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Versions: (draft-chen-mpls-return-path-specified-lsp-ping) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 RFC 7110

Network Working Group                                            M. Chen
Internet-Draft                                                    W. Cao
Intended status: Standards Track            Huawei Technologies Co., Ltd
Expires: February 2, 2013                                        S. Ning
                                                     Tata Communications
                                                               F. Jounay
                                                               Orange CH
                                                               S. Delord
                                                          Alcatel-Lucent
                                                          August 1, 2012


                     Return Path Specified LSP Ping
         draft-ietf-mpls-return-path-specified-lsp-ping-06.txt

Abstract

   This document defines extensions to the failure-detection protocol
   for Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs)
   known as "LSP Ping" that allow selection of the LSP to use for the
   echo reply return path.  Enforcing a specific return path can be used
   to verify bidirectional connectivity and also increase LSP ping
   robustness.  It may also be used by Bidirectional Forwarding
   Detection (BFD) for MPLS bootstrap signaling thereby making BFD for
   MPLS more robust.

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 February 2, 2013.



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Copyright Notice

   Copyright (c) 2012 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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Problem Statements and Solution Overview . . . . . . . . . . .  4
     2.1.  Limitations of Existing Mechanisms for Bidirectional
           LSPs . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  Limitations of Existing Mechanisms for Handling
           Unreliable Return Paths  . . . . . . . . . . . . . . . . .  5
   3.  Extensions . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Reply Via Specified Path mode  . . . . . . . . . . . . . .  7
     3.2.  Reply Path (RP) TLV  . . . . . . . . . . . . . . . . . . .  7
     3.3.  Reply Path sub-TLVs  . . . . . . . . . . . . . . . . . . .  9
       3.3.1.  IPv4 RSVP Tunnel sub-TLV . . . . . . . . . . . . . . .  9
       3.3.2.  IPv6 RSVP Tunnel sub-TLV . . . . . . . . . . . . . . . 10
       3.3.3.  Static Tunnel sub-TLV  . . . . . . . . . . . . . . . . 11
     3.4.  Reply TC TLV . . . . . . . . . . . . . . . . . . . . . . . 12
   4.  Theory of Operation  . . . . . . . . . . . . . . . . . . . . . 13
     4.1.  Sending an Echo Request  . . . . . . . . . . . . . . . . . 13
     4.2.  Receiving an Echo Request  . . . . . . . . . . . . . . . . 14
     4.3.  Sending an Echo Reply  . . . . . . . . . . . . . . . . . . 15
     4.4.  Receiving an Echo Reply  . . . . . . . . . . . . . . . . . 15
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
     6.1.  New TLV  . . . . . . . . . . . . . . . . . . . . . . . . . 16
     6.2.  Sub-TLVs . . . . . . . . . . . . . . . . . . . . . . . . . 16
       6.2.1.  New Sub-TLVs . . . . . . . . . . . . . . . . . . . . . 17
       6.2.2.  New Reply Mode . . . . . . . . . . . . . . . . . . . . 17
     6.3.  Reply Path Return Code . . . . . . . . . . . . . . . . . . 17
   7.  Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 18
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 19



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     9.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19

















































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1.  Introduction

   This document defines extensions to the failure-detection protocol
   for Multiprotocol Label Switching (MPLS) Label Switched Paths (LSPs)
   known as "LSP Ping" [RFC4379] that can be used to specify the return
   paths for the echo reply message, increasing the robustness of LSP
   Ping, reducing the opportunity for error, and improving the
   reliability of the echo reply message.  A new reply mode, which is
   referred to as "Reply via Specified Path", is added and a new Type-
   Length-Value (TLV), which is referred to as Reply Path (RP) TLV, is
   defined in this memo.

   With the extensions described in this document, a bidirectional LSP
   and a pair of unidirectional LSPs (one for each direction) could both
   be tested with a single operational action, hence providing better
   control plane scalability.  The defined extensions can also be
   utilized for creating a single Bidirectional Forwarding Detection
   (BFD)[RFC5880], [RFC5884]session for a bidirectional LSP or for a
   pair of unidirectional LSPs (one for each direction).

