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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(Ed.)
Internet Draft                               Huawei Technologies Co.,Ltd
Category: Standards Track                                     N. So(Ed.)
                                                                 Verizon
Expires: January 11, 2012                                  July 11, 2011


                     Return Path Specified LSP Ping

         draft-ietf-mpls-return-path-specified-lsp-ping-03.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.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

   This Internet-Draft will expire on July 27, 2011.






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Copyright Notice

   Copyright (c) 2011 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.

Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC-2119 [RFC2119].

Table of Contents


   1. Introduction ................................................. 3
   2. Problem Statements and Solution Overview ..................... 3
      2.1. Limitations of Existing Mechanisms for Bidirectional LSPs 4
      2.2. Limitations of Existing Mechanisms for Handling Unreliable
      Return Paths ................................................. 4
   3. Extensions ................................................... 5
      3.1. Reply Via Specified Path mode ........................... 6
      3.2. Reply Path (RP) TLV ..................................... 6
      3.3. RP TLV sub-TLVs.......................................... 9
         3.3.1. IPv4 RSVP Tunnel sub-TLV .......................... 10
         3.3.2. IPv6 RSVP Tunnel sub-TLV .......................... 11
         3.3.3. RP TC sub-TLV ..................................... 12
   4. Theory of Operation ......................................... 13
      4.1. Sending an Echo Request ................................ 14
      4.2. Receiving an Echo Request .............................. 14
      4.3. Sending an Echo Reply .................................. 15
      4.4. Receiving an Echo Reply ................................ 16
   5. Security Considerations ..................................... 17
   6. IANA Considerations ......................................... 17
      6.1. New TLV ................................................ 17
      6.2. Sub-TLVs ............................................... 17
         6.2.1. Sub-TLVs from RFC3479 ............................. 17
         6.2.2. New Sub-TLVs ...................................... 18
         6.2.3. New Reply Mode .................................... 18



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   7. Contributors ................................................ 19
   8. Acknowledgments ............................................. 19
   9. References .................................................. 19
      9.1. Normative References ................................... 19
      9.2. Informative References ................................. 20
   Authors' Addresses ............................................. 21

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) [BFD], [BFD-MPLS] 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].

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.




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   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
   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



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   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
   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 tested LSP 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
   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 (RP) TLV, is defined in this
   document. The RP TLV consists of one or more sub-TLVs 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 notify the remote LSR
   receiving the LSP Ping echo request message to send 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 optionally included in an echo request
   message. It carries the specified return paths that the echo reply
   message is required to follow. The format of RP 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   RP (reply path) TLV Type    |          Length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      RP return code           |              Flag             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Reply Paths                             |
   ~                                                               ~
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   RP 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 RP return code, Flag and Reply Paths fields.

   RP return code is 2 octets in length. It is defined for the egress
   LSR of the forward LSP to report the results of RP TLV processing and
   return path selection. When sending echo request, these codes MUST be
   set to zero. RP return code only used when sending echo reply, and it
   MUST be ignored when processing echo request message. This document
   defines the following RP return codes:



























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   Value      Meaning
   -----      ----------------------
   0          No return code

   1          Malformed RP TLV was received

   2          One or more of the sub-TLVs in RP TLV
              was not understood

   3          The echo reply was sent successfully
              using the specified RP

   4          The specified RP was not found, the echo
              reply was sent via other LSP

   5          The specified RP was not found, the echo
              reply was sent via IP path

   6          The Reply mode in echo request was not set to 5
              (replay via specified path) although RP TLV exists

   7          RP TLV was missing in echo request

   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|E|
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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. If A bit is set, there is
   no need to carry any specific reply path sub-TLVs.

   B (Bidirectional): the return path is required to follow the reverse
   direction of the tested bidirectional LSP.

   E (Exclude): the return path is required to exclude the paths that
   field. This is very useful when one or more previous LSP Ping
   attempts failed. By setting this E bit and carrying the previous
   failed reply path sub-TLVs, a new LSP Ping echo request could be used
   to help the egress LSR to select another candidate path when sending
   echo reply message.




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   A bit MUST NOT be set when any one of other two bits (B bit and E
   bit) set.

   The Reply Paths field is variable in length. It has several nested
   sub-TLVs that describe the specified paths 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 RP TLV MUST be
   present.

3.3. RP TLV sub-TLVs

   Each of the FEC sub-TLVs defined in [RFC4379] is applicable to be a
   sub-TLV for inclusion in the RP 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 RP TC (Traffic Class) sub-TLV.
   Detailed definition is in the following sections.

