Internet Engineering Task Force (IETF) W. Roome
Request for Comments: 8895 Nokia Bell Labs
Category: Standards Track Y. Yang
ISSN: 2070-1721 Yale University
November 2020
Application-Layer Traffic Optimization (ALTO) Incremental Updates Using
Server-Sent Events (SSE)
Abstract
The Application-Layer Traffic Optimization (ALTO) protocol (RFC 7285)
provides network-related information, called network information
resources, to client applications so that clients can make informed
decisions in utilizing network resources. This document presents a
mechanism to allow an ALTO server to push updates to ALTO clients to
achieve two benefits: (1) updates can be incremental, in that if only
a small section of an information resource changes, the ALTO server
can send just the changes and (2) updates can be immediate, in that
the ALTO server can send updates as soon as they are available.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc8895.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terms
2.1. Requirements Language
3. Background
3.1. Incremental Encoding: JSON Merge Patch
3.1.1. JSON Merge Patch Encoding
3.1.2. JSON Merge Patch ALTO Messages
3.2. Incremental Encoding: JSON Patch
3.2.1. JSON Patch Encoding
3.2.2. JSON Patch ALTO Messages
3.3. Multiplexing and Server Push: HTTP/2
3.4. Server Push: Server-Sent Event
4. Overview of Approach and High-Level Protocol Message Flow
4.1. Update Stream Service Message Flow
4.2. Stream Control Service Message Flow
4.3. Service Announcement and Management Message Flow
5. Update Messages: Data Update and Control Update Messages
5.1. Generic ALTO Update Message Structure
5.2. ALTO Data Update Message
5.3. ALTO Control Update Message
6. Update Stream Service
6.1. Media Type
6.2. HTTP Method
6.3. Capabilities
6.4. Uses
6.5. Request: Accept Input Parameters
6.6. Response
6.7. Additional Requirements on Update Stream Service
6.7.1. Event Sequence Requirements
6.7.2. Cross-Stream Consistency Requirements
6.7.3. Multipart Update Requirements
6.8. Keep-Alive Messages
7. Stream Control Service
7.1. URI
7.2. Media Type
7.3. HTTP Method
7.4. IRD Capabilities & Uses
7.5. Request: Accept Input Parameters
7.6. Response
8. Examples
8.1. Example: IRD Announcing Update Stream Services
8.2. Example: Simple Network and Cost Map Updates
8.3. Example: Advanced Network and Cost Map Updates
8.4. Example: Endpoint Property Updates
8.5. Example: Multipart Message Updates
9. Operation and Processing Considerations
9.1. Considerations for Choosing Data Update Messages
9.2. Considerations for Client Processing Data Update Messages
9.3. Considerations for Updates to Filtered Cost Maps
9.4. Considerations for Updates to Ordinal Mode Costs
9.5. Considerations for SSE Text Formatting and Processing
10. Security Considerations
10.1. Update Stream Server: Denial-of-Service Attacks
10.2. ALTO Client: Update Overloading or Instability
10.3. Stream Control: Spoofed Control Requests and Information
Breakdown
11. Requirements on Future ALTO Services to Use This Design
12. IANA Considerations
12.1. application/alto-updatestreamparams+json Media Type
12.2. application/alto-updatestreamcontrol+json Media Type
13. Appendix: Design Decision: Not Allowing Stream Restart
14. References
14.1. Normative References
14.2. Informative References
Acknowledgments
Contributors
Authors' Addresses
1. Introduction
The Application-Layer Traffic Optimization (ALTO) protocol [RFC7285]
provides network-related information, called network information
resources, to client applications so that clients may make informed
decisions in utilizing network resources. For example, an ALTO
server provides network and cost maps, where a network map partitions
the set of endpoints into a manageable number of sets each defined by
a Provider-Defined Identifier (PID) and a cost map provides directed
costs between PIDs. Given network and cost maps, an ALTO client can
obtain costs between endpoints by first using the network map to get
the PID for each endpoint and then using the cost map to get the
costs between those PIDs. Such costs can be used by the client to
choose communicating endpoints with low network costs.
The ALTO protocol defines only an ALTO client pull model without
defining a mechanism to allow an ALTO client to obtain updates to
network information resources, other than by periodically re-fetching
them. In settings where an information resource may be large but
only parts of it may change frequently (e.g., some entries of a cost
map), complete re-fetching can be inefficient.
This document presents a mechanism to allow an ALTO server to push
incremental updates to ALTO clients. Integrating server push and
incremental updates provides two benefits: (1) updates can be small,
in that if only a small section of an information resource changes,
the ALTO server can send just the changes and (2) updates can be
immediate, in that the ALTO server can send updates as soon as they
are available.
While primarily intended to provide updates to GET-mode network and
cost maps, the mechanism defined in this document can also provide
updates to POST-mode ALTO services, such as the ALTO endpoint
property and endpoint cost services. The mechanism can also support
new ALTO services to be defined by future extensions, but a future
service needs to satisfy requirements specified in Section 11.
The rest of this document is organized as follows. Section 3 gives
background on the basic techniques used in this design: (1) JSON
merge patch and JSON patch to allow incremental updates and (2)
Server-Sent Events (SSE) [SSE] to allow server push. With the
background, Section 4 gives a non-normative overview of the design.
Section 5 defines individual messages in an update stream. Section 6
defines the update stream service. Section 7 defines the stream
control service. Section 8 gives several examples to illustrate the
two types of services. Section 9 describes operation and processing
considerations by both ALTO servers and clients. Section 13
discusses a design feature that is not supported. Section 10
discusses security issues. Sections 11 and 12 review the
requirements for future ALTO services to use SSE and IANA
considerations, respectively.
2. Terms
Besides the terminologies as defined in [RFC7285], this document also
uses additional terminologies defined as follows:
Update Stream:
A reliable, in-order connection compatible with HTTP/1.x between
an ALTO client and an ALTO server so that the server can push a
sequence of update messages using [SSE] to the client.
Update Stream Server:
This document refers to an ALTO server providing an update stream
as an ALTO update stream server, or update stream server for
short. Note that the ALTO server mentioned in this document
refers to a general server that provides various kinds of
services; it can be an update stream server or stream control
server (see below). It can also be a server providing ALTO
Information Resource Directory (IRD).
Update Message:
A message that is either a data update message or a control update
message.
Data Update Message:
An update message that is for a single ALTO information resource
and sent from the update stream server to the ALTO client when the
resource changes. A data update message can be either a full-
replacement message or an incremental-change message. Full
replacement is a shorthand for a full-replacement message, and
incremental change is a shorthand for an incremental-change
message.
Full Replacement:
A data update message for a resource that encodes the content of
the resource in its original ALTO encoding.
Incremental Change:
A data update message that specifies only the difference between
the new content and the previous version. An incremental change
can be encoded using either JSON merge patch or JSON patch in this
document.
Stream Control Service:
A service that provides an HTTP URI so that the ALTO client of an
update stream can use it to send stream control requests to the
ALTO server on the addition or removal of resources receiving
update messages from the update stream. The ALTO server creates a
new stream control resource for each update stream instance,
assigns a unique URI to it, and sends the URI to the client as the
first event in the stream. (Note that the stream control service
in ALTO has no association with the similarly named Stream Control
Transmission Protocol [RFC4960].)
Stream Control:
A shorthand for stream control service.
Stream Control Server:
An ALTO server providing the stream control service.
Substream-ID:
An ALTO client can assign a unique substream-id when requesting
the addition of a resource receiving update messages from an
update stream. The server puts the substream-id in each update
event for that resource. The substream-id allows a client to use
one update stream to receive updates to multiple requests for the
same resource (i.e., with the same resource-id in an ALTO IRD),
for example, for a POST-mode resource with different input
parameters.
Data-ID:
A subfield of the "event" field of [SSE] to identify the ALTO data
(object) to be updated. For an ALTO resource returning a
multipart response, the data-id to identify the data (object) is
the substream-id, in addition to the Content-ID of the object in
the multipart response. The data-id of a single-part response is
just the substream-id.
Control Update Message:
An update message for the update stream server to notify the ALTO
client of related control information of the update stream. A
control update message may be triggered by an internal event at
the server, such as server overloading and hence the update stream
server will no longer send updates for an information resource, or
as a result of a client sending a request through the stream
control service. The first message of an update stream is a
control update message that provides a control URI to the ALTO
client. The ALTO client can use the URI to send stream control
requests to the stream control server.
2.1. 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.
3. Background
The design requires two basic techniques: encoding of incremental
changes and server push. For incremental changes, existing
techniques include JSON merge patch and JSON patch; this design uses
both. For server push, existing techniques include HTTP/2 and [SSE];
this design adopts some design features of HTTP/2 but uses [SSE] as
the basic server-push design. The rest of this section gives a non-
normative summary of JSON merge patch, JSON patch, HTTP/2, and [SSE].
3.1. Incremental Encoding: JSON Merge Patch
To avoid always sending complete data, a server needs mechanisms to
encode incremental changes, and JSON merge patch is one mechanism.
[RFC7396] defines the encoding of incremental changes (called JSON
merge patch objects) to be used by the HTTP PATCH method [RFC5789].
From [RFC7396], this document adopts only the JSON merge patch object
encoding and does not use the HTTP PATCH method, as the updates are
sent as events instead of HTTP methods; also, the updates are server
to client, and PATCH semantics are more for client to server. Below
is a non-normative summary of JSON merge patch objects; see [RFC7396]
for the normative definition.
