Total Introduction
This book is top-down.
Why do we need layers?
Computer networks are too complex.
Each layer solves a specific function, and then uses the services provided by the lower layer to provide better and higher-quality services to the upper layer by exchanging protocol messages with the protocol entities of the peer layer.
Application Layer
Many application services run on the application layer, calling the services provided by the transport layer,such as TCP or UDP.
Common application layer services include HTTP, FTP and some streaming services,etc.
Transport Layer
The service provided is process-to-process,which is an enhancement of the host-to-host service provided at the network layer.
Reliability services : TCP (need round trip time : RTT)
TCP takes 2 RTT to establish a connection.
Unreliability services : UDP (but is Real-time)
Network Layer(based on ip)
The service provided is host-to-host (end-to-end). End-to-end services are provided by point-to-point services.
IP
It’s a best effort service.
Best Effort: NO guarantees for data transfer.
Working methods
Traditional method: IP protocol and router protocol
Router protocol compute the routing table,which is handed over to the ip protocol for forwarding.
- Query the routing table to find the entry
- TRUE : forwarding
- FALSE : forwarding to a fixed address
SDN method: (Software-Defined Networking)
The key to SDN is to separate the control plane and data plane of the network, allowing network administrators to control network traffic programmatically without having to rely on the inherent functions of the underlying hardware devices.
Data Link Layer
The service provided is point-to-point.
Physics layer
Transmitting signals through media.