The ALTO Transport Information Publication Service
draft-ietf-alto-new-transport-22
| Document | Type | Active Internet-Draft (alto WG) | |
|---|---|---|---|
| Authors | Kai Gao , Roland Schott , Y. Richard Yang , Lauren Delwiche , Lachlan Keller | ||
| Last updated | 2024-03-14 (Latest revision 2024-01-03) | ||
| Replaces | draft-schott-alto-new-transport, draft-schott-alto-new-transport-push | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Intended RFC status | Proposed Standard | ||
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| Stream | WG state | Submitted to IESG for Publication | |
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| Document shepherd | Mohamed Boucadair | ||
| Shepherd write-up | Show Last changed 2023-10-26 | ||
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| Details |
draft-ietf-alto-new-transport-22
ALTO K. Gao
Internet-Draft Sichuan University
Intended status: Standards Track R. Schott
Expires: 6 July 2024 Deutsche Telekom
Y. R. Yang
L. Delwiche
L. Keller
Yale University
3 January 2024
The ALTO Transport Information Publication Service
draft-ietf-alto-new-transport-22
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 6 July 2024.
Copyright Notice
Copyright (c) 2024 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 5
2. TIPS Overview . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Transport Requirements . . . . . . . . . . . . . . . . . 6
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 . . . . . . . . . . 11
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 . . . 15
5.1. Media Type . . . . . . . . . . . . . . . . . . . . . . . 15
5.2. Capabilities . . . . . . . . . . . . . . . . . . . . . . 15
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5.3. Uses . . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.4. An Example . . . . . . . . . . . . . . . . . . . . . . . 16
6. TIPS Management . . . . . . . . . . . . . . . . . . . . . . . 18
6.1. Open Request . . . . . . . . . . . . . . . . . . . . . . 18
6.2. Open Response . . . . . . . . . . . . . . . . . . . . . . 19
6.3. Open Example . . . . . . . . . . . . . . . . . . . . . . 22
6.3.1. Basic Example . . . . . . . . . . . . . . . . . . . . 22
6.3.2. Example using Digest Authentication . . . . . . . . . 23
6.3.3. Example using ALTO/SSE . . . . . . . . . . . . . . . 25
7. TIPS Data Transfers - Client Pull . . . . . . . . . . . . . . 26
7.1. Request . . . . . . . . . . . . . . . . . . . . . . . . . 26
7.2. Response . . . . . . . . . . . . . . . . . . . . . . . . 26
7.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 27
7.4. New Next Edge Recommendation . . . . . . . . . . . . . . 28
7.4.1. Request . . . . . . . . . . . . . . . . . . . . . . . 28
7.4.2. Response . . . . . . . . . . . . . . . . . . . . . . 28
7.4.3. Example . . . . . . . . . . . . . . . . . . . . . . . 29
8. Operation and Processing Considerations . . . . . . . . . . . 30
8.1. Considerations for Load Balancing . . . . . . . . . . . . 30
8.2. Considerations for Cross-Resource Dependency
Scheduling . . . . . . . . . . . . . . . . . . . . . . . 31
8.3. Considerations for Managing Shared TIPS Views . . . . . . 32
8.4. Considerations for Offering Shortcut Incremental
Updates . . . . . . . . . . . . . . . . . . . . . . . . . 33
9. Security Considerations . . . . . . . . . . . . . . . . . . . 33
9.1. TIPS: Denial-of-Service Attacks . . . . . . . . . . . . . 34
9.2. ALTO Client: Update Overloading or Instability . . . . . 34
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
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
1. Introduction
Application-Layer Traffic Optimization (ALTO) provides means for
network applications to obtain network status information. So far,
the ALTO information can be transported in two ways:
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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 for
the server to transmit 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.
