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rfc:rfc7737

Internet Engineering Task Force (IETF) N. Akiya Request for Comments: 7737 Big Switch Networks Updates: 7110 G. Swallow Category: Standards Track C. Pignataro ISSN: 2070-1721 Cisco Systems

                                                          L. Andersson
                                                               M. Chen
                                                                Huawei
                                                          January 2016

Label Switched Path (LSP) Ping and Traceroute Reply Mode Simplification

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 procedures
 for the "Reply via Specified Path" Reply Mode.  The value of this
 Reply Mode is 5.  The update creates a simple way to indicate that
 the reverse LSP should be used as the return path.  This document
 also adds an optional TLV that can carry an ordered list of Reply
 Mode values.
 This document updates RFC 7110.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7737.

Akiya, et al. Standards Track [Page 1] RFC 7737 LSP Ping Reply Mode Simplification January 2016

Copyright Notice

 Copyright (c) 2016 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
   1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
 2.  Problem Statements  . . . . . . . . . . . . . . . . . . . . .   4
 3.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.1.  "Reply via Specified Path" Reply Mode Update  . . . . . .   5
   3.2.  Reply Mode Order TLV  . . . . . . . . . . . . . . . . . .   6
 4.  Relationships to Other LSP Ping and Traceroute Features . . .   8
   4.1.  Backwards Compatibility with "Reply via Specified Path"
         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 . . . . . . . . . . . . . . . . . . . . . . . . .  10
   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  . . . . .  11
 5.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
 6.  Manageability Considerations  . . . . . . . . . . . . . . . .  11
 7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   7.1.  New Reply Mode Order TLV  . . . . . . . . . . . . . . . .  12
 8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  12
   8.1.  Normative References  . . . . . . . . . . . . . . . . . .  12
   8.2.  Informative References  . . . . . . . . . . . . . . . . .  13
 Appendix A.  Reply Mode Order TLV Beneficial Scenarios  . . . . .  14
   A.1.  Incorrect Forwarding Scenario . . . . . . . . . . . . . .  14
   A.2.  Non-Co-Routed Bidirectional LSP Scenario  . . . . . . . .  15
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  16
 Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  16
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

Akiya, et al. Standards Track [Page 2] RFC 7737 LSP Ping Reply Mode Simplification January 2016

1. Introduction

 Multiprotocol Label Switching (MPLS) Label Switched Path (LSP) Ping,
 as described in [RFC4379], allows an initiator Label Switching Router
 (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) that 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
 operations.  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 of the operators.
 This document describes the following:
 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 and Traceroute
    features;
 o  In Appendix A, examples of scenarios where the mechanism described
    in this document provides benefits.
 This document updates [RFC7110] by allowing the usage of the "Reply
 via Specified Path" (value=5) Reply Mode without including the Reply
 Path TLV.  The update creates a simple way to indicate that the
 reverse LSP should be used as the return path.

1.1. 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].

Akiya, et al. Standards Track [Page 3] RFC 7737 LSP Ping Reply Mode Simplification January 2016

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 that contribute to this complication.
 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 the case of a bidirectional
    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 that 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, MPLS LSP Ping operations with specific TTL values
    and MPLS LSP Traceroute operations 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" (value=2) Reply Mode value in all cases.
 However, some operators prefer the control channel and a reverse LSP
 as the default return path if they are both available, although this
 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 that 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 reply even after the

Akiya, et al. Standards Track [Page 4] RFC 7737 LSP Ping Reply Mode Simplification January 2016

 responder LSR receives the MPLS echo request and is able to verify
 the request, information is still sent back to the user(s); this is
 considered a false failure.
 Many operators prefer particular return path(s) over other return
 path(s) for specific LSP types.  To accommodate operator-preferred
 paths, implementations may default to operator-preferred return paths
 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 an
 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 determine return path availability
 before the operation is initiated.
 This document (1) updates the procedures for the "Reply via Specified
 Path" Reply Mode to easily indicate the reverse LSP and (2) adds one
 optional TLV to describe an ordered list of Reply Modes.  Based on
 operational needs, the TLV can list 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 result in simplified implementations
 across the various vendors and improve both usability and operational
 needs.

