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ICN Research Group                                             H. Asaeda
Internet-Draft                                                   X. Shao
Intended status: Experimental                                       NICT
Expires: May 1, 2018                                         T. Turletti
                                                                   Inria
                                                        October 28, 2017


       Contrace: Traceroute Facility for Content-Centric Network
                     draft-asaeda-icnrg-contrace-04

Abstract

   This document describes the traceroute facility for Content-Centric
   Network (CCN), named "Contrace".  Contrace investigates: 1) the
   routing path information per name prefix, device name, and function/
   application, 2) the Round-Trip Time (RTT) between content forwarder
   and consumer, and 3) the states of in-network cache per name prefix.
   In addition, it discovers a gateway that supports different protocols
   such as CCN and NDN.

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
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   Drafts is at https://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 May 1, 2018.

Copyright Notice

   Copyright (c) 2017 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



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   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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Contrace Message Formats  . . . . . . . . . . . . . . . . . .   7
     3.1.  Request Message . . . . . . . . . . . . . . . . . . . . .   8
       3.1.1.  Request Block . . . . . . . . . . . . . . . . . . . .  10
       3.1.2.  Report Block  . . . . . . . . . . . . . . . . . . . .  13
     3.2.  Reply Message . . . . . . . . . . . . . . . . . . . . . .  14
       3.2.1.  Reply Block . . . . . . . . . . . . . . . . . . . . .  16
         3.2.1.1.  Reply Sub-Block . . . . . . . . . . . . . . . . .  16
   4.  Contrace User Behavior  . . . . . . . . . . . . . . . . . . .  19
     4.1.  Sending Contrace Request  . . . . . . . . . . . . . . . .  19
       4.1.1.  Gateway Discovery . . . . . . . . . . . . . . . . . .  19
       4.1.2.  Routing Path Information  . . . . . . . . . . . . . .  20
       4.1.3.  In-Network Cache Information  . . . . . . . . . . . .  20
     4.2.  Receiving Contrace Reply  . . . . . . . . . . . . . . . .  20
   5.  Router Behavior . . . . . . . . . . . . . . . . . . . . . . .  21
     5.1.  Receiving Contrace Request  . . . . . . . . . . . . . . .  21
       5.1.1.  Request Packet Verification . . . . . . . . . . . . .  21
       5.1.2.  Request Normal Processing . . . . . . . . . . . . . .  21
     5.2.  Forwarding Contrace Request . . . . . . . . . . . . . . .  22
     5.3.  Sending Contrace Reply  . . . . . . . . . . . . . . . . .  23
     5.4.  Forwarding Contrace Reply . . . . . . . . . . . . . . . .  24
   6.  Publisher Behavior  . . . . . . . . . . . . . . . . . . . . .  24
   7.  Contrace Termination  . . . . . . . . . . . . . . . . . . . .  25
     7.1.  Arriving at Publisher or Gateway  . . . . . . . . . . . .  25
     7.2.  Arriving at Router Having Cache . . . . . . . . . . . . .  25
     7.3.  No Route  . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.4.  No Information  . . . . . . . . . . . . . . . . . . . . .  25
     7.5.  No Space  . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.6.  Fatal Error . . . . . . . . . . . . . . . . . . . . . . .  25
     7.7.  Contrace Reply Timeout  . . . . . . . . . . . . . . . . .  26
     7.8.  Non-Supported Node  . . . . . . . . . . . . . . . . . . .  26
     7.9.  Administratively Prohibited . . . . . . . . . . . . . . .  26
   8.  Configurations  . . . . . . . . . . . . . . . . . . . . . . .  26
     8.1.  Contrace Reply Timeout  . . . . . . . . . . . . . . . . .  26
     8.2.  HopLimit in Fixed Header  . . . . . . . . . . . . . . . .  26
     8.3.  Access Control  . . . . . . . . . . . . . . . . . . . . .  26
   9.  Diagnosis and Analysis  . . . . . . . . . . . . . . . . . . .  27
     9.1.  Number of Hops  . . . . . . . . . . . . . . . . . . . . .  27
     9.2.  Caching Router and Gateway Identification . . . . . . . .  27



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     9.3.  TTL or Hop Limit  . . . . . . . . . . . . . . . . . . . .  27
     9.4.  Time Delay  . . . . . . . . . . . . . . . . . . . . . . .  27
     9.5.  Path Stretch  . . . . . . . . . . . . . . . . . . . . . .  27
     9.6.  Cache Hit Probability . . . . . . . . . . . . . . . . . .  27
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  28
     10.1.  Policy-Based Information Provisioning for Request  . . .  28
     10.2.  Filtering of Contrace Users Located in Invalid Networks   28
     10.3.  Topology Discovery . . . . . . . . . . . . . . . . . . .  29
     10.4.  Characteristics of Content . . . . . . . . . . . . . . .  29
     10.5.  Longer or Shorter Contrace Reply Timeout . . . . . . . .  29
     10.6.  Limiting Request Rates . . . . . . . . . . . . . . . . .  29
     10.7.  Limiting Reply Rates . . . . . . . . . . . . . . . . . .  29
     10.8.  Adjacency Verification . . . . . . . . . . . . . . . . .  30
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  30
     12.2.  Informative References . . . . . . . . . . . . . . . . .  30
   Appendix A.  Contrace Command and Options . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

   In Content-Centric Network (CCN) or Named-Data Network (NDN),
   publishers provide content through the network, and receivers
   retrieve content by name.  In this network architecture, routers
   forward content requests by means of their Forwarding Information
   Bases (FIBs), which are populated by name-based routing protocols.
   CCN/NDN also enables receivers to retrieve content from an in-network
   cache.

   In CCN/NDN, while consumers do not generally need to know which
   content forwarder is transmitting the content to them, operators and
   developers may want to identify the content forwarder and observe the
   routing path information per name prefix for troubleshooting or
   investigating the network conditions.

   Traceroute [5] is a useful tool for analyzing the routing conditions
   in IP networks as it provides intermediate router addresses along the
   path between source and destination and the Round-Trip Time (RTT) for
   the path.  However, this IP-based network tool cannot trace the name
   prefix paths used in CCN/NDN.  Moreover, given a source-rooted
   routing path per name prefix, specifying a forwarding source (i.e.,
   router or publisher) for any content is difficult, because we do not
   always know which branch of the source tree the consumer is on.
   Additionally, it is not feasible to flood the entire source-rooted
   tree to find the path from a source to a consumer.  Furthermore, such
   IP-based network tool does not allow the states of the in-network
   cache to be discovered.



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   This document describes the specification of "Contrace", an active
   network measurement tool for investigating the path and caching
   condition in CCN.  Contrace potentially discovers devices and
   functions/applications in CCN.  Contrace is designed based on the
   work originally published in [4].

   Contrace consists of the Contrace user command and the Contrace
   forwarding function implementation on a content forwarder (e.g.,
   router).  The Contrace user (e.g., consumer) invokes the contrace
   command (described in Appendix A) with the name prefix of the
   content, the device name, or the function (or application) name.  The
   Contrace command initiates the Contrace "Request" message (described
   in Section 3.1).  The Request message, for example, obtains routing
   path and cache information.  When an appropriate adjacent neighbor
   router receives the Request message, it retrieves cache information.
   If the router is not the content forwarder for the request, it
   inserts its "Report" block (described in Section 3.1.2) into the
   Request message and forwards the Request message to its upstream
   neighbor router(s) decided by its FIB.  These two message types,
   Contrace Request and Reply messages, are encoded in the CCNx TLV
   format [1].

