Introduction to TCP/IP (Part 3) - Client Server Model

Source: Microchip Developer Help

Client Server Model Defined

The client-server programming model is a distributed computing architecture that segregates information users (clients) from information providers (servers).

Client

• An application that needs something (like a web page or IP address) from a server
• May contact a server at any time
• Information users

Server

• An application that provides information or resources to clients
• Must be always up and running, waiting for requests
• Information providers

Key Principle

• Client applications communicate only with server applications and vice versa
• Clients do not communicate directly with other clients

Alternative: Peer to Peer (P2P)

In P2P architectures, two or more hosts communicate directly with each other (no client-server distinction).

Client Server Examples

Example: DHCP Client Server

The Dynamic Host Configuration Protocol (DHCP) is responsible for requesting and offering IP addresses.

• DHCP server typically exists in a router
• May also run on a network server for larger networks

Example: HTTP Client and Server

Scenario: Using an HTTP client on a PC to control home lights

1. HTTP client runs on a home lighting control board
2. Control board monitors a lighting control website on an Internet web server
3. User browses to the same webpage, enters credentials
4. User changes the webpage settings
5. Control board checks the webpage and controls lights accordingly

Example: HTTP Client and Server in the Same Host

A network host can be both a client and a server simultaneously.

Example:

• Control board runs an HTTP Client (to check lighting control website)
• Control board also runs an HTTP Server (to serve a setup/configuration page)
• Configuration page allows changing the website and login info used by the HTTP client

Example: Local Network HTTP Server

An HTTP server on an embedded device can directly control the device:

• Eliminates need for HTTP client application and Internet web server
• Easiest solution if HTTP client is on the same local network
• Challenge: Accessing from the Internet is not trivial

Internet Server vs. Local Network Server

Internet Server

Accessing an HTTP server on the Internet is effortless.

Architecture:

• Webpage runs on an HTTP server in the Internet
• Embedded HTTP client posts status and polls for commands
• Web browser (PC/smartphone) monitors and controls via the webpage

Hosting Options:

• Shared web hosting (GoDaddy™, Network Solutions®)
• Cloud services (Amazon Web Services, Microsoft Azure®)
• Service providers allow you to choose a website name

Local Network Server

HTTP server located on a local network (running on the embedded device itself).

Architecture:

• Webpage runs directly on the embedded HTTP server
• Web browser accesses the embedded device directly

Challenges that Internet servers don’t have:

• Firewall restrictions
• No website name by default
• Dynamic IP address issues

Local Network Server Obstacles and Solutions

Obstacles

1. Firewall - Router restricts public access to local network
2. No website name - Remote clients must use IP address
3. Dynamic IP address - ISP may change your IP address

Firewall

Internet
    │
    ▼
┌─────────────────────────────┐
│  Office Router + Firewall   │  ← Firewall is HERE (inside the router)
└─────────────────────────────┘
    │
    ▼
┌─────────────────────────────┐
│  Office Network (Private)   │  ← This is BEHIND the firewall
│  - HTTP Server 192.168.1.101│
└─────────────────────────────┘

Solution 1: Port Forwarding

When you receive incoming traffic on port 80 (HTTP), don’t block it. Instead, forward it to the internal device at 192.168.1.101

Port forwarding allows Internet packets destined for a particular port to be forwarded to a specific local network IP address.

Configuration:

• Configure router to forward incoming HTTP server packets to the local IP address
• Require username/password for unauthorized access prevention
• Use SSL encryption to prevent packet sniffing

Note: Properly implemented and secure port forwarding is also called a “firewall pinhole”

Solution 2: Website Name (Domain Name)

Advantages of a website name:

• Easier to remember than IP addresses
• If IP address changes, DNS servers can be automatically updated

Domain name providers:

• GoDaddy
• DynDNS

Solution 3: Dynamic IP Address Handling

Understanding Dynamic vs Static IP:

Static IP addresses cost more from ISPs

Solution for Dynamic IP:

1. Use website name instead of IP address to access your server
2. Update DNS server with new IP address when it changes
3. Some routers have this capability built-in
4. Embedded device can periodically check its IP and update DNS if changed

DNS Service Providers:

• DynDNS
• ZoneEdit

These provide domain names and ability to update DNS servers when IP changes.

Learn More

• All TCP/IP Protocol Suite Topics
• Part 4 - Sockets and Ports