Internet Engineering Task Force (IETF) R. Alimi, Ed.
Request for Comments: 6392 Google
Category: Informational A. Rahman, Ed.
ISSN: 2070-1721 InterDigital Communications, LLC
Y. Yang, Ed.
Yale University
October 2011
A Survey of In-Network Storage Systems
Abstract
This document surveys deployed and experimental in-network storage
systems and describes their applicability for the DECADE (DECoupled
Application Data Enroute) architecture.
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 Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6392.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Alimi, et al. Informational [Page 1]
RFC 6392 DECADE Survey October 2011
Table of Contents
1. Introduction ....................................................3
2. Survey Overview .................................................3
2.1. Terminology and Concepts ...................................3
2.2. Historical Context .........................................3
3. In-Network Storage System Components ............................5
3.1. Data Access Interface ......................................5
3.2. Data Management Operations .................................5
3.3. Data Search Capability .....................................6
3.4. Access Control Authorization ...............................6
3.5. Resource Control Interface .................................6
3.6. Discovery Mechanism ........................................7
3.7. Storage Mode ...............................................7
4. In-Network Storage Systems ......................................7
4.1. Amazon S3 ..................................................7
4.2. BranchCache ................................................9
4.3. Cache-and-Forward Architecture ............................11
4.4. Cloud Data Management Interface ...........................12
4.5. Content Delivery Network ..................................14
4.6. Delay-Tolerant Network ....................................16
4.7. Named Data Networking .....................................18
4.8. Network of Information ....................................19
4.9. Network Traffic Redundancy Elimination ....................22
4.10. OceanStore ...............................................23
4.11. P2P Cache ................................................24
4.12. Photo Sharing ............................................26
4.13. Usenet ...................................................28
4.14. Web Cache ................................................29
4.15. Observations Regarding In-Network Storage Systems ........31
5. Storage and Other Related Protocols ............................32
5.1. HTTP ......................................................32
5.2. iSCSI .....................................................33
5.3. NFS .......................................................34
5.4. OAuth .....................................................36
5.5. WebDAV ....................................................37
5.6. Observations Regarding Storage and Related Protocols ......39
6. Conclusions ....................................................40
7. Security Considerations ........................................40
8. Contributors ...................................................40
9. Acknowledgments ................................................41
10. Informative References ........................................41
Alimi, et al. Informational [Page 2]
RFC 6392 DECADE Survey October 2011
1. Introduction
DECADE (DECoupled Application Data Enroute) is an architecture that
provides applications with access to provider-based in-network
storage for content distribution (hereafter referred to as only
"in-network storage" in this document). With access to in-network
storage, content distribution applications can be designed to place
less load on network infrastructure. As a simple example, a peer of
a Peer-to-Peer (P2P) application may upload to other peers through
its in-network storage, saving its usage of last-mile uplink
bandwidth. See [1] for further discussion.
A major motivation for DECADE is the substantial increase in capacity
and reduction in cost offered by storage systems. For example, over
the last two decades, there has been at least a 30-fold increase in
the amount of storage that a customer can get for a given price (for
flash memory and hard disk drives) [2] [3] [4].
High-capacity and low-cost in-network storage devices introduce
substantial opportunities. One example of in-network storage is
content caches supporting Web and P2P content. DECADE differs from
existing content caches whose control fully resides with the owners
of the caching devices in that DECADE also allows applications to
control access to their allocated in-network storage, as well as the
resources consumed while accessing that storage (bandwidth,
connections, storage space). While designed in the context of P2P
applications, DECADE may be useful to other applications as well.
This document provides details on deployed and experimental
in-network storage solutions, and evaluates their suitability for
DECADE.
We note that the survey presented in this document is only
representative of the research in this area. Rather than trying to
enumerate an exhaustive list, we have chosen some typical techniques
that lead to derivative works.
2. Survey Overview
2.1. Terminology and Concepts
This document uses terms defined in [1].
2.2. Historical Context
In-network storage has been used previously in numerous scenarios to
reduce network traffic and enable more efficient content
distribution. This section presents a brief history of content
distribution techniques and illustrates how DECADE relates to past
Alimi, et al. Informational [Page 3]
RFC 6392 DECADE Survey October 2011
approaches. Systems have been developed with particular use cases in
mind. Thus, this survey is not meant to point out shortcomings of
existing solutions, but rather to indicate where certain capabilities
required in DECADE [5] are not provided by existing systems.
In the early stage of Internet development, most Web content was
stored at a central server, and clients requested Web content from
the central server. In this architecture, the central server was
required to provide a large amount of bandwidth. As more and more
users access Web content, a central server can become overloaded.
The use of Web caches is one technique to reduce load on a central
server. Web caches store frequently requested content and provide
bandwidth for serving the content to clients.
The ongoing growth of broadband technology in the worldwide market
has been driven by the hunger of customers for new multimedia
services as well as Web content. In particular, the use of audio and
video streaming formats has become common for delivery of rich
information to the public, both residential and business.
To overcome this challenge of massive multimedia consumption, just
installing more Web caches will not be enough. Moving content closer
to the consumer results in greater network efficiency, improved
Quality of Service (QoS), and lower latency, while facilitating
personalization of content through broadband content applications.
In these edge technologies, Content Delivery Networks (CDNs) are a
representative technique. CDNs are based on a large-scale
distributed network of servers located closer to customers for
efficient delivery of digital content, including various forms of
multimedia content.
Although CDNs are an effective means of information access and
delivery, there are two barriers to making CDNs a more common
service: cost and replication integrity. Deploying a CDN with its
associated infrastructure is expensive. A CDN also requires
administrative control over nodes with large storage capacity at
geographically dispersed locations with adequate connectivity. CDNs
can be scalable, but due to this administrative and cost overhead,
they are not rapidly deployable for the common user.
The emergence and maturation of P2P has allowed improvements to many
network applications. P2P allows the use of client resources, such
as CPU, memory, storage, and bandwidth, for serving content. This
can reduce the amount of resources required by a content provider.
Multimedia content delivery using various P2P or peer-assisted
frameworks has been shown to greatly reduce the dependence on CDNs
and central content servers. However, the popularity of P2P
applications has resulted in increased traffic on ISP networks. P2P
Alimi, et al. Informational [Page 4]
RFC 6392 DECADE Survey October 2011
caches (both transparent and non-transparent) have been introduced as
a way to reduce the burden. Though they can be effective in reducing
traffic in certain areas of ISP networks, P2P caches have their
shortcomings. In particular, they are application-dependent and thus
difficult to keep up to date with new and evolving P2P application
protocols. Second, applications may benefit from explicit control of
in-network storage, which P2P caches do not provide. See [1] for
further discussion.
DECADE aims to provide a standard protocol allowing P2P applications
(including content providers) to make use of in-network storage to
reduce the traffic burden on ISP networks, while enabling P2P
applications to control access to content they have placed in
in-network storage.
3. In-Network Storage System Components
Before surveying individual technologies, we describe the basic
components of in-network storage. For consistency and for ease of
comparison, we use the same model to evaluate each storage technology
in this document.
Note that the network protocol(s) used by a given storage system are
also an important part of the design. We omit details of particular
protocol choices in this document.
3.1. Data Access Interface
A set of operations is made available to a user for accessing data in
the in-network storage system. Solutions typically allow both read
and write operations, though the mechanisms for doing so can differ
drastically.
3.2. Data Management Operations
Storage systems may provide users the ability to manage stored
content. For example, operations such as delete and move may be
provided to users. In this survey, we focus on data management
operations that are provided to users and omit those provided to
system administrators.
Alimi, et al. Informational [Page 5]
RFC 6392 DECADE Survey October 2011
3.3. Data Search Capability
Some storage systems may provide the capability to search or
enumerate content that has been stored. In this survey, we focus on
search capabilities that are provided to users and omit those
provided to system administrators. An example of a search would be
to find the list of items stored by a given user over a given period
of time.
3.4. Access Control Authorization
Storage systems typically allow a user, content owner, or some other
entity to define the access policies for the in-network storage
system. The in-network storage system then checks the authorization
of a user before it stores or retrieves content. We define three
types of access control authorization: public-unrestricted, public-
restricted, and private.
"Public-unrestricted" refers to content on an in-network storage
system that is widely available to all clients (i.e., without
restrictions). An example is accessing Wikipedia on the Web, or
anonymous access to FTP sites.