   In this way, the Contrace Request message is forwarded by routers
   toward the content publisher, and the Contrace Report record is
   inserted by each intermediate router.  When the Request message
   reaches the content forwarder (i.e., a router or the publisher who
   has the specified cache or content), the content forwarder forms the
   Contrace "Reply" message (described in Section 3.2) and sends it to
   the downstream neighbor router.  The Reply message is forwarded back
   toward the Contrace user in a hop-by-hop manner.  This request-reply
   message flow, walking up the tree from a consumer toward a publisher,
   is inspired by the design of the IP multicast traceroute facility
   [6].

   Contrace supports multipath forwarding.  The Request messages can be
   forwarded to multiple neighbor routers.  When the Request messages
   forwarded to multiple routers, the different Reply messages will be
   forwarded from different routers or publisher.  To support this case,
   PIT entries initiated by Contrace remain until the defined timeout
   value is expired.











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           1. Request    2. Request    3. Request    4. Request
              (+U)          (U+A)         (U+A+B)       (U+A+B+C)
             +----+        +----+        +----+        +----+
             |    |        |    |        |    |        |    |
             |    v        |    v        |    v        |    v
    +--------+    +--------+    +--------+    +--------+    +---------+
    |Contrace|----| Router |----| Router |----| Router |----|Publisher|
    |  user  |    |   A    |    |   B    |    |   C    |    |         |
    +--------+    +--------+    +--------+    +--------+    +---------+
                                         \
                                          \          +-------+
                                3. Request \         | Cache |
                                   (U+A+B)  \ +---------+    |
                                             v| Caching |----+
                                              |  router |
                                              +---------+

       Figure 1: Request messages forwarded by consumer and routers.
      Contrace user and routers (i.e., Router A,B,C) insert their own
   Report blocks into the Request message and forward the message toward
        the content forwarder (i.e., caching router and publisher)

           3. Reply(C)   2. Reply(C)
           4. Reply(P)   3. Reply(P)   2. Reply(P)   1. Reply(P)
             +----+        +----+        +----+        +----+
             |    |        |    |        |    |        |    |
             v    |        v    |        v    |        v    |
    +--------+    +--------+    +--------+    +--------+    +---------+
    |Contrace|----| Router |----| Router |----| Router |----|Publisher|
    |  user  |    |   A    |    |   B    |    |   C    |    |         |
    +--------+    +--------+    +--------+    +--------+    +---------+
                                         ^
                                          \          +-------+
                               1. Reply(C) \         | Cache |
                                            \ +---------+    |
                                             \| Caching |----+
                                              |  router |
                                              +---------+

    Figure 2: Reply messages forwarded by publisher and routers.  Each
     router forwards the Reply message, and finally the Contrace user
     receives two Reply messages: one from the publisher and the other
                         from the caching router.

   Contrace facilitates the tracing of a routing path and provides: 1)
   the RTT between content forwarder (i.e., caching router or publisher)
   and consumer, 2) the states of in-network cache per name prefix, and
   3) the routing path information per name prefix.



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   In addition, Contrace identifies the states of the cache, such as the
   following metrics for Content Store (CS) in the content forwarder: 1)
   size of the cached content, 2) number of the cached chunks of the
   content, 3) number of the accesses (i.e., received Interests) per
   cache or chunk, and 4) lifetime and expiration time per cache or
   chunk.  The number of received Interests per cache or chunk on the
   routers indicates the popularity of the content.

   Furthermore, Contrace implements policy-based information
   provisioning that enables administrators to "hide" secure or private
   information, but does not disrupt the forwarding of messages.  This
   policy-based information provisioning reduces the deployment barrier
   faced by operators in installing and running Contrace on their
   routers.

2.  Terminology

   In this document, the key words "MUST", "MUST NOT", "REQUIRED",
   "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
   and "OPTIONAL" are to be interpreted as described in RFC 2119 [2],
   and indicate requirement levels for compliant Contrace
   implementations.

2.1.  Definitions

   Since Contrace requests flow in the opposite direction to the data
   flow, we refer to "upstream" and "downstream" with respect to data,
   unless explicitly specified.

   Router
      It is a router facilitating name-based content/device/function
      name or characteristic retrieval in the path between consumer and
      publisher.

   Scheme name
      It indicates a URI and protocol such as "ccnx:/" and "ndn:/".
      This document considers the protocol for name-based
      content/device/function name or characteristic retrieval.

   Gateway
      It is a router supporting multiple scheme names in the path
      between consumer and publisher.  The router has multiple FIBs for
      different protocols and establishes the connections with different
      neighbor routers for each protocol.

   Node
      It is a router, gateway, publisher, or consumer.




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   Content forwarder
      It is either a caching router or a publisher that holds the cache
      (or content) and forwards it to consumers.

   Contrace user
      It is a node that invokes the contrace command and initiates the
      Contrace Request.

   Incoming face
      The face on which data is expected to arrive from the specified
      name prefix.

   Outgoing face
      The face to which data from the publisher or router is expected to
      transmit for the specified name prefix.  It is also the face on
      which the Contrace Request messages are received.

3.  Contrace Message Formats

   Contrace uses two message types: Request and Reply.  Both messages
   are encoded in the CCNx TLV format ([1], Figure 3).  The Request
   message consists of a fixed header, Request block TLV Figure 7, and
   Report block TLV(s) Figure 11.  The Reply message consists of a fixed
   header, Request block TLV, Report block TLV(s), and Reply block/sub-
   block TLV(s) Figure 14.

                          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    |  PacketType   |         PacketLength          |
     +---------------+---------------+---------------+---------------+
     |           PacketType specific fields          | HeaderLength  |
     +---------------+---------------+---------------+---------------+
     / Optional Hop-by-hop header TLVs                               /
     +---------------+---------------+---------------+---------------+
     / PacketPayload TLVs                                            /
     +---------------+---------------+---------------+---------------+
     / Optional CCNx ValidationAlgorithm TLV                         /
     +---------------+---------------+---------------+---------------+
     / Optional CCNx ValidationPayload TLV (ValidationAlg required)  /
     +---------------+---------------+---------------+---------------+

                        Figure 3: Packet format [1]

   The Request and Reply Type values in the fixed header are PT_REQUEST
   and PT_REPLY, respectively (Figure 4).  These messages are forwarded
   in a hop-by-hop manner.  When the Request message reaches the content
   forwarder, the content forwarder turns the Request message into a



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   Reply message by changing the Type field value in the fixed header
   from PT_REQUEST to PT_REPLY and forwards back to the node that has
   initiated the Request message.

                      Code         Type name
                    ========      =====================
                        1         PT_INTEREST [1]
                        2         PT_CONTENT [1]
                        3         PT_RETURN [1]
                        4         PT_REQUEST
                        5         PT_REPLY

                      Figure 4: Packet Type Namespace

   Each Contrace message MUST begin with a fixed header with either a
   Request or Reply type value to specify whether it is a Request
   message or Reply message.  Following a fixed header, there can be a
   sequence of optional hop-by-hop header TLV(s) for a Request message.
   In the case of a Request message, it is followed by a sequence of
   Report blocks, each from a router on the path toward the publisher or
   caching router.