   In this document, term bidirectional LSP includes the co-routed
   bidirectional LSP defined in [RFC3945] and the associated
   bidirectional LSP that is constructed from a pair of unidirectional
   LSPs (one for each direction), and which are associated with one
   another at the LSP's ingress/egress points [RFC5654].  The mechanisms
   defined in this document can apply to both IP/MPLS and MPLS-TP
   scenarios.


2.  Problem Statements and Solution Overview

   MPLS LSP Ping is defined in [RFC4379].  It can be used to detect data
   path failures in all MPLS LSPs, and was originally designed for
   unidirectional LSPs.

   LSP are increasingly being deployed to provide bidirectional
   services.  The co-routed bidirectional LSP is defined in [RFC3471]
   and [RFC3473], and the associated bidirectional LSP is defined in
   [RFC5654].  With the deployment of such services, operators have a
   desire to test both directions of a bidirectional LSP in a single
   operation.

   Additionally, when testing a single direction of an LSP (either a
   unidirectional LSP, or a single direction of a bidirectional LSP)
   using LSP Ping, the validity of the result may be affected by the
   success of delivering the echo reply message.  Failure to exchange
   these messages between the egress Label Switching Router (LSR) and
   the ingress LSR can lead to false negatives where the LSP under test



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   is reported as "down" even though it is functioning correctly.

2.1.  Limitations of Existing Mechanisms for Bidirectional LSPs

   With the existing LSP Ping mechanisms as defined in [RFC4379],
   operators have to enable LSP detection on each of the two ends of a
   bidirectional LSP independently.  This not only doubles the workload
   for the operators, but may also bring additional difficulties when
   checking the backward direction of the LSP under the following
   conditions:

   1.  The LSR that the operator logged on to perform the checking
       operations might not have out-of-band connectivity to the LSR at
       the far end of the LSP.  That can mean it is not possible to
       check the return direction of a bidirectional LSP in a single
       operation - the operator must log on to the LSR at the other end
       of the LSP to test the return direction.

   2.  The LSP being tested might be an inter-domain/inter-AS LSP where
       the operator of one domain/AS may have no right to log on to the
       LSR at the other end of the LSP since this LSR resides in another
       domain/AS.  That can make it completely impossible for the
       operator to check the return direction of a bidirectional LSP.

   Associated bidirectional LSPs have the same issues as those listed
   for co-routed bidirectional LSPs.

   This document defines a mechanism to allow the operator to request
   that both directions of a bidirectional LSP be tested by a single LSP
   Ping message exchange.

2.2.  Limitations of Existing Mechanisms for Handling Unreliable Return
      Paths

   [RFC4379] defines 4 reply modes:

   1. Do not reply
   2. Reply via an IPv4/IPv6 UDP packet
   3. Reply via an IPv4/IPv6 UDP packet with Router Alert
   4. Reply via application level control channel.


   Obviously, the issue of the reliability of the return path for an
   echo reply message does not apply in the first of these cases.

   [RFC4379] states that the third mode may be used when the IP return
   path is deemed unreliable.  This mode of operation requires that all
   intermediate nodes must support the Router Alert option and must



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   understand and know how to forward MPLS echo replies.

   This is a rigorous requirement in deployed IP/MPLS networks
   especially since the return path may be through legacy IP-only
   routers.  Furthermore, for inter-domain LSPs, the use of the Router
   Alert option may encounter significant issues at domain boundaries
   where the option is usually stripped from all packets.  Thus, the use
   of this mode may itself introduce issues that lead to the echo reply
   messages not being delivered.

   And in any case, the use modes 2 or 3 cannot guarantee the delivery
   of echo responses through an IP network that is fundamentally
   unreliable.  The failure to deliver echo response messages can lead
   to false negatives making it appear that the LSP has failed.

   Allowing the ingress LSR to control the path used for echo reply
   messages, and in particular forcing those messages to use an LSP
   rather than being sent through the IP network, enables an operator to
   apply an extra level of deterministic process to the LSP Ping test.

