To be able to decode a network coded packet, a node must already know
one of the two coded packets. This is done by buffering skbs before
transmission and buffering packets sniffed with promiscuous mode from
other hosts.

Packets are kept in a buffer similar to the one with forward-skbs: A
hash table, where each entry, which corresponds to a src-dst pair, has a
linked list packets.
Signed-off-by: NMartin Hundebøll <martin@hundeboll.net>
Signed-off-by: NMarek Lindner <lindner_marek@yahoo.de>
Signed-off-by: NAntonio Quartulli <ordex@autistici.org>

Before adding forward-skbs to the coding buffer, the buffer is searched
for a potential coding opportunity. If one is found, the two packets are
network coded and transmitted right away. If not, the forward-skb is
added to the buffer.

Network coded packets are transmitted with information about the two
receivers and the two coded packets. The first receiver is given by the
MAC header, while the second is given in the payload/bat-header. The
second receiver uses promiscuous mode to receive the packet and check
the second destination.
Signed-off-by: NMartin Hundebøll <martin@hundeboll.net>
Signed-off-by: NMarek Lindner <lindner_marek@yahoo.de>
Signed-off-by: NAntonio Quartulli <ordex@autistici.org>

Two be able to network code two packets, one packet must be buffered
until the next is available. This is done in a "coding buffer", which is
essentially a hash table with lists of packets. Each entry in the hash
table corresponds to a specific src-dst pair, which has a linked list of
packets that are buffered.

This patch adds skbs to the buffer just before forwarding them. The
buffer is traversed every 10 ms, where timed skbs are removed from the
buffer and transmitted. To allow experiments with the network coding
scheme, the timeout is tunable through a file in debugfs.
Signed-off-by: NMartin Hundebøll <martin@hundeboll.net>
Signed-off-by: NMarek Lindner <lindner_marek@yahoo.de>
Signed-off-by: NAntonio Quartulli <ordex@autistici.org>

To use network coding efficiently, a relay must know when neighbor nodes
are likely to have enough information to be able to decode a network
coded packet. This is detected by using OGMs from batman-adv to discover
when one neighbor is in range of another neighbor. The relay check the
TLL to detect when an OGM is forwarded from one neighbor by another
neighbor, and thereby knows that the two neighbors are in range and thus
overhear packets sent by each other.

This information is saved in the orig_node struct to be used when
searching for coding opportunities. Two lists are added to the
orig_node struct: One for neighbors that can hear the orig_node
(outgoing nc_nodes) and one for neighbors that the orig_node can hear
(incoming nc_nodes).

Information about nc_nodes is kept for 10 seconds and is available
through debugfs in batman_adv/nc_nodes to use when debugging network
coding.
Signed-off-by: NMartin Hundebøll <martin@hundeboll.net>
Signed-off-by: NMarek Lindner <lindner_marek@yahoo.de>
Signed-off-by: NAntonio Quartulli <ordex@autistici.org>

Network coding exploits the 802.11 shared medium to allow multiple
packets to be sent in a single transmission. In brief, a relay can XOR
two packets, and send the coded packet to two destinations. The
receivers can decode one of the original packets by XOR'ing the coded
packet with the other original packet. This will lead to increased
throughput in topologies where two packets cross one relay.

In a simple topology with three nodes, it takes four transmissions
without network coding to get one packet from Node A to Node B and one
from Node B to Node A:

 1.  Node A  ---- p1 --->  Node R                Node B
 2.  Node A                Node R  <--- p2 ----  Node B
 3.  Node A  <--- p2 ----  Node R                Node B
 4.  Node A                Node R  ---- p1 --->  Node B

With network coding, the relay only needs one transmission, which saves
us one slot of valuable airtime:

 1.  Node A  ---- p1 --->  Node R                Node B
 2.  Node A                Node R  <--- p2 ----  Node B
 3.  Node A  <- p1 x p2 -  Node R  - p1 x p2 ->  Node B

The same principle holds for a topology including five nodes. Here the
packets from Node A and Node B are overheard by Node C and Node D,
respectively. This allows Node R to send a network coded packet to save
one transmission:

   Node A                  Node B

    |     \              /    |
    |      p1          p2     |
    |       \          /      |
    p1       > Node R <       p2
    |                         |
    |         /      \        |
    |    p1 x p2    p1 x p2   |
    v       /          \      v
           /            \
   Node C <              > Node D

More information is available on the open-mesh.org wiki[1].

This patch adds the initial code to support network coding in
batman-adv. It sets up a worker thread to do house keeping and adds a
sysfs file to enable/disable network coding. The feature is disabled by
default, as it requires a wifi-driver with working promiscuous mode, and
also because it adds a small delay at each hop.

[1] http://www.open-mesh.org/projects/batman-adv/wiki/CatwomanSigned-off-by: NMartin Hundebøll <martin@hundeboll.net>
Signed-off-by: NMarek Lindner <lindner_marek@yahoo.de>
Signed-off-by: NAntonio Quartulli <ordex@autistici.org>