PROBE: A Utility for Probing Interfaces
RFC 8335

Internet Engineering Task Force (IETF)                         R. Bonica
Request for Comments: 8335                                     R. Thomas
Updates: 4884                                           Juniper Networks
Category: Standards Track                                     J. Linkova
ISSN: 2070-1721                                                   Google
                                                               C. Lenart
                                                                 Verizon
                                                            M. Boucadair
                                                                  Orange
                                                           February 2018

                PROBE: A Utility for Probing Interfaces

Abstract

   This document describes a network diagnostic tool called PROBE.
   PROBE is similar to PING in that it can be used to query the status
   of a probed interface, but it differs from PING in that it does not
   require bidirectional connectivity between the probing and probed
   interfaces.  Instead, PROBE requires bidirectional connectivity
   between the probing interface and a proxy interface.  The proxy
   interface can reside on the same node as the probed interface, or it
   can reside on a node to which the probed interface is directly
   connected.  This document updates RFC 4884.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8335.

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Copyright Notice

   Copyright (c) 2018 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  ICMP Extended Echo Request  . . . . . . . . . . . . . . . . .   5
     2.1.  Interface Identification Object . . . . . . . . . . . . .   6
   3.  ICMP Extended Echo Reply  . . . . . . . . . . . . . . . . . .   7
   4.  ICMP Message Processing . . . . . . . . . . . . . . . . . . .   9
     4.1.  Code Field Processing . . . . . . . . . . . . . . . . . .  11
   5.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   6.  Updates to RFC 4884 . . . . . . . . . . . . . . . . . . . . .  12
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  The PROBE Application  . . . . . . . . . . . . . . .  17
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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1.  Introduction

   Network operators use PING [RFC2151] to test bidirectional
   connectivity between two interfaces.  For the purposes of this
   document, these interfaces are called the probing and probed
   interfaces.  PING sends an ICMP [RFC792] [RFC4443] Echo Request
   message from the probing interface to the probed interface.  The
   probing interface resides on a probing node while the probed
   interface resides on a probed node.

   If the probed interface receives the ICMP Echo Request message, it
   returns an ICMP Echo Reply.  When the probing interface receives the
   ICMP Echo Reply, it has verified bidirectional connectivity between
   the probing and probed interfaces.  Specifically, it has verified
   that:

   o  The probing node can reach the probed interface.

   o  The probed interface is active.

   o  The probed node can reach the probing interface.

   o  The probing interface is active.

   This document describes a network diagnostic tool called PROBE.
   PROBE is similar to PING in that it can be used to query the status
   of a probed interface, but it differs from PING in that it does not
   require bidirectional connectivity between the probing and probed
   interfaces.  Instead, PROBE requires bidirectional connectivity
   between the probing interface and a proxy interface.  The proxy
   interface can reside on the same node as the probed interface, or it
   can reside on a node to which the probed interface is directly
   connected.  Section 5 of this document describes scenarios in which
   this characteristic is useful.

   Like PING, PROBE executes on a probing node.  It sends an ICMP
   Extended Echo Request message from a local interface, called the
   probing interface, to a proxy interface.  The proxy interface resides
   on a proxy node.

   The ICMP Extended Echo Request contains an ICMP Extension Structure
   and the ICMP Extension Structure contains an Interface Identification
   Object.  The Interface Identification Object identifies the probed
   interface.  The probed interface can reside on or directly connect to
   the proxy node.

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   When the proxy interface receives the ICMP Extended Echo Request, the
   proxy node executes access control procedures.  If access is granted,
   the proxy node determines the status of the probed interface and
   returns an ICMP Extended Echo Reply message.  The ICMP Extended Echo
   Reply indicates the status of the probed interface.

   If the probed interface resides on the proxy node, PROBE determines
   the status of the probed interface as it would determine its oper-
   status [RFC7223].  If oper-status is equal to 'up' (1), PROBE reports
   that the probed interface is active.  Otherwise, PROBE reports that
   the probed interface is inactive.

   If the probed interface resides on a node that is directly connected
   to the proxy node, and the probed interface appears in the IPv4
   Address Resolution Protocol (ARP) table [RFC826] or IPv6 Neighbor
   Cache [RFC4861], PROBE reports interface reachability.  Otherwise,
   PROBE reports that the table entry does not exist.

