event: application/alto-networkmap+json,1
  data: { ... full Network Map message ... }
  
  event: application/alto-costmap+json,2
  data: { ... full Cost Map message ... }
  
  event: application/merge-patch+json,2
  data: { ... Merge Patch update for the Cost Map ... }

Figure 2: Examples of ALTO update events.

6.3. ALTO Control Events

Control events have the media type "application/alto-updatestreamcontrol+json", and the data is of type UpdateStreamControlEvent:

  object {
     [String     control-uri;]
     [String     remove<1..*>;]
  } UpdateStreamControlEvent;

The "control-uri" field is the URI of the Stream Control resource for this Update Stream (Section 8). The ALTO server MUST send a control event with that URI as the first event in an Update Stream.

The "remove" field is a list of client-ids of resources for which the server will no longer send updates. The server sends this event after processing the client's Stream Control request to remove those resources (Section 7.6.2).

7. Update Stream Service

An Update Stream Service returns a stream of SSE messages, as defined in Section 6. An Update Stream resource is used to request a new Update Stream.

A server creates an Update Stream Control resource for each active Update Stream. A client uses the control resource to remove resources from the Update Stream instance, or to request updates for additional resources. Update Stream resources are listed in the server's IRD, but Stream Control resources are not. Instead, the first event that the server sends to the client has the URI for the Update Stream Control resource for that stream (see Section 6.3.

Section 8 describes the Update Stream Control Service.

7.1. Media Type

The media type of an ALTO Update Stream resource is "text/event-stream".

7.2. HTTP Method

An ALTO Update Stream resource is requested using the HTTP POST method.

7.3. Accept Input Parameters

An ALTO Client supplies the Update Stream resource parameters by specifying media type "application/alto-updatestreamparams+json" with an HTTP POST body containing a JSON Object of type UpdateStreamReq, where:

  object {
     [AddUpdatesReq   add;]
     [ClientId        remove<0..*>;]
  } UpdateStreamReq;
  
  object-map {
     ClientId -> AddUpdateReq;
  } AddUpdatesReq;
  
  object {
     String       resource-id;
     [String      tag;]
     [Boolean     incremental-updates;]
     [Object      input;]
  } AddUpdateReq;

The "add" field specifies the resources for which the client wants updates, and has one entry for each resource. The client creates a unique client-id (Section 6.1) for each such resource, and uses those client-ids as the keys in the "add" field.

An Update Stream request MUST have an "add" field specifying one or more resources. If it does not, 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 events.

The "resource-id" field is the resource-id of an ALTO resource, and MUST be in the Update Streams's "uses" list (see Section 7.5). 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 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 Server SHOULD omit sending a full replacement update at the start of the stream (see Section 7.6.2). If that version is not current, the server MUST ignore the "tag" field.

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 7.4). 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 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 events. If the inputs for several POST-mode resources are missing or invalid, the server MUST pick one error response and return it.

The "remove" field is used by the Update Stream Control Service Section 8, and is not allowed in the Update Stream request. If the "remove" field exists, 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 events.

7.4. 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 6, JSON Merge Patch is the only incremental update event type defined by this document. However future extensions may define other types of incremental updates.

7.5. Uses

The "uses" attribute MUST be an array with the resource-ids of every resource for which this stream can provide updates.

This set may be any subset of the ALTO Server's resources, and may include resources defined in linked IRDs. However, it is RECOMMENDED that the ALTO Server 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.

7.6. Response

The response is a stream of SSE update events. Section 6 defines the events, and [SSE] defines how they are encoded into a stream.

An ALTO server SHOULD send updates only when the underlying values change. However, it may be difficult for a server to guarantee that in all circumstances. Therefore a client MUST NOT assume that an SSE update event represents an actual change.

There are additional requirements on the server's response, as described below.

7.6.1. Keep-Alive Messages

In an SSE stream, any line which starts with a colon (U+003A) character is a comment, and an ALTO Client MUST ignore that line ([SSE]). As recommended in [SSE], an ALTO Server SHOULD send a comment line (or an event) every 15 seconds to prevent clients and proxy servers from dropping the HTTP connection.

