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ALTO                                                              K. Gao
Internet-Draft                                        Sichuan University
Intended status: Standards Track                                  Y. Lee
Expires: 15 January 2021
                                                          S. Randriamasy
                                                         Nokia Bell Labs
                                                               Y.R. Yang
                                                         Yale University
                                                                J. Zhang
                                                       Tongji University
                                                            14 July 2020


                      ALTO Extension: Path Vector
                   draft-ietf-alto-path-vector-11

Abstract

   This document is an extension to the base Application-Layer Traffic
   Optimization protocol [RFC7285].  The current ALTO Cost Services only
   allow applications to obtain cost values on an end-to-end path
   defined by its source and destination.  The present extension
   provides abstracted information on particular network components or
   elements traversed by a path between its source and destination.
   Examples of such abstracted components are networks, data centers or
   links.  This is useful for applications whose performance is impacted
   by particular network components they traverse or by their
   properties.  Applications having the choice among several connection
   paths may use this information to select paths accordingly and
   improve their performance.  In particular, they may infer that
   several paths share common links and prevent traffic bottlenecks by
   avoiding such paths.  This document introduces a new cost type called
   Path Vector.  A Path Vector is an array of entities that each
   identifies an abstracted representation of a network part and that
   are called Abstract Network Element (ANE).  Each ANE is defined by a
   set of properties.  ANE properties are conveyed by an ALTO
   information resource called "Property Map", that can be packed
   together with the Path Vectors in a multipart response.  They can
   also be obtained via a separate ALTO request to a Property Map. An
   ALTO Property Map is an extension to the ALTO protocol, that is
   specified in another document entitled "Unified Properties for the
   ALTO Protocol" [I-D.ietf-alto-unified-props-new].

Status of This Memo

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




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   This Internet-Draft will expire on 15 January 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Languages  . . . . . . . . . . . . . . . . . . .   6
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Design Requirements . . . . . . . . . . . . . . . . . . .   7
     4.2.  Recent Use Cases  . . . . . . . . . . . . . . . . . . . .   9
       4.2.1.  Large-scale Data Analytics  . . . . . . . . . . . . .   9
       4.2.2.  Context-aware Data Transfer . . . . . . . . . . . . .  10
       4.2.3.  CDN and Service Edge  . . . . . . . . . . . . . . . .  10
   5.  Path Vector Extension: Overview . . . . . . . . . . . . . . .  10
     5.1.  Abstract Network Element  . . . . . . . . . . . . . . . .  11
       5.1.1.  ANE Domain  . . . . . . . . . . . . . . . . . . . . .  11
       5.1.2.  Ephemeral ANE and Persistent ANE  . . . . . . . . . .  11
       5.1.3.  Property Filtering  . . . . . . . . . . . . . . . . .  12
     5.2.  Path Vector Cost Type . . . . . . . . . . . . . . . . . .  12
     5.3.  Multipart Path Vector Response  . . . . . . . . . . . . .  13
       5.3.1.  Identifying the Media Type of the Root Object . . . .  14
       5.3.2.  References to Part Messages . . . . . . . . . . . . .  14
       5.3.3.  Order of Part Messages  . . . . . . . . . . . . . . .  15
   6.  Specification: Basic Data Types . . . . . . . . . . . . . . .  15



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     6.1.  ANE Name  . . . . . . . . . . . . . . . . . . . . . . . .  15
     6.2.  ANE Domain  . . . . . . . . . . . . . . . . . . . . . . .  15
       6.2.1.  Entity Domain Type  . . . . . . . . . . . . . . . . .  16
       6.2.2.  Entity Identifier Encoding  . . . . . . . . . . . . .  16
       6.2.3.  Hierarchy and Inheritance . . . . . . . . . . . . . .  16
       6.2.4.  Media Type of Defining Resource . . . . . . . . . . .  16
     6.3.  ANE Property Name . . . . . . . . . . . . . . . . . . . .  16
     6.4.  Initial ANE Property Types  . . . . . . . . . . . . . . .  16
       6.4.1.  New ANE Property Type: Maximum Reservable
               Bandwidth . . . . . . . . . . . . . . . . . . . . . .  17
       6.4.2.  New ANE Property Type: Persistent Entity ID . . . . .  18
     6.5.  Path Vector Cost Type . . . . . . . . . . . . . . . . . .  18
       6.5.1.  Cost Metric: ane-path . . . . . . . . . . . . . . . .  18
       6.5.2.  Cost Mode: array  . . . . . . . . . . . . . . . . . .  19
     6.6.  Part Resource ID  . . . . . . . . . . . . . . . . . . . .  19
   7.  Specification: Service Extensions . . . . . . . . . . . . . .  19
     7.1.  Multipart Filtered Cost Map for Path Vector . . . . . . .  19
       7.1.1.  Media Type  . . . . . . . . . . . . . . . . . . . . .  19
       7.1.2.  HTTP Method . . . . . . . . . . . . . . . . . . . . .  19
       7.1.3.  Accept Input Parameters . . . . . . . . . . . . . . .  20
       7.1.4.  Capabilities  . . . . . . . . . . . . . . . . . . . .  21
       7.1.5.  Uses  . . . . . . . . . . . . . . . . . . . . . . . .  21
       7.1.6.  Response  . . . . . . . . . . . . . . . . . . . . . .  21
     7.2.  Multipart Endpoint Cost Service for Path Vector . . . . .  25
       7.2.1.  Media Type  . . . . . . . . . . . . . . . . . . . . .  25
       7.2.2.  HTTP Method . . . . . . . . . . . . . . . . . . . . .  25
       7.2.3.  Accept Input Parameters . . . . . . . . . . . . . . .  25
       7.2.4.  Capabilities  . . . . . . . . . . . . . . . . . . . .  26
       7.2.5.  Uses  . . . . . . . . . . . . . . . . . . . . . . . .  26
       7.2.6.  Response  . . . . . . . . . . . . . . . . . . . . . .  26
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  29
     8.1.  Example: Information Resource Directory . . . . . . . . .  29
     8.2.  Example: Multipart Filtered Cost Map  . . . . . . . . . .  31
     8.3.  Example: Multipart Endpoint Cost Resource . . . . . . . .  32
     8.4.  Example: Incremental Updates  . . . . . . . . . . . . . .  35
   9.  Compatibility . . . . . . . . . . . . . . . . . . . . . . . .  36
     9.1.  Compatibility with Legacy ALTO Clients/Servers  . . . . .  36
     9.2.  Compatibility with Multi-Cost Extension . . . . . . . . .  36
     9.3.  Compatibility with Incremental Update . . . . . . . . . .  36
     9.4.  Compatibility with Cost Calendar  . . . . . . . . . . . .  36
   10. General Discussions . . . . . . . . . . . . . . . . . . . . .  37
     10.1.  Constraint Tests for General Cost Types  . . . . . . . .  37
     10.2.  General Multipart Resources Query  . . . . . . . . . . .  37
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  37
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  38
     12.1.  ALTO Entity Domain Registry  . . . . . . . . . . . . . .  38
     12.2.  ALTO Entity Property Type Registry . . . . . . . . . . .  39
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  40



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   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  40
     14.2.  Informative References . . . . . . . . . . . . . . . . .  41
   Appendix A.  Changes since -10  . . . . . . . . . . . . . . . . .  42
   Appendix B.  Changes since -09  . . . . . . . . . . . . . . . . .  43
   Appendix C.  Changes since -08  . . . . . . . . . . . . . . . . .  43
   Appendix D.  Changes Since Version -06  . . . . . . . . . . . . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  44

1.  Introduction

   Network performance metrics are crucial to the Quality of Experience
   (QoE) of today's applications.  The ALTO protocol allows Internet
   Service Providers (ISPs) to provide guidance, such as topological
   distance between different end hosts, to overlay applications.  Thus,
   the overlay applications can potentially improve the QoE by better
   orchestrating their traffic to utilize the resources in the
   underlying network infrastructure.

   Existing ALTO Cost Map and Endpoint Cost Service provide only cost
   information on an end-to-end path defined by its <source,
   destination> endpoints: The base protocol [RFC7285] allows the
   services to expose the topological distances of end-to-end paths,
   while various extensions have been proposed to extend the capability
   of these services, e.g., to express other performance metrics
   [I-D.ietf-alto-performance-metrics], to query multiple costs
   simultaneously [RFC8189], and to obtain the time-varying values
   [I-D.ietf-alto-cost-calendar].

   While the existing extensions are sufficient for many overlay
   applications, however, the QoE of some overlay applications depends
   not only on the cost information of end-to-end paths, but also on
   some intermediate network components and their properties.  For
   example, job completion time, which is an important QoE metric for a
   large-scale data analytics application, is impacted by shared
   bottlenecks inside the carrier network.

   Predicting such information can be very complex without the help of
   the ISP [AAAI2019].  With proper guidance from the ISP, an overlay
   application may be able to schedule its traffic for better QoE.  In
   the meantime, it may be helpful as well for ISPs if applications
   could avoid using bottlenecks or challenging the network with poorly
   scheduled traffic.

   Despite the benefits, ISPs are not likely to expose details on their
   network paths: first for the sake of confidentiality, second because
   it may result in a huge volume and overhead, and last because it is
   difficult for ISPs to figure out what information and what details an



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   application needs.  Likewise, applications do not necessarily need
   all the network path details and are likely not able to understand
   them.

