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[PATCH 4/5] TCP Vegas support

To: Andrew Morton <akpm@xxxxxxxxx>, "David S. Miller" <davem@xxxxxxxxxxxxx>
Subject: [PATCH 4/5] TCP Vegas support
From: Stephen Hemminger <shemminger@xxxxxxxx>
Date: Fri, 18 Mar 2005 16:26:26 -0800
Cc: netdev@xxxxxxxxxxx
Organization: Open Source Development Lab
Sender: netdev-bounce@xxxxxxxxxxx
Add TCP Vegas support back as a configurable algorithm.
Since it has issues that make it perform poorly in many environments
make it default to off.

Signed-off-by: Stephen Hemminger <shemminger@xxxxxxxx>


diff -Nru a/net/ipv4/Kconfig b/net/ipv4/Kconfig
--- a/net/ipv4/Kconfig  2005-03-18 16:11:05 -08:00
+++ b/net/ipv4/Kconfig  2005-03-18 16:11:05 -08:00
@@ -396,6 +396,16 @@
        TCP Westwood+ significantly increases fairness wrt TCP Reno in 
        wired networks and throughput over wireless links.   
 
+config TCP_CONG_VEGAS
+       tristate "TCP Vegas"
+       default n
+       ---help---
+       TCP Vegas is a sender-side only change to TCP that anticipates
+       the onset of congestion by estimating the bandwidth. TCP Vegas
+       adjusts the sending rate by modifying the congestion
+       window. TCP Vegas should provide less packet loss, but it is
+       not as aggressive as TCP Reno.
+
 endmenu
 
 source "net/ipv4/ipvs/Kconfig"
diff -Nru a/net/ipv4/Makefile b/net/ipv4/Makefile
--- a/net/ipv4/Makefile 2005-03-18 16:11:05 -08:00
+++ b/net/ipv4/Makefile 2005-03-18 16:11:05 -08:00
@@ -26,6 +26,7 @@
 obj-$(CONFIG_IP_TCPDIAG) += tcp_diag.o 
 obj-$(CONFIG_TCP_CONG_BIC) += tcp_bic.o
 obj-$(CONFIG_TCP_CONG_WESTWOOD) += tcp_westwood.o
+obj-$(CONFIG_TCP_CONG_VEGAS) += tcp_vegas.o
 
