Each layer and the set of protocols it uses

1. The functions of each layer are relatively simple

2. Minimize communication between layers

3. The precise definition of the functions of each layer must be independent of the specific method

4. The lower layer provides services to the upper layer

1. The nth layer entity not only uses the services of n-1, but also provides services to layer n1.

2. The lowest layer only provides services; the highest layer provides services to users

3. The upper layer can only use the lower layer services through the inter-layer interface; the lower layer services are transparent to the upper layer

4. There is a logical direct communication channel between the peer layers.

Control rules for communication between peer entities

I. grammar

II. Semantics

III. Synchronize

SAP is provided for upper-level use

Only functions that are "visible" by higher-level entities are called services

ask

instruct

response

Confirm

1. Connection-oriented and connection-less

2. Reliable and unreliable

3. Response and non-response

Bits

Transparent transmission of bitstreams

RJ45

Repeater

hub

Network card, network cable, modem

frame

Encapsulate IP datagrams into frames

PPP

ArpaNet (the world's first computer network)

switch

bridge

Packet/datagram

Responsible for the delivery of data packets from source to sink

Routing

IP

IGP (Internal Gateway)

IPX (packet switching)

router

data segment

End-to-end reliable data transfer

flow control

TCP

gateway

information

Create, manage, terminate sessions, establish synchronization

DNS

gateway

information

Encrypt, decrypt, translate, compress data

JPEG

gateway

information

Interact directly with users

FTP (20 (data), 21 (control))

DNS(53)

DHCP

gateway