Network Coding Research Group                               K. Matsuzono
Internet-Draft                                                 H. Asaeda
Intended status: Informational                                      NICT
Expires: April 27, 2022                                      C. Westphal
                                                        October 24, 2021

 Network Coding for Content-Centric Networking / Named Data Networking:
                     Considerations and Challenges


   This document describes the current research outcomes in Network
   Coding (NC) for Content-Centric Networking (CCNx) / Named Data
   Networking (NDN), and clarifies the technical considerations and
   potential challenges for applying NC in CCNx/NDN.  This document is
   the product of the Coding for Efficient Network Communications
   Research Group (NWCRG) and the Information-Centric Networking
   Research Group (ICNRG).

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   carefully, as they describe your rights and restrictions with respect
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
     2.1.  Definitions related to NC . . . . . . . . . . . . . . . .   4
     2.2.  Definitions related to CCNx/NDN . . . . . . . . . . . . .   6
   3.  CCNx/NDN Basics . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  NC Basics . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Advantages of NC and CCNx/NDN . . . . . . . . . . . . . . . .   8
   6.  Technical Considerations  . . . . . . . . . . . . . . . . . .   9
     6.1.  Content Naming  . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  11
       6.2.1.  Scope of NC . . . . . . . . . . . . . . . . . . . . .  11
       6.2.2.  Consumer Operation  . . . . . . . . . . . . . . . . .  11
       6.2.3.  Forwarder Operation . . . . . . . . . . . . . . . . .  12
       6.2.4.  Producer Operation  . . . . . . . . . . . . . . . . .  13
       6.2.5.  Backward Compatibility  . . . . . . . . . . . . . . .  14
     6.3.  In-network Caching  . . . . . . . . . . . . . . . . . . .  14
     6.4.  Seamless Consumer Mobility  . . . . . . . . . . . . . . .  15
   7.  Challenges  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.1.  Adoption of Convolutional Coding  . . . . . . . . . . . .  15
     7.2.  Rate and Congestion Control . . . . . . . . . . . . . . .  16
     7.3.  Security  . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.4.  Routing Scalability . . . . . . . . . . . . . . . . . . .  17
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   10. Informative References  . . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   Information-Centric Networking (ICN) in general, and Content-Centric
   Networking (CCNx) [16] or Named Data Networking (NDN) [19] in
   particular, have emerged as a novel communication paradigm advocating
   the retrieval of data based on their names.  This paradigm pushes
   content awareness into the network layer.  It is expected to enable
   consumers to obtain the content they desire in a straightforward and
   efficient manner from the heterogenous networks they may be connected
   to.  The CCNx/NDN architecture has introduced innovative ideas and
   has stimulated research in a variety of areas, such as in-network
   caching, name-based routing, multipath transport, content security.
   One key benefit of requesting content by name is that it eliminates

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   the requirement of establishing a session between the client and a
   specific server, and the content can thereby be retrieved from
   multiple sources.

   In parallel, there has been a growing interest in both academia and
   industry for better understanding the fundamental aspects of Network
   Coding (NC) toward enhancing key system performance metrics such as
   data throughput, robustness and reduction in the required number of
   transmissions through connected networks, and redundant transmission
   on broadcast or point-to-multipoint connections.  NC is a technique
   that is typically used for encoding packets to recover from lost
   source packets at the receiver, and for effectively obtaining the
   desired information in a fully distributed manner.  In addition, NC
   can be used for security enhancements [2] [3] [4] [5].

   From the perspective of the NC transport mechanism, NC can be divided
   into two major categories: coherent NC, and non-coherent NC [39].  In
   coherent NC, the source and destination nodes know the exact network
   topology and the coding operations at intermediate nodes.  When
   multiple consumers are attempting to receive the same content such as
   live video streaming, coherent NC could enable optimal throughput by
   sending the content flow over the constructed optimal multicast trees
   [33].  However, it requires a fully adjustable and specific routing
   mechanism, and a large computational capacity for central
   coordination.  In the case of non-coherent NC, that often comprises
   the use of Random Linear Coding (RLC), it is not necessary to know
   the network topology nor the intermediate coding operations [34].  As
   non-coherent NC works in a completely independent and decentralized
   manner, this approach is more feasible especially in the large-scale
   use cases.

   NC combines multiple packets together with parts of the same content,
   and may do this at the source or at other nodes in the network.
   Network coded packets are not associated with a specific server, as
   they may have been combined within the network.  As NC is focused on
   what information should be encoded in a network packet instead of the
   specific host at which it has been generated, it is in line with the
   architecture of the CCNx/NDN core networking layer.  NC has already
   been implemented for information/content dissemination [6] [7] [8].
   Montpetit, et al., first suggested that NC techniques be exploited to
   enhance key aspects of system performance in ICN [9].  NC provides
   CCNx/NDN with the highly beneficial potential of effectively
   disseminating information in a completely topology independent and
   decentralized manner.