3.1.1. JSON Merge Patch Encoding
Informally, a JSON merge patch message consists of a JSON merge patch
object (referred to as a patch in [RFC7396]), which defines how to
transform one JSON value into another using a recursive merge patch
algorithm. Specifically, the patch is computed by treating two JSON
values (first one being the original and the second being the
updated) as trees of nested JSON objects (dictionaries of name/value
pairs), where the leaves are values (e.g., JSON arrays, strings, and
numbers), other than JSON objects, and the path for each leaf is the
sequence of keys leading to that leaf. When the second tree has a
different value for a leaf at a path or adds a new leaf, the patch
has a leaf, at that path, with the new value. When a leaf in the
first tree does not exist in the second tree, the JSON merge patch
tree has a leaf with a JSON "null" value. Hence, in the patch, null
as the value of a name/value pair will delete the element with "name"
in the original JSON value. The patch does not have an entry for any
leaf that has the same value in both versions. See the MergePatch
pseudocode at the beginning of Section 2 of [RFC7396] for the formal
specification of how to apply a given patch. As a result, if all
leaf values are simple scalars, JSON merge patch is a quite efficient
representation of incremental changes. It is less efficient when
leaf values are arrays, because JSON merge patch replaces arrays in
their entirety, even if only one entry changes.
3.1.2. JSON Merge Patch ALTO Messages
To provide both examples of JSON merge patch and a demonstration of
the feasibility of applying JSON merge patch to ALTO, the sections
below show the application of JSON merge patch to two key ALTO
messages.
3.1.2.1. JSON Merge Patch Network Map Messages
Section 11.2.1.6 of [RFC7285] defines the format of an ALTO network
map message. Assume a simple example ALTO message sending an initial
network map:
{
"meta" : {
"vtag": {
"resource-id" : "my-network-map",
"tag" : "da65eca2eb7a10ce8b059740b0b2e3f8eb1d4785"
}
},
"network-map" : {
"PID1" : {
"ipv4" : [ "192.0.2.0/24", "198.51.100.0/25" ]
},
"PID2" : {
"ipv4" : [ "198.51.100.128/25" ]
},
"PID3" : {
"ipv4" : [ "0.0.0.0/0" ],
"ipv6" : [ "::/0" ]
}
}
}
Consider the following JSON merge patch update message, which (1)
adds an ipv4 prefix "203.0.113.0/25" and an ipv6 prefix
"2001:db8:8000::/33" to "PID1", (2) deletes "PID2", and (3) assigns a
new "tag" to the network map:
{
"meta" : {
"vtag" : {
"tag" : "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
},
"network-map": {
"PID1" : {
"ipv4" : [ "192.0.2.0/24", "198.51.100.0/25",
"203.0.113.0/25" ],
"ipv6" : [ "2001:db8:8000::/33" ]
},
"PID2" : null
}
}
Applying the JSON merge patch update to the initial network map is
equivalent to the following ALTO network map:
{
"meta" : {
"vtag": {
"resource-id" : "my-network-map",
"tag" : "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
},
"network-map" : {
"PID1" : {
"ipv4" : [ "192.0.2.0/24", "198.51.100.0/25",
"203.0.113.0/25" ],
"ipv6" : [ "2001:db8:8000::/33" ]
},
"PID3" : {
"ipv4" : [ "0.0.0.0/0" ],
"ipv6" : [ "::/0" ]
}
}
}
3.1.2.2. JSON Merge Patch Cost Map Messages
Section 11.2.3.6 of [RFC7285] defines the format of an ALTO cost map
message. Assume a simple example ALTO message for an initial cost
map:
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-network-map",
"tag": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
],
"cost-type" : {
"cost-mode" : "numerical",
"cost-metric": "routingcost"
},
"vtag": {
"resource-id" : "my-cost-map",
"tag" : "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
}
},
"cost-map" : {
"PID1": { "PID1": 1, "PID2": 5, "PID3": 10 },
"PID2": { "PID1": 5, "PID2": 1, "PID3": 15 },
"PID3": { "PID1": 20, "PID2": 15 }
}
}
The following JSON merge patch message updates the example cost map
so that (1) the "tag" field of the cost map is updated, (2) the cost
of PID1->PID2 is 9 instead of 5, (3) the cost of PID3->PID1 is no
longer available, and (4) the cost of PID3->PID3 is defined as 1.
{
"meta" : {
"vtag": {
"tag": "c0ce023b8678a7b9ec00324673b98e54656d1f6d"
}
}
"cost-map" : {
"PID1" : { "PID2" : 9 },
"PID3" : { "PID1" : null, "PID3" : 1 }
}
}
Hence, applying the JSON merge patch to the initial cost map is
equivalent to the following ALTO cost map:
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-network-map",
"tag": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
],
"cost-type" : {
"cost-mode" : "numerical",
"cost-metric": "routingcost"
},
"vtag": {
"resource-id": "my-cost-map",
"tag": "c0ce023b8678a7b9ec00324673b98e54656d1f6d"
}
},
"cost-map" : {
"PID1": { "PID1": 1, "PID2": 9, "PID3": 10 },
"PID2": { "PID1": 5, "PID2": 1, "PID3": 15 },
"PID3": { "PID2": 15, "PID3": 1 }
}
}
3.2. Incremental Encoding: JSON Patch
3.2.1. JSON Patch Encoding
One issue of JSON merge patch is that it does not handle array
changes well. In particular, JSON merge patch considers an array as
a single object and hence can only replace an array in its entirety.
When the change is to make a small change to an array, such as the
deletion of an element from a large array, whole-array replacement is
inefficient. Consider the example in Section 3.1.2.1. To add a new
entry to the ipv4 array for PID1, the server needs to send a whole
new array. Another issue is that JSON merge patch cannot change a
value to be null, as the JSON merge patch processing algorithm
(MergePatch in Section 3.1.1) interprets a null as a removal
instruction. On the other hand, some ALTO resources can have null
values, and it is possible that the update will want to change the
new value to be null.
JSON patch [RFC6902] can address the preceding issues. It defines a
set of operators to modify a JSON object. See [RFC6902] for the
normative definition.
3.2.2. JSON Patch ALTO Messages
To provide both examples of JSON patch and a demonstration of the
difference between JSON patch and JSON merge patch, the sections
below show the application of JSON patch to the same updates shown in
Section 3.1.2.
3.2.2.1. JSON Patch Network Map Messages
First, consider the same update as in Section 3.1.2.1 for the network
map. Below is the encoding using JSON patch:
[
{
"op": "replace",
"path": "/meta/vtag/tag",
"value": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
},
{
"op": "add",
"path": "/network-map/PID1/ipv4/2",
"value": "203.0.113.0/25"
}
{
"op": "add",
"path": "/network-map/PID1/ipv6",
"value": ["2001:db8:8000::/33"]
},
{
"op": "remove",
"path": "/network-map/PID2"
}
]
3.2.2.2. JSON Patch Cost Map Messages
Compared with JSON merge patch, JSON patch does not encode cost map
updates efficiently. Consider the cost map update shown in
Section 3.1.2.2, the encoding using JSON patch is:
[
{
"op": "replace",
"path": "/meta/vtag/tag",
"value": "c0ce023b8678a7b9ec00324673b98e54656d1f6d"
},
{
"op": "replace",
"path": "/cost-map/PID1/PID2",
"value": 9
},
{
"op": "remove",
"path": "/cost-map/PID3/PID1"
},
{
"op": "replace",
"path": "/cost-map/PID3/PID3",
"value": 1
}
]
3.3. Multiplexing and Server Push: HTTP/2
HTTP/2 [RFC7540] provides two related features: multiplexing and
server push. In particular, HTTP/2 allows a client and a server to
multiplex multiple HTTP requests and responses over a single TCP
connection. The requests and responses can be interleaved on a block
(frame) by block (frame) basis, by indicating the requests and
responses in HTTP/2 messages, avoiding the head-of-line blocking
problem encountered with HTTP/1.1. To achieve the same goal, this
design introduces substream-id to allow a client to receive updates
to multiple resources. HTTP/2 also provides a server-push facility
to allow a server to send asynchronous updates.
Despite the two features of HTTP/2, this design chooses a design
compatible with HTTP/1.x for the simplicity of HTTP/1.x. A design
based on HTTP/2 may more likely need to be implemented using a more
complex HTTP/2 client library. In such a case, one approach for
using server push for updates is for the update stream server to send
each data update message as a separate server-push item and let the
client apply those updates as they arrive. An HTTP/2 client library
may not necessarily inform a client application when the server
pushes a resource. Instead, the library might cache the pushed
resource and only deliver it to the client when the client explicitly
requests that URI. Further, it is more likely that a design based on
HTTP/2 may encounter issues with a proxy between the client and the
server, in that server push is optional and can be disabled by any
proxy between the client and the server. This is not a problem for
the intended use of server push; eventually, the client will request
those resources, so disabling server push just adds a delay. But
this means that Server Push is not suitable for resources that the
client does not know to request.
Thus, this design leaves a design based on HTTP/2 as a future work
and focuses on ALTO updates on HTTP/1.x and [SSE].
3.4. Server Push: Server-Sent Event
Server-Sent Events (SSE) are techniques that can work with HTTP/1.1.
The following is a non-normative summary of SSE; see [SSE] for its
normative definition.
SSE enable a server to send new data to a client by "server push".
The client establishes an HTTP [RFC7230] [RFC7231] connection to the
server and keeps the connection open. The server continually sends
messages. Each message has one or more lines, where a line is
terminated by a carriage return immediately followed by a new line, a
carriage return not immediately followed by a new line, or a new line
not immediately preceded by a carriage return. A message is
terminated by a blank line (two line terminators in a row).
