[PATCH TCP 4/4] Switch out when bandwidth changes abruptly

[Baruch Even <baruch@xxxxxxxxx>]

This is another H-TCP layer patch. This patch adds a bandwidth switcher so that
when a new stream joins we quickly yield bandwidth to it by going back to a
backoff factor of one-half.

For this to work we have a bandwidth estimator and need a new hook for
congestion control algorithms, this is in effect something that can be used
generally and is not necessarily specific to H-TCP. But H-TCP is the only user
for it at this time. A simple use for it is to export it to user-space for a
more accurate bandwidth estimation on the sender side.

Signed-Off-By: Baruch Even <baruch@xxxxxxxxx>

---
 include/net/tcp.h    |    1 
 net/ipv4/tcp_htcp.c  |  138 +++++++++++++++++++++++++++++++++++++++------------
 net/ipv4/tcp_input.c |   10 +++
 3 files changed, 117 insertions(+), 32 deletions(-)

Index: 2.6.11-stephen-htcp/net/ipv4/tcp_htcp.c
===================================================================
--- 2.6.11-stephen-htcp.orig/net/ipv4/tcp_htcp.c
+++ 2.6.11-stephen-htcp/net/ipv4/tcp_htcp.c
@@ -9,6 +9,7 @@
 #include <linux/module.h>
 #include <net/tcp.h>
 
+#define ALPHA_BASE     (1<<7)  /* 1.0 with shift << 7 */
 #define BETA_MIN       (1<<6)  /* 0.5 with shift << 7 */
 #define BETA_MAX       102     /* 0.8 with shift << 7 */
 
@@ -16,24 +17,40 @@ static int use_rtt_scaling = 1;
 module_param(use_rtt_scaling, int, 0644);
 MODULE_PARM_DESC(use_rtt_scaling, "turn on/off RTT scaling");
 
+static int use_bandwidth_switch = 1;
+module_param(use_bandwidth_switch, int, 0644);
+MODULE_PARM_DESC(use_bandwidth_switch, "turn on/off bandwidth switcher");
+
 struct htcp_ca {
-       u32     alpha;          /* Fixed point arith, << 7 */
-       u32     beta;           /* Fixed point arith, << 7 */
-       u32     modeswitch;     /* Delay modeswitch until we had at least one 
congestion event */
-       u32     ccount;         /* Number of RTTs since last congestion event */
+       u16     alpha;          /* Fixed point arith, << 7 */
+       u8      beta;           /* Fixed point arith, << 7 */
+       u8      modeswitch;     /* Delay modeswitch until we had at least one 
congestion event */
+       u8      ccount;         /* Number of RTTs since last congestion event */
+       u8      undo_ccount;
+       u16     packetcount;
        u32     minRTT;
        u32     maxRTT;
+       u32     snd_cwnd_cnt2;
 
-       u32     undo_ccount;
        u32     undo_maxRTT;
+       u32     undo_old_maxB;
+
+       /* Bandwidth estimation */
+       u32     minB;
+       u32     maxB;
+       u32     old_maxB;
+       u32     Bi;
+       u32     lasttime;
 };
 
 static inline void htcp_reset(struct htcp_ca *ca)
 {
        ca->undo_ccount = ca->ccount;
        ca->undo_maxRTT = ca->maxRTT;
+       ca->undo_old_maxB = ca->old_maxB;
 
        ca->ccount = 0;
+       ca->snd_cwnd_cnt2 = 0;
 }
 
 static u32 htcp_cwnd_undo(struct tcp_sock *tp)
@@ -41,6 +58,7 @@ static u32 htcp_cwnd_undo(struct tcp_soc
        struct htcp_ca *ca = tcp_ca(tp);
        ca->ccount = ca->undo_ccount;
        ca->maxRTT = ca->undo_maxRTT;
+       ca->old_maxB = ca->undo_old_maxB;
        return max(tp->snd_cwnd, (tp->snd_ssthresh<<7)/ca->beta);
 }
 
