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ICNRG                                                           M. Mosko
Internet-Draft                                                PARC, Inc.
Intended status: Experimental                                   I. Solis
Expires: March 15, 2018                                         LinkedIn
                                                                 C. Wood
                                         University of California Irvine
                                                      September 11, 2017

                             CCNx Semantics


   This document describes the core concepts of the CCNx architecture
   and presents a minimum network protocol based on two messages:
   Interests and Content Objects.  It specifies the set of mandatory and
   optional fields within those messages and describes their behavior
   and interpretation.  This architecture and protocol specification is
   independent of a specific wire encoding.

   The protocol also uses a Control message called an InterestReturn,
   whereby one system can return an Interest message to the previous hop
   due to an error condition.  This indicates to the previous hop that
   the current system will not respond to the Interest.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on March 15, 2018.

Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
     1.2.  Protocol Overview . . . . . . . . . . . . . . . . . . . .   3
   2.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Message Grammar . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Consumer Behavior . . . . . . . . . . . . . . . . . . . .  10
     2.3.  Publisher Behavior  . . . . . . . . . . . . . . . . . . .  11
     2.4.  Forwarder Behavior  . . . . . . . . . . . . . . . . . . .  12
       2.4.1.  Interest HopLimit . . . . . . . . . . . . . . . . . .  12
       2.4.2.  Interest Aggregation  . . . . . . . . . . . . . . . .  13
       2.4.3.  ContentStore Behavior . . . . . . . . . . . . . . . .  14
       2.4.4.  Interest Pipeline . . . . . . . . . . . . . . . . . .  14
       2.4.5.  Content Object Pipeline . . . . . . . . . . . . . . .  15
   3.  Names . . . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     3.1.  Name Examples . . . . . . . . . . . . . . . . . . . . . .  17
     3.2.  Interest Payload ID . . . . . . . . . . . . . . . . . . .  17
   4.  Cache Control . . . . . . . . . . . . . . . . . . . . . . . .  17
   5.  Content Object Hash . . . . . . . . . . . . . . . . . . . . .  18
   6.  Link  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  18
   7.  Hashes  . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
   8.  Validation  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  Validation Algorithm  . . . . . . . . . . . . . . . . . .  19
   9.  Interest to Content Object matching . . . . . . . . . . . . .  20
   10. Interest Return . . . . . . . . . . . . . . . . . . . . . . .  21
     10.1.  Message Format . . . . . . . . . . . . . . . . . . . . .  22
     10.2.  ReturnCode Types . . . . . . . . . . . . . . . . . . . .  22
     10.3.  Interest Return Protocol . . . . . . . . . . . . . . . .  23
       10.3.1.  No Route . . . . . . . . . . . . . . . . . . . . . .  24
       10.3.2.  HopLimit Exceeded  . . . . . . . . . . . . . . . . .  25
       10.3.3.  Interest MTU Too Large . . . . . . . . . . . . . . .  25
       10.3.4.  No Resources . . . . . . . . . . . . . . . . . . . .  25
       10.3.5.  Path Error . . . . . . . . . . . . . . . . . . . . .  25
       10.3.6.  Prohibited . . . . . . . . . . . . . . . . . . . . .  25
       10.3.7.  Congestion . . . . . . . . . . . . . . . . . . . . .  25
       10.3.8.  Unsupported Content Object Hash Algorithm  . . . . .  26
       10.3.9.  Malformed Interest . . . . . . . . . . . . . . . . .  26

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   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  26
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  26
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  26
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  26
     14.2.  Informative References . . . . . . . . . . . . . . . . .  27
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  27

1.  Introduction

   This document describes the principles of the CCNx architecture.  It
   describes the network protocol based on two message types: Interests
   and Content Objects.  The description is not dependent on a specific
   wire format or particular encodings.  This section introduces the
   main concepts of CCNx, which are further elaborated in the remainder
   of the document.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

1.2.  Protocol Overview

   CCNx is a request and response protocol to fetch chunks of data using
   a name.  The integrity of each chunk may be directly asserted through
   a digital signature or message authentication code (MAC), or,
   alternatively, indirectly via hash chains.  Chunks may also carry
   weaker message integrity checks (MICs) or no integrity protection
   mechanism at all.  Because provenance information is carried with
   each chunk (or larger indirectly protected block), we no longer need
   to rely on host identities, such as those derived from TLS
   certificates, to ascertain the chunk legitimacy.  Data integrity is
   therefore a core feature of CCNx; it does not rely on the data
   transmission channel.  There are several options for data
   confidentiality, discussed later.

   As a request and response protocol, CCNx may be carried over many
   different transports.  In use today are Ethernet, TCP, UDP, 802.15.4,
   GTP, GRE, DTLS, TLS, and others.  While the specific wire format of
   CCNx may vary to some extent based on transport, the core principles
   and behaviors of CCNx outlined in this document should remain fixed.

   CCNx uses hierarchical names to identify bytes of payload.  The Name
   combines a routable prefix with an arbitrary application-dependent
   suffix assigned by the publisher to a piece of content.  The result
   is a "named payload".  This is different from other systems that use

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   only self-certifying names, where the payload name is intrinsically
   derivable from the payload or its realization in a network object
   (e.g., a SHA-256 hash of the payload or network object).  In human-
   readable form, we represent names as a "ccnx:" [CCNxURI] scheme URI
   [RFC3986], though the canonical encoding should be octet strings.  In
   this respect, we speak of a name being made up of hierarchical path
   segments, which is the URI terminology.

   This document only defines the general properties of CCNx names.  In
   some isolated environments, CCNx users may be able to use any name
   they choose and either inject that name (or prefix) into a routing
   protocol or use other information foraging techniques.  In the
   Internet environment, there will be policies around the formats of
   names and assignments of names to publishers, though those are not
   specified here.

