tipc: Major redesign of broadcast link ACK/NACK algorithms

Completely redesigns broadcast link ACK and NACK mechanisms to prevent
spurious retransmit requests in dual LAN networks, and to prevent the
broadcast link from stalling due to the failure of a receiving node to
acknowledge receiving a broadcast message or request its retransmission.

Note: These changes only impact the timing of when ACK and NACK messages
are sent, and not the basic broadcast link protocol itself, so inter-
operability with nodes using the "classic" algorithms is maintained.

The revised algorithms are as follows:

1) An explicit ACK message is still sent after receiving 16 in-sequence
messages, and implicit ACK information continues to be carried in other
unicast link message headers (including link state messages).  However,
the timing of explicit ACKs is now based on the receiving node's absolute
network address rather than its relative network address to ensure that
the failure of another node does not delay the ACK beyond its 16 message
target.

2) A NACK message is now typically sent only when a message gap persists
for two consecutive incoming link state messages; this ensures that a
suspected gap is not confirmed until both LANs in a dual LAN network have
had an opportunity to deliver the message, thereby preventing spurious NACKs.
A NACK message can also be generated by the arrival of a single link state
message, if the deferred queue is so big that the current message gap
cannot be the result of "normal" mis-ordering due to the use of dual LANs
(or one LAN using a bonded interface). Since link state messages typically
arrive at different nodes at different times the problem of multiple nodes
issuing identical NACKs simultaneously is inherently avoided.

3) Nodes continue to "peek" at NACK messages sent by other nodes. If
another node requests retransmission of a message gap suspected (but not
yet confirmed) by the peeking node, the peeking node forgets about the
gap and does not generate a duplicate retransmit request. (If the peeking
node subsequently fails to receive the lost message, later link state
messages will cause it to rediscover and confirm the gap and send another
NACK.)

4) Message gap "equality" is now determined by the start of the gap only.
This is sufficient to deal with the most common cases of message loss,
and eliminates the need for complex end of gap computations.

5) A peeking node no longer tries to determine whether it should send a
complementary NACK, since the most common cases of message loss don't
require it to be sent. Consequently, the node no longer examines the
"broadcast tag" field of a NACK message when peeking.

Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

1 files changed, 6 insertions, 6 deletions

diff --git a/net/tipc/node.h b/net/tipc/node.h
index 90689f48761..c88ce64f8a3 100644
--- a/net/tipc/node.h
+++ b/net/tipc/node.h
@@ -66,9 +66,9 @@
  *    @supported: non-zero if node supports TIPC b'cast capability
  *    @acked: sequence # of last outbound b'cast message acknowledged by node
  *    @last_in: sequence # of last in-sequence b'cast message received from node
- *    @gap_after: sequence # of last message not requiring a NAK request
- *    @gap_to: sequence # of last message requiring a NAK request
- *    @nack_sync: counter that determines when NAK requests should be sent
+ *    @last_sent: sequence # of last b'cast message sent by node
+ *    @oos_state: state tracker for handling OOS b'cast messages
+ *    @deferred_size: number of OOS b'cast messages in deferred queue
  *    @deferred_head: oldest OOS b'cast message received from node
  *    @deferred_tail: newest OOS b'cast message received from node
  *    @defragm: list of partially reassembled b'cast message fragments from node
@@ -91,9 +91,9 @@ struct tipc_node {
 		u8 supported;
 		u32 acked;
 		u32 last_in;
-		u32 gap_after;
-		u32 gap_to;
-		u32 nack_sync;
+		u32 last_sent;
+		u32 oos_state;
+		u32 deferred_size;
 		struct sk_buff *deferred_head;
 		struct sk_buff *deferred_tail;
 		struct sk_buff *defragm;