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Versions: 00 01 02 03 draft-ietf-mpls-lsp-ping-reply-mode-simple

Internet Engineering Task Force                                 N. Akiya
Internet-Draft                                                G. Swallow
Updates: 4379 (if approved)                                 C. Pignataro
Intended status: Standards Track                           Cisco Systems
Expires: January 28, 2015                                   L. Andersson
                                                                 M. Chen
                                                                  Huawei
                                                           July 27, 2014


  Label Switched Path (LSP) Ping/Traceroute Reply Mode Simplification
             draft-akiya-mpls-lsp-ping-reply-mode-simple-03

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 adds one value to the
   Reply Mode field to indicate reverse LSP.  This document also adds an
   optional TLV which can carry ordered list of Reply Mode values.

   This document updates RFC4379.

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 January 28, 2015.






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Copyright Notice

   Copyright (c) 2014 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Problem Statements  . . . . . . . . . . . . . . . . . . . . .   3
   3.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Reply via reverse LSP . . . . . . . . . . . . . . . . . .   5
     3.2.  Reply Mode Order TLV  . . . . . . . . . . . . . . . . . .   5
   4.  Relations to Other LSP Ping/Trace Features  . . . . . . . . .   6
     4.1.  Reply Path TLV  . . . . . . . . . . . . . . . . . . . . .   6
       4.1.1.  Reply Mode Order TLV Usage Example with Reply Path
               TLV . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Proxy LSP Ping  . . . . . . . . . . . . . . . . . . . . .   7
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  New Reply Mode  . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  New Reply Mode Order TLV  . . . . . . . . . . . . . . . .   8
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   8.  Contributing Authors  . . . . . . . . . . . . . . . . . . . .   8
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     9.2.  Informative References  . . . . . . . . . . . . . . . . .   9
   Appendix A.  Reply Mode Order TLV Beneficial Scenarios  . . . . .   9
     A.1.  Incorrect Forwarding Scenario . . . . . . . . . . . . . .   9
     A.2.  Non-Co-Routed Bidirectional LSP Scenario  . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   The MPLS LSP Ping, described in [RFC4379], allows an initiator to
   encode instructions (Reply Mode) on how a responder should send the
   response back to the initiator.  [RFC7110] also allows the initiator
   to encode a TLV (Reply Path TLV) which can instruct the responder to
   use specific LSP to send the response back to the initiator.  Both



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   approaches are powerful as they provide the ability for the initiator
   to control the return path.

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

   This document further describes the problem in Section 2, and
   proposes a solution in Section 3 to minimizes false failure scenarios
   while accommodating operator preferences.  Additionally, Appendix A
   provides examples of scenarios where the mechanism described in this
   document provides benefits.

2.  Problem Statements

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

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

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

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




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

   When specific return path encoding is supplied by users or
   applications, then there are no issues in choosing the return path
   encoding.  When specific return path encoding is not supplied by
   users or applications, then implementations use extra logic to
   compute, and sometimes guess, the default return path encodings.  If
   a responder node receives an MPLS echo request containing return path
   instructions which cannot be accommodated due to unavailability, then
   the responder often drops such packets.  This results in the
   initiator not receiving the MPLS LSP echo reply packets back.  This
   consequence may be acceptable for failure cases (e.g., broken LSPs)
   where the MPLS echo request terminated on unintended target.
   However, the initiator not receiving back MPLS echo reply packets,
   even when the intended target received and verified the requests, is
   not desirable as false failures will be conveyed to users.

   Many operators prefer some return path(s) over others for specific
   LSP types.  To accommodate this, implementations may default to
   operator preferred return path (or allow default return path to be
   configured) for a specific operation.  However, if the sender of MPLS
   echo request knew that preferred return path will not be available at
   the intended target node, then it is not very beneficial to use a
   Reply Mode corresponding to preferred return path (i.e., the sender
   of the MPLS echo request will not receive the MPLS echo reply in the
   successful case).  What would be beneficial, for a given operation,
   is for the sender of the MPLS echo request to determine which return
   path(s) can and cannot be used ahead of time.

