mirror of
https://git.proxmox.com/git/mirror_iproute2
synced 2025-10-31 12:21:10 +00:00
509dcd43c9
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
|
|
.
|
|
.SH NAME
|
|
tc\-stab \- Generic size table manipulations
|
|
.
|
|
.SH SYNOPSIS
|
|
.nf
|
|
tc qdisc add ... stab
|
|
.RS 4
|
|
[ \fBmtu\fR BYTES ] [ \fBtsize\fR SLOTS ]
|
|
[ \fBmpu\fR BYTES ] [ \fBoverhead\fR BYTES ]
|
|
[ \fBlinklayer\fR { adsl | atm | ethernet } ] ...
|
|
.RE
|
|
.fi
|
|
|
|
.SH OPTIONS
|
|
For the description of BYTES \- please refer to the \fBUNITS\fR
|
|
section of \fBtc\fR(8).
|
|
|
|
.IP \fBmtu\fR 4
|
|
.br
|
|
maximum packet size we create size table for, assumed 2048 if not specified explicitly
|
|
.IP \fBtsize\fR
|
|
.br
|
|
required table size, assumed 512 if not specified explicitly
|
|
.IP \fBmpu\fR
|
|
.br
|
|
minimum packet size used in computations
|
|
.IP \fBoverhead\fR
|
|
.br
|
|
per\-packet size overhead (can be negative) used in computations
|
|
.IP \fBlinklayer\fR
|
|
.br
|
|
required linklayer specification.
|
|
.PP
|
|
.
|
|
.SH DESCRIPTION
|
|
.
|
|
Size tables allow manipulation of packet sizes, as seen by the whole scheduler
|
|
framework (of course, the actual packet size remains the same). Adjusted packet
|
|
size is calculated only once \- when a qdisc enqueues the packet. Initial root
|
|
enqueue initializes it to the real packet's size.
|
|
|
|
Each qdisc can use a different size table, but the adjusted size is stored in
|
|
an area shared by whole qdisc hierarchy attached to the interface. The effect is
|
|
that if you have such a setup, the last qdisc with a stab in a chain "wins". For
|
|
example, consider HFSC with simple pfifo attached to one of its leaf classes.
|
|
If that pfifo qdisc has stab defined, it will override lengths calculated
|
|
during HFSC's enqueue; and in turn, whenever HFSC tries to dequeue a packet, it
|
|
will use a potentially invalid size in its calculations. Normal setups will
|
|
usually include stab defined only on root qdisc, but further overriding gives
|
|
extra flexibility for less usual setups.
|
|
|
|
The initial size table is calculated by \fBtc\fR tool using \fBmtu\fR and
|
|
\fBtsize\fR parameters. The algorithm sets each slot's size to the smallest
|
|
power of 2 value, so the whole \fBmtu\fR is covered by the size table. Neither
|
|
\fBtsize\fR, nor \fBmtu\fR have to be power of 2 value, so the size
|
|
table will usually support more than is required by \fBmtu\fR.
|
|
|
|
For example, with \fBmtu\fR\~=\~1500 and \fBtsize\fR\~=\~128, a table with 128
|
|
slots will be created, where slot 0 will correspond to sizes 0\-16, slot 1 to
|
|
17\~\-\~32, \&..., slot 127 to 2033\~\-\~2048. Sizes assigned to each slot
|
|
depend on \fBlinklayer\fR parameter.
|
|
|
|
Stab calculation is also safe for an unusual case, when a size assigned to a
|
|
slot would be larger than 2^16\-1 (you will lose the accuracy though).
|
|
|
|
During the kernel part of packet size adjustment, \fBoverhead\fR will be added
|
|
to original size, and then slot will be calculated. If the size would cause
|
|
overflow, more than 1 slot will be used to get the final size. This of course
|
|
will affect accuracy, but it's only a guard against unusual situations.
|
|
|
|
Currently there are two methods of creating values stored in the size table \-
|
|
ethernet and atm (adsl):
|
|
|
|
.IP ethernet 4
|
|
.br
|
|
This is basically 1\-1 mapping, so following our example from above
|
|
(disregarding \fBmpu\fR for a moment) slot 0 would have 8, slot 1 would have 16
|
|
and so on, up to slot 127 with 2048. Note, that \fBmpu\fR\~>\~0 must be
|
|
specified, and slots that would get less than specified by \fBmpu\fR will get
|
|
\fBmpu\fR instead. If you don't specify \fBmpu\fR, the size table will not be
|
|
created at all (it wouldn't make any difference), although any \fBoverhead\fR
|
|
value will be respected during calculations.
