Internet Research Task Force (IRTF) K. Matsuzono
Request for Comments: 9273 H. Asaeda
Category: Informational NICT
ISSN: 2070-1721 C. Westphal
Huawei
August 2022
Network Coding for Content-Centric Networking / Named Data Networking:
Considerations and Challenges
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).
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Research Task Force
(IRTF). The IRTF publishes the results of Internet-related research
and development activities. These results might not be suitable for
deployment. This RFC represents the consensus of the Coding for
Efficient Network Communications Research Group of the Internet
Research Task Force (IRTF). Documents approved for publication by
the IRSG are not candidates for any level of Internet Standard; see
Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9273.
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Table of Contents
1. Introduction
2. Terminology
2.1. Definitions Related to NC
2.2. Definitions Related to CCNx/NDN
3. CCNx/NDN Basics
4. NC Basics
5. Advantages of NC and CCNx/NDN
6. Technical Considerations
6.1. Content Naming
6.1.1. Unique Naming for NC Packets
6.1.2. Nonunique Naming for NC Packets
6.2. Transport
6.2.1. Scope of NC
6.2.2. Consumer Operation
6.2.3. Forwarder Operation
6.2.4. Producer Operation
6.2.5. Backward Compatibility
6.3. In-Network Caching
6.4. Seamless Consumer Mobility
7. Challenges
7.1. Adoption of Convolutional Coding
7.2. Rate and Congestion Control
7.3. Security
7.4. Routing Scalability
8. IANA Considerations
9. Security Considerations
10. Informative References
Acknowledgments
Authors' Addresses
1. Introduction
Information-Centric Networking (ICN), in general, and Content-Centric
Networking (CCNx) [1] or Named Data Networking (NDN) [2], in
particular, have emerged as a novel communication paradigm that
advocates for 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,
and content security. One key benefit of requesting content by name
is that it eliminates the requirement to establish 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, in
terms of security aspects, NC can be managed using a practical
security scheme that deals with pollution attacks [3] and can be
utilized for preventing eavesdroppers from obtaining meaningful
information [4] or protecting a wireless video stream while retaining
the NC benefits [5] [6].
From the perspective of the NC transport mechanism, NC can be divided
into two major categories: coherent NC and noncoherent NC [7] [8].
In coherent NC, the source and destination nodes know the exact
network topology and the coding operations available 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 [9]. However, it requires a fully adjustable
and specific routing mechanism and a large computational capacity for
central coordination. In the case of noncoherent NC, which often
uses Random Linear Coding (RLC), it is not necessary to know the
network topology nor the intermediate coding operations [10]. As
noncoherent NC works in a completely independent and decentralized
manner, this approach is more feasible in terms of eliminating such a
central coordination.
NC combines multiple packets together with parts of the same content
and may do this at the source and/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 allows for
recovery of missing packets by encoding the information across
several packets. ICN is synergistic with NC, as it allows for
caching of data packets throughout the network. In a typical network
that uses NC, the sender must transmit enough packets to retrieve the
original data. ICN offers an opportunity to retrieve network-coded
packets directly from caches in the network, making the combination
of ICN and NC particularly effective. In fact, NC has already been
implemented for information/content dissemination [11] [12] [13].
Montpetit et al. first suggested that NC techniques be exploited to
enhance key aspects of system performance in ICN [14]. Although
CCNx/NDN excels to exploit the benefits of NC (as described in
Section 5), some technical considerations are needed to combine NC
and CCNx/NDN owing to the unique features of CCNx/NDN (as described
in Section 6).
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
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). This
document was read and reviewed by all the active research group
members. It is not an IETF product and is not a standard.
2. Terminology
2.1. Definitions Related to NC
This section provides the terms related to NC used in this document,
which are defined in RFC 8406 [15] and produced by the NWCRG.
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.
Repair Packet:
A packet containing one or more coded symbols (also called repair
symbol). The coded symbol or repair symbol is a unit of data that
is the result of a coding operation, applied either to source
symbols or (in case of recoding) source and/or coded symbols.
When there is a single repair symbol per repair packet, a repair
symbol corresponds to a repair packet.
Encoding versus Recoding versus Decoding:
Encoding is an operation that takes source symbols as input and
produces encoding symbols (source or coded symbols) as output.
Recoding is an operation that takes encoding symbols as input and
produces encoding symbols as output. Decoding is an operation
that takes encoding symbols as input and produces source symbols
as output.