"Public-restricted" refers to content on an in-network storage system
that is available to a restricted (though still potentially large)
set of clients, but that does not require any confidential
credentials from the client. An example is some content (e.g., a TV
show episode) on the Internet that can only be viewable in selected
countries or networks (i.e., white/black lists or black-out areas).
"Private" refers to content on an in-network storage system that is
only made available to one or more clients presenting the required
confidential credentials (e.g., password or key). This content is
not available to anyone without the proper confidential access
credentials.
Note that a combination of access control types may be applicable for
a given scenario. For example, the retrieval (read) of content from
an in-network storage system may be public-unrestricted, but the
storage (write) to the same system may be private.
3.5. Resource Control Interface
This is the interface through which users manage the resources on
in-network storage systems that can be used by other peers, e.g., the
bandwidth or connections. The storage system may also allow users to
indicate a time for which resources are granted.
Alimi, et al. Informational [Page 6]
RFC 6392 DECADE Survey October 2011
3.6. Discovery Mechanism
Users use the discovery mechanism to find the location of in-network
storage, find an access interface or resource control interface, or
find other interfaces of in-network storage.
3.7. Storage Mode
Storage systems may use the following modes of storage: file system,
object-based, or block-based.
A file system typically organizes files into a hierarchical tree
structure. Each level of the hierarchy normally contains zero or
more directories, each with zero or more files. A file system may
also be flat or use some other organizing principle.
We define an object-based storage mode as one that stores discrete
chunks of data (e.g., IP datagrams or another type of aggregation
useful to an application) without a pre-defined hierarchy or
meta-structure.
We define a block-based storage mode as one that stores a raw
sequence of bytes, with a client being able to read and/or write data
at offsets within that sequence. Data is typically accessed in
blocks for efficiency. A common example for this storage mode is raw
access to a hard disk.
In this survey, we define "storage mode" to refer to how data is
structured within the system, which may not be the same as how it is
accessed by a client. For example, a caching system may cache
objects with hierarchical names, but may internally use an object-
based storage mode.
4. In-Network Storage Systems
This section surveys in-network storage systems using the methodology
defined above. The survey includes some systems that are widely
deployed today, some systems that are just being deployed, and some
experimental systems. The survey covers both traditional client-
server architectures and P2P architectures. The surveyed systems are
listed in alphabetical order. Also, for each system, a brief
explanation of the relevance to DECADE is given.
4.1. Amazon S3
Amazon S3 (Simple Storage Service) [6] provides an online storage
service using Web (HTTP) interfaces. Users create buckets, and each
bucket can contain stored objects. Users are provided an interface
Alimi, et al. Informational [Page 7]
RFC 6392 DECADE Survey October 2011
through which they can manage their buckets. Amazon S3 is a popular
backend storage service for other services. Other related storage
services are the Blob Service provided by Windows Azure [7], Google
Storage for Developers [8], and Dropbox [9].
4.1.1. Applicability to DECADE
Amazon S3 is a very widely used (deployed) example of in-network
storage. Amazon S3 leases the storage to third-party companies for
disparate services. In particular, Amazon S3 has a rich model for
authorization (using signed queries) to integrate with a wide variety
of use cases. A focus for Amazon S3 is scalability. Particular
simplifications that were made are the absence of a general,
hierarchical namespace and the inability to update the contents of
existing data.
4.1.2. Data Access Interface
Users can read and write objects.
4.1.3. Data Management Operations
Users can delete previously stored objects.
4.1.4. Data Search Capability
Users can list contents of buckets to find objects matching desired
criteria.
4.1.5. Access Control Authorization
All methods of access control are supported for clients: public-
unrestricted, public-restricted, and private.
For example, access to stored objects can be restricted by an owner,
a list of other Amazon S3 Web Service users, or all Amazon S3 Web
Service users; or can be open to all users (anonymous access).
Another option is for the owner to generate and sign a query (e.g., a
query to read an object) that can be used by any user until an owner-
defined expiration time.
4.1.6. Resource Control Interface
Not provided.
Alimi, et al. Informational [Page 8]
RFC 6392 DECADE Survey October 2011
4.1.7. Discovery Mechanism
Users are provided a well-known DNS name (either a default provided
by Amazon S3, or one customized by a particular user). Users
accessing S3 storage use DNS to discover an IP address where S3
requests can be sent.
4.1.8. Storage Mode
Object-based, with the extension that objects can be organized into
user-defined buckets.
4.2. BranchCache
BranchCache [10] is a feature integrated into Windows (Windows 7 and
Windows Server 2008R2) that aims to optimize enterprise branch office
file access over WAN links. The main goals are to reduce WAN link
utilization and improve application responsiveness by caching and
sharing content within a branch while still maintaining end-to-end
security. BranchCache allows files retrieved from the Web servers
and file servers located in headquarters or data centers to be cached
in remote branch offices, and shared among users in the same branch
accessing the same content. BranchCache operates transparently by
instrumenting the HTTP and Server Message Block (SMB) components of
the networking stack. It provides two modes of operation:
Distributed Cache and Hosted Cache.
In both modes, a client always contacts a BranchCache-enabled content
server first to get the content identifiers for local search. If the
content is cached locally, the client then retrieves the content
within the branch. Otherwise, the client will go back to the
original content server to request the content. The two modes differ
in how the content is shared.
In the Hosted Cache mode, a locally provisioned server acts as a
cache for files retrieved from the servers. After getting the
content identifiers, the client first consults the cache for the
desired file. If it is not present in the cache, the client
retrieves it from the content server and sends it to the cache for
storage.
In the Distributed Cache mode, a client first queries other clients
in the same network using the Web Services Discovery multicast
protocol [11]. As in the Hosted Cache mode, the client retrieves the
file from the content server if it is not available locally. After
retrieving the file (either from another client or the content
server), the client stores the file locally.
Alimi, et al. Informational [Page 9]
RFC 6392 DECADE Survey October 2011
The original content server always authorizes requests from clients.
Cached content is encrypted such that clients can decrypt the data
only using keys derived from metadata returned by the content server.
In addition to instrumenting the networking stack at clients, content
servers must also support BranchCache.
4.2.1. Applicability to DECADE
BranchCache is an example of an in-network storage system primarily
targeted at enterprise networks. It supports a P2P-like mode
(Distributed Cache) as well as a client-server mode (Hosted Cache).
Integration into the Microsoft OS will ensure wide distribution of
this in-network storage technology.
4.2.2. Data Access Interface
Clients transparently retrieve (read) data from a cache (on a client
or a Hosted Cache), since BranchCache operates by instrumenting the
networking stack. In the Hosted Cache mode, clients write data to
the Hosted Cache once it is retrieved from the content server.
4.2.3. Data Management Operations
Not provided.
4.2.4. Data Search Capability
Not provided.
4.2.5. Access Control Authorization
The access control method for clients is private. For example,
transferred content is encrypted, and can only be decrypted by keys
derived from data received from the original content server. Though
data may be transferred to unauthorized clients, end-to-end security
is maintained by only allowing authorized clients to decrypt the
data.
4.2.6. Resource Control Interface
The storage capacity of caches on the clients and Hosted Caches is
configurable by system administrators. The Hosted Cache further
allows configuration of the maximum number of simultaneous client
accesses. In the Distributed Cache mode, exponential back-off and
throttling mechanisms are utilized to prevent reply storms of popular
content requests. The client will also spread data-block access
among multiple serving clients that have the content (complete or
partial) to improve latency and provide some load balancing.
Alimi, et al. Informational [Page 10]
RFC 6392 DECADE Survey October 2011
4.2.7. Discovery Mechanism
The Distributed Cache mode uses multicast for discovery of other
clients and content within a local network. Currently, the Hosted
Cache mode uses policy provisioning or manual configuration of the
server used as the Hosted Cache. In this mode, the address of the
server may be found via DNS.
4.2.8. Storage Mode
Object-based.
4.3. Cache-and-Forward Architecture
Cache-and-Forward (CNF) [12] is an architecture for content delivery
services for the future Internet. In this architecture, storage can
be exploited on nodes within the network, either directly on routers
or deployed near the routers. CNF is based on the concept of store-
and-forward routers with large storage, providing for opportunistic
delivery to occasionally disconnected mobile users and for in-network
caching of content. The proposed CNF protocol uses reliable hop-by-
hop transfer of large data files between CNF routers in place of an
end-to-end transport protocol such as TCP.
