MPTCP 源碼分析(三) 子路徑選擇

時間 2019-11-17

標籤 mptcp 源碼分析路徑選擇欄目 HTTP/TCP 简体版

原文原文鏈接

簡述：

支持MPTCP的鏈路中存在多條子路徑，所以在發送數據的時候須要選擇最優路徑來進行操做。

MPTCP利用內核通知鏈對MPTCP中各子路徑進行增長路徑、刪除路徑、修改路徑優先級的操做。MPTCP根據

相應的策略進行路徑選擇。

路徑選擇的代碼實現

路徑選擇的關鍵在於從多個子路徑中選擇其中一個進行數據的發送。此過程經過下面的函數實現：

"net/mptcp/mptcp_sched.c" line 114 of 496
114 static struct sock *get_available_subflow(struct sock *meta_sk,
115                       struct sk_buff *skb,
116                       bool zero_wnd_test)
117 {
118     struct mptcp_cb *mpcb = tcp_sk(meta_sk)->mpcb;   //MPTCP內核實現的三個組成部分之一：Multi-path control bock(mpcb) 
119     struct sock *sk, *bestsk = NULL, *lowpriosk = NULL, *backupsk = NULL;
120     u32 min_time_to_peer = 0xffffffff, lowprio_min_time_to_peer = 0xffffffff;
121     int cnt_backups = 0;
122
123     /* if there is only one subflow, bypass the scheduling function */    
124     if (mpcb->cnt_subflows == 1) {
125         bestsk = (struct sock *)mpcb->connection_list;
126         if (!mptcp_is_available(bestsk, skb, zero_wnd_test))
127             bestsk = NULL;
128         return bestsk;
129     }
130
131     /* Answer data_fin on same subflow!!! */
132     if (meta_sk->sk_shutdown & RCV_SHUTDOWN &&
133         skb && mptcp_is_data_fin(skb)) {
134         mptcp_for_each_sk(mpcb, sk) {
135             if (tcp_sk(sk)->mptcp->path_index == mpcb->dfin_path_index &&
136                 mptcp_is_available(sk, skb, zero_wnd_test))
137                 return sk;
138         }
139     }

View Code

第124行的代碼是處理只有一個子路徑的特殊狀況。第132行是對關閉操做進行了處理，在

收包過程當中對接收關閉包的子路徑進行記錄，而後回包的時候使用相同的子路徑。

"net/mptcp/mptcp_input.c" line 876 of 2293
875     /* Record it, because we want to send our data_fin on the same path */
876     if (tp->mptcp->map_data_fin) {
877         mpcb->dfin_path_index = tp->mptcp->path_index;
878         mpcb->dfin_combined = !!(sk->sk_shutdown & RCV_SHUTDOWN);
879     }

第876行是對子路徑關閉的處理，關於data_fin能夠參考https://tools.ietf.org/html/rfc6824#section-3.3.3。

下面的代碼遍歷mpcb下管理的sk，選擇最合適的sk。

"net/mptcp/mptcp_sched.c" line 142 of 496
141     /* First, find the best subflow */
142     mptcp_for_each_sk(mpcb, sk) {
143         struct tcp_sock *tp = tcp_sk(sk);
144
145         if (tp->mptcp->rcv_low_prio || tp->mptcp->low_prio)
146             cnt_backups++;
147
148         if ((tp->mptcp->rcv_low_prio || tp->mptcp->low_prio) &&
149             tp->srtt < lowprio_min_time_to_peer) {
150             if (!mptcp_is_available(sk, skb, zero_wnd_test))
151                 continue;
152
153             if (mptcp_dont_reinject_skb(tp, skb)) {
154                 backupsk = sk;
155                 continue;
156             }
157
158             lowprio_min_time_to_peer = tp->srtt;
159             lowpriosk = sk;
160         } else if (!(tp->mptcp->rcv_low_prio || tp->mptcp->low_prio) &&
161                tp->srtt < min_time_to_peer) {
162             if (!mptcp_is_available(sk, skb, zero_wnd_test))
163                 continue;
164
165             if (mptcp_dont_reinject_skb(tp, skb)) {
166                 backupsk = sk;
167                 continue;
168             }
169
170             min_time_to_peer = tp->srtt;
171             bestsk = sk;
172         }
173     }

View Code

從上面的代碼能夠看出，選擇sk的依據有5條：

1. 「tp->mptcp->rcv_low_prio 」 which stands for priority level in the remote machine.

2. "tp->mptcp->low_prio" which stands for priority level in the local machine.

3. whether the skb has already been enqueued into this subsocket

4. "tp->srtt" stands for smoothed round trip time.

5. mptcp_is_available() check the congestion of this subflow.

"net/mptcp/mptcp_sched.c" line 175 of 496
175     if (mpcb->cnt_established == cnt_backups && lowpriosk) {
176         sk = lowpriosk;  
177     } else if (bestsk) {
178         sk = bestsk;
179     } else if (backupsk) {
180         /* It has been sent on all subflows once - let's give it a
181          * chance again by restarting its pathmask.
182          */
183         if (skb)
184             TCP_SKB_CB(skb)->path_mask = 0;
185         sk = backupsk;
186     }
187
188     return sk;
189 }