path: root/doc/rfc/rfc8609.txt

Internet Research Task Force (IRTF)                             M. Mosko
Request for Comments: 8609                                    PARC, Inc.
Category: Experimental                                          I. Solis
ISSN: 2070-1721                                                 LinkedIn
                                                                 C. Wood
                                         University of California Irvine
                                                               July 2019


        Content-Centric Networking (CCNx) Messages in TLV Format

Abstract

   Content-Centric Networking (CCNx) is a network protocol that uses a
   hierarchical name to forward requests and to match responses to
   requests.  This document specifies the encoding of CCNx messages in a
   TLV packet format, including the TLV types used by each message
   element and the encoding of each value.  The semantics of CCNx
   messages follow the encoding-independent CCNx Semantics
   specification.

   This document is a product of the Information Centric Networking
   research group (ICNRG).  The document received wide review among
   ICNRG participants and has two full implementations currently in
   active use, which have informed the technical maturity of the
   protocol specification.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Research Task
   Force (IRTF).  The IRTF publishes the results of Internet-related
   research and development activities.  These results might not be
   suitable for deployment.  This RFC represents the consensus of the
   Information-Centric Networking Research Group of the Internet
   Research Task Force (IRTF).  Documents approved for publication by
   the IRSG are not candidates for any level of Internet Standard; see
   Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8609.





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Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Type-Length-Value (TLV) Packets . . . . . . . . . . . . . . .   5
     3.1.  Overall Packet Format . . . . . . . . . . . . . . . . . .   7
     3.2.  Fixed Headers . . . . . . . . . . . . . . . . . . . . . .   8
       3.2.1.  Interest Fixed Header . . . . . . . . . . . . . . . .   9
         3.2.1.1.  Interest HopLimit . . . . . . . . . . . . . . . .   9
       3.2.2.  Content Object Fixed Header . . . . . . . . . . . . .   9
       3.2.3.  Interest Return Fixed Header  . . . . . . . . . . . .  10
         3.2.3.1.  Interest Return HopLimit  . . . . . . . . . . . .  10
         3.2.3.2.  Interest Return Flags . . . . . . . . . . . . . .  10
         3.2.3.3.  Return Code . . . . . . . . . . . . . . . . . . .  10
     3.3.  Global Formats  . . . . . . . . . . . . . . . . . . . . .  11
       3.3.1.  Pad . . . . . . . . . . . . . . . . . . . . . . . . .  11
       3.3.2.  Organization-Specific TLVs  . . . . . . . . . . . . .  12
       3.3.3.  Hash Format . . . . . . . . . . . . . . . . . . . . .  12
       3.3.4.  Link  . . . . . . . . . . . . . . . . . . . . . . . .  13
     3.4.  Hop-by-Hop TLV Headers  . . . . . . . . . . . . . . . . .  14
       3.4.1.  Interest Lifetime . . . . . . . . . . . . . . . . . .  14
       3.4.2.  Recommended Cache Time  . . . . . . . . . . . . . . .  15
       3.4.3.  Message Hash  . . . . . . . . . . . . . . . . . . . .  16
     3.5.  Top-Level Types . . . . . . . . . . . . . . . . . . . . .  17
     3.6.  CCNx Message TLV  . . . . . . . . . . . . . . . . . . . .  18
       3.6.1.  Name  . . . . . . . . . . . . . . . . . . . . . . . .  19
         3.6.1.1.  Name Segments . . . . . . . . . . . . . . . . . .  20
         3.6.1.2.  Interest Payload ID . . . . . . . . . . . . . . .  20
       3.6.2.  Message TLVs  . . . . . . . . . . . . . . . . . . . .  21
         3.6.2.1.  Interest Message TLVs . . . . . . . . . . . . . .  21
         3.6.2.2.  Content Object Message TLVs . . . . . . . . . . .  23
       3.6.3.  Payload . . . . . . . . . . . . . . . . . . . . . . .  25
       3.6.4.  Validation  . . . . . . . . . . . . . . . . . . . . .  25
         3.6.4.1.  Validation Algorithm  . . . . . . . . . . . . . .  25
         3.6.4.2.  Validation Payload  . . . . . . . . . . . . . . .  32



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   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  33
     4.1.  Packet Type Registry  . . . . . . . . . . . . . . . . . .  33
     4.2.  Interest Return Code Registry . . . . . . . . . . . . . .  34
     4.3.  Hop-by-Hop Type Registry  . . . . . . . . . . . . . . . .  35
     4.4.  Top-Level Type Registry . . . . . . . . . . . . . . . . .  36
     4.5.  Name Segment Type Registry  . . . . . . . . . . . . . . .  37
     4.6.  Message Type Registry . . . . . . . . . . . . . . . . . .  37
     4.7.  Payload Type Registry . . . . . . . . . . . . . . . . . .  38
     4.8.  Validation Algorithm Type Registry  . . . . . . . . . . .  39
     4.9.  Validation-Dependent Data Type Registry . . . . . . . . .  40
     4.10. Hash Function Type Registry . . . . . . . . . . . . . . .  40
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  41
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  44
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  44
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  44
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  46

1.  Introduction

   This document specifies a Type-Length-Value (TLV) packet format and
   the TLV type and value encodings for CCNx messages.  A full
   description of the CCNx network protocol, providing an encoding-free
   description of CCNx messages and message elements, may be found in
   [RFC8569].  CCNx is a network protocol that uses a hierarchical name
   to forward requests and to match responses to requests.  It does not
   use endpoint addresses; the Internet Protocol does.  Restrictions in
   a request can limit the response by the public key of the response's
   signer or the cryptographic hash of the response.  Every CCNx
   forwarder along the path does the name matching and restriction
   checking.  The CCNx protocol fits within the broader framework of
   Information-Centric Networking (ICN) protocols [RFC7927].

   This document describes a TLV scheme using a fixed 2-byte T and a
   fixed 2-byte L field.  The rational for this choice is described in
   Section 5.  Briefly, this choice avoids multiple encodings of the
   same value (aliases) and reduces the work of a validator to ensure
   compliance.  Unlike some uses of TLV in networking, each network hop
   must evaluate the encoding, so even small validation latencies at
   each hop could add up to a large overall forwarding delay.  For very
   small packets or low-throughput links, where the extra bytes may
   become a concern, one may use a TLV compression protocol, for
   example, [compress] and [CCNxz].

   This document uses the terms CCNx Packet, CCNx Message, and CCNx
   Message TLV.  A CCNx Packet refers to the entire Layer 3 datagram as
   specified in Section 3.1.  A CCNx Message is the ABNF token defined
   in the CCNx Semantics document [RFC8569].  A CCNx Message TLV refers
   to the encoding of a CCNx Message as specified in Section 3.6.



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   This document specifies:

   o  the CCNx Packet format,

   o  the CCNx Message TLV format,

   o  the TLV types used by CCNx messages,

   o  the encoding of values for each type,

   o  top-level types that exist at the outermost containment,

   o  Interest TLVs that exist within Interest containment, and

   o  Content Object TLVs that exist within Content Object containment.

   This document is supplemented by these documents:

   o  [RFC8569], which covers message semantics and the protocol
      operation regarding Interest and Content Object, including the
      Interest Return protocol.

   o  [CCNxURI], which covers the CCNx URI notation.

   The type values in Section 4 conform to the IANA-assigned numbers for
   the CCNx protocol.  This document uses the symbolic names defined in
   that section.  All TLV type values are relative to their parent
   containers.  For example, each level of a nested TLV structure might
   define a "type = 1" with a completely different meaning.

   Packets are represented as 32-bit wide words using ASCII art.  Due to
   the nested levels of TLV encoding and the presence of optional fields
   and variable sizes, there is no concise way to represent all
   possibilities.  We use the convention that ASCII art fields enclosed
   by vertical bars "|" represent exact bit widths.  Fields with a
   forward slash "/" are variable bit widths, which we typically pad out
   to word alignment for picture readability.

   The document represents the consensus of the ICN RG.  It is the first
   ICN protocol from the RG, created from the early CCNx protocol [nnc]
   with significant revision and input from the ICN community and RG
   members.  The document has received critical reading by several
   members of the ICN community and the RG.  The authors and RG chairs
   approve of the contents.  The document is sponsored under the IRTF
   and is not issued by the IETF and is not an IETF standard.  This is
   an experimental protocol and may not be suitable for any specific
   application and the specification may change in the future.




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1.1.  Requirements Language

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

2.  Definitions

   These definitions summarize items defined in [RFC8569].  This
   document defines their encodings.

   o  Name: A hierarchically structured variable-length identifier.  It
      is an ordered list of path segments, which are variable-length
      octet strings.  In human-readable form, it is represented in URI
      format as "ccnx:/path/part".  There is no host or query string.
      See [CCNxURI] for complete details.

   o  Interest: A message requesting a Content Object with a matching
      Name and other optional selectors to choose from multiple objects
      with the same Name.  Any Content Object with a Name and attributes
      that matches the Name and optional selectors of the Interest is
      said to satisfy the Interest.

   o  Content Object: A data object sent in response to an Interest
      request.  It has an optional Name and a content payload that are
      bound together via cryptographic means.

3.  Type-Length-Value (TLV) Packets

   We use 16-bit Type and 16-bit Length fields to encode TLV-based
   packets.  This provides 65,536 different possible types and value
   field lengths of up to 64 KiB.  With 65,536 possible types at each
   level of TLV encoding, there should be sufficient space for basic
   protocol types, while also allowing ample room for experimentation,
   application use, vendor extensions, and growth.  This encoding does
   not allow for jumbo packets beyond 64 KiB total length.  If used on a
   media that allows for jumbo frames, we suggest defining a media
   adaptation envelope that allows for multiple smaller frames.











