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No other man pages do so, hiding netdev is kind of silly and I don't mind having my own address normally visible.
164 lines
6.4 KiB
Groff
164 lines
6.4 KiB
Groff
.TH STAB 8 "31 October 2011" iproute2 Linux
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.
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.SH NAME
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tc\-stab \- Generic size table manipulations
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.
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.SH SYNOPSIS
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.nf
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tc qdisc add ... stab
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.RS 4
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[ \fBmtu\fR BYTES ] [ \fBtsize\fR SLOTS ]
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[ \fBmpu\fR BYTES ] [ \fBoverhead\fR BYTES ]
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[ \fBlinklayer\fR { adsl | atm | ethernet } ] ...
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.RE
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.fi
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.SH OPTIONS
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For the description of BYTES \- please refer to the \fBUNITS\fR
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section of \fBtc\fR(8).
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.IP \fBmtu\fR 4
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.br
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maximum packet size we create size table for, assumed 2048 if not specified explicitly
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.IP \fBtsize\fR
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.br
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required table size, assumed 512 if not specified explicitly
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.IP \fBmpu\fR
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.br
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minimum packet size used in computations
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.IP \fBoverhead\fR
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.br
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per\-packet size overhead (can be negative) used in computations
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.IP \fBlinklayer\fR
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.br
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required linklayer specification.
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.PP
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.
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.SH DESCRIPTION
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.
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Size tables allow manipulation of packet sizes, as seen by the whole scheduler
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framework (of course, the actual packet size remains the same). Adjusted packet
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size is calculated only once \- when a qdisc enqueues the packet. Initial root
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enqueue initializes it to the real packet's size.
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Each qdisc can use a different size table, but the adjusted size is stored in
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an area shared by whole qdisc hierarchy attached to the interface. The effect is
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that if you have such a setup, the last qdisc with a stab in a chain "wins". For
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example, consider HFSC with simple pfifo attached to one of its leaf classes.
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If that pfifo qdisc has stab defined, it will override lengths calculated
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during HFSC's enqueue; and in turn, whenever HFSC tries to dequeue a packet, it
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will use a potentially invalid size in its calculations. Normal setups will
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usually include stab defined only on root qdisc, but further overriding gives
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extra flexibility for less usual setups.
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The initial size table is calculated by \fBtc\fR tool using \fBmtu\fR and
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\fBtsize\fR parameters. The algorithm sets each slot's size to the smallest
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power of 2 value, so the whole \fBmtu\fR is covered by the size table. Neither
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\fBtsize\fR, nor \fBmtu\fR have to be power of 2 value, so the size
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table will usually support more than is required by \fBmtu\fR.
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For example, with \fBmtu\fR\~=\~1500 and \fBtsize\fR\~=\~128, a table with 128
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slots will be created, where slot 0 will correspond to sizes 0\-16, slot 1 to
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17\~\-\~32, \&..., slot 127 to 2033\~\-\~2048. Sizes assigned to each slot
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depend on \fBlinklayer\fR parameter.
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Stab calculation is also safe for an unusual case, when a size assigned to a
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slot would be larger than 2^16\-1 (you will lose the accuracy though).
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During the kernel part of packet size adjustment, \fBoverhead\fR will be added
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to original size, and then slot will be calculated. If the size would cause
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overflow, more than 1 slot will be used to get the final size. This of course
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will affect accuracy, but it's only a guard against unusual situations.
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Currently there are two methods of creating values stored in the size table \-
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ethernet and atm (adsl):
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.IP ethernet 4
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.br
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This is basically 1\-1 mapping, so following our example from above
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(disregarding \fBmpu\fR for a moment) slot 0 would have 8, slot 1 would have 16
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and so on, up to slot 127 with 2048. Note, that \fBmpu\fR\~>\~0 must be
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specified, and slots that would get less than specified by \fBmpu\fR will get
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\fBmpu\fR instead. If you don't specify \fBmpu\fR, the size table will not be
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created at all (it wouldn't make any difference), although any \fBoverhead\fR
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value will be respected during calculations.
