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Versions: (draft-asaeda-icnrg-ccninfo) 00 01 02 03 04 05

ICN Research Group                                             H. Asaeda
Internet-Draft                                                   A. Ooka
Intended status: Experimental                                       NICT
Expires: September 23, 2020                                      X. Shao
                                          Kitami Institute of Technology
                                                          March 22, 2020


CCNinfo: Discovering Content and Network Information in Content-Centric
                                Networks
                      draft-irtf-icnrg-ccninfo-04

Abstract

   This document describes a mechanism named "CCNinfo" that discovers
   information about the network topology and in-network cache in
   Content-Centric Networks (CCN).  CCNinfo investigates: 1) the CCN
   routing path information per name prefix, 2) the Round-Trip Time
   (RTT) between the content forwarder and consumer, and 3) the states
   of in-network cache per name prefix.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 September 23, 2020.

Copyright Notice

   Copyright (c) 2020 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 . . . . . . . . . . . . . . . . . . . . . . . . .   7
     2.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   7
   3.  CCNinfo Message Formats . . . . . . . . . . . . . . . . . . .   8
     3.1.  Request Message . . . . . . . . . . . . . . . . . . . . .   9
       3.1.1.  Request Block . . . . . . . . . . . . . . . . . . . .  11
       3.1.2.  Report Block  . . . . . . . . . . . . . . . . . . . .  14
     3.2.  Reply Message . . . . . . . . . . . . . . . . . . . . . .  15
       3.2.1.  Reply Block . . . . . . . . . . . . . . . . . . . . .  16
         3.2.1.1.  Reply Sub-Block . . . . . . . . . . . . . . . . .  16
   4.  CCNinfo User Behavior . . . . . . . . . . . . . . . . . . . .  19
     4.1.  Sending CCNinfo Request . . . . . . . . . . . . . . . . .  19
       4.1.1.  Routing Path Information  . . . . . . . . . . . . . .  20
       4.1.2.  In-Network Cache Information  . . . . . . . . . . . .  20
     4.2.  Receiving CCNinfo Reply . . . . . . . . . . . . . . . . .  20
   5.  Router Behavior . . . . . . . . . . . . . . . . . . . . . . .  21
     5.1.  User and Neighbor Verification  . . . . . . . . . . . . .  21
     5.2.  Receiving CCNinfo Request . . . . . . . . . . . . . . . .  21
     5.3.  Forwarding CCNinfo Request  . . . . . . . . . . . . . . .  22
       5.3.1.  Regular Request . . . . . . . . . . . . . . . . . . .  22
       5.3.2.  Full Discovery Request  . . . . . . . . . . . . . . .  23
     5.4.  Sending CCNinfo Reply . . . . . . . . . . . . . . . . . .  24
     5.5.  Forwarding CCNinfo Reply  . . . . . . . . . . . . . . . .  24
   6.  CCNinfo Termination . . . . . . . . . . . . . . . . . . . . .  25
     6.1.  Arriving at First-hop Router  . . . . . . . . . . . . . .  25
     6.2.  Arriving at Router Having Cache . . . . . . . . . . . . .  25
     6.3.  Arriving at Last Router . . . . . . . . . . . . . . . . .  25
     6.4.  Invalid Request . . . . . . . . . . . . . . . . . . . . .  25
     6.5.  No Route  . . . . . . . . . . . . . . . . . . . . . . . .  25
     6.6.  No Information  . . . . . . . . . . . . . . . . . . . . .  26
     6.7.  No Space  . . . . . . . . . . . . . . . . . . . . . . . .  26
     6.8.  Fatal Error . . . . . . . . . . . . . . . . . . . . . . .  26
     6.9.  CCNinfo Reply Timeout . . . . . . . . . . . . . . . . . .  26
     6.10. Non-Supported Node  . . . . . . . . . . . . . . . . . . .  26
     6.11. Administratively Prohibited . . . . . . . . . . . . . . .  26
   7.  Configurations  . . . . . . . . . . . . . . . . . . . . . . .  26
     7.1.  CCNinfo Reply Timeout . . . . . . . . . . . . . . . . . .  27
     7.2.  HopLimit in Fixed Header  . . . . . . . . . . . . . . . .  27
     7.3.  Access Control  . . . . . . . . . . . . . . . . . . . . .  27
   8.  Diagnosis and Analysis  . . . . . . . . . . . . . . . . . . .  27
     8.1.  Number of Hops  . . . . . . . . . . . . . . . . . . . . .  27



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     8.2.  Caching Router Identification . . . . . . . . . . . . . .  27
     8.3.  TTL or Hop Limit  . . . . . . . . . . . . . . . . . . . .  27
     8.4.  Time Delay  . . . . . . . . . . . . . . . . . . . . . . .  27
     8.5.  Path Stretch  . . . . . . . . . . . . . . . . . . . . . .  28
     8.6.  Cache Hit Probability . . . . . . . . . . . . . . . . . .  28
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  28
     10.1.  Policy-Based Information Provisioning for Request  . . .  28
     10.2.  Filtering CCNinfo Users Located in Invalid Networks  . .  29
     10.3.  Topology Discovery . . . . . . . . . . . . . . . . . . .  29
     10.4.  Characteristics of Content . . . . . . . . . . . . . . .  29
     10.5.  Computational Attacks  . . . . . . . . . . . . . . . . .  29
     10.6.  Longer or Shorter CCNinfo Reply Timeout  . . . . . . . .  30
     10.7.  Limiting Request Rates . . . . . . . . . . . . . . . . .  30
     10.8.  Limiting Reply Rates . . . . . . . . . . . . . . . . . .  30
     10.9.  Adjacency Verification . . . . . . . . . . . . . . . . .  30
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  31
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  31
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  31
     12.2.  Informative References . . . . . . . . . . . . . . . . .  31
   Appendix A.  ccninfo Command and Options  . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  33

1.  Introduction

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

   In CCN, while consumers do not generally need to know the content
   forwarder that is transmitting the content to them, the 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 [7] is a useful tool for discovering the routing
   conditions in IP networks because it provides intermediate router
   addresses along the path between the 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.  Moreover, such
   IP-based network tools do not obtain the states of the in-network
   cache to be discovered.

