| ALTO WG | W. Roome |
| Internet-Draft | Alcatel-Lucent |
| Intended status: Standards Track | X. Shi |
| Expires: August 3, 2015 | Y. Yang |
| Yale University | |
| January 30, 2015 |
ALTO Incremental Updates Using Server-Sent Events (SSE)
draft-roome-alto-incr-update-sse-01
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 a client to obtain updates to those maps, other than by periodically re-fetching them. Because the maps may be very 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 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.
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].
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/.
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 August 3, 2015.
Copyright (c) 2015 IETF Trust and the persons identified as the document authors. All rights reserved.
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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. Network Maps partition the set of endpoints into a manageable number of Provider-Defined Identifiers (PIDs), and Cost Maps provide 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 very 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 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, an ALTO Server can use the mechanisms defined in this document to 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.
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. Sections 5 and 6 define two new Update Stream Services. Section 7 describes how a client should handle incoming updates, and Section 8 gives an example of the Information Resource Directory (IRD) for an ALTO Server that offers a comprehensive set of Update Services. Section 9 discusses the design decisions behind this update mechanism. The remaining sections review the security and IANA considerations.
This section presents a non-normative overview of the update mechanism.
An ALTO Server can offer one or more Update Stream resources. Each stream presents a continuous sequence of update messages for a set of ALTO resources selected by the server. Each message updates one resource. The messages are Server-Sent Events (SSEs), as defined by [SSE]. An update message is either a complete replacement or else an incremental change. Complete 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 a full replacement or an incremental update. These decisions can vary from resource to resource and from update to update.
There are two types of Update Stream resources: Full Update Streams and Filtered Update Streams. A Full Update Stream is a GET-mode resource that provides updates to a set of GET-mode resources selected by the server.
A Filtered Update Stream is a POST-mode resource, and allows the client to select a subset of the update events offered by the server for that stream. In particular, a client may ask a server to send full updates events instead of incremental updates. A Filtered Update Stream can also provide updates to POST-mode resources such as the Endpoint Property Service.
An ALTO Server may offer any number of Update Stream resources, for any collection of the server's resources. A server may offer updates to the same resource via several different Update Stream resources, provided that the different update messages yield the same net result.
An ALTO Server's Information Resource Directory (IRD) defines its Update Stream resources.
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 server closes it. A client may connect to any number of Update Stream resources. Because each connection 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.
The following is a non-normative summary of Server-Sent Events. 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). Messages may contain three fields: an event type, an id, and data. All fields are strings. The data field may contain new-lines; the other fields cannot. The event type and id fields are optional.
Here 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
In the events defined in this document, the data field is a JSON object. That object is either a complete specificiation 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].
The event type 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].
We do not use the SSE id field.
We encode the event type sub-fields as:
resource-id , media-type
Note that commas (character code 0x2c) are allowed in ALTO resource-ids, but not in media-type names. Hence when parsing the SSE event type into sub-types, a client MUST split the string on the last comma.
Here 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 previous Cost Map ... }
An SSE event with an empty event type is a keep-alive message. An ALTO Server MAY send keep-alive messages as needed. An ALTO Client MUST ignore any keep-alive messages.
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 incremental updates, but uses a different transport mechanism.
The process of applying a Merge Patch is defined by the following recursive algorithm, as specified in [RFC7386]:
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 document.
Section 11.2.1.6 of [RFC7285] defines the format of a Network Map message. Here is a simple example:
{
"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:
{
"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" ]
}
}
}
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 that 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:
{
"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 }
}
}
An Update Stream Service returns a stream of SSE messages, as defined in Section 3.2.
The media type of an ALTO Update Stream resource is "text/event-stream".
An ALTO Update Stream resource is requested using the HTTP GET method.
None.
The capabilities are defined by an object of type UpdateStreamCapabilities:
object {
JSONString update-events<1..*>;
} UpdateEventStreamCapabilities;
The strings in the array are the event types (see Section 3.2) sent by this Update Stream.
If an Update Event Service's "update-events" capability list has an event with a media-type of "application/merge-patch+json" for a resource-id, then the event capability list MUST also have a full-replacement event for that resource-id. For example, suppose "my-costmap" is the resource-id of a Cost Map. Then if the event list has "my-costmap,application/merge-patch+json", it MUST also have the event "my-costmap,application/alto-costmap+json".
An array with the resource-ids of the resources for which this stream sends updates. This array MUST contain the resource-ids of every event type in the "update-events" capability.
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.
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 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.
Here is an example of a client's request and the server's immediate response, using the Update Stream resource "my-routingcost-update-stream" defined in the IRD in Section 8. This assumes the Update Stream service sends updates for a Network Map with resource-id "my-network-map" and an associated Cost Map with resource-id "my-routingcost-map". Note that the server may "chunk" the returned data (see [RFC2616]); for simplicity, we have omitted those details.
GET /updates/routingcost HTTP/1.1
Host: alto.example.com
Accept: text/event-stream
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 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 ... }
The Filtered Update Stream service is similar to the Update Stream service (Section 5), except that the client can select the types of update events.
The media type of an ALTO Update Stream resource is "text/event-stream".
A Filtered ALTO Update Stream resource is requested using the HTTP POST method.
An ALTO Client supplies filtering parameters by specifying media type "application/alto-updatestreamfilter+json" with HTTP POST body containing a JSON object of type ReqFilteredUpdateStream, where:
object {
[UpdateEventType update-events<1..*>;]
[VersionTag vtags<1..*>;]
[ResourceInputs inputs<1..*>;]
} ReqFilteredUpdateStream;
object-map {
ResourceID -> JSONObject;
} ResourceInputs;
The "update-events" field gives the types of the events the ALTO Client wishes to receive. These events MUST be a subset of the "update-events" capability of this resource; the ALTO Server MUST ignore any events not in the resource's capability list. If the "update-events" list is omitted, the ALTO Server MUST send all event types in the "update-events" capability of this resource.