   In this document, we consider how NC can be applied to the CCNx/NDN
   architecture and describe the technical considerations and potential
   challenges for making CCNx/NDN-based communications better using the

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   NC technology.  It should be noted that the presentation of specific
   solutions (e.g., NC optimization methods) for enhancing CCNx/NDN
   performance metrics by exploiting NC is outside the scope of this

   This document represents the collaborative work and consensus of the
   Coding for Efficient Network Communications Research Group (NWCRG)
   and the Information-Centric Networking Research Group (ICNRG).  It is
   not an IETF product and is not a standard.

2.  Terminology

   This section provides the terms related to NC and CCNx/NDN used in
   this document.

2.1.  Definitions related to NC

   The terms regarding NC used in this document are defined as follows.
   These are aligned with RFCs produced by the FEC Framework (FECFRAME)
   IETF Working Groups as well as IRTF Coding for Efficient Network
   Communications Research Group (NWCRG)[22].

   o  Source Packet: A packet originating from the source that
      contributes to one or more source symbols.  The source symbol is a
      unit of data originating from the source that is used as input to
      encoding operations.  For instance, a real-time transport protocol
      (RTP) packet as a whole can constitute a source symbol.  In other
      situations (e.g., to address variable size packets), a single RTP
      packet may contribute to various source symbols.

   o  Coded Packet, or Repair Packet: A packet containing one or more
      coded symbols (also called repair symbol).  The coded symbol is a
      unit of data that is the result of a coding operation, applied
      either to source symbols or (in case of re-coding) source and/or
      coded symbols.  When there is a single coded symbol per coded
      packet, a coded symbol corresponds to a coded packet.

   o  Innovative Packet: A source or coded packet that increases the
      rank of the linear system (also called degrees of freedom) at a

   o  Encoding versus Re-coding versus Decoding: Encoding is an
      operation that takes source symbols as input and produces encoding
      symbols (source or coded symbols) as output.  Re-coding is an
      operation that takes encoding symbols as input and produces
      encoding symbols as output.  Decoding is an operation takes
      encoding symbols as input and produces source symbols as output.

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   The terms regarding coding types are defined as follows:

   o  Random Linear Coding (RLC): A particular form of linear coding
      using a set of random coding coefficients.  Linear coding linearly
      combines a set of input source and/or coded symbols using a given
      set of coefficients and resulting in a coded symbol.  Many linear
      codes exist that differ from the way coding coefficients are drawn
      from a finite field of a given size.

   o  Block Coding: A coding technique wherein the input flow(s) must be
      first segmented into a sequence of blocks; encoding and decoding
      are performed independently on a per-block basis.

   o  Sliding Window Coding or Convolutional Coding: A general class of
      coding techniques that rely on a sliding encoding window.
      Encoding window is a set of source (and coded in the case of re-
      coding) symbols used as input to the coding operations.  The set
      of symbols change over time, as the encoding window slides over
      the input flow(s).  This is an alternative solution to block

   o  Fixed or Elastic Sliding Window Coding: A coding technique that
      generates coded symbol(s) on the fly, from the set of source
      symbols present in the sliding encoding window at that time,
      usually by using linear coding.  The sliding window may be either
      of fixed size or of variable size over the time (also known as
      "Elastic Sliding Window").  For instance, the size may depend on
      acknowledgments sent by the receiver(s) for a particular source
      symbol or source packet (received, decoded, or decodable).

   The terms regarding low-level coding aspects are defined as follows:

   o  Rank of the Linear System or Degrees of Freedom: At a receiver,
      the number of linearly independent equations of the linear system.
      It is also known as "Degrees of Freedom".  The system may be of
      "full rank," wherein decoding is possible, or "partial rank",
      wherein only partial decoding is possible.

   o  Generation, or Block: With block codes, the set of source symbols
      of the input flow(s) that are logically grouped into a block,
      before doing encoding.

   o  Generation Size, or Block Size: With block codes, the number of
      source symbols belonging to a block.  It is equivalent to the
      number of source packets when there is a single source symbol per
      source packet.

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   o  Generation ID, or Block ID: With block codes, the identifier of a
      block to which source and coded symbols belong.  It is also known
      as "Source Block Number (SBN)".

   o  Coding Coefficient: With linear coding, this is a coefficient in a
      certain finite field.  This coefficient may be chosen in different
      ways: for instance, randomly, in a predefined table, or using a
      predefined algorithm plus a seed.

   o  Coding Vector: A set of coding coefficients used to generate a
      certain coded symbol through linear coding.

   o  Finite Field: Finite fields, used in linear codes, have the
      desired property of having all elements (except zero) invertible
      for + and * and no operation over any elements can result in an
      overflow or underflow.  Examples of finite fields are prime fields
      {0..p^m-1}, where p is prime.  Most used fields use p=2 and are
      called binary extension fields {0..2^m-1}, where m often equals 1,
      4 or 8 for practical reasons.

   o  Finite Field size: The number of elements in a finite field.  For
      example the binary extension field {0..2^m-1} has size q=2^m.

