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In this article, we are going to explain how to find the right MTU size but first, we have to understand why MTU size and why it is important.
MTU Size
In computer networking, MTU stands for Maximum Transmission Unit. As the name suggests it is the unit that tells the largest data packet a device can accept or transfer via an internet connection. It is measured typically in bytes.
Eg. When you list the interface in your Linux dist. you will see all the information with ip a or ifconfig command. In the following code section, you see an example showing the commands’ output.
[foofunc@test centos]# ip a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UP group default qlen 1000
link/ether fa:16:3e:3f:69:3c brd ff:ff:ff:ff:ff:ff
inet 10.0.23.12/24 brd 10.0.23.255 scope global dynamic noprefixroute eth0
valid_lft 80247sec preferred_lft 80247sec
inet6 fe80::f816:3eff:fe3f:693c/64 scope link
valid_lft forever preferred_lft forever
[foofunc@test centos]# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST> mtu 1500
inet 10.0.23.12 netmask 255.255.255.0 broadcast 10.0.23.255
inet6 fe80::f816:3eff:fe3f:693c prefixlen 64 scopeid 0x20<link>
ether fa:16:3e:3f:69:3c txqueuelen 1000 (Ethernet)
RX packets 62240 bytes 9129859 (8.7 MiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 29804 bytes 6365796 (6.0 MiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
lo: flags=73<UP,LOOPBACK,RUNNING> mtu 65536
inet 127.0.0.1 netmask 255.0.0.0
inet6 ::1 prefixlen 128 scopeid 0x10<host>
loop txqueuelen 1000 (Local Loopback)
RX packets 13195350 bytes 2170537545 (2.0 GiB)
RX errors 0 dropped 0 overruns 0 frame 0
TX packets 13195350 bytes 2170537545 (2.0 GiB)
TX errors 0 dropped 0 overruns 0 carrier 0 collisions 0
In the above code section, we have seen the output of the ip a and ifconfig commands. Both commands list all the interfaces in the system and their properties. In this article, we will focus on MTU size. First line of the interface details shows mtu size, here in the code section of eth0 MTU size is 1500 bytes. In most of the system, you have seen MTU size is 1500 bytes. The following diagram illustrates what MTU looks like in a typical network data transmission. It is consist of A payload, with 1460 bytes
The TCP and IP headers, with 20 bytes each.
Significance of the MTU size
- Setting the right MTU size is really important. It is the main reason in-network if your internet connection speed isn’t as fast as you would expect. You can also experience other performance-related issues.
- We know that the larger the MTU size is, it can be put more data in packets. That means transmission would be faster and more efficient across the network but if errors occur the packet will take longer to retransmit.
- If the MTU size is too small, which means more bandwidth is unnecessarily wasted on headers. It also leads to more resource consumption because more packets also mean the routers and the operating systems on both ends might have to work harder that takes more CPU time.
How to choose the right MTU size.
You can find the right MTU size through a ping test, it’s the simplest way. In the following code section we have shown few ping commands with different MTU size and showing how mtu size changes the time packet loss and other factors in packet transmission.
To check the right MTU size we used ping command with following options.
- -f: Flood ping, for every ECHO_REQUEST sent a period “.” is printed, while for ever ECHO_REPLY received a backspace is printed. This provides a rapid display of how many packets are being dropped.
- -l: specify size of the MTU in following example we have used 1450, 1000 and 9000 bytes MTU size
- -C : Number of max packets to transmit, in our case we are using 100000.
[foofunc@test centos]#sudo ping -f -l 1450 10.0.25.13 -c 100000
WARNING: probably, rcvbuf is not enough to hold preload.
PING 10.0.25.13 (10.0.25.13) 56(84) bytes of data.
--- 10.0.25.13 ping statistics ---
100000 packets transmitted, 100000 received, 0% packet loss, time 873ms
rtt min/avg/max/mdev = 0.076/1.405/5.108/0.680 ms, pipe 545, ipg/ewma 0.012/0.947 ms
[foofunc@test centos]# sudo ping -f -l 1000 10.0.25.13 -c 100000
PING 10.0.25.13 (10.0.25.13) 56(84) bytes of data.
--- 10.0.25.13 ping statistics ---
100000 packets transmitted, 100000 received, 0% packet loss, time 912ms
rtt min/avg/max/mdev = 0.075/6.869/9.612/3.259 ms, pipe 1000, ipg/ewma 0.009/1.147 ms
[foofunc@test centos]# sudo ping -f -l 9000 10.0.25.13 -c 100000
WARNING: probably, rcvbuf is not enough to hold preload.
PING 10.0.25.13 (10.0.25.13) 56(84) bytes of data.
--- 10.0.25.13 ping statistics ---
100000 packets transmitted, 95043 received, 4.957% packet loss, time 915ms
rtt min/avg/max/mdev = 0.095/4.959/11.337/3.426 ms, pipe 1603, ipg/ewma 0.009/1.855 ms
In the above example I have tested the ping commands on the local network when my MTU size is less or equal to 1500 bytes there were no packet loss and time is less than when MTU size is 9000. You can also look at the round trip times which is minimum in when MTU size is set to 1500 bytes. Therefore, to get the right MTU size look for the minimum packet loss and minimum and less RTT time and overall time.
Summary
In this article we have learned about what is mtu size, what is its significance and how to choose right MTU size. In computer networking we have seen most of the router, computers have standard mtu size set to 1500 bytes but in internal organization you might notice that if organization have machines’ interfaces with higher bandwidth they have increased the MTU size to 9000 bytes (jumbo frames) and get more faster data transmission.
