412 lines
13 KiB
C
412 lines
13 KiB
C
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/*
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* TCP Vegas congestion control
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*
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* This is based on the congestion detection/avoidance scheme described in
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* Lawrence S. Brakmo and Larry L. Peterson.
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* "TCP Vegas: End to end congestion avoidance on a global internet."
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* IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
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* October 1995. Available from:
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* ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
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*
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* See http://www.cs.arizona.edu/xkernel/ for their implementation.
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* The main aspects that distinguish this implementation from the
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* Arizona Vegas implementation are:
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* o We do not change the loss detection or recovery mechanisms of
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* Linux in any way. Linux already recovers from losses quite well,
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* using fine-grained timers, NewReno, and FACK.
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* o To avoid the performance penalty imposed by increasing cwnd
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* only every-other RTT during slow start, we increase during
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* every RTT during slow start, just like Reno.
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* o Largely to allow continuous cwnd growth during slow start,
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* we use the rate at which ACKs come back as the "actual"
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* rate, rather than the rate at which data is sent.
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* o To speed convergence to the right rate, we set the cwnd
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* to achieve the right ("actual") rate when we exit slow start.
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* o To filter out the noise caused by delayed ACKs, we use the
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* minimum RTT sample observed during the last RTT to calculate
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* the actual rate.
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* o When the sender re-starts from idle, it waits until it has
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* received ACKs for an entire flight of new data before making
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* a cwnd adjustment decision. The original Vegas implementation
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* assumed senders never went idle.
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*/
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#include <linux/config.h>
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#include <linux/mm.h>
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#include <linux/module.h>
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#include <linux/skbuff.h>
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#include <linux/tcp_diag.h>
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#include <net/tcp.h>
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/* Default values of the Vegas variables, in fixed-point representation
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* with V_PARAM_SHIFT bits to the right of the binary point.
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*/
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#define V_PARAM_SHIFT 1
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static int alpha = 1<<V_PARAM_SHIFT;
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static int beta = 3<<V_PARAM_SHIFT;
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static int gamma = 1<<V_PARAM_SHIFT;
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module_param(alpha, int, 0644);
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MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)");
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module_param(beta, int, 0644);
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MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)");
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module_param(gamma, int, 0644);
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MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");
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/* Vegas variables */
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struct vegas {
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u32 beg_snd_nxt; /* right edge during last RTT */
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u32 beg_snd_una; /* left edge during last RTT */
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u32 beg_snd_cwnd; /* saves the size of the cwnd */
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u8 doing_vegas_now;/* if true, do vegas for this RTT */
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u16 cntRTT; /* # of RTTs measured within last RTT */
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u32 minRTT; /* min of RTTs measured within last RTT (in usec) */
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u32 baseRTT; /* the min of all Vegas RTT measurements seen (in usec) */
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};
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/* There are several situations when we must "re-start" Vegas:
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*
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* o when a connection is established
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* o after an RTO
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* o after fast recovery
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* o when we send a packet and there is no outstanding
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* unacknowledged data (restarting an idle connection)
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*
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* In these circumstances we cannot do a Vegas calculation at the
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* end of the first RTT, because any calculation we do is using
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* stale info -- both the saved cwnd and congestion feedback are
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* stale.
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*
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* Instead we must wait until the completion of an RTT during
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* which we actually receive ACKs.
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*/
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static inline void vegas_enable(struct tcp_sock *tp)
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{
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struct vegas *vegas = tcp_ca(tp);
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/* Begin taking Vegas samples next time we send something. */
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vegas->doing_vegas_now = 1;
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/* Set the beginning of the next send window. */
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vegas->beg_snd_nxt = tp->snd_nxt;
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vegas->cntRTT = 0;
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vegas->minRTT = 0x7fffffff;
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}
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/* Stop taking Vegas samples for now. */
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static inline void vegas_disable(struct tcp_sock *tp)
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{
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struct vegas *vegas = tcp_ca(tp);
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vegas->doing_vegas_now = 0;
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}
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static void tcp_vegas_init(struct tcp_sock *tp)
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{
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struct vegas *vegas = tcp_ca(tp);
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vegas->baseRTT = 0x7fffffff;
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vegas_enable(tp);
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}
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/* Do RTT sampling needed for Vegas.
