Linux 上的 UDP 或 TCP 缓冲区大小应该有多大？

Question

I need to write a couple of C++ applications on Linux, one to receive data via UDP and the second TCP.

The only thing I'm unsure about is regarding the buffer.

How do I choose what size buffer?

If I make the buffer large enough, am I guaranteed to avoid scenarios where half of a packet is at the end of my buffer and I need to copy the bytes to the beginning and then receive the remaining half of the packet?

I am going to use the Linux socket API functions if it matters.

Answer 1

If I make the buffer large enough, am I guaranteed to avoid scenarios where half of a packet is at the end of my buffer and I need to copy the bytes to the beginning and then receive the remaining half of the packet?

Based on the above paragraph, I'm going to surmise that the buffer you are referring to is the application-space buffer that you pass into your recv() calls, and not the in-kernel buffer that the networking stack maintains on your application's behalf.

For UDP, the answer is simple: Your buffer needs to be large enough to hold the largest possible datagram you expect to receive. Since UDP datagrams are typically less than 1500 bytes (to avoid fragmentation) and in all cases are <= 65507 bytes (since that is the maximum datagram size the UDP protocol supports), you can always make your receive buffer 65507 bytes long, or smaller if you want to save a bit on RAM usage.

For TCP, the protocol is stream-based, so the amount of data written in to your recv-buffer by a given recv() call is unrelated to packet sizes. Another consequence of TCP being stream-based is that it doesn't do any message-framing -- that means you will have to handle partial messages regardless of how big or small you make your buffer. The only advantage of a larger TCP buffer is that it's a bit more efficient to handle more bytes at a time instead of fewer, again at the cost of using a little more RAM.

Answer 2

If I make the buffer large enough, am I guaranteed to avoid scenarios where half of a packet is at the end of my buffer and I need to copy the bytes to the beginning and then receive the remaining half of the packet?

For TCP: It doesn't matter. Packets are an implementation detail. The application doesn't even have to think about them. TCP is a byte-stream protocol and all you ever get from the API is a stream of bytes. Message boundaries are never preserved.

For UDP: Packets are still an implementation detail. You send and receive datagrams. Your read function always gets an entire datagram so long as your buffer is as large as the largest datagram your application protocol supports.