ALTO Transport Information Publication Service

Introduction Application-Layer Traffic Optimization (ALTO) provides means for network applications to obtain network status information. So far, two transport protocols have been designed: (1) The ALTO base protocol , which is designed for the simple use case in which an ALTO client requests a network information resource, and the server sends the complete content of the requested information resource to the client; and (2) ALTO/SSE (ALTO Incremental Updates Using Server-Sent Events) , which is designed for the use case that an ALTO client indicates to the server that it wants to receive updates for a set of resources, and the server can then concurrently, and incrementally push updates to the client whenever monitored resources change. Both protocols are designed for HTTP/1.x and, ideally, they should be able to automatically take advantage of newer HTTP versions such as HTTP/2 and HTTP/3 . However, there are issues with both protocols when HTTP moves to higher versions. First consider the ALTO base protocol, which is designed to transfer only complete information resources. Hence, a client can run the base protocol on top of HTTP/2 or HTTP/3 to request multiple information resources concurrently, in concurrent streams, but each request must be for a complete information resource; there is no capability of transferring incremental updates. Hence, there can be large overhead when the client already has an information resource and then there are small changes to the resource. Next, consider ALTO/SSE. Although ALTO/SSE can transfer incremental updates, it introduces a customized multiplexing protocol on top of HTTP, assuming a total-order message channel from the server to the client. The multiplexing design does not provide naming (i.e., providing resource identifier) to individual incremental updates. Hence, the design cannot use concurrent per-stream server push or non-blocking per-stream client pull, available in HTTP/2 and HTTP/3, because both cases require the resource identifier. Additionally, ALTO/SSE is a push-only protocol, which denies the client flexibility in choosing how and when it receives updates. This document introduces a new ALTO service called the transport information publication service (TIPS), which allows an ALTO server to publish the naming of individual incremental updates to multiple ALTO information resources. TIPS gives an ALTO client the new capability to explicitly, concurrently (non-blocking) request (pull) specific incremental updates using native HTTP/2 or HTTP/3, while still functioning for HTTP/1.x. It also provides an ALTO server the new capability to concurrently push specific incremental updates using native HTTP/2 or HTTP/3 server push.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

TIPS Overview

Component Terminology This document uses the following components:

Network information resource:: A piece of retrievable information about network state, per .
Transport state (ts):: Defined in this document to be the container of transport information that is information about the network information resource. Though the transport state may include other transport information, it has two basic components: updates queue (uq) and receiver set (rs).
Updates queue (uq):: Contains a sequence of incremental updates about a network information resource. Each incremental update is assigned a sequence number, and a URI can be constructed using the sequence number. A static network information resource (e.g., Cost Map, Network Map) may need only a single updates queue. A dynamic network information resource (e.g., Filtered Cost Map) may create an updates queue within the transport state for each unique filter request.
Receiver set (rs):: The receiver set contains the set of clients who have requested to receive server push updates.
Information Resource Directory (IRD):: Requested by an ALTO client to discover available information resources provided by an ALTO server, per . Each entry in an IRD indicates a URI at which an ALTO server accepts requests, and returns either an information resource or an information resource directory that references additional information resources. An ALTO server's IRD MAY define one or more transport information publication services, which ALTO clients use to request new TIPS instances.
Transport Information Publication Service (TIPS):: Allows an ALTO client to create transport state, read transport state, and delete the transport state (from client’s view) for an ALTO information resource.
TIPS Queue State Service (QSS):: Allows an ALTO client to read the updates queue.
TIPS Update Read Service (URS):: Allows an ALTO client to read the content of a an update item from the updates queue
TIPS Update Push Service (UPS):: Allows an ALTO client to receive an element pushed by the ALTO server.

Figure 1 shows an example illustrating the TIPS abstraction. Each ALTO client (Client 1, Client 2, or Client 3) maintains a single HTTP connection with the ALTO server.

ALTO Transport Information.