   At the beginning of PacketPayload TLVs, one top-level TLV type,
   T_TRACE (Figure 5), exists at the outermost level of a CCNx protocol
   message.  This TLV indicates that the Name segment TLV(s) and Reply
   block TLV(s) would follow in the Request or Reply message.

                    Code         Type name
                  ========      =========================
                      1         T_INTEREST [1]
                      2         T_OBJECT [1]
                      3         T_VALIDATION_ALG [1]
                      4         T_VALIDATION_PAYLOAD [1]
                      5         T_PING
                      6         T_TRACE

                    Figure 5: Top-Level Type Namespace

3.1.  Request Message

   When a Contrace user initiates a trace request (e.g., by contrace
   command described in Appendix A), a Contrace Request message is
   created and forwarded to its upstream router through the Incoming
   face(s) determined by its FIB.

   The Contrace Request message format is as shown in Figure 6.  It
   consists of a fixed header, Request block TLV (Figure 7), Report
   block TLV(s) (Figure 11), and Name TLV.  The Type value of Top-Level



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   type namespace is T_TRACE (Figure 5).  The Type value for the Report
   message is PT_REQUEST.

                          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    |  PacketType   |         PacketLength          |
     +---------------+---------------+---------------+---------------+
     |    HopLimit   |   ReturnCode  |Reserved (MBZ) | HeaderLength  |
     +===============+===============+===============+===============+
     |                                                               |
     +                       Request block TLV                       +
     |                                                               |
     +---------------+---------------+---------------+---------------+
     /                       Report block TLV 1                      /
     +---------------+---------------+---------------+---------------+
     /                       Report block TLV 2                      /
     +---------------+---------------+---------------+---------------+
     /                               .                               /
     /                               .                               /
     +---------------+---------------+---------------+---------------+
     /                       Report block TLV n                      /
     +===============+===============+===============+===============+
     |            T_TRACE            |         MessageLength         |
     +---------------+---------------+---------------+---------------+
     |            T_NAME             |             Length            |
     +---------------+---------------+---------------+---------------+
     / Name segment TLVs (name prefix specified by contrace command) /
     +---------------+---------------+---------------+---------------+

    Figure 6: Request message consists of a fixed header, Request block
                  TLV, Report block TLV(s), and Name TLV

   HopLimit: 8 bits

      HopLimit is a counter that is decremented with each hop.  It
      limits the distance a Request may travel on the network.

   ReturnCode: 8 bits

      ReturnCode is used for the Reply message.  This value is replaced
      by the content forwarder when the Request message is returned as
      the Reply message (see Section 3.2).  Until then, this field MUST
      be transmitted as zeros and ignored on receipt.







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  Value  Name             Description
  -----  ---------------  ----------------------------------------------
  0x00   NO_ERROR         No error
  0x01   WRONG_IF         Contrace Request arrived on an interface
                          to which this router would not forward for
                          the specified name/function toward the
                          publisher.
  0x02   INVALID_REQUEST  Invalid Contrace Request is received.
  0x03   NO_ROUTE         This router has no route for the name prefix
                          and no way to determine a potential route.
  0x04   NO_INFO          This router has no cache information for the
                          specified name prefix, device information, or
                          function.
  0x05   NO_SPACE         There was not enough room to insert another
                          Report block in the packet.
  0x06   NO_GATAWAY       Contrace Request arrived on a non-gateway
                          router.
  0x07   INFO_HIDDEN      Information is hidden from this trace because
                          of some policy.
  0x0E   ADMIN_PROHIB     Contrace Request is administratively
                          prohibited.
  0x0F   UNKNOWN_REQUEST  This router does not support/recognize the
                          Request message.
  0x80   FATAL_ERROR      A fatal error is one where the router may
                          know the upstream router but cannot forward
                          the message to it.

   Reserved (MBZ): 8 bits

      The reserved fields in the Value field MUST be transmitted as
      zeros and ignored on receipt.

3.1.1.  Request Block

   When a Contrace user transmits the Request message, it MUST insert
   the Request block TLV (Figure 7) and the Report block TLV (Figure 11)
   of its own to the Request message before sending it through the
   Incoming face(s).













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                          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
     +---------------+---------------+---------------+---------------+
     |          T_TRACE_REQ          |             Length            |
     +---------------+---------------+-----+-------------------------+
     |  SchemeName   | SkipHopCount  |TimeO|      Reserved (MBZ)     |
     +---------------+---------------+-----+-------------------------+
     |           Request ID          |             Flags             |
     +---------------+---------------+-------------------------------+

              Figure 7: Request block TLV (hop-by-hop header)

                      Code           Type name
                  =============     =====================
                        1           T_INTLIFE [1]
                        2           T_CACHETIME [1]
                        3           T_MSGHASH [1]
                      4 - 7         Reserved [1]
                        8           T_TRACE_REQ
                        9           T_TRACE_REPORT
                     %x0FFE         T_PAD [1]
                     %x0FFF         T_ORG [1]
                  %x1000-%x1FFF     Reserved [1]

                    Figure 8: Hop-by-Hop Type Namespace

   Type: 16 bits

      Format of the Value field.  For the single Request block TLV, the
      type value MUST be T_TRACE_REQ.  For all the available types for
      hop-by-hop type namespace, please see Figure 8.

   Length: 16 bits

      Length of Value field in octets.  For the Request block, it MUST
      be 4 in the current specification.

   SchemeName: 8 bits

      Currently, the following scheme names are defined.











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                         Code            Scheme name
                     =============     ===============
                           0               ccnx:/
                           1               ndn:/
                           2               cefore:/
                       %x03-%FF         Not assigned

                          Figure 9: Scheme Names

   SkipHopCount: 8 bits

      Number of skipped routers.  This value MUST be lower than the
      value of HopLimit at the fixed header.

   TimeO: 3 bits

      Timeout value (seconds).  This Timeout value means a [Contrace
      Reply Timeout] value (seconds) requested by the Contrace user
      later described in Section 8.1.  A Contrace user requests routers
      along the path to keep the PIT entry for the Request until this
      timeout value expires.  Note that, because of some security
      concern (Section 10.5), a router along the path may configure the
      shorter timeout value than this requested timeout value.  In that
      case, the Request may be timed out and the Contrace user may not
      receive the Reply as expected.

   Request ID: 16 bits

      This field is used as a unique identifier for this Contrace
      Request so that duplicate or delayed Reply messages can be
      detected.

   Flags: 16 bits

      The trace conditions specified as the contrace command options
      (described in Appendix A) are transferred in the Flags field.  The
      trace conditions depend on the specified name (i.e., name_prefix,
      device_name, or function_name) as shown in Figure 10.  Note that
      code %x01 and %x02 are exclusive options; that is, only one of
      them should be turned on at once.











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       Code        Type name
   ============    =====================================================
       %x01        Cache retrieval allowing partial match (name_prefix)
       %x02        No cache information required (name_prefix)
       %x04        Publisher reachability (name_prefix and device_name)
       %x08        Force trace. Request to multiple upstream routers
                   simultaneously (name_prefix, device_name, and
                   function_name)
       %x16        Discovery of gateway supporting specified scheme
                   name (name_prefix, device_name, and function_name)
       %x32        Function's or application's version number retrieval
                   (function_name)
   %x64-%x32768    Not assigned

            Figure 10: Codes and types specified in Flags field

3.1.2.  Report Block

   A Contrace user and each upstream router along the path would insert
   its own Report block TLV without changing the Type field of the fixed
   header of the Request message until one of these routers is ready to
   send a Reply.  In the Report block TLV (Figure 11), the Request
   Arrival Time and the Node Identifier MUST be inserted.