7.6.2. Event Sequence Requirements

• The first event MUST be a control event with the URI of the Stream Control resource (Section 8) for this Update Stream (Section 6.3).
• 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.
• 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.
• 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.
• When the client uses the Control Stream resource to stop updates for one or more resources Section 8, the ALTO Server MUST send a control event (Section 6.3) whose "remove" field has the client-ids of those resources. If the client uses the Control Stream resource to terminate all active resources and close the stream, the server MUST send a control event whose "remove" field has the client-ids of all active resources.

7.6.3. Cross-Stream Consistency Requirements

If several clients create Update Streams for updates to the same resource, the server MUST send the same updates to all of them. 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 create Update Streams for the same Cost Map, 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.

A server MAY offer several different Update Stream resources that provide updates to the same underlying resource (that is, a resource-id may appear in the "uses" field of more than one Update Stream resource). In this case, those Update Stream resources MUST return the same update data.

8. Update Stream Control Service

The Update Stream Control Service allows a client to remove resources from the set of resources that are monitored by an Update Stream, or add additional resources to that set. The Control Service also allows a client to gracefully shutdown an Update Stream.

The Update Stream Control Service is not listed in the ALTO Server's IRD. Instead, when a client requests a new Update Stream, the server creates a new Update Stream Control resource for that stream, and sends its URI to the client as the first event in the Update Stream (Section 7.6.2).

As described below, each control request adds resources to the set of monitored resources, or removes previously added resources, or does both. Each control request is a separate HTTP request; the client MAY NOT stream multiple control requests in one HTTP request. However, if the client and server support HTTP Keep-Alive ([RFC2616]), the client MAY send multiple HTTP requests on the same TCP/IP connection.

8.1. URI

The URI for a Stream Control resource, by itself, MUST uniquely specify the Update Stream instance which it controls. The server MUST NOT use other properties of an HTTP request, such as cookies or the client's IP address, to determine the Update Stream. Furthermore, a server MUST NOT re-use a Control Stream URI once the associated Update Stream has been closed.

The client MUST evaluate a non-absolute Stream Control URI (for example, a URI without a host, or with a relative path) in the context of the URI used to create the Update Stream. If a Stream Control URI has an explicit host, it may be different from the Update Stream's host.

It is expected that the server will assign a unique stream id to each Update Stream instance, and will embed that id in the associated Control Stream URI. However, the exact mechanism is left to the server. Clients MUST NOT attempt to deduce a stream id from the Control Stream URI.

8.2. Media Type

An ALTO Update Stream Control resource does not have a specific media type. If a request is successful, the server returns an HTTP "200 OK" response with Content-Type "text/plain" and no data. If a request is unsuccessful, the server returns an ALTO error response (Section 8.5.2 of [RFC7285])

8.3. HTTP Method

An ALTO Update Stream Control resource is requested using the HTTP POST method.

8.4. Accept Input Parameters

A Stream Control resource accepts the same input media type and input parameters as the Update Stream Service (Section 7.3). The only difference is that a Stream Control resource also accepts the "remove" field.

If specified, the "remove" field is an array of client-ids the client previously added to this Update Stream. An empty "remove" array is equivalent to a list of all currently active resources; the server responds by removing all resources and closing the stream.

A client MAY use the "add" field to add additional resources. However, the client MUST assign a unique client-id to each resource. Client-ids MUST be unique over the lifetime of this Update Stream: a client MUST NOT re-use a previously removed client-id.

If a request has any error, the server MUST NOT add or remove any resources from the associated Update Stream. In particular,

• Each "add" request must satisfy the requirements in Section 7.3. If not, the server MUST return the error response defined in Section 7.3.
• The server MUST return an E_INVALID_FIELD_VALUE error if a client-id in the "remove" field was not added in a prior request. Thus it is illegal to "add" and "remove" the same client-id in the same request. However, it is legal to remove a client-id twice.
• The server MUST return an E_INVALID_FIELD_VALUE error if a client-id in the "add" field has been used before in this stream.
• The server MUST return an E_INVALID_FIELD_VALUE error if the request has a non-empty "add" field and a "remove" field with an empty list of client-ids (to replace all active resources with a new set, the client MUST explicitly enumerate the client-ids to be removed).
• If the associated Update Stream has been closed, the server MUST return either an ALTO E_INVALID_FIELD_VALUE error, or else an HTTP error, such as "404 Not Found".

8.5. Capabilities & Uses

None (Control Stream Services do not appear in the IRD).