3. Solution

 This document updates the procedures for the "Reply via Specified
 Path" Reply Mode to easily indicate the reverse LSP.  This document
 also adds an optional TLV that can carry an ordered list of Reply
 Modes.

3.1. "Reply via Specified Path" Reply Mode Update

 Some LSP types are capable of having a related LSP in the 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 that are capable
 and allowed to carry the IP encapsulated MPLS echo reply.

Akiya, et al. Standards Track [Page 5] RFC 7737 LSP Ping Reply Mode Simplification January 2016

 [RFC7110] has defined the Reply Mode "Reply via Specified Path",
 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] specifies that if the "Reply via Specified
 Path" Reply Mode is used the Reply Path TLV MUST be present.
 This document updates the procedures for the "Reply via Specified
 Path" Reply Mode as follows:
 o  The "Reply via Specified Path" Reply Mode MAY be used without
    including a Reply Path TLV.
 o  The usage of the "Reply via Specified Path" Reply Mode without the
    inclusion of a Reply Path TLV implies the reverse LSP.  In other
    words, the usage of the "Reply via Specified Path" Reply Mode
    without the inclusion of a Reply Path TLV has the same semantics
    as the usage of the "Reply via Specified Path" Reply Mode with the
    inclusion of a Reply Path TLV with the B bit set in the Flags
    field.
 This document updates the first sentence of Section 5.1 of [RFC7110]
 as follows:
 o  When sending an echo request, in addition to the rules and
    procedures defined in Section 4.3 of [RFC4379], the Reply Mode of
    the echo request MUST be set to "Reply via Specified Path", and a
    Reply Path TLV SHOULD be carried in the echo request message
    correspondingly; if the Reply Path TLV is not carried in the
    message, then it indicates the reverse LSP as the reply path.
 Note that the reverse LSP is in relation to the last Forwarding
 Equivalence Class (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 values in preferred order.  The first Reply Mode value is the
 most preferred, and the last Reply Mode value is the least preferred.
 The following rules apply when using the 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

Akiya, et al. Standards Track [Page 6] RFC 7737 LSP Ping Reply Mode Simplification January 2016

     Mode field of the received MPLS echo reply.  It may be beneficial
     for implementations to provide counters and/or logs, with
     appropriate log dampening, to record this error case.
 2.  The Reply Mode Order TLV MAY be included in MPLS echo requests.
 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 that is 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 the conditions described in items 6, 7,
     8, and 9 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 logs,
     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 specified in the TLV and MUST NOT use the value specified
     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.  The 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.  Reply Mode value 5 (Reply via Specified Path) MAY be included
     more than once in the Reply Mode Order TLV.  However, in such a
     case, a Reply Path TLV MUST be included for all instances of
     Reply Mode value 5 that are included in the Reply Mode Order TLV.
     In other words, three instances of Reply Mode value 5 in the
     Reply Mode Order TLV will each require a Reply Path TLV.
 9.  The Reply Mode value 1 (Do not reply) MUST NOT be used in the
     Reply Mode Order TLV.

Akiya, et al. Standards Track [Page 7] RFC 7737 LSP Ping Reply Mode Simplification January 2016

 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 the 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.  The Reply Mode Order TLV
 Type is 32770.
 The Length field is 2 octets in length.  It defines the length, in
 octets, of the list of Reply Mode values.
 Each Reply Mode field is 1 octet, and there is no padding.

