RFC 8030
Generic Event Delivery Using HTTP Push
Pages: 31
Proposed Standard
Proposed Standard
Part 1 of 2 – Pages 1 to 19
Internet Engineering Task Force (IETF) M. Thomson Request for Comments: 8030 Mozilla Category: Standards Track E. Damaggio ISSN: 2070-1721 B. Raymor, Ed. Microsoft December 2016 Generic Event Delivery Using HTTP PushAbstract
This document describes a simple protocol for the delivery of real- time events to user agents. This scheme uses HTTP/2 server push. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc8030. Copyright Notice Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Conventions and Terminology . . . . . . . . . . . . . . . 4 2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 6 2.1. HTTP Resources . . . . . . . . . . . . . . . . . . . . . 7 3. Connecting to the Push Service . . . . . . . . . . . . . . . 8 4. Subscribing for Push Messages . . . . . . . . . . . . . . . . 8 4.1. Collecting Subscriptions into Sets . . . . . . . . . . . 9 5. Requesting Push Message Delivery . . . . . . . . . . . . . . 10 5.1. Requesting Push Message Receipts . . . . . . . . . . . . 10 5.2. Push Message Time-To-Live . . . . . . . . . . . . . . . . 11 5.3. Push Message Urgency . . . . . . . . . . . . . . . . . . 13 5.4. Replacing Push Messages . . . . . . . . . . . . . . . . . 14 6. Receiving Push Messages for a Subscription . . . . . . . . . 15 6.1. Receiving Push Messages for a Subscription Set . . . . . 17 6.2. Acknowledging Push Messages . . . . . . . . . . . . . . . 18 6.3. Receiving Push Message Receipts . . . . . . . . . . . . . 19 7. Operational Considerations . . . . . . . . . . . . . . . . . 20 7.1. Load Management . . . . . . . . . . . . . . . . . . . . . 20 7.2. Push Message Expiration . . . . . . . . . . . . . . . . . 20 7.3. Subscription Expiration . . . . . . . . . . . . . . . . . 21 7.3.1. Subscription Set Expiration . . . . . . . . . . . . . 21 7.4. Implications for Application Reliability . . . . . . . . 22 7.5. Subscription Sets and Concurrent HTTP/2 Streams . . . . . 22 8. Security Considerations . . . . . . . . . . . . . . . . . . . 22 8.1. Confidentiality from Push Service Access . . . . . . . . 23 8.2. Privacy Considerations . . . . . . . . . . . . . . . . . 23 8.3. Authorization . . . . . . . . . . . . . . . . . . . . . . 24 8.4. Denial-of-Service Considerations . . . . . . . . . . . . 25 8.5. Logging Risks . . . . . . . . . . . . . . . . . . . . . . 25 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26 9.1. Header Field Registrations . . . . . . . . . . . . . . . 26 9.2. Link Relation URNs . . . . . . . . . . . . . . . . . . . 26 9.3. Service Name and Port Number Registration . . . . . . . . 28 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 28 10.1. Normative References . . . . . . . . . . . . . . . . . . 28 10.2. Informative References . . . . . . . . . . . . . . . . . 30 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 31 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 31
1. Introduction
Many applications on mobile and embedded devices require continuous access to network communications so that real-time events -- such as incoming calls or messages -- can be delivered (or "pushed") in a timely fashion. These devices typically have limited power reserves, so finding more efficient ways to serve application requirements greatly benefits the application ecosystem. One significant contributor to power usage is the radio. Radio communications consume a significant portion of the energy budget on a wireless device. Uncoordinated use of persistent connections or sessions from multiple applications can contribute to unnecessary use of the device radio, since each independent session can incur its own overhead. In particular, keep-alive traffic used to ensure that middleboxes do not prematurely time out sessions can result in significant waste. Maintenance traffic tends to dominate over the long term, since events are relatively rare. Consolidating all real-time events into a single session ensures more efficient use of network and radio resources. A single service consolidates all events, distributing those events to applications as they arrive. This requires just one session, avoiding duplicated overhead costs. The W3C Push API [API] describes an API that enables the use of a consolidated push service from web applications. This document expands on that work by describing a protocol that can be used to: o request the delivery of a push message to a user agent, o create new push message delivery subscriptions, and o monitor for new push messages. A standardized method of event delivery is particularly important for the W3C Push API, where application servers might need to use multiple push services. The subscription, management, and monitoring functions are currently fulfilled by proprietary protocols; these are adequate, but do not offer any of the advantages that standardization affords. This document intentionally does not describe how a push service is discovered. Discovery of push services is left for future efforts, if it turns out to be necessary at all. User agents are expected to be configured with a URL for a push service.