 obj-$(CONFIG_XFRM) += xfrm4_policy.o xfrm4_state.o xfrm4_input.o \
                      xfrm4_output.o
diff -Nru a/net/ipv4/tcp_vegas.c b/net/ipv4/tcp_vegas.c
--- /dev/null   Wed Dec 31 16:00:00 196900
+++ b/net/ipv4/tcp_vegas.c      2005-03-18 16:11:05 -08:00
@@ -0,0 +1,409 @@
+/*
+ * TCP Vegas congestion control
+ *
+ * This is based on the congestion detection/avoidance scheme described in
+ *    Lawrence S. Brakmo and Larry L. Peterson.
+ *    "TCP Vegas: End to end congestion avoidance on a global internet."
+ *    IEEE Journal on Selected Areas in Communication, 13(8):1465--1480,
+ *    October 1995. Available from:
+ *     ftp://ftp.cs.arizona.edu/xkernel/Papers/jsac.ps
+ *
+ * See http://www.cs.arizona.edu/xkernel/ for their implementation.
+ * The main aspects that distinguish this implementation from the
+ * Arizona Vegas implementation are:
+ *   o We do not change the loss detection or recovery mechanisms of
+ *     Linux in any way. Linux already recovers from losses quite well,
+ *     using fine-grained timers, NewReno, and FACK.
+ *   o To avoid the performance penalty imposed by increasing cwnd
+ *     only every-other RTT during slow start, we increase during
+ *     every RTT during slow start, just like Reno.
+ *   o Largely to allow continuous cwnd growth during slow start,
+ *     we use the rate at which ACKs come back as the "actual"
+ *     rate, rather than the rate at which data is sent.
+ *   o To speed convergence to the right rate, we set the cwnd
+ *     to achieve the right ("actual") rate when we exit slow start.
+ *   o To filter out the noise caused by delayed ACKs, we use the
+ *     minimum RTT sample observed during the last RTT to calculate
+ *     the actual rate.
+ *   o When the sender re-starts from idle, it waits until it has
+ *     received ACKs for an entire flight of new data before making
+ *     a cwnd adjustment decision. The original Vegas implementation
+ *     assumed senders never went idle.
+ */
+
+#include <linux/config.h>
+#include <linux/mm.h>
+#include <linux/module.h>
+#include <linux/skbuff.h>
+#include <linux/tcp_diag.h>
+
+#include <net/tcp.h>
+
+/* Default values of the Vegas variables, in fixed-point representation
+ * with V_PARAM_SHIFT bits to the right of the binary point.
+ */
+#define V_PARAM_SHIFT 1
+static int alpha = 1<<V_PARAM_SHIFT;
+static int beta  = 3<<V_PARAM_SHIFT;
+static int gamma = 1<<V_PARAM_SHIFT;
+
+module_param(alpha, int, 0644);
+MODULE_PARM_DESC(alpha, "lower bound of packets in network (scale by 2)");
+module_param(beta, int, 0644);
+MODULE_PARM_DESC(beta, "upper bound of packets in network (scale by 2)");
+module_param(gamma, int, 0644);
+MODULE_PARM_DESC(gamma, "limit on increase (scale by 2)");
+
+
+/* Vegas variables */
+struct vegas_ca {
+       u32     beg_snd_nxt;    /* right edge during last RTT */
+       u32     beg_snd_una;    /* left edge  during last RTT */
+       u32     beg_snd_cwnd;   /* saves the size of the cwnd */
+       u8      doing_vegas_now;/* if true, do vegas for this RTT */
+       u16     cntRTT;         /* # of RTTs measured within last RTT */
+       u32     minRTT;         /* min of RTTs measured within last RTT (in 
usec) */
+       u32     baseRTT;        /* the min of all Vegas RTT measurements seen 
(in usec) */
+};
+
+/* There are several situations when we must "re-start" Vegas:
+ *
+ *  o when a connection is established
+ *  o after an RTO
+ *  o after fast recovery
+ *  o when we send a packet and there is no outstanding
+ *    unacknowledged data (restarting an idle connection)
+ *
+ * In these circumstances we cannot do a Vegas calculation at the
+ * end of the first RTT, because any calculation we do is using
+ * stale info -- both the saved cwnd and congestion feedback are
+ * stale.
+ *
+ * Instead we must wait until the completion of an RTT during
+ * which we actually receive ACKs.
+ */
+static void tcp_vegas_enable(struct tcp_sock *tp)
+{
+       struct vegas_ca *vegas = tcp_ca(tp);
+
+       /* Begin taking Vegas samples next time we send something. */
+       vegas->doing_vegas_now = 1;
+
+       /* Set the beginning of the next send window. */
+       vegas->beg_snd_nxt = tp->snd_nxt;
+
+       vegas->cntRTT = 0;
+       vegas->minRTT = 0x7fffffff;
+}
+
+/* Stop taking Vegas samples for now. */
+static inline void tcp_vegas_disable(struct tcp_sock *tp)
+{
+       struct vegas_ca *vegas = tcp_ca(tp);
+
+       vegas->doing_vegas_now = 0;
+}
+
+static void tcp_vegas_start(struct tcp_sock *tp)
+{
+       struct vegas_ca *vegas = tcp_ca(tp);
+
+       vegas->baseRTT = 0x7fffffff;
+       tcp_vegas_enable(tp);
+}
+
+/* Do RTT sampling needed for Vegas.
+ * Basically we:
+ *   o min-filter RTT samples from within an RTT to get the current
+ *     propagation delay + queuing delay (we are min-filtering to try to
+ *     avoid the effects of delayed ACKs)
+ *   o min-filter RTT samples from a much longer window (forever for now)
+ *     to find the propagation delay (baseRTT)
+ */
+static void tcp_vegas_rtt_calc(struct tcp_sock *tp, u32 rtt)
+{
+       struct vegas_ca *vegas = tcp_ca(tp);
+       u32 vrtt = rtt + 1; /* Never allow zero rtt or baseRTT */
+
+       /* Filter to find propagation delay: */
+       if (vrtt < vegas->baseRTT)
+               vegas->baseRTT = vrtt;
+
+       /* Find the min RTT during the last RTT to find
+        * the current prop. delay + queuing delay:
+        */
+       vegas->minRTT = min(vegas->minRTT, vrtt);
+       vegas->cntRTT++;
+}
+
+static void tcp_vegas_ca_state(struct tcp_sock *tp, u8 ca_state)
+{
+       if (ca_state == TCP_CA_Open)
+               tcp_vegas_enable(tp);
+       else
+               tcp_vegas_disable(tp);
+}
+
+/*
+ * If the connection is idle and we are restarting,
