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The ALTO Transport Information Publication Service
draft-ietf-alto-new-transport-15

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9569.
Authors Kai Gao , Roland Schott , Y. Richard Yang , Lauren Delwiche , Lachlan Keller
Last updated 2023-10-11 (Latest revision 2023-10-10)
Replaces draft-schott-alto-new-transport, draft-schott-alto-new-transport-push
RFC stream Internet Engineering Task Force (IETF)
Formats
Reviews
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Associated WG milestone
Jun 2023
RFC for ALTO using HTTP/2 and /3 mechanisms
Document shepherd Mohamed Boucadair
Shepherd write-up Show Last changed 2023-06-16
IESG IESG state Became RFC 9569 (Proposed Standard)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Martin Duke
Send notices to mohamed.boucadair@orange.com
IANA IANA review state Version Changed - Review Needed
draft-ietf-alto-new-transport-15
ALTO                                                              K. Gao
Internet-Draft                                        Sichuan University
Intended status: Standards Track                               R. Schott
Expires: 12 April 2024                                  Deutsche Telekom
                                                              Y. R. Yang
                                                             L. Delwiche
                                                               L. Keller
                                                         Yale University
                                                         10 October 2023

           The ALTO Transport Information Publication Service
                    draft-ietf-alto-new-transport-15

Abstract

   The ALTO Protocol (RFC 7285) leverages HTTP/1.1 and is designed for
   the simple, sequential request-reply use case, in which an ALTO
   client requests a sequence of information resources and the server
   responds with the complete content of each resource one at a time.

   ALTO incremental updates using Server-Sent Events (SSE) (RFC 8895)
   defines a multiplexing protocol on top of HTTP/1.x, so that an ALTO
   server can incrementally push resource updates to clients whenever
   monitored network information resources change, allowing the clients
   to monitor multiple resources at the same time.  However, HTTP/2 and
   later versions already support concurrent, non-blocking transport of
   multiple streams in the same HTTP connection.

   To take advantage of newer HTTP features, this document introduces
   the ALTO Transport Information Publication Service (TIPS).  TIPS uses
   an incremental RESTful design to give an ALTO client the new
   capability to explicitly, concurrently (non-blocking) request (pull)
   specific incremental updates using native HTTP/2 or HTTP/3, while
   still functioning for HTTP/1.1.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Application-Layer
   Traffic Optimization Working Group mailing list (alto@ietf.org),
   which is archived at https://mailarchive.ietf.org/arch/browse/alto/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ietf-wg-alto/draft-ietf-alto-new-transport.

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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 12 April 2024.

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   5
     1.2.  Notations . . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  TIPS Overview . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Transport Requirements  . . . . . . . . . . . . . . . . .   5
     2.2.  TIPS Terminology  . . . . . . . . . . . . . . . . . . . .   7
   3.  TIPS Updates Graph  . . . . . . . . . . . . . . . . . . . . .  10
     3.1.  Basic Data Model of Updates Graph . . . . . . . . . . . .  10
     3.2.  Updates Graph Modification Invariants . . . . . . . . . .  12
   4.  TIPS Workflow and Resource Location Schema  . . . . . . . . .  12
     4.1.  Workflow  . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.2.  Resource Location Schema  . . . . . . . . . . . . . . . .  14
   5.  TIPS Information Resource Directory (IRD) Announcement  . . .  16
     5.1.  Media Type  . . . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Capabilities  . . . . . . . . . . . . . . . . . . . . . .  16

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     5.3.  Uses  . . . . . . . . . . . . . . . . . . . . . . . . . .  17
     5.4.  An Example  . . . . . . . . . . . . . . . . . . . . . . .  17
   6.  TIPS Management . . . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Open Request  . . . . . . . . . . . . . . . . . . . . . .  19
     6.2.  Open Response . . . . . . . . . . . . . . . . . . . . . .  20
     6.3.  Open Example  . . . . . . . . . . . . . . . . . . . . . .  23
     6.4.  Heartbeat Request and Response  . . . . . . . . . . . . .  24
     6.5.  Close Request and Response  . . . . . . . . . . . . . . .  25
   7.  TIPS Data Transfers - Client Pull . . . . . . . . . . . . . .  25
     7.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.2.  Response  . . . . . . . . . . . . . . . . . . . . . . . .  26
     7.3.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  27
     7.4.  New Next Edge Recommendation  . . . . . . . . . . . . . .  27
       7.4.1.  Request . . . . . . . . . . . . . . . . . . . . . . .  27
       7.4.2.  Response  . . . . . . . . . . . . . . . . . . . . . .  28
   8.  Operation and Processing Considerations . . . . . . . . . . .  28
     8.1.  Considerations for Choosing Updates . . . . . . . . . . .  28
     8.2.  Considerations for Load Balancing . . . . . . . . . . . .  29
     8.3.  Considerations for Cross-Resource Dependency
           Scheduling  . . . . . . . . . . . . . . . . . . . . . . .  30
     8.4.  Considerations for Client Processing Updates  . . . . . .  31
     8.5.  Considerations for Updates to Filtered Cost Maps  . . . .  32
     8.6.  Considerations for Updates to Ordinal Mode Costs  . . . .  32
     8.7.  Considerations for Managing Shared TIPS Views . . . . . .  33
     8.8.  Considerations for Offering Shortcut Incremental
           Updates . . . . . . . . . . . . . . . . . . . . . . . . .  33
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
     9.1.  TIPS: Denial-of-Service Attacks . . . . . . . . . . . . .  34
     9.2.  ALTO Client: Update Overloading or Instability  . . . . .  34
     9.3.  Spoofed URI . . . . . . . . . . . . . . . . . . . . . . .  35
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  35
     10.1.  application/alto-tips+json Media Type  . . . . . . . . .  35
     10.2.  application/alto-tipsparams+json Media Type  . . . . . .  36
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  37
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  37
     11.2.  Informative References . . . . . . . . . . . . . . . . .  38
   Appendix A.  A High-Level Deployment Model  . . . . . . . . . . .  38
   Appendix B.  Conformance to "Building Protocols with HTTP" Best
           Current Practices . . . . . . . . . . . . . . . . . . . .  40
   Appendix C.  Push-mode TIPS using HTTP Server Push  . . . . . . .  41
   Appendix D.  Persistent HTTP Connections  . . . . . . . . . . . .  41
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  41

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1.  Introduction

   Application-Layer Traffic Optimization (ALTO) provides means for
   network applications to obtain network status information.  So far,
   two transport protocols have been designed:

   1.  The ALTO base protocol [RFC7285], which is designed for the
       simple use case in which an ALTO client requests a network
       information resource, and the server sends the complete content
       of the requested information (if any) resource to the client.

   2.  ALTO incremental updates using Server-Sent Events (ALTO/SSE)
       [RFC8895], which is designed for an ALTO client to indicate to
       the server that it wants to receive updates for a set of
       resources and the server can then concurrently and incrementally
       push updates to that client whenever monitored resources change.

   Both protocols are designed for HTTP/1.1 [RFC9112], but HTTP/2
   [RFC9113] and HTTP/3 [RFC9114] can support HTTP/1.1 workflows.
   However, HTTP/2 and HTTP/3 provide features that can improve certain
   properties of ALTO and ALTO/SSE.

   *  First, consider the ALTO base protocol, which is designed to
      transfer only complete information resources.  A client can run
      the base protocol on top of HTTP/2 or HTTP/3 to request multiple
      information resources in concurrent streams, but each request must
      be for a complete information resource: there is no capability it
      transmits incremental updates.  Hence, there can be a large
      overhead when the client already has an information resource and
      then there are small changes to the resource.

   *  Next, consider ALTO/SSE [RFC8895].  Although ALTO/SSE can transfer
      incremental updates, it introduces a customized multiplexing
      protocol on top of HTTP, assuming a total-order message channel
      from the server to the client.  The multiplexing design does not
      provide naming (i.e., a resource identifier) to individual
      incremental updates.  Such a design cannot use concurrent data
      streams available in HTTP/2 and HTTP/3, because both cases require
      a resource identifier.  Additionally, ALTO/SSE is a push-only
      protocol, which denies the client flexibility in choosing how and
      when it receives updates.

   To mitigate these concerns, this document introduces a new ALTO
   service called the Transport Information Publication Service (TIPS).
   TIPS uses an incremental RESTful design to provide an ALTO client
   with a new capability to explicitly, concurrently issue non-blocking
   requests for specific incremental updates using native HTTP/2 or
   HTTP/3, while still functioning for HTTP/1.1.

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   Despite the benefits, however, ALTO/SSE [RFC8895], which solves a
   similar problem, has its advantages.  First, SSE is a mature
   technique with a well-established ecosystem that can simplify
   development.  Second, SSE does not allow multiple connections to
   receive updates for multiple objects over HTTP/1.1.

