RFC 8030
Generic Event Delivery Using HTTP Push
Pages: 31
Proposed Standard
Proposed Standard
Part 2 of 2 – Pages 20 to 31
7. Operational Considerations
7.1. Load Management
A push service is likely to have to maintain a very large number of open TCP connections. Effective management of those connections can depend on being able to move connections between server instances. A user agent MUST support the 307 (Temporary Redirect) status code [RFC7231], which can be used by a push service to redistribute load at the time that a new subscription is requested. A server that wishes to redistribute load can do so using HTTP alternative services [RFC7838]. HTTP alternative services allows for redistribution of load while maintaining the same URIs for various resources. A user agent can ensure a graceful transition by using the GOAWAY frame once it has established a replacement connection.7.2. Push Message Expiration
Storage of push messages based on the TTL header field comprises a potentially significant amount of storage for a push service. A push service is not obligated to store messages indefinitely. A push service is able to indicate how long it intends to retain a message to an application server using the TTL header field (Section 5.2). A user agent that does not actively monitor for push messages will not receive messages that expire during that interval. Push messages that are stored and have not been delivered to a user agent are delivered when the user agent recommences monitoring. Stored push messages SHOULD include a Last-Modified header field (Section 2.2 of [RFC7232]) indicating when delivery was requested by an application server. A GET request to a push message subscription resource with only expired messages results in a response as though no push message was ever sent. Push services might need to limit the size and number of stored push messages to avoid overloading. To limit the size of messages, the push service MAY return a 413 (Payload Too Large) status code [RFC7231] in response to requests that include an entity body that is too large. Push services MUST NOT return a 413 status code in responses to an entity body that is 4096 bytes or less in size.
To limit the number of stored push messages, the push service MAY respond with a shorter Time-To-Live than proposed by the application server in its request for push message delivery (Section 5.2). Once a message has been accepted, the push service MAY later expire the message prior to its advertised Time-To-Live. If the application server requested a delivery receipt, the push service MUST return a failure response (Section 6.2).7.3. Subscription Expiration
In some cases, it may be necessary to terminate subscriptions so that they can be refreshed. This applies to both push message subscriptions and receipt subscriptions. A push service MAY expire a subscription at any time. If there are outstanding requests to an expired push message subscription resource (Section 6) from a user agent or to an expired receipt subscription resource (Section 6.3) from an application server, this MUST be signaled by returning a 404 (Not Found) status code. A push service MUST return a 404 (Not Found) status code if an application server attempts to send a push message to an expired push message subscription. A user agent can remove its push message subscription by sending a DELETE request to the corresponding URI. An application server can remove its receipt subscription by sending a DELETE request to the corresponding URI.7.3.1. Subscription Set Expiration
A push service MAY expire a subscription set at any time and MUST also expire all push message subscriptions in the set. If a user agent has an outstanding request to a push subscription set (Section 6.1), this MUST be signaled by returning a 404 (Not Found) status code. A user agent can request that a subscription set be removed by sending a DELETE request to the subscription set URI. This MUST also remove all push message subscriptions in the set. If a specific push message subscription that is a member of a subscription set is expired or removed, then it MUST also be removed from its subscription set.
7.4. Implications for Application Reliability
A push service that does not support reliable delivery over intermittent network connections or failing applications on devices, forces the device to acknowledge receipt directly to the application server, incurring additional power drain in order to establish and maintain (usually secure) connections to the individual application servers. Push message reliability can be important if messages contain information critical to the state of an application. Repairing the state can be expensive, particularly for devices with limited communications capacity. Knowing that a push message has been correctly received avoids retransmissions, polling, and state resynchronization. The availability of push message delivery receipts ensures that the application developer is not tempted to create alternative mechanisms for message delivery in case the push service fails to deliver a critical message. Setting up a polling mechanism or a backup messaging channel in order to compensate for these shortcomings negates almost all of the advantages a push service provides. However, reliability might not be necessary for messages that are transient (e.g., an incoming call) or messages that are quickly superseded (e.g., the current number of unread emails).7.5. Subscription Sets and Concurrent HTTP/2 Streams
If the push service requires that the user agent use push message subscription sets, then it MAY limit the number of concurrently active streams with the SETTINGS_MAX_CONCURRENT_STREAMS parameter within an HTTP/2 SETTINGS frame [RFC7540]. The user agent MAY be limited to one concurrent stream to manage push message subscriptions and one concurrent stream for each subscription set returned by the push service. This could force the user agent to serialize subscription requests to the push service.8. Security Considerations
This protocol MUST use HTTP over TLS [RFC2818] following the recommendations in [RFC7525]. This includes any communications between the user agent and the push service, plus communications between the application server and the push service. All URIs therefore use the "https" scheme. This provides confidentiality and integrity protection for subscriptions and push messages from external parties.
