ICNRG                                                           M. Mosko
Internet-Draft                                                PARC, Inc.
Intended status: Experimental                                   I. Solis
Expires: July 28, 2019                                          LinkedIn
                                                                 C. Wood
                                         University of California Irvine
                                                        January 24, 2019

                             CCNx Semantics


   This document describes the core concepts of the Content Centric
   Networking (CCNx) architecture and presents a 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.

   This document is a product of the Information Centric Networking
   research group (ICNRG).

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
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   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
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 28, 2019.

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Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   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 . . . . . . . . . . . . . . . . . .   4
     1.2.  Architecture  . . . . . . . . . . . . . . . . . . . . . .   4
     1.3.  Protocol Overview . . . . . . . . . . . . . . . . . . . .   5
   2.  Protocol  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     2.1.  Message Grammar . . . . . . . . . . . . . . . . . . . . .   9
     2.2.  Consumer Behavior . . . . . . . . . . . . . . . . . . . .  12
     2.3.  Publisher Behavior  . . . . . . . . . . . . . . . . . . .  14
     2.4.  Forwarder Behavior  . . . . . . . . . . . . . . . . . . .  14
       2.4.1.  Interest HopLimit . . . . . . . . . . . . . . . . . .  15
       2.4.2.  Interest Aggregation  . . . . . . . . . . . . . . . .  16
       2.4.3.  Content Store Behavior  . . . . . . . . . . . . . . .  17
       2.4.4.  Interest Pipeline . . . . . . . . . . . . . . . . . .  18
       2.4.5.  Content Object Pipeline . . . . . . . . . . . . . . .  18
   3.  Names . . . . . . . . . . . . . . . . . . . . . . . . . . . .  19
     3.1.  Name Examples . . . . . . . . . . . . . . . . . . . . . .  20
     3.2.  Interest Payload ID . . . . . . . . . . . . . . . . . . .  21
   4.  Cache Control . . . . . . . . . . . . . . . . . . . . . . . .  21
   5.  Content Object Hash . . . . . . . . . . . . . . . . . . . . .  22
   6.  Link  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   7.  Hashes  . . . . . . . . . . . . . . . . . . . . . . . . . . .  22
   8.  Validation  . . . . . . . . . . . . . . . . . . . . . . . . .  23
     8.1.  Validation Algorithm  . . . . . . . . . . . . . . . . . .  23
   9.  Interest to Content Object matching . . . . . . . . . . . . .  24
   10. Interest Return . . . . . . . . . . . . . . . . . . . . . . .  25
     10.1.  Message Format . . . . . . . . . . . . . . . . . . . . .  25
     10.2.  ReturnCode Types . . . . . . . . . . . . . . . . . . . .  26
     10.3.  Interest Return Protocol . . . . . . . . . . . . . . . .  26
       10.3.1.  No Route . . . . . . . . . . . . . . . . . . . . . .  27
       10.3.2.  HopLimit Exceeded  . . . . . . . . . . . . . . . . .  28
       10.3.3.  Interest MTU Too Large . . . . . . . . . . . . . . .  28

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       10.3.4.  No Resources . . . . . . . . . . . . . . . . . . . .  28
       10.3.5.  Path Error . . . . . . . . . . . . . . . . . . . . .  28
       10.3.6.  Prohibited . . . . . . . . . . . . . . . . . . . . .  28
       10.3.7.  Congestion . . . . . . . . . . . . . . . . . . . . .  29
       10.3.8.  Unsupported Content Object Hash Algorithm  . . . . .  29
       10.3.9.  Malformed Interest . . . . . . . . . . . . . . . . .  29
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  29
   12. Security Considerations . . . . . . . . . . . . . . . . . . .  29
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     13.2.  Informative References . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   This document describes the principles of the CCNx architecture.  It
   describes a network protocol that uses a hierarchical name to forward
   requests and to match responses to requests.  It does not use
   endpoint addresses, such as Internet Protocol.  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 concerns the semantics of the protocol and is not dependent
   on a specific wire format encoding.  The CCNx Messages [CCNMessages]
   document describes a type-length-value (TLV) wire protocol encoding.
   This section introduces the main concepts of CCNx, which are further
   elaborated in the remainder of the document.

