I. Introduction
This article is a summary of the final review knowledge points of the computer network (Xie Xiren version) regular exam. Other students who are interested in computer networks and related professionals can also read it.
2. Chapter 1 Overview
1. The composition of the Internet:
(1) Edge part : It consists of all hosts connected to the Internet. This part is directly used by users for communication and resource sharing.
(2) Core part : consists of a large number of networks and routers connecting these networks. This part serves the edge part (connectivity and switching).
2. The communication method of the end system:
(1) Client-server mode (C/S mode):
The client server method (C/S method) describes the relationship between the service and the serviced process. Both the client and the server refer to the two application processes involved in the communication, the client is the service requester, and the server is the service provider.
Features of client software:
① It runs after being called by the user, and actively initiates communication (request service) to the remote server when it intends to communicate, so the user program must know the address of the server program.
② Second, the client software does not require special hardware and a very complicated operating system.
Features of the server software:
① He is a program specially designed to provide a certain service, which can handle the requests of multiple remote or local customers at the same time.
② After the system is started, it is called automatically and runs continuously, passively waiting for and accepting communication requests from customers from all over the world, so the server program does not need to know the address of the client program.
③ He generally needs powerful hardware and advanced operating system support.
Note: After the communication relationship between the client and the server is established, the communication can be bidirectional, and both the client and the server can send and receive data.
(2) Peer-to-peer (P2P):
Peer-to-peer connection, abbreviated as P2P, means that two hosts do not distinguish between the service requester and the service provider when communicating. As long as both hosts are running peer-to-peer connection software (P2P software), they can communicate on an equal, peer-to-peer connection.
The peer-to-peer connection method still uses the client-server method in essence, but each host in the peer-to-peer connection is both a client and a server.
3. The characteristics of three kinds of data exchange:
(1) Circuit switching: The bit stream of the entire message is continuously transmitted from the source point, as if transmitted in a pipeline.
(2) Message exchange: The entire message is transmitted to the adjacent node first, and after all is stored, the forwarding table is searched and forwarded to the next node.
(3) Packet switching: A single packet (this is only a part of the entire message) is transmitted to the adjacent node, stored and searched for the forwarding table, and forwarded to the next node.
4. According to the scope of computer network classification:
(1) WAN (Wide Area Network) :
The scope of a wide area network is usually tens to thousands of kilometers, so it is sometimes called a long haul network. The WAN is the core part of the Internet, and its task is to carry data sent by hosts over long distances (for example, across different countries). The links connecting the switches of each node in the WAN are generally high-speed links with large communication capacity.
(2) MAN (Metropolitan Area Network) :
The scope of action of the metropolitan area network is generally a city, which can span several blocks or even the entire city, and its action distance is about 5-50km. A metropolitan area network can be owned by one or several units, but it can also be a public facility used to interconnect multiple LANs. At present, many metropolitan area networks adopt Ethernet technology, so it is often discussed in the scope of LAN sometimes.
(3) LAN (Local Area Network) :
A local area network is generally connected by a microcomputer or a workstation through a high-speed communication line (the rate is usually above 10 Mbit/s), but it is geographically limited to a smaller range (such as about 1 km). In the early days of LAN development, a school or factory often had only one LAN, but now LANs are widely used, and most schools or enterprises have many interconnected LANs (such networks are often called campus networks or enterprise networks).
(4) Personal Area Network PAN (Personal Area Network) :
A personal area network is a network that connects personal electronic devices (such as portable computers, etc.) with wireless technology in the place where individuals work, so it is often called a wireless personal area network WPAN (Wireless PAN), and its range is very small. About 10 m or so.
By the way, if the distance between the central processing units is very close (such as only on the order of 1 meter or even smaller), it is generally called a multiprocessor system rather than a computer network.
