1435562d7e
The current CCID-2 RTT estimator code is in parts broken and lags behind the
suggestions in RFC2988 of using scaled variants for SRTT/RTTVAR.
That code is replaced by the present patch, which reuses the Linux TCP RTT
estimator code - reasons for this code duplication are given below.
Further details:
----------------
1. The minimum RTO of previously one second has been replaced with TCP's, since
RFC4341, sec. 5 says that the minimum of 1 sec. (suggested in RFC2988, 2.4)
is not necessary. Instead, the TCP_RTO_MIN is used, which agrees with DCCP's
concept of a default RTT (RFC 4340, 3.4).
2. The maximum RTO has been set to DCCP_RTO_MAX (64 sec), which agrees with
RFC2988, (2.5).
3. De-inlined the function ccid2_new_ack().
4. Added a FIXME: the RTT is sampled several times per Ack Vector, which will
give the wrong estimate. It should be replaced with one sample per Ack.
However, at the moment this can not be resolved easily, since
- it depends on TX history code (which also needs some work),
- the cleanest solution is not to use the `sent' time at all (saves 4 bytes
per entry) and use DCCP timestamps / elapsed time to estimated the RTT,
which however is non-trivial to get right (but needs to be done).
Reasons for reusing the Linux TCP estimator algorithm:
------------------------------------------------------
Some time was spent to find a better alternative, using basic RFC2988 as a first
step. Further analysis and experimentation showed that the Linux TCP RTO
estimator is superior to a basic RFC2988 implementation. A summary is on
http://www.erg.abdn.ac.uk/users/gerrit/dccp/notes/ccid2/rto_estimator/
In addition, this estimator fared well in a recent empirical evaluation:
Rewaskar, Sushant, Jasleen Kaur and F. Donelson Smith.
A Performance Study of Loss Detection/Recovery in Real-world TCP
Implementations. Proceedings of 15th IEEE International
Conference on Network Protocols (ICNP-07). 2007.
Thus there is significant benefit in reusing the existing TCP code.
Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
100 lines
2.7 KiB
C
100 lines
2.7 KiB
C
/*
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* net/dccp/ccids/ccid2.h
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*
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* Copyright (c) 2005 Andrea Bittau <a.bittau@cs.ucl.ac.uk>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
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*/
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#ifndef _DCCP_CCID2_H_
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#define _DCCP_CCID2_H_
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#include <linux/dccp.h>
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#include <linux/timer.h>
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#include <linux/types.h>
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#include "../ccid.h"
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/* NUMDUPACK parameter from RFC 4341, p. 6 */
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#define NUMDUPACK 3
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struct sock;
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struct ccid2_seq {
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u64 ccid2s_seq;
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unsigned long ccid2s_sent;
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int ccid2s_acked;
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struct ccid2_seq *ccid2s_prev;
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struct ccid2_seq *ccid2s_next;
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};
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#define CCID2_SEQBUF_LEN 1024
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#define CCID2_SEQBUF_MAX 128
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/** struct ccid2_hc_tx_sock - CCID2 TX half connection
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*
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* @{cwnd,ssthresh,pipe}: as per RFC 4341, section 5
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* @packets_acked: Ack counter for deriving cwnd growth (RFC 3465)
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* @srtt: smoothed RTT estimate, scaled by 2^3
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* @mdev: smoothed RTT variation, scaled by 2^2
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* @mdev_max: maximum of @mdev during one flight
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* @rttvar: moving average/maximum of @mdev_max
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* @rto: RTO value deriving from SRTT and RTTVAR (RFC 2988)
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* @rtt_seq: to decay RTTVAR at most once per flight
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* @rpseq: last consecutive seqno
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* @rpdupack: dupacks since rpseq
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* @av_chunks: list of Ack Vectors received on current skb
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*/
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struct ccid2_hc_tx_sock {
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u32 cwnd;
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u32 ssthresh;
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u32 pipe;
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u32 packets_acked;
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struct ccid2_seq *seqbuf[CCID2_SEQBUF_MAX];
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int seqbufc;
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struct ccid2_seq *seqh;
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struct ccid2_seq *seqt;
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/* RTT measurement: variables/principles are the same as in TCP */
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u32 srtt,
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mdev,
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mdev_max,
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rttvar,
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rto;
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u64 rtt_seq:48;
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struct timer_list rtotimer;
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u64 rpseq;
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int rpdupack;
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unsigned long last_cong;
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u64 high_ack;
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struct list_head av_chunks;
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};
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static inline bool ccid2_cwnd_network_limited(struct ccid2_hc_tx_sock *hctx)
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{
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return (hctx->pipe >= hctx->cwnd);
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}
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struct ccid2_hc_rx_sock {
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int data;
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};
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static inline struct ccid2_hc_tx_sock *ccid2_hc_tx_sk(const struct sock *sk)
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{
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return ccid_priv(dccp_sk(sk)->dccps_hc_tx_ccid);
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}
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static inline struct ccid2_hc_rx_sock *ccid2_hc_rx_sk(const struct sock *sk)
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{
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return ccid_priv(dccp_sk(sk)->dccps_hc_rx_ccid);
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}
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#endif /* _DCCP_CCID2_H_ */
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