MessagePending

[Paul Fremantle <paul@wso2.com>]

Rereading the spec around MakeConnection, I realize that we defined 
MessagePending as only applicable on a message flowing back on a 
MakeConnection backchannel. It seems to me that we could usefully add 
this message in cases where a response is flowing on a back channel.

For example. Suppose I have an interaction

1. GET1 ----->
<---- RESPONSE1

2. GET2----->
(response lost)

3. GET3----->
<------RESPONSE3

In 3, the Get will have an ack for response1, and the server can 
indicate that response 2 is missing.

We already have logic to decide when piggybacking of acks is 
appropriate. I believe the logic could be extended to decide when it was 
ok to send back a MessagePending header:

* In the sequenceID case, it would be appropriate whenever the message 
being sent back was part of the same Offered response sequence.
* In the URI case, it would be appropriate whenever the replyTo URI 
matched the MessagePending URI.

Paul