   This document defines extensions to LSP Ping that can be used to
   specify the return paths of the echo reply message in an LSP echo
   request message.


3.  Extensions

   LSP Ping defined in [RFC4379] is carried out by sending an echo
   request message.  It carries the Forwarding Equivalence Class (FEC)
   information of the LSP being tested which indicates which MPLS path
   is being verified, along the same data path as other normal data
   packets belonging to the FEC.

   LSP Ping [RFC4379] defines four reply modes that are used to direct
   the egress LSR in how to send back an echo reply.  This document
   defines a new reply mode, the Reply via Specified Path mode.  This
   new mode is used to direct the egress LSR of the tested LSP to send
   the echo reply message back along the path specified in the echo
   request message.

   In addition, a new TLV, the Reply Path TLV, is defined in this
   document.  The Reply Path TLV contains one nested sub-TLV that can be
   used to carry the specified return path information to be used by the
   echo reply message.







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3.1.  Reply Via Specified Path mode

   A new reply mode is defined to be carried in the Reply Mode field of
   the LSP Ping echo request message.

   The recommended value of the Reply via Specified Path mode is 5 (This
   is to be confirmed by the IANA).

       Value    Meaning
       -----    -------
           5     Reply via Specified Path


   The Reply via Specified Path mode is used to request that the remote
   LSR receiving the LSP Ping echo request message sends back the echo
   reply message along the specified paths carried in the Reply Path
   TLV.

3.2.  Reply Path (RP) TLV

   The Reply Path (RP) TLV is an optional TLV, if the Reply via
   Specified Path mode requested, the Reply Path (RP) TLV MUST be
   included in an echo request message.  It carries the specified return
   paths that the echo reply message is required to follow.  The format
   of Reply Path 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 Path TLV Type       |          Length               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |    Reply Path return code     |              Flag             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                       Reply Paths                             |
       ~                                                               ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                   Figure 1 Reply Path TLV

   Reply Path TLV Type field is 2 octets in length, and the type value
   is TBD by IANA.

   The Length field is 2 octets in length.  It defines the length in
   octets of the Reply Path return code, Flag and Reply Paths fields.

   Reply Path return code is 2 octets in length.  It is defined for the
   egress LSR of the forward LSP to report the results of Reply Path TLV
   processing and return path selection.  When sending echo request,



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   these codes MUST be set to zero.  Reply Path return code only used
   when sending echo reply, and it MUST be ignored when processing echo
   request message.  This document defines the following Reply Path
   return codes:

Value         Meaning
------        ----------------------
0x0000        No return code
0x0001        Malformed Reply Path TLV was received
0x0002        One or more of the sub-TLVs in Reply Path TLV was not understood
0x0003        The echo reply was sent successfully using the specified Reply Path
0x0004        The specified Reply Path was not found, the echo reply was sent via
              other LSP
0x0005        The specified Reply Path was not found, the echo reply was sent via
              IP path
0x0006        The Reply mode in echo request was not set to 5(Reply via
              Specified Path) although Reply Path TLV exists
0x0007        Reply Path TLV was missing in echo request
0x0008-0xffff Reserved


   Flag field is also 2 octets in length, it is used to notify the
   egress how to process the Reply Paths field when performing return
   path selection.  The Flag field is a bit vector and has following
   format:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      MUST be zero         |A|B|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   A (Alternative path): the egress LSR MUST select a non-default path
   as the return path.  This is very useful when reverse default path
   problems are suspected which can be confirmed when the echo reply is
   forced to follow a non-default return path.  Here, the default path
   refers to the path that the egress LSR will use to send the echo
   reply when the return path is not explicitly specified as defined in
   this document.  If A bit is set, there is no need to carry any
   specific reply path sub-TLVs, and when received, the sub-TLVs SHOULD
   be ignored.

   B (Bidirectional): the return path is required to follow the reverse
   direction of the tested bidirectional LSP.  If B bit is set, there is
   no need to carry any specific reply path sub-TLVs, and when received,
   the sub-TLVs SHOULD be ignored.

   The A bit and B bit set MUST NOT both be set, otherwise, an echo
   reply with the RP return code set to "Malformed RP TLV was received"
   SHOULD be returned.