While ALTO/SSE [RFC8895] and TIPS both can transport incremental
updates of ALTO information resources to clients, they have different
design goals. The TIPS extension enables more scalable and robust
distribution of incremental updates, but is missing the session
management and built-in server push capabilities of ALTO/SSE. From
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the performance perspective, TIPS is optimizing throughput by
leveraging concurrent and out-of-order transport of data, while ALTO/
SSE is optimizing latency as new events can be immediately
transferred to the clients without waiting for another round of
communication when there are multiple updates. Thus, we do not see
TIPS as a replacement but as a complement of ALTO/SSE. One example
of combining these two extensions is as shown in Section 6.3.3.
Note that future extensions may leverage server push, a feature of
HTTP/2 [RFC9113] and HTTP/3 [RFC9114], as an alternative of SSE. We
discuss why this alternative design is not ready 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.
Some operational complexities that must be taken into consideration
when implementing this extension are discussed in Section 8,
including load balancing Section 8.1, fetching and processing
incremental updates of dependent resources Section 8.2
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.
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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 only maintain and transfer
the "diff" upon changes. Thus, it is more efficient than storing
and transferring the full 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.
Long-polling updates: Long-polling 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.
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The ALTO new transport specified in this document satisfies all the
design requirements:
* This document reuses the data format introduced in [RFC8895] that
enables incremental updates using JSON patches or merge patches.
* This document introduce a unified data model to describe the
changes (snapshots and incremental updates) of an ALTO resource,
referred to as a TIPS view. In the data model, snapshots and
incremental updates are indexed as individual HTTP resources
following a unified naming convention, independent of the HTTP
version. Thus, these updates can be concurrently requested and be
transferred in a non-blocking manner either by using multiple
connections or leveraging multiplexed data transfer offered by
HTTP/2 or HTTP/3.
* The unified naming convention is based on a monotonically
increasing sequence number, making it possible for a client to
construct the URL of a future update and send a long-polling
request.
* The unified naming convention is independent of the HTTP versions
and can operate atop HTTP/1.1, HTTP/2 or HTTP/3.
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
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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. Encoding of a updates graph is
specified in Section 6.1.
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". Version is
encoded as a JSONNumber, as specified in Section 6.1.
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.
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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 | | 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.
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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.
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, which is
identified by a sequence number (defined as a JSONNumber).
Version 0 is reserved as the initial state (empty/null).
* A tag identifies the content of a node. A tag has the same format
as the "tag" field in Section 10.3 of [RFC7285] and is valid only
within the scope of resource.
* 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
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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
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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
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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).
A server MAY close a TIPS view at any time, e.g., under high system
load or due to client inactivity. In the event that a TIPS view is
closed, an edge request will receive error code 404 in response, and
the client will have to request a new TIPS view URI.
If resources allow, a server SHOULD avoid closing TIPS views that
have active polling edge requests or have recently served responses
until clients have had a reasonable interval to request the next
update, unless guided by specific control policies.
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
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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):
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<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
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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-network-map"],
"capabilities": {
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"cost-type-names": ["num-routingcost"]
}
},
"my-hopcount-map": {
"uri": "https://alto.example.com/costmap/hopcount",
"media-type": "application/alto-costmap+json",
"uses": ["my-network-map"],
"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-network-map"],
"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",
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"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"
}
}
},
"tips-sse": {
"uri": "https://alto.example.com/updates/tips",
"media-type": "text/event-stream",
"accepts": "application/alto-updatestreamparams+json",
"uses": [ "update-my-costs-tips" ],
"capabilities": {
"incremental-change-media-types": {
"update-my-costs-tips": "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:
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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":
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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]. An ALTO server
SHOULD use the "https" scheme unless the contents of the TIPS view
are intended to be publicly accessible and does not raise security
concerns. The field MUST contain only ASCII characters. In case
the original URL contains international characters (e.g., in the
domain name), the ALTO server implementation MUST properly encode
the URL into the ASCII format (e.g., using the "urlencode"
function).