   Therefore, it is beneficial for both parties if an ALTO server
   provides ALTO clients with an "abstract network state" that provides
   the necessary details to applications, while hiding the network
   complexity and confidential information.  An "abstract network state"
   is a selected set of abstract representations of intermediate network
   components traversed by the paths between <source, destination> pairs
   combined with properties of these components that are relevant to the
   overlay applications' QoE.  Both an application via its ALTO client
   and the ISP via the ALTO server can achieve better confidentiality
   and resource utilization by appropriately abstracting relevant path
   components.  The pressure on the server scalability can also be
   reduced by abstracting components and their properties and combining
   them in a single response.

   This document extends [RFC7285] to allow an ALTO server convey
   "abstract network state", for paths defined by their <source,
   destination> pairs.  To this end, it introduces a new cost type
   called "Path Vector".  A Path Vector is an array of identifiers of
   so-called Abstract Network Element (ANE).  An ANE represents an
   abstract intermediate component traversed by a path.  It can be
   associated with various properties.  The associations between ANEs
   and their properties are encoded in an ALTO information resource
   called Unified Property Map, which is specified in
   [I-D.ietf-alto-unified-props-new].

   For better confidentiality, this document aims to minimize
   information exposure.  In particular, this document enables and
   recommends that first ANEs are constructed on demand, and second an
   ANE is only associated with properties that are requested by an ALTO
   client.  A Path Vector response involved two ALTO Maps: the Cost Map
   that contains the Path Vector results and the up-to-date Unified
   Property Map that contains the properties requested for these ANEs.
   To enforce consistency and improve server scalability, this document
   uses the "multipart/related" message defined in [RFC2387] to return
   the two maps in a single response.

   The rest of the document are organized as follows.  Section 3
   introduces the extra terminologies that are used in this document.
   Section 4 uses an illustrative example to introduce the additional
   requirements of the ALTO framework, and discusses potential use
   cases.  Section 5 gives an overview of the protocol design.
   Section 6 and Section 7 specify the Path Vector extension to the ALTO
   IRD and the information resources, with some concrete examples
   presented in Section 8.  Section 9 discusses the backward



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   compatibility with the base protocol and existing extensions.
   Security and IANA considerations are discussed in Section 11 and
   Section 12 respectively.

2.  Requirements Languages

   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.

   When the words appear in lower case, they are to be interpreted with
   their natural language meanings.

3.  Terminology

   This document extends the ALTO base protocol [RFC7285] and the
   Unified Property Map extension [I-D.ietf-alto-unified-props-new].  In
   addition to the terms defined in these documents, this document also
   uses the following additional terms:

   *  Abstract Network Element (ANE): An Abstract Network Element is a
      representation of network components.  It can be a link, a
      middlebox, a virtualized network function (VNF), etc., or their
      aggregations.  An ANE can be constructed either statically in
      advance or on demand based on the requested information.  In a
      response, each ANE is represented by a unique ANE Name.  Note that
      an ALTO client must not assume ANEs in different responses but
      with the same ANE Name refer to the same network component(s).

   *  Path Vector: A Path Vector, or an ANE Path Vector, is a JSON array
      of ANE Names.  It conveys the information that the path between a
      source and a destination traverses the ANEs in the same order as
      they appear in the Path Vector.

   *  Path Vector resource: A Path Vector resource refers to an ALTO
      resource which supports the extension defined in this document.

   *  Path Vector cost type: The Path Vector cost type is a special cost
      type, which is specified in Section 6.5.  When this cost type is
      present in an IRD entry, it indicates that the information
      resource is a Path Vector resource.  When this cost type is
      present in a Cost Map or an Endpoint Cost Map, it indicates each
      cost value must be interpreted as a Path Vector.

   *  Path Vector request: A Path Vector request refers to the POST
      message sent to an ALTO Path Vector resource.



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   *  Path Vector response: A Path Vector response refers to the
      multipart/related message returned by a Path Vector resource.

4.  Problem Statement

4.1.  Design Requirements

   This section gives an illustrative example of how an overlay
   application can benefit from the Path Vector extension.

   Assume that an application has control over a set of flows, which may
   go through shared links or switches and share a bottleneck.  The
   application hopes to schedule the traffic among multiple flows to get
   better performance.  The capacity region information for those flows
   will benefit the scheduling.  However, existing cost maps can not
   reveal such information.

   Specifically, consider a network as shown in Figure 1.  The network
   has 7 switches (sw1 to sw7) forming a dumb-bell topology.  Switches
   sw1/sw3 provide access on one side, sw2/sw4 provide access on the
   other side, and sw5-sw7 form the backbone.  Endhosts eh1 to eh4 are
   connected to access switches sw1 to sw4 respectively.  Assume that
   the bandwidth of link eh1 -> sw1 and link sw1 -> sw5 are 150 Mbps,
   and the bandwidth of the rest links are 100 Mbps.

                                 +------+
                                 |      |
                               --+ sw6  +--
                             /   |      |  \
       PID1 +-----+         /    +------+   \          +-----+  PID2
       eh1__|     |_       /                 \     ____|     |__eh2
   1.2.3.4  | sw1 | \   +--|---+         +---|--+ /    | sw2 |  2.3.4.5
            +-----+  \  |      |         |      |/     +-----+
                      \_| sw5  +---------+ sw7  |
       PID3 +-----+   / |      |         |      |\     +-----+  PID4
       eh3__|     |__/  +------+         +------+ \____|     |__eh4
   3.4.5.6  | sw3 |                                    | sw4 |  4.5.6.7
            +-----+                                    +-----+

                       Figure 1: Raw Network Topology

   The single-node ALTO topology abstraction of the network is shown in
   Figure 2.








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                             +----------------------+
                    {eh1}    |                      |     {eh2}
                    PID1     |                      |     PID2
                      +------+                      +------+
                             |                      |
                             |                      |
                    {eh3}    |                      |     {eh4}
                    PID3     |                      |     PID4
                      +------+                      +------+
                             |                      |
                             +----------------------+

              Figure 2: Base Single-Node Topology Abstraction

   Consider an application overlay (e.g., a large-scale data analytics
   system) which wants to optimize the total throughput of the traffic
   among a set of end host <source, destination> pairs, say eh1 -> eh2
   and eh1 -> eh4.  The application can request a cost map providing
   end-to-end available bandwidth, using "availbw" as cost-metric and
   "numerical" as cost-mode.

   The application will receive from the ALTO server that the bandwidth
   of eh1 -> eh2 and eh1 -> eh4 are both 100 Mbps.  But this information
   is not enough to determine the optimal total throughput.  Consider
   the following two cases:

   *  Case 1: If eh1 -> eh2 uses the path eh1 -> sw1 -> sw5 -> sw6 ->
      sw7 -> sw2 -> eh2 and eh1 -> eh4 uses path eh1 -> sw1 -> sw5 ->
      sw7 -> sw4 -> eh4, then the application will obtain 150 Mbps at
      most.

   *  Case 2: If eh1 -> eh2 uses the path eh1 -> sw1 -> sw5 -> sw7 ->
      sw2 -> eh2 and eh1 -> eh4 uses the path eh1 -> sw1 -> sw5 -> sw7
      -> sw4 -> eh4, then the application will obtain only 100 Mbps at
      most.

   To allow applications to distinguish the two aforementioned cases,
   the network needs to provide more details.  In particular:

   *  For eh1 -> eh2, the ALTO server must give more details which is
      critical for the overlay application to distinguish between Case 1
      and Case 2 and to compute the optimal total throughput
      accordingly.

   *  The ALTO server must allow the client to distinguish the common
      network components shared by eh1 -> eh2 and eh1 -> eh4, e.g., eh1
      - sw1 and sw1 - sw5 in Case 1.




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   *  The ALTO server must give details on the properties of the network
      components used by eh1 -> eh2 and eh1 -> eh4, e.g., the available
      bandwidth between eh1 - sw1, sw1 - sw5, sw5 - sw7, sw5 - sw6, sw6
      - sw7, sw7 - sw2, sw7 - sw4, sw2 - eh2, sw4 - eh4 in Case 1.

   In general, we can conclude that to support the multiple flow
   scheduling use case, the ALTO framework must be extended to satisfy
   the following additional requirements:

   AR1:  An ALTO server must provide essential information on
      intermediate network components on the path of a <source,
      destination> pair that are critical to the QoE of the overlay
      application.

   AR2:  An ALTO server must provide essential information on how the
      paths of different <source, destination> pairs share a common
      network component.

   AR3:  An ALTO server must provide essential information on the
      properties associated to the network components.

   The Path Vector extension defined in this document propose a solution
   to provide these details.

4.2.  Recent Use Cases

   While the multiple flow scheduling problem is used to help identify
   the additional requirements, the Path Vector extension can be applied
   to a wide range of applications.  This section highlights some real
   use cases that are recently reported.  See [I-D.bernstein-alto-topo]
   for a more comprehensive survey of use cases where extended network
   topology information is needed.

4.2.1.  Large-scale Data Analytics

   One potential use case of the Path Vector extension is for large-
   scale data analytics such as [SENSE] and [LHC], where data of
   Gigabytes, Terabytes and even Petabytes are transferred.  For these
   applications, the QoE is usually measured as the job completion time,
   which is related to the completion time of the slowest data transfer.
   With the Path Vector extension, an ALTO client can identify
   bottlenecks inside the network.  Therefore, the overlay application
   can make optimal traffic distribution or resource reservation (i.e.,
   proportional to the size of the transferred data), leading to optimal
   job completion time and network resource utilization.






