ALTO WG W. Roome
Internet-Draft Alcatel-Lucent
Intended status: Standards Track Y. Yang
Expires: November 29, 2015 Tongji/Yale University
May 28, 2015
ALTO Incremental Updates Using Server-Sent Events (SSE)
draft-ietf-alto-incr-update-sse-00
Abstract
The Application-Layer Traffic Optimization (ALTO) [RFC7285] protocol
provides network related information to client applications so that
clients may make informed decisions. To that end, an ALTO Server
provides Network and Cost Maps. Using those maps, an ALTO Client can
determine the costs between endpoints.
However, the ALTO protocol does not define a mechanism to allow an
ALTO client to obtain updates to those maps, other than by
periodically re-fetching them. Because the maps may be large
(potentially tens of megabytes), and because only parts of the maps
may change frequently (especially Cost Maps), that can be extremely
inefficient.
Therefore this document presents a mechanism to allow an ALTO Server
to provide updates to ALTO Clients. Updates can be both immediate,
in that the server can send updates as soon as they are available,
and incremental, in that if only a small section of a map changes,
the server can send just the changes.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on November 29, 2015.
Copyright Notice
Copyright (c) 2015 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
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Overview of Approach . . . . . . . . . . . . . . . . . . . . 4
3. Update Events . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Overview of SSEs . . . . . . . . . . . . . . . . . . . . 5
3.2. ALTO Update Events . . . . . . . . . . . . . . . . . . . 6
3.3. Keep-Alive Messages . . . . . . . . . . . . . . . . . . . 7
4. Incremental Update Message Format . . . . . . . . . . . . . . 7
4.1. Overview of JSON Merge Patch . . . . . . . . . . . . . . 7
4.2. JSON Merge Patch Applied to Network Map Messages . . . . 8
4.3. JSON Merge Patch Applied to Cost Map Messages . . . . . . 10
5. Update Stream Service . . . . . . . . . . . . . . . . . . . . 11
5.1. Media Type . . . . . . . . . . . . . . . . . . . . . . . 11
5.2. HTTP Method . . . . . . . . . . . . . . . . . . . . . . . 11
5.3. Accept Input Parameters . . . . . . . . . . . . . . . . . 11
5.4. Capabilities . . . . . . . . . . . . . . . . . . . . . . 13
5.5. Uses . . . . . . . . . . . . . . . . . . . . . . . . . . 13
5.6. Response . . . . . . . . . . . . . . . . . . . . . . . . 14
5.6.1. Event Sequence Requirements . . . . . . . . . . . . . 14
5.6.2. Cross-Stream Consistency Requirements . . . . . . . . 14
5.7. Considerations For Updates To Filtered Cost Maps . . . . 15
5.8. Example: Simple Network and Cost Map Updates . . . . . . 15
5.9. Example: Advanced Network and Cost Map Updates . . . . . 16
5.10. Example: Endpoint Property Updates . . . . . . . . . . . 17
6. Client Actions When Receiving Update Messages . . . . . . . . 19
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7. IRD Example . . . . . . . . . . . . . . . . . . . . . . . . . 20
8. Design Decisions and Discussions . . . . . . . . . . . . . . 21
8.1. HTTP2 Server-Push . . . . . . . . . . . . . . . . . . . . 22
8.2. Not Allowing Stream Restart . . . . . . . . . . . . . . . 22
8.3. Is Incremental Update Useful for Network Maps? . . . . . 23
8.4. Other Incremental Update Message Types . . . . . . . . . 24
9. Security Considerations . . . . . . . . . . . . . . . . . . . 24
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 26
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 26
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
The Application-Layer Traffic Optimization (ALTO) [RFC7285] protocol
provides network related information to client applications so that
clients may make informed decisions. To that end, an ALTO Server
provides Network and Cost Maps, where a Network Map partitions the
set of endpoints into a manageable number of Provider-Defined
Identifiers (PIDs), and a Cost Map provides directed costs between
PIDs. Given Network and Cost Maps, an ALTO Client can obtain costs
between endpoints by using the Network Map to get the PID for each
endpoint, and then using the Cost Map to get the costs between those
PIDs.
However, the ALTO protocol does not define a mechanism to allow a
client to obtain updates to those maps, other than by periodically
re-fetching them. Because the maps may be large (potentially tens of
megabytes), and because parts of the maps may change frequently
(especially Cost Maps), that can be extremely inefficient.