   With the Return Path TLV flags and the sub-TLVs defined in [RFC4379]
   and in this document, it could provide following options for return
   paths specifying:





























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   1. Specify a particular LSP as return path
         - use those sub-TLVs defined in [RFC4379],

   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.

   5. Allow any LSPs except specific or general ones as return path
         - use E bit (Section 3.2 of this document) and combine with
             the specific paths identified by the sub-TLVs carried in
             Reply Path field.


3.3.1. IPv4 RSVP Tunnel sub-TLV

   The IPv4 RSVP Tunnel sub-TLV is used in the RP TLV to allow the
   operator to specify a more generic tunnel FEC other than a particular
   LSP as the return path. The egress LSR chooses any LSP from the LSPs
   that have the same Tunnel attributes and satisfy the conditions
   carried in the Flag field. The format of IPv4 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | IPv4 RSVP Tunnel sub-TLV Type |        Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IPv4 tunnel end point address                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Flag             |     Tunnel ID                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Extended Tunnel ID                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   IPv4 tunnel sender address                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   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.




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   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 have
   the same Tunnel attributes and the LSP MUST be the primary LSP.

   S (Secondary): the return path MUST be chosen from the LSPs that have
   the same Tunnel attributes and the LSP MUST be the secondary LSP.

   P bit and S bit MUST NOT both be set. If P bit and S bit are both not
   set, the return path could be any one of the LSPs that have the same
   Tunnel attributes.

3.3.2. IPv6 RSVP Tunnel sub-TLV

   The IPv6 RSVP Tunnel sub-TLV is used in the RP TLV to allow the
   operator to specify a more generic tunnel FEC other than a particular
   LSP as the return path. The egress LSR chooses an LSP from the LSPs
   that have the same Tunnel attributes and satisfy the conditions
   carried in the Flag field. The format of IPv6 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | IPv6 RSVP Tunnel sub-TLV Type |        Length                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                 IPv6 tunnel end point address                 |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              Flag             |     Tunnel ID                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Extended Tunnel ID                      |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   IPv6 tunnel sender address                  |
   |                                                               |
   |                                                               |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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 recommended type value is 19 (to be confirmed by IANA).

   The Flag field is 2 octets in length and is identical to that
   described in Section 3.3.

3.3.3. RP TC sub-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 sub-TLV: RP TC sub-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 specified return LSP set to the value
   specified in this sub-TLV. Since there may be more than one FEC sub-
   TLVs (return paths) specified in the RP TLV, the relevant RP TC sub-
   TLV MUST directly follow the FEC sub-TLV that identifies the
   corresponding specified return LSP. The format of RP TC 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  RP TC sub-TLV type           |          Length               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | TC  |                    MUST be zero                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The RP TC sub-TLV Type field is 2 octets in length, and the
   recommended type value is 17 (to be confirmed by IANA).

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

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 (RP TLV) which 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(the A bit and E 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. Of course when the echo reply message is
   not intended for testing the specified return path, 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 RP TLV MUST



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   be carried in the echo request message correspondingly. The RP 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 RP
   TLV. If the operator wants the echo reply to be sent along a
   different path other than the reverse direction of the bidirectional
   LSP, "A" bit SHOULD be set or another FEC sub-TLV SHOULD be carried
   in the RP TLV instead, and the B bit MUST be clear.

   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 RP 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 RP TLV, it SHOULD find and select the
   desired return path. If there is a matched path, an echo reply with
   RP TLV that identify the return path SHOULD be sent back to the
   ingress LSR, the RP return code SHOULD be set to "The echo reply was
   sent successfully using the specified RP". If there is no such path,
   an echo reply with RP TLV SHOULD be sent back to the ingress LSR, the
   RP return code SHOULD be set to relevant code (defined Section 3.2)
   for the real situation to reflect the result of RP 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 RP return code SHOULD be set to "The specified RP
   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 RP return code
   SHOULD be set to "The specified RP was not found, the echo reply was
   sent via IP path". If there is unknown sub-TLV in the received RP
   TLV, the RP return code SHOULD be set to "One or more of the sub-TLVs
   in RP TLV was not understood".

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



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   path.

   IF the B bit of the RP 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 E bit of the RP TLV in a received echo request message is set,
   the egress LSR MUST exclude the paths identified by those FEC sub-
   TLVs carried in the RP TLV and select other path to send the echo
   reply.

   If the A and E bit of the RP TLV in a received echo request message
   is not set, the echo reply is REQUIRED not only to send along the
   specified path, but to test the selected return path as well (by
   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 routable 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 (both A and E
   bit are 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. 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 RP TLV that identifies the return path
   MUST be carried, the RP return code SHOULD be set to relevant code
   that reflects results about how the egress processes the RP TLV in a
   previous received echo request message and return path selection. By
   carrying the RP 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.

