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Versions: (draft-randriamasy-alto-multi-cost) 00 01 02 03 04 05 06 07 08 09 10 RFC 8189

Network Working Group                                S. Randriamasy, Ed.
Internet-Draft                                             W. Roome, Ed.
Intended status: Standards Track                          Alcatel-Lucent
Expires: April 21, 2016                                        N. Schwan
                                                      Thales Deutschland
                                                        October 19, 2015


                            Multi-Cost ALTO
                     draft-ietf-alto-multi-cost-01

Abstract

   The ALTO (Application Layer-Traffic Optimization) Protocol
   ([RFC7285]) defines several services that return various metrics
   describing the costs between network endpoints.  For example, when
   downloading a file that is mirrored on several sites, a user
   application may use these ALTO cost metrics to determine the most
   efficient mirror site.

   An ALTO Server may offer a variety of cost metrics, based on latency,
   bandwidth, hop count, jitter, or whatever else the ALTO Server deems
   useful.  When selecting a mirror site, a client may consider more
   than one metric, perhaps trading bandwidth for latency.  While the
   base ALTO Protocol allows a client to use more than one cost metric,
   to do so, the client must request each metric separately.  This
   document defines a new service that allows a client to retrieve
   several cost metrics with one request, which is considerably more
   efficient.  In addition, this document extends the ALTO constraint
   tests to allow a user to specify an arbitrary logical combination of
   tests on several cost metrics.

Requirements Language

   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].

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.




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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 21, 2016.

Copyright Notice

   Copyright (c) 2015 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
   (http://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 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview Of Approach  . . . . . . . . . . . . . . . . . . . .   5
     3.1.  Multi-Cost Data Format  . . . . . . . . . . . . . . . . .   5
     3.2.  Compatibility With Legacy Clients . . . . . . . . . . . .   6
     3.3.  Filtered Multi Cost Map Resources . . . . . . . . . . . .   6
     3.4.  Endpoint Cost Service Resources . . . . . . . . . . . . .   7
     3.5.  Full Cost Map Resources . . . . . . . . . . . . . . . . .   7
     3.6.  Extended Constraint Tests . . . . . . . . . . . . . . . .   8
   4.  Protocol Extensions for Multi-Cost ALTO Transactions  . . . .   9
     4.1.  Filtered Cost Map Extensions  . . . . . . . . . . . . . .   9
       4.1.1.  Accept Input Parameters . . . . . . . . . . . . . . .   9
       4.1.2.  Capabilities  . . . . . . . . . . . . . . . . . . . .  12
       4.1.3.  Response  . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Endpoint Cost Service Extensions  . . . . . . . . . . . .  13
       4.2.1.  Accept Input Parameters . . . . . . . . . . . . . . .  13
       4.2.2.  Capabilities  . . . . . . . . . . . . . . . . . . . .  14
       4.2.3.  Response  . . . . . . . . . . . . . . . . . . . . . .  14
   5.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Information Resource Directory  . . . . . . . . . . . . .  14
     5.2.  Multi-Cost Filtered Cost Map: Example #1  . . . . . . . .  16
     5.3.  Multi-Cost Filtered Cost Map: Example #2  . . . . . . . .  17
     5.4.  Multi-Cost Filtered Cost Map: Example #3  . . . . . . . .  18
     5.5.  Endpoint Cost Service . . . . . . . . . . . . . . . . . .  20



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   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  22
   7.  Privacy And Security Considerations . . . . . . . . . . . . .  22
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  22
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  22
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   IETF has designed a new service called ALTO that provides guidance to
   overlay applications, which have to select one or several hosts from
   a set of candidates that are able to provide a desired resource.
   This guidance is based on parameters such the topological distance,
   that affect performance and efficiency of the data transmission
   between the hosts.  The purpose of ALTO is to improve Quality of
   Experience (QoE) in the application while reducing resource
   consumption in the underlying network infrastructure.  The ALTO
   protocol conveys the Internet View from the perspective of a Provider
   Network region that spans from a region to one or more Autonomous
   System (AS) and is called a Network Map. ALTO may also provide the
   Provider determined Cost Map between locations of the Network Map or
   Endpoint Cost Map between groups of individual endpoints.  Last,
   these costs are provided as numerical or ordinal values.

   Current ALTO Costs and their modes provide values that are seen to be
   stable over a longer period of time, such as hopcount and
   administrative routing cost to reflect ISP routing preferences.
   Recently, new use cases have extended the usage scope of ALTO to
   Content Delivery Networks, Data Centers and applications that need
   additional information to select their Endpoints or handle their
   PIDs.

