Shulou(Shulou.com)06/01 Report--

2. Hierarchical relationship

one。 Physical layer:

Objective: to ensure the error-free transmission of the original data bit stream.

Task: the physical layer defines voltage, interface, cable standard, transmission distance, etc. Physical layer protocols define four basic functional characteristics of an interface:

Mechanical properties: explain the shape and size of the connector used in the interface, the number and arrangement of leads, etc.

Electrical characteristics: state the range of the voltage that appears on which line of the interface cable.

Functional characteristics: explain the meaning of the voltage of a certain level on a certain line.

Discipline characteristics: describe the sequence of possible events for different functions.

two。 Data Link layer and switch:

Functions of the data link layer:

Establishment, maintenance and dismantling of data links

Frame wrapper, frame transmission, frame synchronization

Physical address definition

Network topology (Ethernet frame, token ring frame)

Error control (error recovery of frames), flow control

The data and control information in the frame are separated by transparent transmission.

Switch conversion principle:

Forwarding: the switch broadcasts and forwards data frames according to the MAC address form

Learning: the MAC address table is formed by the switch by learning the source MAC address of the received data frame

Broadcast: if the destination address is not in the MAC address table, the switch broadcasts the data frame to all ports except the port on which it was received

Update: the aging time of the switch MAC address table is 300 seconds. If the switch finds that the ingress port of a frame is different from the port of the source MAC address in the MAC address table, the switch relearns the MAC address to the new port.

three。 Network layer:

Function: define the logical address based on IP protocol, connect different media types, and choose the best path for data to pass through the network.

Routing: the process of forwarding packets across an internetwork from the source host to the destination host

Routing table: a route entry maintained in a router that the router chooses according to the routing table.

Static rout

Features: the routing table is set manually; static routes will not change unless the network administrator intervenes; the formation of the routing table does not need to take up network resources

Applicable environment: generally used in networks with small network size and fixed topology

Default rout

Features: of all route types, the default route has the lowest priority

Applicable environment: generally used in an end network with only one exit or as a supplement to other routes

Dynamic routing: the process by which routers in a network communicate with each other, transmit routing information, update and maintain routing tables using received routing information; based on a routing protocol

Features: reduce management tasks and occupy network bandwidth

LRIP is the first routing protocol standard developed for TCP/IP environments.

RIP is a distance-vector routing protocol

How RIP works: RIP routing protocol sends entire routing table information to neighbors; RIP routing protocol uses hop count as a metric to select the best route according to the number of hops; the maximum hop count is 15 hops, and 16 hops are unreachable; after a series of routing updates, each router in the network has a complete routing table, which is called convergence

Fourth, transport layer protocols and applications:

The role of the transport layer: the IP layer provides point-to-point connectivity while the transport layer provides end-to-end connectivity

TCP (Transmission Control Protocol Transmission Control Protocol)

Reliable, connection-oriented protocol; low transmission efficiency

TCP needs to overcome: network delay and jitter; network packet loss; maximum transmission size of different segments on the network path; and other possible failures.

UDP (User Datagram Protocol user Datagram Protocol)

Unreliable, connectionless protocol (UDP is connectionless and its reliability is basically equivalent to that of the IP layer)

High transmission efficiency: add an 8-byte UDP header directly to the IP packet

Applicable scenarios for UDP: simple request-response services (such as DNS, the IP address of the request www.baidu.com); scenarios where the timeliness of the data is high, but the integrity and correctness are not high

Application layer protocols and applications

Function: work with applications to exchange application-specific data using the basic network

Commonly used application layer protocols: DNS, SMTP and POP3, HTTP and HTTPS, Telnet, FTP and TFTP

DNS (Domain Name System Domain name system)

Complete the mapping between domain name and IP address (nslookup)

Port udp 53 / tcp 53

SMTP (Simple Mail Transfer Protocol- simple Mail transfer Protocol)

Used to send and receive mail

Port number 25

POP3 (Post Office Protocol v3-Post Office Protocol version 3)

For clients to receive mail

Port number 110

HTTP (Hypertext Transfer Protocol- Hypertext transfer Protocol)

Used to transmit data such as plain text, hypertext, audio and video used by Internet browsers

The port number is 80 of TCP

HTTPS (secure Hypertext transfer Protocol)

Development based on HTTP

Provide encryption to ensure the privacy and integrity of messages

The port number is 443

Telnet (Terminal Network)

Used for text remote management of network devices such as computers or routers

The port number is 23 of TCP

FTP (File Transfer Protocol- File transfer Protocol)

Used to transfer files

The port numbers are 21 and 20 of TCP

TFTP (Simple File Transfer Protocol- simple File transfer Protocol)

For file transfer

The port number is 69 of UDP

The corresponding relationship between OSI layer 7 and TCP/ IP layer 4

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