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Reflexive Forwarding for CCNx and NDN Protocols
draft-oran-icnrg-reflexive-forwarding-03

Document Type Active Internet-Draft (individual)
Authors David R. Oran , Dirk Kutscher
Last updated 2022-06-06
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draft-oran-icnrg-reflexive-forwarding-03
ICNRG                                                            D. Oran
Internet-Draft                       Network Systems Research and Design
Updates: 8569, 8609 (if approved)                            D. Kutscher
Intended status: Experimental  University of Applied Sciences Emden/Leer
Expires: 8 December 2022                                     6 June 2022

            Reflexive Forwarding for CCNx and NDN Protocols
                draft-oran-icnrg-reflexive-forwarding-03

Abstract

   Current Information-Centric Networking protocols such as CCNx and NDN
   have a wide range of useful applications in content retrieval and
   other scenarios that depend only on a robust two-way exchange in the
   form of a request and response (represented by an _Interest-Data
   exchange_ in the case of the two protocols noted above).  A number of
   important applications however, require placing large amounts of data
   in the Interest message, and/or more than one two-way handshake.
   While these can be accomplished using independent Interest-Data
   exchanges by reversing the roles of consumer and producer, such
   approaches can be both clumsy for applications and problematic from a
   state management, congestion control, or security standpoint.  This
   specification proposes a _Reflexive Forwarding_ extension to the CCNx
   and NDN protocol architectures that eliminates the problems inherent
   in using independent Interest-Data exchanges for such applications.
   It updates RFC8569 and RFC8609.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 8 December 2022.

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Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Problems with pushing data  . . . . . . . . . . . . . . .   5
     1.2.  Problems with utilizing independent exchanges . . . . . .   5
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   6
   3.  Overview of the Reflexive Forwarding design . . . . . . . . .   6
   4.  Consumer Operation  . . . . . . . . . . . . . . . . . . . . .  11
   5.  Naming of Reflexive Interests . . . . . . . . . . . . . . . .  12
   6.  Producer Operation  . . . . . . . . . . . . . . . . . . . . .  13
   7.  Forwarder Operation . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Forwarder algorithms in pseudocode  . . . . . . . . . . .  15
       7.1.1.  Processing of a normal Interest containing a Reflexive
               Name Prefix TLV . . . . . . . . . . . . . . . . . . .  15
       7.1.2.  Processing of a Reflexive Interest  . . . . . . . . .  15
   8.  State coupling between producer and consumer  . . . . . . . .  16
   9.  Use cases for Reflexive Interests . . . . . . . . . . . . . .  16
     9.1.  Achieving Remote Method Invocation with Reflexive
           Interests . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.2.  RESTful Web Interactions  . . . . . . . . . . . . . . . .  19
     9.3.  Achieving simple data pull from consumers with reflexive
           Interests . . . . . . . . . . . . . . . . . . . . . . . .  19
   10. Implementation Considerations . . . . . . . . . . . . . . . .  23
     10.1.  Forwarder implementation considerations  . . . . . . . .  23
       10.1.1.  Interactions with Input Processing of Interest and
               Data packets  . . . . . . . . . . . . . . . . . . . .  23
       10.1.2.  Interactions with Interest Lifetime  . . . . . . . .  24
       10.1.3.  Interactions with Interest aggregation and multi-path/
               multi-destination forwarding  . . . . . . . . . . . .  25
     10.2.  Consumer Implementation Considerations . . . . . . . . .  26
       10.2.1.  Data objects returned by the consumer to reflexive
               name Interests arriving from a producer . . . . . . .  26
       10.2.2.  Terminating unwanted reflexive Interest exchanges  .  26
       10.2.3.  Interactions with caching  . . . . . . . . . . . . .  27
     10.3.  Producer Implementation Considerations . . . . . . . . .  27
   11. Operational Considerations  . . . . . . . . . . . . . . . . .  27
   12. Mapping to CCNx and NDN packet encodings  . . . . . . . . . .  28

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     12.1.  Packet encoding for CCNx . . . . . . . . . . . . . . . .  29
     12.2.  Packet encoding for NDN  . . . . . . . . . . . . . . . .  29
       12.2.1.  Reflexive Name Component Type  . . . . . . . . . . .  29
       12.2.2.  Reflexive Name Prefix TLV  . . . . . . . . . . . . .  30
       12.2.3.  PIT Tokens for NDNLPv2 . . . . . . . . . . . . . . .  30
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  31
     13.1.  Reflexive Name Prefix TLV  . . . . . . . . . . . . . . .  31
     13.2.  Forward and Reverse PIT-Token hop-by-hop option TLVs . .  31
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  32
     14.1.  Collisions of reflexive Interest names . . . . . . . . .  32
     14.2.  Additional resource pressure on PIT and FIB  . . . . . .  33
     14.3.  Potential Vulnerabilities from the use of PIT Tokens . .  33
     14.4.  Privacy Considerations . . . . . . . . . . . . . . . . .  33
   15. Normative References  . . . . . . . . . . . . . . . . . . . .  34
   16. Informative References  . . . . . . . . . . . . . . . . . . .  34
   Appendix A.  Alternative Designs Considered . . . . . . . . . . .  38
     A.1.  Handling reflexive interests using dynamic FIB entries  .  38
       A.1.1.  Design complexities and performance issues with
               FIB-based design  . . . . . . . . . . . . . . . . . .  39
       A.1.2.  Interactions between FIB-based design and Interest
               Lifetime  . . . . . . . . . . . . . . . . . . . . . .  40
     A.2.  Reflexive forwarding using Path Steering  . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

1.  Introduction

   Current ICN protocols such as CCNx [RFC8569] and NDN [NDN] have a
   wide range of useful applications in content retrieval and other
   scenarios that depend only on a robust two-way exchange in the form
   of a request and response.  These ICN architectures use the terms
   "consumer" and "producer" for the respective roles of the requester
   and the responder, and the protocols directly capture the mechanics
   of the two-way exchange through the "Interest message" carrying the
   request, and the "Data message" carrying the response.  Through these
   constructs, the protocols are heavily biased toward a pure _pull-
   based_ interaction model where requests are small (carrying little or
   no user-supplied data other than the name of the requested data
   object), and responses are relatively large - up to an architecture-
   defined maximum transmission unit (MTU) on the order of kilobytes or
   tens of kilobytes.

   A number of important applications however require interaction models
   more complex than individual request/response interactions in the
   same direction (i.e. between the same consumer and one or more
   producers).  Among these we identify three important classes which
   are the target of the proposed enhancements defined in this
   specification.  These are described in the following paragraphs.

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   *Remote Method Invocation (RMI, aka RPC):*  When invoking a remote
      method, it is common for the method to require arguments supplied
      by the caller.  In conventional TCP/IP style protocols like CORBA
      or HTTP "Post", these are pushed to the server as part of the
      message or messages that comprise the request.  In ICN-style
      protocols there is an unattractive choice between inflating the
      request initiation with pushed arguments, or arranging to have one
      or more independent request/response pairs in the opposite
      direction for the server to fetch the arguments.  Both of these
      approaches have substantial disadvantages.  Recently, a viable
      alternative emerged through the work on RICE [Krol2018] which
      pioneered the main design elements proposed in this specification.

   *Phone-Home scenario:*  Applications in sensing, Internet-of-things
      (IoT) and other types where data is produced unpredictably and
      needs to be _pushed_ somewhere create a conundrum for the pure
      pull-based architectures considered here.  If instead one eschews
      relaxing the size asymmetry between requests and responses, some
      additional protocol machinery is needed.  Earlier efforts in the
      ICN community have recognized this issue and designed methods to
      provoke a cooperating element to issue a request to return the
      data the originator desires to push, essentially "phoning home" to
      get the responder to fetch the data.  One that has been explored
      to some extent is the _Interest-Interest-Data_ exchange
      [Carzaniga2011], where an Interest is sent containing the desired
      request as encapsulated data.  CCNx-1.0 Bidirectional Streams
      [Mosko2017] are also based on a scheme where an Interest is used
      to signal a name prefix that a consumer has registered for
      receiving Interests from a peer in a bidirectional streaming
      session.

   *Peer state synchronization:*  A large class of applications,
      typified by those built on top of reliable order-preserving
      transport protocols, require initial state synchronization between
      the peers.  This is accomplished with a three-way (or longer)
      handshake, since employing a two-way handshake as provided in the
      existing NDN and CCNx protocols exposes a number of well-know
      hazards, such as _half-open connections_. When attempted for
      security-related operations such as key exchange, additional
      hazards such as _man-in-the-middle_ attacks become trivial to
      mount.  Existing alternatives, similar to those used in the two
      examples above, instead utilize either overlapping Interest-Data
      exchanges in opposite directions (resulting in a four-way
      handshake) or by adding initialization data to the initial request
      and employing an Interest-Interest-Data protocol extension as
      noted in the Phone-home scenarios above.

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   All of the above application interaction models present interesting
   challenges, as neither relaxing the architecture to support pushing
   large amounts of data, nor introducing substantial complexities
   through multiple independent Interest-Data exchanges is an attractive
   approach.  The following subsections provide further background and
   justification for why push and/or independent exchanges are
   problematical.

1.1.  Problems with pushing data

   There are two substantial problems with the simple approach of just
   allowing arbitrary amounts of data to be included with requests.
   These are:

   1.  In ICN protocols such as NDN and CCNx, Interest messages are
       intended to be small, on the order the size of a TCP ACK, as
       opposed to the size of a TCP data segment.  This is because the
       hop-by-hop congestion control and forwarder state management
       requires Interest messages to be buffered in expectation of
       returning data, and possibly retransmitted hop-by-hop as opposed
       to end-to-end.  In addition, the need to create and manage state
       on a per-Interest basis is substantially complicated if requests
       in Interest messages are larger than a Path MTU (PMTU) and need
       to be fragmented hop-by-hop.

   2.  If the payload data of a request is used for invoking a
       computation (as in the RMI case described above) then substantial
       bandwidth can be wasted if the computation is either refused or
       abandoned for any number of reasons, including the requestor
       failing an authorization check, or the responder not having
       sufficient resources to execute the associated computation.