   The key concept of CCNx is that a subjective name is
   (cryptographically) bound to a fixed payload.  These (publisher-
   generated) bindings can therefore be (cryptographically) verified.
   For example, a publisher could compute a cryptographic hash over the
   name and payload, sign the hash, and deliver the tuple {Name,
   Payload, Validation}.  Consumers of this data can check the binding
   integrity by re-computing the same cryptographic hash and verifying
   the digital signature in Validation.  Additional information would be
   included as needed by specific validation mechanisms.  Therefore, we
   divide Validation in to a ValidationAlgorithm and a
   ValidationPayload.  The ValidationAlgorithm has information about the
   crypto suite and parameters.  In particular, the ValidationAlgorithm
   usually has a field called KeyId which identifies the public key used
   by the validation, when applicable.  The ValidationPayload is the
   output of the validation algorithm, such as a CRC value, an HMAC
   output, or an RSA signature.

   In addition to the essential Name, Payload, and Validation sections,
   a CCNx user may need to include some other signaling information.
   This could include a hint about the type of Payload (e.g.,
   application data, a cryptographic key, etc.) or cache control
   directives, etc.  We will call this extra signaling information

   A named payload is thus the tuple {{Name, ExtraFields, Payload,
   ValidationAlgorithm}, ValidationPayload}, where all fields in the
   inner tuple are covered by the validation algorithm.

   CCNx specifies a network protocol around Interests (request messages)
   and Content Objects (response messages) to move named payloads.  An
   Interest includes the Name -- which identifies the desired response
   -- and two optional limiting restrictions.  The first restriction on

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   the KeyId to limit responses to those signed with a
   ValidationAlgorithm KeyId field equal to the restriction.  The second
   is the ContentObjectHash restriction, which limits the response to
   one where the cryptographic hash of the entire named payload is equal
   to the restriction.

   The hierarchy of a CCNx Name is used for routing via the longest
   matching prefix in a Forwarder.  The longest matching prefix is
   computed name segment by name segment in the hierarchical path name,
   where each name segment must be exactly equal to match.  There is no
   requirement that the prefix be globally routable.  Within a
   deployment any local routing may be used, even one that only uses a
   single flat (non-hierarchical) name segment.

   Another concept of CCNx is that there should be flow balance between
   Interest messages and Content Object messages.  At the network level,
   an Interest traveling along a single path should elicit no more than
   one Content Object response.  If some node sends the Interest along
   more than one path, that node should consolidate the responses such
   that only one Content Object flows back towards the requester.  If an
   Interest is sent broadcast or multicast on a multiple-access media,
   the sender should be prepared for multiple responses unless some
   other media-dependent mechanism like gossip suppression or leader
   election is used.

   As an Interest travels the forward path following the Forwarding
   Information Base (FIB), it establishes state at each forwarder such
   that a Content Object response can trace its way back to the original
   requester(s) without the requester needing to include a routable
   return address.  We use the notional Pending Interest Table (PIT) as
   a method to store state that facilitates the return of a Content
   Object.  The PIT table is not mandated by the specification.

   The notional PIT table stores the last hop of an Interest plus its
   Name and optional restrictions.  This is the data required to match a
   Content Object to an Interest (see Section 9).  When a Content Object
   arrives, it must be matched against the PIT to determine which
   entries it satisfies.  For each such entry, at most one copy of the
   Content Object is sent to each listed last hop in the PIT entries.

   If multiple Interests with the same {Name, KeyIdRestriction,
   ContentObjectHashRestriction} tuple arrive at a node before a Content
   Object matching the first Interest comes back, they are grouped in
   the same PIT entry and their last hops aggregated (see
   Section 2.4.2).  Thus, one Content Object might satisfy multiple
   pending Interests in a PIT.

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   In CCNx, higher-layer protocols often become so-called "name-based
   protocols" because they operate on the CCNx Name.  For example, a
   versioning protocol might append additional name segments to convey
   state about the version of payload.  A content discovery protocol
   might append certain protocol-specific name segments to a prefix to
   discover content under that prefix.  Many such protocols may exist
   and apply their own rules to Names.  They may be layered with each
   protocol encapsulating (to the left) a higher layer's Name prefix.

   This document also describes a control message called an
   InterestReturn.  A network element may return an Interest message to
   a previous hop if there is an error processing the Interest.  The
   returned Interest may be further processed at the previous hop or
   returned towards the Interest origin.  When a node returns an
   Interest it indicates that the previous hop should not expect a
   response from that node for the Interest, i.e., there is no PIT entry
   left at the returning node for a Content Object to follow.

   There are multiple ways to describe larger objects in CCNx.  Some
   options may use the namespace while others may use a structure such
   as a Manifest.  This document does not address these options at this

   The remainder of this document describes a named payload as well as
   the Interest and Content Object network protocol behavior in detail.

2.  Protocol

   CCNx is a request and response protocol.  A request is called an
   Interest and a response is called a ContentObject.  CCNx also uses a
   1-hop control message called InterestReturn.  These are, as a group,
   called CCNx Messages.

2.1.  Message Grammar

   The CCNx message ABNF [RFC5234] grammar is show in Figure 1.  The
   grammar does not include any encoding delimiters, such as TLVs.
   Specific wire encodings are given in a separate document.  If a
   Validation section exists, the Validation Algorithm covers from the
   Body (BodyName or BodyOptName) through the end of the ValidationAlg
   section.  The InterestLifetime, CacheTime, and Return Code fields
   exist outside of the validation envelope and may be modified.

   The various fields -- in alphabetical order -- are defined as:

   o  AbsTime: Absolute times are conveyed as the 64-bit UTC time in
      milliseconds since the epoch (standard POSIX time).

