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@xxxxxxxxxxxxxx> Acked-by: Ian McDonald <ian.mcdonald@xxxxxxxxxxx> --- net/dccp/ackvec.h | 3 +++ net/dccp/output.c | 22 ++++++++++++++-------- 2 files changed, 17 insertions(+), 8 deletions(-) --- a/net/dccp/ackvec.h +++ b/net/dccp/ackvec.h @@ -21,6 +21,9 @@ /* 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) --- a/net/dccp/output.c +++ b/net/dccp/output.c @@ -153,21 +153,27 @@ unsigned int dccp_sync_mss(struct sock * 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; - To unsubscribe from this list: send the line "unsubscribe dccp" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
