Application Level Gateways Prof. Chuan-Ming Liu Computer Science - - PowerPoint PPT Presentation

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Application Level Gateways

• Prof. Chuan-Ming Liu

Computer Science and Information Engineering National Taipei University of Technology Taipei, TAIWAN

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Introduction

Tunneling allows one protocol suite to use the transport-level delivery service from another protocol suite in place of a physical network Continue discussing the techniques that clients and servers use to communicate across heterogeneous environments

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Constrained Environments

Limited access

Direct, uniform access is not always possible Programmers can not depend on uniform, end-to-end transport-level connectivity

Limited functionality

Limited on memory, CPU, and protocol software

Security

Partition the service into many groups

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Application Gateways

In the restricted environments, programmer usually use application gateway to overcome connectivity constraints Application gateway an intermediate program that runs on intermediate machines to relay information between a client and the desired server. If a machine has been dedicated to running

• ne particular application gateway, the

machine is referred to gateway machine

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Mail Gateway

SMTP E-AIL DOMAIN X.400 E-MAIL DOMAIN OS with TCP/IP and X.400 support Application gateway

Interface to SMTP mail system Interface to X.400 mail system

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Mail Gateway Implementation

Two threads work independently

One handles incoming message The other handles outgoing message

Long output message does not block input processing and long input message does not interfere with output processing

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Application Gateway vs. Tunneling

It is difficult to tell which is better Application gateway

Advantages:

Does not need to modify the OS Allows existing network systems to operate undisturbed

Disadvantages:

Separate application gateway program for each service Additional hardware resources

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Application Gateway vs. Tunneling

Tunneling:

Advantages:

Does not need to change when new services appear Provide uniformity

Disadvantages:

Need to modify OS to install Dramatic impact on users – change the underlying network usually results in changes to the client software

To avoid changing users’ environment, application gateway is preferred

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Limited Internet Connectivity

Application gateway can increase the range of services accessible from computers that have limited connectivity A computer with limited connectivity can run client software because the computer can wait until connectivity is available before executing the client software

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Limited Internet Connectivity

Application gateways offer a solution for situations in which a host cannot run a server

Operates like a server that accepts incoming information on behalf of each of the hosts Allows a host to access the information that has arrived

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E-mail Gateways

IP router O.S.

Appl. gateway portable computers that do not run a mail server to rest of internet large computer that runs application gateway

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Email Gateway

In addition to the standard mail server, the computer must also offer an application gateway service that users to access their mailboxs Application gateway can support heterogeneous protocol

Sending messages – SMTP( Simple Mail Transfer Protocol) Receiving messages – POP (Post Office Protocol)

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Security

Application gateway can also solve the security problem One instance is the restrict remote login

conventional hosts filters remote login except from C C: computer that runs application gateway which controls remote access for remote login O.S. Appl. gateway IP router

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Extra Hop Problem

Extra hop problem:

A situation in which datagrams pass across the same network twice on their way to final destination Caused by incorrect routing tables

Introducing application gateway into existing network can also create a form of the extra hop problem

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Example – extra hop problem

IP router application gateway machine IP router

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Extra Hop

May not be important when the network traffic is not large When the traffic is large, the next hop may make a solution too expensive Designers need to calculate expected load carefully before they adopt the application gateway

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Example Application Gateway

Interconnect the email and FTP services

Users can not access the ftp protocol Users can use the email service

Approach:

Users send requests for comments via email service to the application gateway Application gateway receives the request from the email, parses the email header, and then executes the ftp service for the users, and emails back to the user

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Example Application Gateway

To implement such an application gateway, a programmer needs:

A machine that has access to both email and ftp A program that acts as the application gateway A mechanism that passes each incoming email message for specified destination to the gateway

On LINUX, it is easy to implement The example uses a shell script to implement

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For the rest of this chapter, please read it by yourself!