8.1. Confidentiality from Push Service Access
The protection afforded by TLS does not protect content from the push service. Without additional safeguards, a push service can inspect and modify the message content. Applications using this protocol MUST use mechanisms that provide end-to-end confidentiality, integrity, and data origin authentication. The application server sending the push message and the application on the user agent that receives it are frequently just different instances of the same application, so no standardized protocol is needed to establish a proper security context. The distribution of subscription information from the user agent to its application server also offers a convenient medium for key agreement. For this requirement, the W3C Push API [API] has adopted Message Encryption for WebPush [ENCRYPT] to secure the content of messages from the push service. Other scenarios can be addressed by similar policies. The Topic header field exposes information that allows more granular correlation of push messages on the same subject. This might be used to aid traffic analysis of push messages by the push service.8.2. Privacy Considerations
Push message confidentiality does not ensure that the identity of who is communicating and when they are communicating is protected. However, the amount of information that is exposed can be limited. The URIs provided for push resources MUST NOT provide any basis to correlate communications for a given user agent. It MUST NOT be possible to correlate any two push resource URIs based solely on their contents. This allows a user agent to control correlation across different applications or over time. Of course, this does not prevent correlation using other information that a user agent might expose. Similarly, the URIs provided by the push service to identify a push message MUST NOT provide any information that allows for correlation across subscriptions. Push message URIs for the same subscription MAY contain information that would allow correlation with the associated subscription or other push messages for that subscription. User and device information MUST NOT be exposed through a push or push message URI.
In addition, push URIs established by the same user agent or push
message URIs for the same subscription MUST NOT include any
information that allows them to be correlated with the user agent.
Note: This need not be perfect as long as the resulting anonymity
set ([RFC6973], Section 6.1.1) is sufficiently large. A push URI
necessarily identifies a push service or a single server instance.
It is also possible that traffic analysis could be used to
correlate subscriptions.
A user agent MUST be able to create new subscriptions with new
identifiers at any time.
8.3. Authorization
This protocol does not define how a push service establishes whether
a user agent is permitted to create a subscription, or whether push
messages can be delivered to the user agent. A push service MAY
choose to authorize requests based on any HTTP-compatible
authorization method available, of which there are multiple options
(including experimental options) with varying levels of security.
The authorization process and any associated credentials are expected
to be configured in the user agent along with the URI for the push
service.
Authorization is managed using capability URLs for the push message
subscription, push, and receipt subscription resources ([CAP-URI]).
A capability URL grants access to a resource based solely on
knowledge of the URL.
Capability URLs are used for their "easy onward sharing" and "easy
client API" properties. These properties make it possible to avoid
relying on prearranged relationships or additional protocols between
push services and application servers.
Capability URLs act as bearer tokens. Knowledge of a push message
subscription URI implies authorization to either receive push
messages or delete the subscription. Knowledge of a push URI implies
authorization to send push messages. Knowledge of a push message URI
allows for reading and acknowledging that specific message.
Knowledge of a receipt subscription URI implies authorization to
receive push receipts.
Encoding a large amount of random entropy (at least 120 bits) in the
path component ensures that it is difficult to successfully guess a
valid capability URL.