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   o  CacheTime: The absolute time after which the publisher believes
      there is low value in caching the content object.  This is a
      recommendation to caches (see Section 4).

   o  ConObjField: These are optional fields that may appear in a
      Content Object.

   o  ConObjHash: The value of the Content Object Hash, which is the
      SHA256-32 over the message from the beginning of the body to the
      end of the message.  Note that this coverage area is different
      from the ValidationAlg.  This value SHOULD NOT be trusted across
      domains (see Section 5).

   o  ExpiryTime: An absolute time after which the content object should
      be considered expired (see Section 4).

   o  HopLimit: Interest messages may loop if there are loops in the
      forwarding plane.  To eventually terminate loops, each Interest
      carries a HopLimit that is decremented after each hop and no
      longer forwarded when it reaches zero.  See Section 2.4.

   o  InterestField: These are optional fields that may appear in an
      Interest message.

   o  KeyIdRestr: The KeyId Restriction.  A Content Object must have a
      KeyId with the same value as the restriction.

   o  ObjHashRestr: The Content Object Hash Restriction.  A content
      object must hash to the same value as the restriction using the
      same HashType.  The ObjHashRestr MUST use SHA256-32.

   o  KeyId: An identifier for the key used in the ValidationAlg.  For
      public key systems, this should be the SHA-256 hash of the public
      key.  For symmetric key systems, it should be an identifer agreed
      upon by the parties.

   o  KeyLink: A Link (see Section 6) that names how to retrieve the key
      used to verify the ValidationPayload.  A message SHOULD NOT have
      both a KeyLink and a PublicKey.

   o  Lifetime: The approximate time during which a requester is willing
      to wait for a response, usually measured in seconds.  It is not
      strongly related to the network round trip time, though it must
      necessarily be larger.

   o  Name: A name is made up of a non-empty first segment followed by
      zero or more additional segments, which may be of 0 length.  Path
      segments are opaque octet strings, and are thus case-sensitive if

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      encoding UTF-8.  An Interest MUST have a Name.  A ContentObject
      MAY have a Name (see Section 9).  The segments of a name are said
      to be complete if its segments uniquely identify a single Content
      Object.  A name is exact if its segments are complete.  An
      Interest carrying a full name is one which specifies an exact name
      and the ObjHashRestr of the corresponding Content Object.

   o  Payload: The message's data, as defined by PayloadType.

   o  PayloadType: The format of the Payload.  If missing, assume
      DataType.  DataType means the payload is opaque application bytes.
      KeyType means the payload is a DER-encoded public key.  LinkType
      means it is one or more Links (see Section 6).

   o  PublicKey: Some applications may wish to embed the public key used
      to verify the signature within the message itself.  The PublickKey
      is DER encoded.  A message SHOULD NOT have both a KeyLink and a

   o  RelTime: A relative time, measured in milli-seconds.

   o  ReturnCode: States the reason an Interest message is being
      returned to the previous hop (see Section 10.2).

   o  SigTime: The absolute time (UTC milliseconds) when the signature
      was generated.

   o  Hash: Hash values carried in a Message carry a HashType to
      identify the algorithm used to generate the hash followed by the
      hash value.  This form is to allow hash agility.  Some fields may
      mandate a specific HashType.

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   Message       := Interest / ContentObject / InterestReturn
   Interest      := HopLimit [Lifetime] BodyName [Validation]
   ContentObject := [CacheTime / ConObjHash] BodyOptName [Validation]
   InterestReturn:= ReturnCode Interest
   BodyName      := Name Common
   BodyOptName   := [Name] Common
   Common        := *Field [Payload]
   Validation    := ValidationAlg ValidatonPayload

   Name          := FirstSegment *Segment
   FirstSegment  := 1* OCTET
   Segment       := 0* OCTET

   ValidationAlg := RSA-SHA256 HMAC-SHA256 CRC32C
   ValidatonPayload := 1* OCTET
   RSA-SHA256    := KeyId [PublicKey] [SigTime] [KeyLink]
   HMAC-SHA256   := KeyId [SigTime] [KeyLink]
   CRC32C        := [SigTime]

   AbsTime       := 8 OCTET ; 64-bit UTC msec since epoch
   CacheTime     := AbsTime
   ConObjField   := ExpiryTime / PayloadType
   ConObjHash    := Hash ; The Content Object Hash
   DataType      := "1"
   ExpiryTime    := AbsTime
   Field         := InterestField / ConObjField
   Hash          := HashType 1* OCTET
   HashType      := SHA256-32 / SHA512-64 / SHA512-32
   HopLimit      := OCTET
   InterestField := KeyIdRestr / ObjHashRestr
   KeyId         := 1* OCTET ; key identifier
   KeyIdRestr    := 1* OCTET
   KeyLink       := Link
   KeyType       := "2"
   Lifetime      := RelTime
   Link          := Name [KeyIdResr] [ObjHashRestr]
   LinkType      := "3"
   ObjHashRestr  := Hash
   Payload       := *OCTET
   PayloadType   := DataType / KeyType / LinkType
   PublicKey     := ; DER-encoded public key
   RelTime       := 1* OCTET ; msec
   ReturnCode    := ; see Section 10.2
   SigTime       := AbsTime

                                 Figure 1

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2.2.  Consumer Behavior

   To request a piece of content for a given {Name, [KeyIdRest],
   [ObjHashRestr]} tuple, a consumer creates an Interest message with
   those values.  It MAY add a validation section, typically only a
   CRC32C.  A consumer MAY put a Payload field in an Interest to send
   additional data to the producer beyond what is in the Name.  The Name
   is used for routing and may be remembered at each hop in the notional
   PIT table to facilitate returning a content object; Storing large
   amounts of state in the Name could lead to high memory requirements.
   Because the Payload is not considered when forwarding an Interest or
   matching a Content Object to an Interest, a consumer SHOULD put an
   Interest Payload ID (see Section Section 3.2) as part of the name to
   allow a forwarder to match Interests to content objects and avoid
   aggregating Interests with different payloads.  Similarly, if a
   consumer uses a MAC or a signature, it SHOULD also include a unique
   segment as part of the name to prevent the Interest from being
   aggregated with other Interests or satisfied by a Content Object that
   has no relation to the validation.

   The consumer SHOULD specify an InterestLifetime, which is the length
   of time the consumer is willing to wait for a response.  The
   InterestLifetime is an application-scale time, not a network round
   trip time (see Section 2.4.2).  If not present, the InterestLifetime
   will use a default value (TO_INTERESTLIFETIME).