   This document describes the specifications of "CCNinfo", an active
   networking tool for discovering the path and content caching



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   information in CCN.  CCNinfo is designed based on a previous work
   [6].

   CCNinfo can be implemented with the user commands (such as ccninfo
   described in Appendix A) and forwarding function implementation on a
   content forwarder (e.g., router).  The CCNinfo user (e.g., consumer)
   invokes the ccninfo command with the name prefix of the content.  The
   ccninfo command initiates the "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 the 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 it to its upstream neighbor router(s) decided by
   its FIB.  These two message types, Request and Reply messages, are
   encoded in the CCNx TLV format [1].

   Thus, the Request message is forwarded by routers toward the content
   publisher and the Report record is inserted by each intermediate
   router.  When the Request message reaches the content forwarder
   (i.e., a router that can forward the specified cache or content), the
   content forwarder forms the "Reply" message (described in
   Section 3.2) and sends it to the downstream neighbor router.  The
   Reply message is forwarded back toward the user in a hop-by-hop
   manner.  This request-reply message flow, walking up the tree from a
   consumer toward a publisher, is similar to the behavior of the IP
   multicast traceroute facility [8].

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

   In addition, CCNinfo identifies the states of the cache, such as the
   following metrics for Content Store (CS) in the content forwarder: 1)
   size of cached content objects, 2) number of cached content objects,
   3) number of accesses (i.e., received Interests) per content, and 4)
   elapsed cache time and remaining cache lifetime of content.

   CCNinfo supports multipath forwarding.  The Request messages can be
   forwarded to multiple neighbor routers.  When the Request messages
   are forwarded to multiple routers, the different Reply messages are
   forwarded from different routers or publishers.








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

     Figure 2: Default behavior.  Reply messages forwarded by routers.
      Each router forwards the Reply message along its PIT entry and
     finally, the CCNinfo user receives a Reply message from Router C,
      which is the first-hop router for the Publisher.  Another Reply
      message from the Caching router is discarded at Router B as the
            corresponding Reply message was already forwarded.

   Within a network with multipath condition, there is a case (Figure 3)
   wherein a single CCNinfo Request is split into multiple Requests
   (e.g., at Router A), which are injected into a single router (Router



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   D).  In this case, multiple Replies with the same Request ID and Node
   Identifier including different Report blocks are received by the
   router (Router D).  To recognize different CCNinfo Reply messages,
   the routers MUST distinguish the PIT entries by the Request ID and
   exploiting path labels, which could be a hash value of the
   concatenation information of the cumulate Node Identifiers in the
   hop-by-hop header and the specified content name.  For example, when
   Router D in Figure 3 receives a CCNinfo Request from Router B, its
   PIT includes the Request ID and value such as
   H((Router_A|Router_B)|content_name), where "H" indicates some hash
   function and "|" indicates concatenation.  When Router D receives a
   CCNinfo Request from Router C, its PIT includes the same Request ID
   and value of H((Router_A|Router_C)|content_name).  Two different
   Replies are later received on Router D and each Reply is
   appropriately forwarded to Router B and Router C, respectively.

                               +--------+
                               | Router |
                               |   B    |
                               +--------+
                              /          \
                             /            \
     +--------+    +--------+              +--------+     +---------+
     | CCNinfo|----| Router |              | Router | ... |Publisher|
     |  user  |    |   A    |              |   D    |     |         |
     +--------+    +--------+              +--------+     +---------+
                             \            /
                              \          /
                               +--------+
                               | Router |
                               |   C    |
                               +--------+

                                 Figure 3

   To avoid routing loop, when a router seeks the cumulate Node
   Identifiers of the Report blocks in the hop-by-hop header, it MUST
   examine whether its own Node Identifier is not previously inserted.
   If a router detects its own Node Identifier in the hop-by-hop header,
   the router terminates the Request as will be described in
   Section 6.8.

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




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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 [3];
   they indicate the requirement levels for the compliant CCNinfo
   implementations.

2.1.  Definitions

   This document follows the basic terminologies and definitions
   described in [1].  Although CCNinfo 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 that facilitates CCN-based content retrieval in the
      path between the consumer and publisher.

   Scheme name
      It indicates a URI and protocol.  This document only considers
      "ccn:/" as the scheme name.

   Prefix name
      A prefix name, which is defined in [2], is a name that does not
      uniquely identify a single content object, but rather a namespace
      or prefix of an existing content object name.

   Exact name
      An exact name, which is defined in [2], is one that uniquely
      identifies the name of a content object.

   Node
      It is a router, publisher, or consumer.

   Content forwarder
      It is either a caching router or a first-hop router that forwards
      content objects to consumers.

   CCNinfo user
      It is a node that initiates the CCNinfo Request, which is usually
      invoked by the ccninfo command (described in Appendix A) or other
      similar commands.

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




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

   Upstream router
      The router that connects to an Incoming face of a router, which is
      responsible for forwarding data for the specified name prefix to
      the router.

   First-hop router (FHR)
      The router that matches a FIB entry with an Outgoing face
      referring to a local application or a publisher.

   Last-hop router (LHR)
      The router that is directly connected to a consumer.

