DTN2 Router <-> Convergence Layer Interaction
	    ---------------------------------------------


This is an (incomplete) document that describes the contract between
DTN2 routers and convergence layers. XXX/demmer needs more work

Basic Design
------------

Each link has two BundleLists: the queue_ and the inflight_ lists.
A router indicates that it wants to transmit a bundle by calling
BundleActions::queue_bundle. That in turn puts the bundle on the
queue_ and calls ConvergenceLayer::bundle_queued. 

When the convergence layer initiates transmission of the bundle, it
should call Link::add_to_inflight and Link::del_from_queue on the
bundle. That adjusts the bundle lists to indicate to the rest of the
system that the bundle is actively being transmitted. (May in the
future add an event to indicate this).

Once the bundle transmission has completed, the convergence layer
should post a BundleTransmittedEvent to indicate it. 

Backpressure
------------

Each link queue has a configurable queue depth with a high water and
low water mark. That limits the number of bundles that will be queued
on the link and provides backpressure to the rest of the system. 

The router responds to bundle transmitted events by checking whether
the link queue has space for new bundles, and if so will initiate new
transmissions by queuing more bundles there

Link Closures
-------------

When links close, any bundles that are currently inflight (i.e. on the
inflight list) should be put back onto the link queue by the
convergence layer. Then when the link is re-opened, the convergence
layer should initiate transmission of the bundles that are on the
queue, as it will not get additional bundles_queued() calls when the
link reopens.

Cancelling Bundles
------------------

If routers have queued bundles on a link and want to cancel
transmission, they should call BundleActions::cancel_bundle. If the
bundle is on the link queue, it is simply removed from the queue,
no indication is given to the convergence layer, and a
BundleSendCancelledEvent is posted. If the bundle is on the inflight
queue, then ConvergenceLayer::cancel_bundle is called. If the
convergence layer can cancel transmission, it should. If not, it can
ignore the call and then presumably eventually complete transmission.