Transcript MTU - e-Learning@UTM

MTU
Fragmentation process
MTU
• The Maximum Transmission Unit (MTU) is
– the maximum length of data that can be
transmitted by a protocol in one instance.
– For example, the MTU of Ethernet (by default
1500) is the largest number of bytes that can be
carried by an Ethernet frame (excluding the
header and trailer).
– MTUs are found at various layers of the OSI
model, and can often be tweaked to more
efficiently transport large volumes of data.
MTU
Ethernet Frame
• Internet Minimum MTU: 576 Bytes
• Each router must be able to fragment as needed to handle IP
datagrams up to the size of the largest MTU used by networks to
which they attach.
• Routers are also required, as a minimum, to handle an MTU of at
least 576 bytes.
• This value is specified in RFC 791, and was chosen to allow a
“reasonable sized” data block of at least 512 bytes, plus room for
the standard IP header and options.
• Since it is the minimum size specified in the IP standard, 576 bytes
has become a common default MTU value used for IP datagrams.
• Even if a host is connected over a local network with an MTU larger
than 576, it may choose to use an MTU value of 576 anyway, to
ensure that no further fragmentation will be required by
intermediate routers.