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Network Working Group                              K. Kompella (Juniper)
Internet Draft                                            P. Pan (Ciena)
draft-ietf-mpls-lsp-ping-03.txt                       N. Sheth (Juniper)
Category: Standards Track                    D. Cooper (Global Crossing)
Expires: December 2003                                G. Swallow (Cisco)
                                                     S. Wadhwa (Juniper)
                                                    R. Bonica (WorldCom)
                                                               June 2003

                   Detecting MPLS Data Plane Failures

                             *** DRAFT ***


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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.


Copyright Notice

   Copyright (C) The Internet Society (2003).  All Rights Reserved.












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Abstract

   This document describes a simple and efficient mechanism that can be
   used to detect data plane failures in Multi-Protocol Label Switching
   (MPLS) Label Switched Paths (LSPs).  There are two parts to this
   document: information carried in an MPLS "echo request" and "echo
   reply" for the purposes of fault detection and isolation; and
   mechanisms for reliably sending the echo reply.


Changes since last revision

   (This section to be removed before publication.)

    - Changed title to "Detecting MPLS Data Plane Failures"
    - removed section 5 "Reliable Reply Path"
    - filled in IANA section
      - added new top level TLV for Vendor Enterprise Code
    - Clarified Downstream Router ID and Downstream Interface Address
    - Clarified receiving procedure
    - Example for multipath operation


Issues

   (This section to be removed before publication.)

    - Question: use two bits from the TLV space to indicate
      - Ignore TLV if not understood
      - Reflect TLV in reply
    - Tweak error codes?  Add stack depth?
    - More multipath stuff?



















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1. Introduction

   This document describes a simple and efficient mechanism that can be
   used to detect data plane failures in MPLS LSPs.  There are two parts
   to this document: information carried in an MPLS "echo request" and
   "echo reply"; and mechanisms for transporting the echo reply.  The
   first part aims at providing enough information to check correct
   operation of the data plane, as well as a mechanism to verify the
   data plane against the control plane, and thereby localize faults.
   The second part suggests two methods of reliable reply channels for
   the echo request message, for more robust fault isolation.

   An important consideration in this design is that MPLS echo requests
   follow the same data path that normal MPLS packets would traverse.
   MPLS echo requests are meant primarily to validate the data plane,
   and secondarily to verify the data plane against the control plane.
   Mechanisms to check the control plane are valuable, but are not
   covered in this document.

   To avoid potential Denial of Service attacks, it is recommended to
   regulate the MPLS ping traffic going to the control plane.  A rate
   limiter should be applied to the well-known UDP port defined below.

1.1. Conventions

   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 [KEYWORDS].

1.2. Structure of this document

   The body of this memo contains four main parts: motivation, MPLS echo
   request/reply packet format, MPLS ping operation, and a reliable
   return path.  It is suggested that first-time readers skip the actual
   packet formats and read the Theory of Operation first; the document
   is structured the way it is to avoid forward references.

   The last section (reliable return path for RSVP LSPs) may be removed
   in a future revision.












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2. Motivation

   When an LSP fails to deliver user traffic, the failure cannot always
   be detected by the MPLS control plane.  There is a need to provide a
   tool that would enable users to detect such traffic "black holes" or
   misrouting within a reasonable period of time; and a mechanism to
   isolate faults.

   In this document, we describe a mechanism that accomplishes these
   goals.  This mechanism is modeled after the ping/traceroute paradigm:
   ping (ICMP echo request [ICMP]) is used for connectivity checks, and
   traceroute is used for hop-by-hop fault localization as well as path
   tracing.  This document specifies a "ping mode" and a "traceroute"
   mode for testing MPLS LSPs.

   The basic idea is to test that packets that belong to a particular
   Forwarding Equivalence Class (FEC) actually end their MPLS path on an
   LSR that is an egress for that FEC.  This document proposes that this
   test be carried out by sending a packet (called an "MPLS echo
   request") along the same data path as other packets belonging to this
   FEC.  An MPLS echo request also carries information about the FEC
   whose MPLS path is being verified.  This echo request is forwarded
   just like any other packet belonging to that FEC.  In "ping" mode
   (basic connectivity check), the packet should reach the end of the
   path, at which point it is sent to the control plane of the egress
   LSR, which then verifies that it is indeed an egress for the FEC.  In
   "traceroute" mode (fault isolation), the packet is sent to the
   control plane of each transit LSR, which performs various checks that
   it is indeed a transit LSR for this path; this LSR also returns
   further information that helps check the control plane against the
   data plane, i.e., that forwarding matches what the routing protocols
   determined as the path.

