FW: [Notifications] Setting up email event streams

I apologize for the cross-posting, but the theme has some similarities with discussions that happened in the OPS area and specifically in NETCONF.

Subscribing to the notifications list is probably the right way to follow this discussion or even contribute to it.

Dan

From: Lisa Dusseault [mailto:lisa@osafoundation.org]
Sent: Tuesday, January 16, 2007 1:16 AM
To: Message Notifications interest group discussion list
Subject: [Notifications] Setting up email event streams

I've been meaning to send this out more broadly after a bit of private comment: two quite different models for setting up event streams based on what's easier to authenticate -- the event stream "owner" (in this case the mailbox owner) or the stream recipient. I hope it helps compare different approaches.

Lisa

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The goal is to start (and stop) events flowing from an event source (in our case, an email server) to the event sink, frequently using a system with an explicit event relay. Because there are often three parties required in an event flow , we have to pick some way for the three parties to coordinate.

The three parties are the event source (an email server), the event sink (a client, perhaps not an email client) and an event relay. The event relay has many users thus it has addresses for each user and possibly also addresses for devices/clients. Two kinds of application-layer explicit event relays are starting to be common: XMPP and SIP. For the purposes of considering an event feed setup model, we do not want to be concerned about the mostly orthogonal issue of what protocol to use to talk to event relay. So for now we assume that event relays have all features common to XMPP and SIP (addresses, pub/sub features, event stream aggregation).

Choosing between two event stream setup models requires assumptions about

- what kind of authorization is easier or more important to provide

- whether there are a lot of different event sink addressess

- whether direct access to the event source is always/usually available (can be affected by whether the event source is publicly available)

- whether event flows stop and start frequently, or can simply be turned on for a very long term

MODEL A. SUBSCRIBE Chain

A-1 Description

In this model, when a user decides to get an email event flow to a particular device or piece of software, the user typically works with the event sink interface to indicate where to get the events from. Then a SUBSCRIBE message of some kind gets generated by the event sink, sent to the event relay and is handled by the event source.

A-2 Authorization issue

The authorization problem here is to determine if the address that the SUBSCRIBE message comes from is permitted to receive these events. In the case of a publicly-subscribable resource this problem disappears. In many email use cases, only a small number of event sink addresses will be authorized to receive event notifications -- perhaps the event source (the email server) can simply be configured with a short allowed list of event recipients that doesn't change often (e.g. my personal IM address, my work address, and the one used by my phone provided by my phone service provider).

Some SUBSCRIBE systems have the ability to provide a PENDING response to a subscription. This is used when an out-of-band mechanism is initiated to authorize the subscribing address. In an email event source use case, one obvious out-of-band mechanism is for the email server to create an email with a link saying "Click here to authorize this subscription".

It's also possible to use a mechanism like URLAUTH to prove authorization -- in this approach, the subscriber provides a URL that has a secret token in it authorizing the subscription. It's possible that this would authorize the subscribing address for future subscriptions as well, or only for a limited period.

A-3 Drawbacks:

a) Only some notification protocols support SUBSCRIBE semantics.

b) Complicated filters and event types (to support use cases like "Alert me on event type 'email arrives' if it is from my boss, has my full email address or the address of the developer list on the to line, and the subject does not begin with 'FUNNY'..." ) might need a little extra work. Both SIP and XMPP subscriptions can be extended in such a way that existing notification relays would pass on a filter specification in whatever language/format most appropriate for email. It might even be possible to send SIEVE conditionals in the body of the SUBSCRIBE message. XMPP has notification nodes in XEP-0060 and notification filters in XEP-0163.

A-4 Advantages

b) Allows for frequent starting and stopping of subscriptions, possibly an efficiency/scaling concern

MODEL B. Sender RULES

B-1 Description

In this model, the user typically works with the event source in order to configure it to send events to a given set of addresses. For example, the user might have access to a Web interface to an email server where it can say "Send new message notifications to xmpp://lisa@psg.com" or someday use MANAGESIEVE.

B-2 Authorization issue

The authorization problem here is to determine if the user creating the sending rule is authorized to send an event stream to the receiving address. If anybody can send to the receiving address (e.g. email) then it's publicly addressable and the problem disappears. Many kinds of addresses are publicly addressable and use whitelisting together with asking permission ("do you wish to allow messages from this source in the future").

B-3 Drawbacks:

a) Only some event sources support RULE systems. We are building SIEVE for email clients and servers but it may be harder to generalize this beyond email clients and servers. Even sticking strictly the proposed scope of email server sources, we have to consider whether a client such as a calendar client (subscribing for IMIP messages) or a time-management dashboard (subscribes to "unread" counts but doesn't otherwise read email) are going to be able to implement SIEVE, authenticate to mail servers etc.

b) If sender rules are setup through Web interfaces, users are definitely not going to turn them on and off frequently.

c) Failure feedback: how does the event source let the user know that some address keeps bouncing?