Hello everyone!
Today, we will learn about the OSI reference model and the TCP/IP reference model. Before learning, let's ask you a question: How does our data communicate?
Maybe you think we can communicate because we have network protocols. Yes, our data cannot communicate without network protocols, but our data cannot communicate with each other only by network protocols. A basic data communication system consists of five parts, among which the key part is the network protocol.
What is a network protocol?
In a computer network, a set of formats and conventions that communication parties understand and comply with our pre-defined to enable different devices on the network to communicate with each other. Network protocols are standard descriptions of a series of rules and conventions.
In addition to the rapid growth of computers, vendors have launched their own network architectures and standards to dominate the data communication network field. However, on a network, the sender and receiver must comply with the same standard for communication between different devices. However, a single large protocol makes network design more difficult and makes it difficult to analyze and locate problems. Therefore, the computer model introduces the concept of layering.
To solve the compatibility problem between networks and help vendors produce compatible network devices, the International Organization for Standardization (ISO) proposes the Open Systems Interconnection Reference Model (OSI/RM).
The OSI reference model is designed based on the following principles:
There are clear boundaries between the layers and each layer implements a specific function.
Hierarchy is conducive to the formulation of international standard protocols.
The number of layers is sufficient to avoid duplication of functions among the layers.
OSI seven-layer model
7. Application layer: provides inter-application communication (PDU).
6. Presentation layer: processes data formats and encrypts data (PDU).
5. Session layer: establishes, maintains, and manages sessions (PDUs).
4. Transport layer: Set up end-to-end host connections (segment).
3. Network layer: addressing and routing (packets)
2. Data link layer: provides media access and link management (data frames).
1. Physical layer: bitstream transmission (bitstream)
Layers 1 to 3 are the lower layer, also called the media layer, and are mainly responsible for data transmission in the network. This feature is implemented by combining hardware and software.
The 4 layer is called the transport layer and is responsible for establishing connection-oriented or connectionless transmission channels.
Layers 5 to 7 are called the upper layer, also called the host layer. They are mainly used to ensure correct data segment transmission. They are usually implemented by software.
The OSI reference model has the following features:
Network operations are simplified.
Provides plug-and-play compatibility and standard interfaces between different vendors.
Enables vendors to design interoperable network devices to facilitate standardization. Network changes in one area can be isolated from those in another area. Therefore, each area can be upgraded independently and quickly.
Complicated network problems are divided into simple problems, which are easy to learn and operate.
The OSI 7-layer protocol architecture clearly defines the network structure and has a complete theory. However, it is relatively complex and not practical. With the development of the Internet, the TCP/IP 5-layer architecture is widely used.
TCP/IP layered model
The TCP/IP protocol stack is a collection of data communication protocols, including many protocols. Its protocol stack name derives from the two main protocols, TCP
(Transmission Control Protocol) and IP (Internet Protocol). The TCP/IP protocol stack ensures that network devices can communicate with each other. It is a set of rules that govern how information is transmitted over the network.
The difference between the TCP/IP model and the OSI reference model is that the presentation layer and the session layer are classified into the application layer.
Different layers of the TCP/IP model correspond to different protocols.
The following describes the functions of each layer of the TCP/IP model:
Physical layer
The physical layer transparently transmits bitstreams. When the sender sends 1 or 0, the receiver should receive 1 or 0 instead of 0 or 1. To achieve this purpose, the physical layer needs to implement the following functions:
(1) Specify the media type, interface type, and signaling type.
(2) Defines electrical, mechanical, process, and functional requirements for activating, maintaining, and closing physical links between end systems.
(3) Standardize the voltage level, data transmission rate, maximum transmission distance, and physical connectors.
The physical layer standard specifies the physical medium and the connector used to connect the device to the physical medium.
Common physical layer standards for LANs include the IEEE-defined Ethernet standard 802.3, token bus standard 802.4, token ring network standard 802.5, and FDDI (fiber distributed data interface) defined by ANSI (the American National Standards Institute) X3T9.5 committee.
Common physical layer standards for WANs include the EIA/TIA-232 (RS-232),
V.24 and V.35, and physical and electrical specifications of various digital interfaces. G.703, etc.
Physical layer media include coaxial cables, twisted pairs, optical fibers, and radio waves.
Devices at the physical layer are repeaters and hubs, but these are rarely used as networks develop.
Datalink layer
The data link layer is the second layer in the OSI reference model, between the physical layer and the network layer. It encapsulates data from the network layer into frames from the top-down, divides bitstreams from the physical layer into frames from the bottom up, and controls transmission on the group and object channels, including how to handle transmission errors and adjust the transmission rate to match the receiver, and manages the establishment, maintenance, and release of data link paths between two network entities.
