网络层TCP报文分片
Fragmentation of TCP packet at network layer
假设一个 TCP 消息包含 900 字节的数据和 20
TCP 字节 header。现在，如果我们有一个 link 可以支持最大帧大小为 512 字节，那么我们是否会在传输层有 2 个大小足够小的 TCP 数据包，以便可以通过此 link 发送它们。或者它们会在数据 link 层进一步分散吗？
会发生以下哪项：
Transport layer: h1 data
Network layer: h2 h1 data
data link layer: frame 1 - h3 h2 h1 data-part1
frame 2 - h3 h2 h1 data-part2
或
data link layer: frame 1 - h3 h2 h1 data-part1
frame 2 - h3 data-part2
分段是 IPv4 DNA 的一部分，它发生在 IPv4 的第 3 层。 IPv6 消除了分片，发送主机只发送正确大小的数据包（对数据进行预分片）。
路由器在第 3 层（网络层）运行，分段发生在 IPv4 路由器中。如果 IPv6 数据包对于下一跳来说太大，它们将被路由器随意丢弃，并向源主机发回 ICMPv6 消息。
The internet protocol also provides for fragmentation and reassembly
of long datagrams, if necessary, for transmission through "small
packet" networks.
和
The internet protocol implements two basic functions: addressing and
fragmentation.
和
In the routing of messages from one internet module to another,
datagrams may need to traverse a network whose maximum packet size is
smaller than the size of the datagram. To overcome this difficulty, a
fragmentation mechanism is provided in the internet protocol.
分片过程说明：
Fragmentation
Fragmentation of an internet datagram is necessary when it originates
in a local net that allows a large packet size and must traverse a
local net that limits packets to a smaller size to reach its
destination.
An internet datagram can be marked "don't fragment." Any internet
datagram so marked is not to be internet fragmented under any
circumstances. If internet datagram marked don't fragment cannot be
delivered to its destination without fragmenting it, it is to be
discarded instead.
Fragmentation, transmission and reassembly across a local network
which is invisible to the internet protocol module is called intranet
fragmentation and may be used [6].
The internet fragmentation and reassembly procedure needs to be able
to break a datagram into an almost arbitrary number of pieces that can
be later reassembled. The receiver of the fragments uses the
identification field to ensure that fragments of different datagrams
are not mixed. The fragment offset field tells the receiver the
position of a fragment in the original datagram. The fragment offset
and length determine the portion of the original datagram covered by
this fragment. The more-fragments flag indicates (by being reset) the
last fragment. These fields provide sufficient information to
reassemble datagrams.
The identification field is used to distinguish the fragments of one
datagram from those of another. The originating protocol module of an
internet datagram sets the identification field to a value that must
be unique for that source-destination pair and protocol for the time
the datagram will be active in the internet system. The originating
protocol module of a complete datagram sets the more-fragments flag to
zero and the fragment offset to zero.
To fragment a long internet datagram, an internet protocol module (for
example, in a gateway), creates two new internet datagrams and copies
the contents of the internet header fields from the long datagram into
both new internet headers. The data of the long datagram is divided
into two portions on a 8 octet (64 bit) boundary (the second portion
might not be an integral multiple of 8 octets, but the first must be).
Call the number of 8 octet blocks in the first portion NFB (for Number
of Fragment Blocks). The first portion of the data is placed in the
first new internet datagram, and the total length field is set to the
length of the first datagram. The more-fragments flag is set to one.
The second portion of the data is placed in the second new internet
datagram, and the total length field is set to the length of the
second datagram. The more-fragments flag carries the same value as
the long datagram. The fragment offset field of the second new
internet datagram is set to the value of that field in the long
datagram plus NFB.
This procedure can be generalized for an n-way split, rather than the
two-way split described.
To assemble the fragments of an internet datagram, an internet
protocol module (for example at a destination host) combines internet
datagrams that all have the same value for the four fields:
identification, source, destination, and protocol. The combination is
done by placing the data portion of each fragment in the relative
position indicated by the fragment offset in that fragment's internet
header. The first fragment will have the fragment offset zero, and
the last fragment will have the more-fragments flag reset to zero.
