Network Coding for Content-Centric Networking / Named Data Networking: Requirements and Challenges
draft-irtf-nwcrg-nwc-ccn-reqs-01

Network Coding Research Group                               K. Matsuzono
Internet-Draft                                                 H. Asaeda
Intended status: Informational                                      NICT
Expires: September 12, 2019                                  C. Westphal
                                                                  Huawei
                                                          March 11, 2019

 Network Coding for Content-Centric Networking / Named Data Networking:
                      Requirements and Challenges
                    draft-irtf-nwcrg-nwc-ccn-reqs-01

Abstract

   This document describes the current research outcomes regarding
   Network Coding (NC) for Content-Centric Networking (CCN) / Named Data
   Networking (NDN), and clarifies the requirements and challenges for
   applying NC into CCN/NDN.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   3
     2.2.  NDN/CCN Background  . . . . . . . . . . . . . . . . . . .   5
   3.  Advantage given by NC and CCN/NDN . . . . . . . . . . . . . .   6
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Content Naming  . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Transport . . . . . . . . . . . . . . . . . . . . . . . .   9
       4.2.1.  Scope of Network Coding . . . . . . . . . . . . . . .   9
       4.2.2.  Consumer Operation  . . . . . . . . . . . . . . . . .   9
       4.2.3.  Router Operation  . . . . . . . . . . . . . . . . . .  10
       4.2.4.  Publisher Operation . . . . . . . . . . . . . . . . .  11
       4.2.5.  Backward Compatibility  . . . . . . . . . . . . . . .  11
     4.3.  In-network Caching  . . . . . . . . . . . . . . . . . . .  11
     4.4.  Seamless Mobility . . . . . . . . . . . . . . . . . . . .  12
     4.5.  Security and Privacy  . . . . . . . . . . . . . . . . . .  13
   5.  Challenges  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Adopting Convolutional Coding . . . . . . . . . . . . . .  13
     5.2.  Rate and Congestion Control . . . . . . . . . . . . . . .  14
     5.3.  Security and Privacy  . . . . . . . . . . . . . . . . . .  14
     5.4.  Routing Scalability . . . . . . . . . . . . . . . . . . .  15
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  15
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Introduction

   Information-Centric Networks in general, and Content-Centric
   Networking (CCN) [15] or Named Data Networking (NDN) [16] in
   particular, have emerged as a novel communication paradigm advocating
   to retrieve data through 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
   CCN/NDN architecture has introduced innovative ideas and has
   stimulated research in a variety of areas, such as in-network
   caching, name-based routing, multi-path transport, content security,
   and so on.  One key benefit of requesting content by name is that it
   removes the need to establish a session between the client and a
   specific server, and that content can thereby be retrieved from
   multiple sources.

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   In parallel, there has been a growing interest from both academia and
   industry to better understand 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, point-to-multipoint
   connections, etc.  Typically, NC is a technique mainly used to encode
   packets to recover lost source packets at the receiver, and to
   effectively get the desired information in a fully distributed
   manner.  In addition, NC can be used for security enhancements [2]
   [3] [4] [5].

   NC aggregates multiple packets with parts of the same content
   together, and may do this at the source or at other nodes in the
   network.  As such, network coded packets are not connected to a
   specific server, as they may have evolved within the network.  Since
   NC focuses on what information should be encoded in a network packet,
   rather than the specific host where it has been generated, it is in
   line with the CCN/NDN core networking layer (described in more detail
   later on).  NC has already been implemented for information/content
   dissemination (e.g. [6] [7] [8]).  NC provides CCN/NDN with the
   highly beneficial potential to effectively disseminate information in
   a completely independent and decentralized manner. [9] first
   suggested to exploit NC techniques to enhance key system performances
   in ICN, and others have considered NC in ICN use cases such as
   content dissemination [10], seamless mobility [11], joint caching and
   network coding [12] [13], low-latency video streaming [14], etc.

   In this document, we consider how NC can be applied to the CCN/NDN
   architecture and describe the requirements and potential challenges
   for making CCN/NDN-based communications better using the NC
   technology.  Please note that providing specific solutions (e.g., NC
   optimization methods) to enhance CCN/NDN performance metrics by
   exploiting NC is out of scope of this document.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

2.1.  Definitions

   The terminology regarding NC used in this document is described
   below.  It is aligned with RFCs produced by the FEC Framework
   (FECFRAME) IETF Working Groups as well as recent activities in the
   Network Coding Research Group [18].

