Request for Comments: 6646
DECoupled Application Data Enroute (DECADE) Problem Statement
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. In P2P applications, one technique for reducing the transit and uplink P2P traffic is to introduce storage capabilities within the network. Traditional caches (e.g., P2P and Web caches) provide such storage, but they can be complex (e.g., P2P caches need to explicitly support individual P2P application protocols), and do not allow users to manage resource usage policies for content in the cache. This document discusses the introduction of in-network storage for P2P applications and shows the need for a standard protocol for accessing this storage.
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/rfc6646.
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Table of Contents
1. Introduction ....................................................2 2. Terminology and Concepts ........................................3 3. The Problems ....................................................4 3.1. P2P Infrastructural Stress and Inefficiency ................4 3.2. P2P Cache: A Complex Type of In-Network Storage ............5 3.3. Ineffective Integration of P2P Applications ................6 4. Usage Scenarios .................................................6 4.1. BitTorrent .................................................6 4.2. Content Publisher ..........................................7 5. Security Considerations .........................................8 5.1. Denial-of-Service Attacks ..................................8 5.2. Copyright and Legal Issues .................................8 5.3. Traffic Analysis ...........................................8 5.4. Modification of Information ................................8 5.5. Masquerade .................................................9 5.6. Disclosure .................................................9 5.7. Message Stream Modification ................................9 6. Acknowledgments .................................................9 7. Informative References .........................................10
Peer-to-peer (P2P) applications, including both P2P streaming and P2P file-sharing applications, make up a large fraction of the traffic in many Internet Service Provider (ISP) networks today. One way to reduce bandwidth usage by P2P applications is to introduce storage capabilities in networks. Allowing P2P applications to store and retrieve data from inside networks can reduce traffic on the last- mile uplink, as well as on backbone and transit links.
Existing P2P caches provide in-network storage and have been deployed in some networks. However, the current P2P caching architecture poses challenges to both P2P cache vendors and P2P application developers. For P2P cache vendors, it is challenging to support a number of continuously evolving P2P application protocols, due to lack of documentation, ongoing protocol changes, and rapid introduction of new features by P2P applications. For P2P application developers, closed P2P caching systems limit P2P applications from effectively utilizing in-network storage. In particular, P2P caches typically do not allow users to explicitly store content into in-network storage. They also do not allow applications to specific resource and access control policies over the usage of in-network storage. The challenges, if not addressed, may lead to reduced efficiency for P2P applications, and increased load on the network infrastructure.
The challenges can be effectively addressed by using a standard, open protocol to access in-network storage [Data_Lockers]. P2P applications can store and retrieve content in the in-network storage, as well as control resources (e.g., bandwidth, connections) consumed by peers in a P2P application. As a simple example, a peer of a P2P application may upload to other peers through its in-network storage, saving its usage of last-mile uplink bandwidth.
In this document, we distinguish between two functional components of the native P2P application protocol: signaling and data access. Signaling includes operations such as handshaking and discovering peer and content availability. The data access component transfers content from one peer to another.
In essence, coupling of the signaling and data access makes in-network storage complex to support various application services. However, these applications have common requirements for data access, making it possible to develop a standard protocol.
2. Terminology and Concepts
The following terms have special meaning in the definition of the in-network storage system.
In-network storage: A service inside a network that provides storage and bandwidth to network applications. In-network storage may reduce upload/transit/backbone traffic and improve network application performance. The position of in-network storage is in the core of a network -- for example, co-located with the border router (network attached storage) or inside a data center. P2P cache (peer-to-peer cache): A kind of in-network storage that understands the signaling and transport of specific P2P application protocols. It caches the content for those specific P2P applications in order to serve peers and reduce traffic on certain links.
3. The Problems
The emergence of P2P as a major network application (especially P2P file sharing and streaming) has led to substantial opportunities. The P2P paradigm can be utilized to design highly scalable and robust applications at low cost, compared to the traditional client-server paradigm.
However, P2P applications also face substantial design challenges. A particular challenge facing P2P applications is the additional stress that they place on the network infrastructure. At the same time, lack of infrastructure support can lead to unstable P2P application performance, in particular during peer churns and flash crowds, when a large group of users begin to retrieve the content during a short period of time, leading to stress on bandwidth-constrained access uplinks. A potential way to reduce network stress and improve P2P application performance would be to make it possible for peers that are on bandwidth-constrained access to put data in a place that is free of bandwidth constraints and also accessible by other peers. These problems are now discussed in further detail.
3.1. P2P Infrastructural Stress and Inefficiency
A particular problem of the P2P paradigm is the stress that P2P application traffic places on the infrastructure of ISPs. Multiple measurements (e.g., [ipoque_Internet_Study]) have shown that P2P traffic has become a major type of traffic on some networks. Furthermore, the inefficiency of network-agnostic peering (at the P2P transmission level) leads to unnecessary traversal across network domains or spanning the backbone of a network [RFC5693].
