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Just as in standard mode for FTP, passive mode has two connections: control and data The bottom part of Figure 12-2 shows an example of the setup of these two connections The control connection in passive mode is established in the same manner as standard FTP: the user device chooses an open port greater than 1023 as a source port and uses a destination port of 21 Whenever a data connection is needed, the user establishes the connection to the server this is the opposite of standard-mode FTP For this data connection, the user device again chooses an open port number greater than 1023 as a source port, but acquires from the FTP server what port number to use for the destination port, a number greater than 1023 This number is negotiated on the control connection Client on the Inside of the Appliance When the client is on the inside of the network and initiates an FTP connection to an FTP server on the outside of the network, the appliance allows the connection by default because the connection is traveling from a higher-level interface to a lower-level one This is also true for the data connection Therefore, both connections can be established unless you are filtering with ACLs Client on the Outside of the Appliance In this example, assume that the client is on the outside of your network and that the FTP server is on the inside For the initial client connection to work, you need to configure an ACL that will allow traffic heading to the FTP server for TCP port 21 without this, no type of FTP connection is possible Once the command connection has been established, when the client attempts to retrieve or send a file, the client will attempt to establish a data connection to the server Assuming that application inspection is enabled for FTP, the appliance automatically looks at the FTP commands that the user is entering on the command connection, as well as the connection information being negotiated, and dynamically creates an entry.

BCNF and dependency preservation with 3NF It is worth noting that SQL does not provide a way of specifying functional depen-

follow-up or a questionnaire Be specific; ask if the pay program is competitive Analyze turnover rates to ensure that high-performing personnel are staying and low-performing personnel are departing

12:

dencies, except for the special case of declaring superkeys by using the primary key or unique constraints It is possible, although a little complicated, to write assertions that enforce a functional dependency (see Exercise 715); unfortunately, testing the assertions would be very expensive in most database systems Thus even if we had a dependency-preserving decomposition, if we use standard SQL we would not be able to ef ciently test a functional dependency whose left-hand side is not a key Although testing functional dependencies may involve a join if the decomposition is not dependency preserving, we can reduce the cost by using materialized views, which many database systems support Given a BCNF decomposition that is not dependency preserving, we consider each dependency in a minimum cover Fc that is not preserved in the decomposition For each such dependency , we de ne a materialized view that computes a join of all relations in the decomposition, and projects the result on The functional dependency can be easily tested on the materialized view, by means of a constraint unique ( ) On the negative side, there is a space and time overhead due to the materialized view, but on the positive side, the application programmer need not worry about writing code to keep redundant data consistent on updates; it is the job of the database system to maintain the materialized view, that is, keep up up to date when the database is updated (Later in the book, in Section 145, we outline how a database system can perform materialized view maintenance ef ciently) Thus, in case we are not able to get a dependency-preserving BCNF decomposition, it is generally preferable to opt for BCNF, and use techniques such as materialized views to reduce the cost of checking functional dependencies

in the connection table for the data connection However, if you ve disabled application inspection for FTP, you will have to manually add an ACL to allow traffic to your FTP server at TCP ports greater than 1023 as well as the first filter statement for port 21 that I already mentioned

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