   The CCNx protocol derives from the early ICN work by Jacobson et al.
   [nnc].  Jacobson's version of CCNx is known as the 0.x version ("CCNx
   0.x") and the present work is known as the 1.0 version ("CCNx 1.0").
   There are two active implementations of CCNx 1.0.  The most complete
   implementation is Community ICN (CINC) [cicn], a Linux Foundation
   project hosted at fd.io.  Another active implementation is CCN-lite
   [ccnlite], with support for IoT systems and the RIOT operating
   system.  CCNx 0.x formed the basis of the Named Data Networking [ndn]
   (NDN) university project.

   The current CCNx 1.0 specification diverges from CCNx 0.x in a few
   significant areas.  The most pronounced behavioral difference between
   CCNx 0.x and CCNx 1.0 is that CCNx 1.0 has a simpler response
   processing behavior.  In both versions, a forwarder uses a
   hierarchical longest prefix match of a request name against the
   forwarding information base (FIB) to send the request through the
   network to a system that can issue a response.  A forwarder must then
   match a response's name to a request's name to determine the reverse

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   path and deliver the response to the requester.  In CCNx 0.x, the
   Interest name may be a hierarchical prefix of the response name,
   which allows a form of layer 3 content discovery.  In CCNx 1.0, a
   response's name must exactly equal a request's name.  Content
   discovery is performed by a higher-layer protocol.

   CCNx Selectors [selectors] is an example of using a higher-layer
   protocol on top of the CCNx 1.0 layer-3 to perform content discovery.
   The selector protocol uses a method similar to the original CCNx 0.x
   techniques without requiring partial name matching of a response to a
   request in the forwarder.

   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 draft 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.

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.  Architecture

   We describe the architecture of the network in which CCNx operates
   and introduce certain terminology from [terminology].  The detailed
   behavior of each component and message grammars are in Section 2.

   A producer (also called a publisher) is an endpoint that encapsualtes
   content in Content Objects for transport in the CCNx network.  A
   producer has a public/private keypair and signs (directly or
   indirectly) the content objects.  Usually, the producer's keyid (hash
   of the public key) is well-known or may be derived from the
   producer's namespace via standard means.

   A producer operates within one or more namespaces.  A namespace is a
   name prefix that is represented in the forwarding information base
   (FIB).  This allows a request to reach the producer and fetch a
   response (if one exists).

   The forwarding information base (FIB) is a table that tells a
   forwarder where to send a request.  It may point to a local

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   application, a local cache or content store, or to a remote system.
   If there is no matching entry in the FIB, a forwarder cannot process
   a request.  The detailed rules on name matching to the FIB are given
   in Section 2.4.4.  An endpoint has a FIB, though it may be a simple
   default route.  An intermediate system (i.e. a router) typically has
   a much larger FIB.  A core CCNx forwarder, for example, would know
   all the global routes.

   A consumer is an endpoint that requests a name.  It is beyond the
   scope of this document to describe how a consumer learns of a name or
   publisher keyid -- higher layer protocols build on top of CCNx handle
   those tasks, such as search engines or lookup services or well known
   names.  The consumer constructs a request, called an Interest, and
   forwards it via the endpoint's FIB.  The consumer should get back
   either a response, called a Content Object, that matches the Interest
   or a control message, called an InterestReturn, that indicates the
   network cannot handle the request.

   There are three ways to detect errors in Interest handling.  An
   InterestReturn is a network control message that indicates a low-
   level error like no route or out of resources.  If an Interest
   arrives at a producer, but the producer does not have the requested
   content, the producer should send an application-specific error
   message (e.g. a not found message).  Finally, a consumer may not
   receive anything, in which case it should timeout and, depending on
   the application, retry the request or return an error to the

1.3.  Protocol Overview

   The goal of CCNx is to name content and retrieve the content from the
   network without binding it to a specific network endpoint.  A routing
   system (specified separately) populates the forwarding information
   base (FIB) tables at each CCNx router with hierarchical name prefixes
   that point towards the content producers under that prefix.  A
   request finds matching content along those paths, in which case a
   response carries the data, or if no match is found a control message
   indicates the failure.  A request may further refine acceptable
   responses with a restriction on the response's signer and the
   cryptographic hash of the response.  The details of these
   restrictions are described below.

   The CCNx name is a hierarchical series of path segments.  Each path
   segment has a type and zero or more bytes.  Matching two names is
   done as a binary comparison of the type and value, segment by
   segment.  The human-readable form is defined under a URI scheme
   "ccnx:" [CCNxURI], though the canonical encoding of a name is a
   series of (type, octet string) pairs.  There is no requirement that

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   any path segment be human readable or UTF-8.  The first few segments
   in a name will matched against the FIB and a routing protocol may put
   its own restrictions on the routable name components (e.g. a maximum
   length or character encoding rules).  In principle, path segments and
   names have unbounded length, though in practice they are limited by
   the wire format encoding and practical considerations imposed by a
   routing protocol.  Note that in CCNx path segments use binary
   comparison whereas in a URI the authority uses case-insensitive
   hostname (due to DNS).

