STAB(8) Linux STAB(8)
NAME
tc-stab - Generic size table manipulations
SYNOPSIS
tc qdisc add ... stab
[ mtu BYTES ] [ tsize SLOTS ]
[ mpu BYTES ] [ overhead BYTES ]
[ linklayer { adsl | atm | ethernet } ] ...
OPTIONS
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.
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 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 usu-
ally 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 pro-
vides 48 bytes for payload. Also all the cells must be fully uti-
lized, 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.
TYPICAL OVERHEADS
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:
o IPoA in LLC case requires SNAP, instead of LLC-NLPID (see rfc2684)
- this is the reason why it actually takes more space than PPPoA.
o In rare cases, FCS might be preserved on protocols that include
Ethernet frames (Bridged and PPPoE). In such situation, any Ether-
net 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.
o When the size table is consulted, and you're shaping traffic for
the sake of another modem/router, an Ethernet header (without pad-
ding) 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].
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 ethtool to turn off
offloading features.
SEE ALSO
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 AT vger.org>
AUTHOR
Manpage created by Michal Soltys (soltys AT ziu.info)
iproute2 31 October 2011 STAB(8)
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