dccp: Leave headroom for options when calculating the MPS

The Maximum Packet Size (MPS) is of interest for applications which want
to transfer data, so it is only relevant to the data transfer phase of a
connection (unless one wants to send data on the DCCP-Request, but that is
not considered here).

The strategy chosen to deal with this requirement is to leave room for only 
such options that may appear on data packets.

A special consideration applies to Ack Vectors: this is purely guesswork,
since these can have any length between 3 and 1020 bytes. The strategy
chosen here is to subtract a configurable minimum, the value of 16 bytes
(2 bytes for type/length plus 14 Ack Vector cells) has been found by 
experimentatation. If people experience this as too much or too little,
this could later be turned into a Kconfig option.	

There are currently no CCID-specific header options which may appear on data
packets, hence it is not necessary to define a corresponding CCID field.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>

net/dccp/ackvec.h

@@ -20,6 +20,9 @@
 /* We can spread an ack vector across multiple options */
 #define DCCP_MAX_ACKVEC_LEN (DCCP_SINGLE_OPT_MAXLEN * 2)
 
+/* Estimated minimum average Ack Vector length - used for updating MPS */
+#define DCCPAV_MIN_OPTLEN	16
+
 #define DCCP_ACKVEC_STATE_RECEIVED	0
 #define DCCP_ACKVEC_STATE_ECN_MARKED	(1 << 6)
 #define DCCP_ACKVEC_STATE_NOT_RECEIVED	(3 << 6)

net/dccp/output.c

@@ -161,21 +161,27 @@ unsigned int dccp_sync_mss(struct sock *sk, u32 pmtu)
 	struct inet_connection_sock *icsk = inet_csk(sk);
 	struct dccp_sock *dp = dccp_sk(sk);
 	u32 ccmps = dccp_determine_ccmps(dp);
-	int cur_mps = ccmps ? min(pmtu, ccmps) : pmtu;
+	u32 cur_mps = ccmps ? min(pmtu, ccmps) : pmtu;
 
 	/* Account for header lengths and IPv4/v6 option overhead */
 	cur_mps -= (icsk->icsk_af_ops->net_header_len + icsk->icsk_ext_hdr_len +
 		    sizeof(struct dccp_hdr) + sizeof(struct dccp_hdr_ext));
 
 	/*
-	 * FIXME: this should come from the CCID infrastructure, where, say,
-	 * TFRC will say it wants TIMESTAMPS, ELAPSED time, etc, for now lets
-	 * put a rough estimate for NDP + TIMESTAMP + TIMESTAMP_ECHO + ELAPSED
-	 * TIME + TFRC_OPT_LOSS_EVENT_RATE + TFRC_OPT_RECEIVE_RATE + padding to
-	 * make it a multiple of 4
+	 * Leave enough headroom for common DCCP header options.
+	 * This only considers options which may appear on DCCP-Data packets, as
+	 * per table 3 in RFC 4340, 5.8. When running out of space for other
+	 * options (eg. Ack Vector which can take up to 255 bytes), it is better
+	 * to schedule a separate Ack. Thus we leave headroom for the following:
+	 *  - 1 byte for Slow Receiver (11.6)
+	 *  - 6 bytes for Timestamp (13.1)
+	 *  - 10 bytes for Timestamp Echo (13.3)
+	 *  - 8 bytes for NDP count (7.7, when activated)
+	 *  - 6 bytes for Data Checksum (9.3)
+	 *  - %DCCPAV_MIN_OPTLEN bytes for Ack Vector size (11.4, when enabled)
 	 */
-
-	cur_mps -= ((5 + 6 + 10 + 6 + 6 + 6 + 3) / 4) * 4;
+	cur_mps -= roundup(1 + 6 + 10 + dp->dccps_send_ndp_count * 8 + 6 +
+			   (dp->dccps_hc_rx_ackvec ? DCCPAV_MIN_OPTLEN : 0), 4);
 
 	/* And store cached results */
 	icsk->icsk_pmtu_cookie = pmtu;