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Hence, it is possible to use the same name for these two options that are often used together. There are two types of sockets, listening and connected sockets, and two corresponding sets of options. Thus, it is necessary to modify client behavior.įor Linux-based clients, we can use another option, which is also called TCP_DEFER_ACCEPT. To decide whether this ACK is necessary, a client should know the application protocol and its current state. In this case, delayed ACK will cause a delay in a client-server interaction. If FTP were used instead of HTTP, the client would not send any data until it received a packet with the FTP server prompt. Why would the client send this “useless” ACK packet anyway? A TCP stack has no way of knowing the status of an ACK packet. With HTTP client-server interaction, it may be necessary to change client behavior. More information about this option and its significance for a high-performance Web server is available in the Apache documentation.
![connected by tcp hack connected by tcp hack](https://cdn.arstechnica.net/wp-content/uploads/2021/06/alpaca.jpg)
This feature, called an “accept filter” in FreeBSD, is used in different ways, although in all cases, the effect is the same as TCP_DEFER_ACCEPT-the server will not wait for the final ACK packet, waiting only for a packet carrying a payload. Setsockopt(s, SOL_SOCKET, SO_ACCEPTFILTER, &af, sizeof(af)) For example, in FreeBSD, the same behavior is achieved with the following code: Now, only three packets will be sent over the network, and the connection establishment delay will be significantly reduced, which is typical for HTTP.Įquivalents of this option are available on other operation systems, as well. After sending the SYN/ACK, the server will then wait for a data packet from a client. Set on a server-side listening socket, it instructs the kernel not to wait for the final ACK packet and not to initiate the process until the first packet of real data has arrived. To alleviate these problems, Linux (along with some other OSs) includes a TCP_DEFER_ACCEPT option in its TCP implementation. Note also that the receiver has already been waiting for the information-since before the data was ever sent. But in this case, at least four packets will be sent in both directions, adding considerable delay times. Usually, an HTTP request is quite small and fits into a single packet.
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When the three-way handshake is completed, the client starts to send “useful” data to be transferred to the server. After receiving the SYN/ACK, the packet server wakes up a receiver process while waiting for data. The client then sends an ACK packet to acknowledge receipt of the second packet and to finalize the connection procedure. The server replies by sending a packet with SYN/ACK flags set (a SYN/ACK packet) to acknowledge receipt of the initial packet. The connection procedure in TCP/IP is a so-called “three-way handshake.” First, a client sends a TCP packet with a SYN flag set and no payload (a SYN packet). The server has now created a socket and is waiting for a connection. These packets often introduce unwanted delays and increased overhead and result in overall performance degradation. Often, it's possible to carry payload in the “marked” packet, but sometimes, internal logic forces a TCP/IP stack to send out packets with just a header. A typical example of service information is a set of so-called flags, which mark the packets as having special meaning to a TCP/IP stack, such as acknowledgement of successful packet receiving. A packet always has a header that carries service information, used for internal protocol handling, and it may also carry payload data. On a network, information travels in discrete units called IP packets (or IP datagrams). Consider how the TCP establishes a connection with the goal of transferring data. (This is what it's called in Linux other OSs offer the same option but use different names.) To understand the idea of the TCP_DEFER_ACCEPT option, it is necessary to picture a typical process of the HTTP client-server interaction. The first option we'll consider is TCP_DEFER_ACCEPT. Other TCP options can significantly improve performance and reduce server response latency under certain conditions. While critically important, reducing traffic is just one part of the puzzle of high-performance network data transmission. Last time, we explained how the TCP_CORK option can decrease the number of packets transferred over a network. By Alex Kuznetsov, Alex Plant, and Alexander Tormasov