                          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
     +---------------+---------------+---------------+---------------+
     |         T_TRACE_REPORT        |             Length            |
     +---------------+---------------+---------------+---------------+
     |                     Request Arrival Time                      |
     +---------------+---------------+---------------+---------------+
     /                        Node Identifier                        /
     +---------------+---------------+---------------+---------------+

              Figure 11: Report block TLV (hop-by-hop header)

   Type: 16 bits

      Format of the Value field.  For the Request block TLV(s), the type
      value(s) MUST be T_TRACE_REPORT.

   Length: 16 bits

      Length of Value field in octets.

   Request Arrival Time: 32 bits





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      The Request Arrival Time is a 32-bit NTP timestamp specifying the
      arrival time of the Contrace Request packet at this router.  The
      32-bit form of an NTP timestamp consists of the middle 32 bits of
      the full 64-bit form; that is, the low 16 bits of the integer part
      and the high 16 bits of the fractional part.

      The following formula converts from a UNIX timeval to a 32-bit NTP
      timestamp:

         request_arrival_time
         = ((tv.tv_sec + 32384) << 16) + ((tv.tv_nsec << 7) / 1953125)

      The constant 32384 is the number of seconds from Jan 1, 1900 to
      Jan 1, 1970 truncated to 16 bits.  ((tv.tv_nsec << 7) / 1953125)
      is a reduction of ((tv.tv_nsec / 1000000000) << 16).

      Note that Contrace does not require all the routers on the path to
      have synchronized clocks in order to measure one-way latency.

   Node Identifier: variable length

      This field specifies the Contrace user or the router identifier
      (e.g., IPv4 address) of the Incoming face on which packets from
      the publisher are expected to arrive, or all-zeros if unknown or
      unnumbered.  Since we may not always rely on the IP addressing
      architecture, it would be necessary to define the identifier
      uniqueness (e.g., by specifying the protocol family) for this
      field.  However, defining such uniqueness is out of scope of this
      document.  Potentially, this field may be defined as a new TLV,
      which might be defined in the document for the CCNx TLV format[1].

3.2.  Reply Message

   When a content forwarder receives a Contrace Request message from the
   appropriate adjacent neighbor router, it would insert a Reply block
   TLV and Reply sub-block TLV(s) of its own to the Request message and
   turn the Request into the Reply by changing the Type field of the
   fixed header of the Request message from PT_REQUEST to PT_REPLY.  The
   Reply message (see Figure 12) would then be forwarded back toward the
   Contrace user in a hop-by-hop manner.











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                          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    |  PacketType   |         PacketLength          |
     +---------------+---------------+---------------+---------------+
     |    HopLimit   |   ReturnCode  |Reserved (MBZ) | HeaderLength  |
     +===============+===============+===============+===============+
     |                                                               |
     +                       Request block TLV                       +
     |                                                               |
     +---------------+---------------+---------------+---------------+
     /                               .                               /
     /                               .                               /
     /                      n Report block TLVs                      /
     /                               .                               /
     /                               .                               /
     +===============+===============+===============+===============+
     |            T_TRACE            |         MessageLength         |
     +---------------+---------------+---------------+---------------+
     |            T_NAME             |             Length            |
     +---------------+---------------+---------------+---------------+
     / Name segment TLVs (name prefix specified by contrace command) /
     +---------------+---------------+---------------+---------------+
     /                        Reply block TLV                        /
     +---------------+---------------+---------------+---------------+
     /                     Reply sub-block TLV 1                     /
     +---------------+---------------+---------------+---------------+
     /                     Reply sub-block TLV 2                     /
     +---------------+---------------+---------------+---------------+
     /                               .                               /
     /                               .                               /
     +---------------+---------------+---------------+---------------+
     /                     Reply sub-block TLV k                     /
     +---------------+---------------+---------------+---------------+

    Figure 12: Reply message consists of a fixed header, Request block
   TLV, Report block TLV(s), Name TLV, and Reply block/sub-block TLV(s)














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                      Code           Type name
                  =============     =====================
                        0           T_NAME [1]
                        1           T_PAYLOAD [1]
                        2           T_KEYIDRESTR [1]
                        3           T_OBJHASHRESTR [1]
                        5           T_PAYLDTYPE [1]
                        6           T_EXPIRY [1]
                        8           T_TRACE_REPLY
                      9 - 12        Reserved [1]
                      %x0FFE        T_PAD [1]
                      %x0FFF        T_ORG [1]
                  %x1000-%x1FFF     Reserved [1]

                  Figure 13: CCNx Message Type Namespace

3.2.1.  Reply Block

   The Reply block TLV is an envelope for Reply sub-block TLV(s)
   (explained in Section 3.2.1.1).

                          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
     +---------------+---------------+---------------+---------------+
     |         T_TRACE_REPLY         |             Length            |
     +---------------+---------------+---------------+---------------+

                Figure 14: Reply block TLV (packet payload)

   Type: 16 bits

      Format of the Value field.  For the Report block TLV, the type
      value MUST be T_TRACE_REPLY.

   Length: 16 bits

      Length of Value field in octets.  This length is a total length of
      Reply sub-block(s).

3.2.1.1.  Reply Sub-Block

   In addition to the Reply block, a router on the traced path will add
   one or multiple Reply sub-blocks followed by the Reply block before
   sending the Reply to its neighbor router.

   The Reply sub-block is flexible for various purposes.  For instance,
   operators and developers may want to obtain various characteristics
   of content such as content's ownership and copyright, or other cache



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   states and conditions.  Various information about device or function
   (or application) may be also retrieved by the variety of Reply sub-
   blocks.  In this document, Reply sub-block TLVs for T_TRACE_CONTENT
   and T_TRACE_CONTENT_OWNER (Figure 15) and for T_TRACE_GATEWAY
   (Figure 16) are defined; other Reply sub-block TLVs will be defined
   in separate document(s).

                          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            |
     +---------------+---------------+---------------+---------------+
     |                         Content Size                          |
     +---------------+---------------+---------------+---------------+
     |                         Object Count                          |
     +---------------+---------------+---------------+---------------+
     |                      # Received Interest                      |
     +---------------+---------------+---------------+---------------+
     |                         First Seqnum                          |
     +---------------+---------------+---------------+---------------+
     |                          Last Seqnum                          |
     +---------------+---------------+---------------+---------------+
     |                        Cache Lifetime                         |
     +---------------+---------------+---------------+---------------+
     |                     Remain Cache Lifetime                     |
     +---------------+---------------+---------------+---------------+
     |            T_NAME             |             Length            |
     +---------------+---------------+---------------+---------------+
     /   Name segment TLVs (name prefix partially/exactly matched)   /
     +---------------+---------------+---------------+---------------+

          Figure 15: Reply sub-block TLV for T_TRACE_CONTENT and
                  T_TRACE_CONTENT_OWNER (packet payload)

                          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            |
     +---------------+---------------+---------------+---------------+
     |          Scheme Name          |         Reserved (MBZ)        |
     +---------------+---------------+---------------+---------------+

    Figure 16: Reply sub-block TLV for T_TRACE_GATEWAY (packet payload)








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                   Code           Type name
               =============     ===========================
                     0           T_TRACE_CONTENT
                     1           T_TRACE_CONTENT_OWNER
                     2           T_TRACE_GATEWAY
                     3           T_TRACE_DEVICE
                     4           T_TRACE_FUNCTION
                  %x0FFF         T_ORG
               %x1000-%x1FFF     Reserved (Experimental Use)

                 Figure 17: Contrace Reply Type Namespace

   Type: 16 bits

      Format of the Value field.  For the Reply sub-block TLV, the type
      value MUST be one of the type value defined in the Contrace Reply
      Type Namespace (Figure 17).  T_TRACE_CONTENT is specified when the
      cache information is replied from a caching router.
      T_TRACE_CONTENT_OWNER is specified when the content information is
      replied from a publisher.  T_TRACE_GATEWAY is used to discover a
      gateway that has a FIB for the specified scheme name.