3.  CCNinfo Message Formats

   CCNinfo uses two message types: Request and Reply.  Both messages are
   encoded in the CCNx TLV format ([1], Figure 4).  The Request message
   consists of a fixed header, Request block TLV (Figure 8), 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 4: Packet format [1]

   The Request and Reply Type values in the fixed header are PT_REQUEST
   and PT_REPLY, respectively (Figure 5).  These messages are forwarded
   in a hop-by-hop manner.  When the Request message reaches the content
   forwarder, the content forwarder turns it into a Reply message by



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

                      Code         Type name
                    ========      =====================
                      %x00        PT_INTEREST [1]
                      %x01        PT_CONTENT [1]
                      %x02        PT_RETURN [1]
                      %x03        PT_REQUEST
                      %x04        PT_REPLY

                      Figure 5: Packet Type Namespace

   The CCNinfo Request and Reply messages 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, a top-level TLV type,
   T_DISCOVERY (Figure 6), 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
               =============      =========================
                  %x0000          Reserved [1]
                  %x0001          T_INTEREST [1]
                  %x0002          T_OBJECT [1]
                  %x0003          T_VALIDATION_ALG [1]
                  %x0004          T_VALIDATION_PAYLOAD [1]
                  %x0005          T_DISCOVERY

                    Figure 6: Top-Level Type Namespace

3.1.  Request Message

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

   The Request message format is shown in Figure 7.  It consists of a
   fixed header, Request block TLV (Figure 8), Report block TLV(s)
   (Figure 11), and Name TLV.  The Type value of the Top-Level type



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   namespace is T_DISCOVERY (Figure 6).  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_DISCOVERY          |         MessageLength         |
     +---------------+---------------+---------------+---------------+
     |            T_NAME             |             Length            |
     +---------------+---------------+---------------+---------------+
     / Name segment TLVs (name prefix specified by ccninfo command)  /
     +---------------+---------------+---------------+---------------+

    Figure 7: 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 whenever a
      Request packet is forwarded.  It limits the distance that 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
  -----  ---------------  ----------------------------------------------
  %x00   NO_ERROR         No error
  %x01   WRONG_IF         CCNinfo Request arrived on an interface
                          to which this router would not forward for
                          the specified name/function toward the
                          publisher.
  %x02   INVALID_REQUEST  Invalid CCNinfo Request is received.
  %x03   NO_ROUTE         This router has no route for the name prefix
                          and no way to determine a route.
  %x04   NO_INFO          This router has no cache information for the
                          specified name prefix.
  %x05   NO_SPACE         There was not enough room to insert another
                          Report block in the packet.
  %x06   INFO_HIDDEN      Information is hidden from this discovery
                          owing to some policy.
  %x0E   ADMIN_PROHIB     CCNinfo Request is administratively
                          prohibited.
  %x0F   UNKNOWN_REQUEST  This router does not support/recognize the
                          Request message.
  %x80   FATAL_ERROR      In a fatal error, 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 CCNinfo user transmits the Request message, s/he MUST insert
   her/his Request block TLV (Figure 8) 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_DISC_REQ          |             Length            |
     +---------------+---------------+-------+-------+---------+-+-+-+
     |           Request ID          |SkipHop|      Flags      |F|O|C|
     +---------------+---------------+-------+-------+---------+-+-+-+
     |                     Request Arrival Time                      |
     +---------------+---------------+---------------+---------------+
     /                        Node Identifier                        /
     +---------------+---------------+---------------+---------------+


              Figure 8: Request block TLV (hop-by-hop header)

                   Code           Type name
               =============      =========================
                  %x0000          Reserved [1]
                  %x0001          T_INTLIFE [1]
                  %x0002          T_CACHETIME [1]
                  %x0003          T_MSGHASH [1]
               %x0004-%x0007      Reserved [1]
                  %x0008          T_DISC_REQ
                  %x0009          T_DISC_REPORT
                  %x0FFE          T_PAD [1]
                  %x0FFF          T_ORG [1]
               %x1000-%x1FFF      Reserved [1]

                    Figure 9: Hop-by-Hop Type Namespace

   Type: 16 bits

      Format of the Value field.  For the type value of the first
      Request block TLV MUST be T_DISC_REQ.  For all the available types
      for hop-by-hop type namespace, please see Figure 9.

   Length: 16 bits

      Length of Value field in octets.

   Request ID: 16 bits

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

   SkipHop (Skip Hop Count): 4 bits





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      Number of skipped routers for a Request.  The maximum value of
      this parameter is 15.  This value MUST be lower than that of
      HopLimit at the fixed header.

   Flags: 12 bits

      The Flags field is used to indicate the types of the content or
      path discoveries.  Currently, as shown in Figure 10, three bits,
      "C", "O", and "F", are assigned, and the other 9 bits are reserved
      (MBZ) for the future use.  These flags are set by CCNinfo users
      when they initiate Requests (see Appendix A), and the routers that
      receive the Requests deal with the flags and change the behaviors
      (see Section 5 for details).  The Flag values defined in this
      Flags field correspond to the Reply sub-blocks.

   Flag    Value   Description
   -----   -----   -----------------------------------------------------
     C       0     Path discovery (i.e., no cache information retrieved)
                   (default)
     C       1     Cache information retrieval
     O       0     Request to any content forwarder (default)
     O       1     Publisher reachability (i.e., only FHR can reply)
                   Type of Reply sub-block will be T_DISC_CONTENT_OWNER
     F       0     Request based on FIB's strategy (default)
     F       1     Full discovery request. Request to possible multiple
                   upstream routers specified in FIB simultaneously

            Figure 10: Codes and types specified in Flags field

   Request Arrival Time: 32 bits

      The Request Arrival Time is a 32-bit NTP timestamp specifying the
      arrival time of the CCNinfo Request packet at a specific 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).





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      Note that it is RECOMMENDED for all the routers on the path to
      have synchronized clocks; however, if they do not have
      synchronized clocks, CCNinfo measures one-way latency.

   Node Identifier: variable length

      This field specifies the node identifier (e.g., node name or hash-
      based self-certifying name [9]) or all-zeros if unknown.  This
      document assumes that the Name TLV defined in the CCNx TLV format
      [1] can be used for this field and the node identifier is
      specified in it.

3.1.2.  Report Block

   A CCNinfo user and each upstream router along the path would insert
   their 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 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_DISC_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 Report block TLV, the type
      value(s) MUST be T_DISC_REPORT in the current specification.