4.3.1. Applicability to DECADE
CNF is an example of an experimental in-network storage system that
would require storage space on (or near) a large number of routers in
the Internet if it was deployed. As the name of the system implies,
it would provide short-term caching and not long-term network
storage.
4.3.2. Data Access Interface
Users implicitly store content at CNF routers by requesting files.
End hosts read content from in-network storage by submitting queries
for content.
4.3.3. Data Management Operations
Not provided.
4.3.4. Data Search Capability
Not provided.
Alimi, et al. Informational [Page 11]
RFC 6392 DECADE Survey October 2011
4.3.5. Access Control Authorization
The access control method is public-restricted (to any client that is
part of the CNF network).
4.3.6. Resource Control Interface
Not provided.
4.3.7. Discovery Mechanism
A query including a location-independent content ID is sent to the
network and routed to a CNF router, which handles retrieval of the
data and forwarding to the end host.
4.3.8. Storage Mode
Object-based, with objects representing individual files. The
architecture proposes to cache large files in storage within the
network, though objects could be made to represent smaller chunks of
larger files.
4.4. Cloud Data Management Interface
The Cloud Data Management Interface (CDMI) is a specification to
access and manage cloud storage. CDMI is specified by the Storage
Networking Industry Association (SNIA).
CDMI is a functional interface that applications can use to create,
retrieve, update, and delete data elements from the cloud. As part
of this interface, the client will be able to discover the
capabilities of the cloud storage offering and use this interface to
manage containers and the data that is placed in them. In addition,
metadata can be set on containers and their contained data elements
through this interface [13].
CDMI follows a traditional client-server model, and operates over an
HTTP interface using the Representational State Transfer (REST)
model. Similar to Amazon S3 buckets (see Section 4.1), users may
create containers in which data objects may be stored. Even though
data objects may be accessed via a user-defined name within a
container, it is also possible to access data objects via a storage-
defined Object ID, which is provided in the response upon creation of
a data object.
Alimi, et al. Informational [Page 12]
RFC 6392 DECADE Survey October 2011
4.4.1. Applicability to DECADE
CDMI is an important initiative to standardize storage interfaces for
cloud services, which are rapidly becoming an important type of
storage service. In particular, it specifies a set of operations for
creating, reading, writing, and managing data objects at a remote
server (or set of servers) via HTTP.
4.4.2. Data Access Interface
Users can read and write data objects, and also update data in
existing data objects. CDMI data objects are sent on the wire
embedded as a field in a JavaScript Object Notation (JSON) object.
The protocol also defines interfaces in which the contents of data
objects can be written via simple HTTP GET/PUT operations.
4.4.3. Data Management Operations
Users can delete already-existing data objects. The create operation
also supports modes in which the created object is copied or moved
from an existing data object.
Data system metadata also allows users to configure policies
regarding time-to-live, after which a data object is automatically
deleted, as well as the redundancy with which a data object is
stored.
4.4.4. Data Search Capability
Users may list the contents of containers to locate data objects
matching any desired criteria.
4.4.5. Access Control Authorization
All methods of access control for clients are supported: public-
unrestricted, public-restricted, and private.
In particular, CDMI allows access to data objects to be protected by
Access Control Lists (ACLs) that can allow or restrict access based
on user name, group, administrative status, or whether a user is
authenticated or anonymous.
4.4.6. Resource Control Interface
CDMI supports attributes 'cdmi_max_latency' and 'cdmi_max_throughput'
(set at either the level of containers, or a specific data object),
which control the level of service offered to any users accessing a
particular data object.
Alimi, et al. Informational [Page 13]
RFC 6392 DECADE Survey October 2011
4.4.7. Discovery Mechanism
Users are provided a well-known DNS name. The DNS name is resolved
to determine the IP address to which requests may be sent.
4.4.8. Storage Mode
Object-based, with the extension that objects can be organized into
user-defined containers.
4.5. Content Delivery Network
A CDN provides services that improve performance by minimizing the
amount of data transmitted through the network, improving
accessibility, and maintaining correctness through content
replication. CDNs offer fast and reliable applications and services
by distributing content to cache or edge servers located close to
users. See [14] for an additional taxonomy and survey.
A CDN has some combination of content delivery, request routing,
distribution, and accounting infrastructures. The content-delivery
infrastructure consists of a set of edge servers (also called
surrogates) that deliver copies of content to end users. The
request-routing infrastructure is responsible for directing client
requests to appropriate edge servers. It also interacts with the
distribution infrastructure to keep an up-to-date view of the content
stored in the CDN caches. The distribution infrastructure moves
content from the origin server to the CDN edge servers and ensures
consistency of content in the caches. The accounting infrastructure
maintains logs of client accesses and records the usage of the CDN
servers. This information is used for traffic reporting and usage-
based billing.
In practice, a CDN typically hosts static content including images,
video, media clips, advertisements, and other embedded objects for
Web viewing. A focus for CDNs is the ability to publish and deliver
content to end users in a reliable and timely manner. A CDN focuses
on building its network infrastructure to provide the following
services and functionalities: storage and management of content;
distribution of content among surrogates; cache management; delivery
of static, dynamic, and streaming content; backup and disaster
recovery solutions; and monitoring, performance measurement, and
reporting.
Examples of existing CDNs are Akamai, Limelight, and CloudFront.
Alimi, et al. Informational [Page 14]
RFC 6392 DECADE Survey October 2011
The following description uses the term "content provider" to refer
to the entity purchasing a CDN service, and the term "client" to
refer to the subscriber requesting content via the CDN from the
content provider.
4.5.1. Applicability to DECADE
CDNs are a very widely used (deployed) example of in-network storage
for multimedia content. The existence and operation of the storage
system are totally transparent to the end user. CDNs typically
require a strong business relationship between the content providers
and content distributors, and often the business relationship extends
to the ISPs.
4.5.2. Data Access Interface
A CDN is typically a closed system, and generally provides only a
read (retrieve) access interface to clients. A CDN typically does
not provide a write (store) access interface to clients. The content
provider can access network edge servers and store content on them,
or edge servers can retrieve content from content providers. Client
nodes can only retrieve content from edge servers.
4.5.3. Data Management Operations
A content provider can manage the data distributed in different cache
nodes, such as moving popular data objects from one cache node to
another cache node, or deleting rarely accessed data objects in cache
nodes. User nodes, however, have no right to perform these
operations.
4.5.4. Data Search Capability
A content provider can search or enumerate the data each cache node
stores. User nodes cannot perform search operations.
4.5.5. Access Control Authorization
All methods of access control (for reading) are supported for
clients: public-unrestricted, public-restricted, and private. Some
CDN edge servers allow usage of HTTP basic authentication with the
origin server or restrictions by IP address, or they can use a token-
based technique to allow the origin server to apply its own
authorization criteria.
As mentioned previously, clients typically cannot write to the CDN.
Writing is typically a private operation for the content providers.
Alimi, et al. Informational [Page 15]
RFC 6392 DECADE Survey October 2011
4.5.6. Resource Control Interface
Not provided.
4.5.7. Discovery Mechanism
Content providers can directly find internal CDN cache nodes to store
content, since they typically have an explicit business relationship.
Clients can locate CDN nodes through DNS or other redirection
mechanisms.
4.5.8. Storage Mode
Though the addressing of objects uses URLs that typically refer to
objects in a hierarchical fashion, the storage mode is typically
object-based.
4.6. Delay-Tolerant Network
The Delay-Tolerant Network (DTN) [15] is an evolution of an
architecture originally designed for the Interplanetary Internet.
The Interplanetary Internet is a communication system envisioned to
provide Internet-like services across interplanetary distances in
support of deep space exploration. The DTN architecture can be
utilized in various operational environments characterized by severe
communication disruptions, disconnections, and high delays (e.g., a
month-long loss of connectivity between two planetary networks
because of high solar radiation due to sun spots). The DTN
architecture is thus suitable for environments including deep space
networks, sensor-based networks, certain satellite networks, and
underwater acoustic networks.
A key aspect of the DTN is a store-and-forward overlay layer called
the "Bundle Protocol" or "Bundle Layer", which exists between the
transport and application layers [16]. The Bundle Layer forms a
logical overlay that employs persistent storage to help combat long-
term network interruptions by providing a store-and-forward service.