   Length: 16 bits

      Length of Value field in octets.

   Scheme Name: 8 bits

      The code of the scheme name defined in Figure 9.

   Content Size: 32 bits

      The total size (MB) of the (cached) content objects.  Note that
      the maximum size expressed by 32 bit field is 65 GB.

   Object Count: 32 bits

      The number of the (cached) content objects.

   # Received Interest: 32 bits

      The number of the received Interest messages to retrieve the
      content.

   First Seqnum: 32 bits

      The first sequential number of the (cached) content objects.




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   Last Seqnum: 32 bits

      The last sequential number of the (cached) content objects.  Above
      First Seqnum and this Last Seqnum do not guarantee the
      consecutiveness of the cached content objects.

   Cache Lifetime: 32 bits

      The elapsed time after the oldest content object in the cache is
      stored.  The Cache Lifetime is a 32-bit NTP timestamp, and the
      formula converts from a UNIX timeval to a 32-bit NTP timestamp is
      same as that of Section 3.1.2.

   Remain Cache Lifetime: 32 bits

      The lifetime of a content object, which is removed first among the
      cached content objects.  The Remain Cache Lifetime is a 32-bit NTP
      timestamp.

4.  Contrace User Behavior

4.1.  Sending Contrace Request

   A Contrace user initiates a Contrace Request by sending the Request
   message to the adjacent neighbor router(s) of interest.  As a typical
   example, a Contrace user invokes the contrace command (detailed in
   Appendix A) that forms a Request message and sends it to the user's
   adjacent neighbor router(s).

   When the Contrace user's program initiates a Request message, it MUST
   insert the necessary values, the "Request ID" (in the Request block)
   and the "Node Identifier" (in the Report block), in the Request and
   Report blocks.  Contrace user's program MUST also record the Request
   ID at the corresponding PIT entry.  The Request ID is a unique
   identifier for the Contrace Request.

   After the Contrace user's program sends the Request message, until
   the Reply times out, the Contrace user's program MUST keep the
   following information; Request ID and Flags specified in the Request
   block, Node Identifier and Request Arrival Time specified in the
   Report block, and HopLimit specified in the fixed header.

4.1.1.  Gateway Discovery

   A Contrace Request can be used for gateway discovery; if a Contrace
   user invokes a Contrace Request with a scheme name (e.g., ccnx:/ or
   ndn:/) and the "gateway discovery" flag value (i.e., "%x16" bit as
   seen in Figure 10), s/he could potentially discover a gateway that



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   supports different protocols such as CCN and NDN.  The Contrace
   Request for gateway discovery only indicates the routing path
   information (see Section 4.1.2) and the scheme name whether the
   router is a gateway or not; it does not provide other information,
   e.g., cache information.

4.1.2.  Routing Path Information

   A Contrace user can send a Contrace Request for investigating routing
   path information for the specified named content.  By the Request,
   the legitimate user can obtain; 1) identifiers (e.g., IP addresses)
   of intermediate routers, 2) identifier of content forwarder, 3)
   number of hops between content forwarder and consumer, and 4) RTT
   between content forwarder and consumer, per name prefix.  This
   Contrace Request is terminated when it reaches the content forwarder.
   The contrace command enables user to obtain both the routing path
   information and in-network cache information (see below) in a same
   time.

4.1.3.  In-Network Cache Information

   A Contrace user can send a Contrace Request for investigating in-
   network cache information.  By this Request, the legitimate user can
   obtain; 1) size of the cached content, 2) number of the cached chunks
   of the content, 3) number of the accesses (i.e., received Interests)
   per cache or chunk, and 4) lifetime and expiration time per cache or
   chunk, for Content Store (CS) in the content forwarder.  This
   Contrace Request is terminated when it reaches the content forwarder.

4.2.  Receiving Contrace Reply

   A Contrace user's program will receive one or multiple Contrace Reply
   messages from the adjacent neighbor router that has previously
   received and forwarded the Request message(s).  When the program
   receives the Reply, it MUST compare the kept Request ID and the
   Request ID noted in the Reply.  If they do not match, the Reply
   message SHOULD be silently discarded.

   If the number of the Report blocks in the received Reply is more than
   the initial HopLimit value (which was inserted in the original
   Request) + 1, the Reply SHOULD be silently ignored.

   After the Contrace user has determined that s/he has traced the whole
   path or as much as s/he can expect to, s/he might collect statistics
   by waiting a timeout.  Useful statistics provided by Contrace can be
   seen in Section 9.





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5.  Router Behavior

5.1.  Receiving Contrace Request

5.1.1.  Request Packet Verification

   Upon receiving a Contrace Request message, a router MUST examine
   whether the message comes from a valid adjacent neighbor node.  If it
   is invalid, the Request MUST be silently ignored.  The router next
   examines the value of the "HopLimit" in the fixed header and the
   value of the "SkipHopCount" in the Request block (Figure 7).  If
   SkipHopCount value is equal or more than the HopLimit value, the
   Request MUST be silently ignored.

5.1.2.  Request Normal Processing

   When a router receives a Contrace Request message, it performs the
   following steps.

   1.  HopLimit and SkipHopCount are counters that are decremented with
       each hop.  The router terminates the Contrace Request when the
       HopLimit value becomes zero.  Until the SkipHopCount value
       becomes zero, the router forwards the Contrace Request messages
       to the upstream router(s) (if it knows) without adding its own
       Report block and without replying the Request.  If the router
       does not know the upstream router(s), without depending on the
       SkipHopCount value, it replies the Contrace Reply message with
       NO_ROUTE return code.

   2.  The router examines the Flags field of the Request block of
       received Contrace Request.  If the flag value indicates "%x00" or
       "%x01" bit (as seen in Figure 10) for "cache information
       discovery", the router examines its FIB and CS.  If the router
       caches the specified content, it inserts own Report block to the
       message and sends the Reply message with own Reply block and sub-
       block.  If the router does not cache the specified content but
       knows the neighbor router(s) for the specified name prefix, it
       inserts own Report block and forwards the Request to the upstream
       neighbor(s).  If the router does not cache the specified content
       and does not know the upstream neighbor router(s) for the
       specified name prefix, it replies the Contrace Reply message with
       NO_ROUTE return code.

   3.  If the flag value indicates "%x02" bit for "routing path
       information discovery", the router examines its FIB and CS.  If
       the router caches the specified content, it inserts own Report
       block to the message and sends the Reply message with own Reply
       block.  The router does not insert any Reply sub-block here.  If



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       the router does not cache the specified content but knows the
       neighbor router(s) for the specified name prefix, it inserts own
       Report block and forwards the Request to the upstream
       neighbor(s).  If the router does not cache the specified content
       and does not know the upstream neighbor router(s) for the
       specified name prefix, it replies the Contrace Reply message with
       NO_ROUTE return code.

