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ICNRG                                                           M. Mosko
Internet-Draft                                                  I. Solis
Intended status: Experimental                                    C. Wood
Expires: July 14, 2016                                        PARC, Inc.
                                                        January 11, 2016


                             CCNx Semantics
                   draft-irtf-icnrg-ccnxsemantics-01

Abstract

   This document describes the core concepts of the CCNx architecture
   and presents a minimum network protocol based on two messages:
   Interest and Content Object.  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.  It 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 July 14, 2016.

Copyright Notice

   Copyright (c) 2016 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



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   (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  . . . . . . . . . . . . . . . . . .  5
   2.  Named Payload  . . . . . . . . . . . . . . . . . . . . . . . .  6
   3.  Names  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.1.  Name Examples  . . . . . . . . . . . . . . . . . . . . . .  8
   4.  Interests  . . . . . . . . . . . . . . . . . . . . . . . . . .  9
   5.  Content Objects  . . . . . . . . . . . . . . . . . . . . . . . 11
   6.  Link . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  Hashes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Validation . . . . . . . . . . . . . . . . . . . . . . . . . . 14
     8.1.  Validation Algorithm . . . . . . . . . . . . . . . . . . . 14
   9.  Interest to Content Object matching  . . . . . . . . . . . . . 15
   10. Request/Response Protocol  . . . . . . . . . . . . . . . . . . 16
   11. Interest Return  . . . . . . . . . . . . . . . . . . . . . . . 17
     11.1. Message Format . . . . . . . . . . . . . . . . . . . . . . 17
     11.2. ReturnCode Types . . . . . . . . . . . . . . . . . . . . . 18
     11.3. Interest Return Protocol . . . . . . . . . . . . . . . . . 18
       11.3.1.  No Route  . . . . . . . . . . . . . . . . . . . . . . 19
       11.3.2.  HopLimit Exceeded . . . . . . . . . . . . . . . . . . 20
       11.3.3.  Interest MTU Too Large  . . . . . . . . . . . . . . . 20
       11.3.4.  No Resources  . . . . . . . . . . . . . . . . . . . . 20
       11.3.5.  Path Error  . . . . . . . . . . . . . . . . . . . . . 20
       11.3.6.  Prohibited  . . . . . . . . . . . . . . . . . . . . . 20
       11.3.7.  Congestion  . . . . . . . . . . . . . . . . . . . . . 20
       11.3.8.  Unsupported Content Object Hash Algorithm . . . . . . 21
       11.3.9.  Malformed Interest  . . . . . . . . . . . . . . . . . 21
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 23
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 24
   15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     15.1. Normative References . . . . . . . . . . . . . . . . . . . 25
     15.2. Informative References . . . . . . . . . . . . . . . . . . 25
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 26







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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.

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

   The key concept of CCNx is that a subjective name is bound to a fixed
   payload via cryptographic operations.  This implies that the fact
   that a given publisher bound a certain subjective name to a certain
   payload can be verified via cryptographic means.  For example, a
   publisher could use a cryptographic hash over the name and payload,
   sign the hash, and deliver the tuple {Name, Payload, Validation}.
   Additional information would be included as needed by different
   validation mechanisms are used.  A typical named payload is thus
   {Name, Payload, ValidationAlgorithm}.

   CCNx specifies a network protocol around Interests (request messages)
   and Content Objects (response messages) to move named payloads.  An
   Interest includes the Name, the desired payload, and two optional
   restrictions to limit responses to a specific publisher or a specific
   Content Object.  The Content Object response carries a matching Name
   and the specified payload.  Matching a Content Object to an Interest
   is an exact match on the Name.  The CCNx network protocol of
   Interests and Content Objects imposes a restriction on Names: each
   Name should be hierarchical and is used to route towards an
   authoritative source.  The CCNx Name looks like a URI absolute path
   and we use URI terminology to describe CCN Names as made up of name
   segments.  In practice it has a binary encoding, not a text string.

   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.  Some Forwarders may use
   more advanced matching rules that allow both longest matching prefix
   and shorter prefixes.




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   Another central 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 upstream from
   it to the requester.

   There are additional optional attributes in an Interest message that
   can be used to select between multiple Content Objects with matching
   Names (it is possible that multiple publishers could issue Content
   Objects with the same Name).  An Interest may carry an optional
   KeyIdRestriction and / or an optional ContentObjectHashRestriction.
   If either or both of these are present, a Forwarder must ensure that
   they exactly match the corresponding KeyId and computed
   ContentObjectHash in the Content Object.