   Length: 16 bits

      Length of Value field in octets.

   Request Arrival Time: 32 bits

      Same definition as given in Section 3.1.1.

   Node Identifier: variable length

      Same definition as given in Section 3.1.1.



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3.2.  Reply Message

   When a content forwarder receives a CCNinfo Request message from an
   appropriate adjacent neighbor router, it inserts its own Reply block
   TLV and Reply sub-block TLV(s) to the Request message and turns 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) is then forwarded back toward the CCNinfo
   user in a hop-by-hop manner.

                          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_DISCOVERY          |         MessageLength         |
     +---------------+---------------+---------------+---------------+
     |            T_NAME             |             Length            |
     +---------------+---------------+---------------+---------------+
     / Name segment TLVs (name prefix specified by ccninfo 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
                  ==============     ===================
                      %x0000         T_NAME [1]
                      %x0001         T_PAYLOAD [1]
                      %x0002         T_KEYIDRESTR [1]
                      %x0003         T_OBJHASHRESTR [1]
                      %x0005         T_PAYLDTYPE [1]
                      %x0006         T_EXPIRY [1]
                      %x0007         T_DISC_REPLY
                  %x0008-%x0012      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 the 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_DISC_REPLY          |             Length            |
     +---------------+---------------+---------------+---------------+

                Figure 14: Reply block TLV (packet payload)

   Type: 16 bits

      Format of the Value field.  For the Reply block TLV, the type
      value MUST be T_DISC_REPLY in the current specification.

   Length: 16 bits

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

3.2.1.1.  Reply Sub-Block

   In addition to the Reply block, the 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 or developers may want to obtain various characteristics of




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   content such as content's ownership and various cache states and
   conditions.

   This document describes the Reply sub-block TLVs for T_DISC_CONTENT
   and T_DISC_CONTENT_OWNER (Figure 15) (other Reply sub-block TLVs will
   be discussed in separate document(s)).  Note that some routers may
   not be capable of reporting the following values such as Object Size,
   Object Count, # Received Interest, First Seqnum, Last Seqnum, Elapsed
   Cache Time, and Remain Cache Lifetime, as shown in Figure 15, or do
   not report these values due to their policy.  These values therefore
   MAY be returned with null.

                          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            |
     +---------------+---------------+---------------+---------------+
     |                          Object Size                          |
     +---------------+---------------+---------------+---------------+
     |                         Object Count                          |
     +---------------+---------------+---------------+---------------+
     |                      # Received Interest                      |
     +---------------+---------------+---------------+---------------+
     |                         First Seqnum                          |
     +---------------+---------------+---------------+---------------+
     |                          Last Seqnum                          |
     +---------------+---------------+---------------+---------------+
     |                       Elapsed Cache Time                      |
     +---------------+---------------+---------------+---------------+
     |                      Remain Cache Lifetime                    |
     +---------------+---------------+---------------+---------------+
     |            T_NAME             |             Length            |
     +---------------+---------------+---------------+---------------+
     /                       Name Segment TLVs                       /
     +---------------+---------------+---------------+---------------+

           Figure 15: Reply sub-block TLV for T_DISC_CONTENT and
                   T_DISC_CONTENT_OWNER (packet payload)

                   Code          Type name
               =============     ===========================
                  %x0000         T_DISC_CONTENT
                  %x0001         T_DISC_CONTENT_OWNER
                  %x0FFF         T_ORG
               %x1000-%x1FFF     Reserved (Experimental Use)

                  Figure 16: CCNinfo Reply Type Namespace




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   Type: 16 bits

      Format of the Value field.  For the Reply sub-block TLV, the type
      value MUST be one of the type values defined in the CCNinfo Reply
      Type Namespace (Figure 16).  T_DISC_CONTENT is specified when the
      cache information is replied from a caching router.
      T_DISC_CONTENT_OWNER is specified when the content information is
      replied from a FHR attached to a publisher.

   Length: 16 bits

      Length of the Value field in octets.

   Object Size: 32 bits

      The total size (KB) of the unexpired content objects.  Note that
      the maximum size expressed by the 32-bit field is approximately
      4.29 TB.  If the cache is refreshed after reboot, the counter MUST
      be refreshed (i.e., MUST be set to 0).  If the cache remains after
      reboot, the counter MUST NOT be refreshed (i.e., MUST be kept as
      it is).

   Object Count: 32 bits

      The number of unexpired content objects.  Note that the maximum
      count expressed by the 32-bit field is approximately 4.29 billion.
      If the cache is refreshed after reboot, the counter MUST be
      refreshed (i.e., MUST be set to 0).  If the cache remains after
      reboot, the counter MUST NOT be refreshed (i.e., MUST be kept as
      it is).

   # Received Interest: 32 bits

      The total number of the received Interest messages to retrieve the
      cached content objects.

   First Seqnum: 32 bits

      The first sequential number of the unexpired content objects.

   Last Seqnum: 32 bits

      The last sequential number of the unexpired content objects.  The
      First Seqnum and Last Seqnum do not guarantee the consecutiveness
      of the cached content objects; however, knowing these values may
      help in the analysis of consecutive or discontinuous chunks such
      as [10].




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   Elapsed Cache Time: 32 bits

      The elapsed time (seconds) after the oldest content object of the
      content is cached.

   Remain Cache Lifetime: 32 bits

      The lifetime (seconds) of a content object, which is removed first
      from the cached content objects.

   Specifications of the Name TLV (whose type value is T_NAME) and the
   Name Segment TLVs are described in [1], which are followed by
   CCNinfo.  CCNinfo also enables to specification of the content name
   either with a prefix name (such as "ccn:/news/today") or an exact
   name (such as "ccn:/news/today/Chunk=10").  When a CCNinfo user
   specifies a prefix name, s/he will obtain the summary information of
   the matched content objects in the content forwarder.  In contrast,
   when a CCNinfo user specifies an exact name, s/he will obtain only
   about the specified content object in the content forwarder.  A
   CCNinfo Request message MUST NOT be sent only with a scheme name,
   ccn:/, because it will be rejected and discarded by routers.