4. Relationships to Other LSP Ping and Traceroute Features

4.1. Backwards Compatibility with "Reply via Specified Path" Reply Mode

 [RFC7110] introduces the "Reply via Specified Path" (value=5) Reply
 Mode.  [RFC7110] also specifies that if this Reply Mode is used the
 Reply Path TLV MUST be included.  This document relaxes the semantics
 and specifies that this Reply Mode MAY be used without the Reply Path
 TLV.  This MAY be done to indicate that the reverse LSP SHALL be used
 as the return path.
 If an initiator LSR that sent an MPLS echo request message with the
 "Reply via Specified Path" Reply Mode but without including the Reply
 Path TLV receives back an MPLS echo reply message with a return code
 of "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 specify multiple return paths, then the initiator LSR SHOULD

Akiya, et al. Standards Track [Page 8] RFC 7737 LSP Ping Reply Mode Simplification January 2016

 include multiple "Reply via Specified Path" (value=5) Reply Mode
 values and multiple corresponding Reply Path TLV objects (one Reply
 Path TLV corresponding to a "Reply via Specified Path" Reply Mode and
 one Reply Path TLV identifying a return path).
 As described in Section 3.1, it is valid to use the "Reply via
 Specified Path" Reply Mode without inclusion in a Reply Path TLV.
 For the Reply Mode Order TLV, it is also valid to include a "Reply
 via Specified Path" 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" Reply
 Mode values are included but fewer corresponding Reply Path TLV
 objects exist, the responder LSR SHOULD think that the former "Reply
 via Specified Path" Reply Mode values have corresponding Reply Path
 TLVs.  The latter "Reply via Specified Path" Reply Mode values have
 no corresponding Reply Path TLVs.  For example, if the Reply Mode
 Order TLV carries Reply Modes {5, 5, 5} and only two Reply Path TLVs
 carry FEC X and FEC Y, respectively, then the reply path order is as
 follows:
 1.  Reply via Specified Path (FEC X)
 2.  Reply via Specified Path (FEC Y)
 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 the 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

Akiya, et al. Standards Track [Page 9] RFC 7737 LSP Ping Reply Mode Simplification January 2016

 The encoding specified by the Reply Mode Order TLV and the Reply Path
 TLV in the MPLS echo request message will cause 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 the 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
 o  One Reply Path TLV carrying FEC Y
 The encoding specified by the Reply Mode Order TLV and the Reply Path
 TLV in the MPLS echo request message will cause 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
 as 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 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:

Akiya, et al. Standards Track [Page 10] RFC 7737 LSP Ping Reply Mode Simplification January 2016

 o  The Reply Mode field.
 o  The Reply Mode Order TLV.
 o  The Reply Path TLV(s).  If there is more than one Reply Path TLV,
    then the ordering of the TLVs 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 be ignored and the Reply
 Mode field in the MPLS proxy ping request message be used.

5. Security Considerations

 The security considerations specified in [RFC4379] and [RFC7110] also
 apply for this document.
 In addition, this document introduces the Reply Mode Order TLV.  It
 provides a new way for an unauthorized source to gather more network
 information, especially information regarding the potential return
 path(s) of an LSP.  To protect against unauthorized sources using
 MPLS echo request messages with the Reply Mode Order TLV to obtain
 network information, as also specified in [RFC4379], it is
 RECOMMENDED that implementations provide a means of checking the
 source addresses of MPLS echo request messages against an access list
 before accepting the message.
 Another potential security issue is that the MPLS echo request and
 reply messages are not encrypted; the contents of the MPLS echo
 request and reply messages may therefore be potentially exposed.
 Although the exposure is within the MPLS domain, if such exposure is
 a concern, some encryption mechanisms [MPLS-OPP-ENCR] may be
 employed.

6. Manageability Considerations

 Section 2 described problems that increase complexity with respect to
 operations and implementations.  In order to simplify operations and
 to allow LSP Ping and Traceroute to function efficiently whilst
 preserving code simplicity, it is RECOMMENDED that implementations
 allow devices to have configuration options to set operator-preferred
 Reply Modes.  For example:
 o  For those operators who are more interested in MPLS echo reply
    packets reaching the initiator LSR:
    1.  Reply via an IPv4/IPv6 UDP packet (2)

Akiya, et al. Standards Track [Page 11] RFC 7737 LSP Ping Reply Mode Simplification January 2016