Therefore this document presents a mechanism to allow an ALTO Server
to provide incremental updates to ALTO Clients. Updates can be both
immediate, in that the server can send updates as soon as they are
available, and incremental, in that if only a small section of a map
changes, the server can send just the changes.
While primarily intended to provide updates to Network and Cost Maps,
the mechanism defined in this document can provide updates to any
ALTO resource, including POST-mode services such as Endpoint Property
and Endpoint Cost Services, as well as new ALTO services to be
defined by future extensions.
The rest of this document is organized as follows. Section 2 gives
an overview of the incremental update approach, which is based on
Server-Sent Events (SSEs). Section 3 defines the update events, and
Section 4 defines the format of the incremental update messages.
Section 5 defines the new Update Stream Service, Section 6 describes
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how a client should handle incoming updates, and Section 7 gives an
example of the Information Resource Directory (IRD) for an ALTO
Server that offers a comprehensive set of Update Services. Section 8
discusses the design decisions behind this update mechanism. The
remaining sections review the security and IANA considerations.
2. Overview of Approach
This section presents a non-normative overview of the update
mechanism to be defined in this document.
An ALTO Server can offer one or more Update Stream resources, where
each Update Stream resource (or Update Stream for short) is a POST-
mode service that returns a continuous sequence of update messages
for one or more ALTO resources. An Update Stream can provide updates
to both GET-mode resources, such as Network and Cost Maps, and POST-
mode resources, such as Endpoint Property Services.
Each update message updates one resource, and is sent as a Server-
Sent Event (SSE), as defined by [SSE]. An update message is either a
full replacement or else an incremental change. Full replacement
updates use the JSON message formats defined by the ALTO protocol.
Incremental updates use JSON Merge Patch ([RFC7386]) to describe the
changes to the resource. The ALTO Server decides when to send update
messages, and whether to send full replacements or incremental
updates. These decisions can vary from resource to resource and from
update to update.
An ALTO Server may offer any number of Update Stream resources, for
any subset of the server's resources. An ALTO Server's Information
Resource Directory (IRD) defines the Update Stream resources, and
declares the set of resources for which each Update Stream provides
updates. The server selects the resource set for each stream,
although the set should be closed under the ALTO resource dependency
relationship (i.e., the "uses" relationship). Thus the Update Stream
for a Cost Map should also provide updates for the Network Map upon
which that Cost Map depends.
When an ALTO Client requests an Update Stream resource, the client
establishes a new persistent connection to the server. The
connection remains open, and the server continues to send updates,
until either the client or the server closes it. A client may
request any number of Update Streams simultaneously. Because each
stream consumes resources on the server, a server may limit the
number of open Update Streams, may close inactive streams, may
provide Update Streams via other processors, or may require client
authorization/authentication.
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3. Update Events
3.1. Overview of SSEs
The following is a non-normative summary of Server-Sent Events
(SSEs). See [SSE] for the normative definition.
Server-Sent Events enable a server to send new data to a client by
"server-push". The client establishes an HTTP ([RFC2616]) connection
to the server, and keeps the connection open. The server continually
sends messages. Messages are delimited by two new-lines (this is a
slight simplification; see [SSE] for details). Each line is of the
form "field-name: string value". 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" or "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. Note that
the server may "chunk" the returned data (see [RFC2616]); for
simplicity, we have omitted those details.
GET /stream HTTP/1.1
Host: example.com
Accept: text/event-stream
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: good bye
Figure 1: A Sample SSE stream.
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3.2. ALTO Update Events
In the events defined in this document, the data field is a JSON
object. That object is either a complete specification of an ALTO
resource, or else a JSON Merge Patch object describing changes to
apply to an ALTO resource. We will refer to these as full-
replacement and Merge Patch messages, respectively. The data objects
in full-replacement messages are defined by [RFC7285]; examples are
Network and Cost Map messages. The data objects in Merge Patch
messages are defined by [RFC7386].
To indicate whether the data is a full-replacement or a Merge Patch
object, in our update messages, the SSE "event" field has two sub-
fields: the resource-id of an ALTO resource, and the media-type of
the JSON message in the data field. The media-types for full-
replacement messages are defined by [RFC7285], and include
"application/alto-networkmap+json" for Network Map messages and
"application/alto-costmap+json" for Cost Map messages. The media-
type for a JSON Merge Patch message is "application/merge-
patch+json", and is defined by [RFC7386]. An extension document may
introduce other media-types to indicate new types of update messages.
