General Troubleshooting : How to determine the proper MTU size with ICMP pings

How to determine the proper MTU size with ICMP pings

To find the proper MTU size, you have to run a special ping to the destination address. This is usually the gateway, local server or an IP address domain name internet (e.g. thepacketwizard.com). You probably want to start around 1800 and move down 10 each time until you get to a ping reply. Once you have a ping reply start moving backup by 2-5 bits to get to the fragmented packet size. Take that value and add 28 to the value to account for the various TCP/IP headers. E.g. let’s say that 1452 was the proper packet size (where you first got an ICMP reply to your ping). The actual MTU size would be 1480, which is the optimum for the network we’re working with. Header size varies depending what the packet is traversing.

 

1500 Standard MTU

– 20 IP Header

– 24 GRE Encaps.

– 52 IPSec Encap.

– 8 PPPoE

– 20 TCP Header

 

Windows

ping  (host) (-f) (-l (packet size))

An example would be:

ping  thepacketwizard.com -f -l 1800

(result = "Packet needs to be fragmented but DF set.")

ping thepacketwizard.com -f -l 1472 

(result = reply)

 

The options used are:

      • -f: set “Don’t Fragment” flag in packet
      • -l size: send buffer size

 

Linux

ping (-M do) (-s (packet size)) (host)

An example would be:

ping thepacketwizard.com -M do -s 1800

(result = "Frag needed and DF set" or "message too long")

ping thepacketwizard.com -M do -s 1472

(result = reply)

 

The options used are:

      • -M <hint>: Select Path MTU Discovery strategy. <hint> may be either “do” (prohibit fragmentation, even local one), “want” (do PMTU discovery, fragment locally when packet size is large), or “dont” (do not set DF flag).
      • -s packetsize: Specifies the number of data bytes to be sent. The default is 56, which translates into 64 ICMP data bytes when combined with the 8 bytes of ICMP header data.

 

Mac

ping (-D) (-s (packet size)) (host)

An example would be:

ping thepacketwizard.com -D -s 1800

(result = "sendto: Message too long")

ping thepacketwizard.com -D -s 1462

(result = reply)

 

The options used are:

      • -D: set the “Don’t Fragment” bit
      • -s packetsize: Specify the number of data bytes to be sent. The default is 56, which translates into 64 ICMP data bytes when combined with the 8 bytes of ICMP header data.

There is a lot to know about MTU check it out on Wikipedia : Wikipedia – MTU

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