TIPS Workflow The intended use case of TIPS is to run it as a service inside an HTTP/2 or HTTP/3 connection (though it can run inside an HTTP/1.x connection, without server push functionality). Specifically, an ALTO client opens an HTTP connection with an ALTO server, and uses TIPS to create (or at least receive a view of) a transport state at the server for each network information resource that the client wants to monitor. The transport state provides (1) the base URI that the server will host incremental updates for the information resource and (2) the start sequence number that the server will order the updates. The server will assign each additional update with a consecutive sequence number. In the simplest use case, the transport state provides enough information for the client to continuously pull each additional update, resulting in the following workflow:

ALTO TIPS Workflow Supporting Client Pull. | . | . . | POST to create/receive transport . . | state for resource 1 . . | ------------------------------------> | . | , . . |<------------------------------------- | . | . . | GET /content/ . | ------------------------------------------------->| | update content . . |<------------------------------------------------->| | . . | GET /content/ . | ------------------------------------------------->| | update content . . |<------------------------------------------------->| | | DELETE transport state for resource 1 . |-------------------------------------> | | | Close HTTP connection . |-------------------------------------->| o TIPS: Transport Information Publication Service URS : Update Read Service ]]> A client that prefers server push can use the following workflow:

ALTO TIPS Workflow Supporting Server Push. | . | . . | POST to create/receive transport . . | state for resource 1 and add . . | self to receiver set . . | ------------------------------------> | . | , . . |<------------------------------------- | . | . . | PUSH /content/ . | <------------------------------------------------ | | . . | PUSH /content/ . | <------------------------------------------------ | | | DELETE self from receiver set of . | resource 1 . |-------------------------------------> | | | Close HTTP connection . |-------------------------------------->| o TIPS: Transport Information Publication Service UPS : Update Push Service ]]>

TIPS With HTTP/1.x While TIPS is designed to take advantage of newer HTTP features, TIPS still functions with HTTP/1.x for client pull functionality only, with the limitation that to cancel a long pull request for a resource update, it cannot send a DELETE request. Additionally, only one request can be outstanding at a time due to the blocking nature of HTTP/1.x.

TIPS Information Resources The TIPS framework defines 4 types of service: Transport Information Publication Service (TIPS), the TIPS Queue State Service (QSS), the TIPS Update Read Service (URS), and the TIPS Update Push Service (UPS). This document defines TIPS (Section 4), the QSS (Section 5), the URS (Section 6), and the UPS (Section 7).

Notations This document uses the same syntax and notations as introduced in Section 8.2 of RFC 7285 to specify the extensions to existing ALTO resources and services.

Transport Information Publication Service A TIPS service allows an ALTO client to create transport state, read transport state and delete the transport state for an ALTO information resource.

TIPS Information Resource Directory An ALTO server's IRD (Information Resource Directory) MAY define one or more transport information publication services, which ALTO clients use to request new TIPS instances. An IRD entry defining a TIPS MUST declare the "media-type", and specify "capabilities" and "uses" as follows.

Media Type The media type of the Transport Information Publication Service resource is "application/alto-tips+json".

Capabilities The capabilities declaration of TIPS is modelled on that defined in Section 6.3 of . Specifically, the capabilities are defined as an object of type TIPSCapabilities: String; } IncrementalUpdateMediaTypes; ]]> with field:

incremental-change-media-types:: If a TIPS can provide updates with incremental changes for a resource, the "incremental-change-media-types" field has an entry for that resource-id, and the value is the supported media types of the incremental change separated by commas. For the implementation of this specification, this will be "application/merge-patch+json", "application/json-patch+json", or "application/merge-patch+json,application/json-patch+json". Future extensions may define other types of incremental changes. When choosing the media types to encode incremental updates for a resource, the server MUST consider the limitations of the encoding. For example, when a JSON merge patch specifies that the value of a field is null, its semantics are that the field is removed from the target and hence the field is no longer defined (i.e., undefined). This, however, may not be the intended result for the resource, when null and undefined have different semantics for the resource. In such a case, the server MUST choose JSON patch over JSON merge patch if JSON patch is indicated as a capability of the TIPS. If the server does not support JSON patch to handle such a case, the server then needs to send a full replacement.
support-server-push:: The "support-server-push" field specifies whether the given TIPS supports server push. If the "support-server-push" field is "true", this TIPS will allow a client to start or stop server push.

