Networks....

[the_dial_up_boy]

A protocol is a data structure exchanged and the procedure used to exchange. There exists a hierarchy between protocols because every protocol has integration with others but following level superiors and inferiors. The point is the communication between levels and their exchanges.
Communication interface is the whole functionality that a level can offer.
The communications that work in a same level are named peer to peer (directly connection btw 2 host), it uses the same protocol, said too language, it has structure and modes where the exchange works.
What is exchanged when there is an active communication?
The protocol data unit, PDU, is made by 2 parts:
-----Header: packets info, part active of the protocol, info about the sender
------Payload: that we are sending to the receiver, the "body" not manipulated by the protocol.

All the levels have own rules to manipulate the header:

---Connection and disconnection: mode which they work
---Data transfer: time and modes of work
---Data stream control: it works to synchronize sender and receiver
---Control errors: it works about delicate situations helping to solve them
---Fragmentation: payload division to optimize any possibility of the protocol
---Reconstruction: to assemble again the payload
---Multiplexing and demultiplexing: they work to realize interface condivises in the middle of more agents of a following superior level.
---Routing: ability to determine a route less bad to connect 2 points.

Standard applied in any protocols was OSI but, because it had some points very hard to be solved, ARPANET (1969) took his place (like everyone know, it has been developed by USA defense department).
Arpanet is made by 3 levels:
1. The network interconnection, IP protocol. Arpanet takes the idea about the network like a whole of interconnections in sub-networks made by a connectionless protocol (it means without a constant connection).
2. Transport protocols, they are 2, one connectionless and one connection oriented (UDP and TCP ,lol).
3. Applications

Arpanet - like Internet.
Up the Ethernet cable there is a transreceiver ethernet, hardware able to transfer the data to the pc, after it is the driver (that “speaks” with the pc hardware).
The fundamental internet data is the "8bit" unit, sequence of 8 bit able to realize 256 different values.
Ethernet address is made by 6 "8bit" unit different from that IP, made by 4 in base -10- , included 0 and 255, divided by ".”. At the end there is a port made by 2 "8 bit” unit, with a number included 0 and 65535, it identifies a service internal without errors.
Any level can receive messages from 2 directions, superior and inferior, acting like interface beyond the network and the superior level, using the interface of the inferior level.
It means that a generic application will use in order: UDP or TCP, IP, and DRIVER for the ethernet access.

MULTIPLEXING AND DEMULTIPLEXING.
In the transportation level we find the TCP and UDP .They can act like
multiplexer or demultiplexer , in order if the packet goes from a superior level to an inferior and vice versa.
Example if a node has 2 ethernet connections on 2 different networks;
IP will act like a demultiplexer sending beyond the tcp/udp level a frame from ethernet, opposite it will act like a multiplexer sending in ethernet a datagram or a segment from a superior level.

A last IP application is named IP FORWARDING, where all the frames that arrive won’t be sent beyond a superior level but beyond another network (routing).

thanx to "East king" TECHNOBEAST