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* Basically we:
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* o min-filter RTT samples from within an RTT to get the current
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* propagation delay + queuing delay (we are min-filtering to try to
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* avoid the effects of delayed ACKs)
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* o min-filter RTT samples from a much longer window (forever for now)
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* to find the propagation delay (baseRTT)
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*/
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static void tcp_vegas_rtt_calc(struct tcp_sock *tp, u32 usrtt)
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{
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struct vegas *vegas = tcp_ca(tp);
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u32 vrtt = usrtt + 1; /* Never allow zero rtt or baseRTT */
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/* Filter to find propagation delay: */
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if (vrtt < vegas->baseRTT)
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vegas->baseRTT = vrtt;
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/* Find the min RTT during the last RTT to find
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* the current prop. delay + queuing delay:
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*/
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vegas->minRTT = min(vegas->minRTT, vrtt);
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vegas->cntRTT++;
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}
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static void tcp_vegas_state(struct tcp_sock *tp, u8 ca_state)
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{
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if (ca_state == TCP_CA_Open)
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vegas_enable(tp);
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else
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vegas_disable(tp);
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}
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/*
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* If the connection is idle and we are restarting,
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* then we don't want to do any Vegas calculations
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* until we get fresh RTT samples. So when we
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* restart, we reset our Vegas state to a clean
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* slate. After we get acks for this flight of
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* packets, _then_ we can make Vegas calculations
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* again.
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*/
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static void tcp_vegas_cwnd_event(struct tcp_sock *tp, enum tcp_ca_event event)
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{
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if (event == CA_EVENT_CWND_RESTART ||
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event == CA_EVENT_TX_START)
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tcp_vegas_init(tp);
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}
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static void tcp_vegas_cong_avoid(struct tcp_sock *tp, u32 ack,
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u32 seq_rtt, u32 in_flight, int flag)
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{
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struct vegas *vegas = tcp_ca(tp);
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if (!vegas->doing_vegas_now)
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return tcp_reno_cong_avoid(tp, ack, seq_rtt, in_flight, flag);
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/* The key players are v_beg_snd_una and v_beg_snd_nxt.
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*
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* These are so named because they represent the approximate values
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* of snd_una and snd_nxt at the beginning of the current RTT. More
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* precisely, they represent the amount of data sent during the RTT.
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* At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,
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* we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding
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* bytes of data have been ACKed during the course of the RTT, giving
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* an "actual" rate of:
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*
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* (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration)
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*
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* Unfortunately, v_beg_snd_una is not exactly equal to snd_una,
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* because delayed ACKs can cover more than one segment, so they
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* don't line up nicely with the boundaries of RTTs.
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*
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* Another unfortunate fact of life is that delayed ACKs delay the
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* advance of the left edge of our send window, so that the number
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* of bytes we send in an RTT is often less than our cwnd will allow.
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* So we keep track of our cwnd separately, in v_beg_snd_cwnd.
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*/
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if (after(ack, vegas->beg_snd_nxt)) {
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/* Do the Vegas once-per-RTT cwnd adjustment. */
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u32 old_wnd, old_snd_cwnd;
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/* Here old_wnd is essentially the window of data that was
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* sent during the previous RTT, and has all
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* been acknowledged in the course of the RTT that ended
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* with the ACK we just received. Likewise, old_snd_cwnd
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* is the cwnd during the previous RTT.
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*/
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old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) /
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tp->mss_cache;
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old_snd_cwnd = vegas->beg_snd_cwnd;
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/* Save the extent of the current window so we can use this
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* at the end of the next RTT.