Uses The "uses" attribute MUST be an array with the resource-ids of every network information resource for which this TIPS can provide service. This set may be any subset of the ALTO server's network information resources and may include resources defined in linked IRDs. However, it is RECOMMENDED that the ALTO server selects a set that is closed under the resource dependency relationship. That is, if a TIPS' "uses" set includes resource R1 and resource R1 depends on ("uses") resource R0, then the TIPS' "uses" set SHOULD include R0 as well as R1. For example, a TIPS for a cost map SHOULD also provide transport state for the network map upon which that cost map depends.

Example Extending the IRD example in Section 8.1 of , below is the IRD of an ALTO server supporting ALTO base protocol, ALTO/SSE, and ALTO TIPS. Note that it is straightforward for an ALTO sever to run HTTP/2 and support concurrent retrieval of multiple resources such as "my- network-map" and "my-routingcost-map" using multiple HTTP/2 streams. The resource "update-my-costs-tips" provides an ALTO TIPS based connection, and this is indicated by the media-type "application/alto-tips+json".

Create Transport State using TIPS

Request An ALTO client requests that the server sets up transport state for a given resource by sending an HTTP POST body with the media type "application/alto-tipsparams+json". That body contains a JSON object of type TIPSReq, where: with the following fields:

resource-id/tag:: The resource-id of an ALTO resource and MUST be in the TIPS' "uses" list (Section 4.1). 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 , and the ALTO client has previously retrieved a version of that resource from ALTO, the ALTO client MAY set the "tag" field to the tag part of the client's version of that resource. The server MAY use the tag when creating the updates queue. Note that the client MUST support all incremental methods from the set announced in the server's capabilities for this resource. If a client does not support all incremental methods from the set announced in the server's capabilities, the client MUST NOT use the TIPS service.
input:: If the resource is a POST-mode service that requires input, the ALTO client MUST set the "input" field to a JSON object with the parameters that the resource expects.

Response If the request has any errors, the TIPS service MUST return an HTTP "400 Bad Request" to the ALTO client; the body of the response follows the generic ALTO error response format specified in Section 8.5.2 of . Hence, an example ALTO error response has the format: Note that "field" and "value" are optional fields. If the "value" field exists, the "field" field MUST exist.

If the TIPS request does not have a "resource-id" field, the error code of the error message MUST be E_MISSING_FIELD and the "field" field SHOULD be "resource-id". The TIPS service MUST NOT create any transport state.
If the "resource-id" field is invalid or is not associated with the TIPS, the error code of the error message MUST be E_INVALID_FIELD_VALUE. The "field" field SHOULD be the full path of the "resource-id" field, and the "value" field SHOULD be the invalid resource-id.
If the resource is a POST-mode service that requires input, the client MUST set the "input" field to a JSON object with the parameters that that resource expects. If the "input" field is missing or invalid, TIPS MUST return the same error response that resource would return for missing or invalid input (see ).

The response to a valid request MUST be a JSON object of type AddTIPSResponse, denoted as media type "application/alto-tips+json": with field:

transport-state-uri:: The ALTO client MUST evaluate the URI according to . Hence, the transport state may be located at the same host as the invoking TIPS or on a different host. Let absolute-transport-state-uri be the absolute URI after resolution. When creating the URI for the transport state, the TIPS service MUST NOT use other properties of an HTTP request, such as cookies or the client's IP address, to determine the transport state. Furthermore, TIPS MUST NOT reuse a URI for a different object in the same connection. It is expected that there is an internal mechanism to map a transport-state-uri to the transport state to be accessed. For example, TIPS may assign a unique, internal state id to each transport state instance. However, the exact mechanism is left to the TIPS provider.
transport-state/updates-queue-meta/uri:: This must be a relative URI segment. Specifically, let uq-uri denote transport-state/updates-queue-meta/uri and define absolute-transport-state-uri/uq-uri absolute-transport-state-uri/uq-uri/number absolute-transport-state-uri/uq-uri/content/number. Each of them MUST be a valid URI.
transport-state/updates-queue-meta/state-summary/start-seq:: This MUST be a valid non-negative integer. The start-seq tells the client where to start reading from the updates queue.

