ALTO Extension: Path Vector
draft-ietf-alto-path-vector-14

Document Type Active Internet-Draft (alto WG)
Authors Kai Gao  , Young Lee  , Sabine Randriamasy  , Y. Yang  , Jingxuan Zhang 
Last updated 2021-02-22
Replaces draft-yang-alto-path-vector
Stream Internet Engineering Task Force (IETF)
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Send notices to vijay.gurbani@gmail.com
ALTO                                                              K. Gao
Internet-Draft                                        Sichuan University
Intended status: Standards Track                                  Y. Lee
Expires: 26 August 2021                                          Samsung
                                                          S. Randriamasy
                                                         Nokia Bell Labs
                                                               Y.R. Yang
                                                         Yale University
                                                                J. Zhang
                                                       Tongji University
                                                        22 February 2021

                      ALTO Extension: Path Vector
                     draft-ietf-alto-path-vector-14

Abstract

   This document is an extension to the base Application-Layer Traffic
   Optimization (ALTO) protocol.  It extends the ALTO Cost Map service
   and ALTO Property Map service so that the application can decide
   which endpoint(s) to connect based on not only numerical/ordinal cost
   values but also details of the paths.  This is useful for
   applications whose performance is impacted by specified components of
   a network on the end-to-end paths, e.g., they may infer that several
   paths share common links and prevent traffic bottlenecks by avoiding
   such paths.  This extension introduces a new abstraction called
   Abstract Network Element (ANE) to represent these components and
   encodes a network path as a vector of ANEs.  Thus, it provides a more
   complete but still abstract graph representation of the underlying
   network(s) for informed traffic optimization among endpoints.

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 https://datatracker.ietf.org/drafts/current/.

   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 26 August 2021.

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Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include 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  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Requirements Languages  . . . . . . . . . . . . . . . . . . .   6
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Problem Statement . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Design Requirements . . . . . . . . . . . . . . . . . . .   7
     4.2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.2.1.  Large-scale Data Analytics  . . . . . . . . . . . . .  10
       4.2.2.  Context-aware Data Transfer . . . . . . . . . . . . .  11
       4.2.3.  CDN and Service Edge  . . . . . . . . . . . . . . . .  11
   5.  Path Vector Extension: Overview . . . . . . . . . . . . . . .  11
     5.1.  Abstract Network Element  . . . . . . . . . . . . . . . .  12
       5.1.1.  ANE Domain  . . . . . . . . . . . . . . . . . . . . .  12
       5.1.2.  Ephemeral ANE and Persistent ANE  . . . . . . . . . .  12
       5.1.3.  Property Filtering  . . . . . . . . . . . . . . . . .  13
     5.2.  Path Vector Cost Type . . . . . . . . . . . . . . . . . .  13
     5.3.  Multipart Path Vector Response  . . . . . . . . . . . . .  14
       5.3.1.  Identifying the Media Type of the Root Object . . . .  15
       5.3.2.  References to Part Messages . . . . . . . . . . . . .  16
   6.  Specification: Basic Data Types . . . . . . . . . . . . . . .  16
     6.1.  ANE Name  . . . . . . . . . . . . . . . . . . . . . . . .  16
     6.2.  ANE Domain  . . . . . . . . . . . . . . . . . . . . . . .  16
       6.2.1.  Entity Domain Type  . . . . . . . . . . . . . . . . .  16
       6.2.2.  Domain-Specific Entity Identifier . . . . . . . . . .  16
       6.2.3.  Hierarchy and Inheritance . . . . . . . . . . . . . .  16
       6.2.4.  Media Type of Defining Resource . . . . . . . . . . .  17
     6.3.  ANE Property Name . . . . . . . . . . . . . . . . . . . .  17
     6.4.  Initial ANE Property Types  . . . . . . . . . . . . . . .  17
       6.4.1.  New ANE Property Type: Maximum Reservable
               Bandwidth . . . . . . . . . . . . . . . . . . . . . .  18
       6.4.2.  New ANE Property Type: Persistent Entity ID . . . . .  19
     6.5.  Path Vector Cost Type . . . . . . . . . . . . . . . . . .  19
       6.5.1.  Cost Metric: ane-path . . . . . . . . . . . . . . . .  19

