One document matched: draft-ietf-alto-incr-update-sse-00.xml
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<rfc category="std"
docName="draft-ietf-alto-incr-update-sse-00"
ipr="trust200902">
<front>
<title abbrev="ALTO Incremental Updates">ALTO Incremental Updates Using Server-Sent Events (SSE)</title>
<author initials="W." surname="Roome" fullname="Wendy Roome">
<organization abbrev="Alcatel-Lucent">Alcatel-Lucent/Bell Labs</organization>
<address>
<postal>
<street>600 Mountain Ave, Rm 3B-324</street>
<city>Murray Hill</city>
<region>NJ</region>
<code>07974</code>
<country>USA</country>
</postal>
<phone>+1-908-582-7974</phone>
<email>w.roome@alcatel-lucent.com</email>
</address>
</author>
<!--
<author initials="X." surname="Shi" fullname="Xiao Shi">
<organization abbrev="Yale University">Yale University</organization>
<address>
<postal>
<street>51 Prospect Street</street>
<city>New Haven</city>
<region>CT</region>
<code>06511</code>
<country>USA</country>
</postal>
<email>xiao.shi@yale.edu</email>
</address>
</author>
-->
<author fullname="Y. Richard Yang" initials="Y." surname="Yang">
<organization>Tongji/Yale University</organization>
<address>
<postal>
<street>51 Prospect St</street>
<city>New Haven</city>
<code>CT</code>
<country>USA</country>
</postal>
<email>yang.r.yang@gmail.com</email>
</address>
</author>
<date month="May" day="28" year="2015"/>
<area>Networks</area>
<workgroup>ALTO WG</workgroup>
<keyword>ALTO</keyword>
<abstract>
<t>
The Application-Layer Traffic Optimization (ALTO)
<xref target="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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</abstract>
<note title="Requirements Language">
<t>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 <xref target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>
The Application-Layer Traffic Optimization (ALTO)
<xref target="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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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 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.
</t>
</section>
<section title="Overview of Approach">
<t>
This section presents a non-normative overview of the update mechanism
to be defined in this document.
</t>
<t>
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.
</t>
<t>
Each update message updates one resource, and is sent as a
Server-Sent Event (SSE), as defined by <xref target="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 (<xref target="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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Update Events">
<section title="Overview of SSEs">
<t>
The following is a non-normative summary of Server-Sent Events (SSEs).
See <xref target="SSE"/> for the normative definition.
</t>
<t>
Server-Sent Events enable a server
to send new data to a client by "server-push".
The client establishes
an HTTP (<xref target="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 <xref target="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.
</t>
<t>
<xref target="sse-example"/> 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 <xref target="RFC2616"/>);
for simplicity, we have omitted those details.
</t>
<figure anchor="sse-example" title="A Sample SSE stream.">
<artwork><![CDATA[
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
]]></artwork>
</figure>
</section>
<section title="ALTO Update Events" anchor="ALTO.SSE.Events">
<t>
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 <xref target="RFC7285"/>;
examples are Network and Cost Map messages.
The data objects in Merge Patch messages
are defined by <xref target="RFC7386"/>.
</t>
<t>
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 <xref target="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 <xref target="RFC7386"/>.
<!-- yry add extension -->
An extension document may introduce other media-types to indicate new types of update messages.
</t>
<t>
Specifically, the two sub-fields of the event field are encoded as:
</t>
<figure>
<artwork><![CDATA[
resource-id , media-type
]]></artwork>
</figure>
<t>
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.
</t>
<t>
This document does not use the SSE "id" field.
</t>
<t>
<xref target="alto-sse-example"/> shows some examples of ALTO update events:
<!-- yry give an example of having , in resource id? -->
</t>
<figure anchor="alto-sse-example" title="Examples of ALTO update events.">
<artwork><![CDATA[
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 ... }
]]></artwork>
</figure>
</section>
<section title="Keep-Alive Messages">
<t>
An SSE event with an empty "event" field is a keep-alive message.