   Thus a multitude of new Cost Types that better reflect the
   requirements of these applications are expected to be specified, in
   particular cost values that change more frequently than previously
   assumed.

   The ALTO protocol [RFC7285] restricts ALTO Cost Maps and Endpoint
   Cost services to only one Cost Type and Cost Mode per ALTO request.
   To retrieve information for several Cost Types, an ALTO client must
   send several separate requests to the server.

   It would be far more efficient, in terms of RTT, traffic, and
   processing load on the ALTO client and server, to get all costs with
   a single query/response transaction.  Vector costs provide a robust
   and natural input to multi-variate path computation as well as robust
   multi-variate selection of multiple Endpoints.  In particular, one



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   Cost Map reporting on N Cost Types is less bulky than N Cost Maps
   containing one Cost Type each.  This is valuable for both the storage
   of these maps and their transmission.  Additionally, for many
   emerging applications that need information on several Cost Types,
   having them gathered in one map will save time.  Another potential
   advantage is consistency: provide values on several Cost Types in one
   single batch is useful for Clients needing synchronized ALTO
   information updates.

   Along with multi-cost values queries, the filtering capabilities need
   to be extended to allow constraints on multiple metrics.  The base
   protocol allows a client to provide optional constraint tests for a
   Filtered Cost Map or the Endpoint Cost Service.  In the base
   protocol, the constraint tests are limited to the AND-combination of
   simple comparison tests on the value of the (single) requested Cost
   Type.  It is therefore necessary to allow constraints on multiple
   metrics.  Beyond that, applications that are sensitive to several
   metrics and struggle with complicated network conditions may need to
   arbitrate between conflicting objectives such as routing cost and
   network performance.  To address this issue, this document proposes
   to extend the base protocol by extending constraints to test multiple
   metrics, and by allowing these constraints to be combined with
   logical 'ORs' as well as logical 'ANDs'.  This allows an application
   to make requests such as: "select solutions with either (moderate
   "hopcount" AND high "routingcost") OR (higher "hopcount" AND moderate
   "routingcost")".  To ensure compatibility with legacy ALTO Clients,
   Multi-Cost extensions are specified for the Filtered Cost Map service
   and the Endpoint Cost Map service only, while enabling to provide
   full Cost maps as well.

   This document is organized as follows: Section 2 defines terminology
   used in this document.  Section 3 gives a non-normative overview of
   the multi-cost extensions, and Section 4 gives their formal
   definition.  Section 5 gives several complete examples.  The
   remaining sections describe the IANA and privacy considerations.

2.  Terminology

   This document uses terms defined as follows:

   o  {1.2.3}: References of this form are to sections in the ALTO
      protocol specification [RFC7285].

   o  When refering to a section of this document, the format 1.2.3,
      without brackets, will be used.

   o  Endpoint (EP): can be a Peer, a CDN storage location, a physical
      server involved in a virtual server-supported application, a Party



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      in a resource sharing swarm such as a computation Grid or an
      online multi-party game.

   o  Endpoint Discovery (EP Discovery) : this term covers the different
      types of processes used to discover the eligible endpoints.

   o  Network Service Provider (NSP): includes both ISPs, who provide
      means to transport the data, and Content Delivery Networks (CDNs)
      who care for the dissemination, persistent storage and possibly
      identification of the best/closest content copy.

   o  ALTO transaction: a request/response exchange between an ALTO
      Client and an ALTO Server.

   o  Application Client (AC): this term generalizes the case of a P2P
      client to include the case of a CDN client, a client of an
      application running on a virtual server, a Grid application client
      and any Client having the choice in several connection points for
      data or resource exchange.

3.  Overview Of Approach

   The following is a non-normative overview of the multi-cost
   extensions defined in this document.  It assumes the reader is
   familiar with Cost Map resources in the ALTO Protocol ([RFC7285]).

3.1.  Multi-Cost Data Format

   Formally, the cost entries in an ALTO Cost Map can be any type of
   JSON value (see the DstCosts object in {11.2.3.6}).  However, that
   section also says that an implementation may assume costs are JSON
   numbers, unless the implementation is using an extension which
   signals a different data type.

   Therefore this document extends the definition of a Cost Map to allow
   a cost to be an array of costs, one per metric, instead of just one
   number.  For example, here is a Cost Map with the "routingcost" and
   "hopcount" metrics.  Note that this is identical to a regular ALTO
   Cost Map, except that the values are arrays instead of numbers.