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       6.5.2.  Cost Mode: array  . . . . . . . . . . . . . . . . . .  20
     6.6.  Part Resource ID  . . . . . . . . . . . . . . . . . . . .  20
   7.  Specification: Service Extensions . . . . . . . . . . . . . .  20
     7.1.  Notations . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.2.  Multipart Filtered Cost Map for Path Vector . . . . . . .  20
       7.2.1.  Media Type  . . . . . . . . . . . . . . . . . . . . .  20
       7.2.2.  HTTP Method . . . . . . . . . . . . . . . . . . . . .  21
       7.2.3.  Accept Input Parameters . . . . . . . . . . . . . . .  21
       7.2.4.  Capabilities  . . . . . . . . . . . . . . . . . . . .  22
       7.2.5.  Uses  . . . . . . . . . . . . . . . . . . . . . . . .  23
       7.2.6.  Response  . . . . . . . . . . . . . . . . . . . . . .  23
     7.3.  Multipart Endpoint Cost Service for Path Vector . . . . .  26
       7.3.1.  Media Type  . . . . . . . . . . . . . . . . . . . . .  26
       7.3.2.  HTTP Method . . . . . . . . . . . . . . . . . . . . .  26
       7.3.3.  Accept Input Parameters . . . . . . . . . . . . . . .  26
       7.3.4.  Capabilities  . . . . . . . . . . . . . . . . . . . .  27
       7.3.5.  Uses  . . . . . . . . . . . . . . . . . . . . . . . .  27
       7.3.6.  Response  . . . . . . . . . . . . . . . . . . . . . .  27
   8.  Examples  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
     8.1.  Example: Information Resource Directory . . . . . . . . .  31
     8.2.  Example: Multipart Filtered Cost Map  . . . . . . . . . .  33
     8.3.  Example: Multipart Endpoint Cost Resource . . . . . . . .  34
     8.4.  Example: Incremental Updates  . . . . . . . . . . . . . .  38
   9.  Compatibility with Other ALTO Extensions  . . . . . . . . . .  40
     9.1.  Compatibility with Legacy ALTO Clients/Servers  . . . . .  40
     9.2.  Compatibility with Multi-Cost Extension . . . . . . . . .  40
     9.3.  Compatibility with Incremental Update . . . . . . . . . .  40
     9.4.  Compatibility with Cost Calendar  . . . . . . . . . . . .  40
   10. General Discussions . . . . . . . . . . . . . . . . . . . . .  41
     10.1.  Constraint Tests for General Cost Types  . . . . . . . .  41
     10.2.  General Multi-Resource Query . . . . . . . . . . . . . .  42
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  42
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  44
     12.1.  ALTO Entity Domain Type Registry . . . . . . . . . . . .  44
     12.2.  ALTO Entity Property Type Registry . . . . . . . . . . .  44
   13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  45
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .  45
     14.1.  Normative References . . . . . . . . . . . . . . . . . .  45
     14.2.  Informative References . . . . . . . . . . . . . . . . .  46
   Appendix A.  Changes since -12  . . . . . . . . . . . . . . . . .  48
   Appendix B.  Changes since -11  . . . . . . . . . . . . . . . . .  48
   Appendix C.  Changes since -10  . . . . . . . . . . . . . . . . .  48
   Appendix D.  Changes since -09  . . . . . . . . . . . . . . . . .  49
   Appendix E.  Changes since -08  . . . . . . . . . . . . . . . . .  49
   Appendix F.  Changes Since Version -06  . . . . . . . . . . . . .  49
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  50

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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
   [RFC8896].

   While the existing extensions are sufficient for many overlay
   applications, the QoE of some overlay applications depends not only
   on the cost information of end-to-end paths, but also on particular
   components of a network on the paths 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
   bottleneck links inside the carrier network as link capacity may
   impact the rate of data input/output to the job.  We refer to such
   components of a network as Abstract Network Elements (ANE).

   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 an increase in volume and computation overhead, and
   last because it is difficult for ISPs to figure out what information
   and what details an 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

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   complexity and confidential information.  An "abstract network state"
   is a selected set of abstract representations of Abstract Network
   Elements traversed by the paths between <source, destination> pairs
   combined with properties of these Abstract Network Elements 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 Abstract Network Elements.  The requirements on the server
   scalability can also be reduced by combining Abstract Network
   Elements and their properties in a single response.

   This document extends [RFC7285] to allow an ALTO server to 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 that
   identifies an Abstract Network Element, which 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 involves 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 is 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 extension to the ALTO IRD and the
   information resources, with some concrete examples presented in
   Section 8.  Section 9 discusses the backward compatibility with the
   base protocol and existing extensions.  Security and IANA
   considerations are discussed in Section 11 and Section 12
   respectively.

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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

   NOTE: This document depends on the Unified Property Map extension
   [I-D.ietf-alto-unified-props-new] and should be processed after the
   Unified Property Map document.

   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 an
      abstract representation for a component in a network that handle
      data packets and whose properties can potentially have an impact
      on the end-to-end performance of traffic.  An ANE can be a
      physical device such as a router, a link or an interface, or an
      aggregation of devices such as a subnetwork, or a data center.

      The definition of Abstract Network Element is similar to Network
      Element defined in [RFC2216] in the sense that they both provide
      an abstract representation of particular components of a network.
      However, they have different criteria on how these particular
      components are selected.  Specifically, Network Element requires
      the components to be potentially capable of exercising QoS
      control, while Abstract Network Element only requires the
      components to have an impact on the end-to-end performance.

   *  ANE Name: An ANE can be constructed either statically in advance
      or on demand based on the requested information.  Thus, different
      ANEs may only be valid within a particular scope, either ephemeral
      or persistent.  Within each scope, an ANE is uniquely identified
      by an ANE Name, as defined in Section 6.1.  Note that an ALTO
      client must not assume ANEs in different scopes but with the same
      ANE Name refer to the same component(s) of the network.

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   *  Path Vector: A Path Vector, or an ANE Path Vector, is a JSON array
      of ANE Names.  It is a generalization of BGP path vector.  While
      standard BGP path vector specifies a sequence of autonomous
      systems for a destination IP prefix, the Path Vector defined in
      this extension specifies a sequence of ANEs either for a source
      PID and a destination PID as in a cost map, or for a source
      endpoint and a destination endpoint as in an endpoint cost map.

   *  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.

   *  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 extension defined in this document.

   Assume that an application has control over a set of flows, which may
   go through shared links or switches and share bottlenecks.  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 other links are 100 Mbps.