Read Transport State using TIPS An ALTO client can read the transport state using the returned absolute-transport-state-uri (see Section 4.2.2) using the HTTP GET method. For example, if it is on the same connection as that of the invoking TIPS, it is ]]> The response to a valid request MUST be a JSON object of type TransportState defined in Section 4.2, denoted as media type "application/alto-tips+json".

Delete Transport State using TIPS An ALTO client can delete the transport state (from its view) using the returned absolute-transport-state-uri using the HTTP DELETE method. Whether or not the server actually deletes the transport is implementation dependent. A server will not want to delete a transport state if another client is using it. If the transport state is on the same connection as that of the invoking TIPS, the DELETE request looks like: ]]> The response to a valid request MUST be 200 if success, and the corresponding error code if there is any error.

Example For simplicity, assume that the ALTO server is using the Basic authentication. If a client with username "client1" and password "helloalto" wants to create a transport state of an ALTO Cost Map resource with resource ID "my-routingcost-map", it can send the following request: If the operation is successful, the ALTO server returns the following message:

TIPS Queue State Service (QSS) The TIPS QSS service allows an ALTO client to read the updates queue. It uses the URI returned in Section 4.

Media Type A QSS response uses "application/alto-tipsqueue+json" media type.

HTTP Method An ALTO QSS resource is requested using the HTTP GET method.

IRD Capabilities and Uses None (QSS does not appear in the IRD).

Request An ALTO client reads the queue state using an HTTP GET method, with the path constructed as: := / specified in Section 4. ]]>

Response If the transport state is closed, the ALTO server MUST return an error HTTP message with status code 404 (Not Found) and media type "application/alto-error+json". The response from the server, if no error, is the UpdatesQueue object indicated as "application/alto-tipsqueue+json" media type: UpdateItem; } UpdatesQueue; object-map { JSONString media-type; JSONString tag; [JSONString link;] } UpdateItem; ]]> with field:

Seq:: A required, positive JSON integer indicating the sequence number of the incremental update. As JSON allows a large integer space, when the server reaches the largest integer, the server SHOULD close the incremental updates queue.
media-type:: A required JSON string giving the type of the incremental update (see ).
tag:: A required JSON string which is defined the same way as the "tag" field of VersionTag (see Section 10.3 of ).
link:: An optional JSON string giving an optional link for a client to directly request a resource as a complete snapshot (not through incremental updates).

Example An ALTO client can also read the (meta data) of an individual element in the updates queue. The HTTP method is a GET, and the URI is constructed by appending the corresponding seq to preceding URI: := / ]]>

TIPS Update Read Service (URS) The TIPS URS service allows an ALTO client to read the content of an update item from the updates queue.

Path The path to the content of an update item resource has the following format: := /content/ ]]> where updates-queue-complete-uri is specified in Section 5.4. For example, if the client wants to query the content of the first update item (with sequence number 101), it can send a request to "/tips/2718281828459/uq/content/101".

Media Type The media type of an Update Item resource MUST be the same as the "media-type" field of the update item with sequence number seq in the updates queue. For example, if the client wants to query the content of the first update item (with sequence number 101) whose media type is "application/alto-costmap+json", it must set the "Accept" header to "application/alto-costmap+json, application/alto-error+json" in the request.

HTTP Method The Update Read Service resource is requested using the HTTP GET method.

