Q: What are protocols?
A: Protocols are a set of rules and regulation on how to communicate.
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|
| The networking applications themselves. example: Email-client, browser, Comand Line (CL also provides ways to interact over the web).
| This layer is responsible for supplying an interface for a user to communicate
| with over the web. It deals with showing requested resources (e.g text, images, videos)
Application |
Layer | Used protocol: Http, Https (for data assembling).
| Protocols that have metadata (headers) for request/responses, along with data, for the application to use.
|
|
| Data type (PDU): User Data , user specific data (e.g user input, user requested resources)
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         |
           | This layer is responsible for recieving the encoded data, decoding it, making data ready to use by application layer.
         | On the other hand it also is responsible for encrypting application layer data to be ready to be transported.
         | This layer also deals with compression of formated data to be ready to be sent out.
Presentation| Character encoding also happens at this layer.
Layer       |
| Used protocols: Http (for character encoding)
|
| Data type (PDU): Formated Data
| NOTE!!! many people consider Presentation Layer to be part of the Application Layer
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|
| This layer is responsible for managing all the incomming/outgoing sessions, opening them and destroying them.
| It keeps session data for session oppened.
Session | Sessions could be oppened by Client Softwear such as e-mail, browsers, command line etc...
Layer |
| Used protocols: Does not use protocols, this layer basically keeps session data and add it to the formated data from Presentation Layer
|
| Data type (PDU): Formated Data
| NOTE!!! You can check current sessions using the netstat comand on CMD.
           | NOTE!!! This is the last fully softwear level, all three of the above layers were using softwear to process data.
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|
| This layer is responsible for segmenting the data encoded/encrypted in session layer into small chunks of data.[ ] -> [][][]
| This layer is also responsible for assembling segmented data recieved. [][][] -> [ ]
           | Data type (PDU): Segment
           |
Transport | Each segment must have a sequence-number added to it to keep it in order, this is since the segments might not arrive in order.
Layer |
           | The sequence-number is added into a header thats attached to every segment:
| [segment|header:{seq_num:1}] [segment|header:{seq_num:2}]...
|
| The segment-header also includes the maximum-segment-size attr that tells what is the size of the segment in bits.
| [segment|header:{seq_num:1,mss:256}] [segment|header:{seq_num:2, mss:256}]...
|
           | The type connection relayebility os also determined here, and added to the segment header. Connection relayebility has to with UDP/TCP.
           | TCP (Transfer Control Protocol) - connection oriented, relayable connection. This protocol is used for data that is important.
           | UDP (User Datagram Protocol) - connectionless, un-relayable connection. This protocol is used for un-important data,
           |
           | PORT NUMBERS
           | ------------
           | This layer also adds the protocol corresponding port number.
           | For example: http uses port 80, https uses 443, ftp uses 20,21.
           | If your using TCP or UDP your using these 16bit port numbers.
           | The segments now look as such:
           | [segment|header:{seq_num:1,mss:256,port:80}] [segment|header:{seq_num:2, mss:256, port:80}]...
           |
           | What is connection oriented? Its the use of aknowledgment of the two sides (transmiting/recieving) of each other to promise a secure connection.
           |
           | ------------------------------------------------------------
           | TCP (Transfer Control Protocol)
           | ------------------------------------------------------------
           |
           | There are two phases for transmitting segments over TCP: Three way handshake & transmition with acknowledgment.
           | ================ Three way handsake =====================>
           | 1) Check its the right recipient.
           | 2) agree on the file size and segments to transmit.
           | 3) agree on protocol and other params
           | <=========================================================
           |
           | ---------- segment.1 ------------> sending segment with segment-number
           |<---------- Acknowledge.send.2 ---- segment recieved, sending aknowledgement & next segment-number to send
           | ---------- segment.2 ----\-------> Segment got lost or was corrupted on the way. Recipient does not send acknowledgment.
           | --- wait for acknowledgment ?
           | --- did not recieve, send again !
           | ---------- segment.2 ------------> This time segment was recieved.