5. Three elements of the agreement:
(1) Syntax: the structure or format of data and control information;
(2) Semantics: what kind of control information needs to be sent, what kind of action to complete and what kind of response to make;
(3) Synchronization: the detailed description of the sequence of events.
6. Seven-layer structure and four-layer structure:
(Because considering the end-of-term review of this note, only the structure diagrams of these two structures are placed here)
Chapter 2 Physical Layer
1. Three methods of bandpass modulation:
(1) Frequency modulation (FM) : That is, the frequency of the carrier changes with the baseband digital signal. For example, 0 or 1 corresponds to frequency f1 or f2 respectively.
(2) Amplitude modulation (AM) : That is, the amplitude of the carrier varies with the baseband digital signal. For example, 0 or 1 corresponds to no carrier or carrier output respectively.
(3) Phase Modulation (PM) : That is, the initial phase of the carrier wave changes with the baseband digital signal. For example, 0 or 1 corresponds to a phase of 0 degrees or 180 degrees, respectively.
2. Techniques for converting analog signals into digital data encoding include:
Amplitude Shift Keying (ASK), Frequency Shift Keying (FSK), Phase Shift Keying (PSK)
3. Nye's criterion and Shannon's formula determine the limit transmission rate:
Ney's criterion conclusion: the highest symbol transmission rate in an ideal low-communication channel with a bandwidth of W (Hz) is 2W (symbols/second). Decision (ie identification) of symbols becomes impossible.
Shannon's formula : The limiting information transfer rate of a channel:
C = W log2(1+S/N) (bit/s)
In the formula, W is the bandwidth of the channel (in Hz); S is the average power of the signal transmitted in the channel; N is the Gaussian noise power inside the channel.
3. Twisted pair standard, shielded twisted pair shielding layer function
(1) The standard of twisted pair:
Twisted pair wires can generally be divided into the following categories: three categories, four categories, five categories, super five categories, six categories, seven categories
| Stranded wire category |
bandwidth |
Cable Features |
typical application |
| 3 |
16 MHz |
2 pairs of 4-core twisted pair |
Analog telephone; formerly used for traditional Ethernet (10 Mbit/s) |
| 4 |
20 MHz |
4 pairs of 8-core twisted pair |
Token LAN |
| 5 |
100 MHz |
Increased stranding compared to category 4 |
Applications with transmission rates up to 100 Mbit/s |
| 5E (Category 5e) |
125 MHz |
Less attenuation compared to category 5 |
Applications with transfer rates up to 1 Gbit/s |
| 6 |
250 MHz |
Improved performance such as crosstalk compared to Category 5 |
Applications with transfer rates above 1 Gbit/s |
| 7 |
600 MHz |
Use shielded twisted pair |
Applications with transfer rates above 10 Gbit/s |
(2) Line sequence standard:
Orange and white - 1, orange - 2, green and white - 3, blue - 4, blue and white - 5, green - 6, brown and white - 7, brown - 8
(3) The role of the shielding layer of the shielded twisted pair: to improve the ability of the twisted pair to resist electromagnetic interference
4. CDMA code division multiplexing method
Code Division Multiplexing : CDM (Code Division Multiplexing) is another method of sharing channels. When the code division multiplexing channel is shared by multiple users with different addresses, it becomes code division multiple access (CDMA) .
official:
If the result is 1, 1 is sent, if the result is -1, 0 is sent, otherwise no data is sent.
example:
1. A total of four stations carry out code division multiple access CDMA communication. The chips of the four stations are
A: (-1 -1 -1 +1 +1 -1 +1 +1) B: (-1 -1 +1 -1 +1 +1 +1 -1)
C: (-1 +1 -1 +1 +1 +1 -1 -1) D: (-1 +1 -1 -1 -1 -1 +1 -1 )
Now receive such a chip sequence: (-1 +1 -3 +1 -1 -3 +1 +1) Ask which station sent the data? Did the station sending the data send a 1 or a 0?