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   +--------+------------------+---------------------------------------+
   | Abbrev |       Name       | Description                           |
   +--------+------------------+---------------------------------------+
   | T_ORG  | Vendor Specific  | Information specific to a vendor      |
   |        |   Information    | implementation (Section 3.3.2).       |
   |        |                  |                                       |
   | T_PAD  |     Padding      | Adds padding to a field (Section      |
   |        |                  | 3.3.1).                               |
   |        |                  |                                       |
   |  n/a   |   Experimental   | Experimental use.                     |
   +--------+------------------+---------------------------------------+

                        Table 1: Reserved TLV Types

   There are several global TLV definitions that we reserve at all
   hierarchical contexts.  The TLV types in the range 0x1000 - 0x1FFF
   are Reserved for Experimental Use.  The TLV type T_ORG is also
   Reserved for Vendor Extensions (see Section 3.3.2).  The TLV type
   T_PAD is used to optionally pad a field out to some desired
   alignment.

                        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             |
   +---------------+---------------+---------------+---------------+

                    Figure 1: Type and Length encoding

   The Length field contains the length of the Value field in octets.
   It does not include the length of the Type and Length fields.  The
   Length MAY be zero.

   TLV structures are nestable, allowing the Value field of one TLV
   structure to contain additional TLV structures.  The enclosing TLV
   structure is called the container of the enclosed TLV.

   Type values are context dependent.  Within a TLV container, one may
   reuse previous type values for new context-dependent purposes.












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3.1.  Overall Packet Format

   Each CCNx Packet includes the 8-byte fixed header, described below,
   followed by a set of TLV fields.  These fields are optional hop-by-
   hop headers and the 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
   +---------------+---------------+---------------+---------------+
   |    Version    |  PacketType   |         PacketLength          |
   +---------------+---------------+---------------+---------------+
   |           PacketType-specific fields          | HeaderLength  |
   +---------------+---------------+---------------+---------------+
   / Optional hop-by-hop header TLVs                               /
   +---------------+---------------+---------------+---------------+
   / PacketPayload TLVs                                            /
   +---------------+---------------+---------------+---------------+

                      Figure 2: Overall Packet Format

   The PacketPayload of a CCNx Packet is the protocol message itself.
   The Content Object Hash is computed over the PacketPayload only,
   excluding the fixed and hop-by-hop headers, as those might change
   from hop to hop.  Signed information or similarity hashes should not
   include any of the fixed or hop-by-hop headers.  The PacketPayload
   should be self-sufficient in the event that the fixed and hop-by-hop
   headers are removed.  See Message Hash (Section 3.4.3).

   Following the CCNx Message TLV, the PacketPayload may include
   optional Validation TLVs.

                        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
   +---------------+---------------+---------------+---------------+
   | CCNx Message TLV                                              /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationAlgorithm TLV                         /
   +---------------+---------------+---------------+---------------+
   / Optional CCNx ValidationPayload TLV (ValidationAlg required)  /
   +---------------+---------------+---------------+---------------+

                       Figure 3: PacketPayload TLVs

   After discarding the fixed and hop-by-hop headers, the remaining
   PacketPayload should be a valid protocol message.  Therefore, the
   PacketPayload always begins with 4 bytes of type-length that
   specifies the protocol message (whether it is an Interest, Content
   Object, or other message type) and its total length.  The embedding



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   of a self-sufficient protocol data unit inside the fixed and hop-by-
   hop headers allows a network stack to discard the headers and operate
   only on the embedded message.  It also decouples the PacketType field
   -- which specifies how to forward the packet -- from the
   PacketPayload.

   The range of bytes protected by the Validation includes the CCNx
   Message TLV and the ValidationAlgorithm TLV.

   The ContentObjectHash begins with the CCNx Message TLV and ends at
   the tail of the CCNx Packet.

3.2.  Fixed Headers

   In Figure 2, the fixed header fields are:

   o  Version: defines the version of the packet, which MUST be 1.

   o  HeaderLength: The length of the fixed header (8 bytes) and hop-by-
      hop headers.  The minimum value MUST be 8.

   o  PacketType: describes forwarder actions to take on the packet.

   o  PacketLength: Total octets of packet including all headers (fixed
      header plus hop-by-hop headers) and protocol message.

   o  PacketType-specific Fields: specific PacketTypes define the use of
      these bits.

   The PacketType field indicates how the forwarder should process the
   packet.  A Request Packet (Interest) has PacketType PT_INTEREST, a
   Response (Content Object) has PacketType PT_CONTENT, and an Interest
   Return has PacketType PT_RETURN.

   HeaderLength is the number of octets from the start of the CCNx
   Packet (Version) to the end of the hop-by-hop headers.  PacketLength
   is the number of octets from the start of the packet to the end of
   the packet.  Both lengths have a minimum value of 8 (the fixed header
   itself).

   The PacketType-specific fields are reserved bits whose use depends on
   the PacketType.  They are used for network-level signaling.









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3.2.1.  Interest Fixed Header

   If the PacketType is PT_INTEREST, it indicates that the packet should
   be forwarded following the Interest pipeline in Section 2.4.4 of
   [RFC8569].  For this type of packet, the Fixed Header includes a
   field for a HopLimit as well as Reserved and Flags fields.  The
   Reserved field MUST be set to 0 in an Interest.  There are currently
   no flags defined, so the Flags field MUST be set to 0.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |    Version    |  PT_INTEREST  |         PacketLength          |
   +---------------+---------------+---------------+---------------+
   |   HopLimit    |   Reserved    |     Flags     | HeaderLength  |
   +---------------+---------------+---------------+---------------+

                         Figure 4: Interest Header

3.2.1.1.  Interest HopLimit

   For an Interest message, the HopLimit is a counter that is
   decremented with each hop.  It limits the distance an Interest may
   travel on the network.  The node originating the Interest MAY put in
   any value up to the maximum of 255.  Each node that receives an
   Interest with a HopLimit decrements the value upon reception.  If the
   value is 0 after the decrement, the Interest MUST NOT be forwarded
   off the node.

   It is an error to receive an Interest from a remote node with the
   HopLimit field set to 0.

3.2.2.  Content Object Fixed Header

   If the PacketType is PT_CONTENT, it indicates that the packet should
   be forwarded following the Content Object pipeline in Section 2.4.4
   of [RFC8569].  A Content Object defines a Flags field; however, there
   are currently no flags defined, so the Flags field must be set to 0.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |    Version    |  PT_CONTENT   |         PacketLength          |
   +---------------+---------------+---------------+---------------+
   |            Reserved           |     Flags     | HeaderLength  |
   +---------------+---------------+---------------+---------------+

                      Figure 5: Content Object Header



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3.2.3.  Interest Return Fixed Header

   If the PacketType is PT_RETURN, it indicates that the packet should
   be processed following the Interest Return rules in Section 10 of
   [RFC8569].  The only difference between this Interest Return message
   and the original Interest is that the PacketType is changed to
   PT_RETURN and a ReturnCode is put into the ReturnCode field.  All
   other fields are unchanged from the Interest packet.  The purpose of
   this encoding is to prevent packet length changes so no additional
   bytes are needed to return an Interest to the previous hop.

                        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    |   PT_RETURN   |         PacketLength          |
   +---------------+---------------+---------------+---------------+
   |   HopLimit    |  ReturnCode   |     Flags     | HeaderLength  |
   +---------------+---------------+---------------+---------------+

                     Figure 6: Interest Return Header

3.2.3.1.  Interest Return HopLimit

   This is the original Interest's HopLimit, as received before
   decrement at the node sending the Interest Return.

3.2.3.2.  Interest Return Flags

   These are the original Flags as set in the Interest.

3.2.3.3.  Return Code

   This section maps the Return Code name [RFC8569] to the TLV symbolic
   name.  Section 4.2 maps the symbolic names to numeric values.  This
   field is set by the node creating the Interest Return.

   A return code of "0" MUST NOT be used, as it indicates that the
   returning system did not modify the Return Code field.













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   +-------------------------------------+-----------------------------+
   |             Return Type             | Name in RFC 8569            |
   +-------------------------------------+-----------------------------+
   |          T_RETURN_NO_ROUTE          | No Route                    |
   |                                     |                             |
   |       T_RETURN_LIMIT_EXCEEDED       | Hop Limit Exceeded          |
   |                                     |                             |
   |        T_RETURN_NO_RESOURCES        | No Resources                |
   |                                     |                             |
   |         T_RETURN_PATH_ERROR         | Path Error                  |
   |                                     |                             |
   |         T_RETURN_PROHIBITED         | Prohibited                  |
   |                                     |                             |
   |          T_RETURN_CONGESTED         | Congested                   |
   |                                     |                             |
   |        T_RETURN_MTU_TOO_LARGE       | MTU too large               |
   |                                     |                             |
   | T_RETURN_UNSUPPORTED_HASH_RESTRICTI | Unsupported ContentObjectHa |
   |                  ON                 | shRestriction               |
   |                                     |                             |
   |     T_RETURN_MALFORMED_INTEREST     | Malformed Interest          |
   +-------------------------------------+-----------------------------+

                           Table 2: Return Codes

3.3.  Global Formats

   This section defines global formats that may be nested within other
   TLVs.

3.3.1.  Pad

   The pad type may be used by sources that prefer word-aligned data.
   Padding 4-byte words, for example, would use a 1-byte, 2-byte, and
   3-byte Length.  Padding 8-byte words would use a (0, 1, 2, 3, 5, 6,
   7)-byte Length.