8.6. Response

If a request is successful, the server returns an HTTP "200 OK" response with Content-Type "text/plain" and no data. If there are any errors, the server MUST return the appropriate error code, and MUST NOT add or remove any resources from the Update Stream. Thus control requests are atomic: they cannot partially succeed.

The server MUST process the "add" field before the "remove" field. If the request removes all active resources without adding any additional resources, the server MUST close the Update Stream. Thus an Update Stream cannot have zero resources.

Whenever a server removes resources as a result of a Stream Control request, the server MUST send the corresponding "remove" Control Events (Section 6.3) on the Update Stream. If one control request removes several resources, the server MAY send one Control Event for all those resources, or a separate event for each removed resource, or any combination thereof.

9. Examples

9.1. 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 "update-my-costs" defined in the IRD in Section 9.4. The client requests updates for the Network Map and "routingcost" Cost Map, but not for the "hopcount" Cost Map. The client uses the server's resource-ids as the client-ids. 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.

  POST /updates/costs HTTP/1.1
  Host: alto.example.com
  Accept: text/event-stream,application/alto-error+json
  Content-Type: application/alto-updatestreamparams+json
  Content-Length: ###
  
  { "add": {
      "my-network-map": {
        "resource-id": "my-network-map"
        },
      "my-routingcost-map": {
        "resource-id": "my-routingcost-map"
      }
    }
  }
  
  
  HTTP/1.1 200 OK
  Connection: keep-alive
  Content-Type: text/event-stream
  
  event: application/alto-updatestreamcontrol+json
  data: {"control-uri":
  data: "http://alto.example.com/updates/streams/3141592653589"}
  
  event: application/alto-networkmap+json,my-network-map
  data: { ... full Network Map message ... }
  
  event: application/alto-costmap+json,my-routingcost-map
  data: { ... full routinccost Cost Map message ... }

After sending those 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: application/merge-patch+json,my-routingcost-map
  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: application/alto-networkmap+json,my-network-map
  data: { ... full Network Map message ... }
  
  event: application/alto-costmap+json,my-routingcost-map
  data: { ... full Cost Map message ... }

9.2. Example: Advanced Network and Cost Map Updates

This example is similar to the previous one, except that the client requests updates for the "hopcount" Cost Map as well as the "routingcost" Cost Map, and provides the current version tag of the Network Map, so the server is not required to send the full Network Map update event at the beginning of the stream. In this example, the client uses the client-ids "net", "routing" and "hops" for those resources. The ALTO Server sends the stream id and the full Cost Maps, followed by updates for the Network Map and Cost Maps as they become available:

  POST /updates/costs HTTP/1.1
  Host: alto.example.com
  Accept: text/event-stream,application/alto-error+json
  Content-Type: application/alto-updatestreamparams+json
  Content-Length: ###

  { "add": {
      "net": {
        "resource-id": "my-network-map".
        "tag": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"
        },
      "routing": {
        "resource-id": "my-routingcost-map"
        },
      "hops": {
        "resource-id": "my-hopcount-map"
      }
    }
  }
  
  
  HTTP/1.1 200 OK
  Connection: keep-alive
  Content-Type: text/event-stream
  
  event: =application/alto-updatestreamcontrol+json
  data: {"control-uri":
  data: "http://alto.example.com/updates/streams/2718281828459"}
  
  event: application/alto-costmap+json,routing
  data: { ... full routingcost Cost Map message ... }
   
  event: application/alto-costmap+json,hops
  data: { ... full hopcount Cost Map message ... }
 
     (pause)
     
  event: application/merge-patch+json,routing
  data: {"cost-map": {"PID2" : {"PID3" : 31}}}
     
  event: application/merge-patch+json,hops
  data: {"cost-map": {"PID2" : {"PID3" : 4}}}

If the client wishes to stop receiving updates for the "hopcount" Cost Map, the client can send a "remove" request on the Stream Control URI:

  POST /updates/streams/2718281828459" HTTP/1.1
  Host: alto.example.com
  Accept: text/plain,application/alto-error+json
  Content-Type: application/alto-updatestreamparams+json
  Content-Length: ###

  { 
    "remove": [ "hops" ]
  }
  
  
  HTTP/1.1 200 OK
  Connection: keep-alive
  Content-Type: text/plain
  Content-Length: 0
  
      (stream closed without sending data content)

