STAB(8)                 (31 October 2011)                 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).

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

              required table size, assumed 512 if not specified

              minimum packet size used in computations

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

              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

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          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

          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 bytes.

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              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

          +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 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.

          +o   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

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          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),

          Please direct bugreports and patches to:

          Manpage created by Michal Soltys (

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