   The CCNx name, as used by the forwarder, is purposefully left as a
   general octet-encoded type and value without any requirements on
   human readability and character encoding.  The reason for this is
   that we are concerned with how a forwarder processes names.  We
   expect that applications, routing protocols, or other higher layers
   will apply their own conventions and restrictions on the allowed path
   segment types and path segment values.

   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.

   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.  A
   named payload is thus the tuple {{Name, ExtraFields, Payload,
   ValidationAlgorithm}, ValidationPayload}, where all fields in the
   inner tuple are covered by the validation payload (e.g. signature).
   Consumers of this data can check the binding integrity by re-
   computing the same cryptographic hash and verifying the digital
   signature in ValidationPayload.

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   In addition to digital signatures (e.g.  RSA), CCNx also supports
   message authentication codes (e.g.  HMAC) and message integrity codes
   (e.g.  SHA-256 or CRC).  To maintain the cryptographic binding, there
   should be at least one object with a signature or authentication
   code, but not all objects require it.  For example, a first object
   with a signature could refer to other objects via a hash chain, a
   Merkle tree, or a signed manifest.  The later objects may not have
   any validation and rely purely on the references.  The use of an
   integrity code (e.g.  CRC) is intended for detecting accidental
   corruption in an Interest.

   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 optional matching restrictions.  Restrictions limit the
   possible matching Content Objects.  Two restrictions exist:
   KeyIdRestr and ContentObjectHashRestr.  The first restriction on the
   KeyId limits responses to those signed with a ValidationAlgorithm
   KeyId field equal to the restriction.  The second is the Content
   ObjectHash restriction, which limits the response to one where the
   cryptographic hash of the entire named payload is equal to the

   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

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   a method to store state that facilitates the return of a Content

   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.

   An actual PIT table is not mandated by the specification.  An
   implementation may use any technique that gives the same external
   behavior.  There are, for example, research papers that use
   techniques like label switching in some parts of the network to
   reduce the per-node state incurred by the PIT table [dart].  Some
   implementations store the PIT state in the FIB, so there is not a
   second table.

   If multiple Interests with the same {Name, KeyIdRestr,
   ContentObjectHashRestr} 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.

   In CCNx, higher-layer protocols are often 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.
   Aggregating layer-3 content objects in to larger objects is beyond
   the scope of this document.  One proposed method, FLIC [flic], uses a
   manifest to enumerate the pieces of a larger object.  Manifests are,
   themselves, Content Objects.  Another option is to use a convention

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   in the Content Object name, as in the CCNx Chunking [chunking]
   protocol where a large object is broken in to small chunks and each
   chunk receives a special name component indicating its serial order.

   At the semantic level, described in this document, we do not address
   fragmentation.  One experimental fragmentation protocol, BeginEnd
   Fragments [befrags] uses a multipoint-PPP style technique for use
   over layer-2 interfaces with the CCNx Messages [CCNMessages] TLV wire
   forman specification.

   With these concepts, the remainder of the document specifies the
   behavior of a forwarder in processing Interest, Content Object, and
   InterestReturn messages.

2.  Protocol

   CCNx is a request and response protocol.  A request is called an
   Interest and a response is called a Content Object.  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 shown 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).

   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).

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

   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.

   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  ContentObjectHashRestr: The Content Object Hash Restriction.  A
      content object must hash to the same value as the restriction
      using the same HashType.  The ContentObjectHashRestr MUST use

   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 identifier 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
      encoding UTF-8.  An Interest MUST have a Name.  A Content Object
      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 ContentObjectHashRestr of the corresponding Content

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   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  Vendor: Vendor-specific opaque data.  The Vendor data includes the
      IANA Private Enterprise Numbers [EpriseNumbers], followed by
      vendor-specific information.  CCNx allows vendor-specific data in
      most locations of the grammar.

