大于ｍｔｕ就不得不分片传输，
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ｒｆｃ７９１
The maximum sized datagram that can be transmitted through the
next network is called the maximum transmission unit (MTU).
If the total length is less than or equal the maximum transmission
unit then submit this datagram to the next step in datagram
processing; otherwise cut the datagram into two fragments, the
first fragment being the maximum size, and the second fragment
being the rest of the datagram. The first fragment is submitted
to the next step in datagram processing, while the second fragment
is submitted to this procedure in case it is still too large.
ｒｆｃ１１９１
When one IP host has a large amount of data to send to another host,
the data is transmitted as a series of IP datagrams. It is usually
preferable that these datagrams be of the largest size that does not
require fragmentation anywhere along the path from the source to the
destination. (For the case against fragmentation, see [5].) This
datagram size is referred to as the Path MTU (PMTU), and it is equal
to the minimum of the MTUs of each hop in the path. A shortcoming of
the current Internet protocol suite is the lack of a standard
mechanism for a host to discover the PMTU of an arbitrary path.
The current practice [1] is to use the lesser of 576 and the
first-hop MTU as the PMTU for any destination that is not connected
to the same network or subnet as the source. In many cases, this
results in the use of smaller datagrams than necessary, because many
paths have a PMTU greater than 576. A host sending datagrams much
smaller than the Path MTU allows is wasting Internet resources and
probably getting suboptimal throughput. Furthermore, current
practice does not prevent fragmentation in all cases, since there are
some paths whose PMTU is less than 576.
In this memo, we describe a technique for using the Don't Fragment
(DF) bit in the IP header to dynamically discover the PMTU of a path.
The basic idea is that a source host initially assumes that the PMTU
of a path is the (known) MTU of its first hop, and sends all
datagrams on that path with the DF bit set. If any of the datagrams
are too large to be forwarded without fragmentation by some router
along the path, that router will discard them and return ICMP
Destination Unreachable messages with a code meaning "fragmentation
needed and DF set" [7]. Upon receipt of such a message (henceforth
called a "Datagram Too Big" message), the source host reduces its
assumed PMTU for the path.
【 在 freyoneby (freyoneby) 的大作中提到: 】
: 哪些标准有描述的
: - 来自「最水木 for iPhone 7 Plus」
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