While traditional IP networks are also based on store-and-forward
principles, the amount of time of a packet being kept in "storage" at
a traditional IP router is typically on the order of milliseconds (or
less). In contrast, the DTN architecture assumes that most Bundle
Layer nodes will use some form of persistent storage (e.g., hard
disk, flash memory, etc.) for DTN packets because of the nature of
the DTN environment.
Alimi, et al. Informational [Page 16]
RFC 6392 DECADE Survey October 2011
4.6.1. Applicability to DECADE
The DTN is an example of an experimental in-network storage system
that would require fundamental changes to the Internet protocols.
4.6.2. Data Access Interface
Users implicitly cause content to be stored (until successfully
forwarded) at Bundle Layer nodes by initiating/terminating any
transaction that traverses the DTN.
4.6.3. Data Management Operations
Users can implicitly cause deletion of content stored at Bundle Layer
nodes via a "time-to-live" type of parameter that the user can
control (for transactions originating from the user).
4.6.4. Data Search Capability
Not provided.
4.6.5. Access Control Authorization
The access control method is public-restricted (to any client that is
part of the DTN) or private.
4.6.6. Resource Control Interface
Not provided.
4.6.7. Discovery Mechanism
A Uniform Resource Identifier (URI) approach is used as the basis of
the addressing scheme for DTN transactions (and subsequent store-and-
forward routing through the DTN network).
4.6.8. Storage Mode
Object-based. DTN applications send data to the Bundle Layer, which
then breaks the data into segments. These segments are then routed
through the DTN network, and stored in Bundle Layer nodes as required
(before being forwarded).
Alimi, et al. Informational [Page 17]
RFC 6392 DECADE Survey October 2011
4.7. Named Data Networking
Named Data Networking (NDN) [17] is a research initiative that
proposes to move to a new model of addressing and routing for the
Internet. NDN uses "named data"-based routing and forwarding, to
replace the current IP-address-based model. NDN also uses name-based
data caching in the routers.
Each NDN Data packet will be assigned a content name and will be
cryptographically signed. Data delivery is driven by the requesting
end. Routers disseminate name-based prefix announcements by using
routing protocols such as Intermediate System to Intermediate System
(IS-IS) or the Border Gateway Protocol (BGP). The requester will
send out an "Interest" packet, which identifies the name of the data
that it wants. Routers that receive this Interest packet will
remember the interface it came from and will then forward it on a
name-based routing protocol. Once an Interest packet reaches a node
that has the desired data, a named Data packet is sent back, which
carries both the name and content of the data, along with a digital
signature of the producer. This named Data packet is then forwarded
back to the original requester on the reverse path of the Interest
packet [18].
A key aspect of NDN is that routers have the capability to cache the
named data. If a request for the same data (i.e., same name) comes
to the router, then the NDN router will forward the named data stored
locally to fulfill the request. The proponents of NDN believe that
the network can be designed naturally, matching data delivery
characteristics instead of communication between endpoints, because
data delivery has become the primary use of the network.
4.7.1. Applicability to DECADE
NDN is an example of an experimental in-network storage system that
would require storage space on a large number of routers in the
Internet. Named Data packets would be kept in storage in the NDN
routers and provided to new requesters of the same data.
4.7.2. Data Access Interface
Users implicitly store content at NDN routers by requesting content
(the named Data packets) from the network. Subsequent requests by
different users for the same content will cause the named Data
packets to be read from the NDN routers' in-network storage.
Alimi, et al. Informational [Page 18]
RFC 6392 DECADE Survey October 2011
4.7.3. Data Management Operations
Users do not have the direct ability to delete content stored in the
NDN routers. However, there will be some type of time-to-live
parameter associated with the named Data packets, though this has not
yet been specified.
4.7.4. Data Search Capability
Not provided.
4.7.5. Access Control Authorization
All methods of access control for clients are supported: public-
unrestricted, public-restricted, and private.
The basic security mechanism in NDN is for the sender to digitally
sign the content (the named Data packets) that it sends. It is
envisioned that a complete access control system can be built on top
of NDN, though this has not yet been specified.
4.7.6. Resource Control Interface
Not provided.
4.7.7. Discovery Mechanism
Names are used as the basis of the addressing and discovery scheme
for NDN (and subsequent store-and-forward routing through the NDN
network). NDN names are assumed to be hierarchical and to be able to
be deterministically constructed. This is still an active area of
research.
4.7.8. Storage Mode
Object-based. NDN sends named Data packets through the network.
These Data packets are routed through the NDN network and stored in
NDN routers.
4.8. Network of Information
Similar to NDN (see Section 4.7), Network of Information (NetInf)
[19] is another information-centric approach in which the named data
objects are the basic component of the networking architecture.
NetInf is thus moving away from today's host-centric networking
Alimi, et al. Informational [Page 19]
RFC 6392 DECADE Survey October 2011
architecture where the nodes in the network are the primary objects.
In today's network, the information objects are named relative to the
hosts they are stored on (e.g.,
http://www.example.com/information-object.txt).
The NetInf naming and security framework builds the foundation for an
information-centric security model that integrates security deeply
into the architecture. In this model, trust is based on the
information itself. Information objects (IOs) are given a unique
name with cryptographic properties. Together with additional
metadata, the name can be used to verify the data integrity as well
as several other security properties, such as self-certification,
name persistency, and owner authentication and identification. The
approach also gives some benefits over the security model in today's
host-centric networks, as it minimizes the need for trust in the
infrastructure, including the hosts providing the data, the channel,
or the resolution service.
In NetInf, the information objects are published into the network.
They are registered with a Name Resolution Service (NRS). The NRS is
also used to register network locators that can be used to retrieve
data objects that represent the published IOs. When a receiver wants
to retrieve an IO, the request for the IO is resolved by the NRS into
a set of locators. These locators are then used to retrieve a copy
of the data object from the "best" available source(s). NetInf is
open to use any type of underlying transport network. The locators
can thus be a heterogeneous set, e.g., IPv4, IPv6, Medium Access
Control (MAC), etc.
NetInf will make extensive use of caching of information objects in
the network and will provide network functionality that is similar to
what overlay solutions such as CDNs and P2P distribution networks
(e.g., BitTorrent) provide today.
4.8.1. Applicability to DECADE
NetInf is an example of an experimental information-centric network
architecture that will require storage space for storage and caching
of information objects on a large number of NetInf nodes in the
Internet.
Alimi, et al. Informational [Page 20]
RFC 6392 DECADE Survey October 2011
4.8.2. Data Access Interface
Users will publish IOs with specific IDs into the network. This is
done by the client sending a register message to the NRS stating that
the IO with the specific ID is available. When another user wishes
to retrieve the IO, they will use the given ID to make a request for
the IO. The ID is then resolved by the NRS, and the IO is delivered
from a nearby in-network storage location.
4.8.3. Data Management Operations
Users do not have the direct ability to delete content stored in the
NetInf nodes. However, there can be some type of time-to-live
parameter associated with the information objects, though this has
not yet been specified.
4.8.4. Data Search Capability
Not provided.
4.8.5. Access Control Authorization
All methods of access control for clients are supported: public-
unrestricted, public-restricted, and private. The basic security
mechanism in NetInf is for the publisher to digitally sign the
content of the information object that it publishes. It is
envisioned that a complete access control system can be built on top
of NetInf, though this has not yet been specified.
4.8.6. Resource Control Interface
Not provided.
4.8.7. Discovery Mechanism
NetInf IDs are used for naming and accessing information objects.
The IDs are resolved by the NRS into locators that are used for
routing and transport of data through the transport networks. This
is still an active area of research.
4.8.8. Storage Mode
Object-based. From an application perspective, NetInf can be used
for publishing entire files or chunks of files. NetInf is agnostic
to the application perspective and treats everything as information
objects.
Alimi, et al. Informational [Page 21]
RFC 6392 DECADE Survey October 2011
4.9. Network Traffic Redundancy Elimination
Redundancy Elimination (RE) is used for identifying and removing
repeated content from network transfers. This technique has been
proposed to improve network performance in many types of networks,
such as ISP backbones and enterprise access links. One example of an
RE proposal is SmartRE [20], proposed by Anand et al., which focuses
on network-wide RE. In packet-level RE, forwarding elements are
equipped with additional storage that can be used to cache data from
forwarded packets. Upstream routers may replace packet data with a
fingerprint that tells a downstream router how to decode and
reconstruct the packet based on cached data.
