How to find the largest UDP packet I can send without fragmenting?

I need to know what the largest UDP packet I can send to another computer is without fragmentation.

This size is commonly known as the MTU (Maximum Transmission Unit). Supposedly, between 2 computers, will be many routers and modems that may have different MTUs.

I read that the TCP implementation in windows automatically finds the maximum MTU in a path.

I was also experimenting, and I found out that the maximum MTU from my computer to a server was 57712 bytes+header. Anything above that was discarded. My computer is on a LAN, isn't the MTU supposed to be around 1500 bytes?

The following doesn't answer your question directly but you might find it interesting; it says that IP packets can be disassembled/reassembled, and therefore bigger than limit on the underling media (e.g. 1500-byte Ethernet): Resolve IP Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPSEC

More on this topic:

• Re: UDP fragmentation says you should use ICMP instead of UDP to discover MTU
• Path MTU Discovery says that a TCP connection might include implicit MTU negotiation via ICMP

I don't know about generating ICMP via an API on Windows: at one time such an API was proposed, and was controversial because people argued that would make it easy to write software that implements denial-of-service functionality by generating a flood of ICMP messages.

No, it looks like it is implemented: see for example Winsock Programmer's FAQ Examples: Ping: Raw Sockets Method.

So, to discover MTU, generate ping packets with the 'do not fragment' flag.

Maybe there's an easier API than this, I don't know; but I hope I've given you to understand the underlying protocol[s].

In addition to all the previous answers, quoting the http://www.kohala.com/start/unpv22e/unpv22e.html">classic</a>;:

This pretty much means that you want to limit your datagram size to under 576 if you work over public internet and you control only one side of the exchange - that's what most of the standard UDP-based protocols do.

Also note that PMTU is a dynamic property of the path. This is one of the things TCP deals with for you. Unless you are ready to re-implement lots of sequencing, timing, and retransmission logic, use TCP for any critical networking. Benchmark, test, profile, i.e. prove that TCP is your bottleneck, only then consider UDP.

This is an interesting topic for me. Perhaps some practical results might be of interest when delivering chunky UDP data around the real world internet via UDP, and with a transmission rate of 1 packet a second, data continues to turn up with minimal packet loss up to about 2K. Over this and you start running into issues, but regularly we delivered 1600+ bytes packets without distress - this is over GPRS mobile networks as well as WAN world wide. At ~1K assuming the signal is stable (its not!) you get low packet loss.

Interestingly its not the odd packet, but often a squall of packets for a few seconds - which presumably is why VoIP calls just collapse occasionally.

Your own MTU is available in the registry, but the MTU in practice is going to the smallest MTU in the path between your machine and the destination. Its both variable and can only be determined empirically. There are a number of RFCs showing how to determine it.

LAN's can internally have very large MTU values, since the network hardware is typically homogeneous or at least centrally administrated.

For UDP applications you must handle end-to-end MTU yourself if you want to avoid IP fragmentation or dropped packets. The recommended approach for any application is to do your best to use PMTU to pick your maximum datagram, or send datagrams < minimum PMTU

https://www.rfc-editor.org/rfc/rfc5405#section-3.2

> Unicast UDP Usage Guidelines for Application Designers "SHOULD NOT send datagrams that exceed the PMTU, SHOULD discover PMTU or send datagrams < minimum PMTU

Windows appears to settings and access to PMTU information via it's basic socket options interface:

You can make sure PMTU discover is on via IP_MTU_DISCOVER, and you can read the MTU via IP_MTU.

https://docs.microsoft.com/en-us/windows/desktop/winsock/ipproto-ip-socket-options

Here's a bit of Windows PowerShell that I wrote to check for Path MTU issues. (The general technique is not too hard to implement in other programming languages.) A lot of firewalls and routers are configured to drop all ICMP by people who don't know any better. Path MTU Discovery depends on being able to receive an ICMP Destination Unreachable message with Fragementation Needed set in response to sending a packet with Don't Fragment set. The Resolve IPv4 Fragmentation, MTU, MSS, and PMTUD Issues with GRE and IPsec actually does a really good job of explaining how discovery works.

function Test-IPAddressOrName($ipAddressOrName)
{
    $ipaddress = $null
    $isValidIPAddressOrName = [ipaddress]::TryParse($ipAddressOrName, [ref] $ipaddress)

    if ($isValidIPAddressOrName -eq $false)
    {
        $hasResolveDnsCommand = $null -ne (Get-Command Resolve-DnsName -ErrorAction SilentlyContinue)
        if ($hasResolveDnsCommand -eq $true)
        {
            $dnsResult = Resolve-DnsName -DnsOnly -Name $ipAddressOrName -ErrorAction SilentlyContinue
            $isValidIPAddressOrName = $null -ne $dnsResult
        }
    }

    return $isValidIPAddressOrName
}

function Get-NameAndIPAddress($ipAddressOrName)
{
    $hasResolveDnsCommand = $null -ne (Get-Command Resolve-DnsName -ErrorAction SilentlyContinue)