1.1.  Terminology

   This document uses the following terms:

   o  Probing interface: The interface that sends the ICMP Extended Echo
      Request.

   o  Probing node: The node upon which the probing interface resides.

   o  Proxy interface: The interface to which the ICMP Extended Echo
      Request message is sent.

   o  Proxy node: The node upon which the proxy interface resides.

   o  Probed interface: The interface whose status is being queried.

   o  Probed node: The node upon which the probed interface resides.  If
      the proxy interface and the probed interface reside upon the same
      node, the proxy node is also the probed node.  Otherwise, the
      proxy node is directly connected to the probed node.

1.2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

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2.  ICMP Extended Echo Request

   The ICMP Extended Echo Request message is defined for both ICMPv4 and
   ICMPv6.  Like any ICMP message, the ICMP Extended Echo Request
   message is encapsulated in an IP header.  The ICMPv4 version of the
   Extended Echo Request message is encapsulated in an IPv4 header,
   while the ICMPv6 version is encapsulated in an IPv6 header.

   Figure 1 depicts the ICMP Extended Echo Request message.

       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      |     Code      |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Identifier          |Sequence Number|   Reserved  |L|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |   ICMP Extension Structure

               Figure 1: ICMP Extended Echo Request Message

   IP Header fields:

   o  Source Address: The Source Address identifies the probing
      interface.  It MUST be a valid IPv4 or IPv6 unicast address.

   o  Destination Address: The Destination Address identifies the proxy
      interface.  It MUST be a unicast address.

   ICMP fields:

   o  Type: Extended Echo Request.  The value for ICMPv4 is 42.  The
      value for ICMPv6 is 160.

   o  Code: MUST be set to 0 and MUST be ignored upon receipt.

   o  Checksum: For ICMPv4, see RFC 792.  For ICMPv6, see RFC 4443.

   o  Identifier: An Identifier to aid in matching Extended Echo Replies
      to Extended Echo Requests.  May be 0.

   o  Sequence Number: A Sequence Number to aid in matching Extended
      Echo Replies to Extended Echo Requests.  May be 0.

   o  Reserved: This field MUST be set to 0 and ignored upon receipt.

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   o  L (local): The L-bit is set if the probed interface resides on the
      proxy node.  The L-bit is clear if the probed interface is
      directly connected to the proxy node.

   o  ICMP Extension Structure: The ICMP Extension Structure identifies
      the probed interface.

   Section 7 of [RFC4884] defines the ICMP Extension Structure.  As per
   RFC 4884, the Extension Structure contains exactly one Extension
   Header followed by one or more objects.  When applied to the ICMP
   Extended Echo Request message, the ICMP Extension Structure MUST
   contain exactly one instance of the Interface Identification Object
   (see Section 2.1).

   If the L-bit is set, the Interface Identification Object can identify
   the probed interface by name, index, or address.  If the L-bit is
   clear, the Interface Identification Object MUST identify the probed
   interface by address.

   If the Interface Identification Object identifies the probed
   interface by address, that address can be a member of any address
   family.  For example, an ICMPv4 Extended Echo Request message can
   carry an Interface Identification Object that identifies the probed
   interface by IPv4, IPv6, or IEEE 802 address.  Likewise, an ICMPv6
   Extended Echo Request message can carry an Interface Identification
   Object that identifies the probed interface by IPv4, IPv6, or IEEE
   802 address.

2.1.  Interface Identification Object

   The Interface Identification Object identifies the probed interface
   by name, index, or address.  Like any other ICMP Extension Object, it
   contains an Object Header and Object Payload.  The Object Header
   contains the following fields:

   o  Class-Num: Interface Identification Object.  The value is 3.

   o  C-Type: Values are (1) Identifies Interface by Name, (2)
      Identifies Interface by Index, and (3) Identifies Interface by
      Address.

   o  Length: Length of the object, measured in octets, including the
      Object Header and Object Payload.

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   If the Interface Identification Object identifies the probed
   interface by name, the Object Payload MUST be the interface name as
   defined in [RFC7223].  If the Object Payload would not otherwise
   terminate on a 32-bit boundary, it MUST be padded with ASCII NULL
   characters.

   If the Interface Identification Object identifies the probed
   interface by index, the length is equal to 8 and the payload contains
   the if-index [RFC7223].