4.9.1. Applicability to DECADE
RE is an example of an experimental in-network storage system that
would require a large amount of associated packet processing at
routers if it was ever deployed.
4.9.2. Data Access Interface
RE is typically transparent to the user. Writing into storage is
done by transferring data that has not already been cached. Storage
is read when users transmit data identical to previously transmitted
data.
4.9.3. Data Management Operations
Not provided.
4.9.4. Data Search Capability
Not provided.
4.9.5. Access Control Authorization
The access control method is public-restricted (to any client that is
part of the RE network). Note that the content provider still
retains control over which peers receive the requested data. The
returned data is "compressed" as it is transferred within the
network.
4.9.6. Resource Control Interface
Not provided. The content provider still retains control over the
rate at which packets are sent to a peer. The packet size within the
network may be reduced.
Alimi, et al. Informational [Page 22]
RFC 6392 DECADE Survey October 2011
4.9.7. Discovery Mechanism
No discovery mechanism is necessary. Routers can use RE without the
users' knowledge.
4.9.8. Storage Mode
Object-based, with "objects" being data from packets transmitted
within the network.
4.10. OceanStore
OceanStore [21] is a storage platform developed at the University of
California, Berkeley, that provides globally distributed storage.
OceanStore implements a model where multiple storage providers can
pool resources together. Thus, a major focus is on resiliency, self-
organization, and self-maintenance.
The protocol is resilient to some storage nodes being compromised by
utilizing Byzantine agreement and erasure codes to store data at
primary replicas.
4.10.1. Applicability to DECADE
OceanStore is an example of an experimental in-network storage system
that provides a high degree of network resilience to failure
scenarios.
4.10.2. Data Access Interface
Users may read and write objects.
4.10.3. Data Management Operations
Objects may be replaced by newer versions, and multiple versions of
an object may be maintained.
4.10.4. Data Search Capability
Not provided.
4.10.5. Access Control Authorization
Provided, but specifics for clients are unclear from the available
references.
Alimi, et al. Informational [Page 23]
RFC 6392 DECADE Survey October 2011
4.10.6. Resource Control Interface
Not provided.
4.10.7. Discovery Mechanism
Users require an entry point into the system in the form of one
storage node that is part of OceanStore. If a hostname is provided,
the address of a storage node may be determined via DNS.
4.10.8. Storage Mode
Object-based.
4.11. P2P Cache
Caching of P2P traffic is a useful approach to reduce P2P network
traffic, because objects in P2P systems are mostly immutable and the
traffic is highly repetitive. In addition, making use of P2P caches
does not require changes to P2P protocols and can be deployed
transparently from clients.
P2P caches operate similarly to Web caches (Section 4.14) in that
they temporarily store frequently requested content. Requests for
content already stored in the cache can be served from local storage
instead of requiring the data to be transmitted over expensive
network links.
Two types of P2P caches exist: transparent P2P caches and
non-transparent P2P caches.
For a transparent cache, once a P2P cache is established, the network
will transparently redirect P2P traffic to the cache, which either
serves the file directly or passes the request on to a remote P2P
user and simultaneously caches that data. Transparency is typically
implemented using Deep Packet Inspection (DPI). DPI products
identify and pass P2P packets to the P2P caching system so it can
cache and accelerate the traffic.
A non-transparent cache appears as a super peer; it explicitly peers
with other P2P clients.
To enable operation with existing P2P software, P2P caches directly
support P2P application protocols. A large number of P2P protocols
are used by P2P software and hence are supported by caches, leading
to higher complexity. Additionally, these protocols evolve over
time, and new protocols are introduced.
Alimi, et al. Informational [Page 24]
RFC 6392 DECADE Survey October 2011
4.11.1. Applicability to DECADE
A P2P cache is an example of in-network storage for P2P systems.
However, unlike DECADE, the existence and operation of the storage
system are totally transparent to the end user.
4.11.2. Transparent P2P Caches
4.11.2.1. Data Access Interface
The data access interface allows P2P content to be cached (stored)
and supplied (retrieved) locally such that network traffic is
reduced, but it is transparent to P2P users, and P2P users implicitly
use the data access interface (in the form of their native P2P
application protocol) to store or retrieve content.
4.11.2.2. Data Management Operations
Not provided.
4.11.2.3. Data Search Capability
Not provided.
4.11.2.4. Access Control Authorization
The access control method is typically public-restricted (to any
client that is part of the P2P channel or swarm).
4.11.2.5. Resource Control Interface
Not provided.
4.11.2.6. Discovery Mechanism
The use of DPI means that no discovery mechanism is provided to P2P
users; it is transparent to P2P users. Since DPI is used to
recognize P2P applications' private protocols, P2P cache
implementations must be updated as new applications are added and
existing protocols evolve.
4.11.2.7. Storage Mode
Object-based. Chunks (typically, the unit of transfer among P2P
clients) of content are stored in the cache.
Alimi, et al. Informational [Page 25]
RFC 6392 DECADE Survey October 2011
4.11.3. Non-Transparent P2P Caches
4.11.3.1. Data Access Interface
The data access interface allows P2P content to be cached (stored)
and supplied (retrieved) locally such that network traffic is
reduced. P2P users implicitly store and retrieve from the cache
using the P2P application's native protocol.
4.11.3.2. Data Management Operations
Not provided.
4.11.3.3. Data Search Capability
Not provided.
4.11.3.4. Access Control Authorization
The access control method is typically public-restricted (to any
client that is part of the P2P channel or swarm).
4.11.3.5. Resource Control Interface
Not provided.
4.11.3.6. Discovery Mechanism
A P2P cache node behaves as if it were a normal peer in order to join
the P2P overlay network. Other P2P users can find such a cache node
through an overlay routing mechanism and can interact with it as if
it were a normal neighbor node.
4.11.3.7. Storage Mode
Object-based. Chunks (typically, the unit of transfer among P2P
clients) of content are stored in the cache.
4.12. Photo Sharing
There are a growing number of popular online photo-sharing (storing)
systems. For example, the Kodak Gallery system [22] serves over
60 million users and stores billions of images [23]. Other well-
known examples of photo-sharing systems include Flickr [24] and
ImageShack [25]. There are also a number of popular blogging
Alimi, et al. Informational [Page 26]
RFC 6392 DECADE Survey October 2011
services, such as Tumblr [26], that specialize in sharing large
numbers of photos as well as other multimedia content (e.g., video,
text, audio, etc.) as part of their service. All of these in-network
storage systems utilize both free and paid subscription models.
Most photo-sharing systems are based on a traditional client-server
architecture. However, a minority of systems also offer a P2P mode
of operation. The client-server architecture is typically based on
HTTP, with a browser client and a Web server.
4.12.1. Applicability to DECADE
Photo sharing is a very widely used (deployed) example of in-network
storage where the end user has direct visibility and extensive
control of the system. The typical end-user interface is through an
HTTP-based Web browser.
4.12.2. Data Access Interface
Users can read (view) and write (store) photos.
4.12.3. Data Management Operations
Users can delete previously stored photos.
4.12.4. Data Search Capability
Users can tag photos and/or organize them using sophisticated Web
photo album generators. Users can then search for objects (photos)
matching desired criteria.
4.12.5. Access Control Authorization
The access control method for clients is typically either private or
public-unrestricted. For example, writing (storing) to a photo blog
is typically private to the owner of the account. However, all other
clients can view (read) the contents of the blog (i.e., public-
unrestricted). Some photo-sharing Websites provide private access to
read photos to allow sharing with a limited set of friends.
4.12.6. Resource Control Interface
Not provided.
Alimi, et al. Informational [Page 27]
RFC 6392 DECADE Survey October 2011
4.12.7. Discovery Mechanism
Clients usually log on manually to a central Web page for the service
and enter the appropriate information to access the desired
information. The address to which the client connects is usually
determined by DNS using the hostname from the provided URL.
4.12.8. Storage Mode
File system (file-based). Photos are usually stored as files. They
can then be organized into meta-structures (e.g., albums, galleries,
etc.) using sophisticated Web photo album generators.
4.13. Usenet
Usenet is a distributed Internet-based discussion (message) system.
