Frequently Asked Questions
Q. Can I modify a NDIS packet that I did not allocate myself?
A. No.
NDIS packet descriptors, buffer
descriptors, chained virtual memory (VM) and OOB data that you did not allocate
yourself must be considered to be logically read-only.
Some developers read the DDK documentation and misinterpret references to
packet "ownership". For example, the DDK documentation for
ProtocolReceivePackets includes this comment:
"The NDIS library maintains the reference count for such
an indicated packet, and the protocol retains ownership of the
packet until it has called NdisReturnPackets with that packet as many
times as the value returned by its ProtocolReceivePacket function."
Here the term "ownership" does not infer "exclusive
access" - and does not infer that you can modify the NDIS packet descriptor,
buffer descriptor, VM or OOB data.
In fact, multiple protocols can concurrently have
"ownership" of the same NDIS packet. If any one protocol modified a NDIS packet
concurrently owned by other protocols, the resulting behavior would probably be
unpredictable.
Similarly, NDIS packets passed to a miniport or IM driver
SendPacketsHandler or SendHandler should be considered read-only.
The VM associated with a NDIS packet being sent may very well be mapped versions
of memory managed by the TCP/IP driver or a user mode application. If you modify
a send packet's VM (that you did not allocate yourself) subsequent retries for reliable
transmission pass the modified data instead of the data originally intended to
be transmitted.
In other words: If you did not allocate the NDIS packet
yourself, then you really cannot "guess" or "assume" what the effects of
modifying the packet would be to the component that originally allocated the
object.