Each line in a message is of the form "field-name: string value".
Lines with a blank field name (that is, lines that start with a
colon) are ignored, as are lines that do not have a colon. The
protocol defines three field names: event, id, and data. If a
message has more than one "data" line, the value of the data field is
the concatenation of the values on those lines. There can be only
one "event" and "id" line per message. The "data" field is required;
the others are optional.
Figure 1 is a sample SSE stream, starting with the client request.
The server sends three events and then closes the stream.
(Client request)
GET /stream HTTP/1.1
Host: example.com
Accept: text/event-stream
(Server response)
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: start
id: 1
data: hello there
event: middle
id: 2
data: let's chat some more ...
data: and more and more and ...
event: end
id: 3
data: goodbye
Figure 1: A Sample SSE Stream
4. Overview of Approach and High-Level Protocol Message Flow
With the preceding background, this section now gives a non-normative
overview of the update mechanisms and message flow to be defined in
later sections of this document. Figure 2 gives the main components
and overall message flow.
-------------------------------------------------------------------
| |
| +-------+ +-------+ 1. init request +------+ |
| | | | | <-------------- | | |
| | | | | --------------> | | |
| 3.add/ | | | | 1'. control URI | | |
| remove | | | | | | |
| resource |Stream | |Update | | | |
-------->|Control| private |Stream | 2a. data update |Client| --
|Server |<------->|Server | messages | |
-------- | | | | ---------------> | | <-
| response | | | | ---------------> | | |
| | | | | 2b. control update| | |
| +-------+ +-------+ messages +------+ |
| |
-------------------------------------------------------------------
Figure 2: ALTO SSE Architecture and Message Flow
4.1. Update Stream Service Message Flow
The building block of the update mechanism defined in this document
is the update stream service (defined in Section 6), where each
update stream service is a POST-mode service that provides update
streams.
Note that the lines of the format "** ... **" are used to describe
message flows in this section and the following sections.
** Initial request: client -> update server **:
When an ALTO client requests an update stream service, the ALTO
client establishes a persistent connection to the update stream
server and submits an initial update-stream request (defined in
Section 6.5), creating an update stream. This initial request
creating the update stream is labeled "1. init request" in
Figure 2.
An update stream can provide updates to both GET-mode resources,
such as ALTO network and cost maps, and POST-mode resources, such
as ALTO endpoint property service. Also, to avoid creating too
many update streams, this design allows an ALTO client to use one
update stream to receive updates to multiple requests. In
particular, the client may request to receive updates for the same
resource but with different parameters for a POST-mode resource,
in addition to being able to consolidate updates for multiple
resources into a single stream. The updates for each request is
called a substream and hence the update server needs an identifier
to indicate the substream when sending an update. To achieve this
goal, the client assigns a unique substream-id when requesting
updates to a resource in an update stream, and the server puts the
substream-id in each update.
** Data updates: update server -> client **:
The objective of an update stream is to continuously push (to an
ALTO client) the data value changes for a set of resources, where
the set of resources is specified by the ALTO client's requests.
This document refers to messages sending such data-value changes
as data update messages (defined in Section 5.2). Although an
update stream may update one or more requests, each data update
message updates only one request and is sent as a Server-Sent
Event (SSE), as defined by [SSE]. A data update message is
encoded either as a full replacement or as an incremental change.
A full replacement uses the JSON message format defined by the
ALTO protocol. There can be multiple encodings for incremental
changes. The current design supports incremental changes using
JSON merge patch [RFC7396] or JSON patch [RFC6902] to describe the
changes of the resource. Future documents may define additional
mechanisms for incremental changes. The update stream server
decides when to send data update messages and whether to send full
replacements or incremental changes. These decisions can vary
from resource to resource and from update to update. Since the
transport is a design compatible with HTTP/1.x, data update
messages are delivered reliably and in order, and the lossless,
sequential delivery of its messages allows the server to know the
exact state of the client to compute the correct incremental
updates. Figure 2 shows examples of data update messages (labeled
"2a. data update messages") in the overall message flow.
** Control updates: update server -> client **:
An update stream can run for a long time and hence there can be
status changes at the update stream server side during the
lifetime of an update stream; for example, the update stream
server may encounter an error or need to shut down for
maintenance. To support a robust, flexible protocol design, this
document allows the update stream server to send control update
messages (defined in Section 5.3) in addition to data update
messages to the ALTO client. Figure 2 shows that both data
updates and control updates can be sent by the server to the
client (labeled "2b. control update messages").
4.2. Stream Control Service Message Flow
** Stream control: client -> stream control server **:
In addition to control changes triggered from the update stream
server side, in a flexible design, an ALTO client may initiate
control changes as well, in particular, by adding or removing ALTO
resources receiving updates. An ALTO client initiates such
changes using the stream control service (defined in Section 7).
Although one may use a design that the client uses as the same
HTTP connection to send the control requests, it requires stronger
server support, such as HTTP pipeline. For more flexibility, this
document introduces stream control service. In particular, the
update stream server of an update stream uses the first message to
provide the URI of the stream control service (labeled "1':
control URI" in Figure 2).
The ALTO client can then use the URI to ask the stream control
server specified in the URI to request the update stream server to
(1) send data update messages for additional resources, (2) stop
sending data update messages for previously requested resources,
or (3) gracefully stop and close the update stream altogether.
4.3. Service Announcement and Management Message Flow
** Service announcements: IRD server -> client **:
An update server may provide any number of update stream services,
where each update stream may provide updates for a given subset of
the ALTO server's resources. An ALTO server's Information
Resource Directory (IRD) defines the update stream services and
declares the set of resources for which each update stream service
provides updates. The ALTO server selects the resource set for
each update stream service. It is recommended that if a resource
depends on one or more other resource(s) (indicated with the
"uses" attribute defined in [RFC7285]), these other resource(s)
should also be part of that update stream. Thus, the update
stream for a cost map should also provide updates for the network
map on which that cost map depends.
** Service management (server) **:
An ALTO client may request any number of update streams
simultaneously. Because each update stream consumes resources on
the update stream server, an update stream server may require
client authorization and/or authentication, limit the number of
open update streams, close inactive streams, or redirect an ALTO
client to another update stream server.
5. Update Messages: Data Update and Control Update Messages
This section defines the format of update messages sent from the
server to the client. It first defines the generic structure of
update messages (Section 5.1). It then defines the details of the
data update messages (Section 5.2) and the control update messages
(Section 5.3). These messages will be used in the next two sections
to define the update stream service (Section 6) and the stream
control service (Section 7).
5.1. Generic ALTO Update Message Structure
Both data update and control update messages from the server to the
client have the same basic structure. Each message includes a data
field to provide data information, which is typically a JSON object,
and an event field preceding the data field, to specify the media
type indicating the encoding of the data field.
A data update message needs additional information to identify the
ALTO data (object) to which the update message applies. To be
generic, this document uses a data-id to identify the ALTO data
(object) to be updated; see below.
Hence, the event field of ALTO update message can include two
subfields (media-type and data-id), where the two subfields are
separated by a comma (',', U+002C):
media-type [ ',' data-id ]
According to Section 4.2 of [RFC6838], the comma character is not
allowed in a media-type name so there is no ambiguity when decoding
of the two subfields.
Note that an update message does not use the SSE "id" field.
5.2. ALTO Data Update Message
A data update message is sent when a monitored resource changes. As
discussed in the preceding section, the event field of a data update
message includes two subfields: 'media-type' and 'data-id'.
The 'media-type' subfield depends on whether the data update is a
complete specification of the identified data or an incremental patch
(e.g., a JSON merge patch or JSON patch), if possible, describing the
changes from the last version of the data. This document refers to
these as full replacement and incremental change, respectively. The
encoding of a full replacement is defined by its defining document
(e.g., network and cost map messages by [RFC7285]) and uses the media
type defined in that document. The encoding of JSON merge patch is
defined by [RFC7396], with the media type "application/merge-
patch+json"; the encoding of JSON patch is defined by [RFC6902], with
media type "application/json-patch+json".
The 'data-id' subfield identifies the ALTO data to which the data
update message applies.
First, consider the case that the resource contains only a single
JSON object. For example, since an ALTO client can request data
updates for both a cost map resource (object) and its dependent
network map resource (object) in the same update stream, to
distinguish the updates, the client assigns a substream-id for each
resource receiving data updates. Substream-ids MUST be unique within
an update stream but need not be globally unique. A substream-id is
encoded as a JSON string with the same format as that of the type
ResourceID (Section 10.2 of [RFC7285]). The type SubstreamID is used
in this document to indicate a string of this format. The substream-
id of a single JSON object is the 'data-id'.
As an example, assume that the ALTO client assigns substream-id "1"
in its request to receive updates to the network map and substream-id
"2" to the cost map. Then, the substream-ids are the data-ids
indicating which objects will be updated. Figure 3 shows some
examples of ALTO data update messages:
event: application/alto-networkmap+json,1
data: { ... full network map message ... }
event: application/alto-costmap+json,2
data: { ... full cost map message ... }
event: application/merge-patch+json,2
data: { ... JSON merge patch update for the cost map ... }
Figure 3: Examples of ALTO Data Update Messages
Next, consider the case that a resource may include multiple JSON
objects. This document considers the case that a resource may
contain multiple components (parts), and they are encoded using the
media type "multipart/related" [RFC2387]. Each part of this
multipart response MUST be an HTTP message including a Content-ID
header and a JSON object body. Each component requiring the update
stream service (defined in Section 6) MUST be identified by a unique
Content-ID to be defined in its defining document.