   One MUST NOT pad inside a Name.  Apart from that, a pad MAY be
   inserted after any other TLV in the CCNx Message TLV or in the
   ValidationAlgorithm TLV.  In the remainder of this document, we will
   not show optional Pad TLVs.










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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_PAD             |             Length            |
   +---------------+---------------+---------------+---------------+
   /                 variable-length pad MUST be zeros             /
   +---------------+---------------+---------------+---------------+

                          Figure 7: Pad Encoding

3.3.2.  Organization-Specific TLVs

   Organization-specific TLVs (also known as Vendor TLVs) MUST use the
   T_ORG type.  The Length field is the length of the organization-
   specific information plus 3.  The Value begins with the 3 byte
   organization number derived from the network byte order encoding of
   the IANA "Private Enterprise Numbers" registry [IANA-PEN], followed
   by the organization-specific information.

   A T_ORG MAY be used as a path segment in a Name.  It is treated like
   any other path segment.

                        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_ORG             |     Length (3+value length)   |
   +---------------+---------------+---------------+---------------+
   |   PEN[0]      |    PEN[1]     |     PEN[2]    |               /
   +---------------+---------------+---------------+               +
   /                  Vendor Specific Value                        /
   +---------------+---------------+---------------+---------------+

                   Figure 8: Organization-Specific TLVs

3.3.3.  Hash Format

   Hash values are used in several fields throughout a packet.  This TLV
   encoding is commonly embedded inside those fields to specify the
   specific hash function used and its value.  Note that the reserved
   TLV types are also reserved here for user-defined experimental
   functions.

   The LENGTH field of the hash value MUST be less than or equal to the
   hash function length.  If the LENGTH is less than the full length, it
   is taken as the left LENGTH bytes of the hash function output.  Only
   specified truncations are allowed, not arbitrary truncations.





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   This nested format is used because it allows binary comparison of
   hash values for certain fields without a router needing to understand
   a new hash function.  For example, the KeyIdRestriction is bit-wise
   compared between an Interest's KeyIdRestriction field and a
   ContentObject's KeyId field.  This format means the outer field
   values do not change with differing hash functions so a router can
   still identify those fields and do a binary comparison of the hash
   TLV without need to understand the specific hash used.  An
   alternative approach, such as using T_KEYID_SHA512-256, would require
   each router keeps an up-to-date parser and supporting user-defined
   hash functions here would explode the parsing state-space.

   A CCNx entity MUST support the hash type T_SHA-256.  An entity MAY
   support the remaining hash types.

                  +-----------+------------------------+
                  |   Abbrev  |    Lengths (octets)    |
                  +-----------+------------------------+
                  | T_SHA-256 |           32           |
                  |           |                        |
                  | T_SHA-512 |         64, 32         |
                  |           |                        |
                  |    n/a    | Experimental TLV types |
                  +-----------+------------------------+

                       Table 3: CCNx Hash Functions

                        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_FOO             |              36               |
   +---------------+---------------+---------------+---------------+
   |           T_SHA512            |               32              |
   +---------------+---------------+---------------+---------------+
   /                        32-byte hash value                     /
   +---------------+---------------+---------------+---------------+

                Figure 9: Example nesting inside type T_FOO

3.3.4.  Link

   A Link is the tuple: {Name, [KeyIdRestr], [ContentObjectHashRestr]}.
   It is a general encoding that is used in both the payload of a
   Content Object with PayloadType = "Link" and in a Content Object's
   KeyLink field.  A Link is essentially the body of an Interest.






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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
   +---------------+---------------+---------------+---------------+
   / Mandatory CCNx Name                                           /
   +---------------+---------------+---------------+---------------+
   / Optional KeyIdRestriction                                     /
   +---------------+---------------+---------------+---------------+
   / Optional ContentObjectHashRestriction                         /
   +---------------+---------------+---------------+---------------+

                         Figure 10: Link Encoding

3.4.  Hop-by-Hop TLV Headers

   Hop-by-hop TLV headers are unordered and meaning MUST NOT be attached
   to their ordering.  Three hop-by-hop headers are described in this
   document:

   +-------------+--------------------+--------------------------------+
   |    Abbrev   |        Name        | Description                    |
   +-------------+--------------------+--------------------------------+
   |  T_INTLIFE  | Interest Lifetime  | The time an Interest should    |
   |             |  (Section 3.4.1)   | stay pending at an             |
   |             |                    | intermediate node.             |
   |             |                    |                                |
   | T_CACHETIME | Recommended Cache  | The Recommended Cache Time for |
   |             |   Time (Section    | Content Objects.               |
   |             |       3.4.2)       |                                |
   |             |                    |                                |
   |  T_MSGHASH  |    Message Hash    | A cryptographic hash (Section  |
   |             |  (Section 3.4.3)   | 3.3.3).                        |
   +-------------+--------------------+--------------------------------+

                     Table 4: Hop-by-Hop Header Types

   Additional hop-by-hop headers are defined in higher level
   specifications such as the fragmentation specification.

3.4.1.  Interest Lifetime

   The Interest Lifetime is the time that an Interest should stay
   pending at an intermediate node.  It is expressed in milliseconds as
   an unsigned integer in network byte order.

   A value of 0 (encoded as 1 byte 0x00) indicates the Interest does not
   elicit a Content Object response.  It should still be forwarded, but
   no reply is expected and a forwarder could skip creating a PIT entry.




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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_INTLIFE            |             Length            |
   +---------------+---------------+---------------+---------------+
   /                                                               /
   /                      Lifetime (Length octets)                 /
   /                                                               /
   +---------------+---------------+---------------+---------------+

                   Figure 11: Interest Lifetime Encoding

3.4.2.  Recommended Cache Time

   The Recommended Cache Time (RCT) is a measure of the useful lifetime
   of a Content Object as assigned by a content producer or upstream
   node.  It serves as a guideline to the Content Store cache in
   determining how long to keep the Content Object.  It is a
   recommendation only and may be ignored by the cache.  This is in
   contrast to the ExpiryTime (described in Section 3.6.2.2.2) which
   takes precedence over the RCT and must be obeyed.

   Because the Recommended Cache Time is an optional hop-by-hop header
   and not a part of the signed message, a content producer may re-issue
   a previously signed Content Object with an updated RCT without
   needing to re-sign the message.  There is little ill effect from an
   attacker changing the RCT as the RCT serves as a guideline only.

   The Recommended Cache Time (a millisecond timestamp) is an unsigned
   integer in network byte order that indicates the time when the
   payload expires (as the number of milliseconds since the epoch in
   UTC).  It is a 64-bit field.

                        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_CACHETIME           |               8               |
   +---------------+---------------+---------------+---------------+
   /                                                               /
   /                    Recommended Cache Time                     /
   /                                                               /
   +---------------+---------------+---------------+---------------+

                Figure 12: Recommended Cache Time Encoding







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3.4.3.  Message Hash

   Within a trusted domain, an operator may calculate the message hash
   at a border device and insert that value into the hop-by-hop headers
   of a message.  An egress device should remove the value.  This
   permits intermediate devices within that trusted domain to match
   against a ContentObjectHashRestriction without calculating it at
   every hop.

   The message hash is a cryptographic hash from the start of the CCNx
   Message TLV to the end of the packet.  It is used to match against
   the ContentObjectHashRestriction (Section 3.6.2.1.2).  The Message
   Hash may be of longer length than an Interest's restriction, in which
   case the device should use the left bytes of the Message Hash to
   check against the Interest's value.

   The Message Hash may only carry one hash type and there may only be
   one Message Hash header.

   The Message Hash header is unprotected, so this header is only of
   practical use within a trusted domain, such as an operator's
   autonomous system.

                       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_MSGHASH            |         (length + 4)          |
   +---------------+---------------+---------------+---------------+
   |          hash type            |            length             |
   +---------------+---------------+---------------+---------------+
   /                           hash value                          /
   +---------------+---------------+---------------+---------------+

                      Figure 13: Message Hash Header

















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3.5.  Top-Level Types

   The top-level TLV types listed below exist at the outermost level of
   a CCNx Message TLV.

   +----------------------+------------+-------------------------------+
   |        Abbrev        |    Name    | Description                   |
   +----------------------+------------+-------------------------------+
   |      T_INTEREST      |  Interest  | An Interest MessageType.      |
   |                      |  (Section  |                               |
   |                      |    3.6)    |                               |
   |                      |            |                               |
   |       T_OBJECT       |  Content   | A Content Object MessageType  |
   |                      |   Object   |                               |
   |                      |  (Section  |                               |
   |                      |    3.6)    |                               |
   |                      |            |                               |
   |   T_VALIDATION_ALG   | Validation | The method of message         |
   |                      | Algorithm  | verification such as a        |
   |                      |  (Section  | Message Integrity Check       |
   |                      |  3.6.4.1)  | (MIC), Message Authentication |
   |                      |            | Code (MAC), or cryptographic  |
   |                      |            | signature.                    |
   |                      |            |                               |
   | T_VALIDATION_PAYLOAD | Validation | The validation output, such   |
   |                      |  Payload   | as the CRC32C code or the RSA |
   |                      |  (Section  | signature.                    |
   |                      |  3.6.4.2)  |                               |
   +----------------------+------------+-------------------------------+

                       Table 5: CCNx Top Level Types




















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3.6.  CCNx Message TLV

   This is the format for the CCNx Message itself.  The CCNx Message TLV
   is the portion of the CCNx Packet between the hop-by-hop headers and
   the Validation TLVs.  The figure below is an expansion of the "CCNx
   Message TLV" depicted in the beginning of Section 3.  The CCNx
   Message TLV begins with MessageType and runs through the optional
   Payload.  The same general format is used for both Interest and
   Content Object messages which are differentiated by the MessageType
   field.  The first enclosed TLV of a CCNx Message TLV is always the
   Name TLV, if present.  This is followed by an optional Message TLVs
   and an optional Payload TLV.