The terms regarding coding types are defined as follows:
Random Linear Coding (RLC):
A particular form of linear coding using a set of random coding
coefficients. Linear coding performs a linear combination of a
set of input symbols (i.e., source and/or coded symbols) using a
given set of coefficients and results in a repair symbol.
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.
Sliding Window Coding or Convolutional Coding:
A general class of coding techniques that rely on a sliding
encoding window. An encoding window is a set of source (and coded
in the case of recoding) 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.
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 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:
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.
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.
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.
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.
Coding Vector:
A set of coding coefficients used to generate a certain coded
symbol through linear coding.
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.
2.2. Definitions Related to CCNx/NDN
The terminology regarding CCNx/NDN used in this document is defined
in RFC 8793 [16], which was produced by the ICNRG. They are
consistent with the relevant documents ([17] [18]).
3. CCNx/NDN Basics
We briefly explain the key concepts of CCNx/NDN. In a CCNx/NDN
network, there are two types of packets at the network level:
interest and data packet (defined in Section 3.4 of [16]). The term
"content object", which means a unit of content data, is an alias to
data packet [16]. The ICN consumer (defined in Section 3.2 of [16])
requests a content object by sending an interest that carries the
name of the data.
Once an ICN forwarder (defined in Section 3.2 of [16]) 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 cache
storage, called Content Store (CS) (defined in Section 3.3 of [16]).
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 [16]) 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 [16]) lookup for selecting an
outgoing interface. The FIB lists name prefixes and their
corresponding forwarding interfaces in order to send the interest
toward 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 verifying data integrity and
origin authentication and, in particular, that the data is indeed
that which corresponds to the name [19]. 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 [16]) 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
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 repair 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 repair packets
in the content flow (called systematic coding), where the repair
packets are typically used for recovering lost source packets.
Repair packets can also be used for performing encoding. If the
forwarding nodes know each coding vector and generation identifier
(hereinafter referred to as generation ID) of the received repair
packets, they may perform an encoding operation (called recoding),
which is the most distinctive 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
source and repair packets (possibly only repair 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, recoding should
generate and provide innovative packets. One of the 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 [9].
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 [7], while the latter focuses on the content/information
itself at the networking layer. Because these approaches are
complementary and 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
[1]. 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 [14].
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) [20]. 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 [21]
[22] [23], seamless consumer mobility [24] [25], and low-latency low-
loss video streaming [26]. 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 [19]. In this section, two possible
naming schemes are presented.
6.1.1. Unique Naming for NC Packets
Each source and repair packet (hereinafter referred to as NC packet)
may have a unique name, as each original content object has in CCNx/
NDN and as PIT and CS operations typically require a unique name for
identifying the NC packet. As a method of naming an NC packet that
takes into account the feature of block coding, the coding vector and
the generation ID can be used as a part of the content object name.
As in [21], 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 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 NC 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 an NC 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/NDN). The consumer
is able 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 the interest with only the prefix name matches any NC
packet with the same prefix, the consumer could immediately obtain an
NC packet from a nearby CS (in-network cache) without knowing the
coding vectors of the cached NC packets in advance. In the case
wherein NC packets in transit are modified by in-network recoding
performed at forwarders, the consumer could also receive the modified
NC 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 NC 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 [14]. 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).
6.1.2. Nonunique Naming for NC Packets
An NC packet may have a name that indicates that it is an NC packet
and move the coding information into a metadata field in the payload
(i.e., the name includes the data type, source, or repair 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 NC 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, 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 means that a forwarder or producer cannot
inject unrequested data packets on its own initiative. 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 a data forwarder can perform in-network
recoding 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 recoding is performed to
generate the NC packet at any forwarder that is able to respond to
the interest. This could occur when each NC 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 multiple NC
packets have the same name, the former case may occur; recoding
occurs anywhere in the network where it is possible to modify the
received NC packet and forward it. As CCNx/NDN comprises mechanisms
for ensuring the integrity of the data during transfer, in-network
recoding introduces complexities in the network that needs
consideration for the integrity mechanisms to still work. Similarly,
in-network caching of NC packets at forwarders may be valuable;
however, the forwarders would require some mechanisms to validate the
NC packets (see Section 9).
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 NC 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 an NC packet, the consumer could
appropriately receive an innovative packet if it is available at some
forwarders.
In order to specify the exact name of the NC 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 [17] that enumerates
the content objects, including NC 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 NC 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. This issue could happen when 1) incoming
interests for NC 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 NC packets (possibly in
the CS) to use for recoding. 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 [21]. 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 [25][26] at a
forwarder, as well as a consumer-driven mechanism, could be
indispensable for enabling fast loss recovery and realizing 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
an innovative packet effectively and quickly, an appropriate setting
of the FIB and efficient interest-forwarding strategies should also
be considered.
