architecture", RFC 5960,
              DOI 10.17487/RFC5960, August 2010,
              <http://www.rfc-editor.org/info/rfc5960>.

   [RFC7555]  Swallow, G., Lim, V., and S. Aldrin, "Proxy MPLS Echo
              Request", RFC 7555, DOI 10.17487/RFC7555, June 2015,
              <http://www.rfc-editor.org/info/rfc7555>.

Appendix A.  Reply Mode Order TLV Beneficial Scenarios

   This section lists examples of how the Reply Mode Order TLV can
   benefit.





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A.1.  Incorrect Forwarding Scenario

   As shown in Figure 2, a network has an LSP with the forwarding path:
   A-B-C-D-E.  The LSP has a control channel.

     A------B------C------D------E
                          |
                          |
                          F

     Forward Paths: A-B-C-D-E

           Figure 2: Incorrect Forwarding

   If D is incorrectly label switching to F (instead of E).  In this
   scenario, LSP Traceroute with "Reply via application level control
   channel (4)" will result in following result.

      Success (Reply from B)
      Success (Reply from C)
      Success (Reply from D)
      Timeout...
      Complete

   This is because F does not have a control channel to send the MPLS
   echo reply message.  With the extension described in this document,
   same procedures can be performed with the Reply Mode Order TLV
   carrying {4, 2}. When LSP Traceroute is issued, then following output
   may be displayed without any unnecessary timeout.

      Success (Reply from B, Reply Mode: 4)
      Success (Reply from C, Reply Mode: 4)
      Success (Reply from D, Reply Mode: 4)
      FEC Mismatch (Reply from F, Reply Mode: 2)
      Complete

   The result provides more diagnostic information to the initiator LSR,
   and without any delay (i.e. timeout from one or more downstream
   LSRs).

A.2.  Non-Co-Routed Bidirectional LSP Scenario

   As shown in Figure 3, a network has a bidirectional LSP where the
   forward LSP and the reverse LSP are not fully co-routed.







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              +----C------D----+
             /                  \
     A------B                    G------H
             \                  /
              +----E------F----+

     Forward Paths: A-B-C-D-G-H (upper path)
     Reverse Paths: H-G-F-E-B-A (lower path)

           Figure 3: Non-Co-Routed Bidirectional LSP

   Some operators may prefer and configure the system to default the
   Reply Mode to indicate the reverse LSP when MPLS echo request
   messages are sent on bidirectional LSPs.  Without extensions
   described in this document, following behaviors will be seen:

   o  When LSP Ping is issued from A, the reply will come back on the
      reverse LSP from H.

   o  When LSP Traceroute is issued from A, the replies will come back
      on the reverse LSP from B, G and H, but will encounter a timeout
      from C and D as there are no reverse LSP on those nodes.

   o  When LSP Ping with specific TTL value is issued from A, whether a
      timeout will be encountered depends on the value of the TTL used
      (i.e. whether or not the MPLS echo request terminates on a node
      that has reverse LSP).

   One can argue that the initiator LSR can automatically generate the
   same MPLS echo request with different Reply Mode value to those nodes
   that timeout.  However, such mechanism will result in extended time
   for the entire operation to complete (i.e. multiple seconds to
   multiple minutes).  This is undesirable, and perhaps unacceptable if
   the "user" is an application.

   With the extension described in this document, same procedures can be
   performed with the Reply Mode Order TLV carrying {5, 2}. When LSP
   Traceroute is issued, then following output may be displayed without
   any unnecessary timeout.

      Success (Reply Mode: 5)
      Success (Reply Mode: 2)
      Success (Reply Mode: 2)
      Success (Reply Mode: 5)
      Success (Reply Mode: 5)
      Complete





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Authors' Addresses

   Nobo Akiya
   Big Switch Networks

   Email: nobo.akiya.dev@gmail.com


   George Swallow
   Cisco Systems

   Email: swallow@cisco.com


   Carlos Pignataro
   Cisco Systems

   Email: cpignata@cisco.com


   Loa Andersson
   Huawei

   Email: loa@mail01.huawei.com


   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com





















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