What is the Internet?
The Internet can be understood in two different ways. (its composition. its service)
1. The main components of the Internet
At the edge of the Internet are all hosts on the Internet. These hosts are also called end systems . End systems access the Internet through Internet Service Providers (Internet Service Providers, ISPs). (communication link) and packet switch (packet switch) are connected together. Different links can transmit data at different rates, and the transmission rate of a link is measured in bits per second (bit/s, or bps). When one end system sends data to another end system, the sender divides the data into segments and adds a header byte to each segment. The resulting information packets are called packets, and these packets are sent to the destination end system through the network. There are assembled as initial data.
A packet switch receives an arriving packet on one of its incoming communication links and forwards the packet on one of its outgoing communication links. Two well-known types of packet switches are routers and link-layer switches . Link layer switches are typically used in the access network, while routers are typically used in the network core. From the sending end system to the receiving end system, the series of communication links and packet switches that a packet goes through is called the path through the network .
End systems, packet switches, and other Internet components run a set of protocols that control the receiving and sending of information across the network. TCP (Transmission Control Protocol, Transmission Control Protocol) and IP (Internet Protocol, Internet Protocol) are the two most important protocols in the Internet. The main protocols of the Internet are collectively referred to as the TCP/IP protocol.
2. Internet service description
Looking at the Internet from another perspective—the infrastructure that provides services for applications, it also includes many applications, and these applications involve multiple end systems that exchange data with each other, so they are called distributed applications . Internet applications run on end systems , ie they do not run on packet switches in the core of the network.
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 running on another end system , the socket interface is a set of rules that the sending program must follow. (To illustrate with an example) Alice uses the postal service to send a letter to Bob, Alice can't just write the letter and throw it out the window. Instead, the postal service requires Alice to put the letter in an envelope; write Bob's full name, address, and zip code on the outside of the envelope; seal the envelope, affix a stamp; and finally place the envelope in a letter box. The postal service has its own "postal service interface", and the same Internet also has a socket interface that the program that sends data must follow, so that the Internet delivers data to the program that receives the data.
What is a network protocol?
Protocol : A collection of rules for communication between two or more communicating peer entities.
What is the network edge?
Computers and other devices connected to the Internet are often referred to as end systems, and because they are at the edge of the Internet , they are called end systems.
End systems are also called hosts because they house (run) applications, and hosts are sometimes further divided into: clients and servers
What is an access network?
An access network is the network that physically connects end systems to their edge routers . An edge router is the first router on the path from an end system to any other remote end system.
What are the common physical media?
A bit is transmitted from one end system to another end system, passing through a series of links and routers, and the two routers form a "transmitter-receiver" pair. For each "transmitter-receiver" Yes, the bits are sent by propagating electromagnetic waves or pulses of light across a physical medium. There are two types of physical media: guided media and unguided media . For guided media, radio waves travel along solid media, such as fiber optic cables, copper wires, cables, etc.; for non-guided media, radio waves propagate in the air or outer space, such as in wireless local area networks or digital satellite channels.
Common physical media are: twisted pair copper wire, coaxial cable, fiber optics, terrestrial radio channel, satellite radio channel.
What is network core?
The core of the network is a mesh structure consisting of packet switches and links of the end systems.
Packet switching : In various network applications, end systems exchange messages (messages) with each other, and the messages can contain anything the protocol designer needs. To send a message from a source system to an end system, the source divides the message into smaller chunks of data called packets . Between the source end and the destination end, each packet is transmitted through a communication link and a packet switch (packet switch). (There are two main types of switches: routers and link switches)
Store-and-forward transmission : Most packet switches use a store -and-forward transmission mechanism at the input of the link . Store-and-forward transmission means that the entire packet must be received before the switch can begin transmitting the first bit of the packet to the output link. In other words, only when the switch has completely received all the bits of the packet can it start transmitting (forwarding) the packet out the link.
Queuing delay and packet loss : Each packet 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. Output buffering plays an important role in packet switching, if an arriving packet needs to be transmitted to a certain link, but the link is found to be busy transmitting other packets, the arriving packet must wait in the output buffer. Therefore, in addition to the store-and-forward delay, the packet also bears the queuing delay of the output buffer . These queuing delays vary, depending on how congested the network is. Because the size of the buffer space is limited, if the buffer space is filled with packets waiting to be transmitted, a packet loss (packet loss) will occur at this time, and the arriving packet or one of the queued packets will be discarded .
Forwarding table and routing protocol : A router connected to a communication link gets a packet, and then forwards the packet to another communication link connected to it. But how does a router decide which link it should forward to?
