DECADE R. Alimi
Internet-Draft Google
Intended status: Informational Y. Yang
Expires: April 28, 2011 Yale University
A. Rahman
InterDigital Communications, LLC
D. Kutscher
NEC
L. Chen
H. Liu
Yale University
October 25, 2010
DECADE Architecture
draft-alimi-decade-arch-01
Abstract
Peer-to-peer (P2P) applications have become widely used on the
Internet today and make up a large portion of the traffic in many
networks. One technique to improve the network efficiency of P2P
applications is to introduce storage capabilities within the network.
The DECADE Working Group has been formed with the goal of developing
an architecture to provide this capability. This document presents
an architecture, discusses the underlying principles and identifies
core components and protocols supporting the architecture.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 28, 2011.
Copyright Notice
Copyright (c) 2010 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Entities . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. DECADE Storage Servers . . . . . . . . . . . . . . . . . . 5
2.2. DECADE Storage Provider . . . . . . . . . . . . . . . . . 5
2.3. DECADE Content Providers . . . . . . . . . . . . . . . . . 5
2.4. DECADE Content Consumers . . . . . . . . . . . . . . . . . 5
2.5. Content Distribution Application . . . . . . . . . . . . . 5
2.6. Application End-Point . . . . . . . . . . . . . . . . . . 6
3. Architectural Principles . . . . . . . . . . . . . . . . . . . 6
3.1. Decoupled Control and Data Planes . . . . . . . . . . . . 6
3.2. Immutable Data Objects . . . . . . . . . . . . . . . . . . 7
3.3. Data Object Identifiers . . . . . . . . . . . . . . . . . 8
3.4. Explicit Control . . . . . . . . . . . . . . . . . . . . . 8
3.5. Resource and Data Access Control through User
Delegation . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5.1. Resource Allocation . . . . . . . . . . . . . . . . . 9
3.5.2. User Delegations . . . . . . . . . . . . . . . . . . . 9
4. System Components . . . . . . . . . . . . . . . . . . . . . . 10
4.1. Content Distribution Application . . . . . . . . . . . . . 12
4.1.1. Data Sequencing and Naming . . . . . . . . . . . . . . 12
4.1.2. Native Protocols . . . . . . . . . . . . . . . . . . . 12
4.1.3. DECADE Client . . . . . . . . . . . . . . . . . . . . 13
4.2. DECADE Server . . . . . . . . . . . . . . . . . . . . . . 13
4.2.1. Access Control . . . . . . . . . . . . . . . . . . . . 13
4.2.2. Resource Scheduling . . . . . . . . . . . . . . . . . 14
4.2.3. Data Store . . . . . . . . . . . . . . . . . . . . . . 14
4.3. Protocols . . . . . . . . . . . . . . . . . . . . . . . . 14
4.3.1. DECADE Resource Protocol . . . . . . . . . . . . . . . 14
4.3.2. Standard Data Transports . . . . . . . . . . . . . . . 15
4.4. DECADE Data Sequencing and Naming . . . . . . . . . . . . 15
4.5. In-Network Storage Components Mapped to DECADE
Architecture . . . . . . . . . . . . . . . . . . . . . . . 16
4.5.1. Data Access Interface . . . . . . . . . . . . . . . . 16
4.5.2. Data Management Operations . . . . . . . . . . . . . . 16
4.5.3. Data Search Capability . . . . . . . . . . . . . . . . 16
4.5.4. Access Control Authorization . . . . . . . . . . . . . 16
4.5.5. Resource Control Interface . . . . . . . . . . . . . . 16
4.5.6. Discovery Mechanism . . . . . . . . . . . . . . . . . 16
4.5.7. Storage Mode . . . . . . . . . . . . . . . . . . . . . 17
5. Security Considerations . . . . . . . . . . . . . . . . . . . 17
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7. Informative References . . . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
Peer-to-peer (P2P) applications have become widely used on the
Internet today to distribute contents, and they contribute a large
portion of the traffic in many networks. The DECADE Working Group
has been formed with the goal of developing an architecture to
introduce in-network storage to be used by such applications, to
achieve more efficient content distribution. Specifically, in many
subscriber networks, it is typically more expensive to upgrade
network equipment in the "last-mile", because it can involve
replacing equipment and upgrading wiring at individual homes,
businesses, and devices such as DSLAMs and CMTSs. Thus, it can be
cheaper to upgrade core infrastructure involving fewer components
that are shared by many subscribers. See
[I-D.ietf-decade-problem-statement] for a more complete discussion of
the problem domain and general discussion of the capabilities to be
provided by DECADE.