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*/
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vegas->beg_snd_una = vegas->beg_snd_nxt;
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vegas->beg_snd_nxt = tp->snd_nxt;
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vegas->beg_snd_cwnd = tp->snd_cwnd;
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/* Take into account the current RTT sample too, to
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* decrease the impact of delayed acks. This double counts
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* this sample since we count it for the next window as well,
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* but that's not too awful, since we're taking the min,
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* rather than averaging.
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*/
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tcp_vegas_rtt_calc(tp, seq_rtt*1000);
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/* We do the Vegas calculations only if we got enough RTT
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* samples that we can be reasonably sure that we got
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* at least one RTT sample that wasn't from a delayed ACK.
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* If we only had 2 samples total,
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* then that means we're getting only 1 ACK per RTT, which
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* means they're almost certainly delayed ACKs.
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* If we have 3 samples, we should be OK.
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*/
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if (vegas->cntRTT <= 2) {
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/* We don't have enough RTT samples to do the Vegas
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* calculation, so we'll behave like Reno.
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*/
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if (tp->snd_cwnd > tp->snd_ssthresh)
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tp->snd_cwnd++;
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} else {
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u32 rtt, target_cwnd, diff;
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/* We have enough RTT samples, so, using the Vegas
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* algorithm, we determine if we should increase or
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* decrease cwnd, and by how much.
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*/
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/* Pluck out the RTT we are using for the Vegas
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* calculations. This is the min RTT seen during the
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* last RTT. Taking the min filters out the effects
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* of delayed ACKs, at the cost of noticing congestion
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* a bit later.
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*/
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rtt = vegas->minRTT;
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/* Calculate the cwnd we should have, if we weren't
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* going too fast.
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*
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* This is:
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* (actual rate in segments) * baseRTT
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* We keep it as a fixed point number with
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* V_PARAM_SHIFT bits to the right of the binary point.
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*/
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target_cwnd = ((old_wnd * vegas->baseRTT)
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<< V_PARAM_SHIFT) / rtt;
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/* Calculate the difference between the window we had,
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* and the window we would like to have. This quantity
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* is the "Diff" from the Arizona Vegas papers.
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*
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* Again, this is a fixed point number with
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* V_PARAM_SHIFT bits to the right of the binary
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* point.
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*/
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diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd;
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if (tp->snd_cwnd < tp->snd_ssthresh) {
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/* Slow start. */
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if (diff > gamma) {
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/* Going too fast. Time to slow down
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* and switch to congestion avoidance.
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*/
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tp->snd_ssthresh = 2;
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/* Set cwnd to match the actual rate
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* exactly:
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* cwnd = (actual rate) * baseRTT
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* Then we add 1 because the integer
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* truncation robs us of full link
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* utilization.
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*/
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tp->snd_cwnd = min(tp->snd_cwnd,
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(target_cwnd >>
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V_PARAM_SHIFT)+1);
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}
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} else {
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/* Congestion avoidance. */
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u32 next_snd_cwnd;
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/* Figure out where we would like cwnd
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* to be.
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*/
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if (diff > beta) {
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/* The old window was too fast, so
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* we slow down.
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*/
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next_snd_cwnd = old_snd_cwnd - 1;
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} else if (diff < alpha) {
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/* We don't have enough extra packets
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* in the network, so speed up.
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*/
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next_snd_cwnd = old_snd_cwnd + 1;
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} else {
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/* Sending just as fast as we
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* should be.
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*/
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next_snd_cwnd = old_snd_cwnd;
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}
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/* Adjust cwnd upward or downward, toward the
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* desired value.
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*/
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if (next_snd_cwnd > tp->snd_cwnd)
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tp->snd_cwnd++;
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else if (next_snd_cwnd < tp->snd_cwnd)
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tp->snd_cwnd--;
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}
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}
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/* Wipe the slate clean for the next RTT. */
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vegas->cntRTT = 0;
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vegas->minRTT = 0x7fffffff;
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}
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/* The following code is executed for every ack we receive,
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* except for conditions checked in should_advance_cwnd()
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* before the call to tcp_cong_avoid(). Mainly this means that
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* we only execute this code if the ack actually acked some
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* data.
