To understand the backlog argument, we must realize that for a given listening socket, the kernel maintains two queues :
要明白backlog參數(shù)的含義，我們必須明白對(duì)于一個(gè)listening socket，kernel維護(hù)者兩個(gè)隊(duì)列：

1.An incomplete connection queue, which contains an entry for each SYN that has arrived from a client for which the server is awaiting completion of the TCP three-way handshake. These sockets are in the SYN_RCVD state .
1.一個(gè)未完成連接的隊(duì)列，此隊(duì)列維護(hù)著那些已收到了客戶端SYN分節(jié)信息，等待完成三路握手的連接，socket的狀態(tài)是SYN_RCVD

2.A completed connection queue, which contains an entry for each client with whom the TCP three-way handshake has completed. These sockets are in the ESTABLISHED state
2.一個(gè)已完成的連接的隊(duì)列，此隊(duì)列包含了那些已經(jīng)完成三路握手的連接，socket的狀態(tài)是ESTABLISHED

The backlog argument to the listen function has historically specified the maximum value for the sum of both queues.
backlog參數(shù)歷史上被定義為上面兩個(gè)隊(duì)列的大小之和

Berkeley-derived implementations add a fudge factor to the backlog: It is multiplied by 1.5
Berkely實(shí)現(xiàn)中的backlog值為上面兩隊(duì)列之和再乘以1.5

When a SYN arrives from a client, TCP creates a new entry on the incomplete queue and then responds with the second segment of the three-way handshake: the server's SYN with an ACK of the client's SYN (Section 2.6). This entry will remain on the incomplete queue until the third segment of the three-way handshake arrives (the client's ACK of the server's SYN), or until the entry times out. (Berkeley-derived implementations have a timeout of 75 seconds for these incomplete entries.)
當(dāng)客戶端的第一個(gè)SYN到達(dá)的時(shí)候，TCP會(huì)在未完成隊(duì)列中增加一個(gè)新的記錄然后回復(fù)給客戶端三路握手中的第二個(gè)分節(jié)(服務(wù)端的SYN和針對(duì)客戶端的ACK)，這條記錄會(huì)在未完成隊(duì)列中一直存在，直到三路握手中的最后一個(gè)分節(jié)到達(dá)，或者直到超時(shí)(Berkeley時(shí)間將這個(gè)超時(shí)定義為75秒)

If the queues are full when a client SYN arrives, TCP ignores the arriving SYN (pp. 930–931 of TCPv2); it does not send an RST. This is because the condition is considered temporary, and the client TCP will retransmit its SYN, hopefully finding room on the queue in the near future. If the server TCP immediately responded with an RST, the client's connect would return an error, forcing the application to handle this condition instead of letting TCP's normal retransmission take over. Also, the client could not differentiate between an RST in response to a SYN meaning "there is no server at this port" versus "there is a server at this port but its queues are full."
如果當(dāng)客戶端SYN到達(dá)的時(shí)候隊(duì)列已滿，TCP將會(huì)忽略后續(xù)到達(dá)的SYN，但是不會(huì)給客戶端發(fā)送RST信息，因?yàn)榇藭r(shí)允許客戶端重傳SYN分節(jié)，如果返回錯(cuò)誤信息，那么客戶端將無法分清到底是服務(wù)端對(duì)應(yīng)端口上沒有相應(yīng)應(yīng)用程序還是服務(wù)端對(duì)應(yīng)端口上隊(duì)列已滿這兩種情況