Application-Layer Traffic Optimization (ALTO) Problem Statement

Peer-to-peer (P2P) applications, such as file sharing, real-time communication, and live media streaming, use a significant amount of Internet resources. Different from the client/server architecture, P2P applications access resources such as files or media relays distributed across the Internet and exchange large amounts of data in connections that they establish directly with nodes sharing such resources. One advantage of P2P systems results from the fact that the resources such systems offer are often available through multiple replicas. However, applications generally do not have reliable information of the underlying network and thus have to select among available instances based on information they deduce from empirical measurements that, in some situations, lead to suboptimal choices. For example, one popular metric is an estimation of round-trip time. This choice occurs before actual data transmission begins and thus before the peer can deduce actual throughput. This is one reason why a peer selection algorithm that simply uses round-trip time often results in a sub-optimal choice of peers. Many of today's P2P systems use an overlay network consisting of direct peer connections. Such connections often do not account for the underlying network topology. In addition to having suboptimal performance, such networks can lead to congestion and cause serious inefficiencies. As shown in , traffic generated by popular P2P applications often cross network boundaries multiple times, overloading links which are frequently subject to congestion . Moreover, such transits, besides resulting in a poor experience for the user, can be quite costly to the network operator. Recent studies show a possible solution to this problem. Internet Service Providers (ISP), network operators or third parties can collect reliable network information. This information includes relevant information such as topology or instantaneous bandwidth available. Normally, such information is rather "static", i.e., information which can change over time but on a much longer time scale than information used for congestion control on the transport layer. By providing this information to P2P applications, it would be possible to greatly increase application performance, reduce congestion and optimize the overall traffic across different networks. Presumably, both the application and the network operator can benefit from such information. Thus, network operators have an incentive to provide, either directly themselves or indirectly through a third party, such information and applications have an incentive to use such information. This document gives the problem statement of optimizing traffic generated by P2P applications using information provided by a separate party. introduces the problem. describes some use cases where both P2P applications and network operators benefit from a solution to such a problem. describes the main issues to consider when designing such a solution. Note a companion document to this document, the ALTO Requirements, goes into the details of these issues.

The papers and are examples of contemporary solution proposals that address the problem described in this document. Moreover, these proposals have encouraging simulation and field test results. These and similar, independent, solutions all consist of two essential parts: a discovery mechanism which a P2P application uses to find a reliable information source and a protocol P2P applications use to query such sources in order to retrieve the information needed to perform better-than-random selection of the endpoints providing a desired resource. It is not easy to foresee how such solutions will perform in the Internet. A more accurate evaluation requires representative data collected from real systems by a critical mass of users. However, wide adoption is unlikely without an agreement on a common solution based upon an open standard.

The following terms have special meaning in the definition of the Application-Layer Traffic Optimization (ALTO) problem. A distributed communication system (e.g., file sharing) that uses the ALTO service to improve its performance or quality of experience while optimizing resource consumption in the underlying network infrastructure. Applications may use the P2P model to organize themselves, use the client-server model, or use a hybrid of both. A specific participant in an application. Colloquially, a peer refers to a participant in a P2P network or system, and this definition does not violate that assumption. If the basis of the application is the client-server or hybrid model, then the usage of the terms "client" and "server" disambiguates the peer's role. Peer-to-Peer. Content, such as a file or a chunk of a file or a server process, for example to relay a media stream or perform a computation, which applications can access. In the ALTO context, a resource is often available in several equivalent replicas. In addition, different peers share these resources, often simultaneously. An application layer identifier used to identify a resource, no matter how many replicas exist. For P2P applications, a resource provider is a specific peer that provides some resources. For client-server or hybrid applications, a provider is a server that hosts a resource. For P2P applications, a resource consumer is a specific peer that needs to access resources. For client-server or hybrid applications, a consumer is a client that needs to access resources. All address information that a resource consumer needs to access the desired resource at a specific resource provider. This information usually consists of the resource provider's IP address and possibly other information, such as a transport protocol identifier or port numbers. A virtual network consisting of direct connections on top of another network, established by a group of peers. An entity that is logically separate from the resource consumer that assists a resource consumer to identify a set of resource providers. Some P2P applications refer to the resource directory as a P2P tracker. Information about the location of a host in the network topology. The ALTO service gives recommendations based on this information. A host location attribute may consist of, for example, an IP address, an address prefix or address range that contains the host, an autonomous system (AS) number, or any other localization attribute. These different options may provide different levels of detail. Depending on the system architecture, this may have implications on the quality of the recommendations ALTO is able to provide, on whether recommendations can be aggregated, and on how much privacy-sensitive information about users might be disclosed to additional parties. Several resource providers may be able to provide the same resource. The ALTO service gives guidance to a resource consumer or resource directory about which resource provider(s) to select in order to optimize the client's performance or quality of experience while optimizing resource consumption in the underlying network infrastructure. A logical entity that provides interfaces to query the ALTO service. The logical entity that sends ALTO queries. Depending on the architecture of the application one may embed it in the resource consumer or in the resource directory. A message sent from an ALTO client to an ALTO server, which requests guidance from the ALTO Service. A message sent from an ALTO server to an ALTO client, which contains guiding information from the ALTO service. An ALTO transaction consists of an ALTO query and the corresponding ALTO response. Traffic that stays within the network infrastructure of one Internet Service Provider (ISP). This type of traffic usually results in the least cost for the ISP. Internet traffic exchanged by two Internet Service Providers whose networks connect directly. Apart from infrastructure and operational costs, peering traffic is often free to the ISPs, within the contract of a peering agreement. Internet traffic exchanged on the basis of economic agreements amongst Internet Service Providers (ISP). An ISP generally pays a transit provider for the delivery of traffic flowing between its network and remote networks to which the ISP does not have a direct connection. A protocol used by the application for establishing an overlay network between the peers and exchanging data on it, as well as for data exchange between peers and resource directories if applicable. These protocols play an important role in the overall ALTO architecture. However, defining them is out of the scope of the ALTO WG. The protocol used for sending ALTO queries and ALTO replies between an ALTO client and ALTO Server. A protocol used for populating the ALTO server with topology-related information. The protocol used for synchronization, query forwarding, or referral between ALTO servers that have been provisioned with only partial knowledge of the topology-related information (e.g., on a per-domain basis).