Using network information alone to construct more efficient P2P swarms is not sufficient to reduce P2P traffic in access networks, as the total access upload traffic is equal to the total access download traffic in a traditional P2P system. On the other hand, it is reported that P2P traffic is becoming the dominant traffic on the access networks of some networks, reaching as high as 50-60% on the downlinks and 60-90% on the uplinks [DCIA] [ICNP] [ipoque_P2P_Survey] [P2P_File_Sharing]. Consequently, it becomes increasingly important to reduce upload access traffic, in addition to cross-domain and backbone traffic.
The inefficiency of P2P is also observed when traffic is sent upstream as many times as there are remote peers interested in getting the corresponding information. For example, the P2P application transfer completion times remain affected by potentially (relatively) slow upstream transmission. Similarly, the performance of real-time P2P applications may be affected by potentially (relatively) higher upstream latencies.
3.2. P2P Cache: A Complex Type of In-Network Storage
An effective technique to reduce P2P infrastructural stress and inefficiency is to introduce in-network storage. A survey of existing in-network storage systems can be found in [RFC6392].
In the current Internet, in-network storage is introduced as P2P caches, either transparently or explicitly as a P2P peer. To provide service to a specific P2P application, the P2P cache server must support the specific signaling and transport protocols of the specific P2P application. This can lead to substantial complexity for the P2P cache vendor.
First, there are many P2P applications on the Internet (e.g., BitTorrent, eMule, Flashget, and Thunder for file sharing; Abacast, Kontiki, Octoshape, PPLive, PPStream, and UUSee for P2P streaming). Consequently, a P2P cache vendor faces the challenge of supporting a large number of P2P application protocols, leading to product complexity and increased development cost.
Second, a specific P2P application protocol may evolve continuously to add new features or fix bugs. This in turn forces a P2P cache vendor to continuously monitor application updates to track such changes, leading to product complexity and increased costs.
Third, many P2P applications use proprietary protocols or support end-to-end encryption. This can render P2P caches ineffective. Therefore, these three problems make it difficult to use the P2P cache as a network middlebox to support P2P application distribution.
Finally, an end host has better connectivity and connection quality to a P2P cache than to a remote peer. Without the ability to manage bandwidth usage, the P2P cache may increase the volume of download traffic, which runs counter to the reduction of upload access traffic.
3.3. Ineffective Integration of P2P Applications
As P2P applications evolve, it has become increasingly clear that usage of in-network resources can improve the user's experience. For example, multiple P2P streaming systems seek additional in-network resources during a flash crowd, such as just before a major live streaming event. In asymmetric networks, when the aggregated upload bandwidth of a channel cannot meet the download demand, a P2P application may seek additional in-network resources to maintain a stable system.
However, some P2P applications using in-network infrastructural resources require flexibility in implementing resource allocation policies. A major competitive advantage of many successful P2P systems is their substantial expertise in how to most efficiently utilize peer and infrastructural resources. For example, many live P2P systems have specific algorithms to select those peers that behave as stable, higher-bandwidth sources. Similarly, the higher- bandwidth sources frequently use algorithms to choose to which peers the source should send content. Developers of these systems continue to fine-tune these algorithms over time.
To permit developers to evolve and fine-tune their algorithms and policies, the in-network storage should expose basic mechanisms and allow as much flexibility as possible to P2P applications. This conforms to the end-to-end systems principle and allows innovation and satisfaction of specific business goals. Existing techniques for in-network storage in P2P applications lack these capabilities.
4. Usage Scenarios
Usage scenarios are presented to illustrate the problems in both Content Distribution Network (CDN) and P2P scenarios.
When a BitTorrent client A uploads a block to multiple peers, the block traverses the last-mile uplink once for each peer. After that, the peer B that just received the block from A also needs to upload through its own last-mile uplink to others when sharing this block. This is not an efficient use of the last-mile uplink. With an in-network storage server, however, the BitTorrent client may upload the block to its in-network storage. Peers may retrieve the block from the in-network storage, reducing the amount of data on the last-mile uplink. If supported by the in-network storage, a peer can also save the block in its own in-network storage while it is being retrieved; the block can then be uploaded from the in-network storage to other peers.
As previously discussed, BitTorrent or other P2P applications currently cannot explicitly manage which content is placed in the existing P2P caches, nor can they manage access and resource control policies. Applications need to retain flexibility to control the content distribution policies and topology among peers.
4.2. Content Publisher
Content publishers may also utilize in-network storage. For example, consider a P2P live streaming application. A Content Publisher typically maintains a small number of sources, each of which distributes blocks in the current play buffer to a set of P2P peers.
Some content publishers use another hybrid content distribution approach incorporating both P2P and CDN modes. As an example, Internet TV may be implemented as a hybrid CDN/P2P application by distributing content from central servers via a CDN, and also incorporating a P2P mode amongst end hosts and set-top boxes. In-network storage may be beneficial to hybrid CDN/P2P applications as well to support P2P distribution and to enable content publisher standard interfaces and controls.
However, there is no standard interface for different content publishers to access in-network storage. One streaming content publisher may need the existing in-network storage to support streaming signaling or another such capability, such as transcoding capability, bitmap information, intelligent retransmission, etc., while a different content publisher may only need the in-network storage to distribute files. However, it is reasonable that the application services are only supported by content publishers' original servers and clients, and intelligent data plane transport for those content publishers are supported by in-network storage.