Specifically, the two sub-fields of the event field are encoded as:
resource-id , media-type
Note that a comma (character code 0x2c) is allowed in ALTO resource-
ids, but not in media-type names. Hence when parsing the SSE event
field to obtain the two sub-fields, a client MUST split the string on
the last comma.
This document does not use the SSE "id" field.
Figure 2 shows some examples of ALTO update events:
event: my-network-map,application/alto-networkmap+json
data: { ... full Network Map message ... }
event: my-routingcost-map,application/alto-costmap+json
data: { ... full Cost Map message ... }
event: my-routingcost-map,application/merge-patch+json
data: { ... Merge Patch update for the Cost Map ... }
Figure 2: Examples of ALTO update events.
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3.3. Keep-Alive Messages
An SSE event with an empty "event" field is a keep-alive message. An
ALTO Server MAY send keep-alive messages as needed. An ALTO Client
MUST ignore any keep-alive messages.
4. Incremental Update Message Format
4.1. Overview of JSON Merge Patch
The following is a non-normative summary of JSON Merge Patch. See
[RFC7386] for the normative definition.
JSON Merge Patch is intended to allow applications to update server
resources via the HTTP PATCH method [RFC5789]. This document adopts
the JSON Merge Patch message format to encode the changes, but uses a
different transport mechanism.
Informally, a Merge Patch object is a JSON data structure that
defines how to transform one JSON value into another. Merge Patch
treats the two JSON values as trees of nested JSON Objects
(dictionaries of name-value pairs), where the leaves are values other
than JSON Objects (e.g., JSON Arrays, Strings, Numbers, etc.), 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 Merge Patch tree 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 Merge Patch tree has a leaf with a JSON "null"
value. The Merge Patch tree does not have an entry for any leaf that
has the same value in both versions.
As a result, if all leaf values are simple scalars, JSON Merge Patch
is a very efficient representation of the change. It is less
efficient when leaf values are arrays, because JSON Merge Patch
replaces arrays in their entirety, even if only one entry changes.
Formally, the process of applying a Merge Patch is defined by the
following recursive algorithm, as specified in [RFC7386]:
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define MergePatch(Target, Patch) {
if Patch is an Object {
if Target is not an Object {
Target = {} # Ignore the contents and
# set it to an empty Object
}
for each Name/Value pair in Patch {
if Value is null {
if Name exists in Target {
remove the Name/Value pair from Target
}
} else {
Target[Name] = MergePatch(Target[Name], Value)
}
}
return Target
} else {
return Patch
}
}
Note that null as the value of a name/value pair will delete the
element with "name" in the original JSON value.
4.2. JSON Merge Patch Applied to Network Map Messages
Section 11.2.1.6 of [RFC7285] defines the format of a Network Map
message. Here is a simple example:
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{
"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" ]
}
}
}
When applied to that message, the following Merge Patch update
message adds the ipv6 prefix "2000::/3" to "PID1", deletes "PID2",
and assigns a new "tag" to the Network Map:
{
"meta" : {
"vtag" : {
"tag" : "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
},
"network-map": {
"PID1" : {
"ipv6" : [ "2000::/3" ]
},
"PID2" : null
}
}
Here is the updated Network Map:
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{
"meta" : {
"vtag": {
"resource-id" : "my-network-map",
"tag" : "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
},
"network-map" : {
"PID1" : {
"ipv4" : [ "192.0.2.0/24", "198.51.100.0/25" ],
"ipv6" : [ "2000::/3" ]
},
"PID3" : {
"ipv4" : [ "0.0.0.0/0" ],
"ipv6" : [ "::/0" ]
}
}
}
4.3. JSON Merge Patch Applied to Cost Map Messages
Section 11.2.3.6 of [RFC7285] defines the format of a Cost Map
message. Here is a simple example:
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-network-map",
"tag": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
],
"cost-type" : {
"cost-mode" : "numerical",
"cost-metric": "routingcost"
}
},
"cost-map" : {
"PID1": { "PID1": 1, "PID2": 5, "PID3": 10 },
"PID2": { "PID1": 5, "PID2": 1, "PID3": 15 },
"PID3": { "PID1": 20, "PID2": 15 }
}
}
The following Merge Patch message updates the example cost map so
that PID1->PID2 is 9 instead of 5, PID3->PID1 is no longer available,
and PID3->PID3 is now defined as 1:
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{
"cost-map" : {
"PID1" : { "PID2" : 9 },
"PID3" : { "PID1" : null, "PID3" : 1 }
}
}
Here is the updated cost map:
{
"meta" : {
"dependent-vtags" : [
{"resource-id": "my-network-map",
"tag": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
],
"cost-type" : {
"cost-mode" : "numerical",
"cost-metric": "routingcost"
}
},
"cost-map" : {
"PID1": { "PID1": 1, "PID2": 9, "PID3": 10 },
"PID2": { "PID1": 5, "PID2": 1, "PID3": 15 },
"PID3": { "PID2": 15, "PID3": 1 }
}
}
5. Update Stream Service
An Update Stream Service returns a stream of SSE messages, as defined
in Section 3.2.
