Redshift Academy

Open Systems Interconnection (OSI) Standard - TCP/IP Protocol
-------------------------------------------------------------

Virtually all networks in use today are based in some fashion on OSI. The core of this standard is the OSI Reference Model, a set of seven layers that define the different stages that data must go through to travel from one device to another over a network.

Application Set

Layer 7: Application - This is the layer that actually interacts with the operating system or application whenever the user chooses to transfer files, read messages or perform other network-related activities.
Layer 6: Presentation - Layer 6 takes the data provided by the Application layer and converts it into a standard format that the other layers can understand.
Layer 5: Session - Layer 5 establishes, maintains and ends communication with the receiving device.

Transport Set

Layer 4: Transport - This layer maintains flow control of data and provides for error checking and recovery of data between the devices. Flow control means that the Transport layer looks to see if data is coming from more than one application and integrates each application's data into a single stream for the physical network.
Layer 3: Network - The way that the data will be sent to the recipient device is determined in this layer. Logical protocols, routing and addressing are handled here.
Layer 2: Data - In this layer, the appropriate physical protocol is assigned to the data. Also, the type of network and the packet sequencing is defined.
Layer 1: Physical - This is the level of the actual hardware. It defines the physical characteristics of the network such as connections, voltage levels and timing.

The OSI Reference Model is really just a guideline. Actual protocol stacks often combine one or more of the OSI layers into a single layer. A protocol stack is a group of protocols that all work together to allow software or hardware to perform a function. The TCP/IP protocol stack is a good example. It uses four layers that map to the OSI model as follows:

Layer 1: Network Interface - This layer combines the Physical and Data layers and routes the data between devices on the same network. It also manages the exchange of data between the network and other devices.
Layer 2: Internet - This layer corresponds to the Network layer. The Internet Protocol (IP) uses the IP address, consisting of a Network Identifier and a Host Identifier, to determine the address of the device it is communicating with.
Layer 3: Transport - Corresponding to the OSI Transport layer, this is the part of the protocol stack where the Transport Control Protocol (TCP) can be found. TCP manages the assembling of a message or files into smaller data packets that are transferred to the Network Interface for transmission over the Internet where they can be received by a TCP layer of another device that reassembles the data packets from the originating device.
Layer 4: Application - Layer 4 combines the Session, Presentation and Application layers of the OSI model. Protocols for specific functions such as e-mail (Simple Mail Transfer Protocol, SMTP) and file transfer (File Transfer Protocol, FTP) reside at this level.

A TCP packet consist of:

IP is an Internet protocol (layer 2) that handles the address part of each packet so that it gets to the right destination. Basically, IP is responsible for managing the sending and receiving of data packets over the Internet. An IP packet consists of:

Protocol (the protocol of the packed encapsulated within the packet (i.e. TCP, UDP, etc.)
Trailer (not shown) - The trailer, sometimes called the footer, typically contains a couple of bits that tell the receiving device that it has reached the end of the packet.
It may also have some type of error checking. The most common error checking used in packets is Cyclic Redundancy Check (CRC). CRC is pretty neat. Here is how it works in certain computer networks: It takes the sum of all the 1s in the payload and adds them together. The result is stored as a hexadecimal value in the trailer. The receiving device adds up the 1s in the payload and compares the result to the value stored in the trailer. If the values match, the packet is good. But if the values do not match, the receiving device sends a request to the originating device to resend the packet.

The whole process can be summarized as follows:

The Application layer sends the data (to be transferred to remote destination) to the Transport layer.
The transport layer puts its header in the beginning and sends this complete packet (TCP-header + app-data) to the Iinternet layer.
Along the same lines, The Iinternet layer puts its header in front of the data received from Transport layer (Note that data received from TCP = TCP-header + app-data).
So now the structure of IP datagram becomes IP-header + TCP-header + app-data.
This IP datagram is passed to the Network Interface layer which adds its own header to the IP datagram and then the whole packet is transmitted over network. For an ethernet connection this looks like:

IP Routers
-----------

After the IP datagram has been assembled, the local machine ships it down the wire to a router that picks up the frame. Routers are very smart. They can look at the network type and, if necessary, reformat the Data layer information before passing the data onto the destination or another router. The router has a map of the local internet in its memory and each frame is sent off to its destination via the best available route. In the process the IP datagrams are just re-framed with different headers and frame source and frame destination addresses.

The router uses the technique of packet switching and all of the packets in the message may or may not take the same route. In this way the network can dynamically balance the load across various pieces of equipment. If there is a problem with one piece of equipment in the network while a message is being transferred, packets can be routed around the problem, ensuring the delivery of the entire message. The routers that make up the main part of the Internet can reconfigure the paths that packets take because they look at the information surrounding the data packet, and they tell each other about line conditions, such as delays in receiving and sending data and traffic on various pieces of the network.