   These problems also exist in pure datagram transport protocols such
   as those used for legacy RMI applications like NFS [RFC7530].  More
   usual are application protocols like HTTP(s) which rely on the TCP or
   QUIC 3-way handshake to establish a session and then have congestion
   control and segmentation provided as part of the transport protocol,
   further allowing sessions to be rejected before large amounts of data
   are transmitted or significant computational resources expended.

1.2.  Problems with utilizing independent exchanges

   In order to either complete a three-way handshake, or fetch data via
   a pull from the original requestor, the role of consumer and producer
   need to be reversed and an Interest/Data exchange initiated in the
   direction opposite of the initiating exchange.  When done with an
   independent Interest/Data request and response, a number of
   complications ensue.  Among them are:

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   1.  The originating consumer needs to have a routable name prefix
       that can be used for the exchange.  This means the consumer must
       arrange to have its name prefix propagated in the ICN routing
       system with sufficient reach that the producer issuing the
       interest can be assured it is routed appropriately.  While some
       consumers are generally online and act as application servers,
       justifying the maintenance of this routing information, many do
       not.  Further, in mobile environments, a pure consumer that does
       not need to have a routable name prefix can benefit from the
       inherent consumer mobility support in the CCNx and NDN protocols.
       By requiring a routable name prefix, extra mobile routing
       machinery is needed, such as that proposed in KITE [Zhang2018] or
       MAPME [Auge2018].

   2.  The consumer name prefix in item (1) above must be communicated
       to the producer as a payload, name suffix, or other field of the
       initiating Interest message.  Since this name in its entirety is
       chosen by the consumer, it is highly problematic from a security
       standpoint, as it can recruit the producer to mount a reflection
       attack against the consumer's chosen victim.

   3.  The correlation between the exchanges in opposite directions must
       be maintained by both the consumer and the producer as
       independent state, as opposed to being architecturally tied
       together as would be the case with a conventional 3-way handshake
       finite state machine.  While this can of course be accomplished
       with care by both parties, experience has shown that it is error
       prone (for example see the checkered history of interactions
       between the SIP [RFC3261] and SDP Offer-Answer [RFC6337])
       protocols.  When employed as the wrapper for a key management
       protocol such as with TLS [RFC8446] state management errors can
       be catastrophic for security.

2.  Requirements Language

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

3.  Overview of the Reflexive Forwarding design

   This specification defines a _Reflexive Forwarding_ extension to CCNx
   and NDN that avoids the problems enumerated in Sections 1.1 and 1.2.
   It straightforwardly exploits the hop-by-hop state and symmetric
   routing properties of the current protocols.

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   Figure 1 below illustrates a canonical NDN/CCNx forwarder with its
   conceptual data structures of the Content Store (CS), Pending
   Interest Table (PIT) and Forwarding Information Base (FIB).  The key
   observation involves the relation between the PIT and the FIB.  Upon
   arrival of an Interest, a PIT entry is created which contains state
   recording the incoming interface on which the Interest was received
   on.  If the Interest is not immediately satisfied by cached data in
   the CS, the forwarder looks up the name in the FIB to ascertain the
   _next-hop_ to propagate the Interest onward upstream toward the named
   producer.  Therefore, a chain of forwarding state is established
   during Interest forwarding that couples the PIT entries of the chain
   of forwarders together conceptually as _breadcrumbs_. These are used
   to forward the returning Data Message over the inverse path through
   the chain of forwarders until the Data message arrives at the
   originating consumer.  The state in the PITs is _unwound_ by
   destroying it as each PIT entry is _satisfied_. This behavior is
   *critical* to the feasibility of the reflexive forwarding design we
   propose.

    +-----------------------------------------------------------------+
    |                                                      ICN Node   |
    | Send data to all                                     ========   |
    | interfaces that                                                 |
    | requested it                                                    |
    |                  YES +------------------+                       |
   <------------------------| Pending Interest |  <---------------------
    |              |       |    Table (PIT)   |               Data    |
    |              |       +------------------+  1) Find     (Signed) |
    |              | 2) Save         |              Name              |
    |              V    Data         | NO            in               |
    |   +---------------+            |              PIT?              |
    |   | Content Store |            |                                |
    |   |      (CS)     |            |                                |
    |   +---------------+            |                                |
    |                                |                                |
    |                                V                                |
    |                             Drop Data                           |
    |                                                                 |
    +-----------------------------------------------------------------+

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    +-----------------------------------------------------------------+
    |                                                      ICN Node   |
    |                                                      ========   |
    |                                                                 |
    |                                           +====================+|
    |                                           |Forwarding Strategy ||
    |                                           +====================+|
    |                                                                 |
    |   1) Find name          2) Matching        3) Find matching     |
    |        in CS?              name in PIT?       entry in FIB?     |
    |                    NO                   NO                   YES|
    |  +---------------+   +----------------+   +-------------------+ |
    |  | Content Store |   |   Pending      |   |  Forwarding       | |
   --->|      (CS)     |-->|   Interest     |-->|  Information Base |-->
    |  |               |   |   Table (PIT)  |   |     ( FIB)        | |
    |  +---------------+   +----------------+   +-------------------+ |
    | Return   | YES           YES | NO               NO |            |
    |  Data    |          Add      |   Add               |  Drop      |
    |          |          Incoming |   new               |   or       |
    |   <------|          Itf.     |   Interest          |  NACK      |
    |                              V                     V            |
    |                                                                 |
    +-----------------------------------------------------------------+

                     Figure 1: ICN forwarder structure

   Given the above forwarding properties for Interests, it should be
   clear that while an Interest is outstanding and ultimately arrives at
   a producer who can respond to it, there is sufficient state in the
   chain of forwarders to route not just a returning Data message, but
   potentially another Interest directed through the inverse path to the
   unique consumer who issued the original Interest.  (Section 10.1.3
   describes how Interest aggregation of requests to the same target
   name from multiple consumers interacts with this scheme.)  The key
   question therefore is how to access this state in a way that it can
   be used to forward Interests.

   In order to achieve this _Reflexive Interest_ forwarding on the
   inverse path recorded in the PIT of each forwarder, we need a few
   critical design elements:

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   1.  The Reflexive Interest needs to have a _Name_. This name is what
       the originating consumer will use to match against the Data
       object (or multiple Data objects - more on this later) that the
       producer may request by issuing the Reflexive Interest.  This
       cannot be just any name, but needs to essentially name the state
       already recorded in the PIT and not allow the consumer to
       manufacture an arbitrary name and mount a reflection attack as
       pointed out in Section 1.2, Paragraph 2, Item 2.

   2.  Each forwarder along the inverse path from producer to consumer
       must be able to forward the reflexive Interest towards the
       direction of the Consumer, without relying on global routing
       information, as the Reflexive Name Prefixes are only valid while
       the originating Interest/Data exchange state is present at the
       forwarders.  Essential to this operation is the ability to access
       the PIT entry associated with the original Interest message,
       since that contains the state necessary to identify the ingress
       face of the original Interest, which is the unique (modulo
       aggregation) output face over which the Reflexive Interest needs
       to be forwarded.  The Name assigned by the consumer for Reflexive
       Name Prefix in theory is adequate to the task, but entails an
       expensive and complicated lookup procedure.  In order to make
       this lookup both simple and efficient, we adopt an extended
       version of the "PIT-Token" scheme pioneered by the high-speed
       NIST NDN forwarder [Shi2020].  In this specification, we are
       using _Forward Direction PIT Tokens_ (FPTs) that nodes attach to
       forwarded Interests in the upstream direction, and _Reverse
       Direction PIT Tokens_ (RPTs) that nodes attach to Reflexive
       Interests (as well as regular Data messages) in the downstream
       direction.  We describe the specific processing requirements in
       more detail below.

   3.  There has to be coupling of the state between the originating
       Interest-Data exchange and the enclosed Reflexive Interest-Data
       exchange at both the consumer and the producer.  In our design,
       this is accomplished by the way reflexive interest names are
       chosen.

   The following sections provide the normative details on each of these
   design elements.  The overall interaction flow for reflexive
   forwarding is illustrated below in Figure 2.

+-----------+              +-----------+               +-----------+
| Consumer  |              | Forwarder |               | Producer  |
+-----------+              +-----------+               +-----------+
      |                          |                           |
      | I1                       |                           |
      |------------------------->|                           |

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      |           -------------\ |                           |
      |           | Record RNP |-|                           |
      |           | in PIT(I1) | |                           |
      |           |------------| |                           |
      |                          | --------------------\     |
      |                          |-| Add FPT to I1.FPT |     |
      |                          | |-------------------|     |
      |                          |                           |
      |                          | I1                        |
      |                          |-------------------------->|
      |                          |                           | ------------\
      |                          |                           |-| Check     |
      |                          |                           | | prefix,   |
      |                          |                           | | creating  |
      |                          |                           | | Reflexive |
      |                          |                           | | Interest  |
      |                          |                           | | state     |
      |                          |                           | |-----------|
      |                          |        -----------------\ |
      |                          |        | I1.FPT->I2.RPT |-|
      |                          |        |----------------| |
      |                          |                           |
      |                          |                       RI1 |
      |                          |<--------------------------|
      |                          | --------------------\     |
      |                          |-| use I2.RPTto find |     |
      |                          | | PIT(I1),          |     |
      |                          | | check match       |     |
      |                          | | of PIT(I1).RNP    |     |
      |                          | | create PIT(I2),   |     |
      |                          | | forward I2        |     |
      |                          | |-------------------|     |
      |------------------------\ |                           |
      || add I1.RPT and I2.FPT |-|                           |
      || to PIT(I2)            | |                           |
      ||-----------------------| |                           |
      |                          |                           |
      |                       I2 |                           |
      |<-------------------------|                           |
      |                          |                           |
      | D2 obj                   |                           |
      |------------------------->|                           |
      |------------------------\ |                           |
      || consume PIT(I2) entry |-|                           |
      || and forward D2        | |                           |
      ||-----------------------| |                           |
      |       -----------------\ |                           |
      |       | I2.FPT->D2.RPT |-|                           |