    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 has assigned a new TLV type value from the "TLVs" sub-registry
 within the "Multiprotocol Label Switching Architecture (MPLS) Label
 Switched Paths (LSPs) Ping Parameters" registry, for the Reply Mode
 Order TLV.
 The new TLV Type value has been assigned from the range 32768-49161,
 as specified in Sections 3 and 7.2 of [RFC4379]; this range is for
 optional TLVs that can be silently dropped if not recognized.
   Type    Meaning                            Reference
   -----   -------                            ---------
   32770   Reply Mode Order TLV               RFC 7737

8. References

8.1. 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>.

Akiya, et al. Standards Track [Page 12] RFC 7737 LSP Ping Reply Mode Simplification January 2016

8.2. Informative References

 [MPLS-OPP-ENCR]
            Farrel, A. and S. Farrell, "Opportunistic Security in MPLS
            Networks", Work in Progress, draft-ietf-mpls-
            opportunistic-encrypt-00, July 2015.
 [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>.

Akiya, et al. Standards Track [Page 13] RFC 7737 LSP Ping Reply Mode Simplification January 2016

Appendix A. Reply Mode Order TLV Beneficial Scenarios

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

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 Path: A-B-C-D-E
                    Figure 2: Incorrect Forwarding
 If D is incorrectly label switching to F (instead of E), then LSP
 Traceroute with "Reply via application level control channel (4)"
 will result in the following:
    Success (Reply from B)
    Success (Reply from C)
    Success (Reply from D)
    Timeout...
    Complete
 This is because F does not have a control channel on which to send
 the MPLS echo reply message.  With the extensions described in this
 document, the same procedures can be performed with the Reply Mode
 Order TLV carrying {4, 2}. When LSP Traceroute is issued, then the
 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).

Akiya, et al. Standards Track [Page 14] RFC 7737 LSP Ping Reply Mode Simplification January 2016

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.
                         +----C------D----+
                        /                  \
                A------B                    G------H
                        \                  /
                         +----E------F----+
                Forward Path: A-B-C-D-G-H (upper path)
                Reverse Path: H-G-F-E-B-A (lower path)
               Figure 3: Non-Co-Routed Bidirectional LSP
 Some operators may prefer and configure "Reply via Specified Path" as
 the default Reply Mode but without including the Reply Path TLV, to
 indicate that the reverse LSP is used as the return path when MPLS
 echo request messages are sent on bidirectional LSPs.  Without the
 extensions described in this document, the 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 LSPs on those nodes.
 o  When LSP Ping with a 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 a reverse LSP).
 One can argue that the initiator LSR can automatically generate the
 same MPLS echo request with a different Reply Mode value to those
 nodes that time out.  However, such a mechanism will result in an
 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.

Akiya, et al. Standards Track [Page 15] RFC 7737 LSP Ping Reply Mode Simplification January 2016

 With the extensions described in this document, the same procedures
 can be performed with the Reply Mode Order TLV carrying {5, 2}.  When
 LSP Traceroute is issued, then the 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

Acknowledgements

 The authors especially thank Tom Taylor, who passed away close to the
 time of publication of this RFC.  Tom did a careful review of the
 document and provided useful comments.
 The authors would also like to thank Santiago Alvarez and Faisal
 Iqbal for discussions that motivated the creation of this document;
 Sam Aldrin, Curtis Villamizar, Ross Callon, Jeffrey Zhang, Jeremy
 Whittaker, Mustapha Alissaoui, Qin Wu, Jie Dong, and Adrian Farrel
 for providing valuable comments that influenced the content of the
 document; and Dan Frost, Victor Kuarsingh, and Deborah Brungard for
 reviewing the document and providing useful comments.

Contributors

 Shaleen Saxena
 Brocade
 Email: ssaxena@brocade.com

Akiya, et al. Standards Track [Page 16] RFC 7737 LSP Ping Reply Mode Simplification January 2016

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

Akiya, et al. Standards Track [Page 17]

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