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                                 +-----+
                                 |     |
                               --+ sw6 +--
                              /  |     |  \
        PID1 +-----+         /   +-----+   \          +-----+  PID2
        eh1__|     |_       /               \     ____|     |__eh2
   192.0.2.2 | sw1 | \   +--|--+         +--|--+ /    | sw2 | 192.0.2.3
             +-----+  \  |     |         |     |/     +-----+
                       \_| sw5 +---------+ sw7 |
        PID3 +-----+   / |     |         |     |\     +-----+  PID4
        eh3__|     |__/  +-----+         +-----+ \____|     |__eh4
   192.0.2.4 | sw3 |                                  | sw4 | 192.0.2.5
             +-----+                                  +-----+

   bw(eh1--sw1) = bw(sw1--sw5) = 150 Mbps
   bw(eh2--sw2) = bw(eh3--sw3) = bw(eh4--sw4) = 100 Mbps
   bw(sw1--sw5) = bw(sw3--sw5) = bw(sw2--sw7) = bw(sw4--sw7) = 100 Mbps
   bw(sw5--sw6) = bw(sw5--sw7) = bw(sw6--sw7) = 100 Mbps

                       Figure 1: Raw Network Topology

   The single-node ALTO topology abstraction of the network is shown in
   Figure 2.  Assume the cost map returns a hypothetical cost type
   representing the available bandwidth between a source and a
   destination.

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

              Figure 2: Base Single-Node Topology Abstraction

   Now assume the application wants to maximize the total rate of the
   traffic among a set of end host <source, destination> pairs, say eh1
   -> eh2 and eh1 -> eh4.  Let x denote the transmission rate of eh1 ->
   eh2 and y denote the rate of eh1 -> eh4.  The objective function is

       max(x + y).

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   With the ALTO Cost Map, the cost between PID1 and PID2 and between
   PID1 and PID4 will be 100 Mbps.  And the client can get a capacity
   region of

       x <= 100 Mbps,
       y <= 100 Mbps.

   With this information, the client may mistakenly think it can achieve
   a maximum total rate of 200 Mbps.  However, one can easily see that
   this rate is infeasible, as there are only two potential cases:

   *  Case 1: eh1 -> eh2 and eh1 -> eh4 take different path segments
      from sw5 to sw7.  For example, if eh1 -> eh2 uses path eh1 -> sw1
      -> sw5 -> sw6 -> sw7 -> sw2 -> eh2 and eh1 -> eh4 uses path eh1 ->
      sw1 -> sw5 -> sw7 -> sw4 -> eh4, then the shared bottleneck links
      are eh1 -> sw1 and sw1 -> sw5.  In this case, the capacity region
      is

          x     <= 100 Mbps
          y     <= 100 Mbps
          x + y <= 150 Mbps

      and the real optimal total rate is 150 Mbps.

   *  Case 2: eh1 -> eh2 and eh1 -> eh4 take the same path segment from
      sw5 to sw7.  For example, if eh1 -> eh2 uses path eh1 -> sw1 ->
      sw5 -> sw7 -> sw2 -> eh2 and eh1 -> eh4 also uses path eh1 -> sw1
      -> sw5 -> sw7 -> sw4 -> eh4, then the shared bottleneck link is
      sw5 -> sw7.  In this case, the capacity region is

          x     <= 100 Mbps
          y     <= 100 Mbps
          x + y <= 100 Mbps

      and the real optimal total rate is 100 Mbps.

   Clearly, with more accurate and fine-grained information, the
   application can gain a better prediction of its traffic and may
   orchestrate its own resources accordingly.  However, to provide such
   information, the network needs to expose more details beyond the
   simple cost map abstraction.  In particular:

   *  The ALTO server must give more details about the network paths
      that are traversed by the traffic between a source and a
      destination beyond a simple numerical value, which allows the
      overlay application to distinguish between Case 1 and Case 2 and
      to compute the optimal total rate accordingly.

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

   *  The ALTO server must give details on the properties of the ANEs
      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 ANEs 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 ANE.

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

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

4.2.  Use Cases

   While the multiple flow scheduling problem is used to help identify
   the additional requirements, the extension defined in this document
   can be applied to a wide range of applications.  This section
   highlights some real use cases that are reported.

4.2.1.  Large-scale Data Analytics

   One potential use case of the extension defined in this document 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 all the data
   transfers belonging to the job.  With the extension defined in this
   document, 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 important to know the capabilities of various ANEs between two
   end hosts, especially in the mobile environment.  With the extension
   defined in this document, 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 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 extension defined in this document, 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 capabilities (e.g., storage, GPU) and available Service Level
   Agreement (SLA) plans.  Thus, an ALTO client may leverage the
   information to better conduct CDN request routing or offload
   functionalities from the user equipment to the service edge, with
   considerations on different resource constraints.

5.  Path Vector Extension: Overview

   This section gives a non-normative overview of the extension defined
   in this document.  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
       components in a network whose properties may have an impact on
       the end-to-end performance of the traffic handled by those
       component,

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

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   3.  uses the Unified Property Map to convey the association between
       the ANEs and their properties.

   Thus, an ALTO client can learn about the ANEs that are critical to
   the QoE of a <source, destination> pair by investigating the
   corresponding Path Vector value (AR1), identify common ANEs if an ANE
   appears in the Path Vectors of multiple <source, destination> pairs
   (AR2), and retrieve the properties of the ANEs 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 a component or an
   aggregation of components of a network whose properties have an
   impact on the end-to-end performance for traffic between a source and
   a destination.