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4.2.2.  Context-aware Data Transfer

   It is sometimes important to know how the capabilities of various
   network components between two end hosts, especially in the mobile
   environment.  With the Path Vector extension, an ALTO client may
   query the "network context" information, i.e., whether the two hosts
   are connected to the access network through a wireless link or a
   wire, and the capabilities of the access network.  Thus, the client
   may use different data transfer mechanisms, or even deploy different
   5G User Plane Functions (UPF) [I-D.ietf-dmm-5g-uplane-analysis] to
   optimize the data transfer.

4.2.3.  CDN and Service Edge

   A growing trend in today's applications is to bring storage and
   computation closer to the end user for better QoE, such as Content
   Delivery Network (CDN), AR/VR, and cloud gaming, as reported in
   various recent documents ([I-D.contreras-alto-service-edge],
   [I-D.huang-alto-mowie-for-network-aware-app], and
   [I-D.yang-alto-deliver-functions-over-networks]).

   With the Path Vector extension, an ALTO server can selectively reveal
   the CDNs and service edges that reside along the paths between
   different end hosts, together with their properties such as available
   Service Level Agreement (SLA) plans.  Otherwise, the ALTO client may
   have to make multiple queries and potentially with the complete list
   of CDNs and/or service edges.  While both approaches offer the same
   information, making multiple queries introduces larger delay and more
   overhead on both the ALTO server and the ALTO client.

5.  Path Vector Extension: Overview

   This section gives a non-normative overview of the Path Vector
   extension.  It is assumed that readers are familiar with both the
   base protocol [RFC7285] and the Unified Property Map extension
   [I-D.ietf-alto-unified-props-new].

   To satisfies the additional requirements, this extension:

   1.  introduces Abstract Network Element (ANE) as the abstraction of
       intermediate network components,

   2.  extends the Cost Map and Endpoint Cost Service to convey the
       intermediate network components traversed by the path of a
       <source, destination> pair as Path Vectors,

   3.  uses the Unified Property Map to convey the association between
       the intermediate network components and their properties.



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   Thus, an ALTO client can learn about the intermediate network
   components that are critical to the QoE of a <source, destination>
   pair by investigating the corresponding Path Vector value (AR1),
   identify common network components if an ANE appears in the Path
   Vectors of multiple <source, destination> pairs (AR2), and retrieve
   the properties of the network components by searching the Unified
   Property Map (AR3).

5.1.  Abstract Network Element

   This extension introduces Abstract Network Element (ANE) as an
   indirect and network-agnostic way to specify an aggregation of
   intermediate network components between a source and a destination.
   Specifically, an ANE is a string of type ANEName as specified in
   Section 6.1 and its associated set of properties.

5.1.1.  ANE Domain

   In this extension, the associations between ANE and the properties
   are conveyed in a Unified Property Map. Thus, they must follow the
   mechanisms specified in the [I-D.ietf-alto-unified-props-new].

   Specifically, this document defines a new entity domain called "ane"
   as specified in Section 5.1.1 and defines two initial properties for
   the "ane" domain.

5.1.2.  Ephemeral ANE and Persistent ANE

   For different requests, there can be different ways of grouping
   network components and assigning ANEs.  For example, an ALTO server
   may define an ANE for each aggregated bottleneck link between the
   sources and destinations specified in the request.  As the aggregated
   bottleneck links vary for different combinations of sources and
   destinations, the ANEs are ephemeral and are no longer valid after
   the request completes.  Thus, the scope of ephemeral ANEs are limited
   to the corresponding Path Vector response.

   While ephemeral ANEs returned by a Path Vector response do not exist
   beyond that response, some of them may represent entities that are
   persistent and defined in a standalone Property Map. Indeed, it may
   be useful for clients to occasionally query properties on persistent
   entities, without caring about the path that traverses them.
   Persistent entities have a persistent ID that is registered in a
   Property Map, together with their properties.







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5.1.3.  Property Filtering

   Resource-constrained ALTO clients may benefit from the filtering of
   Path Vector query results at the ALTO server, as an ALTO client may
   only require a subset of the available properties.

   Specifically, the available properties for a given resource are
   announced in the Information Resource Directory as a new capability
   called "ane-property-names".  The selected properties are specified
   in a filter called "ane-property-names" in the request body, and the
   response must return and only return the selected properties for the
   ANEs in the response.

   The "ane-property-names" capability for Cost Map and for Endpoint
   Cost Service are specified in Section 7.1.4 and Section 7.2.4
   respectively.  The "ane-property-names" filter for Cost Map and
   Endpoint Cost Service are specified in Section 7.1.3 and
   Section 7.2.3 accordingly.

5.2.  Path Vector Cost Type

   For an ALTO client to correctly interpret the Path Vector, this
   extension specifies a new cost type called the Path Vector cost type,
   which must be included both in the Information Resource Directory and
   the ALTO Cost Map or Endpoint Cost Map so that an ALTO client can
   correct interpret the cost values.

   The Path Vector cost type must convey both the interpretation and
   semantics in the "cost-mode" and "cost-metric" respectively.
   Unfortunately, a single "cost-mode" value cannot fully specify the
   interpretation of a Path Vector, which is a compound data type.  For
   example, in programming languages such as Java, a Path Vector will
   have the type of JSONArray[ANEName].

   Instead of extending the "type system" of ALTO, this document takes a
   simple and backward compatible approach.  Specifically, the "cost-
   mode" of the Path Vector cost type is "array", which indicates the
   value is a JSON array.  Then, an ALTO client must check the value of
   the "cost-metric".  If the value is "ane-path", meaning the JSON
   array should be further interpreted as a path of ANENames.

   The Path Vector cost type is specified in Section 6.5.









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5.3.  Multipart Path Vector Response

   For a basic ALTO information resource, a response contains only one
   type of ALTO resources, e.g., Network Map, Cost Map, or Property Map.
   Thus, only one round of communication is required: An ALTO client
   sends a request to an ALTO server, and the ALTO server returns a
   response, as shown in Figure 3.

            ALTO client                              ALTO server
                 |-------------- Request ---------------->|
                 |<------------- Response ----------------|

               Figure 3: A Typical ALTO Request and Response

   The Path Vector extension, on the other hand, involves two types of
   information resources: Path Vectors conveyed in a Cost Map or an
   Endpoint Cost Map, and ANE properties conveyed in a Unified Property
   Map. Instead of two consecutive message exchanges, the Path Vector
   extension enforces one round of communication.  Specifically, the
   Path Vector extension requires the ALTO client to include the source
   and destination pairs and the requested ANE properties in a single
   request, and encapsulates both Path Vectors and properties associated
   with the ANEs in a single response, as shown in Figure 4.

            ALTO client                              ALTO server
                 |------------- PV Request -------------->|
                 |<----- PV Response (Cost Map Part) -----|
                 |<--- PV Response (Property Map Part) ---|

          Figure 4: The Path Vector Extension Request and Response

   This design is based on the following considerations:

   1.  Since ANEs may be constructed on demand, and potentially based on
       the requested properties (See Section 5.1 for more details).  If
       sources and destinations are not in the same request as the
       properties, an ALTO server either cannot construct ANEs on-
       demand, or must wait until both requests are received.

   2.  As ANEs may be constructed on demand, mappings of each ANE to its
       underlying network devices and resources can be specific to the
       request.  In order to respond to the Property Map request
       correctly, an ALTO server must store the mapping of each Path
       Vector request until the client fully retrieves the property
       information.  The "stateful" behavior may substantially harm the
       server scalability and potentially lead to Denial-of-Service
       attacks.




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   One approach to realize the one-round communication is to define a
   new media type to contain both objects, but this violates modular
   design.  This document follows the standard-conforming usage of
   "multipart/related" media type defined in [RFC2387] to elegantly
   combine the objects.  Path Vectors are encoded as a Cost Map or an
   Endpoint Cost Map, and the Property Map is encoded as a Unified
   Propert Map. They are encapsulated as parts of a multipart message.
   The modular composition allows ALTO servers and clients to reuse the
   data models of the existing information resources.  Specifically,
   this document addresses the following practical issues using
   "multipart/related".

5.3.1.  Identifying the Media Type of the Root Object

   ALTO uses media type to indicate the type of an entry in the
   Information Resource Directory (IRD) (e.g., "application/alto-
   costmap+json" for Cost Map and "application/alto-endpointcost+json"
   for Endpoint Cost Map).  Simply putting "multipart/related" as the
   media type, however, makes it impossible for an ALTO client to
   identify the type of service provided by related entries.

   To address this issue, this document uses the "type" parameter to
   indicate the root object of a multipart/related message.  For a Cost
   Map resource, the "media-type" in the IRD entry must be "multipart/
   related" with the parameter "type=application/alto-costmap+json"; for
   an Endpoint Cost Service, the parameter must be "type=application/
   alto-endpointcost+json".

5.3.2.  References to Part Messages

   The ALTO SSE extension (see [I-D.ietf-alto-incr-update-sse]) uses
   "client-id" to demultiplex push updates.  However, "client-id" is
   provided for each request, which introduces ambiguity when applying
   SSE to a Path Vector resource.

