Application-Layer Traffic Optimization
Application-Layer Traffic Optimization WG
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The ALTO working group was established in 2008 to devise a
request/response protocol for allowing a host to benefit from a server
that is more cognizant of the network infrastructure than the host
would be. The working group has developed an HTTP-based protocol
to allow hosts to benefit from the network infrastructure
by having access to a pair of maps: a topology map and a cost map.
The origins of the ALTO protocol lie in peer-to-peer (P2P)
applications, where the host is a peer in a P2P network and desires a
rendezvous with other peers for file sharing, real-time
communications, etc. ALTO is now being considered as a solution for
problems outside the P2P domain, such as in datacenter networks and
in content distribution networks (CDN) where exposing abstract
topologies helps applications.
To support the emerging new uses of ALTO, certain extensions are being
sought. These extensions can be classified as follows:
o Protocol extensions for reducing the volume of on-the-wire data
exchange required to align the ALTO server and
clients. Extensions under consideration are mechanisms for
delivering server-initiated notifications and partial updates of
maps. Efforts developed in other working groups such as
Websockets and JSON-patch will be considered, as well as bespoke
mechanisms specific to the ALTO protocol.
o One or more alternatives to the base ALTO server discovery mechanism
(RFC-to-be) to accommodate environments where (1) timely deployment
of existing mechanisms, including the base ALTO server discovery
mechanism, is unlikely, and/or (2) it is desirable for an ALTO
client to be able to discover an ALTO server outside its own
domain. The WG will consider mechanisms that are in use or
defined by other WGs. If such discovery mechanisms can be reused,
the WG will produce one or more documents to specify how they may
be adopted as additional or alternative ALTO server discovery
mechanisms. In the absence of such existing work, the WG will
develop one or more ALTO-specific server discovery mechanisms.
However, developing a general-purpose service discovery mechanism
is not in scope.
o Protocol extensions to convey a richer set of attributes to allow
applications to determine not only "where" to connect but also
"when" to connect. Such additional information will be related
both to endpoints (e.g. conveying server load and cache
geo-location information for CDN use cases) and to
endpoint-to-endpoint costs (e.g. bandwidth calendaring to
represent time-averaged cost values in datacenter networks).
The working group will specify such extension in coordination
with other working groups that have a focus on the related use
cases. The scope of extensions is not limited to those
identified by the WGs, but is limited by the criteria set out
o A document specifying how a graph representation format
(originating, say, from a YANG data model) can be used in ALTO and
optionally be exported by an ALTO server in addition to network
and cost maps. The graph representation will be based on existing
ALTO abstraction (e.g., PIDs) and complement existing
path-based ALTO cost map representation. Together, they provide
a more complete, potentially more compact, but still abstract
representation of networks for informed traffic optimization
among endpoints. In settings with multiple application source-
destination pairs with shared links, such a representation will
help avoid bottleneck (or failed) links. The WG will not
consider, nor will it model, topology internals not affecting
endpoints (e.g., routing protocol internals or RIB data).
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 that service?
- Can a client get that information without excessive privacy and
information leakage concerns? Extensions defining new endpoint
properties should focus on exposing attributes of endpoints
that are related to the goals of ALTO -- optimization of
application-layer traffic -- as opposed to more general
properties of endpoints. privacy and information leakage aspects
of new endpoint properties will in any case be evaluated to the
guidelines provided in the IANA considerations and Security
Considerations of the ALTO protocol specification (RFC-to-be,
sections 14.3 and 15.4 at IESG review time).
- Is it information that a client cannot find easily some other
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
Issues related to the specific content exchanged in systems that make
use of ALTO are also excluded from the WG's scope, as is the issue
dealing with enforcing the legality of the content.