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      |       |----------------| |                           |
      |                          | ------------------\       |
      |                          |-| satisfy PIT(I2) |       |
      |                          | |-----------------|       |
      |                          |                           |
      |                          | D2                        |
      |                          |-------------------------->|
      |                          |                           | -------------\
      |                          |                           |-| all        |
      |                          |                           | | parameters |
      |                          |                           | | received,  |
      |                          |                           | | answer I1  |
      |                          |                           | |------------|
      |                          |                           |
      |                          |                 D1 object |
      |                          |<--------------------------|
      |     -------------------\ |                           |
      |     | satisfy PIT(I1), |-|                           |
      |     | forward D1       | |                           |
      |     |------------------| |                           |
      |       -----------------\ |                           |
      |       | I1.FPT->D1.RPT |-|                           |
      |       |----------------| |                           |
      |                          |                           |
      |                       D1 |                           |
      |<-------------------------|                           |
      |                          |                           |
    Legend:
    I1: Interest #1 containing the Reflexive Name Prefix TLV
    RI: Reflexive Interest with Reflexive Name Prefix Component
    RNP: Reflexive Name Prefix
    FPT: Forward Direction PIT Token
    RPT: Reverse Direction PIT Token
    D1: Data message, answering initiating I1 Interest
    D2: Data message, answering RI

                   Figure 2: Message Flow Overview

4.  Consumer Operation

   A consumer that wants to employ Reflexive Forwarding MUST create an
   Interest (I1) with a Reflexive Name Prefix (RNP) TLV that is used by
   the producer when issuing Reflexive Interests (RI) back to the
   consumer.  Upon receiving a Reflexive Interest (e.g.  RI1 in
   Figure 2) from a Producer in response to the Interest whose first
   name component is the RNP supplied earlier, the consumer SHOULD
   perform a name match against the object specified in the Reflexive
   Name, and return that object to the producer in a conventional Data

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   message, (e.g.  D2 in Figure 2).

5.  Naming of Reflexive Interests

   A consumer may have one or more objects for the producer to fetch,
   and therefore needs to communicate enough information in its initial
   Interest to allow the producer to construct properly formed Reflexive
   Interest names.  For some applications the set of _full names_ (see
   the ICN Terminology RFC [RFC8793]) is known a priori, for example
   through compile time bindings of arguments in interface definitions
   or by the architectural definition of a simple sensor reading.  In
   other cases, the full names of the individual objects must be
   communicated in the original Interest message.

   We define a new typed name component, identified by a registered name
   component type in the IANA registry for [RFC8569].  We call this the
   _Reflexive Interest Name Component type_. It MUST be the first (i.e.
   high order) name component of any Reflexive Interest issued by a
   producer.  Its value is a random 128 bit quantity, assigned by the
   consumer, which provides the entropy required to uniquely identify
   the issuing consumer for the duration of any outstanding Interest-
   Data exchange.  We suggest using a UUID as specified in [RFC4122] but
   any scheme that meets the randomness and entropy requirements can
   suffice.  The consumer SHOULD choose a different random value for
   each Interest message it constructs because:

   1.  If there is insufficient randomness, a name collision on the
       Reflexive Names could occur at any of the intermediate forwarders
       which would result in the same mutability problems generated by
       poor name selection in other contexts; and

   2.  Re-use of the same reflexive interest name over multiple
       interactions might reveal linkability information that could be
       used by surveillance adversaries for tracking purposes.

   This initial name component is either communicated by itself through
   a _Reflexive Name Prefix TLV_ in the originating Interest, or
   prepended to any object names the consumer wishes the producer to
   fetch explicitly where there is more than one object needed by the
   producer for the current Interest-Data interaction.  There are four
   cases to consider:

   1.  The reflexive _fullname_ of a single object to fetch.

   2.  A single reflexive name prefix out of which the producer can (by
       application-specific means) construct a number of _fullnames_ of
       the objects it may want to fetch.

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   3.  The reflexive _fullname_ of a FLIC Manifest [I-D.irtf-icnrg-flic]
       enumerating the suffixes that may be used by the producer to
       construct the necessary names.  We distinguish this from the
       single object fetch in case (1) above because the use of a
       Manifest implies multiple reflexive Interest/Data exchanges with
       the consumer.

   4.  Multiple reflexive name TLVs MAY be included in the Interest
       message if none of the above 3 options covers the desired use
       case.

   The last of the four options above, while not explicitly outlawed,
   SHOULD NOT be used.  This is because it results in a longer Interest
   message and requires extra FIB resources.  Hence, it is more likely a
   forwarder will reject the Interest for lack of resources.  A
   forwarder MAY optimize for the case of a single Reflexive Name TLV at
   the expense of those with more than one.

   A producer, upon receiving an Interest with one or more Reflexive
   Name TLVs, may decide it needs to retrieve the associated data
   object(s).  It therefore can issue one or more Reflexive Interests by
   appending the necessary name components needed to form valid full
   names of the associated objects present at the originating consumer.
   These in fact comprise conventional Interest-Data exchanges, with no
   alteration of the usual semantics with regard to signatures, caching,
   expiration, etc.  When the producer has retrieved the required
   objects to complete the original Interest-Data exchange, it can issue
   its Data response, which unwinds all the established state at the
   producer, the consumer, and the intermediate forwarders.

6.  Producer Operation

   A producer that has received an Interest with a Reflexive Name Prefix
   (RNP) MUST store the supplied RNP and the Forward PIT Token (FPT)
   from the received Interest for subsequent (optional, depending on
   application semantics) Reflexive Interest sending.

   When sending a Reflexive Interest back to the consumer, the producer
   MUST construct a corresponding Interest name based on the RNP and
   insert the received Forward PIT Token (FPT) as the Reverse PIT Token
   (RPT) TLV in the reflexive Interest.

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7.  Forwarder Operation

   The forwarder operation for CCNx and/or NDN is changed in the
   following respects when supporting Reflexive Interests.  The
   requirements are slightly different for a simple forwarder meeting
   the mandatory aspects of the specification, versus a forwarder
   designed for high-performance, as discussed later in Section 10.1.1.
   The main differences are in how PIT lookups are done, and whether the
   forwarder only does the steps necessary to process the PIT Tokens
   supplied by upstream and downstream forwarders, or whether it also
   generates and processes its own PIT Tokens.

   1.  Upon receiving an Interest containing a Reflexive Name Prefix
       (RNP) TLV the forwarder MUST record the RNP as an element of the
       PIT entry for that Interest.  (For interactions with Interest
       aggregation, also see Section 10.1.3).

   2.  When forwarding an Interest with a Reflexive Name Prefix (RNP)
       TLV, the forwarder MAY generate a Forward PIT Token (FPT) and
       append it to the forwarded Interest to be processed by the next
       hop.

   3.  If an Interest contains a Reverse PIT Token (RPT), the forwarder
       MAY use that value to access the corresponding PIT entry, or do a
       direct lookup based on the Reflexive Interest Name Prefix.

   4.  The forwarders MUST check that the high-order Name component of
       the Interest is of type RNP.  If not, while this could strictly
       speaking be considered an error, the forwarder SHOULD simply
       process the Interest as a normal non-reflexive Interest and skip
       the steps below.  A match indicates that this is a Reflexive
       Interest corresponding to the original consumer to producer
       Interest, so execute the following steps.

   5.  Create a new PIT entry for the Reflexive Interest (if resources
       are sufficient).  Also, see Section 10.1.1 for how PIT sharding
       interacts with the location and creation of PIT entries on high-
       speed forwarders.

   6.  Record the Forward PIT-Token (FPT), if any, in this PIT entry, as
       would be done for any received Interest containing an FTP TLV.

   7.  Look up the ingress face from the originating Interest's PIT
       entry, forward the Reflexive Interest on this face, with the
       following changes:

       *  Append the the RPT from the ingress face information of the
          original Interest's PIT entry, if any

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       *  If the downstream forwarder desires the upstream forwarder to
          supply an RPT in any returning Data Packet for this Reflexive
          interest, optionally append a FPT TLV to the Interest.

   The PIT entry for the Reflexive Interest is consumed per regular
   Interest/Data message forwarding requirements.  The PIT entry for the
   originating Interest (that communicated the Reflexive Interest Name)
   is also consumed by a final Data message from the producer to the
   original consumer.

7.1.  Forwarder algorithms in pseudocode

   This section provides some pseudocode examples to further explain the
   details of forwarder operation.  It has separate code paths for
   minimal forwarder operations and those needed by high-performance
   forwarders as is further discussed in Section 10.1.1.

7.1.1.  Processing of a normal Interest containing a Reflexive Name
        Prefix TLV

   Create PIT entry for Interest;
   IF interest contains FPT
       Record FPT along with ingress face to use as RPT later;
       Record RNP in PIT entry;
   EITHER
       Create entry in an RNP look-aside table with RNP value;
   OR
       Generate a FPT for this PIT entry and add to Interest;
   Forward Interest upstream;

7.1.2.  Processing of a Reflexive Interest

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   IF Interest contains an RPT
       use RPT to lookup up PIT entry for original interest;
   ELSE
       Use RNP of Interest's Name TLV to lookup original Interest PIT entry;

   IF PIT entry of original Interest not is found
       Issue an Interest Return with "No Route" error back to the producer;
       RETURN;
   ELSE
       Create PIT entry for Reflexive Interest;

   IF RNP of Reflexive Interest matches RNP in PIT entry
       BEGIN
       Extract FPT from Original Interest PIT entry (if any);
       Add FPT to Reflexive interest as RPT for downstream forwarder;
       Optionally, generate and add FPT for the Reflexive Interest for returning Data
       END;
   ELSE
       Process as a normal Interest;

8.  State coupling between producer and consumer

   A consumer that wishes to use this scheme MUST utilize one of the
   reflexive naming options defined in Section 5 and include it in the
   corresponding Interest message.  The Reflexive Name TLV _and_ the
   full name of the requested data object (that identifies the producer)
   identify the common state shared by the consumer and the producer.
   When the producer responds by sending Interests with the Reflexive
   Name Prefix, the original consumer therefore has sufficient
   information to map these Interests to the ongoing Interest-Data
   exchange.

