The ALTO Transport Information Publication Service
draft-ietf-alto-new-transport-17
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Active".
|
|
|---|---|---|---|
| Authors | Kai Gao , Roland Schott , Y. Richard Yang , Lauren Delwiche , Lachlan Keller | ||
| Last updated | 2023-10-26 (Latest revision 2023-10-21) | ||
| Replaces | draft-schott-alto-new-transport, draft-schott-alto-new-transport-push | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
IOTDIR Telechat review
by Wesley Eddy
Ready w/issues
GENART Last Call review
(of
-15)
by Linda Dunbar
Ready w/issues
ARTART Early review
(of
-07)
by Spencer Dawkins
Ready w/issues
ARTART Early review
(of
-01)
by Spencer Dawkins
Ready w/issues
|
||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestone |
|
||
| Document shepherd | Mohamed Boucadair | ||
| Shepherd write-up | Show Last changed 2023-10-26 | ||
| IESG | IESG state | IESG Evaluation::Revised I-D Needed | |
| Consensus boilerplate | Yes | ||
| Telechat date |
(None)
Needs a YES. Needs 2 more YES or NO OBJECTION positions to pass. |
||
| Responsible AD | Martin Duke | ||
| Send notices to | mohamed.boucadair@orange.com | ||
| IANA | IANA review state | IANA OK - Actions Needed |
draft-ietf-alto-new-transport-17
ALTO K. Gao
Internet-Draft Sichuan University
Intended status: Standards Track R. Schott
Expires: 23 April 2024 Deutsche Telekom
Y. R. Yang
L. Delwiche
L. Keller
Yale University
21 October 2023
The ALTO Transport Information Publication Service
draft-ietf-alto-new-transport-17
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.
Gao, et al. Expires 23 April 2024 [Page 1]
Internet-Draft ALTO TIPS October 2023
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 23 April 2024.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 5
1.2. Notations . . . . . . . . . . . . . . . . . . . . . . . . 5
2. TIPS Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Transport Requirements . . . . . . . . . . . . . . . . . 5
2.2. TIPS Terminology . . . . . . . . . . . . . . . . . . . . 6
3. TIPS Updates Graph . . . . . . . . . . . . . . . . . . . . . 9
3.1. Basic Data Model of Updates Graph . . . . . . . . . . . . 9
3.2. Updates Graph Modification Invariants . . . . . . . . . . 11
4. TIPS Workflow and Resource Location Schema . . . . . . . . . 11
4.1. Workflow . . . . . . . . . . . . . . . . . . . . . . . . 11
4.2. Resource Location Schema . . . . . . . . . . . . . . . . 13
5. TIPS Information Resource Directory (IRD) Announcement . . . 14
5.1. Media Type . . . . . . . . . . . . . . . . . . . . . . . 15
5.2. Capabilities . . . . . . . . . . . . . . . . . . . . . . 15
Gao, et al. Expires 23 April 2024 [Page 2]
Internet-Draft ALTO TIPS October 2023
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 . . . . . . . . . . . . . . . . . . . . . . 21
7. TIPS Data Transfers - Client Pull . . . . . . . . . . . . . . 22
7.1. Request . . . . . . . . . . . . . . . . . . . . . . . . . 22
7.2. Response . . . . . . . . . . . . . . . . . . . . . . . . 23
7.3. Example . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.4. New Next Edge Recommendation . . . . . . . . . . . . . . 24
7.4.1. Request . . . . . . . . . . . . . . . . . . . . . . . 24
7.4.2. Response . . . . . . . . . . . . . . . . . . . . . . 25
8. Operation and Processing Considerations . . . . . . . . . . . 25
8.1. Considerations for Choosing Updates . . . . . . . . . . . 25
8.2. Considerations for Load Balancing . . . . . . . . . . . . 26
8.3. Considerations for Cross-Resource Dependency
Scheduling . . . . . . . . . . . . . . . . . . . . . . . 27
8.4. Considerations for Client Processing Updates . . . . . . 28
8.5. Considerations for Updates to Filtered Cost Maps . . . . 29
8.6. Considerations for Updates to Ordinal Mode Costs . . . . 29
8.7. Considerations for Managing Shared TIPS Views . . . . . . 29
8.8. Considerations for Offering Shortcut Incremental
Updates . . . . . . . . . . . . . . . . . . . . . . . . . 30
9. Security Considerations . . . . . . . . . . . . . . . . . . . 31
9.1. TIPS: Denial-of-Service Attacks . . . . . . . . . . . . . 31
9.2. ALTO Client: Update Overloading or Instability . . . . . 31
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
10.1. application/alto-tips+json Media Type . . . . . . . . . 32
10.2. application/alto-tipsparams+json Media Type . . . . . . 33
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
11.1. Normative References . . . . . . . . . . . . . . . . . . 34
11.2. Informative References . . . . . . . . . . . . . . . . . 35
Appendix A. A High-Level Deployment Model . . . . . . . . . . . 35
Appendix B. Conformance to "Building Protocols with HTTP" Best
Current Practices . . . . . . . . . . . . . . . . . . . . 37
Appendix C. Push-mode TIPS using HTTP Server Push . . . . . . . 38
Appendix D. Persistent HTTP Connections . . . . . . . . . . . . 38
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 38
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:
Gao, et al. Expires 23 April 2024 [Page 3]
Internet-Draft ALTO TIPS October 2023
1. The ALTO base protocol [RFC7285], which is designed for the
simple use case in which an ALTO client requests a network
information resource, and the server sends the complete content
of the requested information (if any) resource to the client.
