dccp ccid-2: Schedule Sync as out-of-band mechanism

The problem with Ack Vectors is that 

  i) their length is variable and can in principle grow quite large,
 ii) it is hard to predict exactly how large they will be.

Due to the second point it seems not a good idea to reduce the MPS; in
particular when on average there is enough room for the Ack Vector and an
increase in length is momentarily due to some burst loss, after which the
Ack Vector returns to its normal/average length.

The solution taken by this patch is to subtract a minimum-expected Ack Vector
length from the MPS (previous patch), and to defer any larger Ack Vectors onto
a separate Sync - but only if indeed there is no space left on the skb.

This patch provides the infrastructure to schedule Sync-packets for transporting
(urgent) out-of-band data. Its signalling is quicker than scheduling an Ack, since
it does not need to wait for new application data.

It can thus serve other parts of the DCCP code as well.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>

1 files changed, 8 insertions, 0 deletions

diff --git a/net/dccp/output.c b/net/dccp/output.c
index 1b3168307586..bfda071559f4 100644
--- a/net/dccp/output.c
+++ b/net/dccp/output.c
@@ -305,6 +305,8 @@ void dccp_write_xmit(struct sock *sk, int block)
 			if (err)
 				DCCP_BUG("err=%d after ccid_hc_tx_packet_sent",
 					 err);
+			if (dp->dccps_sync_scheduled)
+				dccp_send_sync(sk, dp->dccps_gsr, DCCP_PKT_SYNC);
 		} else {
 			dccp_pr_debug("packet discarded due to err=%d\n", err);
 			kfree_skb(skb);
@@ -591,6 +593,12 @@ void dccp_send_sync(struct sock *sk, const u64 ackno,
 	DCCP_SKB_CB(skb)->dccpd_type = pkt_type;
 	DCCP_SKB_CB(skb)->dccpd_ack_seq = ackno;
 
+	/*
+	 * Clear the flag in case the Sync was scheduled for out-of-band data,
+	 * such as carrying a long Ack Vector.
+	 */
+	dccp_sk(sk)->dccps_sync_scheduled = 0;
+
 	dccp_transmit_skb(sk, skb);
 }