    $ipAddress = $null
    $validIPAddress = [ipaddress]::TryParse($ipAddressOrName, [ref] $ipAddress)
    $nameAndIp = [PSCustomObject] @{ 'Name' = $null; 'IPAddress' = $null }

    if ($validIPAddress -eq $false)
    {
        if ($hasResolveDnsCommand -eq $true)
        {
            $dnsResult = Resolve-DnsName -DnsOnly $ipAddressOrName -Type A -ErrorAction SilentlyContinue

            if ($null -ne $dnsResult -and $dnsResult.QueryType -eq 'A')
            {
                $nameAndIp.Name = $dnsResult.Name
                $nameAndIp.IPAddress = $dnsResult.IPAddress
            }
            else
            {
                Write-Error "The name $($ipAddressOrName) could not be resolved."
                $nameAndIp = $null
            }
        }
        else
        {
            Write-Warning "Resolve-DnsName not present. DNS resolution check skipped."
        }
    }
    else
    {
        $nameAndIp.IPAddress = $ipAddress

        if ($hasResolveDnsCommand -eq $true)
        {
            $dnsResult = Resolve-DnsName -DnsOnly $ipAddress -Type PTR -ErrorAction SilentlyContinue

            if ($null -ne $dnsResult -and $dnsResult.QueryType -eq 'PTR')
            {
                $nameAndIp.Name = $dnsResult.NameHost
            }
        }
    }

    return $nameAndIp
}

<#
    .Synopsis
    Performs a series of pings (ICMP echo requests) with Don't Fragment specified to discover the path MTU (Maximum Transmission Unit).

    .Description
    Performs a series of pings with Don't Fragment specified to discover the path MTU (Maximum Transmission Unit). An ICMP echo request 
    is sent with a random payload with a payload length specified by the PayloadBytesMinimun. ICMP echo requests of increasing size are 
    sent until a ping response status other than Success is received. If the response status is PackeTooBig, the last successful packet 
    length is returned as a reliable MTU; otherwise, if the respone status is TimedOut, the same size packet is retried up to the number 
    of retries specified. If all of the retries have been exhausted with a response status of TimedOut, the last successful packet 
    length is returned as the assumed MTU.

    .Parameter UseDefaultGateway
    If UseDefaultGateway is specified the default gateway reported by the network interface is used as the destination host.

    .Parameter DestinationHost
    The IP Address or valid fully qualified DNS name of the destination host.

    .Parameter InitialTimeout
    The number of milliseconds to wait for an ICMP echo reply. Internally, this is doubled each time a retry occurs.

    .Parameter Retries
    The number of times to try the ping in the event that no reply is recieved before the timeout.

    .Parameter PayloadBytesMinimum
    The minimum number of bytes in the payload to use. The minimum MTU for IPv4 is 68 bytes; however, in practice, it's extremely rare 
    to see an MTU size less than 576 bytes so the default value is 548 bytes (576 bytes total packet size minus an ICMP header of 28 
    bytes).

    .Parameter PayloadBytesMaximum
    The maximum number of bytes in the payload to use. An IPv4 MTU for jumbo frames is 9000 bytes. The default value is 8973 bytes (9001 
    bytes total packet size, which is 1 byte larger than the maximum IPv4 MTU for a jumbo frame, minus an ICMP header of 28 bytes).

    .Example
    Discover-PathMTU -UseDefaultGateway

    .Example
    Discover-PathMTU -DestinationHost '192.168.1.1'

    .Example
    Discover-PathMTU -DestinationHost 'www.google.com'
#>
function Discover-PathMtu
{
    [CmdletBinding(SupportsShouldProcess = $false)]
    param
    (
        [Parameter(Mandatory = $true, ParameterSetName = 'DefaultGateway')]
        [switch] $UseDefaultGateway,

        [Parameter(Mandatory = $true, Position = 0, ValueFromPipeline = $true, ParameterSetName = 'IPAddressOrName')]
        [ValidateScript({ Test-IPAddressOrName $_ })]
        [string] $DestinationHost,

        [Parameter(ParameterSetName = 'IPAddressOrName')]
        [Parameter(ParameterSetName = 'DefaultGateway')]
        [int] $InitialTimeout = 3000,

