Networking

OSI Model Layers

7. Application 6. Presentation 5. Session 4. Transport 3. Network 2. Data Link 1. Physical

The 8 steps in a DNS lookup - Step 1:

A user types 'example.com' into a web browser and the query travels into the Internet and is received by a DNS recursive resolver.

4 DNS servers involved in loading a webpage

DNS Recursor Root nameserver TLD nameserver Authoritative nameserver

1. Physical Layer

Define physical means of sending bits of data from one device to another, such as how the cables are attached to the network adapters or how data is transmitted over radio waves. The Physical Layer doesn't change the data in any way. Transmission and reception of raw bit streams over a physical medium

5. Session

Establish/maintain/manage end to end bidirectional flows between endpoints/applications. (data) Managing communication sessions, i.e. continuous exchange of information in the form of multiple back-and-forth transmissions between two nodes

Application of TCP

It helps you to establish/set up a connection between different types of computers. Operates independently of the operating system Supports many routing-protocols. It enables the internetworking between the organizations. It can be operated independently. Supports several routing protocols. TCP can be used to establish a connection between two computers.

DNS Step 6:

Lastly, the recursive resolver sends a query to the domain's nameserver.

7. Application

Provides different services to the application (Data) This is where the higher level protocols operate. Examples: HTTP, SMTP, FTP, DNS, DHCP, Telnet High-level APIs, including resource sharing, remote file access

DNS (Domain Name System)

Resolves domain names to IP so browsers can load internet resources. (Internet phonebook) An IP address is given to each device on the internet and that address is necessary to find the appropriate device. (Street address)

DNS Step 8:

The DNS resolver then responds to the web browser with the IP address of the domain requested initially. Once the 8 steps of the DNS lookup have returned the IP address for example.com, the browser is able to make the request for the web page:

DNS Step 7:

The IP address for example.com is then returned to the resolver from the nameserver.

DNS Step 5:

The TLD server then responds with the IP address of the domain's nameserver, example.com.

DNS Step 9:

The browser makes a HTTP request to the IP address.

DNS Recursor

The recursor can be thought of as a librarian who is asked to go find a particular book somewhere in a library. The DNS recursor is a server designed to receive queries from client machines through applications such as web browsers. Typically the recursor is then responsible for making additional requests in order to satisfy the client's DNS query.

DNS Step 4:

The resolver then makes a request to the .com TLD.

DNS Step 2:

The resolver then queries a DNS root nameserver (.).

Root nameserver

The root server is the first step in translating (resolving) human readable host names into IP addresses. It can be thought of like an index in a library that points to different racks of books - typically it serves as a reference to other more specific locations.

DNS Step 3:

The root server then responds to the resolver with the address of a Top Level Domain (TLD) DNS server (such as .com or .net), which stores the information for its domains. When searching for example.com, our request is pointed toward the .com TLD.

DNS Step 10:

The server at that IP returns the webpage to be rendered in the browser

TLD nameserver

The top level domain server (TLD) can be thought of as a specific rack of books in a library. This nameserver is the next step in the search for a specific IP address, and it hosts the last portion of a hostname (In example.com, the TLD server is "com").

Authoritative nameserver

This final nameserver can be thought of as a dictionary on a rack of books, in which a specific name can be translated into its definition. The authoritative nameserver is the last stop in the nameserver query. If the authoritative name server has access to the requested record, it will return the IP address for the requested hostname back to the DNS Recursor (the librarian) that made the initial request.

2. Data Link

This layer provides the functional and procedural means to transfer data between network entities and to detect and possibly correct errors that may occur in the Physical layer. At this layer, data packets are encoded and decoded into bits. Examples: 802.3 (Ethernet), ATM, Frame Relay, Switch Reliable transmission of data frames between two nodes connected by a physical layer

4. Transport

This layer provides transparent transfer of data between end systems, or hosts, and is responsible for end-to-end error recovery and flow control. It ensures complete data transfer. Segment (TCP) / Datagram (UDP) Examples: TCP, UDP, SPX Reliable transmission of data segments between points on a network, including segmentation, acknowledgement and multiplexing

6. Presentation

This layer tranforms data to provide a standard interface for the application layer. Data compression, encryption etc. (data) Examples: JPEG, MPEG, GIF, ASCII Translation of data between a networking service and an application; including character encoding, data compression and encryption/decryption

3. Network

This layer translates logical network address and names to their physical address. Routers operate at this level. (packet) Examples: IPv4, IPv6, IPX, IPSec, Routers. Structuring and managing a multi-node network, including addressing, routing and traffic control

TCP

Transmission Control Protocol - provides reliable, ordered, and error-checked delivery of a stream of packets on the internet. TCP is tightly linked with IP and usually seen as TCP/IP in writing. The transmission control protocol puts IP packets in sequence and checks for errors in transmission. It is a connection-oriented protocol. (Handshake) TCP reads data as streams of bytes, and the message is transmitted to segment boundaries. The speed for TCP is slower. Header size is 20 bytes Using handshake protocol like SYN, SYN-ACK, ACK TCP is reliable as it guarantees delivery of data to the destination router.

Application of UDP

UDP method is largely used by time-sensitive applications as well as by servers that answer small queries from a larger client base. UDP is compatible with packet broadcasts for sending all over the network and for multicasting sending. It is also used in Domain Name System, Voice over IP, and online games.

UDP

User Datagram Protocol. Used instead of TCP when guaranteed delivery of each packet is not necessary. UDP uses a best-effort delivery mechanism. It is a connectionless protocol. (No handshake) Header size is 8 bytes. UDP protocol has no fixed order because all packets are independent of each other. UDP is faster as error recovery is not attempted. No Acknowledgment segments The delivery of data to the destination can't be guaranteed in UDP. UDP has just a single error checking mechanism which is used for checksums.