1.1. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. This document defines the following terms: application: Both the sender and the ultimate consumer of push messages. Many applications have components that are run on a user agent and other components that run on servers. application server: The component of an application that usually runs on a server and requests the delivery of a push message. push message subscription: A message delivery context that is established between the user agent and the push service, and shared with the application server. All push messages are associated with a push message subscription. push message subscription set: A message delivery context that is established between the user agent and the push service that collects multiple push message subscriptions into a set. push message: A message sent from an application server to a user agent via a push service. push message receipt: A message delivery confirmation sent from the push service to the application server. push service: A service that delivers push messages to user agents. user agent: A device and software that is the recipient of push messages. Examples in this document use the HTTP/1.1 message format [RFC7230]. Many of the exchanges can be completed using HTTP/1.1: o Subscribing for Push Messages (Section 4) o Requesting Push Message Delivery (Section 5) o Replacing Push Messages (Section 5.4) o Acknowledging Push Messages (Section 6.2)
When an example depends on HTTP/2 server push, the more verbose frame format from [RFC7540] is used: o Receiving Push Messages for a Subscription (Section 6) o Receiving Push Messages for a Subscription Set (Section 6.1) o Receiving Push Message Receipts (Section 6.3) All examples use HTTPS over the default port (443) rather than the registered port (1001). A push service deployment might prefer this configuration to maximize chances for user agents to reach the service. A push service might use HTTP alternative services to redirect a user agent to the registered port (1001) to gain the benefits of the standardized HTTPS port without sacrificing reachability (see Section 3). This would only be apparent in the examples through the inclusion of the Alt-Used header field [RFC7838] in requests from the user agent to the push service. Examples do not include specific methods for push message encryption or application server authentication because the protocol does not define a mandatory system. The examples in Voluntary Application Server Identification [VAPID] and Message Encryption for WebPush [ENCRYPT] demonstrate the approach adopted by the W3C Push API [API] for its requirements.
2. Overview
A general model for push services includes three basic actors: a user agent, a push service, and an application (server). +-------+ +--------------+ +-------------+ | UA | | Push Service | | Application | +-------+ +--------------+ | Server | | | +-------------+ | Subscribe | | |--------------------->| | | Monitor | | |<====================>| | | | | | Distribute Push Resource | |-------------------------------------------->| | | | : : : | | Push Message | | Push Message |<---------------------| |<---------------------| | | | | Figure 1: WebPush Architecture At the very beginning of the process, a new message subscription is created by the user agent and then distributed to its application server. This subscription is the basis of all future interactions between the actors. A subscription is used by the application server to send messages to the push service for delivery to the user agent. The user agent uses the subscription to monitor the push service for any incoming message. To offer more control for authorization, a message subscription is modeled as two resources with different capabilities: o A subscription resource is used to receive messages from a subscription and to delete a subscription. It is private to the user agent. o A push resource is used to send messages to a subscription. It is public and shared by the user agent with its application server. It is expected that a unique subscription will be distributed to each application; however, there are no inherent cardinality constraints in the protocol. Multiple subscriptions might be created for the
same application, or multiple applications could use the same subscription. Note, however, that sharing subscriptions has security and privacy implications. Subscriptions have a limited lifetime. They can also be terminated by either the push service or the user agent at any time. User agents and application servers must be prepared to manage changes in the subscription state.2.1. HTTP Resources
This protocol uses HTTP resources [RFC7230] and link relations [RFC5988]. The following resources are defined: push service: This resource is used to create push message subscriptions (Section 4). A URL for the push service is configured into user agents. push message subscription: This resource provides read and delete access for a message subscription. A user agent receives push messages (Section 6) using a push message subscription. Every push message subscription has exactly one push resource associated with it. push message subscription set: This resource provides read and delete access for a collection of push message subscriptions. A user agent receives push messages (Section 6.1) for all the push message subscriptions in the set. A link relation of type "urn:ietf:params:push:set" identifies a push message subscription set. push: An application server requests the delivery (Section 5) of a push message using a push resource. A link relation of type "urn:ietf:params:push" identifies a push resource. push message: The push service creates a push message resource to identify push messages that have been accepted for delivery (Section 5). The push message resource is also deleted by the user agent to acknowledge receipt (Section 6.2) of a push message. receipt subscription: An application server receives delivery confirmations (Section 5.1) for push messages using a receipt subscription. A link relation of type "urn:ietf:params:push:receipt" identifies a receipt subscription.
