Manual: tc-stab
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. Ini‐
tial 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 de‐
fined, 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 parame‐
ter.
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 cal‐
culated. 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 calcu‐
lations.
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.
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:
• 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 sit‐
uation, 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 sub‐
tracting 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:
AUTHOR
Manpage created by Michal Soltys (soltys@ziu.info)
iproute2 31 October 2011 STAB(8)