   This document adds one Reply Mode value to describe the reverse LSP,
   and one optional TLV to describe an ordered list of reply modes.
   Based on operational needs, the TLV can describe multiple Reply Mode
   values in a preferred order to allow the responder to use the first
   available Reply Mode from the list.  This eliminates the need for the
   initiator to compute, or sometimes guess, the default return path
   encoding.  And that will result in simplified implementations across
   vendors, and result in improved usability to fit operational needs.

3.  Solution

   This document adds one reply mode to indicate reverse LSP, to be used
   by the MPLS LSP Ping and Traceroute.  This document also adds an
   optional TLV which can carry ordered list of reply modes.



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3.1.  Reply via reverse LSP

   Some LSP types are capable of having related LSP in reverse
   direction, through signaling or other association mechanisms.
   Examples of such LSP types are RSVP LSPs and TP LSPs.  This document
   uses the term "Reverse LSP" to refer to the LSP in reverse direction
   of such LSP types.  Note that this document restricts the scope of
   "Reverse LSP" applicability to those reverse LSPs which are capable
   and allowed to carry the IP encapsulated MPLS echo reply.

   This document adds one Reply Mode to be used by MPLS LSP Ping and
   Traceroute operations.

     Value   Meaning
     -----   -------
      TBD1   Reply via reverse LSP

   MPLS echo request with TBD1 (Reply via reverse LSP) in the Reply Mode
   field may be used to instruct responder to use reverse LSP to send
   MPLS echo reply.  Reverse LSP is in relation to the last FEC
   specified in the Target FEC Stack TLV.

   When a responder is using this Reply Mode, transmitting MPLS echo
   reply packet MUST use IP destination address of 127/8 for IPv4 and
   0:0:0:0:0:FFFF:7F00/104 for IPv6.

3.2.  Reply Mode Order TLV

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

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

   2.  Reply Mode Order TLV MUST NOT be included in MPLS echo reply.

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

   4.  If a responder node understands the Reply Mode Order TLV and the
       TLV is valid, then the responder MUST consider Reply Mode values




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       described in the TLV and MUST NOT use the value described in the
       Reply Mode field of received MPLS echo request.

   5.  If a responder node understands the Reply Mode Order TLV but the
       TLV is not valid (due to conditions listed below), then the
       responder MUST only use the value described in the Reply Mode
       field of received MPLS echo request.

   6.  Reply Mode Order TLV MUST contain at least one Reply Mode value,
       and SHOULD contain at least two Reply Mode values.

   7.  A Reply Mode value MUST NOT be repeated (i.e.  MUST NOT appear
       multiple times) in the Reply Mode Order TLV.

   8.  Reply Mode value 1 (Do not reply) SHOULD NOT be used in the Reply
       Mode Order TLV.

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

   The format of the TLV is as follows:

      0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Reply Mode Order TLV Type     |          Length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Reply mode 1  | Reply mode 2  | Reply mode 3  | Reply mode 4  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       Figure 1 Reply Mode Order TLV

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

4.  Relations to Other LSP Ping/Trace Features

4.1.  Reply Path TLV

   [RFC7110] defines a new Reply Mode (5 - Reply via Specified Path).
   This Reply Mode specified in MPLS echo request indicates that MPLS
   echo reply be sent on one specific path described by the Reply Path
   TLV.  The Flags field of the Reply Path TLV can indicate B
   (Bidirectional) bit to describe reverse direction of the tested
   bidirectional LSP.  However, it is desired to have a new Reply Mode
   (TBD1 - Reply via reverse LSP) to indicate reverse direction of the
   tested bidirectional LSP without requiring to include additional TLV



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   (i.e.  Reply Path TLV).  Therefore, a new Reply Mode (TBD1 - Reply
   via reverse LSP) has been added.

