I gathered a list today of notification work related to various IETF applications area working groups, past and present. There's a lot.

For the purposes of this discussion, a notification is a message sent to a client from notification server, where the server initiates the connection. The notification was solicited, because the client previously requested a subscription for a specific kind of event from the server. The notification problem is not easy because to solve it properly it requires clients to either have a new kind of server, a notification management server, to manage and receive their notifications, or it requires clients to listen on a port and accept incoming requests (which is forbidden by many firewalls and corporate policies).

General: In Aug 1998 the NOTIFY BOF was held at IETF with much controversy and huge scope problems. GENA (Generic Event Notification Architecture) is one of the more concrete proposals that came out of this. It has received some unofficial discussion and review in the WebDAV WG (which I co-chair), where notifications would be useful to let users know when new documents show up in their folders, when documents they want to edit become unlocked, etc.

ENP (Event Notification Protocol, May 2000) is a notifications proposal based directly on WebDAV, but perhaps no less generic than GENA. This proposal was also discussed in the now-defunct SWAP (Simple Workflow Access Protocol) WG.

HTTP: In 1998 a workshop called WISEN (Workshop on Internet Scale Event Notification) discussed general notification, focusing on the Web. Many of those involved in 1998 were UC Irvine grad students, and UCI professor Richard Taylor advised at least some of those. The Institute for Software Research at UCI frequently works on notifications, distributed systems, event-based systems, etc. Adam Rifkin put together a somewhat-dated but very comprehensive survey of event systems. One of those UCI students, Rohit Khare, left to found KnowNow, which implements HTTP-based notification clients and servers.

SIP (Session Initiation Protocol) isn't applications area (it's transport area) but they recently came out with a reasonably extensible notifications RFC. A recent academic paper acknowledges debts to SIP in the design of a Group Event Notification Protocol.

SNAP ([Smart/Simple] Notification and Alarm Protocol) is another concrete proposal. It has been discussed most in the VPIM (Voice Profile for Internet Mail) WG, but it showed up at the Lemonade BOF at last week's IETF, proposed for use in "unified messaging", to notify mail clients of new types of messages in their unified mail inboxes. SNAP has also been proposed for use in MMS (Multimedia Messaging Service). There are also claims SNAP has been implemented by "major email vendors" and Comverse.

The CalSched WG could use notification in order to let users know when an appointment is coming up.

The OPES WG and the recently-defunct WEBI WG discussed the requirements for a Web notification architecture. WEBI wants Web page cache servers to subscribe to Web content servers, so that rather than poll for new page versions, the content server could send a notification when the cache should be thrown out or updated. They also discussed using GENA.

The LDAP-EXT (Lightweight Directory Access Protocol) WG has discussed notifications and LDAP.

The IPP (Internet Printing Protocol) WG has discussed notifications and has a current Internet-Draft discussing requirements.

Instant messaging even enters the fray. It's long been understood (see the draft I wrote in 1998, or this or this other one) that finding out when somebody is online may best be done with subscriptions and notifications. Last week's Jabber BOF brought to light a protocol for which there exists an Event Notification Service.