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   o  Random Linear Coding (RLC): Particular case of Linear Coding using
      a set of random coding coefficients.

   o  Generation, or (IETF) Block: With Block Codes, the set of content
      data that are logically grouped into a Block, before doing
      encoding.

   o  Generation Size: With Block Codes, the number k of content data
      belonging to a Block.

   o  Encoding Vector: A set of coding coefficients used to generate a
      certain coded packet through linear coding.  The number of nonzero
      coefficients in the Coding Vector defines its density

   o  Finite Field: Finite fields, used in Linear Codes, have the
      desired property of having all elements (except zero) invertible
      for + and * and all operations over any elements do not 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.

   o  Block Coding: Coding technique where the input Flow(s) must be
      first segmented into a sequence of blocks, FEC encoding and
      decoding being performed independently on a per-block basis.

   o  Sliding Window Coding or Convolutional Coding: General class of
      coding techniques that rely on a sliding encoding window.  This is
      an alternative solution to Block Coding.

   o  Fixed or Elastic Sliding Window Coding: Coding technique that
      generates repair data on-the-fly, from the set of source data
      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").

   o  Feedback: Feedback information sent by a decoding node to a node
      (or from a consumer to a publisher in case of End-to-End Coding).
      The nature of information contained in a feedback packet varies,
      depending on the use-case.  It can provide reception and/or
      decoding statistics, or the list of available source packets
      received or decoded, or the list of lost source packets that
      should be retransmitted, or a number of additional repair packet
      needed to have a full rank linear system.

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   Concerning CCN/NDN, the following terminology and definitions are
   used.

   o  Consumer: A node requesting content.  It initiates communication
      by sending an interest packets.

   o  Publisher: A node providing content.  It originally creates or
      owns the content.

   o  Forwarding Information Base (FIB): A lookup table in a content
      router containing the name prefix and corresponding destination
      interface to forward the interest packets.

   o  Pending Interest Table (PIT): A lookup table populated by the
      interest packets containing the name prefix of the requested data,
      and the outgoing interface used to forward the received data
      packets.

   o  Content Store (CS): A storage space for a router to cache content
      objects.  It is also known as in-network cache.

   o  Content Object: A unit of content data delivered through the CCN/
      NDN network.

   o  Content Flow: A sequence of content objects associated with the
      unique content name prefix.

2.2.  NDN/CCN Background

   Armed with the terminology above, we briefly explain the key concepts
   of CCN/NDN.  Both protocols are similar in principle, and different
   on some implementation choices.

   In a CCN network, there are two types of packets at the network
   level: interest and data.  The consumer request a content by sending
   an "interest" message, that carries the name of the data.  On
   difference to note here in CCN and NDN is that in later versions of
   CCN, the interest must carry a full name, while in NDN it may carry a
   name prefix (and receive in return any data with a name matching this
   prefix).

   Once a router receives an "interest" message, it performs a series of
   look-up: first it checks in the Content Store if it has a copy of the
   requested content available.  If it does, it returns the data and the
   transaction has successfully completed.

   If it does not, it performs a look-up of the PIT to see if there is
   already an outgoing request for the same data.  If there is not, then

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   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 used later to send the data back, since interest packets do
   not carry a source field that identifies the requester.  If there is
   already a PIT entry for this name, then it is updated with the
   incoming interface of this new request and the interest is discarded.

   After the PIT look-up, the interest undergoes a FIB lookup to select
   an outgoing interface.  The FIB lists name prefixes and their
   corresponding forwarding interfaces, to send the interface towards a
   router that possesses a copy of the requested data.

   Once a copy of the data is retrieved, it is send back to the
   requester(s) using the trail of PIT entries; intermediate node remove
   the PIT state every time that an interest is satisfied, and may store
   the data in their content store.

   Data packets carry some information to validate the data, in
   particular that the data is indeed the one that corresponds to the
   name.  This is required since authentication of the object is crucial
   in CCN/NDN.  However, this step is optional at intermediate routers,
   so as to speed up the processing.

   The key aspect of CCN/NDN is that the consumer of the content does
   not establish a session with a specific server.  Indeed, the node
   that returns the content is not aware of the network location of the
   requester and the requester 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 totally different networks.