   HTTP/2 [RFC9113] and HTTP/3 [RFC9114] also specify server push, which
   might enhance TIPS.  We discuss push-mode TIPS as an alternative
   design in Appendix C.

   Specifically, this document specifies:

   *  Extensions to the ALTO Protocol for dynamic subscription and
      efficient uniform update delivery of an incrementally changing
      network information resource.

   *  A new resource type that indicates the TIPS updates graph model
      for a resource.

   *  URI patterns to fetch the snapshots or incremental updates.

   Appendix B discusses to what extent the TIPS design adheres to the
   Best Current Practices for building protocols with HTTP [RFC9205].

1.1.  Requirements Language

   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.

1.2.  Notations

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

2.  TIPS Overview

2.1.  Transport Requirements

   The ALTO Protocol and its extensions support two transport
   mechanisms: First, a client can directly request an ALTO resource and
   obtain a complete snapshot of that ALTO resource, as specified in the
   base protocol [RFC7285]; Second, a client can subscribe to
   incremental changes of one or multiple ALTO resources using the
   incremental update extension [RFC8895], and a server pushes the

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   updates to the client through Server Sent Events (SSE).

   However, the current transport mechanisms are not optimized for
   storing, transmitting, and processing (incremental) updates of ALTO
   information resources.  Specifically, the new transport mechanism
   must satisfy the following requirements:

   Incremental updates:  Incremental updates can reduce both the data
      storage on an ALTO server and the transmission time of the
      updates, especially when the change of an ALTO resource is minor.
      The base protocol does not support incremental updates and the
      current incremental update mechanism in [RFC8895] has limitations
      (as discussed below).

   Concurrent, non-blocking update transmission:  When a client needs to
      receive and apply multiple incremental updates, it is desired to
      transmit the updates concurrently to fully utilize the bandwidth
      and to reduce head-of-line blocking.  The ALTO incremental update
      extension [RFC8895], unfortunately, does not satisfy this
      requirement -- even though the updates can be multiplexed by the
      server to avoid head-of-line blocking between multiple resources,
      the updates are delivered sequentially and can suffer from head-
      of-line blocking inside the connection, for example, when there is
      a packet loss.

   Prefetching updates:  Prefetching updates can reduce the time to send
      the request, making it possible to achieve sub-RTT transmission of
      ALTO incremental updates.  In [RFC8895], this requirement is
      fulfilled using server-sent event (SSE) and is still desired in
      the ALTO new transport.

   Backward compatibility:  While some of the previous requirements are
      offered by HTTP/2 [RFC9113] and HTTP/3 [RFC9114], it is desired
      that the ALTO new transport mechanism can work with HTTP/1.1 as
      many development tools and current ALTO implementations are based
      on HTTP/1.1.

   The ALTO new transport specified in this document satisfies all the
   design requirements and hence improves the efficiency of continuous
   dissemination of ALTO information.  The key idea is to introduce a
   unified data model to describe the changes (snapshots and incremental
   updates) of an ALTO resource, referred to as a TIPS view.  Along with
   the data model, this document also specifies a unified naming for the
   snapshots and incremental updates, independent of the HTTP version.
   Thus, these updates can be concurrently requested.  Prefetching is
   realized using long polling.

   This document assumes the deployment model discussed in Appendix A.

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2.2.  TIPS Terminology

   In addition to the terms defined in [RFC7285], this document uses the
   following terms:

   Transport Information Publication Service (TIPS):  Is a new type of
      ALTO service, as specified in this document, to enable a uniform
      transport mechanism for updates of an incrementally changing ALTO
      network information resource.

   Network information resource:  Is a piece of retrievable information
      about network state, per [RFC7285].

   TIPS view (tv):  Is defined in this document to be the container of
      incremental transport information about the network information
      resource.  The TIPS view has one basic component, updates graph
      (ug), but may include other transport information.

   Updates graph (ug):  Is a directed, acyclic graph whose nodes
      represent the set of versions of an information resource, and
      edges the set of update items to compute these versions.  An ALTO
      map service (e.g., Cost Map, Network Map) may need only a single
      updates graph.  A dynamic network information service (e.g.,
      Filtered Cost Map) may create an updates graph (within a new TIPS
      view) for each unique request.

   Version:  Represents a historical content of an information resource.
      For an information resource, each version is associated with and
      uniquely identified by a monotonically and consecutively increased
      sequence number.  This document uses the term "version s" to refer
      to the version associated with sequence number "s".

   Start sequence number (start-seq):  Is the smallest non-zero sequence
      number in an updates graph.

   End sequence number (end-seq):  Is the largest sequence number in an
      updates graph.

   Snapshot:  Is a full replacement of a resource and is contained
      within an updates graph.

   Incremental update:  Is a partial replacement of a resource contained

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      within an updates graph, codified in this document as a JSON Merge
      Patch or JSON Patch.  An incremental update is mandatory if the
      source version (i) and target version (j) are consecutive, i.e., i
      + 1 = j, and optional or a shortcut otherwise.  Mandatory
      incremental updates are always in an updates graph, while
      optional/shortcut incremental updates may or may not be included
      in an updates graph.

   Update item:  Refers to the content on an edge of the updates graph,
      which can be either a snapshot or an incremental update.  An
      update item can be considered as a pair (op, data) where op
      denotes whether the item is an incremental update or a snapshot,
      and data is the content of the item.

   ID#i-#j:  Denotes the update item on a specific edge in the updates
      graph to transition from version i to version j, where i and j are
      the sequence numbers of the source node and the target node of the
      edge, respectively.

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                                   +-------------+
    +-----------+ +--------------+ |  Dynamic    | +-----------+
    |  Routing  | | Provisioning | |  Network    | | External  |
    | Protocols | |    Policy    | | Information | | Interface |
    +-----------+ +--------------+ +-------------+ +-----------+
          |              |                |              |
+----------------------------------------------------------------------+
| ALTO Server                                                          |
| +------------------------------------------------------------------+ |
| |                                              Network Information | |
| | +-------------+                         +-------------+          | |
| | | Information |                         | Information |          | |
| | | Resource #1 |                         | Resource #2 |          | |
| | +-------------+                         +-------------+          | |
| +-----|--------------------------------------/-------\-------------+ |
|       |                                     /         \              |
| +-----|------------------------------------/-----------\-----------+ |
| |     |       Transport Information       /             \          | |
| | +--------+                     +--------+        +--------+      | |
| | |  tv1   |----+          +-----|  tv2   |        |  tv3   |---+  | |
| | +--------+    |          |     +--------+        +--------+   |  | |
| |     |         |          |           |             |          |  | |
| | +--------+ +--------+ +--------+ +--------+ +--------+ +--------+| |
| | | tv1/ug | | tv1/hb | | tv2/ug | | tv2/hb | | tv3/ug | | tv3/hb || |
| | +--------+ +--------+ +--------+ +--------+ +--------+ +--------+| |
| +----|\---------/\---------|---------/---------------|------|------+ |
|      | \       /  \        |        /                |      |        |
+------|--\-----/----\-------|-------/-----------------|------|--------+
       |   \   /      \      |      /                  |      +
       |    +-/-----+  \     |     /                   |     /
       |     /       \  \    |    /                    |    /
       |    /         \  \   |   /                     |   /
       |   /           \  \  |  /                      |  |
   +----------+       +----------+                 +----------+
   | Client 1 |       | Client 2 |                 | Client 3 |
   +----------+       +----------+                 +----------+

tvi   = TIPS view i
tvi/ug = incremental updates graph associated with tvi
tvi/hb = client liveness check for tvi

                   Figure 1: Overview of ALTO TIPS

   Figure 1 shows an example illustrating an overview of the ALTO TIPS
   service.  The server provides the TIPS service of two information
   resources (#1 and #2) where #1 is an ALTO map service, and #2 is a
   filterable service.  There are 3 ALTO clients (Client 1, Client 2,
   and Client 3) that are connected to the ALTO server.

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   Each client uses the TIPS view to retrieve updates.  Specifically, a
   TIPS view (tv1) is created for the map service #1, and is shared by
   multiple clients.  For the filtering service #2, two different TIPS
   views (tv2 and tv3) are created upon different client requests with
   different filter sets.

   A heartbeat mechanism is used to detect the liveness of clients using
   TIPS.  It allows the ALTO server to identify unused TIPS views, e.g.,
   because of accidentally crashed ALTO clients, and release the
   resources of such views.

3.  TIPS Updates Graph

   In order to provide incremental updates for a resource, an ALTO
   server creates an updates graph, which is a directed, acyclic graph
   that contains a sequence of incremental updates and snapshots
   (collectively called update items) of a network information resource.