                        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
   +---------------+---------------+---------------+---------------+
   |         MessageType           |         MessageLength         |
   +---------------+---------------+---------------+---------------+
   / Name TLV       (Type = T_NAME)                                /
   +---------------+---------------+---------------+---------------+
   / Optional Message TLVs   (Various Types)                       /
   +---------------+---------------+---------------+---------------+
   / Optional Payload TLV  (Type = T_PAYLOAD)                      /
   +---------------+---------------+---------------+---------------+

                   Figure 14: CCNx Message TLV Encoding

   +-----------+---------------+---------------------------------------+
   |   Abbrev  |      Name     | Description                           |
   +-----------+---------------+---------------------------------------+
   |   T_NAME  | Name (Section | The CCNx Name requested in an         |
   |           |     3.6.1)    | Interest or published in a Content    |
   |           |               | Object.                               |
   |           |               |                                       |
   | T_PAYLOAD |    Payload    | The message payload.                  |
   |           |    (Section   |                                       |
   |           |     3.6.3)    |                                       |
   +-----------+---------------+---------------------------------------+

                      Table 6: CCNx Message TLV Types











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3.6.1.  Name

   A Name is a TLV encoded sequence of segments.  The table below lists
   the type values appropriate for these name segments.  A Name MUST NOT
   include Pad TLVs.

   As described in CCNx Semantics [RFC8569], using the CCNx URI
   [CCNxURI] notation, a T_NAME with zero length corresponds to "ccnx:/"
   (the default route).  The message grammar does not allow the first
   name segment to have zero length in a CCNx Message TLV Name.  In the
   TLV encoding, "ccnx:/" corresponds to T_NAME with zero length.

                        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_NAME             |            Length             |
   +---------------+---------------+---------------+---------------+
   / Name segment TLVs                                             /
   +---------------+---------------+---------------+---------------+

                         Figure 15: Name Encoding

   +---------------+-------------+-------------------------------------+
   | Symbolic Name |     Name    | Description                         |
   +---------------+-------------+-------------------------------------+
   | T_NAMESEGMENT |     Name    | A generic name segment.             |
   |               |   segment   |                                     |
   |               |   (Section  |                                     |
   |               |   3.6.1.1)  |                                     |
   |               |             |                                     |
   |     T_IPID    |   Interest  | An identifier that represents the   |
   |               |  Payload ID | Interest Payload field. As an       |
   |               |   (Section  | example, the Payload ID might be a  |
   |               |   3.6.1.2)  | hash of the Interest Payload.  This |
   |               |             | provides a way to differentiate     |
   |               |             | between Interests based on their    |
   |               |             | payloads without having to parse    |
   |               |             | all the bytes of the payload        |
   |               |             | itself, and instead using only this |
   |               |             | Payload ID name segment.            |
   |               |             |                                     |
   |   T_APP:00 -  | Application | Application-specific payload in a   |
   |   T_APP:4096  |  Components | name segment.  An application may   |
   |               |   (Section  | apply its own semantics to the 4096 |
   |               |   3.6.1.1)  | reserved types.                     |
   +---------------+-------------+-------------------------------------+

                         Table 7: CCNx Name Types



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3.6.1.1.  Name Segments

   4096 special application payload name segments are allocated.  These
   have application semantics applied to them.  A good convention is to
   put the application's identity in the name prior to using these name
   segments.

   For example, a name like "ccnx:/foo/bar/hi" would be encoded as:

                        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_NAME)           |           0x14 (20)           |
   +---------------+---------------+---------------+---------------+
   |        (T_NAME_SEGMENT)       |           0x03 (3)            |
   +---------------+---------------+---------------+---------------+
   |       f                o               o      |(T_NAME_SEGMENT)
   +---------------+---------------+---------------+---------------+
   |               |            0x03 (3)           |       b       |
   +---------------+---------------+---------------+---------------+
   |      a                r       |           (T_NAME_SEGMENT)    |
   +---------------+---------------+---------------+---------------+
   |           0x02 (2)            |       h       |       i       |
   +---------------+---------------+---------------+---------------+

                     Figure 16: Name Encoding Example

3.6.1.2.  Interest Payload ID

   The InterestPayloadID is a name segment created by the origin of an
   Interest to represent the Interest Payload.  This allows the proper
   multiplexing of Interests based on their name if they have different
   payloads.  A common representation is to use a hash of the Interest
   Payload as the InterestPayloadID.

   As part of the Value of the TLV, the InterestPayloadID contains a
   one-octet identifier of the method used to create the
   InterestPayloadID followed by a variable-length octet string.  An
   implementation is not required to implement any of the methods to
   receive an Interest; the InterestPayloadID may be treated only as an
   opaque octet string for the purposes of multiplexing Interests with
   different payloads.  Only a device creating an InterestPayloadID name
   segment or a device verifying such a segment needs to implement the
   algorithms.

   It uses the encoding of hash values specified in Section 3.3.3.





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   In normal operations, we recommend displaying the InterestPayloadID
   as an opaque octet string in a CCNx URI, as this is the common
   denominator for implementation parsing.

   The InterestPayloadID, even if it is a hash, should not convey any
   security context.  If a system requires confirmation that a specific
   entity created the InterestPayload, it should use a cryptographic
   signature on the Interest via the ValidationAlgorithm and
   ValidationPayload or use its own methods inside the Interest Payload.

3.6.2.  Message TLVs

   Each message type (Interest or Content Object) is associated with a
   set of optional Message TLVs.  Additional specification documents may
   extend the types associated with each.

3.6.2.1.  Interest Message TLVs

   There are two Message TLVs currently associated with an Interest
   message: the KeyIdRestriction selector and the ContentObjectHashRestr
   selector are used to narrow the universe of acceptable Content
   Objects that would satisfy the Interest.

                        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
   +---------------+---------------+---------------+---------------+
   |         MessageType           |         MessageLength         |
   +---------------+---------------+---------------+---------------+
   | Name TLV                                                      |
   +---------------+---------------+---------------+---------------+
   / Optional KeyIdRestriction TLV                                 /
   +---------------------------------------------------------------+
   / Optional ContentObjectHashRestriction TLV                     /
   +---------------------------------------------------------------+

                     Figure 17: Interest Message TLVs















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   +----------------+------------------------------+-------------------+
   |     Abbrev     |             Name             | Description       |
   +----------------+------------------------------+-------------------+
   |  T_KEYIDRESTR  |  KeyIdRestriction (Section   | A representation  |
   |                |          3.6.2.1.1)          | (as per Section   |
   |                |                              | 3.3.3) of the     |
   |                |                              | KeyId             |
   |                |                              |                   |
   | T_OBJHASHRESTR | ContentObjectHashRestriction | A representation  |
   |                |     (Section 3.6.2.1.2)      | (as per Section   |
   |                |                              | 3.3.3) of the     |
   |                |                              | hash of the       |
   |                |                              | specific Content  |
   |                |                              | Object that would |
   |                |                              | satisfy the       |
   |                |                              | Interest.         |
   +----------------+------------------------------+-------------------+

                 Table 8: CCNx Interest Message TLV Types

3.6.2.1.1.  KeyIdRestriction

   An Interest MAY include a KeyIdRestriction selector.  This ensures
   that only Content Objects with matching KeyIds will satisfy the
   Interest.  See Section 3.6.4.1.4.1 for the format of a KeyId.

3.6.2.1.2.  ContentObjectHashRestriction

   An Interest MAY contain a ContentObjectHashRestriction selector.
   This is the hash of the Content Object -- the self-certifying name
   restriction that must be verified in the network, if an Interest
   carried this restriction (see Message Hash (Section 3.4.3)).  The
   LENGTH MUST be from one of the allowed values for that hash (see
   Section 3.3.3).

   The ContentObjectHashRestriction SHOULD be of type T_SHA-256 and of
   length 32 bytes.














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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_OBJHASHRESTR         |           (LENGTH+4)          |
   +---------------+---------------+---------------+---------------+
   |           hash type           |             LENGTH            |
   +---------------+---------------+---------------+---------------+
   /                     LENGTH octets of hash                     /
   +---------------+---------------+---------------+---------------+

             Figure 18: ContentObjectHashRestriction Encoding

3.6.2.2.  Content Object Message TLVs

   The following message TLVs are currently defined for Content Objects:
   PayloadType (optional) and ExpiryTime (optional).