   One way these tools can be used is to periodically ping a FEC to
   ensure connectivity.  If the ping fails, one can then initiate a
   traceroute to determine where the fault lies.  One can also
   periodically traceroute FECs to verify that forwarding matches the
   control plane; however, this places a greater burden on transit LSRs
   and thus should be used with caution.












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3. Packet Format

   An MPLS echo request is a (possibly labelled) UDP packet; the
   contents of the UDP packet have the following format:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Version Number        |         Must Be Zero          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |  Message Type |   Reply mode  |  Return Code  | Return Subcode|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sender's Handle                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Sequence Number                        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    TimeStamp Sent (seconds)                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  TimeStamp Sent (microseconds)                |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  TimeStamp Received (seconds)                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                TimeStamp Received (microseconds)              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                            TLVs ...                           |
      .                                                               .
      .                                                               .
      .                                                               .
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The Version Number is currently 1.  (Note: the Version Number is to
   be incremented whenever a change is made that affects the ability of
   an implementation to correctly parse or process an MPLS echo
   request/reply.  These changes include any syntactic or semantic
   changes made to any of the fixed fields, or to any TLV or sub-TLV
   assignment or format that is defined at a certain version number.
   The Version Number may not need to be changed if an optional TLV or
   sub-TLV is added.)

   The Message Type is one of the following:

       Value    Meaning
       -----    -------
           1    MPLS Echo Request
           2    MPLS Echo Reply

   The Reply Mode can take one of the following values:



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       Value    Meaning
       -----    -------
           1    Do not reply
           2    Reply via an IPv4 UDP packet
           3    Reply via an IPv4 UDP packet with Router Alert

   An MPLS echo request with "Do not reply" may be used for one-way
   connectivity tests; the receiving router may log gaps in the sequence
   numbers and/or maintain delay/jitter statistics.  An MPLS echo
   request would normally have "Reply via an IPv4 UDP packet"; if the
   normal IPv4 return path is deemed unreliable, one may use "Reply via
   an IPv4 UDP packet with Router Alert" (note that this requires that
   all intermediate routers understand and know how to forward MPLS echo
   replies).

   The Return Code is set to zero by the sender.  The receiver can set
   it to one of the following values:

       Value    Meaning
       -----    -------
           0    The error code is contained in the Error Code TLV
           1    Malformed echo request received
           2    One or more of the TLVs was not understood
           3    Replying router is an egress for the FEC
           4    Replying router has no mapping for the FEC
           5    Replying router is not one of the "Downstream Routers"
           6    Replying router is one of the "Downstream Routers",
                and its mapping for this FEC on the received interface
                is the given label
           7    Replying router is one of the "Downstream Routers",
                but its mapping for this FEC is not the given label

   The Return Subcode is unused at present and SHOULD be set to zero.

   The Sender's Handle is filled in by the sender, and returned
   unchanged by the receiver in the echo reply (if any).  There are no
   semantics associated with this handle, although a sender may find
   this useful for matching up requests with replies.

   The Sequence Number is assigned by the sender of the MPLS echo
   request, and can be (for example) used to detect missed replies.

   The TimeStamp Sent is the time-of-day (in seconds and microseconds,
   wrt the sender's clock) when the MPLS echo request is sent.  The
   TimeStamp Received in an echo reply is the time-of-day (wrt the
   receiver's clock) that the corresponding echo request was received.

   TLVs (Type-Length-Value tuples) have the following format:



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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Type              |            Length             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Value                             |
      .                                                               .
      .                                                               .
      .                                                               .
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Types are defined below; Length is the length of the Value field in
   octets.  The Value field depends on the Type; it is zero padded to
   align to a four-octet boundary.

          Type #                           Value Field
          ------                           -----------
               1                           Target FEC Stack
               2                           Downstream Mapping
               3                           Pad
               4                           Error Code
               5                           Vendor Enterprise Code

3.1. Target FEC Stack

   A Target FEC Stack is a list of sub-TLVs.  The number of elements is
   determined by the looking at the sub-TLV length fields.

      Sub-Type #       Length              Value Field
      ----------       ------              -----------
               1            5              LDP IPv4 prefix
               2           17              LDP IPv6 prefix
               3           20              RSVP IPv4 Session Query
               4           56              RSVP IPv6 Session Query
               5                           Reserved; see Appendix
               6           13              VPN IPv4 prefix
               7           25              VPN IPv6 prefix
               8           14              L2 VPN endpoint
               9           10              L2 circuit ID

   Other FEC Types will be defined as needed.