3.1.  Basic Data Model of Updates Graph

   For each resource (e.g., a cost map, a network map), the incremental
   updates and snapshots can be represented using the following directed
   acyclic graph model, where the server tracks the change of the
   resource maps with version IDs that are assigned sequentially (i.e.,
   incremented by 1 each time):

   *  Each node in the graph is a version of the resource, where a tag
      identifies the content of the version (tag is valid only within
      the scope of resource).  Version 0 is reserved as the initial
      state (empty/null).

   *  Each edge is an update item.  In particular, the edge from i to j
      is the update item to transit from version i to version j.

   *  Version is path-independent (different paths arrive at the same
      version/node has the same content)

   A concrete example is shown in Figure 2.  There are 7 nodes in the
   graph, representing 7 different versions of the resource.  Edges in
   the figure represent the updates from the source version to the
   target version.  Thick lines represent mandatory incremental updates
   (e.g., ID103-104), dotted lines represent optional incremental
   updates (e.g., ID103-105), and thin lines represent snapshots (e.g.,
   ID0-103).  Note that node content is path independent: the content of
   node v can be obtained by applying the updates from any path that
   ends at v.  For example, assume the latest version is 105 and a
   client already has version 103.  The base version of the client is
   103 as it serves as a base upon which incremental updates can be

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   applied.  The target version 105 can either be directly fetched as a
   snapshot, computed incrementally by applying the incremental updates
   between 103 and 104, then 104 and 105, or if the optional update from
   103 to 105 exists, computed incrementally by taking the "shortcut"
   path from 103 to 105.

                                                          +======+
                                                    ------|  0   |
                                                   /      +======+
                                          ID0-101 /        |   |
                                                |/__       |   |
                                         +======+          |   |
                         tag: 3421097 -> | 101  |          |   |
                                         +======+          |   |
                                 ID101-102  ||             |   |
                                            \/             |   |
                                         +======+          |   |
                         tag: 6431234 -> | 102  |          |   |
                                         +======+          |   |
                                 ID102-103  ||             |   |
                                            \/             |   |
                                         +======+          /   |
      +--------------+   tag: 0881080 -> | 103  |<--------/    |
      | Base Version |   =======>        +======+ ID0-103      |
      +--------------+             103-104  ||    ..           |
                                            \/     ..          |
                                         +======+  ..          |
                         tag: 6452654 -> | 104  |  .. ID103    |
                                         +======+  .. -105     |
                                 ID104-105  ||     ..          | ID0-105
                                            \/   |._           /
                                         +======+             /
                         tag: 7838392 -> | 105  |<-----------/
                                         +======+
                                 ID105-106  ||
                                            \/
                                         +======+
                         tag: 6470983 -> | 106  |
                                         +======+

                       Figure 2: TIPS Model Example

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3.2.  Updates Graph Modification Invariants

   A server may change its updates graph (to compact, to add nodes,
   etc.), but it must ensure that any resource state that it makes
   available is reachable by clients, either directly via a snapshot
   (that is, relative to 0) or indirectly by requesting an earlier
   snapshot and a contiguous set of incremental updates.  Additionally,
   to allow clients to proactively construct URIs for future update
   items, the ID of each added node in the updates graph must increment
   contiguously by 1.  More specifically, the updates graph MUST satisfy
   the following invariants:

   *  Continuity: At any time, let ns denote the smallest non-zero
      version (i.e., start-seq) in the update graph and ne denote the
      latest version (i.e., end-seq).  Then any version in between ns
      and ne must also exist.  This implies that the incremental update
      from ni to ni + 1 exists for any ns <= ni <= ne, and all versions
      in the update graph (except 0) is an integer interval [ns, ne].

   *  Feasibility: Let ns denote the start-seq in the update graph.  The
      server must provide a snapshot of ns and, in other words, there is
      always a direct link to ns in the update graph.

   *  "Right shift" only: Assume a server provides versions in [n1, n2]
      at time t and versions in [n1', n2'] at time t'.  If t' > t, then
      n1' >= n1 and n2' >= n2.

   For example, consider the case that a server compacts a resource's
   updates graph to conserve space, using the example model in
   Section 3.1.  Assume at time 0, the server provides the versions
   {101, 102, 103, 104, 105, 106}. At time 1, both {103, 104, 105, 106}
   and {105, 106} are valid sets.  However, {102, 103, 104, 105, 106}
   and {104, 105, 106} are not valid sets as there is no snapshot to
   version 102 or 104 in the update graph.  Thus, there is a risk that
   the right content of version 102 (in the first example) or 104 (in
   the second example) cannot be obtained by a client that does not have
   the previous version 101 or 103, respectively.

4.  TIPS Workflow and Resource Location Schema

4.1.  Workflow

   At a high level, an ALTO client first uses the TIPS service (denoted
   as TIPS-F and F is for frontend) to indicate the information
   resource(s) that the client wants to monitor.  For each requested
   resource, the server returns a JSON object that contains a URI, which
   points to the root of a TIPS view (denoted as TIPS-V), and a summary
   of the current view, which contains the information to correctly

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   interact with the current view.  With the URI to the root of a TIPS
   view, clients can construct URIs (see Section 4.2) to fetch
   incremental updates and to report liveness.

   An example workflow is shown in Figure 3.  After the TIPS-F service
   receives the request from the client to monitor the updates of an
   ALTO resource, it creates a TIPS view service and returns the
   corresponding information to the client.  The URI points to that
   specific TIPS-V instance and the summary contains a heartbeat
   interval, the start-seq and end-seq of the update graph, and a
   server-recommended edge to consume first, e.g., from i to j.

   An ALTO client can then continuously pull each additional update with
   the information.  For example, the client in Figure 3 first fetches
   the update from i to j, and then from j to j+1.  Note that the update
   item at <tips-view-uri>/ug/<j>/<j+1> may not yet exist, so the server
   holds the request until the update becomes available (long polling).

   In the meantime, the client must periodically send a heartbeat signal
   by making a POST request to /<tips-view-path>/hb to indicate that the
   client is still active.  Otherwise, the server may close the TIPS
   view to release the resources.  See details in Section 6.4.

   Once the client no longer needs the TIPS view, it can send a DELETE
   request to the TIPS-V service.

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      Client                                 TIPS-F           TIPS-V
        o                                       .                .
        | POST to create/receive a TIPS view    .  Create TIPS   .
        |           for resource 1              .      View      .
        |-------------------------------------> |.-.-.-.-.-.-.-> |
        | <tips-view-uri>, <tips-view-summary>  .                |
        | <-------------------------------------| <-.-.-.-.-.-.-.|
        |                                                        .
        | GET /<tips-view-path>/ug/<i>/<j>                       .
        |------------------------------------------------------> |
        | content on edge i to j                                 |
        | <------------------------------------------------------|
        |                                                        .
        | GET /<tips-view-path>/ug/<j>/<j+1>                     .
        |------------------------------------------------------> |
        |                                                        |
        | POST /<tips-view-path>/hb                              |
        |------------------------------------------------------> |
        |                                                        |
        | content on edge j to j+1                               |
        | <------------------------------------------------------|
        |                                                        .
        | DELETE <tips-view-path>                                .
        |------------------------------------------------------> |
        |                                                        o
        o

            Figure 3: ALTO TIPS Workflow Supporting Client Pull

4.2.  Resource Location Schema

   The resource location schema defines how a client constructs URI to
   fetch incremental updates and to report liveness through heartbeat
   requests.

   To access each update in an updates graph, consider the model
   represented as a "virtual" file system (adjacency list), contained
   within the root of a TIPS view URI (see Section 6.2 for the
   definition of tips-view-uri).  For example, assuming that the update
   graph of a TIPS view is as shown in Figure 2, the location schema of
   this TIPS view will have the format as in Figure 4.

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       <tips-view-path>  // root path to a TIPS view
         |_ ug    // updates graph
         |  |_ 0
         |  |  |_ 101    // full 101 snapshot
         |  |  |_ 103
         |  |  \_ 105
         |  |_ 101
         |  |  \_ 102    // 101 -> 102 incremental update
         |  |_ 102
         |  |  \_ 103
         |  |_ 103
         |  |  |_ 104
         |  |  \_ 105    // optional shortcut 103 -> 105 incr. update
         |  |_ 104
         |  |  \_ 105
         |  \_ 105
         |     \_ 106
         |_ hb           // heartbeat
         \_ ...