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

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

   ValidationAlg := (RSA-SHA256 / HMAC-SHA256 / CRC32C) *Vendor
   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

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   ConObjField   := ExpiryTime / PayloadType
   ConObjHash    := Hash ; The Content Object Hash
   DataType      := "1"
   ExpiryTime    := AbsTime
   Field         := InterestField / ConObjField / Vendor
   Hash          := HashType 1* OCTET
   HashType      := SHA256-32 / SHA512-64 / SHA512-32
   HopLimit      := OCTET
   InterestField := KeyIdRestr / ContentObjectHashRestr
   KeyId         := 1* OCTET ; key identifier
   KeyIdRestr    := 1* OCTET
   KeyLink       := Link
   KeyType       := "2"
   Lifetime      := RelTime
   Link          := Name [KeyIdResr] [ContentObjectHashRestr]
   LinkType      := "3"
   ContentObjectHashRestr  := Hash
   Payload       := *OCTET
   PayloadType   := DataType / KeyType / LinkType
   PublicKey     := ; DER-encoded public key
   RelTime       := 1* OCTET ; msec
   ReturnCode    := ; see Section 10.2
   SigTime       := AbsTime
   Vendor        := PEN 0* OCTET
   PEN           := ; IANA Private Enterprise Number

                                 Figure 1

2.2.  Consumer Behavior

   To request a piece of content for a given {Name, [KeyIdRest],
   [ContentObjectHashRestr]} 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 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.

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   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 (2 seconds).

   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 Content Object.  The consumer SHOULD:

   o  Ensure the content object is properly formatted.

   o  Verify that the returned Name matches a pending request.  If the
      request also had KeyIdRestr or ObjHashRest, it MUST 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

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      traditional PKI methods, a trusted key resolution service, or
      methods like [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
      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 or not be cacheable (i.e. an expiry time of 0).

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

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   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.

   A loop may also terminate because the Interest is aggregated with
   it's previous PIT entry along the loop.  In this case, the Content
   will be sent back along the loop and eventually return to a node that
   already forwarded the content, so it will likely not have a PIT entry
   any more.  When the content reaches a node without a PIT entry, it
   will be discarded.  It may be that a new Interest or another looped
   Interest will return to that same node, in which case the node will
   either return a cached response to make a new PIT entry, as below.

   The HopLimit is the last resort method to stop Interest loops where a
   Content Object chases an Interest around a loop and where the
   intermediate nodes, for whatever reason, no longer have a PIT entry
   and do not cache the Content Object.

   Every Interest MUST carry a HopLimit.  An Interest received from a
   local application MAY have a 0 HopLimit, which restricts the Interest
   to other local sources.

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

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

   A RECOMMENDED HopLimit processing pipeline is below:

   o  If Interest received from a remote system:

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      *  If received HopLimit is 0, optionally send InterestReturn
         (HopLimit Exceeded), and discard Interest.

      *  Otherwise, decrement the HopLimit by 1.

   o  Process as per Content Store and Aggregation rules.

   o  If the Interest will be forwarded:

      *  If the (potentailly decremented) HopLimit is 0, restrict
         forwarding to the local system.

      *  Otherwise, forward as desired to local or remote systems.

2.4.2.  Interest Aggregation

   Interest aggregation is when a forwarder receives an Interest message
   that could be satisfied by the response to another Interest message
   already forwarded by the node, so the forwarder suppresses forwarding
   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 ContentObjectHashRestr.

   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).

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   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
      forwarded).  If this Interest has a larger HopLimit than the
      pending Interest, it MUST be forwarded.

   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
      InterestExpiry time per previous hop.  In the first case, the
      forwarder might send a Content Object 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.  Content Store Behavior

   The Content Store is a special cache that is an integral part 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 attack via cache poisoning, there are
   special rules about how a Content Store behaves.

   1.  A forwarder MAY implement a Content Store.  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 Content Store MUST NOT
       reply unless it knows the signature on the matching Content
       Object is correct.  It may do this by external knowledge (i.e.,
       in a managed network or system with pre-populated caches) or by
       having the public key and cryptographically verifying the
       signature.  A Content Store is NOT REQURIED to verify signatures;
       if it does not, then it treats these cases like a cache miss.

   3.  If a Content Store chooses to verify signatures, then it MAY do
       so as follows.  If the public key is provided in the Content
       Object itself (i.e., in the PublicKey field) or in the Interest,
       the Content Store MUST verify that the public key's SHA-256 hash
       is equal to the KeyId and that it verifies the signature.  A

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       Content Store MAY verify the digital signature of a Content
       Object before it is cached, but it is not required to do so.  A
       Content Store 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.

   4.  If an Interest has an ContentObjectHashRestr, then the Content
       Store MUST NOT reply unless it knows the the matching Content
       Object has the correct hash.  If it cannot verify the hash, then
       it should treat the Interest as a cache miss.

