The software interacts with the network and the OS

     The software interacts with the network and the OS

What are the OSI Layers from top to bottom?

 

·         Application, Presentation, Session, Transport, Network, Data link and Physical

 

What are the TCP/IP Layers?

 

·         Application, Transport, Internet and Network Access

 

What happens at the application layer? (OSI)

 

·         The software interacts with the network and the OS

 

What happens at the Presentation layer?

 

·         Translation: Networks can connect very different types of computers together: PCs, Macintoshes, UNIX systems, AS/400 servers and mainframes can all exist on the same network. These systems have many distinct characteristics and represent data in different ways; they may use different character sets for example. The presentation layer handles the job of hiding these differences between machines.

Compression: Compression (and decompression) may be done at the presentation layer to improve the throughput of data. (There are some who believe this is not, strictly speaking, a function of the presentation layer.)

Encryption: Some types of encryption (and decryption) are performed at the presentation layer. This ensures the security of the data as it travels down the protocol stack. For example, one of the most popular encryption schemes that is usually associated with the presentation layer is the Secure Sockets Layer (SSL) protocol. Not all encryption is done at layer 6, however; some encryption is often done at lower layers in the protocol stack, in technologies such as IPSec.

 

What happens at the Presentation layer?

 

·         Translation: Networks can connect very different types of computers together: PCs, Macintoshes, UNIX systems, AS/400 servers and mainframes can all exist on the same network. These systems have many distinct characteristics and represent data in different ways; they may use different character sets for example. The presentation layer handles the job of hiding these differences between machines.

Compression: Compression (and decompression) may be done at the presentation layer to improve the throughput of data. (There are some who believe this is not, strictly speaking, a function of the presentation layer.)

Encryption: Some types of encryption (and decryption) are performed at the presentation layer. This ensures the security of the data as it travels down the protocol stack. For example, one of the most popular encryption schemes that is usually associated with the presentation layer is the Secure Sockets Layer (SSL) protocol. Not all encryption is done at layer 6, however; some encryption is often done at lower layers in the protocol stack, in technologies such as IPSec.

 

What happens at the Session layer?

 

·         Maintain communication. "Handshake"

 

What happens at the Transport layer?

 

·         Error correction, application separation (ports).

 

What are the 2 main transport protocols?

 

·         TCP, UDP

 

TCP is unreliable

 

·         False. TCP asks for acknowledgement after a packet is sent to its destination (confirms that the packet was delivered).

 

UDP is Unreliable

 

·         True. UDP does not ask for confirmation after a package is delivered. This is mostly used for real-time communication such as VoIP or Live communication.

 

What happens at the Network layer?

 

·         Find the best possible route the get to its destination (end to end). Logical addressing. It never changes.

Logical Addressing: Every device that communicates over a network has associated with it a logical address, sometimes called a layer three address. For example, on the Internet, the Internet Protocol (IP) is the network layer protocol and every machine has an IP address. Note that addressing is done at the data link layer as well, but those addresses refer to local physical devices. In contrast, logical addresses are independent of particular hardware and must be unique across an entire internetwork.

Routing: Moving data across a series of interconnected networks is probably the defining function of the network layer. It is the job of the devices and software routines that function at the network layer to handle incoming packets from various sources, determine their final destination, and then figure out where they need to be sent to get them where they are supposed to go. I discuss routing in the OSI model more completely in this topic on the topic on indirect device connection, and show how it works by way of an OSI model analogy.

Datagram Encapsulation: The network layer normally encapsulates messages received from higher layers by placing them into datagrams (also called packets) with a network layer header.

Fragmentation and Reassembly: The network layer must send messages down to the data link layer for transmission. Some data link layer technologies have limits on the length of any message that can be sent. If the packet that the network layer wants to send is too large, the network layer must split the packet up, send each piece to the data link layer, and then have pieces reassembled once they arrive at the network layer on the destination machine. A good example is how this is done by the Internet Protocol.

Error Handling and Diagnostics: Special protocols are used at the network layer to allow devices that are logically connected, or that are trying to route traffic, to exchange information about the status of hosts on the network or the devices themselves.

 

What happens at the Data Link layer?

 

·         Find the local area network the packet is been sent to. Physical address. Changes as it goes from network to network.