3.  Advantage given by NC and CCN/NDN

   Both NC for large scale content dissemination [7] and CCN/NDN can
   contribute to effective content/information delivery while working
   jointly.  They both bring 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 to combine [17], while the latter focuses on
   content/information itself at the networking layer.  Because these
   approaches are complementary, it is natural to combine them.  The
   CCN/NDN core abstraction at networking layer through name makes
   network stack simple as it enables applications to take maximum
   advantage of multiple simultaneous connectivities due to its simpler
   relationship with the layer 2 [15].

   CCN/NDN itself, however, cannot provide reliable and robust content
   dissemination.  This requires some specific CCN/NDN transport (i.e.,

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   strategy layer) [15].  NC can enable the CCN/NDN transport system to
   effectively distribute and cache data associated with multi-path data
   retrieval.  Furthermore, NC can contribute to improving both caching
   performance and cache privacy that CCN/NDN newly poses at the
   networking layer [23].  In this context, it should be natural that
   there is much room for considering NC integration into CCN/NDN
   transport exploiting in-network caching and multi-path transmission
   [9] and seamless mobility [11] [29].

   From the perspective of NC transport mechanism, NC is divided into
   two major categories: one is coherent NC, and the other is non-
   coherent NC [31].  In coherent NC, source and destination nodes
   exactly know network topology and coding operations at intermediate
   nodes.  When multiple consumers are trying to receive the same
   content such as live video streaming, coherent NC could enable the
   optimal throughput by making the content flow sent over the
   constructed optimal multicast trees [24].

   However, it requires a fully adjustable and specific name-based
   routing mechanism for CCN/NDN, and an intense computational task for
   central coordination.  In the case of non-coherent NC that often
   utilizes RLC, it is not required to know either network topology nor
   intermediate coding operations [25].  Since non-coherent NC works in
   a completely independent and decentralized manner, this approach is
   more feasible especially in the large scale use cases that are
   intended with CCN/NDN.  This document thus focuses on non-coherent NC
   with RLC.

4.  Requirements

   This section presents the NC requirements for ICN/CCN in terms of
   network architecture and protocol.  The current document focuses on
   NC in a block coding manner.

4.1.  Content Naming

   Naming content objects is as important for CCN/NDN as naming hosts is
   for today's Internet [19].  Before performing network coding for
   specified content in CCN/NDN, the overall content should be split
   into small content objects to avoid packet fragmentation that could
   cause unnecessary packet processing and degrade throughput.  The size
   of content objects should be within the allowable packet size so as
   to avoid packet fragmentation in CCN/NDN network, and then network
   coding should be applied into a set of the content objects.

   Each coded packet MAY have a unique name as the original content
   object has in CCN/NDN, since PIT/FIB/CS operations need a unique name
   to identify the coded data.  As a way of naming coded packet, the

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   encoding vector and the identifier of generation can be used as a
   part of the content object name [10].  For instance, when the block
   size (also called generation size) is k and the encoding vector is
   [1,0,0,0], the name would be like /CCN.com/video-A/k/1000.  This
   naming scheme is simple and can support the delivery of coded packets
   with exactly the same operations in the FIB/PIT/CS as for original
   source packets.  However, such a naming way requires the consumer to
   know the naming structure (through a specific name resolution scheme
   for instance) in order for nodes to specify the exact name of
   generated coded data packet to retrieve it.  From this point of view,
   it could shift the generation of the encoding vector from the content
   producer onto the content requester.

   If a naming schema such as above is used, it would be valuable to
   reconsider whether Interests should carry full names (as in CCN) or
   prefixes (as in NDN) as multiple network coded packets could match a
   response to a specific prefix for a given generation, such as
   /CCN.com/video-A/k.  In the latter case allowing partial name
   matching, the content requestor may not be able to obtain degrees of
   freedom.  Thus, extensions in the TLV header of the Interest would be
   used to specify further network coding information so as to limit
   coded packets to be received (for instance, by specifying the encoded
   vectors the content requestor receives (also called decoding matrix)
   as in [9]).  However, it may incur a largely increased size of TLV
   header.  Without such coding information, the forwarding node would
   need to maintain some records regarding interest packets sent before,
   in order to provide new degrees of freedom.