   5.  It must obey 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

   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
       Content Store 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 Content Object 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

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   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.

   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

   Note that a forwarder does not need to know any semantics about a
   name.  It only needs to be able to match a prefix to forward
   Interests and match an exact or full name to forward Content Objects.
   It is not sensitive to the name segment types.

   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.

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   |     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
   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.  Note that as per the message grammar, an Interest must have a
   Name with at least one name segment and that name segment must have
   at least 1 octet of value.  A Content Object must have a similar name
   or no name at all.  The FIB, on the other hand, could have 0-length
   names (a default route), or a first name segment with no value, or a
   regular name.

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   |           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 Content Object 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 Content Object is considered expired and MUST no longer be
   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 MAY discard the content object

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   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 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 Content ObjectHashRestr.  If neither restriction is
   present, then any Content Object with a matching name from any
   publisher could be returned.

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 supports hash agility.

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   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
   KeyIdRestr, then that KeyIdRestr MUST exactly match the Content
   Object's KeyId.

   Validation Algorithms fall into three categories: MICs, MACs, and
   Signatures.  Validators using Message Integrity Code (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

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

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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 KeyIdRestr, if present, and (c) the computed Content
   ObjectHash exactly equals the Interest ContentObjectHashRestr, if

   The matching rules are given by this predicate, which if it evaluates
   true means the Content Object matches the Interest.  Ni = Name in
   Interest (may not be empty), Ki = KeyIdRestr in the interest (may be
   empty), Hi = ContentObjectHashRestr in Interest (may be empty).
   Likewise, No, Ko, Ho are those properties in the Content Object,
   where No and Ko may be empty; Ho always exists (it is an intrinsic
   property of the Content Object).  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 ContentObjectHashRestr in the Interest
   specifies an unsupported hash algorithm, then no Content Object 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 Content
   Object 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 Content ObjectHash as an
   expressed field, it must be calculated in network to match against.
   It is sufficient within an autonomous system to calculate a Content
   ObjectHash 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 KeyIdRestr.

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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.

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

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10.2.  ReturnCode Types

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

   | 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

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   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.

   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.

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   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.

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.

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10.3.7.  Congestion

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

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.  IANA Considerations

   This memo includes no request to IANA.

12.  Security Considerations

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

   The CCNx message format includes the ability to attach MICs (e.g.
   SHA-256 or CRC), MACs (e.g.  HMAC), and Signatures (e.g.  RSA or
   ECDSA) to all packet types.  This does not mean that it is a good
   idea to use an arbitrary ValidationAlgorithm, nor to include
   computationally expensive algorithms in Interest packets, as that
   could lead to computational DoS attacks.  Applications should use an
   explicit protocol to guide their use of packet signatures.  As a
   general guideline, an application might use a MIC on an Interest to
   detect unintentionally corrupted packets.  If one wishes to secure an
   Interest, one should consider using an encrypted wrapper and a
   protocol that prevents replay attacks, especially if the Interest is
   being used as an actuator.  Simply using an authentication code or
   signature does not make an Interests secure.  There are several
   examples in the literature on how to secure ICN-style messaging
   [mobile] [ace].

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

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

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

   The specific encoding of messages will have security implications.
   [CCNMessages] uses a type-length-value (TLV) encoding.  We chose to
   compromise between extensibility and unambiguous encodings of types
   and lengths.  Some TLVs use variable length T and variable length L
   fields to accomodate a wide gamut of values while trying to be byte-
   efficient.  Our TLV encoding uses a fixed length 2-byte T and 2-byte
   L.  Using a fixed-length T and L field solves two problems.  The
   first is aliases.  If one is able to encode the same value, such as
   0x2 and 0x02, in different byte lengths then one must decide if they
   mean the same thing, if they are different, or if one is illegal.  If
   they are different, then one must always compare on the buffers not
   the integer equivalents.  If one is illegal, then one must validate
   the TLV encoding -- every field of every packet at every hop.  If
   they are the same, then one has the second problem: how to specify
   packet filters.  For example, if a name has 6 name components, then
   there are 7 T's and 7 L's, each of which might have up to 4
   representations of the same value.  That would be 14 fields with 4
   encodings each, or 1001 combinations.  It also means that one cannot
   compare, for example, a name via a memory function as one needs to
   consider that any embedded T or L might have a different format.

   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

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   Interest payload as an Interest.  It should also verify that it sent
   the Interest in the Interest Return to that node and not allow anyone
   to negate Interest messages.