   When an ANE is defined by the ALTO server, it is assigned an
   identifier, i.e., string of type ANEName as specified in Section 6.1,
   and a set of associated properties.

5.1.1.  ANE Domain

   In this extension, the associations between ANE and the properties
   are conveyed in a Unified Property Map. Thus, ANEs must constitute an
   entity domain (Section 5.1 of [I-D.ietf-alto-unified-props-new]), and
   each ANE property must be an entity property (Section 5.2 of
   [I-D.ietf-alto-unified-props-new]).

   Specifically, this document defines a new entity domain called "ane"
   as specified in Section 6.2 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
   components of a network 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

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   be useful for clients to occasionally query properties on persistent
   entities, without caring about the path that traverses them.  For
   example, an ALTO server may define an ANE for each service edge
   cluster.  Once a client chooses to use a service edge, e.g., by
   deploying some user-defined functions, it may want to stick to the
   service edge to avoid the complexity of state transition or
   synchronization.  Persistent entities have a persistent ID that is
   registered in a Property Map, together with their properties.  See
   Section 6.2.4 and Section 6.4.2 for more detailed instructions on how
   to identify ephemeral ANEs and persistent ANEs.

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 includes and only includes 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 7.2.4 and Section 7.3.4
   respectively.  The "ane-property-names" filter for Cost Map and
   Endpoint Cost Service are specified in Section 7.2.3 and
   Section 7.3.3 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
   correctly 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 C++, 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

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   value is a JSON array.  Then, an ALTO client must check the value of
   the "cost-metric".  If the value is "ane-path", it means that the
   JSON array should be further interpreted as a path of ANENames.

   The Path Vector cost type is specified in Section 6.5.

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 extension defined in this document, 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
   extension defined in this document enforces one round of
   communication.  Specifically, the ALTO client must include the source
   and destination pairs and the requested ANE properties in a single
   request, and the ALTO server must return a single response containing
   both the Path Vectors and properties associated with the ANEs in the
   Path Vectors, as shown in Figure 4.  Since the two parts are bundled
   together in one response message, their orders are interchangeable.
   See Section 7.2.6 and Section 7.3.6 for details.

            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.

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   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.

   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 is "multipart/
   related" with the parameter "type=application/alto-costmap+json"; for
   an Endpoint Cost Service, the parameter is "type=application/alto-
   endpointcost+json".

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5.3.2.  References to Part Messages

   As the response of a Path Vector resource is a multipart message with
   two different parts, it is important that each part can be uniquely
   identified.  Following the designs of [RFC8895], this extension
   requires that an ALTO server assigns 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), is
   present in the part message's "Content-ID" header.  By concatenating
   the Part Resource ID to the identifier of the Path Vector request, an
   ALTO server/client can uniquely identify the Path Vector Part or the
   Property Map part.

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.

6.2.1.  Entity Domain Type

   ane

6.2.2.  Domain-Specific Entity Identifier

   The entity identifiers are the ANE Names in the associated Property
   Map.

6.2.3.  Hierarchy and Inheritance

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

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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 "pid" MUST
   be a Property Map. The media type of defining resources for the "ane"
   domain is:

   application/alto-propmap+json

   Specifically, for ephemeral ANEs that appear in a Path Vector
   response, their entity domain names MUST be exactly ".ane" and the
   defining resource of these ANEs is the Property Map part of the
   multipart response.  Meanwhile, for persistent ANEs whose entity
   domain name has the format of "PROPMAP.ane" where PROPMAP is the name
   of a Property Map resource, PROPMAP is the defining resource of these
   ANEs.  Persistent entities are "persistent" because standalone
   queries can be made by an ALTO client to their defining resources
   when the connection to the Path Vector service is closed.

   For example, the defining resource of an ephemeral ANE whose entity
   identifier is ".ane:NET1" is the Property Map part that contains this
   identifier.  The defining resource of a persistent ANE whose entity
   identifier is "dc-props.ane:DC1" is the Property Map with the
   resource ID "dc-props".

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
     192.0.2.0/24+-+ +-----------+ +---------+          +--+192.0.4.0/24
                   | |   MEC1    | |         |          |
                   | +-----------+ |   +-----+          |
            PID2   |               |   |     +----------+
     192.0.3.0/24+-+               |   |         NET3
                   |               |   | 15 Gbps
                   |               |   |        \
                   +---------------+   |         -------- L2
                         NET1          |
                                +---------------+
                                | +-----------+ |   PID4
                                | |   MEC2    | +--+192.0.5.0/24
                                | +-----------+ |
                                +---------------+
                                      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 which an ephemeral ANE presents
      (See Section 5.1.2 for details).  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.  Since MEC1 is associated with NET1, the "persistent-
   entity-id" of the ephemeral ANE ".ane:NET1" is the persistent entity
   id "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 extension defined in this document.

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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   An ALTO client MUST interpret the Path Vector as if the traffic
   between a source and a destination logically traverses the ANEs in
   the same order as they appear in the Path Vector.

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]).

   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.  Notations

   This document uses the same syntax and notations as introduced in
   Section 8.2 of RFC 7285 [RFC7285] to specify the extensions to
   existing ALTO resources and services.

7.2.  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.

