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NAME | SYNOPSIS | OPTIONS | DESCRIPTION | TYPICAL OVERHEADS | ETHERNET CARDS CONSIDERATIONS | SEE ALSO | AUTHOR | COLOPHON |
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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
⟨https://git.kernel.org/pub/scm/network/iproute2/iproute2.git⟩ on
2025-08-11. (At that time, the date of the most recent commit
that was found in the repository was 2025-08-08.) 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), tc-taprio(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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