dccp ccid-2: Use feature-negotiation to report Ack Ratio changes

This uses the new feature-negotiation framework to signal Ack Ratio changes,
as required by RFC 4341, sec. 6.1.2.

This raises some problems for CCID-2 since it can at the moment not cope
gracefully with Ack Ratio of e.g. 2. A FIXME has thus been added which
reverts to the existing policy of bypassing the Ack Ratio sysctl.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>

net/dccp/feat.c

@@ -74,6 +74,18 @@ static int dccp_hdlr_seq_win(struct sock *sk, u64 seq_win, bool rx)
 
 static int dccp_hdlr_ack_ratio(struct sock *sk, u64 ratio, bool rx)
 {
+#ifndef __CCID2_COPES_GRACEFULLY_WITH_DYNAMIC_ACK_RATIO_UPDATES__
+	/*
+	 * FIXME: This is required until several problems in the CCID-2 code are
+	 * resolved. The CCID-2 code currently does not cope well; using dynamic
+	 * Ack Ratios greater than 1 caused instabilities. These were manifest
+	 * in hangups and long RTO timeouts (1...3 seconds). Until this has been
+	 * stabilised, it is safer not to activate dynamic Ack Ratio changes.
+	 */
+	dccp_pr_debug("Not changing %s Ack Ratio from 1 to %u\n",
+		      rx ? "RX" : "TX", (u16)ratio);
+	ratio = 1;
+#endif
 	if (rx)
 		dccp_sk(sk)->dccps_r_ack_ratio = ratio;
 	else

net/dccp/proto.c

@@ -189,7 +189,6 @@ int dccp_init_sock(struct sock *sk, const __u8 ctl_sock_initialized)
 	dp->dccps_rate_last	= jiffies;
 	dp->dccps_role		= DCCP_ROLE_UNDEFINED;
 	dp->dccps_service	= DCCP_SERVICE_CODE_IS_ABSENT;
-	dp->dccps_l_ack_ratio	= dp->dccps_r_ack_ratio = 1;
 
 	dccp_init_xmit_timers(sk);