7.2.1.  Media Type

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

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7.2.2.  HTTP Method

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

7.2.3.  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, which is defined as a JSON object
   of type "ReqFilteredCostMap" in Section 11.3.2.3 of RFC 7285
   [RFC7285], 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 selected ANE properties to be included
      in the response.  Each property in this list MUST match one of the
      supported ANE properties indicated in the resource's "ane-
      property-names" capability (See Section 7.2.4).  If the field is
      NOT present, it MUST be interpreted as an empty list.

   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.

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      POST /costmap/pv HTTP/1.1
      Host: alto.example.com
      Accept: multipart/related;type=application/alto-costmap+json,
              application/alto-error+json
      Content-Length: 201
      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" ]
      }

7.2.4.  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

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      returned.  If the field is NOT present, it MUST be interpreted as
      an empty list, indicating the ALTO server cannot provide any ANE
      property.

7.2.5.  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.

7.2.6.  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 [RFC2387] with the following parameters:

   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 "Content-ID" header of the Path
      Vector part.

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

   The body of the response MUST consist of two parts:

   *  The Path Vector part MUST include "Content-ID" and "Content-Type"
      in its header.  The value of "Content-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 of

      resource-id '.' part-resource-id

      where "resource-id" is the resource Id of the Path Vector
      resource, and "part-resource-id" has the same value as the
      "Content-ID" of the Path Vector part.  The "meta" field MUST also

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      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 "Content-ID" and
      "Content-Type" in its header.  The value of "Content-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 is 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 for each ANE
      has one member for each property that is both 1) associated with
      the ANE, and 2) specified in the "ane-property-names" in the
      request.

   A complete and valid response MUST include both the Path Vector part
   and the Property Map part in the multipart message.  If any part is
   NOT present, the client MUST discard the received information and
   send another request if necessary.

   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.  When doing so, an ALTO server MAY NOT set the "start"
   parameter, which implies the first part is the root object.

   Example: Consider the network in Figure 1.  The response of the
   example request in Section 7.2.3 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: 821
   Content-Type: multipart/related; boundary=example-1;
                 type=application/alto-costmap+json

   --example-1
   Content-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
   Content-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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7.3.  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.

7.3.1.  Media Type

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

7.3.2.  HTTP Method

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

7.3.3.  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, which is defined as a
   JSON object of type ReqEndpointCost in Section 11.5.1.3 in RFC 7285
   [RFC7285], 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 7.2.3.

   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: 222
   Content-Type: application/alto-endpointcostparams+json

   {
     "cost-type": {
       "cost-mode": "array",
       "cost-metric": "ane-path"
     },
     "endpoints": {
       "srcs": [ "ipv4:192.0.2.2" ],
       "dsts": [ "ipv4:192.0.3.2" ]
     },
     "ane-property-names": [ "max-reservable-bandwidth" ]
   }

7.3.4.  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 7.2.4.

7.3.5.  Uses

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

7.3.6.  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 7.2.6.

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

   The body MUST consist of two parts:

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   *  The Path Vector part MUST include "Content-ID" and "Content-Type"
      in its header.  The value of "Content-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 of

      resource-id '.' part-resource-id

      where "resource-id" is the resource Id of the Path Vector
      resource, and "part-resource-id" has the same value as the
      "Content-ID" of the Path Vector part.

   *  The Unified Property Map part MUST also include "Content-ID" and
      "Content-Type" in its header.  The value of "Content-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 for each ANE
      has one member for each property that is both 1) associated with
      the ANE, and 2) specified in the "ane-property-names" in the
      request.

   A complete and valid response MUST include both the Path Vector part
   and the Property Map part in the multipart message.  If any part is
   NOT present, the client MUST discard the received information and
   send another request if necessary.

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   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.  When doing so, an ALTO server MAY NOT set the "start"
   parameter, which implies the first part is the root object.

   Example: Consider the network in Figure 1.  The response of the
   example request in Section 7.3.3 is as follows.

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   HTTP/1.1 200 OK
   Content-Length: 810
   Content-Type: multipart/related; boundary=example-1;
                 type=application/alto-endpointcost+json

   --example-1
   Content-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": {
       "ipv4:192.0.2.2": { "ipv4:192.0.3.2": ["ANE1"] }
     }
   }
   --example-1
   Content-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 extension defined in this
   document, we consider the network in Figure 5.  Assume that the 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 extension defined in this document, 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: 153
   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: 818
   Content-Type: multipart/related; boundary=example-1;
                 type=application/alto-costmap+json

   --example-1
   Content-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" ]
       }
     }
   }
   --example-1
   Content-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 for two source and destination pairs:
   192.0.4.2 -> 192.0.2.2 and 192.0.4.2 -> 192.0.5.2.

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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.  As one can
   see in Figure 5, flow 192.0.4.2 -> 192.0.2.2 traverses NET2, L1 and
   NET1, and flow 192.0.4.2 -> 192.0.5.2 traverses NET2, L2 and NET3.

   The second part returns the requested properties of ANEs.  Assume
   NET1, NET2 and NET3 has sufficient bandwidth and their "max-
   reservable-bandwidth" values are set to a sufficiently large number
   (50 Gbps in this case).  On the other hand, assume there are no prior
   reservation on L1 and L2, and their "max-reservable-bandwidth" values
   are the corresponding link capacity (10 Gbps for L1 and 15 Gbps for
   L2).