   The consumer SHOULD set the Interest HopLimit to a reasonable value
   or use the default 255.  If the consumer knows the distances to the
   producer via routing, it SHOULD use that value.

   A consumer hands off the Interest to its first forwarder, which will
   then forward the Interest over the network to a publisher (or
   replica) that may satisfy it based on the name (see Section 2.4).

   Interest messages are unreliable.  A consumer SHOULD run a transport
   protocol that will retry the Interest if it goes unanswered, up to
   the InterestLifetime.  No transport protocol is specified in this

   The network MAY send to the consumer an InterestReturn message that
   indicates the network cannot fulfill the Interest.  The ReturnCode
   specifies the reason for the failure, such as no route or congestion.
   Depending on the ReturnCode, the consumer MAY retry the Interest or
   MAY return an error to the requesting application.

   If the content was found and returned by the first forwarder, the
   consumer will receive a ContentObject.  The consumer SHOULD:

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   o  Ensure the content object is properly formatted.

   o  Verify that the returned Name matches a pending request.  If the
      request also had KeyIdRestr and ObjHashRest, it should also
      validate those properties.

   o  If the content object is signed, it SHOULD cryptographically
      verify the signature.  If it does not have the corresponding key,
      it SHOULD fetch the key, such as from a key resolution service or
      via the KeyLink.

   o  If the signature has a SigTime, the consumer MAY use that in
      considering if the signature is valid.  For example, if the
      consumer is asking for dynamically generated content, it should
      expect the SigTime to not be before the time the Interest was

   o  If the content object is signed, it should assert the
      trustworthiness of the signing key to the namespace.  Such an
      assertion is beyond the scope of this document, though one may use
      traditional PKI methods, a trusted key resolution service, or
      methods like [schematized trust].

   o  It MAY cache the content object for future use, up to the
      ExpiryTime if present.

   o  A consumer MAY accept a content object off the wire that is
      expired.  It may happen that a packet expires while in flight, and
      there is no requirement that forwarders drop expired packets in
      flight.  The only requirement is that content stores, caches, or
      producers MUST NOT respond with an expired content object.

2.3.  Publisher Behavior

   This document does not specify the method by which names populate a
   Forwarding Information Base (FIB) table at forwarders (see
   Section 2.4).  A publisher is either configured with one or more name
   prefixes under which it may create content, or it chooses its name
   prefixes and informs the routing layer to advertise those prefixes.

   When a publisher receives an Interest, it SHOULD:

   o  Verify that the Interest is part of the publishers namespace(s).

   o  If the Interest has a Validation section, verify the
      ValidationPayload.  Usually an Interest will only have a CRC32C
      unless the publisher application specifically accommodates other
      validations.  The publisher MAY choose to drop Interests that

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      carry a Validation section if the publisher application does not
      expect those signatures as this could be a form of computational
      denial of service.  If the signature requires a key that the
      publisher does not have, it is NOT RECOMMENDED that the publisher
      fetch the key over the network, unless it is part of the
      application's expected behavior.

   o  Retrieve or generate the requested content object and return it to
      the Interest's previous hop.  If the requested content cannot be
      returned, the publisher SHOULD reply with an InterestReturn or a
      content object with application payload that says the content is
      not available; this content object should have a short ExpiryTime
      in the future.

2.4.  Forwarder Behavior

   A forwarder routes Interest messages based on a Forwarding
   Information Base (FIB), returns Content Objects that match Interests
   to the Interest's previous hop, and processes InterestReturn control
   messages.  It may also keep a cache of Content Objects in the
   notional Content Store table.  This document does not specify the
   internal behavior of a forwarder -- only these and other external

   In this document, we will use two processing pipelines, one for
   Interests and one for Content Objects.  Interest processing is made
   up of checking for duplicate Interests in the PIT (see
   Section 2.4.2), checking for a cached Content Object in the Content
   Store (see Section 2.4.3), and forwarding an Interest via the FIB.
   Content Store processing is made up of checking for matching
   Interests in the PIT and forwarding to those previous hops.

2.4.1.  Interest HopLimit

   Interest looping is not prevented in CCNx.  An Interest traversing
   loops is eventually discarded using the hop-limit field of the
   Interest, which is decremented at each hop traversed by the Interest.

   Every Interest MUST carry a HopLimit.

   When an Interest is received from another forwarder, the HopLimit
   MUST be positive.  A forwarder MUST decement the HopLimit of an
   Interest by at least 1 before it is forwarded.

   If the HopLimit equals 0, the Interest MUST NOT be forwarded to
   another forwarder; it MAY be sent to a publisher application or
   serviced from a local Content Store.

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2.4.2.  Interest Aggregation

   Interest aggregation is when a forwarder receives an Interest message
   that could be satisfied by another Interest message already forwarded
   by the node so the forwarder suppresses the new Interest; it only
   records the additional previous hop so a Content Object sent in
   response to the first Interest will satisfy both Interests.

   CCNx uses an interest aggregation rule that assumes the
   InterestLifetime is akin to a subscription time and is not a network
   round trip time.  Some previous aggregation rules assumed the
   lifetime was a round trip time, but this leads to problems of
   expiring an Interest before a response comes if the RTT is estimated
   too short or interfering with an ARQ scheme that wants to re-transmit
   an Interest but a prior interest over-estimated the RTT.

   A forwarder MAY implement an Interest aggregation scheme.  If it does
   not, then it will forward all Interest messages.  This does not imply
   that multiple, possibly identical, Content Objects will come back.  A
   forwarder MUST still satisfy all pending Interests, so one Content
   Object could satisfy multiple similar interests, even if the
   forwarded did not suppress duplicate Interest messages.