   To address this issue, an ALTO server must assign a unique identifier
   to each part of the "multipart/related" response message.  This
   identifier, referred to as a Part Resource ID (See Section 6.6 for
   details), must be present in the part message's "Resource-Id" header.
   The MIME part header must also contain the "Content-Type" header,
   whose value is the media type of the part (e.g., "application/alto-
   costmap+json", "application/alto-endpointcost+json", or "application/
   alto-propmap+json").

   If an ALTO server provides incremental updates for this Path Vector
   resource, it must generate incremental updates for each part
   separately.  The client-id must have the following format:




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      pv-client-id '.' part-resource-id

   where pv-client-id is the client-id assigned to the Path Vector
   request, and part-resource-id is the "Resource-Id" header value of
   the part.  The media-type must match the "Content-Type" of the part.

   The same problem applies to the part messages as well.  The two parts
   must contain a version tag, which SHOULD contain a unique Resource
   ID.  This document requires the resource-id in a Version Tag to have
   the following format:

      pv-resource-id '.' part-resource-id

   where pv-resource-id is the resource ID of the Path Vector resource
   in the IRD entry, and the part-resource-id has the same value as the
   "Resource-Id" header of the part.

5.3.3.  Order of Part Messages

   According to [RFC2387], the Path Vector part, whose media type is the
   same as the "type" parameter of the multipart response message, is
   the root object.  Thus, it is the element the application processes
   first.  Even though the "start" parameter allows it to be placed
   anywhere in the part sequence, it is RECOMMENDED that the parts
   arrive in the same order as they are processed, i.e., the Path Vector
   part is always put as the first part, followed by the property map
   part.  It is also RECOMMENDED that when doing so, an ALTO server
   SHOULD NOT set the "start" parameter, which implies the first part is
   the root object.

6.  Specification: Basic Data Types

6.1.  ANE Name

   An ANE Name is encoded as a JSON string with the same format as that
   of the type PIDName (Section 10.1 of [RFC7285]).

   The type ANEName is used in this document to indicate a string of
   this format.

6.2.  ANE Domain

   The ANE domain associates property values with the Abstract Network
   Elements in a Property Map. Accordingly, the ANE domain always
   depends on a Property Map.






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6.2.1.  Entity Domain Type

   ane

6.2.2.  Entity Identifier Encoding

   The entity identifier of the "ane" domain has the same format as
   defined in Section 5.1.3 in [I-D.ietf-alto-unified-props-new], and
   the DomainTypeSpecificEntityID part has the same format as the
   ANEName type.

6.2.3.  Hierarchy and Inheritance

   There is no hierarchy or inheritance for properties associated with
   ANEs.

6.2.4.  Media Type of Defining Resource

   When resource specific domains are defined with entities of domain
   type "ane", the defining resource for entity domain type "ane" MUST
   be a Property Map. The media type of defining resources for the "ane"
   domain is:

   application/alto-propmap+json

   Specifically, the defining resource of ephemeral ANEs is the Property
   Map part of the multipart response.  The defining resource of
   persistent ANEs is the Property Map on which standalone queries for
   properties of persistent ANEs are made.

6.3.  ANE Property Name

   An ANE Property Name is encoded as a JSON string with the same format
   as that of Entity Property Name (Section 5.2.2 of
   [I-D.ietf-alto-unified-props-new]).

6.4.  Initial ANE Property Types

   In this document, two initial ANE property types are specified, "max-
   reservable-bandwidth" and "persistent-entity-id".

   Note that the two property types defined in this document do not
   depend on any information resource, so their ResourceID part must be
   empty.







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                                        ----- L1
                                       /
           PID1   +---------------+ 10 Gbps +----------+    PID3
    1.2.3.0/24+---+ +-----------+ +---------+          +---+3.4.5.0/24
                  | |   MEC1    | |         |          |
                  | +-----------+ |   +-----+          |
           PID2   |               |   |     +----------+
    2.3.4.0/24+---+               |   |         NET3
                  |               |   | 15 Gbps
                  |               |   |        \
                  +---------------+   |         -------- L2
                        NET1          |
                               +---------------+
                               | +-----------+ |   PID4
                               | |   MEC2    | +---+4.5.6.0/24
                               | +-----------+ |
                               +---------------+
                                     NET2

                    Figure 5: Examples of ANE Properties

   In this document, Figure 5 is used to illustrate the use of the two
   initial ANE property types.  There are 3 sub-networks (NET1, NET2 and
   NET3) and two interconnection links (L1 and L2).  It is assumed that
   each sub-network has sufficiently large bandwidth to be reserved.

6.4.1.  New ANE Property Type: Maximum Reservable Bandwidth

   Identifier:  "max-reservable-bandwidth"

   Intended Semantics:  The maximum reservable bandwidth property stands
      for the maximum bandwidth that can be reserved for all the traffic
      that traverses an ANE.  The value MUST be encoded as a non-
      negative numerical cost value as defined in Section 6.1.2.1 of
      [RFC7285] and the unit is bit per second.  If this property is
      requested but not present in an ANE, it MUST be interpreted as
      that the ANE does not support bandwidth reservation.

   Security Considerations:  ALTO entity properties expose information
      to ALTO clients.  ALTO service providers should be made aware of
      the security ramifications related to the exposure of an entity
      property.

   To illustrate the use of "max-reservable-bandwidth", consider the
   network in Figure 5.  An ALTO server can create an ANE for each
   interconnection link, where the initial value for "max-reservable-
   bandwidth" is the link capacity.




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6.4.2.  New ANE Property Type: Persistent Entity ID

   Identifier:  "persistent-entity-id"

   Intended Semantics:  The persistent entity ID property is the entity
      identifier of the persistent ANE associated with an ephemeral ANE.
      The value of this property is encoded with the format defined in
      Section 5.1.3 of [I-D.ietf-alto-unified-props-new].  In this
      format, the entity ID combines:

      *  a defining information resource for the ANE on which a
         "persistent-entity-id" is queried, which is the property map
         defining the ANE as a persistent entity, together with the
         properties

      *  the persistent name of the ANE in this property map

      With this format, the client has all the needed information for
      further standalone query properties on the persistent ANE.

   Security Considerations:  ALTO entity properties expose information
      to ALTO clients.  ALTO service providers should be made aware of
      the security ramifications related to the exposure of an entity
      property.

   To illustrate the use of "persistent-entity-id", consider the network
   in Figure 5.  Assume the ALTO server has a Property Map resource
   called "mec-props" that defines persistent ANEs "MEC1" and "MEC2"
   that represent the corresponding mobile edge computing (MEC)
   clusters.  The "persistent-entity-id" of the ephemeral ANE that is
   associated with MEC1 has the value "mec-props.ane:MEC1".

6.5.  Path Vector Cost Type

   This document defines a new cost type, which is referred to as the
   "Path Vector" cost type.  An ALTO server MUST offer this cost type if
   it supports the Path Vector extension.

6.5.1.  Cost Metric: ane-path

   The cost metric "ane-path" indicates the value of such a cost type
   conveys an array of ANE names, where each ANE name uniquely
   represents an ANE traversed by traffic from a source to a
   destination.







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6.5.2.  Cost Mode: array

   The cost mode "array" indicates that every cost value in a Cost Map
   or an Endpoint Cost Map MUST be interpreted as a JSON array object.

   Note that this cost mode only requires the cost value to be a JSON
   array of JSONValue.  However, an ALTO server that enables this
   extension MUST return a JSON array of ANEName (Section 6.1) when the
   cost metric is "ane-path".

6.6.  Part Resource ID

   A Part Resource ID is encoded as a JSON string with the same format
   as that of the type ResourceID (Section 10.2 of [RFC7285]).

   NOTE: Even though the client-id assigned to a Path Vector request and
   the Part Resource ID may contain up to 64 characters by their own
   definition, their concatenation (see Section 5.3.2) MUST also conform
   to the same length constraint.  The same requirement applies to the
   resource ID of the Path Vector resource, too.  Thus, it is
   RECOMMENDED to limit the length of resource ID and client ID related
   to a Path Vector resource to 31 characters.

7.  Specification: Service Extensions

7.1.  Multipart Filtered Cost Map for Path Vector

   This document introduces a new ALTO resource called multipart
   filtered cost map resource, which allows an ALTO server to provide
   other ALTO resources associated to the cost map resource in the same
   response.

7.1.1.  Media Type

   The media type of the multipart filtered cost map resource is
   "multipart/related;type=application/alto-costmap+json".

7.1.2.  HTTP Method

   The multipart filtered cost map is requested using the HTTP POST
   method.










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7.1.3.  Accept Input Parameters

   The input parameters of the multipart filtered cost map are supplied
   in the body of an HTTP POST request.  This document extends the input
   parameters to a filtered cost map with a data format indicated by the
   media type "application/alto-costmapfilter+json", which is a JSON
   object of type PVReqFilteredCostMap, where:

   object {
     [EntityPropertyName ane-property-names<0..*>;]
   } PVReqFilteredCostMap : ReqFilteredCostMap;

   with fields:

   ane-property-names:  A list of properties that are associated with
      the ANEs.  Each property in this list MUST match one of the
      supported ANE properties indicated in the resource's "ane-
      property-names" capability.  If the field is NOT present, it MUST
      be interpreted as an empty list, indicating that the ALTO server
      MUST NOT return any property in the Unified Property part.

   Example: Consider the network in Figure 1.  If an ALTO client wants
   to query the "max-reservable-bandwidth" between PID1 and PID2, it can
   submit the following request.

      POST /costmap/pv HTTP/1.1
      Host: alto.example.com
      Accept: multipart/related;type=application/alto-costmap+json,
              application/alto-error+json
      Content-Length: [TBD]
      Content-Type: application/alto-costmapfilter+json

      {
        "cost-type": {
          "cost-mode": "array",
          "cost-metric": "ane-path"
        },
        "pids": {
          "srcs": [ "PID1" ],
          "dsts": [ "PID2" ]
        },
        "ane-property-names": [ "max-reservable-bandwidth" ]
      }








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7.1.4.  Capabilities

   The multipart filtered cost map resource extends the capabilities
   defined in Section 11.3.2.4 of [RFC7285].  The capabilities are
   defined by a JSON object of type PVFilteredCostMapCapabilities:

   object {
     [EntityPropertyName ane-property-names<0..*>;]
   } PVFilteredCostMapCapabilities : FilteredCostMapCapabilities;

   with fields:

   cost-type-names:  The "cost-type-names" field MUST only include the
      Path Vector cost type, unless explicitly documented by a future
      extension.  This also implies that the Path Vector cost type MUST
      be defined in the "cost-types" of the Information Resource
      Directory's "meta" field.

   cost-constraints:  If the "cost-type-names" field includes the Path
      Vector cost type, "cost-constraints" field MUST be "false" or not
      present unless specifically instructed by a future document.

   testable-cost-type-names:  If the "cost-type-names" field includes
      the Path Vector cost type, the Path Vector cost type MUST NOT be
      included in the "testable-cost-type-names" field unless
      specifically instructed by a future document.

   ane-property-names:  Defines a list of ANE properties that can be
      returned.  If the field is NOT present, it MUST be interpreted as
      an empty list, indicating the ALTO server cannot provide any ANE
      property.

7.1.5.  Uses

   This member MUST include the resource ID of the network map based on
   which the PIDs are defined.  If this resource supports "persistent-
   entity-id", it MUST also include the defining resources of persistent
   ANEs that may appear in the response.

7.1.6.  Response

   The response MUST indicate an error, using ALTO protocol error
   handling, as defined in Section 8.5 of [RFC7285], if the request does
   no.

   The "Content-Type" header of the response MUST be "multipart/related"
   as defined by [RFC2387] with the following parameters:




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   type:  The type parameter MUST be "application/alto-costmap+json".
      Note that [RFC2387] permits both parameters with and without the
      double quotes.

   start:  The start parameter is as defined in [RFC2387].  If present,
      it MUST have the same value as the "Resource-Id" header of the
      Path Vector part.

   boundary:  The boundary parameter is as defined in [RFC2387].

   The body of the response consists of two parts:

   *  The Path Vector part MUST include "Resource-Id" and "Content-Type"
      in its header.  The value of "Resource-Id" MUST has the format of
      a Part Resource ID.  The "Content-Type" MUST be "application/alto-
      costmap+json".

      The body of the Path Vector part MUST be a JSON object with the
      same format as defined in Section 11.2.3.6 of [RFC7285].  The JSON
      object MUST include the "vtag" field in the "meta" field, which
      provides the version tag of the returned cost map.  The resource
      ID of the version tag MUST follow the format in Section 5.3.2.
      The "meta" field MUST also include the "dependent-vtags" field,
      whose value is a single-element array to indicate the version tag
      of the network map used, where the network map is specified in the
      "uses" attribute of the multipart filtered cost map resource in
      IRD.

   *  The Unified Property Map part MUST also include "Resource-Id" and
      "Content-Type" in its header.  The value of "Resource-Id" has the
      format of a Part Resource ID.  The "Content-Type" MUST be
      "application/alto-propmap+json".

      The body of the Unified Property Map part MUST be a JSON object
      with the same format as defined in Section 4.6 of
      [I-D.ietf-alto-unified-props-new].  The JSON object MUST include
      the "dependent-vtags" field in the "meta" field.  The value of the
      "dependent-vtags" field MUST be an array of VersionTag objects as
      defined by Section 10.3 of [RFC7285].  The "vtag" of the Path
      Vector part MUST be included in the "dependent-vtags".  If
      "persistent-entity-id" is requested, the version tags of the
      dependent resources that may expose the entities in the response
      MUST also be included.  The PropertyMapData has one member for
      each ANEName that appears in the Path Vector part, which is an
      entity identifier belonging to the self-defined entity domain as
      defined in Section 5.1.2.3 of [I-D.ietf-alto-unified-props-new].
      The EntityProps has one member for each property requested by an
      ALTO client if applicable.



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   If the "start" parameter is not present, the Path Vector part MUST be
   the first part in the multipart response.

   Example: Consider the network in Figure 1.  The response of the
   example request in Section 7.1.3 is as follows, where "ANE1"
   represents the aggregation of all the switches in the network.













































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   HTTP/1.1 200 OK
   Content-Length: [TBD]
   Content-Type: multipart/related; boundary=example-1;
                 type=application/alto-costmap+json

   --example-1
   Resource-Id: costmap
   Content-Type: application/alto-costmap+json

   {
     "meta": {
       "vtag": {
         "resource-id": "filtered-cost-map-pv.costmap",
         "tag": "d827f484cb66ce6df6b5077cb8562b0a"
       },
       "dependent-vtags": [
         {
           "resource-id": "my-default-networkmap",
           "tag": "75ed013b3cb58f896e839582504f6228"
         }
       ],
       "cost-type": { "cost-mode": "array", "cost-metric": "ane-path" }
     },
     "cost-map": {
       "PID1": { "PID2": ["ANE1"] }
     }
   }
   --example-1
   Resource-Id: propmap
   Content-Type: application/alto-propmap+json

   {
     "meta": {
       "dependent-vtags": [
         {
           "resource-id": "filtered-cost-map-pv.costmap",
           "tag": "d827f484cb66ce6df6b5077cb8562b0a"
         }
       ]
     },
     "property-map": {
       ".ane:ANE1": { "max-reservable-bandwidth": 100000000 }
     }
   }







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7.2.  Multipart Endpoint Cost Service for Path Vector

   This document introduces a new ALTO resource called multipart
   endpoint cost resource, which allows an ALTO server to provide other
   ALTO resources associated to the endpoint cost resource in the same
   response.

7.2.1.  Media Type

   The media type of the multipart endpoint cost resource is
   "multipart/related;type=application/alto-endpointcost+json".

7.2.2.  HTTP Method

   The multipart endpoint cost resource is requested using the HTTP POST
   method.

7.2.3.  Accept Input Parameters

   The input parameters of the multipart endpoint cost resource are
   supplied in the body of an HTTP POST request.  This document extends
   the input parameters to an endpoint cost map with a data format
   indicated by the media type "application/alto-
   endpointcostparams+json", which is a JSON object of type
   PVEndpointCostParams, where

   object {
     [EntityPropertyName ane-property-names<0..*>;]
   } PVReqEndpointcost : ReqEndpointcost;

   with fields:

   ane-property-names:  This document defines the "ane-property-names"
      in PVReqEndpointcost as the same as in PVReqFilteredCostMap.  See
      Section 7.1.3.

   Example: Consider the network in Figure 1.  If an ALTO client wants
   to query the "max-reservable-bandwidth" between eh1 and eh2, it can
   submit the following request.












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   POST /ecs/pv HTTP/1.1
   Host: alto.example.com
   Accept: multipart/related;type=application/alto-endpointcost+json,
           application/alto-error+json
   Content-Length: [TBD]
   Content-Type: application/alto-endpointcostparams+json

   {
     "cost-type": {
       "cost-mode": "array",
       "cost-metric": "ane-path"
     },
     "endpoints": {
       "srcs": [ "ipv4:1.2.3.4" ],
       "dsts": [ "ipv4:2.3.4.5" ]
     },
     "ane-property-names": [ "max-reservable-bandwidth" ]
   }

7.2.4.  Capabilities

   The capabilities of the multipart endpoint cost resource are defined
   by a JSON object of type PVEndpointcostCapabilities, which is defined
   as the same as PVFilteredCostMapCapabilities.  See Section 7.1.4.

7.2.5.  Uses

   If this resource supports "persistent-entity-id", it MUST also
   include the defining resources of persistent ANEs that may appear in
   the response.

7.2.6.  Response

   The response MUST indicate an error, using ALTO protocol error
   handling, as defined in Section 8.5 of [RFC7285], if the request is
   invalid.

   The "Content-Type" header of the response MUST be "multipart/related"
   as defined by [RFC7285] with the following parameters:

   type:  The type parameter MUST be "application/alto-
      endpointcost+json".

   start:  The start parameter is as defined in Section 7.1.6.

   boundary:  The boundary parameter is as defined in [RFC2387].

   The body consists of two parts:



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   *  The Path Vector part MUST include "Resource-Id" and "Content-Type"
      in its header.  The value of "Resource-Id" MUST has the format of
      a Part Resource ID.  The "Content-Type" MUST be "application/alto-
      endpointcost+json".

      The body of the Path Vector part MUST be a JSON object with the
      same format as defined in Section 11.5.1.6 of [RFC7285].  The JSON
      object MUST include the "vtag" field in the "meta" field, which
      provides the version tag of the returned endpoint cost map.  The
      resource ID of the version tag MUST follow the format in
      Section 5.3.2.