   If the Interface Identification Object identifies the probed
   interface by address, the payload is as depicted in Figure 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
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |            AFI                | Address Length|   Reserved    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Address   ....

       Figure 2: Interface Identification Object - C-Type 3 Payload

   Payload fields are defined as follows:

   o  Address Family Identifier (AFI): This 16-bit field identifies the
      type of address represented by the Address field.  All values
      found in the IANA registry of Address Family Numbers (available
      from <https://www.iana.org/assignments/address-family-numbers>)
      are valid in this field.

   o  Address Length: Number of significant bytes contained by the
      Address field.  (The Address field contains significant bytes and
      padding bytes.)

   o  Reserved: This field MUST be set to 0 and ignored upon receipt.

   o  Address: This variable-length field represents an address
      associated with the probed interface.  If the address field would
      not otherwise terminate on a 32-bit boundary, it MUST be padded
      with zeroes.

3.  ICMP Extended Echo Reply

   The ICMP Extended Echo Reply message is defined for both ICMPv4 and
   ICMPv6.  Like any ICMP message, the ICMP Extended Echo Reply message
   is encapsulated in an IP header.  The ICMPv4 version of the Extended
   Echo Reply message is encapsulated in an IPv4 header, while the
   ICMPv6 version is encapsulated in an IPv6 header.

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   Figure 3 depicts the ICMP Extended Echo Reply message.

       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      |     Code      |          Checksum             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Identifier          |Sequence Number|State|Res|A|4|6|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                Figure 3: ICMP Extended Echo Reply Message

   IP Header fields:

   o  Source Address: Copied from the Destination Address field of the
      invoking Extended Echo Request message.

   o  Destination Address: Copied from the Source Address field of the
      invoking Extended Echo Request message.

   ICMP fields:

   o  Type: Extended Echo Reply.  The value for ICMPv4 is 43.  The value
      for ICMPv6 is 161.

   o  Code: Values are (0) No Error, (1) Malformed Query, (2) No Such
      Interface, (3) No Such Table Entry, and (4) Multiple Interfaces
      Satisfy Query.

   o  Checksum: For ICMPv4, see RFC 792.  For ICMPv6, see RFC 4443.

   o  Identifier: Copied from the Identifier field of the invoking
      Extended Echo Request packet.

   o  Sequence Number: Copied from the Sequence Number field of the
      invoking Extended Echo Request packet.

   o  State: If Code is not equal to 0, this field MUST be set to 0 and
      ignored upon receipt.  Likewise, if the probed interface resides
      upon the proxy node, this field MUST be set to 0 and ignored upon
      receipt.  Otherwise, this field reflects the state of the ARP
      table or Neighbor Cache entry associated with the probed
      interface.  Values are (0) Reserved, (1) Incomplete, (2)
      Reachable, (3) Stale, (4) Delay, (5) Probe, and (6) Failed.

   o  Res: This field MUST be set to 0 and ignored upon receipt.

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   o  A (Active): The A-bit is set if the Code is equal to 0, the probed
      interface resides on the proxy node, and the probed interface is
      active.  Otherwise, the A-bit is clear.

   o  4 (IPv4): The 4-bit is set if the A-bit is also set and IPv4 is
      running on the probed interface.  Otherwise, the 4-bit is clear.

   o  6 (IPv6): The 6-bit is set if the A-bit is also set and IPv6 is
      running on the probed interface.  Otherwise, the 6-bit is clear.

4.  ICMP Message Processing

   When a node receives an ICMP Extended Echo Request message and any of
   the following conditions apply, the node MUST silently discard the
   incoming message:

   o  The node does not recognize ICMP Extended Echo Request messages.

   o  The node has not explicitly enabled ICMP Extended Echo
      functionality.

   o  The incoming ICMP Extend Echo Request carries a Source Address
      that is not explicitly authorized for the L-bit setting of the
      incoming ICMP Extended Echo Request.

   o  The incoming ICMP Extend Echo Request carries a Source Address
      that is not explicitly authorized for the incoming ICMP Extended
      Echo Request type (i.e., by ifName, by IfIndex, or by Address).

   o  The Source Address of the incoming message is not a unicast
      address.

   o  The Destination Address of the incoming message is a multicast
      address.

