tc-stab(8) — Linux manual page


STAB(8)                           Linux                          STAB(8)

NAME         top

       tc-stab - Generic size table manipulations

SYNOPSIS         top

       tc qdisc add ... stab
           [ mtu BYTES ] [ tsize SLOTS ]
           [ mpu BYTES ] [ overhead BYTES ]
           [ linklayer { adsl | atm | ethernet } ] ...

OPTIONS         top

       For the description of BYTES - please refer to the UNITS section
       of tc(8).

           maximum packet size we create size table for, assumed 2048 if
           not specified explicitly

           required table size, assumed 512 if not specified explicitly

           minimum packet size used in computations

           per-packet size overhead (can be negative) used in

           required linklayer specification.

DESCRIPTION         top

       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

       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):

           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

           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.

SEE ALSO         top

       tc(8), tc-hfsc(7), tc-hfsc(8),

       Please direct bugreports and patches to: <>

AUTHOR         top

       Manpage created by Michal Soltys (

COLOPHON         top

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iproute2                     31 October 2011                     STAB(8)

Pages that refer to this page: tc-hfsc(7)tc(8)tc-hfsc(8)tc-taprio(8)