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>
diff --git a/net/dccp/output.c b/net/dccp/output.c
index 22a618a..27c79bc 100644
--- a/net/dccp/output.c
+++ b/net/dccp/output.c
@@ -161,21 +161,27 @@
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;