   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 only data type, original or coded
   packet, etc).  This however would preclude network coding on packets
   without prior decoding them (for instance, in the CS of forwarding
   nodes).  It would not be beneficial for applications or services that
   may not need to understand the packet payload.  Due to the
   possibility that multiple coded packets may have a same name, as
   described above, some mechanism is needed for the content requestor
   to obtain innovative coded packets.A mechanism to insert the multiple
   innovative packets into the CS may be required as well.  If the
   coding information of coded packet are encrypted together with the
   payload (for instance, at source coding), the content requestor or
   forwarding nodes would incur extra computational overhead for
   decryption of the packet to interpret the coding information.

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4.2.  Transport

   The pull-based request-response feature of CCN/NDN is a fundamental
   principle of its transport layer; one Interest retrieves at most one
   Data packet.  It is believed that it is important to not violate this
   rule, as it would open denial of service attacks issues, and thus the
   following basic operation should be considered to apply NC to CCN/
   NDN.  In any case, such security considerations must be addressed if
   this rule were to be violated.

4.2.1.  Scope of Network Coding

   It should be discussed whether the network can update data packets
   that are being received in transit, or if only the data that matches
   an interest can be subject to network coding operations.  In the
   latter case, the network coding is performed on an end-to-end basis
   (where one end is the consumer, and the other end is any node that is
   able to respond to the Interest).  In the former case, NC happens
   anywhere in the network that is able to update the data.  As CCN/NDN
   has mechanisms in place to ensure the integrity of the data during
   transfer, NC in the network introduce complexities that would require
   special consideration for the integrity mechanisms to still work.

   Similarly, caching of network coded packets at intermediate node may
   be valuable, but may prevent the node caching the coded content to
   validate the content.

4.2.2.  Consumer Operation

   To obtain NC benefits associated with in-network caching, consumers
   need to issue interests directing the router (or publisher) to
   forward innovative coded packets if available.  The reason why this
   directive is needed is that delay-sensitive applications such as
   live-video streaming may want to sequentially get original packets
   rather than coded packets cached in routers due to real-time
   constraint.  Issuing such an interest is possible by using optional
   TLV (Type Length Value) header contained in Interest TLV packet
   format which allows network elements to add or modify information on
   the fly.  Consumer can put an instruction into it, and for instance,
   if routers detect that it is better for consumer to get coded packets
   rather than original packets, routers can modify it to do so.  After
   receiving interests having the instruction in optional header, the
   router with useful coded packets forward them.

   As another solution, consumer issues Interests specifying unique
   names for each coded packets.  In this case, a unified naming scheme
   considering both original and coded packets is required.  Moreover,
   in the case of NC end-to-end approach, publishers need to get

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   feedback from the corresponding receivers to adjust some coding
   parameters.  To deal with this, a receiver may have to request a
   specific interest name to reach the corresponding publisher and put
   required information into the optional header.

4.2.3.  Router Operation

   Routers need to appropriately handle PIT entries to accommodate
   interests for coded packets as well as original packets.  Moreover,
   in order to decode as necessary, nodes need to know the coding vector
   used for each coded packet (note: since all the data for a specific
   content may not come through the same path/network, intermediate
   nodes may never be able to decode).  In a typical case, the coding
   vector used for each coded packet is attached to the header of coded
   data.  In regard to this point, the generation size (also called
   block size) for NC should be set to a reasonable value so that the
   total coded packet size including header needed for expressing the
   coding vector information and data message fits into the allowable
   packet size.  It may be useful to use compression techniques for
   coding vectors [20] [21].

   Router may try to forward useful independent coded packets toward
   downstream nodes in order to respond to received interests for coded
   packets.  Routers thus need to determine whether or not they can
   generate useful coded packets for consumers.  Assuming that the size
   of the Finite Field in use is not relatively small, re-encoding using
   enough cached packets has a strong probability of making independent
   coded packets [24].  If router does not have enough cached packets to
   newly produce independent coded packets, it relays received interests
   to upstream nodes to receive a new original or independent coded
   packet and pass it to downstream nodes.  In another possible case,
   when receiving interests for only original packets, routers may try
   to decode and get all the original packets and store them (if there
   are fully available cache capacity), enabling faster response to the
   interests.  Since there is a tradeoff between NC encoding/decoding
   calculation cost and cache capacity, and the usage efficacy of re-
   encoding or decoding at router, router should need to determine how
   to response to receiving interests according to the use case (e.g.,
   delay-sensitive or delay-tolerant application) and the router
   situation such as available cache space and computational capability.