4.4. Receiving an Echo Reply




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   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 RP return code is "The echo reply was sent
   successfully using the specified RP", and if both the A bit and E bit
   are not set. The ingress LSR MUST do FEC validation (based on the FEC
   stack information of the return path carried in the RP 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
   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 RP return code in the RP TLV set to
   "The specified RP 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.

   When the LSP Ping initiator fails after some time to receive the echo
   reply message, the operator MAY initiate another LSP Ping by
   resending a new echo request carrying a RP TLV with E bit set, the
   sub-TLVs and/or B bit (when the tested LSP is a bidirectional LSP)
   identify the previous tried reply paths that are used to notify the
   egress LSR to send echo reply message along any other workable path
   other than these failed return paths. Hence it could improve the
   reliability of the echo reply message.





















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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)

6.2. Sub-TLVs

6.2.1. Sub-TLVs from RFC3479

   The following sub-TLVs to the Target FEC Stack specified in Section
   3.2 of RFC4379 and fournd in the "Multiprotocol Label Switching
   Architecture (MPLS) Label Switched Paths (LSPs) Parameters - TLVs"
   registry, "TLVs and sub-TLVs" sub- registry are also valid for the
   Reply Path TLV.















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     Sub-Type       Value Field                            Reference
     --------       -----------                            ---------
            1       LDP IPv4 prefix                        RFC4379
            2       LDP IPv6 prefix                        RFC4379
            3       RSVP IPv4 LSP                          RFC4379
            4       RSVP IPv6 LSP                          RFC4379
            5       Not Assigned                           RFC4379
            6       VPN IPv4 prefix                        RFC4379
            7       VPN IPv6 prefix                        RFC4379
            8       L2 VPN endpoint                        RFC4379
            9       "FEC 128" Pseudowire (deprecated)      RFC4379
           10       "FEC 128" Pseudowire                   RFC4379
           11       "FEC 129" Pseudowire                   RFC4379
           12       BGP labeled IPv4 prefix                RFC4379
           13       BGP labeled IPv6 prefix                RFC4379
           14       Generic IPv4 prefix                    RFC4379
           15       Generic IPv6 prefix                    RFC4379
           16       Nil FEC                                RFC4379

   The IANA is requested to assign the same values to the sub-TLVs for
   the Reply Path TLV (Type 21)as those used for the sub-TLVs for the
   Target FEC Stack TLV (TLV Type 1) in the same registry.

6.2.2. 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).

     Sub-type      Value Field             Reference
     -------       -----------             ---------
     17            RP TC                   this document (sect 3.3.3)
     18            IPv4 RSVP Tunnel        this document (sect 3.3.2)
     19            IPv6 RSVP Tunnel        this document (sect 3.3.1)

6.2.3. New Reply Mode

   IANA is requested to assign a new reply mode code point from the 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)

7. Contributors



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   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

8. Acknowledgments

   The authors would like to thank Adrian Farrel and Peter Ashwood-
   Smith for their review, suggestion and comments to this document.

9. References

9.1. Normative References

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

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















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9.2. Informative References

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

   [RFC3473] L. Berger, "Generalized Multi-Protocol Label Switching
             (GMPLS) Signaling", RFC 3473, January 2003.

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

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

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

   [BFD-MPLS] Aggarwal, R., Kompella, K., Nadeau, T., and Swallow, G.,
             "BFD For MPLS LSPs", RFC5884, June 2010.






























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Authors' Addresses

   Mach(Guoyi) Chen
   Huawei Technologies Co., Ltd.
   No. 3 Xinxi Road
   Shangdi Information Industry Base
   Hai-Dian District, Beijing 100085
   China

   EMail: mach@huawei.com


   So Ning
   Verizon Inc.
   2400 N. Glenville Rd.,
   Richardson, TX  75082

   Phone: +1 972-729-7905
   EMail: ning.so@verizonbusiness.com


   Frederic Jounay
   France Telecom
   2, avenue Pierre-Marzin
   22307 Lannion Cedex
   FRANCE

   EMail: frederic.jounay@orange-ftgroup.com


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

   EMail: simon.delord@alcatel-lucent.com


   Xinchun Guo
   Huawei Technologies Co., Ltd.
   No. 3 Xinxi Road
   Shangdi Information Industry Base
   Hai-Dian District, Beijing 100085
   China


   EMail: guoxinchun@huawei.com




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   Wei Cao
   Huawei Technologies Co., Ltd.
   No. 3 Xinxi Road
   Shangdi Information Industry Base
   Hai-Dian District, Beijing 100085
   China

   EMail: caoweigne@huawei.com











































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