2.2.  Definitions related to CCNx/NDN

   The terminologies regarding CCNx/NDN used in this document are
   defined in RFC8793 [17] produced by ICNRG.  They are consistent with
   the relevant documents ([1][18]).

3.  CCNx/NDN Basics

   We briefly explain the key concepts of CCNx/NDN.  Both protocols are
   similar in principle, but differ in some architecture and protocol

   In a CCNx network, there are two types of packets at the network
   level: interest and data packet (defined in Section 3.4 of [17]).
   The term of content object, which means a unit of content data, is an
   alias to data packet [17].  The ICN consumer (defined in Section 3.2
   of [17]) requests a content object by sending an interest that
   carries the name of the data.  One difference to note here between
   CCNx and NDN is that in CCNx [18], the interest is required to carry
   a full name, while in NDN [20], it may carry a name prefix (and
   receive in return any data with a name matching this prefix).

   Once an ICN forwarder (defined in Section 3.2 of [17]) receives an
   interest, it performs a series of lookups: first it checks if it has
   a copy of the requested content object available in the Content Store

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   (CS) (defined in Section 3.3 of [17]).  If it does, it returns the
   data, and the transaction is considered to have been successfully

   If it does not have a copy of the requested content object in the CS,
   it performs a lookup of the Pending Interest Table (PIT) (defined in
   Section 3.3 of [17]) to check if there is already an outgoing
   interest for the same content object.  If there is no such interest,
   then it creates an entry in the PIT that lists the name included in
   the interest, and the interfaces from which it received the interest.
   This is later used to send the content object back, as interest
   packets do not carry a source field that identifies the consumer.  If
   there is already a PIT entry for this name, it is updated with the
   incoming interface of this new interest, and the interest is

   After the PIT lookup, the interest undergoes a Forwarding Information
   Base (FIB) (defined in Section 3.3 of [17]) lookup for selecting an
   outgoing interface.  The FIB lists name prefixes and their
   corresponding forwarding interfaces in order to send the interest
   towards a forwarder that possesses a copy of the requested data.

   Once a copy of the data is retrieved, it is sent back to the
   consumer(s) using the trail of PIT entries; forwarders remove the PIT
   state every time that an interest is satisfied, and may store the
   data in their CS.

   Data packets carry some information for validating the data, and in
   particular, that the data is indeed that which corresponds to the
   name.  This is necessary because authentication of the object is
   crucial in CCNx/NDN.  However, this step is optional at forwarders in
   order to speed up the processing.

   The key aspect of CCNx/NDN is that the consumer of the content does
   not establish a session with a specific server.  Indeed, the
   forwarder or producer (defined in Section 3.2 of [17]) that returns
   the content object is not aware of the network location of the
   consumer and the consumer is not aware of the network location of the
   node that provides the content.  This, in theory, allows the
   interests to follow different paths within a network or even to be
   sent over completely different networks.

4.  NC Basics

   While the forwarding node simply relays received data packets in
   conventional IP communication networks, NC allows the node to combine
   some data packets that are already received into one or several
   output packets to be sent.  In this section, we simply describe the

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   basic operations of NC.  Herein, we focus on RLC in a block coding
   manner that is well known as a major coding technique.

   For simplicity, let us consider an example case of end-to-end coding
   wherein a producer and consumer respectively perform encoding and
   decoding for a content object.  This end-to-end coding is regarded as
   a special case of NC.  The producer splits the content into several
   blocks called generations.  Encoding and decoding are performed
   independently on a per-block (per-generation) basis.  Let us assume
   that each generation consists of K original source packets of the
   same size.  When the packets do not have the same size, zero padding
   is added.  In order to generate one coded packet within a certain
   generation, the producer linearly combines K of the original source
   packets, where additions and multiplications are performed using a
   coding vector consisting of K coding coefficients that are randomly
   selected in a certain finite field.  The producer may respond to
   interests to send the corresponding source packets and coded packets
   in the content flow (called systematic coding), where the coded
   packets (also called repair packets) are typically used for repairing
   lost source packets.

   Coded packets can also be used for performing encoding.  If the
   forwarding nodes know each coding vector and generation identifier of
   the received coded packets, they may perform an encoding operation
   (called re-coding), which is the most distinctie feature of NC
   compared to other coding techniques.