   Both NET1 and NET2 have a mobile edge deployed, i.e., MEC1 in NET1
   and MEC2 in NET2.  Assume the ANEName for MEC1 and MEC2 are "MEC1"
   and "MEC2" and their properties can be retrieved from the property
   map "ane-props".  Thus, the "persistent-entity-id" property of NET1
   and NET3 are "ane-props.ane:MEC1" and "ane-props.ane:MEC2"
   respectively.

   POST /endpointcost/pv HTTP/1.1
   Host: alto.example.com
   Accept: multipart/related;
           type=application/alto-endpointcost+json,
           application/alto-error+json
   Content-Length: 278
   Content-Type: application/alto-endpointcostparams+json

   {
     "cost-type": {
       "cost-mode": "array",
       "cost-metric": "ane-path"
     },
     "endpoints": {
       "srcs": [ "ipv4:192.0.4.2" ],
       "dsts": [ "ipv4:192.0.2.2", "ipv4:192.0.5.2" ]
     },
     "ane-property-names": [
       "max-reservable-bandwidth",
       "persistent-entity-id"
     ]
   }

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   HTTP/1.1 200 OK
   Content-Length: 1305
   Content-Type: multipart/related; boundary=example-2;
                 type=application/alto-endpointcost+json

   --example-2
   Content-ID: ecs
   Content-Type: application/alto-endpointcost+json

   {
     "meta": {
       "vtags": {
         "resource-id": "endpoint-cost-pv.ecs",
         "tag": "bb6bb72eafe8f9bdc4f335c7ed3b10822a391cef"
       },
       "cost-type": {
         "cost-mode": "array",
         "cost-metric": "ane-path"
       }
     },
     "endpoint-cost-map": {
       "ipv4:192.0.4.2": {
         "ipv4:192.0.2.2":   [ "NET3", "L1", "NET1" ],
         "ipv4:192.0.5.2":   [ "NET3", "L2", "NET2" ]
       }
     }
   }
   --example-2
   Content-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:MEC1"

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       },
       ".ane:NET2": {
         "max-reservable-bandwidth": 50000000000,
         "persistent-entity-id": "ane-props.ane:MEC2"
       },
       ".ane:NET3": {
         "max-reservable-bandwidth": 50000000000
       },
       ".ane:L1": {
         "max-reservable-bandwidth": 10000000000
       },
       ".ane:L2": {
         "max-reservable-bandwidth": 15000000000
       }
     }
   }

   As mentioned in Section 6.5.1, an advanced ALTO server may obfuscate
   the response in order to preserve its own privacy or conform to its
   own policies.  For example, an ALTO server may choose to aggregate
   NET1 and L1 as a new ANE with ANE name "AGGR1", and aggregate NET2
   and L2 as a new ANE with ANE name "AGGR2".  The "max-reservable-
   bandwidth" of "AGGR1" takes the value of L1, which is smaller than
   that of NET1, and the "persistent-entity-id" of "AGGR1" takes the
   value of NET1.  The properties of "AGGR2" are computed in a similar
   way and the obfuscated response is as shown below.  Note that the
   obfuscation of Path Vector responses is implementation-specific and
   is out of the scope of this document, and developers may refer to
   Section 11 for further references.

   HTTP/1.1 200 OK
   Content-Length: 1157
   Content-Type: multipart/related; boundary=example-2;
                 type=application/alto-endpointcost+json

   --example-2
   Content-ID: ecs
   Content-Type: application/alto-endpointcost+json

   {
     "meta": {
       "vtags": {
         "resource-id": "endpoint-cost-pv.ecs",
         "tag": "bb6bb72eafe8f9bdc4f335c7ed3b10822a391cef"
       },
       "cost-type": {
         "cost-mode": "array",
         "cost-metric": "ane-path"

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       }
     },
     "endpoint-cost-map": {
       "ipv4:192.0.4.2": {
         "ipv4:192.0.2.2":   [ "NET3", "AGGR1" ],
         "ipv4:192.0.5.2":   [ "NET3", "AGGR2" ]
       }
     }
   }
   --example-2
   Content-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:AGGR1": {
         "max-reservable-bandwidth": 10000000000,
         "persistent-entity-id": "ane-props.ane:MEC1"
       },
       ".ane:AGGR2": {
         "max-reservable-bandwidth": 15000000000,
         "persistent-entity-id": "ane-props.ane:MEC2"
       },
       ".ane:NET3": {
         "max-reservable-bandwidth": 50000000000
       }
     }
   }

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".

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   POST /updates/pv HTTP/1.1
   Host: alto.example.com
   Accept: text/event-stream
   Content-Type: application/alto-updatestreamparams+json
   Content-Length: 112

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

   Based on the server-side process defined in [RFC8895], 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=example-3;
          type=application/alto-endpointcost+json,ecspvsub1
   data: --example-3
   data: Content-ID: ecsmap
   data: Content-Type: application/alto-endpointcost+json
   data:
   data: <endpoint-cost-map-entry>
   data: --example-3
   data: Content-ID: propmap
   data: Content-Type: application/alto-propmap+json
   data:
   data: <property-map-entry>
   data: --example-3--

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

   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>

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9.  Compatibility with Other ALTO Extensions

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 incurring any incompatibility problem.