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   The Reply Paths field is variable in length, not more than one sub-
   TLV MUST be carried, which describes the specified path that the echo
   reply message is required to follow.  When the Reply Mode field is
   set to "Reply via Specified Path" in an LSP echo request message, the
   Reply Path TLV MUST be present.

3.3.  Reply Path sub-TLVs

   Each of the FEC sub-TLVs for the Target FEC Stack TLV[RFC4379] is
   applicable to be a sub-TLV for inclusion in the Reply Path TLV for
   expressing a specific return path.

   In addition, three more new sub-TLVs are defined: IPv4 RSVP Tunnel
   sub-TLV, IPv6 RSVP Tunnel sub-TLV and Static Tunnel sub-TLV.
   Detailed definition is in the following sections.

   With the Return Path TLV flags and the sub-TLVs defined for the
   Target FEC Stack TLV and in this document, it could provide following
   options for return paths specifying:

   1.  Specify a particular LSP as return path

          - use those sub-TLVs defined for the Target FEC Stack TLV

   2.  Specify a more generic tunnel FEC as return path

          - use the IPv4/IPv6 RSVP Tunnel sub-TLVs defined in Section
          3.3.1 and Section 3.3.2 of this document

   3.  Specify the reverse path of the bidirectional LSP as return path

          - use B bit defined in Section 3.2 of this document.

   4.  Force return path to non-default path

          - use A bit defined in Section 3.2 of this document.

3.3.1.  IPv4 RSVP Tunnel sub-TLV

   The IPv4 RSVP Tunnel sub-TLV is used in the Reply Path TLV to allow
   the operator to specify a more generic tunnel FEC other than a
   particular LSP as the return path.  According to the bits set in the
   Flag field, the egress LSR will then choose an LSP from the specified
   Tunnel as the return path.  The format of IPv4 RSVP Tunnel sub-TLV is
   as follows:






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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | IPv4 RSVP Tunnel sub-TLV Type |        Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IPv4 tunnel end point address                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Flag             |     Tunnel ID                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Extended Tunnel ID                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   IPv4 tunnel sender address                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 2 IPv4 RSVP Tunnel sub-TLV

   The IPv4 RSVP Tunnel sub-TLV is derived from the RSVP IPv4 FEC TLV
   that is defined in Section 3.2.3 [RFC4379].  All fields have the same
   semantics as defined in [RFC4379] except that the LSP-ID field is
   omitted and a new Flag field is defined.

   The IPv4 RSVP Tunnel sub-TLV Type field is 2 octets in length, and
   the recommended type value is 18 (to be confirmed by IANA).

   The Flag field is 2 octets in length, it is used to notify the egress
   LSR how to choose the return path.  The Flag field is a bit vector
   and has following format:

   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      MUST be zero         |S|P|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   P (Primary): the return path MUST be chosen from the LSPs that belong
   to the specified Tunnel and the LSP MUST be the primary LSP.

   S (Secondary): the return path MUST be chosen from the LSPs that
   belong to the specified Tunnel and the LSP MUST be the secondary LSP.

   P bit and S bit MUST NOT both be set, otherwise, an echo reply with
   the RP return code set to "Malformed RP TLV was received" SHOULD be
   returned.  If P bit and S bit are both not set, the return path could
   be any one of the LSPs from the same Tunnel.

3.3.2.  IPv6 RSVP Tunnel sub-TLV

   The IPv6 RSVP Tunnel sub-TLV is used in the Reply Path TLV to allow
   the operator to specify a more generic tunnel FEC other than a
   particular LSP as the return path.  According to the bits set in the
   Flag field, the egress LSR will then choose an LSP from the specified



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   Tunnel as the return path.  The format of IPv6 RSVP Tunnel sub-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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | IPv6 RSVP Tunnel sub-TLV Type |        Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IPv6 tunnel end point address                 |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Flag             |     Tunnel ID                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Extended Tunnel ID                      |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   IPv6 tunnel sender address                  |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                    Figure 3 IPv6 RSVP Tunnel sub-TLV

   The IPv6 RSVP Tunnel sub-TLV is derived from RSVP IPv6 FEC TLV that
   is defined in Section 3.2.4 of [RFC4379].All fields have the same
   semantics as defined in [RFC4379] except that the LSP-ID field is
   omitted and a new Flag field is defined.

