TCP拥塞控制算法内核实现剖析（二）
TCP拥塞控制算法内核实现剖析（二）内核版本：2.6.37
主要源文件：linux-2.6.37/ net/ ipv4/ tcp_bic.c
======================================================================================================/* BIC TCP Parameters */
struct bictcp {
u32 cnt ; /* increase cwnd by 1 after ACKs */
u32 last_max_cwnd ; /* last maximum snd_cwnd */
u32 loss_cwnd ; /* congestion window at last loss */
u32 last_cwnd ; /* the last snd_cwnd */
u32 last_time ; /* time when updated last_cwnd */
u32 epoch_start ; /* beginning of an epoch */
#define ACK_RATIO_SHIFT 4
u32 delayed_ack ; /* estimate the ratio of Packets/ACKs << 4 */
} ;
/* Scale factor beta calculation
* max_cwnd = snd_cwnd * beta
*/
#define BICTCP_BETA_SCALE 1024
/* In binary search ,
* go to point (max+min) / N
*/
#define BICTCP_B 4 /*并不是真正的二分*/全局变量static int fast_convergence = 1 ; /* BIC能快速的达到一个平衡值，开关*/
static int max_increment = 16 ; /* 每次增加的MSS 不能超过这个值，防止增长太过剧烈*/
static int low_window = 14 ; /* lower bound on congestion window , for TCP friendliness */
static int beta = 819 ; /* = 819 / 1024(BICTCP_BETA_SCALE) ，beta for multiplicative increase 。？*/
static int initial_ssthresh ; /* 初始的阈值 */
static int smooth_part = 20 ; /* log(B/(B*Smin))/log(B/(B-1))+B, # of RTT from Wmax-B to Wmax 。？*/
/* initial_ssthresh的初始值被设置成2^31-1=2147483647 */==========================================================================================================struct inet_connection_sock {
...
u32 icsk_ca_priv ;
#define ICSK_CA_PRIV_SIZE (16*sizeof(u32))
}
static inline void *inet_csk_ca( const struct sock *sk )
{
return (void *)inet_csk(sk)->icsk_ca_priv ;
}============================================================================================================
不明白？!
/* Slow start with delack produces 3 packets of burst , so that it is safe "de facto". This will be
* default - same as the default reordering threshold - but if reordering increases , we must
* be able to allow cwnd to burst at least this much in order to not pull it back when holes
* are filled.
*/
static __inline__ __u32 tcp_max_burst ( const struct tcp_sock *sk )
{
return tp->reordering ;
}
/* u8 reordering ; Packets reordering metric */
/* RFC2681 Check whether we are limited by application or congestion window
* This is the inverse of cwnd check in tcp_tso_should_defer
*/
/* 返回0，不需要增加cwnd ; 返回1，cwnd被限制，需要增加 */
int tcp_is_cwnd_limited ( const struct sock *sk , u32 in_flight )
{
const struct tcp_sock *tp = tcp_sk(sk) ;
u32 left ;
if( in_flight >= tp->snd_cwnd ) /* 不是规定in_flight < snd_cwnd ? */
return 1 ;
left = tp->snd_cwnd - in_flight ;
if( sk_can_gso(sk) &&
left * sysctl_tcp_tso_win_divisor < tp->snd_cwnd &&
left * tp->mss_cache < sk->sk_gso_max_size )
return 1 ;
return left <= tcp_max_busrt( tp ) ;
}=============================================================================================================static void bictcp_cong_avoid ( struct sock *sk , u32 ack , u32 in_flight )
{
struct tcp_sock *tp = tcp_sk(sk) ;
struct bictcp *ca = inet_csk_ca(sk) ;
/* 如果发送拥塞窗口不被限制，不能再增加，则返回 */
if( !tcp_is_cwnd_limited(sk , in_flight))
return ;
if( tp->snd_cwnd < tp->snd_ssthresh )
tcp_slow_start( tp ) ;
else {
bictcp_update(ca , tp->snd_cwnd ) ;
tcp_cong_avoid_ai( tp , ca->cnt ) ;
}
}从以上函数可以看出，BIC的慢启动和reno相同。在拥塞避免阶段，当snd_cwnd <= low_window ，两者也采用相同方法。
只有当snd_cwnd > low时，BIC才开始显示出它的特性。
在include/ net / tcp.h中，/* TCP timestamps are only 32-bits */
#define tcp_time_stamps ((__u32)(jiffies))
/*
* Compute congestion window to use.