5.1. Media Type
The media type of an ALTO Update Stream resource is "text/event-
stream".
5.2. HTTP Method
An ALTO Update Stream resource is requested using the HTTP POST
method.
5.3. Accept Input Parameters
An ALTO Client supplies filtering parameters by specifying media type
"application/alto-updatestreamparams+json" with an HTTP POST body
containing a JSON object of type UpdateStreamReq, where:
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object-map {
ResourceID -> ResourceUpdateReq;
} UpdateStreamReq;
object {
[String tag;]
[Boolean incremental-updates;]
[Object input;]
} ResourceUpdateReq;
The keys are the resource-ids of the resources for which the client
wants updates. Each resource-id MUST be one of those in the Update
Streams's "uses" list (see Section 5.5). The ResourceUpdateReq
values give additional parameters for the updates for each resource.
If any resource-id is invalid, or is not associated with this Update
Stream, the server MUST return an E_INVALID_FIELD_VALUE error
response (see Section 8.5.2 of [RFC7285]), and MUST close the stream
without sending any update events.
If the client wants to receive updates for a resource, but does not
need to set any of the sub-fields described below, the client MUST
provide an entry for that resource-id whose value is an empty JSON
Object (e.g., "{}").
If the "incremental-updates" field for a resource-id is "true", the
server MAY send incremental update events for this resource-id
(assuming the server supports incremental updates for that resource;
see Section 5.4). If the "incremental-updates" field is "false", the
ALTO Server MUST NOT send incremental update events for that
resource. In this case, whenever a change occurs, the server MUST
send a full-replacement update instead of an incremental update. The
ALTO Server SHOULD send the full-replacement message soon after the
change, although the server MAY wait until more changes are
available. Thus an ALTO Client which declines to accept incremental
updates may not get updates as quickly as a client which does.
The default for "incremental-updates" is "true", so to suppress
incremental updates, the client MUST explicitly set "incremental-
updates" to "false". Note that the client cannot suppress full-
replacement update events.
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 if the
client has previously retrieved a version of that resource from the
server, the client MAY set the "tag" field to "tag" part of the
resource's version tag. If that version is still current, the ALTO
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Server SHOULD omit sending a full replacement update at the start of
the stream (see Section 5.6.1).
If the resource-id is a POST-mode service which requires input, the
client MUST set the "input" field to a JSON Object with the
parameters that resource expects. If the "input" field is missing or
invalid, the ALTO Server MUST return the same error response that
that resource would return for missing or invalid input (see
[RFC7285]). In this case, the server MUST close the Update Stream
without sending any update events. If the inputs for several POST-
mode resources are missing or invalid, the server MUST pick one error
response and return it.
5.4. Capabilities
The capabilities are defined by an object of type
UpdateStreamCapabilities:
object {
IncrementalUpdateMediaTypes incremental-update-media-types;
} UpdateStreamCapabilities;
object-map {
ResourceID -> String;
} IncrementalUpdateMediaTypes;
If this Update Stream can provide incremental update events for a
resource, the "incremental-update-media-types" field has an entry for
that resource-id, and the value is the media-type of the incremental
update message. Normally this will be "application/merge-
patch+json", because, as described in Section 3.2, JSON Merge Patch
is the only incremental update event type defined by this document.
However future extensions may define other types of incremental
updates.
