Computer network notes (i) Shehiren Edition __ Computer

One, the important concept of computer network overview

* 1, all Internet standards are published on the Internet in the form of an RFC (Request for Comments), which is a working document issued by the IETF Internet Engineering department under the Internet Association (ISOC)

* 2, the edge of the Internet: made up of all the hosts connected to the Internet, this part is directly used by users.

2.1, the network edge of the end of the system between the communication methods are divided into: c/s,p2p.

* 3, the core of the Internet: consisting of a large number of networks and routers connected to these networks, which provide services for the edge segment (providing connectivity and exchange)

3.1, in the core part of the special role is the router, and the router is the key component of the implementation of packet switching, the task is to forward the packet received, characterized by storage and forwarding. This is the most important function of the network core part

3.2, the concept of exchange: In a way to dynamically allocate the resources of transmission lines

3.3.1, although in the router uses the packet exchange, but has to mention that it is by the circuit exchange originally evolved. The exchange process is to establish a connection (beginning to occupy a communication resource)--call (always occupying communication resources)--releasing the connection (returning the communication Resource), and its disadvantage is that utilization is low in most cases.

3.3.2, because, in the computer network, sends the data, is needs to pass from the application layer to the physical layer between the 7 layer processing, and needs to establish the connection first. If the data sent at a large, and the transmission time than the time to establish a large number of connections, then the utilization of the channel is relatively high, the rest, the utilization rate is very low, in fact, the utilization rate is generally 1%~10%. Message exchange and packet switching do not establish a connection process, they do not need to advance the allocation of transmission bandwidth. and packet switching is to cut packets again on the basis of the packet, so in the sending time delay, processing delay, propagation delay, the sum of the queue delay is shorter than the message exchange. In addition, there is also the point that the exchange of each packet is approximately parallel, meaning that after you send the first packet, you can send a second group (whether it belongs to a message with the first packet), and the first group is probably still on the road to the destination, This is different from the circuit exchange of resources always possession. Packet switching, because of the small data, many groups, more close to the parallel state, the channel idle time is small, the higher the efficiency.

3.3.3, packet switching process: Routers receive a packet, the first temporary storage, lookup forwarding (the table IP is divided by the region), if the destination IP belongs to an area, select the corresponding route. Because the forwarding of each packet is independent, the forwarding path between groups can be different. When forwarding a lot of packets, it can be in a routing node, distribution group to different next route, achieve the effect of concurrent execution. And the target side, only after receiving a complete message, start to adjust the sorting combination.

3.3.4, short grouped storage media is the router's memory (memory)

Features of 3.3.5 and packet switching:

1, in the process of packet transmission dynamic allocation of broadband transmission, the communication link is paragraph by section occupied.

2, each group can select the forwarding route independently

3, do not need to connect to send a group

4, reliable, storage forwarding (not because of a link to a problem and will not be able to transfer, because the topological structure of the internet always have more than one link to reach a destination)

* 4, two communication between the programs, or two of the communication between the host, in fact, the essence of the communication between the processes.

*5, packet (packet) is the data unit transmitted in the Internet

*6, the message is binary, it means that the OSI system, all operations are bit-flow operations

*7, computer storage, are byte byte as the smallest unit, and computer network transmission is bit bit as the smallest unit, and, computer storage k= 2^10,m=2^20,g=2^30, network transmission k=10^3,m=10^6,g=10^9. So there's almost eight times times as many downloads as you can count: bandwidth unit MBps, download traffic unit: Mbps, these two m are not the same

*8, the general bandwidth speed measurement system is to calculate the data of the net load (without the package header, handshake information)

*9, flow is the sum of downloading data and uploading data

*10, WAN is the core part of the Internet

*11, access network is not the core of the Internet, nor is it part of the edge of the Internet. For example, the ISP (Internet Service Provider) is a access network, is the internet edge and the core part of the link bridge

*12, Delay:

Send delay = Data frame Length (b)/send rate (bps)

Propagation delay = Channel length (m ')/propagation speed of electromagnetic waves on the channel (MPS)

Processing delay: Analysis of the first, the extraction of data parts, error detection, to find the appropriate route

Queued delay: Queue latency caused by a large number of packets while in the router

*13, Data transmitter: Network adapter

*14, typing a character on the keyboard is 1 bytes, 8 bits (of course, different encodings have different say, here refers to keyboard input), half-width symbol is a character, the full cross symbol is two characters, such as Chinese characters

*15, bandwidth is how much data can be transmitted on the link

*16, Channel Utilization: D (Network current time delay) =d0 (Network idle time delay)/(1-u (Network Utilization))

*17 and OSI/RM models are proposed by ISO

*18, Application layer: Through the application process interaction between the implementation of specific network applications, such as HTTP, SMTP, FTP, DNS, TFTP, TELNET, WWW, IMAP, MIME, DHCP,,SNMP protocol. This layer of dry matter is actually to tell the receiver (plus the first), how to parse the data received, these two parties agreed that the analytical format is the agreement

18.1, the application layer of interactive data units called packets (by the total size of the content to be transmitted through the size of the specified message, cut from)

*19, Transport layer: responsible for providing common data transmission services for communication between processes in two hosts. This is actually the way to determine the transmission, and then the data corresponding to the processing into the header

19.1, TCP (Transmission Control Protocol), the transmission unit for the message segment (is sent by the message after a certain size cut)

19.2, UDP (User Datagram Protocol), transmission unit for the datagram (equivalent to the message, just different layers of different names)

*20, Network layer: 1, responsible for the transport layer generated by the message segment or packet into groups or packets. 2, choose the right route

20.1, the Internet network layer is using IP (Internet Protocol)

*21, Data Link layer: is responsible for the network layer passed down the packet or packet number, because, in the transmission link, the packet forwarding is independent, then how the final assembly into a complete content. This requires that the link layer add a tail to the group, like a two-way pointer list, with the pre and next pointers. and contains control information (sync information, address information, error control)

21.1, the link Layer Interaction Data unit is called the data frame

*22, Physical layer: Determine the representation of bits (0/1) and identification method, switch to how much voltage to represent 1, how small the voltage represents 0; determine how many pin pins should be connected to the cable and how each pin should be connected. The measurement of electrical frequency is not a physical layer.

*23, the protocol is a peer-to-peer interaction between two hosts, interacting through the PDU (Protocol Data unit). A service is an interaction between the upper and lower layers of a host, interacting with SAP (service Access point) through SDU (service Data unit). A SDU can be made up of several PDUs, and a PDU can be composed of several SDU

*24, everything over IP:TCP/IP agreements can serve a wide range of application physical services

IP over EVERYTHING:IP protocol can be run on a wide range of network-composed internet

*25, service transparency: that is, the lower layer can not see the protocol, and only use the interface provided by the upper layer, do not need to understand the implementation process, which is consistent with the idea of object programming

*26, the three Elements of network protocol:

(1) Semantics. Semantics is the meaning of each part of the interpretation of control information. It sets out what kind of control information needs to be emitted, and what kind of response it takes to complete the action. (2) syntax. syntax is the structure and format of user data and control information, and the order in which data appears. (3) Timing. Timing is a detailed description of the order in which events occur. (also referred to as "Sync").

The difference between the *27, the backbone ISP and the local access network is that the backbone is connected to the light brazing, while the local access network is a 5 line or a 6-class line.