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   {
    "meta" : {
      "dependent-vtags" : [ ... ],
      "multi-cost-types" : [
        {"cost-mode": "numerical", "cost-metric": "routingcost"},
        {"cost-mode": "numerical", "cost-metric": "hopcount"}
      ]
    }
    "cost-map" : {
      "PID1": { "PID1":[1,0],  "PID2":[5,23],  "PID3":[10,5] },
      ...
    }
   }

3.2.  Compatibility With Legacy Clients

   The multi-cost extensions defined in this document should not break
   legacy implementations (that is, clients and servers which are not
   aware of these extensions).  One way to achieve that would be to
   define a new media type for an array-valued Multi Cost Map. However,
   as indicated above, an array-valued Multi Cost Map is almost
   identical to a single-valued Cost Map, so it should be simple to
   write a parser which handles either type of cost map.  Hence defining
   a new media type could result in a lot of wasteful duplication.

   Therefore this document does not define any new media types.
   Instead, as described below, it extends the specifications in the
   ALTO Server's Information Resource Directory (IRD) so that legacy
   clients will not request array-valued Cost Map resources.  This
   relies on the requirement that implementations MUST ignore unknown
   fields ({8.3.7} in [RFC7285]).

3.3.  Filtered Multi Cost Map Resources

   This document extends the Filtered Cost Map service to allow the same
   resource to return either a single-valued Cost Map, as defined in
   [RFC7285], or an array-valued Multi Cost Map, as defined in this
   document.  An extended Filtered Cost Map resource has a new
   capability, "max-cost-types".  The value is the maximum number of
   cost types this resource can return for one request.  The existence
   of this capability means the resource understands the extensions in
   this document.

   For example, the following fragment from an IRD defines an extended
   Filtered Cost Map resource:






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      "filtered-multicost-map" : {
        "uri" : "http://alto.example.com/multi/costmap/filtered",
        "media-types" : [ "application/alto-costmap+json" ],
        "accepts" : [ "application/alto-costmapfilter+json" ],
        "uses" : [ "my-default-network-map" ],
        "capabilities" : {
          "max-cost-types" : 2,
          "cost-type-names" : [ "num-routingcost",
                                "num-hopcount" ],
          ...
        }

   A legacy client will ignore the "max-cost-types" capability, and will
   send a request with the input parameter "cost-type" describing the
   desired cost metric, as defined in [RFC7285].  The ALTO Server will
   return a single-valued legacy Cost Map.

   However, a multi-cost-aware client will realize that this resource
   supports the multi-cost extensions, and can send a POST request with
   the new input parameter "multi-cost-types", whose value is an array
   of cost types.  Because the request has the "multi-cost-types"
   parameter (rather than the "cost-type" parameter defined in the base
   protocol), the server realizes that the client also supports the
   extensions in this document, and hence responds with a Multi Cost
   Map, with the costs in the order listed in "multi-cost-types".

3.4.  Endpoint Cost Service Resources

   This document uses the technique described in Section 3.3 to extend
   the Endpoint Cost Service to return array-valued costs to clients who
   also are aware of these extensions.

3.5.  Full Cost Map Resources

   Full Cost Map resources are GET-mode requests, with no capabilities
   other than the name of the cost type they return.  It is therefore
   not possible to define an array-valued Full Cost Map resource so that
   multi-cost-aware clients can recognize it and legacy clients will
   ignore it.  Indeed, the response for a Full Cost Map conveying
   multiple cost types would include a "meta" field that would itself
   include a "cost-type" field, that would list several values
   corresponding to the cost types of the cost map.  A legacy client
   would not be able to understand this list.  It would not know what
   the cost type of the map is and neither would it be able to interpret
   the cost values array provided by a Multi-Cost full maps.

   However {11.3.2.3} of [RFC7285] requires a Filtered Cost Map to
   return the entire Cost Map if the client omits the source and



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   destination PIDs.  Hence a client can use an extended Filtered Cost
   Map resource to get a full Multi Cost Map.

3.6.  Extended Constraint Tests

   [RFC7285] defines a simple constraint test capability for Filtered
   Cost Maps and Endpoint Cost Services.  If a resource supports
   constraints, the server restricts the response to costs that satisfy
   a list of simple predicates provided by the client.  For example, if
   the client gives the constraints

        "constraints": ["ge 10", "le 20"]

   Then the server only returns costs in the range [10,20].

   To be useful with multi-cost requests, the constraint tests require
   several extensions.  First, because a multi-cost request involves
   more than one cost metric, the simple predicates must be extended to
   specify the metric to test.  Therefore we extend the predicate syntax
   to "[##] op value", where "##" is the index of a cost metric in this
   multi cost request.