5.5. Uses
The "uses" attribute MUST be an array with the resource-ids of every
resource for which this stream can provide updates.
This set can include any subset of the resources proved by the ALTO
Server, and may include resources defined in linked IRDs. However,
it is RECOMMENDED that the ALTO Server select 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
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Map SHOULD also provide updates for the Network Map upon which that
Cost Map depends.
5.6. Response
The response is a stream of SSE update events. Section 3.2 defines
the events, and [SSE] defines how they are encoded into a stream.
There are additional requirements between events in the stream, as
described below.
5.6.1. Event Sequence Requirements
o As soon as possible after the client initiates the connection, the
ALTO Server MUST send a full-replacement update event for each
resource-id requested by the client. The only exception is for a
GET-mode resource with a version tag: the server MAY omit the
initial full-replacement event for that resource if the "tag"
field the client provided for that resource-id matches the tag of
the server's current version.
o If this stream provides updates for resource-ids R0 and R1, and if
R1 depends on R0, then the ALTO Server MUST send the update for R0
before sending the related update for R1. For example, suppose a
stream provides updates to a Network Map and its dependent Cost
Maps. When the Network Map changes, the ALTO Server MUST send the
Network Map update before sending the Cost Map updates.
o If this stream provides updates for resource-ids R0 and R1, and if
R1 depends on R0, then the ALTO Server SHOULD send an update for
R1 as soon as possible after sending the update for R0. For
example, when a Network Map changes, the ALTO Server SHOULD send
update events for the dependent Cost Maps as soon as possible
after the update event for the Network Map.
5.6.2. Cross-Stream Consistency Requirements
If several distinct Update Stream resources offer updates for the
same resource-id, the ALTO Server MUST send the same update data on
all of those Update Streams. Similarly, the server MUST send the
same updates to all clients connected to the that stream. However,
the server MAY pack data items into different Merge Patch events, as
long as the net result of applying those updates is the same.
For example, suppose two different clients open the same Cost Map
Update Stream, and suppose the ALTO 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
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server MAY send a single Merge Patch event (with all three cost
points) to one client, while sending three separate Merge Patch
events (with one cost point per event) to the other client.
5.7. Considerations For Updates To Filtered Cost Maps
If an Update Stream provides updates to a Filtered Cost Map which
allows constraint tests, then a client MAY request updates to a
Filtered Cost Map request with a constraint test. In this case, when
a cost changes, the server MUST send an update if the new value
satisfies the test. If the new value does not, whether the server
sends an update depends on whether the previous value satisfied the
test. If it did not, the server SHOULD NOT send an update to the
client. But if the previous value did, then the server MUST send an
update with a "null" value, to inform the client that this cost no
longer satisfies the criteria.
An ALTO Server can avoid such issues by offering Update Streams only
for Filtered Cost Maps which do not allow constraint tests.
5.8. Example: Simple Network and Cost Map Updates
Here is an example of a client's request and the server's immediate
response, using the Update Stream resource "my-costs-update-stream"
defined in the IRD in Section 7. The client requests updates for the
Network Map and "routingcost" Cost Map, but not for the "hopcount"
Cost Map. Because the client does not provide a "tag" for the
Network Map, the server must send a full update for the Network Map
as well as for the Cost Map. The client does not set "incremental-
updates" to "false", so it defaults to "true". Thus server will send
Merge Patch updates for the Cost Map, but not for the Network Map,
because this Update Stream resource does not provide incremental
updates for the Network Map.
Note that the server may "chunk" the returned data (see [RFC2616]);
for simplicity, we have omitted those details.
