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diff --git a/man/man8/tc-prio.8 b/man/man8/tc-prio.8 new file mode 100644 index 0000000..605f3d3 --- /dev/null +++ b/man/man8/tc-prio.8 @@ -0,0 +1,185 @@ +.TH PRIO 8 "16 December 2001" "iproute2" "Linux" +.SH NAME +PRIO \- Priority qdisc +.SH SYNOPSIS +.B tc qdisc ... dev +dev +.B ( parent +classid +.B | root) [ handle +major: +.B ] prio [ bands +bands +.B ] [ priomap +band band band... +.B ] [ estimator +interval timeconstant +.B ] + +.SH DESCRIPTION +The PRIO qdisc is a simple classful queueing discipline that contains +an arbitrary number of classes of differing priority. The classes are +dequeued in numerical descending order of priority. PRIO is a scheduler +and never delays packets - it is a work-conserving qdisc, though the qdiscs +contained in the classes may not be. + +Very useful for lowering latency when there is no need for slowing down +traffic. + +.SH ALGORITHM +On creation with 'tc qdisc add', a fixed number of bands is created. Each +band is a class, although is not possible to add classes with 'tc qdisc +add', the number of bands to be created must instead be specified on the +command line attaching PRIO to its root. + +When dequeueing, band 0 is tried first and only if it did not deliver a +packet does PRIO try band 1, and so onwards. Maximum reliability packets +should therefore go to band 0, minimum delay to band 1 and the rest to band +2. + +As the PRIO qdisc itself will have minor number 0, band 0 is actually +major:1, band 1 is major:2, etc. For major, substitute the major number +assigned to the qdisc on 'tc qdisc add' with the +.B handle +parameter. + +.SH CLASSIFICATION +Three methods are available to PRIO to determine in which band a packet will +be enqueued. +.TP +From userspace +A process with sufficient privileges can encode the destination class +directly with SO_PRIORITY, see +.BR socket(7). +.TP +with a tc filter +A tc filter attached to the root qdisc can point traffic directly to a class +.TP +with the priomap +Based on the packet priority, which in turn is derived from the Type of +Service assigned to the packet. +.P +Only the priomap is specific to this qdisc. +.SH QDISC PARAMETERS +.TP +bands +Number of bands. If changed from the default of 3, +.B priomap +must be updated as well. +.TP +priomap +The priomap maps the priority of +a packet to a class. The priority can either be set directly from userspace, +or be derived from the Type of Service of the packet. + +Determines how packet priorities, as assigned by the kernel, map to +bands. Mapping occurs based on the TOS octet of the packet, which looks like +this: + +.nf +0 1 2 3 4 5 6 7 ++---+---+---+---+---+---+---+---+ +| | | | +|PRECEDENCE | TOS |MBZ| +| | | | ++---+---+---+---+---+---+---+---+ +.fi + +The four TOS bits (the 'TOS field') are defined as: + +.nf +Binary Decimal Meaning +----------------------------------------- +1000 8 Minimize delay (md) +0100 4 Maximize throughput (mt) +0010 2 Maximize reliability (mr) +0001 1 Minimize monetary cost (mmc) +0000 0 Normal Service +.fi + +As there is 1 bit to the right of these four bits, the actual value of the +TOS field is double the value of the TOS bits. Tcpdump -v -v shows you the +value of the entire TOS field, not just the four bits. It is the value you +see in the first column of this table: + +.nf +TOS Bits Means Linux Priority Band +------------------------------------------------------------ +0x0 0 Normal Service 0 Best Effort 1 +0x2 1 Minimize Monetary Cost 0 Best Effort 1 +0x4 2 Maximize Reliability 0 Best Effort 1 +0x6 3 mmc+mr 0 Best Effort 1 +0x8 4 Maximize Throughput 2 Bulk 2 +0xa 5 mmc+mt 2 Bulk 2 +0xc 6 mr+mt 2 Bulk 2 +0xe 7 mmc+mr+mt 2 Bulk 2 +0x10 8 Minimize Delay 6 Interactive 0 +0x12 9 mmc+md 6 Interactive 0 +0x14 10 mr+md 6 Interactive 0 +0x16 11 mmc+mr+md 6 Interactive 0 +0x18 12 mt+md 4 Int. Bulk 1 +0x1a 13 mmc+mt+md 4 Int. Bulk 1 +0x1c 14 mr+mt+md 4 Int. Bulk 1 +0x1e 15 mmc+mr+mt+md 4 Int. Bulk 1 +.fi + +The second column contains the value of the relevant +four TOS bits, followed by their translated meaning. For example, 15 stands +for a packet wanting Minimal Monetary Cost, Maximum Reliability, Maximum +Throughput AND Minimum Delay. + +The fourth column lists the way the Linux kernel interprets the TOS bits, by +showing to which Priority they are mapped. + +The last column shows the result of the default priomap. On the command line, +the default priomap looks like this: + + 1 2 2 2 1 2 0 0 1 1 1 1 1 1 1 1 + +This means that priority 4, for example, gets mapped to band number 1. +The priomap also allows you to list higher priorities (> 7) which do not +correspond to TOS mappings, but which are set by other means. + +This table from RFC 1349 (read it for more details) explains how +applications might very well set their TOS bits: + +.nf +TELNET 1000 (minimize delay) +FTP + Control 1000 (minimize delay) + Data 0100 (maximize throughput) + +TFTP 1000 (minimize delay) + +SMTP + Command phase 1000 (minimize delay) + DATA phase 0100 (maximize throughput) + +Domain Name Service + UDP Query 1000 (minimize delay) + TCP Query 0000 + Zone Transfer 0100 (maximize throughput) + +NNTP 0001 (minimize monetary cost) + +ICMP + Errors 0000 + Requests 0000 (mostly) + Responses <same as request> (mostly) +.fi + + +.SH CLASSES +PRIO classes cannot be configured further - they are automatically created +when the PRIO qdisc is attached. Each class however can contain yet a +further qdisc. + +.SH BUGS +Large amounts of traffic in the lower bands can cause starvation of higher +bands. Can be prevented by attaching a shaper (for example, +.BR tc-tbf(8) +to these bands to make sure they cannot dominate the link. + +.SH AUTHORS +Alexey N. Kuznetsov, <kuznet@ms2.inr.ac.ru>, J Hadi Salim +<hadi@cyberus.ca>. This manpage maintained by bert hubert <ahu@ds9a.nl> |