/* * TCP Westwood+: end-to-end bandwidth estimation for TCP * * Angelo Dell'Aera: author of the first version of TCP Westwood+ in Linux 2.4 * * Support at http://c3lab.poliba.it/index.php/Westwood * Main references in literature: * * - Mascolo S, Casetti, M. Gerla et al. * "TCP Westwood: bandwidth estimation for TCP" Proc. ACM Mobicom 2001 * * - A. Grieco, s. Mascolo * "Performance evaluation of New Reno, Vegas, Westwood+ TCP" ACM Computer * Comm. Review, 2004 * * - A. Dell'Aera, L. Grieco, S. Mascolo. * "Linux 2.4 Implementation of Westwood+ TCP with Rate-Halving : * A Performance Evaluation Over the Internet" (ICC 2004), Paris, June 2004 * * Westwood+ employs end-to-end bandwidth measurement to set cwnd and * ssthresh after packet loss. The probing phase is as the original Reno. */ #include <linux/mm.h> #include <linux/module.h> #include <linux/skbuff.h> #include <linux/inet_diag.h> #include <net/tcp.h> /* TCP Westwood structure */ struct westwood { u32 bw_ns_est; /*中间变量，经过一次平滑后的带宽值 first bandwidth estimation..not too smoothed 8) */ u32 bw_est; /*最终估计的带宽值 bandwidth estimate */ u32 rtt_win_sx; /*采样周期的起始点，一个RTT后结束 here starts a new evaluation... */ u32 bk; //在某个时间段delta内确认的字节数 u32 snd_una; /*snd_una历史记录，用于计算bk used for evaluating the number of acked bytes */ u32 cumul_ack; //在慢速路径下，一个ACK确认的数据量 cumulative ack u32 accounted; //在慢速路径下，收到的重复数据包个数 u32 rtt; //当前RTT值，以毫秒为单位，每收到一个ACK都更新 u32 rtt_min; /*最小RTT值，以毫秒为单位，inimum observed RTT */ u8 first_ack; /*是否第一个ack包 flag which infers that this is the first ack */ u8 reset_rtt_min; /*是否重置采样周期 Reset RTT min to next RTT sample*/ }; /* TCP Westwood functions and constants */ #define TCP_WESTWOOD_RTT_MIN (HZ/20) /* 50ms */ #define TCP_WESTWOOD_INIT_RTT (20*HZ) /* 20000ms 太保守??? maybe too conservative?! */ /* westwood在丢包率较高的无线网络中表现较好。 Reno对随机丢包和拥塞丢包都较为敏感，随机丢包会导致Reno不必要的降低拥塞窗口和慢启动阈值。 在westwood算法中，需要强调的一点是：丢包对带宽的影响不大。 每个RTT采样一次带宽值，而这次样本只占bw_est的1/64。丢包后进入快速恢复阶段，尽管在快速恢复阶段 中得到的几个采样值较小，但是整体的bw_est却没有太大的减小。 来看一下为什么westwood对随机丢包不敏感。 （1）随机丢包 丢包前：cwnd = bw_est * RTT 丢包后：cwnd = bw_est * RTTmin 因为是随机丢包，所以丢包前的RTT只是比RTTmin略大。丢包后的bw_est也只是微略减小。 所以丢包后的cwnd只是微略的减小。 （2）拥塞丢包 丢包前：cwnd = bw_est * RTTmax => BDP + Queue 丢包后：cwnd = bw_est * RTTmin => BDP 因为是拥塞丢包，所以丢包前的bw_est已经是连接的最大带宽，并且时延也达到了最大值。 这是丢包后就达到了完全利用BDP，同时使Queue为空的效果。 可以看到，westwood对随机丢包和拥塞丢包采取同样的算法来处理，却能达到不同的效果。 但是，westwood不能主动的区分随机丢包和拥塞丢包。 */ /* * @tcp_westwood_create * This function initializes fields used in TCP Westwood+, * it is called after the initial SYN, so the sequence numbers * are correct but new passive connections we have no * information about RTTmin at this time so we simply set it to * TCP_WESTWOOD_INIT_RTT. This value was chosen to be too conservative * since in this way we're sure it will be updated in a consistent * way as soon as possible. It will reasonably happen within the first * RTT period of the connection lifetime. */ static void tcp_westwood_init(struct sock *sk) { //初始化westwood拥塞算法的参数 struct westwood *w = inet_csk_ca(sk); w->bk = 0; w->bw_ns_est = 0; w->bw_est = 0; w->accounted = 0; w->cumul_ack = 0; w->reset_rtt_min = 1; w->rtt_min = w->rtt = TCP_WESTWOOD_INIT_RTT; w->rtt_win_sx = tcp_time_stamp;//jiffies毫秒 w->snd_una = tcp_sk(sk)->snd_una; w->first_ack = 1; } /* * @westwood_do_filter * Low-pass filter. Implemented using constant coefficients. */ static inline u32 