For a resource using the media type "multipart/related", the 'data-
id' subfield MUST be the concatenation of the substream-id, the '.'
separator (U+002E), and the unique Content-ID, in order.
5.3. ALTO Control Update Message
Control update messages have the media type "application/alto-
updatestreamcontrol+json", and the data is of type
UpdateStreamControlEvent:
object {
[String control-uri;]
[SubstreamID started<1..*>;]
[SubstreamID stopped<1..*>;]
[String description;]
} UpdateStreamControlEvent;
control-uri:
the URI providing stream control for this update stream (see
Section 7). The server sends a control update message notifying
the client of the control-uri. This control update message
notifying the control-uri will be sent once and MUST be the first
event in an update stream. If the URI value is NULL, the update
stream server does not support stream control for this update
stream; otherwise, the update stream server provides stream
control through the given URI.
started:
a list of substream-ids of resources. It notifies the ALTO client
that the update stream server will start sending data update
messages for each resource listed.
stopped:
a list of substream-ids of resources. It notifies the ALTO client
that the update stream server will no longer send data update
messages for the listed resources. There can be multiple reasons
for an update stream server to stop sending data update messages
for a resource, including a request from the ALTO client using
stream control (Section 6.7.1) or an internal server event.
description:
a non-normative, human-readable text providing an explanation for
the control event. When an update stream server stops sending
data update messages for a resource, it is RECOMMENDED that the
update stream server use the description field to provide details.
There can be multiple reasons that trigger a "stopped" event; see
above. The intention of this field is to provide a human-readable
text for the developer and/or the administrator to diagnose
potential problems.
6. Update Stream Service
An update stream service returns a stream of update messages, as
defined in Section 5. An ALTO server's IRD (Information Resource
Directory) MAY define one or more update stream services, which ALTO
clients use to request new update stream instances. An IRD entry
defining an update stream service MUST define the media type, HTTP
method, and capabilities and uses as follows.
6.1. Media Type
The media type of an ALTO update stream service is "text/event-
stream", as defined by [SSE].
6.2. HTTP Method
An ALTO update stream service is requested using the HTTP POST
method.
6.3. Capabilities
The capabilities are defined as an object of type
UpdateStreamCapabilities:
object {
IncrementalUpdateMediaTypes incremental-change-media-types;
Boolean support-stream-control;
} UpdateStreamCapabilities;
object-map {
ResourceID -> String;
} IncrementalUpdateMediaTypes;
If this update stream can provide data update messages 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 the incremental change separated by commas.
Normally, this will be "application/merge-patch+json", "application/
json-patch+json", or "application/merge-patch+json,application/json-
patch+json", because, as described in Section 5, they are the only
incremental change types defined by this document. However, future
extensions may define other types of incremental changes.
When choosing the media types to encode incremental changes for a
resource, the update stream 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); see the MergePatch algorithm in Section 3.1.1 on
how null value is processed. 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 update stream server
MUST choose JSON patch over JSON merge patch if JSON patch is
indicated as a capability of the update stream server. If the server
does not support JSON patch to handle such a case, the server then
need to send a full replacement.
The "support-stream-control" field specifies whether the given update
stream supports stream control. If the "support-stream-control"
field is "true", the update stream server will use the stream control
specified in this document; otherwise, the update stream server may
use other mechanisms to provide the same functionality as stream
control.
6.4. Uses
The "uses" attribute MUST be an array with the resource-ids of every
resource for which this update stream can provide updates. Each
resource specified in the "uses" MUST support full replacement; the
update stream server can always send full replacement, and the ALTO
client MUST accept full replacement.
This set may be any subset of the ALTO server's 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 an update stream's "uses" set
includes resource R1 and resource R1 depends on ("uses") resource R0,
then the update stream's "uses" set SHOULD include R0 as well as R1.
For example, an update stream for a cost map SHOULD also provide
updates for the network map upon which that cost map depends.
6.5. Request: Accept Input Parameters
An ALTO client specifies the parameters for the new update stream by
sending an HTTP POST body with the media type "application/alto-
updatestreamparams+json". That body contains a JSON object of type
UpdateStreamReq, where:
object {
[AddUpdatesReq add;]
[SubstreamID remove<0..*>;]
} UpdateStreamReq;
object-map {
SubstreamID -> AddUpdateReq;
} AddUpdatesReq;
object {
ResourceID resource-id;
[JSONString tag;]
[Boolean incremental-changes;]
[Object input;]
} AddUpdateReq;
add:
Specifies the resources (and the parameters for the resources) for
which the ALTO client wants updates. In the scope of the same
update stream, the ALTO client MUST assign a substream-id that is
unique in the scope of the update stream (Section 5.2) for each
entry and use those substream-ids as the keys in the "add" field.
resource-id:
The resource-id of an ALTO resource and MUST be in the update
stream's "uses" list (Section 6.4). If the resource-id is a GET-
mode resource with a version tag (or "vtag"), as defined in
Sections 6.3 and 10.3 of [RFC7285], and the ALTO client has
previously retrieved a version of that resource from the update
stream server, the ALTO client MAY set the "tag" field to the tag
part of the client's version of that resource. If that version is
not current, the update stream server MUST send a full replacement
before sending any incremental changes, as described in
Section 6.7.1. If that version is still current, the update
stream server MAY omit the initial full replacement.
incremental-changes:
The ALTO client specifies whether it is willing to receive
incremental changes from the update stream server for this
substream. If the "incremental-changes" field is "true", the
update stream server MAY send incremental changes for this
substream. In this case, the client MUST support all incremental
methods from the set announced in the server's capabilities for
this resource; see Section 6.3 for the server's announcement of
potential incremental methods. If a client does not support all
incremental methods from the set announced in the server's
capabilities, the client can set "incremental-changes" to "false",
and the update stream server then MUST NOT send incremental
changes for that substream. The default value for "incremental-
changes" is "true", so to suppress incremental changes, the ALTO
client MUST explicitly set "incremental-changes" to "false". An
alternative design of incremental-changes control is a more fine-
grained control, by allowing a client to select a subset of
incremental methods from the set announced in the server's
capabilities. But this alternative design is not adopted in this
document, because it adds complexity to the server, which is more
likely to be the bottleneck. Note that the ALTO client cannot
suppress full replacement. When the ALTO client sets
"incremental-changes" to "false", the update stream server MUST
send a full replacement instead of an incremental change to the
ALTO client. The update stream server MAY wait until more changes
are available and send a single full replacement with those
changes. Thus, an ALTO client that declines to accept incremental
changes may not get updates as quickly as an ALTO client that
does.
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.
remove:
It is used in update stream control requests (Section 7) and is
not allowed in the update stream request. The update stream
server SHOULD ignore this field if it is included in the request.
If a request has any errors, the update stream server MUST NOT create
an update stream. Also, the update stream server will send an error
response to the ALTO client, as specified in Section 6.6.
6.6. Response
If the update stream request has any errors, the update stream server
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:
HTTP/1.1 400 Bad Request
Content-Length: 131
Content-Type: application/alto-error+json
Connection: Closed
{
"meta":{
"code": "E_INVALID_FIELD_VALUE",
"field": "add/my-network-map/resource-id",
"value": "my-networkmap/#"
}
}
Note that "field" and "value" are optional fields. If the "value"
field exists, the "field" field MUST exist.
* If an update stream request does not have an "add" field
specifying one or more resources, the error code of the error
message MUST be E_MISSING_FIELD and the "field" field SHOULD be
"add". The update stream server MUST close the stream without
sending any events.
* If the "resource-id" field is invalid or is not associated with
the update stream, 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 there are more than one invalid resource-
ids, the update stream server SHOULD pick one and return it. The
update stream server MUST close the stream (i.e., TCP connection)
without sending any events.
* 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, the update stream server MUST return the same
error response that that resource would return for missing or
invalid input (see [RFC7285]). In this case, the update stream
server MUST close the update stream without sending any events.
If the input for several POST-mode resources is missing or
invalid, the update stream server MUST pick one and return it.
The response to a valid request is a stream of update messages.
Section 5 defines the update messages, and [SSE] defines how they are
encoded into a stream.
An update stream server SHOULD send updates only when the underlying
values change. However, it may be difficult for an update stream
server to guarantee that in all circumstances. Therefore, a client
MUST NOT assume that an update message represents an actual change.
6.7. Additional Requirements on Update Stream Service
6.7.1. Event Sequence Requirements
* The first event MUST be a control update message with the URI of
the update stream control service (see Section 7) for this update
stream. Note that the value of the control-uri can be "null",
indicating that there is no control stream service.
* As soon as possible, after the ALTO client initiates the
connection, the update stream server checks the "tag" field for
each added update request. If the "tag" field is not specified in
an added update request, the update stream server MUST first send
a full replacement for the request. If the "tag" field is
specified, the client can accept incremental changes, and the
server can compute an incremental update based on the "tag" (the
server needs to ensure that for a POST resource with input, the
"tag" should indicate the correct result for different inputs);
the update stream server MAY omit the initial full replacement.
* If this update stream provides updates for resource-ids R0 and R1
and if R1 depends on R0, then the update stream server MUST send
the update for R0 before sending the related updates for R1. For
example, suppose an update stream provides updates to a network
map and its dependent cost maps. When the network map changes,
the update stream server MUST send the network map update before
sending the cost map updates.
* When the ALTO client uses the stream control service to stop
updates for one or more resources (Section 7), the ALTO client
MUST send a stream control request. The update stream server MUST
send a control update message whose "stopped" field has the
substream-ids of all stopped resources.