   Note that this TLV defines a stack of FECs, the first FEC element
   corresponding to the top of the label stack, etc.

   An MPLS echo request MUST have a Target FEC Stack that describes the
   FEC stack being tested.  For example, if an LSR X has an LDP mapping



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   for 192.168.1.1 (say label 1001), then to verify that label 1001 does
   indeed reach an egress LSR that announced this prefix via LDP, X can
   send an MPLS echo request with a FEC Stack TLV with one FEC in it,
   namely of type LDP IPv4 prefix, with prefix 192.168.1.1/32, and send
   the echo request with a label of 1001.

   Say LSR X wanted to verify that a label stack of <1001, 23456> is the
   right label stack to use to reach a VPN IPv4 prefix of 10/8 in VPN
   foo.  Say further that LSR Y with loopback address 192.168.1.1
   announced prefix 10/8 with Route Distinguisher RD-foo-Y (which may in
   general be different from the Route Distinguisher that LSR X uses in
   its own advertisements for VPN foo), label 23456 and BGP nexthop
   192.168.1.1.  Finally, suppose that LSR X receives a label binding of
   1001 for 192.168.1.1 via LDP.  X has two choices in sending an MPLS
   echo request: X can send an MPLS echo request with a FEC Stack TLV
   with a single FEC of type VPN IPv4 prefix with a prefix of 10/8 and a
   Route Distinguisher of RD-foo-Y.  Alternatively, X can send a FEC
   Stack TLV with two FECs, the first of type LDP IPv4 with a prefix of
   192.168.1.1/32 and the second of type of IP VPN with a prefix 10/8
   with Route Distinguisher of RD-foo-Y.  In either case, the MPLS echo
   request would have a label stack of <1001, 23456>.  (Note: in this
   example, 1001 is the "outer" label and 23456 is the "inner" label.)

3.1.1. LDP IPv4 Prefix

   The value consists of four octets of an IPv4 prefix followed by one
   octet of prefix length in bits.  The IPv4 prefix is in network byte
   order.  See [LDP] for an example of a Mapping for an IPv4 FEC.

3.1.2. LDP IPv6 Prefix

   The value consists of sixteen octets of an IPv6 prefix followed by
   one octet of prefix length in bits.  The IPv6 prefix is in network
   byte order.  See [LDP] for an example of a Mapping for an IPv6 FEC.

3.1.3. RSVP IPv4 Session

   The value has the format below.  The value fields are taken from
   [RFC3209, sections 4.6.1.1 and 4.6.2.1].

       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 tunnel end point address                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |     Tunnel ID                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Extended Tunnel ID                      |



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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   IPv4 tunnel sender address                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |            LSP ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.4. RSVP IPv6 Session

   The value has the format below.  The value fields are taken from
   [RFC3209, sections 4.6.1.2 and 4.6.2.2].

       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 tunnel end point address                 |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |          Tunnel ID            |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Extended Tunnel ID                      |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                   IPv6 tunnel sender address                  |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |            LSP ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.5. VPN IPv4 Prefix

   The value field consists of the Route Distinguisher advertised with
   the VPN IPv4 prefix, the IPv4 prefix and a prefix length, 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IPv4 prefix                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |                 Must Be Zero                  |



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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.6. VPN IPv6 Prefix

   The value field consists of the Route Distinguisher advertised with
   the VPN IPv6 prefix, the IPv6 prefix and a prefix length, 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         IPv6 prefix                           |
      |                                                               |
      |                                                               |
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Prefix Length |                 Must Be Zero                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.7. L2 VPN Endpoint

   The value field consists of a Route Distinguisher (8 octets), the
   sender (of the ping)'s CE ID (2 octets), the receiver's CE ID (2
   octets), and an encapsulation type (2 octets), formatted 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Route Distinguisher                      |
      |                          (8 octets)                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Sender's CE ID        |       Receiver's CE ID        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encapsulation Type       |         Must Be Zero          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.1.8. L2 Circuit ID

   The value field consists of a remote PE address (the address of the
   targetted LDP session), a VC ID and an encapsulation type, 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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Remote PE Address                        |



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      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             VC ID                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      Encapsulation Type       |         Must Be Zero          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.2. Downstream Mapping

   The Downstream Mapping is an optional TLV in an echo request.  The
   Length is 16 + 4*M + 4*N octets, where M is the Multipath Length, and
   N is the number of Downstream Labels.  The Value field of a
   Downstream Mapping has the following format:

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    Downstream IPv4 Address                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               MTU             | Address Type  |   DS Index    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                  Downstream Interface Address                 |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      | Hash Key Type | Depth Limit   |        No of Multipaths       |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                    IP Address or Next Label                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .               (more IP Addresses or Next Labels)              .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      .                                                               .
      .                                                               .
      .                                                               .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |               Downstream Label                |    Protocol   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   If the interface to the downstream LSR is numbered, then the
   Downstream IPv4 Address can either be the downstream LSR's Router ID
   or the interface address of the downstream LSR.  In this case, the
   Address Type is set to IPv4 and the Downstream Interface Address is
   set to the downstream LSR's interface address.  If the interface to
   the downstream LSR is unnumbered, the Downstream IPv4 Address MUST be
   the downstream LSR's Router ID, and the Address Type MUST be
   Unnumbered, and the Downstream Interface Address MUST be the index
   assigned by the upstream LSR to the interface.



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   The MTU is the largest MPLS frame (including label stack) that fits
   on the interface to the Downstream LSR.  The Downstream Interface
   Address Type is one of:

             Type #        Address Type
             ------        ------------
                  1        IPv4
                  2        Unnumbered

   'Protocol' is taken from the following table:

         Protocol #        Signaling Protocol
         ----------        ------------------
                  0        Unknown
                  1        Static
                  2        BGP
                  3        LDP
                  4        RSVP-TE
                  5        Reserved; see Appendix

   The notion of "downstream router" and "downstream interface" should
   be explained.  Consider an LSR X.  If a packet that was originated
   with TTL n>1 arrived with outermost label L at LSR X, X must be able
   to compute which LSRs could receive the packet if it was originated
   with TTL=n+1, over which interface the request would arrive and what
   label stack those LSRs would see.  (It is outside the scope of this
   document to specify how this computation is done.)  The set of these
   LSRs/interfaces are the downstream routers/interfaces (and their
   corresponding labels) for X with respect to L.  Each pair of
   downstream router and interface requires a separate Downstream
   Mapping to be added to the reply, and is given a unique DS Index.
   (Note that there are multiple Downstream Label fields in each TLV as
   the incoming label L may be swapped with a label stack.)

   The case where X is the LSR originating the echo request is a special
   case.  X needs to figure out what LSRs would receive the MPLS echo
   request for a given FEC Stack that X originates with TTL=1.

   The set of downstream routers at X may be alternative paths (see the
   discussion below on ECMP) or simultaneous paths (e.g., for MPLS
   multicast).  In the former case, the Multipath sub-field is used as a
   hint to the sender as to how it may influence the choice of these
   alternatives.  The "No of Multipaths" is the number of IP
   Address/Next Label fields.  The Hash Key Type is taken from the
   following table:

      Hash Key Type             IP Address or Next Label
      --------------------      ------------------------



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      0   no multipath          (nothing; M = 0)
      1   label                 M   labels
      2   IP address            M   IP addresses
      3   label range           M/2 low/high label pairs
      4   IP address range      M/2 low/high address pairs
      5   no more labels        (nothing; M = 0)
      6   All IP addresses      (nothing; M = 0)
      7   no match              (nothing; M = 0)

   The Depth Limit is applicable only to a label stack, and is the
   maximum number of labels considered in the hash; this SHOULD be set
   to zero if unspecified or unlimited.

   IP Address or Next Label is an IP address from the range 127/8 or an
   next label which will exercise this particular path.

   The semantics of the Hash Key Type and IP Address/Next Label are as
   follows:

      type 1 - a list of single labels is provided, any one of which
               will cause the hash to match this MP path.
      type 2 - a list of single IP addresses is provided, any one of
               which will cause the hash to match this MP path.
      type 3 - a list of label ranges is provided, any one of which will
               cause the hash to match this MP path.
      type 4 - a list of IP address ranges is provided, any one of which
               will cause the hash to match this MP path.
      type 5 - if no more labels are provided on the stack, this MP path
               will apply (can only appear once).
      type 6 - Any IP addresses matches.  Underlying labels may go
               elsewhere, but all IP takes only one MP path (can only
               appear once).
      type 7 - no matches are possible given the set of "Multipath
               Exercise TLV" provided by prior hops.

   If prior hops provide a "Downstream Multipath Mapping TLV" the labels
   and IP addresses should be picked from the set provided in prior
   "Multipath Exercise TLV" or "Hash Key Type" of 7 used.