   Otherwise, when a node receives an ICMPv4 Extended Echo Request, it
   MUST format an ICMP Extended Echo Reply as follows:

   o  Don't Fragment (DF) flag is 1

   o  More Fragments flag is 0

   o  Fragment Offset is 0

   o  TTL is 255

   o  Protocol is ICMP

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   When a node receives an ICMPv6 Extended Echo Request, it MUST format
   an ICMPv6 Extended Echo Reply as follows:

   o  Hop Limit is 255

   o  Next Header is ICMPv6

   In either case, the responding node MUST do the following:

   o  Copy the Source Address from the Extended Echo Request message to
      the Destination Address of the Extended Echo Reply.

   o  Copy the Destination Address from the Extended Echo Request
      message to the Source Address of the Extended Echo Reply.

   o  Set the DiffServ codepoint to CS0 [RFC4594].

   o  Set the ICMP Type to Extended Echo Reply.

   o  Copy the Identifier from the Extended Echo Request message to the
      Extended Echo Reply.

   o  Copy the Sequence Number from the Extended Echo Request message to
      the Extended Echo Reply.

   o  Set the Code field as described in Section 4.1.

   o  Set the State field to 0.

   o  Clear the A-bit, the 4-bit, and the 6-bit.

   o  If (1) the Code Field is equal to (0) No Error, (2) the L-bit is
      set, and (3) the probed interface is active, set the A-bit.  Also,
      set the 4-bit and the 6-bit as appropriate.

   o  If the Code field is equal to (0) No Error and the L-bit is clear,
      then set the State field to reflect the state of the ARP table or
      Neighbor Cache entry that represents the probed interface.

   o  Set the Checksum appropriately.

   o  Forward the ICMP Extended Echo Reply to its destination.

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4.1.  Code Field Processing

   The Code field MUST be set to (1) Malformed Query if any of the
   following conditions apply:

   o  The ICMP Extended Echo Request does not include an ICMP Extension
      Structure.

   o  The ICMP Extension Structure does not include exactly one
      Interface Identification Object.

   o  The L-bit is clear and the Interface Identification Object
      identifies the probed interface by ifName or ifIndex.

   o  The query is otherwise malformed.

   The Code field MUST be set to (2) No Such Interface if the L-bit is
   set and the ICMP Extension Structure does not identify an interface
   that resides on the proxy node.

   The Code field MUST be set to (3) No Such Table Entry if the L-bit is
   clear and the address found in the Interface Identification Object
   does not appear in the IPv4 Address Resolution Protocol (ARP) table
   or the IPv6 Neighbor Cache.

   The Code field MUST be set to (4) Multiple Interfaces Satisfy Query
   if any of the following conditions apply:

   o  The L-bit is set and the ICMP Extension Structure identifies more
      than one interface that resides in the proxy node.

   o  The L-bit is clear and the address found in the Interface
      Identification Object maps to multiple IPv4 ARP or IPv6 Neighbor
      Cache entries.

   Otherwise, the Code field MUST be set to (0) No Error.

5.  Use Cases

   In the scenarios listed below, network operators can use PROBE to
   determine the status of a probed interface but cannot use PING for
   the same purpose.  In all scenarios, assume bidirectional
   connectivity between the probing and proxy interfaces.  However,
   bidirectional connectivity between the probing and probed interfaces
   is lacking.

   o  The probed interface is unnumbered.

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   o  The probing and probed interfaces are not directly connected to
      one another.  The probed interface has an IPv6 link-local address
      but does not have a more globally scoped address.

   o  The probing interface runs IPv4 only while the probed interface
      runs IPv6 only.

   o  The probing interface runs IPv6 only while the probed interface
      runs IPv4 only.

   o  For lack of a route, the probing node cannot reach the probed
      interface.

6.  Updates to RFC 4884

   Section 4.6 of [RFC4884] provides a list of extensible ICMP messages
   (i.e., messages that can carry the ICMP Extension Structure).  This
   document adds the ICMP Extended Echo Request message and the ICMP
   Extended Echo Reply message to that list.

7.  IANA Considerations

   IANA has performed the following actions:

   o  Added the following to the "ICMP Type Numbers" registry:

         42 Extended Echo Request

      Added the following to the "Type 42 - Extended Echo Request"
      subregistry:

         (0) No Error

   o  Added the following to the "ICMPv6 'type' Numbers" registry:

         160 Extended Echo Request

         As ICMPv6 distinguishes between informational and error
         messages, and this is an informational message, the value has
         been assigned from the range 128-255.

