The 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 (if any) resource to the client; and (2) ALTO incremental updates using Server-Sent Events (ALTO/SSE) , which is designed for an ALTO client to indicate 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 that 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 higher HTTP versions are used. 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). TIPS uses an incremental RESTful design to provide an ALTO client with a 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. TIPS also provides an ALTO server to concurrently push specific incremental updates using native HTTP/2 or HTTP/3 server push. Specifically, this document specifies the following:

Extensions to the ALTO Protocol to create, update, or remove an incremental representational state transfer resource (iREST).
A new resource type that specifies the iREST graph model for a resource.
URI patterns to fetch the snapshots or incremental updates.

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.

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

TIPS Overview

Basic Data Model: Incremental Representational State Transfer (iREST) For each resource (e.g., a cost map, network map), the incremental updates and snapshots can be represented using the following directed acyclic graph model, where the server maps base resource IDs to incremental update IDs that are assigned sequentially:

Each node in the graph is a version of the resource, where a tag identifies content of version (tag is valid only within the scope of resource). Version 0 is reserved as the initial state (empty/null).
Each edge is transport data (update item). In particular, edge from i to j is the (op, data) to transition version from version i to version j.
Node content is path independent (different paths arrive at the same content)

A concrete example is as shown in . Thick lines represent incremental updates and thin lines represent snapshots. Note that node content is path independent: the content can be obtained by applying the updates from different paths. For example, assume the client already has version at 103: the version at 105 can either be directly fetched as a snapshot or computed incrementally by applying the incremental updates between 103 and 104, then 104 and 105.

iREST Model Example | 101 | | | +======+ | | || | | ID101-102 || | | \/ | | +======+ | | tag: 123566431234 -> | 102 | | | +======+ | | || | | ID102-103 || ID0-103| | \/ | | +======+ / | tag: 123400881080 -> | 103 |<----/ | +---------------+ +======+ | | Base Resource | =====> || | +---------------+ ID103-104 || | \/ | +======+ | tag: 123566452654 -> | 104 | | +======+ | || | ID0-105 ID104-105 || | \/ / +======+ / tag: 123567838392 -> | 105 |<-------/ +======+ || ID105-106 || \/ +======+ tag: 123566470983 -> | 106 | +======+ tag: denotes the version of the resource at node i ID###-###: denotes update item (op, data) to transition from version at node i to version at node j ]]>

iREST Resource Location Schema To access each individual update, consider the model represented as a "virtual" file system (adjacency list).

Location Schema for . ug // updates graph 0 101 // full 101 content 103 105 101 102 // 101 -> 102 patch 102 103 103 104 104 105 105 106 push // server push metadata ... Location -> resource naming (ID assignment) - /ug// Client Pull: GET //ug/0/105 GET //ug/105/106 Server Push: Use the URI as above ]]> Due to the sequential nature of the update patch IDs, a client can long pull a future update that does not yet exist (e.g., the patch from 106 to 107) by constructing the URI: /ug// ]]>

High-level Service Model Conceptually, the TIPS system consists of 3 types of resources:

(R1) TIPS frontend to manage (create/delete) TIPS views.

(R2) TIPS view directory, which provides metadata (e.g., references) about the network resource data.

(R3) The actual network resource data, encoded as complete ALTO network resources (e.g., cost map, network map) or incremental updates.

Service Model | | | | | | | | | incremental | | | | |-------->| | | | | transfer | | | | | | | | | | resource | | | | | | | | | |<------------|-----| | +-----+ +-----+ | |Client| | | | | | | "iget" base | | | | | | resource 2 | | | +-----+ +-----+ | | |-------------|---->| | | | | | | | | incremental | | | | | | | | | | transfer | | | | | ------->| | | | | resource | | | | | | | | | |<------------|-----| | | | | | | +------+ | +-----+ +-----+ +-----+ | | | +------------------------------------------------+ ]]>
Design Point: Component Resource Location

Design 1 (Single): all 3 types at the same, single server (accessed via relative reference)

Design 2 (Flexible): all 3 types can be at their own server (accessed via absolute reference)

Design 3 (Dir + Data): R2 and R3 must remain together, though R1 might not be on the same server

This document specifies Design 1 (keeping R1, R2, and R3 on the same server) in order to simplify session management, though at the expense of maximum load balancing flexibility (see for a discussion on load balancing considerations). A future companion document may extend the protocol to support Design 2 or Design 3.

TIPS Terminology This document uses the following components:

Network information resource:

A piece of retrievable information about network state, per .