                     Figure 4: Location Schema Example

   TIPS uses this directory schema to generate template URIs which allow
   clients to construct the location of incremental updates after
   receiving the tips-view-uri from the server.  The generic template
   for the location of the update item on the edge from node 'i' to node
   'j' in the updates graph is:

       <tips-view-uri>/ug/<i>/<j>

   Due to the sequential nature of the update item IDs, a client can
   long poll a future update that does not yet exist (e.g., the
   incremental update from 106 to 107) by constructing the URI for the
   next edge that will be added, starting from the sequence number of
   the current last node (denoted as end-seq) in the graph to the next
   sequential node (with the sequence number of end-seq + 1):

       <tips-view-uri>/ug/<end-seq>/<end-seq + 1>

   Incremental updates of a TIPS view are read-only.  Thus, they are
   fetched using the HTTP GET method.

   In the meantime, the URI of the heartbeat service of a TIPS view has
   the following format:

       <tips-view-uri>/hb

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   To avoid the message hitting an HTTP cache, the heartbeat request
   must use the POST method.

5.  TIPS Information Resource Directory (IRD) Announcement

   To announce a TIPS information resource in the information resource
   directory (IRD), an ALTO server MUST specify the "media-type",
   "capabilities" and "uses" as follows.

5.1.  Media Type

   The media type of the Transport Information Publication Service
   resource is "application/alto-tips+json".

5.2.  Capabilities

   The capabilities field of TIPS is modeled on that defined in
   Section 6.3 of [RFC8895].

   Specifically, the capabilities are defined as an object of type
   TIPSCapabilities:

        object {
          IncrementalUpdateMediaTypes incremental-change-media-types;
        } TIPSCapabilities;

        object-map {
           ResourceID -> String;
        } IncrementalUpdateMediaTypes;

                         Figure 5: TIPSCapabilities

   with field:

   incremental-change-media-types:  If a TIPS can provide updates with
      incremental changes for a resource, the "incremental-change-media-
      types" field has an entry for that resource-id, and the value is
      the supported media types of incremental changes, separated by
      commas.  For the implementation of this specification, this MUST
      be "application/ merge-patch+json", "application/json-patch+json",
      or "application/ merge-patch+json,application/json-patch+json",
      unless defined by a future extension.

      When choosing the media types to encode incremental updates for a
      resource, the server MUST consider the limitations of the
      encoding.  For example, when a JSON merge patch specifies that the
      value of a field is null, its semantics are that the field is
      removed from the target and hence the field is no longer defined

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      (i.e., undefined).  This, however, may not be the intended result
      for the resource, when null and undefined have different semantics
      for the resource.  In such a case, the server MUST choose JSON
      patch over JSON merge patch if JSON patch is indicated as a
      capability of the TIPS.  If the server does not support JSON patch
      to handle such a case, the server then needs to send a full
      replacement.

5.3.  Uses

   The "uses" attribute MUST be an array with the resource-ids of every
   network information resource for which this TIPS can provide service.

   This set may be any subset of the ALTO server's network information
   resources and may include resources defined in linked IRDs.  However,
   it is RECOMMENDED that the ALTO server selects a set that is closed
   under the resource dependency relationship.  That is, if a TIPS'
   "uses" set includes resource R1 and resource R1 depends on ("uses")
   resource R0, then the TIPS' "uses" set SHOULD include R0 as well as
   R1.  For example, if a TIPS provides a TIPS view for a cost map, it
   SHOULD also provide a TIPS view for the network map upon which that
   cost map depends.

   If the set is not closed, at least one resource R1 in the "uses"
   field of a TIPS depends on another resource R0 which is not in the
   "uses" field of the same TIPS.  Thus, a client cannot receive
   incremental updates for R0 from the same TIPS service.  If the client
   observes in an update of R1 that the version tag for R0 has changed,
   it must request the full content of R0, which is likely to be less
   efficient than receiving the incremental updates of R0.

5.4.  An Example

   Extending the IRD example in Section 8.1 of [RFC8895], Figure 6 is
   the IRD of an ALTO server supporting ALTO base protocol, ALTO/SSE,
   and ALTO TIPS.

     "my-network-map": {
       "uri": "https://alto.example.com/networkmap",
       "media-type": "application/alto-networkmap+json"
     },
     "my-routingcost-map": {
       "uri": "https://alto.example.com/costmap/routingcost",
       "media-type": "application/alto-costmap+json",
       "uses": ["my-networkmap"],
       "capabilities": {
         "cost-type-names": ["num-routingcost"]
       }

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     },
     "my-hopcount-map": {
       "uri": "https://alto.example.com/costmap/hopcount",
       "media-type": "application/alto-costmap+json",
       "uses": ["my-networkmap"],
       "capabilities": {
         "cost-type-names": ["num-hopcount"]
       }
     },
     "my-simple-filtered-cost-map": {
       "uri": "https://alto.example.com/costmap/filtered/simple",
       "media-type": "application/alto-costmap+json",
       "accepts": "application/alto-costmapfilter+json",
       "uses": ["my-networkmap"],
       "capabilities": {
         "cost-type-names": ["num-routingcost", "num-hopcount"],
         "cost-constraints": false
       }
     },
     "update-my-costs": {
       "uri": "https://alto.example.com/updates/costs",
       "media-type": "text/event-stream",
       "accepts": "application/alto-updatestreamparams+json",
       "uses": [
           "my-network-map",
           "my-routingcost-map",
           "my-hopcount-map",
           "my-simple-filtered-cost-map"
       ],
       "capabilities": {
         "incremental-change-media-types": {
           "my-network-map": "application/json-patch+json",
           "my-routingcost-map": "application/merge-patch+json",
           "my-hopcount-map": "application/merge-patch+json"
         },
         "support-stream-control": true
       }
     },
     "update-my-costs-tips": {
       "uri": "https://alto.example.com/updates-new/costs",
       "media-type": "application/alto-tips+json",
       "accepts": "application/alto-tipsparams+json",
       "uses": [
           "my-network-map",
           "my-routingcost-map",
           "my-hopcount-map",
           "my-simple-filtered-cost-map"
       ],

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       "capabilities": {
         "incremental-change-media-types": {
           "my-network-map": "application/json-patch+json",
           "my-routingcost-map": "application/merge-patch+json",
           "my-hopcount-map": "application/merge-patch+json",
           "my-simple-filtered-cost-map": "application/merge-patch+json"
         },
       }
     }

        Figure 6: Example of an ALTO Server Supporting ALTO Base
                   Protocol, ALTO/SSE, and ALTO TIPS

   Note that it is straightforward for an ALTO server to run HTTP/2 and
   support concurrent retrieval of multiple resources such as "my-
   network-map" and "my-routingcost-map" using multiple HTTP/2 streams.

   The resource "update-my-costs-tips" provides an ALTO TIPS service,
   and this is indicated by the media-type "application/ alto-
   tips+json".

6.  TIPS Management

   Upon request, a server sends a TIPS view to a client.  This TIPS view
   may be created at the time of the request or may already exist
   (either because another client has already created a TIPS view for
   the same requested network resource or because the server perpetually
   maintains a TIPS view for an often-requested resource).  The server
   MAY keep track of which clients are subscribing to each TIPS view to
   determine whether or not it should delete a TIPS view and its
   corresponding updates graph and associated data.

6.1.  Open Request

   An ALTO client requests that the server provide a TIPS view for a
   given resource by sending an HTTP POST body with the media type
   "application/alto-tipsparams+json".  That body contains a JSON object
   of type TIPSReq, where:

       object {
          ResourceID   resource-id;
          [JSONString  tag;]
          [Object      input;]
       } TIPSReq;

                             Figure 7: TIPSReq

   with the following fields:

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   resource-id:  The resource-id of an ALTO resource and MUST be in the
      TIPS' "uses" list (Section 5).  If a client does not support all
      incremental methods from the set announced in the server's
      capabilities, the client MUST NOT use the TIPS service.

   tag:  If the resource-id is a GET-mode resource with a version tag
      (or "vtag"), as defined in Section 10.3 of [RFC7285], and the ALTO
      client has previously retrieved a version of that resource from
      ALTO, the ALTO client MAY set the "tag" field to the tag part of
      the client's version of that resource.  The server MAY use the tag
      when calculating a recommended starting edge for the client to
      consume.  Note that the client MUST support all incremental
      methods from the set announced in the server's capabilities for
      this resource.

   input:  If the resource is a POST-mode service that requires input,
      the ALTO client MUST set the "input" field to a JSON object with
      the parameters that the resource expects.

