数据包有时会被串联 [英] Packets sometimes get concatenated

问题描述

我正在尝试使用Erlang创建一个简单的服务器/应用程序。

我的服务器使用 gen_tcp：listen（Port，[list，{active，false} ，{keepalive，true}，{nodelay，true}]），客户端使用 gen_tcp：connect（Server，Port，[list，{active，true}，{ keepalive，true}，{nodelay，true}]）。
从服务器收到的消息由诸如 {tcp，_，[115，58 |数据]} 。

问题是，有时数据包在发送或接收时会串联在一起，因此会导致意外行为，因为警卫将下一个数据包视为

有没有办法确保每个数据包都作为一条消息发送到接收进程？

解决方案

普通TCP是一种没有数据包边界概念的流协议（如Alnitak所说）。

通常，您可以使用成帧协议以UDP（每包大小有限并且可以无序接收）或TCP发送消息。

成帧意味着您为每条消息加上前缀

在erlang中，您可以将{packet，4}添加到套接字选项中以使其成帧TCP上的数据包行为。

假设双方（客户端/服务器）使用{packet，4}，那么您只会收到完整的消息。

<注意：您不会看到size标头，erlang会将其从您看到的消息中删除。因此，您在顶部的示例匹配项仍然可以正常工作

I'm trying to make a simple server/application in Erlang.
My server initialize a socket with gen_tcp:listen(Port, [list, {active, false}, {keepalive, true}, {nodelay, true}]) and the clients connect with gen_tcp:connect(Server, Port, [list, {active, true}, {keepalive, true}, {nodelay, true}]). Messages received from the server are tested by guards such as {tcp, _, [115, 58 | Data]}.

Problem is, packets sometimes get concatenated when sent or received and thus cause unexpected behaviors as the guards consider the next packet as part of the variable.

Is there a way to make sure every packet is sent as a single message to the receiving process?

解决方案

Plain TCP is a streaming protocol with no concept of packet boundaries (like Alnitak said).

Usually, you send messages in either UDP (which has limited per-packet size and can be received out of order) or TCP using a framed protocol.

Framed meaning you prefix each message with a size header (usualy 4 bytes) that indicates how long the message is.

In erlang, you can add {packet,4} to your socket options to get framed packet behavior on top of TCP.

assuming both sides (client/server) use {packet,4} then you will only get whole messages.

note: you won't see the size header, erlang will remove it from the message you see. So your example match at the top should still work just fine

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