dccp: Minimise header option overhead in setting the MPS

This patch resolves a long-standing FIXME to dynamically update the Maximum
Packet Size depending on actual options usage.

It uses the flags set by the feature-negotiation infrastructure to compute
the required header option size.

Most options are fixed-size, a notable exception are Ack Vectors (required
currently only by CCID-2). These can have any length between 3 and 1020
bytes. As a result of testing, 16 bytes (2 bytes for type/length plus 14 Ack
Vector cells) have been found to be sufficient for loss-free situations.

There are currently no CCID-specific header options which may appear on data
packets, thus it is not necessary to define a corresponding CCID field as
suggested in the old comment.

Further changes:
----------------
 Adjusted the type of 'cur_mps' to match the unsigned return type of the
 function.

Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Acked-by: Ian McDonald <ian.mcdonald@jandi.co.nz>
Signed-off-by: David S. Miller <davem@davemloft.net>

2 files changed, 17 insertions, 8 deletions

diff --git a/net/dccp/ackvec.h b/net/dccp/ackvec.h
index 45f95e55f873..7ea557b7c6b1 100644
--- a/net/dccp/ackvec.h
+++ b/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)
diff --git a/net/dccp/output.c b/net/dccp/output.c
index 22a618af4893..27c79bcc6a1e 100644
--- a/net/dccp/output.c
+++ b/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 -= roundup(5 + 6 + 10 + 6 + 6 + 6, 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;