8.4. Denial-of-Service Considerations
A user agent can control where valid push messages originate by limiting the distribution of push URIs to authorized application servers. Ensuring that push URIs are hard to guess ensures that only application servers that have received a push URI can use it. Push messages that are not successfully authenticated by the user agent will not be delivered, but this can present a denial-of-service risk. Even a relatively small volume of push messages can cause battery-powered devices to exhaust power reserves. To address this case, the W3C Push API [API] has adopted Voluntary Application Server Identification [VAPID], which allows a user agent to restrict a subscription to a specific application server. The push service can then identify and reject unwanted messages without contacting the user agent. A malicious application with a valid push URI could use the greater resources of a push service to mount a denial-of-service attack on a user agent. Push services SHOULD limit the rate at which push messages are sent to individual user agents. A push service MAY return a 429 (Too Many Requests) status code [RFC6585] when an application server has exceeded its rate limit for push message delivery to a push resource. The push service SHOULD also include a Retry-After header [RFC7231] to indicate how long the application server is requested to wait before it makes another request to the push resource. A push service or user agent MAY also terminate subscriptions (Section 7.3) that receive too many push messages. A push service is also able to deny service to user agents. Intentional failure to deliver messages is difficult to distinguish from faults, which might occur due to transient network errors, interruptions in user agent availability, or genuine service outages.8.5. Logging Risks
Server request logs can reveal subscription-related URIs or relationships between subscription-related URIs for the same user agent. Limitations on log retention and strong access control mechanisms can ensure that URIs are not revealed to unauthorized entities.
9. IANA Considerations
This protocol defines new HTTP header fields in Section 9.1. New link relation types are identified using the URNs defined in Section 9.2. Port registration is defined in Section 9.39.1. Header Field Registrations
HTTP header fields are registered within the "Message Headers" registry maintained at <https://www.iana.org/assignments/message- headers/>. This document defines the following HTTP header fields, and the following entries have been added to the "Permanent Message Header Field Names" registry ([RFC3864]): +-------------------+----------+----------+--------------+ | Header Field Name | Protocol | Status | Reference | +-------------------+----------+----------+--------------+ | TTL | http | standard | Section 5.2 | | Urgency | http | standard | Section 5.3 | | Topic | http | standard | Section 5.4 | +-------------------+----------+----------+--------------+ The change controller is: "IETF (iesg@ietf.org) - Internet Engineering Task Force".9.2. Link Relation URNs
This document registers URNs for use in identifying link relation types. These have been added to a new "Web Push Identifiers" registry according to the procedures in Section 4 of [RFC3553]; the corresponding "push" sub-namespace has been entered in the "IETF URN Sub-namespace for Registered Protocol Parameter Identifiers" registry. The "Web Push Identifiers" registry operates under the IETF Review policy [RFC5226]. Registry name: Web Push Identifiers URN Prefix: urn:ietf:params:push Specification: RFC 8030 (this document) Repository: http://www.iana.org/assignments/webpush-parameters/
Index Value: Values in this registry are URNs or URN prefixes that
start with the prefix "urn:ietf:params:push". Each is registered
independently.
Registrations in the "Web Push Identifiers" registry include the
following information:
URN: A complete URN or URN prefix.
Description: A summary description.
Contact: Email for the person or group making the registration.
Index Value: As described in [RFC3553]
Reference: A reference to a specification describing the semantics
of the URN or URN prefix.
URN prefixes that are registered include a description of how the
URN is constructed. This is not applicable for specific URNs.
These values are entered as the initial content of the "Web Push
Identifiers" registry.
URN: urn:ietf:params:push
Description: This link relation type is used to identify a resource
for sending push messages.
Contact: The WEBPUSH WG of the IETF (webpush@ietf.org)
Reference: RFC 8030 (this document)
URN: urn:ietf:params:push:set
Description: This link relation type is used to identify a
collection of push message subscriptions.
Contact: The WEBPUSH WG of the IETF (webpush@ietf.org)
Reference: RFC 8030 (this document)
URN: urn:ietf:params:push:receipt
Description: This link relation type is used to identify a resource
for receiving delivery confirmations for push messages.
Contact: The WEBPUSH WG of the IETF (webpush@ietf.org)
Reference: RFC 8030 (this document)9.3. Service Name and Port Number Registration
Service names and port numbers are registered within the "Service Name and Transport Protocol Port Number Registry" maintained at <https://www.iana.org/assignments/service-names-port-numbers/>. In accordance with [RFC6335], IANA has assigned the System Port number 1001 and the service name "webpush". Service Name: webpush Port Number: 1001 Transport Protocol: tcp Description: HTTP Web Push Assignee: The IESG (iesg@ietf.org) Contact: The IETF Chair (chair@ietf.org) Reference: RFC 8030 (this document)10. References
10.1. Normative References
[CAP-URI] Tennison, J., "Good Practices for Capability URLs", W3C First Public Working Draft capability-urls, February 2014, <http://www.w3.org/TR/capability-urls/>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, DOI 10.17487/RFC2818, May 2000, <http://www.rfc-editor.org/info/rfc2818>.