   At the consumer, decoding is performed by solving a set of linear
   equations that are represented by the coding vectors of the received
   coded packets within a certain generation.  In order to obtain all
   the source packets, the consumer requires K linearly independent
   equations.  In other words, the consumer must receive at least K
   linearly independent data packets (called innovative packets).  As
   receiving a linearly dependent data packet is not useful for
   decoding, re-coding should generate and provide innovative packets.
   One of major benefits of RLC is that even for a small-sized finite
   field (e.g., q=2^8), the probability of generating linearly dependent
   packets is negligible [33].

5.  Advantages of NC and CCNx/NDN

   Combining NC and CCNx/NDN can contribute to effective large-scale
   content/information dissemination.  They individually provide similar
   benefits such as throughput/capacity gain and robustness enhancement.
   The difference between their approaches is that, the former considers
   content flow as algebraic information that is to be combined [21],
   while the latter focuses on the content/information itself at the
   networking layer.  Because these approaches are complementary and

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   their combination would be advantageous, it is natural to combine

   The name-based communication in CCNx/NDN enables consumers to obtain
   requested content objects without establishing and maintaining end-
   to-end communication channels between nodes.  This feature
   facilitates the exploitation of the in-network cache and multipath/
   multisource retrieval and also supports consumer mobility without the
   need for updating the location information/identifier during handover
   [16].  Furthermore, the name-based communication intrinsically
   supports multicast communication because identical interests are
   aggregated at the forwarders.

   NC can enable the CCNx/NDN transport system to effectively distribute
   and cache the data associated with multipath data retrieval [9].
   Exploiting multipath data retrieval and in-network caching with NC
   contributes to not only improving the cache hit rate but also
   expanding the anonymity set of each consumer (the set of potential
   routers that can serve a given consumer) [31].  The expansion makes
   it difficult for adversaries to infer the content consumed by others,
   and thus contributes to improving cache privacy.  Others also have
   introduced some use cases of the application of NC in CCNx/NDN, such
   as the cases of content dissemination with in-network caching [10]
   [13] [14], seamless consumer mobility [11] [37], and low-latency low-
   loss video streaming [15].  In this context, it is well worth
   considering NC integration in CCNx/NDN.

6.  Technical Considerations

   This section presents the considerations for CCNx/NDN with NC in
   terms of network architecture and protocol.  This document focuses on
   NC when employed in a block coding manner.

6.1.  Content Naming

   Naming content objects is as important for CCNx/NDN as naming hosts
   is in the current-day Internet [25].  In this section, two possible
   naming schemes are presented.

   Each coded packet may have a unique name as content objects (original
   source packets) has in CCNx/NDN, as PIT/CS operations typically
   require a unique name for identifying the coded packet.  As a method
   of naming a coded packet, the coding vector and the identifier of the
   generation (also called block) can be used as a part of the content
   object name.  As in [10], when the generation ID is "g-id",
   generation size is 4, and coding vector is (1,0,0,0), the name could
   be /  Some other identifiers and/or
   parameters related to the encoding scheme can also be used as name

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   components.  For instance, the encoding ID specifying the coding
   scheme may be used with "enc-id" such as /
   g-id/1000, as defined in the FEC Framework (FECFRAME) [27].  This
   naming scheme is simple and can support the delivery of coded packets
   with exactly the same operations in the PIT/CS as those for the
   content objects.

   If a content-naming schema such as the one presented above is used,
   an interest requesting a coded packet may have the full name
   including a generation id and coding vector (/
   g-id/1000) or only the name prefix including only a generation id
   (/  In the former case, exact name matching to
   the PIT is simply performed at data forwarders (as in CCNx).  The
   consumer is enabled to specify and retrieve an innovative packet
   necessary for the consumer to decode successfully.  This could shift
   the generation of the coding vector from the data forwarder onto the

   In the latter case, partial name matching is required at the data
   forwarders (as in the case of NDN).  As the interest with only the
   prefix name matches any coded packet with the generation ID, the
   consumer could immediately obtain an coded packet from a nearby CS
   (in-network cache) without knowing the coding vectors of the cached
   coded packets in advance.  In the case wherein coded packets in
   transit are modified by in-network re-coding performed at forwarders,
   the consumer could also receive the modified coded packets.  However,
   in contrast to the former case, the consumer may fail to obtain
   sufficient degrees of freedom (see Section 6.2.3).  To address this
   issue, a new TLV type in an interest message may be required for
   specifying further coding information in order to limit the coded
   packets to be received.  For instance, this is enabled by specifying
   the coding vectors of innovative packets for the consumer (also
   called decoding matrix) as in [9].  This extension may incur an
   interest packet of significantly increased size, and it may thus be
   useful to use compression techniques for coding vectors [28] [29].
   Without such coding information provided by the interest, the
   forwarder would be required to maintain some records regarding the
   interest packets that were satisfied previously (See Section 6.2.3).

