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Test Your Knowledge in Computers #576
Difficulty: Medium
All programming languages have automatic garbage collection that monitors the dynamically allocated pieces of memory and determine if any variable in the program still references it. If the memory is no longer referenced, it is 'garbage' and becomes eligible to be 'collected'.
True or False?
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tc(8) [centos man page]

TC(8)								       Linux								     TC(8)

NAME
tc - show / manipulate traffic control settings SYNOPSIS
tc qdisc [ add | change | replace | link | delete ] dev DEV [ parent qdisc-id | root ] [ handle qdisc-id ] qdisc [ qdisc specific parame- ters ] tc class [ add | change | replace | delete ] dev DEV parent qdisc-id [ classid class-id ] qdisc [ qdisc specific parameters ] tc filter [ add | change | replace | delete ] dev DEV [ parent qdisc-id | root ] protocol protocol prio priority filtertype [ filtertype specific parameters ] flowid flow-id tc [ FORMAT ] qdisc show [ dev DEV ] tc [ FORMAT ] class show dev DEV tc filter show dev DEV tc [ -force ] [ -OK ] -b[atch] [ filename ] FORMAT := { -s[tatistics] | -d[etails] | -r[aw] | -p[retty] | -i[ec] } DESCRIPTION
Tc is used to configure Traffic Control in the Linux kernel. Traffic Control consists of the following: SHAPING When traffic is shaped, its rate of transmission is under control. Shaping may be more than lowering the available bandwidth - it is also used to smooth out bursts in traffic for better network behaviour. Shaping occurs on egress. SCHEDULING By scheduling the transmission of packets it is possible to improve interactivity for traffic that needs it while still guaranteeing bandwidth to bulk transfers. Reordering is also called prioritizing, and happens only on egress. POLICING Whereas shaping deals with transmission of traffic, policing pertains to traffic arriving. Policing thus occurs on ingress. DROPPING Traffic exceeding a set bandwidth may also be dropped forthwith, both on ingress and on egress. Processing of traffic is controlled by three kinds of objects: qdiscs, classes and filters. QDISCS
qdisc is short for 'queueing discipline' and it is elementary to understanding traffic control. Whenever the kernel needs to send a packet to an interface, it is enqueued to the qdisc configured for that interface. Immediately afterwards, the kernel tries to get as many packets as possible from the qdisc, for giving them to the network adaptor driver. A simple QDISC is the 'pfifo' one, which does no processing at all and is a pure First In, First Out queue. It does however store traffic when the network interface can't handle it momentarily. CLASSES
Some qdiscs can contain classes, which contain further qdiscs - traffic may then be enqueued in any of the inner qdiscs, which are within the classes. When the kernel tries to dequeue a packet from such a classful qdisc it can come from any of the classes. A qdisc may for example prioritize certain kinds of traffic by trying to dequeue from certain classes before others. FILTERS
A filter is used by a classful qdisc to determine in which class a packet will be enqueued. Whenever traffic arrives at a class with sub- classes, it needs to be classified. Various methods may be employed to do so, one of these are the filters. All filters attached to the class are called, until one of them returns with a verdict. If no verdict was made, other criteria may be available. This differs per qdisc. It is important to notice that filters reside within qdiscs - they are not masters of what happens. CLASSLESS QDISCS
The classless qdiscs are: [p|b]fifo Simplest usable qdisc, pure First In, First Out behaviour. Limited in packets or in bytes. pfifo_fast Standard qdisc for 'Advanced Router' enabled kernels. Consists of a three-band queue which honors Type of Service flags, as well as the priority that may be assigned to a packet. red Random Early Detection simulates physical congestion by randomly dropping packets when nearing configured bandwidth allocation. Well suited to very large bandwidth applications. sfq Stochastic Fairness Queueing reorders queued traffic so each 'session' gets to send a packet in turn. tbf The Token Bucket Filter is suited for slowing traffic down to a precisely configured rate. Scales well to large bandwidths. CONFIGURING CLASSLESS QDISCS
In the absence of classful qdiscs, classless qdiscs can only be attached at the root of a device. Full syntax: tc qdisc add dev DEV root QDISC QDISC-PARAMETERS To remove, issue tc qdisc del dev DEV root The pfifo_fast qdisc is the automatic default in the absence of a configured qdisc. CLASSFUL QDISCS
The classful qdiscs are: CBQ Class Based Queueing implements a rich linksharing hierarchy of classes. It contains shaping elements as well as prioritizing capa- bilities. Shaping is performed using link idle time calculations based on average packet size and underlying link bandwidth. The latter may be ill-defined for some interfaces. HTB The Hierarchy Token Bucket implements a rich linksharing hierarchy of classes with an emphasis on conforming to existing practices. HTB facilitates guaranteeing bandwidth to classes, while also allowing specification of upper limits to inter-class sharing. It con- tains shaping elements, based on TBF and can prioritize classes. PRIO The PRIO qdisc is a non-shaping container for a configurable number of classes which are dequeued in order. This allows for easy prioritization of traffic, where lower classes are only able to send if higher ones have no packets available. To facilitate config- uration, Type Of Service bits are honored by default. THEORY OF OPERATION
Classes form a tree, where each class has a single parent. A class may have multiple children. Some qdiscs allow for runtime addition of classes (CBQ, HTB) while others (PRIO) are created with a static number of children. Qdiscs which allow dynamic addition of classes can have zero or more subclasses to which traffic may be enqueued. Furthermore, each class contains a leaf qdisc which by default has pfifo behaviour, although another qdisc can be attached in place. This qdisc may again contain classes, but each class can have only one leaf qdisc. When a packet enters a classful qdisc it can be classified to one of the classes within. Three criteria are available, although not all qdiscs will use all three: tc filters If tc filters are attached to a class, they are consulted first for relevant instructions. Filters can match on all fields of a packet header, as well as on the firewall mark applied by ipchains or iptables. Type of Service Some qdiscs have built in rules for classifying packets based on the TOS field. skb->priority Userspace programs can encode a class-id in the 'skb->priority' field using the SO_PRIORITY option. Each node within the tree can have its own filters but higher level filters may also point directly to lower classes. If classification did not succeed, packets are enqueued to the leaf qdisc attached to that class. Check qdisc specific manpages for details, however. NAMING
All qdiscs, classes and filters have IDs, which can either be specified or be automatically assigned. IDs consist of a major number and a minor number, separated by a colon. Both major and minor number are limited to 16 bits. There are two special values: root is signified by major and minor of all ones, and unspecified is all zeros. QDISCS A qdisc, which potentially can have children, gets assigned a major number, called a 'handle', leaving the minor number namespace available for classes. The handle is expressed as '10:'. It is customary to explicitly assign a handle to qdiscs expected to have children. CLASSES Classes residing under a qdisc share their qdisc major number, but each have a separate minor number called a 'classid' that has no relation to their parent classes, only to their parent qdisc. The same naming custom as for qdiscs applies. FILTERS Filters have a three part ID, which is only needed when using a hashed filter hierarchy. PARAMETERS
The following parameters are widely used in TC. For other parameters, see the man pages for individual qdiscs. RATES Bandwidths or rates. These parameters accept a floating point number, possibly followed by a unit (both SI and IEC units sup- ported). bit or a bare number Bits per second kbit Kilobits per second mbit Megabits per second gbit Gigabits per second tbit Terabits per second bps Bytes per second kbps Kilobytes per second mbps Megabytes per second gbps Gigabytes per second tbps Terabytes per second To specify in IEC units, replace the SI prefix (k-, m-, g-, t-) with IEC prefix (ki-, mi-, gi- and ti-) respectively. TC store rates as a 32-bit unsigned integer in bps internally, so we can specify a max rate of 4294967295 bps. TIMES Length of time. Can be specified as a floating point number followed by an optional unit: s, sec or secs Whole seconds ms, msec or msecs Milliseconds us, usec, usecs or a bare number Microseconds. TC defined its own time unit (equal to microsecond) and stores time values as 32-bit unsigned integer, thus we can specify a max time value of 4294967295 usecs. SIZES Amounts of data. Can be specified as a floating point number followed by an optional unit: b or a bare number Bytes. kbit Kilobits kb or k Kilobytes mbit Megabits mb or m Megabytes gbit Gigabits gb or g Gigabytes TC stores sizes internally as 32-bit unsigned integer in byte, so we can specify a max size of 4294967295 bytes. VALUES Other values without a unit. These parameters are interpreted as decimal by default, but you can indicate TC to interpret them as octal and hexadecimal by adding a '0' or '0x' prefix respectively. TC COMMANDS
The following commands are available for qdiscs, classes and filter: add Add a qdisc, class or filter to a node. For all entities, a parent must be passed, either by passing its ID or by attaching directly to the root of a device. When creating a qdisc or a filter, it can be named with the handle parameter. A class is named with the classid parameter. delete A qdisc can be deleted by specifying its handle, which may also be 'root'. All subclasses and their leaf qdiscs are automatically deleted, as well as any filters attached to them. change Some entities can be modified 'in place'. Shares the syntax of 'add', with the exception that the handle cannot be changed and nei- ther can the parent. In other words, change cannot move a node. replace Performs a nearly atomic remove/add on an existing node id. If the node does not exist yet it is created. link Only available for qdiscs and performs a replace where the node must exist already. FORMAT
The show command has additional formatting options: -s, -stats, -statistics output more statistics about packet usage. -d, -details output more detailed information about rates and cell sizes. -r, -raw output raw hex values for handles. -p, -pretty decode filter offset and mask values to equivalent filter commands based on TCP/IP. -iec print rates in IEC units (ie. 1K = 1024). -b, -b filename, -batch, -batch filename read commands from provided file or standard input and invoke them. First failure will cause termination of tc. -force don't terminate tc on errors in batch mode. If there were any errors during execution of the commands, the application return code will be non zero. -OK in batch mode, print OK and a new line on standard output after each successfully interpreted command. HISTORY
tc was written by Alexey N. Kuznetsov and added in Linux 2.2. SEE ALSO
tc-bfifo(8), tc-cbq(8), tc-choke(8), tc-codel(8), tc-drr(8), tc-ematch(8), tc-fq_codel(8), tc-hfsc(7), tc-hfsc(8), tc-htb(8), tc-pfifo(8), tc-pfifo_fast(8), tc-red(8), tc-sfb(8), tc-sfq(8), tc-stab(8), tc-tbf(8), User documentation at http://lartc.org/, but please direct bugreports and patches to: <netdev@vger.kernel.org> AUTHOR
Manpage maintained by bert hubert (ahu@ds9a.nl) iproute2 16 December 2001 TC(8)

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