   As an Interest travels the forward path following the Forwarding
   Information Base (FIB), it leaves behind state at each Forwarder.
   This state is stored in the Pending Interest Table (PIT), which
   tracks the ingress and egress ports as well as the Name,
   KeyIdRestriction (if one exists), and ContentObjectHashRestriction
   (if one exists) of each Interest.  When a Content Object arrives at
   the node, it is exactly matched against that tuple to see if it
   satisfies any Interests in the PIT.  If it does, it is returned along
   the matching Interest's reverse path.  If it does not, the Content
   Object is dropped.

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

   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



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   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
   time.

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

   TODO -- we have not adopted the Requirements Language yet.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].
































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2.  Named Payload

   CCNx supports several cryptographic means to bind a Name to a
   payload.  Interest and Content Object messages include a section
   called the ValidationAlgorithm, which specifies the algorithm to use
   to verify the binding.  Several validation algorithms are supported
   including specific Message Integrity Checks (MICs), Message
   Authentication Codes (MACs), and Signature types.  These are over
   specific wire format encodings.  Additional schemes could be added in
   the future.  Interest and Content Object messages also include a
   section called the ValidationPayload which contains the validation
   output.

   The KeyId is an optional field in the ValidationAlgorithm.  It is an
   octet string that identifies the key used to sign the Content Object.
   It uniquely identifies the publisher.  It is similar to a Subject Key
   Identifier from X509 [RFC 3280, Section 4.2.1.2].  It should be
   derived from the key used to sign, for example SHA-256 hash of the
   key.  It applies to both public/private key systems and to symmetric
   key systems using HMAC.

   A PublicKeyLocator is an optional field in the ValidationAlgorithm.
   It may be one of (a) the signer's public key, (b) the signer's
   certificate, or (c) a KeyName that points to the location of the
   signer's key or certificate.

   A Key inside a PublicKeyLocator is a public key corresponding to the
   signer's private key.  Examples would be PEM or DER encodings.  The
   exact encoding is up to the wire format.

   A Certificate is an X.509 certificate of the signer's public key.
   Examples would be PEM or DER encodings of the certificate.  The exact
   encoding is up to the wire format.

   A KeyName is a CCNx Name and optional signer's KeyId of that name's
   publisher.  The CCNx Name points to a Key or Certificate.  The
   KeyName signer KeyId is of the signer of the target name's Content
   Object, not of the target key or certificate.

   A SignatureTime is an optional field in the ValidationAlgorithm.  It
   may be used as part of the signature validation check to ensure the
   signature was generated around the expected time.  In particular, if
   the public key is conveyed in a certificate with a validity period,
   the verifying system may enforce that the signature came from a
   corresponding period.  Some verifiers might determine a signature
   without a SignatureTime to be invalid.





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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.

   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 Labeled Content Information (LCI [LCI]) URI
   representation of CCNx names .

   +-------------------------+-----------------------------------------+
   |           Name          | Description                             |
   +-------------------------+-----------------------------------------+
   |          lci:/          | A 0-length name, corresponds to a       |
   |                         | default route.                          |
   |                         |                                         |
   |        lci:/NAME=       | A name with 1 segment of 0 length,      |
   |                         | distinct from lci:/.                    |
   |                         |                                         |
   | lci:/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




























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4.  Interests

   An Interest is composed of the tuple {Name, Metadata, Payload,
   Validator}.  These fields are defined below.  Only the Name is
   mandatory.  Other fields, if missing, should be interpreted as "do
   not care".

   Name is a hierarchical path that identifies the resource.  It is
   matched as described above.

   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.

   An Interest may contain Validation fields including a
   ValidationAlgorithm section describing the type of validator to use
   and the ValidationPayload fields containing the output of the
   validation.  Typically this would only be a MIC - a crc, checksum, or
   digest.

   An Interest contains additional fields with information about the
   query.  Two fields - the KeyIdRestriction and
   ContentObjectHashRestriction - serve as selectors to help identify
   the specific Content Object that should be returned.

