Embedded Web Server (microcontroller with TCP-IP).pdf

AVR460: Embedded Web Server

Introduction

Intelligent homes will be connected to the Internet and require a microcontroller to

communicate with the other network devices. The AVR ® embedded web server can

simplify the design process for embedded web server applications. The AVR embed-

ded web server design includes a complete web server with TCP/IP support and

Ethernet interface. It also includes support for sending mail, and software for auto-

matic configuration of the web server in the network. The AVR web server reference

design includes complete source code written in C-language. A comprehensive

designers guide describes all web server components and gives embedded systems

designers a quick start to embedded web servers.

Embedded

Web Server

Application

Note

System Description

The AVR embedded web server reference design is designed for integration in digital

equipment. The AVR embedded web server can be plugged into any Ethernet inter-

face and communicate with a standard web browser. Figure 2 illustrates some

situations where the web server can be used.

As shown in Figure 2, various consumer electronics can be controlled from a com-

puter connected to the Internet. The web page is the “Control Center” for the AVR

embedded web server.

Figure 1. AVR Embedded Web Server

Rev. 2396B–11/01

Suppose the AVR embedded web server is embedded in several units in a house. Every

server is connected to the network. A computer located at home as on Figure 2 controls

all devices and can receive requests from other computers on the Internet. The web

server is identified by its unique IP address and can be controlled remotely from any-

where in the world as long as the authorization is in order.

Figure 2. Monitoring Home Equipment from the Office

Workstation

Server

Internet

Hub

Office

Home Computer

Refridgerator

Television

Hub

Home

TCP/IP Protocol Suite The TCP/IP protocol suite allows computers of all sizes, running different operating sys-

tems, to communicate with each other. It forms the basis for what is called the worldwide

Internet, a Wide Area Network (WAN) of several million computers.

TCP/IP Suite Layers

The TCP/IP protocol suite is a combination of different protocols at various layers.

TCP/IP is normally considered to be a 4-layer system as shown in Figure 3.

Figure 3. Four Layers of TCP/IP Protocol Suite

Application

Telnet, FTP, HTTP

Transport

TCP, UDP

Network

IP, ICMP

Link

Device Driver and Interface Card

AVR Embedded Web Server

2396B–11/01

AVR Embedded Web Server

Application Layer

The Application layer handles the details of a particular application. Common TCP/IP

applications include:

• Telnet for remote login

• Browser support for displaying web pages

• File transfer applications

• E-mail applications

The three lower layers do not know anything about the specific application and only take

care of communications details.

Transport Layer

TCP is responsible for a reliable flow of data between two hosts. Typically, TCP divides

data passed to it from the application into appropriately sized chunks for the network

layer below, acknowledging received packets that are sent and retransmits lost packets.

Since this reliable, flow of data is provided by the Transport Layer, the Application Layer

above can ignore these details.

UDP is a much simpler service to the Application Layer. It sends packets of data called

datagrams from one host to the other, but with no guarantee that the datagrams reach

the other end. Desired reliability must be added by the Application Layer.

Network Layer

This layer is sometimes called the Internet Layer. It handles the movements of packets

around the network. Routing of packets, for example, takes place here. IP (Internet Pro-

tocol) and ICMP (Internet Control Message Protocol) provides the Network Layer in the

TCP/IP Protocol Suite.

Link Layer

Data-link or Network Interface Layer is another common name of this layer. The Link

Layer normally includes the device driver in the operating system and the corresponding

network interface (card) in the computer. Together they handle all the hardware details

of physically interfacing with the cable.

Figure 4 shows an example that includes two hosts on a Local Area Network (LAN) such

as Ethernet, using HTTP.

Figure 4. Example with Protocols Involved

Application

HTTP

Client

HTTP

Server

Transport

TCP

Network

Link

Ethernet

Driver

Ethernet

Driver

2396B–11/01

One side represents the client, and the other the server. The server provides some type

of service to clients, in this case, access to web pages on the server host. Each layer

has one or more protocols for communicating with its peer at the same layer. One proto-

col, for example, allows the two TCP layers to communicate, and another protocol lets

the two IP layers communicate.