   *  The Unified Property Map part MUST also include "Resource-Id" and
      "Content-Type" in its header.  The value of "Resource-Id" MUST has
      the format of a Part Resource ID.  The "Content-Type" MUST be
      "application/alto-propmap+json".

      The body of the Unified Property Map part MUST be a JSON object
      with the same format as defined in Section 4.6 of
      [I-D.ietf-alto-unified-props-new].  The JSON object MUST include
      the "dependent-vtags" field in the "meta" field.  The value of the
      "dependent-vtags" field MUST be an array of VersionTag objects as
      defined by Section 10.3 of [RFC7285].  The "vtag" of the Path
      Vector part MUST be included in the "dependent-vtags".  If
      "persistent-entity-id" is requested, the version tags of the
      dependent resources that may expose the entities in the response
      MUST also be included.  The PropertyMapData has one member for
      each ANEName that appears in the Path Vector part, which is an
      entity identifier belonging to the self-defined entity domain as
      defined in Section 5.1.2.3 of [I-D.ietf-alto-unified-props-new].
      The EntityProps has one member for each property requested by the
      ALTO client if applicable.

   If the "start" parameter is not present, the Path Vector part MUST be
   the first part in the multipart response.

   Example: Consider the network in Figure 1.  The response of the
   example request in Section 7.2.3 is as follows.













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   HTTP/1.1 200 OK
   Content-Length: [TBD]
   Content-Type: multipart/related; boundary=example-1;
                 type=application/alto-endpointcost+json

   --example-1
   Resource-Id: ecs
   Content-Type: application/alto-endpointcost+json

   {
     "meta": {
       "vtag": {
         "resource-id": "ecs-pv.costmap",
         "tag": "d827f484cb66ce6df6b5077cb8562b0a"
       },
       "dependent-vtags": [
         {
           "resource-id": "my-default-networkmap",
           "tag": "75ed013b3cb58f896e839582504f6228"
         }
       ],
       "cost-type": { "cost-mode": "array", "cost-metric": "ane-path" }
     },
     "cost-map": {
       "ipv4:1.2.3.4": { "ipv4:2.3.4.5": ["ANE1"] }
     }
   }
   --example-1
   Resource-Id: propmap
   Content-Type: application/alto-propmap+json

   {
     "meta": {
       "dependent-vtags": [
         {
           "resource-id": "ecs-pv.costmap",
           "tag": "d827f484cb66ce6df6b5077cb8562b0a"
         }
       ]
     },
     "property-map": {
       ".ane:ANE1": { "max-reservable-bandwidth": 100000000 }
     }
   }







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8.  Examples

   This section lists some examples of Path Vector queries and the
   corresponding responses.  Some long lines are truncated for better
   readability.

8.1.  Example: Information Resource Directory

   To give a comprehensive example of the Path Vector extension, we
   consider the network in Figure 5.  The example ALTO server provides
   the following information resources:

   *  "my-default-networkmap": A Network Map resource which contains the
      PIDs in the network.

   *  "filtered-cost-map-pv": A Multipart Filtered Cost Map resource for
      Path Vector, which exposes the "max-reservable-bandwidth" property
      for the PIDs in "my-default-networkmap".

   *  "ane-props": A filtered Unified Property resource that exposes the
      information for persistent ANEs in the network.

   *  "endpoint-cost-pv": A Multipart Endpoint Cost Service for Path
      Vector, which exposes the "max-reservable-bandwidth" and the
      "persistent-entity-id" properties.

   *  "update-pv": An Update Stream service, which provides the
      incremental update service for the "endpoint-cost-pv" service.

   Below is the Information Resource Directory of the example ALTO
   server.  To enable the Path Vector extension, the "path-vector" cost
   type (Section 6.5) is defined in the "cost-types" of the "meta"
   field, and is included in the "cost-type-names" of resources
   "filetered-cost-map-pv" and "endpoint-cost-pv".

   {
     "meta": {
       "cost-types": {
         "path-vector": {
           "cost-mode": "array",
           "cost-metric": "ane-path"
         }
       }
     },
     "resources": {
       "my-default-networkmap": {
         "uri" : "https://alto.example.com/networkmap",
         "media-type" : "application/alto-networkmap+json"



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       },
       "filtered-cost-map-pv": {
         "uri": "https://alto.example.com/costmap/pv",
         "media-type": "multipart/related;
                        type=application/alto-costmap+json",
         "accepts": "application/alto-costmapfilter+json",
         "capabilities": {
           "cost-type-names": [ "path-vector" ],
           "ane-property-names": [ "max-reservable-bandwidth" ]
         },
         "uses": [ "my-default-networkmap" ]
       },
       "ane-props": {
         "uri": "https://alto.example.com/ane-props",
         "media-type": "application/alto-propmap+json",
         "accepts": "application/alto-propmapparams+json",
         "capabilities": {
           "mappings": {
             ".ane": [ "cpu" ]
           }
         }
       },
       "endpoint-cost-pv": {
         "uri": "https://alto.exmaple.com/endpointcost/pv",
         "media-type": "multipart/related;
                        type=application/alto-endpointcost+json",
         "accepts": "application/alto-endpointcostparams+json",
         "capabilities": {
           "cost-type-names": [ "path-vector" ],
           "ane-property-names": [
             "max-reservable-bandwidth", "persistent-entity-id"
           ]
         },
         "uses": [ "ane-props" ]
       },
       "update-pv": {
         "uri": "https://alto.example.com/updates/pv",
         "media-type": "text/event-stream",
         "uses": [ "endpoint-cost-pv" ],
         "accepts": "application/alto-updatestreamparams+json",
         "capabilities": {
           "support-stream-control": true
         }
       }
     }
   }





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8.2.  Example: Multipart Filtered Cost Map

   The following examples demonstrate the request to the "filtered-cost-
   map-pv" resource and the corresponding response.

   The request uses the "path-vector" cost type in the "cost-type"
   field.  The "ane-property-names" field is missing, indicating that
   the client only requests for the Path Vector but not the ANE
   properties.

   The response consists of two parts.  The first part returns the array
   of ANEName for each source and destination pair.  There are two ANEs,
   where "L1" represents the interconnection link L1, and "L2"
   represents the interconnection link L2.

   The second part returns an empty Property Map. Note that the ANE
   entries are omitted since they have no properties (See Section 3.1 of
   [I-D.ietf-alto-unified-props-new]).

   POST /costmap/pv HTTP/1.1
   Host: alto.example.com
   Accept: multipart/related;type=application/alto-costmap+json,
           application/alto-error+json
   Content-Length: [TBD]
   Content-Type: application/alto-costmapfilter+json

   {
     "cost-type": {
       "cost-mode": "array",
       "cost-metric": "ane-path"
     },
     "pids": {
       "srcs": [ "PID1" ],
       "dsts": [ "PID3", "PID4" ]
     }
   }

   HTTP/1.1 200 OK
   Content-Length: [TBD]
   Content-Type: multipart/related; boundary=boundary;
                 type=application/alto-costmap+json

   --boundary
   Resource-Id: costmap
   Content-Type: application/alto-costmap+json

   {
     "meta": {



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       "vtag": {
         "resource-id": "filtered-cost-map-pv.costmap",
         "tag": "d827f484cb66ce6df6b5077cb8562b0a"
       },
       "dependent-vtags": [
         {
           "resource-id": "my-default-networkmap",
           "tag": "75ed013b3cb58f896e839582504f6228"
         }
       ],
       "cost-type": {
         "cost-mode": "array",
         "cost-metric": "ane-path"
       }
     },
     "cost-map": {
       "PID1": {
         "PID3": [ "L1" ],
         "PID4": [ "L1", "L2" ]
       }
     }
   }
   --boundary
   Resource-Id: propmap
   Content-Type: application/alto-propmap+json

   {
     "meta": {
       "dependent-vtags": [
         {
           "resource-id": "filtered-cost-map-pv.costmap",
           "tag": "d827f484cb66ce6df6b5077cb8562b0a"
         }
       ]
     },
     "property-map": {
     }
   }

8.3.  Example: Multipart Endpoint Cost Resource

   The following examples demonstrate the request to the "endpoint-cost-
   pv" resource and the corresponding response.

   The request uses the path vector cost type in the "cost-type" field,
   and queries the Maximum Reservable Bandwidth ANE property and the
   Persistent Entity property.




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   The response consists of two parts.  The first part returns the array
   of ANEName for each valid source and destination pair, where "NET1"
   represent sub-network NET1, and "AGGR" is the aggregation of L1 and
   NET3.

   The second part returns the requested properties of ANEs.  Since NET1
   has sufficient bandwidth, it sets the "max-reservable-bandwidth" to a
   sufficiently large number.  It also represents a persistent ANE
   defined in the "ane-props" resource, identified by "ane-
   props.ane:datacenter1".  The aggregated "max-reservable-bandwidth" of
   ane:AGGR is constrained by the link capacity of L1.  The "persistent-
   entity-id" property is omitted as both L1 and NET3 do not represent
   any persistent entity.