Chapter 3 Data Link Layer
1. Three basic issues in the data link layer: encapsulation into frames, transparent transmission and error detection
(1) Encapsulation into a frame : It is to add a header and a tail before and after a piece of data, thus forming a frame. After receiving the bit stream submitted by the physical layer, the receiving end can identify the start and end of the frame from the received bit stream according to the header and tail marks.
(2) Transparent transmission: Transparent transmission means that no matter what kind of bit combination the transmitted data is, it should be able to be transmitted on the link. When the bit combination in the transmitted data happens to be exactly the same as a certain control information, appropriate measures must be taken so that the receiver will not mistake such data as some kind of control information. Only in this way can the transmission of the data link layer be transparent.
(3) Error detection: Error detection refers to the technology of adding appropriate redundancy to the transmitted code sequence (codeword) so that the receiving end can find out whether an error occurs during transmission.
2. CRC test (test calculation, please search for relevant examples)
3. PPP protocol byte padding and zero bit padding (test calculation, please search for relevant examples by yourself)
4. Ethernet: Ethernet is a computer local area network technology.
5. Main points of CSMA/CD work:
At t = 0, A sends data. B detects that the channel is free.
At t = t - d (here t > d > 0), when the data sent by A has not reached B, because B detects that the channel is idle, B sends data.
After time d / 2, that is, at t = t - d / 2, the data sent by A and the data sent by B collided. But neither A nor B knows that a collision has occurred at this time.
At t = t , B detects a collision and stops sending data.
At t = 2 t - d , A also detects a collision and stops sending data.
Both A and B fail to send data, and they both delay sending for a period of time before resending.
6. Truncated Binary Exponential Backoff Algorithm
Truncated Binary Exponential Back-off (TBEB) algorithm, the principle is to let the collided station stop sending data immediately, but back off for a random time to reduce the probability of collision during retransmission .
Contention period: 2 t Specific contention time: 51.2us For 10Mb/s contention period, 512b or 64 bytes can be sent.
7. Ethernet frame type
(1) Unicast (unicast) frame (one-to-one) , that is, the MAC address of the received frame is the same as the hardware address of the station.
(2) Broadcast (broadcast) frame (one-to-all) , that is, a frame (all 1 addresses) sent to all sites on the local area network.
(3) Multicast (multicast) frame (one-to-many) , that is, a frame sent to some sites on the local area network.
8. Ethernet frame format
The MAC frame is relatively simple and consists of five fields. The first two fields are 6-byte destination address and source address fields respectively. The third field is a 2-byte type field, which is used to mark the protocol used by the upper layer, so that the data of the received MAC frame is handed over to the protocol of the upper layer. The fourth field is the data field, whose length is between 46 and 1500 bytes. The last field is the 4-byte Frame Check Sequence FCS (checked with CRC).
9. Extended Ethernet
Scalable at physical layer (hub), scalable at data link layer (switch)
10. Collision Domain Broadcast Domain Bandwidth
Conflict domain: The shared medium is required by multiple devices, then multiple devices form a conflict domain, the smaller the better (preferably one)
Broadcast domain: one-to-all, everyone can initiate a broadcast, the smaller the better, generally one broadcast domain per department (it is not suitable for too large, it will affect the normal communication)
Bandwidth: It is more widely used in digital communication to describe the theoretical maximum rate at which a network or line can transmit data.
11. Switch self-learning function
The sender looks up the MAC address table, if there is no broadcast, the sender's MAC address and the corresponding port are written into the address table during the broadcast, and the cycle is repeated.
12. Vlan definition
Vlan virtual local area network: use Ethernet switches to divide a local area network into some small local area networks to reduce the scope and improve security.
Chapter 4 Network Layer
1. Intermediary devices for network interconnection
① The intermediate device used in the physical layer is called a repeater.
② The intermediate device used by the data link layer is called a bridge or bridge (bridge).
③ The intermediate device used by the network layer is called a router.