6.7.2. Cross-Stream Consistency Requirements
If multiple ALTO clients create multiple update streams from the same
update stream resource and with the same update request parameters
(i.e., same resource and same input), the update stream server MUST
send the same updates to all of them. However, the update stream
server MAY pack data items into different patch events, as long as
the net result of applying those updates is the same.
For example, suppose two different ALTO clients create two different
update streams for the same cost map, and suppose the update stream
server processes three separate cost point updates with a brief pause
between each update. The server MUST send all three new cost points
to both clients. But the update stream server MAY send a single
patch event (with all three cost points) to one ALTO client while
sending three separate patch events (with one cost point per event)
to the other ALTO client.
An update stream server MAY offer several different update stream
resources that provide updates to the same underlying resource (that
is, a resource-id may appear in the "uses" field of more than one
update stream resource). In this case, those update stream resources
MUST return the same update.
6.7.3. Multipart Update Requirements
This design allows any valid media type for full replacement. Hence,
it supports ALTO resources using multipart to contain multiple JSON
objects. This realizes the push benefit but not the incremental
encoding benefit of SSE.
JSON patch and merge patch provide the incremental encoding benefit
but can be applied to only a single JSON object. If an update stream
service supports a resource providing a multipart media type, which
we refer to as a multipart resource, then the update stream service
needs to handle the issue that the message of a full multipart
resource can include multiple JSON objects. To address the issue,
when an update stream service specifies that it supports JSON patch
or merge patch incremental updates for a multipart resource, the
service MUST ensure that (1) each part of a multipart message is a
single JSON object, (2) each part is specified by a static Content-ID
in the initial full message, (3) each data update event applies to
only one part, and (4) each data update specifies substream-
id.content-id as the "event" field of the event, to identify the part
to be updated.
6.8. Keep-Alive Messages
In an SSE stream, any line that starts with a colon (U+003A)
character is a comment, and an ALTO client MUST ignore that line
[SSE]. As recommended in [SSE], an update stream server SHOULD send
a comment line (or an event) every 15 seconds to prevent ALTO clients
and proxy servers from dropping the HTTP connection. Note that
although TCP also provides a Keep-Alive function, the interval
between TCP Keep-Alive messages can depend on the OS configuration
and varies. The preceding recommended SSE Keep-Alive allows the SSE
client to detect the status of the update stream server with more
certainty.
7. Stream Control Service
A stream control service allows an ALTO client to remove resources
from the set of resources that are monitored by an update stream or
add additional resources to that set. The service also allows an
ALTO client to gracefully shut down an update stream.
When an update stream server creates a new update stream and if the
update stream server supports stream control for the update stream,
the update stream server creates a stream control service for that
update stream. An ALTO client uses the stream control service to
remove resources from the update stream instance or to request
updates for additional resources. An ALTO client cannot obtain the
stream control service through the IRD. Instead, the first event
that the update stream server sends to the ALTO client has the URI
for the associated stream control service (see Section 5.3).
Each stream control request is an individual HTTP request. The ALTO
client MAY send multiple stream control requests to the stream
control server using the same HTTP connection.
7.1. URI
The URI for a stream control service, by itself, MUST uniquely
specify the update stream instance that it controls. The stream
control server MUST NOT use other properties of an HTTP request, such
as cookies or the client's IP address, to determine the update
stream. Furthermore, an update stream server MUST NOT reuse a
control service URI once the associated update stream has been
closed.
The ALTO client MUST evaluate a relative control URI reference
[RFC3986] (for example, a URI reference without a host or with a
relative path) in the context of the URI used to create the update
stream. The stream control service's host MAY be different from the
update stream's host.
It is expected that there is an internal mechanism to map a stream
control URI to the unique update stream instance to be controlled.
For example, the update stream service may assign a unique, internal
stream id to each update stream instance. However, the exact
mechanism is left to the update stream service provider.
To prevent an attacker from forging a stream control URI and sending
bogus requests to disrupt other update streams, the service should
consider two security issues. First, if http, not https, is used,
the stream control URI can be exposed to an on-path attacker. To
address this issue, in a setting where the path from the server to
the client can traverse such an attacker, the server SHOULD use
https. Second, even without direct exposure, an off-path attacker
may guess valid stream control URIs. To address this issue, the
server SHOULD choose stream control URIs with enough randomness to
make guessing difficult; the server SHOULD introduce mechanisms that
detect repeated guesses indicating an attack (e.g., keeping track of
the number of failed stream control attempts). Please see the W3C's
"Good Practices for Capability URLs" <https://www.w3.org/TR/
capability-urls/>.
7.2. Media Type
An ALTO stream control response does not have a specific media type.
7.3. HTTP Method
An ALTO update stream control resource is requested using the HTTP
POST method.
7.4. IRD Capabilities & Uses
None (Stream control services do not appear in the IRD).
7.5. Request: Accept Input Parameters
A stream control service accepts the same input media type and input
parameters as the update stream service (Section 6.5). The only
difference is that a stream control service also accepts the "remove"
field.
If specified, the "remove" field is an array of substream-ids the
ALTO client previously added to this update stream. An empty
"remove" array is equivalent to a list of all currently active
resources; the update stream server responds by removing all
resources and closing the stream.
An ALTO client MAY use the "add" field to add additional resources.
The ALTO client MUST assign a unique substream-id to each additional
resource. Substream-ids MUST be unique over the lifetime of this
update stream; an ALTO client MUST NOT reuse a previously removed
substream-id. The processing of an "add" resource is the same as
discussed in Sections 6.5 and 6.6.
If a request has any errors, the update stream server MUST NOT add or
remove any resources from the associated update stream. Also, the
stream control server will return an error response to the client, as
specified in Section 7.6.
7.6. Response
The stream control server MUST process the "add" field before the
"remove" field. If the request removes all active resources without
adding any additional resources, the update stream server MUST close
the update stream. Thus, an update stream cannot have zero
resources.
If the request has any errors, the stream control server MUST return
an HTTP "400 Bad Request" to the ALTO client. The body part of the
response follows the generic ALTO error response format specified in
Section 8.5.2 of [RFC7285]. An error response has the same format as
specified in Section 6.6. Detailed error code and error information
are specified as below.
* If the "add" request does not satisfy the requirements in
Section 6.5, the stream control server MUST return the ALTO error
message defined in Section 6.6.
* If any substream-id in the "remove" field was not added in a prior
request, the error code of the error message MUST be
E_INVALID_FIELD_VALUE, the "field" field SHOULD be "remove", and
the "value" field SHOULD be an array of the invalid substream-ids.
Thus, it is illegal to "add" and "remove" the same substream-id in
the same request. However, it is legal to remove a substream-id
twice. To support the preceding checking, the update stream
server MUST keep track of previously used but now closed
substream-ids.
* If any substream-id in the "add" field has been used before in
this stream, the error code of the error message MUST be
E_INVALID_FIELD_VALUE, the "field" field SHOULD be "add", and the
"value" field SHOULD be an array of invalid substream-ids.
* If the request has a non-empty "add" field and a "remove" field
with an empty list of substream-ids (to replace all active
resources with a new set, the client MUST explicitly enumerate the
substream-ids to be removed), the error code of the error message
MUST be E_INVALID_FIELD_VALUE, the "field" field SHOULD be
"remove", and the "value" field SHOULD be an empty array.
If the request is valid but the associated update stream has been
closed, then the stream control server MUST return an HTTP "404 Not
Found".
If the request is valid and the stream control server successfully
processes the request without error, the stream control server should
return either an HTTP "202 Accepted" response or an HTTP "204 No
Content" response. The difference is that for the latter case, the
stream control server is sure that the update stream server has also
processed the request. Regardless of a 202 or 204 HTTP response, the
final updates of related resources will be notified by the update
stream server using its control update message(s), due to the modular
design.
8. Examples
8.1. Example: IRD Announcing Update Stream Services
Below is an example IRD announcing three update stream services. The
first, which is named "update-my-costs", provides updates for the
network map, the "routingcost" and "hopcount" cost maps, and a
Filtered Cost Map resource. The second, which is named "update-my-
prop", provides updates to the endpoint properties service. The
third, which is named "update-my-pv", provides updates to a
nonstandard ALTO service returning a multipart response.
Note that in the "update-my-costs" update stream shown in the example
IRD, the update stream server uses JSON patch for network map, and it
uses JSON merge patch to update the other resources. Also, the
update stream will only provide full replacements for "my-simple-
filtered-cost-map".
Also, note that this IRD defines two Filtered Cost Map resources.
They use the same cost types, but "my-filtered-cost-map" accepts cost
constraint tests, while "my-simple-filtered-cost-map" does not. To
avoid the issues discussed in Section 9.3, the update stream provides
updates for the second but not the first.
This IRD also announces a nonstandard ALTO service, which is named
"my-pv". This service accepts an extended endpoint cost request as
an input and returns a multipart response, including an endpoint cost
resource and a property map resource. This document does not rely on
any other design details of this new service. In this document, the
"my-pv" service is only used to illustrate how the update stream
service provides updates to an ALTO resource returning a multipart
response.