   POST /endpointcost/pv HTTP/1.1
   Host: alto.example.com
   Accept: multipart/related;
           type=application/alto-endpointcost+json,
           application/alto-error+json
   Content-Length: [TBD]
   Content-Type: application/alto-endpointcostparams+json

   {
     "cost-type": {
       "cost-mode": "array",
       "cost-metric": "ane-path"
     },
     "endpoints": {
       "srcs": [ "ipv4:1.2.3.4", "ipv4:2.3.4.5" ],
       "dsts": [ "ipv4:3.4.5.6" ]
     },
     "ane-property-names": [
       "max-reservable-bandwidth",
       "persistent-entity-id"
     ]
   }

   HTTP/1.1 200 OK
   Content-Length: [TBD]
   Content-Type: multipart/related; boundary=boundary;
                 type=application/alto-endpointcost+json

   --boundary
   Resource-Id: ecs
   Content-Type: application/alto-endpointcost+json

   {
     "meta": {



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       "vtags": {
         "resource-id": "endpoint-cost-pv.ecs",
         "tag": "bb6bb72eafe8f9bdc4f335c7ed3b10822a391cef"
       },
       "cost-type": {
         "cost-mode": "array",
         "cost-metric": "ane-path"
       }
     },
     "endpoint-cost-map": {
       "ipv4:1.2.3.4": {
         "ipv4:3.4.5.6":   [ "NET1", "AGGR" ]
       },
       "ipv4:2.3.4.5": {
         "ipv4:3.4.5.6":   [ "NET1", "AGGR" ]
       }
     }
   }
   --boundary
   Resource-Id: propmap
   Content-Type: application/alto-propmap+json

   {
     "meta": {
       "dependent-vtags": [
         {
           "resource-id": "endpoint-cost-pv.ecs",
           "tag": "bb6bb72eafe8f9bdc4f335c7ed3b10822a391cef"
         },
         {
           "resource-id": "ane-props",
           "tag": "bf3c8c1819d2421c9a95a9d02af557a3"
         }
       ]
     },
     "property-map": {
       ".ane:NET1": {
         "max-reservable-bandwidth": 50000000000,
         "persistent-entity-id": "ane-props.ane:datacenter1",
       },
       ".ane:AGGR": {
         "max-reservable-bandwidth": 10000000000
       }
     }
   }

   After the client obtains "ane-props.ane:datacenter1", it can query
   the "ane-props" resource to get the properties of the persistent ANE.



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8.4.  Example: Incremental Updates

   In this example, an ALTO client subscribes to the incremental update
   for the multipart endpoint cost resource "endpoint-cost-pv".

   POST /updates/pv HTTP/1.1
   Host: alto.example.com
   Accept: text/event-stream
   Content-Type: application/alto-updatestreamparams+json
   Content-Length: [TBD]

   {
     "add": {
       "ecspvsub1": {
         "resource-id": "endpoint-cost-pv",
         "input": <ecs-input>
       }
     }
   }

   Based on the server-side process defined in
   [I-D.ietf-alto-incr-update-sse], the ALTO server will send the
   "control-uri" first using Server-Sent Event (SSE), followed by the
   full response of the multipart message.

   HTTP/1.1 200 OK
   Connection: keep-alive
   Content-Type: text/event-stream

   event: application/alto-updatestreamcontrol+json
   data: {"control-uri": "https://alto.example.com/updates/streams/123"}

   event: multipart/related;boundary=boundary;
          type=application/alto-endpointcost+json,ecspvsub1
   data: --boundary
   data: Resource-ID: ecsmap
   data: Content-Type: application/alto-endpointcost+json
   data:
   data: <endpoint-cost-map-entry>
   data: --boundary
   data: Resource-ID: propmap
   data: Content-Type: application/alto-propmap+json
   data:
   data: <property-map-entry>
   data: --boundary--

   When the contents change, the ALTO server will publish the updates
   for each node in this tree separately.



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   event: application/merge-patch+json, ecspvsub1.ecsmap
   data: <Merge patch for endpoint-cost-map-update>

   event: application/merge-patch+json, ecspvsub1.propmap
   data: <Merge patch for property-map-update>

9.  Compatibility

9.1.  Compatibility with Legacy ALTO Clients/Servers

   The multipart filtered cost map resource and the multipart endpoint
   cost resource has no backward compatibility issue with legacy ALTO
   clients and servers.  Although these two types of resources reuse the
   media types defined in the base ALTO protocol for the accept input
   parameters, they have different media types for responses.  If the
   ALTO server provides these two types of resources, but the ALTO
   client does not support them, the ALTO client will ignore the
   resources without conducting any incompatibility.

9.2.  Compatibility with Multi-Cost Extension

   This document does not specify how to integrate the Path Vector cost
   type with the multi-cost extension [RFC8189].  While it is not
   RECOMMENDED to put the Path Vector cost type with other cost types in
   a single query, there is no compatible issue.

9.3.  Compatibility with Incremental Update

   The extension specified in this document is not compatible with the
   original incremental update extension
   [I-D.ietf-alto-incr-update-sse].  A legacy ALTO client cannot
   recognize the compound client-id, and a legacy ALTO server may use
   the same client-id for updates of both parts.

   ALTO clients and servers must follow the specifications given in this
   document to ensure compatibility with the incremental update
   extension.

9.4.  Compatibility with Cost Calendar

   The extension specified in this document is compatible with the Cost
   Calendar extension [I-D.ietf-alto-cost-calendar].  When used together
   with the Cost Calendar extension, the cost value between a source and
   a destination is an array of path vectors, where the k-th path vector
   refers to the abstract network paths traversed in the k-th time
   interval by traffic from the source to the destination.





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   When used with time-varying properties, e.g., maximum reservable
   bandwidth (maxresbw), a property of a single entity may also have
   different values in different time intervals.  In this case, an ANE
   with different property values must be considered as different ANEs.

   The two extensions combined together can provide the historical
   network correlation information for a set of source and destination
   pairs.  A network broker or client may use this information to derive
   other resource requirements such as Time-Block-Maximum Bandwidth,
   Bandwidth-Sliding-Window, and Time-Bandwidth-Product (TBP) (See
   [SENSE] for details).

10.  General Discussions

10.1.  Constraint Tests for General Cost Types

   The constraint test is a simple approach to query the data.  It
   allows users to filter the query result by specifying some boolean
   tests.  This approach is already used in the ALTO protocol.
   [RFC7285] and [RFC8189] allow ALTO clients to specify the
   "constraints" and "or-constraints" tests to better filter the result.

   However, the current syntax can only be used to test scalar cost
   types, and cannot easily express constraints on complex cost types,
   e.g., the Path Vector cost type defined in this document.

   In practice, developing a language for general-purpose boolean tests
   can be complex and is likely to be a duplicated work.  Thus, it is
   worth looking into the direction of integrating existing well-
   developed query languages, e.g., XQuery and JSONiq, or their subset
   with ALTO.

   Filtering the Path Vector results or developing a more sophisticated
   filtering mechanism is beyond the scope of this document.

10.2.  General Multipart Resources Query

   Querying multiple ALTO information resources continuously may be a
   general requirement.  And the coming issues like inefficiency and
   inconsistency are also general.  There is no standard solving these
   issues yet.  So we need some approach to make the ALTO client request
   the compound ALTO information resources in a single query.

11.  Security Considerations

   This document is an extension of the base ALTO protocol, so the
   Security Considerations [RFC7285] of the base ALTO protocol fully
   apply when this extension is provided by an ALTO server.



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   The Path Vector extension requires additional considerations on two
   security considerations discussed in the base protocol:
   confidentiality of ALTO information (Section 15.3 of [RFC7285]) and
   availability of ALTO service (Section 15.5 of [RFC7285]).

   For confidentiality of ALTO information, a network operator should be
   aware of that this extension may introduce a new risk: the Path
   Vector information may make network attacks easier.  For example, as
   the Path Vector information may reveal more fine-grained internal
   network structures than the base protocol, an ALTO client may detect
   the bottleneck link and start a distributed denial-of-service (DDoS)
   attack involving minimal flows to conduct the in-network congestion.

   To mitigate this risk, the ALTO server should consider protection
   mechanisms to reduce information exposure or obfuscate the real
   information, in particular, in settings where the network and the
   application do not belong to the same trust domain.  But the
   implementation of Path Vector extension involving reduction or
   obfuscation should guarantee the requested properties are still
   accurate.

   For availability of ALTO service, an ALTO server should be cognizant
   that using Path Vector extension might have a new risk: frequent
   requesting for path vectors might conduct intolerable increment of
   the server-side storage and break the ALTO server.  It is known that
   the computation of Path Vectors is unlikely to be cacheable, in that
   the results will depend on the particular requests (e.g., where the
   flows are distributed).  Hence, the service providing Path Vectors
   may become an entry point for denial-of-service attacks on the
   availability of an ALTO server.  To avoid this risk, authenticity and
   authorization of this ALTO service may need to be better protected.

12.  IANA Considerations

12.1.  ALTO Entity Domain Registry

   This document registers a new entry to the ALTO Domain Entity
   Registry, as instructed by Section 12.2 of
   [I-D.ietf-alto-unified-props-new].  The new entry is as shown below
   in Table 1.











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     +------------+----------------+-------------+-------------------+
     | Identifier | Entity Address | Hierarchy & | Media Type of     |
     |            | Encoding       | Inheritance | Defining Resource |
     +============+================+=============+===================+
     | ane        | See            | None        | application/alto- |
     |            | Section 6.2.2  |             | propmap+json      |
     +------------+----------------+-------------+-------------------+

                        Table 1: ALTO Entity Domain

   Identifier:  See Section 6.2.1.

   Entity Identifier Encoding:  See Section 6.2.2.