   4.  If the flag value indicates "%x04" bit for "publisher discovery",
       the node receiving the Request message examines whether it owns
       the requested content as the publisher.  If it is the publisher,
       it sends the Reply message with own Report block and sub-block.
       If the node is not the publisher but know the upstream neighbor
       router(s) for the specified name prefix, it adds the own Report
       block and forwards the Request to the neighbor(s).  If the node
       is not the publisher and does not know the upstream neighbor
       router(s) for the specified name prefix, it replies the Contrace
       Reply message with NO_ROUTE return code.

   5.  When a router receives a Contrace Request in which the "gateway
       discovery" flag (i.e., "%x16") is set in the Flags field and a
       scheme name is specified, the router examines whether it has the
       FIB for the specified scheme name and the connections with the
       neighbor router(s) for the scheme protocol.  If the router is the
       gateway, it sends the Reply message back toward the Contrace
       user.  If the router does not have the FIB for the specified
       scheme name or does not connect to any neighbor router for the
       specified scheme name, the router returns the Reply with
       NO_GATEWAY return code.

5.2.  Forwarding Contrace Request

   When a router decides to forward a Request message with its Report
   block to its upstream router(s), it specifies the Request Arrival
   Time and Node Identifier in the Report block of the Request message.
   The router then forwards the Request message upstream toward the
   publisher or caching router based on the FIB entry.

   When the router forwards the Request message, it MUST record the
   Request ID at the corresponding PIT entry.  The router can later
   decide the PIT entry to correctly forward back the Reply message even
   if it receives multiple Reply messages within the same timeout
   period.  (See below.)

   Contrace supports multipath forwarding.  The Request messages can be
   forwarded to multiple neighbor routers.  Some router may have
   strategy for multipath forwarding; when it sends Interest messages to
   multiple neighbor routers, it may delay or prioritize to send the



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   message to the upstream routers.  The Contrace Request, as the
   default, complies with such strategy; a Contrace user could trace the
   actual forwarding path based on the strategy.  On the other hand,
   there may be the case that a Contrace user wants to discover all
   potential forwarding paths based on routers' FIBs.  If a Contrace
   user invokes a Contrace Request with the force flag value (i.e.,
   "%x08" bit as seen in Figure 10), the forwarding strategy will be
   ignored and the router sends Requests to multiple upstream routers
   simultaneously, and the Contrace user could trace the all potential
   forwarding paths.

   When the Request messages forwarded to multiple routers, the
   different Reply messages will be forwarded from different routers or
   publisher.  To support this case, PIT entries initiated by Contrace
   remain until the configured Contrace Reply Timeout (Section 8.1)
   passes.  In other words, unlike the ordinary Interest-Data
   communications in CCN, the router SHOULD NOT remove the PIT entry
   created by the Contrace Request before the timeout value expires,
   even if the router receives the Contrace Reply.

   Contrace Requests SHOULD NOT result in PIT aggregation in routers
   during the Request message transmission.

5.3.  Sending Contrace Reply

   When a router decides to send a Reply message to its downstream
   neighbor router or the Contrace user with NO_ERROR return code, it
   inserts a Report block having the Request Arrival Time and Node
   Identifier to the hop-by-hop TLV header of the Request message.  And
   then the router inserts the corresponding Reply block and Reply sub-
   block to the payload.  The router does not insert any Reply block/
   sub-block if there is an error.  The router finally changes the Type
   field in the fixed header from PT_REQUEST to PT_REPLY and forwards
   the message back as the Reply toward the Contrace user in a hop-by-
   hop manner.

   When a router decides to send the Reply message for the Request for
   the cache or routing path information discovery, it forms the Reply
   message including a Reply block and a Reply sub-block with the
   T_TRACE_CONTENT type value (Figure 15) and various cache information.
   After the router puts the NO_ERROR return code in the fixed header,
   it sends the Reply back toward the Contrace user.

   When a router decides to send the Reply message for the Request for
   the publisher discovery, it forms the Reply message including a Reply
   block and a Reply sub-block with the T_TRACE_CONTENT_OWNER type value
   (Figure 15) and various cache information.  After the router puts the




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   NO_ERROR return code in the fixed header, it sends the Reply back
   toward the Contrace user.

   When a router decides to send the Reply message for the Request for
   the gateway discovery, it forms the Reply message including a Reply
   block and a Reply sub-block with the T_TRACE_GATEWAY type value
   (Figure 16) and the scheme name (Figure 9).  After the router puts
   the NO_ERROR return code in the fixed header, it sends the Reply back
   toward the Contrace user.

   If a router cannot continue the Request, it MUST put an appropriate
   ReturnCode in the Request message, change the Type field value in the
   fixed header from PT_REQUEST to PT_REPLY, and forward the Reply
   message back toward the Contrace user, to terminate the request.  See
   Section 7.

5.4.  Forwarding Contrace Reply

   When a router receives a Contrace Reply whose Request ID matches the
   one in the original Contrace Request block TLV from a valid adjacent
   neighbor node, it MUST relay the Contrace Reply back to the Contrace
   user.  If the router does not receive the corresponding Reply within
   the [Contrace Reply Timeout] period, then it removes the
   corresponding PIT entry and terminates the trace.

   Contrace Replies MUST NOT be cached in routers upon the Reply message
   transmission.

6.  Publisher Behavior

   Upon receiving a Contrace Request message, a publisher MUST examine
   whether the message comes from a valid adjacent neighbor node.  If it
   is invalid, the Request SHOULD be silently ignored.

   If a publisher cannot accept the Request, it will note an appropriate
   ReturnCode in the Request message, change the Type field value in the
   fixed header from PT_REQUEST to PT_REPLY, and forward the message as
   the Reply back to the Contrace user.  See Section 7 for details.

   If a publisher accepts the Request forwarded by a valid adjacent
   neighbor node, it retrieves the local content information.  The Reply
   message having a Reply block and Reply sub-block is transmitted back
   to the neighbor node that had forwarded the Request message.








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7.  Contrace Termination

   When performing an expanding hop-by-hop trace, it is necessary to
   determine when to stop expanding.  There are several cases an
   intermediate router might return a Reply before a Request reaches the
   caching router or the publisher.

7.1.  Arriving at Publisher or Gateway

   A Contrace Request can be determined to have arrived at the publisher
   or gateway.

7.2.  Arriving at Router Having Cache

   A Contrace Request can be determined to have arrived at the router
   having the specified content cache within the specified HopLimit.

7.3.  No Route

   If the router cannot determine the routing paths or neighbor routers
   for the specified name prefix, device name, or function within the
   specified HopLimit, the router MUST note a ReturnCode of NO_ROUTE in
   the fixed header of the message, and forwards the message as the
   Reply back to the Contrace user.

7.4.  No Information

   If the router does not have any information about the specified name
   prefix, device name, or function within the specified HopLimit, the
   router MUST note a ReturnCode of NO_INFO in the fixed header of the
   message, and forwards the message as the Reply back to the Contrace
   user.

7.5.  No Space

   If appending the Report block would make the Contrace Request packet
   longer than the MTU of the Incoming face, or longer than 1280 bytes
   (especially in the situation supporting IPv6 as the payload [3]), the
   router MUST note a ReturnCode of NO_SPACE in the fixed header of the
   message, and forwards the message as the Reply back to the Contrace
   user.