   A coded packet may have a name that indicates that it is a coded
   packet, and move the coding information into a metadata field in the
   payload (i.e., the name includes the data type, source or coded
   packet).  This would not be beneficial for applications or services
   that may not need to understand the packet payload.  Owing to the
   possibility that multiple coded packets may have the same name, some
   mechanism is required for the consumer to obtain innovative packets.
   As described in Section 6.3, a mechanism for managing the multiple
   innovative packets in the CS would also be required.  In addition,

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   extra computational overhead would be incurred when the payload is
   being encrypted.

6.2.  Transport

   The pull-based request-response feature of CCNx/NDN is a fundamental
   principle of its transport layer; one interest retrieves at most one
   data packet.  This property prevents consumer or forwarder to inject
   large amounts of unrequested data into the network.  It is believed
   that it is important that this rule not be violated, as 1) it would
   open denial-of-service (DoS) attacks, 2) it invalidates existing
   congestion control approaches following this rule, and 3) it would
   reduce the efficiency of existing consumer mobility approaches.
   Thus, the following basic operation should be considered for applying
   NC to CCNx/NDN.  Nevertheless, such security considerations must be
   addressed if this rule were to be violated.

6.2.1.  Scope of NC

   An open question is whether data forwarder can perform in-network re-
   coding with data packets that are being received in transit, or if
   only the data that matches an interest can be subject to NC
   operations.  In the latter case, encoding or re-coding is performed
   to generate the coded packet at any forwarder that is able to respond
   to the interest.  This could occur when each coded packet has a
   unique name and interest has the full name.  On the other hand, if
   interest has a partial name without any coding vector information or
   coded packets have a same name, the former case may occur; re-coding
   occurs anywhere in the network where it is possible to modify the
   received coded packet and forward it.  As CCNx/NDN comprises
   mechanisms for ensuring the integrity of the data during transfer,
   in-network re-coding introduces complexities in the network that
   needs consideration for the integrity mechanisms to still work.
   Similarly, in-network caching of coded packets at forwarders may be
   valuable; however, the forwarders would require some mechanisms to
   validate the coded packets (see Section 8).

6.2.2.  Consumer Operation

   To obtain NC benefits (possibly associated with in-network caching),
   the consumer is required to issue interests that direct the forwarder
   (or producer) to respond with innovative packets if available.  In
   the case where each coded packet may have a unique name (as described
   in Section 6.1), by issuing an interest specifying a unique name with
   g-id and the coding vector for a coded packet, the consumer could
   appropriately receive an innovative packet if it is available at some

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   In order to specify the exact name of the coded packet to be
   retrieved, the consumer is required to know the valid naming scheme.
   From a practical viewpoint, it is desirable for the consumer
   application to automatically construct the right name components
   without depending on any application specifications.  To this end,
   the consumer application may retrieve and refer to a manifest [1]
   that enumerates the content objects including coded packets, or may
   use some coding scheme specifier as a name component to construct the
   name components of interests to request innovative packets.

   Conversely, the consumer without decoding capability (e.g., specific
   sensor node) may want to receive only the source packets.  As
   described in Section 6.1, because the coded packet can have a name
   that is explicitly different from source packets, issuing interests
   for retrieving source packets is possible.

6.2.3.  Forwarder Operation

   If the forwarder constantly responds to the incoming interests by
   returning non-innovative packets, the consumer(s) cannot decode and
   obtain the source packets for all time.  This issue could happen when
   1) incoming interests for coded packets do not specify some coding
   parameters such as the coding vectors to be used, and 2) the
   forwarder does not have a sufficient number of linearly independent
   source or coded packets (possibly in the CS) to use for re-coding.
   In this case, the forwarder is required to determine whether or not
   it can generate innovative packets to be forwarded to the
   interface(s) at which the interests arrived.  An approach to deal
   with this issue is that the forwarder maintains a tally of the
   interests for a specific name, generation ID and the incoming
   interface(s), in order to record how many degrees of freedom have
   already been provided [10].  As such a scheme requires state
   management (and potentially timers) in forwarders, scalability and
   practicality must be considered.  In addition, some transport
   mechanism for in-network loss detection and recovery [15] [37] at
   forwarder as well as a consumer-driven mechanism could be
   indispensable for enabling fast loss recovery and realising NC gains.
   If a forwarder cannot either return a matching innovative packet from
   its local content store, nor produce on-the-fly a recoded packet that
   is innovative, it is important that the forwarder not simply return a
   non-innovative packet but instead do a forwarding lookup in its FIB
   and forward the Interest toward the producer or upstream forwarder
   that can provide an innovative packet.  In this context, to retrieve
   innovative packet effectively and quickly, an appropriate setting of
   the FIB and efficient interest forwarding strategies should also be

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   In another possible case, when receiving interests only for source
   packets, the forwarder may attempt to decode and obtain all the
   source packets and store them (if the full cache capacity are
   available), thus enabling a faster response to the interests.  As re-
   coding or decoding results in an extra computational overhead, the
   forwarder is required to determine how to respond to received
   interests according to the use case (e.g., a delay-sensitive or
   delay-tolerant application) and the forwarder situation, such as
   available cache space and computational capability.

