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

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Table of Contents

1. Introduction

The cost mode attribute indicates how costs should be interpreted when communicated in the Application-Layer Traffic Optimization (ALTO) protocol [RFC7285]. The base ALTO specification includes a provision for only two modes:¶

"numerical":

Indicates that numerical operations can be performed (e.g., normalization) on the returned costs (Section 6.1.2.1 of [RFC7285]).¶

"ordinal":

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 [RFC7285]).¶

Additional cost modes are required for specific ALTO deployment cases (e.g., [I-D.ietf-alto-path-vector]). In order to allow for such use cases, this document relaxes the constraint imposed by the base ALTO specification on allowed cost modes (Section 3) and creates a new ALTO registry to track new cost modes (Section 4).¶

2. Terminology

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 [RFC2119][RFC8174] when, and only when, they appear in all capitals, as shown here.¶

This document makes use of the terms defined in [RFC7285].¶

3. Updates to RFC7285

OLD:
   The cost mode attribute indicates how costs should be interpreted.
   Specifically, the cost mode attribute indicates whether returned
   costs should be interpreted as numerical values or ordinal rankings.

   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.

NEW:

OLD:
   A cost mode is encoded as a string.  The string MUST have a value of
   either "numerical" or "ordinal".

NEW:

4. IANA Considerations

This document requests IANA to create a new subregistry entitled "ALTO Cost Modes" under the "Application-Layer Traffic Optimization (ALTO) Protocol" registry available at [ALTO].¶

The registry is initially populated with the following values:¶

+===========-+=============================+====================+
| Identifier | Description                 | Intended Semantics |
+============+=============================+====================+
| numerical  | Indicates that numerical    | Section 6.1.2.1    |
|            | operations can be performed |  of RFC7285        |
|            | on the returned costs       |                    |
+------------+-----------------------------+--------------------+
| ordinal    | Indicates that the cost     | Section 6.1.2.2    |
|            | values in a cost map        |  of RFC7285        |
|            | represent ranking           |                    |
+------------+-----------------------------+--------------------+

The assignment policy for this registry is "IETF Review" (Section 4.8 of [RFC8126]).¶

Cost modes prefixed with "priv:" are reserved for Private Use (Section 4.1 of [RFC8126]).¶

5. Security Considerations

This document does not introduce new concerns other than those already discussed in Section 15 of [RFC7285].¶

6. Acknowledgements

Many thanks to Benjamin Kaduk for spotting the issue during the review of [I-D.ietf-alto-path-vector].¶

Thanks to Adrian Farrel, Dhruv Dhody, Luis Miguel Contreras Murillo, Sabine Randriamasy, and Qiao Xiang for the review and comments.¶

Special thanks to Kai Gao for Shepherding the document.¶

Authors' Addresses