"my-network-map": {
"uri": "https://alto.example.com/networkmap",
"media-type": "application/alto-networkmap+json",
},
"my-routingcost-map": {
"uri": "https://alto.example.com/costmap/routingcost",
"media-type": "application/alto-costmap+json",
"uses": ["my-networkmap"],
"capabilities": {
"cost-type-names": ["num-routingcost"]
}
},
"my-hopcount-map": {
"uri": "https://alto.example.com/costmap/hopcount",
"media-type": "application/alto-costmap+json",
"uses": ["my-networkmap"],
"capabilities": {
"cost-type-names": ["num-hopcount"]
}
},
"my-filtered-cost-map": {
"uri": "https://alto.example.com/costmap/filtered/constraints",
"media-type": "application/alto-costmap+json",
"accepts": "application/alto-costmapfilter+json",
"uses": ["my-networkmap"],
"capabilities": {
"cost-type-names": ["num-routingcost", "num-hopcount"],
"cost-constraints": true
}
},
"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
}
},
"my-props": {
"uri": "https://alto.example.com/properties",
"media-type": "application/alto-endpointprops+json",
"accepts": "application/alto-endpointpropparams+json",
"capabilities": {
"prop-types": ["priv:ietf-bandwidth"]
}
},
"my-pv": {
"uri": "https://alto.example.com/endpointcost/pv",
"media-type": "multipart/related;
type=application/alto-endpointcost+json",
"accepts": "application/alto-endpointcostparams+json",
"capabilities": {
"cost-type-names": [ "path-vector" ],
"ane-properties": [ "maxresbw", "persistent-entities" ]
}
},
"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-props": {
"uri": "https://alto.example.com/updates/properties",
"media-type": "text/event-stream",
"uses": [ "my-props" ],
"accepts": "application/alto-updatestreamparams+json",
"capabilities": {
"incremental-change-media-types": {
"my-props": "application/merge-patch+json"
},
"support-stream-control": true
}
},
"update-my-pv": {
"uri": "https://alto.example.com/updates/pv",
"media-type": "text/event-stream",
"uses": [ "my-pv" ],
"accepts": "application/alto-updatestreamparams+json",
"capabilities": {
"incremental-change-media-types": {
"my-pv": "application/merge-patch+json"
},
"support-stream-control": true
}
}
8.2. Example: Simple Network and Cost Map Updates
Given the update streams announced in the preceding example IRD, the
section below shows an example of an ALTO client's request and the
update stream server's immediate response, using the update stream
resource "update-my-costs". In the example, the ALTO client requests
updates for the network map and "routingcost" cost map but not for
the "hopcount" cost map. The ALTO client uses the ALTO server's
resource-ids as the substream-ids. Because the client does not
provide a "tag" for the network map, the update stream server must
send a full replacement for the network map as well as for the cost
map. The ALTO client does not set "incremental-changes" to "false",
so it defaults to "true". Thus, the update stream server will send
patch updates for the cost map and the network map.
POST /updates/costs HTTP/1.1
Host: alto.example.com
Accept: text/event-stream,application/alto-error+json
Content-Type: application/alto-updatestreamparams+json
Content-Length: 155
{ "add": {
"my-network-map": {
"resource-id": "my-network-map"
},
"my-routingcost-map": {
"resource-id": "my-routingcost-map"
}
}
}
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: application/alto-updatestreamcontrol+json
data: {"control-uri":
data: "https://alto.example.com/updates/streams/3141592653589"}
event: application/alto-networkmap+json,my-network-map
data: {
data: "meta" : {
data: "vtag": {
data: "resource-id" : "my-network-map",
data: "tag" : "da65eca2eb7a10ce8b059740b0b2e3f8eb1d4785"
data: }
data: },
data: "network-map" : {
data: "PID1" : {
data: "ipv4" : [ "192.0.2.0/24", "198.51.100.0/25" ]
data: },
data: "PID2" : {
data: "ipv4" : [ "198.51.100.128/25" ]
data: },
data: "PID3" : {
data: "ipv4" : [ "0.0.0.0/0" ],
data: "ipv6" : [ "::/0" ]
data: }
data: }
data: }
data: }
event: application/alto-costmap+json,my-routingcost-map
data: {
data: "meta" : {
data: "dependent-vtags" : [{
data: "resource-id": "my-network-map",
data: "tag": "da65eca2eb7a10ce8b059740b0b2e3f8eb1d4785"
data: }],
data: "cost-type" : {
data: "cost-mode" : "numerical",
data: "cost-metric": "routingcost"
data: },
data: "vtag": {
data: "resource-id" : "my-routingcost-map",
data: "tag" : "3ee2cb7e8d63d9fab71b9b34cbf764436315542e"
data: }
data: },
data: "cost-map" : {
data: "PID1": { "PID1": 1, "PID2": 5, "PID3": 10 },
data: "PID2": { "PID1": 5, "PID2": 1, "PID3": 15 },
data: "PID3": { "PID1": 20, "PID2": 15 }
data: }
data: }
After sending those events immediately, the update stream server will
send additional events as the maps change. For example, the
following represents a small change to the cost map. PID1->PID2 is
changed to 9 from 5, PID3->PID1 is no longer available, and
PID3->PID3 is now defined as 1:
event: application/merge-patch+json,my-routingcost-map
data: {
data: "meta" : {
data: "vtag": {
data: "tag": "c0ce023b8678a7b9ec00324673b98e54656d1f6d"
data: }
data: },
data: "cost-map": {
data: "PID1" : { "PID2" : 9 },
data: "PID3" : { "PID1" : null, "PID3" : 1 }
data: }
data: }
As another example, the following represents a change to the network
map: an ipv4 prefix "203.0.113.0/25" is added to PID1. It triggers
changes to the cost map. The update stream server chooses to send an
incremental change for the network map and send a full replacement
instead of an incremental change for the cost map:
event: application/json-patch+json,my-network-map
data: {
data: {
data: "op": "replace",
data: "path": "/meta/vtag/tag",
data: "value" :"a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
data: },
data: {
data: "op": "add",
data: "path": "/network-map/PID1/ipv4/2",
data: "value": "203.0.113.0/25"
data: }
data: }
event: application/alto-costmap+json,my-routingcost-map
data: {
data: "meta" : {
data: "vtag": {
data: "tag": "c0ce023b8678a7b9ec00324673b98e54656d1f6d"
data: }
data: },
data: "cost-map" : {
data: "PID1": { "PID1": 1, "PID2": 3, "PID3": 7 },
data: "PID2": { "PID1": 12, "PID2": 1, "PID3": 9 },
data: "PID3": { "PID1": 14, "PID2": 8 }
data: }
data: }
8.3. Example: Advanced Network and Cost Map Updates
This example is similar to the previous one, except that the ALTO
client requests updates for the "hopcount" cost map as well as the
"routingcost" cost map and provides the current version tag of the
network map, so the update stream server is not required to send the
full network map data update message at the beginning of the stream.
In this example, the client uses the substream-ids "net", "routing",
and "hops" for those resources. The update stream server sends the
stream control URI and the full cost maps, followed by updates for
the network map and cost maps as they become available:
POST /updates/costs HTTP/1.1
Host: alto.example.com
Accept: text/event-stream,application/alto-error+json
Content-Type: application/alto-updatestreamparams+json
Content-Length: 244
{ "add": {
"net": {
"resource-id": "my-network-map",
"tag": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
},
"routing": {
"resource-id": "my-routingcost-map"
},
"hops": {
"resource-id": "my-hopcount-map"
}
}
}
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: application/alto-updatestreamcontrol+json
data: {"control-uri":
data: "https://alto.example.com/updates/streams/2718281828459"}
event: application/alto-costmap+json,routing
data: { ... full routingcost cost map message ... }
event: application/alto-costmap+json,hops
data: { ... full hopcount cost map message ... }
(pause)
event: application/merge-patch+json,routing
data: {"cost-map": {"PID2" : {"PID3" : 31}}}
event: application/merge-patch+json,hops
data: {"cost-map": {"PID2" : {"PID3" : 4}}}
If the ALTO client wishes to stop receiving updates for the
"hopcount" cost map, the ALTO client can send a "remove" request on
the stream control URI:
POST /updates/streams/2718281828459 HTTP/1.1
Host: alto.example.com
Accept: text/plain,application/alto-error+json
Content-Type: application/alto-updatestreamparams+json
Content-Length: 24
{
"remove": [ "hops" ]
}
HTTP/1.1 204 No Content
Content-Length: 0
(stream closed without sending data content)
The update stream server sends a "stopped" control update message on
the original request stream to inform the ALTO client that updates
are stopped for that resource:
event: application/alto-updatestreamcontrol+json
data: {
data: "stopped": ["hops"]
data: }
Below is an example of an invalid stream control request. The
"remove" field of the request includes an undefined substream-id, and
the stream control server will return an error response to the ALTO
client.
POST /updates/streams/2718281828459 HTTP/1.1
Host: alto.example.com
Accept: text/plain,application/alto-error+json
Content-Type: application/alto-updatestreamparams+json
Content-Length: 31
{
"remove": [ "properties" ]
}
HTTP/1.1 400 Bad Request
Content-Length: 89
Content-Type: application/alto-error+json
{
"meta":{
"code": "E_INVALID_FIELD_VALUE",
"field": "remove",
"value": "properties"
}
If the ALTO client no longer needs any updates and wishes to shut the
update stream down gracefully, the client can send a "remove" request
with an empty array:
POST /updates/streams/2718281828459 HTTP/1.1
Host: alto.example.com
Accept: text/plain,application/alto-error+json
Content-Type: application/alto-updatestreamparams+json
Content-Length: 17
{
"remove": [ ]
}
HTTP/1.1 204 No Content
Content-Length: 0
(stream closed without sending data content)
The update stream server sends a final control update message on the
original request stream to inform the ALTO client that all updates
are stopped and then closes the stream:
event: application/alto-updatestreamcontrol+json
data: {
data: "stopped": ["net", "routing"]
data: }
(server closes stream)
8.4. Example: Endpoint Property Updates
As another example, here is how an ALTO client can request updates
for the property "priv:ietf-bandwidth" for one set of endpoints and
"priv:ietf-load" for another. The update stream server immediately
sends full replacements with the property values for all endpoints.