In the Internet, each end system has an address called an IP address. When the source host wants to send a packet to the destination end system, the source includes the IP address of the destination in the header of the packet. When a packet arrives at a router in the network, the router checks a part of the destination address of the packet and sends an IP address to a The neighboring router of the station forwards the packet. Each router has a forwarding table that maps destination addresses to outgoing links. (When a packet arrives at a router, the router examines the address and uses the destination address to search its forwarding table to find the appropriate outgoing link. The router then directs the packet to that outgoing link).
How is the forwarding table set up?
The Internet has some special routing protocols for automatically setting up the above-mentioned forwarding tables.
Latency, Packet Loss and Throughput in Packet Switched Networks
A packet starts at one host (source), travels through a series of routers, and ends its journey at another host (destination). As a packet follows this path from one node (host or router) to subsequent nodes (host or router), the packet experiences several different types of delays at each node along the way. There are node processing delays, queuing delays, transmission delays, and propagation delays .
As part of end-to-end routing between source and destination, a packet is sent from an upstream node to Router B through Router A, which has an outgoing link to Router B preceded by a queue (also called caching). Only when no other packets are being transmitted on the link and no other packets are queued in front of the queue, the packet can be transmitted in this queue; if the link is currently busy or other packets are already queued on the link, Then the newly arrived packets will join the queue.
(1) Processing delay
The time required to examine the packet header and decide where to direct the packet is part of the processing delay .
(2) Queuing delay
In a queue, a packet experiences queuing delay while it waits on the link for transmission. The length of queuing time for a particular packet depends on the number of earlier arriving packets that are queued for transmission on the link.
(3) Transmission delay
Assuming that packets are transmitted on a first-come-first-served basis, newly arrived packets can only be transmitted after all arriving packets have been transmitted. Use L bits to represent the length of the packet, and use Rbps to represent the link transmission rate from router A to router B. Transmission delay is L/R, which is the time it takes to push all of the packet's bits onto the link (i.e., transmit).
(4) Propagation delay
Once a bit is pushed onto the link, that bit needs to be propagated to Router B. The time it takes to propagate from the origin of the link to router B is the propagation delay . The propagation delay is equal to the distance between two routers divided by the propagation rate.
(5) Comparison of transmission delay and propagation delay
Transmission delay is the time required for a router to push out 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 the time it takes for a bit to propagate from one router to another, and it is a function of the distance between the two routers and has nothing to do with packet length or link transmission rate.
Queuing Delay and Packet Loss
The queue in front of a link has only limited capacity, when an arriving packet finds a full queue, since there is no place to store the packet, the router will discard the packet, that is, the packet will be lost.
protocol layering
All protocols at various layers are called a protocol stack. The protocol stack of the Internet consists of five layers: physical layer, link layer, network layer, transport layer and application layer
(1) Application layer
The application layer is where web applications and their application layer protocols reside. The application layer includes many protocols, HTTP (it provides the request and delivery of Web documents), SMTP (it provides the transmission of e-mail messages), FTP (it provides the file transfer between two end systems). The application layer protocol is distributed on multiple end systems, and the application program in one end system uses the protocol to exchange information packets with the application program in another end system. We call this information packet at the application layer message ( message ) ).
(2) Transport layer
The Internet's transport layer transports application-layer messages between application endpoints. There are two transport protocols TCP and UDP, either of which can transport application layer messages. TCP provides connection-oriented services to its applications. This service includes reliable delivery of application layer messages to destinations and flow control. TCP also divides long messages into short messages and provides congestion control mechanisms. The UDP protocol provides connectionless services to its applications, with no reliability, no flow control, and no congestion control. A packet at the transport layer is called a segment.
(3) Network layer
The network layer is responsible for moving network layer packets called datagrams from one host to another. The network layer includes the well-known Internet Protocol IP, which defines the various fields in the datagram and how end systems and routers act on these fields.
(4) Link layer
The network layer routes datagrams through a series of routers between source and destination. To move a packet from one node (host or router) to the next node on the path, the network layer must rely on the services of the link layer, especially At each node, the network layer passes the datagram down to the link layer, and the link layer passes the datagram along the path to the next node, where the link layer passes the datagram up to the network layer.
The services provided by the link layer depend on the specific link layer protocol applied to the link. Link layer packets are called frames .
(5) Physical layer
While the job of the link layer is to move an entire frame from one network element to an adjacent network element, the job of the physical layer is to move individual bits within that frame from one node to the next.