   The Interest Lifetime element specifies the maximum number of
   milliseconds a Forwarder should retain the Interest in its PIT.  A
   lifetime of "0" means the requester does not expect a response from
   the Interest - it serves as a type of notification.  The lifetime is
   only a guideline for a Forwarder, which may keep an Interest for a
   shorter or longer time, based on local conditions and system policy.
   It may change hop to hop if the Interest is delayed for any
   signifiant amount of time.  It is measured in millisecond resolution,
   so in fast switching it normally would not need to change.  This
   field does not affect the matching of Content Objects.

   The Interest HopLimit element is a counter that is decremented with
   each hop.  It limits the distance an Interest may travel.  The node
   originating the Interest may put in any value - up to the maximum -
   in network byte order.  Each node that receives an Interest with a
   HopLimit decrements the value upon reception.  If the value is 0



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   after the decrement, the Interest cannot be forwarded off the node.
   The PIT entry should also track the maximum HopLimit forwarded.  If
   an Interest with a longer HopLimit arrives, it should be forwarded
   even if it is identical to a previously forwarded Interest.

   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.
   Other implementations may define additional optional headers, for
   example Nonces for loop detection, headers for Differentiated
   Services Code Points (DSCP), or Flow Labels.








































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5.  Content Objects

   A Content Object is composed of the same tuple as an Interest: {Name,
   Metadata, Payload, Validator}.

   The Name is an optional field that identifies the contents.  If the
   name does not exist, then the ContentObject can only be matched by a
   ContentObjectHashRestriction.

   The Payload of a Content Object holds the upper layer payload.  It
   may be encrypted, based on outside information or a protocol
   information header.

   The optional Metadata PayloadType field identifies the type of
   Content Object: "DATA" implies an opaque payload; "LINK" is a Content
   Object with an encoded Link as a payload.  "DATA" is the default.

   The optional field ExpiryTime is a millisecond timestamp containing
   the number of milliseconds since the epoch in UTC of when the
   contents will expire.  If it is not present, there is no expiration
   on the Content Object.  The ExpiryTime should be part of the signed
   information covered by the Validator, if present.

   A publisher or upstream node may include a Recommended Cache Time for
   Content Objects.  It is represented as the number of milliseconds
   since the epoch in UTC as the desired minimum time to keep the
   content object in cache.  This recommendation is a guideline as to
   the useful lifetime of a Content Object, but may be ignored.  The
   RecommendedCacheTime should not be covered by the Validator, if
   present, as nodes may change the value based on their caching.  The
   ExpiryTime takes precedence over the RecommendedCacheTime, if both
   exist.

   Other protocols, such as versioning or chunking, could place other
   kinds of metadata in the Content Object.
















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6.  Link

   A Link is the tuple {Name, KeyId, ContentObjectHashRestriction}.  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 a Link.  This
   tuple may be used as a KeyName to identify a specific object with the
   certificate wrapped key.











































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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, which 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.







































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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.  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 actual cryptographic bytes, such as a CRC
   value or an HMAC value or a signature value.

   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 cryptography such as RSA, DSA, or
   Elliptical Curve (EC).  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 KeyName.  MAC and Signature validators may also
   include a SignatureTime, as described above.

   A PublicKeyLocator KeyName points to a Content Object with an 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 given, exactly matches the Interest name, and (b) the
   ValidationAlgorithm KeyId of the Content Object exactly equals the
   Interest KeyIdRestriction, if given, and (c) the computed
   ContentObjectHash exactly equals the Interest
   ContentObjectHashRestriction, if given.

   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 that if a Content Object does not have a Name, then it
   must match the Interest on at least the Hash restriction.

   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
   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.







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10.  Request/Response Protocol

   As an Interest moves through the network following the FIB table
   based on longest matching prefix, it leaves state at each forwarding
   node.  The state is represented in a notional Pending Interest Table
   (PIT).  The PIT tracks the Name, KeyIdRestriction, and
   ContentObjectHashRestriction to be matched by a Content Object.  If a
   second Interest arrives with the same Name and selector values, it
   may be aggregated with the existing pending Interest.  If the second
   Interest extends the Lifetime of the pending Interest, it should be
   forwarded to extend the life of downstream Interests.

   When a Content Object arrives at a Forwarder, it is matched against
   the Interests in the PIT.  For each matching Interest, the Content
   Object is forwarded along the reverse path of that PIT entry and the
   PIT entry is removed.  A Content Object that does not match any
   Interest in the PIT is dropped.

