Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab Application Level Gateways Prof. Chuan-Ming Liu Computer Science and Information Engineering National Taipei University of Technology Taipei, TAIWAN NTUT, TAIWAN 1
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 2
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 3
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 one particular application gateway, the machine is referred to gateway machine NTUT, TAIWAN 4
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab Mail Gateway Application gateway Interface to Interface to SMTP mail system X.400 mail system OS with TCP/IP and X.400 support SMTP E-AIL X.400 E-MAIL DOMAIN DOMAIN NTUT, TAIWAN 5
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 6
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 7
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 8
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 9
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 10
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab E-mail Gateways to rest of large computer internet that runs Appl. application gateway gateway IP router O.S. portable computers that do not run a mail server NTUT, TAIWAN 11
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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) NTUT, TAIWAN 12
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab Security Application gateway can also solve the security problem One instance is the restrict remote login C: computer that runs filters Appl. application gateway remote gateway which controls remote login IP access for remote login except router O.S. from C conventional hosts NTUT, TAIWAN 13
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 14
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab Example – extra hop problem IP router application gateway IP machine router NTUT, TAIWAN 15
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 16
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 17
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab 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 NTUT, TAIWAN 18
Mobile Computing & Software Engineering Lab Mobile Computing & Software Engineering Lab For the rest of this chapter, please read it by yourself! NTUT, TAIWAN 19
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