Application-Layer Traffic Optimization

Document Charter Application-Layer Traffic Optimization WG (alto)
Title Application-Layer Traffic Optimization
Last updated 2020-03-25
State Approved
WG State Active
IESG Responsible AD Martin Duke
Charter Edit AD Martin Duke
Send notices to (None)


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.