*/
static inline void bictcp_update( struct bictcp *ca , u32 cwnd )
{
if ( ca->last_cwnd == cwnd &&
(s32) ( tcp_time_stamp - ca->last_time) <= HZ / 32 )/* 31.25ms以内不更新ca！！！*/
return ;
ca->last_cwnd = cwnd ;
ca->last_time = tcp_time_stamp ;
if ( ca->epoch_start == 0 ) /* recording the beginning of an epoch */
ca->epoch_start = tcp_time_stamp ;
/* start off normal */
if( cwnd <= low_window ) {/*为了保持友好性*/
ca->cnt = cwnd ;/*这样16个以内的ack，可使snd_cwnd++ */
return ;
}
/* binary increase */
if ( cwnd < ca->last_max_cwnd ) {/*上次掉包前一个snd_cwnd */
__u32 dist = (ca->last_max_cwnd - cwnd) / BICTCP_B ; /* 四分之一 */
if ( dist > max_increment ) /* linear increase */
/*dist > 16，处于线性增长阶段，每收到16个ACK，会使snd_cwnd++ */
ca->cnt = cwnd / max_increment ;
else if ( dist <= 1U ) /* binary search increase */
/* dist <=1 ， ca->cnt=5*cwnd，会造成snd_cwnd增长极其缓慢，即处于稳定阶段 */
ca->cnt = (cwnd * smooth_part ) / BICTCP_B ;
else /* binary search increase */
/* 1 < dist <= 16 ，每收到dist个ACK，会使snd_cwnd++，故增长很快 */
ca->cnt = cwnd / dist ;
} else { /* 进入max_probing阶段 */
if ( cwnd < ca->last_max_cwnd + BICTCP_B ) /* cwnd < ca->last_max_cwnd + 4 */
ca->cnt = (cwnd * smooth_part ) / BICTCP_B ; /* ca->cnt = 5*cwnd ; slow start */
else if ( cwnd < ca->last_max_cwnd + max_increment * ( BICTCP_B - 1))
/* 增长率从5/(3*cwnd)~47/(3*cwnd)，snd_cwnd的增长加快*/
ca->cnt = (cwnd * (BICTCP_B - 1)) / (cwnd - ca->last_max_cwnd) ;
else
ca->cnt = cwnd / max_increment ; /* 增长率为16/cwnd ，更快 */
}
/* if in slow start or link utilization is very low */
if ( ca->loss_cwnd == 0 ) {/* 没有发生过丢包，所以snd_cwnd增长应该快点*/
if ( ca->cnt > 20 ) /* increase cwnd 5% per RTT */
ca->cnt = 20 ;
}
/* 相当于乘与delayed_ack的百分比，delayed得越严重，则snd_cwnd应该增加越快*/
/* 这样有无delayed对snd_cwnd的影响不大*/
ca->cnt = (ca->cnt << ACK_RATIO_SHIFT) / ca->delayed_ack ;
/* ca->cnt cannot be zero */
if ( ca->cnt == 0)
ca->cnt = 1 ;
}从以上函数可以看出，和reno相比，BIC在拥塞避免阶段snd_cwnd增长极快。
当ca->last_max_cwnd - snd_cwnd >= 4 时，snd_cwnd最慢的增长率为 1/16 。
而当ca->last_max_cwnd - snd_cwnd <4 时，增长率非常低，可以使当前的snd_cwnd维持很长一段时间，
即以最合适的snd_cwnd发送数据。
这两点使BIC在高带宽、长时延的环境下能达到较高的吞吐量。
1. 搜索阶段(1) cwnd < last_max_cwnd - 64， 则cnt = cwnd / 16
(2) last_max_cwnd - 64 <= cwnd < last_max_cwnd -4 ，则cnt = cwnd / dist
(3) last_max_cwnd - 4 <= cwnd < last_max_cwnd ，则cnt = 5*cwnd总体来说，snd_cwnd增长先快后慢，趋于稳定。
2. max probing阶段
(1) last_max_cwnd <= cwnd < last_max_cwnd + 4，则cnt = 5*cwnd
(2) last_max_cwnd + 4 <= cwnd < last_max_cwnd + 48 ，则cnt = 3*cwnd / (cwnd - last_max_cwnd)
(3) cwnd >= last_max_cwnd + 48 ，则cnt = cwnd / 16总体来说，snd_cwnd的增长先慢后快，越来越快。 很好阿希望于楼主多多交流哦
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