ALTO Extension: Composition Mode of Cost Maps

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Table of Contents

1. Introduction

The Application-Layer Traffic Optimization (ALTO) protocol provides abstractions for application operators and/or end users to query network distance or property information. Specifically, ALTO has defined network map and cost map, which typically are used together, to provide a prediction model of distance information between endpoints in a network.¶

Given the scale of the Internet today, it is unlikely that the prediction model can overfit. Thus, with higher model complexity, an ALTO service tends to provide better accuracy from the same implementation method. As a consequence, operators of the ALTO maps have to make the trade-off between service quality (accuracy of the predicated value) and model complexity (sizes of the maps).¶

Currently, there is no standard way of composing the prediction results from multiple ALTO cost maps. Clients either only request a single pair of network and cost maps, or blindly select ALTO maps and compose the results. These approaches either make inefficient trade-offs, i.e., achieving substantial lower accuracy gains than occupied map sizes, or make incorrect use of the servers' exposed maps, i.e., the composition mode is different from how the server internally constructs the models.¶

This extension is motivated by the ensemble method in machine learning [ENSEMBLE]. Ensemble method uses multiple prediction models to improve the "efficiency" and can typically achieve higher accuracy with the same model complexity. When the models are composed (or "ensembled") using the boosting method [BOOSTING], models are ordered and higher-order models are trained not directly with the samples but residuals (prediction errors) of lower-order models. Thus, model accuracy and model complexity typically grow simultaneously with the number of models -- in the context of ALTO, the number of maps. Thus, an ALTO server may realize differentiated service by controlling the access to higher-order maps.¶

Specifically, this extension defines a new type of ALTO resource called ALTO composition advertisement Section 4. The resource specifies the list of ALTO cost maps and how they are intended to be composed.¶

2. Conventions and Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].¶

All numeric values are in network byte order. Values are unsigned unless otherwise indicated. Literal values are provided in decimal or hexadecimal as appropriate. Hexadecimal literals are prefixed with "0x" to distinguish them from decimal literals.¶

This document reuses the terms defined in RFC 7285 [RFC7285].¶

3. Composition Modes

This document has some requirements on the cost maps that can be composed. For cost maps that satisfy these requirements, 3 different composition modes are specified to define how the results of these maps must be combined.¶

4. ALTO Composition Advertisement

    object {
        JSONString  comp-mode;
        JSONString  cost-type-names<1..*>;
        [JSONNumber weights<1..*>;]
    } CompAdvCapabilities;

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