   The IPv6 RSVP Tunnel sub-TLV Type field is 2 octets in length, and
   the type value is TBD.

   The Flag field is 2 octets in length and is identical to that
   described in Section 3.3.1.

3.3.3.  Static Tunnel sub-TLV

   The Static Tunnel sub-TLV is used in the Reply Path TLV to allow the
   operator to specify a more generic tunnel FEC other than a particular
   LSP as the return path.  According to the bits set in the Flag field,
   the egress LSR will then choose an LSP from the specified Tunnel as
   the return path.  The format of Static RSVP Tunnel sub-TLV is as
   follows.  The value fields are taken from the definitions in
   [RFC6370].




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    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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Static Tunnel sub-TLV Type |        Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Source Global ID                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Source Node ID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Destination Global ID                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Destination Node ID                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       Source Tunnel Num       |     Destination Tunnel Num    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Flag             |     Must Be Zero              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                     Figure 4 Static Tunnel sub-TLV

   The Flag field is 2 octets in length and is identical to that
   described in Section 3.3.1.

3.4.  Reply TC TLV

   Reply TOS Byte TLV [RFC4379] is used by the originator of the echo
   request to request that an echo reply be sent with the IP header TOS
   byte set to the value specified in the TLV.  Similarly, in this
   document, a new TLV: Reply TC TLV is defined and MAY be used by the
   originator of the echo request to request that an echo reply be sent
   with the TC bits of the return path LSP set to the value specified in
   this TLV.  The format of Reply TC 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 TC TLV type        |          Length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TC  |                    MUST be zero                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Reply TC TLV Type field is 2 octets in length, and the type value
   is TBD.

   The Length field is 2 octets in length, the value of length field is
   fixed 4.






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4.  Theory of Operation

   The procedures defined in this document currently only apply to
   "ping" mode.  The "traceroute" mode is out of scope for this
   document.

   In [RFC4379], the echo reply is used to report the LSP checking
   result to the LSP Ping initiator.  This document defines a new reply
   mode and a new TLV (Reply Path TLV) that enable the LSP ping
   initiator to specify or constrain the return path of the echo reply.
   Similarly the behavior of echo reply is extended to detect the
   requested return path by looking at a specified path FEC TLV.  This
   enables LSP Ping to detect failures in both directions of a path with
   a single operation, this of course cuts in half the operational steps
   required to verify the end to end bidirectional connectivity and
   integrity of an LSP.

   When the echo reply message is intended to test the return MPLS LSP
   path(when the A bit is not set in the previous received echo request
   message), the destination IP address of the echo reply message MUST
   never be used in a forwarding decision.  To avoid this possibility
   the destination IP address of the echo reply message that is
   transmitted along the specified return path MUST be set to numbers
   from the range 127/8 for IPv4 or 0:0:0:0:0:FFFF:127/104 for IPv6, and
   the IP TTL MUST be set 1, and the TTL in the outermost label MUST be
   set to 255.  Of course when the echo reply message is not intended
   for testing the specified return path (when the A bit is set in the
   previous received echo request message) , the procedures defined in
   [RFC4379] (the destination IP address is copied from the source IP
   address) apply unchanged.

4.1.  Sending an Echo Request

   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 MUST be carried in the echo request message correspondingly.  The
   Reply Path TLV includes one or several reply path sub-TLV(s) to
   identify the return path(s) the egress LSR should use for its reply.

   For a bidirectional LSP, since the ingress LSR and egress LSR of a
   bidirectional LSP are aware of the relationship between the forward
   and backward direction LSPs, only the B bit SHOULD be set in the
   Reply Path TLV.  If the operator wants the echo reply to be sent
   along a different path other than the reverse direction of the
   bidirectional LSP, the "A" bit SHOULD be set or another FEC sub-TLV
   SHOULD be carried in the Reply Path TLV instead, and the B bit MUST
   be clear.



