[Computer network] - Network layer - Network Interconnection

Interconnect multiple different networks merged into a larger network, a variety of different network long-standing need networking

The difference at the network

May appear on the network layer some differences may be significant differences in different networks, the Internet has become so complicated

When a packet must pass through one or more external networks before reaching the destination network, many problems can occur at a network interface. example
When the connection-oriented network via connectionless networks, data packets may be reordered, which it is undesirable sender, and the receiver is not ready to deal with the issue order
How a maximum of 1500 bytes of the packet network transmission of a data packet subsection 8000
For the different networks, error control, flow control and congestion control is generally not the same

How to connect network

The physical layer
repeater and the hub
is simply to move data from one network to another. They just re-generate an analog signal, it does not understand the content of the agreements related to digital

Data link layer
bridges and switches
may receive the frame, check the MAC address, and forwards the frames to a different network

Network layer
router (Router)
in a storage network between - forwarding packets, the packet format can be converted
router can handle multiple protocol is called multiprotocol router

Transport layer
transport gateway (transport gateway)
interface to the transport layer connection between the two

Application Layer
application gateway (application gateway)
semantics can be translated message

A common network layer used to interconnect different networks

The difference between switches and routers

Switch the entire frame based on the MAC address for transmission based; without having to understand network layer protocol packets used
Router extracts the packets from the frame, then use the network address of the packet to determine his target destination, to be understood that the network layer protocol packets used

Tunneling

The same source and destination host network type is located, which is connected to a different type of network, is connected to the tunnel technique may be employed

Working process
host configuration comprises a packet London, Paris IPv6 address, the packet is sent to the multi-protocol IPv6 network connection Paris router to an IPv4 network
when the multiprotocol router obtain the IPv6 packet, it sends the packet encapsulated with an IPv4 header the other side of the IPv4, the encapsulated IPv4 packets directed multiprotocol router, the network and the IPv6 network is connected London
when the wrapped packets from London router, the original IPv6 packet is taken out and sent to the final destination host

to sum up

Connecting two network packets by an intermediate network is encapsulated in the middle
Tunnel analogy: tunnel is treated as a single connection packets can enter and exit at the end portions.

Packet is fragmented

Each network or links are reasons to limit the maximum packet length:

Hardware, such as Ethernet frame length limit
Operating systems, if all buffers are 512 bytes
Protocol, for example the number of bits in the packet length field
Comply with certain national (or international) standards
We expect the "number of retransmissions due to errors introduced" reduced to a certain extent
Packet desirable to avoid a long time occupation of the channel

Due to many reasons, networks of different packet size limitations, large data packets sent by and recombinant fragments, large data packets through a small network packets, the data packet gateway from the large-divided into several segments (the fragment), each segment as a separate packet transmission.

Segment restructuring strategy

Transparent segment

Recombination subsequent segmentation process along with other transparent network
behind the gateway large packet is fragmented, each outlet must go through the same gateway, and where recombination, for example: ATM network

Problems caused by

Export gateway needs to know when all of the packets in attendance
All packets must leave from the same exit gateway
Large packet network through a series of small packets, segmentation requires repeated recombination, a large overhead

Non-transparent segment

Other network segment recombination process opaque, not recombination intermediate gateway, and made by the destination host

Problems caused by

High requirements for the host, able to restructure
Each segment must have a data header, network overhead increases
When the data packet for dividing the segment to be able to reconstruct the original data stream in accordance with the way number

Segment number

Offset method (IP uses)
defines a substantially longitudinal segment, so that substantially all the segments through the network
when the packet is fragmented, but the last segment is smaller than a length substantially equal to an outer, substantially all of length equal to length
of a data packet may containing a number of basic segments

Data packet header comprising: original data packet number, fragment number, the last segment indicator bits, wherein
the segment number: refers to the current segment offset in the original data packet in the
last segment indication bit: indicates whether a partition is the last paragraph

(a) the original data packet, contains 10 bytes of data;
(B) by a maximum when the packet length is "header + 8 bytes payload" network segments;
(C) as a by the maximum packet length segment of the network 5