TOPEX Bytton (HSPA+ / LTE)
ByttonLTE(full)_genericUsermanual_sw306FAS_revN.1.docx
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You should always bear in mind that the MTU is the
maximum physical value
. Thus, in many instances
the net payload (logical size) must be smaller. For example, when you use tunnels, they add up specific
headers to the packets, so you must reduce the MTU to achieve an overall length no greater that 1500!
The same holds true for PPP over Ethernet, you must take into account the extra encapsulation, the
actual length must be smaller, in order not to exceed the 1500 bytes limit!
The real MTU depends upon the actual carrier that you use to connect to Internet, for instance several
ADSL providers have a MTU larger than 1500, so that the Ethernet packets that are transmitted via ADSL
will not be over this limit.
In practice, trial and error is the only sure way of finding the optimal MTU for each case. It works like this:
you issue a PING command towards your Internet provider with the options “–f” (set Don’t Fragment flag
in the packet) and – l 1472 (specifies the size):
ping -f -l 1472 k.ro
or
ping –f –l 1462 209.123.109.175
and watch for the results:
If the answer is OK (O% loss), you can increase the MTU value (1472) by 10 and try again.
Continue with larger and larger values until you receive the error message:
"Packet needs to be fragmented."
The message is quite clear: the value of 1482 is too high, the packets need to be fragmented, but you
have set the flag “DF” (Do Not Fragment) so the packets cannot be transmitted.
You get “100% loss” for the PING data packets!
Repeat this step by decreasing your current MTU value by 10 and using the PING command again.
Continue until you no longer receive a message that the packets are fragmented.
Verify this by pinging with successive size increments / decrements smaller than ten, as shown:
1473 is too large!