<!--
[yry: Is keep alive already defined by SSE? If so, can we refer to it?]
[wdr: Not that I've seen.]
-->
An ALTO Server MAY send keep-alive messages as needed.
An ALTO Client MUST ignore any keep-alive messages.
</t>
</section>
</section>
<section title="Incremental Update Message Format">
<section title="Overview of JSON Merge Patch">
<t>
The following is a non-normative summary of JSON Merge Patch.
See <xref target="RFC7386"/> for the normative definition.
</t>
<t>
JSON Merge Patch is intended to allow applications to update
server resources via the HTTP PATCH method <xref target="RFC5789"/>.
This document adopts the JSON Merge Patch message format
to encode the changes,
but uses a different transport mechanism.
</t>
<t>
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.
<!--
[yry: union of the two trees, to be safe to say subset?]
[wdr: Union implies everything in both. I just deleted it.
and expanded subsequent description.]
-->
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.
</t>
<t>
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.
</t>
<t>
Formally, the process of applying a Merge Patch is defined by the
following recursive algorithm, as specified in <xref target="RFC7386"/>:
</t>
<figure>
<artwork><![CDATA[
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
}
}
]]></artwork>
</figure>
<t>
Note that null as the value of a name/value pair will
delete the element with "name" in the original JSON value.
</t>
</section>
<section title="JSON Merge Patch Applied to Network Map Messages">
<t>
Section 11.2.1.6 of <xref target="RFC7285"/> defines the format
of a Network Map message.
<!-- yry do we mandate that tag must be included -->
Here is a simple example:
</t>
<figure>
<artwork><![CDATA[
{
"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" ]
}
}
}
]]></artwork>
</figure>
<t>
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:
</t>
<figure>
<artwork><![CDATA[
{
"meta" : {
"vtag" : {
"tag" : "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
},
"network-map": {
"PID1" : {
"ipv6" : [ "2000::/3" ]
},
"PID2" : null
}
}
]]></artwork>
</figure>
<t>
Here is the updated Network Map:
</t>
<figure>
<artwork><![CDATA[
{
"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" ]
}
}
}
]]></artwork>
</figure>
</section>
<section title="JSON Merge Patch Applied to Cost Map Messages">
<t>
Section 11.2.3.6 of <xref target="RFC7285"/> defines the format
of a Cost Map message. Here is a simple example:
</t>
<figure>
<artwork><![CDATA[
{
"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 }
}
}
]]></artwork>
</figure>
<t>
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:
<!-- yry again, the tag issue -->
</t>
<figure>
<artwork><![CDATA[
{
"cost-map" : {
"PID1" : { "PID2" : 9 },
"PID3" : { "PID1" : null, "PID3" : 1 }
}
}
]]></artwork>
</figure>
<t>
Here is the updated cost map:
</t>
<figure>
<artwork><![CDATA[
{
"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 }
}
}
]]></artwork>
</figure>
</section>
</section>
<section title="Update Stream Service" anchor="UpdateStreamService">
<t>
An Update Stream Service returns a stream of SSE messages,
as defined in <xref target="ALTO.SSE.Events"/>.
</t>
<section title="Media Type">
<t>
The media type of an ALTO Update Stream resource is "text/event-stream".
</t>
</section>
<section title="HTTP Method">
<t>
An ALTO Update Stream resource is
<!--
[yry: MUST be?]
[wdr: I used the language of rfc 7285. Eg, Section 11.2.1.2.]
-->
requested using the HTTP POST method.
</t>
</section>
<section title="Accept Input Parameters" anchor="UpdateInput.media-type">
<t>
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:
</t>
<figure>
<artwork><![CDATA[
object-map {
ResourceID -> ResourceUpdateReq;
} UpdateStreamReq;
object {
[String tag;]
[Boolean incremental-updates;]
[Object input;]
} ResourceUpdateReq;
]]></artwork>
</figure>
<t>
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 <xref target="UpdateStreamService.Uses"/>).
