.: Advanced Linux Networking :.

Suppose your Linux system is a router for a busy domain, or is a major server that processes a lot of traffic. In situations like this, it's not uncommon for Linux to find that it has more packets to process than it can send over its network interfaces. Thus, Linux needs some system for scheduling the transmission of outgoing packets. Ordinarily, Linux uses a first-in/first-out (FIFO) strategy, in which each outgoing packet waits in line behind all the others that have already been queued. In some situations, however, you might not want to use this system. You might want to favor certain types of packets, such as those delivered to certain networks or those that involve certain protocols. For instance, you might want to favor packets that carry real-time data, such as Internet telephony protocols. Adjusting packet priorities is the job of the quality of service (QoS) options. These options are all available from the QoS and/or Fair Queueing menu off of the Networking Options menu.

In order to implement a QoS system, you must select the QoS and/or Fair Queueing option in the menu of the same name. This action enables many of the options on this menu. A few others rely upon your selection of one or more other specific options. The most basic features are enabled by the various packet scheduler and queue options, such as CBQ Packet Scheduler and SFQ Queue. These options allow the kernel to schedule packets in more complex ways than the default FIFO. The QoS Support and Packet Classifier API options, as well as their individual suboptions, enable the use of Differentiated Services and the Resource Reservation Protocol (RSVP). These both allow routers to communicate QoS priorities to other routers. If all the routers between two sites implement compatible QoS protocols, the end result can be greatly improved performance for time-critical protocols, at the expense of less time-critical protocols.

Most nonrouter systems don't need any QoS options. If you're configuring a Linux computer as a router, though—particularly a heavily used router—you may want to activate these options. If you activate one, it may make sense to activate them all, because without all options activated, the tools you use to specify QoS criteria won't be as flexible. For instance, if you omit the U32 Classifier option, you won't be able to prioritize traffic according to the destination address.

In practice, using QoS features requires the use of advanced routing tools, such as ip and tc. Chapter 24 touches upon these tools, but they can be extremely complex. The iproute2 + tc Notes (http://snafu.freedom.org/linux2.2/iproute-notes.html) and Differentiated Services on Linux (http://diffserv.sourceforge.net) Web sites contain additional documentation on these tools.