The "vtags" field is an array of version tags, as defined in Section 10.3 of [RFC7285], for any resources which the client already has. At startup, the server SHOULD NOT send the full version of any resource for which the client has the current version.
The "inputs" field gives the client input needed for any POST-mode resources requested by the client. The value is a JSON object. The keys are the resource-ids of the POST-mode resources, and the value for each resource-id is the JSON object that resource requires as its input.
If the "update-events" field includes events for a POST-mode resource, but the "inputs" field for that resource is missing or invalid, then ALTO Server MUST return the same error response that that resource would return if given that 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.
If a client requests Merge Patch update events for a resource-id, the client MUST also request the corresponding full map update events for that resource-id.
If a client requests the full-replacement update event for a resource-id, but does not request the Merge Patch update event for that resource-id, when that resource changes, the ALTO 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 Merge Patch events will not get updates as quickly as a client which does.
The "capabilities" and "uses" fields are the same as for the Full Update Stream Service, as described in Section 5.4 and Section 5.5, respectively.
The format of the response, and the associated rules, are the same as for the Full Update Stream Service (Section 5.6), except that the ALTO Server SHOULD NOT send an initial full-replacement message for any resource for which the version in the "vtags" field of the client's input matches the resource's current version.
Here is an example of a client's request and the server's immediate response, using the Filtered Update Stream resource "my-allresources-update-stream" defined in the IRD in Section 8. The client requests updates for the Network Map and the "routingcost" Cost Map, but does not want updates for the "hopcount" Cost Map. The "vtags" field gives the client's version of the Network Map. Because that version is still current, 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 "routingcost" Cost Map as they become available:
POST /updates/allresources HTTP/1.1
Host: alto.example.com
Accept: text/event-stream
Content-Type: application/alto-updatestreamfilter+json
Content-Length: ###
{ "update-events": [
"my-network-map,application/alto-networkmap+json",
"my-routingcost-map,application/alto-costmap+json",
"my-routingcost-map,application/merge-patch+json"
],
"vtags": [
{"resource-id": "my-network-map",
"tag": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
}
],
}
HTTP/1.1 200 OK
Connection: keep-alive
Content-Type: text/event-stream
event: my-routingcost-map,application/alto-costmap+json
data: { ... full Cost Map message ... }
(pause)
event: my-routingcost-map,application/merge-patch+json
data: {"cost-map": {"PID2" : {"PID3" : 31}}}
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.
POST /updates/allresources HTTP/1.1
Host: alto.example.com
Accept: text/event-stream
Content-Type: application/alto-updatestreamfilter+json
Content-Length: ###
{ "update-events": [
"my-properties,application/alto-endpointprops+json",
"my-properties,application/merge-patch+json"
],
"inputs": {
"my-properties": {
"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: }
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.
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. If the client uses the Filtered Update Stream service, 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.
Here is an example of an IRD that offers both regular and Filtered Update Stream services. The unfiltered Update Stream provides updates for the Network Map and "routingcost" Cost Map. The Filtered Update Stream provides update to both those maps, plus the "hopcount" Cost Map and the Endpoint Properties service.
"my-network-map": {
"uri": "http://alto.example.com/networkmap",
"media-type": "application/alto-networkmap+json",
},
"my-routingcost-map": {
"uri": "http://alto.example.com/costmap",
"media-type": "application/alto-costmap+json",
"uses": ["my-networkmap+json"],
"capabilities": {
"cost-type-names": ["num-routingcost"]
}
},
"my-hopcount-map": {
"uri": "http://alto.example.com/costmap",
"media-type": "application/alto-costmap+json",
"uses": ["my-networkmap+json"],
"capabilities": {
"cost-type-names": ["num-hopcount"]
}
},
"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-routingcost-update-stream": {
"uri": "http://alto.example.com/updates/routingcost",
"media-type": "text/event-stream",
"uses": ["my-network-map", "my-routingcost-map"],
"capabilities": {
"update-events": [
"my-network-map,application/alto-networkmap+json",
"my-routingcost-map,application/alto-costmap+json",
"my-routingcost-map,application/merge-patch+json"
]
}
},
"my-allresources-update-stream": {
"uri": "http://alto.example.com/updates/allresources",
"media-type": "text/event-stream",
"uses": [
"my-network-map",
"my-routingcost-map",
"my-hopcount-map",
"my-properties"
],
"accepts": "application/alto-updatestreamfilter+json",
"capabilities": {
"update-events": [
"my-network-map,application/alto-networkmap+json",
"my-routingcost-map,application/alto-costmap+json",
"my-routingcost-map,application/merge-patch+json"
"my-hopcount-map,application/alto-costmap+json",
"my-hopcount-map,application/merge-patch+json"
"my-properties,application/alto-endpointprops+json",
"my-properties,application/merge-patch+json"
]
}
}
An alternative would be to use HTTP 2 Server-Push [I-D-ietf-http2], 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.
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.
Also, although this is a theoretical inefficiency, in practice it is unlikely to be a problem. 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.
In short, using event ids to avoid resending the full map adds a considerable amount of complexity to avoid a situation which is hopefully very rare. We believe that complexity is not worth the benefit.
The Filtered Update Stream service does allow the client to specify the vtag 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 vtags to Cost Maps might add a new set of complications.
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 occurance. 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.
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.
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.
This extension does not introduce any privacy issues not already present in the ALTO protocol.
This document defines a new media-type, "application/alto-updatestreamfilter+json", as described in Section 6.3. All other media-types used in this document have already been registered, either for ALTO or JSON Merge Patch.