Application Level Gateways

Transcription

1 Application Level Gateways Prof. Chuan-Ming Liu Computer Science and Information Engineering National Taipei University of Technology Taipei, TAIWAN NTUT, TAIWAN 1

2 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

3 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

4 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

5 Mail Gateway Interface to SMTP mail system Application gateway OS with TCP/IP and X.400 support Interface to X.400 mail system SMTP E-AIL DOMAIN X.400 DOMAIN NTUT, TAIWAN 5

6 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

7 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

8 Application Gateway vs. Tunneling: Advantages: Tunneling 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

9 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

10 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

11 Gateways to rest of internet IP router large computer that runs application gateway Appl. gateway O.S. portable computers that do not run a mail server NTUT, TAIWAN 11

12 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

13 Security Application gateway can also solve the security problem One instance is the restrict remote login filters remote login except from C IP router C: computer that runs application gateway which controls remote access for remote login Appl. gateway O.S. conventional hosts NTUT, TAIWAN 13

14 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

15 Example extra hop problem IP router IP router application gateway machine NTUT, TAIWAN 15

16 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

17 Example Application Gateway Interconnect the and FTP services Users can not access the ftp protocol Users can use the service Approach: Users send requests for comments via service to the application gateway Application gateway receives the request from the , parses the header, and then executes the ftp service for the users, and s back to the user NTUT, TAIWAN 17

18 Example Application Gateway To implement such an application gateway, a programmer needs: A machine that has access to both and ftp A program that acts as the application gateway A mechanism that passes each incoming 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

19 For the rest of this chapter, please read it by yourself! NTUT, TAIWAN 19