   The exchange is finished when the producer who received the original
   Interest message responds with a Data message (or an Interest Return
   message in the case of error) answering the original Interest.  After
   sending this Data message, the producer SHOULD destroy the
   corresponding shared state.  It MAY decide to use a timer that will
   trigger a later state destruction.  After receiving this Data
   message, the originating consumer MUST destroy the corresponding
   Interest-Data exchange state.

9.  Use cases for Reflexive Interests

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9.1.  Achieving Remote Method Invocation with Reflexive Interests

   RICE (Remote Method Invocation in ICN) [Krol2018] used a similar
   Reflexive Interest Forwarding scheme that inspired the design
   specified in this document (similar to the original design captured
   in Appendix A.1).

   In RICE, the original Interest denotes the remote method (plus
   potential parameters) to be invoked at a producer (server).  Before
   committing any computing resources, the server can then request
   authentication credentials and (optional) parameters using reflexive
   Interest-Data exchanges.

   When the server has obtained the necessary credentials and input
   parameters, it can decide to commit computing resources, starts the
   compute process, and returns a handle ("Thunk") in the final Data
   message to the original consumer (client).

   The client would later request the computation results using a
   regular Interest-Data exchange (outside the Reflexive-Interest
   transaction), using the Thunk as a name for the computation result.

   Figure 3 depicts an abstract message diagram for RICE.  In addition
   to the 4-way Reflexive Forwarding Handshake (see Figure 2 for the
   details of the interaction), RICE adds another (standard) ICN
   Interest/Data exchange for transmitting the RMI result.  The Thunk
   name is provided to the consumer in the D1 DATA message (answering
   the initial I1 Interest).

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   +-----------+              +-----------+
   | Consumer  |              | Producer  |
   +-----------+              +-----------+
         |                          |
         | I1                       |
         |------------------------->|
         |                          | ---------------------\
         |                          |-| Requesting request |
         |                          | | parameters         |
         |                          | | and credentials    |
         |                          | |--------------------|
         |                          |
         |                      RI1 |
         |<-------------------------|
         |                          |
         | RD1                      |
         |------------------------->|
         |                          | --------------------\
         |                          |-| Commit compute    |
         |                          | | resources,        |
         |                          | | return Thunk name |
         |                          | |-------------------|
         |                          |
         |                       D1 |
         |<-------------------------|
         |                          | ----------------\
         |                          |-| Invoke Remote |
         |                          | | Method        |
         |                          | |---------------|
         | -------------------\     |
         |-| After some time, |     |
         | | request result   |     |
         | |------------------|     |
         |                          |
         | I2                       |
         |------------------------->|
         |                          |
         |                       D2 |
         |<-------------------------|
         |                          |
       Legend:
       I1: Interest #1 containing the Reflexive Name Prefix TLV
       D1: Data message, answering initiating I1 Interest,
           returning Thunk name
       RI1: Reflexive Interest issued by producer
       RD1: Data message, answering RI (parameters, credentials)
       I2: Regular Interest for Thunk (compute result)
       D2: Data message, answering I2

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                        Figure 3: RICE Message Flow

9.2.  RESTful Web Interactions

   In todays HTTP-based web, RESTful (Representational State Transfer)
   web interactions are realized by sending requests in a client/server
   interaction, where the requests provides the application context (or
   a reference to it).  It has been noted in [Moiseenko2014] that
   corresponding requests often exceed the response messages in size,
   and that this raises the problems noted in Section 1.1 when
   attempting to map such exchanges directly to CCNx/NDN.

   Another reason not to include all request parameters in a (possibly
   encrypted) Interest message is the fact that a server (that is
   serving thousands of clients) would be obliged to receive, possibly
   decrypt and parse the complete requests before being able to
   determine whether the requestor is authorized, whether the request
   can be served etc.  Many non-trivial requests could thus lead to
   computational overload attacks.

   Using Reflexive Interest Forwarding for RESTful Web Interactions
   would encode the REST request in the original request, together with
   a Reflexive Interest Prefix that the server could then use to get
   back to the client for authentication credentials and request
   parameters, such as cookies.  The request result (response message)
   could either be transmitted in the Data message answering the
   original request, or - in case of dynamic, longer-running
   computations - in a seperate Interest/Data exchange, potentially
   leveraging the Thunk scheme described in Section 9.1.

   Unlike approaches where clients have to signal a globally routable
   prefix to the network, this approach would not require the client
   (original consumer) to expose its identity to the network (the
   network only sees the temporary Reflexive Name Prefix), but it would
   still be possible to authenticate the client at the server.

9.3.  Achieving simple data pull from consumers with reflexive Interests

   An oft-cited use case for ICN network architectures is _Internet of
   Things_ (IoT), where the sources of data are limited-resource sensor/
   actuators.  Many approaches have been tried (e.g.  [Baccelli2014],
   [Lindgren2016], [Gundogan2018]) with varying degrees of success in
   addressing the issues outlined in Section 1.1.  The reflexive
   forwarding extension may substantially ameliorate the documented
   difficulties by allowing a different model for the basic interaction
   of sensors with the ICN network.

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   Instead of simply acting as a producer (either directly to the
   Internet or indirectly through the use of some form of application-
   layer gateway), the IoT device need only act as a consumer to
   initiate commuhication.  When it has data to provide, it issues a
   "phone-home" Interest message to a pre-configured rendezvous name
   (e.g. an application-layer gateway or ICN Repo [Chen2015]) and
   provides a reflexive name prefix TLV for the data it wishes to
   publish.  The target producer may then issue the necessary reflexive
   Interest message(s) to fetch the data.  Once fetched, validated, and
   stored, the producer then responds to the original Interest message
   with a success indication, possibly containing a Data object if
   needed to allow the originating device to modify its internal state.
   Alternatively, the producer might choose to not respond and allow the
   original Interest to time out, although this is NOT RECOMMENDED
   except in cases where the extra message transmission bandwith is at a
   premium compared to the persistence of stale state in the forwarders.
   We note that this interaction approach mirrors the earlier efforts
   using Interest-Interest-Data designs.

   Figure 4 depicts this interaction with the (optional) D1 message.
   See Figure 2 for the details of the general Reflexive Forwarding
   interaction.

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                +-----------+         +-----------+
                | Consumer  |         | Producer  |
                +-----------+         +-----------+
        ------------\ |                     |
        | new IoT   |-|                     |
        | data item | |                     |
        | produced  | |                     |
        |-----------| |                     |
     ---------------\ |                     |
     | "phone home" |-|                     |
     | by notifying | |                     |
     | producer     | |                     |
     |--------------| |                     |
                      |                     |
                      | I1                  |
                      |-------------------->|
                      |                     | --------------------\
                      |                     |-| generate Interest |
                      |                     | | for IoT data      |
                      |                     | |-------------------|
                      |                     |
                      |                 RI1 |
                      |<--------------------|
   -----------------\ |                     |
   | send requested |-|                     |
   | data object    | |                     |
   |----------------| |                     |
                      |                     |
                      | RD1                 |
                      |-------------------->|
                      |                     | -----------------------\
                      |                     |-| finalize interaction |
                      |                     | | with optional        |
                      |                     | | Data message         |
                      |                     | |----------------------|
                      |                     |
                      |       D1 (optional) |
                      |<--------------------|
                      |                     |
       Legend:
       I1: Interest #1 containing the Reflexive Name Prefix TLV
       D1: Data message (OPTIONAL), finalizing interaction
       RI1: Reflexive Interest requesting the IoT data
       RD1: Data message, answering RI, returning IoT data object
       D1: (optional) Data answering I1

                    Figure 4: "Phone Home" Message Flow

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   There are two approaches that the IoT device can use for its response
   to a reflexive Interest.  It can simply construct a Data Message
   bound through the usual ICN hash name to the reflexive Interest name.
   Since the scope of any data object bound in this way is only the
   duration of the enclosing Interest-Data exchange (see Section 10.2)
   the producer would need to itself construct any persistent Data
   object, name it, and sign it.  This is sometimes the right approach,
   as for some applications the identity of the originating IoT device
   is not important from an operational or security point of view; in
   contrast the identity of the gateway or Repo is what matters.

   If alternatively, the persistent Data object should be bound from a
   naming and security point of view to the originating IoT device, this
   can be easily accomplished.  Instead of directly placing the content
   in a Data object responding to the reflexive Interest as above, the
   consumer encapsulates a complete CCNx/NDN Data message (which
   includes the desired name of the data) in the response to the
   reflexive Interest message.

   The interaction model described above brings a number potential
   advantages, some obvious, some less so.  We enumerate a few of them
   as follows:

   *  By not requiring the IoT device to be actively listening for
      Interests, it can sleep and only wake up if it has something to
      communicate.  Conversely, parties interested in obtaining data
      from the device do not need to be constantly polling in order to
      ascertain if there is new data available.

   *  No forwarder resources are tied up with state apart from the
      actual reflexive forwarding interactions.  All that is needed is
      enough routing state in the FIB to be able to forward the "phone
      home" Interest to an appropriate target producer.  While this
      model does not provide all the richness of a full Pub/Sub system
      (like that described in [Gundogan2018]) we argue it is adequate
      for a large subclass of such applications.

   *  The reflexive interest, through either a name suffix or Interest
      payload, can give the IoT device useful context from which to
      craft its Data object in response.  One highly useful parameter
      would be a robust clock value for the device to use as a timestamp
      of the data, possibly as part of its name, to correctly place it
      in a time seres of sensor readings.  This substantially alleviates
      the need for low-end devices to have a robust time base, as long
      as they trust the producer they contact to provide it.

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10.  Implementation Considerations

   There are a number of important aspects to the reflexive forwarding
   design which affect correctness and performance of existing
   forwarder, consumer, and producer implementations desiring to support
   it.  This section discusses the effect of each of these elements on
   the CCNx/NDN protocol architecture.