6.2.  Open Response

   The response to a valid request MUST be a JSON object of type
   AddTIPSResponse, denoted as media type "application/alto-tips+json":

       object {
         URI               tips-view-uri;
         TIPSViewSummary   tips-view-summary;
       } AddTIPSResponse;

       object {
         UpdatesGraphSummary   updates-graph-summary;
         JSONNumber            heartbeat-interval;
       } TIPSViewSummary;

       object {
         JSONNumber       start-seq;
         JSONNumber       end-seq;
         StartEdgeRec     start-edge-rec;
       } UpdatesGraphSummary;

       object {
         JSONNumber       seq-i;
         JSONNumber       seq-j;
       } StartEdgeRec;

                         Figure 8: AddTIPSResponse

   with the following fields:

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   tips-view-uri:  URI to the requested TIPS view.  The value of this
      field must have the following format:

          schema "://" tips-view-host "/" tips-view-path

          tips-view-host = host [ ":" port]
          tips-view-path = path

      where schema must be "http" or "https" unless specified by a
      future extension, and host, port and path are as specified in
      Sections 3.2.2, 3.2.3, and 3.3 in [RFC3986].  The URI MUST be
      unique per client session and may be de-aliased by the server to
      refer to the actual location of the TIPS view which may be shared
      by other clients.

      When creating the URI for the TIPS view, TIPS MUST NOT use other
      properties of an HTTP request, such as cookies or the client's IP
      address, to determine the TIPS view.  Furthermore, TIPS MUST NOT
      reuse a URI for a different object in the same user session.

      It is expected that there is an internal mechanism to map a tips-
      view-uri to the TIPS view to be accessed.  For example, TIPS may
      assign a unique, internal state id to each TIPS view instance.
      However, the exact mechanism is left to the TIPS provider.

   tips-view-summary:  Contains an updates-graph-summary and a
      heartbeat-interval.

      The updates-graph-summary field contains the starting sequence
      number (start-seq) of the updates graph and the last sequence
      number (end-seq) that is currently available, along with a
      recommended edge to consume (start-edge-rec).  How the server
      calculates the recommended edge depends on the implementation.
      Ideally, if the client does not provide a version tag, the server
      should recommend the edge of the latest snapshot available.  If
      the client does provide a version tag, the server should calculate
      the cumulative size of the incremental updates available from that
      version onward and compare it to the size of the complete resource
      snapshot.  If the snapshot is bigger, the server should recommend
      the first incremental update edge starting from the client's
      tagged version.  Otherwise, the server should recommend the latest
      snapshot edge.

      The heartbeat-interval field contains the interval length of the
      heartbeat.  The time unit is second and the value MUST be
      interpreted as a floating number.

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      The client is expected to send a heartbeat message (see
      Section 6.4) for each TIPS view that it is currently using, at
      least every heartbeat-interval seconds.  It is RECOMMENDED that
      the ALTO server picks an appropriate value for the heartbeat-
      interval field: if the value is too big, it may waste additional
      resources to maintain unused TIPS views; but if the value is too
      small, the normal use of the TIPS service might be disrupted.

   If the request has any errors, the TIPS service must return an HTTP
   "400 Bad Request" to the ALTO client; the body of the response
   follows the generic ALTO error response format specified in
   Section 8.5.2 of [RFC7285].  Hence, an example ALTO error response
   has the format shown in Figure 9.

       HTTP/1.1 400 Bad Request
       Content-Length: 131
       Content-Type: application/alto-error+json

       {
           "meta":{
               "code":  "E_INVALID_FIELD_VALUE",
               "field": "resource-id",
               "value": "my-network-map/#"
           }
       }

                        Figure 9: ALTO Error Example

   Note that "field" and "value" are optional fields.  If the "value"
   field exists, the "field" field MUST exist.

   *  If the TIPS request does not have a "resource-id" field, the error
      code of the error message MUST be E_MISSING_FIELD and the "field"
      field SHOULD be "resource-id".  The TIPS service MUST NOT create
      any TIPS view.

   *  If the "resource-id" field is invalid or is not associated with
      the TIPS, the error code of the error message MUST be
      E_INVALID_FIELD_VALUE.  The "field" field SHOULD be the full path
      of the "resource-id" field, and the "value" field SHOULD be the
      invalid resource-id.

   *  If the resource is a POST-mode service that requires input, the
      client MUST set the "input" field to a JSON object with the
      parameters that that resource expects.  If the "input" field is
      missing or invalid, TIPS MUST return the same error response that
      resource would return for missing or invalid input (see
      [RFC7285]).

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   Furthermore, it is RECOMMENDED that the server uses the following
   HTTP codes to indicate other errors, with the media type
   "application/alto-error+json".

   *  429 (Too Many Requests): when the number of TIPS views open
      requests exceeds the server threshold.  The server may indicate
      when to re-try the request in the "Re-Try After" headers.

6.3.  Open Example

   For simplicity, assume that the ALTO server is using the Basic
   authentication.  If a client with username "client1" and password
   "helloalto" wants to create a TIPS view of an ALTO Cost Map resource
   with resource ID "my-routingcost-map", it can send the request
   depicted in Figure 10.

       POST /tips HTTP/1.1
       Host: alto.example.com
       Accept: application/alto-tips+json, application/alto-error+json
       Authorization: Basic Y2xpZW50MTpoZWxsb2FsdG8K
       Content-Type: application/alto-tipsparams+json
       Content-Length: 41

       {
         "resource-id": "my-routingcost-map"
       }

                          Figure 10: Open Example

   If the operation is successful, the ALTO server returns the message
   shown in Figure 11.

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       HTTP/1.1 200 OK
       Content-Type: application/alto-tips+json
       Content-Length: 287

       {
         "tips-view-uri": "https://alto.example.com/tips/2718281828459",
         "tips-view-summary": {
           "updates-graph-summary": {
             "start-seq": 101,
             "end-seq": 106,
             "start-edge-rec" : {
               "seq-i": 0,
               "seq-j": 105
             }
           },
           "heartbeat-interval": 5
         }
       }

                        Figure 11: Response Example

6.4.  Heartbeat Request and Response

   Upon creating a TIPS view, the ALTO client must indicate its liveness
   to that specific TIPS view by sending a heartbeat request every
   heartbeat-interval second(s).  The heartbeat request MUST have the
   following format:

       POST /<tips-view-path>/heartbeat HTTP/1.1
       HOST: <tips-view-host>

   The body of the request must be empty.

   The response to a heartbeat request must have status code 200 if the
   server has acknowledged the heartbeat message, and the corresponding
   error code if there is any error.  When the request is successful,
   the body of the response must be empty.

   It is RECOMMENDED that the server uses the following HTTP codes to
   indicate errors, with the media type "application/alto-error+json",
   regarding TIPS view heartbeat requests.

   *  404 (Not Found): if the requested TIPS view does not exist or is
      closed.

   An ALTO server should close a TIPS view if multiple consecutive
   heartbeat requests are not received for that TIPS view.  The maximum
   number of missing heartbeat requests is specific to the server

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   implementation, e.g., statically configured for the server, specific
   to the resource and/or client, or dynamically set based on the server
   load, etc.

6.5.  Close Request and Response

   An ALTO client can indicate it no longer desires to pull/receive
   updates for a specific network resource by "deleting" the TIPS view
   using the returned tips-view-uri and the HTTP DELETE method.  The
   DELETE request MUST have the following format:

       DELETE /<tips-view-path> HTTP/1.1
       HOST: <tips-view-host>

   The response to a valid request must be 200 if successful, and the
   corresponding error code if there is any error.

   It is RECOMMENDED that the server uses the following HTTP codes to
   indicate errors, with the media type "application/alto-error+json",
   regarding TIPS view close requests.

   *  404 (Not Found): if the requested TIPS view does not exist or is
      closed.

   It must be noted that whether or not the server releases the
   resources associated with the TIPS view is implementation-dependent.
   For example, an ALTO server may maintain a set of clients that
   subscribe to the TIPS view of a resource: a client that deletes the
   view is removed from the set, and the TIPS view is only removed when
   the dependent set becomes empty.  See other potential implementations
   in Section 8.7.

7.  TIPS Data Transfers - Client Pull

   TIPS allows an ALTO client to retrieve the content of an update item
   from the updates graph, with an update item defined as the content
   (incremental update or snapshot) on an edge in the updates graph.

7.1.  Request

   The client sends an HTTP GET request, where the media type of an
   update item resource MUST be the same as the "media-type" field of
   the update item on the specified edge in the updates graph.

   The GET request MUST have the following format:

       GET /<tips-view-path>/ug/<i>/<j>
       HOST: <tips-view-host>

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   For example, consider the updates graph in Figure 4.  If the client
   wants to query the content of the first update item (0 -> 101) whose
   media type is "application/alto- costmap+json", it must send a
   request to "/tips/2718281828459/ug/0/101" and set the "Accept" header
   to "application/alto- costmap+json, application/alto-error+json".
   See Section 7.3 for a concrete example.

7.2.  Response

   If the request is valid (ug/<i>/<j> exists), the response is encoded
   as a JSON object whose data format is indicated by the media type.