3. Connecting to the Push Service
The push service MUST use HTTP over Transport Layer Security (TLS) [RFC2818] following the recommendations in [RFC7525]. The push service shares the same default port number (443/TCP) with HTTPS, but MAY also advertise the IANA-allocated TCP System Port (1001) using HTTP alternative services [RFC7838]. While the default port (443) offers broad reachability characteristics, it is most often used for web-browsing scenarios with a lower idle timeout than other ports configured in middleboxes. For WebPush scenarios, this would contribute to unnecessary radio communications to maintain the connection on battery-powered devices. Advertising the alternate port (1001) allows middleboxes to optimize idle timeouts for connections specific to push scenarios with the expectation that data exchange will be infrequent. Middleboxes SHOULD comply with REQ-5 in [RFC5382], which states that "the value of the 'established connection idle-timeout' MUST NOT be less than 2 hours 4 minutes".4. Subscribing for Push Messages
A user agent sends a POST request to its configured push service resource to create a new subscription. POST /subscribe HTTP/1.1 Host: push.example.net A 201 (Created) response indicates that the push subscription was created. A URI for the push message subscription resource that was created in response to the request MUST be returned in the Location header field. The push service MUST provide a URI for the push resource corresponding to the push message subscription in a link relation of type "urn:ietf:params:push". An application-specific method is used to distribute the push URI to the application server. Confidentiality protection and application server authentication MUST be used to ensure that this URI is not disclosed to unauthorized recipients (Section 8.3).
HTTP/1.1 201 Created
Date: Thu, 11 Dec 2014 23:56:52 GMT
Link: </push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV>;
rel="urn:ietf:params:push"
Link: </subscription-set/4UXwi2Rd7jGS7gp5cuutF8ZldnEuvbOy>;
rel="urn:ietf:params:push:set"
Location: https://push.example.net/subscription/LBhhw0OohO-Wl4Oi971UG
4.1. Collecting Subscriptions into Sets
Collecting multiple push message subscriptions into a subscription
set can represent a significant efficiency improvement for push
services and user agents. The push service MAY provide a URI for a
subscription set resource in a link relation of type
"urn:ietf:params:push:set".
When a subscription set is returned in a push message subscription
response, the user agent SHOULD include this subscription set in a
link relation of type "urn:ietf:params:push:set" in subsequent
requests to create new push message subscriptions.
A user agent MAY omit the subscription set if it is unable to receive
push messages in an aggregated way for the lifetime of the
subscription. This might be necessary if the user agent is
monitoring subscriptions on behalf of other push message receivers.
POST /subscribe HTTP/1.1
Host: push.example.net
Link: </subscription-set/4UXwi2Rd7jGS7gp5cuutF8ZldnEuvbOy>;
rel="urn:ietf:params:push:set"
The push service SHOULD return the same subscription set in its
response, although it MAY return a new subscription set if it is
unable to reuse the one provided by the user agent.