        [Parameter(ParameterSetName = 'IPAddressOrName')]
        [Parameter(ParameterSetName = 'DefaultGateway')]
        [int] $Retries = 3,

        [Parameter(ParameterSetName = 'IPAddressOrName')]
        [Parameter(ParameterSetName = 'DefaultGateway')]
        $PayloadBytesMinimum = 548,

        [Parameter(ParameterSetName = 'IPAddressOrName')]
        [Parameter(ParameterSetName = 'DefaultGateway')]
        $PayloadBytesMaximum = 8973
    )

    begin
    {
        $ipConfiguration = Get-NetIPConfiguration -Detailed | ?{ $_.NetProfile.Ipv4Connectivity -eq 'Internet' -and $_.NetAdapter.Status -eq 'Up' } | Sort { $_.IPv4DefaultGateway.InterfaceMetric } | Select -First 1
        $gatewayIPAddress = $ipConfiguration.IPv4DefaultGateway.NextHop

        $pingOptions = New-Object System.Net.NetworkInformation.PingOptions
        $pingOptions.DontFragment = $true
        $pinger = New-Object System.Net.NetworkInformation.Ping

        $rng = New-Object System.Security.Cryptography.RNGCryptoServiceProvider
    }

    process
    {
        $pingIpAddress = $null

        if ($UseDefaultGateway -eq $true)
        {
            $DestinationHost = $gatewayIPAddress
        }

        $nameAndIP = Get-NameAndIPAddress $DestinationHost

        if ($null -ne $nameAndIP)
        {
            Write-Host "Performing Path MTU discovery for $($nameAndIP.Name) $($nameAndIP.IPAddress)..."

            $pingReply = $null
            $payloadLength = $PayloadBytesMinimum
            $workingPingTimeout = $InitialTimeout

            do
            {
                $payloadLength++

                # Use a random payload to prevent compression in the path from potentially causing a false MTU report.
                [byte[]] $payloadBuffer = (,0x00 * $payloadLength)
                $rng.GetBytes($payloadBuffer)

                $pingCount = 1

                do
                {
                    $pingReply = $pinger.Send($nameAndIP.IPAddress, $workingPingTimeout, $payloadBuffer, $pingOptions)

                    if ($pingReply.Status -notin 'Success', 'PacketTooBig', 'TimedOut')
                    {
                        Write-Warning "An unexpected ping reply status, $($pingReply.Status), was received in $($pingReply.RoundtripTime) milliseconds on attempt $($pingCount)."
                    }
                    elseif ($pingReply.Status -eq 'TimedOut')
                    {
                        Write-Warning "The ping request timed out while testing a packet of size $($payloadLength + 28) using a timeout value of $($workingPingTimeout) milliseconds on attempt $($pingCount)."
                        $workingPingTimeout = $workingPingTimeout * 2
                    }
                    else
                    {
                        Write-Verbose "Testing packet of size $($payloadLength + 28). The reply was $($pingReply.Status) and was received in $($pingReply.RoundtripTime) milliseconds on attempt $($pingCount)."
                        $workingPingTimeout = $InitialTimeout
                    }

                    Sleep -Milliseconds 10

                    $pingCount++
                } while ($pingReply.Status -eq 'TimedOut' -and $pingCount -le $Retries)
            } while ($payloadLength -lt $PayloadBytesMaximum -and $pingReply -ne $null -and $pingReply.Status -eq 'Success')

            if ($pingReply.Status -eq 'PacketTooBig')
            {
                Write-Host "Reported IPv4 MTU is $($ipConfiguration.NetIPv4Interface.NlMtu). The discovered IPv4 MTU is $($payloadLength + 27)."
            }
            elseif ($pingReply.Status -eq 'TimedOut')
            {
                Write-Host "Reported IPv4 MTU is $($ipConfiguration.NetIPv4Interface.NlMtu). The discovered IPv4 MTU is $($payloadLength + 27), but may not be reliable because the packet appears to have been discarded."    
            }
            else
            {
                Write-Host "Reported IPv4 MTU is $($ipConfiguration.NetIPv4Interface.NlMtu). The discovered IPv4 MTU is $($payloadLength + 27), but may not be reliable, due to an unexpected ping reply status."    
            }

            return $payloadLength + 27
        }
        else
        {
            Write-Error "The name $($DestinationHost) could not be resolved. No Path MTU discovery will be performed."
        }
    }

    end
    {
        if ($null -ne $pinger)
        {
            $pinger.Dispose()
        }

        if ($null -ne $rng)
        {
            $rng.Dispose()
        }
    }
}