internet edge
End systems are connected together by communication link and packet switch . Information travels in packets over communication links ; the two most important types of packet switches in today's Internet are routers and link -layer switches .
The end system accesses the Internet through the Internet Service Provider ( Internet Service Provider, ISP ). ISPs are interconnected with each other and managed independently.
End systems, packet switches, and other Internet components all run a series of protocols ( protocol ), which control the sending and receiving of information in the Internet. The main protocols of the Internet are collectively referred to as TCP/IP , among which TCP ( Transmission Control Protocol , Transmission Control Protocol) and IP ( Internet Protocol , Internet Protocol).
Another perspective :
End systems connected to the Internet provide a socket interface that specifies how a program running on one end system requests the Internet infrastructure to deliver data to a specific destination program on another end system . The Internet Sockets interface is a set of rules that the sending program must follow.
All activity on the Internet involving two or more communicating remote entities is governed by protocols. For example, a hardware-implemented protocol in two physically connected computers controls the flow of bits on the "wire" between two network interface cards; a congestion control protocol in an end system controls the rate at which packets are transmitted between a sender and receiver; Protocols in routers determine the path packets take from source to destination.
A protocol defines the format and sequence of messages exchanged between two or more communicating entities, and the actions to be taken for sending and/or receiving messages or other events .
Access Network
The network edge is various applications and end systems, and the network that physically connects the network edge devices to its edge router ( edge router ) is called the access network . An edge router is the first router on the path from any end system to any other remote end system.
The access network is divided into:
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- Home access network : DSL ( Digital Subscriber Line , digital subscriber line), cable ( cable Internet access ), FTTH ( Fiber To The Home , fiber to the home), dial-up and satellite
- Business (and Home) Access : Ethernet and WIFI
- Wide-area wireless access : 3G and LTE ( Long-Term Eolution , long-term evolution)
Physical media: divided into two types, guided media (guided media) and non-guided media (unguided media).
Twisted pair copper wire, coaxial cable, fiber optics, terrestrial radio channel, satellite radio channel
network core
The network core is a mesh network of packet switches and links interconnecting Internet end systems.
There are two basic methods of moving data through network links and switches: packet switching and circuit switching .
1. Packet switching ( packet switching )
end systems exchange messages ( messages ) with each other. In order to send a message from the source end system to the destination end system, the source end system divides the long message into smaller data blocks, called packets ( packet ), between source and destination, each packet is transmitted through communication links and packet switches. There are two main types of switches: routers and link-layer switches .
Store-and-Forward Transmission ( Store-and-Forward Transmission ), most packet switches use a store-and-forward transmission mechanism at the input end of the link. This means that the entire packet must be received before the switch can forward the first bit of the packet to the output of the link.
Queuing delay and packet loss ( Queuing Delays and Packet Loss ), each switch has multiple links connected to it, for each connected link, the packet switch has an output buffer ( output buffer , also called output queue , output queue ), which is used to store the packets that the router is going to send to that link. If the output buffer is full, there will be packet loss ( packet loss , packet loss).
Forwarding Tables and Routing Protocols ( Forwarding Tables and Routing Protocols ), how does a router decide which link to forward a packet to? Each router has a forwarding table , which is used to map the destination address (or a part of the destination address) into an output link. The Internet has some special routing protocols for automatically setting up forwarding tables.
2. Circuit switching ( circuit switching )
In a circuit switched network, the resources (buffers, link transmission rates) required along the path for communication between end systems are reserved during the communication session between end systems. In a packet-switched network, these resources are not reserved; messages for a session use resources on demand, so may have to wait (ie, queue) to access the communication link.
The multiplexing in the circuit switching network is realized by frequency -division multiplexing and time - division multiplexing .
Frequency division can be understood as parallel, each link uses different frequency spectrum to transmit at the same time; time division can be understood as concurrent, dividing the time domain into frames, each frame has a specific time slot (time slot), different link transmission uses different time slot.