④ The intermediate device used above the network layer is called a gateway. Connecting two incompatible systems with a gateway requires protocol conversion at a higher level.
2. The delivery method of the router
Direct Delivery: Send directly on the same network
Indirect delivery: If you are not on the same network, search for publications and forward them continuously until you perform direct delivery on the same network.
3. IP address, network number, host number classification, dotted decimal notation, subnet mask
IP address: IP address is to assign a worldwide unique 32-bit identifier to each interface of each host (or router) on the Internet.
Network number: indicates the network to which the host (or router) is connected. A network number must be unique across the entire Internet.
Host number: mark the host (or router). A host number must be unique within the network range indicated by the network number preceding it.
Dotted decimal notation: For a host or a router, the IP address is a 32-bit binary code. To improve readability, we often insert a space for every 8 bits in the 32-bit IP address (but there are no such spaces in the machine). For notational convenience, they are represented by their decimal equivalents with a dot between them. This is called dotted decimal notation.
Subnet mask: It is used to indicate which bits of an IP address identify the subnet where the host is located, and which bits identify the bit mask of the host.
4. Special IP address
| network number |
host number |
source address using |
The destination address uses |
representative meaning |
| 0 |
0 |
Can |
can't |
This host on this network (see Section 6.6 DHCP protocol) |
| 0 |
host-id |
Can |
can't |
A host host-id on this network |
| all 1 |
all 1 |
can't |
Can |
Only broadcast on this network (no router forwarding) |
| net-id |
all 1 |
can't |
Can |
broadcast to all hosts on net-id |
| 127 |
Not all 0s or all 1s any number of |
Can |
Can |
For local software loopback testing |
5. Classless Addressing CIDR Subnetting Supernet Convergence Network Prefix
Question type:
Determine the network number and broadcast address given the host and subnet mask
Given the network number and subnet mask (network prefix, network suffix) to determine the number of network hosts and addresses
subnetting
converged supernet
Link: How to quickly divide subnets - Baidu experience (baidu.com) (block size, different from textbooks)
6. ARP protocol function and definition request packet and response packet content
A RP protocol function: find out the corresponding MAC address from the IP address of the machine (host or router).
ARP protocol definition: A protocol to find out its corresponding MAC address from the IP address of a machine (host or router), and store a mapping table from IP address to hardware address in the host ARP cache, and this mapping table is often dynamic Update (addition or timeout deletion).
The process of request grouping and response grouping is as follows:
7. IP datagram format and fixed header and MTU
An IP datagram consists of two parts: header and data. The first part of the header is a fixed length, a total of 20 bytes, which is required for all IP datagrams. Following the fixed part of the header are some optional fields, whose length is variable. The meaning of each field in the header is introduced below.
MTU : The maximum length of the data field, that is, the maximum transmission unit, the most commonly used 1 500 bytes.
8. IP datagram fragmentation to see clearly what the data is, how much is the header, and what is the size of each piece (calculated, check the textbook)
Version: 4 digits; header length: 4 digits; differentiated services: 8 digits; total length: 16 digits (referring to the sum of the radical length of each fragment after fragmentation and the data length of the fragment); identification: 16 digits ; Flag: 3 bits; Fragment offset: 13 bits; Except the last datagram fragment, the length of each fragment must be 8 bytes (64 bits).
(doubtful!)
9. Routing and forwarding table format
The routing table must contain the following three items: destination network address, subnet mask and next-hop address.
| destination network address |
subnet mask |
Next hop |
| 128.30.33.0 |
255.255.255.128 |
interface 0 |
| 128.30.33.128 |
255.255.255.128 |
interface 1 |
| 128.30.36.0 |
255.255.255.0 |
R2 |
10. How to build and apply address aggregation (with calculation, see the question of address aggregation)
11. ICMP four error message functions
① Destination unreachable: When the router or host cannot deliver the datagram, it sends a destination unreachable message to the source.
