Now, these are my personal notes that I took during studying for ICND1. Notes are harsh and if you are beginner, they will not make much sense to you as I took them for my own study principle. Anyway, please be free to read it as you will probably find something interesting and new to learn about.
History leading to TCP/IP
- The world now uses only TCP/IP model.
- Not so long time ago, vendors manufactured and created protocols but supported only that vendor’s computers. For example, IBM published Systems Network Architecture (SNA) networking model in 1974. As a result, if your company bought computers from three vendors, network engineers often had to create three different networks based on the networking models created by each company, and then somehow connect those networks, making the combined networks much more complex.
- Although vendor-defined proprietary networking models often worked well, having an open vendor-neutral networking model would help competition and reduce complexity.
- The International Organization Standardization ISO wanted to create a model-free vendor-neutral networking model. ISO created a model called the OSI Open Systems Interconnection networking model in the 1970s. ISO wanted to standardize protocols and enable communication between computers. However, US Department of Defense DoD wanted to further develop protocols to make communication independent to other vendors and TCP/IP was born. By the end of the 1990s, TCP/IP began to dominate networks.
Overview of TCP/IP Networking model
- To define TCP/IP protocol, Requests For Comments (RFC) are used.
- Everything is connected, already TCP/IP built-in in other computers, phones, cables. When you buy new phone, plug it to old cables, it works because cables work with TCP/IP. New phone works, new PC works, connects to the Internet, etc. That is due to already built in TCP/IP standards and protocols that work together.
- TCP/IP original RFC 1122 breaks TCP in 4 layers
- TCP/IP Updated divides Link into Data Link and Physical.
TCP/IP application layer
- Provides services to the application software.
- Application layer does not define application itself, but the application’s services that needs.
- Application layer provides interfaces between software running on a computer and a network itself.
- The most popular TCP/IP application is web browser.
HTTP Protocol Mechanisms:
- To make a request for a web page and get a response from a server, applications use HTTP.
- HTTP was invented by Tim Berners-Lee in 1990s.
- Uniform Resource Locators URL or Universal Resource Identifiers URI
- Bob requests GET home.html website for him
- Larry responses with HTTP header 200 (OK) and home.html
- Larry sends more home.html data to save space.
NOTE: header code is sent only once and not in every response by Larry
TCP/IP transport layer
- includes smaller number of protocols.
- most common are TCP and UDP
- transport layer protocols provide services to the application layer protocols
TCP Error Recovery
- To recover from errors and lost packets, TCP uses the concept of acknowledgments.
- When the packet is being lost, and the receiver realizes that it is missing, it will send TCP header request for that segment.
Same layer and adjacent layer interactions:
- In the example above, we have seen that application layer (HTTP) needs help and it asks lower layer services to act upon.
- The lower layer provides service to the layer above it.
- In order for two computers to communicate using TCP/IP, they will need same-layer interactions. In example above, Larry uses sequence numbers to help Bob figure out which SEQ is missing, if that happens.
TCP/IP network layer
- IP provides several features, most importantly, addressing and routing.
- Each host that uses TCP/IP needs unique address in order to be identified in the network.
- IP uses dotted-decimal notation DDN to create IPv4 address 18.104.22.168
ROUTERS ARE NETWORKING DEVICES THAT CONNECT THE PARTS OF THE TCP/IP NETWORK TOGETHER FOR THE PURPOSE OF ROUTING IP PACKETS TO THE CORRECT DESTINATION.
TCP/IP link layer
- TCP/IP link layer defines protocols and hardware required to deliver data across some physical network.
- Term link refers to the physical connections, or links, between two devices and the protocols used to control those links.
- Link layer also provides services to the layer above it
- Link layer includes two functions: 1) Functions related to the physical transmission of the data, 2) the protocols and rules that control the use of that physical media.
Data link = encapsulation, addressing Physical = bit transmission