Alcatel Omni Switch/Router, User Manual

Page: 764 / 1100

Path MTU Discovery
Page 25-42
Path MTU Discovery
All Gigabit Ethernet modules and all Mammoth-based Ethernet modules on the Omni
Switch/Router in Release 4.0 and later support path Maximum Transmission Unit (
MTU
)
discovery. In path
MTU
discovery, the Ethernet frame (datagram) size is set to the largest size
that does not require fragmentation anywhere along the path from a source host to its desti-
nation. This frame size, known as a Path
MTU
(
PMTU
), is thus equal to the minimum of the
MTUs
of each hop in the path.
♦
Note
♦
MTU
discovery is not supported on token ring,
FDDI
,
WAN
, or non-Mammoth Ethernet modules. However,
token ring and
FDDI
can be used as intermediate links
(e.g., trunking or bridging) between remote switches.
Path
MTU
discovery is active all of the time and is part of the switch’s operating system; you
do not need configure it.
The source host initially assumes that the
PMTU
of a path is the
MTU
of the first hop. It sends
all datagrams with the “Don’t Fragment” (
DF
) bit set. If a switch/router along the path receives
a datagram that is too large to forward without fragmentation, the following steps will be
executed:
1. The switch/router that cannot forward these datagrams (i.e., the constricting hop) will
discard them.
2. The constricting hop will send
ICMP
destination unreachable messages to the source host
with a code that indicates fragmentation is needed and the “Don’t Fragment” (
DF
) bit in
the Internet Protocol (
IP
) header has been set. This message (known as a “Datagram Too
Big” message) contains the
PMTU
of the constricting hop.
3. After receiving a “Datagram Too Big” message, the source host reduces the size of the
MTU
so it matches the
PMTU
of the constricting hop.
4. The
MTU
discovery process ends when datagrams can be sent without fragmentation.
However, the source host will not reduce the size of a datagram below 68 octets.