4.  CCNinfo User Behavior

4.1.  Sending CCNinfo Request

   A CCNinfo user invokes a CCNinfo user's program (e.g., ccninfo
   command) that initiates a CCNinfo Request message and sends it to the
   user's adjacent neighbor router(s) of interest.  The user later
   obtains both the routing path information and in-network cache
   information simultaneously.

   When the CCNinfo user's program initiates a Request message, it MUST
   insert the necessary values, i.e., the "Request ID" and the "Node
   Identifier", in the Request block.  The Request ID MUST be unique for
   the CCNinfo user until s/he receives the corresponding Reply
   message(s) or the Request is timed out.

   Owing to some policies, a router may want to validate the CCNinfo
   Requests (whether it accepts the Request or not) especially when the
   router receives the "full discovery request" (see Section 5.3.2).
   Accordingly, the CCNinfo user's program MAY require appending the
   user's signature into the CCNx ValidationPayload TLV.  The router
   then forwards the Request message or sends the Reply message whenever
   it approves the Request; otherwise, it rejects the Request message as
   described in Section 6.11.





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   After the CCNinfo user's program sends the Request message, until the
   Reply is timed out or the expected numbers of Replies or a Reply
   message with a non-zero ReturnCode in the fixed header is received,
   the CCNinfo user's program MUST keep the following information:
   HopLimit, specified in the fixed header, Request ID, Flags, Node
   Identifier, and Request Arrival Time, specified in the Request block.

4.1.1.  Routing Path Information

   A CCNinfo user can send a CCNinfo Request for investigating the
   routing path information for the specified named content.  Using the
   Request, a legitimate user can obtain 1) the node identifiers of the
   intermediate routers, 2) node identifier of the content forwarder, 3)
   number of hops between the content forwarder and consumer, and 4) RTT
   between the content forwarder and consumer, per name prefix.  This
   CCNinfo Request is terminated when it reaches the content forwarder.

4.1.2.  In-Network Cache Information

   A CCNinfo user can send a CCNinfo Request for investigating in-
   network cache information.  Using the Request, a legitimate user can
   obtain 1) the size of cached content objects, 2) number of cached
   content objects, 3) number of accesses (i.e., received Interests) per
   content, and 4) lifetime and expiration time of the cached content
   objects, for Content Store (CS) in the content forwarder, unless the
   content forwarder is capable of reporting them (see Section 3.2.1.1).
   This CCNinfo Request is terminated when it reaches the content
   forwarder.

4.2.  Receiving CCNinfo Reply

   A CCNinfo user's program will receive one or multiple CCNinfo 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 Node
   Identifier.  If they do not match, the Reply message MUST be silently
   discarded.

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

   After the CCNinfo user has determined that s/he has traced the whole
   path or the maximum path that s/he can be expected to, s/he might
   collect statistics by waiting for a timeout.  Useful statistics
   provided by CCNinfo are stated in Section 8.





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5.  Router Behavior

5.1.  User and Neighbor Verification

   Upon receiving a CCNinfo Request message, a router MAY examine
   whether a valid CCNinfo user has sent the message.  If the router
   recognizes that the Request sender's signature specified in the
   Request is invalid, it SHOULD terminate the Request, as defined in
   Section 6.4.

   Upon receiving a CCNinfo Request/Reply message, a router MAY examine
   whether the message comes from a valid adjacent neighbor node.  If
   the router recognizes that the Request/Reply sender is invalid, it
   SHOULD silently ignore the Request/Reply message, as specified in
   Section 10.9.

5.2.  Receiving CCNinfo Request

   After a router accepts the CCNinfo Request message, it performs the
   following steps.

   1.  The value of "HopLimit" in the fixed header and that of
       "SkipHopCount" in the Request block are counters that are
       decremented with each hop.  If the HopLimit value is zero, the
       router terminates the Request, as defined in Section 6.5.  If the
       SkipHopCount value is equal to or more than the HopLimit value,
       the router terminates the Request, as defined in Section 6.4.
       Otherwise, until the SkipHopCount value becomes zero, the router
       forwards the Request message to the upstream router(s) without
       adding its own Report block and without replying to the Request.
       If the router does not know the upstream router(s) regarding the
       specified name prefix, it terminates the Request, as defined in
       Section 6.5.  It should be noted that the Request messages are
       terminated at the FHR; therefore, although the maximum value for
       the HopLimit is 255 and that for SkipHopCount is 15, if the
       Request messages reach the FHR before the HopLimit or
       SkipHopCount value becomes 0, the FHR silently discards the
       Request message and the Request is timed out.

   2.  The router examines the Flags field (specified in Figure 10) in
       the Request block of the received CCNinfo Request.  If the "C"
       flag is set but the "O" flag is not set, it is categorized as the
       "cache information discovery".  If both the "C" and "O" flags are
       not set, it is categorized as the "routing path information
       discovery".  If the "O" flag is set, it is categorized as the
       "publisher discovery".





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   3.  If the Request is either "cache information discovery" or
       "routing path information discovery", the router examines its FIB
       and CS.  If the router caches the specified content, it inserts
       its own Report block in the hop-by-hop header and sends the Reply
       message with its own Reply block and sub-block(s) (in the case of
       cache information discovery) or the Reply message with its own
       Reply block without adding any Reply sub-blocks (in the case of
       routing path information discovery).  If the router does not
       cache the specified content but knows the upstream neighbor
       router(s) for the specified name prefix, it inserts its 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 terminates the Request, as defined in
       Section 6.5.

