Internet-Draft ALTO TIPS October 2023
Gao, et al. Expires 20 April 2024 [Page]
Workgroup:
ALTO
Internet-Draft:
draft-ietf-alto-new-transport-16
Published:
Intended Status:
Standards Track
Expires:
Authors:
K. Gao
Sichuan University
R. Schott
Deutsche Telekom
Y. R. Yang
Yale University
L. Delwiche
Yale University
L. Keller
Yale University

The ALTO Transport Information Publication Service

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.

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 20 April 2024.

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Table of Contents

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.

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.

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:

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 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.

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 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.

                                   +-------------+
    +-----------+ +--------------+ |  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 |            | tv2/ug |                 | tv3/ug | | |
| | +--------+            +--------+                 +--------+ | |
| +----|----\----------------|-------------------------|--------+ |
|      |     \               |                         |          |
+------|------\--------------|-------------------------|----------+
       |       +------+      |                         |
       |               \     |                         |
   +----------+       +----------+                 +----------+
   | Client 1 |       | Client 2 |                 | Client 3 |
   +----------+       +----------+                 +----------+

tvi   = TIPS view i
tvi/ug = incremental updates graph associated with 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.

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.

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 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

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 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.

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 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).

It must be noted that a server may close a TIPS view, e.g., under high system load or due to inactivity. It is RECOMMENDED that a client detects the liveness and declares interests of the TIPS view by sending a polling edge request. For example, as long as the polling request to <tips-view-uri>/ug/<j>/<j+1> does not receive error code 404, the TIPS view is still alive.

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>                     .
  |------------------------------------------------------> |
  .                                                        .
  .                                                        .
  | content on edge j to j+1                               |
  | <------------------------------------------------------|
  |                                                        .
  o                                                        .
                                                           .
                                         TIPS View Closed  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.

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.

  <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
    \_ ...
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.

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 (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"]
      }
    },
    "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"
      ],
      "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).

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:

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;
    } 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:

tips-view-uri:

URI to the requested TIPS view. The value of this field MUST have the following format:

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

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

where scheme 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].

A server SHOULD NOT use properties that are not included in the request body to determine the URI of a TIPS view, such as cookies or the client's IP address. Furthermore, TIPS MUST NOT reuse a URI for a different TIPS view (different resources or different request bodies).

tips-view-summary:

Contains an updates-graph-summary.

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.

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]).

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.

    HTTP/1.1 200 OK
    Content-Type: application/alto-tips+json
    Content-Length: 258

    {
      "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
          }
        }
      }
    }
Figure 11: Response Example

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>

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 by the server.
  • 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.

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:

    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 by the server.
  • 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 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 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

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.

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.

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.

  • 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 have sent a polling request 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.
  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 new classes of Denial-of-Service attacks. In particular, for the TIPS server, one or multiple malicious ALTO clients might create an excessive number of TIPS views, to exhaust the server resource and/or to block normal users from the accessing the service.

To avoid such attacks, 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 TIPS view eviction policies, IP reputation, rate-limiting, and compartmentalization of the overall threshold into smaller thresholds that apply to subsets of potential clients. If service availability is a concern, ALTO clients may establish service level agreements with the ALTO server.

9.2. ALTO Client: Update Overloading or Instability

The availability of continuous updates 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, which might help prevent the client 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.

10. IANA Considerations

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

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.

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 Authors' 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.

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 Authors' 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, , <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, , <https://www.rfc-editor.org/rfc/rfc3986>.
[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, , <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, , <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, , <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, , <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, , <https://www.rfc-editor.org/rfc/rfc9112>.
[RFC9113]
Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113, DOI 10.17487/RFC9113, , <https://www.rfc-editor.org/rfc/rfc9113>.
[RFC9114]
Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114, , <https://www.rfc-editor.org/rfc/rfc9114>.

11.2. Informative References

[IANA-Media-Type]
"Media Types", , <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, , <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: 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

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 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:

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, however, 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
Chengdu
610000
China
Roland Schott
Deutsche Telekom
Ida-Rhodes-Straße 2
64295 Darmstadt
Germany
Yang Richard Yang
Yale University
51 Prospect Street
New Haven, CT
United States of America
Lauren Delwiche
Yale University
51 Prospect Street
New Haven, 3408
United States of America
Lachlan Keller
Yale University
51 Prospect Street
New Haven, 3408
United States of America