2. ALTO incremental updates using Server-Sent Events (ALTO/SSE)
[RFC8895], which is designed for an ALTO client to indicate to
the server that it wants to receive updates for a set of
resources and the server can then concurrently and incrementally
push updates to that client whenever monitored resources change.
Both protocols are designed for HTTP/1.1 [RFC9112], but HTTP/2
[RFC9113] and HTTP/3 [RFC9114] can support HTTP/1.1 workflows.
However, HTTP/2 and HTTP/3 provide features that can improve certain
properties of ALTO and ALTO/SSE.
* First, consider the ALTO base protocol, which is designed to
transfer only complete information resources. A client can run
the base protocol on top of HTTP/2 or HTTP/3 to request multiple
information resources in concurrent streams, but each request must
be for a complete information resource: there is no capability it
transmits incremental updates. Hence, there can be a large
overhead when the client already has an information resource and
then there are small changes to the resource.
* Next, consider ALTO/SSE [RFC8895]. Although ALTO/SSE can transfer
incremental updates, it introduces a customized multiplexing
protocol on top of HTTP, assuming a total-order message channel
from the server to the client. The multiplexing design does not
provide naming (i.e., a resource identifier) to individual
incremental updates. Such a design cannot use concurrent data
streams available in HTTP/2 and HTTP/3, because both cases require
a resource identifier. Additionally, ALTO/SSE is a push-only
protocol, which denies the client flexibility in choosing how and
when it receives updates.
To mitigate these concerns, this document introduces a new ALTO
service called the Transport Information Publication Service (TIPS).
TIPS uses an incremental RESTful design to provide an ALTO client
with a new capability to explicitly, concurrently issue non-blocking
requests for specific incremental updates using native HTTP/2 or
HTTP/3, while still functioning for HTTP/1.1.
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.
Gao, et al. Expires 23 April 2024 [Page 4]
Internet-Draft ALTO TIPS October 2023
HTTP/2 [RFC9113] and HTTP/3 [RFC9114] also specify server push, which
might enhance TIPS. We discuss push-mode TIPS as an alternative
design in Appendix C.
Specifically, this document specifies:
* Extensions to the ALTO Protocol for dynamic subscription and
efficient uniform update delivery of an incrementally changing
network information resource.
* A new resource type that indicates the TIPS updates graph model
for a resource.
* URI patterns to fetch the snapshots or incremental updates.
Appendix B discusses to what extent the TIPS design adheres to the
Best Current Practices for building protocols with HTTP [RFC9205].
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
1.2. Notations
This document uses the same syntax and notations as introduced in
Section 8.2 of [RFC7285] to specify the extensions to existing ALTO
resources and services.
