Application Layer Traffic Optimization (ALTO) Cross-Domain Server Discovery

Document Type Replaced Internet-Draft (individual)
Authors Sebastian Kiesel  , Martin Stiemerling 
Last updated 2017-01-08 (latest revision 2016-07-07)
Replaced by RFC 8686
Stream (None)
Intended RFC status (None)
Expired & archived
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Stream Stream state (No stream defined)
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IESG IESG state Replaced by draft-ietf-alto-xdom-disc
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This Internet-Draft is no longer active. A copy of the expired Internet-Draft can be found at


The goal of Application-Layer Traffic Optimization (ALTO) is to provide guidance to applications that have to select one or several hosts from a set of candidates capable of providing a desired resource. ALTO is realized by a client-server protocol. Before an ALTO client can ask for guidance it needs to discover one or more ALTO servers that can provide suitable guidance. In some deployment scenarios, in particular if the information about the network topology is partitioned and distributed over several ALTO servers, it may be needed to discover an ALTO server outside of the own network domain, in order to get appropriate guidance. This document details applicable scenarios, itemizes requirements, and specifies a procedure for ALTO cross-domain server discovery. Technically, the algorithm specified in this document takes one IP address and a U-NAPTR Service Parameter (i.e., "ALTO:http" or "ALTO:https") as parameters. It performs DNS lookups (for NAPTR resource records in the or tree) and returns one or more URI(s) of information resources related to that IP address.


Sebastian Kiesel (
Martin Stiemerling (

(Note: The e-mail addresses provided for the authors of this Internet-Draft may no longer be valid.)