   Caching nodes must take caution when processing content objects.  It
   is essential that the Content Store obey the rules outlined in
   Section 2.4.3 to avoid certain types of attacks.  Unlike NDN, CCNx
   1.0 has no mechanism to work around an undesired result from the
   network (there are no "excludes"), so if a cache becomes poisoned
   with bad content it might cause problems retrieving content.  There
   are three types of access to content from a content store:
   unrestricted, signature restricted, and hash restricted.  If an
   Interest has no restrictions, then the requester is not particular
   about what they get back, so any matching cached object is OK.  In
   the hash restricted case, the requester is very specific about what
   they want and the content store (and every forward hop) can easily
   verify that the content matches the request.  In the signature
   verified case (often used for initial manifest discovery), the
   requester only knows the KeyId that signed the content.  It is this
   case that requires the closest attention in the content store to
   avoid amplifying bad data.  The content store must only respond with
   a content object if it can verify the signature -- this means either
   the content object carries the public key inside it or the Interest
   carries the public key in addition to the KeyId.  If that is not the
   case, then the content store should treat the Interest as a cache
   miss and let an endpoint respond.

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

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

   A CCNx name uses binary matching whereas a URI uses a case
   insensitive hostname.  Some systems may also use case insensitive
   matching of the URI path to a resource.  An implication of this is

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

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

13.  References

13.1.  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-

13.2.  Informative References

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

   [befrags]  Mosko, M. and C. Tschudin, "ICN "Begin-End" Hop by Hop
              Fragmentation", 2017, <https://www.ietf.org/archive/id/

   [ccnlite]  Tschudin, C., et al., University of Basel, "CCN-Lite V2",
              2011-2018, <http://www.ccn-lite.net/>.

              Mosko, M., Solis, I., and C. Wood, "CCNx Messages in TLV
              Format (Internet draft)", 2018, <https://www.ietf.org/id/

   [ccnxke]   Mosko, M., Uzun, E., and C. Wood, "CCNx Key Exchange
              Protocol Version 1.0", 2017,

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

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              Mosko, M., "CCNx Content Object Chunking", 2016,

   [cicn]     Muscariello, L., et al., Cisco Systems, "Community ICN
              (CICN)", 2017-2018, <https://wiki.fd.io/view/Cicn>.

   [dart]     Garcia-Luna-Aceves, J. and M. Mirzazad-Barijough, "A
              Light-Weight Forwarding Plane for Content-Centric
              Networks", 2016, <https://arxiv.org/pdf/1603.06044.pdf>.

              IANA, "IANA Private Enterprise Numbers", 2015,

   [esic]     Mosko, M. and C. Wood, "Encrypted Sessions In CCNx
              (ESIC)", 2017, <https://www.ietf.org/id/draft-wood-icnrg-

   [flic]     Tschudin, C. and C. Wood, "File-Like ICN Collection
              (FLIC)", 2017, <https://www.ietf.org/archive/id/draft-

   [mobile]   Mosko, M., Uzun, E., and C. Wood, "Mobile Sessions in
              Content-Centric Networks", IFIP Networking, 2017,

   [ndn]      UCLA, "Named Data Networking", 2007,

   [nnc]      Jacobson, V., Smetters, D., Thornton, J., Plass, M.,
              Briggs, N., and R. Braynard, "Networking Named Content",
              2009, <http://dx.doi.org/10.1145/1658939.1658941>.

   [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-

   [RFC7927]  Kutscher, D., Eum, S., Pentikousis, K., Psaras, I.,
              Corujo, D., Saucez, D., Schmidt, T., and M. Waehlisch,
              "Information-Centric Networking (ICN) Research
              Challenges", 2016, <https://trac.tools.ietf.org/html/

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   [RFC7945]  Pentikousis, K., Ohlman, B., Davies, E., Spirou, S., and
              G. Boggia, "Information-Centric Networking: Evaluation and
              Security Considerations", 2016,

              Mosko, M., "CCNx Selector Based Discovery", 2017,

              Wissingh, B., Wood, C., Afanasyev, A., Zhang, L., Oran,
              D., and C. Tschudin, "Information-Centric Networking
              (ICN): CCN and NDN Terminology", 2017,

   [trust]    Tschudin, C., Uzun, E., and C. Wood, "Trust in
              Information-Centric Networking: From Theory to Practice",
              2016, <https://doi.org/10.1109/ICCCN.2016.7568589>.

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

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

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

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