Figure 1 shows the scope of the ALTO client protocol: Peers or super-peers can use such a protocol to query an ALTO-service. The mapping of topological information onto an ALTO service as well as the application protocol interaction between peers and super-peers are out of scope for the ALTO client protocol.

Network engineers have been facing the problem of traffic optimization for a long time and have designed mechanisms like MPLS and DiffServ to deal with it. The problem these protocols address consists in finding (or setting) optimal routes for packets traveling between specific source and destination addresses and based on requirements such as low latency, high reliability, and priority. Such solutions are usually implemented at the link and network layers, and tend to be almost transparent. At best, applications can only "mark" the traffic they generate with the corresponding properties. However, P2P applications that are today posing serious challenges to Internet infrastructures do not benefit much from the above route-based techniques. Cooperating with external services aware of the network topology could greatly optimize the traffic the P2P application generates. In fact, when a P2P application needs to establish a connection, the logical target is not a host, but rather a resource (e.g., a file or a media relay) that is often available in multiple instances on different peers. Selection of the closest one -- or, in general, the best from an overlay topological proximity -- has much more impact on the overall traffic than the route followed by its packets to reach the endpoint. Optimization of peer selection is particularly important in the initial phase of the process. Consider a P2P protocol such as BitTorrent, where a querying peer receives a list of candidate destinations where a resource resides. From this list, the peer will derive a smaller set of candidates to connect to and exchange information with. In another example, a streaming video client may be provided with a list of destinations from which it can stream content. In both cases, the use of topology information in an early stage will allow applications to improve their performance and will help ISPs make a better use of their network resources. In particular, an economic goal for ISPs is to reduce the transit traffic on interdomain links. Addressing the Application-Layer Traffic Optimization (ALTO) problem means, on the one hand, deploying an ALTO service to provide applications with information regarding the underlying network and, on the other hand, enhancing applications in order to use such information to perform better-than-random selection of the endpoints they establish connections with.

File sharing applications allow users to search for content shared by other users and download it. Typically, search results consist of many instances of the same file (or chunk of a file) available from multiple sources. The goal of an ALTO solution is to help peers find the best ones according to the underlying networks. On the application side, integration of ALTO functionalities may happen at different levels. For example, in the completely decentralized Gnutella network, selection of the best sources is totally up to the user. In systems like BitTorrent and eDonkey, central elements such as trackers or servers act as mediators. Therefore, in the former case, optimization would require modification in the applications, while in the latter it could just be implemented in some central elements.

Providers of popular content like media and software repositories usually resort to geographically distributed caches and mirrors for load balancing. Selection of the proper mirror/cache for a given user is today based on inaccurate geolocation data, on proprietary network location systems or often delegated to the user himself. An ALTO solution could be easily adopted to ease such a selection in an automated way.

P2P applications for live streaming allow users to receive multimedia content produced by one source and targeted to multiple destinations, in a real-time or near-real-time way. This is particularly important for users or networks that do not support multicast. Peers often participate in the distribution of the content, acting as both receivers and senders. The goal of an ALTO solution is to help peers to find the best sources and the best destinations for media flows they receive and relay.

P2P real-time communications allow users to establish direct media flows for real-time audio, video, and real-time text calls or to have text chats. In the basic case, media flows directly between the two endpoints. However, unfortunately a significant portion of users have limited access to the Internet due to NATs, firewalls or proxies. Thus, other elements need to relay the media. Such media relays are distributed over the Internet with a public addresses. An ALTO solution needs to help peers to find the best relays.