   A client may conduct proactive fetching of future updates, by long
   polling updates that have not been listed in the directory yet.  For
   long-poll prefetch, the client must have indicated the media type
   that may appear.  It is RECOMMENDED that the server allows for at
   least the prefetch of <end-seq> -> <end-seq + 1>

   Hence, the server processing logic SHOULD be:

   *  If ug/<i>/<j> exists: return content using encoding.

   *  Else if ug/<i>/<j> pre-fetch is acceptable: put request in a
      backlog queue.

   *  Else: return error.

   It is RECOMMENDED that the server uses the following HTTP codes to
   indicate errors, with the media type "application/alto-error+json",
   regarding update item requests.

   *  404 (Not Found): if the requested TIPS view does not exist or is
      closed.

   *  410 (Gone): if an update has a seq that is smaller than the start-
      seq.

   *  415 (Unsupported Media Type): if the media type(s) accepted by the
      client does not include the media type of the update chosen by the
      server.

   *  425 (Too Early): if the seq exceeds the server prefetch window

   *  429 (Too Many Requests): when the number of pending (long-poll)
      requests exceeds the server threshold.  The server may indicate
      when to re-try the request in the "Re-Try After" headers.

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7.3.  Example

   Assume the client wants to get the contents of the update item on
   edge 0 to 101.  The format of the request is shown in Figure 12.

       GET /tips/2718281828459/ug/0/101 HTTP/1.1
       Host: alto.example.com
       Accept: application/alto-costmap+json, \
                 application/alto-error+json

                           Figure 12: GET Example

   The response is shown in Figure 13.

       HTTP/1.1 200 OK
       Content-Type: application/alto-costmap+json
       Content-Length: 50

       { ... full replacement of my-routingcost-map ... }

                    Figure 13: Response to a GET Request

7.4.  New Next Edge Recommendation

   While intended TIPS usage is for the client to receive a recommended
   starting edge in the TIPS summary, consume that edge, then construct
   all future URIs by incrementing the sequence count by 1, there may be
   cases in which the client needs to request a new next edge to
   consume.  For example, if a client has an open TIPS view yet has not
   polled in a while, the client may request the next logical
   incremental URI but the server has compacted the updates graph so it
   no longer exists.  Thus, the client must request a new next edge to
   consume based on its current version of the resource.

7.4.1.  Request

   An ALTO client requests that the server provide a next edge
   recommendation for a given TIPS view by sending an HTTP POST request
   with the media type "application/alto-tipsparams+json".  The request
   has the form:

       POST /<tips-view-path>/ug HTTP/1.1
       HOST: <tips-view-host>

   The POST body has the following form, where providing the version tag
   of the resource the client already has is optional:

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       object {
           [JSONString  tag;]
       } TIPSNextEdgeReq;

7.4.2.  Response

   The response to a valid request MUST be a JSON object of type
   UpdatesGraphSummary (defined in Section 6.2 but reproduced in
   Figure 14 as well), denoted as media type "application/alto-
   tips+json":

       object {
         JSONNumber       start-seq;
         JSONNumber       end-seq;
         StartEdgeRec     start-edge-rec;
       } UpdatesGraphSummary;

       object {
         JSONNumber       seq-i;
         JSONNumber       seq-j;
       } StartEdgeRec;

                       Figure 14: UpdatesGraphSummary

   It is RECOMMENDED that the server uses the following HTTP codes to
   indicate errors, with the media type "application/alto-error+json",
   regarding new next edge requests.

   *  404 (Not Found): if the requested TIPS view does not exist or is
      closed.

   *  415 (Unsupported Media Type): if the media type(s) accepted by the
      client does not include the media type application/alto-tips+json.

8.  Operation and Processing Considerations

8.1.  Considerations for Choosing Updates

   When implementing TIPS, a developer should be cognizant of the
   effects of update schedule, which includes both the choice of timing
   (i.e., when/what to trigger an update on the updates graph) and the
   choice of message format (i.e., given an update, send a full
   replacement or an incremental change).  In particular, the update
   schedule can have effects on both the overhead and the freshness of
   information.  To minimize overhead, developers may choose to batch a
   sequence of updates for resources that frequently change by
   cumulative updates or a full replacement after a while.  Developers
   should be cognizant that batching reduces the freshness of

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   information and should also consider the effect of such delays on
   client behaviors.

   For incremental updates, this design allows both JSON patch and JSON
   merge patch for incremental changes.  JSON merge patch is clearly
   superior to JSON patch for describing incremental changes to cost
   maps, endpoint costs, and endpoint properties.  For these data
   structures, JSON merge patch is more space efficient, as well as
   simpler to apply.  There is no advantage in allowing a server to use
   JSON patch for those resources.

   The case is not as clear for incremental changes to network maps.

   First, consider small changes, such as moving a prefix from one PID
   to another.  JSON patch could encode that as a simple insertion and
   deletion, while JSON merge patch would have to replace the entire
   array of prefixes for both PIDs.  On the other hand, to process a
   JSON patch update, the ALTO client would have to retain the indexes
   of the prefixes for each PID.  Logically, the prefixes in a PID are
   an unordered set, not an array; aside from handling updates, a client
   does not need to retain the array indexes of the prefixes.  Hence, to
   take advantage of JSON patch for network maps, ALTO clients would
   have to retain additional, otherwise unnecessary, data.

   Second, consider more involved changes, such as removing half of the
   prefixes from a PID.  JSON merge patch would send a new array for
   that PID, while JSON patch would have to send a list of remove
   operations and delete the prefix one by one.

   Therefore, each TIPS instance may choose to encode the updates using
   JSON merge patch or JSON patch based on the type of changes in
   network maps.

8.2.  Considerations for Load Balancing

   There are two levels of load balancing in TIPS.  The first level is
   to balance the load of TIPS views for different clients, and the
   second is to balance the load of incremental updates.

   Load balancing of TIPS views can be achieved either at the
   application layer or at the infrastructure layer.  For example, an
   ALTO server may set <tips-view-host> to different subdomains to
   distribute TIPS views, or simply use the same host of the TIPS
   service and rely on load balancers to distribute the load.

   TIPS allows clients to make concurrent pulls of incremental updates
   potentially through different HTTP connections.  As a consequence, it
   introduces additional complexities when the ALTO server is being load

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   balanced -- a feature widely used to build scalable and fault-
   tolerant web services.  For example, a request may be incorrectly
   processed if the following two conditions both hold:

   *  the backend servers are stateful, i.e., the TIPS view is created
      and stored only on a single server;

   *  the ALTO server is using layer-4 load balancing, i.e., the
      requests are distributed based on the TCP 5-tuple.

   Thus, additional considerations are required to enable correct load
   balancing for TIPS, including:

   *  Use a stateless architecture: One solution is to follow the
      stateless computing pattern: states about the TIPS view are not
      maintained by the backend servers but are stored in a distributed
      database.  Thus, concurrent requests to the same TIPS view can be
      processed on arbitrary stateless backend servers, which all
      fetches data from the same database.

   *  Configure the load balancers properly: In case when the backend
      servers are stateful, the load balancers must be properly
      configured to guarantee that requests of the same TIPS view always
      arrive at the same server.  For example, an operator or a provider
      of an ALTO server may configure layer-7 load balancers that
      distribute requests based on the tips-view-path component in the
      URI.

8.3.  Considerations for Cross-Resource Dependency Scheduling

   Dependent ALTO resources result in cross-resource dependencies in
   TIPS.  Consider the following pair of resources, where my-cost-map
   (C) is dependent on my-network-map (N).  The updates graph for each
   resource is shown, along with links in between the respective updates
   graphs to show dependency:

                              +---+   +---+   +---+   +---+   +---+
         my-network-map (N)   | 0 |-->|89 |-->|90 |-->|91 |-->|92 |
                              +---+   +---+   +---+   +---+   +---+
                                        |   \       \       \
                                        |    \       \       \
                              +---+   +---+   +---+   +---+   +---+
         my-cost-map (C)      | 0 |-->|101|-->|102|-->|103|-->|104|
                              +---+   +---+   +---+   +---+   +---+
                               |_______________________|

                    Figure 15: Example Dependency Model

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   In Figure 15, the cost-map versions 101 and 102 (denoted as C101 and
   C102) are dependent on the network-map version 89 (denoted as N89).
   The cost-map version 103 (C103) is dependent on the network-map
   version 90 (N90), and so on.

   Thus, the client must decide the order in which to receive and apply
   the updates.  The order may affect how fast the client can build a
   consistent view and how long the client needs to buffer the update.

   *  Example 1: The client requests N89, N90, N91, C101, C102 in that
      order.  The client either gets no consistent view of the resources
      or has to buffer N90 and N91.

   *  Example 2: The client requests C101, C102, C103, N89.  The client
      either gets no consistent view or has to buffer C103.

