Computer Networks and the Internet
1. What is the Internet
Internet: Internet; supported by tcp/ip protocol for transmission
internet: Internet
A network consists of nodes and edges
node:
- Host node:
- Data interaction nodes: divided into repeaters, interactors (link layer), and routers (network layer) by different levels.
Edge: communication link
- Access network link: The link that connects the host to the Internet
- Backbone link: the link between routers
Protocols: Distinguished according to levels
- Application layer: Different applications have different message parsing protocols, such as HTTP and FTP
- Transport layer: Mainly divided into two types: TCP and UDP protocols
- Network layer: mainly based on IP protocol
- Link layer and logistics layer: several
Internet composition
- A bunch of networks are interconnected through network interconnection devices
Network interaction equipment: network interactive machine (link layer), router (network layer)
What is a protocol?
IP Internet Protocol is the core protocol of the Internet network layer. The emergence of virtual Internet networks: The actual computer network is intricate; physical devices shield the differences between physical networks by using IP protocols; when hosts in the network are connected using IP protocols, there is no need to pay attention to network details, thus forming a virtual network.
The IP protocol turns a complex actual network into a virtual interconnected network; and solves the problem of datagram transmission paths in the virtual network.
- PDU: Protocol Data Unit
From a service perspective
- Internet: It is distributed applications and the infrastructure that provides services for distributed application processes
- The services provided are mainly divided into: connection-oriented (TCP) and connectionless (UDP) services
2. Network structure
- It is composed of network edge, network core and access point, so that different host devices can exchange messages and interconnect.
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2.1. Network edge:
- Internet-connected host
- Applications on the host (client and server)
- The main modes of interaction between application processes and application processes are:
- client/server mode
- such as web services
- Peer-to-peer mode (P2P mode)
- Such as Xunlei, and other file distribution systems
- client/server mode
- Use infrastructure to provide communication services to application processes:
- TCP service:
- Reliable, no disorder, no loss; and features flow control and congestion control
- Such as: HTTP, FTP, SMTP
- UDP service:
- Connectionless, unreliable, no flow control and congestion control
- Such as streaming media, remote phone calls, DNS
- TCP service:
2.2. Network core:
- Network of networks (interconnected link nodes)
- interconnected routers
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How is data transmitted over the network?
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Electrical (wire) circuit interaction : exclusive resources
- Not shared (performance is guaranteed once each call is established)
- If no data is sent during the call, the allocated resources will be wasted.
- Often used by traditional telephone networks
- Store and forward in line or time units
- Mainly divided into FDM and TDM
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Group interaction : store-forward in groups
- Store all the packets, forward them to the next link, store them, and forward them again.
- Network bandwidth resources are no longer divided into slices, and the entire bandwidth is used during transmission.
- Data transmitted between hosts is divided into packets
- Advantages: It can share resources and use them on demand.
- Disadvantages: nodes must receive the entire packet before forwarding; delay is greater than line switching, queuing time
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Key Functions of Network Core
- Routing: Determine the source-to-destination path taken by the packet through routing table calculations
- Forwarding: moving packets from a router's input link to an output link
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The packet interaction network is divided into two services
- Datagram Network:
- Datagrams do not need to establish a connection. Data is transmitted as soon as it is available.
- Each packet is routed independently (paths are different and may be out of order)
- The router routes based on the destination address of the packet
- Virtual circuit network:
- Establishing a connection requires creating a virtual circuit table on each node, and each group will use a short virtual circuit number.
- Datagram Network:
2.3. Access network and physical media
- Via wired or wireless communication links
- How to connect end systems to edge routers?
- Residential access network (modem): fiber to the home - cable TV
- unit access network
- Wireless access network: wireless LANs, wide area wireless access
- Physical medium:
- Conductor medium: coaxial optical fiber, optical fiber
- Invisible medium: wireless link
3. Packet delay, loss and throughput
3.1. Packet delay
- Node Processing Latency
- Time spent checking routing table
- queuing delay
- Random, depending on traffic intensity = packet length * average rate of packet arrival queue/bandwidth
- Transmission delay
- The larger the data transmitted, the higher the delay.
- propagation delay
- The delay caused by one node propagating to another node (the longer the distance, the higher the delay)
3.2. Lost
- When lost, three things will happen
- Retransmission from previous link node
- Source host transmits from
- Do not re-upload
Throughput
The size of the throughput depends on the bandwidth of the smallest link
4. Protocol levels and service models
4.1. Protocol level: The implementation of the protocol requires the lower layer to provide services to the upper layer.
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The protocol is implemented through the lower layer interface.
Each layer implements a service or a set of services
**Service: Within an entity, it provides services to the upper layer next time, or the upper layer provides services to the lower layer ; the bottom entity provides the ability of communication between them to the upper layer entity
Service users: specific applications
Service access point: port (socket)
Service provider: such as TCP protocol
Type of service:
Connection-oriented service: TCP
Two peer layer entities do not need to establish a connection before establishing communication.
Connectionless service: UDP
Two peer layer entities do not need to establish a connection before establishing communication.
4.2. Relationship between SDU and PDU
- SDU: Service data unit; also called business data unit, it is a data set of user services at a designated layer. When transmitted to the receiver, the data does not change at the same protocol layer, that is, the business part. After it is sent to the lower layer, the lower layer will Encapsulated in PDU and sent out. Service data units are information units from higher-layer protocols that are transferred to lower-layer protocols. The Nth layer service data unit SDU has a one-to-one correspondence with the protocol data unit (PDU) of the upper layer. Depending on the data in the protocol data unit, it is sent to the designated layer at the receiving end.
- PDU: Protocol data unit; unit information exchanged between peer entities at each layer of the computer network; for example, the PDU of the TCP layer is segment, and the PDU exchanged between the application layers is application data.
PDU(N) = SDU(N-1)
SDU(N) = PDU(N-1)
Simple understanding is:
The PDU of this layer is the SDU of the lower layer; the PDU of this layer plus the header information of this layer is also composed of the PDU of this layer.
The SDU of this layer is the PDU of the upper layer;
4.3. Internet protocol stack (data unit)
- Application layer: web application
- Provide network application services (message) for human users or other application processes
- FTP、SWTP、HTTP、DNS
- Presentation layer (ISO reference model)
- Session layer (ISO reference model)
- Transport layer: data transmission between hosts (segment)
- Based on the end-to-end communication provided by the physical layer, it is subdivided into process-to-process, turning unreliable communication into reliable communication.
- TCP、UDP
- Network layer: Select routing (packet) for datagrams from source to destination (connectionless mode: datagram-datagram)
- Complete the communication between the source host and the target host, end-to-end communication, unreliable
- IP, routing protocol
- Link layer: data transmission between adjacent network nodes (frame)
- Communication between 2 adjacent points, point-to-point communication, unreliable or reliable
- Point-to-point protocol PPP, 802.11 (WIFI), Ethernet
- Physical layer: transmitting bits (bits) on the wire
