Overview of core basic knowledge of computer network

The first chapter--Overview Computer network Performance Index 1. Rate, transmission data rate, also known as data rate, bit rate, units are: b/S, kb/s, MB/s, tb/s 2. Bandwidth, the highest transfer rate, which is the highest rate, with the same unit rate 3. ThroughputThe amount of data per unit of time through a network channel or interface, in units B, KB, Mb, Gb, Tb 4. Delay, transmission delay (time required for the host or router to send data), propagation delay (the time it takes to transmit the electromagnetic wave in the channel), processing delay, queue delay 5. Delay Bandwidth Product= Propagation delay * Bandwidth 6. Roundtrip time RTT 7, utilization of channel utilization and network utilization Computer network Architecture OSI:Open System Interconnect Basic Reference Model, open Systems Interconnection Reference model protocol:The rules, standards, or conventions established for data exchange in the network are called network protocols. Benefits of Tiering:1, each layer independent, the big problem into a number of independent small problems 2, good flexibility, as long as the interface is not changed, the internal implementation can be modified without affecting the upper and lower layer 3, the structure is easy to separate 4, easy to implement and maintain 5, promote standardization work Five-layer structure (seven-layer structure is to add a presentation layer under the application layer to the session layer, more is a theoretical structure, actually more than five-layer structure representation)1, the Application Layer 2, the Transport Layer 3, the network layer 4, the data Link Layer 5, the physical layer
Chapter II-Physical layer the primary task of the physical layer is to identify some of the features associated with the Transport media interface, including: 1, mechanical characteristics 2, electrical characteristics 3, functional characteristics 4, process characteristics How information interacts1, simplex Communication 2, half duplex communication 3, full duplex communication Channel Multiplexing Technology1, Frequency division take-users in the same time to occupy different resources 2, Division multiplexing-users at different times occupy the same resources 3, wavelength division multiplexing-Optical Frequency Division multiplexing 4, Code Division multiplexing
Chapter Three--Data link layer Data Link:In addition to a physical line representing the physical layer, but also includes a number of communication protocols to control data transmission, the combination of the two is the data link Network adapter:The Data Link layer protocol is usually implemented by the network adapter, which realizes the function of the data link layer and the physical Layer two layer. Its main function is to realize the computer and the external network communication. Protocol Data Unit for the Data Link layer:Frame Point-to-point data link Communication steps (Point-to-point data links, data links using broadcast channels are two main data links):1, Node A in the network layer of the node to the IP datagram to add the header and the tail package into frame 2, node A to the encapsulated frame through the physical layer link to the node B3, Node B in check the received frame error-free, to the network layer of the B node, otherwise, discard this frame the three basic issues to be addressed in the Data Link layer protocol:1, package into frame--frame start character Soh, Terminator EOT2, transparent transmission--prevent the transmission of text in the presence of SOH or EOT, resulting in error start or end of error 3, errors detection-Extensive use of CRC cyclic redundancy detection PPP Point-to-Point ProtocolThe data link layer is the most used protocol, he provides unreliable datagram services, because the data link layer does not have to provide more than the network layer IP protocol functions, so PPP protocol does not need to error correction, no serial number, no flow control, simple is the main requirements of PPP protocol and the maximum characteristics Ethernet: The most widely used LAN specification today, using CSMA/CD technology and operating at 10m/s rates on a variety of cables CSMA/CD Protocol:Carrier monitoring multi-point access/collision detection
CSMA/CD Core points:
1, "Multi-point access", many computers connected to a bus with multi-point access, at the same time only one computer can occupy the bus to transfer data
2, "Carrier monitoring", each station (computer) must constantly detect whether the channel is transmitting data, not occupied in order to get the right to send
3, "Collision detection", the side of the sending side monitoring, if the detection of a bus has two stations simultaneously transmit data, immediately stop the transmission
MAC Address:Physical Address of Ethernet Network Bridge:can be forwarded, filtered frames, can be connected to different physical layers, Mac sub-layer and different Ethernet, the data link layer can be extended Ethernet, the disadvantage is to increase time delay. hubs:Can forward bitstream, work on physical layer, extend Ethernet in physical layer