   A Forwarder may implement a Content Object cache called a Content
   Store.  At the core protocol level, the Content Store must obey
   similar rules as the Forwarder.  If an Interest specifies a
   ContentObjectHashRestriction, the Content Store SHOULD NOT respond
   unless it has verified the hash of the Content Object.  If the
   Interest carries a KeyIdRestriction then the Content Store MUST
   cryptographically verify the signature or not respond.

   TODO: Specify what to do in case of failure.
























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11.  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
      message.

   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
   [CCNMessages].

   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.

11.1.  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
   [CCNMessages].




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11.2.  ReturnCode Types

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

   +----------------------+--------------------------------------------+
   | Name                 | Description                                |
   +----------------------+--------------------------------------------+
   | No Route             | The returning Forwarder has no route to    |
   | (Section 11.3.1)     | the Interest name.                         |
   |                      |                                            |
   | HopLimit Exceeded    | The HopLimit has decremented to 0 and need |
   | (Section 11.3.2)     | to forward the packet.                     |
   |                      |                                            |
   | Interest MTU too     | The Interest's MTU does not conform to the |
   | large                | required minimum and would require         |
   | (Section 11.3.3)     | fragmentation.                             |
   |                      |                                            |
   | No Resources         | The node does not have the resources to    |
   | (Section 11.3.4)     | process the Interest.                      |
   |                      |                                            |
   | Path error           | There was a transmission error when        |
   | (Section 11.3.5)     | forwarding the Interest along a route (a   |
   |                      | transient error).                          |
   |                      |                                            |
   | Prohibited           | An administrative setting prohibits        |
   | (Section 11.3.6)     | processing this Interest.                  |
   |                      |                                            |
   | Congestion           | The Interest was dropped due to congestion |
   | (Section 11.3.7)     | (a transient error).                       |
   |                      |                                            |
   | Unsupported Content  | The Interest was dropped because it        |
   | Object Hash          | requested a Content Object Hash            |
   | Algorithm            | Restriction using a hash algorithm that    |
   | (Section 11.3.8)     | cannot be computed.                        |
   |                      |                                            |
   | Malformed Interest   | The Interest was dropped because it did    |
   | (Section 11.3.9)     | not correctly parse.                       |
   +----------------------+--------------------------------------------+

                   Table 3: Interest Return Reason Codes

11.3.  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



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   specification.

   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.

11.3.1.  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
   message.

   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



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   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.

11.3.2.  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.

11.3.3.  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.

11.3.4.  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.

11.3.5.  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.

11.3.6.  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.

11.3.7.  Congestion

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




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11.3.8.  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.

11.3.9.  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.







































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12.  Acknowledgements


















































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13.  IANA Considerations

   This memo includes no request to IANA.

   All drafts are required to have an IANA considerations section (see
   Guidelines for Writing an IANA Considerations Section in RFCs
   [RFC5226] for a guide).  If the draft does not require IANA to do
   anything, the section contains an explicit statement that this is the
   case (as above).  If there are no requirements for IANA, the section
   will be removed during conversion into an RFC by the RFC Editor.









































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14.  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 send
   the Interest in the Interest Return to that node and not allow anyone
   to negate Interest messages.

   If two peers require authenticated messaging, they must use an
   external mechanism.








































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15.  References

15.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

15.2.  Informative References

   [CCN]      PARC, Inc., "CCNx Open Source", 2007,
              <http://www.CCNx.org>.

   [CCNMessages]
              Mosko, M. and I. Solis, "CCNx Messages in TLV Format
              (Internet draft)", 2016, <http://tools.ietf.org/html/
              draft-irtf-icnrg-ccnxmessages-01>.

   [LCI]      Mosko, M., "Labeled Content Information (Internet draft)",
              2015, <http://tools.ietf.org/html/
              draft-mosko-icnrg-ccnxlabeledcontent-01>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <http://www.rfc-editor.org/info/rfc3552>.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,
              <http://www.rfc-editor.org/info/rfc5226>.



















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Authors' Addresses

   Marc Mosko
   PARC, Inc.
   Palo Alto, California  94304
   USA

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


   Ignacio Solis
   PARC, Inc.
   Palo Alto, California  94304
   USA

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


   Christopher Wood
   PARC, Inc.
   Palo Alto, California  94304
   USA

   Phone: +01 650-XXX-YYYY
   Email: christopher.wood@parc.com
























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