A server MUST NOT use the same URI for different TIPS views,
either for different resources or different request bodies to the
same resource. URI generation is implementation specific, for
example, one may compute a Universally Unique Identifier (UUID,
[RFC4122]) or a hash value based on the request, and append it to
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a base URL. For performance considerations, it is NOT RECOMMENDED
to 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, which may result in duplicated TIPS views in cases
such as mobile clients. However, this is not mandatory as a
server may intentionally use client information to compute the
TIPS view URI to provide service isolation between clients.
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). If the client does
NOT provide a version tag, the server MUST recommend the edge of
the latest snapshot available. If the client does provide a
version tag, the server MUST either recommend the first
incremental update edge starting from the client's tagged version
or the edge of the latest snapshot. Which edge is selected
depends on the implementation. For example, a server MAY
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
recommends the first incremental update edge starting from the
client's tagged version. Otherwise, the server recommends 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
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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, if present, MUST 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. If present, the "field" field MUST be the
full path of the "resource-id" field, and the "value" field MUST
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.
It is RECOMMENDED that the server provide the ALTO/SSE support for
the TIPS resource. Thus, the client can be notified of the version
updates of all the TIPS views that it monitors and make better cross-
resource transport decisions (see Section 8.2 for related
considerations).
6.3. Open Example
6.3.1. Basic 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.
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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: Request Example of Opening a TIPS View
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: 255
{
"tips-view-uri": "https://alto.example.com/tips/2718281828",
"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 of Opening a TIPS View
6.3.2. Example using Digest Authentication
Below is another example of the same query using Digest
authentication, a mandatory authentication method of ALTO servers as
defined in Section 8.3.5 of [RFC7285]. The content of the response
is the same as in Figure 11 and thus omitted for simplicity.
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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"
}
HTTP/1.1 401 UNAUTHORIZED
WWW-Authenticate: Digest
realm="alto.example.com",
qop="auth",
algorithm="MD5",
nonce="173b5aba4242409ee2ac3a4fd797f9d7",
opaque="a237ff9ab865379a69d9993162ef55e4"
POST /tips HTTP/1.1
Host: alto.example.com
Accept: application/alto-tips+json, application/alto-error+json
Authorization: Digest
username="client1",
realm="alto.example.com",
uri="/tips",
qop=auth,
algorithm=MD5,
nonce="173b5aba4242409ee2ac3a4fd797f9d7",
nc=00000001,
cnonce="ZTg3MTI3NDFmMDQ0NzI1MDQ3MWE3ZTFjZmM5MTNiM2I=",
response="8e937ae696c1512e4f990fa21c7f9347",
opaque="a237ff9ab865379a69d9993162ef55e4"
Content-Type: application/alto-tipsparams+json
Content-Length: 41
{
"resource-id": "my-routingcost-map"
}
HTTP/1.1 200 OK
Content-Type: application/alto-tips+json
Content-Length: 258
{....}
Figure 12: Open Example with Digest Authentication
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6.3.3. Example using ALTO/SSE
This section gives an example of receiving incremental updates of the
TIPS view summary using ALTO/SSE [RFC8895]. Consider the tips-sse
resource, as announced by the IRD in Figure 6, which provides ALTO/
SSE for the update-my-cost-tips resource, a client may send the
following request to receive updates of the TIPS view (authentication
is omitted for simplicity).
POST /updates/tips HTTP/1.1
Host: alto.example.com
Accept: text/event-stream,application/alto-error+json
Content-Type: application/alto-updatestreamparams+json
Content-Length: 76
{
"add": {
"tips-123": { "resource-id": "update-my-cost-tips" }
}
}
Figure 13: Example of Monitoring TIPS view with ALTO/SSE
Then, the client will be able to receive the TIPS view summary as
follows.