Capabilities and Uses None

Response If the transport state is closed, the ALTO server MUST return an error HTTP message with status code 404 (Not Found) and media type "application/alto-error+json". Otherwise, let LOW_SEQ and HIGH_SEQ represent the smallest and largest sequence number in the updates queue of the transport state respectively. If seq is smaller than LOW_SEQ, the ALTO server MUST return an error HTTP message with status code 410 (Gone) and media type "application/alto-error+json". If seq is greater than HIGH_SEQ, the ALTO server MAY either treat the request as a long polling request and save it as a pending request; if the server wants to conduct resource control, the server can return an error code and indicate the "Re-Try After" headers. If the request is valid, the response is encoded as a JSON object whose data format is indicated by the media type. When a client reads an entry from the updates queue, that entry is not removed from the queue. This is particularly important if two clients have access to the same updates queue, as they may be reading from different places in the queue (depending on the frequency of client pull and the starting seq the server determined for each client).

Example Assume the client wants to get the contents of updates item 101, the request is: And the response will be

TIPS Update Push Service (UPS) The TIPS UPS service allows an ALTO client to receive an element pushed by the ALTO server. If a client registers for UPS, it should not request updates via URS to avoid receiving the same information twice, unless the client does not receive the expected updates (see Section 9.2).

Manage Server Push: Receiver Set

Receiver Set Operations A client starts to receive server push when it is added to the receiver set. A client can add itself to the receiver set when creating the transport queue, or add itself explicitly to the receiver set. A client can read the status of the receiver set and delete itself from the receiver set to stop server push. Implicit Create: As a short cut, when creating a transport queue, an ALTO client can start server push by setting the "incremental-changes" field to be true when creating a transport queue using the HTTP POST method with ALTO SSE AddUpdateReq ( Sec. 6.5) as the parameter: PUT Create: A client can add itself in the receiver set by using the HTTP PUT method: PUT transport-queue/rs/self Read: A client can see only itself in the receiver set. The appearance of self in the receiver set (read does not return "not exists" error) is an indication that push starts. Delete: A client can delete itself (stops receiving push) either explicitly or implicitly.

Explicit delete: A client deletes itself using the HTTP DELETE method: DELETE transport-queue/rs/self.
Implicit delete: Transport queue is connection ephemeral: the close of connection or stream for the transport queue deletes the transport queue (from the view) for the client.

Examples The first example is a client creating a transport queue and starting server push. server request HEADERS - END_STREAM + END_HEADERS :method = POST :scheme = https :path = /tqs host = alto.example.com accept = application/alto-error+json, application/alto-transport+json content-type = application/alto-transport+json content-length = 69 DATA - END_STREAM { "resource-id": "my-routingcost-map", ["incremental-push": true] } ]]> client response: HEADERS - END_STREAM + END_HEADERS :status = 200 content-type = application/alto-transport+json content-length = 28 DATA - END_STREAM {"tq": "/tqs/2718281828459"} ]]> If the client reads the status of the transport queue created above using the read operation (GET) in the same HTTP connection, the client should see itself in the receiver set: server request HEADERS - END_STREAM + END_HEADERS :method = GET :scheme = https :path = /tqs/2718281828459 host = alto.example.com accept = application/alto-error+json, application/alto-transport+json Server -> client response: HEADERS - END_STREAM + END_HEADERS :status = 200 content-type = application/alto-transport+json content-length = 519 DATA - END_STREAM { "uq": [ {"seq": 101, "media-type": "application/alto-costmap+json", "tag": "a10ce8b059740b0b2e3f8eb1d4785acd42231bfe"}, {"seq": 102, "media-type": "application/merge-patch+json", "tag": "cdf0222x59740b0b2e3f8eb1d4785acd42231bfe"}, {"seq": 103, "media-type": "application/merge-patch+json", "tag": "8eb1d4785acd42231bfecdf0222x59740b0b2e3f", "link": "/tqs/2718281828459/snapshot/2e3f"} ], "rs": ["self"] } ]]> A client can stop incremental push updates from the server to itself by sending the request:

Server Push of Incremental Updates

Server Push The work flow of server push of individual updates is the following:

Initialization: the first update pushed from the server to the client MUST be the later of the following two: (1) the last independent update in the incremental updates queue; and (2) the following entry of the entry that matches the tag when the client creates the transport queue. The client MUST set SETTINGS_ENABLE_PUSH to be consistent.
Push state: the server MUST maintain the last entry pushed to the client (and hence per client, per connection state) and schedule next update push accordingly.
Push management: The client MUST NOT cancel (RST_STREAM) a PUSH_PROMISE to avoid complex server state management.