10.1.  Forwarder implementation considerations

10.1.1.  Interactions with Input Processing of Interest and Data packets

   Reflexive Interests are designed specifically to be no different from
   any other Interest other than the use of the Reflexive Interest
   Prefix name component type as their high-order name component.  This
   means that a forwarder does not have to have special handling in
   terms of creation, and destruction, and other Interest processing
   needs such as timeouts, Interest satisfaction, and caching of
   returning data in the CS if desired.  However, this design does
   require additional processing for Reflexive Interests not needed in
   the absence of reflexive forwarding.  The most significant
   requirements are:

   *  In order to locate the corresponding PIT entry for the original
      Interest, the forwarder's packet input processing needs to be able
      to efficiently locate the PIT entry of the original Interest that
      contained the RNP TLV.

   *  Ensure that the high order name component of the Reflexive
      Interest matches the RNP stored in that PIT entry.

   There are a few additional considerations to highlight for high-speed
   forwarders however; these are discussed in the following paragraphs.

   In order to achieve forwarding scalability, high speed forwarders
   need to exploit available parallelism in both CPU (through multiple
   cores) and memory (through multiport DRAM and limiting accesses to
   both DRAM and L3 caches).  One commonly-used technique is _PIT
   sharding_, where the forwarder-global PIT is partitoned among cores
   such that all processing of both Interest and Data for a given Name
   is directed at the same core, optimizing both L1 I-cache utilization
   and L2/L3/DRAM throughput and latency.  This is achieved in a number
   of implementations (e.g.  [So2013]) by hashing the fullname in the
   Interest or Data and using that hash to select the assigned
   processing core (and associated memory banks).  This efficiently
   distributes the load and minimizes the number of memory accesses
   other than to bytes of the input packet.

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   Straightforward input name hashing to achieve a sharded PIT has one
   potentially undesirable side effect: the original Interest containing
   the Reflexive Name Prefix TLV and any resultant reflexive Interests
   issued by the producer will likely hash to different PIT shards,
   making any pointers that need to be traversed across shards or cross-
   shard updates expensive, possibly dramatically so.  One could either
   optimize those accesses (as, for example, suggested in the discussion
   of Interest Lifetime in Section 10.1.2) or add special input handling
   of reflexive interests to steer them to the same shard as the
   original interest.  This latter technique is what we have specified
   by making the use of PIT Tokens similar to those in [Shi2020] an
   important element of the design.

10.1.2.  Interactions with Interest Lifetime

   If and when a producer decides to fetch data from the consumer using
   one or more reflexive Interest-Data exchanges, the total latency for
   the original Interest-Data exchange is inflated, potentially by
   multiple RTTs.  It is difficult for a consumer to predict the
   inflation factor when issuing the original Interest, and hence there
   can be a substantial hazard of that Interest lifetime expiring before
   completion of the full multi-way exchange.  This can result in
   persistent failures, which is obviously highly undesirable.

   There is a fairly straightforward technique that can be employed by
   forwarders to avoid these "false" Interest lifetime expirations.  In
   the absence of a superior alternative technique, it is RECOMMENDED
   that all forwarders implement the following algorithm.

   If and when a reflexive Interest arrives matching the original
   Interest's PIT entry, the forwarder examines the Interest lifetime of
   the arriving reflexive Interest.  Call this value _IL_r_. The
   forwarder computes MAX(_IL_t, (IL_r * 1.5)_), and replaces _IL_t_
   with this value.  This in effect ensures that the remaining Interest
   lifetime of the original Interest accounts for the additional 1.5
   RTTs that may occur as a result of the reflexive Interest-Data
   exchange.

      |  We note that this is not unduly expensive in this design where
      |  the two PIT entries are guaranteed to be in the same PIT shard
      |  on a high speed forwarder.  The earlier design discussed in
      |  Appendix A.1.2 required some additional gymnastics.

   While the above approach of inflating the interest lifetime of the
   original Interest to accommodate the additional RTTs of reflexive
   Interest-Data exchanges avoids the timeout problem, this does
   introduce a new vulnerability that must be dealt with.  A Producer,
   either through a bug or malicious intent, could keep an originating

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   Interest-Data exchange alive by continuing to send reflexive
   Interests back to the consumer, while the consumer had no way to
   terminate the enclosing interaction (there is no "cancel Interest"
   function in either NDN nor CCNx).  To eliminate this hazard, if the
   consumer rejects a reflexive interest with a T_RETURN_PROHIBITED
   error, the forwarder(s), in addition to satisfying the coresponding
   PIT entry, MUST also delete the RNP from the original Interest's PIT
   entry, thereby preventing any further reflexive Interests from
   reaching the consumer.  This allows the enclosing Interest-Data
   exchange to either time out or be correctly ended with a Data message
   or Interest Return from the Producer.

10.1.3.  Interactions with Interest aggregation and multi-path/multi-
         destination forwarding

   As with numerous other situations where multiple Interests for the
   same named object arrive containing different parameters (e.g.
   Interest Lifetime, QoS, payload hash) the same phenomenon occurs for
   the reflexive Name TLV.  If Interests with different reflexive name
   prefix TLVs collide, the forwarder MUST NOT aggregate these Interest
   messages and instead MUST create a separate PIT entry for each.  This
   in turn means that a different Forward PIT-Token (FPT) will be placed
   in the individual forwarded Interests.

   Forwarders supporting multi-path forwarding may of course exploit
   this capability for Interests with identical reflexive name prefix
   TLVs, like any other Interests.  There are two sub-cases of multi-
   next hop behavior; regular multi-path (where the split traffic
   reconverges further upstream) and multi-destination (where it doesn't
   and the Interest reaches multiple producers).

   For multi-path, since the Interests that converge upstream carry
   identical reflexive Interest name TLVs, they will get aggregated.
   The forwarder might, just as for any other Interest, decide to either
   do single or multi-path forwarding of that reflexive Interest.  If
   sent multi-path in parallel, these also will reconverge on the
   inverse path and get aggregated.  The inclusion of the Forward PIT-
   Token (FPT) in the forwarded Interest is unaffected by multi-path
   since it is only used on returning Data messages or Reflexive
   Interests to access the correct PIT entry.

   For multi-destination, reflexive Interests might get issued by
   multiple producers, but they will carry the same reflexive name
   prefix and hence be forwarded using the ingress face of the same
   original Interest PIT entry until reaching the join point, at which
   they will get aggregated and thus handled identically to any other
   Interest(s) subject to aggregation.

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10.2.  Consumer Implementation Considerations

10.2.1.  Data objects returned by the consumer to reflexive name
         Interests arriving from a producer

   The Data objects returned to the producer in response to a reflexive
   Interest are normal CCNx/NDN data objects.  The object returned in
   response to a reflexive Interest is named with its hash as the
   trailing component of the reflexive Interest name, and hence the
   scope of the object is under most circumstances meaningful only for
   the duration of the enclosing Interest-Data interaction.  This
   property is ideal for naming and securing data that is "part of" the
   enclosing interaction - things like method arguments, authenticators,
   and key exchange parameters, but not for the creation and naming of
   objects intended to survive outside the current interaction's scope
   (c.f.  Section 9.3, which describes how to provide globally-named
   objects using encapsulation).  In general, the consumer should use
   the following guidelines in creating Data messages in response to
   reflexive Interest messages from the producer.

   (a)  Set the recommended cache time (T_CACHETIME) either to zero, or
        a value no greater than the Interest lifetime (T_INTLIFE) of the
        original Interest messsage.

   (b)  Set the payload type (T_PAYLDTYPE) according to the type of
        object being returned (e.g. object, link, manifest)

   (c)  Set the expiry time (T_EXPIRY) to a value greater than _now_,
        and less than or equal to the _now_ + Interest lifetime
        (T_INTLIFE) of the original Interest messsage.

10.2.2.  Terminating unwanted reflexive Interest exchanges

   A consumer may wish to stop receiving reflxive Interests due to
   possible erors or malicious behavior on the part of the producer.
   Therefore, if the consumer receives an unwanted reflexive Interest,
   it SHOULD reject that interest with a T_RETURN_PROHIBITED error (See
   section 10.3.6 of [RFC8609] ).  This will provoke the forwarders to
   prevent further reflexive Interests from reaching the consumer, as
   described above in Section 10.1.2, Paragraph 5.

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10.2.3.  Interactions with caching

   The reflexive named objects provide "local", temporary names that are
   only defined for one specific interaction between a consumer and a
   producer.  Corresponding Data objects MUST NOT be shared among
   multiple consumers (violating this would require special gyrations by
   the producer since the reflexive Name utilizes per-consumer/per-
   interaction random values).  A producer MUST NOT issue an Interest
   message for any reflexive name after it has sent the final Data
   message answering the original Interest.

   Forwarders SHOULD still cache reflexive Data objects for
   retransmissions within a transactions, but they MUST invalidate or
   remove them from the content store when they forward the final Data
   message answering the original Interest.

10.3.  Producer Implementation Considerations

   Producers receiving an Interest with a Reflexive Name Component, MAY
   decide to issue Interests for the corresponding Data objects.  All
   Reflexive Interest message that a producer sends MUST be sent over
   the face that the original Interest was received on.

11.  Operational Considerations

   This extension represents a substantial enhancement to the CCNx/NDN
   protocol architecture and hence has important forward and backward
   compatibility effects.  The most important of these is that correct
   operation of the scheme requires an unbroken chain of forwarders
   between the consumer and the desired producer that support the
   Reflexive Name TLV, the Forward and Backward PIT-Token TLVs and the
   corresponding forwarder capabilities specified in Section 7.  When
   this invariant is not satisfied, some means is necessary to detect
   and hopefully recover from the error.  We have identified three
   possible approaches to handling the lack of universal deployment of
   forwarders supporting the reflexive forwarding scheme.

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   The first approach simply lets the producer detect the error by
   getting a "no route to destination" error when trying to send an
   Interest to a reflexive name.  This will catch the error, but only
   after forwarding resources are tied up and the producer has done some
   work on the original Interest message.  Further, the producer would
   need a bit of smarts to determine that this is a permanent error and
   not a transient to be retried.  In order for the consumer to attempt
   recovery, there might be a need for some explicit error returned for
   the original interest to tell the consumer what the likely problem
   is.  This approach does not enable an obvious recovery path for the
   consumer either, since if the producer cannot easily detect the
   error, the consumer has no way to know if a retry has any chance of
   succeeding.