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: application/alto-tips+json,tips-123
data: {
data: "tips-view-uri": "https://alto.example.com/tips/2718281828",
data: "tips-view-summary": {
data: "updates-graph-summary": {
data: "start-seq": 101,
data: "end-seq": 106,
data: "start-edge-rec" : {
data: "seq-i": 0,
data: "seq-j": 105
data: }
data: }
data: }
data: }
When there is an update to the TIPS view, for example, the end-seq is
increased by 1, the client will be able to receive the incremental
update of the TIPS view summary as follows.
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event: application/merge-patch+json,tips-123
data: {
data: "tips-view-summary": {
data: "updates-graph-summary": {
data: "end-seq": 107
data: }
data: }
data: }
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 sends a request to
"/tips/2718281828/ug/0/101" and sets 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 provided in the directory yet.
For such updates, the client MUST indicate all media types that may
appear. It is RECOMMENDED that the server allows for at least the
long polling of <end-seq> -> <end-seq + 1>.
Hence, the server processing logic MUST be:
* If ug/<i>/<j> exists: return content using encoding.
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* Else if long polling ug/<i>/<j> is acceptable: put request in a
backlog queue, then either a response is triggered when the
content is ready or the request is interrupted, e.g., by a network
error.
* 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 update does not exist, or 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 long-polling 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 14.
GET /tips/2718281828/ug/0/101 HTTP/1.1
Host: alto.example.com
Accept: application/alto-costmap+json, \
application/alto-error+json
Figure 14: GET Example
The response is shown in Figure 15.
HTTP/1.1 200 OK
Content-Type: application/alto-costmap+json
Content-Length: 50
{ ... full replacement of my-routingcost-map ... }
Figure 15: Response to a GET Request
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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 MAY 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 URL of
the request MUST have the format of
<tips-view-path>/ug
and the HOST field MUST be the <tips-view-host>.
The POST body has the same format as the TIPSReq Figure 7. The
resource-id MUST be the same as the resource ID used to create the
TIPS view, and the optional input field MUST NOT be present.
7.4.2. Response
The response to a valid request MUST be a JSON merge patch to the
object of type AddTIPSResponse (defined in Section 6.2), denoted as
media type "application/merge-patch+json". The "update-graph-
summary" field MUST be present in the response and hence its parent
field "tips-view-summary" MUST be present as well.
If the tag field is present in the request, the server MUST check if
any version within the range [start-seq, end-seq] has the same tag
value. If the version exists, e.g., denoted as tag-seq, the server
MUST compute the paths from both tag-seq and 0 to the end-seq, and
choose the one with the minimal cost. The cost MAY be implementation
specific, e.g., number of messages, accumulated data size, etc. The
first edge of the selected path MUST be returned as the recommended
next edge.
If the tag field is NOT present, it MUST be interpreted as the tag-
seq is 0.
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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.
7.4.3. Example
We give an example of the new next edge recommendation service.
Assume that a client already creates a TIPS view as in Section 6.3,
whose updates graph is as shown in Figure 2. Now assume that the
client already has tag 0881080 whose corresponding sequence number is
103, and sends the following new next edge recommendation request
(authentication is omitted for simplicity):
POST /tips/2718281828/ug HTTP/1.1
HOST alto.example.com
Accept: application/merge-patch+json, application/alto-error+json
Content-Type: application/alto-tipsparams+json
Content-Length: 62
{
"resource-id": "my-routingcost-map",
"tag": "0881080"
}
According to Figure 2, there are 3 potential paths: 103 -> 104 -> 105
-> 106, 103 -> 105 -> 106, and 0 -> 105 -> 106. Assume that the
server chooses shortest update path by the accumulated data size and
the best path is 103 -> 105 -> 106. Thus, the server responds with
the following message:
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HTTP/1.1 200 OK
Content-Type: application/merge-patch+json
Content-Length: 193
{
"tips-view-summary": {
"updates-graph-summary": {
"start-seq": 101,
"end-seq": 106,
"start-edge-rec": {
"seq-i": 103,
"seq-j": 105
}
}
}
}
8. Operation and Processing Considerations
TIPS has some common operational considerations as ALTO/SSE
[RFC8895], including:
* server choosing update messages (Section 9.1 of [RFC8895]);
* client processing update messages (Section 9.2 of [RFC8895]);
* updates of filtered map services (Section 9.3 of [RFC8895]);
* updates of ordinal mode costs (Section 9.4 of [RFC8895]).