TIPS view (ts):

Defined in this document to be the container of incremental transport information about the network information resource. Though the TIPS view may include other transport information, it has two basic components: updates graph (ug) and receiver set (rs).

Updates graph (ug):

Contains a sequence of incremental updates and snapshots (called update items) of a network information resource, based on the iREST model defined above. Each incremental update is assigned a sequence number, and a URI can be constructed using the sequence numbers. A static network information resource (e.g., Cost Map, Network Map) may need only a single updates graph. A dynamic network information resource (e.g., Filtered Cost Map) may create an updates graph (within a new TIPS view) 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.

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 retrieve a JSON object that contains a URI to a TIPS view of each network information resource that the client wants to monitor, along with a summary of each TIPS view (which provides, at minimum, the start and end sequence number of the updates graph and a server-recommended edge to consume first). In the simplest use case, the TIPS view summary 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 a TIPS view . | for resource 1 . | ------------------------------------> | | , . |<------------------------------------- | | . | GET //ug// . | --------------------------------------| | content on edge i to j . |<--------------------------------------| | . | GET //ug// . | --------------------------------------| | content on edge j to j+1 . |<--------------------------------------| | . | DELETE TIPS view for resource 1 . |-------------------------------------> | | . | Close HTTP connection . |-------------------------------------->| o TIPS: Transport Information Publication Service ]]>
A client that prefers server push can use the following workflow:
ALTO TIPS Workflow Supporting Server Push. | | . | POST to create/receive a TIPS view . | for resource 1 and add . | self to receiver set . | ------------------------------------> | | , . |<------------------------------------- | | . | PUSH /ug// . | <-------------------------------------| | . | PUSH /ug// . | <-------------------------------------| | . | PUT to remove self from receiver . | set of resource 1 . |-------------------------------------> | | . | Close HTTP connection . |-------------------------------------->| o TIPS: Transport Information Publication 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 it cannot cancel any outstanding requests or fetch resources concurrently over the same connection.

TIPS Information Resource Directory (IRD) Announcement 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. If the field is FALSE or not present, this TIPS does not provide 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 a TIPS view 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".

TIPS Open/Close Upon request, a server sends a TIPS view to a client. This TIPS view may be created at the time of the request or may already exist (either because another client has an active connection to a TIPS view for the same requested network resource or because the server perpetually maintains a TIPS view for an often requested resource). The server MAY keep track of which clients have an active connection to each TIPS view to determine whether or not it should delete a TIPS view and its corresponding updates graph and associated data.
Open Request An ALTO client requests that the server provide a TIPS view 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:

The resource-id of an ALTO resource and MUST be in the TIPS' "uses" list (). 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.

tag:

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 calculating a recommended starting edge for the client to consume. Note that the client MUST support all incremental methods from the set announced in the server's capabilities for this resource.

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.

server-push:

Set to TRUE if a client desires to receive updates via server push. If the value is FALSE or not present, the client does not accept server push updates.

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

tips-view-uri:

Relative URI to the TIPS view of a network resource. When creating the URI for the TIPS view, TIPS MUST NOT use other properties of an HTTP request, such as cookies or the client's IP address, to determine the TIPS view. 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 tips-view-uri to the TIPS view to be accessed. For example, TIPS may assign a unique, internal state id to each TIPS view instance. However, the exact mechanism is left to the TIPS provider.

tips-view-summary:

Contains both an updates-graph-summary and an optional server-push boolean value which is set to TRUE if and only if the client indicates server push. The updates-graph-summary field contains the starting sequence number (start-seq) of the updates graph and the last sequence number (end-seq) that is currently available, along with a recommended edge to consume (start-edge-rec). How the server calculates the recommended edge depends on the implementation. Ideally, if the client does not provide a version tag, the server should recommend the edge of the latest snapshot available. If the client does provide a version tag, the server should calculate the cumulative size of the patch updates availabe from that version onward and compare it to the size of the complete resource snapshot. If the snapshot is bigger, the server should recommend the first patch update edge starting from client's tagged version. Else, the server should recommend the latest snapshot edge. If the client indicates server push, the recommended edge will be the first content pushed.

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 TIPS view.

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 ).

Open 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 TIPS view 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:

Close Request An ALTO client can indicate it no longer desires to pull/receive updates for a specific network resource by "deleting" the TIPS view using the returned tips-view-uri and the HTTP DELETE method. Whether or not the server actually deletes the TIPS view is implementation dependent. Likely, a server will remove the client from a dependency set associated with the TIPS view. A server will not want to delete a TIPS view if another client is using it. 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. If the connection between the client and TIPS provider is severed without a DELETE request having been sent, the server MUST treat it as if the client had sent a DELETE request because the TIPS view is, at least from the client view, per-session based.

