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yuanrongxi / razor

A google's congestion Control Algorithm
MIT License
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请教一个关于rtt计算的理论问题 #14

Closed shentu521 closed 5 years ago

shentu521 commented 5 years ago

最近空下来了，看了下袁总的文章， https://mp.weixin.qq.com/s/Ej63-FTe5-2pkxyXoXBUTw

写的相当棒。

晚上看好看到了计算rtt的部分.

每次发送端发送ack后，服务端会发送对应的SIM_SEG_ACK应答.

函数sim_sender_ack代码片段: ` /计算RTT/ now_ts = GET_SYS_MS();

key.u32 = ack->acked_packet_id;
iter = skiplist_search(sender->cache, key);
if (iter != NULL){
    seg = (sim_segment_t*)iter->val.ptr;
    //发送时间为: [ seg->timestamp(帧产生的时间) + seg->send_ts ](帧的发送时间) + sender->first_ts ，转为绝对时间
    //假设接收端是收到这个包了立刻就发送回来的:
    //那么RTT = now_ts - seg->timestamp - seg->send_ts -> sender->first_ts ;
    if (now_ts > seg->timestamp + seg->send_ts + sender->first_ts)//把相对时间转为绝对时间
        sim_session_calculate_rtt(s, (uint16_t)(now_ts - seg->timestamp - seg->send_ts - sender->first_ts));
    skiplist_remove(sender->cache, key);
}`

这里我们sim_session_calculate计算rtt,

`void sim_session_calculate_rtt(sim_session_t* s, uint32_t keep_rtt) { if (keep_rtt < 5) keep_rtt = 5;

s->rtt_var = (s->rtt_var * 3 + SU_ABS(s->rtt, keep_rtt)) / 4;
if (s->rtt_var < 10)
    s->rtt_var = 10;

s->rtt = (7 * s->rtt + keep_rtt) / 8;
if (s->rtt < 10)
    s->rtt = 10;

/*通知RTT更新*/
if (s->sender != NULL)
    sim_sender_update_rtt(s, s->sender);
if (s->receiver != NULL)
    sim_receiver_update_rtt(s, s->receiver);

}`

这里计算rtt的过程中，

s->rtt_var = (s->rtt_var * 3 + SU_ABS(s->rtt, keep_rtt)) / 4; if (s->rtt_var < 10) s->rtt_var = 10;

s->rtt = (7 * s->rtt + keep_rtt) / 8;
if (s->rtt < 10)
    s->rtt = 10;

这个计算公式，对应的理论支撑文档在哪里，袁总方便提示下么?

问的问题比较菜，多多体谅，苦笑脸.

yuanrongxi commented 5 years ago

去看看经典TCP对RTT的计算，RFC文档里面有描述