Tech-invite   3GPPspecs   RFCs   SIP   Search in Tech-invite

868584838281807978777675747372717069686766656463626160595857565554535251504948474645444342414039383736353433323130292827262524232221201918171615141312111009080706050403020100IETF‑orgGroupsStats
in Index   Prev   Next

RFC 8609

Content-Centric Networking (CCNx) Messages in TLV Format

Pages: 46
Group: IRTF
Experimental
Part 1 of 3 – Pages 1 to 16
None   None   Next

Top   ToC   RFC8609 - Page 1
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.
Top   ToC   RFC8609 - Page 2
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
Top   ToC   RFC8609 - Page 3
   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.
Top   ToC   RFC8609 - Page 4
   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.
Top   ToC   RFC8609 - Page 5
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.
Top   ToC   RFC8609 - Page 6
   +--------+------------------+---------------------------------------+
   | 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.
Top   ToC   RFC8609 - Page 7
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
Top   ToC   RFC8609 - Page 8
   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.
Top   ToC   RFC8609 - Page 9
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
Top   ToC   RFC8609 - Page 10
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.
Top   ToC   RFC8609 - Page 11
   +-------------------------------------+-----------------------------+
   |             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.
Top   ToC   RFC8609 - Page 12
                        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.
Top   ToC   RFC8609 - Page 13
   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.
Top   ToC   RFC8609 - Page 14
                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   / 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.
Top   ToC   RFC8609 - Page 15
                        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
Top   ToC   RFC8609 - Page 16
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


Next Section