The data link layer is divided into two sublayers: LLC, Logic Link Control sublayer, MAC, Media Access Control sublayer.
The LLC sublayer is located between the network layer and the MAC sublayer. It identifies the protocol type and encapsulates data for transmission over the network.
The LLC sublayer performs most functions of the data link layer and some functions of the network layer. For example, a frame consists of the data to be transmitted, address, and CRC check. When the frame is received, the frame is split, and the address identification and CRC check are performed. In addition, the frame sequence control, error control, and flow control functions are provided. In addition, it performs some network layer functions such as datagram, virtual-circuit, and multiplexing.
The MAC sublayer specifies how data is transmitted over physical lines and communicates with the physical layer. It defines physical addressing, network topology, line specifications, error notification, in-order delivery, and traffic control.
Data Link Layer Address and Protocols
As if everyone has a name, each network device is identified by a physical address, which is a MAC address. The MAC address of a network device is globally unique. A
MAC address consists of 48 binary bits, which are usually represented by hexadecimal digits. The first six hexadecimal digits are allocated by the IEEE to devise manufacturers, and the last six hexadecimal digits are allocated by each manufacturer.
For example, the first six hexadecimal digits of the MAC address of Huawei network products are 0x00e0fc.
NIC, Network Interface Card, also called network adapter, which has a fixed MAC address. Most NIC vendors burn MAC addresses into the ROM. When the network adapter is initialized, the MAC physical address in the ROM is read into the RAM. If a new network card is inserted into the computer, the physical address of the computer becomes the physical address of the new network card.
It's worth noting that if your computer has two network adapters, it has two MAC addresses. Therefore, some network devices may have multiple MAC addresses.
IEEE 802.2 LLC is a common data link layer protocol for LANs.
Common data link layer protocols used in WANs include HDLC（High-level Data Link Control), PPP（Point-to-Point Protocol), FR（Frame Relay), and so on.
HDLC is a bit-synchronous-oriented data link layer protocol developed by ISO. It specifies the data encapsulation method of synchronous serial links using frame characters and checksum.
PPP is defined in RFC 1661. It consists of the Link Control Protocol (LCP),
Network Control Protocol (NCP), and PPP extended protocol suite. PPP supports synchronous and asynchronous serial links and multiple network-layer protocols.
PPP is the default encapsulation protocol of the data link layer of the serial interface on a VRP router.
FR is an industry-standard switched data link protocol that uses the error-free check to speed up data forwarding.
The most common devices at the data link layer are Ethernet switches.
Network layer
The network layer is responsible for forwarding packets from source to destination between networks.
When sending data, the network layer encapsulates the packet segment generated by the transport layer and the header information of the network layer into a packet for transmission. When receiving the data, the network layer processes the packet according to the header information added by the peer end.
The network layer provides the following functions:
Provide logical addresses: The network layer defines an address that uniquely identifies a network device, if data is transmitted across networks (cross-link), logical addresses are used for addressing.
Network layer addresses are IP addresses in the TCP/IP model. Currently, there are two versions of IP addresses: IPv4 address and IPv6 address. Currently, IPv4 addresses are widely used on live networks. An IPv4 address is usually expressed in dotted-decimal notation, for example, 10.8.2.48.
Routing: forwards data packets from one network to another network.
Routes determine the path through which packets are forwarded from the source to the destination.
Common network layer protocols: Internet Protocol (IP), ICMP (Internet Control Message Protocol), ARP (Address Resolution Protocol), RARP (Reverse Address Resolution Protocol).
IP is the most important protocol at the network layer. It provides logical addressing, routing, packet encapsulation, and de-encapsulation functions. ICMP, ARP, and RARP assist IP.
The IP protocol provides a globally unified addressing mode, which shields the differences of physical network addresses and makes route lookup possible. It also provides a globally unified packet format to shield the differences at the network link layer and make network interconnection possible.
IP is one of the most important protocols in the TCP/IP architecture.
ICMP is a management protocol and provides information services for IP. ICMP messages are carried in IP packets.
ARP implements dynamic mapping between IP addresses and hardware addresses, that is, hardware addresses are obtained based on known IP addresses.
RARP implements dynamic mapping between hardware addresses and IP addresses. That is, RARP obtains IP addresses based on known hardware addresses.
Routers are commonly used at the network layer to forward packets between different networks.
As shown in the figure, Host A and Host B on different networks (different links) communicate with each other. The router interface on the same network (the same link) as Host A receives the data frame sent by Host A. The link layer of the router analyzes the frame header and determines the frame to be sent to itself. Then, the router sends the frame to the network layer. The network layer determines the network segment where the destination address resides according to the network layer packet header. Then, the network layer forwards the frame to the next-hop through the corresponding interface according to the table until the packet reaches the destination Host B.
Thanks for reading!