The fourth chapter--Network layer IP AddressFor knowledge See IP address classification and subnet mask http://blog.csdn.net/zhangliangzi/article/details/51263337 Address Resolution Protocol ARP:Resolves the IP address of the host and router to the hardware address. ARP mechanism:Each host has an ARP cache, which has a local area network host, the router's IP address to the Hardware address mapping table, and this mapping table is also frequently updated dynamically. Internetwork Control Message Protocol ICMP:Provide the host or router with information, report errors, or abnormal conditions. ICMP message type:1. Error report message 2, inquiry message ICMP application:Using the ping command, the application layer crosses the transport layer directly using the ICMP protocol echo request and the Echo reply message to test the connectivity between the hosts. Router packet forwarding algorithm (Network layer operation):The IP address of the destination host is extracted from the datagram header, according to the IP address class (A, B, C, D), extracting the network address 1, if the network address n is the network address connected to the router, the datagram is delivered directly to the destination host of the network. (This includes the network interface software through the ARP protocol to convert the IP address into a MAC address, the datagram encapsulated into a link-layer Mac frame, through the physical layer line to send this frame to the destination host process) 2, if the network address is not connected to the router network address, then query the route table 2.1, if the routing table A specific host route to the datagram IP address, the datagram is sent to the next hop route specified by the routing table 2.2, if there is a route to the destination network address n in the routing table, the datagram is sent to the next hop route specified in the routing table 2.3, if there is a default in the routing table Send datagrams to the default route 2.4 in the routing table, report packet forwarding errors
Fifth-Transport layer Overview:Although the IP layer sends packet data to the destination host, strictly speaking, the two host communication in the computer network is actually the application process communication on two hosts, the endpoint of the communication is not the host but the application process on the host. The network layer provides logical communication between hosts, and the transport layer provides logical communication between ports. The main features of UDP:1, UDP is non-connected Transport layer protocol, send data without establishing a connection, reliability is reduced but improve efficiency 2, UDP is to provide the best delivery service 3, UDP is message-oriented, that is, send a message at a time, do not merge, do not split, but if the data is too long will be in the network layer IP shard transmission, affect the network layer efficiency 4, UDP can not avoid network congestion data loss, but to ensure the stable rate of transmission data 5, UDP support a pair of one or one-to-many, many-to-many communication 6, UDP header shorter than 8 bytes, more than TCP 20 bytes (including fixed word length) reduced overhead. UDP header field:1, the source port 2, the destination port 3, the length, the datagram length 4, the checksum, verifies the transmission information whether is wrong, has the mistake to discard The main features of TCP:1, TCP is a connection-oriented transport layer protocol. TCP connection must be established before using TCP, just like a phone call, after the call 2, the TCP connection is point-to-point, only two Port 3, TCP provides reliable service, error-free, not lost, not duplicated, ordered to arrive, and transport layer below are unreliable efforts to provide the best service. 4, TCP provides full-duplex communication, both ends of the communication with send cache and receive cache, can be sent or received at idle 5, TCP-oriented byte streamSocket (SOCKET)= IP Address: port number TCP message header field:
The main header field explains:1. Serial Number:The first byte of the data segment of the message segment occupies the position of the entire message data segment2. Confirmation Number:Expected to receive the next segment of the sequence number (such as: the previous segment of the sequence number 600, the data segment length is 100, after the correct reception, this expected to accept the data sequence number is 700)3. Window Size:Indicates the amount of data allowed to be sent, which can be changed dynamically4, the checksum:Verify data integrity and correctness5. Emergency pointers:Point out the end position of the emergency data in this paper, note: Emergency data can also be sent when the window is 0 o'clock 6 Control bits explained:1.ACK 1-bit response; The confirmation Number field is valid only when ack=1, and the confirmation number is invalid when ack=02. SYN plays a synchronous role;when Syn=1,ack=0 said: This is a connection request message segment. If the connection is agreed, the syn=1,ack=1 is made in the Response message section. Therefore, syn=1 indicates that this is a connection request, or that the connection accepts a message3.FIN used to release a connection; fin=1 indicates that the sender's data for this segment has been sent, and the TCP connection is required to be released4, PSHindicates that the receiving party should hand over the message to the application layer5. RSTindicates a connection reset6, URGindicates an emergency pointer, urg=1 indicates emergency data. TCP Three-time handshake with four wavesSee TCP three handshake and four wave http://blog.csdn.net/zhangliangzi/article/details/50695611 TCP reliable transmission mechanism 1. Flow control, sliding window mechanism--Let the sender send the rate is not too fast, the receiver is in time to receive, through the sliding window mechanism, that is, the receiver sends the response message multiple times to modify the window value of the header field to control the sender data transmission rate 2. Congestion control, slow start, congestion avoidance, fast retransmission, fast recovery mechanism
The sixth chapter-Application layer DNS: Domain Name System DNS domain name resolution: resolves the domain name to the corresponding IP address DNS name server type:1. root domain Server2. Top-level domain name server3. Privileged Domain Name server4. Local Domain Name server DNS Server domain name resolution process:1, the user in the browser input the domain name of the website to visit, if the operating system check the local Hosts file cache the mapping of this domain name, then call directly, complete the domain name resolution.