+ * then we don't want to do any Vegas calculations
+ * until we get fresh RTT samples.  So when we
+ * restart, we reset our Vegas state to a clean
+ * slate. After we get acks for this flight of
+ * packets, _then_ we can make Vegas calculations
+ * again.
+ */
+static void tcp_vegas_cwnd_event(struct tcp_sock *tp, enum tcp_ca_event event)
+{
+       if (event == CA_EVENT_CWND_RESTART || event == CA_EVENT_TX_START)
+               tcp_vegas_enable(tp);
+}
+
+static void tcp_vegas_cong_avoid(struct tcp_sock *tp, u32 ack,
+                                u32 seq_rtt, u32 in_flight, int flag)
+{
+       struct vegas_ca *vegas = tcp_ca(tp);
+
+       if (!vegas->doing_vegas_now) {
+               tcp_reno_cong_avoid(tp, ack, seq_rtt, in_flight, flag);
+               return;
+       }
+
+       /* The key players are v_beg_snd_una and v_beg_snd_nxt.
+        *
+        * These are so named because they represent the approximate values
+        * of snd_una and snd_nxt at the beginning of the current RTT. More
+        * precisely, they represent the amount of data sent during the RTT.
+        * At the end of the RTT, when we receive an ACK for v_beg_snd_nxt,
+        * we will calculate that (v_beg_snd_nxt - v_beg_snd_una) outstanding
+        * bytes of data have been ACKed during the course of the RTT, giving
+        * an "actual" rate of:
+        *
+        *     (v_beg_snd_nxt - v_beg_snd_una) / (rtt duration)
+        *
+        * Unfortunately, v_beg_snd_una is not exactly equal to snd_una,
+        * because delayed ACKs can cover more than one segment, so they
+        * don't line up nicely with the boundaries of RTTs.
+        *
+        * Another unfortunate fact of life is that delayed ACKs delay the
+        * advance of the left edge of our send window, so that the number
+        * of bytes we send in an RTT is often less than our cwnd will allow.
+        * So we keep track of our cwnd separately, in v_beg_snd_cwnd.
+        */
+
+       if (after(ack, vegas->beg_snd_nxt)) {
+               /* Do the Vegas once-per-RTT cwnd adjustment. */
+               u32 old_wnd, old_snd_cwnd;
+
+
+               /* Here old_wnd is essentially the window of data that was
+                * sent during the previous RTT, and has all
+                * been acknowledged in the course of the RTT that ended
+                * with the ACK we just received. Likewise, old_snd_cwnd
+                * is the cwnd during the previous RTT.
+                */
+               old_wnd = (vegas->beg_snd_nxt - vegas->beg_snd_una) /
+                       tp->mss_cache_std;
+               old_snd_cwnd = vegas->beg_snd_cwnd;
+
+               /* Save the extent of the current window so we can use this
+                * at the end of the next RTT.
+                */
+               vegas->beg_snd_una  = vegas->beg_snd_nxt;
+               vegas->beg_snd_nxt  = tp->snd_nxt;
+               vegas->beg_snd_cwnd = tp->snd_cwnd;
+
+               /* Take into account the current RTT sample too, to
+                * decrease the impact of delayed acks. This double counts
+                * this sample since we count it for the next window as well,
+                * but that's not too awful, since we're taking the min,
+                * rather than averaging.
+                */
+               tcp_vegas_rtt_calc(tp, seq_rtt);
+
+               /* We do the Vegas calculations only if we got enough RTT
+                * samples that we can be reasonably sure that we got
+                * at least one RTT sample that wasn't from a delayed ACK.
+                * If we only had 2 samples total,
+                * then that means we're getting only 1 ACK per RTT, which
+                * means they're almost certainly delayed ACKs.
+                * If  we have 3 samples, we should be OK.
+                */
+
+               if (vegas->cntRTT <= 2) {
+                       /* We don't have enough RTT samples to do the Vegas
+                        * calculation, so we'll behave like Reno.
+                        */
+                       if (tp->snd_cwnd > tp->snd_ssthresh)
+                               tp->snd_cwnd++;
+               } else {
+                       u32 rtt, target_cwnd, diff;
+
+                       /* We have enough RTT samples, so, using the Vegas
+                        * algorithm, we determine if we should increase or
+                        * decrease cwnd, and by how much.
+                        */
+
+                       /* Pluck out the RTT we are using for the Vegas
+                        * calculations. This is the min RTT seen during the
+                        * last RTT. Taking the min filters out the effects
+                        * of delayed ACKs, at the cost of noticing congestion
+                        * a bit later.
+                        */
+                       rtt = vegas->minRTT;
+
+                       /* Calculate the cwnd we should have, if we weren't
+                        * going too fast.
+                        *
+                        * This is:
+                        *     (actual rate in segments) * baseRTT
+                        * We keep it as a fixed point number with
+                        * V_PARAM_SHIFT bits to the right of the binary point.
+                        */
+                       target_cwnd = ((old_wnd * vegas->baseRTT)
+                                      << V_PARAM_SHIFT) / rtt;
+
+                       /* Calculate the difference between the window we had,
+                        * and the window we would like to have. This quantity
+                        * is the "Diff" from the Arizona Vegas papers.