7.6.  Fatal Error

   A Contrace Request has encountered a fatal error if the last
   ReturnCode in the trace has the 0x80 bit set (see Section 3.1).





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7.7.  Contrace Reply Timeout

   If a Contrace user or a router encounters the Request or Reply
   message whose expires its own [Contrace Reply Timeout] value
   (Section 8.1), which is used to time out a Contrace Reply such as the
   case of Section 7.8.

7.8.  Non-Supported Node

   Cases will arise in which a router or a publisher along the path does
   not support Contrace.  In such cases, a Contrace user and routers
   that forward the Contrace Request will time out the Contrace request.

7.9.  Administratively Prohibited

   If Contrace is administratively prohibited, a router or a publisher
   rejects the Request message, and the router or the publisher, or its
   downstream router will reply the Contrace Reply with the ReturnCode
   of ADMIN_PROHIB.

8.  Configurations

8.1.  Contrace Reply Timeout

   The [Contrace Reply Timeout] value is used to time out a Contrace
   Reply.  Both Contrace users and routers can configure their own
   Contrace Reply Timeout values.  Contrace users, for example, can
   configure the timeout value by the contrace command.  The default
   [Contrace Reply Timeout] value is 4 (seconds).  Routers may want to
   configure the short timeout values because of some security concern,
   e.g., Section 10.5.  However, the [Contrace Reply Timeout] value
   SHOULD NOT be larger than 6 (seconds) and SHOULD NOT be lower than 3
   (seconds).

8.2.  HopLimit in Fixed Header

   If a Contrace user does not specify the HopLimit value in a fixed
   header for a Request message as the HopLimit, the HopLimit is set to
   32.  Note that a Contrace user specifies 0 as the HopLimit, it is an
   invalid Request and discarded.

8.3.  Access Control

   A router MAY configure the valid or invalid networks to enable an
   access control.  The access control can be defined per name prefix,
   such as "who can retrieve which name prefix".  See Section 10.2.





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9.  Diagnosis and Analysis

9.1.  Number of Hops

   A Contrace Request message is forwarded in a hop-by-hop manner and
   each forwarding router appended its own Report block.  We can then
   verify the number of hops to reach the content forwarder or the
   publisher.

9.2.  Caching Router and Gateway Identification

   It is possible to identify the caching routers or a gateway in the
   path from the Contrace user to the content forwarder, while some
   routers may hide their identifier (with all-zeros) in the Report
   blocks (Section 10.1).

9.3.  TTL or Hop Limit

   By taking the HopLimit from the content forwarder and forwarding TTL
   threshold over all hops, it is possible to discover the TTL or hop
   limit required for the content forwarder to reach the Contrace user.

9.4.  Time Delay

   If the routers have synchronized clocks, it is possible to estimate
   propagation and queuing delay from the differences between the
   timestamps at successive hops.  However, this delay includes control
   processing overhead, so is not necessarily indicative of the delay
   that data traffic would experience.

9.5.  Path Stretch

   By getting the path stretch "d / P", where "d" is the hop count of
   the data and "P" is the hop count from the consumer to the publisher,
   we can measure the improvement in path stretch in various cases, such
   as different caching and routing algorithms.  We can then facilitate
   investigation of the performance of the protocol.

9.6.  Cache Hit Probability

   Contrace can show the number of received interests per cache or chunk
   on a router.  By this, Contrace measures the content popularity
   (i.e., the number of accesses for each content/cache), and you can
   investigate the routing/caching strategy in networks.







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10.  Security Considerations

   This section addresses some of the security considerations.

10.1.  Policy-Based Information Provisioning for Request

   Although Contrace gives excellent troubleshooting cues, some network
   administrators or operators may not want to disclose everything about
   their network to the public, or may wish to securely transmit private
   information to specific members of their networks.  Contrace provides
   policy-based information provisioning allowing network administrators
   to specify their response policy for each router.

   The access policy regarding "who is allowed to retrieve" and/or "what
   kind of information" can be defined for each router.  For the former
   access policy, routers having the specified content can examine the
   signature enclosed in the Request message and decide whether they
   should notify the content information in the Reply or not.  If the
   routers decide to not notify the content information, they reply the
   Contrace Reply with the ReturnCode of ADMIN_PROHIB without appending
   any Reply (sub-)block TLV.  For the latter policy, the permission,
   whether (1) All (all cache information is disclosed), (2) Partial
   (cache information with the particular name prefix can (or cannot) be
   disclosed), or (3) Deny (no cache information is disclosed), is
   defined at routers.

   On the other hand, we entail that each router does not disrupt
   forwarding Contrace Request and Reply messages.  When a Request
   message is received, the router SHOULD insert Report block.  Here,
   according to the policy configuration, the Node Identifier field in
   the Report block MAY be null (i.e., all-zeros), but the Request
   Arrival Time field SHOULD NOT be null.  At last, the router SHOULD
   forward the Request message to the upstream router toward the content
   forwarder if no fatal error occurs.

10.2.  Filtering of Contrace Users Located in Invalid Networks

   A router MAY support an access control mechanism to filter out
   Requests from invalid Contrace users.  For it, invalid networks (or
   domains) could, for example, be configured via a list of allowed/
   disallowed networks (as seen in Section 8.3).  If a Request is
   received from the disallowed network (according to the Node
   Identifier in the Request block), the Request SHOULD NOT be processed
   and the Reply with the ReturnCode of INFO_HIDDEN may be used to note
   that.  The router MAY, however, perform rate limited logging of such
   events.





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10.3.  Topology Discovery

   Contrace can be used to discover actively-used topologies.  If a
   network topology is a secret, Contrace Requests may be restricted at
   the border of the domain, using the ADMIN_PROHIB return code.

10.4.  Characteristics of Content

   Contrace can be used to discover what publishers are sending to what
   kinds of contents.  If this information is a secret, Contrace
   Requests may be restricted at the border of the domain, using the
   ADMIN_PROHIB return code.

10.5.  Longer or Shorter Contrace Reply Timeout

   Routers can configure the Contrace Reply Timeout (Section 8.1), which
   is the allowable timeout value to keep the PIT entry.  If routers
   configure the longer timeout value, there may be an attractive attack
   vector against PIT memory.  Moreover, especially when the force
   option (Section 5.2) is specified for the Contrace Request, a number
   of Reply messages may come back and cause a response storm.  (See
   Section 10.7 for rate limiting to avoid the storm).  In order to
   avoid DoS attacks, routers may configure the shorter timeout value
   than the user-configured Contrace timeout value.  However, if it is
   too short, the Request may be timed out and the Contrace user does
   not receive the all Replies and only retrieves the partial path
   information (i.e., information about part of the tree).

   There may be the way to allow for incremental exploration (i.e., to
   explore the part of the tree the previous operation did not explore),
   whereas discussing such mechanism is out of scope of this document.

10.6.  Limiting Request Rates

   A router may limit Contrace Requests by ignoring some of the
   consecutive messages.  The router MAY randomly ignore the received
   messages to minimize the processing overhead, i.e., to keep fairness
   in processing requests, or prevent traffic amplification.  No error
   is returned.  The rate limit is left to the router's implementation.

10.7.  Limiting Reply Rates

   Contrace supporting multipath forwarding may result in one Request
   returning multiple Reply messages.  In order to prevent abuse, the
   routers in the traced path MAY need to rate-limit the Replies.  No
   error is returned.  The rate limit function is left to the router's
   implementation.




