Application-Layer Traffic Optimization
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Application-Layer Traffic Optimization WG
||Application-Layer Traffic Optimization
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A significant part of the Internet traffic today is generated by
peer-to-peer (P2P) applications used for file sharing, real-time
communications, and live media streaming. P2P applications exchange
large amounts of data, often uploading as much as downloading. In
contrast to client/server architectures, P2P applications often must
choose one or more suitable candidates from a selection of peers
offering the same resource or service.
One of the advantages of P2P systems comes from redundancy in resource
availability. This requires choosing among a list of peers, yet
applications have at best incomplete information to help the
selection, e.g., topology of the network.
Applications can sometimes obtain network information dynamically or
measure link performance with respect to particular peers, but even
when this is an option it takes time. The application cannot always
start out with an optimal arrangement of peers, thus risking at least
temporary poor performance and excessive cross-domain traffic.
Providing more information for use in peer selection can
improve P2P performance and lower ISP costs.
The Working Group will design and specify an Application-Layer Traffic
Optimization (ALTO) service that will provide applications with
information to perform better-than-random initial peer selection.
ALTO services may take different approaches at balancing factors such
as maximum bandwidth, minimum cross-domain traffic, lowest cost to the
user, etc. The WG will consider the needs of BitTorrent, tracker-less
P2P, and other applications, such as content delivery networks (CDN)
and mirror selection.
The WG will focus on the following items:
- A "problem statement" document providing a description of the
problem and a common terminology.
- A requirements document. This document will list requirements for
the ALTO service, identifying, for example, types of information P2P
applications may need for optimizing their choices.
- A request/response protocol for querying the ALTO service to obtain
information useful for peer selection, and a format for requests and
responses. If the requirements analysis identifies the need to allow
clients to delegate third-parties to query the ALTO service on their
behalf, the WG will ensure that the protocol provides a mechanism to
assert the consent of the delegating client.
- A specification of core request and response formats and semantics
to communicate network preferences to applications. Since ALTO
services may be run by entities with different levels of knowledge
about the underlying network, such preferences may have different
representations. Initially the WG will consider: IP ranges to prefer
and to avoid, ranked lists of the peers requested by the client,
information about topological proximity and approximate geographic
locations. Other usages will be considered as charter additions once
the work for the initial services has been completed.
- In order to query the ALTO server, clients must first know one or
more ALTO servers that might provide useful information. The WG will
look at service discovery mechanisms that are in use, or defined
elsewhere (e.g. based on DNS SRV records or DHCP options). If such
discovery mechanisms can be reused, the WG will produce a document to
specify how they may be adopted for locating such servers. However, a
new, general-purpose service discovery mechanism is not in scope.
- An informational document discussing deployment related issues and
documenting lessons learned from early implementation experiences.
When the WG considers standardizing information that the ALTO server
could provide, the following criteria are important to ensure real
- Can the ALTO service realistically discover that information?
- Is the distribution of that information allowed by the operators of
- Is it information that a client will find useful?
- Can a client get that information without excessive privacy concerns
(e.g. by sending large lists of peers)?
- Is it information that a client cannot find easily some other way?
After these criteria are met, the importance of the data will be
considered for prioritizing standardization work, for example the
number of operators and clients that are likely to be able to provide
or use that particular data. In any case, this WG will not propose
standards on how congestion is signaled, remediated, or avoided, and
will not deal with information representing instantaneous network
state. Such issues belong to other IETF areas and will be treated
accordingly by the specific area.
This WG will focus solely on the communication protocol between
applications and ALTO servers. Note that ALTO services may be useful
in client-server environments as well as P2P environments, although
P2P environments are the first focus. If, in the future, the IETF
considers changes to other protocols for actually implementing ALTO
services (e.g. application-layer protocols for Internet coordinate
systems, routing protocol extensions for ISP-based solutions), such
work will be done in strict coordination with the appropriate WGs.
Issues related to the content exchanged in P2P systems are also
excluded from the WG's scope, as is the issue dealing with enforcing
the legality of the content.