   4.  If the Request is the "publisher discovery", the router examines
       whether it is the FHR for the requested content.  If yes, it
       sends the Reply message with its own Report block and sub-blocks.
       If the router is not the FHR but knows the upstream neighbor
       router(s) for the specified name prefix, it adds its own Report
       block and forwards the Request to the neighbor(s).  If the node
       is not the FHR and does not know the upstream neighbor router(s)
       for the specified name prefix, it terminates the Request, as
       defined in Section 6.5.

5.3.  Forwarding CCNinfo Request

5.3.1.  Regular 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, F flag, and Node Identifier specified in the Request
   block at the corresponding PIT entry.  The router can later check the
   PIT entry to correctly forward back the Reply message(s) back.

   CCNinfo supports multipath forwarding.  The Request messages can be
   forwarded to multiple neighbor routers.  Some routers may have a
   strategy for multipath forwarding; when a router sends Interest
   messages to multiple neighbor routers, it may delay or prioritize to
   send the message to the upstream routers.  The CCNinfo Request, as
   the default, complies with such strategies; a CCNinfo user could




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   trace the actual forwarding path based on the forwarding strategy and
   will receive a single Reply message such as a content object.

   However, CCNinfo can discover all possible content forwarders.  See
   the next section for more information.

5.3.2.  Full Discovery Request

   There may be a case wherein a CCNinfo user wants to discover all
   possible forwarding paths based on the routers' FIBs.  The "full
   discovery request" enables this functionality.  If a CCNinfo user
   sets the F flag in the Request block of the Request message (as seen
   in Figure 10) to request the full discovery, the upstream routers
   forward the Requests to all multiple upstream routers based on the
   FIBs simultaneously.  Then, the CCNinfo user can trace all possible
   forwarding paths.

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

      Figure 17: Full discovery request.  Reply messages forwarded by
   publisher and routers.  Each router forwards the Reply message along
      its PIT entry and finally, the CCNinfo user receives two Reply
   messages: one from the FHR (Router C) and the other from the Caching
                                  router.

   To receive different Reply messages forwarded from different routers,
   the PIT entries initiated by CCNinfo remain until the configured
   CCNinfo Reply Timeout (Section 7.1) is expired.  In other words,
   unlike the ordinary Interest-Data communications in CCN, if routers
   that accept the full discovery request receive the full discovery
   request, the routers SHOULD NOT remove the PIT entry created by the
   full discovery request until the CCNinfo Reply Timeout value expires.




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   Note that the full discovery request is an OPTIONAL implementation of
   CCNinfo; it MAY NOT be implemented on routers.  Even if it is
   implemented on a router, it MAY NOT accept the full discovery request
   from non-validated CCNinfo users or routers or because of its policy.
   If a router does not accept the full discovery request, it rejects
   the full discovery request as described in Section 6.11.  Routers
   that enable the full discovery request MAY rate-limit Replies, as
   described in Section 10.8 as well.

5.4.  Sending CCNinfo Reply

   If there is a caching router or FHR for the specified content within
   the specified hop count along the path, the caching router or FHR
   sends back the Reply message toward the CCNinfo user and terminates
   the Request.

   When a router decides to send a Reply message to its downstream
   neighbor router or the CCNinfo user with NO_ERROR return code, it
   inserts a Report block with the Request Arrival Time and Node
   Identifier to the hop-by-hop TLV header of the Request message.
   Then, the router inserts the corresponding Reply block with an
   appropriate type value (Figure 15) and Reply sub-block(s) to the
   payload.  The router does not insert any Reply block/sub-blocks 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 CCNinfo user in a hop-by-hop manner.

   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 CCNinfo user to terminate the request (see
   Section 6).

5.5.  Forwarding CCNinfo Reply

   When a router receives a CCNinfo Reply whose Request ID and Node
   Identifier match those in the PIT entry, sent from a valid adjacent
   neighbor router, it forwards the CCNinfo Reply back toward the
   CCNinfo user.  If the router does not receive the corresponding Reply
   within the [CCNinfo Reply Timeout] period, then it removes the
   corresponding PIT entry and terminates the trace.

   The Flags field in the Request block TLV is used to indicate whether
   the router keeps the PIT entry during the CCNinfo Reply Timeout even
   after one or more corresponding Reply messages are forwarded.  When
   the CCNinfo user does not set the F flag (i.e., "0"), the
   intermediate routers immediately remove the PIT entry whenever they
   forward the corresponding Reply message.  When the CCNinfo user sets



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   the F flag (i.e., "1"), which means the CCNinfo user chooses the
   "full discovery request" (see Section 5.3.2), the intermediate
   routers keep the PIT entry within the [CCNinfo Reply Timeout] period.
   After this timeout, the PIT entry is removed.

   CCNinfo Replies MUST NOT be cached in routers upon the transmission
   of Reply messages.

6.  CCNinfo Termination

   When performing an expanding hop-by-hop trace, it is necessary to
   determine when to stop expanding.  There are several cases when an
   intermediate router might return a Reply before a Request reaches the
   caching router or the FHR, as described in Section 3.2.

6.1.  Arriving at First-hop Router

   A CCNinfo Request can be determined to have arrived at the first-hop
   router.  To ensure that a router recognizes that it is the FHR for
   the specified content, it needs to have a FIB entry (or attach) to
   the corresponding publisher or the content.

6.2.  Arriving at Router Having Cache

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

6.3.  Arriving at Last Router

   A CCNinfo Request can be determined to have arrived at the last
   router of the specified HopLimit.  If the last router does not have
   the corresponding cache, it MUST send the Reply message with NO_INFO
   return code without inserting the Reply block TLV.

6.4.  Invalid Request

   If the router does not validate the Request, the router MUST note a
   ReturnCode of INVALID_REQUEST in the fixed header of the message and
   forward the message without appending any Reply (sub-)block TLVs as
   the Reply back to the CCNinfo user.  The router MAY, however,
   randomly ignore the received invalid messages.  (See Section 10.7.)