                        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
   +---------------+---------------+---------------+---------------+
   |         MessageType           |         MessageLength         |
   +---------------+---------------+---------------+---------------+
   | Name TLV                                                      |
   +---------------+---------------+---------------+---------------+
   / Optional PayloadType TLV                                      /
   +---------------------------------------------------------------+
   / Optional ExpiryTime TLV                                       /
   +---------------------------------------------------------------+

                  Figure 19: Content Object Message TLVs

   +-------------+-------------+---------------------------------------+
   |    Abbrev   |     Name    | Description                           |
   +-------------+-------------+---------------------------------------+
   | T_PAYLDTYPE | PayloadType | Indicates the type of Payload         |
   |             |   (Section  | contents.                             |
   |             |  3.6.2.2.1) |                                       |
   |             |             |                                       |
   |   T_EXPIRY  |  ExpiryTime | The time at which the Payload         |
   |             |   (Section  | expires, as expressed in the number   |
   |             |  3.6.2.2.2) | of milliseconds since the epoch in    |
   |             |             | UTC.  If missing, Content Object may  |
   |             |             | be used as long as desired.           |
   +-------------+-------------+---------------------------------------+

              Table 9: CCNx Content Object Message TLV Types





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3.6.2.2.1.  PayloadType

   The PayloadType is an octet representing the general type of the
   Payload TLV.

   o  T_PAYLOADTYPE_DATA: Data (possibly encrypted)

   o  T_PAYLOADTYPE_KEY: Key

   o  T_PAYLOADTYPE_LINK: Link

   The Data type indicates that the Payload of the ContentObject is
   opaque application bytes.  The Key type indicates that the Payload is
   a DER-encoded public key.  The Link type indicates that the Payload
   is one or more Links (Section 3.3.4).  If this field is missing, a
   Data type is assumed.

                        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_PAYLDTYPE        |               1               |
   +---------------+---------------+---------------+---------------+
   |  PayloadType  |
   +---------------+

                      Figure 20: PayloadType Encoding

3.6.2.2.2.  ExpiryTime

   The ExpiryTime is the time at which the Payload expires, as expressed
   by a timestamp containing the number of milliseconds since the epoch
   in UTC.  It is a network byte order unsigned integer in a 64-bit
   field.  A cache or end system should not respond with a Content
   Object past its ExpiryTime.  Routers forwarding a Content Object do
   not need to check the ExpiryTime.  If the ExpiryTime field is
   missing, the Content Object has no expressed expiration, and a cache
   or end system may use the Content Object for as long as desired.

                        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_EXPIRY            |               8               |
   +---------------+---------------+---------------+---------------+
   /                          ExpiryTime                           /
   /                                                               /
   +---------------+---------------+---------------+---------------+

                      Figure 21: ExpiryTime encoding



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3.6.3.  Payload

   The Payload TLV contains the content of the packet.  It MAY be of
   zero length.  If a packet does not have any payload, this field
   SHOULD be omitted, rather than being of zero length.

                        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_PAYLOAD           |            Length             |
   +---------------+---------------+---------------+---------------+
   /                        Payload Contents                       /
   +---------------+---------------+---------------+---------------+

                        Figure 22: Payload Encoding

3.6.4.  Validation

   Both Interests and Content Objects have the option to include
   information about how to validate the CCNx Message.  This information
   is contained in two TLVs: the ValidationAlgorithm TLV and the
   ValidationPayload TLV.  The ValidationAlgorithm TLV specifies the
   mechanism to be used to verify the CCNx Message.  Examples include
   verification with a Message Integrity Check (MIC), a Message
   Authentication Code (MAC), or a cryptographic signature.  The
   ValidationPayload TLV contains the validation output, such as the
   CRC32C code or the RSA signature.

   An Interest would most likely only use a MIC type of validation -- a
   CRC, checksum, or digest.

3.6.4.1.  Validation Algorithm

   The ValidationAlgorithm is a set of nested TLVs containing all of the
   information needed to verify the message.  The outermost container
   has type = T_VALIDATION_ALG.  The first nested TLV defines the
   specific type of validation to be performed on the message.  The type
   is identified with the "ValidationType" as shown in the figure below
   and elaborated in the table below.  Nested within that container are
   the TLVs for any ValidationType-dependent data -- for example, a Key
   Id, Key Locator, etc.

   Complete examples of several types may be found in Section 3.6.4.1.5.








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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_VALIDATION_ALG        |      ValidationAlgLength      |
   +---------------+---------------+---------------+---------------+
   |        ValidationType         |            Length             |
   +---------------+---------------+---------------+---------------+
   / ValidationType-dependent data                                 /
   +---------------+---------------+---------------+---------------+

                 Figure 23: Validation Algorithm Encoding

   +-----------------+---------------+---------------------------------+
   |      Abbrev     |      Name     | Description                     |
   +-----------------+---------------+---------------------------------+
   |     T_CRC32C    |     CRC32C    | Castagnoli CRC32 (iSCSI, ext4,  |
   |                 |    (Section   | etc.) with normal form          |
   |                 |   3.6.4.1.1)  | polynomial 0x1EDC6F41.          |
   |                 |               |                                 |
   |  T_HMAC-SHA256  |  HMAC-SHA256  | HMAC (RFC 2104) using SHA256    |
   |                 |    (Section   | hash.                           |
   |                 |   3.6.4.1.2)  |                                 |
   |                 |               |                                 |
   |   T_RSA-SHA256  |   RSA-SHA256  | RSA public-key signature using  |
   |                 |    (Section   | SHA256 digest.                  |
   |                 |   3.6.4.1.3)  |                                 |
   |                 |               |                                 |
   | T_EC-SECP-256K1 |   SECP-256K1  | Elliptic Curve signature with   |
   |                 |    (Section   | SECP-256K1 parameters (see      |
   |                 |   3.6.4.1.3)  | [ECC]).                         |
   |                 |               |                                 |
   | T_EC-SECP-384R1 |   SECP-384R1  | Elliptic Curve signature with   |
   |                 |    (Section   | SECP-384R1 parameters (see      |
   |                 |   3.6.4.1.3)  | [ECC]).                         |
   +-----------------+---------------+---------------------------------+

                      Table 10: CCNx Validation Types

3.6.4.1.1.  Message Integrity Checks

   MICs do not require additional data in order to perform the
   verification.  An example is CRC32C that has a zero-length value.









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3.6.4.1.2.  Message Authentication Codes

   MACs are useful for communication between two trusting parties who
   have already shared secret keys.  An example is the HMAC algorithm.
   A MAC uses the KeyId field to identify which shared secret is in use.
   The meaning of the KeyId is specific to the two parties involved and
   could be simply an integer to enumerate keys.  If a new MAC requires
   an additional field, such as an Initialization Vector, that field
   would need to be defined as part of the updated specification.

3.6.4.1.3.  Signature

   Signature type Validators specify a digest mechanism and a signing
   algorithm to verify the message.  Examples include an RSA signature
   on a SHA256 digest, an Elliptic Curve signature with SECP-256K1
   parameters, etc.  These Validators require a KeyId and a mechanism
   for locating the publisher's public key (a KeyLocator) -- and
   optionally a PublicKey or Certificate or KeyLink.

3.6.4.1.4.  Validation-Dependent Data

   Different Validation Algorithms require access to different pieces of
   data contained in the ValidationAlgorithm TLV.  As described above,
   Key Ids, Key Locators, Public Keys, Certificates, Links, and Key
   Names all play a role in different Validation Algorithms.  Any number
   of Validation-Dependent Data containers can be present in a
   Validation Algorithm TLV.
























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   Below is a table of CCNx ValidationType-dependent data types:

   +-------------+-----------------+-----------------------------------+
   |    Abbrev   |       Name      | Description                       |
   +-------------+-----------------+-----------------------------------+
   |   T_KEYID   |   SignerKeyId   | An identifier of the shared       |
   |             |     (Section    | secret or public key associated   |
   |             |   3.6.4.1.4.1)  | with a MAC or Signature.          |
   |             |                 |                                   |
   | T_PUBLICKEY |    Public Key   | DER-encoded public key.           |
   |             |     (Section    |                                   |
   |             |   3.6.4.1.4.2)  |                                   |
   |             |                 |                                   |
   |    T_CERT   |   Certificate   | DER-encoded X.509 certificate.    |
   |             |     (Section    |                                   |
   |             |   3.6.4.1.4.3)  |                                   |
   |             |                 |                                   |
   |  T_KEYLINK  |     KeyLink     | A CCNx Link object.               |
   |             |     (Section    |                                   |
   |             |   3.6.4.1.4.4)  |                                   |
   |             |                 |                                   |
   |  T_SIGTIME  |  SignatureTime  | A millisecond timestamp           |
   |             |     (Section    | indicating the time when the      |
   |             |   3.6.4.1.4.5)  | signature was created.            |
   +-------------+-----------------+-----------------------------------+

              Table 11: CCNx Validation-Dependent Data Types

3.6.4.1.4.1.  KeyId

   The KeyId for a signature is the publisher key identifier.  It is
   similar to a Subject Key Identifier from X.509 (see Section 4.2.1.2
   of [RFC5280]).  It should be derived from the key used to sign, such
   as from the SHA-256 hash of the key.  It applies to both public and
   private key systems and to symmetric key systems.

   The KeyId is represented using the hash format in Section 3.3.3.  If
   an application protocol uses a non-hash identifier, it should use one
   of the reserved values.












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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_KEYID            |            LENGTH+4           |
   +---------------+---------------+---------------+---------------+
   |          <hash type>          |             LENGTH            |
   +---------------+---------------+---------------+---------------+
   /                     LENGTH octets of hash                     /
   +---------------+---------------+---------------+---------------+

                         Figure 24: KeyId Encoding

3.6.4.1.4.2.  Public Key

   A Public Key is a DER-encoded Subject Public Key Info block, as in an
   X.509 certificate.

                        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_PUBLICKEY          |            Length             |
   +---------------+---------------+---------------+---------------+
   /                Public Key (DER-encoded SPKI)                  /
   +---------------+---------------+---------------+---------------+

                      Figure 25: Public Key Encoding

3.6.4.1.4.3.  Certificate

   A Certificate is a DER-encoded X.509 certificate.  The KeyId
   (Section 3.6.4.1.4.1) is derived from this encoding.