Examples A client can wait for the server for incremental push, where the server first sends PUSH_PROMISE, for the first example in Sec. 2.2: client PUSH_PROMISE in current stream: PUSH_PROMISE - END_STREAM Promised Stream 4 HEADER BLOCK :method = GET :scheme = https :path = /tqs/2718281828459/uq/101 host = alto.example.com accept = application/alto-error+json, application/alto-costmap+json Server -> client content Stream 4: HEADERS + END_STREAM + END_HEADERS :status = 200 content-type = application/alto-costmap+json content-length = 539 DATA + END_STREAM { "meta" : { "dependent-vtags" : [{ "resource-id": "my-network-map", "tag": "da65eca2eb7a10ce8b059740b0b2e3f8eb1d4785" }], "cost-type" : { "cost-mode" : "numerical", "cost-metric": "routingcost" }, "vtag": { "resource-id" : "my-routingcost-map", "tag" : "3ee2cb7e8d63d9fab71b9b34cbf764436315542e" } }, "cost-map" : { "PID1": { "PID1": 1, "PID2": 5, "PID3": 10 }, "PID2": { "PID1": 5, "PID2": 1, "PID3": 15 }, "PID3": { "PID1": 20, "PID2": 15 } } } Server -> client PUSH_PROMISE in current stream: PUSH_PROMISE - END_STREAM Promised Stream 6 HEADER BLOCK :method = GET :scheme = https :path = /tqs/2718281828459/uq/102 host = alto.example.com accept = application/alto-error+json, application/merge-patch+json Server -> client content Stream 6 HEADERS + END_STREAM + END_HEADERS :status = 200 content-type = application/merge-patch+json content-length = 7 DATA + END_STREAM { ... } ]]>

Server Push Stream Management

Server -> Client [PUSH_PROMISE for Transport Queue on Stream SID_tq] The server push MUST satisfy the following requirements:

PUSH_PROMISE MUST be sent in stream SID_tq to serialize to allow the client to know the push order;
Each PUSH_PROMISE chooses a new server-selected stream ID, and the stream is closed after push.

Operation and Processing Considerations

Considerations for Choosing Updates TIPS should be cognizant of the effects of its update schedule, which includes both the choice of timing (i.e., when/what to trigger an update on the updates queue) and the choice of message format (i.e., given an update, send a full replacement or an incremental change). In particular, the update schedule can have effects on both the overhead and the freshness of information. To minimize overhead, TIPS may choose to batch a sequence of updates for resources that frequently change by cumulative updates or a full replacement after a while. TIPS should be cognizant that batching reduces the freshness of information. The server should also consider the effect of such delays on client behaviors. For incremental updates, this design allows both JSON patch and JSON merge patch for incremental changes. JSON merge patch is clearly superior to JSON patch for describing incremental changes to cost maps, endpoint costs, and endpoint properties. For these data structures, JSON merge patch is more space efficient, as well as simpler to apply. There is no advantage allowing a server to use JSON patch for those resources. The case is not as clear for incremental changes to network maps. First, consider small changes, such as moving a prefix from one PID to another. JSON patch could encode that as a simple insertion and deletion, while JSON merge patch would have to replace the entire array of prefixes for both PIDs. On the other hand, to process a JSON patch update, the ALTO client would have to retain the indexes of the prefixes for each PID. Logically, the prefixes in a PID are an unordered set, not an array; aside from handling updates, a client has no need to retain the array indexes of the prefixes. Hence, to take advantage of JSON patch for network maps, ALTO clients would have to retain additional, otherwise unnecessary, data. Second, consider more involved changes, such as removing half of the prefixes from a PID. JSON merge patch would send a new array for that PID, while JSON patch would have to send a list of remove operations and delete the prefix one by one. Therefore, each TIPS may decide on its own whether to use JSON merge patch or JSON patch according to the changes in network maps.