6.5.  No Route

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



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6.6.  No Information

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

6.7.  No Space

   If appending the Report block would make the Request packet longer
   than the MTU of the Incoming face or longer than 1280 bytes (in the
   case of IPv6 as the payload [5]), the router MUST note a ReturnCode
   of NO_SPACE in the fixed header of the message and forward the
   message as the Reply back to the CCNinfo user.

6.8.  Fatal Error

   If a CCNinfo Request has encountered a fatal error, the router MUST
   note a ReturnCode of FATAL_ERROR in the fixed header of the message
   and forward the message as the Reply back to the CCNinfo user.  This
   may happen, for example, when the router detects some routing loop in
   the Request blocks (see Section 1).

6.9.  CCNinfo Reply Timeout

   If a router receives the Request or Reply message that expires its
   own [CCNinfo Reply Timeout] value (Section 7.1), the router will
   silently discard the Request or Reply message.

6.10.  Non-Supported Node

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

6.11.  Administratively Prohibited

   If CCNinfo is administratively prohibited, the router rejects the
   Request message and MUST send the CCNinfo Reply with the ReturnCode
   of ADMIN_PROHIB.  The router MAY, however, randomly ignore the
   Request messages to be rejected (see Section 10.7).

7.  Configurations








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7.1.  CCNinfo Reply Timeout

   The [CCNinfo Reply Timeout] value is used to time out a CCNinfo
   Reply.  The value for a router can be statically configured by the
   router's administrators/operators.  The default value is 3 (seconds).
   The [CCNinfo Reply Timeout] value SHOULD NOT be larger than 4
   (seconds) and SHOULD NOT be lower than 2 (seconds).

7.2.  HopLimit in Fixed Header

   If a CCNinfo user does not specify the HopLimit value in the fixed
   header for a Request message as the HopLimit, the HopLimit is set to
   32.  Note that 0 HopLimit is an invalid Request; hence, the router in
   this case follows the way defined in Section 6.4.

7.3.  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).

8.  Diagnosis and Analysis

8.1.  Number of Hops

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

8.2.  Caching Router Identification

   While some routers may hide their node identifiers with all-zeros in
   the Report blocks, the routers in the path from the CCNinfo user to
   the content forwarder can be identified (Section 10.1).

8.3.  TTL or Hop Limit

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

8.4.  Time Delay

   If the routers have synchronized clocks, it is possible to estimate
   the propagation and queuing delays from the differences between the
   timestamps at the successive hops.  However, this delay includes the



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   control processing overhead; therefore, it is not necessarily
   indicative of the delay that would be experienced by the data
   traffic.

8.5.  Path Stretch

   By obtaining 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 improvements in path stretch in various cases,
   such as in different caching and routing algorithms.  We can then
   facilitate the investigation of the performance of the protocol.

8.6.  Cache Hit Probability

   CCNinfo can show the number of received interests per cache or chunk
   on a router.  Accordingly, CCNinfo measures the content popularity
   (i.e., the number of accesses for each content/cache), thereby
   enabling the investigation of the routing/caching strategy in
   networks.

9.  IANA Considerations

   New assignments can only be made via a Standards Action as specified
   in [4].  This document does not intend to be the standard document.
   However, the new assignments such as the ReturnCode and various type
   values will be considered when this specification becomes the RFC.

10.  Security Considerations

   This section addresses some of the security considerations.

10.1.  Policy-Based Information Provisioning for Request

   Although CCNinfo 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.  CCNinfo provides
   policy-based information provisioning, thereby 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
   type of access policy, routers with the specified content MAY examine
   the signature enclosed in the Request message and decide whether they
   should notify the content information in the Reply.  If the routers
   decide to not notify the content information, they MUST send the
   CCNinfo Reply with the ReturnCode of ADMIN_PROHIB without appending
   any Reply (sub-)block TLVs.  For the latter type of policy, the



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   permission, whether (1) All (all cache information is disclosed), (2)
   Partial (cache information with a particular name prefix can (or
   cannot) be disclosed), or (3) Deny (no cache information is
   disclosed), is defined at the routers.

   In contrast, we entail that each router does not disrupt the
   forwarding of CCNinfo Request and Reply messages.  When a Request
   message is received, the router SHOULD insert the Report block if the
   ReturnCode is NO_ERROR.  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.
   Finally, the router SHOULD forward the Request message to the
   upstream router toward the content forwarder if the ReturnCode is
   kept with NO_ERROR.

10.2.  Filtering CCNinfo Users Located in Invalid Networks

   A router MAY support an access control mechanism to filter out
   Requests from invalid CCNinfo users.  To accomplish this, invalid
   networks (or domains) could, for example, be configured via a list of
   allowed/disallowed networks (as observed in Section 7.3).  If a
   Request is received from a disallowed network (according to the Node
   Identifier in the Request block), the Request MUST NOT be processed
   and the Reply with the ReturnCode of INFO_HIDDEN SHOULD be used to
   note that.  The router MAY, however, perform rate-limited logging of
   such events.

10.3.  Topology Discovery

   CCNinfo can be used to discover actively used topologies.  If a
   network topology is not disclosed, CCNinfo Requests SHOULD be
   restricted at the border of the domain using the ADMIN_PROHIB return
   code.

10.4.  Characteristics of Content

   CCNinfo can be used to discover the type of content being sent by
   publishers.  If this information is a secret, CCNinfo Requests SHOULD
   be restricted at the border of the domain, using the ADMIN_PROHIB
   return code.

10.5.  Computational Attacks

   CCNinfo may impose heavy tasks at content forwarders.  The current
   CCNinfo specification allows to return null values for several
   fields, such as First/Last Seqnum or Elapsed Cache Time fields in the
   Reply sub-block.  As mentioned in Section 3.2.1.1, these values MAY
   be null.  This means that the content forwarder can not only hide



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   these values owing to privacy/security policies, but also skip the
   implementations of the complex functions to report these values.