   A RECOMMENDED Interest aggregation scheme is:

   o  Two Interests are considered 'similar' if they have the same Name,
      KeyIdRestr, and ObjHashRestr.

   o  Let the notional value InterestExpiry (a local value at the
      forwarder) be equal to the receive time plus the InterestLifetime
      (or a platform-dependent default value if not present).

   o  An Interest record (PIT entry) is considered invalid if its
      InterestExpiry time is in the past.

   o  The first reception of an Interest MUST be forwarded.

   o  A second or later reception of an Interest similar to a valid
      pending Interest from the same previous hop MUST be forwarded.  We
      consider these a retransmission requests.

   o  A second or later reception of an Interest similar to a valid
      pending Interest from a new previous hop MAY be aggregated (not

   o  Aggregating an Interest MUST extend the InterestExpiry time of the
      Interest record.  An implementation MAY keep a single
      InterestExpiry time for all previous hops or MAY keep the

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      InterestExpiry time per previous hop.  In the first case, the
      forwarder might send a ContentObject down a path that is no longer
      waiting for it, in which case the previous hop (next hop of the
      Content Object) would drop it.

2.4.3.  ContentStore Behavior

   The ContentStore is a special cache that sits on the fast path of a
   CCNx forwarder.  It is an optional component.  It serves to repair
   lost packets and handle flash requests for popular content.  It could
   be pre-populated or use opportunistic caching.  Because the Content
   Store could serve to amplify an attach via cache poisoning, there are
   special rules about how a Content Store behaves.

   1.  A forwarder MAY implement a ContentStore.  If it does, the
       Content Store matches a Content Object to an Interest via the
       normal matching rules (see Section 9).

   2.  If an Interest has a KeyIdRestr, then the ContentStore MUST NOT
       reply unless it knows the signature on the matching ContentObject
       is correct.  It may do this by external knowledge (i.e., in a
       managed system pre-populating the cachine) or by having the
       public key and cryptographically verifying the signature.  If the
       public key is provided in the ContentObject itself (i.e., in the
       PublicKey field) or in the Interest, the ContentStore MUST verify
       that the public key's SHA-256 hash is equal to the KeyId and that
       it verifies the signature.  A ContentStore MAY verify the digital
       signature of a Content Object before it is cached, but it is not
       required to do so.  A ContentStore SHOULD NOT fetch keys over the
       network.  If it cannot or has not yet verified the signature, it
       should treat the Interest as a cache miss.

   3.  If an Interest has an ObjHashRestr, then the ContentStore MUST
       NOT reply unless it knows the the matching ContentObject has the
       correct hash.  If it cannot verify the hash, then it should treat
       the Interest as a cache miss.

   4.  It must object the Cache Control directives (see Section 4).

2.4.4.  Interest Pipeline

   1.  Perform the HopLimit check (see Section 2.4.1).

   2.  Determine if the Interest can be aggregated, as per
       Section 2.4.2.  If it can be, aggregate and do not forward the

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   3.  If forwarding the Interest, check for a hit in the Content Store,
       as per Section 2.4.3.  If a matching Content Object is found,
       return it to the Interest's previous hop.  This injects the
       ContentStore as per Section 2.4.5.

   4.  Lookup the Interest in the FIB.  Longest prefix match (LPM) is
       performed name segment by name segment (not byte or bit).  It
       SHOULD exclude the Interest's previous hop.  If a match is found,
       forward the Interest.  If no match is found or the forwarder
       choses to not forward due to a local condition (e.g.,
       congestion), it SHOULD send an InterestReturn message, as per
       Section 10.

2.4.5.  Content Object Pipeline

   1.  It is RECOMMENDED that a forwarder that receives a content object
       check that the ContentObject came from an expected previous hop.
       An expected previous hop is one pointed to by the FIB or one
       recorded in the PIT as having had a matching Interest sent that

   2.  A Content Object MUST be matched to all pending Interests that
       satisfy the matching rules (see Section 9).  Each satisfied
       pending Interest MUST then be removed from the set of pending

   3.  A forwarder SHOULD NOT send more then one copy of the received
       Content Object to the same Interest previous hop.  It may happen,
       for example, that two Interest ask for the same Content Object in
       different ways (e.g., by name and by name an KeyId) and that they
       both come from the same previous hop.  It is normal to send the
       same content object multiple times on the same interface, such as
       Ethernet, if it is going to different previous hops.

   4.  A Content Object SHOULD only be put in the Content Store if it
       satisfied an Interest (and passed rule #1 above).  This is to
       reduce the chances of cache poisoning.

3.  Names

   A CCNx name is a composition of name segments.  Each name segment
   carries a label identifying the purpose of the name segment, and a
   value.  For example, some name segments are general names and some
   serve specific purposes, such as carrying version information or the
   sequencing of many chunks of a large object into smaller, signed
   Content Objects.

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   There are three different types of names in CCNx: prefix, exact, and
   full names.  A prefix name is simply a name that does not uniquely
   identify a single Content Object, but rather a namespace or prefix of
   an existing Content Object name.  An exact name is one which uniquely
   identifies the name of a Content Object.  A full name is one which is
   exact and is accompanied by an explicit or implicit ConObjHash.  The
   ConObjHash is explicit in an Interest and implicit in a Content

   The name segment labels specified in this document are given in the
   table below.  Name Segment is a general name segment, typically
   occurring in the routable prefix and user-specified content name.
   Other segment types are for functional name components that imply a
   specific purpose.