Data Link Layer Sublayers: Logical Link Control (LLC) and Media Access Control (MAC)
The data link layer is often conceptually divided into two sublayers: logical link control (LLC) and media access control (MAC). This split is based on the architecture used in the IEEE 802 Project, which is the IEEE working group responsible for creating the standards that define many networking technologies (including all of the ones I mentioned above except FDDI). By separating LLC and MAC functions, interoperability of different network technologies is made easier, as explained in our earlier discussion of networking model concepts.

Data Link Layer Functions
The following are the key tasks performed at the data link layer:

Logical Link Control (LLC): Logical link control refers to the functions required for the establishment and control of logical links between local devices on a network. As mentioned above, this is usually considered a DLL sublayer; it provides services to the network layer above it and hides the rest of the details of the data link layer to allow different technologies to work seamlessly with the higher layers. Most local area networking technologies use the IEEE 802.2 LLC protocol.

Media Access Control (MAC): This refers to the procedures used by devices to control access to the network medium. Since many networks use a shared medium (such as a single network cable, or a series of cables that are electrically connected into a single virtual medium) it is necessary to have rules for managing the medium to avoid conflicts. For example. Ethernet uses the CSMA/CD method of media access control, while Token Ring uses token passing.

Data Framing: The data link layer is responsible for the final encapsulation of higher-level messages into frames that are sent over the network at the physical layer.

Addressing: The data link layer is the lowest layer in the OSI model that is concerned with addressing: labeling information with a particular destination location. Each device on a network has a unique number, usually called a hardware address or MAC address, that is used by the data link layer protocol to ensure that data intended for a specific machine gets to it properly.

Error Detection and Handling: The data link layer handles errors that occur at the lower levels of the network stack. For example, a cyclic redundancy check (CRC) field is often employed to allow the station receiving data to detect if it was received correctly.

 

What happens at the Physical layer?

 

·         Passes the bits (01s) using electric or optical signals.

Definition of Hardware Specifications: The details of operation of cables, connectors, wireless radio transceivers, network interface cards and other hardware devices are generally a function of the physical layer (although also partially the data link layer; see below).

Encoding and Signaling: The physical layer is responsible for various encoding and signaling functions that transform the data from bits that reside within a computer or other device into signals that can be sent over the network.

Data Transmission and Reception: After encoding the data appropriately, the physical layer actually transmits the data, and of course, receives it. Note that this applies equally to wired and wireless networks, even if there is no tangible cable in a wireless network!

Topology and Physical Network Design: The physical layer is also considered the domain of many hardware-related network design issues, such as LAN and WAN topology.

In general, then, physical layer technologies are ones that are at the very lowest level and deal with the actual ones and zeroes that are sent over the network. For example, when considering network interconnection devices, the simplest ones operate at the physical layer: repeaters, conventional hubs and transceivers. These devices have absolutely no knowledge of the contents of a message. They just take input bits and send them as output.

 

What happens at the Application layer (TCP/IP)

 

·         The software interacts with the network or OS, encryption, reformatting, and starts communication.

 

What happens at the Internet layer?

 

·         The data is packed into IP diagrams which contains the source and destination logical addresses.

 

What happens at the Network Access layer?

 

·         Defines how the data is physically sent through the network using hardware devices such as coaxial cables, fiber optic, switches and more.

 

Application layer protocols

 

·         HTTP, HTTPS, FTP, SSH, SMTP, DHCP, NFS, Telnet, SNMP, POP3, NNTP and IRC

 

Some common Presentation layer formats are

 

·         Graphics files JPEG, TIFF, GIF, and so on are graphics file formats that require the data to be formatted in a certain way.

Text and data The presentation layer can translate data into different formats such as American Standard Code for Information Interchange (ASCII) and the Extended Binary Coded Decimal Interchange Code (EBCDIC), or Hypertext Markup Language (HTML).

Sound/video MPEGs, QuickTime video, and MIDI files all have their own data formats to and from which data must be converted.

 

Some common network protocols are:

 

·         ARP, SPX/IPX, Apple Talk, IP (IPv4, IPv6), ICMP, IGMP, RIP, EGP

 

Some common Data Link layer protocols are:

 

·         PPP (Point-to-Point Protocol) and SLIP (Serial Line Internet Protocol).

 

FTP Ports

 

·         20, 21

 

SHH Ports

 

·         22

 

Telnet ports

 

·         23

 

SMTP ports

 

·         25

 

DNS ports

 

·         53