   Some proposed schemes [10]require that the router maintain a tally of
   the interests for a specific name and generation, so as to know how
   many degrees of freedom have been provided already for the NC
   packets.  Scalability and practicality of maintaining such scheme at
   intermediate routers should considered.

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   To enable fast loss recovery cooperating with in-network caching, a
   transport mechanism of in-network loss detection and recovery [29]
   [14] at router as well as consumer-driven mechanism should be
   considered.

4.2.4.  Publisher Operation

   The procedure for splitting an overall content into small content
   objects is responsible for the original publisher.  When applying NC
   for the content, the publisher performs NC over the content objects,
   and naming processing for the coded packets.  If the producer takes
   the lead in determining the used encoding vectors and generating the
   coded packets, there are the two possible end-to-end cases; 1)
   content requestors obtain the names of coded packets through a
   certain mechanism, and send the correspond interests toward the
   publisher to get the coded packets already generated at the
   publisher, and 2) the publisher determines the encoding vectors after
   receiving interests specifying them.  In the former case, although
   content requestors cannot flexibly specify an encoding vector for
   generating the coded packet to retain, but the latency for getting
   the coded data can be reduced compared to the latter case where
   additional NC operations need after receiving interests.  According
   to application requirement for latency, such NC operation strategy
   should be considered.

4.2.5.  Backward Compatibility

   Network Coding operations should be applied in addition to the
   regular network behavior.  As such, nodes should be able to not
   support network coding (either in forwarding the packets, but also in
   the caching mechanism).  Network Coding operations should function
   alongside regular network operations.  A network coding framework
   should be compatible with a regular framework, so as to allow
   backward compatibility and smooth migration from one framework to the
   other.

4.3.  In-network Caching

   Caching is an essential technique to improve throughput and latency
   in various applications.  In-network caching CCN/NDN essentially
   supports at network level is highly beneficial by exploiting NC to
   enable effective multicast transmission [30], multipath data
   retrieval [10] [11], fast loss recovery [14], and so on.  However,
   there are several issues to be considered.

   As a general issue, there are limitations of cache capacity, and
   caching policy affects on consumer's performances [22] [26] [27].  It
   is thus highly significant for routers to determine which packets

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   should be cached and discarded.  Since delay-sensitive applications
   often do not require in-network cache for a long period due to their
   real-time constraints, routers have to know the necessity for caching
   received packets to save the caching volume.  This could be possible
   by putting a flag into optional header of data packets at publisher
   side.  When receiving data packets with the flag meaning no necessity
   for cache, routers just have to forward them to downstream nodes.  On
   the other hand, when receiving original packets or coded packets
   without the flag, router may cache them based on a specified
   replacement policy.

   One key aspect of in-network caching is whether or not intermediate
   nodes can cache NC packets without first decoding them.  If in-
   network caches store coded packets, they need to be able to validate
   that the packets are not compromised, so as to avoid cache pollution
   attacks.  Without having all the packets in a generation, the cache
   cannot decode the packets to check if it is authenticated.  Caching
   of coded packets would require some mechanism to validate coded
   packets.  In addition, when coded packets have a same name, it would
   also require some mechanism to identify them.

4.4.  Seamless Mobility

   This subsection presents how NC can achieve seamless mobility [11]
   [29] and clarify the requirements.  A key feature of CCN/NDN is that
   it is sessionless and that multiple interests can be send to
   different copies of the content in parallel.  CCN/NDN enables a
   consumer to retrieve the content from multiple sources that are
   distributed and asynchronous.

   In this context, network coding provide a mechanism to ensure that
   the Interests sent to multiple copies of the content retrieve
   innovative packets, even in the case of packet losses on some of the
   paths/networks to these copies.  NC adds a reliability layer to CCN
   in a distributed and asynchronous manner.  One key benefit is that
   the link between the consumer and the multiple copies acts as a
   virtual logical link, upon which rate adaptation mechanism (say, for
   video streaming) can be performed.

   This naturally applies to mobility event, where the consumer may
   connect between multiple access points before a mobility event (make-
   before-break handoff).  In such mobility event, the consumer is
   connected first to the previous access point, then to both the
   previous and next access points, then finally only to the next access
   points.  With CCN, the consumer only sends interests on the available
   interfaces.  Requesting network coded packets ensures that during the
   phase where it is connected to the previous and the next APs at the
   same time, it does not receive duplicate data, but does not miss on

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   any content either.  By combining NC with CCN, the consumer receives
   additional degrees of freedom with any innovative packet it receives
   on either interface.  From this point of view, an effective interest
   forwarding strategy with a rate adaptation mechanism should be
   considered for consumer to achieve seamless mobility while improving
   overall throughput or latency.