westwood_do_filter(u32 a, u32 b) { return (((7 * a) + b) >> 3); //返回7a/8与1b/8之和 } static void westwood_filter(struct westwood *w, u32 delta) { //带宽过滤器 /* If the filter is empty fill it with the first sample of bandwidth */ if (w->bw_ns_est == 0 && w->bw_est == 0) { //如果是第一次得到带宽测量样本 w->bw_ns_est = w->bk / delta; w->bw_est = w->bw_ns_est; } else {//如果已经有收到过测量样本了 w->bw_ns_est = westwood_do_filter(w->bw_ns_est, w->bk / delta); w->bw_est = westwood_do_filter(w->bw_est, w->bw_ns_est); } } /* * @westwood_pkts_acked * Called after processing group of packets. * but all westwood needs is the last sample of srtt. */ static void tcp_westwood_pkts_acked(struct sock *sk, u32 cnt, s32 rtt) { //每收到一个ACK时，会更新当前的RTT（w->rtt） struct westwood *w = inet_csk_ca(sk); if (rtt > 0) //这个参数rtt是以微秒为单位的，rtt为-1则不会执行转换 w->rtt = usecs_to_jiffies(rtt); //w->rtt是以毫秒为单位的 } /* * @westwood_update_window * It updates RTT evaluation window if it is the right moment to do * it. If so it calls filter for evaluating bandwidth. */ static void westwood_update_window(struct sock *sk) { //每经过一个RTT后，采集一个新的测量样本，更新带宽估计值。 struct westwood *w = inet_csk_ca(sk); s32 delta = tcp_time_stamp - w->rtt_win_sx; //计算时间差 /* Initialize w->snd_una with the first acked sequence number in order * to fix mismatch between tp->snd_una and w->snd_una for the first * bandwidth sample */ if (w->first_ack) {//如果是第一个ack包 w->snd_una = tcp_sk(sk)->snd_una; w->first_ack = 0; } /* * See if a RTT-window has passed. * Be careful since if RTT is less than * 50ms we don't filter but we continue 'building the sample'. * This minimum limit was chosen since an estimation on small * time intervals is better to avoid... * Obviously on a LAN we reasonably will always have * right_bound = left_bound + WESTWOOD_RTT_MIN */ //如果当前的rtt大于TCP_WESTWOOD_RTT_MIN，也就是超时了 if (w->rtt && delta > max_t(u32, w->rtt, TCP_WESTWOOD_RTT_MIN)) { westwood_filter(w, delta);// 更新带宽估计值 w->bk = 0;//清零确认字节数 w->rtt_win_sx = tcp_time_stamp;//重设取样周期开始时间 } } static inline void update_rtt_min(struct westwood *w) {//更新最小RTT if (w->reset_rtt_min) { //如果非0 //当发生超时后，最小RTT可能不再准确，需要更新 w->rtt_min = w->rtt; w->reset_rtt_min = 0; } else //如果为0 w->rtt_min = min(w->rtt, w->rtt_min);//更新最小RTT } /* * @westwood_fast_bw * It is called when we are in fast path. In particular it is called when * header prediction is successful. In such case in fact update is * straight forward and doesn't need any particular care. */ //快速路径时的带宽估计值更新 //处于快速路径时调用，说明此时收到的数据包是顺序的，此时应该处于Open状态。 //这种状态下，收到新的ACK会使tp->snd_una前进。 //所以，tp->snd_una - w->snd_una能代表此ACK确认的数据量。 static inline void westwood_fast_bw(struct sock *sk) { const struct tcp_sock *tp = tcp_sk(sk); struct westwood *w = inet_csk_ca(sk); westwood_update_window(sk); //更新带宽估计值 w->bk += tp->snd_una - w->snd_una; //累计确认的字节数 w->snd_una = tp->snd_una;//记录当前snd_una update_rtt_min(w); //更新最小RTT } /* * @westwood_acked_count * This function evaluates cumul_ack for evaluating bk in case of * delayed or partial acks. */ //慢速路径时，计算所收到ACK确认的数据量。 //这时候的ACK可能是delayed ACK、partial ACK、duplicate ACK、 //cumulative ACK following a retransmission event. static inline u32 westwood_acked_count(struct sock *sk) { const struct tcp_sock *tp = tcp_sk(sk); struct westwood *w = inet_csk_ca(sk); //计算此ACK确认的字节数 w->cumul_ack = tp->snd_una - w->snd_una; /* If cumul_ack is 0 this is a dupack since it's not moving * tp->snd_una. */ //如果cumul_ack=0，那么此ACK是dupack， //代表接收端收到一个数据包。 