[컴퓨터네트워크] 컴퓨터네트워크 (Computer Network) 2단원 정리
protocol : a language interpreted by network software
set of rules and formats for communicating.
components - format of the message(syntax & semantic), order of message exchange, local action
layer : abstractioni of different levels of related functionality.
benefit - decomposition, modularity(개별 단위. 모듈성)
encapsulation
decomposition : breaks down complex problems (simple service들이 결합됨.), abstraction (complexity를 숨기고, implementation details at individual layers. 오직 필요한 정보만 interface를 통해 communicate 된다.
modularity : Reusability (network driver를 여러 개의 pgm이 공유할 수 있음.), change of technology (Ethernet -> wireless LAN)
protocol stack : set of protocol layers.
OSI (open systems interconnect) layer : (physical, data link, network, transport, session, presentation, application)
Application layer
application : set of programs run on the network
presentation : manages format
session : manages different sessions (download two file)
ex) HTTP, FTP, DNS, SMTP, POP3 (protocol)
Transport layer (OS)
service : reliable delivery, provides convenient interface to app layer via sockets (대부분 transport layer and below belong to kernel)
ex) UDP(no reliability, connetionless), TCP(very strong reliability, connection-oriented)
Connectionless model : No prior setup required for communication - datagram service
Connection-oriented model : connection(call setup) required, and establishes a connection (session) prior to communication - virtual circuit network, circuit switched network => CIRCUIT SWITCH
Main function of transport layer
(1) flow control : match the transmission rate to the rate currently sustainable on the path to the destination and / or the receiver
(2) multiplexing : multiple applications share the same end-to-end connection : adds and application specific identifier so that receiving end system can hand incoming packets to correct application
socket interface : application layer와 transport layer 사이의 interface = function call
Network layer
service : end-to-end packet delivery through routing ("find a path"), provides network-wide global addressing scheme(source and destination can be anywhere in the world. ex : IP address), does not guarantee "reliable delivery" (connectionless)
Data link layer
service : handling data TX/TX on the node's physical link
property : data link is first layer of software, very dependent on underlying physical link properties, usually bundle(묶다.) both physical and datalink layer on host adapter card like Ethernet card.
sublayer : required similar operations for local networks
LLC (Logical link control) sublayer : error control, flow control
MAC (Medium access control) sublayer : addressing, access to physical channel
Physical layer
service : sending and receiving bits on the physical channel
coding, modulation, synchronization. (언제 시작인지)
Socket interface interface between "application" layer and "transport" layer
근본적으로 function call in a program of the application layer
Socket = network API
sockets enables simple interface between application and networking function. (hides complexity of details of all layers)
layer 가 exchange 하는 정보는? packet
packet의 구성요소 : header, payload, encapsulation
Header : meta-data(필요한 data가 아닌 그 data에 대한 정보), contains address
payload : the data to be delivered
encapsulation : act of attaching the header and hides data
상위 계층일수록 packet의 길이가 짧다.