2, if not in the Hosts file, the browser to the local DNS request resolution, if the cache mapping relationship, then return the results, complete the resolution;
3, if the local DNS does not, then send the request to Rootdns (root DNS server), the root DNS server will inform the local server to query the site authorized DNS server. That is, the IP address of the authorized DNS server of the website is sent to the local DNS server (the DNS server authorized by the website is the top level, the right domain name server);
4, the website authorizes the DNS server to send the resolved IP address back to the local DNS, the local DNS cache maps the relationship and sends the IP address back to the user;
5. When the browser obtains an IP address, it sends an HTTP request to it.
FTP:File Transfer Protocol, TCP-based, requires the establishment of two parallel TCP connections, "Control connections" and "Data Connections" World Wide Web (WWW):A large-scale, on-line information store. The browser is the World Wide Web client program, the client program makes a request to the server program, and the server program sends the client program back to the Web document required by the client and becomes the page World Wide Web and Internet, Ethernet relationship and difference:The World Wide Web is what we often say about the internet, and Ethernet is a type of network of the World Wide Web, is the common standard of local area network adoption. URL:Uniform Resource identifiers, the World Wide Web uses URLs to uniquely identify world wide Web documents HTML:Hypertext Markup Language HTTP:The Hypertext Transfer Protocol, which is strictly adhered to by client and server in the World Wide Web, is a stateless, no-connection protocol, but it relies on TCP for data transmission, and TCP is stateful and connected, and HTTP must be known about HTTP http://blog.csdn.net/ zhangliangzi/article/details/51336564 SMTP:Simple Mail delivery protocol, also relies on TCP to implement message transfer POP3:Post Office Protocol POP3 Features:With the client-server mode of operation, the user receiving the mail must run the POP client, and the mail server of the recipient's ISP must run the SMTP and POP server program; The advantage is very simple, the disadvantage is that the POP server deletes the message as long as the user reads the message from the POP server IMAP:Internet Message Access Protocol IMAP Features:The user PC runs the IMAP client program and then establishes a TCP connection with the IMAP server on the recipient's mail server, allowing the user to directly manipulate the mail server's mailbox; The biggest advantage of IMAP is that users can use different computers to process mail in different places. The disadvantage is that multiple views are required to establish a TCP connection multiple times Web-based e-mail:163, GMAIL, Sina Use this kind of mail service; The browser writes the message and sends it to the mail server on the website via the HTTP protocol, then sends the message to the recipient's Web mail server via SMTP and sends the message to the corresponding user's browser mailbox via HTTP. MIME:General Internet Mail Extension, the old standard specifies that the message text cannot contain 7 characters other than the character set of ASCII, MIME defines the format of many message contents, specifies the transmission encoding in various formats, and can convert any content format. Can be defined by the message header field, such as Content-type=text/html;charset=utf-8 DHCP:Dynamic Host Configuration Protocol. Provides a Plug and Play network that allows a single computer to automatically join a new network and obtain an IP address. When the client program is ported to a new network, the configuration information can be obtained automatically using DHCP.

Overview of core basic knowledge of computer network