[MS-PCCRC]:
Peer Content Caching and Retrieval: Content Identification
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Revision Summary
Date / Revision History / Revision Class / Comments /12/5/2008 / 0.1 / Major / Initial Availability
1/16/2009 / 0.1.1 / Editorial / Changed language and formatting in the technical content.
2/27/2009 / 0.1.2 / Editorial / Changed language and formatting in the technical content.
4/10/2009 / 0.2 / Minor / Clarified the meaning of the technical content.
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7/2/2009 / 1.1 / Minor / Clarified the meaning of the technical content.
8/14/2009 / 1.2 / Minor / Clarified the meaning of the technical content.
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Table of Contents
1 Introduction 5
1.1 Glossary 5
1.2 References 7
1.2.1 Normative References 7
1.2.2 Informative References 8
1.3 Overview 8
1.4 Relationship to Protocols and Other Structures 8
1.5 Applicability Statement 9
1.6 Versioning and Localization 9
1.7 Vendor-Extensible Fields 9
2 Structures 10
2.1 Content, Segments, and Blocks 10
2.2 Segment Identifiers (HoHoDk) and Keys 10
2.3 Content Information Data Structure Version 1.0 12
2.3.1 Fields 14
2.3.1.1 SegmentDescription 14
2.3.1.2 SegmentContentBlocks 15
2.4 Content Information Data Structure Version 2.0 15
2.4.1 Fields 16
2.4.1.1 ChunkDescription 16
2.4.1.2 SegmentDescription 17
2.5 Server Secret Key 17
3 Structure Examples 18
3.1 125 KB Content 18
3.2 Version 1.0 Content Information, 125 KB Content 18
3.3 Version 1.0 Content Information, 125 KB Content Range Request 19
3.4 Version 1.0 Content Information, 125 MB Content 20
3.5 Version 1.0 Content Information, 125 MB Content Range Request 21
3.6 Version 2.0 Content Information, 189 KB Content 23
3.7 Version 2.0 Content Information, 189 KB Content Range Request 24
4 Security 26
4.1 Security Considerations for Implementers 26
4.1.1 Download Confidentiality 26
4.1.2 Content Block Validation 26
4.1.3 Content Chunk Validation 26
4.2 Index of Security Fields 26
5 Appendix A: Product Behavior 27
6 Change Tracking 28
7 Index 29
1 Introduction
The Peer Content Caching and Retrieval: Content Identification data structure specifies the Content Information Data Structure format used to uniquely identify content for discovery and retrieval purposes on a peer-to-peer network. This data structure is used within the Peer Content Caching and Retrieval service framework. The Content Information Data Structure is used in the discovery protocol specified in [MS-PCCRD] and the retrieval protocol described in [MS-PCCRR] to identify content for discovery, response, and retrieval requests. The Content Information Data Structure contains all the necessary information to allow a peer to uniquely specify content for discovery, and ensures the confidentiality and integrity of downloaded content. To satisfy these requirements, Content Information uses cryptographic hashing and encryption algorithms to encrypt and generate hashes of the content and includes a shared secret that is specific to that content.
The Peer Content Caching and Retrieval Framework is based on a peer-to-peer discovery and distribution model, where the peers themselves act as caches from which they serve other requesting peers. The framework supports the mode of using pre-provisioned hosted caches in place of peer-based caching. The framework is designed to reduce bandwidth consumption on branch-office wide-area-network (WAN) links by having clients retrieve content from distributed caches, when distributed caches are available, rather than from the content servers, which are often located remotely from branch offices over the WAN links. The main benefit of the framework is to reduce operation costs by reducing WAN link utilization, while providing faster downloads from the local area networks (LANs) in the branch offices.
Sections 1.7 and 2 of this specification are normative. All other sections and examples in this specification are informative.
1.1 Glossary
This document uses the following terms:
authorized client: A client in possession of the segment secret for a particular segment, or, in the context of content, a client in possession of all the segment secrets for a particular piece of content.
big-endian: Multiple-byte values that are byte-ordered with the most significant byte stored in the memory location with the lowest address.
block: A subdivision of a segment. Each segment is divided into blocks of equal size (64 kilobytes (KB)) except for the last block in the last segment, which can be smaller if the content size is not a multiple of the standard segment sizes. In version 2.0 Content Information, segments are not divided into blocks.
block hash: A hash of a content block within a segment. Also known as a block ID.
chunk: (1) A sequence of words that are treated as a single unit by a module that checks spelling.
(2) A collection of one or more segment descriptions along with metadata, such as the chunk type and size.
ciphertext: The encrypted form of a message. Ciphertext is achieved by encrypting the plaintext form of a message, and can be transformed back to plaintext by decrypting it with the proper key. Without that transformation, a ciphertext contains no distinguishable information.
client: For the Peer Content Caching and Retrieval Framework, a client is a client-role peer; that is, a peer that is searching for content, either from the server or from other peers or hosted cashes. In the context of the Retrieval Protocol, a client is a peer that requests a block-range from a server_role_peer. It acts as a Web Services Dynamic Discovery (WS-Discovery) [WS-Discovery] client.
content: (1) Multimedia data. content is always in ASF, for example, a single ASF music file or a single ASF video file. Data in general. A file that an application accesses. Examples of content include web pages and documents stored on either web servers or SMB file servers.
(2) Items that correspond to a file that an application attempts to access. Examples of content include web pages and documents stored on either HTTP servers or SMB file servers. Each content item consists of an ordered collection of one or more segments.
content block: A block of data in the content that can be retrieved from clients.
content information: An opaque blob of data containing a set of hashes for a specific file that can be used by the application to retrieve the contents of the file using the branch cache. The details of content information are discussed in [MS-PCCRC].
content range: The starting offset and length for the content desired. Multipart ranges (that is, non-contiguous) are not supported.
content server: The original server that a peer contacts to obtain either the hashes of the content or the actual content when it is not available from the peers.
dataBlock: See block.
encryption: In cryptography, the process of obscuring information to make it unreadable without special knowledge.
hash: A fixed-size result that is obtained by applying a one-way mathematical function, which is sometimes referred to as a hash algorithm, to an arbitrary amount of data. If the input data changes, the hash also changes. The hash can be used in many operations, including authentication and digital signing.
HoHoDk: A hash that represents the content-specific label or public identifier that is used to discover content from other peers or from the hosted cache. This identifier is disclosed freely in broadcast messages. Knowledge of this identifier does not prove authorization to access the actual content.
hosted cache: A centralized cache comprised of blocks added by peers.
Keyed-Hashing for Message Authentication (HMAC): For more information, see [RFC2104].
little-endian: Multiple-byte values that are byte-ordered with the least significant byte stored in the memory location with the lowest address.
multicast: Allows a host to send data to only those destinations that specifically request to receive the data. In this way, multicasting differs from sending broadcast data, because broadcast data is sent to all hosts. multicasting saves network bandwidth because multicast data is received only by those hosts that request the data, and the data travels over any link only once. multicasting saves server bandwidth because a server has to send only one multicast message per network instead of one unicast message per receiver.
passphrase: One or more words entered as a security setting to enable device or identity authentication.
peer: (1) An additional endpoint that is associated with an endpoint in a session. An example of a peer is the callee endpoint for a caller endpoint.
(2) A node participating in the content caching and retrieval system. A peer is a node that both accesses the content and serves the content it caches for other peers.