   Second, if the client for example wants values either greater than 10
   or smaller than 20, the "AND" of simple predicates is not sufficient;
   to be useful, clients must be able to express "OR" tests.  Hence we
   add a new field, "or-constraints", to the client request.  The value
   is an array of arrays of simple predicates, and represents the OR of
   ANDs of those predicates.

   Thus the following request tells the server to limit its response to
   cost points with "routingcost" <= 100 AND "hopcount" <= 2, OR else
   "routingcost" <= 10 AND "hopcount" <= 6:

      {
        "multi-cost-types": [
            {"cost-metric": "routingcost", "cost-mode": "numerical"},
            {"cost-metric": "hopcount",    "cost-mode": "numerical"}
        ],
        "or-constraints": [
            ["[0] le 100", "[1] le 2"],
            ["[0] le 10",  "[1] le 6"]
        ],
        "pids": {...}
      }

   Finally, a client might want to test a cost type whose actual value
   is irrelevant, as long as it satisfies the tests.  For example, the




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   following request tells the server to return just "routingcost" for
   those source and destination pairs for which "hopcount" is <= 6:

      {
        "multi-cost-types": [
            {"cost-metric": "routingcost", "cost-mode": "numerical"},
        ],
        "testable-cost-types": [
            {"cost-metric": "hopcount", "cost-mode": "numerical"},
        ],
        "constraints": ["[0] le 6"],
        "pids": {...}
      }

   In this example, "[0]" means the constraint applies to "hopcount"
   because that is the first cost type in the "testable-cost-types"
   parameter.

4.  Protocol Extensions for Multi-Cost ALTO Transactions

   This section provides the formal specifications of extensions to
   RFC7285 to support Multi-Cost ALTO transactions.

4.1.  Filtered Cost Map Extensions

   This document extends Filtered Cost Maps, as defined in {11.3.2} of
   [RFC7285], by adding new input parameters and capabilities, and by
   returning JSONArrays instead of JSONNumbers as the cost values.

   The media type (11.3.2.1}, HTTP method (11.3.2.2} and "uses"
   specifications (11.3.2.5} are unchanged.

4.1.1.  Accept Input Parameters

   The ReqFilteredCostMap object in {11.3.2.3} of [RFC7285] is extended
   as follows:















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       object {
          [CostType cost-type;]
          [CostType multi-cost-types<1..*>;]
          [CostType testable-cost-types<1..*>;]
          [JSONString constraints<0..*>;]
          [JSONString or-constraints<0..*>;]
          PIDFilter pids;
       } ReqFilteredCostMap;

       object {
          PIDName srcs<0..*>;
          PIDName dsts<0..*>;
       } PIDFilter;


   cost-type:  If present, as defined in {11.3.2.3} of [RFC7285], with
      the additional requirement that the client MUST provide either
      "cost-type" or "multi-cost-types", but MUST NOT provide both.

   multi-cost-types:  If present, the ALTO Server MUST return array-
      valued costs for the cost types in thlis list.  For each entry,
      the "cost-metric" and "cost-mode" fields MUST match one of the
      supported cost types indicated in this resource's "capabilities"
      field (Section 4.1.2).  The client MUST NOT use this field unless
      this resource's "max-cost-types" capability exists and has a value
      greater than 0.  The client MUST specify either "cost-type" or
      "multi-cost-types", but MUST NOT specify both.

   testable-cost-types:  A list of cost types for extended constraint
      tests, as described for the "constraints" parameter.  If present,
      the cost types must be a subset of the cost types in the
      resource's "testable-cost-type-names" capability (Section 4.1.2).

      This feature allows a client to test a cost type whose actual
      value is irrelevant, as long as it satisfies the tests.  For
      example, a client may want a Filtered Cost Map for cost metric
      "routingcost" that is restricted to those PID pairs that satisfy
      constraints on metric "hopcount".  This feature is also useful for
      a server that does not allow constraints on all the cost types
      indicated in this resource's capabilities.

   constraints:  Unless this resource's "max-cost-types" capability
      (Section 4.1.2) is defined with a value greater than 0, this
      parameter is an array of constraint tests where each test is
      formulated as defined in {11.3.2.3} of [RFC7285].






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      This parameter MUST NOT be specified if the "or-constraints"
      parameter is specified, or if the resource's "cost-constraints"
      capability is false.