if (!w->cumul_ack) { w->accounted += tp->mss_cache;//接收端保存的乱序数据包加一 w->cumul_ack = tp->mss_cache;//代表传输了一个数据包 } //如果cumul_ack > 1，则有可能是多种情况。 if (w->cumul_ack > tp->mss_cache) { /* Partial or delayed ack 表示此ACK为partial ACK */ if (w->accounted >= w->cumul_ack) { w->accounted -= w->cumul_ack; //表示只确认了一个包，其它包已经被dupack确认过了 w->cumul_ack = tp->mss_cache; } else { /* delayed ack or cumulative ack， * 表示被延迟的确认，或者结束Recovery的累积确认 */ w->cumul_ack -= w->accounted; w->accounted = 0; } } w->snd_una = tp->snd_una; //记录当前的snd_una return w->cumul_ack;//返回此ACK确认的字节数 } /* * TCP Westwood * Here limit is evaluated as Bw estimation*RTTmin (for obtaining it * in packets we use mss_cache). Rttmin is guaranteed to be >= 2 * so avoids ever returning 0. */ static u32 tcp_westwood_bw_rttmin(const struct sock *sk) {//此函数在丢包后调用，根据带宽来设置拥塞窗口和慢启动阈值。 const struct tcp_sock *tp = tcp_sk(sk); const struct westwood *w = inet_csk_ca(sk); return max_t(u32, (w->bw_est * w->rtt_min) / tp->mss_cache, 2); } static void tcp_westwood_event(struct sock *sk, enum tcp_ca_event event) {//westwood的"入口函数"，其他函数都是通过这个函数调用的， //不同与其他的TCP拥塞控制算法。 struct tcp_sock *tp = tcp_sk(sk); struct westwood *w = inet_csk_ca(sk); switch (event) { /* 处于快速路径时，用此函数更新w->bw_est和w->rtt_min */ case CA_EVENT_FAST_ACK: westwood_fast_bw(sk); break; /* 退出Recovery或CWR状态时，进行拥塞窗口和慢启动阈值设置*/ case CA_EVENT_COMPLETE_CWR: tp->snd_cwnd = tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk); break; //当RTO超时时，会先进行FRTO检测，这时可设置慢启动阈值， //而拥塞窗口则设置为1. case CA_EVENT_FRTO: tp->snd_ssthresh = tcp_westwood_bw_rttmin(sk); /* Update RTT_min when next ack arrives */ //如果超时了，那么最小RTT可能不准确，需要重新设置 w->reset_rtt_min = 1; break; //处于慢速路径时，这时候的拥塞状态可能是CWR、Recovery、Loss等。 //必须采用westwood_acked_count()来统计此ACK确认的数据量。 //同时也进行w->bw_est和w->rtt_min的更新。 case CA_EVENT_SLOW_ACK: westwood_update_window(sk);//更新带宽估计值 w->bk += westwood_acked_count(sk);//计算所收到ACK确认的数据量 update_rtt_min(w);//更新最小rtt break; default: /* don't care 对其它的事件则不做响应 */ break; } } /* Extract info for Tcp socket info provided via netlink. */ //通过netlink提供信息给tcp_socket static void tcp_westwood_info(struct sock *sk, u32 ext, struct sk_buff *skb) { const struct westwood *ca = inet_csk_ca(sk); if (ext & (1 << (INET_DIAG_VEGASINFO - 1))) { struct tcpvegas_info info = { .tcpv_enabled = 1, .tcpv_rtt = jiffies_to_usecs(ca->rtt), .tcpv_minrtt = jiffies_to_usecs(ca->rtt_min), }; nla_put(skb, INET_DIAG_VEGASINFO, sizeof(info), &info); } } static struct tcp_congestion_ops tcp_westwood = { .init = tcp_westwood_init, .ssthresh = tcp_reno_ssthresh, .cong_avoid = tcp_reno_cong_avoid, .min_cwnd = tcp_westwood_bw_rttmin, .cwnd_event = tcp_westwood_event, .get_info = tcp_westwood_info, .pkts_acked = tcp_westwood_pkts_acked, .owner = THIS_MODULE, .name = "westwood" }; static int __init tcp_westwood_register(void) { //注册westwood算法 BUILD_BUG_ON(sizeof(struct westwood) > ICSK_CA_PRIV_SIZE); return tcp_register_congestion_control(&tcp_westwood); } static void __exit tcp_westwood_unregister(void) { //注销westwood算法 tcp_unregister_congestion_control(&tcp_westwood); } module_init(tcp_westwood_register);//westwood模块的入口函数 module_exit(tcp_westwood_unregister);//westwood模块的出口函数 MODULE_AUTHOR("Stephen Hemminger, Angelo Dell'Aera"); MODULE_LICENSE("GPL"); MODULE_DESCRIPTION("TCP Westwood+");