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*/
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/* If we are in slow start, increase our cwnd in response to this ACK.
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* (If we are not in slow start then we are in congestion avoidance,
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* and adjust our congestion window only once per RTT. See the code
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* above.)
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*/
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if (tp->snd_cwnd <= tp->snd_ssthresh)
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tp->snd_cwnd++;
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/* to keep cwnd from growing without bound */
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tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp);
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/* Make sure that we are never so timid as to reduce our cwnd below
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* 2 MSS.
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*
|
||
|
|
* Going below 2 MSS would risk huge delayed ACKs from our receiver.
|
||
|
|
*/
|
||
|
|
tp->snd_cwnd = max(tp->snd_cwnd, 2U);
|
||
|
|
}
|
||
|
|
|
||
|
|
/* Extract info for Tcp socket info provided via netlink. */
|
||
|
|
static void tcp_vegas_get_info(struct tcp_sock *tp, u32 ext,
|
||
|
|
struct sk_buff *skb)
|
||
|
|
{
|
||
|
|
const struct vegas *ca = tcp_ca(tp);
|
||
|
|
if (ext & (1<<(TCPDIAG_VEGASINFO-1))) {
|
||
|
|
struct tcpvegas_info *info;
|
||
|
|
|
||
|
|
info = RTA_DATA(__RTA_PUT(skb, TCPDIAG_VEGASINFO,
|
||
|
|
sizeof(*info)));
|
||
|
|
|
||
|
|
info->tcpv_enabled = ca->doing_vegas_now;
|
||
|
|
info->tcpv_rttcnt = ca->cntRTT;
|
||
|
|
info->tcpv_rtt = ca->baseRTT;
|
||
|
|
info->tcpv_minrtt = ca->minRTT;
|
||
|
|
rtattr_failure: ;
|
||
|
|
}
|
||
|
|
}
|
||
|
|
|
||
|
|
static struct tcp_congestion_ops tcp_vegas = {
|
||
|
|
.init = tcp_vegas_init,
|
||
|
|
.ssthresh = tcp_reno_ssthresh,
|
||
|
|
.cong_avoid = tcp_vegas_cong_avoid,
|
||
|
|
.min_cwnd = tcp_reno_min_cwnd,
|
||
|
|
.rtt_sample = tcp_vegas_rtt_calc,
|
||
|
|
.set_state = tcp_vegas_state,
|
||
|
|
.cwnd_event = tcp_vegas_cwnd_event,
|
||
|
|
.get_info = tcp_vegas_get_info,
|
||
|
|
|
||
|
|
.owner = THIS_MODULE,
|
||
|
|
.name = "vegas",
|
||
|
|
};
|
||
|
|
|
||
|
|
static int __init tcp_vegas_register(void)
|
||
|
|
{
|
||
|
|
BUG_ON(sizeof(struct vegas) > TCP_CA_PRIV_SIZE);
|
||
|
|
tcp_register_congestion_control(&tcp_vegas);
|
||
|
|
return 0;
|
||
|
|
}
|
||
|
|
|
||
|
|
static void __exit tcp_vegas_unregister(void)
|
||
|
|
{
|
||
|
|
tcp_unregister_congestion_control(&tcp_vegas);
|
||
|
|
}
|
||
|
|
|
||
|
|
module_init(tcp_vegas_register);
|
||
|
|
module_exit(tcp_vegas_unregister);
|
||
|
|
|
||
|
|
MODULE_AUTHOR("Stephen Hemminger");
|
||
|
|
MODULE_LICENSE("GPL");
|
||
|
|
MODULE_DESCRIPTION("TCP Vegas");
|