4.5.  Security and Privacy

   This subsection describes the requirement for security and privacy
   provided by NC in CCN/NDN, such as data integrity especially when
   intermediate nodes perform re-encoding, as in the case of hash
   restrictions for original data packets, and so on.

   Network coding impacts the security mechanisms of CCN/NDN.  In
   particular, CCN/NDN is designed to prevent modification of the Data
   packets.  Because Data packets for a specific name can be self-
   authenticated, they can be validated on the delivery path, and can
   also be cached at untrusted intermediate nodes.  Network coding may
   bring up issues if intermediate nodes are allowed to modify packets
   by performing additional network coding operations.  Intermediate
   nodes may also be caching network coded packets without having the
   ability to perform validation of the content and therefore open
   themselves to cache pollution attacks.

   In CCN/NDN, content objects can be encrypted to support access
   control or privacy.  If the coding information of coded packet is
   included in the encrypted data payload, extra computational overhead
   occurs.  With consideration for low computation overhead, some
   mechanism supporting both NC and access control/privacy should be
   considered.

5.  Challenges

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

5.1.  Adopting Convolutional Coding

   Several block coding approaches have been proposed so far, but there
   is still no sufficient discussion and application of convolutional
   coding approach (e.g., sliding or elastic window coding) in CCN/NDN.
   Convolutional coding is often appropriate to situations where a fully
   or partially reliable delivery of continuous data flows is needed,
   especially when these data flows feature realtime constraints.  As in
   [32] on an end-to-end basis, it would be advantageous for continuous
   content flow to adopt sliding window coding in CCN/NDN.  In this
   case, the publisher needs to appropriately set coding parameters and

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   let content requestor know the information, and content requestor
   needs to send interest (i.e., feedback information) about the data
   reception status.  Since CCN/NDN advocates hop-by-hop communication,
   it would be worth discussing and investigating how convolutional
   coding can be applied in a hop-by-hop fashion and the benefits.  In
   particular, assuming that NC could occur at intermediate nodes with
   some useful data packets stored in the CS as described in the
   previous section, both the encoding window and CS management would be
   required, and the feasibility and practicality should be considered.

5.2.  Rate and Congestion Control

   Adding redundancy using coded packets may cause further network
   congestion and adversely affect overall throughput performance.  In
   particular, in a situation where 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
   indispensable that each content flow cooperatively implements
   congestion control to adjust the consumed bandwidth to stabilize the
   network condition (i.e., to achieve low packet loss rate, delay, and
   jitter).  From this point of view, a router supported approach would
   be effective, but an effective deployment scenario is needed.

5.3.  Security and Privacy

   CCN/NDN introduces new security and privacy issues at the networking
   layer different from IP network, such as cache poisoning and
   pollution attack, DoS attack using interest packets, and so on.

   NC could be utilized to mitigate some security or privacy issues CCN/
   NDN introduces.  For instance, assuming that consumers can utilize
   multipath data retrieval and caching in CCN/NDN with NC, cache
   privacy and anonymity set for consumers can be improved as well as
   caching performance due to the diversity of caching content along
   different paths.  In the case that there is a trusted router or proxy
   that can operate NC for both interest and data packets, consumer
   privacy can be protected and censorship can be mitigated; consumer
   splits the interest into small chunks and encrypts a linear
   combination of the chunks with the public key of the trusted
   intermediate node, and the content provider follows the same approach
   as the client.  This approach however prevents in-network cache
   utilization and involves the high computation cost of many
   cryptographic operations.

   On the other hand, considering NC operations over CCN/NDN, the issues
   related to in-network caching add additional complexity.  In order to
   avoid cache poisoning attack which tries to fill routers cache with
   polluted content, router needs to check whether or not the content is

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   validated.  However, in the case of performing NC and generating a
   new coded data at routers, a validation mechanism to accurately
   verify coded data as quickly as possible should be considered while
   maintaining in-network cache benefits (lower latency and network
   resource saving).  If router can cache some valid coded data, it
   needs to put a great deal of thought into the effectiveness with
   respect to cache pollution attack, since coded data newly generated
   may be unpopular.  Moreover, Denial of Service (DoS) attacks may
   target either the routers or the publishers performing NC to pose
   unnecessary coded data, impose higher NC computation load, and
   increase the number of PIT entries, which requires some careful
   considerations to avoid them.