   For example, suppose LSR X at hop 10 has two downstream LSRs Y and Z
   for the FEC in question.  X could return Hash Key Type 4, with
   low/high IP addresses of 1.1.1.1->1.1.1.255 for downstream LSR Y and
   2.1.1.1->2.1.1.255 for downstream LSR Z.  The head end reflects this
   information to LSR Y.  Y, which has three downstream LSRs U, V and W,
   computes that 1.1.1.1->1.1.1.127 would go to U and 1.1.1.128->
   1.1.1.255 would go to V.  Y would then respond with 3 Downstream
   Mappings: to U, with Hash Key Type 4 (1.1.1.1->1.1.1.127); to V, with
   Hash Key Type 4 (1.1.1.127->1.1.1.255); and to W, with Hash Key Type



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   7.

3.3. Pad TLV

   The value part of the Pad TLV contains a variable number (>= 1) of
   octets.  The first octet takes values from the following table; all
   the other octets (if any) are ignored.  The receiver SHOULD verify
   that the TLV is received in its entirety, but otherwise ignores the
   contents of this TLV, apart from the first octet.

              Value        Meaning
              -----        -------
                  1        Drop Pad TLV from reply
                  2        Copy Pad TLV to reply
              3-255        Reserved for future use

3.4. Error Code

   The Error Code TLV is currently not defined; its purpose is to
   provide a mechanism for a more elaborate error reporting structure,
   should the reason arise.

3.5. Vendor Enterprise Code

   The Length is always 4; the value is the SMI Enterprise code, in
   network octet order, of the vendor with a Vendor Private extension to
   any of the fields in the fixed part of the message, in which case
   this TLV MUST be present.  If none of the fields in the fixed part of
   the message have vendor private extensions, this TLV is OPTIONAL.


4. Theory of Operation

   An MPLS echo request is used to test a particular LSP.  The LSP to be
   tested is identified by the "FEC Stack"; for example, if the LSP was
   set up via LDP, and is to an egress IP address of 10.1.1.1, the FEC
   stack contains a single element, namely, an LDP IPv4 prefix sub-TLV
   with value 10.1.1.1/32.  If the LSP being tested is an RSVP LSP, the
   FEC stack consists of a single element that captures the RSVP Session
   and Sender Template which uniquely identifies the LSP.

   FEC stacks can be more complex.  For example, one may wish to test a
   VPN IPv4 prefix of 10.1/8 that is tunneled over an LDP LSP with
   egress 10.10.1.1.  The FEC stack would then contain two sub-TLVs, the
   first being a VPN IPv4 prefix, and the second being an LDP IPv4
   prefix.  If the underlying (LDP) tunnel were not known, or was
   considered irrelevant, the FEC stack could be a single element with
   just the VPN IPv4 sub-TLV.



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   When an MPLS echo request is received, the receiver is expected to do
   a number of tests that verify that the control plane and data plane
   are both healthy (for the FEC stack being pinged), and that the two
   planes are in sync.

4.1. Dealing with Equal-Cost Multi-Path (ECMP)

   LSPs need not be simple point-to-point tunnels.  Frequently, a single
   LSP may originate at several ingresses, and terminate at several
   egresses; this is very common with LDP LSPs.  LSPs for a given FEC
   may also have multiple "next hops" at transit LSRs.  At an ingress,
   there may also be several different LSPs to choose from to get to the
   desired endpoint.  Finally, LSPs may have backup paths, detour paths
   and other alternative paths to take should the primary LSP go down.

   To deal with the last two first: it is assumed that the LSR sourcing
   MPLS echo requests can force the echo request into any desired LSP,
   so choosing among multiple LSPs at the ingress is not an issue.  The
   problem of probing the various flavors of backup paths that will
   typically not be used for forwarding data unless the primary LSP is
   down will not be addressed here.

   Since the actual LSP and path that a given packet may take may not be
   known a priori, it is useful if MPLS echo requests can exercise all
   possible paths.  This, while desirable, may not be practical, because
   the algorithms that a given LSR uses to distribute packets over
   alternative paths may be proprietary.

   To achieve some degree of coverage of alternate paths, there is a
   certain lattitude in choosing the destination IP address and source
   UDP port for an MPLS echo request.  This is clearly not sufficient;
   in the case of traceroute, more lattitude is offered by means of the
   "Multipath Exercise" sub-TLV of the Downstream Mapping TLV.  This is
   used as follows.  An ingress LSR periodically sends an MPLS
   traceroute message to determine whether there are multipaths for a
   given LSP.  If so, each hop will provide some information how each of
   its downstreams can be exercised.  The ingress can then send MPLS
   echo requests that exercise these paths.  If several transit LSRs
   have ECMP, the ingress may attempt to compose these to exercise all
   possible paths.  However, full coverage may not be possible.