The ResourceUpdateReq values give additional parameters
for the updates for each resource.
</t>
<t>
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 <xref target="RFC7285"/>),
and MUST close the stream without sending any update events.
</t>
<t>
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., "{}").
</t>
<t>
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 <xref target="UpdateStreamService.Capabilities"/>).
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.
</t>
<t>
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.
</t>
<t>
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 <xref target="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 Server SHOULD omit sending
a full replacement update at the start of the stream
(see <xref target="UpdateStreamService.Response.Sequence"/>).
</t>
<t>
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 <xref target="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.
</t>
</section>
<section title="Capabilities" anchor="UpdateStreamService.Capabilities">
<t>
The capabilities are defined by an object of type UpdateStreamCapabilities:
</t>
<figure>
<artwork><![CDATA[
object {
IncrementalUpdateMediaTypes incremental-update-media-types;
} UpdateStreamCapabilities;
object-map {
ResourceID -> String;
} IncrementalUpdateMediaTypes;
]]></artwork>
</figure>
<t>
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 <xref target="ALTO.SSE.Events"/>,
JSON Merge Patch is the only incremental update event type defined by this document.
However future extensions may define other types of incremental updates.
</t>
</section>
<section title="Uses" anchor="UpdateStreamService.Uses">
<t>
The "uses" attribute MUST be an array with the resource-ids of every resource
for which this stream can provide updates.
</t>
<t>
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 Map SHOULD also provide
updates for the Network Map upon which that Cost Map depends.
</t>
</section>
<section title="Response" anchor="UpdateStreamService.Response">
<t>
The response is a stream of SSE update events.
<xref target="ALTO.SSE.Events"/> defines the events,
and <xref target="SSE"/> defines how they are encoded into a stream.
</t>
<t>
There are additional requirements between events
in the stream, as described below.
</t>
<section title="Event Sequence Requirements" anchor="UpdateStreamService.Response.Sequence">
<t>
<list style="symbols">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</list>
</t>
</section>
<section title="Cross-Stream Consistency Requirements">
<t>
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.
</t>
<t>
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 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.
</t>
</section>
</section>
<section title="Considerations For Updates To Filtered Cost Maps"
anchor="UpdateStreamService.FCMConsiderations">
<t>
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.
</t>
<t>
An ALTO Server can avoid such issues
by offering Update Streams only for Filtered Cost Maps
which do not allow constraint tests.
</t>
</section>
<section title="Example: Simple Network and Cost Map Updates">
<t>
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 <xref target="IRD.example"/>.
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.
</t>
<t>
Note that the server may "chunk" the returned data (see <xref target="RFC2616"/>);
for simplicity, we have omitted those details.
</t>
<figure>
<artwork><![CDATA[
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 ... }
]]></artwork>
</figure>
<t>
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:
</t>
<figure>
<artwork><![CDATA[
event: my-routingcost-map,application/merge-patch+json
data: {"cost-map": {"PID1" : {"PID2" : 9}}}
]]></artwork>
</figure>
<t>
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:
</t>
<figure>
<artwork><![CDATA[
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 ... }
]]></artwork>
</figure>
</section>
<section title="Example: Advanced Network and Cost Map Updates">
<t>
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:
</t>
<figure>
<artwork><![CDATA[
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}}}
]]></artwork>
</figure>
</section>
<section title="Example: Endpoint Property Updates">
<t>
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.
</t>
<figure>
<artwork><![CDATA[
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: }
]]></artwork>
</figure>
</section>
</section>
<section title="Client Actions When Receiving Update Messages">
<t>
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.
</t>
<t>
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.
</t>
<t>
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 <xref target="UpdateStreamService.Response.Sequence"/>).
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
Although not as efficient as possible, this recovery method is simple and reliable.
</t>
</section>
<section title="IRD Example" anchor="IRD.example">
<t>
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.