   A forwarding table entry may contain name segments of any type.
   Routing protocol policy and local system policy may limit what goes
   into forwarding entries, but there is no restriction at the core
   level.  An Interest routing protocol, for example, may only allow
   binary name segments.  A load balancer or compute cluster may route
   through additional component types, depending on their services.

   |     Name    | Description                                         |
   |     Name    | A generic name segment that includes arbitrary      |
   |   Segment   | octets.                                             |
   |             |                                                     |
   |   Interest  | An octet string that identifies the payload carried |
   |  Payload ID | in an Interest. As an example, the Payload ID might |
   |             | be a hash of the Interest Payload.  This provides a |
   |             | way to differentiate between Interests based on the |
   |             | Payload solely through a Name Segment without       |
   |             | having to include all the extra bytes of the        |
   |             | payload itself.                                     |
   |             |                                                     |
   | Application | An application-specific payload in a name segment.  |
   |  Components | An application may apply its own semantics to these |
   |             | components.  A good practice is to identify the     |
   |             | application in a Name segment prior to the          |
   |             | application component segments.                     |

                     Table 1: CCNx Name Segment Types

   At the lowest level, a Forwarder does not need to understand the
   semantics of name segments; it need only identify name segment
   boundaries and be able to compare two name segments (both label and

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   value) for equality.  The Forwarder matches paths segment-by-segment
   against its forwarding table to determine a next hop.

3.1.  Name Examples

   This section uses the CCNx URI [CCNxURI] representation of CCNx names

   |           Name           | Description                            |
   |          ccnx:/          | A 0-length name, corresponds to a      |
   |                          | default route.                         |
   |                          |                                        |
   |       ccnx:/NAME=        | A name with 1 segment of 0 length,     |
   |                          | distinct from ccnx:/.                  |
   |                          |                                        |
   | ccnx:/NAME=foo/APP:0=bar | A 2-segment name, where the first      |
   |                          | segment is of type NAME and the second |
   |                          | segment is of type APP:0.              |

                        Table 2: CCNx Name Examples

3.2.  Interest Payload ID

   An Interest may also have a Payload which carries state about the
   Interest but is not used to match a Content Object.  If an Interest
   contains a payload, the Interest name should contain an Interest
   Payload ID (IPID).  The IPID allows a PIT table entry to correctly
   multiplex Content Objects in response to a specific Interest with a
   specific payload ID.  The IPID could be derived from a hash of the
   payload or could be a GUID or a nonce.  An optional Metadata field
   defines the IPID field so other systems could verify the IPID, such
   as when it is derived from a hash of the payload.  No system is
   required to verify the IPID.

4.  Cache Control

   CCNx supports two fields that affect cache control.  These determine
   how a cache or Content Store handles a Content Object.  They are not
   used in the fast path, but only to determine if a ContentObject can
   be injected on to the fast path in response to an Interest.

   The ExpiryTime is a field that exists within the signature envelope
   of a Validation Algorithm.  It is the UTC time in milliseconds after
   which the ContentObject is considered expired and MUST no longer be

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   used to respond to an Interest from a cache.  Stale content MAY be
   flushed from the cache.

   The Recommended Cache Time (RCT) is a field that exists outside the
   signature envelope.  It is the UTC time in milliseconds after which
   the publisher considers the Content Object to be of low value to
   cache.  A cache SHOULD discard it after the RCT, though it MAY keep
   it and still respond with it.  A cache is MAY discard the content
   object before the RCT time too; there is no contractual obligation to
   remember anything.

   This formulation allows a producer to create a Content Object with a
   long ExpiryTime but short RCT and keep re-publishing the same,
   signed, Content Object over and over again by extending the RCT.
   This allows a form of "phone home" where the publisher wants to
   periodically see that the content is being used.

5.  Content Object Hash

   CCNx allows an Interest to restrict a response to a specific hash.
   The hash covers the Content Object message body and the validation
   sections, if present.  Thus, if a Content Object is signed, its hash
   includes that signature value.  The hash does not include the fixed
   or hop-by-hop headers of a Content Object.  Because it is part of the
   matching rules (see Section 9), the hash is used at every hop.

   There are two options for matching the content object hash
   restriction in an Interest.  First, a forwarder could compute for
   itself the hash value and compare it to the restriction.  This is an
   expensive operation.  The second option is for a border device to
   compute the hash once and place the value in a header (ConObjHash)
   that is carried through the network.  The second option, of course,
   removes any security properties from matching the hash, so SHOULD
   only be used within a trusted domain.  The header SHOULD be removed
   when crossing a trust boundary.

6.  Link

   A Link is the tuple {Name, [KeyIdRestr], [ContentObjectHashRestr]}.
   The information in a Link comprises the fields the fields of an
   Interest which would retrieve the Link target.  A Content Object with
   PayloadType = "Link" is an object whose payload is one or more Links.
   This tuple may be used as a KeyLink to identify a specific object
   with the certificate wrapped key.  It is RECOMMENDED to include at
   least one of KeyIdRestr or ContentObjectHashRestr.  If neither
   restriction is present, then any Content Object with a matching name
   from any publisher could be returned.

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7.  Hashes

   Several protocol fields use cryptographic hash functions, which must
   be secure against attack and collisions.  Because these hash
   functions change over time, with better ones appearing and old ones
   falling victim to attacks, it is important that a CCNx protocol
   implementation support hash agility.

   In this document, we suggest certain hashes (e.g., SHA-256), but a
   specific implementation may use what it deems best.  The normative
   CCNx Messages [CCNMessages] specification should be taken as the
   definition of acceptable hash functions and uses.

8.  Validation

8.1.  Validation Algorithm

   The Validator consists of a ValidationAlgorithm that specifies how to
   verify the message and a ValidationPayload containing the validation
   output, e.g., the digital signature or MAC.  The ValidationAlgorithm
   section defines the type of algorithm to use and includes any
   necessary additional information.  The validation is calculated from
   the beginning of the CCNx Message through the end of the
   ValidationAlgorithm section.  The ValidationPayload is the integrity
   value bytes, such as a MAC or signature.

   Some Validators contain a KeyId, identifying the publisher
   authenticating the Content Object.  If an Interest carries a
   KeyIdRestriction, then that KeyIdRestriction MUST exactly match the
   Content Object's KeyId.