4.2. Sending an MPLS Echo Request

   An MPLS echo request is a (possibly) labelled UDP packet.  The IP
   header is set as follows: the source IP address is a routable address
   of the sender; the destination IP address is a (randomly chosen)
   address from 127/8; the IP TTL is set to 1.  The source UDP port is
   chosen by the sender; the destination UDP port is set to 3503



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   (assigned by IANA for MPLS echo requests).  The Router Alert option
   is set in the IP header.

   If the echo request is labelled, one may (depending on what is being
   pinged) set the TTL of the innermost label to 1, to prevent the ping
   request going farther than it should.  Examples of this include
   pinging a VPN IPv4 or IPv6 prefix, an L2 VPN end point or an L2
   circuit ID.  This can also be accomplished by inserting a router
   alert label above this label; however, this may lead to the undesired
   side effect that MPLS echo requests take a different data path than
   actual data.

   In "ping" mode (end-to-end connectivity check), the TTL in the
   outermost label is set to 255.  In "traceroute" mode (fault isolation
   mode), the TTL is set successively to 1, 2, ....

   The sender chooses a Sender's Handle, and a Sequence Number.  When
   sending subsequent MPLS echo requests, the sender SHOULD increment
   the sequence number by 1.  However, a sender MAY choose to send a
   group of echo requests with the same sequence number to improve the
   chance of arrival of at least one packet with that sequence number.

   The TimeStamp Sent is set to the time-of-day (in seconds and
   microseconds) that the echo request is sent.  The TimeStamp Received
   is set to zero.

   An MPLS echo request MUST have a FEC Stack TLV.  Also, the Reply Mode
   must be set to the desired reply mode; the Return Code and Subcode
   are set to zero.

   In the "traceroute" mode, the echo request SHOULD contain one or more
   Downstream Mapping TLVs.  For TTL=1, all the downstream routers (and
   corresponding labels) for the sender with respect to the FEC Stack
   being pinged SHOULD be sent in the echo request.  For n>1, the
   Downstream Mapping TLVs from the echo reply for TTL=(n-1) are copied
   to the echo request with TTL=n; the sender MAY choose to reduce the
   size of a "Downstream Multipath Mapping TLV" when copying into the
   next echo request as long as the Hash Key Type matching the label or
   IP address used to exercise the current MP is still present.

4.3. Receiving an MPLS Echo Request

   An LSR X that receives an MPLS echo request first parses the packet
   to ensure that it is a well-formed packet, and that the TLVs are
   understood.  If not, X SHOULD send an MPLS echo reply with the Return
   Code set to "Malformed echo request received" or "TLV not understood"
   (as appropriate), and the Subcode set to the appropriate value.




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   If the echo request is good, X then checks whether it is a valid
   transit or egress LSR for the FEC in the echo request.  If not, X MAY
   log this fact.  If it is, X notes that interface I over which the
   echo was received, and the label L with which it came.  X checks
   whether it actually advertised L for the FEC in the echo request; X
   MAY further check whether it expects L over interface I or not.

   If the echo request contains a Downstream Mapping TLV, X MUST further
   check whether its Router ID or one of its interface addresses matches
   one of the Downstream IPv4 Address; if the Address Type is
   Unnumbered, X further checks if the interface I has the given
   (upstream) index.  If these check out, X determines whether the given
   Downstream Label is in fact the label that X sent as its mapping for
   the FEC over the downstream interface.  The result of the checks in
   the previous and this paragraph are captured in the Return
   Code/Subcode.

   If the echo request has a Reply Mode that wants a reply, X uses the
   procedure in the next subsection to send the echo reply.

4.4. Sending an MPLS Echo Reply

   An MPLS echo reply is a UDP packet.  It MUST ONLY be sent in response
   to an MPLS echo request.  The source IP address is a routable address
   of the replier; the source port is the well-known UDP port for MPLS
   ping.  The destination IP address and UDP port are copied from the
   source IP address and UDP port of the echo request.  The IP TTL is
   set to 255.  If the Reply Mode in the echo request is "Reply via an
   IPv4 UDP packet with Router Alert", then the IP header MUST contain
   the Router Alert IP option.  If the reply is sent over an LSP, the
   topmost label MUST in this case be the Router Alert label (1) (see
   [LABEL-STACK]).

