计算机网络重要知识点

OSI Model:

7. APPLICATION Layer ：User Interface (Browser)

6. Presentation Layer : Data presentation and encryption  (Common Format)

ensures that information sent by the application layer of one system is readable by the application layer of another system
translates between multiple data representation formats by using a common data representation format
concerned with data structures and negotiation of data transfer syntax

5. Session Layer : Inter-host Connection  (Dialog and Conversation)

establishes, manages, and terminates sessions between communicating hosts
synchronizes dialog between presentation layer entities and manages their data exchange
offers provisions for efficient data transfer, class of service, and exception reporting of session, presentation, and application layer problems

   
4. Transport Layer : End to End Connection   (Reliability, Flow control, Error correction)

segments and reassembles data into a data stream
concerned with how reliable transport over an internetwork is accomplished
responsible for reliable network communication between end nodes and provides mechanisms for the establishment, maintenance, and termination of virtual circuits, transport fault detection and recovery, and information flow control
        Flow control and reliability:  can be compared to talking to a foreigner.   Often you would ask the foreigner to repeat his/her words (reliability) and to speak slowly (flow control)

       TCP:  Connection -oriented
                    Software checking for segment
                    Re-send anything lost or error
                    Uses acknowledgments
                    Provides flow control
       UDP: unreliable
                connectionless 
                provides no software checking for segment
                uses no acknowledgments
                provides no flow control
          
3. Network layer : Addresses and Best Path   (Path selection, Routing and Addressing)

Provide connectivity and path selection between two end systems where routing occurs
These may be located on geographically separated networks

   Responsible for hierarchichal addressing, segment network and control flow of traffic, reduce congestion
   IP Address: 32 bits long, Two components(network ID and host ID)
   
     ARP: 根据IP地址找到对应的MAC地址 

2. Data Link Layer : Access to media    (frame, media access control, naming )

provides reliable transit of data across physical layer
is concerned with physical (as opposed to logical) addressing, network topology, network access, error notification, ordered delivery of frames, and flow control

数据链路层（DataLinkLayer):在物理层提供比特流服务的基础上，建立相邻结点之间的数据链路，通过差错控制提供数据帧（Frame）在信道上无差错的传输，并进行各电路上的动作系列。 在不可靠的物理介质上提供可靠的传输。

分为LLC 和 MAC两层， 分别针对网络层和物理层：
Logical Link Control LLC:　Logical identifies different protocol types and encapsulate, Flow Control (基于滑动窗口，回退或选择重传)
Media Access Control MAC Protocol : Naming, Framing, Physical Addressing  
Carrier Sense Multiple Access with Collision Detection CSMA/CD :发送数据前 先侦听信道是否空闲
,若空闲，则立即发送数据。若信道忙碌，则等待一段时间至信道中的信息传输结束后再发送数据；若在上一段信息发送结束后，同时有两个或两个以上的节点都提出发送请求，则判定为冲突。若侦听到冲突,则立即停止发送数据，等待一段随机时间,再重新尝试。

1. Physical Layer : Binary Transimission    (Signal and Media)

defines the electrical and functional specifications for the link between end systems (including media).
defines voltage（电压） levels, timing of voltage changes, physical data rates, maximum transmission distances, physical connectors, and other, similar attributes

Protocol：










TCP Transmission Control Protocol 

Problems must
4000
be solved in TCP:

Reliable transfer
Flow control
Sliding window
congestion avoidance…
Connection management
Establish connection：three handshakes
Release connection： four handshakes



TCP Establish Connection 



TCP Release Connection:



UDP User Datagram Protocol:

Why do we need UDP?

No connection establishment (which can add delay)  No delay
Simple: no connection state at sender, receiver
Small segment header
No congestion control: UDP can blast away as fast as desired

Features:

Connectionless:
          no handshaking between UDP sender, receiver
          each UDP segment handled independently of others
Often used for streaming multimedia applications:
     loss tolerant
     rate sensitive
UDP are used in:
     RIP: To send the route information periodically
     DNS: Avoid the delay to setup the TCP connection
     SNMP: When congestion, SNMP must still runable. Without the congestion and reliability control mechanism, UDP has better performance than TCP under the circumstances.                                     Other protocols include TFTP, DHCP 
Add reliability at application layer if necessary