      Added the following to the "Type 160 - Extended Echo Request"
      subregistry:

         (0) No Error

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   o  Added the following to the "ICMP Type Numbers" registry:

         43 Extended Echo Reply

      Added the following to the "Type 43 - Extended Echo Reply"
      subregistry:

         (0) No Error
         (1) Malformed Query
         (2) No Such Interface
         (3) No Such Table Entry
         (4) Multiple Interfaces Satisfy Query

   o  Added the following to the "ICMPv6 'type' Numbers" registry:

         161 Extended Echo Reply

         As ICMPv6 distinguishes between informational and error
         messages, and this is an informational message, the value has
         been assigned from the range 128-255.

      Added the following to the "Type 161 - Extended Echo Reply"
      subregistry:

         (0) No Error
         (1) Malformed Query
         (2) No Such Interface
         (3) No Such Table Entry
         (4) Multiple Interfaces Satisfy Query

   o  Added the following to the "ICMP Extension Object Classes and
      Class Sub-types" registry:

         (3) Interface Identification Object

      Added the following C-types to the "Sub-types - Class 3 -
      Interface Identification Object" subregistry:

         (0) Reserved
         (1) Identifies Interface by Name
         (2) Identifies Interface by Index
         (3) Identifies Interface by Address

      C-Type values are assigned on a First Come First Serve (FCFS)
      basis with a range of 0-255.

   All codes mentioned above are assigned on an FCFS basis with a range
   of 0-255.

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8.  Security Considerations

   The following are legitimate uses of PROBE:

   o  to determine the operational status of an interface.

   o  to determine which protocols (e.g., IPv4 or IPv6) are active on an
      interface.

   However, malicious parties can use PROBE to obtain additional
   information.  For example, a malicious party can use PROBE to
   discover interface names.  Having discovered an interface name, the
   malicious party may be able to infer additional information.
   Additional information may include:

   o  interface bandwidth

   o  the type of device that supports the interface (e.g., vendor
      identity)

   o  the operating system version that the above-mentioned device
      executes

   Understanding this risk, network operators establish policies that
   restrict access to ICMP Extended Echo functionality.  In order to
   enforce these policies, nodes that support ICMP Extended Echo
   functionality MUST support the following configuration options:

   o  Enable/disable ICMP Extended Echo functionality.  By default, ICMP
      Extend Echo functionality is disabled.

   o  Define enabled L-bit settings.  By default, the option to set the
      L-bit is enabled and the option to clear the L-bit is disabled.

   o  Define enabled query types (i.e., by name, by index, or by
      address); by default, all query types are disabled.

   o  For each enabled query type, define the prefixes from which ICMP
      Extended Echo Request messages are permitted.

   o  For each interface, determine whether ICMP Echo Request messages
      are accepted.

   When a node receives an ICMP Extended Echo Request message that it is
   not configured to support, it MUST silently discard the message.  See
   Section 4 for details.

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   PROBE must not leak information about one Virtual Private Network
   (VPN) into another.  Therefore, when a node receives an ICMP Extended
   Echo Request and the proxy interface is in a different VPN than the
   probed interface, the node MUST return an ICMP Extended Echo Reply
   with error code equal to (2) No Such Interface.

   In order to protect local resources, implementations SHOULD rate-
   limit incoming ICMP Extended Echo Request messages.

9.  References

9.1.  Normative References

   [RFC792]   Postel, J., "Internet Control Message Protocol", STD 5,
              RFC 792, DOI 10.17487/RFC0792, September 1981,
              <https://www.rfc-editor.org/info/rfc792>.

   [RFC826]   Plummer, D., "Ethernet Address Resolution Protocol: Or
              Converting Network Protocol Addresses to 48.bit Ethernet
              Address for Transmission on Ethernet Hardware", STD 37,
              RFC 826, DOI 10.17487/RFC0826, November 1982,
              <https://www.rfc-editor.org/info/rfc826>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", STD 89,
              RFC 4443, DOI 10.17487/RFC4443, March 2006,
              <https://www.rfc-editor.org/info/rfc4443>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,
              <https://www.rfc-editor.org/info/rfc4861>.