A content publisher also benefits from a standard interface to access in-network storage servers provided by different providers. The standard interface must allow content publishers to retain control over content placed in their own in-network storage and to grant access and resources only to the desired end hosts and peers.
In the hybrid CDN/P2P scenario, if only the end hosts can store content in the in-network storage server, the content must be downloaded and then uploaded over the last-mile access link before another peer may retrieve it from an in-network storage server. Thus, in this deployment scenario, it may be advantageous for a content publisher or CDN provider to store content in in-network storage servers.
5. Security Considerations
There are several security considerations related to in-network storage.
5.1. Denial-of-Service Attacks
An attacker can try to consume a large portion of the in-network storage, or exhaust the connections of the in-network storage through a denial-of-service (DoS) attack. Authentication, authorization, and accounting mechanisms should be considered in the cross-domain environment. Limitation of access from an administrative domain sets up barriers for content distribution.
5.2. Copyright and Legal Issues
Copyright and other laws may prevent the distribution of certain content in various localities. In-network storage operators may adopt system-wide ingress or egress filters to implement necessary policies for storing or retrieving content, and applications may apply Digital Rights Management (DRM) to the data stored in the network storage. However, the specification and implementation of such policies (e.g., filtering and DRM) is not in scope for the problem this document proposes to solve.
5.3. Traffic Analysis
If the content is stored in the provider-based in-network storage, there may be a risk to privacy: a malicious service provider could use some link that a victim user is interested in, estimate that another user accessing the same data may have the same interest, and use this information as a basis to perform a phishing attack on the other user.
5.4. Modification of Information
This type of threat means that some unauthorized entity may alter in-transit in-network storage access messages generated on behalf of an authorized principal in such a way as to effect unauthorized management operations, including falsifying the value of an object. This threat may result in false data being supplied either because the data on a legitimate store is modified or because a bogus store is introduced into the network.
This type of threat means that an unauthorized entity gains access to a system or performs a malicious act by illegitimately posing as an authorized entity. In the context of this specification, when accessing in-network storage, one malicious end host can masquerade as another authorized end host or application server to access a protected resource in the in-network storage.
This type of threat involves the danger of someone eavesdropping on exchanges between in-network storage and application clients. Protecting against this threat may be required as a matter of application policy.
5.7. Message Stream Modification
This type of threat means that messages may be maliciously re-ordered, delayed, or replayed to an extent greater than what would occur in a natural network system, in order to effect unauthorized management operations on in-network storage. If the middlebox (such as a Network Address Translator (NAT)) or proxy between an end host and in-network storage is compromised, it is easy to do a stream modification attack.
We would like to thank the following people for contributing to this document:
Ronald Bonica David Bryan Kar Ann Chew Lars Eggert Roni Even Adrian Farrel Yingjie Gu David Harrington Leif Johansson Francois Le Faucheur Hongqiang Liu Tao Ma Borje Ohlman Akbar Rahman Peter Saint-Andre Robert Sparks Sean Turner Yu-shun Wang Richard Woundy Yunfei Zhang
7. Informative References
[DCIA] Parker, A., "P2P Media Summit presentation", Distributed Computing Industry Association, October 2006, <http://www.dcia.info/activities/p2pmsla2006/ CacheLogic.ppt>.
Yang, Y., "Open Content Distribution using Data Lockers", CoXNet Workshop, Beijing, China, November 2010, <http:// cs-www.cs.yale.edu/homes/yry/projects/p4p/ open-data-lockers-nov-2010-coxnet.pdf>. [ICNP] Wu, H., "Challenges and Opportunities of Internet Developments in China", ICNP 2007 Keynote Speech, October 2007, <http://www.ieee-icnp.org/2007/keynote_D.html>.
Casadesus-Masanell, R. and A. Hervas-Drane, "Peer-to-Peer File Sharing and the Market for Digital Information Goods", Journal of Economics & Management Strategy, Vol. 19, No. 2, pp. 333-373, Summer 2010. [RFC5693] Seedorf, J. and E. Burger, "Application-Layer Traffic Optimization (ALTO) Problem Statement", RFC 5693, October 2009. [RFC6392] Alimi, R., Ed., Rahman, A., Ed., and Y. Yang, Ed., "A Survey of In-Network Storage Systems", RFC 6392, October 2011.
Schulze, H. and K. Mochalski, "Internet Study 2008/2009", 2009, <http://www.ipoque.com/resources/internet-studies>.
"ipoque's 2007 P2P Survey to be presented at Technology Review's Emerging Technologies Conference at MIT", August 2007, <http://www.ipoque.com/en/news-events/ press-center/press-releases/2007/ ipoque%C2%B4s-2007-p2p-survey-to-be-presented-at- technology>.
Haibin Song Huawei 101 Software Avenue, Yuhua District Nanjing, Jiangsu Province 210012 China EMail: firstname.lastname@example.org Ning Zong Huawei 101 Software Avenue, Yuhua District Nanjing, Jiangsu Province 210012 China
Y. Richard Yang