</t>
<t>
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 <xref target="UpdateStreamService.FCMConsiderations"/>,
the Update Stream provides updates for the second,
but not the first.
</t>
<figure>
<artwork><![CDATA[
"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"],
"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"
}
}
}
]]></artwork>
</figure>
</section>
<section title="Design Decisions and Discussions">
<section title="HTTP2 Server-Push">
<t>
An alternative would be to use HTTP 2 Server-Push
<xref target="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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Not Allowing Stream Restart">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Is Incremental Update Useful for Network Maps?" anchor="IncrUpdForNetworkMaps">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
<section title="Other Incremental Update Message Types">
<t>
Other JSON-based incremental update formats have been defined,
in particular JSON Patch (<xref target="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.
</t>
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
</section>
</section>
<section title="Security Considerations" anchor="Security">
<t>
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".
</t>
<t>
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.
</t>
<t>
This extension does not introduce any privacy issues
not already present in the ALTO protocol.
</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>
This document defines a new media-type, "application/alto-updatestreamparams+json",
as described in <xref target="UpdateInput.media-type"/>.
All other media-types used in this document have already been registered,
either for ALTO or JSON Merge Patch.
</t>
<t>
<list style="hanging" hangIndent="3">
<t hangText="Type name:">application</t>
<t hangText="Subtype name:">alto-updatestreamparams+json</t>
<t hangText="Required parameters:">n/a</t>
<t hangText="Optional parameters:">n/a</t>
<t hangText="Encoding considerations:">Encoding considerations are
identical to those specified for the "application/json" media type. See
<xref target="RFC7159" />.</t>
<t hangText="Security considerations:">Security considerations relating
to the generation and consumption of ALTO Protocol messages are
discussed in <xref target="Security"/> of this document
and Section 15 of <xref target="RFC7285"/>.</t>
<t hangText="Interoperability considerations:">This document specifies
format of conforming messages and the interpretation thereof.</t>
<t hangText="Published specification:"><xref target="UpdateInput.media-type"/>
of this document.</t>
<t hangText="Applications that use this media type:">ALTO servers and
ALTO clients either stand alone or are embedded within other
applications.</t>
<t hangText="Additional information:">
<list style="hanging" hangIndent="3">
<t hangText="Magic number(s):">n/a</t>
<t hangText="File extension(s):">This document uses the mime type
to refer to protocol messages and thus does not require a file
extension.</t>
<t hangText="Macintosh file type code(s):">n/a</t>
</list>
</t>
<t hangText="Person & email address to contact for further information:">
See Authors' Addresses section.</t>
<t hangText="Intended usage:">COMMON</t>
<t hangText="Restrictions on usage:">n/a</t>
<t hangText="Author:">See Authors' Addresses section.</t>
<t hangText="Change controller:">Internet Engineering Task Force (mailto:iesg@ietf.org).</t>
</list>
</t>
</section>
</middle>
<back>
<references>
<!--
<reference anchor="RFC5226">
<front>
<title>Guidelines for Writing an IANA Considerations Section in RFCs</title>
<author initials="T." surname="Narten" fullname="T. Narten"/>
<author initials="H." surname="Alvestrand" fullname="H. Alvestrand"/>
<date month="May" year="2008"/>
</front>
<seriesInfo name="RFC" value="5226" />
<seriesInfo name="BCP" value="26" />
</reference>
-->
<reference anchor='RFC2119'>