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10.8.  Adjacency Verification

   Contrace Request and Reply messages MUST be forwarded by adjacent
   neighbor nodes or routers.  Forwarding Contrace messages given from
   non-adjacent neighbor nodes/routers MUST be prohibited.  Such invalid
   messages SHOULD be silently discarded.  Note that defining the secure
   way to verify the adjacency cannot rely on the way specified in CCNx
   message format or semantics.  An adjacency verification mechanism and
   the corresponding TLV for adjacency verification using hop-by-hop TLV
   header will be defined in a separate document.

11.  Acknowledgements

   The authors would like to thank Spyridon Mastorakis, Ilya Moiseenko,
   and David Oran for their valuable comments and suggestions on this
   document.

12.  References

12.1.  Normative References

   [1]        Mosko, M., Solis, I., and C. Wood, "CCNx Messages in TLV
              Format", draft-irtf-icnrg-ccnxmessages-04 (work in
              progress), March 2017.

   [2]        Bradner, S., "Key words for use in RFCs to indicate
              requirement levels", RFC 2119, March 1997.

   [3]        Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

12.2.  Informative References

   [4]        Asaeda, H., Matsuzono, K., and T. Turletti, "Contrace: A
              Tool for Measuring and Tracing Content-Centric Networks",
              IEEE Communications Magazine, Vol.53, No.3, pp.182-188,
              March 2015.

   [5]        Malkin, G., "Traceroute Using an IP Option", RFC 1393,
              January 1993.

   [6]        Asaeda, H., Mayer, K., and W. Lee, "Mtrace Version 2:
              Traceroute Facility for IP Multicast", draft-ietf-mboned-
              mtrace-v2-17 (work in progress), March 2017.







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Appendix A.  Contrace Command and Options

   The contrace command enables the Contrace user to investigate the
   routing path based on the name prefix of the content (e.g.,
   ccnx:/news/today), device name, and function (or application) name.
   The name prefix, device name, and function name (or application name)
   are mandatory but exclusive options; that is, only one of them should
   be used with the contrace command at once.

   The usage of contrace command is as follows:

   Usage: contrace [-P] [-g] [-f] [-n] [-o] [-r hop_count] [-s
          hop_count] [-w wait_time] name_prefix; or,

   Usage: contrace [-r hop_count] [-s hop_count] [-w wait_time]
          device_name | function_name (or application_name)

   name_prefix
      Name prefix of the content (e.g., ccnx:/news/today) the Contrace
      user wants to trace.  If the Contrace user specifies only a scheme
      name, e.g., "ccnx:/", s/he must specify "-g" option (i.e.,
      contrace -g ccnx:/).  In that case, the Contrace user discovers
      the router having the FIB of the specified scheme name and the RTT
      between Contrace user and the router.  The "-P" option allows a
      partial match for the name prefix; otherwise, an exact match is
      required.

   device_name
      Device name (e.g., ccnx:/%device/server-A, ccnx:/%device/sensor-
      123) the Contrace user wants to trace.  Here, we assume the
      contrace command with the "%device" prefix indicates the trace
      request for specified device/server/node, but defining the syntax
      of device name specification is [TBD].

   function_name (or application_name)
      Function name (e.g., ccnx:/%function/firewall,
      ccnx:/%function/transcoding/mpeg2-h.264) or application name
      (e.g., ccnx:/%application/mplayer) the Contrace user wants to
      trace.  Here, we assume the contrace command with the "%function"
      or "%application" prefix indicates the trace request for specified
      function or application, but defining the syntax of function or
      application name specification is [TBD].

   g option
      This option enables to discover a gateway that supports specified
      scheme name and has multiple FIBs.  When a Contrace user specifies
      only a scheme name, e.g., "ccnx:/", this option must be specified
      and other content name prefix is ignored.



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   f option
      This option enables to ignore the forwarding strategy and send
      Contrace Requests to multiple upstream routers simultaneously.
      The Contrace user could then trace the all potential forwarding
      paths.

   n option
      This option can be specified if a Contrace user only needs the
      routing path information to the specified content/cache and RTT
      between Contrace user and content forwarder (i.e., cache
      information is not given).

   o option
      This option enables to trace the path to the content publisher.
      If this option is specified, each router along the path to the
      publisher only forwards the Request message; it inserts each
      Report block but does not send Reply even if it caches the
      specified content.  The publisher (who has the complete set of
      content and is not a caching router) replies the Reply message.
      Specifying only a scheme name is not allowed with this option.

   r option
      Number of traced routers.  If the Contrace user specifies this
      option, only the specified number of hops from the Contrace user
      trace the Request; each router inserts its own Report block and
      forwards the Request message to the upstream router(s), and the
      last router stops the trace and sends the Reply message back to
      the Contrace user.  This value is set in the "HopLimit" field
      located in the fixed header of the Request.  For example, when the
      Contrace user invokes the Contrace command with this option such
      as "-r 3", only three routers along the path examine their path
      and cache information.  If there is a caching router within the
      hop count along the path, the caching router sends back the Reply
      message and terminates the trace request.  If the last router does
      not have the corresponding cache, it replies the Reply message
      with NO_INFO return code (described in Section 3.1) with no Reply
      block TLV inserted.  The Request messages are terminated at
      publishers; therefore, although the maximum value for this option
      a Contrace user can specify is 255, the Request messages should be
      in general reached at the publisher within significantly lower
      than 255 hops.

   s option
      Number of skipped routers.  If the Contrace user specifies this
      option, the number of hops from the Contrace user simply forward
      the Contrace Request messages without adding its own Report block
      and without replying the Request, and the next upstream router
      starts the trace.  This value is set in the "SkipHopCount" field



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      located in the Request block TLV.  For example, when the Contrace
      user invokes the Contrace command with this option such as "-s 3",
      the three upstream routers along the path only forwards the
      Request message, but does not append their Report blocks in the
      hop-by-hop headers and does not send the Reply messages even
      though they have the corresponding cache.  The Request messages
      are terminated at publishers; therefore, although the maximum
      value for this option a Contrace user can specify is 255, if the
      Request messages reaches the publisher, the publisher silently
      discards the Request message and the request will be timed out.

   w option
      This option defines the Contrace timeout value (in seconds) that
      the Contrace user will wait for the Reply.  After the timeout, the
      Contrace user terminates the Request and silently discards the
      Reply message even if s/he receives the Reply.  Note that routers
      along the path can configure the Contrace Reply Timeout
      Section 8.1, which is the allowable timeout value to keep the PIT
      entry.  In order to avoid DoS attacks Section 10, routers MAY
      configure the shorter timeout value than the user-configured
      Contrace timeout value.  If it is shorter, the Request may be
      timed out and the Contrace user may not receive the Reply as
      expected.

Authors' Addresses

   Hitoshi Asaeda
   National Institute of Information and Communications Technology
   4-2-1 Nukui-Kitamachi
   Koganei, Tokyo  184-8795
   Japan

   Email: asaeda@nict.go.jp


   Xun Shao
   National Institute of Information and Communications Technology
   4-2-1 Nukui-Kitamachi
   Koganei, Tokyo  184-8795
   Japan

   Email: x-shao@nict.go.jp









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   Thierry Turletti
   Inria
   2004 Route des Lucioles
   Sophia Antipolis  06902
   France

   Email: thierry.turletti@inria.fr












































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