      If this resource's "max-cost-types" capability is greater than 0,
      then this parameter MUST be an array of extended constraint tests,
      where each test consists of two or three entities separated by
      white space: (1) an optional cost type index, of the form "[#]",
      with default value "[0]", (2) a required operator, and (3) a
      required target value.  The operator and target value are as
      defined in {11.3.2.3} of [RFC7285].  The cost type index specifies
      the cost type to test.  If the "testable-cost-types" parameter is
      present, assuming the index is "i", the test applies to the i'th
      cost type in "testable-cost-types" (starting with index 0).
      Otherwise, if the "multi-cost-types" parameter is present, the
      test applies to the i'th cost type in "multi-cost-types".  If
      neither of those parameters is present, the test applies to the
      cost type in the "cost-type" parameter.  In this case, the index
      MUST be 0.  Regardless of how the tested cost type is selected, it
      MUST be a cost type in the resource's "testable-cost-type-names"
      capability, or, if omitted, the resource's "cost-type-names"
      capability.

      As an example: suppose "multi-cost-types" has the single element
      "routingcost", "testable-cost-types" has the single element
      "hopcount", and "constraints" has the single element "[0] le 5".
      This is equivalent to the database query "SELECT routingcost WHERE
      hopcount <= 5".

      Note that as long as this resource's "max-cost-types" capability
      is greater than 0, a client may use the extended constraint tests
      even on single-valued cost map requests, that is, requests with
      the "cost-type" parameter rather than "multi-cost-types".

   or-constraints:  A JSONArray of JSONArrays of JSONStrings, where each
      string is a constraint test as defined for the "constraints"
      parameter.  The constraint tests are interpreted as the logical OR
      of ANDs.  That is, the ALTO Server should return a cost point only
      if it satisfies all constraints in any one of the sub-arrays.
      This parameter MUST NOT be specified unless this resource's "cost-
      constraints" capability is "true" and its "max-cost-types"
      capability is defined with a value greater than 0 (Section 4.1.2).

      This parameter MUST NOT be specified if the "constraints"
      parameter is specified.

      Note that if the "max-cost-types" capability has a value greater
      than 0, a client MAY use the "or-constraints" parameter together



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      with the "cost-type" parameter.  That is, if the client and server
      are both aware of the extensions in this document, a client MAY
      use an "OR" test for a single-valued cost request.

   pids, srcs, dsts:  As defined in {11.3.2.3} of [RFC7285].

4.1.2.  Capabilities

   The FilteredCostMapCapabilities object in {11.3.2.4} is extended as
   follows:

       object {
          JSONString cost-type-names<1..*>;
          [JSONBool cost-constraints;]
          [JSONNumber max-cost-types;]
          [JSONString testable-cost-type-names<0..*>;]
       } FilteredCostMapCapabilities;

   max-cost-types:  If present with value N greater than 0, this
      resource understands the multi-cost extensions in this document,
      and can return a Multi Cost Map with any combination of N or fewer
      cost types in the "cost-type-names" list.  If omitted, the default
      value is 0.

   testable-cost-type-names:  If present, and if "cost-constraints" is
      true, the resource only allows constraint tests on the cost type
      names in this array.  Each name in "testable-cost-type-names" MUST
      be in "cost-type-names".  If omitted or empty, the default is the
      value of the "cost-type-names" capability.

   cost-type-names and cost-constraints:  As defined in {11.3.2.4} of
      [RFC7285].

   Note that "testable-cost-type-names" allows an ALTO Server to provide
   constraint tests on some, but not all, cost types.

4.1.3.  Response

   If the client specifies the "cost-type" input parameter, the response
   is exactly as defined in {11.2.3.6} of [RFC7285].  If the client
   provides the "multi-cost-types" instead, then the response is changed
   as follows:

   o  In "meta", the field "cost-type" is replaced with the field
      "multi-cost-types", with the same value as the "multi-cost-types"
      input parameter.





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   o  The costs are JSONArrays, instead of JSONNumbers.  All arrays have
      the same cardinality as the "multi-cost-types" input parameter,
      and contain the cost type values in that order.  If a cost type is
      not available for a particular source and destination, the ALTO
      Server MUST use the JSON null value for that array element.  If
      none of the cost types are available for a particular source and
      destination, the ALTO Server MAY omit the entry for that source
      and destination.

4.2.  Endpoint Cost Service Extensions

   This document extends the Endpoint Cost Service, as defined in
   {11.5.1} of [RFC7285], by adding new input parameters and
   capabilities, and by returning JSONArrays instead of JSONNumbers as
   the cost values.

   The media type (11.5.1.1}, HTTP method (11.5.1.2} and "uses"
   specifications (11.5.1.5} are unchanged.