   Validation Algorithms fall into three categories: MICs, MACs, and
   Signatures.  Validators using MIC algorithms do not need to provide
   any additional information; they may be computed and verified based
   only on the algorithm (e.g., CRC32C).  MAC validators require the use
   of a KeyId identifying the secret key used by the authenticator.
   Because MACs are usually used between two parties that have already
   exchanged secret keys via a key exchange protocol, the KeyId may be
   any agreed-upon value to identify which key is used.  Signature
   validators use public key cryptographic algorithms such as RSA, DSA,
   ECDSA.  The KeyId field in the ValidationAlgorithm identifies the
   public key used to verify the signature.  A signature may optionally
   include a KeyLocator, as described above, to bundle a Key or
   Certificate or KeyLink.  MAC and Signature validators may also
   include a SignatureTime, as described above.

   A PublicKeyLocator KeyLink points to a Content Object with a DER-
   encoded X509 certificate in the payload.  In this case, the target

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   KeyId must equal the first object's KeyId.  The target KeyLocator
   must include the public key corresponding to the KeyId.  That key
   must validate the target Signature.  The payload is an X.509
   certificate whose public key must match the target KeyLocator's key.
   It must be issued by a trusted authority, preferably specifying the
   valid namespace of the key in the distinguished name.

9.  Interest to Content Object matching

   A Content Object satisfies an Interest if and only if (a) the Content
   Object name, if present, exactly matches the Interest name, and (b)
   the ValidationAlgorithm KeyId of the Content Object exactly equals
   the Interest KeyIdRestriction, if present, and (c) the computed
   ContentObjectHash exactly equals the Interest
   ContentObjectHashRestriction, if present.

   The matching rules are given by this predicate, which if it evaluates
   true means the ContentObject matches the Interest.  Ni = Name in
   Interest (may not be empty), Ki = KeyIdRestriction in the interest
   (may be empty), Hi = ContentObjectHashRestriction in Interest (may be
   empty).  Likewise, No, Ko, Ho are those properties in the
   ContentObject, where No and Ko may be empty; Ho always exists.  For
   binary relations, we use & for AND and | for OR.  We use E for the
   EXISTS (not empty) operator and ! for the NOT EXISTS operator.

   As a special case, if the ContentObjectHashRestriction in the
   Interest specifies an unsupported hash algorithm, then no
   ContentObject can match the Interest so the system should drop the
   Interest and MAY send an InterestReturn to the previous hop.  In this
   case, the predicate below will never get executed because the
   Interest is never forwarded.  If the system is using the optional
   behavior of having a different system calculate the hash for it, then
   the system may assume all hash functions are supported and leave it
   to the other system to accept or reject the Interest.

   (!No | (Ni=No)) & (!Ki | (Ki=Ko)) & (!Hi | (Hi=Ho)) & (E No | E Hi)

   As one can see, there are two types of attributes one can match.  The
   first term depends on the existence of the attribute in the
   ContentObject while the next two terms depend on the existence of the
   attribute in the Interest.  The last term is the "Nameless Object"
   restriction which states that if a Content Object does not have a
   Name, then it must match the Interest on at least the Hash

   If a Content Object does not carry the ContentObjectHash as an
   expressed field, it must be calculated in network to match against.
   It is sufficient within an autonomous system to calculate a

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   ContentObjectHash at a border router and carry it via trusted means
   within the autonomous system.  If a Content Object
   ValidationAlgorithm does not have a KeyId then the Content Object
   cannot match an Interest with a KeyIdRestriction.

10.  Interest Return

   This section describes the process whereby a network element may
   return an Interest message to a previous hop if there is an error
   processing the Interest.  The returned Interest may be further
   processed at the previous hop or returned towards the Interest
   origin.  When a node returns an Interest it indicates that the
   previous hop should not expect a response from that node for the
   Interest -- i.e., there is no PIT entry left at the returning node.

   The returned message maintains compatibility with the existing TLV
   packet format (a fixed header, optional hop-by-hop headers, and the
   CCNx message body).  The returned Interest packet is modified in only
   two ways:

   o  The PacketType is set to InterestReturn to indicate a Feedback

   o  The ReturnCode is set to the appropriate value to signal the
      reason for the return

   The specific encodings of the Interest Return are specified in

   A Forwarder is not required to send any Interest Return messages.

   A Forwarder is not required to process any received Interest Return
   message.  If a Forwarder does not process Interest Return messages,
   it SHOULD silently drop them.

   The Interest Return message does not apply to a Content Object or any
   other message type.

   An Interest Return message is a 1-hop message between peers.  It is
   not propagated multiple hops via the FIB.  An intermediate node that
   receives an InterestReturn may take corrective actions or may
   propagate its own InterestReturn to previous hops as indicated in the
   reverse path of a PIT entry.

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10.1.  Message Format

   The Interest Return message looks exactly like the original Interest
   message with the exception of the two modifications mentioned above.
   The PacketType is set to indicate the message is an InterestReturn
   and the reserved byte in the Interest header is used as a Return
   Code.  The numeric values for the PacketType and ReturnCodes are in

10.2.  ReturnCode Types

   This section defines the InterestReturn ReturnCode introduced in this
   RFC.  The numeric values used in the packet are defined in

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   | Name                 | Description                                |
   | No Route (Section    | The returning Forwarder has no route to    |
   | 10.3.1)              | the Interest name.                         |
   |                      |                                            |
   | HopLimit Exceeded    | The HopLimit has decremented to 0 and need |
   | (Section 10.3.2)     | to forward the packet.                     |
   |                      |                                            |
   | Interest MTU too     | The Interest's MTU does not conform to the |
   | large (Section       | required minimum and would require         |
   | 10.3.3)              | fragmentation.                             |
   |                      |                                            |
   | No Resources         | The node does not have the resources to    |
   | (Section 10.3.4)     | process the Interest.                      |
   |                      |                                            |
   | Path error (Section  | There was a transmission error when        |
   | 10.3.5)              | forwarding the Interest along a route (a   |
   |                      | transient error).                          |
   |                      |                                            |
   | Prohibited (Section  | An administrative setting prohibits        |
   | 10.3.6)              | processing this Interest.                  |
   |                      |                                            |
   | Congestion (Section  | The Interest was dropped due to congestion |
   | 10.3.7)              | (a transient error).                       |
   |                      |                                            |
   | Unsupported Content  | The Interest was dropped because it        |
   | Object Hash          | requested a Content Object Hash            |
   | Algorithm (Section   | Restriction using a hash algorithm that    |
   | 10.3.8)              | cannot be computed.                        |
   |                      |                                            |
   | Malformed Interest   | The Interest was dropped because it did    |
   | (Section 10.3.9)     | not correctly parse.                       |