After that, the update stream server sends data update messages for
the individual endpoints as their property values change.
POST /updates/properties HTTP/1.1
Host: alto.example.com
Accept: text/event-stream
Content-Type: application/alto-updatestreamparams+json
Content-Length: 511
{ "add": {
"props-1": {
"resource-id": "my-props",
"input": {
"properties" : [ "priv:ietf-bandwidth" ],
"endpoints" : [
"ipv4:198.51.100.1",
"ipv4:198.51.100.2",
"ipv4:198.51.100.3"
]
}
},
"props-2": {
"resource-id": "my-props",
"input": {
"properties" : [ "priv:ietf-load" ],
"endpoints" : [
"ipv6:2001:db8:100::1",
"ipv6:2001:db8:100::2",
"ipv6:2001:db8:100::3"
]
}
}
}
}
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: application/alto-updatestreamcontrol+json
data: {"control-uri":
data: "https://alto.example.com/updates/streams/1414213562373"}
event: application/alto-endpointprops+json,props-1
data: { "endpoint-properties": {
data: "ipv4:198.51.100.1" : { "priv:ietf-bandwidth": "13" },
data: "ipv4:198.51.100.2" : { "priv:ietf-bandwidth": "42" },
data: "ipv4:198.51.100.3" : { "priv:ietf-bandwidth": "27" }
data: } }
event: application/alto-endpointprops+json,props-2
data: { "endpoint-properties": {
data: "ipv6:2001:db8:100::1" : { "priv:ietf-load": "8" },
data: "ipv6:2001:db8:100::2" : { "priv:ietf-load": "2" },
data: "ipv6:2001:db8:100::3" : { "priv:ietf-load": "9" }
data: } }
(pause)
event: application/merge-patch+json,props-1
data: { "endpoint-properties":
data: {"ipv4:198.51.100.1" : {"priv:ietf-bandwidth": "3"}}
data: }
(pause)
event: application/merge-patch+json,props-2
data: { "endpoint-properties":
data: {"ipv6:2001:db8:100::3" : {"priv:ietf-load": "7"}}
data: }
If the ALTO client needs the "priv:ietf-bandwidth" property and the
"priv:ietf-load" property for additional endpoints, the ALTO client
can send an "add" request on the stream control URI:
POST /updates/streams/1414213562373" HTTP/1.1
Host: alto.example.com
Accept: text/plain,application/alto-error+json
Content-Type: application/alto-updatestreamparams+json
Content-Length: 448
{ "add": {
"props-3": {
"resource-id": "my-props",
"input": {
"properties" : [ "priv:ietf-bandwidth" ],
"endpoints" : [
"ipv4:198.51.100.4",
"ipv4:198.51.100.5"
]
}
},
"props-4": {
"resource-id": "my-props",
"input": {
"properties" : [ "priv:ietf-load" ],
"endpoints" : [
"ipv6:2001:db8:100::4",
"ipv6:2001:db8:100::5"
]
}
}
}
}
HTTP/1.1 204 No Content
Content-Length: 0
(stream closed without sending data content)
The update stream server sends full replacements for the two new
resources, followed by incremental changes for all four requests as
they arrive:
event: application/alto-endpointprops+json,props-3
data: { "endpoint-properties": {
data: "ipv4:198.51.100.4" : { "priv:ietf-bandwidth": "25" },
data: "ipv4:198.51.100.5" : { "priv:ietf-bandwidth": "31" },
data: } }
event: application/alto-endpointprops+json,props-4
data: { "endpoint-properties": {
data: "ipv6:2001:db8:100::4" : { "priv:ietf-load": "6" },
data: "ipv6:2001:db8:100::5" : { "priv:ietf-load": "4" },
data: } }
(pause)
event: application/merge-patch+json,props-3
data: { "endpoint-properties":
data: {"ipv4:198.51.100.5" : {"priv:ietf-bandwidth": "15"}}
data: }
(pause)
event: application/merge-patch+json,props-2
data: { "endpoint-properties":
data: {"ipv6:2001:db8:100::2" : {"priv:ietf-load": "9"}}
data: }
(pause)
event: application/merge-patch+json,props-4
data: { "endpoint-properties":
data: {"ipv6:2001:db8:100::4" : {"priv:ietf-load": "3"}}
data: }
8.5. Example: Multipart Message Updates
This example shows how an ALTO client can request a nonstandard ALTO
service returning a multipart response. The update stream server
immediately sends full replacements of the multipart response. After
that, the update stream server sends data update messages for the
individual parts of the response as the ALTO data (object) in each
part changes.
POST /updates/pv HTTP/1.1
Host: alto.example.com
Accept: text/event-stream
Content-Type: application/alto-updatestreamparams+json
Content-Length: 382
{
"add": {
"ecspvsub1": {
"resource-id": "my-pv",
"input": {
"cost-type": {
"cost-mode": "array",
"cost-metric": "ane-path"
},
"endpoints": {
"srcs": [ "ipv4:192.0.2.2" ],
"dsts": [ "ipv4:192.0.2.89", "ipv4:203.0.113.45" ]
},
"ane-properties": [ "maxresbw", "persistent-entities" ]
}
}
}
}
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: application/alto-updatestreamcontrol+json
data: {"control-uri":
data: "https://alto.example.com/updates/streams/1414"}
event: multipart/related;boundary=example-pv;
type=application/alto-endpointcost+json,ecspvsub1
data: --example-pv
data: Content-ID: ecsmap
data: Content-Type: application/alto-endpointcost+json
data:
data: { ... data (object) of an endpoint cost map ... }
data: --example-pv
data: Content-ID: propmap
data: Content-Type: application/alto-propmap+json
data:
data: { ... data (object) of a property map ... }
data: --example-pv--
(pause)
event: application/merge-patch+json,ecspvsub1.ecsmap
data: { ... merge patch for updates of ecspvsub1.ecsmap ... }
event: application/merge-patch+json,ecspvsub1.propmap
data: { ... merge patch for updates of ecspvsub1.propmap ... }
9. Operation and Processing Considerations
9.1. Considerations for Choosing Data Update Messages
The update stream server should be cognizant of the effects of its
update schedule, which includes both the choice of timing (i.e.,
when/what to trigger an update) 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,
the server may choose to batch a sequence of updates for resources
that frequently change by sending cumulative updates or a full
replacement after a while. The update stream server should be
cognizant that batching reduces the freshness of information. The
server should also consider the effect of such delays on client
behaviors (see below on client timeout on waiting for updates of
dependent resources).
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 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
has no 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 update stream server may decide on its own whether to
use JSON merge patch or JSON patch according to the changes in
network maps.
9.2. Considerations for Client Processing Data Update Messages
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 change for
a resource, the ALTO client should apply those patches 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 update stream server MUST
send the network map update message before sending the updates for
the dependent cost maps (see Section 6.7.1).
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 message indicating that it is
based on the new network map version tag.
The update stream server SHOULD send updates for dependent resources
(i.e., the cost maps in the preceding example) in a timely fashion.
However, if the ALTO client does not receive the expected updates, a
simple recovery method is that the ALTO client closes the update
stream connection, discards the dependent resources, and
reestablishes the update stream. The ALTO client MAY retain the
version tag of the last version of any tagged resources and give
those version tags when requesting the new update stream. In this
case, if a version is still current, the update stream server will
not resend that resource.
Although not as efficient as possible, this recovery method is simple
and reliable.
9.3. Considerations for Updates to Filtered Cost Maps
If an update stream 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 update stream server MUST send an update if the
new value satisfies the test. If the new value does not, whether the
update stream server sends an update depends on whether the previous
value satisfied the test. If it did not, the update stream server
SHOULD NOT send an update to the ALTO client. But if the previous
value did, then the update stream server MUST send an update with a
"null" value to inform the ALTO client that this cost no longer
satisfies the criteria.
An update stream server can avoid having to handle such a complicated
behavior by offering update streams only for Filtered Cost Maps that
do not allow constraint tests.
9.4. 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 an update
stream server to offer incremental updates for ordinal mode cost
maps, update stream server implementors should be aware that
incremental updates for ordinal costs are more complicated than for
numerical costs, and ALTO clients should be aware that small changes
may result in large updates.
An update stream server can avoid this complication by only offering
full replacements for ordinal cost maps.
9.5. Considerations for SSE Text Formatting and Processing
SSE was designed for events that consist of relatively small amounts
of line-oriented text data, and SSE clients frequently read input one
line at a time. However, an update stream sends a full cost map as a
single events, and a cost map may involve megabytes, if not tens of
megabytes, of text. This has implications that the ALTO client and
the update stream server may consider.
First, some SSE client libraries read all data for an event into
memory and then present it to the client as a character array.
However, a client may not have enough memory to hold the entire JSON
text for a large cost map. Hence, an ALTO client SHOULD consider
using an SSE library that presents the event data in manageable
chunks, so the ALTO client can parse the cost map incrementally and
store the underlying data in a more compact format.
Second, an SSE client library may use a low-level, generic socket
read library that stores each line of an event data, just in case the
higher-level parser may need the line delimiters as part of the
protocol formatting. A server sending a complete cost map as a
single line may then generate a multi-megabyte data "line", and such
a long line may then require complex memory management at the client.
