How to set pmtu in golang

Go language is a popular programming language, especially in the field of network programming. In network programming, setting PMTU (Path Maximum Transmission Unit) is very important, and the Go language also provides some APIs to set PMTU. This article will introduce how to set PMTU in Go language.

What is PMTU?

PMTU (Path Maximum Transmission Unit) refers to the maximum load that a data packet can carry during network transmission. When transmitting data, the size of the packets cannot be too large, otherwise they may be corrupted or lost. Therefore, the size of the data packet needs to be limited before transmission, which is what PMTU does. PMTU can vary based on network topology and device configuration, so it needs to be adjusted dynamically when transmitting data.

PMTU settings in Go language

When using Go language for network programming, we need to understand how to set PMTU to ensure the transmission of data packets in the network. The Go language provides some APIs to set PMTU. Next, we will introduce two methods of setting PMTU: using the net package and using raw socket.

Use net package

Using net package can easily adjust PMTU. The following is a sample program:

package main

import (
    "net"
    "fmt"
)

func main() {
    conn, err := net.Dial("udp", "127.0.0.1:5555")
    if err != nil {
        fmt.Println(err)
        return
    }
    defer conn.Close()

    conn.SetWriteBuffer(65535)
    conn.SetReadBuffer(65535)

    err = conn.Write([]byte("Hello World!"))
    if err != nil {
        fmt.Println(err)
        return
    }

    buf := make([]byte, 4096)
    n, err := conn.Read(buf)
    if err != nil {
        fmt.Println(err)
        return
    }

    fmt.Println(string(buf[0:n]))
}

In the routine, the size of the write and read buffers is set to 65535 bytes to ensure that data can be transmitted. Of course, this value can be adjusted according to actual conditions.

Using raw socket

Using raw socket to set PMTU is relatively complicated. First, you need to create an original socket, and then set the size of its data packet. The following is a sample program:

package main

import (
    "fmt"
    "syscall"
    "unsafe"
)

const (
    IPPROTO_IP = 0
    IP_MTU_DISCOVER = 10
    IP_PMTUDISC_DO = 2
    IP_MTU = 14
)

func main() {
    fd, err := syscall.Socket(syscall.AF_INET, syscall.SOCK_RAW, syscall.IPPROTO_RAW)
    if err != nil {
        fmt.Printf("Error: %v\n", err)
        return
    }
    defer syscall.Close(fd)

    pmtu, err := syscall.GetsockoptInt(fd, IPPROTO_IP, IP_MTU)
    if err != nil {
        fmt.Printf("Error: %v\n", err)
        return
    }
    fmt.Printf("PMTU=%d\n", pmtu)

    flag := IP_PMTUDISC_DO
    _, _, e := syscall.Syscall6(syscall.SYS_SETSOCKOPT, uintptr(fd), uintptr(IPPROTO_IP), uintptr(IP_MTU_DISCOVER), uintptr(unsafe.Pointer(&flag)), unsafe.Sizeof(flag), 0)
    if e != 0 {
        fmt.Printf("Error: %v\n", e)
        return
    }

    pmtu, err = syscall.GetsockoptInt(fd, IPPROTO_IP, IP_MTU)
    if err != nil {
        fmt.Printf("Error: %v\n", err)
        return
    }
    fmt.Printf("PMTU=%d\n", pmtu)
}

In the routine, since you want to use the original socket, you first need to obtain privileges to obtain sufficient permissions. When setting PMTU, you need to call the systemcall.SetsockoptInt() API to change the socket settings. This process is relatively complicated.

Summary

Whether you use the net package or raw socket, setting PMTU in Go language is very simple. We need to choose the appropriate method to set up according to the actual situation to ensure that data can be transmitted stably in the network. At the same time, we need to pay attention to adjusting the size of the data packet and maintaining a reasonable PMTU value.

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