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POST /updates/costmaps HTTP/1.1
Host: alto.example.com
Accept: text/event-stream,application/alto-error+json
Content-Type: application/alto-updatestreamparams+json
Content-Length: ###
{ "my-network-map": {},
"my-routingcost-map": {}
}
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: my-network-map,application/alto-networkmap+json
data: { ... full Network Map message ... }
event: my-routingcost-map,application/alto-costmap+json
data: { ... full routinccost Cost Map message ... }
After sending those two events immediately, the ALTO Server will send
additional events as the maps change. For example, the following
represents a small change to the Cost Map:
event: my-routingcost-map,application/merge-patch+json
data: {"cost-map": {"PID1" : {"PID2" : 9}}}
If a major change to the Network Map occurs, the ALTO Server MAY
choose to send full Network and Cost Map messages rather than Merge
Patch messages:
event: my-network-map,application/alto-networkmap+json
data: { ... full Network Map message ... }
event: my-routingcost-map,application/alto-costmap+json
data: { ... full Cost Map message ... }
5.9. Example: Advanced Network and Cost Map Updates
This example is similar to the previous one, except that the client
requests updates for the "hopcount" as well as "routingcost" Cost
Map, and provides the current version tag of the Network Map, so the
server does not send the full Network Map update event at the
beginning of the stream. After that, the ALTO Server sends updates
for the Network Map and Cost Maps as they become available:
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POST /updates/costmaps HTTP/1.1
Host: alto.example.com
Accept: text/event-stream,application/alto-error+json
Content-Type: application/alto-updatestreamparams+json
Content-Length: ###
{ "my-network-map": {
"tag": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
},
"my-routingcost-map": {}
"my-hopcount-map": {}
}
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: my-routingcost-map,application/alto-costmap+json
data: { ... full routingcost Cost Map message ... }
event: my-hopcount-map,application/alto-costmap+json
data: { ... full hopcount Cost Map message ... }
(pause)
event: my-routingcost-map,application/merge-patch+json
data: {"cost-map": {"PID2" : {"PID3" : 31}}}
event: my-hopcount-map,application/merge-patch+json
data: {"cost-map": {"PID2" : {"PID3" : 4}}}
5.10. Example: Endpoint Property Updates
As another example, here is how a client can request updates for the
property "priv:ietf-bandwidth" for a set of endpoints. The ALTO
Server immediately sends a full-replacement message with the property
values for all endpoints. After that, the server sends update events
for the individual endpoints as their property values change.
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POST /updates/properties HTTP/1.1
Host: alto.example.com
Accept: text/event-stream
Content-Type: application/alto-updatestreamparams+json
Content-Length: ###
{ "my-properties": {
"input": {
"properties" : [ "priv:ietf-bandwidth" ],
"endpoints" : [
"ipv4:1.0.0.1",
"ipv4:1.0.0.2",
"ipv4:1.0.0.3"
]
}
}
}
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: my-properties,application/alto-endpointprops+json
data: { "endpoint-properties": {
data: "ipv4:1.0.0.1" : { "priv:ietf-bandwidth": "13" },
data: "ipv4:1.0.0.2" : { "priv:ietf-bandwidth": "42" },
data: "ipv4:1.0.0.3" : { "priv:ietf-bandwidth": "27" }
data: } }
(pause)
event: my-properties,application/merge-patch+json
data: { "endpoint-properties":
data: {"ipv4:1.0.0.1" : {"priv:ietf-bandwidth": "3"}}
data: }
(pause)
event: my-properties,application/merge-patch+json
data: { "endpoint-properties":
data: {"ipv4:1.0.0.3" : {"priv:ietf-bandwidth": "38"}}
data: }
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6. Client Actions When Receiving Update Messages
In general, when a client receives a full-replacement update message
for a resource, the client should replace the current version with
the new version. When a client receives a Merge Patch update message
for a resource, the 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 a client can do that. We will 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 ALTO Server MUST send the
Network Map update message before sending the updates for the
dependent Cost Maps (see Section 5.6.1).
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 client receives the update
messages for all dependent Cost Maps. The client then applies all
Network and Cost Map updates atomically.
Alternatively, the client MAY update the Network Map immediately. In
this case, the client MUST mark each dependent Cost Map as
temporarily invalid, and MUST NOT use that map until the client
receives a Cost Map update message with the new Network Map version
tag. Note that the client MUST NOT delete the Cost Maps, because the
server may send Merge Patch update messages.
The ALTO Server SHOULD send updates for dependent resources in a
timely fashion. However, if the client does not receive the expected
updates, the client MUST close the Update Stream connection, discard
the dependent resources, and reestablish the Update Stream. The
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 ALTO Server
will not re-send that resource.
Although not as efficient as possible, this recovery method is simple
and reliable.
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7. IRD Example
Here is an example of an IRD that offers two Update Stream services.
The first provides updates for the Network Map, the "routingcost" and
"hopcount" Cost Maps, and a Filtered Cost Map resource. The second
Update Stream provides updates to the Endpoint Properties service.