HTTP/1.1 201 Created
Date: Thu, 11 Dec 2014 23:56:52 GMT
Link: </push/YBJNBIMwwA_Ag8EtD47J4A>;
rel="urn:ietf:params:push"
Link: </subscription-set/4UXwi2Rd7jGS7gp5cuutF8ZldnEuvbOy>;
rel="urn:ietf:params:push:set"
Location: https://push.example.net/subscription/i-nQ3A9Zm4kgSWg8_ZijV
A push service MUST return a 400 (Bad Request) status code for
requests that contain an invalid subscription set. A push service
MAY return a 429 (Too Many Requests) status code [RFC6585] to reject
requests that omit a subscription set.
How a push service detects that requests originate from the same user agent is implementation-specific but could take ambient information into consideration, such as the TLS connection, source IP address, and port. Implementers are reminded that some heuristics can produce false positives and hence, cause requests to be rejected incorrectly.5. Requesting Push Message Delivery
An application server requests the delivery of a push message by sending an HTTP POST request to a push resource distributed to the application server by a user agent. The content of the push message is included in the body of the request. POST /push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV HTTP/1.1 Host: push.example.net TTL: 15 Content-Type: text/plain;charset=utf8 Content-Length: 36 iChYuI3jMzt3ir20P8r_jgRR-dSuN182x7iB A 201 (Created) response indicates that the push message was accepted. A URI for the push message resource that was created in response to the request MUST be returned in the Location header field. This does not indicate that the message was delivered to the user agent. HTTP/1.1 201 Created Date: Thu, 11 Dec 2014 23:56:55 GMT Location: https://push.example.net/message/qDIYHNcfAIPP_5ITvURr-d6BGt5.1. Requesting Push Message Receipts
An application server can include the Prefer header field [RFC7240] with the "respond-async" preference to request confirmation from the push service when a push message is delivered and then acknowledged by the user agent. The push service MUST support delivery confirmations. POST /push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV HTTP/1.1 Host: push.example.net Prefer: respond-async TTL: 15 Content-Type: text/plain;charset=utf8 Content-Length: 36 iChYuI3jMzt3ir20P8r_jgRR-dSuN182x7iB
When the push service accepts the message for delivery with
confirmation, it MUST return a 202 (Accepted) response. A URI for
the push message resource that was created in response to the request
MUST be returned in the Location header field. The push service MUST
also provide a URI for the receipt subscription resource in a link
relation of type "urn:ietf:params:push:receipt".
HTTP/1.1 202 Accepted
Date: Thu, 11 Dec 2014 23:56:55 GMT
Link: </receipt-subscription/3ZtI4YVNBnUUZhuoChl6omUvG4ZM>;
rel="urn:ietf:params:push:receipt"
Location: https://push.example.net/message/qDIYHNcfAIPP_5ITvURr-d6BGt
For subsequent receipt requests to the same origin [RFC6454], the
application server SHOULD include the returned receipt subscription
in a link relation of type "urn:ietf:params:push:receipt". This
gives the push service the option to aggregate the receipts. The
push service SHOULD return the same receipt subscription in its
response, although it MAY return a new receipt subscription if it is
unable to reuse the one provided by the application server.
An application server MAY omit the receipt subscription if it is
unable to receive receipts in an aggregated way for the lifetime of
the receipt subscription. This might be necessary if the application
server is monitoring receipt subscriptions on behalf of the other
push message senders.
A push service MUST return a 400 (Bad Request) status code for
requests that contain an invalid receipt subscription. If a push
service wishes to limit the number of receipt subscriptions that it
maintains, it MAY return a 429 (Too Many Requests) status code
[RFC6585] to reject receipt requests that omit a receipt
subscription.
5.2. Push Message Time-To-Live
A push service can improve the reliability of push message delivery
considerably by storing push messages for a period. User agents are
often only intermittently connected, and so benefit from having
short-term message storage at the push service.
Delaying delivery might also be used to batch communication with the
user agent, thereby conserving radio resources.
