Application-Layer Traffic Optimization (ALTO) Cross-Domain Server Discovery
RFC 8686

Document Type RFC - Proposed Standard (February 2020; No errata)
Authors Sebastian Kiesel  , Martin Stiemerling 
Last updated 2020-03-09
Replaces draft-kiesel-alto-xdom-disc
Stream Internet Engineering Task Force (IETF)
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Stream WG state Submitted to IESG for Publication (wg milestone: Nov 2017 - Submit alternative s... )
Document shepherd Jan Seedorf
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IESG IESG state RFC 8686 (Proposed Standard)
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Consensus Boilerplate Yes
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Send notices to Jan Seedorf <>
IANA IANA review state Version Changed - Review Needed
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Internet Engineering Task Force (IETF)                         S. Kiesel
Request for Comments: 8686                       University of Stuttgart
Category: Standards Track                                 M. Stiemerling
ISSN: 2070-1721                                                     H-DA
                                                           February 2020

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


   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 necessary to discover an ALTO server outside of
   the ALTO client's 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

   Technically, the procedure specified in this document takes one
   IP address or prefix and a U-NAPTR Service Parameter (typically,
   "ALTO:https") as parameters.  It performs DNS lookups (for NAPTR
   resource records in the "" or "" trees) and
   returns one or more URIs of information resources related to that IP
   address or prefix.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
     1.1.  Terminology and Requirements Language
   2.  ALTO Cross-Domain Server Discovery Procedure: Overview
   3.  ALTO Cross-Domain Server Discovery Procedure: Specification
     3.1.  Interface
     3.2.  Step 1: Prepare Domain Name for Reverse DNS Lookup
     3.3.  Step 2: Prepare Shortened Domain Names
     3.4.  Step 3: Perform DNS U-NAPTR Lookups
     3.5.  Error Handling
   4.  Using the ALTO Protocol with Cross-Domain Server Discovery
     4.1.  Network and Cost Map Service
     4.2.  Map-Filtering Service
     4.3.  Endpoint Property Service
     4.4.  Endpoint Cost Service
     4.5.  Summary and Further Extensions
   5.  Implementation, Deployment, and Operational Considerations
     5.1.  Considerations for ALTO Clients
     5.2.  Considerations for Network Operators
   6.  Security Considerations
     6.1.  Integrity of the ALTO Server's URI
     6.2.  Availability of the ALTO Server Discovery Procedure
     6.3.  Confidentiality of the ALTO Server's URI
     6.4.  Privacy for ALTO Clients
   7.  IANA Considerations
   8.  References
     8.1.  Normative References
     8.2.  Informative References
   Appendix A.  Solution Approaches for Partitioned ALTO Knowledge
     A.1.  Classification of Solution Approaches
     A.2.  Discussion of Solution Approaches
     A.3.  The Need for Cross-Domain ALTO Server Discovery
     A.4.  Our Solution Approach
     A.5.  Relation to the ALTO Requirements
   Appendix B.  Requirements for Cross-Domain Server Discovery
     B.1.  Discovery Client Application Programming Interface
     B.2.  Data Storage and Authority Requirements
     B.3.  Cross-Domain Operations Requirements
     B.4.  Protocol Requirements
     B.5.  Further Requirements
   Appendix C.  ALTO and Tracker-Based Peer-to-Peer Applications
     C.1.  A Generic Tracker-Based Peer-to-Peer Application
     C.2.  Architectural Options for Placing the ALTO Client
     C.3.  Evaluation
     C.4.  Example
   Authors' Addresses

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