6.2.4.  Producer Operation

   Before performing NC for specified content in CCNx/NDN, the producer
   is responsible for splitting the overall content into small content
   objects to avoid packet fragmentation that could cause unnecessary
   packet processing and degraded throughput.  The size of the content
   objects should be within the allowable packet size in order to avoid
   packet fragmentation in CCNx/NDN network.  The producer performs the
   encoding operation for a set of the small content objects, and the
   naming process for the coded packets.

   If the producer takes the lead in determining what coding vectors to
   use in generating the coding packets, there are three general
   strategies for naming and producing the coded packets:

   1.  consumers themselves understand in detail the naming conventions
       used for coded packets and thereby can send the corresponding
       interests toward the producer to obtain coded packets whose
       coding parameters have already been determined by the producer.

   2.  the producer determines the coding vectors and generates the
       coded packets after receiving interests specifying the packets
       the consumer wished to receive.

   3.  The naming scheme for specifying the coding vectors and
       corresponding coded packets is explicitly represented via a
       "Manifest" (e.g., FLIC [24]) that can be obtained by the consumer
       and used to select among the available coding vectors and their
       corresponding packets, and thereby send the corresponding

   In the first case, although the consumers cannot flexibly specify a
   coding vector for generating the coded packet to obtain, the latency
   for obtaining the coded packet is less than in the latter two cases.
   For the second case, there is a latency penalty for the additional NC
   operations performed after receiving the interests.  For the third
   case, the coded packets to be included in the manifest must be pre-
   computed by the producer (since the manifest references coded packets

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   by their hashes, not their names), but the producer can select which
   to include the manifest, and produce multiple manifests either in
   advance or on demand with different coding tradeoffs if so desired.

   A common benefit the first two approaches to end-to-end coding is
   that if the producer adds a signature on the coded packets, data
   validation becomes possible throughout (as is the case with CCNx/NDN
   operation in the absence of NC).  The third approach of using a
   manifest trades off the additional latency incurred by the need to
   fetch the manifest against the efficiency of needing a signature only
   on the manifest and not on each individual coded packet.

6.2.5.  Backward Compatibility

   NC operations should be applied in addition to the regular network
   behavior.  Hence, nodes should be able to not support network coding
   (not only in forwarding the packets, but also in the caching
   mechanism).  NC operations should function alongside regular network
   operations.  An NC framework should be compatible with a regular
   framework in order to facilitate backward compatibility and smooth
   migration from one framework to the other.

6.3.  In-network Caching

   Caching is a useful technique used for improving throughput and
   latency in various applications.  In-network caching in CCNx/NDN
   essentially provides support at network level and is highly
   beneficial owing to the involved exploitation of NC for enabling
   effective multicast transmission [38], multipath data retrieval [10]
   [11], fast loss recovery [15].  However, there remain several issues
   to be considered.

   There generally exist limitations in the CS capacity, and the caching
   policy affects the consumer's performance [30] [35] [36].  It is thus
   crucial for forwarders to determine which content objects should be
   cached and which discarded.  As delay-sensitive applications often do
   not require an in-network cache for a long period owing to their
   real-time constraints, forwarders have to know the necessity for
   caching received content objects to save the caching volume.  In
   CCNx, this could be made possible by setting a Recommended Cache Time
   (RCT) in the optional header of the data packet at the producer side.
   The RCT serves as a guideline for the CS cache in determining how
   long to retain the content object.  When the RCT is set as zero, the
   forwarder recognizes that caching the content object is not useful.
   Conversely, the forwarder may cache it when the RCT has a greater
   value.  In NDN, the TLV type of FreshnessPeriod could be used.

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   One key aspect of in-network caching is whether or not forwarders can
   cache coded packets in their CS.  They may be caching the coded
   packets without having the ability to perform a validation of the
   content objects.  Therefore, the caching of the coded packets would
   require some mechanism to validate the coded packets (see Section 8).
   In the case wherein the coded packets have the same name, it would
   also require some mechanism to identify them.

6.4.  Seamless Consumer Mobility

   A key feature of CCNx/NDN is that it is sessionless, which enables
   the consumer and forwarder to send multiple interests toward
   different copies of the content in parallel, by using multiple
   interfaces at the same time in an asynchronous manner.  Through the
   multipath data retrieval, the consumer could obtain the content from
   multiple copies that are distributed while using the aggregate
   capacity of multiple interfaces.  For the link between the consumer
   and the multiple copies, the consumer can perform a certain rate
   adaptation mechanism for video streaming [11] or congestion control
   for content acquisition [12].