                        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_CERT             |            Length             |
   +---------------+---------------+---------------+---------------+
   /                 Certificate (DER-encoded X.509)               /
   +---------------+---------------+---------------+---------------+

                      Figure 26: Certificate Encoding










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3.6.4.1.4.4.  KeyLink

   A KeyLink type KeyLocator is a Link.

   The KeyLink ContentObjectHashRestr, if included, is the digest of the
   Content Object identified by KeyLink, not the digest of the public
   key.  Likewise, the KeyIdRestr of the KeyLink is the KeyId of the
   ContentObject, not necessarily of the wrapped key.

                        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_KEYLINK            |            Length             |
   +---------------+---------------+-------------------------------+
   / Link                                                          /
   +---------------------------------------------------------------+

                        Figure 27: KeyLink Encoding

3.6.4.1.4.5.  SignatureTime

   The SignatureTime is a millisecond timestamp indicating the time at
   which a signature was created.  The signer sets this field to the
   current time when creating a signature.  A verifier may use this time
   to determine whether or not the signature was created during the
   validity period of a key, or if it occurred in a reasonable sequence
   with other associated signatures.  The SignatureTime is unrelated to
   any time associated with the actual CCNx Message, which could have
   been created long before the signature.  The default behavior is to
   always include a SignatureTime when creating an authenticated message
   (e.g., HMAC or RSA).

   SignatureTime is an unsigned integer in network byte order that
   indicates when the signature was created (as the number of
   milliseconds since the epoch in UTC).  It is a fixed 64-bit field.

                        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_SIGTIME           |               8               |
   +---------------+---------------+-------------------------------+
   /                         SignatureTime                         /
   +---------------------------------------------------------------+

                     Figure 28: SignatureTime Encoding






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3.6.4.1.5.  Validation Examples

   As an example of a MIC-type validation, the encoding for CRC32C
   validation would be:

                        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_VALIDATION_ALG         |               4               |
   +---------------+---------------+---------------+---------------+
   |            T_CRC32C           |               0               |
   +---------------+---------------+---------------+---------------+

                    Figure 29: CRC32C Encoding Example

   As an example of a MAC-type validation, the encoding for an HMAC
   using a SHA256 hash would be:

                        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_VALIDATION_ALG        |               40              |
   +---------------+---------------+---------------+---------------+
   |        T_HMAC-SHA256          |               36              |
   +---------------+---------------+---------------+---------------+
   |             T_KEYID           |               32              |
   +---------------+---------------+---------------+---------------+
   /                            KeyId                              /
   /---------------+---------------+-------------------------------+

                  Figure 30: HMAC-SHA256 Encoding Example




















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   As an example of a Signature-type validation, the encoding for an RSA
   public-key signature using a SHA256 digest and Public Key would be:

                        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_VALIDATION_ALG        |   44 octets + Variable Length |
   +---------------+---------------+---------------+---------------+
   |          T_RSA-SHA256         |   40 octets + Variable Length |
   +---------------+---------------+---------------+---------------+
   |             T_KEYID           |               32              |
   +---------------+---------------+---------------+---------------+
   /                            KeyId                              /
   /---------------+---------------+-------------------------------+
   |          T_PUBLICKEY          |  Variable Length (~160 octets)|
   +---------------+---------------+---------------+---------------+
   /                Public Key (DER-encoded SPKI)                  /
   +---------------+---------------+---------------+---------------+

                  Figure 31: RSA-SHA256 Encoding Example

3.6.4.2.  Validation 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
   +---------------+---------------+---------------+---------------+
   |     T_VALIDATION_PAYLOAD      |  ValidationPayloadLength      |
   +---------------+---------------+---------------+---------------+
   / Type-dependent data                                           /
   +---------------+---------------+---------------+---------------+

                  Figure 32: Validation Payload Encoding

   The ValidationPayload contains the validation output, such as the
   CRC32C code or the RSA signature.
















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4.  IANA Considerations

   This section details each kind of CCNx protocol value that can be
   registered.  Each type registry can be updated by incrementally
   expanding the type space, i.e., by allocating and reserving new
   types.  As per [RFC8126], this section details the creation of the
   "Content-Centric Networking (CCNx)" registry and several
   subregistries.

4.1.  Packet Type Registry

   IANA has created the "CCNx Packet Types" registry and allocated the
   packet types described below.  The registration procedure is RFC
   Required.  The Type value is 1 octet.  The range is 0x00-0xFF.

         +------+-------------+----------------------------------+
         | Type |     Name    |            Reference             |
         +------+-------------+----------------------------------+
         | 0x00 | PT_INTEREST | Fixed Header Types (Section 3.2) |
         |      |             |                                  |
         | 0x01 |  PT_CONTENT | Fixed Header Types (Section 3.2) |
         |      |             |                                  |
         | 0x02 |  PT_RETURN  | Fixed Header Types (Section 3.2) |
         +------+-------------+----------------------------------+

                               Packet Types

























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4.2.  Interest Return Code Registry

   IANA has created the "CCNx Interest Return Code Types" registry and
   allocated the Interest Return code types described below.  The
   registration procedure is Specification Required.  The Type value is
   1 octet.  The range is 0x00-0xFF.

   +------+---------------------------------------+--------------------+
   | Type |                  Name                 |     Reference      |
   +------+---------------------------------------+--------------------+
   | 0x00 |                Reserved               |                    |
   |      |                                       |                    |
   | 0x01 |           T_RETURN_NO_ROUTE           | Fixed Header Types |
   |      |                                       | (Section 3.2.3.3)  |
   |      |                                       |                    |
   | 0x02 |        T_RETURN_LIMIT_EXCEEDED        | Fixed Header Types |
   |      |                                       | (Section 3.2.3.3)  |
   |      |                                       |                    |
   | 0x03 |         T_RETURN_NO_RESOURCES         | Fixed Header Types |
   |      |                                       | (Section 3.2.3.3)  |
   |      |                                       |                    |
   | 0x04 |          T_RETURN_PATH_ERROR          | Fixed Header Types |
   |      |                                       | (Section 3.2.3.3)  |
   |      |                                       |                    |
   | 0x05 |          T_RETURN_PROHIBITED          | Fixed Header Types |
   |      |                                       | (Section 3.2.3.3)  |
   |      |                                       |                    |
   | 0x06 |           T_RETURN_CONGESTED          | Fixed Header Types |
   |      |                                       | (Section 3.2.3.3)  |
   |      |                                       |                    |
   | 0x07 |         T_RETURN_MTU_TOO_LARGE        | Fixed Header Types |
   |      |                                       | (Section 3.2.3.3)  |
   |      |                                       |                    |
   | 0x08 | T_RETURN_UNSUPPORTED_HASH_RESTRICTION | Fixed Header Types |
   |      |                                       | (Section 3.2.3.3)  |
   |      |                                       |                    |
   | 0x09 |      T_RETURN_MALFORMED_INTEREST      | Fixed Header Types |
   |      |                                       | (Section 3.2.3.3)  |
   +------+---------------------------------------+--------------------+

                        CCNx Interest Return Types










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4.3.  Hop-by-Hop Type Registry

   IANA has created the "CCNx Hop-by-Hop Types" registry and allocated
   the hop-by-hop types described below.  The registration procedure is
   RFC Required.  The Type value is 2 octets.  The range is
   0x0000-0xFFFF.

   +---------------+-------------+-------------------------------------+
   |      Type     |     Name    |              Reference              |
   +---------------+-------------+-------------------------------------+
   |     0x0000    |   Reserved  |                                     |
   |               |             |                                     |
   |     0x0001    |  T_INTLIFE  |   Hop-by-hop TLV headers (Section   |
   |               |             |                 3.4)                |
   |               |             |                                     |
   |     0x0002    | T_CACHETIME |   Hop-by-hop TLV headers (Section   |
   |               |             |                 3.4)                |
   |               |             |                                     |
   |     0x0003    |  T_MSGHASH  |   Hop-by-hop TLV headers (Section   |
   |               |             |                 3.4)                |
   |               |             |                                     |
   |    0x0004 -   |   Reserved  |                                     |
   |     0x0007    |             |                                     |
   |               |             |                                     |
   |     0x0FFE    |    T_PAD    |         Pad (Section 3.3.1)         |
   |               |             |                                     |
   |     0x0FFF    |    T_ORG    | Organization-Specific TLVs (Section |
   |               |             |                3.3.2)               |
   |               |             |                                     |
   | 0x1000-0x1FFF |   Reserved  |     Experimental Use (Section 3)    |
   +---------------+-------------+-------------------------------------+

                           CCNx Hop-by-Hop Types


















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4.4.  Top-Level Type Registry

   IANA has created the "CCNx Top-Level Types" registry and allocated
   the top-level types described below.  The registration procedure is
   RFC Required.  The Type value is 2 octets.  The range is
   0x0000-0xFFFF.

     +--------+----------------------+-------------------------------+
     |  Type  |         Name         |           Reference           |
     +--------+----------------------+-------------------------------+
     | 0x0000 |       Reserved       |                               |
     |        |                      |                               |
     | 0x0001 |      T_INTEREST      | Top-Level Types (Section 3.5) |
     |        |                      |                               |
     | 0x0002 |       T_OBJECT       | Top-Level Types (Section 3.5) |
     |        |                      |                               |
     | 0x0003 |   T_VALIDATION_ALG   | Top-Level Types (Section 3.5) |
     |        |                      |                               |
     | 0x0004 | T_VALIDATION_PAYLOAD | Top-Level Types (Section 3.5) |
     +--------+----------------------+-------------------------------+

                           CCNx Top-Level Types





























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4.5.  Name Segment Type Registry

   IANA has created the "CCNx Name Segment Types" registry and allocated
   the name segment types described below.  The registration procedure
   is Specification Required.  The Type value is 2 octets.  The range is
   0x0000-0xFFFF.