4.1.1.  Reply Mode Order TLV Usage Example with Reply Path TLV

   If the initiator was interested in encoding following return paths:

   1.  Reply via application level control channel

   2.  FEC X

   3.  FEC Y

   4.  Reply via an IPv4/IPv6 UDP packet

   Then the MPLS echo request message is to carry:

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

   o  The Reply Path TLV carrying {FEC X, FEC Y}

   Described encoding of the Reply Mode Order TLV and the Reply Path TLV
   in the MPLS echo request message will result in the responder 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.  Proxy LSP Ping

   The mechanism defined in this document will work with Proxy LSP Ping
   defined by [I-D.ietf-mpls-proxy-lsp-ping].  MPLS proxy ping request
   can carry a Reply Mode value and the Reply Mode Order TLV with list
   of Reply Mode values.  Proxy LSR MUST copy both Reply Mode value and
   the Reply Mode Order TLV into MPLS echo request.  Proxy LSR, upon
   receiving MPLS echo reply, MUST copy Reply Mode value into MPLS proxy
   ping reply.  With these procedures, Reply Mode used by the MPLS echo
   reply sender is propagated in the Reply Mode field to the sender of
   MPLS proxy ping request.

5.  Security Considerations

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

6.  IANA Considerations







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6.1.  New Reply Mode

   IANA is requested to assign one reply modes from the "Reply Mode"
   sub-registry within the "Multiprotocol Label Switching Architecture
   (MPLS)" registry.

     Value   Meaning                            Reference
     -----   -------                            ---------
      TBD1   Reply via reverse LSP              this document

6.2.  New Reply Mode Order TLV

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

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

     Type   Meaning                            Reference
     ----   -------                            ---------
     TBD2   Reply Mode Order TLV               this document

7.  Acknowledgements

   Authors would like to thank Santiago Alvarez and Faisal Iqbal for
   discussions which motivated creation of this document.  Authors would
   also like to thank Sam Aldrin, Curtis Villamizar, Ross Callon,
   Jeffrey Zhang, Jeremy Whittaker and Mustapha Alissaoui for providing
   valuable comments to influence the contents of the draft.

8.  Contributing Authors

   Shaleen Saxena
   Cisco Systems
   Email: ssaxena@cisco.com

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



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9.2.  Informative References

   [I-D.ietf-mpls-proxy-lsp-ping]
              Swallow, G., Lim, V., and S. Aldrin, "Proxy MPLS Echo
              Request", draft-ietf-mpls-proxy-lsp-ping-02 (work in
              progress), July 2014.

   [RFC7110]  Chen, M., Cao, W., Ning, S., Jounay, F., and S. Delord,
              "Return Path Specified Label Switched Path (LSP) Ping",
              RFC 7110, January 2014.

Appendix A.  Reply Mode Order TLV Beneficial Scenarios

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

A.1.  Incorrect Forwarding Scenario

   A network has a following LSP, and the LSP has a control channel.

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

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

           Figure 2: Incorrect Forwarding

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

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

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

      Success (Reply from B, Reply Mode: 4)
      Success (Reply from C, Reply Mode: 4)
      Success (Reply from D, Reply Mode: 4)



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      FEC Mismatch (Reply from F, Reply Mode: 2)
      Complete

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

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

   A network has a following bidirectional LSP where the forward LSP and
   the reverse LSP are not fully co-routed.

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

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

           Figure 3: Non-Co-Routed Bidirectional LSP

   Some operators may prefer and configure the system to default the
   Reply Mode to "Reply via reverse LSP (TBD1)" when MPLS echo request
   messages are sent on bidirectional LSPs.  Without extensions
   described in this document, following behaviors will be seen:

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

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

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

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

   With the extension described in this document, same procedures can be
   performed with the Reply Mode Order TLV carrying {TBD1, 2}. When LSP



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   Traceroute is issued, then following output may be displayed without
   any unnecessary timeout.

      Success (Reply Mode: TBD1)
      Success (Reply Mode: 2)
      Success (Reply Mode: 2)
      Success (Reply Mode: TBD1)
      Success (Reply Mode: TBD1)
      Complete

Authors' Addresses

   Nobo Akiya
   Cisco Systems

   Email: nobo@cisco.com


   George Swallow
   Cisco Systems

   Email: swallow@cisco.com


   Carlos Pignataro
   Cisco Systems

   Email: cpignata@cisco.com


   Loa Andersson
   Huawei

   Email: loa@mail01.huawei.com


   Mach(Guoyi) Chen
   Huawei

   Email: mach.chen@huawei.com











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