There are also some operation considerations specific to TIPS, which
we discuss below.
8.1. 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 a client to make concurrent pulls of incremental updates
for the same TIPS view potentially through different HTTP
connections. As a consequence, it introduces additional complexities
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when the ALTO server is being load balanced. For example, a request
may be directed to a wrong backend server and get 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.
8.2. 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 16: Example Dependency Model
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In Figure 16, 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.
To get consistent ALTO information, a client must process the updates
following the guidelines specified in Section 9.2 of [RFC8895]. If
resource permits (i.e., sufficient updates can be buffered), an ALTO
client can safely use long polling to fetch all the updates. This
allows a client to build consistent views quickly as the updates are
already stored in the buffer. Otherwise, it is RECOMMENDED to
request
8.3. 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 and no client
immediately issues a new edge request;
- the TIPS view is never removed from the storage.
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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.
8.4. 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.
Additionally, operators of the ALTO servers MUST follow the
guidelines in [RFC9325] to avoid new TLS vulnerabilities discovered
after [RFC7285] was published.
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.
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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 MAY choose to limit the number of
active views and reject new requests when that threshold is reached.
TIPS allows predictive fetching and the server MAY also choose to
limit the number of pending requests. If a new request exceeds the
threshold, the server MAY log the event and 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]:
* application/alto-tips+json: as described in Section 6.2;
* application/alto-tipsparams+json: as described in Section 6.1;
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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: N/A.
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.
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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: N/A.
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
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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>.
[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>.
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[RFC9114] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.
[RFC9325] Sheffer, Y., Saint-Andre, P., and T. Fossati,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November
2022, <https://www.rfc-editor.org/rfc/rfc9325>.
11.2. Informative References
[IANA-Media-Type]
"Media Types", June 2023,
<https://www.iana.org/assignments/media-types/media-
types.xhtml>.
[RFC4122] Leach, P., Mealling, M., and R. Salz, "A Universally
Unique IDentifier (UUID) URN Namespace", RFC 4122,
DOI 10.17487/RFC4122, July 2005,
<https://www.rfc-editor.org/rfc/rfc4122>.
[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 create TIPS views.
* (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.
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+------------------------------------------------+
| |
+------+ |R1: Frontend/Open R2: Directory/Meta R3: Data |
| | "iget" base | +-----+ +-----+ +-----+ |
| | resource 1 | | | | | | | |
| |-------------|---->| | | | | | |
| | incremental | | | | |-------->| | |
| | transfer | | | | | | | |
| | resource | | | | | | | |
| |<------------|-----------------------| | | | |
|Client| | | | +-----+ +-----+ |
| | "iget" base | | | |
| | resource 2 | | | +-----+ +-----+ |
| |-------------|---->| | | | | | |
| | incremental | | | | | | | |
| | transfer | +-----+ | | ------->| | |
| | resource | | | | | |
| |<------------|-----------------------| | | | |
+------+ | +-----+ +-----+ |
| |
+------------------------------------------------+
Figure 17: 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
balancing. See Section 8.1 for a discussion on load balancing
considerations. Future documents may extend the protocol to support
Design 2.
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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,
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].
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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, Linda Dunbar and Sheng Jiang
for the directorate reviews, Martin Duke for the Area Director
review, Francesca Palombini, Wesley Eddy, Roman Danyliw, Murray
Kucherawy and Zaheduzzaman Sarker for the telechat and IESG reviews,
and Mohamed Boucadair for shepherding the document.
Authors' Addresses
Kai Gao
Sichuan University
No.24 South Section 1, Yihuan Road
Chengdu
610000
China
Email: kaigao@scu.edu.cn
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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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