Considerations for Client Processing Updates In general, when an ALTO client receives a full replacement for a resource, the ALTO client should replace the current version with the new version. When an ALTO client receives an incremental change for a resource, the ALTO client should apply those patches to the current version of the resource. However, because resources can depend on other resources (e.g., cost maps depend on network maps), an ALTO client MUST NOT use a dependent resource if the resource on which it depends has changed. There are at least two ways an ALTO client can do that. The following paragraphs illustrate these techniques by referring to network and cost map messages, although these techniques apply to any dependent resources. Note that when a network map changes, the update stream server MUST send the network map update message before sending the updates for the dependent cost maps. One approach is for the ALTO client to save the network map update message in a buffer and continue to use the previous network map and the associated cost maps until the ALTO client receives the update messages for all dependent cost maps. The ALTO client then applies all network and cost map updates atomically. Alternatively, the ALTO client MAY update the network map immediately. In this case, the cost maps using the network map become invalid because they are inconsistent with the current network map; hence, the ALTO client MUST mark each such dependent cost map as temporarily invalid and MUST NOT use each such cost map until the ALTO client receives a cost map update indicating that it is based on the new network map version tag. When implementing server push, the server SHOULD send updates for dependent resource (i.e., the cost maps in the preceding example) in a timely fashion. However, if the ALTO client does not receive the expected updates, a simple recovery method is that the ALTO client uses client pull to request the missing update. The ALTO client MAY retain the version tag of the last version of any tagged resources and search those version tags when identifying the new updates to pull. Although not as efficient as possible, this recovery method is simple and reliable.

Considerations for Updates to Filtered Cost Maps If TIPS provides updates to a Filtered Cost Map that allows constraint tests, then an ALTO client MAY request updates to a Filtered Cost Map request with a constraint test. In this case, when a cost changes, the updates queue MUST have an update if the new value satisfies the test. If the new value does not, whether there is an update depends on whether the previous value satisfied the test. If it did not, the updates queue SHOULD NOT have an update. But if the previous value did, then the updates queue MUST add an update with a "null" value to inform the ALTO client that this cost no longer satisfies the criteria. TIPS can avoid having to handle such a complicated behavior by offering TIPS only for Filtered Cost Maps that do not allow constraint tests.

Considerations for Updates to Ordinal Mode Costs For an ordinal mode cost map, a change to a single cost point may require updating many other costs. As an extreme example, suppose the lowest cost changes to the highest cost. For a numerical mode cost map, only that one cost changes. But for an ordinal mode cost map, every cost might change. While this document allows TIPS to offer incremental updates for ordinal mode cost maps, TIPS implementors should be aware that incremental updates for ordinal costs are more complicated than for numerical costs, and ALTO clients should be aware that small changes may result in large updates. A TIPS can avoid this complication by only offering full replacements as updates in the updates queue for ordinal cost maps.

Security Considerations The security considerations (Section 15 of ) of the base protocol fully apply to this extension. For example, the same authenticity and integrity considerations (Section 15.1 of ) still fully apply; the same considerations for the privacy of ALTO users (Section 15.4 of ) also still fully apply. The additional services (addition of update read service and update push service) provided by this extension extend the attack surface described in Section 15.1.1 of . Below, we discuss the additional risks and their remedies.

