[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.