MPTCP 源碼分析(五) 接收端窗口值

簡述：

     在TCP協議中影響數據發送的三個因素分別爲：發送端窗口值、接收端窗口值和擁塞窗口值。

本文主要分析MPTCP中各個子路徑對接收端窗口值rcv_wnd的處理。

 

接收端窗口值的初始化

     根據《MPTCP 源碼分析(二) 創建子路徑》中描述服務端在發送完SYN/ACK並接收到ACK的時候創建新的sock。

在內核實現中，針對鏈接請求分爲兩個步驟處理：

1. SYN隊列處理：當服務端收到SYN的時候，此鏈接請求request_sock將被存放於listening socket的SYN隊列，服務端發送SYN/ACK並等待相應的ACK。
2. accept隊列處理：一旦等待的ACK收到，服務端將會建立新的socket，並將鏈接請求從listening socket的SYN隊列移到其accept隊列。

當服務端進入LINSTEN狀態後，收到第一個SYN包後的處理流程以下：

詳細的函數調用爲：

tcp_v4_rcv 

               =》 tcp_v4_do_rcv

                     =》 tcp_rcv_state_process 

                         =》mptcp_conn_request

                              =》tcp_v4_conn_request

                                   =》tcp_conn_request

                                        =》tcp_openreq_init

在函數tcp_conn_request中對鏈接請求request_sock進行了分配內存。

"net/ipv4/tcp_input.c" line 6097 of 6195
6097     req = inet_reqsk_alloc(rsk_ops);
6098     if (!req)
6099         goto drop;

在函數 tcp_openreq_init中對request_sock進行了初始化操做。

1226 static inline void tcp_openreq_init(struct request_sock *req,
1227                     struct tcp_options_received *rx_opt,
1228                     struct sk_buff *skb)
1229 {                   
1230     struct inet_request_sock *ireq = inet_rsk(req);
1231    
1232     req->rcv_wnd = 0;       /* So that tcp_send_synack() knows! */
1233     req->cookie_ts = 0;
1234     tcp_rsk(req)->rcv_isn = TCP_SKB_CB(skb)->seq;
1235     tcp_rsk(req)->rcv_nxt = TCP_SKB_CB(skb)->seq + 1;
1236     tcp_rsk(req)->snt_synack = 0;
1237     req->mss = rx_opt->mss_clamp;
1238     req->ts_recent = rx_opt->saw_tstamp ? rx_opt->rcv_tsval : 0;
1239     ireq->tstamp_ok = rx_opt->tstamp_ok;
1240     ireq->sack_ok = rx_opt->sack_ok;
1241     ireq->snd_wscale = rx_opt->snd_wscale;
1242     ireq->wscale_ok = rx_opt->wscale_ok;
1243     ireq->acked = 0;
1244     ireq->ecn_ok = 0;
1245     ireq->mptcp_rqsk = 0;
1246     ireq->saw_mpc = 0;
1247     ireq->ir_rmt_port = tcp_hdr(skb)->source;
1248     ireq->ir_num = ntohs(tcp_hdr(skb)->dest);
1249 }

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第1232行對request_sock的rcv_wnd進行了初始化爲0。

 

當服務端收到ACK的時候就會創建相應的socket。將會調用tcp_create_openreq_child函數實現，定義以下：

"include/net/tcp.h" line 578 of 1787
struct sock *tcp_create_openreq_child(struct sock *sk,
                      struct request_sock *req,
                      struct sk_buff *skb);

對於rcv_wnd的處理具體以下：

"net/ipv4/tcp_minisocks.c" line 441 of 872
512         newtp->window_clamp = req->window_clamp;
513         newtp->rcv_ssthresh = req->rcv_wnd;
514         newtp->rcv_wnd = req->rcv_wnd;
515         newtp->rx_opt.wscale_ok = ireq->wscale_ok; 

這個階段爲MPTCP的第一條子路徑創建狀況的三次握手，所以此時建立的socket的屬性爲master而非slave.

 

下面的情景爲建立一條子路徑的狀況，當服務端收到第一個SYN包的函數調用狀況以下：

函數mptcp_v4_join_request將會對鏈接請求request_sock進行內存分配並初始化。具體的調用以下：

mptcp_v4_join_request

                                   =》tcp_conn_request

                                        =》inet_reqsk_alloc

                                        =》tcp_openreq_init

當客戶端的ACK到達後，內核會將此鏈接請求request_sock的rcv_wnd賦值給slave subsocket.

