At that point, the original voltage is not recoverable, and the error needs to be corrected. If the corrupted bit travels much farther, however, accumulated noise can change its meaning completely. A repeater must be placed so that a signal reaches it before any noise changes the meaning of any of its bits. The location of a repeater on a link is vital. A repeater can extend the physical length of a LAN. The repeater then sends the refreshed signal. A repeater receives a signal and, before it becomes too weak or corrupted, regenerates the original bit pattern. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. (There have been some models which would utterly break connections instead of 'fixing' them.) For example, disabling H.323 support (an old VoIP protocol) should be fine.A repeater is a device that operates only in the physical layer. You could say ALGs are tools for disguising problems.)Īs for which you can disable: that really depends on which protocols you use, and whether your particular router's ALG is of acceptable quality. (Yes, most of those stop working when encryption is enabled since the ALG can no longer look inside. For example, you can log in to the FTP server, but it timeouts while trying to get the file list. Usually, what happens if the appropriate ALG is not present is that certain connections simply hang in the middle. An ALG tries to do the appropriate rewriting of those FTP commands. But some protocols also send the client's or server's address inside packets themselves – for example, yes, the same FTP does this (in active mode the client sends its own address, in passive mode the server does).
(This includes automagic port forwarding when NAT is in use.)įirewalls with NAT enabled translate IP addresses and TCP/UDP ports within the corresponding headers. So the firewall needs an ALG module that snoops on FTP commands and automatically adds the necessary rules. However, some protocols use additional connections – for example, FTP in 'active' mode makes the server connect back to you on a separate port. That is, you send a packet from port X to server's port Y, and the firewall automatically allows the reverse back in. On a stateful firewall, the "state" is usually tied to just addresses and port numbers.
That is, firewall modules which cope with some peculiarities of those protocols. "ALG" here stands for "Application-layer Gateway".
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