   Hierarchy:  None

   Inheritance:  None

   Media Type of Defining Resource:  See Section 6.2.4.

   Security Considerations:  In some usage scenarios, ANE addresses
      carried in ALTO Protocol messages may reveal information about an
      ALTO client or an ALTO service provider.  Applications and ALTO
      service providers using addresses of ANEs will be made aware of
      how (or if) the addressing scheme relates to private information
      and network proximity, in further iterations of this document.

12.2.  ALTO Entity Property Type Registry

   Two initial entries are registered to the ALTO Domain "ane" in the
   "ALTO Entity Property Type Registry", as instructed by Section 12.3
   of [I-D.ietf-alto-unified-props-new].  The two new entries are shown
   below in Table 2.

             +--------------------------+--------------------+
             | Identifier               | Intended Semantics |
             +==========================+====================+
             | max-reservable-bandwidth | See Section 6.4.1  |
             +--------------------------+--------------------+
             | persistent-entity-id     | See Section 6.4.2  |
             +--------------------------+--------------------+

                 Table 2: Initial Entries for ane Domain in
                  the ALTO Entity Property Types Registry







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13.  Acknowledgments

   The authors would like to thank discussions with Andreas Voellmy,
   Erran Li, Haibin Song, Haizhou Du, Jiayuan Hu, Qiao Xiang, Tianyuan
   Liu, Xiao Shi, Xin Wang, and Yan Luo. The authors thank Greg
   Bernstein (Grotto Networks), Dawn Chen (Tongji University), Wendy
   Roome, and Michael Scharf for their contributions to earlier drafts.

14.  References

14.1.  Normative References

   [I-D.ietf-alto-cost-calendar]
              Randriamasy, S., Yang, Y., WU, Q., Lingli, D., and N.
              Schwan, "Application-Layer Traffic Optimization (ALTO)
              Cost Calendar", Work in Progress, Internet-Draft, draft-
              ietf-alto-cost-calendar-21, 17 March 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-alto-cost-
              calendar-21.txt>.

   [I-D.ietf-alto-incr-update-sse]
              Roome, W. and Y. Yang, "ALTO Incremental Updates Using
              Server-Sent Events (SSE)", Work in Progress, Internet-
              Draft, draft-ietf-alto-incr-update-sse-22, 20 March 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-alto-incr-
              update-sse-22.txt>.

   [I-D.ietf-alto-performance-metrics]
              WU, Q., Yang, Y., Lee, Y., Dhody, D., Randriamasy, S., and
              L. Contreras, "ALTO Performance Cost Metrics", Work in
              Progress, Internet-Draft, draft-ietf-alto-performance-
              metrics-11, 12 June 2020, <http://www.ietf.org/internet-
              drafts/draft-ietf-alto-performance-metrics-11.txt>.

   [I-D.ietf-alto-unified-props-new]
              Roome, W., Randriamasy, S., Yang, Y., Zhang, J., and K.
              Gao, "Unified Properties for the ALTO Protocol", Work in
              Progress, Internet-Draft, draft-ietf-alto-unified-props-
              new-11, 9 March 2020, <http://www.ietf.org/internet-
              drafts/draft-ietf-alto-unified-props-new-11.txt>.

   [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>.

   [RFC2387]  Levinson, E., "The MIME Multipart/Related Content-type",




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              RFC 2387, DOI 10.17487/RFC2387, August 1998,
              <https://www.rfc-editor.org/info/rfc2387>.

   [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>.

   [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>.

   [RFC8189]  Randriamasy, S., Roome, W., and N. Schwan, "Multi-Cost
              Application-Layer Traffic Optimization (ALTO)", RFC 8189,
              DOI 10.17487/RFC8189, October 2017,
              <https://www.rfc-editor.org/info/rfc8189>.

14.2.  Informative References

   [AAAI2019] Xiang, Q., Yu, H., Aspnes, J., Le, F., Kong, L., and Y.R.
              Yang, "Optimizing in the dark: Learning an optimal
              solution through a simple request interface", Proceedings
              of the AAAI Conference on Artificial Intelligence 33,
              1674-1681 , 2019.

   [I-D.bernstein-alto-topo]
              Bernstein, G., Yang, Y., and Y. Lee, "ALTO Topology
              Service: Uses Cases, Requirements, and Framework", Work in
              Progress, Internet-Draft, draft-bernstein-alto-topo-00, 21
              October 2013, <http://www.ietf.org/internet-drafts/draft-
              bernstein-alto-topo-00.txt>.

   [I-D.contreras-alto-service-edge]
              Contreras, L., Perez, D., and C. Rothenberg, "Use of ALTO
              for Determining Service Edge", Work in Progress, Internet-
              Draft, draft-contreras-alto-service-edge-00, 4 November
              2019, <http://www.ietf.org/internet-drafts/draft-
              contreras-alto-service-edge-00.txt>.

   [I-D.huang-alto-mowie-for-network-aware-app]
              Huang, W., Zhang, Y., Yang, R., Xiong, C., Lei, Y., Han,
              Y., and G. Li, "MoWIE for Network Aware Application", Work
              in Progress, Internet-Draft, draft-huang-alto-mowie-for-
              network-aware-app-00, 9 March 2020, <http://www.ietf.org/
              internet-drafts/draft-huang-alto-mowie-for-network-aware-
              app-00.txt>.




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   [I-D.ietf-dmm-5g-uplane-analysis]
              Homma, S., Miyasaka, T., Matsushima, S., and D. Voyer,
              "User Plane Protocol and Architectural Analysis on 3GPP 5G
              System", Work in Progress, Internet-Draft, draft-ietf-dmm-
              5g-uplane-analysis-03, 3 November 2019,
              <http://www.ietf.org/internet-drafts/draft-ietf-dmm-5g-
              uplane-analysis-03.txt>.

   [I-D.yang-alto-deliver-functions-over-networks]
              Yang, S., Cui, L., Xu, M., Yang, Y., and R. Huang,
              "Delivering Functions over Networks: Traffic and
              Performance Optimization for Edge Computing using ALTO",
              Work in Progress, Internet-Draft, draft-yang-alto-deliver-
              functions-over-networks-01, 13 July 2020,
              <http://www.ietf.org/internet-drafts/draft-yang-alto-
              deliver-functions-over-networks-01.txt>.

   [LHC]      "CERN - LHC", 2019, <https://atlas.cern/tags/lhc>.

   [SENSE]    "Services - SENSE", 2019, <http://sense.es.net/services>.

   [TON2019]  Gao, K., Xiang, Q., Wang, X., Yang, Y.R., and J. Bi, "An
              objective-driven on-demand network abstraction for
              adaptive applications", IEEE/ACM Transactions on
              Networking (TON) Vol 27, no. 2 (2019): 805-818., 2019.

Appendix A.  Changes since -10

   Revision -11

   *  replaces "part" with "components" in the abstract;

   *  identifies additional requirements (AR) derived from the flow
      scheduling example, and introduces how the extension addresses the
      additional requirements

   *  fixes the inconsistent use of "start" parameter in multipart
      responses;

   *  specifies explicitly how to handle "cost-constraints";

   *  uses the latest IANA registration mechanism defined in
      [I-D.ietf-alto-unified-props-new];

   *  renames "persistent-entities" to "persistent-entity-id";

   *  makes "application/alto-propmap+json" as the media type of
      defining resources for the "ane" domain;



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   *  updates the examples;

   *  adds the discussion on ephemeral and persistent ANEs.

Appendix B.  Changes since -09

   Revision -10

   *  revises the introduction which

      -  extends the scope where the PV extension can be applied beyond
         the "path correlation" information

   *  brings back the capacity region use case to better illustrate the
      problem

   *  revises the overview to explain and defend the concepts and
      decision choices

   *  fixes inconsistent terms, typos

Appendix C.  Changes since -08

   This revision

   *  fixes a few spelling errors

   *  emphasizes that abstract network elements can be generated on
      demand in both introduction and motivating use cases

Appendix D.  Changes Since Version -06

   *  We emphasize the importance of the path vector extension in two
      aspects:

      1.  It expands the problem space that can be solved by ALTO, from
          preferences of network paths to correlations of network paths.

      2.  It is motivated by new usage scenarios from both application's
          and network's perspectives.

   *  More use cases are included, in addition to the original capacity
      region use case.

   *  We add more discussions to fully explore the design space of the
      path vector extension and justify our design decisions, including
      the concept of abstract network element, cost type (reverted to
      -05), newer capabilities and the multipart message.



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   *  Fix the incremental update process to be compatible with SSE -16
      draft, which uses client-id instead of resource-id to demultiplex
      updates.

   *  Register an additional ANE property (i.e., persistent-entities) to
      cover all use cases mentioned in the draft.

Authors' Addresses

   Kai Gao
   China
   610000
   Chengdu
   No.24 South Section 1, Yihuan Road
   Sichuan University

   Email: kaigao@scu.edu.cn


   Young Lee


   Sabine Randriamasy
   Nokia Bell Labs
   Route de Villejust
   91460 Nozay
   France

   Email: sabine.randriamasy@nokia-bell-labs.com


   Yang Richard Yang
   Yale University
   51 Prospect Street
   New Haven,  CT
   United States of America

   Email: yry@cs.yale.edu


   Jingxuan Jensen Zhang
   China
   201804
   Shanghai
   4800 Caoan Road
   Tongji University

   Email: jingxuan.n.zhang@gmail.com



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