   +--------------+------------------+---------------------------------+
   |     Type     |       Name       |            Reference            |
   +--------------+------------------+---------------------------------+
   |    0x0000    |     Reserved     |                                 |
   |              |                  |                                 |
   |    0x0001    |  T_NAMESEGMENT   |       Name (Section 3.6.1)      |
   |              |                  |                                 |
   |    0x0002    |      T_IPID      |       Name (Section 3.6.1)      |
   |              |                  |                                 |
   |   0x0010 -   |     Reserved     |             RFC 8609            |
   |    0x0013    |                  |                                 |
   |              |                  |                                 |
   |    0x0FFF    |      T_ORG       |    Organization-Specific TLVs   |
   |              |                  |         (Section 3.3.2)         |
   |              |                  |                                 |
   |   0x1000 -   |    T_APP:00 -    | Application Components (Section |
   |    0x1FFF    |    T_APP:4096    |              3.6.1)             |
   +--------------+------------------+---------------------------------+

                          CCNx Name Segment Types

4.6.  Message Type Registry

   IANA has created the "CCNx Message Types" registry and registered the
   message segment types described below.  The registration procedure is
   RFC Required.  The Type value is 2 octets.  The range is
   0x0000-0xFFFF.

















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   +---------------+----------------+----------------------------------+
   |      Type     |      Name      |            Reference             |
   +---------------+----------------+----------------------------------+
   |     0x0000    |     T_NAME     |   Message Types (Section 3.6)    |
   |               |                |                                  |
   |     0x0001    |   T_PAYLOAD    |   Message Types (Section 3.6)    |
   |               |                |                                  |
   |     0x0002    |  T_KEYIDRESTR  |   Message Types (Section 3.6)    |
   |               |                |                                  |
   |     0x0003    | T_OBJHASHRESTR |   Message Types (Section 3.6)    |
   |               |                |                                  |
   |     0x0005    |  T_PAYLDTYPE   |   Content Object Message Types   |
   |               |                |        (Section 3.6.2.2)         |
   |               |                |                                  |
   |     0x0006    |    T_EXPIRY    |   Content Object Message Types   |
   |               |                |        (Section 3.6.2.2)         |
   |               |                |                                  |
   |    0x0007 -   |    Reserved    |             RFC 8609             |
   |     0x000C    |                |                                  |
   |               |                |                                  |
   |     0x0FFE    |     T_PAD      |       Pad (Section 3.3.1)        |
   |               |                |                                  |
   |     0x0FFF    |     T_ORG      |    Organization-Specific TLVs    |
   |               |                |         (Section 3.3.2)          |
   |               |                |                                  |
   | 0x1000-0x1FFF |    Reserved    |   Experimental Use (Section 3)   |
   +---------------+----------------+----------------------------------+

                            CCNx Message Types

4.7.  Payload Type Registry

   IANA has created the "CCNx Payload Types" registry and allocated the
   payload types described below.  The registration procedure is
   Specification Required.  The Type value is 1 octet.  The range is
   0x00-0xFF.

     +------+--------------------+-----------------------------------+
     | Type |        Name        |             Reference             |
     +------+--------------------+-----------------------------------+
     | 0x00 | T_PAYLOADTYPE_DATA | Payload Types (Section 3.6.2.2.1) |
     |      |                    |                                   |
     | 0x01 | T_PAYLOADTYPE_KEY  | Payload Types (Section 3.6.2.2.1) |
     |      |                    |                                   |
     | 0x02 | T_PAYLOADTYPE_LINK | Payload Types (Section 3.6.2.2.1) |
     +------+--------------------+-----------------------------------+

                            CCNx Payload Types



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4.8.  Validation Algorithm Type Registry

   IANA has created the "CCNx Validation Algorithm Types" registry and
   allocated the validation algorithm types described below.  The
   registration procedure is Specification Required.  The Type value is
   2 octets.  The range is 0x0000-0xFFFF.

   +---------------+-----------------+---------------------------------+
   |      Type     |       Name      |            Reference            |
   +---------------+-----------------+---------------------------------+
   |     0x0000    |     Reserved    |                                 |
   |               |                 |                                 |
   |     0x0002    |     T_CRC32C    |  Validation Algorithm (Section  |
   |               |                 |             3.6.4.1)            |
   |               |                 |                                 |
   |     0x0004    |  T_HMAC-SHA256  |  Validation Algorithm (Section  |
   |               |                 |             3.6.4.1)            |
   |               |                 |                                 |
   |     0x0005    |   T_RSA-SHA256  |  Validation Algorithm (Section  |
   |               |                 |             3.6.4.1)            |
   |               |                 |                                 |
   |     0x0006    | T_EC-SECP-256K1 |  Validation Algorithm (Section  |
   |               |                 |             3.6.4.1)            |
   |               |                 |                                 |
   |     0x0007    | T_EC-SECP-384R1 |  Validation Algorithm (Section  |
   |               |                 |             3.6.4.1)            |
   |               |                 |                                 |
   |     0x0FFE    |      T_PAD      |       Pad (Section 3.3.1)       |
   |               |                 |                                 |
   |     0x0FFF    |      T_ORG      |    Organization-Specific TLVs   |
   |               |                 |         (Section 3.3.2)         |
   |               |                 |                                 |
   | 0x1000-0x1FFF |     Reserved    |   Experimental Use (Section 3)  |
   +---------------+-----------------+---------------------------------+

                      CCNx Validation Algorithm Types















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4.9.  Validation-Dependent Data Type Registry

   IANA has created the "CCNx Validation-Dependent Data Types" registry
   and allocated the validation-dependent data types described below.
   The registration procedure is RFC Required.  The Type value is 2
   octets.  The range is 0x0000-0xFFFF.

   +---------------+----------------+----------------------------------+
   |      Type     |      Name      |            Reference             |
   +---------------+----------------+----------------------------------+
   |     0x0000    |    Reserved    |                                  |
   |               |                |                                  |
   |     0x0009    |    T_KEYID     |    Validation-Dependent Data     |
   |               |                |       (Section 3.6.4.1.4)        |
   |               |                |                                  |
   |     0x000A    | T_PUBLICKEYLOC |    Validation-Dependent Data     |
   |               |                |       (Section 3.6.4.1.4)        |
   |               |                |                                  |
   |     0x000B    |  T_PUBLICKEY   |    Validation-Dependent Data     |
   |               |                |       (Section 3.6.4.1.4)        |
   |               |                |                                  |
   |     0x000C    |     T_CERT     |    Validation-Dependent Data     |
   |               |                |       (Section 3.6.4.1.4)        |
   |               |                |                                  |
   |     0x000D    |     T_LINK     |    Validation-Dependent Data     |
   |               |                |       (Section 3.6.4.1.4)        |
   |               |                |                                  |
   |     0x000E    |   T_KEYLINK    |    Validation-Dependent Data     |
   |               |                |       (Section 3.6.4.1.4)        |
   |               |                |                                  |
   |     0x000F    |   T_SIGTIME    |    Validation-Dependent Data     |
   |               |                |       (Section 3.6.4.1.4)        |
   |               |                |                                  |
   |     0x0FFF    |     T_ORG      |    Organization-Specific TLVs    |
   |               |                |         (Section 3.3.2)          |
   |               |                |                                  |
   | 0x1000-0x1FFF |    Reserved    |   Experimental Use (Section 3)   |
   +---------------+----------------+----------------------------------+

                   CCNx Validation-Dependent Data Types

4.10.  Hash Function Type Registry

   IANA has created the "CCNx Hash Function Types" registry and
   allocated the hash function types described below.  The registration
   procedure is Specification Required.  The Type value is 2 octets.
   The range is 0x0000-0xFFFF.




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   +---------------+-----------+---------------------------------------+
   |      Type     |    Name   |               Reference               |
   +---------------+-----------+---------------------------------------+
   |     0x0000    |  Reserved |                                       |
   |               |           |                                       |
   |     0x0001    | T_SHA-256 |      Hash Format (Section 3.3.3)      |
   |               |           |                                       |
   |     0x0002    | T_SHA-512 |      Hash Format (Section 3.3.3)      |
   |               |           |                                       |
   |     0x0FFF    |   T_ORG   |  Organization-Specific TLVs (Section  |
   |               |           |                 3.3.2)                |
   |               |           |                                       |
   | 0x1000-0x1FFF |  Reserved |      Experimental Use (Section 3)     |
   +---------------+-----------+---------------------------------------+

                         CCNx Hash Function Types

5.  Security Considerations

   The CCNx protocol is a Layer 3 network protocol, which may also
   operate as an overlay using other transports such as UDP or other
   tunnels.  It includes intrinsic support for message authentication
   via a signature (e.g., RSA or elliptic curve) or Message
   Authentication Code (e.g., HMAC).  In lieu of an authenticator, it
   may instead use a Message Integrity Check (e.g., SHA or CRC).  CCNx
   does not specify an encryption envelope; that function is left to a
   high-layer protocol (e.g., Encrypted Sessions in CCNx [esic]).