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 5.7, the Update Stream provides
updates for the second, but not the first.
"my-network-map": {
"uri": "http://alto.example.com/networkmap",
"media-type": "application/alto-networkmap+json",
},
"my-routingcost-map": {
"uri": "http://alto.example.com/costmap/routingcost",
"media-type": "application/alto-costmap+json",
"uses": ["my-networkmap"],
"capabilities": {
"cost-type-names": ["num-routingcost"]
}
},
"my-hopcount-map": {
"uri": "http://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": "http://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": "http://alto.example.com/costmap/filtered/simple",
"media-type": "application/alto-costmap+json",
"accepts": "application/alto-costmapfilter+json",
"uses": ["my-networkmap"],
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"capabilities": {
"cost-type-names": ["num-routingcost", "num-hopcount"],
"cost-constraints": false
}
},
"my-properties": {
"uri": "http://alto.example.com/properties",
"media-type": "application/alto-endpointprops+json",
"accepts": "application/alto-endpointpropparams+json",
"capabilities": {
"prop-types": ["priv:ietf-bandwidth"]
}
},
"my-costs-update-stream": {
"uri": "http://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-update-media-types": {
"my-routingcost-map": application/merge-patch+json",
"my-hopcount-map": "application/merge-patch+json"
}
}
},
"my-properties-update-stream": {
"uri": "http://alto.example.com/updates/properties",
"media-type": "text/event-stream",
"uses": [ "my-properties" ],
"accepts": "application/alto-updatestreamparams+json",
"capabilities": {
"incremental-update-media-types": {
"my-properties": "application/merge-patch+json"
}
}
}
8. Design Decisions and Discussions
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8.1. HTTP2 Server-Push
An alternative would be to use HTTP 2 Server-Push [RFC7540], instead
of SSE over HTTP 1.1, as the transport mechanism for update messages.
That would have several advantages: HTTP 2 Server-Push is designed to
allow a server to send asynchronous messages to the client, and HTTP
library packages should make it simple for servers to send those
asynchronous messages, and for clients to receive them.
The disadvantage is HTTP 2 is a new protocol, and it is considerably
more complicated than HTTP 1.1. While there is every reason to
expect that HTTP library packages will eventually support HTTP 2, we
do not want to delay deployment of an ALTO incremental update
mechanism until that time.
Hence we have chosen to base ALTO updates on HTTP 1.1 and SSE. When
HTTP 2 support becomes ubiquitous, a future extension of this
document may define updates via HTTP 2 Server-Push.
8.2. Not Allowing Stream Restart
If an update stream is closed accidentally, when the client
reconnects, the server must resend the full maps. This is clearly
inefficient. To avoid that inefficiency, the SSE specification
allows a server to assign an id to each event. When a client
reconnects, the client can present the id of the last successfully
received event, and the server restarts with the next event.
However, that mechanism adds additional complexity. The server must
save SSE messages in a buffer, in case clients reconnect. But that
mechanism will never be perfect: if the client waits too long to
reconnect, or if the client sends an invalid id, then the server will
have to resend the complete maps anyway.
Furthermore, this is unlikely to be a problem in practice. Clients
who want continuous updates for large resources, such as full Network
and Cost Maps, are likely to be things like P2P trackers. These
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 ALTO Server will
have to retransmit only a small amount of redundant data.
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In short, using event ids to avoid resending the full map adds a
considerable amount of complexity to avoid a situation which we
expect is very rare. We believe that complexity is not worth the
benefit.
The Update Stream service does allow the client to specify the tag of
the last received version of any tagged resource, and if that is
still current, the server need not retransmit the full resource.
Hence 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.
8.3. Is Incremental Update Useful for Network Maps?
It is not clear whether incremental updates (that is, Merge Patch
updates) are useful for Network Maps. For minor changes, such as
moving a prefix from one PID to another, they can be useful. But
more involved changes to the Network Map are likely to be "flag
days": they represent a completely new Network Map, rather than a
simple, well-defined change.
At this point we do not have sufficient experience with ALTO
deployments to know how frequently Network Maps will change, or how
extensive those changes will be. For example, suppose a link goes
down and the network uses an alternative route. This is a frequent
occurrence. If an ALTO Server models that by moving prefixes from
one PID to another, then Network Maps will change frequently.