   The format of the echo reply is the same as the echo request.  The
   Sender's Handle, the Sequence Number and TimeStamp Sent are copied
   from the echo request; the TimeStamp Received is set to the time-of-
   day that the echo request is received (note that this information is
   most useful if the time-of-day clocks on the requestor and the
   replier are synchronized).  The FEC Stack TLV from the echo request
   MAY be copied to the reply.

   The replier MUST fill in the Return Code and Subcode, as determined
   in the previous subsection.

   If the echo request contains a Pad TLV, the replier MUST interpret
   the first octet for instructions regarding how to reply.

   If the echo request contains a Downstream Mapping TLV, the replier



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   SHOULD compute its downstream routers and corresponding labels for
   the incoming label, and add Downstream Mapping TLVs for each one to
   the echo reply it sends back.

4.5. Receiving an MPLS Echo Reply

   An LSR X should only receive an MPLS Echo Reply in response to an
   MPLS Echo Request that it sent.  Thus, on receipt of an MPLS Echo
   Reply, X should parse the packet to assure that it is well-formed,
   then attempt to match up the Echo Reply with an Echo Request that it
   had previously sent, using the destination UDP port and the Sender's
   Handle.  If no match is found, then X jettisons the Echo Reply;
   otherwise, it checks the Sequence Number to see if it matches.  Gaps
   in the Sequence Number MAY be logged and SHOULD be counted.  Once an
   Echo Reply is received for a given Sequence Number (for a given UDP
   port and Handle), the Sequence Number for subsequent Echo Requests
   for that UDP port and Handle SHOULD be incremented.

   If the Echo Reply contains Downstream Mappings, and X wishes to
   traceroute further, it SHOULD copy the Downstream Mappings into its
   next Echo Request (with TTL incremented by one).

4.6. Non-compliant Routers

   If the egress for the FEC Stack being pinged does not support MPLS
   ping, then no reply will be sent, resulting in possible "false
   negatives".  If in "traceroute" mode, a transit LSR does not support
   MPLS ping, then no reply will be forthcoming from that LSR for some
   TTL, say n.  The LSR originating the echo request SHOULD try sending
   the echo request with TTL=n+1, n+2, ..., n+k in the hope that some
   transit LSR further downstream may support MPLS echo requests and
   reply.  In such a case, the echo request for TTL>n MUST NOT have
   Downstream Mapping TLVs, until a reply is received with a Downstream
   Mapping.


Normative References

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

   [LABEL-STACK] Rosen, E., et al, "MPLS Label Stack Encoding", RFC
       3032, January 2001.

   [RSVP]  Braden, R. (Editor), et al, "Resource ReSerVation protocol
       (RSVP) -- Version 1 Functional Specification," RFC 2205,
       September 1997.




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   [RSVP-REFRESH]  Berger, L., et al, "RSVP Refresh Overhead Reduction
       Extensions", RFC 2961, April 2001.

   [RSVP-TE]  Awduche, D., et al, "RSVP-TE: Extensions to RSVP for LSP
       tunnels", RFC 3209, December 2001.


Informative References

   [ICMP]  Postel, J., "Internet Control Message Protocol", RFC 792.

   [LDP]  Andersson, L., et al, "LDP Specification", RFC 3036, January
       2001.


Security Considerations

   There are at least two approaches to attacking LSRs using the
   mechanisms defined here.  One is a Denial of Service attack, by
   sending MPLS echo requests/replies to LSRs and thereby increasing
   their workload.  The other is obfuscating the state of the MPLS data
   plane liveness by spoofing, hijacking, replaying or otherwise
   tampering with MPLS echo requests and replies.

   Authentication will help reduce the number of seemingly valid MPLS
   echo requests, and thus cut down the Denial of Service attacks;
   beyond that, each LSR must protect itself.

   Authentication sufficiently addresses spoofing, replay and most
   tampering attacks; one hopes to use some mechanism devised or
   suggested by the RPSec WG.  It is not clear how to prevent hijacking
   (non-delivery) of echo requests or replies; however, if these
   messages are indeed hijacked, MPLS ping will report that the data
   plane isn't working as it should.

   It doesn't seem vital (at this point) to secure the data carried in
   MPLS echo requests and replies, although knowledge of the state of
   the MPLS data plane may be considered confidential by some.













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5. IANA Considerations

   The TCP and UDP port number 3503 has been allocated by IANA for LSP
   echo requests and replies.