|
|
.IP "atm, adsl"
|
|
.br
|
|
ATM linklayer consists of 53 byte cells, where each of them provides 48 bytes
|
|
for payload. Also all the cells must be fully utilized, thus the last one is
|
|
padded if/as necessary.
|
|
|
|
When the size table is calculated, adjusted size that fits properly into lowest
|
|
amount of cells is assigned to a slot. For example, a 100 byte long packet
|
|
requires three 48\-byte payloads, so the final size would require 3 ATM cells
|
|
\- 159 bytes.
|
|
|
|
For ATM size tables, 16\~bytes sized slots are perfectly enough. The default
|
|
values of \fBmtu\fR and \fBtsize\fR create 4\~bytes sized slots.
|
|
.PP
|
|
.
|
|
.SH "TYPICAL OVERHEADS"
|
|
The following values are typical for different adsl scenarios (based on
|
|
\fB[1]\fR and \fB[2]\fR):
|
|
|
|
.nf
|
|
LLC based:
|
|
.RS 4
|
|
PPPoA \- 14 (PPP \- 2, ATM \- 12)
|
|
PPPoE \- 40+ (PPPoE \- 8, ATM \- 18, ethernet 14, possibly FCS \- 4+padding)
|
|
Bridged \- 32 (ATM \- 18, ethernet 14, possibly FCS \- 4+padding)
|
|
IPoA \- 16 (ATM \- 16)
|
|
.RE
|
|
|
|
VC Mux based:
|
|
.RS 4
|
|
PPPoA \- 10 (PPP \- 2, ATM \- 8)
|
|
PPPoE \- 32+ (PPPoE \- 8, ATM \- 10, ethernet 14, possibly FCS \- 4+padding)
|
|
Bridged \- 24+ (ATM \- 10, ethernet 14, possibly FCS \- 4+padding)
|
|
IPoA \- 8 (ATM \- 8)
|
|
.RE
|
|
.fi
|
|
There are a few important things regarding the above overheads:
|
|
.
|
|
.IP \(bu 4
|
|
IPoA in LLC case requires SNAP, instead of LLC\-NLPID (see rfc2684) \- this is
|
|
the reason why it actually takes more space than PPPoA.
|
|
.IP \(bu
|
|
In rare cases, FCS might be preserved on protocols that include Ethernet frames
|
|
(Bridged and PPPoE). In such situation, any Ethernet specific padding
|
|
guaranteeing 64 bytes long frame size has to be included as well (see RFC2684).
|
|
In the other words, it also guarantees that any packet you send will take
|
|
minimum 2 atm cells. You should set \fBmpu\fR accordingly for that.
|
|
.IP \(bu
|
|
When the size table is consulted, and you're shaping traffic for the sake of
|
|
another modem/router, an Ethernet header (without padding) will already be added
|
|
to initial packet's length. You should compensate for that by subtracting 14
|
|
from the above overheads in this case. If you're shaping directly on the router
|
|
(for example, with speedtouch usb modem) using ppp daemon, you're using raw ip
|
|
interface without underlying layer2, so nothing will be added.
|
|
|
|
For more thorough explanations, please see \fB[1]\fR and \fB[2]\fR.
|
|
.
|
|
.SH "ETHERNET CARDS CONSIDERATIONS"
|
|
.
|
|
It's often forgotten that modern network cards (even cheap ones on desktop
|
|
motherboards) and/or their drivers often support different offloading
|
|
mechanisms. In the context of traffic shaping, 'tso' and 'gso' might cause
|
|
undesirable effects, due to massive TCP segments being considered during
|
|
traffic shaping (including stab calculations). For slow uplink interfaces,
|
|
it's good to use \fBethtool\fR to turn off offloading features.
|
|
.
|
|
.SH "SEE ALSO"
|
|
.
|
|
\fBtc\fR(8), \fBtc\-hfsc\fR(7), \fBtc\-hfsc\fR(8),
|
|
.br
|
|
\fB[1]\fR http://ace\-host.stuart.id.au/russell/files/tc/tc\-atm/
|
|
.br
|
|
\fB[2]\fR http://www.faqs.org/rfcs/rfc2684.html
|
|
|
|
Please direct bugreports and patches to: <netdev@vger.kernel.org>
|
|
.
|
|
.SH "AUTHOR"
|
|
.
|
|
Manpage created by Michal Soltys (soltys@ziu.info)
|