A Cost Mode Registry for the Application-Layer Traffic Optimization (ALTO) Protocol

The cost mode attribute indicates how costs should be interpreted when communicated in the Application-Layer Traffic Optimization (ALTO) protocol . The base ALTO specification includes a provision for only two modes: Indicates that numerical operations can be performed (e.g., normalization) on the returned costs (Section 6.1.2.1 of ). Indicates that the cost values in a cost map represent ranking (relative to all other values in a cost map), not actual costs (Section 6.1.2.2 of ). Additional cost modes are required for specific ALTO deployment cases (e.g., ). In order to allow for such use cases, this document creates a new ALTO registry to track new cost mode values () and relaxes the constraints imposed by the base ALTO specification on allowed cost mode values ().

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here. This document makes use of the terms defined in .

This document updates Section 6.1.2 of as follows:

The cost mode attribute indicates how costs should be interpreted. This document defines two cost modes (numerical values or ordinal rankings), but additional values can be defined in the future. It is important to communicate such information to ALTO clients, as certain operations may not be valid on certain costs returned by an ALTO server. For example, it is possible for an ALTO server to return a set of IP addresses with costs indicating a ranking of the IP addresses. Arithmetic operations that would make sense for numerical values, do not make sense for ordinal rankings. ALTO clients may handle such costs differently. Cost modes are indicated in protocol messages as strings.

This document updates Section 10.5 of as follows:

A cost mode is encoded as a string. The string MUST have a value that is listed in of RFCXXXX. Note to the RFC Editor: Please replace RFCXXXX with the RFC number to be assigned to this document.

This document requests IANA to create a new subregistry entitled "ALTO Cost Modes" under the "Application-Layer Traffic Optimization (ALTO) Protocol" registry available at . The registry is initially populated with the following values:

New values MUST be assigned after IETF Review (Section 4.8 of ).

ALTO-related considerations are discussed in Section 15 of .

Many thanks to Benjamin Kaduk for spotting the issue during the review of .