AI for LC20

Here is a new proposal for dealing with anonymous endpoints and "return
to sender" semantics.  It keeps anonymous roughly as it is, without
mentioning NATs and so forth, and adds "return to sender" as a separate
pseudo-address.  I've also added a note trying to clarify the difference
between the two URIs, and I've explicitly added [destination] as a
property that can be anonymous (we give that as its default value).

The /dev/null pseudo-address would also fit in here.

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Replace the paragraph reading

    Due to the range of network technologies currently in wide-spread
    use (e.g., NAT, DHCP, firewalls), many deployments cannot assign a
    meaningful global IRI to a given endpoint. To allow these
    "anonymous" endpoints to send and receive messages, WS-Addressing
    defines the following pre-defined URI for use by endpoints that
    cannot have a stable, resolvable IRI:
    "http://www.w3.org/@@@@/@@/addressing/anonymous". Messages whose
    [reply endpoint], [source endpoint] and/or [fault endpoint] use this
    address MUST rely on some out-of-band mechanism for delivering
    replies or faults (e.g. returning the reply on the same transport
    connection).

With this text:

    In many cases, the destination to which to send a particular message
    will be clear from context, but it may not be possible to assign
    this destination a valid, stably resolvable IRI.  To allow such
    endpoints to send and receive messages, WS-Addressing defines the
    following pre-defined URI 
    "http://www.w3.org/@@@@/@@/addressing/anonymous". Messages whose
    [destination], [reply endpoint], [source endpoint] and/or [fault
    endpoint] use this address MUST rely on some out-of-band mechanism
    for delivery (e.g. returning the reply on the same transport
    connection, or sending to a previously agreed-upon destination).

    In many cases, the sender of a message may wish to receive responses
    or faults directly, and the underlying transport may support this,
    even though the sender may not have a valid, stably resolvable
    IRI.   To allow such endpoints to receive messages, WS-Addressing
    defines the following pre-defined URI:
    "http://www.w3.org/@@@@/@@/addressing/sender
    <http://www.w3.org/@@@@/@@/addressing/anonymous>".  This URI MAY be
    used as the [address] of the [reply endpoint] and/or [fault
    endpoint] addressing property, but SHOULD NOT be so used if the
    transport binding is known not to provide a return facility.

    Note that in some cases the implicit destination for messages sent
    to the anonymous address may be the sender.  In such a case, the
    /anonymous/ and /sender/ URIs will have the same effect. 
    Nonetheless, the two URIs signal different intents.  The anonymous
    URI requests the message be sent to a previously agreed-upon
    destination, which might or might not be the sender.  The sender URI
    specifically requests the message be sent to the sender, regardless
    of any out-of-band agreement.  The anonymous URI may also be used as
    a [destination] or [source endpoint], while the sender URI should
    not be used for this purpose.