This document presents a potential architecture of providing in-
network storage that can be integrated into content distribution
applications. The primary focus is P2P-based content distribution,
but the architecture may be useful to other applications with similar
characteristics and requirements. In particular, content
distribution applications that may split data into smaller pieces for
distribution may be able to utilize DECADE.
The design philosophy of the DECADE architecture is to provide only
the core functionality that is needed for applications to make use of
in-network storage. With such core functionality, the protocol may
be simple and easier to support by storage providers. If more
complex functionality is needed by a certain application or class of
applications, it may be layered on top of the DECADE protocol.
The DECADE protocol will leverage existing transport and application
layer protocols and will be designed to work with a small set of
alternative IETF protocols.
This document proceeds in two steps. First, it details the core
architectural principles that can guide the DECADE design. Next,
given these core principles, this document presents the core
components of the DECADE architecture and identifies usage of
existing protocols and where there is a need for new protocol
development.
This document will be updated to track the progress of the DECADE
survey [I-D.ietf-decade-survey] and requirements [I-D.gu-decade-reqs]
drafts.
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2. Entities
2.1. DECADE Storage Servers
DECADE storage servers are operated by DECADE storage providers and
provide the DECADE functionality as specified in this memo, including
mechanisms to store, retrieve and manage data. A storage provider
may typically operate multiple storage servers.
2.2. DECADE Storage Provider
A DECADE in-storage provider deploys and/or manages DECADE servers
within a network. A storage provider may also own or manage the
network in which the DECADE servers are deployed.
A DECADE storage provider, possibly in cooperation with one or more
network providers, determines deployment locations for DECADE servers
and determines the available resources for each.
2.3. DECADE Content Providers
DECADE content providers access DECADE storage servers (by way of a
DECADE client) to upload and manage data. A content provider can
access one or more storage servers. A content provider may be a
single process or a distributed application (e.g., in a P2P
scenario).
2.4. DECADE Content Consumers
DECADE content consumers access storage servers (by way of a DECADE
client) to download data that has previously been stored by a content
provider. A content consumer can access one or more storage servers.
A content consumer may be a single process or a distributed
application (e.g., in a P2P scenario). An instance of a distributed
application, such as a P2P application, may both provide content to
and consume content from DECADE storage servers.
2.5. Content Distribution Application
A content distribution application is a distributed application
designed for dissemination of possibly-large data to multiple
consumers. Content Distribution Applications typically divide
content into smaller immutable blocks for dissemination.
The term Application Developer refers to the developer of a
particular Content Distribution Application.
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2.6. Application End-Point
An Application End-Point is an instance of a Content Distribution
Application that makes use of DECADE server(s). A particular
Application End-Point may be a DECADE Content Provider, a DECADE
Content Consumer, or both.
An Application End-Point need not be an active member of a "swarm" to
interact with the DECADE storage system. That is, an End-Point may
interact with the DECADE storage servers as an offline activity.
3. Architectural Principles
We identify the following key principles.
3.1. Decoupled Control and Data Planes
The DECADE infrastructure is intended to support multiple content
distribution applications. A complete content distribution
application implements a set of control functions including content
search, indexing and collection, access control, ad insertion,
replication, request routing, and QoS scheduling. Different content
distribution applications can have unique considerations designing
the control and signaling functions. For example, a major
competitive advantage of many successful P2P systems is their
substantial expertise in how to most efficiently utilize peer and
infrastructural resources. Many live P2P systems have their specific
algorithms in selecting the peers that behave as the more stable,
higher-bandwidth sources. They continue to fine-tune such
algorithms. In other words, in-network storage should export basic
mechanisms and allow as much flexibility as possible to the control
planes to implement specific policies. This conforms to the end-to-
end systems principle and allows innovation and satisfaction of
specific business goals.
