tipc: improve TIPC throughput by Gap ACK blocks

During unicast link transmission, it's observed very often that because
of one or a few lost/dis-ordered packets, the sending side will fastly
reach the send window limit and must wait for the packets to be arrived
at the receiving side or in the worst case, a retransmission must be
done first. The sending side cannot release a lot of subsequent packets
in its transmq even though all of them might have already been received
by the receiving side.
That is, one or two packets dis-ordered/lost and dozens of packets have
to wait, this obviously reduces the overall throughput!

This commit introduces an algorithm to overcome this by using "Gap ACK
blocks". Basically, a Gap ACK block will consist of <ack, gap> numbers
that describes the link deferdq where packets have been got by the
receiving side but with gaps, for example:

      link deferdq: [1 2 3 4      10 11      13 14 15       20]
--> Gap ACK blocks:       <4, 5>,   <11, 1>,      <15, 4>, <20, 0>

The Gap ACK blocks will be sent to the sending side along with the
traditional ACK or NACK message. Immediately when receiving the message
the sending side will now not only release from its transmq the packets
ack-ed by the ACK but also by the Gap ACK blocks! So, more packets can
be enqueued and transmitted.
In addition, the sending side can now do "multi-retransmissions"
according to the Gaps reported in the Gap ACK blocks.

The new algorithm as verified helps greatly improve the TIPC throughput
especially under packet loss condition.

So far, a maximum of 32 blocks is quite enough without any "Too few Gap
ACK blocks" reports with a 5.0% packet loss rate, however this number
can be increased in the furture if needed.

Also, the patch is backward compatible.

Acked-by: Ying Xue <ying.xue@windriver.com>
Acked-by: Jon Maloy <jon.maloy@ericsson.com>
Signed-off-by: Tuong Lien <tuong.t.lien@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 31 insertions, 0 deletions

diff --git a/net/tipc/msg.h b/net/tipc/msg.h
index 528ba9241acc..ec5c511a9c9c 100644
--- a/net/tipc/msg.h
+++ b/net/tipc/msg.h
@@ -117,6 +117,37 @@ struct tipc_msg {
 	__be32 hdr[15];
 };
 
+/* struct tipc_gap_ack - TIPC Gap ACK block
+ * @ack: seqno of the last consecutive packet in link deferdq
+ * @gap: number of gap packets since the last ack
+ *
+ * E.g:
+ *       link deferdq: 1 2 3 4      10 11      13 14 15       20
+ * --> Gap ACK blocks:      <4, 5>,   <11, 1>,      <15, 4>, <20, 0>
+ */
+struct tipc_gap_ack {
+	__be16 ack;
+	__be16 gap;
+};
+
+/* struct tipc_gap_ack_blks
+ * @len: actual length of the record
+ * @gack_cnt: number of Gap ACK blocks in the record
+ * @gacks: array of Gap ACK blocks
+ */
+struct tipc_gap_ack_blks {
+	__be16 len;
+	u8 gack_cnt;
+	u8 reserved;
+	struct tipc_gap_ack gacks[];
+};
+
+#define tipc_gap_ack_blks_sz(n) (sizeof(struct tipc_gap_ack_blks) + \
+				 sizeof(struct tipc_gap_ack) * (n))
+
+#define MAX_GAP_ACK_BLKS	32
+#define MAX_GAP_ACK_BLKS_SZ	tipc_gap_ack_blks_sz(MAX_GAP_ACK_BLKS)
+
 static inline struct tipc_msg *buf_msg(struct sk_buff *skb)
 {
 	return (struct tipc_msg *)skb->data;