Recovery of packets. First you need to know about Packet Ids.
Then we can think of strategies on how to rerequest them and from whom.
The obvious way is to just rerequest them from the source. A protocol for such an obvious strategy is easily written up, we tend to leave it to the person who will first implement this, but if you are that person and wish for some help, lynX or other folks can go ahead and fill this page.
See also Routing.