   NC adds a reliability layer to CCNx in a distributed and asynchronous
   manner, because NC provides a mechanism for ensuring that the
   interests sent to multiple copies of the content in parallel retrieve
   innovative packets, even in the case of packet losses on some of the
   paths/networks to these copies.  This applies to consumer mobility
   events [11], wherein the consumer could receive additional degrees of
   freedom with any innovative packet if at least one available
   interface exists during the mobility event.  An interest forwarding
   strategy at the consumer (and possibly forwarder) for efficiently
   obtaining innovative packets would be required for the consumer to
   achieve seamless consumer mobility.

7.  Challenges

   This section presents several primary challenges and research items
   to be considered when applying NC in CCNx/NDN.

7.1.  Adoption of Convolutional Coding

   Several block coding approaches have been proposed thus far; however,
   there is still not sufficient discussion and application of the
   convolutional coding approach (e.g., sliding or elastic window
   coding) in CCNx/NDN.  Convolutional coding is often appropriate for
   situations wherein a fully or partially reliable delivery of
   continuous data flows is required, and especially when these data
   flows feature realtime constraints.  As in [40], on an end-to-end
   coding basis, it would be advantageous for continuous content flow to

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   adopt sliding window coding in CCNx/NDN.  In this case, the producer
   is required to appropriately set coding parameters and let the
   consumer know the information, and the consumer is required to send
   interests augmented with feedback information regarding the data
   reception and/or decoding status.  As CCNx/NDN utilises hop-by-hop
   forwarding state, it would be worth discussing and investigating how
   convolutional coding can be applied in a hop-by-hop manner and what
   benefits might accrue.  In particular, in the case wherein in-network
   re-coding could occur at forwarders, both the encoding window and CS
   management would be required, and the corresponding feasibility and
   practicality should be considered.

7.2.  Rate and Congestion Control

   The addition of redundancy using repair packets may result in further
   network congestion and could adversely affect the overall throughput.
   In particular, in a situation wherein fair bandwidth sharing is more
   desirable, each streaming flow must adapt to the network conditions
   to fairly consume the available link bandwidth.  It is thus necessary
   that each content flow cooperatively implement congestion control to
   adjust the consumed bandwidth [23].  From this perspective, although
   a forwarder-supported approach would be effective, an effective
   deployment approach that provides benefits under partial deployment
   is required.

   As described in Section 6.4, NC can contribute to seamless consumer
   mobility by obtaining innovative packets without receiving duplicated
   packets through multipath data retrieval.  It can be challenging to
   develop an effective rate and congestion control mechanism in order
   to achieve seamless consumer mobility while improving the overall
   throughput or latency by fully exploiting NC operations.

7.3.  Security

   While CCNx/NDN introduces new security issues at the networking layer
   that are different from the IP network, such as a cache poisoning and
   pollution attacks, a DoS attack using interest packets, some security
   approaches are already provided [25] [26].  The application of NC in
   CCNx/NDN brings two potential security aspects that need to be dealt

   The first is in-network re-coding at forwarders.  Some mechanism for
   ensuring the integrity of the coded packets newly produced by in-
   network re-coding is required in order for consumers or other
   forwarders to deal with valid coded packets.  To this end, there are
   some possible approaches described in Section 8, but there may be
   more effective method with lower complexity and computation overhead.

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   The second is that attackers maliciously request and inject coded
   packets, which could amplify some attacks.  As coded packets are
   unpopular in general use, they could be targeted by a cache pollution
   attack that requests less popular content objects more frequently to
   undermine popularity-based caching by skewing the content popularity.
   Such an attack needs to be dealt with in order to maintain the in-
   network cache efficiency.  By injecting invalid coded packets with
   the goal of filling the CSs at the forwarders with them, the cache
   poisoning attack could be effectual depending on the exact integrity
   coverage on coded packets.  On the assumption that each coded packet
   has the valid signature, the straightforward approach would comprise
   the forwarders verifying the signature within the coded packets in
   transit and only transmitting and storing the validated coded
   packets.  However, as performing a signature verification by the
   forwarders may be infeasible at line speed, some mechanisms should be
   considered for distributing and reducing the load of signature
   verification, in order to maintain in-network cache benefits such as
   latency and network-load reduction.

7.4.  Routing Scalability

   In CCNx/NDN, a name-based routing protocol without a resolution
   process streamlines the routing process and reduces the overall
   latency.  In IP routing, the growth in the routing table size has
   become a concern.  It is thus necessary to use a hierarchical naming
   scheme in order to improve the routing scalability by enabling the
   aggregation of the routing information.