The Usenet messages are arranged as a set of "newsgroups" that are
classified hierarchically by subject. Usenet information is
distributed and stored among a large conglomeration of servers that
store and forward messages to one another in so-called news feeds.
Individual users may read messages from, and post messages to, a
local news server typically operated by an ISP. This local server
communicates with other servers and exchanges articles with them. In
this fashion, the message is copied from server to server and
eventually reaches every server in the network [27].
Traditional Usenet as described above operates as a P2P network
between the servers, and in a client-server architecture between the
user and their local news server. The user requires a Usenet client
to be installed on their computer and a Usenet server account
(through their ISP). However, with the rise of Web browsers, the
Usenet architecture is evolving to be Web-based. The most popular
example of this is Google Groups, where Google hosts all the
newsgroups and client access is via a standard HTTP-based Web
browser [28].
4.13.1. Applicability to DECADE
Usenet is a historically very important and widely used (deployed)
example of in-network storage in the Internet. The use of this
system is rapidly declining, but efforts have been made to preserve
the stored content for historical purposes.
4.13.2. Data Access Interface
Users can read and post (store) messages.
Alimi, et al. Informational [Page 28]
RFC 6392 DECADE Survey October 2011
4.13.3. Data Management Operations
Users sometimes have limited ability to delete messages that they
previously posted.
4.13.4. Data Search Capability
Traditionally, users could manually search through the newsgroups, as
they are classified hierarchically by subject. In the newer Web-
based systems, there is also an automatic search capability based on
key-word matches.
4.13.5. Access Control Authorization
The access control method is either public-unrestricted or private
(to client members of that newsgroup).
4.13.6. Resource Control Interface
Not provided.
4.13.7. Discovery Mechanism
Clients usually log on manually to their Usenet accounts. DNS may be
used to resolve hostnames to their corresponding addresses.
4.13.8. Storage Mode
File system. Messages are usually stored as files that are then
organized hierarchically by subject into newsgroups.
4.14. Web Cache
Web cache [29] has been widely deployed by many ISPs to reduce
bandwidth consumption and Web access latency since the late 1990s. A
Web cache can cache the Web documents (e.g., HTML pages, images)
between server and client to reduce bandwidth usage, server load, and
perceived lag. A Web cache server is typically shared by many
clients, and stores copies of documents passing through it;
subsequent requests may be satisfied from the cache if certain
conditions are met.
Another form of cache is a client-side cache, typically implemented
in Web browsers. A client-side cache can keep a local copy of all
pages recently displayed by a browser, and when the user returns to
one of these Web pages, the local cached copy is reused.
Alimi, et al. Informational [Page 29]
RFC 6392 DECADE Survey October 2011
A related protocol for P2P applications to use Web cache is HPTP
(HTTP-based Peer to Peer) [30]. It proposes sharing chunks of P2P
files/streams using HTTP with cache-control headers.
4.14.1. Applicability to DECADE
Web cache is a very widely used (deployed) example of in-network
storage for the key Internet application of Web browsing. The
existence and operation of the storage system are transparent to the
end user in most cases. The content caching time is controlled by
time-to-live parameters associated with the original content. The
principle of Web caching is to speed up Web page reading by using
(the same) content previously requested by another user to service a
new user.
4.14.2. Data Access Interface
Users explicitly read from a Web cache by making requests, but they
cannot explicitly write data into it. Data is implicitly stored in
the Web cache by requesting content that is not already cached and
meets policy restrictions of the cache provider.
4.14.3. Data Management Operations
Not provided.
4.14.4. Data Search Capability
Not provided.
4.14.5. Access Control Authorization
The access control method for clients is public-unrestricted. It is
important to note that if content is authenticated or encrypted
(e.g., HTTPS, Secure Socket Layer (SSL)), it will not be cached.
Also, if the content is flagged as private (vs. public) at the HTTP
level by the origin server, it will not be cached.
4.14.6. Resource Control Interface
Not provided.
4.14.7. Discovery Mechanism
Web caches can be transparently deployed between a Web server and Web
clients, employing DPI for discovery. Alternatively, Web caches
could be explicitly discovered by clients using techniques such as
DNS or manual configuration.
Alimi, et al. Informational [Page 30]
RFC 6392 DECADE Survey October 2011
4.14.8. Storage Mode
Object-based. Web content is keyed within the cache by HTTP Request
fields, such as Method, URI, and Headers.
4.15. Observations Regarding In-Network Storage Systems
The following observations about the surveyed in-network storage
systems are made in the context of DECADE as defined by [1].
The majority of the surveyed systems were designed for client-server
architectures and do not support P2P. However, there are some
important exceptions, especially for some of the newer technologies
such as BranchCache and P2P cache, that do support a P2P mode of
operation.
The P2P cache systems are interesting, since they do not require
changes to the P2P applications themselves. However, this is also a
limitation in that they are required to support each application
protocol.
Many of the surveyed systems were designed for caching as opposed to
long-term network storage. Thus, during DECADE protocol design, it
should be carefully considered whether a caching mode should be
supported in addition to a long-term network storage mode. There is
typically a trade-off between providing a caching mode and long-term
(and usually also reliable) storage with regards to some performance
metrics. Note that [1] identifies issues with classical caching from
a DECADE perspective, such as the fact that P2P caches typically do
not allow users to explicitly control content stored in the cache.
Certain components of the surveyed systems are outside of the scope
of DECADE. For example, a protocol used for searching across
multiple DECADE servers is out of scope. However, applications may
still be able to implement such functionality if DECADE exposes the
appropriate primitives. This has the benefit of keeping the core
in-network storage systems simple, while permitting diverse
applications to design mechanisms that meet their own requirements.
Today, most in-network storage systems follow some variant of the
authorization model of public-unrestricted, public-restricted, and
private. For DECADE, we may need to evolve the authorization model
to support a resource owner (e.g., end user) authorization, in
addition to the network authorization.
Alimi, et al. Informational [Page 31]
RFC 6392 DECADE Survey October 2011
5. Storage and Other Related Protocols
This section surveys existing storage and other related protocols, as
well as comments on the usage of these protocols to satisfy DECADE's
use cases. The surveyed protocols are listed alphabetically.
5.1. HTTP
HTTP [31] is a key protocol for the World Wide Web. It is a
stateless client-server protocol that allows applications to be
designed using the REST model. HTTP is often associated with
downloading (reading) content from Web servers to Web browsers, but
it also has support for uploading (writing) content to Web servers.
It has been used as the underlying protocol for other protocols, such
as Web Distributed Authoring and Versioning (WebDAV).
HTTP is used in some of the most popular in-network storage systems
surveyed previously, including CDNs, photo sharing, and Web cache.
Usage of HTTP by a storage protocol implies that no extra software is
required in the client (i.e., Web-based client), as all standard Web
browsers are based on HTTP.
5.1.1. Data Access Interface
Basic read and write operations are supported (using HTTP GET, PUT,
and POST methods).
5.1.2. Data Management Operations
Not provided.
5.1.3. Data Search Capability
Not provided.
5.1.4. Access Control Authorization
All methods of access control for clients are supported: public-
unrestricted, public-restricted, and private.
The majority of Web pages are public-unrestricted in terms of reading
but do not allow any uploading of content. In-network storage
systems range from private or public-unrestricted for photo sharing
(described in Section 4.12.5) to public-unrestricted for Web caching
(described in Section 4.14.5).
Alimi, et al. Informational [Page 32]
RFC 6392 DECADE Survey October 2011
5.1.5. Resource Control Interface
Not provided.
5.1.6. Discovery Mechanism
Manual configuration is typically used. Clients typically address
HTTP servers by providing a hostname, which is resolved to an address
using DNS.
5.1.7. Storage Mode
HTTP is a protocol; it thus does not define a storage mode. However,
a non-collection resource can typically be thought of as a "file".
These files may be organized into collections, which typically map
onto the HTTP path hierarchy, creating the illusion of a file system.
5.1.8. Comments
HTTP is based on a client-server architecture and thus is not
directly applicable for the DECADE focus on P2P. Also, HTTP offers
only a rudimentary toolset for storage operations compared to some of
the other storage protocols.
5.2. iSCSI
Small Computer System Interface (SCSI) is a set of protocols enabling
communication with storage devices such as disk drives and tapes;
Internet SCSI (iSCSI) [32] is a protocol enabling SCSI commands to be
sent over TCP. As in SCSI, iSCSI allows an Initiator to send
commands to a Target. These commands operate on the device level as
opposed to individual data objects stored on the device.