9.2.  Compatibility with Multi-Cost Extension

   The extension defined in this document is NOT compatible with the
   multi-cost extension [RFC8189].  The reason is that if a resource
   supports both the extension defined in this document and the multi-
   cost extension, the media type of this resource depends on the
   selection of cost types: if the path vector cost type is selected,
   the media type of the response is either "multipart/related;
   type=application/alto-costmap+json" or "multipart/related;
   type=application/alto-endpointcost+json"; if the path vector cost
   type is not selected, the media type of the response is either
   "application/alto-costmap+json" or "application/alto-
   endpointcost+json".

   Note that this problem may happen when an ALTO information resource
   supports multiple cost types, even if it does not enable the multi-
   cost extension.  Thus, Section 7.2.4 has specified that if an ALTO
   information resource enables the extension defined in this document,
   the path vector cost type MUST be the only cost type in the "cost-
   type-names" capability of this resource.

9.3.  Compatibility with Incremental Update

   ALTO clients and servers MUST follow the specifications given in
   Section 5.2 of {{RFC8895} to support incremental updates for a Path
   Vector resource.

9.4.  Compatibility with Cost Calendar

   The extension specified in this document is compatible with the Cost
   Calendar extension [RFC8896].  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 ANE may also have
   different values in different time intervals.  In this case, if such
   an ANE has different property values in two time intervals, it MUST
   be treated as two different ANEs, i.e., with different entity
   identifiers.  However, if it has the same property values in two time
   intervals, it MAY use the same identifier.

   This rule allows the Path Vector extension to represent both changes
   of ANEs and changes of the ANEs' properties in a uniform way.  The
   Path Vector part is calendared in a compatible way, and the Property
   Map part is not affected by the calendar extension.

   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 bespoke language for general-purpose
   boolean tests can be a complex undertaking, and it is conceivable
   that there are some existing implementations already (the authors
   have not done an exhaustive search to determine whether there are
   such implementations).  One avenue to develop such a language may be
   to explore extending current query languages like XQuery [XQuery] or
   JSONiq [JSONiq] and integrating these with ALTO.

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

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10.2.  General Multi-Resource Query

   Querying multiple ALTO information resources continuously is a
   general requirement.  Enabling such a capability, however, must
   address the general issues like efficiency and consistency.  The
   incremental update extension [RFC8895] supports submitting multiple
   queries in a single request, and allows flexible control over the
   queries.  However, it does not cover the case introduced in this
   document where multiple resources are needed for a single request.

   This extension gives an example of using a multipart message to
   encode two specific ALTO information resources: a filtered cost map
   or an endpoint cost map, and a property map.  By packing multiple
   resources in a single response, the implication is that servers may
   proactively push related information resources to clients.

   Thus, it is worth looking into the direction of extending the SSE
   mechanism as used in the incremental update extension [RFC8895], or
   upgrading to HTTP/2 [RFC7540] and HTTP/3 [I-D.ietf-quic-http], which
   provides the ability to multiplex queries and to allow servers
   proactively send related information resources.

   Defining a general multi-resource query mechanism is out of the scope
   of this document.

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.

   The Path Vector extension requires additional scrutiny on three
   security considerations discussed in the base protocol:
   confidentiality of ALTO information (Section 15.3 of [RFC7285]),
   potential undesirable guidance from authenticated ALTO information
   (Section 15.2 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.

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   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.  For example, in
   the multi-flow bandwidth reservation use case as introduced in
   Section 4, only the available bandwidth of the shared bottleneck link
   is crucial, and the ALTO server may only preserve the critical
   bottlenecks and can change the order of links appearing in the Path
   Vector response.

   However, arbitrary reduction and obfuscation of information exposure
   may potentially introduce a risk on the integrity of the ALTO
   information, leading to infeasible or suboptimal decisions of ALTO
   clients,

   To mitigate this risk, if an ALTO client finds that the traffic
   distribution based on the Path Vector information is not feasible
   (e.g., causing constant congestion) or not better than a distribution
   which does not fully conform to the information (e.g., by randomly
   choosing the source/destination for certain flows), it can follow the
   protection strategies for potential undesirable guidance from
   authenticated ALTO information, specified in Section 15.2.2 of RFC
   7285 [RFC7285].  While repeatedly sending the same query can
   potentially detect the integrity problem for certain obfuscation
   methods (e.g., those based on time or randomness) under certain
   network conditions (e.g., where the routing and ANE properties are
   stable), an ALTO client must be aware that this behavior may be
   considered as a denial-of-service attack on the server and may lead
   to the rejection of further requests from the client.

   On the other hand, this risk can also be mitigated from the server
   side.  While the implementation of an ALTO server is beyond the scope
   of this document, implementations of ALTO servers involving reduction
   or obfuscation of the Path Vector information should consider
   reduction/obfuscation mechanisms that can preserve the integrity of
   ALTO information, for example, by using minimal feasible region
   compression algorithms [TON2019] or obfuscation protocols
   [SC2018][JSAC2019].

   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 computation and storage, which can break the ALTO
   server.  For example, if an ALTO server implementation dynamically
   computes the Path Vectors for each requests, the service providing
   Path Vectors may become an entry point for denial-of-service attacks
   on the availability of an ALTO server.

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   To mitigate this risk, an ALTO server may consider using
   optimizations such as precomputation-and-projection mechanisms
   [JSAC2019] to reduce the overhead for processing each query.  Also,
   an ALTO server may also protect itself from malicious clients by
   monitoring the behaviors of clients and stopping serving clients with
   suspicious behaviors (e.g., sending requests at a high frequency).

