Network Coding Research Group K. Matsuzono
Internet-Draft H. Asaeda
Intended status: Informational NICT
Expires: April 27, 2022 C. Westphal
Huawei
October 24, 2021
Network Coding for Content-Centric Networking / Named Data Networking:
Considerations and Challenges
draft-irtf-nwcrg-nwc-ccn-reqs-07
Abstract
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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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
document.
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
receiver.
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
coding.
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
choices.
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
completed.
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
discarded.
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
them.
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 /CCNx.com/video-A/g-id/1000. 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 /CCNx.com/video-A/enc-id/
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 (/CCNx.com/video-A/
g-id/1000) or only the name prefix including only a generation id
(/CCNx.com/video-A/g-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
consumer.
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
forwarders.
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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
considered.
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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
interests.
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
with.
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
process.
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
[1] Mosko, M. and et al., "Content-Centric Networking (CCNx)
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(ICN): Content-Centric Networking (CCNx) and Named Data
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[18] Mosko, M. and et al., "Content-Centric Networking (CCNx)
Messages in TLV Format", RFC 8609, July 2019,
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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
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and Congestion Control in Transport", Work in Progress,
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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)
Framework", RFC 6363, Oct. 2011.
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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
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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@futurewei.com,
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