Specifically, in the DECADE architecture, the control plane focuses
on the application-specific, complex, and/or processing intensive
functions while the data plane provides storage and data transport
functions.
o Control plane: Signals details of where the data is to be
downloaded from. Also signals the time, quality of service, and
receiver of the download. It also provides higher layer meta-data
management functions such as defining the sequence of data blocks
forming a higher layer content object. These are behaviors
designed and implemented by the Application. By Application, we
mean the broad sense that include other control plane protocols.
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o Data plane: Stores and transfers data as instructed by the
Application's Control Plane.
Decoupling control plane and data plane is not new. For example,
OpenFlow is an implementation of this principle for Internet routing,
where the computation of the forwarding table and the application of
the forwarding table are separated. Google File System applies the
principle to file system design, by utilizing the Master to handle
the meta-data management, and the chunk servers to handle the data
plane (i.e., read and write of chunks of data). NFS4 also implements
this principle.
Note that applications may have different Data Plane implementations
in order to support particular requirements (e.g., low latency). In
order to provide interoperability, the DECADE architecture does not
intend to enable arbitrary data transport protocols. However, the
architecture may allow for multiple data transport protocols to be
used.
Also note that although an application's existing control plane
functions remain implemented within the application, the particular
implementation may need to be adjusted to support DECADE.
3.2. Immutable Data Objects
A property of bulk contents to be distributed is that they typically
are immutable -- once a piece of content is generated, it is
typically not modified. It is not common that bulk contents such as
video frames and images need to be modified after distribution.
Many content distribution applications divide content objects into
blocks for two reasons: (1) multipath: different blocks may be
fetched from different content sources in parallel, and (2) faster
recovery and verification: individual blocks may be recovered and
verified. Typically, applications use a block size larger than a
single packet in order to reduce control overhead.
Common applications whose content matches this model include P2P
streaming (live and video-on-demand) and P2P file-sharing content.
However, other types of applications may additionally match this
model.
DECADE adopts a design in which immutable data objects may be stored
at a storage server. Applications may consider existing blocks as
DECADE data objects, or they may adjust block sizes before storing in
a DECADE server.
Focusing on immutable data blocks in the data plane can substantially
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simplify the data plane design, since consistency requirements can be
relaxed. It also allows effective reuse of data blocks and de-
duplication of redundant data.
Depending on specific application requirements, data objects can be
complete self-contained resources (such as video files) or chunks of
such resources. The DECADE architecture and protocols are agnostic
to the nature of the data objects and do not specify a fixed size for
them.
Note that immutable content may still be deleted. Also note that
immutable data blocks do not imply that contents cannot be modified.
For example, a meta-data management function of the control plane may
associate a name with a sequence of immutable blocks. If one of the
blocks is modified, the meta-data management function changes the
mapping of the name to a new sequence of immutable blocks.
3.3. Data Object Identifiers
Objects that are stored in a DECADE storage server can be accessed by
DECADE content consumers by a resource identifier that has been
assigned within a certain application context.
Because a DECADE content consumer can access more than one storage
server within a single application context, a data object that is
replicated across different storage servers managed by a DECADE
storage provider, can be accessed by a single identifier.
Note that since data objects are immutable, it is possible to support
persistent identifiers for data objects.
3.4. Explicit Control
To support the functions of an application's control plane,
applications must be able to know and control which data is stored at
particular locations. Thus, in contrast with content caches,
applications are given explicit control over the placement (selection
of a DECADE server), deletion (or expiration policy), and access
control for stored data.
Consider deletion/expiration policy as a simple example.
Applications may require a DECADE server to store content for a
relatively short period of time (e.g. for live-streaming data) or may
need to store content long term (e.g., for video-on-demand).
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3.5. Resource and Data Access Control through User Delegation
DECADE provides a shared infrastructure to be used by multiple
tenants of multiple content distribution applications. Thus, it
needs to provide both resource and data access control.
3.5.1. Resource Allocation
There are two primary interacting entities in the DECADE
architecture. First, Storage Providers control where DECADE storage
servers are provisioned and their total available resources. Second,
Applications control data transfers amongst available DECADE servers
and between DECADE servers and end-points. A form of isolation is
required to enable concurrently-running Applications to each
explicitly manage their own content and share of resources at the
available servers.