   The CCNx Packet format includes the ability to attach MICs (e.g.,
   SHA-256 or CRC), MACs (e.g., HMAC), and Signatures (e.g., RSA or
   ECDSA) to all packet types.  Because Interest packets can be sent at
   will, an application should carefully select when to use a given
   ValidationAlgorithm in an Interest to avoid DoS attacks.  MICs, for
   example, are inexpensive and could be used as desired, whereas MACs
   and Signatures are more expensive and their inappropriate use could
   open a computational DoS attack surface.  Applications should use an
   explicit protocol to guide their use of packet signatures.  As a
   general guideline, an application might use a MIC on an Interest to
   detect unintentionally corrupted packets.  If one wishes to secure an
   Interest, one should consider using an encrypted wrapper and a
   protocol that prevents replay attacks, especially if the Interest is
   being used as an actuator.  Simply using an authentication code or
   signature does not make an Interest secure.  There are several
   examples in the literature on how to secure ICN-style messaging
   [mobile] [ace].






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   As a Layer 3 protocol, this document does not describe how one
   arrives at keys or how one trusts keys.  The CCNx content object may
   include a public key embedded in the object or may use the
   PublicKeyLocator field to point to a public key (or public-key
   certificate) that authenticates the message.  One key exchange
   specification is CCNxKE [ccnxke] [mobile], which is similar to the
   TLS 1.3 key exchange except it is over the CCNx Layer 3 messages.
   Trust is beyond the scope of a Layer 3 protocol and is left to
   applications or application frameworks.

   The combination of an ephemeral key exchange (e.g., CCNxKE [ccnxke])
   and an encapsulating encryption (e.g., [esic]) provides the
   equivalent of a TLS tunnel.  Intermediate nodes may forward the
   Interests and Content Objects but have no visibility inside.  It also
   completely hides the internal names in those used by the encryption
   layer.  This type of tunneling encryption is useful for content that
   has little or no cacheability, as it can only be used by someone with
   the ephemeral key.  Short-term caching may help with lossy links or
   mobility, but long-term caching is usually not of interest.

   Broadcast encryption or proxy re-encryption may be useful for content
   with multiple uses over time or many consumers.  There is currently
   no recommendation for this form of encryption.

   The specific encoding of messages will have security implications.
   This document uses a Type-Length-Value (TLV) encoding.  We chose to
   compromise between extensibility and unambiguous encodings of types
   and lengths.  Some TLV encodings use variable-length T and variable-
   length L fields to accommodate a wide gamut of values while trying to
   be byte efficient.  Our TLV encoding uses a fixed length 2-byte T and
   2-byte L.  Using fixed-length T and L fields solves two problems.
   The first is aliases.  If one is able to encode the same value, such
   as 0x02 and 0x0002, in different byte lengths, then one must decide
   if they mean the same thing, if they are different, or if one is
   illegal.  If they are different, then one must always compare on the
   buffers not the integer equivalents.  If one is illegal, then one
   must validate the TLV encoding -- every field of every packet at
   every hop.  If they are the same, then one has the second problem:
   how to specify packet filters.  For example, if a name has 6 name
   components, then there are 7 T fields and 7 L fields, each of which
   might have up to 4 representations of the same value.  That would be
   14 fields with 4 encodings each, or 1001 combinations.  It also means
   that one cannot compare, for example, a name via a memory function,
   as one needs to consider that any embedded T or L might have a
   different format.






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   The Interest Return message has no authenticator from the previous
   hop.  Therefore, the payload of the Interest Return should only be
   used locally to match an Interest.  A node should never forward that
   Interest payload as an Interest.  It should also verify that it sent
   the Interest in the Interest Return to that node and not allow anyone
   to negate Interest messages.

   Caching nodes must take caution when processing content objects.  It
   is essential that the Content Store obey the rules outlined in
   [RFC8569] to avoid certain types of attacks.  CCNx 1.0 has no
   mechanism to work around an undesired result from the network (there
   are no "excludes"), so if a cache becomes poisoned with bad content
   it might cause problems retrieving content.  There are three types of
   access to content from a Content Store: unrestricted, signature
   restricted, and hash restricted.  If an Interest has no restrictions,
   then the requester is not particular about what they get back, so any
   matching cached object is OK.  In the hash restricted case, the
   requester is very specific about what they want, and the Content
   Store (and every forward hop) can easily verify that the content
   matches the request.  In the signature restricted case (which is
   often used for initial manifest discovery), the requester only knows
   the KeyId that signed the content.  This case requires the closest
   attention in the Content Store to avoid amplifying bad data.  The
   Content Store must only respond with a content object if it can
   verify the signature -- this means either the content object carries
   the public key inside it or the Interest carries the public key in
   addition to the KeyId.  If that is not the case, then the Content
   Store should treat the Interest as a cache miss and let an endpoint
   respond.

   A user-level cache could perform full signature verification by
   fetching a public key according to the PublicKeyLocator.  However,
   that is not a burden we wish to impose on the forwarder.  A user-
   level cache could also rely on out-of-band attestation, such as the
   cache operator only inserting content that it knows has the correct
   signature.

   The CCNx grammar allows for hash algorithm agility via the HashType.
   It specifies a short list of acceptable hash algorithms that should
   be implemented at each forwarder.  Some hash values only apply to end
   systems, so updating the hash algorithm does not affect forwarders --
   they would simply match the buffer that includes the type-length-hash
   buffer.  Some fields, such as the ConObjHash, must be verified at
   each hop, so a forwarder (or related system) must know the hash
   algorithm, and it could cause backward compatibility problems if the
   hash type is updated.





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   A CCNx name uses binary matching, whereas a URI uses a case-
   insensitive hostname.  Some systems may also use case-insensitive
   matching of the URI path to a resource.  An implication of this is
   that human-entered CCNx names will likely have case or non-ASCII
   symbol mismatches unless one uses a consistent URI normalization for
   the CCNx name.  It also means that an entity that registers a CCNx-
   routable prefix -- say, "ccnx:/example.com" -- would need separate
   registrations for simple variations like "ccnx:/Example.com".  Unless
   this is addressed in URI normalization and routing protocol
   conventions, there could be phishing attacks.

   For a more general introduction to ICN-related security concerns and
   approaches, see [RFC7927] and [RFC7945].

6.  References

6.1.  Normative References

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

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

6.2.  Informative References

   [ace]      Shang, W., Yu, Y., Liang, T., Zhang, B., and L. Zhang,
              "NDN-ACE: Access control for constrained environments over
              named data networking", NDN Technical Report NDN-0036,
              2015, <http://new.named-data.net/wp-content/uploads/2015/
              12/ndn-0036-1-ndn-ace.pdf>.

   [ccnxke]   Mosko, M., Uzun, E., and C. Wood, "CCNx Key Exchange
              Protocol Version 1.0", Work in Progress, draft-wood-icnrg-
              ccnxkeyexchange-02, March 2017.

   [CCNxURI]  Mosko, M. and C. Wood, "The CCNx URI Scheme", Work in
              Progress, draft-mosko-icnrg-ccnxurischeme-01, April 2016.

   [CCNxz]    Mosko, M., "CCNxz TLV Header Compression Experimental
              Code", commit f1093a2, March 2018,
              <https://github.com/PARC/CCNxz>.






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   [compress] Mosko, M., "Header Compression for TLV-based Packets",
              ICNRG Interim Meeting, 2016,
              <https://datatracker.ietf.org/meeting/interim-2016-icnrg-
              02/materials/slides-interim-2016-icnrg-2-7>.

   [ECC]      Certicom Research, "SEC 2: Recommended Elliptic Curve
              Domain Parameters", 2010,
              <http://www.secg.org/sec2-v2.pdf>.

   [esic]     Mosko, M. and C. Wood, "Encrypted Sessions In CCNx
              (ESIC)", Work in Progress, draft-wood-icnrg-esic-01,
              September 2017.

   [IANA-PEN] IANA, "Private Enterprise Numbers",
              <http://www.iana.org/assignments/enterprise-numbers>.

   [mobile]   Mosko, M., Uzun, E., and C. Wood, "Mobile Sessions in
              Content-Centric Networks", IFIP Networking, 2017,
              <http://dl.ifip.org/db/conf/networking/
              networking2017/1570334964.pdf>.

   [nnc]      Jacobson, V., Smetters, D., Thornton, J., Plass, M.,
              Briggs, N., and R. Braynard, "Networking Named Content",
              Proceedings of the 5th international conference on
              Emerging networking experiments and technologies (CoNEXT
              '09), 2009, <http://dx.doi.org/10.1145/1658939.1658941>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC7927]  Kutscher, D., Ed., Eum, S., Pentikousis, K., Psaras, I.,
              Corujo, D., Saucez, D., Schmidt, T., and M. Waehlisch,
              "Information-Centric Networking (ICN) Research
              Challenges", RFC 7927, DOI 10.17487/RFC7927, July 2016,
              <https://www.rfc-editor.org/info/rfc7927>.

   [RFC7945]  Pentikousis, K., Ed., Ohlman, B., Davies, E., Spirou, S.,
              and G. Boggia, "Information-Centric Networking: Evaluation
              and Security Considerations", RFC 7945,
              DOI 10.17487/RFC7945, September 2016,
              <https://www.rfc-editor.org/info/rfc7945>.







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   [RFC8126]  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>.

   [RFC8569]  Mosko, M., Solis, I., and C. Wood, "Content-Centric
              Networking (CCNx) Semantics", RFC 8569,
              DOI 10.17487/RFC8569, July 2019,
              <https://www.rfc-editor.org/info/rfc8569>.

Authors' Addresses

   Marc Mosko
   PARC, Inc.
   Palo Alto, California  94304
   United States of America

   Phone: +01 650-812-4405
   Email: mmosko@parc.com


   Ignacio Solis
   LinkedIn
   Mountain View, California  94043
   United States of America

   Email: nsolis@linkedin.com


   Christopher A. Wood
   University of California, Irvine
   Irvine, California  92697
   United States of America

   Phone: +01 315-806-5939
   Email: woodc1@uci.edu















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