4.2.1.  Accept Input Parameters

   The ReqEndpointCostMap object in {11.5.1.3} of [RFC7285] is extended
   as follows:

       object {
          [CostType cost-type;]
          [CostType multi-cost-types<1..*>;]
          [CostType testable-cost-types<1..*>;]
          [JSONString constraints<0..*>;]
          [JSONString or-constraints<0..*>;]
          EndpointFilter endpoints;
       } ReqFilteredCostMap;

       object {
          [TypedEndpointAddr srcs<0..*>;]
          [TypedEndpointAddr dsts<0..*>;]
       } EndpointFilter;

   cost-type:  As defined in {11.5.1.3} of [RFC7285], with the
      additional requirement that the client MUST specify either "cost-
      type" or "multi-cost-types", but not both.

   multi-cost-types:  If present, the ALTO Server MUST return array-
      valued costs for the cost types in this list.  For each entry, the
      "cost-metric" and "cost-mode" fields MUST match one of the
      supported cost types indicated in this resource's "capabilities"
      field (Section 4.2.2).  The client MUST NOT use this field unless
      this resource's "max-cost-types" capability exists and has a value



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      greater than 0.  Although optional, the client MUST specify either
      "cost-type" or "multi-cost-types".  The client MUST NOT specify
      both.

   testable-cost-types, constraints, or-constraints:  Defined
      equivalently to the corresponding input parameters for an extended
      Filtered Cost Map (Section 4.1.1).

   endpoints, srcs, dsts:  As defined in {11.5.1.3} of [RFC7285].

4.2.2.  Capabilities

   The extensions to the Endpoint Cost Service capabilities are
   identical to the extensions to the Filtered Cost Map (see
   Section 4.1.2).

4.2.3.  Response

   The extensions to the Endpoint Cost Service response are similar to
   the extensions to the Filtered Cost Map response (Section 4.1.3).
   Specifically, if the client specifies the "cost-type" input
   parameter, the response is exactly as defined in {11.5.1.6} of
   [RFC7285].  If the client provides the "multi-cost-types" instead,
   then the response is changed as follows:

   o  In "meta", the field "cost-type" is replaced with the field
      "multi-cost-types", with the same value as the "multi-cost-types"
      input parameter.

   o  The costs are JSONArrays, instead of JSONNumbers.  All arrays have
      the same cardinality as the "multi-cost-types" input parameter,
      and contain the cost type values in that order.  If a cost type is
      not available for a particular source and destination, the ALTO
      Server MUST use the JSON null value for that array element.  If
      none of the cost types are available for a particular source and
      destination, the ALTO Server MAY omit the entry for that source
      and destination.

5.  Examples

5.1.  Information Resource Directory

   The following is an example of an ALTO Server's Information Resource
   Directory.  In addition to Network and Cost Map resources, it defines
   a Filtered Cost Map and an Endpoint Cost Service, both which
   understand the multi-cost extensions.

   GET /directory HTTP/1.1



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   Host: alto.example.com
   Accept: application/alto-directory+json,application/alto-error+json


   HTTP/1.1 200 OK
   Content-Length: [TODO]
   Content-Type: application/alto-directory+json

   {
     "meta" : {
       "default-alto-network-map" : "my-default-network-map",
       "cost-types" : {
          "num-routing" : {
            "cost-mode" : "numerical",
            "cost-metric" : "routingcost"
          },
          "num-hopcount" : {
            "cost-mode" : "numerical",
            "cost-metric" : "hopcount"
          },
            .....
            Other ALTO cost types as described
            in current ALTO Protocol
            .....
          }
     },
     "resources" : {
         "my-default-network-map" : {
           "uri" : "http://alto.example.com/networkmap",
           "media-type" : "application/alto-networkmap+json"
         },
         "numerical-routing-cost-map" : {
           "uri" : "http://alto.example.com/costmap/num-routing",
           "media-types" : [ "application/alto-costmap+json" ],
           "uses" : [ "my-default-network-map" ],
           "capabilities" : {
             "cost-type-names" : [ "num-routing" ]
           }
         },
         "numerical-hopcount-cost-map" : {
           "uri" : "http://alto.example.com/costmap/num-hopcount",
           "media-types" : [ "application/alto-costmap+json" ],
           "uses" : [ "my-default-network-map" ],
           "capabilities" : {
             "cost-type-names" : [ "num-hopcount" ]
           }
         },
         .........