2. TIPS Overview
2.1. Transport Requirements
The ALTO Protocol and its extensions support two transport
mechanisms: First, a client can directly request an ALTO resource and
obtain a complete snapshot of that ALTO resource, as specified in the
base protocol [RFC7285]; Second, a client can subscribe to
incremental changes of one or multiple ALTO resources using the
incremental update extension [RFC8895], and a server pushes the
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:
Gao, et al. Expires 23 April 2024 [Page 5]
Internet-Draft ALTO TIPS October 2023
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
Gao, et al. Expires 23 April 2024 [Page 6]
Internet-Draft ALTO TIPS October 2023
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,
Gao, et al. Expires 23 April 2024 [Page 7]
Internet-Draft ALTO TIPS October 2023
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
Gao, et al. Expires 23 April 2024 [Page 8]
Internet-Draft ALTO TIPS October 2023
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
Gao, et al. Expires 23 April 2024 [Page 9]
Internet-Draft ALTO TIPS October 2023
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
Gao, et al. Expires 23 April 2024 [Page 10]
Internet-Draft ALTO TIPS October 2023
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
Gao, et al. Expires 23 April 2024 [Page 11]
Internet-Draft ALTO TIPS October 2023
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).
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, servers 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.
Gao, et al. Expires 23 April 2024 [Page 12]
Internet-Draft ALTO TIPS October 2023
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.
Gao, et al. Expires 23 April 2024 [Page 13]
Internet-Draft ALTO TIPS October 2023
<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.
Gao, et al. Expires 23 April 2024 [Page 14]
Internet-Draft ALTO TIPS October 2023
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.
Gao, et al. Expires 23 April 2024 [Page 15]
Internet-Draft ALTO TIPS October 2023
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"]
}
Gao, et al. Expires 23 April 2024 [Page 16]
Internet-Draft ALTO TIPS October 2023
},
"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"
},
}
Gao, et al. Expires 23 April 2024 [Page 17]
Internet-Draft ALTO TIPS October 2023
}
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
Gao, et al. Expires 23 April 2024 [Page 18]
Internet-Draft ALTO TIPS October 2023
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
Gao, et al. Expires 23 April 2024 [Page 19]
Internet-Draft ALTO TIPS October 2023
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
Gao, et al. Expires 23 April 2024 [Page 20]
Internet-Draft ALTO TIPS October 2023
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"
}
Gao, et al. Expires 23 April 2024 [Page 21]
Internet-Draft ALTO TIPS October 2023
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.
Gao, et al. Expires 23 April 2024 [Page 22]
Internet-Draft ALTO TIPS October 2023
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.
Gao, et al. Expires 23 April 2024 [Page 23]
Internet-Draft ALTO TIPS October 2023
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;
Gao, et al. Expires 23 April 2024 [Page 24]
Internet-Draft ALTO TIPS October 2023
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.
Gao, et al. Expires 23 April 2024 [Page 25]
Internet-Draft ALTO TIPS October 2023
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 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
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:
Gao, et al. Expires 23 April 2024 [Page 26]
Internet-Draft ALTO TIPS October 2023
* 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.
Gao, et al. Expires 23 April 2024 [Page 27]
Internet-Draft ALTO TIPS October 2023
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.
Gao, et al. Expires 23 April 2024 [Page 28]
Internet-Draft ALTO TIPS October 2023
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:
Gao, et al. Expires 23 April 2024 [Page 29]
Internet-Draft ALTO TIPS October 2023
* 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.
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.
Gao, et al. Expires 23 April 2024 [Page 30]
Internet-Draft ALTO TIPS October 2023
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.
Gao, et al. Expires 23 April 2024 [Page 31]
Internet-Draft ALTO TIPS October 2023
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;
Note to the RFC Editor: Please replace This-Document with the RFC
number to be assigned to this document.
10.1. application/alto-tips+json Media Type
Type name: application
Subtype name: alto-tips+json
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Encoding considerations are identical to
those specified for the "application/json" media type. See
[RFC8259].
Security considerations: See the Security Considerations section of
This-Document.
Interoperability considerations: This document specifies the format
of conforming messages and the interpretation thereof.
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
Gao, et al. Expires 23 April 2024 [Page 32]
Internet-Draft ALTO TIPS October 2023
Magic number(s): N/A
File extension(s): This document uses the media type to refer to
protocol messages and thus does not require a file extension.
Macintosh file type code(s): N/A
Person and email address to contact for further information: See Aut
hors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: Internet Engineering Task Force
(mailto:iesg@ietf.org).
Provisional registration?: No
10.2. application/alto-tipsparams+json Media Type
Type name: application
Subtype name: alto-tipsparams+json
Required parameters: N/A
Optional parameters: N/A
Encoding considerations: Encoding considerations are identical to
those specified for the "application/json" media type. See
[RFC8259].
Security considerations: See the Security Considerations section of
This-Document.