Packet Switching vs Circuit Switching Comparison:
Packet switching can perform better than circuit switching. Circuit switching pre-allocates the use of transmission links regardless of demand, and link time allocated but not needed is unused. Packet switching, on the other hand, allocates link usage on demand. Link transfer capacity will be shared on a packet-by-packet basis only among users with packets that need to be transmitted over the link. Although packet switching and circuit switching are both prevalent in telecommunications networks today, the trend is definitely in the direction of packet switching. Even many of today's circuit-switched telephone networks are slowly migrating to packet switching. In particular, telephone networks often use packet switching to handle the expensive overseas portion of a telephone call.
network of networks
IXP ( Internet Exchange Point , Internet Exchange Point) is a confluence point where multiple ISPs can peer together ( peer ), that is, a pair of adjacent ISPs at the same hierarchical level directly connect their networks together.
This ecosystem is a network structure 4 consisting of access ISPs , regional ISPs , Tier 1 ISPs , PoPs , multi-home , peer-to-peer and IXPs.
Adding a content provider network on top of network structure 4 creates network structure 5 , which describes the Internet today.
Latency, packet loss, and throughput (amount of data that can be transmitted per second) in packet-switched networks
Delay : node processing delay ( nodal processing delay ), queuing delay ( queuing delay ), transmission delay ( transmission delay ) and propagation delay ( propagation delay ), these delays add up to the total node delay ( total nodal delay ).
Note the difference between transmission delay and propagation delay: transmission delay is the time required for a router to launch a packet, which is a function of packet length and link transmission rate, and has nothing to do with the distance between two routers; propagation delay is a The time it takes for a bit to propagate from one router to another is a function of the distance between the two routers and is independent of the packet length and link transmission rate.
Queuing delay is the most complex and interesting component of node delay. For example: R is the link transmission rate (the rate at which bits are pushed out from the queue, bps, bits per second), a represents the average rate at which packets arrive at the queue (packets/second), and a packet consists of L bits, then the average rate at which bits arrive at the queue The rate is: La bps, the ratio: La/R is called (traffic intensity).
La/R > 1 will cause packet loss.
Throughput is limited by the bottleneck link ( bottleneck link ) in the connection link , that is, the link with the smallest throughput in the transmission path.
Protocol Hierarchy and Service Model
Protocol layering , a protocol layer can be implemented by software, hardware or a combination of both. Application layer and transport layer protocols are generally implemented in software, and network layer protocols are a mixture of software and hardware implementations. All of these are combined and all protocols at each layer are called protocol stacks (protocol stack). The protocol stack of the Internet consists of 5 layers: physical layer, link layer, network layer, transport layer, and application layer.
- Application layer : The application layer protocol is distributed over multiple end systems, and an application on one end system uses the protocol to exchange packets of information with an application on another end system. At the application layer , this information is grouped as a message .
- Transport layer : The transport layer protocols in the Internet are TCP and UDP. TCP provides connection-oriented services, flow control and congestion control. The packets of the transport layer are called segments .
- Network layer : The network layer is responsible for moving network layer packets called datagrams from one host to another, routing datagrams through a series of routers between source and destination . The well-known protocol at the network layer is the IP protocol, and there are many other routing protocols.
- Link Layer : In order to move packets from one node to the next node on the path, the network layer relies on link layer services. There are many protocols in the link layer, such as Ethernet, WiFi and cable access network DOCSIS protocol. A datagram may be handled by different link-layer protocols. Link layer packets are called frames .
- Physical layer : The job of the physical layer is to move individual bits from one node to the next. The protocol is related to the specific link transmission medium. Ethernet has many physical layer protocols.
OSI model : open system interconnection model, application layer, presentation layer, session layer, transport layer, network layer, link layer, physical layer.
Package:
Network attacks:
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A virus is a type of malware that requires some form of user interaction to infect a user's device;
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A worm is a type of malware that enters a device without any apparent user interaction.
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Denial-of-Service ( DoS) attack , most Internet DoS attacks fall into three categories: vulnerability attacks, bandwidth flooding, and connection flooding.
- Vulnerability attack : Sending crafted packets to vulnerable hosts or operating systems .
- Bandwidth flooding : The attacker sends a large number of packets to the target, and legitimate packets cannot reach the server.
- Connection flooding ( Connection flooding ): The attacker creates a large number of half-open or fully-open TCP connections on the target host, and the host is in trouble.
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Distributed DoS ( Distributed DoS, DDoS ), the attacker controls multiple distributed sources to attack.
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Packet sniffing and IP spoofing .
Digression
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