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         And other information resources as described in RFC7285
         .........
         "filtered-multicost-map" : {
           "uri" : "http://alto.example.com/multi/costmap/filtered",
           "media-types" : [ "application/alto-costmap+json" ],
           "accepts" : [ "application/alto-costmapfilter+json" ],
           "uses" : [ "my-default-network-map" ],
           "capabilities" : {
             "cost-constraints" : true,
             "max-cost-types" : 2,
             "cost-type-names" : [ "num-routingcost",
                                   "num-hopcount" ],
             "testable-cost-type-names" : [ "num-routingcost",
                                            "num-hopcount" ]
           }
         },
         "endpoint-multicost-map" : {
           "uri" : "http://alto.example.com/multi/endpointcost/lookup",
           "media-types" : [ "application/alto-endpointcost+json" ],
           "accepts" : [ "application/alto-endpointcostparams+json" ],
           "uses" : [ "my-default-network-map" ],
           "capabilities" : {
             "cost-constraints" : true,
             "max-cost-types" : 2,
             "cost-type-names" : [ "num-routingcost",
                                   "num-hopcount" ],
             "testable-cost-type-names" : [ "num-routingcost",
                                            "num-hopcount" ]
         }
       }
     }
   }

5.2.  Multi-Cost Filtered Cost Map: Example #1

   This example illustrates a static multi-cost ALTO transaction, where
   the utilized Cost Types all have static values.  We assume that the
   Cost Types available at the ALTO Server are "routingcost" and
   "hopcount" and the "numerical" mode is available for both of them.
   The "routingcost" may be based on monetary considerations where as
   the "hopcount" is used to report on the path delay.  We also assume
   that ALTO server does not know the value of the "routingcost" between
   PID2 and PID3, and hence uses 'null' for those costs.








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   POST /multi/costmap/filtered" HTTP/1.1
   Host: alto.example.com
   Accept: application/alto-costmap+json,application/alto-error+json

   {
     "multi-cost-types": [
       {"cost-mode": "numerical", "cost-metric": "routingcost"},
       {"cost-mode": "numerical", "cost-metric": "hopcount"}
     ],
     "pids" : {
       "srcs" : [ ],
       "dsts" : [ ]
     }
   }


   HTTP/1.1 200 OK
   Content-Length: [TODO]
   Content-Type: application/alto-costmap+json

   {
    "meta" : {
      "dependent-vtags" : [
        {"resource-id": "my-default-network-map",
         "tag": "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
        }
      ],
      "multi-cost-types" : [
        {"cost-mode": "numerical", "cost-metric": "routingcost"},
        {"cost-mode": "numerical", "cost-metric": "hopcount"}
      ]
     }
    }
    "cost-map" : {
      "PID1": { "PID1":[1,0],   "PID2":[4,23],   "PID3":[10,5]   },
      "PID2": { "PID1":[15,5],  "PID2":[1,0],    "PID3":[null,9] },
      "PID3": { "PID1":[20,12], "PID2":[null,1], "PID3":[1,0]    }
    }
   }


5.3.  Multi-Cost Filtered Cost Map: Example #2

   This is an example of using constraints to restrict the returned
   source/destination PID pairs to those with "routingcost" between 5
   and 10, or "hopcount" equal to 0.





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   POST multi/multicostmap/filtered HTTP/1.1
   Host: alto.example.com
   Content-Type: application/alto-costmapfilter+json
   Accept: application/alto-costmap+json,application/alto-error+json

   {
     "multi-cost-types" : [
       {"cost-mode": "numerical", "cost-metric": "routingcost"},
       {"cost-mode": "numerical", "cost-metric": "hopcount"}
     ],
     "or-constraints" : [ ["[0] ge 5", "[0] le 10"],
                          ["[1] eq 0"] ]
     "pids" : {
       "srcs" : [ "PID1", "PID2" ],
       "dsts" : [ "PID1", "PID2", "PID3" ]
     }
   }


   HTTP/1.1 200 OK
   Content-Type: application/alto-costmap+json

   {
     "meta" : {
       "dependent-vtags" : [
         {"resource-id": "my-default-network-map",
          "tag": "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
         }
       ],
       "multi-cost-types" : [
         {"cost-mode": "numerical", "cost-metric": "routingcost"},
         {"cost-mode": "numerical", "cost-metric": "hopcount"}
       ]
     }
     "cost-map" : {
       "PID1": { "PID1": [1,0], "PID3": [10,5] },
       "PID2": { "PID2": [1,0]                 }
     }
   }


5.4.  Multi-Cost Filtered Cost Map: Example #3

   This example uses extended constraints to limit the response to cost
   points with ("routingcost" <= 10 and "hopcount" <= 2), or else
   ("routingcost" <= 2 and "hopcount" <= 6).  Unlike the previous
   example, the client is only interested in the "routingcost" cost
   type, and uses the "cost-type" parameter instead of "multi-cost-



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   types" to tell the server to return scalar costs instead of array
   costs:

   POST multi/multicostmap/filtered HTTP/1.1
   Host: alto.example.com
   Content-Type: application/alto-costmapfilter+json
   Accept: application/alto-costmap+json,application/alto-error+json