<front>
<title abbrev='HTTP'>Key words for use in RFCs to Indicate Requirement Levels</title>
<author initials='S.' surname='Bradner' fullname='S. Bradner' />
<date year='1997' month='March' />
</front>
<seriesInfo name='RFC' value='2119' />
<seriesInfo name='BCP' value='14' />
<format type='TXT' target='http://www.rfc-editor.org/rfc/rfc2119.txt' />
</reference>
<reference anchor='RFC2616'>
<front>
<title abbrev='HTTP'>Hypertext Transfer Protocol -- HTTP/1.1</title>
<author initials='R.' surname='Fielding' fullname='N. Freed' />
<author initials='J.' surname='Gettys' fullname='J. Gettys' />
<author initials='J.' surname='Mogul' fullname='J. Mogul' />
<author initials='H.' surname='Frystyk' fullname='H. Frystyk' />
<author initials='L.' surname='Masinter' fullname='L. Masinter' />
<author initials='P.' surname='Leach' fullname='P. Leach' />
<author initials='T.' surname='Burners-Lee' fullname='T. Burners-Lee' />
<date year='1999' month='June' />
</front>
<seriesInfo name='RFC' value='2616' />
<format type='TXT' target='http://www.rfc-editor.org/rfc/rfc2616.txt' />
</reference>
<reference anchor="RFC5789">
<front>
<title>PATCH Method for HTTP</title>
<author initials="L." surname="Dusseault" fullname="L. Dusseault"/>
<author initials="J." surname="Snell" fullname="J. Snell"/>
<date month="March" year="2010"/>
</front>
<seriesInfo name="RFC" value="5789" />
</reference>
<reference anchor="RFC6902">
<front>
<title abbrev='JSON-Patch'>JavaScript Object Notation (JSON) Patch</title>
<author initials='P.' surname='Bryan' fullname='P. Bryan' />
<author initials='M.' surname='Nottingham' fullname='M. Nottingham' />
<date year='2013' month='April' />
</front>
<seriesInfo name='RFC' value='6902' />
<format type='TXT' target='http://www.rfc-editor.org/rfc/rfc6902.txt' />
</reference>
<reference anchor="RFC7159">
<front>
<title>The JavaScript Object Notation (JSON) Data Interchange Format</title>
<author initials="T." surname="Bray" fullname="T. Bray"/>
<date month="March" year="2014"/>
</front>
<seriesInfo name="RFC" value="7159" />
</reference>
<reference anchor="RFC7285">
<front>
<title>Application-Layer Traffic Optimization (ALTO) Protocol</title>
<author initials="R." surname="Almi" fullname="R. Alimi"/>
<author initials="R." surname="Penno" fullname="R. Penno"/>
<author initials="Y." surname="Yang" fullname="Y. Yang"/>
<author initials="S." surname="Kiesel" fullname="S. Kiesel"/>
<author initials="S." surname="Previdi" fullname="S. Previdi"/>
<author initials="W." surname="Roome" fullname="W. Roome"/>
<author initials="S." surname="Shalunov" fullname="S. Shalunov"/>
<author initials="R." surname="Woundy" fullname="R. Woundy"/>
<date month="September" year="2014"/>
</front>
<seriesInfo name="RFC" value="7285" />
</reference>
<reference anchor="RFC7386">
<front>
<title>JSON Merge Patch</title>
<author initials="P." surname="Hoffman" fullname="P. Hoffman"/>
<author initials="J." surname="Snell" fullname="J. Snell"/>
<date month="October" year="2014"/>
</front>
<seriesInfo name="RFC" value="7386" />
</reference>
<reference anchor="RFC7540">
<front>
<title>Hypertext Transfer Protocol Version 2 (HTTP/2)</title>
<author initials="M." surname="Belshe" fullname="M. Belshe"/>
<author initials="R." surname="Peon" fullname="R. Peon"/>
<author initials="M." surname="Thomson" fullname="M. Thomson"/>
<date month="May" year="2015"/>
</front>
<seriesInfo name="RFC" value="7540" />
</reference>
<reference anchor="SSE">
<front>
<title>Server-Sent Events (W3C)</title>
<author initials="I." surname="Hickson" fullname="Ian Hickson"/>
<date month="December" year="2012"/>
</front>
<!-- http://www.w3.org/TR/eventsource/ -->
</reference>
</references>
<section anchor="Acknowledgments" title="Acknowledgments">
<t>Thank you to Xiao Shi (Yale University) for his contributions to an earlier version of this document.</t>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-24 06:04:36 |