Topic 10. Mikrotik Router Mikrotik Hotspot MTU calculations on MikroTik

Mikrotik Router Mikrotik Hotspot MTU calculations on MikroTik

Maximum Transmission Unit is a term in information technology that refers to the largest data packet size that can be transmitted through a network medium. The size of the MTU is varied depending on the network technology used. An example is in a network based on Ethernet technology, the maximum MTU size is 1500 bytes. It is the data-link layer assignment that should determine the size of the MTU.

MTU settings are usually done on networking devices such as switches, routers and so on. Very seldom do the MTU settings manually on the workstation or host. If the ip layer accepts packets that will be forwarded into the network, then the device will calculate the packet size if added with 20 bytes ip header. If it turns out that the packet to be transmitted has a larger MTU ukran than the MTU of the device receiving the packet, then the packet will be fragmented, or cut to a smaller size.

Mikrotik Router Mikrotik Hotspot MTU calculations on MikroTik

Large MTU values allow to transmit data faster, imagine if you have a lot of data then collected into one big box. Then we simply send all the data once using one big box. But if for small box, then we need to send several times.The value of a large MTU does not then always have a good effect. The longer the MTU, the less reliable the data transmission process. If there is a malfunction of the packet in transmission then all damaged packets will be resent by TCP (Transmission Control Protocol) protocol.

So also on the system in MikroTik is RouterOS which has a standard size of each MTU of each interface. In RouterOS for MTU it is divided into several types ie L3 MTU ( IP / Layer-3 ), L2.5 MTU . Of the types of MTU has a value of different standards. 

• L3 MTU = 1500 (Data: 1480 + IP: 20), 
• L2.5 MTU = 1504 (Data: 1480 + IP: 20 + MPLS: 4)
• L2 MTU = 1508 (Data: 1480 + IP: 20 + MPLS: 4 + VLAN: 4)
• FULL Frame MTU = 1522 (Data: 1480 + IP: 20 + MPLS: 4 + VLAN: 4 + ETH: 14).

Mikrotik Router Mikrotik Hotspot MTU calculations on MikroTik

The big difference in the value of each type of MTU depends on the type of traffic that will be handled. If the size of the MTU is only set at 1500 (L3 MTU) without adding the type of MTU The other then for traffic with VLAN service, MPLS, will not be able to pass. Under normal conditions data packets that have a size too large will be fragmented by the device automatically. Too many fragmented packets will cause the packet queue to be long, then the device receiving the packet must also reorder the received packet. The size of MTU supported by Mikrotik products can be seen here . 

Especially if there is an application that requires a static data packet value, then the network admin must be able to determine the size of data packets to be passed in order for the packet to be received properly. Some services that transmit data over the network will usually add headers to the data packet, such as ping.

When data goes out of the device, data packets will be added with some headers, including 20bytes IP Header, and ICMP header 8bytes. To do a test, you can use the ping -f -s command on windows OS, ping -M -s on Linux, or ping -D -s on Mac OSX. Suppose we try ping with the size of 1500 packets, without doing fragmentation.

Mikrotik Router Mikrotik Hotspot MTU calculations on MikroTik

The destination device is unable to accept because the packet size is too large.Why so, when the opponent device has MTU 1500 settings ?. Size 1500 is the size of the data packet before adding the header. Then we can calculate the exact size of the data so that the ping packets can be accepted. 

1500 = 20 (IP header) + 8 (ICMP header) + Data
Data = 1472

If we try ping with size 1472, then the package can be received well 

Mikrotik Router Mikrotik Hotspot MTU calculations on MikroTik

Other services usually also add headers with certain values, such as PPTP VPN and PPPOE. This service will add pptp header and checksum, while PPPoE add PPPoE header, PPP ID, DST & SRC Address. 

PPTP:
The above calculation is also applicable when using other services. It can be concluded that with the increase of ip header size, the size of the data on the transmitted is also smaller. The size of the MTU miminum is 576 bytes.

MTU Path Discovery 

To know the value of opponent device's MTU, devices connected to the network have a mechanism called MTU path discovery. This mechanism does not necessarily require special features or services, but uses a fairly simple way that is by utilizing the error reporting mechanism on ICMP. 

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