Some push messages are not useful once a certain period of time
elapses. Delivery of messages after they have ceased to be relevant
is wasteful. For example, if the push message contains a call
notification, receiving a message after the caller has abandoned the
call is of no value; the application at the user agent is forced to suppress the message so that it does not generate a useless alert. An application server MUST include the TTL (Time-To-Live) header field in its request for push message delivery. The TTL header field contains a value in seconds that suggests how long a push message is retained by the push service. The TTL rule specifies a non-negative integer, representing time in seconds. A recipient parsing and converting a TTL value to binary form SHOULD use an arithmetic type of at least 31 bits of non- negative integer range. If a recipient receives a TTL value greater than the greatest integer it can represent, or if any of its subsequent calculations overflows, it MUST consider the value to be 2147483648 (2^31). TTL = 1*DIGIT A push service MUST return a 400 (Bad Request) status code in response to requests that omit the TTL header field. A push service MAY retain a push message for a shorter duration than requested. It indicates this by returning a TTL header field in its response with the actual TTL. This TTL value MUST be less than or equal to the value provided by the application server. Once the TTL period elapses, the push service MUST NOT attempt to deliver the push message to the user agent. A push service might adjust the TTL value to account for time accounting errors in processing. For instance, distributing a push message within a server cluster might accrue errors due to clock skew or propagation delays. A push service is not obligated to account for time spent by the application server in sending a push message to the push service, or delays incurred while sending a push message to the user agent. An application server needs to account for transit delays in selecting a TTL header field value. A Push message with a zero TTL is immediately delivered if the user agent is available to receive the message. After delivery, the push service is permitted to immediately remove a push message with a zero TTL. This might occur before the user agent acknowledges receipt of the message by performing an HTTP DELETE on the push message resource. Consequently, an application server cannot rely on receiving acknowledgement receipts for zero TTL push messages.
If the user agent is unavailable, a push message with a zero TTL expires and is never delivered.5.3. Push Message Urgency
For a device that is battery-powered, it is often critical that it remains dormant for extended periods. Radio communication in particular consumes significant power and limits the length of time that the device can operate. To avoid consuming resources to receive trivial messages, it is helpful if an application server can communicate the urgency of a message and if the user agent can request that the push server only forwards messages of a specific urgency. An application server MAY include an Urgency header field in its request for push message delivery. This header field indicates the message urgency. The push service MUST NOT forward the Urgency header field to the user agent. A push message without the Urgency header field defaults to a value of "normal". A user agent MAY include the Urgency header field when monitoring for push messages to indicate the lowest urgency of push messages that it is willing to receive. A push service MUST NOT deliver push messages with lower urgency than the value indicated by the user agent in its monitoring request. Push messages of any urgency are delivered to a user agent that does not include an Urgency header field when monitoring for messages. The grammar for the Urgency header field is as follows: Urgency = urgency-option urgency-option = ("very-low" / "low" / "normal" / "high") In order of increasing urgency: +----------+-----------------------------+--------------------------+ | Urgency | Device State | Example Application | | | | Scenario | +----------+-----------------------------+--------------------------+ | very-low | On power and Wi-Fi | Advertisements | | low | On either power or Wi-Fi | Topic updates | | normal | On neither power nor Wi-Fi | Chat or Calendar Message | | high | Low battery | Incoming phone call or | | | | time-sensitive alert | +----------+-----------------------------+--------------------------+ Table 1: Illustrative Urgency Values
Multiple values for the Urgency header field MUST NOT be included in requests; otherwise, the push service MUST return a 400 (Bad Request) status code.5.4. Replacing Push Messages
A push message that has been stored by the push service can be replaced with new content. If the user agent is offline during the time that the push messages are sent, updating a push message avoids the situation where outdated or redundant messages are sent to the user agent. Only push messages that have been assigned a topic can be replaced. A push message with a topic replaces any outstanding push message with an identical topic. A push message topic is a string carried in a Topic header field. A topic is used to correlate push messages sent to the same subscription and does not convey any other semantics. The grammar for the Topic header field uses the "token" rule defined in [RFC7230]. Topic = token For use with this protocol, the Topic header field MUST be restricted to no more than 32 characters from the URL and a filename-safe Base 64 alphabet [RFC4648]. A push service that receives a request with a Topic header field that does not meet these constraints MUST return a 400 (Bad Request) status code to the application server. A push message replacement request creates a new push message resource and simultaneously deletes any existing message resource that has a matching topic. If an attempt was made to deliver the deleted push message, an acknowledgment could arrive at the push service after the push message has been replaced. Delivery receipts for such deleted messages SHOULD be suppressed. The replacement request also replaces the stored TTL, Urgency, and any receipt subscription associated with the previous message in the matching topic. A push message with a topic that is not shared by an outstanding message to the same subscription is stored or delivered as normal.
