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

       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 computations

       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

   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 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),

   Please direct bugreports and patches to: <>


   Manpage created by Michal Soltys (


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