Computer network 1-7

The main protocol in the network is TCP,IP. TCP provides reliable and ordered byte stream data transfer.

Circuit switching: Pre-opening a link for a user, dedicated to exchanging data for you.
Frequency division: Each user divides a certain frequency.
Division: Each user is divided into a certain time, similar to the CPU time division, each with
You can use all the bandwidth in your time.

Packet switching: Different packet-sharing network resources, each grouping using the full link bandwidth
。
Problems caused by packet switching: resource congestion, generating packet queuing, and waiting for link resources.

Telegram Transmission: No matter how big the telegram is, it is transmitted by the whole message, not into
Row packet transport.

To connect the computer to the Internet requires terminating the router, the terminating router is the default gateway.

Wireless access network: Shared bandwidth.
Wireless LAN: The use of radio spectrum instead of network cables, high frequency radio waves, high
Radio frequency transmission range is limited.
Wide Area Wireless access:

Cable media: Cable, optical cable (twisted pair, coaxial cable, optical cable: Propagation of light pulses, speed
Not be disturbed by the airwaves).
Wireless media: Atmospheric, radio, Microwave: Can reach 45m/s, satellite: Earth Synchronous Guardian
The number of 3 satellites and satellites placed 23,000 kilometres above the equator is limited.

Wireless media transmission is affected by the environment,

Delay problem in packet switched networks: Latency is generated on the router and the switching node
, each packet is stored and forwarded on the router.
Send delay:
Node processing delay: Do checksum errors on received packets, route tabular,
Transmission delay: Physical link length, signal transmission speed in the media,
Queued delay: Wait for the link delay, the most difficult to control.

The level of the protocol: a lot of network components: host, router, a variety of media links, applications
Programs, protocols, hardware software.

Application layer: Ftp,smtp,http (Network application), the unit of transmission is a message.
Transport Layer: TCP,UDP (data transfer between host processes), the unit of transmission is segment
。
Network layer: IP (routing protocol, transmission of datagrams from source to message), transfer of single
Bits are groupings or packages.
Link layer: ppp,ethernet (data transmitted between adjacent nodes on the network), units transferred
is frame.
Physical layer: A bit stream transmitted over a channel, in bits 0, 1

The host realizes the 5 layer, the router only realizes 3 layers (network layer, link layer, physical layer).
Each layer encapsulates the data.

Process: A program that runs on a host.

IP and port number to determine the address of the app.

HTTP is a stateless protocol, that is, the server does not retain access to information.

A cookie is present in memory, or it can exist inside a hard drive.

Proxy server (soft proxy, hard proxy): Save access time, the proxy server has slow
If there is no modification, the direct send is not obtained from the actual server.

Mirror site is also a proxy server,
Proxy server can be placed in the LAN, a user access to the information can be placed in the agent
Servers, other users accessing the same content can be obtained from the proxy server.

SMTP Transport phase: not only rely on tcp,smtp themselves also have handshake, transmission, end.

Messages can carry sound, video, Word, PDF.

The protocol used by the user to send mail is SMTP, and the protocol used by the user to download the message is POP3,
IMAP, http.

DNS: is a table, distributed database, used to store the IP domain name conversion table.
Why not focus on DNS and store all the host mapping tables. DNS is usually decentralized and
is a distributed database to store.

Local domain Name server: Each ISP, the enterprise can have the local domain name server, the DNS query first
First to the local domain name server.
Authorized name servers: Each host must register on the licensing server. Each ISP only
can have an authoritative domain name server.

Root name server: When the local domain name server cannot be resolved, it is like a root domain server
Query, if the root name server cannot query, the authoritative name server is queried.

Mediation Domain Name server: If the root name server does not know the authoritative domain name server, the mediation
The domain name server is responsible for helping the root name server contact the authorized name server.

DNS caching: Once a domain name server learns a map, it caches it for a certain amount of time
After the interval, the cached entries expire and are deleted.

Computer network 1-7