5.2.1. Data Access Interface
Read and write commands indicate which data is to be read or written
by specifying the offset (using Logical Block Addressing) into the
storage device. The size of data to be read or written is an
additional parameter in the command.
5.2.2. Data Management Operations
Since commands operate at the device level, management operations are
different than with traditional file systems. Management commands
for SCSI/iSCSI include explicit device control commands, such as
starting, stopping, and formatting the device.
Alimi, et al. Informational [Page 33]
RFC 6392 DECADE Survey October 2011
5.2.3. Data Search Capability
SCSI/iSCSI does not provide the ability to search for particular data
within a device. Note that such capabilities can be implemented
outside of iSCSI.
5.2.4. Access Control Authorization
With respect to access to devices, the access control method is
private. iSCSI uses the Challenge Handshake Authentication Protocol
(CHAP) [33] to authenticate Initiators and Targets when accessing
storage devices. However, since SCSI/iSCSI operates at the device
level, neither authentication nor authorization is provided for
individual data objects. Note that such capabilities can be
implemented outside of iSCSI.
5.2.5. Resource Control Interface
Not provided.
5.2.6. Discovery Mechanism
Manual configuration may be used. An alternative is the Internet
Storage Name Service (iSNS) [34], which provides the ability to
discover available storage resources.
5.2.7. Storage Mode
As a protocol, iSCSI does not explicitly have a storage mode.
However, it provides block-based access to clients. SCSI/iSCSI
provides an Initiator with block-level access to the storage device.
5.3. NFS
The Network File System (NFS) is designed to allow users to access
files over a network in a manner similar to how local storage is
accessed. NFS is typically used in local area networks or in
enterprise settings, though changes made in later versions of NFS
(e.g., [35]) make it easier to operate over the Internet.
5.3.1. Data Access Interface
Traditional file-system operations such as read, write, and update
(overwrite) are provided. Locking is provided to support concurrent
access by multiple clients.
Alimi, et al. Informational [Page 34]
RFC 6392 DECADE Survey October 2011
5.3.2. Data Management Operations
Traditional file-system operations such as move and delete are
provided.
5.3.3. Data Search Capability
The user has the ability to list contents of directories to find
filenames matching desired criteria.
5.3.4. Access Control Authorization
All methods of access control for clients are supported: public-
unrestricted, public-restricted, and private. For example, files and
directories can be protected using read, write, and execute
permissions for the files' owner and group, and for the public
(others). Also, NFSv4.1 has a rich ACL model allowing a list of
Access Control Entries (ACEs) to be configured for each file or
directory. The ACEs can specify per-user read/write access to file
data, file/directory attributes, creation/deletion of files in a
directory, etc.
5.3.5. Resource Control Interface
While disk space quotas can be configured, administrative policy
typically limits the total amount of storage allocated to a
particular user. User control of bandwidth and connections used by
remote peers is not provided.
5.3.6. Discovery Mechanism
Manual configuration is typically used. Clients address NFS servers
by providing a hostname and a directory that should be mounted. DNS
may be used to look up an address for the provided hostname.
5.3.7. Storage Mode
As a protocol, there is no defined internal storage mode. However,
implementations typically use the underlying file-system storage.
Note that extensions have been defined for alternate storage modes
(e.g., block-based [36] and object-based [37]).
Alimi, et al. Informational [Page 35]
RFC 6392 DECADE Survey October 2011
5.3.8. Comments
The efficiency and scalability of the NFS access control method are
concerns in the context of DECADE. In particular, Section 6.2.1 of
[35] states that:
Only ACEs that have a "who" that matches the requester
are considered.
Thus, in the context of DECADE, to specify per-peer access control
policies for an object, a client would need to explicitly configure
the ACL for the object for each individual peer. A concern with this
approach is scalability when a client's peers may change frequently,
and ACLs for many small objects need to be updated constantly during
participation in a swarm.
Note that NFSv4.1's usage of RPCSEC_GSS provides support for multiple
security mechanisms. Kerberos V5 is required, but others, such as
X.509 certificates, are also supported by way of the Generic Security
Service Application Program Interface (GSS-API). Note, however, that
NFSv4.1's usage of such security mechanisms is limited to linking a
requesting user to a particular account maintained by the NFS server.
5.4. OAuth
Open Authorization (OAuth) [38] is a protocol that enriches the
traditional client-server authentication model for Web resources. In
particular, OAuth distinguishes the "client" from the "resource
owner", thus enabling a resource owner to authorize a particular
client for access (e.g., for a particular lifetime) to private
resources.
We include OAuth in this survey so that its authentication model can
be evaluated in the context of DECADE. OAuth itself, however, is not
a network storage protocol.
5.4.1. Data Access Interface
Not provided.
5.4.2. Data Management Operations
Not provided.
5.4.3. Data Search Capability
Not provided.
Alimi, et al. Informational [Page 36]
RFC 6392 DECADE Survey October 2011
5.4.4. Access Control Authorization
Not provided. While similar in spirit to the WebDAV ticketing
extensions [39], OAuth instead uses the following process: (1) a
client constructs a delegation request, (2) the client forwards the
request to the resource owner for authorization, (3) the resource
owner authorizes the request, and finally (4) a callback is made to
the client indicating that its request has been authorized.
Once the process is complete, the client has a set of token
credentials that grant it access to the protected resource. The
token credentials may have an expiration time, and they can also be
revoked by the resource owner at any time.
5.4.5. Resource Control Interface
Not provided.
5.4.6. Discovery Mechanism
Not provided.
5.4.7. Storage Mode
Not provided.
5.4.8. Comments
The ticketing mechanism requires server involvement, and the
discussion relating to WebDAV's proposed ticketing mechanism (see
Section 5.5.8) applies here as well.
5.5. WebDAV
WebDAV [40] is a protocol designed for Web content authoring. It is
developed as an extension to HTTP (described in Section 5.1), meaning
that it can be simpler to integrate into existing software. WebDAV
supports traditional operations for reading/writing from storage, as
well as other constructs, such as locking and collections, that are
important when multiple users collaborate to author or edit a set of
documents.
5.5.1. Data Access Interface
Traditional read and write operations are supported (using HTTP GET
and PUT methods, respectively). Locking is provided to support
concurrent access by multiple clients.
Alimi, et al. Informational [Page 37]
RFC 6392 DECADE Survey October 2011
5.5.2. Data Management Operations
WebDAV supports traditional file-system operations, such as move,
delete, and copy. Objects are organized into collections, and these
operations can also be performed on collections. WebDAV also allows
objects to have user-defined properties.
5.5.3. Data Search Capability
The user has the ability to list contents of collections to find
objects matching desired criteria. A SEARCH extension [41] has also
been specified allowing listing of objects matching client-defined
criteria.
5.5.4. Access Control Authorization
All methods of access control for clients are supported: public-
unrestricted, public-restricted, and private.
For example, an ACL extension [42] is provided for WebDAV. ACLs
allow both user-based and group-based access control policies
(relating to reading, writing, properties, locking, etc.) to be
defined for objects and collections.
A ticketing extension [39] has also been proposed, but has not
progressed since 2001. This extension allows a client to request the
WebDAV server to create a "ticket" (e.g., for reading an object) that
can be distributed to other clients. Tickets may be given expiration
times, or may only allow for a fixed number of uses. The proposed
extension requires the server to generate tickets and maintain state
for outstanding tickets.
5.5.5. Resource Control Interface
An extension [43] allows disk space quotas to be configured for
collections. The extension also allows WebDAV clients to query
current disk space usage. User control of bandwidth and connections
used by remote peers is not provided.
5.5.6. Discovery Mechanism
Manual configuration is typically used. Clients address WebDAV
servers by providing a hostname, which can be resolved to an address
using DNS.
Alimi, et al. Informational [Page 38]
RFC 6392 DECADE Survey October 2011
5.5.7. Storage Mode
Though no storage mode is explicitly defined, WebDAV can be thought
of as providing file system (file-based) storage to a client. A
non-collection resource can typically be thought of as a "file".
Files may be organized into collections, which typically map onto the
HTTP path hierarchy.