假设一个 TCP 消息包含 900 字节的数据和 20 TCP 字节 header。现在，如果我们有一个 link 可以支持最大帧大小为 512 字节，那么我们是否会在传输层有 2 个大小足够小的 TCP 数据包，以便可以通过此 link 发送它们。或者它们会在数据 link 层进一步分散吗？
会发生以下哪项：
Transport layer: h1 data
Network layer: h2 h1 data
data link layer: frame 1 - h3 h2 h1 data-part1
frame 2 - h3 h2 h1 data-part2
或
data link layer: frame 1 - h3 h2 h1 data-part1
frame 2 - h3 data-part2
分段是 IPv4 DNA 的一部分，它发生在 IPv4 的第 3 层。 IPv6 消除了分片，发送主机只发送正确大小的数据包（对数据进行预分片）。
路由器在第 3 层（网络层）运行，分段发生在 IPv4 路由器中。如果 IPv6 数据包对于下一跳来说太大，它们将被路由器随意丢弃，并向源主机发回 ICMPv6 消息。
The internet protocol also provides for fragmentation and reassembly of long datagrams, if necessary, for transmission through "small packet" networks.
和
The internet protocol implements two basic functions: addressing and fragmentation.
和
In the routing of messages from one internet module to another, datagrams may need to traverse a network whose maximum packet size is smaller than the size of the datagram. To overcome this difficulty, a fragmentation mechanism is provided in the internet protocol.
分片过程说明：
Fragmentation
Fragmentation of an internet datagram is necessary when it originates in a local net that allows a large packet size and must traverse a local net that limits packets to a smaller size to reach its destination.
An internet datagram can be marked "don't fragment." Any internet datagram so marked is not to be internet fragmented under any circumstances. If internet datagram marked don't fragment cannot be delivered to its destination without fragmenting it, it is to be discarded instead.
Fragmentation, transmission and reassembly across a local network which is invisible to the internet protocol module is called intranet fragmentation and may be used [6].
The internet fragmentation and reassembly procedure needs to be able to break a datagram into an almost arbitrary number of pieces that can be later reassembled. The receiver of the fragments uses the identification field to ensure that fragments of different datagrams are not mixed. The fragment offset field tells the receiver the position of a fragment in the original datagram. The fragment offset and length determine the portion of the original datagram covered by this fragment. The more-fragments flag indicates (by being reset) the last fragment. These fields provide sufficient information to reassemble datagrams.
The identification field is used to distinguish the fragments of one datagram from those of another. The originating protocol module of an internet datagram sets the identification field to a value that must be unique for that source-destination pair and protocol for the time the datagram will be active in the internet system. The originating protocol module of a complete datagram sets the more-fragments flag to zero and the fragment offset to zero.
To fragment a long internet datagram, an internet protocol module (for example, in a gateway), creates two new internet datagrams and copies the contents of the internet header fields from the long datagram into both new internet headers. The data of the long datagram is divided into two portions on a 8 octet (64 bit) boundary (the second portion might not be an integral multiple of 8 octets, but the first must be). Call the number of 8 octet blocks in the first portion NFB (for Number of Fragment Blocks). The first portion of the data is placed in the first new internet datagram, and the total length field is set to the length of the first datagram. The more-fragments flag is set to one. The second portion of the data is placed in the second new internet datagram, and the total length field is set to the length of the second datagram. The more-fragments flag carries the same value as the long datagram. The fragment offset field of the second new internet datagram is set to the value of that field in the long datagram plus NFB.
This procedure can be generalized for an n-way split, rather than the two-way split described.
To assemble the fragments of an internet datagram, an internet protocol module (for example at a destination host) combines internet datagrams that all have the same value for the four fields: identification, source, destination, and protocol. The combination is done by placing the data portion of each fragment in the relative position indicated by the fragment offset in that fragment's internet header. The first fragment will have the fragment offset zero, and the last fragment will have the more-fragments flag reset to zero.