+                        *
+                        * Again, this is a fixed point number with
+                        * V_PARAM_SHIFT bits to the right of the binary
+                        * point.
+                        */
+                       diff = (old_wnd << V_PARAM_SHIFT) - target_cwnd;
+
+                       if (tp->snd_cwnd < tp->snd_ssthresh) {
+                               /* Slow start.  */
+                               if (diff > gamma) {
+                                       /* Going too fast. Time to slow down
+                                        * and switch to congestion avoidance.
+                                        */
+                                       tp->snd_ssthresh = 2;
+
+                                       /* Set cwnd to match the actual rate
+                                        * exactly:
+                                        *   cwnd = (actual rate) * baseRTT
+                                        * Then we add 1 because the integer
+                                        * truncation robs us of full link
+                                        * utilization.
+                                        */
+                                       tp->snd_cwnd = min(tp->snd_cwnd,
+                                                          (target_cwnd >>
+                                                           V_PARAM_SHIFT)+1);
+
+                               }
+                       } else {
+                               /* Congestion avoidance. */
+                               u32 next_snd_cwnd;
+
+                               /* Figure out where we would like cwnd
+                                * to be.
+                                */
+                               if (diff > beta) {
+                                       /* The old window was too fast, so
+                                        * we slow down.
+                                        */
+                                       next_snd_cwnd = old_snd_cwnd - 1;
+                               } else if (diff < alpha) {
+                                       /* We don't have enough extra packets
+                                        * in the network, so speed up.
+                                        */
+                                       next_snd_cwnd = old_snd_cwnd + 1;
+                               } else {
+                                       /* Sending just as fast as we
+                                        * should be.
+                                        */
+                                       next_snd_cwnd = old_snd_cwnd;
+                               }
+
+                               /* Adjust cwnd upward or downward, toward the
+                                * desired value.
+                                */
+                               if (next_snd_cwnd > tp->snd_cwnd)
+                                       tp->snd_cwnd++;
+                               else if (next_snd_cwnd < tp->snd_cwnd)
+                                       tp->snd_cwnd--;
+                       }
+               }
+
+               /* Wipe the slate clean for the next RTT. */
+               vegas->cntRTT = 0;
+               vegas->minRTT = 0x7fffffff;
+       }
+
+       /* The following code is executed for every ack we receive,
+        * except for conditions checked in should_advance_cwnd()
+        * before the call to tcp_cong_avoid(). Mainly this means that
+        * we only execute this code if the ack actually acked some
+        * data.
+        */
+
+       /* If we are in slow start, increase our cwnd in response to this ACK.
+        * (If we are not in slow start then we are in congestion avoidance,
+        * and adjust our congestion window only once per RTT. See the code
+        * above.)
+        */
+       if (tp->snd_cwnd <= tp->snd_ssthresh)
+               tp->snd_cwnd++;
+
+       /* to keep cwnd from growing without bound */
+       tp->snd_cwnd = min_t(u32, tp->snd_cwnd, tp->snd_cwnd_clamp);
+
+       /* Make sure that we are never so timid as to reduce our cwnd below
+        * 2 MSS.
+        *
+        * 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)
+{
+       if (ext & (1<<(TCPDIAG_VEGASINFO-1))) {
+               struct tcpvegas_info *info
+                       = tcpdiag_put(skb, TCPDIAG_VEGASINFO, sizeof(*info));
+               if (info) {
+                       struct vegas_ca *ca = tcp_ca(tp);
+                       info->tcpv_enabled = ca->doing_vegas_now;
+                       info->tcpv_rttcnt = ca->cntRTT;
+                       info->tcpv_rtt = jiffies_to_usecs(ca->baseRTT);
+                       info->tcpv_minrtt = jiffies_to_usecs(ca->minRTT);
+               }
+       }
+}
+
+static struct tcp_ca_type tcp_vegas = {
+       .start          = tcp_vegas_start,
+       .ssthresh       = tcp_reno_ssthresh,
+       .min_cwnd       = tcp_reno_cwnd_min,
+       .cong_avoid     = tcp_vegas_cong_avoid,
+       .rtt_sample     = tcp_vegas_rtt_calc,
+       .set_state      = tcp_vegas_ca_state,
+       .cwnd_event     = tcp_vegas_cwnd_event,
+       .get_info       = tcp_vegas_get_info,
+
+       .owner          = THIS_MODULE,
+       .name           = "vegas",
+};
+
+static int __init tcp_vegas_init(void)
+{
+       BUG_ON(sizeof(struct vegas_ca) > TCP_CA_PRIV_SIZE);
+       tcp_ca_register(&tcp_vegas);
+       return 0;
+}
+
+static void __exit tcp_vegas_exit(void)
+{
+       tcp_ca_unregister(&tcp_vegas);
+}
+
+module_init(tcp_vegas_init);
+module_exit(tcp_vegas_exit);
+
+MODULE_AUTHOR("Stephen Hemminger");
+MODULE_LICENSE("GPL");
+MODULE_DESCRIPTION("TCP Vegas");

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