TIPS View Metadata Directory An ALTO client can read the metadata directory of the TIPS view to understand its full contents beyond the intial summary provided.
Request An ALTO client requests the updates graph's metadata directory by sending an HTTP GET request with the media type "application/alto-tipsview+json". The GET request looks like: ]]> An ALTO client can also read the metadata directory of just the the updates graph (perhaps to calculate a new shortest path to the end of the graph after not having fetched for a period of time). The HTTP method is a GET, and looks like: /ug ]]> An ALTO client can also read the metadata of an individual edge in the updates graph. The HTTP method is a GET, and looks like: /ug/meta// ]]>

Response If the TIPS view 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, for the entire metadata directory of the TIPS view is the UpdatesDirectory object indicated as "application/alto-tipsview+json" media type: EndSeqUpdates; } UpdatesGraph; object-map { JSONString -> UpdateItemMeta; } EndSeqUpdates; object { JSONString media-type; JSONString tag; JSONNumber size; } UpdateItemMeta; object { Boolean server-push; NextEdge next-edge; } PushState; object { JSONNumber seq-i; JSONNumber seq-j; } NextEdge; ]]> with the following fields:

meta:

Specifies the network resource-id TIPS view to which the directory corresponds and the version tag of the directory itself.

updates-graph:

Contains a map of edges in the updates graph (start-seq to end-seq). Each edge has a corresponding update-item-meta which includes the edge's media type, resource version tag, and size of the content in kilobytes. Note that two different edges will share the same resource version tag if they connect to the same end-seq node in the graph.

push-state:

Contains server-push boolean which is set to TRUE if the client has indicated server push. Otherwise, it is set to FALSE. push-state also contains next-edge which indicates the next edge that will be pushed to the client. If server-push is FALSE, next-edge is NULL.

Example

Refresh Directory An ALTO client can refresh the updates graph directory without re-fetching the whole updates graph object.
Request An ALTO client requests the TIPS view metadata directory by sending an HTTP GET request with the media type "application/alto-tipsview+json" and the version tag of the TIPS view directory it has currently. The GET request looks like:

Response The server may return the whole directory or the incremental changes, using merge-patch. A patch response looks like:

Server Updates Directory Invariants A server may change its updates graph (to compact, etc.), but it must satisfy the following invariants:

Continuity: ns -> ne, anything in between ns and ne also exists (implies ni -> ni + 1 patch exists), where ns is start-seq and ne is end-seq

Feasibility: let n0 denote the smallest version in N (where N is the complete set of nodes in the updates graph), the server can provide a snapshot of n0. In other words, there is always a direct link to ns

"Right shift" only: if a server provides [n1, n2] at time t and [n1', n2'] at time t'. If t' > t and get [n1', n2'], then n1' >= n1 and n2' >= n2.

For example, using the example model in , assume at time 0, the server provides the valid set [101, 102, 103, 104, 105, 106]. At time 1, both [103, 104, 105, 106] and [105, 106] are valid sets. However, [102, 103, 104, 105, 106] is not a valid set as there is no snapshot of the version at 102 and [104, 105, 106] is not a valid set as there is no snapshot of the version at 104. Thus, there is a risk that the right content of the version at 102, in the first example, and 104, in the second example, cannot be obtained by a client that does not have the previous version at 101 or 103, respectively.

TIPS Data Transfers - Client Pull TIPS allows an ALTO client to retrieve the content of an update item from the updates graph, with an update item defined as the content (patch or snapshot) on an edge in the updates graph.
Request The client sends an HTTP GET request, where the media type of an update item resource MUST be the same as the "media-type" field of the update item on the specified edge in the updates graph. For example, if the client wants to query the content of the first update item (0 -> 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. The GET request looks like /ug// ]]> For example, if the client wants to query the content of the first update item (0 -> 101), it can send a request to "/tips/2718281828459/ug/0/101".

Response If the request is valid (ug/<i>/<j> exists), the response is encoded as a JSON object whose data format is indicated by the media type. It is possible that a client conducts proactive fetching of future updates, by long pulling updates that have not been listed in the directory yet. For long-pull prefetch, the client must have indicated the media type which may appear. It is RECOMMENDED that the server allows for at least the prefetch of <end-seq> -> <end-seq + 1> Hence, the server processing logic SHOULD be:

If ug/<i>/<j> exists: return content using encoding.