   NC also offers a new surface of attack; for instance, if the encoding
   vector is exposed at the network layer, it would have to be protected
   (and validated) so as to avoid modifications by an attacker (and
   allow for verification) on the path of the packet.

   In this context, from the perspective of both feasibility and
   practicability, a more effective approach with consideration for
   security and privacy would be needed in order to accelerate the
   deployment of CCN/NDN with NC.

5.4.  Routing Scalability

   In CCN/NDN, a name-based routing protocol without a resolution
   process streamlines the routing process and reduces the overall
   latency.  As in IP routing, the growth in the routing table size has
   become a concern.  This may require a hierarchical naming scheme so
   as to improve the routing scalability by enabling aggregation of
   routing information.  Moreover, in order to effectively leverage NC
   over CCN/NDN to improve throughput or reduce latency, routers need to
   know which node can exactly and quickly provide coded packets.  Such
   routing coordination creates routing scalability issues.

   From another NC perspective, consumers need in advance to know how to
   specify the names of the coded data to appropriately get it in a
   scalable manner.  In this context, it would be challenging to achieve
   effective and scalable routing for Interests requesting coded data as
   well as to simplify the routing process.

6.  Security Considerations

   This document does not impact the security of the Internet.  Security
   considerations related to NC for CCN/NDN are described in the
   previous Section.

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7.  References

7.1.  Normative References

   [1]        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>.

7.2.  Informative References

   [2]        Cai, N. and R. Yeung, "Secure network coding", Proc.
              International Symposium on Information Theory
              (ISIT), IEEE, June 2002.

   [3]        Lima, L., Gheorghiu, S., Barros, J., Mdard, M., and A.
              Toledo, "Secure Network Coding for Multi-Resolution
              Wireless Video Streaming", IEEE Journal of Selected Area
              (JSAC), vol. 28, no. 3, April 2002.

   [4]        Gkantsidis, C. and P. Rodriguez, "Cooperative Security for
              Network Coding File Distribution", Proc. Infocom, IEEE,
              April 2006.

   [5]        Vilea, J., Lima, L., and J. Barros, "Lightweight security
              for network coding", Proc. ICC, IEEE, May 2008.

   [6]        Dimarkis, A., Godfrey, P., Wu, Y., Wainwright, M., and K.
              Ramchandran, "Network Coding for Distributed Storage
              Systems", IEEE Trans. Information Theory, vol. 56, no.9,
              September 2010.

   [7]        Gkantsidis, C. and P. Rodriguez, "Network coding for large
              scale content distribution", Proc. Infocom, IEEE, March
              2005.

   [8]        Seferoglu, H. and A. Markopoulou, "Opportunistic Network
              Coding for Video Streaming over Wireless", Proc. Packet
              Video Workshop (PV), IEEE, November 2007.

   [9]        Montpetit, M., Westphal, C., and D. Trossen, "Network
              Coding Meets Information-Centric Networking: An
              Architectural Case for Information Dispersion Through
              Native Network Coding", Proc. Workshop on Emerging Name-
              Oriented Mobile Networking Design (NoM), ACM, June 2012.

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   [10]       Saltarin, J., Bourtsoulatze, E., Thomos, N., and T. Braun,
              "NetCodCCN: a network coding approach for content-centric
              networks", Proc. Infocom, IEEE, April 2016.

   [11]       Ramakrishnan, A., Westphal, C., and J. Saltarin, "Adaptive
              Video Streaming over CCN with Network Coding for Seamless
              Mobility", Proc. International Symposium on Multimedia
              (ISM), IEEE, December 2016.

   [12]       Wang, J., Ren, J., Lu, K., Wang, J., Liu, S., and C.
              Westphal, "An Optimal Cache Management Framework for
              Information-Centric Networks with Network Coding", Proc.
              Networking Conference, IFIP/IEEE, June 2014.

   [13]       Wang, J., Ren, J., Lu, K., Wang, J., Liu, S., and C.
              Westphal, "A Minimum Cost Cache Management Framework for
              Information-Centric Networks with Network Coding",
              Computer Networks, Elsevier, August 2016.

