SUBSCRIBE uri HTTP/1.1

  Subscribes to notifications on the given uri

  Required headers:

    Notification-Type: update | update/newmember | delete | move |
                       pragma/<http://schemas.microsoft.com/exchange/newmail>
      The kind of notifications to subscribe to:

      Type		Target	Depth	Meaning
      update		message	0	message body/props modified
      update		folder	0	folder props modified
      update		folder	1	folder props modified, or object
					added/removed/modified
      update/newmember	folder	1	object added to folder
      delete		any	0	object was deleted
      delete		folder	1	object deleted from folder
      move		any	0	object was moved
      move		folder	1	object moved in/out of folder
      pragma/...	mailbox	any	special new mail notification

      Depth defaults to 0 for a message or 1 for a folder.

      "mailbox" means the top level of the store. I assume the
      newmail notification triggers whenever a message is delivered
      by the MTA to any folder in the mailbox, but I don't know for
      sure.

      Note that "update/newmember" will notice both newly-created
      objects and objects moved/copied into the folder, but "delete"
      will only notice DELETEs, not MOVEs out of the folder.


  Optional headers:

    Depth: 0 | 1
      see above

    Subscription-Lifetime:
      Lifetime of the subscription, in seconds

    Call-Back: httpu://HOSTNAME:PORT/PATH
      Specifies UDP port for the server to send NOTIFY messages to.
      (See ietf/draft-goland-http-udp-00.txt for details about how
      httpu works.) If no Call-Back is specified, the client must POLL
      for changes.

    Notification-Delay:
      Only valid if using a Call-Back. Theoretically, the server will
      coalesce events that occur within this interval into a single
      NOTIFY, but I don't think it actually does.

    Subscription-Id:
      If specified, then this is a renewal (and Notification-Type,
      Depth, and Call-Back cannot be specified). May be a
      comma-separated list of IDs.


  Status codes:

    200, 400, 401, 404, 415, 501
    207 - See "Response body" below
    412 - Subscription-Id no longer exists or doesn't go with URI


  Response headers:

    Subscription-Id:
      gives the ID of the subscription, which can be used to renew
      or UNSUBSCRIBE

    Subscription-Lifetime:
    Notification-Delay:
      may not be exactly what was requested


  Response body:

    For a 207 response (eg, renewing multiple subscriptions), the
    body looks like:

    <?xml version="1.0"?>
    <a:multistatus xmlns:b="http://schemas.microsoft.com/Exchange/" xmlns:a="DAV:">
      <a:response>
        <a:href>http://shrewdness.xcs.ximian.com/exchange/danw/Inbox</a:href>
        <a:status>HTTP/1.1 200 OK</a:status>
        <b:subscriptionID>
          <li>21</li>
        </b:subscriptionID>
      </a:response>
    </a:multistatus>



UNSUBSCRIBE uri HTTP/1.1

  Unsubscribes from changes on uri. (Otherwise the server will continue
  sending them even after the client exits, until the subscription
  expires.)

  Required headers:

    Subscription-Id:
      the subscription id returned by SUBSCRIBE. May be a
      comma-separated list.

  Status codes:

    200, 400
    207 - As for SUBSCRIBE



POLL uri HTTP/1.1

  Polls for changes to subscriptions. This will stop the server from
  sending NOTIFY messages about the subscription until it is triggered
  again.

  Required headers:

    Subscription-Id:
      the subscription(s) to poll. It seems that the subscriptions do
      not have to correspond to the uri?

  Status codes:

    207 - As for SUBSCRIBE
    406
    412 - As for SUBSCRIBE. The server also returns 412 (inside
          Multi-Status) if you try to POLL without authenticating.

    Inside Multi-Status:
    200 - Events have occurred on the subscription
    204 - Events have not occurred on the subscription
    401, 404


NOTIFY uri HTTP/1.1

  Sent from the server to the Call-Back port on the client to notify
  of a change to the subscription. The server will continue sending
  NOTIFY messages until the client POLLs the subscription or the
  subscription expires.

  Required headers:

    Subscription-Id:
      The subscription(s) that are being notified.

  Status codes:

    N/A: the client responds to the NOTIFY by POLLing