However, an ALTO Server might model that as a change in costs between
PIDs, rather than a change in the PID definitions. If a server takes
that approach, simple routing changes will affect Cost Maps, but not
Network Maps.
So while we allow a server to use Merge Patch on Network Maps, we do
not require the server to do so. Each server may decide on its own
whether to use Merge Patch for Network Maps.
This is not to say that Network Map updates are not useful. Clearly
Network Maps will change, and update events are necessary to inform
clients of the new map. Further, there maybe another incremental
update encoding that is better suited for updating Networks Maps; see
discussions in the next section.
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8.4. Other Incremental Update Message Types
Other JSON-based incremental update formats have been defined, in
particular JSON Patch ([RFC6902]). The update events defined in this
document have the media-type of the update data. JSON Patch has its
own media type ("application/json-patch+json"), so this update
mechanism could easily be extended to allow servers to use JSON Patch
for incremental updates.
However, we think that JSON Merge Patch is clearly superior to JSON
Patch for describing incremental updates 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; we
see no advantage to allowing a server to use JSON Patch for those
resources.
The case is not as clear for incremental updates to Network Maps.
For example, suppose a prefix moves from one PID to another. JSON
Patch could encode that as a simple insertion and deletion, while
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
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, clients would have to retain additional, otherwise
unnecessary, data.
However, it is entirely possible that JSON Patch will be appropriate
for describing incremental updates to new, as yet undefined ALTO
resources. In this case, the extensions defining those new resources
can use the update framework defined in this document, but recommend
using JSON Patch, or some other method, to describe the incremental
changes.
9. Security Considerations
Allowing persistent update stream connections enables a new class of
Denial-of-Service attacks. An ALTO Server MAY choose to limit the
number of active streams, and reject new requests when that threshold
is reached. In this case the server should return the HTTP status
"503 Service Unavailable".
Alternatively an ALTO Server MAY return the HTTP status "307
Temporary Redirect" to redirect the client to another ALTO Server
which can better handle a large number of update streams.
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This extension does not introduce any privacy issues not already
present in the ALTO protocol.
10. IANA Considerations
This document defines a new media-type, "application/alto-
updatestreamparams+json", as described in Section 5.3. All other
media-types used in this document have already been registered,
either for ALTO or JSON Merge Patch.
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
[RFC7159].
Security considerations: Security considerations relating to the
generation and consumption of ALTO Protocol messages are discussed
in Section 9 of this document and Section 15 of [RFC7285].
Interoperability considerations: This document specifies format of
conforming messages and the interpretation thereof.
Published specification: Section 5.3 of this document.
Applications that use this media type: ALTO servers and ALTO clients
either stand alone or are embedded within other applications.
Additional information:
Magic number(s): n/a
File extension(s): This document uses the mime 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
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Restrictions on usage: n/a
Author: See Authors' Addresses section.
Change controller: Internet Engineering Task Force
(mailto:iesg@ietf.org).
11. References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, BCP 14, March 1997.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Burners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC5789] Dusseault, L. and J. Snell, "PATCH Method for HTTP", RFC
5789, March 2010.
[RFC6902] Bryan, P. and M. Nottingham, "JavaScript Object Notation
(JSON) Patch", RFC 6902, April 2013.
[RFC7159] Bray, T., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, March 2014.
[RFC7285] Almi, R., Penno, R., Yang, Y., Kiesel, S., Previdi, S.,
Roome, W., Shalunov, S., and R. Woundy, "Application-Layer
Traffic Optimization (ALTO) Protocol", RFC 7285, September
2014.
[RFC7386] Hoffman, P. and J. Snell, "JSON Merge Patch", RFC 7386,
October 2014.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, "Hypertext Transfer
Protocol Version 2 (HTTP/2)", RFC 7540, May 2015.
[SSE] Hickson, I., "Server-Sent Events (W3C)", December 2012.
Appendix A. Acknowledgments
Thank you to Xiao Shi (Yale University) for his contributions to an
earlier version of this document.
Authors' Addresses
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Wendy Roome
Alcatel-Lucent/Bell Labs
600 Mountain Ave, Rm 3B-324
Murray Hill, NJ 07974
USA
Phone: +1-908-582-7974
Email: w.roome@alcatel-lucent.com
Y. Richard Yang
Tongji/Yale University
51 Prospect St
New Haven CT
USA
Email: yang.r.yang@gmail.com
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