② Time Exceeded: When a router receives a datagram with a lifetime of zero, it will not only discard the datagram, but also send a Time Exceeded message to the source. When the end point cannot receive all the datagram fragments of a datagram within the predetermined time, it discards all the received datagram fragments and sends a time exceeded message to the source point.
③ Parameter problem: When the value of some field in the header of the datagram received by the router or the destination host is incorrect, the datagram is discarded and a parameter problem message is sent to the source.
④ Change route (redirection): The router sends a change route message to the host, letting the host know that the datagram should be sent to another router next time (through a better route).
12. RIP routing table changes
For the RIP packets sent by each neighboring router , perform the following steps:
① For the RIP message sent by the adjacent router with the address X, modify all the items in this message first: change the address in the "next hop" field to X, and change all the "distance" fields to The value is incremented by 1 (see explanation 1 below). Each item has three key data, namely: to the destination network N, the distance is d, and the next-hop router is X.
② Perform the following steps for each item in the modified RIP message:
If there is no destination network N in the original routing table, add this item to the routing table. Otherwise (that is, there is a destination network N in the routing table, then check the next-hop router address)
If the next-hop router address is X, replace the received item with the item in the original routing table. Otherwise (i.e. this item is: to destination network N, but next-hop router is not X)
If the distance d in the received item is less than the distance in the routing table, update it, otherwise do nothing.
If the update routing table of the adjacent router has not been received within 3 minutes, the adjacent router is recorded as an unreachable router, that is, the distance is set to 16 (a distance of 16 means unreachable).
③ Return.
The basis of the distance vector algorithm given above is the Bellman-Ford algorithm (or Ford-Fulkerson algorithm). The gist of this algorithm is this:
Let X be a node on the shortest path from node A to B. If the path A→B is divided into two paths A→X and X→B, each path A→X and X→B is also the shortest path from node A to X and node X to B respectively.
13. OSPF definition, algorithm, autonomous system division area
OSPF Definition: OSPF (Open Shortest Path First ) is an interior gateway protocol (I IGP) for routing decisions within a single autonomous system (AS). It is an implementation of the link state routing protocol , which belongs to the interior gateway protocol (IGP), so it operates inside the autonomous system.
Algorithm: Dijkstra's Algorithm (Shortest Path Algorithm S PF )
Autonomous System Area (AS ) : An autonomous system is a group of routers and networks under the control of an administrative authority.
Chapter 5 Transport Layer
1. The essence of communication--application process communication
The entity that actually communicates is the process in the host, and it is a process in this host that exchanges data with a process in another host (that is, communication). Therefore, strictly speaking, the communication between two hosts means that the application processes in the two hosts communicate with each other . Although the IP protocol can send the packet to the destination host, the packet still stays at the network layer of the host and has not been delivered to the application process in the host. From the perspective of the transport layer, the real endpoint of communication is not the host but the process in the host. In other words, end-to-end communication is communication between application processes.
2. Transport layer protocol
① User Datagram Protocol (UDP): Oriented to the message, there is no need to establish a connection before transmitting data. Does not provide reliable delivery, but is simple.
② Transmission Control Protocol (TCP): Provides connection-oriented (byte stream) services, does not provide broadcast or multicast, occupies resources, and delivers reliably.
3. Port number P215 16 bits, 65535, port number classification
The transport layer of TCP/IP uses a 16-bit port number to identify a port.