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
Gao, et al. Expires 23 April 2024 [Page 33]
Internet-Draft ALTO TIPS October 2023
Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): This document uses the media type to refer to
protocol messages and thus does not require a file extension.
Macintosh file type code(s): N/A
Person and email address to contact for further information: See Aut
hors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: Internet Engineering Task Force
(mailto:iesg@ietf.org).
Provisional registration?: No
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/rfc/rfc3986>.
[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>.
Gao, et al. Expires 23 April 2024 [Page 34]
Internet-Draft ALTO TIPS October 2023
[RFC8259] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", STD 90, RFC 8259,
DOI 10.17487/RFC8259, December 2017,
<https://www.rfc-editor.org/rfc/rfc8259>.
[RFC8895] Roome, W. and Y. Yang, "Application-Layer Traffic
Optimization (ALTO) Incremental Updates Using Server-Sent
Events (SSE)", RFC 8895, DOI 10.17487/RFC8895, November
2020, <https://www.rfc-editor.org/rfc/rfc8895>.
[RFC9112] Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
Ed., "HTTP/1.1", STD 99, RFC 9112, DOI 10.17487/RFC9112,
June 2022, <https://www.rfc-editor.org/rfc/rfc9112>.
[RFC9113] Thomson, M., Ed. and C. Benfield, Ed., "HTTP/2", RFC 9113,
DOI 10.17487/RFC9113, June 2022,
<https://www.rfc-editor.org/rfc/rfc9113>.
[RFC9114] Bishop, M., Ed., "HTTP/3", RFC 9114, DOI 10.17487/RFC9114,
June 2022, <https://www.rfc-editor.org/rfc/rfc9114>.
11.2. Informative References
[IANA-Media-Type]
"Media Types", June 2023,
<https://www.iana.org/assignments/media-types/media-
types.xhtml>.
[RFC9205] Nottingham, M., "Building Protocols with HTTP", BCP 56,
RFC 9205, DOI 10.17487/RFC9205, June 2022,
<https://www.rfc-editor.org/rfc/rfc9205>.
Appendix A. A High-Level Deployment Model
Conceptually, the TIPS system consists of three types of resources:
* (R1) TIPS frontend to 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.
Gao, et al. Expires 23 April 2024 [Page 35]
Internet-Draft ALTO TIPS October 2023
+------------------------------------------------+
| |
+------+ |R1: Frontend/Open R2: Directory/Meta R3: Data |
| | "iget" base | +-----+ +-----+ +-----+ |
| | resource 1 | | | | | | | |
| |-------------|---->| | | | | | |
| | incremental | | | | |-------->| | |
| | transfer | | | | | | | |
| | resource | | | | | | | |
| |<------------|-----------------------| | | | |
|Client| | | | +-----+ +-----+ |
| | "iget" base | | | |
| | resource 2 | | | +-----+ +-----+ |
| |-------------|---->| | | | | | |
| | incremental | | | | | | | |
| | transfer | +-----+ | | ------->| | |
| | resource | | | | | |
| |<------------|-----------------------| | | | |
+------+ | +-----+ +-----+ |
| |
+------------------------------------------------+
Figure 16: Sample TIPS Deployment Model
Design Point: Component Resource Location
* Design 1 (Single): all the three resource types at the same,
single server (accessed via relative reference)
* Design 2 (Flexible): all three resource types can be at their own
server (accessed via absolute reference)
* Design 3 (Dir + Data): R2 and R3 must remain together, though R1
might not be on the same server
This document supports Design 1 and Design 3. For Design 1, the TIPS
service simply needs to always use the same host for the TIPS views.
For Design 3, the TIPS service can set tips-view-host to a different
server. Note that the deployment flexibility is at the logical
level, as these services can be distinguished by different paths and
potentially be routed to different physical servers by layer-7 load
balancing. See Section 8.2 for a discussion on load balancing
considerations. Future documents may extend the protocol to support
Design 2.
Gao, et al. Expires 23 April 2024 [Page 36]
Internet-Draft ALTO TIPS October 2023
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].
Gao, et al. Expires 23 April 2024 [Page 37]
Internet-Draft ALTO TIPS October 2023
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
Email: kaigao@scu.edu.cn
Gao, et al. Expires 23 April 2024 [Page 38]
Internet-Draft ALTO TIPS October 2023
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
Gao, et al. Expires 23 April 2024 [Page 39]