The ALTO Server sends a "remove" control event on the original request stream to inform the client that updates are stopped for that resource:

  event: application/alto-updatestreamcontrol+json
  data: { "remove": ["hops"] }
  

If the client no longer needs any updates, and wishes to shut the Update Stream down gracefully, the client can send a "remove" request with an empty array:

  POST /updates/streams/2718281828459" HTTP/1.1
  Host: alto.example.com
  Accept: text/plain,application/alto-error+json
  Content-Type: application/alto-updatestreamparams+json
  Content-Length: ###

  { 
    "remove": [ ]
  }
  
  
  HTTP/1.1 200 OK
  Connection: keep-alive
  Content-Type: text/plain
  Content-Length: 0
 
      (stream closed without sending data content)

The ALTO Server sends a final "remove" control event on the original request stream to inform the client that all updates are stopped, and then closes the stream:

  event: application/alto-updatestreamcontrol+json
  data: { "remove": ["net", "routing"] }
  
      (server closes stream)  

9.3. Example: Endpoint Property Updates

As another example, here is how a client can request updates for the property "priv:ietf-bandwidth" for one set of endpoints, and "priv:ietf-load" for another. The ALTO Server immediately sends full-replacement messages 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/properties HTTP/1.1
  Host: alto.example.com
  Accept: text/event-stream
  Content-Type: application/alto-updatestreamparams+json
  Content-Length: ###
  
  { "add": {
      "props-1": {
        "resource-id": "my-props",
        "input": {
          "properties" : [ "priv:ietf-bandwidth" ],
          "endpoints" : [
            "ipv4:198.51.100.1",
            "ipv4:198.51.100.2",
            "ipv4:198.51.100.3"
          ]
        }
      },
      "props-2": {
        "resource-id": "my-props",
        "input": {
          "properties" : [ "priv:ietf-load" ],
          "endpoints" : [
            "ipv4:198.51.100.31",
            "ipv4:198.51.100.32",
            "ipv4:198.51.100.33"
          ]
        }
      },
    }
  }
  
  
  HTTP/1.1 200 OK
  Connection: keep-alive
  Content-Type: text/event-stream
  
  event: application/alto-updatestreamcontrol+json
  data: {"control-uri":
  data: "http://alto.example.com/updates/streams/1414213562373"}
  
  event: application/alto-endpointprops+json,props-1
  data: { "endpoint-properties": {
  data:     "ipv4:198.51.100.1" : { "priv:ietf-bandwidth": "13" },
  data:     "ipv4:198.51.100.2" : { "priv:ietf-bandwidth": "42" },
  data:     "ipv4:198.51.100.3" : { "priv:ietf-bandwidth": "27" }
  data:  } }
  
  event: application/alto-endpointprops+json,props-2
  data: { "endpoint-properties": {
  data:     "ipv4:198.51.100.31" : { "priv:ietf-load": "8" },
  data:     "ipv4:198.51.100.32" : { "priv:ietf-load": "2" },
  data:     "ipv4:198.51.100.33" : { "priv:ietf-load": "9" }
  data:  } }
  
     (pause)
  
  event: application/merge-patch+json,props-1
  data: { "endpoint-properties":
  data:   {"ipv4:198.51.100.1" : {"priv:ietf-bandwidth": "3"}}
  data: }
  
     (pause)
  
  event: application/merge-patch+json,props-2
  data: { "endpoint-properties":
  data:   {"ipv4:198.51.100.33" : {"priv:ietf-load": "7"}}
  data: }

9.4. 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 12.1, 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"],
    "capabilities": {
      "cost-type-names": ["num-routingcost", "num-hopcount"],
      "cost-constraints": false
    }
  },
  "my-props": {
    "uri": "http://alto.example.com/properties",
    "media-type": "application/alto-endpointprops+json",
    "accepts": "application/alto-endpointpropparams+json",
    "capabilities": {
      "prop-types": ["priv:ietf-bandwidth"]
    }
  },
  "update-my-costs": {
    "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"
      }
    }
  },
  "update-my-props": {
    "uri": "http://alto.example.com/updates/properties",
    "media-type": "text/event-stream",
    "uses": [ "my-props" ],
    "accepts": "application/alto-updatestreamparams+json",
    "capabilities": {
      "incremental-update-media-types": {
        "my-props": "application/merge-patch+json"
      }
    }
  }
  

10. 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 7.6.2).

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.

11. Design Decisions and Discussions

11.1. 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.