TIPS: Denial-of-Service Attacks Allowing persistent TIPS states enables a new class of Denial-of-Service attacks. In particular, For the TIPS server, an ALTO client might create an unreasonable number of transport states. To avoid these attacks on the TIPS server, the server SHOULD choose to limit the number of active states and reject new requests when that threshold is reached. The Update Read Service allows predictive fetching and the server SHOULD also choose to limit the number of pending requests. In these cases, the update stream server SHOULD return the HTTP status "503 Service Unavailable". It is important to note that the preceding approaches are not the only possibilities. For example, it may be possible for TIPS to use somewhat more clever logic involving IP reputation, rate-limiting, and compartmentalization of the overall threshold into smaller thresholds that apply to subsets of potential clients.

ALTO Client: Update Overloading or Instability The availability of continuous updates, when the client indicates receiving server push, can also cause overload for an ALTO client, in particular, an ALTO client with limited processing capabilities. The current design does not include any flow control mechanisms for the client to reduce the update rates from the server. Under overloading, the client MAY choose to remove the information resources with high update rates. Also, under overloading, the client may no longer be able to detect whether information is still fresh or has become stale. In such a case, the client should be careful in how it uses the information to avoid stability or efficiency issues.

Spoofed URI An outside party that can read the TIPS response or that can observe TIPS requests can obtain the transport state URI and use that to send fraudulent "DELETE" requests, thus disabling the service for the valid ALTO client. This can be avoided by encrypting the requests and responses (see Section 15 of ). In general, as the architecture allows the possibility for TIPS, URS and UPS to be different entities, the additional risks should be evaluated and remedied. For example, the private communication path between the servers may be attacked, resulting in a risk of communications breakdown between them, as well as invalid or spoofed messages claiming to be on that private communications path. Proper security mechanisms, including confidentiality, authenticity, and integrity mechanisms, should be considered.

IANA Considerations IANA is requested to register the following media type under ALTO registry as defined in :

application/alto-tips+json: as described in ;
application/alto-tipsparams+json: as described in ;
application/alto-tipsqueue+json: as described in

application/alto-tips+json Media Type

Type name:

application

Subtype name:

alto-tips+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 .

Security considerations:

Security considerations relating to the generation and consumption of ALTO Protocol messages are discussed in Section 10 of and Section 15 of .

Interoperability considerations:

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

Published specification:

of this document.

Applications that use this media type:

ALTO servers and ALTO clients either stand alone or are embedded within other applications.

Fragment identifier considerations:

N/A

Additional information:

Deprecated alias names for this type:: N/A
Magic number(s):: N/A
File extension(s):: This document uses the media type to refer to protocol messages and thus does not require a file extension.
Macintosh file type code(s):: N/A

Person and 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).

application/alto-tipsparams+json Media Type

Type name:

application

Subtype name:

alto-tipsparams+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 .

Security considerations:

Security considerations relating to the generation and consumption of ALTO Protocol messages are discussed in Section 10 of and Section 15 of .

Interoperability considerations:

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

Published specification:

of this document.

Applications that use this media type:

ALTO servers and ALTO clients either stand alone or are embedded within other applications.

Fragment identifier considerations:

N/A

Additional information:

Deprecated alias names for this type:: N/A
Magic number(s):: N/A
File extension(s):: This document uses the media type to refer to protocol messages and thus does not require a file extension.
Macintosh file type code(s):: N/A

Person and 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).

application/alto-tipsqueue+json Media Type

Type name:

application

Subtype name:

alto-tipsqueue+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 .

Security considerations:

Security considerations relating to the generation and consumption of ALTO Protocol messages are discussed in Section 10 of and Section 15 of .

Interoperability considerations:

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

Published specification:

of this document.

Applications that use this media type:

ALTO servers and ALTO clients either stand alone or are embedded within other applications.

Fragment identifier considerations:

N/A

Additional information:

Deprecated alias names for this type:: N/A
Magic number(s):: N/A
File extension(s):: This document uses the media type to refer to protocol messages and thus does not require a file extension.
Macintosh file type code(s):: N/A

Person and 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).

Acknowledgments The authors of this document would also like to thank Adrian Farrel, Qin Wu, Jordi Ros Giralt, and the many others who reviewed and commented on previous drafts of this document.