   To realize the benefits of NC, consumers need to efficiently obtain
   innovative packets using multipath retrieval mechanisms of CCNx/NDN.
   This would require some efficient routing mechanism to appropriately
   set the FIB and also an efficient interest forwarding strategy.  Such
   routing coordination may create routing scalability issues.  It would
   be challenging to achieve effective and scalable routing for
   interests requesting coded packets as well as to simplify the routing

8.  Security Considerations

   In-network re-coding is a distinguishing feature of NC.  Only valid
   coded packets produced by in-network re-coding must be requested and
   utilized (and possibly stored).  To this end, there exist some
   possible approaches.  First, as a signature verification approach,
   the exploitation of multi-signature capability could be applied.
   This allows not only the original content producer but also some
   forwarders responsible for in-network re-coding to have their own
   unique signing key.  Each forwarder of the group signs newly
   generated coded packet in order for other nodes to be able to

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   validate the data with the signature.  The CS may verify the
   signature within the coded packet before storing it to avoid invalid
   data caching.  Second, as a consumer-dependent approach, the consumer
   puts a restriction on the matching rule using only the name of the
   requested data.  The interest ambiguity can be clarified by
   specifying both the name and the key identifier (the producer's
   public key digest) used for matching to the requested data.  This
   KeyId restriction is built in the CCNx design [1].  Only the
   requested data packet satisfying the interest with the KeyId
   restriction would be forwarded and stored in the CS, thus resulting
   in a reduction in the chances of cache poisoning.  Moreover, in the
   CCNx design, there exists the rule that the CS obeys in order to
   avoid amplifying invalid data; if an interest has a KeyID
   restriction, the CS must not reply unless it knows that the signature
   on the matching content object is correct.  If the CS cannot verify
   the signature, the interest may be treated as a cache miss and
   forwarded to the upstream forwarder(s).  Third, as a certificate
   chain management approach (possibly without certificate authority),
   some mechanism such as [32] could be used to establish a trustworthy
   data delivery path.  This approach adopts the hop-by-hop
   authentication mechanism, wherein forwarding-integrated hop-by-hop
   certificate collection is performed to provide suspension certificate
   chains such that the data retrieval is trustworthy.

   Depending on the adopted caching strategy such as cache replacement
   policies, forwarders should also take caution when storing and
   retaining the coded packets in the CS as they could be targeted by
   cache pollution attacks.  In order to mitigate the cache pollution
   attacks' impact, forwarders should check the content request
   frequencies to detect the attack and may limit requests by ignoring
   some of the consecutive requests.  The forwarders can then decide to
   apply or change to the other cache replacement mechanism.

   The forwarders or producers require careful attention to the DoS
   attacks aiming at provoking the high load of NC operations by using
   the interests for coded packets.  In order to mitigate such attacks,
   the forwarders could adopt a rate-limiting approach.  For instance,
   they could monitor the PIT size growth for coded data per content to
   detect the attacks, and limit the interest arrival rate when
   necessary.  If the NC application wishes to secure an interest
   (considered as the NC actuator) in order to prevent such attacks, the
   application should consider using an encrypted wrapper and an
   explicit protocol.

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9.  Acknowledgements

   The authors would like to thank ICNRG and NWCRG members, especially
   Marie-Jose Montpetit, David Oran, Vincent Roca, and Thierry Turletti,
   for their valuable comments and suggestions on this document.

10.  Informative References

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   [10]       Saltarin, J., Bourtsoulatze, E., Thomos, N., and T. Braun,
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   [18]       Mosko, M. and et al., "Content-Centric Networking (CCNx)
              Messages in TLV Format", RFC 8609, July 2019,

   [19]       Zhang, L., Afanasyev, A., Burke, J., Jacobson, V., Claffy,
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   [20]       NDN Packet Format, "NDN Packet Format Specification 0.3
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   [22]       Adamson, B. and et al., "Taxonomy of Coding Techniques for
              Efficient Network Communications", RFC 8406, June 2018,

   [23]       Kuhn, N., Lochin, E., Michel, F., and M. Welzl, "Coding
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   [25]       Kutscher, D. and et al., "Information-Centric Networking
              (ICN) Research Challenges", RFC 7927, July 2016.

   [26]       Pentikousis, K. and et al., "Information-Centric
              Networking: Evaluation and Security Considerations",
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   [27]       Watson, M. and et al., "Forward Error Correction (FEC)
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   [32]       Li, R., Asaeda, H., and J. Wu, "DCAuth: Data-Centric
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   [38]       Ali, M. and U. Niesen, "Coding for Caching: Fundamental
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Authors' Addresses

   Kazuhisa Matsuzono
   National Institute of Information and Communications Technology
   4-2-1 Nukui-Kitamachi
   Koganei, Tokyo  184-8795


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   Hitoshi Asaeda
   National Institute of Information and Communications Technology
   4-2-1 Nukui-Kitamachi
   Koganei, Tokyo  184-8795


   Cedric Westphal
   2330 Central Expressway
   Santa Clara, California  95050


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