   The following sections detail the new name spaces to be managed by
   IANA.  For each of these name spaces, the space is divided into
   assignment ranges; the following terms are used in describing the
   procedures by which IANA allocates values: "Standards Action" (as
   defined in [IANA]); "Expert Review" and "Vendor Private Use".

   Values from "Expert Review" ranges MUST be registered with IANA, and
   MUST be accompanied by an Experimental RFC that describes the format
   and procedures for using the code point.

   Values from "Vendor Private" ranges MUST NOT be registered with IANA;
   however, the message MUST contain an enterprise code as registered
   with the IANA SMI Network Management Private Enterprise Codes.  For
   each name space that has a Vendor Private range, it must be specified
   where exactly the SMI Enterprise Code resides; see below for
   examples.  In this way, several enterprises (vendors) can use the
   same code point without fear of collision.

5.1. Message Types, Reply Modes, Return Codes

   It is requested that IANA maintain registries for Message Types,
   Reply Modes, Return Codes and Return Subcodes.  Each of these can
   take values in the range 0-255.  Assignments in the range 0-191 are
   via Standards Action; assignments in the range 192-251 are made via
   Expert Review; values in the range 252-255 are for Vendor Private
   Use, and MUST NOT be allocated.

   If any of these fields fall in the Vendor Private range, a top-level
   Vendor Enterprise Code TLV MUST be present in the message.

5.2. TLVs

   It is requested that IANA maintain registries for the Type field of
   top-level TLVs as well as for sub-TLVs.  The valid range for each of
   these is 0-65535.  Assignments in the range 0-32767 are made via
   Standards Action; assignments in the range 32768-64511 are made via
   Expert Review; values in the range 64512-65535 are for Vendor Private
   Use, and MUST NOT be allocated.

   If a TLV or sub-TLV has a Type that falls in the range for Vendor
   Private Use, the Length MUST be at least 4, and the first four octets
   MUST be that vendor's SMI Enterprise Code, in network octet order.
   The rest of the Value field is private to the vendor.



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Acknowledgments

   This document is the outcome of many discussions among many people,
   that include Manoj Leelanivas, Paul Traina, Yakov Rekhter, Der-Hwa
   Gan, Brook Bailey, Eric Rosen and Ina Minei.

   The Multipath Exercise sub-field of the Downstream Mapping TLV was
   adapted from text suggested by Curtis Villamizar.


Appendix

   This appendix specifies non-normative aspects of detecting MPLS data
   plane liveness.

5.1. CR-LDP FEC

   This section describes how a CR-LDP FEC can be included in an Echo
   Request using the following FEC subtype:

      Sub-Type #       Length              Value Field
      ----------       ------              -----------
               5            6              CR-LDP LSP ID

   The value consists of the LSPID of the LSP being pinged.  An LSPID is
   a four octet IPv4 address (a local address on the ingress LSR, for
   example, the Router ID) plus a two octet identifier that is unique
   per LSP on a given ingress LSR.

       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                       Ingress LSR Router ID                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |          Must Be Zero         |            LSP ID             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

5.2. Downstream Mapping for CR-LDP

   If a label in a Downstream Mapping was learned via CR-LDP, the
   Protocol field in the Mapping TLV can use the following entry:

         Protocol #        Signaling Protocol
         ----------        ------------------
                  5        CR-LDP






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Authors' Addresses

   Kireeti Kompella
   Nischal Sheth
   Juniper Networks
   1194 N.Mathilda Ave
   Sunnyvale, CA 94089
   e-mail: kireeti@juniper.net
   e-mail: nsheth@juniper.net

   Ping Pan
   Ciena
   10480 Ridgeview Court
   Cupertino, CA 95014
   e-mail: ppan@ciena.com
   phone: +1 408.366.4700

   Dave Cooper
   Global Crossing
   960 Hamlin Court
   Sunnyvale, CA 94089
   email: dcooper@gblx.net
   phone: +1 916.415.0437

   George Swallow
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA 01824
   e-mail:  swallow@cisco.com
   phone: +1 978.497.8143

   Sanjay Wadhwa
   Juniper Networks
   10 Technology Park Drive
   Westford, MA 01886-3146
   email: swadhwa@unispherenetworks.com
   phone: +1 978.589.0697

   Ronald P. Bonica
   WorldCom
   22001 Loudoun County Pkwy
   Ashburn, Virginia, 20147
   email: ronald.p.bonica@wcom.com
   phone: +1 703.886.1681







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Intellectual Property Rights Notices

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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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