@@ -62,8 +80,53 @@ static inline void measure_rtt(struct tc
        }
 }
 
+static void measure_achieved_throughput(struct tcp_sock *tp, u32 pkts_acked)
+{
+       struct htcp_ca *ca = tcp_ca(tp);
+       u32 now = tcp_time_stamp;
+
+       /* achieved throughput calculations */
+       if (tp->ca_state != TCP_CA_Open && tp->ca_state != TCP_CA_Disorder) {
+               ca->packetcount = 0;
+               ca->lasttime = now;
+               return;
+       }
+
+       ca->packetcount += pkts_acked;
+
+       if (ca->packetcount >= tp->snd_cwnd - (ca->alpha>>7? : 1)
+                       && now - ca->lasttime >= ca->minRTT
+                       && ca->minRTT > 0) {
+               __u32 cur_Bi = ca->packetcount*HZ/(now - ca->lasttime);
+               if (ca->ccount <= 3) {
+                       /* just after backoff */
+                       ca->minB = ca->maxB = ca->Bi = cur_Bi;
+               } else {
+                       ca->Bi = (3*ca->Bi + cur_Bi)/4;
+                       if (ca->Bi > ca->maxB)
+                               ca->maxB = ca->Bi;
+                       if (ca->minB > ca->maxB)
+                               ca->minB = ca->maxB;
+               }
+               ca->packetcount = 0;
+               ca->lasttime = now;
+       }
+}
+
 static inline void htcp_beta_update(struct htcp_ca *ca, u32 minRTT, u32 maxRTT)
 {
+       if (use_bandwidth_switch) {
+               u32 maxB = ca->maxB;
+               u32 old_maxB = ca->old_maxB;
+               ca->old_maxB = ca->maxB;
+
+               if (!between(5*maxB, 4*old_maxB, 6*old_maxB)) {
+                       ca->beta = BETA_MIN;
+                       ca->modeswitch = 0;
+                       return;
+               }
+       }
+
        if (ca->modeswitch && minRTT > max(HZ/100, 1) && maxRTT) {
                ca->beta = (minRTT<<7)/maxRTT;
                if (ca->beta < BETA_MIN)
@@ -76,8 +139,9 @@ static inline void htcp_beta_update(stru
        }
 }
 
-static inline void htcp_alpha_update(struct htcp_ca *ca, u32 minRTT)
+static inline void htcp_alpha_update(struct htcp_ca *ca)
 {
+       u32 minRTT = ca->minRTT;
        u32 factor = 1;
        u32 diff = ca->ccount * minRTT; /* time since last backoff */
 
@@ -86,18 +150,17 @@ static inline void htcp_alpha_update(str
                factor = 1+ ( 10*diff + ((diff/2)*(diff/2)/HZ) )/HZ;
        }
 
-       ca->alpha = 2*factor*((1<<7)-ca->beta);
-       if (!ca->alpha)
-               ca->alpha = 1<<7;
-
        if (use_rtt_scaling && minRTT) {
-               u32 scale;
-               scale = (HZ<<3)/(10*minRTT);
+               u32 scale = (HZ<<3)/(10*minRTT);
                scale = min(max(scale, 1U<<2), 10U<<3); /* clamping ratio to 
interval [0.5,10]<<3 */
-               ca->alpha = (ca->alpha<<3)/scale;
-               if (!ca->alpha)
-                       ca->alpha = 1<<7;
+               factor = (factor<<3)/scale;
+               if (!factor)
+                       factor = 1;
        }
+
+       ca->alpha = 2*factor*((1<<7)-ca->beta);
+       if (!ca->alpha)
+               ca->alpha = ALPHA_BASE;
 }
 
 /* After we have the rtt data to calculate beta, we'd still prefer to wait one
@@ -115,7 +178,7 @@ static void htcp_param_update(struct tcp
        u32 maxRTT = ca->maxRTT;
 
        htcp_beta_update(ca, minRTT, maxRTT);
-       htcp_alpha_update(ca, minRTT);
+       htcp_alpha_update(ca);
 
        /* add slowly fading memory for maxRTT to accommodate routing changes 
etc */
        if (minRTT > 0 && maxRTT > minRTT)
@@ -125,6 +188,7 @@ static void htcp_param_update(struct tcp
 static u32 htcp_recalc_ssthresh(struct tcp_sock *tp)
 {
        struct htcp_ca *ca = tcp_ca(tp);
+       htcp_param_update(tp);
        return max((tp->snd_cwnd * ca->beta) >> 7, 2U);
 }
 
@@ -142,47 +206,57 @@ static void htcp_cong_avoid(struct tcp_s
        } else {
                measure_rtt(tp);
 
+               /* keep track of number of round-trip times since last backoff 
event */
+               if (ca->snd_cwnd_cnt2++ > tp->snd_cwnd) {
+                       ca->ccount++;
+                       ca->snd_cwnd_cnt2 = 0;
+                       htcp_alpha_update(ca);
+               }
+
                 /* In dangerous area, increase slowly.
                 * In theory this is tp->snd_cwnd += alpha / tp->snd_cwnd
                 */
-               if (tp->snd_cwnd_cnt >= tp->snd_cwnd) {
+               if ((tp->snd_cwnd_cnt++ * ca->alpha)>>7 >= tp->snd_cwnd) {
                        if (tp->snd_cwnd < tp->snd_cwnd_clamp)
-                               tp->snd_cwnd += ca->alpha;
+                               tp->snd_cwnd++;
                        tp->snd_cwnd_cnt = 0;
-                       ca->ccount++;
-               } else
-                       tp->snd_cwnd_cnt++;
+               }
        }
 }
 