Else if ug/<i>/<j> pre-fetch is acceptable: put request in a backlog queue.

Else: return error.

Server Processing "Error" Conditions It is RECOMMENDED that the server uses the following HTTP codes to indicate errors, with the media type "application/alto-error+json", regarding update item requests.

404 (Not Found): if the requested TIPS view does not exist or is closed.

410 (Gone): if an update has a seq that is smaller than the start-seq.

415 (Unsupported Media Type): if the media type(s) accepted by the client does not include the media type of the update chosen by the server.

425 (Too Early): if the seq exceeds the server prefetch window

429 (Too Many Requests): when the number of pending (long-pull) requests exceeds server threshold. Server may indicate when to re-try the request in the "Re-Try After" headers.

Example Assume the client wants to get the contents of the update item on edge 0 to 101. The request is: And the response will be

TIPS Data Transfer - Server Push TIPS allows an ALTO client to receive an update item pushed by the ALTO server. If a client registers for server push, it should not request updates via pull to avoid receiving the same information twice, unless the client does not receive the expected updates (see ).
Manage Server Push A client starts to receive server push when it is added to the receiver set. A client can read the status of the push state and remove itself from the receiver set to stop server push.
Start Server Push A client can add itself explicitly to the receiver set or add itself to the receiver set when requesting the TIPS view. Explicit add: A client can explicitly add itself in the receiver set by using the HTTP PUT method with media type "application/alto-tipsview+json", where the client may optionally specify a starting edge (next-edge) from which it would like to receive updates: /push object { Boolean server-push; [NextEdge next-edge;] } PushState; object { JSONNumber seq-i; JSONNumber seq-j; } NextEdge; ]]> with the following fields:

server-push:

Set to true if the client desires to receive server push updates.

next-edge:

Optional field to request a starting edge to be pushed if the client has pulled the updates graph directory and has calculated the path it desires to take. The server is MAY push this edge first if available.

Short cut add: When requesting a TIPS view, an ALTO client can start server push by setting the option "server-push" field to be true using the HTTP POST method defined in . Example of a client requesting a TIPS view and starting server push: server request HEADERS - END_STREAM + END_HEADERS :method = POST :scheme = https :path = /tips host = alto.example.com accept = application/alto-error+json, application/alto-tips+json content-type = application/alto-tips+json content-length = 69 DATA - END_STREAM { "resource-id": "my-routingcost-map", ["server-push": true] } Server -> client response: HEADERS - END_STREAM + END_HEADERS :status = 200 content-type = application/alto-tips+json content-length = 196 DATA - END_STREAM { "tips-view-uri": "/tips/2718281828459", "tips-view-summary": { "updates-graph-summary": { "start-seq": 101, "end-seq": 106, "start-edge-rec" : { "seq-i": 0, "seq-j": 105 } }, "server-push": true } } ]]>

Read Push State A client can use the HTTP GET method, with accept header set to "application/alto-tipsview+json" (defined in ) to check the status of server push. /push ]]> Example: server request HEADERS - END_STREAM + END_HEADERS :method = GET :scheme = https :path = /tips/2718281828459/push host = alto.example.com accept = application/alto-error+json, application/alto-tipsview+json Server -> client response: HEADERS - END_STREAM + END_HEADERS :status = 200 content-type = application/alto-tipsview+json content-length = 519 DATA - END_STREAM { "server-push": true; "next-edge": { "seq-i": 0, "seq-j": 105 } } ]]>

Stop Push A client can stop receiving server push updates either explicitly or implicitly. Explicit stop: A client stops push by using the HTTP PUT method to /<tips-view-uri>/push, with content type "application/alto-tipsview+json" and setting server-push to FALSE: Implicit stop: (1) TIPS view is connection ephemeral: the close of connection or stream for the TIPS view deletes the TIPS view from the view of the client. (2) The client sends a DELETE /<tips-view-uri> request, indicating it no longer is interested in the resource, which also deletes the client from the push receiver set if present. Note that a client may choose to explicitly stop server push for a resource, but may not delete the TIPS view so that it can switch seemlessly from server push to client pull in the case that the server push frequency is undesirable, without having to request a new TIPS view. Example of explicit stop: server request HEADERS - END_STREAM + END_HEADERS :method = PUT :scheme = https :path = /tips/2718281828459/push host = alto.example.com accept = application/alto-error+json content-type = application/alto-tipsview+json content-length = 69 DATA - END_STREAM { "server-push": false } Server-> client response HEADERS - END_STREAM + END_HEADERS :status = 200 ]]>