Management of the resources at a server are delegated by a Storage
Provider to one or more applications. Applications are able to
explicitly and independently manage their own share of resources.
3.5.2. User Delegations
Storage providers have the ability to explicitly manage the entities
allowed to utilize the resources at a DECADE server. This capability
is needed for reasons such as capacity-planning and legal
considerations in certain deployment scenarios.
To provide a scalable way to manage applications granted resources at
a DECADE server, a layer of indirection is added. Instead of
granting resources to an application, the DECADE server grants a
share of the resources to a user. The user may in turn share the
granted resources amongst multiple applications. The share of
resources granted by a storage provider is called a User Delegation.
A User Delegation may be granted to an end-user (e.g., an ISP
subscriber), a Content Provider, or an Application Provider. A
particular instance of an application may make use of the storage
resources:
o granted to the end-user (with the end-user's permission),
o granted to the Content Provider (with the Content Provider's
permission>, and/or
o granted to the Application Provider.
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4. System Components
The current version of the document has primarily focused on the
architectural principles. The detailed system components will be
discussed in the next document revision.
This section presents an overview of the components in the DECADE
architecture.
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.--------------------------------------------------------------.
| Application End-Point |
| .------------. .-----------------. .----------. |
| | App-Layer | ... | App Data Object | | App Data | |
| | Algorithms | | Sequencing | | Naming | |
| `------------' `-----------------' `----------' |
| |
| .----------------------------------------------------------. |
| | DECADE Client | |
| | | |
| | .-------------------------. .--------------------------. | |
| | | Resource Controller | | Data Controller | | |
| | | .--------. .----------. | | .------------. .-------. | | |
Native | | | | Data | | Resource | | | | Data | | Data | | | |
App | | | | Access | | Sharing | | | | Scheduling | | Index | | | |
Protocol(s)| | | | Policy | | Policy | | | | | | | | | |
.--> | | | '--------' `----------' | | `------------' `-------' | | |
| | | `-------------------------' `--------------------------' | |
| | | | ^ | |
| | `------------ | ----------------- | -----------------------' |
| `-------------- | ----------------- | -------------------------'
| | |
v | DECADE | Standard
.-------------. | Resource | Data
| Application | | Protocol (DRP) | Transport (SDT)
| End-Point | | |
`-------------' | | Content Distribution
^ ^ | | Application
= | ===== | ============== | ================= | ==========================
| | | | DECADE Server(s)
| | | |
| | .- | ----------------- | ----------------------.
| | | | v |
| | | | .----------------. |
| | | |----> | Access Control | <--------. |
| DRP | SDT | | `----------------' | |
| | | | ^ | |
| | | | v | |
| | | | .---------------------. | |
| | | `-> | Resource Scheduling | <------| |
v v DRP | `---------------------' | |
.------------. <------> | ^ | |
| DECADE | | v .------------. |
| Server | SDT | .-----------------. | User | |
`------------' <------> | | Data Store | | Delegation | |
| `-----------------' | Management | |
| DECADE Server `------------' |
`----------------------------------------------'
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Figure 1: DECADE Architecture Components
A component diagram of the DECADE architecture is displayed in
Figure 1. The diagram illustrates the major components of a Content
Distribution Application related to DECADE, as well as the functional
components of a DECADE Server.
To keep the scope narrow, we only discuss the primary components
related to protocol development. Particular deployments may require
additional components (e.g., monitoring and accounting at a DECADE
server), but they are intentionally omitted from the current version
of this document.
4.1. Content Distribution Application
Content Distribution Applications have many functional components.
For example, many P2P applications have components to manage overlay
topology management, piece selection, etc. In supporting DECADE, it
may be advantageous to consider DECADE within some of these
components. However, in this architecture document, we focus on the
components directly employed to support DECADE.
4.1.1. Data Sequencing and Naming
DECADE is primarily designed to support applications that can divide
distributed contents into immutable data objects. To accomplish
this, applications include a component responsible for re-assembling
data objects and also creating the individual data objects. We call
this component Application Data Sequencing. The specific
implementation is entirely decided by the application.
In assembling or producing the data objects, an important
consideration is the naming of these objects. We call the component
responsible for assigning and interpreting application-layer names
the Application Data Naming component. The specific implementation
is entirely decided by the application.