   A second approach is to bump the CCNx/NDN protocol version to
   explicitly indicate the lack of compatability.  Such Interests would
   be rejected by forwarders not supporting these protocol extensions.
   A consumer wishing to use the reflexive name TLV together with
   Reverse PIT-Tokens, would use the higher protocol version on those
   Interest messages (but could of course continue to use the current
   version number on other Interest messages).  This is a big hammer,
   but may be called for in this situation because:

   (a)  it detects the problem immediately and deterministically, and

   (b)  one could assume an ICN routing protocol that would only forward
        to a next hop that supports the updated protocol version number.
        The supported forwarder protocol versions would have been
        communicated in the routing protocol ahead of time.

   A third option is to, as a precondition to utilizing the protocol in
   a deployment, create and deploy a neighbor capability exchange
   protocol which will tell a downstream forwarder if the upstream can
   handle the new TLV.  This might avoid the large hammer of updating
   the protocol version, but of course this puts a pretty strong
   dependency on somebody actually designing and publishing such a
   protocol!  On the other hand, a neighbor capability exchange protocol
   for CCNx/NDN would have a number of other substantial benefits, which
   makes it worth seriously considering anyway.

12.  Mapping to CCNx and NDN packet encodings

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12.1.  Packet encoding for CCNx

   For CCNx[RFC8569] this specification defines one new Name Component
   TLV type, and two hop-by-hop option TLVs.

     +==================+================+==========================+
     |      Abbrev      |      Name      |       Description        |
     +==================+================+==========================+
     | T_REFLEXIVE_NAME | Reflexive Name | Name component to use as |
     |                  | Component      | name prefix in Reflexive |
     |                  |                | Interest Messages        |
     +------------------+----------------+--------------------------+

                       Table 1: Reflexive Name TLV

   +========+=========+===============================================+
   | Abbrev |   Name  |                  Description                  |
   +========+=========+===============================================+
   | T_FPT  | Forward | 1-32 byte value chosen by the forwarder for a |
   |        | PIT     | PIT entry communicated upstream toward a      |
   |        | TOKEN   | producer                                      |
   +--------+---------+-----------------------------------------------+
   | T_RPT  | Reverse | 1-32 byte value placed in either a Data       |
   |        | PIT     | packet or a Reflexive Interest packet by a    |
   |        | TOKEN   | producer or forwarder to allow the downstream |
   |        |         | forwarder to access the PIT entry identified  |
   |        |         | by a received forward PIT Token (FPT)         |
   +--------+---------+-----------------------------------------------+

                    Table 2: Hop-by-hop PIT Token TLVs

12.2.  Packet encoding for NDN

   *These are proposed assignments based on [NDNTLV].  Suggestions from
   the NDN team would be greatly appreciated.*

12.2.1.  Reflexive Name Component Type

   The NDN Name component TLVs needs to have a new component type added
   with type RNP (for reflexive name prefix).  We suggest something
   like: *TBD*

      |  *Note:*It seems like the current 0.2.1 packet format only has
      |  allocated two name component types - a _GenericNameComponent_
      |  and a _ImplicitSha256DigestComponent_. Shouldn't there be more
      |  types by now or is this spec out of date?

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12.2.2.  Reflexive Name Prefix TLV

   The Reflexive Name Prefix TLV needs to be added to the NDN Interest
   packet format.  We suggest using [RFC4122], hence something like:

              +---------+----------+------------------------+
              | RNP ::= | RNP-TYPE | TLV-LENGTH(=16) BYTE8) |
              +---------+----------+------------------------+

                Table 3: Proposed Reflexive Name Prefix TLV
                          for NDN Interest Packet

12.2.3.  PIT Tokens for NDNLPv2

   The current NDN Link Protocol current has an assignment for the PIT
   Token mechanism pioneered in [Shi2020].  That approach only needed a
   single field, since PIT Tokens are only used to express one
   "direction" - for consumer-to-producer Interests and producer-to-
   consumer Data messages.  This specification employs PIT Tokens not
   only on enclosing Interest-Data exchanges, but also on Reflexive
   Interests to locate the PIT entry of an enclosing Interest on
   reception by a forwarder.  Therefore we suggest that the existing
   NDNLPv2 assignment of

         +---------------+--------------------------------------+
         | LpHeaderField | PitToken                             |
         +---------------+--------------------------------------+
         | PitToken      | PIT-TOKEN-TYPE TLV-LENGTH 1*32OCTET> |
         +---------------+--------------------------------------+

              Table 4: Current NDNLPv2 PIT Token assignment

   be renamed to indicate its use in the forward direction of consumer
   to producer Interests and returning Data, and a second allocation be
   done for a _Reverse PIT Token_ specifically for inclusion in
   Reflexive Interests as follows:

        +-----------------+--------------------------------------+
        | LpHeaderField   | ReversePitToken                      |
        +-----------------+--------------------------------------+
        | ReversePitToken | PIT-TOKEN-TYPE TLV-LENGTH 1*32OCTET> |
        +-----------------+--------------------------------------+

          Table 5: Proposed NDNLPv2 Reverse PIT Token assignment

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13.  IANA Considerations

13.1.  Reflexive Name Prefix TLV

   Please add the T_REFLEXIVE_NAME component TLV to the CCNx Name types
   TLV types registry of [RFC8609], with Length 16 bytes and type of 128
   bit random value.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |         T_REFLEXIVE_NAME      |               16              |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   |       128 bit value randomly assigned by consumer             |
   +-------------------------------+-------------------------------+

                  Figure 5: Reflexive Name component type

13.2.  Forward and Reverse PIT-Token hop-by-hop option TLVs

   Please add the T_FPT and T_RPT TLVS to the CCNx Hop-by-Hop Type
   Registry of [RFC8609], with Length 1-32 bytes and type of random
   value.

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |             T_FPT             |               1-32            |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   |       1-32 byte value randomly assigned by forwarder          |
   +-------------------------------+-------------------------------+

                 Figure 6: Forward PIT-Token hop-by-hop TLV

                        1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +---------------+---------------+---------------+---------------+
   |             T_RPT             |               1-32            |
   +---------------+---------------+---------------+---------------+
   |                                                               |
   |       1-32 byte value randomly assigned by forwarder          |
   +-------------------------------+-------------------------------+

                 Figure 7: Reverse PIT-Token hop-by-hop TLV

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14.  Security Considerations

   One of the major motivations for the reflexive forwarding extension
   specified in this document is in fact to enable better security and
   privacy characteristics for ICN networks.  The main considerations
   are presented in Section 1, but we briefly recapitulate them here:

   *  Current approaches to authentication and data transfer often use
      payloads in Interest messages, which are clumsy to secure
      (Interest messages must be signed) and as a consequence make it
      very difficult to ensure consumer privacy.  Reflexive forwarding
      moves all sensitive data to the Data messages sent in response to
      reflexive Interests, which are secured in the same manner as all
      other Data messages in the CCNx and NDN protocol designs.

   *  In many scenarios, consumers are forced to also act as producers
      so that data may be fetched by either a particular, or arbitrary
      other party.  The means the consumer must arrange to have a
      routable name prefix and that prefix be disseminated by the
      routing protocol or other means.  This represents both a privacy
      hazard (by revealing possible important information about the
      consumer) and a security concern as it opens up the consumer to
      the full panoply of flooding and crafted Interest Denial of
      Service attacks.

   *  In order to achieve multi-way handshakes, in current designs a
      consumer wishing a producer to communicate back must inform the
      producer of what (globally routable) name to use.  This gives the
      consumer a convenient means to mount a variety of reflection
      attacks by enlisting the producer to send Interests to desired
      victims.

   As a major protocol extension however, this design brings its own
   potential security issues, which are discussed in the following
   subsections.

14.1.  Collisions of reflexive Interest names

   Reflexive Interest names are constructed using 128-bit random
   numbers.  This is intended to ensure an off-path attacker cannot
   easily manufacture a matching reflexive Interest and either
   masquerade as the producer, or mount a denial of service attack on
   the consumer.  It also limits tracking through the linkability of
   Interests containing a re-used random value.

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   Therefore consumers MUST utilize a robust means of generating these
   random values, and it is RECOMMENDED that the [RFC4122] format be
   used, with a pseudo-random number generator (PRNG) approved for use
   with cryptographic protocols.

14.2.  Additional resource pressure on PIT and FIB

   Normal Interest message processing in CCNx and NDN needs to consider
   effect of various resource depletion attacks on the PIT, particularly
   in the form of Interest flooding attacks (see [Gasti2012] for a good
   overview of DoS and DDoS mitigation on ICN networks).  Interest
   messages utilizing this reflexive forwarding extension can place
   additional resource pressure on the PIT.

   While this does not represent a new DoS/DDoS attack vector, the
   ability of a malicious consumer to utilize this extension in an
   attack does represent an increased risk of resource depletion,
   especially if such Interests are given unfair access to PIT and FIB
   resources.  Implementers SHOULD therefore protect PIT and FIB
   resources by weighing requests for reflexive forwarding resources
   appropriately relative to other Interests.

14.3.  Potential Vulnerabilities from the use of PIT Tokens

   By including PIT Tokens in the CCNx or NDN protocol, an attacker has
   the opportunity to manipulate these values by either replacement or
   elision.  So far we do not have enough experimental data nor formal
   security analysis to assess whether useful attacks against the
   protocol via the PIT Tokens can be mounted.  The fields are carried
   differently in CCNx and NDN, but in both cases they are outside the
   cryptographic integrity envelope and not encrypted for
   confidentiality as part of the base protocols.

   For both cases however, the potential vulnerabilities can be foiled,
   at least for point-to-point communication over an L2 hop, by
   employing either link-layer encryption (in the case of CCNx), or by
   encrypting the NDNLPv2 protocol, which carries these fields for NDN.