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 subsequent interests.
As recoding 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 a CCNx/NDN network. The producer performs
the encoding operation for a set of the small content objects and the
naming process for the NC packets.
If the producer takes the lead in determining what coding vectors to
use in generating the NC packets, there are three general strategies
for naming and producing the NC packets:
1. Consumers themselves understand in detail the naming conventions
used for NC packets and thereby can send the corresponding
interests toward the producer to obtain NC packets whose coding
parameters have already been determined by the producer.
2. The producer determines the coding vectors and generates the NC
packets after receiving interests specifying the packets the
consumer wished to receive.
3. The naming scheme for specifying the coding vectors and
corresponding NC packets is explicitly represented via a
"Manifest" (e.g., FLIC [30]) 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 NC packet to obtain, the latency for
obtaining the NC 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 NC packets to be included in the manifest must be pre-
computed by the producer (since the manifest references NC packets by
their hashes, not their names), but the producer can select which to
include in the manifest and produce multiple manifests either in
advance or on demand with different coding tradeoffs, if so desired.
A common benefit of the first two approaches to end-to-end coding is
that, if the producer adds a signature on the NC packets, data
validation becomes possible throughout (as is the case with the 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 NC packet.
6.2.5. Backward Compatibility
NC operations should be applied in addition to the regular ICN
behavior and should function alongside regular ICN operations.
Hence, nodes that do not support NC should still be able to properly
handle packets, not only in being able to forward the NC packets but
also to cache these packets. 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 the network level and is highly
beneficial, owing to the involved exploitation of NC for enabling
effective multicast transmission [31], multipath data retrieval [21]
[24], and fast loss recovery [26]. 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 [32] [33] [34]. 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.
One key aspect of in-network caching is whether or not forwarders can
cache NC packets in their CS. They may be caching the NC packets
without having the ability to perform a validation of the content
objects. Therefore, the caching of the NC packets would require some
mechanism to validate the NC packets (see Section 9). In the case
wherein the NC 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 [24] or congestion control
for content acquisition [35].
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 [24], 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 real-time constraints. As in [36], on an end-to-end
coding basis, it would be advantageous for continuous content flow to
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 utilizes the 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 recoding 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 [37]. From this perspective, an
effective deployment approach (e.g., a forwarder-supported approach
that can provide 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, and avoiding duplicated
packets has congestion control benefits as well. 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,
pollution attacks, and a DoS attack using interest packets, some
security approaches are already provided [19] [38]. The application
of NC in CCNx/NDN brings two potential security aspects that need to
be dealt with.
The first is in-network recoding at forwarders. Some mechanism for
ensuring the integrity of the NC packets newly produced by in-network
recoding is required in order for consumers or other forwarders to
receive valid NC packets. To this end, there are some possible
approaches described in Section 9, but there may be a more effective
method with lower complexity and computation overhead.
The second is that attackers maliciously request and inject NC
packets, which could amplify some attacks. As NC 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 NC 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 NC packets. On the assumption that each NC packet has
the valid signature, the straightforward approach would comprise the
forwarders verifying the signature within the NC packets in transit
and only transmitting and storing the validated NC 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 NC packets, as well as to simplify the routing
process.
8. IANA Considerations
This document has no IANA actions.
9. Security Considerations
In-network recoding is a distinguishing feature of NC. Only valid NC
packets produced by in-network recoding 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 recoding to have their own
unique signing key. Each forwarder of the group signs a newly
generated NC packet in order for other nodes to be able to validate
the data with the signature. The CS may verify the signature within
the NC 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 [17]. 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 [39], could be used to establish
a trustworthy data delivery path. This approach adopts the hop-by-
hop authentication mechanism, wherein the 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 NC 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 aimed at provoking the high load of NC operations by using
the interests for NC 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 NC packets 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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Acknowledgments
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.
Authors' Addresses
Kazuhisa Matsuzono
National Institute of Information and Communications Technology
4-2-1 Nukui-Kitamachi, Tokyo
184-8795
Japan
Email: matsuzono@nict.go.jp
Hitoshi Asaeda
National Institute of Information and Communications Technology
4-2-1 Nukui-Kitamachi, Tokyo
184-8795
Japan
Email: asaeda@nict.go.jp
Cedric Westphal
Huawei
2330 Central Expressway
Santa Clara, California 95050
United States of America
Email: cedric.westphal@futurewei.com,