#Find-MaxMTU.ps1
#Alan dot Kaplan at va dot gov
#10-1-2016
#version 2 adds collection of MTU for each hop
#version 2.1 bug fixes, output to clipboard, pause if from PS cmd
#version 2.2 minor cleanup for posting
#requires -version 3

#http://stackoverflow.com/questions/32434882/powershell-tracert
#I added -d to remove host name, maximum 
Function Invoke-Tracert {
    param([string]$RemoteHost)
    cls
    tracert -d $RemoteHost |
        ForEach-Object{
        if($_.Trim() -match "Tracing route to .*") {
            Write-Host $_ -ForegroundColor Green
        } elseif ($_.Trim() -match "^\d{1,2}\s+") {
            $n,$a1,$a2,$a3,$target,$null = $_.Trim()-split"\s{2,}"
            if ($a1 -eq '*'){$iTimeoutCtr +=1}ELSE{$iTimeoutCtr = 0}
            $Properties = @{
                Hop    = $n;
                First  = $a1;
                Second = $a2;
                Third  = $a3;
                Node   = $target
            }
            New-Object psobject -Property $Properties
            if ($iTimeoutCtr -eq $iMaxTimeOuts){
            [void]{break;cls }
            }
        }
    }
}

#This is an minor adaptation of code here:
#https://rcmtech.wordpress.com/2015/03/08/powershell-find-mtu/
Function Find-MTU($TestAddress){
    Write-Host Finding MTU for $TestAddress -ForegroundColor Green
    $LastMinBuffer=$BufferSizeMin
    $LastMaxBuffer=$BufferSizeMax
    $MaxFound=$false

    #calculate first MTU attempt, halfway between zero and BufferSizeMax
    [int]$BufferSize = ($BufferSizeMax - 0) / 2
    while($MaxFound -eq $false){
        try{
            $Response = ping $TestAddress -n 1 -f -l $BufferSize
            #if MTU is too big, ping will return: Packet needs to be fragmented but DF set.
            if($Response -like "*fragmented*"){throw}
            if($LastMinBuffer -eq $BufferSize){
                #test values have converged onto the highest working MTU, stop here and report value
                $MaxFound = $true
                write-verbose "found."
                break
            } else {
                #it worked at this size, make buffer bigger
                write-verbose "$BufferSize" 
                $LastMinBuffer = $BufferSize
                $BufferSize = $BufferSize + (($LastMaxBuffer - $LastMinBuffer) / 2)
            }
        } catch {
            #it didn't work at this size, make buffer smaller
            write-verbose "$BufferSize" 
            $LastMaxBuffer = $BufferSize
            #if we're getting close, just subtract 1
            if(($LastMaxBuffer - $LastMinBuffer) -le 3){
                $BufferSize = $BufferSize - 1
            } else {
                $BufferSize = $LastMinBuffer + (($LastMaxBuffer - $LastMinBuffer) / 2)
            }
        }
        write-verbose ","
    }
    [pscustomobject]@{
    Address =  $testaddress
    MTU = $BufferSize
    }
}

Function Find-AllMTUs{
    $msg = "Stop trace after this number of timeouts"
    $iMaxTimeOuts=  Run-TxtMsg $msg 2
    if (!$iMaxTimeOuts ){Exit}
 
    [array]$hops = Invoke-Tracert $DestinationAddress
    $nodes = $hops.node
    #If you get a timeout, final is not collected.  Add it back in
    $nodes += ($hops[$hops.getupperbound(0)] ).node
    $nodes |  where {$_ -ne "Request timed out."} |  select  -Unique|% {Find-MTU $_}
}

Function Run-TxtMsg($msg,$defaultval){  
    $retval = Read-Host "$msg,`n[Enter for default value: $DefaultVal]"
    #use default  if not specified
    if ($retVal.Length -eq 0) {$defaultval}ELSE{$retval}
    }

Function Test-HostEntry($lookup){
Try{
    if ([Net.IPAddress]::TryParse($lookup,[ref]$null)){
        $z= [System.Net.Dns]::GetHostByAddress($lookup)
        }ELSE{
        $z= [System.Net.Dns]::GetHostByName($lookup)
        }
    $True
}Catch{$false}
}

Function Test-Command ($sCmdlet){
    $retval = $true
    try{
        get-command $sCmdlet -ea Stop | Out-Null
        }Catch{
        Write-verbose "Command $sCmdlet not available"
        $retval = $false
    }
    $retval 
}
#------------- Script Begins ---------------
#set DestinationAddress to the name or IP address you wish to test against
$DestinationAddress = Read-host  "Run test to this IP or FQDN"
if (!$DestinationAddress){Exit}

$retval = Test-HostEntry $DestinationAddress
if ( $retval -eq $false){
    Write-Warning "Failed to ping $DestinationAddress!"
    Exit
}

$msg = "Set BufferSizeMax to the largest MTU you want to try, `nusually 1500 or up to 9000 if using Jumbo Frames"
$BufferSizeMax =  Run-TxtMsg $msg 1500
if (!$BufferSizeMax ){Exit}

#Ed Wilson's method, https://technet.microsoft.com/en-us/library/ff730939.aspx
$title = "Run Tracert?"
$message = "Do you want to get the MTU for each hop found by by a trace route?  This takes longer. 'No' is just to $destinationAddress`."
$yes = New-Object System.Management.Automation.Host.ChoiceDescription "&Yes", "Get MTU for each hop."
$no = New-Object System.Management.Automation.Host.ChoiceDescription "&No", "Get MTU for $destinationaddress only."
$cancel = New-Object System.Management.Automation.Host.ChoiceDescription "&Cancel", "Exit."
$options = [System.Management.Automation.Host.ChoiceDescription[]]($yes, $no, $cancel)
$result = $host.ui.PromptForChoice($title, $message, $options, 0) 

switch ($result)
{
    0 {
        $MTUList = Find-AllMTUs
        cls
        #Test for Core 
        if ((Test-Command 'Out-GridView') -and (Test-Path "$env:windir\explorer.exe")){
            $MTUList | Out-GridView -Title "MTUs for each hop to $DestinationAddress.  Selected items will be copied to your clipboard" -PassThru | clip
        }ELSE{
            cls
            $MTUList | format-table -auto 
            #Pause if launched from shell with  "Run with PowerShell"
            if (([Environment]::GetCommandLineArgs()) -match '&'){pause}
            }
        }
    1 {Find-MTU $DestinationAddress}
    Default {Exit}
}