10.6.  Longer or Shorter CCNinfo Reply Timeout

   Routers can configure CCNinfo Reply Timeout (Section 7.1), which is
   the allowable timeout value to keep the PIT entry.  If routers
   configure a longer timeout value, there may be an attractive attack
   vector against the PIT memory.  Moreover, especially when the full
   discovery request option (Section 5.3) is specified for the CCNinfo
   Request, several Reply messages may be returned and cause a response
   storm.  (See Section 10.8 for rate limiting to avoid the storm).  To
   avoid DoS attacks, routers MAY configure the timeout value, which is
   shorter than the user-configured CCNinfo timeout value.  However, if
   it is too short, the Request may be timed out and the CCNinfo user
   does not receive all Replies; s/he only retrieves the partial path
   information (i.e., information about a part of the tree).

   There may be a way to enable incremental exploration (i.e., to
   explore the part of the tree that was not explored by the previous
   operation); however, discussing such mechanisms is out of scope of
   this document.

10.7.  Limiting Request Rates

   A router may rate-limit CCNinfo 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.  In such a
   case, no error message is returned.  The rate limit function is left
   to the router's implementation.

10.8.  Limiting Reply Rates

   CCNinfo supporting multipath forwarding may result in one Request
   returning multiple Reply messages.  To prevent abuse, the routers in
   the traced path MAY need to rate-limit the Replies.  In such a case,
   no error message is returned.  The rate limit function is left to the
   router's implementation.

10.9.  Adjacency Verification

   It is assumed that the CCNinfo Request and Reply messages are
   forwarded by adjacent neighbor nodes or routers.  The CCNx message
   format or semantics do not define a secure way to verify the node/
   router adjacency, while HopAuth [9] provides a possible method for an
   adjacency verification and defines the corresponding message format




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   for adjacency verification as well as the router behaviors.  CCNinfo
   MAY use a similar method for node adjacency verification.

11.  Acknowledgements

   The authors would like to thank Spyridon Mastorakis, Ilya Moiseenko,
   David Oran, and Thierry Turletti 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", RFC 8609, July 2019.

   [2]        Mosko, M., Solis, I., and C. Wood, "CCNx Semantics",
              RFC 8569, July 2019.

   [3]        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>.

   [4]        Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [5]        Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 8200, July 2017.

12.2.  Informative References

   [6]        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.

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

   [8]        Asaeda, H., Mayer, K., and W. Lee, "Mtrace Version 2:
              Traceroute Facility for IP Multicast", RFC 8487, October
              2018.






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   [9]        Li, R. and H. Asaeda, "Hop-by-Hop Authentication in
              Content-Centric Networking/Named Data Networking", draft-
              li-icnrg-hopauth-02 (work in progress), February 2020.

   [10]       Li, R., Matsuzono, K., Asaeda, H., and X. Fu, "Consecutive
              Caching and Adaptive Retrieval for In-Network Big Data
              Sharing", Proc. IEEE ICC, Kansas City, USA, May 2018.

   [11]       Asaeda, H., Ooka, A., Matsuzono, K., and R. Li, "Cefore:
              Software Platform Enabling Content-Centric Networking and
              Beyond", IEICE Transaction on Communications, Vol.E102-B,
              No.9, pp.1792-1803, September 2019.

   [12]       "Cefore Home Page", <https://cefore.net/>.

Appendix A.  ccninfo Command and Options

   CCNinfo is implemented in Cefore [11][12].  The ccninfo command in
   Cefore enables the CCNinfo user to investigate the routing path based
   on the name prefix of the content (e.g., ccn:/news/today).  The name
   prefix is mandatory but has exclusive options; that is, only one of
   them should be used with the ccninfo command at once.

   The usage of ccninfo command is as follows:

   Usage: ccninfo [-f] [-c] [-o] [-r hop_count] [-s hop_count]
          name_prefix

   name_prefix
      Prefix name of content (e.g., ccn:/news/today) or exact name of
      content (e.g., ccn:/news/today/Chunk=10) the CCNinfo user wants to
      trace.

   f option
      This option enables the "full discovery request"; routers send
      CCNinfo Requests to multiple upstream faces based on their FIBs
      simultaneously.  The CCNinfo user can then trace all possible
      forwarding paths.

   c option
      This option can be specified if a CCNinfo user needs the cache
      information as well as the routing path information for the
      specified content/cache and RTT between the CCNinfo user and
      content forwarder.

   o option
      This option enables to trace the path to the content publisher.
      Each router along the path to the publisher inserts each Report



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      block and forwards the Request message.  It does not send Reply
      even if it caches the specified content.  FHR that attaches the
      publisher (who has the complete set of content and is not a
      caching router) sends the Reply message.

   r option
      Number of traced routers.  If the CCNinfo user specifies this
      option, only the specified number of hops from the CCNinfo user
      traces the Request.  Each router inserts its own Report block and
      forwards the Request message to the upstream router(s).  The last
      router stops the trace and sends the Reply message back to the
      CCNinfo user.  This value is set in the "HopLimit" field located
      in the fixed header of the Request.  For example, when the CCNinfo
      user invokes the CCNinfo command with this option, such as "-r 3",
      only three routers along the path examine their path and cache
      information.

   s option
      Number of skipped routers.  If the CCNinfo user specifies this
      option, routers corresponding to the value specified in this
      option are skipped and the CCNinfo Request messages are forwarded
      to the next router without the addition of Report blocks; the next
      upstream router then starts the trace.  This value is set in the
      "SkipHopCount" field located in the Request block TLV.  For
      example, when the CCNinfo user invokes the CCNinfo command with
      this option, such as "-s 3", three upstream routers along the path
      only forward the Request message but do not append their Report
      blocks in the hop-by-hop header and do not send Reply messages
      despite having the corresponding cache.

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


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

   Email: a-ooka@nict.go.jp



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   Xun Shao
   Kitami Institute of Technology
   165 Koen-cho
   Kitami, Hokkaido  090-8507
   Japan

   Email: x-shao@mail.kitami-it.ac.jp












































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