OSI Model

OSI（Open System Interconnection）開放式通訊系統互連參考模型，是網路通訊的基礎架構。

七層模型概覽

OSI Layer	Layer Name	Protocols & Technologies	Full English Name
7	Application Layer	HTTP/HTTPS, FTP, SMTP, DNS, Telnet, SNMP, POP3/IMAP	HTTP/HTTPS: Hypertext Transfer Protocol / Secure, FTP: File Transfer Protocol, SMTP: Simple Mail Transfer Protocol, DNS: Domain Name System
6	Presentation Layer	SSL/TLS, ASCII, EBCDIC, JPEG, GIF, PNG, MPEG, MP3	SSL/TLS: Secure Sockets Layer / Transport Layer Security, ASCII: American Standard Code for Information Interchange
5	Session Layer	NetBIOS, PPTP, RPC, SMB	NetBIOS: Network Basic Input/Output System, PPTP: Point-to-Point Tunneling Protocol, RPC: Remote Procedure Call, SMB: Server Message Block
4	Transport Layer	TCP, UDP, SCTP, DCCP	TCP: Transmission Control Protocol, UDP: User Datagram Protocol, SCTP: Stream Control Transmission Protocol
3	Network Layer	IP, ICMP, IGMP, OSPF, RIP, BGP	IP: Internet Protocol, ICMP: Internet Control Message Protocol, OSPF: Open Shortest Path First, BGP: Border Gateway Protocol
2	Data Link Layer	Ethernet (MAC layer), PPP, ARP, HDLC, MPLS, VLAN	PPP: Point-to-Point Protocol, ARP: Address Resolution Protocol, HDLC: High-Level Data Link Control, MPLS: Multiprotocol Label Switching
1	Physical Layer	Ethernet (physical), USB, Bluetooth, DSL, IEEE 802.11	USB: Universal Serial Bus, DSL: Digital Subscriber Line, IEEE 802.11: Wireless LAN/Wi-Fi Standards

Layer 7 - Application Layer

應用層 - 提供網路服務給應用程式，最接近終端使用者。

提供為應用軟體而設計的介面，以設定與另一應用軟體之間的通訊
Protocols: SMTP, HTTP, HTTPS, Telnet, FTP, DNS, DHCP, SNMP

Layer 6 - Presentation Layer

表現層 - 處理資料格式轉換、加密/解密、壓縮。

把數據轉換為能與接收者的系統格式相容並適合傳輸的格式
SSL / TLS

Layer 5 - Session Layer

會議層 - 管理應用程式之間的會話或連線。

負責在數據傳輸中設定和維護電腦網路中兩台電腦之間的通訊連接
Stateful firewall (e.g., AWS Security Groups)

Layer 4 - Transport Layer

傳輸層 - 確保完整的資料傳輸，包含錯誤修正、流量控制和分段。

主要負責電腦整體的資料傳輸及控制。它可以將一個較大的資料切割成多個適合傳輸的資料，替模型頂端的第五、六、七等三個通訊層提供流量管制及錯誤控制。

TCP 協定在傳輸資料內加入驗證碼，當對方收到後，就會依這個驗證碼，回傳對應的確認訊息（ACK），若對方未及時傳回確認訊息，資料就會重新傳遞一次，以確保資料傳輸的完整性。

TCP & UDP Overview

TCP - Transmission Control Protocol

Is a connection based protocol. A Connection is established between devices using a random port on client and a known port on the server. Once established the connection is bi-directional. The connection is a reliable connection, provided via the segments encapsulated in IP Packets.

TCP Segments

3-way Handshake

Frame Types

Unicast Frame: Destination MAC is another host. Switch will flood only if MAC Address is not in MAC address table.
Broadcast Frame: Destination MAC address of FFFF.FFFF.FFFF. Broadcast frames are always flooded.

Layer 3 - Network Layer

網路層 - 管理定址、路由和跨網路資料傳遞。

網路層定義網路路由及定址功能，讓資料能夠在網路間傳遞。資料在傳輸時，該協定將 IP 位址加入傳輸資料內，並把資料組成封包（Packet）。在網路上傳輸時，封包裡面的 IP 位址會告訴網路設備這筆資料的來源及目的地。

End to End - 終端機或使用者端的封包傳送協定
IP address (IPv4/v6) - cross network addressing
ARP - Find the MAC address for this IP
Route - where to forward this packet
Route Tables - Multiple Routes
Router - moves packets from SRC to DST
Devices: Router

Layer 2 → Layer 3

Building a Common L3 Network - a hop occurs when a packet is passed from one network segment to the next.

Datagram / Packet Structure

Layer 4 Protocol: ICMP = 1 / TCP = 6 / UDP = 17

IPv4

Dotted-decimal Notation: 4 x 0-255

Subnet Mask

Allows a Host to determine if IP addresses it needs to communicate with is local or remote. Influences if it needs to use a gateway or can communicate locally.

Route Table & Routes

0.0.0.0/0 is the default route.

Address Resolution Protocol (ARP)

Links a L3 address to a L2 address. The Address Resolution Protocol is a layer 2 protocol used to map MAC addresses to IP addresses.

IP Routing

Layer 3 Problems

Layer 2 - Data Link Layer

資料連結層 - 處理同一區域網路（LAN）內節點之間的資料傳輸。

將實體層的數位訊號封裝成一組符合邏輯傳輸資料，這組訊號稱為資料訊框（Data Frame）。訊框內包含媒體存取控制（Media Access Control，MAC）位址，而資料在傳輸時，這項位址資訊可讓對方主機辨識資料來源。MAC 位址是一組序號，每個網路設備的 MAC 位址都是獨一無二的。

Hop to Hop - 指的是 router 傳送封包的協定
MAC address: 48 bits (e.g., 3e:22:fb:b9:5b:75), 24 bits for manufacturer (OUI)
Collision Detection (CSMA/CD)
Unicast (1:1) / Broadcast (1:ALL)
Devices: Switch

Frame Format

Field	Description
Preamble	一連串的 1010…10，用來同步
SFD	Start of Frame Delimiter，為 10101011，表示資料的起始
DA	Destination Address，目標的 MAC 位址
SA	Source Address，來源的 MAC 位址
ET	EtherType，IP 封包為 0x0800；ARP 封包為 0x0806
Payload	MAC 封包所要傳送的資料內容（46~1500 bytes）
PAD	Padding，補滿至最小長度
FCS	Frame Check Sequence (CRC32)，用來確認傳送資料是否有錯誤

CSMA/CD

Carrier Sense Multiple Access With Collision Detection（載波偵聽多路訪問／碰撞檢測）

Without Layer 2:

With Layer 2 (CSMA/CD):

Layer 2 using a HUB

Still occurs multiple collisions.

Layer 2 using a Switch

Collision occurs on one port only.

Layer 1 - Physical Layer

實體層 - 定義實體連線和原始資料在媒介上的傳輸。

Specifications define the transmission and reception of RAW BIT STREAMS between a device and a SHARED physical medium
Defines voltage levels, timing, rates, distances, modulation and connectors
No access Control
No uniquely identified devices
No device to device communications