网络层TCP数据包的分片

Question

Suppose that a TCP message that contains 900 bytes of data and 20 bytes of TCP header. Now If we have a link that can support a maximum frame size of 512 bytes, will we have 2 TCP packets at transport layer of small enough sizes that they could be sent over this link. Or will they be fragmented further at the Data link layer?

Which of the following will happen:

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

OR

data link layer:  frame 1 -  h3 h2 h1 data-part1
                  frame 2 -  h3 data-part2

Answer 1

Fragmentation is part of the DNA of IPv4, and it happens in layer-3 for IPv4. IPv6 has eliminated fragmentation, and it is up to the sending host to only send packets of the correct size (pre-fragment the data).

Routers operate at layer-3 (Network Layer), and fragmentation happens in routers for IPv4. If IPv6 packets are too large for the next hop, they are unceremoniously dropped by the router, and an ICMPv6 message is sent back to the source host.

RFC 791, INTERNET PROTOCOL :

The internet protocol also provides for fragmentation and reassembly of long datagrams, if necessary, for transmission through "small packet" networks.

and

The internet protocol implements two basic functions: addressing and fragmentation.

and

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.

The description of the fragmentation process:

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.