                   Table 3: Interest Return Reason Codes

10.3.  Interest Return Protocol

   This section describes the Forwarder behavior for the various Reason
   codes for Interest Return.  A Forwarder is not required to generate
   any of the codes, but if it does, it MUST conform to this

   If a Forwarder receives an Interest Return, it SHOULD take these
   standard corrective actions.  A forwarder is allowed to ignore
   Interest Return messages, in which case its PIT entry would go
   through normal timeout processes.

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   o  Verify that the Interest Return came from a next-hop to which it
      actually sent the Interest.

   o  If a PIT entry for the corresponding Interest does not exist, the
      Forwarder should ignore the Interest Return.

   o  If a PIT entry for the corresponding Interest does exist, the
      Forwarder MAY do one of the following:

      *  Try a different forwarding path, if one exists, and discard the
         Interest Return, or

      *  Clear the PIT state and send an Interest Return along the
         reverse path.

   If a forwarder tries alternate routes, it MUST ensure that it does
   not use same same path multiple times.  For example, it could keep
   track of which next hops it has tried and not re-use them.

   If a forwarder tries an alternate route, it may receive a second
   InterestReturn, possibly of a different type than the first
   InterestReturn.  For example, node A sends an Interest to node B,
   which sends a No Route return.  Node A then tries node C, which sends
   a Prohibited.  Node A should choose what it thinks is the appropriate
   code to send back to its previous hop

   If a forwarder tries an alternate route, it should decrement the
   Interest Lifetime to account for the time spent thus far processing
   the Interest.

10.3.1.  No Route

   If a Forwarder receives an Interest for which it has no route, or for
   which the only route is back towards the system that sent the
   Interest, the Forwarder SHOULD generate a "No Route" Interest Return

   How a forwarder manages the FIB table when it receives a No Route
   message is implementation dependent.  In general, receiving a No
   Route Interest Return should not cause a forwarder to remove a route.
   The dynamic routing protocol that installed the route should correct
   the route or the administrator who created a static route should
   correct the configuration.  A forwarder could suppress using that
   next hop for some period of time.

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10.3.2.  HopLimit Exceeded

   A Forwarder MAY choose to send HopLimit Exceeded messages when it
   receives an Interest that must be forwarded off system and the
   HopLimit is 0.

10.3.3.  Interest MTU Too Large

   If a Forwarder receives an Interest whose MTU exceeds the prescribed
   minimum, it MAY send an "Interest MTU Too Large" message, or it may
   silently discard the Interest.

   If a Forwarder receives an "Interest MTU Too Large" is SHOULD NOT try
   alternate paths.  It SHOULD propagate the Interest Return to its
   previous hops.

10.3.4.  No Resources

   If a Forwarder receives an Interest and it cannot process the
   Interest due to lack of resources, it MAY send an InterestReturn.  A
   lack of resources could be the PIT table is too large, or some other
   capacity limit.

10.3.5.  Path Error

   If a forwarder detects an error forwarding an Interest, such as over
   a reliable link, it MAY send a Path Error Interest Return indicating
   that it was not able to send or repair a forwarding error.

10.3.6.  Prohibited

   A forwarder may have administrative policies, such as access control
   lists, that prohibit receiving or forwarding an Interest.  If a
   forwarder discards an Interest due to a policy, it MAY send a
   Prohibited InterestReturn to the previous hop.  For example, if there
   is an ACL that says /parc/private can only come from interface e0,
   but the Forwarder receives one from e1, the Forwarder must have a way
   to return the Interest with an explanation.

10.3.7.  Congestion

   If a forwarder discards an Interest due to congestion, it MAY send a
   Congestion InterestReturn to the previous hop.

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10.3.8.  Unsupported Content Object Hash Algorithm

   If a Content Object Hash Restriction specifies a hash algorithm the
   forwarder cannot verify, the Interest should not be accepted and the
   forwarder MAY send an InterestReturn to the previous hop.

10.3.9.  Malformed Interest

   If a forwarder detects a structural or syntactical error in an
   Interest, it SHOULD drop the interest and MAY send an InterestReturn
   to the previous hop.  This does not imply that any router must
   validate the entire structure of an Interest.

11.  Acknowledgements

12.  IANA Considerations

   This memo includes no request to IANA.


13.  Security Considerations

   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.
   Verifying digital signatures requires a public key operation in the
   data plane.  This can be abused as a denial-of-service vector against
   caching nodes.  Therefore, it is recommended that caching routers
   only cache Content Objects which can be verified by an Interest

   If two adjacent peers require authenticated messaging, they must use
   an external mechanism such as MACSEC.

14.  References

14.1.  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-

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14.2.  Informative References

   [CCN]      PARC, Inc., "CCNx Open Source", 2007,

              Mosko, M., Solis, I., and C. Wood, "CCNx Messages in TLV
              Format (Internet draft)", 2017,

   [CCNxURI]  Mosko, M. and C. Wood, "The CCNx URI Scheme (Internet
              draft)", 2017,

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003, <https://www.rfc-

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008, <https://www.rfc-

Authors' Addresses

   Marc Mosko
   PARC, Inc.
   Palo Alto, California  94304

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

   Ignacio Solis
   Mountain View, California  94043

   Email: nsolis@linkedin.com

Mosko, et al.            Expires March 15, 2018                [Page 27]

Internet-Draft               CCNx Semantics               September 2017

   Christopher A. Wood
   University of California Irvine
   Irvine, California  92697

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

Mosko, et al.            Expires March 15, 2018                [Page 28]

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