           | <--------- Aknowledgment.send.3
           | ---------- segment.3 ------------>
           |
           | NOTE!!! HTTP/HTTPS/FTP/TELENET all use TCP, this is since connection relayebility is needed.
           |
           | ------------------------------------------------------------
           | UDP (User Datagram Protocol)
           | ------------------------------------------------------------
           | UDP is used for unimportent data or data that needs speed of transfer, for examples:
           | Video streaming, needs to transmit 30/40fps at optimal setting (no buffer)
           | Voiceover IP, needs fast data transmition for voice quality.
           | Multiplayer games, needs fast data transmition for game accuracy.
           |
           | ============== NO HANDSHAKE OR ANY CONNECTION RELAYEBILITY ================
           |
           | ------------- segment.1 -------------> transmited into the ether
           | ------------- segment.2 -------------> transmited into the ether
           | ------------- segment.3 -------------> transmited into the ether
           |
           | NOTE!!! Used for LIVE stuff mainly.
           |
           | ---------------- OTHER PROTOCOLS ---------------------------
           | ------------------------------------------------------------
           | RIP (Routing Information Protocol) PROTOCOL
           | ------------------------------------------------------------
           | This is the protocol that router speaks with each other to.
           | RIP uses UDP for fast transfer.
           |
           | ------------------------------------------------------------
           | DNS (Domain Name System)
           | ------------------------------------------------------------
           | Protocol for Domain Name Servers to communicate and translate ip number to domain names.
           | Uses both UDP & TCP.
           |
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|
| This layers responsibility has to do with routing, also known as Best Path Determination.
           | Since the network is devided into many smaller local networks, connected by routers/gateways, this layer build the path
           | of routers between the Source IP and the Destination IP.
           |
| This layer takes the segments from the transport layer, and adds routing ioformation to it, packing up a segment to
Network       | new unit called "packet".
Layer         |
           | Additional data this layer adds to the network layer header is Source IP (own ip), and Destination IP (requested ip).
           | The process of resolution of IPs is called Host Adressing.
           |
           | This layer contains also the TCP/IP model, thought by many to be a seperate layer, though this assumption is misleading since
           | TCP/IP is only one of the protocols supported.
           |
           | After the procedure of Best Path Determination, the packet is issued to the data link layer to be transmited out.
           |
           | Data type (PDU): Packet
           |
|
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|
| This layers whole responsibility is to deliver/send the packets to next router/gateway on the Best Path determined in the network layer.
| The data link layer makes decision based on physical adresses (MAC adresses), and
Data Link | provides a reliable way to transit the data across physical links.
Layer |
| The recieved packet is wrraped again with a Data Link header that contains the MAC adress of the next router on path.
| The devices that can understand MAC adresses, such as switches, are called layer 2 devices, this is since layer 2 (Data Link Layer)
| contain the ability to read/write MAC adresses.
           | The Source IP and and Destination IP from the previous layer are converted to Source MAC (originating device) & Destination MAC (destination            | device), and this is added to the layer
| DL layer header.
| NOTE!!! here the layer deal with MAC & destination devices, previous layer delt with IPs not MAC.
           |
           | The packet wrraped with the DL layer header (also known as Ethernet header) is now called a frame.
           | Data type (PDU): Frames.
|
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|
| In this layer the packets from the previous level are serialized via hardware into binary, and transmited out to the ether.
| This layer is the most complex layer since it deals with serializing higher level data into binary, this means dependens on many types of
Physical | chip and card types (wifi, cable, sattalite).
Layer |
           | The physical layer provides an electrical, mechanical, and procedural interface to the transmission medium.
           | The shapes and properties of the electrical connectors, the frequencies to broadcast on,
           | the line code to use and similar low-level parameters, are specified here.
           |
           | Within the semantics of the OSI model, the physical layer translates logical
           | communications requests from the data link layer into hardware-specific operations to
           | cause transmission or reception of electronic signals.
           |
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The OSI Model

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