5.5.8. Comments
The efficiency and scalability of the WebDAV access control method
are concerns in the context of DECADE, for reasons similar to those
stated in Section 5.3.8 for NFS. The proposed WebDAV ticketing
extension partially alleviates these concerns, but the particular
technique may need further evaluation before being applied to DECADE.
In particular, since DECADE clients may continuously upload/download
a large number of small-size objects, and a single DECADE server may
need to scale to many concurrent DECADE clients, requiring the server
to maintain ticket state and generate tickets may not be the best
design choice. Server-generated tickets can also increase latency
for data transport operations, depending on the message flow used by
DECADE.
5.6. Observations Regarding Storage and Related Protocols
The following observations about the surveyed storage and related
protocols are made in the context of DECADE as defined by [1].
All of the surveyed protocols were primarily designed for client-
server architectures and not for P2P. However, it is conceivable
that some of the protocols could be adapted to work in a P2P
architecture.
Several popular in-network storage systems today use HTTP as their
key protocol, even though it is not classically considered as a
storage protocol. HTTP is a stateless protocol that is used to
design RESTful applications. HTTP is a well-supported and widely
implemented protocol that can provide important insights for DECADE.
The majority of the surveyed protocols do not support low-latency
access for applications such as live streaming. This was one of the
key general requirements for DECADE.
The majority of the surveyed protocols do not support any form of
resource control interface. Resource control is required for users
to manage the resources on in-network storage systems, e.g., the
bandwidth or connections, that can be used by other peers. Resource
control is a key capability required for DECADE.
Alimi, et al. Informational [Page 39]
RFC 6392 DECADE Survey October 2011
Nearly all surveyed protocols did, however, support the following
capabilities required for DECADE: ability of the user to read/write
content, some form of access control, some form of error indication,
and the ability to traverse firewalls and NATs.
6. Conclusions
Though there have been many successful in-network storage systems,
they have been designed for use cases different from those defined in
DECADE. For example, many of the surveyed in-network storage systems
and protocols were designed for client-server architectures and not
P2P. No surveyed system or protocol has the functionality and
features to fully meet the set of requirements defined for DECADE.
DECADE aims to provide a standard protocol for P2P applications and
content providers to access and control in-network storage, resulting
in increased network efficiency while retaining control over content
shared with peers. Additionally, defining a standard protocol can
reduce the complexity of in-network storage, since multiple P2P
application protocols no longer need to be implemented by in-network
storage systems.
7. Security Considerations
This document is a survey of existing in-network storage systems, and
does not introduce any security considerations beyond those of the
surveyed systems.
For more information on security considerations of DECADE, see [1].
8. Contributors
The editors would like to thank the following people for contributing
to the development of this document:
- ZhiHui Lv
- Borje Ohlman
- Pang Tao
- Lucy Yong
- Juan Carlos Zuniga
Alimi, et al. Informational [Page 40]
RFC 6392 DECADE Survey October 2011
9. Acknowledgments
The editors would like to thank the following people for providing
valuable comments to various draft versions of this document: David
Bryan, Tao Mao, Haibin Song, Ove Strandberg, Yu-Shun Wang, Richard
Woundy, Yunfei Zhang, and Ning Zong.
10. Informative References
[1] Song, H., Zong, N., Yang, Y., and R. Alimi, "DECoupled
Application Data Enroute (DECADE) Problem Statement", Work
in Progress, October 2011.
[2] Storage Search, "Flash Memory vs. Hard Disk Drives -- Which
Will Win?", .
[3] Brisken, W., "Hard Drive Price Trends", US VLBI Technical
Meeting, May 2008.
[4] Woundy, R., "TSV P2P Efforts -- From an ISP's Perspective",
IETF 81, Quebec, Canada, July 2011,
.
[5] Gu, Y., Bryan, D., Yang, Y., and R. Alimi, "DECADE
Requirements", Work in Progress, September 2011.
[6] Amazon Web Services, "Amazon Simple Storage Service
(Amazon S3)", .
[9] Dropbox, "Dropbox Features", .
[10] Microsoft Corporation, "BranchCache",
.
[11] Microsoft Corporation, "Web Services Dynamic Discovery
(WS-Discovery)", April 2005, .
Alimi, et al. Informational [Page 41]
RFC 6392 DECADE Survey October 2011
[12] Paul, S., Yates, R., Raychaudhuri, D., and J. Kurose, "The
Cache-and-Forward Network Architecture for Efficient Mobile
Content Delivery Services in the Future Internet", Innovations
in NGN: Future Network and Services, 2008.
[13] SNIA, "Cloud Data Management Interface (CDMI)",
.
[14] Pathan, A.K. and Buyya, R., "A Taxonomy and Survey of Content
Delivery Networks", Grid Computing and Distributed Systems
Laboratory, University of Melbourne, Technical Report,
February 2007.
[15] Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst, R.,
Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant Networking
Architecture", RFC 4838, April 2007.
[16] Scott, K. and S. Burleigh, "Bundle Protocol Specification",
RFC 5050, November 2007.
[17] Named Data Networking, "Named Data Networking Home Page",
.
[19] Network of Information, "NetInf Overview",
.
[23] Wikipedia, "Kodak Gallery",
.
[24] Flickr, "Flickr Home Page", .
[25] ImageShack, "ImageShack Home Page", .
[26] Tumblr, "Tumblr Home Page", .
Alimi, et al. Informational [Page 42]
RFC 6392 DECADE Survey October 2011
[27] Wikipedia, "Usenet", .
[28] Google, "Google Groups", .
[29] Huston, G., Telstra, "Web Caching", The Internet Protocol
Journal Volume 2, No. 3.
[30] Shen, G., Wang, Y., Xiong, Y., Zhao, B., and Z-L. Zhang, "HPTP:
Relieving the Tension between ISPs and P2P", 6th International
Workshop on Peer-To-Peer Systems (IPTPS2007).
[31] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., Masinter, L.,
Leach, P., and T. Berners-Lee, "Hypertext Transfer Protocol --
HTTP/1.1", RFC 2616, June 1999.
[32] Satran, J., Meth, K., Sapuntzakis, C., Chadalapaka, M., and E.
Zeidner, "Internet Small Computer Systems Interface (iSCSI)",
RFC 3720, April 2004.
[33] Simpson, W., "PPP Challenge Handshake Authentication Protocol
(CHAP)", RFC 1994, August 1996.
[34] Tseng, J., Gibbons, K., Travostino, F., Du Laney, C., and J.
Souza, "Internet Storage Name Service (iSNS)", RFC 4171,
September 2005.
[35] Shepler, S., Ed., Eisler, M., Ed., and D. Noveck, Ed., "Network
File System (NFS) Version 4 Minor Version 1 Protocol",
RFC 5661, January 2010.
[36] Black, D., Fridella, S., and J. Glasgow, "Parallel NFS (pNFS)
Block/Volume Layout", RFC 5663, January 2010.
[37] Halevy, B., Welch, B., and J. Zelenka, "Object-Based Parallel
NFS (pNFS) Operations", RFC 5664, January 2010.
[38] Hammer-Lahav, E., Ed., "The OAuth 1.0 Protocol", RFC 5849,
April 2010.
[39] Ito, K., "Ticket-Based Access Control Extension to WebDAV",
Work in Progress, October 2001.
[40] Dusseault, L., Ed., "HTTP Extensions for Web Distributed
Authoring and Versioning (WebDAV)", RFC 4918, June 2007.
[41] Reschke, J., Ed., Reddy, S., Davis, J., and A. Babich, "Web
Distributed Authoring and Versioning (WebDAV) SEARCH",
RFC 5323, November 2008.
Alimi, et al. Informational [Page 43]
RFC 6392 DECADE Survey October 2011
[42] Clemm, G., Reschke, J., Sedlar, E., and J. Whitehead, "Web
Distributed Authoring and Versioning (WebDAV)
Access Control Protocol", RFC 3744, May 2004.
[43] Korver, B. and L. Dusseault, "Quota and Size Properties
for Distributed Authoring and Versioning (DAV) Collections",
RFC 4331, February 2006.
Authors' Addresses
Richard Alimi (editor)
Google
EMail: ralimi@google.com
Akbar Rahman (editor)
InterDigital Communications, LLC
EMail: Akbar.Rahman@InterDigital.com
Yang Richard Yang (editor)
Yale University
EMail: yry@cs.yale.edu
Alimi, et al. Informational [Page 44]
RFC, FYI, BCP