For example, the following message could cause an existing message to be replaced: POST /push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV HTTP/1.1 Host: push.example.net TTL: 600 Topic: upd Content-Type: text/plain;charset=utf8 Content-Length: 36 ZuHSZPKa2b1jtOKLGpWrcrn8cNqt0iVQyroF If the push service identifies an outstanding push message with a topic of "upd", then that message resource is deleted. A 201 (Created) response indicates that the push message replacement was accepted. A URI for the new push message resource that was created in response to the request is included in the Location header field. HTTP/1.1 201 Created Date: Thu, 11 Dec 2014 23:57:02 GMT Location: https://push.example.net/message/qDIYHNcfAIPP_5ITvURr-d6BGt The value of the Topic header field MUST NOT be forwarded to user agents. Its value is neither encrypted nor authenticated.6. Receiving Push Messages for a Subscription
A user agent requests the delivery of new push messages by making a GET request to a push message subscription resource. The push service does not respond to this request; instead, it uses HTTP/2 server push [RFC7540] to send the contents of push messages as they are sent by application servers. A user agent MAY include an Urgency header field in its request. The push service MUST NOT deliver messages with lower urgency than the value of the header field as defined in Table 1 (Illustrative Urgency Values). Each push message is pushed as the response to a synthesized GET request sent in a PUSH_PROMISE. This GET request is made to the push message resource that was created by the push service when the application server requested message delivery. The response headers SHOULD provide a URI for the push resource corresponding to the push message subscription in a link relation of type "urn:ietf:params:push". The response body is the entity body from the most recent request sent to the push resource by the application server.
The following example request is made over HTTP/2:
HEADERS [stream 7] +END_STREAM +END_HEADERS
:method = GET
:path = /subscription/LBhhw0OohO-Wl4Oi971UG
:authority = push.example.net
The push service permits the request to remain outstanding. When a
push message is sent by an application server, a server push is
generated in association with the initial request. The response for
the server push includes the push message.
PUSH_PROMISE [stream 7; promised stream 4] +END_HEADERS
:method = GET
:path = /message/qDIYHNcfAIPP_5ITvURr-d6BGt
:authority = push.example.net
HEADERS [stream 4] +END_HEADERS
:status = 200
date = Thu, 11 Dec 2014 23:56:56 GMT
last-modified = Thu, 11 Dec 2014 23:56:55 GMT
cache-control = private
link = </push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV>;
rel="urn:ietf:params:push"
content-type = text/plain;charset=utf8
content-length = 36
DATA [stream 4] +END_STREAM
iChYuI3jMzt3ir20P8r_jgRR-dSuN182x7iB
HEADERS [stream 7] +END_STREAM +END_HEADERS
:status = 200
A user agent can also request the contents of the push message
subscription resource immediately by including a Prefer header field
[RFC7240] with a "wait" preference set to "0". In response to this
request, the push service MUST generate a server push for all push
messages that have not yet been delivered.
A 204 (No Content) status code with no associated server pushes
indicates that no messages are presently available. This could be
because push messages have expired.