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   In some cases, operators may want to treat two unidirectional LSPs
   (one for each direction) as a pair.  There may not be any binding
   relationship between the two LSPs.  Using the mechanism defined in
   this document, operators can run LSP Ping one time from one end to
   complete the failure detection on both unidirectional LSPs.  To
   accomplish this, the echo request message MUST carry (in the Reply
   Path TLV) a FEC sub-TLV that belongs to the backward LSP.

4.2.  Receiving an Echo Request

   "Ping" mode processing as defined in Section 4.4 of [RFC4379] applies
   in this document.  In addition, when an echo request is received, if
   the egress LSR does not know the reply mode defined in this document,
   an echo reply with the return code set to "Malformed echo request"
   and the Subcode set to zero will be send back to the ingress LSR
   according to the rules of [RFC4379].  If the egress LSR knows the
   reply mode, according to the Reply Path TLV, it SHOULD find and
   select the desired return path.  If there is a matched path, an echo
   reply with Reply Path TLV that identify the return path SHOULD be
   sent back to the ingress LSR, the Reply Path return code SHOULD be
   set to "The echo reply was sent successfully using the specified
   return path".  If there is no such path, an echo reply with Reply
   Path TLV SHOULD be sent back to the ingress LSR, the Reply Path
   return code SHOULD be set to relevant code (defined Section 3.2) for
   the real situation to reflect the result of Reply Path TLV processing
   and return path selection.  For example, if the specified LSP is not
   found, the egress then chooses another LSP as the return path to send
   the echo reply, the Reply Path return code SHOULD be set to "The
   specified reply path was not found, the echo reply was sent via other
   LSP", and if the egress chooses an IP path to send the echo reply,
   the Reply Path return code SHOULD be set to "The specified reply path
   was not found, the echo reply was sent via IP path".  If there is
   unknown sub-TLV in the received Reply Path TLV, the Reply Path return
   code SHOULD be set to "One or more of the sub-TLVs in Reply Path TLV
   was not understood".

   If the A bit of the Reply Path TLV in a received echo request message
   is set, the egress LSR SHOULD send the echo reply along an non-
   default return path.

   IF the B bit of the Reply Path TLV in a received echo request message
   is set, the egress LSR SHOULD send the echo reply along the reverse
   direction of the bidirectional LSP.

   If the A bit of the Reply Path TLV in a received echo request message
   is not set(a.k.a a specific return path sub-TLV is carried or the B
   bit is set), the echo reply is REQUIRED not only to send along the
   specified path, but to test the selected return path as well (by



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   carrying the FEC stack information of the return path).

   In addition, the FEC validate results of the forward path LSP SHOULD
   NOT affect the egress LSR continue to test return path LSP.

4.3.  Sending an Echo Reply

   As described in [RFC4379], the echo reply message is a UDP packet,
   and it MUST be sent only in response to an MPLS echo request.  The
   source IP address is a valid IP address of the replier, the source
   UDP port is the well-know UDP port for LSP ping.

   When the echo reply is intended to test the return path (the A is not
   set in the previous received echo request), the destination IP
   address of the echo reply message MUST never be used in a forwarding
   decision.  To avoid this problem, the IP destination address of the
   echo reply message that is transmitted along the specified return
   path MUST be set to numbers from the range 127/8 for IPv4 or 0:0:0:0:
   0:FFFF:127/104 for IPv6, and the IP TTL MUST be set to 1, the TTL in
   the outermost label MUST be set to 255.  Otherwise, the same as
   defined in [RFC4379], the destination IP address and UDP port are
   copied from the source IP address and source UDP port of the echo
   request.

   When sending the echo reply, a Reply Path TLV that identifies the
   return path MUST be carried, the Reply Path return code SHOULD be set
   to relevant code that reflects results about how the egress processes
   the Reply Path TLV in a previous received echo request message and
   return path selection.  By carrying the Reply Path TLV in an echo
   reply, it gives the Ingress LSR enough information about the reverse
   direction of the tested path to verify the consistency of the data
   plane against control plane.  Thus a single LSP Ping could achieve
   both directions of a path test.  If the return path is pure IP path,
   no sub-TLVs are carried in the Reply Path TLV.

