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dccp: Minimise header option overhead in setting the MPS

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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>
This commit is contained in:
Gerrit Renker 2009-02-27 22:38:28 +00:00 committed by David S. Miller
parent 9ce0146102
commit 361a5c1dd0
2 changed files with 17 additions and 8 deletions
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3
net/dccp/ackvec.h
View file

@ -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)

22
net/dccp/output.c
View file

@ -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;