ActionClient send_goal and cancel retries about actionlib

[sjfwilson]

We have been working on the ActionClient robustness/resend design. During the most recent review of the design, and while discussing strategy for resending goals on a timer, the following question was asked:

"Does anyone use ActionClient/ActionServer with the expectation that goals are processed on the Server in the order that they are sent by the Client?"

We know that using TCP for topic transport in ROS implicitly provides in-order delivery of messages, including the goals. Does this extend into any implicit guarantee provided by the ActionClient interface? Is this something you have any insight into?

Is this something that people would explicitly implement in their application, for example using
Are there any guarantees like this that we need to continue to provide with any changes we would put up for review?