4.1.2. Native Protocols
Applications may still use existing protocols. Existing protocols
used primarily for control/signaling needed by the application, but
may also serve as a data transport as they do today; it is important
that applications still be designed to be robust (e.g., if DECADE
servers are unavailable).
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4.1.3. DECADE Client
An application may be modified to support DECADE. We call the layer
providing the DECADE support to an application the DECADE Client. It
is important to note that a DECADE Client need not be embedded into
an application. It could be implemented alone, or could be
integrated in other entities such as network devices themselves.
4.1.3.1. Resource Controller
Applications may have different Resource sharing policies and Data
access policies to control their resource and data in DECADE servers.
These policies can be existing policies of applications (e.g., tit-
for-tat) or custom policies adapted for DECADE. The specific
implementation is decided by the application.
4.1.3.2. Data Controller
DECADE is designed to decouple the control and the data transport of
applications. Data transport between applications and DECADE servers
uses standard data transport protocols. It may need to schedule the
data being transferred according to network conditions, available
DECADE Servers, and/or available DECADE Server resources. An index
indicates data available at remote DECADE servers. The index (or a
subset of it) may be advertised to other Application End-Points.
4.2. DECADE Server
DECADE server is an important functional component of DECADE. It
stores data from Application End-Points, and provides control and
access of those data to Application End-Points. Note that a DECADE
server is not necessarily a single physical machine, it could also be
implemented as a cluster of machines.
4.2.1. Access Control
An Application End-Point can access its own data or other Application
End-Point's data (provided sufficient authorization) in DECADE
servers. Application End-Points may also authorize other End-Points
to store data. If an access is authorized by an Application End-
Point, the DECADE Server will provide access.
Note that even if an request is authorized, it may still fail to
complete due to insufficient resources by either the requesting
Application End-Point or the providing Application End-Point.
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4.2.2. Resource Scheduling
Applications may apply their existing resource sharing policies or
use a custom policy for DECADE. DECADE servers perform resource
scheduling according to the resource sharing policies indicated by
Application End-Points as well as configured User Delegations.
Access control and resource control are separated in DECADE server.
It is possible that an Application End-Point provides only access to
its data without any resources. In order to access this data,
another Application End-Point may use the granted access along with
its own available resources to store or retrieve data from a DECADE
Server.
4.2.3. Data Store
Data from applications may be stored into disks and explicitly or
automatically (e.g., after a TTL) deleted from disks. It may be
possible to perform optimizations in certain cases, such as avoiding
writing temporary data (e.g., live streaming) to disk.
4.3. Protocols
The DECADE Architecture uses two protocols. First, the DECADE
Resource Protocol is responsible for communicating access control and
resource scheduling policies to the DECADE Server. Second, standard
data transport protocols (e.g., WebDAV or NFS) are used to transfer
data objects to and from a DECADE Server. The DECADE architecture
will specify a small number of Standard Data Transport instances.
Decoupling the protocols in this way allows DECADE to both directly
utilize existing standard data transports and to evolve
independently.
It is also important to note that the two protocols do not need to be
separate on the wire. For example, the DECADE Resource Protocol
messages may be piggybacked within extension fields provided by
certain data transport protocols. However, this document considers
them as two separate functional components for clarity.
4.3.1. DECADE Resource Protocol
The DECADE Resource Protocol is responsible for communicating both
access control and resource sharing policies to DECADE Servers used
for data transport.
The DECADE architecture specification will provide exactly one DECADE
Resource Protocol.
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4.3.2. Standard Data Transports
Existing data transport protocols are used to read and write data
from a DECADE Server. Protocols under consideration are WebDAV and
NFS.
4.4. DECADE Data Sequencing and Naming
We have discussed above that an Application may have its own behavior
for both sequencing and naming data objects. In order to provide a
simple and generic interface, the DECADE Server is only responsible
for storing and retrieving individual data objects.