Server Push of Incremental Updates
Server Push The objective of the server is to push the latest version to the client using the lowest cost (sum of size) of the updates. Hence, it is RECOMMENDED that the server computes the push path using the following algorithm, upon each event computing a push:

Compute client current version (n_c). During initialization, if the TIPS view request has a tag, find that version; otherwise n_c = 0

Compute the shortest path from current version to latest version, n_c, n1, ... n_e (latest version). Note that the shortest path may not involve the tagged version and instead follow the edge from 0 to the latest snapshot.

push /<tips-view-uri>/ug/n_c/n1

Note

Initialization: the client MUST set SETTINGS_ENABLE_PUSH to be consistent. If the client specifically requests a starting edge to be pushed, the server MAY start with that edge, even if it is not the shortest path.

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 Using the example updates graph in , a client can wait on the server for incremental push, where the server first sends PUSH_PROMISE: client PUSH_PROMISE in current stream: PUSH_PROMISE - END_STREAM Promised Stream 4 HEADER BLOCK :method = GET :scheme = https :path = /tips/2718281828459/ug/0/101 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 = /tips/2718281828459/ug/101/102 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 { "meta": { "vtag": { "tag": "c0ce023b8678a7b9ec00324673b98e54656d1f6d" } }, "cost-map": { "PID1": { "PID2": 9 }, "PID3": { "PID1": null, "PID3": 1 } } } ]]>

Server Push Stream Management
Server -> Client [PUSH_PROMISE for updates graph on Stream SID_tq] The server push MUST satisfy the following requirements:

PUSH_PROMISE MUST be sent in stream SID_tq to serialize and 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 Load Balancing TIPS allow clients to make concurrent pulls of the incremental updates potentianlly through different HTTP connections. As a consequence, it introduces additional complexties when the ALTO server is being load balanced -- a feature widely used to build scalable and fault-tolerant web services. For example, a request may be incorrectly processed if

the backend servers are stateful, i.e., the TIPS view is created and stored only on a single server;

the ALTO server is using layer-4 load balancing, i.e., the requests are distributed based on the TCP 5-tuple.

Thus, additional considerations are required to enable correct load balancing for TIPS, including:

Use a stateless architecture: One solution is to follow the stateless computing pattern: states about the TIPS view are not maintained by the backend servers but are stored in a distributed database. Thus, concurrent requests to the same TIPS view can be processed on arbitrary stateless backend servers, which all fetches data from the same database.

Configure the load balancers properly: In case when the backend servers are stateful, the load balancers must be properly configured to guarantee that requests of the same TIPS view always arrive at the same server. For example, the ALTO server may configure layer-7 load balancers that distribute requests based on URL or cookies.

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 graph) 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 Cross-Resource Dependency Scheduling Dependent ALTO resources result in cross-resource dependenies in TIPS. Consider the following pair of resources, where my-cost-map is dependent on my-network-map:
Example Dependency Model |101|-->|102|-->|103|-->|104| +---+ +---+ +---+ +---+ +---+ | \ \ | \ \ +---+ +---+ +---+ +---+ +---+ my-cost-map (C) | 0 |-->|101|-->|102|-->|103|-->|104| +---+ +---+ +---+ +---+ +---+ |_______________________| ]]>
In pull-mode, a client can decide the order in which to receive the updates. In push-mode, the server must decide. Pushing order may affect how fast the client can build a consistent view and how long the client needs to buffer the update.

Example 1: The server pushes N101, N102, N103, C101, C102 in that order. The client either gets no consistent view of the resources or it has to buffer N2 and N3

Example 2: The server pushes C1, C2, C3, N1. The client either gets no consistent view or it has to buffer C3.

Therefore, the server is RECOMMENDED to push updates in the ascending order of the smallest dependent tag, e.g., {C1, C2, N1} before {C3, N2}

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 server SHOULD 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 graph 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 graph SHOULD NOT have an update. But if the previous value did, then the updates graph 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 graph 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 TIPS views enables a new class of Denial-of-Service attacks. In particular, For the TIPS server, an ALTO client might create an unreasonable number of TIPS views. To avoid these attacks on the TIPS server, the server SHOULD choose to limit the number of active views and reject new requests when that threshold is reached. TIPS allows predictive fetching and the server SHOULD also choose to limit the number of pending requests. In these cases, the server SHOULD return the HTTP status "429 Too many requests". 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 TIPS view 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 ).

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-tipsview+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-tipsview+json Media Type

Type name:

application

Subtype name:

alto-tipsview+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.