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Internet Engineering Task Force (IETF)                      M. Boucadair
Request for Comments: 9274                                        Orange
Updates: 7285                                                      Q. Wu
Category: Standards Track                                         Huawei
ISSN: 2070-1721                                                July 2022


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

Abstract

   This document creates a new IANA registry for tracking cost modes
   supported by the Application-Layer Traffic Optimization (ALTO)
   Protocol.  Also, this document relaxes a constraint that was imposed
   by the ALTO specification on allowed cost mode values.

   This document updates RFC 7285.

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
   https://www.rfc-editor.org/info/rfc9274.

Copyright Notice

   Copyright (c) 2022 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
   (https://trustee.ietf.org/license-info) 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 Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Updates to RFC 7285
     3.1.  Updates to Section 6.1.2 of RFC 7285
     3.2.  Updates to Section 10.5 of RFC 7285
   4.  Backward Compatibility Considerations
   5.  IANA Considerations
   6.  Security Considerations
   7.  References
     7.1.  Normative References
     7.2.  Informative References
   Acknowledgements
   Authors' Addresses

1.  Introduction

   The cost mode attribute indicates how costs should be interpreted
   when communicated as described in "Application-Layer Traffic
   Optimization (ALTO) Protocol" [RFC7285], which 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., [ALTO-PV]).  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 5).

   The mechanisms defined in [RFC7285] are used to advertise the support
   of new cost modes for specific cost metrics.  Refer to Section 4 for
   more details.

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 RFC 7285


3.1.  Updates to Section 6.1.2 of RFC 7285

   This document updates Section 6.1.2 of [RFC7285] as follows:

   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:

      The cost mode attribute indicates how costs should be interpreted.
      Two cost modes (numerical values and ordinal rankings) are
      defined, but additional cost modes 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.

      For any future documents that defines a new cost mode, indicating
      whether that new cost mode applies to all or a subset of cost
      metrics is strongly recommended.  This recommendation is meant to
      prevent nondeterministic behaviors that may result in presenting a
      cost mode with a specific metric, while such an association does
      not make sense or can't be unambiguously interpreted by ALTO
      implementations.

      If the definition of a cost mode does not indicate whether that
      cost mode applies to a subset of cost metrics, ALTO
      implementations MUST be prepared to accept that cost mode for any
      cost metric.


3.2.  Updates to Section 10.5 of RFC 7285

   This document updates Section 10.5 of [RFC7285] as follows:

   OLD:

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

   NEW:

      A cost mode is encoded as a string.  The string MUST be no more
      than 32 characters, and it MUST NOT contain characters other than
      US-ASCII alphanumeric characters (U+0030-U+0039, U+0041-U+005A,
      and U+0061-U+007A), the hyphen-minus ('-', U+002D), the colon
      (':', U+003A), or the low line ('_', U+005F).  Cost modes reserved
      for Private Use are prefixed with "priv:" (Section 5).  Otherwise,
      the cost mode MUST have a value that is listed in the registry
      created in Section 5 of [RFC9274].


4.  Backward Compatibility Considerations

   ALTO servers that support new cost modes for specific cost metrics
   will use the mechanism specified in Section 9.2 of [RFC7285] to
   advertise their capabilities.  ALTO clients (including legacy) will
   use that information to specify cost constraints in their requests
   (e.g., indicate a cost metric and a cost mode).  An example of such a
   behavior is depicted in Section 9.2.3 of [RFC7285].

   If an ALTO client includes a cost mode that is not supported by an
   ALTO server, the server indicates such an error with the error code
   E_INVALID_FIELD_VALUE as per Section 8.5.2 of [RFC7285].  In
   practice, legacy ALTO servers will reply with the error code
   E_INVALID_FIELD_VALUE to requests that include a cost type other than
   "numerical" or "ordinal" for the "routingcost" cost metric.

   The encoding constraints in Section 3.2 do not introduce any
   interoperability issue given that currently implemented cost modes
   adhere to these constrains (mainly, those in [RFC7285] and
   [ALTO-PV]).

5.  IANA Considerations

   IANA has created the new "ALTO Cost Modes" subregistry within the
   "Application-Layer Traffic Optimization (ALTO) Protocol" registry
   available at [ALTO].

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

   Requests to register a new ALTO cost mode must include the following
   information:

   Identifier:  The name of the ALTO cost mode.  Refer to Section 3.2
      for more details on allowed encoding.

   Description:  A short description of the requested ALTO cost mode.

   Intended Semantics:  A reference to where the semantic of the
      requested cost mode is defined.

   Reference:  A reference to the document that registers the requested
      cost mode.

   Cost modes prefixed with "priv:" are reserved for Private Use
   (Section 4.1 of [RFC8126]).  IANA has added the following note to the
   new subregistry:

   |  Identifiers prefixed with "priv:" are reserved for Private Use
   |  (see RFC 9274, Section 5).

   The subregistry is initially populated with the following values:

   +============+=============================+============+===========+
   | Identifier | Description                 | Intended   | Reference |
   |            |                             | Semantics  |           |
   +============+=============================+============+===========+
   | numerical  | Indicates that numerical    | Section    | RFC 9274  |
   |            | operations can be performed | 6.1.2.1    |           |
   |            | on the returned costs       | of         |           |
   |            |                             | [RFC7285]  |           |
   +------------+-----------------------------+------------+-----------+
   | ordinal    | Indicates that the cost     | Section    | RFC 9274  |
   |            | values in a cost map        | 6.1.2.2    |           |
   |            | represent ranking           | of         |           |
   |            |                             | [RFC7285]  |           |
   +------------+-----------------------------+------------+-----------+

                          Table 1: ALTO Cost Modes

6.  Security Considerations

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

7.  References

7.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7285]  Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
              Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
              "Application-Layer Traffic Optimization (ALTO) Protocol",
              RFC 7285, DOI 10.17487/RFC7285, September 2014,
              <https://www.rfc-editor.org/info/rfc7285>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2.  Informative References

   [ALTO]     IANA, "Application-Layer Traffic Optimization (ALTO)
              Protocol",
              <https://www.iana.org/assignments/alto-protocol/>.

   [ALTO-PV]  Gao, K., Lee, Y., Randriamasy, S., Yang, Y. R., and J. J.
              Zhang, "An ALTO Extension: Path Vector", Work in Progress,
              Internet-Draft, draft-ietf-alto-path-vector-25, 20 March
              2022, <https://datatracker.ietf.org/doc/html/draft-ietf-
              alto-path-vector-25>.

Acknowledgements

   Many thanks to Benjamin Kaduk for spotting the issue during the
   review of [ALTO-PV].

   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.

   Thanks to Martin Duke for the AD review.

   Thanks to Roni Even for the gen-art review, Jaime Jimenez for the
   artart review, and Stephen Farrell for the secdir review.

   Thanks to Robert Wilton, Lars Eggert, Francesca Palombini, Roman
   Danyliw, Paul Wouters, and Murray Kucherawy for the IESG review.

Authors' Addresses

   Mohamed Boucadair
   Orange
   35000 Rennes
   France
   Email: mohamed.boucadair@orange.com


   Qin Wu
   Huawei
   Yuhua District
   101 Software Avenue
   Nanjing
   Jiangsu, 210012
   China
   Email: bill.wu@huawei.com