+static u32 htcp_cwnd_min(struct tcp_sock *tp)
+{
+       return tp->snd_ssthresh;
+}
+
 static void htcp_start(struct tcp_sock *tp)
 {
        struct htcp_ca *ca = tcp_ca(tp);
-       ca->alpha = 1<<7;
+       memset(ca, 0, sizeof(struct htcp_ca));
+       ca->alpha = ALPHA_BASE;
        ca->beta = BETA_MIN;
-       ca->modeswitch = 0;
-       ca->ccount = 0;
-       ca->minRTT = 0;
-       ca->maxRTT = 0;
+       printk(KERN_INFO "htcp started for %p/%p\n", tp, ca);
 }
 
 static void htcp_ca_state(struct tcp_sock *tp, u8 new_state)
 {
-       if (new_state == TCP_CA_CWR || new_state == TCP_CA_Recovery) {
-               htcp_param_update(tp);
-               htcp_reset(tcp_ca(tp));
-       } else if (new_state == TCP_CA_Loss) {
+       switch (new_state) {
+       case TCP_CA_CWR:
+       case TCP_CA_Recovery:
+       case TCP_CA_Loss:
                htcp_reset(tcp_ca(tp));
+               break;
        }
 }
 
 static struct tcp_ca_type htcp = {
        .start          = htcp_start,
        .ssthresh       = htcp_recalc_ssthresh,
-       .min_cwnd       = tcp_reno_cwnd_min,
+       .min_cwnd       = htcp_cwnd_min,
        .cong_avoid     = htcp_cong_avoid,
        .set_state      = htcp_ca_state,
        .undo_cwnd      = htcp_cwnd_undo,
+       .pkts_acked     = measure_achieved_throughput,
 
        .owner          = THIS_MODULE,
        .name           = "htcp",
@@ -192,6 +266,8 @@ static int __init htcp_init(void)
 {
        BUILD_BUG_ON(sizeof(struct htcp_ca) > TCP_CA_PRIV_SIZE);
        BUILD_BUG_ON(BETA_MIN >= BETA_MAX);
+       if (!use_bandwidth_switch)
+               htcp.pkts_acked = NULL;
        tcp_ca_register(&htcp);
        return 0;
 }
Index: 2.6.11-stephen-htcp/include/net/tcp.h
===================================================================
--- 2.6.11-stephen-htcp.orig/include/net/tcp.h
+++ 2.6.11-stephen-htcp/include/net/tcp.h
@@ -1197,6 +1197,7 @@ struct tcp_ca_type {
 
        void (*cwnd_event)(struct tcp_sock *tp, enum tcp_ca_event ev);
        u32 (*undo_cwnd)(struct tcp_sock *tp);
+       void (*pkts_acked)(struct tcp_sock *tp, u32 num_acked);
 
        struct list_head        list;
        struct module           *owner;
Index: 2.6.11-stephen-htcp/net/ipv4/tcp_input.c
===================================================================
--- 2.6.11-stephen-htcp.orig/net/ipv4/tcp_input.c
+++ 2.6.11-stephen-htcp/net/ipv4/tcp_input.c
@@ -2030,6 +2030,7 @@ static int tcp_clean_rtx_queue(struct so
        __u32 now = tcp_time_stamp;
        int acked = 0;
        __s32 seq_rtt = -1;
+       u32 pkts_acked = 0;
 
        while ((skb = skb_peek(&sk->sk_write_queue)) &&
               skb != sk->sk_send_head) {
@@ -2041,9 +2042,12 @@ static int tcp_clean_rtx_queue(struct so
                 * the other end.
                 */
                if (after(scb->end_seq, tp->snd_una)) {
-                       if (tcp_skb_pcount(skb) > 1)
+                       if (tcp_skb_pcount(skb) > 1) {
+                               u32 pkts = tcp_skb_pcount(skb);
                                acked |= tcp_tso_acked(sk, skb,
                                                       now, &seq_rtt);
+                               pkts_acked += pkts - tcp_skb_pcount(skb);
+                       }
                        break;
                }
 
@@ -2056,6 +2060,7 @@ static int tcp_clean_rtx_queue(struct so
                 */
                if (!(scb->flags & TCPCB_FLAG_SYN)) {
                        acked |= FLAG_DATA_ACKED;
+                       pkts_acked += tcp_skb_pcount(skb);
                } else {
                        acked |= FLAG_SYN_ACKED;
                        tp->retrans_stamp = 0;
@@ -2091,6 +2096,9 @@ static int tcp_clean_rtx_queue(struct so
                tcp_ack_packets_out(sk, tp);
        }
 
+       if (tp->ca_proto->pkts_acked)
+               tp->ca_proto->pkts_acked(tp, pkts_acked);
+
 #if FASTRETRANS_DEBUG > 0
        BUG_TRAP((int)tp->sacked_out >= 0);
        BUG_TRAP((int)tp->lost_out >= 0);