12.  IANA Considerations

12.1.  ALTO Entity Domain Type Registry

   This document registers a new entry to the ALTO Domain Entity Type
   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.

    +============+=========================+=========================+
    | Identifier | Entity Address Encoding | Hierarchy & Inheritance |
    +============+=========================+=========================+
    | ane        | See Section 6.2.2       | None                    |
    +------------+-------------------------+-------------------------+

                Table 1: ALTO Entity Domain Type Registry

   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.

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             +==========================+====================+
             | 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

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-unified-props-new]
              Roome, W., Randriamasy, S., Yang, Y., Zhang, J., and K.
              Gao, "ALTO extension: Entity Property Maps", Work in
              Progress, Internet-Draft, draft-ietf-alto-unified-props-
              new-15, 26 November 2020, <http://www.ietf.org/internet-
              drafts/draft-ietf-alto-unified-props-new-15.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",
              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>.

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   [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>.

   [RFC8895]  Roome, W. and Y. Yang, "Application-Layer Traffic
              Optimization (ALTO) Incremental Updates Using Server-Sent
              Events (SSE)", RFC 8895, DOI 10.17487/RFC8895, November
              2020, <https://www.rfc-editor.org/info/rfc8895>.

   [RFC8896]  Randriamasy, S., Yang, R., Wu, Q., Deng, L., and N.
              Schwan, "Application-Layer Traffic Optimization (ALTO)
              Cost Calendar", RFC 8896, DOI 10.17487/RFC8896, November
              2020, <https://www.rfc-editor.org/info/rfc8896>.

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.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-02, 2 November
              2020, <http://www.ietf.org/internet-drafts/draft-
              contreras-alto-service-edge-02.txt>.

   [I-D.huang-alto-mowie-for-network-aware-app]
              Xiong, C., Zhang, Y., Yang, R., Li, G., Lei, Y., and Y.
              Han, "MoWIE for Network Aware Application", Work in
              Progress, Internet-Draft, draft-huang-alto-mowie-for-
              network-aware-app-02, 5 January 2021,
              <http://www.ietf.org/internet-drafts/draft-huang-alto-
              mowie-for-network-aware-app-02.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-14, 13 January 2021, <http://www.ietf.org/
              internet-drafts/draft-ietf-alto-performance-metrics-
              14.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-04, 2 November 2020,
              <http://www.ietf.org/internet-drafts/draft-ietf-dmm-5g-
              uplane-analysis-04.txt>.

   [I-D.ietf-quic-http]
              Bishop, M., "Hypertext Transfer Protocol Version 3
              (HTTP/3)", Work in Progress, Internet-Draft, draft-ietf-
              quic-http-33, 15 December 2020, <http://www.ietf.org/
              internet-drafts/draft-ietf-quic-http-33.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>.

   [JSAC2019] Xiang, Q., Zhang, J., Wang, X., Liu, Y., Guok, C., Le, F.,
              MacAuley, J., Newman, H., and Y.R. Yang, "Toward Fine-
              Grained, Privacy-Preserving, Efficient Multi-Domain
              Network Resource Discovery", IEEE/ACM IEEE Journal on
              Selected Areas of Communication 37(8): 1924-1940, 2019.

   [JSONiq]   "The JSON Query language", 2020,
              <https://www.jsoniq.org/>.

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

   [RFC2216]  Shenker, S. and J. Wroclawski, "Network Element Service
              Specification Template", RFC 2216, DOI 10.17487/RFC2216,
              September 1997, <https://www.rfc-editor.org/info/rfc2216>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

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   [SC2018]   Xiang, Q., Zhang, J., Wang, X., Liu, Y., Guok, C., Le, F.,
              MacAuley, J., Newman, H., and Y.R. Yang, "Fine-grained,
              multi-domain network resource abstraction as a fundamental
              primitive to enable high-performance, collaborative data
              sciences", Proceedings of the Super Computing 2018,
              5:1-5:13 , 2019.

   [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.

   [XQuery]   "XQuery 3.1: An XML Query Language", 2017,
              <https://www.w3.org/TR/xquery-31/>.

Appendix A.  Changes since -12

   Revision -13

   *  changes the abstract based on the chairs' reviews

   *  integrates Richard's responds to WGLC reviews

Appendix B.  Changes since -11

   Revision -12

   *  clarifies the definition of ANEs in a similar way as how Network
      Elements is defined in [RFC2216]

   *  restructures several paragraphs that are not clear (Sec 3, Path
      Vector bullet, Sec 4.2, Sec 5.1.3, Sec 6.2.4, Sec 6.4.2, Sec 9.3)

   *  uses "ALTO Entity Domain Type Registry"

Appendix C.  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

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   *  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;

   *  updates the examples;

   *  adds the discussion on ephemeral and persistent ANEs.

Appendix D.  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 E.  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 F.  Changes Since Version -06

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

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      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.

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

   Email: kaigao@scu.edu.cn

   Young Lee
   Samsung
   South Korea

   Email: younglee.tx@gmail.com

   Sabine Randriamasy
   Nokia Bell Labs
   Route de Villejust
   91460 Nozay
   France

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

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

   Email: yry@cs.yale.edu

   Jingxuan Jensen Zhang
   Tongji University
   4800 Caoan Road
   Shanghai
   201804
   China

   Email: jingxuan.n.zhang@gmail.com

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