11.2. 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.

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.

11.3. 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 the discussions in the next section.

11.4. 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.

12. Miscellaneous Considerations

12.1. 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.

12.2. Considerations For Incremental Updates To Ordinal Mode Costs

For an ordinal mode cost map, a change to a single cost point may require updating many other costs. As an extreme example, suppose the lowest cost changes to the highest cost. For a numerical mode cost map, only that one cost changes. But for an ordinal mode cost map, every cost might change. While this document allows a server to offer incremental updates for ordinal mode cost maps, server implementors should be aware that incremental updates for ordinal costs are more complicated than for numerical costs, and clients should be aware that small changes may result in large updates.

An ALTO Server can avoid this complication by only offering full replacement updates for ordinal cost maps.

12.3. Considerations Related to SSE Line Lengths

SSE was designed for events that consist of relatively small amounts of line-oriented text data, and SSE clients frequently read input one line-at-a-time. However, an Update Stream sends full cost maps as single events, and a cost map may involve megabytes, if not tens of megabytes, of text. This has implications for both the ALTO Server and Client.

First, SSE clients might not be able to handle a multi-megabyte data "line". Hence when sending a full network map or cost map, an ALTO server SHOULD insert a new-line character periodically. Approximately every 2,000 characters should be sufficient for most SSE clients.

Second, some SSE client packages read all the data for an event into memory, and then present it to the client as a single character array. However, a client computer may not have enough memory to hold the entire JSON text for a large cost map. Hence an ALTO client SHOULD consider using an SSE library which presents the event data in manageable chunks, so the client can parse the cost map incrementally and store the underlying data in a more compact format.

13. Security Considerations

13.1. Denial-of-Service Attacks

Allowing persistent update stream connections enables a new class of Denial-of-Service attacks. A client might create an unreasonable number of update stream connections, or add an unreasonable number of client-ids to one update stream. To avoid those attacks, an ALTO Server MAY choose to limit the number of active streams, and reject new requests when that threshold is reached. A server MAY also chose to limit the number of active client-ids on any given stream, or limit the total number of client-ids used over the lifetime of a stream, and reject any stream control request which would exceed those limits. In these cases, the server SHOULD return the HTTP status "503 Service Unavailable".

While this technique prevents Update Stream DoS attacks from disrupting an ALTO Server's other services, it does make it easier for a DoS attack to disrupt the Update Stream service. Therefore a server may prefer to restrict Update Stream services to authorized clients, as discussed in Section 15 of [RFC7285].

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.

13.2. Spoofed Control Requests

An outside party which can read the update stream response can obtain the control URI and use that to send a fraudulent "remove" requests, thus disabling updates for the valid client. This can be avoided by encrypting the stream (see Section 15 of [RFC7285]). Also, the ALTO Server echoes the "remove" requests on the update stream, so the valid client can detect unauthorized requests.

13.3. Privacy

This extension does not introduce any privacy issues not already present in the ALTO protocol.

14. IANA Considerations

This document defines two new media-types, "application/alto-updatestreamparams+json", as described in Section 7.3, and "application/alto-updatestreamcontrol+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.

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 13 of this document and Section 15 of [RFC7285].

Interoperability considerations:

This document specifies format of conforming messages and the interpretation thereof.

Published specification:

Section 7.3 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

Restrictions on usage:

n/a

Author:

See Authors' Addresses section.

Change controller:

Internet Engineering Task Force (mailto:iesg@ietf.org).

Type name:

application

Subtype name:

alto-updatestreamcontrol+json

Required parameters:

n/a

Optional parameters:

n/a

Encoding considerations:

Encoding considerations are identical to those specified for the "application/json" media type. See [RFC7159].

Security considerations:

Security considerations relating to the generation and consumption of ALTO Protocol messages are discussed in Section 13 of this document and Section 15 of [RFC7285].

Interoperability considerations:

This document specifies format of conforming messages and the interpretation thereof.

Published specification:

Section 6.3 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

Restrictions on usage:

n/a

Author:

See Authors' Addresses section.

Change controller:

Internet Engineering Task Force (mailto:iesg@ietf.org).

15. 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)", W3C Recommendation 03 February 2015, February 2015.

Appendix A. Acknowledgments

Thank you to Xiao Shi (Yale University) for his contributions to an earlier version of this document.

Authors' Addresses