4.4.  Receiving an Echo Reply

   The rules and process defined in Section 4.6 of [RFC4379] apply here.
   When an echo reply is received, if the reply mode is "Reply via
   Specified Path" and the Reply Path return code is "The echo reply was
   sent successfully using the specified retun path", and if the A bit
   is not set.  The ingress LSR MUST perform FEC validation (based on
   the FEC stack information of the return path carried in the Reply
   Path TLV) as an egress LSR does when receiving an echo request, the
   FEC validation process (relevant to "ping" mode) defined in Section
   4.4.1 of [RFC4379] applies here.

   When an echo reply is received with return code set to "Malformed



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   echo request received" and the Subcode set to zero.  It is possible
   that the egress LSR may not know the "Reply via Specified Path" reply
   mode, the operator may choose to re-perform another LSP Ping by using
   one of the four reply modes defined [RFC4379].

   On receipt of an echo reply with Reply Path return code in the Reply
   Path TLV set to "The specified reply path was not found, ...", it
   means that the egress LSR could not find a matched return path as
   specified.  Operators may choose to specify another LSP as the return
   path or use other methods to detect the path further.


5.  Security Considerations

   Security considerations discussed in [RFC4379] apply to this
   document.  In addition to that, in order to prevent using the
   extension defined in this document for "proxying" any possible
   attacks, the return path LSP MUST have destination to the same node
   where the forward path is from.


6.  IANA Considerations

   IANA is requested to assign one new TLV from the "Multiprotocol Label
   Switching Architecture (MPLS) Label Switched Paths (LSPs) Parameters
   - TLVs" registry, "TLVs and sub-TLVs" sub- registry; and a set of
   sub-TLVs under this new TLV; one new Reply Mode from the "Multi-
   Protocol Label Switching (MPLS) Label Switched Paths (LSPs)
   Parameters" registry, the "Reply Mode" subregistry.

6.1.  New TLV

   The IANA is requested to as assign a new TLV from the "Multiprotocol
   Label Switching Architecture (MPLS) Label Switched Paths (LSPs)
   Parameters - TLVs" registry, "TLVs and sub-TLVs" sub- registry.

   Value   Meaning                           Reference
   -----   -------                           ---------
   21      Reply Path TLV                    this document (sect 3.2)
   TBD     Reply TC TLV                      this document (sect 3.4)

6.2.  Sub-TLVs

   Reply Path TLV is designed to share the existing and any future new
   defiend sub-TLVs of the Target FEC Stack TLV.  At the same time,
   Reply Path TLV will define its own dedicated sub-TLV(see Section
   3.3.1, 3.3.2 and 3.3.3).  Reply Path TLV has its own sub-TLV space,
   but for those shared sub-TLVs, the assignment MUST be kept the same



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   as for those sub-TLVs of Target FEC Stack TLV.  So, the sub-TLV range
   of Reply Path TLV are partitioned as following:

   0-31743 - This range MUST be kept as the same as Target FEC Stack TLV
   and sub-TLVs[RFC4379].

   31744-32767 - Standards Action;

   32768-64511 - This range MUST be kept as the same as Target FEC Stack
   TLV and sub-TLVs [RFC4379].

   64512-65535 - Standards Action.

6.2.1.  New Sub-TLVs

   IANA is also requested to assign three new sub-TLV types from
   "Multiprotocol Label Switching Architecture (MPLS) Label Switched
   Paths (LSPs) Parameters - TLVs" registry, "TLVs and sub-TLVs" sub-
   registry for the Reply Path TLV (Type 21).  As explained above, the
   following sub-type MUST be assigned from the range of 31744-32767 or
   64512-65535.
   Sub-type      Value Field             Reference
   -------       -----------             ---------
   TBD           IPv4 RSVP Tunnel        this document (sect 3.3.1)
   TBD           IPv6 RSVP Tunnel        this document (sect 3.3.2)
   TBD           Static Tunnel           this document (sect 3.3.3)


6.2.2.  New Reply Mode

   IANA is requested to assign a new reply mode code point from the
   "Multi-Protocol Label Switching (MPLS) Label Switched Paths (LSPs)
   Parameters" registry, the "Reply Mode" subregistry.
   Value    Meaning                      Reference
   -----    -------                      ----------
   5        Reply via Specified Path     this document (sect 3.1)

6.3.  Reply Path Return Code

   IANA is requested to create a new registry for Reply Path return
   code.