The issue of naming data objects at the DECADE server would benefit
from additional feedback. There are multiple options that have been
considered:
o Self-certifying Name: The name of a data object may be a hash of
its contents. Advantages of this scheme include simplicity and
low probability of naming collisions without requiring any
identifiers or namespaces to be allocated. Disadvantages of this
scheme include collision in identifiers (with low probability) and
introduction of an additional distribution delay due to the
necessity of reading the full object to compute its hash before
advertising its availability.
o Application-specified Name: The name of a data object is specified
by the application itself. For example, this could be a function
of the application's content identifier and index of the data
object. To avoid conflicts, identifiers could be assigned to
particular applications. An advantage of this scheme is that
collisions could be avoided. A disadvantage is that assigning
identifiers introduces additional management complexity.
o Server-specified Name: The name of a data object is specified by
the DECADE server upon initially being stored. An advantage of
this approach is that naming conflicts can be completely avoided
without requiring particular identifiers to be assigned to
applications. A disadvantage is that it introduces additional
latency between the time when a application may upload a data
object and advertise availability of the data object at the DECADE
Server.
The current preferred design is to use self-certifying names.
However, additional feedback is welcomed.
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4.5. In-Network Storage Components Mapped to DECADE Architecture
In this section we evaluate how the basic components of an in-network
storage system identified in Section 3 of [I-D.ietf-decade-survey]
map into the DECADE architecture.
It is important to note that complex and/or application-specific
behavior is delegated to applications instead of tuning the storage
system wherever possible.
4.5.1. Data Access Interface
Users can read and write objects of arbitrary size through the DECADE
Client's Data Controller, making use of a standard data transport.
4.5.2. Data Management Operations
Users can move or delete previously stored objects via the DECADE
Client's Data Controller, making use of a standard data transport.
4.5.3. Data Search Capability
Users can enumerate or search contents of DECADE servers to find
objects matching desired criteria through services provided by the
Content Distribution Application (e.g., buffer-map exchanges, a DHT,
or peer-exchange). In doing so, End-Points may consult their local
data index in the DECADE Client's Data Controller.
4.5.4. Access Control Authorization
All methods of access control are supported: public-unrestricted,
public-restricted and private. Access Control Policies are generated
by a Content Distribution Application and provided to the DECADE
Client's Resource Controller. The DECADE Server is responsible for
implementing the access control checks.
4.5.5. Resource Control Interface
Users can manage the resources (e.g. bandwidth) on the DECADE server
that can be used by other Application End-Points. Resource Sharing
Policies are generated by a Content Distribution Application and
provided to the DECADE Client's Resource Controller. The DECADE
Server is responsible for implementing the resource sharing policies.
4.5.6. Discovery Mechanism
This is outside the scope of the DECADE architecture. However, it is
expected that DNS or some other well known protocol will be used for
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the users to discover the DECADE servers.
4.5.7. Storage Mode
DECADE Servers provide an object-based storage mode. Immutable data
objects may be stored at a DECADE server. Applications may consider
existing blocks as DECADE data objects, or they may adjust block
sizes before storing in a DECADE server.
5. Security Considerations
This document currently does not contain any security considerations
beyond those mentioned in [I-D.ietf-decade-problem-statement].
6. IANA Considerations
This document does not have any IANA considerations.
7. Informative References
[I-D.ietf-decade-problem-statement]
Yongchao, S., Zong, N., Yang, Y., and R. Alimi, "DECoupled
Application Data Enroute (DECADE) Problem Statement",
draft-ietf-decade-problem-statement-00 (work in progress),
August 2010.
[I-D.ietf-decade-survey]
Alimi, R., Rahman, A., and Y. Yang, "A Survey of In-
network Storage Systems", draft-ietf-decade-survey-01
(work in progress), October 2010.
[I-D.gu-decade-reqs]
Yingjie, G., Bryan, D., Yang, Y., and R. Alimi, "DECADE
Requirements", draft-gu-decade-reqs-05 (work in progress),
July 2010.
Authors' Addresses
Richard Alimi
Google
Email: ralimi@google.com
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Y. Richard Yang
Yale University
Email: yry@cs.yale.edu
Akbar Rahman
InterDigital Communications, LLC
Email: akbar.rahman@interdigital.com
Dirk Kutscher
NEC
Email: dirk.kutscher@neclab.eu
Lijiang Chen
Yale University
Email: lijiang.chen@yale.edu
Hongqiang Liu
Yale University
Email: hongqiang.liu@yale.edu
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