   [RFC4884]  Bonica, R., Gan, D., Tappan, D., and C. Pignataro,
              "Extended ICMP to Support Multi-Part Messages", RFC 4884,
              DOI 10.17487/RFC4884, April 2007,
              <https://www.rfc-editor.org/info/rfc4884>.

   [RFC7223]  Bjorklund, M., "A YANG Data Model for Interface
              Management", RFC 7223, DOI 10.17487/RFC7223, May 2014,
              <https://www.rfc-editor.org/info/rfc7223>.

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   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

   [RFC2151]  Kessler, G. and S. Shepard, "A Primer On Internet and TCP/
              IP Tools and Utilities", FYI 30, RFC 2151,
              DOI 10.17487/RFC2151, June 1997,
              <https://www.rfc-editor.org/info/rfc2151>.

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,
              <https://www.rfc-editor.org/info/rfc4594>.

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Appendix A.  The PROBE Application

   The PROBE application accepts input parameters, sets a counter, and
   enters a loop to be exited when the counter is equal to 0.  On each
   iteration of the loop, PROBE emits an ICMP Extended Echo Request,
   decrements the counter, sets a timer, and waits.  The ICMP Extended
   Echo Request includes an Identifier and a Sequence Number.

   If an ICMP Extended Echo Reply carrying the same Identifier and
   Sequence Number arrives, PROBE relays information returned by that
   message to its user.  However, on each iteration of the loop, PROBE
   waits for the timer to expire regardless of whether an Extended Echo
   Reply message arrives.

   PROBE accepts the following parameters:

   o  Count

   o  Wait

   o  Probing Interface Address

   o  Hop Count

   o  Proxy Interface Address

   o  Local

   o  Probed Interface Identifier

   Count is a positive integer whose default value is 3.  Count
   determines the number of times that PROBE iterates through the above-
   mentioned loop.

   Wait is a positive integer whose minimum and default values are 1.
   Wait determines the duration of the above-mentioned timer, measured
   in seconds.

   Probing Interface Address specifies the Source Address of the ICMP
   Extended Echo Request.  The Probing Interface Address MUST be a
   unicast address and MUST identify an interface that resides on the
   probing node.

   The Proxy Interface Address identifies the interface to which the
   ICMP Extended Echo Request message is sent.  It must be an IPv4 or
   IPv6 unicast address.  If it is an IPv4 address, PROBE emits an
   ICMPv4 message.  If it is an IPv6 address, PROBE emits an ICMPv6
   message.

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   Local is a boolean value.  It is TRUE if the proxy and probed
   interfaces both reside on the same node.  Otherwise, it is FALSE.

   The Probed Interface Identifier identifies the probed interface.  It
   is one of the following:

   o  an interface name;

   o  an address from any address family (e.g., IPv4, IPv6, IEEE 802,
      48-bit MAC, or 64-bit MAC); or

   o  an if-index.

   If the Probed Interface Identifier is an address, it does not need to
   be of the same address family as the proxy interface address.  For
   example, PROBE accepts an IPv4 Proxy Interface Address and an IPv6
   Probed Interface Identifier.

Acknowledgments

   Thanks to Sowmini Varadhan, Jeff Haas, Carlos Pignataro, Jonathan
   Looney, Dave Thaler, Mikio Hara, Joel Halpern, Yaron Sheffer, Stefan
   Winter, Jean-Michel Combes, Amanda Barber, and Joe Touch for their
   thoughtful review of this document.

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Authors' Addresses

   Ron Bonica
   Juniper Networks
   2251 Corporate Park Drive
   Herndon, Virginia  20171
   United States of America

   Email: rbonica@juniper.net

   Reji Thomas
   Juniper Networks
   Elnath-Exora Business Park Survey
   Bangalore, Karnataka  560103
   India

   Email: rejithomas@juniper.net

   Jen Linkova
   Google
   1600 Amphitheatre Parkway
   Mountain View, California  94043
   United States of America

   Email: furry@google.com

   Chris Lenart
   Verizon
   22001 Loudoun County Parkway
   Ashburn, Virginia  20147
   United States of America

   Email: chris.lenart@verizon.com

   Mohamed Boucadair
   Orange
   Rennes 35000
   France

   Email: mohamed.boucadair@orange.com

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