   This document (Section 3.2) defines the following return codes:








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Value         Meaning                                                              Reference
------        ----------------------                                               --------------
0x0000        No return code                                                       this document
0x0001        Malformed Reply Path TLV was received                                this document
0x0002        One or more of the sub-TLVs in Reply Path TLV was not understood     this document
0x0003        The echo reply was sent successfully using the specified Reply Path  this document
0x0004        The specified Reply Path was not found, the echo reply was sent via
              other LSP                                                            this document
0x0005        The specified Reply Path was not found, the echo reply was sent via
              IP path                                                              this document
0x0006        The Reply mode in echo request was not set to 5(replay via
              specified path) although Reply Path TLV exists                       this document
0x0007        Reply Path TLV was missing in echo request                           this document
0x0008-0xffff Reserved


   The range of 0x0008-0xffff are reserved for future extensions.


7.  Contributors

   The following individuals also contributed to this document:
   Ehud Doron
   Orckit-Corrigent
   EMail: ehudd@orckit.com

   Ronen Solomon
   Orckit-Corrigent
   EMail: RonenS@orckit.com

   Ville Hallivuori
   Tellabs
   Sinimaentie 6 C
   FI-02630 Espoo, Finland
   EMail: ville.hallivuori@tellabs.com

   Xinchun Guo
   EMail: guoxinchun@huawei.com


8.  Acknowledgements

   The authors would like to thank Adrian Farrel, Peter Ashwood-Smith,
   Sriganesh Kini, Gregory Mirsky and Tom Petch for their review,
   suggestion and comments to this document.


9.  References



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9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

   [RFC6370]  Bocci, M., Swallow, G., and E. Gray, "MPLS Transport
              Profile (MPLS-TP) Identifiers", RFC 6370, September 2011.

9.2.  Informative References

   [RFC3471]  Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Functional Description", RFC 3471,
              January 2003.

   [RFC3473]  Berger, L., "Generalized Multi-Protocol Label Switching
              (GMPLS) Signaling Resource ReserVation Protocol-Traffic
              Engineering (RSVP-TE) Extensions", RFC 3473, January 2003.

   [RFC3945]  Mannie, E., "Generalized Multi-Protocol Label Switching
              (GMPLS) Architecture", RFC 3945, October 2004.

   [RFC5654]  Niven-Jenkins, B., Brungard, D., Betts, M., Sprecher, N.,
              and S. Ueno, "Requirements of an MPLS Transport Profile",
              RFC 5654, September 2009.

   [RFC5880]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD)", RFC 5880, June 2010.

   [RFC5884]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for MPLS Label
              Switched Paths (LSPs)", RFC 5884, June 2010.


Authors' Addresses

   Mach(Guoyi) Chen
   Huawei Technologies Co., Ltd
   Q14 Huawei Campus, No. 156 Beiqing Road, Hai-dian District
   Beijing  100095
   China

   Email: mach@huawei.com





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   Wei Cao
   Huawei Technologies Co., Ltd
   Q14 Huawei Campus, No. 156 Beiqing Road, Hai-dian District
   Beijing  100095
   China

   Email: wayne.caowei@huawei.com


   So Ning
   Tata Communications

   Email: ning.so@tatacommunications.com


   Frederic Jounay
   Orange CH
   4 rue caudray 1020 Renens
   Switzerland

   Email: frederic.jounay@orange.ch


   Simon Delord
   Alcatel-Lucent
   Building 3, 388 Ningqiao Road, Jinqiao, Pudong
   Shanghai  201206
   China

   Email: simon.delord@alcatel-lucent.com





















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