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.IP "atm, adsl"
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.br
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ATM linklayer consists of 53 byte cells, where each of them provides 48 bytes
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for payload. Also all the cells must be fully utilized, thus the last one is
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padded if/as necessary.
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When the size table is calculated, adjusted size that fits properly into lowest
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amount of cells is assigned to a slot. For example, a 100 byte long packet
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requires three 48\-byte payloads, so the final size would require 3 ATM cells
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\- 159 bytes.
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For ATM size tables, 16\~bytes sized slots are perfectly enough. The default
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values of \fBmtu\fR and \fBtsize\fR create 4\~bytes sized slots.
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.PP
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.
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.SH "TYPICAL OVERHEADS"
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The following values are typical for different adsl scenarios (based on
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\fB[1]\fR and \fB[2]\fR):
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.nf
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LLC based:
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.RS 4
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PPPoA \- 14 (PPP \- 2, ATM \- 12)
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PPPoE \- 40+ (PPPoE \- 8, ATM \- 18, ethernet 14, possibly FCS \- 4+padding)
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Bridged \- 32 (ATM \- 18, ethernet 14, possibly FCS \- 4+padding)
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IPoA \- 16 (ATM \- 16)
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.RE
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VC Mux based:
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.RS 4
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PPPoA \- 10 (PPP \- 2, ATM \- 8)
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PPPoE \- 32+ (PPPoE \- 8, ATM \- 10, ethernet 14, possibly FCS \- 4+padding)
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Bridged \- 24+ (ATM \- 10, ethernet 14, possibly FCS \- 4+padding)
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IPoA \- 8 (ATM \- 8)
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.RE
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.fi
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There are a few important things regarding the above overheads:
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.
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.IP \(bu 4
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IPoA in LLC case requires SNAP, instead of LLC\-NLPID (see rfc2684) \- this is
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the reason why it actually takes more space than PPPoA.
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.IP \(bu
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In rare cases, FCS might be preserved on protocols that include Ethernet frames
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(Bridged and PPPoE). In such situation, any Ethernet specific padding
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guaranteeing 64 bytes long frame size has to be included as well (see RFC2684).
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In the other words, it also guarantees that any packet you send will take
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minimum 2 atm cells. You should set \fBmpu\fR accordingly for that.
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.IP \(bu
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When the size table is consulted, and you're shaping traffic for the sake of
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another modem/router, an Ethernet header (without padding) will already be added
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to initial packet's length. You should compensate for that by subtracting 14
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from the above overheads in this case. If you're shaping directly on the router
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(for example, with speedtouch usb modem) using ppp daemon, you're using raw ip
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interface without underlying layer2, so nothing will be added.
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For more thorough explanations, please see \fB[1]\fR and \fB[2]\fR.
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.
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.SH "ETHERNET CARDS CONSIDERATIONS"
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.
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It's often forgotten that modern network cards (even cheap ones on desktop
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motherboards) and/or their drivers often support different offloading
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mechanisms. In the context of traffic shaping, 'tso' and 'gso' might cause
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undesirable effects, due to massive TCP segments being considered during
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traffic shaping (including stab calculations). For slow uplink interfaces,
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it's good to use \fBethtool\fR to turn off offloading features.
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.
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.SH "SEE ALSO"
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.
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\fBtc\fR(8), \fBtc\-hfsc\fR(7), \fBtc\-hfsc\fR(8),
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.br
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\fB[1]\fR http://ace\-host.stuart.id.au/russell/files/tc/tc\-atm/
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.br
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\fB[2]\fR http://www.faqs.org/rfcs/rfc2684.html
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Please direct bugreports and patches to: <netdev@vger.kernel.org>
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.
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.SH "AUTHOR"
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.
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Manpage created by Michal Soltys (soltys@ziu.info)
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