   [14]       Matsuzono, K., Asaeda, H., and T. Turletti, "Low Latency
              Low Loss Streaming using In-Network Coding and Caching",
              Proc. Infocom, IEEE, May 2017.

   [15]       Jacobson, V., Smetters, D., Thornton, J., Plass, M.,
              Briggs, N., and R. Braynard, "Networking Named Content",
              Proc. CoNEXT, ACM, December 2009.

   [16]       Zhang, L., Afanasyev, A., Burke, J., Jacobson, V., Claffy,
              K., Crowley, P., Papadopoulos, C., Wang, L., and B. Zhang,
              "Named data networking", ACM Comput. Commun. Rev., vol.
              44, no. 3, July 2014.

   [17]       Koetter, R. and M. Medard, "An Algebraic Approach to
              Network Coding", IEEE/ACM Trans. on Networking, vol. 11,
              no 5, Oct. 2003.

   [18]       Adamson, B., Adjih, C., Bilbao, J., Firoiu, V., Fitzek,
              F., Lochin, E., Masucci, A., Montpetit, M., Pedersen, M.,
              Peralta, G., Roca, V., Saxena, P., and S. Sivakumar,
              "Network Coding Taxonomy", draft-irtf-nwcrg-network-
              coding-taxonomy-05 (work in progress), September 2017.

   [19]       Kutscher, et al., D., "Information-Centric Networking
              (ICN) Research Challenges", RFC 7927, July 2016.

   [20]       Thomos, N. and P. Frossard, "Toward one Symbol Network
              Coding Vectors", IEEE Communications letters, vol. 16, no.
              11, November 2012.

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   [21]       Lucani, D., Pedersen, M., Heide, J., and F. Fitzek,
              "Fulcrum Network Codes: A Code for Fluid Allocation of
              Complexity",  available at http://arxiv.org/abs/1404.6620,
              April 2014.

   [22]       Perino, D. and M. Varvello, "A reality check for content
              centric networking", Proc. SIGCOMM Workshop on
              Information-centric networking (ICN'11), ACM, August 2011.

   [23]       Wu, Q., Li, Z., Tyson, G., Uhlig, S., Kaafar, M., and G.
              Xie, "Privacy-Aware Multipath Video Caching for Content-
              Centric Networks", IEEE Journal of Selected Area
              (JSAC) vol. 38, no. 8, June 2016.

   [24]       Wu, Y., Chou, P., and K. Jain, "A comparison of network
              coding and tree packing", Proc. ISIT, IEEE, June 2004.

   [25]       Ho, T., Medard, M., Koetter, R., Karger, R., Effros, D.,
              Shi, M., and B. Leong, "A Random Linear Network Coding
              Approach to Multicast", IEEE Trans. Information
              Theory, vol. 52, no.10, October 2006.

   [26]       Podlipnig, S. and L. Osz, "A Survey of Web Cache
              Replacement Strategies", Proc. ACM Computing Surveys vol.
              35, no. 4, December 2003.

   [27]       Rossini, G. and D. Rossi, "Evaluating CCN multi-path
              interest forwarding strategies", Elsevier Computer
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   [28]       Chai, W., He, D., Psaras, I., and G. Pavlou, "Cache Less
              for More in Information-centric Networks", Journal
              Computer Communications, vol. 37. no. 7, April 2013.

   [29]       Carofiglio, G., Muscariello, L., Papalini, M., Rozhnova,
              N., and X. Zeng, "Leveraging ICN In-network Control for
              Loss Detection and Recovery in Wireless Mobile networks",
              Proc. ICN ACM, September 2016.

   [30]       Ali, M. and U. Niesen, "Coding for Caching: Fundamental
              Limits and Practical Challenges", IEEE Communications
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   [31]       Koetter, R. and F. Kschischang, "An algebraic approach to
              network coding", IEEE Trans. Netw. vol.11, no.5, October
              2008.

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   [32]       Tournoux, P., Lochin, E., Lacan, J., Bouabdallah, A., and
              V. Roca, "On-the-Fly Erasure Coding for Real-Time Video
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              2011.

Authors' Addresses

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

   Email: matsuzono@nict.go.jp

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

   Email: asaeda@nict.go.jp

   Cedric Westphal
   Huawei
   2330 Central Expressway
   Santa Clara, California  95050
   USA

   Email: cedric.westphal@huawei.com

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