Table 5-2 Commonly known port numbers
| application |
FTP |
TELNET |
SMTP |
DNS |
TFTP |
HTTP |
SNMP |
SNMP (trap) |
HTTPS |
| well-known port number |
21 |
23 |
25 |
53 |
69 |
80 |
161 |
162 |
443 |
Classification:
(1) Server use: well-known port number (global universal port number) (0-1023), registered port number (1024-49151)
(2) The port number used by the client (ephemeral port number) (49151-65535)
4. The UDP header format and length must be memorized by P217. Given the header data, specific information can be obtained
User datagram UDP has two fields: data field and header field. The header field is very simple, only 8 bytes (Figure 5-5), and consists of four fields, each of which is two bytes long. The meaning of each field is as follows:
| (1) |
source port |
Source port number. Select it when you need a reply from the other party. All 0s can be used when not needed. |
| (2) |
destination port |
Destination port number. This must be used when the endpoint delivers the message. |
| (3) |
length |
The length of the UDP user datagram, the minimum value is 8 (only the header). |
| (4) |
checksum |
Detects errors in transmission of UDP user datagrams. If there is a mistake, discard it. |
5. UDP is packet-oriented ; TCP is byte-oriented
6. Application of continuous ARQ protocol algorithm
The continuous ARQ protocol stipulates that each time the sender receives an acknowledgment, it slides the sending window forward by one group position. Figure 5-13(b) shows that the sender has received the acknowledgment of the first packet, and then moves the sending window forward by one packet. If the first 5 packets have already been sent, the 6th packet in the window can now be sent.
The receiver generally adopts the method of cumulative confirmation. That is to say, the receiver does not need to send acknowledgment to the received packets one by one, but after receiving several packets, it sends acknowledgment to the last packet that arrives in order, which means: all packets up to this packet have been received. Received correctly.
7. TCP packet header format
The first 20 bytes of the TCP segment header are fixed (Figure 5-14), and the following 4n bytes are optional options (n is an integer). Therefore the minimum length of the TCP header is 20 bytes.
8. TCP reliable transmission ( contains too much and messy content, please read the textbook )
9. Four types of TCP congestion control methods and specific algorithms
10. TCP three-way handshake process
1. The first handshake: When the connection is established, the client sends a syn packet (syn=j) to the server, and enters the SYN_SEND state, waiting for the server to confirm; SYN: Synchronize Sequence Numbers (Synchronize Sequence Numbers)
2. The second handshake: the server receives the syn packet and must confirm the client's SYN (ack=j+1), and at the same time, it also sends a SYN packet (syn=k), that is, the SYN+ACK packet. At this time, the server enters SYN_RECV state;
3. The third handshake: the client receives the SYN+ACK packet from the server, and sends an acknowledgment packet ACK (ack=k+1) to the server. After the packet is sent, the client and server enter the ESTABLISHED state and complete the three-way handshake.
Link: Three-way handshake_Baidu Encyclopedia (baidu.com)
Chapter 6 Application Layer
1. Definition of application layer:
The application layer is also called application entity (AE), which consists of several application-specific service elements (SASE) and one or more common application service elements (CASE). Each SASE provides specific application services, such as file transport access and management (FTAM), electronic message processing (MHS), virtual terminal protocol (VAP), etc. CASE provides a set of common application services, such as Contact Control Service Element (ACSE), Reliable Transportation Service Element (RTSE) and Remote Operation Service Element (ROSE), etc.
2. How to convert IP address and domain name in DNS protocol
When an application process needs to resolve the host name to an IP address, the application process calls the resolver (resolver), and becomes a client of DNS, puts the domain name to be resolved in the DNS request message, and uses UDP user data Report to the local domain name server (UDP is used to reduce overhead). After searching the domain name, the local domain name server returns the corresponding IP address in the reply message. The application process can communicate after obtaining the IP address of the destination host.
3. FTP : File transfer protocol FTP (File Transfer Protocol) is the most widely used file transfer protocol on the Internet. FTP provides interactive access, allowing customers to specify the type and format of the file (such as specifying whether to use ASCII codes), and allows the file to have access rights (such as the user who accesses the file must be authorized and enter a valid password). FTP shields the details of each computer system, so it is suitable for transferring files between arbitrary computers in a heterogeneous network.
4. SMTP : Simple Mail Transfer Protocol SMTP
