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NAME | SYNOPSIS | OPTIONS | DESCRIPTION | TYPICAL OVERHEADS | ETHERNET CARDS CONSIDERATIONS | SEE ALSO | AUTHOR | COLOPHON |
STAB(8) Linux STAB(8)
tc-stab - Generic size table manipulations
tc qdisc add ... stab
[ mtu BYTES ] [ tsize SLOTS ]
[ mpu BYTES ] [ overhead BYTES ]
[ linklayer { adsl | atm | ethernet } ] ...
For the description of BYTES - please refer to the UNITS section of
tc(8).
mtu
maximum packet size we create size table for, assumed 2048 if not
specified explicitly
tsize
required table size, assumed 512 if not specified explicitly
mpu
minimum packet size used in computations
overhead
per-packet size overhead (can be negative) used in computations
linklayer
required linklayer specification.
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 tc tool using mtu and tsize
parameters. The algorithm sets each slot's size to the smallest power
of 2 value, so the whole mtu is covered by the size table. Neither
tsize, nor mtu have to be power of 2 value, so the size table will
usually support more than is required by mtu.
For example, with mtu = 1500 and tsize = 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 linklayer 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, overhead 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):
ethernet
This is basically 1-1 mapping, so following our example from
above (disregarding mpu for a moment) slot 0 would have 8, slot 1
would have 16 and so on, up to slot 127 with 2048. Note, that
mpu > 0 must be specified, and slots that would get less than
specified by mpu will get mpu instead. If you don't specify mpu,
the size table will not be created at all (it wouldn't make any
difference), although any overhead value will be respected during
calculations.
atm, adsl
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 mtu and tsize create 4 bytes sized slots.
The following values are typical for different adsl scenarios (based
on [1] and [2]):
LLC based:
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)
VC Mux based:
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)
There are a few important things regarding the above overheads:
· IPoA in LLC case requires SNAP, instead of LLC-NLPID (see
rfc2684) - this is the reason why it actually takes more space
than PPPoA.
· 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 mpu accordingly for that.
· 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 [1] and [2].
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 ethtool
to turn off offloading features.
tc(8), tc-hfsc(7), tc-hfsc(8),
[1] http://ace-host.stuart.id.au/russell/files/tc/tc-atm/
[2] http://www.faqs.org/rfcs/rfc2684.html
Please direct bugreports and patches to: <netdev@vger.kernel.org>
Manpage created by Michal Soltys (soltys@ziu.info)
This page is part of the iproute2 (utilities for controlling TCP/IP
networking and traffic) project. Information about the project can
be found at
⟨http://www.linuxfoundation.org/collaborate/workgroups/networking/iproute2⟩.
If you have a bug report for this manual page, send it to
netdev@vger.kernel.org, shemminger@osdl.org. This page was obtained
from the project's upstream Git repository
⟨git://git.kernel.org/pub/scm/linux/kernel/git/shemminger/iproute2.git⟩
on 2017-07-05. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-to-
date source for the page, or you have corrections or improvements to
the information in this COLOPHON (which is not part of the original
manual page), send a mail to man-pages@man7.org
iproute2 31 October 2011 STAB(8)
Pages that refer to this page: tc-hfsc(7), tc(8), tc-hfsc(8)