   Therefore, the client is RECOMMENDED to request and process updates
   in the ascending order of the smallest dependent tag, e.g., {C101,
   C102, N89} before {C103, N90}

8.4.  Considerations for Client Processing Updates

   In general, when an ALTO client receives a full replacement for a
   resource, the ALTO client should replace the current version with the
   new version.  When an ALTO client receives an incremental update for
   a resource, the ALTO client should apply those updates to the current
   version of the resource.

   However, because resources can depend on other resources (e.g., cost
   maps depend on network maps), an ALTO client must not use a dependent
   resource if the resource on which it depends has changed.  There are
   at least two ways an ALTO client can do that.  The following
   paragraphs illustrate these techniques by referring to network and
   cost map messages, although these techniques apply to any dependent
   resources.

   Note that when a network map changes, the server should send the
   network map update message before sending the updates for the
   dependent cost maps.

   One approach is for the ALTO client to save the network map update
   message in a buffer and continue to use the previous network map and
   the associated cost maps until the ALTO client receives the update
   messages for all dependent cost maps.  The ALTO client then applies
   all network and cost map updates atomically.

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   Alternatively, the ALTO client may update the network map
   immediately.  In this case, the cost maps using the network map
   become invalid because they are inconsistent with the current network
   map; hence, the ALTO client must mark each such dependent cost map as
   temporarily invalid and must not use each such cost map until the
   ALTO client receives a cost map update indicating that it is based on
   the new network map version tag.

   Though a server should send update items sequentially, it is possible
   that a client receives the update items out of order (in the case of
   a retransmitted update item or a result of concurrent fetch).  The
   client must buffer the update items if they arrive out of order and
   then apply them sequentially (based on the sequence numbers) due to
   the operation of JSON merge patch and JSON patch.

8.5.  Considerations for Updates to Filtered Cost Maps

   If TIPS provides updates to a Filtered Cost Map that allows
   constraint tests, then an ALTO client may request updates to a
   Filtered Cost Map request with a constraint test.  In this case, when
   a cost changes, the updates graph MUST have an update if the new
   value satisfies the test.  If the new value does not, whether there
   is an update depends on whether the previous value satisfies the
   test.  If it did not, the updates graph SHOULD NOT have an update.
   But if the previous value did, then the updates graph MUST add an
   update with a "null" value to inform the ALTO client that this cost
   no longer satisfies the criteria.

   TIPS can avoid having to handle such complicated behavior by offering
   TIPS only for Filtered Cost Maps that do not allow constraint tests.

8.6.  Considerations for Updates to Ordinal Mode Costs

   For an ordinal mode cost map, a change to a single cost point may
   require updating many other costs.  As an extreme example, suppose
   the lowest cost changes to the highest cost.  For a numerical mode
   cost map, only that one cost changes.  But for an ordinal mode cost
   map, every cost might change.  While this document allows TIPS to
   offer incremental updates for ordinal mode cost maps, TIPS
   implementors should be aware that incremental updates for ordinal
   costs are more complicated than for numerical costs, and that small
   changes of the original cost value may result in large updates.

   A TIPS implementation can avoid this complication by only offering
   full replacements as updates in the updates graph for ordinal cost
   maps.

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8.7.  Considerations for Managing Shared TIPS Views

   From a client's point of view, it sees only one copy of the TIPS view
   for any resource.  However, on the server side, there are different
   implementation options, especially for common resources (e.g.,
   network map or cost map) that may be frequently queried by many
   clients.  Some potential options are listed below:

   *  An ALTO server creates one TIPS view of the common resource for
      each client.  When the client deletes the view, the server deletes
      the view in the server storage.

   *  An ALTO server maintains one copy of the TIPS view for each common
      resource and all clients requesting the same resources use the
      same copy.  There are two ways to manage the storage for the
      shared copy:

      -  the ALTO server maintains the set of clients that subscribe to
         the TIPS view, and only removes the view from the storage when
         the set becomes empty.

      -  the TIPS view is never removed from the storage.

   Developers may choose different implementation options depending on
   criteria such as request frequency, available resources of the ALTO
   server, the ability to scale, and programming complexity.

8.8.  Considerations for Offering Shortcut Incremental Updates

   Besides the mandatory stepwise incremental updates (from i to i+1),
   an ALTO server may optionally offer shortcut incremental updates, or
   simple shortcuts, between two non-consecutive versions i and i+k (k >
   1).  Such shortcuts offer alternative paths in the update graph and
   can potentially speed up the transmission and processing of
   incremental updates, leading to faster synchronization of ALTO
   information, especially when the client has limited bandwidth and
   computation.  However, implementors of an ALTO server must be aware
   that:

   1.  Optional shortcuts may increase the size of the update graph, in
       the worst case being the square of the number of updates (i.e.,
       when a shortcut is offered for each version to all future
       versions).

   2.  Optional shortcuts require additional storage on the ALTO server.

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   3.  Optional shortcuts may reduce concurrency when the updates do not
       overlap, e.g., when the updates apply to different parts of an
       ALTO resource.  In such a case, the total size of the original
       updates is close to the size of the shortcut, but the original
       updates can be transmitted concurrently while the shortcut is
       transmitted in a single connection.

9.  Security Considerations

   The security considerations (Section 15 of [RFC7285]) of the base
   protocol fully apply to this extension.  For example, the same
   authenticity and integrity considerations (Section 15.1 of [RFC7285])
   still fully apply; the same considerations for the privacy of ALTO
   users (Section 15.4 of [RFC7285]) also still fully apply.

   The additional services (addition of update read service and update
   push service) provided by this extension extend the attack surface
   described in Section 15.1.1 of [RFC7285].  The following sub-sections
   discuss the additional risks and their remedies.

9.1.  TIPS: Denial-of-Service Attacks

   Allowing TIPS views enables a new class of Denial-of-Service attacks.
   In particular, for the TIPS server, an ALTO client might create an
   excessive number of TIPS views.

   To avoid these attacks on the TIPS server, the server SHOULD choose
   to limit the number of active views and reject new requests when that
   threshold is reached.  TIPS allows predictive fetching and the server
   SHOULD also choose to limit the number of pending requests.  If a new
   request exceeds the threshold, the server SHOULD log the event and
   may return the HTTP status "429 Too many requests".

   It is important to note that the preceding approaches are not the
   only possibilities.  For example, it may be possible for TIPS to use
   somewhat more clever logic involving IP reputation, rate-limiting,
   and compartmentalization of the overall threshold into smaller
   thresholds that apply to subsets of potential clients.

9.2.  ALTO Client: Update Overloading or Instability

   The availability of continuous updates, when the client indicates
   receiving server push, can also cause overload for an ALTO client, in
   particular, an ALTO client with limited processing capabilities.  The
   current design does not include any flow control mechanisms for the
   client to reduce the update rates from the server.  For example, TCP,
   HTTP/2 and QUIC provide stream and connection flow control data
   limits, and PUSH stream limits, which might help prevent the client

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   from being overloaded.  Under overloading, the client MAY choose to
   remove the information resources with high update rates.

   Also, under overloading, the client may no longer be able to detect
   whether information is still fresh or has become stale.  In such a
   case, the client should be careful in how it uses the information to
   avoid stability or efficiency issues.

9.3.  Spoofed URI

   An outside party that can read the TIPS response or that can observe
   TIPS requests can obtain the TIPS view URI and use that to send
   fraudulent "DELETE" requests, thus disabling the service for the
   valid ALTO client.  This can be avoided by encrypting the requests
   and responses (Section 15 of [RFC7285]).

10.  IANA Considerations

   IANA is requested to register the following media types from the
   registry available at [IANA-Media-Type]:

   *  application/alto-tips+json: as described in Section 6.2;

   *  application/alto-tipsparams+json: as described in Section 6.1;

      Note to the RFC Editor: Please replace This-Document with the RFC
      number to be assigned to this document.

10.1.  application/alto-tips+json Media Type

   Type name:  application

   Subtype name:  alto-tips+json

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  Encoding considerations are identical to
      those specified for the "application/json" media type.  See
      [RFC8259].

   Security considerations:  See the Security Considerations section of
      This-Document.

   Interoperability considerations:  This document specifies the format
      of conforming messages and the interpretation thereof.

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   Published specification:  Section 6.2 of This-Document.

   Applications that use this media type:  ALTO servers and ALTO clients
      either stand alone or are embedded within other applications.

   Fragment identifier considerations:  N/A

   Additional information:

      Deprecated alias names for this type:  N/A

      Magic number(s):  N/A

      File extension(s):  This document uses the media type to refer to
         protocol messages and thus does not require a file extension.

      Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  See Aut
      hors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See Authors' Addresses section.

   Change controller:  Internet Engineering Task Force
      (mailto:iesg@ietf.org).