   {
     "cost-type" : {
       "cost-mode": "numerical", "cost-metric": "routingcost"
     },
     "testable-cost-types" : [
       {"cost-mode": "numerical", "cost-metric": "routingcost"},
       {"cost-mode": "numerical", "cost-metric": "hopcount"}
     ],
     "or-constraints": [
            ["[0] le 10", "[1] le 2"],
            ["[0] le 3",  "[1] le 6"]
     ],
     "pids" : {
       "srcs" : [ ],
       "dsts" : [ ]
     }
   }


   HTTP/1.1 200 OK
   Content-Type: application/alto-costmap+json

   {
     "meta" : {
       "dependent-vtags" : [
         {"resource-id": "my-default-network-map",
          "tag": "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
         }
       ],
       "cost-type" : {
         "cost-mode": "numerical", "cost-metric": "routingcost"
       }
     }
     "cost-map" : {
       "PID1": { "PID1": 1, "PID3": 10 },
       "PID2": { "PID2": 1 },
       "PID3": { "PID3": 1 }
     }
   }




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5.5.  Endpoint Cost Service

   This example uses the Endpoint Cost Service to retrieve the
   "routingcost" and "hopcount" for selected endpoints, limiting the
   response to costs with either low hopcount and reasonable routingcost
   (hopcount <= 2 and routingcost <= 10), or else low routingcost and
   reasonable hopcount (routingcost <= 3 and hopcount <= 6).












































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   POST /multi/endpointcost/lookup HTTP/1.1
   Host: alto.example.com
   Content-Length: [TODO]
   Content-Type: application/alto-endpoincostparams+json
   Accept: application/alto-endpointcost+json,
           application/alto-error+json

   {
     "multi-cost-types" : [
       {"cost-mode": "numerical", "cost-metric": "routingcost"},
       {"cost-mode": "numerical", "cost-metric": "hopcount"}
     ],
     "or-constraints": [
            ["[0] le 10", "[1] le 2"],
            ["[0] le 3",  "[1] le 6"]
     ],
     "endpoints" : {
       "srcs": [ "ipv4:192.0.2.2" ],
       "dsts": [
         "ipv4:192.0.2.89",
         "ipv4:198.51.100.34",
         "ipv4:203.0.113.45"
       ]
     }
   }


   HTTP/1.1 200 OK
   Content-Length: [TODO]
   Content-Type: application/alto-endpointcost+json

   {
     "meta" : {
       "multi-cost-types" : [
         {"cost-mode": "numerical", "cost-metric": "routingcost"},
         {"cost-mode": "numerical", "cost-metric": "hopcount"}
       ]
     }
     "endpoint-cost-map" : {
       "ipv4:192.0.2.2": {
         "ipv4:192.0.2.89":   [15, 5],
         "ipv4:203.0.113.45": [4, 23]
       }
     }
   }






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6.  IANA Considerations

   This document does define any new media types or introduce any new
   IANA considerations.

7.  Privacy And Security Considerations

   This document does introduce any privacy or security issues not
   already present in the ALTO protocol.

8.  Acknowledgements

   The authors would like to thank Richard Alimi, Vijay Gurbani, Dave
   Mac Dysan, Dhruv Dhodi, Young Lee, Richard Yang and Wang Xin for
   fruitful discussions and feedback on this document and previous
   versions.

9.  References

9.1.  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,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC5693]  "Application Layer Traffic Optimization (ALTO) Problem
              Statement", October 2009.

   [RFC7285]  Almi, R., Penno, R., Yang, Y., Kiesel, S., Previdi, S.,
              Roome, W., Shalunov, S., and R. Woundy, "Application-Layer
              Traffic Optimization (ALTO) Protocol", RFC 7285, September
              2014.

9.2.  Informative References

   [RFC6708]  "Application-Layer Traffic Optimization (ALTO)
              Requirements", February 2012.

Authors' Addresses

   Sabine Randriamasy (editor)
   Alcatel-Lucent/Bell Labs
   Route de Villejust
   NOZAY  91460
   FRANCE

   Email: Sabine.Randriamasy@alcatel-lucent.com



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   Wendy Roome (editor)
   Alcatel-Lucent/Bell Labs
   600 Mountain Ave, Rm 3B-324
   Murray Hill, NJ  07974
   USA

   Phone: +1-908-582-7974
   Email: w.roome@alcatel-lucent.com


   Nico Schwan
   Thales Deutschland
   Lorenzstrasse 10
   Stuttgart  70435
   Germany

   Email: nico.schwan@thalesgroup.com


































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