6.1. Receiving Push Messages for a Subscription Set
There are minor differences between receiving push messages for a subscription and a subscription set. If a subscription set is available, the user agent SHOULD use the subscription set to monitor for push messages rather than individual push message subscriptions. A user agent requests the delivery of new push messages for a collection of push message subscriptions by making a GET request to a push message subscription set resource. The push service does not respond to this request; instead, it uses HTTP/2 server push [RFC7540] to send the contents of push messages as they are sent by application servers. A user agent MAY include an Urgency header field in its request. The push service MUST NOT deliver messages with lower urgency than the value of the header field as defined in Table 1 (Illustrative Urgency Values). Each push message is pushed as the response to a synthesized GET request sent in a PUSH_PROMISE. This GET request is made to the push message resource that was created by the push service when the application server requested message delivery. The synthetic request MUST provide a URI for the push resource corresponding to the push message subscription in a link relation of type "urn:ietf:params:push". This enables the user agent to differentiate the source of the message. The response body is the entity body from the most recent request sent to the push resource by an application server. The following example request is made over HTTP/2. HEADERS [stream 7] +END_STREAM +END_HEADERS :method = GET :path = /subscription-set/4UXwi2Rd7jGS7gp5cuutF8ZldnEuvbOy :authority = push.example.net The push service permits the request to remain outstanding. When a push message is sent by an application server, a server push is generated in association with the initial request. The server push's response includes the push message. PUSH_PROMISE [stream 7; promised stream 4] +END_HEADERS :method = GET :path = /message/qDIYHNcfAIPP_5ITvURr-d6BGt :authority = push.example.net
HEADERS [stream 4] +END_HEADERS
:status = 200
date = Thu, 11 Dec 2014 23:56:56 GMT
last-modified = Thu, 11 Dec 2014 23:56:55 GMT
link = </push/JzLQ3raZJfFBR0aqvOMsLrt54w4rJUsV>;
rel="urn:ietf:params:push"
cache-control = private
content-type = text/plain;charset=utf8
content-length = 36
DATA [stream 4] +END_STREAM
iChYuI3jMzt3ir20P8r_jgRR-dSuN182x7iB
HEADERS [stream 7] +END_STREAM +END_HEADERS
:status = 200
A user agent can request the contents of the push message
subscription set resource immediately by including a Prefer header
field [RFC7240] with a "wait" preference set to "0". In response to
this request, the push service MUST generate a server push for all
push messages that have not yet been delivered.
A 204 (No Content) status code with no associated server pushes
indicates that no messages are presently available. This could be
because push messages have expired.
6.2. Acknowledging Push Messages
To ensure that a push message is properly delivered to the user agent
at least once, the user agent MUST acknowledge receipt of the message
by performing an HTTP DELETE on the push message resource.
DELETE /message/qDIYHNcfAIPP_5ITvURr-d6BGt HTTP/1.1
Host: push.example.net
If the push service receives the acknowledgement and the application
has requested a delivery receipt, the push service MUST return a 204
(No Content) response to the application server monitoring the
receipt subscription.
If the push service does not receive the acknowledgement within a
reasonable amount of time, then the message is considered to be not
yet delivered. The push service SHOULD continue to retry delivery of
the message until its advertised expiration.
The push service MAY cease to retry delivery of the message prior to
its advertised expiration due to scenarios such as an unresponsive
user agent or operational constraints. If the application has
requested a delivery receipt, then the push service MUST return a 410 (Gone) response to the application server monitoring the receipt subscription.6.3. Receiving Push Message Receipts
The application server requests the delivery of receipts from the push service by making an HTTP GET request to the receipt subscription resource. The push service does not respond to this request; instead, it uses HTTP/2 server push [RFC7540] to send push receipts when messages are acknowledged (Section 6.2) by the user agent. Each receipt is pushed as the response to a synthesized GET request sent in a PUSH_PROMISE. This GET request is made to the same push message resource that was created by the push service when the application server requested message delivery. The response includes a status code indicating the result of the message delivery and carries no data. The following example request is made over HTTP/2. HEADERS [stream 13] +END_STREAM +END_HEADERS :method = GET :path = /receipt-subscription/3ZtI4YVNBnUUZhuoChl6omUvG4ZM :authority = push.example.net The push service permits the request to remain outstanding. When the user agent acknowledges the message, the push service pushes a delivery receipt to the application server. A 204 (No Content) status code confirms that the message was delivered and acknowledged. PUSH_PROMISE [stream 13; promised stream 82] +END_HEADERS :method = GET :path = /message/qDIYHNcfAIPP_5ITvURr-d6BGt :authority = push.example.net HEADERS [stream 82] +END_STREAM +END_HEADERS :status = 204 date = Thu, 11 Dec 2014 23:56:56 GMT If the user agent fails to acknowledge the receipt of the push message and the push service ceases to retry delivery of the message prior to its advertised expiration, then the push service MUST push a failure response with a status code of 410 (Gone).
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