   Provisional registration?:  No

10.2.  application/alto-tipsparams+json Media Type

   Type name:  application

   Subtype name:  alto-tipsparams+json

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  Encoding considerations are identical to
      those specified for the "application/json" media type.  See
      [RFC8259].

   Security considerations:  See the Security Considerations section of
      This-Document.

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   Interoperability considerations:  This document specifies the format
      of conforming messages and the interpretation thereof.

   Published specification:  Section 6.1 of This-Document.

   Applications that use this media type:  ALTO servers and ALTO clients
      either stand alone or are embedded within other applications.

   Fragment identifier considerations:  N/A

   Additional information:

      Deprecated alias names for this type:  N/A

      Magic number(s):  N/A

      File extension(s):  This document uses the media type to refer to
         protocol messages and thus does not require a file extension.

      Macintosh file type code(s):  N/A

   Person and email address to contact for further information:  See Aut
      hors' Addresses section.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See Authors' Addresses section.

   Change controller:  Internet Engineering Task Force
      (mailto:iesg@ietf.org).

   Provisional registration?:  No

11.  References

11.1.  Normative References

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

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

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   [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/rfc/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/rfc/rfc8174>.

   [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
              Interchange Format", STD 90, RFC 8259,
              DOI 10.17487/RFC8259, December 2017,
              <https://www.rfc-editor.org/rfc/rfc8259>.

   [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/rfc/rfc8895>.

   [RFC9112]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
              June 2022, <https://www.rfc-editor.org/rfc/rfc9112>.

   [RFC9113]  Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
              DOI 10.17487/RFC9113, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9113>.

   [RFC9114]  Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
              June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.

11.2.  Informative References

   [IANA-Media-Type]
              "Media Types", June 2023,
              <https://www.iana.org/assignments/media-types/media-
              types.xhtml>.

   [RFC9205]  Nottingham, M., "Building Protocols with HTTP", BCP 56,
              RFC 9205, DOI 10.17487/RFC9205, June 2022,
              <https://www.rfc-editor.org/rfc/rfc9205>.

Appendix A.  A High-Level Deployment Model

   Conceptually, the TIPS system consists of three types of resources:

   *  (R1) TIPS frontend to manage (create/delete) TIPS views.

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   *  (R2) TIPS view directory, which provides metadata (e.g.,
      references) about the network resource data.

   *  (R3) The actual network resource data, encoded as complete ALTO
      network resources (e.g., cost map, network map) or incremental
      updates.

                      +------------------------------------------------+
                      |                                                |
 +------+             |R1: Frontend/Open  R2: Directory/Meta  R3: Data |
 |      | "iget" base |     +-----+           +-----+         +-----+  |
 |      | resource 1  |     |     |           |     |         |     |  |
 |      |-------------|---->|     |           |     |         |     |  |
 |      | incremental |     |     |           |     |-------->|     |  |
 |      | transfer    |     |     |           |     |         |     |  |
 |      | resource    |     |     |           |     |         |     |  |
 |      |<------------|-----------------------|     |         |     |  |
 |Client|             |     |     |           +-----+         +-----+  |
 |      | "iget" base |     |     |                                    |
 |      | resource 2  |     |     |           +-----+         +-----+  |
 |      |-------------|---->|     |           |     |         |     |  |
 |      | incremental |     |     |           |     |         |     |  |
 |      | transfer    |     +-----+           |     | ------->|     |  |
 |      | resource    |                       |     |         |     |  |
 |      |<------------|-----------------------|     |         |     |  |
 +------+             |                       +-----+         +-----+  |
                      |                                                |
                      +------------------------------------------------+

               Figure 16: Sample TIPS Deployment Model

   Design Point: Component Resource Location

   *  Design 1 (Single): all the three resource types at the same,
      single server (accessed via relative reference)

   *  Design 2 (Flexible): all three resource types can be at their own
      server (accessed via absolute reference)

   *  Design 3 (Dir + Data): R2 and R3 must remain together, though R1
      might not be on the same server

   This document supports Design 1 and Design 3.  For Design 1, the TIPS
   service simply needs to always use the same host for the TIPS views.
   For Design 3, the TIPS service can set tips-view-host to a different
   server.  Note that the deployment flexibility is at the logical
   level, as these services can be distinguished by different paths and
   potentially be routed to different physical servers by layer-7 load

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   balancing.  See Section 8.2 for a discussion on load balancing
   considerations.  Future documents may extend the protocol to support
   Design 2.

Appendix B.  Conformance to "Building Protocols with HTTP" Best Current
             Practices

   This specification adheres fully to [RFC9205] as further elaborated
   below:

   *  TIPS does not "redefine, refine, or overlay the semantics of
      generic protocol elements such as methods, status codes, or
      existing header fields" and instead focuses on "protocol elements
      that are specific to [the TIPS] application -- namely, [its] HTTP
      resources" (Section 3.1 of [RFC9205]).

   *  There are no statically defined URI components (Section 3.2 of
      [RFC9205]).

   *  No minimum version of HTTP is specified by TIPS which is
      recommended (Section 4.1 of [RFC9205]).

   *  The TIPS design follows the advice that "When specifying examples
      of protocol interactions, applications should document both the
      request and response messages with complete header sections,
      preferably in HTTP/1.1 format" (Section 4.1 of [RFC9205]).

   *  TIPS uses URI templates which is recommended (Section 4.2 of
      [RFC9205]).

   *  TIPS follows the pattern that "a client will begin interacting
      with a given application server by requesting an initial document
      that contains information about that particular deployment,
      potentially including links to other relevant resources.  Doing so
      ensures that the deployment is as flexible as possible
      (potentially spanning multiple servers), allows evolution, and
      also allows the application to tailor the "discovery document" to
      the client" (Section 4.4.1 of [RFC9205]).

   *  TIPS uses existing HTTP schemes (Section 4.4.2 of [RFC9205]).

   *  TIPS defines its errors "to use the most applicable status code"
      (Section 4.6 of [RFC9205]).

   *  TIPS does not "make assumptions about the relationship between
      separate requests on a single transport connection; doing so
      breaks many of the assumptions of HTTP as a stateless protocol and
      will cause problems in interoperability, security, operability,

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      and evolution" (Section 4.11 of [RFC9205]).  The only relationship
      between requests is that a client must first discover where a TIPS
      view of a resource will be served, which is consistent with the
      URI discovery in Section 4.4.1 of [RFC9205].

Appendix C.  Push-mode TIPS using HTTP Server Push

   TIPS allows ALTO clients to subscribe to incremental updates of an
   ALTO resource, and the specification in this document is based on the
   current best practice of building such a service using native HTTP.
   Earlier versions of this document had investigated the possibility of
   enabling push-mode TIPS, i.e., by taking advantage of the server push
   feature in HTTP/2 and HTTP/3.

   In the ideal case, push-mode TIPS can potentially improve performance
   (e.g., latency) in more dynamic environments and use cases, with
   wait-free message delivery.  Using native server push also results in
   minimal changes to the current protocol.  While not adopted due to
   the lack of server push support and increased protocol complexity,
   push-mode TIPS remains a potential direction of protocol improvement.

Appendix D.  Persistent HTTP Connections

   Previous versions of this document use persistent HTTP connections to
   detect the liveness of clients.  This design is replaced with the
   heartbeat mechanism because it does not conform well with the best
   current practice of HTTP.  For example, if an ALTO client is
   accessing a TIPS view over an HTTP proxy, the connection is not
   established directly between the ALTO client and the ALTO server, but
   between the ALTO client and the proxy and between the proxy and the
   ALTO server.  Thus, using persistent connections may not correctly
   detect the right liveness state.

Acknowledgments

   The authors of this document would like to thank Mark Nottingham and
   Spencer Dawkins for providing invaluable reviews of earlier versions
   of this document, Adrian Farrel, Qin Wu, and Jordi Ros Giralt for
   their continuous feedback, Russ White, Donald Eastlake, Martin
   Thomson, Bernard Adoba, Spencer Dawkins, and Sheng Jiang for the
   directorate reviews, Martin Duke for the Area Director review, and
   Mohamed Boucadair for shepherding the document.

Authors' Addresses

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

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   Chengdu
   610000
   China
   Email: kaigao@scu.edu.cn

   Roland Schott
   Deutsche Telekom
   Ida-Rhodes-Straße 2
   64295 Darmstadt
   Germany
   Email: Roland.Schott@telekom.de

   Yang Richard Yang
   Yale University
   51 Prospect Street
   New Haven,  CT
   United States of America
   Email: yry@cs.yale.edu

   Lauren Delwiche
   Yale University
   51 Prospect Street
   New Haven,  3408
   United States of America
   Email: lauren.delwiche@yale.edu

   Lachlan Keller
   Yale University
   51 Prospect Street
   New Haven,  3408
   United States of America
   Email: lachlan.keller@yale.edu

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