14.4.  Privacy Considerations

   ICN architectures like CCNx and NDN provide a rich tapestry of
   interesting privacy issues, which have been extensively explored in
   the research literature.  The fundamental tradeoffs for privacy
   concern the risk of exposing the names of information objects to the
   forwarding elements of the network, which is a necessary property of
   any name-based routing and forwarding design.  Numerous approaches
   have been explored with varying degrees of success, such as onion
   routing ([DiBenedettoGTU12]), name encryption ([Ghali2017]), and name

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   obfuscation ([Arianfar2011]) among others.

   Reflexive forwarding does not change the overall landscape of privacy
   tradeoffs, nor seem to introduce additional hazards.  In fact, the
   privacy exposures are confined to the inverse path of forwarders from
   the producer to the consumer, through which the original Interest
   forwarding may have already exposed names on path.  Similar name
   privacy techniques to those cited above may be equally applied to the
   names in reflexive Interests.

   While the individual reflexive Interest-Data exchanges have similar
   properties to those in any NDN or CCNx exchange, the target usages by
   applications may have interaction patterns that are subject to
   relatively straightforward fingerprinting by adversaries.  For
   example, a particular RMI invocation may fingerprint simply through
   the count of arguments fetched by the producer and their sizes.  The
   attacker must however be on path, which somewhat ameliorates the
   exposure hazards.

15.  Normative References

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

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

   [RFC8609]  Mosko, M., Solis, I., and C. Wood, "Content-Centric
              Networking (CCNx) Messages in TLV Format", RFC 8609,
              DOI 10.17487/RFC8609, July 2019,
              <https://www.rfc-editor.org/info/rfc8609>.

16.  Informative References

   [Arianfar2011]
              Arianfar, S., Koponen, T., Raghavan, B., and S. Shenker,
              "On preserving privacy in content-oriented networks, in
              ICN '11: Proceedings of the ACM SIGCOMM workshop on
              Information-centric networking",
              DOI https://doi.org/10.1145/2018584.2018589, August 2011,
              <https://dl.acm.org/doi/10.1145/2018584.2018589>.

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   [Auge2018] Augé, J., Carofiglio, G., Grassi, G., Muscariello, L.,
              Pau, G., and X. Zeng, "MAP-Me: Managing Anchor-Less
              Producer Mobility in Content-Centric Networks, in IEEE
              Transactions on Network, Volume 15, Issue 2",
              DOI 10.1109/TNSM.2018.2796720, June 2018,
              <https://ieeexplore.ieee.org/document/8267132>.

   [Baccelli2014]
              Baccelli, E., Mehlis, C., Hahm, O., Schmidt, T., and M.
              Wählisch, "Information centric networking in the IoT:
              experiments with NDN in the wild, in ACM-ICN '14:
              Proceedings of the 1st ACM Conference on Information-
              Centric Networking", DOI 10.1145/2660129.2660144,
              September 2014,
              <https://dl.acm.org/doi/abs/10.1145/2660129.2660144>.

   [Carzaniga2011]
              Carzaniga, A., Papalini, M., and A.L. Wolf, "Content-Based
              Publish/Subscribe Networking and Information-Centric
              Networking", DOI 10.1145/2018584.2018599, September 2011,
              <https://conferences.sigcomm.org/sigcomm/2011/papers/icn/
              p56.pdf>.

   [Chen2015] Chen, S., Cao, J., and L. Zhu, "NDSS: A Named Data Storage
              System, in International Conference on Cloud and Autonomic
              Computing", DOI 10.1109/ICCAC.2015.12, September 2014,
              <https://ieeexplore.ieee.org/document/7312154>.

   [DiBenedettoGTU12]
              DiBenedetto, S., Gasti, P., Tsudik, G., and E. Uzun,
              "ANDaNA: Anonymous Named Data Networking Application, in
              NDSS 2012", DOI https://arxiv.org/abs/1112.2205v2, 2102,
              <https://www.ndss-symposium.org/ndss2012/andana-anonymous-
              named-data-networking-application>.

   [Gasti2012]
              Gasti, P., Tsudik, G., Uzun, Ersin., and L. Zhang, "DoS
              and DDoS in Named Data Networking, in 22nd International
              Conference on Computer Communication and Networks
              (ICCCN)", DOI 10.1109/ICCCN.2013.6614127, August 2013,
              <https://ieeexplore.ieee.org/document/6614127>.

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   [Ghali2017]
              Tsudik, G., Ghali, C., and C. Wood, "When encryption is
              not enough: privacy attacks in content-centric networking,
              in ICN '17: Proceedings of the 4th ACM Conference on
              Information-Centric Networking",
              DOI https://doi.org/10.1145/3125719.3125723, September
              2017,
              <https://dl.acm.org/doi/abs/10.1145/3125719.3125723>.

   [Gundogan2018]
              Gündoğan, C., Kietzmann, P., Schmidt, T., and M. Wählisch,
              "HoPP: publish-subscribe for the constrained IoT, in ICN
              '18: Proceedings of the 5th ACM Conference on Information-
              Centric Networking", DOI 10.1145/3267955.3269020,
              September 2018,
              <https://dl.acm.org/doi/abs/10.1145/3267955.3269020>.

   [I-D.irtf-icnrg-flic]
              Tschudin, C., Wood, C. A., Mosko, M., and D. Oran, "File-
              Like ICN Collections (FLIC)", Work in Progress, Internet-
              Draft, draft-irtf-icnrg-flic-03, 7 November 2021,
              <https://datatracker.ietf.org/doc/html/draft-irtf-icnrg-
              flic-03>.

   [I-D.oran-icnrg-pathsteering]
              Moiseenko, I. and D. Oran, "Path Steering in CCNx and
              NDN", Work in Progress, Internet-Draft, draft-oran-icnrg-
              pathsteering-06, 5 May 2022,
              <https://datatracker.ietf.org/doc/html/draft-oran-icnrg-
              pathsteering-06>.

   [Krol2018] Krol, M., Habak, K., Oran, D., Kutscher, D., and I.
              Psaras, "RICE: Remote Method Invocation in ICN, in
              Proceedings of the 5th ACM Conference on Information-
              Centric Networking — ICN '18",
              DOI 10.1145/3267955.3267956, September 2018,
              <https://conferences.sigcomm.org/acm-icn/2018/proceedings/
              icn18-final9.pdf>.

   [Lindgren2016]
              Lindgren, A., Ben Abdessiem, F., Ahlgren, B., Schlelén,
              O., and A.M. Malik, "Design choices for the IoT in
              Information-Centric Networks, in 13th IEEE Annual Consumer
              Communications and Networking Conference (CCNC)",
              DOI 10.1109/CCNC.2016.7444905, January 2016,
              <https://ieeexplore.ieee.org/abstract/document/7444905>.

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   [Moiseenko2014]
              Moiseenko, I., Stapp, M., and D. Oran, "Communication
              patterns for web interaction in named data networking",
              DOI 10.1145/2660129.2660152, September 2014,
              <https://dl.acm.org/doi/10.1145/2660129.2660152>.

   [Mosko2017]
              Mosko, M., "CCNx 1.0 Bidirectional Streams",
              arXiv 1707.04738, July 2017,
              <https://arxiv.org/abs/1707.04738>.

   [NDN]      "Named Data Networking", 2020,
              <https://named-data.net/project/execsummary/>.

   [NDNTLV]   "NDN Packet Format Specification", 2016,
              <http://named-data.net/doc/ndn-tlv/>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC4122]  Leach, P., Mealling, M., and R. Salz, "A Universally
              Unique IDentifier (UUID) URN Namespace", RFC 4122,
              DOI 10.17487/RFC4122, July 2005,
              <https://www.rfc-editor.org/info/rfc4122>.

   [RFC6337]  Okumura, S., Sawada, T., and P. Kyzivat, "Session
              Initiation Protocol (SIP) Usage of the Offer/Answer
              Model", RFC 6337, DOI 10.17487/RFC6337, August 2011,
              <https://www.rfc-editor.org/info/rfc6337>.

   [RFC7530]  Haynes, T., Ed. and D. Noveck, Ed., "Network File System
              (NFS) Version 4 Protocol", RFC 7530, DOI 10.17487/RFC7530,
              March 2015, <https://www.rfc-editor.org/info/rfc7530>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8793]  Wissingh, B., Wood, C., Afanasyev, A., Zhang, L., Oran,
              D., and C. Tschudin, "Information-Centric Networking
              (ICN): Content-Centric Networking (CCNx) and Named Data
              Networking (NDN) Terminology", RFC 8793,
              DOI 10.17487/RFC8793, June 2020,
              <https://www.rfc-editor.org/info/rfc8793>.

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   [Shi2020]  Shi, J., Pesavento, D., and L. Benmohamed, "NDN-DPDK: NDN
              Forwarding at 100 Gbps on Commodity Hardware, in
              Proceedings of the 7th ACM Conference on Information-
              Centric Networking — ICN '20",
              DOI 10.1145/3405656.3418715, September 2020,
              <https://dl.acm.org/doi/10.1145/3405656.3418715>.

   [So2013]   So, W., Narayanan, A., and D. Oran, "Named data networking
              on a router: Fast and DoS-resistant forwarding with hash
              tables, in proceedings of Architectures for Networking and
              Communications Systems", DOI 10.1109/ANCS.2013.6665203,
              October 2013,
              <https://ieeexplore.ieee.org/document/6665203>.

   [Zhang2018]
              Zhang, Y., Xia, Z., Mastorakis, S., and L. Zhang, "KITE:
              Producer Mobility Support in Named Data Networking, in
              Proceedings of the 5th ACM Conference on Information-
              Centric Networking — ICN '18",
              DOI 10.1145/3267955.3267959, September 2018,
              <https://conferences.sigcomm.org/acm-icn/2018/proceedings/
              icn18-final23.pdf>.

Appendix A.  Alternative Designs Considered

   During development of this specification, a number of alternative
   designs were considered and at least partially documented.  This
   appendix explains them for historical purposes, and explains why
   these were considered inferior to the design we settled on to carry
   forward.