[RFC3553] Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An IETF URN Sub-namespace for Registered Protocol Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June 2003, <http://www.rfc-editor.org/info/rfc3553>. [RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration Procedures for Message Header Fields", BCP 90, RFC 3864, DOI 10.17487/RFC3864, September 2004, <http://www.rfc-editor.org/info/rfc3864>. [RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006, <http://www.rfc-editor.org/info/rfc4648>. [RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 5226, DOI 10.17487/RFC5226, May 2008, <http://www.rfc-editor.org/info/rfc5226>. [RFC5382] Guha, S., Ed., Biswas, K., Ford, B., Sivakumar, S., and P. Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142, RFC 5382, DOI 10.17487/RFC5382, October 2008, <http://www.rfc-editor.org/info/rfc5382>. [RFC5988] Nottingham, M., "Web Linking", RFC 5988, DOI 10.17487/RFC5988, October 2010, <http://www.rfc-editor.org/info/rfc5988>. [RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S. Cheshire, "Internet Assigned Numbers Authority (IANA) Procedures for the Management of the Service Name and Transport Protocol Port Number Registry", BCP 165, RFC 6335, DOI 10.17487/RFC6335, August 2011, <http://www.rfc-editor.org/info/rfc6335>. [RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI 10.17487/RFC6454, December 2011, <http://www.rfc-editor.org/info/rfc6454>. [RFC6585] Nottingham, M. and R. Fielding, "Additional HTTP Status Codes", RFC 6585, DOI 10.17487/RFC6585, April 2012, <http://www.rfc-editor.org/info/rfc6585>. [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014, <http://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Semantics and Content", RFC 7231, DOI 10.17487/RFC7231, June 2014, <http://www.rfc-editor.org/info/rfc7231>. [RFC7232] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer Protocol (HTTP/1.1): Conditional Requests", RFC 7232, DOI 10.17487/RFC7232, June 2014, <http://www.rfc-editor.org/info/rfc7232>. [RFC7240] Snell, J., "Prefer Header for HTTP", RFC 7240, DOI 10.17487/RFC7240, June 2014, <http://www.rfc-editor.org/info/rfc7240>. [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015, <http://www.rfc-editor.org/info/rfc7525>. [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015, <http://www.rfc-editor.org/info/rfc7540>. [RFC7838] Nottingham, M., McManus, P., and J. Reschke, "HTTP Alternative Services", RFC 7838, DOI 10.17487/RFC7838, April 2016, <http://www.rfc-editor.org/info/rfc7838>.10.2. Informative References
[API] Beverloo, P., Thomson, M., van Ouwerkerk, M., Sullivan, B., and E. Fullea, "Push API", W3C Editor's Draft push- api, November 2016, <https://w3c.github.io/push-api/>. [ENCRYPT] Thomson, M., "Message Encryption for Web Push", Work in Progress, draft-ietf-webpush-encryption-06, October 2016. [RFC6973] Cooper, A., Tschofenig, H., Aboba, B., Peterson, J., Morris, J., Hansen, M., and R. Smith, "Privacy Considerations for Internet Protocols", RFC 6973, DOI 10.17487/RFC6973, July 2013, <http://www.rfc-editor.org/info/rfc6973>. [VAPID] Thomson, M. and P. Beverloo, "Voluntary Application Server Identification for Web Push", Work in Progress, draft-ietf-webpush-vapid-01, June 2016.
Acknowledgements
Significant technical input to this document has been provided by Ben Bangert, Peter Beverloo, Kit Cambridge, JR Conlin, Lucas Jenss, Matthew Kaufman, Costin Manolache, Mark Nottingham, Idel Pivnitskiy, Robert Sparks, Darshak Thakore, and many others.Authors' Addresses
Martin Thomson Mozilla 331 E Evelyn Street Mountain View, CA 94041 United States of America Email: martin.thomson@gmail.com Elio Damaggio Microsoft One Microsoft Way Redmond, WA 98052 United States of America Email: elioda@microsoft.com Brian Raymor (editor) Microsoft One Microsoft Way Redmond, WA 98052 United States of America Email: brian.raymor@microsoft.com