It is RECOMMENDED that an update stream server limit the lengths of
data lines.
Third, an SSE server may use a library, which may put line breaks in
places that would have semantic consequences for the ALTO updates;
see Section 11. The update stream server implementation MUST ensure
that no line breaks are introduced to change the semantics.
10. 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 streams and stream
control URIs) provided by this extension extend the attack surface
described in Section 15.1.1 of [RFC7285]. Below, we discuss the
additional risks and their remedies.
10.1. Update Stream Server: Denial-of-Service Attacks
Allowing persistent update stream connections enables a new class of
Denial-of-Service attacks.
For the update stream server, an ALTO client might create an
unreasonable number of update stream connections or add an
unreasonable number of substream-ids to one update stream.
To avoid these attacks on the update stream server, the server SHOULD
choose to limit the number of active streams and reject new requests
when that threshold is reached. An update stream server SHOULD also
choose to limit the number of active substream-ids on any given
stream or limit the total number of substream-ids used over the
lifetime of a stream and reject any stream control request that would
exceed those limits. In these cases, the update stream server SHOULD
return the HTTP status "503 Service Unavailable".
It is important to note that the preceding approaches are not the
only possibilities. For example, it may be possible for the update
stream server to use somewhat more clever logic involving IP
reputation, rate-limiting, and compartmentalization of the overall
threshold into smaller thresholds that apply to subsets of potential
clients.
While the preceding techniques prevent update stream DoS attacks from
disrupting an update stream server's other services, it does make it
easier for a DoS attack to disrupt the update stream service.
Therefore, an update stream server MAY prefer to restrict update
stream services to authorized clients, as discussed in Section 15 of
[RFC7285].
Alternatively, an update stream server MAY return the HTTP status
"307 Temporary Redirect" to redirect the client to another ALTO
server that can better handle a large number of update streams.
10.2. 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.
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.
10.3. Stream Control: Spoofed Control Requests and Information
Breakdown
An outside party that can read the update stream response or that can
observe stream control requests can obtain the control URI and use
that to send a fraudulent "remove" requests, thus disabling updates
for the valid ALTO client. This can be avoided by encrypting the
update stream and stream control requests (see Section 15 of
[RFC7285]). Also, the update stream server echoes the "remove"
requests on the update stream, so the valid ALTO client can detect
unauthorized requests.
In general, as the architecture allows the possibility for the update
stream server and the stream control server to be different entities,
the additional risks should be evaluated and remedied. For example,
the private communication path between the servers may be attacked,
resulting in a risk of communications breakdown between them, as well
as invalid or spoofed messages claiming to be on that private
communications path. Proper security mechanisms, including
confidentiality, authenticity, and integrity mechanisms, should be
considered.
11. Requirements on Future ALTO Services to Use This Design
Although this design is quite flexible, it has underlying
requirements.
The key requirements are that (1) each data update message is for a
single resource and (2) an incremental change can be applied only to
a resource that is a single JSON object, as both JSON merge patch and
JSON patch can apply only to a single JSON object. Hence, if a
future ALTO resource can contain multiple objects, then either each
individual object also has a resource-id or an extension to this
design is made.
At the low-level encoding level, new line in SSE has its own
semantics. Hence, this design requires that resource encoding does
not include new lines that can be confused with SSE encoding. In
particular, the data update message MUST NOT include "event: " or
"data: " at a new line as part of data message.
If an update stream provides updates to a Filtered Cost Map that
allows constraint tests, the requirements for such services are
stated in Section 9.3.
12. IANA Considerations
This document defines two new media types: "application/alto-
updatestreamparams+json", as described in Section 6.5, and
"application/alto-updatestreamcontrol+json", as described in
Section 5.3. All other media types used in this document have
already been registered, either for ALTO, JSON merge patch, or JSON
patch.
12.1. application/alto-updatestreamparams+json Media Type
Type name: application
Subtype name: alto-updatestreamparams+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: Security considerations relating to the
generation and consumption of ALTO Protocol messages are discussed
in Section 10 of RFC 8895 and Section 15 of [RFC7285].
Interoperability considerations: RFC 8895 specifies format of
conforming messages and the interpretation thereof.
Published specification: Section 6.5 of RFC 8895.
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): RFC 8895 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 & email address to contact for further information: See
Authors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: Internet Engineering Task Force
(mailto:iesg@ietf.org).
12.2. application/alto-updatestreamcontrol+json Media Type
Type name: application
Subtype name: alto-updatestreamcontrol+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: Security considerations relating to the
generation and consumption of ALTO Protocol messages are discussed
in Section 10 of RFC 8895 and Section 15 of [RFC7285].
Interoperability considerations: RFC 8895 specifies format of
conforming messages and the interpretation thereof.
Published specification: Section 5.3 of RFC 8895.
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): RFC 8895 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 & email address to contact for further information: See
Authors' Addresses section.
Intended usage: COMMON
Restrictions on usage: N/A
Author: See Authors' Addresses section.
Change controller: Internet Engineering Task Force
(mailto:iesg@ietf.org).
13. Appendix: Design Decision: Not Allowing Stream Restart
If an update stream is closed accidentally, when the ALTO client
reconnects, the update stream server must resend the full maps. This
is clearly inefficient. To avoid that inefficiency, the SSE
specification allows an update stream server to assign an id to each
event. When an ALTO client reconnects, the ALTO client can present
the id of the last successfully received event, and the update stream
server restarts with the next event.
However, that mechanism adds additional complexity. The update
stream server must save SSE messages in a buffer in case ALTO clients
reconnect. But that mechanism will never be perfect: If the ALTO
client waits too long to reconnect or if the ALTO client sends an
invalid ID, then the update stream server will have to resend the
complete maps anyway.
Furthermore, this is unlikely to be a problem in practice. ALTO
clients who want continuous updates for large resources, such as full
network and cost maps, are likely to be things like P2P trackers.
These ALTO clients will be well connected to the network; they will
rarely drop connections.
Mobile devices certainly can and do drop connections and will have to
reconnect. But mobile devices will not need continuous updates for
multi-megabyte cost maps. If mobile devices need continuous updates
at all, they will need them for small queries, such as the costs from
a small set of media servers from which the device can stream the
currently playing movie. If the mobile device drops the connection
and reestablishes the update stream, the update stream server will
have to retransmit only a small amount of redundant data.
In short, using event ids to avoid resending the full map adds a
considerable amount of complexity to avoid a situation that is very
rare. The complexity is not worth the benefit.
The update stream service does allow the ALTO client to specify the
tag of the last received version of any tagged resource, and if that
is still current, the update stream server need not retransmit the
full resource. Hence, ALTO clients can use this to avoid
retransmitting full network maps. Cost maps are not tagged, so this
will not work for them. Of course, the ALTO protocol could be
extended by adding version tags to cost maps, which would solve the
retransmission-on-reconnect problem. However, adding tags to cost
maps might add a new set of complications.
14. References
14.1. 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/info/rfc2119>.
[RFC2387] Levinson, E., "The MIME Multipart/Related Content-type",
RFC 2387, DOI 10.17487/RFC2387, August 1998,
<https://www.rfc-editor.org/info/rfc2387>.
[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/info/rfc3986>.
[RFC6838] Freed, N., Klensin, J., and T. Hansen, "Media Type
Specifications and Registration Procedures", BCP 13,
RFC 6838, DOI 10.17487/RFC6838, January 2013,
<https://www.rfc-editor.org/info/rfc6838>.
[RFC6902] Bryan, P., Ed. and M. Nottingham, Ed., "JavaScript Object
Notation (JSON) Patch", RFC 6902, DOI 10.17487/RFC6902,
April 2013, <https://www.rfc-editor.org/info/rfc6902>.
[RFC7285] Alimi, R., Ed., Penno, R., Ed., Yang, Y., Ed., Kiesel, S.,
Previdi, S., Roome, W., Shalunov, S., and R. Woundy,
"Application-Layer Traffic Optimization (ALTO) Protocol",
RFC 7285, DOI 10.17487/RFC7285, September 2014,
<https://www.rfc-editor.org/info/rfc7285>.
[RFC7396] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7396,
DOI 10.17487/RFC7396, October 2014,
<https://www.rfc-editor.org/info/rfc7396>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[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/info/rfc8259>.
[SSE] Hickson, I., "Server-Sent Events", W3C Recommendation,
February 2015, <https://www.w3.org/TR/eventsource/>.
14.2. Informative References
[RFC4960] Stewart, R., Ed., "Stream Control Transmission Protocol",
RFC 4960, DOI 10.17487/RFC4960, September 2007,
<https://www.rfc-editor.org/info/rfc4960>.
[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP",
RFC 5789, DOI 10.17487/RFC5789, March 2010,
<https://www.rfc-editor.org/info/rfc5789>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<https://www.rfc-editor.org/info/rfc7231>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
Acknowledgments
Thank you to Dawn Chen (Tongji University), Shawn Lin (Tongji
University), and Xiao Shi (Yale University) for their contributions
to an earlier version of this document.
Contributors
Sections 2, 5.1, 5.2, and 8.5 of this document are based on
contributions from Jingxuan Jensen Zhang, and he is considered an
author.
Authors' Addresses
Wendy Roome
Nokia Bell Labs (Retired)
124 Burlington Rd
Murray Hill, NJ 07974
United States of America
Phone: +1-908-464-6975
Email: wendy@wdroome.com
Y. Richard Yang
Yale University
51 Prospect St
New Haven, CT
United States of America
Email: yry@cs.yale.edu