A.1.  Handling reflexive interests using dynamic FIB entries

   The original draft specification employed the use of dynamically-
   created FIB entries for forwarding Reflexive Interests.  In this
   approach, at each forwarder along the inverse path from producer to
   consumer, a FIB entry must be present that matches this name via
   Longest Name Prefix Match (LNPM), so that when the reflexive interest
   arrives, the forwarder can forward it downstream toward the
   originating consumer.  This FIB entry would point directly to the
   incoming interface on which the corresponding original Interest
   arrived.  The FIB entry needs to be created as part of the forwarding
   of the original Interest so that it is available in time to catch any
   reflexive Interest issued by the producer.  It would usually make
   sense to destroy this FIB entry when the Data message satisfying the
   original Interest arrives since this avoids any dangling stale state.
   Given the design details discussed below, stale FIB state would not
   represent a correctness hazard and hence could be done lazily if

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   desired in an implementation.

   In this scheme, the forwarder operates as follows:

   1.  The forwarder creates short-lifetime FIB entries for any
       Reflexive Interest Name prefixes communicated in an Interest
       message.  If the forwarder does not have sufficient resources to
       do so, it rejects the Interest with the T_RETURN_NO_RESOURCES
       error - the same error used if the forwarder were lacking
       sufficient PIT resources to process the Interest message.

   2.  Those FIB entries are queried whenever an Interest message
       arrives whose first name component is of the type _Reflexive
       Interest Name Component (RNP)_

   3.  The FIB entry gets removed eventually, after the corresponding
       Data message has been forwarded.  One option would be to remove
       the FIB directly after the Data message has been forwarded.
       However, the forwarder might choose do lazy cleanup.

   There are a number of additional considerations with this design that
   need to be dealt with.

A.1.1.  Design complexities and performance issues with FIB-based design

   When processing an Interest containing the reflexive name TLV and
   creating the necessary FIB entry, the forwarder also creates a _back
   pointer_ from that FIB entry to the PIT entry for the Interest
   message that created it.  This PIT entry contains the current value
   of the remaining Interest lifetime or alternatively a value from
   which the remaining Interest lifetime can be easily computed.  Call
   this value _IL_t_.

   The forwarder input thread could key off the high-order name
   component type (one byte) and if reflexive, do a reflexive FIB lookup
   instead of a full name hash.  The reflexive FIB entry would contain
   the shard identity of the matching Interest (concretely, the core id
   servicing the shard) and steer the reflexive interest there.  The
   reflexive name prefix FIB lookup would have to be competitive
   performance-wise with a full-name hash for this to win, however.
   Experimentation is needed to further evaluate such implementation
   tradeoffs for input packet load balancing in high-speed forwarders.

   The FIB is a performance-critical data structure in any forwarder, as
   it needs to support relatively expensive longest name prefix match
   (LNPM) lookup algorithms.  A number of well-known FIB data structures
   are heavily optimized for read access, since for normal Interest
   message processing the FIB changes slowly - only after topological

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   changes or routing protocol updates.  Support for reflexive names
   using dynamic FIB entries changes this, as FIB entries would be
   created and destroyed rapidly as Interest messages containing
   reflexive name TLVs are processed and the corresponding Data messages
   come back.

   While it may be feasible, especially in low-end forwarders handling a
   low packet forwarding rate to ignore this problem, for high-speed
   forwarders there are a number of hazards, including:

   1.  If the entire FIB needs to be locked in order to insert or remove
       entries, this could cause inflated forwarding delays or in
       extreme cases, forwarding performance collapse.

   2.  A number of high-speed forwarder implementations employ a sharded
       PIT scheme (see Section 10.1.1) to better parallelize forwarding
       across processing cores.  The FIB, however, is still a shared
       data structure which is either read without read locks across
       cores, or explicitly copied such that there is a separate copy of
       the FIB for each PIT shard.  Clearly, a high update rate without
       read locks and/or updating many copies of the FIB are
       unattractive implementation options.  (Note: unlike the adopted
       scheme in the main specification, by just depending on a dynamic
       FIB it is not feasible to force reflexive interests to be hashed
       or be otherwise directed to the PIT shard holding the original
       Interest state).

   There are any number of alternative FIB implementations that can work
   adequately.  The most straightforward would be to simply implement a
   "special" FIB for just reflexive name lookups.  This is feasible
   because reflexive names deterministically contain the distinguished
   high-order name component type of T_REFLEXIVE_NAME, whose content is
   a 64-bit value that can be easily hashed to a FIB entry directly,
   avoiding the more expensive LNPM lookup.  Inserts and deletes then
   devolve to the well-understood problem of hash table maintenance.

A.1.2.  Interactions between FIB-based design and Interest Lifetime

   If Interest lifetime handling is implemented naively, it may run
   afoul of a sharded PIT forwarder implementation, since the PIT entry
   for the reflexive Interest and the PIT entry for the original
   Interest may be in different shards.  Therefore, if the update is
   done cross-shard on each reflexive Interest arrival, performance may
   suffer, perhaps dramatically.  Instead, the following approach to
   updating the Interest lifetime after computing the new value is would
   be needed by this FIB-based design for sharded-PIT forwarders.

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   When creating the reflexive FIB entry as above in Appendix A.1.1,
   copy the remaining Interest lifetime from the PIT entry.  Do the PIT
   update if and only if this value is about to expire, thus paying the
   cross-shard update cost only if the original Interest is about to
   expire.  A further optimization at the cost of modest extra
   complexity is to instead _queue_ the update to the core holding the
   shard of the original PIT entry rather than doing the update
   directly.  If the PIT entry expires or is satisfied, instead of
   removing it the associated core checks the update queue and does the
   necessary update.

A.2.  Reflexive forwarding using Path Steering

   We also considered leveraging Path Steering
   [I-D.oran-icnrg-pathsteering] Path Labels that inform the forwarder
   at each hop which outgoing face to use for for forwarding the
   Reflexive Interest.  In this approach, the producer, when creating
   and issuing the Reflexive Interest with the Reflexive Name Prefix
   includes a Path Label to strictly steer the forwarding at all hops
   from the producer to the consumer (strict mode Path Steering).  This
   means, the Reflexive Interest carries the Reflexive Name Prefix, but
   forwarders do not apply LNPM or any other outgoing face selection
   based on the name.  It also eliminates the need for dynamic FIB
   entries as discussed above in Appendix A.1.  Instead the forwarding
   is strictly steered by the Path Label using regular Path Steering
   semantics.

   The message flow using Path Steering would look like the following:

+-----------+                        +-----------+               +-----------+
| Consumer  |                        | Forwarder |               | Producer  |
+-----------+                        +-----------+               +-----------+
      | -----------------------------\     |                           |
      |-| Create I1 with additional, |     |                           |
      | | emptyPath Label            |     |                           |
      | | data structure             |     |                           |
      | | for reverse discovery      |     |                           |
      | |----------------------------|     |                           |
      |                                    |                           |
      | I1 with Path Label                 |                           |
      | and RNP TLV                        |                           |
      |----------------------------------->|                           |
      |                                    | -----------------\        |
      |                                    |-| Add path label |        |
      |                                    | | for adjacency  |        |
      |                                    | | to Consumer    |        |
      |                                    | |----------------|        |
      |                                    |                           |

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      |                                    | I1                        |
      |                                    |-------------------------->|
      |                                    |       ------------------\ |
      |                                    |       | Create RI state |-|
      |                                    |       |-----------------| |
      |                                    |                           |
      |                                    |               RI with RNP |
      |                                    |            and path label |
      |                                    |             (strict mode) |
      |                                    |<--------------------------|
      |        --------------------------\ |                           |
      |        | perform path label      |-|                           |
      |        | switching (no FIB info) | |                           |
      |        |-------------------------| |                           |
      |                                    |                           |
      |                        RI with RNP |                           |
      |<-----------------------------------|                           |
      |                                    |                           |
      | D2 (RNP)                           |                           |
      |----------------------------------->|                           |
      |                                    | --------------------\     |
      |                                    |-| regular PIT-based |     |
      |                                    | | forwarding        |     |
      |                                    | |-------------------|     |
      |                                    |                           |
      |                                    | D2 (RNP)                  |
      |                                    |-------------------------->|
      |                                    |        -----------------\ |
      |                                    |        | all parameters |-|
      |                                    |        | received,      | |
      |                                    |        | answer orig.   | |
      |                                    |        | I1 Interest    | |
      |                                    |        |----------------| |
      |                                    |                           |
      |                                    |                        D1 |
      |                                    |<--------------------------|
      |              --------------------\ |                           |
      |              | regular PIT-based |-|                           |
      |              | forwarding        | |                           |
      |              |-------------------| |                           |
      |                                    |                           |
      |                                 D1 |                           |
      |<-----------------------------------|                           |
      |                                    |                           |

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    Legend:
    I1: Interest #1 containing the Reflexive Name Prefix TLV
    RI: Reflexive Interest with Reflexive Name Prefix Component
    RNP: Reflexive Name Prefix
    D1: Data message, answering initiating I1 Interest
    D2: Data message, answering RI

         Figure 8: Message Flow Overview using Path Steering

   Path Steering uses Path Label data structures on the downstream path
   (from producer to consumer) to discover and collect hop-by-hop
   forwarding information so that consumers can then specify selected
   paths for subsequent Interests.  Reflexive Forwarding would use the
   same data structure, but for "reverse discovery", i.e., in the
   upstream direction from consumer to producer.

   From an operational perspective the path-steering approach does not
   exhibit good properties with respect to backward compatibility.
   Without a complete path of forwarders between consumer and producer
   that support path steering, reflexive interests cannot reach the
   intended consumer.  While we might envision a way to steer a
   subsequent Interest onto a working path as proposed in
   [I-D.oran-icnrg-pathsteering], there is no capability to force
   Interest routing away from an otherwise working path not supporting
   the reflexive name TLV.

Authors' Addresses

   Dave Oran
   Network Systems Research and Design
   4 Shady Hill Square
   Cambridge, MA 02138
   United States of America
   Email: daveoran@orandom.net

   Dirk Kutscher
   University of Applied Sciences Emden/Leer
   Constantiapl. 4
   26723 Emden
   Germany
   Email: ietf@dkutscher.net
   URI:   https://dirk-kutscher.info

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