 

  

master sock 和 slave sock之間接收端窗口值的關係

     TCP在發包的時候會告訴對方自身的接收端窗口值。MPTCP的實現以下：

"net/mptcp/mptcp_output.c" line 992 of 1667
 992 u16 mptcp_select_window(struct sock *sk)
993 {      
994     u16 new_win     = tcp_select_window(sk);
995     struct tcp_sock *tp = tcp_sk(sk);
996     struct tcp_sock *meta_tp = mptcp_meta_tp(tp);
997    
998     meta_tp->rcv_wnd    = tp->rcv_wnd;
999     meta_tp->rcv_wup    = meta_tp->rcv_nxt;
1000        
1001     return new_win;
1002 }  

第994得到最新的窗口值並返回。第998行將slave sock的rcv_wnd賦值給master sock。

 

第994行的函數tcp_select_window的實現以下：

"net/ipv4/tcp_output.c" line 275 of 3327
275 u16 tcp_select_window(struct sock *sk)
276 {
277     struct tcp_sock *tp = tcp_sk(sk);
278     /* The window must never shrink at the meta-level. At the subflow we
279      * have to allow this. Otherwise we may announce a window too large
280      * for the current meta-level sk_rcvbuf.
281      */
282     u32 cur_win = tcp_receive_window(mptcp(tp) ? tcp_sk(mptcp_meta_sk(sk)) : tp);
283     u32 new_win = tp->__select_window(sk);

對於第283行的__select_window()函數，MPTCP的內核實現以下：

 

"net/mptcp/mptcp_output.c" line 771 of 1667
771 u32 __mptcp_select_window(struct sock *sk)
772 {
773     struct inet_connection_sock *icsk = inet_csk(sk);
774     struct tcp_sock *tp = tcp_sk(sk), *meta_tp = mptcp_meta_tp(tp);
775     struct sock *meta_sk = mptcp_meta_sk(sk);
776     int mss, free_space, full_space, window;
777
778     /* MSS for the peer's data.  Previous versions used mss_clamp
779      * here.  I don't know if the value based on our guesses
780      * of peer's MSS is better for the performance.  It's more correct
781      * but may be worse for the performance because of rcv_mss
782      * fluctuations.  --SAW  1998/11/1
783      */
784     mss = icsk->icsk_ack.rcv_mss;
785     free_space = tcp_space(meta_sk);
786     full_space = min_t(int, meta_tp->window_clamp,
787             tcp_full_space(meta_sk));
788
789     if (mss > full_space)
790         mss = full_space;
791
792     if (free_space < (full_space >> 1)) {
793         icsk->icsk_ack.quick = 0;
794
795         if (tcp_memory_pressure)
796             /* TODO this has to be adapted when we support different
797              * MSS's among the subflows.
798              */
799             meta_tp->rcv_ssthresh = min(meta_tp->rcv_ssthresh,
800                             4U * meta_tp->advmss);
801
802         if (free_space < mss)
803             return 0;
804     }
805
806     if (free_space > meta_tp->rcv_ssthresh)
807         free_space = meta_tp->rcv_ssthresh;
808
809     /* Don't do rounding if we are using window scaling, since the
810      * scaled window will not line up with the MSS boundary anyway.
811      */
812     window = meta_tp->rcv_wnd;
813     if (tp->rx_opt.rcv_wscale) {
814         window = free_space;
815
816         /* Advertise enough space so that it won't get scaled away.
817          * Import case: prevent zero window announcement if
818          * 1<<rcv_wscale > mss.
819          */
820         if (((window >> tp->rx_opt.rcv_wscale) << tp->
821              rx_opt.rcv_wscale) != window)
822             window = (((window >> tp->rx_opt.rcv_wscale) + 1)
823                   << tp->rx_opt.rcv_wscale);
824     } else {
825         /* Get the largest window that is a nice multiple of mss.
826          * Window clamp already applied above.
827          * If our current window offering is within 1 mss of the
828          * free space we just keep it. This prevents the divide
829          * and multiply from happening most of the time.
830          * We also don't do any window rounding when the free space
831          * is too small.
832          */
833         if (window <= free_space - mss || window > free_space)
834             window = (free_space / mss) * mss;
835         else if (mss == full_space &&
836              free_space > window + (full_space >> 1))
837             window = free_space;
838     }
839
840     return window;
841 }

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影響window的計算的因素爲：

1. 收到的MSS( icsk->icsk_ack.rcv_mss)
2. 套接字緩衝區總的空間(tcp_full_space)
3. 套接字緩衝區的空閒空間(tcp_space)
4. meta_tp->rcv_ssthresh  /* Current window clamp */

觀察上面的代碼能夠知道MPTCP的實現和__tcp_select_window的區別是都是依據meta_tp，而非tp。這說明

master sock 和 其他slave sock使用相同的 rcv_wnd。

 

結論：

1.master sock 和 其他slave sock使用相同的接收緩衝區和 rcv_wnd。