Distributed hash tables (DHT) are a class of overlay algorithms used to implement lookup functionalities in popular P2P systems, without using centralized elements. In such systems, peers maintain addresses of other peers participating in the same DHT in a routing table, sorted according to specific criteria. An ALTO solution will provide valuable information for DHT algorithms.

This section introduces some aspects of the problem that some people may not be aware of when they first start studying the problem space. The goal of an ALTO service is to provide applications with information they can use to perform better-than-random peer selection.

At least three different kinds of entities can provide ALTO services: Network operators. Network operators usually have full knowledge of the network they administer and are aware of their network topology and transit and peering traffic policies. Third parties. Third parties are entities separate from network operators, but which may have either collected network information or have arrangements with network operators to learn the network information. Examples of such entities are content delivery networks like Akamai, which control wide and highly distributed infrastructures, or companies providing an ALTO service on behalf of ISPs. User communities. User communities run distributed algorithms, for example for estimating the topology of the Internet.

It is important for the reader to understand there are significant user communities that expect an ALTO Server to be a centralized service. Likewise, there are other user communities that expect a service that services P2P applications be itself a distributed, possibly even a P2P, service. A result of this is one can reasonably expect there to be some sort of service discovery mechanism to go along with the ALTO protocol definition.

On the one hand, there are data elements an ALTO client could provide in its query to an ALTO server that could help increase the level of accuracy in the replies. For example, if the querying client indicates what kind of application it is using (e.g. real-time communications or bulk data transfer), the server will be able to indicate priorities in its replies accommodating the requirements of the traffic the application will generate. On the other hand, applications might consider such information private. In addition, some applications may not know a priori what kind of request they will be making.

Operators, with their intimate knowledge of their network topology, can play an important role in addressing the ALTO problem. However, operators often consider such network information to be confidential.

Caching is a common approach to optimizing traffic generated by applications that require large data transfers. In some cases, such techniques have proven to be extremely effective in both enhancing user experience and saving network resources. A cache, either explicitly or transparently, replaces the content source. Thus, the cache must use the same protocol as the querying peer. That is, if a cache stores web content, it must present an HTTP interface to the web client. Any cache solution for a given protocol needs to present that same protocol to the client. Said differently, each caching solution for a different protocol needs to implement that specific protocol. For this reason, one can only reasonably expect caching solutions for the most popular protocols, such as HTTP and BitTorrent. It is extremely important to realize that caching and ALTO are entirely orthogonal. ALTO, especially if it is aware of caches, can in fact direct clients to nearby caches where the user could get a much better quality of experience.

This document is neither a requirements document nor a protocol specification. However, we believe it is important for the reader to understand areas of security and privacy that will be important for the design and implementation of an ALTO solution. Moreover, issues such as digital rights management are out of scope for ALTO, as they are not technically enforceable at this level. The approach proposed in this document asks P2P applications to delegate a portion of their routing capability to third parties. This gives the third party a significant role in P2P systems. In the case where the network operator deploys an ALTO solution, it the P2P community might consider it hostile because the operator could, for example: use ALTO to prevent content distribution and enforce copyrights; redirect applications to corrupted mediators providing malicious content; track connections to perform content inspection or logging; and apply policies based on criteria other than network efficiency. For example, the service provider may suggest routes sub-optimal from the user's perspective to avoid peering points regulated by inconvenient economic agreements. It is important to note there is no protocol mechanism to require ALTO for P2P applications. If, for some reason, ALTO fails to improve the performance of P2P applications, ALTO will not gain popularity and the P2P community will not use it. At the time of this writing, the privacy issues described in the previous section are relevant for an ALTO solution. Users may be reluctant to disclose sensitive information to an ALTO server. Operators, on the other hand, may not wish to disclose information that would expose details of their interior topology. When exploring the solution space in detail, one needs to consider these issues so that an ALTO protocol does not presume mandatory information disclosure, by either clients or servers.

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The basis of this document is draft-marocco-alto-problem-statement, written by Enrico Marocco and Vijay Gurbani. They continue to provide significant edits and inputs to the current document editors.

Vinay Aggarwal and the P4P working group conducted the research work done outside the IETF. Emil Ivov, Rohan Mahy, Anthony Bryan, Stanislav Shalunov, Laird Popkin, Stefano Previdi, Reinaldo Penno, Dimitri Papadimitriou, Sebastian Kiesel, Greg DePriest and many others provided insightful discussions, specific comments and much needed corrections. Jan Seedorf and Sebastian Kiesel are partially supported by the NAPA-WINE project (Network-Aware P2P-TV Application over Wise Networks, http://www.napa-wine.org), a research project supported by the European Commission under its 7th Framework Program (contract no. 214412). The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the NAPA-WINE project or the European Commission. Thanks in particular to Richard Yang for several reviews.