This file describes the protocol used for communicating packets.
See include/pdp.h and the sources in this directory for more info.

There are 3 kinds of pdp messages:

[pdp register_ro <packet_id>]
[pdp register_rw <packet_id>]
[pdp process]

An object can receive a packet by catching the 3 kinds of messages:

When a register_ro message is received, the object can call 
pdp_packet_copy_ro(packet) to reseve a read only copy for itself.
The only operations on the packet at that time are read only
operations. 

The same goes for handling the register_rw message. You can
reserve a read/write copy by using pdp_packet_copy_rw(packet)

When a process message is received, the object is allowed to start
processing the data.

In the register_ro or register_rw phases, the object can inspect
the packet to see if it wants to register it (using pdp_packet_header
and pdp_packet_data operations) but it is not allowed to allocate other objects
in that phase. This is only allowed in the process phase. (or after
the receiving packet is registerd.) 

An object can send out a pdp message using the outlet_pdp(outlet, packet)
function. This sends out these 3 messages in sequence. It is best to
unregister a packet using pdp_packet_mark_unused(packet) before sending it out,
if it is no longer used by the sending object. This eliminates one extra 
copy operation on the data.