The Application Layer is normally a user-process while the lower three layers are usu-

ally implemented in the kernel (the operating system).

Port Numbers

Different applications can use TCP or UDP at any time. The Transport layer protocols

store an identifier in the headers they generate to identify the application. Both TCP and

UDP use 16-bit port numbers to identify applications. TCP and UDP store the source

port number and the destination port number in those respective headers.

Servers are normally known by their well-known port number. Every TCP/IP implemen-

tation with a FTP server provides that service on TCP port 21. Every Telnet server is on

TCP port 23. Services provided by any implementation of TCP/IP have well-known port

numbers between 1 and 1023. The well-known ports are managed by the Internet

Assigned Numbers Authority (IANA).

A client usually does not care what port number it uses on its end. All it needs to be cer-

tain of is that whatever port number it uses, it must be unique on its host. Client port

numbers are called ephemeral ports (i.e., short lived). This is because a client typically

exists only as long as the user running the client needs its service, while servers typi-

cally run as long as the host is up. Most TCP/IP implementations allocate ephemeral

port numbers between 1024 and 5000. The port numbers above 5000 are intended for

other services (those that are not well known across the Internet).

The combination of an IP address and a port number is called a socket.

Encapsulation

When an application sends data using TCP, the data is sent down the protocol stack,

through each layer, until it is sent as a stream of bits across the network. Each layer

adds information to the data by prepending headers and adding trailers to the data it

receives. Figure 5 shows this process.

Figure 5. Encapsulation of Data as It Goes Down the Protocol Stack

User

Data

HTTP

Client

Appl.

Header

User

Data

TCP

Header

Application

Data

TCP Segment

Header

TCP

Header

Application

Data

IP Datagram

Ethernet

Driver

Ethernet

Header

TCP

Header

Application

Data

Ethernet

Trailer

Ethernet Frame

46 to 1500 Bytes

AVR Embedded Web Server

2396B–11/01

AVR Embedded Web Server

Some abbreviations:

• TCP segment: The unit of data that TCP sends to IP.

• IP datagram: The unit of data that IP sends to the network interface.

• Frame: The stream of bits that flows across the Ethernet.

IP (Internet Protocol) adds an identifier to the IP header it generates to indicate which

layer the data belongs to. IP handles this by storing an 8-bit value in its header called

the protocol field. Similarly, many different applications can be using TCP or UDP at any

time. The Transport Layer protocol stores an identifier in the header they generate to

identify the application. Both TCP and UDP use 16-bit port numbers to identify applica-

tions. The TCP and UDP store the source port number and the destination port number

in their respective headers. The network interface sends and receives frames on behalf

of IP, ARP, RARP. There must be some form of identification in the Ethernet header

indicating which network layer protocol generates the data. To handle this, there is a 16-

bit frame type field in the Ethernet header.

Internet Addresses

Every interface on the internet has a unique Internet Address (IP Address). The

addresses are 32-bit numbers. Figure 6 shows the structure and the five different

classes of Internet addresses.

Figure 6. The Five Different Classes of Internet Addresses

7 Bits

NetID

24 Bits

HostID

1 0

14 Bits

NetID

16 Bits

HostID

1 1 0

21 Bits

NetID

8 Bits

HostID

1 1 1 0

28 Bits

Multicast Group ID

1 1 1 1

28 Bits

(Reserved for Future Use)

The 32-bit addresses are normally written as four decimal numbers, one for each byte of

the address. See Table 1. This notation is called dotted-decimal. Since every interface

on internet must have a unique IP address, there must be one central authority for allo-

cating these addresses for networks connected to the worldwide Internet. Internet

Network Information Center (NIC) is the responsible authority.

Table 1. Ranges of Different Classes of IP Addresses

Class

Range

0.0.0.0 to 127.255.255.255

128.0.0.0 to 191.255.255.255

192.0.0.0 to 223.255.255.255

224.0.0.0 to 239.255.255.255

240.0.0.0 to 255.255.255.255

2396B–11/01

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: