dccp: Deprecate Ack Ratio sysctl

This patch deprecates the Ack Ratio sysctl, since
 * Ack Ratio is entirely ignored by CCID-3 and CCID-4,
 * Ack Ratio currently doesn't work in CCID-2 (i.e. is always set to 1);
 * even if it would work in CCID-2, there is no point for a user to change it:
   - Ack Ratio is constrained by cwnd (RFC 4341, 6.1.2),
   - if Ack Ratio > cwnd, the system resorts to spurious RTO timeouts
     (since waiting for Acks which will never arrive in this window),
   - cwnd is not a user-configurable value.

The only reasonable place for Ack Ratio is to print it for debugging. It is
planned to do this later on, as part of e.g. dccp_probe.

With this patch Ack Ratio is now under full control of feature negotiation:
 * Ack Ratio is resolved as a dependency of the selected CCID;
 * if the chosen CCID supports it (i.e. CCID == CCID-2), Ack Ratio is set to
   the default of 2, following RFC 4340, 11.3 - "New connections start with Ack
   Ratio 2 for both endpoints";
 * what happens then is part of another patch set, since it concerns the
   dynamic update of Ack Ratio while the connection is in full flight.

Thanks to Tomasz Grobelny for discussion leading up to this patch.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/dccp.txt

@@ -125,9 +125,6 @@ send_ndp = 1
 send_ackvec = 1
 	Whether or not to send Ack Vector options (sec. 11.5).
 
-ack_ratio = 2
-	The default Ack Ratio (sec. 11.3) to use.
-
 tx_ccid = 2
 	Default CCID for the sender-receiver half-connection.
 

include/linux/dccp.h

@@ -368,7 +368,6 @@ static inline unsigned int dccp_hdr_len(const struct sk_buff *skb)
   * @dccpms_ccid - Congestion Control Id (CCID) (section 10)
   * @dccpms_send_ack_vector - Send Ack Vector Feature (section 11.5)
   * @dccpms_send_ndp_count - Send NDP Count Feature (7.7.2)
-  * @dccpms_ack_ratio - Ack Ratio Feature (section 11.3)
   * @dccpms_pending - List of features being negotiated
   * @dccpms_conf -
   */
@@ -378,7 +377,6 @@ struct dccp_minisock {
 	__u8			dccpms_tx_ccid;
 	__u8			dccpms_send_ack_vector;
 	__u8			dccpms_send_ndp_count;
-	__u8			dccpms_ack_ratio;
 	struct list_head	dccpms_pending;
 	struct list_head	dccpms_conf;
 };

net/dccp/dccp.h

@@ -98,7 +98,6 @@ extern int  sysctl_dccp_retries2;
 extern int  sysctl_dccp_feat_sequence_window;
 extern int  sysctl_dccp_feat_rx_ccid;
 extern int  sysctl_dccp_feat_tx_ccid;
-extern int  sysctl_dccp_feat_ack_ratio;
 extern int  sysctl_dccp_feat_send_ack_vector;
 extern int  sysctl_dccp_feat_send_ndp_count;
 extern int  sysctl_dccp_tx_qlen;

net/dccp/minisocks.c

@@ -47,7 +47,6 @@ void dccp_minisock_init(struct dccp_minisock *dmsk)
 	dmsk->dccpms_sequence_window = sysctl_dccp_feat_sequence_window;
 	dmsk->dccpms_rx_ccid	     = sysctl_dccp_feat_rx_ccid;
 	dmsk->dccpms_tx_ccid	     = sysctl_dccp_feat_tx_ccid;
-	dmsk->dccpms_ack_ratio	     = sysctl_dccp_feat_ack_ratio;
 	dmsk->dccpms_send_ack_vector = sysctl_dccp_feat_send_ack_vector;
 	dmsk->dccpms_send_ndp_count  = sysctl_dccp_feat_send_ndp_count;
 }

net/dccp/options.c

@@ -26,7 +26,6 @@
 int sysctl_dccp_feat_sequence_window = DCCPF_INITIAL_SEQUENCE_WINDOW;
 int sysctl_dccp_feat_rx_ccid	      = DCCPF_INITIAL_CCID;
 int sysctl_dccp_feat_tx_ccid	      = DCCPF_INITIAL_CCID;
-int sysctl_dccp_feat_ack_ratio	      = DCCPF_INITIAL_ACK_RATIO;
 int sysctl_dccp_feat_send_ack_vector = DCCPF_INITIAL_SEND_ACK_VECTOR;
 int sysctl_dccp_feat_send_ndp_count  = DCCPF_INITIAL_SEND_NDP_COUNT;
 

net/dccp/sysctl.c

@@ -40,13 +40,6 @@ static struct ctl_table dccp_default_table[] = {
 		.mode		= 0644,
 		.proc_handler	= proc_dointvec,
 	},
-	{
-		.procname	= "ack_ratio",
-		.data		= &sysctl_dccp_feat_ack_ratio,
-		.maxlen		= sizeof(sysctl_dccp_feat_ack_ratio),
-		.mode		= 0644,
-		.proc_handler	= proc_dointvec,
-	},
 	{
 		.procname	= "send_ackvec",
 		.data		= &sysctl_dccp_feat_send_ack_vector,